Platinum oxygen evolution reaction

Platinum has also had its share of attention in recent years. The effect of phosphoric acid concentration on the oxygen evolution reaction kinetics at a platinum electrode using 0-7 m-17-5 m phosphoric acid at 25°C has been studied with a rotating disc electrode . The characteristics of the ORR are very dependent on phosphoric acid concentration and H2O2 is formed as an intermediate reaction. Also, platinum dissolution in concentrated phosphoric acid at 176 and 196°C at potentials up to 0-9 (SHE) has been reported . 

At IREQ, besides the participation in the field tests run by the engineers of Hydro-Quebec (12), the main effort has been to tackle fundamental problems in the field of electrocatalysis (18-22) and of anodic oxidation of different potential fuels (23-26). A careful and extensive study of the electrochemical properties of the tungsten bronze has been carried out (18-20) the reported activity of these materials in acid media for the oxygen reduction could not be reproduced and this claim by other workers has been traced back to some platinum impurities in the electrodes. Some novel techniques in the area of electrode preparation are also under study (21,22) the metallic deposition of certain metals on oriented graphite show some interesting catalytic features for the oxygen reduction and also for the oxygen evolution reaction. 

Furthermore, BDD anodes have a high overpotential for the oxygen evolution reaction compared with the platinum anode (Fig. 1.3). This high overpotential for oxygen evolution at BDD electrodes is certainly related to the weak BDD-hydroxyl radical interaction, what results in the formation of H202 near to the electrode s surface (1.14), which is further oxidized at the BDD anode (1.15) ... 

Other approaches have focused upon using non-precious metals and their oxides as alternatives to the platinum catalysts. For example, the mixed oxide catalysts of the binary and ternary alloys of noble metals and transition metals have been investigated for the oxygen evolution reaction in solid polymer electrolyte water electrolyzers. Binary, ternary, and quaternary platinum alloys with base metals of Cu, Ni, and Co have been used as electrocatalysts in liquid acid electrolyte cells. It was also reported that a R-Cu-Cr alloy displayed better activity to oxygen reduction than R and Pt-Cr in liquid electrolyte.The enhanced electrocatalytic activity of these types of alloys has been attributed to various factors, including the decrease of the nearest neighbor distance of platinum,the formation of Raney type... 

The motivation for studying the oxygen evolution reaction (OER) on minute amounts of platinum group metal (PGM) catalysts stems from the necessity to add robusmess to the fuel cell (FC) catalysts during the so-called transient... 

Miles MH, Klaus EA, Gunn BP, Locker JR, Seralin WE, Srinivasan S (1978) The oxygen evolution reaction on platinum, iridium, ruthenium and their alloys at 80 °C in acid solutions. Electrochim Acta 23 521-526... 

The oxygen reactions occur at potentials where most metal surfaces are covered by adsorbed or phase oxide layers. This is particularly true for oxygen evolution, which occurs at potentials of 1.5 to 2.2 V (RHE). At these potentials many metals either dissolve or are completely oxidized. In acidic solutions, oxygen evolution can be realized at electrodes of the platinum group metals, the lead dioxide, and the oxides of certain other metals. In alkaline solutions, electrodes of iron group metals can also be used (at these potentials, their surfaces are practically completely oxidized). 

Similar size effects have been observed in some other electrochemical systems, but by far not in all of them. At platinized platinum, the rate of hydrogen ionization and evolution is approximately an order of magnitude lower than at smooth platinum. Yet in the literature, examples can be found where such a size effect is absent or where it is in the opposite direction. In cathodic oxygen reduction at platinum and at silver, there is little difference in the reaction rates between smooth and disperse electrodes. In methanol oxidation at nickel electrodes in alkaline solution, the reaction rate increases markedly with increasing degree of dispersion of the nickel powders. Such size effects have been reported in many papers and were the subject of reviews (Kinoshita, 1982 Mukerjee, 1990). 

Oxides of Platinum Metals Anodes of platinum (and more rarely of other platinum metals) are used in the laboratory for studies of oxygen and chlorine evolution and in industry for the synthesis of peroxo compounds (such as persulfuric acid, H2S2O8) and organic additive dimerization products (such as sebacic acid see Section 15.6). The selectivity of the catalyst is important for all these reactions. It governs the fraction of the current consumed for chlorine evolution relative to that consumed in oxygen evolution as a possible parallel reaction it also governs the current yields and chemical yields in synthetic electrochemical reactions. 

The character of the oxide layers influences the kinetics and mechanism of the electrochemical reactions occurring on the platinum anode surface. The relation between the rate of oxygen evolution and oxide layer thickness is complex. In the region where the a-oxides exist, the reaction rate decreases with increasing oxide layer thickness. In the region where the P-oxides exist, the reaction rate depends little on oxide layer thickness or, according to some data, increases with increasing oxide layer thickness. 

Since the oxygen overvoltage on a platinum electrode is equal to 0.45 V, the minimum potential required for oxygen evolution is (+ 0.813 + 0.45) V or 1.263 V. Since the voltage required to implement this reaction is lower than that for a normal chlorine electrode, it follows that oxygen will be evolved at the anode in preference to chlorine. In contrast to the two possible reactions at the anode, there are five reactions that are possible at the cathode as shown below ... 

M. Faraday was the first to observe an electrocatalytic process, in 1834, when he discovered that a new compound, ethane, is formed in the electrolysis of alkali metal acetates (this is probably the first example of electrochemical synthesis). This process was later named the Kolbe reaction, as Kolbe discovered in 1849 that this is a general phenomenon for fatty acids (except for formic acid) and their salts at higher concentrations. If these electrolytes are electrolysed with a platinum or irridium anode, oxygen evolution ceases in the potential interval between +2.1 and +2.2 V and a hydrocarbon is formed according to the equation... 

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