Stability of Fluorine Doped Tin Oxide (FTO) Coated Substrates Under Extreme Polarization Conditions

Abstract

Fluorine doped tin oxide (FTO) is a transparent conducting oxide which is widely used in different applications ranging from sensors, digital displays, smart window coatings, catalysts supports, and electrode substrates for various electrochemical applications. The FTO coated glass slides were evaluated for their chemical stability in different pH solutions at room temperature by carrying out potentiodynamic polarization and cyclic voltammetry (CV). Accelerated stress tests were carried out in 0.1 M HCl and 0.1 M NaOH solutions by scanning the potential from -2 V to 2 VRHE at a scan rate of 1 V/s. Ex-situ vibrational spectroscopy and in-situ photo absorption spectroscopy were carried out to evaluate the material degradation when electrochemically polarized. The FTO was more stable in 0.1 M HNO3 and 0.1 M NaCl solutions under both anodic and cathodic polarization conditions. Oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) were the cathodic reactions observed during cathodic polarization of the FTO. The activity of FTO for HER was higher in the acidic solutions than that in the neutral or alkaline solutions. Cathodic polarization in 0.1 M HCl solution resulted in a reduction of SnO2 to lower valent species. The semiconductivity of FTO, which was n-type at low anodic potentials, was p-type at high anodic potentials possibly due to the formation of a SnO-type surface layer by the removal of lattice oxygen during the oxygen evolution reaction. Cyclic stability of the FTO was evaluated by conducting CV between -2.0 and vbn 2.0 VAg/AgCl at 1 V/s. Intergranular corrosion was observed and the stability was completely lost after 215 cycles in the 0.1 M HCl solution, as seen in Fig. 1. The stability of FTO was better in the 0.1 M NaOH solution, wherein the electro-catalytic activity degraded after 500 cycles, the electrical connectivity was maintained even after 4500 cycles. Figure 1