Oxide-derived Copper Research Articles

Enhanced formate production from sulfur modified copper for electrocatalytic CO2 reduction

Producing liquid fuels with high selectivity and low overpotential has long been a goal of electrocatalytic CO2 reduction reaction (CO2RR). Here we present a detailed study on the structural and catalytic properties of sulfur on copper for CO2RR. Cu2S was formed by enabling sulfur to deposit on the copper surface with a simple surface impregnation method. The sulfur modified samples, especially the sulfur modified oxide-derived copper (SM-OD-Cu), have significantly higher selectivity to formate than the pure and oxide-derived copper. At a potential range of -0.51∼-0.75 V vs. RHE, the selectivity of formate for SM-OD-Cu in the liquid phase was nearly 100%. Also, the partial current density of formate reached -4.26 mA/cm2 at -0.75 V vs. RHE. By using density functional theory calculation, we found that Cu2S significantly inhibits the CO pathway and hydrogen evolution reaction, and further improves the selectivity of formate.

Enhanced CO2-to-CH4 conversion via grain boundary oxidation effect in CuAg systems

The authentic active sites of oxide-derived copper (OD-Cu), namely grain boundaries (GBs) and oxidized Cuδ+ species, is still debatable, and their role in governing CH4 conversion remains unclear. Herein, this study answers these questions using bimetallic catalysts by novel electro-shock strategy with controllable GBs for the oxidization of Cuδ+ species by modulating Ag loading. The Ag enrichment at the GBs facilitates the bonding of oxygen with the uncoordinated Cu atoms, resulting in GB oxidation effect. The obtained CH4 selectivity is twice that of GBs or nanoalloy effect. The enhanced performance is attributed to the stable Cuδ+ species and unique electron transfer mechanism from GB oxidation structure. Operando attenuated-total-reflection Fourier-transform-infrared-spectroscopy unveils the reaction pathway of CO2-to-CH4 and the sluggish reversible quenching processes of intermediates. Theoretical calculations indicate that the weak *CO adsorption on GB oxidation structure facilitates *CO hydrogenation, promoting CO2-to-CH4 conversion.

Selective Electrochemical CO2 Reduction to Ethylene and Multi-carbon Products on Oxide-derived Porous CuO Micro-cages

The electrochemical reduction of CO2 (CO2RR) presents a dual benefit: it helps mitigate environmental pollution while producing valuable multi-carbon (C2+) chemicals and storing renewable energy in chemical fuels. However, there is an urgent need for efficient electrocatalysts that can selectively increase the production of ethylene and C2+ products for the wide-scale implementation of CO2RR. Herein, we have facilely synthesized porous micro-caged oxide-derived copper oxide (OD-Cu MC) using a one-pot hydrothermal approach followed by air-annealing at 350 °C. The resulting electrocatalyst exhibited excellent performance for ethylene (C2H4) production, achieving a faradaic efficiency (FE) of 44.8 % for C2H4 and cumulative FE of 73.1 % for C2+ products at current density (ID) of 300 mA cm−2. At an ID of 400 mA cm−2, OD-Cu MC demonstrated a turnover frequency (TOF) of 0.012 s−1 for ethylene, which is 7.8 times higher than the TOF observed for commercially available copper oxide (CuO-CM). Moreover, at an ID of 300 mA cm−2, OD-Cu MC achieved a single-pass CO2 conversion (SPCC) of 35.2 % and a half-cell energy efficiency of 15.6 % for C2+ products. The catalyst also showed good stability, maintaining its performance for over 4 h at an ID of 200 mA cm−2. This straightforward synthesis approach opens new avenues for enhancing C2+ product selectivity in CO2RR.

Al-doped oxide-derived copper catalyst with stable Cu+ site for efficient electrocatalytic CO2 reduction to C2H4

Electrocatalytic carbon dioxide reduction reaction (ECO2RR), a pivotal process converting CO2 into high-value products, plays a crucial role in advancing objectives of carbon neutrality. Within various catalysts, copper-based electrocatalysts hold a unique position as they demonstrate a remarkable aptitude for producing high-carbon (C2+) products. Notably, the well-established Cu+ sites exhibit robust C-C coupling capabilities, particularly in the synthesis of C2H4 products. However, the challenge lies in maintaining the stability of oxide-derived copper under the negative potential operating conditions. In light of this, we use the ion exchange principle to make Al partially replace Cu-BDC, and then get a dopant by high temperature calcination to prepare Al-doped CuO catalyst. It has been demonstrated that the introduction of oxyphilic Al atoms could induce the redistribution of electrons in the CuO matrix and stabilize the presence of Cu+ in the ECO2RR process. This configuration significantly enhances the catalytic performance of CuO for electrochemically converting CO2 into C2H4. Specifically, the optimized Al-CuO catalyst exhibits ∼50 % Faradaic Efficiency for C2H4 (FEC2H4) at −1.377 V vs. RHE, doubling the performance compared to pure CuO. The design and implementation of doping engineering presented in this work propel advancements in the field of electrocatalytic CO2 reduction for C2H4 production, offering a crucial avenue for the development of efficient catalysts.

Highly selective electrocatalytic CO2 reduction to multi-carbon products at CuPd synergistic sites over OD-Cu based catalysts

CO2 reduction is a potential way to kill two birds with one stone by converting greenhouse gas into fuels through clean energy. Cu-based catalysts, with a wide range of products, often have limited selectivity and low activity for specific products. In this study, Cu2O was used as the oxide-derived copper (OD-Cu) substrate, which was loaded with CuPd nanoparticles by using pulsed potential and galvanic replacement. Theoretical calculations revealed that the strong H-affinity Pd sites can reduce the selectivity of formate by stabilizing *COOH and simultaneously lowering the d-band center of neighboring Cu. Additionally, the asymmetric C–C coupling of *CHO+*CO, distributed between Cu and Pd sites, can lower the free energy change of rate-determining step to promote production of the C2+ products. Faradaic efficiency (FE) of the C2+ products over Cu2O-CuPd0.5μM was more than three times higher than that over untreated Cu2O NW, rising from 16.50 % to 54.47 % under −1 V (vs RHE) bias. Moreover, the FE for formate and CO decreased respectively from 28.57 % and 19.30 % to 3.69 % and 11.82 %, accompanied by a simultaneous increase in current density by 71.49 %.

Dynamics of bulk and surface oxide evolution in copper foams for electrochemical CO2 reduction

Oxide-derived copper (OD-Cu) materials exhibit extraordinary catalytic activities in the electrochemical carbon dioxide reduction reaction (CO2RR), which likely relates to non-metallic material constituents formed in transitions between the oxidized and the reduced material. In time-resolved operando experiment, we track the structural dynamics of copper oxide reduction and its re-formation separately in the bulk of the catalyst material and at its surface using X-ray absorption spectroscopy and surface-enhanced Raman spectroscopy. Surface-species transformations progress within seconds whereas the subsurface (bulk) processes unfold within minutes. Evidence is presented that electroreduction of OD-Cu foams results in kinetic trapping of subsurface (bulk) oxide species, especially for cycling between strongly oxidizing and reducing potentials. Specific reduction-oxidation protocols may optimize formation of bulk-oxide species and thereby catalytic properties. Together with the Raman-detected surface-adsorbed *OH and C-containing species, the oxide species could collectively facilitate *CO adsorption, resulting an enhanced selectivity towards valuable C2+ products during CO2RR.

Benzyl alcohol promoted electrocatalytic reduction of carbon dioxide and C2 production by Cu2O/Cu

Oxide-derived copper (OD-Cu)-based materials containing Cuδ+ (0 < δ ≤ 1) have been considered as promising electrocatalysts for electrochemical CO2 reduction reaction (ECO2RR) to produce valuable C2 products, which, however, suffer from poor stability, mainly due to the inevitable reduction of Cuδ+. We report here that the benzyl alcohol (BA) introduced in catholyte presents a markedly positive effect on the durability and C2 selectivity enhancement of ECO2RR when OD-Cu-based material is adopted as cathode electrocatalyst. The critical evidence of ECO2RR intermediates has been provided by in situ Raman and FTIR spectroscopy, which reveals the surface mechanisms of BA-assisted ECO2RR over OD-Cu catalysts. On the one hand, the introduction of BA increases the CO2 supply for ECO2RR by slowing down its neutralizing reaction, which results in the increased intermediate species coverage on the catalytic surface therefore the delayed self-reduction of Cuδ+; On the other hand, the interaction between BA and the intermediate species *CO during ECO2RR brings an enhanced selectivity of C2 products (FE C2 = 85.5 % at −1.38 V vs RHE). Besides, an energy-savings and economic electrolyzer can be achieved by coupling anodic BA oxidation reaction to benzoic acid and ECO2RR to C2 products with much lowered cell voltages.

Unraveling the rate-determining step of C2+ products during electrochemical CO reduction

The electrochemical reduction of CO has drawn a large amount of attention due to its potential to produce sustainable fuels and chemicals by using renewable energy. However, the reaction’s mechanism is not yet well understood. A major debate is whether the rate-determining step for the generation of multi-carbon products is C-C coupling or CO hydrogenation. This paper conducts an experimental analysis of the rate-determining step, exploring pH dependency, kinetic isotope effects, and the impact of CO partial pressure on multi-carbon product activity. Results reveal constant multi-carbon product activity with pH or electrolyte deuteration changes, and CO partial pressure data aligns with the theoretical formula derived from *CO-*CO coupling as the rate-determining step. These findings establish the dimerization of two *CO as the rate-determining step for multi-carbon product formation. Extending the study to commercial copper nanoparticles and oxide-derived copper catalysts shows their rate-determining step also involves *CO-*CO coupling. This investigation provides vital kinetic data and a theoretical foundation for enhancing multi-carbon product production.

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.

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).

Exploring the Influence of Malachite Forming on Oxide-Derived Copper Electrodes on C2+ Product Selectivity

Electrocatalytic activity and C2 product selectivity of two distinguished oxide-derived Cu in electrochemical carbon dioxide reduction reaction (CO2RR) were investigated. Cu oxide layers electrodeposited at different deposition rates exhibited different morphologies, the electrode with a more compact structure was found selective to C2 products with two times higher faradaic efficiencies (40%). Both Cu+ and Cu2+ species have been identified on the surface of oxide-derived Cu electrodes by X-ray photoelectron spectroscopy (XPS). Also, oxide-derived electrodes were investigated by X-ray diffraction (XRD). Results confirmed the presence of Cu oxide phases for primary electrodes, which were then fully reduced to the metallic Cu after CO2RR. Moreover, SEM investigations helped us distinguish the morphology of the two electrodes and monitor morphological differences before and after CO2RR. To shed light on the adsorbed species, intermediate (metastable) phases, and reaction mechanisms during CO2RR, electrochemical surface-enhanced Raman spectroscopy (SERS) was utilized. This method helped us vividly observe the formation of a metastable phase (malachite) on the electrode surface, which showed lower FEs for C2 products. Moreover, the analysis of SERS indicated a strong tie between the presence of the malachite phase and strongly adsorbed CO on electrode surfaces, preventing dimerization and further reduction. This malachite phase terminating the surface can hinder the charge transport and interfere with further reductions in C2 products. Figure 1

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.

Regulating reconstruction of oxide-derived Cu for electrochemical CO2 reduction toward n-propanol.

Oxide-derived copper (OD-Cu) is the most efficient and likely practical electrocatalyst for CO2 reduction toward multicarbon products. However, the inevitable but poorly understood reconstruction from the pristine state to the working state of OD-Cu under strong reduction conditions largely hinders the rational construction of catalysts toward multicarbon products, especially C3 products like n-propanol. Here, we simulate the reconstruction of CuO and Cu2O into their derived Cu by molecular dynamics, revealing that CuO-derived Cu (CuOD-Cu) intrinsically has a richer population of undercoordinated Cu sites and higher surficial Cu atom density than the counterpart Cu2O-derived Cu (Cu2OD-Cu) because of the vigorous oxygen removal. In situ spectroscopes disclose that the coordination number of CuOD-Cu is considerably lower than that of Cu2OD-Cu, enabling the fast kinetics of CO2 reaction and strengthened binding of *C2 intermediate(s). Benefiting from the rich undercoordinated Cu sites, CuOD-Cu achieves remarkable n-propanol faradaic efficiency up to ~17.9%, whereas the Cu2OD-Cu dominantly generates formate.

Oxide-Derived Copper Nanowire Bundles for Efficient CO2 Reduction to Multi-Carbon Products

Cu-based catalysts for efficient C2+ production from CO2 electrocatalytic reduction reaction (CO2ERR) exhibit significant promise, but still suffer from ambiguous mechanisms due to the intrinsic structure instability during electroreduction. Herein, we report an oxide-derived copper nanowire bundle (OD-Cu NWB) for efficient CO2ERR to C2+ products. OD-Cu NWBs with a well-preserved nanowire bundle morphology lead to promoted multi-carbon production compared to commercial copper powders. The formation of OD-Cu NWBs shows a great dependence on the precipitation/calcination temperatures and per-reduction potentials, which further influence the ultimate CO2ERR performance correspondingly. The optimized preparation parameters for the formation of a well-ordered nanowire bundle morphology are found, leading to a preferred C2+ production ability. Besides the nanowire bundle morphology, the oxide-derived Cu essence of OD-Cu NWBs with stabilized Cu+ species from per-reduction also promotes the CO2ERR activity and facilitates the C-C coupling of key intermediates for C2+ production. This work provides a facile strategy and inspiration for CO2ERR catalyst developments targeting high-valued multi-carbon products.

Hydrothermal Fabrication of Carbon-Supported Oxide-Derived Copper Heterostructures: A Robust Catalyst System for Enhanced Electro-Reduction of CO2 to C2 H4.

Anthropogenic CO2 can be converted to alternative fuels and value-added products by electrocatalytic routes. Copper-based catalysts are found to be the star materials for obtaining longer-chain carbon compounds beyond 2e- products. Herein, we report a facile hydrothermal fabrication of a highly robust electrocatalyst: in-situ grown heterostructures of plate-like CuO-Cu2 O on carbon black. Simultaneous synthesis of copper-carbon catalysts with varied amounts of copper was conducted to determine the optimum blend. It is observed that the optimum ratio and structure have aided in achieving the state of art faradaic efficiency for ethylene >45 % at -1.6 V vs. RHE at industrially relevant high current densities over 160 to 200 mA ⋅ cm-2 . It is understood that the in-situ modification of CuO to Cu2 O during the electrolysis is the driving force for the highly selective conversion of CO2 to ethylene through the *CO intermediates at the onset potentials followed by C-C coupling. The excellent distribution of Cu-based platelets on the carbon structure enables rapid electron transfer and enhanced catalytic efficiency. It is inferred that choosing the right composition of the catalyst by tuning the catalyst layer over the gas diffusion electrode can substantially affect the product selectivity and promote reaching the potential industrial scale.

Electro-reduced copper on polymeric C3N4 for photocatalytic reduction of CO2

Coupling light absorber with an efficient cocatalyst is the most common way to fabricate composite photocatalyst. The light absorber supplies electrons and holes, while the cocatalyst domains chemical reactions. Inspired by the results from electrochemical CO2 reduction, Cu-based materials are the most promising cocatalysts for photocatalytic CO2 reduction. However, oxide-derived copper (OD-Cu) hasn't been introduced into photocatalytic systems yet, due to easily undergoing valence changes once exposed to air. Herein, we find that OD-Cu could be decorated on photocatalyst, polymeric C3N4 (PCN), by a self-designed in-situ electroreduction method. The as-prepared OD-Cu/PCN presents better visible light absorption than the one without in-situ electroreduction and prefers multi-carbon products. According to CO2RR measurements, the highest selectivity to C2H5OH is 58%. Density functional theory calculation further indicates that the unique structure between OD-Cu and PCN leads to a low energy barrier of C–C coupling due to the enhanced *CO dimerization and thus promotes the production of C2H5OH. Moreover, this optimized sample achieves a solar-to-fuel efficiency, of up to 0.94%, which is 1.69 times the pristine PCN.

Surface Adsorbed Hydroxyl: A Double-Edged Sword in Electrochemical CO2 Reduction over Oxide-Derived Copper.

Oxide-derived Cu (OD-Cu) featured with surface located sub-20 nm nanoparticles (NPs) created via surface structure reconstruction was developed for electrochemical CO2 reduction (ECO2 RR). With surface adsorbed hydroxyls (OHad ) identified during ECO2 RR, it is realized that OHad , sterically confined and adsorbed at OD-Cu by surface located sub-20 nm NPs, should be determinative to the multi-carbon (C2 ) product selectivity. In situ spectral investigations and theoretical calculations reveal that OHad favors the adsorption of low-frequency *CO with weak C≡O bonds and strengthens the *CO binding at OD-Cu surface, promoting *CO dimerization and then selective C2 production. However, excessive OHad would inhibit selective C2 production by occupying active sites and facilitating competitive H2 evolution. In a flow cell, stable C2 production with high selectivity of ∼60 % at -200 mA cm-2 could be achieved over OD-Cu, with adsorption of OHad well steered in the fast flowing electrolyte.

Understanding the restructuring and degradation of oxide-derived copper during electrochemical CO2 reduction

Renewable energy produced through electrochemical CO2 reduction is promising for mitigating the energy crisis and environmental issues. Notably, oxide-derived copper (OD-Cu) has attracted considerable attention owing to its unique ability to catalyze CO2 electroreduction to >2e− products, especially C2+ products, which are the key feedstocks for the chemical process. However, Cu under operating conditions usually leads to structure reconstruction and degradation, which hinders understanding the origin of its activity. Understanding the restructuring and degradation of OD-Cu catalyst to form C2+ products is critical for understanding the active sites and, thus for designing effective and durable catalysts for CO2 reduction. This paper describes that Cu defect sites are easily dissolved during electrochemical CO2 reduction, and the presence of defect sites is strongly related to the catalytic performance of C2+ products. Here, the C2+ products Faradaic efficiency (FE) of OD-Cu decreased from 39.4% at 1 h to 26.0% at 5 h at −1.5 V vs. RHE, along with a decrease in low-coordinated defect sites. The low-coordinated defect sites facilitate CO adsorption and help to increase the CO coverage, thus promoting CO–CO coupling. The decrease in the number of defect sites and activity degradation is circumvented by promoting the Cu redeposition process. The C2+ products FE of OD-Cu can be maintained at ca. 47.2% without an obvious decrease in CO2-saturated 0.1 M KHCO3 with 1 mM Cu2+. The results of this study help in understanding the activity of OD-Cu and reveal its deactivation mechanism. This knowledge will aid in designing highly active and stable catalysts for CO2 reduction.

Revisiting Reaction Kinetics of CO Electroreduction to C2+ Products in a Flow Electrolyzer

The identification of the rate-determining step (RDS) in the electrochemical CO/CO2 reduction to multi-carbon (C2+) products has been complicated by the deficiency of rigorous reaction kinetic data. This work describes an experimental analysis of the key reaction steps by exploring the effect of CO partial pressure on the activity of C2+ products. With the aid of a flow electrolyzer integrated with a gas diffusion electrode, the distinct reaction orders of CO and reaction mechanisms in forming different C2+ products were determined. Specifically, *CO dimerization is identified as the RDS for ethylene and ethanol production, as evidenced by the gradual transition of measured CO reaction order from second to zero as CO partial pressure increases from 0.05 to 1 atm. The formation of n-propanol is suggested to proceed via the *CO trimerization mechanism. The acetate generation mechanism might involve a critical step of *CO hydrogenation before C–C coupling. Kinetic studies reveal that product-specific active sites are responsible for activity and selectivity toward specific C2+ products over oxide-derived copper.

Hollow Hierarchical Cu2 O-Derived Electrocatalysts Steering CO2 Reduction to Multi-Carbon Chemicals at Low Overpotentials.

The electrochemical reduction of carbon dioxideinto multi-carbon products (C2+ ) using renewably generated electricity provides a promising pathway for energy and environmental sustainability. Various oxide-derived copper (OD-Cu) catalysts have been showcased, but still require high overpotential to drive C2+ production owing to sluggish carbon-carbon bond formation and low CO intermediate (*CO) coverage. Here, the dilemma is circumvented by elaborately devising the OD-Cu morphology. First, computational studies propose a hollow and hierarchical OD-Cu microstructure that can generate a core-shell microenvironment to inhibit CO evolution and accelerate *CO dimerization via intermediate confinement and electric field enhancement, thereby boosting C2+ generation. Experimentally, the designed nanoarchitectures are synthesized through a heteroseed-induced approach followed by electrochemical activation. In situ spectroscopic studies further elaborate correlation between *CO dimerization and designed architectures. Remarkably, the hierarchical OD-Cu manifests morphology-dependent selectivity of CO2 reduction, giving a C2+ Faradaic efficiency of 75.6% at a considerably positive potential of -0.55V versus reversible hydrogen electrode.