Synthesis And Performance Of Ag₄Sn Intermetallic Electrocatalyst

Among many kinds of fuel cells, direct methanol alkaline fuel cell （DMAFC） hasthe advantages of simple structure, easy operation, and low operating temperature. Yetthere are still many problems hindering large scale commercialization of AFC, like theeasily-poisoned electrolyte, low diffusion rate of anions, crossover effect caused bymethanol, and high cost of Pt based electrocatalysts. Reducing platinum content orfinding alternative non precious materials have always been the hotspot inelectrocatalysts research. In this work, we focus on developing high dispersed, smallparticle size Ag₄Sn electrocatalysts for the cathode reaction in DMAFC, as known asthe oxygen reduction reaction （ORR）. In addition, the mechanism of such catalyticreaction is also discussed, in order to provide guidance for future Ag based catalystresearch.Three synthesis procedures are employed in this thesis: high temperature reactionusing NaBH₄as reducing agent, assisted with mechanical stirring （the product isdenoted as S1）; room temperature reaction using NaBH₄as reducing agent, assistedwith magnetic stirring （denoted as S2）; and low temperature reaction using NaBH₄asreducing agent, assisted with mechanical stirring and ultrasonic （denoted as S3）. Theinfluence of different metal precursors, surfactants, and carbon supports towardsproduct composition, morphologies, and electrocatalytic activity are also investigated.S1is prepared under70oC, using SnSO4and AgNO₃as precursors, PVP as surfactant,and the main composition is Ag₄Sn, while a small amount of Ag existed. Yetaccording to TEM results, the nanoparticles of as-prepared S1suffered from severelyagglomeration. S2is prepared at room temperature, using AgNO₃and stannousoctanoate as precursors,1,10-Phenanthroline monohydrate as surfactant. Based onXRD results, the product is also in consistence with JCPDS00-059-1151Ag₄Sn, yetalso suffering from serious particle agglomeration. S3is prepared under0oC, usingEG as capping agent. The high viscosity of EG at low temperature can dramaticallyslow mass transfer therefore prevent nanoparticles from instantaneously growing intolarge size and agglomeration. According to TEM results, S3nanoparticle with averagesize of approximately5.85nm uniformly dispersed on carbon support. Duringelectrochemical measurements, S3exhibits highest onset potential, largest limitingand kinetic current density among three samples. It is worth mentioning that the onset potential of S3is30mV higher than that of Ag/C sample, which prepared by the samemethodology, and the kinetic current density is4times larger than Ag/C at0.6V vs.RHE, indicating a enhanced electrocatalytic performance towards ORR after alloying.In this work, methanol tolerance is also examined. For Ag₄Sn/C, the injection ofmethanol did not arouse any obvious oxidation curve in CA results, which indicatesthat Ag₄Sn/C is high tolerant towards methanol. Meanwhile, i-t curve of Pt/C showeda high responsive current corresponding to methanol oxidation, indicating thatmethanol has a mixed potential effect on Pt/C catalysts. Stability is also with greatvalue for cathode materials. After operating the cell at0.713V for20hours, relativecurrent of Ag₄Sn/C S1, S2and S3droped to45%,68%, and88%, therefore, thesequence of stability is as following: S3>S2>S1. Ag/C prepared by method3exhibitsobvious decrease with a current loss of approximately73%, suggesting enhancedstability of Ag₄Sn/C intermetallic catalyst. Thus, Ag₄Sn/C is a potential cathodecatalyst for DMAFC applications.