Fuel cells represent a promising technology for clean and efficient power generation, and their performance is critically dependent on the design of the membrane-electrode assembly (MEA). The MEA comprises crucial components, including an electrolyte membrane, catalyst layers (CLs), and gas diffusion layers. Among these components, the CLs, composed of carbon-supported Pt (Pt/C) and polymeric ionomers, play a pivotal role in charge and mass transport during oxidation and reduction reactions in fuel cells. The oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) is often slower than the hydrogen oxidation reaction in the anode catalyst layer, necessitating the optimization of the CCL structure to enhance both performance and cost-effectiveness of MEAs.
One key aspect of CCL structure optimization is the control of pore formation within the CLs, categorized as primary pores inside the agglomerates related to catalyst activity and secondary pores between agglomerates related to mass transfer. These pores significantly influence oxygen transport through the CCLs and subsequently impact fuel cell power performance. In a new study published in the Journal Power Sources and conducted by Soonyong So and Dr. Keun-Hwan Oh from the Korea Research Institute of Chemical Technology (KRICT), investigated the effects of dispersants on ionomer-Pt/C agglomerate size and MEA power performance. The new research contributes to a deeper understanding of the relationship between ink rheological properties, CL agglomerate size, and fuel cell performance.
The study begins by selecting a suitable dispersant based on its molecular weight and hydrophile-lipophile balance (HLB) value. The HLB value, calculated using the Davies method, determines the dispersant’s hydrophilic and lipophilic characteristics. In this case, the authors chose 1-propanol as the dispersant due to its favorable HLB value for forming an oil-in-water emulsion with the perfluorinated ionomer used in the study. The chosen dispersant, Zetasperse170, is a non-ionic compound with modified chemical structure.
In their experiments, the addition of dispersant to the catalyst ink resulted in the formation of well-dispersed agglomerates, as observed through laser scattering particle size analysis. The agglomerate size decreases with increasing dispersant content up to a point, beyond which the size distribution becomes wider. This behavior is attributed to the uneven self-formation of micelles due to excessive dispersant. The dispersant’s hydrophobic interaction between main chains and electrostatic interactions of side chains in a high-dielectric solvent like 1-propanol play a crucial role in reducing ionomer-catalyst agglomerate size.
The authors investigated the rheological properties of the catalyst ink, including steady-shear viscosity and yield stress, by varying dispersant content. All samples exhibit shear-thinning behavior, with the ink containing 1% dispersant showing the highest shear thinning index (n), indicating that its viscosity is less affected by shear rate. This behavior is attributed to the formation of more homogenous and smaller agglomerates due to the presence of 1% dispersant.
Soonyong So and Dr. Keun-Hwan Oh conducted a three-interval thixotropy test (3iTT) to simulate the ink-coating process. The ink with 1% dispersant exhibited structural regeneration during the test, indicating the formation of well-dispersed agglomerates. The ink’s moduli are also measured, revealing that the 1% dispersant-loaded ink is the most elastic, leading to improved ink stability during the coating process. The authors used transmission electron microscopy to analyze the internal structure of the catalyst inks with varying dispersant content. The images showed that the 1% dispersant-loaded ink exhibits homogeneous adsorption of ionomer with Pt/C and smaller agglomerate size. This suggests that the ink with 1% dispersant forms a more desirable structure for gas transport through the CLs, leading to improved fuel cell performance. Furthermore, pore size distribution and scanning electron microscopy images of the fabricated CLs further support the importance of dispersant optimization. The CL with 1% dispersant shows larger pore volume in the critical range of 50-100 nm, which positively affects catalytic activation and mass transfer overpotential. In contrast, the CL with 9% dispersant, although having smaller agglomerates, exhibits a dense structure and hinders proton transport due to disconnection of proton-conducting paths. They also conducted electrochemical analyses, including i–V polarization curves and Tafel plots, to evaluate the impact of dispersants on fuel cell performance. The MEA with 1% dispersant content shows the highest performance over the entire current density range, attributed to the thinner ionomer layer and smaller agglomerate size, which reduce oxygen transport resistance. The MEA with 1% dispersant exhibited lower proton transport resistance and charge transfer resistance, indicating enhanced catalytic activity and oxygen transport to the catalyst surface. This results in improved fuel cell performance.
To further investigate performance variations, the authors compared MEAs under different back pressure and flow-field channel conditions. Interestingly, the MEA with 1% dispersant shows the highest performance under specific back pressure conditions, demonstrating the efficacy of dispersant optimization. Additionally, the number of flow-field channels impacts performance, with a higher channel count enhancing water discharge and reactant transport.
In conclusion, the study by Soonyong So and Dr. Keun-Hwan Oh provided valuable insights into the optimization of catalyst ink for improved fuel cell performance. By carefully selecting and controlling dispersant content, the authors achieved well-dispersed agglomerates, enhanced ink rheological properties, and improved CL structure. These factors contribute to higher catalytic activity and mass transport, ultimately leading to superior fuel cell performance. This research highlights the importance of ink formulation and CL structure in optimizing membrane-electrode assemblies for efficient energy conversion.
Soonyong So, Keun-Hwan Oh, Effect of dispersant on catalyst ink properties and catalyst layer structure for high performance polymer electrolyte membrane fuel cells, Journal of Power Sources, Volume 561, 2023, 232664,