Electrochemical Synthesis and Characterisation of Multimetallic Nanostructured Electrocatalysts

Abstract

This thesis concerns investigations on novel multistage electrochemical deposition of nanostructured systems composed of noble metals platinum, ruthenium, and gold. Various electrochemical synthetic pathways were systematically explored producing multilayered nanoscale electrode systems composed of Pt, Ru, or Au on glassy carbon or crystalline gold used as substrates. Electrochemical pathways involved sequential surface-limited redox-replacement (SLRR) reactions of underpotentially-deposited or overpotentially-deposited copper, potentiostatic dealloying, direct spontaneous deposition of noble metals (without intermediary steps involving redox-replacement templating reactions) as well as sequential codeposition of noble metals (with or without SLRR templating reactions). Fundamental studies were conducted using thermodynamic and kinetic models, in situ electrochemical techniques and ex situ microscopic, spectroscopic, or spectrophotometric techniques employed for probing factors controlling electrode dynamics, electrocatalysis, morphology, bulk and surface compositional properties of the noble metal-based electrode systems. Unique multilayered multimetallic nanoclusters synthesized (with binary active sites of Pt with Ru or Au) exhibited superior electrocatalytic activity towards methanol or formic acid oxidation reactions when benchmarked to equivalent monometallic multilayered Pt. Hydrodynamic electrokinetic studies of the oxygen reduction reaction (ORR) on the multilayered monometallic Pt and bimetallic Rucontaining nanoclusters revealed that the monometallic nanoclusters exhibited direct four-electron ORR whereas electrocatalysis on the bimetallic ones could be tuned to proceed via a two-electron reaction pathway. Electrocatalytic bifunctional reaction mechanisms were especially enhanced by the nanostructured systems investigated. Characterisation of multilayered nanoclusters surface and near-surface metal contents revealed interactions between metal centers, carbon and oxygen containing surface functional groups on the glassy carbon, which appeared to have played a significant role in the overall stabilization and catalytic activity of the electrochemically immobilized nanoclusters. Physico chemical models and characteristics of intermediary Cu adlayers in the electrosynthetic pathways revealed role of Cu surface coverage (within the framework of electrochemical isotherms with kinetic and thermodynamic parameters) and heterogeneity effects, adatom substrate interactions as well as adatom adatom lateral interactions within individual adlayers during multilayer Pt growth on crystalline Au. New relationships for computing trends in the apparent Gibbs free energy of the SLRR reaction provided insights on reaction energetics of the interfacial Pt layered growth.