Water vapor as a power generator

Dr. Neil Canter, Contributing Editor | TLT Tech Beat June 2013

A polymer composite can generate electricity upon stimulation by water vapor.

KEY CONCEPTS
• A polymer composite exhibits the ability to generate power from water vapor.
• In a humid air environment, the polymer composite absorbs up to 10% of the water by weight.
• The movement of the composite is characterized by a five-stage process.

THE CONTINUING INCREASE IN THE WORLDWIDE DEMAND for energy is leading researchers to seek out sources that are readily available, can be easily and cost effectively used and will produce minimum emissions. One approach that has been examined is to use a thermoelectric material such as a skutterudite, which has the ability to convert automotive exhaust heat into electricity (1).

A second strategy is known as energy harvesting, which involves using energy sources found in nature. Two examples are windmills that can generate electricity from the wind and solar cells that take advantage of light energy from the sun.

In a previous TLT article, a technology that converts human motion into electrical energy was discussed (2). By placing a device that contains liquid droplets interacting with a dielectric- film-coated electrode in a shoe, researchers believe that people can generate sufficient electricity to power portable electronic devices by walking.

One obvious energy source that has not been harvested in the past is water vapor. Dr. Mingming Ma, postdoctoral researcher at the David H. Koch Institute for Integrative Cancer Research at The Massachusetts Institute of Technology in Cambridge, Mass., has interest in using water vapor as an energy source for artificial muscles. He says, “One way to develop artificial muscles is to use responsive polymeric materials that can be used as a motor because they change structure in response to an external stimulus.”

Polypyrroles have been studied as artificial muscles for three decades, according to Ma. He says, “Polypyrroles change their structure upon application of an electric potential. As polypyrroles are electronically oxidized or reduced, counter ions migrating into and out of the polypyrrole structure cause the polymer’s size to change.”

The problem with polypyrroles is that a certain voltage needs to be applied to stimulate the change in structure. If the voltage is too low, then no deformation is seen. In addition, these polymers are quite brittle and rigid, which means that they can easily break when stretched.

If a way can be found to make polypyrrole more flexible and ductile, then the resulting polymer could become more effective as an artificial muscle. Such an approach has now been developed.

POLYOL-BORATE
Inspired by examples from nature, Ma, in collaboration with Robert Langer, David H. Koch Institute Professor in the chemical engineering department at MIT, and fellow researchers, have developed a polymer composite that can generate electricity upon stimulation by water vapor.

Ma says, “We noted that in nature animal skins are strong and flexible due to the presence of two types of protein fibers. There is a rigid collagen fiber and a more elastic and thinner elastin fiber. Our objective was to use polypyrrole as the rigid polymer and find a flexible resin that is complementary.”

The researchers decided to work with borate esters because they are very flexible and can readily be hydrolyzed and then reform when water is removed. Ma says, “Borate esters can form a very dynamic network where bonds are formed and then broken very quickly.”

Based on this information, the researchers decided to prepare a polyol borate network within the polypyrrole matrix, forming a polymer composite in the form of a film. The polyol borate was prepared by ethoxylating pentaerythritol and then coordinating it with a boron (III) species. Electropolymerization of pyrrole was then conducted to produce the polymer composite.

Ma says, “Electropolymerization is very similar to radical polymerization except that an electric current is used to initiate the process. A molecule is oxidized to generate a radical, two radicals are combined to form a neutral dimer and then the process is repeated again and again.”

The resulting composite was found to be very responsive to water. In the presence of humid air, which corresponds to a water gradient, the composite absorbed up to 10 percent of the water by weight.

Ma says, “We conducted an infrared study using deuterated water to evaluate the response of the composite to water. After immersion of the composite in deuterated water for 10 seconds, a hydrogen-deuterium exchange takes place, causing three peaks related to hydroxyl and amine hydrogen bending to disappear and three new peaks to reappear. The process was reversed after the composite is left in ambient air with a relative humidity of 20 percent for four minutes.”

In effect, the composite was rapidly breathing water from the air. To demonstrate the potential of this polymer composite to generate power from water, the composite film was left in contact with a moist, nonwoven paper substrate. Ma says, “We just used normal paper as any type of paper or clothes will work.”

The movement of the composite is characterized by a five-stage process that starts with asymmetric swelling and curling away from the paper, forcing the film to buckle and then topple over. This is followed by further water absorption on the other side of the film, leading to completion of the cycle. The stages are shown in Figure 3.


Figure 3. A composite containing a polyol borate network within a polypyrrole matrix responds readily to water vapor by undergoing the five-stage process shown. Through this process, this composite polymer can generate electricity. (Courtesy of Dr. Ning Zhang/The Massachusetts Institute of Technology)

The amount of force and stress generated by the composite was measured after it was clamped and preloaded with a force of 0.05 newton when covered with moist paper. Once uncovered, the film generated a contractile force of up to 14 newtons and a maximum stress of 27 megapascals that occurred as the composite shrank and stiffened as the water desorbed. This is about 80 times more powerful than the strongest mammalian skeletal muscle.

By coupling the polymer with a piezoelectric film, the researchers can generate an average power of 5.6 nanowatts with a 0.2 gram device. They also found that a 20-micrometer thick film can lift glass slides 380 times its own weight and transport silver wires as a cargo 10 times its own weight.

Future work will focus on improving the energy transfer of the composite. Ma says, “We can only generate a small amount of electricity that can power a small sensor. Our goal is to increase the efficiency for the conversion from mechanical to electrical energy so that the composite can act as a true actuator to power much bigger devices.”

Additional information can be found in a recent article (3) or by contacting Ma at mmma@mit.edu.

REFERENCES
1. Canter, N. (2011), “Electricity from Automotive Exhaust Heat,” TLT, 67 (3), pp. 10-11.
2. Canter, N. (2011), “Powering your Cell Phone as you Walk,” TLT, 67 (11), pp. 6-7.
3. Ma, M., Guo, L., Anderson, D. and Langer, R., “Bio-Inspired Polymer Composite Actuator and Generator Driven by Water Gradients,” Science, 339 (6116), pp. 186-189.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.