Bipolar membrane electrolyzers enable high single-pass CO2 electroreduction to multicarbon products

A critical appraisal of advances in integrated CO2 capture and electrochemical conversion

This perspective work examines the current advancements in integrated CO2 capture and electrochemical conversion technologies, comparing the emerging methods of (1) electrochemical reactive capture (eRCC) though amine- and (bi)carbonate-mediated processes and (2) direct (flue gas) adsorptive capture and conversion (ACC) with the conventional approach of sequential carbon capture and conversion (SCCC). We initially identified and discussed a range of cell-level technological bottlenecks inherent to eRCC and ACC including, but not limited to, mass transport limitations of reactive species, limitation of dimerization, impurity effects, inadequate in situ generation of CO2 to sustain industrially relevant current densities, and catalyst instabilities with respect to some eRCC electrolytes, amongst others. We followed this with stepwise perspectives on whether these are considered intrinsic challenges of the technologies - otherwise recommendations were disclosed where appropriate. Furthermore, technoeconomic analysis (TEA) was conducted using a net present value (NPV) model to determine the minimum selling prices (MSPs) for CO, HCOOH, CH3OH, C2H5OH, and C2H4 as target products based on cell-performance metrics from contemporary literature for SCCC, eRCC, and ACC. Additionally, sensitivity analyses were performed, focusing on cell-level parameters (voltage requirements, Faradaic efficiencies, current density), production scale factors, and other relevant variables (levelized costs of electricity and stack). This analysis sheds light on the cost-driving factors influencing commercial viability, revealing key techno-economic challenges for eRCC, particularly with liquid products. However, it also identifies optimization opportunities in current designs. By pinpointing critical areas for improvement, this work helps advance electrochemical CO2 reduction technologies towards more sustainable and economically competitive applications at different scales.

Promotion of C─C Coupling in the CO2 Electrochemical Reduction to Valuable C2+ Products: From Micro-Foundation to Macro-Application

The electrochemical CO2 reduction reaction (CO2RR) to valuable C2+ products emerges as a promising strategy for converting intermittent renewable energy into high-energy-density fuels and feedstock. Leveraging its substantial commercial potential and compatibility with existing energy infrastructure, the electrochemical conversion of CO2 into multicarbon hydrocarbons and oxygenates (C2+) holds great industrial promise. However, the process is hampered by complex multielectron-proton transfer reactions and difficulties in reactant activation, posing significant thermodynamic and kinetic barriers to the commercialization of C2+ production. Addressing these barriers necessitates a comprehensive approach encompassing multiple facets, including the effective control of C─C coupling in industrial electrolyzers using efficient catalysts in optimized local environments. This review delves into the advancements and outstanding challenges spanning from the microcosmic to macroscopic scales, including the design of nanocatalysts, optimization of the microenvironment, and the development of macroscopic electrolyzers. By elucidating the influence of the local electrolyte environment, and exploring the design of potential industrial flow cells, guidelines are provided for future research aimed at promoting C─C coupling, thereby bridging microscopic insights and macroscopic applications in the field of CO2 electroreduction.

Reactive carbon capture using electrochemical reactors

The electrolytic upgrading of CO2 presents a promising strategy to mitigate global CO2 emissions while generating valuable carbon-based products such as carbon monoxide, formate, and ethylene. However, the adoption of industrial-scale CO2 electrolyzers is hindered by the high energy and capital costs associated with the purification and pressurization of captured CO2 prior to electrolysis. One promising solution is "reactive carbon capture," which involves the electrolytic conversion of the eluent from CO2 capture units, or the "reactive carbon solution," directly into valuable products. This approach circumvents the energy-intensive processes required for electrolyzers fed with gaseous CO2. This Tutorial Review highlights recent advances for reactive carbon capture, showcasing its potential as a scalable solution for electrolyzers that upgrade CO2 into fuels and products.

Tackling CO2 Loss in Electrocatalytic Carbon Dioxide Reduction by Advanced Material and Electrolyzer Design

Electrocatalytic CO2 reduction (ECO2R) has been considered as a promising approach to convert CO2 into valuable chemicals and fuels. CO2 loss in conventional alkaline electrolyzers has been recognized as a major obstacle that compromising the efficiency of the ECO2R system. This review firstly conducts an in-depth assessment of the origin and influence of CO2 loss. On this basis, this work summarizes electrolyzer configurations based on novel material and structure design that are capable of tackling CO2 loss, including acidic electrolyzer, bipolar membrane (BPM) derived electrolyzer, cascade electrolyzer, liquid-phase-anode electrolyzer, and liquid-fed electrolyzer. The design strategies and challenges of these carbon efficient electrolyzers have been deliberated in detail. By comparing and analyzing the advantages and limitations of various electrolyzer designs, this work aims to provide some guidelines for the development of efficient ECO2R technology toward large-scale industrial application.

Addressing the Carbonate Issue: Electrocatalysts for Acidic CO2 Reduction Reaction

Electrochemical CO2 reduction reaction (CO2RR) powered by renewable energy provides a promising route to CO2 conversion and utilization. However, the widely used neutral/alkaline electrolyte consumes a large amount of CO2 to produce (bi)carbonate byproducts, leading to significant challenges at the device level, thereby impeding the further deployment of this reaction. Conducting CO2RR in acidic electrolytes offers a promising solution to address the "carbonate issue"; however, it presents inherent difficulties due to the competitive hydrogen evolution reaction, necessitating concerted efforts toward advanced catalyst and electrode designs to achieve high selectivity and activity. This review encompasses recent developments of acidic CO2RR, from mechanism elucidation to catalyst design and device engineering. This review begins by discussing the mechanistic understanding of the reaction pathway, laying the foundation for catalyst design in acidic CO2RR. Subsequently, an in-depth analysis of recent advancements in acidic CO2RR catalysts is provided, highlighting heterogeneous catalysts, surface immobilized molecular catalysts, and catalyst surface enhancement. Furthermore, the progress made in device-level applications is summarized, aiming to develop high-performance acidic CO2RR systems. Finally, the existing challenges and future directions in the design of acidic CO2RR catalysts are outlined, emphasizing the need for improved selectivity, activity, stability, and scalability.

Syngas Production Improvement from CO2RR Using Cu-Sn Electrodeposited Catalysts

Electrochemical reduction of CO2 is an efficient and novel strategy to reduce the amount of this greenhouse-effect pollutant gas in the atmosphere while synthesizing value-added products, all of it with an easy synergy with intermittent renewable energies. This study investigates the influence of different ways of combining electrodeposited Cu and Sn as metallic elements in the electrocatalyst. From there, the use of Sn alone or with a small amount of Cu beneath is investigated, and finally, the best catalyst obtained, which has Sn over a slight Cu layer, is evaluated in consecutive cycles to make an initial exploration of the catalyst durability. As a result of this work, after optimization of the Sn and Cu-based catalysts, it is possible to obtain more than 60% of the organic products of interest, predominantly CO, the main component of syngas. Finally, this great amount of CO is obtained under low cell potential (below 3 V), which is a remarkable result in terms of the cost of the process.

Macro- and Nano-Porous Ag Electrodes Enable Selective and Stable Aqueous CO2 Reduction

Electrochemical carbon dioxide (CO2) reduction from aqueous solutions offers a promising strategy to overcome flooding and salt precipitation in gas diffusion electrodes used in gas-phase CO2 electrolysis. However, liquid-phase CO2 electrolysis often exhibits low CO2 reduction rates because of limited CO2 availability. Here, a macroporous Ag mesh is employed and activated to achieve selective CO2 conversion to CO with high rates from an aqueous bicarbonate solution. It is found that activation of Ag surface using oxidation/reduction cycles produces nanoporous surfaces that favor CO2-to-CO conversion. Notably, it is found that a combination of dissolved CO2 in bicarbonate solution with CO2 generated in situ from bicarbonate ions enables increased CO2 availability and a CO2-to-CO conversion rate over 100 mA cm-2. By optimizing the oxidation/reduction cycles to fine-tune the structure of Ag surface, CO2-to-CO conversion is reported from a bicarbonate solution with CO Faradaic efficiency of over 85% at current density of 100 mA cm-2, high concentration of 24.7% at outlet gas stream and stability of over 100 h with maintaining CO FE over 85% during whole reaction time.

Cascade Catalytic Systems for Converting CO2 into C2+ Products

The excessive emission and continuous accumulation of CO2 have precipitated serious social and environmental issues. However, CO2 can also serve as an abundant, inexpensive, and non-toxic renewable C1 carbon source for synthetic reactions. To achieve carbon neutrality and recycling, it is crucial to convert CO2 into value-added products through chemical pathways. Multi-carbon (C2+) products, compared to C1 products, offer a broader range of applications and higher economic returns. Despite this, converting CO2 into C2+ products is difficult due to its stability and the high energy required for C-C coupling. Cascade catalytic reactions offer a solution by coordinating active components, promoting intermediate transfers, and facilitating further transformations. This method lowers energy consumption. Recent advancements in cascade catalytic systems have allowed for significant progress in synthesizing C2+ products from CO2. This review highlights the features and advantages of cascade catalysis strategies, explores the synergistic effects among active sites, and examines the mechanisms within these systems. It also outlines future prospects for CO2 cascade catalytic synthesis, offering a framework for efficient CO2 utilization and the development of next-generation catalytic systems.

Concentrated C2+ Alcohol Production Enabled by Post-Intermediate Modulation and Augmented CO Adsorption in CO Electrolysis

The electrocatalytic synthesis of multicarbon products from CO2/CO feedstock represents a sustainable method for chemical production with a reduced carbon footprint. Traditional copper catalysts predominantly produce alkenes, but generating valuable and versatile C2+ alcohols, especially high-energy-density C3 alcohols, has been challenging due to issues with selectivity, activity, and stability. Here, we present the construction of Ru-doped Cu nanowires that enhance the selectivity of n-PrOH and C2+ alcohols. In situ Raman spectroscopy shows that our approach promotes both *CO binding and availability, particularly facilitating the formation of high-frequency-bound *CO (*COHFB) and maintaining multiple *CO adsorption modes on Ru-modified and bare low-coordinated Cu nanowires. Density-functional theory (DFT) simulations illustrate that introducing Ru species onto a low-coordinated Cu step surface simultaneously stabilizes CO and alcohol-related intermediates, shifting the dominant reaction pathway toward alcohols and facilitating CO-C2 coupling at the expense of ethylene selectivity. In an alkaline gas-diffusion electrolyzer, we attained a maximum Faradaic efficiency (FE) of 35.9% for n-PrOH and 62.4% for the total C2+ alcohols. Optimizing parameters in the membrane electrode assembly (MEA) system enabled the one-pot generation and separation of C2+ alcohols, achieving a record concentration of 18.8 wt % (4.2 wt % n-PrOH and 14.6 wt % EtOH) with nearly 100% purity at 200 mA/cm2 over 100 h. This work not only provides new insights and guidance for the development of future catalysts from the perspectives of surface science and mechanisms but also highlights the importance of coupling material engineering with reactor engineering to optimize the production process of high-value alcohol products.

Alkali metal cations enhance CO2 reduction by a Co molecular complex in a bipolar membrane electrolyzer

The electrochemical reduction of CO2 is a promising pathway for converting CO2 into valuable fuels and chemicals. The local environment at the cathode of CO2 electrolyzers plays a key role in determining activity and selectivity, but currently some mechanisms are still under debate. In particular, alkali metal cations have been shown to enhance the selectivity of metal catalysts, but their role remains less explored for molecular catalysts especially in high-current electrolyzers. Here, we investigated the enhancement effects of cations (Na+, K+, Cs+) on Co phthalocyanine (CoPc) in a state-of-the-art reverse-biased bipolar membrane electrolyzer. When added to the anolyte, these cations increased the Faradaic efficiency for CO, except in the case of Na+ in which the effect was transient, but the effects are convoluted with the transport process through the membrane. Alternatively, these cations can also be added directly to the cathode as chloride salts, allowing the use of a pure H2O anolyte feed, leading to sustained improved CO selectivity (61% at 100 mA cm-2 after 24 h). Our results show that cation addition is a simple yet effective strategy for improving the product selectivity of molecular electrocatalysts, opening up new avenues for tuning their local environment for CO2 reduction.This article is part of the discussion meeting issue 'Green carbon for the chemical industry of the future'.

Substituent tuning of Cu coordination polymers enables carbon-efficient CO2 electroreduction to multi-carbon products

CO2 electroreduction is a potential pathway to achieve net-zero emissions in the chemical industry. Yet, CO2 loss, resulting from (bi)carbonate formation, renders the process energy-intensive. Acidic environments can address the issue but at the expense of compromised product Faradaic efficiencies (FEs), particularly for multi-carbon (C2+) products, as rapid diffusion and migration of protons (H+) favors competing H2 and CO production. Here, we present a strategy of tuning the 2-position substituent length on benzimidazole (BIM)-based copper (Cu) coordination polymer (CuCP) precatalyst - to enhance CO2 reduction to C2+ products in acidic environments. Lengthening the substituent from H to nonyl enhances H+ diffusion retardation and decreases Cu-Cu coordination numbers (CNs), favoring further reduction of CO. This leads to a nearly 24× enhancement of selectivity towards CO hydrogenation and C-C coupling at 60 mA cm-2. We report the highest C2+ product FE of more than 70% at 260 mA cm-2 on pentyl-CuCP and demonstrate a CO2-to-C2+ single-pass conversion (SPC) of ~54% at 180 mA cm-2 using pentyl-CuCP in zero-gap electrolyzers.

A Solid-State Electrolyte Facilitates Acidic CO2 Electrolysis without Alkali Metal Cations by Regulating Proton Transport

Electrochemical CO2 reduction (CO2R) in acidic media provides a pathway to curtail CO2 losses by suppressing the formation of (bi)carbonates. In such systems, a high concentration of alkali metal cations is necessary for mitigating the proton-rich environment and suppressing the competing hydrogen evolution reaction. However, a high cation concentration also promotes salt precipitation within the gas diffusion layer, resulting in poor system durability. Here, we resolve this conundrum by replacing the liquid catholyte with a solid-state proton conductor to regulate H+ transport. This is postulated to allow for a locally alkaline environment at the cathode, enabling selective CO2R even without alkali metal cations. We show that this strategy is effective over a broad range of catalyst systems. For instance, we achieve an 87% CO faradaic efficiency (FE) at 300 mA/cm2 using a composite nanoporous Au and single-atom Ni catalyst, with 0.25 M H2SO4 as the anolyte. Stable operation over 110 h and a high single-pass carbon efficiency of 82.8% were also successfully demonstrated. Importantly, we find that this solid-state system is also particularly effective at converting dilute feedstock (5% CO2) with a CO FE of 47.7%, a factor of 16.4 times higher than a conventional system. Our results introduce a simple yet effective design approach for developing efficient acidic CO2R electrolyzers.

Bipolar Membranes Via Divergent Synthesis: On the Interplay between Ion Exchange Capacity and Water Dissociation Catalysis

Bipolar membranes (BPMs) enable the operation of electrochemical reactors with electrode compartments in different chemical environments or pH. The transport properties at the microscopic scale are dictated by the composition and morphology of the interfacial junctions as well as the specific chemistry of the ion-exchange layers that support the current of protons and hydroxide ions. This work elucidates the relation between water-dissociation efficiency and the physicochemical properties of the individual ion-exchange membrane layers in the poly(styrene-b-poly(ethylene-ran-butylene)-b-polystyrene) (SEBS)-based BPM. The optimal water dissociation performance of three previously reported water-dissociation catalysts in the SEBS-based BPM was examined, with junction thickness of graphene oxide > TiO2 > SnO2, resulting in disparate junction morphologies at the BPM's interface. A hybrid junction system, which included both the effective water dissociation catalyst SnO2 and direct contacting of the ion-exchange membrane layer, exhibited high water dissociation efficiency. This was likely due to the immediate ion transport pathway provided by direct membrane contact around the catalyst, which also improved the interfacial adhesion. A higher ion exchange capacity (IEC) in BPMs substantially enhanced the water dissociation performance in BPMs without water-dissociation catalysts. However, the incorporation of the effective SnO2 catalyst into the BPMs with a lower IEC significantly improved performance, an effect attributed to the hybrid junction system. Additionally, the increase in water uptake and ion conductivity of the cation exchange layer with higher IEC suggested that the cation exchange layer and its interface to the water-dissociation catalyst layer may play a key role in water dissociation. This study identifies the key parameters of individual BPM components and their interactions to water dissociation performance, offering new insights to guide in the construction of future BPMs optimized for enhanced water dissociation efficiency at high current densities.

Ionomer and Membrane Designs for Low-temperature CO2 and CO Electrolysis

Low-temperature electroreduction of CO2 and CO (CO(2)RR) into valuable chemicals and fuels offers a promising pathway to reduce greenhouse gas emissions and achieve carbon neutrality. Today's low-temperature CO(2)RR technology relies on the use of ionomers, polymers with ionized groups, primarily as catalyst layer (CL) additives. In the meantime, ionomers can assemble into ion-exchange membranes (IEMs), serving as important components of electrolyzers. According to the ion-exchange functions, ionomer additives are classified as cation-exchange ionomers (CEIs) and anion-exchange ionomers (AEIs); similarly, IEMs are divided into cation-exchange membranes (CEMs) and anion-exchange membranes (AEMs), as well as the multilayer polymer electrolytes (MPEs). Recent studies show that ionomer additives can regulate the catalytic microenvironment and thereby enhance performance towards desired products. This Review discusses the roles of ionomer additives and IEMs in CO2 and CO reduction reactions, highlighting the latest mechanistic insights and performance advances. It outlines challenges in designing ionomer additives and IEMs to improve product selectivity, energy efficiency (EE), and operational lifetime of CO(2)RR electrolyzers, while also providing perspectives on future research directions. The aim is to connect the current status of ionomer and membrane development with performance metrics analysis, offering insights for the advancement of commercially relevant low-temperature CO(2)RR electrolyzers.

Materials descriptors for advanced water dissociation catalysts in bipolar membranes

The voltage penalty driving water dissociation (WD) at high current density is a major obstacle in the commercialization of bipolar membrane (BPM) technology for energy devices. Here we show that three materials descriptors, that is, electrical conductivity, microscopic surface area and (nominal) surface-hydroxyl coverage, effectively control the kinetics of WD in BPMs. Using these descriptors and optimizing mass loading, we design new earth-abundant WD catalysts based on nanoparticle SnO2 synthesized at low temperature with high conductivity and hydroxyl coverage. These catalysts exhibit exceptional performance in a BPM electrolyser with low WD overvoltage (ηwd) of 100 ± 20 mV at 1.0 A cm-2. The new catalyst works equivalently well with hydrocarbon proton-exchange layers as it does with fluorocarbon-based Nafion, thus providing pathways to commercializing advanced BPMs for a broad array of electrolysis, fuel-cell and electrodialysis applications.

Local ionic transport enables selective PGM-free bipolar membrane electrode assembly

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.

Electrochemical CO2 Reduction in Acidic Electrolytes: Spectroscopic Evidence for Local pH Gradients

Inspired by recent advances in electrochemical CO2 reduction (CO2R) under acidic conditions, herein we leverage in situ spectroscopy to inform the optimization of CO2R at low pH. Using attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and fluorescent confocal laser scanning microscopy, we investigate the role that alkali cations (M+) play on electrochemical CO2R. This study hence provides important information related to the local electrode surface pH under bulk acidic conditions for CO2R, both in the presence and absence of an organic film layer, at variable [M+]. We show that in an acidic electrolyte, an appropriate current density can enable CO2R in the absence of metal cations. In situ local pH measurements suggest the local [H+] must be sufficiently depleted to promote H2O reduction as the competing reaction with CO2R. Incrementally incorporating [K+] leads to increases in the local pH that promotes CO2R but only at proton consumption rates sufficient to drive the pH up dramatically. Stark tuning measurements and analysis of surface water structure reveal no change in the electric field with [M+] and a desorption of interfacial water, indicating that improved CO2R performance is driven by suppression of H+ mass transport and modification of the interfacial solvation structure. In situ pH measurements confirm increasing local pH, and therefore decreased local [CO2], with [M+], motivating alternate means of modulating proton transport. We show that an organic film formed via in situ electrodeposition of an organic additive provides a means to achieve selective CO2R (FECO2R ∼ 65%) over hydrogen evolution reaction in the presence of strong acid (pH 1) and low cation concentrations (≤0.1 M) at both low and high current densities.

Trilayer Polymer Electrolytes Enable Carbon-Efficient CO2 to Multicarbon Product Conversion in Alkaline Electrolyzers

The electrochemical CO2 reduction reaction (CO2RR) is an appealing method for carbon utilization. Alkaline CO2 electrolyzers exhibit high CO2RR activity, low full-cell voltages, and cost-effectiveness. However, the issue of CO2 loss caused by (bi)carbonate formation leads to excessive energy consumption, rendering the process economically impractical. In this study, we propose a trilayer polymer electrolyte (TPE) comprising a perforated anion exchange membrane (PAEM) and a bipolar membrane (BPM) to facilitate alkaline CO2RR. This TPE enables the coexistence of high alkalinity near the catalyst surface and the H+ flux at the interface between the PAEM and the cation exchange layer (CEL) of the BPM, conditions favoring both CO2 reduction to multicarbon products and (bi)carbonate removal in KOH-fed membrane electrode assembly (MEA) reactors. As a result, we achieve a Faradaic efficiency (FE) of approximately 46 % for C2H4, corresponding to a C2+ FE of 64 % at 260 mA cm-2, with a CO2-to-C2H4 single-pass conversion (SPC) of approximately 32 % at 140 mA cm-2-nearly 1.3 times the limiting SPC in conventional AEM-MEA electrolyzers. Furthermore, coupling CO2 reduction with formaldehyde oxidation reaction (FOR) in the TPE-MEA electrolyzer reduces the full-cell voltage to 2.3 V at 100 mA cm-2 without compromising the C2H4 FE.

Toward Durable CO2 Electroreduction with Cu-Based Catalysts via Understanding Their Deactivation Modes

The technology of CO2 electrochemical reduction (CO2ER) provides a means to convert CO2, a waste greenhouse gas, into value-added chemicals. Copper is the most studied element that is capable of catalyzing CO2ER to obtain multicarbon products, such as ethylene, ethanol, acetate, etc., at an appreciable rate. Under the operating condition of CO2ER, the catalytic performance of Cu decays because of several factors that alters the surface properties of Cu. In this review, these factors that cause the degradation of Cu-based CO2ER catalysts are categorized into generalized deactivation modes, that are applicable to all electrocatalytic systems. The fundamental principles of each deactivation mode and the associated effects of each on Cu-based catalysts are discussed in detail. Structure- and composition-activity relationship developed from recent in situ/operando characterization studies are presented as evidence of related deactivation modes in operation. With the aim to address these deactivation modes, catalyst design and reaction environment engineering rationales are suggested. Finally, perspectives and remarks built upon the recent advances in CO2ER are provided in attempts to improve the durability of CO2ER catalysts.

Solid Electrolytes for Low-Temperature Carbon Dioxide Valorization: A Review

One of the most promising approaches to address the global challenge of climate change is electrochemical carbon capture and utilization. Solid electrolytes can play a crucial role in establishing a chemical-free pathway for the electrochemical capture of CO2. Furthermore, they can be applied in electrocatalytic CO2 reduction reactions (CO2RR) to increase carbon utilization, produce high-purity liquid chemicals, and advance hybrid electro-biosystems. This review article begins by covering the fundamentals and processes of electrochemical CO2 capture, emphasizing the advantages of utilizing solid electrolytes. Additionally, it highlights recent advancements in the use of the solid polymer electrolyte or solid electrolyte layer for the CO2RR with multiple functions. The review also explores avenues for future research to fully harness the potential of solid electrolytes, including the integration of CO2 capture and the CO2RR and performance assessment under realistic conditions. Finally, this review discusses future opportunities and challenges, aiming to contribute to the establishment of a green and sustainable society through electrochemical CO2 valorization.

Aryl ether-free polymer electrolytes for electrochemical and energy devices

Anion exchange polymers (AEPs) play a crucial role in green hydrogen production through anion exchange membrane water electrolysis. The chemical stability of AEPs is paramount for stable system operation in electrolysers and other electrochemical devices. Given the instability of aryl ether-containing AEPs under high pH conditions, recent research has focused on quaternized aryl ether-free variants. The primary goal of this review is to provide a greater depth of knowledge on the synthesis of aryl ether-free AEPs targeted for electrochemical devices. Synthetic pathways that yield polyaromatic AEPs include acid-catalysed polyhydroxyalkylation, metal-promoted coupling reactions, ionene synthesis via nucleophilic substitution, alkylation of polybenzimidazole, and Diels-Alder polymerization. Polyolefinic AEPs are prepared through addition polymerization, ring-opening metathesis, radiation grafting reactions, and anionic polymerization. Discussions cover structure-property-performance relationships of AEPs in fuel cells, redox flow batteries, and water and CO2 electrolysers, along with the current status of scale-up synthesis and commercialization.

Rational Design of Local Reaction Environment for Electrocatalytic Conversion of CO2 into Multicarbon Products

The electrocatalytic conversion of CO2 into multi-carbon (C2+) products provides an attractive route for storing intermittent renewable electricity as fuels and feedstocks with high energy densities. Although substantial progress has been made in selective electrosynthesis of C2+ products via engineering the catalyst, rational design of the local reaction environment in the vicinity of catalyst surface also acts as an effective approach for further enhancing the performance. Here, we discuss recent advances and pertinent challenges in the modulation of local reaction environment, encompassing local pH, the choice of the species and concentrations of cations and anions as well as local reactant/intermediate concentrations, for achieving high C2+ selectivity. In addition, mechanistic understanding in the effects of the local reaction environment is also discussed. Particularly, the important progress extracted from in situ and operando spectroscopy techniques provides insights into how local reaction environment affects C-C coupling and key intermediates formation that lead to reaction pathways toward a desired C2+ product. The possible future direction in understanding and engineering the local reaction environment is also provided.

Pure-Water-Fed Forward-Bias Bipolar Membrane CO2 Electrolyzer

Coupling renewable electricity to reduce carbon dioxide (CO2) electrochemically into carbon feedstocks offers a promising pathway to produce chemical fuels sustainably. While there has been success in developing materials and theory for CO2 reduction, the widespread deployment of CO2 electrolyzers has been hindered by challenges in the reactor design and operational stability due to CO2 crossover and (bi)carbonate salt precipitation. Herein, we design asymmetrical bipolar membranes assembled into a zero-gap CO2 electrolyzer fed with pure water, solving both challenges. By investigating and optimizing the anion-exchange-layer thickness, cathode differential pressure, and cell temperature, the forward-bias bipolar membrane CO2 electrolyzer achieves a CO faradic efficiency over 80% with a partial current density over 200 mA cm-2 at less than 3.0 V with negligible CO2 crossover. In addition, this electrolyzer achieves 0.61 and 2.1 mV h-1 decay rates at 150 and 300 mA cm-2 for 200 and 100 h, respectively. Postmortem analysis indicates that the deterioration of catalyst/polymer-electrolyte interfaces resulted from catalyst structural change, and ionomer degradation at reductive potential shows the decay mechanism. All these results point to the future research direction and show a promising pathway to deploy CO2 electrolyzers at scale for industrial applications.

Gas-Induced Structural Damages in Forward-Bias Bipolar Membrane CO2 Electrolysis Studied by Fast X-ray Tomography

Forward-bias bipolar membrane (BPM) CO2 coelectrolysis (CO2ELY) aims at overcoming the issues of salt precipitation and CO2 crossover in anion exchange membrane CO2ELY. Increasing the stability of BPM-CO2ELY is crucial for widespread application of the technique. In this study, we employ time-resolved X-ray tomographic microscopy to elucidate the structural dynamics that occur within the electrochemical cell during operation under various conditions. Using advanced image processing methods, including custom 4D machine learning segmentation, we can visualize and quantify damages in the membrane and anode catalyst layer (CL). We compare our results to a CO2 transport model and hypothesize gaseous CO2 as the cause of the observed damages. At any operation condition, CO2 is formed at the junction in the center of the BPM by recombination of carbonate ions. CO2 migrates to the anode by diffusion and goes into the gas phase at the interface of the membrane and anode CL. After sufficient CO2 accumulation and pressure buildup after only tens of minutes, small irreversible holes break into the CL distributed over the entire active area. Additionally, at higher current densities, the CO2 accumulation leads to membrane delamination at the BPM junction. Despite the clear degradation processes, we do not observe an obvious direct effect on the electrochemical performance. The poor stability of BPM-CO2ELY remains an open question.

Mn-doped Sequentially Electrodeposited Co-based Oxygen Evolution Catalyst for Efficient Anion Exchange Membrane Water Electrolysis

Designing high-performance and durable oxygen evolution reaction (OER) catalysts is important for green hydrogen production through anion exchange membrane water electrolysis (AEMWE). Herein, a series of Mn-doped Co-based OER catalysts supported on FeOxHy (FCMx) are presented to enhance the OER activity. Mn doping effectively reduces the size of the Co oxide particles, thereby augmenting the active surface area. Moreover, Mn doping induces the creation of oxygen vacancies, leading to an efficient structural conversion during the OER, which is confirmed via in situ Raman spectroscopy. Under optimal conditions, the catalyst exhibits an overpotential of 234.4 mV at 10 mA cm-2 and a Tafel slope of 37.2 mV dec-1 under half-cell conditions. The AEMWE single-cell system demonstrates a current density of 1560 mA cm-2 at 1.8 V at 60 °C with a degradation rate of 0.4 mV h-1 for 500 h at 500 mA cm-2. Our development of a robust OER catalyst represents notable progress in the field of nonprecious-metal water electrolysis, marking a step toward cost-effective green hydrogen production.

CO2 Electrolyzers

CO2 electrolyzers have progressed rapidly in energy efficiency and catalyst selectivity toward valuable chemical feedstocks and fuels, such as syngas, ethylene, ethanol, and methane. However, each component within these complex systems influences the overall performance, and the further advances needed to realize commercialization will require an approach that considers the whole process, with the electrochemical cell at the center. Beyond the cell boundaries, the electrolyzer must integrate with upstream CO2 feeds and downstream separation processes in a way that minimizes overall product energy intensity and presents viable use cases. Here we begin by describing upstream CO2 sources, their energy intensities, and impurities. We then focus on the cell, the most common CO2 electrolyzer system architectures, and each component within these systems. We evaluate the energy savings and the feasibility of alternative approaches including integration with CO2 capture, direct conversion of flue gas and two-step conversion via carbon monoxide. We evaluate pathways that minimize downstream separations and produce concentrated streams compatible with existing sectors. Applying this comprehensive upstream-to-downstream approach, we highlight the most promising routes, and outlook, for electrochemical CO2 reduction.

Backbone Engineering of Polymeric Catalysts for High-Performance CO2 Reduction in Bipolar Membrane Zero-Gap Electrolyzer

Bipolar membranes (BPMs) have emerged as a promising solution for mitigating CO2 losses, salt precipitation and high maintenance costs associated with the commonly used anion-exchange membrane electrode assembly for CO2 reduction reaction (CO2RR). However, the industrial implementation of BPM-based zero-gap electrolyzer is hampered by the poor CO2RR performance, largely attributed to the local acidic environment. Here, we report a backbone engineering strategy to improve the CO2RR performance of molecular catalysts in BPM-based zero-gap electrolyzers by covalently grafting cobalt tetraaminophthalocyanine onto a positively charged polyfluorene backbone (PF-CoTAPc). PF-CoTAPc shows a high acid tolerance in BPM electrode assembly (BPMEA), achieving a high FE of 82.6 % for CO at 100 mA/cm2 and a high CO2 utilization efficiency of 87.8 %. Notably, the CO2RR selectivity, carbon utilization efficiency and long-term stability of PF-CoTAPc in BPMEA outperform reported BPM systems. We attribute the enhancement to the stable cationic shield in the double layer and suppression of proton migration, ultimately inhibiting the undesired hydrogen evolution and improving the CO2RR selectivity. Techno-economic analysis shows the least energy consumption (957 kJ/mol) for the PF-CoTAPc catalyst in BPMEA. Our findings provide a viable strategy for designing efficient CO2RR catalysts in acidic environments.

Reactive Carbon Capture Enables CO2 Electrolysis with Liquid Feedstocks

ConspectusThe electrochemical reduction of carbon dioxide (CO2RR) is a promising strategy for mitigating global CO2 emissions while simultaneously yielding valuable chemicals and fuels, such as CO, HCOO-, and C2H4. This approach becomes especially appealing when integrated with surplus renewable electricity, as the ensuing production of fuels could facilitate the closure of the carbon cycle. Despite these advantages, the realization of industrial-scale electrolyzers fed with CO2 will be challenged by the substantial energy inputs required to isolate, pressurize, and purify CO2 prior to electrolysis.To address these challenges, we devised an electrolyzer capable of directly converting reactive carbon solutions (e.g., a bicarbonate-rich eluent that exits a carbon capture unit) into higher value products. This "reactive carbon electrolyzer" operates by reacting (bi)carbonate with acid generated within the electrolyzer to produce CO2 in situ, thereby facilitating CO2RR at the cathode. This approach eliminates the need for expensive CO2 recovery and compression steps, as the electrolyzer can then then coupled directly to the CO2 capture unit.This Account outlines our endeavors in developing this type of electrolyzer, focusing on the design and implementation of materials for electrocatalytic (bi)carbonate conversion. We highlight the necessity for a permeable cathode that allows the efficient transport of (bi)carbonate ions while maintaining a sufficiently high catalytic surface area. We address the importance of the supporting electrolyte, detailing how (bi)carbonate concentration, counter cations, and ionic impurities impact selectivity for products formed in the electrolyzer. We also catalog state-of-the-art performance metrics for reactive carbon electrolyzers (i.e., Faradaic efficiency, full cell voltage, CO2 utilization efficiency) and outline strategies to bridge the gap between these values and those required for commercial operation Collectively, these findings contribute to the ongoing efforts to realize industrial-scale electrochemical reactors for CO2 conversion, bringing us closer to a sustainable and closed-loop carbon cycle.

Designing and Engineering Atomically Dispersed Metal Catalysts for CO2 to CO Conversion: From Single to Dual Metal Sites

The electrochemical CO2 reduction reaction (CO2 RR) is a promising approach to achieving sustainable electrical-to-chemical energy conversion and storage while decarbonizing the emission-heavy industry. The carbon-supported, nitrogen-coordinated, and atomically dispersed metal sites are effective catalysts for CO generation due to their high activity, selectivity, and earth abundance. Here, we discuss progress, challenges, and opportunities for designing and engineering atomic metal catalysts from single to dual metal sites. Engineering single metal sites using a nitrogen-doped carbon model was highlighted to exclusively study the effect of carbon particle sizes, metal contents, and M-N bond structures in the form of MN4 moieties on catalytic activity and selectivity. The structure-property correlation was analyzed by combining experimental results with theoretical calculations to uncover the CO2 to CO conversion mechanisms. Furthermore, dual-metal site catalysts, inheriting the merits of single-metal sites, have emerged as a new frontier due to their potentially enhanced catalytic properties. Designing optimal dual metal site catalysts could offer additional sites to alter the surface adsorption to CO2 and various intermediates, thus breaking the scaling relationship limitation and activity-stability trade-off. The CO2 RR electrolysis in flow reactors was discussed to provide insights into the electrolyzer design with improved CO2 utilization, reaction kinetics, and mass transport.

Electrocatalytic CO2 Reduction to C2+ Products in Flow Cells

Electrocatalytic CO2 reduction into value-added fuels and chemicals by renewable electric energy is one of the important strategies to address global energy shortage and carbon emission. Though the classical H-type electrolytic cell can quickly screen high-efficiency catalysts, the low current density and limited CO2 mass transfer process essentially impede its industrial applications. The electrolytic cells based on electrolyte flow system (flow cells) have shown great potential for industrial devices, due to higher current density, improved local CO2 concentration, and better mass transfer efficiency. The design and optimization of flow cells are of great significance to further accelerate the industrialization of electrocatalytic CO2 reduction reaction (CO2 RR). In this review, the progress of flow cells for CO2 RR to C2+ products is concerned. Firstly, the main events in the development of the flow cells for CO2 RR are outlined. Second, the main design principles of CO2 RR to C2+ products, the architectures, and types of flow cells are summarized. Third, the main strategies for optimizing flow cells to generate C2+ products are reviewed in detail, including cathode, anode, ion exchange membrane, and electrolyte. Finally, the preliminary attempts, challenges, and the research prospects of flow cells for industrial CO2 RR toward C2+ products are discussed.

Electrocatalysis of nitrogen pollution: transforming nitrogen waste into high-value chemicals

On 16 June 2023, the United Nations Environment Programme highlighted the severity of nitrogen pollution faced by humans and called for joint action for sustainable nitrogen use. Excess nitrogenous waste (NW: NO, NO2, NO2-, NO3-, etc.) mainly arises from the use of synthetic fertilisers, wastewater discharge, and fossil fuel combustion. Although the amount of NW produced can be minimised by reducing the use of nitrogen fertilisers and fossil fuels, the necessity to feed seven billion people on Earth limits the utility of this approach. Compared to current industrial processes, electrocatalytic NW reduction or CO2-NW co-reduction offers a potentially greener alternative for recycling NW and producing high-value chemicals. However, upgrading this technology to connect upstream and downstream industrial chains is challenging. This viewpoint focuses on electrocatalytic NW reduction, a cutting-edge technology, and highlights the challenges in its practical application. It also discusses future directions to meet the requirements of upstream and downstream industries by optimising production processes, including the pretreatment and supply of nitrogenous raw materials (e.g. flue gas and sewage), design and macroscopic preparation of electrocatalysts, and upscaling of reactors and other auxiliary equipment.

Protocol for assembling and operating bipolar membrane water electrolyzers

Renewable energy-driven bipolar membrane water electrolyzers (BPMWEs) are a promising technology for sustainable production of hydrogen from seawater and other impure water sources. Here, we present a protocol for assembling BPMWEs and operating them in a range of water feedstocks, including ultra-pure deionized water and seawater. We describe steps for membrane electrode assembly preparation, electrolyzer assembly, and electrochemical evaluation. For complete details on the use and execution of this protocol, please refer to Marin et al. (2023).1.

Advanced Catalyst Design and Reactor Configuration Upgrade in Electrochemical Carbon Dioxide Conversion

Electrochemical carbon dioxide reduction reaction (CO2 RR) driven by renewable energy shows great promise in mitigating and potentially reversing the devastating effects of anthropogenic climate change and environmental degradation. The simultaneous synthesis of energy-dense chemicals can meet global energy demand while decoupling emissions from economic growth. However, the development of CO2 RR technology faces challenges in catalyst discovery and device optimization that hinder their industrial implementation. In this contribution, a comprehensive overview of the current state of CO2 RR research is provided, starting with the background and motivation for this technology, followed by the fundamentals and evaluated metrics. Then the underlying design principles of electrocatalysts are discussed, emphasizing their structure-performance correlations and advanced electrochemical assembly cells that can increase CO2 RR selectivity and throughput. Finally, the review looks to the future and identifies opportunities for innovation in mechanism discovery, material screening strategies, and device assemblies to move toward a carbon-neutral society.

Accelerating CO2 electrochemical conversion towards industrial implementation

Despite significant progress in CO2 conversion field, there remains a significant gap between fundamental research and the industrial demands. This Comment discusses key performance parameters for industrial applications and outlines current limitations in the field.

Microenvironment Regulation Strategies Facilitating High-Efficiency CO2 Electrolysis in a Zero-Gap Bipolar Membrane Electrolyzer

In alkaline and neutral zero-gap CO2 electrolyzers, the carbon utilization efficiency of the electrocatalytic CO2 reduction to CO is less than 50% because of inherently homogeneous reactions. Utilization of the bipolar membrane (BPM) electrolyzer can effectively suppress (bi)carbonate formation and parasitic CO2 losses; however, an excessive concentration of H+ in the catalyst layer (CL) significantly hinders the activity and selectivity for CO2 reduction. Here, we report a microenvironment regulation strategy that controls the CL thickness and ionomer content to regulate local CO2 transport and the local pH within the CL. We report 80% faradaic efficiency of CO at a current density of 400 mA/cm2 without the use of a buffering layer, exceeding that of state-of-the-art catalysts with a buffering layer. A carbon utilization efficiency of 63.6% at 400 mA/cm2 is also obtained. This study demonstrates the significance of regulating the microenvironment of the CL in a BPM system.

Oxidation Chemistry of Bicarbonate and Peroxybicarbonate: Implications for Carbonate Management in Energy Storage

Carbonate formation presents a major challenge to energy storage applications based on low-temperature CO2 electrolysis and recyclable metal-air batteries. While direct electrochemical oxidation of (bi)carbonate represents a straightforward route for carbonate management, knowledge of the feasibility and mechanisms of direct oxidation is presently lacking. Herein, we report the isolation and characterization of the bis(triphenylphosphine)iminium salts of bicarbonate and peroxybicarbonate, thus enabling the examination of their oxidation chemistry. Infrared spectroelectrochemistry combined with time-resolved infrared spectroscopy reveals that the photoinduced oxidation of HCO3- by an Ir(III) photoreagent results in the generation of the short-lived bicarbonate radical in less than 50 ns. The highly acidic bicarbonate radical undergoes proton transfer with HCO3- to furnish the carbonate radical anion and H2CO3, leading to the eventual release of CO2 and H2O, thus accounting for the appearance of H2O and CO2 in both electrochemical and photochemical oxidation experiments. The back reaction of the carbonate radical subsequently oxidizes the Ir(II) photoreagent, leading to carbonate. In the absence of this back reaction, dimerization of the carbonate radical provides entry into peroxybicarbonate, which we show undergoes facile oxidation to O2 and CO2. Together, the results reported identify tangible pathways for the design of catalysts for the management of carbonate in energy storage applications.

Modular Solar-to-Fuel Electrolysis at Low Cell Potentials Enabled by Glycerol Electrooxidation and a Bipolar Membrane Separator

Solar fuel generation through water electrolysis or electrochemical CO2 reduction is thermodynamically limited when it is paired with oxygen evolution reaction (OER). Glycerol electrooxidation reaction (GEOR) is an alternative anodic reaction with lower anodic electrochemical potential that utilizes a renewable coproduct produced during biodiesel synthesis. We show that GEOR on an Au-Pt-Bi ternary metal electrocatalyst in a model alkaline crude glycerol solution can provide significant cell potential reductions even when paired to reduction reactions in seawater and acidic catholytes via a bipolar membrane (BPM). We showed that the combination of GEOR and a BPM separator lowers the total cell potential by 1 V at an electrolysis current of 10.0 mA cm-2 versus an anode performing anode's OER when paired with hydrogen evolution and CO2 reduction cathodes. The observed voltage reduction was steady for periods of up to 80 h, with minimal glycerol crossover observed through the membrane. These results motivate new, high-performance cell designs for photoelectrochemical solar fuel integrated systems based on glycerol electrooxidation.

Multiscale CO2 Electrocatalysis to C2+ Products: Reaction Mechanisms, Catalyst Design, and Device Fabrication

Electrosynthesis of value-added chemicals, directly from CO2, could foster achievement of carbon neutral through an alternative electrical approach to the energy-intensive thermochemical industry for carbon utilization. Progress in this area, based on electrogeneration of multicarbon products through CO2 electroreduction, however, lags far behind that for C1 products. Reaction routes are complicated and kinetics are slow with scale up to the high levels required for commercialization, posing significant problems. In this review, we identify and summarize state-of-art progress in multicarbon synthesis with a multiscale perspective and discuss current hurdles to be resolved for multicarbon generation from CO2 reduction including atomistic mechanisms, nanoscale electrocatalysts, microscale electrodes, and macroscale electrolyzers with guidelines for future research. The review ends with a cross-scale perspective that links discrepancies between different approaches with extensions to performance and stability issues that arise from extensions to an industrial environment.

Maximizing single-pass conversion does not result in practical readiness for CO2 reduction electrolyzers

The authors comment that maximizing product concentration is a more meaningful target for CO2 electrolyzers than maximizing single-pass conversion.

Bridging Trans-Scale Electrode Engineering for Mass CO2 Electrolysis

Electrochemical CO2 upgrade offers an artificial route for carbon recycling and neutralization, while its widespread implementation relies heavily on the simultaneous enhancement of mass transfer and reaction kinetics to achieve industrial conversion rates. Nevertheless, such a multiscale challenge calls for trans-scale electrode engineering. Herein, three scales are highlighted to disclose the key factors of CO2 electrolysis, including triple-phase boundaries, reaction microenvironment, and catalytic surface coordination. Furthermore, the advanced types of electrolyzers with various electrode design strategies are surveyed and compared to guide the system architectures for continuous conversion. We further offer an outlook on challenges and opportunities for the grand-scale application of CO2 electrolysis. Hence, this comprehensive Perspective bridges the gaps between electrode research and CO2 electrolysis practices. It contributes to facilitating the mixed reaction and mass transfer process, ultimately enabling the on-site recycling of CO2 emissions from industrial plants and achieving net negative emissions.

Phase-Inversion Induced 3D Electrode for Direct Acidic Electroreduction CO2 to Formic acid

Direct electrochemical CO₂ reduction to formic acid (FA) instead of formate is a challenging task due to the high acidity of FA and competitive hydrogen evolution reaction. Herein, 3D porous electrode (TDPE) is prepared by a simple phase inversion method, which can electrochemically reduce CO₂ to FA in acidic conditions. Owing to interconnected channels, high porosity, and appropriate wettability, TDPE not only improves mass transport, but also realizes pH gradient to build higher local pH micro-environment under acidic conditions for CO₂ reduction compared with planar electrode and gas diffusion electrode. Kinetic isotopic effect experiments demonstrate that the proton transfer becomes the rate-determining step at the pH of 1.8; however, not significant in neutral solution, suggesting that the proton is aiding the overall kinetics. Maximum FA Faradaic efficiency of 89.2% has been reached at pH 2.7 in a flow cell, generating FA concentration of 0.1 m. Integrating catalyst and gas-liquid partition layer into a single electrode structure by phase inversion method paves a facile avenue for direct production of FA by electrochemical CO₂ reduction.

Confinement of an alkaline environment for electrocatalytic CO2 reduction in acidic electrolytes

Acidic electrochemical CO₂ reduction reaction (CO₂RR) can minimize carbonate formation and eliminate CO₂ crossover, thereby improving long-term stability and enhancing single-pass carbon efficiency (SPCE). However, the kinetically favored hydrogen evolution reaction (HER) is generally predominant under acidic conditions. This paper describes the confinement of a local alkaline environment for efficient CO₂RR in a strongly acidic electrolyte through the manipulation of mass transfer processes in well-designed hollow-structured Ag@C electrocatalysts. A high faradaic efficiency of over 95% at a current density of 300 mA cm^-2 and an SPCE of 46.2% at a CO₂ flow rate of 2 standard cubic centimeters per minute are achieved in the acidic electrolyte, with enhanced stability compared to that under alkaline conditions. Computational modeling results reveal that the unique structure of Ag@C could regulate the diffusion process of OH^- and H⁺, confining a high-pH local reaction environment for the promoted activity. This work presents a promising route to engineer the microenvironment through the regulation of mass transport that permits the CO₂RR in acidic electrolytes with high performance.

Organic Non-Nucleophilic Electrolyte Resists Carbonation during Selective CO2 Electroreduction

The spontaneous reaction of CO₂ with water and hydroxide to form (bi)carbonates in alkaline aqueous electrolytes compromises the energy and carbon efficiency of CO₂ electrolyzers. We hypothesized that electrolyte carbonation could be mitigated by operating the reaction in an aprotic solvent with low water content, while also employing an exogenous non-nucleophilic acid as the proton donor to prevent parasitic capture of CO₂ by its conjugate base. However, it is unclear whether such an electrolyte design could simultaneously engender high CO₂ reduction selectivity and low electrolyte carbonation. We herein report selective CO₂ electroreduction with low carbonate formation on a polycrystalline Au catalyst using dimethyl sulfoxide as the solvent and acetic acid/acetate as the proton-donating medium. CO₂ is reduced to CO with over 90% faradaic efficiency at potentials relative to the reversible hydrogen electrode that are comparable to those in neutral aqueous electrolytes. ¹H and ¹³C NMR studies demonstrate that only millimolar concentrations of bicarbonates are reversibly formed, that the proton activity of the medium is largely unaffected by exposure to CO₂, and that low carbonation is maintained upon addition of 1 M water. This work demonstrates that electrolyte carbonation can be attenuated and decoupled from efficient CO₂ reduction in an aprotic solvent, offering new electrolyte design principles for low-temperature CO₂ electroreduction systems.

Coupling Value-Added Anodic Reactions with Electrocatalytic CO2 Reduction

Electrocatalytic CO₂ reduction features a promising approach to realize carbon neutrality. However, its competitiveness is limited by the sluggish oxygen evolution reaction (OER) at anode, which consumes a large portion of energy. Coupling value-added anodic reactions with CO₂ electroreduction has been emerging as a promising strategy in recent years to enhance the full-cell energy efficiency and produce valuable chemicals at both cathode and anode of the electrolyzer. This review briefly summarizes recent progresses on the electrocatalytic CO₂ reduction, and the economic feasibility of different CO₂ electrolysis systems is discussed. Then a comprehensive summary of recent advances in the coupled electrolysis of CO₂ and potential value-added anodic reactions is provided, with special focus on the specific cell designs. Finally, current challenges and future opportunities for the coupled electrolysis systems are proposed, which are targeted to facilitate progress in this field and push the CO₂ electrolyzers to a more practical level.

Strategies to Enhance CO2 Electrochemical Reduction from Reactive Carbon Solutions

CO₂ electrochemical reduction (CO₂ ER) from (bi)carbonate feed presents an opportunity to efficiently couple this process to alkaline-based carbon capture systems. Likewise, while this method of reducing CO₂ currently lags behind CO₂ gas-fed electrolysers in certain performance metrics, it offers a significant improvement in CO₂ utilization which makes the method worth exploring. This paper presents two simple modifications to a bicarbonate-fed CO₂ ER system that enhance the selectivity towards CO. Specifically, a modified hydrophilic cathode with Ag catalyst loaded through electrodeposition and the addition of dodecyltrimethylammonium bromide (DTAB), a low-cost surfactant, to the catholyte enabled the system to achieve a FE_CO of 85% and 73% at 100 and 200 mA·cm^-2, respectively. The modifications were tested in 4 h long experiments where DTAB helped maintain FE_CO stable even when the pH of the catholyte became more alkaline, and it improved the CO² utilization compared to a system without DTAB.

Neutralization Short-Circuiting with Weak Electrolytes Erodes the Efficiency of Bipolar Membranes

Bipolar membranes (BPMs) are critical components of a variety of electrochemical energy technologies. Many electrochemical applications require the use of buffers to maintain stable, nonextreme pH environments, yet the impact of buffers or weak acids/bases on the electrochemical behavior of BPMs remains poorly understood. Our data for a cell containing weak electrolytes is consistent with internal pH gradients within the anion exchange membrane (AEM) or cation exchange membrane (CEM) component of the BPM that form via ionic short-circuiting processes at open-circuit. Short-circuiting results from the coupling of co-ion crossover and parasitic neutralization and leads to buffering of the bipolar interface. This phenomenon, which we term neutralization short-circuiting, serves to erode BPM efficiency by attenuating the open-circuit membrane voltage and introducing parasitic reverse bias currents associated with weak acid/base dissociation at the interface. These findings establish a mechanistic basis for the operation of BPM cells in the presence of weak acid/base electrolytes.

Enriching Surface-Accessible CO2 in the Zero-Gap Anion-Exchange-Membrane-Based CO2 Electrolyzer

Zero-gap anion exchange membrane (AEM)-based CO₂ electrolysis is a promising technology for CO production, however, their performance at elevated current densities still suffers from the low local CO₂ concentration due to heavy CO₂ neutralization. Herein, via modulating the CO₂ feed mode and quantitative analyzing CO₂ utilization with the aid of mass transport modeling, we develop a descriptor denoted as the surface-accessible CO₂ concentration ([CO₂ ]^SA ), which enables us to indicate the transient state of the local [CO₂ ]/[OH^- ] ratio and helps define the limits of CO₂ -to-CO conversion. To enrich the [CO₂ ]^SA , we developed three general strategies: (1) increasing catalyst layer thickness, (2) elevating CO₂ pressure, and (3) applying a pulsed electrochemical (PE) method. Notably, an optimized PE method allows to keep the [CO₂ ]^SA at a high level by utilizing the dynamic balance period of CO₂ neutralization. A maximum j_CO of 368±28 mA cm_geo ^-2 was achieved using a commercial silver catalyst.

Zero-Gap Electrochemical CO2 Reduction Cells: Challenges and Operational Strategies for Prevention of Salt Precipitation

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.

Regulating Spin States in Oxygen Electrocatalysis

Developing efficient and stable transition metal oxides catalysts for energy conversion processes such as oxygen evolution reaction and oxygen reduction reaction is one of the key measures to solve the problem of energy shortage. The spin state of transition metal oxides is strongly correlated with their catalytic activities. In an octahedral structure of transition metal oxides, the spin state of active centers could be regulated by adjusting the splitting energy and the electron pairing energy. Regulating spin state of active centers could directly modulate the d orbitals occupancy, which influence the strength of metal-ligand bonds and the adsorption behavior of the intermediates. In this review, we clarified the significance of regulating spin state of the active centers. Subsequently, we discussed several characterization technologies for spin state and some recent strategies to regulate the spin state of the active centers. Finally, we put forward some views on the future research direction of this vital field.

Membrane electrode assembly design to prevent CO2 crossover in CO2 reduction reaction electrolysis

To reach a net-zero energy economy by 2050, it is critical to develop negative emission technologies, such as CO₂ reduction electrolyzers, but these devices still suffer from various issues including low utilization of CO₂ because of its cross-over from the cathode to the anode. This comment highlights the recent innovative design of membrane electrode assembly, utilizing a bipolar membrane and catholyte layer that blocks CO₂ cross-over and enables high CO₂ single-pass utilization.