CO2 electrolyzers show promise as a cleaner alternative to produce value-added chemicals. In the last decade, research has shifted from classifying CO2 reduction activity and selectivity as a catalytic property (zero-dimensional [0D]) to one that includes the complex interactions of gas, liquid, and solid species between the cathode and anode (1D). To scale CO2 electrolyzers, however, 2D and 3D spatial variations in product selectivity, activity, and stability arise due to the design of reactor components, as well as concentration variations of the reactants, intermediates, and products. Conventional “black-box” measurement protocols are then insufficient to characterize CO2 electrolyzers. Here, we discuss the critical multi-dimensional phenomena occurring inside these electrochemical systems, which impact the observed performance. Recent literature is used to demonstrate how a spatial perspective is essential for proper data interpretation, designing effective catalysts, and prolonging CO2 electrolyzer lifetimes. Researchers should then define CO2 electrolysis systems in multiple dimensions (2D and 3D).@en

Bipolar membranes in electrochemical CO2 conversion cells enable different reaction environments in the CO2-reduction and O2-evolution compartments. Under ideal conditions, water-splitting in the bipolar membrane allows for platinum-group-metal-free anode materials and high CO2 utilizations. In practice, however, even minor unwanted ion crossover limits stability to short time periods. Here we report the vital role of managing ionic species to improve CO2 conversion efficiency while preventing acidification of the anodic compartment. Through transport modelling, we identify that an anion-exchange ionomer in the catalyst layer improves local bicarbonate availability and increasing the proton transference number in the bipolar membranes increases CO2 regeneration and limits K+ concentration in the cathode region. Through experiments, we show that a uniform local distribution of bicarbonate ions increases the accessibility of reverted CO2 to the catalyst surface, improving Faradaic efficiency and limiting current densities by twofold. Using these insights, we demonstrate a fully platinum-group-metal-free bipolar membrane electrode assembly CO2 conversion system exhibiting <1% CO2/cation crossover rates and 80-90% CO2-to-CO utilization efficiency over 150 h operation at 100 mA cm−2 without anolyte replenishment.@en

Carbon dioxide (CO2) electrolysis on copper (Cu) catalysts has attracted interest due to its direct production of C2+ feedstocks. Using the knowledge that CO2 reduction on copper is primarily a tandem reaction of CO2 to CO and CO to C2+ products, we show that modulating CO concentrations within the liquid catalyst layer allows for a C2+ selectivity of >80% at 200 mA cm−2 under broad conversion conditions. The importance of CO pooling is demonstrated through residence time distribution curves, varying flow fields (serpentine/parallel/interdigitated), and flow rates. While serpentine flow fields require high conversions to limit CO selectivity and maximize C2+ selectivity, the longer CO residence times of parallel flow fields achieve similar selectivity over broad flow rates. Critically, we show that parts of the catalyst area predominantly reduce CO instead of CO2 as supported by CO reduction experiments, transport modelling, and achieving a CO2 utilization efficiency greater than the theoretical limit of 25% for C2+ products.@en

CO2 electrolysis allows the sustainable production of carbon-based fuels and chemicals. However, state-of-the-art CO2 electrolysers employing anion exchange membranes (AEMs) suffer from (bi)carbonate crossover, causing low CO2 utilization and limiting anode choices to those based on precious metals. Here we argue that bipolar membranes (BPMs) could become the primary option for intrinsically stable and efficient CO2 electrolysis without the use of scarce metals. Although both reverse- and forward-bias BPMs can inhibit CO2 crossover, forward-bias BPMs fail to solve the rare-earth metals requirement at the anode. Unfortunately, reverse-bias BPM systems presently exhibit comparatively lower Faradaic efficiencies and higher cell voltages than AEM-based systems. We argue that these performance challenges can be overcome by focusing research on optimizing the catalyst, reaction microenvironment and alkali cation availability. Furthermore, BPMs can be improved by using thinner layers and a suitable water dissociation catalyst, thus alleviating core remaining challenges in CO2 electrolysis to bring this technology to the industrial scale.@en

Carbon dioxide (CO2) electrolysis is an attractive technology for the production of valueadded products and hydrocarbons, establishing a route for leveraging renewable energy sources in the synthesis of fuels and chemicals. Considerable progress has been made in the past decade in advancing CO2 electrolyzers towards commercial scales, however major challenges remain for achieving practical feasibility.....@en

Electrochemical ammonia (NH₃) synthesis from nitrate (NO₃⁻) offers a promising greener alternative to the fossil-fuel-based Haber-Bosch process to support the increasing demand for nitrogen fertilizers while removing environmental waste. Previous studies have mainly focused on designing catalysts to promote the direct conversion (NO₃⁻ → NH₃) while suppressing the two-step pathway (NO₃⁻ → NO₂⁻ → NH₃). We hypothesize that efficient nitrate reduction is possible on simple catalysts by instead promoting the two-step reaction and using chemical reactor principles in a membrane electrode assembly, despite NO₂⁻ intermediates. Here, we use an unmodified copper catalyst and control reactivity through current density, flow rate, and electrolyte recycling. Balancing the electrolyte flow rate with current density results in ideal residence times for NO₂⁻, allowing for 91% FE_NH3 in a 5 cm² electrolyzer with a NO₃⁻ to NH₃ partial current of 1.8 A. This work shows that traditional engineering principles can substantially boost the NO₃ reduction reaction, even for simple catalysts.

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Electrochemical reduction of CO₂ presents an attractive way to store renewable energy in chemical bonds in a potentially carbon-neutral way. However, the available electrolyzers suffer from intrinsic problems, like flooding and salt accumulation, that must be overcome to industrialize the technology. To mitigate flooding and salt precipitation issues, researchers have used super-hydrophobic electrodes based on either expanded polytetrafluoroethylene (ePTFE) gas-diffusion layers (GDL’s), or carbon-based GDL’s with added PTFE. While the PTFE backbone is highly resistant to flooding, the non-conductive nature of PTFE means that without additional current collection the catalyst layer itself is responsible for electron-dispersion, which penalizes system efficiency and stability. In this work, we present operando results that illustrate that the current distribution and electrical potential distribution is far from a uniform distribution in thin catalyst layers (~50 nm) deposited onto ePTFE GDL’s. We then compare the effects of thicker catalyst layers (~500 nm) and a newly developed non-invasive current collector (NICC). The NICC can maintain more uniform current distributions with 10-fold thinner catalyst layers while improving stability towards ethylene (≥ 30%) by approximately two-fold.

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The electrochemical CO₂ reduction reaction (CO₂RR) is an attractive method to produce renewable fuel and chemical feedstock using clean energy sources. Formate production represents one of the most economical target products from CO₂RR but is primarily produced using post-transition metal catalysts that require comparatively high overpotentials. Here a composition of bimetallic Cu–Pd is formulated on 2D Ti₃C₂T_x (MXene) nanosheets that are lyophilized into a highly porous 3D aerogel, resulting in formate production much more efficient than post-transition metals. Using a membrane electrode assembly (MEA), formate selectivities >90% are achieved with a current density of 150 mA cm⁻² resulting in the highest ever reported overall energy efficiency of 47% (cell potentials of −2.8 V), over 5 h of operation. A comparable Cu-Pd aerogel achieves near-unity CO production without the MXene templating. This simple strategy represents an important step toward the experimental demonstration of 3D-MXenes-based electrocatalysts for CO₂RR application and opens a new platform for the fabrication of macroscale aerogel MXene-based electrocatalysts.

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Salt precipitation is a problem in electrochemical CO2 reduction electrolyzers that limits their long-term durability and industrial applicability by reducing the active area, causing flooding and hindering gas transport. Salt crystals form when hydroxide generation from electrochemical reactions interacts homogeneously with CO2 to generate substantial quantities of carbonate. In the presence of sufficient electrolyte cations, the solubility limits of these species are reached, resulting in "salting out"conditions in cathode compartments. Detrimental salt precipitation is regularly observed in zero-gap membrane electrode assemblies, especially when operated at high current densities. This Perspective briefly discusses the mechanisms for salt formation, and recently reported strategies for preventing or reversing salt formation in zero-gap CO2 reduction membrane electrode assemblies. We link these approaches to the solubility limit of potassium carbonate within the electrolyzer and describe how each strategy separately manipulates water, potassium, and carbonate concentrations to prevent (or mitigate) salt formation.

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Electrochemical conversion of gaseous CO₂ to value-added products and fuels is a promising approach to achieve net-zero CO₂ emission energy systems. Significant efforts have been achieved in the design and synthesis of highly active and selective electrocatalysts for this reaction and their reaction mechanism. To perform an efficient conversion and desired product selectivity in practical applications, we need an active, cost-effective, stable, and scalable electrolyzer design. Membrane-electrode assemblies (MEAs) can be an efficient solution to address the key challenges in the aqueous gas diffusion electrodes (GDE), e.g., ohmic resistances and complex reactor design. This review presents a critical overview of recent advances in experimental design and simulation of MEAs for CO₂ reduction reaction, including the shortcomings and remedial strategies. In the last section, the remaining challenges and future research opportunities are suggested to support the advancement of CO₂ electrochemical technologies.

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The electrochemical reduction of carbon dioxide (CO₂) to value-added materials has received considerable attention. Both bulk transition-metal catalysts and molecular catalysts affixed to conductive noncatalytic solid supports represent a promising approach toward the electroreduction of CO₂. Here, we report a combined silver (Ag) and pyridine catalyst through a one-pot and irreversible electrografting process, which demonstrates the enhanced CO₂conversion versus individual counterparts. We find that by tailoring the pyridine carbon chain length, a 200 mV shift in the onset potential is obtainable compared to the bare silver electrode. A 10-fold activity enhancement at -0.7 V vs reversible hydrogen electrode (RHE) is then observed with demonstratable higher partial current densities for CO, indicating that a cocatalytic effect is attainable through the integration of the two different catalytic structures. We extended the performance to a flow cell operating at 150 mA/cm², demonstrating the approach's potential for substantial adaptation with various transition metals as supports and electrografted molecular cocatalysts.

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The electrochemical reduction of CO2 (CO2RR) on silver catalysts has been demonstrated under elevated current density, longer reaction times, and intermittent operation. Maintaining performance requires that CO2 can access the entire geometric catalyst area, thus maximizing catalyst utilization. Here we probe the time-dependent factors impacting geometric catalyst utilization for CO2RR in a zero-gap membrane electrode assembly. We use three flow fields (serpentine, parallel, and interdigitated) as tools to disambiguate cell behavior. Cathode pressure drop is found to play the most critical role in maintaining catalyst utilization at all time scales by encouraging in-plane CO2 transport throughout the gas-diffusion layer (GDL) and around salt and water blockages. The serpentine flow channel with the highest pressure drop is then the most failure-resistant, achieving a CO partial current density of 205 mA/cm2 at 2.76 V. These findings are confirmed through selectivity measurements over time, double-layer capacitance measurements to estimate GDL flooding, and transport modeling of the spatial CO2 concentration.

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The production of value added C1 and C2 compounds within CO2 electrolyzers has reached sufficient catalytic performance that system and process performance-such as CO2 utilization-have come more into consideration. Efforts to assess the limitations of CO2 conversion and crossover within electrochemical systems have been performed, providing valuable information to position CO2 electrolyzers within a larger process. Currently missing, however, is a clear elucidation of the inevitable trade-offs that exist between CO2 utilization and electrolyzer performance, specifically how the faradaic efficiency of a system varies with CO2 availability. Such information is needed to properly assess the viability of the technology. In this work, we provide a combined experimental and 3D modelling assessment of the trade-offs between CO2 utilization and selectivity at 200 mA cm-2 within a membrane-electrode assembly CO2 electrolyzer. Using varying inlet flow rates we demonstrate that the variation in spatial concentration of CO2 leads to spatial variations in faradaic efficiency that cannot be captured using common 'black box' measurement procedures. Specifically, losses of faradaic efficiency are observed to occur even at incomplete CO2 consumption (80%). Modelling of the gas channel and diffusion layers indicated that at least a portion of the H2 generated is considered as avoidable by proper flow field design and modification. The combined work allows for a spatially resolved interpretation of product selectivity occurring inside the reactor, providing the foundation for design rules in balancing CO2 utilization and device performance in both lab and scaled applications. This journal is

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Typically, anion exchange membranes (AEMs) are used in CO2 electrolyzers, but those suffer from unwanted CO2 crossover, implying (indirect) energy consumption for generating an excess of CO2 feed and purification of the KOH anolyte. As an alternative, bipolar membranes (BPMs) have been suggested, which mitigate the reactant loss by dissociating water albeit requiring a higher cell voltage when operating at a near-neutral pH. Here, we assess the direct and indirect energy consumption required to produce CO in a membrane electrode assembly with BPMs or AEMs. More than 2/3 of the energy consumption for AEM-based cells concerns CO2 crossover and electrolyte refining. While the BPM-based cell had a high stability and almost no CO2 loss, the Faradaic efficiency to CO was low, making the energy requirement per mol of CO higher than for the AEM-based cell. Improving the cathode-BPM interface should be the future focus to make BPMs relevant to CO2 electrolyzers.

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Advancing reaction rates for electrochemical CO2 reduction in membrane electrode assemblies (MEAs) have boosted the promise of the technology while exposing new shortcomings. Among these is the maximum utilization of CO2, which is capped at 50% (CO as targeted product) due to unwanted homogeneous reactions. Using bipolar membranes in an MEA (BPMEA) has the capability of preventing parasitic CO2 losses, but their promise is dampened by poor CO2 activity and selectivity. In this work, we enable a 3-fold increase in the CO2 reduction selectivity of a BPMEA system by promoting alkali cation (K+) concentrations on the catalyst's surface, achieving a CO Faradaic efficiency of 68%. When compared to an anion exchange membrane, the cation-infused bipolar membrane (BPM) system shows a 5-fold reduction in CO2 loss at similar current densities, while breaking the 50% CO2 utilization mark. The work provides a combined cation and BPM strategy for overcoming CO2 utilization issues in CO2 electrolyzers.

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Electrochemical reduction of CO₂ has received significant interest for converting CO₂ to value added products and closing the carbon cycle. Recent advances through catalyst development have aided in satisfying the requirements of achieving a high product selectivity, activity and long-term stability. Among various industrially valuable products, formic acid has found numerous applications such asin fuel cells and textile industry. In this work, we report the synthesis of bismuth oxychloride dispersed on nitrogen-doped carbon through a facile ion adsorption process using bismuth acetate, hydrochloric acid and urea as precursors, and discuss its performance as an electrocatalyst for the electrochemical reduction of CO₂ to formate. The results show that bismuth oxychloride dispersed on nitrogen-doped carbon has good catalytic activity for CO₂ reduction to formate in 0.5 M KHCO₃, achieving a maximum faradaic efficiency of 84.3 % at −0.87 V versus RHE. The catalyst is found to be stable for 5 h of continuous operation and achieves a turnover frequency of 146.36 h⁻¹.

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