Abstract
An FeNi (oxy)hydroxide cocatalyst overlayer was photoelectrochemically deposited on a thin-film hematite (α-Fe2O3) photoanode, leading to a cathodic shift of ∼100 mV in the photocurrent onset potential. Operando X-ray absorption spectroscopy (XAS) at the Fe and Ni K-edges was used to study the changes in the overlayer with potential in the dark and under illumination conditions. Potential or illumination only had a minor effect on the Fe oxidation state, suggesting that Fe atoms do not accumulate significant amount of charge over the whole potential range. In contrast, the Ni K-edge spectra showed pronounced dependence on potential in the dark and under illumination. The effect of illumination is to shift the onset for the Ni oxidation because of the generated photovoltage and suggests that holes that are photogenerated in hematite are transferred mainly to the Ni atoms in the overlayer. The increase in the oxidation state of Ni proceeds at potentials corresponding to the redox wave of Ni, which occurs immediately prior to the onset of the oxygen evolution reaction (OER). Linear combination fitting analysis of the obtained spectra suggests that the overlayer does not have to be fully oxidized to promote oxygen evolution. Cathodic discharge measurements show that the photogenerated charge is stored almost exclusively in the Ni atoms within the volume of the overlayer.
Highlights
Since the pioneering work of Fujishima and Honda in the early 1970s,1 photoelectrochemical (PEC) water splitting has been actively investigated for its potential to provide an elegant path for the direct conversion of intermittent solar power into storable hydrogen fuel.[2]
It comes with some serious drawbacks: low mobility of charge carriers, low lifetime of photogenerated charge carriers due to bulk recombination, surface recombination at the photoanode/electrolyte interface, and high external voltage for solar-driven water splitting.[4−6] To address the last two issues, a widely exploited route to improve performance is to deposit various overlayers, which significantly reduce both surface recombination and the overpotential needed for water photo-oxidation.[7−9] NiFehydroxide-based electrocatalysts are considered as favorable materials because of their inexpensive cost, high catalytic activity for the oxygen evolution reaction (OER),[10] and stability under water-oxidation reaction conditions in an alkaline solution.[11]
Light-modulated impedance spectroscopy on Sn-doped hematite revealed that the addition of a NiFehydroxide overlayer caused a decrease of surface recombination as compared to the bare hematite sample, which improved sharply with potential in the vicinity of the photocurrent onset.[19]
Summary
Since the pioneering work of Fujishima and Honda in the early 1970s,1 photoelectrochemical (PEC) water splitting has been actively investigated for its potential to provide an elegant path for the direct conversion of intermittent solar power into storable hydrogen fuel.[2]. Operando XAS, which can directly monitor changes in the oxidation state of catalytic sites in response to applied potential and/or illumination, has been proven to be a powerful method for investigating chargetransfer processes in electrochemical and photoelectrochemical systems.[22,23] Braun et al, in their pioneering operando oxygen K-edge XAS study of a bare hematite photoanode under applied potential and illumination, interpreted pre-edge features as two types of holes at the hematite/electrolyte interface, which reached maximum population in the vicinity of the photocurrent onset.[24,25] A number of measurements were done on the metal absorption edge of overlayers deposited on photoabsorbers.[26−29] Minguzzi et al performed an operando XAS study on an IrOx overlayer deposited on top of a hematite photoanode, at the Ir K-edge, which showed clear oxidation of the Ir sites in response to illumination.[26] Li et al used highenergy-resolution fluorescence detection XAS to study ultrathin (1−3 nm) overlayers of IrOx on top of a Si photoanode.[29] The authors observed a near-linear increase of the Ir oxidation state prior to the current onset, followed by saturation or even decrease a few hundreds of meV above the onset potential These trends were completely identical for anodes that operate under light and dark conditions, aside from being shifted according to the respective onset potentials. The WL intensity is described relative to this jump
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