Nitrogen-doped Carbon Supports And Trimetallic Palladium-based Catalysts For Formic Acid Electrooxidation

With the rapid upgrading of energy structure,direct formic acid fuel cell as a new type of green energy power source has attracted extensive interests.At present,palladium-based catalysts are the hot spots of anode catalyst for direct formic acid fuel cells,since its activity and anti-COad-toxic performance are better than that of platinum.However,due to the high price and poor stability of palladium-based catalysts,researchers have devoted to the modification of palladium-based catalysts.In order to improve the catalytic performance and lifespan of palladium-based catalysts and reduce the cost of direct formic acid fuel cell,modification of carbon supports with nitrogen and synthesizing Pd-based trimetallic catalysts were explored in this study.To investigate the effect of doped-nitrogen type in carbon support on the electrochemical performance of Pd toward formic acid electrooxidation,melamine,dicyandiamide and urea were chosen as nitrogen sources to dope nitrogen into nitric acid treated Vulcan XC-72 carbon black.Then palladium nanoparticles were loaded on the nitrogen-doped carbon supports.The X-ray photoelectron spectroscopy(XPS)analysis shows that the doped nitrogen can be divided into pyridinic nitrogen,pyrrolic nitrogen and graphitic nitrogen.The catalyst with more pyrrolic-nitrogen content exhibits higher formic acid oxidation activity.Among the catalysts synthesized in this study,the catalyst treated with urea(Pd/UR)possesses the largest pyrrolic nitrogen content,and exhibits the best activity toward formic acid oxidation.DFT calculation discloses that the adsorption energy of HCOO*and*COOH on pyrrolic-Pd is the minimum,which indicates that the more pyrrolic-N retains,the faster kinetics of formic acid oxidation is.The quicker refreshment of active site displays,and the more smoothly the reaction proceeds.Such conclusion is consistent with the results of Tafel slope and exchange current density.Moreover,palladium and nitrogen complex revealed catalytic capability for formic acid oxidation.Pd2+/UR catalysts performed the best.To investigate the Pd-based trimetallic catalysts,PdCuNi trimetallic catalysts were supported on the nitric acid treated Vulcan XC-72 carbon black.Palladium content was fixed at 50%of the total metal molar content for all the PdCuNi trimetallic catalysts.The molar content of copper and nickel were adjusted to investigate the effect of copper and nickel content on Pd toward formic acid electrooxidation.It was found by High Resolution Transmission Electron Microscopy(HRTEM)that the metal nanoparticles of PdCuNi trimetallic catalysts were uniformly distributed with almost the same particle size.The formic acid oxidation activity of Pd0.5Cu0.375Ni0.125 is the highest,followed by Pd0.5Cu0.125Ni0.375 catalyst and Pd0.5Cu0.25Ni0.25 catalyst.Moreover,the stability of trimetallic PdCuNi catalysts in both acid and alkaline electrolytes were compaired.As copper and nickel exist on the surface of PdCuNi trimetallic catalysts in alkaline electrolyte,more OHad species is generated than in acid electrolyte to oxidize COad into CO2 and release the active sites.Thus,the stability of PdCuNi trimetallic catalysts in alkaline electrolyte is better than that in acid electrolyte.PdCoNi trimetallic catalysts were prepared with alloy structure and supported on the nitric acid treated Vulcan XC-72 carbon black.The effect of molar ratio between palladium,cobalt,and nickel on the electrocatalytic activity of Pd toward formic acid oxidation were investigated.It was found that 75%is the best molar content of palladium.Then fix the palladium content and adjust the molar ratio of cobalt and nickel.Pd0.75CoO0.125Ni0.125 catalyst performs the highest catalytic activity for formic acid oxidation among the synthesized PdCoNi trimetallic catalysts.