Asymmetric Thermoelectrochemical Cell for Harvesting Low-grade Heat under Isothermal Operation

Low-grade heat is abundant, but efficient low-grade heat recovery is still a great challenge. So here, we propose an asymmetric thermal cell with a high-energy efficiency over 3%The asymmetric thermal cell is thermally charged and electrically discharged under isothermal operation, so it has potential for various applications, due to its flexibility, low cost, and light weight. The performance of asymmetric thermal cell depends heavily on the content of oxygen-functional groups and the quality of electrodes.

Researchers are suggested to strictly follow the protocol. Visual demonstration helps better understand the structure of asymmetric thermocell as a new technique, and ensures the production quality. Demonstrating the procedure will be Mu Kaiyu and Wang Xun, graduate student, and Dr.Huang Yu-Ting, post-doctorate from our lab.

To set up a cold water bath, place a double-walled glass beaker on a magnetic stirrer, and circulate ice water through the external layer. Pour 100 milliliters of sulfuric acid into the beaker, and turn on the magnetic stirrer. Add one gram of sodium nitrate to the beaker.

Add one gram of flake graphite to the beaker containing the sulfuric acid, and stir for one hour in the cold bath. After one hour, gradually add six grams of potassium permanganate to the solution. Stir the mixture for another two hours.

After two hours, replace the ice water in the external layer with the water at a temperature of 35 degrees Celsius. Change the reaction environment to 35 degrees Celsius for further procedure. Continue the oxidation of the graphite by stirring for one half-hour.

Add 46 milliliters of deionized water into the beaker one drop at a time, which will raise the temperature of the beaker to the range of 80 to 90 degrees Celsius. The synthesis of graphene oxide is an intense reaction. Please strictly follow the protocol, and conduct the experiments in the film hood, with proper laboratory safety equipment.

Add 140 milliliters of deionized water, and then 20 milliliters of hydrogen peroxide. Look for golden particles of graphene oxide to appear as a result. Wash the product thoroughly several times with dilute hydrochloric acid and deionized water until the graphene oxide suspension reaches a pH of seven.

Freeze the washed graphene oxide suspension overnight. Dry it in a freeze-dryer until the water evaporates completely. Mix graphene oxide, carbon black, and PVDF in a mass ratio of 75 to 15 to 10, and put them in a glass bottle.

Measure a quantity of solvent N-Methyl-2-pyrrolidone that is four times the mass of the graphene oxide-carbon black-PVDF mixture. Drip the solvent into the solid mixture. Use a mixer to create a paste.

Mix the solvent and solid at 2, 000 RPM for 30 minutes. Then, de-foam at 1, 200 RPM for two minutes. Brush-coat the paste on carbon paper until the coat has a mass loading of eight to 15 milligrams per square centimeter.

Dry it for four hours at 40 degrees Celsius. To prepare a carboxymethylcellulose solution, dissolve CMC powder 1%by weight in deionized water. Stir for 10 hours.

Next, add 50 milligrams of gluco-emeraldine-base polyaniline and 10 milligrams of carbon black to a beaker. Then, add 150 microliters of the carboxymethylcellulose solution to the beaker. Mix with a magnetic stirrer for 12 hours.

To complete preparation of the polyaniline slurry, add six microliters of 40%styrene-butadiene solution to the beaker, and stir for another 15 minutes. Place a piece of carbon paper on the doctor blade coater. Drop the mixed polyaniline slurry at the leading edge of the carbon paper.

Blade-coat the slurry to produce a film 400 micrometers thick on the carbon paper. Dry the coating for four hours at 50 degrees Celsius. Make current collectors by cutting titanium foil into the appropriate size.

Use a 20 kilohertz ultrasonic spot-welding machine to connect each piece of foil to a nickel tab. Place a porous, hydrophilic, polypropylene-based separator between the graphene oxide electrode and a polyaniline electrode. Stack each electrode with one current collector.

Assemble the asymmetric thermal electro-chemical cell pouch, or asymmetric thermocell, by packing the electrodes in aluminum-laminated film. Using a compact vacuum sealer, seal three sides of the aluminum-laminated film for four seconds. Inject 500 microliters of one-molar potassium chloride electrolyte into the pouch, and allow it to equilibrate for 10 minutes.

Then, extrude the excess electrolyte. Seal the last side of the pouch in the vacuum sealer. Apply thermal paste to interfaces of pouch cell, to ensure good thermal contact.

To set up the temperature control system, stack the asymmetric thermocell between two thermo-electric modules. Place thermal couples on the top and bottom sides of the cell. Use a potentiostat to perform electrochemical tests of the asymmetric thermocell.

Conduct the thermal charging in open-circuit mode. Carry out the electrical discharging process in closed mode, at a constant current. A built-in voltage, delta-v not, was observed in open-circuit conditions at room temperature.

When the asymmetric thermocell was heated from room temperature to high temperature, the cell voltage increased as electrons moved to the surface of the graphene oxide. When an external load was connected, the asymmetric thermocell was discharged. When the asymmetric thermocell was heated from room temperature to a high temperature of 70 degrees Celsius, the open circuit potential reached 0.185 volts.

Discharge of the asymmetric thermocell was conducted under a constant current of 0.1 milliamps. The output electrical work was calculated by integrating the discharging voltage over the charge capacity. The asymmetrical thermocell attained an energy conversion efficiency of 3.32%equivalent to 25.3%of the Carnot efficiency.

Compared to other thermoelectrochemical systems, the energy conversion efficiency of the asymmetric thermocell is the highest ever achieved at 70 degrees Celsius. The asymmetric thermocell has the potential to be used to convert waste heat to electricity in a wide variety of scenarios. The oxygen functional groups are essential to the thermal pseudo-compactive effect of graphene oxide.

The quality of the synthesis of graphene oxide, and the materials loading in step 3.4, are important. The efficiency and the securability of asymmetric thermocells can be improved by changing the electrode materials. For example, by using Prussian blue analog as the anode.

This technology first explored heat-to-electricity conversion under isothermal operations, and revolutionized the thermal electrochemical systems.