Direct synthesis of sulfonated mesoporous silica as inorganic fillers of proton-conducting organic–inorganic composite membranes

https://doi.org/10.1016/j.memsci.2010.04.024Get rights and content

Abstract

The mesoporous silica sphere grafted with organic sulfonic acid has been synthesized by a condensation reaction of 1,3-propane sultone and 3-mercaptopropyltrimethoxy silane, followed by hydrothermal reaction with tetramethoxysilane. Proton-conducting organic–inorganic composite membranes were cast together with Nafion® using these functionalized mesoporous silicas as inorganic fillers. The composite membrane showed increased water uptake and higher ionic conductivity relative to recast Nafion membrane under low relative humidity condition. This led to enhanced performance of proton exchange membrane fuel cell employing the composite membrane containing the sulfonated mesoporous silica under the low humidity condition without external humidification of reactant gas. The current density at 0.5 V of the membrane–electrode assembly (MEA) fabricated with composite membrane containing the sulfonated mesoporous silica was ca. 3.3 times higher than that of the MEA fabricated with recast Nafion membrane.

Introduction

Among various fuel cells, polymer electrolyte membrane fuel cell (PEMFC) is suitable for applications including automobiles and residential heat and power supply [1]. The key components that determine the performance and cost of PEMFC are electrocatalysts and the proton exchange membrane. The latter is dictated by polymer electrolyte membranes (PEMs, most commonly Nafion®), which are employed to provide proton conduction from the anode to the cathode and effective separation of the anode (hydrogen) and cathode (oxygen) reactants. The Nafion PEM must be kept well hydrated to retain acceptable proton conductivity. Loss of water content in the PEM leads to a sharp decrease in the proton conductivity and shrinkage of the membrane, which results in mechanical degradation of the membrane–catalyst interface. Thus hydrogen is usually humidified externally to saturation before it is fed to the fuel cell. Yet, it is obviously desirable to run PEMFC without externally humidifying the reactant gases – it reduces cost, size and complexity of the fuel cell system. For this operation, it is vital to improve PEM proton conductivities under low humidity conditions [2].

A common strategy to improve the water retention capacity is to integrate hygroscopic materials such as silicon oxides [3], [4], [5], [6], organically modified silicas [7], [8], metal oxides [9], metal phosphonates [10], and heteropolyacids [11] into the Nafion membranes. However, these composite membranes containing inorganic moieties did not always lead to desired improvement in the performance of membrane–electrode assembly (MEA), mainly because the proton conductivity of the composite membranes containing these less-proton-conductive oxides was markedly lowered compared with that of pristine Nafion® membrane. In previous works, we modified the surface of montmorillonite (MMT) or TiO2 with an organic sulfonic acid group and prepared composite membranes with Nafion® in order to minimize the loss of proton conductivity caused by adding the inorganic oxides while reducing the methanol permeability [12], [13], [14]. Surface sulfonated or organically modified clay/Nafion composite membranes were studied in comparison with the commercial Nafion membrane [15], [16]. The water retention of these composite membranes was enhanced, whereas their proton conductivity was slightly reduced. However, the power densities of the fuel cells using these composite membranes were significantly improved, especially in a non-humidified gas feeding condition. Porous silica is a known proton conductor [17]. Recently, hybrid membranes of Nafion and mesoporous silica grafted with sulfonic acid groups have been synthesized by the sol–gel process [18]. The high surface area of mesoporous silica and presence of hydrophilic sulfonic and silanol groups inside the mesopores enhanced the water uptake.

In this work, we report a new type of directly sulfonated mesoporous silica (HSO3-DSMS) with the sulfonic acid group derived from organic sulfonated methoxy silane and 3-mercaptopropyltrimethoxy silane (3-MPTMS). This directly sulfonated mesoporous silica (HSO3-DSMS) was employed to form composite electrolyte membranes with Nafion®. The characteristics of the membrane were studied in terms of water uptake, proton conductivity and spectroscopic properties, and the performance of MEA fabricated from these membranes was evaluated for PEMFC without external humidifier system.

Section snippets

Synthesis of sulfonated mesoporous silica by direct sulfonation method

1,3-Propane sultone (1,3-PS), 3-mercaptopropyltrimethoxy silane (3-MPTMS), and NaOH were added to absolute methanol. The solution was heated at 50 °C for 24 h and a white precipitate of dialkyl sulfide silane was formed. The mixture containing the white precipitate was dissolved in distilled water. A surfactant template, hexadecyltrimethylammonium bromide (CTABr) and a silica source, tetramethoxysilane (TMOS) were added to the above solution. The resulting gel was maintained under continuous

Synthesis and characterization of sulfonated mesoporous silica

A new type of sulfonated mesoporous silica (HSO3-DSMS) was synthesized by direct sulfonation, i.e., reaction of 1,3-propane sultone (1,3-PS) and 3-mercaptopropyltrimethoxy silane (3-MPTMS) to obtain a silane with dialkyl sulfide functionality (HSO3-RSR–), followed by hydrothermal reaction with tetramethoxysilane (TMOS) as depicted in Scheme 1 (Route 1). For comparison, sulfonation was also carried out in two steps to obtain the HSO3-MS sample. As shown in Route 2, mesoporous silica grafted with

Conclusions

In this work, we have demonstrated a simple and effective method to synthesize mesoporous silica grafted with organic sulfonic acid in a single step process. The mesoporous silica could also been synthesized by two-step method, i.e., thiol grafting followed by sulfonation. The directly sulfonated silica showed higher surface area and pore volumes. The sulfonated mesoporous silica was employed as an effective filler of a composite membrane with Nafion. The fabricated composite membrane showed

Acknowledgements

This work was supported by Korea Center for Artificial Photosynthesis (KCAP) and Brain Korea 21 program funded by the Ministry of Education, Science, and Technology of Korea.

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