Electrocatalytic fuel oxidation

The electrodes must have a high electrocatalytic activity for fuel oxidation or oxidant reduction, a high electrical conductivity (electronic or mixed ionic and electronic), and must be chemically stable to reduction or oxidation, and gas permeable. 

Empiricism in early catalytic studies had shown that combining two or more metals could alter catalyst activity and stability. Electronic and structural interactions between the catalyst components were thought to be responsible for the observed changes by modifying the reactant strength of adsorption (7/5, 270). Electrocatalytic research exploited this idea for fuel oxidations and fundamental studies of electrocatalytic activity (78, 79, 80, 113, 271-274). Primarily, clusters of group VIII and group IB metals have... 

The archetypical direct fuel cell is the DMFC. like the PEMFC, the DMFC uses a proton-conducting membrane to separate the anode and cathode, and protons liberated during electrocatalytic methanol oxidation [Eq. (15.6)] at the anode are involved in oxygen reduction at the cathode. However, whereas in the hydrogen fuel cell the anode reaction is straightforward, the methanol oxidation is comparably sluggish, which is mainly attributed to poisoning effects. 

In addition to improvements in the intrinsic electrocatalytic activity, research has been carried out to modify ( design , possibly) the catalyst surface in order to enhance the catalyst utilization efficiency and ultimately the fuel oxidation superficial current density. The latter can be expressed in terms of catalyst layer physico-chemical properties as [218] ... 

This permits efficient SOFC operation with CH4, and also natural gas, as the fuel. Direct electrocatalytic CH4 oxidation at the tpb is too slow for practical SOFC units with any known electrocatalyst, but, due to the catalytic properties of the Ni surface, CH4 is converted to COj and H2 via Reactions (13.4) and (13.5) and the latter is then easily oxidized electrocatalytically at the tpb. 

Qi, J., Xin, L., Chadderdon, D.J., Qiu, Y., Jiang, Y., Benipal, N., Liang, C.H., and Li, W.Z. (2014) Electrocatalytic selective oxidation of glycerol to tartronate on Au/ C anode catalysts in anion exchange membrane fuel cells with electricity cogeneration. Applied Catalysis B Environmental, 154, 360-368. 

Table 3.1 lists some of the anodic reactions which have been studied so far in small cogenerative solid oxide fuel cells. A more detailed recent review has been written by Stoukides46 One simple and interesting rule which has emerged from these studies is that the selection of the anodic electrocatalyst for a selective electrocatalytic oxidation can be based on the heterogeneous catalytic literature for the corresponding selective catalytic oxidation. Thus the selectivity of Pt and Pt-Rh alloy electrocatalysts for the anodic NH3 oxidation to NO turns out to be comparable (>95%) with the... 

C.G. Vayenas, Catalytic and Electrocatalytic Reactions in Solid Oxide Fuel Cells, Solid State Ionics 28-30, 1521-1539 (1988). 

The electrocatalytic oxidation of methanol has been widely investigated for exploitation in the so-called direct methanol fuel cell (DMFC). The most likely type of DMFC to be commercialized in the near future seems to be the polymer electrolyte membrane DMFC using proton exchange membrane, a special form of low-temperature fuel cell based on PEM technology. In this cell, methanol (a liquid fuel available at low cost, easily handled, stored, and transported) is dissolved in an acid electrolyte and burned directly by air to carbon dioxide. The prominence of the DMFCs with respect to safety, simple device fabrication, and low cost has rendered them promising candidates for applications ranging from portable power sources to secondary cells for prospective electric vehicles. Notwithstanding, DMFCs were... 

Interest in fuel cells has stimulated many investigations into the detailed mechanisms of the electrocatalytic oxidation of small organic molecules such as methanol, formaldehyde, formic acid, etc. The major problem using platinum group metals is the rapid build up of a strongly adsorbed species which efficiently poisons the electrodes. 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

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