  More than half of the waste heat rejected into the environment has
temperatures lower than 100 $^\circ C$, which accounts for nearly 85 PWh/year
worldwide. Efficiently harvesting low-grade heat could be a promising step
toward carbon neutrality. Recent developments of ionic thermoelectrics (i-TE)
with giant thermopower have provoked intensive interest in using ions as energy
and charge carriers for efficient thermal energy harvesting. However, current
literature primarily focuses on improving thermopower only, while the ion
transport and thermodynamics affecting the efficiencies have been largely
neglected. This review article clarifies the fundamentals of electrochemistry
and thermodynamics for developing highly efficient i-TE devices. Two major
types of i-TE devices, thermo-ionic capacitors (TIC) and thermogalvanic cells
(TGC), are discussed in detail. The article analyzes the methods of enhancing
ionic thermopower in the literature by taking an entropic point of view. We
also derived modified thermoelectric factor Z for both TICs and TGCs that fully
incorporate the dynamics of ion transport and electrochemical reactions. Recent
developments of hybrid devices showing improved efficiencies, power density,
and multifunctionality are reviewed. Finally, we comment on the remaining
challenges and provide an outlook on future directions.
