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Photoelectrochemical water splitting is an attractive method to convert solar energy to storable chemical energy in the form of hydrogen, however, the materials requirements to achieve this efficiently are 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, and it is difficult to fully avoid degradation of the material. In addition, the water reduction and oxidation processes need to occur fast, in order to favorably compete with recombination or surface degradation processes; hence, the kinetic rate constants for charge transfer and surface recombination are very important parameters. Intensity-modulated photocurrent spectroscopy (IMPS) is a powerful technique to study the carrier dynamics in a photoelectrochemical cell. The photocurrent admittance corresponds to the frequency-dependent external quantum efficiency, and time constants for charge transfer and surface recombination can be determined, provided a simple model can be applied.1 In this presentation, we focus on the dynamic properties of p-CuBi2O4 in order to elucidate the rate determining steps that determine the efficiency of photocathodic reactions. In inert aqueous solutions at pH 5, an unfavorable balance exists between the rate constants for charge transfer and surface recombination, which limits the conversion efficiency.2 On the other hand, upon adding H2O2 as an electron acceptor, the photoelectron transfer efficiency is improved related to faster electron transfer to the solution or slower surface recombination due to passivation effects. Strategies to improve the applicability of CuBi2O4 in solar water splitting systems are discussed. The authors gratefully acknowledge CONACYT, SENER and CICY for funding through the Renewable Energy Laboratory of South East Mexico (LENERSE; Project 254667; SP-4), and CONACYT under the Basic Sciences (CB) project A1-S-28734. References “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, Alberto Vega-Poot, Geonel Rodríguez Gattorno, Bruce A. Parkinson, and Gerko Oskam. J. Phys. Chem. C, 122, 27169−27179 (2018).

Metal oxide (MO) semiconductors based on abundant, low-cost materials are attractive for application in photoelectrochemical water splitting systems related to their inherent stability, specifically against oxidation. In addition, these materials provide a wide-ranging versatility in terms of composition, physical and chemical properties, and configuration of multi-component architectures. The semiconducting MO must at least comply with three requirements: (i) absorb sunlight efficiently; (ii) the valence and conduction band edges must be situated at the appropriate energies; and (iii) the electrochemical reaction must have favorable kinetics compared with charge separation, transport and recombination kinetics. Many systems have been evaluated for photo-oxidation of water in the past 10 years, with some of the more popular materials being Fe2O3, WO3, BiVO4, TiO2, etc. In order to improve the efficiency of a solar water splitting system, the specific loss mechanisms need to be elucidated; based on these insights strategies may then be developed to improve performance. However, it is not straightforward to determine the precise loss mechanisms and generally non-steady state methods need to be applied in combination with steady-state techniques. Intensity-modulated photocurrent spectroscopy (IMPS) is a powerful technique to study the carrier dynamics in a photoelectrochemical cell: the photocurrent admittance corresponds to the frequency-dependent external quantum efficiency, and time constants for charge separation, charge transfer and surface recombination may be deduced. The kinetic model applicable to the specific system determines the interpretation of the results. On the other hand, theoretical modeling is a very powerful method to validate the interpretation of the results, and to complement the experimental studies in order to obtain a clear picture of the dominant processes and effects. In this presentation, the results from IMPS studies on a variety of systems will be discussed and strategies to improve performance are presented and tested, both experimentally and theoretically: (i) planar, compact WO3 / BiVO4 heterojunction systems are compared with nanorod array WO3 / BiVO4 heterojunctions, with and without a hole scavenger; (ii) CuBi2O4 with and without catalysts and an alternative electron acceptor; and (iii) specific examples of other materials to highlight the proposed interpretations. The results are compared with calculations of the IMPS response using a classic kinetic model and a continuity equation approach in order to incorporate the effects of charge transport and electric field within the electrode. The authors gratefully acknowledge funding from CONACYT, SENER, and CICY through the Renewable Energy Laboratory of South East Mexico (LENERSE; Project 254667; SP-4), and CONACYT under the Basic Sciences (CB) project A1-S-28734. We also thank the Ministerio de Ciencia e Innovación, the Agencia Estatal de Investigación (AEI), and the European Regional Development Fund of the European Union through Project PID2019-110430GB-C22. AR thanks the Spanish Ministry of Education, Culture and Sports via a PhD grant (FPU2017-03684). Finally, we would like thank Dr. Antonio Zapien and the NANOMXCN initiative to foment scientific collaboration between Mexico and China for support.