1. Introduction

The depletion of the global oil reserves and the problem of possible climate changes due to increasing levels of carbon dioxide in the atmosphere have led to an increasing search for both new alternative clean energy sources and, simultaneously, for finding a good way to safely remove the large amount of CO₂ being produced with the actual energy systems. In the particular case of transportation, the main energy-consuming sector in developed countries, it accounted in the European Union (UE) in the early 2000s for over 30% of energy use and almost 70% of the oil-derived fuels demand, these figures being thought to continue increasing in the next few years [1]. A relatively recent report (2011), also from the UE, indicated that the CO₂ emissions from transportation represented 42% of the total emissions from consumers, emissions from cars meaning over half of the total emissions from the transportation sector [2]. The International Energy Agency has reported that the average CO₂ concentration has been recorded to be in 2016 above 400 ppm and that it is growing at a rate of more than 2 ppm per year, with the subsequently expected global climate change [3]. This is shown in Fig. 1.1 [4, 5], where the sharp increase in CO₂ concentration in air since 1960 is clearly noted.