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Energies 2020, 13, 420 18 of 96 with titanium, stainless steel or dense graphite and has to be highly conductive and chemically inert [198]. The electrochemical reactions (Equations (22)–(26)) occur only on triple-phase boundaries (TPBs), that are active sites at which the gas diffusion layer, the active catalyst and the electrolyte are connected. Channels, at the gas diffusion layer, transport water and product gases and conduct electrons for the reaction, at the active layer, reactions are performed, and protons are transported through the electrolyte [210]. 2H+ + 2e− → H2 (Overall HER reaction) (22) H+ + e− → Had (Volmer step) (23) Had + H+ + e− → H2 (Heyrovsky step) (24) 2Had → H2 (Tafel step) (25) H2O → 12O2 + 2H+ + 2e− (Overall OER reaction) (26) High temperature operation reduces the required activation energy and improves the cell performance, leading to high current density with smaller electrode area and hence low investment cost. However, if temperatures are higher than the liquid saturation temperature at the operational pressure, the membrane becomes dry and the proton diffusion inefficient [163]. Two pathways are viable: (i) the slight temperature increase matched with pressure increase to modify the saturation point, and (ii) the elevate increase in temperature modifying the materials used such as the polybenzimidazole membranes (PBI) doped with phosphoric acid which are suitable for operational temperature up to 200 ◦C at ambient pressure [211]. High-pressure operation is attractive for both the practicability to operate at high temperatures and the direct hydrogen storage at high pressure. The main issue is the crossover permeation that is intensified by the pressure gradient between anode and cathode [212]. 3.1.3. High-Temperature Electrolysis Cells Two types of high temperature electrolysis cells exist: with solid oxide and molten carbonate electrolytes. The typical operating temperature ranges between 723 and 1273 K at atmospheric pressure or higher [213]. High temperature electrolyzers are commonly assembled with two electrodes separated by a dense electrolyte that conveys ions. Since both electrolytes conduct anions, the solid oxide O2− and the molten carbonate CO32− ions, they can also electrolyze carbon dioxide to carbon monoxide [157]. The co-electrolysis of H2O and CO2 generates syngas as a product. Carbon dioxide and steam could be electrolyzed separately and afterwards their products mixed to form syngas. However, the co-electrolysis reaction exhibits the highest performance and operational advantages [214]. The high operating temperatures improve the transport phenomena since the kinetics of electrochemical reactions is faster (lower activation overpotential), the ohmic overpotential decreased, and the diffusion conductivity is enhanced [215]. Main drawbacks are the high-quality steel needed for auxiliary components, slow load following and start up (several hours), and the requirement for continuous operation as suffers the thermal cycles [216]. Therefore, high temperature electrolyzers are not well suited to be coupled with fluctuant and intermittent energy sources. Molten carbonate electrolyzers are not researched systematically because of the high corrosivity of the electrolyte and the high-cost post-process treatments for separation and recirculation of CO2 as the mixed product of CO2 and O2 are released jointly [157]. However, their features are based on fuel cell devices with the same electrolyte. Molten carbonate devices are typically fabricated with Ni-based porous electrodes. The overall hydrogen evolution reaction is described by Equation (27). Though the kinetics of CO2 electrolysis is slower than water splitting, CO could be produced through the CO2 reductionPDF Image | Green Synthetic Fuels
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