Exploring the stability and catalytic activity of monoethanolamine functionalized CuO electrode in electrochemical CO2 reduction.

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

Electrochemical CO2 reduction (ECR) to value-added fuels and chemical feedstocks driven by renewable energy sources is one promising approach to alleviating the ever-increasing atmospheric CO2 concentration and the rapid fossil fuels consumption.1,2 Among various products formed, formate is a very attractive product due to its wide applications in textile and pharmaceutical industries, and in fuel cells as the hydrogen carrier. Bi in particular has been widely investigated as advanced catalyst for formate production because of their cost-effectiveness, low toxicity, and high activity. Up to date, various strategies including size, morphology, defect, grain boundary, and heterostructure engineering have been developed to improve the catalytic performances of Bi-based catalysts. Despite some exciting achievements, the performances over Bi-based catalysts still fall behind the commercial requirements. Most existing catalysts suffer from low formate production rate (in H-type cell) because high standard of catalytic activity and product selectivity could be hardly achieved simultaneously. On the other side, desirable formate faradaic efficiency (FE) of ˃90 % can be only achieved in a narrow potential window owing to the competing hydrogen evolution reaction (HER) at high potential. Therefore, developing Bi-based catalysts for formate production via ECR with high formation rate and current density over a broad potential window is a top priority but remains as great challenges. Electronic regulation of electrocatalysts via element doping has been regarded as a powerful strategy to enhance the electrochemical activity of ECR. The introduction of heteroatoms can regulate the electron density and thus, precisely modify the electronic structures of the active sites, resulting in optimal adsorption energy of the reaction intermediates. S modification of catalysts can modulate the electronic structure of catalysts, thus boosting the electrocatalytic activity for HER, 3 oxygen evolution reaction (OER),4 and oxygen reduction reaction (ORR).5 Moreover, S doping can increase the adsorption capacity of CO2 and decrease the energy barrier for the formation of *HCOO intermediate.6 To that end, S doping is expected to be an effective strategy to improve the catalytic performance of ECR by tuning the electronic structure. However, the studies on fine-tuning the electronic structure of Bi-based catalysts for ECR by S doping have rarely been reported so far; a comprehensive understanding of the S doping effect on the ECR activity is also lacking. The study demonstrates S doped Bi2O3 nanosheets coupled with carbon nanotube (S-Bi2O3-CNT) as catalyst for ECR to formate production. The prepared S2-Bi2O3-CNT with the S doping amount of 0.7 at % achieved high FE over a wide potential range, and also achieved high partial current density (48.6 mA cm-2) and good long-term stability. Experimental results and a series of characterizations highlighted the advantages of S doping which facilitates electron transfer, increases the adsorption of CO2 and offers more undercoordinated Bi sites. Moreover, DFT results reveal that S doping induced the electronic delocalization of Bi sites, which optimized the adsorption of *CO2 and *HCOO intermediates while hindering the adsorption of *H. Therefore, S doped Bi2O3 promotes the ECR to formate by supressing the competitive HER simultaneously. This work opens up an attractive avenue for developing highly efficient electrocatalyst for ECR at atomic level.References Nielsen, D. U.; Hu, X.-M.; Daasbjerg, K.; Skrydstrup, T., Chemically and electrochemically catalysed conversion of CO2 to CO with follow-up utilization to value-added chemicals. Nat. Catal. 2018, 1, 244-254.Ross, M. B.; De Luna, P.; Li, Y.; Dinh, C.-T.; Kim, D.; Yang, P.; Sargent, E. H., Designing materials for electrochemical carbon dioxide recycling. Nat. Catal. 2019, 2, 648-658.Yu, J.; Guo, Y.; Miao, S.; Ni, M.; Zhou, W.; Shao, Z., Spherical ruthenium disulfide-sulfur-doped graphene composite as an efficient hydrogen evolution electrocatalyst. ACS Appl. Mater. Interfaces 2018, 10, 34098-34107. Yu, L.; Wu, L.; McElhenny, B.; Song, S.; Luo, D.; Zhang, F.; Yu, Y.; Chen, S.; Ren, Z., Ultrafast room-temperature synthesis of porous S-doped Ni/Fe (oxy) hydroxide electrodes for oxygen evolution catalysis in seawater splitting. Energy Environ. Sci. 2020, 13, 3439-3446.Wang, Y. C.; Lai, Y. J.; Song, L.; Zhou, Z. Y.; Liu, J. G.; Wang, Q.; Yang, X. D.; Chen, C.; Shi, W.; Zheng, Y. P., S‐doping of an Fe/N/C ORR catalyst for polymer electrolyte membrane fuel cells with high power density. Angew. Chem., Int. Ed. 2015, 54, 9907-9910. 6. Tian, W.; Zhang, H.; Sun, H.; Suvorova, A.; Saunders, M.; Tade, M.; Wang, S., Heteroatom (N or N‐S)‐Doping Induced Layered and Honeycomb Microstructures of Porous Carbons for CO2 Capture and Energy Applications. Adv. Funct. Mater. 2016, 26, 8651-8661.

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 reduction of CO2 into liquid fuels is a promising approach to achieving a carbon-neutral energy cycle but remains a great challenge due to the lack of efficient catalysts. Here, the hierarchical architectures assembled by ultrathin and porous S-modified Cu nanoflakes (Cu-S NFs) are designed and constructed as an efficient electrocatalyst for CO2 conversion to formate with high partial current density. Specifically, when integrated into a gas diffusion electrode in a flow cell, Cu-S NFs are capable of delivering the ultrahigh formate current density up to 404.1 mA cm-2 with a selectivity of 89.8%. Electrochemical tests and theoretical calculations indicate that the superior performance of the designed catalysts may be attributed to the unique structure, which can provide abundant active sites, fast charge transfer, and highly active edge sites.

There has been significant progress in the electrochemical reduction of CO2 since the seminal work of Hori et al. in 1985 demonstrating hydrocarbon production at copper cathodes. The introduction of gas diffusion electrode (GDE) and membrane electrode assembly (MEA) flow cells have demonstrated progress to produce C2 products (ethylene, ethanol and acetate) at current densities >150 mA cm-2 with Faradaic Efficiencies (FE) >50% at different types of Cu cathodes. Selectivity depends on several pathways and the intermediate binding strength of species like *CO, *CHO, *OCCO, and *OCCOH. Thus, there are opportunities to engineer or optimize product selectivity.The work advanced ethylene production through Cu-P0.065 electrocatalyst (Cuδ+ = 0.13), achieving 52% FE at 150 mA cm-2 in 0.1 M KHCO3, and remarkably improving to 70% FE in weakly acidic conditions (pH 6) while maintaining 64% FE at 250 mA cm−2. Electrolyte engineering demonstrated that larger alkali cations (Na+ to Cs+) effectively suppress hydrogen evolution from 31% to 4% while promoting C2 formation (45% to 89% FE). K+ concentration optimization (0.1-2M) further enhanced ethylene production, increasing FE from 42% to 65% at 4V while reducing HER from 48% to 20%. Mechanistic studies revealed K+ and OH- species accumulation near the electrocatalyst surface promotes C-C coupling through K+ and *OCCO intermediate interactions, while phosphorus doping enhances *CO generation and coupling.In terms of ethanol our work demonstrates that Cu-Sn0.03 electrocatalyst (Cuδ+ = 0.27) can achieve 48% FE at 350 mA cm-2 under alkaline conditions. A breakthrough in product separation was achieved through a dual-layer membrane configuration incorporating CEM and AEM. This system, using 1M KOH (pH 14) inner layer and 1M KOH+H3PO4 (pH 6) anolyte, reached 51.92% FE for ethanol with minimal anode crossover (<1.07% FE), enabling direct production of >8 wt% ethanol. Additionally, anion identity studies (PO4 2-, SO4 2-, and NO4 -) demonstrated consistent C2 efficiencies (61-65%) at fixed pH.In terms of acetate production, we show Cu2Se electrocatalyst (Cuδ+ = 0.47), demonstrating 40% FE for acetate at 350 mA cm-2. Notably, all electrocatalysts maintained exceptional stability over 250 hours with minimal degradation (0.02% FE loss/hour). These results show structure-function relationships of electrocatalysts in CO2 reduction and high levels of ethylene, ethanol, and acetate production; however, several critical barriers related to operating (viz. Energy) cost and durability remain. We conclude with a review of the state of the art and the needs for commercialization.

Multi-parameter study of CO2 electrochemical reduction from concentrated bicarbonate feed

Introduction A pyranose dehydrogenase from the basidiomycete Coprinopsis cinerea (CcPDH) is an eukaryotic pyrroloquinoline quinone (PQQ)-dependent dehydrogenase consisting three-domains structure [1]. The PQQ and heme b cofactors are located in the 45 kDa and 21 kDa domains, respectively, and a small C-terminal domain is family 1-type carbohydrate-binding module, which these domains are connected by a proline-rich linker region. CcPDH shows the oxidation activity toward monosaccharides in a 1C4 chair conformation such as D-glucosone and L-fucose. The N-terminal cytochrome domain is a 6-coordinated low-spin heme b with Met/His ligands that enables direct electrical contact between the enzyme and the electrode [2]. Electrons are transferred from reduced PQQ in catalytic domain to heme b in cytochrome domain. The previous study reported that intact CcPDH is capable of direct electron transfer (DET)-based bioelectrocatalysis on a glassy carbon (GC) electrode [3]. The catalytic currents were observed at a lower potential than the redox potential of heme b in the cytochrome domain, suggesting that both the PQQ and the cytochrome domains in CcPDH are able to DET with the GC electrode. In present study, to examine DET reaction of the PQQ domain, an isolated PQQ domain was expressed in Pichia expression system. Here, we demonstrated the direct bioelectrocatalysis for the isolated PQQ domain from C. cinerea pyranose dehydrogenase. The bioelectrocatalytic current density of 1.6 mAcm-2 was achieved on a preformed 2-mercaptoethanol (ME) self-assembled monolayer (SAM) modified gold nanoparticles (AuNPs) electrode under optimized conditions. Experimental The direct bioelectrocatalysis of DHPDH was obtained by using glassy carbon (GC) electrode modified with the enzyme as working electrode. An aliquot of 30 μL of 1 μM enzyme solution containing 10 μM PQQ and 1 mM CaCl2 was dropped on the entire surface of the GC electrode and then dried at room temperature. AuNPs-modified and unmodified polycrystalline Au electrodes were used for the direct bioelectrocatalysis of DHPDH. 1 μL of the concentrated AuNPs was dropped onto the surface of the Au electrode, and then air-dried. To prepare electrodes with immobilized DHPDH, AuNPs-modified and unmodified polycrystalline Au electrodes were immersed in 20 mM aqueous solution of 2-mercaptoethanol for 1 h at room temperature (ME-AuNPs electrode and ME-Au electrode). Then they were immersed in 1 μM holo-DHPDH in a 50 mM sodium acetate buffer solution (pH 6.0) at 4 ℃ for over 15 h. Cyclic voltammetry measurements were performed in 50 mM various pH buffer with 100 mM L-fucose. A conventional three-electrode cell was used, in which a platinum wire served as the counter electrode and Ag/AgCl (3 M NaCl) served as the reference electrode. Results and Discussion Initially, a clear catalytic current of L-fucose oxidation by the DHPDH on the GC electrode was observed starting at same potential of intact CcPDH. It is demonstrated that the DHPDH can also directly transfer electrons to the electrodes. The catalytic current density of DHPDH (0.9 μA cm-2, at 0.35 V) was an order of magnitude smaller than that of intact CcPDH. To improve the direct bioelectrocatalysis of DHPDH, a thiol-based SAM coated Au electrodes were studied. Accordingly, the optimal SAM for DHPDH was found to be shorter alkyl chain length and hydroxide-functionalized thiols. The catalytic current density of 11 μA cm-2 (at 0.35 V) was obtained when using ME-Au electrode. DHPDH immobilized on ME-Au electrode showed optimum at pH 6.0 and a value of K m = 35.1 mM, respectively. In order to obtain higher catalytic currents, furthermore, AuNPs were cast on the surface of Au electrode. Increasing the number of AuNP casts from 3 to 15 times on led to increasing catalytic current densities from 125 μA cm-2 to 240 μA cm-2 at 0.35 V. Additionally, we had considered the effects of the electrolyte (ion conductivity) and temperature dependence for catalytic current by AuNPs-Au electrode at the casting cycle three times. The catalytic current at 50 °C was approximately 5-times higher than that at 20 °C. Upon increasing acetate buffer concentration from 50 mM to 500 mM, the current density was enhanced to as 1.7-fold. Figure 1 shows the cyclic voltammogram of L-fucose oxidation on DHPDH-modified ME-AuNPs electrode under optimized conditions of 50 °C, pH 6, 500 mM acetate buffer, and the AuNPs casting cycle 15 times. Finally, the catalytic current reached to a steady-state value of 1.6 mA cm-2.

Electrocatalytic CO2 reduction (CO2RR) has emerged as a promising approach for converting CO2 into valuable chemicals and fuels to achieve a sustainable carbon cycle. However, the development of efficient electrocatalysts with high current densities and superior product selectivity remains a significant challenge. In this study, we present the synthesis of a porous nitrogen-doped carbon nanosheet loaded with heterostructured Ni/Ni3ZnC0.7 nanoparticles through a facile hydrothermal-calcination method (Ni/Ni3ZnC0.7-NC). Remarkably, the Ni/Ni3ZnC0.7-NC catalyst exhibits outstanding performance towards CO2RR in an H-cell, demonstrating a high CO faradaic efficiency of 92.47% and a current density (jCO) of 15.77 mA cm-2 at 0.87 V vs. RHE. To further explore its potential industrial applications, we constructed a flow cell and a rechargeable Zn-CO2 flow cell utilizing the Ni/Ni3ZnC0.7-NC catalyst as the cathode. Impressively, not only does the Ni/Ni3ZnC0.7-NC catalyst achieve an industrial high current density of 254 mA cm-2 at a voltage of -1.19 V vs. RHE in the flow cell, but it also exhibits a maximum power density of 4.2 mW cm-2 at 22 mA cm-2 in the Zn-CO2 flow cell, while maintaining excellent rechargeability. Density functional theory (DFT) calculations indicate that Ni/Ni3ZnC0.7-NC possesses more spontaneous reaction pathways for CO2 reduction to CO, owing to its heterogeneous structure in contrast to Ni3ZnC0.7-NC and Ni-NC. Consequently, Ni/Ni3ZnC0.7-NC demonstrates accelerated CO2RR reaction kinetics, resulting in improved catalytic activity and selectivity for CO2RR.

Electrochemical reduction of carbon dioxide (CO 2 ) to CO is a promising strategy. However, achieving high Faradaic efficiency with high current density using ionic liquid (IL) electrolyte remains a challenge. In this study, the new IL N ‐octyltrimethyl ammonium 1,2,4‐triazole ([N 1118 ][TRIZ]) shows outstanding performance for electrochemical reduction of CO 2 to CO on the commercial Ag electrode, and the current density can be up to 50.8 mAcm −2 with a Faradaic efficiency of 90.6 %. The current density of CO is much higher than those reported in the IL electrolyte. In addition, the density functional theory (DFT) calculation further proved that [N 1118 ][TRIZ] interact with CO 2 to form [N 1118 ] + [TRIZ‐CO 2 ] − complex which played a key role in reducing the activation energy of CO 2 . The formation of [N 1118 ] + [TRIZ‐CO 2 ] − complex was verified by Fourier Transform Infrared (FTIR) spectroscopy. According to the molecular orbital theory, the electrons obtained from IL was filled in the anti‐bonding orbit ( π *) of the CO 2 , resulting in reducing the C=O bond energy. This work provides a new strategy to design novel ILs for high efficiency electrochemical reduction of CO 2 to CO.

Nickel and molybdenum alloy powder was electrodeposited on a titanium cathode from a NiSO4?7H2O and (NH4)6 Mo7O24?4H2O ammonium solution. The desired chemical composition, structure, size and shape of particles in the powder samples were achieved by an appropriate choice of electrolysis parameters (current density, composition and temperature of the solution, cathode material and electrolysis duration). Metal coatings form in the current density range 15 mA cm-2&lt;j&lt;30 mA cm-2. If the current density is greater than 40mA cm-2 then powders form. The chemical composition of powder samples depends on the current density of electrodeposition. The molybdenum content in the powder increases with the increase of current density (in the low current density range), while in the higher current density range the molybdenum content in the alloy decreases with the increase of the current density of deposition. Smaller sized particles form at higher current density. X-ray analysis, differential scanning calorimetric and measurements of the temperature dependence of electric resistance and magnetic permeability of the powder samples were all used to establish a predominantly amorphous structure of the powder samples formed at the current density of j?70mA cm-2. The crystalline particle content in the powder samples increases with the decrease of the current density of deposition. Powder heating causes structural changes. The process of thermal stabilization of nickel and molybdenum amorphous powders takes place in the temperature interval from 463K to 573K and causes a decrease in electrical resistance and increase in magnetic permeability. The crystallization temperature depends on the value of current density of powder electrodeposition. Powder formed at j=180 mA cm-2 begins to crystallize at 573K, while the powder deposited at j=50 mA cm-2 begins to crystallize at 673K. Crystallization of the powder causes a decrease in electric resistivity and magnetic permeability. The Curie temperature of the crystallized powders is about 10 K higher than the Curie temperature of amorphous powders.

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.

Operando characterization is crucial for understanding the selectivity and stability of the electrochemical CO2 reduction reaction (eCO2RR). Existing operando techniques normally use single-compartment cells operating at low currents. However, high current densities on the order of 100 mA cm-2 are required for practical applications. Under a high current, reaction pathways and electrolyte dynamics can change, and stability issues such as salt precipitation and water crossover become more pronounced. Here, we developed an inline operando NMR method that is compatible with high-current reaction conditions. Demonstrating this on a copper-catalyzed eCO2RR at 100 mA cm-2, the operando NMR revealed a fast decrease of Faradaic efficiency for formate and ethanol within half an hour of reaction, accompanied by a pH decrease from 14 to 8 and a continuous accumulation of bicarbonate in the electrolyte. Water crossover was simultaneously observed and quantified via a deuteration technique and became more severe at high currents. This study revealed a highly dynamic electrolyte environment of copper-catalyzed eCO2RR. Using a gas diffusion flow cell and a benchtop NMR system, this operando approach is accessible by non-NMR experts and readily applicable to a wide range of catalysts, electrolyte compositions, and reactor designs for eCO2RR.

The economic feasibility of electrocatalytic carbon dioxide reduction reaction (CO2RR) relies on developing highly selective and efficient catalysts operating at a high current density. Herein, we explore a ligand-engineering strategy involving the use of metal–organic frameworks (MOFs) and combining the desirable features of homogeneous and heterogeneous catalysts for boosting the activity of CO2RR. Zn-based MOFs involving two different azolate functional ligands, i.e., 1,2,4-triazole (Calgary Framework 20, CALF20) and 2-methylimidazole (zeolitic imidazolate framework-8, ZIF-8), were investigated for CO2RR in an alkaline flow cell electrolyzer. The highest CO partial current density of −53.2 mA/cm2 was observed for a Zn-based MOF (CALF20). CALF20 showed the highest reported Faradaic efficiency of Zn-based MOFs for CO production (∼94% at −0.97 V versus reversible hydrogen electrode, RHE), with a turnover frequency (TOF) of 1360.8 h–1 and a partial current density of −32.8 mA/cm2. Experimental and density functional theory (DFT) results indicate that the sp2 carbon atoms in azole ligands coordinated with the metal center in MOFs are the active sites for CO2RR due to the fully occupied 3d orbital of Zn(II) centers. Ab initio investigation shows that both azolate frameworks in CALF20 and ZIF-8 have the most favorable adsorption sites at the N–sp2 C. Adopting the triazole ligand in CALF20 enhances the charge transfer (as compared with the diazole group in ZIF-8), which induces more electrons in the adjacent active sites at the azole ligand and facilitates *COOH formation, boosting current density and Faradaic efficiency toward CO production. This study suggests that ligand engineering in MOFs could be a viable approach to design a highly efficient CO2RR catalyst.

The electrochemical CO2 reduction reaction (CO2RR) is attracting attention as a promising method to convert CO2 into value-added products such as CO, which is used as a feedstock in the Fischer-Tropsch process. Previous research demonstrated that precious metal catalysts, such as Ag or Au, proceed CO2 reduction to CO generation with high efficiency[1]. Despite their high Faradaic efficiency (current efficiency) and current density in using porous gas diffusion electrode (GDE) systems, a higher utilization efficiency of those catalysts is needed due to their high cost of those catalysts. Based on the above background, in the present study, we employed the sputtering process as a direct formation method of catalysts’ nanoparticles on the catalyst layer on GDEs. We used gold as a model catalyst, fabricated Au-supported GDEs (Au-GDEs), and evaluated their CO2RR activity. Au-GDEs were fabricated using an RF sputtering process, and Au nanoparticles were directly deposited onto commercially available GDEs. The thicknesses of the catalysts’ layer were controlled by adjusting the sputtering time. The Au-modified GDEs with the conventional spray-coating method were also prepared using Au nanoparticles synthesized by a conventional solution process using citric acid as the reduction agent(Au_Spray) [2] for comparison. The CO2RR activity was evaluated by galvanostatic measurements using 1 M KHCO3 as electrolytes. Fig. 1(a) shows a cross-section scanning electron microscopy (SEM) image of an Au-GDE with a typical catalyst layer thickness of 100 nm (Au_100). The catalyst layer composed of Au (the white part of the figure) was uniformly formed on the surface of GDE. The X-ray absorption structure (XAS) analyses revealed that the valence state of Au was Au(0), and that the catalyst layer on the GDEs was composed of Au nanoparticles. Next, the CO2RR activity of the Au-GDE was evaluated. Carbon monoxide (CO) was detected as the main CO2RR product, and formate (HCOO−) and hydrogen (H2) as a side reaction product were also detected. The effect of film thickness of Au-GDE on CO2RR selectivity was evaluated at a current density of 50 mA cm-2. Au-GDE with a film thickness of 10 nm (Au_10) and Au_100 produced CO with the Faradaic efficiencies (FEs) of 72.7 % and 79.8 %, respectively. The FEs of hydrogen evolution reaction (HER) of Au_10 and Au_100 were 26.1% and 10.3%, respectively. The CO efficiencies for Au_10 and Au_100 were significantly higher than those of Au_Spray, which had a similar Au loading to that of Au_100. The higher CO selectivity of the Au-GDEs prepared by sputtering might be attributed to the higher coverage of Au on GDEs, which was confirmed by SEM-EDX mapping.The potential dependence of partial current density for CO (j CO) normalized by Au mass loading was evaluated as an indicator of Au utilization efficiency for CO2RR (Fig. 1(b)). The electrode with a catalyst layer thickness of 10 nm showed the 1882 A g-1 (potential of -0.85 V vs. RHE). As far as we know, this is the highest value among the Au-based electrodes using GDEs [3]. The value is about one order of magnitude higher than that of Au_Spray. This high CO production activity can be attributed to the formation of a thin Au catalyst layer composed of Au nanoparticles on the electrode surface by sputtering. [4] The results of the electrode structure analysis and details of the CO2RR activity will be discussed in the presentation.[1]A. Fen wick et. al., ACS Energy Lett., 2022, 7, 871−879 [2] X. Ji et. al., J. Am. Chem. Soc., 2007, 129, 13939–13948 [3] M. Sassenburg et. al., ACS Appl. Energy Mater., 2022, 5, 5983–5994. [4] T. Yamada et. al., in preparation. Figure 1

Cell designs that integrate membrane-electrode assemblies (MEAs) with highly selective catalysts are a promising route to reduce ohmic losses and achieve high energy efficiency in CO2 reduction at industrially relevant current densities. In this work, porous silver filtration membranes are demonstrated as simple and efficient gas-diffusion electrodes for CO2 reduction to CO at high current densities in an MEA-type device. A partial current density for CO of up to ca. 200 mA cm-2 was achieved at a cell voltage of ca. 3.3 V, in tandem with minimal H2 production. However, the analysis of cathodic and anodic outlet streams revealed that CO2 cross-over across the anion-exchange membranes, mostly in the form of CO32- but partially as HCOO- generated over the cathode, actually exceeds the amount of CO2 converted to the target product, resulting in a poor utilization of the reactant and in the early onset of mass transfer limitations. In addition, CO2 cross-over leads to a nonstoichiometric decrease of the outlet flow rate from the cathodic compartment. This effect can lead to a substantial overestimation of catalytic performance if the inlet flow rate of CO2 is used as reference for calculating partial current densities and Faradaic efficiencies. The results of this work highlight the importance of carrying out a carbon balance, in addition to traditional measurements of activity and selectivity, to adequately assess the performance of CO2 reduction devices at high current densities, and inform future efforts aimed at mitigating membrane cross-over in MEA-type electrolyzers for CO2 reduction.