Anode surface poisoning

While poisoning of anodes is rare in view of the strongly oxidizing conditions existing on their surface, poisoning of cathodes is routine in technology. When in-... 

The addition of tungsten by electrochemical deposition has been studied elsewhere [74]. In this study, the lowest amount of carbon monoxide adsorbates were detected together with the largest methanol oxidation currents with a surface coverage of 15%. Since the total anodic charge density for methanol oxidation was less than that obtained on bare platinum, this indicates that the surface does not have a tendency to create surface poisons. 

Abstract Direct formic acid fuel cells offer an alternative power source for portable power devices. They are currently limited by unsustainable anode catalyst activity, due to accumulation of reaction intermediate surface poisons. Advanced electrocatalysts are sought to exclusively promote the direct dehydrogenation pathway. Combination and structure of bimetallic catalysts have been found to enhance the direct pathway by either an electronic or steric mechanism that promotes formic acid adsorption to the catalyst surface in the CH-down orientation. Catalyst supports have been shown to favorably impact activity through either enhanced dispersion, electronic, or atomic structure effects. 

However, the main problem of electrooxidation at a fixed anodic potential before oxygen evolution is a decrease in the catalytic activity, commonly called the poisoning effect, due to the formation of a polymer layer on the anode surface. This deactivation, which depends on the adsorption properties of the anode surface and the concentration and the nature of the organic compounds, is more accentuated in the... 

The tolerance for H2S is usually <0.1 ppm. H2S will decompose on the anode surface and poison the Pt, an effect, which can be irreversible at high potential or with high S concentration when Pt-S is formed (thermodynamically at very low potential already). H2S can migrate to the cathode and form Pt-S there as well. The only way to remove Pt-S is to oxidize it at very high potential (1.2 V vs NHE). Pt-based catalysts promoting S oxidation at low temperature are not yet known [100]. 

H2S reacting with nickel can block electrochemically active sites for the hydrogen oxidation, can change the wettability of the anode toward carbonates, can modify the anode surface and its porous structure, can alter the anode conductivity, can change the carbonate conversion to sulfate and can poison catalytic sites for the water gas shift reaction. 

H2S causes poisoning of the Ni-anode surface by formation of NiS which kineti-cally hinders the oxidation of hydrogen [3, 7, 18]. It may react chemically to form NiS (reaction 5.14), or electrochemically as sulfide in the electrolyte (reaction 5.15), with the nickel anode to form nickel sulfide. 

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. 

Anode Investigations using cyclovoltammetry confirm an important effect of surface oxides (see Vols. 3, 4). A known example of the different anodic activity is the poisoning of platinum by adsorbed carbon monoxide species, for example, in the direct methanol fuel cell (DMFC),... 

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