A promising measure to increase the efficiency of energy conversion is the use of low temperature waste heat being readily available in a vast amount. Thermogalvanic cells (TGC) offer the possibility of electrical energy harvesting from such thermal energy. An externally applied heat source leads to a temperature gradient across the electrolyte inducing a gradient in the chemical potential and a difference in the electrical potential between the electrodes. Due to the aim of approaching a homogeneous equilibrium different coupled molecular transport mechanisms are taking place in the electrolyte impacting the performance of the TGC. In contrast to the common approach of using classical transport correlations such as Fourier’s, Ohm’s or Fick’s law to describe these coupled processes this work applies the theory of non-equilibrium thermodynamics (NET). The method considers all interactions of the driving forces and resulting fluxes occurring in the electrolyte and clearly separates the transport phenomena into their origin. The knowledge of these relations, which can be described by characteristic phenomenological coefficients, gives a better understanding of the TGCs internal processes and the opportunity to determine the entropy production aligned with local heat sources inside its electrolyte. In this work a TGC based on a polymer electrolyte membrane with two gaseous hydrogen electrodes of different temperatures was investigated experimentally. Due to the need of water in the electrolyte to ensure ionic transport the TGC was fed with a humidified hydrogen gas. The experimental setup gives the opportunity of precisely adjusting the temperature and composition of the species at both electrodes thus control the gradients in temperature, concentration i.e. chemical potential and electric potential between the electrodes. By suppressing all but one driving force across the electrolyte and measuring the open circuit voltage (OCV) different coupling coefficients as well as the occurring fluxes in the electrolyte were determined for a wide variety of working conditions. By implementing the measured phenomenological coefficients in an earlier published model of the TGC considered in this work a significantly lower deviation of the modeled and measured OCV values compared to the approach of using classical transport coefficients can be determined. This indicates the importance of applying the NET when considering transport phenomena while different gradients being present. We also show that by an aligned combination of the gradients in temperature and chemical potential of the water their impacts on the electrical potential can sum up leading to a higher OCV or even cause a pole reversal due to contrary effects on the electrical potential difference. Figure 1