Effect of Cathode Proton Conductivity on PGM-free PEM Fuel Cell Performance (2020)

The most promising alternative to Platinum Group Metals (PGMs) Oxygen Reduction Reaction (ORR) catalysts for proton exchange membrane fuel cell (PEMFC) applications is a class of electrocatalysts consisting of atomically dispersed transition metals (Fe and Mn) in a matrix of carbon and nitrogen, so called M-N-C materials [1]. Currently, the emerging novel M-N-C materials demonstrated 30% of PGM catalyst activity [1]. Performance of PGM-free electrocatalysts can be improved by increasing density of M-Nx sites or optimizing their accessibility through proper integration into the triple phase catalyst layer in a membrane electrode assembly (MEA). Since overall MEA’s performance depends on properties of all its components: membrane, electrodes and gas diffusion layers (GDL), it is important to understand impact of these materials on PGM-free fuel cell operation. In this work we report effects of membrane thickness on performance of PGM-free PEMFCs using various electrochemical methods and modelling. The commercial, PGM-free electrocatalyst manufactured by Pajarito Powder and marketed as PMF-011904 was used in the present study. Catalyst coated membranes with active area of 23 cm 2 were manufactured by IRD Fuel Cell by their proprietary digital printing method. For this work we used membranes with thickness of 15 (LYT0008), 25 (N211), 125 (N115) and 175 (N117) μm. The cathode PGM-free catalyst loading was 6.0 mg cm -2 , while anode employed Pt/C catalyst with loading of 0.2 mg Pt cm -2 . Freudenberg H23C8 GDLs were applied for both electrodes. Electrochemical evaluation of PGM–free MEAs was performed using a custom test station, designed at the HNEI and characterized by dynamic response time &lt;0.1 s. The anode and cathode were fed with H 2 and O 2 respectively, at constant flow rates of 0.5 slpm at 100% relative humidity and 150 kPa absolute backpressure for both electrodes, and the cell temperature was 80°C. Fig. 1 a) shows a representative SEM image of the MEA cross-section. The cathode thickness was found to be 140-160 μm for all samples. The IV data clearly showed that an increase in the membrane thickness led to an increase of HFR and caused poor performance (Fig. 1 b). The highest performance was achieved for MEAs with thin membranes 15 and 25 μm. At the same time application of thick membrane allowed us to reach the highest ocv values 0.979 V (175 μm) vs 0.929 V (15 μm). The impedance data were analyzed using physics-based model which includes proton and oxygen transport in the cathode layer [2]. The results demonstrated a rapid decay of proton conductivity from 15-18 to 5-10 mS cm -1 at current densities higher than 100 mA cm -2 . Possible mechanism of the observed decay can be attributed to dissolution of Fe-N-C centers and formation of Fe 3+ cations which reduce ionomer conductivity by screening SO 3- groups [3-4]. Due to relatively low proton conductivity and large electrode thickness, the ORR rate is strongly non–uniform through the cathode depth. Our estimates showed that this non-uniformity leads to 2 K overheat of the catalyst layer at the membrane surface. The overheat could induce pressure gradient, pushing liquid water out of the electrode and clearing void pores for gaseous transport of oxygen. This mechanism might explain explosive growth of the cathode oxygen diffusivity (5-7⋅10 -3 cm 2 s -1 ). Detailed analyses of the PGM-free PEMFC performance and its correlation with AC-impedance results will be presented and discussed. Acknowledgments We gratefully acknowledge funding from ONR (N00014-19-1-2159) and DOE EERE (DE-EE0008419). The authors are thankful to T. Carvalho for assisting with SEM. Reference 1. S.T. Thompson, A.R. Wilson, P. Zelenay, D.J. Myers, K.L. More, K.C. Neyerlin, D. Papageorgopoulos, Solid State Ionics 319 , 68-69 (2018). 2. A. Kulikovsky, J. Electroanal. Chem . 669 , 28-34 (2012). 3. T. Reshetenko, G. Randolf, M. Odgaard, B. Zulevi, A. Serov, A. Kulikovsky, J. Electrochem. Soc. 167 , 084501 (2020). 4. T. Reshetenko, A. Serov, A. Kulikovsky, P. Atanassov, J. Electrochem. Soc. 166 , F653-F660 (2019). Figure 1