Kinetic analysis of the synergistic effect of NaBH4 treatment and Co-Pi coating on Fe2O3 photoanodes for photoelectrochemical water oxidation

Abstract

It is always a challenge to accelerate the surface reaction kinetics of α-Fe2O3 photoanode for photoelectrochemical (PEC) water splitting. Although the NaBH4 treatment and Co-Pi coating have exhibited a synergetic effect on reducing the onset potential and enhancing the photocurrent density of the Fe2O3 photoanode, their intrinsic interaction is still poorly understood. In this work, the synergetic effect has been well-explained by photoelectrochemical tests and the impedance spectroscopy. It is discovered that the surface states locating at the surface of Fe2O3 were mostly passivated by Co-Pi. However, excess surface states formed on the NaBH4 treated Fe2O3 photoanode at a relatively negative potential, which could not be passivated by Co-Pi. Those surface states acted as dual roles, that is, the hole transfer media between Co-Pi and the valence band of Fe2O3 as well as the block layer to prevent the recombination of the photogenerated electrons with Co-Pi. The intuitive result was that surface charge transfer speeded up and the charge recombination slowed down concurrently, which led to a reduced onset potential by 210 mV and an increased photocurrent in the PEC water splitting.