Characterization of Solvent-Dependent Ink Structure and Catalyst Layer Morphology Based on Ink Sedimentation Dynamics and Catalyst-Ionomer Cast Films

Abstract

Ink structure is of great research interest because it influences the catalyst layer microstructure and ultimately determines fuel cell performance. To improve understanding of the ink structure, particle size distribution and ink sedimentation dynamics were characterized in this study by performing acoustic spectroscopy (AS) and analytical centrifugation (AC) analyses, respectively, using actual concentrations of Pt/C inks (10.4 wt% of solid content) without dilution. For the catalyst inks prepared with 1-propanol-water bi-solvents of different compositions, the particle size distributions of the inks were similar when a small amount of ionomer (I/C 0.2) was included in the ink. However, the ink sedimentation dynamics showed significantly different results depending on the solvent composition. The different ink dynamics can be explained in terms of the ink structure, and these solventdependent ink structures were further studied through the fabrication of catalyst-ionomer cast films. The catalyst agglomeration size and the catalyst-ionomer interfacial structure were effectively observed using scanning electron microscopy (SEM) images obtained from the cast films. The present study reveals the effects of the solvent properties on the structures of ink and catalyst agglomerates. It also demonstrates that the AS and AC techniques can be useful for ink studies.

Investigating the effect of solvent composition on ink structure and crack formation in polymer electrolyte membrane fuel cell catalyst layers

“…During the ink preparation process, the components of the ink interact with each other to form a structure. The interaction between the catalyst and solvent determines the size of the catalyst aggregation, the interaction between the ionomer and solvent determines the size and shape of the ionomer, and the interaction between the catalyst and ionomer affects the adsorption form of the ionomer on the catalyst surface, thus altering the ink structure [13,21]. Ultimately, the final structure of the ink is determined as a result of interactions that minimize the free energy of the ink system.…”

“…In fact, the role of polymers as stabilizers in colloid systems has been reported for a long time [27], and research is ongoing to elucidate the interactions between particles, polymers, and solvents in colloid systems [28]. In ink systems for fuel cells, the degree of aggregation of ionomers and adsorption of catalysts vary depending on the solvent [21], but their influence on ink stability appears to be relatively minor. In ink systems including ionomers, there are limitations in examining differences in ink structure depending on the solvent only through ink sedimentation analysis, and therefore, the rheological characteristics of the ink were examined.…”

Solvent Effects on the Catalyst Ink and Layer Microstructure for Anion Exchange Membrane Fuel Cells

“…35,36 For the successfully commercialized perfluoro sulfonic acid (PFSA) ionomers used in PEMFCs, 37 the dielectric constant (ε) and solubility parameter (δ) of the organic dispersion solvent were identified as the key parameters affecting the conformation and surface charge of the PFSAtype Nafion ionomer in the solution. 33,34,38,39 A previous study suggested that the pK a of the solvent can affect the porous structure of membranes cast from phosphonic acid ionomer dispersion through the acid−base interaction between ionomer and solvent. 40 By tuning the solvent, the interactions between the ionomer and catalyst can be optimized, leading to the formation of a three-dimensional (3D) network in ink and, ultimately, a well-connected carbon-ionomer network in the CLs.…”

“…The microstructure of CL is affected by the composition of the catalyst ink and the preparation techniques (such as spray-coating, transfer printing, or screen printing) for both AEMFCs and proton exchange membrane fuel cells (PEMFCs). − When a specific preparation technique is used, the microstructure of the CL is controlled by the interactions between the ionomer and catalyst in ink during the dispersing and drying processes under the strong influence of solvent environment. − In addition, the surface functional groups affect the surface charge of carbon support in dispersion solvent, which in turn significantly affects the dispersibility of ink. , For the successfully commercialized perfluoro sulfonic acid (PFSA) ionomers used in PEMFCs, the dielectric constant (ε) and solubility parameter (δ) of the organic dispersion solvent were identified as the key parameters affecting the conformation and surface charge of the PFSA-type Nafion ionomer in the solution. ,,, A previous study suggested that the p K a of the solvent can affect the porous structure of membranes cast from phosphonic acid ionomer dispersion through the acid–base interaction between ionomer and solvent . By tuning the solvent, the interactions between the ionomer and catalyst can be optimized, leading to the formation of a three-dimensional (3D) network in ink and, ultimately, a well-connected carbon-ionomer network in the CLs. ,,,− The well-connected network structure helps maintain the continuity of the Nafion ionomer and the high porosity of the CL, resulting in high mass-transport rate and proton conductivity. ,, …”