A Study on Effect of Ionomer Content on Catalyst Ink Property and PEM Water Electrolyzer Performance
Authors: Song Chaojie , Tsay Ken , Fisher Elizabeth , Sheibley Nate , Shaigan Nima , Malek Ali , Fatih Khalid 
Published: September 2023

Producing hydrogen from water electrolysis with renewable electricity is essential for a carbon-free and environment-friendly economy. Proton exchange membrane (PEM) water electrolysis has advantages over other types of water electrolysis technologies with respect to compact system, high purity H2, high current density operation, better safety and reliability etc. Catalyst coated membrane (CCM) is the core of the membrane electrode assembly (MEA) and PEM water electrolyzer [1]. The CCMs are prepared by depositing catalyst inks onto the polymer electrolyte membrane. The composition of the catalyst inks plays an important role in determining the CCM and PEMWE performance.
Catalyst ink is prepared from catalyst, ionomer and solvent. Commonly used is IrOx as catalyst, Nafion solution as ionomer (binder and proton conductivity path), and a mixture of organic solvent and water as solvent. One of the key parameters determining the CCM performance is the ionomer content in the catalyst layer.
A wide range of ionomer content was reported in the literature, ranging from 2 to 30wt%. Xu et al. reported an optimal value of 25 wt% ionomer content using Ru0.7Ir0.3O2 [1]. The same Nafion content was used by Su et al. with IrO2 [2]. Bernt and Gasteiger found 11.6 wt% ionomer content showed the best performance with IrO2/TiO2 [3]. Ma et al. concluded that 30 wt% ionomer content was the best using Ir black [4]. P. Holzapfel et al [5] and S. Khandavali et al [6] used 2 wt% of ionomer content in their studies with IrO2. The large variability of the ionomer content indicates that there is a need on fundamental understanding of the effect of ionomer content for PEMWE applications.
Catalyst ink properties may affect the catalyst layer structure and further PEMWE performance. S. Khandavali et al. studied the rheology and microstructure of the catalyst inks [7]. However, no further steps were presented such as fabricating CCMs using the ink and testing the CCMs in PEMWE. How the ink properties affect the catalyst layer structure, and further the PEMWE performance are not studied to our knowledge. In this work, a study on effect of ionomer content on catalyst ink property and further PEMWE performance is presented.
In this work, catalyst ink was prepared from a mixture of isopropanol and water (1:1), Nafion solution, and IrO2. Inks with Nafion concentrations ranging from 1.0 wt. % to 20 wt. % were investigated. Ink properties such as viscosity, Zeta-potential and average particle size were studied. Properties of CCMs developed from the inks by directly coating the catalyst ink on Nafion membrane using ultrasonic spray were also investigated.
The CCMs prepared from inks with 5.0, 7.0, 8.5 and 10% Nafion were tested in PEM water electrolyzer single cell at 80oC and ambient pressure. The CCM with the 7.0% Nafion shows the highest performance, while the 5.0% Nafion shows the lowest. The 8.5 and 10% Nafion CCMs show slightly lower performance than the 7.0%. The PEMWE was diagnosed with AC impedance. Fig. 1 presents the EIS spectra of the four CCMs in PEMWE obtained at 50 mA.cm-2. It can be seen that other than the CCM with 5.0% Nafion, all other 3 CCMs showed similar spectra. This is in agreement with the polarization curves. Impedance data fitting using the modified Randles equivalent circuit (solid lines in Fig. 1) shows that the 5.0% Nafion demonstrated the highest anode charge transfer resistance (oxygen evolution reaction (OER)) and the 7.0% shows the lowest value. Correlation of the ink properties with the PEMWE performance will be presented.
References

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Su, B. J. Bladergroen, V. Linkov, S. Pasupathi, S. Ji, Int. J. Hydrogen Energy, 36 (2011) 1615081 – 15088

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Khandavali, J. H. Park, N. N. Nariuki, S. F. Zaccarine, S. Pylypenko, D. J. Myers, M. Ulsh, S. A. Mauger, ACS Appl. Mater. Interfaces, 11 (2019) 45068 – 45079

Figure 1