Potassium phosphotungstate spheres as an anode material for a solar rechargeable battery

Abstract

Potassium phosphotungstate spheres, prepared by a simple coprecipitation method, are introduced for the first time as an anode material for a solar rechargeable battery. The microstructure of the as-prepared potassium phosphotungstate spheres is characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that the obtained potassium phosphotungstate particles were monodisperse and uniform spheres and present good photo-generated electron storage capacity with suitable redox potentials. Correspondingly, the as-fabricated battery using potassium phosphotungstate as the anode material exhibits satisfactory solar energy conversion and storage as well as rechargeable capability. Therefore, the potassium phosphotungstate spheres can be used as potential anode material for applications in the solar rechargeable battery.

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Exploring renewable and clean energy sources is the practical solution to solving the problem of global warming and the energy crisis.^1,2 Solar energy has been considered as the best substitute for conventional fossil sources, due to its advantage of being abundant, environmentally benign, and inexpensive.³ However, the bottleneck in efficiently using solar energy is the intermittent nature of solar irradiation, which poses significant impacts to the reliability and stability of the electric grid. Therefore, developing affordable solar energy storage devices with the bi-function of converting solar energy to electricity and storing the solar-generated electricity is a feasible way to solve the bottleneck problem of how to efficiently use solar energy.

A solar rechargeable battery is a type of solar energy conversion and storage system. The battery can convert the solar energy into chemical/electrical energy, and the chemical energy can be stored directly inside the batteries.^4–9 In general, a solar rechargeable battery consists of a dye-sensitized TiO₂ electrode, membrane, cathode, and anode. In recent years, various materials have been developed as anode materials to store photo-generated electrons, such as WO₃ and conducting polymer.^10–12 In order to efficiently store photo-generated electrons, the anode material should possess the bi-function of receiving photo-generated electrons and storing energy. The selection of anode materials is highly important to improve the performance of the solar rechargeable battery. Polyoxometalates (POMs) have numerous applications in catalysis, photochromism, electrochromism, and photoluminescence, and promising multifunctional materials have been created for future use by utilizing these phenomena.^13,14 Recently, polyoxometalates have received extensive interest because they are attractive active species used in electrochemical rechargeable batteries, due to their versatile, tunable properties and multielectron redox reactions.^15–17 In particular, the unique electrical properties of polyoxometalates are caused by the reversible intercalation of ions (e.g., H⁺, Li⁺).^13,14 Also, the conduction band of the phosphotungstate is lower compared to TiO₂,^18,19 which makes it suitable for photo-generated electron storage. With this in mind, we focused on phosphotungstate as an anode material for a solar rechargeable battery to store the photo-generated electrons.

In this work, potassium phosphotungstate (K₃PW₁₂O₄₀) spheres are prepared by a simple coprecipitation method and are introduced for the first time as an anode material for a solar rechargeable battery. Our research shows that the as-fabricated solar rechargeable battery using potassium phosphotungstate spheres as an anode material for photo-generated electron storage is an effective, workable strategy for solar energy conversion and storage.

Fig. 1 displays typical SEM and TEM images of the as-prepared K₃PW₁₂O₄₀. As shown in Fig. 1(a), the K₃PW₁₂O₄₀ particles are uniform spheres with a diameter of approximately 300 nm. The TEM image (Fig. 1(b)) shows that the discrete and uniform spherical morphology of K₃PW₁₂O₄₀ is well preserved after calcination. Fig. 1(b) shows K₃PW₁₂O₄₀ spheres with diameters of 300 ± 50 nm, which agrees well with the aforementioned SEM image.

Fig. 1 (a) SEM image of the as-prepared K₃PW₁₂O₄₀ and (b) the TEM image of the as-prepared K₃PW₁₂O₄₀.

The structure of as-prepared K₃PW₁₂O₄₀ and H₃PW₁₂O₄₀ was studied by means of XRD. As displayed in Fig. 2(b), it is clear that H₃PW₁₂O₄₀ shows characteristic diffraction peaks at 10.2°, 21.3°, 25.3°, 29.4°, 34.6° 53.2°, and 59.8°. After the co-precipitation process, it is shown that the XRD pattern of the as-prepared K₃PW₁₂O₄₀ spheres has obvious diffraction peaks at 10.7°, 21.6°, 26.5°, 30.7°, 36.2° 55.9°, and 62.9°, which is similar to the results reported by Haber and Chen et al.^20,21 It is clear that the diffraction peak with the maximum intensity shifted from 25.3° to 26.5° after the exchange of the H⁺ by K⁺. The diffraction patterns of K₃PW₁₂O₄₀ reveal that no traces of pure H₃PW₁₂O₄₀ were detected, which proved the presence of only one phase with good crystallinity.

Fig. 2 (a) XRD pattern of the as-prepared K₃PW₁₂O₄₀ and (b) the XRD pattern of H₃PW₁₂O₄₀.

The structure of the K₃PW₁₂O₄₀ sample was further characterized by FT-IR spectra. As shown in Fig. 3, the absorption peaks of H₃PW₁₂O₄₀ appear at 1080.1, 982.7, 889.0, 802.0, 595.2, and 523.5 cm⁻¹, which correspond to the peaks of absorption frequencies of the typical Keggin anion.²² After the simple co-precipitation process, the peaks of the as-prepared K₃PW₁₂O₄₀ appear at 1080.8, 983.9, 890.6, 810.6, 595.6, and 526.1 cm⁻¹, which were similar to the peaks of H₃PW₁₂O₄₀. It is indicated that the as-prepared K₃PW₁₂O₄₀ maintains a relatively complete Keggin structure with H₃PW₁₂O₄₀. It is clear that the peak of ν_as(W–O_c–W) shifted from 802.0 to 810.6 cm⁻¹. It has been reported that the H⁺ of the H₃PW₁₂O₄₀ was linked to the bridging oxygen (W–O_c–W), and thus, the main reason for the peak shift is probably the exchange of the H⁺ by K⁺.^20,23 It is evident that as-prepared K₃PW₁₂O₄₀ spheres maintain the integrity of the Keggin structure after the co-precipitation process.

Fig. 3 (a) Infrared spectra of the as-prepared K₃PW₁₂O₄₀ and (b) the infrared spectra of H₃PW₁₂O₄₀.

To verify the feasibility of using potassium phosphotungstate spheres as an anode, cyclic voltammograms (CVs) of the cathode/anode were tested. The CV of the K₃PW₁₂O₄₀ is shown in Fig. 4, and it is clear that there are two pairs of peaks appearing at −0.08/−0.28 V and −0.76/−0.87 V. The reduction potential (−0.28 V vs. Ag/Ag⁺) of the K₃PW₁₂O₄₀ is more positive than the potential of the conduction band edge of TiO₂ (−0.68 V vs. Ag/Ag⁺). It has previously been demonstrated that the photo-generated electrons can transport from the TiO₂ to the K₃PW₁₂O₄₀ anode through the external circuit under illumination.^10,12 However, the reduction potential of the second redox peak of the K₃PW₁₂O₄₀ (−0.76 V) is lower than the potential of the conduction band edge of TiO₂. This indicates that the K₃PW₁₂O₄₀ can be only partially reduced by the photo-generated electrons. In the typical CVs of LiI, two pairs of redox peaks appear at 0.17/−0.005 V and −0.29/−0.56 V, which correspond to a two-step reversible reaction.^8,24,25 The redox potential of the I₃⁻/I⁻ couple is lower than that of dye N719 (0.4 V; the CV is shown in Fig. S1, ESI†), which indicates that excited state dye N719 can oxidize I⁻ to a potential as high as 0.40 V under illumination.^23,24 Basically, the K₃PW₁₂O₄₀ used as the anode material would be feasible for photo-generated electron storage on the basis of the energy level or potential.

Fig. 4 Cyclic voltammograms of (a) the as-prepared K₃PW₁₂O₄₀ and (b) LiI (0.01 M LiI in propylene carbonate with 0.1 M LiClO₄).

In order to demonstrate the electrochemical performance of K₃PW₁₂O₄₀, the battery was fabricated with a dye-sensitized TiO₂ electrode, LiI as the cathode, and K₃PW₁₂O₄₀ as the anode. The configuration of the battery based on K₃PW₁₂O₄₀ as the anode is shown in Fig. 5. The working mechanism of the battery has been previously reported.^10,12

Fig. 5 Scheme of the configuration and working mechanism of the solar rechargeable battery.

The fabricated battery is photo-charged for 10 min with an illumination intensity of 100 mW cm⁻², and then discharged in the dark. As shown in Fig. 6(a), after under-illumination for 10 min (the J–V performance of the N719-based DSSCs is shown in Fig. S2 in the ESI†), the battery can be galvanostatically discharged for 9 min at a current density of 50 mA g⁻¹ in the dark, which indicates that the battery can successfully convert chemical energy into electrical energy. After photo-charging, the voltage of the battery reaches 0.65 V, indicating that the redox couples (I⁻ and K₃PW₁₂O₄₀) converted into the redox couples (I₃⁻ and Li_xK₃PW₁₂O₄₀) in the cathode and anode, respectively. This indicates that the solar energy can be converted into chemical energy. It is clear that the average working voltage of the battery is approximately 0.35 V in the discharge process, which corresponds to the average potential difference between the redox couples I⁻/I₃⁻ and K₃PW₁₂O₄₀/Li_xK₃PW₁₂O₄₀. For the first cycle, the discharge capacity of the K₃PW₁₂O₄₀ anode is 7.8 mA h g⁻¹, with the cut-off voltage at 0.1 V. Obviously, a battery based on K₃PW₁₂O₄₀ spheres can repeat the photo-charge and electrochemical discharge processes. After 15 cycles, the anode retains a discharge capacity of 12.6 mA h g⁻¹, demonstrating the good stability of the K₃PW₁₂O₄₀ anode for storing photo-generated electrons. Based on the working mechanism analysis and the experimental results shown above, the K₃PW₁₂O₄₀ spheres as an anode material were successfully applied to a solar rechargeable battery.

Fig. 6 (a) Potential profiles of the solar rechargeable battery with K₃PW₁₂O₄₀ as the anode at photo-charge under 100 mW cm⁻² irradiation and at the current density of 50 mA g⁻¹ for discharging in the dark. (b) Discharge capacities vs. cycle number during 15 cycles.

Conclusions

In summary, K₃PW₁₂O₄₀ spheres were successfully prepared by a simple coprecipitation method. It was demonstrated that the K₃PW₁₂O₄₀ spheres show suitable redox potentials and good electrochemical activity. The as-fabricated battery with K₃PW₁₂O₄₀ spheres as the anode shows excellent solar energy conversion, storage capability, and cycle stability. Therefore, the K₃PW₁₂O₄₀ spheres as an anode material were successfully applied to a solar rechargeable battery. The facile synthesis technique and the good electrochemical performance make K₃PW₁₂O₄₀ spheres a promising candidate as an anode material for solar rechargeable batteries.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

Financial support from the NSFC (21603093) and Natural Science Foundation of Jiangxi (20161BBE50095) are greatly appreciated.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7se00510e

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