The Study Of PtM-Ag/C(M=Cu, Co, Ni) Cathode Catalysts For PEMFCs

Proton exchange membrane fuel cell(PEMFC) is a device that can convert the chemical energy of the hytrogen and oxygen directly into electric energy. The catalysts of PEMFC have seperated as two parts: cathode catalyst and anode catalyst. They can catalyze hytrogen and oxygen separatly in the electrode reactions. Because the cathodic overpotential is higher than the anodic overpotential, the cathodic reaction is harder to start than the anodic reaction. So the cathode catalyst needs to be more active and the loading amount is higher than the anode catalyst. Recently, the most widely used and well-performed cathode catalyst is still the Pt/C catalyst. But as a noble metal Pt is widely used in many other fields, so the cost of Pt catalyst is quite high. The high cost of PEMFC is the main reason that prevents its commercial application. There are two ways to reduce the cost: low Pt-loading alloy and non Pt catalyst. Pt alloy still plays an important role in reducing Pt loading and improving the utilization of Pt. In recent years, the PtCu, PtCo and PtNi alloys had been prepared and all had excellent catalytic activity. By analyzing the binary phase diagrams of the five elements: Pt, Cu, Co, Ni and Ag, ternary alloys among them have been found, which may have better catalytic activty than the binary alloys. So in this paper we prepared three series of PtCuAg/C, PtCoAg/C and PtNiAg/C with different atomic ratios by chemical reducing, and discussed the effects of the addition of Ag into the PtCu/C, PtCo/C and PtNi/C catalysts.First, Pretreatment was done for the Carbon XC-72 to improve its surface activity, enhance the water solubility, dispersibility and the adsorption of the alloy particles.Second, PtCu/C and PtCuAg/C catalysts with different atomic ratio of Pt:Cu:Ag were tested by CV and LSV. When the atomic ratio is 3:10:1, the PtCuAg/C has the best catalytic activity. The size of PtCuAg/C(3:10:1) particle is around 3nm and quite uniform by XRD and TEM. The XPS test shows that PtCuAg/C(3:10:1) catalyst contains more CuO than PtCu/C, and that the CuO can be dissolved with the CV cycling, then more Pt atoms will be exposed on the surface. So the Pt utilization will be improved and the ORR limited current will be enhanced, the catalytic activity is improved. After heat treatment with 700℃ for 2h in Ar, the structures of the PtCuAg/C catalysts damaged heavily, tested by the CV and LSV. Their catalytic activity became very low. It inferred that the Ag atoms imerged on the surface, and the Pt atoms were wrapped in the inner part of the catalysts.So the Pt atoms cannot show any catalytic activity.Third, PtCo/C and PtCoAg/C catalysts with different atomic ratio of Pt:Co:Ag were tested by CV and LSV. When the atomic ratio is 3:10:0.5, the PtCoAg/C has the a higher onset potential(E) and limited current. The size of PtCoAg/C(3:10:0.5)is around 2-3 nm and quite uniform tested by TEM and XRD. The XPS test shows that the Co element on the surfaces of the PtCoAg/C(3:10:0.5) and PtCo/C both are CoO. But the CoO of the PtCoAg/C(3:10:0.5) has lower activity, so it will be disolved less slowly with the CV cycling. It is benefit for the stability of the catalyst. And with the disolution of CoO, the catalytic activity will be improved further.Fourth, PtNi/C and PtNiAg/C catalysts with different atomic ratio of Pt:Ni:Ag were tested by CV and LSV. But it proved that the additon of Ag can not improve the catalytic activity of PtNi/C catalyst. It means the Ag atoms do not appear on the surface of the alloy. Because the XRD pattern is nearly close to the pure Pt, the Ag element is in the inner part of the alloy.Last, the right amount of additon of Ag element can improve the catalytic activity of PtCu/C and PtCo/C and will reduce the catalytic activity of PtNi/C catalyst. The Ag addtion can reduce the alloy particle size, affect the surface composition and the activity to improve the catalytic activity further.