Nanocomposite of Fe2 O3 @C@MnO2 as an Efficient Cathode Catalyst for Rechargeable Lithium-Oxygen Batteries

Electrodeposited Manganese Dioxides and Their Composites as Electrocatalysts for Energy Conversion Reactions

Enhancing the efficiencies of electrochemical reactions for converting renewable energy into clean chemical fuels as well as generating clean energy is critical to achieving carbon neutrality. However, this enhancement can be achieved using materials that are not constrained by resource limitations and those that can be converted into devices in a scalable manner, preferably for industrial applications. This review explores the applications of electrochemically deposited manganese dioxides (MnO2) and their composites as electrochemical catalysts for oxygen evolution (OER) and hydrogen evolution reactions for converting renewable energy into chemical fuels. It also explores their applications as electrochemical catalysts for oxygen reduction reaction (ORR) and bifunctional OER/ORR for the efficient operation of fuel cells and metal-air batteries, respectively. Manganese is the second most abundant transition metal in the Earth's crust, and electrodeposition represents a binder-free and scalable technique for fabricating devices (electrodes). To propose an improved catalyst design, the studies on the electrodeposition mechanism of MnO2 as well as the fabrication techniques for MnO2-based nanocomposites accumulated in the development of electrodes for supercapacitors are also included in this review.

Growth of carbon nanotubes over carbon nanofibers catalyzed by bimetallic alloy nanoparticles as a bifunctional electrode for Zn-air batteries

Design of economical, large-scale, stable, and highly active bifunctional electrocatalysts for Zn-air batteries with enhanced oxygen reduction and oxygen evolution performance is needed. Herein, a series of electrocatalysts were facilely fabricated where in situ formed bimetallic nanoparticles aided in the growth of carbon nanotubes over carbon nanofibers (MM'-CNT@CNF) during thermal treatment. Different combinations of Fe, Ni, Co and Mn metals and melamine as precursor for CNT growth were investigated. The synergistic interaction between bimetallic nanoparticles and N-doped carbon results in greatly improved bifunctional catalytic activity for both oxygen reduction and evolution reactions (ORR, OER) using FeNi-CNT@CNF as catalyst. The half-wave potential (0.80 V vs. RHE) for FeNi-CNT@CNF for ORR was close to that of Pt/C (0.79 V vs. RHE), meanwhile its stability was superior to Pt/C. Likewise, during OER, the FeNi-CNT@CNF reached a current density of 10 mA cm^-2 at a rather low overpotential of 310 mV vs. RHE compared to benchmark RuO₂ (410 mV). The rechargeable Zn-air prototype battery using FeNi-CNT@CNF as an air electrode outperformed the mixture of Pt/C and RuO₂ with discharge/charge overpotential of 0.61 V, power density of 118 mW cm^-2 at 10 mA cm^-2 and an improved cycling stability over 108 hours.

Composite NiCo2 O4 @CeO2 Microsphere as Cathode Catalyst for High-Performance Lithium-Oxygen Battery

The large overpotential and poor cycle stability caused by inactive redox reactions are tough challenges for lithium-oxygen batteries (LOBs). Here, a composite microsphere material comprising NiCo₂ O₄ @CeO₂ is synthesized via a hydrothermal approach followed by an annealing processing, which is acted as a high performance electrocatalyst for LOBs. The unique microstructured catalyst can provide enough catalytic surface to facilitate the barrier-free transport of oxygen as well as lithium ions. In addition, the special microsphere and porous nanoneedles structure can effectively accelerate electrolyte penetration and the reversible formation and decomposition process of Li₂ O₂ , while the introduction of CeO₂ can increase oxygen vacancies and optimize the electronic structure of NiCo₂ O₄ , thereby enhancing the electron transport of the whole electrode. This kind of catalytic cathode material can effectively reduce the overpotential to only 1.07 V with remarkable cycling stability of 400 loops under 500 mA g^-1 . Based on the density functional theory calculations, the origin of the enhanced electrochemical performance of NiCo₂ O₄ @CeO₂ is clarified from the perspective of electronic structure and reaction kinetics. This work demonstrates the high efficiency of NiCo₂ O₄ @CeO₂ as an electrocatalyst and confirms the contribution of the current design concept to the development of LOBs cathode materials.

Sacrificial ZnO nanorods drive N and O dual-doped carbon towards trifunctional electrocatalysts for Zn–air batteries and self-powered water splitting devices

Integrated energy systems (IES) have attracted increasing attention in recent years.

Progress of MOF-Derived Functional Materials Toward Industrialization in Solar Cells and Metal-Air Batteries

The cutting-edge photovoltaic cells are an indispensable part of the ongoing progress of earth-friendly plans for daily life energy consumption. However, the continuous electrical demand that extends to the nighttime requires a prior deployment of efficient real-time storage systems. In this regard, metal-air batteries have presented themselves as the most suitable candidates for solar energy storage, combining extra lightweight with higher power outputs and promises of longer life cycles. Scientific research over non-precious functional catalysts has always been the milestone and still contributing significantly to exploring new advanced materials and moderating the cost of both complementary technologies. Metal-organic frameworks (MOFs)-derived functional materials have found their way to the application as storage and conversion materials, owing to their structural variety, porous advantages, as well as the tunability and high reactivity. In this review, we provide a detailed overview of the latest progress of MOF-based materials operating in metal-air batteries and photovoltaic cells.

Electrodeposition Technologies for Li-Based Batteries: New Frontiers of Energy Storage

Electrodeposition induces material syntheses on conductive surfaces, distinguishing it from the widely used solid-state technologies in Li-based batteries. Electrodeposition drives uphill reactions by applying electric energy instead of heating. These features may enable electrodeposition to meet some needs for battery fabrication that conventional technologies can rarely achieve. The latest progress of electrodeposition technologies in Li-based batteries is summarized. Each component of Li-based batteries can be electrodeposited or synthesized with multiple methods. The advantages of electrodeposition are the main focus, and they are discussed in comparison with traditional technologies with the expectation to inspire innovations to build better Li-based batteries. Electrodeposition coats conformal films on surfaces and can control the film thickness, providing an effective approach to enhancing battery performance. Engineering interfaces by electrodeposition can stabilize the solid electrolyte interphase (SEI) and strengthen the adhesion of active materials to substrates, thereby prolonging the battery longevity. Lastly, a perspective of future studies on electrodepositing batteries is provided. The significant merits of electrodeposition should greatly advance the development of Li-based batteries.

Electrodeposition of (hydro)oxides for an oxygen evolution electrode

Electrochemical water splitting is a promising technology for hydrogen production and sustainable energy conversion, but the electrolyzers that are currently available do not have anodic electrodes that are robust enough and highly active for the oxygen evolution reaction (OER). Electrodeposition provides a feasible route for preparing freestanding OER electrodes with high active site utilization, fast mass transport and a simple fabrication process, which is highly attractive from both academic and commercial points of view. This minireview focuses on the recent electrodeposition strategies for metal (hydro)oxide design and water oxidation applications. First, the intrinsic advantages of electrodeposition in comparison with traditional technologies are introduced. Then, the unique properties and underlying principles of electrodeposited metal (hydro)oxides in the OER are unveiled. In parallel, illustrative examples of the latest advances in materials structural design, controllable synthesis, and mechanism understanding through the electrochemical synthesis of (hydro)oxides are presented. Finally, the latest representative OER mechanism and electrodeposition routes for OER catalysts are briefly overviewed. Such observations provide new insights into freestanding (hydro)oxides electrodes prepared via electrodeposition, which show significant practical application potential in water splitting devices. We hope that this review will provide inspiration for researchers and stimulate the development of water splitting technology.

O2 Adsorption Associated with Sulfur Vacancies on MoS2 Microspheres

MoS₂ is well-known for its catalytic properties, mainly to adsorb hydrogenous or carbonaceous materials. However, the effect of MoS₂ on the oxygen adsorption has been investigated only a few times thus far. In this work, we first studied the adsorbability of O₂ by MoS₂ through the analysis of Li₂O₂ growth on the surface of flower-like MoS₂ microspheres with different concentrations of sulfur vacancies, which can be applied as the highly active electrocatalysts for Li-O₂ batteries. The enhancement of battery performance for the Def-MoS₂@CTs (CTs = carbon textile substrates) with a larger concentration of sulfur vacancies (S/Mo = 1.61) can be achieved. The experimental and theoretical results confirm that the sulfur vacancies play a crucial role in the adsorption process and thus affect the morphology and nucleation of Li₂O₂. In addition, a fundamental catalytic mechanism for this adsorption process is also proposed. These results provide a new insight into the development of a highly active electrocatalyst by introducing a large concentration of defects for Li-O₂ batteries.

Urchin-like NiO-NiCo2O4 heterostructure microsphere catalysts for enhanced rechargeable non-aqueous Li-O2 batteries

Urchin-like NiO-NiCo2O4 microspheres with heterostructures were successfully synthesized through a facile hydrothermal method, followed by thermal treatment. The unique structure of NiO-NiCo2O4 with the synergetic effect between NiCo2O4 and NiO, and the heterostructure favour the catalytic activity towards Li-O2 batteries. NiCo2O4 is helpful for boosting both the oxygen reduction reaction and oxygen evolution reaction for the Li-O2 batteries and NiO is likely to promote the decomposition of certain by-products. The special urchin-like morphology facilitates the continuous oxygen flow and accommodates Li2O2. Moreover, benefitting from the heterostructure, NiO-NiCo2O4 microspheres are able to promote the transport of Li ions and electrons to further improve battery performance. Li-O2 batteries utilizing a NiO-NiCo2O4 microsphere electrode show a much higher specific capacity and a lower overpotential than those with a Super P electrode. Moreover, they exhibit an enhanced cycling stability. The electrode can be continuously discharged and charged without obvious terminal voltage variation for 80 cycles, as the discharge capacity is restricted at 600 mA h g-1, suggesting that NiO-NiCo2O4 is a promising catalyst for Li-O2 batteries.

Nanohybrid structured RuO2/Mn2O3/CNF as a catalyst for Na-O2 batteries

A 3D RuO₂/Mn₂O₃/carbon nanofiber (CNF) composite has been prepared in this study by a facile two step microwave synthesis, as a bi-functional electrocatalyst towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). RuO₂ nanoparticles with the mean size of 1.57 nm are uniformly distributed on Mn₂O₃ nano-rods grown on electrospun CNFs. The electrocatalytic activity of the composites are investigated towards ORR/OER under alkaline condition. The ternary RuO₂/Mn₂O₃/CNF composite showed superior ORR activity in terms of onset potential (0.95 V versus RHE) and Tafel slope (121 mV dec^-1) compared to its RuO₂/CNF and Mn₂O₃/CNF counterparts. In the case of OER, the RuO₂/Mn₂O₃/CNF exhibited 0.34 V over-potential value measured at 10 mA cm^-2 and 52 mV dec^-1 Tafel slope which are lower than those of the other synthesized samples and as compared to state of the art RuO₂ and IrO _x type materials. RuO₂/Mn₂O₃/CNF also exhibited higher specific capacity (9352 mAh [Formula: see text]) than CNF (1395 mAh [Formula: see text]), Mn₂O₃/CNF (3108 mAh [Formula: see text]) and RuO₂/CNF (4859 mAh g _carbon ^-1) as the cathode material in Na-O₂ battery, which indicates the validity of the results in non-aqueous medium. Taking the benefit of RuO₂ and Mn₂O₃ synergistic effect, the decomposition of inevitable side products at the end of charge occurs at 3.838 V versus Na/Na⁺ by using RuO₂/Mn₂O₃/CNF, which is 388 mV more cathodic compared with CNF.

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.

Decorating carbon nanofibers with Mo2C nanoparticles towards hierarchically porous and highly catalytic cathode for high-performance Li-O2 batteries

A facile synthesis of the hierarchically porous cathode with Mo₂C nanoparticles through the electrospinning technique and heat treatment is proposed. The carbonization temperature of the precursors is the key factor for the formation of Mo₂C nanoparticles on the carbon nanofibers (MCNFs). Compared with the Mo₂N nanoparticles embedded into N-doped carbon nanofibers film (MNNFs) and N-doped carbon nanofibers film (NFs), the battery with MCNFs cathode is capable of operation with a high-capacity (10,509 mAh g^-1 at 100 mA g^-1), a much reduced discharge-charge voltage gap, and a long-term life (124 cycles at 200 mA g^-1 with a specific capacity limit of 500 mAh g^-1). These excellent performances are derived from the synergy of the following advantageous factors: (1) the hierarchically self-standing and binder-free structure of MCNFs could ensure the high diffusion flux of Li⁺ and O₂ as well as avoid clogging of the discharge product, bulk Li₂O₂; (2) the well dispersed Mo₂C nanoparticles not only afford rich active sites, but also facilitate the electronic transfer for catalysis.

Modest Oxygen-Defective Amorphous Manganese-Based Nanoparticle Mullite with Superior Overall Electrocatalytic Performance for Oxygen Reduction Reaction

Manganese-based oxides have exhibited high promise as noncoinage alternatives to Pt/C for catalyzing oxygen reduction reaction (ORR) in basic solution and a mix of Mn^3+/4+ valence is believed to be vital in achieving optimum ORR performance. Here, it is proposed that, distinct from the most studied perovskites and spinels, Mn-based mullites with equivalent molar ratio of Mn³⁺ and Mn⁴⁺ provide a unique platform to maximize the role of Mn valence in facile ORR kinetics by introducing modest content of oxygen deficiency, which is also beneficial to enhanced catalytic activity. Accordingly, amorphous mullite SmMn₂ O_5-δ nanoparticles with finely tuned concentration of oxygen vacancies are synthesized via a versatile top-down approach and the modest oxygen-defective sample with an Mn³⁺ /Mn⁴⁺ ratio of 1.78, i.e., Mn valence of 3.36 gives rise to a superior overall ORR activity among the highest reported for the family of Mn-based oxides, comparable to that of Pt/C. Altogether, this study opens up great opportunities for mullite-based catalysts to be a cost-effective alternative to Pt/C in diverse electrochemical energy storage and conversion systems.

MCNTs@MnO2 Nanocomposite Cathode Integrated with Soluble O2-Carrier Co-salen in Electrolyte for High-Performance Li-Air Batteries

Li-air batteries (LABs) are promising because of their high energy density. However, LABs are troubled by large electrochemical polarization during discharge and charge, side reactions from both carbon cathode surface/peroxide product and electrolyte/superoxide intermediate, as well as the requirement for pure O₂. Here we report the solution using multiwall carbon nanotubes (MCNTs)@MnO₂ nanocomposite cathode integrated with N,N'-bis(salicylidene)ethylenediaminocobalt(II) (Co^II-salen) in electrolyte for LABs. The advantage of such a combination is that on one hand, the coating layer of δ-MnO₂ with about 2-3 nm on MCNTs@MnO₂ nanocomposite catalyzes Li₂O₂ decomposition during charge and suppresses side reactions between product Li₂O₂ and MCNT surface. On the other hand, Co^II-salen works as a mobile O₂-carrier and accelerates Li₂O₂ formation through the reaciton of (Co^III-salen)₂-O₂^2- + 2Li⁺ + 2e^- → 2Co^II-salen + Li₂O₂. This reaction route overcomes the pure O₂ limitation and avoids the formation of aggressive superoxide intermediate (O₂^- or LiO₂), which easily attacks organic electrolyte. By using this double-catalyst system of Co-salen/MCNTs@MnO₂, the lifetime of LABs is prolonged to 300 cycles at 500 mA g^-1 (0.15 mA cm^-2) with fixed capacity of 1000 mAh g^-1 (0.30 mAh cm^-2) in dry air (21% O₂). Furthermore, we up-scale the capacity to 500 mAh (5.2 mAh cm^-2) in pouch-type batteries (∼4 g, 325 Wh kg^-1). This study should pave a new way for the design and construction of practical LABs.

Three-Dimensional Ordered Macroporous FePO4 as High-Efficiency Catalyst for Rechargeable Li-O2 Batteries

The Li-O₂ battery is receiving much recent attention because of its superhigh theoretical energy density. However, its performance is limited by the irreversible formation/decomposition of Li₂O₂ on the cathode and the undesired electrolyte decomposition. In this work, low-cost three-dimensional ordered macroporous (3DOM) FePO₄ is synthesized by using polystyrene (PS) spheres template in a facile experimental condition and applied as a high-efficiency catalyst for rechargeable Li-O₂ batteries, including good rate performance, high specific capacity, and perfect cycling stability. The superior performances can be attributed to the unique structure of 3DOM FePO₄ cathodes, which can provide an efficient buffer space for O₂/Li₂O₂ conversion. In addition, it is demonstrated that the Li⁺ intercalation/deintercalation behavior of 3DOM FePO₄ in ether-based electrolyte can contribute to capacity for Li-O₂ batteries over cycling. As a result, when there is no O₂ in the environment, the Li-O₂ cell can also be operated as a rechargeable Li-FePO₄ cell with a perfect cycle capability.

Nanostructured Bifunctional Redox Electrocatalysts

Electrocatalysts are playing a prominent role in the design of renewable energy devices. Benefiting from a long and prosperous history of synthesizing individual hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) electrocatalysts, the development of bifunctional ORR/OER or HER/OER electrocatalysts has recently emerged as a new research hotspot. In this review, a brief account of recent developments of bifunctional electrocatalysts for ORR/OER and HER/OER are introduced, aiming to provide insights into theoretical understanding of these reactions through analysis and comparison of various bifunctional electrocatalysts. The related reaction mechanisms and the associated activity descriptors for aforementioned reactions in the recent literatures are also presented. Different series of bifunctional electrocatalysts with much improved performances are discussed in detail and their design principles are outlined. Finally, the existing challenges and the future effort directions for enhancing the performance of bifunctional electrocatalysts are proposed.

Novel Hydrogel-Derived Bifunctional Oxygen Electrocatalyst for Rechargeable Air Cathodes

The commercialization of Zn-air batteries has been impeded by the lack of low-cost, highly active, and durable catalysts that act independently for oxygen electrochemical reduction and evolution. Here, we demonstrate excellent performance of NiCo nanoparticles anchored on porous fibrous carbon aerogels (NiCo/PFC aerogels) as bifunctional catalysts toward the Zn-air battery. This material is designed and synthesized by a novel K₂Ni(CN)₄/K₃Co(CN)₆-chitosan hydrogel-derived method. The outstanding performance of NiCo/PFC aerogels is confirmed as a superior air-cathode catalyst for a rechargeable Zn-air battery. At a discharge-charge current density of 10 mA cm^-2, the NiCo/PFC aerogels enable a Zn-air battery to cycle steadily up to 300 cycles for 600 h with only a small increase in the round-trip overpotential, notably outperforming the more costly Pt/C+IrO₂ mixture catalysts (60 cycles for 120 h). With the simplicity of the synthetic method and the outstanding electrocatalytic performance, the NiCo/PFC aerogels are promising electrocatalysts for Zn-air batteries.

Highly hierarchical porous structures constructed from NiO nanosheets act as Li ion and O2pathways in long cycle life, rechargeable Li–O2batteries

Synthetic route and properties of flower-like NiO used as a cathodic catalyst in rechargeable Li–O₂batteries.

A soil/Vulcan XC-72 hybrid as a highly-effective catalytic cathode for rechargeable Li–O2batteries

We report for the first time a hybrid of soil and commercial Vulcan XC-72 carbon (labeled as soil/C) as a high-performance cathode catalyst for rechargeable lithium–oxygen batteries. It was found that soil as a low cost and metal-free void volume expander in the hybrid catalyst is promising in the application of rechargeable Li–O₂batteries.