Effectively facilitating the proton conduction of proton exchange membrane by polydopamine modified hollow metal−organic framework
Graphical abstract
An effective proton conduction accelerator, polydopamine (PDA) modified hollow metal−organic framework (DHZIF-8), was incorporated into Nafion matrix to construct composite proton exchange membrane (PEM) with excellent proton conductivities. The proton conduction of composite PEM was greatly facilitated mainly by higher water retention capacity and formed acid-base pairs.
Introduction
Due to the properties of low pollution, high efficiency and moderate operation, proton exchange membrane fuel cell (PEMFC) has been considered as an indispensably green device applied in residential buildings, mobile equipments, and commercial vehicles [1,2]. Proton exchange membrane (PEM) is the heart part of PEMFC, and excellent proton conduction of PEM is a basic guarantee for efficient working of PEMFC. Therefore, many attentions have been put to the exploration of eminent proton conductors for promoting the proton conduction of PEM as more as possible [3,4].
Metal−organic frameworks (MOFs) as one of extremely intriguing porous materials have exhibited charming architectures and attractive applications in separation, adsorption, sensing, and electrode, etc [[5], [6], [7], [8]]. Recently, their application as outstanding proton conductors has been greatly concerned, owing to tunable frameworks, regular pores and controllable functional groups [9]. Such adjustable and regular features of MOFs are not only favorable to the arrangement of proton transfer sites within the frameworks, but also beneficial to the understanding of proton conducting mechanism [10]. Three approaches are usually adopted to enhance the proton conductivities of MOFs. The first approach is encapsulation of inorganic acid (such as H3PO4, H2SO4, etc.) into the pores [11]. The encapsulated inorganic acid can increase the acidity to facilitate the proton conduction. The second method is modification of MOFs with desirable functional groups (such as –NH2, –SO3H, –OH, etc.) [[12], [13], [14], [15]]. The functional groups can improve the hydrophilicity, and act as proton transfer sites. The third approach is introducing guest molecules (such as ammonium cations, imidazole, etc.) into the frameworks [16,17]. The introduced guest molecules can enhance proton concentration, and form hydrogen bonding network for proton conduction. However, the practical utilization of MOFs as proton conductors is restricted to some extent. The effects of bulk phase and particle boundaries result in intermittent conduction between MOFs [9]. Additionally, the high crystallinity and brittleness lead to difficult processing of MOFs [18].
Construction of MOF/polymer composite PEMs is a feasible and effective method for overcoming the aforementioned constraints of MOFs as eminent proton conductors and elevating the proton conductivities of PEMs. For a typical MOF/polymer composite PEM, the high proton-conductive ability of MOFs can be sufficiently realized by ameliorated effects of particle boundaries and bulk phase between the interfaces of MOFs and polymers. Besides, the mechanical property of the prepared PEM can be stabilized after polymer filling between MOF grains. The pioneering MOF/polymer (Ca-MOF/PVP) composite PEM was reported by Zhu's group, which displayed obviously elevated proton conductivity when compared to those of individual PVP matrix and pure Ca-MOF [19]. Whereafter, other MOF/polymer composite PEMs have been successfully prepared, such as Fe-MIL-101-NH2-SPPO, sul-MOF-808/Nafion, MIL-101-NH2-SO3H/SNF-PAEK, phytic@MIL101/Nafion, Cr-MIL-101-NH2-SPES, MIL-53(Al)-NH2/Nafion and so on [[20], [21], [22], [23], [24], [25]]. The resultant PEMs all showed improved proton conductivities. Among the MOFs in the composite membranes, especially those modified by –NH2, could form base–acid pairs with acid matrixes as effective proton transfer pathways.
Besides, these studies mainly focus on regulating the chemical constitution of MOFs by modification of them with specific functional groups or imbuement of their frameworks with particular proton carriers. In practice, besides the chemical constitution, the structure or morphology of fillers is also a vital factor determining the proton conductivities of composite PEMs [26,27]. Especially, it has been reported that hollow structure or morphology of fillers could endow composite membranes with greatly promoted proton conductivities by more water retention and alleviative conduction barrier. High water retention capacity is especially conducive to fast transportation of protons. For instance, sulfonated hollow polystyrene spheres (sul-hPS) were added into Nafion matrix. The obtained composite PEM displayed considerable increase of proton conductivity, benefiting from excellent water retention of sul-hPS and mitigated proton transfer barrier throughout sul-hPS [28]. Mesoporous hollow silica spheres (MHSi) were incorporated into Nafion matrix. Obviously improved proton conductivity of the resultant composite PEM was achieved by the high water retention capacity of MHSi and alleviated proton conduction barrier throughout MHSi [29]. Therefore, construction of hollow MOFs is supposed to be an effective means for proton conductivity promotion of composite membranes. Actually, Sen et al. found comparatively higher proton conductivity of hollow MOF nanoparticles than that of ZIF-8 without hollow structure [30]. However, to the best of our knowledge, the work regarding hollow MOF as filler of polymer matrix to promote the proton conductivity of composite PEM has been barely reported so far.
Based on the above analysis, we hope to design MOF with hollow structure and amino groups, and then incorporate it into polymer matrix to construct a composite PEM with greatly facilitated proton conduction.
As an universal adhesive chemical, dopamine can coat on almost all material surfaces by self-polymerization to polydopamine (PDA) under mild conditions [31]. After PDA coating, the material surfaces bear abundant –NH2/–NH– groups. So specific materials coated by PDA can constitute base–acid pairs with acid polymer matrixes for great proton conductivity boosting of composite PEMs. For example, PDA coated graphene oxide (DGO) was dispersed in sulfonated poly(ether ether ketone) (SPEEK) to prepare DGO/SPEEK composite PEM. The formed acid–base pairs between –SO3H of SPEEK and –NH2/–NH– of DGO worked as effective proton hopping pathways, which greatly increased the proton conductivities under anhydrous condition [32]. PDA coated SiO2 (DSiO2) was embedded into SPEEK to synthesize DSiO2/SPEEK composite PEM. Proton conductivities under anhydrous condition were obviously elevated by the proton jumping sites of the formed base–acid pairs between –NH2/–NH– of DSiO2 and –SO3H of SPEEK [33]. Thus, PDA modification can be considered to functionalize MOFs with –NH2/–NH– groups for proton conduction. In addition, ZIF-8, an easily synthesized MOF by zinc ions and 2-methylimidazole (Hmim), can be etched as hollow structure by tannic acid (TA) during very fast time (within minutes) [34]. Bearing these in mind, we designed and prepared a PDA modified hollow MOF (DHZIF-8) by fist TA etching of ZIF-8 and subsequent PDA modification. DHZIF-8/RN composite PEM was successfully obtained by incorporation of DHZIF-8 into Nafion matrix. Expectantly and satisfactorily, the resultant DHZIF-8/RN composite PEM exhibited highly promoted proton conductivities. The hydrophilic groups on surface and hollow structure of DHZIF-8 were favorable to water adsorption and storage. It endowed the composite PEM with greater water retention capacity, which was greatly conducive to fast transportation of protons. Moreover, the hollow structure adequately alleviated the proton transfer obstruction throughout DHZIF-8. Besides, –NH2/–NH– of DHZIF-8 could not only constitute base-acid pairs with –SO3H of Nafion as effective proton transfer pathways, but also offer extra proton conductive sites for proton conduction. They could also induce –SO3H of Nafion to assemble as ionic clusters along DHZIF-8 to some extent and further mitigate the proton conduction barrier between Nafion matrix and DHZIF-8. These prominently accelerated the proton conduction of the obtained composite PEM. Its proton conductivities were as high as 0.255 S/cm under 80 °C, 95% RH, and 3.66 mS/cm under 120 °C, anhydrous condition, respectively, which were much higher than those of Nafion control-membrane under the same condition (0.104 S/cm and 1.14 mS/cm). Additionally, the composite PEM exhibited a high proton conductivity stability ascribed to the thermal and water stabilities of DHZIF-8 [35]. This work offers a valuable guidance for designing and preparing functionalized MOFs with specific structures to effectively facilitate the proton conduction of PEMs.
Section snippets
Materials
Tannic acid (TA), tris(hydroxymethyl)aminomethane (Tris) and dopamine hydrochloride were obtained from Aladin. Concentrated sulphuric acid, methanol, N,N′-dimethylformamide (DMF) and H2O2 solution (30 wt%) were purchased from Sinopharm Chemical Reagent Co., Ltd. ZIF-8 powers were provided by CHEMSOON. Nafion solution (5 wt%) was received from DuPont.
Preparation of HZIF-8 and DHZIF-8
ZIF-8 (120 mg) was put into aqueous TA solution (4 g/L, 90 mL) under sonication, and kept for 15 min. HZIF-8 was then obtained by thorough washing
Characterization of ZIF-8, HZIF-8 and DHZIF-8
The morphologies of ZIF-8, HZIF-8 and DHZIF-8 are determined by FESEM and TEM images in Fig. 1. As shown in Fig. 1 (a) and (d), ZIF-8 exhibits an approximate cubic structure with the size of about 450 nm. After etching by TA, HZIF-8 presents an obviously hollow structure (Fig. 1 (b) and (e)). The internal cavity of HZIF-8 can be clearly observed compared with solid ZIF-8. The etching process can be understood by the proposed mechanism [34]. In principle, TA is a kind of weak organic acid with
Conclusion
Composite PEM with effectively facilitated proton conduction was achieved by incorporation of DHZIF-8 into Nafion matrix. DHZIF-8 was constructed by first TA etching of ZIF-8 and subsequent PDA modification. The hydrophilic groups on surface and hollow structure of DHZIF-8 caused better water retention of the composite PEM. Besides, effective acid–base pairs could be formed between –SO3H of Nafion and –NH2/–NH– of DHZIF-8. These greatly accelerated the proton conduction of the composite PEM.
CRediT author statement
Zhuang Rao: Conceptualization, Data Curation, Writing-Original Draft.
Minqiu Lan, Zhengyun Wang, Huihai Wan, Guangfang Li, Jiannan Zhu: Validation.
Beibei Tang: Writing-Review & Editing, Supervision.
Hongfang Liu: Writing-Review & Editing, Funding acquisition, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We greatly appreciate the financial support from National Natural Science Foundation of China (22005109), Natural Science Foundation of Hubei Province (2020CFB214), Hubei Key Laboratory of Material Chemistry and Service Failure (2020KMC02), and Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education (2018). We also appreciate the measurement support form Analytical and Testing Center of Huazhong University of Science and Technology. Besides, we are grateful
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