Preparation And Electrocatalytic Performance Of Nitrogen-doped Carbon-based Non-precious Metal Catalyst

Proton exchange membrane fuel cells(PEMFCs)have been considered the ideal clean energy conversion device due to the advantages of high power density,low emissions,low operating temperature,fast start-up,simple system structure.The oxygen reduction reaction(ORR)process at the cathode is much slower than the oxidation reaction process at the anode.So ORR process at the cathode is the main reason for hindering the further development of PEMFCs.The cathode catalysts of PEMFCs mainly adopted the platinum based catalysts at present.However,the major drawback of the platinum based catalysts is their limited availability,high cost and low tolerance to CO poisoning.It is significant to explore non-precious metals catalysts with low cost,high ORR catalytic activity and stability to promote the commercialization process of PEMFCs.Nitrogen-doped carbon based catalysts(such as Fe-N-C catalysts)possess great potential for ORR catalyst in fuel cells application due to their high efficiency,durable performance and low cost among various non-precious metals catalysts.These catalysts are obtained via pyrolysis of the precursor containing nitrogen source,carbon source and transition metals.PANI has widely used as the precursor of nitrogen-doped carbon based catalysts due to its advantages of unique doping characteristics,controllable microstructure and cheap availability.Introduction of transition metals(such as Fe)can promote the generation of ORR catalytic activity site and markedly improve the ORR catalytic activity of nitrogen-doped carbon based catalysts during the heat treatment.Different transition metal have different effects on the ORR performance of nitrogen-doped carbon based catalysts.Metal salts are composed of metal cations and anions.The common cations of iron salts are Fe2+ ions and Fe3+ions.The common anions of iron salts are Cl-,SO42-,NO3 and CO42-.PANI doped by different valence Fe ions can form different PANI-Fe precursors.On the other hand,PANI doped by different anions can form heteroatom(such as Cl,S and N)doped carbon based catalysts during the heat treatment.So different precursors prepared by different Fe salts can achieve the improvement of the ORR catalytic activity of Fe-N-C catalysts through both introduction of Fe and heteroatom doping at the same time during the heat treatment.The Fe-N-C catalysts prepared via different Fe salts precursors have different ORR performance due to the difference of Fe valence and anions..Therefore,it is important to study the influence of different Fe salts on the structure and ORR performance of Fe-N-C catalysts and further explore the ORR mechanism of nitrogen-doped carbon based catalysts.In this paper,different Fe salts were added into the solution and different PANI-Fe polymer were synthesized via chemical oxidation method during the the process of the aniline polymerization.Fe-N-C catalyst has been prepared through heat treatment of the PANI-Fe polymer as the precursors in nitrogen gas.We studied the influence of different Fe valence and anions on the structure and ORR performance of Fe-N-C catalysts.TEM,Raman,XRD and XPS were used to characterize the morphology,the crbon structure,the crystal structure and the surface chemical state of N,of the Fe-N-C catalysts,respectively.The ORR performance of catalysts were evaluated by RDE technology.We further studied the ORR mechanism of Fe-N-C catalysts.In addation,We take the best ORR performance Fe-N-C/H4FeNO8S2 catalyst as an example.PANI-Fe polymer obtained by chemical oxidation method and the mixture of Fe and PANI obtained by physical method have been used as the precursors,respectively.We simply study the influence of both chemical oxidation method and physical mixing method on the the structure and ORR performance of Fe-N-C catalyst.ICP-AES,TEM and XPS were used to characterize the structure of the PANI and PANI-Fe polymer,the iron concentrations,the morphology,and the chemical state of N,respectively.The ORR performance of catalysts were evaluated by RDE technology.We further studied the ORR mechanism of Fe-N-C catalysts.Results indicate that:(1)The electrochemistry measurements of Fe-N-C/FeC/2,Fe-N-C/FeCl3,Fe-N-C/H8FeN2O8S2 and Fe-N-C/H4FeNO8S2 catalysts indicated that the ORR performance of Fe-N-C catalysts obtained via ferric salts was better than that of Fe-N-C catalyst obtained via ferrous salts.However,the electrochemistry measurements of Fe-N-C/FeSO4 and Fe-N-C/Fe2(SO4)3 had the opposite result.(2)The anions had a significant impact on the ORR performance of Fe-N-C catalysts.For ferrous salts,Fe-N-C catalysts toward the ORR as follows:Fe-N-C/FeSO4>Fe-N-C/FeCl2>Fe-N-C/H8FeN2O8S2>Fe-N-C/FeC2O4;For ferric salts,Fe-N-C catalysts toward the ORR as follows:Fe-N-C/H4FeNO8S2>Fe-N-C/FeCl3>Fe-N-C/Fe(NO3)3>Fe-N-C/Fe2(SO4)3.(3)Fe-N-C-p catalysts prepared by physical mixing method contained much Fe.However,Fe-N-C-c catalysts prepared by c chemical oxidation method contained trace Fe.We could find a lot of clear metallic particles with crystallite structure covered by carbon layer in Fe-N-C-p catalysts.While there were not clear crystallite structure of Fe species in Fe-N-C-c catalysts.The Fe-N-C-c catalyst exhibited excellent electrocatalytic performance for ORR in acid media,which can be attributed to higher ratio of graphitic N than that of Fe-N-C-p catalysts.