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Designing Impurity Distributions in Photoelectrodes for Solar Water Oxidation





Berkeley Lab researcher Coleman Kronawitter and colleagues have developed a novel technique that boosts the efficiency of solar water splitting — the electrochemical generation of hydrogen from water using sunlight. At the heart of this new technology are chemically stable photoelectrodes that optimize conversion of light with visible frequencies. Chemical stability and visible light activity are prerequisites for efficient and large-scale production of solar fuels by photoelectrochemical (PEC) processes.

The photoelectrodes contain arrays of nanostructures whose chemical composition promotes decoupling of the required light-absorption and current-conduction processes. The functions of specific regions within these nanostructures are established by their chemical compositions and three-dimensional geometries, which include volume, orientation with respect to the direction of light propagation, as well as proximity to the semiconductor-liquid interface. This separation of material, accomplished during fabrication by segregating the distribution of impurities within a substrate or a pair of substrates, yields a PEC electrode that is optically absorptive, electrochemically stable, and possesses the electronic band alignment required to drive solar water oxidation.

Using the Berkeley Lab technology, a photoelectrode can be made bearing two types of impurities, each of which promotes one of two properties desired for PEC processes. By engineering the distribution of these impurities, or dopants, either in two separate adjacent substrates or in separate regions of a single substrate, the properties of each can be improved. This technique has been demonstrated on a substrate comprised of zinc oxide (ZnO) nanocrystals. The near-surface regions of the nanocrystals were doped with nickel (Ni), an impurity that enhances solar light absorption. The nanocrystal cores were doped with aluminum (Al), an impurity that enhances electronic conductivity. The near-surface location of the Ni dopant facilitates electron transfer from/to an electrolyte; the core location of the Al dopant facilitates the acceptance of electrons excited by solar illumination in the surface/electrolyte region as well as the transport of electrons to electrical contacts at the bottom of the substrate. The result of this segregation — a decoupling of the two major PEC functions — demonstrably provides a direct avenue for optimization of efficient electrodes.  As designed by this technique, chemically stable wide-bandgap oxide-based photoelectrodes, which typically only absorb scarcely-available UV light, were shown to produce a photocurrent driven 44% by visible light.

DEVELOPMENT STAGE:  Bench scale prototype.

STATUS: Patent pending. Available for licensing or collaborative research.


Kronawitter, C.X., Ma, Z., Liu, D., Mao, S.S., Antoun, B.R., “Engineering Impurity Distributions in Photoelectrodes for Solar Water Oxidation,” Advanced Energy Materials, Vol. 2, Issue 1, pp. 52-57, January 2012.


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