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WIREs Energy Environ.
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A review and perspective of efficient hydrogen generation via solar thermal water splitting

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Solar thermal water splitting (STWS) produces renewable hydrogen from water using concentrated sunlight. Because it utilizes energy from the entire solar spectrum to directly drive the redox reactions that split water, it can achieve high theoretical solar‐to‐hydrogen efficiencies. In two‐step STWS, a metal oxide is first heated by concentrated sunlight to high temperatures to reduce it and produce O2. In the second step, the reduced material is exposed to H2O to reoxidize it to its original oxidation state and produce H2. Various aspects of this process are reviewed in this work, including the reduction and oxidation chemistries of the active redox materials, the effects of operating conditions, and the solar thermal reactors in which the STWS reactions occur, and a perspective is given on the future optimization of STWS. WIREs Energy Environ 2016, 5:261–287. doi: 10.1002/wene.174 This article is categorized under: Concentrating Solar Power > Science and Materials Energy and Climate > Science and Materials Energy Research & Innovation > Science and Materials
Methods of concentrating solar irradiance using (a) power tower and heliostats and (b) a parabolic dish concentrator. Reprinted under the creative commons license. (c) A schematic of a generic two‐step solar thermal water splitting cycle.
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A ‘beam‐up’ receiver configuration (left) and the solar field design sounding the tower at a latitude of 34.5°N. Reprinted from Ref with permission ASME.
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The newly proposed solar thermal particle flow reactor. (a) An individual reduction/oxidation reactor unit and (b) receiver configuration containing multiple individual reduction/oxidation reactor units. Reactors are not shown to scale.
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Particle based solar thermal water splitting reactor concepts: (a) rotating cavity particle reactor, reprinted from Ref with permission from Elsevier. (b) Nonrotating particle flow reactor, reprinted from Ref with permission from Elsevier. (c) aerosol flow reactor, reprinted with permission of Ref . Copyright 2014 American Chemical Society. (d) Internally circulating fluidized bed reactor, Reprinted from with permission ASME. (e) moving particle packed bed reactor. Reprinted from with permission ASME. The labels in (a) point out: (1) rotating drum, (2) actuation, (3) aperture, (4) cavity, (5) screw feeder, (6) product outlet port, (7) rotary joint, (8) working fluids, (9) insulation, (10) quartz window and labels in (b) point out: (1) water‐cooled window mount and vortex‐flow generation, (2) water‐cooled cavity aperture, (3) BOP and data‐acquisition cavity access ports, (4) alumina‐tile reaction surface, (5) annular solid ZnO exit, (6) bulk insulation and cavity‐shape support, (7) central product‐vapor and gas exit.
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Monolith‐based solar thermal water splitting reactor concepts: (a) the porous monolith cavity reactor, from Ref reprinted with permission from AAAS; (b) the rotating piston reactor, reproduced by permission of John Wiley and Sons; (c) the CR5, reproduced with permission of Sandia Corporation, copyright 2012 and (d) the SurroundSun reactor, reprinted from Ref with permission from Elsevier.
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Process diagram from Lapp et al., and Ermanoski et al.. (a) Reprinted from Energy 37, Lapp et al., ‘Efficiency of two‐step solar thermochemical nonstoichiometric redox cycles with heat recovery’, Copyright 2012, with permission from Elsevier. (b) Reproduced from Ermanoski et al., with permission from the PCCP Owner Societies.
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Three different methods to achieve low PO2 of oxygen and the associated energy requirements: (a) vacuum pumping, (b) direct inert gas sweep, and (c) recycled inert gas sweep.
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A comparison of the reduced Zn and SnO recovery after thermal ZnO and SnO2 decomposition using the same experimental apparatus and conditions. Reproduced from Ref by permission of Elsevier.
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(a) Perovskite based solar thermal water splitting cycles. SLMA materials are capable of producing significantly more H2 than ceria when reduced at 1350°C and oxidized at 1000°C. This figure is reproduced under the creative commons license. (b) Many other perovskite formulations have shown to be capable of undergoing STWS. From left to right are the H2 production capacities of Ba0.25Sr0.75Co0.80Fe0.20O3, Ba0.50Sr0.50Co0.80Fe0.20O3, La0.60Sr0.40Co0.20Fe0.80O3, LaSrCoO3, La0.65Sr0.35MnO3, La0.50Sr0.50MnO3, and La0.50Sr0.50MnO3 under operating conditions shown in the inset. Reprinted from Ref , copyright 2014, with permission from the American Chemical Society.
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Ceria‐based solar thermal water splitting cycles. (a) H2 and O2 production rates using a cavity reactor, (b) H2 and O2 production over 500 cycles using the cavity reactor, cycled at 1500/800°C with a PO2 = 10−5 atm and PH2O = 0.13–0.15 atm. From Ref reprinted with permission from AAAS. (c) and (d) show the reduction and oxidation reaction extents of doped ceria as measured by TGA experiments under 1500/800°C, PO2 = 10−4 atm and PH2O = 0.4 atm operating conditions. Reprinted from Ref , copyright 2013 with permission from the American Chemical Society.
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Thermodynamic extent of oxygen nonstoichiometry (δ) of ceria, based on temperature and partial pressure of oxygen. Reprinted from Energy 37, Lapp et al., ‘Efficiency of two‐step solar thermochemical nonstoichiometric redox cycles with heat recovery’, copyright 2012, with permission from Elsevier.
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(a) The doped‐hercynite cycle H2 production rates after reduction at various temperatures and oxidation at 1000°C. (b) The doped‐hercynite cycle H2 production rates of CoFe2O4 on ZrO2 after reduction and oxidation under the same conditions as (a). Reproduced by permission of The International Association of H2 Energy. (c) shows the H2 production rates of doped‐hercynite operating under temperature swing water splitting conditions (left two peaks) and isothermal water splitting conditions (right peak). Reproduced from Science 2013 with permission from the AAAS.
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Thermodynamic predictions of the reduction of MFe2O4 where M = Fe, Co, Ni or Zn. Figure (a) shows the simulated MFe2O4 (blue) and O content (red) while (b) shows the reduced metal oxide content in a solid (orange), liquid (black), or gas (green) phase. Reprinted from Ref , copyright 2008, with permission from the American Chemical Society.
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