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Fabricating non-noble metal-based carbon air electrodes with highly efficient bifunctionality is big challenge owing to the sluggish kinetics of oxygen reduction/evolution reaction (ORR/OER). The efficient cathode catalyst is urgently needed to further improve the performance of rechargeable zinc-air batteries. Herein, an activation-doping assisted interface modification strategy is demonstrated based on freestanding integrated carbon composite (CoNiLDH@NPC) composed of wood-based N and P doped active carbon (NPC) and CoNi layer double hydroxides (CoNiLDH). In the light of its large specific surface area and unique defective structure, CoNiLDH@NPC with strong interfacecoupling effect in 2D-3D micro-nanostructure exhibits outstanding bifunctionality. Such carbon composites show half-wave potential of 0.85 V for ORR, overpotential of 320 mV with current density of 10 mA cm-2 for OER, and ultra-low gap of 0.70 V. Furthermore, highly-ordered open channels of wood provide enormous space to form abundant triple-phase boundary for accelerating the catalytic process. Consequently, zinc-air batteries using CoNiLDH@NPC show high power density (aqueous: 263 mW cm-2, quasi-solid-state: 65.8 mW cm-2) and long-term stability (aqueous: 500 h, quasi-solid-state: 120 h). This integrated protocol opens a new avenue for the rational design of efficient freestanding air electrode from biomass resources. [ABSTRACT FROM AUTHOR]

Owing to kinetic‐sluggish nature of electrocatalytic oxygen transformation processes, it is pivotal to develop durable and efficient bifunctional air electrode catalysts for fabricating high‐performance Zn–air batteries (ZABs). In this work, oxygen vacancy (Ov) induced Mn(III) sites optimization is achieved via nano‐micro structure modulation. Protonated carbon nitride (p‐C3N4) is applied as a structure‐stiffening module to immobilize α‐MnO2 on N/P‐doped active carbon (NPAC) and induce Ov construction. X‐ray adsorption spectra (XAS) disclose the formation of Ov and Mn(III) sites in MCC, the unit coordination structure is well maintained with the aid of a dual‐carbon strategy. Mn(III) sites efficiently catalyze oxygen reduction/evolution reaction (ORR/OER), MCC shows high half‐wave potential (E1/2) of 0.88 V for ORR and low potential at 10 mA cm−2 (Ej = 10) of 1.64 V for OER. According to density functional theory (DFT) simulations analysis, the gorgeous bifunctional activity is owing to that optimized charge distribution facilitates the intermediates transformation. Aqueous ZABs based on MCC manifests high peak power density of 452 mW cm−2 and durable cycling stability of 1640 h. Quasi‐solid‐state ZABs based on MCC also show satisfactory performances (175 mW cm−2, 105 h). This work provides the route to develop efficient and durable electrocatalyst for constructing ZABs with long lifespan and high‐power‐density. [ABSTRACT FROM AUTHOR]

A facile design and fabrication of self‐standing metal‐free polyaniline (PANI)@carbon nanotubes (CNTs) composite membrane was initially proposed by straightforward noncovalent wrapping the polymer around pure CNTs. Without introduction of extra heteroatoms into CNTs, the optimized PANI@CNTs composite exhibits a much better electrocatalytic performance for oxygen evolution reaction (OER) than pure CNTs via favorable interfacial modification with PANI to largely expose the active sites of on the surface of pure CNTs. Besides, it displays good oxygen reduction reaction (ORR) performance. When directly utilized as bifunctional air electrode without extra additive agents, the composite membrane‐enabled rechargeable Zn‐air batteries not only deliver a high peak power density (201.9 W g−1) and a large energy density (850.3 Wh kgZn−1), but also present robust cycling performance for 216 cycles with a high energy efficiency of 57.8%. [ABSTRACT FROM AUTHOR]

Abstract Unitized regenerative proton exchange membrane fuel cell (UR-PEMFC) technology has progressed in the recent past and has started appearing towards few applications. However, the UR-PEMFC viability is limited by its lower round-trip efficiency mainly due to several reasons such as sluggish air electrode reactions, lower performance/stability, higher materials cost etc. In this context, many approaches are being implemented for efficiency enhancement including design and development of effective bifunctional air electrodes (oxygen reduction and evolution reactions) materials both for fuel cell and electrolyzer modes as well as for optimization of operating condition for performance stability in real life applications. This review focusses on the recent developments of air electrode active materials design/development for performance improvement in UR-PEMFC. Among all developed electrode materials, the catalysts with Pt- and Ir-based metals still provided the maximum round-trip efficiency of about 50% at 500 mA cm−2 in the unit cell. Highlights • Recent developments in bifunctional air electrodes for UR-PEMFC is reviewed. • Limitations of UR-PEMFC are its material high cost and slow kinetic rate of ORR. • Pt- and Ir-based catalysts gave the maximum round-trip efficiency in the unit cell. • Future research of UR-PEMFC should focus on cost-effective and durable catalysts. [ABSTRACT FROM AUTHOR]

Abstract: Rechargeable zinc-air flow batteries are investigated as possible technology for fast responding large-scale electrical energy storage due to the use of inexpensive, non-toxic and abundant materials, and compact system design. The operating ranges for several parameters such as flow rate (2-8 cm s−1), concentration of electrolyte (6 or 8 M KOH), charge/discharge current densities (up to 100 mA cm−2 mean), and active or passive air supply as well as their influence on the performance and stability of the electrodes are investigated and compared. Bi-catalyzed bifunctional air electrodes are tested by means of half-cell measurements achieving minimum 200 charge/discharge cycles at 50 mA cm−2 with the longest operation time being 800 h. At this current density, charge/discharge efficiencies are in the range of 50% for all tested air electrodes. End-of-life characterization by means of scanning electron microscopy reveals mechanical degradation of the electrode material. On the negative zinc electrode, zinc deposition morphology on different current collector materials (nickel, brass, and steel) is investigated using Rota-Hull cylinders showing nickel to be the most suitable material. The pulse interrupt current method is thereby successfully applied for compact zinc deposition in a broad current density range without any electrolyte additive. Subsequent scale-up of the rechargeable zinc-air flow battery and unit cell operation is finally performed for proof-of-concept.Graphical Abstract: [ABSTRACT FROM AUTHOR]

Abstract: Development of secondary zinc–air batteries goes through a proper specification of the electrolyte formulation adapted to extend the cycle life of the battery. However, defining an optimal formulation is not a trivial work due to the specific requirements for each electrode. At half‐cell level, it has been determined that ZnO‐saturated 4 mol L−1 KOH with 2 mol L−1 KF and 2 mol L−1 K2CO3 (4s‐2) is the most suitable formulation to increase the cycle life of secondary zinc electrode, whereas additive‐free 8 mol L−1 KOH (8‐0) formulation is more beneficial for the bifunctional air electrode (BAE). Through this systematic cycle life assessment, it has been found that the most suitable electrolyte formulation for the full cell system is a compendium for both electrodes requirements. It has determined an optimal electrolyte formulation for the full system consisting of ZnO‐saturated 7 mol L−1 KOH with 1.4 mol L−1 KF and 1.4 mol L−1 K2CO3 (7s‐1.4). This electrolyte composition increases at least 2.5 times the reversibility of the secondary zinc–air battery in comparison with that employing the traditional formulation for primary zinc–air batteries (additive‐free 8 mol L−1 KOH). In addition, the development of a proper cell design or separator is also necessary to further enhance the secondary zinc–air cycle life. [ABSTRACT FROM AUTHOR]

Spent lithium ion batteries are potential environmental hazards and if not re-cycled waste natural resources. One of the difficulties for recovery is the recycling cost high and restricting the economic drivers. In this research, a process is developed to directly reuse nickel-manganese-cobalt oxides from spent lithium ion batteries to prepare air electrodes based on a simple thermal treatment method. 100% value metals (cobalt, nickel and manganese) were regenerated from the cathode. By heated at 600 °C, the Ni-Co-Mn oxides experienced a significant phase change from an α-NaFeO 2 -type structure to spinel structure. Electrochemical tests revealed that the regenerated nickel-manganese-cobalt oxides exhibited remarkable bifunctional catalytic activities towards the oxygen evolution and oxygen reduction reactions in aqueous KOH electrolyte. The nickel-manganese-cobalt oxide powders were applied in an air battery, the energy efficiency was 75% at a current density of 10 mA cm −2 , which was better than commercal zinc-air batteries. [ABSTRACT FROM AUTHOR]

An efficient, durable and low cost air cathode with low polarization between the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is essential for a high performance and durable secondary zinc-air battery. Different valence states and morphologies of Mn x O y catalysts were synthetized via thermal treatment of EMD (generating Mn 2 O 3 and Mn 3 O 4 ) and acid digestion of synthetized Mn 2 O 3 (producing α-MnO 2 ) in order to develop an efficient Bifunctional Air Electrode (BAE). Change in the ratio H + to Mn 2 O 3 during the acid digestion affects the sample microporosity, the crystallographic plane distribution, as well as the physical and chemical adsorbed water which was related to defects, i.e. cation vacancies (Mn 4+ ) and Mn 3+ . These characteristics were discussed and linked to the electrocatalytic activity. The best ORR performing catalyst was that with the higher surface water content (associated to material BET surface area) and a (310) surface as the 2nd more contributing plane (after 211). On the other hand, the catalyst with the higher structural water and with (110) and (200) crystallographic planes being the most intensity contributors (after 211) was the most OER active material. In this work, it was able to find a relationship between catalyst structure and air-efficiency through a volcano-like relationship between air-efficiency and surface water content. Air-efficiency (also take as round-efficiency discharge/charge in battery context) can be taken as a good descriptor of potentially good materials for Zn-Air secondary batteries technology. In this term, we were able to prepare a Bifunctional Air Electrode based on the selected α-MnO 2 sample which demonstrated a round-efficiency of 53%, a ΔV around 1 V and a neglected loss of the charge potential (about 2.1 V) over the entire lifecycle test (more 200 cycles over 30 hours) with a capacity retention superior to 95%. [ABSTRACT FROM AUTHOR]

Designing a bifunctional air electrode which catalyses both the oxygen reduction and oxygen evolution reactions is an essential part of progress towards fully rechargeable metal-air batteries, such as the iron-air battery which is environmentally friendly, low cost, and does not suffer risk of thermal runaway like lithium-ion batteries. This paper reports the development of a lightweight carbon-based bifunctional air electrode, catalysed by a small (0.5 mg cm −2 ) loading of 30 wt.% palladium on carbon. The Pd-catalysed air electrode showed good bifunctional activity, with 0.53 V potential difference between oxygen reduction and evolution. The Pd/C air electrode showed improved catalytic activity at high current densities (≥ 50 mA cm −2 ) and enhanced durability compared with two commercial Pt/C air electrodes produced by Gaskatel GmbH and Johnson Matthey. A stable oxygen evolution potential was maintained over 1,000 charge-discharge cycles. [ABSTRACT FROM AUTHOR]

Pb−Ru pyrochlore has been of interest as bifunctional electrocatalyst for an air electrode. An amorphous citrate precursor (ACP) process has been optimized to prepare Pb−Ru pyrochlore powders with high surface areas with consequent improvement of its electrocatalytic performance in an air electrode. The surface area of the final powder is 30 m2/g, which is larger than the value obtained by the conventional hydroxide method. A PTEF-bonded gas diffusion electrode loaded with pyrochlore catalysts prepared by the ACP method showed good bifunctional performance. The electrode loaded with only 10 mg/cm2 of pyrochlore powder prepared by ACP showed good bifunctional performance, i.e. 100 mA/cm2 for oxygen reduction and 100 mA/cm2 for oxygen evolution, at 0.6 and 1.6 V vs. RHE, respectively. This performance compares well to the results from lower-area pyrochlore at much higher loading. [ABSTRACT FROM AUTHOR]

Large overpotential is one of the critical issues of bifunctional air electrodes for metal–air secondary batteries and alkaline regenerative fuel cells. To overcome this problem, construction of gas diffusion electrodes providing plenty amount of reaction fields for oxygen reduction and evolution are necessary. In this work, we investigate the reaction fields in the gas diffusion electrodes by comparing the oxygen diffusion resistances in the gas diffusion electrodes with different electrolyte solutions and catalyst layer thicknesses to improve the overpotential of the gas diffusion electrode. It is suggested that the reaction fields for oxygen reduction and oxygen evolution concentrate on the air side and the electrolyte sides of the catalyst layer, respectively. Based on this assignment, we fabricate a gas diffusion electrode where the oxygen reduction and evolution catalysts are, respectively arranged to the air and electrolyte sides of the catalyst layer. This electrode showed both lower overpotential and higher cyclability than those loaded the same catalysts with the opposed and mixed configuration. This result verifies the assignment of the reaction fields and shows the impact of the catalyst arrangement on the activity and the durability of the bifunctional electrodes. [Display omitted] [ABSTRACT FROM AUTHOR]