Proton Transport and the Water Environment in Nafion Fuel Cell Membranes and AOT Reverse Micelles

Abstract

The properties of confined water and diffusive proton-transfer kinetics in the nanoscopic water channels of Nafion fuel cell membranes at various hydration levels are compared to water in a series of well-characterized AOT reverse micelles with known water nanopool sizes using the photoacid pyranine as a molecular probe. The side chains of Nafion are terminated by sulfonate groups with sodium counterions that are arrayed along the water channels. AOT has sulfonate head groups with sodium counterions that form the interface with the reverse micelle's water nanopool. The extent of excited-state deprotonation is observed by steady-state fluorescence measurements. Proton-transfer kinetics and orientational relaxation are measured by time-dependent fluorescence using time-correlated single photon counting. The time dependence of deprotonation is related to diffusive proton transport away from the photoacid. The fluorescence reflecting the long time scale proton transport has an approximately t-0.8 power law decay in contrast to bulk water, which has a t-3/2 power law. For a given hydration level of Nafion, the excited-state proton transfer and the orientational relaxation are similar to those observed for a related size AOT water nanopool. The effective size of the Nafion water channels at various hydration levels are estimated by the known size of the AOT reverse micelles that display the corresponding proton-transfer kinetics and orientational relaxation.

Proton transfer in liquid water confined inside graphene slabs

“…[22][23][24][25] When confined in constrained geometries, the microscopical properties of the proton also suffer drastic changes. So, studies of PT near alumina surfaces [26] and in Nafion fuel cell membranes [27] reported changes in frequencies of vibrational motions and orientational relaxation times induced by the presence of the surface. Recently, a work based on multi-state empirical valence bond (EVB) calculations on PT in one-dimensional water chains confined in carbon nanotubes confirmed early results from Hummer et al [29] and revealed that the rate of PT inside the tubes was one order of magnitude faster than in bulk.…”

Long‐Range Proton Conduction across Free‐Standing Serum Albumin Mats

“…The proton can then diffuse to bulk water or recombine with the excited anion to form the ROH* form in what is known as geminate recombination 33. Since the ROH* and RO −* forms have different emission wavelengths (for HPTS, 440 and 535 nm, respectively), it is relatively easy to follow their time‐resolved and steady‐state fluorescence 34, 35, 36, 37. In pure water, the proton diffuses rapidly from the photoanion, which results in a predominant RO −* species, as can be seen in the steady‐state emission spectra (Figure 2a).…”

“…However, the decay of the ROH* form is much slower on the BSA mat, even in fully hydrated samples, which might imply that protons diffuse along the BSA mat and not into bulk water. The time‐resolved emission of the ROH* form can be used to get an estimation of the dimensionality of the proton diffusion space, where the fluorescent tail obeys a power‐law of t −d/2 , where d is the diffusion space dimensionality (see further discussion in Supporting Information) 34, 36, 37. By plotting a log–log plot (Figure S5, Supporting Information) of the lifetime corrected ROH* decay, we could estimate the fractal space dimensionality of the proton diffusion by linear fitting the first nanoseconds of the decay.…”

Potentials of mean force in acidic proton transfer reactions in constrained geometries

“…Often protons are conducted through restricted spaces, such as in the case of protons within water channels like those of nafion membranes in hydrogen fuel cells, [25][26][27] for protons in tiny water nanopools embedded inside proteins and peptides, [28] for protons near alumina surfaces [29] or inside biomembranes. [22] Nevertheless, despite the large scientific and industrial relevance of confined protons, the mechanisms of proton conduction in confined spaces are still a largely unexplored area of research, essentially due to the limited number of experimental techniques accurate enough and also the lack of detailed predictions from the theory and simulation side.…”