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The behavior of V205 single crystals, grown from the melt, during cathodic polarization and recovery, has been investigated. The dependence of potential on pH showed that the only plausible reduction product was a solid oxide or oxyhydroxide, polarization being due to the formation of a solid solution. In the pH range 2.0-5.5, the curve of electrode potential vs. time during both polarization and recovery was accurately predicted by an equation, already applied to MnO2, derived from the assumption that the product is removed from the surface by diffusion into the electrode. Using tritium as a tracer, it was shown that hydrogen is incorporated in the electrode during discharge at pH 4.7. The activation energy for diffusion was found to be 6.7 kcal/mole, which is consistent with bipolar diffusion of protons and electrons.Vanadium pentoxide as a cathode in electrochemical cells has previously been studied to a limited extent. Watson and Scott (1) studied polycrystalline V20~ electrodes in an acid electrolyte saturated with dissolved V205. They found that, for low constant-current discharges, polarization curves for this electrode were qualitatively similar to those for MnO2 in an alkaline medium. These authors were partially successful in applying a theory developed (2) for MnO2 to these discharge curves. This theory is based on the assumption that polarization is caused by an accumulation of reduced product in solid solution on the electrode surface and that recovery is a result of the subsequent diffusion of the product into the electrode. Its application to MnO2 has been demonstrated quantitatively (3).Shukler and Kuz'min (4) studied the cathodic behavior of pressed pellet electrodes made of 80% V205 and 20% graphite. These authors used H2SO4 as an electrolyte and worked at higher apparent current densities than Watson and Scott. They concluded that the V205 electrode is reduced directly to a second solid phase on the electrode surface. This phase was identified as one containing V(IV).Here we attempt to determine the discharge mechanism for single crystalline V205 in NH4C1 buffered solutions and to show under what experimental conditions the theory applicable to MnO2 can be expected to predict the discharge and recovery curves. ExperimentalElectrode preparation.--Single crystals of V205 were grown using a modification of a method originally due to Arsen'eva and Kurchatov (5) and supplied to this laboratory by Boros in a private communication.The crystals were grown in a platinum crucible in an air atmosphere. Baker analyzed (99.5%) V20~ was melted at 690~ and a small seed of crystalline V205 was placed on the melt surface. The temperature was then slowly raised until only a single crystallite remained. At this point the melt was cooled at a rate of approximately 1~ min for a total of 10~176 The single crystal grew as a thin, flat sheet on the surface of the melt with the ac crystallographic plane parallel to the surface of the melt. The crystal was removed before it reached the edge of the crucible by lifting it...

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