Elsevier

Journal of Power Sources

Volume 177, Issue 2, 1 March 2008, Pages 247-253
Journal of Power Sources

ZrO2–SiO2/Nafion® composite membrane for polymer electrolyte membrane fuel cells operation at high temperature and low humidity

https://doi.org/10.1016/j.jpowsour.2007.11.081Get rights and content

Abstract

Recast Nafion® composite membranes containing ZrO2–SiO2 binary oxides with different Zr/Si ratios are investigated for polymer electrolyte membrane fuel cells (PEMFCs) at temperatures above 100 °C. Fine particles of the ZrO2–SiO2 binary oxides, same as an inorganic filter, are synthesized from a sodium silicate and a carbonate complex of zirconium by a sol–gel technique. The composite membranes are prepared by blending a 10% (w/w) Nafion®-water dispersion with the inorganic compound. All composite membranes show higher water uptake than unmodified membranes, and the proton conductivity increases with increasing zirconia content at 80 °C. By contrast, the proton conductivity decreases with zirconia content for the composite membranes containing binary oxides at 120 °C. The composite membranes are tested in a 9-cm2 commercial single cell at both 80 °C and 120 °C in humidified H2/air under different relative humidity (RH) conditions. Composite membrane containing the ZrO2–SiO2 binary oxide (Zr/Si = 0.5) give the best performance of 610 mW cm−1 under conditions of 0.6 V, 120 °C, 50% RH and 2 atm.

Introduction

Most recent research on polymer electrolyte membrane fuel cells (PEMFCs) has been focused on the development of new proton-conducting membranes (CPEMS) for operation at higher temperatures with lower humidification levels [1], [2], [3], [4], [5], [6]. Operation of PEMFCs above 100 °C has many advantages such as faster electrode kinetics, higher tolerance to impurities in the fuel gas, the need for smaller heat exchanger, and easier water and thermal management [7]. Common PEMs are based on a hydrated perfluorosulfonic acid (PFSA) polymer such as Nafion®. This type of polymer requires water for proton conductivity, and the proton conductivity decreases considerably at temperatures above the boiling point of water due to dehydration. In addition, dehydration also causes shrinkage of the membrane and increases the contact resistance at the membrane and electrode interface [8]. Therefore, conventional membranes cannot be used in PEMFC operation at temperatures above 100 °C. Hence, the development of novel membranes that are thermomechanically stable at higher temperature is vital for further commercialization of PEMFCs in a wide range of applications.

Many approaches have been adopted in the search for improved membranes and include the modification of conventional PFSA membranes to improve their water retention properties at higher temperatures through the incorporation of various hygroscopic inorganic particles. Such species improve the mechanical properties of the membranes because specific interactions between the inorganic and organic components help to improve membrane water management [9]. Moreover, the inorganic particles form a new membrane structure that inhibits the direct permeation of reaction gases [1].

The PFSA polymers-based organic/inorganic composite membranes have been investigated using hydrophilic and/or proton-conducting compounds such as SiO2, TiO2, and ZrO2 as inorganic fillers [1], [3], [5]. Saccá et al. [1] developed composite membranes containing different percentages of commercial ZrO2 that were tested in a commercial single cell in the temperature range of 80–130 °C [1]. The composite membranes showed a higher water retention capacity and better single-cell performance than a Nafion® 112 membrane. A membrane containing a 10% (w/w) ZrO2 gave the highest power density of 387 mW cm−2 at 130 °C and 85% RH. Zeng at al. [5] modified PFSA membranes by incorporating a silica sol. The perfluorosulfonic acid ionomer (PFSI)/silica composite membrane exhibited better water uptake, proton conductivity and cell performance [5].

In this study, a ZrO2–SiO2 binary oxide is synthesized as an inorganic filler to modify a PFSA membrane. ZrO2–SiO2 binary oxide is known to be a catalyst for oxidizing cyclohexane [10] and the isomerization of n-hexane [11], because ZrO2–SiO2 is a solid-state acid and has been widely examined in the catalytic field [12], [13]. The acidity of the ZrO2–SiO2 binary oxide can be improved further by post-treatment with a strong acid [14]. Therefore, the ZrO2–SiO2 binary oxide can effectively improve the proton conductivity of the membranes. The binary oxide contains ZrO2 and SiO2 in a single particle and has bonding structures of Zr–O–Zr, Si–O–Si, and some Zr–O–Si [15].

Fine particles of the binary oxide are synthesized at different Zr:Si mole ratios, and ZrO2 and SiO2 are synthesized individually using a sol–gel technique. Composite membranes of SiO2/Nafion®, ZrO2/Nafion® and ZrO2–SiO2/Nafion® are also prepared by means of the doctor-blade casting method. The prepared membranes are characterized both physically and chemically and tested in a commercial single cell to determine the influence of the binary oxide as an inorganic filler on the properties of PEMs in PEMFCs for higher temperature operation.

Section snippets

Synthesis of ZrO2–SiO2 binary oxide

The ZrO2–SiO2 binary oxide was synthesized using the following procedure [14]: ZrOCl2·8H2O was dissolved in a distilled water and a solution of (NH4)2CO3 was added until a white precipitate of ZrOCO3 was formed. The precipitate was washed several times with distilled water to remove the chloride ions. The (NH4)2CO3 solution was added to the precipitate with constant stirring until the pH 8. Subsequently, anionic surfactant sodium dodecyl sulfate was added to the solution. The mixture was

FT-IR spectroscopic analysis of ZrO2–SiO2 binary oxides

The FT-IR spectra of the ZrO2–SiO2 particles with different Zr:Si mole ratios are presented in Fig. 2. The spectra show a sharp peak at 1020 cm−1 and this suggests the formation of a Zr–O–Si bond. By comparison, asymmetric stretching vibration of the Si–O–Si bond is observed at 1100 cm−1 for pure silica. The shift in stretching frequency is due to deterioration of the silica framework after insertion of zirconium atoms [14]. The absorption peak at 1630 cm−1 is attributed to coordinated and

Conclusions

Fine particles of a ZrO2–SiO2 binary oxide are synthesized with different Zr:Si ratios, and composite membranes are prepared using the recasting procedure. The bonding structure of the binary oxide and the Si–O–Zr linkage are verified by FT-IR spectroscopy. Scanning electron micrographs confirm good distribution of inorganic fillers in the polymer matrix, and thermogravimetric analysis shows the presence and amount of inorganic fillers.

Water uptake increases with increasing silica content, and

Acknowledgement

This work was financially supported by the Seoul Research and Business Development Program.

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