Shining a light on solar water splitting.

Abstract

Direct photocatalytic splitting water to hydrogen and oxygen has been the subject of thousands of papers over the past four decades, with many optimistic promises. In his Perspective “A nickel finish protects silicon photoanodes for water splitting” (15 November 2013, p. [811][1]), J. A. Turner highlights progress in finding a stable electrode. It's fascinating science, but could this actually be a successful technology? Although cells with vertical electrodes are usually used for laboratory work, large-scale deployment will require solar cells angled according to latitude. Let's set aside the gas separation and membrane issues required and focus on some simple but critically important questions that must be addressed if direct solar water splitting is to be an important contender. Quantum efficiency is not the only issue. ![Figure][2] CREDIT: WIKIMEDIA COMMONS How large could individual cells be? Would they be in interconnected modules making up panels, as solid-state photovoltaics are now assembled? The need for a transparent cell cover means that low gas pressure is unavoidable. How would cells generating low-pressure hydrogen be connected in modules and then by extensive piping to a hydrogen collection point? Could any leaky cells in the array be sealed off? Installations would have to be in locations where the temperature is never low enough to freeze water and risk fracturing the cells. Eventually, the gas has to be compressed and stored for use. A second network to replenish the water (and electrolyte?) is less problematic but necessary. A typical large solar cell field can span 1 to 2 km2, as shown in the photo on the National Renewable Energy Laboratory (NREL) Web site ([www.nrel.gov/ncpv/][3]). Try to imagine the pipe network needed to bring the gas from hundreds of modules to a collection point. Hydrogen is notoriously difficult to contain, and the cost and maintenance of the extensive pipe network are unattractive. Moreover, the electrode surfaces would often be in hot water for years, likely resulting in degradation, and replacement is not economically feasible. Obviously, implementing photocatalytic water splitting presents a remarkably difficult engineering problem. Sun Catalytix, a startup funded by the Advanced Research Projects—Energy (ARPA-E), abandoned it shortly after beginning work ([ 1 ][4], [ 2 ][5]). NREL has extensively studied alternative paths to solar hydrogen production ([ 3 ][6]), but such alternatives are rarely mentioned in discussions of direct solar hydrogen generation. The simplest possibility is an array of commercial silicon cells (efficiency >16%) coupled to a commercial electrolyzer with about 70% efficient hydrogen generation. These components will produce hydrogen at about 10% efficiency and last a long time. There is also a recent extensive discussion of key components for photosplitting systems ([ 4 ][7]). The advantages of hydrogen generation in a central facility are obvious. Can direct solar water splitting be competitive with already established solar electricity technology and electrolysis? Serious discussion is long overdue. 1. [↵][8] 1. R. V. Noorden , Nat. News 10.1038/nature.2012.10703 (2012). doi:10.1038/nature.2012.10703 [OpenUrl][9][CrossRef][10] 2. [↵][11] 1. E. Waltz , “Sun Catalytix Won't be Raking It in with Artificial Leaf,” IEEE Spectrum (24 May 2012). 3. [↵][12] NREL, Photoelectrochemical Water Splitting ([www.nrel.gov/hydrogen/proj\_production\_delivery.html#split][13]). 4. [↵][14] 1. J. McKone, 2. H. Gray, 3. N. S. Lewis , Chem. Mater. 26, 407 (2014). [OpenUrl][15] [1]: /lookup/doi/10.1126/science.1246766 [2]: pending:yes [3]: http://www.nrel.gov/ncpv/ [4]: #ref-1 [5]: #ref-2 [6]: #ref-3 [7]: #ref-4 [8]: #xref-ref-1-1 View reference 1 in text [9]: {openurl}?query=rft.jtitle%253DNat.%2BNews%26rft_id%253Dinfo%253Adoi%252F10.1038%252Fnature.2012.10703%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [10]: /lookup/external-ref?access_num=10.1038/nature.2012.10703&link_type=DOI [11]: #xref-ref-2-1 View reference 2 in text [12]: #xref-ref-3-1 View reference 3 in text [13]: http://www.nrel.gov/hydrogen/proj_production_delivery.html#split [14]: #xref-ref-4-1 View reference 4 in text [15]: {openurl}?query=rft.jtitle%253DChem.%2BMater.%26rft.volume%253D26%26rft.spage%253D407%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx