Janus (Mo/ß-Mo2C)@C heterostructure as an efficient electrocatalyst for the hydrogen evolution reaction in acidic and alkaline media.

To explore low-cost, high-efficiency, and noble-metal-free catalysts for electrocatalytic water splitting in both acidic and alkaline media, the metal-metal carbide Janus hierarchical structure comprising Mo andß-Mo2C embedded on a carbon layer (Mo/ß-Mo2C)@C is synthesized by a hydrothermal reaction and subsequent low-temperature magnesium thermic process. Systematic characterization by XRD, XPS, Raman scattering, and SEM/TEM reveals the successful formation of metallic Mo andß-Mo2C nanoparticles. The synthesized (Mo/ß-Mo2C)@C has a large specific surface area and boasts highly efficient hydrogen evolution reaction activity including low overpotentials of 152 and 171 mV at a current density of 10 mA cm-2and small Tafel slopes of 51.7 and 63.5 mV dec-1in acidic and alkaline media, respectively. In addition, the catalyst shows outstanding stability for 48 h in both acidic and alkaline media. The excellent catalytic activity originates from more active sites and greater electron conductivity bestowed by the carbon layer, which also improves the long-term stability in both acidic and alkaline solutions.

Role of TiO2coating layer on the performance of Cu2O photocathode in photoelectrochemical CO2reduction.

TiO2is usually employed as a protective layer for Cu2O in photoelectrocatalytic CO2reduction. However, the role of TiO2layer on CO2reduction activity and selectivity is still elusive. In this work, a systematic investigation is carried out to probe the impact of the deposition parameters of TiO2overlayer, including the temperature and thickness, on CO2reduction performance. Compositional and (photo-)electrochemical analysis is performed to explore the property of TiO2overlayers. Carrier behavior, including donor density and electron energy, and stability of TiO2are demonstrated to be influenced by atomic layer deposition conditions and thus play a role in controlling CO2reduction reaction. Specifically, as the thickness of the TiO2layer increases from 2 to 50 nm, the electron energy tends to be lowered accompanying the electron transfer mode from tunneling for TiO2thin layers to type II for thick TiO2, leading to a decrease in CO2reduction selectivity. With an increase of the TiO2deposition temperature, the stability increases with a loss of conductivity. Cu2O coated with 2 nm TiO2at 150 °C is proven to be the optimized candidate in this work for photoelectrochemical reduction of CO2to CO, HCOOH and CH3COOH under an applied bias of -0.4 versus RHE.

A facile synthesis of Cu catalysts with multiple high-index facets for the suppression of competing H2 evolution during electrocatalytic CO2 reduction.

The electrochemical CO2 reduction reaction (CO2RR) over the high-index facets of Cu nanoparticles (NPs) is favourable towards the formation of multi-carbon products, such as hydrocarbons and oxygenates. However, the facile synthesis of Cu NPs with multiple high-index facets remains a great challenge in the research community. Herein, we have prepared numerous Cu catalysts with flat surfaces by electropolishing polycrystalline Cu foils before and after annealing at different temperatures ranging from 200 °C to 1000 °C under an argon atmosphere. The individual electrode crystal orientations were investigated via X-ray diffraction (XRD) and electron backscattering diffraction (EBSD) techniques. As confirmed by the EBSD technique, the formation of high-index facets increases with an increase in the annealing temperature and reaches a high quantity of high-index facets enclosed mainly by (211) and (431) facets with about 94% of those on the electrode annealed at 1000 °C. As a possible application, we have used the different electrodes for CO2RR at -1.0 V vs. RHE with special emphasis on the formation of H2 gas and C1 products. Thus, the electrodes prepared at higher temperatures enable the suppression of competing H2 evolution due to the increased amount of high-index facets. Moreover, the formation rates of C1 products were inhibited as well at the electrodes with increased number of high-index facets. The drops in the formation rates of both H2 and C1 products indicate that they are consumed in the chemical reaction to commence the formation of multi-carbon products. However, further study is still required with superior attention on CO2RR towards the C2+ product formation at a range of applied potentials.

From low to high-index facets of noble metal nanocrystals: a way forward to enhance the performance of electrochemical CO2 reduction.

To date, noble metal nanoparticles, mainly the gold (Au) and silver (Ag) nanoparticles, are the most active and selective heterogeneous catalysts that have revealed a tendency to form CO and directly synthesize syngas as a result of the electrochemical CO2 reduction reaction (CO2RR). The CO2RR activity and selectivity are influenced by a wide range of factors, such as morphology, surface structure, shape, composition, type of electrolyte used, and pH. Most of these issues have been reviewed and evaluated critically. Herein, the CO2RR activity and selectivity to CO formation on the low and high-index facets of Au, Ag, and Pt NCs were evaluated with a greater motive to provide new insights into the field. The author refers to different experimental approaches and the corresponding theoretical methods that have been employed to study the product formation activity and selectivity on low and high-index facet noble metal NCs for the CO2RR. In conclusion, some perspectives have been provided on the future research of the low and high-index facets of noble metal NCs for CO2 reduction.

New aspects of C2 selectivity in electrochemical CO2 reduction over oxide-derived copper.

The electrocatalytic CO2 reduction to yield C2 products is of particular interest in solar-to-fuel conversion schemes. The nanocrystalline oxide derived copper (ODCu) electrodes are specifically attractive due to their high faradaic efficiency towards C2 hydrocarbons like ethylene, ethane, acetate and ethanol. However, the mechanistic understanding of this special selectivity is still an impediment. In this work, ODCu is obtained from Cu2O nanowires and employed for electrocatalytic CO2 reduction, during which ethylene is found to be the major product with a faradaic efficiency of 65% at -0.8 V (vs. RHE). By in situ photoresponse measurement, combined with the ex situ structure and composition analysis, Cu2O is demonstrated to be persistent on the surface of ODCu throughout the CO2 reduction reaction (CO2RR) even at high applied bias (-1.0 V vs. RHE), while Cu2O is not present on the bulk Cu foil. Density functional theory calculations are employed to further investigate the correlation between the surface Cu2O on ODCu and its C2 selectivity performance, which is attributed to the orbital interactions between the persistent oxide and CO2 reduction intermediates. It should be noted that uncovering the active sites is the initial step to understand the surface reaction chemistry in CO2RR; here, we propose that the presence of Cu2O is the key for C2 selectivity during CO2RR in the ODCu system, which may facilitate the development of highly efficient catalysts.