Oscillations of PEM fuel cells at low cathode humidification

Abstract

a b s t r a c tOscillatory fluctuations of a single polymer electrolyte fuel cell appear upon operation with a dry cathode air supply and a fully humidified anode stream. Periodic transitions between a low and a high current operation point of the oscillating state are observed. The transition time of 20-25 s for the change from the low to the high operation is fast and does not depend on the operating parameters. Contrasting with this behavior, the downward transition depends strongly on the operating conditions. Impedance measurements indicate a high ionic resistance with low water content for the low current operation and a low ionic resistance of the membrane with high water content for the high current operation. An insight into the transitions is obtained by current density distributions at distinct times indicating a propagating active area with defined boundaries. The observations are in agreement with assuming a liquid water reservoir at the anode with a downward transition period depending on the operation conditions. The high current operation possesses a high electro-osmotic drag and a high permeation rate (corresponding to liquid-vapor permeation) leading to a large water flux to the cathode. Subsequently, the liquid reservoir at the anode is consumed leading to an anode drying. The system establishes a new quasi-stable operation point associated with a low current, low electro-osmotic drag coefficient, and a low water permeation (corresponding to vapor-vapor permeation). When liquid water is formed at the anode interface after some time the fast transition to the high current operation occurs. This interpretation is supported by conductive atomic force microscopy current images of the membrane showing a strong dependence of the ionic conductivity on the activation procedures with or without liquid water and also showing oscillatory behavior after the membrane is activated. Specifically, activation with liquid water yields a high conductivity with currents larger by three orders of magnitude.

Effect of open circuit voltage on degradation of a short proton exchange membrane fuel cell stack with bilayer membrane configurations

“…18, current measurement performed in contact mode for Cell 3 shows high conductivity, but still exposes non-conductive areas in the 1.5 m image on the left side. The conductivity images are in principle similar to the new membrane samples reported in various previous studies [21][22][23][24]. However, the conductivity areas are large here and the conductivity is higher.…”

Nonlinear dynamics of fuel cells: a review

“…Oscillatory behavior of PEMFCs is observed in several experimental studies (Atkins et al 2004, Kulikovsky et al 2004a, Benziger et al 2007a, Sanchez et al 2010. Atkins et al (2004) measure autonomous oscillations under potentiostatic operation of a PEMFC, when the feed stream humidifi cation is reduced (see Figure 29 ).…”

“…2. Recently, Sanchez et al (2010) studied oscillations under potentiostatic conditions in detailed experimental investigations. They also attribute the oscillatory behavior to the humidity-dependent conductivity of the membrane in combination with the water transport, but, in addition, consider the presence of liquid water on the anode side as essential for the oscillations in their experiments.…”

Proton Conductive Areas on Sulfonated Poly(Arylene Ketone) Multiblock Copolymer Electrolyte Membrane Studied by Current-Sensing Atomic Force Microscopy

“…[21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] This current-sensing AFM (CS-AFM) technique has been used on Nafion [21][22][23][24][25][26][27][28][29][30][31][32]37 as well as HC-based membranes. [34][35][36] Several groups reported that the distribution of the proton conductive domains was correlated to the hydration level of the membranes.…”