Enhanced Ethanol Oxidation Activity for a Direct Fuel Cell with Silica-Carbon Composite Catalyst Support

Introduction

Direct ethanol fuel cells (DEFCs) are expected as compact power sources for small electric devices including mobile applications because of their simple system without a reformer and high volumetric energy densities, i.e., 6.3 kWh L⁻¹. One of the technical issues of DEFCs is slow electrooxidation rate and low selectivity of CO₂. In order to improve the electrocatalytic reaction rate, Pt-based binary systems, such as Pt-Ru, Pt-Sn [1,2], were proposed. However, these catalysts could improve the partial oxidation of ethanol, but the selectivity of carbon dioxide was not increased compared to that of Pt [1,2].

Recently, Kavanagh and coworkers insisted that adsorbed OH on Pt is suspected to increase activation energy of the C–C split reaction [3]. Here, SiO₂ shows relatively high electronegativity, thus SiO₂ is expected to reduce electronic interaction between the OH species and the C2 intermediate. Hydrogen spillover of SiO₂, which can renew active site of Pt, was reported [4]. Thereby, to overcome abovementioned issues, SiO₂embedded carbon nanofiber (SECNF) is proposed as a catalyst support for ethanol electrooxidation. Fibrous catalyst layers are expected to improve mass transfer rate because of its high porosity [5]. In the present work, we focused on the ethanol oxidation activity of Pt/SECNF with several Si/C ratio.

Experimental

The Pt/SECNFs and the Pt/carbon nanofiber (CNF) were prepared by electrospinning technique, heat treatments, steam activation, and microwave-assisted polyol reduction. The diameters of the fibers were measured by scanning electron microscope (SEM). The crystal structure of the Pt/SECNF was analyzed by X-ray diffraction (XRD) by using Scherrer’s equation.

The Si/C weight ratio of the Pt/SECNF was evaluated by the energy dispersive X-ray tecnique (EDX). The electrocatalytic activity of the catalysts was measured using a glassy carbon electrode, Ag/AgCl (KCl sat.), and Pt mesh as the working, reference, and counter, respectively. The catalyst was mounted on the working electrode at 1.0 mg cm⁻². The measurement was conducted in a water solution of 0.5 M ethanol with 0.5 M H₂SO₄ at scan speed of 20 mV s⁻¹. Electrochemical active surface area (ECSA [m_-Pt² g_-Pt⁻¹]) was measured hydrogen adsorption/desorption profile in 0.5 M H₂SO₄solution.

Results and Discussion

The mean diameter of the fiber catalysts were around 340 nm from SEM observations. The crystalline size of Pt (111) were almost same, around 5 nm, for all catalysts from XRD results. Figure 1 shows relationship between Si/C weight ratio, and ECSA or mass activity at 0.70 V vs. RHE. The ECSA value of the catalysts were almost constant and this results are consistent with the crystalline sizes of Pt. The mass activity was successfully improved by SiO₂addition to the catalyst support. The high mass activity of the Pt/SECNFs is attributed to the hydrogen spillover effect. Also if carbon dioxide production rate is improved by the SECNF, the current density should be increased because complete ethanol oxidation gives twelve electrons which is 6 times more than production of acetaldehyde (partial oxidation).

Conclusion

Pt/SECNF was proposed as a catalyst for ethanol electrooxidation to improve reaction rate. The mass activity was successfully improved by SiO₂ addition to the catalyst support. This activity enhancement is attributed to the interaction between Pt and SiO₂, i.e., hydrogen spillover. Pt/SECNF is a promising catalyst for DEFCs which can reduce Pt loading.

Acknowledgement

This research was supported by JSPS KAKENHI Grant Number 26870093, the research aid from Tokyo Tech Fund (26-102). We significantly thank this foundation.

References

[1] N. Nakagawa et al., J. Power Sources, 199, 103–109 (2012)

[2] S. Rousseau et al., J. Power Sources, 158, 18–24 (2006)

[3] R. Kavanagh et al., Angew. Chem. Int. Ed., 51, 1572–1575 (2012)

[4] W. C. Conner and J. L. Falconer, Chem. Rev., 95, 759–788 (1995)

[5] C. Feng et al., J. Power Sources, 242, 57–64 (2013)