Research

Supercapacitors

Development of electrochemical capacitors (ECCs) or supercapacitor devices that provide high power and energy densities and have the ability to be rapidly and repeatedly cycled through their charging/discharging processes. In collaboration with the MacDiarmid/Manohar Group and the NanoTechnology Institute, we fabricate and evaluate coin cell and fiber supercapacitor devices, which can be woven into textiles, using conducting polymer nanofibers and carbon nanotube fiber electrodes, respectively.

Fuel Powered Artificial Muscles

We experimentally demonstrate artificial muscles that convert the energy of a high-energy-density fuel to mechanical energy. These muscles are fuel cells that in some versions store electrical charge and use changes in stored charge for mechanical actuation.

Fig. 1.  Nanotube-based fuel cell muscles. (A) Schematic illustration of the apparatus used for demonstration of a cantilever-based nanotube fuel cell muscle. Element a is the membrane electrode assembly (comprising porous carbon sheet layer, Pt-C-ionomer layer, and Nafion-117 membrane) that is the counter-electrode to the actuating Pt-containing nanotube cantilever strip (element b). (B) Schematic illustration of a one compartment cell mounted in a dynamic mechanical analyzer (DMA) for tensile measurements during either fuel driven or electrically driven actuation. Elements a, b, and c are electrical wires connecting to the fuel cell muscle working electrode (catalyst-containing nanotube sheet), carbon felt counter electrode, and the Ag/AgCl reference electrode, respectively. Element d is the measurement probe assembly of the DMA. (C) Potential and actuator strain versus time for a tensile nanotube actuator that is alternately exposed to pure O2 (red) or a mixture of 5 volume percent H2 in inert gas (blue).  A N2 purge between the O2 and H2 purges has negligible duration on this time scale. The slow actuator response results from the present need to dissolve different gases in relatively massive amounts of electrolyte in different parts of the actuation cycle. Creep, which is also a problem for electrically powered nanotube sheet actuators, causes the irreversible component of actuator strain. (D) Measured tensile actuator strain versus potential and injected charge for an electrically powered nanotube actuator, indicating the measured hydrogen and oxygen potentials for the chemical actuator experiment. Note the agreement between the strain change on going between these potentials in the fuel-powered and electrically-powered actuator experiments. 

The highest demonstrated actuator generated strains and mechanical output power densities for fuel cell muscles are comparable to natural skeletal muscle, and the actuator generated stresses are over a hundred times higher than for natural skeletal muscle. Important possible applications of this research are artificial arms and legs, which have the ability to move and manipulate objects—both for amputees and robots.

Fig. 2.  Continuously shorted fuel cell muscle based on a NiTi shape-memory alloy. (A) Schematic illustration, with cut-away to reveal details, of the fuel-powered artificial muscle mounted in the dynamic mechanical analyzer used for measurements. (B) Actuator strain versus time during exposure of the chemically powered actuator to a mixture of N2, 2.5% by volume hydrogen and 50 % oxygen (red curves) and during exposure to pure oxygen (blue curves). (C) Actuator strain versus time for different volume percents of hydrogen for the experiment in B. The insert shows the dependence of actuator strain on the H2 volume % in the fuel.

Fuel Cells

Fuel cells (FC) are electrochemical devices that convert chemical energy into electricity and heat without a combustion process. The converting process is efficient and environmentally friendly. In addition, unlike batteries, which generate limited power because finite amounts of chemicals are stored inside, fuel cells can continue to produce electricity as long as the fuels (such as hydrogen) and oxygen/air are being supplied.

Among a variety of FC systems, Polymer electrolyte membrane (PEM) fuel cells - also called proton exchange membrane fuel cells is the most promising system for transportation as well as small scale stationary power generation applications. The advantages of PEMFC include low operating temperature (typically, > 85°C, allowing to start quickly because of less warm-up time), high power density, and immediate response for power demand change.

The proton exchange membrane (PEM) is the central part in a PEMFC system and has a major influence on the system’s overall performance and operating conditions. Current PEMs (such as Dupont’s Nafion) function well only under high humidity conditions. PEMFCs based on these membranes are limited to operating temperatures of 60-80°C and require external humidification to maintain optimum performance. Maintaining these temperatures under automotive conditions, especially at peak power, requires over-sized cooling equipment. In addition, the humidification requirements add increased volume, weight, and complexity to the system. These issues would be reduced or eliminated if a PEM could be operated at higher temperatures (approximately 120°C) and low humidity conditions. Additional benefits of operation at elevated temperatures and reduced humidity are a reduction in the occurrence of cathode flooding at peak power, a possible improvement in cell performance due to increased rate of the oxygen reduction reaction, and direct use of reformed fuel derived from an alcohol or hydrocarbon fuels (typically containing small amount of CO) by enhancing Pt catalyst tolerance to CO.

High temperature, low humidity membranes are expected to improve thermal management and ease or eliminate the need for membrane water management in automotive systems. Higher temperature operation will also aid in achieving success in combined heat and power applications for stationary fuel cells.

UTD Fuel Cell Research Group seeks novel polymer and organic/inorganic hybrid candidate materials for PEMs that conduct protons at low relative humidity (i.e., 25-50% RH) and temperatures ranging from room temperature to 120°C.

How does a PEM Fuel Cell work?

A key part of a PEMFC system is a membrane electrode assembly (MEA), which consists of a proton exchange membrane (PEM) and two porous Pt catalyst layers coated onto either side of the membrane, working as cathode and anode (illuminated in the Figure below). Under operating conditions, hydrogen is fed into the anode side and oxidized into protons H+ (Equation 1), and oxygen (air) is fed into the cathode side and reduced into H2O (Equation 2). Protons formed on the anode pass through the membrane and combine with oxygen on the cathode to form H2O. Electrons generated at the anode pass through an external circuit to the cathode and supply power to the external circuit.

Electrospun Conjugated Polymers

Development, production and characterization of our electrospun, nano-scale fibers based on processible PPV (poly-phenylenevinylene) derivatives as emissive layer in polymeric light emitting diodes (PLED). The use of these polymers in PLEDs provides the capability of creating a display with a perfect viewing angle at low cost; high contrast and brightness; fast switching times; and low driving voltages.

Block Copolymers

Synthesis and characterization of functionalized poly-p-phenylene vinylenes (PPVs) block copolymers. The block copolymers are designed to have two segments with side chains containing different electron donating or withdrawing capability, resulting in polymers with characteristic optoelectronic properties. Target applications for these block copolymers are in polymer light emitting diodes (PLEDs) or photovoltaic devices (solar cells).