An efficient tandem photoelectrochemical cell composed of FeOOH/TiO2/BiVO4 and Cu2O for self-driven solar water splitting

https://doi.org/10.1016/j.ijhydene.2018.11.032Get rights and content

Highlights

  • The FeOOH modified TiO2/BiVO4 photoanode was synthesized.

  • A BiVO4–Cu2O unbiased PEC tandem cell with competitive J-V performance was build.

  • The tandem cell has a photoconversion efficiency of 0.46% at zero bias.

Abstract

An integrated solar water splitting tandem cell without external bias was designed using a FeOOH modified TiO2/BiVO4 photoanode as a photoanode and p-Cu2O as a photocathode in this study. An apparent photocurrent (0.37 mA/cm2 at operating voltage of +0.36 VRHE) for the tandem cell without applied bias was measured, which is corresponding to a photoconversion efficiency of 0.46%. Besides, the photocurrent of FeOOH modified TiO2/BiVO4–Cu2O is much higher than the operating point given by pure BiVO4 and Cu2O photocathode (∼0.07 mA/cm2 at +0.42 VRHE). Then we established a FeOOH modified TiO2/BiVO4–Cu2O two-electrode system and measured the current density-voltage curves under AM 1.5G illumination. The unassisted photocurrent density is 0.12 mA/cm−2 and the corresponding amounts of hydrogen and oxygen evolved by the tandem PEC cell without bias are 2.36 μmol/cm2 and 1.09 μmol/cm2 after testing for 2.5 h. The photoelectrochemical (PEC) properties of the FeOOH modified TiO2/BiVO4 photoanode were further studied to demonstrate the electrons transport process of solar water splitting. This aspect provides a fundamental challenge to establish an unbiased and stabilized photoelectrochemical (PEC) solar water splitting tandem cell with higher solar-to-hydrogen efficiency.

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).

References (58)

  • S. Zhang et al.

    Electrodeposition of polyhedral Cu2O on TiO2 nanotube arrays for enhancing visible light photocatalytic performance

    Electrochem Commun

    (2011)
  • J.-N. Nian et al.

    Electrodeposited p-type Cu2O for H2 evolution from photoelectrolysis of water under visible light illumination

    Int J Hydrogen Energy

    (2008)
  • L. Ge

    Novel Pd/BiVO4 composite photocatalysts for efficient degradation of methyl orange under visible light irradiation

    Mater Chem Phys

    (2008)
  • T. Yamashita et al.

    Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials

    Appl Surf Sci

    (2008)
  • Z. Bai et al.

    A Cu2O/Cu2S-ZnO/CdS tandem photoelectrochemical cell for self-driven solar water splitting

    J Alloy Comp

    (2017)
  • F. Zhan et al.

    In situ formation of CuWO4/WO3 heterojunction plates array films with enhanced photoelectrochemical properties

    Int J Hydrogen Energy

    (2015)
  • M. Shao et al.

    Hierarchical nanowire arrays based on ZnO core-layered double hydroxide shell for largely enhanced photoelectrochemical water splitting

    Adv Funct Mater

    (2014)
  • J.H. Kim et al.

    Wireless solar water splitting device with robust cobalt-catalyzed, dual-doped BiVO4 photoanode and perovskite solar cell in tandem: a dual absorber artificial leaf

    ACS Nano

    (2015)
  • X. Zhang et al.

    A perovskite solar cell-TiO2@BiVO4 photoelectrochemical system for direct solar water splitting

    J Mater Chem A

    (2015)
  • X. Wang et al.

    Silicon/hematite core/shell nanowire array decorated with gold nanoparticles for unbiased solar water oxidation

    Nano Lett

    (2013)
  • W. Raja et al.

    Perovskite nanopillar array based tandem solar cell

    ACS Photonics

    (2017)
  • S. Chen et al.

    In-situ and green method to prepare Pt-free Cu2ZnSnS4 (CZTS) counter electrodes for efficient and low cost dye-sensitized solar cells

    ACS Sustainable Chem Eng

    (2015)
  • W.S. Dos Santos et al.

    A hole inversion layer at the BiVO4/Bi4V2O11 interface produces a high tunable photovoltage for water splitting

    Sci Rep

    (2016)
  • J. Luo et al.

    Cu2O nanowire photocathodes for efficient and durable solar water splitting

    Nano Lett

    (2016)
  • J. Guo et al.

    Reduced titania@layered double hydroxide hybrid photoanodes for enhanced photoelectrochemical water oxidation

    J Mater Chem A

    (2017)
  • Z. Li et al.

    Photoelectrochemical cells for solar hydrogen production: current state of promising photoelectrodes, methods to improve their properties, and outlook

    Energy Environ Sci

    (2013)
  • K. Zhang et al.

    Water splitting progress in tandem devices: moving photolysis beyond electrolysis

    Adv Energy Mater

    (2016)
  • Y.-H. Lai et al.

    Multifunctional coatings from scalable single source precursor chemistry in tandem photoelectrochemical water splitting

    Adv Energy Mater

    (2015)
  • K. Tolod et al.

    Recent advances in the BiVO4 photocatalyst for sun-driven water oxidation: top-performing photoanodes and scale-up challenges

    Catal

    (2017)
  • Cited by (42)

    • Recent progress in hydrogen: From solar to solar cell

      2024, Journal of Materials Science and Technology
    View all citing articles on Scopus
    1

    Contributed equally to this work.

    View full text