Fig. 1. The degradation mechanism of the photoelectrode. Process 1: chemical corrosion of a semiconductor light absorber exposed to an electrolyte. Process 2: The anodic photocorrosion reaction. The hole that is generated in the semiconductor and migrated to the surface is captured by the two-electron surface back-bond. It induces the generation of surface radical intermediates and then the dissolution of semiconductor atoms. Process 3: Variation of semiconductor defect concentration under operating conditions, e.g., oxygen vacancies. Process 4: The cathodic photocorrosion reaction. Electrons generated in the semiconductor and migrating to the surface reduce the semiconductor atom and dissolve it. Process 5: Pinholes in the protection layer. Process 6: Leaching of co-catalyst active species. Process 7: Agglomeration of co-catalysts. Process 8: Particle detachment of co-catalysts.

Fig. 2. The corrosion process of semiconductor light absorbers. (a) Schematic diagram of the b, arrangement of p-type photocathode and n-type photoanode semiconductors with respect to the redox potential of water. Φox shows the oxidation potential of the photoanode in aqueous solution, and Φre shows the reduction potential of the photocathode. (b) The change in stability of the n-type material with increasing Φox for the photoanode (left panel), the change in stability of the p-type material with increasing Φre for the photocathode (right panel). Reprinted with permission from Ref. [30]. Copyright 2012, American Chemical Society. (c) Surface oxidation of a silicon-based photoelectrode in an acidic solution under illumination. (d) Surface oxidation and dissolution of a silicon-based photoelectrode in an alkaline solution under illumination. Reprinted with permission from Ref. [16]. Copyright 2019, Royal Society of Chemistry. (e) Calculated oxidation potential Φox (red bars) and reduction potential Φre (black bars) relative to the NHE and vacuum level for a series of semiconductors in solution at pH = 0, ambient temperature 298.15 K, and pressure 1 bar. Reprinted with permission from Ref. [30]. Copyright 2012, American Chemical Society. (f) Photocorrosion of GaN under illimitation (upper panel). Photoexcitation of a GaN semiconductor. Light absorption will induce electron excitation from SN to SGa (bottom left). Electron and hole excitation in GaN resulted in a bond-breaking charge transfer from surface N to Ga (bottom right). Reprinted with permission from Ref. [65]. Copyright 2017, American Chemical Society. (g) The Materials Project Pourbaix diagram of 50%-50% Bi-V system in aqueous solution, assuming a Bi ion concentration at 10?5 mol kg?1 and, a V ion concentration at 10?5 mol kg?1. Reprinted with permission from Ref. [68]. Copyright 2016, Springer Nature.

Fig. 3. (a) Pinholes in the protection layer. SEM images of a Si-based photoelectrode with and without TiO2 protection layer at 400 °C after immersion in 1 mol L?1 KOH electrolyte for 3 days (left panel). Schematic of the etch profiles of silicon with TiO2 protection layer during the etching process in KOH solution (right panel). Reprinted with permission from Ref. [33]. Copyright 2016, Elsevier. Problems encountered in the integration of co-catalysts with light absorbers. (b) A composite PEC photoanode where the co-catalyst thin film attenuates the solar flux that reaches the semiconductor light absorbers. (c) Photocatalytic efficiency Φo?c plots for the catalysts as a function of film thickness, t. Reprinted with permission from Ref. [88]. Copyright 2013, American Chemical Society. (d) Schematic of failure-process pathways for Ni islands/Si during OER in 1 mol L?1 KOH. Reprinted with permission from Ref. [90]. Copyright 2018, Royal Society of Chemistry.

Fig. 4. Schematic depiction of self-healing mechanisms. (a) Intrinsic self-healing, where the active species or elements remain unchanged before and after repair. (b) Extrinsic self-healing, which requires the involvement of external healing agents.

Fig. 5. Intrinsic self-healing of semiconductor light absorbers. (a) Scheme of self-healing after photocorrosion of a CuRhO2 electrode. (b) XPS characterization of CuRhO2 electrodes surface after electrolysis under air, Ar. Reprinted with permission from Ref. [96]. Copyright 2014, American Chemical Society. (c) Oxygen vacancy (Vo) self-healing on TiO2 photocatalysts in the water splitting. (d) Photocatalytic H2 evolution tests of Pt/TiO2 samples in the methanol solution under UV/Vis light irradiation. Vo-L TiO2 and Vo-R TiO2 refer to less, rich Vo rutile TiO2 nanocrystals, respectively. (e) Typical SI-XPS spectra and fitting patterns of Vo-L TiO2 samples and their interacting with chemically absorbed water molecules before, after light irradiation, respectively. (f) HR-TEM images of pristine TiO2 (left panel), Vo-L TiO2 (middle panel), water absorption on Vo-L TiO2 (right panel). Reprinted with permission from Ref. [97]. Copyright 2019, Wiley-VCH.

Fig. 6. Extrinsic self-healing of protection layer. (a) Schematic of the passivation mechanisms for the Si photoanode decorated with Ni islands in the dark at the open circuit with the addition of [Fe(CN)6]3? to the alkaline electrolyte. (b) Schematic of the passivation mechanisms for the Si photoanode decorated with NiOx films in the dark at the open circuit with the addition of O2 to the alkaline electrolyte. Reprinted with permission from Ref. [95]. Copyright 2022, Royal Society of Chemistry. (c) Chronoamperometric stability of np+-Si/μNi electrodes in 1.0 mol L?1 KOH with and without, respectively, 10 mmol L?1 [Fe(CN)6]3?. Reprinted with permission from Ref. [94]. Copyright 2020, Royal Society of Chemistry. (d) Open-circuit potential (Eoc) vs. time in the dark of p+-Si, p+-Si/Ni (5 nm), p+-Si/NiOx (60 nm) electrodes in contact with O2-, N2-saturated 1 mol L?1 KOH. Reprinted with permission from Ref. [95]. Copyright 2022, Royal Society of Chemistry.

Fig. 7. In situ regeneration of co-catalysts with intrinsic self-healing properties on photoelectrodes. (a) Schematic illustration of self-generation and in situ regeneration of NiFe co-catalysts on the Mo:BiVO4 photoelectrode. (b) Stability of a NiFe co-catalysts/Mo:BiVO4/Ti/Sn electrode in a fresh 1 mol L?1 borate buffer at 0.6V vs. RHE under AM1.5G illumination. Reprinted with permission from Ref. [101]. Copyright 2016, Springer Nature. (c) Stability of V-NiOOH/BiVO4 at 0.8 V vs. RHE in 1? mol L?1 KBi with the addition of Fe2+ under AM1.5G illumination. Inset: Schematic illustration of self-regeneration of NiFe co-catalyst in Fe2+ ion-containing electrolyte for V-NiOOH/BiVO4 photoelectrode. Reprinted with permission from Ref. [102]. Copyright 2020, Wiley-VCH. (d) Stability of NiB/BiVO4 applied at 0.8 V vs. RHE in NaB with the addition of Ni2+ under AM1.5G illumination with back side. Inset: Schematic representation of self-regeneration of co-catalysts for B/BiVO4, NiB/BiVO4. Reprinted with permission from Ref. [103]. Copyright 2023, American Association for the Advancement of Science.

Fig. 8. Intrinsic self-healing of co-catalyst films on photoelectrodes. (a) Proposed mechanism for the failure of self-healing in NiFe-based catalysts. (b) Deposition rates of Fe hydroxides on EQCM sensors at different potentials in Fe-containing KBi electrolyte. The gap between the blue, green regions illustrates the mismatch between the OER operational potentials, the potentials required for efficient Fe redeposition. (c) Stability test of the NiFe-Bi catalyst measured at constant current density of 10?mA cm-2 for 100 h in KBi electrolyte at pH 14 with 50 μmol L-1 Fe2+ ions. (d) The proposed Co-catalyzed self-healing mechanism of the NiCoFe-Bi catalyst. (e) Chronopotentiometry tests of the NiCoFe-Bi catalysts on FTO substrate at 10 mA cm-2 for 1000?h in KBi electrolyte at pH 14 with 50 μmol L-1 Fe2+ ions. (f) Chronoamperometric curve of the Si-based photoanode measured at 1.2 V vs. RHE under AM 1.5 G for 100 h (g) Chronopotentiometric curves of NiCoFe-Bi catalyst films measured in KBi electrolyte at pH 14 with 30 μmol L-1 Ni(II), Co(II), or Fe(II) ions added after 0.5 h, respectively. (h) Morphological changes of the NiCoFe-Bi catalysts after the tests. Corresponding cross-sectional SEM images of the four samples. The thickening of the catalyst layer after testing in Co(II)-containing KBi electrolyte can be clearly observed. The scale bars are 200 nm. Reprinted with permission from Ref. [104]. Copyright 2021, Springer Nature.