Novel Methodology for Ex-Situ Characterization of Catalysts in Reversal Tolerant PEM-FCs

Abstract

To reduce damage to carbon containing components of fuel cell stacks during start-up and shutdown, extensive research has been carried out to produce reversal tolerant anodes (RTAs) using oxygen evolution reaction (OER) catalysts [1-4]. However, most of these results were obtained using resource intensive in-situ testing that suffers from long experimental times. To address this, a series of ex-situ experiments was devised to characterize the activity and durability of OER catalysts in a simulated polymer electrolyte fuel cell (PEMFC) environment. The dissolution/re-deposition mechanism of the OER catalysts were investigated using a combination of linear sweep voltammetry and potential stepping experiments within the normal operating range of PEMFCs (0 V to 1.2 V vs RHE). During normal fuel cell operation, IrO2 based catalysts form soluble Ir3+ species as an intermediate between metallic Ir and IrO2. Ir3+ ions can be washed out of the cell which diminishes the reversal tolerance of the anode. After our electrochemical testing, dissolved Ir3+ concentrations in the electrolyte were determined using the inductively coupled plasma mass spectrometry (ICP-MS) method. An in-line ICP-MS technique was previously used by Cherevko et al. to determine the potential resolved dissolution in real time [5]. To validate the ex-situ accelerated testing protocol, experimental reversal tolerance tests were carried out at four different temperatures (20, 40, 60 and 80 °C) and the results showed an increase in the concentration of Ir detected in the electrolyte solutions (Fig. 1). Additionally, different OER catalysts were tested and the results were correlated with in-situ reversal tolerance tests. The effect of OER catalyst structure and support interactions on catalyst stability was investigated by a combination of surface analytical and electrochemical techniques. The surface analysis revealed that there was dissolution and re-deposition occurring during accelerated degradation testing. SEM-EDX imaging of the catalyst layer after testing, showed relocation of the OER catalyst to the cracks of the gas diffusion layer supporting a microporous layer (GDL/MPL) substrate (Fig. 1). Taniguchi A, Akita T, Yasuda K, Miyazaki Y (2004) Analysis of electrocatalyst degradation in PEMFC caused by cell reversal during fuel starvation. J Power Sources 130:42–49. doi: 10.1016/j.jpowsour.2003.12.035Ralph TR, Hudson S, Wilkinson DP (2006) Electrocatalyst stability in PEMFCs and the role of fuel starvation and cell reversal tolerant anodes. ECS Trans 1:67–84. doi: 10.1149/1.2214545Mandal P, Litster S (2016) Investigation and mitigation of degradation in polymer electrolyte fuel cell due to cell reversal using oxygen evolution catalyst. Meet Abstr MA2016-01:1419–1419.Jung J, Park B, Kim J (2012) Durability test with fuel starvation using a Pt/CNF catalyst in PEMFC. Nanoscale Res Lett 7:34. doi: 10.1186/1556-276X-7-34Cherevko S, Geiger S, Kasian O, Kulyk N, Grote J-P, Savan A, Shrestha BR, Merzlikin S, Breitbach B, Ludwig A, Mayrhofer KJJ (2016) Oxygen and hydrogen evolution reactions on Ru, RuO2, Ir, and IrO2 thin film electrodes in acidic and alkaline electrolytes: A comparative study on activity and stability. Catalysis Today 262:170–180. doi: 10.1016/j.cattod.2015.08.014 Figure 1. Images: backscattered electron (BSE) detector images at 500x magnification of OER catalysts deposited on a GDL/MPL (a) before ex-situ testing and (b) after ex-situ testing. The bright dots were confirmed by EDX to contain Ir atoms. Bar graph: Electrolyte Ir concentration after accelerated degradation protocol. Performed in a flooded, N2 purged, 0.09 M H2SO4, three electrode cell for 30,000 cycles from 0.05 V to 1.2 V vs. RHE with a 1 s hold at each potential. OER catalysts were prepared by sonication in 2-propanol overnight and then 400 μg deposited dropwise onto the GDL/MPL substrate. Three trials were carried at each temperature and the error bars are the standard error of the trials. Figure 1