Electrode potential oxygen-reduction reaction

The mechanism of the oxygen reduction reaction is by no means as fully understood as the h.e.r., and a major experimental difficulty is that in acid solutions (pH = 0) E02/H20 = 1 23, which means that oxygen will start to be reduced at potentials at which most metals anodically dissolve. For this reason accurate data on kinetics is available only for the platinum metals. In the case of an iridium electrode at which oxygen reduction is relatively rapid, a number of reaction sequences have been proposed, of which the most acceptable appear to be the following ... 

The third aspect to consider is the electrochemical stability of the material used. For the oxygen reduction reaction, the electrode potential is highly anodic and at this potential, most metals dissolve actively in acid media or form passive oxide films that will Inhibit this reaction. The oxide forming metals can form non-conducting or semi-conducting oxide films of variable thickness. In alkaline solutions, the range of metals that can be used is broader and can include non-precious or semi-precious metals (Ni, Ag). 

In particular, Yeager and co-workers (42) have described a = a 4- j3 T for the reduction of oxygen on platinum in concentrated phosphoric acid in the temperature range 25—250°C with cathodic Tafel slopes nearly independent of temperature (a — 0.08 and j3 = 0.0012/K). The effect of electrode potential on the reaction kinetics is mainly through the entropy of activation [43]. 

Anderson and his coworker carried out a series quantum chemistry studies of oxygen reduction reactions.52-57 Anderson and Abu first studied reversible potential and activation energies for uncatalyzed oxygen reduction to water and the reverse oxidation reaction using the MP2/6-31G method. The electrode was modeled by a non-interacting electron donor molecule with a chosen ionization potential (IP). The primary assumption is that when the reactant reaches a point on the reaction path where its electron affinity (EA) matched the donor IP, an electron transfer is initialized. The donor s IP or reactant s EA was related to the electrode potential by,... 

Figure 5.3. Electrode potential effects on ORR impedance spectra using GDE AC frequency range 6 x 104 to 6 x 10 3 Hz. Electrode potentials (versus SCE) ( ) 0.54 V (+) 0.49 V (x) 0.44 V (o) 0.39 V [6], (Reprinted from Journal of Electroanalytical Chemistry, 499, Antoine O, Bultel Y, Durand R. Oxygen reduction reaction kinetics and mechanism on platinum nanoparticles inside Nafion , 85-94, 2001, with permission from Elsevier.)...

Figure 5.5. Experimental Nyquist diagrams for the ORR on a Pt nanoparticle catalyst (40% Pt/Pt+C) in alkaline solution (1 M NaOH) using an active layer on an RDE at 25°C (EIS frequency range 5 mHz-105 Hz). Electrode potentials (versus Hg/HgO electrode) ( ) -0.02 V ( ) -0.03 V (A) -0.06 V and ( ) -0.08 V [4], (Reprinted from Electrochimica Acta, 48(25-6), Genies L, Bultel Y, Faure R, Durand R. Impedance study of the oxygen reduction reaction on platinum nanoparticles in alkaline media, 3879-90, 2003, with permission from Elsevier.)...

Figure 6.5. Impedance spectra for the oxygen reduction reaction at three different electrode potentials a 0.8 V b 0.7 V c 0.6 V. The microporous layer (loading 3.5 mg/cm2) of the electrode has varying PTFE content ( ) 10 ( ) 20 (A) 30 (+) 40 wt% [5], (Reprinted from Journal of Power Sources, 94(1), Song JM, Cha SY, Lee WM. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method, 78-84, 2001, with permission from Elsevier and the authors.)...

Prior to investigating the oxygen reduction reaction itself, the morphology and composition of the catalyst surface must be better understood under realistic conditions. In electrochemical cells, the electrolyte surrounding the electrodes is mainly water. Under certain potential conditions, water might dissociate and lead to oxygen or OH adsorption. At higher electrode potentials, water may even cause a surface- or bulk-oxide to be formed. We first... 

Methanol oxidation on CNF- and CNT-supported Pt-Ru particles in liquid electrolytes has been studied using cyclic voltammetry, chronoamperometry [13,231-237], and electrochemical impedance spectroscopy [236]. Rotating disk electrode and linear potential sweep voltammetry in liquid electrolytes were used to study the oxygen reduction reaction on Pt supported on CNTs and CNFs (see,... 

Thus, the oxygen reduction reaction offers the possibility of saving 1.23 V based on the difference in the reversible electrode potentials. This voltage saving corresponds to an energy reduction of >900kWh/ston of Cl2.57 58... 

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