Starvation, fuel

More current can be sustained by the electrolysis reaction if more water is available at the anode. However, if not consumed in the electrolysis of water, current is instead used in the corrosion of the anode components. If the supply of water at the anode runs out, the anode potential rises further and the corrosion rate of the anode components increases. Thus, there is preferably an ample supply of water at the anode in order to prevent degradation of the anode components during reversal. 

3 Electrocatalyst Degradation in PEM Fuel Cells Caused by Cell Voltage Reversal During Fuel Starvation 

Surface area loss of eathode platinum due to catalyst agglomeration was also detected by transmission electron microscopy (TEM) analysis and cyclic voltammetry. The behaviors of both eleetrodes were measured and anode degradation eould be attributed to the high anode potential. Fuel starvation eaused severe and permanent damage to the eleetroeatalysts of the PEMFC. 

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According to the different exchange current densities, i0, for hydrogen oxidation and hydrogen evolution on Ni and Pt, the catalytic activity of platinum is by a factor of several hundred to a thousand higher than that of nickel. Therefore, if the utilization of Raney-nickel particles below 10 jum size approaches 100%, it is clear that Pt-activated porous soot particles must be by a factor of from 10 to 30 smaller than Raney-nickel particles to achieve full utilization, that is, vanishing fuel starvation of the catalyst. This happens to be the case with soot agglomerates that are by their very nature of correct size (dv < 0.1 /im) (150, 151). 

A current operational problem is damaging anode voltage reversal following fuel starvation. A flow of patents covers the topic (Knights etal., 2000 2001a Colbow etal., 2001 Taylor etal., 2001). The problem, relevant to both the PEFC and the DMFC, and to all manufacturers, is discussed by Johnson Matthey in Ralph etal. (2003). 

Substantial carbon corrosion occurs in a PEM fuel cell when a reverse current is imposed on one of its electrodes. This can happen at the anode during fuel starvation, in which a reverse current is imposed by either an electric load or normal fuel cells adjacent to a starving cell. It can also happen on the cathode during a fuel cell start-up or shutdown, in which a fuel-air front is formed on the... 

Many start-up/shutdown procedures have aimed to reduce the corrosion current through practices such as anode purging and shunting. Others have attempted to nse more stable carbon materials such as graphitized carbon and carbon nanotnbes (CNTs). - To delay the onset of the COR during fuel starvation, WOR catalysts have been incorporated into the anode electrodes. Increasing anode ionomer content has also been recommended for a fnel starvation-resistant anode because it increases the amount of water available for the WOR. - In general, the materials approaches are applicable to all kinds of carbon corrosion. 

Taniguchi, A. et ah. Analysis of electrocatalyst degradation in PEMFC caused by cell reversal during fuel starvation, J. Power Sources, 130, 42, 2004. 

Patterson, T.W. and Darling, R.M., Damage to the cathode catalyst of a PEM fuel cell caused by localized fuel starvation, Electrochem. Solid-State Lett., 9, A183, 2006. 

As diesel ages a fine sediment and gum forms in the fuel brought about by the reaction of diesel components with oxygen from the air. The fine sediment and gum will block fuel filters, leading to fuel starvation and engine failure. Frequent filter changes are then... 

Another critical mechanism of electrochemical activity loss is due to carbon corrosion. Carbon corrosion has been cited as a major concern in higher temperature environments, such as phosphoric acid fuel cells [40] but is relatively benign at potentials less than IV RHE at the temperatures at which PEM fuel cells normally operate. Several papers have noted that in the case of gross fuel starvation, cell voltages can become negative, as the anode is elevated to very positive potentials, and the carbon is consumed instead of the absent fuel [41]. 

A recent paper by Reiser et al. [42] suggests, however, that transient conditions or localized fuel starvation can induce local potentials on the air electrode significantly higher than IV and thereby induee eorrosion of the carbon supports that results in permanent loss of eleetroehemieally aetive area. The mechanism they describe suggests that the highly conductive bipolar plates of the fuel cell allow for sufficient redistribution of current in the plane of the current collectors and that all regions of the cell experience the same potential difference. 

Perry ML, Patterson TW, Reiser C (2006) Systems strategies to mitigate carbon corrosion in fuel cells durability— fuel starvation and start/stop degradation. ECS Trans 3 783-795... 

Baumgartner WR, Wallnofer E, Schaffer T, Besenhard JO, Hacker V, Peinecke V, Prenninger P (2006) Electrocatalytic corrosion of carbon support in PEMFC at fuel starvation. ECS Trans 3 811-825... 

The activity loss of a operating fuel cell is due to different reasons, including the called operational effects that include exposure to impurities, exposure to and startup from subfreezing conditions, and other operating conditions as potential cycling, fuel starvation, start/stop cycling, and changes in temperature and/or relative humidity. 

Cell reversal during operation with fuel starvation Air-air start-up, platinum crystallite precipitation... 

Overused fuel, U > 90% (fuel starvation and permanent damages to cells). 

One of the issues associated with the use of carbon materials for catalyst supports is that they can corrode. Although the rate of electrochemically driven carbon corrosion (with an equilibrium potential of 0.207 V vs. SHE) is very low under normal fuel-ceU operating conditions (the exchange current density is of the order of 10 A cm [52]), significant rates of corrosion can occur under certain higher potential conditions relevant to fuel-ceU operation, such as during start-up and shut-down [53] and anode fuel starvation. A comparison of carbon corrosion rates of various commercial carbons has been reported by Yu et al. [54], which corroborates that this is a significant problem need resolution. 

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]. 

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