Electrocatalyst reversible electrode potential

In redox flow batteries such as Zn/Cl2 and Zn/Br2, carbon plays a major role in the positive electrode where reactions involving Cl2 and Br2 occur. In these types of batteries, graphite is used as the bipolar separator, and a thin layer of high-surface-area carbon serves as an electrocatalyst. Two potential problems with carbon in redox flow batteries are (i) slow oxidation of carbon and (ii) intercalation of halogen molecules, particularly Br2 in graphite electrodes. The reversible redox potentials for the Cl2 and Br2 reactions [Eq. (8) and... 

Based upon analogies between surface and molecular coordination chemistry outlined in Table 1, we have recently set forth to investigate the interaction of surface-active and reversibly electroactive moieties with the noble-metal electrocatalysts Ru, Rh, Pd, Ir, Pt and Au. Our interest in this class of compounds is based on the fact that chemisorption-induced changes in their redox properties yield important information concerning the coordination/organometallic chemistry of the electrode surface. For example, alteration of the reversible redox potential brought about by the chemisorption process is a measure of the surface-complex formation constant of the oxidized state relative to the reduced form such behavior is expected to be dependent upon the electrode material. In this paper, we describe results obtained when iodide, hydroquinone (HQ), 2,5-dihydroxythiophenol (DHT), and 3,6-dihydroxypyridazine (DHPz), all reversibly electroactive... 

Figure 6.19. Experimental cyclic voltammograms of carbon-supported high surface area nanoparticle electrocatalysts in deaerated perchloric acid electrolyte. Solid curve pure Pt dashed curve Pt5oCo5o alloy electrocatalyst. Inset blow up of the peak potential region of Pt—OH and Pt— formation. Scan rate 100 mV/s. Potentials are referenced with respect to the reversible hydrogen electrode potential (RHE).

A typical PIml electrocatalyst is prepared by placing a mcMioIayer of Pt atoms rai metal nanoparticles using galvanic displacement of an underpotentially deposited (UPD) Cu monolayer by a Pt monolayer [3,7]. Underpotential deposition describes the formatiOTi of a submonolayer or monolayer of a metal on a foreign metallic substrate at potentials positive to the reversible Nemst potential, that is, before bulk deposition can occur [8]. The experimental setup for PtML deposition on an electrode consists of a cell with several compartments one used for Cu UPD, one for rinsing the electrode after emersion from CUSO4 solution, and one for displacement of a Cu monolayer upon electrode immersion in Pt " solution. The cell is under an inert gas (Ar or N2) and facilitates all operations in an 02-free environment. 

Under concentration control, the reversible hydrogen electrode exhibits Nemstian reversibility. This provides for a potential shift of 29.75 mV at room temperature, which translates to a shift of 46.8 mV at 200 °C for each decade of change in hydrogen concentration. Under fuel-cell operating conditions with highly dispersed electrocatalysts, it is possible to approach the kinetic rate determined by the dual-site dissociation of the hydrogen molecule, viz. ... 

Often analytes are irreversibly (slowly) oxidized or reduced at an electrode, that is, require a substantial overpotential to be applied beyond the thermodynamic redox potential (E°) for electrolysis to occur. This problem of slow electron transfer kinetics has spawned much research in the development of electrocatalysts, which may be covalently attached to the electrode, chemisorbed, or trapped in a polymer layer. The basis of electrocatalytic CMEs is illustrated in Figure 15.5. Red is the analyte in the reduced form, which is irreversibly oxidized, and Ox is its oxidized form. The redox mediator is electrochemically reversible and is oxidized at a lower... 

Fig. 9-2 Cyclic voltammetry of two electrocatalysts a) Pt in acidic electrolyte solution, b) Au in alkaline electrolyte solution. The potential scan rate 1 = 50 mV/s two different reference electrodes were used, RHE reversible hydrogen electrode, Hg/HgO mercury/mercury oxide electrode. (Courtesy V. M. Schmidt, Mannheim, Germany)...

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