Hybrid approach combining multiple characterization techniques and simulations for microstructural analysis of proton exchange membrane fuel cell electrodes
Introduction
Proton exchange membrane fuel cell (PEMFC) technology is a promising alternative to replace fossil fuel-dependent internal combustion engines (ICEs) for automotive propulsion power. In order to be competitive in the market, the cost and lifetime of PEMFC systems must be within close proximity of those of ICEs. The ultimate U.S. Department of Energy (DOE) target is to decrease the cost of an automotive PEMFC system from $55/kW to $30/kW, and to increase the durability from ∼3900 to >5000 h [1]. The breakdown of the overall cost for an automotive fuel cell stack shows that the Pt group metal (PGM) catalyst is the major cost-driver [1]. Besides developing PGM-free catalysts, the main strategy in decreasing the fuel cell cost is to lower the amount of PGM without sacrificing performance.
A typical PGM electrode consists of a carbon (C)-supported Pt catalyst, perfluorosulfonic acid ionomer binder, pore space, and under certain conditions, liquid water. The C, Pt, and ionomer binder form agglomerates as illustrated in Fig. 1. This agglomeration results in large pores (secondary pores) between the agglomerates and small pores (primary pores) inside them. The electrochemical reactions occur at the triple phase (C, Pt, ionomer/water) boundaries where electrons (e−), protons (H+), and reactants meet at the Pt catalyst surface. Within this complex geometrical and chemical environment, C support and ionomer binder create networks for conduction of e− and H+, respectively, and the pore space generates pathways for reactant transport and product removal, as illustrated in Fig. 1.
Literature presents many studies reporting either an optimum ionomer [2], [3], [4], [5], [6], [7] or Pt loading [4], [6] but determination of an optimum electrode composition is quite challenging. The electrode microstructure and its effects on fuel cell performance are not as trivial as the functions of constituents, because various components and transport processes influence each other. Therefore, a comprehensive insight into electrode microstructure and its interactions with the corresponding transport mechanisms is required to improve the performance of the electrodes under a variety of conditions and also to lower the amount of PGM catalyst without minimal loss in performance. This insight will also establish a foundation for determining the effects of the variables such as ionomer content, catalyst-ionomer ink solvent, and catalyst type (e.g., Pt alloy versus Pt) on the microstructure, transport properties, and resulting electrode performance.
There are several analysis techniques to investigate the PEMFC electrode microstructure. Mercury intrusion porosimetry (MIP) and Brunauer-Emmett-Teller (BET) gas adsorption porosimetry are the commonly used methods for evaluation of pore sizes and specific surface areas of porous materials such as PEMFC catalysts and electrodes. Gas adsorption porosimetry is preferred over MIP for measuring smaller pores because the high intrusion pressure of mercury may be destructive for the microstructure, and MIP is not successful at resolving pores smaller than 3 nm. Uchida et al. [2] were one of the first to report the bimodal pore size distributions corresponding to primary and secondary pores by using MIP. They also showed that the pore sizes are strongly dependent on the type of C support. In another study, Sobolyeva et al. [8] studied the microstructures of C supports by employing the nitrogen gas adsorption technique. It is important to note that both MIP and BET methods rely on the calculation of pore sizes based on surface tension, capillary forces, and pressure. Therefore, these approaches are limited to provide only bulk property data, whereas the anisotropic heterogeneous electrode microstructure requires more detailed quantification.
Besides the standard porosimetry methods, techniques that can capture the internal microstructure are critical in gaining insight into the three-dimensional distribution of the electrode components. The two-dimensional electron microscopy techniques, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), are popular tools for characterization of PEMFC electrodes [9]. SEM is commonly applied to resolve macro structures while TEM [10], [11], [12], [13], [14], with its high resolution (<1 nm), serves as a powerful tool to characterize size, shape, distribution and crystalline structure of C supported nano-scale catalyst particles in the electrode. Besides the catalyst and support characterization, using atomic force microscopy (AFM) and TEM, Xie et al. [15] showed that the ionomer content changes the sizes of C aggregates and the thickness of the ionomer film covering the C surface. In addition, recently advanced TEM techniques such as aberration-corrected scanning transmission electron microscopy (STEM) with energy-dispersive spectroscopy (EDS) [16], and electron tomography (ET) [17], coupled with cesium ion exchange, were able to provide information about the microstructure of the ionomer phase in the electrode.
The two-dimensional electron microscopy images are the representations of three-dimensional nanostructures. To be able to investigate the whole electrode with TEM, 100–150 nm thick slices need to be extracted from the electrode [5], [9]. However, slicing limits the spatial integrity between consecutive images and this method suffers from not being fully representative of the three-dimensional microstructure, i.e. the porosity is generally underestimated [18]. Electron tomography allows the observation of three-dimensional nanostructures. In this technique, two-dimensional images of the sample are taken at each angle step (i.e. between −70° and +70°, separated by one degree) as it rotates; these are then aligned and reconstructed into a three-dimensional image sequence by using various numerical algorithms. Lopez-Haro et al. [17] employed ET and investigated the ionomer coverage on the C support for different compositions. They determined that a 7 nm film of ionomer is formed on the carbon with the coverage of ionomer on the carbon increasing with increasing ionomer content. This technique is quite successful in providing detailed, high-resolution three-dimensional images of nanostructures of C, Pt, and ionomer by focusing on a few C particles. While ET provides important information regarding the ionomer-carbon interface, in the modeling of electrode transport properties it is important to determine the nano and microstructure over the larger length scales typical of PEMFC electrode thickness (10–15 μm). In addition, considering the heterogeneous morphology of the electrode, this type of ET results may not be representative of the whole electrode.
Focused ion beam-scanning electron microscopy (FIB-SEM) is a commonly-used technique for three-dimensional quantification of the electrode microstructure. In this method, FIB operates as a milling tool and extracts a series of sections from the sample, which are subsequently characterized by SEM. The captured images are reconstructed into a three-dimensional representation of the electrode. This technique has anisotropic resolution, so the high in-plane resolution of SEM is bounded by the sensitivity of the milling operation in the cutting direction. Thiele et al. [19], [20] used the FIB-SEM technique to image the microstructure of the PEMFC electrode with 30 nm increments of serial-sectioning and 5 nm resolution for SEM imaging. They described the whole reconstruction procedure in detail and reported pore size distributions for the electrode microstructure. Recently, they [21] addressed the lack of contrast between pores and catalyst layer material and tried to overcome this problem by filling the pores with ZnO. It is worth mentioning that the application of FIB-SEM technique to PEMFC electrodes is limited by the vacuum environment requirement and its destructive nature.
Nano-scale X-ray computed tomography (nano-CT) offers non-destructive quantification of three-dimensional microstructures. In this technique, an X-ray beam passes through the sample and the transmitted beam is recorded as the sample is rotated between 0 and 180°. The recorded projection images are reconstructed into the three-dimensional electrode microstructure. Employing nano-CT with 32.5 nm voxel size, Epting et al. [18] studied the morphology of PGM-based electrodes. They reported volumetric size distributions of secondary pores (larger than 32.5 nm) and solid phase (C, Pt, ionomer and primary pores corresponds to agglomerates) size distributions. Litster et al. [22] used the same technique to study the morphological and transport properties of the electrodes. They calculated effective reactant diffusivities based on Fick's law and numerical solution of the diffusion equation in the extracted pore volume from X-ray data. Application of this technique is limited by the resolution and the presence of low atomic number materials in the electrode microstructure. The electrode nano-structural features, such as 2–10 nm catalyst particles and primary pores, cannot be disseminated from the nano-CT images because of the low resolution, i.e., 20 nm. Furthermore, the similar atomic numbers of the constituent materials result in insufficient contrast in the intensity map and makes it quite challenging to distinguish C and ionomer. State-of-the-art nano-CT provides three-dimensional images of the secondary pore (pores larger than resolution) morphology, whereas the details of primary pores, C, Pt, and ionomer are mingled within the solid volume. Recently, Babu et al. [23] ion exchanged the protons in Nafion® with cesium (Cs+) and imaged the ionomer in thick PGM-free electrodes using absorption contrast nano-CT. Even though the state-of-the-art resolution of nano-CT is rather low, unlike FIB-SEM it doesn't require a vacuum environment. This allows nano-CT to work under various operating conditions i.e. it was applied to solid oxide fuel cell (SOFC) electrodes at operating temperatures of 700–850 °C with a special stage design [24].
Besides the aforementioned experimental methods, numerous numerical studies [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37] have been conducted to investigate effect of electrode microstructure on the performance and transport properties. Many of them [25], [26], [27], [28], [29], [30] assumed an ideal agglomerate microstructure and applied it to macroscale transport models with various techniques, while others [31], [32], [33], [34], [35], [36], [37] focused on pore scale phenomenon by numerically generating electrode microstructures. In these studies, the electrode microstructure was numerically regenerated with various techniques such as Gaussian random field method [31], [32], random sphere packing [33], [34], [35], and statistical generation of distinct phases within a regular grid [36], [37]. Using Gaussian random field method, secondary pore network of electrode layer was reconstructed with 100 nm voxels based on the two-point correlation function that they obtained from TEM images [31], [32]. They considered C, Pt and ionomer as a mixture while pores smaller than 100 nm are neglected. Other work [33], [34], [35] represented the electrode as randomly-packed C spheres surrounded by a constant thickness ideal ionomer film and neglected the existence of Pt particles. Kim and Pitsch [33] employed simulated-annealing technique and 60 nm spheres were randomly moved until the desired porosity was obtained. Similarly, Lange et al. [34], [35] randomly packed C spheres by using rules regarding placement of each new C particle and a specified overlapping tolerance to reconstruct the electrode structure. Zhang et al. [36] reconstructed the electrode from two phases; 50 nm voxels representing C supported Pt particles and ionomer-pore mixture. Siddique et al. [37] generated all the nanostructures in the electrode, including the Pt phase, based on growing agglomerates of C cells with the help of a quartet structure generation set (QSGS) algorithm [38].
To the best of our knowledge, neither nano-CT nor FIB-SEM was successful in separately resolving individual C, Pt particles, or ionomer within the overall three-dimensional electrode morphology. Individual components of the electrode microstructure can be generated numerically as in Siddique and Liu's study [37], however the random nature of this method lacks experimental support and validation. In this study, an innovative technique is introduced for microstructure characterization. This technique combines the pore morphology from nano-CT data with USAXS, TEM and porosimetry data by employing a numerical algorithm to reconstruct detailed electrode microstructure (including C, Pt, and ionomer phases). Unlike aforementioned numerical models, experimentally-measured size distributions of the carbon primary particles and the catalyst nanoparticles are taken into account and these particles are placed in a morphology obtained directly from the nano-CT. This hybrid experimental-numerical method offers to approximate the primary pores, ionomer microstructure, C and Pt particles that are all not available from the nano-CT data alone. In this context, nano-CT is used to extract the three-dimensional morphology of the cathode. A series of TEM images were taken to analyze the size distributions of C and Pt particles. In order to compare global applicability of the TEM analysis, ultra-small angle and small-angle synchrotron X-ray scattering (USAXS and SAXS) were used to determine the C and Pt particle size distributions, respectively. The porosimetry data was used to approximately determine the amount of primary pores not captured in the nano-CT analysis. Combining all the experimental data, individual C, Pt and ionomer phases were discretely regenerated with the numerical algorithm introduced in this study. The resulting structure is validated by comparing the pore size distributions against MIP and BET techniques. Subsequently, to demonstrate the capabilities of the model in determining charge and reactant transport properties, simulations were conducted for various sub-volumes with different porosities.
Section snippets
Hybrid approach methodology
The hybrid approach combines the strengths of the aforementioned microstructure characterization techniques within one geometric representation and resolves the electrode nanostructures in various length scales. The general approach can be described as the following: three-dimensional secondary pore morphology and the solid phase (unresolved mixture of Pt, C, ionomer and primary pores) geometry are extracted by use of nano-CT. Then, based on a numerical algorithm, which uses TEM, X-ray
Transport simulations
Multiple transport processes occur within the electrode microstructure during the PEMFC operation. It is crucial to understand the interactions between the microstructure and the transport processes. In this section, transport-related characteristic properties are investigated by performing simulations within the pore scale nano-CT geometry and the corresponding hybrid microstructure.
The porosity in the nano-CT data changes depending on the location and the size of the region of interest. The
Summary and conclusions
Incorporating different experimental techniques, a hybrid approach is presented to reconstruct the detailed PEMFC electrode microstructure. Nano-CT was performed to extract the morphology of the secondary pore space. TEM images, X-ray scattering, and porosimetry techniques were used to obtain statistical information about the nano scale structures within the electrode. Information from these different sources were combined with an algorithm proposed in this study, to regenerate the electrode
Acknowledgements
This work is part of a collaborative project with Johnson Matthey Fuel Cells, United Technologies Research Center, the University of Texas-Austin, and Indiana University-Purdue University of Indianapolis. The authors wish to thank the U.S. Department of Energy's Fuel Cell Technologies Office (Nancy Garland, program manager) for support. This research used resources of the Advanced Photon Source (APS), a U.S. Department of Energy Office of Science User Facility operated for the DOE Office of
References (41)
- et al.
J. Power Sources
(1999) - et al.
Electrochim. Acta
(2001) - et al.
J. Power Sources
(2003) - et al.
J. Power Sources
(2004) - et al.
Electrochim. Acta
(2010) - et al.
J. Power Sources
(2011) - et al.
J. Power Sources
(2015) - et al.
Electrochim. Acta
(2005) - et al.
Electrochim. Acta
(2007) - et al.
Int. J. Hydrogen Energy
(2012)
J. Power Sources
J. Power Sources
Electrochim. Acta
J. Power Sources
Electrochim. Acta
Electrochim. Acta
Fuel Cell Technical Team Roadmap
J. Electrochem. Soc.
J. Appl. Electrochem.
ACS Appl. Mater. Interfaces
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