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

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.

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.

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.

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.

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 .

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.