Journal of Power Sources 285 (2015) 580e587

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Journal of Power Sources

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A dual power cell for storing electricity in metal

Gregory X. Zhang e-Zn Inc, Toronto, Canada highlights graphical abstract

We report a new electrochemical technology for storing energy in zinc metal. This technology enables storing en- ergy for as low as $15/kWh. The power and energy capacity of the cell can be independently scaled. It is named as a dual power cell as it can charge and discharge at the same time. Functionality of this dual power cell was demonstrated with a prototype cell. article info abstract

Article history: This article reports a novel electrochemical cell system for storing electrical energy in zinc metal. In this Received 16 November 2014 system the cell is structured with three distinctive sections for energy absorption, storage and delivery. Received in revised form The energy is absorbed into zinc metal through electro deposition in the top section and is delivered 23 February 2015 through zinc dissolution in the bottom section. The zinc metal is contained in the middle section be- Accepted 12 March 2015 tween the sections for deposition and dissolution. Because the zinc metal is contained separately from Available online 14 March 2015 the electrodes this system allows storage of energy at a low cost. Also, having deposition and dissolution of zinc in separate spaces allows independent operation of charging and discharging. This cell is capable Keywords: Dual power cell receiving and outputting power at the same time, based on which it is thus termed as a dual power cell. Energy storage The cell design and results of the prototype testing are reported. The characteristics, potential applica- Zinc tions and cost of stored electricity of the system are also discussed. Metal © 2015 Elsevier B.V. All rights reserved.

1. Introduction electrical power, for which there are technologies in the early stage of commercial application. Long duration storage is needed in ap- Electricity storage is an important enabling technology for plications to separate the times between generation and use of effective use of renewable energy sources such solar and wind [1]. electricity for economical benefits. At present there is a lack of There are two broad categories of electricity storage applications commercially viable technology for long duration storage except for based on the duration of storage: short durations, from a fraction of pumped hydro. However, pumped hydro is limited by availability of second to about one hour, and long durations, from a few hours to suitable lands due to geological and environmental constraints. ten or hundred of hours [2e4]. Short duration storage is typically The technologies for energy storage can be categorized into two used for power support to ensure the reliability and quality of distinctive groups: device-based and material-based. Device-based technology is one in which energy storage and energy conversion are physically in the same space as shown in Fig. 1a. Material-based

E-mail address: [email protected]. technology is one in which energy storage and energy conversion http://dx.doi.org/10.1016/j.jpowsour.2015.03.084 0378-7753/© 2015 Elsevier B.V. All rights reserved. G.X. Zhang / Journal of Power Sources 285 (2015) 580e587 581

Fig. 1. Differences between device-based and material-based technologies. are at different spaces (Fig. 1b). By this definition, fly wheel, super other technologies are also discussed. capacitor and batteries belong to device-based. On the other hand, pumped hydro, compressed air, fuel cells, flow batteries, 2. Design and operation principle and metal air fuel cells belong to material-based because the en- ergy is stored in water, air, hydrogen, redox couple and metal. Fig. 2 illustrates the basic structure of the electrochemical cell A key difference between the two kinds of storage technologies system. In the embodiment, there are a discharging assembly and a is that the power and energy can be independently scaled with charging assembly that is positioned above the discharging as- material-based technologies, while they cannot with device-based sembly. They are housed in the same body of electrolyte in a single technologies. As illustrated in Fig. 1, to scale up the storage capacity container forming an electrochemical cell. The charging assembly for a device-based technology, more units (e.g., batteries), of the consists of a plurality of anodes and cathodes for electro deposition device need to be added, which increase also the power. However, of zinc. There is a mechanism to dislodge periodically the deposited in the case of material-based technology, to scale up storage ca- zinc metallic material on the cathodes. The dislodging mechanism pacity, only the storage capacity for material needs to change. As consists of wipers which are rotated by a motor. The discharging illustrated in Fig. 1c, material-based energy storage technologies assembly comprises a plurality of air cathodes with spaces between have a clear advantage over device-based technologies for long the cathodes. duration applications. This, in fact, explains why pumped hydro has The zinc metal deposits dislodged from the charging assembly been the only economically viable technology for bulk energy fall naturally downward into the anode spaces forming the anodes storage to date. in the discharging assembly. During discharge the zinc anodes are Zinc has long been considered as a promising material for low dissolved and the zinc dissolved in the electrolyte is circulated by cost energy storage [5e7]. In particular, regenerative zinc fuel cells pumping from the discharging assembly back to the charging as- are ideal for economical long duration energy storage for three sembly. The space between the changing assembly and the dis- fundamental reasons: 1) power generation and energy storage are charging assembly serves the function of storing the zinc metal separated such that energy can be stored at low cost; 2) zinc has a deposits when the anode spaces in the discharging assembly are high energy density, highest among the common metals that can be filled. reduced in aqueous electrolytes and 3) zinc is inexpensive and The electrochemical cell so designed and operated has three abundant, being one of the lowest cost metals in the market. distinctive sections to serve the functions for receiving energy from Numerous attempts with various designs and operation prin- an energy source, for storing the energy and for delivering to a load. ciples have been made in the past to develop regenerative zinc fuel By storing electrical energy in metallic material that is physically cells but without success [8]. The most challenging problems have detached from the electrodes, the economics of long duration en- been jamming during transporting the zinc metal into and out the ergy storage is fundamentally changed. electrochemical cells and uneven distribution of the materials The cathode in the charging assembly is an important element within a cell and between individual cells. with a unique design. It is made of a conductive substrate that has e-Zn has successfully resolved the challenging issues of jam- low adherence to zinc deposits and has a plurality of discrete active ming and material distribution based on a novel concept and surface areas as illustrated in Fig. 3. Using substrate material for the operation principle. Functional prototype cells have been con- cathode, e.g. , on which the adhesion of zinc metal structed and successfully demonstrated, based on which a patent deposit is low, the discrete metal deposits can be easily dislodged application has been filed [9]. In this article, the design of the from the cathode surface and distributed evenly into the anode electrochemical cell system and results of the prototype testing are spaces in the discharging assembly. reported. The potential applications of this technology with its The air cathode is another important element with a unique unique features and the economical advantages in comparison to structure. It has a cavity enclosed between the oxygen membrane 582 G.X. Zhang / Journal of Power Sources 285 (2015) 580e587

Fig. 2. Schematic illustration of the basic structural design of the electrochemical cell, the pipes for conducting air and, the electrical leads connecting to the air cathodes are not illustrated for simplicity. electrodes for passing air as illustrated in Fig. 4a. In this design air cathodes can be put in a cell and each cathode can be indepen- flows inside the enclosed cavity and the active surfaces of the ox- dently removed from cell without affecting the integrity of the cell, ygen membrane electrodes face outside of the cavity. (In the design which is beneficial in assembling, service and maintenance. of a metal air cell in the past the active surfaces of the oxygen Another beneficial feature is a chamber beneath the discharging membrane electrodes face inside of the cell enclosure while air assembly for electrolyte circulation by pumping the electrolyte enters from outside of the cell as illustrated in Fig. 4b). Multiple air from the chamber at bottom to the top of the cell. The chamber is separated with a separator material from the anode spaces in the discharging assembly allowing electrolyte but not solid metal to pass through. The electrolyte circulation through a forced convec- tion can help the process of concentration homogenization of the electrolyte in different locations within the cell and can improve the performance of the electrochemical system, particularly at high current densities. At high current densities, the metal concentra- tion may be depleted near the cathode surfaces of the charging assembly during charging and may be highly concentrated in the anode spaces between air cathodes of the discharging assembly during discharging. Electrolyte circulation can help increase metal concentration near the cathode surfaces of the charging assembly and remove the dissolved metal in the discharging assembly. The description of the technology above is based on zinc, but the principle of the technology is applicable to other metals that can be reversibly reduced and oxidized electrochemically in an electrolyte.

3. Prototype and testing results

A prototype cell, shown in Fig. 5, was made according to the design illustrated in Fig. 2. The cell container, made of Plexiglas, has an interior dimension of 26.5 cm in length, 11.5 cm in width and 55 cm in height. The charging assembly had one cathode and two anodes. The anodes were flat sheets of stainless steel of 1 mm in thickness. The cathode was made of magnesium, on which zinc deposits have very low adhesion and can be readily removed. The magnesium cathode has a plurality of discrete active surface areas of 2 mm 2 mm that were separated by 5 mm inactive zones. The Fig. 3. Schematic illustration of the cathode with discrete active areas for metal inactive zones and the edges of the cathode were covered with deposition. epoxy resin. The dimension of the cathode was 21 cm 21 cm. The G.X. Zhang / Journal of Power Sources 285 (2015) 580e587 583

Fig. 4. Schematic illustration of the air cathode structure of the present system (a), and the conventional metal air cell structure of the past (b). distance between the cathode and anodes was 1.5 cm. The wipers, openings were joined with plastic tubes for passing air that was made of plastic material Delrin, were mounted on the horizontal supplied with an air pump. A copper strip, a part of which was laid shaft, which was mechanically joined to a set of drive worm gears on the bottom of each anode space in the discharging assembly, that was connected to a motor mounted on top of the cell container. serves as current collector for the zinc anode. The electrolyte was The discharging assembly had three air cathodes separated by 34% KOH containing 1 M of zinc. 2.5 cm from each other forming two anode spaces. The cathodes Fig. 6 shows the result of a test with 5 h charge and 5 h discharge had a cavity with a dimension of 26 cm 14 cm 1 cm with a cycling in a controlled current mode. The charging current and 24 cm 12 cm 0.9 cm. The cavity was enveloped by the oxygen discharging current was 10 A. The voltage was nearly constant membrane electrodes on a frame made of Plexiglas. Both sides of during each discharging cycle while there are spikes on the voltage the air cathodes in the middle were covered by a membrane oxygen data during charging cycles. The spikes were due to the periodical electrode while only one side of the two cathodes on the sides was wiping events to remove the zinc deposits on the surface of the covered. The air cathodes were fully covered with a separator cathodes; a significantly higher voltage is required for zinc depo- material. There was an opening on each end of the cathode and the sition on the fresh areas of the magnesium cathode. Fig. 7 shows voltage curves during a concurrent charge and discharge. The charging current was varied between 1 and 20 A that resulted in a varied charging voltage. At the same time, the discharge was carried out with a current that was set at10 A, which resulted in a largely constant discharging voltage. The data shows that the electrochemical system can perform sustained charging and discharging at the same time and convert noisy power into a stable power.

4. Discussion

The results from the prototype cell testing show that the elec- trochemical cell system can perform charging and discharging repeatedly as a regenerative fuel cell without failure due to jam- ming and thus demonstrated the principle and functionality of this system for storing electrical energy in a metallic material. An important feature of the electrochemical technology is that the solid active material is detached from the inactive electrode structure. This removes the requirement for maintaining the physical and morphological form of the active material on the electrodes that is critically important for the function and per- formance of rechargeable batteries. In the present system, because the zinc metal deposit is renewed after each charge and discharge cycle, the particular morphology of the metal deposit during each deposition has limited effect on the discharging behavior. Also, the current distribution across the surface of the electrodes does not need to be uniform as the surface condition is refreshed after each wiping event. It is not the case with a rechargeable battery where non-uniform current distribution on electrodes is detrimental to the health of the battery. This is particularly true with recharge- Fig. 5. A photograph of a prototype cell made according to the design illustrated in able zinc batteries; the morphological change of the zinc electrode Fig. 2. due to charge/discharge is one of the most important reasons why 584 G.X. Zhang / Journal of Power Sources 285 (2015) 580e587

Fig. 6. Charge and discharge results of the prototype cell with a controlled current mode. the cycle life of zinc based batteries remains limited despite of of zinc air chemistry is inherently low due to the sluggish nature of decades of research and development. In essence, the present oxygen reactions; the efficiency of the various types of regenerative approach has removed the limitation on the life of the zinc anode zinceair systems has been found to be between 40% and 65% [8]. by changing the issue from a chemical nature to a mechanical The relatively low efficiency in comparison to conventional batte- nature. Since moving parts are involved, the way this system ries is an inherent drawback for the zinceair chemistry. Secondly, operates is mechanically more complicated than normal the results were obtained from a prototype cell, in which the rechargeable batteries. However, the merits of low cost expansion various components and materials as well as the operation condi- of storage capacity and removing the life limit of the zinc anodes tions were not in optimized conditions. It is expected that the could significantly outweigh the drawbacks the mechanical com- cycling energy efficiency of 60%e65% can be obtained when the plications, particularly for large scale and large capacity system is adequately engineered and optimized. There are parasitic applications. efficiency losses due to periodic pumping of air and electrolyte as The zinc metal deposits formed in concentrated KOH may have a well as wiping off zinc deposits. It is estimated that the parasitic range of morphologies but it is most typically dendrite type under a efficiency losses for an adequately engineered system would be less prolonged deposition [10]. Dendritic zinc deposits are electro- than 2e3%. Efficiency more than 60e65% would require advance- chemically very active and can be discharged with a good kinetics ments in electrode technologies for the oxygen reactions, both the at a wide range of currents. The deposits are also physically loose anode and the cathode. In particular, there are rooms for due to the dendritic nature and can be easily removed from the improvement of the air cathode; there has been a lack of surface of the cathode. commercially available air cathodes that have good kinetics and are The voltage efficiency shown in Fig. 6 was about 50%. The cur- long lasting. rent efficiency was between 97% and 99%. The cycling energy effi- One particular novel feature of the present invention is that the ciency of the prototype cell was thus less than 50%. This low system can be used for absorbing and delivering electricity at the efficiency was mainly attributed to two factors. First, the efficiency same time, that is, concurrent charging and discharging, which is not possible with conventional batteries. Fig. 8 shows two charging and discharging profiles: the charging and discharging are continuous with time in Fig. 8a, and are alternating with time in Fig. 8b. This system can function with both profiles while conven- tional rechargeable batteries can only function with the alternating charging and discharging profile. The result in Fig. 7 is a demon- stration, for the first time in history, of a functional electrochemical cell that underwent sustained charging and discharging simulta- neously. This cell can thus be termed as a dual power cell because it can receive input power and provide output power at the same time. The structural design of the cell allows flexible and independent scaling of power and storage capacity. As shown in Fig. 9, the power can be scaled up by adding more electrodes in the charging and discharging assemblies and increasing the size of the container in the horizontal direction. The storage capacity can be scaled up by only increasing the size of the container in the vertical direction and adding more metal containing electrolyte without change anything else. As a potential variation, the charging assembly, discharging as- sembly and the storage section can be positioned in separate con- tainers to make independent charging and discharging cells and a Fig. 7. Charge and discharge voltages as a function of time during a concurrent charging and discharging test of the prototype cell. storage facility as illustrated in Fig. 10. In such an embodiment the G.X. Zhang / Journal of Power Sources 285 (2015) 580e587 585

Charging Charging Power Power

Discharging Discharging

Dual power cell Batteries

Time Time (b) (a)

Fig. 8. A schematic illustration of power profiles for (a) concurrent charging and discharging (b) and alternating charging and discharging. charging and discharging cells can be located in different locations material. The option of using simple containers (as shown in for certain applications. Thus, the discharging cells will have a Fig. 10b) to contain only the active material also allows economic higher power density and energy density and can be used as an energy storage for long times, i.e. days, weeks, or even months. independent power sources in the applications where weight and The electrochemical system can be made at low cost due to its volume are of important consideration. After being fully dis- simple structure and using of common materials. It is estimated charged, the discharging cells can be transported back to the that the cost for the physical structure of the system (power com- location of the charging facility for refilling with fresh metallic ponents) would be in the range of $1000/kW to $1500/kW in mass

Fig. 9. Schematic illustration of the possibility of scaling the cell (a) for power capacity (b) and energy capacity (c). 586 G.X. Zhang / Journal of Power Sources 285 (2015) 580e587

(a)

(b)

(c)

Fig. 10. Schematic illustration of the possibility of positioning the sections of energy absorption, storage and delivery into three separate containers and thus allows them to be used separately. production. The cost of the material and containment would be the runtime. Fig. 11 shows the result of calculation indicating that about $15/kWh, in which roughly $5 for zinc (2 kg), $5 for KOH the cost drastically reduces with increasing storage capacity due to (5 kg), and $5 for containment. The unit cost of the system can be the low cost of the material storage. At a cost of $1500/kW for the calculated with respect to the runtime (energy capacity) of the power components, the cost of energy storage becomes lower than system using a simple equation: $100/kWh for a runtime longer than 20 h, which means it could be more economical even than pumped hydro [3]. Cs ¼ P þ M*t, A number of potential materials have been explored for energy storage as material-based technologies are shown in Table 1: water where Cs is the unit cost of energy storage, P the cost of power through pumped hydro, air through compressed air, hydrogen components, M the cost of energy material and containment and t through and fuel cells, vanadium through flow battery and zinc through electroplating and zinc air cells. For economical energy storage, both the energy material and containment need to be low cost. In the case of water and air, the cost of material is low but not containment, which is limited by geographic and envi- ronmental conditions. For hydrogen, its storage is expensive because it requires special devices or mediums for containment to obtain sufficient energy density and to ensure safety. For vanadium, the raw material, vanadium pentoxide, is expansive; at $63/kWh the cost of energy material is more than twenty times of that for zinc [23,24]. Zinc has a clear advantage over these other materials in energy density, cost of material and containment. Zinc has a favorable set of properties among the common metals for energy storage based on metal fuel cell technology: 1) high energy density, 2) reversible electrochemical reactions in an aqueous solution, and 3) high solubility in the solution. As can be seen in Table 2, metals, such as Al and Mg, that have a higher energy density than zinc, are not suitable because they are not electro- chemically reversible in aqueous solutions. A few metals, such as iron, nickel, copper and lead, can be reversibly oxidized and reduced in aqueous solutions, but are not suitable because they Fig. 11. Calculation result for the unit cost of energy storage as a function of runtime for three cost scenarios of the power components. have a low energy density and a low solubility. Combining with the G.X. Zhang / Journal of Power Sources 285 (2015) 580e587 587

Table 1 tons as discovered resources [21]. Thus, there is sufficient zinc for Characteristics of potential materials for electricity storage. energy storage long into the future. Materiala kWh/kgb kWh/lb $/kWhc Efficiency, % Containment

Water 2.6 10 4d 2.6 10 4d 5e 70e80f Limited availabilityo 5. Conclusions Air 0.1g 0.03g Free 62e70h Limited p availability A novel concept and operation principle for an electrochemical Hydrogen 32.7i 0.003i N/Aj 21e43h Costlyq system and method to store electrical energy in zinc metal has been Vanadium 0.66k 4k 63k 65e75f Low costr Zinc 1.35l 9.6l 1.7m 55e65n Low costr described. In this system the cell is structured with three distinctive sections for energy absorption, storage and delivery in which en- a Water based on pumped hydro technology; air based on compressed air tech- nology; hydrogen based on water electrolysis and fuel cell technology; vanadium ergy is absorbed into zinc metal though electro deposition and is based on flow battery technology; zinc based on metal deposition and zinc air fuel delivered though dissolution of the zinc metal. The deposition and cell technology. dissolution of zinc at different spaces allow independent operation b Theoretical values for active material. of charging and discharging and independent scaling of power and c Active materials only. energy capabilities. The storage capacity of the system can be scaled d Gravitational energy between two reservoirs with 100 m difference in elevation. e 5 cubic meters at $1/cubic; cost of supply water in the developed world varies up at low cost as the zinc metal is stored separately from the active between $0.4 to $2 [12]. components of the electrochemical cell. f [2]. Functional prototype cells have been constructed and repeated g 3 The potential energy generated when 1 m of ambient air is very slowly com- cycling of charging and discharging of long hours successfully pressed into a 5 L bottle at 20 MPa under an isothermal condition [13]. h [4]. demonstrated. Also, sustained charging and discharging simulta- i H2-air chemistry, hydrogen gas at room temperature and atmospheric pressure neously in a single cell were demonstrated experimentally, for the [15]. first time in history. This cell is termed as a dual power cell for its j Hydrogen is consumed once the energy is released (unlike other materials) [15]. þ þ þ þ ability to undertake charging and discharging powers at the same k Assuming V5 V4 /V3 V2 chemistry (0.525 Ah/g, cell voltage ¼ 1.26 V time. [16]), 3 kg of vanadium (5.4 kg V2O5) is required for storing 1kWh leading to $63/ The principle and test results show that electricity can be elec- kWh with a current price of $11.8/kg for V2O5 [23]. l Zinceair chemistry [14]. trochemically stored in zinc metal, which has the most favorable set m $2.2/kg for zinc metal [24]. of technical and economical attributes: high energy density, n [8]. reversible electrochemical reactions in aqueous electrolytes, safe, o Cost of water reservoirs are typically not accounted separately from power stations, but it is most likely to be a significant portion of the system cost which is at and abundant. The cost of zinc material and storage in the tech- least $100/kWh [2]. nology system is less than $20/kWh, which is much lower than p Similar to water but with less availability of suitable lands [2,3]. other potential materials, and thus the technology is well suited for q $40e70/kWh, Capital cost of container and compression equipment only [15]. long time storage at very large scales. r Made of common plastic materials. good reaction kinetics and high stability in alkaline electrolytes, References zinc has the most balanced set of electrochemical properties, which [1] “Packing Some Power”, the Economist, March 3rd, 2012. is why zinc has been used as a favorable anode material for bat- [2] http://www.electricitystorage.org/technology/storage_technologies/ teries of all kinds since the invention of the battery two hundred technology_comparison [accessed on 23.10.14]. years ago [14,18]. [3] D. 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