Green hydrogen: rust as a photoanode and its limitations

Sandra Loyd

The energy system of the future will require large amounts of hydrogen as an energy carrier and raw material. However, this requires the production of hydrogen in a climate-neutral way, for example by so-called photoelectrolysis, ie the breakdown of water into hydrogen and oxygen by means of sunlight. As a light electrode, there is a need for semiconductor materials that convert sunlight into electric current and remain stable in water. Metal oxides are among the best candidates for stable and inexpensive photoelectrodes. Some of these metal oxides also have a catalytically active surface that accelerates hydrogen formation at the cathode and oxygen formation at the anode.

Research has long focused on hematite (α-Fe2O3), widely known as rust. Hematite is stable in water, extremely inexpensive, and useful as a photoanode, and has been shown to have catalytic activity in oxygen evolution. Although research on hematite photoanodes has been ongoing for about 50 years, the efficiency of photocurrent conversion is less than 50 percent of the theoretical maximum value. By comparison, the photocurrent efficiency of silicon semiconductor material, which dominates nearly nine-tenths of the photovoltaic market, is 90 percent of the theoretical maximum value.

However, in a recent study in the journal Nature Materials, Daniel Grave (Ben Gurion University), Dennis Friedrich Helmholtz Zentrum Berlin (HZB) and Avner Rothschild ( Technion) led an explanation as to why the hematite lags so far behind the calculated maximum value. The Technion group investigated how the wavelength of light absorbed in hematite thin films affects photoelectrochemical properties, while the HZB team determined wavelength-dependent charge carrier properties in rust thin films by combining their time-resilient microwave measurements with

E they were able to determine a basic physical property of the material that was generally neglected in the study of inorganic solar sinks: the photogeneration yield spectrum. “By and large, this means that only a portion of the energy of the light absorbed by hematite creates moving charge carriers, the rest creates more localized excited states and is thus lost,” Grave explained.

“This new approach provides experimental insight it provides a light-matter interaction in hematite and allows the optical absorption spectrum to be distinguished between productive absorption and non-productive absorption, ”explained Professor Rothschild. Grave added, they were able to show that the actual upper limit of the conversion efficiency of hematite photoanodes was significantly lower than what would be expected based on the absorption above the band gap. According to the new calculation, today’s “champion” hematite photoanodes are already quite close to the theoretically possible maximum. Much better cannot be achieved

The approach has also been successfully applied to the model material TiO2 and BiVO4, which is currently the best performing metal oxide photoanode material. “With this new approach, we have expanded our arsenal with an effective tool to identify the available potential of photoelectrode materials. Applying this to new materials will hopefully accelerate the discovery and development of a photoelectrode ideal for solar water decomposition. It would also allow us to ‘fail quickly’. which is arguably just as important when developing new absorbent materials, “Friedrich said.

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