Protonic nanoenvironment engineering for tuning the electrocatalytic efficiency and product selectivity of O2 reduction

Abstract

Precise control over the activity and selectivity of oxygen reduction reaction (ORR) catalysts is key to the development of efficient and durable cathodes for proton-exchange membrane (PEM) fuel cells. Recently, hybrid bilayer membrane (HBM) has emerged as a nanoscale electrochemical platform for investigating proton-coupled electron transfer (PCET) reactions, with particular emphasis on ORR thermodynamics and kinetics. In this work, we have developed a unique HBM incorporating a new self-assembled monolayer (SAM) design, deviating from the established nanoconstructs in prior studies. The new design integrates a custom-synthesized tridentate ligand, 2,2′:6′,2′′-terpyridine-4′-oxy-hexane-1-thiol (TPY), for hosting first-row transition metals (M) beyond Cu(II), including Ni(II) and Mn(II), resulting in a SAM decorated with terminal mononuclear M-TPY complexes. Among the observed ORR activity and selectivity, Cu-TPY SAM showed distinctive characteristics in contrast to Ni-TPY SAM and Mn-TPY SAM. Cu-TPY SAM exhibited significantly higher ORR activity via a dissociative 4-electron ORR mechanism, while Ni-TPY SAM and Mn-TPY SAM displayed lower ORR activity employing an associative 2-electron ORR pathway. We attributed these differences to the formation of distinct M–O intermediates, specifically end-on metal-superoxo adducts (η¹ M–O₂⁻) and side-on metal-superoxo adducts (η² M–O₂⁻), upon O₂ binding to the metal center. By appending a 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid monolayer onto M-TPY SAM to form M-TPY HBM, the catalyst–nanoenvironment interface transitions from one with facile proton transfer to one with protons depleted. With the incorporation of dodecyl boronic acid (DBA) as a proton carrier (PC) into the lipid monolayer to form M-TPY HBM DBA, the nanoenvironment switches to one with regulated proton transfer kinetics, ultimately achieving systematic modulation of the ORR activity and selectivity of the embedded M-TPY catalytic site. The mechanistic insights gained on steering the PCET pathways have implications for boosting the activity and selectivity of electrocatalysts tailored for facilitating other redox reactions central to renewable energy schemes and sustainable resource utilization.