Patrick S. Grant

The role of nanomaterials in redox-based supercapacitors for next generation energy storage devices

The development of more efficient electrical storage is a pressing requirement to meet future societal and environmental needs. This demand for more sustainable, efficient energy storage has provoked a renewed scientific and commercial interest in advanced capacitor designs in which the suite of experimental techniques and ideas that comprise nanotechnology are playing a critical role. Capacitors can be charged and discharged quickly and are one of the primary building blocks of many types of electrical circuit, from microprocessors to large-sale power supplies, but usually have relatively low energy storage capability when compared with batteries. The application of nanostructured materials with bespoke morphologies and properties to electrochemical supercapacitors is being intensively studied in order to provide enhanced energy density without comprising their inherent high power density and excellent cyclability. In particular, electrode materials that exploit physical adsorption or redox reactions of electrolyte ions are foreseen to bridge the performance disparity between batteries with high energy density and capacitors with high power density. In this review, we present some of the novel nanomaterial systems applied for electrochemical supercapacitors and show how material morphology, chemistry and physical properties are being tailored to provide enhanced electrochemical supercapacitor performance.

A novel hybrid supercapacitor with a carbon nanotube cathode and an iron oxide/carbon nanotube composite anode

A scalable spray deposition technique has been adopted to the fabrication of a flexible nanostructured hybrid supercapacitor based on thin film multi-walled carbon nanotube (MWNT) cathodes and hematite (α-Fe2O3)/MWNT composite anodes. On the basis of the total weight of the binder-free MWNT and α-Fe2O3/MWNT thin film composite electrodes, the hybrid supercapacitor provided a very high specific energy density of 50 Wh kg−1 at a specific power density of 1000 W kg−1 over the potential range 0–2.8 V, which was an energy density 8 times that of identically prepared symmetric supercapacitors with MWNT only electrodes under the same conditions. The superior electrochemical performance of the hybrid supercapacitor arrangement can be attributed to the incorporation of MWNTs into the α-Fe2O3 anodes, which leads to a decrease of internal resistance and an improvement in both the ion diffusion behaviour and the integrity of the α-Fe2O3 containing films.

Modelling of droplet dynamic and thermal histories during spray forming. I: Individual droplet behaviour

Abstract A computer model has been developed to describe the in-flight dynamic and thermal histories of gas atomised droplets as a function of distance during spray forming. Modifications of the model have been used to evaluate the relative importance of different physical assumptions in controlling droplet cooling and solidification behaviour, and ultimately the evolution of spray formed billet microstructures. Gas atomised droplet dynamic and thermal behaviour is strongly affected by the distribution of droplet diameters in the two phase spray and the momentum transfer between the droplets and the atomising gas, but is relatively unaffected by the details of the droplet solidification mechanism.

Isothermal grain coarsening of spray formed alloys in the semi-solid state

Grain coarsening of alloys in the semi-solid state is important in controlling grain sizes in spray forming and in other processes in which an alloy is formed or cast in the semi-solid state. Until now, this coarsening has been analysed in terms of classic LSW theory or in terms of the migration of grain boundary liquid films. These analyses suggest that the coarsening rate should increase with increasing solid fraction, fS, but this suggestion conflicts with previous experimental results which show that coarsening rate decreases with increasing fS for fS>0.7. This paper shows, for the first time, that coarsening rate does increase with fS for fS less than approximately 0.75 in agreement with the liquid film migration model, and then decreases again with further increasing fS for fS greater than approximately 0.75. A modified model of liquid film migration is proposed which takes into account the reducing area of the liquid film as fS increases for high fS. The formation of intragranular liquid droplets, and the pinning of grain boundary liquid films by dispersoids during coarsening are also discussed.

Spray deposition of steam treated and functionalized single-walled and multi-walled carbon nanotube films for supercapacitors

Steam purified, carboxylic and ester functionalized single-walled carbon nanotube (SWNT) and multi-walled carbon nanotube (MWNT) films with homogeneous distribution and flexible control of thickness and area were fabricated on polymeric and metallic substrates using a modified spray deposition technique. By employing a pre-sprayed polyelectrolyte, the adhesion of the carbon nanotube (CNT) films to the substrates was significantly enhanced by electrostatic interaction. Carboxylic and ester functionalization improved electrochemical performance when immersed in 0.1 M H(2)SO(4) and the specific capacitance reached 155 and 77 F g(-1) for carboxylic functionalized SWNT and MWNT films respectively. Compared with existing techniques such as hot pressing, vacuum filtration and dip coating, the ambient pressure spray deposition technique is suggested as particularly well suited for preparing CNT films at large scale for applications including providing electrodes for electrochemical supercapacitors and paper batteries.

Printable magnetite and pyrrole treated magnetite based electrodes for supercapacitors

Flexible magnetite (Fe3O4) based film electrodes were fabricated by combining facile wet chemistry approaches with a cost-effective spray deposition route. One-dimensional Fe3O4 nanowire and pyrrole treated Fe3O4 nanowire electrodes composed of randomly oriented interpenetrating networks exhibited a high capacitance of 106 F g−1 and 190 F g−1 respectively along with excellent electrochemical stability in an aqueous electrolyte. The supercapacitor configurations and the scalable fabrication technique provides a new solution for large-scale production of printable and flexible energy storage devices.

MODELLING OF DROPLET DYNAMIC AND THERMAL HISTORIES DURING SPRAY FORMING--II. EFFECT OF PROCESS PARAMETERS

Abstract A computer model has been developed to describe the in-flight dynamic and thermal histories of gas atomised droplets as a function of distance during spray forming. The model has been used to investigate the effects of the dynamic and thermal behaviour of individual gas atomised droplets and the cooling and solidification behaviour of the overall spray. The most influential parameters for a given alloy system, in order of importance, are: (i) droplet diameter and, therefore, the droplet size distribution within the spray; (ii) initial axial gas velocity at the point of atomisation and the subsequent gas velocity decay profile; (iii) melt mass flow rate; (iv) melt superheat at the point of atomisation; and (v) alloy composition. Experimental measurements of gas velocities and droplet size distributions during spray forming allow the spray solid fraction at deposition to be calculated and used in a subsequent computer model of billet heat flow to predict the billet top surface temperatures and solid fractions.

One-step spray processing of high power all-solid-state supercapacitors

Aqueous suspensions of multi-wall carbon nanotubes (MWNTs) in dilute H2SO4 were sprayed onto both sides of a Nafion membrane and dried to fabricate flexible solid-state supercapacitors. A single cell with MWNT-only electrodes had a capacitance of 57 F g−1 per electrode at 2 mV s−1 and 44 F g−1 at 150 mV s−1 but with low H+ mobility. Cells with MWNT + ionomer hybrid electrodes showed higher H+ mobility, and the electric double layer (EDL) capacitance increased to 145 F g−1 at 2 mV s−1 and 91 F g−1 at 150 mV s−1. The energy and power densities of one electrode charged to 1 V at 1 A g−1 were 12.9 Wh kg−1 and 3.3 kW kg−1 respectively. Three solid-state supercapacitor cells connected in series charged to 3 V at 1 and 2 A g−1 provided a device power density of 8.9 kW kg−1 at 1 A g−1 and 9.4 kW kg−1 at 2 A g−1, the highest for all-solid-state EDL supercapacitors.

SnS/PbS nanocrystal heterojunction photovoltaics.

We report advances in the growth, characterization and photovoltaic properties of SnS nanocrystals, with controlled < 10 nm size, and their inclusion into a lead chalcogenide solar cell. The SnS/PbS nanocrystalline film heterojunction is shown to display a type II band alignment, in which the direction of flow of the photocurrent depends on the order of the layers and not the relative work functions of the contacts. On placing the SnS layer next to the indium tin oxide (ITO) cathode we observe a dramatic increase in V(oc) to as much as 0.45 V. Our results suggest that SnS nanocrystal films can be used in multi-junction solar cells, that a SnS/PbS heterojunction on its own shows photovoltaic behaviour, and that a SnS layer in an ITO/SnS/PbS/Al device is acting to suppress the flow of an electron injection current.

Control of temperature profile for a spray deposition process

Spray forming is a novel method of rapidly manufacturing tools and dies for stamping and injection operations. The process sprays molten tool steel from a set of arc spray guns onto a ceramic former to build up a thick steel shell. The volumetric contraction that occurs as the steel cools is offset by a volumetric expansion taking place within the sprayed steel, which allows the dimensional accurate tools to be produced. To ensure that the required phase transformation takes place, the temperature of the steel is regulated during spraying. The sprayed metal acts both as a source of mass and a source of heat and by adjusting the rate at which metal is sprayed, the surface temperature profile over the surface of the steel shell can be controlled. The temperature profile is measured using a thermal imaging camera and regulated by adjusting the rate at which the guns spray the steel. Because the temperature is regulated by adjusting the feed rate to an actuator that is moving over the surface, this is an example of mobile control, which is a class of distributed parameter control. The paper describes the design of a time-varying H/sub 2/ controller for the process and presents results from the implementation of the controller on the spray forming process.