Inorganic–organic membranes based on Nafion, [(ZrO2)·(HfO2)0.25] and [(SiO2)·(HfO2)0.28]. Part I: Synthesis, thermal stability and performance in a single PEMFC
Highlights
► Two new “core–shell” nanofillers, [(ZrO2)·(HfO2)0.25] and [(SiO2)·(HfO2)0.28] were prepared. ► The nanofillers were dispersed into Nafion to obtain two series of proton-conducting membranes. ► The two nanofillers affect differently the thermomechanical features and the fuel cell performance of the samples. ► The data are rationalized in a model correlating the different Nafion-nanofiller interactions with the experimental trends.
Introduction
Fuel cells (FCs) are advanced electrochemical devices for energy conversion that obtain electric power from the chemical energy of their reagents with a very high yield, up to 55% or more [1]. One family of FCs that has drawn considerable attention from both the academic world and industry is based on two porous electrodes coated with suitable electrocatalysts, where the reactions involved in the operation of the device occur and separated by a proton-exchange membrane (PEM) [2], [3], [4]. Systems based on PEMs are known as proton-exchange membrane fuel cells (PEMFCs) and have been extensively studied as power sources for a variety of applications from portable electronic devices to light-duty vehicles owing to their low operation temperature (T < 130 °C), high conversion efficiency and high power density in comparison to competing technologies such as lithium batteries and conventional thermal engines [1], [4], [5]. PEMs can be manufactured from a number of materials, including sulfonated polyetherether sulfones, polyetherethereketones, polysiloxanes, and others [6], [7], [8]. However, perfluorinated ionomers, such as Nafion®, Aquivion, Aciplex, among others, are still the most widely applied systems owing to their high proton conductivity and good chemical and electrochemical stability2 [9], [10], [11]. Nevertheless, perfluorinated ionomers suffer from important drawbacks, including the necessity of a very high degree of hydration for proton conduction. This fact ultimately prevents them from operating effectively at temperatures higher than ca. 80–90 °C and low humidification [7], [9], [12]. These are important technological limitations as: (a) the required water and thermal management modules are very bulky and expensive; and (b) it is necessary to use very pure fuels because at temperatures lower than 120–140 °C the platinum-based electrocatalysts used in PEMFCs show a very poor tolerance towards even small traces of common contaminants such as CO and H2S [13] that typically are found in hydrogen obtained from steam-reforming processes [14], [15]. One way to address the limitations of perfluorinated ionomers is to devise hybrid inorganic–organic proton-conducting materials, where a suitable filler of micrometer to nanometer size is added to the polymer host [16], [17], [18]. In recent years, this approach has been focused on a variety of hybrid systems, which utilize the following families of nanofillers [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]: (a) heteropolyacids, including silicotungstic acid, phosphotungstic acid, molybdophosphoric acid and others; (b) zirconium phosphate; (c) organically-modified silicates and silane-based fillers; (d) zeolites; and (e) Pt, Pt–SiO2 and Pt–TiO2. Generally, the filler is a material characterized by a strongly acidic and/or hydrophilic behaviour. This choice is based on the assumption that these properties would possibly provide extra mobile protons as charge carriers and help the resulting hybrid system retain water at high temperatures and low hydration levels. In the past few years, this research group has conducted experiments on a number of different hybrid systems, to obtain doped Nafion ionomers containing: (a) both pristine and “core–shell” oxoclusters [34], [35], [36], [37], [38], [39], [40]; (b) single inorganic oxoclusters doped with an ionic liquid [41]; (c) oxoclusters functionalized with perfluoroalkylated chains [42]; and (d) proton-conducting ionic liquids (PCILs) [43]. These continuing efforts allow better elucidation of the effects arising from the complex interplay between the perfluorinated ionomer and the dopants in terms of structure and the proton conduction mechanism of hybrid proton-conducting materials. Among the investigated materials, systems including nanofillers with a basic character such as HfO2 have shown an improved proton conductivity and interesting structural features [35], [36], [37]. However, the use of pristine hafnium oxide as a filler in hybrid membranes should be avoided because of its very high cost. Recently, it was shown that it is possible to develop oxide nanofillers characterized by “core–shell” morphology. The “core” is made of particles with a high Mohs hardness or crystallinity, while the “shell” consists of the softer oxide [38], [39]. This approach is followed here to prepare nanofillers characterized by a HfO2 “shell” or “core”. The ultimate goal is to reduce the loading of hafnium in the resulting hybrid inorganic–organic membranes. ZrO2 and SiO2 are used as the “core” and the “shell” to obtain nanofillers with the formulae [(ZrO2)·(HfO2)0.25] and [(SiO2)·(HfO2)0.28], respectively, which are characterized by HR-TEM, EDX and ED measurements. Two families of hybrid Nafion-based inorganic–organic proton-conducting membranes are prepared with [(ZrO2)·(HfO2)0.25] and [(SiO2)·(HfO2)0.28] nanofillers and are labelled [Nafion/(ZrHf)x] and [Nafion/(SiHf)x], respectively. In both families, the mass fraction of the nanofiller x is 0.05, 0.10 or 0.15. This study is focused on elucidating the effect of the nanofillers on the proton exchange capacity, water uptake, thermal stability and fuel cell performance of the Nafion host polymer. Particular attention is dedicated to creating a coherent picture that relates the effects of the different Nafion-nanofiller interactions to the water uptake and proton conductivity of the hybrid membranes as a function of the water vapour activity (aH2O) in the environment.
Section snippets
Reagents
A 5 wt% solution of Nafion® ionomer with a proton exchange capacity (PEC) of 0.9 mequiv g−1 (Alfa Aesar, ACS) was used as received. Amorphous silica with a particle size of 9 μm and a porosity of 1.8 mL g−1 was provided by Silysiamont S.p.A., Italy. Zirconium oxide, hafnium oxide and all the solvents were acquired from Sigma–Aldrich and used as received. Nafion 115™ membranes were obtained from Sigma–Aldrich and activated by the standard procedure reported elsewhere [42]. The C2-20
Preparation of the nanofillers
To obtain the nanofillers described here, two oxides characterized by a different Mohs hardness or crystallinity are suspended in dimethylformamide (DMF). The resulting mixture is ground extensively in a planetary ball mill. There are two main objectives of this preparation protocol: (a) to segregate the oxide characterized by the lower hardness or crystallinity, i.e. HfO2 or SiO2, on the surface of the resulting “core–shell” inorganic biphasic nanoparticles; and (b) to obtain nanoparticles
Discussion
The data presented in this work can be rationalized in a coherent picture by considering the Nafion-nanofiller interactions occurring due to the presence of the nanofiller particles in the structure of pristine Nafion. It is well-known that Nafion is characterized by hydrophilic domains dispersed in a highly hydrophobic matrix [10]. Water is absorbed in the hydrophilic domains, where it causes the dissociation of the –SO3H groups found at the domain boundaries [49]. Thus, H3O+ ions are produced
Conclusions
In this work, two “core–shell” nanofillers are obtained by grinding either SiO2 or ZrO2 with HfO2. The resulting [(ZrO2)·(HfO2)0.25] and [(SiO2)·(HfO2)0.28] nanofillers are used in the preparation of two families of three hybrid inorganic–organic membranes each. Nafion is used as the host polymer, and the mass fraction of nanofiller x is set as 0.05, 0.10 and 0.15. The membranes are obtained with a solvent-casting procedure and are subsequently purified and activated. The nanofillers do not
Acknowledgements
Research was funded by the Italian MURST project PRIN2007, “Passive direct methanol fuel cells: electrocatalysts for the oxygen reduction reaction based on carbon nitride supports and hybrid inorganic–organic membranes based on fluorinated ionomers and nanoparticles of mixed oxoclusters”. The author N. B. thanks Texa S.p.A. for the Ph. D. grant. The authors extend their most sincere thanks to the staff of the mechanical workshop of the Department of Chemical Sciences of the University of Padova
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