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Energies 2020, 13, 420 31 of 96 mediators are generated. Electrons of the conduction-band reduce H+ ions, and the holes of the valence band oxidize the redox mediators. On the O2-evolution photocatalyst, the mediator is reduced by Energies 2020, 13, x FOR PEER REVIEW 30 of 95 photogenerated electrons, and H2O is oxidized by holes of the valence band [393]. Photons are absorbed and converted on a broader energy spectrum. Indeed, the H2-evolution reaction step requires reaction step requires a semiconductor with the minimum energy level of the conduction band higher a semiconductor with the minimum energy level of the conduction band higher than the reduction than the reduction potential of H+ ions, while the O2-evolution semiconductor should have the potential of H+ ions, while the O2-evolution semiconductor should have the maximum energy level of maximum energy level of the valence band lower than H2O oxidation potential [394]. Since each the valence band lower than H O oxidation potential [394]. Since each semiconductor should meet 2 semiconductor should meet the energy requirement of only half-reaction much more semiconductor the energy requirement of only half-reaction much more semiconductor materials can be used in materials can be used in the z-scheme device, and a higher light absorption could be achieved the z-scheme device, and a higher light absorption could be achieved through photo-sensitizer [395]. through photo-sensitizer [395]. For instance, WO3 is a suitable material for water oxidation, but the Forinstance,WO isasuitablematerialforwateroxidation,buttheconductionbandhasalowenergy 3 conduction band has a low energy level, on the other side non-oxide photocatalyst (sulfides, nitrides level, on the other side non-oxide photocatalyst (sulfides, nitrides and dye) can reduce protons but are and dye) can reduce protons but are not able to oxidize water [394]. not able to oxidize water [394]. Moreover, also redox mediators need to be efficiently coupled with the photosystem. Shuttle Moreover, also redox m2+edia3+tors need to be− e3ffi− ciently coupled with the photosystem. Shuttle redox redox mediators (e.g., Fe /Fe [396–398], I /I [399]) have been developed in recent years, but mediators (e.g., Fe2+/Fe3+ [396–398], I−/I3− [399]) have been developed in recent years, but drawback drawback such as the low diffusion rates of ions in a liquid media and the competition with water- such as the low diffusion rates of ions in a liquid media and the competition with water-splitting splitting reactions induce researched to focus on solid-state mediators (e.g., noble metals [400], reactions induce researched to focus on solid-state mediators (e.g., noble metals [400], transition transition metals [391], carbonaceous materials [401,402]) [403]. metals [391], carbonaceous materials [401,402]) [403]. Recently, photobiological hydrogen production through semi-artificial systems has been Recently, photobiological hydrogen production through semi-artificial systems has been investigated [404]. The photocatalytic action of an enzyme hybrid photoelectrochemical cell is investigated [404]. The photocatalytic action of an enzyme hybrid photoelectrochemical cell is realized realized coupling enzymes with a semiconductor substrate [405]. Hydrogenases are active microbial coupling enzymes with a semiconductor substrate [405]. Hydrogenases are active microbial enzymes for enzymes for the catalytic reduction of protons to hydrogen, whereas carbon monoxide the catalytic reduction of protons to hydrogen, whereas carbon monoxide dehydrogenases (CODH) and dehydrogenases (CODH) and formate dehydrogenase (FDH) are specific enzymes for CO2 reduction formatedehydrogenase(FDH)arespecificenzymesforCO reduction[406].Instead,thelightabsorbers 2 [406]. Instead, the light absorbers of a material-cell hybrid are realized with synthetic materials of of a material-cell hybrid are realized with synthetic materials of photosynthetic microorganisms, photosynthetic microorganisms, and photocatalyst with whole cells (e.g., Escherichia Coli cells and photocatalyst with whole cells (e.g., Escherichia Coli cells expressing [FeFe]-Hydrogenase expressing [FeFe]-Hydrogenase enzymes [407]) avoiding costly manipulations for cell disruption and enzymes [407]) avoiding costly manipulations for cell disruption and protein purification [405]. protein purification [405]. Figure 15. (a) Natural photosynthesis process in which electrons of reaction centers P680 and P700 Figure 15. (a) Natural photosynthesis process in which electrons of reaction centers P680 and P700 are are injected into charge-carrier molecules; (b) single-step photoelectrochemical reactions; (c) Z- injected into charge-carrier molecules; (b) single-step photoelectrochemical reactions; (c) Z-scheme scheme photoelectrochemical reactions. Reprinted with permission from [408]. photoelectrochemical reactions. Reprinted with permission from [408]. 3.3. Technology Readiness Level 3.3. Technology Readiness Level 3.3.1. Electrolysis 3.3.1. Electrolysis Electrolysis for hydrogen production is a mature technology: suitable industrially manufactured Electrolysis for hydrogen production is a mature technology: suitable industrially manufactured alkaline electrolyzers are available, and demonstration plants are built worldwide. PEM electrolysis alkaline electrolyzers are available, and demonstration plants are built worldwide. PEM electrolysis cell is extensively researched as the efficiency is higher than alkaline performance, but the high cost cell is extensively researched as the efficiency is higher than alkaline performance, but the high cost and earth scarcity of electrocatalysts hinder the large-scale application. SOECs and microbial fuel cells and earth scarcity of electrocatalysts hinder the large-scale application. SOECs and microbial fuel cells are still in at the laboratory scale and scale-up research status. The IEA hydrogen project database summarizes all worldwide demonstration plant projects realized since 2000 [409].PDF Image | Green Synthetic Fuels
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