Investigation of the effects of SPEEK and its clay composite membranes on the performance of Direct Borohydride Fuel Cell

https://doi.org/10.1016/j.ijhydene.2019.09.203Get rights and content

Highlights

  • Polymer electrolyte membranes were fabricated for DBFC.

  • Peak power densities were found to be close to Nafion.

Abstract

In this study, composite cation exchange membranes (CEM) were developed. With the experience from widely studied proton exchange membrane fuel cells (PEMFC), sulfonated polyether ether ketone (SPEEK) was prepared to be a more effective and cheaper ionomer alternative to the industry standard Nafion ®. SPEEK polymer membrane can reach sufficient ionic conductivities but have some mechanical and chemical stability problems (at a high degree of sulfonations (DS)). Therefore, in order to optimize the membrane, composite mixing with a well-known organic/inorganic clays called Cloisite® 15A, Cloisite ® 30B and MMT were used. Test cells for both single-cell and conductivity were designed and constructed. The ionic conductivity cell was different than the ones used in most studies, measuring conductivity in-plane with 4 probes using EIS. The membranes were characterized for their proton conductivity with electrochemical impedance spectroscopy (EIS), for DS with H NMR, water uptake, and fuel cell performance tests. First results showed that the acidic sulfonic groups of SPEEK interacted with organic/inorganic clays and as a result of partial barrier the ionic conductivity was decreased but power densities were increased. SPEEK-Cloisite® 30B composite membrane has given 40 mW/cm2 power density value which is higher than pure SPEEK membrane (35 mW/cm2). The proton conductivities of the final composite membranes were close to bare SPEEK membranes which are 0,065 and 0,075 S/cm for SPEEK-Cloisite ® 30B and pristine SPEEK, respectively.

Introduction

Direct Borohydride Fuel Cell (DBFC) is one of the most promising sub-class fuel cell type that has operation method and materials similar to Proton Exchange Fuel Cell (PEMFC) but instead, the fuels such as sodium borohydride or potassium borohydride solutions used. This difference, eliminates the risks of storage and the transportation of hydrogen fuel. Moreover, the sodium borohydride and potassium borohydride fuels have higher energy and higher theoretical power density. Also for Turkey, in particular, technologies utilizing boron-containing compounds are important since the vast share of boron sources in the world are located in Turkey. Sodium borohydride (NaBH4) has broad application areas as a reducing agent in some organic and inorganic reactions.

Direct borohydride fuel cells (DBFCs) experience increased attention recently as a sub-class of polymer electrolyte fuel cells (PEFC). The reasons behind this attention can be briefly stated as the advantage of the oxidation potential of fuel's – 1.24 V over SHEs. Reactions in the Borohydride Fuel cell are given as follows [[1], [2], [3]]:Anode: NaBH4 + 8OH = NaBO2 + 6H2O + 8e (E0anode = −1.24 V)Cathode: 2O2 + 4H2O + 8e = 8OH (E0cathode = 0.40 V)Overall: NaBH4 + 2O2 = NaBO2 + 2H2O (E0cell = 1.64 V)

Cation and anion exchange membranes (CEM, AEM) could be utilized in borohydride fuel cells. Logically, since the DBFC fuel is alkaline, AEM seems to be the best membrane type but, as can be seen in various studies, CEM gives a better overall performance. Some researchers investigated and compared the performance of the CEM and the AEM group membranes on a NaBH4/H2O2 fuel cell and concluded that the CEM membranes have a better overall effect on the overall performance of the fuel cell [4]. The main reason for this is the high fuel crossover of AEMs to the alkaline fuel, borohydride. Among the CEMs, Nafion ® is by far still the most common membrane in the DBFC studies.[5] Specific to electrolyte polymer membranes, the ionic activity is the major feature which has to be developed. As the operation gets to the highest temperature point in the reaction; ionic conductivity increases but it can cause dryness in the membrane [[6], [7], [8]].

Cloisite ® particles (a modified Montmorillonite (MMT) organoclay) with composite membranes, which is prepared by using a SPEEK hydrocarbon-based polymer, will supply higher power outcome in terms of proton conductivity on DMFC performance. This composite membrane has the potential for drastic improvements for the DMFC systems as a PEM [[14], [15], [16], [17], [18]].

According to numerous researches about Polymers, using SPEEK for the development of membranes and utilization as an electrolyte at DMFCs is accepted as the most promising alternative option [10,12,19]. These sulfonated polymers have been used for a wide variety of fuel cell applications under varying conditions to achieve improved performances, such as nanocomposites or mixed nanocomposites of SPEEK (Fig. 1) [22].

In the last years, Polymer-clay composites have attracted attention both from the sindustry and the academy [24]. A good deal of research was made on the polymer and clay mixings. Results show that clay is a fuel crossover barrier. This discovery has proved that the conductivity of composite membranes is lower than the undisturbed basic polymers [25]. Also, addition of clay to the polymer will lower the improper proton conductivity in DMFC applications [26].

Despite the advantages the MMT clay exhibits, it has some disadvantages as well because of the chemical microstructural behavior. Because the silicate clays are hydrophobic, they do not show affinity to polymers. Because of this, an organic modification made on the clay surface should enhance the compatibility with the polymeric materials [27]. With this chemical modification, the clay could be made compatible with numerous polymers. When the modified clays merge with polymeric matrixes, it is determined that the general mechanical and barrier properties of composite membranes are enhanced [28]. Modification of inorganic clays is an implementation which is done to improve the features of MMT clays [20,23]. To enlarge the implementation furthermore, it should be prepared with organic modifiers like alkylammonium cations. Cloisite ®15A and 30B are two of the MMT clays that are modified to be organic structured. This has promising features such as high distance among layers and high height modulus [21].

Section snippets

Sulfonation SPEEK and membrane preparation

Polyether-ether-ketone that was provided from Sigma-Aldrich, was dissolved in a concentrated sulfuric acid. The reaction was progressed for total 1 week in ambient conditions. SPEEK polymer was decanted and washed with fresh deionizing water frequently to eliminate the excess sulfuric acid. This process was repeated thoroughly until the ph reached to values of 6–7. 5 wt% of clays were added to the SPEEK in Dimethylacetamide (DMAc) and stirred for 12 h. Four sample membranes (SPEEK membrane and

Ion exchange capacity

On IEC calculations, IEC values did not show linearity with the clay percentage. Highest IEC obtained on Cloisite ® 30B membrane, followed by Cloisite ® 15A and MMT membranes, respectively. But this conclusion can lead us to that the structural mechanism of various Cloisite ® might increase IEC drastically. Results were given in Fig. 2.

Ionic conductivity

Ionic conductivity results can be seen in Fig. 3. The conductivity of pristine SPEEK is 0.07 S/cm and 0,075 S/cm at room temperature and at 80 °C, respectively.

Conclusions

Membranes are the most critical and significant component of the fuel cells. Because of this, development of membranes that are highly ionic conductive, lower fuel exchange and chemically stable, also that are low cost and exhibit long working life is very important in the commercialization of fuel cells. DBFCs are getting attention as their membranes are resistant and exhibit long life in presence of alkaline and corrosive fuel. Consequently, literature research has been done and a sulfonated

Acknowledgments

Financial assistance and interest of the TÜBİTAK in Turkey for 215M255 coded project and the BAP-2015-038 coded Kocaeli University projects are gratefully acknowledged. This article was funded by The Scientific and Technological Research Concuil in Turkey (TUBİTAK) and Kocaeli University Scientific Research Project Coordination Department (BAP). TUBITAK is in Gebze, Kocaeli. BAP is also in Kocaeli. Both organizations are in Turkey. TUBİTAK project number is 215M255, BAP project number is

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