Graeme A. Snook

Storing energy in plastics: a review on conducting polymers & their role in electrochemical energy storage

Conducting polymers have become the focus of research due to their interesting properties, such as a wide range of conductivity, facile production, mechanical stability, light weight and low cost and the ease with which conducting polymers can be nanostructured to meet the specific application. They have become valuable materials for many applications, such as energy storage and generation. Recently, conducting polymers have been studied for use in supercapacitors, batteries and fuel cells. This article is to briefly discuss the background & theory behind their conductivity as well as to highlight the recent contributions of conducting polymers to the field of energy. Furthermore, the methods of production of the conducting polymers in addition to the different ways utilised to nano-engineer special morphologies are discussed.

Co-deposition of conducting polymers in a room temperature ionic liquid

This work focuses on the co-deposition of two different conducting polymers in a room temperature ionic liquid. The combination of poly(pyrrole) (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) is investigated in 1-butyl-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (C4mpyr TFSI). In the past, the deposition of the homopolymers from ionic liquids has proven difficult with the resultant layers exhibiting poor kinetics of both the charge and discharge reactions. Combining the two polymers as one layer, using a mixed monomer solution in an ionic liquid as the deposition media, creates a superior polymer layer. Desirable properties of the poly(pyrrole) are high specific capacitance and good mechanical properties while the desirable properties of the poly(thiophene) derivative are high electrical conductivity, good mechanical properties and high porosity. Characterisation of the co-deposited polymer layers is carried out using cyclic voltammetry, capacitance measurements, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) imaging. The co-deposited film is found to exhibit an improved morphology and higher ionic transport, while maintaining a similar specific capacitance to the homo-polymer PPy.

A comparative study of the electrodeposition of polyaniline from a protic ionic liquid, an aprotic ionic liquid and neutral aqueous solution using anilinium nitrate

A novel approach has been developed for the electrochemical deposition of polyaniline in room temperature ionic liquids. Conventionally, the deposition has required a highly proton rich electrolyte to achieve polymerisation of aniline to polyaniline. Here we report on an alternative approach where we pre-protonated the aniline to form anilinium nitrate, which could then be dissolved in electrolyte media for deposition. Three electrolyte media consisting of a protic ionic liquid, a neutral aqueous solution, and an aprotic ionic liquid were trialled. Thick yet porous deposition was achieved in the protic ionic liquid, ethylammonium nitrate, while reasonable deposition was achieved using neutral aqueous media. The use of the aprotic ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (C4mpyrTFSI), only led to deposition of thin films.

Detection of Oxygen Evolution from Nickel Hydroxide Electrodes Using Scanning Electrochemical Microscopy

An important parameter for judging the performance of the nickel hydroxide electrode in electrochemical energy storage devices is the difference between the oxygen evolution potential and the nickel hydroxide oxidation potential. Scanning electrochemical microscopy (SECM) has been investigated as an accurate method for the determination of the potential at which nickel hydroxide formulations evolve oxygen. Oxygen evolution from a variety of nickel hydroxide substrate electrodes in KOH was studied in situ by SECM. The SECM probe tip, positioned just above the substrate surface, was used to detect the oxygen evolved while cyclic voltammetry was also performed on the substrate, thus separating the detection of oxygen evolution from the nickel hydroxide oxidation process. A range of supporting substrates were investigated: conducting glass (indium tin oxide), glassy carbon, and nickel metal, with either nickel hydroxide formulations pasted or microparticles abrasively attached onto the support. Graphite, as a conductive additive to Ni(OH) 2 paste formulations, gave more positive oxygen-evolution overpotentials than similar formulations containing filamentous nickel as the conductive additive. Unmilled nickel hydroxide samples outperformed milled samples; higher rates of oxygen evolution and lower capacities were obtained from the milled nickel hydroxide samples as determined by SECM and high-rate charge/discharge cycling measurements.

A furnace and environmental cell for the in situ investigation of molten salt electrolysis using high-energy X-ray diffraction

This paper describes the design, construction and implementation of a relatively large controlled-atmosphere cell and furnace arrangement. The purpose of this equipment is to facilitate the in situ characterization of materials used in molten salt electrowinning cells, using high-energy X-ray scattering techniques such as synchrotron-based energy-dispersive X-ray diffraction. The applicability of this equipment is demonstrated by quantitative measurements of the phase composition of a model inert anode material, which were taken during an in situ study of an operational Fray-Farthing-Chen Cambridge electrowinning cell, featuring molten CaCl(2) as the electrolyte. The feasibility of adapting the cell design to investigate materials in other high-temperature environments is also discussed.

Quantification of passivation layer growth in inert anodes for molten salt electrochemistry by in situ energy-dispersive diffraction

An in situ energy-dispersive X-ray diffraction experiment was undertaken on operational titanium electrowinning cells to observe the formation of rutile (TiO2) passivation layers on Magneli-phase (TinO2n−1; n = 4–6) anodes and thus determine the relationship between passivation layer formation and electrolysis time. Quantitative phase analysis of the energy-dispersive data was undertaken using a crystal-structure-based Rietveld refinement. Layer formation was successfully observed and it was found that the rate of increase in layer thickness decreased with time, rather than remaining constant as observed in previous studies. The limiting step in rutile formation is thought to be the rate of solid-state diffusion of oxygen within the anode structure.

Reference Electrodes for Ionic Liquids and Molten Salts

Ionic liquids show promise as electrolytes for a host of electrochemical processes due to their favourable physical and electrochemical properties. However, use of conventional aqueous or non-aqueous reference electrodes with ionic liquids poses problems due to the existence of large junction potentials and possible contamination of the test solution.

Role of H+ in Polypyrrole and Poly(3,4-ethylenedioxythiophene) Formation Using FeCl36H2O in the Room Temperature Ionic Liquid, C4mpyrTFSI

The polymerisation reaction of pyrrole and 3,4-ethylenedioxythiophene using the chemical oxidant FeCl3·6H2O in the room temperature ionic liquid butyl-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (C4mpyrTFSI) has been investigated using cyclic voltammetry, UV/vis and IR spectroscopy. The voltammetric data for the Fe2+/3+ reaction is complicated by the presence of H+ introduced upon dissolution of the iron salt by deprotonation of the coordinated waters. The voltammetric and chemical reaction studies show that H+ itself, introduced to solution as trifluoromethanesulfonic acid (HTFSI), can act as the chemical oxidant for the polymerisation reaction. Voltammetric data also implies that in this system the Fe2+/3+ redox couple may not actually be involved in the polymerisation reaction and that the H+ introduced upon dissolution of the FeCl3·6H2O may be the sole cause of the oxidation reaction.

Electrochemical Tailoring of Fibrous Polyaniline and Electroless Decoration with Gold and Platinum Nanoparticles

Presented in this work is a facile and quick electrochemical method for controlling the morphology of thick polyaniline (PANi) films, without the use of templates. By stepping the polymerization potential from high voltages to a lower (or series of lower) voltage(s), we successfully controlled the morphology of the polymer, and fibrous structures, unique to each potential step, were achieved. In addition, the resultant film was tested electrochemically for its viability as an electrode material for flexible batteries and supercapacitors. Furthermore, the PANi film was decorated with gold and platinum nanoparticles via an electroless deposition process for possible electrocatalytic applications, whereby the oxidation of hydrazine at the composite was investigated.

Increasing Cycle Life of Nickel Hydroxide Electrodes at High Currents

The effect of high current (>10C) cycling on the capacity of nickel hydroxide was investigated in aqueous KOH with the objective of increasing cycle life at high currents. Capacity loss at high voltages has been attributed to excess oxygen evolution which occurs when charging is conducted at high rates without voltage limits. Cyclic voltammetry of nickel hydroxide microparticles, attached to glassy carbon electrodes, enabled the separation of the oxidation process from oxygen evolution. With large pasted electrodes, due to their large uncompensated resistance (Ru), these processes are poorly resolved. An alternate method for the discrimination of oxygen evolution has been developed using Scanning Electrochemical Microscopy (SECM) to map the evolution of oxygen against potential. With no control of upper voltage, poor cycle-life was confirmed at 15C (40% capacity loss in 50 cycles). By contrast, voltage control based on SECM studies (Vmax = 1.45V) allowed thousands of cycles with little capacity loss.