Clean production and utilisation of hydrogen in molten salts

Abstract

Green and low cost production of strategic materials such as steel and graphene at large scale is a critical step towards sustainable industrial developments. Hydrogen is a green fuel for the future, and a key element for the clean production of steel. However, the sustainable and economic production of hydrogen is a barrier towards its large scale utilisation in iron and steelmaking, and other possible applications. As a key challenge, the water electrolysis, which is commonly used for the carbon-free production of hydrogen, is uneconomic and involves various problems including the corrosion of equipment, the use of expensive catalysts and high over-potentials, limiting its viability. Moreover, the hydrogen transportation from the electrolyser to the utilisation unit is problematic in terms of cost and safety. From a thermodynamic point of view, the potential and efficiency of the water splitting process can greatly be improved at high temperatures. Therefore, a practical approach to resolve the above-mentioned shortcomings can be based on the electro-generation of hydrogen in high temperature molten salts, and the utilisation of the generated hydrogen in situ to produce metals, alloys or other commercially valuable materials. Clean production of alloy powders is particularly interesting due to the rising of advanced manufacturing methods like additive manufacturing. The hydrogen produced in molten salts can also be used for the large scale preparation of high value advanced carbon nanostructures such as single and multi-layer high quality graphene and nanodiamonds. The combination of these findings can lead to the fabrication of hybrid structures with interesting energy and environmental applications. Surprisingly, the production of a large variety of materials such as Fe, Mo, W, Ni and Co-based alloys should be achievable by the electrolytic hydrogen produced in molten salts at a potential of around 1 V, which can easily be powered by advanced photovoltaic cells. This review discusses the recent advancements on these topics.