Balancing the specific surface area and mass diffusion property of electrospun carbon fibers to enhance the cell performance of vanadium redox flow battery
Graphical abstract
There is a trade-off between the surface area and mass diffusion when employing the electrospun carbon fibers as the electrode for vanadium redox flow battery. Our experimental results demonstrate that when the carbon fiber diameter increases from 0.28 μm to 1.82 μm, the limiting current density boosts from 0.76 A cm−2 to 2.50 A cm−2 without sacrificing the electroactivity for vanadium redox reactions.
Introduction
Vanadium redox flow batteries (VRFBs) considered as one of viable large-scale electrical energy storage systems for the utilization of intermittent and unstable renewables have drawn considerable attention in recent decades [[1], [2], [3], [4]]. Besides the common advantages of flow batteries, including the flexible design, high reliability, and long lifespan, VRFBs possess unique merit that is endowed by the application of the vanadium in different oxidation states. Therefore, the notorious issue for conventional redox flow batteries, cross-contamination between the two reaction components, can be prevented [5,6]. However, the extensive commercialization of VRFBs is still impeded by the high system cost nowadays. One of the approaches to reduce the system cost is to improve the stack performance, for instance, operating the stack at a high current density without compromising the energy efficiency and electrolyte utilization, thereby largely reducing the stack size and the corresponding cost [[7], [8], [9], [10], [11]].
The stack performance is highly depended on the porous electrodes, on which the vanadium redox reactions take place. The surface states of porous electrodes, including wettability and grated functional groups, determine the redox kinetics while their transport properties, including the porosity and pore distribution, determine the transport of different species [[12], [13], [14], [15], [16]]. Hence, optimizing the porous electrodes will significantly promote the stack performance. However, the commonly-used carbon electrodes, including carbon (graphite) felts, carbon papers and carbon cloths, with an average fiber diameter of 10 μm are suffered from either the poor electroactivity or the unsatisfied mass transport [11,[17], [18], [19]].
To address this issue, electrospun carbon fibers (ECFs) have been introduced to fabricate high-surface-area electrodes for VRFBs recently. Versatile electrode compositions and tailorable electrode structures can be readily made through varying several parameters of the electrospinning technique [20]. Normally, the diameter of carbon fibers can be reduced to several hundreds of nanometers, which will largely boost the specific reaction area [21]. Yan's group and Flox's group have done some pioneering work in the fabrication of ECFs with nanoscale diameters as the electrodes for VRFBs [[22], [23], [24], [25], [26]]. Besides the reduction of fiber diameter, the carbonization temperature [22] and carbonization time [23] were also demonstrated as two critical parameters to influence the surface composition and microstructure of ECFs, thereby affecting the vanadium redox activity [24]. To further increase the electroactive sites and modify the physicochemical properties of ECFs, particularly the wettability, different types of carbonaceous materials [27,28] and transition metal oxides [[29], [30], [31], [32], [33]] were introduced during the fabrication process.
However, the nanosized fiber diameter of ECFs will bring about a small pore size distribution and low porosity, which will lead to a severe mass-transport polarization when the free-standing ECFs are used as the electrodes. The poor mass transport behavior was realized in recently published work, particularly when VRFBs were operated at high current densities. For instance, Xu and coworkers [34] directly employed an ultrathin free-standing ECFs with a fiber diameter of 200–300 nm and a thickness of around 50 μm as the electrode for VRFBs. Despite the reduced internal resistance, it was proved that a severe concentration polarization was taken place due to their small pore size. Even at 40 mA cm−2, the energy efficiency was only 74.1%, and the electrolyte utilization was even lower than 70%, which clearly demonstrated a severe mass transport polarization. To tackle this issue, the same group fabricated another ECFs with enlarged pore sizes by a horizontally-opposed blending electrospinning method, in which polyvinylpyrrolidone was simultaneously electrospun at an opposite direction and removed by solvents to create the large pore sizes in the final ECFs [35]. It was shown that the mass transport of VRFBs became better when the as-prepared ECFs were employed as electrodes. Nevertheless, fewer studies have addressed the mass transport properties of ECFs for the flow situations of VRFBs [36].
Herein, in the present contribution, we fabricated ECFs with improved mass transport properties, which was realized by electrospinning the concentrated polyacrylonitrile solution, thereby significantly augmenting the pore diameter and enhancing the mass transport properties. Not only the electroactivity related properties, including the specific surface area and redox activity, were measured, but also the mass-transport related properties, including the pore sizes, porosity, pore distribution, were comprehensively analyzed. Finally, the cell performance of VRFBs with different ECFs was measured in a full flow battery.
Section snippets
Materials
Polyacrylonitrile (PAN, MW = 150,000), N, N-dimethylformamide (DMF) and sulfuric acid (95%) were obtained from Sigma-Aldrich. Vanadyl sulfate (VOSO4·3H2O, ≥99%) and Nafion 212 were respectively obtained from Shenyang Haizhongtian Fine Chemical Factory and Ion Power. Deionized (DI) water (resistivity≤18.2 MΩ) was used for the preparation of the electrolytes. All the other chemicals were used as received.
Electrospun carbon fiber fabrication
PAN powers mixed with DMF was heated at 60 °C with magnetically stirring. PAN solution with
Morphology and transport property
ECFs with the PAN concentration ranged from 9 wt% to 18 wt% were prepared in this work. The final morphologies of ECFs with different polymer concentrations are presented in Fig. 1. The fiber diameter is significantly increased with an increase in the PAN concentration. The average fiber diameter was estimated based on the evaluation of 150 randomly selected fibers, which was processed with ImageJ software. The distribution of fiber diameter is illustrated in Fig. 1e, from which it is observed
Conclusions
In summary, ECFs with different structural properties were fabricated by adjusting the precursor concentration and employed as the electrodes for VRFBs. When the precursor concentration increased from 9 wt% to 18 wt%, ECFs with an average pore diameter ranging from 1.32 μm to 9.05 μm were obtained. The structure and transport properties were then comprehensively investigated. Experimental results proved that the mass transport in the ECFs with enlarged fiber diameter was significantly improved
Acknowledgments
The work described in this paper was supported by the grant from Research Grants Council (Project No. T23-601/17-R) and the Innovation and Technology Commission (Project No. ITS/177/17FP) of the Hong Kong Special Administrative Region, China.
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These authors contributed equally to this work.