Non-precious metal-based electrospun carbon nanofibers as air-electrodes for metal-air batteries

nerative fuel cells and metal-air batteries [1]. These devices represent a promising future for a clean and sustainable conversion and storage of energy. Regenerative fuel cells produce H2 and O2 when acting as electrolyser, and provide electricity when acting as fuel cell (being fed with the produced H2) [1]. On the other hand, metal-air batteries are envisaged as next-generation batteries, with extraordinary high energy densities (depending on the metal used, Li, Na, Zn, Al, Mg, Fe...), among other advantages [2,3]. Furthermore, the use of an alkaline medium as the electrolyte allows the use of non-precious metals, reducing the cost in comparison to the devices using acidic electrolytes (fuel cells and electrolysers usually employ Pt, Pd, Ir, Ru, Au...) [1]. In the last few years, research was focused on the development of highly efficient oxygen reduction/evolution catalysts based on transition metals, such as Co, Fe, Mn, La, etc, in the form of mixed oxides (perovskites, spinels, etc.) [4], or advanced carbon materials [5]. Co-based catalysts have been thoroughly investigated for the ORR/OER, and, usually, Co3O4 is the most employed formulation [6,7]. However, the electrical conductivity of bare Co3O4 is not appropriate for the application in most of these devices. Ni and other transition metals can partially replace Co atoms in the Co3O4 spinel structure, improving its electronic conductivity while promoting oxygen evolution [8,9]. Another approach is to combine these oxides with carbon nanostructures, in particular graphitic ones, in order to maintain suitable stability under cycling operation. In fact, composite catalysts based on transition metal oxides and carbon have received increasing attention owing to their synergistic catalytic activity. Recently, carbon nanofibers (CNF) synthesized by electrospinning, modified with a combination of cobalt oxide and metallic cobalt (CoO-Co/CNF) [10] or loaded with nickel and cobalt (both in the metallic and oxide forms) [11], were investigated in our laboratories as bifunctional air electrodes showing good reversibility and stability. In the present work, the latter catalyst (NiCo-loaded CNFs [11]), after the electrospinning preparation, has been oxidized with the aim of synthesizing the pure spinel structure (NiCo2O4) supported on CNFs. The electrochemical behavior of the spinel has been compared with the previously developed catalyst to elucidate the catalytic activity of the two systems for ORR and OER in relation with their physico-chemical properties.