The influence of electrolyte identity upon the electro-reduction of CO2

AbstractThe influence of supporting electrolyte cations on the voltammetric behaviour and product distribution in N-methylpyrrolidone-based carbon dioxide electroreduction systems is investigated. The reduction potentials associated with TBABF4 (0.1M) and corresponding alkali metal (M+) electrolytes; LiBF4, NaBF4 and RbBF4 (focussing mainly on the reduction of the widely employed Li+ species) were established in both the presence and absence of CO2 at polycrystalline noble metal working electrodes. In situ and ex situ Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and qualitative element identification via flame testing were used to aid the assignment of reduction processes. It was established that CO2 reduction products in the metal cationic systems were formed at a much less negative potential than those found with the non-metal cation (−1.5V vs. Ferrocene, c.f. −2.2V), however the resultant alteration of the surface environment was found to deactivate the electrode to further CO2 reduction. The presence of CO2 in solution was found to affect the potential required for the bulk deposition of metal from the electrolyte through the same process. Where TBA+ and M+ were employed simultaneously in the system, the resultant voltammetry shared the majority of features with the pure M+ system with CO2 reduction suppressed at more negative potentials therefore supporting the conclusion that any ‘catalytic effect’ associated with TBA+ is in fact a lack of deactivation given by the M+ system, rather than any enhancement offered by the former

Au Electrodeposition at the Liquid-Liquid Interface: mechanistic aspects

The deposition mechanism of metallic gold was investigated based on charge transfer voltammetry at the water/1,2-dichloroethane (W/DCE) interface, and the corresponding redox voltammetry of the metal precursor in W and the reductant, triphenylamine (TPA), in DCE. The metal precursor was present as Au(III) (AuCl_4^[−]), or Au(I) (AuCl_2^[−]) in W or DCE. Electron transfer could be observed voltammetrically at the interface between W containing both Au precursors and DCE containing TPA. Au particles, formed by constant potential electrolysis at the W/DCE interface, were examined by transmission electron microscopy. It was shown that deposit size could be controlled via the applied potential and time, with specific conditions to form particles of less than 10 nm identified

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

In situ Electron Paramagnetic Resonance Spectroelectrochemical Study of Graphene-based Supercapacitors: Comparison between Chemically Reduced Graphene Oxide and Nitrogen-doped Reduced Graphene Oxide

An in situ electrochemical electron paramagnetic resonance (EPR) spectroscopic study of N-doped reduced graphene oxide (N-rGO) is reported with the aim of understanding the properties of this material when employed as an electrical double-layer capacitor. N-rGO shows a capacitance of 100 F g−1 in 6 M KOH, which is twice that found for reduced graphene oxide (rGO). The temperature dependence of the rGO EPR signal revealed two different components: a narrow component, following the Curie law, was related to defects; and a broad curve with a stronger Pauli law component was attributed to the spin interaction between mobile electrons and localised π electrons trapped at a more extended aromatic structure. The N-rGO sample presented broader EPR signals, indicative of additional contributions to the resonance width. In situ EPR electrochemical spectroscopy was applied to both samples to relate changes in unpaired electron density to the enhanced capacitance. The narrow and broad components increased and diminished reversibly with potential. The potential-dependent narrow feature was related to the generated radical species from corresponding functional groups: e.g. O- and N-centred radicals. Improved capacitance seen for the N-modified basal graphene planes can be accordingly suggested to underlie the enhanced capacitance of N-rGO in basic electrolytes

Porosity Study of Hybrid Silica Mesostructure in Aluminium Oxide Membrane Columnar by Cyclic Voltammetry Method

Silica mesostructure has been grown within with a porous aluminium oxide membrane columnar material (hybrid-AOM). This was prepared using a sol-gel technique with Pluronic P123 triblock copolymer as the structure-directing agent and tetraethyl orthosilicate as the inorganic source. The porosity of the hybrid-AOM after ethanol extraction was calculated from the cyclic voltammetry response of a neutral probe (FcMeOH), using Randles-SevÄik equation

Comparison of Two-Dimensional Transition Metal Dichalcogenides for Electrochemical Supercapacitors

Layered two-dimensional (2D) materials such as transition metal dichalcogenides (TMDCs) are receiving increased interest for applications in energy storage due to their high specific surface area and versatile electronic structure. In this work, we prepare solvent stabilised dispersions of a variety of few-layer thick TMDC crystals (MoS2, MoSe2, WS2, and TiS2) by ultrasonication. The exfoliated materials were first characterised by a variety of techniques to determine their quality. These dispersions were then used to form supercapacitor electrodes by filtration, without use of any further conductive additives or polymeric binders. These thin layer TMDC electrodes were assembled into symmetrical coin-cell devices for comparative electrochemical testing. It was found that despite being the most widely studied material, MoS2 suffers from inferior charge storage properties compared to the much higher conductivity and lower density TiS2. Impedance spectroscopy was used to investigate the charge storage mechanisms inside the coin cells, which were found to consist of a combination of both rapid, but low magnitude, electric double layer capacitance and much slower, but higher magnitude, ion adsorption pseudocapacitance

Use of voltammetry for in vitro equilibrium and transport studies of ionisable drugs

In this review, we will briefly outline the voltammetric investigations of the transfer of ionisable drugs at the interface between two immiscible electrolyte solutions. The voltammetric techniques enable the determination of some key in vitro properties of ionisable drugs, including partition coefficient, diffusion coefficient and membrane permeability. Some successful applications will be highlighted, together with the background methodologies

An electrochemical investigation of electroless deposition : the copper-DMAB system

An electrochemical study of the copper electroless deposition process, using dimethylamine borane as a reducing agent, has been performed, in order to gain further understanding of the mechanism and kinetics of electroless deposition. An in-depth study of the electro-oxidation of dimethylamine borane (DMAB) was additionally carried out, due to its increasing relevance, not only in electroless deposition, but also in fuel cell technology. DMAB oxidation was studied using different experimental techniques such as voltammetry, chronoamperometry, single-crystal electrochemistry and in-situ Fourier Transform infrared spectroscopy, which demonstrated that adsorption of the DMAB molecule, and its intermediates, plays an important role in the oxidation mechanism and kinetics. The initial dissociation process is catalysed by the presence of metallic surfaces and the applied potential. On gold surfaces, DMAB undergoes a three-electron transfer at low overpotentials, with a further oxidation process of up to six electrons occurring at high overpotentials. Chemical interactions with gold oxide produce further oxidation of the DMAB molecule. In the potential region of gold oxide formation, in highly alkaline media, the dimethylamine is also oxidised. The voltammetric behaviour of bipolar cells was studied using model reversible and quasi-reversible redox couples, in conjunction with numerical simulations of the system. DMAB oxidation and copper electrodeposition were studied separately and together using the bipolar cell, providing useful information of the 'coupling' effects between the cathodic and anodic processes of electroless deposition. The ability to quantify side reactions associated with electroless plating, namely hydrogen evolution in the copper-DMAB system, was also demonstrated. The kinetics of the copper-DMAB electroless system was studied in detail, using the electroless bath and a galvanic cell configuration. The fact that the rate of deposition decreased upon the physical separation of the two half-reactions, as well as the observed catalysis of the oxidation of DMAB by copper surfaces, lead to the conclusion that the mixed potential theory (MPT) does not apply to this system. Faradaic efficiencies never reached 100% due to the parasitic side reactions mentioned above; the latter were especially prominent in the early stages of deposition. Crystalline copper films were obtained, with a higher fraction of Cu (111) than expected for polycrystalline copper, while the roughness of the deposits was found to increase with deposition time.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Resolution of Li deposition vs. intercalation of graphite anodes in lithium ion batteries - an in situ electron paramagnetic resonance study

In situ electrochemical electron paramagnetic resonance (EPR) spectroscopy is used to understand the mixed lithiation/deposition behavior on graphite anodes during the charging process. The conductivity, degree of lithiation, and the deposition process of the graphite are reflected by the EPR spectroscopic quality factor, the spin density, and the EPR spectral change, respectively. Classical over‐charging (normally associated with potentials ≤0 V vs. Li(+)/Li) are not required for Li metal deposition onto the graphite anode: Li deposition initiates at ca. +0.04 V (vs. Li(+)/Li) when the scan rate is lowered to 0.04 mV s(−1). The inhibition of Li deposition by vinylene carbonate (VC) additive is highlighted by the EPR results during cycling, attributed to a more mechanically flexible and polymeric SEI layer with higher ionic conductivity. A safe cut‐off potential limit of +0.05 V for the anode is suggested for high rate cycling, confirmed by the EPR response over prolonged cycling