Improvement of fuel cell performances through the enhanced dispersion of the PTFE binder in electrodes for use in high temperature polymer electrolyte membrane fuel cells

https://doi.org/10.1016/j.ijhydene.2020.03.095Get rights and content

Highlights

  • Bar-coated electrodes were prepared in the presence and absence of a surfactant.

  • The surfactant improved the dispersion of the polytetrafluoroethylene binder.

  • The reproducibility and efficiency of the triple phase boundary were improved.

  • The use of a surfactant gave superior electrode performances in single cell system.

  • This system was applicable for the preparation of large-scale bar-coated electrodes.

Abstract

In high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs), it is important that the structure of the electrode catalyst layer is formed uniformly. To achieve this, the binder must be well dispersed; however, polytetrafluoroethylene (PTFE), which is commonly employed in the preparation of HT-PEMFCs, is difficult to disperse during electrode manufacture due to its high hydrophobicity. In this study, we fabricate electrodes containing a surfactant to improve the dispersion of the PTFE binder and to enhance reproducibility during electrode manufacture. The electrodes are commonly prepared via a bar coating method, which is known to exhibit poor dispersion due to the small amounts of solvent employed compared to the spraying method. We then compare the properties of the obtained electrodes prepared in the presence and absence of the surfactant through physical and electrochemical characterization. It is found that the electrode containing the surfactant is structurally superior, and its single cell performance is significantly higher (i.e., 0.65 V at 0.2 Am cm−2). The single cells are suitable for operation at 150 °C using H2/air at atmospheric pressure and a total platinum loading of 2.0 mg cm−2.

Introduction

Polymer electrolyte membrane fuel cells (PEMFCs) are known to exhibit high energy conversion efficiencies and low environmental pollution. As such, numerous studies have been conducted into PEMFCs based on Nafion and their successful commercialization [1,2]. However, Nafion-based PEMFCs operate at temperatures below 80 °C, and these low-temperature PEMFCs (LT-PEMFCs) are known to exhibit a poor tolerance to CO [[3], [4], [5]], in addition to requiring a humidification device to function [6]. To address these issues, high-temperature PEMFCs (HT-PEMFCs) that operate at 150–200 °C have been investigated, which exhibit high CO resistances [[7], [8], [9]] and can be operated under humidifier-free conditions, thereby simplifying the overall setup. HT-PEMFCs also can utilize waste heat to render their operation highly efficient [[10], [11], [12]]. However, despite such advantages, HT-PEMFCs tend to exhibit lower performances than LT-PEMFCs, and so it is necessary to improve the performances of HT-PEMFCs to allow their practical application. One reason for the lower performance of HT-PEMFCs is that the phosphoric acid present in the catalyst layer can affect the catalytic activity [[13], [14], [15]]. More specifically, in HT-PEMFCs, phosphoric acid from the membrane acts as a proton conductor and is an essential component of the triple phase boundary (TPB). If the amount of phosphoric acid is insufficient in the catalyst layer, the performance of the fuel cell is lowered due to reduced TPB formation. Conversely, if there is an excess of phosphoric acid, Pt particles may become covered by phosphoric acid, thereby blocking their pores and obstructing gas transportation. This, in turn, may interfere with TPB formation and lower the performance of the fuel cell. As such, both the distribution of phosphoric acid and the formed structural and pore network must be uniform within the catalyst layer to ensure the high efficiency of the TPB [16].

Thus, to improve the distribution of phosphoric acid and the formation of the structural network in the catalyst layer, polymeric binders can be added. Such polymeric materials bind the catalyst particles and form a structural network in the catalyst layer, where the hydrophobicity of the polymeric binder allows control of the content and the distribution of phosphoric acid in the catalyst layer [[17], [18], [19]]. Among the various polymeric binders reported to date, polytetrafluoroethylene (PTFE) is widely used in HT-PEMFCs due to its high chemical and thermal stability, in addition to its hydrophobicity [20,21]. However, this hydrophobicity renders the dispersion of PTFE in solvents rather challenging, thereby resulting in agglomeration and subsequent separation from the solvent layer [22]. Therefore, if an electrode is fabricated using a catalyst slurry containing poorly dispersed PTFE, the PTFE distribution in the catalyst layer is uneven, and so the distribution of pores and phosphoric acid in the catalyst layer is also uneven (Fig. 1). This results in reduced TPB formation, thereby inhibiting the performance and reproducibility of the fuel cell. Sufficient dispersion of the PTFE binder in the HT-PEMFC electrode is therefore of particular importance. In this context, ultrasonic spray systems have been investigated using spray nozzles capable of sonication to effectively disperse the particles within the catalyst slurry [20,23,24].

Among the many electrode manufacturing methods reported to date, the bar coating method is more efficient in mass production and for the preparation of larger electrodes than the spraying method. However, to obtain the desired Pt loading in a single casting, a bar coating catalyst slurry is employed that contains a relatively low amount of solvent to ensure a high Pt concentration. As such, the viscosity of the slurry is significantly higher than that employed in the spraying method, and so the dispersion of PTFE is often poor.

Thus, we herein report our study into the manufacture of an electrode in the presence of a surfactant to improve the PTFE dispersion in the bar coating catalyst slurry. For this purpose, we employ 3M NOVEC FC-4430, a non-ionic surfactant with low reactivity and a fluoro-based moiety. Since the surfactant contains both hydrophilic and hydrophobic groups, it is expected that dispersion of the hydrophobic PTFE in the solvent can be enhanced. Furthermore, the characteristics of the bar-coated electrodes prepared both in the presence and absence of the surfactant are compared.

Section snippets

Electrode preparation

The catalyst slurry was prepared by mixing a carbon-supported Pt catalyst (TANAKA, Pt 46.2 wt%), a PTFE solution (Sigma Aldrich, 60 wt% dispersion in H2O) as the binder, and FC-4430 (3M) as the surfactant, in addition to isopropyl alcohol (IPA) and deionized water (DIW). The component weight ratio for the catalyst slurry is as follows Pt/C:PTFE:IPA:DIW:FC-4430 = 3:1:8.24:8.24:0.78. The amount of PTFE in the catalyst layer was 25 wt% [25]. The catalyst slurry was dispersed using an ultrasonic

Comparison of the degree of PTFE dispersion

Initially, the degree of PTFE dispersion in solution (IPA:DIW = 1:1) was examined over time both in the presence and absence of the surfactant. For this purpose, the PTFE solutions were left to stand following the dispersion process, and the degree of dispersion was observed over time (see Fig. 2). As shown, following dispersion, the PTFE solution containing the surfactant maintained a constant PTFE dispersion after 60 min, while in the absence of the surfactant, the PTFE was found to

Conclusions

We herein reported the successful preparation of bar-coated electrodes both in the presence and absence of a surfactant to improve the dispersion of the polytetrafluoroethylene (PTFE) binder, and to enhance reproducibility during electrode manufacture. It was found that in the presence of a surfactant during electrode manufacture, the dispersion of PTFE was maintained for a longer period of time, and this even dispersion led to the formation of a catalyst layer with a uniform structure and

Acknowledgment

This work was partially supported by the Korea Institute of Science and Technology (KIST) Institutional Program (2E29600) and by a National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science (Grant No. 2016M 1A2A2937136), Korea.

References (30)

Cited by (23)

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    Thus, in order to improve the catalyst layer durability in the presence of H3PO4, polymer binder is used to prevent H3PO4 flooding over the thin catalyst layers in the electrodes as demonstrated by Mazúr and co-workers [22]. For such purpose of protecting catalyst layer, various polymers such as polytetrafluoroethylene (PTFE) [23–30], polyvinylidene difluoride (PVDF) [23], PBI [23,31–34], Nafion [23], and PBI-PVDF blends [23,35,36] has been used as binder, and accordingly, the mechanical and gas diffusion properties of catalyst layer have been broadly ranged. In this context, choosing a proper type of polymer binder with an optimal amount in use should be another critical factor determining HT-PEMFC performance [28,29,37,38].

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