A new strategy for designing high-performance sulfonated poly(ether ether ketone) polymer electrolyte membranes using inorganic proton conductor-functionalized carbon nanotubes
Graphical abstract
Introduction
Polymer electrolyte membranes (PEMs) are a critical component in energy conversion devices, including proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) [1], [2]. Their main functions are to transport protons and block the direct contact of fuels and oxidants. The most widely used PEMs are Dupont’s Nafion-based perfluorinated sulfonic acid membranes, which possess inert chemical properties and high proton conductivity in the fully hydrated state. However, the high cost and unstable proton conductivity at high temperatures are the major obstacles for its large-scale commercial application.
Seeking alternatives to Nafion has been the major research focus in recent decades. Sulfonated aromatic polymers, such as sulfonated polyether sulfone [3], [4], sulfonated polyimide [5], [6], and sulfonated poly(phenylene oxide) [7], [8], have been considered to be promising candidates. Among these sulfonated aromatic polymers, sulfonated poly(ether ether ketone) (SPEEK) has drawn considerable attention due to its relative high proton conductivity, excellent thermo-chemical properties, simple preparation procedures and low cost [9]. However, similar to the other sulfonated aromatic polymers, one of the shortcomings of SPEEK is the fewer number of connected sulfonic acid-aggregated hydrophilic ion channels when compared with perfluorinated sulfonic acid polymers. Although increasing the sulfonation degree of SPEEK is a simple solution to increase its proton conductivity, excessive water swelling followed by deteriorated mechanical properties unfortunately emerge at the same time [1], [9]. Among the several strategies explored, fabricating composite membranes of SPEEK and inorganic additives was an effective way to retain the proton conductivity without compromising the mechanical properties [10].
Currently, carbon nanotubes (CNTs) have emerged as a novel type of advanced inorganic fillers due to their unique properties including an extremely high tensile modulus and strength, a large surface-to-volume ratio, high flexibility, excellent electrochemical activity and potential utilization in the field of energy materials [11] as high-performance nanocomposites [12]. Since the first report by Liu et al. [13] in 2006 explained that the risk of short-circuiting in PEMs caused by the excellent electrical conductivity of CNTs could be eliminated by controlling the CNT concentration below the percolation threshold, many studies have focused on the use of CNTs to enhance the performance of PEMs.
Except for the inert proton conduction nature of pristine CNTs, their poor dispersion in PEMs has limited their functions and wide application. Several modification methods have been proposed to improve their dispersion and maximize their functions in PEMs. Pillai’s group demonstrated the use of sulfonic acid-functionalized single-walled CNTs [14] and multiwalled CNTs [15] for enhancing the proton conductivity and mechanical strength of Nafion. Subsequently, they further verified that phosphonated CNTs can effectively prevent the leaching of phosphoric acid in phosphoric acid-doped polybenzimidazole (PBI) membranes during sustained operation [16], [17]. Apart from modifying CNTs through these simple acid groups, ion-conductive polymers were also used to modify CNTs to enhance their proton conductivity as well as compatibility in PEMs. Liu et al. [18] prepared Nafion-functionalized CNTs using an ozone-mediated process and used them as additives in the fabrication of Nafion-based composite membranes. The composite membrane with a Nafion-functionalized CNT content of 0.05 wt% exhibited a five-fold increase in proton conductivity and a 1.5-fold increase in tensile strength compared to a pristine Nafion membrane. They proposed that Nafion chain-coated layers could make the functionalized CNTs act as a direct proton-conducting pathway for rapid proton transport through the composite membranes. In their following work, they used Nafion-, PBI- and poly(styrene sulfonic acid)-functionalized CNTs to modify PBI [19] and chitosan [20] membranes and confirmed the availability of these functionalized CNTs. However, these authors also highlighted that the concentration of functionalized CNTs should be controlled at a low value, such as 1.0 wt%, to prevent CNT aggregation and a sharp increase in electrical conductivity in the PEMs. To disperse high concentrations of CNTs uniformly inside the PEM matrix without causing short-circuiting, polysiloxane-functionalized CNTs were synthesized by covalently grafting hydrophilic layers composed of poly(oxyalkylene) diamines and tetraethyl orthosilicate-reinforced polysiloxane in a layer-by-layer manner onto the surface of CNTs and were then used as an additive to modify Nafion [21]. The results showed that the polysiloxane coating layer can effectively prohibit electron conduction through the composite membranes even with high CNT contents (5–20 wt%). However, the preparation process for these polysiloxane-functionalized CNTs was complex and time consuming, representing the main obstacle for their practical applications in PEMs. Therefore, effective coating materials and facile coating method are crucial for fully playing the functional modification role of CNTs in the PEMs field.
Recently, we prepared insulated and hydrophilic silica-coated CNTs (SiO2@CNTs) using a facile sol-gel process and used these CNTs as a new additive to modify SPEEK [22] and natural polymer chitosan [23] to fabricate PEMs. Silica-coated CNTs not only eliminated the risk of short-circuiting but also enhanced the interfacial interaction between the CNTs and matrix and thus promoted the homogeneous dispersion of the CNTs in the polymer matrix. Although they had improved mechanical properties and high thermal and oxidative stability, the composite membranes exhibited decreased proton conductivities in both cases because of the inert proton conduction activity of silica. To address this issue, in this research, boron phosphate (BPO4), which is a super inorganic proton conductor that has been widely proved in other studies [24], [25], [26] and our previous work [3], was first utilized to modify CNTs using a facile polydopamine-assisted sol-gel method to simultaneously enhance the proton conductivity and mechanical properties of SPEEK. The SPEEK/BPO4-coated CNT (BPO4@CNTs) composite membrane was extensively characterized to understand how the BPO4@CNTs effect the structure, morphology, thermal and mechanical properties and proton conduction behavior of the composite membranes.
Section snippets
Materials
Poly(ether ether ketone) (PEEK) victrex® 450 PF was purchased from Victrex Inc. (England) and was dried under high vacuum at 100 °C for 10 h before use. Multi-walled CNTs (MWCNTs) (diameters 20–40 nm) were purchased from Shenzhen Nanotech Port Co. Ltd. (China). Dopamine hydrochloride (A.R.) was purchased from Acros Organics Inc. (Belgium). Trihydroxymethyl aminomethane (tris, A.R.) was purchased from Shanghai Ruji Biotech Co. Ltd. (China). Hydrochloric acid, tripropylborate (C3H7O)3B (guarantee
Characterization of BPO4@CNTs
The BPO4@CNTs were prepared using a facile polydopamine-assisted sol-gel route, as shown in Fig. 1. Polydopamine (PDA), which has a molecular structure similar to that of the adhesive proteins secreted by mussels, is a well-known bio-inspired polymer possessing extraordinary adhesion ability. After Messersimith’s group reported that PDA can easily deposit or coat on nearly all types of organic and inorganic substrates (even polytetrafluoroethylene) with a controllable coating thickness and
Conclusions
The aim of the present work was to study the functionalization of CNTs using BPO4, which is a super inorganic proton conductor, using a facile polydopamine-assisted sol-gel method to prepare a new class of SPEEK-based composite membranes. The SEM and TEM results showed the homogeneous coating of BPO4 nanoparticles on the surface of the CNTs with the aid of the superior adhesion of polydopamine. The limitation of the short-circuiting caused by the CNTs was also overcome, as verified by the
Acknowledgements
We are grateful to the National Natural Science Foundation of China (Grant Nos. 51303048 and 51403058) and Hubei Provincial Department of Education (Grant No: T201216) for support of this work. We also thank Dr. Shangfeng Jiang (Fuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences) for single cell tests.
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