Electrochemical CO2 reduction reaction (CO2RR) provides a promising approach to curbing harmful emissions contributing to global warming. However, several challenges hinder the commercialization of this technology, including high overpotentials, electrode instability, and low Faradic efficiencies of desirable products. Several materials have been developed to overcome these challenges. This mini-review discusses the recent performance of various cobalt (Co) electrocatalysts, including Co-single atom, Co-multi metals, Co-complexes, Co-based metal–organic frameworks (MOFs), Co-based covalent organic frameworks (COFs), Co-nitrides, and Co-oxides. These materials are reviewed with respect to their stability of facilitating CO2 conversion to valuable products, and a summary of the current literature is highlighted, along with future perspectives for the development of efficient CO2RR.
Molybdenum phosphide (MoP) is viewed as a potential electrocatalyst for the electrochemical hydrogen evolution reaction (HER). However, crystallization of MoP occurs at rather high temperature (>600 °C). At this temperature, coalescence and agglomeration, which affect the performance severely, become inevitable. Herein, an oxalate-guided nonhydrolytic method is demonstrated for the preparation of MoP with smaller particle size and better dispersion qualities onto the surface of carbon nanotubes (CNTs). Molybdenum is coordinated with the oxalate group using oxalic acid, which modifies the self-assembling of molybdenum at the molecular level and renders discrete nucleation and growth of MoP on CNTs. Phosphoric acid (crystalline) was used as a source of phosphorus. The method is simple with the potential to scale-up. A probable mechanism for the growth of MoP on CNTs is proposed. The as-derived MoP/CNT electrode exhibits excellent performance, outperforming most of the MoP-based electrocatalysts, for hydrogen evolution in both acidic and basic media. In addition, the electrode possesses excellent stability. The higher performance of the electrode is rationalized in terms of small particle size with uniform dispersion, high specific and electrochemically active surface area, electrical conductivity, interfacial charge transfer kinetics, and turnover frequency. Estimation of Tafel slope is consistent with electrochemical desorption of hydrogen gas following the Volmer−Heyrovsky mechanism as the rate-determining step.
Transition-metal phosphides are deemed as potential alternative to platinum for large-scale and sustainable electrocatalytic hydrogen production from water. In this study, facile preparation of interconnected hollow cobalt monophosphide (CoP) supported on carbon nanotubes is demonstrated and evaluated as a low-cost electrocatalyst for hydrogen evolution reaction. Hexamethylenetetramine is used as a structure-directing agent to guide the formation of interconnected cobalt oxide, which further grows into interconnected hollow CoP. Interconnected and hollow microstructural artifacts impart benign attributes, such as enhanced specific and electrochemically active surface area, low intrinsic charge transfer resistance, high interfacial charge transfer kinetics, and improved mass transport, to the electrocatalyst. As a result, the as-prepared electrode exhibits remarkable electrocatalytic performance, low onset (18 mV) and overpotential (η = 73 mV); small Tafel slope (54.6 mV dec); and high turnover frequency (0.58 s at η = 73 mV). In addition, the electrode shows excellent electrochemical stability.
The study evaluates the dependency of the HER performance on the microstructural attributes of carbon supports, and correlates the performance variation to crucial features of the as-prepared electrode.
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