Synthesis and Characterization of Pd–Ni Nanoalloy Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells

Proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) are appealing for a variety of energy needs ranging from portable to transportation applications as they offer clean energy with high efficiencies. However, a widespread commercialization of these technologies is hampered by several challenges such as the sluggish oxygen reduction and methanol oxidation reactions as well as the high cost of Pt catalysts. In this regard, extensive efforts have been focused on Pt-based alloy catalysts, and optimum alloy catalysts have been found to offer higher catalytic activity than Pt while lowering the cost. However, the dissolution of non-noble metal components and the consequent performance loss during cell operation remain a concern. With an aim to lower the cost and find platinum-free alternatives, Pd-based catalysts have drawn much attention in recent years. Also, Pd is known to be inactive for methanol oxidation, offering high tolerance to methanol poisoning as a cathode catalyst in DMFC. However, the catalytic activity of Pd for the oxygen reduction reaction (ORR) is lower than that of Pt and its alloys. Also, the stability of Pd is not as good as Pt in acidic, oxidative, and high-temperature environments. To overcome these problems, efforts have been made to alloy Pd with other elements such as Ti, Fe, Co, Au, Mo, and W. It has also been suggested that alloying Pd with other metals having smaller atomic size such as V, Cr, Fe, and Co is particularly effective in enhancing the catalytic activity. Ni is another possible choice to alloy with Pd and enhance its catalytic activity. Accordingly, we present here the synthesis of carbon-supported nanostructured Pd-Ni catalysts with various atomic ratios by a modified polyol reduction process, followed by heat-treatment at various temperatures. The synthesized Pd-Ni alloys are characterized by X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and rotating disk electrode (RDE) and single-cell PEMFC measurements for ORR. XRD and TEM data reveal an increase in the degree of alloying and particle size with increasing heat treatment temperature. XPS data indicate surface segregation with Pd enrichment on the surface of Pd₈₀Ni₂₀ after heat treatment at ≥ 500 °C, suggesting possible lattice strains in the outermost layers. Electrochemical data based on CV, RDE, and single cell PEMFC measurement show that Pd₈₀Ni₂₀ heated at 500 °C exhibits the highest mass catalytic activity for ORR among the Pd-Ni samples investigated, with stability and catalytic activity significantly higher than that found with Pd.  With a lower cost, the Pd-Ni catalysts exhibit much higher tolerance to methanol than Pt, offering an added advantage in DMFC.