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Review
. 2022 Feb 8;13(10):2824-2840.
doi: 10.1039/d1sc06015e. eCollection 2022 Mar 9.

Bridging electrocatalyst and cocatalyst studies for solar hydrogen production via water splitting

Affiliations
Review

Bridging electrocatalyst and cocatalyst studies for solar hydrogen production via water splitting

Masaki Saruyama et al. Chem Sci. .

Abstract

Solar-driven water-splitting has been considered as a promising technology for large-scale generation of sustainable energy for succeeding generations. Recent intensive efforts have led to the discovery of advanced multi-element-compound water-splitting electrocatalysts with very small overpotentials in anticipation of their application to solar cell-assisted water electrolysis. Although photocatalytic and photoelectrochemical water-splitting systems are more attractive approaches for scaling up without much technical complexity and high investment costs, improving their efficiencies remains a huge challenge. Hybridizing photocatalysts or photoelectrodes with cocatalysts has been an effective scheme to enhance their overall solar energy conversion efficiencies. However, direct integration of highly-active electrocatalysts as cocatalysts introduces critical factors that require careful consideration. These additional requirements limit the design principle for cocatalysts compared with electrocatalysts, decelerating development of cocatalyst materials. This perspective first summarizes the recent advances in electrocatalyst materials and the effective strategies to assemble cocatalyst/photoactive semiconductor composites, and further discusses the core principles and tools that hold the key in designing advanced cocatalysts and generating a deeper understanding on how to further push the limits of water-splitting efficiency.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. Application of materials active for solar-driven water splitting via the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).
Fig. 2
Fig. 2. (a) Volcano plot relating the ECs' HER activities and ΔGH*; (b) HER free–energy diagram for P and S sites in the basal plane of pristine and P-doped MoS2; (c) OER volcano plot for metal oxides; (d) OH adsorption energetics (ΔGOH*) as a function of the material's composition obtained by interpolation between the calculated pure phases of WO3 (001), CoOOH (01−12), FeOOH (010), and CoWO4 (010) planes. Adapted with permission from (a and c) ref. , (b) ref. , and (d) ref. ; copyright 2017 American Association for the Advancement of Science (AAAS), 2017 the American Chemical Society (ACS), and 2016 AAAS, respectively.
Fig. 3
Fig. 3. (a) Typical synthetic strategies for powdered ECs and the corresponding electrode preparation; (b) MOF-derived NiCoFe–P NPs containing porous nanocages; (c) direct EC (P–CoMoO4) growth on a Ni foam electrode through a solvothermal method and phosphidation via annealing; (d) elemental mapping of a Ni foam/P–CoMoO4 composite electrode; (e) scanning electron microscopy (SEM) and (f) elemental mapping images, (g) CV, and (h) overall water-splitting experiment using a bifunctional SrNb0.1Co0.7Fe0.2O3−δ EC. Adapted with permission from (b) ref. , (c and d) ref. , and (e–h) ref. ; copyright 2021 ACS, 2020 Wiley, and 2017 Wiley, respectively.
Fig. 4
Fig. 4. (a) Schematic diagram illustrating the in situ oxidation of an EC; (b) Pourbaix diagram of Co–H2O system; (c–e) transmission electron microscope (TEM) images of the Co4N EC surface before and after 20 and 500 cycles of CV for OER; (f) structural differences between crystalline and amorphous NiFe and (g) their corresponding OER polarization curves. Adapted with permission from (b) ref. , (c–e) ref. , and (f and g) ref. ; copyright 2019 the Royal Society of Chemistry (RSC), 2015 Wiley, and 2020 ACS, respectively.
Fig. 5
Fig. 5. (a) Representative strategies for assembling CC/PC composites; (b) VN loaded on CdS via ball-milling; (c) CdS grown on BP via a hydrothermal method; (d) Ca2FeCoO5 loaded on TiO2via impregnation; (e) NiCo2O4 grown on a TiO2/BiVO4 photoelectrode via a hydrothermal method; (f) CoOx-FeOx coated on Ta3N5:Mg + Zr via electrodeposition; (g) MoS2 and CoPi loaded on CdS nanowires via photodeposition. Adapted with permission from (b) ref. , (c) ref. , (d) ref. , (e) ref. , (f) ref. , and (e) ref. ; copyright 2019 ACS, 2020 ACS, 2020 Wiley, 2020 ACS, 2015 ACS, and 2019 ACS, respectively.
Fig. 6
Fig. 6. (a) Colloidal synthesis of monodisperse NPs; (b) NP loading via adsorption and subsequent calcination to remove protective organic ligands; (c) ligand exchange method to replace bulky organic ligands with small inorganic ions; (d) spontaneous ligand removal during catalysis.
Fig. 7
Fig. 7. (a) Hydrogen-bonding mediated NP adsorption on PC and size-controlled PVP-coated Rh NPs (top) and Rh NP-loaded GaN:ZnO after calcination treatment (bottom); (b) photocatalytic activity of Cr2O3 shell-coated Rh NPs/GaN:ZnO; (c) oleylamine coated-Ni3S4 NPs; (d) S2−-stabilized Ni3S4 NPs obtained through (e) ligand exchange; (f) photocurrent scanning of bare, oleylamine coated-Ni3S4, and S2−-stabilized Ni3S4 NP-loaded CdS/Cu(In,Ga)Se2 PEs. Adapted with permission from (a and b) ref. , (c–f) ref. ; copyright 2012 ACS and 2017 Wiley, respectively.
Fig. 8
Fig. 8. (a) TEM image of NiPx@FePyOz NPs; (b) photocurrent of BiVO4 PEs with/without loading NiPx@FePyOz NPs; SEM images of NiPx@FePyOz (c) before and (d) after CV on a fluorine-doped tin oxide (FTO) electrode; (e) transmittance spectra and (f) Fourier transform infrared (FTIR) spectra that confirm oleylamine ligand removal from NiPx@FePyOz NPs; (g) schematic diagram illustrating the transformation of NPs on BiVO4. Adapted with permission from ref. ; copyright 2018 RSC.
Fig. 9
Fig. 9. Energy level matching of a PC semiconductor with a CC for (a) metal CC and (b) semiconductor CC [red and blue cases create ohmic contact (Schottky barrier) for photogenerated electrons (holes) and holes (electrons), respectively]; (c) CV comparison of electrochemical HER activity of Pt and MoS2 ECs in 0.5 M H2SO4; (d) photocatalytic HER activity of Pt/CdS and MoS2/CdS composites under visible-light (>400 nm) irradiation; (e) schematic reaction profile showing the activation energy from light absorption to H2 evolution. Adapted with permission from (c–e) ref. ; copyright 2019 ACS.
Fig. 10
Fig. 10. Decay of transient absorption at (a) 17 000 cm−1 and (b) 2000 cm−1 for bare, CoOx-loaded, and Pt-loaded LaTiO2N after excited by 500 nm laser pulses; (c and d) schematics showing the change in carrier dynamics upon CoOx CC loading. Adapted with permission from ref. ; copyright 2014 ACS.
Fig. 11
Fig. 11. Co-doping effect on the overall water-splitting activity of SrTiO3: (a) TEM images of Mn3O4 doped with various Co concentrations; (b) schematic of CC-loaded photocatalyst; (c) photocatalytic trend as a function of Co dopant concentration in Mn3O4 CCs; (d) electrochemical OER activities of CoxMn3−xO4 NPs in a neutral electrolyte (0.1 M PBS); (e) calculated band structures of Mn3O4 and CoxMn3−xO4 NPs. Adapted with permission from ref. ; copyright 2018 RSC.
Fig. 12
Fig. 12. (a) Current–potential curves of RhZrOx (Zr/Rh: 0–7 w/w)-loaded carbon supports in O2-saturated 1 M Na2SO4 (pH 7); (b) gas evolution over time by RhZrOx/SrTiO3:Al (Rh = 0.1 wt% and Zr = 0.5 wt% vs. SrTiO3 : Al) during and after light irradiation (300 W Xe lamp). Adapted with permission from ref. ; copyright 2020 RSC.
Fig. 13
Fig. 13. (a) Operando XAS experimental setup; (b) Cu K-edge and (c) Ni K-edge XAS spectra of 5Ni5Cu–TiO2 in a water−ethanol under UV irradiation for various exposure times; (d) Cu and (e) Ni phase contents determined by operando XAS for 5Ni5Cu–TiO2; SEM images of 5Ni5Cu–TiO2 (f) before and (g) after photocatalysis (a yellow circle indicates metallic NPs deposited under the UV irradiation). Adapted with permission from ref. ; copyright 2020 ACS.
None
Masaki Saruyama
None
Christian Mark Pelicano
None
Toshiharu Teranishi

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