Activated Carbon Fiber Paper Based Electrodes with High

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Activated carbon fiber paper based electrodes with high electrocatalytic activity for vanadium flow batteries with improved power density Tao Liu, Xianfeng Li, Chi Xu, and Huamin Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14478 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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ACS Applied Materials & Interfaces

Activated carbon fiber paper based electrodes with high electrocatalytic activity for vanadium flow batteries with improved power density Tao Liua, Xianfeng Lia, b ∗, Chi Xua, c, Huamin Zhanga, b * a

Division of energy storage, Dalian Institute of Chemical Physics, Chinese Academy

of Sciences, Zhongshan Road 457, Dalian 116023, China b

Collaborative Innovation Center of Chemistry forEnergy Materials( iChEM), Dalian

116023, China c

University of Chinese Academy of Sciences, Beijing 100039, China

Keywords: vanadium flow battery, carbon paper, activation, electrode, electrocatalytic activity Abstract Vanadium flow batteries (VFBs) have received high attention for large scale energy storage due to their advantages of flexibility design, long cycle life, high efficiency and high safety. However, commercial progress of VFBs has so far been limited by its high cost induced by its low power density. Ultrathin carbon paper is believed to be a very promising electrode for VFB since it illustrates super low ohmic polarization, however, is limited by its low electrocatalytic activity. In this paper, a kind of carbon paper (CP) with super high electrocatalytic activity was fabricated via a universal and *

Corresponding authors. Tel.: +86 411 84379669; Fax: +86 411 84665057; E-mail addresses:

[email protected] (Xianfeng Li), [email protected] (Huamin Zhang).

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simple CO2 activation method. The porosity and oxygen functional groups can be easily tuned via this method. The charge transfer resistance (denoting the electrochemical polarization) of a VFB with CP electrode after CO2 activation decreased dramatically from 970 to 120 mΩcm2. Accordingly, the energy efficiency of a VFB with activated carbon paper as the electrode increased by 13% as compared with one without activation and reaches nearly 80% when the current density is 140 mAcm-2. This paper provides an effective way to prepare high performance porous carbon electrodes for VFBs and even for other battery systems.

1. Introduction The development of the social economy causes the increasing demand for energy, and consequently, the environmental problem and energy shortage issues induced by the consumption of fossil fuels are becoming more serious. The wide application of renewable energy is an inevitable choice to achieve sustainable social and economic development. However, the intermittent nature of renewable energy from sources such as wind and solar power makes it quite challenging for its use and dispatch through the grid.1,

2

Large-scale electrical energy storage systems offer a well-

established approach to improve grid reliability and utilization. Among various energy storage techniques, the vanadium flow battery (VFB) is well suited for large-scale energy storage because of its perfect combination of flexibility design, long cycle life, high efficiency and high safety. 3-5 2 ACS Paragon Plus Environment

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A large amount of progresses have been achieved on VFB technology over the past few decades, which is currently at demonstration stage. Even the availability and the merits of VFBs in many application fields have been well confirmed, the high cost is still one of the major factors that prevent the VFBs technology from broader market penetration.6 Improving the working current density of VFBs is an effective method to reduce the installed cost of VFBs, because the consequential enhanced power density can lead to a reduced material consumption and stack size. Similar to any other electrochemical systems, the increase of current density in a VFB will increase the polarization, resulting in a declined overall performance.7 Therefore, under the premise of keeping the overall performance constant, the key of improving the current density of VFBs is to reduce the polarization of VFBs as low as possible. As one of the key components, an electrode in the VFB plays a role of providing the active sites for electrochemical reactions and has an important effect on the polarization of a VFB.8 Its electrical conductivity will largely affect the body resistance of the electrode and the contact resistance between the electrode and the bipolar plate, and thus greatly influence the ohmic polarization of a VFB. Its electrocatalytic activity and reversibility for redox reactions will determine the electrochemical polarization, and its pore structure has a significant impact on the concentration polarization. Therefore, an ideal electrode for a VFB should possess excellent electrical conductivity, electrocatalytic activity, and appropriate pore structures. 3 ACS Paragon Plus Environment


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Currently, the typical electrode materials for VFBs are mainly carbon felt and graphite felt, which normally have a three-dimensional structure, good stability, high surface area and electrical conductivity.9, 10 Normally the thickness of carbon felt or graphite felt electrode is larger than 3mm, which causes a high ohmic resistance in a VFB. Currently, the proportion of ohmic polarization of a VFB, employing carbon felts or graphite felts, is as high as 60%.11 Furthermore, since the ohmic overvoltage is proportional to the operating current, the ohmic loss can be expected to become more serious when operating at a high power/current region.12 Therefore, for the purpose of improving the current density of VFBs, the thickness of the electrode should be decreased to reduce the ohmic polarization. Mench’s group in the University of Tennessee12-15 has introduced the “zero-gap” structure of fuel cell into the VFB structure design. The ohmic resistance of the VFB decreased dramatically to only 0.5 Ωcm2 by using carbon paper as the electrode. However, the active area of the carbon paper electrode is small due to the thinner thickness and the pristine carbon paper normally exhibits poor electrocatalytic activity for vanadium ions redox reaction. Therefore, to obtain the higher power density, 3 layers carbon paper (400 µm for 1 layer) were stacked together by Mench et al to avoid the large electrochemistry polarization. Nevertheless, the multilayered carbon paper increased the electrode thickness to above 1mm, inducing an increase of the ohmic polarization as well. Therefore, improving the electrocatalytic activity of carbon paper per volume is very necessary. Up to now, several methods have been reported to improve the 4 ACS Paragon Plus Environment

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electrochemical activity of carbon based electrodes. An effective method is to form oxygen or nitrogen functional groups that were confirmed to enable faster charge transfer by oxidation or nitrogenization treatment on the surface of carbon.16-24 Yue et al.18 introduced hydroxyl groups on the carbon papers via treating them with the mixed acids. The electrocatalytic activity of the treated carbon paper can be significantly improved due to the fact that the hydroxyl functional groups can act as active sites. Nevertheless, the reported VFB single cell performance seems to still have limited energy efficiency. Another modification method is to introduce a metal or metallic derivation, such as Ir25, 26, Bi7, 27, CuPt328, Nb2O529, Mn3O430 and WO331, etc., on the surface of the electrode material as an electrocatalyst to reduce the activation barrier for the redox conversion. In our previous study, the carbon paper electrode was coated with supported WO3 to improve carbon fiber’s electrocatalytic performance towards the vanadium redox reactions. Although the electrocatalytic activity of carbon paper electrode was improved, the real VFB performance employing carbon paper modified with WO3 as the electrodes was still lower than does of carbon felts electrodes due to the limited catalytic activity. Therefore, to further improve the electrocatalytic activity of carbon paper based electrode is vital to get an acceptable high power density VFB system. In addition, Flox et al.32 and Fetyan et al. 33 reported electrospun carbon nanofibers as the electrode materials of VFB. Although the high surface area can be obtained by the introduction of nanofibers to improve the electrocatalytic activity, the large concentration polarization 5 ACS Paragon Plus Environment


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caused by the smaller space among the nanofibers restricts it from obtaining a high battery performance. For example, the VFB single cell with electrospun carbon nanofibers as the electrodes reported by Fetyan et al. only exhibits 66% of energy efficiency value at 15 mAcm-2, far below than that (≥80% of EE value at 80 mAcm-2) of commercial VFB. Thus, the thin electrode material with nanostructure for VFB still need to be further studied to improve the cell performance.

Figure 1. Schematic principle of electrode preparation

In this paper, we will present a versatile and simple way to modify carbon paper based electrodes, which can decrease the electrochemistry polarization dramatically, while keeping the electrode thickness at a micron level. The method is based on CO2 activation, not introducing any impurities. During the activation process, the gasification agent, CO2, could etch carbon paper to form pores with tunable morphology on the surface of carbon fiber by reacting with carbon atoms, at the same time, abundant oxygen functional groups will be formed (Figure 1). The increased surface area induced by the produced pores together with the oxygen functional groups will be highly beneficial to decrease the electrochemistry polarization. To investigate the effect of CO2 activation on the electrochemical performance of CP, the microstructure and surface chemistry of CP and the alteration of electrocatalytic 6 ACS Paragon Plus Environment

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activity and VFB performance for CP before and after activation by CO2 have been investigated in detail.

2. Experimental 2.1 Materials Carbon paper used in this study was purchased from SGL Group, Germany. CO2 activation was conducted by exposing the as received carbon paper (CP) to a mixture of N2 and CO2 (V/V = 4:1) for 1 h at 1100, 1300 and 1500 oC (heating rate: 10oC min-1), respectively. The resulted products are denoted as CPC11, CPC13 and CPC15, respectively. 2.2 Characterization The surface morphology of CP before and after activation were observed by scanning electron microscopy (SEM, JSM-7800F). Digital four-probe bulk resistivity measurement equipment (SX1934, Baishen Technology) was employed to test the electrical conductivity along the plane direction of a carbon paper. The area resistance in the through-plane direction was tested with the setup in Figure S1. The resistance across two talmigold plates, which sandwiched carbon paper sample, was measured by low resistance measuring instrument (Hioki 3540). The area resistance of the carbon paper sample was calculated by multiplying above tested resistance by the area of carbon paper sample (20cm2). X-ray diffraction (XRD) analyses of CP and activated CP were 7 ACS Paragon Plus Environment


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performed on a X-ray diffractometer (DX-2700, Dandong Haoyuan Instrument Co., China) with a Cu Kα source (λ=1.54056 Å). The surface chemistry of CP and activated CP was analysed by applying X-ray photoelectron spectroscopy (XPS, ESCALAB250) with Al Kα monochromatic (1486.6 eV). N2 adsorption isotherms were measured using an ASAP2010 system. The electrochemical performance of CP and activated CPs were evaluated by the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements, which were conducted on an electrochemical workstation (CHI 604e, CH Instruments, USA). A graphite plate serves as the counter electrode, and a saturated calomel electrode (SCE) serves as the reference electrode. The working electrode was prepared by hot pressing a piece of CP between two plastic sheets, on one of which there was an open circular hole of 0.28 cm2 as the working area, as shown in Figure S2. A piece of Cu foil contacted to the CP served as the current collector. 0.05 M VO2+/ 0.05 M VO2+ in 3 M H2SO4 was used as the positive half-cell electrolyte, and 0.05 M V2+/ 0.05 M V3+ in 3 M H2SO4 was used as the negative half-cell electrolyte. EIS measurement was conducted at 0.9 V for the positive reaction and -0.55V for the negative reaction, respectively, by applying an AC voltage of 5 mV amplitude over a frequency range from 106 Hz to 10 mHz. 2.3 Single cell performance tests


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The structure of a VFB single cell was shown in Figure S3. The positive or negative electrode was a piece of CP with an area of 12 cm2 (3.0 × 4.0 cm) and a thickness of about 0.3 mm. The current collector was the graphite plate with interdigitated channels. The separator was an ion exchange membrane (N115, DuPont, USA). The cell was sealed with rubber washers. A battery test system (Arbin-BT 2000 instrument, Arbin Co., USA) was adopted for the single cell performance test. The test cell was charged to 1.65 V and discharged to 0.9 V with a constant current density of 60, 80, 100, 120 and 140 mA cm-2, respectively. The negative and positive electrolytes were 40 ml 1.5 M V3+ in 3.0 M H2SO4 solution and 40 ml 1.5 M VO2+ in 3.0 M H2SO4 solution, respectively. The flow rate of the electrolyte is 20 ml min-1. The EIS and polarization curve of VFB single cell were measured by using the testing methods as reported previously11, 34 on a KFM2030 (Kikusui electronics Co., Japan) impedance meter. 3. Results and discussion 3.1 Characterization of CP as-received and after activation To realize our idea, CP with the thickness of about 300 µm was activated at various temperatures from 1100 to 1500 oC (The CPs activated at 1100, 1300 and 1500 oC are denoted as CPC11, CPC13 and CPC15 respectively). The overall morphology of CP was indicated in Figure S4 (See supporting information), CP composed of carbon fibers with the diameter of about 8-10 9 ACS Paragon Plus Environment


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µm and pyrocarbon from resin, which plays a role of binder to combine the carbon fibers. Super macropores with the size of around 100 µm, which are beneficial for the transport of the active species, are distributed among the carbon fibers in a disordered arrangement. The surface of carbon fibers in the as-received CP is overall smooth and some small particles located on the surface of carbon fibers (Figure 2a). After activation, the overall architecture of CP still kept (Figure S4 in supporting information), however, the small particles on the surface of carbon fibers were burn off, and pores with the size ranging from nanometer to micrometer scale were clearly observed on the fibers’ surface (Figure 2b-2d). With the increase of activation temperature, the amount of pores gradually increases and the pore diameter increases as well. For CPC11 activated at 1100 oC (Inserted Figure 2b), only little mesopores with the size of several tens nanometers can be found. While, for CPC15 which is activated at 1500 oC, a large amount of super macropores at micrometer level are evenly distributed on the surface of carbon fibers, and there are many mesopores at the size of several tens nanometers on the surface of the macropores. The changes of pore size are mainly attributed to the faster reaction rate between carbon and CO2 at the higher temperature. As expected, the created pores by CO2 activation dramatically increased the BET specific surface area of CP from 0.1 to 6.5 m2g−1 (Table 1), with the increase of activation temperature from 1100 to 1500 oC, confirming that CO2 activation 10 ACS Paragon Plus Environment

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can effectively tune the surface morphology and surface area of carbon based electrodes. XRD patterns (Figure 3) of CPs before and after activation were carried out to further investigate the effect of activation on the micro-crystalline structure. It shows decreasing intensity of peak (002) with the increase of activation temperature, which indicates that more disordered amorphous structure was formed after CO2 activation.

Figure 2. SEM images of CP as-received (a) and activated by CO2 at various temperatures (b: CPC11, c: CPC13, d: CPC15)



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Figure 3. XRD patterns of CP as-received and activated by CO2 at various temperatures Table 1. Physical and chemical performance index of CP as-received and activated by CO2 at various temperatures sample

Surface area 2 -1

Electrical conductivity

Area resistance 2

C

O

(m g )

(Ωcm, // )

(mΩcm , ⊥)

(at. %)

(at. %)

CP

0.1

0.026

140

96.55

3.45

CPC11

3.1

0.035

156

96.45

3.55

CPC13

5.0

0.046

204

95.39

4.61

CPC15

6.5

0.061

252

93.75

6.25

The surface chemistry of CPs before and after activation was investigated by X-ray photoelectron spectroscopy (XPS) measurement. Figure 4a shows the wide scan XPS spectra of various samples. As expected, the oxygen content of CP was improved obviously after activation. With the increase of activation temperature, carbon content decreases from 96.55 to 93.75 % and oxygen content increases from 3.45 to 6.25 % due to the increase of oxidation degree. For further analysis, we deconvoluted the C1s spectra and the O1s spectra, as shown in Figure 4 b and c. It is worth to note that, comparing with CP, the content of defective carbon in CP after activation increases


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obviously, with the increasing activation temperature from 1100 to 1500 oC. This is consistent with the XRD test results, indicating the enhanced surface defective sites on the surface of carbon fiber. The oxygen functional group on the surface of CP is mainly -OH, which increases with the increase of activation temperature as well. The introduced -OH is expected to serve as the reaction active sites to improve the electrocatalytic activity of the carbon paper. However, the higher oxygen content together with more defective carbon reduce the electrical conductivity in the both parallel and perpendicular directions of CP (Table 1), which is unfavorable for the electrochemical performance of CP electrode.

Figure 4. a) XPS spectra of CP as-received and activated by CO2 at various temperatures, b) C1s and c) O1s XPS spectra.

3.2 Electrochemistry performance of CPs To verify the effect of CO2 activation on the electrochemical performance of CP, cyclic voltammetry (CV) was conducted to assess the electrocatalytic activity of CPs 13 ACS Paragon Plus Environment


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before and after activation toward vanadium ions redox reactions (Figure 5). Compared with CP, CPs after activation exhibited the lower oxidation onset potential, the higher reduction onset potential and the higher peak current both at cathode and anode, implying that the electrocatalytic activity of CP is improved dramatically after CO2 activation. The improved surface area, the defective carbon content and oxygen functional groups by CO2 activation can lead to the increase of electrochemical active surface area, which is responsible for the improvement of the electrocatalytic activity. Among the activated samples, CPC13 shows the highest electrocatalytic activity. The anodic and cathodic peak current for VO2+ /VO2+ and V2+ /V3+ reactions on CPC13 electrode increased significantly from Ipa = 1.46/ Ipc = -0.29 mAcm-2 and Ipa = 2.21/ Ipc = -1.82 mAcm-2 to Ipa = 9.46/ Ipc = -9.14 mAcm-2 and Ipa = 7.96/ Ipc = -8.18 mAcm-2, respectively, compared with the as-received CP, suggesting that the electron transfer kinetics for VO2+ /VO2+ and V2+ /V3+ reactions are both greatly enhanced via CO2 activation. It is worth to note that, although CPC15 possesses the highest surface area and the highest content of oxygen functional group, the lowest conductivity makes its electrocatalytic performance inferior to that of CPC13. The reversibility of vanadium ion redox reaction on the CPs electrode surface was assessed by calculating the oxidation and reduction peak potential separations (∆Eр) and the ratio of the peak current (Ipa/Ipc). Compared with pristine CP electrode, ∆Eр of CPC13 electrode corresponding to VO2+ /VO2+ and V2+ /V3+ reactions decreased by about 380 and 340 mV, respectively, and Ipa/Ipc corresponding to VO2+ /VO2+ and 14 ACS Paragon Plus Environment

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V2+ /V3+ reactions on CPC13 electrode is 1.15 and 0.95, respectively (Table 2), which are close to value 1. The results indicated the high reversibility of CPC13 for vanadium redox couples. All of these results demonstrated that the electrocatalytic activity and the electrochemical reversibility of CP are improved dramatically by CO2 activation.

Figure 5. a) a) Cyclic voltammograms (CVs) of CP as-received and activated by CO2 at various temperatures at a scan rate of 10 mV s−1. b) CVs of CP as-received and activated by CO2 at various temperatures at a scan rate of 10 mV s−1. Table 2. Parameters obtained from CV curves for CPs as-received and after activation at a scan rate of 10 mV s-1. VO2+/VO2+ Electrode CP

Epa (V)

Epc

Ipa

V2+/V3+

Ipc -2

△Ep -2

(V) (mAcm ) (mAcm ) (mV)

Ipa/Ipc

Epa (V)

Epc

Ipa

Ipc -2

△Ep -2

(V) (mAcm ) (mAcm ) (mV)

Ipa/Ipc

1.159 0.552

1.46

-0.29

607 5.13 -0.200 -0.790

2.21

-1.82

590 1.22

CPC11 1.086 0.607

14.61

-8.11

479 1.80 -0.301 -0.959

2.89

-8.18

658 0.35

CPC13 0.972 0.748

18.32

-15.89

224 1.15 -0.424 -0.670

12.29

-12.96

246 0.95

CPC15 0.944 0.764

9.46

-9.14

180 1.04 -0.285 -0.795

7.96

-8.18

510 0.97

pa: anodic current peak, pc: cathodic current peak.



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Figure 6. Nyquist plots of CP as-received and activated by CO2 at various temperatures in 0.05 M VO2+ / 0.05 M VO2+ +3 M H2SO4 solution at 0.9 V(a) and in 0.05 M V2+ / 0.05 M V3+ + 3 M H2SO4 solution at -0.55 V(b).

The electrochemical impedance spectroscopy (EIS) measurement results agree with the results from CV well. As shown in Figure 6, the charge transfer resistances, calculated according to the equivalent circuit (Figure S5), on CP electrode for positive and negative redox reactions are as high as 480 and 790 Ω, respectively, due to the slow kinetics process of the active species across electrode/solution interface. After activation, the charge transfer resistance is largely reduced. CPC13 shows the lowest charge transfer resistances, which are about 20 Ω, for both positive and negative redox reactions, indicating a fast charge transfer process on this electrode. The faster reaction kinetics of vanadium redox couples on CP activated by CO2 can be attributed to abundant exposed carbon edge atoms (defective carbon) and -OH groups as active sites facilitating vanadium ions adsorption. 3.3 VFB single cell performance As described above, the activated carbon fiber paper combines a very high electrocatalytic activity and a super low omhic resistance, which are expected to 16 ACS Paragon Plus Environment

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achieve a high VFB performance. Figure 7 shows the performance of single cells with as received CP and activated CPs as the electrodes at various current densities. From the charge/discharge profiles of the VFB single cells at 60 mAcm-2, as shown in Figure 7 a, the charge/discharge overpotentials for a VFB with CP after activation as the electrodes decrease remarkably compared with those of a VFB with CP electrodes, mainly due to the reduce of the electrochemical polarization, that is a lower charge voltage and a higher discharge voltage. Figure 7 b and c show the coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of VFB single cells at the current densities from 60 to 140 mAcm-2. The VFB single cells with CP after activation exhibit similar CE values with that of CP as-received, but much higher VE value. At the same charge/discharge current density, as the activation temperature increasing, the VE values first increase and then decrease, and this is consistent with their electrocatalytic activity. A VFB assembled with CPC13 electrode exhibits the highest VE value of 92.6%, which is about 13% higher than that with CP electrode, at 60 mAcm-2. Even at 140 mA cm-2, the CE, VE and EE of a VFB single cell with CPC13 electrode reach as high as 94.7 %, 82.4 % and 78.1 %, respectively, which are among the highest values for the reported carbon paper based electrodes. Figure 7 d shows the cyclic performance of a VFB with CPC13 as the electrodes at the current density of 120 mAcm-2. No obvious decay on CE, VE and EE was observed over 200 cycles, suggesting excellent stability of the prepared electrodes over repetitive cycling. 17 ACS Paragon Plus Environment


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Figure 7. Charge/discharge performance of VFB single cells with CP as received and activated by CO2 as electrodes, respectively: (a) charge/discharge profiles at 60 mAcm-2; (b) CE and VE; (c) EE and (d) cyclic performance.

Figure 8. (a) Nyquist plots of VFB single cells with CP as-received and activated at 1300oC, respectively, at SOC of 50%; (b) polarization curves and power density curves for VFB single cells assembled with CP and CPC13.

EIS measurement was conducted to further clarify the effect of the ultrathin porous carbon fiber paper prepared in this study on the polarizations of VFB single cells. 18 ACS Paragon Plus Environment

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Figure 8(a) shows Nyquist plots for VFBs with various CP electrodes obtained at the state of charge (SOC) of 50%. An equivalent circuit (EC) model, as shown in the inset of Figure 8, is used to fit the Nyquist plots. According to the fitting results, we can find that the cell with CP exhibits R2 (charge transfer resistance) of 970 and R3 (finite diffusion resistance) of 565 mΩ cm2, respectively, inducing the inferior charge/discharge performance, although its R1 (ohmic resistance) is as low as 496 mΩcm2. While the VFB with CPC13 electrode exhibits the remarkably decreased R2 and R3 values, which are 120 and 290 mΩ cm2 respectively, together with the low ohmic resistance as 505 mΩcm2, interpreting the dramatically increase on the VE of the VFB with CP electrode well. The decreased charge transfer resistance can be attributed to the increased electrocatalytic activity by CO2 activation due to the high electrochemical active surface area. While the decreased finite diffusion resistance could be ascribed to the developed pore structure with a large amount of macropores created on the surface of carbon fiber and improved hydrophilicity caused by the increase of oxygen functional groups. Therefore, activated ultrathin porous carbon fiber paper perfectly resolved the conflict between electrochemical polarization and ohmic polarization of VFBs with traditional carbon paper as the electrodes. As shown in Figure 8(b), the VFB single cell assembled with CPC13 obtained the higher peak power density of 540 mW cm-2, increasing by 35% than that with CP. Although such activation treatment will cause an increase of the electrode material cost, but the current density of the VFB is nearly doubled. And the consequential enhanced power 19 ACS Paragon Plus Environment


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density can lead to much lower material consumption for a stack, including electrode, membrane, bipolar plate, etc. Therefore, the total cost of the VFB system will be reduced remarkably as well.

4. Conclusion In summary, a kind of carbon paper (CP) with super high electrochemical activity was fabricated via a universal and simple CO2 activation method. The porosity or surface area and oxygen functional groups can be easily tuned through changing activation condition. Compared with pristine CP, the activated carbon papers, especially the sample CPC13, demonstrated much higher electrocatalytic activity toward vanadium redox reactions, attributed to the improvement in the electrochemical active surface area caused by the increase of content of defective carbon and oxygen functional group. The charge transfer resistance of VFB with CP electrode decreased dramatically from 970 to 120 mΩcm2 after CO2 activation. Consequently, the energy efficiency of a VFB with activated carbon paper as the electrode increased by 13% due to the faster charge transfer as compared with that without activation and reaches 78.1% at the high current density of 140 mA·cm-2. These results suggest that CO2 activated carbon paper could be one of the promising electrode materials for VFB with high power density.

Acknowledgements


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The authors greatly acknowledge the financial support from the Natural Science Foundation of China (Grant Nos. 21506210 and 51361135701) and the Outstanding Young Scientist Foundation, Chinese Academy of Sciences (CAS), Dalian Municipal Outstanding Young Talent Foundation (2014J11JH131).

Supporting information Supplementary data associated with this article is available free of charge. Figure S1 shows the sketch map of the experimental setup for measuring area resistance in the through-plane direction of the carbon paper sample. The resistance across two talmigold plates, which sandwiched carbon paper sample, was measured by low resistance measuring instrument (Hioki 3561). Then the area resistance of the carbon paper sample was calculated by multiplying above tested resistance value by the area of carbon paper sample (20 cm2). Figure S2 is the sketch map of the working electrode for CV and EIS measurements in this study. It was prepared by hot pressing a piece of CP between two plastic sheets, on one of which there was an open circular hole of 0.28 cm2 as the working area. A piece of Cu foil contacted to the CP served as the current collector. The structure of a VFB single cell was as shown in Figure S3. The positive or negative electrode was a piece of CP with an area of 12 cm2 (3.0 × 4.0 cm) and a thickness of about 0.3 mm. The current collector was the graphite plate with interdigitated channels. The separator was an ion exchange membrane (N115, DuPont, USA). The cell was sealed with rubber washers. Figure S4 shows the overall 21 ACS Paragon Plus Environment


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morphology of CP as-received (a) and activated by CO2 at various temperatures. CP composed of carbon fibers with the diameter of about 8-10 µm and pyrocarbon from resin, which plays a role of binder to combine the carbon fiber. After activation, the overall architecture of CP still kept constant. Figure S5 is the equivalent circuit used to fit the electrochemical impedence spectra data for the electrode processes. RI is the electrolyte resistance, Rct is the chrage transfer resistance, Cd is the double layer capacitance and Zw is the Warburg impedance.

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Table of Contents Graphic


Activated Carbon Fiber Paper Based Electrodes with High

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