Vanadium(III) Acetylacetonate as an Efficient Soluble Catalyst for Lithium–Oxygen Batteries

Elucidation of Design Criteria for V-based Redox Mediators: Structure-Function Relationships that Dictate Rates of Heterogeneous Electron Transfer

Redox mediators are attractive solutions for addressing the stringent kinetic stipulations required for efficient energy conversion processes. In this work, we compare the electrochemical properties of four vanadium complexes, namely [V(acac)3], [V6O7(OMe)12], [nBu4N]3[V6O13(TRISNO2)2], and [nBu4N]5[V18O46(NO3)] in non-aqueous solutions on glassy carbon electrodes. The goal of this study is to investigate the electron transfer kinetics and diffusivity of these compounds under identical experimental conditions to develop an understanding of structure-function relationships that dictate the physicochemical properties of vanadium oxide assemblies. Complex selection was dictated by two criteria - (1) nuclearity of the transition metal complexes (2) distribution of electron density in the native electronic configuration. Our analyses establish that electronic communication between metal centers significantly impacts charge transfer kinetics of these vanadium-based compounds.

Aprotic Lithium-Oxygen Batteries Based on Nonsolid Discharge Products

Aprotic lithium-oxygen (Li-O2) batteries are considered to be a promising alternative option to lithium-ion batteries for high gravimetric energy storage devices. However, the sluggish electrochemical kinetics, the passivation, and the structural damage to the cathode caused by the solid discharge products have greatly hindered the practical application of Li-O2 batteries. Herein, the nonsolid-state discharge products of the off-stoichiometric Li1-xO2 in the electrolyte solutions are achieved by iridium (Ir) single-atom-based porous organic polymers (termed as Ir/AP-POP) as a homogeneous, soluble electrocatalyst for Li-O2 batteries. In particular, the numerous atomic active sites act as the main nucleation sites of O2-related discharge reactions, which are favorable to interacting with O2-/LiO2 intermediates in the electrolyte solutions, owing to the highly similar lattice-matching effect between the in situ-formed Ir3Li and LiO2, achieving a nonsolid LiO2 as the final discharge product in the electrolyte solutions for Li-O2 batteries. Consequently, the Li-O2 battery with a soluble Ir/AP-POP electrocatalyst exhibits an ultrahigh discharge capacity of 12.8 mAh, an ultralow overpotential of 0.03 V, and a long cyclic life of 700 h with the carbon cloth cathode. The manipulation of nonsolid discharge products in aprotic Li-O2 batteries breaks the traditional growth mode of Li2O2, bringing Li-O2 batteries closer to being a viable technology.

Interface engineering of CeO2 nanoparticle/Bi2WO6 nanosheet nanohybrids with oxygen vacancies for oxygen evolution reactions under alkaline conditions

Because of the interactive combination synergy effect, hetero interface engineering is used way for advancing electrocatalytic activity and durability. In this study, we demonstrate that a CeO₂/Bi₂WO₆ heterostructure is synthesized by a hydrothermal method. Electrochemical measurement results indicate that CeO₂/Bi₂WO₆ displays not only more OER catalytic active sites with an overpotential of 390 mV and a Tafel slope of 117 mV dec^-1 but also durability for 10 h (97.57%). Such outstanding characteristics are primarily attributed to (1) the considerable activities by CeO₂ nanoparticles uniformly distributed on Bi₂WO₆ nanosheets and (2) the plentiful Bi-O-Ce and W-O-Ce species playing the role of strong couples between CeO₂ nanoparticles and Bi₂WO₆ nanosheets and oxygen vacancy existence in CeO₂ nanoparticles, which can improve the electrochemical active surface area (ECSA) and activity, and enhance the conductivity for OERs. This CeO₂/Bi₂WO₆ consists of the heterojunction engineering that can open a modern method of thinking for high effective OER electrocatalysts.

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

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

Self-Formation CoO Nanodots Catalyst in Co(TFSI)2 -Modified Electrolyte for High Efficient Li-O2 Batteries

The major challenges for Li-O₂ batteries are sluggish reaction kinetics and large overpotentials due to the cathode passivation resulting from insulative and insoluble Li₂ O₂ . Here, a novel nanodot (ND)-modified electrolyte is designed by employing cobalt bis(trifluoromethylsulfonyl)imide (Co(TFSI)₂ ) as an electrolyte additive. The Co(TFSI)₂ additive can react with discharge intermediate LiO₂ and product Li₂ O₂ to form CoO NDs. The generated CoO NDs are well dispersed in electrolyte, which integrates both the high catalytic activity of solid catalyst and the good wettability of soluble catalyst. Under the catalytis of CoO NDs, Li₂ O₂ is produced and deposits on the cathode together with them. At the recharge process, these well dispersed CoO NDs help to decompose solid Li₂ O₂ at a lower overpotential. The Li-O₂ cells with Co(TFSI)₂ exhibit a long cycle life of 200 cycles at a current density of 200 mA g^-1 under a cutoff capacity of 1000 mAh g^-1 , as well as a superior reversibility associated with the Li₂ O₂ formation and decomposition. The study is expected to broaden the range of electrolyte additives and provide a new view to developing highly dispersed NDs-based catalysts for Li-O₂ batteries.

Soluble and Perfluorinated Polyelectrolyte for Safe and High-Performance Li-O2 Batteries

The severe performance degradation of high-capacity Li-O₂ batteries induced by Li dendrite growth and concentration polarization from the low Li⁺ transfer number of conventional electrolytes hinder their practical applications. Herein, lithiated Nafion (LN) with the sulfonic group immobilized on the perfluorinated backbone has been designed as a soluble lithium salt for preparing a less flammable polyelectrolyte solution, which not only simultaneously achieves a high Li⁺ transfer number (0.84) and conductivity (2.5 mS cm^-1 ), but also the perfluorinated anion of LN produces a LiF-rich SEI for protecting the Li anode from dendrite growth. Thus, the Li-O₂ battery with a LN-based electrolyte achieves an all-round performance improvement, like low charge overpotential (0.18 V), large discharge capacity (9508 mAh g^-1 ), and excellent cycling performance (225 cycles). Besides, the fabricated pouch-type Li-air cells exhibit promising applications to power electronic equipment with satisfactory safety.

Internal Electric Field and Interfacial Bonding Engineered Step-Scheme Junction for a Visible-Light-Involved Lithium-Oxygen Battery

Li-O₂ batteries have aroused considerable interest in recent years, however they are hindered by high kinetic barriers and large overvoltages at cathodes. Herein, a step-scheme (S-scheme) junction with hematite on carbon nitride (Fe₂ O₃ /C₃ N₄ ) is designed as a bifunctional catalyst to facilitate oxygen redox for a visible-light-involved Li-O₂ battery. The internal electric field and interfacial Fe-N bonding in the heterojunction boost the separation and directional migration of photo-carriers to establish spatially isolated redox centers, at which the photoelectrons on C₃ N₄ and holes on Fe₂ O₃ remarkably accelerate the discharge and charge kinetics. These enable the Li-O₂ battery with Fe₂ O₃ /C₃ N₄ to present an elevated discharge voltage of 3.13 V under illumination, higher than the equilibrium potential 2.96 V in the dark, and a charge voltage of 3.19 V, as well as superior rate capability and cycling stability. This work will shed light on rational cathode design for metal-O₂ batteries.

OUP accepted manuscript

Aprotic lithium–oxygen (Li–O₂) batteries are receiving intense research interest by virtue of their ultra-high theoretical specific energy. However, current Li–O₂ batteries are suffering from severe barriers, such as sluggish reaction kinetics and undesired parasitic reactions. Recently, molecular catalysts, i.e. redox mediators (RMs), have been explored to catalyse the oxygen electrochemistry in Li–O₂ batteries and are regarded as an advanced solution. To fully unlock the capability of Li–O₂ batteries, an in-depth understanding of the catalytic mechanisms of RMs is necessary. In this review, we summarize the working principles of RMs and their selection criteria, highlight the recent significant progress of RMs and discuss the critical scientific and technical challenges on the design of efficient RMs for next-generation Li–O₂ batteries.

Spin-State Manipulation of Two-Dimensional Metal-Organic Framework with Enhanced Metal-Oxygen Covalency for Lithium-Oxygen Batteries

Aprotic Li-O 2 battery has attracted extensive attention in the past decade owing to the high theoretical energy density, however it is obstructed by the sluggish reaction kinetics at cathodes and large voltage hysteresis. Herein, we regulate the spin state of partial Ni 2+ metal centers ( t 2g 6 e g 2 ) of conductive nickel catecholate framework (Ni II -NCF) nanowire arrays to high-valence Ni 3+ ( t 2g 6 e g 1 ) for Ni III -NCF. The spin-state modulation enables enhanced nickel-oxygen covalency in Ni III -NCF, which facilitates electron exchange between the Ni sites and oxygen adsorbates and accelerates the oxygen redox kinetics. The high affinity of Ni 3+ sites with the intermediate LiO 2 promotes formation of nanosheet-like Li 2 O 2 in the void space among Ni III -NCF nanowires upon discharging. These merit the Li-O 2 battery based on Ni III -NCF with remarkably reduced discharge/charge voltage gaps, superior rate capability, and long cycling stability of over 200 cycles. This work highlights the domination of electron spin state on the redox kinetics and will shed insights into electronic structure regulation of electrocatalysts for Li-O 2 battery and beyond.

Simultaneously Enhancing the Redox Potential and Stability of Multi-Redox Organic Catholytes by Incorporating Cyclopropenium Substituents

High redox potential, two-electron organic catholytes for nonaqueous redox flow batteries were developed by appending diaminocyclopropenium (DAC) substituents to phenazine and phenothiazine cores. The parent heterocycles exhibit two partially reversible oxidations at moderate potentials [both at lower than 0.7 V vs ferrocene/ferrocenium (Fc/Fc⁺)]. The incorporation of DAC substituents has a dual effect on these systems. The DAC groups increase the redox potential of both couples by ∼300 mV while simultaneously rendering the second oxidation (which occurs at 1.20 V vs Fc/Fc⁺ in the phenothiazine derivative) reversible. The electron-withdrawing nature of the DAC unit is responsible for the increase in redox potential, while the DAC substituents stabilize oxidized forms of the molecules through resonance delocalization of charge and unpaired spin density. These new catholytes were deployed in two-electron redox flow batteries that exhibit voltages of up to 2.0 V and no detectable crossover over 250 cycles.

Two Competing Reactions of Sulfurized Polyacrylonitrile Produce High-Performance Lithium-Sulfur Batteries

Sulfurized polyacrylonitrile (SPAN) is an attractive cathode candidate for the advanced lithium-sulfur (Li-S) batteries owing to its outstanding cyclic stability. Nevertheless, SPAN suffers from inadequate initial Coulombic efficiency (CE) induced by the sluggish reaction kinetics, which is primarily ascribed to the low Li-ion diffusivity and high electronic resistivity of the Li₂S product. In this work, an optimal trace amount of soluble lithium polysulfide of Li₂S₈ is introduced as a redox mediator for a freestanding fibrous SPAN cathode to enhance the reversible oxidation efficiency of Li₂S. During the delithiation process, the chemical interactions between Li₂S and Li₂S₈ additive facilitate the electrochemical oxidation of Li₂S, resulting in the transformation of not only C-S/S-S bonds in SPAN but also elemental sulfur. Benefiting from the synergistic effect of the two competing reactions, a high initial CE of 82.9% could be achieved at a current density of 200 mA g^-1. Moreover, a superior capacity retention along with a high capacity of 1170 mAh g^-1 up to the 400th cycle is available at 1000 mA g^-1. The study offers a feasible approach for Li-S batteries toward the practical applications of SPAN.

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

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

Organogermanium Nanowire Cathodes for Efficient Lithium-Oxygen Batteries

We report a technique for effectively neutralizing the generation of harmful superoxide species, the source of parasitic reactions, in lithium-oxygen batteries to generate stable substances. In organic electrolytes, organogermanium (Propa-germanium, Ge-132) nanowires can suppress solvated superoxide and induce strong surface-adsorption reaction due to their high anti-superoxide disproportionation activity. Resultantly, the effect of organogermanium nanowires mitigate toxic oxidative stress to stabilize organic electrolytes and promote good Li₂O₂ growth. These factors led to long duration of the electrolytes and impressive rechargeability of lithium-oxygen batteries.

High-Capacity and Stable Li-O2 Batteries Enabled by a Trifunctional Soluble Redox Mediator

Li-O₂ batteries with ultrahigh theoretical energy densities usually suffer from low practical discharge capacities and inferior cycling stability owing to the cathode passivation caused by insulating discharge products and by-products. Here, a trifunctional ether-based redox mediator, 2,5-di-tert-butyl-1,4-dimethoxybenzene (DBDMB), is introduced into the electrolyte to capture reactive O₂ ^- and alleviate the rigorous oxidative environment of Li-O₂ batteries. Thanks to the strong solvation effect of DBDMB towards Li⁺ and O₂ ^- , it not only reduces the formation of by-products (a high Li₂ O₂ yield of 96.6 %), but also promotes the solution growth of large-sized Li₂ O₂ particles, avoiding the passivation of cathode as well as enabling a large discharge capacity. Moreover, DBDMB makes the oxidization of Li₂ O₂ and the decomposition of main by-products (Li₂ CO₃ and LiOH) proceed in a highly effective manner, prolonging the stability of Li-O₂ batteries (243 cycles at 1000 mAh g^-1 and 1000 mA g^-1 ).

Efficient Carbon Dioxide Electroreduction over Ultrathin Covalent Organic Framework Nanolayers with Isolated Cobalt Porphyrin Units

Electrochemical CO₂ reduction represents a sustainable approach for the conversion of CO₂ into valuable fuels and chemicals. Here, we fabricated a series of composite nanomaterials through template-oriented polymerization of covalent organic frameworks (COFs) with isolated cobalt porphyrin units on amino-functionalized carbon nanotubes for efficient electrocatalytic CO₂ reduction reaction (CO₂RR). Compared with pure COFs, the hybrid form of ultrathin COF nanolayers wrapped on the conductive scaffold leads to distended current density and stable Faradaic efficiency for CO₂-to-CO conversion over a wide potential range. Specifically, the catalytic performances of the system can be finely optimized by the modification of the reticular structure with different functional groups. Our work gives a new strategy for the preparation of highly active and selective electrocatalysts for CO₂RR.

Efficient recovery of Cu(II) by LTA-zeolites with hierarchical pores and their resource utilization in electrochemical denitrification: Environmentally friendly design and reutilization of waste in water

Water pollution seriously endangers human health and the environment. Here we prepared and tested mesoporous LTA zeolites for the adsorption of Cu(II) from aqueous media and the captured copper was further used for electrochemical nitrate reduction. The prepared hierarchically porous LTA exhibited a high capacity (341.5 mg g^-1) for Cu(II) adsorption, following the pseudo-second-order kinetic and Freundlich adsorption isotherm models well. The Cu-LTA sample was characterised by various analytical methods, and Cu(I) species were identified as the active sites for nitrate electrochemical reduction. Based on the spectral characterization and reducibility, strong metal-support interaction was found between copper and LTA, which is beneficial to the dispersion of active sites and their contacts with nitrates. In total, 10.1 g-N-NO₃ g^-1-Cu was reduced over the Cu-LTA-modified cathode in a three-electrode system with high N₂ selectivity (92.1 %). Compared to purely microporous zeolites, mesoporous LTA has a higher capacity for Cu(II) removal and nitrate reduction. The mesoporous structure allows easy access to the inner active sites with low diffusion resistance. The low Tafel slope and high current density confirm the high activity of the mesoporous Cu-LTA, making it a promising and efficient material for the removal and reuse of heavy metal ions.

Ligand-assisted capping growth of self-supporting ultrathin FeNi-LDH nanosheet arrays with atomically dispersed chromium atoms for efficient electrocatalytic water oxidation

Self-supporting ultrathin FeNi-layered double hydroxide nanosheet arrays with atomically dispersed Cr atoms were firstly fabricated from stainless steel mesh by a facile ligand-assisted capping growth approach. Their unique nanostructure and a strong synergetic effect between the atomically dispersed Cr dopants and the active sites afford an exceptional OER activity.

Effective Oxygen Reduction Reaction Performance of FeCo Alloys In Situ Anchored on Nitrogen-Doped Carbon by the Microwave-Assistant Carbon Bath Method and Subsequent Plasma Etching

Electrocatalysts with strong stability and high electrocatalytic activity have received increasing interest for oxygen reduction reactions (ORRs) in the cathodes of energy storage and conversion devices, such as fuel cells and metal-air batteries. However, there are still several bottleneck problems concerning stability, efficiency, and cost, which prevent the development of ORR catalysts. Herein, we prepared bimetal FeCo alloy nanoparticles wrapped in Nitrogen (N)-doped graphitic carbon, using Co-Fe Prussian blue analogs (Co₃[Fe(CN)₆]₂, Co-Fe PBA) by the microwave-assisted carbon bath method (MW-CBM) as a precursor, followed by dielectric barrier discharge (DBD) plasma treatment. This novel preparation strategy not only possessed a fast synthesis rate by MW-CBM, but also caused an increase in defect sites by DBD plasma treatment. It is believed that the co-existence of Fe/Co-N sites, rich active sites, core-shell structure, and FeCo alloys could jointly enhance the catalytic activity of ORRs. The obtained catalyst exhibited a positive half-wave potential of 0.88 V vs. reversible hydrogen electrode (RHE) and an onset potential of 0.95 V vs. RHE for ORRs. The catalyst showed a higher selectivity and long-term stability than Pt/C towards ORR in alkaline media.

Ru-Coated metal–organic framework-derived Co-based particles embedded in porous N-doped carbon nanocubes as a catalytic cathode for a Li–O2 battery

Ru-Co₄N/Co-NC is prepared and used as a cathode for a Li–O₂ battery, which shows high-capacity, low overpotential and long cycle-life.