Investigation Of Graphene Supported Nickel Matrix Composites Catalysts For The Oxygen Reduction

With the aggravation of the energy crisis and environmental pollution,scientists have been making great efforts to explore new clean energy systems to decrease the consumption of conventional fossil fuels.Fuel cells,due to their high energy density,low operating temperature,low emission of pollutants,have been regarded as one of the most promising power sources.But the high cost of platinum based catalysts for fuel cells is an obstacle for the scale application of fuel cells.On the other hand,the platinum is easily poisoned by CO and other intermediate of the fuels.To decrease the cost of catalysts and promote the commercialization of fuel cells,non-platinum based catalysts have aroused great interests among scientists.Until now,great efforts have been taken to explore alternative catalysts for oxygen reduction reaction（ORR）in fuel cells.In view of the excellent properties of graphene,in this work,we regard it as the supported material and combined it with transition metal compounds in order to explore the high activity and high stability catalysts for the oxygen reduction.In this work,the NiO is selected as the research object,and the GO supported Ni（OH）₂ is taken as the precursor for Ni O/rGO.The structure and morphology of the synthetized catalysts were characterized by X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,Raman specture,scaning electron microscope（SEM）and transmission electron microscopy（TEM）.The electrochemical catalytic performance of the catalysts for ORR were investigated by cyclic voltammetry（CV）,linear sweeping voltammetry（LSV）,Tafel,i-t curves and the mechanism of oxygen reduction reaction of the catalysts were tested with AC impedance spectroscopy,rotating disc electrode（RDE）and rotating ring disc electron（RRDE）tests.The specific content summarized as follows:（1）Firstly,the influences of treating atmosphere（air,argon and ammonia）on the structure and morphology of Ni（OH）₂/GO were investigated,so does the electrocatalytic performance for ORR.The results show that the obtain NiO by pyrolysising Ni（OH）₂/GO under the air was sheet-like while under argon and ammonia atmosphere was globular,otherwise,nitrogen atoms was doped on the reduced graphene oxide in the ammonia atmosphere.Electrochemical tests show that the current density and onset potential（about-0.13 V）of g-Ni O/N-rGO are close to that of commercial Pt/C（20%）catalysts and the catalytic performance of NiO/rGO is enhanced due to the doping of nitrogen which originates from the treatment atmosphere of ammonia.Morphology has little effect on the process of the oxygen reduction reduction.Both RDE and RRDE tests prove that the ORR major happens through 4-electron style in alkaline electrolyte with the catalysis of the g-NiO/N-r GO.（2）Secondly,to obtain higher catalytic activity,the catalyst of NiO/rGO modified with Pd was investigated,the main method is to disperse NiO/rGO in the aqueous solution of Pd salt,after that NaBH₄ was employed to restore the Pd to the NiO/rGO.Results show that the Pd and NiO nanoparticles were distributed uniformly on the reduced graphene oxide sheets,but compared with NiO,Pd nanoparticles are larger,mainly due to the reduction of NaBH₄ is too strong.The electrochemical tests show that after Pd modified NiO/rGO,both onset potential or current density have a certain increase according to the doped Pd nanopaticles which improving its conductivity and reducing the contact resistance of the electrode,hence,the catalytic activity for oxygen reduction was improved.The RDE and RRDE tests proved that the ORR mainly through 4-electron style in alkaline electrolyte but there was also a small amount of 2-electron style in alkaline medium.