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Spin-Coated Heterogenous Stacked Electrodes for Performance Enhancement in CMOS-Compatible On-Chip Microsupercapacitors
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  • Agin Vyas*
    Agin Vyas
    Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Kemivägen 9, 41296, Gothenburg, Sweden
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-9064-6280
  • Simin Zare Hajibagher
    Simin Zare Hajibagher
    Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Kemivägen 9, 41296, Gothenburg, Sweden
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  • Ulises Méndez-Romero
    Ulises Méndez-Romero
    Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemigården 4, 41296, Gothenburg, Sweden
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  • Shameel Thurakkal
    Shameel Thurakkal
    Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemigården 4, 41296, Gothenburg, Sweden
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  • Qi Li
    Qi Li
    Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Kemivägen 9, 41296, Gothenburg, Sweden
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    Orcidhttps://orcid.org/0000-0001-6422-5020
  • Mazharul Haque
    Mazharul Haque
    Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Kemivägen 9, 41296, Gothenburg, Sweden
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  • R. K. Azega
    R. K. Azega
    Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Kemivägen 9, 41296, Gothenburg, Sweden
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  • Ergang Wang
    Ergang Wang
    Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemigården 4, 41296, Gothenburg, Sweden
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    Orcidhttps://orcid.org/0000-0002-4942-3771
  • Xiaoyan Zhang
    Xiaoyan Zhang
    Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemigården 4, 41296, Gothenburg, Sweden
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  • Per Lundgren
    Per Lundgren
    Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Kemivägen 9, 41296, Gothenburg, Sweden
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    Orcidhttps://orcid.org/0000-0003-3234-1670
  • Peter Enoksson
    Peter Enoksson
    Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Kemivägen 9, 41296, Gothenburg, Sweden
    Enoaviatech AB, 112 26 Stockholm, Sweden
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  • Anderson Smith
    Anderson Smith
    Department of Electrical Engineering, Chalmers University of Technology, Hörsalsvägen 7, 41296, Gothenburg, Sweden
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ACS Applied Energy Materials

Cite this: ACS Appl. Energy Mater. 2022, 5, 4, 4221–4231
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https://doi.org/10.1021/acsaem.1c03745
Published March 24, 2022

Copyright © 2022 The Authors. Published by American Chemical Society. This publication is licensed under

CC-BY 4.0 .

Abstract

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Integration of microsupercapacitors (MSCs) with on-chip sensors and actuators with nanoenergy harvesters can improve the lifetime of wireless sensor nodes in an Internet-of-Things (IoT) architecture. However, to be easy to integrate with such harvester technology, MSCs should be fabricated through a complementary-metal-oxide-semiconductor (CMOS) compatible technology, ubiquitous in electrode choice with the capability of heterogeneous stacking of electrodes for modulation in properties driven by application requirements. In this article, we address both these issues through fabrication of multielectrode modular, high energy density microsupercapacitors (MSC) containing reduced graphene oxide (GO), GO-heptadecane-9-amine (GO-HD9A), rGO-octadecylamine (rGO-ODA), and rGO-heptadecane-9-amine (rGO-HD9A) that stack through a scalable, CMOS compatible, high-wafer-yield spin-coating process. Furthermore, we compare the performance of the stack with individual electrode MSCs fabricated through the same process. The individual electrodes, in the presence of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfony)imide (EMIM-TFSI), demonstrate a capacitance of 38, 30, 36, and 105 μF cm–2 at 20 mV s–1 whereas the fabricated stack of electrodes demonstrates a high capacitance of 280 μF cm–2 at 20 mV s–1 while retaining and enhancing the material-dependent capacitance, charge retention, and power density.

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Introduction

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Rapid steps in integration of microelectromechanical systems (MEMS) and complementary-metal-oxide-semiconductor (CMOS) technology based integrated circuits (IC) have led to an exponentially increasing demand for electronics and, in particular, mobile electronics. (1) As electronic devices continue to shrink in size, while having better performance and expanding functionality, (2) their growing abundance rapidly increases the total power requirements, traditionally relying on batteries as their primary power supply. (3) However, in spite of extensive research on new sustainable materials (4−6) for battery electrodes, their lifetimes are considerably lower than the requirements for a self-reliant sensor power supply, a problem which can be solved by integrating an energy harvesting/storage system into the device. Such integration would allow the device to be potentially self-powering.
Harvesting systems have traditionally relied on mechanisms such as sunlight, contact, or vibration to provide electrical energy. (7) To power microelectronic circuits such as wireless sensor nodes, the energy requirement typically ranges from 100 μW to 10 mW. (8) Silicon photovoltaics have commercial products available that can generate power for entire households. (9) They have a conversion efficiency of 15% for 10 mW cm–2 of solar power present in the surroundings. MEMS vibrational piezoelectric energy harvesters and triboelectric energy harvesters have also demonstrated a potential to be on-chip devices that can support a self-reliant power supply functionality (10,11) with harvesting capabilities of 4–100 μW cm–2. These devices are traditionally manufactured through CMOS compatible fabrication processes. A microsupercapacitor (MSC) is an ideal candidate for an energy storage unit for self-powering systems as it has the potential for a cycle life far superior to that of current state-of-the-art batteries. (12) Furthermore, the high-power density of the MSCs is well suited for rapid charge/discharge cycles typical for a nanoenergy harvesters. However, harnessing the full potential of these technologies requires integration of MSCs through CMOS compatible technology for a seamlessly integrated self-reliant sensor node.
CMOS compatible processes are limited to certain selection of fabrication technologies such as sputtering, thermal annealing, physical and chemical vapor deposition, electron beam evaporation, photolithography, doping, and reactive ion etching. Devices that can be fabricated through CMOS compatible processes are viable for on-chip integration with integrated circuits. (13) To integrate MSCs into CMOS and MEMS technology in a front-end-of-line (FEOL) process, we need to address two major needs of integration. First, they must be fabricated through scalable methodologies compatible with CMOS and MEMS technology and second, the process must be ubiquitous for deposition of electrode material engineered for high energy, low self-discharge, or any other special characteristics tailor-synthesized to fulfill application necessities.
A literature survey addressing the needs of scalability and MEMS compatibility shows that silicon substrate based MSCs can be fabricated through CMOS compatible technologies such as chemical vapor deposition, (14) laser scribing, (15) inkjet printing, (16) doping, (17) layer-by-layer assembly, (18) pyrolysis, (19) and spin coating. (20,21) Furthermore, several scalable techniques such as vacuum filtration, (22) xurography, (23) spray coating, (24) and sputtering (25) have been applied for fabrication of solid state and flexible MSCs. Among all these processes, spin coating is one process that can be considered truly material versatile while being CMOS compatible. Spin coating is not particularly dependent on material choice; several electrode materials such as rGO, (26) carbon nanotubes (CNTs), (15) carbon nanofibers (CNFs), (14) onion-like carbon (OLC), (27) carbide-derived carbon (CDC), (28) graphene nanoparticles (GNPs), (29) and exfoliated graphene (30) can be synthesized into aqueous organic/inorganic solutions; these are critical for good spin coating. (31) Among these, rGO has demonstrated a capacity for high-energy and -power density MSC, with graphene with large specific surface area (2630 m2 g–1). Furthermore, rGO can be combined with several functionalization agents to improve the capability of MSCs in the above-mentioned terms. (32) Spin-coated MSCs geared toward CMOS compatibility have been fabricated by Huang et al. (33) through laser cutting patterns on a Kapton tape mask on a Si substrate. The material is spin coated on the exposed surfaces to fabricate interdigitated electrodes. Similarly, Zhang et al. (34) have also demonstrated fabrication of graphene-based solid state MSC through spin coating. Both these processes, however, use Kapton tape as a masking agent. Recently, graphene-based composites mixed with Ag nanowires or single walled CNTs have been used as spin-coating material for plotter-assisted fabrication of all solid MSCs. (23) Similarly, graphene has been used with a combination of MXenes with CNTs for low pass filter application. (35) Composites of 3D-laser-induced graphene foams mixed with ZnP nanosheets have also been used for integration with triboelectric nanogenerators (TENG) in a scalable fabrication process through spin coating. (36) Another set of spin-coated MSCs were fabricated by Mai et al., (37) where the electrodes were composed of alternated stack of rGO–CNT photoresist composites. The MSCs further illustrated spin-coating with graphene as base electrode material for property modulation as a perfect candidate for a low cost, high energy density MSC fabrication process also compatible for integration with microenergy harvesting techniques. However, to be integrated in applications such as the IoT, body implants, and RFID systems, the fabrication of MSCs through spin coating requires a standardized process plan whose process steps are incorporable in a current CMOS/MEMS fabrication facility.
We have previously demonstrated a spin-coating process for the fabrication of MSCs in a CMOS compatible spin-coating process by fabricating graphene-based rGO electrodes through an aluminum hard mask. (38) These devices however suffered from issues with electrode adhesion, wafer coverage of material, and uniformity in thickness of spin-coated electrodes. The spin-coating process was improved by utilizing surface roughening through Fe nanoparticles for enhanced coverage, adhesion, and uniformity. (21) However, until now, this process has not been tested for stacking a variety of electrode materials to yield modular properties such as optimizing for energy density while demonstrating good power density.
Thus, this article demonstrates the fabrication of a modular composite stack of functionalized rGO based on four different materials. The electrode materials include postannealed reduced graphene oxide (GO), postannealed branched-alkylamino GO-heptadecane-9-amine (GO-HD9A), prereduced linear rGO-octadecylamine (rGO-ODA), and prereduced rGO-heptadecane-9-amine (rGO-HD9A), spin-coated layer-by-layer in a bottom-up approach. Organic groups such as heptadecane-9-amine (HD9A) and octadecylamine (ODA) chains improve rGO’s solution stability (39) while preserving its latent characteristics. HD9A and ODA are branched and linear alkylamine chains, respectively. These functional groups can also allow rGO to be combined with several different materials in order to modulate the MSC characteristics. This stack of individual rGO based materials have an additive response to the capacitance characteristics of the Stack-MSC. The critical evaluation of singular-material MSCs has also been performed for each constituent of the electrode stack. We show that the MSC containing the stack of electrode materials conforms to the properties of the MSCs fabricated with individual constituent elements. The MSCs -GO-, GO-HD9A, rGO-ODA, and rGO-HD9A in the presence of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfony)imide (EMIM-TFSI), demonstrate a capacitance of 38, 30, 36, and 105 μF cm–2 at 20 mV s–1 respectively; the fabricated stack (Stack-MSC) demonstrates a capacitance of 280 μF cm–2 at 20 mV s–1 while retaining and enhancing material dependent capacitance, charge retention, and power density.

Fabrication of MSCs

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The MSCs with stacked electrode materials are fabricated on a 2-in. Si substrate (Figure 1a). The substrate is cleaned in 1:1 NH3:H2O2 and 1:1 HCl:H2O2 at 80 °C for 10 min, respectively. A 400 nm SiO2 layer is thermally grown on the substrate surface to make the devices insulating. The SiO2 surface is roughened through a thin Cr (2 nm) film annealed at 500 °C (21) (Figure 1b). Au/Cr (100/20 nm) current collectors are deposited on the surface through a lift-off photoresist mask. The photoresist, LOR-3A (Microchem) and S1813 (MicroChem) is spin coated on the surface roughened substrate at 4000 pm and 2000 rpm s–1 (Figure 1c). Soft baking at 180 °C for 5 min and 120 °C at 1 min 20 s is conducted. The spin-coated photoresist is exposed in ultraviolet mask aligner under a custom chromium hard mask with current collector patterns. The photoresist is developed in MF-319 developer for 90 s. The substrate is descummed in O2 for 1 min at 100 W. Later, gold and titanium are evaporated on the surface-enhanced substrate through electron beam evaporation (Figure 1d). The lift-off photoresist was developed in mr-REM 400 (MicroChem) remover solution at 35 kHz sonication for 55 min. Once the current collectors are fabricated (Figure 1e), we spin coat the electrode solutions at varying spin speeds acquired through rigorous testing of spinning all the solutions at different spin speeds to acquire the most uniform coverage on a Cr covered Si chip. For GO, GO-HD9A, rGO-ODA, and rGO-HD9A-MSCs, five layers of each were spin coated, while in the stacked MSC, all the four layers of different material are spin coated five times between intervals of vacuum drying at 100 °C (Figure 1f). The resultant thickness achieved for each layer is 180 ± 20, 130 ± 20, 170 ± 20, 595 ± 40, and 1.2 ± 0.1 μm for GO, GO-HD9A, rGO-ODA, rGO-HD9A, and Stack-MSCs, respectively. The representation of layer order in Stack MSC is shown in Figure 1k. Once the layers have dried, poly(methyl methacrylate) (PMMA) polymer coating (20 nm) was spin coated on the surface to improve surface uniformity. An Al layer of 200 nm is deposited through electron beam evaporation (Figure 1g) on the surface of the polymer and then etched in the gaps of the MSC pattern through reactive ion etching via a Cl2/SiCl4 mixture in the presence of Ar (Figure 1h). The electrode stack was then etched away using an O2 plasma recipe at 100 W at 100 mTorr (Figure 1i). Finally, the Al hard mask is etched and the samples are annealed at 500 °C to convert GO to rGO for improving the device resistance by uniformizing the stack (40) (Figure 1j).

Figure 1

Figure 1. (a–j) Schematic MSC fabrication process through spin coating of electrode solution and subsequent hard masking via an aluminum layer. (k) Schematic orientation of Stack MSC electrodes during the spin-coated deposition process step.

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Parts a–d of Figure 2 show the optical micrographs of the wafer substrate after each respective fabrication step. As visible in the images, the process is conformal to photoresist development with the final electrode stack taking the exact form in Figure 2d of the current collectors shown in Figure 2a. This demonstrates the feasibility of taking the device design down to micrometers, even nanometers if the lithography process enables such resolution. The wafer yield can be seen in Figure 2e, where 18 out of 24 devices fabricated on the wafer are functional. Observations from cross sectional SEM micrographs suggest a stacking of rGO sheets over a planar area in Figure 2f. The optical micrographs in parts g and h of Figure 2 show that the interdigitated fingers do not have any unetched material that can act as shorts.

Figure 2

Figure 2. (a) Au/Cr current collectors post lift-off.(b) Spin coated mixture of solutions on the wafer substrate.(c) Aluminum hard mask.(d) Stack electrodes on current collectors at the final step. (e) Camera capture of the fabricated Stack-MSC wafer (2 in.). (f) Cross section of the fabricated MSC with conductive rGO layers of approximately 1.1 μm. (g) Optical micrograph of the Stack-MSC. (h) High resolution optical image of the finger geometry of the Stack-MSC post GO reduction.

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Results

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In this section, we will first discuss the performance of individual electrodes based MSCs, i.e., GO-MSC, GO-HD9A, rGO-ODA, rGO-HD9A and then describe the electrochemical performance of the Stack-MSC, all fabricated through the process described in the Fabrication of MSCs. Finally, we present a comparison of Stack-MSC with its constituent MSCs.
The effects of surface roughening on spin-coating and performance of individual MSCs, GO, GO-HD9A, rGO-ODA, and rGO-HD9A are shown in Figure 3a–d. In Figure 3i, the molecular structures of different materials are outlined. The GO layer in Figure 3a–i is a layer of graphene with various oxidation sites, marked as red. GO-HD9A, similarly, is graphene functionalized with a branched alkane with an amine functional group, heptadecane-9-amine, and specific bonding sites. This functionalization has been achieved to increase the surface adhesion of GO and to facilitate thickness uniformity. rGO-ODA is a similar functionalization with a linear alkane group instead of branched with amine end. Moreover, it is also prefunctionalized in the solution and thus does not require post-process annealing for GO reduction. Finally, rGO-HD9A is prereduced GO-HD9A with similar properties as its precursor. The effects of surface roughening on the electrode solutions can be observed in Figure 3ii, more specifically for functionalized GO solutions where we see an extremely high change in surface coverage compared to nonroughened substrates. The electrochemical results of the fabricated MSCs can be seen in Figure 3iii. At first, it is not intuitive to see that rGO-HD9A holding the highest areal capacitance, as we would expect rGO-ODA to demonstrate a higher specific surface area due to its unbranched alkane chain. However, since the amount of deposited rGO-HD9A, increased through higher steric interaction, is substantially higher than for the other materials, we see a higher capacity to store charge. The steric hindrance of the molecules increases the area of interaction for the subsequent spin coating of layers. The amount of deposited rGO-HD9A is approximately 3 times higher than that for GO, GO-HD9A, and rGO-ODA. However, on normalization of capacitance with respect to device volume, we notice that rGO-HD9A is among the worst performing materials. This too can be attributed to the presence of steric hindrance within the molecules.

Figure 3

Figure 3. (i) Schematic representation of the atomic configuration. (ii) Improvement in coverage of spin-coated electrodes after addition of a Cr nanoparticle layer (experiments conducted on a 1 cm2 Si chips). (iii) cyclic voltammograms of MSC with 20 fingers with 40 μm spacing fabricated by process described in 1 at different scan rates ranging from 100–500 mV s–1. (iv) Bode plot for fabricated MSCs, for 3 g/L (a) GO in H2O (reduced postprocess), (b) GO-HD9A in ODCB (reduced postprocess), (c) rGO-ODA in ODCB, and (d) rGO-HD9A in ODCB.

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GO-MSC in Figure 3a-iii shows a stronger quasi-rectangular behavior (determined by R) compared to other materials. R is the ratio to an ideal capacitor with a rectangular cyclic voltammogram. This can be attributed to the post annealing step that removes all the impurities from the GO flakes while reducing GO to rGO. Similarly, we see the equivalent response for GO-HD9A in Figure 3b-iii. In Figure 3iv, we evaluate the EIS spectra of the individual materials. As visible, postprocess GO demonstrates a stronger resistive behavior due to issues related to vertically oriented electrical conductivity of the film. GO-HD9A, rGO-ODA, and rGO-HD9A improve the frequency behavior of the MSC substantially through their modulated structural van der Waals bonding in the alkane chains. GO-HD9A does not store a large amount of charge, however, it shows the lowest Resr among the fabricated MSCs while demonstrating the highest uniformity in performance over a large number of tested MSCs in the same batch. In summary, the chosen four materials had unique properties such as GO for strong EDL behavior, GO-HD9A for uniformity, rGO-ODA for capacitance improvement and rGO-HD9A for improved thickness.
The MSC stack constituted a layer of GO-HD9A at the bottom to improve uniformity, followed by GO for strong EDL behavior, followed by rGO-ODA for vertical conductivity and rGO-HD9A for higher electrode deposition and adhesion improvement during post processing steps. A further reason for the particular choice of material stack was obtained through EIS analysis of the fabricated MSCs. The stack design is based on a layer by layer approach of depositing material with the highest conductance first, followed by subsequent layers, i.e., GO-HD9A > GO > rGO-ODA > rGO-HD9A. The high parasitic resistance of prereduced MSCs can be attributed to the presence of amine groups and their tail ends interacting with EMIM-TFSI electrolyte, causing higher charge transfer resistance. (41) The chosen materials were spin-coated in a quasi layer-by-layer process in the order GO-HD9A, GO, rGO-ODA, and rGO-HD9A, from bottom-to-top. The stacking process was repeated five times to achieve the fabricated Stack-MSC. It is important to note that the stacking was not just four layers in total with one material per layer, but a mixed stack with five layers of heterogeneous stacks. Figure 4 shows the electrochemical performance the Stack-MSC at various input signals.

Figure 4

Figure 4. Electrochemical characterization of Stack-MSC: (a) Cyclic voltammetry at various scan-rates and (b) GCD measurement. EIS performance of the Stack-MSC represented as (c) Nyquist, (d) Bode with Creal and ϕ as functions of frequency, and (e) Bode in terms of Cimag over a range of operational frequencies. (f) Normalized capacitance of Stack-MSC over several cyclic charge–discharge cycles.

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In Figure 4b, it can be noted that the Stack-MSC demonstrates a longer time for charging at 1 μA cm–2 leading to an increased curvature in the GCD cycle. This shows that the leakage current influences the rate of MSC charging. Figure 4c shows the Nyquist plot of the MSC as a function of Zreal and −Zimag obtained from eqs 4 and 5 (Section S3, Device Characterization, Supporting Information: equations for real and imaginary capacitance). The device reveals an equivalent series resistance (Resr) of 1.2 kΩ. This can be corroborated with the IR drop from the GCD measurements. The charge-transfer resistance of the Stack-MSC is larger as the transfer resistance of each material adds on successively in a serial combination. In-spite of its high resistance, the power density of the device is equivalent to the power density observed in GO-MSC. The Warburg impedance observed through the slanted line (after semicircular charge-transfer resistance) is indicative of a porous electrode medium. The real capacitance (Creal) and phase angle (ϕ) from eq 7 (Section S3, Device Characterization, Supporting Information: phase response of the output) are shown in Figure 4d, while Cimag is shown in Figure 4e. Here, we can see that as the frequency increases above 10 Hz, the Cimag component of Stack-MSC tends to zero, giving rise to a highly resistive device behavior. Finally, in Figure 4f, we see the cyclic charge–discharge nature of the Stack-MSC over 3000 cycles of charging and discharging at 5 μA cm–2. As we can see, the device demonstrates 81% of its maximum capacitance at this specific current density after only 3k cycles. The performance is however comparable to several state-of-the-art MSCs presented in literature. (42)
The performance of Stack-MSC in comparison to individual material MSCs namely GO-, GO-HD9A-, rGO-ODA-, and rGO-HD9A-MSCs is shown in Figure 5. Parts a–c of Figure 5 show the cyclic voltammograms of the spin-coated MSCs at 20, 50, and 100 mV s–1. At 20 mV s–1 charging, the behavior for rGO- and HD9A-MSCs appears to be quasi rectangular in shape. However, rGO-ODA- and rGO-HD9A-MSCs show a resistive behavior while scanning toward potentials over 0.5 V. This behavior is inherited by the Stack-MSC. Another interesting feature is that the sum of the capacitances of single electrode material MSCs is 361 μF cm–2 while the Stack-MSC displays an areal capacitance of 307 μF cm–2. The slight acuteness can be attributed to material loading at specific sites on the spin-coated wafer. Figure 5d further elucidates the finding by showing the trends for areal capacitance of different fabricated MSCs over increasing scan-rates. At 1000 mV s–1, the areal capacitance of the Stack-MSC is comparable to single material MSCs. Figure 5e shows GCD measurements of MSCs at 5 μA cm–2. As seen also in Figure 4b, the Stack-MSC has a larger Resr compared to other material MSCs. Figure 5f shows the areal capacitance of MSCs at varying current densities. As visible, at higher current density of 10 μA cm–2, the capacitance retention for Stack-MSC is about 50% of its original capacitance. The large drop in areal capacitance at higher current densities can be attributed to presence of the high parasitic resistance derived from the material’s internal resistance. Also, the current supplied to the MSCs is calculated using the nominal total device area of 1 cm2. As the actual MSC electrode area is only 0.11 cm2, the current density in the electrode should be scaled up to 2 orders of magnitude accordingly.

Figure 5

Figure 5. (a–c) Cyclic voltammograms of the four materials, GO, GO-HD9A, rGO-ODA, and rGO-HD9A-MSCs. in comparison to Stack-MSC at (a) 20, (b) 50, and (c) 100 mV s–1. (d) Comparison of areal capacitance of fabricated MSCs over a range of scan-rates, (e) chronopotentiometric scan of MSCs at 5 μA cm–2, and (f) areal capacitance of the MSCs when charge–discharged at various current densities.

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Figure 6a shows the Creal for MSCs calculated from eq 4 and 5 (Section S3, Device Characterization, Supporting Information: equations for real and imaginary capacitance) from the EIS measurements. This form of Bode plot is useful in evaluating the confluence of capacitive and resistive device behavior through Creal trends. At frequencies higher than 100 Hz, the capacitive performance appears to track the strong Creal traits of rGO-ODA- and rGO-HD9A-MSCs. The behavior of Stack-MSC is strongly affected by the slowest response in the stack. Figure 6b shows the Nyquist plot of the fabricated MSCs. Here, we can clearly see a trend of series resistance from the materials in the Stack-MSC. The total resistance of individual MSCs’ was Resr = 975 Ω while the Stack-MSC has an Resr = 1.2 kΩ. GO-HD9A-, rGO-ODA-, and rGO-HD9A-MSCs show a porous electrode medium with their Nyquist slopes tending more toward the ideal capacitor behavior, while the GO-MSC reveals a high charge-transfer resistance. The Stack-MSC also shows a high charge-transfer resistance, which further strengthens the inference that individual material properties can be directly determining the properties of the combined stack.

Figure 6

Figure 6. (a) Bode plot of Creal vs frequency for Stack, GO, GO-HD9A, rGO-ODA, and rGO-HD9A. (b) Nyquist plot of the fabricated MSCs. (c) Comparison of volumetric capacitance and areal capacitance of individual material with Stack. (d) Areal power density of MSCs over increasing current density. (e) Ragone plot of energy and power density of the MSCs fabricated in the current manuscript in comparison to MSCs with EDLC material published in the recent literature.

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After analyzing the material performance further, we evaluate the individual volumetric capacitance of the MSCs alongside their areal capacitances at 20 mV s–1 in Figure 6c through information on the electrode thickness acquired from surface profiler measurements. The error bars correspond to standard deviation observed during thickness measurement. GO as a material demonstrates the highest volumetric capacitance followed by rGO-ODA, GO-HD9A, and then rGO-HD9A. The stack material reveals a volumetric capacitance comparable to other electrodes, so it can be assumed that the material has not undergone any severe transformation during spin coating of the stack. The average volumetric capacitance of the combined materials is 1.7 F cm–3 compared to volumetric capacitance of the stack as 2.3 F cm–3. Both the metrics are equivalent within the measured accuracy range. Figure 6d shows the power density and areal capacitance of the MSCs over a range of current densities. The material MSCs individually manage to deliver higher power densities at larger currents, especially GO-HD9A-, rGO-ODA-, and rGO-HD9A-MSCs. The resistive nature of Stack and GO leads to a poor power density for these devices. Finally, in Figure 6e, we compare the performance of the MSCs fabricated as a plot displaying their individual energy, power, capacitance densities, and equivalent series resistance.

Discussion

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Based on our analysis of the electrochemical results for surface enhanced spin-coating fabrication for Stack, GO, GO-HD9A, rGO-ODA, and rGO-HD9A MSCs, we present an equivalent circuit analysis for the devices in this section. The rationale for our material choices will also be discussed. Finally, we will compare the performance of the fabricated MSCs with devices in literature using either the same material or a combination of various related meta-materials.
Figure 7a shows the EIS Bode spectrum of the Creal for the fabricated devices using the standard equivalent circuit model constant phase element with diffusion (CPE-diff). The CPE-diff model proposed by Bisquert et al. (43) is considered as a standard referential tool in literature. The model has been explained in Huang et al. (44) to model impedance responses in disordered mediums such as thick rGO films. (45) In this circuit, Resr is shown by Ru in series with a constant phase element in parallel with the combination of a Rp (charge-transfer resistance) and a Warburg admittance in series, where α0 is the variable that is representative of the electric response related to porosity. In Table 1, we see the resulting parameter values when fitting the model for the different devices─Stack, GO, GO-HD9A, rGO-ODA, and rGO-HD9A MSCs. After investigating the equivalent circuit parameter values for MSCs, we see that the net resulting Ru for the Stack is not far off the sum of the constituents’ Ru values (within 20%). Similarly, the stack demonstrates a correlation to four constant phase elements in series (Y0α0). Both these factors fit the explanation to the behavior of the stacked electrode. For the Stack-MSC, Wd is higher which shows its improved charge retention compared to individual materials. The Warburg admittance parameter also reflects the low frequency response where the stack shows a higher capacitive value. Similarly, Rp for the Stack-MSC is lower than all MSCs excluding GO-MSC, another important consideration for charge retention through lower leakage current. The major inference is that the Stack-MSC demonstrates a combination of capacitance for all the materials at low frequencies, but it is heavily influenced by the behavior of the material with smallest Creal capacitance, namely GO, in this case at high frequencies. The main advantage of the stacked-based fabrication technique is related to its versatility with regards to deposition of different materials. We have tested the deposition of a variety of pseudocapacitive electrodes through spin coating. However, during our trials, spin coating of such electrode materials on a substrate cannot experimentally lead to a good wafer yield. One possible alternative is to mix them in a solution of graphene-based material.

Figure 7

Figure 7. (a) Equivalent circuit CPE with diffusion model (Bisquert et al. (43)). (b) Analysis of the fabricated MSCs: Stack, GO, and rGO-HD9A. Table 1 shows the calculated values for different components for the equivalent circuit.

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Table 1. Equivalent Circuit Modelling Results for the Fabricated MSCs
elementStackGOGO-HD9ARGO-ODARGO-HD9AError (±)Units
Ru12216331092822125Ω
Y00.4μ3.7μ1.3μ3.2μ1.7μ19nS* sα
α00.690.660.820.810.837m–
Wd17μ1.2p0.82μ0.710.230.2μS* s0.5
Rp27.4k18.9k0.7M1.9M2.9M407Ω
For the MSC fabrication process to be compatible with CMOS fabrication, MSCs need a process plan that can be incorporated within a MEMS or CMOS process scheme without affecting the material or design quality for the other devices. While keeping these constraints in mind, the MSCs discussed in this article are fabricated using conventional photolithography, electron beam evaporation, reactive ion etching, and low temperature annealing. MEMS and CMOS devices generally require several UV lithography steps for their fabrication. The process plan for the MSCs can be incorporated at any point in the MEMS process level sequence without requiring the wafers to be transported out of the cleanroom, any Kapton masking step or using tools such as focused ion beam for RIE, to count a few general practices applied in MSC fabrication in the recent literature. (46) Furthermore, the issue of material loss during typical cleanroom processes such as photoresist development and ultrasonication has been mitigated through application of surface-roughened Cr NPs, (21) as discussed in our previous publication.
There are a few issues regarding the CMOS compatibility of MSC fabrication that still needs to be addressed before the process can be implemented for integrated systems on-chip. First, graphene-based materials are often highly conductive flakes that can cause unwanted short circuit if they fly off and stick to other devices being fabricated in a FEOL process. Incorporation of pseudocapacitive materials such as MnO2, V2O5, Co3O4, and others with graphene-based inks can also potentially lead to similar problems. Therefore, further work must be conducted on issues related to flaking of bulk graphene layers during fabrication in low pressure and high temperature environments. Second, application of Au/Ti as current collectors is not an optimum solution from a compatibility perspective. Using Pd, Pt, Mo, W, or Cr as current collectors can also be interesting options for future work as these metals are considered CMOS compatible, and they demonstrate high conductance. However, when we fabricated MSCs using Pd/Ti as current collectors and vertically oriented carbon nanofibers as the electrode material, we observed a 10 times reduction in the device capacitance and a considerable increase in device resistance while using H2SO4/PVA as an electrolyte. (47) Finally, using aqueous or gel electrolyte with CMOS circuit fabrication is not advisable, so there also needs to be a study on proper packaging solutions for such materials.
Increasing the number of spin-coated layers results in a higher thickness. By increasing the substrate surface roughness, we can achieve 1.1 μm thick rGO spin-coated electrodes as discussed in our previous publication. (21) The capacitance of those devices was 110 μF cm–2 (comparable to rGO-HD9A in this study). These values for rGO were achieved employing Fe nanoparticles for surface roughening. Fe nanoparticles are not CMOS compatible as Fe’s boiling point is extremely low compared and furthermore, Fe is a reactive metal when in contact with ionic solutions. Therefore, in this study, we have used Cr nanoparticles which have a higher boiling point and lower reactivity, albeit with slightly lower surface roughness. Cr is a material also used in other CMOS compatible fabrication procedures. (48)
MSCs fabricated through spin coating show relatively low energy densities due to utilization of EDLC material only. In comparison to various silicon based MSCs which are fabricated through hard masking technique, the Stack-MSC performs comparatively well. Graphene-based MSC was fabricated by Wu et al. (49) using graphene transfer and current collector hard mask as electrode deposition and etching process, respectively. The MSCs delivered a performance of 80 μF cm–2 with an electrode thickness of 15 nm. Similarly, Beidaghi et al. combined rGO with CNTs to produce MSCs through transportation of electrode ink in SU-8 channels. The resultant device yielded a capacitance of 6.1 mF cm–2 in the most optimum rGO–CNT composition. The high areal capacitance can be attributed to thicker electrodes deposited. Li et al. (50) showed on-chip MSCs using MnO2 in porous carbon mixture. Graphene based electrode materials have been combined with various materials such as V2O5, MoS2, CNTs, and various EDLC or pseudocapacitive materials for higher energy densities. For example, Boruah et al. (24) fabricated a sandwich-type MSC using mask assisted spray deposition of electrodes on a flexible substrate. As discussed previously, Yang et al. (19) demonstrated a stack of MoS2, rGO, and CNT mixed with photoresist (pyrolyzed later) for extremely high-energy densities through spin-coating. GO has also been combined with CNT powder in laser-scribed flexible MSCs for higher energy densities as described by Wen et al. (15) Laser scribing is a viable alternative to spin coating. However, the laser scribed devices, when combined with different materials show a large Ru because of the high-power discharge of lasers during GO to rGO conversion and etching of interdigitated patterns. On comparison with purely pseudocapacitive MSC fabricated through the CMOS compatible process shown in Si et al., (51) we note that EDLC materials demonstrate a higher energy and power density in a stacked combination.
For future work in improving MSCs, the pseudocapacitive materials can be tested for spin-coating and stacking using graphene oxide inks. As discussed before, the performance of Stack-MSC resembles the behavior of material with lowest power density, use of pseudocapacitive material would need to be optimized in the carbon material solution. Furthermore, there is a need for investigation in proper packaging techniques for CMOS compatibility for proper encapsulation of ionic liquids. Proper choice of electrolyte is another important consideration for high performance MSCs. MSC performance in devices discussed in the literature survey generally utilize H2SO4/PVA electrolyte. This is an aqueous electrolyte which has small ions compatible with microporous electrodes. Utilization of H2SO4/PVA on Stack-MSCs would yield a capacitive performance is expected to be 2–3 times better than EMIM-TFSI. However, using any acid is detrimental to a CMOS fabrication process, as SiO2 is reactive to them. Therefore, to ensure CMOS compatibility, we have used an ionic liquid electrolyte. Another direction of work can concern the implementation and advancement of solid state electrolytes with high ionic conductivity.

Conclusion

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In summary, we have presented a heterogeneous stacked multielectrode spin-coated CMOS compatible MSC with four different types of EDLC material based on graphene composites optimized for spin coating. The materials, GO, GO-HD9A, rGO-ODA, and rGO-HD9A, reveal electrochemical and processing qualities such as high capacitance, spin-coated uniformity, and low Resr. We have demonstrated that individual electrode characteristics translate into comparatively high energy density stacked MSCs having a capacitance of 280 μF cm–2 while retaining GO’s relatively high power density of 0.1 mW cm–2. Hence, we have verified the applicability of a fabrication method that can be used for different materials with controlled spin-coating parameters for the applied electrode solution on a Cr nanoparticle roughened surface for improved adhesion, uniformity, and coverage. The robustness of the fabrication process furthermore enables a more standardized evaluation of electrode material performance for substrate-based planar solid state MSCs.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsaem.1c03745.

  • Experimental section and characterization information (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Simin Zare Hajibagher - Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Kemivägen 9, 41296, Gothenburg, Sweden
    • Ulises Méndez-Romero - Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemigården 4, 41296, Gothenburg, Sweden
    • Shameel Thurakkal - Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemigården 4, 41296, Gothenburg, Sweden
    • Qi Li - Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Kemivägen 9, 41296, Gothenburg, Sweden;  Orcidhttps://orcid.org/0000-0001-6422-5020
    • Mazharul Haque - Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Kemivägen 9, 41296, Gothenburg, Sweden
    • R. K. Azega - Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Kemivägen 9, 41296, Gothenburg, Sweden
    • Ergang Wang - Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemigården 4, 41296, Gothenburg, Sweden;  Orcidhttps://orcid.org/0000-0002-4942-3771
    • Xiaoyan Zhang - Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemigården 4, 41296, Gothenburg, Sweden
    • Per Lundgren - Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Kemivägen 9, 41296, Gothenburg, Sweden;  Orcidhttps://orcid.org/0000-0003-3234-1670
    • Peter Enoksson - Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Kemivägen 9, 41296, Gothenburg, Sweden;  Enoaviatech AB, 112 26 Stockholm, Sweden
    • Anderson Smith - Department of Electrical Engineering, Chalmers University of Technology, Hörsalsvägen 7, 41296, Gothenburg, Sweden
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This project has received funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 101006963 (GreEnergy). The authors acknowledge the funding from the projects Vinnova, Area of Advance (AoA) MESSIAH, and SAAB Hogeffektradar. The authors also thank Chalmers NanoFabLab for their cleanroom expertise and Dr. Jinhua Sun for his inputs.

References

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    Advanced Materials (Weinheim, Germany) (2014), 26 (26), 4552-4558CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Authors describe the development of large-area highly uniform, ultrathin, nitrogen and boron co-doped graphene (BNG) films for high-performance micro-supercapacitors (MSCs). The BNG film was prepd. using a layer-by-layer (LBL) assembly of anionic graphene oxide (GO) nanosheets and cationic poly-L-Lysine (PLL) as a nitrogen-contg. precursor, followed by intercalation of H3BO3 within the layers and annealing treatment. The PLL and H3BO3 incorporated into the assembled multilayer films not only serve as nitrogen- and boron-rich precursors, resp., but also generate micropore fillers to promote the formation of porous, yet densely packed BNG films during the thermal treatment.
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    Yang, W.; He, L.; Tian, X.; Yan, M.; Yuan, H.; Liao, X.; Meng, J.; Hao, Z.; Mai, L. Carbon-MEMS-based alternating stacked MoS2@ rGO-CNT micro-supercapacitor with high capacitance and energy density. Small 2017, 13, 1700639,  DOI: 10.1002/smll.201700639
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    Vyas, A.; Li, Q.; Cornaglia, F.; Wang, K.; Anderson, A.; Haque, M.; Kuzmenko, V.; Smith, A.; Lundgren, P.; Enoksson, P. Surface Roughening with Iron Nanoparticles for Promoted Adhesion of Spin Coated Microsupercapacitor Electrodes. MRS Advances 2019, 4, 1335– 1340,  DOI: 10.1557/adv.2019.5
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    Surface Roughening with Iron Nanoparticles for Promoted Adhesion of Spin Coated Microsupercapacitor Electrodes
    Vyas, A.; Li, Q.; Cornaglia, F.; Wang, K.; Anderson, A.; Haque, M.; Kuzmenko, V.; Smith, A. D.; Lundgren, P.; Enoksson, P.
    MRS Advances (2019), 4 (23), 1335-1340CODEN: MARDCQ; ISSN:2059-8521. (Cambridge University Press)
    Microsupercapacitors (MSCs) are miniaturized energy storage devices that can be integrated in an on-chip platform as a component of a power supply for Internet of things' sensors. Integration of these on-chip MSCs require them to be fabricated through CMOS compatible fabrication techniques such as spin coating. One of the biggest challenges in spin coated MSCs is the poor surface adhesion. In this work, we present a CMOS compatible electrode deposition process with enhanced adhesion and retention for reduced graphene oxide (rGO) using spin coating. In order to improve the adhesion and surface uniformity of the deposited electrode material, the surface of Si/SiO2 wafers was subjected to roughening through Fe nanoparticle formation. A 4 nm thick Fe layer deposition substantially magnified the av. mean surface roughness of the substrates. In comparison with substrates without the Fe deposition, the treated ones have more than 300% improvement in surface coverage and rGO mass retention after sonication testing. These results suggest that the surface roughening has a pos. influence on electrode deposition via a spin-coating method.
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    Vyas, A.; Wang, K.; Anderson, A.; Velasco, A.; van den Eeckhoudt, R.; Haque, M. M.; Li, Q.; Smith, A.; Lundgren, P.; Enoksson, P. Enhanced Electrode Deposition for On-Chip Integrated Micro-Supercapacitors by Controlled Surface Roughening. ACS omega 2020, 5, 5219– 5228,  DOI: 10.1021/acsomega.9b04266
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    Enhanced Electrode Deposition for On-Chip Integrated Micro-Supercapacitors by Controlled Surface Roughening
    Vyas, Agin; Wang, Kejian; Anderson, Alec; Velasco, Andres; Van den Eeckhoudt, Ruben; Haque, Mohammad Mazharul; Li, Qi; Smith, Anderson; Lundgren, Per; Enoksson, Peter
    ACS Omega (2020), 5 (10), 5219-5228CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
    On-chip micro-supercapacitors (MSCs), integrated with energy harvesters, hold substantial promise for developing self-powered wireless sensor systems. However, MSCs have conventionally been manufd. through techniques incompatible with semiconductor fabrication technol., the most significant bottleneck being the electrode deposition technique. Utilization of spin-coating for electrode deposition has shown potential to deliver several complementary metal-oxide-semiconductor (CMOS)-compatible MSCs on a silicon substrate. Yet, their limited electrochem. performance and yield over the substrate have remained challenges obstructing their subsequent integration. We report a facile surface roughening technique for improving the wafer yield and the electrochem. performance of CMOS-compatible MSCs, specifically for reduced graphene oxide as an electrode material. A 4 nm iron layer is deposited and annealed on the wafer substrate to increase the roughness of the surface. In comparison to std. nonroughened MSCs, the increase in surface roughness leads to a 78% increased electrode thickness, 21% improvement in mass retention, 57% improvement in the uniformity of the spin-coated electrodes, and a high yield of 87% working devices on a 2'' silicon substrate. Furthermore, these improvements directly translate to higher capacitive performance with enhanced rate capability, energy, and power d. This technique brings us one step closer to fully integrable CMOS-compatible MSCs in self-powered systems for on-chip wireless sensor electronics.
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    Zhao, J.; Shi, Q.; Guo, Y.; Wang, X.; Wang, D.; Tan, F.; Jiang, L.; Yu, Y. Flash foam stamp-inspired fabrication of flexible in-plane graphene integrated micro-supercapacitors on paper. J. Power Sources 2019, 433, 226703,  DOI: 10.1016/j.jpowsour.2019.226703
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    Flash foam stamp-inspired fabrication of flexible in-plane graphene integrated micro-supercapacitors on paper
    Zhao, Jiang; Shi, Qingling; Guo, Yanyan; Wang, Xiangfu; Wang, Debo; Tan, Furui; Jiang, Li; Yu, Ying
    Journal of Power Sources (2019), 433 (), 226703CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    Developing patterning techniques with practicability, cost-effectiveness and safety are essential for shape-confirmable integrated energy storage devices. Herein we design a novel flash foam stamp-inspired technique for simple fabrication of flexible high energy in-plane micro-supercapacitors based on interdigital electrodes of self-depositing electrochem. exfoliated graphene on paper substrates. This technique is an attractive and novel bottom-up microfabrication technol. due to its advantages of simplicity and low-cost. These novelly developed all-solid-state, paper-based in-plane graphene micro-supercapacitors exhibit excellent electrochem. performances, such as high area specific capacitance of ∼4.02 mF cm-2 and excellent cycling stability (95.6% retention at 0.08 mA cm-2 for 5000 cycles). And these solid-state flexible micro-supercapacitors are obsd. non-obvious structural change and loss of performance under bending state, suggesting remarkable flexibility and mech. stability. Therefore, such all-solid-state, flexible paper-based graphene micro-supercapacitors can potentially be used for wearable and portable electronics.
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    Kim, S.-W.; Kang, K.-N.; Min, J.-W.; Jang, J.-H. Plotter-assisted integration of wearable all-solid-state micro-supercapacitors. Nano Energy 2018, 50, 410– 416,  DOI: 10.1016/j.nanoen.2018.05.051
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    Plotter-assisted integration of wearable all-solid-state micro-supercapacitors
    Kim, Sung-Wook; Kang, Kyeong-Nam; Min, Jin-Wook; Jang, Ji-Hyun
    Nano Energy (2018), 50 (), 410-416CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
    We introduce the use of xurog. as a facile plotter-assisted integration technique for an all-solid-state flexible micro-supercapacitors (mSCs), which is cost-effective and time-saving, by which 162 mSCs can be simultaneously produced on an A4 sized PET film. 3D-graphene networks (3DGNs) grown on a SiO2 template exhibited large surface area and outstanding elec. cond. as active materials for mSCs due to the interconnected 3D structure. By simply mixing SWNTs and Ag NWs with 3DGNs, an excellent specific areal capacitance of 19 mF cm-2 was obtained from an as-prepd. 3DGN/SWNT/AgNW mSC on a flexible substrate and 84% of the initial capacitance was maintained after 10,000 cycles. The device exhibited stable electrochem. performances under various deformation conditions. The subsequent transfer onto a textile further allowed the fabrication of a wearable particle detector, which confirmed a practical application of a mSC with good performance integrated via a plotter-assisted technique.
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    Boruah, B. D.; Nandi, S.; Misra, A. Layered assembly of reduced graphene oxide and vanadium oxide heterostructure supercapacitor electrodes with larger surface area for efficient energy-storage performance. ACS Applied Energy Materials 2018, 1, 1567– 1574,  DOI: 10.1021/acsaem.7b00358
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    Layered Assembly of Reduced Graphene Oxide and Vanadium Oxide Heterostructure Supercapacitor Electrodes with Larger Surface Area for Efficient Energy-Storage Performance
    Boruah, Buddha Deka; Nandi, Sukanta; Misra, Abha
    ACS Applied Energy Materials (2018), 1 (4), 1567-1574CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
    The architecture of a supercapacitor (SC) electrode plays a crucial role in defining the overall energy-storage performance of the SC. Layer-by-layer assembly of a reduced graphene oxide (rGO) and vanadium oxide (V2O5) (rGO/V2O5)-based heterostructure is patterned in interdigitated electrodes (IDEs) deposited directly on a flexible conducting current collector for the SC. The IDE pattern offers efficient accessibility to the electrolyte ions and a synergistic contribution for energy storage. An as-fabricated solid-state flexible sandwich-type SC with IDEs displays a more efficient energy-storage performance than a conventional solid-state flexible sandwich-type SC composed of rGO/V2O5 electrodes. Moreover, a solid-state flexible in-plane microsupercapacitor (MSC) is fabricated, which offers much higher capacitance (24 mF/cm2 and 34.28 F/cm3) and energy d. (3.3 μWh/cm2 and 4.7 mWh/cm3). The as-fabricated flexible in-plane MSC displays a negligible capacitance loss of about 6.3% after 10000 charge-discharge cycles and a superior stability of energy-storage performance towards mech. deformation.
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    Göhlert, T.; Siles, P. F.; Päßler, T.; Sommer, R.; Baunack, S.; Oswald, S.; Schmidt, O. G. Ultra-thin all-solid-state micro-supercapacitors with exceptional performance and device flexibility. Nano Energy 2017, 33, 387– 392,  DOI: 10.1016/j.nanoen.2017.01.054
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    Gholami Laelabadi, K.; Moradian, R.; Manouchehri, I. One-Step Fabrication of Flexible, Cost/Time Effective, and High Energy Storage Reduced Graphene Oxide@ PANI Supercapacitor. ACS Applied Energy Materials 2020, 3, 5301– 5312,  DOI: 10.1021/acsaem.0c00317
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    One-Step Fabrication of Flexible, Cost/Time Effective, and High Energy Storage Reduced Graphene Oxide@PANI Supercapacitor
    Gholami Laelabadi, Katayoon; Moradian, Rostam; Manouchehri, Iraj
    ACS Applied Energy Materials (2020), 3 (6), 5301-5312CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
    Microsupercapacitors, as an energy storage device, have unique electrochem. performance, such as high power d., fast charging, long cycle life, and high safety. The redn. time and cost in fabrication processes of microsupercapacitors are important factors in microfabrication technol. In this work, a simple, scalable, and cost-effective fabrication of interdigitated reduced graphene oxide@polyaniline flexible microsupercapacitors is presented. It was found that in fabricating the interdigitated microelectrode patterns on PET substrate; the redn. of graphene oxide and growth of conducting polymer are rapidly performed simultaneously in one step by laser irradn. The capacitance was 72 mF/cm2 at a 35μA/cm2 c.d. These high-capacitance microsupercapacitors demonstrate good stability and > 93.5% of the capacitance is retained after 1000 cycles at a 0.7 mA/cm2 c.d.
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    Pech, D.; Brunet, M.; Durou, H.; Huang, P.; Mochalin, V.; Gogotsi, Y.; Taberna, P.-L.; Simon, P. Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon. Nature Nanotechnol. 2010, 5, 651– 654,  DOI: 10.1038/nnano.2010.162
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    Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon
    Pech, David; Brunet, Magali; Durou, Hugo; Huang, Peihua; Mochalin, Vadym; Gogotsi, Yury; Taberna, Pierre-Louis; Simon, Patrice
    Nature Nanotechnology (2010), 5 (9), 651-654CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Electrochem. capacitors, also called supercapacitors, store energy in 2 closely spaced layers with opposing charges, and are used to power hybrid elec. vehicles, portable electronic equipment and other devices. By offering fast charging and discharging rates, and the ability to sustain millions of cycles, electrochem. capacitors bridge the gap between batteries, which offer high energy densities but are slow, and conventional electrolytic capacitors, which are fast but have low energy densities. Here, the authors demonstrate microsupercapacitors with powers per vol. that are comparable to electrolytic capacitors, capacitances that are 4 orders of magnitude higher, and energies per vol. that are an order of magnitude higher. The authors also measured discharge rates of up to 200 V s-1, which is 3 orders of magnitude higher than conventional supercapacitors. The microsupercapacitors are produced by the electrophoretic deposition of a several-micrometre-thick layer of nanostructured C onions with diams. of 6-7 nm. Integration of these nanoparticles in a microdevice with a high surface-to-vol. ratio, without the use of org. binders and polymer separators, improves performance because of the ease with which ions can access the active material. Increasing the energy d. and discharge rates of supercapacitors will enable them to compete with batteries and conventional electrolytic capacitors in a no. of applications.
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    Chmiola, J.; Largeot, C.; Taberna, P.-L.; Simon, P.; Gogotsi, Y. Monolithic carbide-derived carbon films for micro-supercapacitors. science 2010, 328, 480– 483,  DOI: 10.1126/science.1184126
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    Monolithic Carbide-Derived Carbon Films for Micro-Supercapacitors
    Chmiola, John; Largeot, Celine; Taberna, Pierre-Louis; Simon, Patrice; Gogotsi, Yury
    Science (Washington, DC, United States) (2010), 328 (5977), 480-483CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Microbatteries with dimensions of tens to hundreds of micrometers that are produced by common microfabrication techniques are poised to provide integration of power sources onto electronic devices, but they still suffer from poor cycle lifetime, as well as power and temp. range of operation issues that are alleviated with the use of supercapacitors. There have been a few reports on thin-film and other micro-supercapacitors, but they are either too thin to provide sufficient energy or the technol. is not scalable. By etching supercapacitor electrodes into conductive titanium carbide substrates, we demonstrate that monolithic carbon films lead to a volumetric capacity exceeding that of micro- and macroscale supercapacitors reported thus far, by a factor of 2. This study also provides the framework for integration of high-performance micro-supercapacitors onto a variety of devices.
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    Zhang, L.; DeArmond, D.; Alvarez, N. T.; Malik, R.; Oslin, N.; McConnell, C.; Adusei, P. K.; Hsieh, Y.-Y.; Shanov, V. Flexible micro-supercapacitor based on graphene with 3D structure. Small 2017, 13, 1603114,  DOI: 10.1002/smll.201603114
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    Lu, Y.; Zheng, Y.; Zhang, H.; He, X.; Yang, Q.; Wu, J. A high performance and flexible in-plane asymmetric micro-supercapacitor (MSC) fabricated with functional electrochemical-exfoliated graphene. J. Electroanal. Chem. 2020, 866, 114169,  DOI: 10.1016/j.jelechem.2020.114169
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    A high performance and flexible in-plane asymmetric micro-supercapacitor (MSC) fabricated with functional electrochemical-exfoliated graphene
    Lu, Yuanchao; Zheng, Yawen; Zhang, Hao; He, Xiangxiang; Yang, Qunqing; Wu, Jian
    Journal of Electroanalytical Chemistry (2020), 866 (), 114169CODEN: JECHES; ISSN:1873-2569. (Elsevier B.V.)
    With the increasing development of miniaturized, integrated, wearable and portable elec. devices, in-plane micro-supercapacitors (MSCs), as one of most potential next-generation energy storage setups, have attracted abundant interests from the public. A flexible in-plane asym. MSC, prepd. by the mask-assisted vacuum filtration, was demonstrated. Also, the electrochem.-exfoliated functional graphene oxide (FGO) and chem.-reduced functional reduced graphene oxide (FrGO) were configured as pos. and neg. electrode of the MSCs, coated with the solid PVA/Na2SO4 electrolyte. The result presented that the as-assembled FGO-FrGO MSCs performed the highest areal capacitance of 7.3 mF cm-2. It was assumed that the high performance of the asym. MSCs was attributed to the asym. in-plane interdigitated structure, the abundant functional groups doping and heteroatoms replacement with N, O, P, S elements in graphene. In summary, the functional graphene-based asym. MSCs provide a potential inspiration for the next-generation flexible and wearable energy storage devices.
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    Xiong, G.; Meng, C.; Reifenberger, R. G.; Irazoqui, P. P.; Fisher, T. S. A review of graphene-based electrochemical microsupercapacitors. Electroanalysis 2014, 26, 30– 51,  DOI: 10.1002/elan.201300238
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    A Review of Graphene-Based Electrochemical Microsupercapacitors
    Xiong, Guoping; Meng, Chuizhou; Reifenberger, Ronald G.; Irazoqui, Pedro P.; Fisher, Timothy S.
    Electroanalysis (2014), 26 (1), 30-51CODEN: ELANEU; ISSN:1040-0397. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. The rapid development of miniaturized electronic devices has led to a growing need for rechargeable micropower sources with high performance. Among different sources, electrochem. microcapacitors or microsupercapacitors provide higher power d. than their counterparts and are gaining increased interest from the research and engineering communities. To date, little work has appeared on the integration of microsupercapacitors onto a chip or flexible substrates. This paper provides an overview of research on microsupercapacitors, with particular emphasis on state-of-the-art graphene-based electrodes and solid-state devices on both flexible and rigid substrates. The advantages, disadvantages, and performance of graphene-based microsupercapacitors are summarized and new trends in materials, fabrication and packaging are identified.
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    Lee, S. H.; Lee, J.; Jung, J.; Cho, A. R.; Jeong, J. R.; Dang Van, C.; Nah, J.; Lee, M. H. Enhanced Electrochemical Performance of Micro-Supercapacitors Via Laser-Scribed Cobalt/Reduced Graphene Oxide Hybrids. ACS Appl. Mater. Interfaces 2021, 13, 18821– 18828,  DOI: 10.1021/acsami.1c02102
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    Enhanced Electrochemical Performance of Micro-Supercapacitors Via Laser-Scribed Cobalt/Reduced Graphene Oxide Hybrids
    Lee, Sang Hwa; Lee, Jungjun; Jung, Jaemin; Cho, A. Ra; Jeong, Jae Ryeol; Dang Van, Cu; Nah, Junghyo; Lee, Min Hyung
    ACS Applied Materials & Interfaces (2021), 13 (16), 18821-18828CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    The evolution of "smart life," which connects all internet-of-things (IoT) microdevices and microsensors under wireless communication grids, requires microscale energy storage devices with high power and energy d. and long-term cyclability to integrate them with sustainable power generators. Instead of Li-ion batteries with a short lifetime, pseudocapacitors with longer or infinite cyclability and high-power d. have been considered as efficient energy storage devices for IoT. However, the design and fabrication of microscale pseudocapacitors have difficulties in patterning microscale electrodes when loading active materials at specific points of the electrodes using conventional microfabrication methods. Here, we developed a facile, one-step fabrication method of micro-supercapacitors (MSCs) through the in situ formation of Co metals and the reduced graphene oxides (rGOs) in a one-pot laser scribing process. The prepd. Co/rGO MSC thus exhibited four times higher capacitance than the rGO MSC, due to the Faradaic charge capacitance behavior of the Co/rGO composites.
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    Huang, K. Laser printer patterned sacrificed layer for arbitrary design and scalable fabrication of the all-solid-state interdigitated in-planar hydrous ruthenium oxide flexible micro supercapacitors. J. Power Sources 2019, 417, 108– 116,  DOI: 10.1016/j.jpowsour.2019.02.016
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    Laser printer patterned sacrificed layer for arbitrary design and scalable fabrication of the all-solid-state interdigitated in-planar hydrous ruthenium oxide flexible micro supercapacitors
    Huang, Kai-Chen; Lin, Che-Hsien; S, Anuratha K.; Huang, Tsung-Yu; Lin, Jeng-Yu; Tseng, Fan-Gang; Hsieh, Chien-Kuo
    Journal of Power Sources (2019), 417 (), 108-116CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    We demonstrate the successful fabrication of all-solid-state flexible micro-supercapacitors (μSCs) on com. polyethylene terephthalate (PET) sheet using a com. laser printer. The interdigitated in-planar electrodes of the μSC are fabricated by com. toner cartridge and their self-aligned stacking configurations are made by depositing Pt thin films and hydrous ruthenium oxide (RuO·xH2O, hRuO2) layers on the PET sheets using a post-low-temp. annealing process. The μSC cells can be combined in parallel or series by an arbitrary printing process operated on a personal computer, thereby avoiding unnecessary external connections and improving the capacitive behavior. The fabricated hRuO2-based μSCs exhibit excellent electrochem.-energy storage, along with ultrahigh volumetric power d. (73460 mW cm-3) and energy d. (24.9 mWh cm-3). This facile and simplified strategy constructs μSCs with tailored self-aligned electrodes of arbitrary design, excellent mech. stability, and high flexibility. Therefore, it offers a high-productivity, low-cost, photolithog.-free microfabrication of μSCs for microscale supercapacitor applications.
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    Zhang, L.; Liu, L.; Liu, C.; Li, X.; Liu, F.; Zhao, W.; Wang, S.; Wu, F.; Zhang, G. Photolithographic fabrication of graphene-based all-solid-state planar on-chip microsupercapacitors with ultrahigh power characteristics. J. Appl. Phys. 2019, 126, 164308,  DOI: 10.1063/1.5109691
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    Photolithographic fabrication of graphene-based all-solid-state planar on-chip microsupercapacitors with ultrahigh power characteristics
    Zhang, Ludi; Liu, Linjing; Liu, Chunfeng; Li, Xiang; Liu, Fangshuo; Zhao, Wenqiang; Wang, Shu; Wu, Fengmin; Zhang, Guangyu
    Journal of Applied Physics (Melville, NY, United States) (2019), 126 (16), 164308/1-164308/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    The fabrication of all-solid-state planar on-chip microsupercapacitors with ultrahigh power characteristics was demonstrated by reduced graphene oxide and photolithog. technol. In this paper, an ultrathin reduced graphene oxide film with a thickness of 8 nm was prepd. by a modified spin coating method that makes the microsupercapacitors have higher power characteristics which appear in high cond. and fast charge and discharge rates. The electrodes, with a width of 150 μm, are narrower than existing electrodes in the field and fabricated by photolithog. In terms of performance, these microsupercapacitors have an extremely short time const. which is 0.03 ms, a high power d. which is 17.94 W cm-3, and an excellent cycle stability with a capacitance retention of 94.6% after 10 000 cycles. (c) 2019 American Institute of Physics.
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    Xu, S.; Liu, W.; Hu, B.; Wang, X. Circuit-integratable high-frequency micro supercapacitors with filter/oscillator demonstrations. Nano Energy 2019, 58, 803– 810,  DOI: 10.1016/j.nanoen.2019.01.079
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    Circuit-integratable high-frequency micro supercapacitors with filter/oscillator demonstrations
    Xu, Sixing; Liu, Wei; Hu, Bingmeng; Wang, Xiaohong
    Nano Energy (2019), 58 (), 803-810CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
    Micro supercapacitor (MSC) featured by high capacitance d., has great potential for replacing bulky electrolytic capacitors in circuits to make electronics of miniaturization, yet suffers from sharp capacitance drop under a.c. Here, we report a circuit-integratable high-frequency MSC with hybrid architecture electrode, in which 2D pseudocapacitive MXene served as the active material provides large capacitance and multi-walled carbon nanotube applied as interlayer support offers fast ion transport paths. Such design brings the device an excellent frequency response and a capacitance d. much higher than com. tantalum capacitors at 120 Hz. A novel transfer-free filtration technol. greatly eliminates the equiv. series resistance and the consequent thermal loss, and makes the MSC a more circuit-integratable planar geometry. Moreover, such high-frequency MSC is applied in a low-pass filtering circuit and a relaxation oscillator circuit, displaying advantages in function, size and intergratability compared with electrolytic capacitors.
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    Zhang, C.; Peng, Z.; Huang, C.; Zhang, B.; Xing, C.; Chen, H.; Cheng, H.; Wang, J.; Tang, S. High-energy all-in-one stretchable micro-supercapacitor arrays based on 3D laser-induced graphene foams decorated with mesoporous ZnP nanosheets for self-powered stretchable systems. Nano Energy 2021, 81, 105609,  DOI: 10.1016/j.nanoen.2020.105609
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    High-energy all-in-one stretchable micro-supercapacitor arrays based on 3D laser-induced graphene foams decorated with mesoporous ZnP nanosheets for self-powered stretchable systems
    Zhang, Cheng; Peng, Zhixiang; Huang, Chunlei; Zhang, Bingwen; Xing, Chao; Chen, Huamin; Cheng, Huanyu; Wang, Jun; Tang, Shaolong
    Nano Energy (2021), 81 (), 105609CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
    Micro-supercapacitors are promising energy storage devices that can complement or even replace lithium-ion batteries in wearable and stretchable microelectronics. However, they often possess a relatively low energy d. and limited mech. stretchability. Here, we report an all-in-one planar micro-supercapacitor arrays (MSCAs) based on hybrid electrodes with ultrathin ZnP nanosheets anchored on 3D laser-induced graphene foams (ZnP@LIG) arranged in island-bridge device architecture. The hybrid electrodes with a large sp. surface area demonstrate excellent ionic and elec. conductivities, impressive gravimetric (areal) capacitance of 1425 F g-1 (7.125 F cm-2) at 1 A g-1, and long-term stability. In addn. to high energy (245 m Wh cm-2) and power (12.50 mW kg-1 at 145 m Wh cm-2) densities, the MSCAs with excellent cycling stability also showcase adjustable voltage and current outputs through serial and parallel connections of MSC cells in the island-bridge design, which also allows the system to be reversibly stretched up to 100%. Meanwhile, theor. calcns. validated by UV-vis absorption spectra partially suggest that the enhanced capacitance and rate capability may result from the improved elec. cond. and no. of adsorbed charged ions (Na+ in Na2SO4 aq. electrolyte and K+ in PVA/KCl gel electrolyte) on the pseudocapacitive non-layered ultrathin ZnP nanosheets. The integration of the all-in-one stretchable MSCAs with a crumpled Au-based triboelec. nanogenerator and stretchable crumpled graphene-based strain sensor demonstrates a self-powered stretchable system. The coupled design principle of electronic materials and device architecture provides a promising method to develop high-performance wearable/stretchable energy storage devices and self-powered stretchable systems for future bio-integrated electronics.
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    Yang, Y.; He, L.; Tang, C.; Hu, P.; Hong, X.; Yan, M.; Dong, Y.; Tian, X.; Wei, Q.; Mai, L. Improved conductivity and capacitance of interdigital carbon microelectrodes through integration with carbon nanotubes for micro-supercapacitors. Nano Research 2016, 9, 2510– 2519,  DOI: 10.1007/s12274-016-1137-3
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    Improved conductivity and capacitance of interdigital carbon microelectrodes through integration with carbon nanotubes for micro-supercapacitors
    Yang, Yanjuan; He, Liang; Tang, Chunjuan; Hu, Ping; Hong, Xufeng; Yan, Mengyu; Dong, Yixiao; Tian, Xiaocong; Wei, Qiulong; Mai, Liqiang
    Nano Research (2016), 9 (8), 2510-2519CODEN: NRAEB5; ISSN:1998-0000. (Springer GmbH)
    In the last decade, pyrolyzed-carbon-based composites have attracted much attention for their applications in micro-supercapacitors. Although various methods have been investigated to improve the performance of pyrolyzed carbons, such as cond., energy storage d. and cycling performance, effective methods for the integration and mass-prodn. of pyrolyzed-carbonbased composites on a large scale are lacking. Here, we report the development of an optimized photolithog. technique for the fine micropatterning of photoresist/chitosan-coated carbon nanotube (CHIT-CNT) composite. After subsequent pyrolysis, the fabricated carbon/CHIT-CNT microelectrode-based micro-supercapacitor has a high capacitance (6.09 mF·cm-2) and energy d. (4.5 mWh·cm-3) at a scan rate of 10 mV·s-1. Addnl., the micro-supercapacitor has a remarkable long-term cyclability, with 99.9% capacitance retention after 10,000 cyclic voltammetry cycles. This design and microfabrication process allow the application of carbon microelectromech. system (C-MEMS)-based micro-supercapacitors due to their high potential for enhancing the mech. and electrochem. performance of micro-supercapacitors.[Figure not available: see fulltext.].
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    The materials intended for org. photovoltaics and org. electronics require specific properties, such as conjugation along backbones, good soly. in org. solvents and a suitable band gap. Graphene oxide holds a high indirect optical band gap of 3 eV and a hydrophilic behavior. However, based on the fact that the graphene oxide band gap is related to the C/O ratio, chem. redn. and functionalization must be performed to obtain the proper characteristics that are required for org. applications. Here an efficient approach for chem. modified graphene oxide as a low-cost alternative for org. semiconductor materials was developed. The two synthesized materials (functionalized reduced graphene oxide and functionalized reduced graphene oxide decorated with silver nanoparticles) exhibited a 65% and 52% band gap decrease resp., compared to graphene oxide. Moreover, both materials were sol. in org. solvents at high concns. [10 mg mL-1], turning them suitable for many applications, including org. electronics, with the addnl. advantage of being a soln. process avoiding restacking of layers.
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    On chip, all solid-state and flexible micro-supercapacitors with high performance based on MnOx/Au multilayers
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  • Abstract

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    Figure 1

    Figure 1. (a–j) Schematic MSC fabrication process through spin coating of electrode solution and subsequent hard masking via an aluminum layer. (k) Schematic orientation of Stack MSC electrodes during the spin-coated deposition process step.

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    Figure 2

    Figure 2. (a) Au/Cr current collectors post lift-off.(b) Spin coated mixture of solutions on the wafer substrate.(c) Aluminum hard mask.(d) Stack electrodes on current collectors at the final step. (e) Camera capture of the fabricated Stack-MSC wafer (2 in.). (f) Cross section of the fabricated MSC with conductive rGO layers of approximately 1.1 μm. (g) Optical micrograph of the Stack-MSC. (h) High resolution optical image of the finger geometry of the Stack-MSC post GO reduction.

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    Figure 3

    Figure 3. (i) Schematic representation of the atomic configuration. (ii) Improvement in coverage of spin-coated electrodes after addition of a Cr nanoparticle layer (experiments conducted on a 1 cm2 Si chips). (iii) cyclic voltammograms of MSC with 20 fingers with 40 μm spacing fabricated by process described in 1 at different scan rates ranging from 100–500 mV s–1. (iv) Bode plot for fabricated MSCs, for 3 g/L (a) GO in H2O (reduced postprocess), (b) GO-HD9A in ODCB (reduced postprocess), (c) rGO-ODA in ODCB, and (d) rGO-HD9A in ODCB.

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    Figure 4

    Figure 4. Electrochemical characterization of Stack-MSC: (a) Cyclic voltammetry at various scan-rates and (b) GCD measurement. EIS performance of the Stack-MSC represented as (c) Nyquist, (d) Bode with Creal and ϕ as functions of frequency, and (e) Bode in terms of Cimag over a range of operational frequencies. (f) Normalized capacitance of Stack-MSC over several cyclic charge–discharge cycles.

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    Figure 5

    Figure 5. (a–c) Cyclic voltammograms of the four materials, GO, GO-HD9A, rGO-ODA, and rGO-HD9A-MSCs. in comparison to Stack-MSC at (a) 20, (b) 50, and (c) 100 mV s–1. (d) Comparison of areal capacitance of fabricated MSCs over a range of scan-rates, (e) chronopotentiometric scan of MSCs at 5 μA cm–2, and (f) areal capacitance of the MSCs when charge–discharged at various current densities.

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    Figure 6

    Figure 6. (a) Bode plot of Creal vs frequency for Stack, GO, GO-HD9A, rGO-ODA, and rGO-HD9A. (b) Nyquist plot of the fabricated MSCs. (c) Comparison of volumetric capacitance and areal capacitance of individual material with Stack. (d) Areal power density of MSCs over increasing current density. (e) Ragone plot of energy and power density of the MSCs fabricated in the current manuscript in comparison to MSCs with EDLC material published in the recent literature.

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    Figure 7

    Figure 7. (a) Equivalent circuit CPE with diffusion model (Bisquert et al. (43)). (b) Analysis of the fabricated MSCs: Stack, GO, and rGO-HD9A. Table 1 shows the calculated values for different components for the equivalent circuit.

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      Aq. Zn-based batteries are attractive because of the low cost and high theor. capacity of the Zn metal anode. However, the Zn-based batteries developed so far utilize an excess amt. of Zn (i.e., thick Zn metal anode), which decreases the energy d. of the whole battery. Herein, we demonstrate an anode-free design (i.e., zero-excess Zn), which is enabled by employing a nanocarbon nucleation layer. Electrochem. studies show that this design allows for uniform Zn electrodeposition with high efficiency and stability over a range of current densities and plating capacities. Using this anode-free configuration, we showcase a Zn-MnO2 battery prototype, showing 68.2% capacity retention after 80 cycles. Our anode-free design opens a new direction for implementing aq. Zn-based batteries in energy storage systems.
    6. 6
      Sun, J.; Sadd, M.; Edenborg, P.; Grönbeck, H.; Thiesen, P. H.; Xia, Z.; Quintano, V.; Qiu, R.; Matic, A.; Palermo, V. Real-time imaging of Na+ reversible intercalation in âĂIJJanusâĂİ graphene stacks for battery applications. Science advances 2021, 7, eabf0812,  DOI: 10.1126/sciadv.abf0812
      There is no corresponding record for this reference.
    7. 7
      Vyas, A.; Staaf, H.; Rusu, C.; Ebefors, T.; Liljeholm, J.; Smith, A. D.; Lundgren, P.; Enoksson, P. A micromachined coupled-cantilever for piezoelectric energy harvesters. Micromachines 2018, 9, 252,  DOI: 10.3390/mi9050252
      There is no corresponding record for this reference.
    8. 8
      Vullers, R.; van Schaijk, R.; Doms, I.; Van Hoof, C.; Mertens, R. Micropower energy harvesting. Solid-State Electron. 2009, 53, 684– 693,  DOI: 10.1016/j.sse.2008.12.011
      8
      Micropower energy harvesting
      Vullers, R. J. M.; van Schaijk, R.; Doms, I.; Van Hoof, C.; Mertens, R.
      Solid-State Electronics (2009), 53 (7), 684-693CODEN: SSELA5; ISSN:0038-1101. (Elsevier Ltd.)
      A review. More than a decade of research in the field of thermal, motion, vibration and electromagnetic radiation energy harvesting has yielded increasing power output and smaller embodiments. Power management circuits for rectification and DC-DC conversion are becoming able to efficiently convert the power from these energy harvesters. This paper summarizes recent energy harvesting results and their power management circuits.
    9. 9
      Green, M. A. Silicon photovoltaic modules: a brief history of the first 50 years. Progress in Photovoltaics: Research and applications 2005, 13, 447– 455,  DOI: 10.1002/pip.612
      There is no corresponding record for this reference.
    10. 10
      Todaro, M. T.; Guido, F.; Mastronardi, V.; Desmaele, D.; Epifani, G.; Algieri, L.; De Vittorio, M. Piezoelectric MEMS vibrational energy harvesters: Advances and outlook. Microelectron. Eng. 2017, 183, 23– 36,  DOI: 10.1016/j.mee.2017.10.005
      10
      Piezoelectric MEMS vibrational energy harvesters: Advances and outlook
      Todaro, Maria Teresa; Guido, Francesco; Mastronardi, Vincenzo; Desmaele, Denis; Epifani, Gianmichele; Algieri, Luciana; De Vittorio, Massimo
      Microelectronic Engineering (2017), 183-184 (), 23-36CODEN: MIENEF; ISSN:0167-9317. (Elsevier B.V.)
      Piezoelec. MEMS energy harvesters based on thin films are compact and cost-effective microgenerators for scavenging environmental vibrations. This technol. is promising for the replacement of electrochem. batteries in low power autonomous sensors and microdevices capturing vibrations in the μW-mW range. Most of piezoelec. MEMS devices, reported in the last few years, exhibit low generated power/voltage and are not suitable for practical applications. This work reviews the current status of MEMS energy harvesters based on piezoelec. thin films, highlighting approaches/strategies to face the two main challenges to be addressed for high performance devices, namely generated power and frequency bandwidth. The paper introduces the theor. principles and the main figures of merit of energy conversion in piezoelec. thin films and devices. After an overview on piezoelec. thin films for energy harvesting applications, highlighting their key properties, the manuscript reports a comprehensive survey on the state of the art for this device technol. The last section summarizes the review, highlighting key issues to be addressed and providing an insight into the future outlook to realize devices for practical applications.
    11. 11
      Hamid, H. A.; Çelik-Butler, Z. A novel MEMS triboelectric energy harvester and sensor with a high vibrational operating frequency and wide bandwidth fabricated using UV-LIGA technique. Sensors and Actuators A: Physical 2020, 313, 112175,  DOI: 10.1016/j.sna.2020.112175
      11
      A novel MEMS triboelectric energy harvester and sensor with a high vibrational operating frequency and wide bandwidth fabricated using UV-LIGA technique
      Hamid, H. M. Ashfiqul; Celik-Butler, Zeynep
      Sensors and Actuators, A: Physical (2020), 313 (), 112175CODEN: SAAPEB; ISSN:0924-4247. (Elsevier B.V.)
      A novel triboelec. energy harvesting (TEH) and sensing system scaled down to microelectromech. systems (MEMS) size is presented. The design is structurally optimized for harvesting the highest av. power and powerd. while ensuring the structural robustness. Unlike traditional triboelec. energy harvesters, this design results in a high operating frequency with a wide bandwidth. Adoption of MEMS fabrication techniques including the use of spin-coated Teflon AF rather than a Teflon sheet, adaptation of UV-LIGA (Ultra-Violet Lithographie, Galvanoformung, Abformung) with modifications and implementation of a thick polyimide sacrificial layer make the fabrication process unique. If excited by ±9.33 g external vibration with a frequency of 1.15 kHz, the TEH can generate 0.179μW av. power and 0.597μW peak power at an optimum resistive load of 256 kΩ. The peak surface power d., volumetric power d. and acceleration-normalized volumetric power d. reach 3.98μWcm-2, 2.64 mWcm-3 and 30.3μWcm-2/g2, resp. While the surface power d. of the presented TEH is moderate, the volumetric power d. and acceleration-normalized volumetric power d. are quite competitive among the state-of-the-art designs. The TEH also demonstrates a wide operating frequency bandwidth of 920 Hz. If operated as an accelerometer, the device shows a linear sensitivity of 43 mV/g. Although the simulation predicts the optimum operating frequency and load resistance of the system to be at 800 Hz and 10 MΩ, resp., the exptl. results demonstrate these values to be at 1150 Hz and 256 KΩ. A few fabrication anomalies, most notably the notching in the Teflon layer and bowing of the proof-mass, are responsible for this deviation. In addn., a distortion is obsd. in the simulated output voltage profile which is not present in the exptl. output voltage profile due to the presence of the parasitic capacitance in the exptl. circuit. The aforementioned triboelec. energy harvester can have specific applications in the sensor and actuator systems in the aircraft industry as well as in the automobile industry, micro-robotic systems, prosthetic systems, and sensor nodes in the internet of things (IoT) due to its operating frequency and bandwidth range.
    12. 12
      Smith, A.; Li, Q.; Vyas, A.; Haque, M. M.; Wang, K.; Velasco, A.; Zhang, X.; Thurakkal, S.; Quellmalz, A.; Niklaus, F. Carbon-based electrode materials for microsupercapacitors in self-powering sensor networks: present and future development. Sensors 2019, 19, 4231,  DOI: 10.3390/s19194231
      12
      Carbon-based electrode materials for microsupercapacitors in self-powering sensor networks: present and future development
      Smith, A. D.; Li, Qi; Vyas, Agin; Haque, Mohammad Mazharul; Wang, Kejian; Velasco, Andres; Zhang, Xiaoyan; Thurakkal, Shameel; Quellmalz, Arne; Niklaus, Frank; Gylfason, Kristinn; Lundgren, Per; Enoksson, Peter
      Sensors (2019), 19 (19), 4231CODEN: SENSC9; ISSN:1424-8220. (MDPI AG)
      A review. There is an urgent need to fulfill future energy demands for micro and nanoelectronics. This work outlines a no. of important design features for carbon-based microsupercapacitors, which enhance both their performance and integration potential and are crit. for complimentary metal oxide semiconductor (CMOS) compatibility. Based on these design features, we present CMOS-compatible, graphene-based microsupercapacitors that can be integrated at the back end of the line of the integrated circuit fabrication. Electrode materials and their interfaces play a crucial role for the device characteristics. As such, different carbon-based materials are discussed and the importance of careful design of current collector/electrode interfaces is emphasized. Electrode adhesion is an important factor to improve device performance and uniformity. Addnl., doping of the electrodes can greatly improve the energy d. of the devices. As microsupercapacitors are engineered for targeted applications, device scaling is critically important, and we present the first steps toward general scaling trends. Last, we outline a potential future integration scheme for a complete microsystem on a chip, contg. sensors, logic, power generation, power management, and power storage. Such a system would be self-powering.
    13. 13
      Sze, S. M.; Li, Y.; Ng, K. K. Physics of semiconductor devices; John Wiley & Sons: 2021.
      There is no corresponding record for this reference.
    14. 14
      Vyas, A.; Wang, K.; Li, Q.; Saleem, A. M.; Bylund, M.; Andersson, R.; Desmaris, V.; Smith, A.; Lundgren, P.; Enoksson, P. Impact of electrode geometry and thickness on planar on-chip microsupercapacitors. RSC Adv. 2020, 10, 31435– 31441,  DOI: 10.1039/D0RA05488G
      14
      Impact of electrode geometry and thickness on planar on-chip microsupercapacitors
      Vyas, Agin; Wang, Kejian; Li, Qi; Saleem, Amin M.; Bylund, Maria; Andersson, Rickard; Desmaris, Vincent; Smith, Anderson; Lundgren, Per; Enoksson, Peter
      RSC Advances (2020), 10 (52), 31435-31441CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      We report an assessment of the influence of both finger geometry and vertically-oriented carbon nanofiber lengths in planar micro-supercapacitors. Increasing the finger no. leads to an up-scaling in areal power densities, which increases with scan rate. Growing the nanofibers longer, however, does not lead to a proportional growth in capacitance, proposedly related to limited ion penetration of the electrode.
    15. 15
      Wen, F.; Hao, C.; Xiang, J.; Wang, L.; Hou, H.; Su, Z.; Hu, W.; Liu, Z. Enhanced laser scribed flexible graphene-based micro-supercapacitor performance with reduction of carbon nanotubes diameter. Carbon 2014, 75, 236– 243,  DOI: 10.1016/j.carbon.2014.03.058
      15
      Enhanced laser scribed flexible graphene-based micro-supercapacitor performance with reduction of carbon nanotubes diameter
      Wen, Fusheng; Hao, Chunxue; Xiang, Jianyong; Wang, Limin; Hou, Hang; Su, Zhibin; Hu, Wentao; Liu, Zhongyuan
      Carbon (2014), 75 (), 236-243CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
      By making use of the induced transformation of graphite oxide (GO) to laser scribed graphene (LSG), the coated layers of GO/carbon nanotubes (CNTs) hybrid powders on flexible polyethylene terephthalate sheets were patterned into LSG/CNTs micro-supercapacitors (LSG/CNTs-MSCs). In the presence of CNTs with a smaller diam., the laser-scribed LSG/CNTs-MSC was found to yield the better energy storage performance. This dependence on the CNT diam. can be attributed to the role of CNTs in prevention of restacking of LSG layers and thus the increase of ion-accessible surface area. The CNTs with a smaller diam. can be more easily inserted between the LSG layers, thus inhibiting the restacking phenomenon more effectively. By using the single-wall CNTs (SWCNTs) of 1-2 nm diam., the laser-scribed LSG/SWCNTs-MSC was obsd. to exhibit the best electrochem. properties: the volumetric capacitance of 3.10 F cm-3 at a c.d. of 1000 mA cm-3, the volumetric energy d. of 0.84 mW h cm-3 and power d. of 1.0 W cm-3, and long-term cycling stability.
    16. 16
      Singh, M.; Haverinen, H. M.; Dhagat, P.; Jabbour, G. E. Inkjet printing-process and its applications. Advanced materials 2010, 22, 673– 685,  DOI: 10.1002/adma.200901141
      16
      Inkjet Printing - Process and Its Applications
      Singh, Madhusudan; Haverinen, Hanna M.; Dhagat, Parul; Jabbour, Ghassan E.
      Advanced Materials (Weinheim, Germany) (2010), 22 (6), 673-685CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. In this Progress Report the authors provide an update on recent developments in inkjet printing technol. and its applications, which include org. thin-film transistors, light-emitting diodes, solar cells, conductive structures, memory devices, sensors, and biol./pharmaceutical tasks. Various classes of materials and device types are in turn examd. and an opinion is offered about the nature of the progress that was achieved.
    17. 17
      Wu, Z.-S.; Tan, Y.-Z.; Zheng, S.; Wang, S.; Parvez, K.; Qin, J.; Shi, X.; Sun, C.; Bao, X.; Feng, X. Bottom-up fabrication of sulfur-doped graphene films derived from sulfur-annulated nanographene for ultrahigh volumetric capacitance micro-supercapacitors. J. Am. Chem. Soc. 2017, 139, 4506– 4512,  DOI: 10.1021/jacs.7b00805
      17
      Bottom-Up Fabrication of Sulfur-Doped Graphene Films Derived from Sulfur-Annulated Nanographene for Ultrahigh Volumetric Capacitance Micro-Supercapacitors
      Wu, Zhong-Shuai; Tan, Yun-Zhi; Zheng, Shuanghao; Wang, Sen; Parvez, Khaled; Qin, Jieqiong; Shi, Xiaoyu; Sun, Chenglin; Bao, Xinhe; Feng, Xinliang; Mullen, Klaus
      Journal of the American Chemical Society (2017), 139 (12), 4506-4512CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Heteroatom doping of nanocarbon films can efficiently boost the pseudocapacitance of micro-supercapacitors (MSCs), however, wafer-scale fabrication of sulfur-doped graphene films with a tailored thickness and homogeneous doping for MSCs remains a great challenge. Here we demonstrate the bottom-up fabrication of continuous, uniform, and ultrathin sulfur-doped graphene (SG) films, derived from the peripherical trisulfur-annulated hexa-peri-hexabenzocoronene (SHBC), for ultrahigh-rate MSCs (SG-MSCs) with landmark volumetric capacitance. The SG film was prepd. by thermal annealing of the spray-coated SHBC-based film, with assistance of a thin Au protecting layer, at 800 °C for 30 min. SHBC with 12 phenylthio groups decorated at the periphery is crit. as a precursor for the formation of the continuous and ultrathin SG film, with a uniform thickness of ∼10.0 nm. Notably, the as-produced all-solid-state planar SG-MSCs exhibited a highly stable pseudocapacitive behavior with a volumetric capacitance of ∼582 F/cm3 at 10 mV/s, excellent rate capability with a remarkable capacitance of 8.1 F/cm3 even at an ultrahigh rate of 2000 V/s, ultrafast frequency response with a short time const. of 0.26 ms, and ultrahigh power d. of ∼1191 W/cm3. It is noteworthy that these values obtained are among the best values for carbon-based MSCs reported to date.
    18. 18
      Wu, Z.-S.; Parvez, K.; Winter, A.; Vieker, H.; Liu, X.; Han, S.; Turchanin, A.; Feng, X.; Müllen, K. Layer-by-layer assembled heteroatom-doped graphene films with ultrahigh volumetric capacitance and rate capability for micro-supercapacitors. Adv. Mater. 2014, 26, 4552– 4558,  DOI: 10.1002/adma.201401228
      18
      Layer-by-layer Assembled Heteroatom-Doped Graphene Films with Ultrahigh Volumetric Capacitance and Rate Capability for Micro-Supercapacitors
      Wu, Zhong-Shuai; Parvez, Khaled; Winter, Andreas; Vieker, Henning; Liu, Xianjie; Han, Sheng; Turchanin, Andrey; Feng, Xinliang; Muellen, Klaus
      Advanced Materials (Weinheim, Germany) (2014), 26 (26), 4552-4558CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Authors describe the development of large-area highly uniform, ultrathin, nitrogen and boron co-doped graphene (BNG) films for high-performance micro-supercapacitors (MSCs). The BNG film was prepd. using a layer-by-layer (LBL) assembly of anionic graphene oxide (GO) nanosheets and cationic poly-L-Lysine (PLL) as a nitrogen-contg. precursor, followed by intercalation of H3BO3 within the layers and annealing treatment. The PLL and H3BO3 incorporated into the assembled multilayer films not only serve as nitrogen- and boron-rich precursors, resp., but also generate micropore fillers to promote the formation of porous, yet densely packed BNG films during the thermal treatment.
    19. 19
      Yang, W.; He, L.; Tian, X.; Yan, M.; Yuan, H.; Liao, X.; Meng, J.; Hao, Z.; Mai, L. Carbon-MEMS-based alternating stacked MoS2@ rGO-CNT micro-supercapacitor with high capacitance and energy density. Small 2017, 13, 1700639,  DOI: 10.1002/smll.201700639
      There is no corresponding record for this reference.
    20. 20
      Vyas, A.; Li, Q.; Cornaglia, F.; Wang, K.; Anderson, A.; Haque, M.; Kuzmenko, V.; Smith, A.; Lundgren, P.; Enoksson, P. Surface Roughening with Iron Nanoparticles for Promoted Adhesion of Spin Coated Microsupercapacitor Electrodes. MRS Advances 2019, 4, 1335– 1340,  DOI: 10.1557/adv.2019.5
      20
      Surface Roughening with Iron Nanoparticles for Promoted Adhesion of Spin Coated Microsupercapacitor Electrodes
      Vyas, A.; Li, Q.; Cornaglia, F.; Wang, K.; Anderson, A.; Haque, M.; Kuzmenko, V.; Smith, A. D.; Lundgren, P.; Enoksson, P.
      MRS Advances (2019), 4 (23), 1335-1340CODEN: MARDCQ; ISSN:2059-8521. (Cambridge University Press)
      Microsupercapacitors (MSCs) are miniaturized energy storage devices that can be integrated in an on-chip platform as a component of a power supply for Internet of things' sensors. Integration of these on-chip MSCs require them to be fabricated through CMOS compatible fabrication techniques such as spin coating. One of the biggest challenges in spin coated MSCs is the poor surface adhesion. In this work, we present a CMOS compatible electrode deposition process with enhanced adhesion and retention for reduced graphene oxide (rGO) using spin coating. In order to improve the adhesion and surface uniformity of the deposited electrode material, the surface of Si/SiO2 wafers was subjected to roughening through Fe nanoparticle formation. A 4 nm thick Fe layer deposition substantially magnified the av. mean surface roughness of the substrates. In comparison with substrates without the Fe deposition, the treated ones have more than 300% improvement in surface coverage and rGO mass retention after sonication testing. These results suggest that the surface roughening has a pos. influence on electrode deposition via a spin-coating method.
    21. 21
      Vyas, A.; Wang, K.; Anderson, A.; Velasco, A.; van den Eeckhoudt, R.; Haque, M. M.; Li, Q.; Smith, A.; Lundgren, P.; Enoksson, P. Enhanced Electrode Deposition for On-Chip Integrated Micro-Supercapacitors by Controlled Surface Roughening. ACS omega 2020, 5, 5219– 5228,  DOI: 10.1021/acsomega.9b04266
      21
      Enhanced Electrode Deposition for On-Chip Integrated Micro-Supercapacitors by Controlled Surface Roughening
      Vyas, Agin; Wang, Kejian; Anderson, Alec; Velasco, Andres; Van den Eeckhoudt, Ruben; Haque, Mohammad Mazharul; Li, Qi; Smith, Anderson; Lundgren, Per; Enoksson, Peter
      ACS Omega (2020), 5 (10), 5219-5228CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
      On-chip micro-supercapacitors (MSCs), integrated with energy harvesters, hold substantial promise for developing self-powered wireless sensor systems. However, MSCs have conventionally been manufd. through techniques incompatible with semiconductor fabrication technol., the most significant bottleneck being the electrode deposition technique. Utilization of spin-coating for electrode deposition has shown potential to deliver several complementary metal-oxide-semiconductor (CMOS)-compatible MSCs on a silicon substrate. Yet, their limited electrochem. performance and yield over the substrate have remained challenges obstructing their subsequent integration. We report a facile surface roughening technique for improving the wafer yield and the electrochem. performance of CMOS-compatible MSCs, specifically for reduced graphene oxide as an electrode material. A 4 nm iron layer is deposited and annealed on the wafer substrate to increase the roughness of the surface. In comparison to std. nonroughened MSCs, the increase in surface roughness leads to a 78% increased electrode thickness, 21% improvement in mass retention, 57% improvement in the uniformity of the spin-coated electrodes, and a high yield of 87% working devices on a 2'' silicon substrate. Furthermore, these improvements directly translate to higher capacitive performance with enhanced rate capability, energy, and power d. This technique brings us one step closer to fully integrable CMOS-compatible MSCs in self-powered systems for on-chip wireless sensor electronics.
    22. 22
      Zhao, J.; Shi, Q.; Guo, Y.; Wang, X.; Wang, D.; Tan, F.; Jiang, L.; Yu, Y. Flash foam stamp-inspired fabrication of flexible in-plane graphene integrated micro-supercapacitors on paper. J. Power Sources 2019, 433, 226703,  DOI: 10.1016/j.jpowsour.2019.226703
      22
      Flash foam stamp-inspired fabrication of flexible in-plane graphene integrated micro-supercapacitors on paper
      Zhao, Jiang; Shi, Qingling; Guo, Yanyan; Wang, Xiangfu; Wang, Debo; Tan, Furui; Jiang, Li; Yu, Ying
      Journal of Power Sources (2019), 433 (), 226703CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Developing patterning techniques with practicability, cost-effectiveness and safety are essential for shape-confirmable integrated energy storage devices. Herein we design a novel flash foam stamp-inspired technique for simple fabrication of flexible high energy in-plane micro-supercapacitors based on interdigital electrodes of self-depositing electrochem. exfoliated graphene on paper substrates. This technique is an attractive and novel bottom-up microfabrication technol. due to its advantages of simplicity and low-cost. These novelly developed all-solid-state, paper-based in-plane graphene micro-supercapacitors exhibit excellent electrochem. performances, such as high area specific capacitance of ∼4.02 mF cm-2 and excellent cycling stability (95.6% retention at 0.08 mA cm-2 for 5000 cycles). And these solid-state flexible micro-supercapacitors are obsd. non-obvious structural change and loss of performance under bending state, suggesting remarkable flexibility and mech. stability. Therefore, such all-solid-state, flexible paper-based graphene micro-supercapacitors can potentially be used for wearable and portable electronics.
    23. 23
      Kim, S.-W.; Kang, K.-N.; Min, J.-W.; Jang, J.-H. Plotter-assisted integration of wearable all-solid-state micro-supercapacitors. Nano Energy 2018, 50, 410– 416,  DOI: 10.1016/j.nanoen.2018.05.051
      23
      Plotter-assisted integration of wearable all-solid-state micro-supercapacitors
      Kim, Sung-Wook; Kang, Kyeong-Nam; Min, Jin-Wook; Jang, Ji-Hyun
      Nano Energy (2018), 50 (), 410-416CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
      We introduce the use of xurog. as a facile plotter-assisted integration technique for an all-solid-state flexible micro-supercapacitors (mSCs), which is cost-effective and time-saving, by which 162 mSCs can be simultaneously produced on an A4 sized PET film. 3D-graphene networks (3DGNs) grown on a SiO2 template exhibited large surface area and outstanding elec. cond. as active materials for mSCs due to the interconnected 3D structure. By simply mixing SWNTs and Ag NWs with 3DGNs, an excellent specific areal capacitance of 19 mF cm-2 was obtained from an as-prepd. 3DGN/SWNT/AgNW mSC on a flexible substrate and 84% of the initial capacitance was maintained after 10,000 cycles. The device exhibited stable electrochem. performances under various deformation conditions. The subsequent transfer onto a textile further allowed the fabrication of a wearable particle detector, which confirmed a practical application of a mSC with good performance integrated via a plotter-assisted technique.
    24. 24
      Boruah, B. D.; Nandi, S.; Misra, A. Layered assembly of reduced graphene oxide and vanadium oxide heterostructure supercapacitor electrodes with larger surface area for efficient energy-storage performance. ACS Applied Energy Materials 2018, 1, 1567– 1574,  DOI: 10.1021/acsaem.7b00358
      24
      Layered Assembly of Reduced Graphene Oxide and Vanadium Oxide Heterostructure Supercapacitor Electrodes with Larger Surface Area for Efficient Energy-Storage Performance
      Boruah, Buddha Deka; Nandi, Sukanta; Misra, Abha
      ACS Applied Energy Materials (2018), 1 (4), 1567-1574CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
      The architecture of a supercapacitor (SC) electrode plays a crucial role in defining the overall energy-storage performance of the SC. Layer-by-layer assembly of a reduced graphene oxide (rGO) and vanadium oxide (V2O5) (rGO/V2O5)-based heterostructure is patterned in interdigitated electrodes (IDEs) deposited directly on a flexible conducting current collector for the SC. The IDE pattern offers efficient accessibility to the electrolyte ions and a synergistic contribution for energy storage. An as-fabricated solid-state flexible sandwich-type SC with IDEs displays a more efficient energy-storage performance than a conventional solid-state flexible sandwich-type SC composed of rGO/V2O5 electrodes. Moreover, a solid-state flexible in-plane microsupercapacitor (MSC) is fabricated, which offers much higher capacitance (24 mF/cm2 and 34.28 F/cm3) and energy d. (3.3 μWh/cm2 and 4.7 mWh/cm3). The as-fabricated flexible in-plane MSC displays a negligible capacitance loss of about 6.3% after 10000 charge-discharge cycles and a superior stability of energy-storage performance towards mech. deformation.
    25. 25
      Göhlert, T.; Siles, P. F.; Päßler, T.; Sommer, R.; Baunack, S.; Oswald, S.; Schmidt, O. G. Ultra-thin all-solid-state micro-supercapacitors with exceptional performance and device flexibility. Nano Energy 2017, 33, 387– 392,  DOI: 10.1016/j.nanoen.2017.01.054
      There is no corresponding record for this reference.
    26. 26
      Gholami Laelabadi, K.; Moradian, R.; Manouchehri, I. One-Step Fabrication of Flexible, Cost/Time Effective, and High Energy Storage Reduced Graphene Oxide@ PANI Supercapacitor. ACS Applied Energy Materials 2020, 3, 5301– 5312,  DOI: 10.1021/acsaem.0c00317
      26
      One-Step Fabrication of Flexible, Cost/Time Effective, and High Energy Storage Reduced Graphene Oxide@PANI Supercapacitor
      Gholami Laelabadi, Katayoon; Moradian, Rostam; Manouchehri, Iraj
      ACS Applied Energy Materials (2020), 3 (6), 5301-5312CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
      Microsupercapacitors, as an energy storage device, have unique electrochem. performance, such as high power d., fast charging, long cycle life, and high safety. The redn. time and cost in fabrication processes of microsupercapacitors are important factors in microfabrication technol. In this work, a simple, scalable, and cost-effective fabrication of interdigitated reduced graphene oxide@polyaniline flexible microsupercapacitors is presented. It was found that in fabricating the interdigitated microelectrode patterns on PET substrate; the redn. of graphene oxide and growth of conducting polymer are rapidly performed simultaneously in one step by laser irradn. The capacitance was 72 mF/cm2 at a 35μA/cm2 c.d. These high-capacitance microsupercapacitors demonstrate good stability and > 93.5% of the capacitance is retained after 1000 cycles at a 0.7 mA/cm2 c.d.
    27. 27
      Pech, D.; Brunet, M.; Durou, H.; Huang, P.; Mochalin, V.; Gogotsi, Y.; Taberna, P.-L.; Simon, P. Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon. Nature Nanotechnol. 2010, 5, 651– 654,  DOI: 10.1038/nnano.2010.162
      27
      Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon
      Pech, David; Brunet, Magali; Durou, Hugo; Huang, Peihua; Mochalin, Vadym; Gogotsi, Yury; Taberna, Pierre-Louis; Simon, Patrice
      Nature Nanotechnology (2010), 5 (9), 651-654CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Electrochem. capacitors, also called supercapacitors, store energy in 2 closely spaced layers with opposing charges, and are used to power hybrid elec. vehicles, portable electronic equipment and other devices. By offering fast charging and discharging rates, and the ability to sustain millions of cycles, electrochem. capacitors bridge the gap between batteries, which offer high energy densities but are slow, and conventional electrolytic capacitors, which are fast but have low energy densities. Here, the authors demonstrate microsupercapacitors with powers per vol. that are comparable to electrolytic capacitors, capacitances that are 4 orders of magnitude higher, and energies per vol. that are an order of magnitude higher. The authors also measured discharge rates of up to 200 V s-1, which is 3 orders of magnitude higher than conventional supercapacitors. The microsupercapacitors are produced by the electrophoretic deposition of a several-micrometre-thick layer of nanostructured C onions with diams. of 6-7 nm. Integration of these nanoparticles in a microdevice with a high surface-to-vol. ratio, without the use of org. binders and polymer separators, improves performance because of the ease with which ions can access the active material. Increasing the energy d. and discharge rates of supercapacitors will enable them to compete with batteries and conventional electrolytic capacitors in a no. of applications.
    28. 28
      Chmiola, J.; Largeot, C.; Taberna, P.-L.; Simon, P.; Gogotsi, Y. Monolithic carbide-derived carbon films for micro-supercapacitors. science 2010, 328, 480– 483,  DOI: 10.1126/science.1184126
      28
      Monolithic Carbide-Derived Carbon Films for Micro-Supercapacitors
      Chmiola, John; Largeot, Celine; Taberna, Pierre-Louis; Simon, Patrice; Gogotsi, Yury
      Science (Washington, DC, United States) (2010), 328 (5977), 480-483CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Microbatteries with dimensions of tens to hundreds of micrometers that are produced by common microfabrication techniques are poised to provide integration of power sources onto electronic devices, but they still suffer from poor cycle lifetime, as well as power and temp. range of operation issues that are alleviated with the use of supercapacitors. There have been a few reports on thin-film and other micro-supercapacitors, but they are either too thin to provide sufficient energy or the technol. is not scalable. By etching supercapacitor electrodes into conductive titanium carbide substrates, we demonstrate that monolithic carbon films lead to a volumetric capacity exceeding that of micro- and macroscale supercapacitors reported thus far, by a factor of 2. This study also provides the framework for integration of high-performance micro-supercapacitors onto a variety of devices.
    29. 29
      Zhang, L.; DeArmond, D.; Alvarez, N. T.; Malik, R.; Oslin, N.; McConnell, C.; Adusei, P. K.; Hsieh, Y.-Y.; Shanov, V. Flexible micro-supercapacitor based on graphene with 3D structure. Small 2017, 13, 1603114,  DOI: 10.1002/smll.201603114
      There is no corresponding record for this reference.
    30. 30
      Lu, Y.; Zheng, Y.; Zhang, H.; He, X.; Yang, Q.; Wu, J. A high performance and flexible in-plane asymmetric micro-supercapacitor (MSC) fabricated with functional electrochemical-exfoliated graphene. J. Electroanal. Chem. 2020, 866, 114169,  DOI: 10.1016/j.jelechem.2020.114169
      30
      A high performance and flexible in-plane asymmetric micro-supercapacitor (MSC) fabricated with functional electrochemical-exfoliated graphene
      Lu, Yuanchao; Zheng, Yawen; Zhang, Hao; He, Xiangxiang; Yang, Qunqing; Wu, Jian
      Journal of Electroanalytical Chemistry (2020), 866 (), 114169CODEN: JECHES; ISSN:1873-2569. (Elsevier B.V.)
      With the increasing development of miniaturized, integrated, wearable and portable elec. devices, in-plane micro-supercapacitors (MSCs), as one of most potential next-generation energy storage setups, have attracted abundant interests from the public. A flexible in-plane asym. MSC, prepd. by the mask-assisted vacuum filtration, was demonstrated. Also, the electrochem.-exfoliated functional graphene oxide (FGO) and chem.-reduced functional reduced graphene oxide (FrGO) were configured as pos. and neg. electrode of the MSCs, coated with the solid PVA/Na2SO4 electrolyte. The result presented that the as-assembled FGO-FrGO MSCs performed the highest areal capacitance of 7.3 mF cm-2. It was assumed that the high performance of the asym. MSCs was attributed to the asym. in-plane interdigitated structure, the abundant functional groups doping and heteroatoms replacement with N, O, P, S elements in graphene. In summary, the functional graphene-based asym. MSCs provide a potential inspiration for the next-generation flexible and wearable energy storage devices.
    31. 31
      Xiong, G.; Meng, C.; Reifenberger, R. G.; Irazoqui, P. P.; Fisher, T. S. A review of graphene-based electrochemical microsupercapacitors. Electroanalysis 2014, 26, 30– 51,  DOI: 10.1002/elan.201300238
      31
      A Review of Graphene-Based Electrochemical Microsupercapacitors
      Xiong, Guoping; Meng, Chuizhou; Reifenberger, Ronald G.; Irazoqui, Pedro P.; Fisher, Timothy S.
      Electroanalysis (2014), 26 (1), 30-51CODEN: ELANEU; ISSN:1040-0397. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. The rapid development of miniaturized electronic devices has led to a growing need for rechargeable micropower sources with high performance. Among different sources, electrochem. microcapacitors or microsupercapacitors provide higher power d. than their counterparts and are gaining increased interest from the research and engineering communities. To date, little work has appeared on the integration of microsupercapacitors onto a chip or flexible substrates. This paper provides an overview of research on microsupercapacitors, with particular emphasis on state-of-the-art graphene-based electrodes and solid-state devices on both flexible and rigid substrates. The advantages, disadvantages, and performance of graphene-based microsupercapacitors are summarized and new trends in materials, fabrication and packaging are identified.
    32. 32
      Lee, S. H.; Lee, J.; Jung, J.; Cho, A. R.; Jeong, J. R.; Dang Van, C.; Nah, J.; Lee, M. H. Enhanced Electrochemical Performance of Micro-Supercapacitors Via Laser-Scribed Cobalt/Reduced Graphene Oxide Hybrids. ACS Appl. Mater. Interfaces 2021, 13, 18821– 18828,  DOI: 10.1021/acsami.1c02102
      32
      Enhanced Electrochemical Performance of Micro-Supercapacitors Via Laser-Scribed Cobalt/Reduced Graphene Oxide Hybrids
      Lee, Sang Hwa; Lee, Jungjun; Jung, Jaemin; Cho, A. Ra; Jeong, Jae Ryeol; Dang Van, Cu; Nah, Junghyo; Lee, Min Hyung
      ACS Applied Materials & Interfaces (2021), 13 (16), 18821-18828CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The evolution of "smart life," which connects all internet-of-things (IoT) microdevices and microsensors under wireless communication grids, requires microscale energy storage devices with high power and energy d. and long-term cyclability to integrate them with sustainable power generators. Instead of Li-ion batteries with a short lifetime, pseudocapacitors with longer or infinite cyclability and high-power d. have been considered as efficient energy storage devices for IoT. However, the design and fabrication of microscale pseudocapacitors have difficulties in patterning microscale electrodes when loading active materials at specific points of the electrodes using conventional microfabrication methods. Here, we developed a facile, one-step fabrication method of micro-supercapacitors (MSCs) through the in situ formation of Co metals and the reduced graphene oxides (rGOs) in a one-pot laser scribing process. The prepd. Co/rGO MSC thus exhibited four times higher capacitance than the rGO MSC, due to the Faradaic charge capacitance behavior of the Co/rGO composites.
    33. 33
      Huang, K. Laser printer patterned sacrificed layer for arbitrary design and scalable fabrication of the all-solid-state interdigitated in-planar hydrous ruthenium oxide flexible micro supercapacitors. J. Power Sources 2019, 417, 108– 116,  DOI: 10.1016/j.jpowsour.2019.02.016
      33
      Laser printer patterned sacrificed layer for arbitrary design and scalable fabrication of the all-solid-state interdigitated in-planar hydrous ruthenium oxide flexible micro supercapacitors
      Huang, Kai-Chen; Lin, Che-Hsien; S, Anuratha K.; Huang, Tsung-Yu; Lin, Jeng-Yu; Tseng, Fan-Gang; Hsieh, Chien-Kuo
      Journal of Power Sources (2019), 417 (), 108-116CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      We demonstrate the successful fabrication of all-solid-state flexible micro-supercapacitors (μSCs) on com. polyethylene terephthalate (PET) sheet using a com. laser printer. The interdigitated in-planar electrodes of the μSC are fabricated by com. toner cartridge and their self-aligned stacking configurations are made by depositing Pt thin films and hydrous ruthenium oxide (RuO·xH2O, hRuO2) layers on the PET sheets using a post-low-temp. annealing process. The μSC cells can be combined in parallel or series by an arbitrary printing process operated on a personal computer, thereby avoiding unnecessary external connections and improving the capacitive behavior. The fabricated hRuO2-based μSCs exhibit excellent electrochem.-energy storage, along with ultrahigh volumetric power d. (73460 mW cm-3) and energy d. (24.9 mWh cm-3). This facile and simplified strategy constructs μSCs with tailored self-aligned electrodes of arbitrary design, excellent mech. stability, and high flexibility. Therefore, it offers a high-productivity, low-cost, photolithog.-free microfabrication of μSCs for microscale supercapacitor applications.
    34. 34
      Zhang, L.; Liu, L.; Liu, C.; Li, X.; Liu, F.; Zhao, W.; Wang, S.; Wu, F.; Zhang, G. Photolithographic fabrication of graphene-based all-solid-state planar on-chip microsupercapacitors with ultrahigh power characteristics. J. Appl. Phys. 2019, 126, 164308,  DOI: 10.1063/1.5109691
      34
      Photolithographic fabrication of graphene-based all-solid-state planar on-chip microsupercapacitors with ultrahigh power characteristics
      Zhang, Ludi; Liu, Linjing; Liu, Chunfeng; Li, Xiang; Liu, Fangshuo; Zhao, Wenqiang; Wang, Shu; Wu, Fengmin; Zhang, Guangyu
      Journal of Applied Physics (Melville, NY, United States) (2019), 126 (16), 164308/1-164308/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      The fabrication of all-solid-state planar on-chip microsupercapacitors with ultrahigh power characteristics was demonstrated by reduced graphene oxide and photolithog. technol. In this paper, an ultrathin reduced graphene oxide film with a thickness of 8 nm was prepd. by a modified spin coating method that makes the microsupercapacitors have higher power characteristics which appear in high cond. and fast charge and discharge rates. The electrodes, with a width of 150 μm, are narrower than existing electrodes in the field and fabricated by photolithog. In terms of performance, these microsupercapacitors have an extremely short time const. which is 0.03 ms, a high power d. which is 17.94 W cm-3, and an excellent cycle stability with a capacitance retention of 94.6% after 10 000 cycles. (c) 2019 American Institute of Physics.
    35. 35
      Xu, S.; Liu, W.; Hu, B.; Wang, X. Circuit-integratable high-frequency micro supercapacitors with filter/oscillator demonstrations. Nano Energy 2019, 58, 803– 810,  DOI: 10.1016/j.nanoen.2019.01.079
      35
      Circuit-integratable high-frequency micro supercapacitors with filter/oscillator demonstrations
      Xu, Sixing; Liu, Wei; Hu, Bingmeng; Wang, Xiaohong
      Nano Energy (2019), 58 (), 803-810CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
      Micro supercapacitor (MSC) featured by high capacitance d., has great potential for replacing bulky electrolytic capacitors in circuits to make electronics of miniaturization, yet suffers from sharp capacitance drop under a.c. Here, we report a circuit-integratable high-frequency MSC with hybrid architecture electrode, in which 2D pseudocapacitive MXene served as the active material provides large capacitance and multi-walled carbon nanotube applied as interlayer support offers fast ion transport paths. Such design brings the device an excellent frequency response and a capacitance d. much higher than com. tantalum capacitors at 120 Hz. A novel transfer-free filtration technol. greatly eliminates the equiv. series resistance and the consequent thermal loss, and makes the MSC a more circuit-integratable planar geometry. Moreover, such high-frequency MSC is applied in a low-pass filtering circuit and a relaxation oscillator circuit, displaying advantages in function, size and intergratability compared with electrolytic capacitors.
    36. 36
      Zhang, C.; Peng, Z.; Huang, C.; Zhang, B.; Xing, C.; Chen, H.; Cheng, H.; Wang, J.; Tang, S. High-energy all-in-one stretchable micro-supercapacitor arrays based on 3D laser-induced graphene foams decorated with mesoporous ZnP nanosheets for self-powered stretchable systems. Nano Energy 2021, 81, 105609,  DOI: 10.1016/j.nanoen.2020.105609
      36
      High-energy all-in-one stretchable micro-supercapacitor arrays based on 3D laser-induced graphene foams decorated with mesoporous ZnP nanosheets for self-powered stretchable systems
      Zhang, Cheng; Peng, Zhixiang; Huang, Chunlei; Zhang, Bingwen; Xing, Chao; Chen, Huamin; Cheng, Huanyu; Wang, Jun; Tang, Shaolong
      Nano Energy (2021), 81 (), 105609CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
      Micro-supercapacitors are promising energy storage devices that can complement or even replace lithium-ion batteries in wearable and stretchable microelectronics. However, they often possess a relatively low energy d. and limited mech. stretchability. Here, we report an all-in-one planar micro-supercapacitor arrays (MSCAs) based on hybrid electrodes with ultrathin ZnP nanosheets anchored on 3D laser-induced graphene foams (ZnP@LIG) arranged in island-bridge device architecture. The hybrid electrodes with a large sp. surface area demonstrate excellent ionic and elec. conductivities, impressive gravimetric (areal) capacitance of 1425 F g-1 (7.125 F cm-2) at 1 A g-1, and long-term stability. In addn. to high energy (245 m Wh cm-2) and power (12.50 mW kg-1 at 145 m Wh cm-2) densities, the MSCAs with excellent cycling stability also showcase adjustable voltage and current outputs through serial and parallel connections of MSC cells in the island-bridge design, which also allows the system to be reversibly stretched up to 100%. Meanwhile, theor. calcns. validated by UV-vis absorption spectra partially suggest that the enhanced capacitance and rate capability may result from the improved elec. cond. and no. of adsorbed charged ions (Na+ in Na2SO4 aq. electrolyte and K+ in PVA/KCl gel electrolyte) on the pseudocapacitive non-layered ultrathin ZnP nanosheets. The integration of the all-in-one stretchable MSCAs with a crumpled Au-based triboelec. nanogenerator and stretchable crumpled graphene-based strain sensor demonstrates a self-powered stretchable system. The coupled design principle of electronic materials and device architecture provides a promising method to develop high-performance wearable/stretchable energy storage devices and self-powered stretchable systems for future bio-integrated electronics.
    37. 37
      Yang, Y.; He, L.; Tang, C.; Hu, P.; Hong, X.; Yan, M.; Dong, Y.; Tian, X.; Wei, Q.; Mai, L. Improved conductivity and capacitance of interdigital carbon microelectrodes through integration with carbon nanotubes for micro-supercapacitors. Nano Research 2016, 9, 2510– 2519,  DOI: 10.1007/s12274-016-1137-3
      37
      Improved conductivity and capacitance of interdigital carbon microelectrodes through integration with carbon nanotubes for micro-supercapacitors
      Yang, Yanjuan; He, Liang; Tang, Chunjuan; Hu, Ping; Hong, Xufeng; Yan, Mengyu; Dong, Yixiao; Tian, Xiaocong; Wei, Qiulong; Mai, Liqiang
      Nano Research (2016), 9 (8), 2510-2519CODEN: NRAEB5; ISSN:1998-0000. (Springer GmbH)
      In the last decade, pyrolyzed-carbon-based composites have attracted much attention for their applications in micro-supercapacitors. Although various methods have been investigated to improve the performance of pyrolyzed carbons, such as cond., energy storage d. and cycling performance, effective methods for the integration and mass-prodn. of pyrolyzed-carbonbased composites on a large scale are lacking. Here, we report the development of an optimized photolithog. technique for the fine micropatterning of photoresist/chitosan-coated carbon nanotube (CHIT-CNT) composite. After subsequent pyrolysis, the fabricated carbon/CHIT-CNT microelectrode-based micro-supercapacitor has a high capacitance (6.09 mF·cm-2) and energy d. (4.5 mWh·cm-3) at a scan rate of 10 mV·s-1. Addnl., the micro-supercapacitor has a remarkable long-term cyclability, with 99.9% capacitance retention after 10,000 cyclic voltammetry cycles. This design and microfabrication process allow the application of carbon microelectromech. system (C-MEMS)-based micro-supercapacitors due to their high potential for enhancing the mech. and electrochem. performance of micro-supercapacitors.[Figure not available: see fulltext.].
    38. 38
      Smith, A.; Li, Q.; Anderson, A.; Vyas, A.; Kuzmenko, V.; Haque, M.; Staaf, L.; Lundgren, P.; Enoksson, P. Toward CMOS compatible wafer-scale fabrication of carbon-based microsupercapacitors for IoT. Journal of Physics: Conference Series 2018, 1052, 012143,  DOI: 10.1088/1742-6596/1052/1/012143
      38
      Toward CMOS compatible wafer-scale fabrication of carbon based micro-super-capacitors for IoT
      Smith, A. D.; Li, Q.; Anderson, A.; Vyas, A.; Kuzmenko, V.; Haque, M.; Staaf, L. G. H.; Lundgren, P.; Enoksson, P.
      Journal of Physics: Conference Series (2018), 1052 (17th International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications, 2017), 012143/1-012143/4CODEN: JPCSDZ; ISSN:1742-6588. (IOP Publishing Ltd.)
      This work presents a wafer-scale method of micro-super-capacitor (MSC) fabrication. Deposition of the electrode precursor, i.e. graphene oxide, is accomplished through spin-coating which allows for potential application in CMOS compatible processes for future integrated on-chip energy storage systems. Our MSCs have an areal capacitance of 0.4 mF/cm2 at 10μA, which is a very promising result. Further, the MSC has good rate capability as its capacitance decreases by only 0.03 mF/cm2 when the current increases to 50μA. The MSCs have a max. energy d. of 0.04μWh/cm2 and a max. power d. as high as 96μW/cm2. Addnl., the wafer-scale process demonstrates industrial viability.
    39. 39
      Mendez-Romero, U. A.; Pérez-García, S. A.; Xu, X.; Wang, E.; Licea-Jiménez, L. Functionalized reduced graphene oxide with tunable band gap and good solubility in organic solvents. Carbon 2019, 146, 491– 502,  DOI: 10.1016/j.carbon.2019.02.023
      39
      Functionalized reduced graphene oxide with tunable band gap and good solubility in organic solvents
      Mendez-Romero, Ulises A.; Perez-Garcia, Sergio A.; Xu, Xiaofeng; Wang, Ergang; Licea-Jimenez, Liliana
      Carbon (2019), 146 (), 491-502CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
      The materials intended for org. photovoltaics and org. electronics require specific properties, such as conjugation along backbones, good soly. in org. solvents and a suitable band gap. Graphene oxide holds a high indirect optical band gap of 3 eV and a hydrophilic behavior. However, based on the fact that the graphene oxide band gap is related to the C/O ratio, chem. redn. and functionalization must be performed to obtain the proper characteristics that are required for org. applications. Here an efficient approach for chem. modified graphene oxide as a low-cost alternative for org. semiconductor materials was developed. The two synthesized materials (functionalized reduced graphene oxide and functionalized reduced graphene oxide decorated with silver nanoparticles) exhibited a 65% and 52% band gap decrease resp., compared to graphene oxide. Moreover, both materials were sol. in org. solvents at high concns. [10 mg mL-1], turning them suitable for many applications, including org. electronics, with the addnl. advantage of being a soln. process avoiding restacking of layers.
    40. 40
      Renteria, J. D.; Ramirez, S.; Malekpour, H.; Alonso, B.; Centeno, A.; Zurutuza, A.; Cocemasov, A. I.; Nika, D. L.; Balandin, A. A. Strongly anisotropic thermal conductivity of free-standing reduced graphene oxide films annealed at high temperature. Adv. Funct. Mater. 2015, 25, 4664– 4672,  DOI: 10.1002/adfm.201501429
      40
      Strongly Anisotropic Thermal Conductivity of Free-Standing Reduced Graphene Oxide Films Annealed at High Temperature
      Renteria, Jackie D.; Ramirez, Sylvester; Malekpour, Hoda; Alonso, Beatriz; Centeno, Alba; Zurutuza, Amaia; Cocemasov, Alexandr I.; Nika, Denis L.; Balandin, Alexander A.
      Advanced Functional Materials (2015), 25 (29), 4664-4672CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Thermal cond. of free-standing reduced graphene oxide films subjected to a high-temp. treatment of up to 1000 °C is investigated. The high-temp. annealing dramatically increases the in-plane thermal cond., K, of the films from ≈3 to ≈61 W m-1 K-1 at room temp. The cross-plane thermal cond., K.perp., reveals an interesting opposite trend of decreasing to a very small value of ≈0.09 W m-1 K-1 in the reduced graphene oxide films annealed at 1000 °C. The obtained films demonstrate an exceptionally strong anisotropy of the thermal cond., K/K.perp. ≈ 675, which is substantially larger even than in the high-quality graphite. The elec. resistivity of the annealed films reduces to 1-19 Ω .box.-1. The obsd. modifications of the in-plane and cross-plane thermal cond. components resulting in an unusual K/K.perp. anisotropy are explained theor. The theor. anal. suggests that K can reach as high as ≈500 W m-1 K-1 with the increase in the sp2 domain size and further redn. of the oxygen content. The strongly anisotropic heat conduction properties of these films can be useful for applications in thermal management.
    41. 41
      Ghilane, J.; Martin, P.; Randriamahazaka, H.; Lacroix, J.-C. Electrochemical oxidation of primary amine in ionic liquid media: Formation of organic layer attached to electrode surface. Electrochemistry communications 2010, 12, 246– 249,  DOI: 10.1016/j.elecom.2009.12.005
      41
      Electrochemical oxidation of primary amine in ionic liquid media: Formation of organic layer attached to electrode surface
      Ghilane, Jalal; Martin, Pascal; Randriamahazaka, Hyacinthe; Lacroix, Jean-Christophe
      Electrochemistry Communications (2010), 12 (2), 246-249CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
      Electrochem. oxidn. of primary amine in ionic liq. media has been investigated. The ionic liq. chosen for this study was 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Two primary amine compds. are used for this study; 4-nitrobenzylamine and 2-aminoethylferrocenylmethylether. The oxidn. of the amino compds. in ionic liq. conduces to the modification of the electrode surface. The modified electrodes were characterized by cyclic voltammetry and XPS anal. Both techniques support the presence of an org. layer strongly attached onto the electrode surface. The surface concn. of the attached group obtained in this media was found to be around 1 to 3 × 10-10 mol cm-2. The use of ionic liq. as media for the grafting leads to decrease of the surface concn. of the grafted layer; and the formation of less dense layer compared with classical solvent such as acetonitrile.
    42. 42
      Liang, J.; Mondal, A. K.; Wang, D.-W.; Iacopi, F. Graphene-Based Planar Microsupercapacitors: Recent Advances and Future Challenges. Advanced Materials Technologies 2019, 4, 1800200,  DOI: 10.1002/admt.201800200
      There is no corresponding record for this reference.
    43. 43
      Bisquert, J.; Garcia-Belmonte, G.; Bueno, P.; Longo, E.; Bulhöes, L. O. S. Impedance of constant phase element (CPE)-blocked diffusion in film electrodes. J. Electroanal. Chem. 1998, 452, 229– 234,  DOI: 10.1016/S0022-0728(98)00115-6
      43
      Impedance of constant phase element (CPE)-blocked diffusion in film electrodes
      Bisquert, J.; Garcia-Belmonte, G.; Bueno, P.; Longo, E.; Bulhoes, L. O. S.
      Journal of Electroanalytical Chemistry (1998), 452 (2), 229-234CODEN: JECHES ISSN:. (Elsevier Science S.A.)
      The authors construct a model of a.c. impedance response to blocked linear diffusion that has a sloped low frequency region in the impedance plot. The approach is based on the transmission line analogy to linear diffusion, and it is equiv. to solving Fick's law with a boundary condition that allows the authors to set an arbitrary impedance response at low frequencies. The authors argue that roughness at the blocking interface gives rise to const. phase element (CPE) response at low frequencies, and the authors give an impedance model function that can fit data along the whole frequency range when such a CPE is found. This is tested in an expt. of Li+ insertion in Nb2O5. The model should be of significance for metal oxide thin film electrodes and modified polymer electrodes.
    44. 44
      Huang, J.; Gao, Y.; Luo, J.; Wang, S.; Li, C.; Chen, S.; Zhang, J. Review–Impedance Response of Porous Electrodes: Theoretical Framework, Physical Models and Applications. J. Electrochem. Soc. 2020, 167, 166503,  DOI: 10.1149/1945-7111/abc655
      44
      Editors' choice-review-impedance response of porous electrodes: theoretical framework, physical models and applications
      Huang, Jun; Gao, Yu; Luo, Jin; Wang, Shangshang; Li, Chenkun; Chen, Shengli; Zhang, Jianbo
      Journal of the Electrochemical Society (2020), 167 (16), 166503CODEN: JESOAN; ISSN:1945-7111. (IOP Publishing Ltd.)
      A review. Porous electrodes are prevalent in electrochem. devices. Electrochem. impedance spectroscopy (EIS) is widely used as a noninvasive, in situ characterization tool to investigate multi-phase (electronic, ionic, gaseous) transport and coupling interfacial reactions in porous electrodes. Interpretation of EIS data needs model and fitting which largely det. the type and amt. of information that could possibly be obtained, and thereby the efficacy of the EIS method. This review focuses on physics-based models, as such models, compared to elec. circuit models, are more fundamental in the understanding of the porous electrodes, hence more reliable and more informative. Readers can have a glimpse of the long history of porous electrode theory and in particular its impedance variants, acquaint themselves with the celebrated de Levie model and a general theor. framework, retrace the journey of extending the de Levie model in three directions, namely, incorporating new physico-chem. processes, treating new structural effects, and considering high orders. Afterwards, a wealth of impedance models developed for lithium-ion batteries and polymer electrolyte fuel cells are introduced. Prospects on remaining and emerging issues on impedance modeling of porous electrodes are presented. When introducing theor. models, a "hands-on" approach is adopted by providing substantial math. details and even computation codes in some cases. Such an approach not only enables readers to understand the assumptions and applicability of the models, but also acquaint them with math. techniques involved in impedance modeling, which are instructive for developing their own models.
    45. 45
      Kim, H.-J.; Kim, D.; Jung, S.; Yi, S. N.; Yun, Y. J.; Chang, S. K.; Ha, D. H. Charge transport in thick reduced graphene oxide film. J. Phys. Chem. C 2015, 119, 28685– 28690,  DOI: 10.1021/acs.jpcc.5b10734
      45
      Charge transport in thick reduced graphene oxide film
      Kim, Ho-Jong; Kim, Daehee; Jung, Suyong; Yi, Sam Nyung; Yun, Yong Ju; Chang, Soo Kyung; Ha, Dong Han
      Journal of Physical Chemistry C (2015), 119 (51), 28685-28690CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The authors have investigated temp.-dependent charge transport behavior in thick reduced graphene oxide (RGO) film. Their results show that charges transport through two parallel percolating conducting pathways. One contains large disordered regions as one of its constituents, so its conductance is detd. dominantly by variable range hopping (VRH). The other is composed of small and medium disordered regions and cryst. sp2 domains, so its conductance is detd. by a serial connection of quantum tunneling and thermal activation. The more oxygen functional groups are removed from GO film upon progressive redn., the lower the potential barriers between the cryst. sp2 domains and disordered regions become. The contribution of thermal activation to total conductance does not appear evidently for highly reduced GO film having low potential barriers, but thermal activation causes the conductance of moderately reduced film to change continuously, even at low temps. where the VRH is almost frozen out.
    46. 46
      Wang, Y.; Zhao, Y.; Qu, L. Laser fabrication of functional micro-supercapacitors. Journal of Energy Chemistry 2021, 59, 642– 665,  DOI: 10.1016/j.jechem.2020.12.002
      There is no corresponding record for this reference.
    47. 47
      Vyas, A.; Cornaglia, F.; Rattanasawatesun, T.; Li, Q.; Haque, M.; Sun, J.; Kuzmenko, V.; Smith, A.; Lundgren, P.; Enoksson, P. Investigation of palladium current collectors for vertical graphene-based microsupercapacitors. Journal of Physics: Conference Series 2019, 1319, 012007,  DOI: 10.1088/1742-6596/1319/1/012007
      47
      Investigation of palladium current collectors for vertical graphene-based microsupercapacitors
      Vyas, A.; Cornaglia, F.; Rattanasawatesun, T.; Li, Q.; Haque, M.; Sun, J.; Kuzmenko, V.; Smith, A. D.; Lundgren, P.; Enoksson, P.
      Journal of Physics: Conference Series (2019), 1319 (29th Micromechanics and Microsystem Europe Workshop, 2018), 012007CODEN: JPCSDZ; ISSN:1742-6588. (IOP Publishing Ltd.)
      As microsystems are reduced in size and become integrated in the Internet of Things (IoT), they require an adequate power supply which can be integrated at the same size scale. Microsupercapacitors (MSCs), if coupled with on-chip harvesters, can offer solns. for a self-sustaining, on-chip power supply. However, the implementation of reliable MSC wafer-scale prodn. compatible with CMOS technol. remains a challenge. Palladium (Pd) is known as a CMOS compatible metal and, in this paper, we investigate the use of Pd as a contact material for vertical graphene (VG) electrodes in wafer-scale MSC fabrication. We show that a Ti diffusion barrier is required to prevent short-circuiting for the successful employment of Pd contacts. The fabricated MSCs demonstrate a capacitance of 1.3μF/cm2 with an energy d. of 0.42μJ/cm2. Thus, utilization of a Ti diffusion barrier with a CMOS compatible Pd metal electrode is a step towards integrating MSCs in semiconductor microsystems.
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      Kim, M.; Saia, R. Mo/Cr Metallization for Silicon Device Interconnection. MRS Online Proceedings Library (OPL) 1986, 71, 325,  DOI: 10.1557/PROC-71-325
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      Wu, Z.-S.; Parvez, K.; Feng, X.; Müllen, K. Graphene-based in-plane micro-supercapacitors with high power and energy densities. Nat. Commun. 2013, 4, 1– 8,  DOI: 10.1038/ncomms3487
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      Li, S.; Wang, X.; Shen, C. High-energy-density on-chip supercapacitors using manganese dioxide-decorated direct-prototyped porous carbon electrodes. 2014 IEEE 27th International Conference on Micro Electro Mechanical Systems (MEMS) 2014, 405– 408,  DOI: 10.1109/MEMSYS.2014.6765662
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    51. 51
      Si, W.; Yan, C.; Chen, Y.; Oswald, S.; Han, L.; Schmidt, O. G. On chip, all solid-state and flexible micro-supercapacitors with high performance based on MnOx/Au multilayers. Energy Environ. Sci. 2013, 6, 3218– 3223,  DOI: 10.1039/c3ee41286e
      51
      On chip, all solid-state and flexible micro-supercapacitors with high performance based on MnOx/Au multilayers
      Si, Wenping; Yan, Chenglin; Chen, Yao; Oswald, Steffen; Han, Luyang; Schmidt, Oliver G.
      Energy & Environmental Science (2013), 6 (11), 3218-3223CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      In this work, we introduce a new concept to fabricate on chip, all solid-state and flexible micro-supercapacitors based on MnOx/Au multilayers, which are compatible with current microelectronics. The micro-supercapacitor exhibits a max. energy d. of 1.75 mW h cm-3 and a max. power d. of 3.44 W cm-3, which are both much higher than the values obtained for other solid-state supercapacitors. At a scan rate of 1 V s-1, a volumetric capacitance of 32.8 F cm-3 is obtained for MnOx/Au multilayer electrodes, which is much higher than the bare MnOx electrode. Electrochem. impedance spectroscopy (EIS) and evolution complex capacitance confirm that the elec. cond. of MnOx is improved due to the incorporation of gold, and a low relaxation time const. around 5 ms is obsd. The MnOx/Au multilayer micro-supercapacitor also shows good long-term cycling stability, with a capacitance retention rate of 74.1% after a large cycling no. of 15 000 times. Compared with other supercapacitors, which are not portable and are relatively bulky, the device demonstrated here allows fast and reliable applications in a portable and smart fashion. Furthermore, the nature of the process allows the micro-supercapacitor to be integrated with other micro-devices, to meet the need for micro-scale energy storage.

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