On the Efficiency of Electrochemical Devices from the Perspective of Endoreversible Thermodynamics

Frantisek Marsik,Bernhard Weigand,Pavel Novotny,Martin Tomas,Ondrej Tucek

doi:10.1515/jnet-2018-0076

Abstract

Abstract The current work presents a concept that deals with the production of entropy generated by non-equilibrium processes in consequence of mass and energy transfer. The often used concept of endoreversible thermodynamics is based on the non-realistic conjecture that the entire entropy production is realized at the system boundary. In this contribution, an open system in a thermodynamically non-equilibrium state is assumed. Production of entropy is generated due to non-equilibrium processes accompanied by energy conversion. The steady state of the system is maintained by a negative entropy flux. The conclusions for expansion energy conversion, i. e., thermal machines, confirm the general outcomes of the endoreversible thermodynamics. However, the presented conclusions related to non-expansion energy conversion offer a new perspective on the principle of minimum entropy production and the corresponding stability conditions at steady state. The analysis of the energy conversion in closed cycles is presented for fuel cells, i. e., non-expansion energy conversion. The efficiency of the energy conversion is maximal at zero power output. Moreover, the efficiency of the fuel cells, and consequently the efficiency of all non-expansion energy conversion processes, depends on the load and then the maximal possible efficiency can be determined.

Highlights

Transformation of energy from one form into another can be generally performed in two ways
The basic assumption is that, at steady state, the system is stable so that all deviations of the state parameters from this steady state must be reduced. It is Prigogine’s theory [7, 8] extended with the conditions to entropy changes at the system boundaries. This approach allows one to suggest a connection between the efficiency, the transport properties, and the electrolytic membrane properties related to electrochemical devices, e. g., fuel cells and electrolyzers
The anode hydrogen flow was set to 0.21 nlpm and the cathode air flow was set to 0.66 nlpm; measurement of the polarization curve for the mentioned constant reactants flows was carried out according to JRC Scientific and Technical Reports Test Module PEFC SC 5-2 Polarisation curve for a PEFC single cell [18]

Summary

Introduction

Transformation of energy from one form into another can be generally performed in two ways. The second possible transformation of energy can be realized by so-called non-expansion, expressed usually via the term Vdp [1] This transformation is applicable in the analysis of hydraulic pumps, by the term Vdp, or fuel cells and batteries, by the term Wel, i. The basic assumption is that, at steady state, the system is stable so that all deviations of the state parameters from this steady state must be reduced In principle, it is Prigogine’s theory [7, 8] extended with the conditions to entropy changes at the system boundaries. It is Prigogine’s theory [7, 8] extended with the conditions to entropy changes at the system boundaries This approach allows one to suggest a connection between the efficiency, the transport properties, and the electrolytic membrane properties related to electrochemical devices, e. The manuscript ends with a summary and some general conclusions

Balance of energy and balance of entropy

Energy conversion through non-expansion work

Fuel cell efficiency

Maximal fuel cell efficiency

Conclusion