Researchers from Kanazawa University, the University of Tokyo, and HORIBA, Ltd. have shown that the initial state of platinum-on-carbon (Pt/C) catalyst particles before they are mixed with an ionomer strongly influences how a polymer electrolyte fuel cell (PEFC) catalyst ink subsequently develops.
Catalyst layers are among the most important components of PEFCs because the electrochemical reactions responsible for power generation occur there. They are commonly fabricated from catalyst inks containing Pt/C catalyst, ionomer, and solvents. In Pt/C catalysts, platinum nanoparticles are supported—that is, finely dispersed and immobilized—on the surface of carbon particles.
Previous studies of catalyst-ink preparation have mainly focused on factors such as solvent composition, ionomer content, dispersion method, and mixing time. However, much less attention has been paid to the state of the Pt/C catalyst before the ionomer is added.
The research team therefore introduced a controlled pre-mixing step in which Pt/C catalyst was mixed with deionized water for different periods before ionomer addition. This produced different initial Pt/C aggregate states. After adding the ionomer and ethanol, the researchers prepared catalyst inks using different main-mixing times and evaluated their particle-size distributions, rheological properties, elemental composition by SEM-EDX, and electrochemical surface area (ECSA).
Without pre-mixing, the Pt/C catalyst initially contained relatively large agglomerates. These structures were gradually broken down during subsequent main mixing, and the ECSA increased as mixing continued.
In contrast, short-time pre-mixing produced smaller and relatively uniform Pt/C aggregates before ionomer addition. This initial state facilitated subsequent dispersion, and the highest ECSA in the study, 64.42 m² gPt−1, was obtained after 1 hour of main mixing.
Long-time pre-mixing produced a broader particle-size distribution consistent with re-agglomeration. Even after prolonged main mixing, a fraction of the larger Pt/C structures remained resistant to further breakup. The combined particle-size, rheological, SEM-EDX, and electrochemical results indicate that these persistent structures limited the electrochemical accessibility of Pt surfaces and resulted in lower ECSA.
The findings demonstrate that catalyst-ink processing is determined not only by how the ink is mixed after all components are combined, but also by the initial state of the catalyst before ionomer addition. Controlling this initial Pt/C aggregate state therefore provides an additional process parameter for designing catalyst inks with reproducible microstructural and electrochemical properties.
Researcher Quote
"Catalyst-ink research has traditionally focused on how the ink is mixed after all of its components are combined," said Takuya Tsujiguchi, Professor at Kanazawa University. "Our results show that what happens before that stage also matters. By controlling the initial state of the catalyst, we can influence how the ink evolves during subsequent mixing. This gives us a new process variable for understanding and eventually optimizing catalyst-ink manufacturing."
Toward autonomous optimization of catalyst-ink processing
The findings will also contribute to the development of Mixing and Dispersion ROPES (Robotic Objective Process Exploration System), which is being jointly developed by Kanazawa University, the University of Tokyo, and HORIBA, Ltd. under a NEDO-funded project.
Mixing and Dispersion ROPES is designed to evaluate the dispersion and aggregation states of particles in fuel-cell catalyst inks using multiple analytical and measurement techniques and to use the resulting data to automatically and autonomously explore optimal mixing and dispersion conditions. Kanazawa University is responsible for elucidating the underlying mixing and dispersion mechanisms.
The present study provides mechanistic insight into one important process variable—the initial Pt/C aggregate state—and how it affects subsequent dispersion and electrochemical Pt accessibility. Integrating such mechanistic understanding with particle-size, rheological, and other measurement data is expected to help refine the process variables and evaluation criteria used in autonomous exploration. Ultimately, this approach aims to accelerate and improve the development of manufacturing processes for fuel-cell catalyst layers.
【Glossary】
Polymer electrolyte fuel cell (PEFC)
A fuel cell that generates electricity through electrochemical reactions between hydrogen and oxygen. PEFCs operate at relatively low temperatures and are being developed for applications including vehicles and stationary power systems.
Pt/C catalyst
A catalyst consisting of platinum nanoparticles supported on carbon. "Supported" means that the platinum nanoparticles are finely dispersed and immobilized on the surface of the carbon support.
Ionomer
An ion-conducting polymer used in the catalyst layer. In PEFC catalyst inks, the ionomer helps provide proton-conduction pathways around catalyst particles.
Aggregate and agglomerate
Clusters formed by multiple catalyst particles. In this study, "aggregate" refers to relatively small Pt/C particle clusters, whereas "agglomerate" refers to larger structures formed by the clustering of aggregates.
Rheological properties
Properties describing how a liquid or ink flows and deforms. In this study, viscosity and elastic behavior were measured to evaluate changes in the internal particle network and microstructure of the catalyst ink.
SEM-EDX analysis
A technique combining scanning electron microscopy (SEM), which observes microscopic surface structures, with energy-dispersive X-ray spectroscopy (EDX), which identifies and quantifies elements in the observed region. In this study, SEM-EDX was used to evaluate Pt and F compositions and to support interpretation of the catalyst and ionomer states.
Electrochemical surface area (ECSA)
A measure of the platinum surface area that is electrochemically accessible. A larger ECSA indicates that a greater amount of Pt surface is available for electrochemical reactions.
【Funding】
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO), project JPNP25002.
