Visible Light Responsive Metal Oxide Photoanodes for Photoelectrochemical Water Splitting: a Comprehensive Review on Rational Materials Design

doi:10.15541/jim20170352

Abstract

Abstract:

Photoelectrochemical (PEC) water splitting provides a “green” approach for hydrogen production. Photoanodes for water oxidation reactions are the bottleneck for PEC water splitting due to the involved thermodynamic and kinetic challenges. To obtain the target of 10% solar-to-hydrogen (STH) efficiency toward practical applications, efficient photoanodes should be developed. Owing to the intrinsic advantages of low cost, good light harvesting, low toxicity, and excellent (photo)-electrochemical stability, visible light responsive metal oxides such as WO₃, α-Fe₂O₃ and BiVO₄ have attracted great attention for potential photoanodes and significant achievements have been made in the past decades. In this review, the sate-of-the-art progresses of WO₃, α-Fe₂O₃ and BiVO₄ photoanodes are summarized with an emphasis on the rational materials design toward efficient PEC water splitting. Moreover, their applications in unassisted PEC water splitting systems are briefly introduced. The perspectives on the challenges and future development of visible light responsive metal oxide photoanodes are presented.

Key words: solar energy conversion, semiconductor metal oxides, photoanodes, photoelectrochemical water splitting, solar hydrogen

CLC Number:

TQ174

WANG Song-Can, TANG Feng-Qiu, WANG Lian-Zhou. Visible Light Responsive Metal Oxide Photoanodes for Photoelectrochemical Water Splitting: a Comprehensive Review on Rational Materials Design[J]. Journal of Inorganic Materials, 2018, 33(2): 173-197.

Figures/Tables 19

Fig. 1 Schematic illustration of a full PEC cell

Fig. 3 I-t curves of a bare WO3 photoanode (a) and a Co-Pi modified WO3 photoanode (b) Inset: SEM image demonstrating charge transfer and separation between the Co-Pi layer, WO3, and FTO substrate Reproduced with permission from Ref. [55]. Copyright 2011 American Chemical Society

Fig. 4 FESEM images of the as-prepared WO3 films (a) Nanowires; (b) Nanoflakes-1 (NF1); (c) Nanoflakes-2 (NF2) Insets: film cross section; (d) Photocurrent density vs applied potential curves Reproduced with permission from Ref. [60]. Copyright 2011 American Chemical Society

Fig. 5 (a) Schematic illustration of crystal facet orientation of the 1-step-16 h, 1-step-8 h and 2-step-16 h WO3 photoanodes; (b) Photocurrent density vs applied potential curves; (c) I-t curves of the 1-step-16 h, 1-step-8 h and 2-step-16 h WO3 photoanodes Reproduced with permission from Ref. [73]. Copyright 2016 Elsevier

Fig. 6 (a) Diffuse reflectance spectra and digital images of (i) pure monoclinic WO3, (ii) 0.034N2·WO3, (iii) 0.039N2·WO3, and (iv) 0.039N2·WO3; (b) A model of electron density difference for monoclinic WO3 with incorporating N2. Yellow: electron loss; blue: electron gain; silver sphere: W; red sphere: O; green sphere: N Reproduced with permission from Ref. [76]. Copyright 2012 American Chemical Society

Fig. 7 Dark-current (dotted) and photocurrent (solid) densities of bare α-Fe2O3 and Co-Pi/α-Fe2O3 photoanodes modified by photo-assisted electrodeposition Reproduced with permission from Ref. [112]. Copyright 2011 Royal Society of Chemistry

Fig. 8 (a) Photocurrent density vs applied potential curves of α-Fe2O3 photoanodes sintered at 400, 700, and 800℃ in the dark (dotted curves) and under AM 1.5 G illumination (solid curves) with inset showing enlarged curves photocurrent density vs applied potential curves of α-Fe2O3 photoanodes sintered at 700 and 800℃; (b) XPS survey data for the α-Fe2O3 photoanodes sintered at 400, 700, and 800℃; (c) Absorption coefficient as a function of wavelength and the digital images of the α-Fe2O3 photoanodes sintered at 400, 700, and 800℃ Reproduced with permission from Ref. [120]. Copyright 2010 American Chemical Society

Fig. 9 (a) I-t curve of a Co-Pi/Ag/α-Fe2O3 photoanode with inset showing schematic of a Co-Pi/Ag/α-Fe2O3 photoanode for efficient PEC water splitting; (b) Schematic of an AFF photoanode for PEC water splitting; (c) Schematic of Co based OECs decorated on the surfaces of an AFF photoanode; (d) Photocurrent density vs applied potential curves of a Co based OECs modified AFF photoanode with inset showing IPCE curve measured at 1.23 V (vs RHE) Reproduced with permission from Ref. [130-131]. Copyright 2016 and 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. 10 Band diagram schematic of the BiVO4 samples (a) 1% W-doped BiVO4; (b) W doped BiVO4 homojunction; (c) W doped BiVO4 reverse homojunction; (d) Gradient W doped BiVO4; (e) Carrier separation efficiency curves Reproduced with permission from Ref. [147]. Copyright 2013 Nature Publishing Group

Fig. 11 (a) I-t curve of nanoporous BiVO4/FeOOH/NiOOH photoanode measured at 0.6 V (vs counter electrode) with insets showing: surface and cross-sectional SEM images of nanoporous BiVO4. Reproduced with permission from Ref.[153]. Copyright 2014, American Association for the Advancement of Science. (b) Applied bias photon-to-current efficiency (ABPE) of N2-treated BiVO4/FeOOH/NiOOH photoanode obtained using a two-electrode configuration (CE: counter electrode). Reproduced with permission from Ref. [154]. Copyright 2015 Nature Publishing Group. Distributed under a CC-BY 4.0 license. (c) H2 (black circle) and O2 evolution (red square) using two BiVO4/NiFeOx-Bi photoanodes. Reproduced with permission from Ref. [155]. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Photocurrent density vs applied potential curves of BiVO4 and molecular Co4O4 cubane modified BiVO4 photoanodes under AM 1.5 G illumination. Reproduced with permission from Ref. [156]. Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. 12 (a) Schematic of the WO3 NRs/BiVO4/Co-Pi junction, the charge transfer and separation in the junction for PEC water splitting; (b) Cross-sectional SEM image of a WO3 NRs/BiVO4 photoanode; (c) Photocurrent density (vs applied potential) curves of the WO3 NRs/BiVO4/Co-Pi junction under 1 sun and 3 sun illumination, respectively Reproduced with permission from Ref. [166]. Copyright 2015 Nature Publishing Group. Distributed under a CC-BY 4.0 license

Fig. 13 SEM images of BVO-{040}-V: (a) top view and (b) cross-sectional view, (c) photocurrent density vs applied potential curves and (d) the corresponding PEC water splitting efficiency curves of I: BVO-{040}-V, II: electrochemically treated BVO-{040}-V, and III: electrochemically treated BVO-{040}-V/CoBi photoanodes Reproduced with permission from Ref. [151]. Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. 14 (a) Photostability measurement of the 800℃-BM-700℃ and the previously reported nanoworm BiVO4 photoanodes; (b) Photostability and electrochemical stability of the 800℃- BM-700℃ photoanode; (c) Long-term stability measurement of the NiFe-OECs modified 800℃-BM-700℃ photoanode at 0.6 V (vs RHE) under AM 1.5 G illumination Reproduced with permission from Ref. [196]. Copyright 2016 Nature Publishing Group

Fig. 15 (a) Mechanism of a PEC tandem system with Ohmic contact, a schematic of the wired PEC tandem system in (b) a parallel illumination mode (Mode P) and (c) a tandem illumination mode (Mode T) Reproduced with permission from Ref. [197]. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. 16 (a) Current density-voltage (J-V) curves of the BiVO4 photoanodes with (solid lines) and without (dashed lines) Co-Pi OECs under AM 1.5 G illumination. The |J|-V curves of the Cu2O photocathode is measured with the coverage of the corresponding BiVO4 photoanode (dotted line with the same colour). Reproduced with permission from Ref. [199]. Copyright 2014 American Chemical Society. (b) J-V curves of various α-Fe2O3 photoanodes. The |J|-V curve of the Si photocathode is measured behind the α-Fe2O3 photoanode. (c) Schematic of overall unassisted water splitting by α-Fe2O3 photoanode and amorphous Si photocathode in wired Mode T. (d) Photocurrent stability measurement of the PEC tandem system in (c). Reproduced with permission from Ref. [202]. Copyright 2015 Nature Publishing Group. Distributed under a CC-BY 4.0 license

Fig. 17 General schemes of a WO3/DSSC tandem system (a) and a Fe2O3/DSSC tandem system (b); Working mechanism of the WO3/DSSC and Fe2O3/DSSC tandem systems (c) Reproduced with permission from Ref. [207]. Copyright 2012 Nature Publishing Group conducting substrate for the front photoanode to enhance solar light utilization, an impressive STH efficiency of 7.1% was achieved[209].

Fig. 18 (a) Scheme of a PEC/PV tandem cell with modified BiVO4/α-Fe2O3 dual photoanodes and two parallel-connected c-Si cells; (b) Stability of unassisted water splitting with inset showing: data recorded for short term illumination Reproduced with permission from Ref. [211]. Copyright 2016 Nature Publishing Group. Distributed under a CC-BY 4.0 license

Fig. 19 Schematics of (a) BiVO4/PSC tandem device with PSC behind the BiVO4 photoanode in air, (b) BiVO4/PSC tandem device with a beam splitter, and (c) wireless artificial leaf of BiVO4/PSC tandem device, (d) stability measurement of the BiVO4/PSC tandem device in (b) Reproduced with permission from Ref. [212-214]. Copyright 2015 American Chemical Society. Distributed under an ACS AuthorChoice License. Copyright 2016 American Association for the Advancement of Science. Distributed under a CC BY-NC 4.0 license Copyright 2015 American Chemical Society

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