Solar Water Splitting: Putting an Extra “Eye” on Surface Reactions that Store Sunlight as Fuel

The Science

Mimicking photosynthesis, water-splitting cells absorb sunlight and produce fuel. A challenge in designing such cells is pairing the semiconductor that absorbs sunlight and generates electrons with the catalyst that uses those electrons to produce fuel. Researchers introduced a novel way to study the flow of electrons at the interface of the two materials. Using this capability, they found that ion-permeable catalysts form interfaces that yield more energy relative to comparable – but denser – catalysts.

The Impact

Water splitting provides a potential mechanism for the large-scale conversion and storage of solar energy in the form of a renewable chemical fuel, such as hydrogen. The invention of direct methods to probe charge-separating water-splitting interfaces enables the development of more efficient devices that produce hydrogen from sunlight and water. The discovery also sheds light on fundamental questions regarding charge-transfer at modified interfaces.

Summary

A bottleneck in the development of high-efficiency water-splitting solar devices has been a lack of direct, quantitative information regarding the electronic behavior of the interface between the catalyst and semiconductor. To better understand catalysts, researchers electrically contacted a single-crystal titanium dioxide electrode and coated it with various catalyst films. The semiconductor-catalyst interfaces were directly probed as they operated using a new dual-electrode photoelectrochemistry technique to independently monitor and control the voltage and current at both the materials. Using this approach, researchers watched the charge accumulate in the catalyst and change the catalyst's voltage. Redox-active ion-permeable catalysts, such as nickel hydroxide/nickel oxyhydroxide (Ni(OH)₂/NiOOH), yielded “adaptive” semiconductor-catalyst junctions where the effective Schottky barrier height changed with the oxidation level of the catalyst. In contrast, dense, ion-impermeable iridium oxide-based catalysts yielded constant-barrier-height “buried” junctions. Conversion of dense, thermally deposited nickel oxides on titanium dioxide into ion-permeable Ni(OH)₂/NiOOH correlated with increased apparent photovoltage and fill-factor. The researchers proposed a new theory of adaptive junctions and applied the theory via numerical simulation. While the system used in the study is not efficient, these results provide fundamental insight into the dynamic behavior of interfaces that will help guide the design of efficient semiconductor-catalyst devices. They also illustrate a new class of adaptive semiconductor junctions.

Funding

This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Science, Geosciences, and Biosciences Division, Solar Photochemistry Program, under Grant DE-FG02-12ER16323. S.W.B. acknowledges support from the DuPont Young Professor Program.

Publications

F. Lin, and S.W. Boettcher, “Adaptive semiconductor/electrocatalyst junctions in water splitting photoanodes.” Nature Materials 13, 81-86 (2014). DOI: 10.1038/nmat3811 T.J. Mills, F. Lin, and S.W. Boettcher, “Theory and simulations of electrocatalyst-coated semiconductor electrodes for solar water splitting.” Physical Review Letters 112, 148304 (2014). DOI: 10.1103/PhysRevLett.112.148304