Synthesis And Performance Of The Pd-based Modified Catalysts Used In Fuel Cell

Direct fuel cell has the advantage of being clean,highly efficient,safe,portable and mild operation condition requiring,however,as most direct fuel cells are based on noble metal Pt,the high cost has become the main problem in marketization of fuel cell.Hence,to study and prepare a kind of catalyst with both high catalytic ability and low load of noble metal is of great importance of widespread utilization of fuel cell.Because Pd has the similar physical properties with Pt while less expensive in price,Pd and Pd-based catalyst has been viewed as the ideal replacement of Pt catalyst.Considering the pure Pd metallic particles are prone to aggregate,catalytic activity and utilization of pure Pd metal materials are relatively low.To develop modified Pd-based catalysts with high dispersion and favorable performance is indispensable to promote the development and commercialization of fuel cell.This article aims to develop a simple and mild method to prepare Pd-based composite catalysts(Material A:Pd/p-TiO2/MWCNT and Material B:Pd/SnO2-TiO2/MWCNT)through introduction of conducting metallic oxides(TiO2 and SnO2).The prepared catalytic materials were characterized by methods including TEM,XRD and XPS to study the crystalline structure,size of particle,surface morphology and microstructure of these two materials.Effect of introducing metallic oxides to catalytic material on diameter and dispersion of nano Pd particles and ESA was analyzed.Combined with characterization and methods like CV,Chronoamperometric and EIS,explanations were made about the interaction between microstructure properties and macro-electrochemical activity of catalyst material in fuel cell,and the influence of introducing conducting metallic oxides precursor on catalytic activity,stability,durability and CO tolerance of composite catalyst material.On the basis of this,the synergistic effect between SnO2 and nano Pd particles were discussed.Material A was prepared by multi-wall carbon nanotube(MWCNT)as the supporter,TiO2 as the precursor and PVP as the protective agent for Pd(Ⅱ).This material was then used as anode catalyst for direct formic acid fuel cell.Physical characterizations showed that the Pd metallic nanoparticles were well dispersed in composite materials and the diameter of Pd metallic particle decreased from 7.7 nm for material Pd/MWCNT to 4.07 nm for material Pd/p-TiO2/MWCNT.Electrochemical study revealed that,after the modification of TiO2,the electroactive surface area(ESA)of composite material was increased from 46.11 m2(Pd/MWCNT)g-1 to 83.53 m2 g-1(Pd/p-TiO2/MWCNT)and utilization efficiency of Pd was promoted from 72.07%(Pd/MWCNT)to 76.99%(Pd/p-TiO2/MWCNT).Compared with Pd/MWCNT material,Material A showed higher catalytic activity(peak current value was 100.91 mA cm-2and 37.63 mA cm-2 for Pd/p-TiO2/MWCNT and Pd/MWCNT,respectively)and better electrochemical stability in anodic oxidation of formic acid.However,through the study of material performance,despite the elevated dispersion of Pd metallic nanoparticle,the durability of TiO2 modifiedcomposite material was lowered.Furthermore,this article investigated the electrochemical performance,stability,durability and CO tolerance of Material A in DMFC,DEFC and DEGFC.Through the analysis of ESA,active surface area of Pd/p-TiO2/MWCNT was enlarged to 71.34 m2 g-1,which was 2.26 times that of Pd/MWCNT(31.60 m2 g-1);the utilization efficiency of Pd metallic nanoparticle was 65.75%for Material A,obviously higher than 50.02%of Pd/MWCNT catalyst.The results of oxidation for three fuels revealed that:(1)For oxidation of methanol,catalytic activity of Material A increased 3.34 times higher than that of Pd/MWCNT catalyst(oxidation peak current value for Pd/p-TiO2/MWCNT and Pd/MWCNT was 41.01 and 13.06 mA cm-2,respectively).Raito of If/Ib showed that the CO tolerance of this catalyst was also enhanced markedly,the value of this ratio was 34.7 for Pd/p-TiO2/MWCNT and 5.63 for Pd/MWCNT,respectively;in addition,the modification by TiO2 changed the onset potential,this onset potential was decreased to-0.408 V for Pd/p-TiO2/MWCNT from-0.316 V for Pd/MWCNT.(2)For oxidation of ethanol,catalytic activity of Material A increased 2.48 times higher than that of Pd/MWCNT catalyst(oxidation peak current value for Pd/p-TiO2/MWCNT and Pd/MWCNT was 76.38 and 30.83 mA cm-2,respectively).Raito of If/Ib increased to 3.35 for Material A from 1.27 for Pd/MWCNT;onset potential was decreased to-0.512 V for Material A from-0.472 V for Pd/MWCNT.(3)For oxidation of ethylene glycol,catalytic activity of Material A increased 2.36 times higher than that of Pd/MWCNT catalyst(oxidation peak current value for Pd/p-Ti02/MWCNT and Pd/MWCNT was 67.85 and 28.76 mA cm2,respectively).Raito of If/Ib increased to 5.48 for Material A from 3.37 for Pd/MWCNT;onset potential was decreased to-0.366 V for Material A from-0.274 V for Pd/MWCNT.The durability tests showed that composite catalysts displayed higher stability and CO tolerance in all the three alcohol systems after being modified by TiO2,however,durability showed slight decrease,this result is similar with that in formic acid system.Therefore,further modification is needed.In order to overcome the drawbacks of low durability in Material A,Material B(Pd/SnO2-TiO2/MWCNT)was prepared and applied in direct formic acid fuel cell and direct alcohol fuel cell.Physical properties characterization showed that Pd metallic nanoparticles were also evenly distributed in Material B,this is mainly attributed to the fact that Sn(II)could be adsorbed and dispersed well on the surface of TiO2 particle and then acted as reducing agent to effectively control the reduction of Pd(Ⅱ),making the Pd metallic nanoparticle could only be reduced in-situ on the surface of TiO2 particle.Compared with Material A,although Pd metallic nanoparticles in Material B were a bit larger(an average diameter of 5.8 nm,larger than 4.07nm for Material A),distribution of Pd particles was more compact,which led to similar ESA values in acid system of these two catalysts(83.53 m2 g-1 for Material A and 81.84 m2g-1for Material B),while utilization efficiency of Pd increased to 90.19%for Material B from 65.75%for Material A.In oxidation of formic acid,the catalytic activity of Material B was improved greatly due to the synergistic effect of SnO2-Pd:oxidation peak current was increased to 149.25 mA cm-2,which was 3.96 times that of Pd/MWCNT catalyst and 47.9%higher than Material A.Despite the slightly lower stability of composite material by introduction of SnO2,ESA decrease ratio was decreased from 30.92%for Material A to 22.54%for Material B,which revealed the durability of material could indeed be enhanced by introduction of SnO2.Meanwhile,the electrochemical performance,stability,durability and CO tolerance of Material B adapted on the basis of Material A in alkaline systems(methanol system,ethanol system and ethylene glycol system)were studied.Results showed that:(1)In alkaline alcohol systems,ESA of Material B was 98.60 m2 g-1,that is 38.2%higher than that of Material A,and Pd utilization efficiency was also enhanced greatly.(2)For oxidation of methanol,catalytic activity of Material B increased.Oxidation peak current value for Material B was 84.70 1A cm-2,which is 18.7%higher than that of Material A.Although the introduction of SnO2 led to slight lower CO tolerance and onset potential compared with Material A,its stability anddurability improved dramatically(ESA decrease ratio decreased from 19.2%for Material B from 65.8%for Material A).(3)For oxidation of ethanol,similar to result of methanol oxidation study,oxidation peak current value was increased to 114.56 mA cm-2 after being modified by SnO2,which is 50%higher than that of Material A.Though the SnO2 modified composite material exhibited slight lower CO tolerance and onset potential than Material A,its stability and durability improved greatly(ESA decrease ratio decreased from 12.5%for Material B from 64.11%for Material A).(4)For oxidation of ethylene glycol,after the introduction of SnO2,the oxidation peak current value was decreased to 53.60 mA cm-2 for Material B from 67.85 mA cm-2 for Material A,while CO tolerance(If/Ib)and onset potential were slightly elevated than Material A,this indicated the difference caused by synergistic effect between SnO2 and Pd in ethylene glycol compared with methanol and methanol.Nevertheless,the ESA decrease ratio tests showed that the introduction of SnO2 could greatly enhance the stability of composite material in alkaline ethylene glycol system.Two materials(Material A:Pd/p-TiO2/MW CNT and Material B:Pd/SnO2-TiO2/MWCNT)were prepared and compared with traditional Pd/MWCNT material,which both showed improved electrochemical performance,especially Material B modified on the basis of Material A.Therefore,the synergistic effect of SnO2-Pd of Material B in alkaline alcohol system was discussed:Due to the bi-functional mechanism,the introduction of SnO2 could enhance the concentration of OHads on the surface of Pd catalyst,which thus improved the catalytic activity and CO tolerance of the catalyst material.Through the coupling of acylads-OHads,acetaldehyde could be oxidized to acetic acid and active sites occupied by intermediate products(like acyl and CO)on the surface of Pd catalyst could realize regeneration,catalytic activity,durability and CO tolerance of catalyst are therefore improved.Study of synergistic effect between semiconductor metal oxides(TiO2 and SnO2,etc.)and noble metallic material(Pd and Pt,etc.)is promising to provide basic theoretical foundation and data support for future widespread utilization of direct fuel cell.