Iridium-Based Catalysts for Electro-Oxidation of Ammonia in Alkaline Media
Authors: Song Liang , Liang Zhixiu , Wang Jia X. 
Published: February 2020

Ammonia is an attractive carbon-free liquid fuel for alkaline membrane fuel cells. While ethanol has higher energy density than ammonia, its product, CO2, reacts with OH- to form carbonate in alkaline media, which can cause kinetic loss and cell failure. Thus, carbon-free is an advantage of ammonia.[1, 2] In the 6-electron ammonia oxidation reaction (AOR), 2NH­3 +6OH- = N2 + 3H2O + 6e-, formation of the N-N bond is a rate-limiting step with rather high activation barrier. Most of research focused on Pt-based catalysts because Pt is essential for high current peak at ambient temperature. Ir is known for its low onset potential for AOR. As our recent temperature-dependent kinetic study showed, alloy PtIr/C is currently the best commercial catalyst for AOR at 60 oC and the temperature enhancement factor between 60 and 25 oC is in the order of Ir (5.5) > IrPt (4.2) > Pt (2.0).[3] Thus, developing Ir-based catalysts holds the promise for improving anode performance in fuel cells operating at elevated temperatures.
In this talk, we show that Fe nitride can promote the AOR activity on Ir, Pt, and IrPt catalysts. Three carbon-supported catalysts, Ir4Fe2Nx/C, Ir3Pt1Fe2Nx/C, and Pt3Fe2Nx/C, were prepared by reducing and nitriding metal chloride precursors on Vulcan XC-72 carbon in a tube furnace with mixed hydrogen and ammonia gases. We dispersed the catalysts on gas diffusion electrodes with a loading of platinum group metals (PGM) of 100 μg/cm2 and plot the AOR polarization curves with the currents normalized to PGM loadings in Figure 1. Even though Pt3Fe2Nx/C achieved the highest peak current density, its activity at low overpotential range (0.35- 0.5 V) is much lower than those containing Ir. The compression of the Ir lattice induced by the small alloyed Fe atoms results in slightly higher AOR peak current density of Ir4Fe2Nx/C than Ir/C, while the peak potentials remain unchanged. As the Pt: platinum-group-metal ratio increases from 0 in Ir/C and Ir4Fe2N/C to 100% in Pt3Fe2Nx/C, the peak potential increase gradually, which correspond well with the peak position for Ir3Pt1Fe2Nx/C catalysts which located between Ir/C and IrPt/C. When comparing AOR activity of Ir3Pt1Fe2Nx/C with Ir4Fe2Nx/C, substitution of 1/4 Ir atoms in Ir4Fe2Nx/C with Pt atoms leads to an optimal activity for AOR among the tested samples as a consequence of combining the low onset potentials of Ir4Fe2Nx/C and high peak current density of Pt3Fe2Nx/C. More detailed results of this work will be discussed in the presentation.
Figure 2 shows the X-ray diffraction patterns of these three catalysts in comparison with two commercial catalysts of Ir/C and IrPt/C. There is no peak corresponding to any Fe or Fe nitride structures, indicating that Fe is well alloyed with Ir and/or Pt. It appears that AOR activity on Ir-based catalysts prefers neither expansion or contraction of Ir lattice.
The diffraction peak for Ir3Pt1Fe2Nx/C locates at the same angle as the Ir/C which may be as an integrated effect from Pt (bigger than Ir) and Fe (smaller than Ir) atoms.

Acknowledgements

This research was funded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0000805 and by the Division of Chemical Sciences, Geosciences and Biosciences Division, US Department of Energy under contract DE-SC0012704.

References

[1] Kumar Siddharth, Yattung Chan, Lu Wang, Minhua Shao. Ammonia electro-oxidation reaction: Recent development in mechanistic understanding and electrocatalyst design. Current Opinion in Electrochemistry 2018, 9: 151- 157
[2] C. Zhong, W. B. Hu, Y. F. Cheng. Recent advances in electrocatalysts for electro-oxidation of ammonia. Journal of Materials Chemistry A, 2013, 1: 3216- 3238
[3] Liang Song, Zhixiu Liang, Zhong Ma, Yu Zhang, Jingyi Chen, Radoslav R. Adzic, Jia X. Wang. Temperature-Dependent Kinetics and Reaction Mechanism of Ammonia Oxidation on Pt, Ir, and PtIr Alloy Catalysts. Journal of The Electrochemical Society, 2018, 165 (15): J3095- J3100

Figure 1