Thermally-enhanced minority carrier collection in hematite during photoelectrochemical water and sulfite oxidation

Abstract

Many metal-oxide light absorbing semiconductors, such as a-Fe 2 O 3 (hematite), exhibit localized small polaron carrier conduction. The low electron/hole mobility hinders minority carrier transport, and is not readily modified through doping or nanostructuring. In this work, we demonstrate that thermal energy, which is available in moderately concentrated sunlight, enhances the minority carrier mobility and photoelectrochemical (PEC) water and sulfite oxidation of 30 nm-thick Ti-doped hematite thin-film photoanodes. In NaOH aqueous electrolyte, the instantaneous Tafel slope at 3 mA cm À2 decreases remarkably from 480 mV dec À1 (at 7 C) to 240 mV dec À1 (at 72 C) under 9 suns illumination, representing a substantial increase in the fill factor, which also depends on the doping level. Though the photovoltage decreases with temperature expectedly, we show that it can be mitigated by increasing the light intensity. In the presence of a Na 2 SO 3 hole scavenger, the photocurrent at 1.23 V vs. reversible hydrogen electrode increased from 3.1 (at 7 C) to 5.0 mA cm À2 (at 72 C) under 8 suns illumination, and the onset potential was shown to depend weakly on the temperature. The strong increase in the photocurrent with temperature in the limit of fast reaction kinetics suggests that it arises from an improvement in the collection of minority carriers in the diffusion region of hematite. We show that room temperature and 1 sun illumination intensity is not the optimal reaction operating condition for hematite photoanodes. The thermally-enhanced minority carrier transport are likely generalizable to other small-polaron-type light absorbers for PEC and solar cells.

An integrated thermoelectric-assisted photoelectrochemical system to boost water splitting

“…3d). These two features are in agreement with the reports on the influence of temperature on PEC water splitting performance [36][37][38][39] The reduced photogenerated voltage of the photoelectrodes, prolonged diffusion distance of minority carriers and improved surface catalytic activity with the increase of electrolyte temperature have been proposed to be main reasons. These features suggest that the increased V oc of the thermoelectric-PEC cell with the temperature increase is caused by the voltage input from the thermoelectric device.…”

Plasmon‐Enhanced Photoelectrochemical Water Splitting for Efficient Renewable Energy Storage

“…Chueh's group has evaluated the temperature-dependent photocurrent of α-Fe 2 O 3 , [144] BiVO 4 , [145] and reduced TiO 2 [146] and found that the minority carrier diffusion length, L D , increased with heating of the semiconductor material. Figure 11a shows that the diameter-dependent photocurrent generated by TiO 2 nanowires increased monotonically with increasing temperature (from 10°C to 70°C).…”

The “Rust” Challenge: On the Correlations between Electronic Structure, Excited State Dynamics, and Photoelectrochemical Performance of Hematite Photoanodes for Solar Water Splitting

“…[55,61,62] An activated polaron hopping process is supported by temperature-dependent electrical conductivity measurements as shown in Figure 2c. [54,61,63,64] Recently, a change in the activation energy of electrical conductivity below the Morin transition was reported, further suggesting that charge transport is affected by the magnetic state. [54] Spin interactions as well as polaronic transport in hematite make the Hall effect anomalous, [54,61,65] which complicates using the standard method for measuring charge carrier concentration and mobility.…”