Scientists at the National Institute of Technology, Gunma College and the University of Tsukuba, Japan, are developing a novel, more stable thermocell that turns environmental heat into electricity.
Their recent article, published on February 4, 2020, in the open-access Nature journal Scientific Reports, proposes a new type of battery that is more stable at room temperature than its predecessors. To this purpose, the Japanese scientists who collaborated on the project addressed the drawbacks of traditional, semiconductor-based thermoelectric devices by using instead phase-transition materials as electrodes. The idea is to use electrodes that undergo changes at the crystalline level, which in turn generate an electric potential that is large and stable enough to be useful in IoT devices (smartwatches, smart mobiles, fitness trackers, etc.).
One of the biggest challenges today is to harvest and use nonpolluting sources of energy as much as possible. Collecting residual heat (e.g., lost energy due to the day-night temperature change, waste heat near room temperature, or human body heat) and transforming it into electricity is one way to achieve this goal. For this purpose, there are two main approaches:
- Semiconductor-based thermoelectric devices that use the Seebeck effect. Some applications include Peltier cooling and thermal power generation in space vehicles.
- Thermocells having electrodes with different temperature coefficients and that work via the thermal charging effect. They are also known as “tertiary batteries.”
Within the next subsections, I will explain in more detail what the Seebeck effect is, the complexity of thermocells, and the element of novelty of the Japanese scientists’ energy-harvesting cell.
Semiconductor-based thermoelectric devices
Broadly speaking, a semiconductor is a solid material that conducts electricity better than an insulator but worse than a metal. However, the physics of semiconductors is very rich. Some of them can become superconductors (perfect conductors) when cooled, while others can generate a potential difference (meaning the potential to generate electricity) if they are placed in an environment with a temperature difference. The latter is called the Seebeck effect.
Therefore, if the Seebeck effect is generating electricity by placing the ends of a slab at different temperatures, Peltier cooling — an important application — describes the opposite phenomenon: by applying an electric potential between the ends of a semiconducting material, you can cool one end and heat up the other one.
So then what is a tertiary battery?
Also called thermocells, these batteries have — just like regular batteries — a positive and a negative electrode (the anode and cathode, respectively). What is special about them is that the anode and the cathode are made of different materials, such that they react in a different way to temperature. More precisely, their temperature coefficients are different, and this is important for generating electricity using very small temperature fluctuations.
The temperature coefficient of a material describes how its redox potential changes with temperature. The redox potential represents a material’s affinity to accept or release electrons; it is closely related to the capability of the overall device to generate electricity. Specifically, an electrode with a large temperature coefficient means that a small change in temperature determines a large change of the redox potential.
Why is this important?
This configuration allows a thermocell to operate similarly to a heat engine. Thermal energy is converted to electric energy within a thermal cycle between a high and low temperature. This is strikingly different from how semiconductor-based thermoelectric devices work — they need a stable, permanent temperature difference to generate an electric potential. This makes the old technology unfeasible for harvesting human body heat, which is generated from fluctuations in our body temperature (between getting sweaty at the gym or feeling cold at the office, for example).
Electrodes that undergo a phase transition
So if tertiary batteries are so awesome, why is there a need to complicate things with fancy phase transitions? The first disadvantage of existing prototypes is that the output potential is too low (a few millivolts) to power any smart device. As a reference, a smartwatch needs 1.3 volts to function, which is about a thousand times larger than the current capabilities of these cells. Second, the output voltage is also temperature-dependent, which means the thermocell cannot be used as an independent power supply.
In this context, Takayuki Shibata et al. came up with a new thermocell prototype with electrodes whose crystalline structures change with temperature. The microscopic crystalline structure of a material describes how the atoms in the material are arranged (their specific repeating, ordered pattern). This might make you think of crystals, whose quasi-ordered shapes are related to the microscopic crystalline order.
“So what is a phase transition, Gianina? Get to the point.”
Well, bear with me because we’re almost there. The Japanese scientists were able to synthesize two materials which change their microscopic arrangements of atoms when the temperature fluctuates slightly. Remember that the change in the atomic pattern causes a change in the redox potential of the electrode (this is related to how much electricity we can harvest from the cell ). If the changes are just right, then a thermal cycle can be designed (similar to that of a heat engine) such that there is a net energy gain at the end.
Notice that we are now speaking of energy harvesting. These are not simple batteries, which store energy and need to be charged later on. Nor do you toss them and buy new ones. These are independent power supplies. Used in a smartwatch, for example, they could power it indefinitely (because of the slight changes in your body’s temperature) as long as you wear the watch.
Of course, there are still limitations to the prototype. The output voltage is not there yet (about 120 millivolts, although this is closer to the needed value of 1.3 volts). Furthermore, the authors claim that the need for “chemical and physical uniformity in the electrode material” is much stronger for these types of electrodes than in tertiary batteries. Nevertheless, the idea is very promising for powering a “smart” society.
Original research: https://www.nature.com/articles/s41598-020-58695-z