Preparation And Electro-Catalytic ORR Properties Of Ni_core-Pt_shell Nano-Particle

The use of methanol as energy carrier and its direct electrochemical oxidation in a direct methanol fuel cell (DMFC) represents an important challenge for the polymer electrolyte fuel cell technology, since the complete system without a reformer and further gas treatment steps. However, their commercial viability is still hindered by some factors only in athode. One is the poor kinetics of the cathodic reaction and the high cost of Pt-Based electrocatalysts, The other is the methanol penetrated from the anode to the cathode through the polymer electrolyte leads to a mixed potential at cathanol. This effect led an increase in overpotential of ca. 0.2-0.3V. And the efficiency of fuel cell decrease to 3/4. Now, the research of new electrocatalysts for oxygen reduction reaction was very important, and must have two characteristic, firstly, the catalysts should show the lower cost and a higher catalytic activity of oxygen reduction. Secondly, the catalyst should possess the property ofmethanol tolerance(selectivity), to eliminate the effect of methanol.The M-Pt (M-transition metal) core-shell nanoparticles, which covered with Pt-shell, possess the property of Pt, the inductive effect of M, the characteristic of metal alloy(reaction activity, selectivity). And they were stable in strong acid electrolyte because of Pt shell. So the study on M_core-Pt_shell catalyst material is necessary.In this dissertation, Ni_core-Pt_shell nanoparticles with different thickness of Pt shell were prepared by two-step chemical reduction and nickel core nanoparticle was prepared by two reducing agents (NaBH₄ and N₂H₄·H₂O). TEM and SEM were used to observe the morphology of particles. Then the powders of products were characterized by EDS,XRD and XPS to study the elemental composition of surface and the microstructure. The catalytic activities and the methanol tolerance were characterized by electro-chemistry methods.The results show that:â‘ Ni powers prepared by NaBH₄ reducing agent were presence in amorphous or combining state and elemental crystal Ni weren't find; But elemental crystal Ni about 17nm was gained successfully by N₂H₄·H₂O reducing agent and it showed face-centred cubic structure. Pt shell was chemical deposited successfully on surface of the powers prepared by two reducing agents. Order of the electrocatalysis performance showed Ni₁-Pt₁(N)> Pt> Ni₁-Pt₂(B)> Ni₁-Pt₁(B) and this indicated that Ni(N₂H₄·H₂O) core was helpful to developed into an Ni_core-Pt_shell catalyst for electrocatalysis ORR activity and the methanol tolerance as compared with Ni(NaBH₄).â‘¡the globoid core-shell Ni-Pt nanoparticals prepared by N₂H₄·H₂O reducing agent were about 28nm in diameter. In the nanoparticals Ni core diameter and Pt shell thickness were about 17nm and 8nm respectively. Compared with Pt/C, Ni_core-Pt_shell/C have a higher electrocatalysis ORR activity and the methanol tolerance under the same acidic and methanol condition; The performance of Nicore-Ptshell/C with different thickness of Pt shell exhibited 'volcano-type' behaviour by the influences of Ni-core and it was accorded with the theory of "the d-electron inductive effect ". In the same electrolyte media, the order of the activities of catalysts is Ni₁-Pt₂> Ni₁-Pt₃> Ni₁-Pt₁> Pt> Ni₁-Pt_0.5 and Ni1-Pt2 was excellent.â‘¢The Ni_core-Pt_shell/C, which Ni/Pt atom ratio was 1:2, catalytic activity rise with density of sulfuric acid without methanol. when the methanol density was increased, catalytic activity and the methanol tolerance of Ni_core-Pt_shell/C were decreased under the same acidic condition.