Proton Transfer in Perfluorosulfonic Acid Fuel Cell Membranes with Differing Pendant Chains and Equivalent Weights

Abstract

Proton transfer in the nanoscopic water channels of polyelectrolyte fuel cell membranes was studied using a photoacid, 8-hydroxypyrene-1,3,6-trisulfonic acid sodium salt (HPTS), in the channels. The local environment of the probe was determined using 8-methoxypyrene-1,3,6-trisulfonic acid sodium salt (MPTS), which is not a photoacid. Three fully hydrated membranes, Nafion (DuPont) and two 3M membranes, were studied to determine the impact of different pendant chains and equivalent weights on proton transfer. Fluorescence anisotropy and excited state population decay data that characterize the local environment of the fluorescent probes and proton transfer dynamics were measured. The MPTS lifetime and anisotropy results show that most of the fluorescent probes have a bulk-like water environment with a relatively small fraction interacting with the channel wall. Measurements of the HPTS protonated and deprotonated fluorescent bands' population decays provided information on the proton transport dynamics. The decay of the protonated band from ∼0.5 ns to tens of nanoseconds is in part determined by dissociation and recombination with the HPTS, providing information on the ability of protons to move in the channels. The dissociation and recombination is manifested as a power law component in the protonated band fluorescence decay. The results show that equivalent weight differences between two 3M membranes resulted in a small difference in proton transfer. However, differences in pendant chain structure did significantly influence the proton transfer ability, with the 3M membranes displaying more facile transfer than Nafion.

Effect of Annealing on Proton Conductivity of Aquivion-Like Proton-Exchange Membrane

“…We suggest that the proton transport in Aquivion-like membranes, as in other types of polymer membranes, proceeds through water, confined in the nanostructured channels (the diameter is about several nanometers) according to the Grotthuss mechanism. Moreover, the works [15,16] showed the similarity of proton transport mechanism for different PFSA membranes. The specificity of the proposed model is the special condition of water in channels with such small dimensions, first studied in [17] and used to describe the quasi-liquid surface layer of ice in ref.…”

Proton Transfer from a Photoacid to Water: First Principles Simulations and Fast Fluorescence Spectroscopy

“…[22][23] At short time, theory indicates that α = 1.4, which has been observed experimentally. 12,21 The data in Figure 3 were fit in the following manner. The functional form starts as an exponential and then makes a smooth transition into the power law, which grows in as the exponential decays.…”

“…This is in contrast to prior experiments where the power law was the focus and data were taken to much longer time. 12,21,23 As the power exponent was found to be 1.4 from both experiment and theory, in the fits, α was fixed at 1.4. The exponential decay constants are 78 ± 2 ps and 219 ± 2 ps in H2O and D2O, respectively.…”

“…[12][13][14][15][16][17][18][19] Upon the photo-excitation of an HPTS molecule in water, several fast and slow processes have been observed by transient absorption and time resolved fluorescence spectroscopies. 13,[18][19][20][21] The fastest process is the Stokes shift, which takes place between a few hundred femtoseconds and a picosecond. 13,17 This Stokes shift is not directly related to proton transfer.…”

“…By repopulating the associated state to a small extent, the fluorescence intensity of the "associated" peak decays non-exponentially with an ~90 ps time constant. 12,21 It has been shown theoretically and experimentally that the recombination causes the final complete decay of the associated state to occur as a power law, t -α , where α = ~1.5. [22][23] This process has been discussed in detail using a Smoluchowski type diffusion-reaction theory.…”