Characterization of Inkjet Printed Electrodes with Improved Porosity
Authors: Shukla Shantanu , Stanier David , Saha Madhu S , Zahiri Beniamin , Tam Mickey , Stumper Jürgen , Secanell Marc 
Published: February 2020

Since a considerable amount of polymer electrolyte fuel cell (PEFC) stack cost is associated with platinum (Pt) that serves as a catalyst, reduction of Pt loading in the catalyst layers (CLs) without affecting the performance is a major research goal of the fuel cell industry. One of the essential factors affecting the Pt loading in the CLs is the fabrication technique used. Inkjet printing (IJP) is one such technique that can produce CLs with low Pt loading and a high Pt utilization [1-3]. However, a drawback for the IJP CLs, was found to be a reduction in porosity at higher Pt loadings, which corresponds to the increased number of deposited layers [2]. This was a major limiting factor affecting the performance of IJP electrodes, when comparing to conventionally fabricated electrodes [2]. In this work, a modified catalyst ink formulation is used, resulting in CLs that have a higher porosity compared to previous layers. Further, a methodology to determine the CL porosity, based on Archimedes law, is reported.
The catalyst ink consisted of 50 wt. % Pt/C catalyst dispersed in a mixture of Isopropanol, 5 wt. % ionomer solution and a diol solvent to increase the ink viscosity. The improvement in porosity was achieved using a higher volatility solvent in the ink. A Fujifilm Dimatix inkjet printer was used for the CL fabrication process. Details regarding the fabrication process can be found in our earlier work [2]. The Pt loading in the CLs was determined by X-ray fluorescence analyzer (Themo Scientific Niton XL3t). The target Pt loading was 0.15 mg/cm2, corresponding to approximately 16 printed layers. For comparison, electrodes using the previous ink recipe were also fabricated and analyzed [2]. The CLs were imaged using a laser scanning microscope to estimate the crack density and a scanning electron microscope (SEM) to determine the CL thickness. In-situ characterization of the electrodes was done in a 40 cm2 cell to compare the performance at different conditions, electrochemical active area and oxygen transport resistance using AFCC testing protocols.
For determination of CL porosity, a commercially available density determination kit (Sartorius mechatronics) was modified in-house and used in tandem with a Python based GUI to monitor the sample weight. The setup resembled the one reported by Rashapov et al. [4]. A PTFE substrate was used for the CLs when measuring porosity. Weight of the sample (CL + substrate) was taken in air, n-octane and deionized water. The total pore volume is obtained with the hypothesis that octane intrudes all the pores in the sample, thus reflecting its solid volume fraction, whereas water does not intrude any pores owing to its high surface tension, reflecting the CL bulk volume. To reduce the measurement error, an average of at least three weight readings was taken in each medium. The overall porosity of the IJP CL using the modified recipe was found to be 69.5±2.3 % compared to 51.3±2.2 % using the previous ink recipe, indicating a substantial improvement in the porosity [2]. This is reflected in the preliminary in-situ testing where a gain between 20 – 50 mV is achieved at different operating conditions compared to previous IJP electrodes. CLs using the modified ink recipe showed a power density of 1 W/cm2 at 0.6 V at normal operating conditions (680C, 70% RH), indicating that inkjet printing appears to be a feasible technique for electrode fabrication. Further testing of the CLs is underway.

References:

1. Shukla, S., Domican, K., Karan, K., Bhattacharjee, S., & Secanell, M. Electrochimica Acta, 156, 289-300 (2015)
2. Shukla, S., Stanier, D., Saha, M. S., Stumper, J., & Secanell, M. Journal of The Electrochemical Society, 163(7), F677-F687 (2016)
3. Saha, M. S., Malevich, D., Halliop, E., Pharoah, J. G., Peppley, B. A., & Karan, K. Journal of The Electrochemical Society, 158(5), B562-B567 (2011)
4. Rashapov, R. R., Unno, J., & Gostick, J. T. Journal of The Electrochemical Society, 162(6), F603-F612 (2015)

Figure 1