Ionics

, Volume 19, Issue 7, pp 1021–1026

Carbon paper modified by hydrothermal ammoniated treatment for vanadium redox battery

Authors

  • Zhangxing He
    • Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, School of Chemistry and Chemical EngineeringCentral South University
  • Anqun Su
    • Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, School of Chemistry and Chemical EngineeringCentral South University
  • Chao Gao
    • Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, School of Chemistry and Chemical EngineeringCentral South University
  • Zhi Zhou
    • Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, School of Chemistry and Chemical EngineeringCentral South University
  • Chunyue Pan
    • Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, School of Chemistry and Chemical EngineeringCentral South University
    • Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, School of Chemistry and Chemical EngineeringCentral South University
Original Paper

DOI: 10.1007/s11581-012-0827-4

Cite this article as:
He, Z., Su, A., Gao, C. et al. Ionics (2013) 19: 1021. doi:10.1007/s11581-012-0827-4

Abstract

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.

Keywords

Carbon paperModificationVanadium redox batteryHydrothermal ammoniated treatment

Introduction

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 [15]. 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 [2], acid treatment [11], electrochemical oxidation [12], metals doping [13], and introducing other groups to electrode materials [1416]. 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. [16] 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.

Experimental

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.

Characterizations

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).

Electrochemical tests

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

FTIR analysis

The FTIR spectra of carbon paper untreated and treated at 220 °C for 15 h were recorded in the range of 4,000–750 cm–1, which are shown in Fig. 1. As for untreated sample, the peaks at 3,446 and 2,920 cm−1 are attributed to –OH and C–H stretching vibration, respectively. The peaks at 1,620 and 1,090 cm−1 are assigned to C=C stretching vibration and C–O stretching vibration, respectively. The peak at 1,391 cm−1 corresponded to the C–H bending vibration. Compared with the untreated sample, the characteristic absorption peak of treated samples shows significant change. A stronger and wider absorption peak within 3,100–3,500 cm−1 appears due to the overlap of stretching vibration of –NH2 and –OH [17]. The new peaks at 1,650, 1,605 and 1,410 cm−1 corresponded to N–H in-plane bending vibration, N–H deformation vibration, and C–N stretching vibration, respectively [18]. On the basis of the FTIR analysis, nitrogenous groups are successfully introduced on the surface of carbon paper by hydrothermal ammoniated treatment.
https://static-content.springer.com/image/art%3A10.1007%2Fs11581-012-0827-4/MediaObjects/11581_2012_827_Fig1_HTML.gif
Fig. 1

FTIR spectra of carbon paper untreated and hydrothermal ammoniated at 220 °C for 15 h; a untreated, b hydrothermal ammoniated

Element analysis

The element analyses of untreated and treated carbon paper electrode are listed in Table 1. After ammonia treatment at 220 °C for 15 h, the content of nitrogen increases from 2.957 to 6.432 %, the ratio of C/N decreases from 31.72 to 14.13, and the content of C, H, S elements decreases. The reason may be that the sulfur-containing groups decomposed at high temperature and high pressure, and the nitrogen element was introduced on the surface of carbon paper because of the reaction between NH3 and oxygen-containing groups in the NH3 condition.
Table 1

Element analysis results of untreated and treated carbon paper electrode

Sample

C (%)

S (%)

H (%)

N (%)

C/N

Untreated

93.795

0.826

0.649

2.957

31.72

Ammonia treated

90.871

0.578

0.392

6.432

14.13

SEM analysis

The surface morphology of carbon paper untreated and treated at 220 °C for 15 h are shown in Fig. 2. There is no obvious change on the surface morphology of the carbon paper before and after ammonia treatment. It can be known that no obvious effect occurs on the corrosion and surface morphology of carbon paper at high temperature and pressure. There are fewer contaminants on the surface of carbon paper after hydrothermal ammoniated treatment. This phenomenon may be caused by the weak reaction between NH3 and carbon fiber and removing the surface contaminants that may hinder electron transfer [2]. Thus, compared with electrochemical oxidation treatment, the hydrothermal ammoniated treatment does not affect the surface smoothness and surface morphology of carbon paper. The conclusion is consistent with the results of Wutao [16].
https://static-content.springer.com/image/art%3A10.1007%2Fs11581-012-0827-4/MediaObjects/11581_2012_827_Fig2_HTML.gif
Fig. 2

SEM images of carbon paper untreated (a), and hydrothermal ammoniated at 220 °C for 15 h (b)

Wetting property

The wetting property of the carbon paper was characterized by water contact angle test. The cross-sectional views of water droplets on the surface of untreated and treated carbon paper are presented in Fig. 3. With the enhancement of wetting property, the contact angle becomes smaller. It can be seen that the contact angle of treated sample is 103.0°, smaller than that of the untreated sample (120.0°). It indicated that the hydrophilicity of sample becomes stronger after ammoniated treatment. Because there is no obvious surface change between the untreated and treated sample, the enhanced hydrophilicity of treated carbon paper can be explained that the introduced nitrogenous groups such as –NH2 can combine with the H2O molecule easily.
https://static-content.springer.com/image/art%3A10.1007%2Fs11581-012-0827-4/MediaObjects/11581_2012_827_Fig3_HTML.gif
Fig. 3

Cross-sectional views of water droplets on carbon paper untreated (a), and hydrothermal ammoniated at 220 °C for 15 h (b)

Cyclic voltammetry

Figure 4 presents the CV curves of V(IV)/V(V) on carbon paper untreated and treated in different treatment temperature in 0.1 M VOSO4 + 3 M H2SO4 solution. The electrochemical parameters obtained from Fig. 4 are listed in Table 2. Where Ipa is anodic peak current, Ipc is cathodic peak current, Epa is anodic peak potential, Epc is cathodic peak potential, and ΔEp is peak potential interval. It can be known that both the anodic and cathodic peak currents increase with the increase of treatment temperature below 220 °C. The peak potential interval becomes larger, and the peak current becomes smaller at the temperature above 220 °C. It indicates that the sample treated at 220 °C for 15 h exhibits the best electrochemical activity for V(IV)/V(V) redox reaction. This phenomenon can be explained as follows. For one thing, the lone pair electrons of nitrogen atom in nitrogenous groups can enhance the alkalinity and electrical conductivity of carbon fiber. Because nitrogen atom have strong electron withdrawing ability, the carbon atom connected to nitrogen atom are positively charged [16]. This carbon atoms are the active sites for VO2+/VO2+ couple redox reaction, and the nitrogenous groups and vanadium ions can combine easily to generate the C–N–V bond to increase the charge transfer speed of vanadium ions between electrolyte and electrode. For another thing, the enhancement of the wetting property of electrode is in favor of the adsorption of vanadium ions on the surface of electrode. With the increase of temperature, the reaction between NH3 and carbon paper enhances and more nitrogenous groups can be introduced on the surface of carbon paper. With further increase of temperature, high temperature and pressure can reach the destruction limit of mechanical property, and the conductive network of carbon paper electrode was damaged [14]. The actual reaction area of VO2+/VO2+ couple redox reaction decreased and the active sites decreased appropriately. So the overly high temperature can decrease the electrochemical activity of carbon paper electrode.
https://static-content.springer.com/image/art%3A10.1007%2Fs11581-012-0827-4/MediaObjects/11581_2012_827_Fig4_HTML.gif
Fig. 4

CV curves recorded at 5 mV/s in 0.1 M VOSO4 + 3 M H2SO4. a Untreated, b 160 °C; c 180 °C, d 200 °C, e 220 °C, f 240 °C

Table 2

Parameters obtained from the CV curves for V(IV)/V(V) on different carbon paper electrodes

Temperature/°C

Anodic peak

Cathodic peak

ΔEp (V)

Ipa (mA)

Epa (V)

Ipc (mA)

Epc (V)

Untreated

8.59

1.19

4.74

0.71

0.48

160

11.63

1.13

6.67

0.69

0.44

180

13.63

1.09

8.50

0.71

0.38

200

13.53

1.07

9.66

0.69

0.38

220

14.47

1.03

10.36

0.74

0.29

240

10.99

1.21

5.50

0.58

0.63

Charge–discharge test

Figure 5 shows charge–discharge curves for VRB using carbon paper untreated and treated at different treatment temperature as electrode at current density of 10 mA/cm2. As long as the cell geometry and membrane kept constant for all the tests, the changes in energy efficiency would reflect changes in electrode activity. Compared with cell using untreated sample, the cell using treated sample as electrode has higher capacity, lower charge plateau, and higher discharge plateau. The cell using the sample treated at 220 °C for 15 h shows the best charge–discharge performance. This phenomenon is exactly corresponded to the CV results which are related to the wettability of electrode materials and the electrochemical activity of the samples.
https://static-content.springer.com/image/art%3A10.1007%2Fs11581-012-0827-4/MediaObjects/11581_2012_827_Fig5_HTML.gif
Fig. 5

The twentieth charge–discharge curves of the cells using different carbon paper electrodes at a current density of 10 mA/cm2. a Untreated, b 160 °C, c 180 °C, d 200 °C, e 220 °C, f 240 °C

Figure 6 shows the typical charge–discharge curves of the cell using carbon paper electrode treated at 220 °C for 15 h at different current density. The average coulombic efficiency (CE), voltage efficiency (VE), and overall energy efficiency (EE) values are listed in Table 3. As shown in Table 3, the CE of cells increase with the increase of current density while the EE show a completely opposite trend. For CE, the high current density shortened charge–discharge time so that the crossover of vanadium ions can be reduced and a higher CE is obtained. With the increased current density, the EE decreased because of the increase of charge transfer resistance and overpotentials led by the increase of current density. The CE, VE, and EE of cell at the current density of 20 mA/cm2 after 50th cycles are 97.2, 85.3, and 82.9 %, respectively. When the current density increases to 50 mA/cm2, the values of CE, VE, and EE decrease slightly. Therefore, the battery succeeds in maintaining almost cell efficiencies at different charge–discharge current density.
https://static-content.springer.com/image/art%3A10.1007%2Fs11581-012-0827-4/MediaObjects/11581_2012_827_Fig6_HTML.gif
Fig. 6

Charge–discharge curves of the cell using the carbon paper electrodes hydrothermal ammoniated at 220 °C for 15 h at different current density. j/(mA/cm2); a 20, b 30, c 40, d 50

Table 3

Efficiency values for carbon paper electrodes hydrothermal ammoniated at 220 °C for 15 h at various current density

j/(mA/cm2)

Cycle number

Cell efficiency

CE (%)

VE (%)

EE (%)

20

1

97.8

85.0

83.1

20

97.6

85.9

83.8

50

97.2

85.3

82.9

30

1

96.8

77.5

75

20

97.2

80.0

77.8

50

97.5

79.8

77.8

40

1

99.3

71.4

70.9

20

99.6

69.0

68.7

50

99.3

69.1

68.6

50

1

99.0

61.7

61.1

20

98.6

64.1

63.2

50

98.8

63.0

62.2

Conclusions

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.

Acknowledgments

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).

Copyright information

© Springer-Verlag Berlin Heidelberg 2012