Water-gas shift reaction electrochemical

As evident from Eqns (15.8) and (15.9), when electrons are produced in an electrochemical reaction (i.e., Vpe > 0) as in the electrochemical water-gas shift reaction (eWGSR), the forward rate constant increases, the reverse rate constant decreases, and the equilibrium constant, being the ratio of the two, increases dramatically with an increase in potential. As a result, the equilibrium CO levels for eWGSR are lower than those for the conventional (thermal) WGS reaction. In other words, even if WGS catalysts could be found that are adequately active at lower temperatures, the thermodynamics of eWGSR are far more favorable in achieving lower levels of CO in H2. 

Figure 15.19 A schematic of the electrochemical water gas—shift reaction (eWGSR), in which A = anode, E = electrol3he, and C = cathode.

G) In reality, CO with H20 shifts H2 and C02, and CH4 with H20 reforms to H2 and CO faster than reaction as a fuel at the electrode. CO is a poison for lower temperature fuel cells, but is used as a fuel in the high-temperature cells (e.g., SOFC, MCFC). CO may not actually react electrochemically within these cells. It is commonly understood that CO is consumed in the gas phase through the water-gas shift reaction as CO + H20 = C02 + H2. The H2 formed in this reaction is subsequently consumed electrochemically. 

CO is not directly used by electrochemical oxidation, but produces additional H2 when combined with water in the water gas shift reaction. 

Note that CO, when presents is not directly used by the electrochemical oxidation, but rather produces additional H2 by the water gas shift reaction CO + H2O H2 + CO2. A fuel such as natural gas can also be used in MCFCs. However, it has to be processed either externally or within the cell in the presence of a suitable catalyst to form H2 and CO by the reaction CH4 + H2O < 3H2 + CO (and when CO + H2O - H2 + CO2). 

Li, Metal-semiconductor catalyst Photocatalytic and electrochemical behavior of Pt-Ti02 for the water-gas shift reaction./. Mol. Catal. 1983, 23, 275-289. 

Also corrosion problems of the carbon support have been considered as a cause of electrocatalyst durabihty loss [32], in particular carbon oxidation can occur through electrochemical oxidation at the cathode, with formation of CO2 (C -I- 2H2O = CO2 -I- 4H -F 4e ), or through water gas shift reaction, with the production of CO (C H2O = CO H2). Both these routes are catalyzed by Pt [56, 57] and subtract caibon useful for platinum loading, with consequent metal sintering and decrease of the electrochemical surface area [58]. 

C. Yixuan, W. Zhaobin, C. Y. Anxin, L. Huaxin, H. Zupei, L. Huiqing, D. Yonglei, Y. Chunying, L. Wenzhao, Metal-semiconductor catalyst photocatalytic and electrochemical behavior of Pt-Ti02 for the water-gas shift reaction, J. Mol. Catal. 21 (1983) 275-289. 

The formed hydrogen by the water gas shift reaction can be electrochenticaUy oxidized in the fuel cell to water, electrical energy and heat. In solid oxide fuel cells the product water of the electrochemical oxidation of hydrogen is formed oti the anode site. This product water is available for the water gas shift reactimi on the anode side. At 650 to 850 °C reaction kinetics allows the water gas shift reaction without any catalyst or promoter. So carbon monoxide can be converted directly rai the anode side of the SOFC without any extra catalyst for promoting the water gas shift reaction. No extra converter is needed for the water gas shift reaction in SOFC fuel-cell heating appliances, which reduces the system effort. 

Another solution is to store hydrogen by forming chemical bonds. This can be in the form of metal hydrides, such as MgH2, LaNisHs, and LiBELj, or by using hydrogen and CO2 to make chemical fuels. The latter is a much easier route than the direct photochemical or electrochemical activation of CO2. For example, CO2 and hydrogen can be converted into CO via the slightly endothermic reverse water-gas shift reaction ... 

Although much less significant than the catalyst poisoning by CO, anode performance is adversely affected by the reaction of CO2 with adsorbed hydrides on platinum. This reaction is the electrochemical equivalent of the water gas shift reaction. 

MCFCs operate more efficiently with CO2 containing bio-fuel derived gases. Performance loss on the anode due to fuel dilution is compensated by cathode side performance enhancement resulting from CO2 emichment. CO is not directly used by electrochemical oxidation, but produces additional H2 when combined with water in the water gas shift reaction. 

All of the above-mentioned solid electrolytes are oxygen conductors. An automatic consequence of this is that, as in molten carbonate fuel cells, the products of electrochemical reactions all end up on the anode side. While is beneficial for internal reforming and water gas shift reaction (which utilizes the water produced as a reactant), it dilutes the fuel, and at high utilization it can significantly reduce the Nernst potential. 

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