Rational Design and Construction of Cocatalysts for Semiconductor-Based Photo-Electrochemical Oxygen Evolution: A Comprehensive Review

Abstract

Photo-electrochemical (PEC) water splitting, as an essential and indispensable research branch of solar energy applications, has achieved increasing attention in the past decades. Between the two photoelectrodes, the photoanodes for PEC water oxidation are mostly studied for the facile selection of n-type semiconductors. Initially, the efficiency of the PEC process is rather limited, which mainly results from the existing drawbacks of photoanodes such as instability and serious charge-carrier recombination. To improve PEC performances, researchers gradually focus on exploring many strategies, among which engineering photoelectrodes with suitable cocatalysts is one of the most feasible and promising methods to lower reaction obstacles and boost PEC water splitting ability. Here, the basic principles, modules of the PEC system, evaluation parameters in PEC water oxidation reactions occurring on the surface of photoanodes, and the basic functions of cocatalysts on the promotion of PEC performance are demonstrated. Then, the key progress of cocatalyst design and construction applied to photoanodes for PEC oxygen evolution is emphatically introduced and the influences of different kinds of water oxidation cocatalysts are elucidated in detail. Finally, the outlook of highly active cocatalysts for the photosynthesis process is also included.

Keywords: cocatalysts; oxygen evolution reaction; photoanodes; solar energy; water splitting.

Figure 1

The number of publications and citations in the field of photo‐electrochemical water splitting from 2000 to 2018. During this period, these data climbed up exponentially. The raw data were collected from a literature search in the Web of Science database on 6 October 2018 with the key words “photo‐electrochemical water splitting.”

Figure 2

A sketch of the PEC cell for water splitting.

Figure 3

a) The structure model of the n‐type semiconductor/electrolyte interface under equilibrium conditions and the potential (ϕ) profile across this interface (blue line). Δϕ_SC, Δϕ_H, and Δϕ_G are the potential drops in the space charge layer (I), Helmholtz layer (II), and Gouy layer (III), respectively. b) The electron energy (E) profile of an n‐type semiconductor in equilibrium with the electrolyte in dark. E _C is the energy of the conduction band, E _V is the energy of the valence band, and V _barrier is the barrier height caused by upward band bending. Reproduced with permission.17 Copyright 2016, American Chemical Society.

Figure 4

Energy diagram of PEC components: anode (n‐type semiconductor), electrolyte, and cathode (metal) with a) galvanic contact (dark), b) galvanic contact under light, and c) galvanic contact, external bias, and light, respectively . Reproduced with permission.8 Copyright 2002, Elsevier.

Figure 5

Scanning electron microscope (SEM) images of typical substrates before and after growth of GDY nanowalls: a,d) on 1D silicon nanowires, b,e) on 2D Au foil, and c,f) on 3D GF grown on Ni Foam. Reproduced with permission.52 Copyright 2016, Wiley‐VCH.

Figure 6

Schematic elucidation of bandgap positions of typical photoanodes.

Figure 7

a) J–V plots of Co‐Pi/BiVO₄/ZnO ND, BiVO₄/ZnO ND, and BiVO₄/ZnO ND with H₂O₂ in the electrolyte. Reproduced with permission.68 Copyright 2016, Elsevier. b) Schematic representation of photoelectrode after sun light irradiation. c) The STH efficiency (calculated from the average current in 2 h) grows from 1.27% to 1.51% when the area of cathode is decreased from 1 to 0.2 cm². Reproduced with permission.81 Copyright 2014, Royal Society of Chemistry. d) The differences between the (APCE) (○ and △) and the IPCE (□ and ◇) of bare WO₃ and WO₃ incorporated with cocatalyst, respectively. Reproduced with permission.82 Copyright 2011, Wiley‐VCH. e) ABPE (%) of pure hematite on FTO glass (black), NF1‐P (green), and NF1‐NSP (blue) versus applied potential. Reproduced with permission.83 Copyright 2018, American Chemical Society. f) J–t plots of the BiVO₄/FeOOH/NiOOH film in KB (blue) and KB+V (red) at 0.6 V _RHE (AM 1.5G, 100 mW cm⁻²; scanning rate:10 mV s⁻¹). Reproduced with permission.84 Copyright 2017, Springer Nature.

Figure 8

a) A typical Nyquist plot of a photoelectrode in the photo‐electrochemical process. ω is the angular frequency. b) The constitutes of total impedance. c) A simplified equivalent resistor‐capacitor (RC) circuit model. d) Nyquist plots of the sample P and sample D at pH 8. Reproduced with permission.88 Copyright 2017, Elsevier.

Figure 9

a) Schematic diagram of band bending with and without Co–Pi on TiO₂ for PEC water oxidation. Reproduced with permission.102 Copyright 2015, Royal Society of Chemistry. b) Photovoltaic response on BiVO₄ (left) and BiVO₄/CoOOH (right) under dark (black lines) and light (grey lines). After coating CoOOH, the surface trap states of BiVO₄ is eliminated. V _oc,L is open‐circuit voltage in the dark; V _oc,D is open‐circuit voltage under irradiation; E _redox is the potential of water splitting redox reaction. V _ph is the relevant photovoltage; E _Fp and E _Fn are quasi‐Fermi level of holes and electrons respectively. Reproduced with permission.103 Copyright 2018, American Chemical Society.

Figure 10

a) Charge separation and transfer mechanism in the C–Co–Pi/Fe₂O₃ photoanode. Reproduced with permission.114 Copyright 2018, Elsevier. b) Schematic of Co–Pi/BiVO₄/ZnO ND photoanode. Insets are the band diagrams of trunk and branch of Co–Pi/BiVO₄/ZnO ND. Reproduced with permission.68 Copyright 2016, Elsevier. c) Schematic diagram of the fabrication process and mechanism of α‐Fe₂O₃/NFP nanoarrays. d) J−V curves of pure α‐Fe₂O₃ and α‐Fe₂O₃ loaded with different catalysts 0.1 m KOH solution under AM 1.5G (100 mW cm⁻²). Reproduced with permission.117 Copyright 2017, American Chemical Society. e) The structure of TiO₂/FeMnP core/shell photoanode for PEC water oxidation. f) The photoconversion efficiency of pure TiO₂ and TiO₂/FeMnP. Reproduced with permission.118 Copyright 2017, American Chemical Society.

Figure 11

a) Top‐view and b) side‐view SEM images of Fe₂O₃ with 25 cycles of ALD CoO_x coating. Reproduced with permission.139 Copyright 2017, Royal Society of Chemistry. c) Schematic procedure to synthesize NiO/ZnO core–shell NR arrays on the FTO glass. Reproduced with permission.67 Copyright 2016, Royal Society of Chemistry.

Figure 12

The schematic diagram of the band alignment on the surface of a) Ni _0.75Fe_0.25O_y and b) Ni_0.25Fe_0.75O_y. Interface trap states exist when coating Ni_0.75Fe_0.25O_y. Reproduced with permission.152 Copyright 2017, American Chemical Society. c) Composition maps of electrochemical power generation (P _max) for all three discrete composition libraries (L1, L2, L3), Each library contains 286 compositions of Ni–La–Co–Ce oxides. Reproduced with permission.154 Copyright 2016, Royal Society of Chemistry. d) Composition maps of electrochemical power generation (P _max) for all three discrete composition libraries (L1, L2, L3), Each library contains 286 compositions of Ni–Fe–Co–Ce oxides. Reproduced with permission.155 Copyright 2016, American Chemical Society.

Figure 13

a) Schematic diagram of the proposed catalytic mechanism of Ni(OH)₂ on hematite for PEC water oxidation. b) J–V plots of α‐Fe₂O₃ and Ni(OH)₂/Fe₂O₃ in the dark (dashed line) and under light illumination (solid line) in 1.0 m KOH solution at a scan rate of 50 mV s⁻¹. Reproduced with permission.167 Copyright 2013, Royal Society of Chemistry. c) Photocurrent of Ti‐Fe₂O₃ (blue), Ti‐Fe₂O₃/Ni(OH)₂ (black), and Ti‐Fe₂O₃/Ni(OH)₂/IrO₂ (black) under stepped potential (green, dashed curve). d) Scheme for the charge transfer from hematite to H₂O through Ni(OH)₂ and/or IrO₂. Reproduced with permission.168 Copyright 2015, American Chemical Society.

Figure 14

Crystal models of a) bulk FeOOH and b) ultrathin FeOOH with different oxygen vacancies. c) J–V curves of β‐FeOOH/BiVO₄ photoanodes measured with 0.2 m Na₂SO₄ with and without (inset) light under AM 1.5G (100 mW cm⁻²). Reproduced with permission.73 Copyright 2018, Wiley‐VCH. d) HR‐TEM images of Ti/flame‐d hematite NRs before (top) and after (bottom) the OA etching. Reproduced with permission.33 Copyright 2015, Wiley‐VCH. e) Interfacial energetics of Co‐based cocatalyst coated Ti‐Fe₂O₃ photoanodes under illumination. The photogenerated electrons (e⁻) are collected by the FTO substrate, and the holes go through the charge transfer and injection into electrolyte for water oxidation. V _ph is associated with the quasi‐Fermi level of holes (E _F,_p) and electrons (E _F,_n). The wider arrow means a faster charge‐transfer rate. f) Chopped J–V plots of Ti‐Fe₂O₃ coupled with Co‐L (blue), Co‐M (green), or Co‐H (red) under light. Reproduced with permission.178 Copyright 2018, American Chemical Society.

Figure 15

a) The structure model of carbonate‐intercalated LDHs with different M²⁺/M³⁺ molar ratios showing the metal hydroxide octahedra stacked along the crystallographic c‐axis. Water and anions are present between the interlayers. Reproduced with permission.186 Copyright 2014, Royal Society of Chemistry. b) The morphology model of TiO₂/rGO/NiFe‐LDH system. Reproduced with permission.195 Copyright 2016, Royal Society of Chemistry. c) Cross‐sectional schematic and energy band structure of n⁺p‐Si/SiO_x/Ni/NiO_x/NiFe‐LDH photoanode for water oxidation. Reproduced with permission.196 Copyright 2018, American Chemical Society. d) J–V curves of Ti‐TiO_{2‐ x}@LDH deposited for 40 s with three kinds of LDH (CoAl‐LDH, CoCr‐LDH, and CoFe‐LDH) under illumination. Reproduced with permission.201 Copyright 2017, Royal Society of Chemistry. e) The proposed mechanism of water splitting on NiCoAl‐LDH coated α‐Fe₂O₃ photoanode and Pt cathode. f) LSV curves of α‐Fe₂O₃, CoAl‐LDH/α‐Fe₂O₃, NiAl‐LDH/α‐Fe₂O₃, and NiCoAl‐LDH/α‐Fe₂O₃ under AM 1.5G radiation in the 0.5 m K‐Pi solution (pH 7) with a scan rate of 20 mV s⁻¹. Reproduced with permission.48 Copyright 2018, Elsevier.

Figure 16

a) J–V curves of Fe₂O₃ with and without Ni–Bi under AM 1.5G illumination at 0.5 m KBi electrolyte (pH 9.2). b) Schematic illustration of charge transfer before (left) and after (right) adding Ni–Bi cocatalyst on the Fe₂O₃ photoanode.210 c) J–V curves of bare BiVO₄, Ni–Bi coated BiVO₄, and NiB coated BiVO₄ under AM 1.5G in 1 m Na₂SO₃. d) Schematic of NiB and Ni–Bi‐decorated BiVO₄ and the mechanism of charge separation. Reproduced with permission.216 Copyright 2017, Royal Society of Chemistry.

Figure 17

a) J–V curves of the bare hematite and eRGO hematite‐i nanocomposites. b) Schematic of charge transfer in the eRGO‐hematite nanocomposite for water splitting. Reproduced with permission.222 Copyright 2017, Elsevier. c) Schematic illuminating of the exfoliation and acidification process for fabricating ultrathin g‐C₃N₄‐NS and BiVO₄/g‐C₃N₄‐NS photoanodes. Reproduced with permission.217 Copyright 2017, Elsevier. d) Schematic illustration of electrodeposition process of hematite nanocomposites photoanodes. Reproduced with permission.234 Copyright 2017, Wiley‐VCH.

Figure 18

a) Tilted‐view SEM image of oxide‐passivated p⁺n‐Si photoanode with the micropatterned Ni IOs. b) Schematic illustrating of water oxidation on Ni IO coated Si‐based photoanode. Inset is the charge transfer inside the photoanode after irradiation. c) PEC J–V curves of the Si‐based photoanode integrated with micropatterned planar Ni (blue line), NiIO₆ (red line), and NiFeIO₇ (black line). Reproduced with permission.245 Copyright 2017, American Chemical Society. d) Top scanning electron microscope (SEM) images of NiFe inverse opal (IO) structures. The inset shows mass transport channels with a diameter of ≈100 nm. e) Cross‐section of NiFeIO structures with 2.5 (above left), 5 (above right), and 10 thickness layers (below left) and NiFe planar film for comparison (below right). f) Scheme of the electrolyzer (EZ)‐photovoltaic (PV) combined system for overall water oxidation. Reproduced with permission.246 Copyright 2017, Elsevier.

Figure 19

a) Schematic representation of an electron transfer chain in mesoITO and cocatalysts. Reproduced with permission.248 Copyright 2015, American Chemical Society. b) Chopped LSV curves of TiO₂|PMI|20ALD|IrSil without (black) and with (red) catalyst in 0.1 m KNO₃ at pH 5.8. c) The schematic diagram of TiO₂|dye|Al₂O₃|IrSil electrodes. Reproduced with permission.253 Copyright 2017, American Chemical Society.

Figure 20

a) Comparison of photocurrent densities at 1.23 V _RHE for various photoanodes. b) Schematic illumination of the charge transfer in “hole‐depletion” layer of α‐Fe₂O₃/FeOOH/Au NFs. Reproduced with permission.173 Copyright 2017, Elsevier. c) Schematic illustration of the fast and slow reaction processes on Co₃O₄ cocatalyst and the two‐step‐two‐electron reaction pathway for water oxidation on the CDs/Co₃O₄‐Fe₂O₃ photoanode. d) IPCE (%) of Fe₂O₃, C‐Fe₂O₃, Co₃O₄‐Fe₂O₃, and C/Co₃O₄‐Fe₂O₃ photoanodes. Reproduced with permission.258 Copyright 2016, Wiley‐VCH. e) Schematic representation of the oxygen production initiated by MAl_e[Fe(CN)₆]|PSII (M = Mg or Co) photoanodes under visible light. Reproduced with permission.261 Copyright 2017, Elsevier.

Figure 21

a) Illustration of the charge separation and transfer in Fe/W codoped BiVO₄ coupled with MIL‐100(Fe). b) J–t curves of pure BiVO₄, Fe/W codoped BiVO₄, and MIL‐100 (Fe) coated BiVO₄ under 1 sun irradiation. Reproduced with permission.262 Copyright 2016, Wiley‐VCH. c) Schematic diagram of BiVO₄/CoFe‐PB applied for PEC water oxidation. d) IPCE (%) at 1.23 V _RHE for bare BiVO₄ (black circles) and CoFe‐PB‐coated BiVO₄ (red triangles) at pH 7. Reproduced with permission.263 Copyright 2017, American Chemical Society.

Figure 22

a) Overpotential versus thickness at 1 mA cm⁻² (hollow circles) and 10 mA cm⁻² (filled circles) for α‐FeOOH and Co‐Bi in 1 m Na₂CO₃. b) Overpotential versus pH at 1 mA cm⁻² (black circles) and 10 mA cm⁻² (red circles) for α‐FeOOH and Co‐Bi in 1 m Na₂CO₃. Reproduced with permission.101 Copyright 2014, American Chemical Society. Schematics of the c) planar and d) 3D nanostructured cocatalysts deposited onto a photoelectrode. The PEC J–V curves of a photoelectrode with e) planar and f) 3D‐structured cocatalysts. A _cat indicates the proportion of the metal cocatalysts on the whole surface of a photoelectrode. A _3D is the increased surface area of the 3D catalyst structure with the given A _cat. η_half‑STH is the half cell solar‐to‐hydrogen efficiency. As for traditional planar cocatalysts, an optimum A _cat exists for the maximum PEC performance. By contrast, a photoelectrode with the 3D structured cocatalysts can achieve the ultimate PEC performance of the semiconductor and cocatalyst materials. Reproduced with permission.245 Copyright 2017, American Chemical Society.