The electrochemical thermocell based on a PEM fuel cell with two hydrogen electrodes

Thermoelectric energy converters enable the direct conversion of heat into electrical energy. This allows low-temperature heat, which is generated as a by-product in many technical processes, to be utilized, thereby increasing the overall efficiency of the process. Thermoelectric energy converters are basically divided into two categories: semiconductor-based thermogenerators, which are mainly used at high temperatures and large temperature differences, and electrochemical thermocells, which are mainly used at low temperatures and small temperature differences.

The structure of such an electrochemical thermocell is similar to that of a PEM fuel cell, but both electrodes are supplied with a water vapor/hydrogen mixture. The imposed temperature gradient (between the heat source and the environment) leads to an electrical potential difference between the anode and the cathode, as the chemical potential of the fluids changes depending on the respective temperature. Therefore, in the thermocell, the temperature gradient, the gradient in electrical potential, and the gradient in chemical potential are closely linked as driving forces of the transport processes (heat, mass, and charge transport). These coupled transport processes can be described and characterized using the thermodynamics of irreversible processes. It is assumed that the flows occurring in the thermocell represent a linear combination of all driving forces. This linear relationship is described by means of so-called phenomenological coefficients. In contrast to purely empirical approaches such as Fick's, Fourier's, or Ohm's laws, the phenomenological coefficients take into account all interactions and superpositions of the driving forces and flows occurring in the thermocell.

The Institute of Thermodynamics conducts both experimental and theoretical investigations into PEM thermocells. This includes the development of detailed models to describe operating behavior and experimental measurements to determine the phenomenological coefficients in order to parameterize and validate the models. In addition, U-i characteristics of the thermocell under load are determined for different membrane types. Based on the theoretical and experimental findings, a prototype of a closed thermocell system is finally developed.