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An experimental and modeling/simulation-based evaluation of the efficiency and operational performance characteristics of an integrated, membrane-free, neutral pH solar-driven water-splitting system

Jin, Jian and Walczak, Karl and Singh, Meenesh R. and Karp, Chris and Lewis, Nathan S. and Xiang, Chengxiang (2014) An experimental and modeling/simulation-based evaluation of the efficiency and operational performance characteristics of an integrated, membrane-free, neutral pH solar-driven water-splitting system. Energy and Environmental Science, 7 (10). pp. 3371-3380. ISSN 1754-5692. https://resolver.caltech.edu/CaltechAUTHORS:20140820-110452543

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Abstract

The efficiency limits, gas-crossover behavior, formation of local pH gradients near the electrode surfaces, and safety characteristics have been evaluated experimentally as well as by use of multi-physics modeling and simulation methods for an integrated solar-driven water-splitting system that operates with bulk electrolyte solutions buffered at near-neutral pH. The integrated membrane-free system utilized a triple-junction amorphous hydrogenated Si (a-Si:H) cell as the light absorber, Pt and cobalt phosphate (Co–Pi) as electrocatalysts for the hydrogen-evolution reaction (HER) and oxygen-evolution reaction (OER), respectively, and a bulk aqueous solution buffered at pH = 9.2 by 1.0 M of boric acid/borate as an electrolyte. Although the solar-to-electrical efficiency of the stand-alone triple-junction a-Si:H photovoltaic cell was 7.7%, the solar-to-hydrogen (STH) conversion efficiency for the integrated membrane-free water-splitting system was limited under steady-state operation to 3.2%, and the formation of pH gradients near the electrode surfaces accounted for the largest voltage loss. The membrane-free system exhibited negligible product-recombination loss while operating at current densities near 3.0 mA cm^(−2), but exhibited significant crossover of products (up to 40% H_2 in the O_2 chamber), indicating that the system was not intrinsically safe. A system that contained a membrane to minimize the gas crossover, but which was otherwise identical to the membrane-free system, yielded very low energy-conversion efficiencies at steady state, due to low transference numbers for protons across the membranes resulting in electrodialysis of the solution and the consequent formation of large concentration gradients of both protons and buffer counterions near the electrode surfaces. The modeling and simulation results showed that despite the addition of 1.0 M of buffering agent to the bulk of the solution, during operation significant pH gradients developed near the surfaces of the electrodes. Hence, although the bulk electrolyte was buffered to near-neutral pH, the electrode surfaces and electrocatalysts experienced local environments under steady-state operation that were either highly acidic or highly alkaline in nature, changing the chemical form of the electrocatalysts and exposing the electrodes to potentially corrosive local pH conditions. In addition to significant pH gradients, the STH conversion efficiency of both types of systems was limited by the mass transport of ionic species to the electrode surfaces. Even at operating current densities of <3 mA cm^(−2), the voltage drops due to these pH gradients exceeded the combined electrocatalyst overpotentials for the hydrogen- and oxygen-evolution reactions at current densities of 10 mA cm^(−2). Hence, such near-neutral pH solar-driven water-splitting systems were both fundamentally limited in efficiency and/or co-evolved explosive mixtures of H_2(g) and O_2(g) in the presence of active catalysts for the recombination of H_2(g) and O_2(g).


Item Type:Article
Related URLs:
URLURL TypeDescription
http://pubs.rsc.org/en/Content/ArticleLanding/2014/EE/C4EE01824APublisherArticle
http://dx.doi.org/10.1039/c4ee01824aDOIArticle
ORCID:
AuthorORCID
Singh, Meenesh R.0000-0002-3638-8866
Lewis, Nathan S.0000-0001-5245-0538
Xiang, Chengxiang0000-0002-1698-6754
Additional Information:© 2014 The Royal Society of Chemistry. Received 16th June 2014. Accepted 24th July 2014. First published online 25 Jul 2014. This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.
Group:JCAP
Funders:
Funding AgencyGrant Number
Department of Energy (DOE)DE-SC0004993
Issue or Number:10
Record Number:CaltechAUTHORS:20140820-110452543
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20140820-110452543
Official Citation:Jin, J., Walczak, K., Singh, M. R., Karp, C., Lewis, N. S., & Xiang, C. (2014). An experimental and modeling/simulation-based evaluation of the efficiency and operational performance characteristics of an integrated, membrane-free, neutral pH solar-driven water-splitting system. [10.1039/C4EE01824A]. Energy & Environmental Science, 7(10), 3371-3380. doi: 10.1039/c4ee01824a
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48727
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:20 Aug 2014 19:17
Last Modified:09 Mar 2020 13:19

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