Toyota Central Research Laboratory: Solvent composition seriously affects the aggregate structure in catalyst slurry

The solvent composition of the catalyst slurry significantly affects the pore structure of the catalyst layer and its scale production efficiency. The pore structure of the catalyst layer is affected by many factors, such as material properties and process parameters. The ionomer adsorption ratio is the main factor that dominates the aggregate structure in the slurry. This article shares Toyota Central Research Laboratory's research on the influence of solvent composition on the rheological properties, ionomer adsorption rate and structural characteristics of aggregates in catalyst slurry.

01

Technical background

The catalyst layer of automotive fuel cells consists of carbon-supported catalyst particles and ionomers that transfer protons. The energy conversion efficiency of the fuel cell is deeply affected by the porous structure of the catalyst layer. In the porous electrode, electrons are conducted in the Pt/C catalyst, protons are conducted in the ionomer, and oxygen molecules diffuse and penetrate in the pores and ionomers. The three substances generate water through ORR reaction on the surface of the Pt catalyst. In order to maximize the energy conversion efficiency of the fuel cell, it is necessary to regulate the position and structure of the Pt/C particles and ionomers to optimize the three-phase interface.

In large-scale production, due to the high production efficiency, the catalyst layer is usually coated by slit coating process. The slit coating method is a high-precision coating method. The coating slurry is pressed from the storage device to the nozzle through the supply pipeline, and the slurry is sprayed from the nozzle to transfer to the coated substrate. In the slit coating method, the catalyst slurry composed of Pt/C particles, ionomer and water-alcohol solvent is pressed from the storage device to the nozzle through the supply pipeline, and the slurry is sprayed from the nozzle to transfer to the coated substrate. After drying the catalyst slurry, the porous catalyst layer is transferred to the proton exchange membrane by hot pressing (such as the transfer method for the cathode catalyst layer of Toyota's second-generation Mirai fuel cell). The structure of the catalyst layer prepared by the above process is affected by many factors, including material properties, such as the type and dispersion state of carbon carrier, platinum, and ionomer; process parameters in the catalyst slurry preparation process, such as solvent composition, I/C ratio, temperature and dispersion method. Among them, the solvent composition significantly affects the performance of the catalyst layer.

Existing studies have revealed the existence of rigid aggregates in the catalyst layer, with a size range of 100-300 nm, mainly composed of Pt/C catalyst particles of 20-40 nm in size. Depending on the content and composition of the ionomer, these aggregates further agglomerate to form aggregates of 1-10 μm in size. In order to better understand the effect of solvent composition on performance, it is necessary to clarify how the solvent composition affects the structure of Pt/C particle aggregates (aggregates form the main framework of the catalyst layer) in the catalyst slurry. This article introduces the study of the effect of solvent composition on the structural characteristics of aggregates in the catalyst slurry conducted by Toyota Central Research Laboratory.

02

Research preparation

The solvent composition used in the study is ethanol, 1-propanol, and diacetone alcohol. The solvent polarity can be controlled over a large range through the three solvent compositions, and the solvent polarity is characterized by Hansen solubility. As polarity increases, the polar solvent repels the main chain of water transport in the ionomer, resulting in the adsorption of the ionomer on the carbon surface, and the ionomer adsorption ratio Γ (the ratio of ionomer adsorbed on the Pt/C catalyst to the total ionomer) increases.

03

Result analysis

The following Figure 1 shows the curves of the steady-state flow viscosity η of the catalyst slurry with shear rate, the storage modulus and the loss modulus with strain, and all data points are color-coded based on the adsorption ratio Γ of the ionomer in the catalyst slurry. Studies have shown that shear thinning is observed in almost all catalyst slurries, indicating that the aggregates formed in the catalyst slurry are shear-destroyed. As shown in Figure 3 below, as the ionomer adsorption ratio Γ increases from 0 to 20%, all characteristic values decrease, indicating that when the ionomer adsorption ratio Γ increases to 20%, the Pt/C aggregates are gradually broken.

Figure 1 (a) Viscosity vs. shear rate, (b) Storage modulus vs. strain, (c) Loss modulus vs. strain. The color of the data points indicates the ionomer adsorption ratio Γ (see the color bar at the bottom of the figure)

The fractal dimension is a measure of the irregularity of complex shapes, generally ranging from 0 to 3, with 0 representing dispersed particles, 1 representing rod-like aggregates, 2 representing flat or branched networks, and 3 representing dense aggregates. The results show that as the ionomer adsorption ratio Γ increases, the agglomerates separate into smaller aggregates, and the indecomposable aggregates maintain their structure. The diameter of the aggregates is about 200 nm. At the first viscoelastic transition point of the ionomer adsorption ratio Γ~0%, the fractal dimension D2 drops sharply from 2 to 1. At the second transition point Γ~15%, D2 gradually changes from 1 to 0.5. The consistency of the turning point of the fractal dimension and the rheological properties indicates that the change in rheological properties is attributed to the change in the aggregate structure.

Based on the rheological properties and structural characteristics observed above, Toyota Central Research Institute proposed the decomposition mechanism of aggregates in the catalyst slurry. For convenience, the two structural transitions at Γ~0% and ~15wt% are called T1 and T2, respectively. When the ionomer adsorption ratio Γ is lower than the first transition point Γ~0%, the fractal dimension D2 is close to 2, indicating the formation of a colloidal gel network structure. In this state, due to the adsorption of a small amount of ionomer on the Pt/C aggregates, the electrostatic repulsion between particles is small, so an aggregate network structure is formed. Due to the existence of the colloidal gel network structure, the viscosity and equilibrium storage modulus are both high.

At the structural transition point T1, the fractal dimension D2 drops sharply from 2 to 1, a decrease of one order of magnitude. The sharp change in the D2 value indicates that the network structure is decomposed into smaller rod-like fragments. This state is represented here as state II. After the sharp transition point T1, the D2 value gradually decreases, indicating that the length of the rod gradually shortens with the increase of ionomer Γ. Toyota Central Research Laboratory speculates that this length is determined by the balance between the electrostatic repulsion of the adsorbed ionomer and the hydrophobic (or dissipative attraction) force.

With the further increase of the ionomer adsorption ratio Γ, the D2 value gradually decreases from 1 to 0.5 or less. This means that the fragments collapse to form isolated aggregates through the enhanced electrostatic repulsive interaction caused by further ionomer adsorption. This highly dispersed state is defined as state III. At this stage, there is no network structure. Therefore, the catalyst slurry behaves as a Newtonian liquid.

To determine which specific solvent properties cause the changes, Toyota Central Research Laboratory studied the correlation between slurry characteristics and solvent characteristics. It can be seen that the ionomer adsorption ratio Γ increases with the increase in the water weight fraction. It is speculated that this is because the hydrophilic solvent repels the hydrophobic carbon fluorine backbone in the ionomer and adsorbs to the hydrophobic carbon surface. This also reasonably explains the small effect of platinum loading on the ionomer adsorption. The effect of the solvent on the catalyst slurry structure can be effectively characterized by the Hansen solubility parameter HSP-δP.

Due to the above mechanism, the increase in HSP-δP leads to an increase in the ionomer adsorption ratio Γ. As a result, the aggregates collapse by repulsive interactions, resulting in a decrease in the fractal dimension D2 of the aggregates. Ultimately, the viscosity decreases with increasing HSP-δP. It is noteworthy that the observed correlation with HSP-δP can be approximately represented by a single line regardless of the type of alcohol present in the solvent, indicating that HSP-δP is a solvent characteristic parameter that effectively controls the aggregate structure and viscoelasticity of the catalyst slurry.

04

Summary

In this study, Toyota investigated the effects of solvent on the viscoelasticity, ionomer adsorption rate and structural characteristics of aggregates in catalyst slurries by changing the solvent composition, and proposed the following formation mechanism of aggregates in catalyst slurries.

In polar solvents such as water, the solvent repels the hydrophobic carbon-fluorine backbone in the ionomer, resulting in the adsorption of many ionomers onto the catalyst particles on the hydrophobic carbon surface. In this case, the sulfonic acid groups in the adsorbed ionomers produce electrostatic repulsive interactions, resulting in the formation of well-dispersed, rigid, and separated aggregates of Pt/C catalysts with a size of approximately 200 nm. Even if uniformly dispersed, these aggregates cannot be further mechanically subdivided into smaller particles. As the polarity decreases with increasing alcohol content, ionomers desorb from the surface of the aggregates, resulting in the formation of relatively short rod-like aggregates with a mass fractal dimension approaching 1. As the polarity decreases further, the ionomers continue to desorb, forming a colloidal gel network structure with a fractal dimension approaching 2. As the elastic gel network develops, both viscosity and elasticity increase. All of these transitions can be characterized by the Hansen solubility HSP-δP, which represents the polarity of the solvent. The above studies indicate that the aggregate structure and viscosity of catalyst slurries for proton exchange membrane fuel cells can be designed by controlling the solvent polarity characterized by HSP-δP.