Alkaline water electrolysis is an effective method for the production of high purity hydrogen without the emission of environmental contaminants [1,2]. A dimensionally stable anode (DSA) is widely used for alkaline water electrolysis [3,4]. However, the fabrication of a DSA is complicated and expensive because of the use of noble metal oxides, such as IrO₂ and RuO₂. Many approaches have reduced the required amount of noble metals and simplified the manufacturing process of DSA [5-9]. Ti metal substrate can be covered with a thin oxide film consisting of amorphous or low crystalline TiO₂. TiO₂ is a non-toxic, chemically inert, and corrosion-resistant material [6]. TiO₂ has been extensively studied under ultraviolet light for photocatalysis because it has semiconductor properties (anatase: 3.2 eV bandgap). Reduced titania (TiO_2-x), which contains oxygen vacancies (V_O-Ti³⁺), has recently been reported to exhibit improved photocatalytic activity under visible light. Black titania nanotubes with a substantial amount of Ti³⁺ were prepared by melted aluminum reduction and applied as the anode for photoelectrochemical water splitting [10]. Schumuki et al. reported that anatase black TiO₂ nanotubes with a stable Ti³⁺ defect-structure (formed by high-pressure H₂ treatment) exhibited a high photocatalytic behavior for cathodic hydrogen evolution [11].

The low electrical conductivity (10⁴-10⁷ Ω·cm) and electrocatalytic activity of TiO₂ prohibit its use as an electrocatalyst without a co-catalyst. Ti³⁺ may control the conductivity of TiO₂, and the thermally reduced anatase TiO₂ has a conductivity of 10^-1 Ω·cm. Ti⁴⁺ inside the TiO₂ lattice can be reduced to Ti³⁺ when a cathodic potential is applied, and the cathodic coloration of TiO₂ is the result of the double injection of a monovalent ion and an electron into the oxide matrix,

where M is the monovalent ion [12]. Yoon et al. reported the preparation of a blue TiO₂ nanotube array (NTA) by cathodic polarization and applied it to the anodic generation of chlorine and the hydroxyl radical [8]. In line with the effect of the Ti³⁺ defect-structure on the electrocatalytic behavior of TiO₂, the increase of the TiO₂ surface area is a critical issue because it determines the extent of the reaction. Therefore, either TiO₂ nanoparticles or nanotubes could be applied as the anode for alkaline water electrolysis.

In this work, a one-dimensional TiO₂ nanotube was electrochemically fabricated and applied to the oxygen evolution reaction (OER) in alkaline water electrolysis. The durability of the TiO₂ nanotube structure and its effect on the OER were investigated with electrochemical methods. Surface analysis was performed using scanning electron microscopy and X-ray photoelectron spectroscopy.

Ti foil (99.7 %, 0.127 mm thickness, Sigma Aldrich) was degreased by ultra-sonication in a solution of acetone, ethanol, and deionized (DI) water for 30 min. The degreased Ti foil was anodized in an electrolyte (0.2 wt% NH₄F, 2.5 wt% DI water in ethylene glycol). An amorphous TiO₂ NTA was fabricated by the application of a constant voltage at room temperature using a DC power supply (HP/Agilent 6035A). The electrochromism of the TiO₂ NTA was tested in a 0.1 M KH₂PO₄ buffer solution at pH 7.2 with cyclic voltammetry (CV). The blue TiO₂ NTA was obtained by cathodic polarization at -3 mA/cm² for 90 s. Pt wire was used as a counter electrode and Ag/AgCl/sat. KCl was used as a reference electrode.

The crystallinity of the TiO₂ NTA changed from amorphous to anatase by annealing at 450 ℃ for 1 h. The surface characterization of the TiO₂ NTA was investigated with a scanning electron microscope (SEM, S-4300, HITACHI), X-ray diffractometer (XRD, DMAX 2200, RIGAKU), and X-ray photoelectron spectroscope (XPS, K-Alpha, Thermo Scientific).

The electrochemical activity of the TiO₂ NTA as an anode in 1 M KOH was analyzed with a potentiostat/galvanostat (Autolab, PGSTAT301N, METROHM) in a three-electrode system. Pt wire and Hg/HgO/1 M NaOH were used as a counter electrode and reference electrode in 1 M KOH solution, respectively.

Fig. 1 shows the effect of the heat treatment on the crystallinity of the electrochemically fabricated anodized TiO₂ NTA. After the heat treatment at 450 ℃ for 1 h, the amorphous TiO₂ changed to the gray crystalline anatase phase. Fig. 2 shows the CVs of the amorphous and anatase TiO₂ NTA in a 0.1 M KH₂PO₄ buffer solution at pH 7.2. The potential was anodically scanned from 0.0 V to 3.0 V, reversed to -2.0 V, and repeated. Both specimens changed to the blue TiO₂ at around -1.5 V by the double injection reaction (1). During the repetitive CV, the amorphous TiO₂ underwent reversible electrochromism reactions (i.e., a cathodic coloration and anodic bleaching, as shown in Fig. 2a) but did not show any anodic reactivity above 1.5 V. After the first cycling for the anatase TiO₂, the anodic current started to flow above 1.5 V (due to the reaction: 2H₂O→O₂ + 4H⁺ + 4e^-) and continued until the end of the CV (Fig. 2b), which is in contrast to the amorphous TiO₂ NTA. During the first CV between -1.5 V and 0.0 V, the anatase TiO₂ NTA experienced a cathodic color change to dark blue caused by reaction (1), but bleaching did not occur despite the anodic current flow, which suggests an irreversible electrochromism. The blue anatase TiO₂ NTA persisted during the entire cycling in Fig. 2b and its catalytic activity for the anodic reaction was investigated. The blue anatase TiO₂ NTA was cathodically fabricated at -3 mA/cm² for 90 s in a pH 7.2 buffer solution and applied as the anode for the OER in 1 M KOH. Fig. 3 indicates that the blue anatase TiO₂ NTA has an excellent catalytic activity for the OER (4OH^-→O₂ + 2H₂O + 4e^-), but the anatase TiO₂ NTA did not participate in the reaction. The XPS spectra in Fig. 4 show that the Ti³⁺ peaks (457.7 eV, 462.9 eV) decreased and the Ti⁴⁺ peaks (459.5 eV, 464.7 eV) increased after the repeated 100 cycles of linear sweep voltammetry (LSV) for the OER. The Ti³⁺ in the blue anatase TiO₂ NTA, which can be K_xTi³⁺O₂, generated by cathodic polarization is responsible for the OER. However, the oxidation of K_xTi³⁺O₂ to TiO₂ (Ti³⁺→Ti⁴⁺) during the OER, may cause the loss of its catalytic activity as the anodic OER continues.

Fig. 5 presents the surface morphology of the anatase TiO₂ NTA prepared at 45 V of anodization voltage with reaction times of 4 h, 8 h, and 16 h. Fig. 6 indicates that the length (l) and diameter (D) of the TiO₂ nanotubes increased with the anodization time. Since the inner surface area of the 1-D nanotube is expressed as N·π·D·l (where N is the number density of the nanotubes), the surface area for the OER increases with the anodization time. The LSV in Fig. 7a coincides with the expected surface area increase, i.e., the specimen prepared at 45 V over 16 h shows the highest catalytic activity for the OER. However, the OER activity decreased with each LSV iteration, and the color of the blue anatase TiO₂ NTA gradually tarnished because of the oxidation of Ti³⁺ to Ti⁴⁺, as suggested by the XPS spectra in Fig. 4.

Fig. 7b shows that the extent of the catalytic activity degradation, i_n/i₁, increases with the increase in the number of LSV cycles, where i_n/i₁ is defined as the ratio of the anodic current density of the n^th cycle and 1^st cycle at 2.5 V. Irrespective of the anodization time, 35-40 % of the initial activity of the blue anatase TiO₂ NTA was lost after 10 LSV cycles. To investigate the effect of the different TiO₂ NTA fabrication methods on the catalytic activity for the OER, the anodization voltage was varied from 35 V to 45 V and 55 V with the same anodization time of 16 h. Fig. 8 indicates that, as the anodization voltage increased from 35 V to 55 V, the tube diameter increased by ~25 %, but the tube length increased to nearly 4 times its initial length (from 6 μm to 22 μm). Fig. 9 (in agreement with the data in Figs. 6 and 7) suggests that the blue anatase TiO₂ NTA with a large surface area has high OER activities and their activity losses reach 35-42 % of the initial activities.

To overcome the catalytic activity degradation, the tarnished blue TiO₂ NTA was regenerated by cathodic polarization after repeated LSVs (-3 mA/cm² for 90 s in a pH 7.2 buffer solution). Regeneration occurred after every 10 LSV cycles. Fig. 10 shows the effect of regeneration on the OER, where is the current density at 2.5 V after 10 cycles. Although the catalytic activity of the blue anatase TiO₂ NTA for the OER was not completely recovered during the regeneration, its degradation rate was significantly reduced. This suggests that the blue anatase TiO₂ NTA has good catalytic activity for the OER and its degradation can be recovered by an intermittent cathodic polarization.

This work showed the development of an alternative material for the OER in alkaline water electrolysis. The anatase TiO₂ NTA was fabricated by anodization and successive heat treatments. In contrast to the amorphous TiO₂ NTA, the anatase TiO₂ NTA showed an irreversible electrochromism (its color changed to blue by the cathodic polarization) and can be used as an electrochemically active anode for the OER. XPS spectra suggested that this catalytic activity was closely related to the presence of Ti³⁺. The surface area of the anatase TiO₂ NTA was controlled by the anodization voltage and time, and the analysis of the nanotube length and diameter indicated that the largest surface area was obtained at 55 V during 16 h of anodization. The catalytic activity increased with an increase of the surface area. The activity of the blue anatase TiO₂ NTA gradually degraded with the continuation of the OER and its blue color tarnished because the electrochemically active Ti³⁺ was partially oxidized to Ti⁴⁺ during the anodic reaction. To overcome the degradation of the blue anatase TiO₂ NTA activity, it was continually regenerated by cathodic polarization, which resulted in the reduction of Ti⁴⁺ to Ti³⁺. A cathodic regeneration process significantly regenerated the catalytic activity of the blue anatase TiO₂ NTA. The blue anatase TiO₂ NTA could be an alternative anode for use in alkaline water electrolysis.