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Highly functionalized nanoporous thin carbon paper electrodes for high energy density of zero-gap vanadium redox flow battery
Chemical Engineering Journal 378 (2019) 122190
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Highly functionalized nanoporous thin carbon paper electrodes for high energy density of zero-gap vanadium redox flow battery Saleem Abbasa,b, Sheeraz Mehbooba,b, Hyun-Jin Shina,c, Oc Hee Hand, Heung Yong Haa,b,
T
⁎
a
Center for Energy Storage Research, Korea Institute of Science and Technology (KIST), 14-gil 5, Hwarang-ro, Seongbuk-gu, Seoul 02792, Republic of Korea Division of Energy & Environmental Technology, Korea University of Science & Technology (UST) – KIST School, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea c Department of Chemical & Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea d Western Seoul Center, Korea Basic Science Institute (KBSI), Seoul 03759, Republic of Korea b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
of low energy density of VRFB is • Issue addressed by modifying electrode surface.
functionalized nanopores are • Highly formed on the surface of thin carbon papers.
area, wettability and elec• Surface trode/electrolyte contact is highly improved.
utilization for cell having • Electrolyte modified electrodes is improved by 110%.
27 Wh L more energy is harvested • with 29% higher energy efficiency at −1
50 mA cm−2.
A R T I C LE I N FO
A B S T R A C T
Keywords: Vanadium redox flow battery Nanopores Surface functionalization Carbon paper Electrolyte utilization Energy density
Low energy density of a vanadium redox flow battery (VRFB) due to limited solubility and stability of vanadium ions constrains its wide spread applications and this issue becomes more critical by low active surface areas of electrodes and mass transport limitations of active species on the electrodes that lead to low electrolyte utilization. In this study the issue of low energy density is addressed by improving the electrode performance through modifying the surface properties and morphology of thin (0.19 mm thick) carbon paper electrodes instead of commonly used several millimeters thick carbon felts. Surface functionalization and pore formation of carbon paper are carried out using a catalytic etching method at high temperaturewhere the diameters of nanopores are controlled by tuning the etching conditions. The synergistic effects of thin, nanoporous and functionalized carbon paper result in more effective electrode/electrolyte interaction and the less mass transport resistance. Therefore, the zero-gap VRFB cell employing the nanoporous electrodes displays remarkable performance improvement in terms of electrolyte utilization by 110%, discharge energy density by 155% and energy efficiency by 29% as compared to the one using pristine electrodes at a current density of 50 mA cm−2. The results imply that the more energy can be harvested by employing nanoporous and functionalized carbon paper electrodes having larger active surface areas.
⁎ Corresponding author at: Center for Energy Storage Research, Korea Institute of Science and Technology (KIST), 14-gil 5, Hwarang-ro, Seongbuk-gu, Seoul 02792, Republic of Korea. E-mail address: [email protected] (H.Y. Ha).
https://doi.org/10.1016/j.cej.2019.122190 Received 18 March 2019; Received in revised form 21 May 2019; Accepted 8 July 2019 Available online 09 July 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Chemical Engineering Journal 378 (2019) 122190
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1. Introduction
chemistry of electrodes. In this study, in order to improve the performance of zero-gap VRFBs, carbon paper electrodes have been modified using a catalytic etching method. This method is advantageous over others as it is facile and could provide uniform and well-defined nanopores as well as surface functional groups leading to much increased active surface area and surface wettability available for redox reactions of VRFB. These nanoporous CPs are then subjected to various electrochemical tests such as cyclic voltammetry, electrochemical impedance spectroscopy, charge-discharge (CD) tests by employing as electrode materials for a zero-gap cell at varying current density and a long-term CD cycling test. Many physicochemical analysis techniques, such as SEM, TEM, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and wettability test have been employed to characterize the surface morphologies and properties of the nanoporous carbon papers and the analytical data are used to interpret the relations between the surface properties and the performance of the CP electrodes for vanadium redox reactions.
Renewable energy sources have drawn much attention of researchers in recent years due to the limitations and adverse effects of fossil fuels on the environment [1,2]. However, the efficient integration of these renewable energy sources with electric grids is limited because of their intermittent nature [3–5]. To overcome this issue, energy storage systems (ESSs) are used as means to store electrical energy in various reversible forms [6–8]. The redox flow batteries (RFBs) due to their design flexibility, decoupled scaling of power and energy and long cycle life are most suitable options of ESS [7,9,10]. Among others, allvanadium redox flow battery (VRFB) is one of the most advanced and the only commercialized candidates of RFBs, with employing four different oxidation states of vanadium, V4+/V5+ (VO2+/VO2+, catholyte) and V2+/V3+ (anolyte) as active materials, thus minimizing crossover effect through membrane. In addition to commonly used sulfuric acid as supporting electrolyte, mixed acid electrolytes and various additives have also been reported [4,11–14]. During charge process V4+ is oxidized to V5+ on the cathode while V3+ is reduced to V2+ on the anode and the charged species are reversed to original states during discharge process [15–19]. Carbon or graphite felts are commonly used as electrode materials in VRFB due to their high surface area and chemical stability in acidic environment [20,21]. Thin carbon papers (CP) have also been reported as electrode materials in VRFB as they offer reduced transport path to active species resulting in lower areal specific resistance (ASR) compared to thick felt electrodes [22–26]. Recently, zero-gap flow field design of VRFB cell employing thin carbon electrodes has also been reported to reduce the internal resistances of the conventionally used cells and achieve significantly high power densities [23]. Despite these recent developments in the power density of VRFB system with zero-gap cells, the low energy density of the system is still one of the major hurdles for its broad market penetration. The low energy density of VRFB is mainly due to three reasons: the first is the low solubility of vanadium ions in the electrolyte solutions (H2SO4 + H2O) and the second is the instability of some active species especially V5+ at high temperatures as it is prone to precipitate into irreversible V2O5 leading to a loss in energy density [4,27]. Many works have been done to address these issues; for example, by using mixed acid solutions as electrolyte and by adding stabilizing agents to the electrolytes [4,10,28–31]. The third reason of low energy density is the partial utilization of the active species in the electrolyte due to the mass transport polarizations and poor electrochemical activity of carbon electrodes especially at high current densities. By this, a significant amount of vanadium ions cannot actually take part in the redox reactions during charge-discharge (CD) processes and thus the theoretical energy density is not sufficiently achieved. As the electrolyte is a major component of system cost, i.e. 37% [32], its utilization needs to be improved to reduce the overall cost of VRFB system. The active involvement of electrolyte is normally estimated by a performance indicator known as electrolyte utilization (EU) which is the ratio of actual charge or discharge capacity to the theoretical capacity under the same conditions, i.e.
EU (%) =
Actual capacity (charge or discharge) × 100 Theoretical capacity
2. Experimental section 2.1. Materials Toray carbon papers (TGP-H-060, unteflonized with thickness of 0.19 mm) were used as electrode materials in this study. Cobalt acetate tetrahydrate (98%, Junsei) as metal precursor, isopropanol (IPA, 99.9%, SAMCHUN) as washing agent and ethanol (99.9%, SAMCHUN) as solvent for precursor were used. All reagents were of analytical grade and were directly used without any purification. 2.2. Electrode preparation Thin CP electrodes with functionalized nanopores were prepared by a catalytic etching process in air atmosphere [44]. Initially, pieces of pristine carbon paper (P-CP) were washed with IPA, dried and thermally treated at 500 °C for 4 h and denoted as thermally treated carbon paper (TT-CP). The TT-CPs were immersed in a 4 mM solution of cobalt acetate tetrahydrate in ethanol and sonicated for 1 h followed by drying in an oven at 60 °C. The samples were then heated in a muffle furnace for 1 h at 300 °C to form Co3O4 particles on the surface of carbon fibers of CPs. Nanoporous electrodes were prepared by further heating the catalyst loaded CPs at 500 °C for 0.5–2 h and denoted as 0.5NP-CP, 1NP-CP and 2NP-CP for the carbon papers treated for 0.5, 1 and 2 h, respectively. Atmospheric air was used in all heating steps. 2.3. Characterization The CPs were characterized by a field emission scanning electron microscope (FESEM, Hitachi JSM-7000F), transmission electron microscope (TEM, FEI Tecnai G2 F20) to analyze the surface morphology of CPs, and Raman spectroscopy to examine the structural changes before and after modifications. The X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Sigma Probe) was used to analyze the surface chemistry of the CPs. The surface areas and average pore diameters of CPs were obtained using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) techniques, respectively, on a N2 adsorption device (Micromeritics ASAP 2010 analyzer). Surface wettability of the CP samples was examined with vanadium electrolytes instead of conventionally used deionized (DI) water.
(1)
High EU is favorable for high energy density and low electrolyte volume required for practical use in VRFB leading to a significant reduction in overall cost of the system [33,34]. The higher EU can be obtained by improving the electrode performance for vanadium redox reactions. Though many methods have been reported to improve the performance of carbon electrodes, mainly including thermal treatment, chemical treatment, electrochemical activation, plasma treatment, metal or metal oxide loading, activation by KOH, and so on [12,33,35–43], there is still much room to improve the performance by simultaneously increasing the active surface area and the surface
2.4. Electrochemical measurements All the CPs were electrochemically examined by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). For CV and EIS, a piece of CP with a diameter of 1 cm was used as a working electrode while a Pt wire and an Ag/AgCl electrode were used as counter and reference electrodes, respectively. Concentrations of V4+ 2
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Fig. 1. Design of the zero-gap VRFB cell used in this study.
Fig. 2. SEM images of carbon papers; P-CP (a) TT-CP (b) 0.5NP-CP (c) 1NP-CP (d) and 2NP-CP (e).
and V3+ electrolytes were 0.1 M in 2 M H2SO4. For measuring the double layer capacitances and effective surface areas of the nanoporous carbon paper electrodes, CV was conducted in a 1 M H2SO4 solution. The electrochemical performance of the CPs was further evaluated through CD tests in a VRFB cell. The single-cell used in the CD tests was assembled by sandwiching a Nafion 117 membrane between two layers of CP electrodes which were placed in between two graphite plates having interdigitated pattern of flow fields for electrolyte flow. The area of each piece of CP was 25 cm2 while 3 pieces were layered on each bipolar plate as electrodes [23]. The positive electrolyte was 1.5 M V4+ in 3 M H2SO4 and a negative electrolyte solution, V3+, of the same concentration was prepared by electrochemical reduction of V4+. 50 ml of each electrolyte stored in separate glass reservoirs was purged with nitrogen gas for 10 min to minimize oxidation of the active species and circulated through each compartment of the VRFB cell using a peristaltic pump at a flow rate of 2.5 ml s−1. The theoretical capacity of 1.5 M vanadium electrolyte used for VRFB testing was 40.2 Ah L−1 (2.01 Ah) considering 50 ml of V4+ or V3+. The zero-gap flow field structure of VRFB cell used in this study is shown in Fig. 1. The current, voltage and energy efficiencies were calculated by using the equations given below:
Current efficiency (%) =
∫ Id dt Total discharge current × 100 = × 100 Total charge current ∫ Ic dt (2)
Energy efficiency (%) =
∫ Vd Id dt Total discharge energy × 100 = × 100 Total charge energy ∫ Vc Ic dt (3)
Voltage efficiency (%) =
Energy efficiency × 100 Current efficiency
(4)
where Ic and Id are charge current and discharge current, respectively, while Vc and Vd are charge voltage and discharge voltage, respectively [45]. Discharge polarization curves were measured galvanostatically by discharging the fully charged cell (100% state of charge (SoC)). The 100% SoC was assumed by observing the current less than 4 mA cm−2 when the VRFB cell was held potentiostatically at 1.8 V. Then discharging was started initially at an applied current density of 25 mA cm−2 which was increased stepwise and maintained for 30 s at each step until the cell potential became zero where a limiting current was recorded. 3
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3. Results and discussion
with disordered or defected carbon structure, meaning that the higher is the intensity of its peak (ID), the more will be the disordered structure. On the other hand, the G-band is associated with ordered graphitic structure and its peak intensity, IG, is an indication of degree of graphitization. Therefore, the higher ID/IG values of the nanoporous CPs indicate that the degree of disordered surface carbon has increased due to the catalytic etching treatment [38,50]. Another small peak observed at ~1620 cm−1 as a shoulder peak of the G-band which is generally referred as D′-band also implies structural disorder [51]. This D′-band is only observed in the treated CPs and its intensity increases with increasing etching time. The P-CP has the lowest ID/IG value of 0.05 indicating that initially the ordered graphitic structure is dominant before any treatment. After simple thermal treatment in air atmosphere, the ID/IG ratio increased to 0.10 for TT-CP, but the increment by thermal treatment is not so significant. In the case of 0.5NP-CP, the ID/IG increased up to 0.22, which is more than 4 times of that for P-CP. Further increase in treatment time caused higher degree of disordered surface carbon leading to the ID/IG value of 0.29 for 1NP-CP and 0.34 for 2NPCP. This dependence of the ID/IG ratio on the treatment time also indicates the controllability of the etching process. The surface chemistry of CPs was investigated by X-ray photoelectron spectroscopy (XPS) as shown in Figs. 3b–c, S6 and Table 1. The C1s spectra were deconvoluted to 4 peaks at the binding energies of ~284, ~286, ~288 and ~ 291 eV, corresponding to graphitic carbon (sp2/ C]C), defective carbon (sp3/CeC), carbonyl (C]O) and carboxyl acid (COOH) groups, respectively [40,52,53]. The results show that the graphitic or ordered sp2 carbon (C]C) is significantly decreased while defective or disordered sp3 carbon (CeC) is increased after etching, indicating successful formation of defects or pores on the surface of CP fibers due to catalytic etching [40,42]. Also the portions of carbonyl and carboxyl acid groups are increased mainly due to the oxidation during etching process at high temperatures in air atmosphere. These enhanced oxygen surface functional groups are generally considered to
The surface morphology of CPs was examined by SEM. The surface of P-CP (Fig. 2a) is smooth but contains some loose particles or other impurities. Therefore IPA washing was conducted before any further treatment. The thermal treatment of the IPA-washed CP at 500 °C for 4 h results in a smoother surface, free from these impurities without any significant morphological change (Fig. 2b) coinciding with the literature findings that simple thermal treatment in air atmosphere causes no significant structural changes [46,47]. On the other hand, heating of a catalyst-loaded CP just for 0.5 h at 500 °C shows clear signs of structural change with increased number of pores on its surface (Fig. 2c). Pores are formed on the surface of CP due to repetitive oxidation and reduction of cobalt oxide that penetrates into its body. The number and size of pores apparently increase with increasing the treatment time as shown in Fig. 2d and e. The morphological changes observed by TEM are presented in Fig. S5. Specific surface areas of the CPs were obtained by BET formula and average pore diameters by BJH desorption technique. The results show that the surface area of the P-CP is 0.02 m2 g−1 which is the lowest among all CPs tested in this study and in compliance with already reported values [48,49]. The surface area of TT-CP is 0.06 m2 g−1 with average pore diameter of 8.4 nm. The catalytically modified 0.5NP-CP, however, shows huge increment in surface area (4.96 m2 g−1). As far as average pore size is concerned, an increasing trend is noticed with increasing the treatment time as summarized in Table S3. This time-dependence indicates that the average pore size can be controlled directly by adjusting the treatment time. For the investigation of near-to-surface region of CPs, Raman spectroscopy was carried out and the first order Raman spectra were recorded in the range of 1000–2000 cm−1 (Fig. 3a). Two major peaks of interest appeared in the Raman spectra are D-band and G-band at ~1370 and ~1590 cm−1, respectively. The D-band is generally linked
Fig. 3. Raman spectra of various CPs (a) and XPS C1s spectra of P-CP (b) and 1NP-CP (c). 4
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represent relative surface area [57–60]. Notably, the relative surface areas of CPs are greatly improved after treatment (Table S4) which is considered useful for enhancing electrochemical activities. The results of EIS further confirm the improved electrochemical activities of the nanoporous CPs for vanadium redox reactions. Fig. 4c displays the Nyquist plots of all of the CPs in the V4+ electrolyte solution and the magnified plots are given in the inset. Generally, the semicircle portion of a Nyquist plot at high frequencies represents charge transfer process and the linear part at low frequencies reflects the ionic diffusion process in porous structure of the electrode. The diameter of semicircle denotes charge transfer resistance [61,62]. It can be seen that P-CP has the largest diameter amongst all of the CPs referring to the highest charge transfer resistance (RCT) on its surface while there is a dramatic decrease in this resistance for the nanoporous CPs showing the improved charge transfer process on their surface. Fig. 4d represents the Nyquist plots of all of the CPs in the V3+ electrolyte solution and the inset shows magnified graphs. The results show that the improved performance of the nanoporous CPs in the negative electrolyte solution is even more than in the positive electrolyte solution. This demonstrates that the nanoporous CPs are effective materials for both electrode sides of VRFB. This improvement is most probably due to the nanopores and active surface functional groups on the nanoporous CPs which are generated during the catalytic etching process. The 2NP-CP shows larger RCT than 1NP-CP, which may be due to the extensive structural and active site damage on the surface of 2NP-CP during excessive treatment [33]. The parameters obtained from CV and EIS are summarized in Table 2. These physical and electrochemical observations indicate that 1NP-CP is the best among all CPs made in this study. This is further confirmed by CD experiments with VRFBs as presented below. To investigate the performance of the VRFBs employing the nanoporous CPs as electrodes, CD experiments were carried out at various applied current densities. As discussed earlier with the CV results showing improved onset potentials for vanadium redox reactions by employing the nanoporous CPs, the VRFB systems also display similar trends in performance. Fig. 5a represents the relationship between cell potential and CD energy density for the VRFB cells having P-CP, TT-CP or 1NP-CP as electrodes at an applied current density of 50 mA cm−2. There is a significant decrease of about 238 mV in charging overpotential and about 304 mV in discharging overpotential for the VRFB assembled with 1NP-CP as compared to the one with P-CP. The lower charge and higher discharge potential for the 1NP-CP cell cause a larger available potential window leading to additional energy density gains of 25 Wh L−1 and 27 Wh L−1 during the first CD steps, respectively than the P-CP cell. Fig. 5b displays the discharge energy densities at various applied current densities while the comparison of charge energy densities is given in Fig. S12. As the applied current density increases the CD energy densities decrease for all of the CPs, which is a normal phenomenon due to increased overpotentials at high current densities [56]. Initially at 50 mA cm−2, the 1NP-CP cell attains almost 155% and 20% higher discharge energy density than the cells composed of P-CP and TT-CP, respectively. The degree of improvement gets higher at higher current densities, particularly at 150 mA cm−2 where the cells composed of P-CP and TT-CP show no performance but the 1NP-CP cell still has a discharge energy density about 38 Wh L−1 and an energy efficiency of 86% (Fig. 5c). The1NP-CP cell shows similar margins of improvements in terms of discharge capacity (Fig. 5d). As the 1NP-CP cell shows significantly better performance at 150 mA cm−2 than the cells composed of P-CP and TT-CP, the cell was subjected to extended CD cycles from 200 to 400 mA cm−2. The 1NP-CP cell exhibits a considerable discharge energy density of 5 Wh L−1 and an energy efficiency of 55% even at a very high current density of 400 mA cm−2 (Fig. 5e and f), which is still a reasonable performance. All this improved performance of 1NP-CP may be attributed to presence of active surface functional groups and nanopores that facilitate redox
Table 1 Fitting results of XPS C1s spectra in Fig. 3b and c. Position [eV]
Peak Assignment
P-CP [%]
TT-CP [%]
0.5NPCP [%]
1NP-CP [%]
2NP-CP [%]
284.48 286.33 288.41
C]C (sp2) CeC (sp3) C]O (Carbonyl) COOH (Carboxyl acid)
84.8 7.98 2.57
82.44 9.72 2.59
77.32 12.76 4.73
78.04 11.85 4.76
77.46 12.15 5.27
4.68
5.24
5.19
5.36
5.12
290.97
play a positive role in vanadium redox reactions [20,54,55]. Surface wettability of carbon electrode materials in VRFB is an indication of electrode/electrolyte interaction and generally hydrophilicity is recommended in this aqueous environment. Untreated carbon papers or felts, however, are normally highly hydrophobic causing poor electrochemical activity towards vanadium redox reactions due to inferior electrode/electrolyte interaction. Therefore, in order to examine the surface hydrophilicity of the CPs used in this study, wettability test was also done and, instead of conventionally used DI water, the real VRFB electrolytes were used for the test as shown in Fig. S7. On the surface of P-CP a V4+ droplet was fully absorbed in one hour and a V3+ droplet stayed almost 72 h while, in case of TT-CP, a V4+ droplet was absorbed in 5 s and a V3+ droplet took 10 min for complete absorption. On the other hand, the droplets of both V4+ and V3+ electrolytes were absorbed immediately on the surface of 0.5NP-CP, 1NP-CP and 2NP-CP. This improved surface wettability of nanoporous CPs as compared to P-CP or TT-CP is due to presence of numerous nanopores and hydrophilic functional groups collectively leading to much improved electrode/electrolyte interaction on their surface. Electrochemical activities of the CPs for vanadium redox reactions were investigated initially by CV. Fig. 4a represents the voltammograms for all of the CPs in V4+ electrolyte for the potential window of 0.2–1.5 V against Ag/AgCl at a scan rate of 5 mV s−1. The CV results show that all of the nanoporous CPs have significant improvements in both oxidation and reduction onset potentials compared to P-CP and TT-CP. These improved onset potentials cause enhanced reaction kinetics as well as increased energy storage efficiency due to reduced overpotentials for VRFB, meaning more facilitated vanadium redox reactions on the surface of nanoporous electrodes than on P-CP and TTCP [56]. Although there is not much difference in the amount of anodic peak currents (Ipa) of all of the CPs, yet a significant improvement can be noticed in the cathodic peak currents (Ipc) for the nanoporous CPs indicating their enhanced reversibility as compared to P-CP and TT-CP. The reversibility of redox reactions on the surface of each electrode can be further estimated by calculating the potential separation between the oxidation and the reduction peaks (ΔE) and also by the ratio of Ipc and Ipa. All of the nanoporous CPs show Ipc/Ipa values more close to unity and smaller ΔE than P-CP and TT-CP. The ΔE value for 1NP-CP is found the smallest among all of the CPs. In the case of CV in the V3+ electrolyte solution where redox peaks are normally hard to observe due to the sluggish kinetics of V3+/V2+ redox reaction, the redox peaks for nanoporous CPs are found more pronounced and taller than those of P-CP and TT-CP. This indicates their improved kinetics for V3+/V2+ redox reaction also (Fig. S9f). Cyclic voltammetry in a 1 M H2SO4 solution was also carried out to estimate the double layer capacitances (Cdl) of all CPs to correlate the improved activity of nanoporous CPs with the effective surface area, assuming that the two quantities are linearly proportional: the CV was conducted in the region of 0.0–0.6 V vs Ag/AgCl where the currents are mainly due to the charging of double layer (Fig. S11). The Cdl of CPs can be calculated from the plot of charging current density differences (ΔJ) at a middle potential (0.3 V) against scan rates (Fig. 4b). The linear slope is equivalent to twice of the Cdl which can be further used to 5
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Fig. 4. Cyclic voltammograms of various CPs at 5 mV s−1 in a V4+ electrolyte solution (a), charging current density differences (ΔJ = ja − jc) from CV measurements in 1 M H2SO4 (Fig. S11) as a function of the scan rate. The linear slope is equivalent to twice of the Cdl that are used to represent the effective surface areas of CPs (b), Nyquist plots of various CPs at 0.9 V in the V4+ electrolyte solution (c) and at −0.25 V in the V3+ electrolyte solution (d). The insets of (c) and (d) simply show the magnified plots.
P-CP electrodes throughout the test of 50 cycles. The detailed comparison is given in Figs. S16 and S17. Moreover, the EU for each VRFB cell was also measured as per Eq. (1) to estimate how much electrolyte participated actively during the CD process. As shown in Fig. 6c, the EUs for all VRFB cells decrease with increasing applied current densities, however, the decrease is very rapid in the case of the VRFB assembled with P-CP. Initially at the current density of 50 mA cm−2 the EU for the 1NP-CP cell is almost 87% whereas it is only 41% and 76% for the P-CP and TT-CP cells, respectively, during charging. In other words, almost 59% of the active species are not utilized in the charge process in the case of P-CP due to its hydrophobic and less active surface causing poor electrode/electrolyte interaction. The difference becomes even more significant when the current density is increased to 150 mA cm−2, with an EU of 72% for the 1NP-CP cell while 0% for the P-CP and TT-CP cells during charging. This enhanced EU due to the presence of nanopores and active surface functional groups on the nanoporous CPs is the reason for increased energy density. In order to confirm the enhanced mass transport on the surface of
reactions and ionic diffusion on the electrode surface resulting in higher EU. The CD results of the nanoporous CPs are compared in Fig. S13 and it can be seen that the 1NP-CP cell performs better than the cells composed of 0.5NP-CP and 2NP-CP. Although CV and EIS results show that nanoporous CPs are effective as both anode and cathode, there are indications (Fig. 4c and d) that their catalytic effects are more prominent towards the anode side reactions. In order to confirm this, two sets of asymmetric CD tests were conducted by employing 1NP-CP as electrode firstly only on the cathode side and then only on the anode side while using P-CP on the counter side in both tests. Interestingly, the results (Fig. 6a) confirm that the improvement is more substantial when 1NP-CP is used on the anode side than that on the cathode side. The comparison of Fig. 6a with Fig. 5b shows that nanoporous CPs are useful as electrode materials on both sides however, relatively more effective for the anode side. To compare the stability of the modified and unmodified CPs as electrode materials, a long term CD test was also carried out for 50 cycles at 50 mA cm−2 (Fig. 6b). The results show that the VRFB with 1NP-CP electrodes has higher and stable energy efficiency than that of
Table 2 Electrochemical parameters obtained from CV and EIS analyses. Electrode
Vpa [V]
Vpc [V]
ΔE [mV]
Ipa [mA cm−2]
Ipc [mA cm−2]
Ipc/Ipa
Rs In V4+ [Ω]
RCT In V4+ [Ω]
Rs In V3+ [Ω]
RCT In V3+ [Ω]
P-CP TT-CP 0.5NP-CP 1NP-CP 2NP-CP
1.05 1.04 0.97 0.96 0.98
0.69 0.71 0.81 0.82 0.81
369 329 167 142 172
10.83 10.70 10.48 10.83 10.90
−5.62 −6.85 −9.56 −9.92 −10.24
0.52 0.64 0.91 0.92 0.94
1.41 1.36 1.40 1.33 1.37
164.0 22.0 2.97 1.47 2.23
1.53 1.62 1.52 1.47 1.52
32,826 102 23 5.51 6.53
6
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Fig. 5. Charge-discharge performance of VRFB cells employing P-CP, TT-CP or 1NP-CP as electrodes; cell potential and charge-discharge energy densities at 50 mA cm−2 (a), discharge energy densities (b), energy efficiencies (c) and discharge capacities (d) at various applied current densities. The performance of 1NP-CPassembled cell at high current densities; discharge energy densities (e) and energy efficiency (f) from 200 to 400 mA cm−2.
Fig. 6. Comparison of the discharge energy densities of the VRFB cells assembled with asymmetric electrodes at various discharge current densities (a), energy efficiency of VRFBs employing P-CP or 1NP-CP electrodes for a long run of 50 cycles at 50 mA cm−2 (b), average electrolyte utilization of the VRFBs assembled with various CP electrodes at current densities from 50 to 150 mA cm−2 and constant potential window of 0.7–1.6 V (c) and discharge polarization curves of the VRFBs employing P-CP, TT-CP or 1NP-CP as electrodes with 100% initial SoC (d). 7
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References
1NP-CP, discharging polarization curves were studied referring to the recent studies that successfully demonstrated the importance of polarization curves in estimating the overpotentials in VRFB systems [23,63–65]. Generally, a discharge polarization curve for a cell has 3 major regions denoting various sources of overpotentials dominant in each region; the first region at low current densities where potential drops relatively fast, the intermediate region where the potential decreases gradually and the last region where the curve bends downward at high currents are associated dominantly with activation, ohmic and mass transport losses, respectively. The peak powers and limiting currents can be compared to estimate the performances of the electrodes used in the cells from the view point of mass transport resistance. If the limiting current and peak power of a VRFB cell shift to higher values by only changing electrode material while keeping all of the other parameters the same, the mass transport rate on the electrode surface can be considered as improved [63,66]. As shown in the polarization curves in Fig. 6d, the limiting current densities of VRFBs employing P-CP, TT-CP and 1NP-CP are 550, 575 and 600 mA cm−2, respectively, indicating significantly reduced mass transport resistances in the case of 1NP-CP electrodes. A similar improvement in peak power density is observed, i.e., 349 mW cm−2 for 1NP-CP, significantly higher than 234 and 268 mW cm−2 for P-CP and TT-CP electrodes, respectively. The improved limiting current and peak power densities are indications of reduced mass transport resistance on the surface of 1NP-CP electrodes due to the enhanced wettability and presence of nanopores formed by a catalytic etching. Furthermore, this work has also been compared with some of the previous studies in this field and a brief comparison is given in Table S6 showing that much improved performance is obtained by this work.
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4. Conclusion Our approach to modify thin carbon papers using a catalytic etching method was confirmed to effectively generate functionalized nanopores that led to much enhanced performance of a zero-gap VRFB employing the CPs. The nanoporous CPs provided increased active sites and hydrophilicity for the vanadium electrolytes, which facilitated the redox reactions and mass transport of reactant species on the electrode surface, leading to enhanced EU, energy density and energy efficiency. The VRFB using optimally modified CPs displayed much improved performance compared to the pristine carbon paper (P-CP): the improvements in terms of EU, discharge energy density and energy efficiency as measured at 50 mA cm−2 were 110%, 155% and 29%, respectively. The performance enhancement was even more prominent at high current density of 150 mA cm−2 where the pristine and thermally-treated CPs exhibited almost no activity. The improvement was also confirmed by increased limiting current and peak power densities in discharge polarization curves. In conclusion, the nanoporous carbon papers with active surface functional groups are very promising electrode materials to enhance the performance of VRFBs through increased active surface area, hydrophilicity and reduced mass transport resistance.
Acknowledgements The work was supported by the KIST institutional program on the development of next generation batteries (Project code: 2E29641) and by the Korea CCS R&D Center (Korea CCS 2020 Project) grant funded by the Korea government (Ministry of Science and ICT) (KCRC2014M1A8A1049293).
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122190. 8
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9
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Highly functionalized nanoporous thin carbon paper electrodes for high energy density of zero-gap vanadium redox flow battery Saleem Abbasa,b, Sheeraz Mehbooba,b, Hyun-Jin Shina,c, Oc Hee Hand, Heung Yong Haa,b,
T
⁎
a
Center for Energy Storage Research, Korea Institute of Science and Technology (KIST), 14-gil 5, Hwarang-ro, Seongbuk-gu, Seoul 02792, Republic of Korea Division of Energy & Environmental Technology, Korea University of Science & Technology (UST) – KIST School, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea c Department of Chemical & Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea d Western Seoul Center, Korea Basic Science Institute (KBSI), Seoul 03759, Republic of Korea b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
of low energy density of VRFB is • Issue addressed by modifying electrode surface.
functionalized nanopores are • Highly formed on the surface of thin carbon papers.
area, wettability and elec• Surface trode/electrolyte contact is highly improved.
utilization for cell having • Electrolyte modified electrodes is improved by 110%.
27 Wh L more energy is harvested • with 29% higher energy efficiency at −1
50 mA cm−2.
A R T I C LE I N FO
A B S T R A C T
Keywords: Vanadium redox flow battery Nanopores Surface functionalization Carbon paper Electrolyte utilization Energy density
Low energy density of a vanadium redox flow battery (VRFB) due to limited solubility and stability of vanadium ions constrains its wide spread applications and this issue becomes more critical by low active surface areas of electrodes and mass transport limitations of active species on the electrodes that lead to low electrolyte utilization. In this study the issue of low energy density is addressed by improving the electrode performance through modifying the surface properties and morphology of thin (0.19 mm thick) carbon paper electrodes instead of commonly used several millimeters thick carbon felts. Surface functionalization and pore formation of carbon paper are carried out using a catalytic etching method at high temperaturewhere the diameters of nanopores are controlled by tuning the etching conditions. The synergistic effects of thin, nanoporous and functionalized carbon paper result in more effective electrode/electrolyte interaction and the less mass transport resistance. Therefore, the zero-gap VRFB cell employing the nanoporous electrodes displays remarkable performance improvement in terms of electrolyte utilization by 110%, discharge energy density by 155% and energy efficiency by 29% as compared to the one using pristine electrodes at a current density of 50 mA cm−2. The results imply that the more energy can be harvested by employing nanoporous and functionalized carbon paper electrodes having larger active surface areas.
⁎ Corresponding author at: Center for Energy Storage Research, Korea Institute of Science and Technology (KIST), 14-gil 5, Hwarang-ro, Seongbuk-gu, Seoul 02792, Republic of Korea. E-mail address: [email protected] (H.Y. Ha).
https://doi.org/10.1016/j.cej.2019.122190 Received 18 March 2019; Received in revised form 21 May 2019; Accepted 8 July 2019 Available online 09 July 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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1. Introduction
chemistry of electrodes. In this study, in order to improve the performance of zero-gap VRFBs, carbon paper electrodes have been modified using a catalytic etching method. This method is advantageous over others as it is facile and could provide uniform and well-defined nanopores as well as surface functional groups leading to much increased active surface area and surface wettability available for redox reactions of VRFB. These nanoporous CPs are then subjected to various electrochemical tests such as cyclic voltammetry, electrochemical impedance spectroscopy, charge-discharge (CD) tests by employing as electrode materials for a zero-gap cell at varying current density and a long-term CD cycling test. Many physicochemical analysis techniques, such as SEM, TEM, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and wettability test have been employed to characterize the surface morphologies and properties of the nanoporous carbon papers and the analytical data are used to interpret the relations between the surface properties and the performance of the CP electrodes for vanadium redox reactions.
Renewable energy sources have drawn much attention of researchers in recent years due to the limitations and adverse effects of fossil fuels on the environment [1,2]. However, the efficient integration of these renewable energy sources with electric grids is limited because of their intermittent nature [3–5]. To overcome this issue, energy storage systems (ESSs) are used as means to store electrical energy in various reversible forms [6–8]. The redox flow batteries (RFBs) due to their design flexibility, decoupled scaling of power and energy and long cycle life are most suitable options of ESS [7,9,10]. Among others, allvanadium redox flow battery (VRFB) is one of the most advanced and the only commercialized candidates of RFBs, with employing four different oxidation states of vanadium, V4+/V5+ (VO2+/VO2+, catholyte) and V2+/V3+ (anolyte) as active materials, thus minimizing crossover effect through membrane. In addition to commonly used sulfuric acid as supporting electrolyte, mixed acid electrolytes and various additives have also been reported [4,11–14]. During charge process V4+ is oxidized to V5+ on the cathode while V3+ is reduced to V2+ on the anode and the charged species are reversed to original states during discharge process [15–19]. Carbon or graphite felts are commonly used as electrode materials in VRFB due to their high surface area and chemical stability in acidic environment [20,21]. Thin carbon papers (CP) have also been reported as electrode materials in VRFB as they offer reduced transport path to active species resulting in lower areal specific resistance (ASR) compared to thick felt electrodes [22–26]. Recently, zero-gap flow field design of VRFB cell employing thin carbon electrodes has also been reported to reduce the internal resistances of the conventionally used cells and achieve significantly high power densities [23]. Despite these recent developments in the power density of VRFB system with zero-gap cells, the low energy density of the system is still one of the major hurdles for its broad market penetration. The low energy density of VRFB is mainly due to three reasons: the first is the low solubility of vanadium ions in the electrolyte solutions (H2SO4 + H2O) and the second is the instability of some active species especially V5+ at high temperatures as it is prone to precipitate into irreversible V2O5 leading to a loss in energy density [4,27]. Many works have been done to address these issues; for example, by using mixed acid solutions as electrolyte and by adding stabilizing agents to the electrolytes [4,10,28–31]. The third reason of low energy density is the partial utilization of the active species in the electrolyte due to the mass transport polarizations and poor electrochemical activity of carbon electrodes especially at high current densities. By this, a significant amount of vanadium ions cannot actually take part in the redox reactions during charge-discharge (CD) processes and thus the theoretical energy density is not sufficiently achieved. As the electrolyte is a major component of system cost, i.e. 37% [32], its utilization needs to be improved to reduce the overall cost of VRFB system. The active involvement of electrolyte is normally estimated by a performance indicator known as electrolyte utilization (EU) which is the ratio of actual charge or discharge capacity to the theoretical capacity under the same conditions, i.e.
EU (%) =
Actual capacity (charge or discharge) × 100 Theoretical capacity
2. Experimental section 2.1. Materials Toray carbon papers (TGP-H-060, unteflonized with thickness of 0.19 mm) were used as electrode materials in this study. Cobalt acetate tetrahydrate (98%, Junsei) as metal precursor, isopropanol (IPA, 99.9%, SAMCHUN) as washing agent and ethanol (99.9%, SAMCHUN) as solvent for precursor were used. All reagents were of analytical grade and were directly used without any purification. 2.2. Electrode preparation Thin CP electrodes with functionalized nanopores were prepared by a catalytic etching process in air atmosphere [44]. Initially, pieces of pristine carbon paper (P-CP) were washed with IPA, dried and thermally treated at 500 °C for 4 h and denoted as thermally treated carbon paper (TT-CP). The TT-CPs were immersed in a 4 mM solution of cobalt acetate tetrahydrate in ethanol and sonicated for 1 h followed by drying in an oven at 60 °C. The samples were then heated in a muffle furnace for 1 h at 300 °C to form Co3O4 particles on the surface of carbon fibers of CPs. Nanoporous electrodes were prepared by further heating the catalyst loaded CPs at 500 °C for 0.5–2 h and denoted as 0.5NP-CP, 1NP-CP and 2NP-CP for the carbon papers treated for 0.5, 1 and 2 h, respectively. Atmospheric air was used in all heating steps. 2.3. Characterization The CPs were characterized by a field emission scanning electron microscope (FESEM, Hitachi JSM-7000F), transmission electron microscope (TEM, FEI Tecnai G2 F20) to analyze the surface morphology of CPs, and Raman spectroscopy to examine the structural changes before and after modifications. The X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Sigma Probe) was used to analyze the surface chemistry of the CPs. The surface areas and average pore diameters of CPs were obtained using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) techniques, respectively, on a N2 adsorption device (Micromeritics ASAP 2010 analyzer). Surface wettability of the CP samples was examined with vanadium electrolytes instead of conventionally used deionized (DI) water.
(1)
High EU is favorable for high energy density and low electrolyte volume required for practical use in VRFB leading to a significant reduction in overall cost of the system [33,34]. The higher EU can be obtained by improving the electrode performance for vanadium redox reactions. Though many methods have been reported to improve the performance of carbon electrodes, mainly including thermal treatment, chemical treatment, electrochemical activation, plasma treatment, metal or metal oxide loading, activation by KOH, and so on [12,33,35–43], there is still much room to improve the performance by simultaneously increasing the active surface area and the surface
2.4. Electrochemical measurements All the CPs were electrochemically examined by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). For CV and EIS, a piece of CP with a diameter of 1 cm was used as a working electrode while a Pt wire and an Ag/AgCl electrode were used as counter and reference electrodes, respectively. Concentrations of V4+ 2
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Fig. 1. Design of the zero-gap VRFB cell used in this study.
Fig. 2. SEM images of carbon papers; P-CP (a) TT-CP (b) 0.5NP-CP (c) 1NP-CP (d) and 2NP-CP (e).
and V3+ electrolytes were 0.1 M in 2 M H2SO4. For measuring the double layer capacitances and effective surface areas of the nanoporous carbon paper electrodes, CV was conducted in a 1 M H2SO4 solution. The electrochemical performance of the CPs was further evaluated through CD tests in a VRFB cell. The single-cell used in the CD tests was assembled by sandwiching a Nafion 117 membrane between two layers of CP electrodes which were placed in between two graphite plates having interdigitated pattern of flow fields for electrolyte flow. The area of each piece of CP was 25 cm2 while 3 pieces were layered on each bipolar plate as electrodes [23]. The positive electrolyte was 1.5 M V4+ in 3 M H2SO4 and a negative electrolyte solution, V3+, of the same concentration was prepared by electrochemical reduction of V4+. 50 ml of each electrolyte stored in separate glass reservoirs was purged with nitrogen gas for 10 min to minimize oxidation of the active species and circulated through each compartment of the VRFB cell using a peristaltic pump at a flow rate of 2.5 ml s−1. The theoretical capacity of 1.5 M vanadium electrolyte used for VRFB testing was 40.2 Ah L−1 (2.01 Ah) considering 50 ml of V4+ or V3+. The zero-gap flow field structure of VRFB cell used in this study is shown in Fig. 1. The current, voltage and energy efficiencies were calculated by using the equations given below:
Current efficiency (%) =
∫ Id dt Total discharge current × 100 = × 100 Total charge current ∫ Ic dt (2)
Energy efficiency (%) =
∫ Vd Id dt Total discharge energy × 100 = × 100 Total charge energy ∫ Vc Ic dt (3)
Voltage efficiency (%) =
Energy efficiency × 100 Current efficiency
(4)
where Ic and Id are charge current and discharge current, respectively, while Vc and Vd are charge voltage and discharge voltage, respectively [45]. Discharge polarization curves were measured galvanostatically by discharging the fully charged cell (100% state of charge (SoC)). The 100% SoC was assumed by observing the current less than 4 mA cm−2 when the VRFB cell was held potentiostatically at 1.8 V. Then discharging was started initially at an applied current density of 25 mA cm−2 which was increased stepwise and maintained for 30 s at each step until the cell potential became zero where a limiting current was recorded. 3
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3. Results and discussion
with disordered or defected carbon structure, meaning that the higher is the intensity of its peak (ID), the more will be the disordered structure. On the other hand, the G-band is associated with ordered graphitic structure and its peak intensity, IG, is an indication of degree of graphitization. Therefore, the higher ID/IG values of the nanoporous CPs indicate that the degree of disordered surface carbon has increased due to the catalytic etching treatment [38,50]. Another small peak observed at ~1620 cm−1 as a shoulder peak of the G-band which is generally referred as D′-band also implies structural disorder [51]. This D′-band is only observed in the treated CPs and its intensity increases with increasing etching time. The P-CP has the lowest ID/IG value of 0.05 indicating that initially the ordered graphitic structure is dominant before any treatment. After simple thermal treatment in air atmosphere, the ID/IG ratio increased to 0.10 for TT-CP, but the increment by thermal treatment is not so significant. In the case of 0.5NP-CP, the ID/IG increased up to 0.22, which is more than 4 times of that for P-CP. Further increase in treatment time caused higher degree of disordered surface carbon leading to the ID/IG value of 0.29 for 1NP-CP and 0.34 for 2NPCP. This dependence of the ID/IG ratio on the treatment time also indicates the controllability of the etching process. The surface chemistry of CPs was investigated by X-ray photoelectron spectroscopy (XPS) as shown in Figs. 3b–c, S6 and Table 1. The C1s spectra were deconvoluted to 4 peaks at the binding energies of ~284, ~286, ~288 and ~ 291 eV, corresponding to graphitic carbon (sp2/ C]C), defective carbon (sp3/CeC), carbonyl (C]O) and carboxyl acid (COOH) groups, respectively [40,52,53]. The results show that the graphitic or ordered sp2 carbon (C]C) is significantly decreased while defective or disordered sp3 carbon (CeC) is increased after etching, indicating successful formation of defects or pores on the surface of CP fibers due to catalytic etching [40,42]. Also the portions of carbonyl and carboxyl acid groups are increased mainly due to the oxidation during etching process at high temperatures in air atmosphere. These enhanced oxygen surface functional groups are generally considered to
The surface morphology of CPs was examined by SEM. The surface of P-CP (Fig. 2a) is smooth but contains some loose particles or other impurities. Therefore IPA washing was conducted before any further treatment. The thermal treatment of the IPA-washed CP at 500 °C for 4 h results in a smoother surface, free from these impurities without any significant morphological change (Fig. 2b) coinciding with the literature findings that simple thermal treatment in air atmosphere causes no significant structural changes [46,47]. On the other hand, heating of a catalyst-loaded CP just for 0.5 h at 500 °C shows clear signs of structural change with increased number of pores on its surface (Fig. 2c). Pores are formed on the surface of CP due to repetitive oxidation and reduction of cobalt oxide that penetrates into its body. The number and size of pores apparently increase with increasing the treatment time as shown in Fig. 2d and e. The morphological changes observed by TEM are presented in Fig. S5. Specific surface areas of the CPs were obtained by BET formula and average pore diameters by BJH desorption technique. The results show that the surface area of the P-CP is 0.02 m2 g−1 which is the lowest among all CPs tested in this study and in compliance with already reported values [48,49]. The surface area of TT-CP is 0.06 m2 g−1 with average pore diameter of 8.4 nm. The catalytically modified 0.5NP-CP, however, shows huge increment in surface area (4.96 m2 g−1). As far as average pore size is concerned, an increasing trend is noticed with increasing the treatment time as summarized in Table S3. This time-dependence indicates that the average pore size can be controlled directly by adjusting the treatment time. For the investigation of near-to-surface region of CPs, Raman spectroscopy was carried out and the first order Raman spectra were recorded in the range of 1000–2000 cm−1 (Fig. 3a). Two major peaks of interest appeared in the Raman spectra are D-band and G-band at ~1370 and ~1590 cm−1, respectively. The D-band is generally linked
Fig. 3. Raman spectra of various CPs (a) and XPS C1s spectra of P-CP (b) and 1NP-CP (c). 4
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represent relative surface area [57–60]. Notably, the relative surface areas of CPs are greatly improved after treatment (Table S4) which is considered useful for enhancing electrochemical activities. The results of EIS further confirm the improved electrochemical activities of the nanoporous CPs for vanadium redox reactions. Fig. 4c displays the Nyquist plots of all of the CPs in the V4+ electrolyte solution and the magnified plots are given in the inset. Generally, the semicircle portion of a Nyquist plot at high frequencies represents charge transfer process and the linear part at low frequencies reflects the ionic diffusion process in porous structure of the electrode. The diameter of semicircle denotes charge transfer resistance [61,62]. It can be seen that P-CP has the largest diameter amongst all of the CPs referring to the highest charge transfer resistance (RCT) on its surface while there is a dramatic decrease in this resistance for the nanoporous CPs showing the improved charge transfer process on their surface. Fig. 4d represents the Nyquist plots of all of the CPs in the V3+ electrolyte solution and the inset shows magnified graphs. The results show that the improved performance of the nanoporous CPs in the negative electrolyte solution is even more than in the positive electrolyte solution. This demonstrates that the nanoporous CPs are effective materials for both electrode sides of VRFB. This improvement is most probably due to the nanopores and active surface functional groups on the nanoporous CPs which are generated during the catalytic etching process. The 2NP-CP shows larger RCT than 1NP-CP, which may be due to the extensive structural and active site damage on the surface of 2NP-CP during excessive treatment [33]. The parameters obtained from CV and EIS are summarized in Table 2. These physical and electrochemical observations indicate that 1NP-CP is the best among all CPs made in this study. This is further confirmed by CD experiments with VRFBs as presented below. To investigate the performance of the VRFBs employing the nanoporous CPs as electrodes, CD experiments were carried out at various applied current densities. As discussed earlier with the CV results showing improved onset potentials for vanadium redox reactions by employing the nanoporous CPs, the VRFB systems also display similar trends in performance. Fig. 5a represents the relationship between cell potential and CD energy density for the VRFB cells having P-CP, TT-CP or 1NP-CP as electrodes at an applied current density of 50 mA cm−2. There is a significant decrease of about 238 mV in charging overpotential and about 304 mV in discharging overpotential for the VRFB assembled with 1NP-CP as compared to the one with P-CP. The lower charge and higher discharge potential for the 1NP-CP cell cause a larger available potential window leading to additional energy density gains of 25 Wh L−1 and 27 Wh L−1 during the first CD steps, respectively than the P-CP cell. Fig. 5b displays the discharge energy densities at various applied current densities while the comparison of charge energy densities is given in Fig. S12. As the applied current density increases the CD energy densities decrease for all of the CPs, which is a normal phenomenon due to increased overpotentials at high current densities [56]. Initially at 50 mA cm−2, the 1NP-CP cell attains almost 155% and 20% higher discharge energy density than the cells composed of P-CP and TT-CP, respectively. The degree of improvement gets higher at higher current densities, particularly at 150 mA cm−2 where the cells composed of P-CP and TT-CP show no performance but the 1NP-CP cell still has a discharge energy density about 38 Wh L−1 and an energy efficiency of 86% (Fig. 5c). The1NP-CP cell shows similar margins of improvements in terms of discharge capacity (Fig. 5d). As the 1NP-CP cell shows significantly better performance at 150 mA cm−2 than the cells composed of P-CP and TT-CP, the cell was subjected to extended CD cycles from 200 to 400 mA cm−2. The 1NP-CP cell exhibits a considerable discharge energy density of 5 Wh L−1 and an energy efficiency of 55% even at a very high current density of 400 mA cm−2 (Fig. 5e and f), which is still a reasonable performance. All this improved performance of 1NP-CP may be attributed to presence of active surface functional groups and nanopores that facilitate redox
Table 1 Fitting results of XPS C1s spectra in Fig. 3b and c. Position [eV]
Peak Assignment
P-CP [%]
TT-CP [%]
0.5NPCP [%]
1NP-CP [%]
2NP-CP [%]
284.48 286.33 288.41
C]C (sp2) CeC (sp3) C]O (Carbonyl) COOH (Carboxyl acid)
84.8 7.98 2.57
82.44 9.72 2.59
77.32 12.76 4.73
78.04 11.85 4.76
77.46 12.15 5.27
4.68
5.24
5.19
5.36
5.12
290.97
play a positive role in vanadium redox reactions [20,54,55]. Surface wettability of carbon electrode materials in VRFB is an indication of electrode/electrolyte interaction and generally hydrophilicity is recommended in this aqueous environment. Untreated carbon papers or felts, however, are normally highly hydrophobic causing poor electrochemical activity towards vanadium redox reactions due to inferior electrode/electrolyte interaction. Therefore, in order to examine the surface hydrophilicity of the CPs used in this study, wettability test was also done and, instead of conventionally used DI water, the real VRFB electrolytes were used for the test as shown in Fig. S7. On the surface of P-CP a V4+ droplet was fully absorbed in one hour and a V3+ droplet stayed almost 72 h while, in case of TT-CP, a V4+ droplet was absorbed in 5 s and a V3+ droplet took 10 min for complete absorption. On the other hand, the droplets of both V4+ and V3+ electrolytes were absorbed immediately on the surface of 0.5NP-CP, 1NP-CP and 2NP-CP. This improved surface wettability of nanoporous CPs as compared to P-CP or TT-CP is due to presence of numerous nanopores and hydrophilic functional groups collectively leading to much improved electrode/electrolyte interaction on their surface. Electrochemical activities of the CPs for vanadium redox reactions were investigated initially by CV. Fig. 4a represents the voltammograms for all of the CPs in V4+ electrolyte for the potential window of 0.2–1.5 V against Ag/AgCl at a scan rate of 5 mV s−1. The CV results show that all of the nanoporous CPs have significant improvements in both oxidation and reduction onset potentials compared to P-CP and TT-CP. These improved onset potentials cause enhanced reaction kinetics as well as increased energy storage efficiency due to reduced overpotentials for VRFB, meaning more facilitated vanadium redox reactions on the surface of nanoporous electrodes than on P-CP and TTCP [56]. Although there is not much difference in the amount of anodic peak currents (Ipa) of all of the CPs, yet a significant improvement can be noticed in the cathodic peak currents (Ipc) for the nanoporous CPs indicating their enhanced reversibility as compared to P-CP and TT-CP. The reversibility of redox reactions on the surface of each electrode can be further estimated by calculating the potential separation between the oxidation and the reduction peaks (ΔE) and also by the ratio of Ipc and Ipa. All of the nanoporous CPs show Ipc/Ipa values more close to unity and smaller ΔE than P-CP and TT-CP. The ΔE value for 1NP-CP is found the smallest among all of the CPs. In the case of CV in the V3+ electrolyte solution where redox peaks are normally hard to observe due to the sluggish kinetics of V3+/V2+ redox reaction, the redox peaks for nanoporous CPs are found more pronounced and taller than those of P-CP and TT-CP. This indicates their improved kinetics for V3+/V2+ redox reaction also (Fig. S9f). Cyclic voltammetry in a 1 M H2SO4 solution was also carried out to estimate the double layer capacitances (Cdl) of all CPs to correlate the improved activity of nanoporous CPs with the effective surface area, assuming that the two quantities are linearly proportional: the CV was conducted in the region of 0.0–0.6 V vs Ag/AgCl where the currents are mainly due to the charging of double layer (Fig. S11). The Cdl of CPs can be calculated from the plot of charging current density differences (ΔJ) at a middle potential (0.3 V) against scan rates (Fig. 4b). The linear slope is equivalent to twice of the Cdl which can be further used to 5
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Fig. 4. Cyclic voltammograms of various CPs at 5 mV s−1 in a V4+ electrolyte solution (a), charging current density differences (ΔJ = ja − jc) from CV measurements in 1 M H2SO4 (Fig. S11) as a function of the scan rate. The linear slope is equivalent to twice of the Cdl that are used to represent the effective surface areas of CPs (b), Nyquist plots of various CPs at 0.9 V in the V4+ electrolyte solution (c) and at −0.25 V in the V3+ electrolyte solution (d). The insets of (c) and (d) simply show the magnified plots.
P-CP electrodes throughout the test of 50 cycles. The detailed comparison is given in Figs. S16 and S17. Moreover, the EU for each VRFB cell was also measured as per Eq. (1) to estimate how much electrolyte participated actively during the CD process. As shown in Fig. 6c, the EUs for all VRFB cells decrease with increasing applied current densities, however, the decrease is very rapid in the case of the VRFB assembled with P-CP. Initially at the current density of 50 mA cm−2 the EU for the 1NP-CP cell is almost 87% whereas it is only 41% and 76% for the P-CP and TT-CP cells, respectively, during charging. In other words, almost 59% of the active species are not utilized in the charge process in the case of P-CP due to its hydrophobic and less active surface causing poor electrode/electrolyte interaction. The difference becomes even more significant when the current density is increased to 150 mA cm−2, with an EU of 72% for the 1NP-CP cell while 0% for the P-CP and TT-CP cells during charging. This enhanced EU due to the presence of nanopores and active surface functional groups on the nanoporous CPs is the reason for increased energy density. In order to confirm the enhanced mass transport on the surface of
reactions and ionic diffusion on the electrode surface resulting in higher EU. The CD results of the nanoporous CPs are compared in Fig. S13 and it can be seen that the 1NP-CP cell performs better than the cells composed of 0.5NP-CP and 2NP-CP. Although CV and EIS results show that nanoporous CPs are effective as both anode and cathode, there are indications (Fig. 4c and d) that their catalytic effects are more prominent towards the anode side reactions. In order to confirm this, two sets of asymmetric CD tests were conducted by employing 1NP-CP as electrode firstly only on the cathode side and then only on the anode side while using P-CP on the counter side in both tests. Interestingly, the results (Fig. 6a) confirm that the improvement is more substantial when 1NP-CP is used on the anode side than that on the cathode side. The comparison of Fig. 6a with Fig. 5b shows that nanoporous CPs are useful as electrode materials on both sides however, relatively more effective for the anode side. To compare the stability of the modified and unmodified CPs as electrode materials, a long term CD test was also carried out for 50 cycles at 50 mA cm−2 (Fig. 6b). The results show that the VRFB with 1NP-CP electrodes has higher and stable energy efficiency than that of
Table 2 Electrochemical parameters obtained from CV and EIS analyses. Electrode
Vpa [V]
Vpc [V]
ΔE [mV]
Ipa [mA cm−2]
Ipc [mA cm−2]
Ipc/Ipa
Rs In V4+ [Ω]
RCT In V4+ [Ω]
Rs In V3+ [Ω]
RCT In V3+ [Ω]
P-CP TT-CP 0.5NP-CP 1NP-CP 2NP-CP
1.05 1.04 0.97 0.96 0.98
0.69 0.71 0.81 0.82 0.81
369 329 167 142 172
10.83 10.70 10.48 10.83 10.90
−5.62 −6.85 −9.56 −9.92 −10.24
0.52 0.64 0.91 0.92 0.94
1.41 1.36 1.40 1.33 1.37
164.0 22.0 2.97 1.47 2.23
1.53 1.62 1.52 1.47 1.52
32,826 102 23 5.51 6.53
6
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Fig. 5. Charge-discharge performance of VRFB cells employing P-CP, TT-CP or 1NP-CP as electrodes; cell potential and charge-discharge energy densities at 50 mA cm−2 (a), discharge energy densities (b), energy efficiencies (c) and discharge capacities (d) at various applied current densities. The performance of 1NP-CPassembled cell at high current densities; discharge energy densities (e) and energy efficiency (f) from 200 to 400 mA cm−2.
Fig. 6. Comparison of the discharge energy densities of the VRFB cells assembled with asymmetric electrodes at various discharge current densities (a), energy efficiency of VRFBs employing P-CP or 1NP-CP electrodes for a long run of 50 cycles at 50 mA cm−2 (b), average electrolyte utilization of the VRFBs assembled with various CP electrodes at current densities from 50 to 150 mA cm−2 and constant potential window of 0.7–1.6 V (c) and discharge polarization curves of the VRFBs employing P-CP, TT-CP or 1NP-CP as electrodes with 100% initial SoC (d). 7
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References
1NP-CP, discharging polarization curves were studied referring to the recent studies that successfully demonstrated the importance of polarization curves in estimating the overpotentials in VRFB systems [23,63–65]. Generally, a discharge polarization curve for a cell has 3 major regions denoting various sources of overpotentials dominant in each region; the first region at low current densities where potential drops relatively fast, the intermediate region where the potential decreases gradually and the last region where the curve bends downward at high currents are associated dominantly with activation, ohmic and mass transport losses, respectively. The peak powers and limiting currents can be compared to estimate the performances of the electrodes used in the cells from the view point of mass transport resistance. If the limiting current and peak power of a VRFB cell shift to higher values by only changing electrode material while keeping all of the other parameters the same, the mass transport rate on the electrode surface can be considered as improved [63,66]. As shown in the polarization curves in Fig. 6d, the limiting current densities of VRFBs employing P-CP, TT-CP and 1NP-CP are 550, 575 and 600 mA cm−2, respectively, indicating significantly reduced mass transport resistances in the case of 1NP-CP electrodes. A similar improvement in peak power density is observed, i.e., 349 mW cm−2 for 1NP-CP, significantly higher than 234 and 268 mW cm−2 for P-CP and TT-CP electrodes, respectively. The improved limiting current and peak power densities are indications of reduced mass transport resistance on the surface of 1NP-CP electrodes due to the enhanced wettability and presence of nanopores formed by a catalytic etching. Furthermore, this work has also been compared with some of the previous studies in this field and a brief comparison is given in Table S6 showing that much improved performance is obtained by this work.
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4. Conclusion Our approach to modify thin carbon papers using a catalytic etching method was confirmed to effectively generate functionalized nanopores that led to much enhanced performance of a zero-gap VRFB employing the CPs. The nanoporous CPs provided increased active sites and hydrophilicity for the vanadium electrolytes, which facilitated the redox reactions and mass transport of reactant species on the electrode surface, leading to enhanced EU, energy density and energy efficiency. The VRFB using optimally modified CPs displayed much improved performance compared to the pristine carbon paper (P-CP): the improvements in terms of EU, discharge energy density and energy efficiency as measured at 50 mA cm−2 were 110%, 155% and 29%, respectively. The performance enhancement was even more prominent at high current density of 150 mA cm−2 where the pristine and thermally-treated CPs exhibited almost no activity. The improvement was also confirmed by increased limiting current and peak power densities in discharge polarization curves. In conclusion, the nanoporous carbon papers with active surface functional groups are very promising electrode materials to enhance the performance of VRFBs through increased active surface area, hydrophilicity and reduced mass transport resistance.
Acknowledgements The work was supported by the KIST institutional program on the development of next generation batteries (Project code: 2E29641) and by the Korea CCS R&D Center (Korea CCS 2020 Project) grant funded by the Korea government (Ministry of Science and ICT) (KCRC2014M1A8A1049293).
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122190. 8
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