The proton conductance of Nafion 117 was measured as a function of water content and temperature and compared to changes in the phase state of water. Conductance was measured using a direct current four-point probe technique, while the water phase was determined from differential scanning calorimetry of the melting transitions. Arrhenius plots of conductance show a crossover in the activation energy for proton transport for temperatures coinciding with the melting and freezing of water. This crossover temperature depends on the membrane's water content per acid group, , and displays hysteresis between heating and cooling. Using calorimetry to estimate the fraction of the frozen water phase, both the crossover temperature and the hysteresis are found to correlate with the phase state of the water. For membranes starting with water contents above ϳ 8, the calorimetry and conductivity curves merge at low temperature, suggesting the formation of a common acid hydrate with similar network connectivity; for lower starting water contents, the low-temperature conductivity drops rapidly with . Based on Poisson-Boltzmann models, differences between the conductivity and calorimetry are attributed to gradients in the proton concentration that result in a proton-depleted core in the hydrated pores, which freezes first and contributes minimally to conductivity.

3M's Nano Structured Thin Film (NSTF) electrode, being a core-shell catalyst, offers a novel mean to enhance the performance and lower Pt cost in a polymer electrolyte fuel cell (PEFC). In the present work, fuel cell performance of NSTF is reported and the underlying physics dictating NSTF behavior is probed. It was found that NSTF with 0.15 mg Pt /cm 2 Pt loading shows comparable performance to that of a conventional Pt/C electrode with 0.4 mg Pt /cm 2 loading in a highly humidified condition at 80 C. However, the NSTF performs poorly under dry conditions. A single-phase model was developed to elucidate the underlying phenomenon governing NSTF performance under partially-humidified conditions. NSTF proton conductivity as a function of relative humidity (RH) was determined and the model predictions were compared against a range of experimental data. Detailed results suggest that poor NSTF performance under dry operation is due to low proton conductivity over Pt surface, which reduces catalyst utilization. The importance of water management is highlighted to improve NSTF performance. The high cost of Pt in a fuel cell stack is one of the key barriers to the commercialization of fuel cell vehicle technology. Current state-of-the-art fuel cell vehicles use approximately 72-94 grams of Pt, but it has been estimated that a Pt loading of less than 15 g is required to achieve cost-competiveness with internal combustion engine technology.

An experimental procedure using isothermal galvanostatic operation was developed to quantify the charge ͑water͒ accumulation in proton exchange membrane ͑PEM͒ fuel cells at subfreezing conditions prior to voltage failure ͑i.e., zero cell voltage͒. The charge passed until voltage failure was compared to charge ͑water͒ storage estimates in the membrane phase and the cathode electrode void volume. Cryo-scanning electron microscope images of electrodes following voltage failure were used to assess ice filling of the cathode electrode void volume. At very low current densities, the membrane absorbs a maximum of Ϸ14 to Ϸ15 water molecules/per sulfonate group ͑ max Ϸ 14-15͒ and cathode electrode voids are completely ice filled. It is shown that the maximum charge storage of a membrane electrode assembly increases with electrode void volume and the difference between max and initial . With increasing current densities, decreasing fractions of the maximum charge storage can be utilized, which is shown to be related both to water transport resistances in the membrane phase and to reduced ice filling of the electrode void volume. Experimental results show that the charge storage utilization is mainly controlled by the current density and is less dependant on initial water content or electrode thickness.Proton exchange membrane ͑PEM͒ fuel cells are presently being investigated as an alternative power source for vehicles that require cold climate operation. At low temperatures, PEM fuel cell ͑PEMFC͒ performance suffers from the same irreversibilities that occur during operation at normal temperatures, namely, ohmic, kinetic, and mass transport losses. Our previous studies investigated the ohmic losses due to membrane conductivity 1 and oxygen reduction reaction ͑ORR͒ kinetic losses 2 at temperatures below 0°C. During operation at subfreezing temperatures, the amount of product water that can be produced prior to voltage failure ͑i.e., zero cell voltage͒ is essentially limited by the maximum water uptake of the membrane and the cathode electrode void volume, because the loss of water vapor into the reactant gases is negligible significantly below 0°C. This maximum water uptake can also be expressed in terms of a maximum charge storage ͑in coulombs͒. The latter is a more convenient measure because it corresponds to the maximum time integral of the current by which a frozen fuel cell must reach 0°C before voltage failure occurs as a result of reactant blockage by ice. This concept of maximum charge storage and its relation to mass transport of product water into the membrane as well as into the cathode electrode void volume is the focus of this study. Please note that the concept of charge storage in this study is synonymous with the quantity of water produced electrochemically; it is not to be confused with the platinum and double-layer charge capacity of the electrode, which is negligibly small 3 compared to the charge quantities discussed in this paper.During subfreezing operation, ice formation from product water in electrod...