Mar 07 2011
Polymer electrolyte membrane fuel cells are gaining great attention as a source of power generation for automotive applications. One key requirement for automotive applications is that the fuel cell must be tolerant of frequent start-stop cycling. A new decay mechanism related to local hydrogen starvation, which may be present during start-stop procedures, has been discovered.
Before the start up of the power plant, air is present on both the anode and cathode due to leakage from outside air and/or crossover through the membrane. When hydrogen is introduced into the anode during start up, a condition is created where hydrogen occupies only part of the anode. This creates a high interfacial potential difference in the region where hydrogen is absent, causing carbon corrosion and oxygen evolution at the cathode electrode. A similar transition can occur during the shut down procedure, when the air, introduced to the anode from the outside or through the membrane, replaces the hydrogen. This mechanism, hereafter referred to as reverse-current, is also possible during operation when localized hydrogen starvation occurs, even for a short time. The performance of PEMFC was also decreased by uneven gas distribution in flow channel.
Potential inversion of one or more cells in a PEMFC stack could drastically reduce its performance or, in the worst case, seriously damage the entire stack, owing to thermal deterioration and perforation of the membrane electrode assembly (MEA). The causes of potential inversion are numerous, including inhomogeneous gas distribution, drying of membranes and flooding of electrodes. However, the symptoms are always the same: the polarization curve of such a malfunctioning cell is very steep and rapidly reaches the short-circuit voltage before the other cells of the stack.
Water condensation in the flow field, which can increase the input/output pressure difference, can prevent homogeneous hydrogen distribution in one or more cells (homogeneous gas supply in a fuel cell stack is very important in keeping the voltage of all cells at a similar value, particularly when the stack is operating at high current). These cells can reach negative voltage, long before other correctly supplied cells. Depending on the duration and degree of negative voltage, the fuel cells can recover or be rendered completely ineffective. The surface area loss of the cathode platinum particle by cell reversal was also detected.
One possible mitigation strategy is to employ system changes and stack design practices that can effectively mitigate this decay, without necessarily making any changes to the electrodes structures (e.g. the development of non-carbon-based catalyst supports, that are more corrosion resistant). These strategies include: (1) minimizing the time that the adverse conditions exist, and/or (2) controlling the potentials during start up and/or shut down using external loads, and/or (3) minimizing the number of adverse cycles that occur in a given application. These solutions are relatively simple to implement and are not overly expensive. Furthermore, they can enable one to meet the performance and durability requirements of almost any application, including those with a relatively large number of idle time and start-stop cycles (e.g. automotive), using existing materials.
Do you think these systems strategies are enough to mitigate the electrocatalyst degradation in PEMFC caused by cell reversal?
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