Electrodeposition of small sized NiM2O4 spinels (M: Co, Mn) as bifunctional nanomaterials for rechargeable zinc–air batteries

In this work, the electrodeposition conditions, and the variation of the second metal of Nibased spinels were investigated with the aim of increasing the Zn-Air battery performance/rechargeability. SEM micrographs revealed that nanosheets were obtained for NiCo2O4 and NiMn2O4, while TEM showed that these structures were composed of nanoparticles with sizes of 5.7 and 5.9 nm. TEM also revealed the presence of many surface defects like vacancies, located amorphousness, dislocations, and lattice expansions. For ORR, the NiMn2O4 spinel had a similar current density than Pt/C with a halfwave potential difference of only 60 mV. For the OER, the NiCo2O4 spinel presented the same onset potential as the benchmarked IrO2/C, having a potential difference to achieve 10 mA cm−2 of only 40 mV. In the ZAB, both spinels presented a similar battery voltage (1.3 V), while power densities of 52 and 72 mW cm−2 were found using NiCo2O4 and NiMn2O4 spinels, respectively. Therefore, highly active bifunctional materials were obtained by defects engineering through an easy synthesis method, where the activity was also analyzed by theoretical calculations, indicating that the activity improvement can be related to a superior density of states of the with a higher degree of hybridization of transition metals into the spinel structure.