Vibrating Powders: Electrochemical Quartz Crystal Microbalance Measurement of Potential Dependent IrO2 and Pt/C Mass Changes for Reversal Tolerant PEMFC Anodes
Authors: Moore Colin Edward , Afsahi Foroughazam , Young Alan P , Knights Shanna D , Gyenge Előd L. 
Published: February 2020

Oxygen evolution reaction (OER) catalysts are used to protect carbon components of polymer electrolyte membrane fuel cell (PEMFC) anodes during transient fuel starvation conditions such as start-up/shut-down [1]. OER catalysts such as IrO2 or RuO2 promote the oxidation of water instead of the corrosion of carbon components [1, 2]. However, the conditions in a PEMFC during normal operation (ca. 0 V and low pH) can induce instability in these metal oxides. IrO2 in particular can form soluble Ir3+ species [3] that migrate across the membrane electrode assembly and deposit on the cathode causing poisoning of the oxygen reduction reaction (ORR) catalyst (typically Pt/C).

Currently, OER catalysts for reversal tolerant anodes (RTAs) are investigated using prototype MEA single unit cell testing which is time consuming and expensive to perform [4]. Previously, our group investigated the dissolution of different IrO2 based powder catalysts using a potential stepping stress test. The dissolution of Ir3+ from this test was compared with reversal tolerance of a membrane electrode assembly fabricated using the same IrO2 catalyst [5]. To further investigate the potential dependent mass changes of IrO2 powder layers, we have devised a series of experiments (with potential cycling between 0.05 to 1.2, 1.4 and 1.6 V vs. RHE, 30 times each at 100 mV s-1 and 298 K) using the electrochemical quartz crystal microbalance (EQCM) technique. The EQCM uses a vibrating quartz crystal and measures its frequency (inversely proportional to crystal mass) as a function of potential. Our method was used to investigate a commercially available IrO2 powder (Alfa Aesar) heat treated in air at 350, 450 and 550 °C. This heat treatment improves durability while reducing OER activity [6]. The powders were deposited (24 μg cm-2) onto a gold EQCM crystal to investigate the dissolution behavior as a function of particle size, crystallinity and electrode potential.

We observed that the frequency of the untreated IrO2 increased by ca. 80 Hz (-570 ng cm-2) during the initial polarization from 1.15 V to 0.05 V vs. RHE (Figure a). The electrolyte was tested for dissolved Ir by inductively coupled plasma mass spectrometry (ICP-MS) but it was below the detection limit. This suggests that the first cycle frequency increase is caused by decreased rigidity of the IrO2 particle aggregates. During subsequent cycles, the untreated IrO2 frequency profile was consistent with previous reports. Birss et al. reported the formation of an iridium oxo-hydroxide at low potentials which causes a mass decrease (frequency increase) when polarizing to from 0 to 1.0 V vs. RHE [7]. Conversely, the frequency of heat treated layers first decreases by ca. 70 Hz (500 ng cm-2) before increasing again when polarizing to from 0.05 to 1.6 V vs. RHE (Figure b). Over the course of 90 CV cycles, there was an increase in film mass from the formation of iridium oxo-hydroxide species; confirmed by XPS. A platinum on carbon catalyst (Pt/C) was added to the IrO2 powders and the same experiment was performed (Figure c and d). This produced a dramatic increase in frequency which corresponded to a mass increase larger than the total deposited mass. Therefore, this mass decrease cannot be caused by dissolution of Pt or carbon corrosion alone and is instead attributed to loss of adhesion and changes in the viscoelasticity of the Pt/C aggregates. [8]

The main challenge to using EQCM for powder catalyst investigation is producing layers with consistent and repeatable frequency behavior. In this work, the IrO2 and Pt/C deposition conditions were tightly controlled and each experiment was repeated to ensure the consistency of the results. It can also be difficult to interpret results and various analytical techniques should be used to supplement the EQCM technique. Despite these drawbacks, EQCM is a rapid and powerful tool to learn about potential dependent reactions using a myriad of powder catalysts.

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