Preparation And Study Of Tungsten-based Composite Enhanced Catalysts For DFAFC And Their Electrocatalytic Properties

Direct formic acid fuel cell(DFAFC)system is considered to be one of the developing direction of fuel cells and has been received much concern.Compared with direct methanol fuel cell(DMFC),DFAFC has multiple advantages.For example,formic acid is non-toxic,inflammable,storage and transportation are safe,and it has higher electromotive force,lower crossover effect through the Nafion membrane.However,DFAFC can not meet the demands of the large-scale commercial production up to date.There still exist certain problems in both anode catalysts and cathode catalysts.On the one hand,one of the disadvantages of Pd as anodic catalyst is its long-term instability,and its catalytic activity needs to be improved as well.On the other hand,the high cost of platinum,low efficiency and the carbon support corrosion hamper their practical applications.So it is still a challenging job to develop a new type of anode catalysts with good catalytic activity,excellent tolerance to CO poisoning and high stability,and to develop a new type of cathode catalysts with good catalytic activity,low cost and high efficiency for oxygen reduction.There exists synergistic effect between Pt,Pd and WO3,so tungsten oxide(WO3)has its promotion effect on catalytic performance of catalysts,and tungsten oxide can reduce the cost of catalyst.In addition,the dispersion degree and particle size of the kinds of precious metal catalyst is related to the surface properties of carbon carrier material,which is closely related to the catalytic performance.Carbon nanotubes(CNTs)and graphene,as new type of carbon materials,have attracted considerable interests because of their unique properties such as high specific surface area,electrical conductivity,and good thermal and chemical stability.If being modified by WO3,CNTs and graphene can play multiple roles as a remarkable catalyst support candidate.They can not only promote WO3 particles with better distribution and more interfaces between WO3 and noble metals,but also largely enhance the overall conductivity and corrosion stability.The main achievements can be summarized as follows:(1)The composite WO3-CNTs supported Pt electrocatalysts were prepared by microwave-assisted polyol method.TEM results indicate fine Pt particles are deposited homogeneously on the surface of WO3-CNTs.The interaction between Pt and WO3 can bring about the high dispersion of the Pt nanoparticles and a narrow size distribution.The performances of the Pt/WO3-CNTs catalysts for oxygen reduction reaction(ORR)were studied and evaluated in acid media.Compared to Pt/CNTs catalyst,Pt/WO3-CNTs catalysts show higher electrocatalytic activities for ORR with lower overpotential,higher current densities and faster dynamic process(exchange current densities are higher).The enhancement of ORR activity on Pt/WO3-CNTs electrocatalyst can be attributed to the synergistic or promotion effect of WO3 on Pt.On one hand,the interaction between Pt and WO3 can bring about the high dispersion of the Pt nanoparticles and a narrow size distribution.Also,the modification of the electronic structure of Pt could decrease the adsorption strength of oxygen containing species,which can promote the ORR kinetics leading to an increase in the catalytic activity for the ORR.On the other hand,the hydrogen spillover effect between WO3 and Pt can accelerate the protonation of the O2 molecule,leading to enhance the rate of oxygen reduction on the platinum.Finally,the rotating disk electrode measurements suggest that a 4-electron reduction process takes place on the surfaces of the Pt/WO3-CNTs catalyst as well as Pt/CNTs.(2)The composite WO3-CNTs supported Pd catalysts were prepared by microwave-assisted polyol method,and the performances for formic acid electro-oxidation were studied.The effect of different contents of WO3 and carrier prepared under different calcination temperatures on the catalytic activity of Pd/WO3-CNTs catalysts were investigated.The electrochemical results show Pd/20%WO3-CNTs catalysts with carrier prepared at 400℃ show the best catalytic activity and stability.The peak current density of Pd/20%WO3-CNTs catalysts is 2.8 times as that on Pd/CNTs catalyst.Chronoamperometric curves and galvanostatic polarization curves show the dramatically improved stability of Pd/WO3-CNTs catalysts.CO stripping cyclic voltammograms show the excellent tolerance to CO poisoning as well.In addition,the charge-transport rate within the liquid film near the Pd/20%WO3-CNTs electrode surface is much higher than that on Pd/CNTs.The results illustrate that the addition of WO3 can promote the catalytic activity of Pd for the formic acid electro-oxidation.On the one hand,the hydrogen spillover effect of WO3 will accelerate the dehydrogenation of HCOOH on Pd and lead to the higher rates of formic acid electro-oxidation on the Pd/WO3-CNTs than on the Pd/CNTs.On the other hand,WO3 can decrease the adsorption strength of intermediate such as COad on Pd,and prevent the accumulation of poisoning intermediates,which promotes the oxidation of formic acid in the direct pathway.(3)Based on the above results,reduction graphene oxide(RGO)with higher specific surface area and more outstanding electrical conductivity was used as cabon material instead of CNTs.The composite WO3-RGO supported Pt catalysts were prepared and the performances for ORR were studied.Also,the effects of different contents of WO3 on the catalytic activity of Pt/WO3-RGO catalysts were investigated.Compared with Pt/RGO catalyst,Pt/WO3-RGO catalysts show enchanced catalytic activity for ORR in acid media,with lower overpotential,higher current densities and faster dynamic process.The enhancement of ORR activity on Pt/WO3-RGO electrocatalyst can be attributed to the synergistic or promotion effect of WO3 on Pt.Pt/20%WO3-RGO catalysts has the best performance,then Tafel kinetics,and the rotating disk and the Koutecky-Levich curve of Pt/20%WO3-RGO catalyst for oxygen reduction reaction mechanism research were investigated.Pt/20%WO3-RGO and Pt/20%WO3-CNTs catalysts were prepared by the same method and their performances for ORR in acid media were investigated.As a result,Pt/20%WO3-RGO catalyst shows better catalytic activity and stability in comparison with Pt/20%WO3-CNTs catalyst.The results show the RGO are better support than CNTs.On one hand,the two-dimensional structure of graphene with open surface as well as the formation of the open aperture system provide more transmission channel for reactant and product.On the other hand,a small amount of oxygen containing groups on the surface of RGO will provide a synergistic catalytic effect on Pt catalyst,so as to promote the oxygen reduction reaction.RGO surface groups on graphene may function as anchoring sites for Pt precursor to prevent the aggregation of the Pt nanoparticles during the catalytic process.Also,the π bonding strength of grapheme may lead to a strong metal-support interaction.In summary,Pt/WO3-RGO and Pt/WO3-CNTs catalysts have low cost and unique activity for ORR,making them interesting for the use in the cathode of the fuel cells.(4)Based on the above results,reduction graphene oxide(RGO)with higher specific surface area and more outstanding electrical conductivity was used as cabon material instead of CNTs.The composite WO3-RGO supported Pd catalysts were prepared by using potassium borohydride as reducing agent.The electrochemical results show significantly enhanced electrocatalytic performances for formic acid oxidation on Pd/WO3-RGO catalysts.The peak current density on the best catalyst Pd/20%WO3-RGO catalyst is 2.5 times as that on Pd/RGO catalyst.And more importantly,Pd/WO3-RGO catalysts show its dramatically improved stability and excellent tolerance to CO poisoning.The enhancement of activity on Pd/WO3-RGO electrocatalyst can be ascribed to the interaction between Pd and WO3.Pd/20%WO3-RGO and Pd/20%WO3-CNTs catalysts were prepared by the same method and their catalytic performance for formic acid electro-oxidation were investigated.It is found that Pd/20%WO3-RGO catalyst exhibits better catalytic activity and stability than dose Pd/20%WO3-CNTs catalyst.The results show the RGO are better support than CNTs.RGO surface groups on graphene can play a catalytic role as accelerating the removal of adsorbed intermediate products,so as to release more Pd activity,improve the utilization rate of Pd,resulting in enhanced theactivity and stablity of the catalyst.In summary,Pd/WO3-RGO and Pd/WO3-CNTs catalysts show the great potential as less expensive and more efficient electrocatalyst for DFAFCs.