Effect of sulfated metal oxides on the performance and stability of sulfonated poly (ether ether ketone) nanocomposite proton exchange membrane for fuel cell applications
Graphical abstract
In this study with the aim of increasing proton conductivity, mechanical and chemical properties of SPEEK-based membranes at the same time as an alternative to Nafion for high temperatures, optimized amounts of Sulfated titania (SO42−/TiO2) (STi) and Sulfated zirconium-titanate (SO42−/ZrO2 − TiO2) (SZrTi) binary oxides have been introduced into SPEEK polymer, and the physicochemical properties of resulted composite membranes were investigated.
Introduction
Polymeric membranes are the core component of the proton exchange membrane fuel cells (PEMFCs), which are the energy generator with the lowest pollutant emissions. Nafion is the most used material in this field because of high proton conductivity and significant chemical and mechanical stabilities up to 80 °C [1]. Despite, working at temperatures above 100°C has many advantages such as (i) enhancement of electrochemical kinetics reactions, (ii) facilitation of water management due to one phase formation, (iii) simplification of cooling system because of the temperature gradient increment, (iv) facile recovery of waste heat, (v) the increase of CO tolerance to subside the poisoning of platinum electrocatalyst and usage of low-quality fuels [2]. So, several kinds of research have been devoted to finding an alternative for this commercial material. The current research on the materials explores different methods such as polymers blending [3], crosslinking [4], incorporation of ionic liquids [5,6], block-copolymers [7], fluorination [8], thermal annealing [9], and using organic/inorganic hybrid materials [10].
Organic-inorganic composite membranes are attractive since high mechanical and thermal stability, electrical and magnetic activities are provided by inorganic segments, while the organic moieties provide flexibility and multi-functional reactivity [11]. Increased proton conductivity [12], enhanced mechanical properties [13], significant water retention, and improved chemical stability [14] are mentioned as some of the possible superiority of incorporating the inorganic material into composite membranes for fuel cell applications. A wide range of fillers such as silica [15], aluminum phosphate [16], clays [17], zirconia [18], titania [19], functionalized carbon [20], and polymeric nanoparticles [21] are used for the preparation of the composite proton exchange membranes. Although, in some cases, proton conductivity decreases with an increase in filler content due to the relatively low proton conductivity of some fillers [10]. The conductivity was also affected by the interaction between the additives and proton-exchange groups in the polymer matrix [22]. One of the most attracting polymers, which have been considered as polymer host in organic-inorganic composites, is sulfonated poly ether ether ketone (SPEEK) due to its high chemical and thermo-mechanical stabilities, low cost, and with some modifications, it has superior proton conductivity especially at high temperatures compared with Nafion [23]. Also, these kinds of materials are more eco-friendly since they do not have any fluorinated components.
Up to now, most studies have been devoted to increasing the proton-conductivity of SPEEK but at the price of dropping in mechanical or/and chemical stability and vice versa. For instance, Di Vona et al. [24] prepared two different model SPEEK-based composite membranes with a hydrophobic and hydrophilic surface modifier. They found the composite with a hydrophobic modifier has a smooth fine structure, reproducible mechanical attributes, less water uptake, and proton conductivity, while the composite with the hydrophilic modifier presents a rough structure, lower flexibility, more water uptake, and proton conductivity. Besides, Ozawa et al. [25] synthesized hybrid membranes consisting of an aliphatic backbone and amorphous TiO2 nanoparticles from tetra-allyloxytitanium and phosphonic acid methacrylate by copolymerization-hydrolysis. The membranes exhibited good thermal stability up to 180°C. The tensile modulus of the composite membrane with a composition of 95:5 improved up to 505 MPa and its conductivity was 4.1× 10−2 S cm−1at 130 °C and 100% relative humidity. Also, the peak power density of 4.8 mW cm−2 at 140 °C and 30% RH was recorded by the composite membrane. So, the best combination of method and material must be used to have conductivity and durability at high level simultaneously.
The goal of this study is to prepare an efficient and low-cost membrane, which possess high performance as well as acceptable durability for application in PEMFCs at moderate temperatures. The sulfated form of TiO2 and ZrO2-TiO2 nanoparticles were selected to incorporate into SPEEK matrix, because the sulfated form of titania shows the higher surface area, increased acidity and delayed transformation from amorphous to crystalline state (which is necessary for doping sulfate ions), in comparison to that of the pure form [26]. Also, incorporation of titania to zirconia would improve acid-base properties [27] as well as thermo-mechanical stabilities [28,29]. Hence, these two important superacid nanoparticles: (i) Sulfated titania (SO42−/TiO2) (STi), a conductive nanoparticle with the capability of enhancing thermo-mechanical characteristics of the polymer, and (ii) sulfated zirconia-titania (SO42−/ZrO2 − TiO2) (SZrTi), a binary metal oxide that has superior properties than the single oxide has been introduced into the SPEEK polymer matrix. The properties of prepared nanocomposites were investigated and compared with Nafion and some of the SPEEK-based membranes.
Section snippets
Materials
Poly (oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene), (PEEK), average Mw ~20,800, average Mn ~10,300, and melting point of 322 °C obtained from Sigma-Aldrich. 40 wt% platinum on Vulcan (Pt/C) was purchased from Fuel Cell Store, and Nafion solution (EC-NS-05) was prepared by ElectroChem. The other used chemicals were collected from Merck Co.
Preparation of the sulfated nanoparticles
The nanocrystalline titanium oxide reported in this study was synthesized by the sol–gel method using tetra-n-butyl orthotitanate as a metal
Nanoparticle characterization
The synthesized TiO2 and ZrO2-TiO2 nanoparticles were analyzed by XRD before and after sulfonation, and their corresponding patterns are shown in Fig. 1. The results associated with the reference code, crystal system, chemical formula, and crystallite size are reported in Table 1. As observed, samples become more amorphous after sulfonation. The crystallite size was diminished since the sulfates reduced the crystallinity of the samples. Especially, the monoclinic and cubic phase will not be
Conclusion
SPEEK-based self-humidifying nanocomposite membranes with superacids such as acid-modified titania and zirconia-titania were prepared in this study through the solution-casting method. A comprehensive study was conducted to investigate the performance of the nanocomposite membranes. Also, the results of a single cell at 120°C and 70–80% RH were recorded to survey the performance of the nanocomposite based MEA in a real device. Detailed investigation revealed that:
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Incorporation of sulfated metal
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.
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