Electrodialysis is a state-of-the-art technique for water desalination, acid recovery, and neutralization of water streams. The efficiency of electrodialysis suffers from energy losses due to the large overpotential needed to drive water electrolysis and unintended electrochemical side products. Water electrolysis systems use passive ion-exchange membranes, such as Nafion, which provide the ability for selective migration of ions in between chambers. If these ion exchange membranes could be adapted to drive photodialysis and avoid electrolysis of water there could be up to an 80% increase in efficiency of these processes. Toward this, we doped poly(perfluorosulfonic acid) (cPFSA) membranes with organic hydroxypyrene-based photoacid molecules to serve as sensitizers that convert the energy in sunlight into a change in chemical potential of protons. Visible-light illumination of doped cPFSA resulted in photovoltaic action with a maximum short-circuit ionic photocurrent of ~100 µA/cm2and a maximum open-circuit photovoltage of ~1 mV, under effectively 20 Suns excitation. Adding an anion selective contact to the doped cPFSA in the form of an anion exchange membrane increased the photovoltage almost ten-fold to 20 mV. This behavior differed from that observed using ionically associated tris(bipyridine)ruthenium(II) chloride, which exhibited negligible photo-effects. The non-unity emission quantum yield from these [Ru(bpy)3]2+ dyes also implied that local heating due to non-radiative excited-state decay. The poor photoresponses were supported by fundamental TCSPC measurements which suggest that the dyes in the sulfonic acid channels are not deprotonating likely due to the highly acid nature in the polymer membrane. These results represent the first demonstration of photovoltaic action from an ion-exchange membrane, offering promise for supplementing the power demands of electrochemical processes with renewable sunlight-driven ion transport.

Insights into the mechanism of photovoltaic action from photoacid-functionalized ion-exchange materials

253rd National Meeting (2017)

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