Design And Synthesis Of Highly Active Catalysts For Fuel Cells

In recent years,as fossil energy crisis and environmental degradation caused by burning fossil fuels are getting increasingly worse,the demand of human beings for sustainable energy which can replace the fossil energy becomes stronger.Therefore,fuel cells have attracted considerable attention because of their high energy conversion efficiencies and outstanding environmental friendliness.However,the existing commercial catalysts are far behind the demand for commercialization of fuel cells.Hence,it is critical for the future development of fuel cells to develop a new class of catalysts at low cost with outstanding activities and excellent stabilities.In this regard,a new kind of highly active fuel cell catalysts were designed and synthesized in this paper.On one hand,for catalyzing the oxygen reduction reaction in the cathode of proton exchange membrane fuel cells,monodispersed,uniform and spherical Pd_xNi_y nanoparticles were synthesized using a surfactant-based size-and composition-tunable protocol.The above Pd_xNi_y nanoparticles were also found to be pseudo-core-shell-structured which possessed Ni cores and Pd-enriched surface layers.Then,a Pt monolayer shell was generated on the surface of these nanoparticles employing a Cu underpotential deposition method followed by the displacement of Cu by Pt.Hence,a new class of catalysts?Pd_xNi_y@Pt/C?possessing Pd-Ni core and Pt monolayer shell was formed through the above two steps.Based on the results of our experiments,this kind of catalysts has extremely high ORR activity and admirable stability.After optimizing the composition of the catalysts,its area specific,Pt mass specific and noble-metal specific activity showed,respectively,2.8,11.2 and 3.3 times enhancement as compared with those of the commercial Pt/C catalysts.Electrochemical stability of the catalysts was tested in highly acidic environments utilizing the accelerated degradation tests,showing excellent performance.After 6000 and 12000 potential cycles of the accelerated degradation tests,the electrochemical surface area value of the catalysts remained 97%and 91%of the initial measured value,respectively.And the oxygen reduction reaction activity hardly declined even after 12000potential cycles.On the other hand,for catalyzing the formic acid oxidation in the anode of direct formic acid fuel cells,we designed and successfully synthesized carbon-supported Pd monolayer shell-Pt₃Ni core cubic nanoparticles?Pt₃Ni@Pd/C?by a two-step method,which mainly involved the CO-assisted preparation of cubic Pt₃Ni nanoparticles?NPs?and the Pd monolayer deposition through underpotential deposition of a Cu monolayer followed by the displacement of Cu with Pd.Inductively coupled plasma elemental analysis,X-ray diffraction and transmission electron microscopy measurements were performed to characterize the as-synthesized Pt₃Ni/C catalyst,which reveals that most Pt₃Ni NPs possess cubic nanostructure enclosed by{100}facets,on which Pd monolayer shells were deposited epitaxially via the electrodeposition method,thus acquiring the crystallographic orientation of{100}facets for the Pd monolayer.The performance of Pt₃Ni@Pd/C as the electrocatalyst for formic acid oxidation?FAO?was compared with those of the commercial Pd/C and the Pt₃Ni/C catalysts.Results showed that the Pt₃Ni@Pd/C possessed superior electrocatalytic performance towards FAO,owing to both its monolayer structure and exposed Pd{100}facets.The deposition of Pd monolayer shell on Pt₃Ni/C led to an improvement of 7.5 times in the noble-metal mass activity compared to the original Pt₃Ni/C catalyst.Moreover,the area specific and Pd mass activities of Pt₃Ni@Pd/C reached,respectively,2.5 and8.3 times that of the commercial Pd/C catalyst.