Proton Conductivity Study of a Fuel Cell Membrane with Nanoscale Resolution

Abstract

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...

Correlating Humidity-Dependent Ionically Conductive Surface Area with Transport Phenomena in Proton-Exchange Membranes

“…The surface pattern of the hydrophilic paths and their dynamic behavior on the surface of Nafion ® membrane were revealed and characterized from the time-resolved ionic conductivity mapping by CSAFM. 49 Grazing-incidence small-angle X-ray scattering (GISAXS) studies of Nafion ® confirmed the hydrophobic nature of the membrane surface vs.…”

Investigation of Localized Catalytic and Electrocatalytic Processes and Corrosion Reactions with Scanning Electrochemical Microscopy (SECM)

“…If i S can be confined to a small area close to the UME, local surface modification or analysis can be performed [120][121][122][123] (Figure 9). The confinement of the reaction is achieved by one of the following concepts: -The experiment is performed with a probe penetrating or touching a thin polymeric electrolyte [120,124]. -Dynamic concentration changes of reagents in specifically designed pulse sequences are exploited [125,126].…”

“…The mode differs from the FB mode not only in the connection of the UME and sample to the voltage source, but also by the fact that the reactions at the UME and sample do not need to be oxidation and reduction reactions of the same redox couple. Recently this mode was employed in a combined electrochemical scanning force microscopy (SFM) experiment to monitor the local proton conductivity [124,130] (see Section 3 for details). The direct mode is very likely operative as well in some surface modification processes observed or intentionally performed in STM or conductive SFM experiments.…”

Proton Conductive Areas on Sulfonated Poly(Arylene Ketone) Multiblock Copolymer Electrolyte Membrane Studied by Current-Sensing Atomic Force Microscopy

“…[21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] This current-sensing AFM (CS-AFM) technique has been used on Nafion [21][22][23][24][25][26][27][28][29][30][31][32]37 as well as HC-based membranes. [34][35][36] Several groups reported that the distribution of the proton conductive domains was correlated to the hydration level of the membranes.…”

“…[34][35][36] Several groups reported that the distribution of the proton conductive domains was correlated to the hydration level of the membranes. 22,26,[30][31][32][35][36][37] In our previous papers, each proton conductive spot on the membrane surface of SPE-bl-1 was successfully imaged by CS-AFM under the hydrogen atmosphere at various temperatures and humidities. 36,40 The size of the proton conducing spots was almost unchanged regardless of the temperature and humidity, whereas the number of the spots increased at higher humidity; the total area of the proton conducting spots increased accordingly on the membrane surface.…”