Carbon-Supported Pd-Based Nanoparticle Electrocatalysts For Methanol Tolerant Oxygen Reduction Reaction

The direct methanol fuel cell(DMFC)is a promising new clean power source because methanol is an abundant,inexpensive liquid fuel,easily handled,transported, and stored,and with a high theoretical energy density.However,the DMFC performance is still impacted by several factors,including the poor kinetics of the oxygen reduction reaction(ORR),the high costs of the Pt-based electrocatalysts and the high methanol crossover.Thus,it is highly desirable to develop novel alternative electrocatalysts with high ORR activity,high methanol tolerance as well as without Pt and/or low Pt content.In this dissertation,carbon-supported Pd-Co bimetallic and Pd-Co-Au ternary nanoparticle electrocatalysts were prepared by a modified polyol reduction and then heat-treated at various temperatures.Pd-Co/C catalysts before and after heat treatment evidenced a Pd single phase face centered cubic(fcc)disordered structure.The effects of the composition,structure and particle size on the ORR activity were evaluated in the absence and presence of methanol.The Pd₂Co₁/C catalyst heat-treated at 500℃showed the highest ORR activity among all the Pd-Co/C catalysts in pure acidic solution,which could be attributed to synergistic effect between alloying and particle size(ca.8.0nm),an optimal alloy composition of ca.67 at.%Pd and the favorable structural parameters such as a Pd-Pd mean bond distance of ca.0.2730nm.As-prepared Pd-Co-Au/C catalysts have both Pd and Au fcc structures.However, the Pd ternary alloys with Pd single phase structure were formed when the catalysts were heat-treated at the temperatures higher than 800℃.The maximum ORR activity was found on a Pd₇Co₂Au₁/C alloy catalyst heat-treated at 800℃,which is very close to that on the commercial Pt/C catalyst.Moreover,the Pd-Co-Au/C alloy catalysts exhibited an enhanced ORR activity compared to Pd-Co/C catalysts,and the overpotential for the ORR on the Pd₇Co₂Au₁/C alloy catalyst heat-treated at 800℃was 32mV lower than that on the Pd₂Co₁/C catalyst heat-treated at 500℃at a current density of 0.5mA cm^-2.The enhanced ORR activity on the ternary alloy catalysts could be ascribed to an electronic effect adjusted by the addition of Au species.Palladium-rich carbon-supported Pd-Pt bimetallic nanoparticle electrocatalysts synthesized via a facile,one step reduction route.The surface composition,structure and particle size of the catalysts could be easily adjusted by controlling the amount of trisodium citrate.The Pd₃Pt₁/C catalyst with a Pt rich skin presented higher ORR activity than other Pd₃Pt₁/C catalysts with Pd rich skins and slightly higher ORR activity than the commercial Pt/C catalyst in pure acidic solution.Moreover,the Pd₃Pt₁/C catalyst with Pd rich skin showed the highest specific activity and mass activity for the ORR among different Pd-rich Pd-Pt alloy catalysts with a similar particle size(ca.4.8nm).Such a Pd₃Pt₁/C alloy catalyst showed a comparative ORR activity in comparison with the commercial Pt/C catalyst in pure acidic solution, which could be due to an optimal alloy composition of ca.75 at.%Pd.It is worth mentioning that the Pd-Pt/C catalysts showed higher ORR activity than the Pd-Co/C and Pd-Co-Au/C catalysts,which may be likely associated with the Pd-Pt synergistic effect due to the fact that Pd and Pt have similar catalytic behavior to the ORR and that Co is inactive for the ORR.All the Pd-based catalysts investigated here exhibited higher ORR activity than the Pt/C catalyst in methanol-containing solution.High methanol tolerance during the ORR may be due to the composition effect and to the fact that Pd is inactive for the adsorption and oxidation of methanol.Furthermore,kinetics analysis revealed that the ORR on Pd-based catalysts undergoes an ideal 4e process in forming water.Thus,our studies demonstrate that the Pd-based catalysts,especially Pd-Pt/C catalyst,may function as a promising methanol-tolerant ORR catalyst in a DMFC.