Performance Of Oxygen Electrocatalytic Reduction On Core-Shell M-Pt(M=Fe, Co, Ni) Nanocatalyst

The combustion of fossil fuel causes serious pollution around the world. The fuel cell because of its high energy transforming efficiency and environmental friendship has been paid more and more attention. Among the fuel cells, people attach more important to direct methanol fuel cells. However, in DMFC the methanol crossover from the anode to the cathode through the polymer electrolyte leads to a mixed potential at the cathode. This effect causes a significant decrease in performance for oxygen reduction. Even under the open-circuit condition, the over-potential for oxygen cathode in DMFC is around 0.2⁰.3 V due to the mixed potential effect. Their commercial viability is still hindered by some factors, including the poor kinetics of the cathodic reaction. Aiming to increase catalytic of oxygen reduction and to lower the cost of the catalysts, or to enhance the methanol tolerance, it is very important to study the the oxygen reduction reaction at present.Compared with alloy catalysts, M_core-P_tshell/C catalysts have higher catalytic activity for the oxygen reduction. On the one hand, the Pt at the shell can prevent the dissolution of transition metals from M_core-P_tshell/C, increase the surface area and improve the utility of Pt; on the other hand, using M_core-P_tshell/C as the electrocatalyst of ORR, the decay of the electroactivity may be inhibited, which may be attributed its unique electric characteristic. In this work, The M_core-P_tshell(M=Fe, Co, Ni) catalysts with various shell thicknesses were prepared by two-step chemical reduction and redox-transmetalation(electroless deposition) two methods, their electrocatalytic activity for oxygen reduction and methanol resistance were tested, and the difference of their structures were compared and analyzed. The obtained results were shown as follows:M_core-P_tshell(M=Fe, Co, Ni) with different shell thickness was prepared by two-step chemical reduction; their structure and morphology were characterized by XRD, EDS and TEM. The test results showed that Fe_core-P_tshell, Co_core-P_tshell and Ni_core-P_tshell had narrow size distribution, with a mean particle size of about 50 nm, the Pt shell showed face-centered cubic(fcc) crystal, and the Pt shell thickness decreased with the decrease of the precursor atomic ratio. M_core-P_tshell/C with various _core elements had an optimum atomic ratio in this experiment. With the decreasing of Pt shell thickness, the oxygen reduction activity of Fe_core,x-P_tshell,y/C was first ascended and then descended, especially Fe_core,1-P_tshell,0.5/C was the best, the peak current of Fe_core,1-P_tshell,0.5/C reached 184.7 mA?mg-1, which was 1.5-fold higher than that of Pt/C electrocatalyst; the activity of Co_core,x-P_tshell,y/C presented a decreasing tendency, the peak current of Co_core,1-P_tshell,2/C was 179.1 mA?mg-1, which was 1.4-fold higher than that of Pt/C; the activity of Ni_core-P_tshell/C showed an increasing tendency, Ni_core,1-P_tshell,0.5/C was the best, its peak current was 1.1-fold higher than that of Pt/C. With the decrease of shell thickness, the methanol resistance of Fe_core-P_tshell/C, Co_core-P_tshell/C, and Ni_core-P_tshell/C increased, especially Ni_core,1-P_tshell,0.5/C and Fe_core,1-P_tshell,0.5/C had the best results.Aming to decrease the partical size and reduce the utility of Pt, this paper put forward the redox-transmetalation method which could prepare _core-shell bimetallic nanocatalyst. M_core-P_tshell(M=Fe, Co, Ni) with different shell thickness was prepared and characterized by XRD, SEM and TEM. The test results showed the M_core-P_tshell nano-particles were spherical, the mean particle size of M_core-P_tshell was about 10 nm, which was consistent with the CV results, and the shell thickness of Fe_core-P_tshell, Co_core-P_tshell and Ni_core-P_tshell were 3.27 nm, 1.52 nm and 2.34 nm. The results showed that the ORR peak current density of Fe_core,1-P_tshell,0.5/C was 193.7 mA?mg-1, the peak current density of Co_core,1-P_tshell,2/C and Ni_core,1-P_tshell,0.5/C were 228.4 mA?mg-1, 202.3 mA?mg-1. Relatively speaking, the catalytic activity of Cocore,x-P_tshell,y/C was the best. The peak current density for methanol oxidization on M_core-P_tshell/C was only around 10 mA?mg-1, they all had methanol-resistant property. The difference of their catalytic activity related to geometrical effect and electronic effect between the core elements and Pt shell. Compared with the previous method, M_core-P_tshell/C catalysts showed an enhanced activity and methanol resistance performance. The higher electro-activity on M_core-P_tshell/C was deduced to its larger Pt electrical surface area.The methanol concentration had a distinct impact on the activity of oxygen reduction on M_core-P_tshell/C. The results indicated that the higher the methanol concentration, the lower activity of oxygen reduction on M_core-P_tshell/C, which indicated that the presence of methanol have certain constraints on activity of oxygen reduction. Compared the condition without methanol, the ORR peak current density of Fecore,1-P_tshell,0.5/C prepared by two-step chemical reduction decreased by 32.8%, while the peak current density of Fecore,1-P_tshell,0.5/C prepared by redox-transmetalation decreased by 20.1% in the solution with 1.0 mol·L-1 CH3OH.