Design And Construction Of Nitrogen-doped Porous Carbon/Transition Metal Composites For Electrocatalysts

As a new type of energy storage and conversion device,the low temperature fuel cell?LTFC?has been extensively used in aerospace,transportation,communication,portable power supply and other fields.For LTFCs,oxygen reduction reaction?ORR?which happens at cathode is considered to be the key steps in determining fuel cell performance,however the sluggish kinetic ORR requires noble metal catalysts?usually platinum and its alloys?to accelerate the catalytic process.It is well known that the reserves of platinum and other precious metals are limited,and thus the usage of expensive Pt metals prohibits the large-scale application of LTFCs.Besides,under harsh electrochemical environments,platinum catalysts are difficult to maintain the electrochemical stability at high potential,and the impurity gas in fuel can further reduce the activity of the platinum catalyst.In the past decades,researches on non-precious metal catalysts have made a significant contribution to the development of LTFCs,among which transition metal/nitrogen co-doped carbon materials are thought to be one of the most promising platinum-like catalysts for their excellent performance and stability.In this work,cobalt and molybdenum is selected as transition metal precursor separately,and then combined with the corresponding carbon materials.Finally,nitrogen doped porous carbon/transition metal composites for electrocatalysts are synthesized by high temperature heat treatment.First,assisted with Mg?OH?₂ nanocasting method,Co and N co-doped porous graphene-like carbon nanosheets?Co@N-PGCS?is successfully fabricated by using chitosan as both carbon and N sources.Based on the test results,Co@N-PGCS showsaremarkablehighspecificsurfacearea?1716cm²/g?with mesopore-dominatedstructure.Co@N-PGCSalsoactsasbifunctional electrocatalyst towards both the ORR and oxygen evolution reaction?OER?.Compared with commercial Pt/C,the onset potential and half-wave potential of Co@N-PGCS is-0.075 V?vs.Ag/AgCl?and-0.151 V?vs.Ag/AgCl?,separately,in0.1 M KOH solutions.As for OER performance,the potential of Co@N-PGCS is0.76 V,slightly lower than that of commercial IrO₂?0.70 V?at a current density of10 mA/cm².Extensive experimental and analytical results show that the electrocatalytic activity of Co@N-PGCS is derived from the introduction of Co elements,rich mesoporous as well as few-layer carbon coated structure.Later on,molybdate tetrahydrate,melamine and graphene oxide is chosen as matrix precursor,by using H₂O₂-assisted synthesis routes combining hydrothermal and high temperature pyrolysis methods,the porous N-doped graphene/molybdenum nitride composite?MoN@N-C?is obtained.The introduction of H₂O₂ plays an important role in regulating the particle size of MoN and the microstructure of the composite to achieve different configuration,accordantly enhancing the catalytic performance.When increasing the addition amount of H₂O₂ to 3 mL,MoN@N-C exhibits the best ORR performance in 0.1 M KOH solutions:the onset potential and half-wave potential of MoN@N-C is 0.921 V?vs.RHE?and 0.810 V?vs.RHE?,respectively,which is comparable to Pt/C.Besides,MoN@N-C shows higher electrochemical stability and better resistance to CH₃OH crossover and CO poisoning compared with Pt/C.