Electrostatic Interactions In Electrocatalysts And Electrodes Of Polymer Electrolyte Fuel Cells

Polymer electrolyte fuel cells(PEFCs)are envisioned as the ultimate power source of future vehicles owing to the unrivaled efficiency and environmental friendliness.Cost and durability are two major bottlenecks keeping fuel cell vehicles from large-scale commercialization.Ionomer-free ultra-thin CLs(UTCLs),by eliminating ionomer from the catalyst layer(CL)and largely reducing the CL thickness,are promising to improve catalyst utilization,effectiveness and durability.However,ionomer-free UTCLs are challenging the old belief that ionomer is essential to efficient proton transport and high catalyst utilization and effectiveness in the CL.How do protons transport in ionomer-free UTCLs? What is the proton conductivity thereof? How efficiently are the Pt catalysts utilized therein? These open questions represent critical theoretical challenges facing the development of UTCLs.Efforts in this direction can also direct the design of conventional ionomer-impregnated CLs,as more and more evidences are suggesting that many Pt nanoparticles(can be up to 60%)are trapped in ionomer-free regions therein.Ionomer-free micropores(2-20 nm)with walls made of Pt are prone to be waterfilled due to capillary condensation and thereby liquid water can act as an efficient proton transport medium via structural diffusion(Grotthuss mechanism).In this scenario,electrostatic interactions between the electrode surface charge and protons in water are the driving force of proton conduction and the dominating factor of proton concentration.Following this line of reasoning,describing the surface charging behavior,interfacial behaviors in general,of the electrode surface represents the key to deciphering the problem.Consecutive transitions from negative to positive and further to negative surface charges with increasing electrode potential,as revealed by experiments,defy the canonical linear charging relation parameterized with the potential of zero charge as a sufficient description for Pt electrodes.This situation thrusts us into pursuing a deeper understanding of the metal(Pt)-solution interface.The central contribution of the present thesis is a self-consistent treatment of the metal-solution interface involving formation of surface adsorbed intermediates and associated structural changes of the interface,surface charge on the metal surface,fielddependent behaviors of interfacial water molecules,ion and potential distribution in solution,and thermodynamics and kinetics of the oxygen reduction reaction(ORR)at the interface.This framework provides both fundamental insights of Pt electrocatalysis and technological implications for PEFCs.The thesis is broadly divided into a theory part and an application part.The theory part concerns about a self-consistent treatment of the metal-solution interface.Chapter 2 presents a modified surface charging relation of Pt via refining the structural model for Pt-solution interface that considers the effects of oxide formation.The analytical solution of the model reveals a peculiar non-monotonic charging behavior.The Pt surface may(depending on solution pH)exhibit a negative effective charge in a low potential region,a positive charge in an intermediate potential region and a negative charge in a high potential region due to surface oxide dipoles.This non-monotonic behavior is in agreement with seminal experimental works that had remained hitherto unexplained.In Chapter 3,a unified kinetic and thermodynamic ORR model that self-consistently treats the trinity of adsorbed intermediates,surface charge and the ORR is developed.The model sheds new lights on the role of adsorbed intermediates in the ORR.In addition to the common perception of site-blockers,adsorbed intermediates can have positive effects on the ORR activity by enhancing protophilicity of Pt through shifting the surface charge density to negative values.The model also contributes new insights into root causes of the potential-dependent Tafel slope and the volcano plot.The application part consists of three chapters.Chapter 4 presents a pore-scale model for the ionomer-free UTCL.The refined structural model of the Pt-solution interface is applied and a new surface charging relation of the curved Pt wall is derived.The waterfilled pore with one opening interfacing with a polymer electrolyte membrane as a proton reservoir always possesses negative surface charges in the potential range of 0-1.0 V(RHE).Therefore,its proton conductivity can be 2-5 orders higher than that of pure water.An analytical expression for the ORR activity of the nanopore is obtained and parameterized using polarization data of an ionomer-free thin-film Pt electrode.This model resolves the long-standing puzzle of proton conduction in ionomer-free UTCLs.Extending from continuous Pt films to discrete Pt nanoparticles,Chapter 5 focuses on treating electrostatic reaction conditions around Pt nanoparticles located on a carbon surface in a discrete manner,taking into consideration specific surface charging properties of Pt and the carbon support.The model reveals a double-layer overlap regime in which the surface-specific ORR activity is significantly enhanced by decreasing the interparticle spacing,termed as “particle proximity effect”.This model presents the first theoretical explanation for the particle proximity effect.Chapter 6 presents physical impedance models of a Pt-solution interface,a waterfilled Pt nanopore and a porous Pt electrode,aimed at building diagnostic capabilities for determining structural and physico-chemical parameters of Pt electrodes from impedance data.The key message conveyed in this work is that electrostatic effects at the scale of the Debye length(1~100 nm)play an essential role in the understanding of mechanisms,processes and structure-activity relations of PEFC electrocatalysis,in addition to geometric and electronic factors at the atomic scale(0.1~1 nm),and mass transport factors at the porous electrode scale(>100 nm).