The Construction Of Pd-RE Alloys For Methanol Resistance Oxygen Reduction Reaction

Owing to their high efficiency,high energy density,and incredibly low emissions,direct methanol fuel cells（DMFCs）have gained much interest recently as a clean source of energy.However,the sluggishness of oxygen reduction reaction（ORR）kinetics in DMFCs has hindered their mass commercialization.Although,Pt/C recently served as a benchmark cathode catalyst for DMFCs.Nevertheless,the high cost of Pt and the issue of methanol crossover from the anode to the cathode in a DMFC leads to poor fuel cell performance.Thus,developing alternative cathode catalysts with excellent ORR activity and outstanding methanol resistance to replace Pt-based catalysts is significant but remains challenging.Pd-based catalysts have been considered the best option due to their similar properties to Pt.However,a single Pd catalyst’s ORR activity is significantly lower than Pt/C catalysts.The alloy of Pd with other metals,particularly transition metals（TMs）,has been proven to be a good option;nonetheless,most Pd-TMs alloys could not simultaneously achieve desirable methanol resistance and high ORR activity.Here,we demonstrated the alloy of Pd-RE as the outstanding ORR catalyst for DMFC.By using molten salt as the confinement agent and polyethyleneimine functionalized carbon（PEI/C）as the carrier,we successfully synthesized Pd-RE alloys under an argon/hydrogen atmosphere at 700 ^oC.The as-received Pd-RE catalysts show an excellent ORR and superior methanol resistance compared with Pt/C and Pd/C.The specific achievements are as follows:（1）The catalysts with a mild amount of confinement agent reveal the best ORR activity and methanol resistance properties;thus,the addition of0.1 molar ratio of Mg Cl₂（0.0813 g Mg Cl₂ and 0.5 g Mg）as compared to0.6 molar ratio of Mg Cl₂（0.1834 g Mg Cl₂ and 0.5 g Mg）.（2）Notably,the catalysts synthesized without Mg demonstrated inferior ORR activity,indicating that the inclusion of Mg creates a strong reducing condition and lowers the formation temperature of alloys,thereby improving the electrochemical performance of the Pd-RE alloys.（3）The XRD,TEM,and XPS analyses validated the successful formation of the Pd-RE alloy.Specifically,three series of Pd-RE ratios（1:1,2:1,and 3:1）were prepared in which the catalysts with 1:1 show the most remarkable structural features revealing it as the optimal composition for ORR.Noticeably,a shift in the d-band center of Pd 3d,led to the weakening of oxygen species on the surfaces of the Pd-RE alloys compared to Pd/C.This is likely due to the synergy between Pd and RE elements,resulting in improved electrochemical performance of the Pd-RE alloys.（4）Among the three Pd-Y series（Pd Y,Pd₂Y,and Pd₃Y）employed in the premise of the entire study,Pd Y exhibited excellent activity and methanol resistance.This unequivocally validates the effects of the Pd-RE atomic ratio on ORR performance in DMFCs.For instance,the Pd Y obtained a half-wave of 0.861V,which is 60 and 7 m V higher than 0.801V of Pd/C and 0.854 V of Pt/C,respectively.The ECSA value of the PY is higher than those of Pd/C and Pt/C,respectively.Moreover,the mass and specific activities of Pd Y at 0.9 V vs RHE were considerably higher than those of Pd/C and Pt/C.Most importantly,the Pd Y showed superior methanol resistance at a high methanol concentration;thus,at 2 M methanol concentration,the half-wave of Pd Y was 95 and 306 m V higher than that of Pd/C and Pt/C,respectively.（5）Inspired by the outstanding properties of Pd Y,other RE elements were synthesized.Pd Er revealed the most sterling performance in both ORR activities and the methanol resistance property.Briefly,the Pd Er shows an ECSA value of 101.2 m² g^-1PGM,2.5 and 1.5 times higher than65.4 and 39.7 m² g^-1PGM of Pd/C and Pt/C,respectively.Moreover,the E_1/2potential of Pd Er was 0.901 V,which is 100 and 47 m V more positively shift than 0.801 and 0.854 V of Pd/C and Pt/C.The mass and specific activities of Pd Er were（5.5;6.0）and（3.8;3.9）times higher than those of Pd/C and Pt/C,respectively.Prominently,the Pd Er showed remarkable methanol resistance at 2 M methanol concentration.The E_1/2of Pd Er at 2M methanol concentration shifted positively which was 144 and 355 m V higher than Pd/C and Pt/C,respectively.（6）Lastly,other Pd-RE catalysts in this thesis include Pd:RE（Ho,Pr,Sc,Eu,Ce,Tb,Nd,Tm,Yb,Dy,Gd,Sm,and Lu）.Unfortunately,the majority of these alloys exhibited poor electrochemical performance,which may be attributed to error/contamination during synthesis and the limited time that deterred repeated testing.However,three of these catalysts,Pd Ho,Pd Pr,and Pd Sc,outperformed the Pd/C and Pt/C reference catalysts.Consequently,the ECSA values,half-wave potentials,mass,specific activities,and methanol resistance capacities of these catalysts were significantly greater than those of Pd/C and Pt/C,respectively.Based on electrochemical tests,the trend in electrochemical properties is as follows:Pd Er>Pd Ho>Pd Pr>Pd Sc>Pd Y>Pd/C>Pt/C>Eu>Ce>Tb>Gd>Sm>Lu>Yb>Dy>Nd>Tm.Hence,the superior ORR activity and excellent methanol resistance of Pd-RE alloy catalysts render them promising electrocatalysts for DMFC.33Figures,20 Tables,and 215 References...