Elsevier

Journal of Membrane Science

Volume 454, 15 March 2014, Pages 12-19
Journal of Membrane Science

Influence of the size and shape of silica nanoparticles on the properties and degradation of a PBI-based high temperature polymer electrolyte membrane

https://doi.org/10.1016/j.memsci.2013.12.004Get rights and content

Highlights

  • Incorporation of cross-linked silica particles in PBI-membranes by the sol–gel process.

  • Statistical analysis of silica nanoparticles using TEM.

  • Correlation of particle size, shape and distribution to the membrane properties.

  • The composite membranes show an enhanced chemical stability.

  • Improved lifetime stability in 1300 h constant fuel cell operation.

Abstract

The life time stability of membrane material is one of the major parameters regarding reliability of high temperature polymer electrolyte membrane fuel cells. Present work has improved fuel cell durability and chemical stability by incorporating cross-linked silica particles in phosphoric acid doped poly(2,2′-m-phenylene-5,5′-bibenzimidazole) membranes. Three different silica particle contents were generated in membranes by in-situ sol–gel reaction from the precursor tetraethoxy silane and cross-linked to the polymer chains by using (3-glycidoxypropyl)-methyldiethoxysilane. The size, shape and distribution of the silica nanoparticles were examined by transmission electron microscopy. The amorphous characteristics and the chemical composition of the silica particles were investigated using X-ray diffraction, electron diffraction and energy dispersive X-ray spectroscopy. Detailed statistical analysis showed that by increasing the tetraethoxy silane content, the particle size was reduced while the amount of particles was increased. Ex-situ membrane characterization and in-situ membrane electrode assembly testing revealed a high influence of the silica content on the mechanical stability and start–stop-cycling behavior. The improved lifetime durability of the organic–inorganic composite membrane was proven in comparison to the pure polybenzimidazole membrane in membrane electrode assemblies over 1300 h under constant fuel cell operation in reformate.

Introduction

Polymer electrolyte membrane fuel cells (PEMFC) have gained considerable attention in fuel cell applications over the last several years [1]. The limitation of low temperature PEMFC membranes like Nafion® leads the main focus of future development on membranes which can be operated at temperatures above 100 °C [2], [3]. Savinell et al. presented phosphoric acid doped polybenzimidazole (PBI) for fuel cell operation from 150 °C to 200 °C without humidification [4], [5], [6]. These high temperature PEMFCs (HT-PEMFC) benefit from faster reaction kinetics on the electrodes, increased catalytic activity and simplified heat management [7], [8], [9], [10]. In spite of these advantages, the lifetime of HT-PEMFC still remains the major concern [11]. Striving for the goals of performance, lifetime and cost on the membrane side, different approaches for ruggedized membranes have been reported in the literature [12], [13], [14], [15]. Besides ionic and covalent cross-linking, the third main concept is the application of organic–inorganic composite membranes.

The incorporation of inorganic nanofillers in the polymer matrix is often realized by an in-situ sol–gel creation of the particles during membrane casting [16], [17], [18], [19], [20], [21]. This method prohibits silica nanoparticle agglomeration and therefore enables higher inorganic contents [22]. Chuang et al. prepared silica nanoparticles in-situ from tetraethoxy silane (TEOS) with (3-isocyanatopropyl)triethoxysilane as a cross-linker by the sol–gel technique [23]. The procedure is driven by a combination of hydrolysis and condensation reactions. The resulting silica particles are surrounded by silanole end groups and cross-linked to the polymer chains [24]. By using the sol–gel technique the size, shape and amount of particles are controlled by the concentration and type of additives as well as the reaction parameters such as temperature or time [22], [23], [25], [26]. The smaller the resulting particles are, the more surface sites are available which can interact with the polymer chains and therefore influence the membrane properties [1]. The membrane benefits in various ways such as better retention for liquids, higher mechanical and thermal stability and lower gas permeability [4], [23], [27], [28]. In some cases even better proton conductivity was gained [4], [29], [30]. Originally various different combinations of inorganic fillers and polymer materials were tested and reviewed for low temperature PEMFCs and direct methanol fuel cells [20], [22], [31]. It turned out that silica nanoparticles play an outstanding role because of their high pore volume and thermal stability [21], [32]. These particles were also incorporated in phosphoric acid doped PBI-membranes for HT-PEMFC [24], [33], [34], [35]. This general concept of organic–inorganic composite membranes addresses the weaknesses of PBI-based membranes by improving the mechanical stability in doped status plus reducing the phosphoric acid loss during operation [21]. While the mechanical strength of these membranes is reported to be increased, the proton conductivity is frequently too low [21]. Lifetime analysis of phosphoric acid PBI-based HT-PEMFC have been reported previously [36]. Since the publications in the field of organic–inorganic PBI-based composite membranes concentrate mainly on ex-situ membrane properties, the important question of this materials' long term stability still remains open.

In this work, silica nanoparticles are formed from TEOS by the in-situ sol–gel procedure and cross-linked to the PBI polymer chains with (3-glycidyloxypropyl)-trimethoxysilane (GPTMS) (see Fig. 1). The influence of three different TEOS contents on the size, shape and distribution of the nanoparticles are investigated by transmission electron microscopy (TEM). By applying local energy dispersive X-ray spectroscopy (EDS) measurements at a nanometer regime and electron diffraction the observed particles were proven to be amorphous silica. Their influences on the thermal, chemical and mechanical properties are studied using thermogravimetric analysis (TGA), solvent extraction analysis and stress–strain measurements. Besides in-situ performance testing, the main goal of this work is to prove lifetime and cycle stability of the developed silica stabilized PBI-membrane electrode assemblies (MEA) in long-term fuel cell operation.

Section snippets

Membrane and MEA preparation

Three different membranes with TEOS contents of 40, 80 and 120 wt% of the polymer PBI were prepared. PBI was dissolved in N,N-dimethylacetamide (DMAc, Merck) and stirred for 3 h at 200 °C under pressure. The solution was filtered over a 20 µm filter and added in a glass reactor. A solution of TEOS (Alpha Aesar) and GPTMS (Alpha Aesar) in DMAc was added to the PBI solution under stirring. To the resulting viscous solution, potassium hydroxide (Sigma Aldrich) is added. The reaction product was

Structural analysis

The TEM investigation of the membrane samples containing 40%, 80% and 120% of TEOS and a constant amount of 10% GPTMS reveals the successful formation of silica particles during the in-situ sol–gel reaction. Fig. 2 shows HAADF STEM micrographs of the three different systems. The intensity in these images is proportional to the atomic number squared. Therefore, the obtained silica structures appear bright in the micrographs. The inhomogeneous brightness and the channel-like structure of the gray

Conclusion

By in-situ sol–gel reaction PBI-based membranes with 40%, 80% and 120% of the inorganic silica precursor TEOS was produced. The particles were bound to the polymer PBI by adding a constant amount of 10% GPTMS as a cross-linking agent. Structural analysis by STEM, EDS and XRD proved the formation of amorphous silica particles in the membrane. Their shape, size and distribution were shown to be highly affected by the precursor content. Whereas lower silica contents lead to few large, fibrous

Acknowledgments

Financial support from the German Federal Ministry for Economy and Technology within the program ZIM-KOOP is gratefully acknowledged.

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