Carbon paper modified by hydrothermal ammoniated treatment for vanadium redox battery
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- He, Z., Su, A., Gao, C. et al. Ionics (2013) 19: 1021. doi:10.1007/s11581-012-0827-4
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Modification of carbon paper by hydrothermal ammoniated treatment for vanadium redox battery was investigated in this paper. The content of nitrogen in the carbon paper improved from 2.957 to 6.432 % due to the introduced of nitrogenous groups. The surface smoothness and morphology of carbon fiber did not change after the hydrothermal ammoniated treatment. In the mean time, the hydrophilicity has been enhanced because of the introduction of nitrogenous groups to the surface of carbon paper. The sample, which was treated at 220 °C for 15 h, shows the best performance in electrochemical activity and charge–discharge among all the samples. At the current density of 20 mA/cm2 after 50th cycles, the coulombic efficiency, voltage efficiency, as well as energy efficiency of the fabricated cell has reached up to 97.2, 85.3, and 82.9 %, respectively. It indicates the hydrothermal ammoniated treatment might be a promising approach to modify carbon paper for vanadium redox battery.
KeywordsCarbon paperModificationVanadium redox batteryHydrothermal ammoniated treatment
Vanadium redox battery (VRB), proposed by Skyllas-Kazacos et al. is believed to be one of the most important energy storage systems of intermittently renewable energy (like wind energy, solar energy, etc.) due to its many advantages, i.e., long cycle life, high energy efficiency, separated battery capacity, flexible design, and environmental friendship [1–5]. Thus, VRB have been paid much attention by many scientific research institutions and corporation recently. In the VRB system, VO2+/VO2+ and V2+/V3+ redox couples are employed as the positive and negative active species, respectively, which are separated by ion exchange membrane, and a standard voltage of 1.25 V is produced by the VRB system [3, 6].
Electrode plays an important role in VRB, which provides the reaction place of charge transfer of VO2+/VO2+ and V2+/V3+ redox couples. Currently, the most widely used electrodes for VRB are polyacrylonitrile-based carbon fibers such as graphite felt, carbon paper due to their porous structure, high electronic conductivity, and low cost [7, 8]. However, the poor electrochemical activity and hydrophobicity of these carbon fibers greatly limit the power density and energy efficiency of VRB system [9, 10]. Up to now, a variety of surface treatment methods have been reported for improving the electrochemical activity of carbon fibers, such as heat treatment , acid treatment , electrochemical oxidation , metals doping , and introducing other groups to electrode materials [14–16]. However, these methods are not advantageous to commercial application for the use of noble materials, dangerous concentrated acid, or tedious treatment time. Wu et al.  introduced nitrogenous groups to PAN-graphite felt via hydrothermal ammoniated treatment and opened up a wider application of hydrothermal treatment. The electrochemical properties of the treated graphite felt improved due to the increase of polar nitrogenous groups on the carbon fiber surface, which facilitated charge transfer between electrode and vanadium ions. Carbon paper can reduce the volume of cell stack at high current density and increase power density of vanadium redox flow battery. But the carbon paper also has low wettability and low activity. In this paper, the effect of hydrothermal ammoniated treatment on carbon paper was examined. Carbon paper with high electrochemical activity was obtained. And the structure, wettability, and high performance of the treated carbon paper was investigated.
Chemicals and materials
Carbon paper (HCP030N) with a thickness of 0.29 mm was purchased from Shanghai Hesen Ltd. Co., China. A 25 % NH3 (AR) was purchased from Guangdong Xilong Chemical Industry Co. Ltd., China. Carbon paper was treated thermally in a sealed 50 mL Teflon-lined stainless steel autoclave containing 15 mL 25 % ammonia solution at different temperature for 15 h. The treated samples were washed with deionized water until the pH of the rinsed water was neutral, than dried in vacuum oven at 110 °C for 5 h.
Fourier transform infrared spectroscopy (FTIR) was recorded from dried KBr disks containing the crushed carbon fiber powder on an AVATAR-360 instrument (Livolet Co. Ltd., USA) to characterize groups on carbon fibers. Element analysis of carbon paper before and after treated was characterized by a Vario EL cube CHNOS carbon–sulfur analyzer (ELEMENTAR, Germany). The surface morphology of the samples was characterized by scanning electron microscopy (SEM; JEOL, JSM-6380LV, Japan) at an acceleration voltage of 25 kV. The wetting properties were characterized by water contact angle measurement which was carried on contact angle goniometer (JC2001).
Cyclic voltammeter measurements were carried out by using electrochemical workstation (Shanghai Chenhua Instrument Co. Ltd., China) at a scan rate of 5.0 mV s−1 between 0 and 1.5 V. A three-electrode system was used with carbon paper (1.0 × 1.0 cm2) as a working electrode, Pt as a counter electrode and standard calomel electrode as a reference electrode.
The VRB charge–discharge tests were performed on a battery test system CT2001C-10 V/2A (Wuhan Land Co., China). Two pieces of carbon paper (2 × 2 cm2) were served as positive and negative electrodes, separately. Before the cell was assembled, carbon paper was soaked in original electrolyte for 24 h at an ambient temperature. A perfluorinated ion-exchange membrane (Best Industrial & Trade Co., Ltd., China) was served as the VRB’s separator.
Results and discussion
Element analysis results of untreated and treated carbon paper electrode
Parameters obtained from the CV curves for V(IV)/V(V) on different carbon paper electrodes
Efficiency values for carbon paper electrodes hydrothermal ammoniated at 220 °C for 15 h at various current density
Nitrogenous groups can be successfully introduced on carbon paper surface by the hydrothermal ammoniated treatment. The content of nitrogen on the carbon paper increases after treatment and the hydrophilicity of carbon paper treated can be enhanced by introduction of nitrogenous groups such as –NH2 on the surface of carbon paper. The electrochemical activity enhanced with the increase of treatment temperature below 220 °C. However, due to the damaged conductive network of carbon paper at above 220 °C the electrochemical activity shows the decrease trends. The cell using the sample treated at 220 °C for 15 h exhibits excellent performance. The coulombic efficiency reaches up to 97.2 % and the corresponding energy efficiency is 82.9 % at the current density of 20 mA/cm2 after 50th cycles.
This work was financially supported by the Major State Basic Research Development Program of China (973 Program; no. 2010CB227201) and National Natural Science Foundation of China (no. 51072234).