High membrane conductivity is one of the key parameters in polymer electrolyte fuel cell applications. We introduce an electrochemical atomic force microscopy method that provides simultaneously the surface topography of a Nafion 112 membrane and the conductivity of ion channels with an unprecedented resolution of ca. 10 nm. For given conditions, a large fraction of the channel ports is found to conduct exactly the same number of protons per unit time. This is taken as evidence for an optimum pore size and structure for proton conduction, or alternatively, for an efficient connectivity of the ion channel network, so that the same conductivity is measured at all exit pores. The time response following a potential step and the influence of the relative humidity on the transport properties is investigated. The method will be of relevance for tailoring the production technology to yield an optimised micromorphology, and it permits detailed tests of membrane models and provides data for theoretical modelling of proton conductivity.

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Polymer electrolyte membrane fuel cells (PEMFCs) have attracted enormous attention as promising and environmentally friendly energy conversion devices for stationary and mobile applications due to their ability of attaining high power density and high energy conversion efficiency. One of the key components is the proton exchange (PEM) membrane which has to satisfy numerous demands such as high ionic conductivity, chemical, electrochemical and mechanical stability, and low permeability to reactants over a wide range of water content and temperature. [1][2][3][4] The fuel cell performance is directly related to ion conductivity in the membrane, which in turn depends strongly upon its degree of hydration and on the distribution of the ion transport channels which are a result of polymer microphase separation into hydrophilic and hydrophobic domains.[5] Nanoscale information is essential to understand the performance limiting features. Here we introduce an electrochemical atomic force microscopy (EC-AFM) method that provides, simultaneously, the surface topology of a Nafion 112 membrane and its proton conductivity with an unprecedented resolution of ca. 10 nm. They reveal discrete ion channels and suggest that an optimum pore size and structure governs proton conduction.Nafion, consisting of a hydrophobic polytetrafluoroethylene backbone and hydrophilic ÀSO 3 À H + acid groups connected to the backbone via ÀOÀCFÀCF 3 ÀCF 2 ÀOÀCF 2 ÀCF 2 À side chains, is the most widely used membrane material. It shows excellent chemical stability and proton conductivity when soaked with water which is the medium for proton transport. [6,7] One problem of present fuel cells is that not all catalyst particles are involved in the electrochemical reaction due to the fact that they are not in direct contact with the ionic network on the membrane surface. The inhomogeneity of the ion channel distribution causes an uneven current distribution and therefore gives rise to enhanced local dissipation of heat of reaction that leads to "hot spots" at places with very high reactant turnover.[8] This causes local membrane drying and higher resistance and can initiate free radical formation which accelerates membrane degradation. Especially in applications that involve numerous start-stop cycles, drying-swelling fatigue cycles lead to internal stress in the membrane and shear stress at the interface to the electrodes, which may promote aging and deterioration of the membrane-electrode interface. In order to increase the fuel cell lifetime, more information on the interface between ion-exchange membranes and catalyst particles is needed, preferably under conditions near practical fuel cell operation. In-situ studies are essential in order to reduce the gap between the "real world" and model systems.Several groups have extensively investigated the structure of the Nafion membranes using various electrochemical and instrumental techniques. Small angle X-ray and neutron scattering are well suited. However, their analysis is based on models which are na...

SUMMARYThe properties of the components of a membrane electrode assembly in a polymer electrolyte fuel cell (PEFC) determine its efficiency and performance. This paper aims at demonstrating the importance of nanoscale properties of PEFC membranes and electrodes and discussing the information obtained by various experimental techniques. The nanostructure and conductivity of freshly prepared as well as artificially degraded Nafion membranes and Pt/C electrodes are investigated by contact atomic force microscopy (AFM), conductive AFM, pulsed force mode (PFM)-AFM, in situ scanning tunnelling microscopy (STM), and scanning electron microscopy. The different techniques can provide complementary information on structure and conductivity. With in situ STM on Pt catalyst covered graphite, a layer of very small Pt particles between the catalyst particles is imaged, which is probably not visible with TEM and can explain a systematic discrepancy between TEM and XRD in particle size distribution. Conductive AFM is used to investigate the conductivity of Nafion. The images show a quite inhomogeneous distribution of current at the surface. The percentage of conductive surface increases with humidity, but regions without any current still present up to 80% of relative humidity (RH). Comparison with PFM-AFM images, where differences in adhesion forces are measured, indicates that hydrophobic regions are present at the surface with comparable dimensions, which are attributed to non-conductive PTFE-like polymer backbone. The changes in hydrophilic and hydrophobic parts after artificial degradation by plasma etching in air plasma can be imaged by PFM. High-resolution current images of the membrane were used to directly compare the measured nanostructure of the single conductive channels with model predictions from the literature. Recent models in the literature propose the formation of water-filled inverted micelles, with a mean diameter of 2.4 nm, and their agglomeration into clusters agrees well with the current images.