# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 5 Metal-Air Batteries  
A single review cannot cover in detail all kinds of EESs utilizing ENMs. In the following, metal-air batteries (MABs) are briefly mentioned, even if, for sure, they are not the unique EES that would deserve to be presented, besides LIBs, SIBs, and RFBs. Thanks to the affordable cost, extremely high theoretical energy density, environmental friendliness, and safety, MABs are considered as promising next-generation EESs for portable, mobile, or stationary applications [157]. They employ a negative electrode composed of a light metal such as lithium, sodium, aluminum, iron, or zinc, and a positive electrode, which takes oxygen from atmospheric air to generate energy. Several exhaustive review papers on MABs are available in the literature (see f.i. [158; 159; 160] just to cite a few of them). In spite of their huge potential, some drawbacks, such as large overpotentials associated with charge and discharge processes, still hinder their commercialization. Relevant efforts have been devoted to the development of inexpensive and highly effective bifunctional cathode catalysts able to promote both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR), which directly govern the MAB rechargeability. Up to now, materials based on precious metals (platinum or iridium) supported on carbon are still regarded as the best ORR and OER catalysts [161], but their high prices represent a serious concern in view of the use on a large-scale. In the last few years, several alternative materials including graphene [162], transition-metal oxides (e.g., perovskites and spinels) [163; 164], and porous carbon-based materials [165] have been evaluated as bifunctional catalysts. Electrospinning that allows for the preparation of porous graphite-like carbon fibers embedding or supporting metal and metal oxide NPs strongly interacting with the carbonaceous matrix has received great attention and has become a popular and efficient technique widely adopted to produce bifunctional catalysts [161; 166; 167; 168; 169; 170; 171; 172; 173]. Just to cite some examples, [PERSON] et al. [173] have recently designed highly stable bifunctional catalysts for air electrodes consisting of a combination of metallic cobalt and cobaltous oxide NPs finely dispersed on the surface of ECNFs. The Co/CoO/ECNFs outperform the state-of-the-art catalysts in the O\({}_{2}\) evolution. Also, self-standing Co\({}_{3}\)O\({}_{4}\)/ECNF mats prepared by [PERSON] et al. [167], by using PAN and zeolite imidazolate frameworks as precursors, exhibit superior performance as a cathode for MABs. Besides, neither binder nor conductive metal foam is required for their use. Churros-like metal-free nitrogen-doped ECNFs, featured by internal nanochannels, have been obtained by [PERSON] et al. [166] from a bicomponent polymer blend, with PAN and PS respectively acting as carbon-source and pore-forming sacrificial agent, as frequently reported also for different applications [153]. The ORR activity of these fibers strongly depends on the temperature at which carbonization is operated (800-1100 \({}^{\circ}\)C), which affects not only the specific surface area but also the concentration of beneficial surface graphitic-N species. Electrospinning has then been utilized to produce the perovskite component of a hybrid bifunctional catalyst consisting of La\({}_{0.5}\)Sr\({}_{0.5}\)Co\({}_{0.8}\)Fe\({}_{0.2}\)O\({}_{3}\) porous nanorods (LSCF-PR) and nitrogen-doped reduced graphene oxide (NRGO), active towards both ORR and OER [161]. As schematically depicted in Figure 18a, the LSCF-PR/NRGO composite is obtained by dispersing in Nafion/ethanol solution LSCF-PR synthesized via electrospinningning followed by calcination and GO reduced and doped with nitrogen through exposure to NH\({}_{3}\) atmosphere at high temperature. LSCF-PR (Figure 18a,e), uniformly distributed throughout the NRGO sheets (Figure 18d), is responsible for the most of OER activity, whereas the NRGO component (Figure 18c) primarily contributes to the ORR activity.