Passivating surface states on water splitting hematite photoanodes with alumina overlayers
AuthorsLe Formal, Florian
KAUST Grant NumberKUS-C1-015-21
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AbstractHematite is a promising material for inexpensive solar energy conversion via water splitting but has been limited by the large overpotential (0.5-0.6 V) that must be applied to afford high water oxidation photocurrent. This has conventionally been addressed by coating it with a catalyst to increase the kinetics of the oxygen evolution reaction. However, surface recombination at trapping states is also thought to be an important factor for the overpotential, and herein we investigate a strategy to passivate trapping states using conformal overlayers applied by atomic layer deposition. While TiO2 overlayers show no beneficial effect, we find that an ultra-thin coating of Al2O3 reduces the overpotential required with state-of-the-art nano-structured photo-anodes by as much as 100 mV and increases the photocurrent by a factor of 3.5 (from 0.24 mA cm-2 to 0.85 mA cm-2) at +1.0 V vs. the reversible hydrogen electrode (RHE) under standard illumination conditions. The subsequent addition of Co2+ ions as a catalyst further decreases the overpotential and leads to a record photocurrent density at 0.9 V vs. RHE (0.42 mA cm-2). A detailed investigation into the effect of the Al2O3 overlayer by electrochemical impedance and photoluminescence spectroscopy reveals a significant change in the surface capacitance and radiative recombination, respectively, which distinguishes the observed overpotential reduction from a catalytic effect and confirms the passivation of surface states. Importantly, this work clearly demonstrates that two distinct loss processes are occurring on the surface of high-performance hematite and suggests a viable route to individually address them. © The Royal Society of Chemistry 2011.
CitationLe Formal F, Tétreault N, Cornuz M, Moehl T, Grätzel M, et al. (2011) Passivating surface states on water splitting hematite photoanodes with alumina overlayers. Chem Sci 2: 737–743. Available: http://dx.doi.org/10.1039/c0sc00578a.
SponsorsWe gratefully acknowledge the Swiss Federal Office of Energy (Project number 102326, PECHouse), the Marie Curie Research Network "Hydrogen" (Contract number: MRTN-CT-2006-032474), the King Abdullah University of Science and Technology (KAUST, Award No KUS-C1-015-21), and Toyota Motor Corporation for financial support.
PublisherRoyal Society of Chemistry (RSC)