  The kinetics of the oxygen reduction reaction (ORR) on the practical air
cathode in a Lithium air cell, which is conventionally composed of porous
carbon with or without catalysts supported on it, was investigated. The
mechanism and kinetics of the oxygen reduction reaction (ORR) was studied on a
porous carbon electrode in an oxygen saturated solution of 0.1M Lithium
bis-trifluoromethanesulfonimidate (LiTFSI) in Dimethoxyethane (DME) using
cyclic voltammetery (CV) and the rotating ring-disk electrode (RRDE) technique.
The oxygen reduction and evolution reactions were found to occur at similar
potentials to those observed on a smooth, planar glassy carbon (GC) electrode.
The effect of the porosity and the resultant increase in surface area were
readily observed in the increase in the transient time required for the
intermediates to reach the ring and the much larger disk currents (compared to
smooth, planar GC) recorded respectively. The RRDE data was analyzed using a
kinetic model previously developed by us and the rate constant of the
elementary reactions calculated. The rates constant for the electrochemical
reactions were found to be similar in magnitude to the rate constants
calculated for smooth GC disks. The porosity of the electrode was found to
decrease the rate of desorption of the intermediate and the product and delay
their diffusion by shifting it from a Fickian regime in the electrolyte bulk to
the Knudsen regime in the film pores. Thus, it is shown that the effect of the
electrode porosity on the kinetics of the ORR is physical rather than
electrochemical.
