Xiaobai Lia,
Hongwei Maa,
Hailong Wanga,
Shitong Zhanga,
Zhenhua Jianga,
Baijun Liu*a and
Michael D. Guiverb
aAlan G. MacDiarmid Institute, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, P. R. China. E-mail: liubj@jlu.edu.cn; Fax: +86-431-85168022; Tel: +86-431-85168022
bState Key Laboratory of Engines, School of Mechanical Engineering, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China. E-mail: michael.guiver@outlook.com; Fax: +86-22-27404479; Tel: +86-22-27404479
First published on 11th June 2015
A novel high temperature proton exchange membrane (HT-PEM) with a high phosphoric acid (PA) doping level of 24.6, high proton conductivity of 0.217 S cm−1 at 200 °C, as well as excellent mechanical-dimensional stability was prepared based on a structure-designed polybenzimidazole (PBI).
In the 1990s, researchers at Case Western Reserve University first used phosphoric acid (PA)-doped poly(2,2′-(1,3-phenylene)-5,5′-bibenzimidazole) (m-PBI) as an electrolyte membrane for HT-PEMFCs.4 In this PA-imidazole system, protons transfer from one site to another through the formation and breaking of hydrogen bonds, and the main proton-conduction mechanism changes from vehicle-type to Grotthuss-type.5 As promising membranes for HT-PEMs, PA-PBIs possess many attractive properties and thus have received a great deal of contemporary interest. Until now, however, relatively few PBI structures, such as m-PBI and poly(2,5-benzimidazole) (ABPBI),2a have been studied for PA-imidazole systems. Based on m-PBI or ABPBI, some strategies including grafting,6 crosslinking,7 inorganic doping,8 polymer blending,9 and sol–gel method10 have been adopted to improve the performance of the PEMs. For example, Lee and Choi7c prepared a PA-doped cross-linked benzoxazine–benzimidazole membrane and achieved a proton conductivity of 0.12 S cm−1 at 150 °C under anhydrous conditions. Unfortunately, after immersed the membrane in PA solution at 160 °C for 5 h, the membrane partially dissolved and lost its shape. He and Li6a reported a series of PA-doped benzimidazole-grafted-polybenzimidazole membranes, which could have the highest PA doping level (ADL) of 22.5 and exhibited the highest conductivity of 0.295 S cm−1 at 180 °C. However, its tensile strength was only 2 MPa at room temperature, which was too weak to be handled during the MEA preparation. In addition, the area and volume swelling were as high as 157% and 360%, respectively. As one can see from the above representative PA-PBIs membranes, high acid doping levels (ADLs) usually led to high conductivity but were also accompanied by sacrificing too much mechanical and dimensional stability.11 No markedly successful approaches have thus far improved the overall desired balance of properties for HT-PEMs based on traditional PBIs. This motivated us to explore PBIs with novel structures, in order to expand the scope of study with the aim of improving the overall balance of properties of HT-PEMs.
For the purpose of achieving PA-PBI membranes with high conductivity as well as high mechanical and dimensional stability, a novel PBI was investigated. We hypothesize that the preferred polymer membrane should have a more open “sponge-like” structure which can both hold considerable liquid and maintain original shape at a high level of liquid absorption. For the corresponding molecular design, the desired PBI should have two main features to construct a “sponge-like” microstructure of membrane that can meet the performance requirements. That is, the ideal microstructure should provide “free volume” to hold excess acid molecules and a framework to maintain mechanical strength.
Accordingly, we introduced flexible ether linkages (–O–) and side groups (phenyl) onto the PBI backbone. The selected PBI in this communication is denoted as Ph-PBI. The introduction of ether linkages may facilitate the entanglement between molecular chains thus forming intertwined polymer networks. Also, the interactions of imidazole groups will be helpful to form a stable framework. The side groups with large steric hindrance will disrupt the close polymer chain packing and reduce molecular chain packing density, to increase the “free volume” in the supposed “sponge-like” structure. Furthermore, to make a direct comparison, poly[2,2′-(p-oxydiphenylene)-5,5′-bibenzimidazole] (OPBI)12 having linear molecular structure was prepared by the same polymerization method. The PA-PBI samples were obtained by immersing the membranes (OPBI and Ph-PBI) in a dish with 85 wt% PA solution at 160 °C for 3, 12, 36, 72 and 108 h, respectively. The samples are named as O-3, O-12, and Ph-3, Ph-12, Ph-36, Ph-72, Ph-108, respectively.
Table S2† shows the ADLs and corresponding area, thickness and volume swellings of PA-PBI membranes. The ADLs, volume and area swellings increased with increasing doping time for all samples, as shown in Fig. 1(I)–(III). It could be observed that OPBI membranes had a faster PA doping rate at the initial stage compared to Ph-PBI membranes. At the same doping time, OPBI membranes exhibited ADLs about 1.5 times higher than those of Ph-PBI ones. However, it was surprising to find that the volume and area swelling ratios of OPBI membranes were about 2–3 times higher than those of Ph-PBI membranes. As a result, the swelling of the OPBI membrane was too large to be measured and lost the capability to maintain the original regular shape after 36 h treatment (Fig. 1(IV)), and the 72 and 108 h treated OPBI membranes could not be tested. However, Ph-PBI exhibited quite different absorption process and results. The 72 h treated Ph-PBI (Ph-72) reached an ADL of 19.1, which was similar to that of the 12 h treated OPBI (O-12). It is worth noting that after absorption for the long time of 108 h, Ph-108 achieved extremely high ADLs of 24.6. However, it had acceptably low dimensional change (Fig. 1(IV)). In contrast, O-12 had an ADL of 19.4, which was lower than that of Ph-108. However, O-12 exhibited the volume swelling about 1.4 times and an area swelling about 1.7 times higher than those of Ph-108. Given the importance of the dimensional stability of PA-doped membranes, a wider comparison with other reported work has been made in Table S2.† At similar ADLs, the membranes reported by others showed much higher volume swelling, and some even possessed the volume swelling about 2–3 times higher than our Ph-PBI.13 To further explain the excellent dimensional stability of these novel membranes from the perspective of molecular structure, the molecular simulation of m-PBI, OPBI and Ph-PBI of 30 repeating units was made using the UFF method (Universal Force Field) on Gaussian 09 Version D01, and the results are shown in Fig. 1(VII). Ph-PBI had loose helical molecular chains, which were prone to pack and form the “sponge-like” structure (Fig. 1(V) and (VI)). These were highly related with chemical structure of Ph-PBI with two flexible ether linkages and one rigid biphenyl moiety per repeating unit, which would partially disrupt close chain packing caused by H-bonding interactions of the imidazole groups. This is proved by its relatively low density of 1.25 g cm−3, large FFV (0.138) and large dsp (4.91 Å) (Table S1†). In comparison, conventional m-PBI with more rigid molecular chains had a trend to form a tight structure, and OPBI with more regular and less flexible molecular chains may form a denser structure with less entanglement. The enlarged images of molecular simulation can be seen in Fig. S4.† Another interesting observation is that the Ph-PBI membranes display anisotropic swelling behaviour, and the area swelling ratios were much lower than that of the thickness (Table S2†). This preferred behaviour may minimize the structural breakage of MEAs and, thereby, enhance fuel cell durability and performance.14
Good mechanical properties are important for the membranes to withstand high temperatures and pressure when they are utilized in HT-PEMFCs. Ph-PBI membranes had high tensile strengths of 106.7 MPa. Polymer chain intermolecular forces are reduced by the plasticizing effect of the doping acid, resulting in a reduction in tensile strength with increasing amounts of ADLs.6a In this study, Ph-PBI membranes showed higher tensile stress in comparison with other reported PA-PBI membranes at similar ADLs (Table S2†).13 After 3 h treatment in PA, the tensile strengths of Ph-3 was 22.3 MPa. In contrast, the tensile strength of the OPBI series sharply decreased with doping time, and O-3 only had a tensile strength of 2.8 MPa (Table S2†). That is to say, Ph-PBI membranes treated with PA have much better mechanical stability than O-PBI membranes.
Proton conductivity is one of the most important performance parameters for PEMs to be effectively applied in fuel cells. As expected, the proton conductivities under anhydrous conditions increased for all membranes with increasing doping time and test temperature (Table S2† and Fig. 2). Although OPBI membranes had high conductivities (e.g. O-12 0.142 S cm−1 at 200 °C) owing to their high ADLs within short doping times, their tensile strengths were insufficient. On the contrary, Ph-108 membrane had sufficiently high ADL and accordingly high conductivity, which was much higher than those of O-12. In particular, Ph-108 membrane achieved high conductivity of 0.217 S cm−1 at 200 °C, and exhibited good dimensional stability and strength under these conditions. Excitingly, the conductivity of Ph-108 is considerably higher than that reported for other analogues,13 as shown in Fig. 2. It should be mentioned that the U.S. Department of Energy has a currently established guideline that the target conductivity at 120 °C should reach 0.1 S cm−1 for PEMs in automotive applications.15 In our study, the conductivities of Ph-108 was 0.119 S cm−1 at 120 °C. Hence, the PBI material is possible candidate for HT-PEMs.
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Fig. 2 Conductivities of the OPBI (blue) and Ph-PBI (red) membranes. The dotted columns (blue, purple and orange) are representative comparative data reported before.13 |
Membranes with sufficient mechanical properties (O-3 and Ph-72) were fabricated into MEAs and tested in fuel cells at 160 °C and atmospheric pressure. The polarization curves and power density curves are shown in Fig. 3. At a current density of 0.2 A cm−2, Ph-72 and O-3 membranes show voltages of 0.59 V and 0.49 V, respectively. The maximum power densities of Ph-72 and O-3 membranes are 279 and 144 mW cm−2, respectively. As seen from these results, Ph-72 membrane exhibits much better fuel cell performance than that of O-3 membrane. This much improved performance demonstrates that PA-doped Ph-PBI membranes have the feasibility of being used in HT-PEMs.
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Fig. 3 Fuel cell performance of Ph-72 and O-3 membranes at 160 °C: polarization curves (filled symbols) and power density curves (open symbols). |
In summary, the proton conductivity of HT-PEMs is usually increased by raising ADLs, but at the expense of sacrificing the mechanical and dimensional stability. In the present work, we prepared novel HT-PEMs which simultaneously have high acid doping levels, high proton conductivity, high stability and excellent fuel cell performances. We suggest the excellent overall performance is related to a more open “sponge-like” membrane structure, which is supported by the molecular modelling, the low tested density, large FFV and large dsp results of membranes. This work provides a new insight for the structural design of PBIs for HT-PEMs.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05953d |
This journal is © The Royal Society of Chemistry 2015 |