Aspects of zinc electrochemistry
thesisposted on 26.11.2010 by Peter C. Morgan
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The electrochemistry of porous, and planar zinc electrodes has been examined in connection with the application of zinc in electrochemical power sources. Planar, horizontal, upward-facing zinc electrodes have been galvanostatically oxidised in a convection-free system containing KOH under a series of different conditions. Many additives have been reported to increase T, but in the convection-free system no significant increase has been observed. Porous electrodes have been oxidised in the same system. The formation of duplex films has been confirmed optically, and the expansion of an oxidising electrode has been followed using a travelling microscope. A reaction plane parallel to the external surface has been observed penetrating the electrode. Rotating disc electrode experiments have been used to study the oxidation of zinc in a number of electrolytes. In NaC104 semiconducting films were formed. In KOH a solution soluble region was identified. Linear sweep voltammograms were recorded in KOH, KOH containing polymaleic acid, and NaCl04/ZnBr2 solutions. The techniques have been combined to show that the cathodic peak observed on reversing an anodic sweep in KOH is due to the reduction of soluble species from a Type I film, rather than reduction of a solid state film. The A. C. impedance of zinc in KOH, and in KOH with a carboxymethylcellulose, has been measured, and a theory proposed to explain the experimental results. A practical cell has been modelled. The current distribution along the porous zinc electrodes has been measured. Cadmium ring counter electrodes have been developed of greater recoverable charge capacity, and longer cycle life than the zinc electrode. These have been assembled in a 3-tier stack with separators and zinc electrodes in the central cavity. This complex cell was cycled under electrolyte-starved conditions, and the current passing in each segmental ring has been measured, and the charge recoverable calculated. The differences between oxidative and reductive charge in each-cycle was attributed to zinc relocation. An electrode which only partially filled the cavity showed rapid shape-change. Electrodes that filled the cavity showed an expansion above the level of the rings during cycling, and ultimately failed due to oxygen evolution. The implications of these researches to contemporary porous zinc electrode technology are discussed.