Vapor phase water splitting for practical solar hydrogen production

Poster, Jan Rongé, 003
Publication date: 16th April 2014

After years of development, solar water splitting systems are finally reaching maturity. Recently, a number of promising concepts were presented in which well-known photovoltaics are adapted for application in photoelectrochemical cells [1-3]. Even though these do not parallel the record-breaking efficiencies achieved in the late ‘90s [4], they could be produced at much lower cost. Besides cost and efficiency, one hurdle down the path of commercial development remains almost ignored: the incorporation into practically operatable reactors. Whereas photovoltaics can make the step from lab-scale prototype to commercial device rather easily, photoelectrochemical cells require attention to feed management, product separation and transport of reaction intermediates, rather than light management alone [5].
At the current stage of developments, it seems the only satisfying strategy to achieve product separation is the implementation of an ion exchange membrane [6]. A drawback often overlooked in such systems is the requirement of very high purity water, to avoid membrane poisoning by dissolved salts. This increases operational costs, evidenced by the higher cost of PEM electrolyzers. These complications can be avoided by operating a water splitting device in the vapor phase [7].
We built a two-compartment cell comprising a proton exchange membrane for vapor phase operation. Water is oxidized at a sunlight-illuminated photoanode consisting of TiO₂ conformally deposited on multi-walled carbon nanotube forests by atomic layer deposition. In a second compartment protons from the oxidation reaction are reduced to hydrogen gas on highly dispersed Pt nanoparticles. Proton conduction is achieved by a thin surface layer of Nafion on the electrodes. To keep the membrane hydrated, upconcentration of water from the vapor phase is assisted by a hygroscopic layer of zeolite implanted on the membrane surface. We observed a stable photocurrent without signs of membrane dehydration. Possible schemes to collect and convert water vapor were investigated for large-scale and decentralized applications.
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5. Rongé, J., et al. (2014). Chem. Soc. Rev. doi:10.1039/c3cs60424a
6. Haussener, S., et al. (2012). Energy Environ. Sci., 5, 9922.
7. Xiang, C., et al. (2013). Energy Environ. Sci., 6, 3713.

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