## 3 Results and Discussion
The instantaneous photocurrent under illumination corresponds to the immediate hole arrival at  
Figure 2: (a) LSV of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) under front (F) and back (B) illumination; (b) Photocurrent density under back and front illumination for SnO\({}_{2}\)/BiVO\({}_{4}\) in comparison with SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\); (c) LSV of SnO\({}_{2}\)/WO\({}_{3}\) photoanode measured using a three-electrode configuration set up in aqueous phosphate buffer (pH 7.0) with 0.5 M Na\({}_{2}\)SO\({}_{3}\). the surface. It is interesting to note that the transient photocurrent spikes are suppressed in the LSV and also in the stability measurement under chopped illumination. Usually, spikes in the transient photocurrent measurements indicate surface recombination, due to the presence of the surface states or slow water oxidation kinetics. SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) photoanode shows negligible spikes and a steady photocurrent, which demonstrates that the holes accumulated at the surface readily react with the water and hence improve photocurrent density without producing transient spikes [31]. Further, charge transfer properties have been investigated by Nyquist plots. Figure 3 depicts the Nyquist plots of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) photoanodes. EIS were measured at 1.23 V vs. RHE under 1 Sun illumination. Nyquist plots were fitted with the equivalent circuit, using Zview software; they are presented in Figure 3 and Supplementary Material S2. Parameters extracted from the Nyquist plots are listed in Supplementary Material Table S1. In the equivalent circuit, R\({}_{8}\) represents solution resistance, and R\({}_{\text{ct}}\) can be ascribed to charge transfer resistance at the electrode/electrolyte interface. Generally, a smaller semicircle represents the higher ability of charge transfer property of the photoanode. It is clearly evident from Figure 3 and Table S1 that the charge transfer resistance for SnO\({}_{2}\) 50 nm/WO\({}_{3}\) 50 nm/BiVO\({}_{4}\) is lesser than that of SnO\({}_{2}\) 50 nm/BiVO\({}_{4}\) and other thicknesses of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) photoanodes. It demonstrates that, as the SnO\({}_{2}\)/WO\({}_{3}\) layer is introduced beneath the BiVO\({}_{4}\) layer, the charge transfer resistance dramatically decreases compared to the pristine BiVO\({}_{4}\). This trend is in line with pec performance. As the thickness of WO\({}_{3}\) and SnO\({}_{2}\) increases from 50 nm, the electrical resistance increases. It indicates that the SnO\({}_{2}\) 50 nm/WO\({}_{3}\) 50 nm/BiVO\({}_{4}\) has high charge separation efficiency and high water oxidation kinetics. The stability of SnO\({}_{2}\) 50 nm/WO\({}_{3}\) 50 nm/BiVO\({}_{4}\) has been measured in the three electrode configuration and presented in Figure 4a. It shows considerably good stability for 5 h, with negligible decrease in the photocurrent. Post mortem analysis was carried out to investigate any change in the morphology of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) photoelectrode (Figure S3) after photostability measurement. As is evident from the SEM images of SnO\({}_{2}\)/WO\({}_{3}\)/BiVO\({}_{4}\) (Figure S3), no serious damage to the morphology of the photoelectrode was found after photostability measurement. This result aligns with the previous reports that the photoelectrode used for the sulfite oxidation shows considerable photostability, although there could be slight dissolution of V\({}^{5+}\) from BiVO\({}_{4}\)[32; 33]. In the present case, photostability has been studied for sulfite oxidation, as sulfite ions quickly consume the holes available on the surface of BiVO\({}_{4}\), it does not allow holes to accumulate and trigger the photoanodic corrosion of BiVO\({}_{4}\) very easily. It is also reported that the high quality BiVO\({}_{4}\) can show the photostability of 500 h without undergoing any major compositional and morphological changes [32]. In addition, the WO\({}_{3}\)/BiVO\({}_{4}\) system also exhibits considerable photostability [34].