Modelling and Numerical Simulations of a Membrane Electrode Assembly for Fuel Cell Applications

Modelling and Numerical Simulations of a Membrane Electrode Assembly for Fuel Cell Applications: A hydrogen crossover study based on Non-Equilibrium Thermodynamics

Ripepi, D.

Koper, G.J.M. (mentor)

Applied Sciences

ChemE/Chemical Engineering

Sustainable Energy Technology

2017-06-28

In this study a one-dimensional, steady-state, non-isothermal numerical model was developed in order to investigate the transport phenomena occurring in a membrane electrode assembly for fuel cell applications and to provide an insight on the effects of the hydrogen crossover. The hydrogen that permeates through the membrane is consumed without generation of useful work. Moreover, the effects caused by the hydrogen crossover are so far unclear. This complex phenomenon is usually neglected in fuel cell modelling and the results of experimental measurements are often not in agreement. The proposed model consists of a set of transport equations, based on non-equilibrium thermodynamic. The latter derives from irreversible thermodynamics and it provides a systematic way to study heterogeneous systems, like fuel cells. The numerical results are thermodynamically consistent with the second law of thermodynamics. Initially the isothermal boundary condition was applied to the system in order to first study the coupled charge and mass transport, including the hydrogen crossover. However, it was found out that the thermal effects are not negligible in the simulated system and thus the constant temperature assumption was removed. The model described with the NET method provides useful information about the driving forces profiles (total pressure, hydrogen partial pressure, electric potential and temperature) and entropy production. This allows a more accurate estimation of non-constant parameters and also a more reliable prediction on the hydrogen crossover flux over a wide range of operating conditions. An optimization procedure was carried out showing that even though the set of optimal parameters produced an increment of almost four times in crossover, the power output increased without being affected by the variations hydrogen permeation. Although the results of the simulations show that hydrogen crossover has no significant effects on fuel cell performance in term of power output, the adopted method of non-equilibrium thermodynamics permitted to identify that part of the reduction in cell voltage in near open circuit conditions is attributable to the hydrogen crossover. The effects of hydrogen permeation are only noticeable at very low current densities, so when the crossover flux is comparable to the hydrogen reacting at the catalyst layer. A final analysis was carried out with increased permeability (ten, fifty and a hundred time). This verified that considerable hydrogen crossover flux lead to effects on net heat flux, entropy production, temperature distribution and power output.

http://resolver.tudelft.nl/uuid:bb1ee32c-9daf-4c4d-80f9-6937862c3637

Student theses

master thesis

(c) 2017 Ripepi, D.