Activity and Microstructure of Ultra-Low Iridium Loading Catalyst for PEM Electrolyzer MEA
Authors: Yu Haoran , Danilovic Nemanja , Wang Yang , Capuano Chris , Palmieri Alessandro , Mustain William E , Ayers Katherine E , Maric Radenka 
Published: February 2020

The reduction of catalyst loading is a major barrier to the path of large-scale production of PEM water electrolysis (PEMWE).[1] The state-of-the-art anode catalysts in conventional PEMWEs are Iridium oxides (IrOx) and its mixture with Ruthenium oxide [2]. Typical electrodes for commercial applications have IrOx loadings from 1 to 3 mg cm-2 [1]. While dispersing IrOx on high-surface-area supports, such as Sb-doped SnO2 and Tin-doped Indium oxide (ITO) can improve catalyst utilization and reduce loading, the IrOx loading typically ranged from 0.5 to 1.0 mg cm-2 in the literature.[3] Further reduction is required to meet the DOE target ($4-5/kg H2 production).
Our previous work [4]showed the feasibility of reducing the Ir loading to 0.1 mg cm-2 using the reactive spray deposition technology (RSDT). The RSDT process combines catalyst synthesis and processing into a single step where catalyst particles are synthesized through nucleation in the gas-phase followed by deposition on the catalyst support or binder. [5,6].
We further explored the Iridium oxide catalyst layer (CL) integrity and stability by carefully choosing the catalyst binder and controlling the CL thickness. Perfluorosulfonic acid (Nafion®, DuPont) was found to have more stable performance at 1.8 A cm-2 current hold than Polytetrafluoroethylene (PTFE, DuPont). The IrOx/PTFE cell potential increased by 50 mV within one hour while the IrOx/Nafion cell potential increased by 50 mV after 6.5 hours (Figure 1 a). This can be attributed to the enhanced adhesion between IrOx/Nafion and membrane since the binder was the same type of polymer as the membrane electrolyte (Nafion ® N117). The comparison of CL cross-section supported this hypothesis (Figure 2 a,b). The IrOx/Nafion CL showed column-like agglomerates and the interface of IrOx/Nafion was adhered to the membrane more tightly than the IrOx/PTFE CL which comprised of spherical agglomerates. Figure 2c shows the TEM bright field image of IrOx/Nafion agglomerates where the IrOx particles (dark) were connected by Nafion (light grey).
The CL stability at 1.8 A cm-2 current hold can be further improved by increasing the CL thickness as shown in Figure 1a and b. The Ir loading increased accordingly from 0.1 to 0.2 mg cm-2. IrOx/Nafion with doubled thickness displayed stable performance up to 25 hours at 1.8 A cm-2 current hold with < 10 mV change of cell potential. For the polarization performance, the cell potential of IrOx/Nafion with doubled thickness was slightly higher at the same current density, probably caused by the higher ohmic resistance with thicker CL.
We also investigated supported IrOx catalyst to improve the IrOx dispersion at ultra-low Ir loading. Figure 2d showed the TEM bright field image of IrOx supported on Tin oxide (SnO2, Sigma Aldrich) nanopowders with Nafion® binder. The IrOx particles appeared as dark particles with ~2 nm size dispersed evenly on the surface of SnO2 particles (20-50 nm). The performance of IrOx/SnO2 was comparable to that of IrOx/Nafion but the stability at 1.8 A cm-2 current hold was not as good. Further optimization of the CL will be conducted through optimizing the binder/support ratio, CL thickness, and investigating other corrosion-resistive supports.
[1] K.E. Ayers, C. Capuano, E.B. Anderson, ECS Transactions. 41 (2012) 15-22.
[2] E. Fabbri, A. Habereder, K. Waltar, R. Kotz, T.J. Schmidt, Catal.Sci.Technol. 4 (2014) 3800-3821.
[3] J. Xu, D. Aili, Q. Li, E. Christensen, J.O. Jensen, W. Zhang, M.K. Hansen, G. Liu, X. Wang, N.J. Bjerrum, Energy Environ.Sci. 7 (2014) 820-830.
[4] K.E. Ayers, J.N. Renner, N. Danilovic, J.X. Wang, Y. Zhang, R. Maric, H. Yu, Catalysis Today. 262 (2016) 121-132.
[5] H. Yu, J.M. Roller, W.E. Mustain, R. Maric, J. Power. Sources. 283 (2015) 84-94.
[6] J.M. Roller, M.A. Arellano-Jimenez, R. Jain, H. Yu, C.B. Carter, R. Maric, Journal of the Electrochemical Society. 160 (2013) F716-F730.

Figure 1