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Molecules 2020, 25, 1712 3 of 44 1.1.2. DMFC Methanol can be used in PEM fuel cells as a replacement fuel to Hydrogen. The first physical advantage is the liquid natural state of this compound that facilitates its transport, avoiding delicate compressed gas infrastructure development or heavy metal hydrides. It has higher volumetric energy density than compressed hydrogen at 350bar, has low volatility and is almost environmentally neutral in its degradation [13] [14]. In addition, Methanol can be produced from syngas (carbon monoxide and hydrogen compound), allowing to primarily exploit renewable feedstocks as biomass or solid wastes. In recent years, this idea has attracted a lot of attention to find a possible carbon-neutral energy cycle [15]. DMFCs could find application as alternative power sources for vehicle propulsion [16] but are mainly considered for portable applications [17]. Reactions occur at the two electrodes of a DMFC upon the catalyst active area as shown Equations (4)–(6). Again, the flow of electrons through the external circuit accompanied by the flow of protons through the polymer electrolyte allows the reactions to occur and electrical power to be produced [18]. Attheanode: CH3OH+H2O→6H+ +6e− +CO2 (4) Atthecathode: 32O2+6H++6e−→3H2O (5) Overall reaction : CH3OH + 32O2 → 2H2O + CO2 (6) However, DMFCs suffer from low performance due to methanol crossover. Methanol crossover happens when methanol molecules diffuse through the membrane and are directly oxidized by oxygen on the cathode, causing a mixed potential so, consequently, a decrease in cell performance [19]. Although methanol has a high energy density (about 1.8 kWh kg−1 or 1.7 kWh L−1), it must be diluted in order to reduce methanol crossover. A consequence of dilution is that the cell stack dimensions must be proportionally increased, making it challenging to utilize on small and portable devices. Moreover, it was found that methanol crossover slows down the rate of reaction at the cathode. It has been reported that the rate of reactions not only can be accelerated by adding more platinum–ruthenium catalyst that has a negative effect on the cost, but also by selecting proper membranes and oxygen tolerant cathodes [20]. Also the carbon dioxide generated during the methanol oxidation can further increase the concentration losses: CO2 bubbles can obstruct GDL pores, reducing the available volume aimed to transport methanol towards the anode catalyst [21] causing a decrease in generated power by more than 40% [22], especially at small flow rate and high current density. 1.1.3. Electrolysers Electrolysis is an electrochemical process where water molecules split into hydrogen and oxygen gases using the supplied direct electric current. In PEMWEs, the reverse of the PEMFCs reactions described above Equations. (1)–(3) take place, see Equations (7)–(9). At the anode : H2O → 2H+ + 12O2 +2e− (7) Atthecathode:2H++2e− →H2 (8) Overall reaction : H2O → H2 + 12O2 (9) PEMWE is one of the favorable methods for conversion of renewable energy to high purity hydrogen. The technology has great advantages such as compact design, high current density (above 2 A cm−2), fast response, small footprint, low operating temperatures (20–80 ◦C) and the ability to produce ultrapure hydrogen [23]. However, high energy consumption and low hydrogen evolution rate are two important issues that hinder the large diffusion of this technology. Therefore, in order toPDF Image | Composite Polymers for Electrolyte Membrane Technologies
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