Cathode Electrocatalysts Of Direct Methanol Fuel Cell Based On Highly Stable Core-shell Nanoparticles

Direct methanol fuel cell (DMFC) are considered as the most promising energyconverting devices due to their environmentally friendly characteristics, high powerdensity and low operating temperatures. The electroatalysts are the key components forDMFC, which greatly influence working life, manufacturing costs and output power ofDMFC. Platinum (Pt) nanoparticles loaded on carbon supports (Pt/C) are commonly usedas the anode and cathode electrocatalysts for the DMFC. Nevertheless, the low durabilityof Pt/C, particularly under the cathodic conditions including high oxygen content andhigh potential, is considered as one of the primary limiting factors of commercializationof DMFC. The demands for expensive Pt also enhance the cost of DMFC.It is widely agreed that the Pt electrocatalysts loaded on highly stable supports withstrong anchoring effect for Pt nanoparticles can increase the durability of Ptelectrocatalysts. While preparing non-platinum electrocatalysts for cathode reaction(oxygen reduction reaction, ORR) with excellent activity and high stability is an effectivedirection for lowering the cost of DMFC. In this paper, novel core-shell structuralnanopaticles were prepared for highly stable cathode electrocatalysts using nanodiamond(ND) and nano-silicon carbide (nano-SiC) as basic materials, which have excellentphysical and chemical stabilities.1) Graphene layers on ND (ND@G) and nano-SiC (SiC@G) were produced under avacuum higher above10-2Pa at1300-1600C for1-2h. Owing to highly stable ND ornano-SiC as a core and few-layer graphene as a shell, the ND@G and SiC@G bothexhibited high stability and good electrical conductivity. The Pt electrocatalysts wereprepared using the ND@G and SiC@G as supports (Pt/ND@G and Pt/SiC@G). Theresults of accelerated degradation test (ADT) measured in0.5mol/L sulfuric acid (H2SO4)indicated that the Pt/ND@G and Pt/SiC@G showed better stability in comparison withPt/C because of high anti-oxidation and structural stability of the surpports. The catalyticactivities for ORR of the Pt/ND@G and Pt/SiC@G were similar with or slightly betterthan the Pt/C with tha same Pt loading. 2）An oxidized amorphous carbon nanoshell covered nano-SiC was prepared usingan acid-etching method at room temperature. Silicon atoms were selectively etched awayby the mixed concentrated hydrofluoric acid (HF) and nitric acid (HNO3). The surfaceoxygen functional groups absorbed on carbon nanoshell would provide more nucleationsites for Pt NPs and a strong interaction with them, which can limited the migration andagglomeration of platinum nanoparticles. In addition, highly stable SiC core can inhibitcorrosion and oxidation of supports. So the Pt electrocatalysts supported on the O-C/SiCshowed an excellent stability during ADT experiments.3) Novel core-shell structural non-platinum electrocatalysts with a nitrogen(N)-doped or iron, nitrogen (Fe-N)-codoped grephene shell covered on an ND core(N-ND@G or Fe-N-ND@G) were produced. The N-ND@G was obtained using a postdoping process for ND@G. The Fe-N-ND@G was directly prepared using ND as rawmaterial. The NDs deposited Fe(OH)3nanoparticles were mixed with melamine andtreated at850C in N2atmosphere. The Fe element can catalyze the graphitization of ND,and the codoping of Fe, N was realized during the graphitization process. Theelectrochemical results showed that the N-ND@G and Fe-N-ND@G exhibited highcatalytic activity for ORR in alkaline condition. The numbers of electrons transferred perO2molecule (n) were calculated as3.7and3.9at-0.2V (vs. Hg/HgO) for N-ND@G andFe-N-ND@G, respectively, indicative of a four electron process at high potential region.The half-wave potential of the N-ND@G electrocatalyst for ORR was0.12V (vs.Hg/HgO), which is lower than that of the20wt%Pt/C (0.052V) but far higher than theND@G. The enhancement in the catalytic activity for ORR was attributed to thesynergistic effect of the pyridinic N, pyrrolic N, and graphitic N dopings. The half-wavepotential of the Fe-N-ND@G electrocatalyst shifted to more positive direction (0.097Vvs. Hg/HgO), indicating a further improved catalytic activity, which is caused by the Fe,N codoping. Moreover, both core-shell N-ND@G and Fe-N-ND@G possess a highstability, so they are very promising non-platinum cathode electrocatalysts.