Effect of Co Addition in Amorphous Ni-Based Alloys for the Alkaline Oxygen Evolution Reaction

Abstract

Traditional alkaline water electrolysis has been successful industrially because of the use of low cost, stable, nickel based materials as electrocatalysts. The major drawback to these systems is that they use aqueous KOH as the electrolyte which is not desirable from a corrosion and packaging perspective. The use of novel anion exchange membranes (AEM) in alkaline water electrolysers is now of particular interest since it eliminates the aqueous KOH electrolyte and can provide form factors and much higher current densities that rival PEM-based electrolysers. Conventional Ni electrocatalysts for the oxygen evolution reaction (OER) still display large overpotentials and slower reaction kinetics. The addition of Co to Ni-based alloys has been noted to increase the activity by stabilizing the β-NiOOH phase over γ-NiOOH, but this often leads to an increase in the oxygen overpotential [1]. The issue of increased overpotential can be mitigated with the use of amorphous structures to reduce oxygen overpotentials with the addition of Co [2]. This previous research has been limited to thin films which are not ideal for catalysts in AEM water electrolysis [2]. In this work, amorphous Ni79.2-xCoxNb12.5Y8.3 (x = 0, 5, 10 at.% Co) are synthesized using mechanical alloying as a means to produce high surface area powders. The effect of milling temperature and time on the resulting powder microstructure was characterized using a combination of x-ray diffraction and electron microscopy. Performing mechanical alloying at cryogenic temperatures resulted in an amorphous microstructure for alloys containing 0, 5, and 10 at.% Co upon 6 hours of milling. The role of Co on the alkaline oxygen evolution reaction was investigated by cyclic voltammetry and Tafel measurements. Tafel plots for the amorphous alloys, along with crystalline Ni can be seen in Fig. 1 with the corresponding kinetic properties being summarized in Table 1. These results show that all of the amorphous alloys possessed a lower Tafel slope compared to crystalline Ni while the amount of Co within these amorphous alloys had little effect on Tafel values. The kinetic behaviour of the amorphous alloys towards the OER was a function of the amount of Co. When 5 at.% Co was added to Ni79.2-xCoxNb12.5Y8.3, the overpotential decreased by 30 mV with a three-fold increase in exchange current density. In contrast, when 10 at.% Co was added, only a minor decrease in overpotential was observed accompanied by a nearly six-fold increase in exchange current density. These features, along with the high surface area achieved during mechanical alloying, provide a cost-effective and simple method for producing stable and active electrocatalysts for the oxygen evolution reaction in AEM water electrolysis. [1] T. N. Lambert, J. A. Vigil, S. E. White, D. J. Davis, S. J. Limmer, P. D. Burton, E. N. Coker, T. E. Beechem and M. T. Brumbach, Chem. Commun., 51(46), 9511 (2015) [2] R. D. L. Smith, M. S. Prévot, R. D. Fagan, S. Trudel and C. P. Berllinguette, J. Am. Chem. Soc., 135(31), 11580 (2013) Figure 1