Development of IrO2 Nanosheet–Pt/C Composite as Reversal Tolerant Anode Catalysts for Polymer Electrolyte Fuel Cell

Abstract

Introduction: During the start-up and shutdown cycles of a polymer electrolyte fuel cell (PEFC), certain areas of the anode flow channel may not receive sufficient hydrogen due to water blockage or sluggish diffusion. This can lead to hydrogen starvation, which in turn causes a shortage of protons during cell reaction. The result is an increase in anode potential, which triggers the carbon oxidation reaction (COR) and damages the catalyst layer.Promoting the oxygen evolution reaction (OER) to alleviate the COR is a promising strategy to develop reversal-tolerant anodes (RTAs). Incorporating OER-active materials, such as IrO2, as a co-catalyst is an effective way to mitigate the rise in anode potential and prevent COR [1].In this study, a novel OER-active material, iridium oxide nanosheet (IrO2(ns)), was used as the co-catalysts and combined with commercial Pt/C in three different ways; mixing Pt/C with normal- and small-sized IrO2 nanosheets (IrO2(ns)/Pt/C), and Pt/C mixed with carbon supported IrO2 nanosheet (Pt/C+IrO2(ns)/KB). The OER activity and tolerance to cell reversal conditions were characterized using a 3-electrode rotating disk electrode (RDE) setup. Experiment: IrO2(ns)(250 nm) colloid was synthesized following literature [2]. The resulting IrO2(ns)(250 nm) colloid was subjected to ultrasonication to obtain small-sized nanosheets (IrO2(ns)(100 nm)) [3]. Nanosheet-carbon composite (IrO2(ns)(100 nm)/KB) was prepared by mixing IrO2(ns)(100 nm) colloid and KB (Ketjen black EC-300J) and physically mixed with Pt/C. Commercially available IrO2 nanoparticles (c-IrO2) were purchased from Tokuriki Honten Co., Ltd. and added to Pt/C (c-IrO2-Pt/C) and used as a reference sample.The composite catalyst was prepared by mixing Pt/C with different IrO2 nanomaterials and will be denoted as IrO2(ns)(x mass%)-Pt/C or c-IrO2 (x mass%)-Pt/C depending on the source of the IrO2 material used. RDE studies were conducted using a 3-electrode half-cell setup with 0.1 M HClO4 as the electrolyte. The counter electrode was carbon fiber, and a reversible hydrogen electrode (RHE) was used as the reference electrode.Except for the OER activity, the electrodes for all measurement were prepared with a Pt loading of 8.0 μg/cm2. For hydrogen oxidation reaction (HOR) activity measurements, the electrolyte was purged with H2. For chronopotentiometry (CP) and electrochemically active Pt surface area (ECSA) measurement, the electrolyte was purged with N2. For OER activity measurements, the electrolyte was purged with N2, and the working electrode was prepared with a Pt loading of 17.3 μg/cm2. Except for ECSA measurement, all data were accumulated at an electrolyte temperature of 60oC. Results and Discussion: Figure 1 shows the OER activity (10 mV/s) of Pt/C and composite catalysts prepared with c-IrO2(5 mass%), IrO2(ns)(250 nm, 5 mass%), IrO2(ns)(100 nm, 5 mass%), IrO2(ns)(100 nm, 2.5 mass%) and IrO2(ns)(100 nm, 1 mass%). At 1.5 V vs. RHE, IrO2(ns)(100 nm, 2.5 mass%)-Pt/C exhibited a 2-fold increase in OER activity compared to IrO2(ns)(250 nm, 5 mass%)-Pt/C and a 25 % increase compared to c-IrO2 (5 mass%)-Pt/C. All composite catalysts displayed significantly higher OER activity than the pristine Pt/C sample.To simulate the degradation behavior in the fuel cell anode, a series of CP tests were performed at 16, 48, 80, and 112 μA/cm2. The HOR mass activity was used as a quantitative factor to present the decay caused by cell reversal degradation. The HOR activity before and after CP was evaluated using linear sweep voltammetry (LSV) from -0.08 V to 0.5 V vs. RHE. The HOR mass activity at 20 mV vs. RHE was calculated based on the Koutecky-Levich equation. After CP, the Pt/C sample showed a decrease in HOR activity from 851 A/g to 621 A/g (72% retention). In contrast, IrO2(ns)(100 nm, 2.5 mass%)-Pt/C showed an initial HOR activity comparable to Pt/C (1420 A/g) and still retained a high HOR activity of 1400 A/g after CP (100% retention). The addition of IrO2(ns) as a co-catalyst effectively suppressed the carbon oxidation reaction under cell reversal conditions.