Direct Detection of the Superoxide Anion as a Stable Intermediate in the Electroreduction of Oxygen in a Non-Aqueous Electrolyte Containing Phenol as a Proton Source

From 2e- to 4e- pathway in the alkaline oxygen reduction reaction on Au(100): Kinetic circumvention of the volcano curve

We report the free energy barriers for the elementary reactions in the 2e- and 4e- oxygen reduction reaction (ORR) steps on Au(100) in an alkaline solution. Due to the weak adsorption energy of O2 on Au(100), the barrier for the association channel is very low, and the 2e- pathway is clearly favored, while the barrier for the O-O dissociation channel is significantly higher at 0.5 eV. Above 0.7 V reversible hydrogen electrode (RHE), the association channel becomes thermodynamically unfavorable, which opens up the O-O dissociation channel, leading to the 4e- pathway. The low adsorption energy of oxygenated species on Au is now an advantage, and residue ORR current can be observed up to the 1.0-1.2 V region (RHE). In contrast, the O-O dissociation barrier on Au(111) is significantly higher, at close to 0.9 eV, due to coupling with surface reorganization, which explains the lower ORR activity on Au(111) than that on Au(100). In combination with the previously suggested outer sphere electron transfer to O2 for its initial adsorption, these results provide a consistent explanation for the features in the experimentally measured polarization curve for the alkaline ORR on Au(100) and demonstrate an ORR mechanism distinct from that on Pt(111). It also highlights the importance to consider the spin state of O2 in ORR and to understand the activation barriers, in addition to the adsorption energies, to account for the features observed in electrochemical measurements.

Single-zinc vacancy unlocks high-rate H2O2 electrosynthesis from mixed dioxygen beyond Le Chatelier principle

Le Chatelier's principle is a basic rule in textbook defining the correlations of reaction activities and specific system parameters (like concentrations), serving as the guideline for regulating chemical/catalytic systems. Here we report a model system breaking this constraint in O2 electroreduction in mixed dioxygen. We unravel the central role of creating single-zinc vacancies in a crystal structure that leads to enzyme-like binding of the catalyst with enhanced selectivity to O2, shifting the reaction pathway from Langmuir-Hinshelwood to an upgraded triple-phase Eley-Rideal mechanism. The model system shows minute activity alteration of H2O2 yields (25.89~24.99 mol gcat-1 h-1) and Faradaic efficiencies (92.5%~89.3%) in the O2 levels of 100%~21% at the current density of 50~300 mA cm-2, which apparently violate macroscopic Le Chatelier's reaction kinetics. A standalone prototype device is built for high-rate H2O2 production from atmospheric air, achieving the highest Faradaic efficiencies of 87.8% at 320 mA cm-2, overtaking the state-of-the-art catalysts and approaching the theoretical limit for direct air electrolysis (~345.8 mA cm-2). Further techno-economics analyses display the use of atmospheric air feedstock affording 21.7% better economics as comparison to high-purity O2, achieving the lowest H2O2 capital cost of 0.3 $ Kg-1. Given the recent surge of demonstrations on tailoring chemical/catalytic systems based on the Le Chatelier's principle, the present finding would have general implications, allowing for leveraging systems "beyond" this classical rule.

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.

Direct Capturing and Regulating Key Intermediates for High-Efficiency Oxygen Evolution Reactions

Developing efficient oxygen evolution reaction (OER) electrocatalysts can greatly advance the commercialization of proton exchange membrane (PEM) water electrolysis. However, the unclear and disputed reaction mechanism and structure-activity relationship of OER pose significant obstacles. Herein, the active site and intermediate for OER on AuIr nanoalloys are simultaneously identified and correlated with the activity, through the integration of in situ shell-isolated nanoparticle-enhanced Raman spectroscopy and X-ray absorption spectroscopy. The AuIr nanoalloys display excellent OER performance with an overpotential of only 246 mV to achieve 10 mA cm-2 and long-term stability under strong acidic conditions. Direct spectroscopic evidence demonstrates that * OO adsorbed on IrOx sites is the key intermediate for OER, and it is generated through the O-O coupling of adsorbed oxygen species directly from water, providing clear support for the adsorbate evolution mechanism. Moreover, the Raman information of the * OO intermediate can serve as a universal "in situ descriptor" that can be obtained both experimentally and theoretically to accelerate the catalyst design. It unveils that weakening the interactions of * OO on the catalysts and facilitating its desorption would boost the OER performance. This work deepens the mechanistic understandings on OER and provides insightful guidance for the design of more efficient OER catalysts.

Recent advances and interfacial challenges in solid-state electrolytes for rechargeable Li-air batteries

Among the promising batteries for electric vehicles, rechargeable Li-air (O2) batteries (LABs) have risen keen interest due to their high energy density. However, safety issues of conventional nonaqueous electrolytes remain the bottleneck of practical implementation of LABs. Solid-state electrolytes (SSEs) with non-flammable and eco-friendly properties are expected to alleviate their safety concerns, which have become a research focus in the research field of LABs. Herein, we present a systematic review on the progress of SSEs for rechargeable LABs, mainly focusing on the interfacial issues existing between the SSEs and electrodes. The discussion highlights the challenges and feasible strategies for designing suitable SSEs for LABs.

A copper-seamed coordination nanocapsule as a semiconductor photocatalyst for molecular oxygen activation

Here we report that a Cu²⁺-seamed coordination nanocapsule can serve as an efficient semiconductor photocatalyst for molecular oxygen activation. This capsule was constructed through a redox reaction facilitated self-assembly of cuprous bromide and C-pentyl-pyrogallol[4]arene. Photophysical and electrochemical studies revealed its strong visible-light absorption and photocurrent polarity switching effect. This novel molecular solid material is capable of activating molecular oxygen into reactive oxygen species under simulated sunlight irradiation. The oxygen activation process has been exploited for catalyzing aerobic oxidation reactions. The present work provides new insights into designing nonporous discrete metal-organic supramolecular assemblies for solar-driven molecular oxygen activation.

Reversible Conversion between Lithium Superoxide and Lithium Peroxide: A Closed “Lithium–Oxygen” Battery

Lithium–air batteries have become a desirable research direction in the field of green energy due to their large specific capacity and high energy density. The current research mainly focuses on an open system continuously supplying high-purity oxygen or air. However, factors such as water and CO2 in the open system and liquid electrolytes’ evaporation will decrease battery performance. To improve the practical application of lithium–air batteries, developing a lithium–oxygen battery that does not need a gaseous oxygen supply is desirable. In this study, we designed a closed lithium–oxygen battery model based on the conversion of lithium superoxide and lithium peroxide (LiO2 + e− + Li+ ↔ Li2O2). Herein, the Pd-rGO as a catalyst will produce the LiO2 in the pre-discharge process, and the closed battery can cycle over 57 cycles stably. In addition to in situ Raman spectra, electrochemical quartz crystal microbalance (EQCM) and differential electrochemical mass spectrometry (DEMS) have been applied to explanation the conversion between LiO2 and Li2O2 during the charge–discharge process. This work paves the way to introduce a new closed “lithium–oxygen” battery system for developing large-capacity green energy.

Advances in Lithium–Oxygen Batteries Based on Lithium Hydroxide Formation and Decomposition

The rechargeable lithium-oxygen (Li–O2) batteries have been considered one of the promising energy storage systems owing to their high theoretical energy density. As an alternative to Li−O2 batteries based on lithium peroxide (Li2O2) cathode, cycling Li−O2 batteries via the formation and decomposition of lithium hydroxide (LiOH) has demonstrated great potential for the development of practical Li−O2 batteries. However, the reversibility of LiOH-based cathode chemistry remains unclear at the fundamental level. Here, we review the recent advances made in Li−O2 batteries based on LiOH formation and decomposition, focusing on the reaction mechanisms occurring at the cathode, as well as the stability of Li anode and cathode binder. We also provide our perspectives on future research directions for high-performance, reversible Li−O2 batteries.

First-Order or Second-Order? Disproportionation of Lithium Superoxide in Li-O2 Batteries

The disproportionation of LiO₂ to Li₂O₂ is a key step in Li-O₂ batteries, and it is regarded as a second-order reaction. However, its mechanism is not well addressed, and its kinetics is rarely studied due to the difficulties of quantifying the rate constants, particularly for high concentrations of superoxide (>10 mM). Here, we quantified the kinetic rate constant by a microkinetic model using a microelectrode tip with a thin diffusion layer and fast response. We report that the reaction order of LiO₂ transitions from 1 at high concentrations of superoxide (∼20 mM) to 2 at low concentrations of superoxide (∼1 mM). LiO₂ is chemically reduced by free superoxides to form Li₂O₂ and O₂, instead of reacting with another LiO₂ via a disproportionation step. This chemical-reduction mechanism explained the change of reaction order and the kinetics profile. As a rate-determining step, this step restricts the overall kinetics of the discharging process and should be the focus of future catalyst design.

Real-Time Monitoring of Surface Effects on the Oxygen Reduction Reaction Mechanism for Aprotic Na-O2 Batteries

Discharging of aprotic sodium-oxygen (Na-O₂) batteries is driven by the cathodic oxygen reduction reaction in the presence of sodium cations (Na⁺-ORR). However, the mechanism of aprotic Na⁺-ORR remains ambiguous and is system dependent. In-situ electrochemical Raman spectroscopy has been employed to study the aprotic Na⁺-ORR processes at three atomically ordered Au(hkl) single-crystal surfaces for the first time, and the structure-intermediates/mechanism relationship has been identified at a molecular level. Direct spectroscopic evidence of superoxide on Au(110) and peroxide on Au(100) and Au(111) as intermediates/products has been obtained. Combining these experimental results with theoretical simulation has revealed that the surface effect of Au(hkl) electrodes on aprotic Na⁺-ORR activity is mainly caused by the different adsorption of Na⁺ and O₂. This work enhances our understanding of aprotic Na⁺-ORR on Au(hkl) surfaces and provides further guidance for the design of improved Na-O₂ batteries.

In Situ Surface-Enhanced Raman Spectroscopy Characterization of Electrocatalysis with Different Nanostructures

As energy demands increase, electrocatalysis serves as a vital tool in energy conversion. Elucidating electrocatalytic mechanisms using in situ spectroscopic characterization techniques can provide experimental guidance for preparing high-efficiency electrocatalysts. Surface-enhanced Raman spectroscopy (SERS) can provide rich spectral information for ultratrace surface species and is extremely well suited to studying their activity. To improve the material and morphological universalities, researchers have employed different kinds of nanostructures that have played important roles in the development of SERS technologies. Different strategies, such as so-called borrowing enhancement from shell-isolated modes and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS)-satellite structures, have been proposed to obtain highly effective Raman enhancement, and these methods make it possible to apply SERS to various electrocatalytic systems. Here, we discuss the development of SERS technology, focusing on its applications in different electrocatalytic reactions (such as oxygen reduction reactions) and at different nanostructure surfaces, and give a brief outlook on its development.

Porous Materials Applied in Nonaqueous Li-O2 Batteries: Status and Perspectives

Porous materials possessing high surface area, large pore volume, tunable pore structure, superior tailorability, and dimensional effect have been widely applied as components of lithium-oxygen (Li-O₂ ) batteries. Herein, the theoretical foundation of the porous materials applied in Li-O₂ batteries is provided, based on the present understanding of the battery mechanism and the challenges and advantageous qualities of porous materials. Furthermore, recent progress in porous materials applied as the cathode, anode, separator, and electrolyte in Li-O₂ batteries is summarized, together with corresponding approaches to address the critical issues that remain at present. Particular emphasis is placed on the importance of the correlation between the function-orientated design of porous materials and key challenges of Li-O₂ batteries in accelerating oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) kinetics, improving the electrode stability, controlling lithium deposition, suppressing the shuttle effect of the dissolved redox mediators, and alleviating electrolyte decomposition. Finally, the rational design and innovative directions of porous materials are provided for their development and application in Li-O₂ battery systems.

Cobalt-Catalyzed Selective Unsymmetrical Dioxidation of gem-Difluoroalkenes

gem-Difluoroalkenes represent valuable synthetic handles for organofluorine chemistry; however, most reactions of this substructure proceed through reactive intermediates prone to eliminate a fluorine atom and generate monofluorinated products. Taking advantage of the distinct reactivity of gem-difluoroalkenes, we present a cobalt-catalyzed regioselective unsymmetrical dioxygenation of gem-difluoroalkenes using phenols and molecular oxygen, which retains both fluorine atoms and provides β-phenoxy-β,β-difluorobenzyl alcohols. Mechanistic studies suggest that the reaction operates through a radical chain process initiated by Co(II)/O₂/phenol and quenched by the Co-based catalyst. This mechanism enables the retention of both fluorine atoms, which contrasts most transition-metal-catalyzed reactions of gem-difluoroalkenes that typically involve defluorination.

Oxygen Reduction Reaction on Au Revisited at Different pH Values using in situ Surface-Enhanced Raman Spectroscopy

The mechanisms of the oxygen reduction reaction (ORR) on Au surfaces are revisited in electrolytes with different pH values by using a combination of electrochemical and in situ surface-enhanced Raman scattering spectroscopy. Surprisingly, the in situ Raman signal of the O-O stretching vibration was detected during the ORR on a Au surface by using a λ=785 nm laser. Both the intermediate products O₂ ^- and H₂ O₂ could be detected, which indicates the difficulty of the further reduction H₂ O₂ and results in a lower electron transfer number, especially in neutral and acid electrolytes. The weak absorption ability of HO₂ on the Au surface may explain the poor ORR in neutral and acid electrolytes. This work not only provides a deep insight to understand the reduction mechanisms of O₂ on Au in electrolytes with different pH values but also supplies a new idea for the selection and optimization of electrolytes and efficient electrocatalysts for oxygen reduction.

Potassium Doping Facilitated Formation of Tunable Superoxides in Li2O2 for Improved Electrochemical Kinetics

Superoxide (O₂^-) species play a crucial role in determining the charge kinetics for aprotic lithium-oxygen (Li-O₂) batteries. However, the growth of O₂^--rich lithium peroxide (Li₂O₂) is challenging since O₂^- is thermodynamically unfavorable and unstable in an O₂ atmosphere. Herein, we reported the synthesis of defective Li₂O₂ with tunable O₂^- via K⁺ doping. The K⁺ dopants can successfully stabilize O₂^- species and induce the coordination of Li⁺ with O₂^-, leading to increased Li vacancies. Compared to the pristine Li₂O₂, the as-prepared defective Li₂O₂ can be charged at a lower overpotential in Li-O₂ batteries, which is ascribed to further increased Li vacancies contributed by the depotassiation process at the onset of the charge process. Our findings suggest a new strategy to better control O₂^- species in Li₂O₂ by K⁺ dopants and provide insights into the K⁺ effects on charge mechanism in Li-O₂ batteries.

Electrolytes for Rechargeable Lithium-Air Batteries

Lithium-air batteries are promising devices for electrochemical energy storage because of their ultrahigh energy density. However, it is still challenging to achieve practical Li-air batteries because of their severe capacity fading and poor rate capability. Electrolytes are the prime suspects for cell failure. In this Review, we focus on the opportunities and challenges of electrolytes for rechargeable Li-air batteries. A detailed summary of the reaction mechanisms, internal compositions, instability factors, selection criteria, and design ideas of the considered electrolytes is provided to obtain appropriate strategies to meet the battery requirements. In particular, ionic liquid (IL) electrolytes and solid-state electrolytes show exciting opportunities to control both the high energy density and safety.

In situ Spectroscopic Insight into the Origin of the Enhanced Performance of Bimetallic Nanocatalysts towards the Oxygen Reduction Reaction (ORR)

It is vital to understand the oxygen reduction reaction (ORR) mechanism at the molecular level for the rational design and synthesis of high activity fuel-cell catalysts. Surface enhanced Raman spectroscopy (SERS) is a powerful technique capable of detecting the bond vibrations of surface species in the low wavenumber range, however, using it to probe practical nanocatalysts remains extremely challenging. Herein, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was used to investigate ORR processes on the surface of bimetallic Pt₃ Co nanocatalyst structures. Direct spectroscopic evidence of *OOH suggests that ORR undergoes an associative mechanism on Pt₃ Co in both acidic and basic environments. Density functional theory (DFT) calculations show that the weak *O adsorption arise from electronic effect on the Pt₃ Co surface accounts for enhanced ORR activity. This work shows SHINERS is a promising technique for the real-time observation of catalytic processes.

Relieving the "Sudden Death" of Li-O2 Batteries by Grafting an Antifouling Film on Cathode Surfaces

The "sudden-death" phenomenon has been frequently encountered during discharging of Li-O₂ batteries and has been ascribed to the growth of a blocking film of Li₂O₂ on the cathode surface. Recent fundamental study revealed that this dilemma could be addressed by discharging Li₂O₂ in the electrolyte solution rather than on the cathode surface. However, even for Li-O₂ batteries operated under the conditions favorable for the solution growth of Li₂O₂, sudden death still persists and its origin remains incompletely understood. Herein, by using a combination of in situ spectroscopy and theoretical calculation, we reveal that sudden death of Li-O₂ batteries operated under the conditions (e.g., low discharge current density and high donor number electrolyte solvent) favorable for discharging Li₂O₂ in the electrolyte solutions is caused by adventitious adsorption of a minor quantity of Li₂O₂, which triggers a rapid transition of Li₂O₂ growth mode from solution- to surface-mediated growth. Moreover, a cathode surface modification strategy has been developed to effectively retard the Li₂O₂ adsorption and therefore significantly alleviate the sudden death of Li-O₂ batteries.

In Situ Analysis of Surface Catalytic Reactions Using Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy

Electrochemistry and heterogeneous catalysis continue to attract enormous interest. In situ surface analysis is a dynamic research field capable of elucidating the catalytic mechanisms of reaction processes. Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is a nondestructive technique that has been cumulatively used to probe and analyze catalytic-reaction processes, providing important spectral evidence about reaction intermediates produced on catalyst surfaces. In this perspective, we review recent electrochemical- and heterogeneous-catalysis studies using SHINERS, highlight its advantages, summarize the flaws and prospects for improving the SHINERS technique, and give insight into its future research directions.

Promoting Li-O2 Batteries With Redox Mediators

Li-O₂ batteries have a high theoretical specific energy, 3500 Wh kg^-1 ; however, its practical capacity is far below this value and limited by the passivation with the insulating discharge product Li₂ O₂ . The nonconductive nature of Li₂ O₂ also impedes the charging process, leading to a low coulombic efficiency and high overpotential on charge even at a moderate rate. To address these challenges, redox mediators could be used both during discharge and charge to transfer electrons between O₂ /electrode surface or Li₂ O₂ /electrode surface to overcome the passivation of Li₂ O₂ , which would facilitate the discharge and charge process. The capacity and current density were significantly improved using the redox mediators, thus representing a promising strategy to achieve a high energy density for Li-O₂ batteries.

CO2 electrochemical reduction at thiolate-modified bulk Au electrodes

Simple modification of polycrystalline bulk Au by an appropriate thiol can selectively enhance electrochemical CO₂RR at the expense of HER.

The Salt Matters: Enhanced Reversibility of Li-O2 Batteries with a Li[(CF3 SO2 )(n-C4 F9 SO2 )N]-Based Electrolyte

The safety hazards and cycle instability of lithium metal anodes (LMA) constitute significant barriers to progress in lithium metal batteries. This situation is worse in Li-O₂ batteries because the LMA is prone to be chemically attacked by O₂ shuttled from the cathode. Notwithstanding, efforts on LMA are much sparse than those on the cathode in the realm of Li-O₂ batteries. Here, a novel lithium salt of Li[(CF₃ SO₂ )(n-C₄ F₉ SO₂ )N] (LiTNFSI) is reported, which can effectively suppress the parasitic side reactions and dendrite growth of LMA during cycling and thereby significantly enhance the overall reversibility of Li-O₂ batteries. A variety of advanced research tools are employed to scrutinize the working principles of the LiTNFSI salt. It is revealed that a stable, uniform, and O₂ -resistive solid electrolyte interphase is formed on LMA, and hence the "cross-talk" between the LMA and O₂ shuttled from the cathode is remarkably inhibited in LiTNFSI-based Li-O₂ batteries.

Functional and stability orientation synthesis of materials and structures in aprotic Li–O2batteries

This review presents the recent advances made in the functional and stability orientation synthesis of materials/structures for Li–O₂batteries.

Estimation of electric field effects on the adsorption of molecular superoxide species on Au based on density functional theory

Superoxide species are key intermediates in the oxygen reduction reactions (ORR) that occur at the cathodes of aprotic metal-air batteries. Herein we report a DFT study of the effects of an externally applied electric field (ε) on the stability of various molecular superoxide species, including MO₂ (M = Li, Na, K) and O₂^-, on gold surfaces, which shows that the stability of such species on Au electrodes can be materially affected by the presence of an electric field and solvent molecules, suggesting that such effects should be included in the first-principles modeling of cathode reactions in metal-O₂ cells. In the ε range of ±0.4 V Å^-1, the stability of MO₂ species is found to vary by up to |0.4| eV on Au(111) and |0.2| eV on Au(211) in vacuo, which is larger than the field effects on the stability of O and OH, key intermediates in the ORR by hydrogen. An aprotic solvent such as dimethyl sulfoxide (DMSO), considered here via the inclusion of explicit DMSO molecules above the Au surfaces, stabilizes all three MO₂ species at zero fields and amplifies the field effects on the stability of MO₂, on both Au surfaces. The variations in the stability of the molecular MO₂ species with ε, which have small polarizabilities, are closely approximated by the first-order Stark effect (μ₀·ε, where μ₀ is the static surface dipole moment induced by adsorption at ε = 0 V Å^-1). The superoxide anion (O₂^-) that has been identified on an under-coordinated Au site has a larger polarizability than the MO_x species and a μ₀ that is opposite in sign to those of the metal MO₂ species, which results in larger errors by the first-order approximation, although its stability varies only moderately under positive electric fields of up to 0.4 V Å^-1.

Phenol-Catalyzed Discharge in the Aprotic Lithium-Oxygen Battery

Discharge in the lithium-O2 battery is known to occur either by a solution mechanism, which enables high capacity and rates, or a surface mechanism, which passivates the electrode surface and limits performance. The development of strategies to promote solution-phase discharge in stable electrolyte solutions is a central challenge for development of the lithium-O2 battery. Here we show that the introduction of the protic additive phenol to ethers can promote a solution-phase discharge mechanism. Phenol acts as a phase-transfer catalyst, dissolving the product Li2 O2 , avoiding electrode passivation and forming large particles of Li2 O2 product-vital requirements for high performance. As a result, we demonstrate capacities of over 9 mAh cm-2areal , which is a 35-fold increase in capacity compared to without phenol. We show that the critical requirement is the strength of the conjugate base such that an equilibrium exists between protonation of the base and protonation of Li2 O2 .

Highly Active and Stable Pt-Pd Alloy Catalysts Synthesized by Room-Temperature Electron Reduction for Oxygen Reduction Reaction

Carbon-supported platinum (Pt) and palladium (Pd) alloy catalyst has become a promising alternative electrocatalyst for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. In this work, the synthesis of highly active and stable carbon-supported Pt-Pd alloy catalysts is reported with a room-temperature electron reduction method. The alloy nanoparticles thus prepared show a particle size around 2.6 nm and a core-shell structure with Pt as the shell. With this structure, the breaking of O-O bands and desorption of OH are both promoted in electrocatalysis of ORR. In comparison with the commercial Pt/C catalyst prepared by conventional method, the mass activity of the Pt-Pd/C catalyst for ORR is shown to increase by a factor of ≈4. After 10 000-cycle durability test, the Pt-Pd/C catalyst is shown to retain 96.5% of the mass activity, which is much more stable than that of the commercial Pt/C catalyst.

Recent advances in understanding of the mechanism and control of Li2O2formation in aprotic Li–O2batteries

This review systematically summarizes the recent advances in the mechanism studies and control strategies of Li₂O₂formation in aprotic Li–O₂batteries.

Mechanistic origin of low polarization in aprotic Na–O2 batteries

The mechanistic difference between Li–O₂ and Na–O₂ batteries has been revealed by in situ spectroscopy coupled with theory calculations.

Spectroscopic Identification of the Au-C Bond Formation upon Electroreduction of an Aryl Diazonium Salt on Gold

Electroreduction of aryl diazonium salts on gold can produce organic films that are more robust than their analogous self-assembled monolayers formed from chemical adsorption of organic thiols on gold. However, whether the enhanced stability is due to the Au-C bond formation remains debated. In this work, we report the electroreduction of an aryl diazonium salt of 4,4'-disulfanediyldibenzenediazonium on gold forming a multilayer of Au-(Ar-S-S-Ar)_n, which can be further degraded to a monolayer of Au-Ar-S^- by electrochemical cleavage of the S-S moieties within the multilayer. By conducting an in situ surface-enhanced Raman spectroscopic study of both the multilayer formation/degradation and the monolayer reduction/oxidation processes, coupled to density functional theory calculations, we provide compelling evidence that an Au-C bond does form upon electroreduction of aryl diazonium salts on gold and that the enhanced stability of the electrografted organic films is due to the Au-C bond being intrinsically stronger than the Au-S bond for a given phenylthiolate compound by ca. 0.4 eV.

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 Na2 O2 : 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.

A delicate case of unidirectional proton transfer from water to an aromatic heterocyclic anion

One of the hydroxyl modes of the surrounding water molecules with the lowest stretching frequency facilitates the proton transfer from water to an anion.