Passivating surface states on water splitting hematite photoanodes with alumina overlayers

Abstract

Hematite 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 TiO 2 overlayers show no beneficial effect, we find that an ultra-thin coating of Al 2 O 3 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 À2 to 0.85 mA cm À2 ) at +1.0 V vs. the reversible hydrogen electrode (RHE) under standard illumination conditions. The subsequent addition of Co 2+ 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 À2 ). A detailed investigation into the effect of the Al 2 O 3 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.

Water Oxidation at Hematite Photoelectrodes: The Role of Surface States

“…Similar looking impedance spectra have recently been reported for hematite electrodes, and a variety of equivalent circuits put forth interpret these spectra. 11,45 In these analyses, the low frequency semicircle is generally attributed to the series arrangement of the depletion capacitance of the semiconductor SC C and the Helmholtz capacitance at the electrode surface, and the role of surface states has largely been ignored.…”

Nanoscale Effects in Water Splitting Photocatalysis

“…For example, reaction of silicon with HF can suppress surface recombination and improve the performance of photovoltaic devices [138]. For Fe 2 O 3 photoanodes it has been shown that application of Al 2 O 3 [151] or SnO 2 [152] overlayers also improves the performance. For BiVO 4 photoanodes, both application of an SnO 2 underlayer [153], and incorporation of W dopants boost the electrical power output, which is attributed to reduction of surface recombination Fig.…”

Immobilization of a molecular cobalt electrocatalyst by hydrophobic interaction with a hematite photoanode for highly stable oxygen evolution

“…Illumination on hematite induces the accumulation of photogenerated charges at the interfaces, in particular, hole congregation at the semiconductorelectrolyte junction due to the low mobilities for the charge carriers and the slow kinetics of the oxygen generation reaction. 21,22 Small transient response with our bare hematite may be due to dense nature of film with low surface area. For the Co-Pi/hematite sample (Figure 3), an anodic current spike is also observed during this charge accumulation phase, which decreases rapidly with time and forward potential sweep because the accumulated holes disorder the charge dissemination in the space charge layer.…”

“…The current linearly increases with increasing potential up to ~1.35 V and begins to decline before sharply increasing at 1.65 V vs. RHE, consistent with the behavior reported for hematite photoanodes. 21 On the Co-Pi/hematite, the photocurrent onset occurs at ~ 0.8 V vs. RHE. In addition, the current increases along with a positive sweep and begins to decline once a higher potential regime is reached (Figure 3a).…”