Cell, electro-oxidation

Considering as an example a DEFC, electro-oxidation of ethanol takes place at the anode (negative pole of the cell),... 

At this stage, it should be pointed out that modihcation of a Pt-Sn catalyst by Ru atoms increases cell performance (and hence catalytic activity with regard to ethanol electro-oxidation), but has no effect on the OCV or on product distribution [Rousseau et al., 2006]. It seems, then, that the oxidation mechanism is the same on Pt-Sn and Pt-Sn-Ru, which supports the proposition that Ru allows OH species to be produced when the anode potential is increased and noncatalytically active tin oxides are formed. 

Figure 14.9 CO bulk electro-oxidation at bare Ru(OOOl) in flow cell dotted line, CO fi ee electrolyte solid lines flow of CO saturated electrolyte, with varied upper scan limits (see key on flgure). (See color insert.)...

Figure 14.10 CO bulk electro-oxidation at Ru(OOOl) and Ru(OOOl) modified by 0.05, 0.23, and 0.9 ML Pt, measured at 10 mV in a flow cell with a CO-saturated electrolyte (0.1 M HCIO4) (a) expanded current scale to visualize the onset behavior (b) entire current region.

Figure 14.12 CO bulk electro-oxidation at PtRu alloys, (a, b) PcRui j /Ru(0001) (x = 0.07, 0.25, 0.47) surface alloys measured in a flow cell with a CO-saturated electrolyte, (c) Freshly sputtered Pto.sRuo.s bulk alloy in a rotating disk electrode setup (data from Gasteiger et al. [1995]), compared with a Pto.53Ruo,47/Ru((X)01) surface alloy.

The electrodes in the direct methanol fuel cell (DMFC) (i.e. the anode for oxidising the fuel and the cathode for the reduction of oxygen) are based on finely divided Pt dispersed onto a porous carbon support, and the electro-oxidation of methanol at a polycrystalline Pt electrode as a model for the DMFC has been the subject of numerous electrochemical studies dating back to the early years ot the 20th century. In this particular section, the discussion is restricted to the identity of the species that result from the chemisorption of methanol at Pt in acid electrolyte. This is principally because (i) the identity of the catalytic poison formed during the chemisorption of methanol has been a source of controversy for many years, and (ii) the advent of in situ IR culminated in this controversy being resolved. 

Examples of surface-immobilized mediators are electropolymerized azines for electro-oxidation of The extreme form of this approach is formation of biocatalytic monolayer, comprising a surface-bound mediator species that is itself bound to a single enzyme molecule. Katz et al. report a complete cell based on novel architecture at both electrodes (Figure 7). On the anode side, the FAD center of glucose oxidase is removed from the enzyme shell and covalently attached to a pyrroloquinoline quinone (PQQ) mediator species previously immobilized on a gold surface. The GOx apoenzyme (enzyme with active center removed) is reintroduced in solution and selectively binds to FAD, resulting in a PQQ-... 

Indirect electro-oxidation of primary amines to nitriles is achieved using halogen ion as mediator [93]. The reaction is typically carried out in an undivided cell... 

Electrons liberated at the anode (negative pole of the cell) by the electro-oxidation of the fuel pass through the external circuit (producing electric energy equal to —AG) and arrive at the cathode (positive pole), tvhere they reduce oxygen (from air). Inside the fuel cell, the electric current is transported by migration and diffusion of the electrolyte ions (H, OH, CO ), for example, in a PEMFC. 

Hydrogen is a secondary fuel and, like electricity, is an energy carrier. It is the most electroactive fuel for fuel cells operating at low and intermediate temperatures. Methanol and ethanol are the most electroactive alcohol fuels, and, when they are electro-oxidized directly at the fuel cell anode (instead of being transformed in a hydrogen-rich gas in a fuel processor), the fuel cell is called a DAFC either a DMFC (with methanol) or a DEFC (with ethanol). 

For these low-temperature fuel cells, the development of catalytic materials is essential to activate the electrochemical reactions involved. This concerns the electro-oxidation of the fuel (reformate hydrogen containing some traces of CO, which acts as a poisoning species for the anode catalyst methanol and ethanol, which have a relatively low reactivity at low temperatures) and the electroreduction of the oxidant (oxygen), which is still a source of high energy losses (up to 30-40%) due to the low reactivity of oxygen at the best platinum-based electrocatalysts. 

This section addresses the role of chemical surface bonding in the electrochemical oxidation of carbon monoxide, CO, formic acid, and methanol as examples of the electrocatalytic oxidation of small organics into C02 and water. The (electro)oxidation of these small Cl organic molecules, in particular CO, is one of the most thoroughly researched reactions to date. Especially formic acid and methanol [130,131] have attracted much interest due to their usefulness as fuels in Polymer Electrolyte Membrane direct liquid fuel cells [132] where liquid carbonaceous fuels are fed directly to the anode catalyst and are electrocatalytically oxidized in the anodic half-cell reaction to C02 and water according to... 

The SOFC, like other fuel cells, is an electrochemical device for the conversion of chemical energy of a fuel into electricity and heat. The fuel, for example, hydrogen is not combusted but electro-oxidized at the anode (fuel electrode) by oxygen ions conducted across the electrolyte according to the following overall reaction... 

Mallouk, T. E., Reddington, E., Pu, C., Ley, K. L., Smotkin, E.S., Discovery of methanol electro-oxidation catalysts by combinatorial analysis. In Fuel Cell Semi-... 

Methanol is the most electro-reactive organic fuel, and, when it is electro-oxidized directly at the fuel anode (instead of to be transformed by steam reforming in a hydrogen-rich gas), the fuel cell is called a DMFC. More generally if the direct oxidation of a given fuel (alcohols, borohydrides, etc.)... 

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