The investigation and regulation of oxygen reduction reaction by using hybrid bilayer membranes

Abstract

Rising energy demands and worsening environmental pollution are problems humans must face and solve in the 21st century. Significant advancements in sustainable development are needed to overcome these critical techno-economic issues and build a green society for the well-being of future generations. Some non-traditional power conversion remains challenging. Simple, efficient, and pollution-free proton exchange membrane fuel cells (PEMFC) are especially suited for primary power conversion devices for vehicles and portable electronics. However, the performance of fuel cells is currently restricted by the cathode reaction, which is the sluggish kinetics O2 reduction reaction (ORR). Recently, hybrid bilayer membranes (HBMs) consisting of an artificial selfassembled monolayer (SAM) appended by a lipid monolayer with proton agents have evolved as an electrochemical platform to regulate the catalytic activity and selectivity of electrocatalysts towards the ORR. In the early part of this thesis, an HBM platform is constructed to study the electrochemical behaviors of the Cu(I)/Cu (II) couple attached to the terminus of a SAM with and without a lipid monolayer covered on top. An alkyl phosphate proton agent is incorporated into the lipid monolayer to examine how the redox properties of the Cu moiety change with solution pH. The surface morphology and elemental composition are characterized by atomic force microscopy and X-ray photoelectron spectroscopy. The ORR performance of the Cu catalyst embedded in HBMs is pH-dependent and can be regulated by the number of proton molecules in the lipid monolayer. The next portion of the thesis focuses on designing a proton carrier that can turn on transmembrane proton delivery in the base. In general, faster ORR kineticson non-precious catalysts is observed in alkaline conditions. However, at present, proton transport in the base is challenging. Here, a proton ferrying molecule bearing nitrile groups in protonophores is synthesized to overcome this technological barrier. A protonophore-inspired alkyl-CN can trigger and promote transmembrane proton transport within an HBM under alkaline conditions through hydrogen-bonding networks. This proton regulator can switch between the “Off” state in acid and the “On” state in alkali in real-time in a controllable fashion, thereby overcoming a significant constraint on developing HBMs. The last portion of this thesis aims to broaden the transition metal choice for HBMs beyond Cu and Fe. To explore the use of other first-row metals, an HBM is fabricated with a new ligand framework formed on tris(2-pyridylmethyl)amine that can coordinate a wide range of metal ions into a SAM. The catalytic activity and selectivity of the ORR on the Cu-, Fe-, Ni-, and Co-HBMs are investigated in three proton carriers. Interestingly, a synergistic effect in a bimetallic HBM is observed for the ORR, enabling the HBM to act as a screening tool to search for dual active sites with enhanced ORR activity and desirable selectivity for H2O. These results deepen the understanding of electrochemical behavior in HBMs and fill the knowledge gap to promote proton transfer under alkaline conditions, thereby providing mechanistic insights for optimizing proton-coupled electron transfer processes instrumental to electrochemical energy conversion and storage.