Preparation of Transition Metal Nitride Based Catalysts as Low-Pt and Non-platinum Catalysts for PEMFC

Author:Tang Hai Bo

Supervisor:liao shi jun


Degree Year:2018





Proton exchange membrane fuel cells(PEMFC)with high energy yield,start quickly at low tempertuer,not rely on fossil fuels and low environmental impact has been focused by a huge number of researches,which are considered to be the most promising candidates for transport applications.However,the slow kinetics of the oxygen reduction reaction(ORR)at the cathode causes a large over-potential that significantly reduces its performance.A highly activity cathode electrocatalyst can promote the ORR and,therefore,result in a higher efficiency and improved fuel cell performance.Noble metals,especially Pt-based catalysts,are the most efficient catalysts for the ORR due to their high catalytic activity and ultralow overpotential.However,the tremendous scarcity and consequent high cost of platinum remain major barriers to the commercialization of polymer electrolyte membrane fuel cells.To address these problems,low-platinum and non-platinum catalysts have been widely studied in the past decade and great achievements have been obtained.Among them,transition metal nitrides(TMNs)have emerged as possible alternatives for the ORR or as supports for precious metals,in virtue of their excellent conductivity,and high corrosion resistance,which can effectively solve the poor stability of catalyst.However,the TMNs NPs lack the desired size and morphology controls,leading to low surface area and inefficient electron transport,so their catalytic potential may not be fully realized.Hence,new strategies to develop transition metal nitride based ORR catalysts remain greatly needed.(1)A type of Nb nitride material with a porous nanogrid morphology was prepared using a novel approach whereby a complex of Nb was prepared,followed by a nitridation process in NH3 flow.The catalyst exhibited good catalytic activity and high stability towards oxygen reduction.The effect that doping with a second transition metal had on the performance of the catalyst were also intensively investigated.We found that the ORR activity of NbN could be enhanced significantly by enriching the d electrons of Nb through doping with a second transition metal,and that doping with cobalt resulted in the best improvement.Our optimal catalyst,Nb0.95Co0.05N,had an ORR activity4.6 times that of NbN(current density@0.6V vs.RHE).XPS results revealed that Co doping increased the proportion of Nb in a low-valence state,which may be one of the most important reasons for the enhanced performance.Another important reason is the high surface area resulting from the porous nanogrid morphology.(2)A three-dimensional(3D)nitrogen-doped carbon-coated hierarchical Ti0.95Co0.05N composite was created by adding ethyl cellulose in nitrides precursor followed by nitriding.The catalyst exhibited significantly enhanced catalytic activity and excellent stability towards oxygen reduction.Our optimal catalyst,20%C-Ti0.95Co0.05N,had an ORR performance almost comparable to that of commercial 20%Pt/C in an alkaline medium,and a four-electron pathway was dominant during the ORR reaction.The significantly enhanced ORR performance of this catalyst is attributable to the following factors:the synergic effect of a carbon coating on the nitride nanoparticles(NPs),including the higher surface area and more active-sites exposure caused by the well-defined 3D structure resulting from the carbon coating;better mass transfer and electron transfer due to the carbon coating;the contribution of the doped carbon;and enhancement of the nitride’s activity by doping with a second transition metal.(3)3D porous Co-doped vanadium nitride(VN)nanosheet-assembled microflowers are prepared with a solvothermal approach followed by nitridation in NH3.It is found that the microflower morphology and the Co doping both significantly enhance the ORR performance of the materials.Since the unique 3D porous structure provides higher specific surface area and more active sites as well as enriching the d electrons of V via doping,Co also improves the intrinsic activity of VN.Our optimal V0.95Co0.05N microflowers achieve a half-wave potential for the ORR of up to 0.80 V in 0.1 M KOH solution,which is almost comparable to that of commercial 20%Pt/C.More importantly,the catalysts show superior durability with little current decline(less than 12%)during chronoamperometric evaluation for over 25,000 s.(4)NbMN@Pt was successfully prepared with a pulse deposition method and the influence of different preparation conditions on the performance of catalyst were also studied.On the above works,an ultra-thin Pt layer was deposited successfully on the NbN nanogrid with a pulse deposition method,and core-shell structured NbN@Pt were successfully prepared.It was demonstrated that the catalyst exhibited outstanding ORR activity and stability.In addition,the ORR activity of NbMN@Pt(M=Zn,Co,Ni ect.)and different deposition of platinum on NbMN were also investigated.The result shows that NbCoN@Pt exhibited best ORR activity,which was 2 times higher than that of JM Pt/C in mass activity.The Pt mass activity of nitride-based core–shell catalyst was completely comparable to activities reported for most core–shell catalysts with precious metal cores.The mechanism of performance improvement remain to be further in depth study.