Towards a Fundamental Understanding of the Oxygen Reduction Mechanism

The kinetics of oxygen reduction are thermally activated, therefore as solid oxide fuel cell (SOFC) operating temperature is lowered, the rates of chemical reactions drop exponentially, and activation polarization at the cathode dominates SOFC performance. Hence, cathode polarization is widely regarded to be the key issue hindering reduced temperature SOFC development. In order to understand the various mechanistic contributions to cathode polarization and apply this knowledge to development of lower-polarization/lower-temperature SOFC cathodes, we have embarked on a multi-faceted, multi-disciplinary approach to deconvolute the various contributions to SOFC cathode polarization. This approach includes FIB/SEM to quantify the cathode microstructure (in terms of tortuosity and porosity for gas diffusion, solid-phase surface area for gas adsorption/surface diffusion, and triple phase boundaries for the charge transfer reaction) and heterogeneous catalysis techniques (temperature programmed desorption and reaction) and O-isotope exchange to evaluate the O2 reduction mechanism at the gas-solid reaction interface. These results are then combined (and contrasted) with the more conventional electrochemical polarization techniques (impedance spectroscopy and I-V behavior) to try and elucidate each of the mechanisms as a function of composition and microstructure. The progress to date is summarized