Time/Space-Resolved Studies of the Nafion Membrane Hydration Profile in a Running Fuel Cell By Valerio Rossi Albertini, Barbara Paci, * Francesco Nobili, Roberto Marassi, and Marco Di Michiel The determination of the amount and spatial distribution of water in a polymeric membrane of a proton-exchange-membrane fuel cell (PEMFC) under working conditions is a fundamental task to address in PEMFC technology. Indeed, since proton transfer in such polymeric materials is known to be assisted by water, the fuel-cell (FC) performances depend on the proton-exchange membrane (PEM) hydration degree. However, the hydration degree is influenced not only by the electrochemical conditions the FC is submitted to, but also by many other independent parameters, such as the constriction exerted on the membrane by the other FC components, the electrical current flowing across it, the actual temperature, aging effects, etc, which are hard to take into account in theoretical calculations. In this work, an original method based on very-high-energy synchrotron-radiation X-ray diffraction is applied to carry out the first space/time-resolved measurements of the PEM hydration profile in a running FC. Due to their capability of effectively converting chemical into electrical energy, PEMFCs play a major role in the development of future environmentally friendly hydrogen-based technologies. Indeed, PEMFCs are considered promising candidates for auto- motive propulsion and for stationary applications. [1–5] To optimize the performance and lifetime of a PEMFC, one major problem that must be solved is the water management, because the PEM proton conductivity is highly dependent upon its water content. [6] On the other hand, an excess of water is detrimental, as it may produce cathode flooding, and a consequent reduction of gas supply. During the running of the cell, the membrane both absorbs water, which is produced by oxygen reduction at the cathode or carried by the humidified gas stream, and releases it, because of the evaporation induced by the gas flow and by heating occurring under operative conditions. Water transport through the membrane is caused mainly by the electro-osmotic drag of water by protons moving from the anode to the cathode, and by back-diffusion of the water produced at the cathode, towards the anode, as a consequence of the concentration gradients that build up upon operation. In steady conditions, equilibrium among these competitive mechanisms is reached. However, when the operative parameters are changed, complex water dynamics are observed. Several theoretical investigations [7–16] have been carried out to describe water intake, release, and transport through Nafion membranes. Nevertheless, uncertainties are also present in calculations, due to several experimental effects and constraints, which further complicate the water dynamics and are difficult to model. On the other hand, only a few experimental techniques aimed at measuring the water distribution in the membrane are available, [17] due to the intrinsic difficulty of isolating the signal coming from water molecules inside a relatively thick membrane assembled in a working cell (as required by an in situ investigation). X-ray techniques, [18,19] small-angle neutron scattering (SANS), [20] magnetic-resonance imaging [21–23] and micro-Raman, [24] neutron radiography, [25] electrical-resistance measurements, [26–29] infrared absorption [30] and fluorescence spectroscopy [31] have been used for this purpose. Unfortunately, if one excludes micro-Raman measurements, [24] all of these techniques exhibit rather low spatial resolutions (if any). Moreover, they have other severe limitations, such as slow response to the hydration-degree variations, weak signals (resulting in poor accuracy), and only indirect dependence on the quantity of interest. Here, we propose an alternative approach, based on very- high-energy (about 90 keV) X-ray diffraction (VHEXD), [18,32,33] to measure the hydration degree of PEMs in a working cell in situ. The method consists of vertical stratigraphy of the membrane from one electrode to the other, corresponding to ideally ‘‘slicing’’ the membrane itself in a stack of layers. As a result, the time-dependence of the hydration degree in each layer has been determined at the highest accuracy ever achieved, in all the experimental conditions in which PEMFCs may operate, and in the presence of all the concomitant effects mentioned above. To apply the method discussed in the experimental section, preliminary tests were required to identify the right inclination of the cell (parallelism between the PEM plane and the X-ray beam) and the height at which the beam intersects only the membrane. With these tests, spurious contributions from the other com- ponents of the cell to the diffraction patterns can be prevented. The parallelism condition was met by taking a sequence of radiographies of the cell during a scan of the rocking angle, carried out to visualize its inner parts. Figure 1a shows the first of these radiographies, collected after the cell was placed in the beam trajectory. The components of the cell around the membrane can be easily distinguished. The COMMUNICATION www.advmat.de [*] Dr. B. Paci, Dr V. Rossi Albertini Istituto di Struttura della Materia, C.N.R. Via Fosso del Cavaliere 100, 00133 Roma (Italy) E-mail: Barbara.Paci@ism.cnr.it Dr F. Nobili, Prof. R. Marassi Dipartimento di Scienze Chimiche Universita ` di Camerino Via S.Agostino 1, 62032 Camerino (MC) (Italy) Dr M. Di Michiel European Synchrotron Radiation Facility 6, Jules Horowitz, 38000 Grenoble (France) DOI: 10.1002/adma.200801652 578 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 578–583