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• Measurement of 1.5 surface reactive sites/nm2 on BiVO 4 powder by chemical titration. • Passivation of BiVO 4 electrodes surface states by atomic layer deposition of Al overlayer. • 54 % enhanced photocurrent density on the Al- modified BiVO 4 photoelectrode. • 15 % improvement on BiVO 4 stability with 2 cycles of deposited Al overlayer. Bismuth vanadate (BiVO 4) has surface states that give rise to defect levels that mediate electron-hole recombination. In order to minimize the inefficiencies, an ultrathin Al overlayer was deposited on the BiVO 4 electrodes. A 54 % improvement on the photocurrent density was obtained using the Al-modified BiVO 4 electrode, accompanied by a 15 % increase in stability over 7.5 h of continuous irradiation. Moreover, surface capacitance measurements showed that the Al overlayer was indeed passivating the surface states. We also shed light on the deposition of an Al overlayer on the surface of BiVO 4 , by investigating the process on model BiVO 4 powders. This study presents useful, previously unreported information about the surface chemistry of BiVO 4 based on experimental methods and gives unique insights on the characterization of the BiVO 4 surface. The existence of surface reactive sites on BiVO 4 was confirmed and quantified (1.5 reactive sites/nm2) via chemical titration. [ABSTRACT FROM AUTHOR]