Tethered Alkylammonium Dications as Electrochemical Interface Modifiers: Chain Length Effect on CO2 Reduction Selectivity at Industry-Relevant Current Density

The electrochemical reduction of CO2 (CO2RR) has the potential to be an economically viable method to produce platform chemicals synergistically with renewable energy sources. Copper is one of the most commonly used electrocatalysts for this purpose, as it allows C–C bond formation, yielding a broad product distribution. Controlling selectivity is a stepping stone on the way to its industrial application. The kinetics of the reaction can be modified to favor the rates of certain products quickly and inexpensively by applying additives such as ionic liquids and coelectrolytes that directly affect the electrocatalytic interface. In this work, we propose tethered tetraalkylammonium salts as double-charged cationic modifiers of the electrochemical double layer to control CO2RR product selectivity. A novel setup comprising a gas diffusion electrode (GDE) flow cell coupled with real-time mass spectroscopy was used to study the effect of a library of the selected salts. We emphasize how the length of an alkyl linker effectively controls the selectivity of the reaction toward C1, C2, or C3 products at high relevant current densities (Jtotal = −400 mA cm–2) along with the inhibition of the parasitic hydrogen evolution reaction. Standard long-term experiments were performed for quantitative validation and stability evaluation. These results have broad implications for further tailoring an effective catalytic system for selective CO2 reduction reaction.


Calibration PTR-GDE setup
The high-resolution mass spectrometer proton transfer reaction (PTR) utilizes H3O + molecules as an ionization source.It can efficiently ionize various compounds, particularly volatile organic compounds (VOCs), as long as their proton affinity is higher than that of water.For instance, the ethylene is ionized according to the following reaction: The product quantification in concentration units (C) is achieved by multiplying the ratio of the primary (H 3 O + ) and the targeted compound's "i" ions multiplied and constants related to the sample acquisition and other kinetic parameters according to the equation: Here, t and k are the drift tube's reaction time and the ionization reaction rate, respectively. is the collection efficiency, which represents how much of the products from the stream are actually being measured.
A calibration gas provided by Air liquid with 5 %m of ethylene and pure CO 2 was mixed using Bronkhorst mass flow meters(El-flow) to produce different ethylene concentrations.The concentration mixtures ranged from 1 to 1100 ppm of Ethylene, keeping the total flow rate constant at 20 ml min −1 .The PTR signal response of the various mixtures is displayed in Figure S1.Fitting the experimental points in Figure S1 with a line enabled the quantification of ethylene at any given time, allowing real-time measurements.However, using the non-linear range was avoided to not deplete the primary ions and prevent instrument damage.
The partial current density of ethylene was calculated as follows: (2) Where n is the number of electrons needed to convert CO 2 to ethylene, V is the flow rate, and F is the Faraday constant.The quantification of organic volatile compounds was not considered as they were not the focus of the study.

Calibration EI-GC-GDE set-up
Unlike the high-resolution PTR-MS, an electron impact mass spectrometer can ionize hydrogen and methane.To obtain quantitative results, the mass spectrometer and the gas chromatography unit were run simultaneously, as shown in Figure S1.The flame ionization detector's signal responses determined the hydrogen concentration and the ethylene and methane thermal conductivity detector.Each experiment was calibrated, forming a calibration curve based on the GC quantification.For example, a long-term experiment with 15 GC injections.As shown in the figure, plotting the signal responses of both instruments together allows the line fitting, enabling real-time product quantification.The concentration   is proportional to the normalized mass spectrometer signal ( -  ).Considering the stabilized values, the FE increased by approximately 10 % for ethylene, and the hydrogen evolution reaction was reduced by 3 % after the injection.However, it is known that the selectivity towards H2 is steadily increasing during regular CO 2 electrolysis due to carbonation and GDE flooding.Had the partial current density of hydrogen been more stable, the real effect would have been more evident.

Figure S1 .
Figure S1.Ethylene calibration curve for multiple ethylene concentrations using the PTR GDE setup.The dotted line fits the ethylene-water signal ratio within a linear region, and the green curve fits all points by cubic splines.The ratio corresponds to the ratio of the ethylene C 2 H 2 + (m/z=26) and H 3 O + (m/z=21) signal of mass.H 3 O + with a mass of 21 was used, corresponding to only 0.206% of the total isopical abundance of water molecules.

Figure S2 .Figure S3 .
Figure S2.A simplified diagram of the EI-GC-GDE setup during a CO 2 RR experiment shows the arrows denoting the gas products' flow from the GDE cell outlet to the electrode impact mass spectrometer and the gas chromatography.

Figure S8 .
Figure S8.Scanning Electrode Microscopy (SEM) and Energy Dispersive X-Ray (EDX) images of unused catalysts (before operation) to the ones used for an hour-long operation at 400 mA cm -2 in 1M KHCO3 (After operation).The diammonium cation used as as additive was the one with an octylene linker.

TABLE S1
Performance of various Cu-based catalyst for the electroreduction of CO 2 .The choice of studies was driven by keeping Cu as the base-material itselft, so oxides and metal alloys or mixtures were excluded.

.8 400 This work Bis(triethylammonium)octane dibromide 27 19 38 <-0.8 400 This work
TABLE S2 Comparison of the performance of Cu catalyst modified with quaternary-ammonium salts present in the electrolyte for the electrochemical reduction of CO 2: Study Results vs. Literature