ABSTRACT Constant phase elements (CPE) are routinely used to describe the frequency response of e... more ABSTRACT Constant phase elements (CPE) are routinely used to describe the frequency response of electrochemical systems. However, this approach is often scientifically unsatisfactory because the physical origin of the phase is unclear. Here we observe CPE-like behaviour in a conducting polymer poly(3,4-ethylenedioxythiophene)/poly(styrene-4-sulfonate) (PEDOT:PSS) film that was inkjet printed onto paper to form a flexible electrochemical double layer capacitor electrode. We show that the response of the electrochemically active film can also be described using a physical model with multiple parallel finite RC (resistor–capacitor) transmission lines whose lengths and time constants are determined by the distribution of the measured film thickness. The modeled volumetric capacitance and ionic conductivity match those determined experimentally, suggesting that the physical origin of the constant phase response is a distribution of mass transport limited time constants.
The high cost of powerful, large-stroke, high-stress artificial muscles has combined with perform... more The high cost of powerful, large-stroke, high-stress artificial muscles has combined with performance limitations such as low cycle life, hysteresis, and low efficiency to restrict applications. We demonstrated that inexpensive high-strength polymer fibers used for fishing line and sewing thread can be easily transformed by twist insertion to provide fast, scalable, nonhysteretic, long-life tensile and torsional muscles. Extreme twisting produces coiled muscles that can contract by 49%, lift loads over 100 times heavier than can human muscle of the same length and weight, and generate 5.3 kilowatts of mechanical work per kilogram of muscle weight, similar to that produced by a jet engine. Woven textiles that change porosity in response to temperature and actuating window shutters that could help conserve energy were also demonstrated. Large-stroke tensile actuation was theoretically and experimentally shown to result from torsional actuation.
ABSTRACT A large low frequency inductance is found in a Schottky diode composed of regioregular p... more ABSTRACT A large low frequency inductance is found in a Schottky diode composed of regioregular poly(3-hexylthiophene) and aluminum. This apparent inductance is evident in response to both swept frequency sinusoidal, ramp and step voltage inputs above a threshold voltage. The constant slope of the current in response to a voltage step suggests an incredibly large inductance (a few hundred megahenry) in a device that is only 2000 μm3 in size. A number of potential mechanisms including chemical reactions, barrier modulation, and memory effects are evaluated in order to find a suitable explanation for the inductive behavior. Similarity in the dc characteristics of the organic Schottky diode and organic bistable devices that are being applied as memory suggests that the current leads the voltage due to increments in tunneling current that occur as charges are gradually stored in localized states.
ABSTRACT In spite of a high quantum efficiency in the bacterial photosynthetic reaction center (R... more ABSTRACT In spite of a high quantum efficiency in the bacterial photosynthetic reaction center (RC) the overall efficiency in a RC-based photovoltaic device is very poor partly because of an inefficient collection of charges by electrodes. To explain charge transport between the RC and an electrode a diffusion model is proposed. The numerical solution of the diffusion process describes the measured photocurrent well. An approximation of the initial condition is also made to obtain analytical expressions for the photocurrent. The model suggests that the slow transient response of the photocurrent is due to the diffusion in a biological photovoltaic device.
ABSTRACT The high quantum efficiency (~100%) in the bacterial photosynthetic reaction center (RC)... more ABSTRACT The high quantum efficiency (~100%) in the bacterial photosynthetic reaction center (RC) has inspired research on the application of RCs to build protein based solar cells. Conventionally, applying RCs as the photosensitive layer on the surface of a carbon electrode has shown poor photocurrents in the cells. The low photocurrent is partly due to the weak absorption of light in the monolayer of RCs. Also, an Atomic Force Microscopy image of the electrode shows lots of defects on the immobilized RCs at the electrode surface. In this work, we have built a bio-photoelectrochemical cell in which the RCs are floating in the electrolyte instead of being attached to the surface of an electrode. Despite the simple structure of the cell, the photocurrent is significantly higher in the new cell compared to when RCs are attached to an electrode. The amplitude of current reached to ~40 nA for free floating RCs, about five times larger than that in the cell with attached RCs. The aging effect was studied in both cells in a course of a week. The lifetime of attached RCs on electrode surface was slightly better than solubilized RCs in the electrolyte. Also, it is found that the mechanism which governs the charge transfer from RCs to the electrodes is the same in both bio-photoelectrochemical cells.
... ARASH TAKSHI* AND JOHN D. MADDEN Department of Electrical and Computer Engineering and Advanc... more ... ARASH TAKSHI* AND JOHN D. MADDEN Department of Electrical and Computer Engineering and Advanced Materials & Process Engineering Lab, University of British Columbia (UBC), Vancouver BC V6T 1Z1, Canada ... 7. Princy, KG, Joseph, R. and Kartha, S. (1998). ...
ABSTRACT Bacterial photosynthetic reaction centers (RCs) are promising materials for solar energy... more ABSTRACT Bacterial photosynthetic reaction centers (RCs) are promising materials for solar energy harvesting, due to their high quantum efficiency. A simple approach for making a photovoltaic device is to apply solubilized RCs and charge carrier mediators to the electrolyte of an electrochemical cell. However, the adsorption of analytes on the electrodes can affect the charge transfer from RCs to the electrodes. In this work, photovoltaic devices were fabricated incorporating RCs from purple bacteria, ubiquinone-10 (Q2), and cytochrome c (Cyt c) (the latter two species acting as redox mediators). The adsorption of each of these three species on the gold working electrode was investigated, and the roles of adsorbed species in the photocurrent generation and the cycle of charge transfer were studied by a series of photochronoamperometric, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and cyclic voltammetry (CV) tests. It was shown that both redox mediators were required for photocurrent generation; hence, the RC itself is likely unable to inject electrons into the gold electrode directly. The reverse redox reactions of mediators at the electrodes generates electrical current. Cyclic voltammograms for the RC-exposed gold electrode revealed a redox couple due to the adsorbed RC at +0.5 V (vs NHE), which confirmed that the RC was still redox active, upon adsorption to the gold. Photochronoamperometric studies also indicated that RCs adsorb, and are strongly bound to the surface of the gold, retaining functionality and contributing significantly to the process of photocurrent generation. Similar experiments showed the adsorption of Q2 and Cyt c on unmodified gold surfaces. It was indicated by the photochronoamperometric tests that the photocurrent derives from Q2-mediated charge transfer between the RCs and the gold electrode, while solubilized Cyt c mediates charge transfer between the P-side of adsorbed RC and the Pt counter electrode. Also, the stability of the adsorbed RCs and mediators was evaluated by measuring the photocurrent response over a period of 1 week. It is found that 46% of the adsorbed RCs remain active after a week under aerobic conditions. A significantly extended lifetime is expected by removing oxygen from the electrolyte and sealing the device.
... Arash Takshi1, John D. Madden1, Chi Wah Eddie Fok1, Mya Warren2 1Electrical and Computer Engi... more ... Arash Takshi1, John D. Madden1, Chi Wah Eddie Fok1, Mya Warren2 1Electrical and Computer Engineering, The University of British Columbia (UBC) Vancouver, BC V6T 1Z4, Canada ... 9. AM Aleshin, H. Sandberg, H. Stubb, Synthetic Metals 121 (2001) 1449-1450. 10. ...
The large-ion-accessible surface area of carbon nanotubes (CNTs) and graphene sheets formed as ya... more The large-ion-accessible surface area of carbon nanotubes (CNTs) and graphene sheets formed as yarns, forests, and films enables miniature high-performance supercapacitors with power densities exceeding those of electrolytics while achieving energy densities equaling those of batteries.1-7 Capacitance and energy density can be enhanced by depositing highly pseudocapacitive materials such as conductive polymers on them.3,8-15 Yarns formed from carbon nanotubes are proposed for use in wearable supercapacitors.3,16 In this work, we show that high power, energy density, and capacitance in yarn form are not unique to carbon materials, and we introduce niobium nanowires as an alternative. These yarns show higher capacitance and energy per volume and are stronger and 100 times more conductive than similarly spun carbon multiwalled nanotube (MWNT) and graphene yarns.6,17-22 The long niobium nanowires, formed by repeated extrusion and drawing,17 achieve device volumetric peak power and energ...
ABSTRACT Constant phase elements (CPE) are routinely used to describe the frequency response of e... more ABSTRACT Constant phase elements (CPE) are routinely used to describe the frequency response of electrochemical systems. However, this approach is often scientifically unsatisfactory because the physical origin of the phase is unclear. Here we observe CPE-like behaviour in a conducting polymer poly(3,4-ethylenedioxythiophene)/poly(styrene-4-sulfonate) (PEDOT:PSS) film that was inkjet printed onto paper to form a flexible electrochemical double layer capacitor electrode. We show that the response of the electrochemically active film can also be described using a physical model with multiple parallel finite RC (resistor–capacitor) transmission lines whose lengths and time constants are determined by the distribution of the measured film thickness. The modeled volumetric capacitance and ionic conductivity match those determined experimentally, suggesting that the physical origin of the constant phase response is a distribution of mass transport limited time constants.
The high cost of powerful, large-stroke, high-stress artificial muscles has combined with perform... more The high cost of powerful, large-stroke, high-stress artificial muscles has combined with performance limitations such as low cycle life, hysteresis, and low efficiency to restrict applications. We demonstrated that inexpensive high-strength polymer fibers used for fishing line and sewing thread can be easily transformed by twist insertion to provide fast, scalable, nonhysteretic, long-life tensile and torsional muscles. Extreme twisting produces coiled muscles that can contract by 49%, lift loads over 100 times heavier than can human muscle of the same length and weight, and generate 5.3 kilowatts of mechanical work per kilogram of muscle weight, similar to that produced by a jet engine. Woven textiles that change porosity in response to temperature and actuating window shutters that could help conserve energy were also demonstrated. Large-stroke tensile actuation was theoretically and experimentally shown to result from torsional actuation.
ABSTRACT A large low frequency inductance is found in a Schottky diode composed of regioregular p... more ABSTRACT A large low frequency inductance is found in a Schottky diode composed of regioregular poly(3-hexylthiophene) and aluminum. This apparent inductance is evident in response to both swept frequency sinusoidal, ramp and step voltage inputs above a threshold voltage. The constant slope of the current in response to a voltage step suggests an incredibly large inductance (a few hundred megahenry) in a device that is only 2000 μm3 in size. A number of potential mechanisms including chemical reactions, barrier modulation, and memory effects are evaluated in order to find a suitable explanation for the inductive behavior. Similarity in the dc characteristics of the organic Schottky diode and organic bistable devices that are being applied as memory suggests that the current leads the voltage due to increments in tunneling current that occur as charges are gradually stored in localized states.
ABSTRACT In spite of a high quantum efficiency in the bacterial photosynthetic reaction center (R... more ABSTRACT In spite of a high quantum efficiency in the bacterial photosynthetic reaction center (RC) the overall efficiency in a RC-based photovoltaic device is very poor partly because of an inefficient collection of charges by electrodes. To explain charge transport between the RC and an electrode a diffusion model is proposed. The numerical solution of the diffusion process describes the measured photocurrent well. An approximation of the initial condition is also made to obtain analytical expressions for the photocurrent. The model suggests that the slow transient response of the photocurrent is due to the diffusion in a biological photovoltaic device.
ABSTRACT The high quantum efficiency (~100%) in the bacterial photosynthetic reaction center (RC)... more ABSTRACT The high quantum efficiency (~100%) in the bacterial photosynthetic reaction center (RC) has inspired research on the application of RCs to build protein based solar cells. Conventionally, applying RCs as the photosensitive layer on the surface of a carbon electrode has shown poor photocurrents in the cells. The low photocurrent is partly due to the weak absorption of light in the monolayer of RCs. Also, an Atomic Force Microscopy image of the electrode shows lots of defects on the immobilized RCs at the electrode surface. In this work, we have built a bio-photoelectrochemical cell in which the RCs are floating in the electrolyte instead of being attached to the surface of an electrode. Despite the simple structure of the cell, the photocurrent is significantly higher in the new cell compared to when RCs are attached to an electrode. The amplitude of current reached to ~40 nA for free floating RCs, about five times larger than that in the cell with attached RCs. The aging effect was studied in both cells in a course of a week. The lifetime of attached RCs on electrode surface was slightly better than solubilized RCs in the electrolyte. Also, it is found that the mechanism which governs the charge transfer from RCs to the electrodes is the same in both bio-photoelectrochemical cells.
... ARASH TAKSHI* AND JOHN D. MADDEN Department of Electrical and Computer Engineering and Advanc... more ... ARASH TAKSHI* AND JOHN D. MADDEN Department of Electrical and Computer Engineering and Advanced Materials & Process Engineering Lab, University of British Columbia (UBC), Vancouver BC V6T 1Z1, Canada ... 7. Princy, KG, Joseph, R. and Kartha, S. (1998). ...
ABSTRACT Bacterial photosynthetic reaction centers (RCs) are promising materials for solar energy... more ABSTRACT Bacterial photosynthetic reaction centers (RCs) are promising materials for solar energy harvesting, due to their high quantum efficiency. A simple approach for making a photovoltaic device is to apply solubilized RCs and charge carrier mediators to the electrolyte of an electrochemical cell. However, the adsorption of analytes on the electrodes can affect the charge transfer from RCs to the electrodes. In this work, photovoltaic devices were fabricated incorporating RCs from purple bacteria, ubiquinone-10 (Q2), and cytochrome c (Cyt c) (the latter two species acting as redox mediators). The adsorption of each of these three species on the gold working electrode was investigated, and the roles of adsorbed species in the photocurrent generation and the cycle of charge transfer were studied by a series of photochronoamperometric, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and cyclic voltammetry (CV) tests. It was shown that both redox mediators were required for photocurrent generation; hence, the RC itself is likely unable to inject electrons into the gold electrode directly. The reverse redox reactions of mediators at the electrodes generates electrical current. Cyclic voltammograms for the RC-exposed gold electrode revealed a redox couple due to the adsorbed RC at +0.5 V (vs NHE), which confirmed that the RC was still redox active, upon adsorption to the gold. Photochronoamperometric studies also indicated that RCs adsorb, and are strongly bound to the surface of the gold, retaining functionality and contributing significantly to the process of photocurrent generation. Similar experiments showed the adsorption of Q2 and Cyt c on unmodified gold surfaces. It was indicated by the photochronoamperometric tests that the photocurrent derives from Q2-mediated charge transfer between the RCs and the gold electrode, while solubilized Cyt c mediates charge transfer between the P-side of adsorbed RC and the Pt counter electrode. Also, the stability of the adsorbed RCs and mediators was evaluated by measuring the photocurrent response over a period of 1 week. It is found that 46% of the adsorbed RCs remain active after a week under aerobic conditions. A significantly extended lifetime is expected by removing oxygen from the electrolyte and sealing the device.
... Arash Takshi1, John D. Madden1, Chi Wah Eddie Fok1, Mya Warren2 1Electrical and Computer Engi... more ... Arash Takshi1, John D. Madden1, Chi Wah Eddie Fok1, Mya Warren2 1Electrical and Computer Engineering, The University of British Columbia (UBC) Vancouver, BC V6T 1Z4, Canada ... 9. AM Aleshin, H. Sandberg, H. Stubb, Synthetic Metals 121 (2001) 1449-1450. 10. ...
The large-ion-accessible surface area of carbon nanotubes (CNTs) and graphene sheets formed as ya... more The large-ion-accessible surface area of carbon nanotubes (CNTs) and graphene sheets formed as yarns, forests, and films enables miniature high-performance supercapacitors with power densities exceeding those of electrolytics while achieving energy densities equaling those of batteries.1-7 Capacitance and energy density can be enhanced by depositing highly pseudocapacitive materials such as conductive polymers on them.3,8-15 Yarns formed from carbon nanotubes are proposed for use in wearable supercapacitors.3,16 In this work, we show that high power, energy density, and capacitance in yarn form are not unique to carbon materials, and we introduce niobium nanowires as an alternative. These yarns show higher capacitance and energy per volume and are stronger and 100 times more conductive than similarly spun carbon multiwalled nanotube (MWNT) and graphene yarns.6,17-22 The long niobium nanowires, formed by repeated extrusion and drawing,17 achieve device volumetric peak power and energ...
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