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Large-scale electrical energy storage with high energy density and round-trip efficiency is important to the resilience of power grids and the effective use of intermittent renewable energy such as solar and wind. Lithium-oxygen battery, due to its high energy density, is believed to be one of the most promising energy storage systems for the future. However, large overpotentials, poor cycling stability, and degradation of electrolytes and cathodes have been hindering the development of lithium-oxygen batteries. Numerous heterogeneous oxygen electrocatalysts have been investigated to lower the overpotentials and enhance the cycling stability of lithium-oxygen batteries. Unfortunately, the prevailing issues of electrode passivation and clogging remain. Over the past few years, redox mediators were explored as homogenous catalysts to address the issues, while only limited success has been achieved for these soluble catalysts. In conjunction with a flowing electrolyte system, a new redox flow lithium-oxygen battery (RFLOB) has been devised to tackle the aforementioned issues. The working mechanism and schematic processes will be elaborated in this review. In addition, the performance gap of RFLOB with respect to practical requirements will be analysed. With the above, we anticipate RFLOB would be a credible solution for the implementation of lithium-oxygen battery chemistry for the next generation energy storage. Keywords: Lithium-air battery, Redox catalysis, Oxygen reduction reaction, Oxygen evolution reaction, Redox flow cell

Neutral chloride-based electrolyte and directly grown manganese oxide on carbon paper are used as the electrolyte and air cathode respectively for rechargeable Zn-air batteries. Oxygen reduction and oxygen evolution reactions on manganese oxide show dependence of activities on the pH of the electrolyte. Zn-air batteries with chloride-based electrolyte and manganese oxide catalyst exhibit satisfactory voltage profile (discharge and charge voltage of 1 and 2 V at 1 mA cm−2) and excellent cycling stability (≈90 days of continuous cycle test), which is attributed to the reduced carbon corrosion on the air cathode and decreased carbonation in neutral electrolyte. This work describes a robust electrolyte system that improves the cycle life of rechargeable Zn-air batteries.

The development of rechargeable Li–air batteries has been confronted by the critical challenges of large overpotential loss, low achievable capacity, and prohibitively poor cycling and power performance. Surface passivation and pore clogging of the cathode due to the formation of Li2O2 during discharge result in sluggish interfacial charge transfer and have an impact on the mass transport of Li+ ions and O2 in the electrode, consequently giving rise to large voltage hysteresis and premature termination of discharge with low power performance. Here we report a redox flow lithium–oxygen cell with a modified redox electrolyte to tackle these issues. With the assistance of redox mediators, the cell presents substantially enhanced power performance in O2 and dry air during discharge. Through in situ spectroelectrochemical measurements and theoretical calculations, an oxygen reduction intermediate was unequivocally identified. By judiciously optimizing the redox electrolyte, the cell operates at near complete utilization of Li metal upon multiple refueling. The redox flow lithium–oxygen cell demonstrated here is envisaged to provide a pragmatic approach for the implementation of lithium–oxygen battery chemistry and to pave the way for advanced large-scale energy storage.

The oxygen reduction reaction (ORR) is an important electrode reaction for energy storage and conversion devices based on oxygen electrocatalysis. This paper introduces the thermodynamics, reaction kinetics, reaction mechanisms, and reaction pathways of ORR in aqueous alkaline media. Recent advances of the catalysts for ORR were extensively reviewed, including precious metals, nonmetal-doped carbon, carbon–transition metal hybrids, transition metal oxides with spinel and perovskite structures, and so forth. The applications of those ORR catalysts to zinc–air batteries and alkaline fuel cells were briefly introduced. A concluding remark summarizes the current status of the reaction pathways, advanced catalysts, and the future challenges of the research and development of ORR.

Bifunctional electrocatalysts for rechargeable metal–air batteries often encounter catalyst leaching-resultant performance degradation upon cycling of the batteries, which requires an improvement in stability of the catalyst nanoparticles via immobilization onto conductive supports. Herein, we report in situ growth of Co3O4 nanoparticles onto concurrently synthesized N-doped Vulcan carbon (NVC) to produce Co3O4/NVC powders with tuneable loading density as scalable, stable and efficient hybrid bifunctional electrocatalysts. With an optimized composition the hybrid catalyst exhibited satisfactory ORR and OER activity, giving a voltage difference as small as 0.10 V between the onset potential and half-wave potential at discharge. The good performance of the rechargeable zinc–air batteries constructed using Co3O4/NVC as air-cathodes suggests such a hybrid bifunctional electrocatalyst is a practical and cost-effective solution for applications which demand a large quantity of materials, e.g. in grid-scale energy storage and electric vehicles.

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential reactions for energy-storage and -conversion devices relying on oxygen electrochemistry. High-performance, nonprecious metal-based hybrid catalysts are developed from postsynthesis integration of dual-phase spinel MnCo2O4 (dp-MnCo2O4) nanocrystals with nanocarbon materials, e.g., carbon nanotube (CNT) and nitrogen-doped reduced graphene oxide (N-rGO). The synergic covalent coupling between dp-MnCo2O4 and nanocarbons effectively enhances both the bifunctional ORR and OER activities of the spinel/nanocarbon hybrid catalysts. The dp-MnCo2O4/N-rGO hybrid catalysts exhibited comparable ORR activity and superior OER activity compared to commercial 30 wt % platinum supported on carbon black (Pt/C). An electrically rechargeable zinc–air battery using dp-MnCo2O4/CNT hybrid catalysts on the cathode was successfully operated for 64 discharge–charge cycles (or 768 h equivalent), significantly outperforming the Pt/C counterpart, which could only survive up to 108 h under similar conditions. ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

Near-neutral electrolytes based on zinc chloride and ammonium chloride are examined for rechargeable zinc-air battery application.TheeffectsofpHvalue,saltconcentration,andpolyethyleneglycolandthioureaadditivesareinvestigatedandachlorideelectrolyteisdeveloped.Thereversiblezincdepositionandzincstrippingprocessesarestudiedbycyclicvoltammetrywithrotating-discelectrodetechnique. The zinc anode and air cathode behaviors in near-neutral chloride electrolyte are characterized by quasi steady-statepolarization and impedance spectroscopy. Prototyped zinc-air battery with near-neutral chloride electrolytes can sustain more than1000 hours and hundreds of discharge-charge cycles with minimized zinc dendrite formation and no carbonate formation problem,underdischarge-chargecapacityrangingfrom20to120mAh.Thenear-neutralchlorideelectrolyteprovidesasaferandmorerobustalternative to traditional alkaline electrolyte for rechargeable zinc-air batteries.© The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative CommonsAttribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in anymedium, provided the original work is properly cited. [DOI: 10.1149/2.0311414jes] All rights reserved.Manuscript submitted June 17, 2014; revised manuscript received August 27, 2014. Published October 1, 2014.

In this paper, we report the synthesis, characterization and application of an inexpensive yet efficient bifunctional catalyst composed of Ag nanocrystals (∼11 nm) anchored on α-MnO2 nanorods. The nanostructured Ag–MnO2 catalysts exhibit improved oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performance in aqueous alkaline media, in terms of onset potential, generated current density and Tafel slopes. Rotating disk electrode results show that near-four electrons per oxygen molecule were transferred during ORR of Ag–MnO2. A zinc–air battery prototype employing Ag–MnO2 in the air electrode was successfully operated for 270 cycles under light discharge–charge condition. Ag–MnO2 is an efficient bifunctional catalyst for electrochemical devices such as metal–air batteries and alkaline fuel cells.

Durable electrocatalysts with high catalytic activity toward oxygen reduction reaction (ORR) are crucial to high-performance primary zinc-air batteries (ZnABs) and direct methanol fuel cells (DMFCs). An efficient composite electrocatalyst, Co@Co3 O4 core@shell nanoparticles (NPs) embedded in pyrolyzed polydopamine (PPD) is reported, i.e., in Co@Co3 O4 @PPD core@bishell structure, obtained via a three-step sequential process involving hydrothermal synthesis, high temperature calcination under nitrogen atmosphere, and gentle heating in air. With Co@Co3 O4 NPs encapsulated by ultrathin highly graphitized N-doped carbon, the catalyst exhibits excellent stability in aqueous alkaline solution over extended period and good tolerance to methanol crossover effect. The integration of N-doped graphitic carbon outer shell and ultrathin nanocrystalline Co3 O4 inner shell enable high ORR activity of the core@bishell NPs, as evidenced by ZnABs using catalyst of Co@Co3 O4 @PPD in air-cathode which delivers a stable voltage profile over 40 h at a discharge current density of as high as 20 mA cm(-2) .

The increasing global energy demand and the depletion of fossil fuels have stimulated intense research on fuel cells and batteries. Oxygen electrocatalysis plays essential roles as the electrocatalytic reduction and evolution of di-oxygen are always the performance-limiting factors of these devices relying on oxygen electrochemistry. A novel perovskite with the formula La(Co0.55Mn0.45)0.99O3−δ (LCMO) is designed from molecular orbital principles. The hydrothermally synthesized LCMO nanorods have unique structural and chemical properties and possess high intrinsic activities for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The synergic covalent coupling between LCMO and NrGO enhances the bifunctional ORR and OER activities of the novel LCMO/NrGO hybrid catalyst. The ORR activity of LCMO/NrGO is comparable to the state-of-the-art Pt/C catalyst and its OER activity is competitive to the state-of-the-art Ir/C catalyst. LCMO/NrGO generally outperforms Pt/C and Ir/C with better bifunctional ORR and OER performance and operating durability. LCMO/NrGO represents a new class of low-cost, efficient and durable electrocatalysts for fuel cells, water electrolysers and batteries. ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

Manganese oxide is grown directly on carbon paper through a simple immersion process, and used as a catalyst-modified air cathode for rechargeable zinc–air batteries. The manganese oxide is distributed evenly within the porous carbon paper, which promotes a rapid three-phase reaction and high utilization of the active materials. Zinc–air batteries with the manganese oxide catalyst directly grown on the carbon paper exhibit improved performance compared with zinc–air batteries fabricated by using manganese oxide powder catalyst coated on carbon paper. The directly grown catalyst reduces the contact resistance and enhances the discharge/charge profile of the zinc–air batteries. Zinc–air batteries with the directly grown catalyst show a discharge voltage of 1.2 V at a current density of 15 mA cm−2 and deliver a power density as high as 108 mW cm−2 at an applied current of 168 mA cm−2. Furthermore, good cycling stability for up to 500 cycles is achievable during continuous discharge–charge tests without the need to replace the zinc anode or replenish the electrolyte; this outperforms most currently available bifunctional catalysts for rechargeable zinc–air batteries. This study illustrates a promising platform to enhance the cycle life of rechargeable metal–air batteries.

An efficient, durable and low cost air-cathode is essential for a high performance metal-air battery for practical applications. Herein, we report a composite bifunctional catalyst, Co3O4 nanoparticles-decorated carbon nanofibers (CNFs), working as an efficient air-cathode in high performance rechargeable Zn-air batteries (ZnABs). The particles-on-fibers nanohybrid materials were derived from electrospun metal-ion containing polymer fibers followed by thermal carbonization and a post annealing process in air at a moderate temperature. Electrochemical studies suggest that the nanohybrid material effectively catalyzes oxygen reduction reaction via an ideal 4-electron transfer process and outperforms Pt/C in catalyzing oxygen evolution reactions. Accordingly, the prototype ZnABs exhibit a low discharge-charge voltage gap (e.g. 0.7 V, discharge-charge at 2 mA cm(-2)) with higher stability and longer cycle life compared to their counterparts constructed using Pt/C in air-cathode. Importantly, the hybrid nanofiber mat readily serves as an integrated air-cathode without the need of any further modification. Benefitting from its efficient catalytic activities and structural advantages, particularly the 3D architecture of highly conductive CNFs and the high loading density of strongly attached Co3O4 NPs on their surfaces, the resultant ZnABs show significantly improved performance with respect to the rate capability, cycling stability and current density, promising good potential in practical applications.