Substrate-dependent proton transport and nanostructural orientation of perfluorosulfonic acid polymer thin films on Pt and carbon substrate
Introduction
Development of energy generation, conversion, and storage technologies has been desired because the global demand for sustainable energy is rapidly increasing [[1], [2], [3], [4]]. Among these new energy sources, polymer electrolyte fuel cells (PEFCs) are known for their high current density and use in portable energy solutions [[5], [6], [7], [8]]. In typical polymer electrolyte fuel cells, the electrochemical reactions occur on the carbon-supported platinum (Pt/C) covered by a solid-state proton conducting electrolyte. Perfluorinated sulfonic acid ionomer (Nafion) is one of the most widely used proton-conducting materials owing to its high proton conductivity and thermal and chemical stability [[9], [10], [11]]. On the Pt/C catalyst, Nafion forms a semi-continuous film with nanoscale thickness, through which the transport of protons, water, and oxygen takes place [[12], [13], [14], [15], [16]]. Controlling the structure of the Nafion thin film on the Pt/C catalyst is important as the mass transport properties depend on it.
Nafion is composed of a hydrophobic polytetrafluoroethylene backbone (-CF2-) with perfluorinated ether side-chains terminated by hydrophilic sulfonic acid groups (-SO3H). The sulfonic acid groups exhibit high proton conductivity in a hydrophilic ionic domain [14,[17], [18], [19]]. Free-standing Nafion membranes, which are few hundred micrometers thick, have been widely investigated in the past decades [[20], [21], [22]]. Several studies have used cluster-network or the parallel water channel model to describe the nanostructure in the Nafion membranes. These membranes exhibit proton conductivity ranging from ~10−1 to ~102 mS/cm under 10–100% relative humidity at 25 °C [14,23].
However, Nafion thin films on the Pt/C catalyst have a structure different from that of the free-standing Nafion membrane because of a strong confinement interaction between the thin film and the catalyst [[24], [25], [26], [27], [28]]. The proton conductivity of the Nafion thin films is one to two orders of magnitude lower than that of a free-standing Nafion membrane and shows strong thickness-dependence under a certain relative humidity [13,18]. The confinement effect is known to influence the phase behavior of uncharged block-copolymer systems [[29], [30], [31], [32], [33]]. In case of the thin films, as the thickness approach to the characteristic domain size of the block copolymers, the ionomer/substrate interactions can cause anisotropy in the orientation of the domains, resulting in that the morphology of the Nafion thin film differs from that of the free-standing Nafion membrane [30,34,35].
Grazing-incidence small-angle X-ray scattering (GISAXS) / grazing-incidence wide-angle X-ray scattering (GIWAXS) is a powerful tool to analyze the morphology of Nafion thin films [14,[36], [37], [38]]. GISAXS profiles of Nafion thin films, with thickness of 32–270 nm, cast on Pt, Au or carbon substrate, showed a scattering peak that was attributed to the hydrophilic domains [29]. The Nafion thin film on metallic substrate showed the sharpest scattering peak. In contrast, thin films on carbon substrate exhibited more isotropic behavior than the films on Au and Pt substrates. In the Si supported Nafion thin films with thickness of 25–100 nm was explored which showed that the thinner films show the lower degree of phase separation [30]. These results suggest that film thickness and substrate-ionomer interactions control the reorganization and alignment of the phase-separated nanostructure, thereby altering the water-transport and swelling properties [14,29,30,[39], [40], [41]].
Although many studies have focused on understanding the morphology or proton conductivity of the Nafion thin films, the quantitative relationship between morphology and proton conductivity of the Nafion thin films on Pt and/or carbon substrates has not been clarified. Especially, the proton conductivity of carbon supported Nafion thin films is still not explored. Our group previously reported the relationship between proton conductivity and morphology of Nafion thin films on Pt substrate with various thicknesses after annealing treatment via electrochemical impedance spectroscopy (EIS) and GISAXS/GIWAXS method [38]. In the process, we have developed the measurement method of proton conductivity of Nafion thin films on various substrate using self-designed interdigitated array electrodes.
The present study systematically analyzes the relationship between proton transport properties and morphology relationship of Pt and amorphous carbon supported cast thin films with thicknesses of 50–200 nm. Furthermore, the difference between Pt and carbon supported Nafion thin films in confinement effect / anisotropic behavior was discussed. The film thickness of Nafion thin films used in this study are thicker than the actual film thickness on membrane electrolyte assembly (MEA) with thickness of 2–5 nm [11,19], and the crystallinity of the carbon substrate is also different from the actual carbon support, such as Vulcan [2,3,33]. However, we believe that this paper will provide useful knowledge as a model of clarifying the actual proton transport in MEA.
Section snippets
Sample preparations
5 wt% Nafion solution (Aldrich, EW = 1100) was diluted to desired concentration 0.1–1.6 wt% with 99.5% 1,1,1,3,3,3-hexafluoro-2-propanol (Wako Pure Chemical Industries, Ltd.).
The detailed information of pre-treatment of self-designed interdigitated array electrodes (Fig. S1) were described in our previous work [38]. Interdigitated array electrodes over carbon substrates were fabricated using the same method. The carbon substrate of was characterized by Raman microscope shown in Fig. S2. The
Results and discussion
Fig. 1 shows the proton conductivity of Nafion thin films cast on Pt and carbon substrates at 25 °C and various RH values. The collected Nyquist plot are shown in Fig. S5 in the supporting information. Both Pt- and carbon-supported Nafion thin films exhibit thickness dependence. Compared to the carbon-supported films, the Pt-supported films showed higher conductivity values at the same RH. In the case of 10-nm-thick films, proton conductivity of the Pt-supported specimen was more than an order
Conclusion
We focused on a series of Nafion films with thickness ranging from 50 to 200 nm to reveal differences in confinement effect and impacts from type of substrate. This was done in terms of morphology and proton transport property via GISAXS/GIWAXS and the electrochemical impedance method. Within this range of film thickness, substrate/ionomer interaction are expected to have a significant impact on the structure of the ionomer. Thin films on Pt exhibit completely different anisotropic behavior
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.
Acknowledgement
This research is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). This study was partially supported by the Synchrotron radiation experiments performed at BL40B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2016B1013, 2017A1020, 2017B1044, 2018A1020, 2018B1034, 2019A1025, 2019B1023, 2019B1024).
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2022, Journal of Power SourcesCitation Excerpt :By comparing the integrated intensity along meridional and equatorial directions, it is intuitive to learn the anisotropy of Nafion thin film. Another instrument applied as a supplementary test method for GISAXS is GIWAXS [82], which investigates the semi-crystalline (–CF2–) backbone polymerization in the hydrophobic domains of the Nafion films, as shown in Fig. 6(c). The scattering rings similar to those in GISAXS indicate the crystalline component, which induces the physical crosslink of the –CF2- backbone and resists proton transportation.