Indium Tin Oxide as Catalyst Support for PEM Fuel Cell: RDE and MEA Performance

Introduction

Conventional carbon supports suffer from catastrophic failure during start-up/shut-down events, which result in high cathode interfacial potentials, which, in turn, lead to carbon corrosion and catalyst degradation. Carbon corrosion is a major problem in fuel cells because it leads to loss of catalyst activity and collapse of electrode structure. Nissan, as an automotive OEM, has always approached the issue of catalyst support durability at the materials level, rather than at the systems-level, since system-level mitigation is an added cost to the balance of plant (BOP), and presents one additional factor that can fail during operation. NTCNA and IIT have been working on the development of high-surface-area, corrosion-resistant non-carbon supports, and we have previously demonstrated a class of highly stable non-carbon supports, e.g. ruthenium-titanium oxide (RTO) (1). In this work, we present the results of our work on a second class of non-carbon supports - tin-doped indium oxide (ITO).

Materials and Methods

RDE Testing – RDE tests were performed in a 3-compartment electrochemical cell containing 0.1 M HClO₄ electrolyte. A glassy carbon disk (0.196 cm²) uniformly covered with Pt/C or Pt/ITO catalyst served as the working electrode. The counter and reference electrodes were Pt foil and RHE, respectively.

Membrane Electrode Assembly (MEA) Fabrication - Catalyst inks were prepared by mixing Pt/C or Pt/ITO with water, n-propanol, and a Nafion ionomer dispersion. The mass-based ionomer/support (I/S) ratio in the ink was kept constant at 0.9.  The cathode catalyst layers were formed on GDLs using an automated robotic spray system.  MEAs (25 cm²) were prepared by hot pressing cathode GDEs (0.35 mg_Pt/cm²) and commercial anode GDEs (0.4 mg_Pt/cm²) onto Nafion® NR211 membranes. 

Results and Discussion

Figure 1 shows the RDE performance of Pt/ITO vs. a commercially available Pt/C benchmark catalyst of comparable Pt particle size. The ECA obtained for Pt/ITO (22 m²/g_Pt) was lower than Pt/C (45 m²/g_Pt) due to the low surface area of ITO which causes Pt particle agglomeration. Even with a lower ECA, Pt/ITO shows promising performance since its mass activity of ~150 mA/mg_Pt matches that of the Pt/C benchmark catalyst.  

However, we have repeatedly observed that Pt/ITO performs very poorly in MEA testing. Figure 2 shows a typical H₂/O₂ polarization curve obtained for Pt/ITO at a high Pt loading of 0.35 mg/cm². The  only difference between the two MEAs is the cathode catalyst. Other than the dramatic difference between the two polarization curves, it can also be seen that Pt/ITO shows a very high HFR (high frequency resistance) of ~220 mΩ·cm². The HFR represents the ohmic resistance due to proton transport in the membrane and the electronic contact resistances in the cell. The high HFR is attributed to the poor electronic conductivity of the ITO support since the two MEAs in Figure 2 have the same membrane, cell hardware, and cell components. It is hypothesized that hydroxylated species form on the ITO surface under fuel cell operating conditions. Hydroxide and oxy-hydroxide species may readily form on the surface of ITO due to hydrolysis reactions. Hydroxylated species such as In(OH)₃ have low solubilities, hence they may remain adsorbed on the ITO surface, forming a passivating layer that will increase ohmic resistance (2). Furthermore, the Pourbaix diagram for indium shows that under fuel cell conditions, the formation of In³⁺ is favorable, and it is possible that In³⁺ can act as a poison that covers the Pt active sites.  This is supported by the changes observed in the CV profile in MEA (loss of H_upd features and resistive behavior), suggesting some changes in the chemical properties of ITO and Pt poisoning. The promising RDE results did not translate to good performance in MEA, and this is hypothesized to be caused by cross-over hydrogen in MEA. H₂ crossing over from anode to cathode may accelerate the formation of hydroxylated species. Experiments are underway to verify this.

References

1. J. Parrondo, T. Han, E. Niangar, C. Wang, N. Dale, K. Adjemian and V. Ramani, Proceedings of the National Academy of Sciences, 111 (2014) 45-50.

2. B. Michael, P. A. Veneman, F. S. Marrikar, T. Schulmeyer, A. Simmonds, W. Xia, P. Lee, and N. R. Armstrong.  Langmuir, 23(22), 11089-11099 (2007).