Hybrid composite membranes of chitosan/sulfonated polyaniline/silica as polymer electrolyte membrane for fuel cells

Highlights

• Hybrid membranes based on CS-PAni/SiO₂ were prepared using solvent casting technique.

• CS-PAni/SiO₂ membranes exhibit excellent mechanical and thermal stability.

• Chitosan membranes with 3 wt% PAni/SiO₂ shows high electrochemical selectivity.

Abstract

A series of novel ionic cross-linked chitosan (CS) based hybrid nanocomposites were prepared by using polyaniline/nano silica (PAni/SiO₂) as inorganic filler and sulfuric acid as an ionic cross-linking agent. The CS-PAni/SiO₂ nanocomposites show enhanced mechanical properties and improved oxidative stabilities. These nanocomposites can be effectively used as environmental friendly proton exchange membranes. Incorporation of PAni/SiO₂ into CS matrix enhances water uptake and facilitates the phase separation which enables the formation of hydrophilic domains and improves the proton transport. Moreover, the doped polyaniline also provides some additional pathways for proton conduction. The membrane containing 3 wt% loading of PAni/SiO₂ in chitosan (CS-PAni/SiO₂-3) exhibits high proton conductivity at 80 °C (8.39 × 10⁻³ S cm⁻¹) in fully hydrated state due to its excellent water retention properties. Moreover, methanol permeability of the ionic cross-linked CS-PAni/SiO₂ nanocomposite membranes significantly reduces with the addition of PAni/SiO₂ nano particles. The CS-PAni/SiO₂-3 composite membrane displays the best overall performance as a polymer electrolyte membrane.

Introduction

Nanohybrid materials are part of an interdisciplinary field among material science, nanotechnology, and life science (Al Sagheer & Muslim, 2010; Hou, Shi, & Yu, 2009). They are gaining importance in various applications such as biosensors, structural materials, catalysts, separation membranes, regenerative medicine, food packing, energy conversion and storage, etc. Chitosan is one of the promising natural polymers with characteristics such as biodegradability, chemical inertness, biocompatibility, hydrophilicity, good film-forming properties, and low cost (Liu, Hsu, Su, & Lai, 2005; Yavuz, Uygun, & Bhethanabotla, 2009). Chitosan based membranes have gained considerable attention as polymer electrolyte membrane (PEM) owing to their excellent thermal and chemical stability, good mechanical properties with low methanol permeability and low cost, etc. However, the pristine chitosan has low conductivity due to the absence of mobile hydrogen ions in the structure (10⁻⁹ S cm⁻¹) (Xiao, Xiang, Xiu, & Lu, 2013). A variety of modification approaches such as cross − linking, doping and blending have been explored to prepare efficient membranes based on chitosan.

The polymer electrolyte membrane is also known as proton exchange membrane (PEM). It is the core component of a polymer electrolyte membrane fuel cell (PEMFC), acting as an electrolyte for proton conduction as well as a barrier for fuel crossover (Ge et al., 2015). Among all the strategies that have been explored to prepare efficient PEM materials, much research attentions have been paid to the incorporation of nano-sized inorganic fillers into the polymeric matrix to fabricate hybrid composite membranes (P. Chen, Hao, Wu, Li, & Wang, 2016). The incorporation of hygroscopic inorganic nanomaterials such as silica, titanium dioxide, zirconium dioxide, nanoclays, etc into the polymer matrix has shown improvement in properties of composite membranes like water retention capacity and ionic conductivity at low to medium temperatures (Albu, Maior, Nicolae, & Bocaneala, 2016; Di Noto, Boaretto, Negro, & Pace, 2010). These hydrophilic fillers can provide numerous hydrogen bonding sites for high water uptake of membranes. However the weak interaction between the organic polymer and inorganic filler in the hybrid membrane always results in poor interfacial interaction and hence reduction in conductivity with increased addition of filler (Wu, Shen, Cao, Li, & Jiang, 2014; Yang, Shen, Varcoe, & Wei, 2009). Nafion − titanium dioxide composites were reported to exhibit the maximum conductivity at 1 wt% filler loading (Barbora, Acharya, & Verma, 2009). The composite membranes were developed from sulfonated poly(ether ether ketone) and zirconium titanium phosphate (ZTP) for DMFC applications. It was noticed that the conductivity increased with the increase in filler content up to 5 wt% of ZTP, beyond which both conductivity and ion exchange capacity were decreased due to poor interaction between polymer matrix and the filler (Sangeetha Rani, Beera & Pugazhenthi, 2012). The modifications of the fillers with functional groups such as amine, sulfonic acid group, etc. or through grafting using macromolecules are interesting methods to suppress the fuel permeation and improve the morphology, water retention capacity at higher temperature and proton conduction of the material (Chen et al., 2016; Gupta, Madhukar, & Choudhary, 2013).

Among different metal oxides, the hydrophilic silica has been considered as an attractive material for preparation of proton exchange membranes owing to its unique ability to impart reinforcement and thermal stability. Sagheer et al. reported improvement in thermal and mechanical stability of chitosan membranes with silica filler (Al-Sagheer & Muslim, 2010). Cross linked PVA-SiO₂ hybrid membranes were successfully utilized by Kim et al. to achieve conductivity in the range of 10⁻³–10⁻² S cm⁻¹ and methanol permeability of 10⁻⁸–10⁻⁷ cm² s⁻¹ (Kim, Park, Rhim, & Lee, 2004). The high surface area and hydrophilicity of silica when added to chitosan may provide more proton channels and retain more water in membrane, which would be beneficial for higher proton conductivity (Devanathan, 2008). In addition, the presence of hydroxyl groups on the surface of the silica particles allow the desired chemical modification (Laberty Robert, Valle, Pereira, & Sanchez, 2011). Nikje et al. prepared hybrid membranes based on chitosan and organically modified nano silica and reported better thermal stability and dispersion with 3 wt% filler (Nikje & Tehrani, 2009). It is believed that hybridizing inorganic silica with proton conducting groups can improve the structural/thermal stability and proton conductivity at higher temperature. Polyaniline is an intrinsically conducting polymer with specific interest for ion exchange membranes due to its excellent stability, redox reversibility and unique electrochemical properties. Jian Li et al. prepared polyaniline grafted chitosan membrane and reported remarkable improvement in cation exchange capacity (Li et al., 2015). SPEEK/Polyaniline composite membranes reported by Nagarale et al. shows improvement in proton conductivity and fuel cross over resistance after the addition of polyaniline (Nagarale, Gohil, & Shahi, 2006). Tan and Belenger carried out extensive characterization of Nafion/polyaniline composite membranes for proton conduction in fuel cell technology (Chen et al., 2007). Silica and polyaniline has been studied in different polymer matrices such as Nafion and SPEEK and proved to be synergistic filler for DMFC applications (Nagarale et al., 2006, Sonpingkam and Pattavarakorn, 2014, Tripathi and Shahi, 2011). Moreover, the presence of dopants through physico-chemical interactions can control and stabilize the conductivity of polyaniline (Albu et al., 2016). The electronic structure of polyaniline doped with camphor sulfonic acid and 2–propanesulphic acid has been reported by Magnuson et al. (Magnuson, Guo, & Butorin, 1999). It has been shown that the use of sulfuric acid and p-toluene sulfonic acid dopants enhances both ac conductivity and electrochemical properties of polyaniline (Bhandari & Khastgir, 2015).

This paper reports the preparation of organic – inorganic hybrid nanocomposites of chitosan – polyaniline/silica (PAni/SiO₂). This article also deals with the modification of hydrophilic silica using polyaniline specially to improve the morphology and proton conductivity at low water content. The long conjugated polyaniline chains are expected to promote the proton mobility in the composite membrane (Kumar, Khan, Al Othman, & Siddiqui, 2013; Yang et al., 2009). These hybrid nanocomposites may effectively be used as proton conducting membrane for fuel cell. These membranes prepared with different compositions are characterized to check their applicability in the fuel cell. PAni/SiO₂ was prepared by electrochemical synthesis of sulfonated PAni followed by mixing with SiO₂ and the membrane materials were prepared by simplistic solvent casting technique. The prepared membranes were cross linked using sulfuric acid. The main idea with the use of sulfonated polyaniline sheet is to increase the number of charge carriers which will facilitate conduction and reduce methanol permeability. In addition, cross-linking is done to provide not only significant improvement in hydrolytic stability but also better mechanical integrity through reaction of sulfuric acid and chitosan. Scrutiny of available literature reveals that there is no report available on chitosan (CS) − PAni/SiO₂ composite membrane electrolytes for polymer electrolyte membrane fuel cell (PEMFC) applications. The present investigation reveals that PAni/SiO₂ can improve the overall ionic conductivity, mechanical properties and fuel cross over resistance of chitosan based polymer electrolyte membranes. As the matrix polymer used (chitosan) is natural and biodegradable, these composite membranes may also be considered as environmental friendly cost effective materials.