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Energies 2020, 13, 420 14 of 96 produced [158]. The coverage of the active sites reduces the area of the electrodes available for the reaction, forming an insulating layer. Further, when the detachment of bubbles from the electrode occurs, the mass transport of electrolyte is reduced, and the diffusion overvoltage increases due to bubble dispersion and rapid movement [159]. Therefore, the rapid detachment and removal of the bubble are needed to reduce overpotential and ohmic losses and increase ion conductivity [160]. The process intensification by external fields such as rotating electrodes, centrifugal or supergravity, magnetic and ultrasonic fields and zero-gap configuration is an attractive approach to overcome bubble problems and enhance mass transport and, thus, electrolyzers performances [161]. The total mass production rate (m. the real hydrogen flow-rate to its theoretical value), the number of cell stack (nc) and the current (I) divided by the Faraday constant multiplied by two [162]. m. =η ncI (16) H2 F 2F The electrolyzer efficiency depends on operational parameters. If the current density decreases, efficiency rises due to fewer overpotential losses, but the mass rate of the hydrogen production decreases and the active surface area required increases [163]. The temperature has a substantial impact on process efficiency that increases with temperature growth since the mass transfer phenomena are enhanced, but materials and degradation issues restrict the feasible operational range [164]. Instead, the increase in pressure slightly influences efficiency that declines but is attractive to avoid the subsequent compression stage that enlarges energy and economic costs [165]. The electrolysis module is established connecting cells in series or parallel. When cells are connected in parallel, each electrode is connected to the power generator, so has a single polarity and the module is called monopolar. Otherwise, when cells are connected in series, the current from the power supply is driven into the first cell and circulates through all electrodes. Each interconnector is subjected to two polarities, so in series-configuration is named bipolar module or cell stack and the materials used must be chemically stable in both oxidizing and reducing environments [152]. Unipolar design is simple, reliable and flexible. The bipolar configuration is more compact, induces lower ohmic losses and are more suitable for high-pressure electrolyzers. Still, the requirement of electrolyte circulation and, therefore, of channels or the need for gas/electrolyte separators increases losses due to parasitic currents [166]. 3.1.1. Alkaline Electrolysis Cells Alkaline electrolysis is a mature technology, characterized by good stability, long lifetime and cost-effectiveness. The large-scale hydrogen production via alkaline electrolysis started in the first years of the 20th century [154]. Alkaline cells are typically connected in series through a bipolar plate establishing a stack. However, the drawback is the low maximum power density due to the limited mass transport in a liquid electrolyte and the bubble formation [167]. Low-cost nickel-based electrodes are tolerated as a result of the non-corrosiveness of the electrolyte. Electrodes are fully immersed in an aqueous solution containing a strong base compound, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH) [166]. The ionic transfer and electric conductivity of the electrolyte strongly depend on type and concentration [168]. Typically, 20%–40%wt of KOH or NaOH is adopted, since a higher concentration reduces the water activity [169]. The partition of the product gases is ensured by a separator that generally is a porous diaphragm or a polymeric membrane. The characteristics required to a porous separator are high porosity and wettability to be filled by the electrolyte as well as high electrical resistance, high selective ionic conductivity and low ohmic drop [170]. Diaphragm materials are porous ceramics (e.g., asbestos and glass frits), and porous polymers (e.g., porous polyvinyl chloride (PVC), polyolefins, and PTFE). Polymeric membranes are exclusively permeable to either anion or H2 ) is defined as the product of the Faraday efficiency (ratio ofPDF Image | Green Synthetic Fuels
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