Heteroatom-doped electrodes for all-vanadium redox flow batteries with ultralong lifespan

Abstract

The phosphorus and fluorine codoped graphite felt electrodes with prominent hydrophilicity present excellent electroactivity towards V²⁺/V³⁺ and VO²⁺/VO₂⁺, elevate the discharging ability up to 250 mA cm⁻² and dramatically extend the energy efficiency of vanadium redox flow batteries towards 1000 cycles with 0.003% reduction per cycle.

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Renewable and clean energy (such as wind and solar energy, etc.) is considered as the choice of next-generation energy, whose utilization is hindered by its unstable and intermittent nature.^1–4 The all-vanadium redox flow battery (VRFB) is regarded as one of the most promising large-scale energy storage devices, due to its high safety, long cycle life and low cost of maintenance.^5–14 However, currently available VRFBs are still impeded by a low energy efficiency (EE) and short lifetime.^15–17

Among the VRFB components, the electrode is the dominant factor that constrains VRFB performance.^18,19 The carbon-based electrode materials^20–23 that function as the reaction zone for the redox reactions of V²⁺/V³⁺ and VO²⁺/VO₂⁺ have been studied extensively. The strategies of increasing electrode electro-activity can be generalized into morphology-changed and morphology-preserved modification. The morphology-changed modifications include the growth of metal and metal oxides (Ir,²⁴ Pt,²⁵ Bi,²⁶ Cu,²⁷ Mn₃O₄,^28,29 PbO₂,³⁰ WO₃,³¹ MoO₂,³² CeO₂ (ref. 33)), carbon nanotubes^21,34–36 and graphene,^37–40 but their stability and cost should be taken into consideration, in addition to degradation of battery lifetime. Heteroatom-doping precisely avoids the aforementioned drawbacks, usually adopting non-metal elements as the doping choice, and is the main avenue for morphology-preserved modification. Much progress based on nitrogen,^41,42 chalcogen⁴³ and halogen⁴⁴ doping has been achieved in previous works. However, the battery durability is barely satisfactory. Consequently, it is still of urgency to explore an ultra-stable electrode with high efficiency.

Herein, the phosphorus and fluorine co-doped graphite felt (PF-GF) electrode was developed via dipping and high-temperature carbonization, and applied in the VRFB. Compared with the pristine GF electrode based VRFB, the VRFB based on the PF-GF electrode operates well at a high current density (250 mA cm⁻²) and exhibits an EE of 79% after 1000 cycles at 120 mA cm⁻² with a 0.003% reduction per cycle. The structural integrity of the PF-GF electrode is preserved after such a long-time circulation. The outstanding performances demonstrate that P and F co-doping results in high-efficiency and can promote the stability of the VRFB.

The PF-GF electrode is acquired through pyrolysis of GF deposited with KPF₆; the preparation procedure is schematically illustrated in Fig. 1a. The hydrophilicity was dramatically improved after P and F co-doping compared with that of the pristine GF (Fig. 1a), and no morphology change was observed in the PF-GF from the scanning electron microscopy (SEM) images (Fig. 1c–f) and transmission electron microscopy (TEM) images (Fig. S1a–c†).

Fig. 1 (a) Scheme illustration for preparing the PF-GF electrode. (b) Photograph showing the hydrophilicity of PF-GF and GF. SEM images of GF (c and d) and PF-GF (e and f) with different magnifications. The scale bars in (c) and (e) are 1 μm and the scale bars in (d) and (f) are 100 nm.

To further gain understanding of the chemical composition and functional groups, X-ray photoelectron spectroscopy (XPS) was carried out, and the C 1s and O 1s peaks appear at 284.2 and 532 eV (Fig. 2a), respectively. The O/C ratio of the PF-GF is 0.36, which is higher than that of the GF (0.19) (Fig. S2†). The C 1s spectrum could be deconvoluted into C=O (286.5 eV), C–O (286.2 eV) and C=C (284.3 eV)⁴⁵ peaks (Fig. 2b), and F and P signals are also detected in the PF-GF spectrum (Fig. 2c and d). The results possibly originate from the pinning effects of phosphorus and fluorine elements.⁴⁶ The electro-catalytic effects of P/F atoms can also appear in the carbon materials for other batteries.^47,48 Oxygen-containing groups offer more active sites for vanadium ion redox reactions.³⁸ Additionally, the high oxygen content is an explanation of the good hydrophilicity of the PF-GF.

Fig. 2 (a) XPS spectrum of the PF-GF and (b) C 1s, (c) F 1s and (d) P 2p spectra.

The P and F bring in plentiful defects. From the Raman spectra (Fig. 3), the intensity ratio of the D and G bands, which represents the degree of graphitization, increases from 0.82 in the GF to 0.99 in the PF-GF. This implies an increment in the number of defects, facilitating many more active sites.

Fig. 3 Raman spectra of the PF-GF (red) and GF (black).

The increased number of oxygen-containing groups and defects caused by P and F dramatically promote the reaction of VO²⁺/VO₂⁺ and V²⁺/V³⁺. The cyclic voltammetry curves of the PF-GF electrode show obvious redox reactions. The cathodic and anodic peak potential differences (ΔE) are 457 and 316 mV, respectively (Fig. 4a and b). However, only one peak for the VO²⁺/VO₂⁺ reaction couple was observed for the GF, and the ΔE is 619 mV. The related parameters are listed in Table 1 and can be found in the ESI.† Meanwhile, the reduction (I_pc) and oxidation (I_pa) peak current densities of the PF-GF electrode are improved, and its absolute value of I_pc/I_pa is closer to 1 for the VO²⁺/VO₂⁺ couple at various scanning rates (Fig. S3a–d†), which coincides with the ΔE results. In addition, the peak current value density of the PF-GF rises vastly compared with that of the GF. According to the Randles–Sevcik equation (i_p = 2.99 × 10⁵α^1/2AFnD^1/2ν^1/2C), the diffusion coefficient (D) of the PF-GF is better than that of the GF, especially for V³⁺/V²⁺ (Fig. 4c). The electrochemical impedance spectra of the PF-GF and GF include a semi-circle at the high-frequency region and an oblique line at the low-frequency range, which suggests that the vanadium redox reactions were dominated by charge transfer and diffusion processes. The tremendous reduction of charge transfer resistance demonstrates more favourable electron transfer for the redox reaction (Fig. 4d). The aforementioned results point out that the PF-GF with high reversibility accelerates the redox reaction kinetics.

Fig. 4 The CV profiles of (a) VO₂⁺/VO²⁺ and (b) V³⁺/V²⁺ redox reactions with the PF-GF and GF electrodes at 10 mV s⁻¹. (c) The plots of peak current density versus the square root of the scanning rate for the VO²⁺/VO₂⁺ redox couples. (d) Impedance tests of the PF-GF and GF electrode in the 0.05 M VOSO₄ + 3 M H₂SO₄ electrolyte with 5 mV amplitude.

Table 1 Electrochemical properties obtained from cyclic voltammetry tests of the VO₂⁺/VO²⁺ reactions on the GF and PF-GF electrodes

	I pa (mA cm^−2)	I pc (mA cm^−2)	E pa (mV)	E pc (mV)	−Ipc/Ipa	ΔE (mV)
GF	88.7	−52.2	1198	579	0.59	619
PF-GF	106.4	−89.8	1125	668	0.84	457

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The charging/discharging capabilities of the VRFB based on the PF-GF and GF electrodes were examined in the range from 50 mA cm⁻² to 250 mA cm⁻². The GF electrode based VRFB fails above 150 mA cm⁻². However, the PF-GF electrode has both enhanced energy and voltage efficiency up to 250 mA cm⁻² of 54.9% and 56.1% (Fig. 5a and b), respectively. Compared with that of the GF based VRFB, the overpotential of the PF-GF based VRFB reduces by 94 mV, and the discharging capacity increases by 32% from 14.67 to 19.34 Ah L⁻¹ at 100 mA cm⁻² (Fig. 5c). During the overall charging/discharging process, the overpotential increases with the increment of current density (Fig. 5d). The increase in overpotential of the PF-GF is not as steep, signifying a lower resistance in the VRFB. The results reveal that the PF-GF electrode with enhanced hydrophilicity has vastly reduced interface resistance with the electrolyte, and adds sites with high electroactivity to accelerate vanadium ion redox reactions.

Fig. 5 The (a) VE and (b) EE of the PF-GF and GF electrodes at various current densities, (c) galvanostatic charging/discharging curves of the PF-GF and GF electrodes at 100 mA cm⁻², (d) the plot of overpotential versus current density, and (e) the cycling performance of the PF-GF and GF electrodes at 120 mA cm⁻².

After 1000 cycles, the heteroatom doping with oxygen and fluorine plays the main role in stabilizing the electrode, and no morphology change has been observed (Fig. S4 and S5†). As a result, the heteroatom co-doped electrode enables the VRFB to operate well for 1000 cycles at 120 mA cm⁻², and the average EE of the cell when using the heteroatom co-doped electrode is 79.2% with 0.003% recession per cycle with slow capacity decay (Fig. 5d and S6†), which indicates the remarkable stability. The heteroatom co-doped electrode displays much better stability and rate capability than GF or other modified GF electrodes in many previous works (Table S1†).

In conclusion, the heteroatom-doped electrode, which acquires P and F from a single dopant, has been demonstrated, and exhibits high electrochemical reversibility and excellent electrocatalytic activity, especially for the V²⁺/V³⁺ redox reaction. The VRFB with the heteroatom co-doped electrode reveals splendid rate capability, improved energy and voltage efficiency, and cycling stability. This feasible electrode treatment strategy will inevitably push the VRFB field in a more promising direction.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Ministry of Science and Technology of the People’s Republic of China (Grant No. 2016YFA0202500) and the National Natural Science Foundation of China (Grant No. 51772093).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta07358e

‡ The authors equally contributed to this work.

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