(141e) Alkaline Hydrogen Evolution from Ni–Mo Intermetallics with High Mo Content

The transition from acidic to alkaline conditions promises considerable cost savings for water electrolysis systems based on polymer ion-exchange membrane electrolytes. Binary transition metal alloys containing Ni and Mo have been known for decades to exhibit excellent performance as alkaline water electrolyzer cathodes,¹ but the specific reason that these alloys are more active toward the hydrogen evolution reaction (HER) than either Ni or Mo alone remains the subject of some considerable debate.² Moreover, previously reported mass-specific activities for Ni–Mo alloys are still at least 100-fold lower than that of Pt, which makes them difficult to deploy in a practical electrolysis system that operates at suitably high energy efficiency and power density.

Prior work on Ni–Mo electrocatalysts has suggested that the alloy phase exhibits monotonically increasing HER activity as the bulk Mo mole fraction is increased.³ However, the accessible composition range for binary Ni–Mo alloys is constrained by the solid solubility of Mo in the Ni fcc lattice, which is only ~25 mol% at 1000 °C. Surface-science experiments on bulk Ni_0.8Mo_0.2 alloys have indicated that the topmost atomic layer becomes considerably enriched in Mo, up to approximately equimolar Mo and Ni.⁴ Thus, the question remains as to how much Mo is present in the most active Ni–Mo binary HER electrocatalyst surface, let alone what is the mechanism for enhanced activity.

In this presentation, I will discuss recent and ongoing work aimed at understanding the influence of bulk composition on the activity of nanostructured Ni–Mo alkaline HER catalysts.⁵ We synthesized unsupported powders of Ni, Ni_0.92Mo_0.08 alloy, and the intermetallic compound Mo₇Ni₇. Mass-specific activity was found to be highest for the 8 mol% Mo alloy, but these data were clearly convoluted by catalyst morphologies, as the 50 mol% Mo intermetallic exhibited very low surface roughness. Attempts to account for electrochemically active surface area using a straightforward method based on electrochemical impedance spectroscopy (EIS) yielded unexpected results that conflicted with morphological analysis from electron microscopy. We have therefore postulated that Mo undergoes a pseudocapacitive redox transition near the onset potential for the HER, leading to erroneous results from EIS measurements. These results further implicate a plausible bifunctional catalytic mechanism that explicitly involves Mo redox transitions.

(1) Brown, D. E.; Mahmood, M. N.; Man, M. C. M.; Turner, A. K. Electrochimica Acta 1984, 29 (11), 1551–1556.

(2) McKone, J. R.; Marinescu, S. C.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Chem. Sci. 2014, 5 (3), 865–878.

(3) McKone, J. R.; Sadtler, B. F.; Werlang, C. A.; Lewis, N. S.; Gray, H. B. ACS Catal. 2013, 3 (2), 166–169.

(4) Soriaga, M. P.; Baricuatro, J. H.; Cummins, K. D.; Kim, Y.-G.; Saadi, F. H.; Sun, G.; McCrory, C. C.; McKone, J. R.; Velazquez, J. M.; Ferrer, I. M.; I, C., Azhar. Surf. Sci. 2015, 631, 285–294.

(5) Csernica, P. M.; McKone, J. R.; Mulzer, C. R.; Dichtel, W. R.; Abruña, H. D.; DiSalvo, F. J. ACS Catal. 2017, 3375–3383.