Effect of the Sulfated Zirconia Nanostructure Characteristics on Physicochemical and Electrochemical Properties of SPEEK Nanocomposite Membranes for PEM Fuel Cell Applications (original) (raw)

2018, Macromolecular Materials and Engineering

when operating at elevated temperatures, researchers have focused on heat-resistant materials as electrolytes for application at intermediate temperatures. [3-7] One of such materials is a type of hydrocarbon polymers known as poly(ether ether ketone) (PEEK) which is converted to sulfonated poly(ether ether ketone) (SPEEK) during an electrophilic sulfonation reaction. [8-15] Due to the fact that pristine SPEEK suffers from major drawbacks at higher temperatures, that is, above 100 °C, the fillers are introduced into polymeric matrix to overcome them. [16-18] For instance, numerous additives such as zeolite, [8] aluminum oxide, [19] single-walled carbon nanotubes, [20] and zirconium oxide [21,22] were embedded into SPEEK matrix and possessed great properties. Besides, Du et al. [23] studied the compatibility of silica sulfuric acid (SSA) particles with SPEEK by fabricating a series of SSA/ SPEEK membranes. The proton conductivity, distribution of ionic clusters, water retention, and thermal behavior of the prepared SSA/SPEEK membranes were promoted. [23] In addition, the composites of SPEEK and surface-functionalized TiO 2 (hydrophilic and hydrophobic TiO 2) additives were investigated by Di Vona et al. [24] It was proved that different types of functionalities on the surface of the filler make variation in physicochemical properties of prepared nanocomposite membranes. Likewise, titania nanosheet (TNS)/SPEEK membranes were another interesting candidates for electrolytes of fuel cells working at medium temperatures (T > 100 °C). [25] Sulfated zirconia (SZ) superacid with favorable properties including high proton conductivity, thermal stability, and simple synthesis procedure, is considered a primary interest filler within composites. [26] The SZ fillers by representing a high conductivity of 3 mS cm −1 in Nafion/SZ composite membranes under completely dry condition received much attention for application in high-temperature fuel cells. [27] Zhang et al. [28] employed sulfated zirconia supported platinum catalyst (Pt-SZ) as a self-humidifying agent in SPEEK matrix for PEMFC application under low relative humidity condition. The Pt-SZ additives by providing additional acidic sites within the SPEEK/ Pt-SZ membrane made a general enhancement in terms of proton conductivity, power density, and water uptake (WU) at Sulfated Zirconia Fillers This study expresses that the characteristics of sulfated zirconia (SZ) nanostructure in sulfonated poly(ether ether ketone) (SPEEK)-based membranes is the key to optimize their properties for fuel cell applications. Two types of SZ treating in different thermal conditions are produced by precipitation and analyzed by X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, energy-dispersive X-ray analysis, and transmission electron microscopy. Sulfate concentration on both types of SZ is changing from 0.54 to 1.45 wt% and thus the SZ samples differ in particle size and sulfate content. The nanocomposite membranes are prepared by incorporating 6 wt% of SZ samples into SPEEK matrix in casting procedure followed by performing electrochemical characterizations and impedance spectroscopy. Moreover, morphology, mechanical, and chemical stability of the membranes are investigated by field emission scanning electron microscopy, stress-strain, and ex situ Fenton's tests. The incorporation of SZ sample having more surface sulfate groups in the SPEEK matrix results in not only excessive oxidative stability and tensile strength but also more acidic sites for ion transport, promoting conductivity. Furthermore, both types of nanocomposite membranes show improved ionic conductivity and water affinity with a lower tendency to swell rather than the plain SPEEK membrane. It is proved that desired consequences of doping SZ into SPEEK matrix can be intensified by changing the physicochemical properties of sulfated zirconia nanoparticles.