Nitrogen-doped graphene supported highly dispersed palladium-lead nanoparticles for synergetic enhancement of ethanol electrooxidation in alkaline medium

Highlights

• A novel N-doped graphene supported PdPb nanocatalyst was prepared.

• N-doped graphene facilitates more uniform dispersion of metal particles than graphene.

• Current for ethanol oxidation of PdPb/NG (152.3 mA cm⁻²) is 4 times higher than Pd/G.

• PdPb/NG catalyst shows excellent catalytic durability among the catalysts.

• Catalytic performance was enhanced by the bifunctional mechanism and electronic effect.

Abstract

In this work, a series of palladium and palladium-lead nanoparticles supported on active carbon, graphene and nitrogen-doped graphene are synthesized via a one-step reduction method. Atomic absorption spectroscopy, X-ray powder diffraction, transmission electron microscope and X-ray photoelectron spectroscopy are used to characterize the catalysts. The results indicate that metal nanoparticles are more uniformly dispersed on the surface of N-doped graphene than those on graphene, without any aggregation. Various electrochemical techniques are carried out to evaluate the electrocatalytic ethanol oxidation activity and durability. The peak current for ethanol electrooxidation of Pd/N-doped graphene increases to 70.2 mA cm⁻², obviously higher than that of Pd/Graphene (38.0 mA cm⁻²) and even surpasses that of Pd/C (51.9 mA cm⁻²). N-doped graphene support not only possesses faster dehydrogenation but provides an electron effect to Pd. Introduction of Pb into the catalyst causes the formation of abundant oxygenated species on the catalyst surface at low potential. Based on the synergistic effect of N and Pb towards Pd particles, the PdPb/N-doped graphene catalyst (Pd:Pb = 8:1.0) exhibits remarkably enhanced activity up to 152.3 mA cm⁻² for ethanol oxidation, which is 4.0 and 2.9 times higher than that of Pd/Graphene and Pd/C, respectively. The catalytic durability and stability are also greatly improved.

Introduction

As a kind of attractive energy conversion device for electronics vehicles, direct ethanol fuel cells (DEFCs) have attracted much attention owing to their low emission, high efficiency and no toxicity [1], [2], [3]. Currently, the ethanol oxidation reaction (EOR) in alkaline medium of these catalysts involves release of twelve electrons and cleavage of the C–C bond. However, the C–C bond cleavage is difficult to be implemented at low temperature [4], [5]. To address this issue, lots of researches have been devoted to electrochemical oxidation of ethanol in alkaline medium because of the faster oxidation kinetics in alkaline medium relative to acid medium [6], [7]. Besides, lots of possible derivatives such as CH₃COH and CHCO are produced on catalyst surface during ethanol electrooxidation [8] and prevent further adsorption and electrooxidation of ethanol molecules in solution, leading to a decrease in the catalyst efficiency. These drawbacks have become a bottleneck in the commercialization of DEFCs. Consequently, anode catalysts with high catalytic activity and high durability are extremely desired [9], [10].

Pd catalyst is considered as an excellent alternative for the application of alkaline ethanol fuel cells as the following two reasons: one is that Pd gives the higher electrocatalytic activity and the less poisoning effect compared with Pt for ethanol oxidation in alkaline medium [11], [12], [13], [14]; the other is that Pd is much more abundant than Pt on the earth, which makes Pd less expensive than Pt [11]. The anodes used Pd–based electrocatalysts needed further improvement in catalytic activity and stability to fulfill commercialization of DEFCs. Much effort has been devoted to improve the performance of Pd catalysts by introducing other one or more elements. For example, Au, Ag, Ni, Cu, Sn and Pb were selected as promoting elements to enhance the activity of Pd catalyst for ethanol electrooxidation [15], [16], [17], [18], [19], [20]. Among these elements, Pb seems to have stronger promoting effects. A number of studies have devoted to enhanced electrooxidation of small organic molecules by Pb addition [21], [22], [23], [24], [25]. The promoting effects of Pb in the Pd–based catalysts can be attributed to the geometric effect, the electronic effect and the bifunctional mechanism [21].

Aside from active carbon, carbon nanocoils [26], carbon nanohorns [27], graphite nanofiber (GNF) [28], carbon nanotubes (CNT) [29], [30], [31] and graphene (G) [32], [33] were also used to support Pd–based nanoparticles. Due to the high electric conductivity and special structural properties, graphene have been considered as the most promising support material in fuel cell electrodes [33]. Chemical doping is an important way to modulate the surface structure and physicochemical property of graphene [34], [35], [36]. The incorporation of electron-rich nitrogen atoms into graphene can not only improve the dispersion state of the nanoparticles on the graphene surface [37], [38], but also modify the surface structure of carbon materials and strengthen interaction between metal nanoparticles and supports [39], [40], [41], [42]. However, the synergetic effect of N-doping and second active component towards noble metal on small organic molecule electrooxidation have been rarely reported until now, which will be revealed in this study.

Based on the above considerations, herein, N-doped graphene was selected as a support and PdPb nanoparticles were used as the active component for synthesis of the novel PdPb/NG nanocatalyst. The PdPb/NG nanocatalyst was prepared via simply reducing metal ions on N-doped graphene surface. The electrocatalytic performance of PdPb/NG towards ethanol oxidation was systemically investigated, compared with Pd/C, PdPb/C (8:1.0), Pd/G, Pd/NG, PdPb/G (8:1.0). The origin of high performance of the catalysts was also revealed.