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Energies 2020, 13, 420 12 of 96 3. Power-to-Gas Routes Electrochemical processes convert water into hydrogen and oxygen, exploiting an external circuit that transfers the photogenerated electrons or electricity provided by renewable sources (photovoltaic, wind, geothermal, hydroelectric). 3.1. Electrolysis Water electrolysis consists of splitting water molecules into hydrogen and oxygen through the application of an electric potential. Main components of an electrolyzer are two electrodes soaked in a liquid or solid electrolyte and a separator [151]. Electrodes are made of semiconductor materials that catalyze the water-splitting reaction. Electrons provided by the external power generator are injected into the negatively polarized cathode in which the reduction reaction occurs, and hydrogen is generated. The oxidation reaction takes place at the anode in which oxygen and electrons are delivered. The electrons are injected into the external circuit and the anode is positively polarized. Electrode materials have to be resistant to corrosion, provide high electric conductivity and catalytic activity. The electrolyte enhances the ion conductivity, electrically isolates electrodes preventing short-circuit and partitions the product gas streams, avoiding the formation of a flammable mixture. If the electrolyte is in the liquid phase, a separator is introduced for electrical isolation and products division. Both electrolyte and separator have to be highly stable [152]. Electrolyzers are classified in three leading electrolysis technologies (see Figure 5): (i) the alkaline electrolyzer characterized by an aqueous alkaline electrolyte with sodium or potassium hydroxides; (ii) the proton exchange membrane electrolyzer Energies 2020, 13, x FOR PEER REVIEW 12 of 95 denoted by the polymeric membrane, typically Nafion®, and (iii) the solid oxide electrolyzer that ◦ Figure 5. Schematic of the operational principle of alkaline, proton exchange membrane and solid oxide electrolysis cells. Reprinted with permission from [153]. The overall energy demand (∆H) for the thermodynamic decomposition of water can be supplied The overall energy demand (ΔH) for the thermodynamic decomposition of water can be partially by electric energy (∆G) and the remaining by thermal energy (∆Q) (Equation (12)). At standard supplied partially by electric energy (ΔG) and the remaining by thermal energy (ΔQ) (Equation (12)). temperature and pressure, the reaction requires a minimum theoretical voltage (reversible voltage, ompiecraotbeisaaltehleigchtrotelmyspisercaetullr,ea(6n0e0w–90t0echCn)o[1lo8g].yAbdadsietdionoanllym, iincroecoerngtanyeisamrs,amniacerroobbiaicl erlecstpriorlaytsioisnc,eilsl, aexntewnsitveechlynroelosegayrcbhaesded. onmicroorganismanaerobicrespiration,isextensivelyresearched. At standard temperature and pressure, the reaction requires a minimum theoretical voltage U ) of 1.23 V that is related to the change in Gibbs free energy as expressed in Equation (13), where (reversible voltage, Urev) of 1.23 V that is related to the change in Gibbs free energy as expressed in Equation (13), where z is the number of moles of electrons per mole of products and F is the Faraday constant. Instead, the thermoneutral voltage (Uth) is the minimum voltage for the ideal water-splitting reaction without heat integration. The thermoneutral voltage is proportional to the enthalpy of the overall reaction (Equation (14)) and is approximately 1.47–1.48 V if the water inlet temperature is below 100 °C and 1.26–1.29 V if steam is supplied at temperatures up to 1000 °C. If the cell operates revPDF Image | Green Synthetic Fuels
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