Finally, to probe chemical nature of the rate-limiting step in the OER cycle, we utilized H/D and 16 O/ 18 O isotope measurements. In comparing the CV measurements (Fig. ) and the resultant Tafel slopes (Fig. ), there were minimal changes between the catalyst in KOH and KOD electrolytes, implying that these species were not involved in the rate limiting step. However, when using K 18 OH, the resultant current density dropped significantly and the Tafel slope increased by approximately 15 mV/dec, to 42 mV/dec which indicates that oxygen atoms are the sole species involved in the rate limiting step and the catalytic cycle is pushed away from the conventional one with proton-coupled electron transfer (PCET) steps often involving the breaking of a O-H bond 6 .