Characterization and enhancement of carbon nanotube-supported PtRu electrocatalyst for direct methanol fuel cell applications

Abstract

In this study, a carbon nanotube-supported PtRu electrocatalyst (PtRuCNT) was prepared, characterized and investigated for methanol electro-oxidation by catalytic activity enhancement using an electrochemical treatment. From XPS analyses, the as-prepared catalyst was found to mainly composed of Pt(0)/Pt(II) states for the Pt element and Ru(0)/Ru(IV) states for the Ru element. When the electrocatalyst was subjected to an enhancement treatment, the Ru(IV) state increased substantially from 29.50% to 44.11%. Both CO-stripping experiments and open-circuit cell voltage measurements indicated that the treated PtRuCNT has given rise to an improved performance on methanol electro-oxidation caused mainly by the increase of the Ru(IV) state in this particular case. The single-cell test also revealed that a direct methanol fuel cell (DMFC) can be put into its full operation in a short time. A direct application of this finding is to significantly shorten the activation time of a new DMFC stack. However, the electrochemically treated PtRuCNT catalyst still needs a continuous enhancement mechanism to sustain its enhanced activity. A promotional model is proposed to explain the phenomenon observed and a remedial approach is also suggested to solve the problem for practical applications.

Introduction

Direct methanol fuel cells (DMFCs) have several advantages over other types of fuel cell to be used as dependable and long-lasting portable power sources to replace batteries in a variety of electronic products, including laptop computers and cellular phones. Although significant advances have been achieved for the DMFC systems in recent years, considerable efforts are still needed to put them into real commercialization with smaller sizes, lower costs and better durability. In addition to a variety of nanocatalysts currently being developed, the choice of suitable carbon support materials for the electrocatalysts is also an important factor that can significantly affect the performances of supported electrocatalysts owing to interactions, which modify the catalytic activities, between metal catalysts and carbon support materials [1]. Conductive carbon black powders, such as Vulcans, are commonly used as membrane fuel cell catalyst support materials. However, these conventional carbon materials have been investigated for a long time and substantial information has already been accumulated.

With the advancement of nanotechnology, currently the development of DMFCs has stressed on the applications of nanomaterials to the syntheses of state-of-the-art nanocatalysts. The use of carbon-based nanomaterials [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], e.g., carbon nanotubes, carbon nanofibers, carbon nanocoils, and carbon nanohorns, as a new generation of catalyst supports is the most common practice. This is due to the distinctive characteristics of such new carbon nanomaterials [12], [13], such as more crystalline structures with high electrical conductivities, excellent corrosion resistances, and high purities containing less catalyst poisons, compared to conventional carbon black powders. The fabricated electrode is also more favorable for the transfer of reactants in the catalyst layer leading to improved fuel cell reactions. In general, the prepared catalysts were well-controlled within a suitable nanosize range. Various synthesis methods are used to prepare the catalysts with the treatment of carbon nanometarials. Such treatment uses strong acids to form surface functional groups as a prerequisite.

Since the pioneering work of Bockris and Wroblowa [14], it has been realized that the capability of PtRu electrocatalyst to exhibit a high methanol electro-oxidation activity is due to the ability of the Ru element to form active hydroxyl species with water at a low potential range, which provides the needed oxygen to CO_ad for the formation of CO₂ as a complete methanol electrochemical oxidation reaction. This has solved most of the catalyst-poisoning problems for the DMFC anode. At present, PtRu is the most used anode catalyst in a DMFC. Although a variety of modified multi-component catalysts [15], [16], [17], [18], [19], [20], [21], such as PtRuIr, PtRuP, PtRuWC, PtRuCo, PtRuRhNi, and PtRuSnW, as well as some interesting alternative catalysts [22], [23], e.g., PtCeO₂ and PtVO, have been proposed and actively investigated, their long-term performances are still waited to be seen. However, the PtRu anode catalyst is, in fact, not a true panacea for the DMFCs as it was thought to be. Studies have shown that there are still some serious problems with this binary anode catalyst. For instance, there is a ruthenium dissolution and crossover problem [24], [25], an activity–decay problem, i.e., the durability issue, and a need to significantly increase the electrocatalytic activity of PtRu for more practical applications.

It was reported [26], [27] that the oxidation states of the PtRu catalyst elements play important roles in the process of methanol electro-oxidation, although the conclusions can be very conflicting caused by the pretreatment of the catalysts. It was also found [28], [29], [30] that the X-ray photoelectron spectroscopy (XPS) has provided a useful tool to the investigation of oxidation states of catalyst elements. In this study, we focused on attempting to improve the activity of the PtRuCNT anode catalyst using a simple electrochemical approach. The main idea was to convert the catalyst elements to their higher oxidation states, so it would enhance the catalytic reactions of the anode reactant on the catalyst surfaces. This is deemed to be very useful for significant improvement in the performance of a DMFC.