Structure- and Electrolyte-Sensitivity in CO2 Electroreduction

Monolithic Nitrogen-Doped Carbon Electrode with Hierarchical Porous Structure for Efficient Electrochemical CO2 Reduction

N-doped carbon materials have garnered extensive development in electrochemical CO2 reduction due to their abundant sources, high structural plasticity, and excellent catalytic performance. However, the use of powder carbon materials for electrocatalytic reactions limits their current density and mechanical strength, which pose challenges for industrial applications. In this study, we synthesized a monolithic N-doped carbon electrode with high mechanical strength for efficient electrochemical reduction of CO2 to CO through a simple pyrolysis method, using phenolic resin as the precursor and ZIF-8 as the sacrificial template. At 900 °C, the decomposition of ZIF-8 and the volatilization of Zn atoms promote the formation of a hierarchical porous structure in the carbon matrix, characterized by macropores with extended mesoporous channels. Simultaneously, N active species derived from ZIF-8 are effectively generated around the pores and fully exposed. The efficient mass transfer facilitated by the hierarchical porous structure and high activity of exposed nitrogen species enables efficient conversion of CO2 to CO. When the ZIF-8 content is 30%, the catalyst achieves a Faradaic efficiency of 88.9% for CO at a low potential of -0.7 V (vs RHE), with a CO production rate of 244.05 μmol h-1 cm-2. After 50 h of potentiostatic electrolysis, the current density and FECO remain stable. This work not only provides a strategy for the synergistic effects of hierarchical porous structures and nitrogen doping but also offers an effective method to avoid using powder binders and prepare integrated, stable monolithic electrodes.

The Effect of the Tetraalkylammonium Cation in the Electrochemical CO2 Reduction Reaction on Copper Electrode

Aprotic organic solvents such as acetonitrile offer a potential solution to promote electrochemical CO2 reduction over the competing hydrogen evolution reaction. Tetraalkylammonium cations (TAA+) are widely used as supporting electrolytes in organic media due to their high solubility and conductivity. The alkyl chain length of TAA+ cations is known to influence electron transfer processes in electrochemical systems by the adsorption of TAA+, causing modifications of the double layer. In this work, we elucidate the influence of the cation chain length on the mechanism and selectivity of the CO2RR reaction under controlled dry and wet acetonitrile conditions on copper cathodes. We find that the hydrophobic hydration character of the cation, which can be tuned by the chain length, has an effect on product distribution, altering the reaction pathway. Under dry conditions, smaller cations (TEA+) preferentially promote oxalate production via dimerization of the CO2 ·- intermediate, whereas formate is favored in the presence of water via protonation reaction. Larger cations (TBA+ > TPA+ > TEA+) favor the generation of CO regardless of water content. In situ FTIR analysis showed that TBA+ cations are able to stabilize adsorbed CO more effectively than TEA+, explaining why larger cations generate a higher proportion of CO. Our findings also suggest that higher cation concentrations suppress hydrogen evolution, particularly with larger cations, highlighting the role of cation chain length size and hydrophobic hydration shell.

Bifunctional Catalysts for CO2 Reduction and O2 Evolution: A Pivotal for Aqueous Rechargeable Zn-CO2 Batteries

The quest for the advancement of green energy storage technologies and reduction of carbon footprint is determinedly rising toward carbon neutrality. Aqueous rechargeable Zn-CO2 batteries (ARZCBs) hold the great potential to encounter both the targets simultaneously, i.e., green energy storage and CO2 conversion to value-added chemicals/fuels. The major descriptor of ARZCBs efficiency is allied with the reactions occurring at cathode during discharging (CO2 reduction) and charging (O2 evolution) which own different fundamental mechanisms and hence mandate the employment of two different catalysts. This presents an overall complex and expensive battery system which requires a concrete solution, while the development and application of a bifunctional cathode catalyst toward both reactions could reduce the complexity and cost and thus can be a pivotal for ARZCBs. However, despite the increasing research interest and ongoing research, a systematic evaluation of bifunctional catalysts is rarely reported. In this review, the need of bifunctional cathode catalysts for ARZCBs and associated challenges with strategies have been critically assessed. A detailed progress examination and understanding toward designing of bifunctional catalyst for ARZCBs have been provided. This review will enlighten the future research approaching boosted performance of ARZCBs through the development of efficient bifunctional cathode catalysts.

Effects of Ionic Interferents on Electrocatalytic Nitrate Reduction: Mechanistic Insight

Nitrate, a prevalent water pollutant, poses substantial public health concerns and environmental risks. Electrochemical reduction of nitrate (eNO3RR) has emerged as an effective alternative to conventional biological treatments. While extensive lab work has focused on designing efficient electrocatalysts, implementation of eNO3RR in practical wastewater settings requires careful consideration of the effects of various constituents in real wastewater. In this critical review, we examine the interference of ionic species commonly encountered in electrocatalytic systems and universally present in wastewater, such as halogen ions, alkali metal cations, and other divalent/trivalent ions (Ca2+, Mg2+, HCO3-/CO32-, SO42-, and PO43-). Notably, we categorize and discuss the interfering mechanisms into four groups: (1) loss of active catalytic sites caused by competitive adsorption and precipitation, (2) electrostatic interactions in the electric double layer (EDL), including ion pairs and the shielding effect, (3) effects on the selectivity of N intermediates and final products (N2 or NH3), and (4) complications by the hydrogen evolution reaction (HER) and localized pH on the cathode surface. Finally, we summarize the competition among different mechanisms and propose future directions for a deeper mechanistic understanding of ionic impacts on eNO3RR.

Reversible metal cluster formation on Nitrogen-doped carbon controlling electrocatalyst particle size with subnanometer accuracy

Copper and nitrogen co-doped carbon catalysts exhibit a remarkable behavior during the electrocatalytic CO2 reduction (CO2RR), namely, the formation of metal nanoparticles from Cu single atoms, and their subsequent reversible redispersion. Here we show that the switchable nature of these species holds the key for the on-demand control over the distribution of CO2RR products, a lack of which has thus far hindered the wide-spread practical adoption of CO2RR. By intermitting pulses of a working cathodic potential with pulses of anodic potential, we were able to achieve a controlled fragmentation of the Cu particles and partial regeneration of single atom sites. By tuning the pulse durations, and by tracking the catalyst's evolution using operando quick X-ray absorption spectroscopy, the speciation of the catalyst can be steered toward single atom sites, ultrasmall metal clusters or large metal nanoparticles, each exhibiting unique CO2RR functionalities.

Self-Organizing Sub-μm Surface Structures Stimulated by Microplasma Generated Reactive Species and Short-Pulsed Laser Irradiation

Catalysts are critical components for chemical reactions in industrial applications. They are able to optimize selectivity, efficiency, and reaction rates, thus enabling more environmentally friendly processes. This work presents a novel approach to catalyst functionalization for the CO2 reduction reaction by combining the reactive species of an atmospheric pressure plasma jet with the electric fields and energy input of a laser. This leads to both a nanoscale structuring as well as a controllable chemical composition of the surface, which are important parameters for optimizing catalyst performance. The treatment is conducted on thin copper layers deposited by high power pulsed magnetron sputtering on silicon wafers. Because atomic oxygen plays a key role in oxidizing copper, two photon absorption fluorescence is used to investigate the atomic oxygen density in the interaction zone of the COST plasma jet and a copper surface. The used atmospheric pressure plasma jet provides an atomic oxygen density at the surface in a distance of 8 mm to the jet nozzle of approximately or a flux of . Pulsed laser-induced dewetting is used to form nanoparticles from the deposited copper layer to enhance catalytic performance. Varying the layer thickness allows control of the size of the particles. A gas flow directed on the sample during the combined treatment disturbs the particle formation. This can be prevented by increasing the laser energy to compensate for the cooling effect of the gas flow. Investigating the surface using X-ray photoemission spectroscopy reveals that the untreated copper layer surface consists mostly of metallic copper and Cu(I) oxide. Irradiating the sample only with the laser did not change the composition. The combination of plasma and laser treatment is able to produce Cu(II) species such as CuO, whose concentration increases with treatment time. The presented process allows the tuning of the ratio of C2O/CuO, which is an interesting parameter for further studies on copper catalyst performance.

Intricate Ionic Behaviors in High-Performance Self-Powered Hydrothermal Chemical Generator Using Water and Iron (III) Gate

Utilizing the ionic flux to generate voltage output has been confirmed as an effective way to meet the requirements of clean energy sources. Different from ionic thermoelectric (i-TE) and hydrovoltaic devices, a new hydrothermal chemical generator is designed by amorphous FeCl3 particles dispersing in MWCNT and unique ferric chloride or water gate. In the presence of gate, the special ion behaviors enable the cell to present a constant voltage of 0.60 V lasting for over 96 h without temperature difference. Combining the differences of cation concentration, humidity and temperature between the right and left side of sample, the maximum short-circuit current and power output can be obtained to 168.46 µA and 28.11 µW, respectively. The generator also can utilize the low-grade heat to produce electricity wherein Seebeck coefficient is 6.79 mV K-1. The emerged hydrothermal chemical generator offers a novel approach to utilize the low-grade heat, water and salt solution resources, which provides a simple, sustainable and low-cost strategy to realize energy supply.

Composition effects of electrodeposited Cu-Ag nanostructured electrocatalysts for CO2 reduction

The electrochemical carbon dioxide reduction (CO2RR) on Cu-based catalysts is a promising strategy to store renewable electricity and produce valuable C2+ chemicals. We investigate the CO2RR on Cu-Ag nanostructures that have been electrodeposited in a green choline chloride and urea deep eutectic solvent (DES). We determine the electrochemically active surface area (ECSA) using lead underpotential deposition (UPD) to investigate the CO2RR intrinsic activity and selectivity. We show that the addition of Ag on electrodeposited Cu primarily suppresses the production of hydrogen and methane. While the production of carbon monoxide slightly increases, the partial current of the total C2+ products does not considerably increase. Despite that the production rate of C2+ is similar on Cu and Cu-Ag, the addition of Ag enhances the formation of alcohols and oxygenates over ethylene. We highlight the potential of metal electrodeposition from DES as a sustainable strategy to develop bimetallic Cu-based nanocatalysts for CO2RR.

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.

Studying the cation dependence of CO2 reduction intermediates at Cu by in situ VSFG spectroscopy

The nature of the electrolyte cation is known to have a significant impact on electrochemical reduction of CO2 at catalyst|electrolyte interfaces. An understanding of the underlying mechanism responsible for catalytic enhancement as the alkali metal cation group is descended is key to guide catalyst development. Here, we use in situ vibrational sum frequency generation (VSFG) spectroscopy to monitor changes in the binding modes of the CO intermediate at the electrochemical interface of a polycrystalline Cu electrode during CO2 reduction as the electrolyte cation is varied. A CObridge mode is observed only when using Cs+, a cation that is known to facilitate CO2 reduction on Cu, supporting the proposed involvement of CObridge sites in CO coupling mechanisms during CO2 reduction. Ex situ measurements show that the cation dependent CObridge modes correlate with morphological changes of the Cu surface.

Local reaction environment in electrocatalysis

Beyond conventional electrocatalyst engineering, recent studies have unveiled the effectiveness of manipulating the local reaction environment in enhancing the performance of electrocatalytic reactions. The general principles and strategies of local environmental engineering for different electrocatalytic processes have been extensively investigated. This review provides a critical appraisal of the recent advancements in local reaction environment engineering, aiming to comprehensively assess this emerging field. It presents the interactions among surface structure, ions distribution and local electric field in relation to the local reaction environment. Useful protocols such as the interfacial reactant concentration, mass transport rate, adsorption/desorption behaviors, and binding energy are in-depth discussed toward modifying the local reaction environment. Meanwhile, electrode physical structures and reaction cell configurations are viable optimization methods in engineering local reaction environments. In combination with operando investigation techniques, we conclude that rational modifications of the local reaction environment can significantly enhance various electrocatalytic processes by optimizing the thermodynamic and kinetic properties of the reaction interface. We also outline future research directions to attain a comprehensive understanding and effective modulation of the local reaction environment.

Recent insights on the use of modified Zn-based catalysts in eCO2RR

Converting CO2 into valuable chemicals can provide a new path to mitigate the greenhouse effect, achieving the aim of "carbon neutrality" and "carbon peaking". Among numerous electrocatalysts, Zn-based materials are widely distributed and cheap, making them one of the most promising electrocatalyst materials to replace noble metal catalysts. Moreover, the Zn metal itself has a certain selectivity for CO. After appropriate modification, such as oxide derivatization, structural reorganization, reconstruction of the surfaces, heteroatom doping, and so on, the Zn-based electrocatalysts can expose more active sites and adjust the d-band center or electronic structure, and the FE and stability of them can be effectively improved, and they can even convert CO2 to multi-carbon products. This review aims to systematically describe the latest progresses of modified Zn-based electrocatalyst materials (including organic and inorganic materials) in the electrocatalytic carbon dioxide reduction reaction (eCO2RR). The applications of modified Zn-based catalysts in improving product selectivity, increasing current density and reducing the overpotential of the eCO2RR are reviewed. Moreover, this review describes the reasonable selection and good structural design of Zn-based catalysts, presents the characteristics of various modified zinc-based catalysts, and reveals the related catalytic mechanisms for the first time. Finally, the current status and development prospects of modified Zn-based catalysts in eCO2RR are summarized and discussed.

Polymer Modification Strategy to Modulate Reaction Microenvironment for Enhanced CO2 Electroreduction to Ethylene

Modulation of the microenvironment on the electrode surface is one of the effective means to improve the efficiency of electrocatalytic carbon dioxide reduction (eCO2 RR). To achieve high conversion rates, the phase boundary at the electrode surface should be finely controlled to overcome the limitation of CO2 solubility in the aqueous electrolyte. Herein, we developed a simple and efficient method to structure electrocatalyst with a superhydrophobic surface microenvironment by one-step co-electrodeposition of Cu and polytetrafluoroethylene (PTFE) on carbon paper. The super-hydrophobic Cu-based electrode displayed a high ethylene (C2 H4 ) selectivity with a Faraday efficiency (FE) of 67.3 % at -1.25 V vs. reversible hydrogen electrode (RHE) in an H-type cell, which is 2.5 times higher than a regular Cu electrode without PTFE. By using PTFE as a surface modifier, the activity of eCO2 RR is enhanced and water (proton) adsorption is inhibited. This strategy has the potential to be applied to other gas-conversion electrocatalysts.

Operando Electron Microscopy of Catalysts: The Missing Cornerstone in Heterogeneous Catalysis Research?

Heterogeneous catalysis in thermal gas-phase and electrochemical liquid-phase chemical conversion plays an important role in our modern energy landscape. However, many of the structural features that drive efficient chemical energy conversion are still unknown. These features are, in general, highly distinct on the local scale and lack translational symmetry, and thus, they are difficult to capture without the required spatial and temporal resolution. Correlating these structures to their function will, conversely, allow us to disentangle irrelevant and relevant features, explore the entanglement of different local structures, and provide us with the necessary understanding to tailor novel catalyst systems with improved productivity. This critical review provides a summary of the still immature field of operando electron microscopy for thermal gas-phase and electrochemical liquid-phase reactions. It focuses on the complexity of investigating catalytic reactions and catalysts, progress in the field, and analysis. The forthcoming advances are discussed in view of correlative techniques, artificial intelligence in analysis, and novel reactor designs.

Bioinspired Hydrophobicity for Enhancing Electrochemical CO2 Reduction

Electrochemical carbon dioxide reduction (CO2 R) is a promising pathway for converting greenhouse gasses into valuable fuels and chemicals using intermittent renewable energy. Enormous efforts have been invested in developing and designing CO2 R electrocatalysts suitable for industrial applications at accelerated reaction rates. The microenvironment, specifically the local CO2 concentration (local [CO2 ]) as well as the water and ion transport at the CO2 -electrolyte-catalyst interface, also significantly impacts the current density, Faradaic efficiency (FE), and operation stability. In nature, hydrophobic surfaces of aquatic arachnids trap appreciable amounts of gases due to the "plastron effect", which could inspire the reliable design of CO2 R catalysts and devices to enrich gaseous CO2 . In this review, starting from the wettability modulation, we summarize CO2 enrichment strategies to enhance CO2 R. To begin, superwettability systems in nature and their inspiration for concentrating CO2 in CO2 R are described and discussed. Moreover, other CO2 enrichment strategies, compatible with the hydrophobicity modulation, are explored from the perspectives of catalysts, electrolytes, and electrolyzers, respectively. Finally, a perspective on the future development of CO2 enrichment strategies is provided. We envision that this review could provide new guidance for further developments of CO2 R toward practical applications.

Controlling the C1/C2+ product selectivity of electrochemical CO2 reduction upon tuning bimetallic CuIn electrocatalyst composition and operating conditions

Electrochemical carbon dioxide (CO2) reduction (eCO2R) over Cu-based bimetallic catalysts is a promising technique for converting CO2 into value-added multi-carbon products, such as fuels, chemicals, and materials. For improving the process efficiency, electrocatalyst development for the eCO2R must be integrated with tuning of operating conditions. For example, CuIn-based materials typically lead to preferential C1 product selectivity, which delivers the desired C2+ products upon varying the In/Cu ratio and operating conditions (i.e., in 0.1 M KHCO3 electrolytes using an H-type cell with a cation exchange membrane vs. in 1 M KOH electrolytes using a flow cell with an anion exchange membrane). At lower Cu-loading (i.e., InCu5Ox material), the maximum faradaic efficiency of HCOOH (FEHCOOH) of 70% was achieved at -1 V versus the reversible hydrogen electrode (vs. RHE) in an H-type cell. However, upon increasing the Cu loading, the preferential product selectivity could be altered: the InCu73Ox material led to a high CO selectivity (maximum FE of 51%) in the H-type cell at -0.8 V vs. RHE and delivered a current density of 100 mA cm-2 with a FEC2+ of up to 37% at -0.8 V vs. RHE in the flow cell configuration. Various characterization tools were also employed to probe the catalytic materials to rationalize the electrocatalytic performance.

Local compressive strain regulation of atomically dispersed NiN4 sites for enhancing CO2 electroreduction to CO

Electroreduction of CO2 to valuable chemicals powered by renewable electricity provides a sustainable approach to reduce the environmental issues originating from CO2 emission. However, insufficient current density and production selectivity hinder its further application. In this case, precisely regulating the CO2 reduction reaction (CO2RR) active sites is an excellent strategy to simultaneously reduce the reaction barrier and suppress the hydrogen evolution reaction (HER) pathway. Herein, the strain regulation of atomically dispersed NiN4 active sites is investigated in helical carbon. Ni-N coordination in the curved carbon lattice displays a reduced distance compared to that in a straight lattice, inflicting local compressive strain on NiN4. The resultant catalyst shows the highest CO selectivity of up to 99.4% at -1.4 V (vs. RHE), the FECO is maintained at over 85% over a wide potential range from -0.8 to -1.8 V (vs. RHE), and the maximum partial current density for CO reaches a high of 458 mA cm-2 at -1.8 V (vs. RHE). Theoretical investigations show the superior CO2 electroreduction performance of curved NiN4 stems from its remarkable ability to generate the *COOH intermediate and to suppress the hydrogen combination simultaneously. Our findings offer a novel strategy to rationally regulate the local three-dimensional structure of single-atom sites for efficient electrocatalysis.

Dicopper(I) Sites Confined in a Single Metal-Organic Layer Boosting the Electroreduction of CO2 to CH4 in a Neutral Electrolyte

It is challenging and important to achieve high performance for an electrochemical CO2 reduction reaction (eCO2RR) to yield CH4 under neutral conditions. So far, most of the reported active sites for eCO2RR to yield CH4 are single metal sites; the performances are far below the commercial requirements. Herein, we reported a nanosheet metal-organic layer in single-layer, namely, [Cu2(obpy)2] (Cuobpy-SL, Hobpy = 1H-[2,2']bipyridinyl-6-one), possessing dicopper(I) sites for eCO2RR to yield CH4 in a neutral aqueous solution. Detailed examination of Cuobpy-SL revealed high performance for CH4 production with a faradic efficiency of 82(1)% and a current density of ∼90 mA cm-2 at -1.4 V vs. reversible hydrogen electrode (RHE). No obvious degradation was observed over 100 h of continuous operation, representing a remarkable performance to date. Mechanism studies showed that compared with the conventional single-copper sites and completely exposed dicopper(I) sites, the dicopper(I) sites in the confined space formed by the molecular stacking have a strong affinity to key C1 intermediates such as *CO, *CHO, and *CH2O to facilitate the CH4 production, yet inhibiting C-C coupling.

Solvent Effect on Electrochemical CO2 Reduction Reaction on Nanostructured Copper Electrodes

The electrochemical reduction of CO₂ (CO₂RR) is a sustainable alternative for producing fuels and chemicals, although the production of highly desired hydrocarbons is still a challenge due to the higher overpotential requirement in combination with the competitive hydrogen evolution reaction (HER). Tailoring the electrolyte composition is a possible strategy to favor the CO₂RR over the HER. In this work we studied the solvent effect on the CO₂RR on a nanostructured Cu electrode in acetonitrile solvent with different amounts of water. Similar to what has been observed for aqueous media, our online gas chromatography results showed that CO₂RR in acetonitrile solvent is also structure-dependent, since nanocube-covered copper (CuNC) was the only surface (in comparison to polycrystalline Cu) capable of producing a detectable amount of ethylene (10% FE), provided there is enough water present in the electrolyte (>500 mM). In situ Fourier Transform Infrared (FTIR) spectroscopy showed that in acetonitrile solvent the presence of CO₂ strongly inhibits HER by driving away water from the interface. CO is by far the main product of CO₂RR in acetonitrile (>85% Faradaic efficiency), but adsorbed CO is not detected. This suggests that in acetonitrile media CO adsorption is inhibited compared to aqueous media. Remarkably, the addition of water to acetonitrile has little quantitative and almost no qualitative effect on the activity and selectivity of the CO₂RR. This indicates that water is not strongly involved in the rate-determining step of the CO₂RR in acetonitrile. Only at the highest water concentrations and at the CuNC surface, the CO coverage becomes high enough that a small amount of C₂₊ product is formed.

Spatiotemporal Mapping of Local Heterogeneities during Electrochemical Carbon Dioxide Reduction

The activity and selectivity of a copper electrocatalyst during the electrochemical CO₂ reduction reaction (eCO₂RR) are largely dominated by the interplay between local reaction environment, the catalyst surface, and the adsorbed intermediates. In situ characterization studies have revealed many aspects of this intimate relationship between surface reactivity and adsorbed species, but these investigations are often limited by the spatial and temporal resolution of the analytical technique of choice. Here, Raman spectroscopy with both space and time resolution was used to reveal the distribution of adsorbed species and potential reaction intermediates on a copper electrode during eCO₂RR. Principal component analysis (PCA) of the in situ Raman spectra revealed that a working electrocatalyst exhibits spatial heterogeneities in adsorbed species, and that the electrode surface can be divided into CO-dominant (mainly located at dendrite structures) and C-C dominant regions (mainly located at the roughened electrode surface). Our spectral evaluation further showed that in the CO-dominant regions, linear CO was observed (as characterized by a band at ∼2090 cm^-1), accompanied by the more classical Cu-CO bending and stretching vibrations located at ∼280 and ∼360 cm^-1, respectively. In contrast, in the C-C directing region, these three Raman bands are suppressed, while at the same time a band at ∼495 cm^-1 and a broad Cu-CO band at ∼2050 cm^-1 dominate the Raman spectra. Furthermore, PCA revealed that anodization creates more C-C dominant regions, and labeling experiments confirmed that the 495 cm^-1 band originates from the presence of a Cu-C intermediate. These results indicate that a copper electrode at work is very dynamic, thereby clearly displaying spatiotemporal heterogeneities, and that in situ micro-spectroscopic techniques are crucial for understanding the eCO₂RR mechanism of working electrocatalyst materials.

Nanograin-Boundary-Abundant Cu2O-Cu Nanocubes with High C2+ Selectivity and Good Stability during Electrochemical CO2 Reduction at a Current Density of 500 mA/cm2

Surface and interface engineering, especially the creation of abundant Cu⁰/Cu⁺ interfaces and nanograin boundaries, is known to facilitate C₂₊ production during electrochemical CO₂ reductions over copper-based catalysts. However, precisely controlling the favorable nanograin boundaries with surface structures (e.g., Cu(100) facets and Cu[n(100)×(110)] step sites) and simultaneously stabilizing Cu⁰/Cu⁺ interfaces is challenging, since Cu⁺ species are highly susceptible to be reduced into bulk metallic Cu at high current densities. Thus, an in-depth understanding of the structure evolution of the Cu-based catalysts under realistic CO₂RR conditions is imperative, including the formation and stabilization of nanograin boundaries and Cu⁰/Cu⁺ interfaces. Herein we demonstrate that the well-controlled thermal reduction of Cu₂O nanocubes under a CO atmosphere yields a remarkably stable Cu₂O-Cu nanocube hybrid catalyst (Cu₂O(CO)) possessing a high density of Cu⁰/Cu⁺ interfaces, abundant nanograin boundaries with Cu(100) facets, and Cu[n(100)×(110)] step sites. The Cu₂O(CO) electrocatalyst delivered a high C₂₊ Faradaic efficiency of 77.4% (56.6% for ethylene) during the CO₂RR under an industrial current density of 500 mA/cm². Spectroscopic characterizations and morphological evolution studies, together with in situ time-resolved attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) studies, established that the morphology and Cu⁰/Cu⁺ interfacial sites in the as-prepared Cu₂O(CO) catalyst were preserved under high polarization and high current densities due to the nanograin-boundary-abundant structure. Furthermore, the abundant Cu⁰/Cu⁺ interfacial sites on the Cu₂O(CO) catalyst acted to increase the *CO adsorption density, thereby increasing the opportunity for C-C coupling reactions, leading to a high C₂₊ selectivity.

Recent Progress and Perspectives on Photocathode Materials for CO2 Catalytic Reduction

The continuous consumption of fossil energy and excessive emissions of carbon dioxide (CO₂) have caused a serious energy crisis and led to the greenhouse effect. Using natural resources to convert CO₂ into fuel or high-value chemicals is considered to be an effective solution. Photoelectrochemical (PEC) catalysis utilizes abundant solar energy resources, combined with the advantages of photocatalysis (PC) and electrocatalysis (EC), to achieve efficient CO₂ conversion. In this review, the basic principles and evaluation criteria, of PEC catalytic reduction to CO₂ (PEC CO₂RR), are introduced. Next, the recent research progress on typical kinds of photocathode materials for CO₂ reduction are reviewed, and the structure-function relationships between material composition/structure and activity/selectivity are discussed. Finally, the possible catalytic mechanisms and the challenges of using PEC to reduce CO₂ are proposed.

Probing the Roles of Indium Oxides on Copper Catalysts for Enhanced Selectivity during CO2-to-CO Electrochemical Reduction

The electrochemical CO₂ reduction reaction (CO₂RR) provides an alternative protocol to producing industrial chemicals with renewable electricity sources, and the highly selective, durable, and economic catalysts should expedite CO₂RR applications. Here, we demonstrate a composite Cu-In₂O₃ catalyst in which a trace amount of In₂O₃ decorated on Cu surface greatly improves the selectivity and stability for CO₂-to-CO reduction as compared to the counterparts (Cu or In₂O₃), realizing a CO faradaic efficiency (FE_CO) of 95% at -0.7 V (vs RHE) and no obvious degradation within 7 h. In situ X-ray absorption spectroscopy reveals that In₂O₃ undergoes the redox reaction and preserves the metallic state of Cu during the CO₂RR process. Strong electronic interaction and coupling occur at the Cu/In₂O₃ interface which serves as the active site for selective CO₂RR. Theoretical calculation confirms the roles of In₂O₃ in preventing oxidation and altering the electronic structure of Cu to assist COOH* formation and demote CO* adsorption at the Cu/In₂O₃ interface.

Copper-Based Catalysts for Electrochemical Reduction of Carbon Dioxide to Ethylene

Electrochemical reduction of CO₂ into high energy density multi-carbon chemicals or fuels (e. g., ethylene) via new renewable energy storage has extraordinary implications for carbon neutrality. Copper (Cu)-based catalysts have been recognized as the most promising catalysts for the electrochemical reduction of CO₂ to ethylene (C₂ H₄ ) due to their moderate CO adsorption energy and moderate hydrogen precipitation potential. However, the poor selectivity, low current density and high overpotential of the CO₂ RR into C₂ H₄ greatly limit its industrial applications. Meanwhile, the complex reaction mechanism is still unclear, which leads to blindness in the design of catalysts. Herein, we systematically summarized the latest research, proposed possible conversion mechanisms and categorized the general strategies to adjust of the structure and composition for CO₂ RR, such as tip effect, defect engineering, crystal plane catalysis, synergistic effect, nanoconfinement effect and so on. Eventually, we provided a prospect of the future challenges for further development and progress in CO₂ RR. Previous reviews have summarized catalyst designs for the reduction of CO₂ to multi-carbon products, while lacking in targeting C₂ H₄ alone, an important industrial feedstock. This Review mainly aims to provide a comprehensive understanding for the design strategies and challenges of electrocatalytic CO₂ reduction to C₂ H₄ through recent researches and further propose some guidelines for the future design of copper-based catalysts for electroreduction of CO₂ to C₂ H₄ .

Electroreduction of carbon dioxide to multi-electron reduction products using poly(ionic liquid)-based Cu-Pd catalyst

Electrocatalytic reduction of CO2 (CO2RR) to multi-electron (> 2e-) products provides a green and sustainable route for producing fuels and chemicals. Introducing the second metal element is a feasible strategy for "managing" the key intermediate on Cu-based materials to further improve the CO2RR catalytic performance. In this work, palladium, which promises the generation of CO, was introduced into the poly(ionic liquid)-based copper hybrid (Cu@PIL) to construct a novel Cu-Pd bimetallic electrocatalyst (Cu@PIL@Pd). Remarkably, with a small dosage of palladium (2.0 mol% compared with Cu), a high faradaic efficiency (FE) for C2+ products (68.7%) was achieved at -1.01 V (with respect to the reversible hydrogen electrode (RHE), the same below) with a high partial current density of 178.3 mA cm-2. Meanwhile, high selectivity towards CH4 (FE = 42.5%) and corresponding partial current density of 172.8 mA cm-2 were obtained on the same catalyst at -1.24 V, signifying a significant potential-dependent selectivity. Mechanistic studies reveal that both copper and palladium oxides are reduced to metallic states during the CO2RR. The presence of the adjoint copper phase and the highly dispersed electrostatic layer promote the generation of CO on the palladium components (both the PdO2 phase and the Pd(II) site). Besides, the local CO* was enriched by the significant diffusion resistance of CO in the PIL layer. The spillover of CO* from Pd sites to the adjoint Cu sites, accompanied by the increased local concentration of CO* around Cu sites, accounted for the observed good CO2RR catalytic performance, especially the high C2+ product selectivity.

Triple-Phase Interface Engineering over an In2O3 Electrode to Boost Carbon Dioxide Electroreduction

The electrocatalytic reduction of CO₂ is deemed to be a promising method to ease environmental and energy issues. However, achieving high efficiency and selectivity of CO₂ electroreduction remains a bottleneck due to huge limitation of CO₂ mass transfer and competition of hydrogen evolution reaction (HER) in aqueous solution. In this work, we propose to utilize triple-phase interface engineering over an In₂O₃ electrode to enhance its CO₂ reduction reaction (CO₂RR) performance. Notably, distinguishing from other research studies (doping, defect introduction, and heterojunction construction) that regulate the nature of In₂O₃-based catalysts themselves, we herein tune interfacial wettability of In₂O₃ using facile fluoropolymer coating for the first time. In contrast to the hydrophilic In₂O₃ electrode [Faraday efficiency (FE)_HCOOH ∼ 62.7% and FE_H2 ∼ 24.1% at -0.67 V versus RHE], the hydrophobic fluoropolymer (taking polyvinylidene fluoride as an example)-coated In₂O₃ electrode delivers a significantly enhanced FE_HCOOH of 82.3% and a decreased FE_H2 of 5.7% at the same potential. Upon combining contact angle measurements, density functional theory calculation, and ab initio molecular dynamics simulation, the enhanced CO₂RR performance is revealed to be attributed to the rich triple-phase interfaces formed after fluoropolymer coating as an "aerophilic sponge", which increases the local concentration of CO₂ near In₂O₃ active sites to improve CO₂ reduction and meanwhile reduces the accessible water molecules to suppress competitive HER. This work presents a feasible approach for the enhanced selectivity of HCOOH yield over In₂O₃ by triple-phase interface engineering, which also provides a convenient and effective method for developing other materials used in gas-consumption reactions.

Superbase and Hydrophobic Ionic Liquid Confined within Ni Foams as a Free-Standing Catalyst for CO2 Electroreduction

Access to high-performance and cost-effective catalyst materials is one of the crucial preconditions for the industrial application of electrochemical CO₂ reduction (ECR). In this work, a facile and simple strategy is proposed for the construction of a free-standing electrocatalyst via confining a superbase and hydrophobic ionic liquid (IL, [P₆₆₆₁₄][triz]) into Ni foam pores, denoted as [P₆₆₆₁₄][triz]@Ni foam. These ILs can modulate the surface of Ni foam and create a microenvironment with high CO₂ concentration around the electrode/electrolyte interface, which successfully suppresses the hydrogen evolution reaction (HER) of Ni foam. Consequently, the synthesized [P₆₆₆₁₄][triz]@Ni foam sample can obtain a CO product with 63% Faradaic efficiency from the ECR procedure, while no detectable CO can be found on pristine Ni foam. Owing to the superbase IL, the valency of Ni species retains Ni(I)/Ni(0) during electrolysis. Furthermore, the strikingly high CO₂ capacity by [P₆₆₆₁₄][Triz] (0.91 mol CO₂ per mole of IL) offers a high CO₂ local concentration in the reaction region. Theoretical calculations indicated that the neutral CO₂ molecule turned to be negatively charged with -0.546 e and changed into a bent geometry, thus rendering CO₂ activation and reduction in a low-energy pathway. This study provides a new method of electrode interface modification for the design of efficient ECR catalysts.

A review on recent advances in the electrochemical reduction of CO2 to CO with nano-electrocatalysts

The electrochemical reduction (ECR) of CO2 is a powerful strategy to reduce the world's carbon footprint by converting CO2 to useful products such as CH3OH and CO. Recent techno-economic analysis has found that for the electro-conversion of CO2 to be adapted for practical use, the main products formed from this reaction need to be low-order, such as CO. This review summarizes recent progress in the ECR of CO2 to CO on nano-electrocatalysts (noble, non-noble metals and carbon nanomaterials) and provides the limitations and challenges that each electrocatalyst faces. It discusses the mechanism behind the performance of the electrocatalysts and offers the potential future prospects of the ECR process.

Understanding the role of Cu+/Cu0 sites at Cu2O based catalysts in ethanol production from CO2 electroreduction -A DFT study

Cu₂O based electrocatalysts generally exhibit better selectivity for C₂ products (ethylene or ethanol) in electrochemical carbon dioxide reduction. The surface characteristic of the mixed Cu⁺ and Cu⁰ chemical state is believed to play an essential role that is still unclear. In the present study, density functional theory (DFT) calculations have been performed to understand the role of copper chemical states in selective ethanol formation using a partially reduced Cu₂O surface model consisting of adjacent Cu⁺/Cu⁰ sites. We mapped out the free energy diagram of the reaction pathway from CO intermediate to ethanol and discussed the relation between the formation of critical reduction intermediates and the configuration of Cu⁺/Cu⁰ sites. The results showed that Cu⁺ sites facilitate the adsorption and stabilization of *CO, as well as its further hydrogenation to *CHO. More importantly, as compared to the high reaction energy (1.23 eV) of the dimerization of two *CO on Cu⁺/Cu⁰ sites, the preferable formation of *CHO on the Cu⁺ site makes the C-C coupling reaction with *CO on the Cu⁰ site happen under a relatively lower energy barrier of 0.58 eV. Furthermore, the post C-C coupling steps leading to the formation of the key intermediate *OCHCH₂ to C₂ compound are all thermodynamically favoured. Noteworthily, it is found that *OCHCH₂ inclines to the ethanol formation because the coordinatively unsaturated Cu⁺ site could maintain the C-O bond of *OCHCH₂, and the weak binding between *O and Cu⁺/Cu⁰ sites helps inhibit the pathway toward ethylene. These findings may provide guidelines for the design of CO and CO₂ reduction active sites with enhanced ethanol selectivity.

Electrochemical CO2 reduction on Cu single atom catalyst and Cu nanoclusters: an ab initio approach

The electrochemical CO₂ reduction presents a sustainable route to the production of chemicals such as ethylene or ethanol, however the design of selective catalysts is still challenging. The use of single site copper nitrogen doped carbon materials with porphyrin-like Cu graphene structures have shown a significant improvement towards the production of multi carbon products, particularly ethanol. Nonetheless, during reaction the porphyrin like Cu sites transiently convert into metallic copper nanoclusters in a reversible process, making difficult to understand the actual role of each phase. Here, we present a computational study, where adequate structural models to describe the experimentally determined phases of the single atom catalyst (Cu-N-C material) have been constructed. Moreover, the electrochemical reduction of CO₂ to ethanol and ethylene has been addressed via periodic DFT calculations on each of the systems. On the basis of the computed free energies of reaction, it was found that the Cu nanoclusters exhibit a superior performance for the CO reduction in comparison with the single site. Moreover, they possess a high activity towards the production of ethanol, suggesting them as the active phase responsible for the catalytic performance of the studied material.

Main-Group Metal Single-Atomic Regulators in Dual-Metal Catalysts for Enhanced Electrochemical CO2 Reduction

Single-atom sites can not only act as active centers, but also serve as promising catalyst regulators and/or promoters. However, in many complex reaction systems such as electrochemical CO₂ reduction reaction (CO₂ RR), the introduction of single-atom regulators may inevitably induce the competitive hydrogen evolution reaction (HER) and thus reduce the selectivity. Here, the authors demonstrate that introducing HER-inert main-group metal single atoms adjacent to transition-metal single atoms can modify their electronic structure to enhance the CO₂ RR to CO without inducing the HER side reaction. Dual-metal Cu and In single-site atoms anchored on mesoporous nitrogen-doped carbon (denoted as Cu-In-NC) are prepared by the pyrolysis of a multimetallic metal-organic framework. Cu-In-NC shows a high faradic efficiency of 96% toward CO formation at -0.7 V versus reversible hydrogen electrode, superior to that of its monometallic single-atom counterparts. Density functional theory studies reveal that the HER-inert In sites can activate the adjacent Cu sites through electronic modifications, strengthening the binding of *COOH intermediate and thus boosting the electrochemical reduction of CO₂ to CO.

Real-Time In Situ Monitoring of CO2 Electroreduction in the Liquid and Gas Phases by Coupled Mass Spectrometry and Localized Electrochemistry

The mechanism and dynamics of the CO₂ reduction reaction (CO₂RR) remain poorly understood, which is largely caused by mass transport limitations and lack of time-correlated product analysis tools. In this work, a custom-built gas accessible membrane electrode (GAME) system is used to comparatively assess the CO₂RR behavior of Au and Au−Cu catalysts. The platform achieves high reduction currents (∼ – 50 mA cm^–2 at 1.1 V vs RHE) by creating a three-phase boundary interface equipped with an efficient gas-circulation pathway, facilitating rapid mass transport of CO₂. The GAME system can also be easily coupled with many other analytical techniques as exemplified by mass spectrometry (MS) and localized ultramicroelectrode (UME) voltammetry to enable real-time and in situ product characterization in the gas and liquid phases, respectively. The gaseous product distribution is explicitly and quantitatively elucidated with high time resolution (on the scale of seconds), allowing for the independent assessment of Tafel slope estimates for the hydrogen (159/168 mV decade^–1), ethene (160/170 mV decade^–1), and methane (96/100 mV decade^–1) evolution reactions. Moreover, the UME is used to simultaneously measure the local pH shift during CO₂RR and assess the production of liquid phase species including formate. A positive shift of 0.8 pH unit is observed at a current density of −11 mA cm^–2 during the CO₂RR.

Carbon-Confined Indium Oxides for Efficient Carbon Dioxide Reduction in a Solid-State Electrolyte Flow Cell

Developing robust electrocatalysts and advanced devices is important for electrochemical carbon dioxide (CO₂ ) reduction toward the generation of valuable chemicals. We present herein a carbon-confined indium oxide electrocatalyst for stable and efficient CO₂ reduction. The reductive corrosion of oxidative indium to the metallic state during electrolysis could be prevented by carbon protection, and the applied carbon layer also optimizes the reaction intermediate adsorption, which enables both high selectivity and activity for CO₂ reduction. In a liquid-phase flow cell, the formate selectivity exceeds 90 % in a wide potential window from -0.8 V to -1.3 V vs. RHE. The continuous production of ca. 0.12 M pure formic acid solution is further demonstrated at a current density of 30 mA cm^-2 in a solid-state electrolyte mediated reactor. This work provides significant concepts in the parallel development of electrocatalysts and devices for carbon-neutral technologies.

Understanding the complementarities of surface-enhanced infrared and Raman spectroscopies in CO adsorption and electrochemical reduction

In situ/operando surface enhanced infrared and Raman spectroscopies are widely employed in electrocatalysis research to extract mechanistic information and establish structure-activity relations. However, these two spectroscopic techniques are more frequently employed in isolation than in combination, owing to the assumption that they provide largely overlapping information regarding reaction intermediates. Here we show that surface enhanced infrared and Raman spectroscopies tend to probe different subpopulations of adsorbates on weakly adsorbing surfaces while providing similar information on strongly binding surfaces by conducting both techniques on the same electrode surfaces, i.e., platinum, palladium, gold and oxide-derived copper, in tandem. Complementary density functional theory computations confirm that the infrared and Raman intensities do not necessarily track each other when carbon monoxide is adsorbed on different sites, given the lack of scaling between the derivatives of the dipole moment and the polarizability. Through a comparison of adsorbed carbon monoxide and water adsorption energies, we suggest that differences in the infrared vs. Raman responses amongst metal surfaces could stem from the competitive adsorption of water on weak binding metals. We further determined that only copper sites capable of adsorbing carbon monoxide in an atop configuration visible to the surface enhanced infrared spectroscopy are active in the electrochemical carbon monoxide reduction reaction.

Infrared and Raman spectroscopies are often assumed to provide similar insights into heterogeneous reaction mechanisms. This study shows that these techniques provide similar data when CO is strongly bound to a surface, yet distinct subpopulations of CO are probed when binding is weaker.

Structure-Function Correlation and Dynamic Restructuring of Cu for Highly Efficient Electrochemical CO2 Conversion

The increasing global demand for sustainable energy sources and emerging environmental issues have pushed the development of energy conversion and storage technologies to the forefront of chemical research. Electrochemical carbon dioxide (CO₂ ) conversion provides an attractive approach to synthesizing fuels and chemical feedstocks using renewable energy. On the path to deploying this technology, basic and applied scientific hurdles remain. Copper, as the only metal catalyst that is capable to produce C₂₊ fuels from CO₂ reduction (CO₂ R), still faces challenges in the improvement of electrosynthesis pathways for highly selective fuel production. In this regard, mechanistically understanding CO₂ R on Cu-based electrocatalysts, particularly identifying the structure-function correlation, is crucial. Here, a broad view of the variable structural parameters and their complex interplay in CO₂ R catalysis on Cu was given, with the purpose of providing deep insights and guiding the future rational design of CO₂ R electrocatalysts. First, this Review described the progress and recent advances in the development of well-defined nanostructured catalysts and the mechanistic understanding on the influences from a particular structure of a catalyst, such as facet, defects, morphology, oxidation state, composition, and interface. Next, the in-situ dynamic restructuring of Cu was presented. The importance of operando characterization methods to understand the catalyst structure-sensitivity was also discussed. Finally, some perspectives on the future outlook for electrochemical CO₂ R were offered.

Shaping Copper Nanocatalysts to Steer Selectivity in the Electrochemical CO2 Reduction Reaction

The carbon-neutral production of fuels and chemical feedstocks is one of the grand challenges for our society to solve. The electrochemical conversion of CO₂ is emerging as a promising technology contributing to this goal. Despite the huge amount of progress made over the past decade, selectivity still remains a challenge. This Account presents an overview of recent progress in the design of selective catalysts by exploiting the structural sensitivity of the electrochemical CO₂ reduction reaction (CO₂RR). In particular, it shows that the accurate and precise control of the shape and size of Cu nanocatalysts is instrumental in understanding and in discovering the structure-selectivity relationships governing the reduction of CO₂ to valuable hydrocarbons, such as methane and ethylene. It further illustrates the use of faceted Cu nanocatalysts to interrogate catalytic pathways and to increase selectivity toward oxygenates, such as ethanol, in the framework of tandem schemes. The last part of the Account highlights the role of well-defined nanocatalysts in identifying reconstruction mechanisms which might occur during operation. An outlook for the emerging paradigms which will empower the design of novel catalysts for CO₂RR concludes the Account.

Acidic Electrocatalytic CO2 Reduction Using Space-Confined Nanoreactors

Electrochemical CO₂ reduction reaction (CO₂RR) is an attractive strategy for sustainable production of chemicals and has mainly been implemented in alkaline or neutral electrolytes. However, part of input CO₂ is consumed by the formation of carbonate under these conditions. Herein, a space-confined strategy is proposed for CO₂RR in acidic media, and Ni nanoparticles are encapsulated inside N-doped carbon nanocages as yolk-shell nanoreactors. By confining CO₂RR in the cavities of nanoreactors, a Faradaic efficiency (FE) of 93.2% for CO is achieved at pH 7.2 and 84.3% FE for CO at pH 2.5. The inhibited proton diffusion within the Nernst layer of a nanoreactor is responsible for suppression of competing hydrogen evolution in acid. Moreover, CO₂RR in an acidic flow electrolysis system offers enhanced current density and sustainable operation, in comparison with the conventional neutral pH system. This work shows that steering of mass transport via a unique structure is a viable avenue toward selective CO₂ conversion, and it provides a further understanding of the structure-performance relationship of electrocatalysts.

Strong Correlation between the Dynamic Chemical State and Product Profile of Carbon Dioxide Electroreduction

Utilizing renewable electricity energy to convert CO2 into fuels and chemicals, namely, CO2 electrocatalytic reduction reaction (CO2RR), is becoming increasingly significant yet challenged by low activity and selectivity. Recently, a growing number of studies have demonstrated that oxidized species can surprisingly survive on the catalyst surface under highly cathodic CO2RR conditions and play crucial roles in affecting the product selectivity. However, dynamic evolutions of the surface chemical state together with its real correlation to the product selectivity are still unclear, which is one of the most controversial topics for CO2RR. Herein, we particularly resurvey recent CO2RR researches that are all based on advanced in situ/operando methodologies, aiming to clearly reveal the realistic variations in surface chemical state under the working conditions. Then, recent progress in the regulation of the surface chemical state toward specific CO2RR products in current state-of-the-art catalysts with varying metal centers is systematically summarized, which shows an impressive relation between the dynamic chemical state and product profile. Next, we further highlight the developed strategies to regulate the surface chemical state in catalysts and discuss the debates over the effects of chemical state on product profile during CO2RR. Finally, on the basis of previous achievements, we present major challenges and some perspectives for the exploration of the imperative chemical state sensitivity to product profile during CO2RR.

Chemical Batteries with CO2

Efforts to obtain raw materials from CO2 by catalytic reduction as a means of combating greenhouse gas emissions are pushing the boundaries of the chemical industry. The dimensions of modern energy regimes, on the one hand, and the necessary transport and trade of globally produced renewable energy, on the other, will require the use of chemical batteries in conjunction with the local production of renewable electricity. The synthesis of methanol is an important option for chemical batteries and will, for that reason, be described here in detail. It is also shown that the necessary, robust, and fundamental understanding of processes and the material science of catalysts for the hydrogenation of CO2 does not yet exist.

CO2 Electrolysis System under Industrially Relevant Conditions

ConspectusCarbon dioxide emissions from consumption of fossil fuels have caused serious climate issues. Rapid deployment of new energies makes renewable energy driven CO₂ electroreduction to chemical feedstocks and carbon-neutral fuels a feasible and cost-effective pathway for achieving net-zero emission. With the urgency of the net-zero goal, we initiated our research on CO₂ electrolysis with emphasis on industrial relevance.The CO₂ molecules are thermodynamically stable due to high activation energy of the two C═O bonds, and efficient electrocatalysts are required to overcome the sluggish dynamics and competitive hydrogen evolution reaction. The CO₂ electrocatalysts that we have explored include molecular catalysts and nanostructured catalysts. Molecular catalysts are centered on earth abundant elements such as Fe and Co for catalyzing CO₂ reduction, and using Fe catalysts, we proposed an amidation strategy for reduction of CO₂ to methanol, bypassing the inactive formate pathway. For nanostructured catalysts, we developed a carbon enrichment strategy using nitrogen-rich nanomaterials for selective CO₂ reduction.Direct CO₂ electroreduction from the flue gas stream represents the "holy grail" in the field, because typical CO₂ concentration in flue gas is only 6-15%, posing a significant challenge for CO₂ electrolysis. On the other hand, direct electroreduction of CO₂ in the flue gas eliminates the carbon capture process and simplifies the overall carbon capture and utilization (CCU) scheme. However, direct flue gas reduction is frustrated by the reactive oxygen (5-8%), low CO₂ concentration (6-15%), and potentially toxic impurities. Surface CO₂ enrichment catalysts with high O₂ tolerance could be viable for achieving direct CO₂ electroreduction for decarbonization of flue gas.In addition to the electrocatalysts, the incorporation of catalysts into the electrolyzer and development of a suitable process was also investigated to meet industrial demands. A membrane electrode assembly (MEA) is a zero-gap configuration with cathode and anode catalysts coated on either side of an ion exchange membrane. We adopted the MEA configuration due to the structural simplicity, low ohmic resistance, and high efficiency. The electrode factors (for example, membrane type, catalyst layer porosity, and MEA fabrication method) and the electrolyzer factors (for example, flow channels, gas diffusion layer) are critical to highly efficient operation. We separately developed an anion-exchange membrane-based system for CO production and cation-exchange membrane-based system for formate production. The optimized electrolyzer configuration can generate uniform current and voltage distribution in a large-area electrolyzer and operate using an industrial CO₂ stream. The optimized process was developed with the targets of long-term continuous operation and no electrolyte consumption.

Porous organic polymers for electrocatalysis

Porous organic polymers (POPs) composed of organic building units linked via covalent bonds are a class of lightweight porous network materials with high surface areas, tuneable pores, and designable components and structures. Owing to their well-preserved characteristics in terms of structure and composition, POPs applied as electrocatalysts have shown promising activity and achieved considerable advances in numerous electrocatalytic reactions, including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO₂ reduction reaction, N₂ reduction reaction, nitrate/nitrite reduction reaction, nitrobenzene reduction reaction, hydrogen oxidation reaction, and benzyl alcohol oxidation reaction. Herein, we present a systematic overview of recent advances in the applications of POPs in these electrocatalytic reactions. The synthesis strategies, specific active sites, and catalytic mechanisms of POPs are summarized in this review. The fundamental principles of some electrocatalytic reactions are also concluded. We further discuss the current challenges of and perspectives on POPs for electrocatalytic applications. Meanwhile, the possible future directions are highlighted to afford guidelines for the development of efficient POP electrocatalysts.