Solar energy storage by a microfluidic all-vanadium photoelectrochemical flow cell with self-doped TiO₂ photoanode

Highlights

• The self-doped TiO₂ photoanode is developed for the μVPFC.

• The self-doped TiO₂ is simply prepared by annealing TiO₂ with NaBH₄.

• Light absorption region is extended and electron-hole pair separation is enhanced.

• Excellent photoelectrochemical performance and conversion are achieved.

Abstract

All-vanadium photoelectrochemical flow cell, which combines the vanadium redox flow battery and the photoelectrochemical flow cell, is a promising technology to store solar energy in reversible redox pairs. The development of a high-performance photoanode is vital to promote the storage of solar energy. In this work, we developed a self-doped TiO₂ photoanode and applied it to a microfluidic all-vanadium photoelectrochemical flow cell (μVPFC). The self-doped TiO₂ photoanode was simply prepared by annealing the TiO₂ photoanode with NaBH₄ in the nitrogen atmosphere, by which the self-dopant led to the formation of a disordered layer on the surface and created a mid-band. As a result, the light absorption region was extended and the electron-hole pair separation efficiency was enhanced, promoting the capability of the μVPFC with the self-doped TiO₂ photoanode. The superiority of the self-doped TiO₂ photoanode was confirmed by its excellent photoelectrochemical performance and vanadium ion conversion rate. The μVPFC with the self-doped TiO₂ photoanode yielded an average photocurrent density as high as 0.064 mA·cm⁻² in 6-h operation, which was much higher than the reported TiO₂ and Ti₂O₃ photoanodes and presented the improvements by approximately 167% and 60%, respectively. In addition, intensifying the intensity of light and concentration of vanadium ion enabled the performance of the μVPFC with the self-doped TiO₂ photoanode to be promoted.

Introduction

Global energy demand is sharply increasing because of rapid development of economy. Although fossil fuels are still the primary energy sources, the overuse of fossil fuels poses grand challenges of energy crisis and terrible environmental issues [1], [2], [3], [4]. For this reason, various renewable energy sources have been put forward to alleviate the energy and environmental issues, such as solar energy and wind energy, etc. [5,6] Among them, solar energy has been deemed to be the most important renewable energy source because it is clean, abundant and widely distributed. According to the statistics, the solar energy hitting the earth in each hour can meet the demand of the global energy consumption in one year. No doubt, the energy and environmental issues can be well addressed provided that solar energy is efficiently utilized. However, the solar energy also has its intrinsic defect, that is, it is intermittent. Therefore, the solar energy storage becomes essential in widespread utilization of solar energy. Right now, there exist several solar energy storage strategies by converting solar energy into the heat [7,8] and chemical energy [9], [10], [11]. Vanadium redox flow battery, which stores solar energy in chemical substances (such as the reversible redox vanadium ion pairs of VO₂⁺/VO²⁺ and V³⁺/V²⁺, etc.), is competitive for large-scale energy storage [12,13], because this technology offers several merits, such as long life cycle, large storage capacity, and avoidance of cross contamination and so on [14], [15], [16], [17].

In recent, Liu et al. [18] proposed an all-vanadium photoelectrochemical cell, which could directly convert solar energy into chemical energy stored in vanadium redox pairs via photoelectrochemical reactions. Typically, the semiconductor photocatalysts are employed to prepare the photo-responsive anode (photoanode), while Pt is used as the cathode, which are separated by a membrane. The electrolyte contained VO²⁺is supplied to the photoanode and the electrolyte contained V³⁺ is supplied to the cathode. In principle, the electron-hole pairs are generated in the photoanode upon solar irradiation. The generated holes that have strong oxidation ability can oxidize VO²⁺ to VO₂⁺ at the photoanode. The electrons go to the cathode, at which V³⁺ can be reduced to V²⁺. As such, the solar energy storage in the redox couples by the all-vanadium photoelectrochemical cell can be realized [19], [20], [21]. Clearly, the performance of the all-vanadium photoelectrochemical cell highly depends on the properties of the photocatalysts. Usually, TiO₂ is used at the photoanode because it is stable, nontoxic and abundant [22], [23], [24], [25], [26], [27]. However, big bandgap (3.2 eV) limits wide applications of TiO₂ because it can only respond to UV light and suffers from serious recombination of the electron-hole pairs. As a result, extensive efforts have been devoted to broadening the light response range of TiO₂, such as metal and non-metal doping [28], [29], [30], [31], [32], heterojunction and co-catalyst [33], [34], [35], etc. Choi et al. [36] have reported the effect of the doped materials and amounts of the dopant on the photocatalytic activity. As reported, when the doping amount of Ru³⁺, Fe³⁺, Mo⁵⁺, Os³⁺, V⁴⁺, Re⁵⁺ and Rh³⁺ ions was within 0.1–0.5%, the photocatalytic performance of the TiO₂ could be enhanced. However, doping Al³⁺ and Co³⁺ metal ions reduced the photocatalytic activity. Asahi and co-workers [31] have also reported the N-doped TiO₂ photocatalyst after annealing in the N₂ atmosphere at high temperature resulted in the absorption of visible light.

More recently, the black TiO₂ (such as Ti³⁺-doped TiO₂ and hydrogenated TiO₂) has also been proposed to address this issue [37,38]. As for Ti³⁺-doped TiO₂, due to the existence of the oxygen vacancies and Ti³⁺, a highly disordered surface layer is generated. The bandgap becomes small as the disordered lattice yield the mid-gap state between the valance band and conduction band. In this case, the black TiO₂ can fully respond to solar irradiation, improving solar energy utilization. In addition, the performance of the all-vanadium photoelectrochemical cell is also greatly affected by the cell design. Feng et al. [4] and Jiao et al. [5] have reported that the miniaturized reactor design can boost the cell performance because the intrinsic large surface-to-volume ratio can enhance mass transfer and the light distribution can be more uniform. In line with these ideas, Lin et al. [39] proposed a full-spectrum-responsive photoanode with commercial Ti₂O₃ powders and applied it in a microfluidic all-vanadium photoelectrochemical flow cell (μVPFC). Although better performance has been achieved, large particle size of commercial products (usually tens of micrometers) still limits further improvement in the performance due to less active sites for photoelectrochemical reactions and more serious bulk recombination. In this work, therefore, we prepared a self-doped TiO₂ photoanode for a μVPFC by a simple method. Here, the TiO₂ nanoparticles stacked on the conducting glass were annealed with NaBH₄. After reduction at high temperature, the TiO₂ nanoparticles could be doped with Ti³⁺ and the oxygen vacancies could be generated, forming a self-doped TiO₂ photoanode. Such treatment also led to a core-shell structure, which was represented by a crystalline TiO₂ nanocrystal in the core and a highly-disordered surface layer at the shell [37,40,41]. The light absorption range of the developed photoanode could then be broadened to visible light. Moreover, the nanostructures could be remained to ensure high specific surface area. The prepared self-doped TiO₂ photoanode was evaluated by incorporating it into a μVPFC in terms of the photocurrent density and vanadium ion conversion rate.