Efficient water electrolyzers are constrained by the lack of low-cost and earth-abundant hydrogen evolution reaction (HER) catalysts that can operate at industry-level conditions and be prepared with a facile process. Here we report a self-standing MoC–Mo₂C catalytic electrode prepared via a one-step electro-carbiding approach using CO₂ as the feedstock. The outstanding HER performances of the MoC–Mo₂C electrode with low overpotentials at 500 mA cm⁻² in both acidic (256 mV) and alkaline electrolytes (292 mV), long-lasting lifetime of over 2400 h (100 d), and high-temperature performance (70 ^oC) are due to the self-standing hydrophilic porous surface, intrinsic mechanical strength and self-grown MoC (001)–Mo₂C (101) heterojunctions that have a ΔG_H* value of −0.13 eV in acidic condition, and the energy barrier of 1.15 eV for water dissociation in alkaline solution. The preparation of a large electrode (3 cm × 11.5 cm) demonstrates the possibility of scaling up this process to prepare various carbide electrodes with rationally designed structures, tunable compositions, and favorable properties.

A molten salt electrochemical system comprising a eutectic mixture of Li–Na–K carbonates, a Ni cathode, and a SnO₂ inert anode is proposed for the capture and electrochemical conversion of CO_2. It is demonstrated that CO₂ can be effectively captured by molten carbonates, and subsequently electrochemically split into amorphous carbon on the cathode, and oxygen gas at the anode. The carbon materials generated at the cathode exhibit high BET surface areas of more than 400 m² g⁻¹ and as such, represent value-added products for a variety of applications such as energy storage and pollutant adsorption. In the carbonate eutectic (500 °C), the presence of Li₂CO₃ is shown to be required for the deposition of carbon from the melt, wherein O²⁻ or Li₂O serves as the intermediate for CO₂ capture and electrochemical conversion. SnO₂ proved to be an effective anode for the electrochemical evolution of oxygen. Electrochemical reactions were found to proceed at relatively high current efficiencies, even though the current densities exceed 50 mA cm⁻². The intrinsic nature of alkaline oxides for CO₂ capture, the conversion of CO₂ to value-added products, and the ability to drive the process with renewable energy sources such as solar power, enables the technology to be engineered for high flux capture and utilization of CO₂.

Solid electrode processes fall in the central focus of electrochemistry due to their broad-based applications in electrochemical energy storage/conversion devices, sensors and electrochemical preparation. The electrolytic production of metals, alloys, semiconductors and oxides via the electrochemical reduction of solid compounds (especially solid oxides) in high temperature molten salts has been well demonstrated to be an effective and environmentally friendly process for refractory metal extraction, functional materials preparation as well as spent fuel reprocessing. The (electro)chemical reduction of solid compounds under cathodic polarizations generally accompanies a variety of changes at the cathode/melt electrochemical interface which result in diverse electrolytic products with different compositions, morphologies and microstructures. This report summarizes various (electro)chemical reactions taking place at the compound cathode/melt interface during the electrochemical reduction of solid compounds in molten salts, which mainly include: (1) the direct electro-deoxidation of solid oxides; (2) the deposition of the active metal together with the electrochemical reduction of solid oxides; (3) the electro-inclusion of cations from molten salts; (4) the dissolution–electrodeposition process, and (5) the electron hopping process and carbon deposition with the utilization of carbon-based anodes. The implications of the forenamed cathodic reactions on the energy efficiency, chemical compositions and microstructures of the electrolytic products are also discussed. We hope that a comprehensive understanding of the cathodic processes during the electrochemical reduction of solid compounds in molten salts could form a basis for developing a clean, energy efficient and affordable production process for advanced/engineering materials.

Electroreduction, or electrodeoxidation, of pelleted SiO₂ powder (left image) or its mixture with other metal oxide powders in molten CaCl₂ produces pure Si powder (right image), or the respective silicon alloy powder. Being advantageous by simplicity and resulting in less CO₂ emission, the electrochemical approach has an energy consumption that is below 13 kWh (kg of Si)⁻¹.