Adsorption, surface relaxation and electrolyte structure at Pt(111) electrodes in non-aqueous and aqueous acetonitrile electrolytes.

In situ electrochemical surface X-ray diffraction was employed to investigate the atomic scale structure of the electrochemical double layer and the relaxation at the Pt(111) electrode surface in non-aqueous and aqueous acetonitrile electrolytes under potential control. The X-ray measurements provide insight into the potential-dependence of the interface structure by combining potentiodynamic measurements (X-ray voltammetry) with potentiostatic measurements (crystal truncation rod data) to probe both the metal and electrolyte sides of the interface. The crystal truncation rod measurements are consistent with the potential dependent reorientation of acetonitrile in the absence of water and a parallel arrangement in the presence of water. As acetonitrile concentration increases, the electron density closest to the electrode surface also increases. Finally, Pt surface relaxation in a range of aqueous and non-aqueous solvents is discussed in general with regards to the structure of the electrochemical double layer.

Shell isolated nanoparticles for enhanced Raman spectroscopy studies in lithium-oxygen cells.

A critical and detailed assessment of using Shell Isolated Nanoparticles for Enhanced Raman Spectroscopy (SHINERS) on different electrode substrates was carried out, providing relative enhancement factors, as well as an evaluation of the distribution of shell-isolated nanoparticles upon the electrode surfaces. The chemical makeup of surface layers formed upon lithium metal electrodes and the mechanism of the oxygen reduction reaction on carbon substrates relevant to lithium-oxygen cells are studied with the employment of the SHINERS technique. SHINERS enhanced the Raman signal at these surfaces showing a predominant Li2O based layer on lithium metal in a variety of electrolytes. The formation of LiO2 and Li2O2, as well as degradation reactions forming Li2CO3, upon planar carbon electrode interfaces and upon composite carbon black electrodes were followed under potential control during the reduction of oxygen in a non-aqueous electrolyte based on dimethyl sulfoxide.

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.

Potential Dependence of Surfactant Adsorption at the Graphite Electrode/Deep Eutectic Solvent Interface.

Atomic force microscopy and cyclic voltammetry are used to probe how ionic surfactant adsorbed layer structure affects redox processes at deep eutectic solvent (DES)/graphite interfaces. Unlike its behavior in water, sodium dodecyl sulfate (SDS) in DESs only adsorbs as a complete layer of hemicylindrical hemimicelles far above its critical micelle concentration (CMC). Near the CMC it forms a tail-to-tail monolayer at open-circuit potential (OCP) and positive potentials, and it desorbs at negative potentials. In contrast, cetyltrimethylammonium bromide (CTAB) adsorbs as hemimicelles at low concentrations and remains adsorbed at both positive and negative potentials. The SDS horizontal monolayer has little overall effect on redox processes at the graphite interface, but hemimicelles form an effective and stable barrier. The stronger solvophobic interactions between the C16 versus C12 alkyl chains in the DES allow CTAB to self-assemble into a robust coating at low concentrations and illustrate how the structure of the DES/electrode interface and electrochemical response can be engineered by controlling surfactant structure.

Growth and dissolution of NaO2 in an ether-based electrolyte as the discharge product in the Na-O2 cell.

The deposition and dissolution of sodium superoxide (NaO2) was investigated by atomic force microscopy. Rectangular prisms consisting of 8 smaller sub-structures grew from NaO2 platelets, when discharged in 0.5 M NaClO4, diethylene glycol dimethyl ether on highly ordered pyrolytic graphite. During oxidation the 8 sub-structures are conserved. Ring-like structures of Na2CO3 of 200 nm diameter remain at the end of oxidation.

A highly active nickel electrocatalyst shows excellent selectivity for CO2 reduction in acidic media.

The development of selective electrocatalysts for CO2 reduction in water offers a sustainable route to carbon based fuels and feedstocks. However, molecular catalysts are typically studied in non-aqueous solvents, in part to avoid competitive H2 evolution. [Ni(cyclam)]2+ (1) is one of the few known electrocatalysts that operate in water and 30 years after its report its activity remains a rarely surpassed benchmark. Here we report that [Ni(cyclam-CO2H)]2+ (cyclam-CO2H = 1,4,8,11-tetraazacyclotetradecane-6-carboxylic acid (2)) shows greatly enhanced activity versus1 for CO production. At pHs < pKa of the pendant carboxylic acid a large increase in catalytic activity occurs. Remarkably, despite the high proton concentration (pH 2), 2 maintains selectivity for CO2 reduction and is believed to be unique in operating selectively in such acidic aqueous solutions.

Three-dimensional protonic conductivity in porous organic cage solids.

Proton conduction is a fundamental process in biology and in devices such as proton exchange membrane fuel cells. To maximize proton conduction, three-dimensional conduction pathways are preferred over one-dimensional pathways, which prevent conduction in two dimensions. Many crystalline porous solids to date show one-dimensional proton conduction. Here we report porous molecular cages with proton conductivities (up to 10(-3) S cm(-1) at high relative humidity) that compete with extended metal-organic frameworks. The structure of the organic cage imposes a conduction pathway that is necessarily three-dimensional. The cage molecules also promote proton transfer by confining the water molecules while being sufficiently flexible to allow hydrogen bond reorganization. The proton conduction is explained at the molecular level through a combination of proton conductivity measurements, crystallography, molecular simulations and quasi-elastic neutron scattering. These results provide a starting point for high-temperature, anhydrous proton conductors through inclusion of guests other than water in the cage pores.

Influence of Tetraalkylammonium Cation Chain Length on Gold and Glassy Carbon Electrode Interfaces for Alkali Metal-Oxygen Batteries.

Fundamental studies of dioxygen electrochemistry relevant to metal-air batteries commonly require conductive supporting salts, such as tetraalkylammonium, to sustain redox processes in nonaqueous electrolytes. Electrochemical analysis of the formation and oxidation of superoxide on glassy carbon and gold working electrodes has shown a decrease in reversibility and lowering of the oxygen reduction rate constant when tetraalkylammonium cation alkyl chain length is increased. Probing interfacial regions on Au using in situ surface enhanced Raman spectroscopy (SERS) provides evidence that this is caused by the changing adsorption characteristics of tetralkylammonium cations under negative potentials. These effects are heightened with longer alkyl chain lengths, therefore reducing the reversibility of superoxide formation and dioxygen evolution. From these observations it can be established that shorter chain tetraalkylammonium cations while retaining necessary conductive support: (1) enhance reversibility and rate of superoxide formation and oxidation and (2) for in situ SERS, have lower preference for adsorption, thus improving experimental detection of superoxide at the Au electrode interface.