Intrinsic electrocatalytic activity combined with oxygen preconcentration at non-metalated hypercrosslinked polymers for selective oxygen reduction reaction

The oxygen reduction reaction (ORR) remains a pivotal process in sustainable energy conversion and electrochemical synthesis. However, the development of efficient and cost-effective metal-free catalysts remains a challenge. In the last few years, it has become clear that polymers are a promising option. However, relatively easy-to-synthesize porous materials, such as non-metalated, non-pyrolyzed hyper-crosslinked polymers (HCPs), remain unexplored for the ORR. Here, we have focused on the ORR performance of HCPs synthesized by solvent knitting of triphenylamine (TPA) and triphenylbenzene (TPB) using different catalysts: FeCl₃ (Fe-TPBA) and AlCl₃ (Al-TPBA). Both polymers exhibit a hierarchical pore structure, with the latter showing a predominant contribution from micropores. HCP particles were immobilized in a Nafion® film deposited on the electrode surface, and their electrocatalytic activity towards the ORR was evaluated in both acidic and alkaline electrolytes. Both HCPs exhibit a catalytic effect and promote the 2-electron ORR. It was found that oxygen is preconcentrated in the catalyst film, contributing to the increase in its flux as compared to the bulk electrolyte, thereby increasing the ORR current. Three-phase junctions (electrode|nonconductive polymeric catalyst|ionomer filled with aqueous electrolyte) are suggested as the reaction site. DFT quantum chemical calculations indicate a weak interaction between HCPs' molecular motifs and O₂, making it a suitable O₂ flux promoter. Moreover, the calculations indicate that the presence of nitrogen-containing TPA units enhances the oxygen reduction rate as compared to TPB. This study paves the way for exploration of other non-metalated HCPs prepared from a wide array of precursors for ORR electrocatalysis.