Advanced materials: breaking cocoon to turn Butterfly
proton exchange membrane fuel cell (PEMFC) which uses renewable hydrogen as fuel is a promising energy conversion device with the advantages of high energy density high energy conversion efficiency zero emission. In recent years nitrogen doped carbon based catalysts (m-n-c M: Fe Co or Mn) supported on single metal sites have become a research hotspot because of their excellent activity in acidic systems. However in practical hydrogen / air fuel cells the power density durability of MEA based on m-n-c catalyst are still difficult to meet the practical application requirements. The asymmetry of the performance is mainly due to the low utilization of effective active sites severe carbon corrosion poor mass transfer rate in the thick cathode layer. On the other h the Fenton reaction (Fe2 H2O2) initiated by Fe2 released from fe-n-c catalyst which has attracted much attention will cause the degradation of organic ionomers in the electrode proton membrane of PEMFC. Therefore the development of efficient PGM free Fe free catalysts the reasonable design of electrode structure are very important urgent for the large-scale application of PEMFC.
Professor Wu Gang's team of State University of New York at Buffalo Professor Feiling of University of Louisiana at Lafayette Dr. David A. Cullen of Oak Ridge National Laboratory Shawn of Carnegie Mellon University In order to solve the above problems Professor litster innovated the traditional electrospinning method selected ZIF polyacrylonitrile / polyvinylpyrrolidone (Pan / PVP) as the precursors successfully "weaved" a multi-stage porous nanofiber electrocatalyst with high-speed network structure. The catalyst not only showed excellent orr activity stability in acidic electrolyte but also showed excellent high power density high stability in fuel cell cathode.
systematically study the porosity morphology control. The author emphasizes the importance of polymer selection metal bonding heat treatment process for the preparation of efficient co-n-c catalysts. Firstly during the carbonization process pan forms a carbon skeleton while PVP acts as a sacrificial template porogen to promote the formation of interconnected multistage pore structure in the carbon skeleton. In addition PVP can be used as an additional nitrogen source to provide more n coordination to stabilize the Co atom; secondly a two-step heat treatment method is used. In the first step of low temperature heat treatment zif-8 pan were partially graphitized to form carbon fibers. In the second step of high temperature thermal activation more active con4 sites were embedded into the carbon fiber support. At the same time the graphitization degree graphite nitrogen content of the catalyst were increased thus improving the catalytic activity stability. By using the advanced nano CT imaging technology the authors have proved that the ionic polymer in the catalyst is uniformly dispersed in the whole electrode structure thus improving the utilization rate of effective active sites mass transfer rate of the catalyst. In different forms of accelerated stability tests the catalyst showed amazing stability durability. This improvement may be due to the strong interaction between the con4 site the carbon support as well as the highly graphitized corrosion-resistant carbon structure produced by the electrospun polymer. Based on the first principles DFT calculation it is found that the graphite nitrogen doping around the con4 site can increase the orr intrinsic activity of the co-n4 site by inducing higher charge density on the adjacent carbon atoms thus reducing the activation energy of o = O bond breaking during * ooh dissociation. The researchers of
believe that this study provides new ideas new concepts for the design synthesis of PGM free catalysts opens a door for the synthesis of new materials by traditional methods. All scientific research exploration is a vast bitter history; all challenges innovations will eventually break through the cocoon become a butterfly. These new materials will eventually realize the commercial application of PEMFC.
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