Replacing passive ion-exchange membranes, like Nafion, with membranes that use light to drive ion transport would allow membranes in photoelectrochemical technologies to serve in an active role. Toward this, we modified perfluorosulfonic acid ionomer membranes with organic pyrenol-based photoacid dyes to sensitize the membranes to visible light and initiate proton transport. Covalent modification of the membranes was achieved by reacting Nafion sulfonyl fluoride poly-(perfluorosulfonyl fluoride) membranes with the photoacid 8-hydroxypyrene-1,3,6-tris(2-aminoethylsulfonamide). The modified membranes were strongly colored and maintained a high selectivity for cations over anions. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and ionexchange measurements together provided strong evidence of covalent bond formation between the photoacids and the polymer membranes. Visible-light illumination of the photoacid-modified membranes resulted in a maximum power-producing ionic photoresponse of ∼100 μA/cm 2 and ∼1 mV under 40 Suns equivalent excitation with 405 nm light. In comparison, membranes that did not contain photoacids and instead contained ionically associated Ru II −polypyridyl coordination compound dyes, which are not photoacids, exhibited little-to-no photoeffects (∼1 μA/cm 2 ). These disparate photocurrents, yet similar yields for nonradiative excited-state decay from the photoacids and the Ru II dyes, suggest temperature gradients were not likely the cause of the observed photovoltaic action from photoacid-modified membranes. Moreover, spectral response measurements supported that light absorption by the covalently bound photoacids was required in order to observe photoeffects. These results represent the first demonstration of photovoltaic action from an ion-exchange membrane and offer promise for supplementing the power demands of electrochemical processes with renewable sunlight-driven ion transport.

A composite ion-exchange polymer membrane was shown to mimic a traditional solar cell and exhibit photovoltaic action. Visible-light excitation of the membrane produced ionic power when photoacid dyes were covalently bonded to the polymer and ion-selective contacts were used. The photogenerated ionic power can be used to decrease power demands in any electrochemical device and could be particularly useful for direct light-driven desalination of salt water.

The immobilization of molecular species onto electrodes presents a direct route to modifying surface properties with molecular fidelity. Conventional methods include direct covalent attachment and physisorption of pyrene-appended molecular compounds to electrodes with aromatic character through π-π interactions. A recently reported hybrid approach extends the synthetic flexibility of the latter to a broader range of electrode materials. We report an application of this approach to immobilization of pyrene-appended ferrocene onto pyrene-functionalized indium tin oxide (ITO). The modified ITO surfaces were characterized using X-ray photoelectron spectroscopy, fluorescence spectroscopy, and electrochemical techniques. An electron-transfer rate constant ( k) of 100 ± 8 s was measured between the electrode and immobilized ferrocene using electrochemical methods. For comparison, a ferrocene-modified electrode using conventional covalent attachment of vinylferrocene was also prepared, and k was measured to be 9 ± 2 s.

We previously reported photovoltaic action from photoacid-dye-modified ionexchange membranes. A more controlled model system for those materials are photoacid-modified nanopores in poly(ethylene terephthalate) reported herein. Photoacids bound to the sub-10-nm-sized tips of these nanoporous poly(ethylene terephthalate) materials exhibited decreases in ground-state and excited-state acidity versus the same photoacid dyes dissolved in solution. The data indicate that nano-confinement and local electrostatics are important considerations when designing light-to-ionic energy conversion devices with possible applications in energy conversion and neuron triggering.

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