Heat-treated cobalt–tripyridyl triazine (Co–TPTZ) electrocatalysts for oxygen reduction reaction in acidic medium

Abstract

In this paper, carbon-supported cobalt–tripyridyl triazine (Co–TPTZ) complexes were synthesized by a simple chemical method, then heat-treated at 600, 700, 800, and 900 °C to optimize their activity for the oxygen reduction reaction (ORR). The resulting catalysts (Co–N/C) all showed strong catalytic activity toward the ORR, but the catalyst heat-treated at 700 °C yielded the best ORR activity. Co–N/C catalysts with several Co loadings – 0.64, 2.0, 2.96, 3.33, 5.28, and 7.18 wt% – were also synthesized and tested for ORR activity. X-ray diffraction and energy dispersive X-ray analysis were used to characterize these catalysts in terms of their structure and composition. To achieve further quantitative evaluation of the catalysts in terms of their ORR kinetics and mechanism, rotating disk electrode and rotating ring-disk electrode techniques were used with the Koutecky–Levich theory to obtain several important kinetic parameters: overall ORR electron transfer number, electron transfer coefficiency in the rate-determining step (RDS), chemical reaction rate constant, electron transfer rate constant in the RDS, exchange current density, and mole percentage of H 2O 2 produced in the catalyzed ORR. The overall electron transfer number for the catalyzed ORR was determined to be ∼3.5 with 14% H 2O 2 production, suggesting that the ORR catalyzed by Co–N/C catalysts is a mixture of 2- and 4-electron transfer pathways, dominated by a 4-electron transfer process; based on these measurements, an ORR mechanism is proposed based on the literature and our understanding, to facilitate further investigation. The stability of a Co–N/C catalyst was also tested by fixing a current density to record the change in electrode potential with time. For comparison, two other catalysts, Fe–N/C and TPTZ/C, were also tested for stability under the same conditions as the Co–N/C catalyst. Among these three, the 5 wt% Co–N/C was most stable.