## 2 Strategies and Device Designs for Efficient PEC Water Splitting
### Kinetics of Water Redox Reactions  
The basic water splitting reactions may be expressed as following water redox reactions:  
\[2\mathrm{H}_{2}\mathrm{O}\ \mathrm{(_{liquid})}+4\mathrm{h}^{+}\leftrightarrow 4 \mathrm{H}^{+}+\mathrm{O}_{2\ \mathrm{(gas)}} \tag{1}\]  
\[4\mathrm{H}^{+}+4\mathrm{e}^{-}\leftrightarrow 2\mathrm{H}_{2\ \mathrm{(gas)}} \tag{2}\]  
\[2\mathrm{H}_{2}\mathrm{O}\ \mathrm{(_{liquid})}\leftrightarrow 2\mathrm{H}_{2\ \mathrm{(gas)}}+ \mathrm{O}_{2\ \mathrm{(gas)}} \tag{3}\]Reaction (1) represents the water oxidation half reaction at the anode, also called the oxygen evolution reaction (OER). In the same manner, the water reduction half reaction in reaction (2) is called the hydrogen evolution reaction (HER). The overall water redox reaction in reaction (3) shows that the water is split into hydrogen and oxygen gases. In the redox reaction, the bottleneck lies in the OER (reaction (1)) since the delicate bond rearrangements of the two water molecules should take place to transform into the oxygen gas instead of its high-energy intermediates [46]. Thus, it induces the slow water oxidation kinetics, and unused holes are accumulated on the electrode surface. The accumulation of photoexcited holes is known as the reason for the photocorrosion of BiVO\({}_{4}\), which reduces long-term stability as the photoanode [47]. In order to solve the problem, some solutions have been introduced, such as increasing the density of surface active sites or combining them with oxygen evolution co-catalysts (OECs). Recently, our group reported on the deposition of a high quality polycrystalline BiVO\({}_{4}\) photoanode with a controllable surface area [10]. The BiVO\({}_{4}\) film was grown by pulsed laser deposition (PLD), and the surface area of film was optimized by controlling process temperatures during growth of the BiVO\({}_{4}\). As shown in Figure 4a-c, the morphology of the BiVO\({}_{4}\) film could be controlled, since the growth condition satisfies a 3-dimensional island growth mode (the Volmer-Weber growth). At a low temperature of 230 \({}^{\circ}\)C, diffusion of Bi was suppressed and it resulted in the small BiVO\({}_{4}\) grains being separated by voids (Figure 4a). When the temperature was increased to 330 \({}^{\circ}\)C and 430 \({}^{\circ}\)C, the voids disappeared by intergrain diffusion, and eventually, the coalescence of BiVO\({}_{4}\) grain took place (Figure 4b,c). Therefore, the surface area of BiVO\({}_{4}\) film decreased and it affected the PEC performance. As shown in Figure 4d-f, as the process temperature decreased to 230 \({}^{\circ}\)C, the interfacial area between the BiVO\({}_{4}\) grains and the electrolyte increased. Moreover in the case, the photoexcited electrons in the BiVO\({}_{4}\) grains were more effectively transferred to the bottom FTO electrode without recombination in bulk. Therefore, the PEC performance of the BiVO\({}_{4}\) grown at 230 \({}^{\circ}\)C exhibited the highest current density of \(\sim\)3.0 mA/cm\({}^{2}\) at 1.23 V\({}_{\rm{RHE}}\), and the incident photon-to-current efficiency (IPCE) approaching 35% at the wavelength of 400 nm (Figure 4g,i). To elucidate the effect of the differences in light absorption, UV-vis-NIR absorption spectra were measured (Figure 4h). Note that the films grown at 230 and 330 \({}^{\circ}\)C clearly showed the visible light absorption. Consequently, it was implied that the improved PEC performance of the film grown at 230 \({}^{\circ}\)C was dominated by the increased surface area itself. The results of this study were quite interesting, since the excellent PEC performance of \(\sim\)3.0 mA/cm\({}^{2}\) was exhibited from a single layer of the BiVO\({}_{4}\). Along with the surface area control, the deposition of the OECs on the surface of photoelectrode has been one of the promising approaches for boosting the OER and increasing the stability of BiVO\({}_{4}\)[21]. So far, the OECs have been deposited mostly by electrodeposition [48]. However, recently, [PERSON] et al. introduced another method for OEC synthesis [49].