John Madden


Research Interests

artificial muscle
electrochemical devices
electronic skin
Functional and Intelligent Materials
medical devices
smart materials

Relevant Degree Programs



Master's students
Doctoral students
Any time / year round

We are developing new wearable technologies for medical, robotic and consumer applications.

I support public scholarship, e.g. through the Public Scholars Initiative, and am available to supervise students and Postdocs interested in collaborating with external partners as part of their research.
I support experiential learning experiences, such as internships and work placements, for my graduate students and Postdocs.
I am open to hosting Visiting International Research Students (non-degree, up to 12 months).

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Postdoctoral Fellows

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2020)
Actualizing fast conducting polymer actuators: design optimization, fabrication, and encapsulation (2019)

Conducting polymer actuators offer large strain (> 1%) and high work density, operate at low voltages and can resonate at tens to hundreds of Hertz. Unfortunately, they dry out in air if a solvent-based electrolyte is used, and exchange ions in wet environments, both of which cause their performance to change over time. They also lack a scalable fabrication process through which devices with reproducible performance (especially with fast actuation) are achieved. In this work, we show that a 100 µm poly(styrene-b-isobutylene-b-styrene) encapsulation helps these devices to retain 80% of their stored solvent more than 1000 times longer compared to when there is no encapsulation. The shelf life of the encapsulated device, which is around 4 days when there is no encapsulation, is expected to improve by 600 times with encapsulation. We also developed a new, easily reproducible, and scalable fabrication process through which conducting polymer films as thin as 400 nm can be obtained. High electronic and ionic conductivities of 4 × 10^4 S/m and 4 × 10^-3 S/m, volumetric capacitance of 2.4 × 10^7 F/m3, and strain difference of ~0.65 %, were obtained from thin sprayed films of poly(2,3-dihydro-thieno-1,4-dioxin)-poly(styrene-sulfonate) on porous polyvinylidene fluoride membranes with thicknesses of ~3.5 µm. Using this technique, we showed that 10 mm long, 2 mm wide and 0.125 mm thick trilayers with a steady state peak to peak displacement of ~4.5 mm, and cut off frequency of ~2 Hz, produce a ~0.5 mm displacement up to 50 Hz.In this work, we also modified the already developed 2D transmission line model of trilayer conducting polymer actuators to take into account the effect of contact electrodes and the non-uniform charge-induced strain throughout the volume of the conducting polymer layers. Based on this model, we created a web-based graphical user-interface tool, named ActuaTool, to facilitate the design and modeling of trilayer conducting polymer actuators.This work is paving the way to employ fast conducting polymer actuators in real applications through developing a new fabrication process, their encapsulation and creating a design optimization tool.

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Soft capacitive sensors for proximity, touch, pressure and shear measurements (2019)

Sensors are devices that convert a physical stimulus into an electrical signal. Mechanical stimuli such as touch, pressure, strain and shear are very important for a plethora of applications. A lot of these application areas, including consumer electronics, sports, health care and robotics, require the sensor to be soft, stretchable and even transparent. In this thesis we demonstrate three capacitive sensors that are each an evolution of the preceding version. The first sensor is a flexible, transparent, proximity and touch sensor based on mutual capacitance technology - the conventional technology used in most touch-screen devices. The novelty in this research is the sensor’s ability to operate while being deformed. This is important for applications where the device is expected to experience a bend or stretch while being interacted with such as in a wearable device and smart clothing. The second sensor in this thesis adds the ability to detect pressure and strain to enable its use in further applications. The sensor uses both mutual capacitance and overlap capacitance to detect the range of stimuli mentioned. The dielectric has cylindrical air gaps that enhance the pressure sensitivity. A 4 X 4 array structure is implemented that demonstrates the detection and differentiation of the different stimuli. However, for artificial skin applications, the ability to sense shear is extremely valuable, for example for helping robots grasp objects. The third sensor developed in this thesis is able to detect proximity and light touch similar to the previous iteration, but with 10X increase in pressure sensitivity (1.3% change in capacitance per kPa applied pressure, compared to 0.13% change for the second sensor) and the ability to detect localized shear (2.2% change in capacitance per kPa of shear stress). The novelty is a patterned dielectric architecture with pillars and sliding supports that enable the top surface of the sensor to slide and buckle like real skin and therefore enable the detection of localized shear. All the sensors use readily available materials (silicone, carbon black and/or polyacrylamide), along with conventional molding and bonding techniques and should be easy to produce in large quantities at low cost.

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Ultrathin conducting polymer transducers: fabrication, characterization, and modeling (2019)

Recently, ultrathin poly (3,4-ethylenedioxythiophene) (PEDOT) – based ionic actuators have overcome some initial obstacles to increase the potential for applications in microfabricated devices. While microfabrication processing of trilayer actuators that involve no manual handling has been demonstrated, their mechanical performances remain limited for practical applications. The goal of this thesis is to optimize the transducers in thin films fabrication by micro technologies, fully characterize the electrochemomechanical properties of the resulting trilayers, and develop a model to simulate their bidirectional electromechanical ability (actuation and sensing). At first, ultrathin PEDOT-based trilayer actuators are fabricated via the vapor phase polymerization of 3,4-ethylenedioxythiophene combining with the layer by layer synthesis process. This constitutes the first full characterization of ionic PEDOT-based microactuators operating in air of such a small thickness (17 µm) having bending deformation and output force generation of 1% and 12 µN respectively. Secondly, electrical, electrochemical and mechanical properties of the resulting microactuators have been thoroughly studied. Non-linear characterization was extended to volumetric capacitance dependence on voltage window. Damping coefficient was characterized for the first time. Thirdly, a nonlinear multi-physics model was proposed as a method of simulating actuator and sensor responses in trilayers, represented using a Bond Graph formalism, and was able to implement all of the characterized parameters. The concordance between the simulations and the measurements confirmed the accuracy of the model in predicting the non-linear dynamic behavior of the actuators. In addition, the information extracted from the model also provided an insight into the critical parameters of the actuators and how they affect the actuator efficiency, as well as the energy distribution.Finally, a nouveau bidirectional electromechanical linear model was introduced to simulate the sensing ability of the trilayer transducer and was confirmed via experimental results in both frequency and time domains of a sinusoidal input displacement. The resulting actuators and the proposed models are promising for designing, optimizing, and controlling of the future soft microsystem devices where the use of polymer actuators should be essential.

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Fabrication and Non-Linear Modeling of Conducting Polymer-Based Actuators: Toward Catheter and Tactile Display Applications (2016)

The low voltage operation and relatively high strain response of conducting polymer actuators have made their use in different applications of great interest. In this thesis, modeling and characterization of the chemoelectromechanical behaviour of the linear freestanding and bending trilayer conducting polymer-based actuators are presented. In the modeling approach, a combination of state space representation and a two-dimensional RC transmission line was employed to develop the time domain model. Electrical and ionic conductivities and also Young’s modulus versus oxidation state were measured and incorporated into the model. Significant changes in conductivity and Young’s modulus make using a non-linear model necessary for accurate modeling. Implementation of the non-linear functions for electrical and mechanical properties in the model is one of the major advantages of the modeling approach. Capability of the model to predict the linear strain and radius of curvature for bending trilayer actuators versus time and position with good agreement with experiments are shown in this thesis. Voltage drop along the length of the film, away from the attachment point and the variation in electrical conductivity with state of charge along this length necessitated the use of a 2D non-linear model to obtain effective predictions of response for the film dimension used.Tubular actuators using conducting polymers as the active material for a catheter application are developed. Laser micromachining to pattern the actuators is demonstrated. A 0.95 mm diameter device is shown to achieve a 22 mm radius of curvature under activation of 2 V. A closed form beam bending model for trilayer actuators with tubular and rectangular cross sections is derived. These formulations predict the radius of curvature as a function of applied voltage and free strain considering different Young’s modulus for conducting polymer layers. This derivation is also useful for other multilayer actuators.The force generated by trilayer actuators is an important parameter which is investigated in this work. Mathematical derivation and simulations are employed to determine this parameter. Some solutions and their effects on force generated by trilayer actuators are presented to show how the force can be enhanced for tactile interface application.

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Simulation of lithium-ion batteries based on pulsed current characterization (2016)

Simulation of lithium-ion and other cells is important for basic understanding, design of cells, application in devices including automotive, and as part of system simulations for control and safety purposes. This thesis proposes a new cell equivalent circuit model, called the distributed state of charge model, which consists of a series resistance, and a non-linear RC transmission line. The circuit model components are dependent on the SOC, with the circuit being unique in considering the local (depth dependent) charge state. A pulsed discharge and charge technique is put forth for extracting the model parameters, and their dependence on cell state of charge. The extraction method is applied to commercial lithium-ion cells. It is shown that the extracted parameters are largely independent of magnitude of the pulsed currents. This distinguishes the model from other widely used equivalent circuits in which parameter extraction is generally performed as a function of current. Therefore, this approach is promising for reducing time required for this extraction phase. Validation experiments are performed using both static discharge and a variable-current profile. Two versions of the model are developed based on the governing diffusion mechanism – planar or spherical. Simulations using the planar model matched experimental results well for large current pulses (up to 2.0 C discharge) with slow average discharge (0.20 C discharge) – root-mean-square error typically within 0.84%, and maximum error within 3.7%. On the other hand, the spherical model performs well for higher continuous discharge current (up to 0.50 C discharge), but for lower current pulses (up to 0.67 C discharge) – root-mean-square error typically within 0.95%, and maximum error within 3.3%. This tradeoff may be attributed to the distribution of capacitances in the corresponding electrode models. Parameters of the proposed equivalent-circuit are also extracted for a lithium-alloying tin electrode. Tin is an electrode of interest due to its high specific and volumetric capacity. The response is much different from those obtained from the commercial lithium ion cells, including apparent drops in effective diffusion coefficient by three orders of magnitude over narrow regions of SOC. These characteristics are explained qualitatively using the phase transformation effect.

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Integrated Solar Energy Harvesting and Storage Devices (2015)

Large scale storage of electricity is a vital requirement for the realization of a carbon-neutral electricity grid. This thesis provides a study of integrated solar energy conversion and storage systems in order to increase the efficiency and reduce the utilization cost of solar energy. The efficient performance of photogalvanic cells relies on high dye solubility and selective electrodes with fast electron transfer kinetics. A new configuration is proposed for photogalvanic cells that removes these impractical requirements. Instead of illuminating the device through the electrode a new vertical configuration is employed with light coming between the two electrodes. This way, the light absorption and hence electron generation is spread through the depth of the device which can be adjusted according to the concentration of the dyes to absorb all the incoming photons even with low solubility dyes and slow electrode kinetics. The proposed configuration is mathematically studied and a numerical model is built for detailed analysis that gives practical guidelines for working towards device parameters with high power conversion efficiency. The analysis suggests that upon the realization of highly selective electrodes and an improved dye/mediator couple, an efficiency higher than 13 % should be achievable from the new configuration compared to 3.7 % at best using the conventional approach. Storage however in this system will be challenging due to the characteristic recombination times of dyes and mediators in the same phase.For significant and long-lived storage we designed and demonstrated an integrated solar-battery structure based on two relatively well established technologies of the redox flow battery and the dye-sensitized solar cell. The cell consists of a sensitized electrode in a redox flow battery structure. The design enables independent scaling of power and energy rating of the system thus it is applicable for large scale storage purposes. An areal energy capacity of 52 μWhcm−², charge capacity of 1.2 mAhL−¹, energy efficiency of 78 % and almost perfect Coulombic efficiency are observed for the integrated cell. These values show a 35 times increase in charge capacity and 13 times improvement in areal energy density compared to similar devices.

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Carbon Nanotube Yarn Actuators (2010)

The first demonstration of electromechanical actuation in carbon nanotubes (CNTs), aligned in the form of a twisted yarn, is presented in this thesis. Sheets of CNTs have been known to actuate when charged electrochemically. When an electric potential is applied between a sheet of CNTs and another electrode, both submersed in an electrolyte, the sheet expands. Actuation loads and stresses are low (
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Engineering aspects of polypyrrole activators and their application in active catheters (2010)

Polypyrrole has shown potential as an electrochemically driven artificial muscle. It has also been studied as an electromechanical sensor. Despite its potential as an engineering material, its actuation and sensing behaviours have not been fully characterized and modelled. In this thesis, polypyrrole is characterized in terms of electrochemical stability, mechanical stiffness and sensing capability. A link between actuation and sensing is also presented, suggesting a new mechanism of electromechanical coupling. An analytical model is developed to predict the dynamic actuation response. Finally, polypyrrole is applied to actively deform a catheter.Characterization studies were performed on a PF₆- (hexafluorophosphate) doped polypyrrole inside an aqueous solution of sodium hexafluorophosphate (NaPF₆) - a combination that has shown large repeatable actuation. Polypyrrole is found to be electrochemically stable from -0.4 V to 0.8 V versus an Ag/AgCl reference electrode. Its stiffness is a function of actuation voltage as well as the amplitude and the frequency of the applied load. Its sensitivity as a load sensor is ~ 4 x 10ˉ¹¹ V/Pa and it responds up to at least 100 Hz.A 2D transmission line model representing polypyrrole electrochemical properties (e.g. ionic and electronic conductivities and charge storage) is used to determine charging and hence actuation as a function of time and position. This model is coupled with a mechanical model to predict deflection and is used to design a polypyrrole driven catheter. The capability of polypyrrole to (1) manoeuvre catheters inside arteries and (2) scan catheter tips for imaging were evaluated by fabricating in vitro devices and testing their degree of bending and actuation speed. The feasibility of using the polypyrrole sensor as a feedback loop element on the catheter was also studied and the sensitivity was found to be insufficient for practical use.Polypyrrole driven catheters are able to provide the degree of bending needed for manoeuvring; however actuation speed needs to be improved for the imaging application investigated, which requires operation at frequencies > 10 Hz. According to the model polypyrrole electrodes with thin conductive backings on a flexible catheter can provide the required scanning speed. Further work is required to create encapsulated designs which contain the electrolyte needed for actuation.

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Master's Student Supervision (2010 - 2018)
Characterizing the Behavior of Nylon Actuators and Exploring Methods for Manufacturing Them to Get the Highest Amount of Output (2018)

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.

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Investigation of aqueous droplet-based electrostatic transduction (2017)

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.

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Characterization of ionic polymers: towards applications as soft sensors in medicine (2016)

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.

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A Treatise on Highly Twisted Artificial Muscle: Thermally Driven Shape Memory Alloy and Coiled Nylon Actuators (2015)

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.

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Evaporation cast thin film carbon nanotube strain guages (2013)

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.

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Niobium nonowire yarns and their application as artificial muslces (2013)

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.

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Characterization of Poly(3-Hexylthiophene) Based Schottky Diodes (2012)

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.

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Gold electrode electrochemistry in protein based solar cells (2012)

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.

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"Influences of Growth Conditions and Porosity on Polypyrrole Supercapacitor Electrode Performance" RT 120387 (2011)

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.

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Application of electrospun carbon nanofibres for batteries and supercapacitors (2011)

With the great demand for energy storage devices with much higher energy density, better power performance and longer cycle life, researchers are looking into nano-structured battery and supercapacitor electrodes due to the higher accessibility of ions to electrodes, improved specific capacitance, and reduced chance of mechanical degradation compared to bulk materials. In this study, a composite system of conductive carbon nanofibres with active materials (polypyrrole and silicon nanoparticles) are fabricated for supercapacitor and lithium ion battery electrode applications. The aim is to use carbon as a strong mechanical and electrical support and the high energy storage capability of active materials to develop new generation electrode systems.Carbon nanofibres with 3.9±0.5×10² nm in diameter are fabricated from a copolymer precursor, poly(acrylonitrile-co-acrylamide), through electrospinning and carbonization. The mild exothermic heat reaction of this copolymer and the enhanced heat flow into nano-scaled fibres during stabilization permits fabrication of high quality and large-scale carbon nanofibre mats with reasonable conductivity (18±1 S/cm), high porosity, and high accessible surface area. Carbon nanofibres are subsequently deposited on electrochemically with polypyrrole for 4 and 8 hours for use as a supercapacitor electrode. Capacity is compared with that of a bulk polypyrrole film. Both systems possess a gravimetric capacity of ca. 150 F/g, but an enhanced volumetric capacity in the nanofibrous electrode (9±1×10⁷ F/m³ for nanofibres vs. 6.0±0.5×10⁷ F/m³ for film) at greater amount of polypyrole deposition (8 hour deposition) is observed. The porous nanofibrous system also reduces the ionic resistance from that of the pure polypyrrole film, which is at least 1.9±0.8×10²Ω, to just 2 – 3 Ω at the highly reduced state.In lithium ion battery applications, a core-shell electrospinning method is used to fabricate carbon nanofibres containing silicon nanoparticles in the core. The core-shell structural advantage over non-core-shell structure observed to be in the prevention of silicon particle fusion and reduced breakage after interacting with lithium ions. At 30 wt% nanoparticle loading, both systems can reach over 1000 mAh/g initial capacity at 100 – 600 mA/g cycle rates. After 20 cycles, the capacity retention of Si in most core-shell systems are significantly higher than that of the non-core-shell system by ca. 10 – 20 %. Further optimization is required to improve the cycle life and electrode stability.

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Fabrication and modelling of an all-printed PEDOT: PSS supercapacitor on a commercial paper (2011)

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.

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Feasibility of polyaniline electrodes for lithium titanate based energy storage devices (2011)

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.

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Influence of porosity on chargin speed of polypyrrole supercapacitors (2011)

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.

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