Before mixing starts, initial catalyst structure governs fuel-cell ink dispersion

Researchers from Kanazawa University, the University of Tokyo and HORIBA, Ltd. have revealed that the initial state of platinum-on-carbon (Pt/C) catalyst particles before mixing with an ionomer significantly affects the dispersion of a polymer electrolyte fuel cell (PEFC) catalyst ink. Catalyst layers, containing catalyst inks with Pt/C, ionomer and solvents, are crucial components of PEFCs, where electrochemical reactions occur.

Previous research primarily focused on factors such as solvent composition, ionomer content, dispersion methods and mixing time. However, the initial state of the Pt/C catalyst before ionomer addition received less attention. To investigate this, researchers introduced a controlled pre-mixing step, where Pt/C catalyst was mixed with deionized water for varying periods before adding the ionomer.

Different initial Pt/C aggregate states were produced based on the pre-mixing duration. After adding the ionomer and ethanol, the researchers evaluated the particle-size distributions, rheological properties, elemental composition, and electrochemical surface area (ECSA) of the catalyst inks prepared with different main-mixing times.

Without pre-mixing, the Pt/C catalyst initially contained larger agglomerates, which gradually broke down during subsequent main mixing, increasing the ECSA. Short-time pre-mixing, however, resulted in smaller and more uniform Pt/C aggregates before ionomer addition, leading to the highest ECSA (64.42 m² gPt−1) after 1 hour of main mixing.

In contrast, long-time pre-mixing produced a broader particle-size distribution due to re-agglomeration, with some larger structures remaining resistant to further breakup. These persistent structures limited the electrochemical accessibility of Pt surfaces, resulting in lower ECSA. The findings demonstrate that controlling the initial Pt/C aggregate state provides an additional process parameter for designing catalyst inks with reproducible microstructural and electrochemical properties.