Electrochemical CO2 Reduction Reaction Research Articles

Selective electroreduction of CO2 to C2+ products on cobalt decorated copper catalysts

Cu-catalyzed electrochemical CO2 reduction reaction (CO2RR) to multi-carbon (C2+) products is often plagued by low selectivity because the adsorption energies of different reaction intermediates are in a linear scaling relationship. Development of Cu-based bimetallic catalysts has been considered as an attractive strategy to address this issue; however, conventional bimetallic catalysts often avoid metals with strong CO adsorption energies to prevent surface poisoning. Herein, we demonstrated that limiting the amount of Co in CuCo bimetallic catalysts can enhance C2+ product selectivity. Specifically, we synthesized a series of CuCox catalysts with trace amounts of Co (0.07-1.8 at%) decorated on the surface of Cu nanowires using a simple dip coating method. Our results revealed a volcano-shaped correlation between Co loading and C2+ selectivity, with the CuCo0.4% catalyst exhibiting a 2-fold increase in C2+ selectivity compared to the Cu nanowire sample. In situ Raman and Infrared spectroscopies suggested that an optimal amount of Co could stabilize the Cu oxide/hydroxide species under the CO2RR condition and promote the adsorption of CO, thus enhancing the C2+ selectivity. This work expands the potential for developing Cu-based bimetallic catalysts for CO2RR.

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

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

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

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

Promotion of CO2 Electroreduction on Bismuth Nanosheets with Cerium Oxide nanoparticles.

Formic acid (HCOOH) is a highly energy efficient product of the electrochemical CO2 reduction reaction (CO2RR). Bismuth-based catalysts have shown promise in the conversion of CO2 to formic acid, but there is still a great need for further improvement in selectivity and activity. Herein, we report the preparation of Bi nanosheets decorated by cerium oxide nanoparticles (CeOx) with high Ce3+/Ce4+ ratio and rich oxygen vacancies. The CeOx nanoparticles affect the electronic structures of bismuth, enhance CO2 adsorption, and thus promote the CO2RR properties of Bi nanosheets. Compared with elemental Bi nanosheets, the hetero-structured CeOx/Bi nanosheets exhibit much higher activity over a wide potential window, showing a current density of 16.1 mA cm-2 with a Faradaic efficiency of 91.1% at -0.9 V vs. reversible hydrogen electrode.

Intrinsically Conductive and Cu-Functionalized Polymer-Composite Membranes as Gas Diffusion Electrodes for CO2 Electroreduction.

We introduced a new class of gas diffusion electrodes (GDEs) with adjustable pore morphology. We fabricated intrinsically conductive polymer-composite membranes containing carbon filler, enabling a pore structure variation through film casting cum phase separation protocols. We further selectively functionalized specific pore regions of the membranes with Cu by a NaBH4-facilitated coating strategy. The as-obtained GDEs can facilitate the electrochemical CO2 reduction reaction (CO2RR) at Cu active sites that are presented inside a defined and electrically conductive pore system. When employing them as free-standing cathodes in a CO2 flow electrolyzer, we achieved >70% Faradaic efficiencies for CO2RR products at up to 200 mA/cm2. We further demonstrated that deposition of a dense Cu layer on top of the membrane leads to obstruction of the underlying pore openings, inhibiting an excessive wetting of the pore pathways that transport gaseous CO2. However, the presentation of Cu inside the pore system of our novel membrane electrodes increased the C2H4/CO selectivity by a factor of up to 3 compared to Cu presented in the dense layer on top of the membrane. Additionally, we found that gaseous CO2 could still access Cu in macropores after wetting with electrolyte, while CO2RR was completely suppressed in wetted nm-scale pores.

Porous Bi Nanosheets Derived from β-Bi2O3 for Efficient Electrocatalytic CO2 Reduction to Formate.

The electrochemical CO2 reduction reaction (ECO2RR) is a promising strategy for converting CO2 into high-value chemical products. However, the synthesis of effective and stable electrocatalysts capable of transforming CO2 into a specified product remains a huge challenge. Herein, we report a template-regulated strategy for the preparation of a Bi2O3-derived nanosheet catalyst with abundant porosity to achieve the expectantly efficient CO2-to-formate conversion. The resultant porous bismuth nanosheet (p-Bi) not only exhibited marked Faradaic efficiency of formate (FEformate), beyond 91% in a broad potential range from -0.75 to -1.1 V in the H-type cell, but also demonstrated an appreciable FEformate of 94% at a high current density of 262 mA cm-2 in the commercially important gas diffusion cell. State-of-the-art X-ray absorption near edge structure spectroscopy (XANES) and theoretical calculation unraveled the distinct formate production performance of the p-Bi catalyst, which was cocontributed by its smaller size, plentiful porous structure, and stronger Bi-O bond, thus accelerating the absorption of CO2 and promoting the subsequent formation of intermediates. This work provides an avenue to fabricate bismuth-based catalysts with high planar and porous morphologies for a broad portfolio of applications.

Hierachical Aerogel-Supported Cu-Sn-Ox Solid Solutions for Highly Selective CO2 Electroreduction and Zn-CO2 Batteries.

The electrochemical CO2 reduction reaction (CO2RR) into high-value carbon compounds such as CO and HCOOH is a promising strategy for the utilization and conversion of emitted CO2. However, the selectivity of the CO2RR for HCOOH is typically less than 90% and operates within a narrow voltage range, which limits its practical application. Herein, we propose a novel heterostructural aerogel as a highly efficient electrocatalyst for CO2RR to HCOOH. This catalyst consists of Cu-Sn-Ox solid solutions embedded in a reduced graphene oxide matrix (Cu-Sn-Ox/rGO). The incorporation of Cu2+ into the SnO2 matrix enhances HCOOH production by improving the adsorption of the *OCHO intermediate and inhibiting H2 evolution, as confirmed by in situ measurements and computational studies. As a result, Cu-Sn-Ox/rGO achieves a remarkable Faradaic efficiency (FE) of up to 91.4% for HCOOH and maintains high selectivity over a broad operating voltage range (-0.8 to -1.1 V). Additionally, the assembled Zn-CO2 batteries demonstrated an excellent power density of 1.14 mW/cm2 and exceptional stability for over 25 h.

Corrosion and Enhanced Hydrogen Evolution in Electrochemical Reduction of Ammonium Carbamate on Transition Metal Surfaces.

Experiments and theory are combined to search for catalyst activity and stability descriptors for the direct reactive capture and conversion (RCC) of CO2 in ammonia capture solutions using Cu, Ag, Au, Sn, and Ti electrodes. Two major phenomena emerge in RCC that are not predominant in the electrochemical CO2 reduction (CO2R) reaction, namely, the rapid corrosion and restructuring of the catalyst in the presence of the CO2-ammonia adducts and the promotion of the competing hydrogen evolution reaction (HER). The prevalence of HER in RCC is correlated to the electrostatic attraction of the protonated amine to the electrode and the repulsion of the captured CO2, using the potential of zero charge (PZC). The stability of catalysts under RCC conditions is a function of the applied potential and cannot be readily predicted using binding energy descriptors commonly used in the prediction of CO2R activity. A direct correlation between calculated binding energies of CO2R intermediates, atomic oxygen, hydrogen, and ammonia and the activity and stability of transition metals for RCC cannot be found, highlighting the need for descriptors beyond those known for CO2R.

Stable Cu+/Cu2+ species derived from in-situ growing Cu-S-V bonds in CuVxS electrocatalysts enables high efficiency CO2 electroreduction to methanol

Regulating oxidation state of Cu-based chalcogenides catalysts during electrolysis is of great significance for efficient CO2 electrochemical reduction reaction (CO2RR) to methanol, which still remains a dramatic challenge. Herein, we construct a heterogeneous Cu3VS4/CuS electrocatalyst (denoted as CuVxS), in which the in-situ growing Cu-S-V bonds avoid the accumulated electrons on oxidized Cu sites and inhibit the deep reduction of CuS to metallic Cu during electrolysis, maintaining stable and abundant reactive Cu+/Cu2+ species to promote CO2RR performance to methanol. By this design, the CuV0.1S catalyst achieves an ultrahigh methanol selectivity of 79.8 %. In-situ FTIR measurement reveals that the synergistic effect of Cu+/Cu2+ species and Cu-S-V bonding facilitates the *CO adsorption and the subsequent *CO hydrogenation process, which improve the generation of *CHO and *OCH3 intermediates for enhanced methanol selectivity. This work provides a new insight into stabilizing high oxidation state of Cu-based electrocatalyst to maintain sufficient active sites for CO2RR.

Low-temperature fabrication of morphology-controllable Cu2O for electrochemical CO2 reduction

Cu2O has been successfully synthesized in different morphologies/sizes (nanoparticles and octahedrons) via a low-temperature chemical reduction method. Trapping metal ions in an ice cube and letting them slowly melt in a reducing agent solution is the simplest way to control the nanostructure. Enhancement of charge transfer and transportation of ions by Cu2O nanoparticles was shown by cyclic voltammetry and electrochemical impedance spectroscopy measurements. In addition, nanoparticles exhibited higher current densities, the lowest onset potential, and the Tafel slope than others. The Cu2O electrocatalyst (nanoparticles) demonstrated the Faraday efficiencies (FEs) of CO, CH4, and C2H6 up to 11.90, 76.61, and 1.87%, respectively, at −0.30 V versus reference hydrogen electrode, which was relatively higher FEs than other morphologies/sizes. It is mainly attributed to nano-sized, more active sites and oxygen vacancy. In addition, it demonstrated stability over 11 h without any decay of current density. The mechanism related to morphology tuning and electrochemical CO2 reduction reaction was explained. This work provides a possible way to fabricate the different morphologies/sizes of Cu2O at low-temperature chemical reduction methods for obtaining the CO, CH4, and C2H6 products from CO2

Zn-Cu bimetallic gas diffusion electrodes for electrochemical reduction of CO2 to ethylene

The application of renewable energy to drive the electroreduction of carbon dioxide (CO2) into valuable compounds serves as a means of energy storage and simultaneously aids in the mitigation of climate change. In this study, a bimetallic catalyst consisting of Zinc and Copper is synthesized using the Zinc-doped HKUST-1 metal-organic framework. The catalyst is then fabricated on gas diffusional electrode and investigated in both H-Cell and Flow cell configurations for electrochemical CO2 reduction reaction (ECO2RR). In the H-Cell, the catalysts that have been developed exhibit an ethylene faradaic efficiency (FE) up to 40 % when operated at a potential of −0.8 V relative to the reversible hydrogen electrode (RHE). The Zn-Cu bimetallic gas diffusion electrodes (GDE) exhibit an impressive ethylene FE up to 45 % when operating at a current density of −200 mA cm−2 at −1.0 V versus RHE in the flow cell. This research offers valuable insights into the strategic development of copper-based bimetallic catalysts with the aim of enhancing the efficiency of electrochemical reduction of CO2 to yield multicarbon compounds.

Oxygen Vacancy-Driven Heterointerface Breaks the Linear-Scaling Relationship of Intermediates toward Electrocatalytic CO2 Reduction.

Smart metal-metal oxide heterointerface construction holds promising potentials to endow an efficient electron redistribution for electrochemical CO2 reduction reaction (CO2RR). However, inhibited by the intrinsic linear-scaling relationship, the binding energies of competitive intermediates will simultaneously change due to the shifts of electronic energy level, making it difficult to exclusively tailor the binding energies to target intermediates and the final CO2RR performance. Nonetheless, creating specific adsorption sites selective for target intermediates probably breaks the linear-scaling relationship. To verify it, Ag nanoclusters were anchored onto oxygen vacancy-rich CeO2 nanorods (Ag/OV-CeO2) for CO2RR, and it was found that the oxygen vacancy-driven heterointerface could effectively promote CO2RR to CO across the entire potential window, where a maximum CO Faraday efficiency (FE) of 96.3% at -0.9 V and an impressively high CO FE of over 62.3% were achieved at a low overpotential of 390 mV within a flow cell. The experimental and computational results collectively suggested that the oxygen vacancy-driven heterointerfacial charge spillover conferred an optimal electronic structure of Ag and introduced additional adsorption sites exclusively recognizable for *COOH, which, beyond the linear-scaling relationship, enhanced the binding energy to *COOH without hindering *CO desorption, thus resulting in the efficient CO2RR to CO.

Amphipathic Surfactant on Reconstructed Bismuth Enables Industrial-Level Electroreduction of CO2 to Formate.

Developing efficient electrocatalysts for selective formate production via the electrochemical CO2 reduction reaction (CO2RR) is challenged by high overpotential, a narrow potential window of high Faradaic efficiency (FEformate), and limited current density (Jformate). Herein, we report a hierarchical BiOBr (CT/h-BiOBr) with surface-anchored cetyltrimethylammonium bromide (CTAB) for formate-selective large-scale CO2RR electrocatalysis. CT/h-BiOBr achieves over 90% FEformate across a wide potential range (-0.5 to -1.1 V) and an industrial-level Jformate surpassing 100 mA·cm-2 at -0.7 V. In situ investigations uncover the reconstructed Bi(110) surface as the active phase, with CTAB playing a dual role: its hydrophobic alkyl chains create a CO2-enriching microenvironment, while its polar head groups fine-tune the electronic structure, fostering a highly active phase. This work provides valuable insights into the role of surfactants in electrocatalysis and guides the design of electrocatalysts for the large-scale CO2RR.

A First-Principles Study on the Reaction Mechanisms of Electrochemical CO2 Reduction to C1 and C2 Products on Cu(110)

The mechanism of the electrochemical CO2 reduction reaction on a Cu(110) surface has yet to be fully revealed. In this work, based on first-principles calculations, we investigate the mechanisms of the CO2 reduction reaction to produce C1 (including one C atom) and C2 (including two C atoms) products on a Cu(110) surface. The results show that CH4 and C2H5OH are the main C1 and C2 products on the Cu(110) surface, respectively. CH4 is produced along the pathway CO2 → COOH* → CO* → CHO* → CH2O* → CH3O* → CH4. C2H5OH is produced via the C-C coupling pathway between CO* and CH2O* intermediates, which is the key reaction step. This is because CO* and CH2O* coupling to CO-CH2O* has the lowest barrier among the CHxO* (x = 0–2) coupling pathways. Therefore, it is the most likely C-C coupling pathway. Further, CO-CH2O* is gradually hydrogenated to C2H5OH along the following pathway: CO-CH2O* → CHO-CH2O* → CHOH-CH2* → CH2OH-CH2* → CH2OH-CH3* → C2H5OH.

Decoupling and quantifying the mass transfer resistance of the gas diffusion electrode for CO2 electrochemical reduction reaction

Mass transfer capacity ultimately dominates and limits the performance of CO2 electrochemical reduction reaction (CO2RR), which is closely related to industrial applications. Hence, the quantification of CO2 transport resistance is urgent and necessary. In addition, the transport of CO2 involves the entire process from the gas phase source to the catalytic site, resulting in the superposition of transport resistance from each component of the electrolyzer, which makes the transport resistance decoupling challenging. Based on the limiting current method, an experimental method is developed here to quantify the transport resistance of the complex porous electrode of the carbon fiber substrate (CFS), microporous layer (MPL) and catalyst layer (CL). Additionally, a dummy catalyst layer (DCL) is designed, which has a similar pore structure to CL but no catalytic sites for CO2RR, to further quantify the resistance in CL formed from pore structures and ionomer properties. The experimental results can provide the optimization direction and experimental basis for the subsequent electrode design.

Multiscale Modeling of CO2 Electrochemical Reduction on Copper Electrocatalysts: A Review of Advancements, Challenges, and Future Directions.

Although CO2 contributes to global warming, it also offers potential as a raw material for the production of hydrocarbons (CH4, C2H4 and CH3OH). Electrochemical CO2 reduction reaction (eCO2RR) is an emerging technology that utilizes renewable energy to convert CO2 into valuable fuels, solving environmental and energy problems simultaneously. Insights gained at any individual scale can only provide a limited view of that specific scale. Multiscale modeling, which involves coupling atomistic-level insights (DFT) and (MD), with mesoscale (KMCand MK) and macroscale (CFD) simulations, has received significant attention recently. While multiscale modeling of eCO2RR on electrocatalysts across all scales is limited due to its complexity, this review offers an overview of recent works on single scales and the coupling of two and three scales, such as "DFT+MD", "DFT+KMC", "DFT+MK", "KMC/MK+CFD" and "DFT+MK/KMC+CFD", focusing particularly on Cu-based electrocatalysts. This sets it apart from other reviews that solely focus exclusively on a single scale or only on a combination of DFT and MK/KMC scales. Furthermore, this review offers a concise overview of machine learning (ML) applications for eCO2RR, an emerging approach that has not yet been reviewed. Finally, this review highlights the key challenges, research gaps and perspectives of multiscale modeling for eCO2RR.

Constructing Favorable Microenvironment on Copper Grain Boundaries for CO2 Electro-conversion to Multicarbon Products.

The electrochemical CO2 reduction reaction (eCO2RR) to multicarbon chemicals provides a promising avenue for storing renewable energy. Herein, we synthesized small Cu nanoparticles featuring enriched tiny grain boundaries (RGBs-Cu) through spatial confinement and in situ electroreduction. In-situ spectroscopy and theoretical calculations demonstrate that small-sized Cu grain boundaries significantly enhance the adsorption of the *CO intermediate, owing to the presence of abundant low-coordinated and disordered atoms. Furthermore, these grain boundaries, generated in situ under high current conditions, exhibit excellent stability during the eCO2RR process, thereby creating a stable *CO-rich microenvironment. This high local *CO concentration around the catalyst surface can reduce the energy barrier for C-C coupling and significantly increase the Faradaic efficiency (FE) for multicarbon products across both neutral and alkaline electrolytes. Specifically, the developed RGBs-Cu electrocatalyst achieved a peak FE of 77.3% for multicarbon products and maintained more than 134 h stability at a constant current density of -500 mA cm-2.

Distinct CO2-run-out regime from steric effect of electric double layer in electrochemical CO2 reduction

The field of electrochemical CO2 reduction reaction (eCO2RR) is pursuing high operating current densities, eventually controlled by CO2 transport. Here, we develop a new multiscale modeling approach that is able to more generally describe the effects of the electric double layer (EDL) on CO2 transport over a wide potential window extending to utmost potentials. By leveraging it, we identify a distinct CO2-run-out regime where the supply of CO2 runs out due to the EDL steric effect from a dense layer of solvated cations with the maximum layer thickness equal to the solvated cation size. Consequently, CO2RR current density drops at a relatively negative transition potential generating a bell-shaped polarization curve, which is in contrast to the CO2-transport-limited regime where the current density reaches a plateau. Furthermore, we develop a graphical method, verified by experimental data, to generally predict the transition to the CO2-run-out regime. This work sheds new light on the EDL effects for catalyst design and electrolyzer engineering.

Advancing electrochemical CO2 reduction with group 11 metal nanoclusters for renewable energy solutions

AbstractUnderstanding the intricate relationship between structure and properties is paramount in distinguishing nanocluster (NC) materials from their counterparts. Despite the progress in synthesizing new NCs, the sluggish exploration of their potential applications persists due to the difficulty in stabilizing these materials. However, recent investigations have unveiled their remarkable efficacy as catalysts in electrochemical CO2 reduction reactions, surpassing traditional materials. This discovery, addressing urgent global concerns, has quickly drawn significant attention to this field, leading to its rapid expansion. Hence, there is an urgent need to outline this research landscape and pinpoint effective strategies, marking a significant advancement. In this context, our endeavor is dedicated to offering researchers a thorough understanding of recently synthesized NC materials. We aim to elucidate their distinct structural architectures and associated properties essential for catalyst design. We envision that this systematic review will serve as a guiding beacon for future research endeavors in this burgeoning field.

Understanding the Temperature Effect on Carbon‐Carbon Coupling during CO2 and CO Electroreduction in Zero‐Gap Electrolyzers†

Comprehensive SummaryCu‐catalyzed electrochemical CO2 reduction reaction (CO2RR) and CO reduction reaction (CORR) are of great interest due to their potential to produce carbon‐neutral and value‐added multicarbon (C2+) chemicals. In practice, CO2RR and CORR are typically operated at industrially relevant current densities, making the process exothermal. Although the increased operation temperature is known to affect the performance of CO2RR and CORR, the relationship between temperatures and kinetic parameters was not clearly elaborated, particularly in zero‐gap reactors. In this study, we detail the effect of the temperature on Cu‐catalyzed CO2RR and CORR. Our electrochemical and operando spectroscopic studies show that high temperatures increase the activity of CO2RR to CO and CORR to C2H4 by enhancing the mass transfer of CO2 and CO. As the rates of these two processes are highly influenced by reactant diffusion, elevating the operating temperature results in high local CO2 and CO availability to accelerate product formation. Consequently, the *CO coverage in both cases increases at higher temperatures. However, under CO2RR conditions, *CO desorption is more favorable than carbon‐carbon (C—C) coupling thermodynamically at high temperatures, causing the reduction in the Faradaic efficiency (FE) of C2H4. In CORR, the high‐temperature‐augmented CO diffusion overcomes the unfavorable adsorption thermodynamics, increasing the probability of C—C coupling.