PdNi electrocatalyst for oxygen reduction in acid media

doi:10.1016/j.ijhydene.2008.03.004

International Journal of Hydrogen Energy

Volume 33, Issue 13, July 2008, Pages 3596-3600

https://doi.org/10.1016/j.ijhydene.2008.03.004 Get rights and content

Abstract

The PdNi nanoparticles dispersed on Nafion solution was used as an ink-type electrode for oxygen reduction reaction (ORR), in acid media. This bimetallic catalyst was synthesized via the metallic reductive precipitations of the chloride precursors with ${NaBH}_{4}$ in tetrahydrofuran (THF) at $25^{\circ} C$ . The characterization of the electrocatalyst using X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed bimetallic nanoparticles having a narrow size ranged $8 - 10 nm$ with face-centered cubic (fcc) hexagonal structure. Cyclic voltammograms and polarization curves from rotating-disk electrode measurements showed significantly higher activity on PdNi than observed on Pd catalyst, with a mechanism involving four electrons transfer to water formation. The incorporation of Ni, a less electronegative metal to Pd shifts the onset potential for the ORR approximately 110 mV more positive, and at $0.5 mA {cm}^{- 2}$ the cathode overpotential is reduced by 260 mV in relation to that obtained on Pd, in a 0.5 M $H_{2} {SO}_{4}$ solution at $25^{\circ} C$ .

Introduction

Electrocatalysts used for polymer electrolyte membrane fuel cells, PEMFCs, have been studied for a long time aimed at achieving catalytic activity and stability at low cost [1], [2], [3], [4], [5]. Among the cathode catalysts in PEMFC, bimetallic and alloy catalysts have an active area of research particularly Pt-based materials which have been extensively prepared and evaluated in order to enhance the activity of the oxygen reduction reaction (ORR). Even with the widely used Pt-based catalyst, the ORR is kinetically slow and leads to a higher reduction overpotential, associated by the fact that the oxygen–oxygen bond (O–O) in dioxygen molecule requires higher bond dissociation energy. The enhanced catalytic activity of bimetallic surfaces in comparison to pure metal surfaces is usually ascribed to two effects [6], [7], [8], [9]: the bifunctional effect in which the unique catalytic properties of each of the elements in the compound combine in a synergetic fashion to yield a more active surface than each of the elements alone, and the electronic effect in which one of the elements alters the electronic properties of the other just as to yield a more active catalytic surface. A series of Ru-based binary and ternary catalysts have been synthesized and kinetically investigated for ORR by our group, which include: ${Ru}_{x} {Fe}_{y}$ [10], ${Ru}_{x} {Se}_{y}$ [11], ${Ru}_{x} {Fe}_{y} {Se}_{z}$ [12] and ${Ru}_{x} {Cr}_{y} {Se}_{z}$ [13]. However, it is quite difficult to obtain compositional homogeneity when synthesized these catalysts in organic media because thermal condensation is not a selective synthesis, and a mixture of lower- and higher-nuclearity cluster compounds is produced.

The aim of this work is to synthesize a nanostructured PdNi catalyst in order to improve the electrochemical performance reported on Pd catalysts alone [14]. The bimetallic catalyst was synthesized through reduction of the transition metal chlorides with ${NaBH}_{4}$ in tetrahydrofuran (THF) media. Physical and electrochemical characterizations were performed in order to evaluate phases, particle size and electrochemical activity of the electrocatalyst for the ORR in a sulfuric acid electrolyte.

Section snippets

Electrocatalyst preparation

Nanometric PdNi catalyst has been synthesized by a ${NaBH}_{4}$ reduction of ${PdCl}_{2}$ (0.91 mM) and ${NiCl}_{2}$ (0.91 mM) in a 100 mL THF, following the methodology reported in literature [15], [16], [17]. A chemical reactor was charged with ${PdCl}_{2}$ and ${NiCl}_{2}$ in a previously dried THF. The reducing agent ( ${NaBH}_{4}$ , 1.8 mM) was slowly added while maintaining stirring the solution at $25^{\circ} C$ under $N_{2}$ atmosphere per 16 h. Afterwards the reaction product was washed several times to eliminate the produced sodium chloride and

Catalyst characterization

Fig. 1 shows the XRD patterns of the Pd and PdNi catalysts. As indicated in this figure all the XRD patterns clearly show the five mean characteristic peaks of the face-centered cubic (fcc) crystalline Pd (JCPDS card 046-1043), demonstrating that the alloy catalyst resemble a single phase structure. The diffraction peaks in the PdNi catalyst are slightly shifted to higher $2 θ$ values with respect to the corresponding peaks in the Pd catalyst, revealing the alloy formation between Pd and Ni. The

Conclusions

The synthesis, physical and electrochemical characterization reported in this article indicates that the PdNi electrocatalyst could be a suitable candidate as cathode, through an oxygen reduction reaction (ORR) in a PEMFC. The kinetic analysis revealed that the addition of Ni to Pd reduces the cathode overpotential. Therefore the catalytic activity of the PdNi is enhanced for the ORR, with a four-electron transfer process to water formation.

Acknowledgments

This research project was financially supported by the National Science and Technology Council of Mexico, CONACYT, under Grant no. 46094 and ICY TDF (OCF-OSF). One of the authors (G.R.S.) would like to thank CONACYT for providing him the doctoral fellowship. The authors thank Carlos Flores for assistance in TEM measurements.

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PdNi electrocatalyst for oxygen reduction in acid media

Abstract

Introduction

Section snippets

Electrocatalyst preparation

Catalyst characterization

Conclusions

Acknowledgments

Science

Surf Sci Rep

Int J Electrochem Sci

Int J Hydrogen Energy

Electrochim Acta

Electrochem Commun

Int J Hydrogen Energy

J Power Sources

J Power Sources