A new thermogalvanic device uses nanoparticles to control ion flow, allowing it to convert waste heat to electricity and store the energy without external batteries.
(Nanowerk Spotlight) Heat is everywhere, and most of it goes to waste. From the warmth radiating off industrial pipes to the heat flushed out of vehicle engines and the hum of data centers, enormous amounts of low-temperature thermal energy are released into the environment without ever doing useful work. In a world increasingly concerned with energy efficiency and sustainability, this is more than just a missed opportunity. It is a design flaw built into nearly every modern system.
Scientists have tried to reclaim this overlooked energy using thermoelectric devices, which convert temperature differences into electricity. These systems are compact and solid-state, with no moving parts, making them appealing for use in locations where space is tight and maintenance is difficult. But they suffer from a critical problem. They do not generate much power, and what little they do produce must be used immediately or stored in a separate battery or capacitor. Without a way to store the electricity they generate, their usefulness in real-world settings is sharply limited.
This challenge—turning ambient heat into usable, storable electricity—sits at the intersection of two technological problems. The first is how to improve the efficiency of heat-to-electricity conversion. The second is how to combine conversion and storage within a single device.
Some researchers have tried to address this by pairing thermoelectric systems with micro-supercapacitors or hydrogels that can store small amounts of energy. These hybrid setups tend to be complex, slow, and limited in power output.
A team of researchers in China has taken a different approach. In a new study published in (“Precipitation-Driven Thermoelectric Conversion and Energy Storage Integrated Device”), they describe a device that both generates electricity from low-grade heat and stores it for later use. The mechanism that makes this possible is neither a battery nor a capacitor, but precipitation.
a) Schematic illustration of the original device based on Thermogalvanic effect; b) Schematic illustration of the novel precipitation-driven integrated thermoelectric conversion and energy storage device based on precipitation-driven Thermogalvanic effect. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
At the center of the device is a well-known redox system based on iron-containing compounds called potassium ferricyanide and potassium ferrocyanide. When placed in an aqueous solution and exposed to a temperature gradient, these compounds can participate in electron transfer reactions that create voltage. This approach is known as thermogalvanic energy conversion. It has been explored for decades but suffers from the same limitation as other thermoelectric systems. Once the temperature difference is removed, the voltage disappears and the energy cannot be stored.
The team’s innovation lies in disrupting that equilibrium. By introducing copper nanoparticles into the electrolyte, they triggered a subtle but important change. The copper partially precipitates out of the solution and settles at the bottom of the device. This precipitate does not passively sit in the system. Instead, it interferes with how the redox ions diffuse through the liquid.
As a result, after heat drives the redox reactions and creates a voltage difference, the ions cannot quickly redistribute and cancel out the gradient. The system holds onto the chemical imbalance, making it possible to store energy internally and release it later.
The researchers outline four operational states that describe the behavior of the system. In State I, a temperature difference is applied. At the hot electrode, ferrocyanide is oxidized to ferricyanide, releasing electrons that accumulate at the electrode surface. At the same time, ferricyanide ions begin to diffuse toward the cold side. In State II, an external circuit is connected. Electrons flow from the hot to the cold electrode through the circuit, while ferricyanide at the cold end is reduced back to ferrocyanide. This creates a chemical and electrical imbalance that sustains the voltage.
In traditional thermogalvanic devices, this imbalance would quickly dissipate once the temperature gradient is removed. But in this system, the presence of copper precipitate slows down ion movement. In State III, after the heat and external circuit are removed, the remaining concentration gradient continues to drive redox reactions. These reactions generate a reversed potential difference, allowing the system to retain charge internally. Finally, in State IV, reconnecting the circuit allows the device to release stored energy like a battery.
The result is a compact, integrated device that performs both energy conversion and storage using a single electrolyte. The researchers tested several formulations and found that a copper concentration of 2.1 percent by weight provided the best performance. At a temperature difference of 50 degrees Celsius, this system achieved a Seebeck coefficient of 2.01 millivolts per Kelvin and a power density of 3.51 watts per square meter. Both values exceed those of similar thermogalvanic systems without precipitate. The energy density reached 459 joules per square meter during the charging phase, more than twice the highest previously reported value.
To understand how the copper affects performance, the researchers used cyclic voltammetry and molecular dynamics simulations. These tests showed that the precipitate changes the diffusion rates of the redox ions, creating an asymmetry that favors prolonged energy storage.
Simulations revealed that the copper disrupts the hydration shells surrounding the ions, particularly around ferrocyanide. This disruption reduces the energy barrier for redox reactions, making the system more efficient at generating voltage under heat.
They also confirmed that the effect is not unique to copper. When they used silver nanoparticles instead, they observed similar improvements, though less pronounced. This suggests that the mechanism depends on the physical presence of the precipitate rather than any specific chemical interaction.
The device’s energy storage capabilities were tested under various thermal and electrical conditions. Even at a modest temperature gradient of 17.8 degrees Celsius, the device reached an energy density of 459 joules per square meter. It retained roughly half of this energy during discharge. This represents a substantial improvement over previous integrated systems, which typically operate at energy densities below 200 joules per square meter and often below 1.
The researchers then built a larger module consisting of 32 individual PITCS units connected in series. This integrated device achieved an open-circuit voltage of 1.7 volts at a 30-degree temperature differential and was able to power both LEDs and nixie tubes directly, without external voltage boosters. The energy density of the module reached 463.9 joules per square meter under a temperature difference of just 14 degrees Celsius.
One of the most promising aspects of the design is its stability. The device was tested over multiple charge and discharge cycles without significant performance loss. It also avoids the fragility and environmental sensitivity of hydrogel-based systems, which can degrade under changes in humidity or temperature. The PITCS device relies on simple physical processes that do not require complex fabrication or maintenance.
By rethinking how ions move through a redox system, the researchers have introduced a strategy for coupling energy conversion and storage in a compact, passive device. Their use of precipitation to maintain ion concentration gradients opens new possibilities for low-temperature thermal energy harvesting. This approach may be applicable to other redox couples or materials and could provide a path forward for systems that need to operate autonomously using waste heat as their power source.
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