Influence of the size and shape of silica nanoparticles on the properties and degradation of a PBI-based high temperature polymer electrolyte membrane
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.
References (43)
- et al.
High temperature proton exchange membranes based on polybenzimidazoles for fuel cells
Prog. Polym. Sci.
(2009) Emerging membranes for electrochemical systems: Part II. High temperature composite membranes for polymer electrolyte fuel cell (PEFC) applications
J. Power Sources
(2004)- et al.
Transient evolution of carbon monoxide poisoning effect of PBI membrane fuel cells
J. Power Sources
(2007) - et al.
Experimental characterization and modeling of commercial polybenzimidazole-based MEA performance
J. Power Sources
(2006) - et al.
Polymer membranes for high temperature proton exchange membrane fuel cell: recent advances and challenges
Prog. Polym. Sci.
(2011) - et al.
High temperature PEM fuel cells
J. Power Sources
(2006) - et al.
Synthesis and properties of fluorine-containing polybenzimidazole/silica nanocomposite membranes for proton exchange membrane fuel cells
J. Membr. Sci.
(2007) - et al.
Proton conductivity of phosphoric acid doped polybenzimidazole and its composites with inorganic proton conductors
J. Membr. Sci.
(2003) - et al.
Polymer–ceramic composite protonic conductors
J. Power Sources
(2003) - et al.
Improving the proton conductivity and water uptake of polybenzimidazole-based proton exchange nanocomposite membranes with TiO2 and SiO2 nanoparticles chemically modified surfaces
J. Power Sources
(2011)
PBI-based composite membranes for polymer fuel cells
J. Power Sources
Polybenzimidazole (PBI)-functionalized silica nanoparticles modified PBI nanocomposite membranes for proton exchange membranes fuel cells
J. Membr. Sci.
Poly(2,5-benzimidazole)–silica nanocomposite membranes for high temperature proton exchange membrane fuel cell
J. Membr. Sci.
Enhancement of the gas separation properties of polybenzimidazole (PBI) membrane by incorporation of silica nanoparticles
J. Membr. Sci.
Simulated start–stop as a rapid aging tool for polymer electrolyte fuel cell electrodes
J. Power Sources
Proton-conducting membranes based on benzimidazole polymers for high-temperature PEM fuel cells. A chemical quest
Chem. Soc. Rev.
Oxygen reduction on carbon supported platinum catalysts in high temperature polymer electrolytes
Electrochim. Acta
Polymer electrolyte fuel cells
Adv. Electrochem. Sci. Eng. (Wiley-VCH Verl. GmbH)
Approaches and recent development of polymer electrolyte membranes for fuel cells operating above 100 °C
Chem. Mater.
Kinetics of O2 reduction on a Pt electrode covered with a thin film of solid polymer electrolyte
J. Electrochem. Soc.
Conductivity of PBI membranes for high-temperature polymer electrolyte fuel cells
J. Electrochem. Soc.
Cited by (0)
- 1
These authors contributed equally.