A very practical, 3D air electrode architecture based on a bifunctional cobalt oxide nanowire array that is active towards both the oxygen reduction and oxygen evolution reactions is introduced. This advanced carbon‐ and binder‐free electrode exhibits remarkable electrochemical durability, with 600 h of operation under ambient conditions in a rechargeable zinc‐air battery.

Free-standing layer-by-layer assembled hybrid graphene-MnO2 nanotube (NT) thin films were prepared by an ultrafiltration technique and studied as anodes for lithium ion batteries. Each thin layer of graphene provides not only conductive pathways accelerating a conversion reaction of MnO2 but also buffer layers to maintain electrical contact with MnO2 NT during lithium insertion/extraction. In addition, the unique structures of the thin film provide porous structures that enhance Li ion diffusion into the structure. The graphene-MnO2 NT films as anode present excellent cycle and rate capabilities with a reversible specific capacity based on electrode composite mass of 495 mAh/g at 100 mA/g after 40 cycles with various current rates from 100 to 1600 mA/g. On the contrary, graphene-free MnO2 NT electrodes demonstrate only 140 mAh/g at 80 mA/g after 10 cycles. Furthermore, at a high current rate of 1600 mA/g, the charge capacity of graphene-MnO2 NT film reached 208 mAh/g.

Advanced morphology of intertwined core-corona structured bifunctional catalyst (IT-CCBC) is introduced where perovskite lanthanum nickel oxide nanoparticles (LaNiO3 NP) are encapsulated by high surface area network of nitrogen-doped carbon nanotubes (NCNT) to produce highly active and durable bifunctional catalyst for rechargeable metal-air battery applications. The unique composite morphology of IT-CCBC not only enhances the charge transport property by providing rapid electron-conduction pathway but also facilitates in diffusion of hydroxyl and oxygen reactants through the highly porous framework. Confirmed by electrochemical half-cell testing, IT-CCBC in fact exhibits very strong synergy between LaNiO3 NP and NCNT demonstrating bifunctionality with significantly improved catalytic activities of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Furthermore, when compared to the state-of-art catalysts, IT-CCBC outperforms Pt/C and Ir/C in terms of ORR and OER, respectively, and shows improved electrochemical stability compared to them after cycle degradation testing. The practicality of the catalyst is corroborated by testing in a realistic rechargeable zinc-air battery utilizing atmospheric air in ambient conditions, where IT-CCBC demonstrates superior charge and discharge voltages and long-term cycle stability with virtually no battery voltage fading. These improved electrochemical properties of the catalyst are attributed to the nanosized dimensions of LaNiO3 NP controlled by simple hydrothermal technique, which enables prolific growth of and encapsulation by highly porous NCNT network. The excellent electrochemical results presented in this study highlight IT-CCBC as highly efficient and commercially viable bifunctional catalyst for rechargeable metal-air battery applications.

No abstract

Perovskite oxides (ABO 3 ) have recently attracted attention since tailoring their chemical compositions has resulted in remarkable activity toward oxygen evolution reaction (OER) which governs rechargeability of recently spotlighted metal−air batteries and regenerative fuel cells. For further development of highly OER active perovskite oxides, however, the exact mechanism the OER must be well understood. Herein, we introduce investigation of the OER mechanism of perovskite oxides by ab initio analysis based on well-defined model systems of LaMnO 3 (LMO), LaCoO 3 (LCO), and La 0.5 Sr 0.5 CoO 3 (LSCO). In addition, we have systematically conducted electrochemical experiments from which we have observed an increasing trend in the OER activity in the order of LSCO > LCO > LMO based on the cyclic voltammetry (CV) results obtained in the alkaline medium. To validate the experimental results, free-energy diagrams have been constructed for oxygen intermediates on the surface of the defined models to find the limiting step by changing the B site atom (e.g., Mn and Co) and the partial displacement of Sr atoms in La site. The oxygen adsorption energy of perovskite oxides is found to increase with decreasing number of outer electrons as well as upshifting of the position of the d z 2 orbital toward the Fermi level of B site element. This work demonstrates that highly active OER perovskite oxides can be obtained by modifying the chemical composition to finely tune the oxygen adsorption energy on the catalyst's surface, confirmed by synergetic approaches of using both experimental and ab initio computational studies. KEYWORDS: metal−air battery, oxygen evolution reaction, perovskite oxide, electrocatalyst, density functional theory, ab initio calculation D evelopment of highly active electrocatalysts that are cost competitive takes the center stage in research fields for next-generation electrochemical energy conversion and storage systems. 1−5 These systems have extremely important environmental implications on reducing global warming and facilitating sustainable energy generation. 1−3 Among many electrochemical processes, oxygen evolution reaction (OER) is a significantly important process since it directly governs rechargeability of recently spotlighted metal-air batteries and regenerative fuel cells. 1−3 However, the intrinsically sluggish kinetics of OER significantly limits performance of these advanced energy devices. 6,7 Perovskite oxide, which has the chemical formula ABO 3 consisting of rare earth and alkaline earth in the A site and 3d transition metal in the B site, has drawn tremendous attention due to its remarkable OER activity that is comparable to that of precious metal based catalysts such as iridium and ruthenium supported on carbon. 8,9 Despite their excellent electrocatalytic activity, its origin is still unclear, which must be clearly understood in order to further improve the catalysts. Due to the lack of clear mechanistic understanding, current catalyst research largely progresses in a trial-and-error ...