Tailored Combination of Low Dimensional Catalysts for Efficient Oxygen Reduction and Evolution in Li-O2 Batteries

Carbon Tube-Based Cathode for Li-CO2 Batteries: A Review

Metal–air batteries are considered the research, development, and application direction of electrochemical devices in the future because of their high theoretical energy density. Among them, lithium–carbon dioxide (Li–CO₂) batteries can capture, fix, and transform the greenhouse gas carbon dioxide while storing energy efficiently, which is an effective technique to achieve “carbon neutrality”. However, the current research on this battery system is still in the initial stage, the selection of key materials such as electrodes and electrolytes still need to be optimized, and the actual reaction path needs to be studied. Carbon tube-based composites have been widely used in this energy storage system due to their excellent electrical conductivity and ability to construct unique spatial structures containing various catalyst loads. In this review, the basic principle of Li–CO₂ batteries and the research progress of carbon tube-based composite cathode materials were introduced, the preparation and evaluation strategies together with the existing problems were described, and the future development direction of carbon tube-based materials in Li–CO₂ batteries was proposed.

Synthesis of Co/CeO2 hetero-particles with abundant oxygen-vacancies supported by carbon aerogels for ORR and OER

It is highly significant for the fabrication of rechargeable metal-air batteries to develop cost-efficient and high-performance electrocatalysts of bifunctionality for both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, we demonstrate a hybrid composed of CeO₂-decorated Co nanoparticles supported on three-dimensionally porous carbon aerogels (Co-CeO₂/C aerogels) as a superior bifunctional electrocatalyst. The preparation of Co-CeO₂/C aerogels depends on the formation of a novel CeCl₃/K₃Co(CN)₆-chitosan (CS) hydrogel, during which the cyanide groups of K₃Co(CN)₆ combines the hydroxyls in CS by hydrogen bridges, accompanying with the substitution of chloride groups in CeCl₃ by cyanide groups in K₃Co(CN)₆. The electron spin resonance offers a convincing proof that numerous oxygen vacancies were found in Co-CeO₂/C aerogels after the introduction of CeO₂. The developed Co-CeO₂/C aerogels showed an outstanding electrochemical performance for both OER and ORR in comparsion with RuO₂ and Pt/C catalysts in 0.1 M KOH solution. A small overpotential (380 mV) and a low Tafel slope (99 mV dec^-1) were observed for OER, while the half-wave potential (0.75 V) and the onset potential (0.92 V) were high for ORR. The superior performance could be put down to the multihole heterostructure, multiple components and abundant oxygen vacancies. It was very helpful for the adsorption and the catalyzation of the reactants and the efficient mass transport of reagent/product. This work paves a neoteric method to synthesize a bifunctional hybrid catalyst with a highly efficient performance of energy conversion and storage.

Universal Synthesis of Porous Inorganic Nanosheets via Graphene-Cellulose Templating Route

Two-dimensional (2D) inorganic nanomaterials have attracted enormous interest in diverse research areas because of their intriguing physicochemical properties. However, reliable method for the synthesis and composition manipulation of polycrystalline inorganic nanosheets (NSs) are still considered grand challenges. Here, we report a robust synthetic route for producing various kinds of inorganic porous NSs with desired multiple components and precise compositional stoichiometry by employing tunicin, i.e., cellulose extracted from earth-abundant marine invertebrate shell waste. Cellulose fibrils can be tightly immobilized on graphene oxide (GO) NSs to form stable tunicin-loaded GO NSs, which are used as a sacrificial template for homogeneous adsorption of diverse metal precursors. After a subsequent pyrolysis process, 2D metallic or metal oxide NSs are formed without any structural collapse. The rationally designed tunicin-loaded GO NS templating route paves a new path for the simple preparation of multicompositional inorganic NSs for broad applications, including chemical sensing and electrocatalysis.

Free-Standing Three-Dimensional CuCo2S4 Nanosheet Array with High Catalytic Activity as an Efficient Oxygen Electrode for Lithium-Oxygen Batteries

In this work, a novel free-standing CuCo₂S₄ nanosheet cathode (CuCo₂S₄@Ni) with high catalytic activity is fabricated for aprotic lithium-oxygen (Li-O₂) battery. This deliberately designed oxygen electrode is found to yield lower overpotential (0.82 V), improved specific capacity (9673 mA h g^-1 at 100 mA g^-1), and enhanced cycle life (164 cycles) as compared to the traditional carbonaceous electrode. The improved performance can be ascribed to the superb spinel structure of CuCo₂S₄, in which both Cu and Co exhibit more abundant redox properties, improving oxygen reduction reaction and oxygen evolution reaction kinetics effectively and boosting the electrochemical reactions. Furthermore, the well-designed architecture also plays a critical role in the improved performance. Encouraged by the excellent catalytic activity of this free-standing cathode, large-scale pouch-type Li-O₂ cell based on CuCo₂S₄@Ni cathode is fabricated and can work under different bending and twisting conditions. This free-standing electrode provides a new strategy for developing Li-O₂ batteries with excellent performance and flexible wearable devices.

Strategies toward High-Performance Cathode Materials for Lithium-Oxygen Batteries

Rechargeable aprotic lithium (Li)-O₂ batteries with high theoretical energy densities are regarded as promising next-generation energy storage devices and have attracted considerable interest recently. However, these batteries still suffer from many critical issues, such as low capacity, poor cycle life, and low round-trip efficiency, rendering the practical application of these batteries rather sluggish. Cathode catalysts with high oxygen reduction reaction (ORR) and evolution reaction activities are of particular importance for addressing these issues and consequently promoting the application of Li-O₂ batteries. Thus, the rational design and preparation of the catalysts with high ORR activity, good electronic conductivity, and decent chemical/electrochemical stability are still challenging. In this Review, the strategies are outlined including the rational selection of catalytic species, the introduction of a 3D porous structure, the formation of functional composites, and the heteroatom doping which succeeded in the design of high-performance cathode catalysts for stable Li-O₂ batteries. Perspectives on enhancing the overall electrochemical performance of Li-O₂ batteries based on the optimization of the properties and reliability of each part of the battery are also made. This Review sheds some new light on the design of highly active cathode catalysts and the development of high-performance lithium-O₂ batteries.

Bifunctional Hybrid Catalysts with Perovskite LaCo0.8Fe0.2O3 Nanowires and Reduced Graphene Oxide Sheets for an Efficient Li-O2 Battery Cathode

In this paper, bifunctional catalysts consisting of perovskite LaCo_0.8Fe_0.2O₃ nanowires (LCFO NWs) with reduced graphene oxide (rGO) sheets were prepared for use in lithium-oxygen (Li-O₂) battery cathodes. The prepared LCFO@rGO composite was explored as a cathode catalyst for Li-O₂ batteries, resulting in an outstanding discharge capacity (ca. 7088.2 mAh g^-1) at the first cycle. Moreover, a high stability of the O₂-cathode with the LCFO@rGO catalyst was achieved over 56 cycles under the capacity limit of 500 mAh g^-1 with a rate of 200 mA g^-1, as compared to the Ketjenblack carbon and LCFO NWs. The enhanced electrochemical performance suggests that these hybrid composites of perovskite LCFO NWs with rGO nanosheets could be a perspective bifunctional catalyst for the cathode oxygen reduction and oxygen evolution reactions in the development of next-generation Li-O₂ battery cathodes.

Brush-Like Cobalt Nitride Anchored Carbon Nanofiber Membrane: Current Collector-Catalyst Integrated Cathode for Long Cycle Li-O2 Batteries

To achieve a high reversibility and long cycle life for lithium-oxygen (Li-O₂) batteries, the irreversible formation of Li₂O₂, inevitable side reactions, and poor charge transport at the cathode interfaces should be overcome. Here, we report a rational design of air cathode using a cobalt nitride (Co₄N) functionalized carbon nanofiber (CNF) membrane as current collector-catalyst integrated air cathode. Brush-like Co₄N nanorods are uniformly anchored on conductive electrospun CNF papers via hydrothermal growth of Co(OH)F nanorods followed by nitridation step. Co₄N-decorated CNF (Co₄N/CNF) cathode exhibited excellent electrochemical performance with outstanding stability for over 177 cycles in Li-O₂ cells. During cycling, metallic Co₄N nanorods provide sufficient accessible reaction sites as well as facile electron transport pathway throughout the continuously networked CNF. Furthermore, thin oxide layer (<10 nm) formed on the surface of Co₄N nanorods promote reversible formation/decomposition of film-type Li₂O₂, leading to significant reduction in overpotential gap (∼1.23 V at 700 mAh g^-1). Moreover, pouch-type Li-air cells using Co₄N/CNF cathode stably operated in real air atmosphere even under 180° bending. The results demonstrate that the favorable formation/decomposition of reaction products and mediation of side reactions are hugely governed by the suitable surface chemistry and tailored structure of cathode materials, which are essential for real Li-air battery applications.

Carbon nanotube/Co3O4 nanocomposites selectively coated by polyaniline for high performance air electrodes

We herein report the preparation of carbon nanotube (CNT)/Co₃O₄ nanocomposites selectively coated with polyaniline (PANI) via an electropolymerization method, for use as an effective electrode material for Li-air (Li-O₂) batteries. The Co₃O₄ catalyst attached to the CNTs facilitated the dissociation of reaction products and reduced the overpotential of the cells. As the carbon surface activates the side reactions, the PANI coating on the carbon surface of the electrode suppressed the side reaction at the electrode/Li₂O₂ and electrode/electrolyte interfaces, thus enhancing the cycle performance of the electrode. In addition, the catalytic activity of Co₃O₄ on the CNT/Co₃O₄ nanocomposites remained unaffected, as the Co₃O₄ surface was not covered with a PANI layer due to the nature of the electropolymerization method. Overall, the synergic effect of the PANI layer and the Co₃O₄ catalyst leads to a superior cyclic performance and a low overpotential for the electrode based on selectively PANI-coated CNT/Co₃O₄ nanocomposites.

Perovskite/Carbon Composites: Applications in Oxygen Electrocatalysis

Oxygen electrocatalysis, i.e., oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), plays an extremely important role in oxygen-based renewable-energy technologies such as rechargeable metal-air batteries, regenerative fuel cells and water splitting. Perovskite oxides have recently attracted increasing interest and hold great promise as efficient ORR and OER catalysts to replace noble-metal-based catalysts, owing to their high intrinsic catalytic activity, abundant variety, low cost, and rich resources. The introduction of perovskite-carbon interfaces by forming perovskite/carbon composites may bring a synergistic effect between the two phases, thus benefiting the oxygen electrocatalysis. This review provides a comprehensive overview of recent advances in perovskite/carbon composites for oxygen electrocatalysis in alkaline media, aiming to provide insights into the key parameters that influence the ORR/OER performance of the composites, including the physical/chemical properties and morphologies of the perovskites, the multiple roles of carbon, the synthetic method and the synergistic effect. A special emphasis is placed on the origin of the synergistic effect associated with the interfacial interaction between the perovskite and the carbon phases for enhanced ORR/OER performance. Finally, the existing challenges and the future directions for the synthesis and development of more efficient oxygen catalysts based on perovskite/carbon composites are proposed.

Polyimide-coated carbon electrodes combined with redox mediators for superior Li-O2 cells with excellent cycling performance and decreased overpotential

We report an air electrode employing polyimide-coated carbon nanotubes (CNTs) combined with a redox mediator for Li-O₂ cells with enhanced electrochemical performance. The polyimide coating on the carbon surface suppresses unwanted side reactions, which decreases the amount of accumulated reaction products on the surface of the air electrode during cycling. The redox mediators lower the overpotential of the Li-O₂ cells because they can easily transfer electrons from the electrode to the reaction products. The low overpotential can also decrease the side reactions that activate at a high potential range. Specifically, the CsI redox mediator effectively interrupted dendrite growth on the Li anode during cycling due to the shielding effect of its Cs⁺ ions and acted as a redox mediator due to its I⁻ ions. LiNO₃ also facilitates the decrease in side reactions and the stabilization of the Li anode. The synergic effect of the polyimide coating and the electrolyte containing the LiNO₃/CsI redox mediator leads to a low overpotential and excellent cycling performance (over 250 cycles with a capacity of 1,500 mAh·g_electrode⁻¹).

Air electrode based on poly(3,4-ethylenedioxythiophene) microflower/graphene composite for superior Li–O2batteries with excellent cycle performance

A PEDOT microflower/graphene composite was introduced as a potential electrode material for Li–O₂batteries with enhanced electrochemical performance.