Deciphering Electrolyte Selection for Electrochemical Reduction of Carbon Dioxide and Nitrogen to High-Value-Added Chemicals

Electrochemical reduction of CO₂ (CO₂RR) and nitrogen (NRR) constitute alternatives to fossil fuel-based technologies for the production of high-value-added chemicals. Yet their practical application is still hampered by the low energy and Faradaic efficiencies although numerous efforts have been paid to overcome the fatal shortcomings. To date, most studies have focused on designing and developing advanced electrocatalysts, while the understanding of electrolyte, which would significantly influence the reaction microenvironment, are still not enough to provide insight to construct highly active and selective electrochemical systems. Here, a comprehensive review of the different electrolytes participating in the CO₂RR and NRR is provided, including acidic, neutral, alkaline, and water-in-salt electrolyte as aqueous electrolytes, as well as organic electrolyte, ionic-liquids electrolyte, and the mixture of the two as non-aqueous electrolytes. Through the discussion of the roles of these various electrolytes, it is aimed to grasp their essential function during the electrochemical process and how these functions can be used as design parameters for improving electrocatalytic performance. Finally, priorities for future studies are suggested to support the in-depth understanding of the electrolyte effects and thus guide efficient selection for next-generation gas-involving electrochemical reactions.     

Operando Reconstruction of Porous Carbon Supported Copper Selenide Promotes the C2 Production from CO2RR

Precisely regulating surface reconstruction of copper (Cu) chalcogenides-based catalysts to promote the multicarbon (C₂₊)selectivity of the electrochemical CO₂ reduction reaction (CO₂RR) is hampered by the challenging control of the intractable anions and the optimal Cu^δ+ reduction (0 < δ < 1). Herein, a porous carbon-supported copper selenides electrocatalyst that can remarkably improve the C₂-product yield and especially unveil the time-revolved electrochemical CO₂RR reconstruction process to enable the high C₂-selectivity, most notably for ethanol is constructed. The Faradic efficiency (FE) of C₂-products achieved is as high as ≈85.2% with a partial current density of 229.5 mA cm⁻². Operando infrared spectroscopy and density functional theory (DFT) calculations unravel that the surface Se vacancies (V_Se) formation brings closer the neighboring Cu⁺ atoms and activates the Cu sites, thereby rendering efficient generation of the key intermediates (^*CO and ^*CHO) and lowering the C–C coupling barrier for C₂ production. The appearance of metallic Cu can shorten the next-nearest Cu⁰–Cu⁺ distance for O atom to bridge in, leading to the preferential formation of ^*OC₂H₄ towards ethanol instead of C–O bond cleavage to form ethylene. This work opens the avenue of designing suitable local atomic structures catalysts to engage the intermediates for targeted CO₂RR products.     

Engineering the Pore Structure and Functionality of Ionic Porous Polymers for Separating Acetylene over Carbon Dioxide

Precise engineering of organic porous polymers to realize the selective separation of structurally similar gases presents a great challenge. In this study, a new class of ionic porous polymers P(Ph3Im-Br-nDVB) with a high ionic density and microporous surface area are constructed through a facile copolymerization strategy, providing an efficient path to rational control over pore structure and functionality. The first example of ionic porous organic polymers is reported to address the challenge of discriminating the subtle difference of C₂H₂ and CO₂, which have almost identical molecular sizes and similar physicochemical properties, which achieve the highest C₂H₂/CO₂ selectivity (17.9) among porous organic polymers. These ionic porous polymers exhibit high stability and excellent dynamic breakthrough performance for binary C₂H₂/CO₂ mixtures, indicating their practical feasibility. Modeling studies reveal that anions are the specific binding sites for preferential C₂H₂ capture because of Br⁻···HCCH interactions. This study not only demonstrates an efficient strategy to build novel ionic porous polymers integrating abundant micropores and ionic sites but also sheds some light on the development of functionalized materials for the separation of structurally similar gas molecules.     

Multiple Tuning of the Local Environment Enables Selective CO2 Electroreduction to Ethylene in Neutral Electrolytes

Recently, vigorous progress is made in the selective and high-current reduction of carbon dioxide (CO₂) to ethylene (C₂H₄) by using a flow cell. In most cases, however, the reduction is only achieved in strong alkaline electrolytes, which results in substantial deactivation of electrocatalysts due to the accumulation of precipitates. Here, porous Cu nanowires (NWs) is prepared with abundant atomic defects, which create a synergy with the pore-induced electric field to comprehensively tune the local microenvironment of the electrode surface, thus enabling efficient and stable C₂H₄ production from the CO₂ reduction reaction (CO₂RR) in neutral media. In particular, the enhanced electric field effect increases the local K⁺ concentration for the generation of *CO intermediates; while the atomic defects stabilize OH⁻ and *CO, leading to high local pH and *CO coverage. Such synergy can provide a favorable local environment and high *CO coverage for significantly decreasing the energy barrier of the C─C coupling step. Consequently, a large partial C₂H₄ current density of 222.3 mA cm⁻² with excellent stability is achieved in a neutral electrolyte. Altogether, this work paves new pathways to promote C₂H₄ production in the neutral CO₂RR through multiple tuning of the local environment.     

Engineering Fe–Fe3C@Fe–N–C Active Sites and Hybrid Structures from Dual Metal–Organic Frameworks for Oxygen Reduction Reaction in H2–O2 Fuel Cell and Li–O2 Battery

Dual metal–organic frameworks (MOFs, i.e., MIL‐100(Fe) and ZIF‐8) are thermally converted into Fe–Fe₃C‐embedded Fe–N‐codoped carbon as platinum group metal (PGM)‐free oxygen reduction reaction (ORR) electrocatalysts. Pyrolysis enables imidazolate in ZIF‐8 rearranged into highly N‐doped carbon, while Fe from MIL‐100(Fe) into N‐ligated atomic sites concurrently with a few Fe–Fe₃C nanoparticles. Upon precise control of MOF compositions, the optimal catalyst is highly active for the ORR in half‐cells (0.88 V in base and 0.79 V versus RHE in acid in half‐wave potential), a proton exchange membrane fuel cell (0.76 W cm^?2 in peak power density) and an aprotic Li–O₂ battery (8749 mAh g^?1 in discharge capacity), representing a state‐of‐the‐art PGM‐free ORR catalyst. In the material, amorphous carbon with partial graphitization ensures high active site exposure and fast charge transfer simultaneously. Macropores facilitate mass transport to the catalyst surface, followed by oxygen penetration in micropores to reach the infiltrated active sites. Further modeling simulations shed light on the true Fe–Fe₃C contribution to the catalyst performance, suggesting Fe₃C enhances oxygen affinity, while metallic Fe promotes *OH desorption as the rate‐determining step at the nearby Fe–N–C sites. These findings demonstrate MOFs as model system for rational design of electrocatalyst for energy‐based functional applications.