Sdudy On Non-precious Metal TM-N/C Catalyst For Oxygen Reduction Reaction In Sulfuric Acid

A key to the large scale use of PEMFC is to develop an excellent ORR catalyst with low cost, better electrocatalytic activity, better methanol tolerance and good stability. Recently, much attention has been paid to TM-N/C catalyst because of its good ORR catalytic activity. The TM-N/C catalyst was first made from heat-treating transition metal macrocycle compounds. However, the macrocycle compound itself is expensive and the preparation process is complex in the traditional method. The active site in TM-N/C catalyst for ORR is controversial, too. In this paper, several cheap materials were innovatively used to synthesize the TM-N/C catalyst by a simple and convenient“total solid reaction”method. The effect of precursor composition, reaction temperature, reaction duration on the ORR catalyst was investigated in details. The structure of the catalyst was thoroughly characterized to analyze the nature of ORR catalytic activity site of the resulted catalyst.（1） Metallic Co was preciously reduced and deposited on the surface of carbon black, then mechanically mixed with urea and heat-treated at 800℃for 2 hours under N2 atmosphere to synthesize a Co-N/C catalyst. The results showed that the Co-N/C is more active than the single carbon black. The active site was formed in the heat-treating process through the modification of nitrogen atom to the carbon black. The process was facilitated by metallic Co. Both metallic Co and cobalt oxides are not the active site of the catalyst. The catalyst is good at the methanol tolerance.（2） Carbon black, CoCl2·6H2O and urea was mechanically mixed directly and heat-treated at high temperature to synthesize a CoUr/C catalyst. The Co（II） ion was reduced to metallic Co by carbon black in the heat-treating process, and the metallic Co with“fresh”surface is active to facilitate the conformation of ORR active sites. The activity of CoUr/C catalyst is excellent than Co-N/C catalyst. Heat-treating temperature obviously influences the ORR activity of the CoUr/C catalyst. The activity of catalyst heat-treated at a low temperature is worse. Because the formation rate of metallic Co is slow at a low temperature, which makes the formation point of ORR active site is lack and the ORR activity of the resulted catalyst is worse. At a suitable high temperature, the formation rate of metallic Co increases, the formation rate of active site for ORR also increases and the catalytic activity enhances. But the formed active site will be decomposed at a higher temperature. The formation of active site is controlled by the Co content. With a constant Co content in precursor, the activity of a catalyst from a lesser urea content is bad, because the formed active site is lesser. The activity of a catalyst from a excessive urea content is also bad. The reason is that the decomposed substances of urea covers the formed active site in the surface of carbon black and affects the performance of catalyst.（3） Urea was replaced by melamine as nitrogen source to synthesize CoMe/C catalyst by the same procedure as in CoUr/C catalyst. The results show that melamine is a better nitrogen source than urea. The ORR activity of CoMe/C is more excellent than the CoUr/C catalyst. The formation of ORR active site of CoMe/C catalyst is also facilitated by the reduced metallic Co. There exists pyrrolic form and pyridinic form C-N structures in the CoMe/C catalyst according to the information from N1s high resolution XPS spectrum. The two forms of C-N structure are the catalytic active sites for ORR. However, the pyrrolic form C-N structure is formed more easily than the pyridinic form C-N structure. The pyrrolic form C-N structure is also soluble in diluted H2SO4 solution and will easily be decomposed at higher temperature. At a higher temperature or in a longer heat-treating duration, there is much pyridinic form C-N structure generated in CoMe/C catalyst, and the catalytic performance improves, too. The metallic Co mainly promotes the formation of pyridinic form C-N structure. That is the reason why the activity of CoMe/C catalyst is more super than Me/C which was synthesized just by heat-treated the mixture of melamine and carbon black. The catalytic activity loss of the CoMe/C after leached in sulfuric acid solution is the result of the dissolution of pyrrolic form C-N structure. The stability of catalytic activity of the CoMe/C in diluted sulfuric acid is assured by the insoluble pyridinic form C-N structure. The pyrrolic and pyridinic form C-N structures in the CoMe/C catalyst will decomposed at a higher temperature or longer heat-treating time. The best condition synthesizing CoMe/C catalyst is heat-treating the precusor at 600℃for 2 hours.（4） Melamine was replaced by hexamethylenetetramine （HMTA） as nitrogen source to synthesize CoHMTA/C catalyst by the same procedure as CoMe/C catalyst. ORR activity of the resulted CoHMTA/C is more excellent than the CoMe/C catalyst. Similarly, nitrogen atom mostly exists as Co-N structure, pyrrolic and pyridinic form C-N structure in the CoHMTA/C catalyst heat-treated at a low temperature, the latter two C-N structure are both the active site for ORR. TM-N/C catalyst can be synthesized using different nitrogen source, which shows the“total solid reaction”method is interchangeable. （5） CoCl₂·6H₂O was replaced by NiCl₂·6H₂O to synthesize NiUr/C and NiMe/C catalysts using urea and melamine as nitrogen source respectively. The Ni（II） ion can be reduced to metallic Ni, but the metallic Ni can not effectively facilitate the formation of C-N active site of ORR. The NiUr/C and NiMe/C are not promising ORR catalysts.（6） CoCl₂·6H₂O was replaced by FeCl₃·6H₂O to synthesize FeHMTA/C catalyst using HMTA as nitrogen source. The results show that the FeHMTA/C is good at the ORR catalytic activity. At a low temperature or in a short heat-treating duration, the iron in precursor was first reduced to cementite Fe3C. The metallicα-Fe can well facilitate the formation of ORR active site by rising heat-treating temperature or prolonging heat-treating duration with the Fe3C transforming toα-Fe. The metallic Fe is more difficult to be reduced than metallic Co. The formation of active site of FeHMTA/C catalyst was controlled by the reducing of Fe（III）. A higher temperature or a longer reaction duration is necessary to get a better activity of FeHMTA/C catalyst. Increasing the content of iron in precursor can also increase the catalytic activity of FeHMTA/C. Although difficult reduce process, metallic Fe can better facilitate the formation of active site than metallic Co, and the catalytic activity of FeHMTA/C is better than CoHMTA/C.