Improved performance of sulfonated polyimide composite membranes with rice husk ash as a bio-filler for application in direct methanol fuel cells
Introduction
Membrane technology has become a magnificent separation technology in recent decades, as it works without the aid of additional chemicals and with a very low energy use. There are many different and very characteristic separation processes in membrane technology. These processes can be classified as the same type because a membrane is used in each of them. The structure and composition of the membrane are the most important aspects of the system, as they will determine how the process plays out. Generally, the membranes used in separation processes can be divided into two types: synthetic polymer-based and natural polymer-based (bio-membrane). Synthetic membranes have long dominated the membrane industry due to their high performance, thermal stability and chemical resistance compared to bio-membranes.
Recently, several attempts at developing high-performance bio-membranes have been made as the evolution of synthetic polymer-based membranes has reached a bottleneck and due to the shifting of research towards a more environmentally friendly production method. Bio-membranes have received tremendous attention as promising membrane replacements due to their better biocompatibility and low cost compared to synthetic polymer membranes. Chitosan, alginate and cellulose, which are extracted from natural raw materials, have been studied intensively for their feasibility in membrane separation processes.
Chitosan is a cheap, non-hazardous and environmentally friendly biopolymer [1]. Chitosan has been widely used in water treatment, dye removal, and heavy metal removal applications. For instance, Qin et al. [2] fabricated a short nanofibre porous chitosan membrane for low temperature thermally induced phase separation processes. The results of their experiment showed that the fabricated chitosan membrane was able to remove Cu2+ ions effectively (adsorption capacity: 2.57 mmol g−1) and retained 90% efficiency after 6 cycles of the separation process. In addition, different fabrications will affect the performance of chitosan membrane. Li et al. [3] produced a pure chitosan nanofibrous membrane using the electrospinning method. They claimed that the membrane was able to remove dye (acid blue-113) adequately, and the adsorption capacity was 1377 mg/g, which was 3.34 times higher than that of a normal chitosan membrane.
Moreover, alginate biopolymers are mixed with other polymers or composite materials to form a hybrid membrane. This membrane combines both the advantages of alginate and the properties of the composite materials. For instance, Moulik et al. [4] synthesized a sodium alginate/polyaniline (SA/PAni) composite membrane for pervaporation separation of acetic acid. The results proved that the composite membrane had high potential for commercialization in the application of acetic acid dehydration to a very high purity level of more than 99%. The bio-membrane has a high potential and could replace current synthetic membranes, as they display performances comparable to that which the synthetic membranes can achieve. Nevertheless, bio-membranes are facing some critical issues, which are the limitations of poor thermomechanical stability and low productivity [5]. These problems need to be solved before bio-membranes can be widely used in industrial applications.
Incorporating a filler into the matrix of the polymer has been utilized to increase the performance of a membrane, especially in fuel cell applications. Ionic conductor fillers, such as silica [6] and titanium oxide [7], as well as electronic conductor fillers, such as Pd [8] and PtRu, are incorporated into the membrane matrix in order to enhance the direct methanol fuel cell membrane performances, such as water retention, proton conductivity and methanol permeability. However, these fillers are sometimes very expensive and not environmentally friendly. The high cost of a filler may lead to an increase in the fuel cell production cost, and hence, it may lengthen the time for commercialization of direct methanol fuel cell as alternative portable power sources.
One of the challenges that normally being faced by the proton exchange membrane in a direct methanol fuel cell is the methanol crossover problem. High methanol permeability will lead to poor fuel cell performance [9]. Methanol permeability is sometimes affected by the water uptake of the membrane because of the enlarged channel size during the swelling of the membrane at high water uptake [10]. Several strategies have been introduced in order to navigate out this problem. Preparation of two different polymers via direct blend or sol-gel process will produce bi-functional proton exchange membrane for fuel cell. Addition of a polymer with high methanol rejecting ability such as poly (vinyl) alcohol will enhance the methanol permeability of the membrane [11]. Cai et al. [12] reported that polymer nano-sieves incorporated into Nafion were able to block the methanol crossover under low content due to their small pore size. Morphology of a composite membrane had a significant effect towards the methanol permeability. Smooth cross-sectional surface will have high methanol blocking efficiency [13]. Besides, introducing the proton conductive macromolecules (PCM) will enhance the methanol permeation of composite membrane due to the dense structure, strong electrostatic interaction and ionic cross-linking between the PCM and polymer matrix [14], [15], [16].
Bio-filler, in which the fillers are derived from natural raw materials such as agricultural wastes, is strongly believed to be able to act as a cheaper substitute for the current available inorganic fillers. Their low cost (usually obtained from wastes) [17] and eco-friendliness have led to a new pathway in the development of membrane technology. Currently, eggshells are a common bio-filler that have been investigated in a large number of studies. Eggshell bio-fillers have been utilized in various applications, such as rubber production [18], thermoplastics [19] and coating technology [20], [21]. For instance, Hassen et al. [19] synthesized an eggshell-polypropylene composite thermoplastic, and the results showed that the addition of eggshell bio-filler into the polymer matrix enhanced the mechanical and thermal properties of the polypropylene membrane. Bio-filler has the potential to increase the performance of membranes in a cheaper way. However, there are a limited number of studies being done on the effect of different types of bio-fillers towards membrane performance. In fuel cell applications, no work has been reported on the study of bio-filler being used for a direct methanol fuel cell. Therefore, in this study, the effect of bio-filler on the performance of fuel cell membranes has been carried out.
Rice husk ash (RHA) has attracted the attention of the semiconductor industry and other researchers for use as a bio-filler due to its high silica content (85–95%). Disposing of rice husk agricultural waste has been challenging worldwide, as 1.28 × 108 tons of rice husk are produced annually [22]. RHA-based silica has caught the attention of researchers as raw materials or support for different applications [23], such as a filler for filtration membranes [24], adsorbents, photoluminescent materials [25] and high-capacitance activated carbons [26].
Apparently, there is still no work reported on the application of bio-filler RHA in proton exchange membranes for DMFC applications, although RHA possesses high potential as a filler to increase membrane performance. Hence, giving the rice husk a second life would be one way to recycle the rice husk waste for valuable applications. Thus, the aim of this study is to investigate the feasibility of using RHA as a bio-filler for a fuel cell membrane instead of using a synthetic filler, which has a higher cost and is more time-consuming. Bio-filler RHA was used and fabricated as the filler for the sulfonated polyimide (SPI) membrane to produce a composite membrane and evaluated based on the membrane performance for DMFC applications. RHA was prepared via open-air oxidation at 600 °C. A series of SPI-RHA composite membranes with different RHA loadings were produced using direct mixing with the ultrasonication method. The morphology and properties of these composite membranes were investigated in detail. The effects of the incorporation of RHA into the SPI matrix were studied and evaluated in terms of ion exchange capacity, water uptake, proton conductivity and the methanol permeability of the composite membranes. The passive single-cell performance of DMFC for each composite membrane was evaluated. The output of this work showed that the bio-filler RHA improved the performance of the membrane and that this was the first time in fuel cell history that a bio-filler was utilized in a fuel cell membrane.
Section snippets
Materials
Rice husk was obtained from the rice mill Satu Cita Sdn Bhd, Semanggol, Perak, Malaysia. Chemicals such as 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) (Sigma-Aldrich, Germany), 4,4-diaminodiphenyl ether (ODA) (Sigma-Aldrich, Germany), sodium hydroxide, concentrated hydrochloric acid, fuming sulfuric acid (SO3, 65%) (Sigma-Aldrich, Germany), concentrated sulfuric acid (98%), benzoic acid, acetone, dimethyl sulfoxide (DMSO) (Sigma-Aldrich, Germany) were used as received.
Preparation of rice husk ash
Rice husk was
FTIR analysis
Composite membranes with different compositions of RHA have been produced. Fig. 1 shows the FTIR spectra of RHA, pristine SPI membrane and composite membranes with 3 different RHA loading. Absorption peaks at 1710 cm−1 and 1670 cm−1 for the SPI and composite SPI-RHA membranes represented the stretching vibrations of the carbonyl groups present in the imido rings. The following peak at 1471 cm−1 corresponded to the characteristic absorption of aromatic rings, whereas peak at 1350 cm−1 was the
Conclusion
RHA was produced through an oxidation process at 600 °C and successfully enhanced the performance of the SPI-RHA composite membranes compared to pure SPI membrane. The addition of RHA filler into the SPI matrix improved these composite membranes in terms of ion exchange capacity, water uptake, proton conductivity and methanol permeability. A passive DMFC single-cell test verified that RHA had the ability to acts as a membrane additive to increase the maximum power density by more than 50%
Acknowledgements
The authors appreciatively acknowledge the financial support given by Universiti Kebangsaan Malaysia under DIP-2017-021.
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