Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells
Introduction
The search for renewable and sustainable energy has become one of significant challenges in our society, both for the transport sector and power generation [1]. Solid polymer electrolyte membrane fuel cell, which has recently received considerable attention, is a promising technology to meet today's energy needs [2]. These membrane fuel cells are particularly attractive because the fuel cell is clean energy equipment, eliminating many of the air pollutants from traditional energy [3]. Fuel cells, especially proton exchange membrane fuel cells (PEMFCs) have attracted considerable attention because of potential advantages such as rapid start-up, high energy efficiency, low environmental impact, excellent durability and wide range of applications [4], [5].
Proton exchange membrane (PEM), with a solid polymer electrolyte, is a critical component of PEMFCs. The ideal PEMs must satisfy the requirements of chemical stability, thermal stability, electrical insulation, excellent mechanical properties, and low cost for useful applications [6]. Today, a large number of polymers have been examined for this application, all of which offer both advantages and disadvantages [7]. The most used fuel cell membranes are those based on perflurosulphonic acid (PFSA), such as Nafion® [3]. However, these membranes are limited by their high cost, water dependent proton conductivity, and humidification requirement for regular operation [8]. Therefore, many polymers electrolytes and composite electrolytes with high proton conductivity at high temperatures have been found in the past two decades [9], [10], [11]. A few have been even used at above150 °C [12]. Phosphoric acid (PA)-doped polybenzimidazole (PBI) and its derivatives are promising candidates for anhydrous PEMs developed for intermediate temperatures [13]. The most successful example of these polymers is poly [2, 2′-(m-phenylene)-5, 5′-bibenzimidazole] (mPBI) [14]. Some studies showed that these polymer electrolyte membranes had high proton conductivity at low humidity and gas permeability, excellent thermal stability at temperatures up to 200 °C, and a water resistance coefficient close to zero [13]. Methods used to prepare polybenzimidazoles with high proton conductivities include the introduction of protogenic group in the processing of the polymerization [15], [16], or chemical grafting at nitrogen atom of the imidazole ring [17]. However, there are two challenges for mPBI in fuel cell applications. One is that mPBI is a rigid aromatic polymer whose main chains have strong intermolecular hydrogen bonding interactions which result in poor solubility of PBI in organic solvent and hence poor processability of mPBI [18]. There have been studies to modify mPBI in order to improve the solubility of the polymer [19], by introducing sulfone, ketone, ether or aliphatic units [16], [19], [20], [21], [22]. Kang et al. [14] recently reported a way to introduce ether groups to the structure of mPBI, and the resulting membrane with an acid doping level (ADL) of 18.73 mol of H2SO4 for OPBI per repeat unit showed high proton conductivity (190 mS/cm) at 160 °C and anhydrous conditions.
The other challenge is that the synthesis of mPBI by the polycondensation in liquid-phase (i.e., solution polycondensation) is usually complex and takes a long time, generally 10 h or more [9], [23], [24]. The slow reaction [25] does not meet the increasing demand of the compound. This challenge turns out to be a major problem. Various investigators [26], [27], [28] reported that the use of microwave radiation does not affect the yield, but accelerates the synthesis process by a factor of ten. Such unconventional energy can reduce the polycondensation time by molecular internal uniform heating, but prevents thermal decomposition and other side reactions which are caused by local overheating [14]. Therefore, in this study we also used the microwave radiation method to synthesis some new PBIs.
In addition, the new PBI we prepared in this study was doped with graphene oxide (GO) and its derivatives to improve the performance. Graphite oxide has received a great deal of attention because of its fascinating features [29]. It has different oxygen-containing functional groups, such as carboxyl, hydroxyl and epoxy groups. Because of the oxygen-containing functional groups, GO is easy to hydrate. As GO itself is an electronic insulator with differential conductivity, the high proton conductivity of the composite membrane is attributed to the hydrogen bonds in GO [30]. The acidic functional groups such as carboxylic acid and intermolecular hydrogen bonding can even provide additional proton conducting paths [31]. However, the dispersion of GO prepared by the Hummer method is poor in organic solvents such as DMF [32]. To avoid GO aggregates in organic solvents, one of the most effective ways is to graft active groups on the GO surface. Such functionalization of GO has been shown to be possible by Lerf et al., who prepared and studied a number of chemically modified GO derivatives [33]. Stankovich et al. have functionalized GO with isocyanates [34]. The isocyanate (-NCO) groups were introduced onto the surface of GO layers through the reaction of diisocyanates with –OH and –COOH groups [34], [35], [36].
In this work, we used 3, 3′-diaminobenzidine (DAB) and 5-tert-butyl isophthalic acid (TBIA) to prepare a polybenzimidazole(BuIPBI) containing large alkyl groups to improve the solubility in organic solvent such as DMAc by loosening its chain packing and reducing its high rigidity [37]. The process of polymerization can finish within 3 h by the use of microwave radiation. Besides we treated the surface of GO with tert-butyl isocyanate (TBI) to improve the dispersibility of modified GO (iGO) in both water and organic media. We then prepared GO/BuIPBI and iGO/BuIPBI composite membranes with different filler contents (1–15 wt%) and compared them with neat BuIPBI membrane. We found that the incorporation of iGO in the BuIPBI membrane can improve the chemical stability of the membrane so that it can be immersed in phosphoric acid with a higher concentration(85 wt%) without dissolve or swelling, and the higher concentration can increase the acid contents in the membrane and hence the proton conductivity of the membrane.
Section snippets
Materials
3, 3′-diaminobenzidine (DAB, 99%) was obtained from Acros Organics, New Jersey, USA. 5-tert-butyl isophthalic acid (TBIA) and tert-butyl isocyanate (TBI) were purchased from J&K Scientific Ltd., Beijing, China. Polyphosphoric acid (PPA, >84% P2O5) and graphite oxide (prepared from purified natural graphite (flake, natural, 325 mesh, 99.8%)) were purchased from Alfa Aesar Company, Tianjin, China. Sodium bicarbonate (NaHCO3), N, N′-dimethylacetamide (DMAc), N,N′-dimethylformamide (DMF), potassium
Characterization of BuIPBI, GO, and iGO
The FT-IR spectrum of BuIPBI is shown in Fig. 1. The absence of the carboxyl peak at 1650–1700 cm−1 indicates that the cyclization to form the imidazole ring is complete in BuIPBI [40]. The peaks from 1450 cm−1 to 1600 cm−1 indicate the existence of the benzene ring. The characteristic peaks at 1618 cm−1 is due to the stretching vibration of CN groups. The peaks appearing in the wavenumber range 2800–2900 cm−1 are ascribed to the C–H stretching of tert-butyl groups [41]. The absorption peak at
Conclusions
A series of composite membranes made from GO or iGO and BuIPBI have been prepared for PEMFC applications. Due to the good dispersibility of iGO in organic solvents, BuIPBI membranes with a highly uniform structure with iGO were prepared. Because of the hydrogen bonding between iGO and BuIPBI, the membranes had high chemical stability. These iGO/BuIPBI membranes with higher stability could absorb more acid but undergo only a small amount of swelling. With a high acid content, the iGO/BuIPBI
Acknowledgment
The authors gratefully acknowledge the financial supports from the National High Technology Research and Development Program of China (No. 2011AA11A273), the National Natural Science Foundation of China (No. 21176022, 21176023 and 21376022), the International S&T Cooperation Program of China (No. 2009DFA63120 and 2013DFA51860), and the Fundamental Research Funds for the Central Universities (ZY1328), the Program for Changjiang Scholars and Innovative Research Team in University (IRT1205).
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