Lee Johnson

The role of LiO2 solubility in O2 reduction in aprotic solvents and its consequences for Li-O2 batteries.

When lithium-oxygen batteries discharge, O2 is reduced at the cathode to form solid Li2O2. Understanding the fundamental mechanism of O2 reduction in aprotic solvents is therefore essential to realizing their technological potential. Two different models have been proposed for Li2O2 formation, involving either solution or electrode surface routes. Here, we describe a single unified mechanism, which, unlike previous models, can explain O2 reduction across the whole range of solvents and for which the two previous models are limiting cases. We observe that the solvent influences O2 reduction through its effect on the solubility of LiO2, or, more precisely, the free energy of the reaction LiO2(*) ⇌ Li(sol)(+) + O2(-)(sol) + ion pairs + higher aggregates (clusters). The unified mechanism shows that low-donor-number solvents are likely to lead to premature cell death, and that the future direction of research for lithium-oxygen batteries should focus on the search for new, stable, high-donor-number electrolytes, because they can support higher capacities and can better sustain discharge.

Synthesis of platinum nanoparticles using cellulosic reducing agents

Platinum nanoparticles were formed by reduction of H2PtCl6 using nanocrystalline cellulose from cotton as the reducing agent.

Synthesis of carbon-supported Pt nanoparticle electrocatalysts using nanocrystalline cellulose as reducing agent

Pt nanoparticles have been synthesized at relatively low temperatures in aqueous solution from hexachloroplatinic acid using cellulose nanocrystals (CNXLs) from cotton as reducing agents. The Pt nanoparticles were characterised using X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis. X-ray diffraction and X-ray photoelectron spectroscopy showed that the particles have a metallic Pt core and an oxidised surface layer. TEM analysis showed that the nanoparticles have an average diameter of approximately 2 nm, which is independent of the reactant concentrations. By performing the reduction reaction in the presence of a carbon-black support (Vulcan XC-72R), and removing the cellulosic material by heating in air, it was possible to produce carbon black supported Pt nanoparticles. Electrochemical analysis revealed that this Pt/C was highly active towards electrocatalysis of the oxygen reduction reaction, suggesting that this method may be very useful for fabricating Pt/C electrocatalysts.

Nanocomposite oxygen reduction electrocatalysts formed using bioderived reducing agents

Crystalline cellulose nanofibrils from cotton were used as reducing agents for the synthesis of nanostructured silver. The hydrothermal synthesis involved heating an AgNO3 solution containing suspended cellulose nanofibrils at 80 °C for 2 h. The formation of metallic silver was verified using UV/Visible spectroscopy, X-ray diffraction and transmission electron microscopy (TEM). Cellulose/silver nanocomposite films were formed at glassy carbon surfaces by drop coating with the product suspension and scanning electron microscopy (SEM) was used to characterise the modified surfaces. The film morphology depended on the ratio of silver to cellulose in the films. Cyclic voltammetry and rotating-disk electrode voltammetry were used to study the electrochemical and electrocatalytic behavior of these films. The nanocomposite films formed using this approach were highly active electrocatalysts for the reduction of oxygen in alkaline media.

Deposition of silver nanobowl arrays using polystyrene nanospheres both as reagents and as the templating material

There is considerable interest in the development of nanostructured metal surfaces for applications in chemical sensing, optics, catalysis and magnetics. For a number of years, the use of templates based on polystyrene nanospheres has been particularly successful for the construction of meso- and macroporous surfaces. We have discovered that redox groups on the surfaces of commercially available polystyrene nanospheres reduce Ag+ ions directly. When the nanospheres are arranged as a close packed template on a glassy carbon surface, the reduced Ag preferentially deposits in the pores of the polystyrene template. After removing the template, very well defined Ag nanobowl arrays remain on the carbon surface. This discovery may open the door to the production of highly inexpensive polystyrene-templated metal nanostructures for photonic and sensing applications.

LiO2: Cryosynthesis and Chemical/Electrochemical Reactivities

The reduction of O2 to solid Li2O2, via the intermediates O2- and LiO2, is the desired discharge reaction at the positive electrode of the aprotic Li-O2 batteries. In practice, a plethora of byproducts are identified together with Li2O2 and have been assigned to the side reactions between the reduced oxygen species (O2-, LiO2, and Li2O2) and the battery components (the cathode and electrolyte). Understanding the reactivity of these reduced oxygen species is critical for the development of stable battery components and thus high cycle life. O2- and Li2O2 are readily available, and their reactivities have been studied in depth both experimentally and theoretically. However, little is known about LiO2, which readily decomposes to Li2O2 and is thus unavailable under usual laboratory conditions. Here we report the synthesis and reactivity of LiO2 in liquid NH3 at cryogenic temperatures and conclude that LiO2 is the most reactive oxygen species in Li-O2 batteries.

Operando Monitoring of the Solution-Mediated Discharge and Charge Processes in a Na–O2 Battery Using Liquid-Electrochemical Transmission Electron Microscopy

Although in sodium-oxygen (Na-O2) batteries show promise as high-energy storage systems, this technology is still the subject of intense fundamental research, owing to the complex reaction by which it operates. To understand the formation mechanism of the discharge product, sodium superoxide (NaO2), advanced experimental tools must be developed. Here we present for the first time the use of a Na-O2 microbattery using a liquid aprotic electrolyte coupled with fast imaging transmission electron microscopy to visualize, in real time, the mechanism of NaO2 nucleation/growth. We observe that the formation of NaO2 cubes during reduction occurs by a solution-mediated nucleation process. Furthermore, we unambiguously demonstrate that the subsequent oxidation of NaO2 of which little is known also proceeds via a solution mechanism. We also provide insight into the cell electrochemistry via the visualization of an outer shell of parasitic reaction product, formed through chemical reaction at the interface between the growing NaO2 cubes and the electrolyte, and suggest that this process is responsible for the poor cyclability of Na-O2 batteries. The assessment of the discharge-charge mechanistic in Na-O2 batteries through operando electrochemical transmission electron microscopy visualization should facilitate the development of this battery technology.

Kinetics of lithium peroxide oxidation by redox mediators and consequences for the lithium–oxygen cell

Lithium–oxygen cells, in which lithium peroxide forms in solution rather than on the electrode surface, can sustain relatively high cycling rates but require redox mediators to charge. The mediators are oxidised at the electrode surface and then oxidise lithium peroxide stored in the cathode. The kinetics of lithium peroxide oxidation has received almost no attention and yet is crucial for the operation of the lithium–oxygen cell. It is essential that the molecules oxidise lithium peroxide sufficiently rapidly to sustain fast charging. Here, we investigate the kinetics of lithium peroxide oxidation by several different classes of redox mediators. We show that the reaction is not a simple outer-sphere electron transfer and that the steric structure of the mediator molecule plays an important role. The fastest mediator studied could sustain a charging current of up to 1.9 A cm–2, based on a model for a porous electrode described here.The kinetics of Li2O2 oxidation is of high importance to the operation of Li–O2 batteries. Here the authors work on different types of mediators revealing the dependence of the kinetics on the nature of the redox active site and its steric hindrance.