Gas Diffusion Electrode Design for CO2 Electroreduction Beyond 1 a cm-2

Gas diffusion electrodes (GDEs) are extensively used for high current density electrochemical CO2 electrolysis (ECO2R), enabled by significantly reducing mass transfer resistance of CO2 to the catalyst layer. Conventionally, these GDEs are based upon hydrophobic carbon-based gas-diffusion layers (GDLs) that facilitate the gas transport; however, these supports are prone to flood with electrolyte during electrolysis. This potential-induced flooding, known as electrowetting, is related to the inherent conductivity of carbon and limits the activity of ECO2R. To investigate the effect of electrical conductivity more carefully, a GDE is constructed based on a Cu mesh with a nonconductive microporous GDL applied to this substrate, the latter composed of a mixture of metal oxide and polytetrafluoroethylene. With alumina as the metal oxide, a stable operation is obtained at -200 mA cm-2 with 70% selectivity for ECO2R (with over half toward C2+ products) without flooding as observed by in situ microscopy. On the contrary, with a Vulcan carbon-based GDL, the initial activity is rapidly lost as severe flooding ensues. It is reasoned that electrowetting is averted by virtue of the nonconductive nature of alumina, providing a new perspective on alternative GDL compositions and their influence on ECO2R performance.

ConspectusAs the demand for a carbon-neutral society grows rapidly, research on CO2 electrolysis has become very active. Many catalysts are reported for converting CO2 into value-added products by electrochemical reactions, which have to perform at high Faradaic and energy efficiency to become commercially viable. Various types of CO2 electrolyzers have been used in this effort, such as the H-cell, flow cell, and zero-gap membrane-electrode assembly (MEA) cell. H-cell studies are conducted with electrodes immersed in CO2-saturated electrolyte and have been used to elucidate reaction pathways and kinetic parameters of electrochemical CO2 reduction on many types of catalytic surfaces. From a transport phenomenological perspective, the low solubility and diffusion of CO2 to the electrode surface severely limit the maximum attainable current density, and this metric has been shown to have significant influence on the product spectrum. Flow and MEA cells provide a solution in the form of gas-diffusion electrodes (GDEs) that enable gaseous CO2 to closely reach the catalyst layer and yield record-high current densities. Because GDEs involve a complicated interface consisting of the catalyst surface, gaseous CO2, polymer overlayers, and liquid electrolyte, catalysts with high intrinsic activity might not show high performance in these GDE-based electrolyzers. Catalysts showing low overpotentials at low current densities may suffer from poor electron conductivity and mass transfer limitations at high current densities. Furthermore, the stability of the GDE-based catalysts is often compromised as CO2 electrolysis is pursued with high activity, most notoriously by electrolyte flooding.In this Account, we introduce recent examples where the electrocatalysts were integrated in GDEs, achieving high production rates. The performance of such systems is contingent on both GDE and cell design, and various parameters that affect the cell performance are discussed. Gaseous products, such as carbon monoxide, methane, and ethylene, and liquid products, such as formate and ethanol, have been mainly reported with high partial current density using the flow or MEA cells. Different strategies to this end are described, such as controlling microenvironments by the use of polymers mixed within the catalyst layer or the functionalization of catalyst surfaces with ligands to increase local concentrations of intermediates. Unique CO2 electrolyzer designs are also treated, including the incorporation of light-responsive plasmonic catalysts in the GDE, and combining the electrolyzer with a fermenter utilizing a microbial biocatalyst to synthesize complex multicarbon products. Basic conditions which the catalyst should satisfy to be adapted in the GDEs are listed, and our perspective is provided.

Electrochemical conversion of CO2 to fuels and valuable products is one pathway to reduce CO2 emissions. Electrolyzers using gas diffusion electrodes (GDEs) show much higher current densities than aqueous phase electrolyzers, yet models for multi-physical transport remain relatively undeveloped, often relying on volume-averaged approximations. Many physical phenomena interact inside the GDE, which is a multiphase environment (gaseous reactants and products, liquid electrolyte, and solid catalyst), and a multiscale problem, where "pore-scale" phenomena affect observations at the "macro-scale". We present a direct (not volume-averaged) pore-level transport model featuring a liquid electrolyte domain and a gaseous domain coupled at the liquid-gas interface. Transport is resolved, in 2D, around individual nanoparticles comprising the catalyst layer, including the electric double layer and steric effects. The GDE behavior at the pore-level is studied in detail under various idealized catalyst geometries configurations, showing how the catalyst layer thickness, roughness, and liquid wetting behavior all contribute to (or restrict) the transport necessary for CO2 reduction. The analysis identifies several pathways to enhance GDE performance, opening the possibility for increasing the current density by an order of magnitude or more. The results also suggest that the typical liquid-gas interface in the GDE of experimental demonstrations form a filled front rather than a wetting film, the electrochemical reaction is not taking place at a triple-phase boundary but rather a thicker zone around the triple-phase boundary, the solubility reduction at high electrolyte concentrations is an important contributor to transport limitations, and there is considerable heterogeneity in the use of the catalyst. The model allows unprecedented visualization of the transport dynamics inside the GDE across multiple length scales, making it a key step forward on the path to understanding and enhancing GDEs for electrochemical CO2 reduction.

Toward abiotic sugar synthesis from CO2 electrolysis

Anion exchange membrane water electrolysis (AEMWE) is operated in an alkaline environment and has a similar membrane-electrode assembly (MEA) structure to that of proton exchange membrane water electrolysis (PEMWE). The use of non-noble metals as the electrocatalysts and operation at high current densities are inherently possible, so they are expected to be low-cost, high-performance candidates for next-generation water electrolysis systems. Improving the anion conductivity of the AEM and the activity of the non-precious metal catalysts is essential for the practical application of AEMWEs.1 The non-noble electrocatalysts used in the MEA in this study were Ni0.8Co0.2O2 and Ni0.8Fe0.2O,3 which have a fused-aggregate network structure, and the electrolyte was a hydrocarbon-based AEM (QPAF-4)4, all of which were developed at the University of Yamanashi.The catalyst ink for the anodes was prepared by mixing the Ni0.8Co0.2O catalyst (University of Yamanashi) with solvent (water/methanol) and QPAF-4 (IEC = 2.0 meq g-1, University of Yamanashi) binder solution. The anode ink was coated by using pulse-swirl-spray (PSS, Nordson) on the QPAF-4 membranes (IEC = 1.5 meq g-1) to make the catalyst-coated membranes (CCMs) with the anode. Catalyst inks for the cathodes was prepared by mixing Ni0.8Fe0.2O catalyst (University of Yamanashi) or Pt/CB catalyst (TEC10E50E, Tanaka Kikinzoku) with solvent (water/methanol) and QPAF-4 (IEC = 2.0 meq g-1) binder solution. The cathode ink was coated by using PSS on the gas diffusion layer (GDL, TGP-H-120, Toray) to make the gas diffusion electrodes (GDEs) for the cathodes. Ni mesh (Bekaert.co.jp) was used for the anode GDLs. The single cell (Figure 1, cell structure developed by Yokohama National University) performances were measured while supplying 1 M KOH at 80 °C to both electrodes.Figure 2 (a) shows the I-V performance of cells using Ni0.8Co0.2O for the anode and Ni0.8Fe0.2O or Pt/CB for the cathode. The electrolysis voltage observed for the cell with non-noble metal catalysts on both electrodes was higher than that for the cell with Pt/C on the cathode but was below 2 V at a current density of 1.2 A cm-2. The value of the onset electrolysis voltage was nearly the same as that of the corresponding rotating disk electrode,3 suggesting that the intrinsic performance of the cathode catalyst is being realized in the MEA. Increasing the amount of the cathodic catalyst loading scarcely affected the polarization. It is supposed that the fused-aggregate network structure of the Ni0.8Fe0.2O catalyst contributes to the favorable mass transfer. Figure 2 (b) shows the time dependence of the cell voltage of these cells. The increased loading amount improved the durability of the cathode. It is considered that the increase in the thickness of the cathode catalyst layer alleviated the damage of the GDL fibers to the membrane.These results indicate the possibility of adopting non-precious metal catalysts for both electrodes in AEMWE and thereby significantly reducing the catalyst cost. Acknowledgement This work was partially supported by NEDO of Japan through the AWE-1 project. References 1) A. Lim et al., J. Ind. Eng. Chem., 76, 410 (2019).2) G. Shi et al., ACS Catal., 12, 14209 (2022).3) G. Shi et al., ACS Omega, 8, 13068 (2023).4) H. Ono et al., J. Mater. Chem. A, 5, 24804 (2017). Figure 1

The electrochemical CO2 or CO reduction to chemicals and fuels using renewable energy is a promising way to reduce anthropogenic carbon emissions. The gas diffusion electrode (GDE) design enables low-carbon manufacturing of target products at a current density (e.g., 500 mA cm−2) relevant to industrial requirements. However, the long-term stability of the GDE is restricted by poor water management and flooding, resulting in a significant hydrogen evolution reaction (HER) within almost an hour. The optimization of water management in the GDE demands a thorough understanding of the role of the gas diffusion layer (GDL) and the catalyst layer (CL) distinctively. Herein, the hydrophobicity of the GDL and CL is independently adjusted to investigate their influence on gas transport efficiency and water management. The gas transport efficiency is more enhanced with the increase in hydrophobicity of the GDL than the CL. Direct visualization of water distribution by optical microscope and micro-computed tomography demonstrates that the water flow pattern transfers from the stable displacement to capillary fingering as GDL hydrophobicity increases. Unfortunately, only increasing the hydrophobicity is not sufficient to prevent flooding. A revolutionary change in the design of the GDE structure is essential to maintain the long-term stability of CO2/CO reduction.

Polymer electrolyte fuel cell (PEFC) is promising as a next generation power source for vehicles due to the high efficiency. To make the PEFC commercially competitive, the reduction of the cost is necessary. For the reduction of the cost of PEFC, it is required to increase the maximum current density. Under the high current density operation, the water produced in a catalyst layer (CL) accumulates in a gas diffusion layer (GDL) and the supply of oxygen is suppressed. Therefore, the water transport in the GDL is important to increase the maximum current density of PEFC. To improve the water transport in the GDL, the wettability distribution is important factor. Utaka el al. showed that better cell performance is obtained by forming the hydrophilic region in the hydrophobic GDL because the water transport in the GDL is controlled by the hydrophilic region (1). Similarly, Forner-Cuenca et al. showed that the cell performance is improved by the GDL with the hydrophilic region (2). There are some reports which showed the water transport is improved by the control of the wettability of the GDL. However, these reports are focused on the planer distribution in the GDL and the effect of the wettability distribution in the thickness direction is not clear. For the optimization of GDL structure, it is necessary to clarify the effect of the wettability distribution in the thickness direction. Therefore, in this study, the effect of the wettability distribution in the thickness direction was investigated. To evaluate the water transport in the GDL with the wettability distribution in the thickness direction, a lattice Boltzmann method (LBM) was used (3). Figure 1 shows the simulation domain and boundaries. The size of GDL is 80μm × 80μm × 80μm and the porosity of GDL is 70%. The bottom of the simulation domain is assumed the micro-porous layer (MPL) and it is treated as a solid wall. MPL crack as inlet boundary is set in the MPL. The top of the simulation domain is outlet boundary. The four sides of the simulation domain are solid walls. The contact angle of GDL, MPL and side walls is set 130º. The density of the two-phase in the simulation is set identical to water, ρ = 978kg/m3. The interfacial tension between the two-phase is σ = 6.40×10-2N/m, and liquid and gas viscosities are μL = 4.04×10-4Pa∙s and μG = 2.30×10-5Pa∙s respectively. Figure 2 shows simulation results of water transport in GDL. Figure 2 (a) shows the result for the GDL without the wettability distribution. The contact angle of the GDL is uniform (130 º). Figure 2 (b) shows the result for the GDL with the wettability distribution in the thickness direction. The contact angle from the GDL surface to the depth of 16 μm is 50º and the other regions of the GDL is 130º. In these figures, the water is shown in red. With the water transport in the GDL without the wettability distribution (Fig. 2 (a)), the water was inflowed to the large pore in the center of the GDL (0.32ms). The water was spread in the large pore (0.96ms). After the much water was accumulated in the GDL, the water was reached at the GDL surface (1.9ms). With the water transport in the GDL with the wettability distribution in the thickness direction (Fig. 2 (b)), the water was inflowed to the large pore in the center of the GDL and the water accumulated in the GDL as well as the GDL without the wettability distribution (0.32ms, 0.96ms). When the water was reached at the hydrophilic region in the top of GDL, the water in the GDL was pulled up to the GDL surface and the amount of the water in the center of the GDL was reduced (1.2ms). Compared with the GDL without wettability distribution, the time when the water reached the GDL surface was earlier in the GDL with wettability distribution. Therefore, the water transport in the thickness direction is improved by the wettability distribution in the thickness direction. In the final full paper, the large-scale simulation including the rib and channel will be carried out and the effect of the wettability in the thickness direction will be discussed. Reference (1) Y. Utaka, R. Koresawa, J. Power Sources, 363, 227-233 (2017). (2) A. Forner-Cuenca, J. Biesdorf, V. Manzi-Orezzoli, L. Gubler, T. J. Schmidt, P. Boillat, J. Electrochem. Soc., 163 (13), F1389 (2016). (3) S. Sakaida, Y. Tabe, and T. Chikahisa, J. Power Sources, 361, 133-143 (2017). Figure 1

Neutron radiography was used to measure water concentrations through the cross-sections of sprayed gas diffusion electrodes (GDE) and electrospun perfluorosulfonic acid-platinum nanofiber (NF) electrodes during fuel cell operation. The performance of Generation 1 poly(acrylic acid)-based NF electrodes is improved at high current densities compared to the baseline given by a GDE cell. Through-thickness water profiles reveal that at high current densities, the water concentrations within the membrane electrode assembly (MEA) and gas diffusion layers are around 2x lower in the nanofiber-containing MEA compared to the baseline GDE, commensurate with the observed polarization performance. In Generation 2 electrospun nanofiber electrodes, poly(ethylene oxide)-based electrodes show higher performance than poly(acrylic acid) electrodes, while retaining less water in the MEA. These results imply that the electrospun nanofiber electrodes have an improved ability to maintain hydration without flooding, which leads to improved performance over a range of relative humidities.

Boosting Electrocatalytic CO ₂ Reduction with Conjugated Bimetallic Co/Zn Polyphthalocyanine Frameworks

A novel method for preparing proton exchange membrane fuel cell electrodes by the ultrasonic-spray technique

As one of the most dominant greenhouse gases, CO2 plays a critical role in climate change. Electrochemical CO2 reduction (eCO2R) is a promising approach to mitigate this impact, as it is capable of converting the excess gas into valuable chemicals, especially concerning electrical energy storage from renewable sources at peak times. However, due to high overpotentials, the technology is still in a rather early stage of development and not yet feasible for economic implementation on industrial scale. One of these limitations results from the poor solubility of CO2 in water or commonly used aqueous electrolytes, leading to insufficient transport of the reactant gas to the cathode in classical H-cell setups. Therefore, gas diffusion electrodes (GDEs) are typically employed to reduce the according mass transport resistances [1]. Furthermore, ohmic resistances, among others caused by the electrolyte gaps in the system, contribute to overpotentials. To decrease these constraints, conducting CO2 reduction at membrane electrode assemblies (MEAs) is an interesting opportunity that has been gaining more and more attention in research throughout the past years. Exemplarily, fig. 1 shows that the elimination of only the catholyte compartment and thus implementation of an exchange MEA can already decrease the cell potential by more than 30 % at 2 kA m-2 in comparison to GDE [2] setup.Depending on the cathodic catalyst, there are various reaction routes of eCO2R, one of which aims to synthesize the target product carbon monoxide. This is possible e. g. at silver catalysts, providing high CO selectivity and hydrogen as the only byproduct [3]. The combination of these products, syngas, is an important feedstock for many chemical processes such as Fischer-Tropsch synthesis. Although the majority of studies regarding silver-based MEAs for eCO2R uses pure silver as a catalyst, the configuration of the material is varying: While we usually disperse a mix of powder and flakes to obtain our catalyst suspension, nanoparticles are more frequently used in literature [3-5]. Moreover, porous silver membranes [6, 7] or silver deposited from solutions such as AgNO3 [8] and various other structures have already been investigated. Anyways, when comparing the results yielded with the different configurations of silver (cf. fig. 1), it becomes clear that there is no obvious trend in which material generates the best performances. A closer look at the conditions of the corresponding experiments reveals that this is quite plausible, as there is, on the one hand, no uniform MEA manufacturing procedure: Not only are there deviations in the employed material form, but also the gas diffusion layer, ionomer and membrane as well as ink fabrication and processing. On the other hand, the electrochemical measurements are alternating between galvanostatic and potentiostatic methods with no constant active electrode area, let alone type of test cell or setup.Therefore, this study aims to investigate the influence of differing catalyst configuration used for MEAs while maintaining other parameters concerning manufacturing and characterization procedure. The base of all MEAs is a carbon-based gas diffusion layer of the same supplier and type. An anion exchange ionomer is chosen to enhance the mass transport of the reactants to the catalyst. Only the form of the catalyst, e. g. powder, nanoparticles or fixed structures, is varied. The catalyst and ionomer are attached to the gas diffusion layer which is then sandwiched with the membrane corresponding to the ionomer. Conducting galvanostatic step experiments in exchange MEA setup with a fixed type of anode, the performance of the resulting MEAs is evaluated. This way, important effects on parameters such as Faradaic efficiency for the target product and cell potential can be unraveled.Literature:[1] T. Burdyny, W. A. Smith, Energy Environ. Sci. 12 (5), 2019.[2] I. Moussallem et al., Chem. Eng. and Proc. 52 (2012).[3] Y. Hori et al., Chem. Lett. 14 (11), 1985.[4] Z. Liu et al., J. Electrochem. Soc. 165 (15), 2018.[5] R. B. Kutz et al., Energy Technol. 5 (6), 2017.[6] Y. C. Li et al., ACS Energy Lett. 1 (6), 2016.[7] G. O. Larrazábal et al., ACS Appl. Mater. Interfaces 11 (44), 2019.[8] W. H. Lee et al., J. Mater. Chem. 9 (29), 2021.[9] Y. Hori et al., Electrochim. Acta 48 (18), 2003. Figure 1

The objective of this work is to gain the mechanical understanding of the water management in the proton exchange membrane fuel cell (PEMFC). Water is a by-product of the fuel cell reaction and its amount is proportion to the current of fuel cell output. Water is used to retain the proper hydration level in the PEM. At the same time, condensed water in the flowfield and the gas diffusion layer (GDL) reduces oxygen transport to the oxygen reduction reaction (ORR) area. Therefore, excess accumulation of condensed water causes to lower the performance, particularly at the higher current densities. Detailed simulation of two-phase water in the GDL is significantly important to understand the water management.This work shows the successful in development of a multi-scale calculation technique that incorporates various scales of numerical model in a fuel cell and simultaneously performed a prediction. The outcomes of this work are, (i) development of two-phase fluid model in the micro-scale porous structure of GDL, and (ii) integration of this micro-scale GDL fluid model with a state-of-the-art macro-scale flowfield fluid model to predict two-phase water in the fuel cell. Figure 1 gives the concept of modeling approach at each component of the fuel cell. The flowfield in the bipolar plate and MEA models are calculated using traditional computational fluid dynamics (CFD) method with existing PEMFC model [1-3]. The two-phase fluid in the micro-scale porous structure of GDL is numerically predicted by Lattice Boltzmann Method (LBM) [4-5]. The solution of each iteration from CFD and LBM are required to simultaneously exchange for the next iteration until all solutions are converged, especially at the interfaces. In this figure, it shows the flowfield is transformed into the macro-scale CFD model and the image of detail structured GDL is converted into micro-scale LBM model. Actual porous structure imaging of GDL is obtained by the nano-scale high special resolution x-ray computed tomography. Figure 2 shows an example of the predictions from the simulation when the macroscopic flowfield model is combined with the microscopic GDL model while performing the calculation. The predictions give the detail distributions of variables in every components of the multiscale model. In this figure, the current density distribution on MEA surface, the liquid water transport inside the GDL, and the temperature of solid and fluid phases inside the GDL are presented.

The electrochemical reduction of CO2 has the potential to enable a circular economy where the captured CO2 can be utilized to produce valuable chemicals. However, existing CO2 reduction electrolyzers suffer from limited performance (i.e. low current densities) and short operation times (1-200 hours)1. The gas diffusion electrode (GDE) is a key component which determines system durability and enables higher current density operation. Yet, commercially available GDEs are carbon fiber-based materials, which were developed to fulfill the requirements of polymer electrolyte fuel cells and were repurposed for emerging CO2 electrolyzers. While functional, the commercial GDEs microstructure and surface properties do not meet the requirements of CO2 electrolysis flow cells which leads to their failure within short operation hours. These GDEs fail due to flooding through different mechanisms such as electrowetting2, capillary imbibition3, salt precipitation3, and uneven pressure distribution4. Therefore, there is a need to develop novel GDEs with engineered microstructures and wettability for the unique challenges of CO2 electrolyzers.In this research, we aim to understand the influence of the GDE microstructure on the durability and performance of CO2 electrolyzers. Instead of leveraging commercially available carbon-fiber based substrates, we conceptualize and deploy a scalable technique to manufacture novel GDEs with tunable microstructure. We hypothesize that tackling the multiphase transport challenges in CO2 electrolyzers requires electrode formats containing a plurality of pore sizes and highly controlled microstructures. Thus, we aim to develop a synthetic methodology affording a high degree of microstructural versatility and low manufacturing costs. We investigate the effect of synthetic parameters on the resulting microstructural (e.g. porosity, pore size distribution) and wetting properties. Furthermore, we aim to correlate the synthetic parameters with the resulting GDE properties (e.g. microstructure, porosity) and the corresponding transport properties (e.g. permeability, capillarity, diffusivity). These properties will be further correlated to the flow cell durability and performance. We hypothesize that tuning the pore size distribution and surface chemistry of the GDEs can enhance mass transport rates and resistance to flooding, which could enable longer operating times and higher current density. References D. Wakerley et al., Nat Energy, 7, 130–143 (2022).K. Yang, R. Kas, W. A. Smith, and T. Burdyny, ACS Energy Lett, 6, 33–40 (2021).M. E. Leonard, L. E. Clarke, A. Forner‐Cuenca, S. M. Brown, and F. R. Brushett, ChemSusChem, 13, 400–411 (2020).L. M. Baumgartner, C. I. Koopman, A. Forner-Cuenca, and D. A. Vermaas, ACS Sustain Chem Eng, 10, 4683–4693 (2022).

Introduction In recent years, there has been growing interest in converting carbon dioxide (CO2), a greenhouse gas, into a valuable resource. The electrolytic reduction method, which uses only water and electricity, offers a particularly clean approach to CO2 conversion. Its high conversion efficiency at ambient temperature and pressure, facilitated by gas diffusion electrodes (GDE), makes it a promising option for widespread adoption. Electrocatalytic CO2 reduction using GDE selectively yields carbon monoxide, ethylene, and formic acid with Ag, Cu, or Sn catalysts, respectively 1). These catalysts are often supported on conductive carbon black (CB), valued for its high electrical conductivity and porosity. However, CB-based catalysts encounter challenges like redox-induced degradation and high CO2 overvoltage. So, we turned to plasma in liquid as a new method for making electrocatalysts. Plasma in liquid is generated by subjecting a metal in solution to high-frequency, high-voltage conditions. This process enables the sputtering of metal onto powder samples dispersed in solution, with the metal surface instantly heated to high temperatures. This process can improve electrochemical properties like oxygen deficiency 2). Consequently, plasma treatment in liquid presents an opportunity to utilize highly durable and chemically resistant metal oxides as catalyst base materials. The objective of this study was to synthesize a new catalyst for electrocatalytic CO2 reduction using titanium dioxide as the catalyst base via in-liquid plasma. Titanium dioxide offers advantages of affordability, durability, and chemical resistance. Moreover, oxygen-deficient TiO2 with enhanced conductivity finds application as a base material for batteries.Experimental In the experiments, high-frequency high voltage (4 kV, 100 kHz, 0.5 µs-Pulse Width) was applied under the same conditions to various metal electrodes in a 5.0 wt%-NH4 solution (200 mL) containing dispersed TiO2 (P25: 0.5 g) to generate plasma in the liquid. Common plasma electrodes included tungsten (W), silver (Ag), and copper (Cu). Each gas diffusion electrode (GDE) was prepared by washing the powder after in-liquid plasma treatment with pure water, followed by spray coating on a gas diffusion layer with Nafion dispersion as a binder. Electrocatalytic CO2 reduction was evaluated in a conventional three-chamber gas diffusion cell at 500 mA/cm2 for 10 minutes. The CO2 supplied to the gas chamber was collected in a Tedlar bag, and the CO2 reduction products were analyzed by gas chromatography. Results and Discussion Figure shows the CO2 reduction products and Faraday efficiency of P25 treated with various liquid plasma electrodes. In pristine P25, small amounts of CO and CH4 were detected due to the redox cycle of Ti4+ and Ti3+. The use of tungsten (W), silver (Ag), and copper (Cu) as in-liquid plasma electrodes increased the production of H2, CO, and CH4, respectively. The difference in conversion can be attributed to the fact that the amount of etched Cu is less than that of Ag. Additionally, black nanoparticles were dispersed in the filtered solution when Ag was used, while the solution turned a clear blue color with Cu, suggesting the formation of a complex with ammonia. Generally, oxidized metal nanoparticles are synthesized when plasma is generated in solution. In this study, since an alkaline solution was used, it can be considered that the oxidized Ag nanoparticle surfaces were reduced, resulting in highly dispersed reduced Ag nanoparticles being deposited. In contrast, the generated Cu nanoparticles impregnated TiO2 by dissolving in ammonia and exhibited catalytic activity. CH4 significantly affects Faraday efficiency relative to the amount produced because CO and H are two-electron reactions, while CH4 is an eight-electron reaction, requiring four times as many electrons. Although C2H4 is produced by Cu nanoparticles at sufficient current density, the present results show increased CH4 production. This may be due to TiO2 promoting the hydrogenation of CO2 before Cu forms the C-C bond. Further investigation is required. Conclusions In this study, TiO2 underwent treatment with various in-liquid plasma electrodes for electrocatalytic CO2 reduction. As a result, changing the electrode allowed for successful support of metal nanoparticles on TiO2, resulting in selectivity of CO2 reduction products. During the presentation, we will discuss the comparison with conductive carbon black (CB) and the changes in electrocatalytic CO2 reduction characteristics under different plasma conditions in liquid. Compared to pristine P25, the production of CO increased by 9.5-fold from 40 to 380 μmol, and CH4 increased by 2.0-fold from 20 to 40 μmol.Reference1) Sassenburg et al., ACS Appl. Energy Mater., 5, 5, 5983 (2022).2) Takagi et al., Sci. Total Environ., 902 (13), 166018 (2023). Figure 1

The gas diffusion electrode (GDE) consists of a carbon-based gas diffusion layer (GDL) and an active layer (AL).In this study GDE with bimetallic AL containing Ag/-MnO2 was investigated and compared with previously optimized GDE with Ag/Co3O4 composition.The catalytic structure was created by depositing of a mixture of catalysts containing -MnO2 and Ag with a weight ratio between the two components of 1:1.The prepared electrodes were examined before and after work by XRD phase analysis.The morphology of the above-studied ALs and GDL was investigated with scanning electron microscopy (SEM) and BET analysis, respectively.The polarization, charge/discharge tests and impedance analysis were carried out in a half-cell configuration in 6M KOH.Long-term tests of the two GDEs showed good mechanical and electrochemical stability of over 300-500 charge/discharge cycles.