Nanoporous silicon photocathodes for photoelectrochemical water splitting

Nathan Neale^a, Yixin Zhao^a, Howard Branz^a, Kai Zhu^a

^aNational Renewable Energy Laboratory, Golden, Colorado, 1617 Cole Boulevard, Golden, Colorado, 80401, United States

Proceedings of International Conference on New Advances in Materials Research for Solar Fuels Production (SolarFuel14)

Montréal, Canada, 2014 June 25th - 26th

Organizer: Thomas Hamann

Invited Speaker, Nathan Neale, presentation 034
Publication date: 16th April 2014

Using sunlight to generate fuels such as H₂ through direct photoelectrochemical (PEC) processes is a promising approach to extend solar energy conversion into the transportation sector or provide storage of solar energy for electricity generation. Although the 1.1 eV indirect band gap of silicon is too low for overall water splitting, this inexpensive, Earth-abundand, and industrially mature semiconductor has many attractive features as a photocathode material for PEC water splitting, including a low band gap that uses much of the solar spectrum and a conduction band energy level suitable for hydrogen generation. Recnetly, nanostructured Si photocathodes have been developed for hydrogen production due to their advantages of low reflectivity and a higher surface area compared to bulk planar Si, which improves cahrge collection, exchange current density, and hydrogen gas surface-desorption. Our group and others have focused on the metal-assisted solution etching of planar Si to prepare nanostructured Si – also called "black Si" – a process that is suitable for large-scale manufacture and has been applied to making high-efficiency Si solar cells. However, the performance of Si electrodes used in any aqueous PEC system normally deteriorates from surface oxidation, an effect that appears to be unavoidable because of the high reactivity of Si with O₂ and H2O. Here, we present data on stable, high-performance nanoporous black Si photoelectrodes prepared via electrolessly deposited Pt nanoparticle catalysts made with two metal-assisted etching steps. This second etch causes a significant anodic onset potential shift resulting from reduced charge-transfer resistance. This high PEC performance owing to good interfacial contact between the black Si surface and the Pt NP catalysts is stable for at least two months.

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