Accurate determination of the charge transfer efficiency of photoanodes for solar water splitting

Abstract

The oxygen evolution reaction (OER) at the surface of semiconductor photoanodes is critical for photoelectrochemical water splitting. This reaction involves photo-generated holes that oxidize water via charge transfer at the photoanode/electrolyte interface. However, a certain fraction of the holes that reach the surface recombine with electrons from the conduction band, giving rise to the surface recombination loss. The charge transfer efficiency, η, defined as the ratio between the flux of holes that contribute to the water oxidation reaction and the total flux of holes that reach the surface, is an important parameter that helps to distinguish between bulk and surface recombination losses. However, accurate determination of η by conventional voltammetry measurements is complicated because only the total current is measured and it is difficult to discern between different contributions to the current. Chopped light measurement (CLM) and hole scavenger measurement (HSM) techniques are widely employed to determine η, but they often lead to errors resulting from instrumental as well as fundamental limitations. Intensity modulated photocurrent spectroscopy (IMPS) is better suited for accurate determination of η because it provides direct information on both the total photocurrent and the surface recombination current. However, careful analysis of IMPS measurements at different light intensities is required to account for nonlinear effects. This work compares the η values obtained by these methods using heteroepitaxial thin-film hematite photoanodes as a case study. We show that a wide spread of η values is obtained by different analysis methods, and even within the same method different values may be obtained depending on instrumental and experimental conditions such as the light source and light intensity. Statistical analysis of the results obtained for our model hematite photoanode show good correlation between different methods for measurements carried out with the same light source, light intensity and potential. However, there is a considerable spread in the results obtained by different methods. For accurate determination of η, we recommend IMPS measurements in operando with a bias light intensity such that the irradiance is as close as possible to the AM1.5 Global solar spectrum.

Radiative and Non-Radiative Recombination Pathways in Mixed-Phase TiO2 Nanotubes for PEC Water-Splitting

“…IMPS is a perturbation method used to extract charge transfer kinetics across the semiconductor/ electrolyte interface by a sinusoidal variation of the illumination light intensity (see Figure S3). The rate constants corresponding to both charge transfer and surface recombination were derived from the normalized IMPS spectra [68][69][70]. The complex Nyquist photocurrent response plots were recorded using a small sinusoidal perturbation light signal.…”

Key Strategies to Advance the Photoelectrochemical Water Splitting Performance of α‐Fe2O3 Photoanode

“…(b) Block diagram for different steps involved in water oxidation on photoanode. Reproduced with permission . Copyright 2017, PCCP Owner Societies.…”

“…However, the holes undergo loss processes by charge recombination in the bulk as well as on the surface, and only a fraction of photogenerated holes participate in water oxidation, which ultimately diminishes the solar-to-hydrogen (STH) conversion efficiency of the PEC cell. [29] The block diagram in Figure 3b outlines the efficiency of the different steps involved in water oxidation on a photoanode contributing to the generated photocurrent; [30,31] sunlight absorption (η abs ), bulk charge separation (η tr ), charge transfer to the solid-electrolyte interface (η ct ), and reaction efficiency i. e., Faradaic efficiency (η F ). All these efficiencies and incident photon flux (Φ) are directly proportional to photocurrent (J) as: J = q.Φ.…”

The Spatial Collection Efficiency of Charge Carriers in Photovoltaic and Photoelectrochemical Cells

“…47 The values of f(0), as shown in Figure 3C, are lower than the charge transfer efficiencies, h t , obtained by time and frequency domain techniques for heteroepitaxial hematite photoanodes. 48 This discrepancy stems from differences between the definitions of f(0) and h t . The SCE analysis gives information on the fate of photogenerated charge carriers that were born at distance z from the surface.…”