Partial fuel starvation

Schematic of the fuel cell environment and electrochemical potentials during a partial fuel starvation. (From Lauritzen, M.V., et al. 2007. /. New Mat. Electrochem. Sys. 10 143-145. With permission.

Failure modes studied included fuel starvation and cell reversal, partial fuel starvation, carbon corrosion, membrane thinning and hole formation oxidant starvation excessive drying and flooding, air and fuel contamination, freeze-start and freeze-thaw degradation. 

In addition to the increased water content of the electrodes, the membrane gas cross-over is also increased at a high membrane water content. This may accelerate electrode degradation mechanisms such as carbon corrosion due to partial fuel starvation, which is influenced by the cross-over of oxygen to the anode (see Section 6.4.2). 

If partial fuel starvation (see Section 6.4.2) occurs under idle operation, for example due to water flooding issues, the corrosive conditions on the cathode will be maximized due to the higher cell voltage. 

Figure 4b shows an AB-VDR chart from endurance tests using a lOOppm CO reformate. It shows a distinct minimum near 10% AB. The minimum FRR is between 9 and 10% (see Table 1). At 12% AB, the drastic increase in VDR is attributed to the stoichiometry effect the actual anode stoichiometry is approximately 1.09, significantly lower than the 1.2 set point because of significant consumption by the excess O from the AB. At an anode stoichiometry of 1.09, the cell is at the edge of fuel starvation. Indeed, the cell tripped multiple times because of low cell voltage as a result of partial fuel starvation. 

Various parameters have been identified that influence carbon corrosion, including humidity, with higher humidity enhancing corrosion [94], reactant partial pressure, fuel starvation, and also the carbon characteristics, including surface area and degree of graphitization [94]. As expected, both potential and variations in potential have also a strong influence on carbon corrosion [95]. 

Dttring high overall stack reactarrt rttifizatiorr, uneven flow sharing between cells can result in partial fuel artd/or air starvation conditions in individrral cells. This situation can be exacerbated by the presence of Uqtrid or frozen water in chaimels or other blockages, resulting in further flow-sharing difficrrlties that, in extreme cases, can lead to complete starvation conditions. 

Partial, or local, fuel starvation or the presence of both hydrogen and oxygen simultaneously on the anode. 

Carbon corrosion can also arise from a nonuniform distribution of fuel on the anode side (partial hydrogen coverage) and from crossover of reactant gas through the membrane. Local fuel starvations can cause this type of carbon corrosion. Because of its complexity and consequence to the durability of the fuel cell catalyst layer, local fuel starvation is both a widely studied and researched phenomena. 

Sun et al. (2007) and Hottinen et al. (2003) studied the humidity conditions and the effects of humidity on local fuel starvation and learned that when the fuel cell membrane was under-humidified the current densities increased monotonicaUy across the flow channel due to the progressive hydration of the membrane and increased proton conductivity of the Nafion membrane. In a cell operating with an over-hydrated membrane, the current distribution decreased monotonicaUy due to increased partial pressure and decreased as water content increased in the catalyst layer due to blockage of active sites or water flooding. Thus, water content within the membrane strongly affects the current distribution within the fuel cell and can aggravate localized fuel starvation. 

At CO concentrations above lOOppm, the hydrogen flammabihty limit and fuel starvation (stoichiometry) limit the AB selection. Under these circumstances, the maximum AB allowed can only partially mitigate the poisoning effect of CO. This leads to unstable cell performance, elevated FRR, and, in turn, shortened cell life expectancy (Baldwin et al. 1990). 

Air from the compressor is split into two streams primary air is premixed with the fuel and then fed to the catalyst, which is operated under O2 defect conditions secondary air is used first as a catalyst cooling stream and then mixed with the partially converted stream from the catalyst in a downstream homogeneous section where ignition of gas-phase combustion occurs and complete fuel burnout is readily achieved. The control of the catalyst temperature below 1000 Cis achieved by means of O2 starvation to the catalyst surface, which leads to the release of reaction heat controlled by the mass transfer rate of O2 in the fuel-rich stream and of backside cooling of the catalyst with secondary air. To handle both processes, a catalyst/heat exchanger module has been developed, which consists of a bundle of small tubes externally coated with an active catalyst layer, with cooling air and fuel-rich stream flowing in the tube and in the shell side, respectively [24]. 

Phase 5, Prolonged Starvation In the later part of phase 4 and in phase five, plasma levels of ketone bodies increase and ketone bodies partially replace glucose as a fuel supply for the brain. This, in turn, decreases the demand for hepatic gluconeogenesis and acts to conserve muscle protein. 

The cumulative effect of an anodic ORR (induced by O2 crossover from the cathode to the anode) on cathode carbon catalyst-support loss (C corrosion) under fuel starved and ordinary (e.g. constant current) PEMFC operating eonditions were studied.The effects of C corrosion at constant cmrent are less severe than start-up and fiill H2 partial starvation, but they are large enough to affeet the cell performance after a long-time operation. The design factors of the MEA and operational factors such as humidity and temperature also affect carbon loss. The influence of these parameters is not always simple, and the coupling of these factors was addressed with MEMEPhy s to elucidate the rate of carbon loss under normal PEMFC operation... 

Two hydrogen-deficient conditions are possible one is partial hydrogen coverage that damages the air electrode and the other is fnel starvation that damages the fuel electrode. These two conditions can be distingnished by the relationship between fuel flow and current. Specifically, if... 

---