Study On Electrocatalytic Activity And Reaction Mechanism Of M-apyr/C Catalysts In Alkaline Medium For Oxygen Reduction Reaction

Human beings are facing energy crisis and environmental pollution issues nowadays.Polymer electrolyte membrane fuel cells have two dominant advantages which are high energyconversion efficiency and zero emission, giving itself the potential to be new generation powersource and replace internal-combustion engine. Due to noble metal metal Pt applied on thecathode, high costs have became a key factor bloking large-scale practical application of fuel cells.Breakthroughs should be made in developing non-noble catalysts to replace Pt catylysts. Amongthose non-noble catalysts, carbon supported nitrogen doping catalysts has shown outstandingactivities and durabilites. Thus, a lot of interests have been focused on this class of catalyst.With commercial carbon black BP2000as carbon support, aminopyrine as nitrogen sourceand different kinds of metal salts as transition metal precursors, a series of M-Apyr/C catalystshave been successfully synthesized. ORR catalytic performance and reaction mechanism weredetected by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and rotating ring diskelectrode (RRDE) techniques. Furthermore, X-ray diffraction (XRD), Transmission electronmicroscopy (TEM) and X-ray photoelectron spectroscopy (XPS) has been applied to discover thefactors which can improve or reduce ORR activity. ORR mechanism can be further revealedaccordingly.Firstly, by comparing effects of different carbon supports on ORR activity, BP2000waschosen as the target carbon support. By optimizing carbon support proportion, catalyst sythesisedat the carbon support mass ratio of60%get the highest activity. By optimizing transition metalproportion, catalyst sythesised at the transition metal mass ratio of5%get the highest activity. Bycomparing ORR activity of catalysts synthesized at different temperatures,700℃was found to bethe optimal heat treatment temperature for M-Apyr/C.After that, Mn, Fe, Co, Ni, Cu doped catalysts C-Mn, C-Fe, C-Co, C-Ni, C-Cu and metalfree catalyst C-Metal free were studied. As a result, transition metal doped to enhance ORRactivity should follow the order of Co>> Fe~Cu> Mn>> Ni. Transition metal doped to enhanceselectivity should follow the order of Fe> Mn> Co>> Cu> Ni. XRD、TEM、XPS tests announced four main results:(ⅰ) During pyrolysis process, transiton metal elements bond to Sfollows the order of: Mn、Fe> Co> Ni、Cu;(ⅱ) During pyrolysis process, there existscompetition machanism between N and S, which means more transiton metal elements combinedwith S would reduce it’s combination with N.(ⅲ) How transiton metal elements are dispersedand distributed on the surface of the catslysts, and how were them combined with carbon supportshave critical impact on ORR activity.(ⅳ) Transiton metal elements and pyridinic-N formedM-Nx/C structures are believed to be the catalytic active centers of M-Apyr/C catalysts.Further more, Fe and Co are chosen as target transiton metal elements to study anion dopingeffets on Fe-Apyr/C and Co-Apyr/C catalysts. For Fe-Apyr/C catalysts, ORR activity follows theorder of: C-Fe(NO3)3> C-FeAc> C-FeSO4> C-FeCl2. Selectivity follows the order of: C-FeAc>C-Fe(NO3)3> C-FeSO4> C-FeCl2. For Co-Apyr/C catalysts, ORR activity and selectivity followsthe order of: C-Co(NO3)2> C-CoSO4~C-CoCl2. For anion doping effets on catalyst surfacestructure, ORR activity and selectivity, two main results could be drawn:(ⅰ) NO3-can iomprovethe metal elements dispersion and distribution, and facilitate their combination with carbonsupports. Thus, M-Nx/C structures are easier to form and ORR activity could be enhanced.(ⅱ)SO42-doping would bring competition between N and S, and depress the formation of M-Nx/Cstructures and further bring ORR activity loss.At last, C-Co(NO3)2was chosen as target catalyst to study catalyst loading effects. It wasfound that400μg cm-2is the optimal loading for ORR activity and600μg cm-2is the optimalloading for ORR selectivity. Durability test shows that after10000CV cycles, half wave potentialof C-Co(NO3)2just declined for31mV, indicating it’s favourable durability. Single cell testindicates that, MEA applied C-Co(NO3)2as cathode gives a power density of36.5mW cm-2atroom temperature and alkaline condition, which is comparable with the commercial Pt/C catalystbehavior obtained by our laboratory in the same condition.