Efficient multicarbon formation in acidic CO2 reduction via tandem electrocatalysis

Photoelectrocatalytic CO2 Reduction to Methanol by Molecular Self-Assemblies Confined in Covalent Polymer Networks

Inspired by the porous structures of photosynthetic organelles, we report here a new type of photoelectrode based on a standalone macroporous conjugated polymer network (MCN) that converts sunlight into high-energy electrons for CO2 reduction to CH3OH. The MCN provides supramolecular cavities with sufficient functional groups that control the structures of photocatalytic assemblies, which circumvents the geometric limitations of traditional inorganic counterparts. Stabilized interfacial contact between MCN and photocatalysts is achieved by strong chemical linkages throughout the network. Solar irradiation of MCN with a cobalt-based catalyst generates highly reducing electrons for the reduction of CO2 to CH3OH at a conversion efficiency of 70%. Production of CH3OH sustains for at least 100 h, with a small decrease in yield rates. Scaling up the photoelectrode from 1 to 100 cm2 results in photocurrent generation stabilized at 0.25 A and continuous CH3OH production at a conversion efficiency of 85%, demonstrating the scalability and high performances.

Flow-electrode capacitive separation of organic acid products and recovery of alkali cations after acidic CO2 electrolysis

Acidic CO2 electrolysis, enhanced by the introduction of alkali cations, presents a strategic approach for improving carbon efficiency compared to processes conducted in neutral and alkaline environments. However, a significant challenge arises from the dissolution of both organic acids and alkali cations in a strongly acidic feed stream, resulting in a considerable energy penalty for downstream separation. In this study, we investigate the feasibility of using flow-electrode capacitive deionization (FCDI) technology to separate organic acids and recover alkali cations from a strongly acidic feed stream (pH ~ 1). We show that organic acids, such as formic acid and acetic acid, are retained in molecular form in the separation chamber, achieving a rejection rate of over 90% under all conditions. Alkali cations, such as K+ and Cs+, migrate to the cathode chamber in ionic form, with their removal and recovery significantly influenced by their concentration and the pH of the feed stream, but responding differently to the types and concentrations of organic acids. The energy consumption for the removal and recovery of K+ is 4 to 8 times higher than for Cs+, and the charge efficiency is significantly influenced by the types of organic acid products and alkali cations. We conduct a series of electrochemical measurements and analyze the impedance spectroscopy, identifying that hindered mass transfer governed the electrode process. Our findings underscore the potential of FCDI as an advanced downstream separation technology for acidic electrocatalysis processes.

Anionic Ionomer: Released Surface-Immobilized Cations and an Established Hydrophobic Microenvironment for Efficient and Durable CO2-to-Ethylene Electrosynthesis at High Current over One Month

Electrosynthesis of multicarbon products, such as C2H4, from CO2 reduction on copper (Cu) catalysts holds promise for achieving carbon neutrality. However, maintaining a steady high current-level C2H4 electrosynthesis still encounters challenges, arising from unstable alkalinity and carbonate precipitation caused by undesired ion migration at the cathode under a repulsive electric field. To address these issues, we propose a universal "charge release" concept by incorporating tiny amounts of an oppositely charged anionic ionomer (e.g., perfluorinated sulfonic acid, PFSA) into a cationic covalent organic framework on the Cu surface (cCOF/PFSA). This strategy effectively releases the hidden positive charge within the cCOF, enhancing surface immobilization of cations to impede both outward migration of generated OH- and inward migration of cations, inhibiting carbonate precipitation and creating a strong alkaline microenvironment. Meanwhile, the ionomer's hydrophobic chains create a hydrophobic environment within the cCOF, facilitating efficient gas transport. In situ characterizations and theoretical calculations demonstrate that the cCOF/PFSA catalyst establishes a hydrophobic strong alkaline microenvironment, optimizing the adsorption strength and configuration of *CO intermediates to promote the C2H4 formation. The optimized catalyst achieves a 70.5% Faradaic efficiency for C2H4 with a partial current density over 470 mA cm-2. Notably, it delivers a high single-pass carbon efficiency of 96.5% for CO2RR and sustains an exceptional stability over 760 h. When implemented in a large-area MEA electrolyzer and a 5-cell MEA stack, the system achieves an industrial current of 15 A and continuous C2H4 production exceeding 19 mL min-1, marking a significant step toward industrial feasibility in CO2RR-to-C2H4 conversion.

Stabilized Cu0 -Cu1+ dual sites in a cyanamide framework for selective CO2 electroreduction to ethylene

Electrochemical reduction of carbon dioxide to produce high-value ethylene is often limited by poor selectivity and yield of multi-carbon products. To address this, we propose a cyanamide-coordinated isolated copper framework with both metallic copper (Cu0) and charged copper (Cu1+) sites as an efficient electrocatalyst for the reduction of carbon dioxide to ethylene. Our operando electrochemical characterizations and theoretical calculations reveal that copper atoms in the Cuδ+NCN complex enhance carbon dioxide activation by improving surface carbon monoxide adsorption, while delocalized electrons around copper sites facilitate carbon-carbon coupling by reducing the Gibbs free energy for *CHC formation. This leads to high selectivity for ethylene production. The Cuδ+NCN catalyst achieves 77.7% selectivity for carbon dioxide to ethylene conversion at a partial current density of 400 milliamperes per square centimeter and demonstrates long-term stability over 80 hours in membrane electrode assembly-based electrolysers. This study provides a strategic approach for designing catalysts for the electrosynthesis of value-added chemicals from carbon dioxide.

Accelerating acidic CO2 electroreduction: strategies beyond catalysts

Carbon dioxide electrochemical reduction (CO2RR) into high-value-added chemicals offers an alternative pathway toward achieving carbon neutrality. However, in conventional neutral or alkaline electrolyte systems, a significant portion of CO2 is converted into (bi)carbonate due to the thermodynamically favorable acid-base neutralization reaction between CO2 and hydroxide ions. This results in the single-pass carbon efficiency (SPCE) being theoretically capped at 50%, presenting challenges for practical applications. Acidic CO2RR can completely circumvent the carbonate issue and theoretically achieve 100% SPCE, garnering substantial attention from researchers in recent years. Nevertheless, acidic CO2RR currently lags behind traditional neutral/alkaline systems in terms of product selectivity, stability, and energy efficiency, primarily because the abundance of H+ ions exacerbates the hydrogen evolution reaction (HER). Encouragingly, significant breakthroughs have been made to address these challenges, with numerous studies indicating that the regulation of the local catalytic environment may be more crucial than the catalyst itself. In this review, we will discuss the main challenges and latest strategies for acidic CO2RR, focusing on three key aspects beyond the catalyst: electrolyte regulation, local catalytic environment modification, and novel designs of gas diffusion electrodes (GDEs)/electrolyzers. We will also conclude the current advancement for acidic CO2RR and provide an outlook, with the hope that this technology will contribute to achieving carbon neutrality and advance towards practical application.

Multivalent Cu sites synergistically adjust carbonaceous intermediates adsorption for electrocatalytic ethanol production

Copper (Cu)-based catalysts show promise for electrocatalytic CO2 reduction (CO2RR) to multi-carbon alcohols, but thermodynamic constraints lead to competitive hydrocarbon (e.g., ethylene) production. Achieving selective ethanol production with high Faradaic efficiency (FE) and current density is still challenging. Here we show a multivalent Cu-based catalyst, Cu-2,3,7,8-tetraaminophenazine-1,4,6,9-tetraone (Cu-TAPT) with Cu2+ and Cu+ atomic ratio of about 1:2 for CO2RR. Cu-TAPT exhibits an ethanol FE of 54.3 ± 3% at an industrial-scale current density of 429 mA cm-2, with the ethanol-to-ethylene ratio reaching 3.14:1. Experimental and theoretical calculations collectively unveil that the catalyst is stable during CO2RR, resulting from suitable coordination of the Cu2+ and Cu+ with the functional groups in TAPT. Additionally, mechanism studies show that the increased ethanol selectivity originates from synergy of multivalent Cu sites, which can promote asymmetric C-C coupling and adjust the adsorption strength of different carbonaceous intermediates, favoring hydroxy-containing C2 intermediate (*HCCHOH) formation and formation of ethanol.

Atomically Dispersed Metal Catalysts for the Conversion of CO2 into High-Value C2+ Chemicals

The conversion of carbon dioxide (CO2) into value-added chemicals with two or more carbons (C2+) is a promising strategy that cannot only mitigate anthropogenic CO2 emissions but also reduce the excessive dependence on fossil feedstocks. In recent years, atomically dispersed metal catalysts (ADCs), including single-atom catalysts (SACs), dual-atom catalysts (DACs), and single-cluster catalysts (SCCs), emerged as attractive candidates for CO2 fixation reactions due to their unique properties, such as the maximum utilization of active sites, tunable electronic structure, the efficient elucidation of catalytic mechanism, etc. This review provides an overview of significant progress in the synthesis and characterization of ADCs utilized in photocatalytic, electrocatalytic, and thermocatalytic conversion of CO2 toward high-value C2+ compounds. To provide insights for designing efficient ADCs toward the C2+ chemical synthesis originating from CO2, the key factors that influence the catalytic activity and selectivity are highlighted. Finally, the relevant challenges and opportunities are discussed to inspire new ideas for the generation of CO2-based C2+ products over ADCs.

In Situ Electropolymerizing Toward EP-CoP/Cu Tandem Catalyst for Enhanced Electrochemical CO2-to-Ethylene Conversion

Electrochemical CO2 reduction has garnered significant interest in the conversion of sustainable energy to valuable fuels and chemicals. Cu-based bimetallic catalysts play a crucial role in enhancing *CO concentration on Cu sites for efficient C─C coupling reactions, particularly for C2 product generation. To enhance Cu's electronic structure and direct its selectivity toward C2 products, a novel strategy is proposed involving the in situ electropolymerization of a nano-thickness cobalt porphyrin polymeric network (EP-CoP) onto a copper electrode, resulting in the creation of a highly effective EP-CoP/Cu tandem catalyst. The even distribution of EP-CoP facilitates the initial reduction of CO2 to *CO intermediates, which then transition to Cu sites for efficient C─C coupling. DFT calculations confirm that the *CO enrichment from Co sites boosts *CO coverage on Cu sites, promoting C─C coupling for C2+ product formation. The EP-CoP/Cu gas diffusion electrode achieves an impressive current density of 726 mA cm-2 at -0.9 V versus reversible hydrogen electrode (RHE), with a 76.8% Faraday efficiency for total C2+ conversion and 43% for ethylene, demonstrating exceptional long-term stability in flow cells. These findings mark a significant step forward in developing a tandem catalyst system for the effective electrochemical production of ethylene.

Strategies for overcoming challenges in selective electrochemical CO2 conversion to ethanol

The electrochemical conversion of carbon dioxide (CO2) to valuable chemicals is gaining significant attention as a pragmatic solution for achieving carbon neutrality and storing renewable energy in a usable form. Recent research increasingly focuses on designing electrocatalysts that specifically convert CO2 into ethanol, a desirable product due to its high-energy density, ease of storage, and portability. However, achieving high-efficiency ethanol production remains a challenge compared to ethylene (a competing product with a similar electron configuration). Existing electrocatalytic systems often suffer from limitations such as low energy efficiency, poor stability, and inadequate selectivity toward ethanol. Inspired by recent progress in the field, this review explores fundamental principles and material advancements in CO2 electroreduction, emphasizing strategies for ethanol production over ethylene. We discuss electrocatalyst design, reaction mechanisms, challenges, and future research directions. These advancements aim to bridge the gap between current research and industrialized applications of this technology.

General synthesis of high-entropy single-atom nanocages for electrosynthesis of ammonia from nitrate

Given the growing emphasis on energy efficiency, environmental sustainability, and agricultural demand, there's a pressing need for decentralized and scalable ammonia production. Converting nitrate ions electrochemically, which are commonly found in industrial wastewater and polluted groundwater, into ammonia offers a viable approach for both wastewater treatment and ammonia production yet limited by low producibility and scalability. Here we report a versatile and scalable solution-phase synthesis of high-entropy single-atom nanocages (HESA NCs) in which Fe and other five metals-Co, Cu, Zn, Cd, and In-are isolated via cyano-bridges and coordinated with C and N, respectively. Incorporating and isolating the five metals into the matrix of Fe resulted in Fe-C5 active sites with a minimized symmetry of lattice as well as facilitated water dissociation and thus hydrogenation process. As a result, the Fe-HESA NCs exhibited a high selectivity toward NH3 from the electrocatalytic reduction of nitrate with a Faradaic efficiency of 93.4% while maintaining a high yield rate of 81.4 mg h-1 mg-1.

A synthetic cell-free pathway for biocatalytic upgrading of one-carbon substrates

Biotechnological processes hold tremendous potential for the efficient and sustainable conversion of one-carbon (C1) substrates into complex multi-carbon products. However, the development of robust and versatile biocatalytic systems for this purpose remains a significant challenge. In this study, we report a hybrid electrochemical-biochemical cell-free system for the conversion of C1 substrates into the universal biological building block acetyl-CoA. The synthetic reductive formate pathway (ReForm) consists of five core enzymes catalyzing non-natural reactions that were established through a cell-free enzyme engineering platform. We demonstrate that ReForm works in a plug-and-play manner to accept diverse C1 substrates including CO2 equivalents. We anticipate that ReForm will facilitate efforts to build and improve synthetic C1 utilization pathways for a formate-based bioeconomy.

In situ Generation of Cyclohexanone Drives Electrocatalytic Upgrading of Phenol to Nylon-6 Precursor

Coupling in situ generated intermediates with other substrates/intermediates is a viable approach for diversifying product outcomes of catalytic reactions involving two or multiple reactants. Cyclohexanone oxime is a key precursor for caprolactam synthesis (the monomer of Nylon-6), yet its current production uses unsustainable carbon sources, noble metal catalysts, and harsh conditions. Herein, we report the first work to synthesize cyclohexanone oxime through electroreduction of phenol and hydroxylamine. The Faradaic efficiency reached 69.1 % over Cu catalyst, accompanied by a corresponding cyclohexanone oxime formation rate of 82.0 g h-1 gcat -1. In addition, the conversion of phenol was up to 97.5 %. In situ characterizations, control experiments, and theoretical calculations suggested the importance of balanced activation of water, phenol, and hydroxylamine substrates on the optimal metallic Cu catalyst for achieving high-performance cyclohexanone oxime synthesis. Besides, a tandem catalytic route for the upgrading of lignin to caprolactam has been successfully developed through the integration of thermal catalysis, electrocatalysis, and Beckmann rearrangement, which achieved the synthesis of 0.40 g of caprolactam from 4.0 g of lignin raw material.

Robust Electrocatalytic CO2 Reduction in Acid Enabled by "Molecularly Charged" Cobalt Phthalocyanine: A Profound Understanding from Electric Double Layer

Electrocatalytic CO2 reduction (eCO2R) in acid holds promise in renewable electricity-powered CO2 utilization with high efficiency, but the hydrogen evolution reaction (HER) often prevails and results in a low eCO2R selectivity. Here, using cobalt phthalocyanine/Ketjen black (CoPc/KB) as the model catalysts, we systematically study the effect of active site density, operational current density, and hydrated cations on the acidic eCO2R selectivity and decipher it through the componential dynamics of electric double layer (EDL). The optimal CoPc-4/KB demonstrates a near-unity CO Faradaic efficiency from 50 to 400 mA cm-2 and superb operational stability (>120 h) at 100 mA cm-2. Aided by in situ Raman and infrared spectroscopies, we reveal that the proper cations establish an electrostatic shield for mitigating bulk H+ penetration and mediate the interfacial water structure for suppressing HER. This study should elicit further profound thinking on robust eCO2R system design from the perspective of multiphasic and dynamic EDL.

Recent development and applications of differential electrochemical mass spectrometry in emerging energy conversion and storage solutions

Electrochemical energy conversion and storage are playing an increasingly important role in shaping the sustainable future. Differential electrochemical mass spectrometry (DEMS) offers an operando and cost-effective tool to monitor the evolution of gaseous/volatile intermediates and products during these processes. It can deliver potential-, time-, mass- and space-resolved signals which facilitate the understanding of reaction kinetics. In this review, we show the latest developments and applications of DEMS in various energy-related electrochemical reactions from three distinct perspectives. (I) What is DEMS addresses the working principles and key components of DEMS, highlighting the new and distinct instrumental configurations for different applications. (II) How to use DEMS tackles practical matters including the electrochemical test protocols, quantification of both potential and mass signals, and error analysis. (III) Where to apply DEMS is the focus of this review, dealing with concrete examples and unique values of DEMS studies in both energy conversion applications (CO2 reduction, water electrolysis, carbon corrosion, N-related catalysis, electrosynthesis, fuel cells, photo-electrocatalysis and beyond) and energy storage applications (Li-ion batteries and beyond, metal-air batteries, supercapacitors and flow batteries). The recent development of DEMS-hyphenated techniques and the outlook of the DEMS technique are discussed at the end. As DEMS celebrates its 40th anniversary in 2024, we hope this review can offer electrochemistry researchers a comprehensive understanding of the latest developments of DEMS and will inspire them to tackle emerging scientific questions using DEMS.

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.

Hydrophobic SiO2 Armor: Stabilizing Cuδ+ to Enhance CO2 Electroreduction toward C2+ Products in Strong Acidic Environments

Electroreduction of CO2 in highly acidic environments holds promise for enhancing CO2 utilization efficiency. Due to the HER interference and structural instability, however, challenges in improving the selectivity and stability toward multicarbon (C2+) products remain. In this study, we proposed an "armor protection" strategy involving the deposition of ultrathin, hydrophobic SiO2 onto the Cu surface (Cu/SiO2) through a simple one-step hydrolysis. Our results confirmed the effective inhibition of HER by a hydrophobic SiO2 layer, leading to a high Faradaic efficiency (FE) of up to 76.9% for C2+ products at a current density of 900 mA cm-2 under a strongly acidic condition with a pH of 1. The observed high performance surpassed the reported performance for most previously studied Cu-based catalysts in acidic CO2RR systems. Furthermore, the ultrathin hydrophobic SiO2 shell was demonstrated to effectively prevent the structural reconstruction of Cu and preserve the oxidation state of Cuδ+ active sites during CO2RR. Additionally, it hindered the accumulation of K+ ions on the catalyst surface and diffusion of in situ generated OH- ions away from the electrode, thereby favoring C2+ product generation. In situ Raman analyses coupled with DFT simulations further elucidated that the SiO2 shell proficiently modulated *CO adsorption behavior on the Cu/SiO2 catalyst by reducing *CO adsorption energy, facilitating the C-C coupling. This work offers a compelling strategy for rationally designing and exploiting highly stable and active Cu-based catalysts for CO2RR in highly acidic environments.

Atomic Design of Copper Active Sites in Pristine Metal-Organic Coordination Compounds for Electrocatalytic Carbon Dioxide Reduction

Electrocatalytic carbon dioxide reduction reaction (CO2RR) has emerged as a promising and sustainable approach to cut carbon emissions by converting greenhouse gas CO2 to value-added chemicals and fuels. Metal-organic coordination compounds, especially the copper (Cu)-based coordination compounds, which feature well-defined crystalline structures and designable metal active sites, have attracted much research attention in electrocatalytic CO2RR. Herein, the recent advances of electrochemical CO2RR on pristine Cu-based coordination compounds with different types of Cu active sites are reviewed. First, the general reaction pathways of electrocatalytic CO2RR on Cu-based coordination compounds are briefly introduced. Then the highly efficient conversion of CO2 on various kinds of Cu active sites (e.g., single-Cu site, dimeric-Cu site, multi-Cu site, and heterometallic site) is systematically discussed, along with the corresponding catalytic reaction mechanisms. Finally, some existing challenges and potential opportunities for this research direction are provided to guide the rational design of metal-organic coordination compounds for their practical application in electrochemical CO2RR.

Oxygen-Bridged Cu Binuclear Sites for Efficient Electrocatalytic CO2 Reduction to Ethanol at Ultralow Overpotential

Electrocatalytic CO2 reduction (CO2RR) to alcohols offers a promising strategy for converting waste CO2 into valuable fuels/chemicals but usually requires large overpotentials. Herein, we report a catalyst comprising unique oxygen-bridged Cu binuclear sites (CuOCu-N4) with a Cu···Cu distance of 3.0-3.1 Å and concomitant conventional Cu-N4 mononuclear sites on hierarchical nitrogen-doped carbon nanocages (hNCNCs). The catalyst exhibits a state-of-the-art low overpotential of 0.19 V (versus reversible hydrogen electrode) for ethanol and an outstanding ethanol Faradaic efficiency of 56.3% at an ultralow potential of -0.30 V, with high-stable Cu active-site structures during the CO2RR as confirmed by operando X-ray adsorption fine structure characterization. Theoretical simulations reveal that CuOCu-N4 binuclear sites greatly enhance the C-C coupling at low potentials, while Cu-N4 mononuclear sites and the hNCNC support increase the local CO concentration and ethanol production on CuOCu-N4. This study provides a convenient approach to advanced Cu binuclear site catalysts for CO2RR to ethanol with a deep understanding of the mechanism.