[M(CO)4(2,2′-bipyridine)] (M = Cr, Mo, W) as efficient catalysts for electrochemical reduction of CO2 to CO at a gold electrode

Group 6 complexes of the type [M(CO)4(bpy)] (M=Cr, Mo, W) are capable of behaving as electrochemical catalysts for the reduction of CO2 at potentials less negative than those for the reduction of the radical anions [M(CO)4(bpy)].−. Cyclic voltammetric, chronoamperometric and UV/Vis/IR spectro-electrochemical data reveal that five-coordinate [M(CO)3(bpy)]2− are the active catalysts. The catalytic conversion is significantly more efficient in N-methyl-2-pyrrolidone (NMP) compared to tetrahydrofuran, which may reflect easier CO dissociation from 1e−-reduced [M(CO)4(bpy)].− in the former solvent, followed by second electron transfer. The catalytic cycle may also involve [M(CO)4(H-bpy)]− formed by protonation of [M(CO)3(bpy)]2−, especially in NMP. The strongly enhanced catalysis using an Au working electrode is remarkable, suggesting that surface interactions may play an important role, too

Energy dispersive-EXAFS of Pd nucleation at a liquid/liquid interface

Energy dispersive extended X-ray absorption fine structure (EDE) has been applied to Pd nanoparticle nucleation at a liquid/liquid interface under control over the interfacial potential and thereby the driving force for nucleation. Preliminary analysis focusing on Pd K edge-step height determination shows that under supersaturated conditions the concentration of Pd near the interface fluctuate over a period of several hours, likely due to the continuous formation and dissolution of sub-critical nuclei. Open circuit potential measurements conducted ex-situ in a liquid/liquid electrochemical cell support this view, showing that the fluctuations in Pd concentration are also visible as variations in potential across the liquid/liquid interface. By decreasing the interfacial potential through inclusion of a common ion (tetraethylammonium, TEA+) the Pd nanoparticle growth rate could be slowed down, resulting in a smooth nucleation process. Eventually, when the TEA+ ions reached an equilibrium potential, Pd nucleation and particle growth were inhibited

Electrochemically controlled ion exchange: proton ion exchange with sodium zeolite X and A

Structural characterisation of proton-exchanged zeolites, prepared using ion-transfer at the liquid–liquid interface, is reported. Specifically, electrochemical exchange of protons for sodium with zeolites X and A is described: the structural integrity of the resultant materials was probed by solid-state NMR spectroscopy and temperature-dependent powder X-ray diffraction. It is shown that replacement of ca. 40 % of the Na+ can be achieved using this approach for both zeolites; however, the results indicate that exchange is accompanied by significant structural degradation in the case of zeolite A, with proton exchange occurring at the amorphous regions of the sample. In contrast, zeolite X retains its structure, and the level of proton exchange is comparable with the highest levels reported using conventional chemical methods, highlighting the utility of the electrochemical approach

In situ XAFS Study of Palladium Electrodeposition at the Liquid/Liquid Interface

We report the use of XAFS (X-ray absorption fine structure) as an in situ method to follow the electrochemically driven deposition of palladium nanoparticles at a liquid/liquid interface. A novel glass/plastic hybrid electrochemical cell was used to enable control of the potential applied to the liquid/liquid interface. In situ measurements indicate that the nucleation of metallic nanoparticles can be triggered through chronoamperometry or cyclic voltammetry. In contrast to spontaneous nucleation at the liquid/liquid interface, whereby fluctuations in Pd oxidation state and concentration are observed, under a fixed interfacial potential the growth process occurs at a steady rate leading to a build-up of palladium at the interface. Raman spectroscopy of the deposit suggests that the organic electrolyte binds directly to the surface of the deposited nanoparticles. It was found that the introduction of citric acid results in the formation of spherical nanoparticles at the interface

Mechanical stability of substrate-bound graphene in contact with aqueous solutions

We report on the damage caused to mechanically exfoliated monolayer graphene, bound to silicon dioxide substrate, upon contact with liquids. This phenomenon is of significant importance for a wide range of applications where monolayer graphene sheets are used with liquids, especially as an electrode material in electrochemical applications such as energy storage and conversion. Liquid-induced damage to SiO2-bound graphene was previously observed with a range of solvents. A recently developed microdroplet system, used for a detailed examination of this behaviour, reveals that fewlayer graphene flakes down to a bi-layer are stable with respect to aqueous electrolyte droplet formation, but the stability of these droplets is significantly reduced on monolayer graphene and irreversible rupture of the underlying graphene flake occurs. This damage, which we attribute to the presence of nanoscale defects and high adhesion between the graphene and the substrate, seems specific to plasma-cleaned SiO2 substrates and is not observed on flakes transferred to other substrates. Furthermore, the introduction of impurities, in the form of both polymer residues and native impurities between the flake and the SiO2 substrate, significantly enhance graphene's immunity to external strain as shown by optical microscopy, atomic force microscopy, and Raman spectroscopy.</p

Electrochemical Interaction of Few-Layer Molybdenum Disulfide Composites vs Sodium: New Insights on the Reaction Mechanism

The direct observation of real time electrochemical processes is of great importance for fundamental research on battery materials. Here, we use electron paramagnetic resonance (EPR) spectroscopy to monitor the electrochemical reaction of sodium ions with few-layer MoS2 and its composite with carbon nanotubes (CNTs), thereby uncovering new details of the reaction mechanism. We propose that the sodiation reaction takes place initially in structural defects at the MoS2 surface that have been created during the synthetic process (ultrasonic exfoliation), leading to a decrease in the density of Mo5+ at low symmetry sites that can be related to the electrochemical irreversibility of the process. In the case of the few-layer MoS2/CNTs composite, we found metallic-type conduction behavior for the electrons associated with the Mo paramagnetic centers and improved electrochemical reversibility. The reversible nature of the EPR spectra implies that adsorption/desorption of Na+ ions occurs on the Mo5+ defects, or that they are neutralized during sodiation and subsequently created upon Na+ extraction. These effects help us to understand the higher capacities obtained in the exfoliated samples, as the sum of electrosorption of ions and faradaic effects, and support the suggestion of a different reaction mechanism in the few-layer chalcogenide, which is not exclusively an insertion process

Automated analysis of XANES: A feasibility study of Au reference compounds

With the advent of high-throughput and imaging core level spectroscopies (including X-ray absorption spectroscopy, XAS, as well as electron energy loss spectroscopy, EELS), automated data processing, visualisation and analytics will become a necessity. As a first step towards these objectives we examined the possibilities and limitations of a simple automated XANES peak fitting procedure written in MATLAB, for the parametrisation of XANES features, including ionisation potentials as well as the energies and intensities of electronic transitions. Using a series of Au L3-edge XANES reference spectra we show that most of the relevant information can be captured through a small number of rules applied to constrain the fits. Uncertainty in this strategy arises mostly when the ionisation potential (IP) overlaps with weak electronic transitions or features in the continuum beyond the IP, which can result in ambiguity through multiple equally good fits

Electrical double layer supercapacitors based on graphene nanoplatelets electrodes in organic and aqueous electrolytes: Effect of binders and scalable performance

The effect of some commercial binders, carboxymethyl cellulose, polyvinylidene fluoride, modified styrene-butadiene copolymer and vinylidene fluoride-hexafluoropropene copolymer on the capacitive performance of symmetric Graphene nanoplatelets (GnP) electrodes has been investigated in both organic and aqueous electrolytes. Surface area, pore size distribution, resistance and wetting behaviour of the electrodes have been measured and the effects of these factors have been studied to rationalise their electrical double layer capacitor (EDLC) performances. The most homogeneous distribution and lowest aggregation of the GnP electrode is obtained with CMC binder. Among the electrodes, it has also shown better capacitive behaviour in both aqueous and organic electrolyte due to its higher surface area, and higher mesoporous and microporous distributions than the other electrodes. To investigate scalable performance of the GnP electrodes, CMC binder electrodes have been scaled up from coin cell to pouch cell size. The coin cell electrodes showed higher capacitance than the pouch cell up to current densities of 50 A g−1. The pouch cell has exhibited the higher capacitance at 50 A g−1 and better capacitance retention, which shows that the pouch cell is a better charge storage device than coin cell at high current densities

Optimisation of Electrolytic Solvents for Simultaneous Electrochemical Exfoliation and Functionalisation of Graphene with Metal Nanostructures

The development of a simple, simultaneous electrochemical exfoliation and functionalisation of graphene with metal nanostructures in a one-pot, single step process is reported. This approach is useful in terms of the reduction in processing time and cost, as well as aiding the control of the aggregation of graphene sheets. This first part of this work compares the efficiency of electrochemical graphite exfoliation in dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP) and in a mixture of dimethyl carbonate (DMC) and ethylene carbonate (EC) in an electrolyte consisting of LiClO4 and tetraethylammonium tetrafluoroborate. In the second part, the best performing electrolytic solvent was used for in-situ functionalisation of graphene sheets with gold or cobalt nanostructures. The formation of solid layer electrolyte interface in the DMC/EC system is believed to stabilise the graphite from premature exfoliation and allowed the ions to intercalate efficiently to produce a relatively high yield of monolayer graphene sheets. By contrast, the electrochemical exfoliation of graphite in the other two solvents (DMSO and NMP) produced lower yields of few layer graphene. In particular, the co-intercalation of DMSO fragments the electrode by its decomposition by-products (sulfur/carbon oxides) before sufficient cation intercalation occurs. The simulations electrochemical exfoliation and functionalisation of graphene at a single applied potential in the presence of Au salt in DMC/EC solution resulted in the functionalisation of graphene sheets with a variety of high surface area Au nanowhiskers, nanodendrites, nanowires and lamellar nanoparticles. Alternatively, the use of Co(II) salt in the exfoliation solution resulted in the co-deposition of uniformly grown Co nanoparticles on graphene sheets. The metal-functionalised graphene sheets showed high catalytic activity and stability when used as an electrocatalyst for hydrogen evolution reactions. This process could be extended to other metal salts, or mixtures of metal salts, to form graphene-metal alloy composites for use in various applications

Electron Paramagnetic Resonance Investigation of the Structure of Graphene Oxide: pH-Dependence of the Spectroscopic Response

The time-dependence of the electron paramagnetic resonance (EPR) signal arising from purified graphene oxide (GO) in various solvents has been investigated. The prepared GO was sequentially base and acid (ba) treated to remove manganese impurities. The EPR signal of ba-GO was found to be pH-dependent when exposed to different aqueous solutions, which is related to the decarboxylation process the material undergoes in solution. This process involves the fragmentation of the carbonaceous framework and occurs most rapidly in alkaline conditions. Under acidic conditions, fragmentation is much slower, leading to a gradual increase in the EPR signal from ba-GO in the presence of oxygen. Inferred structural changes were correlated with those deduced from X-ray photoelectron spectroscopy to explain the observed pH- and time-dependent effects. Comparative experiments showed that the oxygen molecule was the key to the increase of unpaired electron density. Exposure to superoxide anions in situ confirmed that the scavenging ability of ba-GO was related to the oxidation of the sp2-carbon structure, which led to an increase of the EPR signal. Overall, the results demonstrate changes of the structure and stability of GO at different pH values