Study On Graphene Supported Anode Catalysts For Direct Borohydride Fuel Cell

Direct borohydride fuel cell is attracting more interest recently due to its highenergy density and fast anode kinetics. And borohydride is a fuel that is easily storedand distributed, chemically stable, and non-combustible. Besides, its oxidationproduct borate is environmental friendly and can be recycled to produce BH4–. Themost important factor to enhance the performance of fuel cell is to improve thecatalytic activity. Usually, the catalytic activity is depend on the catalyst itself, and itsparticle size, shape, size distribution, and dispersion on the catalyst support.Apparently, the catalyst support is an important aspect that will significantly influencecatalytic behavior of the catalyst.Graphene is the name given to a single layer of carbon atoms densely packedinto a benzene ring structure. Graphene sheets are one atom thick,2D layers ofsp2-bonded carbon atoms and it is a new type of carbon material. Graphene has highsurface area, superior electrical conductivity and low manufacturing cost, whichmakes it a promising candidate for fuel cell catalyst supports. As a result, we preparedgraphene supported anode catalysts for sodium borohydride fuel cell. And the papermainly focuses on:Au/G nanocomposites are prepared via a one-pot chemical reduction process inwater medium from graphene oxide and chloroauric acid precursors, using NaBH4asreducing agent. The obtained Au/G nanocomposites are characterized with physicaltechniques, such as XPS. Besides, the influence of sodium borohydride concentrationon the electroactvity of the synthesized Au/G nanocomposites is also discussed. Theresults show that the current density and stability of Au/G nanocomposites areenhanced with increasing NaBH4concentration. Although borohydride hydrolysis willbe enhanced in the process, the mass transfer and kinetics of NaBH4oxidation will beimproved. This indicates that the latter plays a dominating part.Graphene supported Pt nanoparticles are prepared in ethylene glycol solutionusing graphene oxide and H2PtCl6as precursors. The Pt nanoparticles areface-centered-cubic structure with mean diameter of3nm. Electrochemical testsindicate that the ECSA of Pt/G catalyst is larger than that of Pt/C, and the degradationof the Pt/G electrode is less after100cycles. Moreover, there is no difference for thetwo catalysts towards borohydride hydrolysis. But the corresponding peak of NaBH4oxidation on Pt/G electrode shifts to a more negative potential, and the current density is higher. The current density on Pt/G catalyst is1.3times of that on the Pt/C catalyst,indicating better performance of Pt/G catalyst toward borohydride oxidation. Whenemploying Pt/G or Pt/C as the anode catalyst in a direct borohydride-hydrogenperoxide fuel cell, the limited current density and maximum power density on Pt/Gnanocomposites are112mA cm-2and42mW cm-2, which are higher than those onPt/C nanoparticles (75mA cm-2and34mW cm-2).By changing the molar ratio of HAuCl4and H2PtCl6, Pt50Au50/G, Pt67Au33/G,Pt75Au25/G, and Pt/G nanocomposites are synthesized in ethylene glycol-watermedium. TEM results show that the addition of Au nanoparticles could lead to theformation of finer particles on graphene. Electrochemical tests indicate thatPt75Au25/G catalyst shows higher ECSA, larger current density, and better long-termstability. It is also found that only small incorporation of Au into Pt can improve thecatalytic performance of Pt-Au alloy. When employing Pt75Au25/G or Pt/G as theanode catalyst in a direct borohydride-hydrogen peroxide fuel cell, the open circuitpotential and maximum power density of Pt75Au25/G catalyst are1.7789V and48mW cm-2, respectively. But for Pt/G catalyst, the open circuit potential and maximumpower density are only1.757V and42mW cm-2, respectively.