Studies of extended area tin dioxide anodes

Lipp, Ludwig (1996) Studies of extended area tin dioxide anodes. University of Southampton, Doctoral Thesis.

Abstract

Doped tin dioxide coatings on titanium have been prepared by the thermal decomposition method and characterised by cyclic voltammetry with the Fe(CN)₆^4-/Fe(CN)₆^3- couple. Electron transfer was shown to be fast and fully reversible, and the IR drop in the film was low. High capacitive currents indicate that the desired high area surfaces were obtained. Large current densities of up to 8 kA m^-2 were passed without altering the coating. The anodes have been analysed by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Analysis (EDAX) and the best coating conditions were found to be a sandblasted and chemically etched Ti substrate, a precursor solution containing 20 % w/v SnCl₄ + 0.2 % w/v SbCl₃ in 2-propanol and a decomposition temperature of 773 K.

Accelerated long-term tests were carried out in a temperature-controlled beaker cell at 303 K at an applied potential of +2.2 V vs SCE in acid solution, where oxygen evolution occurred on the anode. After an initial decay in current, typical for DSA-type anodes, currents were constant. Cyclic voltammetry, SEM and EDAX before and after the lifetime tests revealed no change in coating after more than 1000 hours of continuous operation.

SnO₂ was deposited onto expanded titanium metal meshes and fabricated into 3-dimensional anode stacks. Mass transport properties of two different anode mesh sizes were studied using a batch recycle system including an undivided cell with the anode operating in the flow-by mode. The ferro-/ferricyanide couple was used in conjunction with linear sweep voltammetry to characterise the mass transport regime within the cell as a function of electrolyte velocity. It was confirmed that the use of stacks of SnO₂ coated Ti meshes leads to a substantial enhancement of the mass transport limited current compared to a flat plate anode, and the increase in current was higher with the finer of the two types of mesh used. The increase in limiting current was proportional to the number of meshes in the stack (up to at least 8 meshes); therefore, the current scaled with the anode area. The mass transport controlled current was >200 mA cm^-3 for a stack of 8 fine meshes and a reactant concentration of 5 mM Fe(CN)₆^4-. Mass transport coefficients compare well with those reported in the literature for similar materials. Velocity exponents of 0.4 - 0.6 confirm that there was turbulent flow throughout the range of velocities and different anode stacks tested.

High current densities for the oxidation of EDTA were found, although the oxidation of other organic compounds (phenol, formaldehyde) showed more complex behaviour.

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