Highly catalytic hollow Ti3C2Tx MXene spheres decorated graphite felt electrode for vanadium redox flow batteries
Introduction
Nowadays, renewable energies such as solar and wind are essential to economic and social development [[1], [2], [3], [4], [5]]. However, the fluctuant and intermittent nature makes them less appealing for applications since the introduction of more than 20% intermittent energy from renewables without storage could destabilize the grid with looming threats of frequency and voltage fluctuations [6]. Underlying these considerations, rechargeable grid-scale energy storage systems with inherent safety and flexible operation such as redox flow batteries provide a potential solution to address this issue [[7], [8], [9], [10], [11], [12]]. Among various kinds of redox flow batteries, the all-vanadium redox flow batteries (VRFBs) proposed by Skyllas-Kazacos et al. have received the most attention owing to the advantages of significantly low cross-contamination and a long lifetime by enlisting the same element, vanadium, in both negative and positive electrolytes [[13], [14], [15], [16]].
Although compelling merits and continuous developments of VRFBs have been made over past decades, the widespread commercialization of this technology is still hindered by both economic and technical barriers [17,18]. One of the most severe issues is the high cost induced by the low operating power density of the cell stack and costly components. A commonly recognized way to address this problem is to operate the VRFBs under high charge/discharge current density without sacrificing the efficiencies, which is tantamount to decreasing the size of the cell stack for the given power requirement, thus reducing the amount of cell component materials including bipolar plates, membranes, and electrodes [[19], [20], [21]]. However, similar to any other electrochemical systems, increase of operating current density in a VRFB would also increase the polarization losses including the activation loss, ohmic loss, and concentration loss, consequently resulting in the declined overall performance [[22], [23], [24], [25], [26]].
As a core component of VRFBs, the electrode served as a platform for electrochemical redox reactions plays a crucial role in determining the battery performance. Up to now, graphite felt and carbon felt have been mostly used as electrodes for VRFBs due to their wide operating potential range, satisfactory stability, high electrical conductivity, high corrosion resistance, and relatively low cost. However, the pristine graphite or carbon felt electrode exhibits a very inferior battery performance, primarily due to its inherent problems of poor electrochemical activity and a low specific area leading to insufficient active sites for vanadium redox reactions [27]. To improve the kinetics of vanadium redox reactions on the electrode surface, a variety of modification methods have been carried out to tune the surface properties of the electrodes, which can be generally classified into three categories: 1) Introducing functional groups onto the electrode surface. In VRFBs, the surface functional groups can facilitate improving the electrocatalytic activity of carbonaceous materials, and therefore, surface treatment methods including thermal treatment [28], nitrogen treatment [29], acid treatment [30], plasma treatment [31], electrochemical oxidation treatment [32], H2O2 treatment [33] and some other treatment methods [34,35] have been developed to improve the battery performance; 2) Decorating electrocatalysts on the fiber surface. Up to now, a variety of carbon-based and metal-based nanostructured electrocatalysts such as carbon nanoparticles [[36], [37], [38]], graphene [39,40], graphene oxide [41,42], carbon nanotubes [43], carbon sheet [44,45], carbon network [46], carbon spheres [47,48], Bi [6,49,50], Sn [51], Cu [52], TiC [53], TiN [[54], [55], [56]], B4C [57], WO3 [58,59], TiO2 [60,61], CeO2 [62], ZrO2 [29], Mn3O4 [63], and etc, have been employed to grow or adhere on the carbon fiber to improve the battery performance; 3) Creating pores on the electrode fibers via surface etching methods including CO2 etching [64,65], water etching [66], KOH etching [67,68], NiO etching [69], ZnO etching [70], and FeOOH etching [71]. These etching methods are capable of creating abundant secondary pores on the fiber surfaces, providing ample active sites for electrochemical reactions.
All of the methods mentioned above are effective and can promote the development of high-performance electrodes for VRFBs. Nevertheless, a common drawback still existing is that the battery performance, especially for energy efficiency, quickly deteriorates during long-term cycling, which is usually induced by the disintegration of the modified electrodes. Another issue worth paying attention to is that the reaction kinetics of positive and negative redox reactions as well as their need for catalysis efforts are different. For instance, in previous research, Mench et al. investigated the VRFB via a dynamic hydrogen electrode and found that the negative electrode comprising the V3+/V2+ redox couple, contributed approximately 80% of the total cell overpotentials [72]. As a result, they suggested the catalysis efforts should be taken to the V3+/V2+ reaction in the negative reaction rather than the VO2+/VO2+ reaction in the positive electrode. Recently, Ha et al. employed PAN- and Rayon-based felts for VRFBs through various sophisticated physical and electrochemical analyses and elucidates the V3+/V2+ redox reaction in the negative side is performance limiting ones for both electrodes [73]. Although the rate-determining step of vanadium reactions during VRFBs operation still exists ambiguity and different opinions, more and more recent researches indicate that developing high-performance negative electrode for V3+/V2+ redox reactions is crucial to state-of-the-art VRFB technology.
In this work, hollow Ti3C2Tx MXene spheres belonging to the family of transition metal carbides and nitrides [74,75], are investigated as novel electrocatalysts for negative V3+/V2+ redox reactions for the first time. As a rare kind of flexible material with high electrical conductivity and excellent hydrophilicity among numerous MXenes (Nb2C, Ti3C2, Ta4C3, etc.), Ti3C2 with different kinds of termination groups (Tx) on its surface (such as fluorine, oxygen, and hydroxyl groups), has unique structure and surface chemistry with a combination of properties such as excellent mechanical stability, metallic conductivity, and low cost. These superior properties distinguish it from other materials and render it as a promising candidate as electrocatalyst for vanadium redox reactions. Our density functional theory shows that Ti3C2 has high electrical conductivity with abundant exposed Ti atoms. Then, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests demonstrate that these hollow MXene spheres have high stability and catalytic activity towards V3+/V2+ redox reaction. In the charge-discharge performance tests, the present battery with MXene decorated graphite felt electrode achieves an energy efficiency of 81.3% at 200 mA cm−2 and 75.0% at 300 mA cm−2, which are 15.7% higher than the pristine electrode at 200 mA cm−2 and 12.8% higher than the XC-72 decorated electrode at 300 mA cm−2. More importantly, the MXene decorated electrode exhibits outstanding durability during long-term cycle tests. All these superior results indicate that the MXene decorated graphite felt could open up a new avenue for the application of high-performance electrodes for VRFBs.
Section snippets
Computational methods
Spin-polarized density functional theory (DFT) based first-principles calculations were carried out adopting ABINIT code [76]. The exchange-correlation functional was dealt by adopting generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) type [77]. The electron-ion interactions were modeled by Projector-augmented-wave (PAW) potentials [78]. To ensure a satisfied convergence for wave-basis expansion, the energy cutoff of 24 Ha was used. The Brillouin zones with 10 × 10 × 1
Results and discussion
The atomic structure and electronic properties of Ti3C2 are firstly evaluated by first-principle calculations, as shown in Fig. 2. From the atomic structure in Fig. 2a, it is seen that two kinds of Ti exist in the Ti3C2: one is in the center, and the other in the surface. The Ti–C bond lengths for that in the center and in the surface are 2.22 and 2.06 Å, respectively, which is caused by the surface effect. More importantly, the layered structured Ti3C2 has abundant exposed and unsaturated Ti
Conclusion
In summary, we synthesized hollow Ti3C2Tx spheres decorated graphite felt electrode for VRFBs. The as-prepared hollow Ti3C2Tx spheres, in the form of 3D nanostructured MXene, exhibited high catalytic activity towards V3+/V2+ redox reaction according to the CV and EIS tests. Meanwhile, a substantial enhancement of battery performance was observed with the incorporation of MXene electrocatalysts. At 200 mA cm−2, the present battery enabled the electrolyte utilization efficiency and energy
Acknowledgements
The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R).
References (87)
- et al.
Appl. Energy
(2019) - et al.
Prog. Energy Combust. Sci.
(2018) - et al.
J. Power Sources
(2017) - et al.
J. Energy Chem.
(2018) - et al.
J. Power Sources
(2018) - et al.
Appl. Therm. Eng.
(2019) - et al.
Appl. Energy
(2017) - et al.
J. Membr. Sci.
(2019) - et al.
Appl. Energy
(2017) - et al.
J. Power Sources
(2017)
Appl. Energy
J. Power Sources
J. Power Sources
Appl. Energy
Batteries
Electrochim. Acta
Electrochim. Acta
J. Power Sources
J. Power Sources
Appl. Surf. Sci.
Electrochim. Acta
J. Power Sources
J. Electroanal. Chem.
Appl. Energy
Energy Storage Mater.
Surf. Coat. Technol.
Electrochim. Acta
Electrochim. Acta
Carbon
J. Power Sources
J. Power Sources
Nano Energy
J. Power Sources
Ionics
Appl. Energy
J. Power Sources
Electrochim. Acta
J. Power Sources
J. Power Sources
Solid State Ion.
Carbon
Appl. Energy
J. Power Sources
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These authors contributed equally to this work.