High-performance Non-noble Cathode Catalysts Forfuel Cells

Fuel cells, as a clean and efficient energy conversion device, is apromisingtechnique in tackling future global energy crisis. However,the sluggish cathode oxygenreduction reaction (ORR) not only limitsthe performance but also relies on precious andscarce platinum groupmetals (PGM), which hampers the further development andgenuinelypractical application of this technology. Much progress has been madetoreduce cost and improve performance of the catalysts by alloyingPGM with transitionmetals, but it will still eventually encounter thecost problem on the way ofcommercialization. Therefore, extensiveattentions gradually move to developnon-precious catalysts to replacecommercial state-of-the-art Pt/C catalysts.In view of low cost and facile preparation method, this work developed two kind ofnon-noble ORR catalysts with high performance and durability.Firstly, we designed a new N-dopedcarbon material with nanoporous coaxialnanocable structure asmetal-free ORR catalyst. CNTs were firstcontrollably coated witha carbonaceous layer via glucosepolymerization reaction under hydrothermal condition,then the product was then mixed withnitrogen-rich doping source (melamine),followedby heattreatment at certain temperature to obtain resulting catalysts. The nanoporouscarbon sheath not onlyimparts high surface area to the catalyst, but also allows thereactants toeasily access the catalytically active sites and facilitates mass transfer.Moreimportantly, the N-doping was in-situ achieved during theformation of nanopores athigh temperature, which allow us to achievehigh concentration of N-doping. The carbonnanotube cores provide athree-dimensional conductive network and guarantee thesufficientelectron conductivity for fast electron transfer. As a result,the designed catalystwith high nitrogen content and surface area was characterized by scanning electronmicroscopy (SEM), transmission electron microscopy (TEM), nitrogenadsorption-desorption isotherm and x-ray photoelectron spectroscopy(XPS)measurements. Moreover, it alsoshowed excellent electrocatalytic activity forORRcomparable to commercial Pt/C catalysts, as well as better durability and is one ofmost active metal-free catalysts.Then, although researches on non-noble catalysts have been studied extensively,the catalysts with comparative or even better catalytic performance than commercialPt/C were seldom reported. Herein, based on our understanding on excellent ORR catalysts, we introduced transition metal iron element into catalysts system andwell-known FeN4active sites were formed by heat treatment. Graphene-encapsulatedmetallic iron or iron carbide nanoparticles were created when iron content was changed.Thus, the resulting catalyst with FeN4active sites and these nanoparticlescompositionsshowed surpassing performance in alkaline electrolyte than that ofcommercial Pt/C. After applyingx-ray diffraction (XRD), TEM, XPS, and extendedx-ray absorption fine structure(EXAFS) characterization methods to explore themorphology, composition and local structure of catalysts, we inferred that metallic ironor iron carbide nanoparticles could modifythe electron structure of venerable FeN4sitesto ameliorate their activity. This mechanism could also elucidate the sources of highactivity of some reported catalysts, which consist of analogous composition andstructure to our catalysts. This may bring new sight into design promising catalysts asalternativefor Pt-based catalysts, even thoughit is still to be validated by theorycalculation.