Synthesis And Characterization Of Nanostructured Palladium Based Electrocatalysts For Oxygen Reduction Reaction In Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cells(PEMFCs) and direct methanol fuel cells(DMFCs) are expected to become the future clean energy sources due to their low temperature operation and minimal environmental impact. However, the sluggish kinetics of the oxygen reduction reaction(ORR) at the cathode and the methanol crossover from the anode to the cathode under the use of methanol as a fuel causes high voltage losses and low conversion efficiency of the fuel cells. The most practical cathode catalysts to speed up the slow ORR kinetics are platinum and its alloys. Despite the significant improvement in performance on these catalysts, the demand of a high loading of the scarce and expensive platinum increases cost of the fuel cells and impedes their widespread commercialization. Furthermore, several limitations have been identified over the years in the application of these catalysts such as high susceptibility to contaminants, poor selectivity towards ORR in the presence of methanol and active site degradation due to formation of H2O2 resulted from incomplete reduction of O2 to H2 O. Pd is five times more abundant in the earth’s crust than Pt and its price is almost one quarter of Pt. The electrocatalytic activity of Pd is the second highest among pure metals, next to Pt and has high selectivity to ORR in the presence of methanol. Therefore, palladium alloys are considered as promising candidates for replacing Pt with an advantage of higher global availability, lower cost and better methanol tolerance. This study aims on the development of palladium based nanostructured electrocatalysts with enhanced activity and high selectivity towards ORR at a reduced cost. On account of this, three types of Pd based nanostructured catalysts such as ordered intermetallic Pd3Fe/C nanoparticles(O-Pd3Fe/C), ternary PdIr-Fe/C alloys and concentration gradient Pd-Ir-Ni/C(CG Pd-Ir-Ni/C) core-shell electrocatalysts have been synthesized and characterized. The most common analysis methods including X-ray diffraction(XRD), transmission electron microscopy coupled with energy dispersive X-ray diffraction spectroscopy(TEM-EDS), high resolution TEM(HRTEM), X-ray photoelectron spectroscopy(XPS), cyclic voltammetry(CV) and rotating disc electrode(RDE) have been employed to explore their physical and electrochemical properties.The ordered intermetallic carbon supported palladium-iron(O-Pd3Fe/C) nanoparticles are synthesized using impregnation method followed by annealing at a predetermined temperature under inert and reducing atmosphere. The O-Pd3Fe/C nanoparticles exhibit higher ORR activity than those of Pd/C and disordered Pd3Fe/C(D-Pd3Fe/C) catalysts, which is attributed to the formation of ordered intermetallic phases with consistent active sites throughout the crystal structure. Furthermore, the O-Pd3Fe/C catalyst outperforms that of Pd3Fe/C prepared using microwave polyol reduction method followed by annealing under similar conditions. This suggests that the impregnation method provides alloys with higher degree of alloying and atomic ordering than that of microwave polyol method. The accelerated durability test after 1000 potential cycles in 0.1 mol L-1 HClO4 solution also revealed that the O-Pd3Fe/C nanoparticles are more stable than that of D-Pd3Fe/C alloy due to the intermetallically stacked atoms in the crystal lattice resulted from high enthalpy of mixing.The other strategy, with which activity and stability of the inexpensive Pd-based alloys could be improved, is through addition of a relatively stable third metal into binary alloys. Platinum group metals are supposed to exhibit high electrochemical stability and among these, Ir shows the highest stability in acid medium due to its high reduction potential above the typical operating fuel cell voltage. Moreover, Ir plays an important role in modifying the surface electronic structure of the alloys and further enhances their catalytic activity towards ORR. Therefore, the second nanostructured Pd based ORR electrocatalysts investigated in this study are a series of carbon supported ternary Pd-Ir-Fe alloy electrocatalysts prepared by microwave polyol reduction method with nominal atomic ratios of 1:1:1, 2:1:1 and 3:1:1(represented as Pd IrFe/C, Pd2IrFe/C and Pd3IrFe/C, respectively). The results of the RDE measurement for ORR indicate superior ORR performance of the ternary alloys to pure Pd/C. Furthermore, the ternary Pd3IrFe/C alloy electrocatalyst exhibits higher mass activity and better stability than that of bimetallic Pd3Fe/C catalyst in 0.1 mol L-1 HClO4 solution. This activity enhancement is attributed to surface segregation of Pd and modification of the electronic structure induced by subsurface Ir, which weakens the binding strength of oxygen containing intermediates and further enhances their removal from the catalyst surface. The ORR test under methanol crossover condition also demonstrates higher methanol tolerance of the ternary Pd3IrFe/C alloy electrocatalyst than that of Pt/C catalyst. The most active Pd3IrFe/C catalyst among the as-prepared ternary alloys contains minimal content of Ir, which offers an added advantage of reducing the cost.Finally, a third Pd based electrocatalyst possessing concentration gradient coreshell nanostructure, whose composition transits from NiIr core to IrPd shell, has been prepared by a modified polyol reduction method in two sequential steps. The structural analysis suggests a certain amount of alloying between Pd and IrNi alloy core during addition of the Pd precursors into the pre-formed IrNi particles, resulting in a gradient growth of IrPd shell with an increase in concentration of Pd towards the shell. The electrochemical testing confirms enhanced ORR activity of this catalyst relative to pure Pd/C and bimetallic Pd4Ni/C catalysts and higher methanol tolerance than that of asprepared Pt/C. This performance improvement could be ascribed to the concentration gradient core-shell nanostructure that results in modification of the surface geometric structure induced by the alloy core substrate.