A new technique for exclusive visualization of platinum particles in catalysts used in fuel cells（Press Release）
- Release Date
- 02 Feb, 2024
- BL40B2 (SAXS BM)
2 February 2024
Japan Synchrotron Radiation Research Institute (JASRI)
Albert Mufundirwa and other members of researchers at the Japan Synchrotron Radiation Research Center, in collaboration with Masazumi Arao and other members of researchers of FC-Cubic, have developed a method to visualize only platinum particles in carbon-supported platinum catalysts used in polymer electrolyte fuel cells. This achievement was accomplished by utilizing a novel contrast-variation [1] small angle X-ray scattering[2] method to render carbon invisible to X-rays, thus enabling accurate estimation of the platinum particle size. While the contrast-variation approach is traditionally employed in small angle neutron scattering [3] and less frequently in small angle X-ray scattering, particularly in protein solution scattering experiments, this marks the first demonstration of its practical application in the field of materials science, specifically for catalyst nanoparticles. The research findings are expected to significantly contribute to the advancement of high-performance catalysts for fuel cells, crucial for the realization of a carbon-neutral society. This development took place at the BL40B2 beamline of the large-scale synchrotron radiation facility SPring-8 [4] . The results of this research were published in Scientific Reports on 27 January 2024. 【Publication information】 |
Research background
A fuel cell is a device that converts the chemical energy of hydrogen and oxygen into electricity via an electrochemical reaction [5] , with only water and heat as byproducts, ensuring no generation of carbon dioxide. Catalysts are employed to facilitate this electrochemical process, with the state-of-the-art catalyst being platinum nanoparticles deposited on the surface of high-surface-area carbon. The performance of the catalyst is directly linked to the size of the Pt nanoparticles, with an optimized nanoscale range being desirable, typically around a diameter of 3 nanometers (1 nanometer is one billionth of a meter). Therefore, it is important measure the particle size of the platinum particles.
The size of platinum particles can be determined by transmission electron microscopy. However, using small-angle X-ray scattering method allows for the simultaneous measurement of up to 100 million platinum particles, resulting in statistically highly accurate results. Challenges arise when platinum particles are deposited onto a carbon support whose microstructure closely resembles that of the platinum particles. A variety of carbon materials are utilized as support for platinum, with recent emphasis placed on highly porous carbon. This form of carbon, known as mesoporous carbon, is valued for its porous structure, which enhances reactant flow and provides additional accommodation for platinum particles, thereby contributing to higher performance. However, since these pores are similar in size to the platinum particles, errors arise in size estimation. Moreover, with the new trend in sustainable use of critical raw materials [6] , reducing the amount of platinum used in these catalysts is becoming fashionable. Consequently, the X-ray scattering signal from platinum decreases, leading to even larger errors.
To the best of our knowledge, the only method for measuring the particle size of platinum particles, while excluding the influence of other components, has been anomalous small angle X-ray scattering [7] . This method requires very precise measurements, which are not easy to achieve. Therefore, we thought there was a need to develop a simpler method to accurately measure only the particle size of platinum.
Research results
The research team decided to apply the concept of contrast-variation, commonly used in neutron scattering experiments, to X-rays. This method involves adding and adjusting the density of the surrounding solvent (electron density in the case of X-rays) in a two-component system to match the density of one component, rendering that component invisible to X-rays and only the other component observable (Figure 1). This method is applicable because of the significant difference in electron density between carbon and platinum. In other words, by adding a solvent with the same density as carbon to a carbon-supported platinum catalyst system, the carbon becomes invisible to X-rays, allowing for accurate determination of the size of the platinum particles.
Figure 1: Principle of contrast variation. (A) Sample in air. The large gray circle represents a carbon particle, and the small black circles represent platinum particles. (B) When immersed in a solvent of low density. (C) When immersed in a solvent whose density matches carbon. The carbon particle becomes invisible.
The first step is finding a suitable solvent. For biomolecules, such as protein aqueous solutions, since the density of protein and water does not differ much, simply dissolving glycerin or sucrose in water can render the protein invisible. The story is different when using carbon, whose density is considerably higher, making neither glycerin nor sucrose suitable for use. Therefore, we utilized a highly dense solvent, denser than carbon, called tetrabromoethane. We adjusted the solvent density by mixing tetrabromoethane with a lower density solvent, dimethyl sulfoxide, in various ratios. Tetrabromoethane has an unusually high density and is a liquid at room temperature, making it suitable for separating mineral ores (as they sink) from their supporting rock (as it floats) via preferential flotation mechanism.
Figure 2 represents the intensity of X-ray scattering from a solid carbon particle called Vulcan, plotted against the concentration of tetrabromoethane solvent. If the solvent density perfectly matched that of carbon, the X-ray scattering intensity should be zero. However, due to the heterogeneous density of carbon sample, the scattering intensity did not completely reach zero. Nevertheless, at concentrations of 50-60% tetrabromoethane, the scattering intensity from carbon became a few percent of that in air, which practically removes scattering from carbon.
Figure 2: Dependence of the intensity of X-ray scattering from Vulcan on the concentration of tetrabromoethane (TBE) in the solvent. The intensity is relative to that in air. Blue represents the measured value and red represents the theoretical values assuming carbon density is uniform. Gray represents the Gaussian distribution of carbon density obtained from literature.
Using this solvent, we recorded scattering of a model mesoporous carbon supported low-platinum catalyst and determined solely the size of the platinum particles. This model catalyst comprised a mixture of a mesoporous carbon called CNovel and a small amount of platinum particles. In air, determining the platinum particle size was not feasible due to additional scattering from the small pores in the carbon. However, in 50% tetrabromoethane, accurate determination was possible (Figure 3).
Figure 3: X-ray scattering of a sample consisting of mesoporous carbon mixed with a small amount of platinum particles. The left figure shows measurements in air, while right figure displays measurement in 50% tetrabromoethane. The shoulder-like feature indicated by the red arrow indicates the presence of a spherical structure. Analysis of this region yields the particle size distribution shown in the insert. The average particle size in air (3.46 nanometers) represents a combination of mesopores in carbon and platinum particles. However, in 50% tetrabromoethane the contribution of mesopores is removed, allowing for accurate determination of the platinum particle size (2.6 nanometers).
Future developments
This contrast variation method has the advantage of its simplicity. Anomalous small angle X-ray scattering [7] requires the use of very precise X-ray wavelengths, limiting measurements to synchrotron radiation facilities. However, the contrast variation method does not have stringent requirements on wavelength, enabling measurements even with laboratory X-ray generators. This makes the method accessible to more catalyst developers, including those without easy access to synchrotron radiation facilities, thereby increasing the number of participants interested in contributing to the development of high-performance catalysts for fuel cells.
Terminology
1) Contrast-variation is a technique used in various scientific fields, particularly in materials science, biology, and chemistry. It involves systematically altering the contrast between different components or features within a sample to enhance the visibility of specific structures under investigation by small angle neutron scattering or small angle X-ray scattering.
2) Small angle X-ray scattering (SAXS) is a technique used to study the structure of materials at the nanometer scale. It involves irradiating X-rays onto a sample and analyzing the scattered X-rays at low angles (typically between 0.1 and 10 degrees) relative to the incident beam. It is suitable for analyzing relatively large objects compared to the wavelength of the X-rays (here, for example, platinum particles with a diameter of 3 nanometers relative to an X-ray wavelength of 0.1 nanometers).
3) Small angle neutron scattering (SANS) is a method used to investigate the structure of a sample by irradiating it with neutrons and recording the neutrons scattered by the sample. Neutrons scatter well even from light elements that do not scatter X-rays as effectively, making SANS suitable for examining samples containing light elements.
4) SPring-8 is a large-scale synchrotron radiation facility located in Harima Science Park City in Hyogo Prefecture, owned by RIKEN, and user support is provided by JASRI. The name SPring-8 is derived from Super Photon ring-8 GeV. At SPring-8, a wide range of research is conducted using synchrotron radiation, including nanotechnology, biotechnology, and industrial applications.
5) An electrochemical reaction is a chemical reaction that involves electron transfer between reactants, usually facilitated by an electric current through an electrolyte. It can convert chemical energy directly into electrical energy (e.g., in batteries and fuel cells) or drive non-spontaneous chemical reactions using electrical energy (e.g., in electrolysis).
6) Critical raw materials refer to natural resources or substances that are essential for industrial processes, technological applications, and economic activities, but are subject to high supply risk due to their scarcity, geopolitical issues, or environmental concerns.
7) Anomalous small angle X-ray scattering extracts element-specific structural data from multi-element materials by leveraging anomalous dispersion, where X-ray scattering intensity changes with energy near the absorption edge. This method demands measurements at three or more wavelengths and is exclusive to synchrotron radiation facilities.
Funding
This research was supported by the New Energy and Industrial Technology Development Organization (NEDO) (JPNP20003).
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