Developing Catalysts Integrated in Gas-Diffusion Electrodes for CO2 Electrolyzers.

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

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).

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

While proton exchange membrane fuel cells (PEMFC) are already commercial, developing highly-efficient oxygen reduction reaction (ORR) electrocatalysts, is still needed. The scientific community is successful in developing new (in principle more active) electrocatalysts but still struggles to obtain the best performance of these materials in operating PEMFCs: their practical performance in membrane electrode assembly (MEA) do not reach those expected from an analytical approach performed using the rotating disk electrode setup, RDE (the most widely-used setup to assess a material’s ORR performance) [1-6].Several reasons explain why RDE data may not be relevant to account for MEA data. Firstly, in RDE, the O2 reactant is fed as gas dissolved in the electrolyte (slow diffusivity and solubility), hence the mass-transfer rate and related limiting current density are ca. 3 orders of magnitude lower than in real PEMFC conditions. Secondly, the activity data can only be extracted at high potential values in RDE (E > 0.85 V vs. RHE), although PEMFC optimal performance is reached at 0.6 < E < 0.8 V vs. RHE (cathode potential). Thirdly, the proton conduction can be limiting in the thin layer of ionomer covering the electrocatalyst nanoparticles in MEA, which could limit their operation, especially at the high current densities at stake in MEA. In essence, one is not sure that the electrocatalysts are used in optimized conditions in MEA, as emphasized mass-transport of reactants (both H+ and O2) to the catalytic sites is not granted for present ionomers and MEA structure [7, 8]. Recent results obtained in newly-developed electrochemical characterization setups like the floating electrode (FE) [9], the gas-diffusion electrode (GDE) [10] or differential cell (DC), could enable to reach much higher ORR current densities at the lab scale and therefore better match MEA data, a prerequisite for relevant benchmarking.In the present contribution, the GDE (Fig. 1a), that enables high current density measurements with a small amount of electrocatalyst [11], will be particularly studied. The combination of experimental measurements with numerical simulations is a promising approach to understand the factors affecting the performance of an electrocatalyst in a given electrode and assembly. In this study, a 3D multiphysics and multicomponent model is developed on the commercial software COMSOL Multiphysics® to study the ORR kinetics in a GDE, considering an acidic electrolyte. The model includes mass, species and momentum transports, as well as the Butler-Volmer's law for the electrochemical reaction, current distribution and transport of charged ions. The numerical results are then compared to the experimental data for three widely-used commercial benchmark ORR electrocatalysts: Pt/Vulcan XC72 carbon, PtCo/Vulcan XC72 carbon and Pt/graphitized carbon black (Fig. 1b). These results will be discussed, which will enable to isolate what phenomenon is the most limiting in this GDE configuration.This work was funded by the Centre of Excellence of Multifunctional Architectured Materials “CEMAM” no. AN-10-LABX-44-01 and by France’s “Programme d’Investissements d’Avenir” operated by the French National Research Agency (grant ANR-22-PEHY-0005). Some of the work was performed in the frame of the CNRS-Michelin joint Laboratory Alcal’HyLab. Keywords: Proton exchange membrane fuel cells, Oxygen reduction reaction, Gas diffusion electrode, Kinetics Modelling

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.

The long-term operation of CO 2 electrolyzers using membrane electrode assemblies (MEAs) is limited by challenges related to water management. However, water balance in CO 2 electrolyzer cells still has not been fully understood, and conflicting observations have been reported in the literature. In this study, a one-dimensional non-isothermal multiphysics model of an exchange MEA CO 2 electrolyzer with a Tokuyama A201 anion exchange membrane is developed to investigate the role of different physical and chemical phenomena on the water balance. The relative contributions of these processes vary with current density and membrane transport properties, which shift the dominant water transport mechanism in the cell. Our results highlight the significant contribution of homogeneous reactions, particularly OH − , to the water balance across the membrane. At low currents ( i < 130 mA cm −2 ), homogeneous buffer reactions dominate the water balance and result in net water production near the catalyst layer. At higher currents ( i > 130 mA cm −2 ), the flux is governed by electro-osmotic drag and a temperature gradient over the cathode gas diffusion electrode (GDE) with their relative contributions depending on membrane properties. Homogeneous buffering can re-emerge as the dominant mechanism at high currents if the hydroxide ion concentration in the membrane increases, for example under CO 2 -limited cathode conditions, allowing hydroxide ions to react with depleted bicarbonate near the anode. • Heat, electro-osmosis, and buffer reactions can govern water flux. • Net water consumption depends on hydroxide concentration. • For current densities <130 mAcm −2 , buffer reactions govern water flux. • Above 130 mA cm −2 , water balance depends on membrane properties and hydroxide concentration. • Results provide guidelines for water management in CO 2 electrolyzers.

Electrochemical CO2 conversion to fuels and chemicals addresses the CO2 emission and renewable energy storage problems simultaneously. Existing infrastructure can be leveraged to take advantage of these fuels, and recycling CO2 results in a low- or zero-carbon fuel cycle. Converting CO2 into chemical feedstock enables the sequestration of CO2 into valuable products such as polymers.Electrochemical CO2 conversion can be performed in aqueous- or gas-phase systems. Aqueous electrolysis, wherein the gaseous reactants are dissolved in the electrolyte, can achieve high selectivity. However, due to the low solubility of the gas reactants in the aqueous electrolyte and the long diffusion distance, current densities remain below few tens of mA/cm2—far from the >500 mA/cm2 regime needed to make ECC economically viable.Gas-phase electrolysis of CO2 could overcome the diffusion limitation of the aqueous system, thereby enabling a much higher current density. In a gas-phase ECC system, a catalyst to reduce CO2 is deposited onto a porous hydrophobic substrate (gas diffusion layer) to form a gas diffusion electrode. The CO2 reaches the catalyst through the gas diffusion layer, reducing the required diffusion length and improving the mass transport. Current densities in the hundreds of mA/cm2 have been achieved using a gas-phase electrolysis system. However, other liquid-phase electrochemical technologies such as water electrolysis achieve multi Amperes/cm2. Also, critical challenges related to stability at high current densities remain.Here, we present the gas diffusion electrode design that drives CO2 electroreduction to ethylene and ethanol at a current density > 100 mA/cm2 for > 100 hours. We then design the catalyst layer to achieve CO2 gas-phase electrolysis at current densities in the > 1 A/cm2 regime, and with the generation of multicarbon products. We report C2+ partial currents exceeding 1.3 A/cm2 at cathodic energy efficiencies above 40% – a sixfold increase relative to the best previously-reported comparable catalysts. These result in a full-cell energy efficiency towards C2 products of 20% above 1 A/cm2 operating currents.

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.

To achieve widespread commercialization of proton exchange membrane (PEM) water electrolyzers, the optimization of iridium (Ir) utilization is crucial. Traditional full-cell-based approaches are time-consuming and labor-intensive. In this work, the feasibility of using a gas diffusion electrode (GDE) half-cell as an alternative to full-cell setups for accelerated investigation of Ir-oxide-containing anode catalyst layers (CLs) is scrutinized. Using CLs composed of Ir oxides of different intrinsic oxygen evolution reaction (OER) activity as a probe, we show that a GDE can successfully reveal the differences in the performance of the CLs. Comparison of the results obtained in the GDE to those from rotating disk electrode (RDE) and full-cell membrane electrode assembly (MEA) measurements indicate that GDE data can closely mimic both setups. However, essential discrepancies are observed between GDE and MEA, which are linked to differences in the catalyst layer | membrane interface and the presence of liquid electrolyte in the GDE setup. Our findings reveal that even though the direct comparison of the OER performance to full-cell measurements is still partially hampered, GDE half-cell setups can already be used for fundamental assessments and accelerated screening of electrocatalysts and CLs at relevant current densities up to 1.5 A cm−2.

Here, we detail how the catalytic behavior of immobilized molecular electrocatalysts for the CO2 reduction reaction (CO2RR) can be impacted by catalyst aggregation. Operando Raman spectroscopy was used to study the CO2RR mediated by a layer of cobalt phthalocyanine (CoPc) immobilized on the cathode of an electrochemical flow reactor. We demonstrate that during electrolysis, the oxidation state of CoPc in the catalyst layer is dependent upon the degree of catalyst aggregation. Our data indicate that immobilized molecular catalysts must be dispersed on conductive supports to mitigate the formation of aggregates and produce meaningful performance data. We leveraged insights from this mechanistic study to engineer an improved CO-forming immobilized molecular catalyst─cobalt octaethoxyphthalocyanine (EtO8-CoPc)─that exhibited high selectivity (FECO ≥ 95%), high partial current density (JCO ≥ 300 mA/cm2), and high durability (ΔFECO < 0.1%/h at 150 mA/cm2) in a flow cell. This work demonstrates how to accurately identify CO2RR active species of molecular catalysts using operando Raman spectroscopy and how to use this information to implement improved molecular electrocatalysts into flow cells. This work also shows that the active site of CoPc during CO2RR catalysis in a flow cell is the metal center.

Polymer electrolyte membrane (PEM) electrolysis is widely considered a promising technology for energy storage and clean hydrogen production, due to the advantages it has over state-of-the-art alkaline water electrolysis.1 Typically, PEM electrolysers can operate at higher current densities than their alkaline counterparts, have a more robust design, and due to the zero-gap design, exhibit smaller Ohmic losses. Furthermore, PEM electrolysers can yield hydrogen at elevated pressures, eliminating the need for compression. Nevertheless, a series of challenges hinder the large-scale commercial implementation of PEM electrolysers. One of the most critical drawbacks is the need for noble metal electrocatalysts, especially at the anode of the cell, such as iridium or ruthenium, to catalyze the oxygen evolution reaction (OER) (1).2H2O → O2 + 4H+ + 4e- E° = 1.23 V vs NHE (1)These metals are used due to their high catalytic activity and long-term stability in the corrosive acidic environment of the PEM cells and their replacement by more available materials is difficult. Over the years, the main efforts of the scientific community towards solving this challenge were focused on increasing the noble metal dispersion, by using various high surface area catalyst supports, using core-shell catalysts, with the aim to minimize the amount of noble metal, modifying the morphology of the electrode, to improve mass transport limitations and electric contact between components and maximize noble metal utilization.2-4 Antimony-doped tin oxide has been reported as a stable OER catalyst support, offering sufficient electronic conductivity at the same, in order to minimize Ohmic losses.5 Furthermore, recent reports indicate that SnO2 does not only act as a catalyst support but takes part in catalysis.6 Building upon literature reports, we prepared a series of Sn/Ir OER catalysts by a two-step sodium borohydride reduction method, in which Sn nanoparticles were deposited over the surface of iridium black particles, decorating it. Catalysts with varying Sn content were incorporated in membrane electrode assemblies (MEAs) and their cell performance was evaluated in a single cell PEM electrolyser. We found that catalysts with up to 20 wt% tin content exhibited an improved cell performance compared to MEAs with similar iridium black loading. We modeled Ohmic, kinetic and mass transport overpotential contributions to experimentally acquired polarization curves and identified that the better cell performance arises mainly from the higher intrinsic catalytic activity, reflected by a higher apparent exchange current density of Sn/Ir OER catalysts compared to Ir black catalysts. The Sn/Ir MEAs also displayed lower mass transport overpotentials, which indicate improved transport in the catalyst layer. This can be due to a more porous structure and less agglomeration of iridium particles induced by the presence of tin over their surface.Interestingly, this performance improvement was only observed for iridium black catalysts. We prepared a series of IrO2-SnO2 catalysts by solution combustion synthesis method, which is known to yield high surface area disperse oxide materials. Comparing the performance of these catalysts to commercial iridium oxide catalysts in the PEM electrolysis cell, the MEAs with SnO2-containing catalysts displayed worse cell performance than MEAs with commercial iridium oxide. Modelling the overpotential contributions showed a lower apparent exchange current density, indicating a lower intrinsic catalytic activity of IrO2-SnO2 catalysts, as well as a higher Ohmic resistance, due to SnO2 addition. These observations indicate the importance of iridium oxidation state for harnessing the promotion effect of tin addition and give important insights into designing more efficient OER catalysts for PEM electrolysis cells.

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.

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

ConspectusAs electrochemical CO2 reduction studies progress from beaker or H-cell devices operating at low current densities to gas diffusion electrode (GDE)-based devices that sustain high reaction rates and provide an avenue toward commercialization, the overall system becomes significantly more complex. While the current densities may vary for the different approaches, it is essential to maintain the same scientific rigor when analyzing these systems. The mass transfer optimizations used in GDE based approaches necessarily add complexity and provide new challenges that need to be analyzed and overcome in terms of both engineering as well as analysis techniques. This Account puts into perspective our recent works analyzing high current density CO2 electrolysis performance via a comprehensive investigation of the entire system.In particular, we show the importance of monitoring (i) the gas flow rates at the outlet of the cathodic compartment, (ii) the anodic gas composition for CO2/O2 ratio, and (iii) pH variations in the electrolyte. A rigorous analysis of these parameters allows us to achieve a complete carbon balance, in addition to accounting for a total of 100% Faradaic efficiency. By analyzing both the cathode outlet and anodic CO2:O2 ratio, we demonstrate that these methods can be used to self-validate results providing robustness. We show that this analysis approach holds for both a zero-gap membrane electrode assembly device and a flowing-catholyte device. In addition, a comprehensive monitoring approach reveals that having an alkaline environment in the vicinity of the cathode can absorb substantial amounts of CO2, which may greatly distort Faradaic efficiencies if not accounted for. While monitoring the outlet flow rate of a reactor appears a simple task, the mixed gases and small flow rates in lab-scale reactors can add challenges and we discuss various methods to measure these flow rates.While pH is well-known to play a role in the activity and selectivity of CO2 reduction, we demonstrate that (i) the operational pH is not necessarily the pH of the initial electrolyte, (ii) there are long transients in pH before steady state is reached (on the order of hours), and (iii) the pH of the anolyte and catholyte can be significantly different over the duration of the electrolysis.By varying the membrane type in a flowing-catholyte reactor (anion exchange, cation exchange, or bipolar membrane), we can use this monitoring approach to quantitatively identify the major differences in CO2 reduction performance related to these distinct membrane types. The overall conclusion is that complex engineering processes entail that a thorough monitoring of parameters is necessary to accurately analyze the performance of high current density electrochemical CO2 reduction devices.

Rising atmospheric carbon dioxide (CO2) concentrations has led to an increased focus on electrochemical CO2 reduction using low-cost renewable electricity to produce low-carbon-intensity fuels and chemical feedstocks. The electrosynthesis of multicarbon (C2+) hydrocarbons and oxygenates, such as ethylene and ethanol, from CO2 feedstocks is appealing due to the large global market demand of these high-value products. Industrial-scale implementation of this technology requires operation at reaction rates above 100 mA/cm2 to minimize electrolyzer capital costs. Gas diffusion electrodes (GDEs) feed CO2 directly to the electrocatalyst surface, alleviating mass transport limitations faced in traditional aqueous environments and enabling operation at industrially relevant reaction rates. However, when implemented in conventional liquid flow cells with alkaline or neutral electrolytes, major challenges emerge in maintaining electrolyte stability with detrimental carbonate and bicarbonate formation, consequently having adverse effects on energy efficiency. Here we introduce therefore a membrane electrode assembly (MEA) based CO2 electrolyzer for C2+ production. We investigate the extent to which it can overcome the electrolyte stability limitations by eliminating alkaline electrolyte degradation and neutral electrolyte salt precipitation typically observed in the conventional approach. The MEA design maintains its performance while operating at elevated temperatures and low partial pressures of CO2, important considerations for industrial reactors. We further design a copper catalyst that takes advantage of the high CO2 concentrations afforded by the MEA. This catalyst design strategy achieves more than 50% Faradaic efficiency towards ethylene and more than 80% Faradaic efficiency towards C2+ products while operating at more than 300 mA/cm2. We demonstrate the stability of the system design through the continuous generation of ethylene while operating at industrially-relevant currents for more than 100 hours. The outcome is an engineered system that provides the longest stable ethylene production amongst all CO2 electrolyzers at reaction rates greater than 100 mA/cm2. To reduce costs associated with gas product separation from dilute streams, maximizing CO2 utilization is important. We investigate alternative flow field designs for the MEA reactor to achieve uniform reactant distribution, leading to improved current densities and reduced hydrogen evolution by at least 25% each. Finally, we demonstrate the process and system scalability through C2+ production in a large-scale MEA electrolyzer as part of the NRG COSIA Carbon XPRIZE.