Investigation of fumed silica as a platinum support for methanol oxidation reaction

https://doi.org/10.1016/j.ijhydene.2020.06.006Get rights and content

Highlights

  • Fumed silica (FS) was examined as a support material for Pt for the first time.

  • The highest activity for methanol oxidation was obtained in the presence of 12% FS.

  • FS prominently increased the electron transfer-rate and active surface area.

  • Pt-nanoparticles formed a brain-like morphology having helical nanoparticles.

  • FS has a prominent effect on the hydrophilicity and the stability of the electrode.

Abstract

In this study, we have examined fumed silica (FS) as a support material for platinum (Pt) catalyst used in methanol fuel cells for the first time. For this purpose, the carbon paste electrode (CPE) was modified with different amounts of FS to prepare CPE/xFS (x: 3, 6, 9, 12%), and then these electrodes were electrochemically coated with Pt nanoparticles to obtain CPE/xFS@Pt. The presence of FS as a support material prominently increased the methanol oxidation activity of the prepared electrodes. The highest activity was obtained in the presence of 12% FS. Characterizations were performed with cyclic voltammetry (CV), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. SEM and EDX analyses revealed that FS and electroplated-Pt homogeneously distributed on the electrode surface. SEM analyses also showed that the synthesized Pt nanoparticles formed a brain-like structure having inter-connected helical particles. The presence of FS increased the electron transfer rate, the effective surface area, and the hydrophilicity of the electrode surface. The CPE/12FS@Pt preserved its methanol oxidation activity for long periods. The results showed that FS was a highly efficient and durable supporting material for Pt-based catalysts which were used in direct methanol fuel cells.

Introduction

Today, clean, efficient, and sustainable energy technologies are needed because of the problems such as increasing energy demand, depletion of fossil fuels, and global warming. Instead of fossil energy resources whose reserves are decreasing and polluting the environment, the search for environmentally friendly and renewable energy sources has come up [[1], [2], [3], [4], [5]]. Moreover, there is a growing interest in portable power applications. With the technological advances in portable devices such as cameras and laptops, power supplies that can provide safe and long-term use have gained importance. In these cases, fuel cells are considered to be the most suitable technology [6,7]. Among the different types of fuel cells, direct methanol fuel cells (DMFCs) have attracted much more attention because of their promising advantages such as low operating temperature, high power density, and relatively quick start-up [[8], [9], [10], [11], [12], [13], [14]]. Due to its low carbon emission, non-harmful character, and ease of use, methanol is preferred as a fuel.

The two main factors that limit the commercialization of DMFCs are low productivity and high cost. The most important reason for the high cost of DMFCs is the requirement to use Pt-based catalysts for methanol electrooxidation because of their high catalytic activity [[15], [16], [17]]. Another reason is the poisoning of Pt-based catalysts with CO gas produced in fuel cells at low-temperature conditions which decreases the activity [18,19]. Supporting materials are used to reduce the poisoning of Pt catalyst, to increase the active surface area, and to reduce the cost of the system [20,21]. The support materials also reduce the loss of catalyst and help to improve catalyst performance [22,23]. For this purpose, a wide variety of carbon, polymer, and hybrid support materials are used. The studies conducted in recent years showed that carbon materials such as carbon black [24,25], multi-walled carbon nanotube [20,26], carbon nanofiber [27,28], and reduced graphene oxide [29] and polymer materials like polypyrrole [9,30] and polyaniline [31,32] had high activities as catalyst support materials.

Fumed silica (FS) is a type of silicon oxide and is also known as pyrogenic silica. FS is obtained by pyrolysis of silicon tetrachloride in flame or evaporating quartz sand at 3000 °C in an electric arc. The obtained product has a large specific surface area and a high amount of surface silanol (Si–OH) groups [33]. FS attracts attention among various materials with its features such as nano-sized character, non-porosity, chain-like three-dimensional morphology, easy functionalization, and high surface area. Thanks to these properties, FS can be used in many applications such as catalysis, biosensors, and adsorption [34,35]. In our previous studies, we have investigated the effects of FS in biosensor applications and we have obtained promising results [[36], [37], [38], [39]]. However, its use as a catalyst support directly in methanol fuel cells has not yet become widespread. Therefore, in this study, we have investigated FS as a support material for the Pt catalyst in methanol fuel cells. FS improved the surface area, electron-transfer rate of CPE and it also increased the adsorption of methanol onto the electrode surface via its surface silanol (Si–OH) groups, which provides improved methanol oxidation activity. Modification of CPE with 12% FS increased 7 times the oxidation current densities of methanol.

Section snippets

Chemicals and instruments

Fumed silica (powder, 14 nm), PtCl4, and H2SO4 were obtained from Aldrich. Nujol, LiClO4, graphite powder, and methanol were purchased from Alfa Aesar, Fluka, Riedel de Haen and Merck, respectively. The aqueous solutions were prepared with ultrapure water obtained from a Sartorius-Arium Comfort I-1-UV-T water purification system.

Voltammetry measurements were conducted with a three-naked electrochemical cell. A saturated calomel electrode was used as a reference electrode and a platinum wire was

Preparation of Pt-loaded carbon paste electrodes

This study aims to investigate the effect of fumed silica (FS) as support material on the performance of platinum (Pt) electrodeposited carbon paste electrodes in methanol oxidation. For this purpose, firstly, FS modified carbon paste (CPE/xFS; x: 3, 6, 9, 12%) electrodes were prepared by incorporating different amounts of FS into the electrode matrix and after that electrodeposition of Pt onto the electrodes were performed. Cyclic voltammetry was used in the electrodeposition process. The

Conclusions

The modification of CPE with FS was performed to evaluate its effect as a support material in direct methanol fuel cells. The methanol oxidation activity of CPE/12FS@Pt is 7 times higher than that of CPE@Pt. Cyclic voltammetry measurements showed that the electron-transfer rate of redox couples prominently increased in the presence of FS and Pt. The amount of electrodeposited Pt and ECSA values increased 1.88 and 2.52 times in the presence of FS, respectively. XRD patterns and XPS spectrums

Acknowledgements

Financial support of Eskişehir Technical University Scientific Research Projects Commission (Project No: 1804F091) is gratefully acknowledged.

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