Electrochemical reduction of oxygen on palladium nanocubes in acid and alkaline solutions

Abstract

The electrocatalytic properties of cubic palladium nanoparticles towards the electrochemical reduction of oxygen were studied in acid and alkaline solutions and compared with those of spherical nanoparticles and bulk Pd. The synthesised Pd nanoparticles were characterised by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Electrooxidation of pre-adsorbed CO was employed for cleaning the palladium catalyst surface. Oxygen reduction was studied using the rotating disk electrode (RDE) method and enhanced electrocatalytic activity of Pd nanocubes was revealed both in acid and alkaline solutions, which was attributed to the prevalence of Pd(1 0 0) facets. The mechanism of oxygen reduction on Pd nanoparticles was similar to that on bulk Pd, the first electron transfer being the rate-limiting step, and the reaction predominantly followed a four-electron pathway in both solutions.

Introduction

Nanostructured palladium catalysts have received much attention in recent years as rather active catalysts for the electrochemical reduction of oxygen [1], [2]. Pd has properties similar to those of platinum and the mechanism of O₂ reduction in both acid and alkaline solutions is the same on these metals [3], [4]. As compared to Pt, Pd is more abundant on the Earth and considerably cheaper [2], but the activity of pure Pd towards O₂ reduction is lower and Pd is less stable in acid media [1]. Bimetallic Pd-based catalysts, such as alloys and core–shell particles with non-noble metals (Co, Fe, etc.), have been shown to possess higher electrocatalytic activity for oxygen reduction than pure Pd [1], [2]. A further advantage of Pd and its alloys is their good selectivity for oxygen reduction reaction (ORR) in the presence of alcohols, which makes them attractive catalysts for direct alcohol fuel cells [2].

There are several studies of oxygen reduction on nanostructured Pd catalysts in acid media [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. The activity of Pd/C catalysts is generally considerably lower than that of Pt/C [8], [9]. However, very high catalytic activity has been observed for Pd nanorods prepared by electrodeposition [10]. For vacuum-evaporated thin Pd films, the specific activity of O₂ reduction was similar to that of bulk Pd and slightly decreased with decreasing the film thickness in H₂SO₄ solution [11], [12], but it was rather constant in HClO₄ [11]. Improved electrocatalytic properties for O₂ reduction have been observed on Pd nanoparticles (PdNPs) supported on carbon nanotubes [13], [14], [15], [16], [17]. For Pd/C catalysts, the activity can also be influenced by pre-treatment of the carbon support [18].

As on platinum, oxygen reduction on Pd is a structure-sensitive reaction [1]. It has been demonstrated that in HClO₄ solution, the ORR activity on Pd single crystals increases in the order of Pd(1 1 0) < Pd(1 1 1) < Pd(1 0 0) [20]. In contradiction to that, high catalytic activity of Pd nanorods as compared to spherical Pd nanoparticles has been attributed to the prevalence of Pd(1 1 0) facets [10]. Very recently, the oxygen reduction studies on Pd nanocubes with a preferential (1 0 0) surface orientation have been published [6], [7]. The specific activity of cubic Pd particles was comparable to that of Pt, but the activity of Pd octahedra, however, was 10 times lower [6]. The higher ORR activity of Pd nanocubes was attributed to the lower coverage of chemisorbed OH, which in turn offers more available reaction sites [6], or, in H₂SO₄ solution, to the structure-dependent adsorption of (bi)sulphate ions [7]. In contrast, studies of the ORR on stepped surfaces of n(1 1 1)–(1 0 0) series of Pd in HClO₄ revealed that the activity increases with increasing the terrace atom density, although the oxide coverage also increases. It has been proposed that on Pd, the active sites for the ORR are the terraces [21].

In alkaline solutions, Pd catalysts have shown remarkably high activity and good stability and are therefore considered as promising electrocatalysts for alkaline fuel cells [19], [22], [23], [24], [25], [26], [27], [28], [29]. In some cases, activities comparable to that of Pt [23] or even higher [25], [28] have been observed. The specific activity of the Pd/C catalysts depends on the Pd particle size, decreasing by a factor of about three with decreasing particle size from 16.7 to 3 nm [24]. However, graphene-supported Pd nanoparticles with a mean diameter of only 1.8 nm showed significantly high catalytic activity for the ORR [28].

The aim of this study was to synthesise cubic Pd nanoparticles and to compare their electrocatalytic activity towards oxygen reduction with that of spherical Pd nanoparticles and bulk Pd in acidic as well as in alkaline solutions.