Surface Phosphorus-Induced CoO Coupling to Monolithic Carbon for Efficient Air Electrode of Quasi-Solid-State Zn-Air Batteries

A bifunctional catalyst of CoO/NBC composite for high-performance rechargeable flexible zinc-air battery

Rechargeable flexible zinc-air batteries (ZABs) represent a promising energy-supply device for wearable electronics due to their low cost, safety and high energy density, but their electrochemical performance often suffers from the sluggish reaction kinetics of air electrode and poor moisture-retention ability of polymer electrolytes. Here, we report a type of high-performance rechargeable flexible ZABs endowed by an efficient bifunctional catalyst for air electrode and a high moisture-retention hydrogel electrolyte. The designed nitrogen-boron co-doped carbon nanotube arrays loaded with cobalt oxide nanoparticles (CoO/NBC) with abundant catalytic active sites and oriented structure provide its excellent electrochemical catalytic activities for both oxygen reduction reaction and oxygen evolution reaction. Based on the bifunctional catalyst of CoO/NBC, the developed rechargeable ZABs exhibit a high open-circuit voltage of 1.44 V and a high energy density of 920.0 Wh kg-1, which are superior than commercial Pt/C + RuO2 and most reported non-precious metal catalysts. Furthermore, a trehalose modified polyacrylamide hydrogel electrolytes (trehalose/PAAm) with high moisture-retention has been synthesized to construct flexible ZABs, which not only exhibit outstanding electrochemical performance (1.39 V and 824.8 Wh kg-1), but also show excellent stability even after 400 charge/discharge cycles or being bent to any angle.

Dual Fe/I Single-Atom Electrocatalyst for High-Performance Oxygen Reduction and Wide-Temperature Quasi-Solid-State Zn-Air Batteries

Oxygen reduction reaction (ORR) electrocatalysts are essential for widespread application of quasi-solid-state Zn-air batteries (ZABs), but the well-known Fe-N-C single-atom catalysts (SACs) suffer from low activity and stability because of unfavorable strong adsorption of oxygenated intermediates. Herein, the study synthesizes dual Fe/I single atoms anchored on N-doped carbon nanorods (Fe/I-N-CR) via a metal-organic framework (MOF)-mediated two-step tandem-pyrolysis method. Atomic-level I doping modulates the electronic structure of Fe-Nx centers via the long-range electron delocalization effect. Benefitting from the synergistic effect of dual Fe/I single-atom sites and the structural merits of 1D nanorods, the Fe/I-N-CR catalyst shows excellent ORR activity and stability, superior to Pt/C and Fe or I SACs. When the Fe/I-N-CR is employed as cathode for quasi-solid-state ZABs, a high power density of 197.9 mW cm-2 and an ultralong cycling lifespan of 280 h at 20 mA cm-2 are both achieved, greatly exceeding those of commercial Pt/C+IrO2 (119.1 mW cm-2 and 47 h). In addition, wide-temperature adaptability and superior stability from -40 to 60 °C are realized for the Fe/I-N-CR-based quasi-solid-state ZABs. This work provides a MOF-mediated two-step tandem-pyrolysis strategy to engineer high-performance dual SACs with metal/nonmetal centers for ORR and sustainable ZABs.

Electron Spin Broken-Symmetry of Fe-Co Diatomic Pairs to Promote Kinetics of Bifunctional Oxygen Electrocatalysis for Zinc-Air Batteries

Designing bifunctional catalysts to reduce the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) reaction barriers while accelerating the reaction kinetics is perceived to be a promising strategy to improve the performance of Zinc-air batteries. Unsymmetric configuration in single-atom catalysts has attracted attention due to its unique advantages in regulating electron orbitals. In this work, a seesaw effect in unsymmetric Fe-Co bimetallic monoatomic configurations is proposed, which can effectively improve the OER/ORR bifunctional activity of the catalyst. Compared with the symmetrical model of Fe-Co, a strong charge polarization between Co and Fe atoms in the unsymmetric model is detected, in whom the spin-down electrons around Co atoms are much higher than those spin-up electrons. The seesaw effect occurred between Co atoms and Fe atoms, resulting in a negative shift of the d-band center, which means that the adsorption of oxygen intermediates is weakened and more conducive to their dissociation. The optimized reaction kinetics of the catalyst leads to excellent performance in ZABs, with a peak power density of 215 mW cm-2 and stable cycling for >1300 h and >4000 cycles. Flexible Zinc-air batteries have also gained excellent performance to demonstrate their potential in the field of flexible wearables.

NiCo2O4 Nanowires Immobilized on Nitrogen-Doped Ti3C2Tx for High-Performance Wearable Magnesium-Air Batteries

Flexible magnesium (Mg)-air batteries provide an ideal platform for developing efficient energy-storage devices toward wearable electronics and bio-integrated power sources. However, high-capacity bio-adaptable Mg-air batteries still face the challenges in low discharge potential and inefficient oxygen electrodes, with poor kinetics property toward oxygen reduction reaction (ORR). Herein, spinel nickel cobalt oxides (NiCo2O4) nanowires immobilized on nitrogen-doped Ti3C2Tx (NiCo2O4/N-Ti3C2Tx) are reported via surface chemical-bonded effect as oxygen electrodes, wherein surface-doped pyridinic-N-C and Co-pyridinic-N moieties accounted for efficient ORR owing to increased interlayer spacing and changed surrounding environment around Co metals in NiCo2O4. Importantly, in polyethylene glycol (PVA)-NaCl neutral gel electrolytes, the NiCo2O4/N-Ti3C2Tx-assembled quasi-solid wearable Mg-air batteries delivered high open-circuit potential of 1.5 V, good flexibility under various bent angles, high power density of 9.8 mW cm-2, and stable discharge duration to 12 h without obvious voltage drop at 5 mA cm-2, which can power a blue flexible light-emitting diode (LED) array and red smart rollable wearable device. The present study stimulates studies to investigate Mg-air batteries involving human-body adaptable neutral electrolytes, which will facilitate the application of Mg-air batteries in portable, flexible, and wearable power sources for electronic devices.

Catalytically Active Carbon for Oxygen Reduction Reaction in Energy Conversion: Recent Advances and Future Perspectives

The shortage and unevenness of fossil energy sources are affecting the development and progress of human civilization. The technology of efficiently converting material resources into energy for utilization and storage is attracting the attention of researchers. Environmentally friendly biomass materials are a treasure to drive the development of new-generation energy sources. Electrochemical theory is used to efficiently convert the chemical energy of chemical substances into electrical energy. In recent years, significant progress has been made in the development of green and economical electrocatalysts for oxygen reduction reaction (ORR). Although many reviews have been reported around the application of biomass-derived catalytically active carbon (CAC) catalysts in ORR, these reviews have only selected a single/partial topic (including synthesis and preparation of catalysts from different sources, structural optimization, or performance enhancement methods based on CAC catalysts, and application of biomass-derived CACs) for discussion. There is no review that systematically addresses the latest progress in the synthesis, performance enhancement, and applications related to biomass-derived CAC-based oxygen reduction electrocatalysts synchronously. This review fills the gap by providing a timely and comprehensive review and summary from the following sections: the exposition of the basic catalytic principles of ORR, the summary of the chemical composition and structural properties of various types of biomass, the analysis of traditional and the latest popular biomass-derived CAC synthesis methods and optimization strategies, and the summary of the practical applications of biomass-derived CAC-based oxidative reduction electrocatalysts. This review provides a comprehensive summary of the latest advances to provide research directions and design ideas for the development of catalyst synthesis/optimization and contributes to the industrialization of biomass-derived CAC electrocatalysis and electric energy storage.

A twisted carbonaceous nanotube as the air-electrode for flexible Zn-Air batteries

Exploring electrocatalysts with high-efficiency oxygen reduction reaction (ORR) is significant for practical applications of fuel cells and metal-air batteries. In this work, a twisted core@shell material has been prepared with helical polypyrrole nanotubes (HPPys) as the core and coordination polymers (CPs) as the shell. After the pyrolysis process, a dense twisted carbon layer was formed by the reaction of CP and HPPy at its interface under Ar. The derived twisted carbonaceous nanotube exhibits good performance in both electrocatalytic ORR and OER. When used as the air-electrode in a flexible Zn-air battery, the battery shows good performance and stability.

An Ultrastable Bifunctional Electrocatalyst Derived from a Co2+-Anchored Covalent-Organic Framework for High-Efficiency ORR/OER and Rechargeable Zinc-Air Battery

It remains a great challenge to develop alternative electrocatalysts with high stability for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, a bifunctional electrocatalyst composed of hollow CoOx (Co3O4/CoO) nanoparticles embedded in lamellar carbon nanofibers is derived from a Co2+-anchored covalent-organic framework. The as-fabricated electrocatalyst (CoOx@NC-800) exhibits a half-wave potential (E1/2) of 0.89 V with ultrahigh long-term stability (100% current retention after 3000 CV cycles). Together with promising OER performance, the CoOx@NC-800 based reversible Zn-air battery displays a small potential gap (0.70 V), superior to that of the commercial 20% Pt/C + RuO2. The density functional theory (DFT) calculations reveal that the remarkable electrocatalytic performance and stability of CoOx@NC-800 are attributed to the optimized adsorption of the *OOH intermediate and reduced free energy of the potential-limiting step. This study establishes the functionalization of COF structure for fabrication of high-performance carbon-based electrocatalysts.

Sulfonic-Pendent Vinylene-Linked Covalent Organic Frameworks Enabling Benchmark Potential in Advanced Energy

Both proton exchange membrane fuel cells and uranium-based nuclear techniques represent two green and advanced energies. However, both of them still face some intractable scientific and industrial problems. For the former, established proton-conduction materials always suffer one or another defect such as low proton conductivity, high activation energy, bad durability, or just small-scale product; while for the later, there still lacks available adsorbent to selectively recover of UO₂ ²⁺ from concentrated nitric acid (>1 M) during the spent fuel reprocessing due to the deactivation of the adsorption site or the decomposition of adsorbent under such rigorous conditions. It is found that the above two issues can be well solved by the construction of sulfonic-pendent vinylene-linked covalent organic frameworks (COFs), since these COFs contain abundant sulfonic units for both intrinsic proton conduction and UO₂ ²⁺ capture through strong coordination fixation and vinylene linkage that enhances the stability up to 12 M nitric acid (one of the best materials surviving in 12 M HNO₃ ).

NaCl sealing Strategy-Assisted synthesis CoO-Co heterojunctions as efficient oxygen electrocatalysts for Zn air batteries

Developing highly efficient, low-cost, and stable bifunctional oxygen electrocatalysts is essential for the wide popularization of rechargeable Zn-air batteries. Combining zero-dimensional metal nanoparticles with two-dimensional metal oxide nanosheets is an appealing strategy to balance performance and cost. However, the precise construction of these composites remains a great challenge, and their interaction mechanisms lack thorough study. Herein, a cobalt-oxide-based bifunctional oxygen electrocatalyst comprising a rich Co-CoO heterointerface (CoO/Co@NG) was synthesized via a NaCl sealing-assisted pyrolysis strategy. The NaCl crystals played the role of a closed nanoreactor, which facilitated the formation of a CoO-Co heterojunction. Experimental results and theoretical calculations confirmed that the ingeniously constructed heterojunction expedited the oxygen reduction reaction and oxygen evolution reaction kinetics, which is superior to Pt/C. When serving as the air electrode in an assembled liquid-state Zn-air battery, the battery shows high power density (215 mW cm^-2), specific capacity (710 mAh g_zn^-1), and outstanding durability (720 h at 10 mA cm^-2). This work provides an innovative avenue to design high-performance heterojunction electrocatalysts for perdurable Zn-air batteries.

Interfacial Engineering of CoN/Co3 O4 Heterostructured Hollow Nanoparticles Embedded in N-Doped Carbon Nanowires as a Bifunctional Oxygen Electrocatalyst for Rechargeable Liquid and Flexible all-Solid-State Zn-Air Batteries

The design of economical, efficient, and robust bifunctional oxygen electrocatalysts is greatly imperative for the large-scale commercialization of rechargeable Zn-air battery (ZAB) technology. Herein, the neoteric design of an advanced bifunctional electrocatalyst composed of CoN/Co₃ O₄ heterojunction hollow nanoparticles in situ encapsulated in porous N-doped carbon nanowires (denoted as CoN/Co₃ O₄ HNPs@NCNWs hereafter) is reported. The simultaneous implementation of interfacial engineering, nanoscale hollowing design, and carbon-support hybridization renders the synthesized CoN/Co₃ O₄ HNPs@NCNWs with modified electronic structure, improved electric conductivity, enriched active sites, and shortened electron/reactant transport pathways. Density functional theory computations further demonstrate that the construction of a CoN/Co₃ O₄ heterojunction can optimize the reaction pathways and reduce the overall reaction barriers. Thanks to the composition and architectural superiorities, the CoN/Co₃ O₄ HNPs@NCNWs exhibit distinguished oxygen reduction reaction and oxygen evolution reaction performance with a low reversible overpotential of 0.725 V and outstanding stability in KOH medium. More encouragingly, the homemade rechargeable liquid and flexible all-solid-state ZABs utilizing CoN/Co₃ O₄ HNPs@NCNWs as the air-cathode deliver higher peak power densities, larger specific capacities, and robust cycling stability, exceeding the commercial Pt/C + RuO₂ benchmark counterparts. The concept of heterostructure-induced electronic modification herein may shed light on the rational design of advanced electrocatalysts for sustainable energy applications.

Ionic Covalent Organic Frameworks-Derived Cobalt Single Atoms and Nanoparticles for Efficient Oxygen Electrocatalysis

Metal single atoms show outstanding electrocatalytic activity owing to the abundant atomic reactive sites and superior stability. However, the preparation of single atoms suffers from inexorable metal aggregation which is harmful to electrocatalytic activity. Here, ionic covalent organic frameworks (iCOFs) are employed as the sacrificial precursor to mitigate the metal aggregation and subsequent formation of bulky particles. Molecular dynamics simulation shows that iCOFs can trap and confine more Co ions as compared to neutral COFs, resulting in the formation of a catalyst composed of Co single atoms and uniformly distributed Co nanoparticles (Co_SA &Co_NP-10 ). However, the neutral COFs derive a catalyst composed of Co atomic clusters and large Co nanoparticles (Co_AC &Co_NP-25 ). The Co_SA &Co_NP-10 catalyst exhibits higher oxygen bifunctional electrocatalytic activities than Co_AC &Co_NP-25 , coinciding with the density functional theory results. Taking the Co_SA &Co_NP-10 as the air cathode in Zn-air batteries (ZABs), the aqueous ZAB presents a high power density of 181 mW cm^-2 , a specific capacity of 811 mAh g^-1 as well as a long cycle life of 407 h at a current density of 10 mA cm^-2 , while the quasi-solid state ZAB displays a power density of 179 mW cm^-2 and the cycle life of 30 h.

Self-Promoting Energy Storage in Balsa Wood-Converted Porous Carbon Coupled with Carbon Nanotubes

For most electrodes fabricated with carbon, transition metal compounds, or conductive polymers, the capacitance may deteriorate with cyclic charging and discharging. Thus, an electrochemically stable supercapacitor has long been pursued by researchers. In this work, the hierarchical structure of balsa wood is preserved in the converted carbon which is used as a supporting framework to fabricate electrodes for supercapacitors. Well-grown carbon nanotubes (CNTs) on interior and exterior surfaces of balsa carbon channels provide two advantages including 1) offering more specific surface area to boost capacitance via electric double layer capacitance and 2) offering more active Fe and Ni sites to participate in the redox reaction to enhance capacitance of the balsa carbon/CNTs electrode. The balsa carbon/CNTs demonstrate an excellent area capacitance of 1940 mF cm^-2 . As active sites on Ni and Fe catalysts and inner walls of CNTs are gradually released, the capacitance increases 66% after 4000 charge-discharge cycles. This work brings forward a strategy for the rational design of high-performance biomass carbon coupled with advanced nanostructures for energy storage.

Wood-Derived Monolithic Catalysts with the Ability of Activating Water Molecules for Oxygen Electrocatalysis

Oxygen reduction reaction (ORR) is the key reaction on cathode of rechargeable zinc-air batteries (ZABs). However, the lack of protons in alkaline conditions limits the rate of ORR. Herein, an activating water strategy is proposed to promote oxygen electrocatalytic activity by enhancing the proton production from water dissociation. FeP nanoparticles (NPs) are coupled on N-doped wood-derived catalytically active carbon (FeP-NWCC) to associate bifunctional active sites. In alkaline, FeP-NWCC possesses outstanding catalytic activities toward ORR (E_1/2  = 0.86 V) and Oxygen evolution reaction (OER) (overpotential is 310 mV at 10 mA cm^-2 ). The liquid ZABs assembled by FeP-NWCC deliver superior peak power density (144 mW cm^-2 ) and cycle stability (over 450 h). The quasi-solid-state ZABs based on FeP-NWCC also display excellent performances. Theoretical calculation illustrates that the superb bifunctional performance of FeP-NWCC results from the elevated dissociation efficiency of water via FeP NPs to assist the oxygen catalytic process. The strategy of activating water provides a new perspective for the design of ORR/OER bifunctional catalysts. This work is a model for the application of forest biomass.

Single-atom palladium anchored N-doped carbon towards oxygen electrocatalysis for rechargeable Zn-air batteries

Herein, an atomically dispersed palladium catalyst on a hierarchical porous structure of N-doped carbon (Pd₁/N-C) is prepared using a facile freeze-drying-assisted strategy. Freeze-drying methods not only suppress the aggregation of Pd atoms but also successfully produce abundant nanopores. HAADF-STEM confirms that Pd single atoms are uniformly anchored on the N-C surface. The Pd₁/N-C electrocatalyst enhances the ORR and OER activity and durability compared to N-C and Pd-NPs/N-C. Rechargeable Zn-air batteries (ZABs) based on novel Pd₁/N-C exhibit a peak power density of 113.7 mW cm^-2 and maintain a voltage efficiency of 64.0% after 495 cycles at a discharge current density of 5 mA cm^-2. Besides, two ZABs in series can supply an LED light for at least 170 h.

Coupling Fe3 C Nanoparticles and N-Doping on Wood-Derived Carbon to Construct Reversible Cathode for Zn-Air Batteries

Electrochemical reduction of oxygen plays a critical role in emerging electrochemical energy technologies. Multiple electron transfer processes, involving adsorption and activation of O₂ and generation of protons from water molecules, cause the sluggish kinetics of the oxygen reduction reaction (ORR). Herein, a double-active-site catalyst of Fe₃ C nanoparticles coupled to paulownia wood-derived N-doped carbon (Fe₃ C@NPW) is fabricated via an active-site-uniting strategy. One site on Fe₃ C nanoparticles contributes to activating water molecules, while another site on N-doped carbon is responsible for activating oxygen molecules. Benefiting from the synergistic effect of double active sites, Fe₃ C@NPW delivers a remarkable catalytic activity for ORR with a half-wave potential of 0.87 V (vs. RHE) in alkaline electrolyte, outperforming commercial Pt/C catalyst. Moreover, zinc-air batteries (ZABs) assembled with Fe₃ C@NPW as a catalyst on cathode achieve a large specific capacity of 804.4 mA h g_Zn^-1 and a long-term stability of 780 cycles. The model solid-state ZABs also display satisfactory performances with an open-circuit voltage of 1.39 V and a high peak power density of 78 mW cm^-2 . These outstanding performances reach the level of first-rank among the non-noble metal electrode materials. This work offers a promising approach to creating double-active-site catalysts by the active-site-uniting strategy for energy conversion fields.

Phase-Reconfiguration-Induced NiS/NiFe2 O4 Composite for Performance-Enhanced Zinc-Air Batteries

Constructing composite structures is an essential approach for obtaining multiple functionalities in a single entity. Available synthesis methods of the composites need to be urgently exploited; especially in situ construction. Here, a NiS/NiFe₂ O₄ composite through a local metal-S coordination at the interface is reported, which is derived from phase reconstruction in the highly defective matrix. X-ray absorption fine structure confirms that long-range order is broken via the local metal-S coordination and, by using electron energy loss spectroscopy, the introduction of NiS/NiFe₂ O₄ interfaces during the irradiation of plasma energy is identified. Density functional theory (DFT) calculations reveal that in situ phase reconfiguration is crucial for synergistically reducing energetic barriers and accelerating reaction kinetics toward catalyzing the oxygen evolution reaction (OER). As a result; it leads to an overpotential of 230 mV @10 mA cm^-2 for the OER and a half-wave potential of 0.81 V for the oxygen reduction reaction (ORR); as well as an excellent zinc-air battery (ZAB) performance with a power density of 148.5 mW cm^-2 . This work provides a new compositing strategy in terms of fast phase reconstruction of bifunctional catalysts.

Incorporated O-CoP nanosheets with an O–P interpenetrated interface as electrocatalytic cathodes for rechargeable Li–CO2 batteries

An electrocatalytic material based on oxygen-decorated CoP nanosheets with an O–P interpenetrated interface was prepared for rechargeable Li–CO2 battery cathodes.