Improving the efficiency of CO2 electrolysis by using a bipolar membrane with a weak-acid cation exchange layer

Enhancing photocatalytic CO2 reduction with TiO2-based materials: Strategies, mechanisms, challenges, and perspectives

The concentration of atmospheric CO2 has exceeded 400 ppm, surpassing its natural variability and raising concerns about uncontrollable shifts in the carbon cycle, leading to significant climate and environmental impacts. A promising method to balance carbon levels and mitigate atmospheric CO2 rise is through photocatalytic CO2 reduction. Titanium dioxide (TiO2), renowned for its affordability, stability, availability, and eco-friendliness, stands out as an exemplary catalyst in photocatalytic CO2 reduction. Various strategies have been proposed to modify TiO2 for photocatalytic CO2 reduction and improve catalytic activity and product selectivity. However, few studies have systematically summarized these strategies and analyzed their advantages, disadvantages, and current progress. Here, we comprehensively review recent advancements in TiO2 engineering, focusing on crystal engineering, interface design, and reactive site construction to enhance photocatalytic efficiency and product selectivity. We discuss how modifications in TiO2's optical characteristics, carrier migration, and active site design have led to varied and selective CO2 reduction products. These enhancements are thoroughly analyzed through experimental data and theoretical calculations. Additionally, we identify current challenges and suggest future research directions, emphasizing the role of TiO2-based materials in understanding photocatalytic CO2 reduction mechanisms and in designing effective catalysts. This review is expected to contribute to the global pursuit of carbon neutrality by providing foundational insights into the mechanisms of photocatalytic CO2 reduction with TiO2-based materials and guiding the development of efficient photocatalysts.

Reaction Environment Regulation for Electrocatalytic CO2 Reduction in Acids

The electrocatalytic CO2 reduction reaction (CO2RR) is a sustainable route for converting CO2 into value-added fuels and feedstocks, advancing a carbon-neutral economy. The electrolyte critically influences CO2 utilization, reaction rate and product selectivity. While typically conducted in neutral/alkaline aqueous electrolytes, the CO2RR faces challenges due to (bi)carbonate formation and its crossover to the anolyte, reducing efficiency and stability. Acidic media offer promise by suppressing these processes, but the low Faradaic efficiency, especially for multicarbon (C2+) products, and poor electrocatalyst stability persist. The effective regulation of the reaction environment at the cathode is essential to favor the CO2RR over the competitive hydrogen evolution reaction (HER) and improve long-term stability. This review examines progress in the acidic CO2RR, focusing on reaction environment regulation strategies such as electrocatalyst design, electrode modification and electrolyte engineering to promote the CO2RR. Insights into the reaction mechanisms via in situ/operando techniques and theoretical calculations are discussed, along with critical challenges and future directions in acidic CO2RR technology, offering guidance for developing practical systems for the carbon-neutral community.

In situ/Operando Synchrotron Radiation Analytical Techniques for CO2/CO Reduction Reaction: From Atomic Scales to Mesoscales

Electrocatalytic carbon dioxide/carbon monoxide reduction reaction (CO(2)RR) has emerged as a prospective and appealing strategy to realize carbon neutrality for manufacturing sustainable chemical products. Developing highly active electrocatalysts and stable devices has been demonstrated as effective approach to enhance the conversion efficiency of CO(2)RR. In order to rationally design electrocatalysts and devices, a comprehensive understanding of the intrinsic structure evolution within catalysts and micro-environment change around electrode interface, particularly under operation conditions, is indispensable. Synchrotron radiation has been recognized as a versatile characterization platform, garnering widespread attention owing to its high brightness, elevated flux, excellent directivity, strong polarization and exceptional stability. This review systematically introduces the applications of synchrotron radiation technologies classified by radiation sources with varying wavelengths in CO(2)RR. By virtue of in situ/operando synchrotron radiationanalytical techniques, we also summarize relevant dynamic evolution processes from electronic structure, atomic configuration, molecular adsorption, crystal lattice and devices, spanning scales from the angstrom to the micrometer. The merits and limitations of diverse synchrotron characterization techniques are summarized, and their applicable scenarios in CO(2)RR are further presented. On the basis of the state-of-the-art fourth-generation synchrotron facilities, a perspective for further deeper understanding of the CO(2)RR process using synchrotron radiation analytical techniques is proposed.

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.

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.

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.

Stabilizing *CO2 Intermediates at the Acidic Interface using Molecularly Dispersed Cobalt Phthalocyanine as Catalysts for CO2 Reduction

CO2 electroreduction (CO2 R) operating in acidic media circumvents the problems of carbonate formation and CO2 crossover in neutral/alkaline electrolyzers. Alkali cations have been universally recognized as indispensable components for acidic CO2 R, while they cause the inevitable issue of salt precipitation. It is therefore desirable to realize alkali-cation-free CO2 R in pure acid. However, without alkali cations, stabilizing *CO2 intermediates by catalyst itself at the acidic interface poses as a challenge. Herein, we first demonstrate that a carbon nanotube-supported molecularly dispersed cobalt phthalocyanine (CoPc@CNT) catalyst provides the Co single-atom active site with energetically localized d states to strengthen the adsorbate-surface interactions, which stabilizes *CO2 intermediates at the acidic interface (pH=1). As a result, we realize CO2 conversion to CO in pure acid with a faradaic efficiency of 60 % at pH=2 in flow cell. Furthermore, CO2 is successfully converted in cation exchanged membrane-based electrode assembly with a faradaic efficiency of 73 %. For CoPc@CNT, acidic conditions also promote the intrinsic activity of CO2 R compared to alkaline conditions, since the potential-limiting step, *CO2 to *COOH, is pH-dependent. This work provides a new understanding for the stabilization of reaction intermediates and facilitates the designs of catalysts and devices for acidic CO2 R.

Acidic CO2 electroreduction for high CO2 utilization: catalysts, electrodes, and electrolyzers

The electrochemical carbon dioxide (CO2) reduction reaction (CO2RR) is considered a promising technology for converting atmospheric CO2 into value-added compounds by utilizing renewable energy. The CO2RR has developed in various ways over the past few decades, including product selectivity, current density, and catalytic stability. However, its commercialization is still unsuitable in terms of economic feasibility. One of the major challenges in its commercialization is the low single-pass conversion efficiency (SPCE) of CO2, which is primarily caused by the formation of carbonate (CO32-) in neutral and alkaline electrolytes. Notably, the majority of CO2RRs take place in such media, necessitating significant energy input for CO2 regeneration. Therefore, performing the CO2RR under conditions that minimize CO32- formation to suppress reactant and electrolyte ion loss is regarded an optimal strategy for practical applications. Here, we introduce the recent progress and perspectives in the electrochemical CO2RR in acidic electrolytes, which receives great attention because of the inhibition of CO32- formation. This includes the categories of nanoscale catalytic design, microscale microenvironmental effects, and bulk scale applications in electrolyzers for zero carbon loss reactions. Additionally, we offer insights into the issue of limited catalytic durability, a notable drawback under acidic conditions and propose guidelines for further development of the acidic CO2RR.

Durable CO2 conversion in the proton-exchange membrane system

Electrolysis that reduces carbon dioxide (CO2) to useful chemicals can, in principle, contribute to a more sustainable and carbon-neutral future1-6. However, it remains challenging to develop this into a robust process because efficient conversion typically requires alkaline conditions in which CO2 precipitates as carbonate, and this limits carbon utilization and the stability of the system7-12. Strategies such as physical washing, pulsed operation and the use of dipolar membranes can partially alleviate these problems but do not fully resolve them11,13-15. CO2 electrolysis in acid electrolyte, where carbonate does not form, has therefore been explored as an ultimately more workable solution16-18. Herein we develop a proton-exchange membrane system that reduces CO2 to formic acid at a catalyst that is derived from waste lead-acid batteries and in which a lattice carbon activation mechanism contributes. When coupling CO2 reduction with hydrogen oxidation, formic acid is produced with over 93% Faradaic efficiency. The system is compatible with start-up/shut-down processes, achieves nearly 91% single-pass conversion efficiency for CO2 at a current density of 600 mA cm-2 and cell voltage of 2.2 V and is shown to operate continuously for more than 5,200 h. We expect that this exceptional performance, enabled by the use of a robust and efficient catalyst, stable three-phase interface and durable membrane, will help advance the development of carbon-neutral technologies.

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.

A Bipolar Membrane-Integrated Electrochlorination Process for Highly Efficient Ammonium Removal in Mature Landfill Leachate: The Importance of ClO• Generation

Electrochemical oxidation has been demonstrated to be a useful method for removing biorefractory organic pollutants in mature landfill leachate but suffers from low efficiency in eliminating ammonium because of its resistance to being oxidized by HO• or free chlorine (FC) at decreased pH. Here, we propose a new bipolar membrane-electrochlorination (BPM-EC) process to address this issue. We found that the BPM-EC system was significantly superior to both the undivided and divided reactors with monopolar membranes in terms of elevated rate of ammonium removal, attenuated generation of byproducts (e.g., nitrate and chloramines), increased Faradaic efficiency, and decreased energy consumption. Mechanistic studies revealed that the integration of BPM was helpful in creating alkaline environments in the vicinity of the anode, which facilitated production of surface-bound HO• and FC and eventually promoted in situ generation of ClO•, a crucial reactive species mainly responsible for accelerating ammonium oxidation and selective transformation to nitrogen. The efficacy of BPM-EC in treating landfill leachates with different ammonium concentrations was verified under batch and continuous-flow conditions. A kinetic model that incorporates the key parameters was developed, which can successfully predict the optimal number of BPM-EC reactors (e.g., 2 and 5 for leachates containing 589.4 ± 5.5 and 1258.1 ± 9.6 mg L-1 NH4+-N, respectively) necessary for complete removal of ammonium. These findings reveal that the BPM-EC process shows promise in treating ammonium-containing wastewater, with advantages that include effectiveness, adaptability, and flexibility.

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.

Boosting the Proton-coupled Electron Transfer via Fe-P Atomic Pair for Enhanced Electrochemical CO2 Reduction

Single-atom catalysts exhibit superior CO2 -to-CO catalytic activity, but poor kinetics of proton-coupled electron transfer (PCET) steps still limit the overall performance toward the industrial scale. Here, we constructed a Fe-P atom paired catalyst onto nitrogen doped graphitic layer (Fe1 /PNG) to accelerate PCET step. Fe1 /PNG delivers an industrial CO current of 1 A with FECO over 90 % at 2.5 V in a membrane-electrode assembly, overperforming the CO current of Fe1 /NG by more than 300 %. We also decrypted the synergistic effects of the P atom in the Fe-P atom pair using operando techniques and density functional theory, revealing that the P atom provides additional adsorption sites for accelerating water dissociation, boosting the hydrogenation of CO2 , and enhancing the activity of CO2 reduction. This atom-pair catalytic strategy can modulate multiple reactants and intermediates to break through the inherent limitations of single-atom catalysts.

Probing Cation Effects on *CO Intermediates from Electroreduction of CO2 through Operando Raman Spectroscopy

Cations in an electrolyte modulate microenvironments near the catalyst surface and affect product distribution from an electrochemical CO2 reduction reaction, and thus, their interaction with intermediate states has been tried to be probed. Herein, we directly observed the cation effect on *CO intermediates on the Cu(OH)2-derived catalyst in real time through operando surface-enhanced Raman spectroscopy at high overpotentials (-1.0 VRHE). Atop *CO peaks are composed of low-frequency binding *CO (*COLFB) and high-frequency binding *CO (*COHFB) because of their adsorption sites. These two *CO intermediates are found to have different sensitivities to the cation-induced field, and each *CO is proposed to be suitably stabilized for efficient C-C coupling. The proportions between *COHFB and *COLFB are dependent on the type of alkali cations, and the increases in the *COHFB ratio have a high correlation with selective C2H4 production under K+ and Cs+, indicating that *COHFB is the dominant and fast active species. In addition, as the hydrated cation size decreases, *COLFB is more sensitively red-shifted than *COHFB, which promotes C-C coupling and suppresses C1 products. Through time-resolved operando measurements, dynamic changes between the two *CO species are observed, showing the rapid initial adsorption of *COHFB and subsequently reaching a steady ratio between *COLFB and *COHFB.

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.

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.

Acidic CO2 Electrolysis Addressing the "Alkalinity Issue" and Achieving High CO2 Utilization

Electrochemical CO2 reduction reaction (CO2 RR) provides a promising approach for sustainable chemical fuel production of carbon neutrality. Neutral and alkaline electrolytes are predominantly employed in the current electrolysis system, but with striking drawbacks of (bi)carbonate (CO3 2- /HCO3 - ) formation and crossover due to the rapid and thermodynamically favourable reaction between hydroxide (OH- ) with CO2 , resulting in low carbon utilization efficiency and short-lived catalysis. Very recently, CO2 RR in acidic media can effectively address the (bi)carbonate issue; however, the competing hydrogen evolution reaction (HER) is more kinetically favourable in acidic electrolytes, which dramatically reduces CO2 conversion efficiency. Thus, it is a big challenge to effectively suppress HER and accelerate acidic CO2 RR. In this review, we begin by summarizing the recent progress of acidic CO2 electrolysis, discussing the key factors limiting the application of acidic electrolytes. We then systematically discuss addressing strategies for acidic CO2 electrolysis, including electrolyte microenvironment modulation, alkali cations adjusting, surface/interface functionalization, nanoconfinement structural design, and novel electrolyzer exploitation. Finally, the new challenges and perspectives of acidic CO2 electrolysis are suggested. We believe this timely review can arouse researchers' attention to CO2 crossover, inspire new insights to solve the "alkalinity problem" and enable CO2 RR as a more sustainable technology.

Integrated CO2 capture and electrochemical upgradation: the underpinning mechanism and techno-chemical analysis

Coupling post-combustion CO2 capture with electrochemical utilization (CCU) is a quantum leap in renewable energy science since it eliminates the cost and energy involved in the transport and storage of CO2. However, the major challenges involved in industrial scale implementation are selecting an appropriate solvent/electrolyte for CO2 capture, modeling an appropriate infrastructure by coupling an electrolyser with a CO2 point source and a separator to isolate CO2 reduction reaction (CO2RR) products, and finally selection of an appropriate electrocatalyst. In this review, we highlight the major difficulties with detailed mechanistic interpretation in each step, to find out the underpinning mechanism involved in the integration of electrochemical CCU to achieve higher-value products. In the past decades, most of the studies dealt with individual parts of the integration process, i.e., either selecting a solvent for CO2 capture, designing an electrocatalyst, or choosing an ideal electrolyte. In this context, it is important to note that solvents such as monoethanolamine, bicarbonate, and ionic liquids are often used as electrolytes in CO2 capture media. Therefore, it is essential to fabricate a cost-effective electrolyser that should function as a reversible binder with CO2 and an electron pool capable of recovering the solvent to electrolyte reversibly. For example, reversible ionic liquids, which are non-ionic in their normal forms, but produce ionic forms after CO2 capture, can be further reverted back to their original non-ionic forms after CO2 release with almost 100% efficiency through the chemical or thermal modulations. This review also sheds light on a focused techno-economic evolution for converting the electrochemically integrated CCU process from a pilot-scale project to industrial-scale implementation. In brief, this review article will summarize a state-of-the-art argumentation of challenges and outcomes over the different segments involved in electrochemically integrated CCU to stimulate urgent progress in the field.

Direct Imaging of Local pH Reveals Bubble-Induced Mixing in a CO2 Electrolyzer

Electrochemical CO2 reduction poses a promising pathway to produce hydrocarbon chemicals and fuels without relying on fossil fuels. Gas diffusion electrodes allow high selectivity for desired carbon products at high current density by ensuring a sufficient CO2 mass transfer rate to the catalyst layer. In addition to CO2 mass transfer, the product selectivity also strongly depends on the local pH at the catalyst surface. In this work, we directly visualize for the first time the two-dimensional (2D) pH profile in the catholyte channel of a gas-fed CO2 electrolyzer equipped with a bipolar membrane. The pH profile is imaged with operando fluorescence lifetime imaging microscopy (FLIM) using a pH-sensitive quinolinium-based dye. We demonstrate that bubble-induced mixing plays an important role in the Faradaic efficiency. Our concentration measurements show that the pH at the catalyst remains lower at -100 mA cm-2 than at -10 mA cm-2, implying that bubble-induced advection outweighs the additional OH- flux at these current densities. We also prove that the pH buffering effect of CO2 from the gas feed and dissolved CO2 in the catholyte prevents the gas diffusion electrode from becoming strongly alkaline. Our findings suggest that gas-fed CO2 electrolyzers with a bipolar membrane and a flowing catholyte are promising designs for scale-up and high-current-density operation because they are able to avoid extreme pH values in the catalyst layer.

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.

Organic-moiety-engineering on copper surface for carbon dioxide reduction

Electrochemical conversion of carbon dioxide (CO₂) into value-added products powered by sustainable electricity is considered as one of the most promising strategies for carbon neutrality. Among the products, hydrocarbons, especially ethylene and ethanol are the most desired species due to their wide industrial applications. Copper-based catalysts are currently the very limited option available for catalyzing the reduction of CO₂ to multi-carbon products. How to enhance the selectivity and current density is the focus in both academia and industry. In recent years, some organic molecules, oligomers and polymers with well-defined structures have been applied and demonstrated to be effective on enhancing electrocatalytic activity of copper catalysts. However, the molecular/copper interaction and CO₂ molecules' behavior at the hetero-interface remain unclear. In this review, we classify the different organic materials which have been applied in the field of electrochemical CO₂ reduction. We focus on the regulation of local microenvironment on the copper surface by organic compounds, including surface hydrophobicity, local electric field, local pH, and coverage of intermediates etc. The relationship between local microenvironment and catalytic activity is specifically discussed. This review could provide guidance for the development of more organic/inorganic hybrid catalysts for further promoting CO₂ reduction reaction.

Continuous ammonia electrosynthesis using physically interlocked bipolar membrane at 1000 mA cm-2

Electrosynthesis of ammonia from nitrate reduction receives extensive attention recently for its relatively mild conditions and clean energy requirements, while most existed electrochemical strategies can only deliver a low yield rate and short duration for the lack of stable ion exchange membranes at high current density. Here, a bipolar membrane nitrate reduction process is proposed to achieve ionic balance, and increasing water dissociation sites is delivered by constructing a three-dimensional physically interlocked interface for the bipolar membrane. This design simultaneously boosts ionic transfer and interfacial stability compared to traditional ones, successfully reducing transmembrane voltage to 1.13 V at up to current density of 1000 mA cm^-2. By combining a Co three-dimensional nanoarray cathode designed for large current and low concentration utilizations, a continuous and high yield bipolar membrane reactor for NH₃ electrosynthesis realized a stable electrolysis at 1000 mA cm^-2 for over 100 h, Faradaic efficiency of 86.2% and maximum yield rate of 68.4 mg h^-1 cm^-2 with merely 2000 ppm NO₃^- alkaline electrolyte. These results show promising potential for artificial nitrogen cycling in the near future.

Electricity-Driven Microbial Metabolism of Carbon and Nitrogen: A Waste-to-Resource Solution

Electricity-driven microbial metabolism relies on the extracellular electron transfer (EET) process between microbes and electrodes and provides promise for resource recovery from wastewater and industrial discharges. Over the past decades, tremendous efforts have been dedicated to designing electrocatalysts and microbes, as well as hybrid systems to push this approach toward industrial adoption. This paper summarizes these advances in order to facilitate a better understanding of electricity-driven microbial metabolism as a sustainable waste-to-resource solution. Quantitative comparisons of microbial electrosynthesis and abiotic electrosynthesis are made, and the strategy of electrocatalyst-assisted microbial electrosynthesis is critically discussed. Nitrogen recovery processes including microbial electrochemical N₂ fixation, electrocatalytic N₂ reduction, dissimilatory nitrate reduction to ammonium (DNRA), and abiotic electrochemical nitrate reduction to ammonia (Abio-NRA) are systematically reviewed. Furthermore, the synchronous metabolism of carbon and nitrogen using hybrid inorganic-biological systems is discussed, including advanced physicochemical, microbial, and electrochemical characterizations involved in this field. Finally, perspectives for future trends are presented. The paper provides valuable insights on the potential contribution of electricity-driven microbial valorization of waste carbon and nitrogen toward a green and sustainable society.

Coverage-driven selectivity switch from ethylene to acetate in high-rate CO2/CO electrolysis

Tuning catalyst microenvironments by electrolytes and organic modifications is effective in improving CO₂ electrolysis performance. An alternative way is to use mixed CO/CO₂ feeds from incomplete industrial combustion of fossil fuels, but its effect on catalyst microenvironments has been poorly understood. Here we investigate CO/CO₂ co-electrolysis over CuO nanosheets in an alkaline membrane electrode assembly electrolyser. With increasing CO pressure in the feed, the major product gradually switches from ethylene to acetate, attributed to the increased CO coverage and local pH. Under optimized conditions, the Faradaic efficiency and partial current density of multicarbon products reach 90.0% and 3.1 A cm^-2, corresponding to a carbon selectivity of 100.0% and yield of 75.0%, outperforming thermocatalytic CO hydrogenation. The scale-up demonstration using an electrolyser stack achieves the highest ethylene formation rate of 457.5 ml min^-1 at 150 A and acetate formation rate of 2.97 g min^-1 at 250 A.

Utilizing CO2 as a Reactant for C3 Oxygenate Production via Tandem Reactions

One possible solution to closing the loop on carbon emissions is using CO₂ as the carbon source to generate high-value, multicarbon products. In this Perspective, we describe four tandem reaction strategies for converting CO₂ into C₃ oxygenated hydrocarbon products (i.e., propanal and 1-propanol), using either ethane or water as the hydrogen source: (1) thermocatalytic CO₂-assisted dehydrogenation and reforming of ethane to ethylene, CO, and H₂, followed by heterogeneous hydroformylation, (2) one-pot conversion of CO₂ and ethane using plasma-activated reactions in combination with thermocatalysis, (3) electrochemical CO₂ reduction to ethylene, CO, and H₂, followed by thermocatalytic hydroformylation, and (4) electrochemical CO₂ reduction to CO, followed by electrochemical CO reduction to C₃ oxygenates. We discuss the proof-of-concept results and key challenges for each tandem scheme, and we conduct a comparative analysis of the energy costs and prospects for net CO₂ reduction. The use of tandem reaction systems can provide an alternative approach to traditional catalytic processes, and these concepts can be further extended to other chemical reactions and products, thereby opening new opportunities for innovative CO₂ utilization technologies.

Aqueous OH- /H+ Dual-Ion Zn-Based Batteries

Aqueous Zn-based batteries hold multiple advantages of eco-friendliness, easy accessibility, high safety, easy fabrication, and fast kinetics, while their widespread applications have been greatly limited by the relatively narrow thermodynamically stable potential windows (i. e., 1.23 V) of water and the mismatched pH conditions between cathode and anode, which presents challenges regarding how to maximize the output voltage and the energy density. Recently, aqueous OH^- /H⁺ dual-ion Zn-based batteries (OH^- /H⁺ -DIZBs), where the Zn anode reacts with hydroxide ions (OH^- ) in alkaline electrolyte while hydrogen ions (H⁺ ) are involved in the cathode reaction in the acidic electrolyte, have been reported to be capable of broadening the working voltage and improving the energy density, which offers practical feasibility toward overcoming the above limitations. This Review thus takes this chance to investigate the recent progress on aqueous OH^- /H⁺ -DIZBs. First, the concept and the history of such OH^- /H⁺ -DIZBs are introduced, and then special emphasis is put on the working mechanisms, the progress of the development of new batteries, and how the electrolytes improve their performance. Finally, the challenges and opportunities in this field are discussed.

Operando characterization of continuous flow CO2 electrolyzers: current status and future prospects

The performance of continuous-flow CO₂ electrolyzers has substantially increased in recent years, achieving current density and selectivity (particularly for CO production) meeting the industrial targets. Further improvement is, however, necessary in terms of stability and energy efficiency, as well as in high-value multicarbon product formation. Accelerating this process requires deeper understanding of the complex interplay of chemical-physical processes taking place in CO₂ electrolyzer cells. Operando characterization can provide these insights under working conditions, helping to identify the reasons for performance losses. Despite this fact, only relatively few studies have taken advantage of such methods up to now, applying operando techniques to characterize practically relevant CO₂ electrolyzers. These studies include X-ray absorption- and Raman spectroscopy, fluorescent microscopy, scanning probe techniques, mass spectrometry, and radiography. Their objective was to characterize the catalyst structure, its microenviroment, membrane properties, etc., and relate them to the device performance (reaction rates and product distribution). Here we review the current state-of-the-art of operando methods, associated challenges, and also their future potential. We aim to motivate researchers to perform operando characterization in continuous-flow CO₂ electrolyzers, to understand the reaction mechanism and device operation under practically relevant conditions, thereby advancing the field towards industrialization.

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.

A unifying mechanism for cation effect modulating C1 and C2 productions from CO2 electroreduction

Electrocatalysis, whose reaction venue locates at the catalyst-electrolyte interface, is controlled by the electron transfer across the electric double layer, envisaging a mechanistic link between the electron transfer rate and the electric double layer structure. A fine example is in the CO₂ reduction reaction, of which rate shows a strong dependence on the alkali metal cation (M⁺) identity, but there is yet to be a unified molecular picture for that. Using quantum-mechanics-based atom-scale simulation, we herein scrutinize the M⁺-coupling capability to possible intermediates, and establish H⁺- and M⁺-associated ET mechanisms for CH₄ and CO/C₂H₄ formations, respectively. These theoretical scenarios are successfully underpinned by Nernstian shifts of polarization curves with the H⁺ or M⁺ concentrations and the first-order kinetics of CO/C₂H₄ formation on the electrode surface charge density. Our finding further rationalizes the merit of using Nafion-coated electrode for enhanced C2 production in terms of enhanced surface charge density.

Grave-to-cradle upcycling of Ni from electroplating wastewater to photothermal CO2 catalysis

Treating hazardous waste Ni from the electroplating industry is mandated world-wide, is exceptionally expensive, and carries a very high CO₂ footprint. Rather than regarding Ni as a disposable waste, the chemicals and petrochemicals industries could instead consider it a huge resource. In the work described herein, we present a strategy for upcycling waste Ni from electroplating wastewater into a photothermal catalyst for converting CO₂ to CO. Specifically, magnetic nanoparticles encapsulated in amine functionalized porous SiO₂, is demonstrated to efficiently scavenge Ni from electroplating wastewater for utilization in photothermal CO₂ catalysis. The core-shell catalyst architecture produces CO at a rate of 1.9 mol·g_Ni^-1·h^-1 (44.1 mmol·g_cat^-1·h^-1), a selectivity close to 100%, and notable long-term stability. This strategy of upcycling metal waste into functional, catalytic materials offers a multi-pronged approach for clean and renewable energy technologies.

Design principles for water dissociation catalysts in high-performance bipolar membranes

Water dissociation (WD, H₂O → H⁺ + OH⁻) is the core process in bipolar membranes (BPMs) that limits energy efficiency. Both electric-field and catalytic effects have been invoked to describe WD, but the interplay of the two and the underlying design principles for WD catalysts remain unclear. Using precise layers of metal-oxide nanoparticles, membrane-electrolyzer platforms, materials characterization, and impedance analysis, we illustrate the role of electronic conductivity in modulating the performance of WD catalysts in the BPM junction through screening and focusing the interfacial electric field and thus electrochemical potential gradients. In contrast, the ionic conductivity of the same layer is not a significant factor in limiting performance. BPM water electrolyzers, optimized via these findings, use ~30-nm-diameter anatase TiO₂ as an earth-abundant WD catalyst, and generate O₂ and H₂ at 500 mA cm⁻² with a record-low total cell voltage below 2 V. These advanced BPMs might accelerate deployment of new electrodialysis, carbon-capture, and carbon-utilization technology.

It is important yet challenging to elucidate the mechanism of water dissociation in bipolar membrane electrolysers. Here the authors show how water dissociation is accelerated by electric-field-focusing and catalytic effects and uncover design principles to optimize the performance.

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

In alkaline and neutral MEA CO₂ electrolyzers, CO₂ rapidly converts to (bi)carbonate, imposing a significant energy penalty arising from separating CO₂ from the anode gas outlets. Here we report a CO₂ electrolyzer uses a bipolar membrane (BPM) to convert (bi)carbonate back to CO₂, preventing crossover; and that surpasses the single-pass utilization (SPU) limit (25% for multi-carbon products, C₂₊) suffered by previous neutral-media electrolyzers. We employ a stationary unbuffered catholyte layer between BPM and cathode to promote C₂₊ products while ensuring that (bi)carbonate is converted back, in situ, to CO₂ near the cathode. We develop a model that enables the design of the catholyte layer, finding that limiting the diffusion path length of reverted CO₂ to ~10 μm balances the CO₂ diffusion flux with the regeneration rate. We report a single-pass CO₂ utilization of 78%, which lowers the energy associated with downstream separation of CO₂ by 10× compared with past systems.

Industrial-Current-Density CO2-to-C2+ Electroreduction by Anti-swelling Anion-Exchange Ionomer-Modified Oxide-Derived Cu Nanosheets

CO₂ electroreduction to high-energy-density C₂₊ products is highly attractive, whereas the C₂₊ selectivity under industrial current densities is still unsatisfying. Here, an anti-swelling anion exchange ionomer (AEI) was first proposed to optimize the local environment for promoting industrial-current-density CO₂-to-C₂₊ electroreduction. Taking the anti-swelling AEI-modified oxide-derived Cu nanosheets as an example, in situ Raman spectroscopy and contact angle measurements revealed that the OH^--accumulated -N(CH₃)₃⁺ groups and anti-swelling backbone of AEI could synergistically regulate the local pH level and water content. In situ Fourier-transform infrared spectroscopy and theoretical calculations demonstrated that the higher local pH value could lower the energy barrier for the rate-limiting COCO* hydrogenated to COCOH* from 0.08 to 0.04 eV, thereby boosting the generation of C₂₊ products. Owing to the anti-swelling backbone, the optimized water content of 3.5% could suppress the competing H₂ evolution and hence facilitate the proton-electron transfer step for C₂₊ production. As a result, the anti-swelling AEI-modified oxide-derived Cu nanosheets achieved a C₂₊ Faradaic efficiency of 85.1% at a current density up to 800 mA cm^-2 with a half-cell power conversion efficiency exceeding 50%, outperforming most reported powder catalysts.

Zero-Gap Bipolar Membrane Electrolyzer for Carbon Dioxide Reduction Using Acid-Tolerant Molecular Electrocatalysts

The scaling-up of electrochemical CO₂ reduction requires circumventing the CO₂ loss as carbonates under alkaline conditions. Zero-gap cell configurations with a reverse-bias bipolar membrane (BPM) represent a possible solution, but the catalyst layer in direct contact with the acidic environment of a BPM usually leads to H₂ evolution dominating. Here we show that using acid-tolerant Ni molecular electrocatalysts selective (>60%) CO₂ reduction can be achieved in a zero-gap BPM device using a pure water and CO₂ feed. At a higher current density (100 mA cm^-2), CO selectivity decreases, but was still >30%, due to reversible product inhibition. This study demonstrates the importance of developing acid-tolerant catalysts for use in large-scale CO₂ reduction devices.

Enabling Acidic Oxygen Reduction Reaction in a Zinc-Air Battery with Bipolar Membrane

Zinc-air batteries are a promising alternative to lithium ion batteries due to their large energy density, safety, and low production cost. However, the stability of the zinc-air battery is often low due to the formation of dendrite which causes short circuiting and the CO₂ adsorption from the air which causes carbonate formation on the air electrode. In this work, we demonstrate a zinc-air battery design with acidic oxygen reduction reaction for the first time via the incorporation of a bipolar membrane. The bipolar membrane creates a locally acidic environment in the air cathode which could lead to a higher oxygen reduction reaction activity and a better 4-electron selectivity toward water instead of the 2-electron pathway toward peroxide. Locally acidic air cathode is also effective at improving the cell's durability by preventing carbonate formation. Gas chromatography confirms that CO₂ adsorption is 7 times lower in the bipolar membrane compared to a conventional battery separator. A stable cycling of 300+ hours is achieved at 5 mA/cm². Dendrite formation is also mitigated due to the mechanical strength of the membrane. The insights from this work could be leveraged to develop a better zinc-air battery design for long-term energy storage applications.

Electrochemical Approaches for CO2 Conversion to Chemicals: A Journey toward Practical Applications

Carbon capture, utilization, and sequestration play an essential role to address CO₂ emissions. Among all carbon utilization technologies, CO₂ electroreduction has gained immense interest due to its potential for directly converting CO₂ to a variety of valuable commodity chemicals using clean, renewable electricity as the sole energy source. The research community has witnessed rapid advances in CO₂ electrolysis technology in recent years, including highly selective catalysts, larger-scale reactors, specific process modeling, as well as a mechanistic understanding of the CO₂ reduction reaction. The rapid advances in the field brings promise to the commercial application of the technology and the rapid rollout of the CO₂ electroreduction for chemical manufacturing.This Account focuses on our contributions in both fundamental and applied research in the field of electrocatalytic CO₂ and CO reduction reactions. We first discuss (1) the development of novel electrocatalysts for CO₂/CO electroreduction to enhance the product selectivity and lower the energy consumption. Specifically, we synthesized nanoporous Ag and homogeneously mixed Cu-based bimetallic catalysts for the enhanced production of CO from CO₂ and multicarbon products from CO, respectively. Then, we review our efforts in (2) the field of reactor engineering, including a dissolved CO₂ H-type cell, vapor-fed CO₂ three-compartment flow cell, and vapor-fed CO₂ membrane electrode assembly, for enhancing reaction rates and scalability. Next, we describe (3) the investigation of reaction mechanisms using in situ and operando techniques, such as surface-enhanced vibrational spectroscopies and electrochemical mass spectroscopy. We revealed the participation of bicarbonate in CO₂ electroreduction on Au using attenuated total-reflectance surface-enhanced infrared absorption spectroscopy, the presence of an "oxygenated" surface of Cu under CO electroreduction conditions using surface-enhanced Raman spectroscopy, and the origin of oxygen in acetaldehyde and other CO electroreduction products on Cu using flow electrolyzer mass spectrometry. Lastly, we examine (4) the commercial potential of the CO₂ electrolysis technology, such as understanding pollutant effects in CO₂ electroreduction and developing techno-economic analysis. Specifically, we discuss the effects of SO₂ and NO_x in CO₂ electroreduction using Cu, Ag, and Sn catalysts. We also identify technical barriers that need to be overcome and offer our perspective on accelerating the commercial deployment of the CO₂ electrolysis technology.

Assessing the economic potential of large-scale carbonate-formation-free CO2 electrolysis

A techno-economic assessment unraveling the quantitative correlation between carbonate formation and the cost of CO2 electroreduction.

Identification of Cu(100)/Cu(111) Interfaces as Superior Active Sites for CO Dimerization During CO2 Electroreduction

The electrosynthesis of valuable multicarbon chemicals using carbon dioxide (CO₂) as a feedstock has substantially progressed recently but still faces considerable challenges. A major difficulty lines in the sluggish kinetics of forming carbon-carbon (C-C) bonds, especially in neutral media. We report here that oxide-derived copper crystals enclosed by six {100} and eight {111} facets can reduce CO₂ to multicarbon products with a high Faradaic efficiency of 74.9 ± 1.7% at a commercially relevant current density of 300 mA cm^-2 in 1 M KHCO₃ (pH ∼ 8.4). By combining the experimental and computational studies, we uncovered that Cu(100)/Cu(111) interfaces offer a favorable local electronic structure that enhances *CO adsorption and lowers C-C coupling activation energy barriers, performing superior to Cu(100) and Cu(111) surfaces, respectively. On this catalyst, no obvious degradation was observed at 300 mA cm^-2 over 50 h of continuous operation.

Spectroelectrochemical Detection of Water Dissociation in Bipolar Membranes

The potentials at which water dissociation occurs in bipolar membranes (BPM) and the relationship between water dissociation and current-voltage curve characteristics are explored using a novel spectroelectrochemical approach in which an anion exchange membrane is doped with a pH indicator. Using this method, we visually detect a pH change in the BPM resulting from OH^- formed during the water dissociation reaction. The color change is measured with a UV/vis spectrometer, while electrochemical characterization of the BPM is performed simultaneously. Additional measurements were performed on BPMs with varying anion and cation exchange membrane layer thickness. Our measurements provide direct evidence of water dissociation occurring within a BPM at cross-membrane potentials below 0.5 V, within the first limiting current density region. We also show that the effects of changing bulk anion and cation exchange layer thickness is highly dependent on the permselectivity of these layers.