New high-performance bulky N-heterocyclic group functionalized poly(terphenyl piperidinium) membranes for HT-PEMFC applications

Highlights

• Bulky basic groups grafted ether-free poly(terphenyl piperidinium) membranes are developed for HT-PEMFC.

• Membranes exhibit excellent PA uptake, high conductivity and modest tensile strength simultaneously.

• The cell with PTP-41%BeIm/215%PA has a high power density of around 1 W cm-2 at 180 °C without backpressure.

Abstract

The development of high-performance high temperature proton exchange membranes (HT-PEMs) is a huge challenge that impedes the application of HT-PEM fuel cell (HT-PEMFC). Herein, novel bulky basic group grafted ether-free poly(terphenyl piperidinium) membranes are proposed for HT-PEMFC. Through a simple one-step polymerization, the poly(terphenyl piperidine) (PTP) polymer is synthesized. In order to improve the phosphoric acid (PA) uptake and solubility of PTP in organic solvents, both benzimidazole and pyridine side chains are functionalized into the PTP backbone. Comparing with methyl and methylene pyridine grafted PTP membranes (PTP-Me and PTP-Py), the methylene benzimidazole side chain grafted membrane (PTP-BeIm) exhibits an excellent PA doping content of 215 wt% and a high conductivity of 0.088 S cm⁻¹ at 180 °C under anhydrous condition. The H₂–O₂ fuel cell based on PTP-41%BeIm/215%PA displays a high peak power density of around 1 W cm⁻² at 180 °C without any backpressure, which is about 1.5 times higher than that of PBI/342 wt%PA based cell. Thus, this work develops a facile and low-cost approach on the preparation of high-performance HT-PEMs.

Introduction

The proton exchange membrane fuel cell (PEMFC) is a sustainable clean energy conversion system, which provides favorable properties such as fast start, high power density and efficiency [1,2]. Nafion® is a successfully commercialized perfluorinated sulfonic acid membrane and displays excellent performance below 80 °C under the full wet condition [2,3]. However, due to the complex production process, Nafion type membranes suffer from high cost, while the relatively low operating temperature of Nafion type PEMFCs also causes several technical problems including poor CO tolerance, complex water-thermal management and low electrode kinetics [4]. Thus, researchers have made great efforts to explore non-fluorinated high temperature proton exchange membranes (HT-PEMs) operating at 100–200 °C [[5], [6], [7]]. As the core material of HT-PEMFC, the HT-PEM plays a role in conducting proton at elevated temperatures and separating fed gases in electrodes [8,9]. Up to now, a large number of studies mainly focus on non-volatile inorganic acids (i.e. phosphoric acid (PA)) doped alkaline polymer membranes. The state-of-the-art representative is the PA doped polybenzimidazole (PBI) membrane, which was first developed by Wainright et al., in 1995 [10]. As the heterocyclic polymer, PBI consists of two benzimidazole groups per polymer unit, which can be doped with PA molecules via acid-base and hydrogen bonding interactions to achieve proton transfer [8]. Owing to its good thermal stability and superior proton conductivity under anhydrous condition, new chemistries and various derivatives of PBIs have been explored [[11], [12], [13], [14], [15]] and reviewed [16]. However, the use of toxicity and carcinogenic reagent of 3,3′,4,4′-tetraaminobiphenyl, harsh synthetic conditions and limited organic solubility of PBI polymers limit the further development of PBI membranes in HT-PEMFC [8,16]. Alternatively, developing new membrane materials with superior performance for HT-PEMFC is motivated.

As potential alternatives for PBI, various aromatic polymers containing pyridine [17] and quinolone [18] in the main chain have been synthesized, while polyolefinic polymers containing pyrrolidone [19,20] and imine [21,22] repeat unit have been chosen as HT-PEMs as well. Besides the main-chain type HT-PEMs, side-chain basic group grafted HT-PEMs have been developed based on quaternary ammonium (QA) grafted polymers (i.e. polysulfone (PSF) [23], poly(arylene ether ketone) (PAEK) [24] and poly(arylene ether sulfone) (PAES) [25,26]), imidazolium functionalized polymers (including PSF [27,28], PAES [29], PAEK [30], poly(epichlorohydrin) (PECH) [31], poly(phenylene oxide) (PPO) [32] and poly(vinyl chloride) (PVC) [33,34]) and triazole functionalized polymers (such as PSF [35] and PAEK [36]). For side chain grafted HT-PEMs, poly(arylene ether)s are first chloromethylated or brominated, and then substituted by various basic groups. Nonetheless, some drawbacks need to be overcome during above procedure, including the usage of highly toxic halogenomethyl reagents and difficulty in precisely designing the grafting degree. Very recently, a new class of poly(arylene piperidine)s (PAPs) were synthesized via superelectrophilic activation by Jannasch et al. [37] They grafted different pendant N-alkyl chains into PAPs and employed poly(arylene piperidinium)s as anion exchange membranes (AEMs), which displayed excellent alkaline stability due to the absence of aryl ether bonds and benzylic sites. Based on above membranes, Peng et al. [38] and Wang et al. [39] separately assembled the membrane electrode assembly (MEA), which achieved remarkably high peak power densities of 1.5 W cm⁻² and 0.92 W cm⁻² at 80 °C, respectively. Since there is N-methyl piperidine moiety in the polymer repeat unit, PAPs are expected to be doped with PA via the acid-base interaction, which inspired us to apply PAP type membranes as HT-PEMs. When we prepared this draft, Lu et al. reported a work on pure PAP membranes for HT-PEMs [40]. They reported that PA doped poly(p-terphenyl-co-N-methyl-piperidine) and poly(p-biphenyl-co-N-methyl- piperidine) membranes exhibited a high proton conductivity of nearly 0.1 S cm⁻¹ at 160 °C and a H₂–O₂ peak power density of 1.2 W cm⁻² with a backpressure of 0.15 MPa. These excellent properties have strengthened our determination to develop PAP based membrane materials for HT-PEMs.

In this work, the poly(p-terphenyl-co-N-methyl-piperidine) (PTP) polymer was synthesized by the polymerization of terphenyl and N-methyl-4-piperidone via a superacid catalysis reaction. However, it was found that the pristine PTP polymer exhibited limited solubility in organic solvents, while the protonated and quaternized PTP polymers displayed good solubility in polar solvents. In order to simultaneously improve the organic solubility and PA doping uptake, the quaternization of PTP was carried out through the Menshukin reaction between PTP and 2-chloromethylbenzimidazole (or 4-(bromomethyl)pyridine). The iodomethane quaternized PTP, protonated PTP and pure PBI membranes were also synthesized as the benchmark. The influence of grafted side chains on the properties of PA doped poly(arylene piperidinium)s was investigated systematically including the fuel cell performance.