CO2 Reduction Reaction Selectivity Research Articles

Electrochemical CO2 Reduction to Ethanol on Zn‐Coordinated Cu Sites Formed by Atmosphere‐Induced Surface Reconstruction

AbstractElectrochemical CO2 reduction reaction (CO2RR) to produce chemicals and fuels is a promising strategy to achieve carbon neutrality. However, due to the slow C─C coupling kinetics and the fact that C2H5OH and C2H4 products share the same *HCCOH intermediate, achieving high activity and selectivity for C2H5OH remains challenging. This paper describes an atmosphere‐induced reconstruction method to optimize the surface composition and coordination structure of the CuxZny bimetallic alloy catalysts for C2H5OH production. Specifically, the CuxZny alloy catalyst treated within CO atmosphere (CO‐CuxZny) enriches with low‐coordinated Cu sites, which are favorable for the adsorption of the *CO intermediates for promoted C2H5OH production. A C2+ Faradaic efficiency (FE) of 85.1% and a C2H5OH FE of 59.5% are achieved by the CO‐Cu84Zn16 at a current density of 300 mA cm−2. In‐situ spectroscopic studies and DFT calculations demonstrate that the enhanced *CO adsorption promotes the C─C coupling process. At the same time, the hydrogenation of *HCCOH is more favorable on CO‐Cu84Zn16 to inhibit the C2H4 pathway, thus enhancing the generation of C2H5OH. This study provides an effective strategy to regulate the selectivity of CO2RR through the control of the surface coordination environment of the active sites.

Impact of gas diffusion layer architecture on the performances of Cu-based electrodes for CO2 electroreduction to ethylene in flow reactors

Electrocatalytic CO2 reduction reaction (CO2RR) is a promising strategy to convert CO2 into fuels and building blocks for chemicals, exploiting renewable energy sources. Flow cells using gas diffusion electrodes (GDEs) instead of simple H-cells are required to achieve higher production rates. In a GDE, CO2 diffuses through a porous gas diffusion layer (GDL) and reaches the active sites located in the catalyst layer; the GDL is a crucial component that can affect the performance of the GDE. In this work, we aimed to establish the parameters of the GDL that influence the selectivity toward the different CO2 reduction products in an alkaline flow reactor. We employed commercial Cu nanoparticles as electrocatalyst and selected a range of market-available GDLs: the structure, composition, and type of cracks of the GDL greatly influence the Faradaic Efficiency (FE) of CO, H2, and ethylene (C2H4). With the emphasis on C2H4, it was found that using a crack-free GDE is essential to get higher FEC2H4. At the same time, larger and more abundant cracks are detrimental to C2H4 production and promote hydrogen evolution: for instance, the FEC2H4 varies in the wide range of 13–32%, from the highly cracked to the crack-free GDL. The presence of both sulfur and oxygen in the microporous layer (MPL) of the GDL helps shift the CO2RR selectivity toward CO. The (in)stability of the GDEs at high current density has been correlated with a change in the wetting properties of the GDE and a loss in hydrophobicity.

Activating *CO by Strengthening Fe–CO π‐Backbonding to Enhance Two‐Carbon Products Formation toward CO2 Electroreduction on Fe–N4 Sites

AbstractMany non‐precious metal‐nitrogen (M–Nx)‐containing catalysts are highly efficient for electrochemical reduction of CO2 to CO and yet encounter challenges in further converting CO to more valuable two‐carbon products (C2), such as ethanol and acetic acid. The ambiguous structure‐activity relationship of the M–Nx moieties toward CO2 reduction reaction (CO2RR) results in difficulties in regulating the CO2RR product selectivity on the M–Nx‐containing catalysts. Herein, by using fluorinated iron phthalocyanines with axial‐coordinated ligands (L–FePc–F) as an M–N4‐based model electrocatalyst for CO2RR, a correlation between the electronic structure and C2 selectivity of Fe–N4 is revealed and a comprehensive descriptor based on the Fe–CO π‐backbonding is proposed for guiding the regulation of M–N4 toward higher C2 selectivity. Based on the regulation principle, the Br‐axial‐coordinated FePc–F (Br–FePc–F) remarkably increases the Faradic efficiency (FE) of C2 products from 0% (i.e., the FEC2 of FePc–F) to 34% due to the strengthened Fe–CO π‐backbonding stemming from the elevated 3dxz/dyz orbital energy and enhanced electron‐donating ability of the Fe centers of Fe–N4. This work provides a strategy and mechanism insights on the regulation of the C2 selectivity of CO2RR on the Fe–N4 moieties, which may be inspiring for precise construction and regulation of the M–Nx‐containing catalysts for specific CO2RR products.

Cr2O3 promotes the catalytic performance of Bi-based catalysts for electrochemical CO2 reduction to HCOOH

Electrochemical CO2 reduction reaction (CO2RR) to formic acid is a much-anticipated way of converting excess CO2 to high-value-added products. Bismuth (Bi)-based materials are promising catalysts, but remain challenging in terms of activity and selectivity. Herein, we reported a composition manipulation strategy to achieve simultaneous high activity and selectivity for CO2RR towards formate by incorporating Lewis acid Cr2O3 into Bi. Specifically, a novel Bi-Cr2O3 crystalline-amorphous catalyst was prepared by electroreduction of bismuth chromate (Bi2(CrO4)3). The as-prepared catalyst shows nano-dendrites structure, promising the maximum exposure of active sites. The obtained Bi-Cr2O3 catalyst exhibits a high Faradic efficiency (FEformate, 93.3 %) at −1.1 V vs reversible hydrogen electrode (RHE), and can achieve a remarkable current density of −40.7 mA cm−2 at the same potential. Meanwhile, the FEformate remains above 90 % over a wide potential window (−0.8 ∼ −1.2 V vs RHE) and the stability can maintain for 13 h at −0.9 V vs RHE with high selectivity (>90 %) and stable physical phase. Furthermore, the Zn-CO2 battery assembled with a Bi-Cr2O3 catalyst achieves the greatest power density of 4.63 mW cm−2 and could maintain cyclic charge/discharge stability for 40 h.

Selectivities of Cu edges surface environment for C1 and C2 pathways

Copper (Cu) is an effective catalytic material for the electrochemical CO2 reduction reaction (CO2RR). The final products are highly reliant on surface conditions, and edge models can effectively expose different surface environments, which provide a large number of representative adsorption sites. Compared with the traditional Cu(111), the change of coordination number (CN) and the formation of OCu bond greatly increase the number of electrons transferred during CO2 activation. First-principles calculations have found that the adsorption site at the outer edge (6-site) of the step surface exhibits a significant advantage in both the C1 and C2 pathways. Due to its minimum CN, there is deep p-d hybridization, resulting in the lower Gibbs free energy (G) at the rate-determining steps (RDS). Specifically, the overpotentials are only 0.74 and 0.66 V during C1 and C2 pathways, respectively, indicating good catalytic performance. Differences in surface conditions will inevitably induce better adsorption state of intermediates, optimizing the selectivity of CO2RR. Our current work potentially provides a new avenue for designing more efficient and selective Cu electrocatalysts.

Targeted C-O bond cleavage of *CH2CHO at copper active sites for efficient electrosynthesis of ethylene from CO2 reduction

*CH2CHO, a pivotal intermediate in CO2 reduction reaction (CO2RR) on copper-based catalysts, hinges on the strength of Cu-C and C-O bonds for the selective production of ethanol and ethylene. However, the targeted cleavage of these bonds at Cu active sites presents a formidable challenge. In this study, we manipulated the selective C-O bond breaking at Cu through an electron enrichment strategy, steering the reaction towards ethylene synthesis. Both experimental and theoretical investigations reveal that Gd incorporation elevates electron density at Cu sites, thereby enhancing Cu-O interaction and concurrently weakening the C-O bond at the critical Cu-*O-CHCH2 bifurcation point. Notably, Gd-doped Cu2O (Gd-Cu2O) demonstrated a 1.43-fold increase in the ethylene/CO ratio relative to undoped Cu2O. This alteration steers the reaction mechanism towards ethylene generation. Our study highlights the pivotal role of regulating reaction intermediates in optimizing activity and selectivity of CO2RR in copper-based catalysts, providing valuable insights for future catalyst development.

Electrochemical CO2 Reduction by Urea Hangman Mn Terpyridine species.

Based on our previous study in chemical subtleties of the proton tunneling distance for metal hydride formation (PTD-MH) to regulate the selectivity of CO2 reduction reaction (CO2RR), we have developed a family of Mn terpyridine derivatives, in which urea groups functions as multipoint hydrogen-bonding hangman to accelerate the reaction rate. We found that such changes to the second coordination sphere significantly increased the turnover frequency (TOF) for CO2 reduction to ca. 360 with this family of molecular catalysts while maintaining high selectivity (ca. 100 %±3) for CO even in the presence of a large amount of phenol as proton source. Notably, the compounds studied in this manuscript all exhibit large value for as that achieved by Fe porphyrins derivates, while saving up to 0.55 V in overpotential with respect to the latter.

Backbone Engineering of Polymeric Catalysts for High-Performance CO2 Reduction in Bipolar Membrane Zero-Gap Electrolyzer.

Bipolar membranes (BPMs) have emerged as a promising solution for mitigating CO2 losses, salt precipitation and high maintenance costs associated with the commonly used anion-exchange membrane electrode assembly for CO2 reduction reaction (CO2RR). However, the industrial implementation of BPM-based zero-gap electrolyzer is hampered by the poor CO2RR performance, largely attributed to the local acidic environment. Here, we report a backbone engineering strategy to improve the CO2RR performance of molecular catalysts in BPM-based zero-gap electrolyzers by covalently grafting cobalt tetraaminophthalocyanine onto a positively charged polyfluorene backbone (PF-CoTAPc). PF-CoTAPc shows a high acid tolerance in BPM electrode assembly (BPMEA), achieving a high FE of 82.6 % for CO at 100 mA/cm2 and a high CO2 utilization efficiency of 87.8 %. Notably, the CO2RR selectivity, carbon utilization efficiency and long-term stability of PF-CoTAPc in BPMEA outperform reported BPM systems. We attribute the enhancement to the stable cationic shield in the double layer and suppression of proton migration, ultimately inhibiting the undesired hydrogen evolution and improving the CO2RR selectivity. Techno-economic analysis shows the least energy consumption (957 kJ/mol) for the PF-CoTAPc catalyst in BPMEA. Our findings provide a viable strategy for designing efficient CO2RR catalysts in acidic environments.

Backbone Engineering of Polymeric Catalysts for High‐Performance CO2 Reduction in Bipolar Membrane Zero‐Gap Electrolyzer

AbstractBipolar membranes (BPMs) have emerged as a promising solution for mitigating CO2 losses, salt precipitation and high maintenance costs associated with the commonly used anion‐exchange membrane electrode assembly for CO2 reduction reaction (CO2RR). However, the industrial implementation of BPM‐based zero‐gap electrolyzer is hampered by the poor CO2RR performance, largely attributed to the local acidic environment. Here, we report a backbone engineering strategy to improve the CO2RR performance of molecular catalysts in BPM‐based zero‐gap electrolyzers by covalently grafting cobalt tetraaminophthalocyanine onto a positively charged polyfluorene backbone (PF‐CoTAPc). PF‐CoTAPc shows a high acid tolerance in BPM electrode assembly (BPMEA), achieving a high FE of 82.6 % for CO at 100 mA/cm2 and a high CO2 utilization efficiency of 87.8 %. Notably, the CO2RR selectivity, carbon utilization efficiency and long‐term stability of PF‐CoTAPc in BPMEA outperform reported BPM systems. We attribute the enhancement to the stable cationic shield in the double layer and suppression of proton migration, ultimately inhibiting the undesired hydrogen evolution and improving the CO2RR selectivity. Techno‐economic analysis shows the least energy consumption (957 kJ/mol) for the PF‐CoTAPc catalyst in BPMEA. Our findings provide a viable strategy for designing efficient CO2RR catalysts in acidic environments.

Promoting Selective CO2 Electroreduction to Formic Acid in Acidic Medium with Low Potassium Concentrations under High CO2 Pressure

AbstractElectrocatalytic CO2 reduction reaction (CO2RR) offers a sustainable pathway for the production of chemicals and fuels. Acidic electrolysis has been shown to be a promising strategy in order to avoid CO2 loss via the formation (bi)carbonate during reaction. Previous studies have been carried out in ambient CO2 pressure systems and have stressed the importance of adding high concentration of alkali cation (K+) in the catholyte to inhibit the hydrogen evolution reaction (HER) and achieve higher selectivity of CO2RR. Herein, CO2 reduction to HCOOH was performed in strong acid (pH 1) using a dendritic bismuth catalyst in a home‐designed high‐pressure electrochemical cell. At a CO2 pressure of 30 bar, we could achieve a high Faradaic efficiency of 100 % at 100 mA cm−2 at a KCl concentration of 3.0 M. With this first system that combines high pressure of CO2 and highly acidic catholyte, we show that pressurization offers an appropriate strategy to limit both HER and K+ dependence. Indeed we obtained a Faradaic efficiency of 34 % in the absence of K+ cations and 75–80 % in the presence of 1.0 M KCl under an applied current density of 100 mA cm−2.

Cobalt phthalocyanine promoted copper catalysts toward enhanced electro reduction of CO2 to C2: Synergistic catalysis or tandem catalysis?

The activity and selectivity of electrocatalytic CO2 reduction reaction (CO2RR) to C2 products on metal catalysts can be regulated by molecular surfactants. However, the mechanism behind it remains elusive and debatable. Herein, copper nanowires (Cu NWs) were fabricated and decorated with cobalt phthalocyanine (CoPc). The electronic interaction between the Cu NWs, CoPc, CO2 and CO2RR intermediates were explored by density functional theory (DFT) calculations. It was found that the selectivity and activity of CO2RR towards C2 products on Cu NWs were considerably enhanced from 35.2% to 69.9% by surface decoration of CoPc. DFT calculations revealed that CO2RR can proceed in the interphase between Cu substrate and CoPc, and the CO2RR intermediates could synergistically bond with both Cu and Co metal centre in CuNWs-CoPc, which favours the adsorption of CO2, CO and CO2RR intermediates, thus reducing the free energy for CO-CO coupling towards C2 products. The synergistic interaction was further extended to phthalocyanine (Pc) and other metal phthalocyanine derivatives (MPc), where a relatively weaker synergistic interaction of CO intermediates with MPc and Cu substrate and only a slight enhancement of CO2RR towards C2 products were observed. This study demonstrates a synergistic catalysis pathway for CO2RR, a novel perspective in interpreting the role of CoPc in enhancing the activity and selectivity of CO2RR on Cu NWs, in contrast to the conventional tandem catalysis mechanism.

Composite interfaces of g-C3N4 fragments loaded on a Cu substrate for CO2 reduction.

Designing an electrocatalyst with high efficiency and product selectivity is always crucial for an electrocatalytic CO2 reduction reaction (CO2RR). Inspired by the great progress of two-dimensional (2D) nanomaterials growing on Cu surfaces and their promising CO2RR catalytic efficiencies at their interfaces, the unique performance of Cu-based 2D materials as high-efficiency and low-cost CO2RR electrocatalysts has attracted extensive attention. Herein, based on density functional theory (DFT) calculations, we proposed a composite structure of graphitic carbon nitride (g-C3N4) fragments loaded on a Cu surface to explore the CO2RR catalytic property of the interface between g-C3N4 and the Cu surface. Three composite interfaces of C3N4/Cu(111), C3N4/Cu(110) and C3N4/Cu(100) have been studied by considering the reaction sites of vertex nitrogen atoms, edge nitrogen atoms and the nearby Cu atoms. It was found that the C3N4/Cu interfaces where nitrogen atoms contact the Cu substrate present competitive CO2RR activity. Among them, C3N4/Cu(111)-N3 exhibited a better activity for CH3OH production, with a low overpotential of 0.38 V. For HCOOH and CH4 production, C3N4/Cu(111)-Cu and C3N4/Cu(100)-N1 have overpotentials of 0.26 V and 0.44 V. The electronic analysis indicates the electron transfer from the Cu substrate to the g-C3N4 fragment and mainly accumulates on the nitrogen atoms of the interface. Such charge accumulation can activate the adsorbed CO bond of CO2 and lead to lower energetic barriers of CO2RR. DFT calculations indicate that the boundary nitrogen sites reduced the energy barrier of *CHO, which is crucial for CO2RR, compared with that of the pristine Cu surface. Our study explores a new Cu-based electrocatalyst and indicates that the C3N4/Cu interface can enhance the activities and selectivity of CO2RR and open a new strategy to design high-efficiency electrocatalysts for CO2RR.

Visualizing the Effect of Temperature on Carbonate Salt Production in CO2 Electrolysis

Carbon dioxide (CO2) electrolysis is a novel and promising technology to tackle anthropogenic climate change by reducing our dependency on fossil fuels and creating a sustainable carbon cycle. However, current CO2 electrolysis suffers from low CO2 reduction reaction (CO2RR) selectivity and rapid gas diffusion electrode (GDE) degradation and flooding due to carbonate precipitation. As the hydrophilic precipitate accumulates in the porous structure of the cathode GDE, the liquid anolyte invades the pores, blocking the catholyte from reaching the catalyst layer. This effect promotes the adverse hydrogen evolution reaction and thus decreases the CO2RR selectivity (1). Current literature focuses on limiting carbonate precipitate formation as opposed to understanding the factors affecting its growth (2). While carbonate precipitation is dependent on the specific electrolyte in a flow cell, the impact of parameters, such as operating temperature and GDE porosity, on the nature of carbonate precipitation still need to be investigated (3).In this study, we examine the relationship between cell temperature with carbonate precipitate accumulation. A zero-gap alkaline membrane electrode assembly (MEA) was used with potassium carbonate as the anolyte and humidified CO2 as the catholyte. Electrochemical impedance spectroscopy was used to calculate the activation, ohmic, and mass transport losses of the cell before and after a constant current density phase where faradaic efficiency was measured to quantify the CO2RR selectivity degradation due to salt accumulation. Furthermore, to characterize precipitate growth, the cathode GDEs were imaged before and after electrochemical testing using scanning electron microscopy (SEM) and wavelength dispersive spectroscopy (WDS). To quantify precipitate-driven flooding over operation time, identical MEAs were imaged using operando X-ray synchrotron radiography over a range of temperatures. Through-plane images of the operating cell were processed to find the electrolyte thickness in the cathode GDE as a function of time. Radiography results revealed progressive, cyclical flooding as well as preferential gas pathways. Moreover, after four hours of constant current operation and at a cell temperature of 60℃, the catalyst layer exhibited large amounts of uniformly distributed precipitates, while the GDE contained large, localized precipitate crystals. This indicates that at high temperatures, carbonate precipitates significantly block the catalyst layer and cause flooding into the substrate layer. This work provides new insight into the effect of temperature on carbonate precipitate buildup, deepening the understanding of the mechanism responsible for precipitate degradation. M. E. Leonard, L. E. Clarke, A. Forner-Cuenca, S. M. Brown, and F. R. Brushett, ChemSusChem, 13 (2020).Y. Xu et al., ACS Energy Lett, 6, 809–815 (2021).E. R. Cofell, U. O. Nwabara, S. S. Bhargava, D. E. Henckel, and P. J. A. Kenis, ACS Appl Mater Interfaces, 13, 15132–15142 (2021).

In Situ Characterization of Atomic-Scale Surface Restructuring of Copper Electrodes Under CO2 Electroreduction Conditions

The electrochemical CO2 reduction reaction (CO2RR) by copper-based heterogeneous catalysts has attracted widespread attention due to its potential social, economic, and environmental benefits, as it utilizes electricity from renewable sources to transform CO2 into value-added chemicals, thus closing the carbon cycle[1]. However, because the energy efficiency, selectivity and lifetime of CO2RR on Cu are still far away from those required at industrially conditions, a knowledge-based catalyst design to optimization of the catalytic system is urgently needed. The reactivity and selectivity of CO2RR depends on the catalyst material as well as on surface and interfacial structure/composition and the nanoscale morphology of the catalyst[2]. With the development of in situ and operando characterization techniques, the fundamental understanding of the structure-reactivity/selectivity relationships of catalysts under reaction conditions can be obtained. Here we monitor the CO-induced surface reconstruction of Cu(100) electrodes during CO2RR in 0.1M KHCO3 solution by in situ scanning tunneling microscopy (STM), surface X-ray diffraction (SXRD), and Raman spectroscopy. In situ STM images during step-wise potential changes show reversible cluster formation. The nanosized clusters start to appear at a potential ≤ -0.2 V vs. RHE and slowly disappear once the potential is changed back to more positive potentials, the clusters slowly disappear. However, the formation of Cu nanocluster leads to an irreversible modification of the surface structure on the molecular level. According to our previous research [3],freshly prepared Cu(100) electrodes are covered by a well-ordered (bi)carbonate adlayer in the double layer regime. After a potential excursion to the range of cluster formation, a disordered adlayer phase with only short-range order is observed in STM images, which can be explained by the presence of Cu adatoms and surface vacancies that remain on the electrode after dissolution of the nanoclustersIn the in situ SXRD studied, crystal truncation rods (CTRs) were recorded at different potentials from the double layer (0 V) to the CO2RR (-1.1 V) regime and then back to 0 V. Quantitative analysis based on a surface adatom/vacancy pairs model exhibits a clear increase in the surface defects density at potentials ≤ -0.2 V and a change in the defects coverage of ≈ 4%. After increasing the potential back to 0 V, the coverage of these defects decreases only slightly, indicating that some Cu adatoms remain on the surface. These results are in good agreement with our STM studies. The vertical expansion of the topmost atomic Cu layer exhibits a distinctly different potential dependence. This surface relaxation is not related to cluster formation, but can be attributed to changes in the type of chemisorbed species on the surface.To identify the adsorbate species on copper surface, in situ surface-enhanced Raman was employed. In the double layer range, bidentate carbonates and (bi)carbonate anions are dominant surface species. Below -0.04 V, the characteristic bands of carbonate become weak or disappear. In the meantime, new bands assigned to adsorbed carboxylate or carboxyl groups emerge, which can be attributed to the first reduction intermediates of CO2[4]. These bands disappear at potential below -0.34 V. Adsorbed CO appears on the surface starting from -0.24 V in the same potential range as the Cu nanocluster. In the reverse potential scan back to more positive values, only weak carbonate bands reappear and formate bands remain constant up to 0.26 V. These observed behavior suggests that some intermediates are formed irreversibly, in accordance with the irreversible changes in the molecular structure of the adlayer.[1] S. Nitopi et. al., Chem. Rev. 2019, 119(12), 7610–7672[2] Y. Y. Birdja at. al, Nature Energy 2019, 4, 732–745[3] R. Amirbeigiarab et. al., Angew. Chemie Int. Ed. 2022, 61 (46), No. e202211360[4] W. Shan et. al., ACS Nano 2020, 14(9), 11363–11372

Tandem Catalysis for Enhanced CO2 to Ethylene Conversion in Neutral Media

AbstractSelective electrochemical CO2 reduction reaction (CO2RR) into value‐added hydrocarbon products such as C2H4 provides a sustainable approach to producing carbon chemicals, which however remains a great challenge owing to the multi‐electron transfer process during CO2 electroreduction. Herein, a tandem catalyst a‐Ni/Cu‐NP@CMK is developed by encapsulating Cu nanoparticles (Cu NPs) into hydrophobic cubic mesoporous carbon with doped atomic Ni‐N4 moieties. Electrochemical tests demonstrate the outstanding C2H4 selectivity of a‐Ni/Cu‐NP@CMK with a high Faraday efficiency (FE) of 72.3% for C2H4 at a large current density of 406.1 mA cm−2 in a flow cell under a neutral medium. Moreover, when used as the cathode catalyst in membrane electrode assembly, a‐Ni/Cu‐NP@CMK stably delivers a current density of 200 mA cm−2 with a FEC2H4 of 63% at ‐2.8 V for 30 h, providing a full‐cell energy efficiency of 28.3% for C2H4 production. Comparative studies disclose that the hydrophobic microenvironment of the Cu NPs in a‐Ni/Cu‐NP@CMK successfully suppresses the competitive hydrogen evolution reaction and improves the CO2RR selectivity. Additionally, in situ spectroscopic investigations and theoretical calculations reveal that the efficient CO2‐to‐CO conversion on the Ni‐N4 moieties feeds Cu NPs with enriched adsorbed CO (*CO), which facilitates the C─C coupling between adjacent *CO to form C2H4.

Steering the Dynamics of Reaction Intermediates and Catalyst Surface during Electrochemical Pulsed CO2 Reduction for Enhanced C2+ Selectivity.

Developing alternative electrolysis techniques is crucial for advancing electrocatalysis in addition to tremendous efforts of material developments. Recently, pulse electrochemical CO2 reduction reaction (CO2RR) has demonstrated dramatic selectivity improvement toward multicarbon (C2+) products compared to potentiostatic electrochemical CO2RR, yet the underlying mechanisms remain little understood. Herein, we develop a fast time-resolved in situ Raman spectroscopic method with a time resolution of 0.25 s. We reveal that pulse electrolysis improves the C2+ selectivity of CO2RR through dynamic controls of the surface CuxO/Cu composition that would be unachievable under potentiostatic electrolysis. The population of the surface-adsorbed CO intermediate (COads) is characterized to be the determining factor in controlling reaction selectivity, which depicts the C2+/C1 selectivity of CO2RR under pulse conditions. Meanwhile, the vibrational character of COads, despite transforming dynamically between the low-frequency and high-frequency modes is characterized not to be the key factor in controlling the reaction selectivity. Such an active control of catalyst surface compositions and reaction intermediates enabled by pulse electrolysis offer a general way of regulating the electrocatalysis performance of broad electrochemical reactions beyond CO2RR.

(Invited) Electrochemical Carbon Dioxide Reduction on Titanium Nitride Mxenes

Carbon dioxide (CO2) is one of the leading causes of environmental issues, including global warming, with a major path of release being the burning of fossil fuels as energy sources. For this reason, the capture and reduction of CO2 to value-added chemicals and fuels via electrocatalysis is on the forefront of the 21st century grand challenges, but the technology is facing a severe low selectivity towards higher hydrocarbon products. Among the various catalysts explored for CO2reduction reaction (CO2RR), two-dimensional (2D) carbides and nitrides based on early transition metals are gaining much attention due to their unique properties including high surface area, high electronic conductivity, and tunable surface chemistries. Compared to carbide MXenes, nitrides have been less explored despite their superior properties. In this work, we leverage the high electronic conductivity of 2D titanium nitride MXenes to tune the selectivity of CO2RR towards higher hydrocarbon products. Specifically, we report on an oxygen-assisted molten salt fluoride etching method to produce large-lateral, few-to-single layer Ti4N3 and Ti2N nitride MXene flakes, and how varying the Ti:N ratio influences the activity and selectivity of CO2RR electrocatalysis. On these nitride MXenes, we observe carbon monoxide as the main product with high selectivity values. To take advantage of this key intermediate for C2+ products, we deposit single atom Cu particles on the surface of the MXene to foster dimerization. This approach led to the formation of higher order carbonaceous products. We found that the selectivity can be tuned by changing the applied current. We also utilize in-situ/operando spectroelectrochemical techniques, including Raman and Fourier-transform infrared (FTIR) spectroscopies, to elucidate the mechanism of product formation on these MXene materials. These findings will be of great interest to the scientific community and ultimately to the broad society as they report on an efficient catalyst design for transforming CO2 into value-added fuels and chemicals.

Efficient CO2 Reduction to Formate on CsPbI3 Nanocrystals Wrapped with Reduced Graphene Oxide

HighlightsA rational design of metal halide perovskites for achieving efficient CO2 reduction reaction was demonstrated.The stability of CsPbI3 perovskite nanocrystal (NCs) in aqueous electrolyte was improved by compositing with reduced graphene oxide (rGO).The CsPbI3/rGO catalyst exhibited > 92% Faradaic efficiency toward formate production with high current density which was associated with the synergistic effects between the CsPbI3 NCs and rGO.Transformation of greenhouse gas (CO2) into valuable chemicals and fuels is a promising route to address the global issues of climate change and the energy crisis. Metal halide perovskite catalysts have shown their potential in promoting CO2 reduction reaction (CO2RR), however, their low phase stability has limited their application perspective. Herein, we present a reduced graphene oxide (rGO) wrapped CsPbI3 perovskite nanocrystal (NC) CO2RR catalyst (CsPbI3/rGO), demonstrating enhanced stability in the aqueous electrolyte. The CsPbI3/rGO catalyst exhibited > 92% Faradaic efficiency toward formate production at a CO2RR current density of ~ 12.7 mA cm−2. Comprehensive characterizations revealed the superior performance of the CsPbI3/rGO catalyst originated from the synergistic effects between the CsPbI3 NCs and rGO, i.e., rGO stabilized the α-CsPbI3 phase and tuned the charge distribution, thus lowered the energy barrier for the protonation process and the formation of *HCOO intermediate, which resulted in high CO2RR selectivity toward formate. This work shows a promising strategy to rationally design robust metal halide perovskites for achieving efficient CO2RR toward valuable fuels.

Post-synthetic modification of covalent organic frameworks for CO2 electroreduction

To achieve high-efficiency catalysts for CO2 reduction reaction, various catalytic metal centres and linker molecules have been assembled into covalent organic frameworks. The amine-linkages enhance the binding ability of CO2 molecules, and the ionic frameworks enable to improve the electronic conductivity and the charge transfer along the frameworks. However, directly synthesis of covalent organic frameworks with amine-linkages and ionic frameworks is hardly achieved due to the electrostatic repulsion and predicament for the strength of the linkage. Herein, we demonstrate covalent organic frameworks for CO2 reduction reaction by modulating the linkers and linkages of the template covalent organic framework to build the correlation between the catalytic performance and the structures of covalent organic frameworks. Through the double modifications, the CO2 binding ability and the electronic states are well tuned, resulting in controllable activity and selectivity for CO2 reduction reaction. Notably, the dual-functional covalent organic framework achieves high selectivity with a maximum CO Faradaic efficiency of 97.32% and the turnover frequencies value of 9922.68 h−1, which are higher than those of the base covalent organic framework and the single-modified covalent organic frameworks. Moreover, the theoretical calculations further reveal that the higher activity is attributed to the easier formation of immediate *CO from COOH*. This study provides insights into developing covalent organic frameworks for CO2 reduction reaction.

Efficient Electromethanation from CO2 on Monodisperse Cu-Based Catalysts

The electrochemical CO2 reduction reaction (CO2RR) is a promising approach to upgrading CO2 into methane (CH4) and other high-value-added chemicals, critical for lowering carbon emissions and mitigating the greenhouse effect. However, the selectivity of CO2RR to methane remains low at the current densities required for commercialization. Herein, we report a simple one-step synthesis of monodisperse copper-based catalysts for efficient electromethanation of CO2. Through rational regulation of active sites, the selectivity for methane production can reach as high as 57.02% under the current density of 300 mA cm–2 as a result of suppressed C–C coupling for improved *CO hydrogenation. This work offers an effective strategy to regulate Cu-based catalysts to manage the hydrogenation of *CO for CO2RR toward CH4 formation.