## 1 Introduction  
The importance of oceans and seas is and has always been vital for human beings. They contribute to weather regulation, CO\({}_{2}\) capture, and O\({}_{2}\) production; constitute a rich source of food and raw materials; have been fundamental for communication and trading for many centuries; and have also been the scene of military conflicts. In accordance with the data published by the European Union in its report on Blue Economy, almost 5 million jobs and a gross added value of just under \(\upcels294\) billion per year are linked to sea activities [1]. Autonomous Underwater Vehicles (AUVs) represent a valuable and strategic tool insofar as the study and conservation of these marine environments. Since the SPURV [2], the first AUV built by the University of Washington (USA) in 1957, these vehicles have evolved in a way that enable them to perform complex missions, for example, search and intervention in pipelines laying on the seabed with no human intervention [3; 4]. These advances are due to the development of powerful sensors and computing sciences, such as Artificial Intelligence, but to support them a power source is always needed. [PERSON] et al. [5] reveal a great variety of AUV configurations and sizes, and two conclusions arise regarding the energy content and power source of the AUVs available today. First, almost all the AUVs on the market are powered by batteries and, second, the endurance (understood as the run-time of the power plant under nominal conditions) is limited to tens of hours. The longest commercially available endurance is 60 h at 2.06 m/s (4 knots) provided by the AUV HUGIN 3000/4500, which uses a semi-fuel cell [6]. There are reports of longer values of endurance, such as a maximum of 125 h at 2.06 m/s (4 knots), achieved by the heavy experimental vehicle, Seahorse, from the Applied Research Laboratory at Pennsylvania State University, USA, [7], but it must be taken into account that the displacement of this AUV is approximately 5 t with a total length equal to 8.53 m and a diameter of 965 mm. Short endurance is a drawback that hinders the development of AUVs. In 2004, the US Navy highlighted this matter in its UUV Master Plan [8] and made recommendations for increasing the energy stored onboard in order to enlarge mission times through the development of new power plant concepts. At the present time, the objectives of the U.S. Navy for heavy AUVs, that is, AUVs with diameters wider than 533 mm and displacements above 1360 kg, have not yet been met. To address the autonomy limitation, the application of fuel cell systems is worth investigation in order to take advantage of their higher energy density and specific energy compared to battery packs with the same power rate and endurance [9]. Different initiatives have tested the applicability of Proton Exchange Membrane Fuel Cells (PEMFCs) [10; 11] and Solid Oxide Fuel Cells (SOFCs) [12; 13] using hydrogen as fuel. One example is AUV Uroshima, capable of cruising 220 km at an average speed of 1.44 m/s (2.8 knots), that is, 43.7 h, powered by a PEMFC-based power plant in 2004 [11]. However, the use of hydrogen as a fuel presents disadvantages related to its handling and storage. Hydrogen has a high specific energy, but poor energy density. To load enough fuel, it must be stored in pressurized tanks, in liquid state under cryogenic conditions, embedded in a reversible Metal Hydride or as a Liquid Organic Hydrogen Carrier. To avoid these problems, other solutions should be considered. One of them is the on-site production of hydrogen through hydrocarbon reforming. In this regard, the Naval Undersea Warfare Center of the US Navy proposed on-board diesel-oil reforming to feed a SOFC [12]. This solution solves the energy storage problem, but it must be noted that the reforming process presents a series of drawbacks that hinder its application on-board AUVs.