This protocol provides an easy, low-cost method of creating a zinc electrode that suppresses dendrites when cycled in an alkaline battery. The main advantage of this technique is that one can easily tune the structure of the zinc electrode by changing zinc particle size, type of origin and its size, or heat treatment protocol. The metal sponge created using this protocol falls into the relative density and cell size that can be difficult to achieve using standard metal foam manufacturing methods.
Begin by adding 2.054 milliliters of deionized water and 4.565 milliliters of decane to a 100 milliliter glass beaker. Then dissolve 0.1 gram of SDS by stirring the solution. Next, add five milligrams of water soluble medium viscosity CMC sodium salt to the beaker and stir for five minutes or until dissolved.
Use plastic or plastic-coated stirring tools because a metallic surface can adversely affect resulting zinc sponges. Add and stir 0.844 grams of water insoluble pre-swollen carboxymethyl cellulose resin to the beaker. Keep the beaker on an overhead paddle stirrer equipped with a plastic paddle and stir the entire mixture at 1, 000 RPM for five minutes.
Add 50 grams of zinc powder and continue stirring for five more minutes. After five minutes, stop the stirrer and remove the beaker. Place the beaker in a desiccator for five minutes at room temperature to outcast the mixture and its contents under vacuum.
Portion the zinc paste into polypropylene molds of approximately 10 millimeter diameter and five millimeter height to air-dry the paste overnight. After drying, carefully remove the zinc paste from the molds and place them into a mesh casing that rests on a notched alumina holder. Place the assembly into a tube furnace with ports to flow gas in and out of the tube.
Pipe nitrogen gas into the furnace for 30 minutes at a rate of 5.7 centimeters per minute to purge the furnace of air. After 30 minutes of purging, throttle the nitrogen gas to a constant rate of 2.8 centimeters per minute. Program the temperature of the furnace to increase linearly from 20 to 369 degrees Celsius over the course of 68 minutes, hold at 369 degrees Celsius for five hours, rise linearly from 369 to 584 degrees Celsius over the course of 105 minutes, and then turn off.
Start the furnace program as the nitrogen continues to flow. Stop the nitrogen gas flow manually after the five hours of temperature hold and pipe in breathing air at 2.8 centimeters per minute. Let the furnace cool to room temperature once the heating program stops while keeping the breathing air flowing.
Finally, remove the cooled zinc sponges and either saw or sand them to the desired dimensions. Add 10.5 milliliters of deionized water to a 100 milliliter glass beaker. Then add 0.12 grams of water soluble high-viscosity cellulose gum and stir using a plastic-coated stirring tool.
Vortex and stir this mixture by hand for five minutes or until the cellulose gum is dissolved. Add 2.4 grams of cornstarch while vortexing the beaker for two minutes and then add 120 grams of prepared zinc powder while vortexing for an additional two minutes. Once done, press the resulting zinc paste into the desired mold cavities and leave them to dry overnight at 70 degrees Celsius in open air in a furnace.
After drying, carefully remove the zinc paste from the molds and place them into a mesh casing that rests on a notched alumina holder. Place the assembly into a tube furnace with ports to flow gas in and out of the tube. Pipe nitrogen gas into the furnace for 30 minutes at a rate of 5.7 centimeters per minute to purge the furnace of air.
After 30 minutes of purging, throttle the nitrogen gas to a constant rate of 2.8 centimeters per minute. Program the temperature of the furnace to increase linearly from 20 to 369 degrees Celsius over the course of 68 minutes, hold at 369 degrees Celsius for five hours, rise linearly from 369 to 584 degrees Celsius over the course of 105 minutes, and then turn off. Start the furnace program as the nitrogen continues to flow.
After five hours of temperature hold, stop the nitrogen gas flow manually and pipe in breathing air at 2.8 centimeters per minute. Allow the furnace to cool to room temperature once the heating program stops while keeping the breathing air flowing. Finally, remove the cooled zinc sponges and saw or sand them to the desired dimensions.
This protocol was used to produce emulsion-based zinc sponges with densities of 2.8 grams per cubic centimeter and aqueous-based sponges with densities of 3.3 grams per cubic centimeter. The zinc sponges should be rigid and brittle. Their cross-sections should look similar to the ones shown here.
All properties of the resulting zinc sponges should fall within the ranges provided in the text manuscript. The zinc sponge electrode shows a cycling stability at a gravimetric capacity of 328 milliampere hours per gram of zinc, which maps to a 43%depth of discharge. After extensive electrochemical cycling of the constructed zinc electrodes to estimate their efficiency, no dendrites were observed in scanning electron micrographs of the zinc sponge electrodes.
The surface of the zinc sponge undergoes restructuring during cycling. The deeper the level of discharge and the greater the cycle life, the greater the amount of restructuring. When attempting this protocol, remember to stir the mixture with a plastic-coated tool.
Similar fabrication processes could be used to create other types of metal foams based on different metals for numerous applications.
