Novel Ni-Based Bifunctional Oxygen Catalysts for Metal Air Batteries and Alkaline Fuel Cells

In recent years there has been a marked increased interest in alkaline based energy storage and conversion systems such as anion exchange membrane fuel cells and metal air batteries. Both technologies rely on the reduction and/or evolution of oxygen. The slow kinetics of the oxygen reduction (ORR) and evolution reactions (OER) in alkaline environments necessitate the use of electro catalysts to increase the reaction rate and increase cell efficiency. Precious metals are most commonly used due to their high activities and relative stabilities.

There has been an extensive search for non-precious metal catalysts (NPMCs) that demonstrate comparable activity and stability for ORR and OER in alkaline environments, but at lower costs. Among the families of materials studied are precious metals, transition metal oxides, transition metal minerals (perovskites, spinels, pyrochlores, etc.), and organometallic complexes. The latter is the focus of this work.

Organometallic NPMCs are generally synthesized by pyrolysis of a combination of metal salts, a nitrogen source (macromolecules or reactive gas) and a carbon support. State of the art NPMCs are based on Fe and Co and are synthesized in elaborate multi-step processes combining high temperature treatment (900^oC to 1050^oC) with acid wash or reactive gas (NH₃) pyrolysis¹. These catalysts have proven to have good ORR activity but limited durability.

In this study we report the synthesis and characterization of a new class of NPMCs for ORR and OER based on organometallic complexes of nickel (Ni) and bimetallic complexes of nickel with cobalt (Co), copper (Cu), or iron (Fe). The complexes comprised of metals immobilized by ligands, covalently attached to a carbon black, yielding a phthalocyanine-like molecule.

The as-synthesized catalysts show low catalytic activity for the oxygen reduction reaction when compared to Pt. However, after a one step pyrolysis at 700^oC in inert atmosphere, the activity of catalysts was found to improve dramatically. The NiFe bimetallic catalyst had an onset potential of 1.03V vs. RHE (measured at 50µA/cm²). The as-synthesized catalysts generally demonstrate decent activity for the oxygen evolution reaction, vastly surpassing platinum. The activity was also found to increase after pyrolysis.

The samples were characterized using fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), thermo gravimetric analysis – mass spectrometry (TGA-MS) and x-ray powder diffraction (XRD). RRDE experiments were used to study the effect of catalyst loading, oxygen concentration, to determine the ORR reaction order, and catalyst stability. Optimization methods to enhance the catalytic performance will also be described. In situ testing of catalysts in Zinc-air batteries was conducted and the cell performance was compared to precious metals and perovskites.

Acknowledgements:

We would like to thank the EPSCORE program for providing funding for this work.

Figure 1: RDE voltammogram of Ni TrPc catalysts for ORR and OER compared to platinum (25µg Pt/cm²)

References:

1. X. Li, B. N. Popov, T. Kawahara, H. Yanagi. J. Power Sources 196 (2011) 1717-1722