Organic/Inorganic Composite Membrane Using a Proton-Conducting SnP2O7 Electrolyte and PFSA Polymers for Intermediate Temperature PEMFC

Current fuel cells for transportation and residential applications use polymer electrolyte membrane fuel cells (PEMFCs) which require platinum catalysts and operate below 80 °C due to water management issues. However, PEMFCs operating at 100 °C or higher temperatures in anhydrous conditions do not need a humidifier, and do not suffer from water management issues created by the presence of liquid water. Excessive amount of liquid water in the electrode can flood the backing and catalysts layers at lower temperatures leading to poorer fuel cell performance. Furthermore, such intermediate temperature PEMFCs can solve the problem of carbon monoxide (CO) poisoning of the anode electrocatalyst leading to relaxation of the hydrogen fuel quality standards, thus lowering infrastructure costs. Finally, higher operating temperatures allow for simpler cooling systems in transportation applications, which results in smaller and lighter radiators. These advantages mean that a paradigm shift in fuel cells for transportation and residential applications can be achieved with an intermediate temperature proton conducting solid-electrolyte that can operate above 150 °C.

Unfortunately, electrolyte membranes and ionomers developed so far do not have satisfactory conductivity and performance in the above temperature ranges, at low-humidity, or in non-humidified conditions. The further advancement of higher temperature PEMFCs this relies on the development of membranes that process the desired electrochemical and mechanical properties in dry (< 30% RH) hot (> 100 ^oC) conditions.

In this study, we have developed an organic/inorganic composite membrane using a SnP₂O₇ (TPP) inorganic conductor and evaluated the fuel cell performance under various condition. Figure 1 illustrates the conductivity of this membrane under anhydrous conditions at various temperatures. The conductivity is a strong function of the % of TPP in the composite and reach as high as 0.1 S/cm at T ≥ 250^oC using ≈ 90wt % TPP.

Membrane electrode assemblies (MEA) were prepared from these membranes using either standard PEMFC Gas diffusion electrodes or Pt/C/TPP based electrodes painted into to the membrane. The performance of one of these MEAs is shown in Figure 2. A maximum power density of 300mW/cm² (not shown) has been obtained for fuel cell operation at ≈ 250 ^oC and < 5 % relative humidity. While these preliminary results are promising, there are still challenges with low open circuit voltage (OCV) due to membrane porosity and high kinetic losses due to insufficient electrode morphology optimization. In this talk we will address these issues and discuss the progress made towards better performing intermediate temperature fuel cells.

[1]    Nagao et al., J Electrochem Soc, 2006

[2]    Nagao et al. Electrochem Solid State Lett, 2006.