## 3 Thermo(-Electro-)Chemical Processes for the Production of Hydrogen and Syngas  
For several decades, the use of hydrogen as an energy carrier, and not only as a raw material in the process industry, has been considered as a potential key element in facilitating the transition process towards the decarbonization of energy systems. This interest is motivated by the fact that the combustion of hydrogen is not associated with the production of carbon dioxide, and the same gas can be used in fuel cells to produce electrochemical electricity, with much higher efficiencies than thermal combustion. Currently, most hydrogen is used in industrial processes and is produced from fossils, with an associated huge amount of CO\({}_{2}\) emissions (about 8 tons per 1 ton of obtained hydrogen) [82]. These emissions are due both to the chemical reactions that characterize the hydrogen formation process and to the combustion of carbonaceous sources, which is necessary to thermally support the process. For the use of hydrogen as an energy vector to be fully sustainable, carbon dioxide emissions must be reduced, and the integration of renewable sources into production processes is a fundamental requirement to achieve this. In particular, the use of solar energy is one of the main options currently being considered. Figure 2 shows a diagram of the main processes for producing hydrogen and syngas using water and/or a carbon source (fossil or biological) as the primary source and can be operated with high-temperature heat and/or electricity, depending on the type of process considered. Figure 2: Processes for hydrogen and syngas production. There are two main approaches for hydrogen production, depending on the primary source used. On the one hand, conventional production processes based on the transformation of carbonaceous feedstocks (e.g., steam reforming of natural gas) with RESs are used to thermally support the process. Unless the carbon source is of renewable origin (e.g., biogas or biomethane) and a CO\({}_{2}\) capture system is provided, this approach does not completely solve the emissions problem, but it does, however, allow for important environmental sustainability benefits. First, emissions are significantly reduced by replacing fossil fuels with RES. Furthermore, by carrying out fuel decarbonization at the centralized level where it is easier to achieve CO\({}_{2}\) capture, distributed emissions at the level of individual utilities are avoided. Another key aspect is related to the possibility of using processes widely established in the chemical industry which are suitable for large-scale production. In this way, the use of RESs in industrial hydrogen production can be encouraged with the aim of a more radical process transformation. The second approach focuses on producing hydrogen exclusively from water using RES, resulting in a completely carbon-free fuel. Hydrogen production processes can be classified into three main categories based on the type of energy used: (1) The first is thermochemical processes, which require thermal energy typically in the range of 500 to 1000 \({}^{\circ}\)C. Conventional methods like steam reforming and gasification of carbonaceous feedstocks fall under this category. Another promising thermal route is Water Splitting Thermochemical Cycles (WSTCs), which have been developed since the 1970s and offer significant potential. (2) The second is electrochemical processes, which primarily use electrical energy. An example is the electrolysis of liquid water, a well-established industrial process, although currently not widely deployed at a large scale. Alkaline electrolysis and Proton Exchange Membrane (PEM) cells are the most commercially viable options, often coupled directly or indirectly with photovoltaic (PV) systems. (3) The third is hybrid processes, which utilize a combination of both electrical power and heat. They are often integrated with PV and CSP systems.