High-temperature molten-salt electrochemical technologies for carbon neutralization

To achieve the net-zero society, the renewable energy-driven electrification of materials production and processing are at the heart of combating climate change. To this end, developing efficient and large-scale electrolyzers plays an important role in bringing clean energy to provoke electrochemical reactions in terms of converting electrical energy to chemical energies that are stored in the electrolytic products. Among various electrolyzers, high-temperature molten-salt electrolyzers have been applied to produce metals in large scale. However, innovations and technique advances are still needed to further reduce the carbon and environmental footprints. Looking back at the history of high-temperature molten-salt electrochemistry, we first should thank Prof. Alessandro Volta who invented the voltaic pile that can supply stable electricity for driving chemical reactions in 1799. From that time on, we entered the era of electrochemistry. Later, Humphry Davy's pioneering work led to the electrolytic isolation of sodium (Na), potassium (K), magnesium (Mg) and calcium (Ca) in a relatively pure form. After that, Davy's student Michael Faraday in 1834 discovered the Faraday Law which is a milestone to underpinning the development of electrolysis. Because the invention of the Hall-Heroult cell in 1886, Al products can be used widely today. Since then, Li, Na, Ca, Sr, and Ba have been gradually produced at a large scale in the 20th century, which significantly influence the field of metallurgy and materials science, as well as our daily lives. Herein, we review the development of high temperature molten salt electrochemistry at Wuhan University in the past two decades, including the electrochemical reduction of solid compounds to metal/alloys, molten-salt capture and electrochemical conversion of CO2, production of functional materials by molten-salt electrolysis, and recycling of spent energetic meals by molten-salt electrolysis. In doing so, we developed the theory of three-phase interline (3PI) to explore the kinetics of solid-compound reduction, supplemented fundamental database of screening materials for inert anodes and revealed the mechanism of stabilizing the protective oxide scale, invented the novel "anode electrolysis" for green metal extraction, and proposed the "electrolyte basicity-electrode reaction modulation" strategy. Accordingly, we refine the future research direction with the aim to close the materials supply chain of renewable energies in terms of developing green extractive methods for producing raw materials at the starting point and recycling/upcycling end-of-life renewable energy devices. Finally, we summarize the opportunities and challenges of HTMS electrochemistry in the context of carbon neutralization, discuss the future development trend of fundamental studies and applied technologies, and then provide insights into the potential contribution of renewable-driven HTMS electrolysis technologies on the achievement of the goal of reaching carbon emission peak and then realizing carbon neutralization in near future.