Thermoelectric Converters Based on Ionic Conductors

Abstract

Thermoelectric materials represent a new paradigm for harvesting low-grade heat, which would otherwise be dissipated to the environment uselessly. Relative to conventional thermoelectric materials generally composed of semiconductors or semi-metals, ionic thermoelectric materials are rising as an alternative choice which exhibit higher Seebeck coefficient and lower thermal conductivity. The ionic thermoelectric materials own a completely different thermoelectric conversion mechanism, in which the ions do not enter the electrode but rearrange on the electrode surface to generate a voltage difference between the hot and cold electrodes. This unique character has inspired worldwide interests on the design of ionic-type thermoelectric converters with attractive advantages of high flexibility, low cost, limited environmental pollution, and self-healing capability. Referring to the categories of ionic thermoelectric conversion, some representative ionic thermoelectric materials with their respective characteristics are summarized in this minireview. In addition, examples of applying ionic thermoelectric materials in supercapacitors, wearable devices, and fire warning system are also discussed. Insight into the challenges for the further development of ionic thermoelectric materials is finally provided.

Potentially Wearable Thermo‐Electrochemical Cells for Body Heat Harvesting: From Mechanism, Materials, Strategies to Applications

“…TCCs also consists of two electrodes connected by an ionic conductive electrolyte, in which the electrolyte generates thermovoltage and the capacitive electrode store the generated energy (Figure 8). [40,47] The thermovoltage is raised from the distinct mobility of cations and anions in the electrolyte under a temperature gradient due to the Soret effect. [98] The resulting gradient of ion concentration induces the potential difference between two electrodes and the charge induced by the potential difference at the electrode/electrolyte can then be stored by capacitive electrode materials.…”

“…[98] The resulting gradient of ion concentration induces the potential difference between two electrodes and the charge induced by the potential difference at the electrode/electrolyte can then be stored by capacitive electrode materials. [40,47] As the power output is not constant in TCCs, the energy storage is an important parameter in addition to the figure-of-merit (ZT value) of ionic conductive electrolytes. Energy storage in an idea TCC could be simply defined as:…”

“…Meanwhile, the thermal conductivity and ionic conductivity of the electrolytes also play a role in the effective utilization of thermal energy and electric energy, so the ionic figure‐of‐merit ( ZT i ) can also be applied here to evaluate the performance of ionic conductive electrolytes as below: where S i is ionic seebeck coefficient, σ is the ionic conductivity, T is the absolute temperature and κ is the thermal conductivity. [ 40,47 ]…”

“…Thermo-electrochemical cells (TECs), which can convert heat to electricity via the temperature dependence of the electrochemical process, display Seebeck coefficients hundreds of times higher than flexible TEGs. [39][40][41] This high performance, coupled to cheap and environmentally friendly electrode and electrolyte materials and simple device construction, is leading to increasing interest in TECs for low-grade heat harvesting. While all TECs have the same basic configuration of two electrodes sandwiching an electrolyte, there exist multiple subclasses of TECs defined by their different operating mechanisms including thermogalvanic cells (TGCs), [41][42][43][44] thermally chargeable capacitors (TCCs), [45][46][47] and thermally regenerative batteries.…”

Quasi-solid conductive gels with high thermoelectric properties and high mechanical stretchability consisting of a low cost and green deep eutectic solvent

“…Recently, ionic TE materials (ITE) such as liquid electrolytes, ionic hydrogels and ionogels emerged as next generation TE materials due to their high TE properties particularly high ionic Seebeck coefficient. 34–37 For example, ionogels consisting of an ionic liquid (IL) can exhibit an ionic Seebeck coefficient of tens of mV K −1 , 38,39 higher than that of the electronic TE materials by 2–3 orders of magnitude. 14,40,41 Additionally, the ionogels using a stretchable gelator can exhibit high mechanical stretchability, 42 and they can be thus used as a wearable energy generator by harvesting body heat.…”