A thorough photoelectrochemical investigation reveals the bulk and surface characteristics limiting the performance of CuFe2O4 photoanodes for solar water oxidation.

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The effect of the electrolyte pH on the performance of metal oxide photoanodes for solar water oxidation has not been fully resolved, and contrasting views have been presented in recent reports. Herein, a comprehensive set of spectroelectrochemical techniques (impedance spectroscopy, intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS), and operando UV–vis spectroscopy) are deployed to clearly uncover the role of pH on the performance of hematite photoanodes. Our results reveal that, despite the presence of high-valent iron-oxo active sites over a wide pH range (7–13.6), the observed performance improvement with increasing pH is mainly driven by the reduction of surface accumulated charges, in the form of reactive intermediate species, that alleviates Fermi level pinning (FLP). Interestingly, IMPS data provides compelling evidence that the mitigation of FLP originates from changes in the reaction mechanism which boost the rate of charge transfer reducing, in turn, the surface charging. Additionally, we present a phenomenological analysis of the IMVS response which brings to light the additional impact of the electrolyte pH on the surface-related recombination dynamics. Our work identifies the pH-dependent kinetics of water oxidation as the key step governing the performance, defining not only the efficiency of charge transfer across the interface but also the degree of FLP that determines both the photocurrent magnitude and onset potential.

As organic semiconductors attract increasing attention to application in the fields of bioelectronics and artificial photosynthesis, understanding the factors that determine their robust operation in direct contact with aqueous electrolytes becomes a critical task. Herein we uncover critical factors that influence the operational stability of donor:acceptor bulk heterojunction photocathodes for solar hydrogen production and significantly advance their performance under operational conditions. First, using the direct photoelectrochemical reduction of aqueous Eu 3+ and impedance spectroscopy, we determine that replacing the commonly used fullerene-based electron acceptor with a perylene diimide-based polymer drastically increases operational stability and identify that limiting the photogenerated electron accumulation at the organic/water interface to values of ca. 100 nC cm −2 is required for stable operation (>12 h). These insights are extended to solar-driven hydrogen production using MoS 3 , MoP, or RuO 2 water reduction catalyst overlayers where it is found that the catalyst morphology strongly affects performance due to differences in charge extraction. Optimized performance of bulk heterojunction photocathodes coated with a MoS 3 :MoP composite gave 1 Sun photocurrent density up to 8.7 mA cm −2 at 0 V vs RHE (pH 1). However, increased stability was gained with RuO 2 where initial photocurrent density (>8 mA cm −2 ) deceased only 15% or 33% during continuous operation for 8 or 20 h, respectively, thus demonstrating unprecedented robustness without a protection layer. This performance represents a new benchmark for organic semiconductor photocathodes for solar fuel production and advances the understanding of stability criteria for organic semiconductor/water-junction-based devices.