A study of new anhydrous, conducting membranes based on composites of aprotic ionic liquid and cross-linked SPEEK for fuel cell application

Highlights

• New composite membranes based on SPEEK/EG/IL were fabricated.

• Composite membranes exhibit good thermal stability than neat SPEEK and XSPEEK membrane.

• Proton conductivity of all composite membranes increased with temperature and amount of ionic liquid.

• Proton conductivity was measured under anhydrous condition in the temperature ranging from 30–140 °C.

Abstract

The present study describe the preparation and characterisation of anhydrous proton conducting composite membranes based on sulfonated poly(ether ether ketone) [SPEEK–degree of sulfonation 70–72%]/ethylene glycol [EG]/ionic liquid by solution casting method using water: ethanol (50:50) as solvent. For this purpose several composite membranes were prepared by mixing solution of SPEEK/ethylene glycol (67:33 wt %) in water:ethanol with varying amounts of 1-butyl-3-methyl-imidazolium trifluromethanesulfonate [bmim][OTf] ionic liquid. The cross-linking of SPEEK was carried out by thermal treatment i.e. by heating in vacuum oven at 80 °C (2 h), 100 °C (2 h), 120 °C (2 h) and 135 °C for 16 h. Ethylene glycol was used as a cross-linker for SPEEK to reduce the leaching out of ionic liquid and enhance the mechanical strength of SPEEK membranes. The membranes were characterized for thermal [thermogravimetry analysis], structural [FTIR–ATR], proton conductivity, morphology (XRD, SEM) and leaching out of ionic liquid with water. FTIR studies clearly showed the interactions between SPEEK, EG and ionic liquid. The proton conductivity and dynamic mechanical properties of the composite membranes were investigated at elevated temperature and under anhydrous conditions. Proton conductivity of all the membranes measured in the temperature range of 30–140 °C under anhydrous conditions was in the range of 10⁻³ Scm⁻¹ which showed an increase with increase in temperature and amount of ionic liquid.

Introduction

Great efforts have been made in the past to solve the world’s energy demand and fuel cell (FC) becomes the first choice for direct conversion of chemical energy into electrical energy because of its ecofriendly nature [1], [2]. Polymer electrolyte membrane fuel cells (PEMFCs) have received much attention among different types of fuel cell due to its prominent feature like – high energy density, low or zero emission of pollutant, quick start up time, low operating temperature etc. Polymer electrolyte membrane ideally permits only transfer of protons between the two electrodes – anode and cathode. Commonly used PEMs are based on perfluorosulfonated polymers i.e. Nafion from Dupont, because of its high chemical, thermal and mechanical stability and have good proton conductivity at temperature below 100 °C. Some critical drawbacks associated with Nafion are: low conductivity at high temperature, high fuel crossover, CO poisoning of electrode Pt/catalyst, complex water management, high cost and engineering issue related to manufacture [3], [4], [5]. Some alternative electrolyte membranes based on polymers like – poly(ether ether ketone)s, poly(imide)s, poly(phenylene)s, poly(sulfone)s etc., have been studied in the past and different Approaches have been adopted to incorporate ionic groups in these materials [6], [7], [8], [9].

The objective of the research was to develop high temperature polymer membranes (>120 °C) to overcome the technical challenges such as tolerance of CO impurity in fuel and complex water and heat management [10], [11]. Various approaches have been adopted by authors to develop high temperature fuel cell membranes and very common approach is to improve water retention and to create effective water channels in the membrane at medium temperature by incorporation of inorganic filler such as SiO₂ [12] and TiO₂ [13], acid- base composites [14], [15], membranes based on proton conductors such as heteropolyacids, zirconium phosphate [16], [17] and chemical cross-linking [18], [19]. Recently there is an increasing demand of PEM for fuel cell which can be operated at high temperature under dry condition: i.e. (1) to replace with an alternative protic solvent to develop anhydrous proton conducting membranes at higher temperature like – phosphoric acid doped poly(benzimidazole) composite membranes [20], [21], [22] but it was found that phosphoric acid was polymerized to pyrophosphoric acid or higher oligomers so proton conductivity decreased drastically. (2) Ionic liquids [ILs] have attracted much attention due to their low-volatility, non-flammability, excellent chemical and thermal stability, wide electrochemical windows, good ionic conductivity and wide temperature liquid range [23], [24], [25]. ILs based composite membranes have been investigated by either solution casting or IL swollen method [26], [27], [28]. Protic ILs [29], [30] were used more frequently because of high ionic conductivity but aprotic ILs [31], [32] have not received much attention due to its low conductivity. Various ILs/polymer composite membranes were studied for PEMFC such as Nafion, SPEEK [33], [34], poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP) [35] and polyacrylonitrile. Sun et al. [36] prepared composite membranes based on Nafion and imidazolium trifluoroacetate (ImH⁺A⁻) and obtained conductivity in range of 10⁻³ S/cm at 120 °C. However the major drawback of ILs/polymer composite membranes was leaching out of ILs over a period of time and decline in mechanical properties of the membranes. For solving the problem of leaching out of ILs, inorganic filler have been incorporated in the composite membranes [37]. However to best of our knowledge no reports are available for investigating leaching of ILs by cross-linking SPEEK using EG.

The present studies were therefore undertaken with an aim to investigate the effect of cross-linking on the performance properties of SPEEK in the presence of varying amount of ionic liquid. We prepared and characterized composite membranes based on SPEEK/EG/IL by solution casting method using water: ethanol (50:50) mixture as solvent for its application in fuel cell which can be operated at high temperature under anhydrous conditions. For this purpose, we used aprotic hydrophilic ionic liquid i.e. 1-butyl-3-methyl imidazolium trifluromethanesulfonate ([bmim][OTf]), SPEEK (Degree of sulfonation 70–72% prepared in the laboratory by post sulfonation method) as matrix and ethylene glycol as cross-linker. Several membranes were prepared by keeping ratio of SPEEK:EG constant (67:33 (wt%)) and varied the amount of ionic liquid (30, 40, 50, 60 and 70 wt %). The effect of ionic liquid content on proton conductivity, structural, mechanical, thermal properties was evaluated.