Fast Water Oxidation Kinetics in Li-Doped TiO2 Nanotubes
Authors: Tsui Lok-kun , Zangari Giovanni 
Published: February 2020

Photoelectrochemical production of hydrogen provides a solution to the need for a storable source of energy converted from sunlight. For this application, TiO2 nanotube arrays are an attractive photoanode system due to their stability, high surface area, and 1D electronic transport properties.1 In photoelectrochemical water splitting, the water oxidation reaction is known to be the slowest process, and usually necessitates the addition of water oxidation catalysts in metal-oxide photoanode systems.2,3 We have recently demonstrated that Li doping of TiO2 nanotubes can improve their photoelectrochemical performance by up to 2-3x by suppression of recombination processes through trap state passivation.4 To further improve the performance of the TiO2 nanotubes, addition of Co3O4, Co3O4 + CoOOH, or Co-Pi catalysts obtained by electrodeposition, chemical precipitation, and photodeposition, respectively, was investigated. The modification by these catalysts were equally successful in shifting the photoelectrochemical response to simulated sunlight in 1M KOH electrolytes cathodically at potentials below 1.0 VRHE (Figure 1(a)).
In order to rationalize the effect of the OER catalysts, the water oxidation reaction efficiency as a function of potential was studied. We modeled the photocurrent using the following equation: JPC = JMAX * ΦCS * ΦWS-Reaction. In this equation JPC is the photocurrent, JMAX is the maximum theoretical photocurrent determined by the light absorption characteristics, ΦCS is the efficiency of the charge separation process in the bulk, and ΦWS is the efficiency of water oxidation at the interface. By substituting water oxidation reaction with the fast oxidation of the hole scavenger Na2SO3, the photoelectrochemical reaction can be assumed to proceed with ΦSulfite-Reaction = 100%.3 Assuming that the absorption and charge separation parameters are the same, the water oxidation reaction efficiency can be calculated by dividing the photocurrent in the absence of Na2SO3 with the photocurrent in the presence of 1M Na2SO3. As seen in Figure 1(b), the Li doped TiO2 nanotubes exhibit near unity water oxidation reaction efficiency at 1.0 VSCE in a solution of 0.2M Na2SO4 + 0.1M NaCH3COO solution at pH = 7. These results demonstrate that the photoelectrochemical water oxidation reaction proceeds quickly on TiO2 nanotubes doped with Li, provided that the holes can be supplied from the bulk to the interface sufficiently quickly.

References
1. L. Tsui and G. Zangari, J. Electrochem. Soc., 161, D3066–D3077 (2014).
2. D. K. Zhong, M. Cornuz, K. Sivula, M. Grätzel, and D. R. Gamelin, Energy Env. Sci., 4, 1759 (2011).
3. T. W. Kim and K.-S. Choi, Science, 343, 990–4 (2014).
4. L. Tsui, M. Saito, T. Homma, and G. Zangari, J. Mater. Chem. A, 3, 360–367 (2015).

Figure 1. (a) Photoelectrochemical response of Co3O4+CoOOH modified Li-doped TiO2 nanotubes in alkaline electrolyte showing a cathodic photocurrent shift at small biases. (b) Water splitting reaction efficiency in Li-doped nanotubes can reach near 100% demonstrating fast kinetics for oxygen evolution, while undoped TiO2 nanotubes can only reach 25% reaction efficiency.

Figure 1