Rational Designing of Co–N–C Electrocatalysts for Comprehensive Elucidation of Intrinsic and Extrinsic Activities in the Oxygen Reduction Reaction

Abstract

The catalytic efficacy of an electrocatalyst is mostly apprised by the intrinsic activity of individual active sites and/or by extrinsic activity, which is dependent on the active site density. In oxygen electrochemistry, extrinsic activity can influence the concomitant current density, while intrinsic activity can potentially influence the onset potential, exchange current density, and half-wave potential besides the current density. Modulation of intrinsic activity is mostly limited as it is mainly dependent on the electronic and chemical structure of active sites. The extrinsic activity can be improved by employing the post-synthesis modifications without disturbing the inherent catalytic nature (i.e., intrinsic activity) of individual active sites to ensure the utility of maximum instituted centers. In this work, we have made an attempt to elucidate the intrinsic and extrinsic activities of Co–N–C-based nanostructures to promote the oxygen reduction reaction (ORR) efficiency, which is further elucidated by X-ray absorption spectroscopic studies. The experimental finding suggests that an increase in the N content (∼9 at %) in the host medium, that is, carbon, decisively impacts the intrinsic nature of electrocatalysts with a positive shift in the ORR onset potential of ∼ 131 mV and a nearly 1.5-fold increase in the exchange current density, a matrix that gives a measure of the ability to support catalytic activity inherently. The targeted removal of electrochemically less effective Co nanoparticles (which block/obscure the active centers Co–Nx in their vicinity) ensures the structural intactness as the C 1s full width at half maximum of the host matrix along with ORR onset potentials and Tafel slopes remains unchanged. The experimental finding further indicates that the current density can be improved by a factor of 2.6 (i.e., from 2.13 to 5.65 mA/cm2), ensuring the effective utilization of electrocatalysts. Overall, the rational combination of intrinsic and extrinsic activities can be valuable to design a variety of transition metal-carbon based electrocatalysts for diverse electrochemical energy devices.