High-Throughput Designed and Laser-Etched NiFeCrVTi High-Entropy Alloys with High Catalytic Activities and Corrosion Resistance for Hydrogen Evolution in Seawater.

Electrocatalytic hydrogen evolution from seawater through wind or solar energy is a cost-effective way to produce green hydrogen fuel. However, the lack of highly active and anti-corrosive electrocatalysts in seawater severely hinders the industrial application. Herein, a novel Ni1.1FeCr0.4V0.3Ti0.3 high-entropy alloy (HEA) is designed through high throughput computing and prepared via powder metallurgy with the surface treated by laser etching under different laser power. The laser-etched NiFeCrVTi high-entropy alloys exhibit a unique periodically ordered structure with multiple active centers and high porosity. The Ni-HEA-30 displays remarkable hydrogen evolution reaction (HER) performance with an overpotential of 55.9 mV and a Tafel slope of 47.3 mV dec-1 in seawater. Density functional theory (DFT) calculations are applied to identify the real active sites for HER on the HEA surface as the key factor for both proton and intermediate transformation, which also reveals that the Cr atom promotes the adsorption energy of water molecules, and the modulation of the electronic structure plays a crucial role in optimizing the hydrogen binding capabilities of the Ni atoms within the alloy. Additionally, the electrocatalyst displays high corrosion resistance in seawater, contributing to the good durability for hydrogen production. This work uncovers a new paradigm to develop novel electrocatalysts with superior reaction activity in seawater.

Intermolecular Hydrogen Bonding Networks Stabilized Organic Supramolecular Cathode for Ultra-High Capacity and Ultra-Long Cycle Life Rechargeable Aluminum Batteries.

Rechargeable aluminum batteries (RABs) are a promising candidate for large-scale energy storage, attributing to the abundant reserves, low cost, intrinsic safety, and high theoretical capacity of Al. However, the cathode materials reported thus far still face challenges such as limited capacity, sluggish kinetics, and undesirable cycle life. Herein, we propose an organic cathode benzo[i] benzo[6,7] quinoxalino [2,3-a] benzo [6,7] quinoxalino [2,3-c] phenazine-5,8,13,16,21,24-hexaone (BQQPH) for RABs. The six C=O and six C=N redox active sites in each molecule enable BQQPH to deliver a record ultra-high capacity of 413 mAh g-1 at 0.2 A g-1. Encouragingly, the intermolecular hydrogen bonding network and π-π stacking interactions endow BQQPH with robust structural stability and minimal solubility, enabling an ultra-long lifetime of 100,000 cycles. Moreover, the electron-withdrawing carbonyl group induces a reduction in the energy level of the lowest unoccupied molecular orbital and expands the π-conjugated system, which considerably enhances both the discharge voltage and redox kinetics of BQQPH. In situ and ex situ characterizations combined with theoretical calculations unveil that the charge storage mechanism is reversible coordination/dissociation of AlCl2 + with the N and O sites in BQQPH accompanied by 12-electron transfer. This work provides valuable insights into the design of high-performance organic cathode materials for RABs.

Aqueous Binders Compatible with Ionic Liquid Electrolyte for High-Performance Aluminum-Ion Batteries.

The incompatibility of poly(vinylidene difluoride) (PVDF) with acidic ionic liquid electrolytes and the use of toxic and high-cost N-methyl pyrrolidone (NMP) solvents hinder the wide application of aluminum-ion batteries (AIBs). In this work, sodium alginate (Na-Alg) is developed as an aqueous binder for the fabrication of graphite positive electrodes in AIBs. The compatibility of various binders with the ionic liquid electrolyte is evaluated, and interaction between various binders and graphite particles before and after cycling is compared and discussed. The results demonstrate that the well compatibility of Na-Alg in ionic liquids and its reasonable distribution on the graphite surface facilitate fast charge transfer and ion diffusion, reduce electrode polarization, and thus contributing to significantly improved cycling stability and rate capability of AIBs. This work provides a new insight into the development of low-cost, eco-friendly, and high-performance binders for AIBs.

Phenoxazine Polymer-based p-type Positive Electrode for Aluminum-ion Batteries with Ultra-long Cycle Life.

Aluminum-ion batteries (AIBs) are a promising candidate for large-scale energy storage due to the abundant reserves, low cost, good safety, and high theoretical capacity of Al. However, AIBs with inorganic positive electrodes still suffer from sluggish kinetics and structural collapse upon cycling. Herein, we propose a novel p-type poly(vinylbenzyl-N-phenoxazine) (PVBPX) positive electrode for AIBs. The dual active sites enable PVBPX to deliver a high capacity of 133 mAh g-1 at 0.2 A g-1 . More impressively, the expanded π-conjugated construction, insolubility, and anionic redox chemistry without bond rearrangement of PVBPX for AIBs contribute to an amazing ultra-long lifetime of 50000 cycles. The charge storage mechanism is that the AlCl4 - ions can reversibly coordinate/dissociate with the N and O sites in PVBPX sequentially, which is evidenced by both experimental and theoretical results. These findings establish a foundation to advance organic AIBs for large-scale energy storage.

Regulation of Atomic Fe-Spin State by Crystal Field and Magnetic Field for Enhanced Oxygen Electrocatalysis in Rechargeable Zinc-Air Batteries.

Highly-active and low-cost bifunctional electrocatalysts for oxygen reduction and evolution are essential in rechargeable metal-air batteries, and single atom catalysts with Fe-N-C are promising candidates. However, the activity still needs to be boosted, and the origination of spin-related oxygen catalytic performance is still uncertain. Herein, an effective strategy to regulate local spin state of Fe-N-C through manipulating crystal field and magnetic field is proposed. The spin state of atomic Fe can be regulated from low spin to intermediate spin and to high spin. The cavitation of dxz and dyz orbitals of high spin FeIII can optimize the O2 adsorption and promote the rate-determining step (*O2 to *OOH). Benefiting from these merits, the high spin Fe-N-C electrocatalyst displays the highest oxygen electrocatalytic activities. Furthermore, the high spin Fe-N-C-based rechargeable zinc-air battery displays a high power density of 170 mW cm-2 and good stability.

Structure and Interface Modification of Carbon Dots for Electrochemical Energy Application.

Carbon dots (CDs) as new nanomaterials have attracted much attention in recent years due to their unique characteristics. Notably, structure and interface modification (carbon core, edge, defects, and functional groups) of CDs have been considered as valid methods to regulate their properties, which contain electron transfer effect, electrochemical activity, fluorescence luminescent, and so on. Additionally, CDs with ultrasmall size, excellent dispersibility, high specific surface area, and abundant functional groups can guarantee positive and extraordinary effects in electrical energy storage and conversion. Therefore, CDs are used to couple with other materials by constructing a special interface structure to enhance their properties. Here, diverse structural and interfacial modifications of CDs with various heteroatoms and synergy effects are systematically analyzed. And not only several main syntheses of CDs-based composites (CDs/X) are summarized but also the merit and demerit of CDs/X in electrical energy storage are discussed. Finally, the applications of CDs/X in energy storage devices (supercapacitors, batteries) and electrocatalysts for practical applications are discussed. This review mainly provides a comprehensive summary and future prospect for synthesis, modification, and electrochemical applications of CDs.

Insights on the fundamental capacitive behavior: a case study of MnO2.

In this work, an insightful study on the fundamental capacitive behavior of MnO2 based electrodes is carried out using MnO2 hierarchical spheres (MHSs) and MnO2 nanoneedles (MNs) as examples. An overall understanding of the relationship between the capacitive performance and the electrode configuration as well as the morphology of active material, loading density, porosity of electrode, and electrolyte concentration is investigated comprehensively. Our analyses show that MnO2 with thin structure is of advantage to increase the utility of active material and to deliver higher specific capacitance, as the faradic reaction happens at/near the surface. Creation of an efficient path for the transport of electrons and ions is crucial to achieve high rate capabilities. Cycling stability could be improved by suppressing the side reaction. It is also important to shed light on the charge contribution from a graphite paper (GP) substrate since it may cause a misinterpretation of the capacitive behavior. This study provides a comprehensive understanding on the fundamental capacitive behavior of MnO2 based electrodes and gives useful clues for designing high performance supercapacitors.

High-Entropy Oxides for Rechargeable Batteries.

High-entropy oxides (HEOs) have garnered significant attention within the realm of rechargeable batteries owing to their distinctive advantages, which encompass diverse structural attributes, customizable compositions, entropy-driven stabilization effects, and remarkable superionic conductivity. Despite the brilliance of HEOs in energy conversion and storage applications, there is still lacking a comprehensive review for both entry-level and experienced researchers, which succinctly encapsulates the present status and the challenges inherent to HEOs, spanning structural features, intrinsic properties, prevalent synthetic methodologies, and diversified applications in rechargeable batteries. Within this review, the endeavor is to distill the structural characteristics, ionic conductivity, and entropy stabilization effects, explore the practical applications of HEOs in the realm of rechargeable batteries (lithium-ion, sodium-ion, and lithium-sulfur batteries), including anode and cathode materials, electrolytes, and electrocatalysts. The review seeks to furnish an overview of the evolving landscape of HEOs-based cell component materials, shedding light on the progress made and the hurdles encountered, as well as serving as the guidance for HEOs compositions design and optimization strategy to enhance the reversible structural stability, electrical properties, and electrochemical performance of rechargeable batteries in the realm of energy storage and conversion.

Rational Design of Organic Electrocatalysts for Hydrogen and Oxygen Electrocatalytic Applications.

Efficient electrocatalysts are pivotal for advancing green energy conversion technologies. Organic electrocatalysts, as cost-effective alternatives to noble-metal benchmarks, have garnered attention. However, the understanding of the relationships between their properties and electrocatalytic activities remains ambiguous. Plenty of research articles regarding low-cost organic electrocatalysts started to gain momentum in 2010 and have been flourishing recently though, a review article for both entry-level and experienced researchers in this field is still lacking. This review underscores the urgent need to elucidate the structure-activity relationship and design suitable electrode structures, leveraging the unique features of organic electrocatalysts like controllability and compatibility for real-world applications. Organic electrocatalysts are classified into four groups: small molecules, oligomers, polymers, and frameworks, with specific structural and physicochemical properties serving as activity indicators. To unlock the full potential of organic electrocatalysts, five strategies are discussed: integrated structures, surface property modulation, membrane technologies, electrolyte affinity regulation, and addition of anticorrosion species, all aimed at enhancing charge efficiency, mass transfer, and long-term stability during electrocatalytic reactions. The review offers a comprehensive overview of the current state of organic electrocatalysts and their practical applications, bridging the understanding gap and paving the way for future developments of more efficient green energy conversion technologies.

Facile self-assembly synthesis of PdPt bimetallic nanotubes with good performance for ethanol oxidation in an alkaline medium.

PdPt bimetallic nanotubes were prepared by the self-assembly of Pt and Pd on Te nanowires at room temperature. The morphologies of the as-prepared PdPt nanotubes were investigated by scanning electron microscopy and transmission electron microscopy, and the results display a large amount of PdPt bimetallic nanotubes with a diameter of 10-20 nm and a length of several micrometers. The composition and structure of the nanotubes were characterized by X-ray diffraction, high-resolution transmission electron microscopy, scanning transmission electron microscopy, and energy spectrum analysis, and the results display uniform compositional distributions of both elements (Pd and Pt). The mechanism of the formation of the nanotube structure was supposed. The electrocatalytic performance of PdPt nanotubes were studied by cyclic voltammetry and chronoamperometry. Electrochemical results show that the as-prepared PdPt nanotube catalysts have not only high activity but also good stability for ethanol oxidation in alkaline medium.

Construction of a Co/MnO Mott-Schottky Heterostructure to Achieve Interfacial Synergy in the Oxygen Reduction Reaction for Aluminum-Air Batteries.

The rational design of non-noble metal-based electrocatalysts for an efficient oxygen reduction reaction (ORR) is an important research topic to promote the advancement of aluminum-air batteries. In this work, heterostructural Co/MnO nanoparticles encapsulated in a N-doped carbon electrocatalyst were prepared via one-step pyrolysis utilizing different reduction potentials of Co and Mn ions, and the heterointerface between the two phases was confirmed. The prepared catalyst displays Pt/C competitive ORR performance because of the interfacial synergy of a Co/MnO Mott-Schottky (M-S) heterostructure, which leads to boosted conductivity, formation of an M-S barrier, and a reduced oxygen reduction energy barrier for excited electrons. Furthermore, the Co/MnO-based aluminum-air battery displays good discharge performance, demonstrating good feasibility for practical application.

Air-Stable Binary Hydrated Eutectic Electrolytes with Unique Solvation Structure for Rechargeable Aluminum-Ion Batteries.

Aluminum-ion batteries (AIBs) have been highlighted as a potential alternative to lithium-ion batteries for large-scale energy storage due to the abundant reserve, light weight, low cost, and good safety of Al. However, the development of AIBs faces challenges due to the usage of AlCl3-based ionic liquid electrolytes, which are expensive, corrosive, and sensitive to humidity. Here, we develop a low-cost, non-corrosive, and air-stable hydrated eutectic electrolyte composed of aluminum perchlorate nonahydrate and methylurea (MU) ligand. Through optimizing the molar ratio to achieve the unique solvation structure, the formed Al(ClO4)3·9H2O/MU hydrated deep eutectic electrolyte (AMHEE) with an average coordination number of 2.4 can facilely realize stable and reversible deposition/stripping of Al. When combining with vanadium oxide nanorods positive electrode, the Al-ion full battery delivers a high discharge capacity of 320 mAh g-1 with good capacity retention. The unique solvation structure with a low desolvation energy of the AMHEE enables Al3+ insertion/extraction during charge/discharge processes, which is evidenced by in situ synchrotron radiation X-ray diffraction. This work opens a new pathway of developing low-cost, safe, environmentally friendly and high-performance electrolytes for practical and sustainable AIBs.

A Low-Cost and Air-Stable Rechargeable Aluminum-Ion Battery.

Rechargeable aluminum-ion batteries (AIBs) are promising for large-scale energy storage due to the abundant reserves, low cost, and high capacity of the Al anode. However, the development of AIBs is currently hindered by the usage of AlCl3 /1-ethyl-3-methylimidazolium chloride electrolyte, which is expensive, highly corrosive, and extremely air-sensitive. Herein, a new hydrated eutectic electrolyte (HEE) composed of hydrated aluminum perchlorate and succinonitrile for low-cost, noncorrosive, and air-stable AIBs is reported. Crystal water in the hydrated aluminum perchlorate plays a vital role in forming the HEE, in which one H2 O and five succinonitrile molecules coordinate with one Al3+ ion. The optimized ratio of Al(ClO4 )3 ·9H2 O to succinonitrile is 1:12. When combining with the self-doped polyaniline cathode, the associated AIB delivers a high discharge capacity of 185 mAh g-1 over 300 cycles; and the charge/discharge mechanism in the HEE is studied experimentally and theoretically. The HEE is nonflammable, air-stable, and noncorrosive, thus enabling good air tolerance and facile fabrication of AIBs.

Atomically Dispersed Iron Active Sites Promoting Reversible Redox Kinetics and Suppressing Shuttle Effect in Aluminum-Sulfur Batteries.

Rechargeable aluminum-sulfur (Al-S) batteries have been considered as a highly potential energy storage system owing to the high theoretical capacity, good safety, abundant natural reserves, and low cost of Al and S. However, the research progress of Al-S batteries is limited by the slow kinetics and shuttle effect of soluble polysulfides intermediates. Herein, an interconnected free-standing interlayer of iron single atoms supported on porous nitrogen-doped carbon nanofibers (FeSAs-NCF) on the separator is developed and used as both catalyst and chemical barrier for Al-S batteries. The atomically dispersed iron active sites (Fe-N4) are clearly identified by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption near-edge structure. The Al-S battery with the FeSAs-NCF shows an improved specific capacity of 780 mAh g-1 and enhanced cycle stability. As evidenced by experimental and theoretical results, the atomically dispersed iron active centers on the separator can chemically adsorb the polysulfides and accelerate reaction kinetics to inhibit the shuttle effect and promote the reversible conversion between aluminum polysulfides, thus improving the electrochemical performance of the Al-S battery. This work provides a new way that can not only promote the conversion of aluminum sulfides but also suppress the shuttle effect in Al-S batteries.

Challenges and Strategies of Low-Cost Aluminum Anodes for High-Performance Al-Based Batteries.

The ever-growing market of electric vehicles and the upcoming grid-scale storage systems have stimulated the fast growth of renewable energy storage technologies. Aluminum-based batteries are considered one of the most promising alternatives to complement or possibly replace the current lithium-ion batteries owing to their high specific capacity, good safety, low cost, light weight, and abundant reserves of Al. However, the anode problems in primary and secondary Al batteries, such as, self-corrosion, passive film, and volume expansion, severely limit the batteries' practical performance, thus hindering their commercialization. Herein, an overview of the currently emerged Al-based batteries is provided, that primarily focus on the recent research progress for Al anodes in both primary and rechargeable systems. The anode reaction mechanisms and problems in various Al-based batteries are discussed, and various strategies to overcome the challenges of Al anodes, including surface oxidation, self-corrosion, volume expansion, and dendrite growth, are systematically summarized. Finally, future research perspectives toward advanced Al batteries with higher performance and better safety are presented.

The kinetics of ferrocene volatilisation from an ionic liquid.

The volatilisation of ferrocene (Fc), dissolved in the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [C(4)mpyrr][NTf(2)], to the gas phase has been indirectly monitored by cyclic voltammetry and chronoamperometry. Simulation of the observed trends in concentration with time using a simple model allowed quantification of the process. Volatilisation of dissolved Fc under flowing wet and dry dinitrogen gas (N(2)) was found to be kinetically limited with a rate constant in the region of 2×10(-7) cm s(-1). The activation energy of diffusion for Fc was found to be 28.2±0.7 kJ mol(-1), while the activation energy of volatilisation of Fc from [C(4)mpyrr][NTf(2)] to dry N(2) was found to be 85±2 kJ mol(-1).

Plasma-Assisted Synthesis of Defect-Rich O and N Codoped Carbon Nanofibers Loaded with Manganese Oxides as an Efficient Oxygen Reduction Electrocatalyst for Aluminum-Air Batteries.

The oxygen reduction reaction (ORR) with sluggish kinetics on the cathode of aluminum-air (Al-air) batteries greatly limits their further development. Here, a new strategy is proposed to synthesize oxygen and nitrogen codoped carbon nanofibers loaded with manganese oxides (MnO/Mn2O3/ONCNF-n) as an efficient electrocatalyst for ORR by using oxygen plasma surface etching. The MnO/Mn2O3/ONCNF-3 exhibit superior ORR performance in an alkaline electrolyte, which is attributed to various active sites including N and O heteroatoms, vacancies, and manganese oxides. Additionally, the fabricated homemade Al-air battery (AAB) with MnO/Mn2O3/ONCNF-3 exhibits a maximum power density of 129.7 mW cm-2, demonstrating comparable performance to AABs based on the commercial Pt/C catalyst. This work provides a new approach of using O2 plasma for enhancing the ORR catalytic activities of carbon materials.

Heterostructural Interface in Fe3C-TiN Quantum Dots Boosts Oxygen Reduction Reaction for Al-Air Batteries.

Oxygen reduction electrocatalysts play important roles in metal-air batteries. Herein, Fe3C-TiN heterostructural quantum dots loaded on carbon nanotubes (FCTN@CNTs) are prepared as electrocatalysts for the oxygen reduction reaction (ORR) through a one-pot pyrolysis. The Fe3C-TiN quantum dots with a diameter of 2-5 nm show the unique characteristic of heterostructural interface. The as-prepared FCTN@CNTs display Pt/C comparable ORR performance (Eonset 1.06 and E1/2 0.95 V) in alkaline medium, which is ascribed to the heterostructural interface between TiN and Fe3C. Furthermore, the Al-air batteries with the FCTN@CNT catalyst display superior discharge performance, demonstrating good feasibility for practical application. This work provides an effective new method to synthesize affordable and efficient oxygen reduction reaction catalysts.

Integrated and Binder-Free Air Cathodes of Co3 Fe7 Nanoalloy and Co5.47 N Encapsulated in Nitrogen-Doped Carbon Foam with Superior Oxygen Reduction Activity in Flexible Aluminum-Air Batteries.

All-solid-sate Al-air batteries with features of high theoretical energy density, low cost, and environmental-friendliness are promising as power sources for next-generation flexible and wearable electronics. However, the sluggish oxygen reduction reaction (ORR) and poor interfacial contact in air cathodes cause unsatisfied performance. Herein, a free-standing Co3 Fe7 nanoalloy and Co5.47 N encapsulated in 3D nitrogen-doped carbon foam (Co3 Fe7 @Co5.47 N/NCF) is prepared as an additive-free and integrated air cathode for flexible Al-air batteries in both alkaline and neutral electrolytes. The Co3 Fe7 @Co5.47 N/NCF outperforms commercial platinum/carbon (Pt/C) toward ORR with an onset potential of 1.02 V and a positive half-wave potential of 0.92 V in an alkaline electrolyte (0.59 V in sodium chloride solution), which is ascribed to the unique interfacial structure between Co3 Fe7 and Co5.47 N supported by 3D N-doped carbon foam to facilitate fast electron and mass transfer. The high ORR performance is also supported by in-situ electrochemical Raman spectra and density functional theory calculation. Furthermore, the fabricated Al-air battery displays good flexibility and delivers a power density of 199.6 mW cm-2 , and the binder-free and integrated cathode shows better discharge performance than the traditionally slurry casting cathode. This work demonstrates a facile and efficient approach to develop integrated air cathode for metal-air batteries.

In Situ Self-Template Synthesis of Fe-N-Doped Double-Shelled Hollow Carbon Microspheres for Oxygen Reduction Reaction.

Herein, we reported a special Fe-N-doped double-shelled hollow carbon microsphere (Fe-N-DSC) which was prepared by a facile, in situ polymerization followed by pyrolysis. With porous ferroferric oxide (Fe3O4) hollow microspheres as the templates, where pyrrole monomers were dispersed around the outer surface and prefilled the interior space. By adding hydrochloric acid, Fe3+ ions were released to initiate polymerization of pyrrole on both the outer and inner surfaces of Fe3O4 microspheres until they were completely dissolved, resulting in the Fe-containing polypyrrole double-shelled hollow carbon microspheres (Fe-PPY-DSC). The Fe-PPY-DSC was then pyrolyzed to generate the Fe-N-DSC. The Fe3O4 hollow microspheres played trifunctional roles, i.e., the template to prepare a double-shelled hollow spherical structure, the initiator (i.e., Fe3+ ions) for the polymerization of pyrrole, and the Fe source for doping. The Fe-N-DSC exhibited a superior catalytic activity for oxygen reduction as comparable to commercial Pt/C catalysts in both alkaline and acidic media. The high catalytic performance was ascribed to the special porous double-shelled hollow spherical structure, which provided more active sites and was beneficial to a high-flux mass transportation.