Interest in delafossite (CuFeO2) as a candidate p-type photocathode for photoelectrochemical (PEC) solar fuel production has recently been increasing, mainly due to its excellent stability in aqueous environments and favorable light absorption properties. However, its PEC performance has remained poor for reasons that have not yet been specifically determined. Herein, we report a detailed investigation on sol–gel-processed CuFeO2 with a range of spectroscopic, PEC, and microscopy techniques aimed at unraveling the material properties governing photogenerated charge carrier harvesting in this v. An analysis of the bulk transport properties using microwave conductivity measurements reveals a good charge carrier mobility (0.2 cm2 V–1 s–1) and a relatively long lifetime (200 ns) for photogenerated charge carriers. Conversely, systematic PEC measurements with varied redox systems reveal the existence of a high density of surface states (1014 cm–2) positioned 0.35 eV above the conduction band, inducing Fermi level pinning at the semiconductor–liquid junction. X-ray photoelectron spectroscopy suggests the presence of a thin layer of metal hydroxide at the surface of the material. These surface states were found to behave as electron traps, correlated with an inversion of polarity at the surface of the semiconductor, and thereby promoting charge recombination and limiting the photovoltage developed at the junction. These findings suggest that if the detrimental effects of the surface states can be eliminated, CuFeO2 would provide a sufficiently high photovoltage to be combined with other solution-processed and stable photoanodes into an easily scalable tandem PEC cell.
The development of solution‐processable routes to prepare efficient photoelectrodes for water splitting is highly desirable to reduce manufacturing costs. Recently, sulfide chalcopyrites (Cu(In,Ga)S2) have attracted attention as photocathodes for hydrogen evolution owing to their outstanding optoelectronic properties and their band gap—wider than their selenide counterparts—which can potentially increase the attainable photovoltage. A straightforward and all‐solution‐processable approach for the fabrication of highly efficient photocathodes based on Cu(In,Ga)S2 is reported for the first time. It is demonstrated that semiconductor nanocrystals can be successfully employed as building blocks to prepare phase‐pure microcrystalline thin films by incorporating different additives (Sb, Bi, Mg) that promote the coalescence of the nanocrystals during annealing. Importantly, the grain size is directly correlated to improved charge transport for Sb and Bi additives, but it is shown that secondary effects can be detrimental to performance even with large grains (for Mg). For optimized electrodes, the sequential deposition of thin layers of n‐type CdS and TiO2 by solution‐based methods, and platinum as an electrocatalyst, leads to stable photocurrents saturating at 8.0 mA cm–2 and onsetting at ≈0.6 V versus RHE under AM 1.5G illumination for CuInS2 films. Electrodes prepared by our method rival the state‐of‐the‐art performance for these materials.
Utilizing renewable sources of energy is very attractive to provide the growing population on earth in the future but demands the development of efficient storage to mitigate their intermittent nature. Chemical storage, with energy stored in the bonds of chemical compounds such as hydrogen or carbon-containing molecules, is promising as these energy vectors can be reserved and transported easily. In this review, we aim to present the advantages and drawbacks of the main water electrolysis technologies available today: alkaline and PEM electrolysis. The choice of electrode materials for utilization in very basic and very acid conditions is discussed, with specific focus on anodes for the oxygen evolution reaction, considered as the most demanding and energy consuming reaction in an electrolyzer. State-of-the-art performance of materials academically developed for two alternative technologies: electrolysis in neutral or seawater, and the direct electrochemical conversion from solar to hydrogen are also introduced.
A natural material of extra-terrestrial origin yields a high-performance electrocatalyst for alkaline water oxidation.
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