| The direct formic acid fuel cell (DFAFC) is considered as a promising technology for automotive and portable electronic devices owing to its high power density and low operating temperature. The anode catalyst is core for DFAFC, so the development of high-efficiency anode catalysts for commercialization of DFAFC is very important. Pd catalysts are superior in formic acid electrooxidation compared with that of Pt catalysts. However, the catalytic activity and durability of the Pd-based catalysts still need to be substantially improved. The development of catalyst support materials is a effective method to improve the activity and stability of fuel cell electrocatalysts. This research is focused on the unfavorable factors, such as the high cost, poor activity and stability of electrocatalyst for DFAFC. Different catalyst supports on the catalyst performance are studied for developing anode Pd catalysts with high activity and high stability activity.The performance of metal carbides and nitrides as the Pd catalyst support are studied for formic acid electrooxidation. Metal carbides TiC, ZrC and metal nitride, TiN optimum ratios in the support are studied. It is found that the addition of metal carbides TiC, ZrC and nitride TiN in the carbon support improves the Pd nanoparticles dispersion on the support, makes the metal particle size smaller. It is found that strong metal-support interactions occur between metal Pd and support TiC, ZrC, TiN through XRD and XPS analyses. As the metal carbides and nitride TiC, ZrC and TiN have high resistance to oxidation and corrosion, good electrical conductivity, the activity and stability of the catalysts are improved significantly for formic acid electrocatalytic oxidation. The maximal catalytic activity and stability for formic acid eletrooxidation is observed on Pd/TiC-C catalyst with Pd:TiC mass ratio of1:2. The optimal mass ratio of ZrC to XC-72carbon is1:1for Pd/ZrC-C catalyst. The peak current density of formic acid electrooxidation on Pd/ZrC-C electrode is nearly1.61times of that on Pd/C. The optimal mass ratio of TiN to XC-72carbon is1:1. The peak current density of formic acid electrooxidation on Pd/TiN-C electrode is nearly1.54times of that on Pd/C.In order to further improve the activity and stability of Pd catalysts, metal oxide materials including α-Al2O3, WO3, TiO2and ZrO2are used to modify the carbon support. The Pd catalysts with high activity have been synthesized by microwave-assisted polyol process. The optimum ratio of the metal oxide and carbon supports is determined. The performance of the catalysts with same metal oxides and carbide as the suppot has been tested for the comparision. The activity of the catalyst with single metal oxide as the support is poor, owing to poor electrical conductivity of metal oxide. The mixture of metal oxide and carbon black used as catalyst support both improve the electronic conductivity of the oxide, but also enhance the hydrophilicity, wettability and corrosion resistance of the carbon material. So the catalyst performance is evidently improved. The addition of metal oxide enhances the Pd nanoparticles dispersion on the support, decreases the particle size of Pd particles. The activity and the stability of the catalyst with metal oxide α-Al2O3, WO3, TiO2, ZrO2and Vulcan XC-72carbon black as a mixture support is higher than that of Pd/C catalyst. The optimal mass ratios of α-Al2O3and ZrO2to XC-72carbon are1:2and1:1, respectively. The best content of WO3is20%of the support mass. It is found that the activity and stability of Pd catalyst supported on WO3/C composite support is better than that of Pd catalyst supported on WO3and carbon black mixed support for formic acid oxidation. By comparing with the activity and stability of the Pd catalysts, it is concluded that Pd catalyst supported on WO3/C composite support has the best catalytic performance among the above four oxide supports. The catalytic activity and stability of metal carbide/carbon supported Pd catalysts are better than that of the oxide/carbon supported Pd catalysts.Commercial ATO, ITO and CNTs, ie ATO/CNTs and ITO/CNTs are used as Pd catalyst supports. We studied the effect of doped metal oxide ATO and ITO in the support on the catalyst performance, and optimize ATO and ITO optimum contents in the supports. The peak current densities of formic acid electrooxidation on Pd/ATO with single doped metal oxide ATO as the support is comparative with that of Pd/CNTs catalyst, indicating doped metal oxide ATO has significantly improved conductivity. The catalytic activity, stability and resistance to CO poisoning capacity of Pd/ATO-CNTs catalyst for formic acid electrooxidation are significantly higher than those of Pd/CNTs. Aging test results reveal stronger stability of ATO support than CNTs, stronger stability of Pd/ATO-CNTs catalyst than Pd/CNTs. ATO addition inhibits the migration and agglomeration of Pd nanoparticles. The desired content of ATO is90wt.%of the support mass. When the ITO content is50wt.%of the support mass, the peak current density of formic acid electrooxidation on the Pd/ITO-CNTs is maximal,1.5times than that of Pd/CNTs. The addition of ITO significantly improves the stability of the Pd catalyst. After500cycles, the peak current density on the Pd/ITO-CNTs decreases by only13.98%, while56.13%on the Pd/CNTs. For comparison, it is found Pd/ITO-CNTs catalyst has better activity and stability for formic acid oxidation than Pd/ATO-CNTs.The metal oxide TiO2support is modified as TixWyO2to increase conductivity by doping transition metal tungsten ions. The optimum ratio of Ti and W atoms in the TixWyO2support is studied, and the calcination time in the support synthesis process is optimized. The catalytic performance of TixWyO2supported Pd catalyst for formic acid electrooxidation is studied. The Pd catalyst supported on Ti0.7W0.3O2prepared with Ti and W atomic ratio of7:3by the calcination time of5h exhibits the best catalytic activity and stability for formic acid electrooxidation. W ions doping has great impact on Ti electronic structure, leading to significantly improved conductivity of TiO2. The interaction between metal Pd and Ti0.7W0.3O2support is stronger than that between Pd and TiO2. The catalytic activity and stability of the Pd/Ti0.7W0.3O2catalyst, compared to Pd/TiO2, have greatly improved. The peak current density of formic acid oxidation on the Pd/Ti0.7W0.3O2electrode is5.47times of that on Pd/TiO2,1.24times of that on Pd/C. The significant negative shifts of the onset potential and peak potential indicate that formic acid on Pd/Ti0.7W0.3O2electrodes is more easily oxidized. The peak current decay rate on the Pd/Ti0.7W0.3O2catalyst is only0.5times as much as on the Pd/C after500cycles. |
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