Full length articlePhosphoric acid doped high temperature proton exchange membranes based on comb-shaped polymers with quaternized graft architectures
Introduction
Proton exchange membrane fuel cells (PEMFCs) are considered as one of the most potential energy conversion technologies due to their high energy density, high energy efficiency and low greenhouse-gas emissions, thus providing the clean and efficient energy for stationary, automotive and portable electronics [[1], [2]]. As the critical component in PEMFCs, proton exchange membrane (PEM), has attracted particular attention in the past decades. So far, perfluorosulfonic membranes, represented by Nafion® (produced by DuPont), are still the main commercial PEMs owing to their excellent chemical stability and high proton conductivity [[3], [4], [5]]. It should be noted that PEMFCs using Nafion® membranes are operated only at a relative low temperatures (below 100 °C) and high hydration levels. This is because that Nafion suffers from quick dehydration from membrane system at the temperatures above 100 °C, thus resulting in a significant reduction of proton conductivity. Furthermore, Nafion has a relative low glass transition temperature of 130 °C, thus leading to a great loss of mechanical properties at high temperatures [[6], [7]]. However, there are many advantages for PEMFCs operating at temperatures above 100 °C (HT-PEMFCs), including less demand of humidification and cooling units, enhanced reaction kinetics of oxygen reduction reaction, utilization of a non‑platinum catalyst and high CO tolerance [[8], [9], [10], [11], [12]]. These features could lead to a significant decrease in the fabrication and operation cost of fuel cells [[13], [14], [15], [16]].
Phosphoric acid (PA) doped polybenzimidazole (PBI) membrane, formed by the interactions between PA and N-heterocycles, was first demonstrated to be applied for HT-PEMFCs (100–200 °C) in the mid 1990s [[17], [18], [19]]. Then it stimulated a series of studies on the synthesis, modification and cross-linking of PBI and its derivative membrane systems [[20], [21], [22], [23], [24], [25], [26]]. Li et al. prepared a series of cross-linked PBI membranes for achieving high PA doping level and high proton conductivity. However, cross-linked PBI membrane with the highest cross-linking degree of 13.0% still had a high volume swelling of 62%. The obtained high proton conductivity was at the expense of deterioration in mechanical strength [25]. Guan et al. synthesized a new kind of poly[2,2′-(4,4′-(2,6-bis(phenoxy) benzonitrile))-5,5′-bibenzimidazole] and doped with PA [26]. Although the introduction of benzonitrile group enhanced the acid doping ability and oxidation stability, the proton conductivity of PBI-CN/3.5H3PO4 was only 10−3 S cm−1 at 170 °C under anhydrous conditions. These results demonstrated that PA doped PBI membranes could not easily disentangle the dilemma between the proton conductivity and the stability, including dimensional stability, mechanical stability and oxidative stability.
Recently, quaternized poly (arylene ether)s have been reported as the potential PA doped high temperature PEMs. The tethered quaternary ammonium (QA) groups on the aromatic backbone have a good capacity to immobilize PA molecules based on a strong QA+⋯H2PO4− interaction [[27], [28], [29]]. Furthermore, quaternized poly(arylene ether)s could be easily obtained by conventional benzyl bromide reaction or chloromethylation reaction [8,30]. Moreover, the amount of absorbed PA is feasibly controlled by the contents of QA groups. In general, these membranes with higher PA doping level exhibit higher proton conductivity as well as lower mechanical and dimensional stability. Therefore, it is a great challenge for the researchers to improve the stability of these PA doped quaternized poly (arylene ether) without sacrificing proton conductivity. For this purpose, we have prepared a series of dual-cross-linked organic-inorganic hybrid membranes using (3-aminopropyl)triethoxysilane as a cross-linker [31]. The results indicate that the modification by cross-linking enhances their oxidative and mechanical stability to a certain extent. However, the oxidative stability and the tensile strength of these cross-linked membranes are still lower than 15.3 h and 16 MPa, respectively, which are not so attractive for the long-term operation of HT-PEMFCs.
In this work, for simultaneously enhancing the proton conductivity, dimensional stability, and chemical stability of PA doped membranes, we attempted to prepare a series of poly(arylene ether ketone)-g-quaternary 4-vinylbenzyl chloride comb-shaped copolymers via atom transfer radical polymerization (ATRP). They comprised both hydrophobic rigid main chains and hydrophilic flexible quaternized side chains. The PA molecules were subsequently tethered to the quaternized side chains based on a strong QA+⋯H2PO4− interaction. Compared to PBI and other main-chain-type quaternized poly (arylene ether)s, the comb-shaped quaternized copolymers could significantly restrict the negative effects of absorbed PA on the membrane volume swelling and the mechanical stability. Moreover, they could form micro-phase aggregated morphology, which facilitates the protons hopping between PA clusters. Therefore, the properties of these PA doped membranes could be regulated by precisely designing the architecture of comb-shaped copolymers, such as the grafting length of quaternized side chains. The comprehensive properties of PA doped membranes based on these comb-shaped copolymers, such as PA doping level, volume swelling ratio, proton conductivity, mechanical and oxidative stability, morphology and preliminary single fuel cell performance, are described here.
Section snippets
Materials
Poly(arylene ether ketone) containing benzyl groups was prepared according to a previous method [28]. N-bromosuccinimide (NBS; 99%) and benzoyl peroxide (BPO; AR) were purchased from Sigma-Aldrich. 4-Vinylbenzyl chloride (90%; VBC), cuprous bromide (CuBr; 99%), and 2,2′-bipyridine (bpy; 99%) were obtained from Aladdin Industrial Corporation (China). Trimethylamine methanol solution (TMA; 3.2 M) was purchased from TCI. PA solution (85 wt%) was purchased from Beijing Chemical works (China). All
Monomer synthesis
The quaternized monomer QVBC for graft polymerization was synthesized via a typical SN2 alkylation reaction under mild condition (Scheme 1). As the reaction proceeded, the color of the mixture became deeper and the temperature of the mixture increased. As shown in Fig. 1, 1H NMR spectrum was used to confirm the chemical structure of QVBC. The characteristic peaks at 7.62–7.54 ppm are assigned to the protons of benzyl group. Meanwhile, the characteristic peaks at 4.65 ppm and 3.08 ppm are
Conclusions
A series of PAEK-g-QVBC-x polymers containing locally and densely quaternary ammonium groups on grafted polystyrene side chains were synthesized by an ATRP technique. PA-PAEK-g-QVBC-6.4 membrane exhibited low PA doping level and volume swelling ratio without sacrificing proton conductivity. The Wdoping, Vdoping and Vswelling values were 101.7%, 6.29 mmol cm−3 and 7.8%, respectively. It still possessed the highest proton conductivity of 65 mS cm−1 at 200 °C. Therefore, PA-PAEK-g-QVBC-x membranes
Acknowledgements
We acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21474036 and 21875088).
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Present address: Endodontics Department of Stomatological Hospital, Jilin University, Changchun 130021, People's Republic of China.