Bi_xTi_(1−x)O_z Functionalized Heterojunction Anode with an Enhanced Reactive Chlorine Generation Efficiency in Dilute Aqueous Solutions
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.
© 2015 American Chemical Society. Received: January 29, 2015; Revised: February 17, 2015. Publication Date (Web): February 20, 2015. The authors would like to acknowledge the financial support of the Bill and Melinda Gates Foundation (BMGF RTTC Grant OPP109500C). Korea Institute of Science and Technology Institutional Program (2E25312) provided support for the graduate study of K.C. This study benefited from the use of instrumentation made available by the Caltech Environmental Analysis Center.
Supplemental Material - cm5b00376_si_001.pdf