Electrochemical Reactions in the Cathode

The occupants of ABO3 at A site can be A, V a and SFa (for simplicity, assume Sr is the low valence element substimting at the A site). The occupants at the B site are Bg, B b, Bg and V b, where Bg indicates a majority of B cations are in valence of 3+, B b represents that some B site cations are in 2+ state and Bg shows some B site cations are in 4+ valence state. The oxygen site can have two types of occupants, Vq and Oq. Details on these notations can be found from [2, 18, 19]. 

The main defect reaction for generation of oxygen ion vacancies is 

For the compounds in this study, i.e., ABO3, we emphasize the regime where the cation vacancies are minor, thus the electroneutrality condition becomes  

Three assumptions must be made to analytically solve the equations  

Assume that the positive hole is the higher valence transition metal ion, Bg and negative electron is the lower valence ion, B b, thus 

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While the electrochemical reaction in the cathodic regime is similar for most commonly used aqueous electrolytes, the anodic reaction depends on composition and pH of the electrolyte. 

In the Bacon ceU, the first successful modem fuel cell, porous nickel was used as a material for the electrodes, doing double duty as conductor and catalyst for the current-producing electrochemical reactions (in the cathode the nickel was lithiated). A little later, Justi introduced Raney nickel and Raney silver as the catalytic electrodes nickel for the hydrogen anode and silver for the oxygen... 

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