Copper Electrocatalysts Research Articles

Controlling the Speciation in Electroreduction of Furanics for Production of Chemicals and Fuels

There is a significant opportunity to improve the sustainability of chemical manufacturing through the pairing of biomass-derived feedstocks with renewable electricity driven processes. Furanics are promising platform chemicals that can be applied to a broad range of chemical and fuel products. Both furfural and HMF can be electrochemically reduced through hydrogenation or hydrogenolysis using copper electrocatalysts. One of the main challenges that needs to be overcome is the control of selectivity to desired products when multiple products are possible from both the electrochemistry and the homogeneous side reactions in the electrolyte. A better mechanistic understanding of the electrochemical and homogeneous reactions is necessary to enable selectivity control. Utilizing microkinetic models fed from bulk electrolysis, in situ FTIR experiments and characterization of copper electrodes after electrochemical reaction, we have been able to elucidate pH-dependent mechanisms for the electroreduction of furfural. Learnings have been extended to HMF as well.

Selectivity and Durability in the Electrochemical Reduction of CO2 to C2 Products Using Cu-P, Cu-Sn and Cu-Se Electrocatalysts

Ethylene is typically reported as the primary product from CO2 reduction at copper electrocatalysts in MEA-type cell configurations with alkaline anolytes. In this work, we evaluate CO2 reduction selectivity at Cu-P, Cu-Sn, and Cu2Se electrocatalysts that were synthesized via a common one-pot approach. Electron microscopy, X-ray diffraction and inductively coupled plasma optical emission spectrometry show 100-110 nm nanoparticles with uniform distributions of P, Sn, or Se. When using neutral (0.1 M KHCO3) anolytes, electrocatalysts with P-doping yield increases in ethylene Faradaic efficiency (up to 52 % at 150 mA cm-2) while Cu-Sn and Cu-Se electrocatalysts result in increased oxygenate selectivity. Cu-Sn electrocatalysts yield an FE of 24 % ethanol at 150 mAcm-2 and Cu2Se electrocatalysts yield an FE of 32 % to acetate at 150 mAcm-2. More alkaline anolytes (1 M KOH) further increased oxygenate formation, with 48 % ethanol Faradaic efficiency on Cu-Sn and 40 % acetate Faradaic efficiency on Cu2Se at 350 mA cm-2. Galvanostatic experiments were conducted at 150 mA cm-2 in 0.1 M KHCO3 electrolyte for over 200 h for the three electrocatalysts. Figure 1 shows the slow losses in FEs to C2 products (0.02 % per h) and increasing cell potentials (1 mV per h). In this talk, we consider the mechanisms for selectivity and the nature of durability improvements. Figure 1

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.

Boosting *CO coverage on Cu octahedra enclosed by Cu(1 1 1) for efficient CO2 electroreduction to C2H5OH

Copper (Cu) electrocatalysts for carbon dioxide reduction reaction (CO2RR) have attracted much attention due to their ability to produce high value-added C2+ products (e.g., C2H4, C2H5OH, and n-propanol). However, achieving high selectivity of Cu electrocatalysts for C2H5OH in aqueous-based electrolytes remains a significant challenge. In this paper, we report Cu octahedra modified with low-coordinated Cu nanoclusters (Cu oct @Cu_NCs) as an efficient electrocatalyst for CO2RR to C2H5OH. Notably, the defect-site-rich Cu nanoclusters can promote the adsorption of *CO and increase the *CO coverage on Cu octahedra enclosed by Cu(111), thus promoting CO2RR to C2H5OH. We achieved a high Faradaic efficiency (FE) of 48.16 % for C2H5OH, and that of the C2+ products is 83.31 % at −1.17 V vs. RHE. This study provides a surface nanostructured route for rationalizing efficient and selective copper electrocatalysts for CO2RR.

Steering Selectivity in Electrocatalytic Furfural Reduction via Electrode-Electrolyte Interface Modification.

Electrocatalytic reduction of biomass-derived furfural (FF) represents a sustainable route to produce furfuryl alcohol (FA) and 2-methylfuran (MF) as a value-added chemical and a biofuel, respectively. However, achieving high selectivity for MF as well as tuning the selectivity between FA and MF within one reaction system remain challenging. Herein, we have reported an electrode-electrolyte interface modification strategy, enabling FA and MF selectivity steering under the same reaction conditions. Specifically, by modifying copper (Cu) electrocatalysts with butyl trimethylammonium bromide (BTAB), we achieved a dramatic shift in selectivity from producing FA (selectivity: 83.8%; Faradaic efficiency, FE: 68.9%) to MF (selectivity: 80.1%; FE: 74.8%). We demonstrated that BTAB adsorption over Cu modulates the electrical double layer (EDL) structure, which repels interfacial water and weakens the hydrogen-bond (H-bond) network for proton transfer, thus impeding FF-to-FA conversion by suppression of the hydrogen atom transfer (HAT) process. On the contrary, FF-to-MF conversion was less affected. This work shows the potential of engineering of the electrode-electrolyte interface for selectivity control in electrocatalysis.

Copper Electrocatalyst Produced by Cu2(OH)2CO3-Mediated In Situ Deposition for Diluted CO2 Reduction to Multicarbon Products.

Pure CO2 is commonly used in most of the current studies for electrochemical CO2 reduction which will need a further cost of gas purification and separation. However, the limited works on diluted CO2 reduction are focused on CO or CH4 production other than C2 products. In this work, copper electrocatalysts were prepared by Cu2(OH)2CO3-mediated in situ deposition for diluted CO2 reduction to multicarbon products. Using in situ Raman spectroscopy, constant amounts of CO and OH* were observed on the catalyst surface, which could effectively suppress the high kinetics of hydrogen evolution and promote C-C coupling, especially under the condition of diluted CO2 reduction. The optimized Cu catalyst achieves a C2 Faradaic efficiency as high as 60.72% in the presence of merely 25% CO2, which is almost equivalent to that observed with pure CO2.

In Situ Infrared Spectroscopic Evidence of Enhanced Electrochemical CO2 Reduction and C-C Coupling on Oxide-Derived Copper.

The reaction mechanism of CO2 electroreduction on oxide-derived copper has not yet been unraveled even though high C2+ Faradaic efficiencies are commonly observed on these surfaces. In this study, we aim to explore the effects of copper anodization on the adsorption of various CO2RR intermediates using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) on metallic and mildly anodized copper thin films. The in situ SEIRAS results show that the preoxidation process can significantly improve the overall CO2 reduction activity by (1) enhancing CO2 activation, (2) increasing CO uptake, and (3) promoting C-C coupling. First, the strong *COO- redshift indicates that the preoxidation process significantly enhances the first elementary step of CO2 adsorption and activation. The rapid uptake of adsorbed *COatop also illustrates how a high *CO coverage can be achieved in oxide-derived copper electrocatalysts. Finally, for the first time, we observed the formation of the *COCHO dimer on the anodized copper thin film. Using DFT calculations, we show how the presence of subsurface oxygen within the Cu lattice can improve the thermodynamics of C2 product formation via the coupling of adsorbed *CO and *CHO intermediates. This study advances our understanding of the role of surface and subsurface conditions in improving the catalytic reaction kinetics and product selectivity of CO2 reduction.

Solution-based Cu+ transient species mediate the reconstruction of copper electrocatalysts for CO2 reduction

Solution-based Cu+ transient species mediate the reconstruction of copper electrocatalysts for CO2 reduction

Durability of CO and CO2 Electrolyzers with Copper Electrocatalysts, Gas Diffusion Electrodes, and Anion Exchange Membranes

Recent works have shown that the electrochemical reduction of CO2 or CO to C2 products may be carried out at high rates and selectivities using electrolyzers with copper catalysts, gas diffusion electrodes (GDEs), and anion exchange membranes (AEMs). The current densities and Faradaic efficiencies reported for CO2 reduction are similar to those used in the commercial production of chlorine via the Chlor-alkali process; however, other performance metrics related to the energetic efficiency, carbon efficiency, single-pass conversion, crossover, and durability or operating lifetime are also important considerations.Work in our labs indicates that operating lifetimes are typically related to flooding, precipitation, or catalyst deactivation. As shown in Figure 1a, flooding in CO2 electrolyzers is typically observed with a sharp increase in hydrogen production and increased cathode gas flows. Figure 1b shows evidence of carbonate precipitation including variations in cell potential and lower cathode flow.In this talk, we consider the relationships between AEM properties (conductivity, water transport, counter ion transport), binder properties (viz. hydrophobicity), and catalysts on operating lifetimes of CO2 electrolyzers. Figure 1

Investigating the Electrochemical CO2 Reduction Mechanism on Oxide-Derived Copper Using in Situ FTIR Spectroscopy

Oxide-derived copper (OD-Cu) electrocatalysts have been extensively used for CO2 reduction reaction (CO2RR) applications in recent years due to their capability to selectively produce valuable C2 compounds. The mechanism of CO2 reduction on these surfaces, however, is still not clearly understood. Using in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), we show how the copper pre-oxidation process can significantly influence the adsorption of various CO2RR intermediates. Upon mildly anodizing a Cu thin film, we observed a redshift on the *CO2 – band, indicating that the initial CO2 adsorption and activation step is promoted on the oxide-derived surface. Additionally, we observed a faster growth of the *COatop band for the anodized copper film at low overpotentials, which demonstrates how a high *CO coverage can be achieved on OD-Cu. Finally, for the first time, we observed the simultaneous formation of two bands at 1327 cm–1 and 1425 cm–1 on the anodized Cu thin film. Using DFT calculations, we show that these bands match the theoretical vibrational frequencies of the *COCHO dimer, a key intermediate of the C2 pathway. Furthermore, our calculations show that the thermodynamics of *CO and *CHO coupling can be improved in the presence of the subsurface oxygen sites in OD-Cu. Our findings reveal the different aspects through which the CO2 reduction mechanism can be improved on OD-Cu electrocatalysts as compared to metallic copper.This work was financially supported by the Research Grants Council (16310419, 16309418, and 16304821) and the Innovation and Technology Commission (grant no. ITC-CNERC14EG03).

Maximizing Roughness Factors in Oxide-Derived Copper Coatings through Electrodeposition Parameters for Enhanced Electrocatalytic Performance

The pursuit of novel techniques for obtaining dispersed copper-based catalysts is crucial in addressing environmental issues like decarbonization. One method for producing nanostructured metals involves the reduction of their oxides, a technique that has found widespread use in CO2 electroreduction. Currently, the intrinsic activities of oxide-derived copper electrocatalysts produced via different routes cannot be compared effectively due to the lack of information on electrochemically active surface area values, despite the availability of electrochemical methods that enable estimation of surface roughness for highly dispersed copper coatings. In this study, we aim to explore the potential of oxide-derived copper to achieve a high electrochemically active surface area by examining samples obtained from acetic and lactic acid deposition solutions. Our results revealed that Cu2O oxides had distinct morphologies depending on the electrodeposition solution used; acetate series samples were dense films with a columnar structure, while electrodeposition from lactic acid yielded a fine-grained, porous coating. The roughness factors of the electroreduced films followed linear relationships with the deposition charge, with significantly different slopes between the two solutions. Notably, a high roughness factor of 650 was achieved for samples deposited from lactic acid solution, which represents one of the highest estimates of electrochemically active surface area for oxide-derived copper catalysts. Our results highlight the importance of controlling the microstructure of the electrodeposited oxide electrocatalysts to maximize surface roughness.

CO electroreduction on single-atom copper.

Electroreduction of carbon dioxide (CO2) or carbon monoxide (CO) toward C2+ hydrocarbons such as ethylene, ethanol, acetate and propanol represents a promising approach toward carbon-negative electrosynthesis of chemicals. Fundamental understanding of the carbon─carbon (C-C) coupling mechanisms in these electrocatalytic processes is the key to the design and development of electrochemical systems at high energy and carbon conversion efficiencies. Here, we report the investigation of CO electreduction on single-atom copper (Cu) electrocatalysts. Atomically dispersed Cu is coordinated on a carbon nitride substrate to form high-density copper─nitrogen moieties. Chemisorption, electrocatalytic, and computational studies are combined to probe the catalytic mechanisms. Unlike the Langmuir-Hinshelwood mechanism known for copper metal surfaces, the confinement of CO adsorption on the single-copper-atom sites enables an Eley-Rideal type of C-C coupling between adsorbed (*CO) and gaseous [CO(g)] carbon moxide molecules. The isolated Cu sites also selectively stabilize the key reaction intermediates determining the bifurcation of reaction pathways toward different C2+ products.

Localized Geometry Determined Selectivity of Iodide‐Derived Copper for Electrochemical CO2 Reduction

AbstractTwo iodide‐derived copper (ID‐Cu) electrocatalysts (E‐ID‐Cu and W‐ID‐Cu) are prepared by electrochemical/wet chemical iodination of Cu foil and subsequent in situ electrochemical reduction reaction. In comparison to electropolished Cu (EP‐Cu), both E‐ID‐Cu and W‐ID‐Cu can produce multicarbon (C2+) products with much‐improved selectivity, with Faradic efficiency (FE) reaching 64.39% for E‐ID‐Cu and 71.16% for W‐ID‐Cu at −1.1 V versus reversible hydrogen electrodes (RHE), which can be attributed to their localized geometry features with high defect density and high surface roughness. Given the well‐determined FEs towards C2+ products, the partial current densities for C2+ production can be estimated to be 251.8 mA cm−2 for E‐ID‐Cu and 290.0 mA cm−2 for W‐ID‐Cu at −1.2 V versus RHE in a flow cell. In situ characterizations and theoretical calculations reveal that the high‐density defects and high surface roughness can promote *CO adsorption by raising the d band center and then facilitate C–C coupling, contributing to the high selectivity of C2+ products for ID‐Cu. Interestingly, the high surface roughness can increase the residence time of *C–H intermediates and decrease the formation energy of the *OCCO and*CH3CH2O intermediates, thus favoring C2+ production, with a unique C2H6 product observed over W‐ID‐Cu with FE of 10.14% at −0.7 V versus RHE.

Guiding catalytic CO2 reduction to ethanol with copper grain boundaries.

The grain boundaries (GBs) in copper (Cu) electrocatalysts have been suggested as active sites for CO2 electroreduction to ethanol. Nevertheless, the mechanisms are still elusive. Herein, we describe how GBs tune the activity and selectivity for ethanol on two representative Cu-GB models, namely Cu∑3/(111) GB and Cu∑5/(100) GB, using joint first-principles calculations and experiments. The unique geometric structures on the GBs facilitate the adsorption of bidentate intermediates, *COOH and *CHO, which are crucial for CO2 activation and CO protonation. The decreased CO-CHO coupling barriers on the GBs can be rationalized via kinetics analysis. Furthermore, when introducing GBs into Cu (100), the product is selectively switched from ethylene to ethanol, due to the stabilization effect for *CH3CHO and inapposite geometric structure for *O adsorption, which are validated by experimental trends. An overall 12.5 A current and a single-pass conversion of 5.18% for ethanol can be achieved over the synthesized Cu-GB catalyst by scaling up the electrode into a 25 cm2 membrane electrode assembly system.

A novel tandem reactor design based on nano-Cu electrocatalysts and microbial biocatalysts for converting CO2 into ethylene and acetate

An electron-bio tandem reactor is assembled with Cu catalysts and microorganisms. Ethylene is produced in the gas phase, and the byproducts such as HCOOH, CO, and H2 can be recycled as electron donors for acetate biosynthesis via the Wood–Ljungdahl pathway.

Chlorine-mediated electrodeposition of hierarchical and hydrophobic copper electrocatalysts for efficient CO2 electroreduction to ethylene.

Hierarchical Cu dendrites fabricated via Cl-mediated electrodeposition afford high C2H4 efficiency (58% faradaic efficiency at -0.9 V vs. RHE) for CO2 electroreduction thanks to not only the optimal hydrophobicity/aerophilicity, but also the dominant distribution of active (100) and (110) facets.

PdCu Electrocatalysts for Selective Nitrate and Nitrite Reduction to Nitrogen

Electrocatalytic conversion of nitrate in waste can enable efficient waste remediation (NO3– to N2) or waste valorization (NO3– to NH4+) depending on the selectivity of the catalyst. Palladium and copper electrocatalysts typically exhibit ideal nitrate and nitrite binding properties, allowing for effective destruction of nitrate. However, rational steering of selectivity through material design remains a critical challenge for PdCu electrocatalyst. Here, we use the electrochemical underpotential deposition method to synthesize palladium nanocube electrocataysts with controlled copper surface coverage (e.g., partial and full copper coatings). We then examine the potential for NO3– destruction (conversion) and NO2– reduction reaction. We identify that partial copper-coated Pd nanocubes not only effectively facilitate the reduction of 95% of NO3– but also increase the reduction of NO2– to N2 with 89% selectivity over 20 consecutive cycles (80 h). We also show that under these conditions, the Pd(100) surface facet is exposed. Complete copper-covered Pd nanocubes effectively facilitate the reduction of ∼99% of NO3–. Complete coverage of copper; however, prevented exposure of Pd(100) surface facet, promoting the selective reduction of NO2– to NH4+ with a 70% selectivity over 20 consecutive cycles (80 h). Density functional theory (DFT) calculations show that NO3– and NO2– adsorb more strongly on the Cu(100) surface compared to the Pd(100) surface, while the NO* intermediate generated from NO3– or NO2– reduction adsorbs more strongly on the Pd surface. Barrier calculations show that NO* can readily migrate from the Cu domain to the Pd domain and that the N–N coupling barrier on Pd is significantly diminished at high NO* coverage. Together, these results suggest that the high N2 selectivity observed on the PdCu electrocatalyst is caused by the spillover of NO* from the Cu domains to the Pd domains.

Mechanistic In Situ ATR-FTIR Studies on the Adsorption and Desorption of Major Intermediates in CO2 Electrochemical Reduction on CuO Nanoparticles.

Increasing levels of carbon dioxide (CO2) from human activities is affecting the ecosystem and civilization as we know it. CO2 removal from the atmosphere and emission reduction by heavy industries through carbon capture, utilization, and storage (CCUS) technologies to store or convert CO2 to useful products or fuels is a popular approach to meet net zero targets by 2050. One promising process of CO2 removal and conversion is CO2 electrochemical reduction (CO2ER) using metal and metal oxide catalysts, particularly copper-based materials. However, the current limitations of CO2ER stem from the low product selectivity of copper electrocatalysts due to existing knowledge gaps of the reaction mechanisms using surfaces that normally have native oxide layers. Here, we report systematic control studies of the surface interactions of major intermediates in CO2ER, formate, bicarbonate, and acetate, with CuO nanoparticles in situ and in real time using attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR). Spectra were collected as a function of concentration, pH, and time in the dark and the in absence of added electrolytes. Isotopic exchange experiments were also performed to elucidate the type of surface complexes from H/D exchange. Our results show that the organics and bicarbonate form mostly outer-sphere complexes mediated by hydrogen bonding with CuO nanoparticles with Gibbs free energy of adsorption of about -25 kJ mol-1. The desorption kinetics of the surface species indicated relatively fast and slow regions reflective of the heterogeneity of sites that affect the strength of hydrogen bonding. These results suggest that hydrogen bonding, whether intermolecular or with surface sites on CuO nanoparticles, might be playing a more important role in the CO2ER reaction mechanism than previously thought, contributing to the lack of product selectivity.

Proving ligand structure-reactivity correlation on multinuclear copper electrocatalysts supported on carbon black for the oxygen reduction reaction

Bioinspired transition-metal catalysts seek to mimic the specific active site of metalloenzymes, as the multicopper oxidase, that can efficiently reduce dioxygen to water via a complete 4-electrons mechanism at low overpotential. However, the multicopper oxidase enzymes lack stability under operando conditions, hampering their application in fuel cell electrodes. Bioinspired multicopper catalysts present a remarkable electrocatalytic activity for the oxygen reduction reaction (ORR), where the structure and electronic properties of the ligands play a fundamental role. In this work, we explore the instantaneous catalytic activity and its evolution under operando conditions of two multicopper catalysts with different ligand flexibility, pyridyl ligand (CuL1), and pyridylmethyl ligand (CuL2), by conventional electrochemical techniques and scanning electrochemical microscopy (SECM). Both catalysts present similar instantaneous electrocatalytic activity with no significant role of the ligand, but there is a change in the mechanism. While the rigidity of CuL1 reduces the dioxygen via direct 4e-, the catalyst with higher flexibility (CuL2) follows a 2e−x 2e− mechanism. The production of H2O2 as ORR byproduct evaluated by rotating ring-disk electrode (RRDE) and stability test evaluated under ORR operating conditions by SECM imaging of both catalysts demonstrated a higher decrease in catalytic activity and higher H2O2 production in CuL2 than in CuL1, which evidences a ligand structure-reactivity correlation. These results contribute to the rational design of next generation of copper catalysts and open the door to a new methodology to evaluate the activity evolution under operando conditions by SECM.

Selective CO2 electroreduction to ethanol on encapsulated nickel nanoparticles by N-doped carbon nanotubes

The electrosynthesis of ethanol by CO2 reduction is mostly achieved on metallic copper electrocatalysts, but confronts low Faradic efficiency (FE) and selectivity. Herein, we report novel electrocatalysts in which Ni nanoparticles are encapsulated in N-rich carbon nanotubes (Ni@NCNT) by chemical vapor deposition method. The optimized Ni@NCNT-700 exhibits a high ethanol FE of 38.5% at −0.5 V and remains over 30% in a wide potential range of −0.5 ∼ −1.2 V. Notably, ethanol is the only liquid product and the total FE for CO and ethanol keeps above 90% in a potential range of −0.6 ∼ −1.2 V vs. reversible hydrogen electrode. A high current density of 128 mA cm−2 is obtained in a 1 M KOH electrolyte at −2.0 V in a flow-cell device. Moreover, In-situ Raman and density functional theory calculations demonstrate that the confinement and synergistic effects of Ni NPs and NCNTs sufficiently lower the energy barrier for C–C coupling and suppressed the *CO desorption.