Solvent‐Mediated Control of the Electrochemical Discharge Products of Non‐Aqueous Sodium–Oxygen Electrochemistry

The reduction of dioxygen in the presence of sodium cations can be tuned to give either sodium superoxide or sodium peroxide discharge products at the electrode surface. Control of the mechanistic direction of these processes may enhance the ability to tailor the energy density of sodium–oxygen batteries (NaO₂: 1071 Wh kg^?1 and Na₂O₂: 1505 Wh kg^?1). Through spectroelectrochemical analysis of a range of non‐aqueous solvents, we describe the dependence of these processes on the electrolyte solvent and subsequent interactions formed between Na⁺ and O₂^?. The solvents ability to form and remove [Na⁺‐O₂^?]_ads based on Gutmann donor number influences the final discharge product and mechanism of the cell. Utilizing surface‐enhanced Raman spectroscopy and electrochemical techniques, we demonstrate an analysis of the response of Na‐O₂ cell chemistry with sulfoxide, amide, ether, and nitrile electrolyte solvents.     

From K‐O2 to K‐Air Batteries: Realizing Superoxide Batteries on the Basis of Dry Ambient Air

Although using an air cathode is the goal for superoxide‐based potassium‐oxygen (K‐O₂) batteries, prior studies were limited to pure oxygen. Now, the first K‐air (dry) battery based on reversible superoxide electrochemistry is presented. Spectroscopic and gas chromatography analyses are applied to evaluate the reactivity of KO₂ in ambient air. Although KO₂ reacts with water vapor and CO₂ to form KHCO₃, it is highly stable in dry air. With this knowledge, rechargeable K‐air (dry) batteries were successfully demonstrated by employing dry air cathode. The reduced partial pressure of oxygen plays a critical role in boosting battery lifespan. With a more stable environment for the K anode, a K‐air (dry) battery delivers over 100 cycles (>500 h) with low round‐trip overpotentials and high coulombic efficiencies as opposed to traditional K‐O₂ battery that fails early. This work sheds light on the benefits and restrictions of employing the air cathode in superoxide‐based batteries.     

Superoxide Stabilization and a Universal KO2 Growth Mechanism in Potassium–Oxygen Batteries

Rechargeable potassium–oxygen (K‐O₂) batteries promise to provide higher round‐trip efficiency and cycle life than other alkali–oxygen batteries with satisfactory gravimetric energy density (935 Wh kg^?1). Exploiting a strong electron‐donating solvent, for example, dimethyl sulfoxide (DMSO) strongly stabilizes the discharge product (KO₂), resulting in significant improvement in electrode kinetics and chemical/electrochemical reversibility. The first DMSO‐based K‐O₂ battery demonstrates a much higher energy efficiency and stability than the glyme‐based electrolyte. A universal KO₂ growth model is developed and it is demonstrated that the ideal solvent for K‐O₂ batteries should strongly stabilize superoxide (strong donor ability) to obtain high electrode kinetics and reversibility while providing fast oxygen diffusion to achieve high discharge capacity. This work elucidates key electrolyte properties that control the efficiency and reversibility of K‐O₂ batteries.     

Crystallographic Evidence for Direct Metal–Metal Bonding in a Stable Open‐Shell La2@Ih‐C80 Derivative

Endohedral metallofullerenes (EMFs) have novel structures and properties that are closely associated with the internal metallic species. Benzyl radical additions have been previously shown to form closed‐shell adducts by attaching an odd number of addends to open‐shell EMFs (such as Sc₃C₂@I_h‐C₈₀) whereas an even number of groups are added to closed‐shell EMFs (for example Sc₃N@I_h‐C₈₀). Herein we report that benzyl radical addition to the closed‐shell La₂@I_h‐C₈₀ forms a stable, open‐shell monoadduct instead of the anticipated closed‐shell bisadduct. Single‐crystal X‐ray diffraction results show the formation of a stable radical species. In this species, the La?La distance is comparable to the theoretical value of a La?La covalent bond and is shorter than reported values for other La₂@I_h‐C₈₀ derivatives, providing unambiguous evidence for the formation of direct La?La bond.     

Concentrated Electrolyte for the Sodium–Oxygen Battery: Solvation Structure and Improved Cycle Life

Alkali metal–oxygen batteries are of great interests for energy storage because of their unparalleled theoretical energy densities. Particularly attractive is the emerging Na–O₂ battery because of the formation of superoxide as the discharge product. Dimethyl sulfoxide (DMSO) is a promising solvent for this battery but its instability towards Na makes it impractical in the Na–O₂ battery. Herein we report the enhanced stability of Na in DMSO solutions containing concentrated sodium trifluoromethanesulfonimide (NaTFSI) salts (>3 mol kg^?1). Raman spectra of NaTFSI/DMSO electrolytes and ab initio molecular dynamics simulation reveal the Na⁺ solvation number in DMSO and the formation of Na(DMSO)₃(TFSI)‐like solvation structure. The majority of DMSO molecules solvating Na⁺ in concentrated solutions reduces the available free DMSO molecules that can react with Na and renders the TFSI anion decomposition, which protects Na from reacting with the electrolyte. Using these concentrated electrolytes, Na–O₂ batteries can be cycled forming sodium superoxide (NaO₂) as the sole discharge product with improved long cycle life, highlighting the beneficial role of concentrated electrolytes for Na‐based batteries.     

Photo‐excited Oxygen Reduction and Oxygen Evolution Reactions Enable a High‐Performance Zn–Air Battery

The storage of solar energy in battery systems is pivotal for a sustainable society, which faces many challenges. Herein, a Zn–air battery is constructed with two cathodes of poly(1,4‐di(2‐thienyl))benzene (PDTB) and TiO₂ grown on carbon papers to sandwich a Zn anode. The PDTB cathode is illuminated in a discharging process, in which photoelectrons are excited into the conduction band of PDTB to promote oxygen reduction reaction (ORR) and raise the output voltage. In a reverse process, holes in the valence band of the illuminated TiO₂ cathode are driven for the oxygen evolution reaction (OER) by an applied voltage. A record‐high discharge voltage of 1.90 V and an unprecedented low charge voltage of 0.59 V are achieved in the photo‐involved Zn–air battery, regardless of the equilibrium voltage. This work offers an innovative pathway for photo‐energy utilization in rechargeable batteries.     

The Structure and Torsional Dynamics of Two Methyl Groups in 2‐Acetyl‐5‐methylfuran as Observed by Microwave Spectroscopy

The molecular‐beam Fourier transform microwave spectrum of 2‐acetyl‐5‐methylfuran is recorded in the frequency range 2–26.5 GHz. Quantum chemical calculations calculate two conformers with trans or cis configuration of the acetyl group, both of which are assigned in the experimental spectrum. All rotational transitions split into quintets due to the internal rotations of two nonequivalent methyl groups. By using the program XIAM, the experimental spectra can be simulated with standard deviations within the measurement accuracy, and yield well‐determined rotational and internal rotation parameters, inter alia the V₃ potentials. Whereas the V₃ barrier height of the ring‐methyl rotor does not change for the two conformers, that of the acetyl‐methyl rotor differs by about 100 cm^?1. The predicted values from quantum chemistry are only on the correct order of magnitude.     

Gas‐Phase Synthesis and Reactivity of the Lithium Acetate Enolate Anion, −CH2CO2Li

Aerial pingpong : The lithium acetate enolate anion, the prototypical lithium salt of an α‐deprotonated carboxylate, was prepared in the gas phase by electrospray ionization (ESI) and collision‐induced ionization (CID). Its structure, reactivity, and energetics are presented along with the results of high‐level computations.

     

Capturing Transient Endoperoxide in the Singlet Oxygen Oxidation of Guanine

The chemistry of singlet O₂ toward the guanine base of DNA is highly relevant to DNA lesion, mutation, cell death, and pathological conditions. This oxidative damage is initiated by the formation of a transient endoperoxide through the Diels–Alder cycloaddition of singlet O₂ to the guanine imidazole ring. However, no endoperoxide formation was directly detected in native guanine or guanosine, even at ?100 °C. Herein, gas‐phase ion–molecule scattering mass spectrometry was utilized to capture unstable endoperoxides in the collisions of hydrated guanine ions (protonated or deprotonated) with singlet O₂ at ambient temperature. Corroborated by results from potential energy surface exploration, kinetic modeling, and dynamics simulations, various aspects of endoperoxide formation and transformation (including its dependence on guanine ionization and hydration states, as well as on collision energy) were determined. This work has pieced together reaction mechanisms, kinetics, and dynamics data concerning the early stage of singlet O₂ induced guanine oxidation, which is missing from conventional condensed‐phase studies.     

Stable Non‐IPR C60 and C70 Fullerenes Containing a Uniform Distribution of Pyrenes and Adjacent Pentagons

The most abundant fullerenes, C₆₀ and C₇₀, and all the pure carbon fullerenes larger than C₇₀, follow the isolated‐pentagon rule (IPR). Non‐IPR fullerenes containing adjacent pentagons (APs) have been stabilized experimentally in cases where, according to Euler’s theorem, it is topologically impossible to isolate all the pentagons from each other. Surprisingly, recent experiments have shown that a few endohedral fullerenes, for which IPR structures are possible, are stabilized in non‐IPR cages. We show that, apart from strain, the physical property that governs the relative stabilities of fullerenes is the charge distribution in the cage. This charge distribution is controlled by the number and location of APs and pyrene motifs. We show that, when these motifs are uniformly distributed in the cage and well‐separated from one other, stabilization of non‐IPR endohedral and exohedral derivatives, as well as pure carbon fullerene anions and cations, is the rule, rather than the exception. This suggests that non‐IPR derivatives might be even more common than IPR ones.     

The Quest for Polysulfides in Lithium–Sulfur Battery Electrolytes: An Operando Confocal Raman Spectroscopy Study

Confocal Raman spectra of a lithium–sulfur battery electrolyte are recorded operando in a depth‐of‐discharge resolved manner for an electrochemical cell with a realistic electrolyte/sulfur loading ratio. The evolution of various possible polysulfides is unambiguously identified by combining Raman spectroscopy data with DFT simulations.     

Difluoroacetaldehyde N‐Triftosylhydrazone (DFHZ‐Tfs) as a Bench‐Stable Crystalline Diazo Surrogate for Diazoacetaldehyde and Difluorodiazoethane

Despite the growing importance of volatile functionalized diazoalkanes in organic synthesis, their safe generation and utilization remain a formidable challenge because of their difficult handling along with storage and security issues. In this study, we developed a bench‐stable difluoroacetaldehyde N‐triftosylhydrazone (DFHZ‐Tfs) as an operationally safe diazo surrogate that can release in situ two low‐molecular‐weight diazoalkanes, diazoacetaldehyde (CHOCHN₂) or difluorodiazoethane (CF₂HCHN₂), in a controlled fashion under specific conditions. DFHZ‐Tfs has been successfully employed in the Fe‐catalyzed cyclopropanation and Doyle–Kirmse reactions, thus highlighting the synthetic utility of DFHZ‐Tfs in the efficient construction of molecule frameworks containing CHO or CF₂H groups. Moreover, the reaction mechanism for the generation of CHOCHN₂ from CF₂HCHN₂ was elucidated by density functional theory (DFT) calculations.     

Biradicaloid Character of Thiophene‐Based Heterophenoquinones: The Role of Electron–Phonon Coupling

The quinoidal versus biradicaloid character of the ground state of a series of thiophene‐based heterophenoquinones is investigated with quantum‐chemical calculations. The role of the ground‐state electronic character on molecular structure and vibrational properties is emphasized. The vibrational activities are experimentally determined and their analysis is performed by taking advantage of the definition of a collective vibrational coordinate (the

     

Tuning Proton Conductivity by Interstitial Guest Change in Size‐Adjustable Nanopores of a CuI‐MOF: A Potential Platform for Versatile Proton Carriers

By exploiting the breathing behavior of nanopores, we have studied for the first time the dependency of the guest‐induced proton conductivity of an interpenetrated Cu^I metal–organic framework (Cu^I‐MOF, [ 1 ]) on various guest molecules. Proton conductivities of over 10^?3 S cm^?1 under humid conditions were induced by a series of guest molecules, namely N,N‐dimethylformamide, dimethyl sulfoxide, diethylamine, 1,4‐dinitrobenzene, nitrobenzene, pyridine, and 1H‐1,2,4‐triazole. A detailed investigation of the guest‐incorporated complexes revealed that low‐energy proton conduction occurs under humid conditions through the Grotthuss mechanism in [ 1 ?NB] and through the vehicle mechanism in the rest of the complexes. Single‐point energy computations revealed considerable stabilization upon guest encapsulation. To the best of our knowledge, [ 1 ] represents the first example in which considerably high protonic conductivity is triggered upon the facile incorporation of small molecules of such a variety. The investigation portrayed herein may be a stepping stone towards the rational design of proton‐conducting materials for practical applications.     

Singlet Oxygen Formation during the Charging Process of an Aprotic Lithium–Oxygen Battery

Aprotic lithium–oxygen (Li–O₂) batteries have attracted considerable attention in recent years owing to their outstanding theoretical energy density. A major challenge is their poor reversibility caused by degradation reactions, which mainly occur during battery charge and are still poorly understood. Herein, we show that singlet oxygen (¹Δ_g) is formed upon Li₂O₂ oxidation at potentials above 3.5 V. Singlet oxygen was detected through a reaction with a spin trap to form a stable radical that was observed by time‐ and voltage‐resolved in operando EPR spectroscopy in a purpose‐built spectroelectrochemical cell. According to our estimate, a lower limit of approximately 0.5 % of the evolved oxygen is singlet oxygen. The occurrence of highly reactive singlet oxygen might be the long‐overlooked missing link in the understanding of the electrolyte degradation and carbon corrosion reactions that occur during the charging of Li–O₂ cells.     

Direct Visualization of the Chemical Mechanism in SERRS of 4‐Aminothiophenol/Metal Complexes and Metal/4‐Aminothiophenol/Metal Junctions

Direct visualization of photoinduced tunneling charge transfer (TCT) in an Au₅/para‐aminothiophenol (PATP)/Ag₆ junction in which Au and Ag clusters form the first and second layer, respectively, is provided by the charge difference density (see picture; green and red stand for holes and electrons, respectively).