An efficient tandem photoelectrochemical cell composed of FeOOH/TiO2/BiVO4 and Cu2O for self-driven solar water splitting
Introduction
As a kind of renewable energy source, hydrogen can replace the fast depleting fossil fuel energy sources [1], [2]. The ability to find an efficient and environmentally friendly way to split water into hydrogen without any external bias potential is of great meaning to society. Recent solar tandem cells are principally composed of photovoltaic/photoelectrode (PV/PEC) device or photoanode/photocathode design [3], [4]. Photovoltaic cells based on silicon [5], perovskite solar cell [6], or copper indium gallium selenide [7], already have obtained relatively high photoelectric conversion efficiency. But, the photocorrosion and photoinactivation of the narrow gap semiconductors (eg. Silicon) are still seriously. Besides, the critical technological and economical drawbacks have limited the copper indium gallium selenide or perovskite solar cell for commercial popularizing.
Compared with the photovoltaic cells, the alternative PEC cells composed of n-type photoanodes and p-type photocathodes are superior in cost and preparation, have attracted considerable attention over the recent years [8], [9], [10], [11], [12]. With the illumination, there will be a self-driven bias in such a system for the difference in Fermi levels of photoanodes and photocathodes. Meanwhile, the H2 and O2 can spontaneously generated at corresponding photoelectrode/electrolyte interfaces [13]. For effective STH conversion in the tandem system, there are two basic requirements that must be considered. Firstly, the 1.6–1.8 V of photopotential is necessary to overcome the thermodynamic and kinetic barriers for water splitting. However, with such a large external bias, it is still hard to find a single semiconductor junction with high STH conversion efficiency [14]. Secondly, the conduction band minimum (CBM) location of the photoanode must lie at lower or similar potentials than the valence band maximum (VBM) location of the photocathode. In addition, the semiconductor materials with smaller band gap are beneficial to the absorption of wider visible light and make the most of photon flux from the sun [15]. For a tandem cell, the generated photocurrents of both photoanodes and photocathodes are equivalent at the run time. The theoretical operating current of the tandem PEC cells could be obtained from the intersection of current-voltage curves of photoanodes and photocathodes [16]. Therefore, the most direct way to increase the operating current is reducing the turn-on voltage or enhancing the photocurrent. As a result, it is particularly important to find a way to both reduce the turn-on voltage and improve the photocurrent after selecting the proper electrodes.
In an unbiased tandem PEC cell, BiVO4 with comparatively low band gap energy of 2.4 eV and suitable band edge positions of valence and conduction band, which requires less bias potential, has been chosen as a preferred material for photoanode [17], [18]. However, the turn-on voltage of BiVO4 is about 0.6 V vs. RHE and the limitation such as the low separation efficiency of photogenerated electrons and holes are disadvantage to the photoconversion efficiency of self-driven tandem cell. It has been reported that coating with TiO2 can build a tunneling barrier for photogenerated holes thus reliably prevent pinholes of the films and suppress active corrosion over macroscopic areas, which can efficiently overcome the essential defect of BiVO4 [19], [20], [21], [22]. Furthermore, to reduce the turn-on voltage and achieve a relatively high photocurrent density of the film at the lower potential, photoanode coupling with oxygen evolution catalyst (OEC), such as Co3O4 [23], NiB [24] and iron oxyhydroxide (FeOOH), is considered an efficient strategy and has been widely investigated. Notably, FeOOH is known to be excellent in evolving O2 at moderate overpotentials for its weak adsorption of O2 or other intermediates in the oxygen reduction [25], [26].
Besides, as one of the few metal oxides that naturally show p-type conductivity, Cu2O has a direct band gap of ca. 2.0 eV which means a considerable light absorption capability in the visible-light region [27], [28]. The conduction band of Cu2O (−0.7 eV) is negative than the potential of hydrogen evolution (E = 0 eV), which means the Cu2O has a powerful driving force to drive water reduction theoretically [29]. Generally, the turn-on potential of Cu2O is 0.4–0.6 V vs. RHE, which is positive than that of the BiVO4 photoanodes. There must be a point of intersection between their current-voltage curves. Thus, put n-BiVO4 and p-Cu2O electrodes in solar water splitting tandem cell could be carried out at zero bias.
In this study, we construct a simple and efficient PEC tandem cell with the FeOOH modified TiO2/BiVO4 as photoanode and Cu2O as photocathode for self-driven water splitting. The cooperation of the TiO2 and FeOOH can both increase the photocurrent and reduce the turn-on voltage of BiVO4 electrode. The unassisted photocurrent density and corresponding hydrogen production of the tandem PEC cell were also detected to further confirm its photoelectric property and stability.
Section snippets
Preparation of photoanodes
All chemicals used in this study were of analytical grade and without further purification. The BiVO4 photoanodes were prepared by electrochemical deposition [30]. A typical three-electrode cell was used for electrodeposition with a fluorine-doped tin oxide (FTO) glass as working electrode (WE), a Ag/AgCl (4 M KCl) electrode as the reference electrode (RE) and a platinum electrode as the counter electrode (CE). The BiOI precursor solution was as the electrolyte and the potentiostatically −0.3 V
Structure of BiVO4–Cu2O PEC tandem cell
The simplified schematic diagram of BiVO4–Cu2O PEC tandem cell for overall water splitting is shown in Fig. 1. To establish a theoretical feasibility PEC tandem cell, the conduction band of photoanode (BiVO4) must be lower than the valence band maximum of the selected photocathode (Cu2O). The equilibrated Fermi energy of both photoelectrodes as the reported values can generate sufficient photopotential to overcome the required 1.23 V plus electrochemical over potentials for water splitting. A
Conclusions
In summary, a facile and costless route to improve the output photocurrent in the tandem cell has been demonstrated by FeOOH modified TiO2/BiVO4 photoanode and Cu2O photocathode simultaneously. The introduce of TiO2 has inhibited the photogenerated electron-hole surface recombination, and the holes from the BiVO4 layer is efficiently collected by FeOOH for facilitating water oxidation into O2. In the meantime, the remaining electrons were transiting to the photocathode through an external
Acknowledgements
This study was supported by the National Nature Science Foundation of China (No. 21471054), the Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007),the Hunan Provincial Natural Science Foundation of China (Grant No. 2017JJ2326), the Natural Science Foundation of Chongqing, China (No. cstc2018jcyjAX0733).
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Contributed equally to this work.