## 7 Conclusions  
In this review, extensive investigations were focused on detailed studies on molten salt mixtures and electrochemical and thermochemical processes for hydrogen and syngas production. This research provides a detailed overview of available molten salt mixtures and their thermophysical properties, with a particular focus on their application in thermoelectrochemical processes. Among the various alternatives, nitrate-based (binary and ternary) salt mixtures are a conventional choice for numerous applications and emerge first as the most promising option for thermochemical applications. On the other hand, quaternary, quinary, and senary reciprocal mixtures, characterized by low melting temperatures (from 90 \({}^{\circ}\)C down to 56 \({}^{\circ}\)C), are appropriate for low-temperature applications; whereas for high temperatures (more than 600 \({}^{\circ}\)C), chloride and fluoride mixtures can be used, but with regard to corrosion, they present particularly severe issues and demand high costs. This work also reviewed three main types of processes employed for hydrogen and syngas production: thermochemical processes, which include conventional methods, such as thermolysis or steam reforming, cracking, and gasification of carbonaceous feedstocks; electrochemical processes, predominantly involving the electrolysis of water; and hybrid processes, like electrolysis of steam and hybrid sulfur cycles. Therefore, for each process, the use of a specific type of molten salt was proposed based on the operating conditions of the cycle. In this analysis, it has been pointed out that solar salt and the ternary nitrate-based mixture can mainly feed the processes of gasification, supercritical water gasification, slow and intermediate pyrolysis, and hydrothermal liquefaction, where heat between 200 and 500 \({}^{\circ}\)C is required. With appropriate process modifications, solar salt can supply heat at 550 \({}^{\circ}\)C for the steam reforming process. Among the thermochemical processes, molten salt mixtures can only be used to supply heat at specific steps, such as the hydrolytic acid decomposition (endothermic stage at 300-450 \({}^{\circ}\)C) in the sulfur-iodine cycle and the thermolysis step (endothermic step at 530 \({}^{\circ}\)C) in the Cu-Cl cycle. In conclusion, this research represents a milestone for future work directed towards developing a ground-breaking concept for molten salt-driven hydrogen and syngas production. The aim is to utilize molten salts as a dynamic interface for the continuous delivery of high-temperature renewable heat from multiple sources, thereby advancing the state of the art in industrial hydrogen and syngas processes. This novel approach promises to reshape the landscape of sustainable energy production, enhancing efficiency and reducing carbon footprints across various sectors. This study's insights are pivotal for the development of molten salt-based energy storage systems. Such systems are envisioned as crucial, scalable, and sustainable interconnection elements between different energy sources, consistent with the EU's long-term objectives for energy security and environmental sustainability. Consequently, future research activities should concentrate on creating flexible interconnections that link renewable power generation and distribution, the electricity grid, and the gas grid. This will facilitate the generation of electricity, the provision of renewable heat at medium and high temperatures, and the production of renewable fuels, leading to enhanced dispatchability and resilience within interconnected energy networks. Conceptualization, L.T., P.H. and M.L. ; methodology, M.D., L.T., P.H. and M.L. ; investigation, M.D., A.C.T., F.R., S.S., L.T., D.C. and J.M. ; data curation, M.D., S.S., A.C.T. and D.C.; writing--original draft preparation, M.D., A.C.T., F.R., S.S., L.T., D.C. and J.M. ; writing--review and editing, F.R. and M.D. ; supervision, S.S., L.T., P.H. and M.L. ; project administration, M.D. and P.H. ; funding acquisition, P.H.