Electrocatalytic CO2 Reduction Research Articles

Altering preferential product selectivity in electrocatalytic CO2 reduction over fluorine-modified CuIn bimetallic materials

Integrating carbon dioxide (CO2)-derived renewable technologies into the petrochemical industry presents a promising avenue for carbon neutrality. The renewable-energy-powered electrochemical CO2 reduction (eCO2R) process, particularly over Cu-based electrocatalysts, is a valuable asset for our decarbonization efforts to avoid climate consequences. Intending to enhance process efficiency, this work is a distinctive case study emphasizing the need to develop electrocatalysts in conjunction with adjusting operating conditions. A representative case is presented using fluorine-modified CuIn bimetallic electrocatalysts with tunable In/Cu ratios, demonstrating how to alter selectivity preferences based on reactor configuration: Our optimized catalyst system led to the highest faradaic efficiency (FE) for C2+ products of 68 % and the highest C1 FE of 80 % using gas diffusion electrodes (GDE)- and H-cell-type based electrolyzer, respectively. Moreover, the controllable In/Cu ratio and the degree of fluoride doping played a pivotal role in governing the physicochemical properties of the bimetallic electrocatalysts and their eCO2R performance. Diverse characterization tools offered systematic insights into the relationships between catalyst structure and properties, reactor configuration, electrolyte, and performance, aiming to enhance the Technology Readiness Level of the eCO2R process.

CO2 Reduction by Transition‐Metal Complex Systems: Effect of Hydrogen Bonding on the Second Coordination Sphere

AbstractHomogeneous electrocatalysts typified by transition‐metal complex show transcendent potency in efficient energy catalysis through molecular design. For example, metal complexes with elaborate design performed wonderful activity and selectivity for electrocatalytic CO2 reduction. Primary coordination sphere of metal complexes plays a key role in regulating its intrinsic redox properties and catalytic activity. However, the overall reduction efficiency of CO2 is also bound up with the substrate activation process. Transition‐metal complexes are hoped to exhibit reasonable redox potential, reactive activity, and stability, while binding and activating CO2 molecules to achieve efficient CO2 reduction. Construction of second coordination sphere, especially hydrogen‐bonding network of transition‐metal complexes, is reported to be the “kill two birds with one stone” strategy to realize efficient CO2 reduction catalysis via systematic catalyst properties modulation and substrate activation. Herein, we present recent progress on the construction of hydrogen‐bonding network in the second coordination sphere of metal complexes by ligand modification or the introduction of exogenous organic ligand, and the resulted productive enhancement of the catalytic performance of metal complexes by the improvement of adsorption capacity and activation of CO2, proton transfer rate, and stability of reaction intermediates, and so forth.

Efficient CO2 Reduction Reaction on Cu-Decorated Biphenylene.

Developing efficient electrocatalysts for CO2 reduction into value-added products is crucial for a green economy. Inspired by the recent experimental synthesis of biphenylene (BPH) and the excellent catalytic activity of copper dispersed on two-dimensional (2D) materials, we chose to systematically investigate the pristine, defective, and Cu-decorated BPH for the electrocatalytic CO2 reduction to value-added hydrocarbons. It is observed that the CO2 molecules bind weakly to the pristine BPH, indicating their chemical inertness. Carbon single-vacancy defects facilitate CO2 adsorption with a strong binding energy (Eb) of -3.23 eV, detrimental to the CO2 reduction reaction (CRR) mechanism. We have further investigated the binding energy and kinetic stability of Cu-decorated BPH as a single-atom-catalyst (SAC). The molecular dynamics simulations confirm the kinetic stability, revealing that the Cu-atom avoids agglomeration under low metal dispersal conditions. The CO2 molecule gets adsorbed horizontally on the Cu-BPH surface with a ΔEb of -0.52 eV. The CRR mechanism is investigated using two pathways beginning with two different initial states, formate (*OCOH) and carboxylic (*COOH). The formate pathway confirms the conversion of *OCOH to *HCOOH with the rate-limiting potential (UL) of 0.39 eV for the production of HCOOH, while for the carboxylic pathway, the conversion of *COH to *CHOH has a UL of 0.32 eV, eventually producing CH3OH. Our findings highlight the role of Cu-BPH as an efficient SAC for CO2 catalytic activity to C1 products, as compared to the state-of-the-art Cu, and holds promise as an electrocatalyst for CRR.

Selective CO2 Reduction Electrocatalysis Using AgCu Nanoalloys Prepared by a "Host-Guest" Method.

Multimetallic nanoalloy catalysts have attracted considerable interest for enhancing the efficiency and selectivity of many electrochemically driven chemical processes. However, the preparation of homogeneous bimetallic alloy nanoparticles remains a challenge. Here, we present a room-temperature and scalable, host-guest approach for synthesis of dilute Cu in Ag alloy nanoparticles. In this approach, an ionic silver bromide precursor harboring exogenous Cu cations is reduced to yield ∼20 nm diameter AgCu alloy nanoparticles wherein the % Cu loading can be tuned precisely. AgCu nanoparticles with a 5% nominal loading of Cu exhibit peak activity (-0.23 mA/cm2 normalized partial current density) and selectivity (83.2% faradaic efficiency) for CO product formation from electrocatalytic reduction of CO2 at mild overpotentials. These AgCu nanoalloys exhibit a higher mass activity compared to Ag- and Cu-containing nanomaterials used for similar electrocatalytic transformations. Our host-guest synthesis platform holds promise for production of other nanoalloys with relevance in electrocatalysis and optics.

Electrocatalytic CO2 reduction to HCO2H by protic NHC-Ir complexes

In electrochemical CO2-conversion strategies, various transition metal-based molecular electrocatalysts are employed to reduce CO2 into products like CO and HCO2H, with H2 as a competitive side product. However, achieving selectivity towards HCO2H remains challenging, and suitable catalysts for the same are limited. Herein we report a normal and an abnormal protic-NHC-based Cp*Ir(III)-half sandwich complexes for catalytic CO2 electroreduction in an aqueous acetonitrile solvent. Both the catalysts predominantly produced HCO2H as the CO2-reduced product at an applied potential of –2.66 V vs Fc+/Fc with 5 % H2O as the proton source; however, the normal protic-NHC-bound complex achieved a Faradic efficiency (FE) of 86±4 %, while the complex with the abnormal protic-NHC ligand furnished FE up to 72±4 %. The protic proton of the protic NHC ligand in these complexes was proposed to participate in a proton relay process, facilitating generation of the crucial Ir–H intermediate, which reacts with CO2 to produce HCO2H through stabilization of the Ir–OCHO intermediate.

In-situ imaging and time-resolved investigation of local pH in electrocatalytic CO2 reduction

Local pH near the catalyst surface exerts a substantial influence on both catalytic selectivity and activity in electrocatalytic CO2 reduction (CO2RR). While the investigation of local pH remains challenging in the aspect of in-situ imaging and monitoring. In this study, a novel lateral-type surface-enhanced Raman spectroscopy (L-SERS) system with a wide pH testing range from acidic to alkaline, is developed for the in-situ observation of local pH distributions with high spatial and temporal resolution during CO2RR. Excellent in-situ imaging of local pH near the catalyst surface is obtained, showing good consistency with simulation results. Besides, the gradual pH elevation and stabilization can be monitored via this time-resolved local pH investigation. This study presents a direct visualization of local pH distribution and variation at the Nernst layer during CO2RR, providing direct evidence in local pH for the feasibility of acidic electrolyte and offering theoretical guidance for optimizing CO2RR conditions.

Operando Spectroelectrochemistry Unravels the Mechanism of CO2 Electrocatalytic Reduction by an Fe Porphyrin.

Iron porphyrins are molecular catalysts recognized for their ability to electrochemically and photochemically reduce carbon dioxide (CO2). The main reduction product is carbon monoxide (CO). CO holds significant industrial importance as it serves as a precursor for various valuable chemical products containing either a single carbon atom (C1), like methanol or methane, or multiple carbon atoms (Cn), such as ethanol or ethylene. Despite the long-established efficiency of these catalysts, optimizing their catalytic activity and stability and comprehending the intricate reaction mechanisms remain a significant challenge. This article presents a comprehensive investigation of the mechanistic aspects of the selective electroreduction of CO2 to CO using an iron porphyrin substituted with four trimethylammonium groups in the para position [(pTMA)FeIII-Cl]4+. By employing infrared and UV/Visible spectroelectrochemistry, changes in the electronic structure and coordination environment of the iron center can be observed in real-time as the electrochemical potential is adjusted, offering new insights into the reaction mechanisms. Catalytic species were identified, and evidence of a secondary reaction pathway was uncovered, potentially prompting a re-evaluation of the nature of the catalytically active species.

Coupling CO2 Electroreduction to CO with alkyne Alkoxycarbonylation.

The alkyne alkoxycarbonylation to α- or β-substituted acrylates was coupled with the electrocatalytic reduction of CO2 to CO. The CO-enriched gaseous mixture produced from the electrocatalytic reduction of CO2 was collected and directly used in the alkyne alkoxycarbonylation. The CO content was found to be critical to the process of carbonylation, and satisfying results were attained by using the gas mixture containing >15 vol % CO. This method offered an efficient but simple CO source from CO2 electroreduction to the alkoxycarbonylation reaction in two-compartment manner.

Scalable Electro‐Biosynthesis of Ectoine from Greenhouse Gases

Converting greenhouse gases into valuable products has become a promising approach for achieving a carbon‐neutral economy and sustainable development. However, the conversion efficiency depends on the energy yield of the substrate. In this study, we developed an electro‐biocatalytic system by integrating electrochemical and microbial processes to upcycle CO2 into a valuable product (ectoine) using renewable energy. This system initiates the electrocatalytic reduction of CO2 to methane, an energy‐dense molecule, which then serves as an electrofuel to energize the growth of an engineered methanotrophic cell factory for ectoine biosynthesis. The scalability of this system was demonstrated using an array of ten 25 cm2 electrochemical cells equipped with a high‐performance carbon‐supported isolated copper catalyst. The system consistently generated methane at the cathode under a total partial current of approximately −37 A (~175 mmolCH4 h–1) and O2 at the anode under a total partial current of approximately 62 A (~583 mmolO2 h–1). This output met the requirements of a 3‐L bioreactor, even at maximum CH4 and O2 consumption, resulting in the high‐yield conversion of CO2 to ectoine (1146.9 mg L–1). This work underscores the potential of electrifying the biosynthesis of valuable products from CO2, providing a sustainable avenue for biomanufacturing and energy storage.

In Situ Oxidative Ring-Opening of Calix[8]arene to Construct Stable Bismuth-Oxo Clusters with Exposed Catalytic Sites for Specific Electroreduction of CO2 to HCOOH.

Nanocluster catalysts typically face challenges in balancing stability with catalytic efficiency. This study introduces a unique bismuth-oxo cluster, solely protected by two ring-opened calixarenes, which demonstrates not only enhanced structural stability but also superior catalytic performance in the sustained conversion of CO2 to HCOOH via electrocatalysis. For the first time, we reveal that under specific solvothermal conditions, tert-butylcalix[8]arene (TBC[8]) can undergo in situ oxidative cleavage of its C-C bond, leading to ring-opened polyphenolic molecules. These molecules serve as protective ligands for the bismuth-oxo cluster, bestowing exceptional structural stability and offering a more flexible and diverse configuration compared to intact TBC[8]. This adaptability promotes the exposure of active bismuth sites on the cluster surface, enhancing catalytic efficiency. Notably, the Bi10 cluster, featuring a monobismuth active site, achieves an exceptional formate production efficiency of 98.79% at -1.25 V vs RHE while maintaining superb durability over 8 h. The stability and catalytic processes of Bi10 surpass those of the Bi13 cluster, which is structurally reinforced by two intact TBC[8] molecules and stabilized by four benzoic ligands. Through in situ infrared spectroscopy and density functional theory calculations, we demonstrate that the monobismuth active site in Bi10 more effectively stabilizes the *OCHO intermediate, thereby promoting the electrocatalytic reduction of CO2 to HCOOH compared to Bi13. This comparative performance underscores the potential of ring-opened calixarene ligands in enhancing the functionality of nanocluster catalysts.

Scalable Electro-Biosynthesis of Ectoine from Greenhouse Gases.

Converting greenhouse gases into valuable products has become a promising approach for achieving a carbon-neutral economy and sustainable development. However, the conversion efficiency depends on the energy yield of the substrate. In this study, we developed an electro-biocatalyticsystem by integrating electrochemical and microbial processes to upcycle CO2 into a valuable product (ectoine) using renewable energy. This system initiates the electrocatalytic reduction of CO2 to methane, an energy-dense molecule, which then serves as an electrofuel to energize the growth of an engineered methanotrophic cell factory for ectoine biosynthesis. The scalability of this system was demonstrated using an array of ten 25 cm2 electrochemical cells equipped with a high-performance carbon-supported isolated copper catalyst. The system consistently generated methane at the cathode under a total partial current of approximately -37 A (~175 mmolCH4 h-1) and O2 at the anode under a total partial current of approximately 62 A (~583 mmolO2 h-1). This output met the requirements of a 3-L bioreactor, even at maximum CH4 and O2 consumption, resulting in the high-yield conversion of CO2 to ectoine (1146.9 mg L-1). This work underscores the potential of electrifying the biosynthesis of valuable products from CO2, providing a sustainable avenue for biomanufacturing and energy storage.

Exploring Electrocatalytic CO2 Reduction Over Materials Derived from Cu‐Based Metal‐Organic Frameworks

AbstractThe direct valorization of carbon dioxide (CO2) into value‐added chemicals offers an efficient and attractive approach to promoting carbon neutrality. Among the available methods, the electrocatalytic CO2 reduction reaction (eCO2RR) for producing multicarbon products (C2+) is gaining attention owing to its simplicity. However, achieving selective control over product formation remains a challenge. One key issue is the lack of a reliable correlation between the physicochemical properties of electrocatalytic materials and their activity and selectivity. To address this gap, we conducted a model study in which carbonized CuxZny@C materials, derived from metal‐organic frameworks (MOFs), were synthesized with varying Cu/Zn ratios. The pyrolyzed bimetallic MOFs retained key properties of the original MOFs while also developing new characteristics. These subtle changes in physicochemical properties influenced product selectivity. The findings of our study revealed that higher Zn doping favors the formation of single‐carbon (C1) products, whereas it is less favorable for multicarbon (C2+) products. Optimizing the Cu/Zn ratio was emphasized through characterization techniques, which will help guide the design of improved electrocatalytic systems for the eCO2RR process.

Efficient electrocatalytic CO2 reduction to ethylene using cuprous oxide derivatives.

Copper-based materials play a vital role in the electrochemical transformation of CO2 into C2/C2+ compounds. In this study, cross-sectional octahedral Cu2O microcrystals were prepared in situ on carbon paper electrodes via electrochemical deposition. The morphology and integrity of the exposed crystal surface (111) were meticulously controlled by adjusting the deposition potential, time, and temperature. These cross-sectional octahedral Cu2O microcrystals exhibited high electrocatalytic activity for ethylene (C2H4) production through CO2 reduction. In a 0.1M KHCO3 electrolyte, the Faradaic efficiency for C2H4 reached 42.0% at a potential of -1.376V vs. RHE. During continuous electrolysis over 10h, the FE (C2H4) remained stable around 40%. During electrolysis, the fully exposed (111) crystal faces of Cu2O microcrystals are reduced to Cu0, which enhances C-C coupling and could serve as the main active sites for catalyzing the conversion of CO2 to C2H4.

Cu−based bimetallic sites' p-d orbital hybridization promotes CO asymmetric coupling conversion to C2 products

Hampered by sluggish C−C coupling kinetics, the low selectivity and efficiency have limited industrial applications of CO2 reduction into valuable multi-carbon products. A direct coupling of CO molecules or their coupling after hydrogenation, followed by the final synthesis of C2 products, can help to overcome these limitations potentially. In this study, a detailed high-throughput screening of bimetallic site catalysts comprising copper (Cu) and 28 other metal (M) atoms was conducted. The metal atoms Cu and M were anchored on a carbon nanotube (CNT) with six nitrogen (N6) defects (CuMN6@CNT), which possesses effective dual active sites for C−C coupling. The calculated results demonstrate that the CuGaN6@CNT catalyst exhibited favorable selectivity, with low theoretical overpotentials of −0.23 and −0.34 eV for ethanol and ethylene, respectively, surpassing most reported catalysts. The synergistic effect of Ga and Cu sites, along with their p-d states hybridization, results in an enhancement of Cu's d state dispersion and energy barriers reduction for C−C coupling. Additionally, the strain effect of the substrate CNT exhibits a direct correlation with the catalytic performance of CuGaN6@CNT by adjusting the d-band center of the Cu site and p-band center of the Ga site. These findings provide a novel insights into the electrocatalytic reduction of CO into valuable C2 products using bimetallic single atom catalyst, offering significant guidance for future research endeavors in this field.

Study on reaction mechanism of CO2 electro-reduction to CO in organic medium: Revealed by experimental and spectroscopic Investigations

CO2 electrocatalytic reduction (CO2RR) to CO in organic electrolytes has received considerable attention due to its potential industrial application prospect. However, the reaction mechanism is not fully understood. In this study, we employed experimental observation, in-situ ATR-IRAS and Raman spectroscopy to elucidate the reaction mechanism of CO2 electro-reduction to CO in an organic medium. The reaction intermediates *CO2•-, *(CO2)2•- and *CO has been identified during CO2RR. Based on these results, the reaction mechanism has been discussed extensively. Density Functional Theory (DFT) analysis further indicates that the electrolytic CO2 reduction to CO in organic electrolyte is relatively more difficult than that in aqueous solutions.

Microenvironment regulation for high-performance acidic CO2 electroreduction on Poly(ionic liquid)-modified Cu surface

Electrocatalytic reduction of CO2 (CO2RR) under acidic conditions provides a feasible way for converting CO2 to multi-carbon products (C2+) with high feedstock conversion and high energetic efficiency (EE). In this work, poly(ionic liquid) (PIL)-Cu hybrids were employed as excellent acidic CO2RR electrocatalysts. A high faradaic efficiency towards C2+ products (FEC2+) of more than 61.0 % was achieved within a wide pH range from 2 to 14 at –700 mA cm–2. The optimum FEC2+ of 83.1 % was obtained at –700 mA cm–2 and pH = 2, corresponding to an EEC2+ of 37.6 %. By reducing the inlet CO2 flow rate to 1.59 mL min–1, a very high CO2 single-pass conversion of 98.7 % was achieved at –300 mA cm–2. The confinement of PIL layer caused by the semi-rigid abundant porous structure and dense cationic-anionic network enables the maintenance of a moderate local alkaline microenvironment and enrichment of K+ cations at the active sites. These effects jointly promote the C–C coupling at the Cu-PIL interface under acidic conditions. Remarkably, the CO2RR pathway can be regulated by functionality and morphology of the PIL layer.

Engrossing structural developments of double perovskites for viable energy applications

Investigation for novel functional catalysts illustrates an essential direction in advancing and researching renewable energy. On account of their specific compositional and structural tunability and remarkable stability, double perovskites have been extensively examined as a category of versatile compounds for applications in photocatalysis and electrocatalysis, highlighting them as a promising candidate for catalytic performance and stability. In this review article, we have elaborated on the tunability of double perovskites in terms of their compositions and structures, which proved to be tremendous features for double perovskites in diverse applications. Furthermore, the fabrication methods and the performance optimization comprising band gap tuning of double perovskites have been discussed. The current status of double perovskites for photocatalytic and electrocatalytic H2 production and CO2 reduction, with recent reports, has also been reviewed. Lastly, the challenges and future outlook are put forward.

Enhanced electrocatalytic CO2 reduction into formate: Unleashing the effect of engineered bimetallic oxides supported on activated carbon

The excessive concentration of anthropogenic CO2 continuously harms the atmosphere. Consequently, deploying CO2 via the electrochemical reduction approach has become a highly sought-after research thrust area. In this study, a facile, one-pot solution combustion synthesis method (SCSM) was employed for the synthesis of various mixed-metal oxides supported on porous activated carbon (SnO2-CuO@AC). The prepared materials are characterized by analytical and spectroscopic techniques. Electrochemical studies were performed for the SnO2-CuO@AC catalyst to reduce CO2 into formic acid via an electrochemical approach. The in-depth properties of the designed catalysts were analysed by theoretical calculations using FT-IR and XRD data. The SnO2-CuO@AC exhibited improved electrochemical performance with a high current density of −10.77 mAcm−2, a low overpotential of −1.29 V, a lower Tafel slope value of 100 mV/dec. The faradaic efficiency for formic acid was 78.8 % at 1.3 V vs. RHE, with high stability for 9 h. The presence of coordinated active sites and oxygen vacancies, and a lower Tafel slope, also endorses CO2 adsorption and activation, faster electron transfer and boosts CO2 reduction. The significant reactivity toward CO2 reduction is attributed to the intensity of interaction between CO2∗- and the electrocatalyst, followed by kinetic activation toward protonation to obtain the desired product.

P-Block Aluminum Single-Atom Catalyst for Electrocatalytic CO2 Reduction with High Intrinsic Activity.

Atomically dispersed transition metal sites on nitrogen-doped carbon catalysts hold great potential for the electrochemical CO2 reduction reaction (CO2RR) to CO due to their encouraging selectivity. However, their intrinsic activity is restricted by the hurdle of the high energy barrier of either *COOH formation or *CO desorption due to the scaling relationship. Herein, we discover a p-block aluminum single-atom catalyst (Al-NC) featuring an Al-N4 site that enables disentangling this hurdle, which endows a moderate reaction kinetic barrier for *COOH formation and *CO desorption, as validated by in situ attenuated total reflection infrared spectroscopy and theoretical simulations. As a result, the developed Al-NC shows a CO Faradaic efficiency (FECO) of up to 98.76% at -0.65 V vs RHE and an intrinsic catalytic turnover frequency of 3.60 s-1 at -0.99 V vs RHE, exceeding those of the state-of-the-art Ni-NC and Fe-NC counterparts. Moreover, it also delivers a partial CO current of 309 mA·cm-2 at 93.65% FECO and 605 mA at >85% FECO in a flow cell and membrane electrode assembly (MEA), respectively. Strikingly, when using low-concentration CO2 (30%) as the feedstock, this catalyst can still deliver a partial CO current of 240 mA at >80% FECO in the MEA. Considering the earth-abundant character of the Al element and the high intrinsic activity of the Al-NC catalyst, it is a promising alternative to today's transition metal-based single-atom catalysts.

Boosting the Efficiency and Selectivity of Electrocatalytic Reduction of CO2 to C1 Products by Mn-Porphyrin via Axial Ligation

Boosting the Efficiency and Selectivity of Electrocatalytic Reduction of CO₂ to C1 Products by Mn-Porphyrin via Axial Ligation