Shaun M. Alia,¹ Sarah Shulda,² Chilan Ngo,² Svitlana Pylypenko,² Bryan S. Pivovar¹

¹ Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, CO 80401

² Department of Chemistry, Colorado School of Mines, Golden, CO 80401

shaun.alia@nrel.gov

In the United States, 2% of energy used goes through the hydrogen pathway, to produce ammonia in agriculture and upgrade oil in transportation. [1, 2] While most hydrogen is produced by steam methane reformation, electrolysis can become competitive with the use of low-cost renewable power sources. Further reductions in the cost of renewables may increase hydrogen use as an energy intermediate between the electric grid and transportation and industrial processes. [3]

Electrolyzers today operate at high capacity and with constant power input. A shift toward intermittent renewables and a focus on hydrogen production cost, however, increases the importance of catalyst thrifting and durability in the oxygen evolution reaction (OER). [2] Iridium or iridium oxide nanoparticles are typically used as OER catalysts, but are limited in activity and durability, particularly at low loading. [4]

Iridium-nickel and iridium-cobalt nanowires have been developed as OER electrocatalysts for proton exchange membrane-based electrolyzers. These catalysts use similar templates and synthesis routes previously used in the development of fuel cell oxygen reduction electrocatalysts. [5] A previous comparison between polycrystalline iridium and nanoparticles suggests that extended structures can potentially benefit from higher site-specific activity. [4] Differences between the nanowire templates affect catalyst composition, structure, and OER activity. While these materials exceed the performance of iridium nanoparticles, acid leaching is necessary to minimize template dissolution and improve durability. The acid leached catalysts exceed the half-cell activity of nanoparticles by an order of magnitude, and the half-cell mass activity of any catalyst available in literature by 4 times. In membrane electrode assemblies, the nanowires also outperform traditional iridium catalysts by 4‒5 times in single-cell electrolyzers.

Figure caption. Surface areas (ECA, x-axis) and specific OER activity (i_s, y-axis) of acid leached iridium-nickel (Ir-Ni) nanowires, iridium-cobalt (Ir-Co) nanowires, iridium nanoparticles (Ir), and iridium oxide nanoparticles (IrO₂). Solid black lines correspond to constant mass activities of 0.3, 1, and 3 A mg_Ir^‒1 at 1.55 V. Nanowire compositions are listed as-synthesized (in parentheses) and following acid leaching (immediately following parentheses).

References

[1] A. Milbrandt and M. Mann, ed. U. S. Department of Energy, http://www.nrel.gov/docs/fy09osti/42773.pdf, 2009.

[2] U. S. Department of Energy, https://www.hydrogen.energy.gov/pdfs/review16/2016_amr_h2_at_scale.pdf, 2016.

[3] P. Denholm, M. O'Connell, G. Brinkman and J. Jorgenson, ed. U. S. Department of Energy, http://www.nrel.gov/docs/fy16osti/65023.pdf, 2015, vol. NREL/TP-6A20-65023, ch. NREL/TP-6A20-65023.

[4] S. M. Alia, B. Rasimick, C. Ngo, K. C. Neyerlin, S. S. Kocha, S. Pylypenko, H. Xu and B. S. Pivovar, Journal of The Electrochemical Society, 2016, 163, F3105-F3112.

[5] S. M. Alia, Y. S. Yan and B. S. Pivovar, Catalysis Science & Technology, 2014, 4, 3589-3600.