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Continuous operation of an electrochemical flow capacitor
Electrochemistry Communications 48 (2014) 178–181
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Continuous operation of an electrochemical flow capacitor S. Porada a, J. Lee a,b, D. Weingarth a, V. Presser a,b,⁎ a b
INM - Leibniz Institute for New Materials, 66123 Saarbrücken, Germany Saarland University, 66123 Saarbrücken, Germany
a r t i c l e
i n f o
Article history: Received 1 August 2014 Received in revised form 26 August 2014 Accepted 26 August 2014 Available online 3 September 2014 Keywords: Electrochemical energy storage Flow capacitor Supercapacitor
a b s t r a c t The electrochemical flow capacitor (EFC) has been recently introduced as a new concept for rapid and capacitive energy storage using flowable carbon-electrolyte suspensions. In our study, we investigate the EFC under static and constant flow conditions. Unlike in static carbon suspensions where poor particle-particle-contact and particle settling yield a highly resistive and time-dependent behavior, we show that flow operation of carbon suspensions reaches high Coulombic efficiency and stable energy density performance. Our results also indicate that the specific capacitance per total mass of carbon electrodes in flow operation is comparable to conventional bound carbon film electrodes. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors have emerged as a promising class of highly efficient devices for high power applications [1]. Albeit recent advances, supercapacitors have a moderate energy density compared to batteries and their suitability for large scale (grid level) applications remains limited [2]. Recently [3], we have introduced the electrochemical flow capacitor (EFC) which potentially combines the cost effectiveness of flow batteries with the high power handling of supercapacitors. This novel energy storage technology employs suspensions of porous carbon particles in an electrolyte instead of solid carbon film electrodes which are used in conventional supercapacitors. Research on semi-solid flow batteries [4–7] and EFCs has made significant progress over the last two years and also led to a new technology level for continuous capacitive deionization [8–10]. However, electrochemical operation with EFC slurries has mostly been investigated in a non-flow configuration [11,12] one-time intermittent operation mode [3,13,14] or semi-continuous, bidirectional pumping [15]. In our paper, we explore fully continuous operation of a closed-loop EFC system with unidirectional flow. Our work highlights that even in flow mode the full capacitance of carbon can be utilized. We show that testing carbon suspensions under static configuration leads to settling of carbon particles which then translates to significant changes in the EFC system performance. 2. Experimental Type YP50-F activated carbon (AC) powder was purchased from Kuraray Chemicals Co, Japan, and dispersed in 1 M Na2SO4 aqueous ⁎ Corresponding author. E-mail address: [email protected] (V. Presser).
http://dx.doi.org/10.1016/j.elecom.2014.08.023 1388-2481/© 2014 Elsevier B.V. All rights reserved.
electrolyte. The carbon content was 10 mass% (≈ 5 vol%) for the flow electrode and 95 mass% for polytetrafluorethylene-bound (PTFE) film electrodes (1.13 cm2 size with 150 μm thickness). The latter were prepared following the procedure outlined in Ref. [16]. Electrochemical measurements were carried out using a VSP300 from Bio-Logic, France, employing cyclic voltammetry (CV), galvanostatic cycling with potential limitation (GCPL), and chronoamperometry (CAM). The specific capacitance was derived from discharge data using either GCPL or CAM. In case of GCPL, the voltage was corrected by the IR drop, while in case of CAM, the leakage current was subtracted. All data were normalized by the total dry carbon mass, that is, not considering binder in case of the static film electrode measurement. The Coulombic efficiency was calculated as the ratio between the charge invested during charging and the recovered charge during discharging. A closed-loop EFC system encompassing 13 cm3 of capacitive flow electrode material per side with two symmetric and identical electrochemical cells was used in this study. Each cell employs graphite plate current collectors (15.4 ×6.2×1.0 cm3) and an anion exchange membrane (AEM; Neosepta AMX, thickness = 140 μm) as separator. The flow capacitor concept does not require the use of an ion exchange membrane; our choice was motivated by the positive effect of a nonporous, smooth separator to the flow of electrode suspension. A 1 cm wide flow channel (volume: ≈ 0.47 cm3) was created by placing 0.59 mm silicone gaskets between AEM and the graphite current collector. The carbon suspensions entered the flow channel through polymer tubes inserted on the sides of the current collector with a constant speed of 13 cm3/min mediated by a peristaltic pump. A constant cell voltage, Vcell, of 0.6, 0.9, or 1.2 V was applied to one of the electrochemical cells (Cell 1), until the system had reached a certain open cell voltage, OCV, measured by the second identical electrochemical cell (Cell 2; see Fig. 1A).
S. Porada et al. / Electrochemistry Communications 48 (2014) 178–181
Some experiments were carried out with one cell without flow to investigate the influence on sedimentation (settling of the suspension) on the electrochemical performance; for this, the carbon mass loading was also 10 mass%. For these experiments, the in- and outlets were thoroughly closed to only account for carbon inside the flow channel. Nitrogen gas sorption measurements at −196 °C were carried out with an Autosorb iQ system (Quantachrome, USA) following the routine outlined in Ref. [16]. The specific surface area was calculated using the Brunauer Emmett Teller equation in the linear relative pressure range 0.01–0.20. We also calculated the cumulative pore size distribution via quenched-solid density functional theory with assuming slit shaped pores. Viscosity analysis of the carbon flow electrodes was performed using a modular compact rheometer (Physica MCR 300, Austria) in a double gap concentric cylinder at room temperature. All measurements were performed using a carbon mass loading of 10 mass% dispersed in various concentrations of electrolyte. 3. Results and discussion 3.1. Setup concept and bulk properties of the carbon flow electrode As seen from Fig. 1A, the cells were separated from each other using carbon suspension reservoirs designed as drip chambers known from intravenous therapy in a valve-free continuous flow setup. This setup concept simplifies the system design and allows to operate the EFC in a fully continuous fashion. Also, this setup operates in a unidirectional fashion, meaning the carbon suspension does not have to be pumped back for discharging by reverting the flow direction [15]. The carbon used for this study was commercially available activated carbon with a specific surface area of 1614 m2/g, a total pore volume of 0.71 cm3/g, and an average pore size of 1.0 nm (Fig. 1B). These values are related to the unprocessed powder; for use in supercapacitors, carbon powders are commonly consolidated to film electrodes by polymer binder, such as PTFE [17]. This process limits the access to the pore volume depending on the amount of binder and the way of application. In contrast, capacitive flow electrodes employ unprocessed powder which allows to capitalize, in theory, the entire ion-
179
accessible pore volume of the dry powder. For the activated carbon under investigation, the pore volume reduction can be quantified with ≈15% when employing 5 mass% PTFE (Fig. 1B); which is in agreement with data presented in Ref. [18]. With the purpose of a proof-of-concept for continuous closed-loop system operation, we kept the carbon mass loading (10 mass%) and electrolyte composition (1 M Na2SO4) constant and obtained stable energy density system performance over several days without clogging of the tubes or the electrochemical cell. Beyond the amount of carbon in the flow electrode suspension [10,11], the viscosity is also influenced by the salt concentration (Fig. 1C). It is important to note that while a high carbon mass loading and salt concentration are attractive to enable high specific capacitance, both parameters should be minimized from a viscosity point of view [10]. 3.2. Electrochemical behavior under static conditions When prepared as conventional PTFE-bound film electrode, AC shows typical supercapacitor-like behavior. In Fig. 2A cyclic voltammograms between 1 and 50 mV/s are presented when cycling up to 0.6 V. At a low scan rate (1 mV/s), a maximum specific capacitance of ≈100 F/g is observed which significantly decreases at high scan rates. The same trend can be seen when investigating the specific capacitance as a function of current density from galvanostatic discharging (Fig. 2B): at 1 A/g, only approximately half of the initial equilibrium capacitance can be used, namely 45 F/g instead of 99 F/g (measured at 10 mA/g; inset in Fig. 2B). At low current density, a system resistance of 4.2 mΩm2 can be derived from the IR drop during galvanostatic discharging. When charging the capacitive carbon suspension in static mode to 0.6 V, the response is highly resistive and time-dependent (Fig. 2C–D). We find that the specific capacitance constantly increases as a function of settling time; in fact, the capacitance measured after 1 h of sedimentation is only 29% (8 F/g) of what was measured was found after 50 h of settling (28 F/g; Fig. 2D). We note that the value of 28 F/g is significantly lower than the capacitance found for the consolidated film electrode, namely 99 F/g. Even charging and discharging in chronoamperometric mode for 10 h only yields a low specific capacitance of 66 F/g. We explain this result by settling of carbon particles which leads to
Fig. 1. (A) Schematic setup of the continuous flow cell system. (B) Comparison of the cumulative pore volume of activated carbon powder versus a film electrodes from activated carbon mixed with 5 mass% PTFE. (C) Viscosity of aqueous media with varying salt content (Na2SO4) with 10 mass% activated carbon.
S. Porada et al. / Electrochemistry Communications 48 (2014) 178–181
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Fig. 2. Electrochemical performance of PTFE-bound carbon film electrodes in 1 M Na2SO4 (A + B). (A) Cyclic voltammograms at 1, 10, and 50 mV/s sweep rate. (B) Capacitance versus current density obtained from galvanostatic charge / discharge cycling between 10 and 1000 mA/g (inset: galvanostatic data at 10 mA/g). Electrochemical performance of a static carbon suspension (C + D). (C) Galvanostatic charge and discharge at 10 mA/g after 1, 10, and 50 h settling time (i.e., consolidation of carbon particles in the flow chamber). (D) Time dependency of the capacitance and Coulombic efficiency. Inset shows chronoamperometric charge and discharge.
improved particle-particle contact over time, but obviously a significant number of the carbon particles does not participate in the charge storage because of loose contact. The variability of performance over time makes it very difficult to rationalize static mode testing of carbon suspensions as a reliable measure for the electrochemical evaluation of flow electrode materials [15]. 3.3. Electrochemical behavior under continuous flow Flowing carbon particles ensure statistic contact with other particles and the current collector inside the electrochemical cell. We have to consider for the chronoamperometric data a leakage resistance of the system which can be quantified as around 0.7 Ωm2 when operating at 0.9 V (Fig. 3A). After 6 h of charging at 0.6 V, an OCV of 0.57 V was obtained, but much longer charging times were required for higher voltages (Fig. 3B). We see that all three studied voltage settings yield a high Coulombic efficiency between 93 and 99% and a specific capacitance between 96 and 106 F/g (Fig. 3C). It is important to note that the capacitance values are similar to the conventional PTFE-bound carbon film electrode (Fig. 3D) when normalized to the mass of dry carbon. When we consider the entire volume of the carbon suspension, these values correspond to ca. 11 F/cm3 which converts into an energy density of approximately 0.6 Wh/dm3. With a measured charge transfer resistance of ca. 26 Ω per device (normalized to membrane area: 21 mΩm2), this is also equivalent to 17 W/m2 at a cell voltage of 1.2 V.
4. Conclusions We have presented data on the continuous operation of an EFC system. This has been accomplished by using a novel valve-free system and a carbon loading of 10 mass% ensured operation without clogging. Highly stable energy density system performance was observed over several days of continuous operation. Interestingly, the specific capacitance (per mass of dry carbon) in the EFC system is similar to conventional film electrodes albeit the resistance in flow configuration is significantly higher. The performance of flow electrodes is also significantly better and not prone to sedimentation effects compared to static-operation of carbon suspensions. Further improvements of the system operation will be subject to future work entailing improving the carbon mass loading and flow channel design. Also, as known from previous work (e.g., Ref. [3]), voltage-loss and self-discharge over time have to be considered as notorious challenges for aqueous supercapacitors and electrochemical flow capacitors alike. Acknowledgements The INM (www.inm-gmbh.de) is part of the Leibniz Research Alliance Energy Transition (LVE). We acknowledge funding from the German Federal Ministry for Research and Education (BMBF, award number 03EK3013). SP acknowledges financial support of the Alexander von Humboldt Foundation. The authors thank Prof. Eduard Arzt
S. Porada et al. / Electrochemistry Communications 48 (2014) 178–181
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Fig. 3. Electrochemical performance of continuous electrochemical flow capacitor in 1 M Na2SO4 (A + B). (A) Current density versus time obtained from chronoamperometry experiment at Vcell = 0.9 V and flow rate of 13 cm3/min. (B) Open cell voltage versus time measured in Cell 2 for several cycles obtained at Vcell = 0.6, 0.9 and 1.2 V applied to Cell 1 during charging and Vcell = 0 V during discharging, flow rate of 13 cm3/min. (C) Specific capacitance and Coulombic efficiency versus cell voltage. (D) Comparison of measured specific capacitance for electrodes under continuous flow, film electrodes and flow electrodes without flow.
for his continuing support and Nicolas Jäckel for gas sorption analysis measurements (both at the INM). References [1] Z. Yang, J. Zhang, M.C.W. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Electrochemical energy storage for green grid, Chem. Rev. 111 (2011) 3577–3613. [2] F. Beguin, V. Presser, A. Balducci, E. Frackowiak, Carbons and electrolytes for advanced supercapacitors, Adv. Mater. 26 (2014) 2219–2251. [3] V. Presser, C.R. Dennison, J. Campos, K.W. Knehr, E.C. Kumbur, Y. Gogotsi, The electrochemical flow capacitor: a New concept for rapid energy storage and recovery, Adv. Energy Mater. 2 (2012) 895–902. [4] M. Duduta, B. Ho, V.C. Wood, P. Limthongkul, V.E. Brunini, W. Craig Carter, Y.-M. Chiang, Semi-solid lithium rechargeable flow battery, Adv. Energy Mater. 1 (2011) 511–516. [5] M. Youssry, L. Madec, P. Soudan, M. Cerbelaud, D. Guyomard, B. Lestriez, Nonaqueous carbon black suspensions for lithium-based redox flow batteries: rheology and simultaneous rheo-electrical behavior, Phys. Chem. Chem. Phys. 15 (2013) 14476–14486. [6] Q. Huang, H. Li, M. Gratzel, Q. Wang, Reversible chemical delithiation/lithiation of LiFePO4: towards a redox flow lithium-ion battery, Phys. Chem. Chem. Phys. 15 (2013) 1793–1797. [7] F. Pan, J. Yang, Q. Huang, X. Wang, H. Huang, Q. Wang, Redox targeting of anatase TiO2 for redox flow lithium-Ion batteries, Adv. Energy Mater. (2014) (in press), http://dx.doi.org/10.1002/aenm.201400567. [8] K.B. Hatzell, E. Iwama, A. Ferris, B. Daffos, K. Urita, T. Tzedakis, F. Chauvet, P.-L. Taberna, Y. Gogotsi, P. Simon, Capacitive deionization concept based on suspension electrodes without ion exchange membranes, Electrochem. Commun. 43 (2014) 18–21.
[9] S.I. Jeon, H.R. Park, J.G. Yeo, S. Yang, C.H. Cho, M.H. Han, D.K. Kim, Desalination via a new membrane capacitive deionization process utilizing flow-electrodes, Energy Environ. Sci. 6 (2013) 1471–1475. [10] S. Porada, D. Weingarth, H.V.M. Hamelers, M. Bryjak, V. Presser, P.M. Biesheuvel, Carbon flow electrodes for continuous operation of capacitive deionization and capacitive mixing energy generation, J. Mater. Chem. A 2 (2014) 9313–9321. [11] J.W. Campos, M. Beidaghi, K.B. Hatzell, C.R. Dennison, B. Musci, V. Presser, E.C. Kumbur, Y. Gogotsi, Investigation of carbon materials for Use as a flowable electrode in electrochemical flow capacitors, Electrochim. Acta 98 (2013) 123–130. [12] K.B. Hatzell, M. Beidaghi, J.W. Campos, C.R. Dennison, E.C. Kumbur, Y. Gogotsi, A high performance pseudocapacitive suspension electrode for the electrochemical flow capacitor, Electrochim. Acta 111 (2013) 888–897. [13] M. Boota, K.B. Hatzell, M. Beidaghi, C.R. Dennison, E.C. Kumbur, Y. Gogotsi, Activated carbon spheres as a flowable electrode in electrochemical flow capacitors, J. Electrochem. Soc. 161 (2014) A1078–A1083. [14] C.R. Dennison, M. Beidaghi, K.B. Hatzell, J.W. Campos, Y. Gogotsi, E.C. Kumbur, Effects of flow cell design on charge percolation and storage in the carbon slurry electrodes of electrochemical flow capacitors, J. Power Sources 247 (2014) 489–496. [15] C.R. Dennison, Y. Gogotsi, E.C. Kumbur, In situ distributed diagnostics of flowable electrode systems: resolving spatial and temporal limitations, Phys. Chem. Chem. Phys. 16 (2014) 18241–18252. [16] D. Weingarth, M. Zeiger, N. Jäckel, M. Aslan, G. Feng, V. Presser, Graphitization as a universal tool to tailor the potential-dependent capacitance of carbon supercapacitors, Adv. Energy Mater. (2014) (in press), http://dx.doi.org/10.1002/ aenm.201400316. [17] F. Beguin, E. Frackowiak, Supercapacitors, Wiley, Weinheim, 2013. [18] N. Jäckel, D. Weingarth, M. Zeiger, M. Aslan, I. Grobelsek, V. Presser, Comparison of carbon onions and carbon blacks as conductive additives for carbon supercapacitors in organic electrolytes, J. Power Sources (2014) (in press), http://dx.doi.org/10. 1016/j.jpowsour.2014.08.090.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Continuous operation of an electrochemical flow capacitor S. Porada a, J. Lee a,b, D. Weingarth a, V. Presser a,b,⁎ a b
INM - Leibniz Institute for New Materials, 66123 Saarbrücken, Germany Saarland University, 66123 Saarbrücken, Germany
a r t i c l e
i n f o
Article history: Received 1 August 2014 Received in revised form 26 August 2014 Accepted 26 August 2014 Available online 3 September 2014 Keywords: Electrochemical energy storage Flow capacitor Supercapacitor
a b s t r a c t The electrochemical flow capacitor (EFC) has been recently introduced as a new concept for rapid and capacitive energy storage using flowable carbon-electrolyte suspensions. In our study, we investigate the EFC under static and constant flow conditions. Unlike in static carbon suspensions where poor particle-particle-contact and particle settling yield a highly resistive and time-dependent behavior, we show that flow operation of carbon suspensions reaches high Coulombic efficiency and stable energy density performance. Our results also indicate that the specific capacitance per total mass of carbon electrodes in flow operation is comparable to conventional bound carbon film electrodes. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors have emerged as a promising class of highly efficient devices for high power applications [1]. Albeit recent advances, supercapacitors have a moderate energy density compared to batteries and their suitability for large scale (grid level) applications remains limited [2]. Recently [3], we have introduced the electrochemical flow capacitor (EFC) which potentially combines the cost effectiveness of flow batteries with the high power handling of supercapacitors. This novel energy storage technology employs suspensions of porous carbon particles in an electrolyte instead of solid carbon film electrodes which are used in conventional supercapacitors. Research on semi-solid flow batteries [4–7] and EFCs has made significant progress over the last two years and also led to a new technology level for continuous capacitive deionization [8–10]. However, electrochemical operation with EFC slurries has mostly been investigated in a non-flow configuration [11,12] one-time intermittent operation mode [3,13,14] or semi-continuous, bidirectional pumping [15]. In our paper, we explore fully continuous operation of a closed-loop EFC system with unidirectional flow. Our work highlights that even in flow mode the full capacitance of carbon can be utilized. We show that testing carbon suspensions under static configuration leads to settling of carbon particles which then translates to significant changes in the EFC system performance. 2. Experimental Type YP50-F activated carbon (AC) powder was purchased from Kuraray Chemicals Co, Japan, and dispersed in 1 M Na2SO4 aqueous ⁎ Corresponding author. E-mail address: [email protected] (V. Presser).
http://dx.doi.org/10.1016/j.elecom.2014.08.023 1388-2481/© 2014 Elsevier B.V. All rights reserved.
electrolyte. The carbon content was 10 mass% (≈ 5 vol%) for the flow electrode and 95 mass% for polytetrafluorethylene-bound (PTFE) film electrodes (1.13 cm2 size with 150 μm thickness). The latter were prepared following the procedure outlined in Ref. [16]. Electrochemical measurements were carried out using a VSP300 from Bio-Logic, France, employing cyclic voltammetry (CV), galvanostatic cycling with potential limitation (GCPL), and chronoamperometry (CAM). The specific capacitance was derived from discharge data using either GCPL or CAM. In case of GCPL, the voltage was corrected by the IR drop, while in case of CAM, the leakage current was subtracted. All data were normalized by the total dry carbon mass, that is, not considering binder in case of the static film electrode measurement. The Coulombic efficiency was calculated as the ratio between the charge invested during charging and the recovered charge during discharging. A closed-loop EFC system encompassing 13 cm3 of capacitive flow electrode material per side with two symmetric and identical electrochemical cells was used in this study. Each cell employs graphite plate current collectors (15.4 ×6.2×1.0 cm3) and an anion exchange membrane (AEM; Neosepta AMX, thickness = 140 μm) as separator. The flow capacitor concept does not require the use of an ion exchange membrane; our choice was motivated by the positive effect of a nonporous, smooth separator to the flow of electrode suspension. A 1 cm wide flow channel (volume: ≈ 0.47 cm3) was created by placing 0.59 mm silicone gaskets between AEM and the graphite current collector. The carbon suspensions entered the flow channel through polymer tubes inserted on the sides of the current collector with a constant speed of 13 cm3/min mediated by a peristaltic pump. A constant cell voltage, Vcell, of 0.6, 0.9, or 1.2 V was applied to one of the electrochemical cells (Cell 1), until the system had reached a certain open cell voltage, OCV, measured by the second identical electrochemical cell (Cell 2; see Fig. 1A).
S. Porada et al. / Electrochemistry Communications 48 (2014) 178–181
Some experiments were carried out with one cell without flow to investigate the influence on sedimentation (settling of the suspension) on the electrochemical performance; for this, the carbon mass loading was also 10 mass%. For these experiments, the in- and outlets were thoroughly closed to only account for carbon inside the flow channel. Nitrogen gas sorption measurements at −196 °C were carried out with an Autosorb iQ system (Quantachrome, USA) following the routine outlined in Ref. [16]. The specific surface area was calculated using the Brunauer Emmett Teller equation in the linear relative pressure range 0.01–0.20. We also calculated the cumulative pore size distribution via quenched-solid density functional theory with assuming slit shaped pores. Viscosity analysis of the carbon flow electrodes was performed using a modular compact rheometer (Physica MCR 300, Austria) in a double gap concentric cylinder at room temperature. All measurements were performed using a carbon mass loading of 10 mass% dispersed in various concentrations of electrolyte. 3. Results and discussion 3.1. Setup concept and bulk properties of the carbon flow electrode As seen from Fig. 1A, the cells were separated from each other using carbon suspension reservoirs designed as drip chambers known from intravenous therapy in a valve-free continuous flow setup. This setup concept simplifies the system design and allows to operate the EFC in a fully continuous fashion. Also, this setup operates in a unidirectional fashion, meaning the carbon suspension does not have to be pumped back for discharging by reverting the flow direction [15]. The carbon used for this study was commercially available activated carbon with a specific surface area of 1614 m2/g, a total pore volume of 0.71 cm3/g, and an average pore size of 1.0 nm (Fig. 1B). These values are related to the unprocessed powder; for use in supercapacitors, carbon powders are commonly consolidated to film electrodes by polymer binder, such as PTFE [17]. This process limits the access to the pore volume depending on the amount of binder and the way of application. In contrast, capacitive flow electrodes employ unprocessed powder which allows to capitalize, in theory, the entire ion-
179
accessible pore volume of the dry powder. For the activated carbon under investigation, the pore volume reduction can be quantified with ≈15% when employing 5 mass% PTFE (Fig. 1B); which is in agreement with data presented in Ref. [18]. With the purpose of a proof-of-concept for continuous closed-loop system operation, we kept the carbon mass loading (10 mass%) and electrolyte composition (1 M Na2SO4) constant and obtained stable energy density system performance over several days without clogging of the tubes or the electrochemical cell. Beyond the amount of carbon in the flow electrode suspension [10,11], the viscosity is also influenced by the salt concentration (Fig. 1C). It is important to note that while a high carbon mass loading and salt concentration are attractive to enable high specific capacitance, both parameters should be minimized from a viscosity point of view [10]. 3.2. Electrochemical behavior under static conditions When prepared as conventional PTFE-bound film electrode, AC shows typical supercapacitor-like behavior. In Fig. 2A cyclic voltammograms between 1 and 50 mV/s are presented when cycling up to 0.6 V. At a low scan rate (1 mV/s), a maximum specific capacitance of ≈100 F/g is observed which significantly decreases at high scan rates. The same trend can be seen when investigating the specific capacitance as a function of current density from galvanostatic discharging (Fig. 2B): at 1 A/g, only approximately half of the initial equilibrium capacitance can be used, namely 45 F/g instead of 99 F/g (measured at 10 mA/g; inset in Fig. 2B). At low current density, a system resistance of 4.2 mΩm2 can be derived from the IR drop during galvanostatic discharging. When charging the capacitive carbon suspension in static mode to 0.6 V, the response is highly resistive and time-dependent (Fig. 2C–D). We find that the specific capacitance constantly increases as a function of settling time; in fact, the capacitance measured after 1 h of sedimentation is only 29% (8 F/g) of what was measured was found after 50 h of settling (28 F/g; Fig. 2D). We note that the value of 28 F/g is significantly lower than the capacitance found for the consolidated film electrode, namely 99 F/g. Even charging and discharging in chronoamperometric mode for 10 h only yields a low specific capacitance of 66 F/g. We explain this result by settling of carbon particles which leads to
Fig. 1. (A) Schematic setup of the continuous flow cell system. (B) Comparison of the cumulative pore volume of activated carbon powder versus a film electrodes from activated carbon mixed with 5 mass% PTFE. (C) Viscosity of aqueous media with varying salt content (Na2SO4) with 10 mass% activated carbon.
S. Porada et al. / Electrochemistry Communications 48 (2014) 178–181
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Fig. 2. Electrochemical performance of PTFE-bound carbon film electrodes in 1 M Na2SO4 (A + B). (A) Cyclic voltammograms at 1, 10, and 50 mV/s sweep rate. (B) Capacitance versus current density obtained from galvanostatic charge / discharge cycling between 10 and 1000 mA/g (inset: galvanostatic data at 10 mA/g). Electrochemical performance of a static carbon suspension (C + D). (C) Galvanostatic charge and discharge at 10 mA/g after 1, 10, and 50 h settling time (i.e., consolidation of carbon particles in the flow chamber). (D) Time dependency of the capacitance and Coulombic efficiency. Inset shows chronoamperometric charge and discharge.
improved particle-particle contact over time, but obviously a significant number of the carbon particles does not participate in the charge storage because of loose contact. The variability of performance over time makes it very difficult to rationalize static mode testing of carbon suspensions as a reliable measure for the electrochemical evaluation of flow electrode materials [15]. 3.3. Electrochemical behavior under continuous flow Flowing carbon particles ensure statistic contact with other particles and the current collector inside the electrochemical cell. We have to consider for the chronoamperometric data a leakage resistance of the system which can be quantified as around 0.7 Ωm2 when operating at 0.9 V (Fig. 3A). After 6 h of charging at 0.6 V, an OCV of 0.57 V was obtained, but much longer charging times were required for higher voltages (Fig. 3B). We see that all three studied voltage settings yield a high Coulombic efficiency between 93 and 99% and a specific capacitance between 96 and 106 F/g (Fig. 3C). It is important to note that the capacitance values are similar to the conventional PTFE-bound carbon film electrode (Fig. 3D) when normalized to the mass of dry carbon. When we consider the entire volume of the carbon suspension, these values correspond to ca. 11 F/cm3 which converts into an energy density of approximately 0.6 Wh/dm3. With a measured charge transfer resistance of ca. 26 Ω per device (normalized to membrane area: 21 mΩm2), this is also equivalent to 17 W/m2 at a cell voltage of 1.2 V.
4. Conclusions We have presented data on the continuous operation of an EFC system. This has been accomplished by using a novel valve-free system and a carbon loading of 10 mass% ensured operation without clogging. Highly stable energy density system performance was observed over several days of continuous operation. Interestingly, the specific capacitance (per mass of dry carbon) in the EFC system is similar to conventional film electrodes albeit the resistance in flow configuration is significantly higher. The performance of flow electrodes is also significantly better and not prone to sedimentation effects compared to static-operation of carbon suspensions. Further improvements of the system operation will be subject to future work entailing improving the carbon mass loading and flow channel design. Also, as known from previous work (e.g., Ref. [3]), voltage-loss and self-discharge over time have to be considered as notorious challenges for aqueous supercapacitors and electrochemical flow capacitors alike. Acknowledgements The INM (www.inm-gmbh.de) is part of the Leibniz Research Alliance Energy Transition (LVE). We acknowledge funding from the German Federal Ministry for Research and Education (BMBF, award number 03EK3013). SP acknowledges financial support of the Alexander von Humboldt Foundation. The authors thank Prof. Eduard Arzt
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Fig. 3. Electrochemical performance of continuous electrochemical flow capacitor in 1 M Na2SO4 (A + B). (A) Current density versus time obtained from chronoamperometry experiment at Vcell = 0.9 V and flow rate of 13 cm3/min. (B) Open cell voltage versus time measured in Cell 2 for several cycles obtained at Vcell = 0.6, 0.9 and 1.2 V applied to Cell 1 during charging and Vcell = 0 V during discharging, flow rate of 13 cm3/min. (C) Specific capacitance and Coulombic efficiency versus cell voltage. (D) Comparison of measured specific capacitance for electrodes under continuous flow, film electrodes and flow electrodes without flow.
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