Atomic Layer Deposition of ZnO on CuO Enables Selective and Efficient Electroreduction of Carbon Dioxide to Liquid Fuels

The spatial distribution of cobalt phthalocyanine and copper nanocubes controls the selectivity towards C2 products in tandem electrocatalytic CO2 reduction

The coupling of CO-generating molecular catalysts with copper electrodes in tandem schemes is a promising strategy to boost the formation of multi-carbon products in the electrocatalytic reduction of CO₂. While the spatial distribution of the two components is important, this aspect remains underexplored for molecular-based tandem systems. Herein, we address this knowledge gap by studying tandem catalysts comprising Co-phthalocyanine (CoPc) and Cu nanocubes (Cu_cub). In particular, we identify the importance of the relative spatial distribution of the two components on the performance of the tandem catalyst by preparing CoPc-Cu_cub/C, wherein the CoPc and Cu_cub share an interface, and CoPc-C/Cu_cub, wherein the CoPc is loaded first on carbon black (C) before mixing with the Cu_cub. The electrocatalytic measurements of these two catalysts show that the faradaic efficiency towards C₂ products almost doubles for the CoPc-Cu_cub/C, whereas it decreases by half for the CoPc-C/Cu_cub, compared to the Cu_cub/C. Our results highlight the importance of a direct contact between the CO-generating molecular catalyst and the Cu to promote C-C coupling, which hints at a surface transport mechanism of the CO intermediate between the two components of the tandem catalyst instead of a transfer via CO diffusion in the electrolyte followed by re-adsorption.

Imparting CO2 Electroreduction Auxiliary for Integrated Morphology Tuning and Performance Boosting in a Porphyrin-based Covalent Organic Framework

Strategies that enable simultaneous morphology-tuning and electroreduction performance boosting are much desired for the exploration of covalent organic frameworks in efficient CO₂ electroreduction. Herein, a kind of functionalizing exfoliation agent has been selected to simultaneously modify and exfoliate bulk COFs into functional nanosheets and investigate their CO₂ electroreduction performance. The obtained nanosheets (Cu-Tph-COF-Dct) with large-scale (≈1.0 μm) and ultrathin (≈3.8 nm) morphology enable a superior FE_CH4 (≈80 %) (almost doubly enhanced than bare COF) with large current-density (-220.0 mA cm^-2 ) at -0.9 V. The boosted performance can be ascribed to the immobilized functionalizing exfoliation agent (Dct groups) with integrated amino and triazine groups that strengthen CO₂ absorption/activation, stabilize intermediates and enrich the CO concentration around the Cu active sites as revealed by DFT calculations. The point-to-point functionalization strategy for modularly assembling Dct-functionalized COF catalyst for CO₂ electroreduction will open up the attractive possibility of developing COFs as efficient CO₂ RR electrocatalysts.

Tandem Electrocatalytic CO2 Reduction with Efficient Intermediate Conversion over Pyramid-Textured Cu-Ag Catalysts

If combined with renewably generated electricity, electrochemical CO₂ reduction (E-CO₂R) could be used as a sustainable source of chemicals and fuels. Tandem catalysis approaches are attractive for providing the product selectivity, which would be required for commercial applications. Here, we demonstrate a two-step tandem electrocatalytic E-CO₂R with efficient conversion of the intermediate species. The catalyst scaffold is Si(100), which is etched to form a textured surface consisting of micron-sized pyramid structures with the {111} facets. Two metals are used in the electrocatalytic cascade: Ag is employed to perform a two-electron reduction of CO₂ to the intermediate CO, and Cu performs conversion to more reduced products. Using high-angle physical vapor deposition, we form separated, micron-scale areas of the two electrocatalysts on opposite sides of the pyramids, with their relative surface coverages being tunable with the deposition angle. Compared to the textured scaffolds with blanket Ag and Cu used as controls, bimetallic pyramid tandem catalysts have higher current densities and much lower faradic efficiencies (FE) for CO. These effects are due to efficient conversion of the CO formed on Ag to more reduced products on Cu. Methane is the main product to be enhanced by the cascade pathway: a bimetallic catalyst with approximately equal coverages of Ag and Cu produces methane with a FE of 62% at -1.1 V_RHE, corresponding to a partial current density of 12.7 mA cm^-2. We estimate an intermediate conversion yield for the CO intermediate of 80-90%, which is close to the mass-transport limited value predicted by reaction-diffusion simulations.

CdS-Enhanced Ethanol Selectivity in Electrocatalytic CO2 Reduction at Sulfide-Derived Cu-Cd

The development of Cu-based catalysts for the electrochemical CO₂ reduction reaction (eCO₂ RR) is of major interest for generating commercially important C₂ liquid products such as ethanol. Cu is exclusive among the eCO₂ RR metallic catalysts in that it facilitates the formation of a range of highly reduced C₂ products, with a reasonable total faradaic efficiency but poor product selectivity. Here, a series of new sulfide-derived copper-cadmium catalysts (SD-Cu_x Cd_y ) was developed. An excellent faradaic efficiency of around 32 % but with a relatively low current density of 0.6 mA cm^-2 for ethanol was obtained using the SD-CuCd₂ catalyst at the relatively low overpotential of 0.89 V in a CO₂ -saturated aqueous 0.10 m KHCO₃ solution with an H-cell. The current density increased by an order of magnitude under similar conditions using a flow cell where the mass transport rate for CO₂ was greatly enhanced. Ex situ spectroscopic and microscopic, and voltammetric investigations pointed to the role of abundant phase boundaries between CdS and Cu⁺ /Cu sites in the SD-CuCd₂ catalyst in enhancing the selectivity and efficiency of ethanol formation at low potentials.

Engineering Bismuth-Tin Interface in Bimetallic Aerogel with a 3D Porous Structure for Highly Selective Electrocatalytic CO2 Reduction to HCOOH

Electrochemical reduction of CO₂ (CO₂ RR) into valuable hydrocarbons is appealing in alleviating the excessive CO₂ level. We present the very first utilization of metallic bismuth-tin (Bi-Sn) aerogel for CO₂ RR with selective HCOOH production. A non-precious bimetallic aerogel of Bi-Sn is readily prepared at ambient temperature, which exhibits 3D morphology with interconnected channels, abundant interfaces and a hydrophilic surface. Superior to Bi and Sn, the Bi-Sn aerogel exposes more active sites and it has favorable mass transfer properties, which endow it with a high FE_HCOOH of 93.9 %. Moreover, the Bi-Sn aerogel achieves a FE_HCOOH of ca. 90 % that was maintained for 10 h in a flow battery. In situ ATR-FTIR measurements confirmed that the formation of *HCOO is the rate-determining step toward formic acid generation. DFT demonstrated the coexistence of Bi and Sn optimized the energy barrier for the production of HCOOH, thereby improving the catalytic activity.

Coupling of Cu(100) and (110) Facets Promotes Carbon Dioxide Conversion to Hydrocarbons and Alcohols

Copper can efficiently electro-catalyze carbon dioxide reduction to C₂₊ products (C₂ H₄ , C₂ H₅ OH, n-propanol). However, the correlation between the activity and active sites remains ambiguous, impeding further improvements in their performance. The facet effect of copper crystals to promote CO adsorption and C-C coupling and consequently yield a superior selectivity for C₂₊ products is described. We achieve a high Faradaic efficiency (FE) of 87 % and a large partial current density of 217 mA cm^-2 toward C₂₊ products on Cu(OH)₂ -D at only -0.54 V versus the reversible hydrogen electrode in a flow-cell electrolyzer. With further coupled to a Si solar cell, record-high solar conversion efficiencies of 4.47 % and 6.4 % are achieved for C₂ H₄ and C₂₊ products, respectively. This study provides an in-depth understanding of the selective formation of C₂₊ products on Cu and paves the way for the practical application of electrocatalytic or solar-driven CO₂ reduction.

Metformin-Templated Nanoporous ZnO and Covalent Organic Framework Heterojunction Photoanode for Photoelectrochemical Water Oxidation

Photoelectrochemical water-splitting offers unique opportunity in the utilization of abundant solar light energy and water resources to produce hydrogen (renewable energy) and oxygen (clean environment) in the presence of a semiconductor photoanode. Zinc oxide (ZnO), a wide bandgap semiconductor is found to crystallize predominantly in the hexagonal wurtzite phase. Herein, we first report a new crystalline triclinic phase of ZnO by using N-rich antidiabetic drug metformin as a template via hydrothermal synthesis with self-assembled nanorod-like particle morphology. We have fabricated a heterojunction nanocomposite charge carrier photoanode by coupling this porous ZnO with a covalent organic framework, which displayed highly enhanced photocurrent density of 0.62 mA/cm² at 0.2 V vs. RHE in photoelectrochemical water oxidation and excellent photon-to-current conversion efficiency at near-neutral pH vis-à-vis bulk ZnO. This enhancement of the photocurrent for the porous ZnO/COF nanocomposite material over the corresponding bulk ZnO could be attributed to the visible light energy absorption by COF and subsequent efficient charge-carrier mobility via porous ZnO surface.

Copper sulfide as the cation exchange template for synthesis of bimetallic catalysts for CO2 electroreduction

Among metals used for CO₂ electroreduction in water, Cu appears to be unique in its ability to produce C2+ products like ethylene. Bimetallic combinations of Cu with other metals have been investigated with the goal of steering selectivity via creating a tandem pathway through the CO intermediate or by changing the surface electronic structure. Here, we demonstrate a facile cation exchange method to synthesize Ag/Cu electrocatalysts for CO₂ reduction using Cu sulfides as a growth template. Beginning with Cu_2−xS nanosheets (C-nano-0, 100 nm lateral dimension, 14 nm thick), varying the Ag⁺ concentration in the exchange solution produces a gradual change in crystal structure from Cu₇S₄ to Ag₂S, as the Ag/Cu mass ratio varies from 0.3 to 25 (CA-nano-x, x indicating increasing Ag fraction). After cation exchange, the nanosheet morphology remains but with increased shape distortion as the Ag fraction is increased. Interestingly, the control (C-nano-0) and cation exchanged nanosheets have very high faradaic efficiency for producing formate at low overpotential (−0.2 V vs. RHE). The primary effect of Ag incorporation is increased production of C2+ products at −1.0 V vs. RHE compared with C-nano-0, which primarily produces formate. Cation exchange can also be used to modify the surface of Cu foils. A two-step electro-oxidation/sulfurization process was used to form Cu sulfides on Cu foil (C-foil-x) to a depth of a few 10 s of microns. With lower Ag⁺ concentrations, cation exchange produces uniformly dispersed Ag; however, at higher concentrations, Ag particles nucleate on the surface. During CO₂ electroreduction testing, the product distribution for Ag/Cu sulfides on Cu foil (CA-foil-x-y) changes in time with an initial increase in ethylene and methane production followed by a decrease as more H₂ is produced. The catalysts undergo a morphology evolution towards a nest-like structure which could be responsible for the change in selectivity. For cation-exchanged nanosheets (CA-nano-x), pre-reduction at negative potentials increases the CO₂ reduction selectivity compared to tests of as-synthesized material, although this led to the aggregation of nanosheets into filaments. Both types of bimetallic catalysts are capable of selective reduction of CO₂ to multi-carbon products, although the optimal configurations appear to be metastable.

Cu sulfides as a template for Ag/Cu sulfide catalysts for electrochemical CO₂. With the introduction of Ag, nanosheet show increased C2+ product generation. The catalysts undergo a morphology evolution as CO₂ reduction proceeds.

Simultaneously Regulating the Ion Distribution and Electric Field to Achieve Dendrite-Free Zn Anode

Rechargeable aqueous Zn-ion batteries are promising candidates for large-scale energy storage systems. However, there are many unresolved problems in commercial Zn foils such as dendrite growth and structural collapse. Herein, Cu mesh modified with CuO nanowires is constructed to simultaneously coordinate the ion distribution and electric field during Zn nucleation and growth. Owing to the improved uniformity of Zn plating and the confined Zn growth in the 3D framework, the prepared Zn anodes can be operated steadily in symmetrical cells for 340 h with a low voltage hysteresis (20 mV). This work can provide a new strategy to design the dendrite-free Zn anodes for practical application.

Selective electrochemical reduction of carbon dioxide to ethylene on a copper hydroxide nitrate nanostructure electrode

Electrochemical carbon dioxide reduction (CO2 RR) is a promising technology to convert CO2 into valuable carbon-based fuels and chemicals. Copper (Cu) is a unique catalyst for this reaction as it yields substantial hydrocarbon products, but still suffers from low selectivity in aqueous solution. Here, we present a nanostructure Cu@Cu2(OH)3NO3 electrode using a facile molten salt decomposition method (MSDM). Both XPS and XRD data indicate that Cu2(OH)3NO3 is converted into metallic Cu when employed in CO2 electroreduction in KHCO3 solution, leaving abundant defects on the dendritic rough surface. Benefiting from the defects and rough surface, this electrode exhibited a high selectivity for C2H4 production with a faradaic efficiency (FE) of 31.80% and a high stability for 20 h.

Fundamentals of Gas Diffusion Electrodes and Electrolysers for Carbon Dioxide Utilisation: Challenges and Opportunities

Electrocatalysis plays a prominent role in the development of carbon dioxide utilisation technologies. Many new and improved CO2 conversion catalysts have been developed in recent years, progressively achieving better performance. However, within this flourishing field, a disconnect in catalyst performance evaluation has emerged as the Achilles heel of CO2 electrolysis. Too often, catalysts are assessed in electrochemical settings that are far removed from industrially relevant operational conditions, where CO2 mass transport limitations should be minimised. To overcome this issue, gas diffusion electrodes and gas-fed electrolysers need to be developed and applied, presenting new challenges and opportunities to the CO2 electrolysis community. In this review, we introduce the reader to the fundamentals of gas diffusion electrodes and gas-fed electrolysers, highlighting their advantages and disadvantages. We discuss in detail the design of gas diffusion electrodes and their operation within gas-fed electrolysers in both flow-through and flow-by configurations. Then, we correlate the structure and composition of gas diffusion electrodes to the operational performance of electrolysers, indicating options and prospects for improvement. Overall, this study will equip the reader with the fundamental understanding required to enhance and optimise CO2 catalysis beyond the laboratory scale.