## 2 Materials and Devices
So, the electric current is generated, as long as the reactants continue to be provided to the device with no need to be recharged, in contrast to what happens with batteries. Moreover, like in a battery, these electrodes are separated by an electrolyte (ionic conductor) that assures the flow of ionic charge inside the device. However, these electrodes are quite different in terms of their chemical nature and structure to that of batteries, as they are not consumed during the oxidation-reduction (also called redox) processes but must be permeable to the gases involved. In this way, they provide a high surface area that leads to a greater contact area between catalysts on their surface and reactants. This results in enhancing the reactions kinetics and providing efficient transport of reactants and products. Consequently (when operating the devices), the occurring electrical inefficiencies are the ones responsible for the heat generated, which, depending on the form of energy recovery and on the type of employed fuel cell, can be captured and used for heating or in steam turbines [42]. Hence, there are a variety of fuel cells which operate at low (\(<\)100 \({}^{\circ}\)C) or high (200-900 \({}^{\circ}\)C) temperatures [43]. However, the only low-temperature devices with strong development support currently are the polymer electrolyte membrane fuel cells (PEMFCs), also known as proton exchange membrane fuel cells, since they typically use Nafion(r) as the electrolyte, a perfluorinated sulfonic acid ion-exchange membrane developed by DuPont (Wilmington, DE, USA), which possesses a high proton conductivity. As aforementioned, PEMFCs are expected to be used primarily for transportation applications (e.g., heavy, medium, and light-duty vehicles; material handling equipment and trains), and they have the potential to be used to generate electricity both as a backup power source and as a dispatchable generator for the grid. PEMFCs can produce electricity exclusively or as combined heat and power, when the heat can be used. One of the main drawbacks of Nafionr is the accelerated dehydration at temperatures around 100 \({}^{\circ}\)C, leading to a decrease of conductivity. Another drawback is its contribution to the higher cost of this type of device, given that they also contain precious metals catalysts (typically platinum) that are neededin both electrodes to overcome the sluggish kinetics of the reactions, principally at the oxygen side [44; 45; 46]. An alternative has been to develop anion exchange membrane fuel cells (AEMFCs), which are similar devices but using existing commercial membranes as the electrolyte, such as Tokuyama A201 (Tokuyama Corporation, Tokyo, Japan) and Fumasep(r) FAA-3 (FUMATECH BWT GmbH, Biettigheim-Bissingen, Germany). These polymeric membranes, due to their chemical structure, present anion selectivity, namely to the hydroxyl ions (OH\({}^{-}\)) predominant in alkaline medium. In turn, this has also allowed the use of non-precious metal catalysts in the electrodes in order to promote the electrochemical reactions, which significantly lowers the cost of this type of device [47; 48; 49; 50]. However, despite a flurry of recent research activity and the improved performance of AEMFCs, their durability is still lower than that of PEMFCs. Thus, further advances in AEMFCs are still required to achieve highly effective, stable, and durable devices [44; 51]. Schemes of both devices (PEMFCs and AEMFCs), along with the electrochemical reactions supporting their operation, are illustrated in Figure 4.  
As mentioned before, PEMFCs have been investigated for longer than AEMFCs, including (among others) the study of the impact of reactants utilization on the performance of the devices for over 10,000 h [52]. In fact, their advanced technological status--able to reach a practical efficiency as high as 65%--is such that most major automakers involved in fuel cell electric vehicle development use this technology [53; 54; 55].