Opportunities for PEC in IoT PE cell collects and stores electricity in remote areas

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New research results show that the pyroelectrochemical cell (PEC) uses changes in ambient temperature to charge itself, demonstrating its potential for applications in the IoT.

Nowadays, there are more and more wirelessly connected devices that monitor the condition of engines and machines, for example, or perform remote sensing in agriculture. Systems known as the "Internet of Things" (IoT) hold great potential for improving the efficiency and safety of devices. However, there are still stumbling blocks to the IoT that thwart many potential applications. How to power these devices in situations where reliable power sources are virtually unavailable?

Research from the College of Engineering at the University of Utah points to a possible solution in the form of a new type of battery, the pyroelectrochemical cell (PEC). The cell was developed and tested in the research laboratories of professors Roseanne Warren and Shad Roundy.

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Using thermal energy from the environment

"It's our idea for an integrated device that captures thermal energy from the environment and converts it directly into stored electrochemical energy in the form of a supercapacitor or battery, with applications for the Internet of Things and distributed sensors," said Warren, lead author of a new proof-of-concept study.

"We're talking about a very small energy harvest here, but the ability to have sensors that can be distributed and don't need to be charged on site is the main advantage," she added. "We have researched the basic physics and found that if there is a rise or fall in temperature, a charge can be generated."

The cell is charged by temperature fluctuations in the environment - be it in a car or airplane or directly in the soil of an agricultural environment. In theory, the PEC could power sensors for IoT applications that would otherwise be impractical to charge. According to Roundy, a co-author of the study, a solar cell would work reliably in some situations.

"But in many environments you run into two problems," says Roundy. "One is that it gets dirty over time. Solar cells need to be kept clean. So in those applications, they get dirty and their performance degrades. There are also many applications where there is simply no sunlight available. For example, we are working on sensors that we place directly under the surface of the ground. They won't get any sunlight."

The PEC uses a pyroelectric composite material as a separator in an electrochemical cell. The material consists of porous polyvinylidene fluoride (PVDF) and barium titanate nanoparticles. The electrical properties of this material change when it is heated or cooled, reducing or increasing the polarization of the pyroelectric separator. The changing temperatures create an electric field inside the cell that moves ions around and allows the cell to store energy.

"It stores electricity in what's called an electrical double layer, which stores the charge in positive and negative ion layers. It's a better capacitor," says lead author Tim Kowalchik, a PhD student in Warren's lab.

"When you heat or cool the system and store electrochemical energy, the amount of positive or negative ions in these layers changes." In the new study, the laboratory's theory of how the cell works was tested.

Does it work in the lab—but also outside?

"We had a predicted functional model that contained a so-called 'orientation effect'," says Kowalchik. "If we change the reverse orientation of the separator in the cell, this should drive the ions in the other direction. This is a change we make to the system that leads to a different result that we can collect."

The team's experiments were designed to show whether the cells would react as predicted. In addition to the alignment effect, the effects of heating and cooling also had to be tested.

"If you heat the thing in one direction, something should happen. If you cool it down first, something should happen, and it should show up differently," Kowalchik said. "We did this with a technique called amperometry. You apply a voltage, keep it constant and measure the current. If nothing changes, the energy flowing into the system remains constant; if energy flows into the system, the current changes."

The cell reacted as the team had imagined, but can it also work outside a laboratory? That's the next question Warren wants to investigate. One of her students is now working on circuit modelling to design a cell and optimize its function.

"We are now starting to change various parameters," said Warren. "How can we improve energy generation and storage and the combination of the two? And then after that, a real field demonstration would be possible."

Enough energy for IoT purposes

The cell could generate up to 100 microjoules per square centimeter from a single heating/cooling cycle. That's not a lot of energy, but according to the researchers, enough to be useful for IoT purposes.

"They want to monitor the condition of their car, the condition of machinery, the condition of plants and soil and things like that. These types of sensors tend to use less power than a smartwatch or phone, which have a screen and transmit a lot of data," says Roundy. "The sensors we're talking about here might just give periodic updates and work autonomously. They don't have an interface or a screen." The study, titled "Direct Conversion of Thermal Energy to Stored Electrochemical Energy via a Self-Charging Pyroelectrochemical Cell," was funded by the National Science Foundation. (sb)

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Link: Direct conversion of thermal energy to stored electrochemical energy via a self-charging pyroelectrochemical cell

Link: University of Utah: College of Engineering