Research On Pt/C Electrocatalyst And Membrane Electrode Assembly In Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cell (PEMFC) is one of the most promising cellsused for transportation and portable power sources, as it has prominent merits such ashigh efficiency, cleanness and et al. However, as the key materials and technologies ofPEMFC, Pt/C electrocatalyst which is widely used nowadays and the membraneelectrode assembly (MEA) are of high cost and show limited performance, which arethe main factors that hinder the industrialization and commercialization of PEMFC.So there are important values of practicality and theory for the research ofelectrocatalyst and MEA for PEMFC.In this paper, we have investigated a new reverse micelles method of Pt/Celectrocatalyst preparation for PEMFC. Owing to the use of a more scientific route,we have prepared Pt/C electrocatalysts with small particles of Pt crystallites,homogenously distributed Pt particles, low crystallinity of Pt crystallites and highelectrocatalytic activities. Compared with the old method, new method can preparePt/C electrocatalysts with improved practical performance.In this paper, many advanced methods are used to study and optimize theformation and characterization of the reverse micelles system, the preparation andcharacterization of Pt/C electrocatalysts. Many key factors which have influence onthe physical, chemical and electrochemical performance of the Pt/C electrocatalystsare investigated in this paper, which include types of surfactants, the mol ratio ofwater to surfactant, the surfactant concentration, the concentration of H2PtCl6 solution,the reduction reaction styles and et al.This paper also compares the Pt/C electrocatalyst prepared by the new reversemicelles method to that of the commercialization catalyst from Johnson Matthey thatstands for the leading level of the world. The results show that Pt/C electrocatalystsby self-made possess higher content of crystal facet of Pt (111), larger pore volume,larger specific surface area and pore diameter than that of the commercializationcatalysts. They show rather similar performance of electrocatalytic activity andworking performance of MEA.Using the new reverse micelles method, the particle diameter (3 – 8 nm) of Ptcrystallites in Pt/C electrocatalyst can be prepared and controlled effectively, by theadjustment of the mol ratio of water to surfactant and the surfactant concentration.The effect of particle diameters of Pt crystallites on the electrocatalyticperformance of Pt/C electrocatalysts is investigated in this paper. When Pt/C catalystsare used as cathode electrocatalysts for oxygen reduction reaction (ORR), theelectrocatalytic activities of electrocatalysts and working performance of MEAsimprove as the particle diameters of Pt crystallites decrease. However, when they areused as anode electrocatalysts, there is little influence on the electrocatalytic activitiesof electrocatalysts and working performance of MEA.We have disclosed the possible mechanism of particle size effect of Pt crystallitesin this paper. The results show that when particle diameter of Pt crystallites in Pt/Celectrocatalyst decreases, the relative crystallinity decreases, the defects on crystalsurface increase, the peak current of ORR increases, the peak potential of ORR movesto more positive, the electrochemical specific surface area of Pt particles increases,the resistance of electrochemical reaction of ORR decreases and thus theelectrocatalytic activity improves.Under the conditions of controlling the particle size of Pt crystallites, we haveinvestigated the effect of Pt content in Pt/C electrocatalyst on its electrocatalyticactivity and working performance of MEA. The results show that when Pt contentreaches 30%, Pt/C electrocatalyst possesses large electrochemical specific surfacearea and the MEA prepared with this catalyst shows good working performance.We have investigated the mechanism of the effect of some operation conditionsof MEA on its working performance by electrochemical impedance spectroscopy(EIS). Through the analysis of measured data and mathematical simulation ofequivalent circuit, we have investigated the dependence and its mechanism of theelectrode parameter, i.e. resistance of electrochemical reaction for both oxygen andhydrogen electrodes of MEA, ohm resistance and double layer capacity related to thestructure of MEA on the operation conditions, i.e. cell temperature, reaction gaspressure and cell working voltage.We have investigated the mechanism of activation for MEA in this paper. EISresults show that activation process reduces the resistance of electrochemical reactionfor the oxygen electrode and ohm resistance and increases the double layer capacityof both the oxygen and hydrogen electrodes greatly. So activation process increasesthe effective electrode area of MEA dramatically. XRD results show that the crystalstructure of Pt/C electrocatalyst has changed during the process of activation, i.e. thecrystal facet space distance of Pt (111) decreases, which favors for the improvementof the electrocatalytic activity of catalyst towards ORR.