electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between Electric potential, electrical potential difference and identifiable chemical change. These reactions involve Electron, electrons moving via an electronic ...

, a thermogalvanic cell is a kind of

galvanic cell A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous oxidation–reduction reactions. An example of a ...

in which

heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

is employed to provide

electrical power Electric power is the rate of transfer of electrical energy within a electric circuit, circuit. Its SI unit is the watt, the general unit of power (physics), power, defined as one joule per second. Standard prefixes apply to watts as with oth ...

directly. These cells are electrochemical cells in which the two

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...

s are deliberately maintained at different temperatures. This temperature difference generates a

potential difference Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge ...

between the electrodes. The electrodes can be of identical composition and the

electrolyte An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...

solution homogeneous. This is usually the case in these cells. This is in contrast to galvanic cells in which electrodes and/or solutions of different compositions provide the electromotive potential. As long as there is a difference in temperature between the electrodes a current will flow through the circuit. A thermogalvanic cell can be seen as analogous to a concentration cell but instead of running on differences in the concentration/pressure of the reactants they make use of differences in the "concentrations" of thermal energy. The principal application of thermogalvanic cells is the production of electricity from low-temperature heat sources (

waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility ...

and solar heat). Their energetic efficiency is low, in the range of 0.1% to 1% for conversion of heat into electricity.

History

The use of heat to empower galvanic cells was first studied around 1880. However it was not until the 1950s that more serious research was undertaken in this field.Agar, JN (1963). “Thermogalvanic cells”. Advances in electrochemistry and electrochemical engineering (Ed. Delahay, P, and Tobias, CW) Interscience, New York; vol. 3 pp. 31–121.

Working mechanism

Thermogalvanic cells are a kind of

heat engine A heat engine is a system that transfers thermal energy to do mechanical or electrical work. While originally conceived in the context of mechanical energy, the concept of the heat engine has been applied to various other kinds of energy, pa ...

. Ultimately the driving force behind them is the transport of

entropy Entropy is a scientific concept, most commonly associated with states of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamics, where it was first recognized, to the micros ...

from the high temperature source to the low temperature sink. Therefore, these cells work thanks to a thermal gradient established between different parts of the cell. Because the rate and

enthalpy Enthalpy () is the sum of a thermodynamic system's internal energy and the product of its pressure and volume. It is a state function in thermodynamics used in many measurements in chemical, biological, and physical systems at a constant extern ...

of chemical reactions depend directly on the temperature, different temperatures at the electrodes imply different

chemical equilibrium In a chemical reaction, chemical equilibrium is the state in which both the Reagent, reactants and Product (chemistry), products are present in concentrations which have no further tendency to change with time, so that there is no observable chan ...

constants. This translates into unequal chemical equilibrium conditions on the hot side and on the cold side. The thermocell tries to approach a homogeneous equilibrium and, in doing so, produces a flow of chemical species and electrons. The electrons flow through the path of least resistance (the outer circuit) making it possible to extract power from the cell.

Types

Different thermogalvanic cells have been constructed attending to their uses and properties. Usually they are classified according to the electrolyte employed in each specific type of cell.

Aqueous electrolytes

In these cells the electrolyte between the electrodes is a water solution of some salt or hydrophylic compound. An essential property of these compounds is that they must be able to undergo

redox reaction Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

s in order to shuttle electrons from one electrode to the other during the cell operation.

Non-aqueous electrolytes

The electrolyte is a solution of some other solvent different from water. Solvents like

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...

acetone Acetone (2-propanone or dimethyl ketone) is an organic compound with the chemical formula, formula . It is the simplest and smallest ketone (). It is a colorless, highly Volatile organic compound, volatile, and flammable liquid with a charact ...

, dimethyl sulphoxide and dimethyl formamide have been successfully employed in thermogalvanic cells running on copper sulfate.

Molten salts

In this type of thermocell the electrolyte is some kind of salt with a relatively low melting point. Their use solves two problems. On one hand the temperature range of the cell is much larger. This is an advantage as these cells produce more power the larger the difference between the hot and cold sides. On the other hand, the liquid salt directly provides the anions and cations necessary for sustainment of a current through the cell. Therefore, no additional current-carrying compounds are necessary as the melted salt is the electrolyte itself. Typical hot source temperatures are between 600–900 K, but can get as high as 1730 K. Cold sink temperatures are in the 400–500 K range.

Solid electrolytes

Thermocells in which the electrolyte connecting the electrodes is an ionic material have been considered and constructed too. The temperature range is also elevated as compared to liquid electrolytes. Studied systems fall in the 400–900 K. Some solid ionic materials that have been employed to construct thermogalvanic cells are AgI, PbCl₂ and PbBr₂.

Uses

The main application of thermogalvanic cells is electricity production under conditions where excess heat is available. In particular thermogalvanic cells are being used in the following areas.

Solar energy

The heat collected from this process generates steam, which can be used in a conventional steam turbine system to make electricity. In contrast to the low-temperature solar thermal systems that are used for air or water heating in domestic or commercial buildings, these solar thermal electricity plants operate at high temperatures, requiring both concentrated sunlight and a large collection area, making the Moroccan desert an ideal location. This is an alternative approach to the more widely used “photovoltaic” technology for producing electricity from sunlight. In a photovoltaic system, the sunlight is absorbed in the photovoltaic device (commonly called a solar cell) and energy is passed to electrons in the material, converting the solar energy directly into electricity. Sometimes, solar thermal electricity and photovoltaics are portrayed as competing technologies and, while this may be true when deciding on the way forward for a specific site, in general they are complementary, using solar energy as extensively as possible.

Thermal generators

Waste heat sources

Thermogalvanic cells can be used to extract a useful quantity of energy from waste heat sources even when the temperature gradient is less than 100C (sometimes only a few tens of degrees). This is often the case in many industrial areas. Research has suggested that thermogalvanic hydrogel could be used to generate electricity from the heat produced by a mobile phone battery when the phone is using power, while also cooling the battery.

Cooling

Thermogalvanism uses chemical energy to move heat from a colder to a hotter body. It was claimed in 2025 that thermogalvanic cooling technology could be a sustainable alternative to the vapour compression technology used in

refrigerator A refrigerator, commonly shortened to fridge, is a commercial and home appliance consisting of a thermal insulation, thermally insulated compartment and a heat pump (mechanical, electronic or chemical) that transfers heat from its inside to ...

s. It was claimed that the cooling power of hydrogalvanic cells could be improved by 70% by optimising electrolytes, and that this made cooling viable.

References

{{Galvanic cells Electrochemistry