Platinum electrodes overpotential

The overpotential required for the evolution of O2 from dilute solutions of HCIO4, platinum electrodes is approximately 0.5 V. 

The nature and the physical state of the metal employed for the electrodes. The fact that reactions involving gas evolution usually require less overpotential at platinised than at polished platinum electrodes is due to the much larger effective area of the platinised electrode and thus the smaller current density at a given electrolysis current. 

D.Y. Wang, and A.S. Nowick, Cathodic and anodic polarization phenomena at platinum electrodes with doped Ce02 as electrolyte. I. Steady-state overpotential, J. Electrochem. Soc. 126(7), 1155-1165 (1979). 

V) is actually required to electrolyze water when platinum electrodes are used. Much contemporary research on electrochemical cells involves attempts to reduce the overpotential and hence to increase the efficiency of electrolytic processes. 

Electrode processes can be retarded (i.e. their overpotential is increased) by the adsorption of the components of the electrolysed solution, of the products of the actual electrode reaction and of other substances formed at the electrode. Figure 5.43 depicts the effect of the adsorption of methanol on the adsorption of hydrogen at a platinum electrode (see page 353). 

It is clear from the calculated limiting-current curves in Fig. 3a that the plateau of the copper deposition reaction at a moderate limiting-current level like 50 mA cm 2 is narrowed drastically by the surface overpotential. On the other hand, the surface overpotential is small for reduction of ferri-cyanide ion at a nickel or platinum electrode (Fig. 3b). At noble-metal electrodes in well-supported solutions, the exchange current density appears to be well above 0.5 A/cm2 (Tla, S20b, D6b, A3e). At various types of carbon, the exchange current density is appreciably smaller (Tla, S17a, S17b). 

In this review, the reference electrode used is defined as a platinum metal electrode exposed to hydrogen at the same temperature and electrolyte (e.g.. Nation) as the solution of interest. With this reference electrode, the electrode overpotential defined in eq 10 is the same as having the reference electrode located next to the reaction site but exposed to the reference conditions (i.e., it carries its own extraneous phases with it). Typical values for the reference conditions are those in the gas channels. If the reference electrode is exposed to the conditions at the reaction site, then a surface overpotential can be defined... 

Deposition of mercury at boron-doped diamond (BDD) and platinum electrodes has also been studied [33]. Deposition and oxidation of mercury was performed by cyclic voltammetry from the solution of 1 mM Hg2 ( 104)2 in 1 M Na l04. In order to learn more about this deposition, it was carried out also under chronoamperometric conditions. The results obtained are shown in Fig. 2 in the form of dimensionless current-time transients. Experimental curves obtained at two different overpotentials were compared with the theoretical curves calculated for instantaneous and progressive nucleation. A good agreement of experimental plots with the instantaneous nucleation mechanism was... 

An I/E polarization experiment run at 298.15 K on a 0.6-cm2 active-area platinum electrode at small overpotentials in four aqueous solutions containing a fixed small amount of Fe2+and increasing amounts of Fe3+gave the following results ... 

Figure 4.8 shows the potential windows obtained at a bright platinum electrode, based on the Fc+/Fc (solvent-independent) potential scale. Because of the overpotentials, the window in water is 3.9 V, which is much wider than the thermodynamic value (2.06 V). The windows for other solvents also contain some overpotentials for the reduction and the oxidation of solvents. However, the general tendency is that the negative potential limit expands to more negative values with the decrease in solvent acidity, while the positive potential limit expands to more positive values with the decrease in solvent basicity. This means that solvents of weak acidity are difficult to reduce, while those of weak basicity are difficult to oxidize. This is in accordance with the fact that the LUMO and HOMO of solvent molecules are linearly related with the AN and DN, respectively, of solvents [8]. 

One of the most important reasons for the application of mercury to the construction of working electrodes is the very high overpotential for hydrogen evolution on such electrodes. Relative to a platinum electrode, the overpotential of hydrogen evolution under comparable conditions on mercury will be -0.8 to -1.0 V. It is therefore possible in neutral or (better) alkaline aqueous solutions... 

For reductions, hanging mercury drop electrodes or mercuryfilm electrodes are frequently used owing to their microscopic smoothness and because of the large overpotential for hydrogen evolution characteristic for this electrode material. Mercury film electrodes are conveniently prepared by the electrochemical deposition of mercury on a platinum electrode from an acidic solution of an Hg2+ salt, e.g. the nitrate. However, the oxidation of mercury metal to mercury salts or organomercurials at a low potential, 0.3-0.4 V versus the saturated calomel electrode (SCE), prevents the use of these electrodes for oxidations. 

An in depth study of the deposition mechanism was carried out by Sun et al. who studied the 1 1 [EMIMJCl/ZnCf system at various temperatures on glassy carbon (GC), nickel and platinum electrodes [106], The GC electrode required the largest overpotential for deposition. The stripping process showed a single peak on GC, whereas on Ni two oxidation processes were observed, separated by ca. 0.6V. Itwas proposed that the more positive oxidation process corresponded to the dissolution of an intermetallic compound formed during electrodeposition. 

The electrochemical reaction proceeds most effectively in the presence of a catalyst, and the nature of the catalyst can have a significant effect upon the electrode overpotentials. As a matter of convenience, all of the early work in the electrolyzer development used platinum as both the anode (SO2 oxidation electrode) and cathode (H2 generation electrode) catalyst. It was recognized, however, that although platinum might be a technically satisfactory catalyst for the cathode, it was only marginally suitable as the anodic catalyst. 

The Cat, or its product of electrode oxidation or reduction Cat, is immobilized at the electrode surface and decreases the overpotential for oxidation or reduction of the S, without being involved in the chemical redox reaction with the S. Typical example is the catalytic effect of underpotential deposited layer of lead on a platinum electrode, on anodic oxidation of methanol [v]. 

Figure 68. The exchange current density as a function of oxygen partial pressure for different temperatures confirming the electrode kinetical model given in the text.256 (Reprinted from D. Y. Wang, A. S. Nowick, Cathodic and Anodic Polarization Phenomena at Platinum Electrodes with Doped CeC>2 as Electrolyte. I. Steady-State Overpotential. , J. Electrochem. Soc., 126, 1155-1165. Copyright 1979 with permission from The Electrochemical Society, Inc.)...

The Tafel equation also describes the evolution of oxygen at a platinum anode. Bockris and Huq found that, with solutions carefully purified by preelectrolysis, the oxygen electrode exhibits reversible behavior (E = 1.24 V, compared with the theoretical 1.23 V). The exchange current density, however, is only of the order of 10" to 10" °A/cm in dilute sulfuric acid so polarization occurs readily, and relatively large overpotentials are observed at moderate current densities. In solutions of ordinary chemical purity the Nemst relation fails for the oxygen electrode because of mixed-potential behavior. Criddle, using platinum electrodes in highly purified 1 M KOH, obtained a rest potential of 1.59 V. The potential is reduced by peroxide, which may be formed with impurities such as metals, protein, or carbon. 

This result is consistent with the observed effective poisoning of the CO oxidation reaction as reflected in the increased potential induced by bismuth in the cyclic voltammetry on the supported platinum electrodes (Figure 10a). The voltammetry of CO stripping on the supported catalysts indicates a similar behavior to that found on Pt(llO) in that bismuth results in a higher overpotential for CO oxidation. One must conclude that the morphology of the supported platinum catalyst results in facets more akin to the more open-packed Pt(l 10) surface than the Pt(lll) surface, a conclusion supported by comparison of the bismuth redox chemistry on the supported catalyst and the single-crystal surfaces [77]. 

Fig. 13 Current-voltage diagram showingthe meaning of activation overpotential for the electrolysis of water using platinum electrodes in alkaline solution. (Reprinted with permission from Ref. 8, Copyright 1998 by Wiley-VCH).

The same polarization characteristics exhibit the platinum electrode modified with copper dendrites formed by the use of the PO regime described in caption of Fig. 26a. It can be seen from Fig. 27 that the process on the electrode with increased surface roughness takes place at considerably lower overpotential than on the smoother one. 

The calculated value E° = +1.23 V for the O2, 4H /2H20 electrode implies that electrolysis of water using this applied potential difference at pH 0 should be possible. Even with a platinum electrode, however, no O2 is produced. The minimum potential for O2 evolution to occur is about 1.8V. The excess potential required ( 0.6V) is the overpotential of O2 on platinum. For electrolytic production of H2 at a Pt electrode, there is no overpotential. For other metals as electrodes, overpotentials are observed, e.g. 0.8 V for Hg. In general, the overpotential depends on the gas evolved, the electrode material and the current density. It may be thought of as the activation energy for conversion of the species discharged at the electrode into that liberated from the electrolytic cell, and an example is given in worked example 16.3. Some metals do not liberate H2 from water or acids because of the overpotential of H2 on them. 

Reoxidation of the cosubstrate at an appropriate electrode surface will lead to the generation of a current that is proportional to the concentration of the substrate, hence the coenzyme can be used as a kind of mediator. The formal potential of the NADH/NAD couple is - 560 mV vs. SCE (KCl-saturated calomel electrode) at pH 7, but for the oxidation of reduced nicotinamide adenine dinucleotide (NADH) at unmodified platinum electrodes potentials >750 mV vs. SCE have to be applied [142] and on carbon electrodes potentials of 550-700 mV vs. SCE [143]. Under these conditions the oxidation proceeds via radical intermediates facilitating dimerization of the coenzyme and forming side-products. In the anodic oxidation of NADH the initial step is an irreversible heterogeneous electron transfer. The resulting cation radical NADH + looses a proton in a first-order reaction to form the neutral radical NAD, which may participate in a second electron transfer (ECE mechanism) or may react with NADH (disproportionation) to yield NAD [144]. The irreversibility of the first electron transfer seems to be the reason for the high overpotential required in comparison with the enzymatically determined oxidation potential. 

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