# Biohydrogen from Microalgae: Production and Applications
## 6 Fuel Cell  
Hydrogen is an energy carrier with a high calorific value of 122 kJ/g, which is about 2.75 folds than fossil fuels, and it is also an environmentally friendly molecule since it only gives water as a by-product of its combustion [62]. For this reason, hydrogen environmental damage ratio has been estimated as 1, compared to that of several hydrocarbon fuels about 20 times higher [8; 63]. Hence, hydrogen is considered as the best alternative to fossil fuels, which usage is supposed to be drastically reduced by 2050, according to the 2015 Paris Climate Agreement. Several technologies are currently available for hydrogen production (Figure 2), which can be classified according to the starting material: on the one hand, from fossil fuels, hydrogen can be obtained by thermochemical conversions, such as partial oxidation, autothermal reforming, or steam reforming. On the other hand, exploiting renewable sources, hydrogen is made by water electrolysis or biomass thermochemical conversion [64]. However, none of these processes is sustainable: they all are energy-intensive process, usually requiring high temperatures. For example, water electrolysis requires temperatures ranging from 20 \({}^{\circ}\)C to 100 \({}^{\circ}\)C [65], while thermochemical processes can reach 2000 \({}^{\circ}\)C [66]. Moreover, fossil thermochemical conversions release high amounts of CO\({}_{2}\)[62]. Theoretically, it could be possible to convert these technologies into green ones by pairing them to mechanism that prevent CO\({}_{2}\) releasing into the atmosphere. This is the CSS approach, according to whom CO\({}_{2}\) can be stored into adequate geological sites, or reutilized for the chemical synthesis of useful products such as CO, urea, methanol, polymer, and carbonates [67]. Nevertheless, CSS approach has high design and operational costs that make his application unfeasible on the long term [68]. To implement a completely green sustainable economy, it is necessary to switch to biohydrogen, the biological hydrogen produced by microorganisms, including microalgae, according to the methods described in the previous paragraphs. An interesting technology that has raised attention in recent years is that of fuel cells, in which hydrogen is often used as fuel. A fuel cell is an electrochemical technology capable of energy conversion. It is indeed capable to transform the chemical energy of a fuel into electricity [69]. A fuel cell may vary for it architecture, for the kind of fuel or for its catalyst; but it always consists of a few simple main parts (Figure 3): electrodes (an anode and a cathode), electrolytes, and an external circuit [70]. Figure 2: Major hydrogen production technologies. The reactions that happen in a fuel cell are simple: the fuel (usually hydrogen) at the anode is oxidized; then the electrons, through the circuit, reach the cathode, where (usually) oxygen is reduced to water [71]. Despite the simplicity of the processes, fuel cells currently present numerous criticalities that limit their use on a large scale. Their main problems are durability of the materials and high costs, often both related to the catalyst used. The most commonly used catalyst is platinum, both pure and alloyed, due to its maximum activity and chemical stability. In some cases, platinum group metals (PGMs) have even been used, including palladium, ruthenium, rhodium, iridium, and osmium. However, all of these materials have high costs due to their global scarcity [72]. The second issue of this technology concerns how to get hydrogen to the fuel cell itself. Storage is, at the moment, the main limitation to the development of an effective hydrogen economy. Many technologies are being studied in order to reach the highest volumetric density possible: physical methods, such as high-pressure cylinders for gaseous hydrogen or cryogenic tanks for liquid hydrogen, chemical reaction with metal and alloys, new materials, such as carbonaceous nanostructures for hydrogen absorption [73; 74; 75].