The efficiency of photoelectrocatalytic (PEC) water splitting is limited by the serious recombination of photogenerated charges, high overpotential, and sluggish kinetics of surface reaction. Herein we describe the recent progress on engineering the electrode–electrolyte and semiconductor–cocatalyst interfaces with cocatalysts, electrolytes, and interfacial layers (interlayers) to increase the PEC efficiency. Introducing cocatalysts has been demonstrated to be the most efficient way to lower the reaction barrier and promote charge injection to the reactants. In addition, it has been found that electrolyte ions can influence the surface catalysis remarkably. Electrolyte cations on the surface can influence the water splitting and backward reactions, and anions may take part in the proton transfer processes, indicating that fine-tuning of the electrolyte parameters turns out to be an important strategy for enhancing the PEC efficiency. Moreover, careful modification of the interface between the cocatalysts and the semiconductor via suitable interlayers is critical for promoting charge separation and transfer, which can indirectly influence the surface catalysis. The mechanisms of surface catalysis are assumed to involve transfer of photogenerated holes to the surface active sites to form high-valent species, which then oxidize the water molecules. Many key scientific issues about the generation of photovoltage, the separation, storage, and transfer of carriers, the function of cocatalysts, the roles of electrolyte ions, and the influences of other parameters during PEC water splitting will be discussed in detail with some perspective views.

Photoelectrochemical (PEC) water splitting is an ideal approach for renewable solar fuel production. One of the major problems is that narrow bandgap semiconductors, such as tantalum nitride, though possessing desirable band alignment for water splitting, suffer from poor photostability for water oxidation. For the first time it is shown that the presence of a ferrihydrite layer permits sustainable water oxidation at the tantalum nitride photoanode for at least 6 h with a benchmark photocurrent over 5 mA cm(-2) , whereas the bare photoanode rapidly degrades within minutes. The remarkably enhanced photostability stems from the ferrihydrite, which acts as a hole-storage layer. Furthermore, this work demonstrates that it can be a general strategy for protecting narrow bandgap semiconductors against photocorrosion in solar water splitting.

The integrated architecture enables the Ta3N5photoanode to approach the theoretical photocurrent limit for solar water splitting.

Hematite is a promising photoanode material for renewable solar fuel production via photoelectrochemical (PEC) water splitting. However, the fast electron−hole recombination and sluggish surface reaction retard it from getting satisfied performance. Herein, hematite nanorod arrays doped with titanium (Ti−Fe 2 O 3 ) on the surface were prepared by a solutionbased process. Because of one-dimension anisotropy and improved charge transfer property, the photocurrent density is doubled compared to pure Fe 2 O 3 at 1.50 V vs RHE under simulated sunlight (AM 1.5 G) irradiation. Loading conjugated Ni(OH) 2 /IrO 2 cocatalyst further leads to about 200 mV negative shift of the onset potential and dramatic increase of the applied bias photon-to-current efficiency (ABPE). We find that Ni(OH) 2 can efficiently capture the photogenerated holes from hematite as a hole-storage layer (HSL) to improve the charge transfer process across the interface of hematite and IrO 2 electrocatalyst. Furthermore, the stored photogenerated holes in Ni(OH) 2 can be utilized by IrO 2 for water oxidation more facilely. This synergetic effect along with the efficient surface doping are proposed to be responsible for the enhanced performance.

Charge transfer has been demonstrated to have a fundamental role in particulate Ta3N5 electrode for achieving high efficient photoelectrochemical water oxidation.

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Ultra-high onset potential hinders the application of hematite for photoelectrochemical (PEC) water splitting. Herein, a hematite photoanode with an unprecedentedly low onset potential of 0.50 V vs. the reversible hydrogen electrode for PEC water oxidation is reported. The drastically reduced onset potential is mainly ascribed to the passivation of the hematite surface states and the gradient structure made by H2-O2 flame at high temperature.

The electrode-electrolyte interface chemistry is highly important for photoelectrochemical (PEC) and electrocatalytic water splitting where cations in the electrolyte are often crucial. However, the roles of cations in an electrolyte are much debated and not well-understood. This work reports that the PEC and electrocatalytic water oxidation (WO) activities in basic electrolytes with different cations follow an unexpected trend (Li(+) > K(+) > Na(+)) especially for long-term reaction. Such an abnormal order of activity is found to be the balance effect of two factors: the distinct extents of the weakening of O-H bond on electrode surface after interacting with cations in different electrolytes and the different rates of oxygen reduction reaction (ORR) which turns out to be dominant. Li(+) not only brings the most significant decrease of O-H bond strength but also is most effective for avoiding back reaction, while Na(+) shows the most detrimental effect on WO because of ORR. Our results provide important insight into the roles of cations in WO and demonstrate a new strategy of tailoring the electrode-electrolyte interface via judicious choice of cations in electrolyte for more efficient PEC and electrocatalytic water splitting.