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Energies 2020, 13, 420 13 of 96 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 at a voltage above the thermoneutrality, external cooling is needed [154]. ∆H = ∆G+∆Q (12) Urev = ∆G (13) zF Uth = ∆H (14) zF Nevertheless, in real condition, an extra voltage is involved in overcoming energy losses due to irreversibility. The cell voltage is defined as the sum of the reversible cell voltage and all the overpotentials that may occur in the cell. U = Urev +ηohm +ηact +ηdif f (15) Energy losses are mainly due to: (i) ohmic overvoltage (ηohm) that is related to resistances to ion conduction in the electrolyte and through the membrane, bubbles formation and resistances to electrons transfer in electrodes and external circuit; (ii) activation overvoltage (ηact) that is related to the kinetics of electrochemical reactions at both electrodes, and (iii) diffusion overvoltage (ηdiff) Energies 2020, 13, x FOR PEER REVIEW 13 of 95 due to mass transport of molecules that diffuses through the electrodes pores [155]. The polarization curve (Figure 6) is the plot of the voltage across the electrodes and the current density. The slope of curve (Figure 6) is the plot of the voltage across the electrodes and the current density. The slope of the polarization curve provides information about the overpotential losses [156]. the polarization curve provides information about the overpotential losses [156]. Figure 6. Comparison between typical ranges of the polarization curves for a different type of Figure 6. Comparison between typical ranges of the polarization curves for a different type of electrolytic electrolytic cells at the actual state of art. The thermoneutral voltage of the alkaline electrolysis cells at the actual state of art. The thermoneutral voltage of the alkaline electrolysis involves low current, involves low current, and therefore, the operational point is generally in the exothermic regime and therefore, the operational point is generally in the exothermic regime (higher current density). (higher current density). Solid oxide is performing better due to the high temperature operation that Solid oxide is performing better due to the high temperature operation that reduces the overvoltage reduces the overvoltage losses. Reprinted with permission from [157]. losses. Reprinted with permission from [157]. A relevant issue is the formation of bubbles that promotes high overpotential and ohmic voltage drop. Bubbles nucleate and grow on electrodes surface through the supersaturation of H2 and O2 22 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,PDF Image | Green Synthetic Fuels
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