using electrochemical impedance spectroscopy, it has been found that the rate determining step of the oxygen reduction reaction changes between 700 and 800°c. this is important because it proves the existence of a critical temperature, previously only theorised, for activation of the entire composite cathode volume close to 750°c. below this temperature, ionic transport is slower; as the temperature increases, so does ionic activity through the cathode. additionally, gjl researchers have demonstrated that the volume ratio between lanthanum strontium manganite (lsm) and yttria stabilised zirconia (ysz) composite cathodes which is close to 1:1 gives the best electrochemical activity because of the extension of the three phase boundary (tpb) in the electrode. by means of impedance analysis and potentiodynamic polarisation, this finding has strong implications for optimal sofc design.