(45f) 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.