Modelling Electrocatalytic Reactions With A Concerted Treatment Of Multistep Electron Transfer Kinetics And Local Reaction Conditions

Oxygen reduction reaction(ORR),the cathode reaction of hydrogen-oxygen proton exchange membrane fuel cells,a classical electrocatalytic reaction,happens in the electric double layer(EDL)region at the electrode-electrolyte interface.The reaction kinetics is influenced by the local reaction conditions,including the adsorption energy of the reaction intermediates,the reactant concentration and the electric potential at the reaction plane,the solvent environment and so on.A physicochemical model,combined local reaction conditions with multistep electron transfer kinetics,is required to understand ORR mechanism and improve ORR kinetics for industrializing the fuel cells.In this paper,a physicochemical model is developed for electrocatalytic reactions,involving multiple electron transfer steps occurring in the EDL.Self-consistent numerical implementation of the model is demonstrated for ORR at Pt(111)in acidic solution,and the aptness of the model is verified by comparison with experimental data.Chapter 2 presents the physicochemical model developed for electrocatalytic reactions.The local reaction conditions are calculated using a mean-field EDL model,which is derived from a comprehensive grand potential that considers the steric effects,solvent polarization,and chemisorption-induced surface dipoles.Macroscopic mass transport in the so-called diffusion layer is controlled by the same set of controlling equations of the EDL model without imposing the electroneutrality assumption as usual.The Gerischer’s formulation of electron transfer theory,corrected with local reaction conditions,is used to describe the kinetics of elementary steps.Multistep kinetics of the electrocatalytic reaction is treated using microkinetic modelling,without resorting to the usual rate-determining step approximation.In formal analysis of the model,we retrieve canonical models with additional assumptions.In Chapter 3,the model is employed for the case of the ORR at Pt(111)in acidic solution.The agreement between the simulation and the experiment is good.A comparative study of the full model and its simplified versions allows us to examine how the ORR is influenced by asymmetric steric effects,finite concentration of ions,solvent polarization,surface charge effects,and metal electronic structure effects.The metal electronic structure effects have the biggest impact,followed by the surface charge effects,the asymmetric steric effects,the finite concentration of ions,and finally the solvent polarization.Chapter 4 focuses on the classical double layer effects of anion adsorption.The adsorbed anion induces the surface dipole,which alters the electrochemical driving force and the concentration of the reactant,promotes the kinetics of cation oxidation,cation reduction and neutral substance oxidation reactions.The enhancement factor of the reaction current is obtained by combining analytical expression and numerical simulation.The key message conveyed in this work is that the complex electrocatalytic reactions can be described by the physicochemical model.The map between the microscopic parameter space of catalyst composition and interfacial properties,and the macroscopic parameter space of electrode performance can be built.The model framework established in this paper can be extended to a variety of electrocatalytic reaction systems.