Anode degradation

Fuel starvation polarization. Humidified anode/cathode feed streams nitrogen/air. 3 bara, 75°C, 4 mg/cm Pt on each of cathode and anode. (From Knights, S.D., et al. 2004. /. Power Sources 127,127-134. With permission.) 

This failure mechanism can have significant impact on the ability of the anode to tolerate adsorbed contaminants. Similar to the impact of carbon corrosion on the cathode, the reduced electrochemically active catalyst surface area becomes very sensitive to the presence of contaminants. This is very important, for example, for operation on reformate where even small amounts of carbon monoxide can result in significant performance loss. 

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The overwhelming majority of alcohol oxidations (including those of carbohydrates) have been run in SSE s of relatively high water contents, often with strong acid present, under constant current conditions 123 Selective oxidation of an alcohol to an aldehyde cannot be accomplished under such conditions instead the carboxylic acid and its degradation product is formed. The cpe approach in SSE s of low water contents should no doubt pay rich devidends in this area. The same applies to the oxidation of secondary alcohols, in which the acid SSE s previously used seem to promote anodic degradation of the ketone formed. 

In former investigations the use of organic solvent was inferior because of the preferential anodic degradation [91]. Therefore, more redox-stable additives, e.g., fluorinated alcohols, were envisioned. Screening of appropriate solvents for the conversion of 37 to 38 excluded simple acids and tert-butanol as potential additives [27]. Primary alcohols with fluorous moieties in [1 position like 41 can be employed. 

Electrophoretic evaluation of the mucous substances of the gastric juice is complicated by the somewhat similar electrophoretic mobilities of serum proteins and their degradation products. Serum albumin has a mobility only slightly more negative than that of mucoprotein in M2 (Fig. 4). The first anodic degradation product of albumin has a mobility identical with that of component M3a (Fig. 4). The second anodic degradation product of serum is localized on the electrophoretic partition very close to component M3b (Fig. 5). Finally, the mobility of y-globulin is identical with that of component XI. 

One way of avoiding an extensive anodic degradation of organic compounds is to use acetone as a modifying medium, which is also well known from chemical oxidations, as with KMn04. Benzoin [33] is thus, in sulfuric acid-acetone containing some water cleaved mainly to benzaldehyde (35%) and benzoic acid (45%) a small amount of benzil (5%) and tar (10%) is also formed. Benzil is apparently not an intermediate, as only benzoic acid (67%) was obtained from oxidation of benzil under similar conditions [33] ... 

The formation of deposits on platinised anodes can cause anode degradation. Thus dissolved impurities present in water which are liable to oxidation to insoluble oxides, namely Mn, Fe, Pb and Sn, can have a detrimental effect on anode life. In the case of MnOj films it has been stated that MnOj may alter the relative proportions of Cl and O produced and thus increase the Pt dissolution rate ". Fe salts may be incorporated into the TiO oxide film and decrease the breakdown potential or form thick sludgy deposits. The latter may limit electrolyte access and iead to the development of localised acidity, at concentrations sufficient to attack the underlying substrate ". 

Ferreira-Aparicio P, Chaparro AM, Gallardo B, Folgado M, Daza L (2009) Anode degradation effects in PEMFC stacks by localized fuel starvation, 2009 fuel cell seminar. Courtesy Associates, Washington... 

Fig. 18.12 Systematic investigation on the anode degradation as functions of current density and water content in fuels with a parameter of electrical potential (From [83, 84])...

The presence of contaminants in both fuel and air results in an additive contamination effect PEMFC performance degradation is the sum of the eathode and the anode degradation. 

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. 

CV is frequently used to determine EPSA changes during fuel cell operation, in order to monitor catalyst surface area decrease due to Pt particle agglomeration or to Pt dissolution or migration into the membrane, and to analyze eleetrode contamination and anode degradation. These uses of CV will be discussed in later sections. 

Lussier A, Sofie S, Dvorak J, Idzerda YU (2008) Mechanism for SOFC anode degradation from hydrogen sulfide exposure. Int J Hydrog Energy 33 3945-3951... 

Madi et al. [42] also investigated the effect of D4 (octamethylcyclotetrasilox-ane) siloxane (at ppb(v) level) on the performance of the SOFC Ni anode. The mechanism of Ni anode degradation was hypothesized. The study was extended to the cell stack. A commercial Ni-YSZ anode-supported SOFC, with the active area of 12.5 cm (TOPSOE Fuel CeU) was used. The cell was heated from room temperature to 800 °C at a rate of 100 °C/h. 

Faes A, Hessler-Wyser A, Presvytes D, Vayenas CG, Van Herle J (2009) Nickel-Zirconia anode degradation and triple phase botmdary quantification from microstmctural analysis. Fuel Cells 9 841-851... 

Madi H, Lanzini A, Diethebn S, PapureUo D, Van Herle J, Lualdi M, Larsen JG, SantareUi M (2015) Solid oxide fuel cell anode degradation by the effect of sHoxanes. J Power Sources 279 460-471... 

Taniguchi et al. found degradation caused by air starvation to be much smaller than under fuel starvation because of the large difference in anode degradation. However, long-term operation (for a few tens of minutes) under the air-starved condition resulted in accelerated degradation in the electrode and in PEMFC performance (Taniguchi, 2008). Therefore, air starvation also has to be avoided to prevent a dramatic decrease in PEMFC performance. 

The performance of H2 SOFCs is limited by cathode performance. In other words, the Faradaic resistance of the current cathode materials is higher than that of the anode and is also higher than ohmic resistance resulting from the electrolyte layer. In contrast, a CH4 SOFC (fed with CH4 or natural gas) is limited by anode performance due to anode degradation by carbon deposition [19]. 

Platinized anodes are fabricated in the form of wire, mesh, rods, tubes, and strips. For soil usage, they are usually embedded in a groundbed of carbonaceous material that provides a high surface area and lowers the anode/earth resistance [11]. Limitations include current attenuation in long sections of wire. Uneven current distribution results in premature localized anode degradation, especially near the connection to a single current feed point. Multiple feed points improve the current distribution and provide system redundancy in the event of excess local anode dissolution. 

A summary of recent studies on the anode versus cathode degradation for PEMFCs is shown in Table 3. All three scenarios, from (1) anode degradation less than cathode degradation, (2) uniform degradation to (3) dominating anode degradation, have been observed by different studies. 

Anode degradation less than cathode 35 (Gore) PtRu/C 0.45 Anode 0.6 Cathode H/air= Ambient 70 100% 800 Cleghorn et al. (2006)... 

To further verify that cathode degradation is faster than anode degradation, a Nafion 101 membrane was laminated on the cathode side of the PSSA membrane (Yu et al. 2003). It was found that membrane stability was significantly increased. Similar results were observed when the Nafion 101 membrane was replaced by a 15- om recast perfluorinated membrane. 

Compared with conventional FTIR, micro-FTIR can offer additional spatial resolution (Endoh et al. 2007). The composition change from the anode to the cathode side can be monitored on membrane cross sections. By monitoring of the carboxylic add concentration (l,690cm ), it was found that anode degradation is more severe than cathode degradation (Fig. 12). 

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