Photoelectrochemical water splitting using solar energy is an attractive way to produce hydrogen as a clean fuel and energy storage material; use of hydrogen either in fuel cells or a combustion engine only produces water as reaction product. This process requires semiconductor electrodes capable of providing rapid charge transfer at the semiconductor/aqueous solution interface, which exhibit long-term stability and efficiently harvest a large portion of the solar spectrum¹. However, the success of efficient water splitting has proved to be an elusive goal, and no material has yet been found that fulfill all criteria for efficient solar water splitting. Some of the main limitations have been the fast recombination of charge carriers, either in the bulk or at the surface, and slow transfer kinetics from the semiconductor to the electrolyte solution². Unfortunately, the kinetic constants that describe these processes are parameters that are not easily accessible from conventional measurements.

Frequency-dependent techniques, such as electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent spectroscopy (IMPS), are two powerful tools to study the main carrier dynamic mechanisms inside the photoelectrochemical cell. Bertoluzzi and Bisquert have recently proposed an equivalent circuit to describe the main mechanisms in water splitting electrochemical cells³. Ponomarev and Peter proposed a generalized analytical model for IMPS in electrochemical systems⁴ and showed that EIS and IMPS analysis should provide identical values for rate constants⁵.

In this work, we describe the general employment of small-signal perturbation techniques to determine the rate-determining steps in the charge carrier dynamics in water splitting photoelectrochemical cells, and we apply the theoretical approach to obtain the rate constants of charge transfer and recombination in different systems. The analysis shows how the kinetics of the recombination reaction and charge transfer to the electrolyte solution affect the photocurrent obtained in each case.

References

[1] Walter, M. G. et al. Solar Water Splitting Cells. Chem. Rev 110, 6446–6473 (2010).

[2] Cachet, H.; Sutter, E. M. M. Kinetics of Water Oxidation at TiO2 Nanotube Arrays at Different pH Domains Investigated by Electrochemical and Light-Modulated Impedance Spectroscopy. J. Phys. Chem. C 2015, 119 (45), 25548–25558.

[3] Bertoluzzi, L.; Bisquert, J. Equivalent Circuit of Electrons and Holes in Thin Semiconductor Films for Photoelectrochemical Water Splitting Applications. J. Phys. Chem. Lett. 2012, 3 (17), 2517–2522.

[4] Ponomarev, E. a.; Peter, L. M. A Generalized Theory of Intensity Modulated Photocurrent Spectroscopy (IMPS). J. Electroanal. Chem. 1995, 396 (1–2), 219–226.

[5] Ponomarev, E. A.; Peter, L. M. A Comparison of Intensity Modulated Photocurrent Spectroscopy and Photoelectrochemical Impedance Spectroscopy in a Study of Photoelectrochemical Hydrogen Evolution at P-InP. J. Electroanal. Chem. 1995, 397 (1–2), 45–52.