**Armin Feldhoff**

Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover, Callinstraße 3A, D-30167 Hannover, Germany; armin.feldhoff@pci.uni-hannover.de; Tel.: +49-511-762-2940

Received: 26 June 2020; Accepted: 18 July 2020; Published: 22 July 2020

**Abstract:** The basic principles of thermoelectrics rely on the coupling of entropy and electric charge. However, the long-standing dispute of energetics versus entropy has long paralysed the field. Herein, it is shown that treating entropy and electric charge in a symmetric manner enables a simple transport equation to be obtained and the power conversion and its efficiency to be deduced for a single thermoelectric material apart from a device. The material's performance in both generator mode (thermo-electric) and entropy pump mode (electro-thermal) are discussed on a single voltage-electrical current curve, which is presented in a generalized manner by relating it to the electrically open-circuit voltage and the electrically closed-circuited electrical current. The electrical and thermal power in entropy pump mode are related to the maximum electrical power in generator mode, which depends on the material's power factor. Particular working points on the material's voltage-electrical current curve are deduced, namely, the electrical open circuit, electrical short circuit, maximum electrical power, maximum power conversion efficiency, and entropy conductivity inversion. Optimizing a thermoelectric material for different working points is discussed with respect to its figure-of-merit *zT* and power factor. The importance of the results to state-of-the-art and emerging materials is emphasized.

**Keywords:** thermoelectrics; power conversion; efficiency; voltage-electrical current curve; working point; entropy pump mode; generator mode; power factor; figure of merit; Altenkirch-Ioffe model
