Storing the energy of sunlight into feedstock chemicals or energy-rich compounds, such hydrogen, appears as an enticing option to broaden the utilization of solar energy far beyond classical photovoltaics. Among different emerging technologies, photoelectrochemical (PEC) water splitting stands out with the promise of competitive solar-to-hydrogen conversion efficiencies (predicted to be above 20 %) and a conveniently simple design.¹ These devices, comprising two photoactive electrodes wired-stacked together, to leverage their complementary light absorption, and directly immersed in an aqueous electrolyte, generate under illumination a voltage greater than 1.23 V, driving the photoelectrosynthetic reactions of H₂ and O₂ separately at the different electrode-electrolyte interfaces. Unfortunately, current conversion efficiencies are far below the expectations. In recent years, there has been an encouraging progress on the refinement of the bulk properties of the semiconducting electrodes (viz. nanostructuring, doping, band gap engineering, etc.) as well as on the design of more active electrocatalyst, both contributing to an improved performance. However, another key component of these devices, the semiconductor-liquid junction (SCLJ), where the complex reactions occur and therefore, whose electrochemical and catalytic properties (namely, surface potential, overpotential, kinetics of charge transfer and recombination) control the conversion efficiency, remains hazy posing a major bottleneck for enhancing the conversion efficiency. A better understanding on the electrocatalytic properties of the SCLJ could not only nail down the processes limiting the performance of these devices but also provide specific routes to patch them.

Over the last few years, a wide variety of in-operando electrochemical based techniques have burst in the field of electrochemical-based solar fuel production offering new insights on the nature of intermediate species, the functioning of electrocatalysts, the carrier dynamics within the electrodes, etc.² Here, we introduce a new technique where a network-type electrical-contact at the electrode-electrolyte interface afford direct probing of the surface carrier dynamics when combined with a transient photocurrent/photovoltage technique. This technique applied to model semiconducting materials incorporating state-of-the-art electrocatalyst demonstrates to provide unprecedented access to steady-state (interfacial energetics) and transient (interfacial kinetics) electrochemical information of the SCLJ in-operando. We believe that these new tools will help to forge a better understanding on the SCLJ and to establish the designing principles for a next generation of more efficient electrodes.

[1] M. S. Prévot, K. Sivula. J. Phys. Chem. C, 2013, 117, 17879-17893

[2] W. A. Smith, I. D. Sharp, N. C. Strandwitz, J. Bisquert. Energy Environ. Sci. 2015, 8, 2851-2862