Localized hydrogen starvation

The ruthenium complex is such an exceedingly active catalyst that, at higher catalyst concentrations, orthomet-alation of the catalyst by the alkene present may occur if local hydrogen starvation is encountered, (eqna-tion 46). 

Local hydrogen starvation not only occurs at operation close to A,a = 1 as shown above, but it can also be caused by local blockage of the gas pathways at the anode side. Accumulation of Hquid water within the diffusion media or a foreign object in the gas channel can prevent hydrogen from being evenly distributed over the entire catalyst layer during operation of a fuel ceU [89]. In order to analyze experimentaUy local blocking of the anode under weU-defined conditions, Liu et al. 

Figure 20.7 Gas and potential distribution at local hydrogen starvation resulting in the reverse current decay mechanism.

Local hydrogen starvation is usually observed in the cell before cell reversal occurs. By measurement of the CO2 concentration in the cathode exhaust gas, carbon corrosion can be detected. However, it is extremely difficult to distinguish between CO2 evolution within the cell and its amount present in air due to low concentrations. Alternatively, critical conditions can be detected by integration of a reference electrode (see Section 20.2.1). Since the reference electrode has to be positioned within the starved region, several reference electrodes have to be used per cell in order to make detection of the starved region probable. This can only be realized in a laboratory cell and is not practical for automotive stacks. 

Localized hydrogen starvation at a PEFCs anode can lead to the formation of local cells as presented on the Fig. 3a and takes place mainly during the start-stop cycling when a H2/O2 front is moving through the anode compartment of the cell [65]. In short, due to the presence of both H2 and O2 in the anode of a fuel ceU, the cell becomes internally shorted with the result of a cathodic potential excursion to approx. 1.5 V [67]. 

The consequences of carbon corrosion at the cathode due to local hydrogen starvation have been confirmed with start-stop experiments and with experiments involving the deliberate blocking of hydrogen flow field channels. The main cause for local hydrogen starvation in real PEFCs is associated to water droplet formation inside flow field channels. 

Fig. 3 (a) Reverse current decay mechanism at local hydrogen starvation in the rear part of a PEM cell (Reproduced by permission from [9]). (b) Effect of gross hydrogen starvation on the half-cell potential at P the anode side (Reproduced by permission from [10])... 

Under normal fuel cell operating conditions, the highest oxidative potentials in the cathode range between 0.6 V (vs. RHE) at high-current density and 0.95 V (vs. RHE) at open circuit (the anode potential remains always near 0 V vs. RHE), so that carbon-support corrosion is negligible. However, under start/stop conditions or in the case of localized hydrogen starvation, the cathode potential significantly exceeds 1 V versus RHE and the associated rapid carbon-support corrosion leads to... 

Fig. 11.8 SEM micrographs of freeze fractured sections of the cathode electrodes of MEAs. Left nondegraded MEA right MEA aged by localized hydrogen starvation. SEM analysis was done without mounting of the MEAs in epoxy (Reproduced from R.N. Carter et al. [23] by permission from John Wiley Sons)...

The negative effect of cell reversal under local hydrogen starvation can be mitigated by using a thicker membrane. The equivalent current density of oxygen crossover, through the membrane, is inversely proportional to the membrane thickness. Thus, larger Ipem lowers the carbon corrosion current density and increases the cell lifetime. 

Gu, W., Makharia, R., Yu, P.T, and Gasteiger, H.A. (2006) Prediction of Local Hydrogen Starvation in a PEM Fuel Cell Origin and Materials Impact, American Chemictil Society 232nd National Meeting, Div. Fuel Chem. Fuel Chemistry Preprints Vol. 52/No. 2, San Francisco, CA. 

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