Optimizing Eg Orbital Occupancy of Transition Metal Sulfides by Building Internal Electric Fields to Adjust the Adsorption of Oxygenated Intermediates for Li-O2 Batteries

Li-O2 batteries are acknowledged as one of the most promising energy systems due to their high energy density approaching that of gasoline, but the poor battery efficiency and unstable cycling performance still hinder their practical application. In this work, hierarchical NiS2 -MoS2 heterostructured nanorods are designed and successfully synthesized, and it is found that heterostructure interfaces with internal electric fields between NiS2 and MoS2 optimized eg orbital occupancy, effectively adjusting the adsorption of oxygenated intermediates to accelerate reaction kinetics of oxygen evolution reaction and oxygen reduction reaction. Structure characterizations coupled with density functional theory calculations reveal that highly electronegative Mo atoms on NiS2 -MoS2 catalyst can capture more eg electrons from Ni atoms, and induce lower eg occupancy enabling moderate adsorption strength toward oxygenated intermediates. It is evident that hierarchical NiS2 -MoS2 nanostructure with fancy built-in electric fields significantly boosted formation and decomposition of Li2 O2 during cycling, which contributed to large specific capacities of 16528/16471 mAh g-1 with 99.65% coulombic efficiency and excellent cycling stability of 450 cycles at 1000 mA g-1 . This innovative heterostructure construction provides a reliable strategy to rationally design transition metal sulfides by optimizing eg orbital occupancy and modulating adsorption toward oxygenated intermediates for efficient rechargeable Li-O2 batteries.

Dispersed Nickel Phthalocyanine Molecules on Carbon Nanotubes as Cathode Catalysts for Li-CO2 Batteries

The Li-CO2 battery has great potential for both CO2 utilization and energy storage, but its practical application is limited by low energy efficiency and short cycle life. Efficient cathode catalysts are needed to address this issue. Herein, this work reports on molecularly dispersed electrocatalysts (MDEs) of nickel phthalocyanine (NiPc) anchored on carbon nanotubes (CNTs) as the cathode catalyst for Li-CO2 batteries. The dispersed NiPc molecules efficiently catalyze CO2 reduction, while the conductive and porous CNTs networks facilitate CO2 evolution reaction, leading to enhanced discharging and charging performance compared to the NiPc and CNTs mixture. Octa-cyano substitution on NiPc (NiPc-CN) further enhances the interaction between the molecule and CNTs, resulting in better cycling stability. The Li-CO2 battery with the NiPc-CN MDE cathode shows a high discharge voltage of 2.72 V and a small discharging-charging potential gap of 1.4 V, and can work stably for over 120 cycles. The reversibility of the cathode is confirmed by experimental characterizations. This work lays a foundation for the development of molecular catalysts for Li-CO2 battery cathodes.

Boosting Li-O2 Battery Performance via Coupling of P-N Site-Rich N, P Co-Doped Graphene-Like Carbon Nanosheets with Nano-CePO4

Development of efficient and robust cathode catalysts is critical for the commercialization of Li-O₂ batteries (LOBs). Herein, a well-designed CePO₄ @N-P-CNSs cathode catalyst for LOBs via coupling P-N site-rich N, P co-doped graphene-like carbon nanosheets (N-P-CNSs) with nano-CePO₄ via a novel "in situ derivation" coupling strategy by in situ transforming the P atoms of P-C sites in N-P-CNSs to CePO₄ is reported. The CePO₄ @N-P-CNSs exhibit superior bifunctional ORR/OER activity relative to commercial Pt/C-RuO₂ with an overall overpotential of 0.64 V (vs RHE). Moreover, the LOB with CePO₄ @N-P-CNSs as the cathode catalyst delivers a low charge overpotential of 0.67 V (vs Li/Li⁺ ), high discharge capacity of 29774 mAh g^-1 at 100 mA g^-1 and long cycling stability of 415 cycles, respectively. The remarkably enhanced LOB performance is attributable to the in situ derived CePO₄ nanoparticles and the P-N sites in N-P-CNSs, which facilitate increased bifunctional ORR/OER activity, promote the rapid and effective decomposition of Li₂ O₂ and inhibit the formation of Li₂ CO₃ . This work may provide new inspiration for designing efficient, durable, and cost-effective cathode catalysts for LOBs.

Single-Atom Pd-N4 Catalysis for Stable Low-Overpotential Lithium-Oxygen Battery

The critical challenge for Li-O₂ batteries lies in the large charge overpotential, leading to undesirable side reactions and inferior cycle stability. Single-atom catalysts have shown promising prospects in expediting the kinetics of oxygen evolution reaction (OER) for Li-O₂ batteries. However, a present practical drawback is the limited understanding of the correlation between the unique atomic structures and the OER mechanism. Herein, a template-assisted strategy is reported to synthesize atomically dispersed Pd anchored on N-doped carbon spheres as cathode catalysts. Benefiting from the well-defined Pd-N₄ moiety, the morphology and distribution of Li₂ O₂ products are distinctly regulated with optimized decomposition reversibility. Theoretical simulations reveal that the unique configuration of Pd-N₄ will contribute to the electron transfer from Pd atoms to the adjacent N atoms, which turns the originally electroneutral Pd into positively charged and downshifts the d-band center and therefore weakens its adsorption energy with the intermediates. The Li-O₂ batteries with Pd SAs/NC cathode achieve a charge overpotential of only 0.24 V and sustainable low-overpotential cycling stability (500 mA g^-1 ), and can retain a low charge voltage to a very high capacity of 10 000 mAh g^-1 . This work provides some insights into designing efficient single-atom catalysts for stable low-overpotential Li-O₂ batteries.

Highly Dispersed Ru-Co Nanoparticles Interfaced With Nitrogen-Doped Carbon Polyhedron for High Efficiency Reversible Li-O2 Battery

The lithium-oxygen (Li-O₂ ) battery with high energy density of 3860 Wh kg^-1 represents one of the most promising new secondary batteries for future electric vehicles and mobile electronic devices. However, slow oxygen reduction/oxygen evolution (ORR/OER) reaction efficiency and unstable cycling performance restrain the practical applications of the Li-O₂ battery. Herein, Ru-modified nitrogen-doped porous carbon-encapsulated Co nanoparticles (Ru/Co@CoN_x -C) are synthesized through reduction of Ru on metal-organic framework (MOFs) pyrolyzed derivatives strategies. Porous carbon polyhedra provide channels for reactive species and stable structure ensures the cyclic stability of the catalyst; abundant Co-N_x sites and high specific surface area (353 m² g^-1 ) provide more catalytically active sites and deposition sites for reaction products. Theoretical calculations further verify that Ru/Co@CoN_x -C can regulate the growth of Li₂ O₂ to improve reversibility of Li-O₂ batteries. Li-O₂ batteries with Ru/Co@CoN_x -C as cathode catalyst achieve small voltage gaps of 1.08 V, exhibit excellent cycle stability (205 cycles), and deliver high discharge specific capacity (17050 mAh g^-1 ). Furthermore, pouch-type Li-O₂ batteries that maintain stable electrochemical performance output even under conditions of bending deformation and corner cutting are successfully assembled. This study demonstrates Ru/Co@CoN_x -C catalyst's great application potential in Li-O₂ batteries.

NiCo Alloy Nanoparticles on a N/C Dual-Doped Matrix as a Cathode Catalyst for Improved Microbial Fuel Cell Performance

The cathode material properties of the microbial fuel cell (MFC) have a quite important effect on their power generation capacity. Excellent oxygen reduction reaction (ORR) performance is the key to obtaining the remarkable capability of MFC. In this study, a series of catalysts are successfully prepared by a simple step-by-step hydrothermal, in situ growth, solution polymerization, and pyrolysis procedure. Here, the NiCo nanoparticles loading on nitrogen/carbon dual-doped matrix annealing at 800 °C (NiCo@DNC-800) under Ar shows good ORR activity with a maximum power density of 2325.60 ± 41.96 mW m^-2 in the case of the 2 mg cm^-2 minimal catalyst loading, and which is about 2.16 times more than that achieved by 20% Pt/C (1074.21 ± 39.36 mW m^-2 ). The unique N/C duel-doped matrix provides more graphitic-N and pyridinic-N that can reduce the resistance of electron diffusion and transport, together with the synergistic catalysis of NiCo active sites improving the oxygen reduction reaction performance of MFC greatly. In addition, the NiCo@DNC-800 cathode catalyst demonstrates that composite materials have great application potential in water pollution treatment and new green energy strategies.

MnCo2 S4 -CoS1.097 Heterostructure Nanotubes as High Efficiency Cathode Catalysts for Stable and Long-Life Lithium-Oxygen Batteries Under High Current Conditions

Constructing the heterostructures is considered to be one of the most effective methods to improve the poor electrical conductivity and insufficient electrocatalytic properties of metal sulfide catalysts. In this work, MnCo2 S4 -CoS1.097 nanotubes are successfully prepared via a reflux- hydrothermal process. This novel cathode catalyst delivers high discharge/charge specific capacities of 21 765/21 746 mAh g-1 at 200 mA g-1 and good rate capability. In addition, a favorable cycling stability with a fixed specific capacity of 1000 mAh g-1 at high current density of 1000 mA g-1 (167 cycles) and 2000 mA g-1 (57 cycles) are delivered. It is proposed that fast transmission of ions and electrons accelerated by the built-in electric field, multiple active sites from the heterostructure, and nanotube architecture with large specific surface area are responsible for the superior electrochemical performance. To some extent, the rational design of this heterostructured metal sulfide catalyst provides guidance for the development of the stable and efficient cathode catalysts for Li-O2 batteries that can be employed under high current conditions.

Oxygen Vacancy-Rich RuO2-Co3O4 Nanohybrids as Improved Electrocatalysts for Li-O2 Batteries

Lithium oxygen (Li-O₂) batteries have shown great potential as new energy-storage devices due to the high theoretical energy density. However, there are still substantial problems to be solved before practical application, including large overpotential, low energy efficiency, and poor cycle life. Herein, we have successfully synthesized a RuO₂-Co₃O₄ nanohybrid with a rich oxygen vacancy and large specific surface area. The Li-O₂ batteries based on the RuO₂-Co₃O₄ nanohybrid shown obviously reduced overpotential and improved circulatory property, which can cycle stably for more than 100 cycles at a current density of 200 mA g^-1. Experimental results and density function theory calculation prove that the introduction of RuO₂ can increase oxygen vacancy concentration of Co₃O₄ and accelerate the charge transfer. Meanwhile, the hollow and porous structure leads to a large specific surface area about 104.5 m² g^-1, exposing more active sites. Due to the synergistic effect, the catalyst of the RuO₂-Co₃O₄ nanohybrid can significantly reduce the adsorption energy of the LiO₂ intermediate, thereby reducing the overpotential effectively.

Monodispersed Ruthenium Nanoparticles on Nitrogen-Doped Reduced Graphene Oxide for an Efficient Lithium-Oxygen Battery

Lithium-oxygen batteries with ultrahigh energy densities have drawn considerable attention as next-generation energy storage devices. However, their practical applications are challenged by sluggish reaction kinetics aimed at the formation/decomposition of discharge products on battery cathodes. Developing effective catalysts and understanding the fundamental catalytic mechanism are vital to improve the electrochemical performance of lithium-oxygen batteries. Here, uniformly dispersed ruthenium nanoparticles anchored on nitrogen-doped reduced graphene oxide are prepared by using an in situ pyrolysis procedure as a bifunctional catalyst for lithium-oxygen batteries. The abundance of ruthenium active sites and strong ruthenium-support interaction enable a feasible discharge product formation/decomposition route by modulating the surface adsorption of lithium superoxide intermediates and the nucleation and growth of lithium peroxide species. Benefiting from these merits, the electrode provides a drastically increased discharge capacity (17,074 mA h g^-1), a decreased charge overpotential (0.51 V), and a long-term cyclability (100 cycles at 100 mA g^-1). Our observations reveal the significance of the dispersion and coordination of metal catalysts, shedding light on the rational design of efficient catalysts for future lithium-oxygen batteries.

Conducting Polymer-Based Nanohybrids for Fuel Cell Application

Carbon materials such as carbon graphitic structures, carbon nanotubes, and graphene nanosheets are extensively used as supports for electrocatalysts in fuel cells. Alternatively, conducting polymers displayed ultrahigh electrical conductivity and high chemical stability havegenerated an intense research interest as catalysts support for polymer electrolyte membrane fuel cells (PEMFCs) as well as microbial fuel cells (MFCs). Moreover, metal or metal oxides catalysts can be immobilized on the pure polymer or the functionalized polymer surface to generate conducting polymer-based nanohybrids (CPNHs) with improved catalytic performance and stability. Metal oxides generally have large surface area and/or porous structures and showed unique synergistic effects with CPs. Therefore, a stable, environmentally friendly bio/electro-catalyst can be obtained with CPNHs along with better catalytic activity and enhanced electron-transfer rate. The mass activity of Pd/polypyrrole (PPy) CPNHs as an anode material for ethanol oxidation is 7.5 and 78 times higher than that of commercial Pd/C and bulk Pd/PPy. The Pd rich multimetallic alloys incorporated on PPy nanofibers exhibited an excellent electrocatalytic activity which is approximately 5.5 times higher than monometallic counter parts. Similarly, binary and ternary Pt-rich electrocatalysts demonstrated superior catalytic activity for the methanol oxidation, and the catalytic activity of Pt24Pd26Au50/PPy significantly improved up to 12.5 A per mg Pt, which is approximately15 times higher than commercial Pt/C (0.85 A per mg Pt). The recent progress on CPNH materials as anode/cathode and membranes for fuel cell has been systematically reviewed, with detailed understandings into the characteristics, modifications, and performances of the electrode materials.

Heterostructured NiS2/ZnIn2S4 Realizing Toroid-like Li2O2 Deposition in Lithium-Oxygen Batteries with Low-Donor-Number Solvents

The aprotic lithium-oxygen (Li-O₂) battery has triggered tremendous efforts for advanced energy storage due to the high energy density. However, realizing toroid-like Li₂O₂ deposition in low-donor-number (DN) solvents is still the intractable obstruction. Herein, a heterostructured NiS₂/ZnIn₂S₄ is elaborately developed and investigated as a promising catalyst to regulate the Li₂O₂ deposition in low-DN solvents. The as-developed NiS₂/ZnIn₂S₄ promotes interfacial electron transfer, regulates the adsorption energy of the reaction intermediates, and accelerates O-O bond cleavage, which are convincingly evidenced experimentally and theoretically. As a result, the toroid-like Li₂O₂ product is achieved in a Li-O₂ battery with low-DN solvents via the solvation-mediated pathway, which demonstrates superb cyclability over 490 cycles and a high output capacity of 3682 mA h g^-1. The interface engineering of heterostructure catalysts offers more possibilities for the realization of toroid-like Li₂O₂ in low-DN solvents, holding great promise in achieving practical applications of Li-O₂ batteries as well as enlightening the material design in catalytic systems.

Conjugated Cobalt Polyphthalocyanine as the Elastic and Reprocessable Catalyst for Flexible Li-CO2 Batteries

Li-CO₂ batteries represent an attractive solution for electrochemical energy storage by utilizing atmospheric CO₂ as the energy carrier. However, their practical viability critically depends on the development of efficient and low-cost cathode catalysts for the reversible formation and decomposition of Li₂ CO₃ . Here, the great potential of a structurally engineered polymer is demonstrated as the cathode catalyst for rechargeable Li-CO₂ batteries. Conjugated cobalt polyphthalocyanine is prepared via a facile microwave heating method. Due to the crosslinked network, it is intrinsically elastic and has improved chemical, physical, and mechanical stability. Electrochemical measurements show that cobalt polyphthalocyanine facilitates the reversible formation and decomposition of Li₂ CO₃ , and therefore enables high-performance Li-CO₂ batteries with large areal capacity and impressive cycling performance. In addition, the elastic and reprocessable property of the polymeric catalyst renders it possible to fabricate flexible batteries.

Three-Dimensional Interconnected Network Architecture with Homogeneously Dispersed Carbon Nanotubes and Layered MoS2 as a Highly Efficient Cathode Catalyst for Lithium-Oxygen Battery

The structure and catalytic activity of the oxygen electrode determine the overall electrochemical performance of lithium-oxygen (Li-O₂) batteries. Here, a three-dimensional (3D) porous interconnected network structure combined with ultrathin MoS₂ nanosheets with homogeneously dispersed CNTs (MoS₂/CNTs) was synthesized via a one-step hydrothermal reaction. The 3D interconnected architecture can efficiently promote the diffusion of O₂ and Li ions as well as impregnation of electrolyte and provide more abundant storage space for the accommodation of discharge products, while the incorporation of uniformly dispersed CNTs improves the electronic conductivity and maintains the integrity of the cathode structure. Therefore, the Li-O₂ battery based on MoS₂/CNTs achieves improved performance with the low overpotentials (discharge/charge overpotentials of approximately 0.29 and 1.05 V), a high discharge specific capacity of 6904 mA h g^-1 at a rate of 200 mA g^-1, and excellent cycling stability (132 cycles). Experimental studies reveal that the improved electrochemical performance can be ascribed to the synergistic advantages of electronic conductive CNTs and excellent catalytic activity of the MoS₂ nanosheets. Moreover, the unique 3D interconnected network structure can effectively facilitate fast charge transfer kinetics and a facile mass transport pathway. These encouraging performances demonstrate the metal sulfide catalyst as a promising catalytic material of oxygen electrodes for Li-O₂ batteries.

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.