Oxygen evolution, perovskites

Our recent our works show that even higher activity and stability can be demonstrated by the three-layer electrodes with nickel layer, active in the oxygen evolution, middle layer with catalyst, active in the oxygen reduction (Mn02, pyropolymer or a perovskite), and a diffusion (waterproof) layer,... 

Fig. 7.111. Current density (based on real surface area) for oxygen evolution on perovskites at an overpotential of 0.3 V vs. M—OH bond strength. The transition-metal ions (M) in perovskites are indicated with different symbols. (Reprinted from J. O M. Bock-ris and T. Ottagawa, J. Electrochem. Soc. 131 2965,1984. Reproduced by permission of The Electrochemical Society, Inc.)...

The considerations of this reaction of Oz evolution on an oxide catalyst again show the importance of electronic factors and bonding. However, the discussion covers only the essentials the reality of the catalysis of perovskites in oxygen evolution involves several other factors that can be referred to here only briefly. 

An increasing amount of attention is being given to oxides as possible anodes for oxygen evolution because of the importance of this reaction in water electrolysis. In this connection, numerous studies have been carried out on noble metal oxides, spinel and perovskite type oxides, and other oxides such as lead and manganese dioxide. Kinetic parameters for the oxygen evolution reaction at a variety of single oxides and mixed oxides are shown in Table 3. 

The kinetics of oxygen evolution have been investigated at a variety of perovskite oxides, mainly in alkaline solution. Notwithstanding the work of Bockris and co-workers [269] on the electrocatalytic activity of the perovskite analog oxide Nax W03 for oxygen reduction, the first report of a study of the electrocatalytic activity of perovskite oxides was by Meadowcroft [270] for oxygen reduction on La(1 l)SrICo03. 

Bockris et al. [87, 290, 291] have recently reported results of a comprehensive program of surface characterization of a large number of perovskite oxide electrodes in oxygen evolution investigations. Anodic and cathodic oxygen reactions were studied in detail as a function of the solid-state surface properties of these materials. Capacity-potential curves were analysed in terms of the Mott-Schottky treatment and indicated that the potential distribution in the oxide corresponds to a depletion of electrons at the oxide electrode surface in the potential region where oxygen reduction... 

Several oxides with perovskite related stmctures can also be intercalated with oxygen ions by an electrochemical method. The oxide Sr2Fe20s with the brownmillerite stmcture has been electrochemically oxidized to SrFeOs. The reaction was carried out by controlled potential electrolysis at a potential below that for oxygen evolution in 1 M aqueous KOH at room temperature. Bulk oxidation was confirmed by Mossbauer spectroscopy and X-ray difflaction. Similar results have been obtained for electrochemical oxidation... 

A Specific Example of Electrocatalysis Oxygen Evolution on Perovskites... 

What has been written so far in this chapter has come primarily from the thoughts of the author, and has arisen from the ideas of many physical electrochemists and their students and collaborators, not forgetting my own. But now I want to tell you about some electrocatalysts called perovskites and their electrocatalytic action on oxygen evolution from alkaline solution. One of the reasons I want... 

FIGURE 1.14 Current density related to real surface area for the oxygen evolution reaction on perovskites at 0.30 V of overpotential versus the M-OH bond strength. The transition metals, M, of the perovskites are indicated in the plot. This plot is a simplified representation of that in [21]. 

FIGURE 1.15 Schematic representation of the first and second (rate-determining) steps of the mechanism of oxygen evolution in perovskites. 

In addition to the mechanisms of oxygen evolution at nickel anodes [44], the surface properties of C03O4 electrodes have also been investigated [45]. Another possibility is the use of mixed oxides of a spinel or perovskite type for anode oxygen evolution examples are Lao.sSro.sCoOs, NiCo204-PTFE, and C03O4 [46]. Representative results of electrochemical measurements in an alkahne electrolyzer are given in Table 8.5. 

Figure 2 depicts the oxygen evolution profiles of bulk and supported perovskites. The rate of oxygen desorption per mol Mn in the samples is plotted as a function of the catalyst temperature. Prior to the measurements, the samples were heated to 1220 K under a stream of air, and then cooled to room temp ture under the same atmosphere. It should be noted that the pure supports did not exhibit any detectable oxygen desorption up to 1370 K. 

Contemporary with the proposal of the enthalpy of the redox transition, Matsumoto and coworkers suggested the overlap between the eg orbital (or a band) and the orbital of the hydroxide ion could explain the trends in the oxygen evolution of perovskites [34, 35]. Recently, researchers predicted and verified a volcano relationship with an 6g occupancy close to unity for optimum oxygen evolution activity for perovskites [10]. In addition, they suggested that the covalency of the metal-oxygen bond could serve as a secondary predictor for the catalytic activity. 

Bockris JO, Otagawa T (1984) The electrocatalysis of oxygen evolution on perovskites. J Electrochem Soc 131 290-302... 

Suntivich J, May KJ, Gasteiger HA, Goodenough JB, Shao-Hom Y (2011) A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles. Science 334 1383-1385... 

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