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Energies 2020, 13, 420 24 of 96 hydrogen to avoid the contamination by CO2, CH4, H2S, and other compounds, the consumption of hydrogen by methanogenesis and the hydrogen crossover to the anode [290]. A critical issue reported [283,291] in long term operation of single-chamber microbial electrolysis cells is the methane contamination. Indeed, methane production increases due to methanogens growth promoted by high hydrogen concentrations. Methanogens (e.g., Methanosarcina) compete with exoelectrogenic bacteria (e.g., Geobacter) at the expense of organic substrates at the anode and hydrogen produced at the cathode [292,293]. Anyhow, the production of biomethane via CO2 reduction (Equation (40)) elicits interest for the favorable process of carbon capture into a fuel [294]. A microbial electrolyzer can also be combined with an anaerobic digestor to produce a fuel with a high fraction of both hydrogen and methane [295]. Indeed, CO2 reduction enables the biogas upgrading, avoiding expensive post-treatment process such as amine scrubbing or pressure swing adsorption [296]. 8H+ +CO2 +4e− → CH4 +2H2O (40) In conclusion, microbial electrolysis cells are highly sensitive to environmental and operating condition. The optimal condition to rapid microbial growth and metabolism is at a temperature of 25–35 ◦C, a near-neutral pH between 6 and 7, and the external resistance of approximately 100–1000 Ω [297]. Wang et al. [298] investigate the initial anolyte condition for high hydrogen production yield. Low values of pH and high temperatures enhance the hydrolysis of cellulose, while at high pH levels and low temperatures the device environment is favorable to hydrogen production. 3.2. Photoelectrochemical Cell Photoelectrochemical cells (PECs) exploit the energy of solar irradiance to enhance the water-splitting reaction in the solution. Recently, PECs have also been developed for the production of syngas from water and CO2 [299]. Photoelectrochemical cells are realized with a photo-electrode, and a counter-electrode dipped in an electrolyte solution (see Figure 10). The photo-electrode is assembled with a thin film of a photocatalyst material that is deposited on a transparent substrate. The photocatalyst promotes water-splitting chemical reactions induced by photoirradiation without being consumed or transformed [300]. The electrodes are connected by an external electrical circuit that transfers photogenerated electrons from the photo-anode to the counter-electrode. Typically, semiconductor materials (e.g., titanium dioxide) are utilized in an electrochemical cell for the photo-anode and a conductive catalyst (e.g., platinum) for the cathode [301]. When the photo-electrode is irradiated by photons with energies that exceed the semiconductor bandgap (BG), electrons are excited from the valence band (VB) to the conduction band (CB), inducing electron (e−) and hole (p+) pairs in the semiconductor particles and thus charge separation (43). The potential difference due to charge separation allows the transfer of photogenerated electrons from the conduction band to the counter-electrode through the external circuit. Whereas, holes chemically react with water on the semiconductor surface. The oxidation of H2O molecules gives O2 and H+ (44). H+ ions migrate through the electrolyte and are reduced on the cathode surface by photogenerated electrons to generate H2 (45) [302]. The electrolyte is an aqueous solution with suitable ionic conductivity and inert in contact with the photoanode. For electrodes that need an acidic electrolyte, H2SO4 is used; for alkaline electrolytes, KOH and NaOH are used; Na2SO4, K2SO4 or KH2PO4/K2HPO4 are used for neutral electrolytes [303]. Since H2 and O2 generation occurs on different electrodes, products are collected separately, avoiding the need of post-treatments.PDF Image | Green Synthetic Fuels
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