What’s a thermogalvanic cell good for?

Thermogalvanic energy harvesting (EH) cells use an electrochemical cell that generates electricity directly from heat by maintaining a temperature difference between two electrodes immersed in an electrolyte solution containing a redox chemical couple. In addition to EH, thermogalvanic cells (TGCs) are being proposed for thermal management and cooling applications.

A TGC is primarily designed to convert low levels of waste heat into electricity. It can generate power from small temperature differences in applications like body heat or industrial waste heat EH applications with small heat gradients. They have typical conversion efficiencies of 0.1% to 1% for converting heat into electricity.

The two electrodes can be made with the same material and are held at different temperatures. Common electrode materials include platinum (Pt) and conducting polymers, like poly(3,4-ethylenedioxythiophene)-tosylate (PEDOT-Tos). The electrolyte is a homogeneous material like a salt solution in water.

In a TGC, electrons flow from the hotter electrode (anode), where an oxidation reaction occurs, to the cooler electrode (cathode), where a reduction reaction occurs, following the same general principle of operation as other galvanic cells (Figure 1).

Figure 1. Structure of a typical TGC showing the temperature gradient used to generate electricity. (Image: Wikipedia)

TGC classifications

TGCs for EH are often classified by their electrolyte. Aqueous electrolytes consist of salt or a similar hydrophilic compound dissolved in water. The electrolyte must be able to support redox reactions to generate electron flow. These systems operate from around room temperature to about 100 °C (depending on the specific salt solution).

A variety of non-aqueous electrolytes are also used in TGC. Examples include solvents like methanol, acetone, dimethyl sulphoxide, dimethyl formamide, and copper sulfate. Depending on the chemistry used, these designs can operate at higher temperatures ranging from 135 to 165 °C.

For applications with a higher temperature heat source, molten salt electrolytes, including mixtures of sodium nitrate (NaNO3) and potassium nitrate (KNO3), are being investigated. Those same salts are currently used in concentrated solar thermal power plants. Typical hot source temperatures are between 325 and 625 °C (600 to 900 K) but can reach 1450 °C (1730 K) in some systems. Typical cold side temperatures range from 125 to 225°C (400 to 500 K).

Experimental TGCs have been fabricated with solid ionic materials used for the electrolyte. Materials considered include iodides, chlorides, and bromides like AgI, PbCl2, and PbBr2. Like the molten salt systems, solid electrolyte designs tend to run at higher temperatures like 125 to 625 °C (400 to 900 K).

TGCs for cooling

Some TGCs have been designed to work in reverse. Instead of generating electricity from heat, these cooling TGCs use an external electric current source to cause a reaction that produces a cooling effect. To support continuous cooling, the heat captured in the electrolyte is dissipated through an external heat sink.

TGC coolers are being investigated to support more environmentally friendly refrigeration systems and for cooling and thermal management in microelectronics and other systems.

A proposed TGC electrochemical refrigeration system is depicted in Figure 2. It has four basic elements: a heat source, heat sink, endothermic half-cell, and exothermic half-cell. It operates using a series of four steps:

Figure 2. Proposed thermogalvanic system for electrochemical refrigeration. The blue oval represents the endothermic reaction, and the red oval represents the exothermic one. The gray rectangle in the middle is the porous separator that suppresses heat transfer. The pump drives the electrolyte circulation to the left of the heat source. (Image: Joule)

During the first step (1 to 2), the electrolyte undergoes an endothermic reaction next to the cathode.
The second step (2 to 3) is when the electrolyte absorbs heat through the heat source and temperature increases.
The electrolyte then undergoes an exothermic reaction (3 to 4), releasing the absorbed heat.
Finally, the electrolyte dissipates heat through the heat sink, cooling and returning to its initial state when the process starts over (steps 4 to 1).

Summary

TGCs are primarily designed to harvest energy from low levels of waste heat and convert it into electricity. They can generate power from small temperature differences in applications like body heat or industrial waste heat. Depending on the materials used, mostly on the type of electrolyte, they can operate from room temperature up to over 1,450 °C. When operated in reverse, the same technology has been proposed for heat removal in an environmentally friendly refrigeration system.