Degradation reverse current cell

The described experiments and simulations support Reiser et al. s theory of the RCD mechanism. This mechanism occurs during start-up and shutd-own of the cell provided that the anode is not completely filled with hydrogen or air. Carbon and water are oxidized in the cathode catalyst layer, resulting in a reverse current and cell degradation. 

A small amount of sulfur in the fuel dramatically degrades the performance of Ni-YSZ anodes due to the adsorption of sulfur on Ni surfaces. The extent of sulfur poisoning, as measured by the relative increase in cell resistance, always increases with H2S concentration in the fuel, but decreases with cell operating temperature and cell current density. Sulfur poisoning of Ni-based anode is generally more reversible as the cell temperature increases and as H2S concentration or exposure time is reduced. 

It appears that the ADP-ribosylation of arginine residues is a reversible modification of protein (8-10). Enzymatic activities have been found that catalyze the degradation of ADP-ribosylarginine or ADP-ribosyl (arginine) protein with release of the free amino acid or protein (8-10). Such activities have been found in avian, bovine, porcine and human cells or tissues (8-11, data not shown). To understand the mechanism of action of these enzymes, currently termed ADP-ribosylarginine hydrolases, they have been extensively purified from turkey erythrocytes and characterized with regard to their kinetic and physical properties. 

Measurement of the single cell voltages in a stack allows the detection of cell reversals due to overall hydrogen or oxygen starvation. As soon as the minimum cell voltage falls below a given lower limit, it is possible to prevent cell degradation by reduction of the cell current. 

Different channels of the cell can be cmisid-ered as placed in parallel, both concerning the gas flow and the electrical current flow. Thus, the complete absence of flow in one section of the cell will not lead to cell reversal, but to the absence of current production in this section. Nevertheless, long-term degradation effects can be expected, in particular in the absence of hydrogen. 

Figure 17.7. The change in current-voltage performance of a PEMFC by the cell voltage reversal experiment (a) before experiment (b) after experiment for 3 min and (c) after experiment for 7 min [19]. (Reprinted from Journal of Power Sources, 130(1-2), Taniguchi A, Akita T, Yasuda K, Miyazaki Y, Analysis of electrocatalyst degradation in PEMFC caused by cell reversal during fuel starvation, 42-9, 2004, with permission fi om Elsevier.)...

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