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Measurements of the electric conductivity of an electrode as it transitions between static and flowable modes
Electrochemistry Communications 99 (2019) 61–64
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Full communication
Measurements of the electric conductivity of an electrode as it transitions between static and flowable modes Elad B. Halfona, Matthew E. Sussa,b, a b
T
⁎
Stephen & Nancy Grand Technion Energy Program, Technion – Israel Institute of Technology, Haifa, Israel Faculty of Mechanical Engineering, Technion – Israel Institute of Technology, Haifa, Israel
ARTICLE INFO
ABSTRACT
Keywords: Electrochemistry Energy conversion Flowable electrodes Flow batteries Capacitive deionization
Electrochemical systems which store energy or desalinate water employ either static electrodes or flowable suspension electrodes. Flowable electrodes enable important functionalities not available to static electrodes, but suffer from significantly lower electric conductivity, typically of order 1 or 10 mS/cm. To combine the benefits of static and flowable electrodes, we here propose and demonstrate an electrode which can be switched between a flow-through static mode and flowable mode in operando. We provide first-time measurements of the electrode's electric conductivity as it undergoes velocity cycling and transitions between modes. The electrode achieves a gigantic conductivity of over 10,000 mS/cm while in static mode, and demonstrates repeatable switching between static and flowable modes at a tunable transition velocity. Such switchable electrodes can in the future enable novel, highly versatile electrochemical systems.
1. Introduction Flowable electrodes consist of flowing suspensions of conductive particles in an electrolyte, and upon flow form dynamic percolating networks for electron transport across the electrode [1]. Such electrodes have recently been widely investigated towards energy storage and water desalination systems such as redox flow batteries (RFBs) and capacitive deionization (CDI) cells [2–6]. In previous decades, flowable electrodes were studied for use in applications such as electrowinning, fuel cells, and electrodialysis [7–10]. In RFBs, flowable electrodes have been used in order to fully decouple spatially the energy stored (in tanks) and power delivered (in the battery) for battery chemistries relying on metal electrodeposition or ion intercalation [4,11–13]. In capacitive deionization (CDI), flowable electrodes allow for continuous desalination of the feedwater, as the particles can be discharged downstream rather than in the cell itself [6,14–17]. Thus, the use of flowable electrodes has enabled novel functionalities in RFB and CDI cells, not available to cells employing static (non-flowable) electrodes. While enabling important functionalities, flowable electrodes suffer from sluggish electron transport through the percolating network formed by the particles. The electric conductivity of flowable electrodes is typically of order 1 or 10 mS/cm, which is generally orders of magnitude lower than that of a static electrode with identical active material, and can be significantly lower than the electrolyte ionic
⁎
conductivity [18–22]. The low electrode electric conductivity limits the applicability of such flowable electrode systems. For example, in CDI, the low electric conductivity results in much of the desalination occurring via electrodialytic mechanism rather than capacitive storage on particles [23], while in flow batteries low electric conductivity strongly limits particle utilization [24,25]. Thus, an electrode which could combine the high electric conductivity of static electrodes with the ability to flow would allow for high-performance systems with wideranging functionality. We here propose and demonstrate an electrode which possesses the capability to switch between the static and flowable modes in operando, by varying the pump velocity (Fig. 1). We measure the electrode's conductivity as it undergoes velocity cycling, demonstrating that it can achieve very high electric conductivities (> 10,000 mS/cm) in static mode and can transition repeatably between static and flowable modes. Further, we show the system has a well-defined transition velocity, which is influenced by interparticle forces, and that this transition velocity can be tuned. While other works measure the electric conductivity of flowable electrodes, [1,16,18,19,21,22,26] to our knowledge, the measurements reported here represent the first characterizations of electric conductivity of a switchable electrode before, during and after transitioning between static and flowable modes. Thus, our work demonstrates the basic feasibility of switchable electrodes, which can serve in the future as the platform for highly versatile electrochemical systems.
Corresponding author at: Faculty of Mechanical Engineering, Technion – Israel Institute of Technology, Haifa, Israel. E-mail address: [email protected] (M.E. Suss).
https://doi.org/10.1016/j.elecom.2018.12.016 Received 8 December 2018; Received in revised form 26 December 2018; Accepted 31 December 2018 Available online 03 January 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Electrochemistry Communications 99 (2019) 61–64
E.B. Halfon, M.E. Suss
Fig. 1. Schematics of a) a static electrode with particles held in place via binder, and b) a flowable electrode where suspended particles form a dynamic percolating network containing tortuous pathways for electron transport. c) A schematic of our proposed switchable electrode, where at low superficial velocities, particles self-assemble and are held in place via gravitational (Fg) and cohesive forces (Fc), enabling high electric conductivity. In the flowable state attained at higher velocities, drag forces acting on particles break the structure and allow for particles to flow out of the cell.
2. Materials & methods To demonstrate the concept of an electrode which can be switched between flowable and static modes, we used 140 g of spheroidal copper particles with nominal sizes between 14 and 25 μm, and 99% purity (Sigma-Aldrich, MO, USA). Thus, the electrode presented here can be utilized directly in a copper-based flow battery [27–29] or in a metalbased flow battery anode by coating metal onto the copper particles during battery operation [25,30]. Other particle sizes and materials can in principle achieve similar switchability, and will be explored in a future work. The particles were placed in 400 ml of 0.1 M HCl and mixed for 1 min in order to remove any surface oxide layer. Afterwards, the particles were washed in deionized (DI) water, filtered, and then added to fresh DI water to form a 5.5 vol% suspension. The suspension was then placed into a tank and mixed at 400 rpm. The tank was fluidically connected to a four electrode conductivity measurement cell identical to the one used in Cohen et al. [19]. To measure electrode conductivity, four-electrode potentiostatic electrochemical impedance spectroscopy (EIS) measurements were performed as the suspension was pumped (Master Flex, Gelsenkirchen, Germany) through the measurement cell. For the velocity cycles, we started by dwelling at a superficial velocity of ~8 mm/s and then reduced velocity by intervals of 0.3 mm/s, with a dwell time at each velocity of 10 min (solid lines in Fig. 2). Superficial velocity was calculated as the pump flowrate divided by the measurement cell cross-sectional area. When reaching the lowest velocity of 1.5 mm/s, a velocity upscan was initiated with the same velocity intervals and dwell times to complete the full cycle (dashed lines in Fig. 2). Upon switching velocity, a mechanical vibration of the cell was initiated at 60 Hz for 60 s with 0.7 V amplitude (TMS – Smart shaker K2004E01, OH, USA). The vibrations were energetic enough to completely break apart any large-scale static structure observable by eye which had formed in the conductivity cell. At each velocity, EIS measurements were performed, with 5 mV amplitude and 10 Hz–100 kHz frequency scan using twisted pair cabling for both working and sense electrodes (Gamry Reference 3000, PA, USA). EIS scans were executed every 1 min, and only the results from the last recorded scan for each velocity step was used. The resistance of the flowable electrode was taken as the high frequency intercept of the measured impedance with the real axis on the Nyquist plot. Conversion to conductivity was made by dividing the measured cell constant of 0.75 cm−1 by the measured resistance [19]. At the lowest flow velocities tested, the measured resistance reached very low values (down to ~30 mOhm) so here experiments were conducted with a more limited frequency range of 10 Hz–10 kHz. At various intervals we extracted and measured the liquid phase's ionic conductivity (Metroham 856, Switzerland).
Fig. 2. a) Measured conductivity of the electrode during three consecutive velocity cycles. Solid lines connect data points taken during downscans of velocity, beginning at ~8 mm/s and ending at 1.5 mm/s, while dashed lines connect data points taken during upscans from 1.5 mm/s to ~8 mm/s. The inset shows the measured impedance of the electrode over the entire frequency range tested for the electrode while in flowable state (at 7.9 mm/s, black full diamonds), and in the static state (1.5 mm/s, green hollow diamonds). b) Measured conductivity of the same electrode but for different values of electrolyte ionic conductivity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Results and discussion In Fig. 2a, we show the results of conductivity measurements of our electrode as it undergoes velocity cycling. At the higher superficial velocities, between about 7–8 mm/s, we measured an electrode conductivity of roughly 1 mS/cm. At these velocities, we also visually observed a largely homogeneous flowable electrode, with particles generally moving in the direction of the flow (Video S1). As velocity was lowered during the downscan (solid lines), the measured conductivity was roughly constant until about 5 mm/s, where a sudden dramatic rise in the measured conductivity to order 10 mS/cm was observed. As velocity was lowered further, the measured electrode conductivity continued to slowly rise to order 100 mS/cm, and at about 2.3 mm/s another substantial leap in conductivity occurred to order 10,000 mS/cm. This first rise in conductivity coincided with the visual observation that the flow becomes inhomogeneous with intermittent solid lump formation and dispersion (Video S2). The second leap corresponded with particles settling to form a static structure with particles stationary while liquid was pumped through the cell (Video S3). The measured conductivity on the subsequent upscan (dashed lines) approximately matched that of the downscan except near to the regions of conductivity leaps. Here, there was a significant and repeatable hysteresis between the up and downscan, where the transition between conductivity states on the upscans required ~0.8–1.5 mm/s higher velocity than the transition between states on the downscans. The inset of Fig. 2a shows a representative measured impedance scan from our fourelectrode cell, where the impedance in the static state (green diamonds) was largely consistent with a simple resistor circuit model, while in the flowable state (black diamonds), a significantly larger resistance is seen 62
Electrochemistry Communications 99 (2019) 61–64
E.B. Halfon, M.E. Suss
together with a capacitive response at lower frequencies. From measurements shown in Fig. 2a of the electrode's conductivity, and separate measurements of the conductivity of the liquid phase alone, we can make several inferences on the electric conductivity of the electrode. In the non-homogeneous flowable and static states, the overall conductivity measured (~100–10,000 mS/cm) is several orders of magnitude above the measured liquid phase conductivity at the end of the experiment of 0.97 mS/cm (higher than the initial DI water likely due to copper corrosion). Thus, we can infer that in these states, the measured conductivity is about equal to the electric conductivity of the electrode, with negligible impact of ionic transport. By contrast, in the flowable state, the measured conductivity is approximately equal to that of the liquid phase conductivity (~1 mS/cm), which indicates that the flowable structure formed either a poor percolation network with electric conductivity significantly < 1 mS/cm or did not percolate electric charge. In Fig. 2b, we show measurements of conductivity during velocity cycling for varying electrolyte ionic strength. When we reduce the ionic conductivity of the electrode's liquid phase from ~1 mS/cm to 0.05 mS/cm via dilutions with DI water, the measured conductivity in the flowable state is reduced to ~0.1 mS/ cm. The velocity where we observe a jump in conductivity is significantly reduced to ~2 mm/s, and there is only a single measured jump from ~0.1 mS/cm to over 104 mS/cm with reduced hysteresis. Visual observations from the diluted system showed only a clear transition from packed structure to uniform flowable state at around 2 mm/ s, without an observable intermediate, inhomogeneous flowable state. Increasing the ionic conductivity of the liquid phase to 35.3 mS/cm by adding NaCl salt results in ~300 mS/cm measured conductivity in the flowable state. A sharp transition between low and high conductivity was not measured and instead, a continuous rise in conductivity can be seen until reaching the packed bed state at which conductivity once again reached well over 104 mS/cm. The observations in Fig. 2a and b demonstrate that the observed switchability and electrode performance is dictated by various forces acting on the particles. When in the flowable state, our electrode could be classified as an upflow fluidized bed [14,19]. In the classical descriptions of fluidized beds, the particle's gravitational force balances fluidic drag and buoyancy force. Further, the transition between packed and fluidized states occurs at a well-known minimum fluidization velocity, Vmf, which we calculate to be 0.02 mm/s for the 20 μm copper particles used here [31,32]. From Fig. 2a, the measured transition velocity between static and flowable states during the downscan, ~2.5 mm/s, is two orders of magnitude higher than the predicted Vmf. Furthermore, although a distinct hysteresis is observed in Fig. 2a near to transition velocities, hysteresis is generally not observed near transition velocities when measuring pressure in a fluidized-packed bed system behaving according to the classical description [33]. We hypothesize that the high observed minimum fluidization velocity, and the observed hysteresis are due to the role of interparticle cohesive forces, notably Van der Waals (VdW) attractive forces. VdW attraction forces promote particle agglomeration, which can result in a larger effective particle size, and a higher minimum fluidization velocity. Furthermore, interparticle VdW forces are known to play a significant role in gas-solid fluidized beds only with small (sub-100 μm) particles, causing hysteresis in the form of a pressure overshoot on upscans [34,35]. We took white light images (Leica DMS1000, Wetzlar, Germany, 300× magnification) of our as-received copper particles suspended in DI water (Fig. 3a), and of the flowable electrode after the experiments of Fig. 2a (Fig. 3b). As can be seen, in Fig. 3a the particles are roughly the nominal size of 20 μm but in Fig. 3b there are numerous large agglomerates with sizes of order 100 μm, indicating significant interparticle attraction in our electrode. The effect of ionic strength on the particles' behavior and transition velocity can be explained via the interplay of VdW attractive and electrostatic repulsive forces acting on particles. Such interplay is captured by the well-known Derjaguin-Landau-Verwey-Overbeek
Fig. 3. White light microscope images of a) as received copper particles suspended in deionized water, with liquid phase ionic conductivity of 0.02 mS/cm, b) the switchable electrode at the end of the velocity cycling experiment of Fig. 2a, with ionic conductivity of 0.97 mS/cm, c) the switchable electrode after diluting to an ionic conductivity of 0.05 mS/cm, and d) the switchable electrode after adding NaCl salt to raise the ionic conductivity to 35 mS/cm. e) The measured particle size distribution of the samples pictured in panels a–d, where volume fraction represents the volume of particles characterized by a specific diameter (using a sphere equivalent volume) out of the entire volume of particles measured.
(DLVO) theory, wherein repulsive forces increase (decrease) at decreasing (increasing) ionic strength due to electric double layer expansion (contraction) [36]. As can be seen in Fig. 3, when we decreased the electrode ionic conductivity to 0.05 mS/cm, this lowered the average size of the particle agglomerate (Fig. 3c), consistent with relatively larger electrostatic repulsion forces between particles. Meanwhile, increasing salt concentration and conductivity to 35 mS/cm resulted in substantially larger agglomerates (Fig. 3d). A quantitative particle size distribution measurement of the samples pictured in Fig. 3a–d was performed via a laser diffraction particle size analyzer (Malvern Mastersizer 2000, Worcestershire, UK) and presented in Fig. 3e. The results of Figs. 2b and 3c–d demonstrate that the interplay between attractive and repulsive interparticle forces is crucial to the performance of the switchable electrode, and that the transition velocity between flowable and static states can be tuned via this interplay. 4. Conclusions We proposed and demonstrated a switchable electrode to overcome the problem of low electric conductivity of flowable electrodes. Such an electrode can be flowable in order to allow for energy storage or particle discharge outside the cell, but can be switched on-demand to a more conductive inhomogeneous flow state or self-assembled into a highly conductive static structure permitting electrolyte flow. We provided first measurements of the electric conductivity of a switchable electrode as it undergoes a transition between a static state with up to order 10,000 mS/cm conductivity and a flowable state. Evidence points to an important effect of interparticle forces on electrode behavior, and we show that such forces can be tuned to affect the transition velocity between states. In the future this concept can be explored with other 63
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particle materials, such as other metals and carbons, and implemented in prototype electrochemical system. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.elecom.2018.12.016.
Desalination 46 (1983) 291–299. [11] T.J. Petek, N.C. Hoyt, R.F. Savinell, J.S. Wainright, J. Power Sources 294 (2015) 620–626. [12] E. Ventosa, D. Buchholz, S. Klink, C. Flox, L.G. Chagas, C. Vaalma, W. Schuhmann, S. Passerini, J.R. Morante, Chem. Commun. 51 (2015) 7298–7301. [13] H. Chen, Y.C. Lu, Adv. Energy Mater. 6 (2016) 1502183. [14] G.J. Doornbusch, J.E. Dykstra, P.M. Biesheuvel, M.E. Suss, J. Mater. Chem. A 4 (2016) 3642–3647. [15] Y. Gendel, A.K.E. Rommerskirchen, O. David, M. Wessling, Electrochem. Commun. 46 (2014) 152–156. [16] K.B. Hatzell, M.C. Hatzell, K.M. Cook, M. Boota, G.M. Housel, A. McBride, E.C. Kumbur, Y. Gogotsi, Environ. Sci. Technol. 49 (2015) 3040–3047. [17] J. Ma, D. He, W. Tang, P. Kovalsky, C. He, C. Zhang, T.D. Waite, Environ. Sci. Technol. 50 (2016) 13495–13501. [18] T.J. Petek, N.C. Hoyt, R.F. Savinell, J.S. Wainright, J. Electrochem. Soc. 163 (2015) A5001–A5009. [19] H. Cohen, S.E. Eli, M. Jogi, M.E. Suss, ChemSusChem 9 (2016) 3045–3048. [20] S. Porada, D. Weingarth, H.V.M. Hamelers, M. Bryjak, V. Presser, P.M. Biesheuvel, J. Mater. Chem. A 2 (2014) 9313–9321. [21] B. Akuzum, L. Agartan, J. Locco, E.C. Kumbur, J. Appl. Electrochem. 47 (2017) 369–380. [22] H. Parant, G. Muller, T. Le Mercier, J.M. Tarascon, P. Poulin, A. Colin, Carbon N. Y. 119 (2017) 10–20. [23] P. Nativ, Y. Badash, Y. Gendel, Electrochem. Commun. 76 (2017) 24–28. [24] N.C. Hoyt, J.S. Wainright, R.F. Savinell, J. Electrochem. Soc. 162 (2015) A652–A657. [25] T. Huh, G. Savaskan, J.W. Evans, J. Appl. Electrochem. 22 (1992) 916–921. [26] B.J. Sabacky, J.W. Evans, Metall. Trans. B 8 (1977) 5–13. [27] D. Lloyd, T. Vainikka, K. Kontturi, Electrochim. Acta 100 (2013) 18–23. [28] L. Sanz, D. Lloyd, E. Magdalena, J. Palma, K. Kontturi, J. Power Sources 268 (2014) 121–128. [29] D. Lloyd, E. Magdalena, L. Sanz, L. Murtomäki, K. Kontturi, J. Power Sources 292 (2015) 87–94. [30] L.F. Arenas, A. Loh, D.P. Trudgeon, X. Li, C. Ponce de León, F.C. Walsh, Renew. Sust. Energ. Rev. 90 (2018) 992–1016. [31] G.B. Wallis, One-dimensional Two-phase Flow, McGraw-Hill, 1969. [32] D. Kunii, O. Levenspiel, Fluidization Engineering, Butterworth-Heinemann, 1991. [33] L.G. Gibilaro, Fluidization Dynamics, Butterworth-Heinemann, 2001. [34] K. Rietema, H.W. Piepers, Chem. Eng. Sci. 45 (1990) 1627–1639. [35] C.C. Xu, J. Zhu, Chem. Eng. Commun. 196 (2009) 499–517. [36] Y. Liang, N. Hilal, P. Langston, V. Starov, Adv. Colloid Interf. Sci. 134–135 (2007) 151–166.
Declarations of interest None. Acknowledgments Funded with the Support of the Nancy and Stephen Grand Technion Energy Program (GTEP), Israel Chemicals Ltd., and the Israel Science Foundation in the framework of the Israel National Research Center for Electrochemical Propulsion (INREP). The authors would like to acknowledge Boris Shvartsev, Leon Rosentsvit, Robert Glouckhovski and Ariel Vardi for the insightful discussions. References [1] K.B. Hatzell, J. Eller, S.L. Morelly, M.H. Tang, J. Alvarez, Y. Gogotsi, Faraday Discuss. 199 (2017) 511–524. [2] M.E. Suss, S. Porada, X. Sun, P.M. Biesheuvel, J. Yoon, V. Presser, Energy Environ. Sci. 8 (2015) 2296–2319. [3] K.B. Hatzell, M. Boota, Y. Gogotsi, Chem. Soc. Rev. 44 (2015) 8664–8687. [4] M. Duduta, B. Ho, V.C. Wood, P. Limthongkul, V.E. Brunini, W.C. Carter, Y.M. Chiang, Adv. Energy Mater. 1 (2011) 511–516. [5] V. Presser, C.R. Dennison, J. Campos, K.W. Knehr, E.C. Kumbur, Y. Gogotsi, Adv. Energy Mater. 2 (2012) 895–902. [6] S. Jeon, H. Park, J. Yeo, S. Yang, C.H. Cho, M.H. Han, D.K. Kim, Energy Environ. Sci. 6 (2013) 1471–1475. [7] J.R. Backhurst, E. Goodridge, R.E. Plimley, M. Fleischmann, Nature 221 (1969) 55–57. [8] B. Kastening, W. Schiel, M. Henschel, J. Electroanal. Chem. 191 (1985) 311–328. [9] F. Goodridge, C.J. Vance, The electrowinning of zinc using a circulating bed electrode, Electrochim. Acta 22 (1977) 1073–1076. [10] O. Kedem, J. Cohen, A. Warshawsky, N. Kahana, EDS - Sealed cell electrodialysis,
64
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Full communication
Measurements of the electric conductivity of an electrode as it transitions between static and flowable modes Elad B. Halfona, Matthew E. Sussa,b, a b
T
⁎
Stephen & Nancy Grand Technion Energy Program, Technion – Israel Institute of Technology, Haifa, Israel Faculty of Mechanical Engineering, Technion – Israel Institute of Technology, Haifa, Israel
ARTICLE INFO
ABSTRACT
Keywords: Electrochemistry Energy conversion Flowable electrodes Flow batteries Capacitive deionization
Electrochemical systems which store energy or desalinate water employ either static electrodes or flowable suspension electrodes. Flowable electrodes enable important functionalities not available to static electrodes, but suffer from significantly lower electric conductivity, typically of order 1 or 10 mS/cm. To combine the benefits of static and flowable electrodes, we here propose and demonstrate an electrode which can be switched between a flow-through static mode and flowable mode in operando. We provide first-time measurements of the electrode's electric conductivity as it undergoes velocity cycling and transitions between modes. The electrode achieves a gigantic conductivity of over 10,000 mS/cm while in static mode, and demonstrates repeatable switching between static and flowable modes at a tunable transition velocity. Such switchable electrodes can in the future enable novel, highly versatile electrochemical systems.
1. Introduction Flowable electrodes consist of flowing suspensions of conductive particles in an electrolyte, and upon flow form dynamic percolating networks for electron transport across the electrode [1]. Such electrodes have recently been widely investigated towards energy storage and water desalination systems such as redox flow batteries (RFBs) and capacitive deionization (CDI) cells [2–6]. In previous decades, flowable electrodes were studied for use in applications such as electrowinning, fuel cells, and electrodialysis [7–10]. In RFBs, flowable electrodes have been used in order to fully decouple spatially the energy stored (in tanks) and power delivered (in the battery) for battery chemistries relying on metal electrodeposition or ion intercalation [4,11–13]. In capacitive deionization (CDI), flowable electrodes allow for continuous desalination of the feedwater, as the particles can be discharged downstream rather than in the cell itself [6,14–17]. Thus, the use of flowable electrodes has enabled novel functionalities in RFB and CDI cells, not available to cells employing static (non-flowable) electrodes. While enabling important functionalities, flowable electrodes suffer from sluggish electron transport through the percolating network formed by the particles. The electric conductivity of flowable electrodes is typically of order 1 or 10 mS/cm, which is generally orders of magnitude lower than that of a static electrode with identical active material, and can be significantly lower than the electrolyte ionic
⁎
conductivity [18–22]. The low electrode electric conductivity limits the applicability of such flowable electrode systems. For example, in CDI, the low electric conductivity results in much of the desalination occurring via electrodialytic mechanism rather than capacitive storage on particles [23], while in flow batteries low electric conductivity strongly limits particle utilization [24,25]. Thus, an electrode which could combine the high electric conductivity of static electrodes with the ability to flow would allow for high-performance systems with wideranging functionality. We here propose and demonstrate an electrode which possesses the capability to switch between the static and flowable modes in operando, by varying the pump velocity (Fig. 1). We measure the electrode's conductivity as it undergoes velocity cycling, demonstrating that it can achieve very high electric conductivities (> 10,000 mS/cm) in static mode and can transition repeatably between static and flowable modes. Further, we show the system has a well-defined transition velocity, which is influenced by interparticle forces, and that this transition velocity can be tuned. While other works measure the electric conductivity of flowable electrodes, [1,16,18,19,21,22,26] to our knowledge, the measurements reported here represent the first characterizations of electric conductivity of a switchable electrode before, during and after transitioning between static and flowable modes. Thus, our work demonstrates the basic feasibility of switchable electrodes, which can serve in the future as the platform for highly versatile electrochemical systems.
Corresponding author at: Faculty of Mechanical Engineering, Technion – Israel Institute of Technology, Haifa, Israel. E-mail address: [email protected] (M.E. Suss).
https://doi.org/10.1016/j.elecom.2018.12.016 Received 8 December 2018; Received in revised form 26 December 2018; Accepted 31 December 2018 Available online 03 January 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Electrochemistry Communications 99 (2019) 61–64
E.B. Halfon, M.E. Suss
Fig. 1. Schematics of a) a static electrode with particles held in place via binder, and b) a flowable electrode where suspended particles form a dynamic percolating network containing tortuous pathways for electron transport. c) A schematic of our proposed switchable electrode, where at low superficial velocities, particles self-assemble and are held in place via gravitational (Fg) and cohesive forces (Fc), enabling high electric conductivity. In the flowable state attained at higher velocities, drag forces acting on particles break the structure and allow for particles to flow out of the cell.
2. Materials & methods To demonstrate the concept of an electrode which can be switched between flowable and static modes, we used 140 g of spheroidal copper particles with nominal sizes between 14 and 25 μm, and 99% purity (Sigma-Aldrich, MO, USA). Thus, the electrode presented here can be utilized directly in a copper-based flow battery [27–29] or in a metalbased flow battery anode by coating metal onto the copper particles during battery operation [25,30]. Other particle sizes and materials can in principle achieve similar switchability, and will be explored in a future work. The particles were placed in 400 ml of 0.1 M HCl and mixed for 1 min in order to remove any surface oxide layer. Afterwards, the particles were washed in deionized (DI) water, filtered, and then added to fresh DI water to form a 5.5 vol% suspension. The suspension was then placed into a tank and mixed at 400 rpm. The tank was fluidically connected to a four electrode conductivity measurement cell identical to the one used in Cohen et al. [19]. To measure electrode conductivity, four-electrode potentiostatic electrochemical impedance spectroscopy (EIS) measurements were performed as the suspension was pumped (Master Flex, Gelsenkirchen, Germany) through the measurement cell. For the velocity cycles, we started by dwelling at a superficial velocity of ~8 mm/s and then reduced velocity by intervals of 0.3 mm/s, with a dwell time at each velocity of 10 min (solid lines in Fig. 2). Superficial velocity was calculated as the pump flowrate divided by the measurement cell cross-sectional area. When reaching the lowest velocity of 1.5 mm/s, a velocity upscan was initiated with the same velocity intervals and dwell times to complete the full cycle (dashed lines in Fig. 2). Upon switching velocity, a mechanical vibration of the cell was initiated at 60 Hz for 60 s with 0.7 V amplitude (TMS – Smart shaker K2004E01, OH, USA). The vibrations were energetic enough to completely break apart any large-scale static structure observable by eye which had formed in the conductivity cell. At each velocity, EIS measurements were performed, with 5 mV amplitude and 10 Hz–100 kHz frequency scan using twisted pair cabling for both working and sense electrodes (Gamry Reference 3000, PA, USA). EIS scans were executed every 1 min, and only the results from the last recorded scan for each velocity step was used. The resistance of the flowable electrode was taken as the high frequency intercept of the measured impedance with the real axis on the Nyquist plot. Conversion to conductivity was made by dividing the measured cell constant of 0.75 cm−1 by the measured resistance [19]. At the lowest flow velocities tested, the measured resistance reached very low values (down to ~30 mOhm) so here experiments were conducted with a more limited frequency range of 10 Hz–10 kHz. At various intervals we extracted and measured the liquid phase's ionic conductivity (Metroham 856, Switzerland).
Fig. 2. a) Measured conductivity of the electrode during three consecutive velocity cycles. Solid lines connect data points taken during downscans of velocity, beginning at ~8 mm/s and ending at 1.5 mm/s, while dashed lines connect data points taken during upscans from 1.5 mm/s to ~8 mm/s. The inset shows the measured impedance of the electrode over the entire frequency range tested for the electrode while in flowable state (at 7.9 mm/s, black full diamonds), and in the static state (1.5 mm/s, green hollow diamonds). b) Measured conductivity of the same electrode but for different values of electrolyte ionic conductivity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Results and discussion In Fig. 2a, we show the results of conductivity measurements of our electrode as it undergoes velocity cycling. At the higher superficial velocities, between about 7–8 mm/s, we measured an electrode conductivity of roughly 1 mS/cm. At these velocities, we also visually observed a largely homogeneous flowable electrode, with particles generally moving in the direction of the flow (Video S1). As velocity was lowered during the downscan (solid lines), the measured conductivity was roughly constant until about 5 mm/s, where a sudden dramatic rise in the measured conductivity to order 10 mS/cm was observed. As velocity was lowered further, the measured electrode conductivity continued to slowly rise to order 100 mS/cm, and at about 2.3 mm/s another substantial leap in conductivity occurred to order 10,000 mS/cm. This first rise in conductivity coincided with the visual observation that the flow becomes inhomogeneous with intermittent solid lump formation and dispersion (Video S2). The second leap corresponded with particles settling to form a static structure with particles stationary while liquid was pumped through the cell (Video S3). The measured conductivity on the subsequent upscan (dashed lines) approximately matched that of the downscan except near to the regions of conductivity leaps. Here, there was a significant and repeatable hysteresis between the up and downscan, where the transition between conductivity states on the upscans required ~0.8–1.5 mm/s higher velocity than the transition between states on the downscans. The inset of Fig. 2a shows a representative measured impedance scan from our fourelectrode cell, where the impedance in the static state (green diamonds) was largely consistent with a simple resistor circuit model, while in the flowable state (black diamonds), a significantly larger resistance is seen 62
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together with a capacitive response at lower frequencies. From measurements shown in Fig. 2a of the electrode's conductivity, and separate measurements of the conductivity of the liquid phase alone, we can make several inferences on the electric conductivity of the electrode. In the non-homogeneous flowable and static states, the overall conductivity measured (~100–10,000 mS/cm) is several orders of magnitude above the measured liquid phase conductivity at the end of the experiment of 0.97 mS/cm (higher than the initial DI water likely due to copper corrosion). Thus, we can infer that in these states, the measured conductivity is about equal to the electric conductivity of the electrode, with negligible impact of ionic transport. By contrast, in the flowable state, the measured conductivity is approximately equal to that of the liquid phase conductivity (~1 mS/cm), which indicates that the flowable structure formed either a poor percolation network with electric conductivity significantly < 1 mS/cm or did not percolate electric charge. In Fig. 2b, we show measurements of conductivity during velocity cycling for varying electrolyte ionic strength. When we reduce the ionic conductivity of the electrode's liquid phase from ~1 mS/cm to 0.05 mS/cm via dilutions with DI water, the measured conductivity in the flowable state is reduced to ~0.1 mS/ cm. The velocity where we observe a jump in conductivity is significantly reduced to ~2 mm/s, and there is only a single measured jump from ~0.1 mS/cm to over 104 mS/cm with reduced hysteresis. Visual observations from the diluted system showed only a clear transition from packed structure to uniform flowable state at around 2 mm/ s, without an observable intermediate, inhomogeneous flowable state. Increasing the ionic conductivity of the liquid phase to 35.3 mS/cm by adding NaCl salt results in ~300 mS/cm measured conductivity in the flowable state. A sharp transition between low and high conductivity was not measured and instead, a continuous rise in conductivity can be seen until reaching the packed bed state at which conductivity once again reached well over 104 mS/cm. The observations in Fig. 2a and b demonstrate that the observed switchability and electrode performance is dictated by various forces acting on the particles. When in the flowable state, our electrode could be classified as an upflow fluidized bed [14,19]. In the classical descriptions of fluidized beds, the particle's gravitational force balances fluidic drag and buoyancy force. Further, the transition between packed and fluidized states occurs at a well-known minimum fluidization velocity, Vmf, which we calculate to be 0.02 mm/s for the 20 μm copper particles used here [31,32]. From Fig. 2a, the measured transition velocity between static and flowable states during the downscan, ~2.5 mm/s, is two orders of magnitude higher than the predicted Vmf. Furthermore, although a distinct hysteresis is observed in Fig. 2a near to transition velocities, hysteresis is generally not observed near transition velocities when measuring pressure in a fluidized-packed bed system behaving according to the classical description [33]. We hypothesize that the high observed minimum fluidization velocity, and the observed hysteresis are due to the role of interparticle cohesive forces, notably Van der Waals (VdW) attractive forces. VdW attraction forces promote particle agglomeration, which can result in a larger effective particle size, and a higher minimum fluidization velocity. Furthermore, interparticle VdW forces are known to play a significant role in gas-solid fluidized beds only with small (sub-100 μm) particles, causing hysteresis in the form of a pressure overshoot on upscans [34,35]. We took white light images (Leica DMS1000, Wetzlar, Germany, 300× magnification) of our as-received copper particles suspended in DI water (Fig. 3a), and of the flowable electrode after the experiments of Fig. 2a (Fig. 3b). As can be seen, in Fig. 3a the particles are roughly the nominal size of 20 μm but in Fig. 3b there are numerous large agglomerates with sizes of order 100 μm, indicating significant interparticle attraction in our electrode. The effect of ionic strength on the particles' behavior and transition velocity can be explained via the interplay of VdW attractive and electrostatic repulsive forces acting on particles. Such interplay is captured by the well-known Derjaguin-Landau-Verwey-Overbeek
Fig. 3. White light microscope images of a) as received copper particles suspended in deionized water, with liquid phase ionic conductivity of 0.02 mS/cm, b) the switchable electrode at the end of the velocity cycling experiment of Fig. 2a, with ionic conductivity of 0.97 mS/cm, c) the switchable electrode after diluting to an ionic conductivity of 0.05 mS/cm, and d) the switchable electrode after adding NaCl salt to raise the ionic conductivity to 35 mS/cm. e) The measured particle size distribution of the samples pictured in panels a–d, where volume fraction represents the volume of particles characterized by a specific diameter (using a sphere equivalent volume) out of the entire volume of particles measured.
(DLVO) theory, wherein repulsive forces increase (decrease) at decreasing (increasing) ionic strength due to electric double layer expansion (contraction) [36]. As can be seen in Fig. 3, when we decreased the electrode ionic conductivity to 0.05 mS/cm, this lowered the average size of the particle agglomerate (Fig. 3c), consistent with relatively larger electrostatic repulsion forces between particles. Meanwhile, increasing salt concentration and conductivity to 35 mS/cm resulted in substantially larger agglomerates (Fig. 3d). A quantitative particle size distribution measurement of the samples pictured in Fig. 3a–d was performed via a laser diffraction particle size analyzer (Malvern Mastersizer 2000, Worcestershire, UK) and presented in Fig. 3e. The results of Figs. 2b and 3c–d demonstrate that the interplay between attractive and repulsive interparticle forces is crucial to the performance of the switchable electrode, and that the transition velocity between flowable and static states can be tuned via this interplay. 4. Conclusions We proposed and demonstrated a switchable electrode to overcome the problem of low electric conductivity of flowable electrodes. Such an electrode can be flowable in order to allow for energy storage or particle discharge outside the cell, but can be switched on-demand to a more conductive inhomogeneous flow state or self-assembled into a highly conductive static structure permitting electrolyte flow. We provided first measurements of the electric conductivity of a switchable electrode as it undergoes a transition between a static state with up to order 10,000 mS/cm conductivity and a flowable state. Evidence points to an important effect of interparticle forces on electrode behavior, and we show that such forces can be tuned to affect the transition velocity between states. In the future this concept can be explored with other 63
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particle materials, such as other metals and carbons, and implemented in prototype electrochemical system. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.elecom.2018.12.016.
Desalination 46 (1983) 291–299. [11] T.J. Petek, N.C. Hoyt, R.F. Savinell, J.S. Wainright, J. Power Sources 294 (2015) 620–626. [12] E. Ventosa, D. Buchholz, S. Klink, C. Flox, L.G. Chagas, C. Vaalma, W. Schuhmann, S. Passerini, J.R. Morante, Chem. Commun. 51 (2015) 7298–7301. [13] H. Chen, Y.C. Lu, Adv. Energy Mater. 6 (2016) 1502183. [14] G.J. Doornbusch, J.E. Dykstra, P.M. Biesheuvel, M.E. Suss, J. Mater. Chem. A 4 (2016) 3642–3647. [15] Y. Gendel, A.K.E. Rommerskirchen, O. David, M. Wessling, Electrochem. Commun. 46 (2014) 152–156. [16] K.B. Hatzell, M.C. Hatzell, K.M. Cook, M. Boota, G.M. Housel, A. McBride, E.C. Kumbur, Y. Gogotsi, Environ. Sci. Technol. 49 (2015) 3040–3047. [17] J. Ma, D. He, W. Tang, P. Kovalsky, C. He, C. Zhang, T.D. Waite, Environ. Sci. Technol. 50 (2016) 13495–13501. [18] T.J. Petek, N.C. Hoyt, R.F. Savinell, J.S. Wainright, J. Electrochem. Soc. 163 (2015) A5001–A5009. [19] H. Cohen, S.E. Eli, M. Jogi, M.E. Suss, ChemSusChem 9 (2016) 3045–3048. [20] S. Porada, D. Weingarth, H.V.M. Hamelers, M. Bryjak, V. Presser, P.M. Biesheuvel, J. Mater. Chem. A 2 (2014) 9313–9321. [21] B. Akuzum, L. Agartan, J. Locco, E.C. Kumbur, J. Appl. Electrochem. 47 (2017) 369–380. [22] H. Parant, G. Muller, T. Le Mercier, J.M. Tarascon, P. Poulin, A. Colin, Carbon N. Y. 119 (2017) 10–20. [23] P. Nativ, Y. Badash, Y. Gendel, Electrochem. Commun. 76 (2017) 24–28. [24] N.C. Hoyt, J.S. Wainright, R.F. Savinell, J. Electrochem. Soc. 162 (2015) A652–A657. [25] T. Huh, G. Savaskan, J.W. Evans, J. Appl. Electrochem. 22 (1992) 916–921. [26] B.J. Sabacky, J.W. Evans, Metall. Trans. B 8 (1977) 5–13. [27] D. Lloyd, T. Vainikka, K. Kontturi, Electrochim. Acta 100 (2013) 18–23. [28] L. Sanz, D. Lloyd, E. Magdalena, J. Palma, K. Kontturi, J. Power Sources 268 (2014) 121–128. [29] D. Lloyd, E. Magdalena, L. Sanz, L. Murtomäki, K. Kontturi, J. Power Sources 292 (2015) 87–94. [30] L.F. Arenas, A. Loh, D.P. Trudgeon, X. Li, C. Ponce de León, F.C. Walsh, Renew. Sust. Energ. Rev. 90 (2018) 992–1016. [31] G.B. Wallis, One-dimensional Two-phase Flow, McGraw-Hill, 1969. [32] D. Kunii, O. Levenspiel, Fluidization Engineering, Butterworth-Heinemann, 1991. [33] L.G. Gibilaro, Fluidization Dynamics, Butterworth-Heinemann, 2001. [34] K. Rietema, H.W. Piepers, Chem. Eng. Sci. 45 (1990) 1627–1639. [35] C.C. Xu, J. Zhu, Chem. Eng. Commun. 196 (2009) 499–517. [36] Y. Liang, N. Hilal, P. Langston, V. Starov, Adv. Colloid Interf. Sci. 134–135 (2007) 151–166.
Declarations of interest None. Acknowledgments Funded with the Support of the Nancy and Stephen Grand Technion Energy Program (GTEP), Israel Chemicals Ltd., and the Israel Science Foundation in the framework of the Israel National Research Center for Electrochemical Propulsion (INREP). The authors would like to acknowledge Boris Shvartsev, Leon Rosentsvit, Robert Glouckhovski and Ariel Vardi for the insightful discussions. References [1] K.B. Hatzell, J. Eller, S.L. Morelly, M.H. Tang, J. Alvarez, Y. Gogotsi, Faraday Discuss. 199 (2017) 511–524. [2] M.E. Suss, S. Porada, X. Sun, P.M. Biesheuvel, J. Yoon, V. Presser, Energy Environ. Sci. 8 (2015) 2296–2319. [3] K.B. Hatzell, M. Boota, Y. Gogotsi, Chem. Soc. Rev. 44 (2015) 8664–8687. [4] M. Duduta, B. Ho, V.C. Wood, P. Limthongkul, V.E. Brunini, W.C. Carter, Y.M. Chiang, Adv. Energy Mater. 1 (2011) 511–516. [5] V. Presser, C.R. Dennison, J. Campos, K.W. Knehr, E.C. Kumbur, Y. Gogotsi, Adv. Energy Mater. 2 (2012) 895–902. [6] S. Jeon, H. Park, J. Yeo, S. Yang, C.H. Cho, M.H. Han, D.K. Kim, Energy Environ. Sci. 6 (2013) 1471–1475. [7] J.R. Backhurst, E. Goodridge, R.E. Plimley, M. Fleischmann, Nature 221 (1969) 55–57. [8] B. Kastening, W. Schiel, M. Henschel, J. Electroanal. Chem. 191 (1985) 311–328. [9] F. Goodridge, C.J. Vance, The electrowinning of zinc using a circulating bed electrode, Electrochim. Acta 22 (1977) 1073–1076. [10] O. Kedem, J. Cohen, A. Warshawsky, N. Kahana, EDS - Sealed cell electrodialysis,
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