Ir-based Catalysts For Hydrogen Oxidation Reaction Of Fuel Cells

The proton exchange membrane fuel cell(PEMFC) is considered to be the most promising chemical power source for electric vehicles and other civil occasion owing to its high energy efficiency, high power density and environmental compatibility. Currently, the most efficient and widely used PEMFC catalysts are Pt-based materials. The prohibitive cost and scarcity of Pt-based materials has stimulated extensive exploration of alternative low-cost non-platinum metal catalysts(NPMCs) with attractive performance for the commercialization of PEMFC. In recent years, a variety of NPMCs with high activity and stability have been reported for oxygen reduction reaction. Among various alternative NPMCs, the materials containing transition metal-nitrogen coordination sites(M-NX, M = Fe, Co, etc.) have been generally considered to be the most promising substitutes for Pt-based catalysts. For the anode side, however, few non-platinum catalysts with high efficiency for hydrogen oxidation reaction(HOR) have been reported. To ultimately solve the Pt-reliance of the PEMFC technology, developing a high efficiency and low cost non-Pt anode electrocatalyst is urgently needed.Ir has attracted considerable attentions as a promising Pt-alternative anode catalyst owing to its cheaper price and stability in acidic media. However, its catalytic activity is much lower than that of Pt. An approach to enhance the HOR catalytic activity of Ir-based nanocrystals is to appropriately modify their electronic structure and geometric structure by alloying with some transition metals, such as Co, Fe. A current challenge to alkaline polymer electrolyte fuel cells(APEFCs) is the unexpectedly sluggish kinetics of the hydrogen oxidation reaction(HOR). A recently proposed resolution is to enhance the oxophilicity of the catalyst, so as to remove the Had intermediate through the reaction with OHad, but this approach is questioned by other researchers. Here we report a clear and convincing test on this problem.In this thesis, we have carried out the following researches on basis of the above mentioned object.(1)We studied the alloying effect in Ir-based alloys on the catalysis of the hydrogen oxidation reaction(HOR) in both acidic and alkaline medium. IrFe, IrNi and IrCo alloy catalysts with similar alloying degree and average nanoparticle size of <5 nm were obtained by our “solvent-vaporization plus hydrogen reduction” method. Among them, the specific activity of 152 A/g Ir at 0.1 V versus RHE was obtained on IrNi/C, which was higher than that on IrFe/C(146 A/g Ir), IrCo/C(133 A/g Ir) and E-TEK Pt/C(116 A/gPt). In the catalysis of HOR in alkaline medium, the performance of the Ir-alloy catalysts was not better than that in acidic medium. The smallest lattice parameter and shortest Ir–Ir bond among these alloys were observed for the IrFe alloy and increased for IrNi and IrCo. The highest HOR activity on IrNi alloy in both acidic and alkaline medium among these alloy catalysts was mainly due to the mid-sized Ni-induced lattice contraction and the modified surface electronic structure that leads to an optimal interaction between the catalyst and hydrogen intermediates(Ir-Had or IrOH). In acidic medium, the weakening of the Ir–Had interaction caused by the electronic effect of alloying element, M(M=Fe,Ni,Co), is responsible for the enhancement of HOR activity. The oxophilic effect of the catalytic metal surface which affects OHad adsorption and desorption and surface Had coverage has a large impact on the HOR activity in the case of alkaline medium.(2)Two dealloying methods, electrochemical and chemical, were implemented to control the surface structure and composition of IrFe alloy nanoparticles and to improve performance for the HOR. We found that both dealloying methods yielded enhanced mass activities toward the HOR relative to Ir Fe/C and Ir/C. Dealloying has posed an electronic effect on weakening the Ir–Had interaction, as revealed by the voltammetric behavior and CO stripping, thus benefiting the oxidative desorption of Had which has been confirmed as the rate determining step of HOR in alkaline media. By using Ir/C, IrFe/C and dealloy IrFe/C(D-IrFe/C) as the HOR catalysts, the specific activity on D-IrFe/C reached 103 A/g Ir(at 0.1 V versus RHE) in alkaline medium, and it was 95 A/gIr and 71 A/g Ir on Ir/C and IrFe/C, respectively.