Although historically used as a tool to probe the structure of PEM materials, recently neutron techniques and sample environments have been developed to probe the transport of water in these materials by enabling structural changes to be monitored as a function of time. Kim and co-workers developed an in situ vapour sorption apparatus for SANS that is capable of controlling the vapour pressure of a given solvent and have employed it to investigate the effects of water vapour sorption in Nafion® films [62, 63]. A French group has developed an in situ, in operando SANS experiment and analysis method to observe the structure of Nafion® and determine the water profile across the thickness of the PEM [64—69]. This technique has also

been used by this group and others to investigate the behaviour of water in a working fuel-cell environment and will be discussed in greater detail below [70, 71]. More recently, Gebel and co-workers [72] used a similar cell and have demonstrated the ability to measure the kinetics of water sorption in Nafion® along with the water concentration-profiles across the thickness of the membrane using neutron scattering.

Kim and co-workers were able to measure structural changes in Nafion® under various relative humidity conditions, ranging from dry to hydrated, using an in situ vapour-sorption SANS (iVSANS) cell. Scattering intensity was measured over the Q range 0.1-0.3 A-1 as a function of time as shown in Fig. 10.8. The position and intensity of the ionomer peak were determined from the scattering profiles to measure the structural evolution of pretreated and as-received Nafion® films after being exposed to water vapour. The humidity changes investigated included dry to 20, 35, 50, 65, 80, and 95 % relative humidity. The results for the as-received Nafion® film can be seen in Fig. 10.9 for a humidity change from dry to 95 % relative humidity at 23 °C. Over the course of the sorption experiment, the ionomer peak increases in intensity and the position of the peak shifts to lower Q, as detailed in Fig. 10.9. The macroscopic scattering intensity (differential cross-section on an absolute scale), d2/dG, of the ionomer peak was correlated with the water uptake (Fig. 10.9a) and increased rapidly during the early stages of sorption and levelled off upon reaching equilibrium for each of the target humidity values. The rate of water sorption and the intensity, both related to the water uptake, were found to increase with increasing relative humidity. The ionomer peak position was found to follow the same trend, with the domain spacing increasing with time during the early stages and plateauing at later times. The equilibrium spacing increases with increasing relative humidity. The time-resolved (kinetic) data of the time-evolution

of the macroscopic scattering intensity was modelled with a solution to Fick’s second law to determine the diffusion coefficient for both as-received and pretreated Nafion® membranes. More recently, Gebel and co-workers performed a similar set of experiments [72]. In addition to obtaining kinetic data to determine the diffusion coefficient, Gebel and co-workers used an established technique to determine the water concentration-profile across the membrane during the sorption process [64—68, 70, 71]. Scattering data from Nafion® equilibrated at various relative humidity values, were recorded and served as a reference to reconstruct the scat­tering obtained during the equilibration process. It is thought that the scattering data taken during the sorption process could be considered as a sum of slices with varying thickness and water contents. These slices can be considered to be repre­sented by the recorded reference spectra and the total in situ scattering from the

membrane in the operating fuel-cell can be recreated by a linear combination of said reference spectra.

One of the most innovative uses of SANS to date has been the in situ, operando technique developed by a group in France [64—68, 70, 71]. In a working PEM fuel

cell, the membrane is not uniformly hydrated across the thickness of the cell and there is usually a water gradient from the anode to the cathode, with the water concentration being higher at the cathode. Ideally, one would want to know the water concentration profile through the membrane thickness as a function of the operating conditions in order to optimize fuel-cell performance and water man­agement. The group in France was the first to develop a fuel cell that was neutron — transparent, which enabled them to measure the scattering from the membrane during cell operation. The premise behind this technique is the water gradient across the fuel cell and the varying membrane nanostructure as a consequence of the different amounts of water. The features of the neutron-scattering data that arise from the nanostructure of Nafion® (i. e. the ionomer peak, incoherent background, etc.) are sensitive to the amount of water in the membrane. Thus, the membrane in the working fuel cell was considered to consist of a series of slices, each with different water content. Reference spectra were obtained for Nafion® membranes that were considered to be at equilibrium with respect to swelling over a range of water contents. The shape of the ionomer peak at a given X served as a reference of the scattering for that particular water content. The scattering intensity obtained during operation, Itotal(Q), was considered to be a linear combination of the scat­tering intensity of the reference spectrum, lYf(Q), where the weight (or coefficient) associated with each individual reference spectrum, a,, was directly correlated with the thickness of the corresponding hydration layer.

Itotai (Q) . агТ[е/ (Q) + k; with^a, = 1 (10.4)

The cell, the reference spectra, and a typical measured water profile can be seen in Fig. 10.10. This technique has been used to study the water gradient profiles during fuel-cell operation as a function of current density, H2/O2 gas ratio, and with differing gas-diffusion layers and gas-flow configurations. Details of this technique and the neutron-transparent fuel cell can be found in the literature [64—67, 73].