COMPLEMENTARY IMPEDANCE ANALYSIS OF HEMATITE PHOTOANODES WITH ELECTRICAL AND OPTICAL PERTURBATION

Hematite is a prospective photoanode material for photoelectrochemical cells (PEC) for solar water splitting. A deeper understanding of the processes involved in the oxygen evolution reaction (OER) and the role of light is essential for guiding the efforts to improve the efficiency of hematite photoanodes.

This work examines model thin film hematite photoanodes using complementary characterization techniques that are well-suited to probe the charge carrier dynamics of complex photoelectrochemical reactions: photoelectrochemical impedance spectroscopy (PEIS), intensity modulated photocurrent spectroscopy (IMPS), and intensity modulated photovoltage spectroscopy (IMVS).

First, we verify the interrelation between PEIS, IMPS and IMVS where the electrical response to periodic oscillations in electrical potential and light intensity are measured in operando.

The IMPS measurements on hematite show two well-defined semicircles with different signs that are assigned to the static hole current (positive) and the recombination current (negative). Series of IMPS measurements at different bias light intensities enable us to reconstruct the elusive recombination current in any operating point.

A deconvolution of the measured PEIS and IMVS spectra is achieved by calculating the corresponding distribution of relaxation times (DRT). With the DRT we are able to separate the overall polarization into different contributions, and we examine how these contributions depend on electrical potential and bias light intensity. The analysis reveals distinct common patterns in PEIS and IMVS that only differ by a constant factor that can be linked to the quantum efficiency of the PEC and enable us to isolate the losses caused by the OER without any a priori assumptions.

In sum, this contribution introduces PEIS, IMPS and IMVS as valuable photoelectrochemical measurement techniques providing new insights into the rate limiting processes in PECs.