Abstract
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 ion-exchange 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/cm2 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 RuII-polypyridyl coordination compound dyes, which are not photoacids, exhibited little-to-no photoeffects (∼1 μA/cm2). These disparate photocurrents, yet similar yields for nonradiative excited-state decay from the photoacids and the RuII 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.
Highlights
Covalent bonding of photoacids in Covalently modified PFSA (cPFSA) were supported by data shown in Figure 4, which contains the Fourier transform infrared (FTIR) spectra and X-ray photoelectron spectroscopy (XPS) spectra for Nafion, PFSF, cPFSA, and ionomer membranes containing ionically associated photoacids
The characteristic sulfonyl fluoride peaks at 795, 823, and 1467 cm−1 present in FTIR spectra of PFSF were undetectable in spectra of cPFSA, which was synthesized from PFSF.[33−36] This suggests that most sulfonyl fluoride groups were modified to sulfonates/sulfonic acids or covalently bound dyes in cPFSA.[36−41] Two small peaks were present in the 2800−2950 cm−1 range for iPFSA and cPFSA, which based on previous literature reports are consistent with assignment to C−H stretches in the pyrene core of the photoacids (Supporting Information, Figures S2 and S3).[42]
Using perfluorosulfonic acid ionomer membrane (PFSA) modified with photoacid dye molecules, 8hydroxypyrene-1,3,6-tris(2-aminoethylsulfonamide), a first-ofits-kind synthetic polymer membrane light-driven proton pump was demonstrated
Summary
Proton pumps are ubiquitous in biology, where light or adenosine triphosphate drives the proton-pumping process to generate a difference in proton activity across a lipid bilayer.[1,2]When these nonequilibrium conditions are generated using light, the photoconversion process can be termed “photovoltaic” because light is responsible for the generation of a voltage across the membrane.[3−10] When proton transport is accompanied by the transport of other ions to maintain charge neutrality, the energy storage process is chemical like that in batteries, whereas when protons are the only species that are transported, the energy storage process is mostly electric like that in capacitors.[1,2] The most efficient and well-studied solar cells utilize semiconductors with pn-junctions and exhibit photovoltaic action by a capacitive mechanism. Several demonstrations of artificial light-driven proton pumps have been reported.[11−15] Most reports utilized a nanometers-thick lipid bilayer membrane containing molecular dyes, which initiated the proton pumping process by a photoinduced proton-coupled electron-transfer reaction.[4−6,8,9] The report by Bakker and colleagues was unique, because it used a 30 μm thick microporous polyethylene membrane impregnated with merocyanine photoacid dye molecules to sensitize the light-to-ionic energy conversion process.[15] The authors observed a ∼210 mV photovoltage using bidirectional excitation from a Xe arc lamp This consisted of visible-light illumination from one side of the membrane and ultravioletlight illumination from the other side of the membrane. This proton-pumping process utilized an external optical asymmetry, and not an internal asymmetry like that present in many semiconductors, the magnitude of the photovoltage was independent of the bulk pH, suggesting that the voltage was capacitive, like that observed in state-ofthe-art electronic solar cells
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