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Solid-State-Electrolyte Reactor: New Opportunity for Electrifying Manufacture
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  • Chunxiao Liu
    Chunxiao Liu
    School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China
    More by Chunxiao Liu
  • Yuan Ji
    Yuan Ji
    School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China
    More by Yuan Ji
  • Tingting Zheng*
    Tingting Zheng
    School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China
    *E-mail: [email protected]
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  • Chuan Xia*
    Chuan Xia
    School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China
    *E-mail: [email protected]
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    Orcidhttps://orcid.org/0000-0003-4526-159X
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JACS Au

Cite this: JACS Au 2025, 5, 2, 521–535
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https://doi.org/10.1021/jacsau.4c01183
Published February 6, 2025

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Abstract

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Electrocatalysis, which leverages renewable electricity, has emerged as a cornerstone technology in the transition toward sustainable energy and chemical production. However, traditional electrocatalytic systems often produce mixed, impure products, necessitating costly purification. Solid-state electrolyte (SSE) reactors represent a transformative advancement by enabling the direct production of high-purity chemicals, significantly reducing purification costs and energy consumption. The versatility of SSE reactors extends to applications such as CO2 capture and tandem reactions, aligning with the green and decentralized production paradigm. This Perspective provides a comprehensive overview of SSE reactors, discussing their principles, design innovations, and applications in producing pure chemicals─such as liquid carbon fuels, hydrogen peroxide, and ammonia─directly from CO2 and other sources. We further explore the potential of SSE reactors in applications such as CO2 capture and tandem reactions, highlighting their compatibility with versatile production systems. Finally, we outline future research directions for SSE reactors, underscoring their role in advancing sustainable chemical manufacturing.

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1. Introduction

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Chemical manufacturing has long been the backbone of modern civilization, underpinning critical industries such as agriculture, healthcare, energy, and materials. (1−4) However, this sector remains one of the most energy-intensive and carbon-emitting, heavily reliant on fossil fuels as both feedstocks and energy sources. (5,6) This has spurred the emergence of electrocatalysis, a revolutionary platform that bridges the gap between renewable energy and chemical manufacturing. (7) By leveraging electricity─especially from solar, wind, and hydropower─electrocatalysis enables the conversion of simple, abundant molecules into high-value chemicals and fuels, such as hydrogen, ammonia, formic acid, and hydrogen peroxide. (8−11) These products are essential as both energy carriers and feedstocks for a wide range of industries. Importantly, the declining costs of renewable energy have made the electrochemical route increasingly competitive, with electricity prices projected to fall below $0.03 per kWh in the near future. (12) This economic shift underscores the potential of electrocatalysis to replace fossil-derived processes, aligning with global efforts to achieve net-zero emissions. In an increasingly decentralized and intermittent energy landscape, electrocatalysis plays a pivotal role by transforming variable renewable energy inputs into stable, storable chemical outputs, which provides a scalable and sustainable solution for industrial production. (13,14)
Despite their vast potential, traditional electrocatalytic processes grapple with a fundamental challenge: the generation of impure product streams. In electrocatalysis, complex reaction pathways often lead to the simultaneous formation of multiple products, complicating the separation and purification processes. For example, in the carbon dioxide electroreduction reaction (CO2RR), diverse products, such as formic acid, carbon monoxide, ethylene, and ethanol, are generated depending on the catalyst and operating conditions. (15,16) This product diversity, while showcasing the versatility of electrocatalysis, necessitates energy-intensive and costly downstream separation methods. The reliance on liquid-phase electrolytes in conventional systems further compounds this issue. These electrolytes, typically aqueous salt solutions such as KHCO3 or KOH, are indispensable for maintaining ionic conductivity but inevitably introduce ionic impurities into the liquid products. As a result, the liquid chemicals produced in these systems are often contaminated with salts, requiring additional purification steps such as distillation, ion exchange, or electrodialysis. (17,18) These processes can account for a substantial fraction of the total production cost, posing a major barrier to the widespread industrial adoption of electrocatalysis.
In addressing these limitations, while the optimization of electrocatalysts is crucial, innovation in reactor design is equally imperative. Recent advances have given rise to the development of solid-state electrolyte (SSE) reactors, a breakthrough in electrochemical technology that fundamentally redefines the boundaries of product purity and process efficiency. (10,19,20) By replacing traditional liquid electrolytes with solid ionic conductors, SSE reactors decouple ion transport from product collection. This innovative architecture prevents the contamination of liquid products with electrolyte salts, enabling the direct synthesis of high-purity chemicals. By eliminating the need for postreaction purification, SSE reactors drastically reduce both operational costs and energy consumption, positioning electrocatalysis as a competitive alternative to conventional chemical manufacturing routes.
The versatility of SSE reactors is further demonstrated by their applicability across a wide range of electrocatalytic reactions. (9,11,21,22) Beyond CO2RR, SSE reactors have been successfully employed in the production of pure hydrogen peroxide via the oxygen reduction reaction (ORR), offering a sustainable and efficient alternative to conventional hydrogen peroxide (H2O2) synthesis. (11,21) Additionally, they have shown promise in electrocatalytic ammonia synthesis through nitrate reduction, addressing both environmental and agricultural needs. (9,22) Recent advancements have also expanded the scope of SSE technology to CO2 capture and tandem reactions, where intermediate products from one reaction seamlessly feed into subsequent processes without the need for separation. (23−25) This capability holds immense potential for the integrated production of complex chemicals, reducing energy inputs and streamlining the overall process.
Beyond economic and technical advantages, the ability of SSE reactors to produce pure products unlocks new paradigms in chemical production, particularly in decentralized manufacturing. This capability challenges the conventional model of large-scale, centralized production facilities by enabling localized, on-demand synthesis of critical chemicals. Such flexibility is invaluable in remote or resource-constrained regions, where centralized industrial infrastructure may be inaccessible or insufficient. The COVID-19 pandemic underscored the importance of decentralized production, as supply chain disruptions highlighted vulnerabilities in the availability of essential chemicals such as disinfectants and medical oxygen. In these scenarios, onsite electrocatalytic production of pure products such as hydrogen peroxide could provide rapid, localized solutions, enhancing resilience and self-sufficiency. (26,27)
In this perspective, we delve into the principles, advancements, and diverse applications of SSE reactors. We examine their role in overcoming the limitations of traditional electrocatalytic systems, particularly in achieving high-purity product synthesis. Recent breakthroughs in SSE reactor design and their extended application in CO2 capture and integration with downstream reactions will be highlighted, highlighting their potential to revolutionize chemical manufacturing. Finally, we discuss the current challenges and broader implications of SSE technology in enabling decentralized and sustainable production models, offering a transformative vision for the future of the chemical industry.

2. Principle and Design of a Solid-State Electrolyte Reactor

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The evolution of electrochemical reactor technologies has led to the development of several configurations, including single-chamber membrane-free reactors, H-cells, flow cells, and membrane electrode assemblies (MEAs), each tailored to optimize the reaction environment and efficiency for specific applications. (28) However, while these designs offer distinct advantages, they also have inherent limitations, particularly when employed in gas-involved reactions (Table 1). For example, single-chamber reactors can facilitate synergistic interactions between cathodic and anodic processes, (29,30) but have limited applications in aqueous electrolysis. Therefore, we will focus primarily on other types of electrolyzer configurations that are more relevant to the scope of discussion.
Table 1. Merits and Limitations of Different Electrolyzer Configurations
Electrolyzer typesMeritsLimitations
Single-chamber membrane-free reactorsimple design; avoid the use of ion exchange membrane and gaskets; allow the synergy of cathodic and anodic processesmixed catholyte and anolyte; product consumption or unwanted side reactions occurring at the opposite electrode
H-cellavoid the interference between cathodic and anodic compartments; convenient for mechanism studylimited electrode surface area; large interelectrode distance; accumulation of liquid products; restricted mass transport of gaseous feedstock
Gas diffusion flow cellimprove the mass transport of gaseous feedstock by facilitating a triple-phase interface; avoid the accumulation of liquid products by flowing the electrolytepoor stability of the triple-phase interface because of water flooding and salt deposition
Membrane electrode assemblywith catalyst layers integrated with the ion exchange membrane; minimize the distance for ion transport and reduce the ohmic losseslower efficiency toward liquid products due to crossover; severer salt deposition
Solid-state electrolyte reactorwith a solid-electrolyte layer for the collection of pure products; avoid the energy-intensive postreaction purification processes; prevent the product crossover; more stable reaction interface, avoid the water floodinglarger ohmic loss and energy consumption; suboptimal performance under industrially relevant conditions
H-cells are among the simplest and most widely used reactor designs in laboratory research (Figure 1a). They offer a straightforward setup for studying fundamental reaction mechanisms and catalyst performance within the kinetically controlled region. For example, the Jaramillo group investigated the CO2RR pathways over copper across a range of potentials in the H-cell, revealing the formation of multicarbon products (including aldehydes, ketones, alcohols, and carboxylic acids) involving enol-like surface intermediates as a possible pathway. (31) However, H-cells are significantly limited by the low solubility of gaseous feedstock (e.g., CO2) in aqueous electrolytes, which is approximately 33 mM for CO2 under ambient conditions (1 atm, 25 °C). (32) This solubility constraint, combined with the limited diffusion coefficients of CO2, leads to insufficient reactant delivery to the catalyst surface, resulting in low reaction rates, diminished product yields, and poor energy efficiency. (33) Attempts to increase CO2 solubility through increased pressure or decreased temperature have yielded only marginal improvements and have failed to address the underlying mass transport bottleneck. (34,35) For example, the Dai group reported a Cu(OH)2-derived Cu/CuOx catalyst achieving 87% Faradaic efficiency for CO2-to-acetate in borate-containing electrolytes under 58 atm CO2(g). However, the current densities are still not promising (<100 mA cm–2) under constrained mass transfer conditions. (36) Moreover, the limited electrode area and large interelectrode distance would lead to increased ohmic polarization, thus restricting the current density in the H-cell. (37) Moreover, the accumulation of liquid products also accelerates product crossover (38) or affects the membrane. (39)

Figure 1

Figure 1. Configurations of the electrolyzer design commonly utilized in CO2 electrolysis. (a) H-cell. (b) Gas diffusion flow cell. (c) Membrane electrode assembly. (d) Solid-state electrolyte reactor. To obtain pure liquid products, ions/molecules electrochemically generated from the cathode, such as HCOO–, move across the AEM toward the middle solid electrolyte layer, and protons generated from anodic reactions (water or hydrogen oxidation) move across the CEM into the middle chamber to compensate for the charge. The target molecules (such as HCOOH) are thus formed within the middle layer via ionic recombination, and carried out via the deionized water stream that flows through this porous layer.

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To overcome these limitations, flow cells incorporating gas diffusion electrodes (GDEs) have been introduced to facilitate a triple-phase interface where gaseous CO2, liquid electrolytes, and solid catalysts converge (Figure 1b). This design significantly improves mass transport, allowing for higher reaction kinetics and increased current densities. (40,41) Xing et al. reported an order of magnitude increase in the current density over that of a Cu catalyst from ∼6 mA cm–2 in a H-cell to ∼250 mA cm–2 in a flow cell under the same potential, indicating the vital role of mass transport. (42) However, flow cells are prone to performance degradation because of “water flooding” at high current densities. This occurs when the liquid electrolyte infiltrates the porous GDE, disrupting the delicate balance of the triple-phase interface and reducing the available catalytic surface area. (43) As a result, long-term stability and operational efficiency are compromised. For instance, the Jiao group reported a slight decrease in the total C2+ Faradaic efficiency after 30 min of CO electrolysis at 500 mA cm–2 in a flow cell owing to flooding caused by the condensation of water vapor. (44)
Membrane electrode assemblies (MEAs) represent a further advancement, integrating the catalyst layer directly with an ion exchange membrane (Figure 1c). This design minimizes the distance for ion transport, reduces ohmic losses, and enhances the overall reaction efficiency, which is more convenient for scaling production. (45) For example, Zheng et al. realized practical 3.34-L-h–1-CO production in a 10 × 10 cm2 MEA at more than 8 A with a low cell voltage of only 2.8 V. (46) Despite these benefits, MEAs face critical challenges, particularly with respect to liquid product handling. Under the influence of concentration gradients and electric fields, liquid products such as formic acid and acetic acid tend to permeate through the ion exchange membrane to the anode, where they are oxidized. This product crossover results in substantial efficiency losses and undermines the benefits offered by the MEA configuration. (47) Additionally, ion transport limitations across the membrane can result in salt deposition in the cathodic GDE and flow channels, further exacerbating system performance degradation. (48) Endrődi et al. observed that although lowering the anolyte concentration could improve stability, the crossover and consequent precipitation of alkali metal cations still occur, leading to performance degradation. (49)
In response to these challenges, SSE reactors have emerged as transformative innovations, addressing key limitations of conventional reactor designs. SSE reactors are specifically engineered to prevent product crossover and enable the direct synthesis of pure liquid products that are free from electrolyte contamination. Their advanced architecture comprises a structured assembly of catalyst-coated gas diffusion layers, ion-selective membranes, and a crucial middle layer─the porous solid electrolyte (Figure 1d). This middle layer serves dual functions: as an ionic conductor, facilitating the selective transport of ions such as protons (H+) or formate ions (HCOO–), and as a product separator, ensuring that liquid products remain isolated from the bulk electrolyte and do not migrate to the anode compartment. This design not only prevents product loss but also allows the generation of pure, concentrated liquid products. The choice of materials for the porous solid electrolyte is critical to the performance of SSE reactors.
Based on the working mechanism of the SSE reactor, the adopted solid electrolyte needs to satisfy the following three properties: high ionic conductivity for efficient ion transfer, proper geometric factors (such as surface area and porosity), and sufficient chemical stability. Generally, polymer-based ion conductors, such as sulfonic acid-functionalized styrene-divinylbenzene copolymers, are commonly employed because of their high ionic conductivity and stability under mild operating conditions. Moreover, the current carrier of polymer ion conductors also changes when modified with different functional groups on the surface. For example, sulfonic acid groups lead to efficient proton transportation, whereas quaternary amino groups lead to migration of anions. Xia et al. compared the CO2RR performance when a H+-conductor and a HCOO–-conductor were used in the SSE reactor. As a result, the H+-conductor SSE achieved a distinctly higher current density (100 mA cm–2 vs. 50 mA cm–2) at a lower cell voltage (3.27 V vs. 3.47 V) than did the HCOO–-conductor owing to the higher electromobility of the protons. Furthermore, they also analyzed the electrochemical impedance spectra of SSE powders of different sizes. These authors reported that the 50-μm-SSE exhibited greater ionic conductivity (0.018 S cm–1) and lower charge transfer (12.5 Ω cm2) resistivity than did the 300-μm-SSE (0.0064 S cm–1 and 24.5 Ω cm2) because of the improved surface area and porosity. For more demanding environments, such as high temperatures or chemically aggressive settings, inorganic ion conductors such as cesium-substituted heteropolyacids (CsxH3–xPW12O40) offer superior thermal and chemical stability. (50)
Compared with traditional systems, SSE reactors demonstrate significant advantages. By eliminating the crossover of liquid products and achieving high product purity, they reduce the need for energy-intensive postreaction purification processes. This improves the overall sustainability and economic viability of the system. The inherent stability of the SSE reactor’s reaction interface prevents flooding, allowing for consistent performance even at industrially relevant current densities. For example, in hydrogen peroxide production via the two-electron oxygen reduction reaction (2e– ORR), SSE reactors achieve Faradaic efficiencies exceeding 90% at current densities up to 400 mA cm–2, with stable operation over extended periods. (21) However, SSE reactors are not without challenges. The assembly of these systems is more complex than traditional designs are, and the increased internal resistance due to the SSE layer can limit energy efficiency. Scaling SSE reactors for industrial applications remains a significant hurdle, requiring further advances in material science and reactor engineering to optimize performance and reduce costs. (18)
Despite these challenges, the unique advantages of SSE reactors make them highly promising platforms for a wide range of electrocatalytic applications. Their ability to produce high-purity liquid products, prevent product loss, and maintain stable operation under demanding conditions positions them as a key technology in the drive toward sustainable chemical manufacturing. As research continues to refine these reactors and expand their compatibility with diverse reaction systems, their role in sustainable chemical manufacturing is expected to grow, driving innovations in areas ranging from CO2 reduction to synthetic fuel and chemical production.

3. Applications in Pure Chemical Production

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3.1. Liquid Carbon Fuels from the CO2RR

Facing the challenge of achieving a carbon-neutral energy cycle, the electrochemical CO2RR has emerged as a transformative technology, capable of converting CO2 into valuable fuels and chemicals using renewable electricity. (51,52)
Liquid fuels such as formic acid, ethanol, and acetic acid are particularly promising products of the CO2RR because of their high energy densities, ease of transport, and compatibility with existing infrastructure. These fuels can serve as both energy carriers and feedstocks for various industrial processes, contributing to a circular carbon economy. (52) Despite these advantages, the production of liquid fuels from traditional CO2 electrolysis faces significant challenges. The main factor is the purity of the liquid fuel, which is often mixed with ionic impurities from the aqueous electrolyte.
Technoeconomic analysis (TEA) provides a comprehensive framework for assessing the economic feasibility of CO2RR technologies. On the basis of an existing calculation model for evaluating the cost of the CO2RR process, we investigated the distribution of the operating cost of common CO2RR-derived liquid products under different electricity unit costs (Figure 2). (17) The operating cost of CO2RR-derived liquid fuels is influenced by three primary factors: electricity input, separation costs, other cost containing materials and system maintenance. Among these, electricity expenses represent a significant portion of the overall operation cost. For simple products such as formic acid, the operating cost can already be competitive with market prices (∼0.73 USD kg–1) under optimal conditions. For more complex products such as ethanol or acetic acid, which require multiple electron transfers and exhibit lower Faradaic efficiencies, electricity costs dominate the overall expenses.

Figure 2

Figure 2. Technoeconomic analysis of the operating cost of the production of common liquid products (formic acid, methanol, ethanol, and n-propanol) in the CO2RR under different unit costs of electricity (0.03, 0.02, and 0.01 USD kWh–1, respectively).

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However, the separation cost incurred during downstream purification should not be ignored. In conventional CO2 electrolysis systems, liquid products are typically mixed with ions such as K+ or Na+ from the electrolyte. Separating these impurities to achieve commercial-grade purity levels involves energy-intensive processes that significantly increase the final cost of the product. For example, the purification of formic acid requires multiple steps, including ion exchange and distillation, to remove residual salts and concentrate the product. These steps can increase the overall cost by 50% or more, undermining the economic benefits of electrochemical production (Figure 2). (18)
As above, TEA highlights that the costs associated with downstream purification significantly impair the economic viability of CO2RR products. While the falling costs of renewable electricity offer a favorable outlook for CO2 electrolysis, the economic viability of this technology depends heavily on the purity and concentration of the resulting products. The global market for liquid fuels and chemicals is vast, with formic acid alone resulting in an annual demand of over 800,000 t. (53) High-purity, high-concentration products are critical for tapping into this market, ensuring that CO2RR technologies can scale to meet industrial demand without sacrificing competitiveness. In this context, solid electrolyte reactors represent a key advancement in this direction, offering a viable route to produce high-value liquid fuels directly from CO2.
In the initial study, Xia et al. demonstrated the feasibility of continuous pure liquid carbon fuel production from CO2 conversion using a solid electrolyte reactor (Figure 3a). (10) By employing a solid ion conductor layer, the system decouples the functions of ion transport and product collection, enabling the generation of concentrated formic acid solutions. This design achieved an impressive 12 M concentration of formic acid and demonstrated operational stability for over 100 h with minimal degradation. The authors compared several types of solid electrolytes with different current carriers (H+ and HCOO–), sizes (50 and 300 μm), and compositions (functionalized polymers and inorganic CsxH3–xPW12O40). The use of a 50-μm-styrene-divinylbenzenesulfonated copolymer as a proton conductor was demonstrated to be the best option due to its high porosity and fast ion conduction. These parameters are significant for achieving better HCOOH selectivity, fewer hydrogen evolution side reactions, lower cell voltages, and higher energy efficiency. However, owing to the limited ion conduction in solid electrolytes and the contact resistance between the solid electrolyte and the ion exchange membrane, the cell voltage was still higher than that of general MEA reactors. The authors further improved the energy efficiency by replacing the oxygen evolution anodic reaction with hydrogen oxidation, largely reducing the cell voltage below 2 V. Building on this foundation, further efforts have refined solid electrolyte reactors for higher product concentrations and efficiencies. The incorporation of an inert gas carrier system not only facilitated the efficient removal of formic acid vapors but also allowed their flexible condensation into concentrated liquid fuels. Coupled with a high activity grain boundary-enriched bismuth catalyst, this SSE reactor demonstrated ultrahigh concentrations of pure formic acid solutions (up to nearly 100 wt %) condensed from generated vapors (Figure 3b). (19) In addition, further engineering over bismuth catalysts to promote the efficiency of pure formic acid production has been fully researched. (54−57) However, the larger overpotential of the main group of metal catalysts in the CO2RR still limits the enhancement of performance.

Figure 3

Figure 3. SSE reactors utilized for liquid carbon fuel production in the CO2RR. (a) Schematic illustration of the CO2 reduction cell with a solid electrolyte. (b) Dependence of formic acid product concentration on the N2 gas flow rate at a fixed overall current density of 200 mA cm–2. Reproduced from ref (19). Available under a CC-BY 4.0 license. Copyright 2020 The Authors. (c) FEs of all the products under different cell currents, along with the corresponding HCOOH partial current density over Pb1Cu SAAs in an SSE reactor. Reproduced with permission from ref (20). Copyright 2021 Springer Nature. (d) Long-term operation of the SSE reactor over Pb1Cu SAAs for pure HCOOH solution production at −3.45 V. Reproduced with permission from ref (20). Copyright 2021 Springer Nature.

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Further advances were made with the introduction of a single-atom alloy catalyst designed to increase the C1 selectivity and efficiency in the SSE reactor. A Pb single-atom alloyed Cu catalyst (Pb1Cu) was developed to optimize the CO2-to-formate conversion pathway. (20) The Pb atoms effectively modulated the electronic structure of the Cu matrix, favoring the formation of HCOO* intermediates and suppressing competing reaction pathways, such as C–C coupling and hydrogen evolution. This catalyst achieved near-unity Faradaic efficiency toward formic acid and high current densities exceeding 1 A cm–2. When integrated into an SSE reactor, the system continuously produced pure formic acid solutions (0.1 M, 8 L in total) for 180 h with minimal performance decay (Figure 3c,d).
Expanding beyond C1 products, this technology was also extended to the electrochemical reduction of carbon monoxide (CO) into C2+ products, notably acetic acid. By optimizing the edge to (100) surface ratio of Cu nanocube catalysts by controlling their size, researchers achieved an unprecedented acetic acid Faradaic efficiency of 43%, with relative product purities exceeding 98 wt %. (58) The reactor continuously produced pure acetic acid solutions with concentrations up to 2 wt % (0.33 M) and an operational stability exceeding 150 h. This outcome demonstrates the ability of SSE systems to produce a diverse range of liquid fuels directly from CO2 or CO, positioning them as versatile platforms for sustainable chemical production.
The development of SSE reactors represents a transformative milestone in the CO2RR, enabling the direct production of high-purity, high-concentration liquid carbon fuels. With ongoing improvements in reactor design and catalyst efficiency, the pathway to commercial deployment appears increasingly viable.

3.2. Hydrogen Peroxide from the 2e– ORR

The current production of H2O2, a widely used and green oxidant, relies on energy- and resource-intensive anthraquinone processes. (59,60) The electrochemical synthesis of H2O2 via two-electron oxygen reduction (2e– ORR) provides a promising energy-efficient and low-waste alternative. (61) However, both traditional and electrocatalytic methods typically yield H2O2 at low concentrations, requiring additional concentration and purification steps to avoid self-decomposition during storage and transportation, raising both economic and safety concerns. Therefore, the development of a scalable, safe, and environmentally friendly method for producing high-purity H2O2 could revolutionize its industrial applications and supply chain.
The method based on SSE reactor represents a significant departure from conventional H2O2 production. Traditional processes require mixing H2 and O2 gases under high pressure─a risky setup due to the flammability of H2 and the reactivity of both gases. To mitigate these risks, H2 is often diluted with inert gases, which limits the achievable H2O2 concentration. Additionally, the reactions are usually conducted in organic solvents such as methanol, adding postreaction purification steps to yield usable aqueous H2O2. (62) In contrast, Xia et al. proposed an electrosynthesis method that innovatively splits the reaction into two separate half-reactions, eliminating the need for high-pressure mixing and hazardous solvents. (11) At the anode, H2 is oxidized to release protons (H+), whereas at the cathode, O2 undergoes a 2e– ORR to generate hydroperoxide ions (HO2–). These ions then combine in an SSE layer to form H2O2, which dissolves directly into flowing deionized (DI) water, producing a pure, aqueous H2O2 solution without further purification (Figure 4a).

Figure 4

Figure 4. SSE reactors utilized for pure H2O2 production via the 2e–-ORR. (a) Schematic illustration of the O2 reduction cell with a solid electrolyte. (b) Dependence of the H2O2 concentration (up to ∼20 wt %) on the DI water flow rate at a constant overall current of 8 A in an SSE reactor. Reproduced with permission from ref (11). Copyright 2019 The American Association for the Advancement of Science. (c) Schematic illustration of the 2e–-ORR SSE reactor with a double-PEM configuration. (d) The I–V curve and corresponding FEs for the production of H2O2 using the SSE reactor with a double-PEM configuration through the flow of 0.03 M Na2SO4 in the middle chamber. Reproduced from ref (21). Available under a CC-BY 4.0 license. Copyright 2022 The Authors.

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In this system, carbon black was chosen for its high surface area, which enhances the catalytic activity and facilitates gas diffusion. To further improve H2O2 selectivity, Xia et al. treated carbon black with nitric acid to introduce oxygen-containing functional groups, which help direct the reaction toward H2O2 generation rather than complete reduction of O2 to water, achieving selectivity levels above 90%. (11) With this optimized reactor and catalyst design, the SSE reactor delivered sustained performance over 100 h of continuous operation, achieving high-purity H2O2 concentrations of up to 20% by weight (Figure 4b). In this work, styrene-divinylbenzene copolymer microspheres functionalized with sulfonic acid groups were utilized as a proton-conducting solid electrolyte. The inorganic SSE (CsxH3–xPW12O40) and HO2–-conducting SSE were also tested. However, the inorganic CsxH3–xPW12O40 proton conductor-equipped cell exhibited a relatively high voltage at the same current density, perhaps due to its poor ion conductivity. Moreover, the anion-conducting SSE-equipped reactor presented a relatively low H2O2 FE owing to the self-decomposition of the product in a highly local alkaline environment in the HO2–-conducting SSE. Notably, in the long term stability test, a reinforced AEM (AMI-7001, Membranes International Inc.) was used for its robust mechanical properties, resulting in better interfacial contact between the SSE and AEM. Moreover, the chemical stability of polymer-based ion conductors and membranes in the oxidative environment of the 2e– ORR should also be considered.
Despite progress in solid electrolyte reactors for pure H2O2 production, challenges still remain in optimizing the catalyst and operating environment. In systems with anion exchange membranes (AEMs), catalysts struggle in alkaline conditions where H2O2 easily deprotonates (pKa > 11) and degrades, reducing stability. (21) Additionally, AEMs are typically less durable than proton exchange membranes (PEMs), such as Nafion, especially when exposed to air. Acidic H2O2 solutions, which are more versatile and in demand because of their stronger oxidative properties, motivate research on efficient H2O2 generation in acidic media. While noble metal catalysts (e.g., Pt- and Pd-based) are effective and stable in acidic environments, their high cost and toxicity limit their scalability. In acidic media, carbon-based catalysts offer a nontoxic alternative but generally require high overpotentials (>300 mV) owing to slow ORR kinetics. (21) The development of efficient and stable carbon catalysts for H2O2 production in acidic environments could unlock practical applications. (63)
To address this issue, Zhang et al. first explored whether alkali metal cations could migrate across PEMs to regulate the catalyst/membrane interface, enhancing H2O2 selectivity. (21) Initial tests using a PEM-MEA setup with a carbon black and H2SO4 anolyte showed limited H2O2 selectivity, as proton flux interfered with cation migration. Using Na2SO4 as an anolyte was also ineffective because of proton transport from the anode to the cathode. In light of this, Zhang et al. developed a new SSE reactor with two PEMs (Nafion-117) to harness cation effects for improved H2O2 production (Figure 4c). The SSE layer between the cathode and anode facilitated ion transfer and maintained low resistance. Oxygen and water flowed continuously at the cathode to drive the 2e– ORR, whereas the anode oxidized water. A dilute cation solution flows through the SSE layer, enhancing H2O2 selectivity by optimizing the local catalyst/PEM interfacial environment. Continuous H2O2 production was achieved with high Faradaic efficiency (∼90%) and stability (>500 h) when only 0.03 M Na2SO4 was used as the cation source, demonstrating a robust approach to sustainable H2O2 synthesis (Figure 4d).
To enable a continuous supply of cation solution in the SSE layer, Zhang et al. further developed a closed-loop system where the cation solution of the SSE layer circulates through the cathode and returns, allowing H2O2 accumulation. In this configuration, the cation mixture mixes with oxygen, flows to the cathode for H2O2 production, and cycles back to the solid electrolyte layer. This design enables cation reuse, eliminates constant resupply, and neutralizes OH– ions at the cathode with SE layer protons, maintaining stability. This setup achieved a steady H2O2 concentration, reaching ∼5,000 ppm in 13 h, and operated stably for over 200 h, producing 3.7 L of 5,000 ppm of H2O2. (21) The closed-loop solid electrolyte design exemplifies a practical approach to cation tuning, presenting an efficient, scalable method for H2O2 production in acidic media.
The design of the SSE reactor for the 2e– ORR promises to bring H2O2 synthesis closer to end users, potentially eliminating the need for bulk transportation and allowing localized production in facilities such as water treatment plants, hospitals, or industrial sites. By enabling safe, on-demand H2O2 generation, SSE technology is promising for enhancing both environmental sustainability and economic efficiency, setting a new standard for decentralized H2O2 production.

3.3. Ammonia from Nitrate Reduction

Ammonia (NH3), a vital chemical with wide applications, is traditionally produced via the Haber-Bosch process. This process is energy-intensive with high carbon emissions, spurring interest in sustainable alternatives that could leverage renewable energy. (9) The electrochemical nitrate reduction reaction (NO3RR) offers a promising pathway for the mild conversion of nitrate (NO3–) to ammonia while managing waste by targeting nitrate contaminants in industrial and agricultural wastewater. (64) However, the NO3RR typically requires high-concentration supporting electrolytes to achieve efficiency and selectivity, which are costly and unsustainable for large-scale use, especially in low-salinity wastewater. Achieving efficient NO3RR in low-electrolyte environments remains a critical challenge because of competition from the HER, which reduces the ammonia yield. (9)
A recent study by the Wang group presented a solution to this problem through an innovative SSE reactor design. (9) This SSE reactor introduces a cation shielding mechanism that enables efficient NO3RR without the need for added electrolytes. The authors first evaluated the performance of a conventional MEA setup using a Ru-dispersed Cu nanowire (Ru-CuNW) catalyst for the NO3RR. Although the MEA was capable of operating without extra electrolytes, it yielded low ammonia selectivity (FE < 20%) because of high proton flux at the catalytic interface. This influx of protons lowered the local pH, favoring the HER over the NO3RR. When the authors switched to an anion exchange membrane, they reported that it relieved the low pH issue but led to nitrate ion loss to the anode side, which hindered complete nitrate conversion to ammonia. These results demonstrated the limitations of conventional MEA systems in achieving both high selectivity and efficient nitrate utilization.
To overcome these limitations, an SSE reactor structure was proposed that incorporates two cation exchange membranes with an additional middle SSE layer between the cathode and anode. (9) With sodium ions flowing through the SSE layer, the authors were able to investigate and optimize the effect of cation shielding, inspired by its role in suppressing the HER in CO2 reduction processes. Sodium ions, which are abundant in wastewater, effectively maintain a high local pH near the catalyst and improve NO3RR selectivity, resulting in a notable FE increase from 25% to 92% at a current density of 100 mA cm–2. Considering the demand for Na+ conduction, the solid electrolyte should conduct cations efficiently. Therefore, the authors continued to use the styrene-divinylbenzenesulfonated copolymer, minimizing the ohmic drop between the cathode and anode.
Following these promising results, the authors developed a complete process design for the SSE system (Figure 5a). (9) First, the nitrate stream flows into the cathode chamber where the NO3RR begins, while sodium ions migrate from the middle SSE layer into the cathode, creating a favorable environment for ammonia production. After the NO3RR, ammonia gas was separated from the catholyte via an air-stripping process. The remaining solution, still containing Na+ ions, was then recirculated into the SSE layer to sustain the cation flow. In the final step, the OH– ions produced from the NO3RR neutralize H+ ions generated in the oxygen evolution reaction (OER) at the anode, maintaining a balance across the reactor chambers and supporting consistent NO3RR performance. This closed-loop design minimizes cation and pH imbalances and eliminates the need for supplemental electrolytes, using only the nitrate solution as the input for ammonia synthesis.

Figure 5

Figure 5. SSE reactors utilized for ammonia production from nitrate reduction. (a) Schematic illustration of the nitrate reduction setup with a solid electrolyte. (b) The air-stripping efficiency of purifying the NH3 product out from the catholyte after each cycle of reactions in the above ten cycles of the stability test. Reproduced with permission from ref (9). Copyright 2024 Springer Nature.

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By fine-tuning several operational parameters, including the flow rate, current density, and nitrate concentration, this system can be further optimized in a single-pass flow configuration. (9) These adjustments helped establish optimal conditions for achieving the highest FE of ammonia, which exceeded 90% in batch conversion tests for 100 mL of 2,000 ppm nitrate solution, meeting the World Health Organization’s nitrate limits for drinking water. Furthermore, the system demonstrated excellent long-term stability; it effectively converted 10 sequential batches of nitrate solution with residual nitrate concentrations under 0.001 M and sodium levels below 0.05 M (Figure 5b).
In addition to its technical achievements, this new SSE reactor design holds substantial promise for industrial and environmental applications. A TEA assessment revealed the potential of the SSE reactor for industrial applications in both waste treatment and sustainable ammonia production. (9) Although the cost is currently above that of the Haber-Bosch process, the dual functionality of nitrate removal and ammonia production offers substantial environmental and operational benefits, underscoring the viability of SSE reactors in decentralized, renewable-powered setups.
While this study provides a compelling blueprint for electrochemical ammonia production, challenges still remain. Although the SSE reactor reduces the need for supporting electrolytes, the cation loop requires a sufficient source of cations, which may limit its application in low-salinity water bodies. Future research should focus on optimizing the cation shuttle mechanism to improve ion transfer without compromising reactor durability. Experimentation with cation selection and the SSE reactor architecture could unlock further improvements in efficiency, yield, and stability, enhancing the suitability for a broader range of water sources and nitrate concentrations.

4. Extended Applications

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4.1. Electrocatalytic CO2 Capture

As climate change concerns grow, efficient carbon capture technologies are increasingly essential. Traditional methods such as amine scrubbing and CaCO3 cycling are effective but highly energy-intensive, requiring high temperatures for CO2 desorption, which makes them costly and challenging to scale. For example, thermal cycling often requires temperatures above 900 °C, which consumes large amounts of fossil fuel-derived energy and offsets CO2 emission reductions. (65)
Emerging electrochemical carbon capture technologies offer promising, energy-efficient alternatives. (66) These systems utilize electricity to drive CO2 capture and release reactions under ambient conditions, making them well-suited for decentralized applications and renewable energy integration. Typically, these methods rely on redox reactions or pH-switching processes: redox-active systems use molecules such as quinones to bind and release CO2 but are often limited by low capture rates and sensitivity to O2, whereas pH-swing methods use electrolytic processes to create localized acid and alkaline conditions for CO2 capture and release. In this context, SSE reactors were further developed to simplify this process by integrating continuous capture and release without additional chemicals, offering a streamlined and efficient solution.
In the conventional CO2RR, carbonate ions formed on the cathode side can migrate toward the anode, where they mix with generated O2, leading to carbon losses and decreased efficiency. SSE reactors mitigate this issue by employing a buffer layer that recombines carbonate ions with protons, converting them back to high-purity CO2 gas within the reactor before any O2 exposure occurs. The modified SSE reactors use a CO2RR at the cathode and an OER at the anode to establish a pH gradient that captures CO2 at the cathode and releases it as high-purity CO2 gas in the middle solid-electrolyte layer (Figure 6a). (23) This electrochemical setup involves a cathode where CO2 is absorbed, forming carbonate or bicarbonate ions. These ions then migrate through the reactor and combine with protons at the central SSE layer, where a styrene-divinylbenzenesulfonated copolymer is used for proton conduction. In the middle chamber, CO2 gas is regenerated and expelled in pure form. This configuration enables the device to achieve high CO2 purity without O2 contamination, as the regenerated CO2 is completely separated from O2. Impressively, this SSE reactor achieved high-purity CO2 recovery (up to 99%) and a continuous CO2 conversion efficiency of over 90%, even at high current densities.

Figure 6

Figure 6. SSE reactors utilized for CO2 capture. (a) Schematic illustration of the SSE reactor with a CO2RR/OER redox configuration for CO2 capture. CO2 is captured by OH– generated in situ at the reaction interface of the CO2RR and forms carbonate ions. These ions then migrate across the AEM to the middle SSE chamber. SSE conducts protons from anodic water oxidation and releases CO2 from carbonate. (b) Schematic illustration of the SSE reactor with an ORR/OER redox configuration for CO2 capture in dilute sources. CO2 is converted into carbonate ions by the high-alkaline environment of the ORR reacting interface. The SSE conducts anode-generated protons and regenerates CO2. (c) Schematic illustration of the SSE reactor with a HER/HOR redox configuration for CO2 capture. The carbonate or bicarbonate solution directly flows through the SSE chamber. SSE conducts protons from anodic hydrogen oxidation to combine with carbonate or bicarbonate, releasing CO2. Moreover, SSE conducts sodium ions to the cathode, leading to the regeneration of NaOH for further CO2 capture.

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Further work by Zhu et al. shifted the focus from CO2 recovery in CO2 electrolysis to broader application for continuous CO2 capture from dilute sources, such as flue gases. (24) This SSE reactor design combines ORR/OER redox electrolysis to enable high-efficiency, continuous CO2 capture without the need for additional chemicals or complex processing. In this design, the SSE reactor captured CO2 at the cathode by converting it into carbonate ions under a high-alkaline environment created by the ORR. These carbonate ions were then driven by the electric field across an anion-exchange membrane into the SSE layer. The SSE adopted a styrene-divinylbenzenesulfonated copolymer for proton conduction supplied by the OER at the anode, where carbonate ions recombined with protons to release high-purity CO2 in the middle layer (Figure 6b). Notably, by reducing the SSE layer thickness from 2.5 mm to 1.5 mm, the authors conserve approximately 200 mV of cell voltage under 100 mA cm–2 at similar carbon-capture FEs. This result indicates a promising strategy for the development of energy-efficient SSE reactors. This device delivered a CO2 capture rate of 86.7 kgCO2 per day per square meter at 500 mA cm–2 and a Faradaic efficiency exceeding 90%. The reactor was operated with minimal energy input (approximately 150 kJ mol–1 CO2), demonstrating the potential of SSE reactors for decentralized carbon capture applications.
The most recent advancement further refined SSE reactor technology by focusing on the efficient regeneration of high-purity CO2 from carbon-containing solutions, specifically (bi)carbonates, in a modular SSE reactor. This study addressed two key issues in carbon capture loops: the energy-intensive regeneration process and capture capacity retention. In this system, the SSE reactor uses hydrogen evolution and oxidation redox reactions to selectively split bicarbonate (NaHCO3) and carbonate (Na2CO3) solutions, regenerating the NaOH absorbent in the catholyte and releasing pure CO2 in the SSE layer (Figure 6c). (25) The sodium form styrene-divinylbenzenesulfonated copolymer is used as the SSE, undertaking dual tasks to conduct both Na+ (from the SSE layer to the cathode for NaOH regeneration) and protons (from the anode to the SSE layer for releasing CO2). The authors also suggest the prospect of further improving the energy efficiency by improving the SSE ionic conductivity and reducing the thickness of the SSE layer. This process, which avoids chemical consumption and byproducts, results in high Na+ transport numbers (∼90%) and demonstrates both high capture capacity retention (∼90%) and low energy consumption (50 kJ mol–1 CO2 at 1 mA cm–2 and 118 kJ mol–1 CO2 at 100 mA cm–2).
These advancements in SSE technology underscore its potential to play a crucial role in future carbon management, offering an efficient, scalable, and sustainable pathway for both CO2 capture and conversion. Expanding SSE reactor functionality to handle a variety of CO2 sources, including low-concentration CO2 from ambient air, will further broaden their application scope and make them viable for direct air capture.

4.2. Tandem Reactions

The increasing global demand for sustainable chemical processes has spurred the development of tandem reaction systems, where intermediate products from one reaction are directly utilized in subsequent processes. These systems are particularly promising when powered by renewable electricity and integrated with solid electrolyte reactors, which offer the advantage of producing high-purity, concentrated liquid products. These intermediates serve as critical reactants in a wide array of chemical and biological processes. (67−75) The seamless transition between electrocatalysis and downstream reactions not only simplifies the overall production workflow but also enhances the sustainability and efficiency of chemical manufacturing.
The success of tandem systems hinges on the availability of high-purity intermediates. Traditional electrocatalytic methods often yield products mixed with ionic impurities, which complicates the integration of downstream processes. (76) Pure liquid products, however, provide a game-changing solution. In addition to their intrinsic chemical value, the compatibility of pure intermediates with downstream reactions makes them ideal for integrated production platforms. For example, pure hydrogen peroxide is essential for oxidation reactions, (59) whereas pure acetic acid is a key substrate in biosynthetic processes. (77) Direct integration of these intermediates eliminates extensive purification, lowering operational costs and improving system efficiency. Producing and utilizing pure intermediates also enhances the overall sustainability of tandem systems. By minimizing energy-intensive purification and improving downstream efficiency, these systems align with the principles of green chemistry and sustainable development. High-purity intermediates act as the nexus of electrocatalysis and downstream reactions, which maximizes resource utilization while minimizing the environmental impact. (73)
Recent advances in tandem reaction systems underscore the vast potential of integrating pure liquid products from solid electrolyte reactors into downstream processes. Among these systems, the tandem electro-biocatalysis system stands out. (67−69) Electrocatalysis, despite its scalability and efficiency, is inherently limited in its ability to propagate carbon–carbon bonds, restricting its product range to C1–C3 species. On the other hand, biological carbon utilization, which leverages sophisticated metabolic pathways, excels in synthesizing long-chain compounds but suffers from slow reaction rates and limited substrate flexibility. By combining the strengths of both approaches, hybrid systems have emerged as a powerful solution, leveraging the speed and precision of electrocatalysis to generate key intermediates, which are then biologically upgraded into complex, long-chain products.
The work by Zheng et al. exemplified this integration through a hybrid system that bridges electrochemical and biological processes. (67) In their setup, an SSE reactor equipped with a grain-boundary-rich Cu catalyst converted CO2 to pure acetic acid with a Faradaic efficiency of 46% and a partial current density of 413 mA cm–2. This ultrapure acetic acid, with a relative purity of ∼97 wt %, is directly utilized in the fermentation of genetically engineered Saccharomyces cerevisiae to produce glucose (Figure 7a). A key innovation here is the spatial decoupling of the electrochemical and biological modules, allowing each to operate under optimal conditions: the SSE reactor maximizes acetic acid purity at precise potentials, whereas the bioreactor operates at a specific pH and temperature ideal for yeast metabolism. This independent optimization minimizes cross-module interference, ensuring high efficiencies in the respective operations. Building on this concept, a similar system uses SSE reactors to produce pure acetic and formic acids, which serve as the carbon source and reducing equivalents, respectively. Formic acid plays a crucial role as a reducing agent, driving anabolic pathways in engineered microbes and regenerating essential cofactors such as NAD(P)H. This enables efficient production of β-farnesene, a valuable biofuel precursor, significantly enhancing the system’s capacity to synthesize long-chain hydrocarbons. (68)

Figure 7

Figure 7. SSE reactors utilized for tandem reactions. (a) Schematic illustration of the in vitro artificial sugar synthesis system with spatial decoupling of the electrochemical and biological modules. Reproduced with permission from ref (67). Copyright 2022 Springer Nature. (b) The present cascade catalytic system coupling electrogenerated hydrogen peroxide with ethylene oxidation. Reproduced with permission from ref (73). Copyright 2023 Springer Nature.

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This approach further extends to Bioplastic production, as demonstrated by Wi et al., who developed an integrated platform combining an SSE reactor with a bioreactor for polyhydroxybutyrate (PHB) synthesis. (70) Unlike systems relying on pure acetic acid, this method generates an optimized biocompatible electrolyte medium containing acetate and biocompatible salts, enabling direct microbial fermentation without additional processing. The SSE reactor, which employs a silver-doped Cu2O nanocube catalyst, electrochemically converts CO into acetate with a Faradaic efficiency of 55% and acetate concentrations exceeding 150 mM. The biocompatible acetate solution is fed into a bioreactor containing Ralstonia eutropha, a microorganism engineered to convert acetate into PHB.
The concept also applies to a tandem electro-thermal catalytic system. For example, Fan et al. reported ethylene glycol production by coupling electrogenerated hydrogen peroxide with ethylene oxidation. (71) Traditionally, ethylene glycol synthesis relies on energy-intensive thermocatalytic routes with high CO2 emissions. This system uses an SSE reactor to produce pure H2O2 via a two-electron oxygen reduction reaction, achieving a Faradaic efficiency of over 90% at current densities up to 500 mA cm–2. The pure H2O2 stream is then fed into a catalytic reactor, where it oxidizes ethylene over a titanium silicalite-1 (TS-1) catalyst under ambient conditions, achieving near-complete selectivity for ethylene glycol (Figure 7b). Similarly, a study by the Wang group explored interfacial electrochemical-chemical coupling for efficient olefin oxidation. (72) This demonstrates an SSE reactor design that uses a high local concentration of H2O2 at the membrane interface to drive ethylene oxidation to ethylene glycol. By integrating this interfacial reaction within the reactor, the system achieves a 3-fold improvement in ethylene glycol production rates compared with traditional tandem systems.
With the advancement of SSE reactors producing pure intermediates as a central intermediate, the versatility of tandem reaction systems extends far beyond examples of food, biofuels and bioplastics. By optimizing the compatibility between electrocatalytic and downstream processes, these systems can be tailored to produce a wide range of valuable compounds. For example, the production of olefins and other hydrocarbons through the coupling of electrocatalytic hydrogenation and chemical synthesis represents a significant opportunity for the petrochemical industry. Similarly, the synthesis of specialty chemicals, such as pharmaceutical precursors, can be enhanced by using pure intermediates generated through electrocatalysis.

5. Future Perspectives

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The advent of SSE reactors marked a significant milestone in electrocatalysis, addressing several critical challenges in modern chemical manufacturing. By enabling the direct production of high-purity chemical products, SSE reactors bypass costly and energy-intensive purification processes associated with traditional systems, thus reshaping the landscape of chemical production. The versatility of SSE reactors extends beyond CO2 reduction and hydrogen peroxide synthesis. Future applications could target more chemical reactions, sustainable production of specialty chemicals, and even direct air capture and conversion of CO2. One particularly promising avenue is the deployment of SSE reactors in tandem reactions, where intermediate products from one reaction serve as feedstock for subsequent processes. However, realizing their transformative potential requires overcoming key scientific, technological, and industrial challenges.
A fundamental component of SSE reactors is the solid electrolyte, which facilitates ion transport while simultaneously acting as a product separator. Current solid electrolyte materials, including polymer-based ion conductors and inorganic oxides, have shown impressive ionic conductivities and chemical stabilities. However, their performance under industrially relevant conditions, such as high temperatures, extreme pH environments, and elevated current densities, remains suboptimal. Future research should prioritize the development of next-generation solid electrolytes with ultrahigh ionic conductivity, tailored ion selectivity, and long-term stability in harsh operating environments. Hybrid materials, which combine the flexibility and processability of polymers with the structural stability and conductivity of inorganic components, offer promising options for achieving these goals. Moreover, the exploration of multifunctional solid electrolytes capable of modulating local reaction environments or participating directly in catalytic processes could redefine the role of solid electrolytes, enhancing overall reactor performance.
Compared with traditional liquid electrolyte systems, SSE reactors inherently experience greater internal resistance. This increased resistance stems from the inherent properties of solid electrolytes and the complex interfaces between different reactor components. As a result, the systems exhibit greater ohmic losses, leading to higher energy consumption and reduced efficiency. Addressing this issue will require innovations in the design and engineering of low-resistance solid electrolytes and advanced interfacial structures.
For the optimization of solid electrolytes, both charge transfer and mass transport are important. Previous reports have demonstrated the greater efficiency of functionalized-polymer-based proton conductors, which use mainly sulfonic groups. Charge conducting could be further improved by trying more types of functional groups, such as phosphonic groups, which are also widely used in proton conductors at room temperature. Moreover, the types of polymer backbone could also be further screened to increase the chemical and mechanical stability of SSE powders. Doping with organic proton conductors such as heteropolyacid or protonated montmorillonite also promises to tune the physicochemical properties of SSE. As for the geometric aspects, small and high porosity powders can efficiently accelerate mass transport. The thinner and more compact packing of the SSE powders would also present an enhanced charge transfer. However, dense integration would also result in greater flow resistance in the SSE chamber, affecting the collection efficiency of pure products, which is expected to be balanced with proton conduction.
Enhanced interfacial engineering could also minimize resistance at critical junctions, especially for SSE-ion exchange membrane interfaces, improve overall energy efficiency and reduce operational costs. From chemical aspects, strategies to increase the affinity between SSE and the membrane should be developed. For example, specific chemical modifications of the main chain of the SSE or the surface of an ion exchange membrane with similar functional groups may increase the degree of charge transfer at these interfaces. Novel preparation processes could also be developed to realize the epitaxial growth of solid-electrolyte-components on the unilateral surface of ion exchange membranes, achieving efficient integration of the SSE and the membrane. From engineering aspects, increasing the contact area at the interface may have an effect. Both improving the roughness of the membrane surface and hybrid packing of SSE powders of different sizes improved the degree of contact. Moreover, advanced hot-pressing techniques are expected to be developed to enhance the interfacial contact, with sufficient moisture remaining for charge conduction.
Moreover, the energy efficiency of SSE reactors could be further optimized by matching proper anodic reactions for proton supply. Although the hydrogen oxidation reaction has been reported to lower the cell voltage effectively, potential security issues in the transportation, storage, and flow control of H2 still limit its large-scale application. Therefore, exploring safe, environmentally friendly, less energy-intensive, and economical anodes as highly efficient proton sources would become a vital research direction for developing energy-efficient SSE reactors. The electro-oxidation of organic molecules such as alcohol and aldehyde, with lower anodic potential, safe operation conditions, and higher value-added products, would become ideal options for the anode of the SSE reactor.
The assembly of SSE reactors poses another technical challenge. Key components such as GDEs and ion exchange membranes must be precisely integrated to ensure optimal performance. However, these components often degrade under continuous operation, leading to decreased efficiency and reliability over time. Advances in GDE design, focusing on enhanced hydrophobicity and optimized pore structures, could prevent flooding and improve gas transport. Similarly, ion exchange membranes need to be tailored for greater durability and selectivity, ensuring consistent performance during long-term operation.
The scale-up of the SSE reactor increases the demand for component consistency, flatness, and assembling precision. The geometric structure of the current collector, gas/liquid flow field, pipelines, and connection mode of circuits should be rationally designed to guarantee the uniform distribution of key factors such as current, mass, temperature, and moisture. Multiphysics simulations with the finite element method could also offer guidance for component design by revealing the parts-structure-dependent field distribution. Moreover, the automation and intelligentization of the SSE reactor assembly process has become a developing tendency toward industrial application. Several strategies could be adopted in the future to further standardize assembly technology, largely to reduce difficulties and risks in cell construction. For example, intelligent sensors could be introduced to monitor the compactness of assembled components and offer feedback in real time. Setting attachments for assisted location would further improve the accuracy of assembly. Exploring the preintegration techniques of solid electrolyte powder, ion exchange membranes, and electrodes would also simplify the assembly process. For example, the mold of SSE powders into sheets and hot-pressed with ion exchange membranes as an individual component would be more convenient and flexible for the building and maintaining of SSE stacks.
A deeper understanding of the reaction mechanisms at the catalytic interface within SSE reactors is also imperative. Unlike traditional systems, SSE reactors create highly localized and dynamic microenvironments that are influenced by ionic gradients and the accumulation of reaction products. These unique conditions can alter the catalytic behavior and reaction pathways, making them difficult to predict and optimize. However, the limited accessibility of in situ and operando characterization techniques currently hinders the detailed exploration of these mechanisms. Solid electrolyte layers restrict direct observation of catalytic processes, complicating efforts to identify active sites, intermediate species, and reaction pathways. To address this, advancements in emerging in situ characterization methods, such as X-ray absorption spectroscopy, neutron scattering, and advanced electron microscopy, are urgently needed to provide deeper insights into the catalytic processes within SSE reactors.
The successful deployment of SSE reactors will also rely on interdisciplinary collaboration across material science, catalysis, engineering, and policy. Governments, industries, and research institutions must work together to establish supportive policies, including incentives for sustainable technologies and increased funding for research and development. Conducting life cycle assessments is essential for evaluating the environmental impact of SSE reactors and ensuring that their deployment aligns with sustainability goals.
Looking ahead, SSE reactors hold immense potential to revolutionize chemical manufacturing, offering a flexible and sustainable solution to some of the most pressing challenges in industry. Their ability to produce high-purity chemicals efficiently and on demand could decentralize production, reduce reliance on centralized facilities, and increase the resilience of supply chains. As the technology matures, SSE reactors could become the cornerstone of a decentralized, resilient, and environmentally friendly industrial ecosystem, empowering communities and industries to adapt to the evolving demands of the future. Through continued innovation and collaboration, SSE reactor technology offers a transformative vision for a greener, more sustainable future.

Author Information

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  • Corresponding Authors
  • Authors
    • Chunxiao Liu - School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China
    • Yuan Ji - School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We acknowledge the National Key Research and Development Program of China (2024YFB4105700), NSFC (22322201, 22278067, 22102018, 52171201), the Fundamental Research Funds for the Central Universities (ZYGX2022J012), and the Natural Science Foundation of Sichuan Province (2025NSFJQ0017, 2023NSFSC0094, 2023NSFSC0911).

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    Tang, C.; Zheng, Y.; Jaroniec, M.; Qiao, S.-Z. Electrocatalytic refinery for sustainable production of fuels and chemicals. Angew. Chem., Int. Ed. 2021, 60 (36), 19572– 19590,  DOI: 10.1002/anie.202101522
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    13
    Electrocatalytic Refinery for Sustainable Production of Fuels and Chemicals
    Tang, Cheng; Zheng, Yao; Jaroniec, Mietek; Qiao, Shi-Zhang
    Angewandte Chemie, International Edition (2021), 60 (36), 19572-19590CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Compared to modern fossil-fuel-based refineries, the emerging electrocatalytic refinery (e-refinery) is a more sustainable and environmentally benign strategy to convert renewable feedstocks and energy sources into transportable fuels and value-added chems. A crucial step in conducting e-refinery processes is the development of appropriate reactions and optimal electrocatalysts for efficient cleavage and formation of chem. bonds. However, compared to well-studied primary reactions (e.g., O2 redn., water splitting), the mechanistic aspects and materials design for emerging complex reactions are yet to be settled. To address this challenge, herein, we first present fundamentals of heterogeneous electrocatalysis and some primary reactions, and then implement these to establish the framework of e-refinery by coupling in situ generated intermediates (integrated reactions) or products (tandem reactions). We also present a set of materials design principles and strategies to efficiently manipulate the reaction intermediates and pathways.
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  14. 14
    Tian, C.; Dorakhan, R.; Wicks, J.; Chen, Z.; Choi, K.-S.; Singh, N.; Schaidle, J. A.; Holewinski, A.; Vojvodic, A.; Vlachos, D. G. Progress and roadmap for electro-privileged transformations of bio-derived molecules. Nat. Catal. 2024, 7, 350– 360,  DOI: 10.1038/s41929-024-01131-6
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  15. 15
    Birdja, Y. Y.; Pérez-Gallent, E.; Figueiredo, M. C.; Göttle, A. J.; Calle-Vallejo, F.; Koper, M. T. M. Advances and challenges in understanding the electrocatalytic conversion of carbon dioxide to fuels. Nat. Energy 2019, 4, 732– 745,  DOI: 10.1038/s41560-019-0450-y
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    15
    Advances and challenges in understanding the electrocatalytic conversion of carbon dioxide to fuels
    Birdja, Yuvraj Y.; Perez-Gallent, Elena; Figueiredo, Marta C.; Gottle, Adrien J.; Calle-Vallejo, Federico; Koper, Marc T. M.
    Nature Energy (2019), 4 (9), 732-745CODEN: NEANFD; ISSN:2058-7546. (Nature Research)
    The electrocatalytic redn. of carbon dioxide is a promising approach for storing (excess) renewable electricity as chem. energy in fuels. Here, we review recent advances and challenges in the understanding of electrochem. CO2 redn. We discuss existing models for the initial activation of CO2 on the electrocatalyst and their importance for understanding selectivity. Carbon-carbon bond formation is also a key mechanistic step in CO2 electroredn. to high-d. and high-value fuels. We show that both the initial CO2 activation and C-C bond formation are influenced by an intricate interplay between surface structure (both on the nano- and on the mesoscale), electrolyte effects (pH, buffer strength, ion effects) and mass transport conditions. This complex interplay is currently still far from being completely understood. In addn., we discuss recent progress in in situ spectroscopic techniques and computational techniques for mechanistic work. Finally, we identify some challenges in furthering our understanding of these themes.
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  16. 16
    Saha, P.; Amanullah, S.; Dey, A. Selectivity in electrochemical CO2 reduction. Acc. Chem. Res. 2022, 55 (2), 134– 144,  DOI: 10.1021/acs.accounts.1c00678
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    16
    Selectivity in Electrochemical CO2 Reduction
    Saha, Paramita; Amanullah, Sk; Dey, Abhishek
    Accounts of Chemical Research (2022), 55 (2), 134-144CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. Conspectus: Electrocatalytic CO2 redn. reaction (CO2RR) to generate fixed forms of carbons that have com. value is a lucrative avenue to ameliorate the growing concerns about the detrimental effect of CO2 emissions as well as generate C-based feed chems., which are generally obtained from the petrochem. industry. The area of electrochem. CO2RR has seen substantial activity in the past decade, and several good catalysts are reported. While the focus was initially on the rate and overpotential of electrocatalysis, it is gradually shifting toward a more chem. challenging issue of selectivity. CO2 can be partially reduced to produce several C1 products like CO, HCOOH, MeOH, etc., before its complete 8e-/8H+ redn. to CH4. In addn. to that, the low valent electron-rich metal centers deployed to activate CO2, a Lewis acid, is prone to reduce protons, which is a substrate for CO2RR, leading to competing H evolution reaction (HER). Similarly, the low valent metal is prone to oxidn. by atm. O2 (i.e., catalyze O redn. reaction, ORR), necessitating strictly anaerobic conditions for CO2RR. Not only is the requirement of O2 free reaction conditions impractical, but also it leads to the release of partially reduced O2 species, like O2-, H2O2 etc., which are reactive and result in oxidative degrdn. of the catalyst. In this account, mechanistic studies of CO2RR by detecting and, often, chem. trapping and characterizing reaction intermediates were used to understand the factors that det. selectivity in CO2RR. The spectroscopic data obtained from different intermediates were identified in different CO2RR catalysts to develop an electronic structure selectivity relation which is deemed important for deciding the selectivity of 2e-/2H+ CO2RR. The roles played by spin state, H bonding, heterogenization in detg. the rate and selectivity of CO2RR (producing only CO, only HCOOH, and only CH4) are discussed using examples of both Fe porphyrin and nonheme bioinspired artificial mimics. Strategies are demonstrated where the competition between CO2RR and HER as well as CO2RR and ORR could be skewed overwhelmingly favoring CO2RR in both these cases.
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  17. 17
    Jouny, M.; Luc, W.; Jiao, F. General techno-economic analysis of CO2 electrolysis systems. Ind. Eng. Chem. Res. 2018, 57, 2165– 2177,  DOI: 10.1021/acs.iecr.7b03514
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    17
    General Techno-Economic Analysis of CO2 Electrolysis Systems
    Jouny, Matthew; Luc, Wesley; Jiao, Feng
    Industrial & Engineering Chemistry Research (2018), 57 (6), 2165-2177CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
    The electrochem. redn. of carbon dioxide (CO2) has received significant attention in academic research, although the techno-economic prospects of the technol. for the large-scale prodn. of chems. are unclear. In this work, we briefly reviewed the current state-of-the-art CO2 redn. figures of merit, and performed an economic anal. to calc. the end-of-life net present value (NPV) of a generalized CO2 electrolyzer system for the prodn. of 100 tons/day of various CO2 redn. products. Under current techno-economic conditions, carbon monoxide and formic acid were the only economically viable products with NPVs of $13.5 million and $39.4 million, resp. However, higher-order alcs., such as ethanol and n-propanol, could be highly promising under future conditions if reasonable electrocatalytic performance benchmarks are achieved (e.g., 300 mA/cm2 and 0.5 V overpotential at 70% Faradaic efficiency). Herein, we established performance targets such that if these targets are achieved, electrochem. CO2 redn. for fuels and chems. prodn. can become a profitable option as part of the growing renewable energy infrastructure.
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  18. 18
    Zhu, P.; Wang, H. High-purity and high-concentration liquid fuels through CO2 electroreduction. Nat. Catal. 2021, 4, 943– 951,  DOI: 10.1038/s41929-021-00694-y
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    18
    High-purity and high-concentration liquid fuels through CO2 electroreduction
    Zhu, Peng; Wang, Haotian
    Nature Catalysis (2021), 4 (11), 943-951CODEN: NCAACP; ISSN:2520-1158. (Nature Portfolio)
    Liq. fuels generated from the electrochem. CO2 redn. reaction (CO2RR) are of particular interest due to their high energy densities and ease of storage and distribution. Unfortunately, they are typically formed in low concns. and mixed with impurities due to the current limitations of traditional CO2 electrolyzers as well as CO2RR catalysts. In this Perspective, we emphasize that while the declining renewable electricity price can greatly lower the formation cost of liq. fuels, the downstream purifn. process will add an extra layer of cost that greatly harms their economic feasibility for large-scale applications. Different strategies in reactor engineering and catalyst improvement are proposed to realize the direct and continuous generation of high-purity and high-concn. liq. fuels from CO2RR electrolyzers, allowing this electrochem. route to become more competitive compared with the traditional chem. engineering industry in the future.
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  19. 19
    Fan, L.; Xia, C.; Zhu, P.; Lu, Y.; Wang, H. Electrochemical CO2 reduction to high-concentration pure formic acid solutions in an all-solid-state reactor. Nat. Commun. 2020, 11, 3633,  DOI: 10.1038/s41467-020-17403-1
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    19
    Electrochemical CO2 reduction to high-concentration pure formic acid solutions in an all-solid-state reactor
    Fan, Lei; Xia, Chuan; Zhu, Peng; Lu, Yingying; Wang, Haotian
    Nature Communications (2020), 11 (1), 3633CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Electrochem. CO2 redn. reaction (CO2RR) to liq. fuels is currently challenged by low product concns., as well as their mixt. with traditional liq. electrolytes, such as KHCO3 soln. Here we report an all-solid-state electrochem. CO2RR system for continuous generation of high-purity and high-concn. formic acid vapors and solns. The cathode and anode were sepd. by a porous solid electrolyte (PSE) layer, where electrochem. generated formate and proton were recombined to form mol. formic acid. The generated formic acid can be efficiently removed in the form of vapors via inert gas stream flowing through the PSE layer. Coupling with a high activity (formate partial current densities ~ 450 mA cm-2), selectivity (maximal Faradaic efficiency ~ 97%), and stability (100 h) grain boundary-enriched bismuth catalyst, we demonstrated ultra-high concns. of pure formic acid solns. (up to nearly 100 wt.%) condensed from generated vapors via flexible tuning of the carrier gas stream.
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  20. 20
    Zheng, T.; Liu, C.; Guo, C.; Zhang, M.; Li, X.; Jiang, Q.; Xue, W.; Li, H.; Li, A.; Pao, C.-W. Copper-catalysed exclusive CO2 to pure formic acid conversion via single-atom alloying. Nat. Nanotechnol. 2021, 16, 1386– 1393,  DOI: 10.1038/s41565-021-00974-5
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    20
    Copper-catalysed exclusive CO2 to pure formic acid conversion via single-atom alloying
    Zheng, Tingting; Liu, Chunxiao; Guo, Chenxi; Zhang, Menglu; Li, Xu; Jiang, Qiu; Xue, Weiqing; Li, Hongliang; Li, Aowen; Pao, Chih-Wen; Xiao, Jianping; Xia, Chuan; Zeng, Jie
    Nature Nanotechnology (2021), 16 (12), 1386-1393CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
    Converting CO2 emissions, powered by renewable electricity, to produce fuels and chems. provides an elegant route towards a carbon-neutral energy cycle. Progress in the understanding and synthesis of Cu catalysts has spurred the explosive development of electrochem. CO2 redn. (CO2RR) technol. to produce hydrocarbons and oxygenates; however, Cu, as the predominant catalyst, often exhibits limited selectivity and activity towards a specific product, leading to low productivity and substantial post-reaction purifn. Here, we present a single-atom Pb-alloyed Cu catalyst (Pb1Cu) that can exclusively (∼96% Faradaic efficiency) convert CO2 into formate with high activity in excess of 1 A cm-2. The Pb1Cu electrocatalyst converts CO2 into formate on the modulated Cu sites rather than on the isolated Pb. In situ spectroscopic evidence and theor. calcns. revealed that the activated Cu sites of the Pb1Cu catalyst regulate the first protonation step of the CO2RR and divert the CO2RR towards a HCOO* path rather than a COOH* path, thus thwarting the possibility of other products. We further showcase the continuous prodn. of a pure formic acid soln. at 100 mA cm-2 over 180 h using a solid electrolyte reactor and Pb1Cu.
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  21. 21
    Zhang, X.; Zhao, X.; Zhu, P.; Adler, Z.; Wu, Z.-Y.; Liu, Y.; Wang, H. Electrochemical oxygen reduction to hydrogen peroxide at practical rates in strong acidic media. Nat. Commun. 2022, 13, 2880,  DOI: 10.1038/s41467-022-30337-0
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    21
    Electrochemical oxygen reduction to hydrogen peroxide at practical rates in strong acidic media
    Zhang, Xiao; Zhao, Xunhua; Zhu, Peng; Adler, Zachary; Wu, Zhen-Yu; Liu, Yuanyue; Wang, Haotian
    Nature Communications (2022), 13 (1), 2880CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
    Electrochem. oxygen redn. to hydrogen peroxide (H2O2) in acidic media, esp. in proton exchange membrane (PEM) electrode assembly reactors, suffers from low selectivity and the lack of low-cost catalysts. Here we present a cation-regulated interfacial engineering approach to promote the H2O2 selectivity (over 80%) under industrial-relevant generation rates (over 400 mA cm-2) in strong acidic media using just carbon black catalyst and a small no. of alkali metal cations, representing a 25-fold improvement compared to that without cation additives. Our d. functional theory simulation suggests a "shielding effect" of alkali metal cations which squeeze away the catalyst/electrolyte interfacial protons and thus prevent further redn. of generated H2O2 to water. A double-PEM solid electrolyte reactor was further developed to realize a continuous, selective (≈90%) and stable (over 500 h) generation of H2O2 via implementing this cation effect for practical applications.
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  22. 22
    Wang, Y.; Zhang, B. Solid electrolyte reactor for nitrate-to-ammonia. Nat. Catal. 2024, 7, 959– 960,  DOI: 10.1038/s41929-024-01223-3
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    There is no corresponding record for this reference.
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    Kim, Y. J.; Zhu, P.; Chen, F.-Y.; Wu, Z.-Y.; Cullen, D. A.; Wang, H. Recovering carbon losses in CO2 electrolysis using a solid electrolyte reactor. Nat. Catal. 2022, 5, 288– 299,  DOI: 10.1038/s41929-022-00763-w
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    23
    Recovering carbon losses in CO2 electrolysis using a solid electrolyte reactor
    Kim, Jung Yoon 'Timothy'; Zhu, Peng; Chen, Feng-Yang; Wu, Zhen-Yu; Cullen, David A.; Wang, Haotian
    Nature Catalysis (2022), 5 (4), 288-299CODEN: NCAACP; ISSN:2520-1158. (Nature Portfolio)
    The practical implementation of electrochem. CO2 redn. technol. is greatly challenged by notable CO2 crossover to the anode side, where the crossed-over CO2 is mixed with O2, via interfacial carbonate formation in traditional CO2 electrolyzers. Here we report a porous solid electrolyte reactor strategy to efficiently recover these carbon losses. By creating a permeable and ion-conducting sulfonated polymer electrolyte between cathode and anode as a buffer layer, the crossover carbonate can combine with protons generated from the anode to re-form CO2 gas for reuse without mixing with anodic O2. Using a silver nanowire catalyst for CO2 redn. to CO, we demonstrated up to 90% recovery of the crossover CO2 in an ultrahigh gas purity form (>99%), while delivering over 90% CO Faradaic efficiency under a 200 mA cm-2 current. A high continuous CO2 conversion efficiency of over 90% was achieved by recycling the recovered CO2 to the CO2 input stream.
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  24. 24
    Zhu, P.; Wu, Z.-Y.; Elgazzar, A.; Dong, C.; Wi, T.-U.; Chen, F.-Y.; Xia, Y.; Feng, Y.; Shakouri, M.; Kim, Y. J. Continuous carbon capture in an electrochemical solid-electrolyte reactor. Nature 2023, 618, 959– 966,  DOI: 10.1038/s41586-023-06060-1
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    24
    Continuous carbon capture in an electrochemical solid-electrolyte reactor
    Zhu, Peng; Wu, Zhen-Yu; Elgazzar, Ahmad; Dong, Changxin; Wi, Tae-Ung; Chen, Feng-Yang; Xia, Yang; Feng, Yuge; Shakouri, Mohsen; Kim, Jung Yoon; Fang, Zhiwei; Hatton, T. Alan; Wang, Haotian
    Nature (London, United Kingdom) (2023), 618 (7967), 959-966CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
    Electrochem. carbon-capture technologies, with renewable electricity as the energy input, are promising for carbon management but still suffer from low capture rates, oxygen sensitivity or system complexity1-6. Here we demonstrate a continuous electrochem. carbon-capture design by coupling oxygen/water (O2/H2O) redox couple with a modular solid-electrolyte reactor7. By performing oxygen redn. reaction (ORR) and oxygen evolution reaction (OER) redox electrolysis, our device can efficiently absorb dil. carbon dioxide (CO2) mols. at the high-alk. cathode-membrane interface to form carbonate ions, followed by a neutralization process through the proton flux from the anode to continuously output a high-purity (>99%) CO2 stream from the middle solid-electrolyte layer. No chem. inputs were needed nor side products generated during the whole carbon absorption/release process. High carbon-capture rates (440 mA cm-2, 0.137 mmolCO2 min-1 cm-2 or 86.7 kgCO2 day-1 m-2), high Faradaic efficiencies (>90% based on carbonate), high carbon-removal efficiency (>98%) in simulated flue gas and low energy consumption (starting from about 150 kJ per molCO2) were demonstrated in our carbon-capture solid-electrolyte reactor, suggesting promising practical applications.
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    Zhang, X.; Fang, Z.; Zhu, P.; Xia, Y.; Wang, H. Electrochemical regeneration of high-purity CO2 from (bi)carbonates in a porous solid electrolyte reactor for efficient carbon capture. Nat. Energy 2025, 10, 55– 65,  DOI: 10.1038/s41560-024-01654-z
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    Becker, T.; Bruns, B.; Lier, S.; Werners, B. Decentralized modular production to increase supply chain efficiency in chemical markets. J. Bus. Econ. 2021, 91, 867– 895,  DOI: 10.1007/s11573-020-01019-4
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    Tonelli, D.; Rosa, L.; Gabrielli, P.; Parente, A.; Contino, F. Cost-competitive decentralized ammonia fertilizer production can increase food security. Nat. Food 2024, 5, 469– 479,  DOI: 10.1038/s43016-024-00979-y
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    Fan, L.; Xia, C.; Yang, F.; Wang, J.; Wang, H.; Lu, Y. Strategies in catalysts and electrolyzer design for electrochemical CO2 reduction toward C2+ products. Sci. Adv. 2020, 6 (8), aay3111  DOI: 10.1126/sciadv.aay3111
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    Lee, K. M.; Jang, J. H.; Balamurugan, M.; Kim, J. E.; Jo, Y. I.; Nam, K. T. Redox-neutral electrochemical conversion of CO2 to dimethyl carbonate. Nat. Energy 2021, 6, 733– 741,  DOI: 10.1038/s41560-021-00862-1
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    29
    Redox-neutral electrochemical conversion of CO2 to dimethyl carbonate
    Lee, Kyu Min; Jang, Jun Ho; Balamurugan, Mani; Kim, Jeong Eun; Jo, Young In; Nam, Ki Tae
    Nature Energy (2021), 6 (7), 733-741CODEN: NEANFD; ISSN:2058-7546. (Nature Portfolio)
    The electrochem. redn. of CO2 to value-added products is a promising approach for using CO2. However, the products are limited to reduced forms, such as CO, HCOOH and C2H4. Decreasing the anodic overpotential and designing membrane-sepd. systems are important determinants of the overall efficiency of the process. In this study we explored the use of redox-neutral reactions in electrochem. CO2 redn. to expand the product scope and achieve higher efficiency. We combined the CO2 redn. reaction with two redox cycles in an undivided cell so that the input electrons are carried through the electrolyte rather than settling in CO2. As a result, di-Me carbonate-a useful fuel additive-has been synthesized directly from CO2 in methanol solvent with a Faradaic efficiency of 60% at room temp. Our study shows that the formation of methoxide intermediates and the cyclic regeneration of the uniformly dispersed palladium catalyst by in situ-generated oxidants are important for di-Me carbonate synthesis at room temp. Furthermore, we successfully synthesized di-Et carbonate from CO2 and ethanol, demonstrating the generality and expandability of our system.
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    Sun, G.-Q.; Yu, P.; Zhang, W.; Zhang, W.; Wang, Y.; Liao, L.-L.; Zhang, Z.; Li, L.; Lu, Z.; Yu, D.-G. Electrochemical reactor dictates site selectivity in N-heteroarene carboxylations. Nature 2023, 615, 67– 72,  DOI: 10.1038/s41586-022-05667-0
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    Electrochemical reactor dictates site selectivity in N-heteroarene carboxylations
    Sun, Guo-Quan; Yu, Peng; Zhang, Wen; Zhang, Wei; Wang, Yi; Liao, Li-Li; Zhang, Zhen; Li, Li; Lu, Zhipeng; Yu, Da-Gang; Lin, Song
    Nature (London, United Kingdom) (2023), 615 (7950), 67-72CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
    Here the development of an electrochem. strategy for the direct carboxylation of pyridines using CO2 was reported. The choice of the electrolysis setup gives rise to divergent site selectivity: a divided electrochem. cell leads to C5 carboxylation, whereas an undivided cell promotes C4 carboxylation. The undivided-cell reaction was proposed to operate through a paired-electrolysis mechanism in which both cathodic and anodic events play crit. roles in altering the site selectivity. Specifically, anodically generated iodine preferentially reacts with a key radical anion intermediate in the C4-carboxylation pathway through hydrogen-atom transfer, thus diverting the reaction selectivity by means of the Curtin-Hammett principle. The scope of the transformation was expanded to a wide range of N-heteroarenes, including bipyridines and terpyridines, pyrimidines, pyrazines and quinolines.
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    Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy Environ. Sci. 2012, 5, 7050– 7059,  DOI: 10.1039/c2ee21234j
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    New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces
    Kuhl, Kendra P.; Cave, Etosha R.; Abram, David N.; Jaramillo, Thomas F.
    Energy & Environmental Science (2012), 5 (5), 7050-7059CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    We report new insights into the electrochem. redn. of CO2 on a metallic copper surface, enabled by the development of an exptl. methodol. with unprecedented sensitivity for the identification and quantification of CO2 electroredn. products. This involves a custom electrochem. cell designed to maximize product concns. coupled to gas chromatog. and NMR for the identification and quantification of gas and liq. products, resp. We studied copper across a range of potentials and obsd. a total of 16 different CO2 redn. products, five of which are reported here for the first time, thus providing the most complete view of the reaction chem. reported to date. Taking into account the chem. identities of the wide range of C1-C3 products generated and the potential-dependence of their turnover frequencies, mechanistic information is deduced. We discuss a scheme for the formation of multi-carbon products involving enol-like surface intermediates as a possible pathway, accounting for the obsd. selectivity for eleven distinct C2+ oxygenated products including aldehydes, ketones, alcs., and carboxylic acids.
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    Dodds, W. S.; Stutzman, L. F.; Sollami, B. J. Carbon dioxide solubility in water. Ind. Eng. Chem. Chem. Eng. Data Ser. 1956, 1, 92– 95,  DOI: 10.1021/i460001a018
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    Carbon dioxide solubility in water
    Dodds, W. S.; Stutzman, L. F.; Sollami, B. J.
    (1956), 1 (No. 1), 92-5 ISSN:.
    The data reported in the literature are collated in units of lbs. CO2 per 100 lbs. of H2O at the reported CO2 temps. and pressures. A family of curves is plotted giving the soly. with respect to temp. at various pressures. The plot indicates that for CO2 pressures up to 200 atm., the soly. decreases with increasing temp. but that above 200 atm. the soly. decreases to a min. at about 70-80° and then increases with increasing temp. For any given temp., the soly. increases with increasing pressure.
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    Sun, Z.; Ma, T.; Tao, H.; Fan, Q.; Han, B. Fundamentals and challenges of electrochemical CO2 reduction using two-dimensional materials. Chem. 2017, 3, 560– 587,  DOI: 10.1016/j.chempr.2017.09.009
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    Fundamentals and Challenges of Electrochemical CO2 Reduction Using Two-Dimensional Materials
    Sun, Zhenyu; Ma, Tao; Tao, Hengcong; Fan, Qun; Han, Buxing
    Chem (2017), 3 (4), 560-587CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
    Electrochem. CO2 redn. (ECR) to value-added fuels and chems. provides a "clean" and efficient way to mitigate energy shortages and to lower the global carbon footprint. The unique structures of two-dimensional (2D) nanosheets and their tunable electronic properties make these nanostructured materials intriguing in catalysis. Various 2D nanosheets are showing promise for CO2 redn., depending on the preferred reaction product (HCOOH, CO, CH4, CH3OH, or CH3COOH). In this review, we focus on recent progress that has been achieved in using these 2D materials for ECR. We highlight procedures available for tuning catalytic activities of 2D materials and describe the fundamentals and future challenges of CO2 catalysis by 2D nanosheets.
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    Li, J.; Kuang, Y.; Meng, Y.; Tian, X.; Hung, W.-H.; Zhang, X.; Li, A.; Xu, M.; Zhou, W.; Ku, C.-S. Electroreduction of CO2 to formate on a copper-based electrocatalyst at high pressures with high energy conversion efficiency. J. Am. Chem. Soc. 2020, 142 (16), 7276– 7282,  DOI: 10.1021/jacs.0c00122
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    34
    Electroreduction of CO2 to Formate on a Copper-Based Electrocatalyst at High Pressures with High Energy Conversion Efficiency
    Li, Jiachen; Kuang, Yun; Meng, Yongtao; Tian, Xin; Hung, Wei-Hsuan; Zhang, Xiao; Li, Aowen; Xu, Mingquan; Zhou, Wu; Ku, Ching-Shun; Chiang, Ching-Yu; Zhu, Guanzhou; Guo, Jinyu; Sun, Xiaoming; Dai, Hongjie
    Journal of the American Chemical Society (2020), 142 (16), 7276-7282CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Electrocatalytic CO2 redn. (CO2RR) to valuable fuels is a promising approach to mitigate energy and environmental problems, but controlling the reaction pathways and products remains challenging. Here a novel Cu2O nanoparticle film was synthesized by square-wave (SW) electrochem. redox cycling of high-purity Cu foils. The cathode afforded ≤ 98% Faradaic efficiency for electroredn. of CO2 to nearly pure formate under ≥ 45 atm CO2 in bicarbonate catholytes. When this cathode was paired with a newly developed NiFe hydroxide carbonate anode in KOH/borate anolyte, the resulting two-electrode high-pressure electrolysis cell achieved high energy conversion efficiencies of ≤ 55.8% stably for long-term formate prodn. While the high-pressure conditions drastically increased the soly. of CO2 to enhance CO2 redn. and suppress hydrogen evolution, the (111)-oriented Cu2O film was found to be important to afford nearly 100% CO2 redn. to formate. The results have implications for CO2 redn. to a single liq. product with high energy conversion efficiency.
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    Zong, S.; Chen, A.; Wiśniewski, M.; Macheli, L.; Jewell, L. L.; Hildebrandt, D.; Liu, X. Effect of temperature and pressure on electrochemical CO2 reduction: A mini review. Carbon Capture Sci. T. 2023, 8, 100133  DOI: 10.1016/j.ccst.2023.100133
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    Li, J.; Kuang, Y.; Zhang, X.; Hung, W.-H.; Chiang, C.-Y.; Zhu, G.; Chen, G.; Wang, F.; Liang, P.; Dai, H. Electrochemical acetate production from high-pressure gaseous and liquid CO2. Nat. Catal. 2023, 6, 1151– 1163,  DOI: 10.1038/s41929-023-01046-8
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    Lu, X.; Leung, D. Y. C.; Wang, H.; Leung, M. K. H.; Xuan, J. Electrochemical reduction of carbon dioxide to formic acid. ChemElectroChem. 2014, 1, 836– 849,  DOI: 10.1002/celc.201300206
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    Electrochemical Reduction of Carbon Dioxide to Formic Acid
    Lu, Xu; Leung, Dennis Y. C.; Wang, Huizhi; Leung, Michael K. H.; Xuan, Jin
    ChemElectroChem (2014), 1 (5), 836-849CODEN: CHEMRA; ISSN:2196-0216. (Wiley-VCH Verlag GmbH & Co. KGaA)
    This Review provides an overview of electrochem. techniques that are implemented in addressing gaseous CO2 towards the synthesis of a particular fuel (i.e. formic acid). The electrochem. reaction mechanism, as well as the advancement of electrodes, catalyst materials, and reactor designs are reviewed and discussed. To date, the electrolytic cell is the dominant reaction site and, based on which, various catalysts have been proposed and researched. In addn., relevant work regarding reactor design optimization for the purpose of alleviating restrictions of the current CO2 electrochem. redn. system are summarized, including low reactant-transfer rate, high reaction overpotential, and low product selectivity. The use of microfluidic techniques to build microscale electrochem. reactors is identified to be highly promising to largely increase the electrochem. performance. Finally, future challenges and opportunities of electrochem. redn. of CO2 are discussed.
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    Li, Y. C.; Yan, Z.; Hitt, J.; Wycisk, R.; Pintauro, P. N.; Mallouk, T. E. Bipolar membranes inhibit product crossover in CO2 electrolysis cells. Adv. Sustain. Syst. 2018, 2, 1700187  DOI: 10.1002/adsu.201700187
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    Sahin, B.; Raymond, S. K.; Ntourmas, F.; Pastusiak, R.; Wiesner-Fleischer, K.; Fleischer, M.; Simon, E.; Hinrichsen, O. Accumulation of liquid byproducts in an electrolyte as a critical factor that compromises long-term functionality of CO2-to-C2H4 electrolysis. ACS Appl. Mater. Interfaces 2023, 15, 45844– 45854,  DOI: 10.1021/acsami.3c08454
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    Weekes, D. M.; Salvatore, D. A.; Reyes, A.; Huang, A.; Berlinguette, C. P. Electrolytic CO2 reduction in a flow cell. Acc. Chem. Res. 2018, 51, 910– 918,  DOI: 10.1021/acs.accounts.8b00010
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    Electrolytic CO2 Reduction in a Flow Cell
    Weekes, David M.; Salvatore, Danielle A.; Reyes, Angelica; Huang, Aoxue; Berlinguette, Curtis P.
    Accounts of Chemical Research (2018), 51 (4), 910-918CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. Electrocatalytic CO2 conversion at near ambient temps. and pressures offers a potential means of converting waste greenhouse gases into fuels or commodity chems. (e.g., CO, formic acid, methanol, ethylene, alkanes, and alcs.). This process is particularly compelling when driven by excess renewable electricity because the consequent prodn. of solar fuels would lead to a closing of the carbon cycle. However, such a technol. is not currently com. available. While CO2 electrolysis in H-cells is widely used for screening electrocatalysts, these expts. generally do not effectively report on how CO2 electrocatalysts behave in flow reactors that are more relevant to a scalable CO2 electrolyzer system. Flow reactors also offer more control over reagent delivery, which includes enabling the use of a gaseous CO2 feed to the cathode of the cell. This setup provides a platform for generating much higher current densities (J) by reducing the mass transport issues inherent to the H-cells.In this Account, we examine some of the systems-level strategies that have been applied in an effort to tailor flow reactor components to improve electrocatalytic redn. Flow reactors that have been utilized in CO2 electrolysis schemes can be categorized into two primary architectures: Membrane-based flow cells and microfluidic reactors. Each invoke different dynamic mechanisms for the delivery of gaseous CO2 to electrocatalytic sites, and both have been demonstrated to achieve high current densities (J > 200 mA cm-2) for CO2 redn. One strategy common to both reactor architectures for improving J is the delivery of CO2 to the cathode in the gas phase rather than dissolved in a liq. electrolyte. This phys. facet also presents a no. of challenges that go beyond the nature of the electrocatalyst, and we scrutinize how the judicious selection and modification of certain components in microfluidic and/or membrane-based reactors can have a profound effect on electrocatalytic performance. In membrane-based flow cells, for example, the choice of either a cation exchange membrane (CEM), anion exchange membrane (AEM), or a bipolar membrane (BPM) affects the kinetics of ion transport pathways and the range of applicable electrolyte conditions. In microfluidic cells, extensive studies have been performed upon the properties of porous carbon gas diffusion layers, materials that are equally relevant to membrane reactors. A theme that is pervasive throughout our analyses is the challenges assocd. with precise and controlled water management in gas phase CO2 electrolyzers, and we highlight studies that demonstrate the importance of maintaining adequate flow cell hydration to achieve sustained electrolysis.
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    Endrődi, B.; Bencsik, G.; Darvas, F.; Jones, R.; Rajeshwar, K.; Janáky, C. Continuous-flow electroreduction of carbon dioxide. Prog. Energy Combust. Sci. 2017, 62, 133– 154,  DOI: 10.1016/j.pecs.2017.05.005
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    Xing, Z.; Hu, L.; Ripatti, D. S.; Hu, X.; Feng, X. Enhancing carbon dioxide gas-diffusion electrolysis by creating a hydrophobic catalyst microenvironment. Nat. Commun. 2021, 12, 136,  DOI: 10.1038/s41467-020-20397-5
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    Enhancing carbon dioxide gas-diffusion electrolysis by creating a hydrophobic catalyst microenvironment
    Xing, Zhuo; Hu, Lin; Ripatti, Donald S.; Hu, Xun; Feng, Xiaofeng
    Nature Communications (2021), 12 (1), 136CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Electroredn. of carbon dioxide (CO2) over copper-based catalysts provides an attractive approach for sustainable fuel prodn. While efforts are focused on developing catalytic materials, it is also crit. to understand and control the microenvironment around catalytic sites, which can mediate the transport of reaction species and influence reaction pathways. Here, we show that a hydrophobic microenvironment can significantly enhance CO2 gas-diffusion electrolysis. For proof-of-concept, we use com. copper nanoparticles and disperse hydrophobic polytetrafluoroethylene (PTFE) nanoparticles inside the catalyst layer. Consequently, the PTFE-added electrode achieves a greatly improved activity and Faradaic efficiency for CO2 redn., with a partial c.d. >250 mA cm-2 and a single-pass conversion of 14% at moderate potentials, which are around twice that of a regular electrode without added PTFE. The improvement is attributed to a balanced gas/liq. microenvironment that reduces the diffusion layer thickness, accelerates CO2 mass transport, and increases CO2 local concn. for the electrolysis.
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    Yang, K.; Kas, R.; Smith, W. A.; Burdyny, T. Role of the carbon-based gas diffusion layer on flooding in a gas diffusion electrode cell for electrochemical CO2 reduction. ACS Energy Lett. 2021, 6 (1), 33– 40,  DOI: 10.1021/acsenergylett.0c02184
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    43
    Role of the Carbon-Based Gas Diffusion Layer on Flooding in a Gas Diffusion Electrode Cell for Electrochemical CO2 Reduction
    Yang, Kailun; Kas, Recep; Smith, Wilson A.; Burdyny, Thomas
    ACS Energy Letters (2021), 6 (1), 33-40CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    The deployment of gas diffusion electrodes (GDEs) for the electrochem. CO2 redn. reaction (CO2RR) has enabled current densities an order of magnitude greater than those of aq. H cells. The gains in prodn., however, have come with stability challenges due to rapid flooding of GDEs, which frustrate both lab. expts. and scale-up prospects. Here, we investigate the role of carbon gas diffusion layers (GDLs) in the advent of flooding during CO2RR, finding that applied potential plays a central role in the obsd. instabilities. Electrochem. characterization of carbon GDLs with and without catalysts suggests that the high overpotential required during electrochem. CO2RR initiates hydrogen evolution on the carbon GDL support. These potentials impact the wetting characteristics of the hydrophobic GDL, resulting in flooding that is independent of CO2RR. Findings from this work can be extended to any electrochem. redn. reaction using carbon-based GDEs (CORR or N2RR) with cathodic overpotentials of less than -0.65 V vs. a reversible hydrogen electrode.
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    Jouny, M.; Luc, W.; Jiao, F. High-rate electroreduction of carbon monoxide to multi-carbon products. Nat. Catal. 2018, 1, 748– 755,  DOI: 10.1038/s41929-018-0133-2
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    High-rate electroreduction of carbon monoxide to multi-carbon products
    Jouny, Matthew; Luc, Wesley; Jiao, Feng
    Nature Catalysis (2018), 1 (10), 748-755CODEN: NCAACP; ISSN:2520-1158. (Nature Research)
    Carbon monoxide electrolysis has previously been reported to yield enhanced multi-carbon (C2+) Faradaic efficiencies of up to ∼55%, but only at low reaction rates. This is due to the low soly. of CO in aq. electrolytes and operation in batch-type reactors. Here, we present a high-performance CO flow electrolyzer with a well controlled electrode-electrolyte interface that can reach total current densities of up to 1 A cm-2, together with improved C2+ selectivities. Computational transport modeling and isotopic C18O redn. expts. suggest that the enhanced activity is due to a higher surface pH under CO redn. conditions, which facilitates the prodn. of acetate. At optimal operating conditions, we achieve a C2+ Faradaic efficiency of ∼91% with a C2+ partial c.d. over 630 mA cm-2. Further investigations show that maintaining an efficient triple-phase boundary at the electrode-electrolyte interface is the most crit. challenge in achieving a stable CO/CO2 electrolysis process at high rates.
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    Wu, J.; Risalvato, F. G.; Sharma, P. P.; Pellechia, P. J.; Ke, F.-S.; Zhou, X.-D. Electrochemical reduction of carbon dioxide II. Design, assembly, and performance of low temperature full electrochemical cells. J. Electrochem. Soc. 2013, 160, F953– F957,  DOI: 10.1149/2.030309jes
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    Electrochemical reduction of carbon dioxide: II. Design, assembly, and performance of low temperature full electrochemical cells
    Wu, Jingjie; Risalvato, Frank G.; Sharma, Pranav P.; Pellechia, Perry J.; Ke, Fu-Sheng; Zhou, Xiao-Dong
    Journal of the Electrochemical Society (2013), 160 (9), F953-F957CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    In this paper, we report the performance of a full electrochem. cell which directly converts carbon dioxide to fuels at room temp. and ambient pressure. The design of this cell features a buffer layer of liq.-phase electrolyte circulating between the ion exchange membrane and the cathode Sn catalyst layer. In the absence of the buffer layer, hydrogen was the predominant product with a faradaic efficiency nearly ∼100%. Incorporating a buffer layer with an electrolyte, e.g. 0.1 M KHCO3, substantially promoted the formation of formate and CO, while suppressing hydrogen prodn. When the anode was fed with hydrogen, the onset of formate prodn. occurred at -0.8 V, with a faradaic efficiency of 65% and a partial c.d. of -1 mA cm-2; at which the energy efficiency toward formate prodn. was 50%. The highest faradaic efficiency obsd. toward formate formation was over 90% at -1.7 V corresponding to a partial c.d. of -9 mA cm-2. When the anode was fed with aq. reactant (e.g. 1 M KOH soln.), the formate prodn. began at -1.2 V with a partial c.d. of -1 mA cm-2, corresponding to a faradaic efficiency of 70% and an energy efficiency of 60%. In this case, the highest faradaic efficiency toward formate formation was 85% at -2.0 V with a partial c.d. of -6 mA cm-2. Our studies show that this full electrochem. cell with a circulating liq.-phase electrolyte buffer layer enables prodn. of formate at an overpotential of ∼-0.2 V regardless of the gaseous or aq. reactants at the anode side.
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    Zheng, T.; Jiang, K.; Ta, N.; Hu, Y.; Zeng, J.; Liu, J.; Wang, H. Large-scale and highly selective CO2 electrocatalytic reduction on nickel single-atom catalyst. Joule 2019, 3, 265– 278,  DOI: 10.1016/j.joule.2018.10.015
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    Large-Scale and Highly Selective CO2 Electrocatalytic Reduction on Nickel Single-Atom Catalyst
    Zheng, Tingting; Jiang, Kun; Ta, Na; Hu, Yongfeng; Zeng, Jie; Liu, Jingyue; Wang, Haotian
    Joule (2019), 3 (1), 265-278CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
    The scaling up of electrocatalytic CO2 redn. for practical applications is still hindered by a few challenges: low selectivity, small c.d. to maintain a reasonable selectivity, and the cost of the catalytic materials. Here we report a facile synthesis of earth-abundant Ni single-atom catalysts on com. carbon black, which were further employed in a gas-phase electrocatalytic reactor under ambient conditions. As a result, those single-at. sites exhibit an extraordinary performance in reducing CO2 to CO, yielding a c.d. above 100 mA cm-2, with nearly 100% selectivity for CO and around 1% toward the hydrogen evolution side reaction. By further scaling up the electrode into a 10 × 10-cm2 modular cell, the overall current in one unit cell can easily ramp up to more than 8 A while maintaining an exclusive CO evolution with a generation rate of 3.34 L hr-1 per unit cell.
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    Lees, E. W.; Bui, J. C.; Romiluyi, O.; Bell, A. T.; Weber, A. Z. Exploring CO2 reduction and crossover in membrane electrode assemblies. Nat. Chem. Eng. 2024, 1, 340– 353,  DOI: 10.1038/s44286-024-00062-0
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    Sassenburg, M.; Kelly, M.; Subramanian, S.; Smith, W. A.; Burdyny, T. Zero-gap electrochemical CO2 reduction cells: Challenges and operational strategies for prevention of salt precipitation. ACS Energy Lett. 2023, 8 (1), 321– 331,  DOI: 10.1021/acsenergylett.2c01885
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    Zero-gap electrochemical carbon dioxide reduction cells, challenges and operational strategies for prevention of salt precipitation
    Sassenburg, Mark; Kelly, Maria; Subramanian, Siddhartha; Smith, Wilson A.; Burdyny, Thomas
    ACS Energy Letters (2023), 8 (1), 321-331CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    Salt pptn. is a problem in electrochem. CO2 redn. electrolyzers that limits their long-term durability and industrial applicability by reducing active area, causing flooding and hindering gas transport. Salt crystals form when hydroxide generation from electrochem. reactions interacts homogeneously with CO2 to generate substantial quantities of carbonate. In the presence of sufficient electrolyte cations, the soly. limits of these species are reached, resulting in "salting out" conditions in cathode compartments. Detrimental salt pptn. is regularly obsd. in zero-gap membrane electrode assemblies, esp. when operated at high current densities. This Perspective briefly discusses the mechanisms for salt formation, and recently reported strategies for preventing or reversing salt formation in zero-gap CO2 redn. membrane electrode assemblies. We link these approaches to the soly. limit of potassium carbonate within the electrolyzer, and describe how each strategy sep. manipulates water, potassium and carbonate concns. to prevent (or mitigate) salt formation.
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    Endrődi, B.; Samu, A.; Kecsenovity, E.; Halmágyi, T.; Sebők, D.; Janáky, C. Operando cathode activation with alkali metal cations for high current density operation of water-fed zero-gap carbon dioxide electrolysers. Nat. Energy 2021, 6, 439– 448,  DOI: 10.1038/s41560-021-00813-w
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    Operando cathode activation with alkali metal cations for high current density operation of water-fed zero-gap carbon dioxide electrolysers
    Endrodi, B.; Samu, A.; Kecsenovity, E.; Halmagyi, T.; Sebok, D.; Janaky, C.
    Nature Energy (2021), 6 (4), 439-448CODEN: NEANFD; ISSN:2058-7546. (Nature Portfolio)
    Continuous-flow electrolyzers allow CO2 redn. at industrially relevant rates, but long-term operation is still challenging. One reason for this is the formation of ppts. in the porous cathode from the alk. electrolyte and the CO2 feed. Here we show that while ppt. formation is detrimental for the long-term stability, the presence of alkali metal cations at the cathode improves performance. To overcome this contradiction, we develop an operando activation and regeneration process, where the cathode of a zero-gap electrolyzer cell is periodically infused with alkali cation-contg. solns. This enables deionized water-fed electrolyzers to operate at a CO2 redn. rate matching those using alk. electrolytes (CO partial c.d. of 420 ± 50 mA cm-2 for over 200 h). We deconvolute the complex effects of activation and validate the concept with five different electrolytes and three different com. membranes. Finally, we demonstrate the scalability of this approach on a multicell electrolyzer stack, with an active area of 100 cm2 per cell.
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    Okuhara, T.; Watanabe, H.; Nishimura, T.; Inumaru, K.; Misono, M. Microstructure of cesium hydrogen salts of 12-tungstophosphoric acid relevant to novel acid catalysis. Chem. Mater. 2000, 12, 2230– 2238,  DOI: 10.1021/cm9907561
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    Microstructure of Cesium Hydrogen Salts of 12-Tungstophosphoric Acid Relevant to Novel Acid Catalysis
    Okuhara, Toshio; Watanabe, Hiromu; Nishimura, Toru; Inumaru, Kei; Misono, Makoto
    Chemistry of Materials (2000), 12 (8), 2230-2238CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    A comprehensive interpretation of the microstructure and mechanism of the formation of a versatile solid acid catalyst, Cs2.5H0.5PW12O40, has been attempted by combining the new results obtained with solid-state NMR, XRD, SEM, and N2 porosimetry with the data reported previously. The ppts. of Cs2.5H0.5PW12O40 just formed from aq. solns. of H3PW12O40 and Cs2CO3 consist of ultrafine crystallites in which the acid form, H3PW12O40, is epitaxially deposited on the surface of Cs3PW12O40 crystallites. Calcination of the ppts. brings about the migration of H+ and Cs+ in the solid to form a nearly uniform solid soln. in which protons distribute randomly through the entire bulk, as revealed by XRD and 31P solid-state NMR. Impregnation of Cs3PW12O40 with the aq. soln. of H3PW12O40 also gives the uniform salt after calcination. Pore-size distribution evaluated by the anal. of N2 desorption isotherm showed that Cs2.5H0.5PW12O40 has mesopores as well as micropores that are interparticle voids of the crystallites. The initial heat of NH3 sorption indicated the presence of very strong acid sites on Cs2.5H0.5PW12O40. High catalytic activity of Cs2.5H0.5PW12O40 reported for solid-liq. reaction systems is thus principally attributed to the strength and no. of acid sites and the mesoporous structure appropriate for the rapid diffusion of mols.
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    Mikkelsen, M.; Jørgensen, M.; Krebs, F. C. The teraton challenge. A review of fixation and transformation of carbon dioxide. Energy Environ. Sci. 2010, 3, 43– 81,  DOI: 10.1039/B912904A
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    The teraton challenge. A review of fixation and transformation of carbon dioxide
    Mikkelsen, Mette; Jorgensen, Mikkel; Krebs, Frederik C.
    Energy & Environmental Science (2010), 3 (1), 43-81CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    A review. The increase in atm. carbon dioxide is linked to climate changes; hence there is an urgent need to reduce the accumulation of CO2 in the atm. The utilization of CO2 as a raw material in the synthesis of chems. and liq. energy carriers offers a way to mitigate the increasing CO2 buildup. This review covers six important CO2 transformations namely: chem. transformations, photochem. redns., chem. and electrochem. redns., biol. conversions, reforming and inorg. transformations. Furthermore, the vast research area of carbon capture and storage is reviewed briefly. This review is intended as an introduction to CO2, its synthetic reactions and their possible role in future CO2 mitigation schemes that has to match the scale of man-made CO2 in the atm., which rapidly approaches 1 teraton.
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    Zhu, D. D.; Liu, J. L.; Qiao, S. Z. Recent advances in inorganic heterogeneous electrocatalysts for reduction of carbon dioxide. Adv. Mater. 2016, 28, 3423– 3452,  DOI: 10.1002/adma.201504766
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    Recent Advances in Inorganic Heterogeneous Electrocatalysts for Reduction of Carbon Dioxide
    Zhu, Dong Dong; Liu, Jin Long; Qiao, Shi Zhang
    Advanced Materials (Weinheim, Germany) (2016), 28 (18), 3423-3452CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. In view of the climate changes caused by the continuously rising levels of atm. CO2, advanced technologies assocd. with CO2 conversion are highly desirable. In recent decades, electrochem. redn. of CO2 has been extensively studied since it can reduce CO2 to value-added chems. and fuels. Considering the sluggish reaction kinetics of the CO2 mol., efficient and robust electrocatalysts are required to promote this conversion reaction. Here, recent progress and opportunities in inorg. heterogeneous electrocatalysts for CO2 redn. are discussed, from the viewpoint of both exptl. and computational aspects. Based on elemental compn., the inorg. catalysts presented here are classified into four groups: metals, transition-metal oxides, transition-metal chalcogenides, and carbon-based materials. However, despite encouraging accomplishments made in this area, substantial advances in CO2 electrolysis are still needed to meet the criteria for practical applications. Therefore, in the last part, several promising strategies, including surface engineering, chem. modification, nanostructured catalysts, and composite materials, are proposed to facilitate the future development of CO2 electroredn.
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    H2O2 is a valuable, environmentally friendly oxidizing agent with a wide range of uses from the provision of clean H2O to the synthesis of valuable chems. The on-site electrolytic prodn. of H2O2 would bring the chem. to applications beyond its present reach. The successful commercialization of electrochem. H2O2 prodn. requires cathode catalysts with high activity, selectivity, and stability. In this Perspective, the authors highlight the authors' current understanding of the factors that control the cathode performance. The authors review the influence of catalyst material, electrolyte, and the structure of the interface at the mesoscopic scale. The authors provide original theor. data on the role of the geometry of the active site and its influence on activity and selectivity. The authors have also conducted original expts. on (i) the effect of pH on H2O2 prodn. on glassy C, pure metals, and metal-Hg alloys, and (ii) the influence of cell geometry and mass transport in liq. half-cells in comparison to membrane electrode assemblies.
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    Hydrogen peroxide (H2O2) as a highly efficient and green oxidant has become one of the 100 most important chems. in the world. Some current research progress in the direct synthesis of H2O2 from H2 and O2 by noble-metal catalyst, fuel cell and plasma methods has been reviewed systematically in this paper. Perspectives about the development direction and application prospect of the above-mentioned three methods have also been discussed.
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    Nature Catalysis (2022), 5 (5), 388-396CODEN: NCAACP; ISSN:2520-1158. (Nature Portfolio)
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    Engineering Microbiology (2022), 2 (4), 100036CODEN: EMNICH; ISSN:2667-3703. (Elsevier B.V.)
    The prodn. of biofuels and biochems. derived from microbial fermn. has received a lot of attention and interest in light of concerns about the depletion of fossil fuel resources and climatic degeneration. However, the economic viability of feedstocks for biol. conversion remains a barrier, urging researchers to develop renewable and sustainable low-cost carbon sources for future bioindustries. Owing to the numerous advantages, acetate has been regarded as a promising feedstock targeting the prodn. of acetyl-CoA-derived chems. This review aims to highlight the potential of acetate as a building block in industrial biotechnol. for the prodn. of bio-based chems. with metabolic engineering. Different alternative approaches and routes comprised of lignocellulosic biomass, waste streams, and C1 gas for acetate generation are briefly described and evaluated. Then, a thorough explanation of the metabolic pathway for biotechnol. acetate conversion, cellular transport, and toxin tolerance is described. Particularly, current developments in metabolic engineering of the manuf. of biochems. from acetate are summarized in detail, with various microbial cell factories and strategies proposed to improve acetate assimilation and enhance product formation. Challenges and future development for acetate generation and assimilation as well as chems. prodn. from acetate is eventually shown. This review provides an overview of the current status of acetate utilization and proves the great potential of acetate with metabolic engineering in industrial biotechnol.
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  • Abstract

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    Figure 1

    Figure 1. Configurations of the electrolyzer design commonly utilized in CO2 electrolysis. (a) H-cell. (b) Gas diffusion flow cell. (c) Membrane electrode assembly. (d) Solid-state electrolyte reactor. To obtain pure liquid products, ions/molecules electrochemically generated from the cathode, such as HCOO–, move across the AEM toward the middle solid electrolyte layer, and protons generated from anodic reactions (water or hydrogen oxidation) move across the CEM into the middle chamber to compensate for the charge. The target molecules (such as HCOOH) are thus formed within the middle layer via ionic recombination, and carried out via the deionized water stream that flows through this porous layer.

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    Figure 2

    Figure 2. Technoeconomic analysis of the operating cost of the production of common liquid products (formic acid, methanol, ethanol, and n-propanol) in the CO2RR under different unit costs of electricity (0.03, 0.02, and 0.01 USD kWh–1, respectively).

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    Figure 3

    Figure 3. SSE reactors utilized for liquid carbon fuel production in the CO2RR. (a) Schematic illustration of the CO2 reduction cell with a solid electrolyte. (b) Dependence of formic acid product concentration on the N2 gas flow rate at a fixed overall current density of 200 mA cm–2. Reproduced from ref (19). Available under a CC-BY 4.0 license. Copyright 2020 The Authors. (c) FEs of all the products under different cell currents, along with the corresponding HCOOH partial current density over Pb1Cu SAAs in an SSE reactor. Reproduced with permission from ref (20). Copyright 2021 Springer Nature. (d) Long-term operation of the SSE reactor over Pb1Cu SAAs for pure HCOOH solution production at −3.45 V. Reproduced with permission from ref (20). Copyright 2021 Springer Nature.

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    Figure 4

    Figure 4. SSE reactors utilized for pure H2O2 production via the 2e–-ORR. (a) Schematic illustration of the O2 reduction cell with a solid electrolyte. (b) Dependence of the H2O2 concentration (up to ∼20 wt %) on the DI water flow rate at a constant overall current of 8 A in an SSE reactor. Reproduced with permission from ref (11). Copyright 2019 The American Association for the Advancement of Science. (c) Schematic illustration of the 2e–-ORR SSE reactor with a double-PEM configuration. (d) The I–V curve and corresponding FEs for the production of H2O2 using the SSE reactor with a double-PEM configuration through the flow of 0.03 M Na2SO4 in the middle chamber. Reproduced from ref (21). Available under a CC-BY 4.0 license. Copyright 2022 The Authors.

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    Figure 5

    Figure 5. SSE reactors utilized for ammonia production from nitrate reduction. (a) Schematic illustration of the nitrate reduction setup with a solid electrolyte. (b) The air-stripping efficiency of purifying the NH3 product out from the catholyte after each cycle of reactions in the above ten cycles of the stability test. Reproduced with permission from ref (9). Copyright 2024 Springer Nature.

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    Figure 6

    Figure 6. SSE reactors utilized for CO2 capture. (a) Schematic illustration of the SSE reactor with a CO2RR/OER redox configuration for CO2 capture. CO2 is captured by OH– generated in situ at the reaction interface of the CO2RR and forms carbonate ions. These ions then migrate across the AEM to the middle SSE chamber. SSE conducts protons from anodic water oxidation and releases CO2 from carbonate. (b) Schematic illustration of the SSE reactor with an ORR/OER redox configuration for CO2 capture in dilute sources. CO2 is converted into carbonate ions by the high-alkaline environment of the ORR reacting interface. The SSE conducts anode-generated protons and regenerates CO2. (c) Schematic illustration of the SSE reactor with a HER/HOR redox configuration for CO2 capture. The carbonate or bicarbonate solution directly flows through the SSE chamber. SSE conducts protons from anodic hydrogen oxidation to combine with carbonate or bicarbonate, releasing CO2. Moreover, SSE conducts sodium ions to the cathode, leading to the regeneration of NaOH for further CO2 capture.

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    Figure 7

    Figure 7. SSE reactors utilized for tandem reactions. (a) Schematic illustration of the in vitro artificial sugar synthesis system with spatial decoupling of the electrochemical and biological modules. Reproduced with permission from ref (67). Copyright 2022 Springer Nature. (b) The present cascade catalytic system coupling electrogenerated hydrogen peroxide with ethylene oxidation. Reproduced with permission from ref (73). Copyright 2023 Springer Nature.

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      Ye, D.; Tsang, S. C. E. Prospects and challenges of green ammonia synthesis. Nat. Synth. 2023, 2, 612– 623,  DOI: 10.1038/s44160-023-00321-7
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    2. 2
      Langer, R.; Peppas, N. A. A bright future in medicine for chemical engineering. Nat. Chem. Eng. 2024, 1, 10– 12,  DOI: 10.1038/s44286-023-00016-y
      There is no corresponding record for this reference.
    3. 3
      Dong, Q.; Hu, S.; Hu, L. Electrothermal synthesis of commodity chemicals. Nat. Chem. Eng. 2024, 1, 680,  DOI: 10.1038/s44286-024-00134-1
      There is no corresponding record for this reference.
    4. 4
      Cheetham, A. K.; Seshadri, R.; Wudl, F. Chemical synthesis and materials discovery. Nat. Synth. 2022, 1, 514– 520,  DOI: 10.1038/s44160-022-00096-3
      There is no corresponding record for this reference.
    5. 5
      Foster, V.; Trotter, P. A.; Werner, S.; Niedermayer, M.; Mulugetta, Y.; Achakulwisut, P.; Brophy, A.; Dubash, N. K.; Fankhauser, S.; Hawkes, A. Development transitions for fossil fuel-producing low and lower-middle income countries in a carbon-constrained world. Nat. Energy 2024, 9, 242– 250,  DOI: 10.1038/s41560-023-01440-3
      There is no corresponding record for this reference.
    6. 6
      Guillén-Gosálbez, G.; Francisco dos Santos, L. Integrating industrial sectors in the transition to more sustainable chemicals. Nat. Chem. Eng. 2024, 1, 336– 337,  DOI: 10.1038/s44286-024-00067-9
      There is no corresponding record for this reference.
    7. 7
      De Luna, P.; Hahn, C.; Higgins, D.; Jaffer, S. A.; Jaramillo, T. F.; Sargent, E. H. What would it take for renewably powered electrosynthesis to displace petrochemical processes?. Science 2019, 364 (6438), eaav3506  DOI: 10.1126/science.aav3506
      There is no corresponding record for this reference.
    8. 8
      Tao, H. B.; Liu, H.; Lao, K.; Pan, Y.; Tao, Y.; Wen, L.; Zheng, N. The gap between academic research on proton exchange membrane water electrolysers and industrial demands. Nat. Nanotechnol. 2024, 19, 1074– 1076,  DOI: 10.1038/s41565-024-01699-x
      There is no corresponding record for this reference.
    9. 9
      Chen, F.-Y.; Elgazzar, A.; Pecaut, S.; Qiu, C.; Feng, Y.; Ashokkumar, S.; Yu, Z.; Sellers, C.; Hao, S.; Zhu, P.; Wang, H. Electrochemical nitrate reduction to ammonia with cation shuttling in a solid electrolyte reactor. Nat. Catal. 2024, 7, 1032– 1043,  DOI: 10.1038/s41929-024-01200-w
      There is no corresponding record for this reference.
    10. 10
      Xia, C.; Zhu, P.; Jiang, Q.; Pan, Y.; Liang, W.; Stavitski, E.; Alshareef, H. N.; Wang, H. Continuous production of pure liquid fuel solutions via electrocatalytic CO2 reduction using solid-electrolyte devices. Nat. Energy 2019, 4, 776– 785,  DOI: 10.1038/s41560-019-0451-x
      10
      Continuous production of pure liquid fuel solutions via electrocatalytic CO2 reduction using solid-electrolyte devices
      Xia, Chuan; Zhu, Peng; Jiang, Qiu; Pan, Ying; Liang, Wentao; Stavitsk, Eli; Alshareef, Husam N.; Wang, Haotian
      Nature Energy (2019), 4 (9), 776-785CODEN: NEANFD; ISSN:2058-7546. (Nature Research)
      Electrocatalytic CO2 redn. is often carried out in a soln. electrolyte such as KHCO3(aq), which allows for ion conduction between electrodes. Therefore, liq. products that form are in a mixt. with the dissolved salts, requiring energy-intensive downstream sepn. Here, we report continuous electrocatalytic conversion of CO2 to pure liq. fuel solns. in cells that utilize solid electrolytes, where electrochem. generated cations (such as H+) and anions (such as HCOO-) are combined to form pure product solns. without mixing with other ions. Using a HCOOH-selective (Faradaic efficiencies > 90%) and easily scaled Bi catalyst at the cathode, we demonstrate prodn. of pure HCOOH solns. with concns. up to 12 M. We also show 100 h continuous and stable generation of 0.1 M HCOOH with negligible degrdn. in selectivity and activity. Prodn. of other electrolyte-free C2+ liq. oxygenate solns., including acetic acid, ethanol and n-propanol, are also demonstrated using a Cu catalyst. Finally, we show that our CO2 redn. cell with solid electrolytes can be modified to suit other, more complex practical applications.
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    11. 11
      Xia, C.; Xia, Y.; Zhu, P.; Fan, L.; Wang, H. Direct electrosynthesis of pure aqueous H2O2 solutions up to 20% by weight using a solid electrolyte. Science 2019, 366, 226– 231,  DOI: 10.1126/science.aay1844
      11
      Direct electrosynthesis of pure aqueous H2O2 solutions up to 20% by weight using a solid electrolyte
      Xia, Chuan; Xia, Yang; Zhu, Peng; Fan, Lei; Wang, Haotian
      Science (Washington, DC, United States) (2019), 366 (6462), 226-231CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
      Hydrogen peroxide synthesis generally requires substantial post reaction purifn. Here, we report a direct electrosynthesis strategy that delivers sep. hydrogen (H2) and oxygen (O2) streams to an anode and cathode sepd. by a porous solid electrolyte, wherein the electrochem. generated H+ and HO2- recombine to form pure aq. H2O2 solns. By optimizing a functionalized carbon black catalyst for two-electron oxygen redn., we achieved >90% selectivity for pure H2O2 at current densities up to 200 mA per square centimeter, which represents an H2O2 productivity of 3.4 mmol per square centimeter per h (3660 mol per kg of catalyst per h). A wide range of concns. of pure H2O2 solns. up to 20 wt. % could be obtained by tuning the water flow rate through the solid electrolyte, and the catalyst retained activity and selectivity for 100 h.
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    12. 12
      Shin, H.; Hansen, K. U.; Jiao, F. Techno-economic assessment of low-temperature carbon dioxide electrolysis. Nat. Sustain. 2021, 4, 911– 919,  DOI: 10.1038/s41893-021-00739-x
      There is no corresponding record for this reference.
    13. 13
      Tang, C.; Zheng, Y.; Jaroniec, M.; Qiao, S.-Z. Electrocatalytic refinery for sustainable production of fuels and chemicals. Angew. Chem., Int. Ed. 2021, 60 (36), 19572– 19590,  DOI: 10.1002/anie.202101522
      13
      Electrocatalytic Refinery for Sustainable Production of Fuels and Chemicals
      Tang, Cheng; Zheng, Yao; Jaroniec, Mietek; Qiao, Shi-Zhang
      Angewandte Chemie, International Edition (2021), 60 (36), 19572-19590CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Compared to modern fossil-fuel-based refineries, the emerging electrocatalytic refinery (e-refinery) is a more sustainable and environmentally benign strategy to convert renewable feedstocks and energy sources into transportable fuels and value-added chems. A crucial step in conducting e-refinery processes is the development of appropriate reactions and optimal electrocatalysts for efficient cleavage and formation of chem. bonds. However, compared to well-studied primary reactions (e.g., O2 redn., water splitting), the mechanistic aspects and materials design for emerging complex reactions are yet to be settled. To address this challenge, herein, we first present fundamentals of heterogeneous electrocatalysis and some primary reactions, and then implement these to establish the framework of e-refinery by coupling in situ generated intermediates (integrated reactions) or products (tandem reactions). We also present a set of materials design principles and strategies to efficiently manipulate the reaction intermediates and pathways.
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    14. 14
      Tian, C.; Dorakhan, R.; Wicks, J.; Chen, Z.; Choi, K.-S.; Singh, N.; Schaidle, J. A.; Holewinski, A.; Vojvodic, A.; Vlachos, D. G. Progress and roadmap for electro-privileged transformations of bio-derived molecules. Nat. Catal. 2024, 7, 350– 360,  DOI: 10.1038/s41929-024-01131-6
      There is no corresponding record for this reference.
    15. 15
      Birdja, Y. Y.; Pérez-Gallent, E.; Figueiredo, M. C.; Göttle, A. J.; Calle-Vallejo, F.; Koper, M. T. M. Advances and challenges in understanding the electrocatalytic conversion of carbon dioxide to fuels. Nat. Energy 2019, 4, 732– 745,  DOI: 10.1038/s41560-019-0450-y
      15
      Advances and challenges in understanding the electrocatalytic conversion of carbon dioxide to fuels
      Birdja, Yuvraj Y.; Perez-Gallent, Elena; Figueiredo, Marta C.; Gottle, Adrien J.; Calle-Vallejo, Federico; Koper, Marc T. M.
      Nature Energy (2019), 4 (9), 732-745CODEN: NEANFD; ISSN:2058-7546. (Nature Research)
      The electrocatalytic redn. of carbon dioxide is a promising approach for storing (excess) renewable electricity as chem. energy in fuels. Here, we review recent advances and challenges in the understanding of electrochem. CO2 redn. We discuss existing models for the initial activation of CO2 on the electrocatalyst and their importance for understanding selectivity. Carbon-carbon bond formation is also a key mechanistic step in CO2 electroredn. to high-d. and high-value fuels. We show that both the initial CO2 activation and C-C bond formation are influenced by an intricate interplay between surface structure (both on the nano- and on the mesoscale), electrolyte effects (pH, buffer strength, ion effects) and mass transport conditions. This complex interplay is currently still far from being completely understood. In addn., we discuss recent progress in in situ spectroscopic techniques and computational techniques for mechanistic work. Finally, we identify some challenges in furthering our understanding of these themes.
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    16. 16
      Saha, P.; Amanullah, S.; Dey, A. Selectivity in electrochemical CO2 reduction. Acc. Chem. Res. 2022, 55 (2), 134– 144,  DOI: 10.1021/acs.accounts.1c00678
      16
      Selectivity in Electrochemical CO2 Reduction
      Saha, Paramita; Amanullah, Sk; Dey, Abhishek
      Accounts of Chemical Research (2022), 55 (2), 134-144CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. Conspectus: Electrocatalytic CO2 redn. reaction (CO2RR) to generate fixed forms of carbons that have com. value is a lucrative avenue to ameliorate the growing concerns about the detrimental effect of CO2 emissions as well as generate C-based feed chems., which are generally obtained from the petrochem. industry. The area of electrochem. CO2RR has seen substantial activity in the past decade, and several good catalysts are reported. While the focus was initially on the rate and overpotential of electrocatalysis, it is gradually shifting toward a more chem. challenging issue of selectivity. CO2 can be partially reduced to produce several C1 products like CO, HCOOH, MeOH, etc., before its complete 8e-/8H+ redn. to CH4. In addn. to that, the low valent electron-rich metal centers deployed to activate CO2, a Lewis acid, is prone to reduce protons, which is a substrate for CO2RR, leading to competing H evolution reaction (HER). Similarly, the low valent metal is prone to oxidn. by atm. O2 (i.e., catalyze O redn. reaction, ORR), necessitating strictly anaerobic conditions for CO2RR. Not only is the requirement of O2 free reaction conditions impractical, but also it leads to the release of partially reduced O2 species, like O2-, H2O2 etc., which are reactive and result in oxidative degrdn. of the catalyst. In this account, mechanistic studies of CO2RR by detecting and, often, chem. trapping and characterizing reaction intermediates were used to understand the factors that det. selectivity in CO2RR. The spectroscopic data obtained from different intermediates were identified in different CO2RR catalysts to develop an electronic structure selectivity relation which is deemed important for deciding the selectivity of 2e-/2H+ CO2RR. The roles played by spin state, H bonding, heterogenization in detg. the rate and selectivity of CO2RR (producing only CO, only HCOOH, and only CH4) are discussed using examples of both Fe porphyrin and nonheme bioinspired artificial mimics. Strategies are demonstrated where the competition between CO2RR and HER as well as CO2RR and ORR could be skewed overwhelmingly favoring CO2RR in both these cases.
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    17. 17
      Jouny, M.; Luc, W.; Jiao, F. General techno-economic analysis of CO2 electrolysis systems. Ind. Eng. Chem. Res. 2018, 57, 2165– 2177,  DOI: 10.1021/acs.iecr.7b03514
      17
      General Techno-Economic Analysis of CO2 Electrolysis Systems
      Jouny, Matthew; Luc, Wesley; Jiao, Feng
      Industrial & Engineering Chemistry Research (2018), 57 (6), 2165-2177CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
      The electrochem. redn. of carbon dioxide (CO2) has received significant attention in academic research, although the techno-economic prospects of the technol. for the large-scale prodn. of chems. are unclear. In this work, we briefly reviewed the current state-of-the-art CO2 redn. figures of merit, and performed an economic anal. to calc. the end-of-life net present value (NPV) of a generalized CO2 electrolyzer system for the prodn. of 100 tons/day of various CO2 redn. products. Under current techno-economic conditions, carbon monoxide and formic acid were the only economically viable products with NPVs of $13.5 million and $39.4 million, resp. However, higher-order alcs., such as ethanol and n-propanol, could be highly promising under future conditions if reasonable electrocatalytic performance benchmarks are achieved (e.g., 300 mA/cm2 and 0.5 V overpotential at 70% Faradaic efficiency). Herein, we established performance targets such that if these targets are achieved, electrochem. CO2 redn. for fuels and chems. prodn. can become a profitable option as part of the growing renewable energy infrastructure.
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    18. 18
      Zhu, P.; Wang, H. High-purity and high-concentration liquid fuels through CO2 electroreduction. Nat. Catal. 2021, 4, 943– 951,  DOI: 10.1038/s41929-021-00694-y
      18
      High-purity and high-concentration liquid fuels through CO2 electroreduction
      Zhu, Peng; Wang, Haotian
      Nature Catalysis (2021), 4 (11), 943-951CODEN: NCAACP; ISSN:2520-1158. (Nature Portfolio)
      Liq. fuels generated from the electrochem. CO2 redn. reaction (CO2RR) are of particular interest due to their high energy densities and ease of storage and distribution. Unfortunately, they are typically formed in low concns. and mixed with impurities due to the current limitations of traditional CO2 electrolyzers as well as CO2RR catalysts. In this Perspective, we emphasize that while the declining renewable electricity price can greatly lower the formation cost of liq. fuels, the downstream purifn. process will add an extra layer of cost that greatly harms their economic feasibility for large-scale applications. Different strategies in reactor engineering and catalyst improvement are proposed to realize the direct and continuous generation of high-purity and high-concn. liq. fuels from CO2RR electrolyzers, allowing this electrochem. route to become more competitive compared with the traditional chem. engineering industry in the future.
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    19. 19
      Fan, L.; Xia, C.; Zhu, P.; Lu, Y.; Wang, H. Electrochemical CO2 reduction to high-concentration pure formic acid solutions in an all-solid-state reactor. Nat. Commun. 2020, 11, 3633,  DOI: 10.1038/s41467-020-17403-1
      19
      Electrochemical CO2 reduction to high-concentration pure formic acid solutions in an all-solid-state reactor
      Fan, Lei; Xia, Chuan; Zhu, Peng; Lu, Yingying; Wang, Haotian
      Nature Communications (2020), 11 (1), 3633CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Electrochem. CO2 redn. reaction (CO2RR) to liq. fuels is currently challenged by low product concns., as well as their mixt. with traditional liq. electrolytes, such as KHCO3 soln. Here we report an all-solid-state electrochem. CO2RR system for continuous generation of high-purity and high-concn. formic acid vapors and solns. The cathode and anode were sepd. by a porous solid electrolyte (PSE) layer, where electrochem. generated formate and proton were recombined to form mol. formic acid. The generated formic acid can be efficiently removed in the form of vapors via inert gas stream flowing through the PSE layer. Coupling with a high activity (formate partial current densities ~ 450 mA cm-2), selectivity (maximal Faradaic efficiency ~ 97%), and stability (100 h) grain boundary-enriched bismuth catalyst, we demonstrated ultra-high concns. of pure formic acid solns. (up to nearly 100 wt.%) condensed from generated vapors via flexible tuning of the carrier gas stream.
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    20. 20
      Zheng, T.; Liu, C.; Guo, C.; Zhang, M.; Li, X.; Jiang, Q.; Xue, W.; Li, H.; Li, A.; Pao, C.-W. Copper-catalysed exclusive CO2 to pure formic acid conversion via single-atom alloying. Nat. Nanotechnol. 2021, 16, 1386– 1393,  DOI: 10.1038/s41565-021-00974-5
      20
      Copper-catalysed exclusive CO2 to pure formic acid conversion via single-atom alloying
      Zheng, Tingting; Liu, Chunxiao; Guo, Chenxi; Zhang, Menglu; Li, Xu; Jiang, Qiu; Xue, Weiqing; Li, Hongliang; Li, Aowen; Pao, Chih-Wen; Xiao, Jianping; Xia, Chuan; Zeng, Jie
      Nature Nanotechnology (2021), 16 (12), 1386-1393CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
      Converting CO2 emissions, powered by renewable electricity, to produce fuels and chems. provides an elegant route towards a carbon-neutral energy cycle. Progress in the understanding and synthesis of Cu catalysts has spurred the explosive development of electrochem. CO2 redn. (CO2RR) technol. to produce hydrocarbons and oxygenates; however, Cu, as the predominant catalyst, often exhibits limited selectivity and activity towards a specific product, leading to low productivity and substantial post-reaction purifn. Here, we present a single-atom Pb-alloyed Cu catalyst (Pb1Cu) that can exclusively (∼96% Faradaic efficiency) convert CO2 into formate with high activity in excess of 1 A cm-2. The Pb1Cu electrocatalyst converts CO2 into formate on the modulated Cu sites rather than on the isolated Pb. In situ spectroscopic evidence and theor. calcns. revealed that the activated Cu sites of the Pb1Cu catalyst regulate the first protonation step of the CO2RR and divert the CO2RR towards a HCOO* path rather than a COOH* path, thus thwarting the possibility of other products. We further showcase the continuous prodn. of a pure formic acid soln. at 100 mA cm-2 over 180 h using a solid electrolyte reactor and Pb1Cu.
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    21. 21
      Zhang, X.; Zhao, X.; Zhu, P.; Adler, Z.; Wu, Z.-Y.; Liu, Y.; Wang, H. Electrochemical oxygen reduction to hydrogen peroxide at practical rates in strong acidic media. Nat. Commun. 2022, 13, 2880,  DOI: 10.1038/s41467-022-30337-0
      21
      Electrochemical oxygen reduction to hydrogen peroxide at practical rates in strong acidic media
      Zhang, Xiao; Zhao, Xunhua; Zhu, Peng; Adler, Zachary; Wu, Zhen-Yu; Liu, Yuanyue; Wang, Haotian
      Nature Communications (2022), 13 (1), 2880CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
      Electrochem. oxygen redn. to hydrogen peroxide (H2O2) in acidic media, esp. in proton exchange membrane (PEM) electrode assembly reactors, suffers from low selectivity and the lack of low-cost catalysts. Here we present a cation-regulated interfacial engineering approach to promote the H2O2 selectivity (over 80%) under industrial-relevant generation rates (over 400 mA cm-2) in strong acidic media using just carbon black catalyst and a small no. of alkali metal cations, representing a 25-fold improvement compared to that without cation additives. Our d. functional theory simulation suggests a "shielding effect" of alkali metal cations which squeeze away the catalyst/electrolyte interfacial protons and thus prevent further redn. of generated H2O2 to water. A double-PEM solid electrolyte reactor was further developed to realize a continuous, selective (≈90%) and stable (over 500 h) generation of H2O2 via implementing this cation effect for practical applications.
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    22. 22
      Wang, Y.; Zhang, B. Solid electrolyte reactor for nitrate-to-ammonia. Nat. Catal. 2024, 7, 959– 960,  DOI: 10.1038/s41929-024-01223-3
      There is no corresponding record for this reference.
    23. 23
      Kim, Y. J.; Zhu, P.; Chen, F.-Y.; Wu, Z.-Y.; Cullen, D. A.; Wang, H. Recovering carbon losses in CO2 electrolysis using a solid electrolyte reactor. Nat. Catal. 2022, 5, 288– 299,  DOI: 10.1038/s41929-022-00763-w
      23
      Recovering carbon losses in CO2 electrolysis using a solid electrolyte reactor
      Kim, Jung Yoon 'Timothy'; Zhu, Peng; Chen, Feng-Yang; Wu, Zhen-Yu; Cullen, David A.; Wang, Haotian
      Nature Catalysis (2022), 5 (4), 288-299CODEN: NCAACP; ISSN:2520-1158. (Nature Portfolio)
      The practical implementation of electrochem. CO2 redn. technol. is greatly challenged by notable CO2 crossover to the anode side, where the crossed-over CO2 is mixed with O2, via interfacial carbonate formation in traditional CO2 electrolyzers. Here we report a porous solid electrolyte reactor strategy to efficiently recover these carbon losses. By creating a permeable and ion-conducting sulfonated polymer electrolyte between cathode and anode as a buffer layer, the crossover carbonate can combine with protons generated from the anode to re-form CO2 gas for reuse without mixing with anodic O2. Using a silver nanowire catalyst for CO2 redn. to CO, we demonstrated up to 90% recovery of the crossover CO2 in an ultrahigh gas purity form (>99%), while delivering over 90% CO Faradaic efficiency under a 200 mA cm-2 current. A high continuous CO2 conversion efficiency of over 90% was achieved by recycling the recovered CO2 to the CO2 input stream.
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    24. 24
      Zhu, P.; Wu, Z.-Y.; Elgazzar, A.; Dong, C.; Wi, T.-U.; Chen, F.-Y.; Xia, Y.; Feng, Y.; Shakouri, M.; Kim, Y. J. Continuous carbon capture in an electrochemical solid-electrolyte reactor. Nature 2023, 618, 959– 966,  DOI: 10.1038/s41586-023-06060-1
      24
      Continuous carbon capture in an electrochemical solid-electrolyte reactor
      Zhu, Peng; Wu, Zhen-Yu; Elgazzar, Ahmad; Dong, Changxin; Wi, Tae-Ung; Chen, Feng-Yang; Xia, Yang; Feng, Yuge; Shakouri, Mohsen; Kim, Jung Yoon; Fang, Zhiwei; Hatton, T. Alan; Wang, Haotian
      Nature (London, United Kingdom) (2023), 618 (7967), 959-966CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
      Electrochem. carbon-capture technologies, with renewable electricity as the energy input, are promising for carbon management but still suffer from low capture rates, oxygen sensitivity or system complexity1-6. Here we demonstrate a continuous electrochem. carbon-capture design by coupling oxygen/water (O2/H2O) redox couple with a modular solid-electrolyte reactor7. By performing oxygen redn. reaction (ORR) and oxygen evolution reaction (OER) redox electrolysis, our device can efficiently absorb dil. carbon dioxide (CO2) mols. at the high-alk. cathode-membrane interface to form carbonate ions, followed by a neutralization process through the proton flux from the anode to continuously output a high-purity (>99%) CO2 stream from the middle solid-electrolyte layer. No chem. inputs were needed nor side products generated during the whole carbon absorption/release process. High carbon-capture rates (440 mA cm-2, 0.137 mmolCO2 min-1 cm-2 or 86.7 kgCO2 day-1 m-2), high Faradaic efficiencies (>90% based on carbonate), high carbon-removal efficiency (>98%) in simulated flue gas and low energy consumption (starting from about 150 kJ per molCO2) were demonstrated in our carbon-capture solid-electrolyte reactor, suggesting promising practical applications.
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    25. 25
      Zhang, X.; Fang, Z.; Zhu, P.; Xia, Y.; Wang, H. Electrochemical regeneration of high-purity CO2 from (bi)carbonates in a porous solid electrolyte reactor for efficient carbon capture. Nat. Energy 2025, 10, 55– 65,  DOI: 10.1038/s41560-024-01654-z
      There is no corresponding record for this reference.
    26. 26
      Becker, T.; Bruns, B.; Lier, S.; Werners, B. Decentralized modular production to increase supply chain efficiency in chemical markets. J. Bus. Econ. 2021, 91, 867– 895,  DOI: 10.1007/s11573-020-01019-4
      There is no corresponding record for this reference.
    27. 27
      Tonelli, D.; Rosa, L.; Gabrielli, P.; Parente, A.; Contino, F. Cost-competitive decentralized ammonia fertilizer production can increase food security. Nat. Food 2024, 5, 469– 479,  DOI: 10.1038/s43016-024-00979-y
      There is no corresponding record for this reference.
    28. 28
      Fan, L.; Xia, C.; Yang, F.; Wang, J.; Wang, H.; Lu, Y. Strategies in catalysts and electrolyzer design for electrochemical CO2 reduction toward C2+ products. Sci. Adv. 2020, 6 (8), aay3111  DOI: 10.1126/sciadv.aay3111
      There is no corresponding record for this reference.
    29. 29
      Lee, K. M.; Jang, J. H.; Balamurugan, M.; Kim, J. E.; Jo, Y. I.; Nam, K. T. Redox-neutral electrochemical conversion of CO2 to dimethyl carbonate. Nat. Energy 2021, 6, 733– 741,  DOI: 10.1038/s41560-021-00862-1
      29
      Redox-neutral electrochemical conversion of CO2 to dimethyl carbonate
      Lee, Kyu Min; Jang, Jun Ho; Balamurugan, Mani; Kim, Jeong Eun; Jo, Young In; Nam, Ki Tae
      Nature Energy (2021), 6 (7), 733-741CODEN: NEANFD; ISSN:2058-7546. (Nature Portfolio)
      The electrochem. redn. of CO2 to value-added products is a promising approach for using CO2. However, the products are limited to reduced forms, such as CO, HCOOH and C2H4. Decreasing the anodic overpotential and designing membrane-sepd. systems are important determinants of the overall efficiency of the process. In this study we explored the use of redox-neutral reactions in electrochem. CO2 redn. to expand the product scope and achieve higher efficiency. We combined the CO2 redn. reaction with two redox cycles in an undivided cell so that the input electrons are carried through the electrolyte rather than settling in CO2. As a result, di-Me carbonate-a useful fuel additive-has been synthesized directly from CO2 in methanol solvent with a Faradaic efficiency of 60% at room temp. Our study shows that the formation of methoxide intermediates and the cyclic regeneration of the uniformly dispersed palladium catalyst by in situ-generated oxidants are important for di-Me carbonate synthesis at room temp. Furthermore, we successfully synthesized di-Et carbonate from CO2 and ethanol, demonstrating the generality and expandability of our system.
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    30. 30
      Sun, G.-Q.; Yu, P.; Zhang, W.; Zhang, W.; Wang, Y.; Liao, L.-L.; Zhang, Z.; Li, L.; Lu, Z.; Yu, D.-G. Electrochemical reactor dictates site selectivity in N-heteroarene carboxylations. Nature 2023, 615, 67– 72,  DOI: 10.1038/s41586-022-05667-0
      30
      Electrochemical reactor dictates site selectivity in N-heteroarene carboxylations
      Sun, Guo-Quan; Yu, Peng; Zhang, Wen; Zhang, Wei; Wang, Yi; Liao, Li-Li; Zhang, Zhen; Li, Li; Lu, Zhipeng; Yu, Da-Gang; Lin, Song
      Nature (London, United Kingdom) (2023), 615 (7950), 67-72CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
      Here the development of an electrochem. strategy for the direct carboxylation of pyridines using CO2 was reported. The choice of the electrolysis setup gives rise to divergent site selectivity: a divided electrochem. cell leads to C5 carboxylation, whereas an undivided cell promotes C4 carboxylation. The undivided-cell reaction was proposed to operate through a paired-electrolysis mechanism in which both cathodic and anodic events play crit. roles in altering the site selectivity. Specifically, anodically generated iodine preferentially reacts with a key radical anion intermediate in the C4-carboxylation pathway through hydrogen-atom transfer, thus diverting the reaction selectivity by means of the Curtin-Hammett principle. The scope of the transformation was expanded to a wide range of N-heteroarenes, including bipyridines and terpyridines, pyrimidines, pyrazines and quinolines.
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    31. 31
      Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy Environ. Sci. 2012, 5, 7050– 7059,  DOI: 10.1039/c2ee21234j
      31
      New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces
      Kuhl, Kendra P.; Cave, Etosha R.; Abram, David N.; Jaramillo, Thomas F.
      Energy & Environmental Science (2012), 5 (5), 7050-7059CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      We report new insights into the electrochem. redn. of CO2 on a metallic copper surface, enabled by the development of an exptl. methodol. with unprecedented sensitivity for the identification and quantification of CO2 electroredn. products. This involves a custom electrochem. cell designed to maximize product concns. coupled to gas chromatog. and NMR for the identification and quantification of gas and liq. products, resp. We studied copper across a range of potentials and obsd. a total of 16 different CO2 redn. products, five of which are reported here for the first time, thus providing the most complete view of the reaction chem. reported to date. Taking into account the chem. identities of the wide range of C1-C3 products generated and the potential-dependence of their turnover frequencies, mechanistic information is deduced. We discuss a scheme for the formation of multi-carbon products involving enol-like surface intermediates as a possible pathway, accounting for the obsd. selectivity for eleven distinct C2+ oxygenated products including aldehydes, ketones, alcs., and carboxylic acids.
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    32. 32
      Dodds, W. S.; Stutzman, L. F.; Sollami, B. J. Carbon dioxide solubility in water. Ind. Eng. Chem. Chem. Eng. Data Ser. 1956, 1, 92– 95,  DOI: 10.1021/i460001a018
      32
      Carbon dioxide solubility in water
      Dodds, W. S.; Stutzman, L. F.; Sollami, B. J.
      (1956), 1 (No. 1), 92-5 ISSN:.
      The data reported in the literature are collated in units of lbs. CO2 per 100 lbs. of H2O at the reported CO2 temps. and pressures. A family of curves is plotted giving the soly. with respect to temp. at various pressures. The plot indicates that for CO2 pressures up to 200 atm., the soly. decreases with increasing temp. but that above 200 atm. the soly. decreases to a min. at about 70-80° and then increases with increasing temp. For any given temp., the soly. increases with increasing pressure.
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    33. 33
      Sun, Z.; Ma, T.; Tao, H.; Fan, Q.; Han, B. Fundamentals and challenges of electrochemical CO2 reduction using two-dimensional materials. Chem. 2017, 3, 560– 587,  DOI: 10.1016/j.chempr.2017.09.009
      33
      Fundamentals and Challenges of Electrochemical CO2 Reduction Using Two-Dimensional Materials
      Sun, Zhenyu; Ma, Tao; Tao, Hengcong; Fan, Qun; Han, Buxing
      Chem (2017), 3 (4), 560-587CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
      Electrochem. CO2 redn. (ECR) to value-added fuels and chems. provides a "clean" and efficient way to mitigate energy shortages and to lower the global carbon footprint. The unique structures of two-dimensional (2D) nanosheets and their tunable electronic properties make these nanostructured materials intriguing in catalysis. Various 2D nanosheets are showing promise for CO2 redn., depending on the preferred reaction product (HCOOH, CO, CH4, CH3OH, or CH3COOH). In this review, we focus on recent progress that has been achieved in using these 2D materials for ECR. We highlight procedures available for tuning catalytic activities of 2D materials and describe the fundamentals and future challenges of CO2 catalysis by 2D nanosheets.
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    34. 34
      Li, J.; Kuang, Y.; Meng, Y.; Tian, X.; Hung, W.-H.; Zhang, X.; Li, A.; Xu, M.; Zhou, W.; Ku, C.-S. Electroreduction of CO2 to formate on a copper-based electrocatalyst at high pressures with high energy conversion efficiency. J. Am. Chem. Soc. 2020, 142 (16), 7276– 7282,  DOI: 10.1021/jacs.0c00122
      34
      Electroreduction of CO2 to Formate on a Copper-Based Electrocatalyst at High Pressures with High Energy Conversion Efficiency
      Li, Jiachen; Kuang, Yun; Meng, Yongtao; Tian, Xin; Hung, Wei-Hsuan; Zhang, Xiao; Li, Aowen; Xu, Mingquan; Zhou, Wu; Ku, Ching-Shun; Chiang, Ching-Yu; Zhu, Guanzhou; Guo, Jinyu; Sun, Xiaoming; Dai, Hongjie
      Journal of the American Chemical Society (2020), 142 (16), 7276-7282CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Electrocatalytic CO2 redn. (CO2RR) to valuable fuels is a promising approach to mitigate energy and environmental problems, but controlling the reaction pathways and products remains challenging. Here a novel Cu2O nanoparticle film was synthesized by square-wave (SW) electrochem. redox cycling of high-purity Cu foils. The cathode afforded ≤ 98% Faradaic efficiency for electroredn. of CO2 to nearly pure formate under ≥ 45 atm CO2 in bicarbonate catholytes. When this cathode was paired with a newly developed NiFe hydroxide carbonate anode in KOH/borate anolyte, the resulting two-electrode high-pressure electrolysis cell achieved high energy conversion efficiencies of ≤ 55.8% stably for long-term formate prodn. While the high-pressure conditions drastically increased the soly. of CO2 to enhance CO2 redn. and suppress hydrogen evolution, the (111)-oriented Cu2O film was found to be important to afford nearly 100% CO2 redn. to formate. The results have implications for CO2 redn. to a single liq. product with high energy conversion efficiency.
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    35. 35
      Zong, S.; Chen, A.; Wiśniewski, M.; Macheli, L.; Jewell, L. L.; Hildebrandt, D.; Liu, X. Effect of temperature and pressure on electrochemical CO2 reduction: A mini review. Carbon Capture Sci. T. 2023, 8, 100133  DOI: 10.1016/j.ccst.2023.100133
      There is no corresponding record for this reference.
    36. 36
      Li, J.; Kuang, Y.; Zhang, X.; Hung, W.-H.; Chiang, C.-Y.; Zhu, G.; Chen, G.; Wang, F.; Liang, P.; Dai, H. Electrochemical acetate production from high-pressure gaseous and liquid CO2. Nat. Catal. 2023, 6, 1151– 1163,  DOI: 10.1038/s41929-023-01046-8
      There is no corresponding record for this reference.
    37. 37
      Lu, X.; Leung, D. Y. C.; Wang, H.; Leung, M. K. H.; Xuan, J. Electrochemical reduction of carbon dioxide to formic acid. ChemElectroChem. 2014, 1, 836– 849,  DOI: 10.1002/celc.201300206
      37
      Electrochemical Reduction of Carbon Dioxide to Formic Acid
      Lu, Xu; Leung, Dennis Y. C.; Wang, Huizhi; Leung, Michael K. H.; Xuan, Jin
      ChemElectroChem (2014), 1 (5), 836-849CODEN: CHEMRA; ISSN:2196-0216. (Wiley-VCH Verlag GmbH & Co. KGaA)
      This Review provides an overview of electrochem. techniques that are implemented in addressing gaseous CO2 towards the synthesis of a particular fuel (i.e. formic acid). The electrochem. reaction mechanism, as well as the advancement of electrodes, catalyst materials, and reactor designs are reviewed and discussed. To date, the electrolytic cell is the dominant reaction site and, based on which, various catalysts have been proposed and researched. In addn., relevant work regarding reactor design optimization for the purpose of alleviating restrictions of the current CO2 electrochem. redn. system are summarized, including low reactant-transfer rate, high reaction overpotential, and low product selectivity. The use of microfluidic techniques to build microscale electrochem. reactors is identified to be highly promising to largely increase the electrochem. performance. Finally, future challenges and opportunities of electrochem. redn. of CO2 are discussed.
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    38. 38
      Li, Y. C.; Yan, Z.; Hitt, J.; Wycisk, R.; Pintauro, P. N.; Mallouk, T. E. Bipolar membranes inhibit product crossover in CO2 electrolysis cells. Adv. Sustain. Syst. 2018, 2, 1700187  DOI: 10.1002/adsu.201700187
      There is no corresponding record for this reference.
    39. 39
      Sahin, B.; Raymond, S. K.; Ntourmas, F.; Pastusiak, R.; Wiesner-Fleischer, K.; Fleischer, M.; Simon, E.; Hinrichsen, O. Accumulation of liquid byproducts in an electrolyte as a critical factor that compromises long-term functionality of CO2-to-C2H4 electrolysis. ACS Appl. Mater. Interfaces 2023, 15, 45844– 45854,  DOI: 10.1021/acsami.3c08454
      There is no corresponding record for this reference.
    40. 40
      Weekes, D. M.; Salvatore, D. A.; Reyes, A.; Huang, A.; Berlinguette, C. P. Electrolytic CO2 reduction in a flow cell. Acc. Chem. Res. 2018, 51, 910– 918,  DOI: 10.1021/acs.accounts.8b00010
      40
      Electrolytic CO2 Reduction in a Flow Cell
      Weekes, David M.; Salvatore, Danielle A.; Reyes, Angelica; Huang, Aoxue; Berlinguette, Curtis P.
      Accounts of Chemical Research (2018), 51 (4), 910-918CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. Electrocatalytic CO2 conversion at near ambient temps. and pressures offers a potential means of converting waste greenhouse gases into fuels or commodity chems. (e.g., CO, formic acid, methanol, ethylene, alkanes, and alcs.). This process is particularly compelling when driven by excess renewable electricity because the consequent prodn. of solar fuels would lead to a closing of the carbon cycle. However, such a technol. is not currently com. available. While CO2 electrolysis in H-cells is widely used for screening electrocatalysts, these expts. generally do not effectively report on how CO2 electrocatalysts behave in flow reactors that are more relevant to a scalable CO2 electrolyzer system. Flow reactors also offer more control over reagent delivery, which includes enabling the use of a gaseous CO2 feed to the cathode of the cell. This setup provides a platform for generating much higher current densities (J) by reducing the mass transport issues inherent to the H-cells.In this Account, we examine some of the systems-level strategies that have been applied in an effort to tailor flow reactor components to improve electrocatalytic redn. Flow reactors that have been utilized in CO2 electrolysis schemes can be categorized into two primary architectures: Membrane-based flow cells and microfluidic reactors. Each invoke different dynamic mechanisms for the delivery of gaseous CO2 to electrocatalytic sites, and both have been demonstrated to achieve high current densities (J > 200 mA cm-2) for CO2 redn. One strategy common to both reactor architectures for improving J is the delivery of CO2 to the cathode in the gas phase rather than dissolved in a liq. electrolyte. This phys. facet also presents a no. of challenges that go beyond the nature of the electrocatalyst, and we scrutinize how the judicious selection and modification of certain components in microfluidic and/or membrane-based reactors can have a profound effect on electrocatalytic performance. In membrane-based flow cells, for example, the choice of either a cation exchange membrane (CEM), anion exchange membrane (AEM), or a bipolar membrane (BPM) affects the kinetics of ion transport pathways and the range of applicable electrolyte conditions. In microfluidic cells, extensive studies have been performed upon the properties of porous carbon gas diffusion layers, materials that are equally relevant to membrane reactors. A theme that is pervasive throughout our analyses is the challenges assocd. with precise and controlled water management in gas phase CO2 electrolyzers, and we highlight studies that demonstrate the importance of maintaining adequate flow cell hydration to achieve sustained electrolysis.
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    41. 41
      Endrődi, B.; Bencsik, G.; Darvas, F.; Jones, R.; Rajeshwar, K.; Janáky, C. Continuous-flow electroreduction of carbon dioxide. Prog. Energy Combust. Sci. 2017, 62, 133– 154,  DOI: 10.1016/j.pecs.2017.05.005
      There is no corresponding record for this reference.
    42. 42
      Xing, Z.; Hu, L.; Ripatti, D. S.; Hu, X.; Feng, X. Enhancing carbon dioxide gas-diffusion electrolysis by creating a hydrophobic catalyst microenvironment. Nat. Commun. 2021, 12, 136,  DOI: 10.1038/s41467-020-20397-5
      42
      Enhancing carbon dioxide gas-diffusion electrolysis by creating a hydrophobic catalyst microenvironment
      Xing, Zhuo; Hu, Lin; Ripatti, Donald S.; Hu, Xun; Feng, Xiaofeng
      Nature Communications (2021), 12 (1), 136CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Electroredn. of carbon dioxide (CO2) over copper-based catalysts provides an attractive approach for sustainable fuel prodn. While efforts are focused on developing catalytic materials, it is also crit. to understand and control the microenvironment around catalytic sites, which can mediate the transport of reaction species and influence reaction pathways. Here, we show that a hydrophobic microenvironment can significantly enhance CO2 gas-diffusion electrolysis. For proof-of-concept, we use com. copper nanoparticles and disperse hydrophobic polytetrafluoroethylene (PTFE) nanoparticles inside the catalyst layer. Consequently, the PTFE-added electrode achieves a greatly improved activity and Faradaic efficiency for CO2 redn., with a partial c.d. >250 mA cm-2 and a single-pass conversion of 14% at moderate potentials, which are around twice that of a regular electrode without added PTFE. The improvement is attributed to a balanced gas/liq. microenvironment that reduces the diffusion layer thickness, accelerates CO2 mass transport, and increases CO2 local concn. for the electrolysis.
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    43. 43
      Yang, K.; Kas, R.; Smith, W. A.; Burdyny, T. Role of the carbon-based gas diffusion layer on flooding in a gas diffusion electrode cell for electrochemical CO2 reduction. ACS Energy Lett. 2021, 6 (1), 33– 40,  DOI: 10.1021/acsenergylett.0c02184
      43
      Role of the Carbon-Based Gas Diffusion Layer on Flooding in a Gas Diffusion Electrode Cell for Electrochemical CO2 Reduction
      Yang, Kailun; Kas, Recep; Smith, Wilson A.; Burdyny, Thomas
      ACS Energy Letters (2021), 6 (1), 33-40CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      The deployment of gas diffusion electrodes (GDEs) for the electrochem. CO2 redn. reaction (CO2RR) has enabled current densities an order of magnitude greater than those of aq. H cells. The gains in prodn., however, have come with stability challenges due to rapid flooding of GDEs, which frustrate both lab. expts. and scale-up prospects. Here, we investigate the role of carbon gas diffusion layers (GDLs) in the advent of flooding during CO2RR, finding that applied potential plays a central role in the obsd. instabilities. Electrochem. characterization of carbon GDLs with and without catalysts suggests that the high overpotential required during electrochem. CO2RR initiates hydrogen evolution on the carbon GDL support. These potentials impact the wetting characteristics of the hydrophobic GDL, resulting in flooding that is independent of CO2RR. Findings from this work can be extended to any electrochem. redn. reaction using carbon-based GDEs (CORR or N2RR) with cathodic overpotentials of less than -0.65 V vs. a reversible hydrogen electrode.
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    44. 44
      Jouny, M.; Luc, W.; Jiao, F. High-rate electroreduction of carbon monoxide to multi-carbon products. Nat. Catal. 2018, 1, 748– 755,  DOI: 10.1038/s41929-018-0133-2
      44
      High-rate electroreduction of carbon monoxide to multi-carbon products
      Jouny, Matthew; Luc, Wesley; Jiao, Feng
      Nature Catalysis (2018), 1 (10), 748-755CODEN: NCAACP; ISSN:2520-1158. (Nature Research)
      Carbon monoxide electrolysis has previously been reported to yield enhanced multi-carbon (C2+) Faradaic efficiencies of up to ∼55%, but only at low reaction rates. This is due to the low soly. of CO in aq. electrolytes and operation in batch-type reactors. Here, we present a high-performance CO flow electrolyzer with a well controlled electrode-electrolyte interface that can reach total current densities of up to 1 A cm-2, together with improved C2+ selectivities. Computational transport modeling and isotopic C18O redn. expts. suggest that the enhanced activity is due to a higher surface pH under CO redn. conditions, which facilitates the prodn. of acetate. At optimal operating conditions, we achieve a C2+ Faradaic efficiency of ∼91% with a C2+ partial c.d. over 630 mA cm-2. Further investigations show that maintaining an efficient triple-phase boundary at the electrode-electrolyte interface is the most crit. challenge in achieving a stable CO/CO2 electrolysis process at high rates.
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    45. 45
      Wu, J.; Risalvato, F. G.; Sharma, P. P.; Pellechia, P. J.; Ke, F.-S.; Zhou, X.-D. Electrochemical reduction of carbon dioxide II. Design, assembly, and performance of low temperature full electrochemical cells. J. Electrochem. Soc. 2013, 160, F953– F957,  DOI: 10.1149/2.030309jes
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      Electrochemical reduction of carbon dioxide: II. Design, assembly, and performance of low temperature full electrochemical cells
      Wu, Jingjie; Risalvato, Frank G.; Sharma, Pranav P.; Pellechia, Perry J.; Ke, Fu-Sheng; Zhou, Xiao-Dong
      Journal of the Electrochemical Society (2013), 160 (9), F953-F957CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      In this paper, we report the performance of a full electrochem. cell which directly converts carbon dioxide to fuels at room temp. and ambient pressure. The design of this cell features a buffer layer of liq.-phase electrolyte circulating between the ion exchange membrane and the cathode Sn catalyst layer. In the absence of the buffer layer, hydrogen was the predominant product with a faradaic efficiency nearly ∼100%. Incorporating a buffer layer with an electrolyte, e.g. 0.1 M KHCO3, substantially promoted the formation of formate and CO, while suppressing hydrogen prodn. When the anode was fed with hydrogen, the onset of formate prodn. occurred at -0.8 V, with a faradaic efficiency of 65% and a partial c.d. of -1 mA cm-2; at which the energy efficiency toward formate prodn. was 50%. The highest faradaic efficiency obsd. toward formate formation was over 90% at -1.7 V corresponding to a partial c.d. of -9 mA cm-2. When the anode was fed with aq. reactant (e.g. 1 M KOH soln.), the formate prodn. began at -1.2 V with a partial c.d. of -1 mA cm-2, corresponding to a faradaic efficiency of 70% and an energy efficiency of 60%. In this case, the highest faradaic efficiency toward formate formation was 85% at -2.0 V with a partial c.d. of -6 mA cm-2. Our studies show that this full electrochem. cell with a circulating liq.-phase electrolyte buffer layer enables prodn. of formate at an overpotential of ∼-0.2 V regardless of the gaseous or aq. reactants at the anode side.
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    46. 46
      Zheng, T.; Jiang, K.; Ta, N.; Hu, Y.; Zeng, J.; Liu, J.; Wang, H. Large-scale and highly selective CO2 electrocatalytic reduction on nickel single-atom catalyst. Joule 2019, 3, 265– 278,  DOI: 10.1016/j.joule.2018.10.015
      46
      Large-Scale and Highly Selective CO2 Electrocatalytic Reduction on Nickel Single-Atom Catalyst
      Zheng, Tingting; Jiang, Kun; Ta, Na; Hu, Yongfeng; Zeng, Jie; Liu, Jingyue; Wang, Haotian
      Joule (2019), 3 (1), 265-278CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
      The scaling up of electrocatalytic CO2 redn. for practical applications is still hindered by a few challenges: low selectivity, small c.d. to maintain a reasonable selectivity, and the cost of the catalytic materials. Here we report a facile synthesis of earth-abundant Ni single-atom catalysts on com. carbon black, which were further employed in a gas-phase electrocatalytic reactor under ambient conditions. As a result, those single-at. sites exhibit an extraordinary performance in reducing CO2 to CO, yielding a c.d. above 100 mA cm-2, with nearly 100% selectivity for CO and around 1% toward the hydrogen evolution side reaction. By further scaling up the electrode into a 10 × 10-cm2 modular cell, the overall current in one unit cell can easily ramp up to more than 8 A while maintaining an exclusive CO evolution with a generation rate of 3.34 L hr-1 per unit cell.
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    47. 47
      Lees, E. W.; Bui, J. C.; Romiluyi, O.; Bell, A. T.; Weber, A. Z. Exploring CO2 reduction and crossover in membrane electrode assemblies. Nat. Chem. Eng. 2024, 1, 340– 353,  DOI: 10.1038/s44286-024-00062-0
      There is no corresponding record for this reference.
    48. 48
      Sassenburg, M.; Kelly, M.; Subramanian, S.; Smith, W. A.; Burdyny, T. Zero-gap electrochemical CO2 reduction cells: Challenges and operational strategies for prevention of salt precipitation. ACS Energy Lett. 2023, 8 (1), 321– 331,  DOI: 10.1021/acsenergylett.2c01885
      48
      Zero-gap electrochemical carbon dioxide reduction cells, challenges and operational strategies for prevention of salt precipitation
      Sassenburg, Mark; Kelly, Maria; Subramanian, Siddhartha; Smith, Wilson A.; Burdyny, Thomas
      ACS Energy Letters (2023), 8 (1), 321-331CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Salt pptn. is a problem in electrochem. CO2 redn. electrolyzers that limits their long-term durability and industrial applicability by reducing active area, causing flooding and hindering gas transport. Salt crystals form when hydroxide generation from electrochem. reactions interacts homogeneously with CO2 to generate substantial quantities of carbonate. In the presence of sufficient electrolyte cations, the soly. limits of these species are reached, resulting in "salting out" conditions in cathode compartments. Detrimental salt pptn. is regularly obsd. in zero-gap membrane electrode assemblies, esp. when operated at high current densities. This Perspective briefly discusses the mechanisms for salt formation, and recently reported strategies for preventing or reversing salt formation in zero-gap CO2 redn. membrane electrode assemblies. We link these approaches to the soly. limit of potassium carbonate within the electrolyzer, and describe how each strategy sep. manipulates water, potassium and carbonate concns. to prevent (or mitigate) salt formation.
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    49. 49
      Endrődi, B.; Samu, A.; Kecsenovity, E.; Halmágyi, T.; Sebők, D.; Janáky, C. Operando cathode activation with alkali metal cations for high current density operation of water-fed zero-gap carbon dioxide electrolysers. Nat. Energy 2021, 6, 439– 448,  DOI: 10.1038/s41560-021-00813-w
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      Operando cathode activation with alkali metal cations for high current density operation of water-fed zero-gap carbon dioxide electrolysers
      Endrodi, B.; Samu, A.; Kecsenovity, E.; Halmagyi, T.; Sebok, D.; Janaky, C.
      Nature Energy (2021), 6 (4), 439-448CODEN: NEANFD; ISSN:2058-7546. (Nature Portfolio)
      Continuous-flow electrolyzers allow CO2 redn. at industrially relevant rates, but long-term operation is still challenging. One reason for this is the formation of ppts. in the porous cathode from the alk. electrolyte and the CO2 feed. Here we show that while ppt. formation is detrimental for the long-term stability, the presence of alkali metal cations at the cathode improves performance. To overcome this contradiction, we develop an operando activation and regeneration process, where the cathode of a zero-gap electrolyzer cell is periodically infused with alkali cation-contg. solns. This enables deionized water-fed electrolyzers to operate at a CO2 redn. rate matching those using alk. electrolytes (CO partial c.d. of 420 ± 50 mA cm-2 for over 200 h). We deconvolute the complex effects of activation and validate the concept with five different electrolytes and three different com. membranes. Finally, we demonstrate the scalability of this approach on a multicell electrolyzer stack, with an active area of 100 cm2 per cell.
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    50. 50
      Okuhara, T.; Watanabe, H.; Nishimura, T.; Inumaru, K.; Misono, M. Microstructure of cesium hydrogen salts of 12-tungstophosphoric acid relevant to novel acid catalysis. Chem. Mater. 2000, 12, 2230– 2238,  DOI: 10.1021/cm9907561
      50
      Microstructure of Cesium Hydrogen Salts of 12-Tungstophosphoric Acid Relevant to Novel Acid Catalysis
      Okuhara, Toshio; Watanabe, Hiromu; Nishimura, Toru; Inumaru, Kei; Misono, Makoto
      Chemistry of Materials (2000), 12 (8), 2230-2238CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      A comprehensive interpretation of the microstructure and mechanism of the formation of a versatile solid acid catalyst, Cs2.5H0.5PW12O40, has been attempted by combining the new results obtained with solid-state NMR, XRD, SEM, and N2 porosimetry with the data reported previously. The ppts. of Cs2.5H0.5PW12O40 just formed from aq. solns. of H3PW12O40 and Cs2CO3 consist of ultrafine crystallites in which the acid form, H3PW12O40, is epitaxially deposited on the surface of Cs3PW12O40 crystallites. Calcination of the ppts. brings about the migration of H+ and Cs+ in the solid to form a nearly uniform solid soln. in which protons distribute randomly through the entire bulk, as revealed by XRD and 31P solid-state NMR. Impregnation of Cs3PW12O40 with the aq. soln. of H3PW12O40 also gives the uniform salt after calcination. Pore-size distribution evaluated by the anal. of N2 desorption isotherm showed that Cs2.5H0.5PW12O40 has mesopores as well as micropores that are interparticle voids of the crystallites. The initial heat of NH3 sorption indicated the presence of very strong acid sites on Cs2.5H0.5PW12O40. High catalytic activity of Cs2.5H0.5PW12O40 reported for solid-liq. reaction systems is thus principally attributed to the strength and no. of acid sites and the mesoporous structure appropriate for the rapid diffusion of mols.
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    51. 51
      Mikkelsen, M.; Jørgensen, M.; Krebs, F. C. The teraton challenge. A review of fixation and transformation of carbon dioxide. Energy Environ. Sci. 2010, 3, 43– 81,  DOI: 10.1039/B912904A
      51
      The teraton challenge. A review of fixation and transformation of carbon dioxide
      Mikkelsen, Mette; Jorgensen, Mikkel; Krebs, Frederik C.
      Energy & Environmental Science (2010), 3 (1), 43-81CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      A review. The increase in atm. carbon dioxide is linked to climate changes; hence there is an urgent need to reduce the accumulation of CO2 in the atm. The utilization of CO2 as a raw material in the synthesis of chems. and liq. energy carriers offers a way to mitigate the increasing CO2 buildup. This review covers six important CO2 transformations namely: chem. transformations, photochem. redns., chem. and electrochem. redns., biol. conversions, reforming and inorg. transformations. Furthermore, the vast research area of carbon capture and storage is reviewed briefly. This review is intended as an introduction to CO2, its synthetic reactions and their possible role in future CO2 mitigation schemes that has to match the scale of man-made CO2 in the atm., which rapidly approaches 1 teraton.
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    52. 52
      Zhu, D. D.; Liu, J. L.; Qiao, S. Z. Recent advances in inorganic heterogeneous electrocatalysts for reduction of carbon dioxide. Adv. Mater. 2016, 28, 3423– 3452,  DOI: 10.1002/adma.201504766
      52
      Recent Advances in Inorganic Heterogeneous Electrocatalysts for Reduction of Carbon Dioxide
      Zhu, Dong Dong; Liu, Jin Long; Qiao, Shi Zhang
      Advanced Materials (Weinheim, Germany) (2016), 28 (18), 3423-3452CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. In view of the climate changes caused by the continuously rising levels of atm. CO2, advanced technologies assocd. with CO2 conversion are highly desirable. In recent decades, electrochem. redn. of CO2 has been extensively studied since it can reduce CO2 to value-added chems. and fuels. Considering the sluggish reaction kinetics of the CO2 mol., efficient and robust electrocatalysts are required to promote this conversion reaction. Here, recent progress and opportunities in inorg. heterogeneous electrocatalysts for CO2 redn. are discussed, from the viewpoint of both exptl. and computational aspects. Based on elemental compn., the inorg. catalysts presented here are classified into four groups: metals, transition-metal oxides, transition-metal chalcogenides, and carbon-based materials. However, despite encouraging accomplishments made in this area, substantial advances in CO2 electrolysis are still needed to meet the criteria for practical applications. Therefore, in the last part, several promising strategies, including surface engineering, chem. modification, nanostructured catalysts, and composite materials, are proposed to facilitate the future development of CO2 electroredn.
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    53. 53
      Virtue Market Research. Global formic acids market research report. 2023. https://virtuemarketresearch.com/report/formic-acid-market.
      There is no corresponding record for this reference.
    54. 54
      Nankya, R.; Xu, Y.; Elgazzar, A.; Zhu, P.; Wi, T.; Qiu, C.; Feng, Y.; Che, F.; Wang, H. Cobalt-doped bismuth nanosheet catalyst for enhanced electrochemical CO2 reduction to electrolyte-free formic acid. Angew. Chem., Int. Ed. 2024, 63 (36), e202403671  DOI: 10.1002/anie.202403671
      There is no corresponding record for this reference.
    55. 55
      Zhu, J.; Li, J.; Lu, R.; Yu, R.; Zhao, S.; Li, C.; Lv, L.; Xia, L.; Chen, X.; Cai, W. Surface passivation for highly active, selective, stable, and scalable CO2 electroreduction. Nat. Commun. 2023, 14, 4670,  DOI: 10.1038/s41467-023-40342-6
      There is no corresponding record for this reference.
    56. 56
      Zhang, G.; Tan, B.; Mok, D. H.; Liu, H.; Ni, B.; Zhao, G.; Ye, K.; Huo, S.; Miao, X.; Liang, Z. Electrifying HCOOH synthesis from CO2 building blocks over Cu-Bi nanorod arrays. Proc. Natl. Acad. Sci. U.S.A. 2024, 121 (29), e2400898121  DOI: 10.1073/pnas.2400898121
      There is no corresponding record for this reference.
    57. 57
      Elgazzar, A.; Zhu, P.; Chen, F.-Y.; Hao, S.; Wi, T.-U.; Qiu, C.; Okatenko, V.; Wang, H. Electrochemical CO2 reduction to formic acid with high carbon efficiency. ACS Energy Lett. 2025, 10, 450– 458,  DOI: 10.1021/acsenergylett.4c02773
      There is no corresponding record for this reference.
    58. 58
      Zhu, P.; Xia, C.; Liu, C.-Y.; Jiang, K.; Gao, G.; Zhang, X.; Xia, Y.; Lei, Y.; Alshareef, H. N.; Senftle, T. P. Direct and continuous generation of pure acetic acid solutions via electrocatalytic carbon monoxide reduction. Proc. Natl. Acad. Sci. U.S.A. 2021, 118 (2), e2010868118  DOI: 10.1073/pnas.2010868118
      58
      Direct and continuous generation of pure acetic acid solutions via electrocatalytic carbon monoxide reduction
      Zhu, Peng; Xia, Chuan; Liu, Chun-Yen; Jiang, Kun; Gao, Guanhui; Zhang, Xiao; Xia, Yang; Lei, Yongjiu; Alshareef, Husam N.; Senftle, Thomas P.; Wang, Haotian
      Proceedings of the National Academy of Sciences of the United States of America (2021), 118 (2), e2010868118CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Electrochem. CO2 or CO redn. to high-value C2+ liq. fuels is desirable, but its practical application is challenged by impurities from cogenerated liq. products and solutes in liq. electrolytes, which necessitates cost- and energy-intensive downstream sepn. processes. By coupling rational designs in a Cu catalyst and porous solid electrolyte (PSE) reactor, here we demonstrate a direct and continuous generation of pure acetic acid solns. via electrochem. CO redn. With optimized edge-to-surface ratio, the Cu nanocube catalyst presents an unprecedented acetate performance in neutral pH with other liq. products greatly suppressed, delivering a maximal acetate Faradaic efficiency of 43%, partial current of 200 mA·cm-2, ultrahigh relative purity of up to 98 wt%, and excellent stability of over 150 h continuous operation. D. functional theory simulations reveal the role of stepped sites along the cube edge in promoting the acetate pathway. Addnl., a PSE layer, other than a conventional liq. electrolyte, was designed to sep. cathode and anode for efficient ion conductions, while not introducing any impurity ions into generated liq. fuels. Pure acetic acid solns., with concns. up to 2 wt% (0.33 M), can be continuously produced by employing the acetate-selective Cu catalyst in our PSE reactor.
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    59. 59
      Yang, S.; Verdaguer-Casadevall, A.; Arnarson, L.; Silvioli, L.; Čolić, V.; Frydendal, R.; Rossmeisl, J.; Chorkendorff, I.; Stephens, I. E. L. Toward the decentralized electrochemical production of H2O2: A focus on the catalysis. ACS Catal. 2018, 8, 4064– 4081,  DOI: 10.1021/acscatal.8b00217
      59
      Toward the Decentralized Electrochemical Production of H2O2: A Focus on the Catalysis
      Yang, Sungeun; Verdaguer-Casadevall, Arnau; Arnarson, Logi; Silvioli, Luca; Colic, Viktor; Frydendal, Rasmus; Rossmeisl, Jan; Chorkendorff, Ib; Stephens, Ifan E. L.
      ACS Catalysis (2018), 8 (5), 4064-4081CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      H2O2 is a valuable, environmentally friendly oxidizing agent with a wide range of uses from the provision of clean H2O to the synthesis of valuable chems. The on-site electrolytic prodn. of H2O2 would bring the chem. to applications beyond its present reach. The successful commercialization of electrochem. H2O2 prodn. requires cathode catalysts with high activity, selectivity, and stability. In this Perspective, the authors highlight the authors' current understanding of the factors that control the cathode performance. The authors review the influence of catalyst material, electrolyte, and the structure of the interface at the mesoscopic scale. The authors provide original theor. data on the role of the geometry of the active site and its influence on activity and selectivity. The authors have also conducted original expts. on (i) the effect of pH on H2O2 prodn. on glassy C, pure metals, and metal-Hg alloys, and (ii) the influence of cell geometry and mass transport in liq. half-cells in comparison to membrane electrode assemblies.
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    60. 60
      Yi, Y.; Wang, L.; Li, G.; Guo, H. A review on research progress in the direct synthesis of hydrogen peroxide from hydrogen and oxygen: Noble-metal catalytic method, fuel-cell method and plasma method. Catal. Sci. Technol. 2016, 6, 1593– 1610,  DOI: 10.1039/C5CY01567G
      60
      A review on research progress in the direct synthesis of hydrogen peroxide from hydrogen and oxygen: noble-metal catalytic method, fuel-cell method and plasma method
      Yi, Yanhui; Wang, Li; Li, Gang; Guo, Hongchen
      Catalysis Science & Technology (2016), 6 (6), 1593-1610CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      Hydrogen peroxide (H2O2) as a highly efficient and green oxidant has become one of the 100 most important chems. in the world. Some current research progress in the direct synthesis of H2O2 from H2 and O2 by noble-metal catalyst, fuel cell and plasma methods has been reviewed systematically in this paper. Perspectives about the development direction and application prospect of the above-mentioned three methods have also been discussed.
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    61. 61
      Siahrostami, S.; Verdaguer-Casadevall, A.; Karamad, M.; Deiana, D.; Malacrida, P.; Wickman, B.; Escudero-Escribano, M.; Paoli, E. A.; Frydendal, R.; Hansen, T. W. Enabling direct H2O2 production through rational electrocatalyst design. Nat. Mater. 2013, 12, 1137– 1143,  DOI: 10.1038/nmat3795
      61
      Enabling direct H2O2 production through rational electrocatalyst design
      Siahrostami, Samira; Verdaguer-Casadevall, Arnau; Karamad, Mohammadreza; Deiana, Davide; Malacrida, Paolo; Wickman, Bjoern; Escudero-Escribano, Maria; Paoli, Elisa A.; Frydendal, Rasmus; Hansen, Thomas W.; Chorkendorff, Ib; Stephens, Ifan E. L.; Rossmeisl, Jan
      Nature Materials (2013), 12 (12), 1137-1143CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Future generations require more efficient and localized processes for energy conversion and chem. synthesis. The continuous on-site prodn. of H2O2 would provide an attractive alternative to the present state-of-the-art, which is based on the complex anthraquinone process. The electrochem. redn. of O to H2O2 is a particularly promising means of achieving this aim. However, it would require active, selective and stable materials to catalyze the reaction. Although progress was made in this respect, further improvements through the development of new electrocatalysts are needed. Using d. functional theory calcns., the authors identify Pt-Hg as a promising candidate. Electrochem. measurements on Pt-Hg nanoparticles show more than an order of magnitude improvement in mass activity, i.e., A g-1 precious metal, for H2O2 prodn., over the best performing catalysts in the literature.
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    62. 62
      Dulo, B.; Phan, K.; Githaiga, J.; Raes, K.; Meester, S. D. Natural quinone dyes: A Review on structure, extraction techniques, analysis and application potential. Waste Biomass Valor. 2021, 12, 6339– 6374,  DOI: 10.1007/s12649-021-01443-9
      There is no corresponding record for this reference.
    63. 63
      Adler, Z.; Zhang, X.; Feng, G.; Shi, Y.; Zhu, P.; Xia, Y.; Shan, X.; Wang, H. Hydrogen peroxide electrosynthesis in a strong acidic environment using cationic surfactants. Precis. Chem. 2024, 2 (4), 129– 137,  DOI: 10.1021/prechem.3c00096
      There is no corresponding record for this reference.
    64. 64
      van Langevelde, P. H.; Katsounaros, I.; Koper, M. T. M. Electrocatalytic nitrate reduction for sustainable ammonia production. Joule 2021, 5, 290– 294,  DOI: 10.1016/j.joule.2020.12.025
      There is no corresponding record for this reference.
    65. 65
      Jiang, Y.; Chen, J.; Chen, F.; Wang, M.; Li, Y.; Li, M.; Qian, B.; Wang, Z.; Zhang, S.; Zhou, H. Synergistic enhancement of Ca-based materials via CeO2 and Al2O3 co-doping for enhanced CO2 capture and thermochemical energy storage in calcium looping technology. Sep. Purif. 2025, 358, 130264  DOI: 10.1016/j.seppur.2024.130264
      There is no corresponding record for this reference.
    66. 66
      Zito, A. M.; Clarke, L. E.; Barlow, J. M.; Bím, D.; Zhang, Z.; Ripley, K. M.; Li, C. J.; Kummeth, A.; Leonard, M. E.; Alexandrova, A. N. Electrochemical carbon dioxide capture and concentration. Chem. Rev. 2023, 123 (13), 8069– 8098,  DOI: 10.1021/acs.chemrev.2c00681
      There is no corresponding record for this reference.
    67. 67
      Zheng, T.; Zhang, M.; Wu, L.; Guo, S.; Liu, X.; Zhao, J.; Xue, W.; Li, J.; Liu, C.; Li, X. Upcycling CO2 into energy-rich long-chain compounds via electrochemical and metabolic engineering. Nat. Catal. 2022, 5, 388– 396,  DOI: 10.1038/s41929-022-00775-6
      67
      Upcycling CO2 into energy-rich long-chain compounds via electrochemical and metabolic engineering
      Zheng, Tingting; Zhang, Menglu; Wu, Lianghuan; Guo, Shuyuan; Liu, Xiangjian; Zhao, Jiankang; Xue, Weiqing; Li, Jiawei; Liu, Chunxiao; Li, Xu; Jiang, Qiu; Bao, Jun; Zeng, Jie; Yu, Tao; Xia, Chuan
      Nature Catalysis (2022), 5 (5), 388-396CODEN: NCAACP; ISSN:2520-1158. (Nature Portfolio)
      Upcycling of carbon dioxide (CO2) into value-added products represents a substantially untapped opportunity to tackle environmental issues and achieve a circular economy. Compared with easily available C1/C2 products, nevertheless, efficient and sustainable synthesis of energy-rich long-chain compds. from CO2 still remains a grand challenge. Here we describe a hybrid electro-biosystem, coupling spatially sep. CO2 electrolysis with yeast fermn., that efficiently converts CO2 to glucose with a high yield. We employ a nanostructured copper catalyst that can stably catalyze pure acetic acid prodn. with a solid-electrolyte reactor. We then genetically engineer Saccharomyces cerevisiae to produce glucose in vitro from electro-generated acetic acid by deleting all defined hexokinase genes and overexpression of heterologous glucose-1-phosphatase. In addn., we showcase that the proposed platform can be easily extended to produce other products like fatty acids using CO2 as the carbon source. These results illuminate the tantalizing possibility of a renewable-electricity-driven manufg. industry.
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    68. 68
      Bi, H.; Wang, K.; Xu, C.; Wang, M.; Chen, B.; Fang, Y.; Tan, X.; Zeng, J.; Tan, T. Biofuel synthesis from carbon dioxide via a bio-electrocatalysis system. Chem. Catal. 2023, 3 (3), 100557  DOI: 10.1016/j.checat.2023.100557
      There is no corresponding record for this reference.
    69. 69
      Zheng, T.; Xia, C. Electrifying biosynthesis for CO2 upcycling. Trends Chem. 2023, 5 (1), 7– 10,  DOI: 10.1016/j.trechm.2022.10.006
      There is no corresponding record for this reference.
    70. 70
      Wi, T.-U.; Xie, Y.; Levell, Z. H.; Feng, D.; Kim, J. Y.; Zhu, P.; Elgazzar, A.; Jeon, T. H.; Shakouri, M.; Hao, S. Upgrading carbon monoxide to bioplastics via integrated electrochemical reduction and biosynthesis. Nat. Synth. 2024, 3, 1392– 1403,  DOI: 10.1038/s44160-024-00621-6
      There is no corresponding record for this reference.
    71. 71
      Fan, L.; Zhao, Y.; Chen, L.; Chen, J.; Chen, J.; Yang, H.; Xiao, Y.; Zhang, T.; Chen, J.; Wang, L. Selective production of ethylene glycol at high rate via cascade catalysis. Nat. Catal. 2023, 6, 585– 595,  DOI: 10.1038/s41929-023-00977-6
      71
      Selective production of ethylene glycol at high rate via cascade catalysis
      Fan, Lei; Zhao, Yilin; Chen, Lei; Chen, Jiayi; Chen, Junmei; Yang, Haozhou; Xiao, Yukun; Zhang, Tianyu; Chen, Jingyi; Wang, Lei
      Nature Catalysis (2023), 6 (7), 585-595CODEN: NCAACP; ISSN:2520-1158. (Nature Portfolio)
      Ethylene glycol is currently produced via an energy-intensive two-step thermocatalytic process that results in substantial CO2 emissions. Sustainable ethylene glycol prodn. via ethylene electro-oxidn. powered by renewable electricity is desirable; however, direct ethylene electro-oxidn. suffers from unsatisfactory product selectivity, particularly at high prodn. rates. Here we report a cascade strategy for efficient and selective prodn. of pure ethylene glycol soln. under ambient conditions with no detectable byproduct formation. Specifically, ethylene is converted to ethylene glycol on a catalyst/solid-acid composite using electrochem. generated hydrogen peroxide as the oxidant. Using an integrated solid-electrolyte reactor, we achieved high electron utilization efficiency of 60-70% at industrially relevant current densities (100-500 mA cm-2) for ethylene glycol prodn. with full product selectivity (∼100%). We further integrated this system with a CO2 electroredn reactor and demonstrated the sustainable prodn. of pure ethylene glycol using CO2 and water as the only feedstocks.
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    72. 72
      Zhang, S.-K.; Feng, Y.; Elgazzar, A.; Xia, Y.; Qiu, C.; Adler, Z.; Sellers, C.; Wang, H. Interfacial electrochemical-chemical reaction coupling for efficient olefin oxidation to glycols. Joule 2023, 7, 1887– 1901,  DOI: 10.1016/j.joule.2023.06.022
      72
      Interfacial electrochemical-chemical reaction coupling for efficient olefin oxidation to glycols
      Zhang, Shou-Kun; Feng, Yuge; Elgazzar, Ahmad; Xia, Yang; Qiu, Chang; Adler, Zachary; Sellers, Chase; Wang, Haotian
      Joule (2023), 7 (8), 1887-1901CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
      Coupling electrochem. and chem. reactions has been demonstrated in traditional tandem reactor systems, but their practical applications are still distd. down to individual reactor optimizations. Here, we demonstrate a fully integrated system that presents significantly improved catalytic performance when compared with traditional tandem systems. Using electrosynthesis of hydrogen peroxide followed by olefin epoxidn. reaction as a representative example, we demonstrated that, by confining the chem. reaction right at the electrode/electrolyte interface in our solid electrolyte reactor, we can fully leverage the interfacial high concn. of H2O2 product from electrocatalysis to boost the following ethylene epoxidn. reaction, which represented a 3-fold improvement in electrolyte-free ethylene glycol generation when compared with a tandem reactor system. This integration strategy can be extended to other electrochem.-chem. coupling reactions, esp. when the coupled reaction is sensitive to reactant concns., which could avoid energy-intensive sepn. or concn. steps typically needed between the electrochem. and chem. reactions.
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    73. 73
      Xia, C. Nexus molecule outputs pure ethylene glycol. Nat. Catal. 2023, 6, 559– 560,  DOI: 10.1038/s41929-023-00989-2
      There is no corresponding record for this reference.
    74. 74
      Wang, P.; Wang, X.; Chandra, S.; Lielpetere, A.; Quast, T.; Conzuelo, F.; Schuhmann, W. Hybrid enzyme-electrocatalyst cascade modified gas-diffusion electrodes for methanol formation from carbon dioxide. Angew. Chem., Int. Ed. 2025,  DOI: 10.1002/anie.202422882
      There is no corresponding record for this reference.
    75. 75
      Fan, L.; Zhu, Z.; Zhao, S.; Panda, S.; Zhao, Y.; Chen, J.; Chen, L.; Chen, J.; He, J.; Zhou, K. Blended nexus molecules promote CO2 to L-tyrosine conversion. Sci. Adv. 2024, 10 (36), eado1352  DOI: 10.1126/sciadv.ado1352
      There is no corresponding record for this reference.
    76. 76
      Orella, M. J.; Brown, S. M.; Leonard, M. E.; Román-Leshkov, Y.; Brushett, F. R. A general technoeconomic model for evaluating emerging electrolytic processes. Energy Technol. 2020, 8, 1900994  DOI: 10.1002/ente.201900994
      There is no corresponding record for this reference.
    77. 77
      Gong, G.; Wu, B.; Liu, L.; Li, J.; Zhu, Q.; He, M.; Hu, G. Metabolic engineering using acetate as a promising building block for the production of bio-based chemicals. Eng. Microbiol. 2022, 2 (4), 100036  DOI: 10.1016/j.engmic.2022.100036
      77
      Metabolic engineering using acetate as a promising building block for the production of bio-based chemicals
      Gong, Guiping; Wu, Bo; Liu, Linpei; Li, Jianting; Zhu, Qili; He, Mingxiong; Hu, Guoquan
      Engineering Microbiology (2022), 2 (4), 100036CODEN: EMNICH; ISSN:2667-3703. (Elsevier B.V.)
      The prodn. of biofuels and biochems. derived from microbial fermn. has received a lot of attention and interest in light of concerns about the depletion of fossil fuel resources and climatic degeneration. However, the economic viability of feedstocks for biol. conversion remains a barrier, urging researchers to develop renewable and sustainable low-cost carbon sources for future bioindustries. Owing to the numerous advantages, acetate has been regarded as a promising feedstock targeting the prodn. of acetyl-CoA-derived chems. This review aims to highlight the potential of acetate as a building block in industrial biotechnol. for the prodn. of bio-based chems. with metabolic engineering. Different alternative approaches and routes comprised of lignocellulosic biomass, waste streams, and C1 gas for acetate generation are briefly described and evaluated. Then, a thorough explanation of the metabolic pathway for biotechnol. acetate conversion, cellular transport, and toxin tolerance is described. Particularly, current developments in metabolic engineering of the manuf. of biochems. from acetate are summarized in detail, with various microbial cell factories and strategies proposed to improve acetate assimilation and enhance product formation. Challenges and future development for acetate generation and assimilation as well as chems. prodn. from acetate is eventually shown. This review provides an overview of the current status of acetate utilization and proves the great potential of acetate with metabolic engineering in industrial biotechnol.
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