Preparation and Active Sites Studies of the New-Type Electrocatalysts for Fuel Cells

Author:Wang Zuo

Supervisor:chen shao wei


Degree Year:2019





Nowadays,the depletion of fossil fuels and environmental pollution that arises from the combustion of fossil fuels have rapidly become wordwide issues.The emergence of fuel cells can not only mitigate the environmental polution,but also serve as a sustaintable energy technology.In Proton Exchange Membrane Fuel Cells(PEMFCs),it directly converts the chemical energy of fuel molecules into electrical energy by oxidizing small molecular fuels at the anode and concurrently reducing oxygen molecules reaction at the cathode.Meanwhile,PEMFCs come with many advantages,such as high specific energy,mild working conditions,cleanliness,rapid start-up,easy assembly and maintenance,and have been hailed as a unique,effective solution for electric vehicles.Yet,for practical applications,a sufficiently high current density is needed.Noble metals(e.g.,Pt,Pd,Ru,etc.)and their alloy nanoparticles have been used extensively as the catalysts of choice for both the anodic and cathodic reactions.Of these,the sluggish kinetics of the oxygen reduction reaction(ORR)at cathode greatly limits the energy conversion efficiency of fuel cells.However,the high cost(catalysts account for more than 50%of the cost of the fuel cells),low durability and low poison resistance of these noble metal-based catalysts have been the main bottlenecks hindering the wide commercialization of PEMFCs.Therefore,it is of great significance to develop low-cost,high-performance catalysts with abundant materials,so as to alleviate the problem of large-scale application of PEMFC.The thesis includes the following three parts:In the first part,indium tin oxide(ITO)nanoparticles are synthesized in benzyl alcohol by hydrothermal reaction.Cl atoms in o-dichlorobenzene(ODCB)are activated by UV photoirradiation to form free-radicals,and react with hydroxyl groups on the surface of ITO nanoparticles.The photoconductive reaction results in the formation of chlorine-doped ITO nanoparticles(Cl-ITO).TEM measurements show that a series of physical parameters such as the size and structure of nanoparticles remain largely unchanged after the photoinduction reaction.Electrochemical measurements show that the electrocatalytic activity of Cl-ITO in ORR is substantially enhanced,as compared with that of the pristine ITO,within the context of diffusion-limiting current and the number of electron transfer involved in ORR.This is likely due to the formation of quasi oxygen-deficient sites on Cl-ITO surfaces that are advantageous for oxygen adsorption and binding on the surface of catalysts,which is the most important rate-determining step for ORR.Compared with the traditional oxygen vacancies formed by high temperature heat treatment or addition of other heteroatoms,results from the present work suggest a novel and facile method for the improvement of the ORR activity of a variety of metal oxides and regulation of the catalytic activity of metal oxides.In the second study,nitrogen and sulfur-codoped graphene composites with Co9S8(NS/rGO-Co)are synthesized by facile thermal annealing of graphene oxides with cobalt nitrate and thiourea in an ammonium atmosphere.Signifcantly,in the same electrolyte(0.1 M KOH aqueous solution),the best sample NS/rGO-Co4 exhibits an oxygen evolution reaction(OER)activity that is even superior to that of benchmark RuO2 catalysts.The sample also shows an oxygen reduction reaction(ORR)activity that is comparable to that of commercial Pt/C,and an overpotential of only-0.193 V to reach 10 mA cm-2 for hydrogen evolution reaction(HER).With this single catalyst for oxygen reversible electrocatalysis,a potential difference of only 0.700 V is observed in 0.1 M KOH solution between the half-wave potential in ORR and the potential to reach 10 mA cm-2 in OER.In addition,an overpotential of only 450 mV is needed to reach 10 mA cm-2 for full water splitting in the same electrolyte.The present trifunctional catalytic activities are markedly better than leading results reported in recent literature,where the remarkable trifunctional activity is attributted to the synergetic effects between N,S-codoped rGO,and Co9S8 nanoparticles.In the third study,three representative metal-free N-doped carbon materials are prepared according to the literature,including N-dopedporous carbon nanosheets(N-PC),three-dimensional N-doped graphene nanoribbonnetworks(N-GRW),and edge-rich N-doped graphene mesh(NGM).In the present study,prolonged potential cycling is conducted in O2-saturated 0.1 M KOH aqueous solution,where apparent attenuation of the ORR activity is observed,within the context of limiting current and onset potential.The attenuation trend of the limiting current is closely correlated with the diminishing content of graphitic N,as manifested in X-ray photoelectron spectroscopy measurements and Mott-Schottky analysis.In addition,the specific activity per graphitic N is found to be almost invariant within a wide range of potentials during prolonged potential cycling for all three catalysts.This is in good agreement with theoretical prediction,whereas no such a correlation is observed with pyrrolic or pyridinic N.Results from density functional theory(DFT)calculations show that the first-electron reduction on carbon atoms adjacent to graphitic N,which is the rate-determining step for the 4e-ORR process,exhibit a much smaller Gibbs free-energy change than that on carbons neighboring pyrrolic or pyridinic N.These results strongly suggest that graphitic N is responsible for the ORR activity of N-doped carbons in alkaline electrolytes.Results in the present work may offer a generic,effective paradigm in the determination of catalytic active sites in heteroatom-doped carbons and be exploited as a fundamental framework for the rational design and engineering of effective carbon catalysts.More importantly,it provides guidance for the design and optimization of efficient and long-life carbon-based catalysts for important electrochemical reactions in energy conversion and storage devices.