John Madden

The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.

View record

Fast-acting, thin, flexible, bending-type actuators are an emerging technology for applications, such as in tactile feedback devices. Conducting polymer (CP) tri-layer actuators are a category of electroactive material that exhibit relatively high strain (>1 %), high work density, operate at low voltages (
View record

The continuing demand for wearable electronics requires the development of lightweight and low-volume energy storage systems as the source of power. We consider the integration of a solar cell and a battery as a continuous power source for wearables. This thesis focuses only on the battery. The current collector of batteries currently used in wearables are heavy and bulky; thus, a metalized nanofibrous network is proposed to retain functionality (high electrical conductivity and support) of the current collector, while possessing lower weight and volume. Furthermore, compared to state-of-the-art lithium-ion batteries, zinc – manganese dioxide (Zn-MnO₂) batteries are of interest due to their lower cost, environmentally-friendliness, and abundance of materials. To fabricate the metalized nanofibrous network, lignin, a natural polymer, is used as the precursor for electrospinning. The produced fibers are thermally stabilized and electroless plated with copper to meet the conductivity requirements. Finally, a layer of MnO₂ paste is brush-coated on the copper plated fibers and the electrochemical performance of the assembled Zn-MnO₂ battery is analyzed.This work harnesses the optimization of the electrospinning solution, thermal stabilization, and electroless copper plating solution. The electrospinning solution is optimized by varying its viscosity. In general, higher viscosity results in larger diameters and fewer beads. Thermal stabilization optimization involves changing the final temperature of the process. Higher final temperature allows for more cross-linking and cleavage of bonds; however, above thermal degradation, fiber fusion is observed. These two optimizations allow for achievement of smallest fiber diameter for lightweight and low-volume applications. The plating solution is optimized for attainment of highest conductivity, by adjusting the reducing agent amount, sonication time, and plating time. Generally, conductivity increases by increasing these parameters. However, above a certain threshold, higher formaldehyde amount reduces the reaction rate and longer sonication can break down the sample. This lignin current collector is assessed with respect to the currently used carbon paper current collector. It is evident that lignin current collector has a higher conductivity and longer cycle life, while possessing smaller initial capacity. For wearable batteries, lifespan (cycle life) is a significant factor. Hence, the lignin current collector shows promise for wearable batteries.

View record

Diabetic pressure ulcers (DFU) are one of the most common complications related todiabetes, a disease that has become a global epidemic affecting many countries, especiallymodern and rich ones. The financial cost of treating DFUs this disease is monumental, costing$550 million yearly in Canada alone to treat. the DFUs being one of the costliest outcomes ofdiabetes, can lead to a sedimentary lifestyle for individuals that could benefit from physicalactivity to combat their diabetes. There is a need to develop technology that can sense andmonitor the condition of feet at this crucial crossroad. This thesis builds on a capacitive sensordeveloped in our lab that can measure normal and shear stress simultaneously, made from soft,comfortable, and affordable materials which could be implemented into an insole or modifiedshoe device. The sensor was characterized using modified protocols of existing methodology toestablish sensitivity, repeatability, and proper calibration. Overall, the sensor can measurestresses in the prescribed ranges for normal (0-1000 kPa) and shear (0-200 kPa) and isresponsive, in the lab as well as real-life testing, to the different time regimes it is being designedfor (standing and walking). We show that the sensor is suited well for measuring displacementchange in the foot to capture anatomy change in the foot and swelling. While the forcecharacterization has been described, there is still a good amount of work to establish this sensingparameter to coupe with hysteresis and creep (in the worst case 24% of the full scale) present inthe deformation of the materials being used. There is a tradeoff to contend with that combinesthe comfort and softness of the sensor to its ability to withstand high forces and how themodeling of these deformations is accurate and relevant to our clinical considerations. Thistechnology could be a game-changer for the common diabetic and here we lay out the frameworkto make soft normal and shear stress in-shoe sensors a reality.

View record

Modern advances in medical robotics have brought them into many facets of day to day life, often directly interacting with humans. Some key applications of these robotics are human-interactive nursing robots and active prosthetics, where artificial ‘e-skins’ provide a soft interface with the environment. This thesis presents an investigation of long-term conductance stability for a flexible, stretchable conductive material, as well as two significant developments to a previously established flexible and stretchable combined pressure and shear sensor. The conductivity investigation was successful in reducing the resistance of flexible conductive elements by an order of magnitude, to approximately 1.8 kΩ. The two sensor development branches are aimed at creating a sensor that better resembles human skin in both form and function. Both sensors use a capacitive sensing approach to detect both pressure and shear using low-cost materials. The first area of development is exchanging the previously rigid and inflexible sensor base with a novel flexible one to allow for sensor function on surfaces with radii of curvature between 10 mm-100 mm, and updating the sensor characterization setup to accommodate curved devices. This work was successful in developing a sensor with a functional range of 0.1 N-1.6 N of normal force and 0-0.8 mm shear displacement at radii of curvature from 100 mm to 10 mm. The second is a novel sensor dielectric design which increases the functional range of the sensor to operate from a minimum functioning force of 0.05 N to a maximum functioning force of 50 N. This range increase is accomplished using a two-stage dielectric pillar shape, allowing the low normal force pressure and shear sensitivity of the original sensor design to be preserved while increasing the maximum allowable force significantly. Next steps in sensor design are integrating the high-force and flex designs, creating array-format sensors, and testing the device in practical environments such as basic robotic hands.

View record

A pressure ulcer, also commonly referred to as a pressure injury or bedsore, is a localized injury or damage to the skin and/or underlying soft tissue, usually over bony prominences, due to prolonged pressure that is sufficient enough to impair blood supply. It is a wound that can disrupt an individual’s life abruptly, affecting not only the person’s physical and mental wellbeing, but also impacting the healthcare system significantly. Most susceptible to the development of a pressure ulcer are individuals confined to beds and/or wheelchairs. In the spinal cord injury (SCI) community, pressure ulcers remain one of the most prevalent and costly preventable secondary complications. There is a need for methods and/or devices that can reduce the incidences or even prevent the formation of such wounds, especially in bedridden patients and wheelchair users. We propose a soft, flexible, and stretchable capacitive pressure sensor array, made out of low-cost materials. It is scalable in size, robust and capable of measuring pressure in the desired range (0 – 200 mmHg) over extended periods of time (>12h) with low repeatability error (0.5%), hysteresis (0.57%), and non-linearity error (0.52%), while fully conforming around various contours (e.g. wheelchair cushions). The pressure readings are displayed on a generated pressure heatmap with the data being wirelessly transmitted to mobile devices (e.g. laptop, smartphone, tablet). A first iteration alert system design enables the sensor to be implemented for potential pressure ulcer prevention, warning the users and healthcare professionals prior to a formation. The working principle of the sensor is based on the well-established capacitance sensing technology, where the relative position of two electrodes is changed by applied force. The sensor array is made of soft, rubbery material and the dielectric contains air to enable relatively large deformations. Further work is required to improve the life-span of the sensor before it is ready to be tested with patients.

View record

In soft electronics technology, stretchable displays move away from conventional rigid devices to adopt mechanical properties more similar to organic materials while preserving function, leading to more conformable and biointegrable devices. Progress has been made on luminescent stretchable displays; however, electrochromic materials that change colour instead of emitting light provide key advantages of low power operation, stability and high contrast. Despite this, development of stretchable electrochromic displays has been limited. In this thesis we demonstrate the first intrinsically stretchable fully integrated electrochromic display device. A poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) conducting polymer and (1-butyl-3-methylimidazolium octyl sulfate) ionic liquid conductive composite is used as the intrinsically stretchable electrochromic electrodes, with a stretchable polyvinyl alcohol and phosphoric acid-based electrolyte and transparent styrene-based elastomer substrate to form a single pixel device. Both a transparent and reflective opaque version of the device are made. High contrasts up to ΔTmax=15% for transparent and ΔRmax=20% for opaque cells are shown, and optical modulation degradation as low as ΔT or ΔR=1-3% is shown when the samples are strained from 0% to 30% length. Under 30% strain they show fast switch times
View record

Today’s prostheses provide numerous intuitive features including several gripping patterns and movements guided by the user’s intentions. Nevertheless, users remain quite dependent on visual and auditory cues when trying to adjust grip forces, and are still unable to use their touchscreens with prosthetic fingertips. To approach this problem, researchers have proven that sensory feedback can help amputees to be faster and more accurate when accomplishing a task. However, there is still a need for a low-cost and adaptable sensory feedback add-on, requiring minimal modifications to the hand. This thesis presents the development of a low-cost sensory feedback system, including a stimulation device and a force sensing fingertip that comprises a capacitive sensor. The novelty of this research is the ability of the fingertip to activate touchscreens, and the full fingertip integration to the prosthetic hand, under the prosthetic glove – easy for the user to maintain. The glove integrated sensor capacitively detects forces over an area of 0.64 cm2. This detection is linear up to 1.6 N, with a resolution of 0.01 N. The second part of this thesis presents the implementation of shear detection, in addition to touch and pressure, using a modified sensor on a flexible printed circuit board. Able to detect touch, pressure, and shear simultaneously, the sensitive fingertip can send sensory feedback to the user. The last part of this thesis presents the implementation of two sensory feedback: one using an armband including a servo motor pushing on the user’s stump (mechanotactile feedback), and one using vibration motors hidden in the socket (vibrotactile feedback) – two feedbacks that are preferred based on a survey that we conducted. It was found that vibrotactile feedback would be more efficient due to its complete integration to the hand and its ability to send shear feedback. The next steps are the implementation of Bluetooth connections for wireless stimulation and the development of a userivinterface for real-time calibration. An ethics approval to test the device on patients was granted and will be put into practice to understand the benefits of the device and gather users’ opinions.

View record

Coiled nylon thermal actuators are polymer based artificial muscles introduced in 2014. Although there are many applications that can benefit from nylon actuators, there has not been much work on the methods of producing the actuators. To maximize the output of the actuators, various factors can be altered during the production phase. These factors are pre-anneal stretch, coiling tensile stress and annealing temperature. Results show that with a pre-anneal stretch of 70% and a coiling stress of 48 MPa, the highest amount of force generated can be reached without substantial failures. Also, it is found that the annealing temperature should be in the range of 170°C to 180°C. Annealing temperature needs to be in this range since the actuator should be above Brill transition temperature. Upon annealing in this temperature range, the actuator is fixed in its configuration, and it will not uncoil after it has been annealed.Additionally, the creep of the actuators and its effect on the output, has been studied. A model for the creep is proposed. The model consists of a Kelvin and a Maxwell model. The proposed model for the creep includes two damper elements, which account for the short term and long-term creep. The model fits the observed response well for the short-term creep, but for the long-term creep the goodness of fit decreases. Additionally, a model for active deformation of the actuator behavior as the result of changes in tensile stress is proposed. In particular, shape memory effects are added to account for load history dependence of the response.The active deformation model and the creep model are essential for developing a control system for using the actuators. The findings of this work can be used by researchers and device designers to bring the nylon actuators to real life applications.

View record

With the effects of climate change being felt more and more each day, any form of alternative energy production, no matter how small, must be investigated. Each new transduction mechanism discovered opens up new possibilities of harnessing energy that was previously untapped or underutilized, possibly shifting some of the burden on carbon-based power plants to non-emitting sources. Even if the mechanism is inefficient or unsuitable for energy conversion, the technology may still be useful as a sensor. This thesis examines a recently discovered mechanical-to-electrical transduction mechanism that can be as simple as a water droplet sandwiched between vibrating electrodes. The mechanism in question is analogous to electrostatic transduction where motion is converted to electricity by pulling apart the two plates of a charged capacitor. Conventional technology, however, has plateaued as their performance is limited by the breakdown potential of air. One drawback of using this transduction mechanism is the necessity of having a biasing source, a requirement not shared by electromagnetic and piezoelectric transducers. By utilizing an electrical double layer capacitor’s (EDLC) high capacitance per area and inherent self-biasing, performance can be improved and one disadvantage can be avoided. In this work, we demonstrate such a device using a droplet of water between two Indium Tin Oxide (ITO) electrodes with one electrode being coated with poly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole-co-tetrafluoroethylene] (PTFE-AF). We investigate its frequency response, an important parameter for sensing and generation applications, and show an improved frequency response of up to 100 Hz, surpassing literature’s best, with a maximum peak-to-peak voltage of 892 mV. We present a linear approximation model that can be used for further optimization of such a system and correctly predicts the point of maximum power transfer. We also investigate how and why the technology self-biases, proposing an alternative theory to those posed in literature. We finally evaluate the system as both a sensor and generator in its current state and ideas that could make this technology competitive.

View record

A phenomenon termed the piezoionic effect is described and characterized in various ionic polymers including polymer networks containing aqueous electrolytes (hydrogels) and organic electrolytes. Initial observations suggest that when an ion containing polymer is compressed, a concentration gradient is induced by the pressure differential, leading to an electrical potential difference detectable at electrodes placed at compressed and uncompressed portions of the polymer. The work focuses on the fundamental characterization of the nature of the piezoionic transduction to probe the effects of relative mobilities of the ions present in the system. The effective ion radii due to ion-solvent interactions and electrostatic ion-polymer interactions have been investigated for their contribution in dictating the piezoionic behavior by NMR measurements of the self-diffusion coefficients. The results are qualitatively correlated to the voltage response to mechanical compression of the polymer samples. Following the experiments, a numerical model is developed which incorporates a number of contributing events believed to be taking place in a concerted manner to cause the piezoionic effect. The deformation induced solvent flow is modeled by means of Biot’s constitutive equations on poroelasticity, a combination of thermodynamic equilibrium and Darcy’s law. The Darcy’s flow induced is then used as the input to model transport of dilute species. Here, the convective factor is being continuously modulated by Darcy’s flow, while Fickian diffusion concurrently takes place. The ionic species experience different displacements due to Stokes' drag experienced by the solvation spheres of the ionic species and solvent molecules and the electrostatic interactions between the charged polymer chains and the mobile ions. Furthermore, this non-homogeneous ionic charge distribution yields a voltage distribution via the Poisson’s equation. This voltage distribution is used to account for the migration of ionic species. The following chapter is dedicated to a novel electrochemical method and modelling approach designed to probe various ionic polymers, some electronically conductive and others interpenetrated, to determine the phase-wise contributions to ionic conductivities. Finally, potential applications of the piezoionic polymers as soft sensors in medicine, particularly in unobtrusive and longitudinal monitoring of physical parameters, are discussed and some preliminary prototypes are introduced and ultimate feasibility is assessed.

View record

A new perspective in the field of actuators was opened by the demonstration of multiwall carbon nanotube based actuators by Foroughi et al, in 2011. The approach involves applying a high degree of twist to create large torsional actuation in carbon nanotube yarns, and more recently in coiled nylon filaments. In this thesis torsional actuation is further studied in nylon, and extended to shape memory alloys (SMA).Torsional actuation is demonstrated using 25 μm diameter micro strands of shape memory alloy (SMAs) that are twisted together. These form yarns with Young’s modulus of 13.5 GPa in the Martensitic phase and 18 GPa in the Austenite state. In torsion, the SMA yarns show more than 8,000 rpm peak rotational speed with 11 reversible rotations for an 8 cm long yarn. This is observed upon applying 0.47 W/cm electrical input power. Providing more than 5 N.m/kg torque, SMA yarns may be of interest in biomedical and other applications. The mechanical behaviour of coiled nylon actuators is studied by testing elastic modulus and by investigating tensile stroke as a function of temperature. Loads that range from 35 MPa to 155 MPa were applied. For the nylon and the coiling conditions used, active thermal contraction totals 19.5 % when the temperature is raised from -40 ⁰C to 160 ⁰C, with most contraction above the glass transition temperature. Introducing various cooling methods was shown to enable increased rate of actuation up to several Hertz. Nylon coiled actuators potentially provide affordable and viable solutions for driving mechanical devices as recently demonstrated in robotic hands and arms. A new biomimetic arrangement of the nylon actuator is presented that imitates the human pennate muscle in structure, including the ability to vary stiffness by a factor of 9 and to increase isometric force from 19 N to 37 N by recruiting additional fibers.

View record

This work describes the research performed on synthesising and measuring the gauge factor of evaporation cast thin film carbon nanotube strain gauges. The main characteristics pursued of the strain gauges are inexpensive, easily manufactured and reasonably sensitive. Carbon nanotubes have exhibited a high gauge factor due to their intrinsic piezoresistivity and were incorporated into evaporation cast films to try to take advantage of the high sensitivity. Another direction taken to improve the sensitivity is alignment of carbon nanotubes in the thin film.Previous work produced an evaporation cast carbon nanotube strain gauge with a relatively high gauge factor. However, it was not reproducible and the research encompassed extends from the previous work. A number of ink compositions with different carbon nanotube and surfactant loadings were used to synthesise thin films of carbon nanotubes on a polyimide substrate. Variations of evaporation casting were used to decrease the evaporation rate in attempts of carbon nanotube alignment through a self-organising liquid crystal phase during evaporation. Other methods of inkjet printing and air flow evaporation casting were also attempted to achieve alignment. Electrical connections using a conductive polymer and metal wires were fabricated onto the samples for electrical measurements. A four-point probe resistance measurement under the application of strain was used to elicit the gauge factors.The strain gauge design was modified from previous work for more reliable electrical connections and for higher applied strains. A procedure for electrical measurements coupled with the application of strain was devised and the gauge factors achieved varied between 0.1 and 4.0 with a median of 1.1 ±0.1. The median gauge factor was reproducible and exhibited by several samples fabricated with different types of evaporation casting. The decrease in evaporation rate did not result in either alignment or relatively high gauge factors. In general, alignment was not achieved with the other methods of air flow evaporation and inkjet printing.

View record

Since the discovery of carbon nanotubes, various devices have been made in differentfields of science and engineering. The mechanical and electrical properties that carbon nanotubesoffer make them a great candidate for use in the structure of artificial muscles. In this thesis, forthe first time, we have demonstrated that metallic nanowires can be engineered to become strongand comparable to the CNT yarns in mechanical and electrical properties. The niobium yarnsoffer conductivity of up to 3×10⁶ S m-¹, tensile strength of up to 1.1 GPa and Young’s modulusof 19 GPa. The niobium nanowire fibres are fabricated by extracting the niobium nanowiresfrom copper-niobium nano-composite matrix, which was made by using a severe plasticdeformation process. As a practical application, torsional artificial muscles were made out of theniobium yarns by twisting and impregnating them with paraffin wax. Upon applying voltage tothe twisted yarn the wax melts and expands due to the heat generated by the current. Thermalexpansion of wax untwists the yarn, which translated to torsional actuation. Torsional speeds of7,200 RPM (in a destructive test) and 1,800 RPM (continuous) were achieved. In addition totorsional actuation, niobium yarns also can provide up to 0.24% of isobaric tensile actuationalong the yarn’s axis at 20 MPa load. Due to the high conductivity of the niobium yarns, theactuator can be made to actuate by even one single 1.5 V battery (for a 1 cm of niobium yarn).The electrochemical capacitance of niobium yarns was measured to be 1.3×10⁷ F m-³ at a scanrate of 25 mV s-¹ in 0.2 M TBAPF₆ salt dissolved in acetonitrile. This value is comparable to theelectrochemical capacitance of the carbon multi-walled nanotube yarns.

View record

This thesis describes the fabrication and electrical characterization of Schottky diodes based on the polymer semiconductor poly(3-hexylthiophene). Printed electronics may not be able to benefit from high vacuum processing, either for economic or technical reasons. The aim was to observe the effects on performance when Schottky diodes were built at atmospheric pressure. 200 nm thick films of poly(3-hexylthiophene) were formed on glass substrates by spinning a 1 wt% polymer solution in chloroform. Vacuum deposited aluminum and gold where used for the Schottky and ohmic contacts respectively. Two types of diodes were manufactured. One type (Au bottom) had its Schottky junction formed by evaporating aluminum onto the polymer under high vacuum. The other (Al bottom) had its Schottky junction formed by depositing the polymer onto aluminum at atmospheric pressure. The final yield of usable devices was 35% for Au bottom and 22% for Al bottom. The hole density and bulk mobility were derived from both DC and AC measurements. The bulk mobility was found to range from 2×10⁻⁵ cm²V⁻¹s⁻¹ to 6×10⁻⁵ cm²V⁻¹s⁻¹. The hole density was determined to be between 5×10¹⁶ cm⁻³ and 3x10¹⁷ cm⁻³. DC measurements showed that Au bottom devices had a current rectification ratio of 2×10⁴ at ±2 V, 100 times greater than Al bottom devices. The space charge limited current (SCLC) had to be considered to successfully model the DC behaviour. The small signal behaviour was modeled with a 2nd order series/parallel circuit, which was determined through impedance spectroscopy. Small signal performance of both device types was predicted to be poor. The corner frequency was determined to be less than 100 Hz for Al bottom devices, and less than 1 kHz for Au bottom devices. Large signal frequency performance of the diodes was tested with a half-wave peak rectifier. The maximum operating frequency was measured to be 40 kHz for Au bottom devices and 10 kHz for Al bottom devices.

View record

The high quantum efficiency of photosynthetic reaction centers makes them candidates for use in solar energy harvesting devices. A bio-photovoltaic cell can be made by dissolving reaction centers and two mediators, such as quinone and cytochrome c, in the conductive electrolyte of an electrochemical cell. The mediators transfer the photo-generated charges to the electrodes upon illumination. So far such protein-based devices have shown low overall power conversion efficiency. Previously it has been shown that slow charge transfer limits the efficiency of these devices. Moreover, it has been observed that the cell response is dependent on the electrode materials and their interactions with the proteins and the mediators. In this thesis, the importance of the cleanliness of the system, the adsorption of two types of reaction centers: Wild-type and cysteineless, two mediators: quinone and cytochrome c, and detergents (used to make the reaction center water soluble) on a gold electrode are investigated. It is shown that common cleaning methods such as sonication in a mixture of deionized water and ethyl alcohol, sulfuric acid potential cycling, and piranha solution may not be practical and sufficient to remove the reaction centers and mediators from the electrodes and the container. Additionally, it is shown that eliminating oxygen can result in the reduction of unwanted parasitic reactions in the cell, which could lower the generated photocurrent and hence the overall efficiency of the cell. Therefore, new methods for cleaning and a new cell design are proposed and used throughout the experiments. Capacitance measurements, using cyclic voltammetry and AC voltammetry techniques, in the absence and the presence of each of the cell analytes suggest that unmodified reaction centers, detergents, and the mediators bind to the surface of the gold electrode irreversibly. Finally, it is shown that cysteineless reaction centers also adsorb irreversibly on the surface of the gold, demonstrating that cysteine S-Au bonding is not the only irreversible binding mechanism.

View record

Supercapacitors are electrical energy storage devices that offer high power density and high energy density. The current energy density of supercapacitors is, however, not sufficient to meet the requirements of many applications. By using polypyrrole (PPy) as an alternate electrode material, the energy density of supercapacitors can be increased. Approaches for simplifying and speeding the production of PPy electrodes are investigated, as are means of increasing power density.The tradeoffs in performance of PPy are investigated when electrochemical deposition conditions – current density and temperature -- are modified to reduce costs. Although the surface morphology changes according to deposition conditions, PPy’s performance in capacitance and charging time is not greatly affected. The best electrode performance is obtained using electrodeposition conditions in which a current density of 0.125 mA/cm² is used and the temperature is held at -30°C. Higher temperatures and faster deposition rates can lead to more voluminous films which are lower in density, volumetric and specific capacitances. Further work is needed to investigate the impact of growth conditions on cycle and shelf life.To decrease the charging time of PPy the hypothesis is that additional porosity will help by creating channels of high ionic mobility. The porosity is achieved by polymerizing onto carbonized polyacrylonitrile nanofibres (NF). PPy-coated NF samples with a density of 1.2 g/cm³ exhibit similar volumetric (160 F/cm³) and specific capacitances (130 F/g) similar to that of pure PPy. The use of NF can increase the apparent ionic conductivities of PPy, allowing NF/PPy samples to charge just as quickly as pure polypyrrole electrodes that are four times less capacitive. However, based on the current model, the advantages of increasing porosity should be more dramatic, suggesting that other mechanisms such as uncompensated resistances and ion depletion may also influence charging time. As such, further work on NF/PPy is needed to determine and hopefully to mitigate the effects of such mechanisms.

View record

Supercapacitors demonstrate substantial improvement in charge storage capability compared to the conventional capacitors. With the emergence of printed electron- ics such as RFID tags, smart cards, electronic paper, and wearable electronics, printed energy storage devices are desirable. Therefore, a flexible and printable supercapacitor with a PEDOT:PSS electrode is fabricated with inkjet micropatterning technology. Electroanalytical measurement techniques are employed to characterize the performance of the printed supercapacitor. It has been found that addition of the surfactant (Triton X-100) increases the porosity of the PE- DOT:PSS electrode. A volume capacitance of 9.36 F/cm₃ (adding surfactant) and 9.09 F/cm₃ (without adding surfactant) are measured with cyclic voltammetry. The two devices have different capacitor charging times, e.g., 50.46 s for electrode added surfactant and 112.9 s for the electrode without adding surfactant.In order to investigate the rate limiting factors of capacitor charging, electro- chemical impedance measurements and equivalent circuit modelling is utilized. Instead of using a constant phase element (CPE), a multiple time constant model is proposed in order to explain the physical origin of the distributed time constant behaviour. Thickness variation of the PEDOT:PSS electrode is assumed as a primary reason for the distributed time constants and thus actual thickness variation is incorporated in the modelling. Data fitting with the measured impedance are consistent with this assumption. However. it also has been found that there are more factors distributing capacitances than just variations in thickness. A lognormal distribution function (LNDF) is utilized in order to further investigate the relationship between the distributed capacitance and the capacitor charging. It is found that the capacitance distribution likely influence the charging.Previous experimental results demonstrate that the distributed capacitance is the physical cause of the distributed time constant behaviour in electrochemical impedance measurement. However, this is the first analytical report proving the relationship between the distributed time constant behaviour and a thickness de- pendent capacitance distribution.

View record

A long lasting energy storage device with energy density rivalling batteries would be very useful in many applications, especially ones where device replacement is difficult or expensive. Devices based on lithium titanate electrodes are considered promising in this regard, as lithium titanate electrodes have very long cycle lives. In this thesis, the feasibility of using the conducting polymer polyaniline in conjunction with a lithium titanate electrode to build a battery-supercapacitor combination energy storage device is considered, since polyaniline is also expected to have a high cycle life, due to its supercapacitor-like charge storage mechanism. Various methods for fabricating a polyaniline electrode are considered, and the deposition of polyaniline onto a stainless steel substrate from an aqueous solution was used. The polyaniline electrode, upon being tested in a non-aqueous solution containing lithium ions, was found to have a specific capacitance and a specific capacity of roughly 220 F/g and 85 F/g respectively. Nuclear magnetic resonance tests were used to find that the lithium ions do not dope the polyaniline and drive its oxidation state changes; therefore, the electrolyte in the proposed device must accommodate all the lithium ions emitted from the lithium titanate electrode. A simulation is presented, based on experimental data from each electrode tested separately, which estimates the energy density of the complete device to be 22.8 Wh/kg and the cost to be $560/kWh. This energy density is more than two-thirds that of a lead-acid battery and the cost is competitive with lithium-ion batteries, so the device is considered viable in applications where long-lasting devices are of utmost importance.

View record

In the field of energy storage, two main factors are essential for storage devices: the power density and energy density, both of which can be provided by supercapacitors. Supercapacitors offer a power to mass and cycle life greater than batteries and an energy density that is much greater than capacitors, making them appropriate for use in portable electronics, hybrid vehicles, and similar applications. Power to mass and discharge time are still not fast enough, however, for use in, for example flash cameras or cell phones or power quality applications. Charging time and power in these devices are often limited by the rate of ion transport into the electrodes. The hypothesis proposed in this thesis is that making electrodes porous increases their speed and hence power, but may reduce the capacitance at the same time. So in order to investigate the hypothesis various electrodes (e.g. pure polypyrrole (PPy) and its composites (carbon nanofiber (CNF) plus PPy) with varying porosities are made. Techniques used to investigate these samples are Cyclic Voltammetry (CV), Ionic Conductivity (IC) measurements, Electrochemical Impedance Spectroscopy (EIS) and Nuclear Magnetic Resonance (NMR) measurements. Through these techniques, it is found that the time constant reduces significantly (by ~ 1 x10⁴ times) for very porous electrodes as expected from hypothesis, and the capacitance reduces by a small factor (by ~ 7 times) compared to that. Even for least porous samples a huge time constant reduction (by ~ 37 times) compared to pure PPy is achieved with only ~ 2 times reduction in volumetric capacitance. The plausibility of these improvements is checked by analyzing the rate-limiting factors in ion transport and it is found that ionic time constants at very high porosities are not representative of the speed of the full cell. The reason for this is due to solution resistance becoming a rate-limiting factor for porosities more than ~ 50%. In this case, any improvements in speed (power) can be achieved by reducing that resistance. Other methods for further improving the power densities are also suggested and they include reducing the separator and electrode thicknesses for instance.

View record