Numerical Simulation of a PEM Fuel Cell: Electrochemical Double Layer Capacity

PABLO D. GIUNTA; FEDERICO J. NORES PONDAL; NICOLÁS FORTUNATO

Buenos Aires

Conferencia; 20th Topical Meeting of the International Society of Electrochemistry; 2017

In this work the surface charge of the cathode of a PEM fuel cell was calculated by means of a one-dimensional and stationary model based on first principles. It was found that, for an increasing current sweep, the surface electric charge raises, passes through a maximum and then decreases. This behavior is based on the increase of the capacity of the double electrochemical layer, due to the increasing adsorption of water dipoles on the catalytic surface that in turn raises the equivalent electrical permittivity of the medium.The electrochemical double layer is described by the Poisson-Nernst-Planck model for the diffuse layer and a water adsorption model (dipoles oriented towards the electrode or towards the electrolyte) on the catalytic surface. The kinetics is described by the Butler-Volmer equation, for the dependence of the reaction rate with the potential difference of the compact layer.It was observed that, as the current increases (from equilibrium at zero current, to the limiting current), the surface charge (per unit area) of the cathode presents a maximum for low currents, close to equilibrium.As the current increases, the electrical potential of the cathode descends monotonously, forming the characteristic polarization curve. It is then expected that the surface electric charge will also decrease with the increase in the current flowing through the cell. However, this discharge of the electrode is observed after a maximum that occurs to currents close to the equilibrium. For low currents, then, as the surface charge raises and the potential difference decreases, the system capacity must increase.The physical basis of this behavior observed at low currents is found in the growing occupation of the active sites of the catalyst by water molecules. The water molecules adsorbed on the catalytic surface modify the capacity of the double electrochemical layer: the greater the number of active sites the water occupies, the greater the total electrical equivalent permittivity of the medium and therefore the greater the electric charge accumulates at the electrode-electrolyte interface (Eq. 1). (1)where the permittivity of the compact layer (formed by the species adsorbed on the catalytic surface) is constant. The second term of the equation represents the additional potential drop due to the dipolar nature of the water molecules. The water fraction at the surface is proportional to the variable X. Therefore, the equivalent electrical permittivity of the compact layer is increased by water adsorption.The pronounced augmentation in the amount of water molecules that occupy the active sites of the catalytic surface as the current raises, causes an increase in the capacity of the double electrochemical layer, increasing the electric charge that accumulates at the electrode-electrolyte interface as the current raises.