Preparation Of PT Nanoparticles Based On Reduced Graphene Oxides And Study On Their Methanol Oxidation Reaction

Recently, direct methanol fuel cells （DMFCs） are booming as a future energy supplier, which is precisely to cater for the demand on energy and environment. Although great effort has been put into DMFC development,to date, DMFCs have two main obstacles that limit their commercialization:low efficiency and high cost.As it is well-known, Pt is one of the most effective catalysts used for the methanol oxidation reaction （MOR） in DMFCs. However, MOR is sluggish because it takes numerous paths to convert methanol into CO2 via several electrons transfer. In addition, many paths will produce CO intermediates due to incomplete oxidation of methanol.This CO species will be adsorbed on the Pt plane and quickly inactivate Pt catalysts. The CO generation is thermo-dynamically favored at low potentials, and its oxidation will be associated with Pt crystalline structure. Among low index crystal Pt planes, the onset potential of CO oxidation on Pt（100） facets is lower than that on Pt （111） facets. However, the Pt （100） facet is more susceptible to inactivation due to stronger adsorption of CO intermediates on Pt （100） facet. Therefore, high prominence of the Pt （111） facet is highly recommended for the structure design of Pt catalysts to improve MOR activity and CO tolerance.On the other hand, Pt nanoparticles will be loaded on carbon materials to make Pt catalysts more active and economical. Among carbon supports, graphene is of grea application interest and theoretical significance due to its low cost, high elasticity, excellent conductivity, huge surface area and robust electrochemical stabilities. Thus, graphene can be used as an atomic-thick carbon support to load Pt catalysts. However, graphene is unstable and non-dispersible in most solvent. Graphene functionalization is indispensable to deposit Pt nanoparticles uniformly on graphene with a tunable size. Non-covalent adsorption is one of effective strategies in graphene modification. It can supply adequate anchoring points for Pt without harm to graphene, which is important for Pt catalysts to avoid catalysts failure from the corrosion of graphene caused by covalent grafting or oxidation. Moreover, Pt/graphene hybrids will restack to form sandwich structure in the air-drying process, which can prevent Pt nanoparticles detaching from or agglomerating on graphene surface during long-term operation.In order to improve the electrocatalytic activity of Pt-based catalyst for DMFC, we add stabilizer or transition metal to control the size and morphology of Pt nanoparticles supported on reduced graphene oxide （RGOs）。The main points were described as follows:（l）We demonstrate a one-pot thermoreduction approach towards the preparation of single-crystal Pt nanoplates, which were uniformly deposited on the reduced graphene oxide （RGO） using polyvinylpyrrolidone （PVP） as a stabilizer. The size of Pt nanoplates can be tuned from 6.8 to 10.1 nm by controlling Pt loading. The as-prepared Pt/PVP/RGO catalysts show high stability and activity towards the methanol oxidation reaction （MOR）. Their MOR current can reach up to 401 mA mg-1Pt and MOR current can maintain 89.4% of its initial value after 10000 potential cycle.（2）Pt nanoparticles （NPs） were uniformly deposited on the reduced graphene oxides （RGOs） with assistant of MoO₃ by one-pot thermo reduction strategy. Comparison is made between Pt-MoO₃/RGO and Pt/RGO catalysts. MoO₃ can reduce the size of Pt NPs on RGOs, and Pt NPs can be averaged to be 3.0 to 4.1 nm depending on MoO₃ loading. Without MoO₃ the size of Pt NPs can reach up to 15.2 nm.MoO₃ in Pt-MoO₃/RGO catalysts conducts a surface-confined reversible electron transfer, which can make Pt/RGO catalysts more resistant to CO poisoning and more active towards methanol oxidation reaction （MOR）. The Pt-MoO₃/RGO catalysts with 16.5% MoO₃ loading show a largest MOR current up to 610 mA·mg-1 Pt with a smallest deteriorate rate of 0.000425 s-1 polarizing for 5000 s at 0.65 V.（3）A one-pot thermos-reduction strategy was developed to prepare ultrafine PtNi alloy nanoparticles （NPs）, which were uniformly deposited onto reduced graphene oxides （RGOs） in the presence of polyvinylpyrrolidone （PVP）. The size of Pt NPs on RGOs can be tuned from 2.5 to 5.4 nm by Ni alloying with Pt/Ni atomic ratio from 2:1 to 5:1. The electrochemical investigation demonstrates that Ni modification can promote the electrocatalytic activity of Pt catalysts on oxidation of methanol with a mass current density up to 1065 mA·mg-1, which is 2.65 times that of Pt catalysts without Ni modification, and comparable to and even better than those of the Pt-based catalysts reported recently.