Performance of Nano Structured Thin Film (NSTF) Electrodes under Partially-Humidified Conditions

Abstract

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.

Understanding the electrocatalysis of oxygen reduction on platinum and its alloys

“…Even so, water management is more challenging on these thin films than on conventional nanoparticulate Pt/C catalysts. 148 …”

Recent Progress on the Key Materials and Components for Proton Exchange Membrane Fuel Cells in Vehicle Applications

“…In addition, it is further postulated that the proton concentration distribution within the water-flooded pores is determined by the electrostatic interactions between the protons and the surface charge of the pore walls [141][142][143]. However, Sinha et al [138] pointed out that, in their study, the NSTF electrode was found to be free of bulk liquid water. They also argued that previous studies using other surfaces such as gold or carbon in liquid water did not show similar proton conduction.…”

“…In the NSTF, the catalyst support consists of highly-oriented crystalline organic whiskers, onto which the Pt or Pt-alloy catalyst is sputter-coated as a polycrystalline thin film [124,138]. Figure 5 shows cross-sectional SEM images of an NSTF electrode.…”

“…The NSFT whiskers are coated with a Pt film, and there is no ionomer in the NSTF electrode, which is analogous to Pt black electrodes. Figure 6 shows the proton conductivity of the Pt whisker, which was estimated by fitting the experimental data with a numerical model [138] as a function of the relative humidity. One can see an enhancement in the proton conductivity in the NSTF electrode compared with pure water and a significant effect of the relative humidity on the proton conductivity.…”

“…Figure 5. Cross-sectional SEM images of (a) conventional Pt/C electrode and (b) NSTF electrode; (c) Detailed structure of the NSTF electrode at higher magnification [138].…”