Fe-Ni Core-Shell Hydroxide Nanoparticles As an Active Oxygen Evolution Reaction (OER) Catalyst

Hydrogen production through the electrolysis of water continues to be a key next-generation energy conversion and storage strategy with advantages including minimal greenhouse gas emissions, the lack of fossil fuels, and the potential as an alternative fuel source for fuel cells and reactant for industries such as ammonia production. Recent work by the Boettcher research group and others has demonstrated that iron incorporation into nickel hydroxide based catalysts can significantly enhance the activity of the catalyst for the oxygen evolution reaction (OER) [1-3], which is the half reaction of water electrolysis known to limit the overall process with high overpotential and slow kinetics. With these recent developments in understanding the role of iron incorporation into nickel hydroxide and the importance of iron as the catalytic site for OER within these bimetallic electrocatalysts [1], there is now an opportunity to further develop nanostructured bimetallic iron-nickel hydroxide catalysts for improved OER performance in an alkaline electrochemical environment.

In this presentation, our results on the synthesis, characterization, and electrochemical testing of an iron-nickel core-shell hydroxide nanoparticle catalyst will be presented. Bimetallic iron-nickel nanoparticles were synthesized using a multi-step procedure in water under ambient conditions. When compared to monometallic iron and nickel nanoparticles, the Fe-Ni nanoparticles show enhanced catalytic activity for OER under alkaline conditions (1 M NaOH). The bimetallic nanoparticles demonstrated an improvement in OER overpotential as well as a significant increase in maximum measured current density, as compared to the monometallic iron and nickel nanoparticles. At 1 mA/cm², the overpotential for the monometallic iron and nickel nanoparticles was 421 mV and 476 mV, respectively, while the bimetallic Fe-Ni nanoparticles had a greatly reduced overpotential of only 256 mV. At 10 mA/cm², bimetallic Fe-Ni nanoparticles had an overpotential of 311 mV. Electron microscopy and elemental analysis results will be presented with a detailed discussion of unique aspects of the FeNi nanoparticle catalyst. Results suggest that while the nanoparticles are nominally in a core-shell morphology, there is significant migration of iron into the nickel shell as well as incorporation of some phosphorus into the nanoparticle shell, likely originating from the phosphonate-based stabilizer used during nanoparticle synthesis. X-ray photoelectron spectroscopy suggests that the primary phase of nickel is nickel hydroxide, and x-ray absorption spectroscopy characterization suggests that the primary phase of nickel is the more disordered alpha phase of nickel hydroxide. The presence of a small amount of nickel (oxy)hydroxide is also likely present, based on characterization results.

References

[1] M.S. Burke, L.J. Enman, A.S. Batchellor, S.H. Zou, S.W. Boettcher, Oxygen evolution reaction electrocatalysis on transition metal oxides and (oxy)hydroxides: Activity trends and design principles, Chemistry of Materials, 27 (2015) 7549-7558.

[2] M.S. Burke, S.H. Zou, L.J. Enman, J.E. Kellon, C.A. Gabor, E. Pledger, S.W. Boettcher, Revised oxygen evolution reaction activity trends for first-row transition-metal (oxy)hydroxides in alkaline media, Journal of Physical Chemistry Letters, 6 (2015) 3737-3742.

[3] L. Trotochaud, S.L. Young, J.K. Ranney, S.W. Boettcher, Nickel-iron oxyhydroxide oxygen-evolution electrocatalysts: The role of intentional and incidental iron incorporation, J. Am. Chem. Soc., 136 (2014) 6744-6753.