Molecular Unravelling of the Structural Effect of Quinone Redox Mediators on Oxygen Reduction Reaction in Aprotic Lithium-Oxygen Batteries.

Redox mediators (RMs) provide tantalizing solutions to unlock the energy capabilities of aprotic lithium-oxygen (Li-O2) batteries by driving solution-mediated Li2O2 growth. However, the structural effect of RMs on the catalytic efficiency of the oxygen reduction reaction remains incompletely understood. Herein, we present the interplay between the structure of RMs and their discharge capabilities by a comparative study of model quinone (Q)-based RMs. Specifically, at low current densities, incorporating electron-withdrawing groups onto the Q ring can positively move the discharge potential and deliver larger discharge capacity by extending the lifespan of the LiQO2 intermediate and allowing for Li2O2 growth into deeper electrolyte regions. Conversely, at high current densities, the absence of electron-withdrawing groups facilitates homogeneous reaction kinetics from LiQ to regenerate Q (i.e., decreased lifespan of LiQO2), mitigating electrode potential polarization and preserving catalytic activity of Q for higher discharge capacity. The work establishes structure-property relationships that guide the rational design of RMs toward next-generation Li-O2 batteries.

Unlock CO2 Reduction Reaction Pathways in Aprotic Li-CO2 Batteries with In Situ Isotope-Labeled Spectroscopy and Theoretical Calculations.

The CO2 reduction reaction (CO2RR) pathway significantly dictates the reversibility and overpotential of aprotic Li-CO2 batteries; however, it has remained incompletely understood due to the lack of direct in situ spectroscopic evidence. Herein, the Li-CO2RR pathways at the model Au | dimethyl sulfoxide (DMSO) interface are interrogated using a combination of in situ isotope-labeled spectroscopy techniques and theoretical calculations. This obtained direct spectroscopic evidence presents that the primary CO2RR proceeds through the CO2-to-CO pathway (i.e., 2Li+ + 2CO2 + 2e- → CO + Li2CO3) initiated at a low overpotential (ca. 2.1 V vs Li/Li+), and the CO2-to-Li2C2O4 pathway (i.e., 2Li+ + 2CO2 + 2e- → Li2C2O4) initiated at a high overpotential (ca. 1.7 V vs Li/Li+), where the potential-dependent pathways critically depend on the coverage of LiCO2 intermediates. Simultaneously, the entire Li-CO2RR process is also accompanied by parasitic reactions to form gaseous C2H4 with COOH* as the crucial intermediate, which is induced by the H+-abstraction reaction between the reactive LiCO2 intermediate and the DMSO solvent. These fundamental insights enable us to establish a molecular picture for Li-CO2RR pathways in aprotic media and will serve as a crucial guideline for reversible Li-CO2 electrochemistry.

Correlating Thickness and Phase of Single Co(OH)2 Micro-Platelets to the Intrinsic Activity of Oxygen Evolution Electrocatalysis.

Morphology, crystal phase, and its transformation are important structures that frequently determine electrocatalytic activity, but the correlations of intrinsic activity with them are not completely understood. Herein, using Co(OH)2 micro-platelets with well-defined structures (phase, thickness, area, and volume) as model electrocatalysts of oxygen evolution reaction, multiple in situ microscopy is combined to correlate the electrocatalytic activity with morphology, phase, and its transformation. Single-entity morphology and electrochemistry characterized by atomic force microscopy and scanning electrochemical cell microscopy reveal a thickness-dependent turnover frequency (TOF) of α-Co(OH)2. The TOF (≈9.5 s-1) of α-Co(OH)2 with ≈14 nm thickness is ≈95-fold higher than that (≈0.1 s-1) with ≈80 nm. Moreover, this thickness-dependent activity has a critical thickness of ≈30 nm, above which no thickness-dependence is observed. Contrarily, ß-Co(OH)2 reveals a lower TOF (≈0.1 s-1) having no significant correlation with thickness. Combining single-entity electrochemistry with in situ Raman microspectroscopy, this thickness-dependent activity is explained by more reversible Co3+/Co2+ kinetics and larger ratio of active Co sites of thinner α-Co(OH)2, accompanied with faster phase transformation and more extensive surface restructuration. The findings highlight the interactions among thickness, ratio of active sites, kinetics of active sites, and phase transformation, and offer new insights into structure-activity relationships at single-entity level.

K-O2 electrochemistry at the Au/DMSO interface probed by in situ spectroscopy and theoretical calculations.

The reaction mechanism underpinning the operation of K-O2 batteries, particularly the O2 reactions at the positive electrode, is still not completely understood. In this work, by combining in situ Raman spectroelectrochemistry and density functional theory calculations, we report on a fundamental study of K-O2 electrochemistry at a model interface of Au electrode/DMSO electrolyte. The key products and intermediates (O2-, KO2 and K2O2) are identified and their dependency on the electrode potential is revealed. At high potentials, the first reduction intermediate of O2-* radical anions (* denotes the adsorbed state) can desorb from the Au electrode surface and combine with K+ cations in the electrolyte producing KO2via a solution-mediated pathway. At low potentials, O2 can be directly reduced to on the Au electrode surface, which can be further reduced to at extremely low potentials. The fact that K2O2 has only been detected in the very high overpotential regime indicates a lack of KO2 disproportionation reaction both on the Au electrode surface and in the electrolyte solution. This work addresses the fundamental mechanism and origin of the high reversibility of the aprotic K-O2 batteries.

Boosting Electrocatalytic Ethylene Epoxidation by Single Atom Modulation.

The electrochemical synthesis of ethylene oxide (EO) using ethylene and water under ambient conditions presents a low-carbon alternative to existing industrial production process. Yet, the electrocatalytic ethylene epoxidation route is currently hindered by largely insufficient activity, EO selectivity, and long-term stability. Here we report a single atom Ru-doped hollandite structure KIr4O8 (KIrRuO) nanowire catalyst for efficient EO production via a chloride-mediated ethylene epoxidation process. The KIrRuO catalyst exhibits an EO partial current density up to 0.7 A cm-2 and an EO yield as high as 92.0 %. The impressive electrocatalytic performance towards ethylene epoxidation is ascribed to the modulation of electronic structures of adjacent Ir sites by single Ru atoms, which stabilizes the *CH2CH2OH intermediate and facilitates the formation of active Cl2 species during the generation of 2-chloroethanol, the precursor of EO. This work provides a single atom modulation strategy for improving the reactivity of adjacent metal sites in heterogeneous electrocatalysts.

Rational Design of Diatomic Active Sites for Elucidating Oxygen Evolution Reaction Performance Trends.

Diatomic catalysts, especially those with heteronuclear active sites, have recently attracted significant attention for their advantages over single-atom catalysts in reactions with relatively high energy barrier, e.g. oxygen evolution reaction. Rational design and synthesis of heteronuclear diatomic catalysts are of immense significance but have so far been plagued by the lack of a definitive correlation between structure and catalytic properties. Here, we report macrocyclic precursor constrained strategy to fabricate series of transition metal (MT, Ni, Co, Fe, Mn, or Cu)-noble (MN, Ir or Ru) centers in carbon material. One notable performance trend is observed in the order of Cu-MN < Mn-MN < Fe-MN < MN < Co-MN < Ni-MN. However, the pathway has been not altered, still following the traditional adsorption reaction mechanism. The effect of the MT atoms on the performances could possibly originate from the distinct adsorption/desorption behaviors of key intermediates (i.e. *OH, *O and/or *OOH), strongly implying that ΔG*OOH-ΔG*OH could be used as the performance descriptor. We believe that our work provides useful strategy for synthesis of diatomic active sites with sole coordination configuration and varied composition, and in-depth insight to their catalytic mechanism, which could be used for further optimization of diatomic catalysts towards oxygen electrocatalysis.

Unraveling the Asymmetric O─O Radical Coupling Mechanism on Ru─O─Co for Enhanced Acidic Water Oxidation.

Heterogeneous oxides with multiple interfaces provide significant advantages in electrocatalytic activity and stability. However, controlling the local structure of these oxides is challenging. In this work, unique heterojunctions are demonstrated based on two oxide types, which are formed via pyrolysis of a ruthenocene metal-organic framework (Ru-MOF) at specific temperatures. The resulted Ru-MOF-400 exhibits excellent electrocatalytic activity, with an overpotential of 190 mV at a current density of 10 mA cm-2 in 0.1 m HClO4 , and a mass activity of 2557 A gRu -1 , three orders of magnitude higher than commercial RuO2 . The Ru─O─Co bond formed by the incorporation of Co into the rutile lattice of RuO2 inhibits the disolution of Ru. Operando electrochemical investigations and density functional theory results reveal that the Ru-MOF-400 undergo asymmetric dual-active site oxide path mechanism during the acidic oxygen evolution reaction process, which is predominantly mediated by the asymmetric Ru─Co dual active site present at the interfaces between Co3 O4 and CoRuOx .

Disentangling the Li-Ion transport and Boundary Phase Transition Processes in Li10 GeP2 S12 Electrolyte by In-Operando High-Pressure and High-Resolution NMR Spectroscopy.

Li-ion transport and phase transition of solid electrolytes are critical and fundamental issues governing the rate and cycling performances of solid-state batteries. In this work, in-operando high-pressure nuclear magnetic resonance (NMR) spectroscopy for the solid-state battery is developed and applied, in combination with 6 Li-tracer NMR and high-resolution NMR spectroscopy, to investigate the Li10 GeP2 S12 electrolyte under true-to-life operation conditions. The results reveal that the Li10 GeP2 S12 phase may become more disordered and a large amount of conductive metastable ß-Li3 PS4 as the glassy matrix in the electrolyte transforms into less conductive phases, mainly γ-Li3 PS4 , when high current densities (e.g., ≥0.5 mA cm-2 ) are applied to the electrolyte. The overall Li-transport also varies and shows a tendency of boundary phases and Li10 GeP2 S12 synergistic dominant conduction at high currents. Accordingly, a mechanism of structural change induced by stress variation due to the drastic morphological change during Li-In alloying at high currents, and the local Li+ diffusion coefficient discrepancy is proposed. These new findings of Li-ion transport and boundary phase transition in Li10 GeP2 S12 solid electrolyte under high-pressure and high current density are first reported and will help provide previously lacking insights into the relationship of structure and performance of Li10 GeP2 S12 .

Hyphenated DEMS and ATR-SEIRAS techniques for in situ multidimensional analysis of lithium-ion batteries and beyond.

A wide spectrum of state-of-the-art characterization techniques have been devised to monitor the electrode-electrolyte interface that dictates the performance of electrochemical devices. However, coupling multiple characterization techniques to realize in situ multidimensional analysis of electrochemical interfaces remains a challenge. Herein, we presented a hyphenated differential electrochemical mass spectrometry and attenuated total reflection surface enhanced infrared absorption spectroscopy analytical method via a specially designed electrochemical cell that enables a simultaneous detection of deposited and volatile interface species under electrochemical reaction conditions, especially suitable for non-aqueous, electrolyte-based energy devices. As a proof of concept, we demonstrated the capability of the homemade setup and obtained the valuable reaction mechanisms, by taking the tantalizing reactions in non-aqueous lithium-ion batteries (i.e., oxidation and reduction processes of carbonate-based electrolytes on Li1+xNi0.8Mn0.1Co0.1O2 and graphite surfaces) and lithium-oxygen batteries (i.e., reversibility of the oxygen reaction) as model reactions. Overall, we believe that the coupled and complementary techniques reported here will provide important insights into the interfacial electrochemistry of energy storage materials (i.e., in situ, multi-dimensional information in one single experiment) and generate much interest in the electrochemistry community and beyond.

Direct In Situ Spectroscopic Evidence for Solution-Mediated Oxygen Reduction Reaction Intermediates in Aprotic Lithium-Oxygen Batteries.

A fundamental understanding of the reaction process is essential to predict and enhance the performance of electrochemical devices. As a central reaction in aprotic lithium-oxygen (Li-O2) batteries, the oxygen reduction reaction (ORR) has been confronted with the "sudden-death" phenomenon caused by the cathode passivation from discharge product Li2O2. The soluble catalyst (e.g., reduction mediator) promoted solution-mediated ORR represents an elegant solution. However, no direct molecular evidence is available so far, and its link to Li-O2 batteries performance remains hypothetical. Here, we present in situ surface-enhanced Raman spectroscopy and obtain direct spectroscopic evidence (i.e., LiAQ and LiAQO2) of the solution-mediated ORR on a model anthraquinone (AQ, a typical reduction mediator)-immobilized Au electrode. With the assistance of density functional theory calculations and differential electrochemical mass spectrometry, the related elementary reaction steps of the solution-mediated ORR are proposed. This work provides intuitive insights into the AQ-catalyzed solution-mediated ORR mechanism that is helpful in the optimization and tailor-design of soluble catalysts for excellent next-generation Li-O2 batteries.

Ultrathin CuF2 -Rich Solid-Electrolyte Interphase Induced by Cation-Tailored Double Electrical Layer toward Durable Sodium Storage.

Solid-electrolyte interphase (SEI) seriously affects battery's cycling life, especially for high-capacity anode due to excessive electrolyte decomposition from particle fracture. Herein, we report an ultrathin SEI (3-4 nm) induced by Cu+ -tailored double electrical layer (EDL) to suppress electrolyte consumption and enhance cycling stability of CuS anode in sodium-ion batteries. Unique EDL with SO3 CF3 -Cu complex absorbing on CuS in NaSO3 CF3 /diglyme electrolyte is demonstrated by in situ surface-enhanced Raman, Cyro-TEM and theoretical calculation, in which SO3 CF3 -Cu could be reduced to CuF2 -rich SEI. Dispersed CuF2 and F-containing compound can provide good interfacial contact for formation of ultrathin and stable SEI film to minimize electrolyte consumption and reduce activation energy of Na+ transport. As a result, the modified CuS delivers high capacity of 402.8 mAh g-1 after 7000 cycles without capacity decay. The insights of SEI construction pave a way for high-stability electrode.

Tailoring P2/P3 Biphases of Layered Nax MnO2 by Co Substitution for High-Performance Sodium-Ion Battery.

P-type layered oxide is a promising cathode candidate for sodium-ion batteries (SIBs), but faces the challenge of simultaneously realizing high rate capability and long cycle life. Herein, Co-substituted Nax MnO2 nanosheets with tunable P2/P3 biphase structures are synthesized by a novel dealloying-annealing strategy. The optimized P2/P3-Na0.67 Mn0.64 Co0.30 Al0.06 O2 cathode delivers an excellent rate capability of 83 mA h g-1 at a high current density of 1700 mA g-1 (10 C), and an outstanding cycling stability over 500 cycles at 1000 mA g-1 . This excellent performance is attributed to the unique P2/P3 biphases with stable crystal structures and fast Na+ diffusion between open prismatic Na sites. Moreover, operando X-ray diffraction is applied to explore the structural evolution of Na0.67 Mn0.64 Co0.30 Al0.06 O2 during the Na+ extraction/insertion processes, and the P2-P2' phase transition is effectively suppressed. Operando Raman technique is utilized to explore the structural superiority of P2/P3 biphase cathode compared with pure P2 or P3 phase. This work highlights precisely tailoring the phase composition as an effective strategy to design advanced cathode materials for SIBs.

Formation, lithium storage properties, and mechanism of nanoporous germanium fabricated by dealloying.

Germanium (Ge) has become a promising anode material for lithium-ion batteries (LIBs) due to its high theoretical capacity and decent electron/ion conductivity, but it exhibits inferior lifespan caused by dramatic volume variations during the (de)lithiation process. Herein, hierarchically, nanoporous Ge (np-Ge) was fabricated by the combination of selective phase corrosion with chemical dealloying. As an anode for LIBs, the np-Ge electrode exhibits marvelous cycling stability with capacity retentions of 1060.0 mA h g-1 at 0.2 A g-1 and 767.1 mA h g-1 at 1 A g-1 after 100 cycles. Moreover, the electrode shows excellent rate capability with a capacity retention of 844.2 mA h g-1 at 5 A g-1. Noticeably, the (de)lithiation mechanisms of np-Ge and porous Si-Ge (p-Si6Ge4) were unveiled by operando X-ray diffraction.

Boosting Potassium-Ion Battery Performance by Encapsulating Red Phosphorus in Free-Standing Nitrogen-Doped Porous Hollow Carbon Nanofibers.

Potassium-ion batteries (KIBs) are a promising alternative to lithium-ion batteries (LIBs) for large-scale renewable energy storage owning to the natural abundance and low cost of potassium. However, the biggest challenge for KIBs application lies in the lack of suitable electrode materials that can deliver long cycle life and high reversible capacity. In this work, we realized unprecedented long cycle life with high reversible capacity (465 mAh g-1 at 2 A g-1 after 800 cycles) as well as outstanding rate capability (342 mAh g-1 at 5 A g-1) for KIBs by embedding red P into free-standing nitrogen-doped porous hollow carbon nanofibers (red P@N-PHCNFs). This design circumvents the problems of pulverization and aggregation of P particles. The  in situ transmission electron microscopy (TEM) investigation reveals the structural robustness of the composite fibers during potassiation. The formation of P-C chemical bonds as well as nitrogen doping in the carbon matrix can facilitate the sturdy contact and enhance the adsorption energy of P atoms evidenced by DFT results. In situ Raman and ex situ XRD demonstrate that the final discharge product of the red P@N-PHCNFs is K4P3.

Advanced Lithium Metal-Carbon Nanotube Composite Anode for High-Performance Lithium-Oxygen Batteries.

The low Coulombic efficiency and hazardous dendrite growth hinder the adoption of lithium anode in high-energy density batteries. Herein, we report a lithium metal-carbon nanotube (Li-CNT) composite as an alternative to the long-term untamed lithium electrode to address the critical issues associated with the lithium anode in Li-O2 batteries, where the lithium metal is impregnated in a porous carbon nanotube microsphere matrix (CNTm) and surface-passivated with a self-assembled monolayer of octadecylphosphonic acid as a tailor-designed solid electrolyte interphase (SEI). The high specific surface area of the Li-CNT composite reduces the local current density and thus suppresses the lithium dendrite formation upon cycling. Moreover, the tailor-designed SEI effectively separates the Li-CNT composite from the electrolyte solution and prevents the latter's further decomposition. When the Li-CNT composite anode is coupled with another CNTm-based O2 cathode, the reversibility and cycle life of the resultant Li-O2 batteries are drastically elevated.

Three-Dimensional Ordered Macroporous Metal-Organic Framework Single Crystal-Derived Nitrogen-Doped Hierarchical Porous Carbon for High-Performance Potassium-Ion Batteries.

The biggest challenge of potassium-ion batteries (KIBs) application is to develop high-performance electrode materials to accommodate the potassium ions large size. Herein, by rational design, we carbonize three-dimensional (3D) ordered macroporous ZIF-8 to fabricate 3D interconnected nitrogen-doped hierarchical porous carbon (N-HPC) that shows excellent rate performance (94 mAh g-1 at 10.0 A g-1), unprecedented cycle stability (157 mA g-1 after 12000 cycles at 2.0 A g-1), and superior reversible capacity (292 mAh g-1 at 0.1 A g-1). The 3D hierarchical porous structure diminishes the diffusion distance for both ions/electrons, while N-doping improves the reactivity and electronic conductivity via producing more defects. In addition, the bicontinuous structure possesses a large specific surface area, decreasing the current density, again improving the rate performance. In situ Raman spectra analysis confirms the potassiation and depotassiation in the N-HPC are highly reversible processes. The galvanostatic intermittent titration measurement and first-principles calculations reveal that the interconnected macropores are more beneficial to the diffusion of the K+. This 3D interpenetrating structure demonstrates a superiority for energy storage applications.

Rechargeable Aluminium-Sulfur Battery with Improved Electrochemical Performance by Cobalt-Containing Electrocatalyst.

The rechargeable aluminium-sulfur (Al-S) battery is regarded as a potential alternative beyond lithium-ion battery system owing to its safety, promising energy density, and the high earth abundance of the constituent electrode materials, however, sluggish kinetic response and short life-span are the major issues that limit the battery development towards applications. In this article, we report CoII,III as an electrochemical catalyst in the sulfur cathode that renders a reduced discharge-charge voltage hysteresis and improved capacity retention and rate capability for Al-S batteries. The structural and electrochemical analysis suggest that the catalytic effect of CoII,III is closely associated with the formation of cobalt sulfides and the changes in the valence states of the CoII,III during the electrochemical reactions of the sulfur species, which lead to improved reaction kinetics and sulfur utilization in the cathode. The Al-S battery, assembled with the cathode consisting of CoII,III decorated carbon matrix, demonstrates a considerably reduced voltage hysteresis of 0.8 V, a reversible specific capacity of ≈500 mAh g-1 at 1 A g-1 after 200 discharge-charge cycles and of ≈300 mAh g-1 at 3 A g-1 .

Compactly Coupled Nitrogen-Doped Carbon Nanosheets/Molybdenum Phosphide Nanocrystal Hollow Nanospheres as Polysulfide Reservoirs for High-Performance Lithium-Sulfur Chemistry.

Lithium-sulfur (Li-S) batteries have been disclosed as one of the most promising energy storage systems. However, the low utilization of sulfur, the detrimental shuttling behavior of polysulfides, and the sluggish kinetics in electrochemical processes, severely impede their application. Herein, 3D hierarchical nitrogen-doped carbon nanosheets/molybdenum phosphide nanocrystal hollow nanospheres (MoP@C/N HCSs) are introduced to Li-S batteries via decorating commercial separators to inhibit polysulfides diffusion. It acts not only as a polysulfides immobilizer to provide strong physical trapping and chemical anchoring toward polysulfides, but also as an electrocatalyst to accelerate the kinetics of the polysulfides redox reaction, and to lower the Li2 S nucleation/dissolution interfacial energy barrier and self-discharge capacity loss in working Li-S batteries, simultaneously. As a result, the Li-S batteries with MoP@C/N HCS-modified separators show superior rate capability (920 mAh g-1 at 2 C) and stable cycling life with only 0.04% capacity decay per cycle over 500 cycles at 1 C with nearly 100% Coulombic efficiency. Furthermore, the Li-S battery can achieve a high area capacity of 5.1 mAh cm-2 with satisfied capacity retention when the cathode loading reaches 5.5 mg cm-2 . This work offers a brand new guidance for rational separator design into the energy chemistry of high-stable Li-S batteries.

Tackling Grand Challenges of the 21st Century with Electroanalytical Chemistry.

Modern electroanalytical chemistry, based on an intimate coupling of electrochemical techniques and spectroscopic and imaging methodologies, contains a broad range of very powerful research tools that are invariably characterized by descriptors of in situ, in vivo, real-time, online, etc. This Perspective offers a viewpoint of the authors on part of the grand challenges facing society and our planet in the 21st century, where contemporary analysts, armed with modern electroanalytical chemistry, have the ability to tackle these challenges with unprecedented levels. Prominent research topics such as energy, environment and biology are highlighted to manifest the capability and indispensability of modern electroanalytical chemistry when addressing these pressing challenges and beyond.