Oxygen reduction reaction platinum oxide formation

The standard potential for the anodic reaction is 1.19 V, close to that of 1.228 V for water oxidation. In order to minimize the oxygen production from water oxidation, the cell is operated at a high potential that requires either platinum-coated or lead dioxide anodes. Various mechanisms have been proposed for the formation of perchlorates at the anode, including the discharge of chlorate ion to chlorate radical (87—89), the formation of active oxygen and subsequent formation of perchlorate (90), and the mass-transfer-controUed reaction of chlorate with adsorbed oxygen at the anode (91—93). Sodium dichromate is added to the electrolyte ia platinum anode cells to inhibit the reduction of perchlorates at the cathode. Sodium fluoride is used in the lead dioxide anode cells to improve current efficiency. 

It is also very important to monitor the effects of the upper potential limit since the potential at which oxygen reduction begins implies hydroxide or oxide co-formation on platinum. There are many studies of the reaction on the three low-index platinum surfaces [95,98]. The catalytic activity of these surfaces decreases in the order of Pt(l 10) > Pt(l 11) > Pt(100) in perchloric acid solution [96], while the order is Pt(110) > Pt(100) > Pt(l 11) in sulfuric acid solution [93]. In the case of Pt(lll), the formation of a two-dimensional ordered ad-layer of specifically adsorbed (bi) sulfate anions is the main reason for the inhibition of oxygen reduction. Moreover, the direct four-electron mechanism was found for the three surfaces in acidic media, while the reaction mechanism varied to a two-electron reduction on the Pt(lll) and Pt(100) due to the shielding of the hydrogen adatoms. 

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... 

Various anodic reactions have been proposed (1) oxidation of impurities (mostly organic) from the solution (2) oxidation of H2O2 accumulated in solution as a result of the two-electron oxygen reduction (3) the formation of surface oxides and (4) platinum anodic dissolution/ " ... 

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