Hirani, M; (2015) Electrochemical Reactions Catalysed by Insulators. Doctoral thesis , UCL (Univetsity College London).

Abstract

Recent years have seen research into non-metal electrocatalysts emerge, as many metallic sources become scarce and associated environmental issues are considered. This thesis explores the electrochemical ability of two metal-free electrocatalysts: carbocatalyst nanodiamond and biocatalyst Saccharomyces cerevisiae (baker’s yeast) to catalyse electrode reactions when immobilised on a boron-doped diamond electrode. The electrode reaction of interest is the Fe(II)/Fe(III) redox cycle in the form of ferro/ferricyanide and the ferrocene derivatives: -methanol, -dimethanol, -carboxylic acid, and -dicarboxylic acid. The electrochemistry of the ferrocene derivatives at a nanodiamond modified electrode yielded significantly enhanced catalytic currents compared to those observed previously with highly charged inorganic complexes; a ten-fold current enhancement compared to the four-fold increase seen for Fe(CN) 4 – 6 with 5 nm nanodiamond. This is a direct result of adsorption of the ferrocenium cation onto the nanodiamond surface which participates in an adsorption-mediated catalytic cycle. The magnitude of the catalytic current was found to decrease in the order FcMeOH > Fc(MeOH)2 > FcCOOH Fc(COOH)2. A trend influenced by the ferrocene derivatives redox potential and its physical attractive or repulsive interaction with nanodiamond surface. Nanodiamond particle size and hence the surface functionality density limits the maximum achievable catalytic currents; with current enhancement decreasing in the order 5 nm > 10 nm > 100 nm > 250 nm > 1000 nm. Mediated extracellular electron transfer from S. cerevisiae using FcMeOH was able to distinguish between respiratory and fermentation metabolism and the effects of pressure on yeast fermentation. However, attempts at in-situ measurements were hindered by the weak yeast-electrode attachment. Finally, SECM was used to quantify electron transfer between the catalyst and solution redox species. While successful for nanodiamond, experimental design issues prevented the quantification of reduced mediator production rate from a single yeast cell.

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