Proton exchange membrane water electrolysis (PEMWE) offers several advantages over other electrolysis technologies including greater energy efficiency, higher production rates, dynamic responses and more compact design. [1] High loading is a major barrier to the path of large-scale production of PEMWE. [2] The state-of-the-art anode catalysts in conventional PEMWEs are mixed iridium and iridium oxides (IrOx) with typical iridium loading from 1 to 3 mg cm^-2, which is too high to meet the long term cost target for energy markets [2]. Meanwhile, translation of catalyst development from lab scale to the megawatt scale remains challenging in terms of electrode stability. [3]

Herein, a composite catalyst layer of iridium oxide/Nafion (IrOx/Nafion) is developed using reactive spray deposition technology (RSDT) which combines the catalyst synthesis and catalyst-coated membrane (CCM) fabrication into one step. The RSDT-derived IrOx/Nafion showed distinctive features that are beneficial to the oxygen evolution reaction (OER) activity and durability. The iridium particles were about 1-2 nm with very high surface area due to the intrinsic property of resistance to sintering [4]. This resulted in high anodic charge proportional to the oxide surface area and superior activity over planar and electrodeposited IrO₂thin films. The OER activity can be further enhanced by increasing the amount of surface oxide of iridium which was achieved by increasing the flame length, flame temperature and oxygen flow rate. In Figure 1, F-1 and F-2 corresponds two different flame conditions with iridium surface oxide percentage of 28% and 100%, respectively. Higher surface oxide percentage (F-2) resulted in superior anodic charge (Figure 1a) and OER activity (Figure 1 b,c). The electrolyzer durability of the RSDT-derived IrOx/Nafion catalyst layer was optimized by increasing the amount of iridium oxide on the iridium particle surface and by decreasing the number of IrOx/Nafion agglomerates on the catalyst layer surface. Ongoing studies of CCMs fully fabricated by the RSDT process showed promising electrolyzer durability of 3300 hours (Figure 1d). Analysis of post-durability test samples will be conducted to investigate the effect of long-term operation on catalyst layer integrity and iridium dissolution and migration in the electrolyte membrane.

Figure 1. Cyclic voltammetry (CV) of RSDT-derived IrOx/Nafion catalyst with two flame conditions (a). CV recorded in nitrogen-purged 0.1M HClO₄ at 25 ^oC with 20 mV/s scan rate from 0.0 V to 1.5V (vs. RHE). Quasi-stationary polarization curves for OER recorded from 1.45-1.69 V vs. RHE with 20 mV step and 5 min dwell time (b). (c) Compares the OER mass activity and specific activity of RSDT-derived IrOx/Nafion catalysts and pure IrO₂ (adapted from [5]) tested with the same procedure. (d) Shows the electrolyzer MEA durability test of RSDT-derived cathode and anode. Test performed using a circular 86 cm² cell at 80 ^oC with 1.8 A cm^-2current load and 400 psi hydrogen pressure.

References

[1] M. Carmo, D.L. Fritz, J. Mergel, D. Stolten, Int. J. Hydrogen Energy. 38 (2013) 4901-4934.

[2] K.E. Ayers, C. Capuano, E.B. Anderson, ECS Transactions. 41 (2012) 15-22.

[3] N. Danilovic, K.E. Ayers, C. Capuano, J.N. Renner, L. Wiles, M. Pertoso, ECS Transactions. 75 (2016) 395-402.

[4] J. Lu, C. Aydin, N.D. Browning, L. Wang, B.C. Gates, Catalysis Letters. 142 (2012) 1445-1451.

[5] T. Reier, Z. Pawolek, S. Cherevko, M. Bruns, T. Jones, D. Teschner, S. Selve, A. Bergmann, H.N. Nong, R. Schlogl, K.J.J. Mayrhofer, P. Strasser, J.Am.Chem.Soc. 137 (2015) 13031-13040.