A Review of Carbon-Composited Materials as Air-Electrode Bifunctional Electrocatalysts for Metal–Air Batteries

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.

Hetero-Diatomic CoN4-NiN4 Site Pairs with Long-Range Coupling as Efficient Bifunctional Catalyst for Rechargeable Zn-Air Batteries

In this study, Co/Ni-NC catalyst with hetero-diatomic Co/Ni active sites dispersed on nitrogen-doped carbon matrix is synthesized via the controlled pyrolysis of ZIF-8 containing Co2+ and Ni2+ compounds. Experimental characterizations and theoretical calculations reveal that Co and Ni are atomically and uniformly dispersed in pairs of CoN4-NiN4 with an intersite distance ≈0.41 nm, and there is long-range d-d coupling between Co and Ni with more electron delocalization for higher bifunctional activity. Besides, the in situ grown carbon nanotubes at the edges of the catalyst particles allow high electronic conductivity for electrocatalysis process. Electrochemical evaluations demonstrate the superior ORR and OER bifunctionality of Co/Ni-NC catalyst with a narrow potential gap of only 0.691 V and long-term durability, significantly prevailing over the single-atom Co-NC and Ni-NC catalysts and the benchmark Pt/C and RuO2 catalysts. Co/Ni-NC catalyzed Zn-air batteries achieve a high specific capacity of 771 mAh g-1 and a long continuous operation period up to 340 h with a small voltage gap of ≈0.65 V, also much superior to Pt/C-RuO2.

Aluminum-air batteries: current advances and promises with future directions

Owing to their attractive energy density of about 8.1 kW h kg-1 and specific capacity of about 2.9 A h g-1, aluminum-air (Al-air) batteries have become the focus of research. Al-air batteries offer significant advantages in terms of high energy and power density, which can be applied in electric vehicles; however, there are limitations in their design and aluminum corrosion is a main bottleneck. Herein, we aim to provide a detailed overview of Al-air batteries and their reaction mechanism and electrochemical characteristics. This review emphasizes each component/sub-component including the anode, electrolyte, and air cathode together with strategies to modify the electrolyte, air-cathode, and even anode for enhanced performance. The latest advancements focusing on the specific design of Al-air batteries and their rechargeability characteristics are discussed. Finally, the constraints and prospects of their use in mobility applications are also covered in depth. Thus, the present review may pave the way for researchers and developers working in energy storage solutions to look beyond lithium/sodium ion-based storage solutions.

In-Situ Nanoarchitectonics of Fe/Co LDH over Cobalt-Enriched N-Doped Carbon Cookies as Facile Oxygen Redox Electrocatalysts for High-Rate Rechargeable Zinc-Air Batteries

The reality of long-term rechargeable and high-performance zinc-air batteries relies majorly on cost-effective and eminent bifunctional electrocatalysts, which can perform both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Herein, we demonstrate a new approach for the synthesis of in-situ-grown layered double hydroxide of iron and cobalt over a cobalt nanoparticle-enriched nitrogen-doped carbon frame (CoL 2:1) by a simple coprecipitation reaction with facile scale-up and explore its electrocatalytic ORR and OER activity for an electrically rechargeable zinc-air battery. Consequently, the developed composite displays excellent ORR and OER activity with an ORR half-wave potential of 0.84 V, a limiting current density of 5.85 mA/cm2, and an OER overpotential of 320 mV with exceptional stability. The outstanding bifunctionality index of the catalyst (ΔE = 0.72 V) inspired us to utilize it as a cathode catalyst in an in-house developed prototype zinc-air battery. The battery could easily supply a specific capacity of 804 mAh/g with a maximum peak power density of 161 mW/cm2. The battery exhibits an attractive charge-discharge profile with a lesser voltage gap of 0.76 V at 10 mA/cm2 with durability for a period of 200 h and a voltage efficiency of 97%, which surpassed the corresponding Pt/C + RuO2-based zinc-air battery. Further, a maximum load of 50 mA/cm2 could easily be sustained during cycling, revealing its outstanding stability. A series-connected two CoL 2:1-based zinc-air batteries effortlessly enlighten a pinwheel fan and LED panel simultaneously, revealing its practicality. The high electrical conductivity and greater specific surface area of Co/N-C and its robust attachment with Fe/Co LDH preserves both active sites, thereby resulting in exceptional performance. Our method is capable of being flexible enough to create various bifunctional Co/N-C-based composite electrodes, opening up a feasible pathway to rechargeable zinc-air batteries with maximum energy density.

Commercially Accessible High-Performance Aluminum-Air Battery Cathodes through Electrodeposition of Mn and Ni Species on Fuel Cell Cathodes

This study presents a cost-effective method for producing high-performance cathodes for aluminum-air batteries. Commercial fuel cell cathodes are modified through electrodeposition of nickel and manganese species. The optimal conditions for electrodeposition are determined using a combination of structural (Raman, SEM, TEM) and electrochemical (LSV, EI, discharge curves) characterization techniques. The structural analysis confirms successful incorporation of nickel and manganese species onto the cathode surface. Electrochemical tests demonstrate enhanced electrochemical activity compared to unmodified cathodes. By combining the favorable properties of electrodeposited manganese species with nickel species, a high-performance cathode is obtained. The developed cathode exhibits capacities of 50 mA h cm-2 in aluminum-air batteries across a wide range of current densities. The electrodeposition method proves effective in improving electrochemical performance. A key advantage of this method is its simplicity and cost-effectiveness. The use of commercially available materials and well-established electrodeposition techniques allows for easy scalability and commercialization. This makes it a viable option for large-scale production of high-performance cathodes for the next-generation energy storage devices.

Reconstructing 3d-Metal Electrocatalysts through Anionic Evolution in Zinc-Air Batteries

As one of the promising sustainable energy storage systems, academic research on rechargeable Zn-air batteries has recently been rejuvenated following development of various 3d-metal electrocatalysts and identification of their dynamic reconstruction toward (oxy)hydroxide, but performance disparity among catalysts remains unexplained. Here, this uncertainty is addressed through investigating the anionic contribution to regulate dynamic reconstruction and battery behavior of 3d-metal selenides. Comparing with the alloy counterpart, anionic chemistry is identified as a performance promoter and further exploited to empower Zn-air batteries. Based on theoretical modeling, Se-resolved operando spectroscopy, and advanced electron microscopy, a three-step Se evolution is established, consisting of oxidation, leaching, and recoordination. The process generates an amorphous (oxy)hydroxide with O-sharing bonded Se motifs that triggers charge redistribution at metal sites and lowers the energetic barrier of their current-driven redox. A pervasive concept of Se back-feeding is then proposed to describe the underlying chemistry for 3d-metal selenides with diversity in crystals or compositions, and the feasibility to fine-tune their behavior is also presented.

Insights into the Electrochemical Behavior of Manganese Oxides as Catalysts for the Oxygen Reduction and Evolution Reactions: Monometallic Core-Shell Mn/Mn3 O4

Overcoming the sluggish electrode kinetics of both oxygen reduction and evolution reactions (ORR/OER) with non-precious metal electrocatalysts will accelerate the development of rechargeable metal-air batteries and regenerative fuel cells. The authors investigated the electrochemical behavior and ORR/OER catalytic activity of core-porous shell Mn/Mn₃ O₄ nanoparticles in comparison with other manganese dioxides (β- and γ-MnO₂ ), and benchmarked against Pt/C and Pt/C-IrO₂ . Under reversible operation in O₂ -saturated 5 M KOH at 22 °C, the early stage activity of core-shell Mn/Mn₃ O₄ shows two times higher ORR and OER current density compared to the other MnO₂ structures at 0.32 and 1.62 V versus RHE, respectively. It is revealed that Mn(III) oxidation to Mn(IV) is the primary cause of Mn/Mn₃ O₄ activity loss during ORR/OER potential cycling. To address it, an electrochemical activation method using Co(II) is proposed. By incorporating Co(II) into MnO_x , new active sites are introduced and the content of Mn(II) is increased, which can stabilize the Mn(III) sites through comproportionation with Mn(IV). The Co-incorporated Mn/Mn₃ O₄ has superior activity and durability. Furthermore, it also surpassed the activity of Pt/C-IrO₂ with similar durability. This study demonstrates that cost-effective ORR/OER catalysis is possible.

Reversible Discharge Products in Li-Air Batteries

Lithium-air (Li-air) batteries stand out among the post-Li-ion batteries due to their high energy density, which has rapidly progressed in the past years. Regarding the fundamental mechanism of Li-air batteries that discharge products produced and decomposed during charging and recharging progress, the reversibility of products closely affects the battery performance. Along with the upsurge of the mainstream discharge products lithium peroxide, with devoted efforts to screening electrolytes, constructing high-efficiency cathodes, and optimizing anodes, much progress is made in the fundamental understanding and performance. However, the limited advancement is insufficient. In this case, the investigations of other discharge products, including lithium hydroxide, lithium superoxide, lithium oxide, and lithium carbonate, emerge and bring breakthroughs for the Li-air battery technologies. To deepen the understanding of the electrochemical reactions and conversions of discharge products in the battery, recent advances in the various discharge products, mainly focusing on the growth and decomposition mechanisms and the determining factors are systematically reviewed. The perspectives for Li-air batteries on the fundamental development of discharge products and future applications are also provided.

Metal-organic framework derived FeNi alloy nanoparticles embedded in N-doped porous carbon as high-performance bifunctional air-cathode catalysts for rechargeable zinc-air battery

Developing efficient and durable bifunctional air-cathode catalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is one of the key efforts promoting the practical rechargeable zinc-air batteries (ZABs). In this paper, high-performance bifunctional air-cathode catalysts by a two-step strategy: atomically dispersed Ni on N-doped carbon is first derived from MOF to form uniformly dispersed NiNC, which are pyrolyzed together with Fe source at different high-temperatures to form FeNi@NC-T (T = 800, 900, and 1000 °C) catalysts. The as-synthesized non-noble metal FeNi@NC-900 catalyst exhibits a considerably small potential gap (ΔE) of 0.72 V between ORR and OER, which is as the same as commercial noble metal Pt/C + Ir black mixed catalyst. The performance of the ZABs using FeNi@NC-900 as the air-cathode catalyst displays a power density of 119 mW·cm^-2 and a specific capacity of 830.1 mAh·g^-1, which is superior to that of Pt/C + Ir black mixed catalyst. This work provides a guideline for designing alloy electrocatalysts with uniform size and nanoparticle distribution for metal-air batteries with bifunctional air-cathodes.

A review on development of metal-organic framework-derived bifunctional electrocatalysts for oxygen electrodes in metal-air batteries

Worldwide demand for oil, coal, and natural gas has increased recently because of odd weather patterns and economies recovering from the pandemic. By using these fuels at an astonishing rate, their reserves are running low with each passing decade. Increased reliance on these sources is contributing significantly to both global warming and power shortage problems. It is vital to highlight and focus on using renewable energy sources for power production and storage. This review aims to discuss one of the cutting-edge technologies, metal-air batteries, which are currently being researched for energy storage applications. A battery that employs an external cathode of ambient air and an anode constructed of pure metal in which an electrolyte can be aqueous or aprotic electrolyte is termed as a metal-air battery (MAB). Due to their reportedly higher energy density, MABs are frequently hailed as the electrochemical energy storage of the future for applications like grid storage or electric car energy storage. The demand of the upcoming energy storage technologies can be satisfied by these MABs. The usage of metal-organic frameworks (MOFs) in metal-air batteries as a bi-functional electrocatalyst has been widely studied in the last decade. Metal ions or arrays bound to organic ligands to create one, two, or three-dimensional structures make up the family of molecules known as MOFs. They are a subclass of coordination polymers; metal nodes and organic linkers form different classes of these porous materials. Because of their modular design, they offer excellent synthetic tunability, enabling precise chemical and structural control that is highly desirable in electrode materials of MABs.

Metal ferrites-based nanocomposites and nanohybrids for photocatalytic water treatment and electrocatalytic water splitting

Photocatalytic degradation is one of the most promising technologies available for removing a variety of synthetic and organic pollutants from the environmental matrices because of its high catalytic activity, reduced energy consumption, and low total cost. Due to its acceptable bandgap, broad light-harvesting efficiency, significant renewability, and stability, Fe₂O₃ has emerged as a fascinating material for the degradation of organic contaminants as well as numerous dyes. This study thoroughly reviewed the efficiency of Fe₂O₃-based nanocomposite and nanomaterials for water remediation. Iron oxide structure and various synthetic methods are briefly discussed. Additionally, the electrocatalytic application of Fe₂O₃-based nanocomposites, including oxygen evolution reaction, oxygen reduction reaction, hydrogen evolution reaction, and overall water splitting efficiency, was also highlighted to illustrate the great promise of these composites. Finally, the ongoing issues and future prospects are directed to fully reveal the standards of Fe₂O₃-based catalysts. This review is intended to disseminate knowledge for further research on the possible applications of Fe₂O₃ as a photocatalyst and electrocatalyst.

Facile synthesis approach of bifunctional Co–Ni–Fe oxyhydroxide and spinel oxide composite electrocatalysts from hydroxide and layered double hydroxide composite precursors†

Zinc–air batteries (ZABs) are promising candidates for the next-generation energy storage systems, however, their further development is severely hindered by kinetically sluggish oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Facile synthesis approaches of highly active bifunctional electrocatalysts for OER and ORR are required for their practical applications. Herein, we develop a facile synthesis procedure for composite electrocatalysts composed of OER-active metal oxyhydroxide and ORR-active spinel oxide containing Co, Ni and Fe from composite precursors consisting of metal hydroxide and layered double hydroxide (LDH). Both hydroxide and LDH are simultaneously produced by a precipitation method with a controlled molar ratio of Co²⁺, Ni²⁺ and Fe³⁺ in the reaction solution, and calcination of the precursor at a moderate temperature provides composite catalysts of metal oxyhydroxides and spinel oxides. The composite catalyst shows superb bifunctional performances with a small potential difference of 0.64 V between a potential of 1.51 V vs. RHE at 10 mA cm⁻² for OER and a half-wave potential of 0.87 V vs. RHE for ORR. The rechargeable ZAB assembled with the composite catalyst as an air-electrode exhibits a power density of 195 mA cm⁻² and excellent durability of 430 hours (1270 cycles) of a charge–discharge cycle test.

Simple and durable: the multi-metal oxyhydroxide and spinal oxide composite catalyst containing Co, Fe and Ni are synthesized from hydroxide and layered double hydroxide composite precursors and shows excellent bifunctional ORR/OER activities.

Metal-organic framework derived vanadium oxide supported nanoporous carbon structure as a bifunctional electrocatalyst for potential application in metal air batteries

High-efficiency, sustainable, non-precious metal-based electrocatalysts with bifunctional catalytic activity for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are essential for metal-air batteries. In this research, a bifunctional electrocatalyst is developed by synthesizing a novel nanoporous vanadium oxide/carbon composite (NVC-900) through pyrolysis of a highly efficient vanadium metal-organic framework, MIL-101 (V). The fabrication process was conveniently carried out by pyrolyzing the synthesized MIL-101 (V) at 900 °C, producing vanadium oxide nanoparticles embedded in the extensively distributed pores of the carbon network. The evenly distributed nanopores substantially improve the performance of the efficient electrocatalyst for both the oxygen reduction reaction and oxygen evolution reactions (ORR/OER) by increasing surface area and facilitating access to stable catalytic active sites. The unique structure was characterized by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). For oxygen reduction reaction (ORR), the electrocatalyst established a promising limiting current density (J _L) of 5.2 mA cm^-2 at 1600 rpm at an onset potential of 1.18 V and a half-wave potential of 0.82 V, and for OER, a current density of 10 mA cm^-2 was delivered at a potential of 1.48 V. In comparison to 10% Pt/C, the synthesized bifunctional electrocatalyst being almost equally active towards bifunctional activity, showed much better long-term cyclic stability. The one-step thermal pyrolysis strategy to synthesize the nanoporous functional material and the proposed electrocatalytic material's long-term bifunctional activity and durability make it an ideal fit for next-generation portable green metal-air batteries.

High-Energy Batteries: Beyond Lithium-Ion and Their Long Road to Commercialisation

Rechargeable batteries of high energy density and overall performance are becoming a critically important technology in the rapidly changing society of the twenty-first century. While lithium-ion batteries have so far been the dominant choice, numerous emerging applications call for higher capacity, better safety and lower costs while maintaining sufficient cyclability. The design space for potentially better alternatives is extremely large, with numerous new chemistries and architectures being simultaneously explored. These include other insertion ions (e.g. sodium and numerous multivalent ions), conversion electrode materials (e.g. silicon, metallic anodes, halides and chalcogens) and aqueous and solid electrolytes. However, each of these potential "beyond lithium-ion" alternatives faces numerous challenges that often lead to very poor cyclability, especially at the commercial cell level, while lithium-ion batteries continue to improve in performance and decrease in cost. This review examines fundamental principles to rationalise these numerous developments, and in each case, a brief overview is given on the advantages, advances, remaining challenges preventing cell-level implementation and the state-of-the-art of the solutions to these challenges. Finally, research and development results obtained in academia are compared to emerging commercial examples, as a commentary on the current and near-future viability of these "beyond lithium-ion" alternatives.

Stabilizing Cobalt Single Atoms via Flexible Carbon Membranes as Bifunctional Electrocatalysts for Binder-Free Zinc-Air Batteries

Single-atom catalysts with high activity and efficient atom utilization have great potential in the electrocatalysis field, especially for rechargeable zinc-air batteries (ZABs). However, it is still a serious challenge to rationally construct a single-atom catalyst with satisfactory electrocatalytic activity and long-term stability. Here, we simultaneously realize the atomic-level dispersion of cobalt and the construction of carbon nanotube (CNT)-linked N-doped porous carbon nanofibers (NCFs) via an electrospinning strategy. In this hierarchical structure, the Co-N₄ sites provide efficient oxygen reduction/evolution electrocatalytic activity, the porous architectures of NCFs guarantee the active site's accessibility, and the interior CNTs enhance the flexibility and mechanical strength of porous fibers. As a binder-free air cathode, the as-prepared catalysts deliver superdurability of 600 h at 10 mA cm^-2 for aqueous ZABs and considerable flexibility and a small voltage gap for all-solid-state ZABs. This work provides an effective single-atom design/nanoengineering for superdurable zinc-air batteries.

Heteroatom-Doped Metal-Free Carbon Nanomaterials as Potential Electrocatalysts

In recent years, heteroatom-incorporated specially structured metal-free carbon nanomaterials have drawn huge attention among researchers. In comparison to the undoped carbon nanomaterials, heteroatoms such as nitrogen-, sulphur-, boron-, phosphorous-, etc., incorporated nanomaterials have become well-accepted as potential electrocatalysts in water splitting, supercapacitors and dye-sensitized solar cells. This review puts special emphasis on the most popular synthetic strategies of heteroatom-doped and co-doped metal-free carbon nanomaterials, viz., chemical vapor deposition, pyrolysis, solvothermal process, etc., utilized in last two decades. These specially structured nanomaterials' extensive applications as potential electrocatalysts are taken into consideration in this article. Their comparative enhancement of electrocatalytic performance with incorporation of heteroatoms has also been discussed.

Carbon-based metal-free electrocatalysts: from oxygen reduction to multifunctional electrocatalysis

Since the discovery of N-doped carbon nanotubes as the first carbon-based metal-free electrocatalyst (C-MFEC) for oxygen reduction reaction (ORR) in 2009, C-MFECs have shown multifunctional electrocatalytic activities for many reactions beyond ORR, such as oxygen evolution reaction (OER), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO₂RR), nitrogen reduction reaction (NRR), and hydrogen peroxide production reaction (H₂O₂PR). Consequently, C-MFECs have attracted a great deal of interest for various applications, including metal-air batteries, water splitting devices, regenerative fuel cells, solar cells, fuel and chemical production, water purification, to mention a few. By altering the electronic configuration and/or modulating their spin angular momentum, both heteroatom(s) doping and structural defects (e.g., atomic vacancy, edge) have been demonstrated to create catalytic active sites in the skeleton of graphitic carbon materials. Although certain C-MFECs have been made to be comparable to or even better than their counterparts based on noble metals, transition metals and/or their hybrids, further research and development are necessary in order to translate C-MFECs for practical applications. In this article, we present a timely and comprehensive, but critical, review on recent advancements in the field of C-MFECs within the past five years or so by discussing various types of electrocatalytic reactions catalyzed by C-MFECs. An emphasis is given to potential applications of C-MFECs for energy conversion and storage. The structure-property relationship for and mechanistic understanding of C-MFECs will also be discussed, along with the current challenges and future perspectives.

Low content Ru-incorporated Pd nanowires for bifunctional electrocatalysis

This paper reports the facile synthesis and characterization of carbon supported Pd nanowires with low Ru contents (nRuPd/C). An anti-galvanic replacement reaction involving the reduction of Ru(iii) ions by nanoporous Pd nanowires to form nRuPd alloy nanowires was observed. A series of nRuPd/C materials with various Ru/Pd ratios were prepared by the spontaneous deposition of a Ru cluster on a Pd nanowire core using different Ru precursor concentrations (RuCl₃ = 0.5, 1.0, 5.0 mM). The successful formation of low content Ru-incorporated Pd nanowires without individual Ru clusters were confirmed using physicochemical characterization. The electrocatalytic activity of the nRuPd/C for the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) in alkaline media was measured by RDE polarization experiments. The electrocatalytic activity varied greatly depending on the Ru content on the Pd nanowires. Among the catalysts, the prepared Pd nanowires incorporated with a very small amount of Ru (ca. 1.4 wt%) exhibited excellent electrocatalytic activity toward the ORR and HER: positive ORR/HER onset and E_1/2 potentials, higher n value, and lower Tafel slope.

The catalytic activity of Pd nanowires with low Ru contents showed superior bifunctional electrocatalytic performance towards both ORR and HER compared to the benchmarking Pt/C.

Zeolitic imidazole framework derived N-doped porous carbon/metal cobalt nanoparticles hybrid for oxygen electrocatalysis and rechargeable Zn-air batteries

Bifunctional electrocatalysts with high catalytic property for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are vital for high-performance zinc-air batteries (ZnABs). In this study, an efficient bifunctional electrocatalyst with hollow structure (C-N/Co (1/2)) has been successfully prepared through carbonization of ZIF-8@ZIF-67 and evaporation of Zn ions at high temperature. With Co nanoparticles encapsulated by an N-doped porous carbon matrix, the catalyst exhibits excellent stability in aqueous alkaline solution over an extended period and good tolerance to the methanol crossover effect. The integration of an N-doped graphitic carbon outer shell and Co nanoparticles enables high ORR and OER activity, as evidenced by ZnAB using the catalyst C-N/Co (1/2) in an air cathode.

Co/N-Doped hierarchical porous carbon as an efficient oxygen electrocatalyst for rechargeable Zn-air battery

Developing efficient electrocatalysts for ORR/OER is the key issue for the large-scale application of rechargeable Zn-air batteries. The design of Co and N co-doped carbon matrices has become a promising strategy for the fabrication of bi-functional electrocatalysts. Herein, the surface-oxidized Co nanoparticles (NPs) encapsulated into N-doped hierarchically porous carbon materials (Co/NHPC) are designed as ORR/OER catalysts for rechargeable Zn-air batteries via dual-templating strategy and pyrolysis process containing Co2+. The fabricated electrocatalyst displays a core-shell structure with the surface-oxidized Co nanoparticles anchored on hierarchically porous carbon sheets. The carbon shells prevent Co NP cores from aggregating, ensuring excellent electrocatalytic properties for ORR with a half-wave potential of 0.82 V and a moderate OER performance. Notably, the obtained Co/NHPC as a cathode was further assembled in a zinc-air battery that delivered an open-circuit potential of 1.50 V, even superior to that of Pt/C (1.46 V vs. RHE), a low charge-discharge voltage gap, and long cycle life. All these results demonstrate that this study provides a simple, scalable, and efficient approach to fabricate cost-effective high-performance ORR/OER catalysts for rechargeable Zn-air batteries.

Acid-treated multi-walled carbon nanotubes as additives for negative active materials to improve high-rate-partial-state-of-charge cycle-life of lead-acid batteries

In this work, a trace amount of acid-treated multi-walled carbon nanotubes (a-MWCNTs) is introduced into the negative active materials (NAMs) of a lead acid battery (LAB) by simply dispersing a-MWCNTs in the water, which is then added into the dry mixture of lead oxide powder, expanders and carbon black for lead paste preparation. The abundant oxygen-containing groups on the a-MWCNTs show significant influence on the chemical reactions happening during the curing process, leading to the improved properties of NAMs. Specifically, after formation, the NAMs containing 100 ppm a-MWCNTs display a spongy-like structure comprised of interconnected domino-like Pb slices, giving favorable porosity and electroactive surface area of the NAMs. Moreover, the quasi-rod structure of Pb slices provides the channels for fast electron transfer. These two features greatly accelerate the electrochemical reaction between Pb and PbSO₄, and hence hinder the accumulation of PbSO₄ crystals. As a result, the high-rate partial-state-of-charge (HRPSoC) cycle-life of the simulated cell constructed from the a-MWCNTs-containing negative plate achieves a HRPSoC cycle-life more than 1.5 times longer than the cell constructed when the negative plate contains only carbon black. Since our method is of great convenience and low-cost, it is expected to have a great feasibility in the LAB industry.

100 ppm aMWCNTs were incorporated into the NAMs of LABs. The oxygen-containing groups on the aMWCNTs induced the formation of interconnected domino-like Pb slices in the NAMs. Thus, the HRPSoC cycle-life of the simulated cell was improved by 50%.

Advanced Electrocatalysis for Energy and Environmental Sustainability via Water and Nitrogen Reactions

Clean and efficient energy storage and conversion via sustainable water and nitrogen reactions have attracted substantial attention to address the energy and environmental issues due to the overwhelming use of fossil fuels. These electrochemical reactions are crucial for desirable clean energy technologies, including advanced water electrolyzers, hydrogen fuel cells, and ammonia electrosynthesis and utilization. Their sluggish reaction kinetics lead to inefficient energy conversion. Innovative electrocatalysis, i.e., catalysis at the interface between the electrode and electrolyte to facilitate charge transfer and mass transport, plays a vital role in boosting energy conversion efficiency and providing sufficient performance and durability for these energy technologies. Herein, a comprehensive review on recent progress, achievements, and remaining challenges for these electrocatalysis processes related to water (i.e., oxygen evolution reaction, OER, and oxygen reduction reaction, ORR) and nitrogen (i.e., nitrogen reduction reaction, NRR, for ammonia synthesis and ammonia oxidation reaction, AOR, for energy utilization) is provided. Catalysts, electrolytes, and interfaces between the two within electrodes for these electrocatalysis processes are discussed. The primary emphasis is device performance of OER-related proton exchange membrane (PEM) electrolyzers, ORR-related PEM fuel cells, NRR-driven ammonia electrosynthesis from water and nitrogen, and AOR-related direct ammonia fuel cells.

Atomic/molecular layer deposition for energy storage and conversion

Energy storage and conversion systems, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting, have played vital roles in the reduction of fossil fuel usage, addressing environmental issues and the development of electric vehicles. The fabrication and surface/interface engineering of electrode materials with refined structures are indispensable for achieving optimal performances for the different energy-related devices. Atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques, the gas-phase thin film deposition processes with self-limiting and saturated surface reactions, have emerged as powerful techniques for surface and interface engineering in energy-related devices due to their exceptional capability of precise thickness control, excellent uniformity and conformity, tunable composition and relatively low deposition temperature. In the past few decades, ALD and MLD have been intensively studied for energy storage and conversion applications with remarkable progress. In this review, we give a comprehensive summary of the development and achievements of ALD and MLD and their applications for energy storage and conversion, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting. Moreover, the fundamental understanding of the mechanisms involved in different devices will be deeply reviewed. Furthermore, the large-scale potential of ALD and MLD techniques is discussed and predicted. Finally, we will provide insightful perspectives on future directions for new material design by ALD and MLD and untapped opportunities in energy storage and conversion.

Biochar Nanocomposite Derived from Watermelon Peels for Electrocatalytic Hydrogen Production

Water splitting is the most potential method to produce hydrogen energy, however, the conventional electrocatalysts encounter the hindrances of high overpotential and low hydrogen production efficiency. Herein, we report a carbon-based nanocomposite (denoted as CCW-x, x stands for the calcination temperature) derived from watermelon peels and CoCl₂, and the as-synthesized CCW-x is used as the electrocatalyst. The overpotential and the Tafel slope of CCW-700 for oxygen evolution reaction (OER) is 237 mV at 10 mA cm^-2 and 69.8 mV dec^-1, respectively, both of which are lower than those of commercial RuO₂. For hydrogen evolution reaction (HER), the overpotential of CCW-700 (111 mV) is higher than that of the widely studied Pt/C (73 mV) but still lower than those of lots of carbon-based nanomaterials (122-177 mV). In the light of CCW-700 is highly active for both OER and HER, we assembled a water-splitting electrocatalyst by employing nickel foam loaded with CCW-700 as the anode and cathode in 1 M KOH. The water-splitting voltage is only 1.54 V for the CCW-700//CCW-700 electrodes and 1.62 V for the RuO₂//Pt/C ones. Therefore, the so-denoted CCW-x powder possesses good electrocatalytic hydrogen production efficiency.

Ultrafine Fe nanoparticles embedded in N-doped carbon nanotubes derived from highly dispersed g-C3N4 nanofibers for the oxygen reduction reaction

The improved performance was attributed to the synergetic interaction between Fe, h-CN and CNTs.

Understanding the mechanisms and design principles for oxygen evolution and oxygen reduction activity on perovskite catalysts for alkaline zinc–air batteries

The high cost and limited availability of the precious metal catalysts required for catalysing the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in metal–air batteries restrict the marketing of these clean energy technologies.

Amino-1H-tetrazole-regulated high-density nitrogen-doped hollow carbon nanospheres for long-life Zn-air batteries

High-density nitrogen-doped porous carbon catalysts have been regarded as promising alternatives to precious metals in proton-exchange membrane fuel cells (PEMFC) and metal-air batteries based on the oxygen reduction reaction (ORR). We herein synthesized high-density pyridinic and graphitic N-doped hollow carbon nanospheres (G&P N-HCS) using a high-yield amino-1H-tetrazole (ATTZ) via a self-sacrificial-template method. The synthesized G&P N-HCS shows a high N content (15.2 at%), in which pyridinic (Pr) and graphitic (Gr) N are highly reactive for the ORR catalysis. We found that the half-wave potential and limiting current density of G&P N-HCS are comparable to the state-of-the-art Pt/C, whereas its cyclic durability is much superior to that of Pt/C. Experimental results indicate that an optimal ratio (1 : 1) between Gr N and Pr N in G&P N-HCS exhibits the highest ORR performances, rather than Gr N-dominated N-HCS or Pr N-dominated N-HCS. Notably, N-HCS containing only Gr N and Pr N has poor catalytic performance for ORR in alkaline electrolytes. Density functional theory (DFT) simulations untangle the catalytic nature of Pr and Gr N and decipher the relations between the N type(s) and total N content required for the ORR catalysis. This study provides a new way to design efficient N-doped porous carbon-enriched active sites, and solves the cathode catalyst in the commercialization of PEMFC and metal-air batteries.

Energy-Efficient Rational Designing of Multifunctional Nanocomposites by Preferential Anchoring of Metal Ions via Fermi Level Positioning of Carbon Nanostructures

Despite the availability and dedicated studies on a variety of carbon nanostructures, amorphous carbon is still a preferred support for a wide range of commercially available metal catalysts. In order to shed some light on this, we carried out electroless deposition of metal nanoparticles on various carbon nanostructures such as amorphous carbon (a-C), carbon nanotubes (CNTs), and nitrogen-doped CNTs (NCNTs) under similar experimental conditions. The main objective is to elucidate the preferable deposition on a particular carbon nanostructure, if any, and understand the underlying mechanism. Experimental results unveil preferred electroless deposition of metal nanoparticles on a-C over CNTs and NCNTs. Notably, the deposition is nicely correlated with the position of the Fermi level (E_F) with respect to the Mⁿ⁺ ↔ M⁰ redox level (E₀). Remarkably, E_F is found to be in the following order NCNT > CNT > a-C and the smaller gap (E₀-E_F) favors the faster electron transfer, resulting in the preferential reduction of Mⁿ⁺, yielding finer nanoparticles on a-C. We believe that this approach can pave the way for designing noble metal-based carbon nanocomposites for a variety of applications, ranging from environmental redemption to electrochemical energy harvesting. As case studies, we have explored the nanocomposites for various catalytic activities and found them to be very competent with recently reported various state-of-the-art electrocatalysts and their commercial counterparts.

Co-doped carbon materials synthesized with polymeric precursors as bifunctional electrocatalysts

The design of stable and high performance metal free bifunctional electrocatalysts is a necessity in alkaline zinc–air batteries for oxygen reduction and evolution reaction. In the present work co-doped carbon materials have been developed from polymeric precursors with abundant active sites to achieve bifunctional activity. A 3-dimensional microporous nitrogen–carbon (NC) and co-doped nitrogen–sulfur–carbon (NSC) and nitrogen–phosphorus–carbon (NPC) were synthesized using poly(2,5-benzimidazole) as an N containing precursor. The obtained sheet like structure shows outstanding ORR and OER performance in alkaline systems with excellent stability compared to Pt/C catalyst. The doped heteroatom in the carbon is expected to have redistributed the charge around heteroatom dopants lowering the ORR potential and modifying the oxygen chemisorption mode thereby weakening the O–O bonding and improving the ORR activity and overall catalytic performance. The bifunctional activity (ΔE = E_j=10 − E_1/2) of an air electrode for NPC, NSC, NC and Pt/C is 0.82 V, 0.87 V, 1.06 V and 1.03 V respectively, and the NPC value is smaller than most of the reported metal and non-metal based electrocatalysts. The ORR (from onset potential) and OER (10 mA cm⁻²) overpotential for NPC, NSC, and NC is (290 mV, 410 mV), (310 mV, 450 mV) and (340 mV, 600 mV) respectively. In the prepared catalyst the NPC exhibited higher ORR and OER activity (NPC > NSC > NC). The doping of P in NPC is found to have a great influence on the microstructure and therefore on the ORR and OER activity.

Metal free bifunctional catalysts based on co-doped carbon materials synthesized from polymeric precursors via a simple pyrolysis route with high cyclic stability and low polarization for Zn–air batteries.

A Flow-Through Membraneless Microfluidic Zinc-Air Cell

In this work, the proof of concept of a functional membraneless microfluidic Zn-air cell (μZAC) that operates with a flow-through arrangement is presented for the first time, where the activity and durability can be modulated by electrodepositing Zn on porous carbon electrodes. For this purpose, Zn electrodes were obtained using chronoamperometry and varying the electrodeposition times (20, 40, and 60 min), resulting in porous electrodes with Zn thicknesses of 3.3 ± 0.3, 11.6 ± 2.4, and 34.8 ± 5.1 μm, respectively. Pt/C was initially used as the cathode to analyze variables, such as KOH concentration and flow rate, and then, two manganese-based materials were evaluated (α-MnO₂ and MnMn₂O₄ spinel, labeled as Mn₃O₄) to determine the effect of inexpensive materials on the cell performance. According to the transmission electron microscopy (TEM) results, α-MnO₂ has a nanorod-like shape with a diameter of 11 ± 1.5 nm, while Mn₃O₄ presented a hemispherical shape with an average particle size of 22 ± 1.8 nm. The use of α-MnO₂ and Mn₃O₄ cathodic materials resulted in cell voltages of 1.39 and 1.35 V and maximum power densities of 308 and 317 mW cm^-2, respectively. The activities of both materials were analyzed through density of state calculations; all manganese species in the α-material MnO₂ presented an equivalent density of states with a reduced orbital occupation to the left of the Fermi energy, which allowed for better global performance above Mn₃O₄/C and Pt/C.

Unraveling the Promotion Effects of a Soluble Cobaltocene Catalyst with Respect to Li-O2 Battery Discharge

The discharge process of a nonaqueous Li-O₂ battery at the cathode is the direct oxygen reduction reaction (ORR) with the formation of discharge product, e.g., Li₂O₂, deposits on the cathode surface. The aggressive superoxide intermediate generated during the ORR severely degrades the organic electrolyte and carbon-based cathodes. To avoid the formation of superoxide species and promote the growth of Li₂O₂ in the electrolyte solution, we employ a soluble cobaltocene [Co(C₅H₅)₂, Cp₂Co] as a homogeneous molecule catalyst to boost the discharge performance of Li-O₂ batteries. Owing to the unique chemical reactivity of Cp₂Co with molecular oxygen, the electrochemistry of the discharge process at the cathode is the (Cp₂Co)₂^II-O₂^2- adduct-mediated process rather than direct electrochemical oxygen reduction, thereby avoiding the formation of aggressive superoxide intermediate. In addition, the strong intermolecular attraction between Cp₂Co and the newly formed Li₂O₂ promotes the solution phase growth of Li₂O₂, which effectively suppresses electrode passivation.

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.

Sustainable Battery Materials from Biomass

Sustainable sources of energy have been identified as a possible way out of today's oil dependency and are being rapidly developed. In contrast, storage of energy to a large extent still relies on heavy metals in batteries. Especially when built from biomass-derived organics, organic batteries are promising alternatives and pave the way towards truly sustainable energy storage. First described in 2008, research on biomass-derived electrodes has been taken up by a multitude of researchers worldwide. Nowadays, in principle, electrodes in batteries could be composed of all kinds of carbonized and noncarbonized biomass: On one hand, all kinds of (waste) biomass may be carbonized and used in anodes of lithium- or sodium-ion batteries, cathodes in metal-sulfur or metal-oxygen batteries, or as conductive additives. On the other hand, a plethora of biomolecules, such as quinones, flavins, or carboxylates, contain redox-active groups that can be used as redox-active components in electrodes with very little chemical modification. Biomass-based binders can replace toxic halogenated commercial binders to enable a truly sustainable future of energy storage devices. Besides the electrodes, electrolytes and separators may also be synthesized from biomass. In this Review, recent research progress in this rapidly emerging field is summarized with a focus on potentially fully biowaste-derived batteries.

Rechargeable Zn-MnO2 batteries: advances, challenges and perspectives

As one type of advanced alternative batteries, zinc-ion batteries (ZIBs) have attracted increasing attention because of their advantages of cost-effectiveness, high safety and environmentally benign features. However, the performance of cathode materials has become a bottleneck for the future application of ZIBs. In recent years, manganese dioxide (MnO₂)-based materials as cathodes for ZIBs have been intensively explored. In this review, recent advances in MnO₂-based cathode materials for ZIBs are comprehensively reviewed with a discussion about the reaction mechanisms for the fundamental understanding of the electrochemical processes. Furthermore, several challenges hindering the technology maturity are also analyzed with corresponding strategies to further improve the electrochemical performance of such Zn-MnO₂ batteries.

Bifunctional Catalysts for Reversible Oxygen Evolution Reaction and Oxygen Reduction Reaction

Metal-air batteries (MABs) and reversible fuel cells (RFCs) rely on the bifunctional oxygen catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Finding efficient bifunctional oxygen catalysts is the ultimate goal and it has attracted a great deal of attention. The dilemma is that a good ORR catalyst is not necessarily efficient for OER, and vice versa. Thus, the development of a new type of bifunctional oxygen catalysts should ensure that the catalysts exhibit high activity for both OER and ORR. Composites with multicomponents for active centers supported on highly conductive matrices could be able to meet the challenges and offering new opportunities. In this Review, the evolution of bifunctional catalysts is summarized and discussed aiming to deliver high-performance bifunctional catalysts with low overpotentials.

Evaluation and Analysis of Battery Technologies Applied to Grid-Level Energy Storage Systems Based on Rough Set Theory

Interest in the development of grid-level energy storage systems has increased over the years. As one of the most popular energy storage technologies currently available, batteries offer a number of high-value opportunities due to their rapid responses, flexible installation, and excellent performances. However, because of the complexity, multifunctionality, and wide deployment of power grids, trade-offs in battery performance exist, especially when considering economics, environmental effects, and safety. Therefore, establishing a comprehensive assessment of battery technologies is an urgent undertaking. In this work, we present an analysis of rough sets to evaluate the integration of battery systems (e.g., lead–acid batteries, lithium-ion batteries, nickel/metal–hydrogen batteries, zinc–air batteries, and Na–S batteries) into a power grid. Specifically, technological properties, economic significance, environmental effects, and safety of these battery systems are evaluated on the basis of rough set theory. In addition, some perspectives are provided to promote the development of battery technologies for grid-level energy storage.

Battery Technologies for Grid-Level Large-Scale Electrical Energy Storage

Grid-level large-scale electrical energy storage (GLEES) is an essential approach for balancing the supply–demand of electricity generation, distribution, and usage. Compared with conventional energy storage methods, battery technologies are desirable energy storage devices for GLEES due to their easy modularization, rapid response, flexible installation, and short construction cycles. In general, battery energy storage technologies are expected to meet the requirements of GLEES such as peak shaving and load leveling, voltage and frequency regulation, and emergency response, which are highlighted in this perspective. Furthermore, several types of battery technologies, including lead–acid, nickel–cadmium, nickel–metal hydride, sodium–sulfur, lithium-ion, and flow batteries, are discussed in detail for the application of GLEES. Moreover, some possible developing directions to facilitate efforts in this area are presented to establish a perspective on battery technology, provide a road map for guiding future studies, and promote the commercial application of batteries for GLEES.