Towards Active and Stable Bifunctional NiCo2O4 Catalysts for O2 Evolution and Reduction in Alkaline Media
Authors: Duarte Ricardo , Rao Reshma , Durrant James , Stephens Ifan , Ryan Mary , M. Bonastre Alejandro , Sharman Jonathan 
Published: December 2020

It is particularly challenging to find a bifunctional catalyst capable of accelerating both the O2 evolution reaction (OER) and O2 reduction reaction (ORR) in aqueous solutions. The discovery of such a material would bring reversible fuel cells and metal air batteries much closer to technological fruition. However, in a real device, such a catalyst will need to retain its activity across an enormous potential window of at least 1 V over several years. To the best of our knowledge, little is known about the factors controlling the stability of bifunctional catalysts.

In this contribution, we investigate the electrochemical activity and stability of a NiCo2O4 catalyst, synthesized at Johnson Matthey as part of EU funded project FLOWCAMP. Using post-mortem techniques, such as inductively coupled plasma mass spectrometry (ICP-MS), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), we provide experimental evidence about catalyst deactivation mechanisms. Results will also include a spectroelectrochemical in-situ UV-Vis study, that provides insights to the reaction mechanism for the OER. Lastly, the activity and stability of NiCo2O4 catalysts is also evaluated in oxygen-electrode prototypes and characterized in a half-cell configuration.

In figure 1 is shown the ORR and OER activity of NiCo2O4 before and after accelerated degradation tests (ADT), in rotating disk electrode (RDE) configuration, employing three different electrochemical potential windows: i) 0.6 VRHE - 1 VRHE; ii) 0.6 VRHE - 1.7 VRHE; and iii) VRHE - 1.7 VRHE. As seen by the blue lines, significant ORR deactivation is observed after cycling across both ORR and OER potentials, whereas a 6% decrease in ORR current at 0.7 VRHE is observed after cycling for 1 hour within the ORR potential window. At the same time, degradation is accompanied by significant redox peak changes, as observed in the inset of Figure 1, which could indicate structural changes occurring during the OER. Post-mortem ICP-MS results indicate that catalyst dissolution accompanies this transformation during the OER. Alongside, in-situ UV-vis experiments identified changes in redox species as a function of applied potential, which are further correlated with the electrochemical current to determine the reaction order with respect to the amount of surface oxidized states.

In this presentation, I will clarify the factors that control the activity and stability of NiCo2O4 catalysts, by coupling electrochemical measurements with in-situ UV-vis, and post mortem characterization techniques. In my talk, I will focus on understanding the causes for deactivation of the ORR current, which will aid further design of stable ORR and OER catalysts in alkaline media.

A
cknowledgements: This work is supported by the project ’’FLOWCAMP’’ (www.flowcamp-project.eu) under the European union’s innovation training network (grant agreement 765289).

[1] L. Francàs, S. Corby, S. Selim, L. Dongho, C. A. Mesa, R. Godin, E. Pastor, I. E. L. Stephens, K. Choi, J. R. Durrant “Spectroelectrochemical study of water oxidation on nickel and iron oxyhydroxide electrocatalysts,” Nat. Commun., vol. 10, no. 5208, 2019.

Figure 1