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Solar water splitting: Putting an extra 'eye' on surface reactions that store sunlight as fuel

Date:
December 4, 2015
Source:
Department of Energy, Office of Science
Summary:
Water-splitting cells absorb sunlight and produce fuel. Creating such cells means pairing a material to absorb sunlight and generate electrons with the one that uses those electrons to produce fuel. Scientists introduced a novel way to study the flow of electrons where the materials meet.
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To one day design cells that mimic trees ability to turn sunlight and water into fuel, scientists at the University of Oregon devised a new technique that allows them to “see” a key interface in the cells – the interface between the semiconductor that absorbs sunlight and generates electricity with the catalyst that uses the electricity to create fuel.
Credit: Image courtesy of Fuding Lin and Shannon Boettcher

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.

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.

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.


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Materials provided by Department of Energy, Office of Science. Note: Content may be edited for style and length.


Journal Reference:

  1. Thomas J. Mills, Fuding Lin, Shannon W. Boettcher. Theory and Simulations of Electrocatalyst-Coated Semiconductor Electrodes for Solar Water Splitting. Physical Review Letters, 2014; 112 (14) DOI: 10.1103/PhysRevLett.112.148304

Cite This Page:

Department of Energy, Office of Science. "Solar water splitting: Putting an extra 'eye' on surface reactions that store sunlight as fuel." ScienceDaily. ScienceDaily, 4 December 2015. <www.sciencedaily.com/releases/2015/12/151204121403.htm>.
Department of Energy, Office of Science. (2015, December 4). Solar water splitting: Putting an extra 'eye' on surface reactions that store sunlight as fuel. ScienceDaily. Retrieved September 26, 2016 from www.sciencedaily.com/releases/2015/12/151204121403.htm
Department of Energy, Office of Science. "Solar water splitting: Putting an extra 'eye' on surface reactions that store sunlight as fuel." ScienceDaily. www.sciencedaily.com/releases/2015/12/151204121403.htm (accessed September 26, 2016).