(Invited) Photoelectrochemistry of Semiconducting Oxide Materials for Solar Water Splitting
Authors: Oskam Gerko 
Published: February 2020

There is a large family of binary, ternary, quaternary, etc. metal oxides that exhibit semiconducting properties, which makes them possible candidates to be applied in solar water splitting. With the recent advances in the controlled and economical synthesis of a large variety of metal oxides and their physical properties, new efforts in the photoelectrochemical characterization of such materials has become an important focus of research in the solar water splitting community.1,2 Although photoelectrochemical water splitting is an attractive method to convert solar energy to storable chemical energy in the form of hydrogen, the materials requirements to achieve this efficiently are very challenging. The semiconducting material needs to absorb sunlight efficiently, must to be capable of reducing and/or oxidizing water, and has to be stable under illumination under current flow in an aqueous electrolyte solution. In particular, for small bandgap semiconductors, stability is often an issue. In addition, the water reduction and oxidation reactions need to occur fast, in order to effectively compete with recombination or surface degradation processes.
The kinetic rate constants for charge transfer and surface recombination therefore are important parameters, but these are difficult to obtain from steady-state measurements. Intensity-modulated photocurrent spectroscopy (IMPS) is a powerful option to study the carrier dynamics in a photoelectrochemical cell. The frequency-dependent photocurrent admittance corresponds to the frequency-dependent external quantum efficiency, and time constants for charge transfer and surface recombination can be determined using a simple model, depending on the complexity of the system under study.3

We have used IMPS on a variety of systems including WO3, p-CuBi2O4 and WO3-BiVO4 heterojunctions, in order to elucidate the rate determining steps in the charge carrier dynamics. For CuBi2O4 photocathodes, an unfavorable balance between the rate constants for charge transfer and surface recombination is found to limit the conversion efficiency.4 On the other hand, IMPS analysis of screen-printed WO3 photoanodes shows that the recombination rate constant is significantly smaller than the charge transfer rate constant; however, the film thickness plays an important role in collection efficiency.5 Recent results on WO3-BiVO4 heterojunctions, CuWO4, and electrocatalyzed Sn-doped Fe2O3 photoanodes are also discussed.

References

“Solar Hydrogen Generation: Toward a Renewable Energy Future”. K. Rajeshwar, R. McConnell and S. Licht, Editors, Kluwer Academic, New York (2008).

“Solution combustion synthesis of oxide semiconductors for solar energy conversion and environmental remediation”. K. Rajeshwar and N. R. de Tacconi, Chem. Soc. Rev. 38(7) 1984-1998 (2009).

“Photoelectrochemical Water Splitting at Semiconductor Electrodes: Fundamental Problems and New Perspectives”. L. M. Peter and K. G. Upul Wijayantha, ChemPhysChem, 15, 1983–1995 (2014).

“Charge Transfer and Recombination Dynamics at Inkjet-Printed CuBi2O4 Electrodes for Photoelectrochemical Water Splitting”. Ingrid Rodríguez-Gutiérrez, Rodrigo García-Rodríguez, Manuel Rodríguez-Pérez, AlbertoVega-Poot, Geonel Rodríguez Gattorno, Bruce A. Parkinson, and Gerko Oskam. J. Phys. Chem. C, 122, 27169−27179 (2018).

“Charge Transfer and Recombination Kinetics at WO3 for Photoelectrochemical Water Oxidation”. Manuel Rodríguez-Pérez, Ingrid Rodríguez-Gutiérrez, Alberto Vega-Poot, Rodrigo García-Rodríguez, Geonel Rodríguez-Gattorno, Gerko Oskam. Electrochim. Acta, 258, 900-908 (2017).