Hematite-based water splitting with low turn-on voltages.

Sunlight-driven photoelectrochemical (PEC) water splitting offers promise as a method for effective solar-energy harvesting and storage. To transform the reaction into economically competitive technology, we need materials that can absorb sunlight broadly, transfer the energy to excited charges at high efficiencies, and catalyze specific reduction and oxidation reactions. Furthermore, the materials should be inexpensive and stable against photocorrosion. To date, an ideal material that satisfies all of these considerations remains elusive. This challenge can, in principle, be addressed by combining various material components, each purposedesigned to offer desired properties with respect to photovoltage generation, charge transport, and catalytic activity. For example, it has recently been shown that the performance of hematite (a-Fe2O3)-based water splitting can indeed be improved by introducing dedicated charge collectors, buried homoand heterojunctions, and oxygen-evolution catalysts. Hematite was chosen as a prototypical system for these proof-of-concept demonstrations because it is an earth-abundant material with great promise for high-efficiency, low-cost water splitting. To realize the potential of hematite, however, we still need to address a key issue concerning its low photovoltage (Vph, typically 0.4 V), which is unreasonably low given that the bandgap of hematite is 2.0 eV. For successful integration with a small-bandgap photocathode, the photovoltage generated at the photoanode needs to be significantly higher so that a total (combined) photovoltage of 1.61 V (or greater, with a minimum overpotential of 0.38 V) is produced. Herein we show that this issue may be addressed by modifying the hematite surface. When decorated with an amorphous NiFeOx layer (Figure 1), hematite produces photovoltages as high as 0.61 V, which enable the observation of turn-on voltages (Von) as low as 0.62 V (versus the reversible hydrogen electrode, RHE) without the need for a second absorber (unless otherwise noted, all electrochemical potentials reported herein are relative to RHE). When a second absorber, Si, was added, a record-low turn-on voltage of 0.32 V was measured. The basis for our approach is illustrated schematically in Figure 2. The fundamental reason for the observed limited photovoltage generation by hematite lies in the relatively positive positions of its valenceand conduction-band edges. However, even within these limits, the Vph value of 0.6–0.8 V calculated for reported flat-band potentials (Vfb) of 0.4–0.6 V has not been reached.We understand the cause of this discrepancy to be a partial Fermi level pinning effect. That is, owing to the existence of surface states, a nonnegligible potential drop takes place within the Helmholtz layer (hH, Figure 2a). [22] The effect is manifested as a more positive Von value, since a significant portion of the applied potential is used to overcome the overpotential hH (Figure 2c). Appropriate surface modification enables the hH to be minimized or eliminated (Figure 2b) and a less positive Von value to be measured (Figure 2d). The effect of the NiFeOx overlayer was profound: it led to a Von shift from approximately 1.0 V to approximately 0.6 V (Figure 1b). Although the apparent effect of the cathodic Von shift is similar to the effect of reducing the kinetic over[*] C. Du, Dr. X. Yang, Dr. M. T. Mayer, H. Hoyt, J. Xie, Dr. G. McMahon, G. Bischoping, Prof. Dr. D. Wang Department of Chemistry Merkert Chemistry Center, Boston College 2609 Beacon Street, Chestnut Hill, MA, 20467 (USA) E-mail: dunwei.wang@bc.edu Homepage: http://www2.bc.edu/dunwei-wang [] These authors contributed equally to this work.