Proton conductivity of phosphoric acid doped polybenzimidazole and its composites with inorganic proton conductors

Abstract

Phosphoric acid doped polybenzimidazole (PBI) and PBI composite membranes have been prepared in the present work. The PBI composites contain inorganic proton conductors including zirconium phosphate (ZrP), (Zr(HPO₄)₂·nH₂O), phosphotungstic acid (PWA), (H₃PW₁₂O₄₀·nH₂O) and silicotungstic acid (SiWA), (H₄SiW₁₂O₄₀·nH₂O). The conductivity of phosphoric acid doped PBI and PBI composite membranes was found to be dependent on the acid doping level, relative humidity (RH) and temperature. A conductivity of 6.8×10⁻² S cm⁻¹ was observed for PBI membranes with a H₃PO₄ doping level of 5.6 (mole number of H₃PO₄ per repeat unit of PBI) at 200 °C and 5% RH. A higher conductivity of 9.6×10⁻² S cm⁻¹ was obtained by composite of 15 wt.% of ZrP in a PBI membrane under the same conditions. Homogeneous membranes with good mechanical strength were prepared by introducing PWA (20–30 wt.%) and SiWA (20–30 wt.%) into PBI, and their conductivity were found to be higher than or comparable with that of the PBI membrane at temperatures up to 110 °C.

Introduction

Polymer electrolytes have received extensive attention, especially for polymer electrolyte membrane fuel cells (PEMFCs) operating at temperatures above 100 °C. As alternatives to the currently available perfluorosulfonic acid (PFSA) membranes, the newly developed polymer membrane electrolytes can be classified according to the way they are prepared. Most of the conventional polymers can be modified by attaching charged units within their structures and in this way obtain the ionic conductivity. The charged unit is commonly an anion, typically sulfonate (–SO₃⁻) as in the sulfonated hydrocarbon polymers, e.g. polysulfone (PSF) [1], polyetheretherketone (PEEK) [2] and polybenzimidazole (PBI) [3], [4], [5], [6].

The second method is to incorporate a polymer matrix with solid inorganic compounds, the so-called inorganic–organic composites or hybrids. Typical polymers in this group include polymers without functional groups such as polyethylene oxides (PEO) [7] and PBI [8], [9], [10] and polymers with functional groups such as Nafion^® [11], sulfonated polysulfone (SPSF) [12], sulfonated polyetheretherketone (SPEEK) [13]. The solid inorganic compounds include oxides such as amorphous silica and inorganic proton conductors such as zirconium phosphate (ZrP), (Zr(HPO₄)₂·nH₂O), phosphotungstic acid (PWA), (H₃PW₁₂O₄₀·nH₂O) and silicotungstic acid (SiWA), (H₄SiW₁₂O₄₀·nH₂O).

The third method is via chemical interactions between basic polymers and strong acids [14], [15], [16] or polymeric acids [17]. Earlier studies employed basic polymers such as PEO, polyvinyl acetate (PVA), polyacrylamide (PAAM) and polyethyleneimine (PEI) [18]. Most of these acid doped polymers exhibit proton conductivity less than 10⁻³ S cm⁻¹ at room temperature. High acid contents result in high conductivity but sacrifice mechanical stability, especially at temperatures above 70–80 °C. In order to improve the conductive, thermostable and mechanical properties of the acid/polymer membranes, cross-linked polymers (e.g. PEI [19]), thermally stable polymers (e.g. polyoxadiazole (POD) [20] and PBI [14], [15], [16]) and introduction of inorganic fillers or/and plasticizers [8], [21] have recently been investigated.

PBI has excellent textile fiber properties and thermal stability and is one of the most promising polymers for these developments. The proton conductivity of PBI was first studied more than 20 years ago [22], [23]. An attempt to graft functional groups onto PBI was made 10 years ago [24] and has continued in recent years [3], [4], [5]. The conductivity of benzylsulfonate grafted PBI was reported to be as high as 2×10⁻² S cm⁻¹ at 25 °C, however, the polymer films should be maintained in an environment of high relative humidity (RH) in order to avoid shrinking and brittleness [6]. Composites of PBI with PWA/SiO₂ [8] and SiWA/SiO₂ [9] have also been developed with a conductivity about 10⁻³ S cm⁻¹ at temperatures above 100 °C under a RH as high as 100%.

Phosphoric acid doped PBI membranes were first suggested for fuel cell applications in 1995 [14]. Thereafter numerous studies have been performed concerning proton conductivities [14], [15], [16], [25], [26], [27], methanol crossover rate [28], thermal stability [26], water drag coefficient [28], [29], [30] and fuel cell tests [31], [32], [33]. Recently PBI blends with SPSF [17], [34], [35], [36] have also been investigated.

The conductivity measurement is one of the key characterizations of these membranes. Wainright et al. [14] reported a conductivity of 2.2×10⁻² S cm⁻¹ at 190 °C for a PBI membrane with a H₃PO₄ doping level of 5.01. The H₃PO₄ doping level is defined as the mole number of H₃PO₄ per repeat unit of PBI, and the same to all the composite membranes discussed below. Fontanella et al. [27] obtained a conductivity of 4.5×10⁻⁵ S cm⁻¹ for dry PBI with a H₃PO₄ doping level of 6.0 at room temperature. Bouchet and Siebert [15] reported an anhydrous conductivity of 7×10⁻⁶ S cm⁻¹ at a temperature of 30 °C and a H₃PO₄ doping level of 3.05. Li et al. [30] obtained a conductivity of 4.5×10⁻³ S cm⁻¹ for hydrous PBI/H₃PO₄ membranes at a 4.5 doping level of H₃PO₄ at 25 °C.

The large discrepancies of reported conductivity measurements can most probably be attributed to the effect of the water content in both membranes and the atmosphere, especially at high temperatures. The relative humidity (RH) in a hydrogen atmosphere containing different water contents changes dramatically with temperature. For example, an hydrogen stream containing 69% (by mole) water gives a RH of 100% at 90 °C, but only 15% at 150 °C and 5% at 200 °C under atmospheric pressure. In other words, a RH of 70% can only be obtained under pressures of about 5 atm at 150 °C or above 15 atm at 200 °C.

In the present work, special attention was paid to control the RH, especially at high temperatures. Three types of PBI composite membranes were prepared by composite of polybenzimidazole with inorganic proton conductors including ZrP, PWA and SiWA. The conductivity of phosphoric acid doped PBI and its composite membranes were measured in a temperature range up to 200 °C.