Pyrolysis-free synthesis of single-atom cobalt catalysts for efficient oxygen reduction

Nitrogen-coordinated single-atom catalysts (SACs) have emerged as one of the most promising alternatives to noble metal-containing benchmarks for highly efficient oxygen reduction reaction (ORR). However, the commonly required high-temperature pyrolysis usually results in undesirable structural changes and randomly produced active sites which gives rise to great challenges to the structure-property relationships, inevitably hindering the understanding of the reaction mechanisms. Herein, we demonstrate a simple yet robust pyrolysis-free route to craft single-atom cobalt catalysts with high electrocatalytic ORR activity via judiciously in situ wrapping an electrocatalytically active porphyrin-based thiophene-sulfur site-containing covalent organic polymer (PTS-COP) shell around a highly conductive multiwalled CNT (MWCNT) core, followed by accurately anchoring single-atom Co-N-4 sites onto the macrocyclic porphyrin structure. The resulting Co-PTS-COPs@MWCNTs was exploited as an ORR electrocatalyst and displayed outstanding performance as a direct consequence of the advantageous architecture (i.e., 1D core@shell heterostructures with few-layer-thick PTS-COP shells) and unique active site configurations (i.e., atomically anchored Co-N-4 sites and homogeneously dispersed thiophene-sulfur sites). Remarkably, an alkaline electrolyte capitalizing on Co-PTS-COPs@MWCNTs achieved excellent ORR activity (E-onset, 0.930 V; E-1/2, 0.835 V), and favourable long-term durability, comparable to that of state-of-the-art carbon-based electrocatalysts.