Bi_xTi_(1−x)O_z Functionalized Heterojunction Anode with an Enhanced Reactive Chlorine Generation Efficiency in Dilute Aqueous Solutions

Abstract

Ir_(0.7)Ta_(0.3)O_y/Bi_xTi_(1–x)O_z heterojunction anodes have been developed and characterized for reactive chlorine species (RCS) generation in dilute aqueous solution (50 mM NaCl). The primary objective of the research was to control the electro-stationary speciation of hydrous metal oxides between hydroxyl radical (>MO_x(·OH)) and higher valence-state oxides (>MO_(x+1)). An underlying layer of the mixed-metal oxide, Ir_(0.7)Ta_(0.3)O_y, was synthesized to serve as a primary Ohmic contact and electron shuttle. Binary thin films of Bi_xTi_(1–x)O_z were prepared from the thermal decomposition of an aqueous solution mixture of Ti/Bi complexes. With these core components, the measured current efficiency for RCS generation (η_(RCS)) was enhanced where the values observed for x = 0.1 or 0.3 were twice of the η_(RCS) of the Ir_(0.7)Ta_(0.3)O_y anode. At the same time, the rates of RCS generation were enhanced by factors of 20–30%. Partial substitution of Ti with Bi results in a positive shift in surface charge allowing for stronger interaction with anions, as confirmed by FTIR-ATR analysis. A kinetic model to describe the formate ion degradation showed that an increasing fraction of Bi in the composite promotes a redox transition of >MO_x(·OH) to >MO_(x+1). In accelerated life tests under conditions corresponding to a service life of 2 years under an operational current density of 300 A m^(–2), dissociation of the Ti component from Ir_(0.7)Ta_(0.3)O_y/TiO_2 was found to be minimal, while Bi_xTi_(1–x)O_z in the surface layers undergoes oxidation and a subsequent dissolution.

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