Purely Protonic Solar Cells Fabricated from Photoacid-Modified Ion-Exchange Membranes

Abstract

Analogies have been drawn between electronic semiconductors and water since 1956 when Fuller explained the similarity between silicon ionization, which generates an electron and a hole as mobile charge carriers, and water ionization, which yields a hydroxide and a proton as mobile charge carriers.1 Since then our group has taken steps to produce a water-based pn-junction solar cell that uses water as a protonic semiconductor. Toward this, we have demonstrated photovoltaic action using photoacid-modified Nafion membranes alone2 and when laminated to an anion-exchange membrane,3 yet each demonstration required the membrane to be wetted by an aqueous acid–base gradient. In this report, a liquid-electrolyte-free photoacid-modified ion-exchange membrane–electrode–assembly (MEA) driving reversible H2 redox chemistry is described. The cell architecture consists of a three-layer membrane (cation-selective Nafion membrane, aminopyrene-functionalized polyphenylene oxide, anion-selective X37 membrane) that was characterized using direct membrane contacts consisting of platinized platinum mesh. The MEA achieved V oc = 10 mV and J sc = 7.5 μA/cm2 under low-intensity 405 nm illumination, resulting in an internal quantum efficiency of ~0.2%. The reported composite MEA overcomes the problem of co-ion crossover by eliminating co-ions altogether while still exhibiting photovoltaic action due to the charge-separating potential across the membrane interfaces. Because photovoltaic performance persists in the absence of electrolyte gradients and because the potentiometric measurements conducted here are only sensitive to the electrochemical potential of protons, we can conclude that the light-induced voltage and current originate from protons and/or hydroxides, and no other ionic species. This MEA thus serves as a robust proof-of-concept of water’s role as a protonic semiconductor. It also offers a promising format for fundamental studies on the origin of photovoltaic action in photoacid–polymer-electrolyte membranes in the absence of the convoluting effects of liquid electrolyte. Acknowledgments: This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Early Career Research Award under award number DE-SC0019162 from the Solar Photochemistry Program. Fuller, C. S. Chem. Prog. 1956, 17, 75-93.White, W.; Sanborn, C. D.; Reiter, R. S.; Fabian, D. M.; Ardo, S. J. Am. Chem. Soc. 2017, 139, 11726–11733.White, W.; Sanborn, C. D.; Fabian, D. M.; Ardo, S. Joule 2018, 2, 94-109.