Poly(2,5-benzimidazole)/sulfonated sepiolite composite membranes with low phosphoric acid doping levels for PEMFC applications in a wide temperature range
Introduction
During the last few decades, proton exchange membrane fuel cells (PEMFCs) have attracted significant attention as green and efficient energy devices, featuring high energy conversion efficiency, environment-friendliness, and quick start-up [1], [2], [3]. Two types of PEMFCs exist according to the operating temperatures: low temperature (LT) (<100 °C) and high temperature PEMFC (HT-PEMFC) (100–200 °C). The traditionally developed PEMFC based on the use of perfluorosulfonated polymer membranes (e.g. Nafion) is assigned to LT-PEMFC and suffers from two main limitations: critical operating conditions (e.g., an operating temperature below 100 °C and a relative humidity (RH) above 80%) and high cost [1], [4]. In contrast, phosphoric acid doped polybenzimidazole (PA-PBI) is the only membrane material that meets the criteria for HT-PEMFC applications required by the US Department of Energy (DOE). Nonetheless, a PA-PBIs (PBI and its derivatives) proton exchange membrane (PEM) based PEMFC is still limited to applications in a constrictive temperature range of 120–180 °C to avoid PA leaching in hydrous atmosphere and degradation above 180 °C [5], [6], [7], [8]. PEMFCs operating at wider temperatures, especially covering LT and HT ranges under various relative humidity (RH) (or without any external humidification system) are highly desirable [9], [10], [11]. Recently, Lee et al. [12] reported a novel class of PEMs based on PA-doped QAPOH (quaternary ammonium-biphosphate ion-pair-coordinated polyphenylene) and these fuel cells exhibit stable performance at 80–160 °C. However, PEMs with high performances over wider temperatures (i.e., from room temperature to 200 °C under a low RH or uncontrolled humidity conditions) have not been reported to date.
To realize a PEM with high performances under such demanding conditions, it is necessary to consider two design aspects: high performance polymer and optimal proton conducting path design. In general, the long-term operating temperature for a PEM material is below its glass transition temperature (Tg). Nafion and most aromatic polymers (as PEM candidates) have Tg values below 200 °C, and their thermomechanical and chemical oxidation stabilities deteriorate after sulfonation treatment [13]. A reasonable proton conductivity for a PEM at the wide temperature range with various RH is extremely important [14]. Proton conducting through a PEM is explained by vehicle (H3O+) and Grotthuss (proton hopping) mechanisms. The former is mainly observed to be proton conducting as a H3O+ vehicle through hydrophilic tunnels in a sulfonated polymer membrane under hydration conditions [15]. The latter is mainly considered as proton hopping between acid molecules in an acid doped membrane (typically PA-PBI) under anhydrous conditions [16]. Since proton conduction is mediated through portable water molecules via vehicle mechanism, sulfonated polymer membranes face at least two critical challenges. Firstly, they must remain humid to obtain high proton conductivity. However, on the one hand, with the operation temperature increasing to 200 °C, the consequently required saturated water vapor pressure as high as 15 atm, will greatly increase the complexity of the system construction if high RH is to be achieved during operation [6]; on the other hand, the hydrophilic channels for water migration will be destroyed when a PEM operates at elevated temperatures close to their Tg, resulting in dramatically decreased proton conductivity [13]. Secondly, they must obtain a maximum possible sulfonation degree to ensure reasonably high proton conductivity which is vital to run at high temperature and low humidity conditions. Unfortunately, both conductivity and water uptake heavily depend on the concentration of proton conducting units (most commonly sulfonic acid) and a high sulfonation degree will greatly damage the mechanical and thermal stability as well as fuel impermeability. As a result, such sulfonated polymer membranes are inefficient for HT-PEMFC operation up to 200 °C.
Compared to sulfonated polymer candidates, PA-PBIs have better comprehensive properties for wider temperature applications although it still has defects. It is worth mentioning that the operating temperature range of 120–180 °C for the PA-PBIs PEM is not limited by PBIs but free PA, since PBIs as a kind of fully aromatic heterocyclic resins have adequate thermal and chemical stabilities for HT application. Only when doped with PA with high DLs, the PBIs-based membranes exhibit reasonable proton conductivity for HT-PEMFC. Theoretically, an imidazole (Im) group in a PBI chain binds a PA molecule, in other words, the saturated PA DL value of PBI is 2 [17]. The practical PA DL of PA-PBI membrane is much higher than 2 and this is due to the so-called free PA. For an ideal temperature range of 120–180 °C for PA-PBIs based HT-PEMFC, the lower boundary temperature of 120 °C is suggested to avoid the formation of water, which would result in free PA leaching. However, the upper boundary level of 180 °C avoids free PA degradation, which will both dramatically decrease the proton conductivity and deteriorate the performance of the resulting fuel cells [18], [19]. It has also been reported that the mechanical strength and dimensional stability of PBI membranes greatly decreased with high PA DLs [20].
Many reviews have comprehensively summarized the development of PBI-based PEMs [6], [19], [21], [22], [23]. Significant efforts have been focused on increasing the PA retention capability while reducing PA leaching to improve the PA-PBI based HT-PEMFC performance. For instance, incorporating insoluble proton conductors [24], [25] or anchoring the soluble proton conductors onto the insoluble matrix [26], [27] can solve the PA leaching problem to some extent. However, the purpose of PA-PBI based PEMs for application in LT conditions have rarely been concerned reported. As discussed above, the real obstacle is the free PA leaching problem discussed above and the relatively low conductivity under LT conditions. Schmidt [28], [29] and Molleo et al. [5] have summarized the effects of PA leaching on membrane electrode assembly (MEA), component degradation, and system performance. Firstly, the free PA leaching out of a membrane can not only dramatically decrease the proton conductivity and mechanical strength of a membrane, but also cause fuel loss and short cuts due to membrane thinning and pin-hole formation. Secondly, the exudative PA will poison the Pt-metal catalyst and destroy the interface between the membrane and the catalyst layer, resulting in an irreversible damage of the MEA. Finally, PA dissolving in the aqueous solution will corrode the components of a PEMFC including carbon, the gas diffusion layer, and metal parts. Therefore, to drive PA-PBI-based PEM spreading in LT applications, it is necessary to design additive proton conducting paths that are independent of conducting protons through massive-free PA.
Amongst these approaches on PEMs for HT-PEMFCs, organic-inorganic composite membranes have attracted significant interest mainly due to their improved proton conductivity and enhanced thermomechanical properties, as well as favorable commercializing prospects such as industrial manufacture and low cost [30]. Kim et al. [22] have summarized the previous efforts in this field. Through incorporation of inorganic fillers into an sulfonated polymer or an acid-doped PBI matrix, such as hygroscopic inorganic oxides (SiO2, TiO2, ZrO2, and Al2O3), polyhedral oligomeric silsesquioxane (POSS), heteropolyacids, and carbonaceous materials, the obtained organic-inorganic composite membranes generally show good thermomechanical strength and enhanced water and/or acid retention capability, thus improving proton conductivity at HT under low RH conditions [6], [21], [31]. Since clay-reinforced polymer composites are renowned for their good mechanical strength, modulus, as well as dimensional, thermal, and chemical stability, these composites are expected to produce more dimensionally stable membranes at low cost [32]. In relevant studies, natural minerals with micro- or nano-size porous, laminar, or channeled structures, such as sepiolite [33], [34], [35], [36], [37], bentonite [38], laponite [32], [39], montmorilonite [40], [41], and palygorskite [42] have been reported for the preparation of PEMs. The results indicate that high proton conductivities can be achieved by additional proton transfer through the absorbed or bound water molecules in clays via vehicle and/or Grotthuss mechanisms.
Here, a proton conduction multipath was used based on PA doped ABPBI incorporated with sulfonated sepiolite particles (S-Sep) composite membranes (ABPBI/S-Sep). Sep is a type of porous fibrous clay mineral that has attracted significant interest due to its unique physical structure that includes a high specific surface area, micro-porous channels (0.35–1.06 nm) that run parallel to its length [43], and excellent water absorbability and maintenance [34]. Due to its natural capacity for excellent hygroscopicity, Sep has been used to improve the mechanical properties and proton conductivity of Nafion/Sep composite membranes at high temperature and low RH [33], [34]. For its application in a wider temperature range without humidity control, several potential advantages are available with this approach. Through incorporation of the hygroscopic Sep with excellent water absorbability and maintenance at HT conditions, S-Sep particles in composite membranes provide new proton paths through internally bound water to offset the insufficient proton conductivity under low PA DLs. While, proton paths crossing the boundary of inorganic and organic components are provided by hydrogen links between sulfonic acid grafted onto Sep surfaces and PA and Im groups in PA-ABPBI. Additionally, proton can transfer through absorbed moistures at LT with high RH conditions. As a result, protons are expected to be efficiently delivered through the composite membrane under various temperature and humidity conditions.
In the present work, a series of ABPBI/S-Sep composite membranes with low PA DLs were successfully utilized to improve proton conductivity and reduce PA leaching to achieve wide temperature operation of PEMFCs. A schematic illustration of the preparation of ABPBI/S-Sep composites is presented in Fig. 1. The rich combined water with high temperature maintenance that exists in zeolitic channels (as shown in the partial view of Sep in Fig. 1A) is expected to form additional proton paths. In step 1, the enlarged internal and external surface area was achieved via acid activation treatment with effects of defibration and magnesium removal [44], [45], [46] (Fig. 1B). Typically, the incorporation of inorganic nanoparticles into a PBI-based membrane may cause phase separation, thus resulting in decreased mechanical strength and proton conductivity [47]. Therefore, the nano-sized Sep particle after defibration was modified by a silane coupling agent in step 2, followed by sulfonation treatment to achieve S-Sep according to Beauger et al. [34] (Fig. 1C). In step 3, ABPBI/S-Sep composites with uniformly dispersed S-Sep were achieved via in-situ synthesis. Finally, ABPBI/S-Sep composite membranes with low PA DLs will be obtained by using a solution casting method followed by PA doping and washing procedures. The effects of acid activation and sulfonation treatments on Sep particles, as well as the structures and properties of ABPBI/S-Sep composite membranes were systematically investigated. Due to the combination of the detailed relevant functions of the S-Sep filler, this work realized that composite membranes can operate in a wider temperature range covering LT and HT. In fact, the excellent performance of ABPBI/S-Sep composite membranes with thermomechanical and oxidation stabilities and comparable proton conductivities with commercial Nafion at LT and PBIs membranes with high PA DLs at HT was achieved. This study contributes a feasible method for the fabrication of a new class of PEMs for operation in a wide temperature range and without the requirement of additional humidifiers.
Section snippets
Raw materials
Sep, a natural fibrous clay mineral, was kindly supplied by Xiangtan Yuanyuan Sepiolite New Material Co., Ltd., China. 3,4-Diaminobenzoic acid (DABA, 98.0%) was purchased from Beijing J&K Technology Co., Ltd., China. Methane sulfonic acid (MSA, 99.0%) and phenyltriethoxysilane (PTES, 99.0%) were purchased from the Aladdin Industrial Corporation, USA. P2O5 (98.0%), PA (85.0%), NaOH (85.0%), isopropanol (99.7%), HCl (36.0–38.0%), fuming sulfuric acid (H2SO4, 20% free SO3 basis), H2O2 (30.0%), FeSO
Modification of Sep particles
A strategy for Sep functionalization was employed mainly according to Beauger et al. [34]. This strategy based on three steps is presented in Fig. 1. Step 1 is the acid activation treatment that achieves Sep fibrous particles with extended surface area and a large number of exposed Si–OH groups. Step 2 introduces phenyltriethoxy groups onto Sep via silanol condensation between Sep and PTES. Step 3 is a sulfonation process that forms a hydrophilic sulfonated benzene ring to promote the
Conclusions
ABPBI-based composite PEMs with the incorporation of sulfonated Sep were successfully prepared via in-situ synthesis followed by a solution casting technique. Through the treatment of acid activation, the Sep fibrous particles achieved a greatly increased specific surface area, pore volume of the micro-pores, amount of exposed Si–OH groups. Subsequent surface modification promoted the compatibility and provided a proton conducting pathway between Sep and ABPBI components, thus resulting in
Acknowledgments
This work was financially supported by the Natural Science Foundation of Hubei Province, China Project (Grant No. 2017CFB575, 2018CFB412) and the Laboratory of Green Materials for Light Industry (Grant Nos. 201710A14 and 201806A07). We would like to thank Dr. Chenxi Xu of Hefei University of Technology is for his help with the fuel cell tests.
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