Effect of Co-Catalyst Support Toward the Cell Reversal Tolerance of IrO2 Nanosheets-Pt/C Catalyst
Authors: Huang TingWei , Sugimoto Wataru 
Published: December 2024

Introduction

During the operation of a polymer electrolyte fuel cell (PEFC), certain parts of the anode flow channel may not receive sufficient supply of hydrogen due to water blockage or sluggish diffusion. This can lead to hydrogen starvation, which further causes a shortage of protons during cell reaction. Such phenomenon causes an increase in anode potential, which triggers the carbon oxidation reaction (COR) and damages the catalyst layer. This destructive chained reaction is known as cell reversal degradation.
Promoting the oxygen evolution reaction (OER) to alleviate the COR is a promising strategy to develop reversal-tolerant anodes (RTAs). Incorporating OER-active co-catalyst such as IrO2 is an effective way to mitigate the rise in anode potential and prevent COR [1]. We developed a new reversal tolerant anode (RTA) by directly coating a novel OER-active material, iridium oxide nanosheets (IrO2(ns)), as a co-catalyst to commercial Pt/C (c-Pt/C) [2]. With only 2.5 mass% of IrO2(ns) coating, the IrO2(ns)-c-Pt/C catalyst achieved an improved cell reversal resistance.
The OER current of IrO2(ns) plays an important role in enhancing the reversal tolerance. In this study, we focus on the degradation behavior of IrO2(ns)-supporting material. IrO2(ns) supported on carbon was physically mixed with c-Pt/C (IrO2(ns)/KB+c-Pt/C) and compared with IrO2(ns) coated on c-Pt/C (IrO2(ns)-c-Pt/C).

Experimental

IrO2(ns) colloid was synthesized following literature [3] [4]. The directly coated catalyst, IrO2(ns)-c-Pt/C, was prepared by mixing IrO2(ns) colloid and Pt/C. The samples will be denoted as Coated-x, where x is the IrO2 mass% in the catalyst. The mixed catalyst, IrO2(ns)/KB+c-Pt/C, was prepared by physically mixing IrO2(ns) /KB, which was prepared by mixing IrO2(ns) colloid and KB (Ketjen black EC-300J), and c-Pt/C. The sample will be denoted as Mixed-x, where x is the IrO2 mass% in the catalyst.
Rotating disk electrode (RDE) studies were conducted using a 3-electrode half-cell setup with 0.1 M HClO4 as the electrolyte. 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. All data were accumulated at 60 oC.

Results and Discussion:

To simulate the degradation behavior in the fuel cell anode, accelerated stress test (AST) was conducted by applying a series of CP (178 μA/cm2, 20 mins) and CV (0.05-1.2 V, 50 mV/s, 10 cycles) tests. The HOR mass activity was measured after 5 CP-CV tests. The HOR activity was evaluated using linear sweep voltammetry (LSV) from -0.08 V to 0.5 V vs. RHE and calculated at 20 mV vs. RHE based on the Koutecky-Levich equation. After AST, the Coated-5 sample showed a decrease in HOR activity from 1575 A/g to 272 A/g (17.3 % retention). In contrast, the Mixed-4.6 sample showed a decrease in HOR from 1840 A/g to 575 A/g (31.2 % retention).
Samples with higher IrO2(ns) content ratio were used for morphology observation by TEM and treated with higher AST current (CP current 1120 μA/cm2). Figures 1(A) and 1(B) show the initial state of Coated-15 and Mixed-10.7 and Figures 1(C) and 1(D) show the morphology after AST. In Coated-15 sample, agglomerated clusters were found, suggesting the dissolution/re-precipitation of Pt nearby IrO2(ns) (Figure (C)). On the other hand, in Mixed-10.7 sample, instead of forming large cluster, the damage of carbon substrate without Pt particle was found, suggesting the degradation of IrO2(ns)/KB due to high OER current (Figure (D)).
According to the results above, the separate coating of IrO2(ns) onto KB helps in preventing the high OER current that damages the Pt/C structure and provides a high HOR retention after AST.

Acknowledgments

This work was supported in part by funds for the "R & D of novel anode catalyst project" in "Collaborative industry-academia-government R&D project for solving common challenges toward dramatically expanded use of fuel cells” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. We appreciate valuable discussions with Prof. Hiroyuki Uchida, Yamanashi University.
References
[1] P. Mandal, B. K. Hong, J. -G. Oh, and S. Litster, J. Power Sources, 397, 2018, 397-404.
[2] T.W. Huang, W. Sugimoto, 2023 ECS Meet. Abstr., MA2023-02, 2023, 1972.
[3] D. Takimoto, K. Fukuda, S. Miyasaka, T. Ishida, Y. Ayato, D. Mochizuki, W. Shimizu, and W. Sugimoto, Electrocatal., 8, 2017, 144-150.
[4] D. Takimoto, Y. Ayato, D. Mochizuki, and W. Sugimoto, Electrochemistry, 85(12), 2017, 779-783.

Figure 1