Influence of Composition of Ti/SnO2-Sb2O5 on the Kinetics of Electro-Oxidation
Authors: Ni Qing , Kirk Donald W. , Thorpe Steven J. 
Published: February 2020

The Ti/SnO2-Sb2O5 anode is a promising material for the anodic oxidation of organic compounds for the application of wastewater treatment. The material consists of a thin film of Sb-doped SnO2 on the titanium substrate. The influence of Sb doping on the conductivity of SnO2 thin films has been well documented, but its effect on the catalysis of electro-oxidation remains largely unknown. The current investigation presents a systematic study addressing this knowledge gap. A series of Ti/SnO2-Sb2O5 anodes were prepared using thermal deposition and electro-deposition. These films contained a wide range of doping levels, expressed as atomic fraction Sb/(Sn+Sb), essentially from SnO2 dominant to SbOx dominant. The surface composition of deposited coatings was determined by X-ray photoelectron spectroscopy. The electrochemical activities of the electrodes were evaluated through the use of polarization scans performed in a simulated wastewater system containing oxalic acid as an organic analog target for destruction. All electrodes exhibited Tafel behavior. Different anodes were compared based on their activity, defined as the anodic current at 1.7VSHE normalized by the surface roughness of the electrode. Electrochemical impedance spectroscopy was used to estimate the effective surface area, by means of effective double-layer capacitance.

Scanning voltammetry in 0.01mol/L oxalic acid showed Tafel slopes varying between 180~250mV/dec (Figure 1). Some samples with very high and low doping levels of antimony showed even higher Tafel slopes. The above findings were related to the conductivity of the doped films.  Herrmann et al., reported that the conductivity of the mixed antimony-tin oxide was highly dependent on the level of Sb doping in the film1. According to their study, the SnO2-Sb2O5 mixed oxide was highly conductive with Sb doping level between 1.7% and 40%. Oxides with doping levels outside this range exhibited a drop in conductivity by three orders of magnitude. In the present investigation, the uncompensated resistance of the coating films in the electrodes can results in abnormally high Tafel slopes as shown in Figure 1.

The activities of the anodes were compared at 1.7VSHE. The current levels were normalized to the double-layer capacitance of the electrode and plotted against the doping level as shown in Figure 2. A volcano curve is evident from the figure, which suggested that the electrode had an optimum doping level at about 2% Sb doping. This level of doping was different from the composition corresponding to the highest conductivity that was identified by Herrmann et al.
1. The beneficial increase in the performance of the electrode at this doping level of could be related to the surface arrangement of Sb atoms in the SnO2 lattice. Ongoing work is being conducted to characterize the surface features of the Sb-doped SnO2materials.

Reference:

1. J-M. J. Herrmann, J-L. Portefaix, M. Forissier, F. Figueras, P. Pichat, Electrical Behavior of Powdered Tin-Antimony Mixed Oxide Catalysts.
J. Chem. Soc. Farad. Trans. I, 75, 1346 (1979).

Figure 1