16 Magnetic Fluids for Thermoelectricity
383
are molten salts that are liquid at room temperature and stay liquid up to temperatures much higher than 100 °C (some can exceed 300 °C). ILs possess large ionic
conductivity values and wider electrochemical windows compared to other liquid
electrolytes [8, [9] making them a promising candidate for a variety of low-grade
waste heat recovery applications.
Today, the most widely studied TE property in liquid electrolytes is that of thermogalvanic effects, i.e. the temperature dependent electrochemical reactions between
the redox couple molecules and the electrodes. The highest Seebeck coefficient
reported is found with Cobalt-based redox couple mixed in ionic liquids larger than
2 mV/K over a wide temperature range extending well above 100 °C. For interested
readers, Dupont et al. [10] has compiled a review of thermoelectrochemical cells
containing a variety of ionic liquids, redox couples, electrode materials and their
combinations.
The Seebeck coefficient in liquid electrolytes was also found to increase by
inclusion of colloidal magnetic nanoparticles (ferrofluids) [11]. These nanoparticles are “charge”-stabilized and their thermodiffusion under a thermal gradient and
the adsorption by the electrodes are believed to influence the fluid’s thermoelectric
potential. In the following sections, brief and salient descriptions of three most dominant physical origins of thermoelectric potential production in complex liquids are
given.
3 Then recent experimental evidences on the combined thermoelectrochemical and thermodiffusion effects in ferrofluids are recounted. As it will be clear,
our current understanding of thermoelectrochemical nature of complex fluids is
far from complete, encouraging further experimental and theoretical research and
development efforts in this exciting field of liquid thermoelectrics.
16.1.1 Basic Mechanisms of Thermoelectric Conversion
in Fluids (3 Pages)
In solid materials, the production of thermoelectric potential is well understood from
the out-of-equilibrium thermodynamics of heat and charge flows (of electrons or
holes), expressed in terms of Onsager relations [12]. The situation is quite different for
liquid thermoelectric materials. First, there are multiple types of charge carriers, i.e.
electrolyte ions and other solutes such as colloidal particles or macromolecules. These
carriers are all susceptible to thermodiffusion as well as interactions among themselves, and certain ions react electrochemically with the electrodes, hugely complicating the experimental data interpretation of such systems [13, 14]. Furthermore, a
liquid sample must be placed inside a container (called thermocell) to be examined,
thus additional precautions must be taken in order to minimize experimental errors
due to thermal and electrical losses to its surroundings.
3 Note that in-depth introduction on the physics and electrochemistry of these phenomena are beyond
the scope of the current article, but references to useful articles and books are given for interested
readers.
383
are molten salts that are liquid at room temperature and stay liquid up to temperatures much higher than 100 °C (some can exceed 300 °C). ILs possess large ionic
conductivity values and wider electrochemical windows compared to other liquid
electrolytes [8, [9] making them a promising candidate for a variety of low-grade
waste heat recovery applications.
Today, the most widely studied TE property in liquid electrolytes is that of thermogalvanic effects, i.e. the temperature dependent electrochemical reactions between
the redox couple molecules and the electrodes. The highest Seebeck coefficient
reported is found with Cobalt-based redox couple mixed in ionic liquids larger than
2 mV/K over a wide temperature range extending well above 100 °C. For interested
readers, Dupont et al. [10] has compiled a review of thermoelectrochemical cells
containing a variety of ionic liquids, redox couples, electrode materials and their
combinations.
The Seebeck coefficient in liquid electrolytes was also found to increase by
inclusion of colloidal magnetic nanoparticles (ferrofluids) [11]. These nanoparticles are “charge”-stabilized and their thermodiffusion under a thermal gradient and
the adsorption by the electrodes are believed to influence the fluid’s thermoelectric
potential. In the following sections, brief and salient descriptions of three most dominant physical origins of thermoelectric potential production in complex liquids are
given.
3 Then recent experimental evidences on the combined thermoelectrochemical and thermodiffusion effects in ferrofluids are recounted. As it will be clear,
our current understanding of thermoelectrochemical nature of complex fluids is
far from complete, encouraging further experimental and theoretical research and
development efforts in this exciting field of liquid thermoelectrics.
16.1.1 Basic Mechanisms of Thermoelectric Conversion
in Fluids (3 Pages)
In solid materials, the production of thermoelectric potential is well understood from
the out-of-equilibrium thermodynamics of heat and charge flows (of electrons or
holes), expressed in terms of Onsager relations [12]. The situation is quite different for
liquid thermoelectric materials. First, there are multiple types of charge carriers, i.e.
electrolyte ions and other solutes such as colloidal particles or macromolecules. These
carriers are all susceptible to thermodiffusion as well as interactions among themselves, and certain ions react electrochemically with the electrodes, hugely complicating the experimental data interpretation of such systems [13, 14]. Furthermore, a
liquid sample must be placed inside a container (called thermocell) to be examined,
thus additional precautions must be taken in order to minimize experimental errors
due to thermal and electrical losses to its surroundings.
3 Note that in-depth introduction on the physics and electrochemistry of these phenomena are beyond
the scope of the current article, but references to useful articles and books are given for interested
readers.