Investigation of the electrospun carbon web as the catalyst layer for vanadium redox flow battery
Introduction
The VRFB has recently received considerable attentions as a large scale energy storage system for intermittent renewable energy (such as solar and wind energy, etc.) due to its outstanding properties such as long cycle life, high reliability, flexible design and environmental friendliness [1], [2], [3]. The VRFB employs and V2+/V3+ redox couples as positive and negative half cell, respectively. Its open circuit voltage is approximately 1.26 V at 100% stage of charge [4], [5]. Due to ions of the same chemical element are used in both half cells, the common problem of cross-contamination in other redox flow battery does not appear in VRFB. As an important part of VRFB, the electrode supplies the redox reaction with places and facilitates the reaction. Therefore, an ideal electrode should possess high electrochemical activity, high electrical conductivity, large surface area, appropriate wettability and stability in the concentrated acid solution. Up to now, carbon based materials especially PAN-based carbon felt (CF) or graphite felt are practically used in VRFB because of their large surface area, large porosity, high electrical conductivity and low cost. However, due to the poor kinetics and reversibility of the vanadium redox couples on these materials, the energy efficiency (EE) of VRFB is limited greatly. Therefore, considerable efforts have been devoted to enhance their electrochemical activity. The most common methods include thermal treatment, acid oxidation and active material modification [6], [7], [8], [9], while these methods just focus on the activity of the surface function groups (oxygen or nitrogen function groups) and the effect of surface area, influence of carbon structure on the electrochemical performance of the PAN-based carbon fibers is in lack of investigation.
Nowadays, electrospinning has been adopted as an efficient approach to prepare nanofibers and high surface area fibrous web. With this technique and subsequent carbonization process, PAN polymer and its composite can be easily made into carbon nanofibers (CNFs) or carbon fibrous web [10], [11], [12], [13]. By controlling the preparation process, PAN-based CNFs with different morphology, composition and structure can be obtained and their structure–function relationship can be studied systematically. Further more, because the electrospun CNFs are generally prepared in the form of nonwoven web, it can be used as electrode catalyst without any binder, which brings more convenience to its application in VRFB. Based on the reasons mentioned above, the PAN-based ECW consisting of nanofibers has been developed in our previous work and the effect of carbonization temperature on the ECW also has been investigated. It is found that electrochemical properties of the ECW really depend on the carbon structure and the electrical conductivity [14]. In order to further study the effect of carbon structure on the electrochemical activity of the PAN-based carbon fibers, the ECW was carbonized at 1000 °C for different times in this research. The conversion of carbon structure and improvement of electrical conductivity relating to the carbonization time were studied, and their effects on the electrochemical properties were also investigated. Besides, the ECW was used as catalyst layer in VRFB practically by two approaches. Cyclic voltammetry (CV) measurement and single cell test demonstrated the effect of ECW introduction.
Section snippets
Preparation of the ECW and ECW/CF composite electrode
Electrospun nonwoven web consisting of nanofibers was prepared by the process reported in the previous work [14]. Then the web was pre-oxidized at 280 °C for 30 min in air. After that, the stabilized web was carbonized by heating them to 1000 °C at a rate of 5 °C min−1 and holding for 30 min, 60 min, 90 min and 120 min in nitrogen flow. For the prepared samples, the ECW carbonized for different times are denoted as 30 min-ECW, 60 min-ECW, 90 min-ECW, 120 min-ECW, respectively. The thickness of
Morphology and composition of the ECW
The morphologies of the ECWs carbonized for different times are presented in Fig. 3. With the increasing of carbonization time, diameters of the CNFs change rarely, which are mainly distributed with the range of 100–200 nm. Besides, cross section of the 90 min-ECW showed in Fig. 3e indicates the CNFs have a solid core structure. The compositions of the ECWs carbonized for different times are analyzed by XPS. Fig. 4 illustrates the evolution of the elements and their relative functional groups.
Conclusions
PAN-based ECW has been developed by electrospinning and subsequent carbonization process at 1000 °C for different times in this paper. With the increasing of carbonization time, the electrochemical reversibility of vanadium redox couples on the ECW is improved step by step. The increasing electrochemical activity of the ECW can be attributed to the promoted electron transfer rate and enhanced mass transport process, which results from the conversion of fibers carbon structure. Forming of the
Acknowledgments
This work is funded by National Basic Research Program of China (No. 2010CB227203).
References (33)
Renew. Sustain. Energy Rev.
(2013)- et al.
Electrochim. Acta
(2013) - et al.
Renew. Sustain. Energy Rev.
(2014) - et al.
Electrochim. Acta
(1992) - et al.
Electrochim. Acta
(1992) - et al.
Electrochim. Acta
(2007) - et al.
Compos. Sci. Technol.
(2003) - et al.
Carbon
(2011) - et al.
J. Power Sources
(2013) - et al.
Carbon
(1995)