Electrochemical deposition of Pt–Ru on diamond electrodes for the electrooxidation of methanol

https://doi.org/10.1016/j.jelechem.2011.01.034Get rights and content

Abstract

The dependence of the electrocatalytic properties of Pt–Ru alloy nanoparticles supported on boron-doped diamond electrodes, on the electrodeposition conditions used to prepare them with respect to the oxidation of methanol was studied. Routes involving sequential and simultaneous deposition of the two elements were studied, and the effect of deposition potential was also examined. The morphology of the electrodeposits was characterized by scanning electron microscopy, and shows spherical Pt–Ru nanoparticles for simultaneous deposition and a dendritic structure for sequential deposition. The Pt–Ru binary catalysts from both deposition methods exhibit higher electrocatalytic activity than Pt alone. However, deposits from simultaneous deposition show higher chemical stability and catalytic activity for methanol oxidation than those prepared using sequential deposition, for which significant electrodissolution of Ru is a problem. The optimal Pt:Ru ratio in simultaneous deposition is seen for intermediate deposition potentials. The different morphologies and microstructures for the two deposition methods are also reflected in differing rate-determining steps for methanol oxidization as judged from Tafel analysis. Overall, a sensitive dependence of catalytic properties on electrodeposition parameters is indicated.

Graphical abstract

Electrochemically deposited Pt-Ru clusters exhibit cyclic voltammetry that is dependent on particle geometry as imaged by electron microscopy.

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Research highlights

► We prepare PtRu nanoparticle clusters on diamond by electrodeposition routes. ► Sequential deposition of the two elements is compared with a simultaneous deposition route. ► Sequential deposition produces a dendritic structure which is unstable because of loss of Ru. ► Simultaneous Pt and Ru electrodeposition gives the best electrocatalyst for methanol oxidation.

Introduction

The direct methanol fuel cell (DMFC) has been recognized as one of the most promising alternative energy conversion sources, due to its low operation temperature (below 100 °C), high energy density (4.4 Wh/mL) and durability [1]. Pt is a highly active electrocatalyst anode for this application [2]. However, as Pt electrocatalysts are prone to poisoning by the intermediates of methanol oxidation, such as carbon monoxide [3], the addition of a second metal to Pt is used to promote the oxidization of Pt–COads and thus reduce the poisoning effect. Pt–Ru binary metallic catalysts are successful in this regard [4], [5], [6]. The addition of ruthenium significantly improves the CO tolerance of Pt, for the reason that ruthenium can oxidize adsorbed water to Ru-OH and Ru-OH then reacts with the CO poisoning species [7].

Pt and Pt–Ru binary electrocatalysts are deposited conventionally by electroless deposition, impregnation reduction, sol–gel and microemulsion methods, the Pechini method, hydrothermal routes and electrochemical deposition [8], [9]. With regard to this latter approach, the sequential electrodeposition of Pt–Ru on HOPG substrates was compared to simultaneous deposition of the two elements using linear sweep deposition [10]. The binary electrocatalytic deposit from sequential deposition exhibited higher catalytic activity and less Ru electrodissolution than simultaneous deposition, although both electrodes showed enhanced electrocatalytic performance compared to Pt.

Various carbon electrodes have been used as electrocatalyst supports such as glassy carbon, highly ordered pyrolytic graphite (HOPG), carbon black, carbon nanotubes, and carbon fibers [9]. However the chemical stability of these supports in hostile environments limits the durability of the devices [11], [12], [13]. In contrast Boron-doped diamond (BDD) has excellent electrochemical and chemical properties suitable for electrode supports, such as a wide electrochemical window, a low background current and a high chemical and electrochemical stability [14], [15], [16].

The deposition of Pt on BDD electrodes has thus been investigated using different routes for fuel cell and sensor applications. For example, Pt was co-deposited with BDD during CVD growth [17], implanted into BDD [18], thermally decomposed on BDD [19] and electrochemically deposited on BDD [20], [21]. For this latter approach, it is notable that the morphology of the deposited Pt is influenced by the electrochemical method used in the deposition, as smooth Pt particles are observed after linear potential sweep deposition [22] in contrast to dendritic structures after potentiostatic deposition [21].

Pt–Ru alloys have also been deposited on diamond by a sequential linear potential sweep method in which Ru deposition followed Pt deposition. The material exhibited higher oxidation currents and lower onset potentials for methanol oxidation than pure Pt. [22] However there is at present no information as to the electrodeposition approach which can provide the best Pt–Ru nanoparticle decorated BDD electrodes. In particular the relative merits of sequential vs. simultaneous deposition of the Pt and Ru elements, and potentiostatic vs. potential sweep methods are not known. The electrochemical properties of BDD are quite different to HOPG. The former tends to be dominated by heterogeneous electroactivity associated with the distribution of the boron dopant [23], while the latter depends on the presence of defects and the distribution of basal and edge planes [10]. It is thus of interest therefore to compare the two carbon forms with regard to the influence of the electrodeposition method employed.

In this study, we thus use a potentiostatic method to simultaneously deposit Pt–Ru at different potentials, and the deposits were compared with those from sequential deposition. The deposits were characterized by XPS and SEM. The electrocatalytic performance of the decorated electrodes from the two deposition methods were compared to the findings on the HOPG electrode, and significant differences were found [10], although the overall general conclusion that electrocatalytic properties depends significantly on electrodeposition conditions remains a valid one for these BDD electrode supports.

Section snippets

Experimental

BDD wafers ([B]>1020 cm−3) of 10 × 10 × 0.6 mm were obtained from Element Six Co. The BDD wafers were mounted in a home-built PTFE holder with a circular area of 0.317 cm2 exposed to the electrolyte. Ruthenium (III) chloride hydrate (RuCl3·xH2O) was purchased from Sigma–Aldrich, and dihydrogen hexachloroplatinate (IV) hexahydrate (H2PtCl6·6H2O) was purchased from Alfa Aesar.

The electrochemical deposition and characterization of Pt–Ru particles was conducted at room temperature (∼21 °C) using a CHI900B

Morphology and XPS characterization of Pt–Ru deposits on BDD

Pt–Ru electrocatalytic particles were simultaneously or sequentially deposited on BDD electrodes. Sequential deposition was conducted by a potentiostatic method at −0.2 V for 10 min, starting with Pt deposition in 0.5 mM H2PtCl6 in 0.5M H2SO4 solution followed by Ru deposition in 0.5 mM RuCl3 in 0.5M H2SO4 solution for a similar time. A reverse procedure, Ru deposition followed by Pt deposition, was also explored, but proved to be hard to control because of the significant loss of Ru from the

Conclusions

The electrodeposition of Pt–Ru electrocatalytic particles was studied by comparing simultaneous and sequential deposition on BDD supports using a potentiostatic method. Smooth cluster morphologies were observed for simultaneous deposition and a dendritic structure was observed for sequential deposition from SEM studies. The morphology of sequential deposition is dominated by Pt deposition in the first step, while a change of aggregate morphology due to the presence of Ru is observed for

Acknowledgement

The authors would like to thank EPSRC (Grant no. EP/F025513/1) and Xiao acknowledges China Scholarship Council for financial support. We also acknowledge support from the European Commission Marie Curie Initial Training Project MATCON (FP7-PEOPLE-ITN-2008-238201).

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