Flow Batteries by Trung Nguyen and Robert F. Savinell enewable energy sources including Batteries and flow batteries/fuel cells Flow Batteries Classification wind and solar can supply a differ in two main aspects. First, in a Rsignificant amount of electrical battery, the electro-active materials are A flow battery is an electrochemical energy in the United States and around stored internally, and the electrodes at device that converts the chemical the world. However, because of their which the energy conversion reactions energy in the electro-active materials intermittent nature, the potential of occur are themselves part of the directly to electrical energy, similar to a these two energy sources can be fully electrochemical fuel. The characteristics conventional battery and fuel cells. The exploited only if efficient, safe, and of the negative and positive electrodes electro-active materials in a flow battery, reliable electrical energy storage (EES) determine both the power density however, are stored mostly externally systems are provided. EES will also be (e.g., electrical, transport, and catalytic in an electrolyte and are introduced critical to improving the robustness properties of the active material and into the device only during operation.3 and efficiency of the grid by reducing non-reactive materials) and the energy True flow batteries have all the reactants power surges and balancing the load density (e.g., mass of active materials) and products of the electro-active over time. A recent report from Sandia of the battery. As a battery converts its chemicals stored external to the power identifies over 17 major opportunities of chemical energy to electrical energy, conversion device. Systems in which all EES to make an impact on the U.S. grid electrodes are consumed and undergo the electro-active materials are dissolved 1 and implementation of renewables. For significant physical and chemical in a liquid electrolyte are called redox very large energy storage applications, changes which affect its electrical (for reduction/oxidation) flow batteries only pumped hydro and compressed performance. Second, because of (RFBs). A schematic of a redox flow gas are cost effective at this time. These the dual functions of the electrodes battery system is shown in Fig. 2. technologies, however, are limited described above, a conventional battery Other true flow batteries might have a by geography, while electrochemical has minimal or no scale-up advantages. gas species (e.g., hydrogen, chlorine) energy storage devices such as Instead, it can only be scaled-out. and liquid species (e.g., bromine). batteries, fuel cells/flow batteries, and That is, if more energy is needed, then Rechargeable fuel cells like H -Br and electrochemical capacitors are among 2 2 more battery modules with identical H2-Cl2 could be thought of as true the leading EES technologies for the components are required. As the amount flow batteries. Systems in which one future because of their scale-ability of electro-active materials increases or more electro-active components are and versatility. Their power and energy in a battery, more current collecting stored internally are called hybrid flow density characteristics are shown in materials, electrolyte, separators, and batteries. Examples include the zinc– 2 Fig. 1. Capacitors, with their very high enclosure materials are also needed. bromine and zinc–chlorine batteries. power densities, low energy densities, Consequently, a battery can never Similarly to conventional batteries, the and sub-second response times, are more approach its theoretical energy density. energy densities of these hybrid flow suitable for power quality management. Furthermore, increasing the capacity batteries are limited by the amount Batteries and flow batteries/fuel cells of a battery almost always increases of electro-active materials that can be have the energy densities needed for internal resistances and consequently stored within the batteries and they large-scale electrical energy storage. decreases power density and efficiency. have limited scale-up advantages. Table I shows some of the more well-known flow battery systems. Although much flow battery research dates back to the 1970s, some research has continued over the past several decades and the state-of-the art has been reviewed in the recent literature.4 Most redox flow batteries consist of two separate electrolytes, one storing the electro-active materials for the negative electrode reactions and the other for the positive electrode reactions. (To prevent confusion, the negative electrode is the anode and the positive electrode is the cathode during discharge. It is understood that they will be reversed during charge.) Both the fresh and spent electrolytes may be circulated and stored in a single storage tank as shown in Fig. 2 or separately to control the concentrations of the electro-active material. An ion selective membrane is often used to prevent mixing or cross-over of the electro- active species which result in chemical short-circuit of electro-active materials. Only the common counter ion carrier is allowed to cross the membrane. For example, in the bromine-polysulfide system, as Na2S2 is converted to Na2S4 at Fig. 1. Power and energy densities of various EES systems. 54 The Electrochemical Society Interface • Fall 2010 - the anode and Br2 is converted to 2Br at the cathode, the excess Na+ ions at the Table I. Characteristics of Some Flow Battery Systems. o anode are allowed to cross to the cathode System Reactions Ecell Electrolyte to maintain electroneutrality condition. Similarly, in the vanadium system, as Redox Anode/Cathode V+2 is oxidized to V+3 at the anode and 3 All Vanadium 2+ 3+ 1.4 V H2SO4/H2SO4 V+5 is reduced to V+4 at the cathode, Anode: V V + e- hydronium ions are transported across + 2+ Cathode: VO + e- VO a proton conducting membrane from 2 the anode to the cathode. In this case, Vanadium- 2+ 3+ 1.3 V VCl -HCl/NaBr- Anode: V V + e- 3 however, sometimes a microporous Polyhalide 5 HCl non-selective membrane separator can - be used since most of the current might Cathode: ½ Br2 + e- Br be carried by high mobility protons in Bromine- 2- 2- 1.5 V NaS /NaBr Anode: 2 S S + 2e- 2 the acid electrolyte and since the cross- Polysulfide 6 2 4 over of the common vanadium cation - lowers efficiency but does not cause Cathode: Br2 + 2e- 2 Br a permanent loss of capacity. (Note: 7 Iron-Chromium 2+ 3+ 1.2 V HCl/HCl the distinction of the flow electrolyte Anode: Fe Fe + e- does not take into account the fixed 3+ 2+ electrolyte within the membrane Cathode: Cr + e- Cr separator of many flow batteries.) 8 H2-Br2 + 1.1 V PEM*- H Br A fuel cell might be considered as a Anode: H2 2H + 2e- type of flow battery in that the power - conversion component is independent Cathode: Br2 + 2e- 2Br of the chemical energy capacity of the device. Most fuel cells, however, cannot Hybrid be reversed electrically efficiently, and Zinc-Bromine 2+ 1.8 V ZnBr2/ZnBr2 therefore cannot be used effectively as Anode: Zn Zn + 2e- an electrical energy storage device; so Cathode: Br + 2e- 2 Br- they are considered as a chemical energy 2 conversion device only. 9 Zinc-Cerium 2+ 2.4 V CH3SO3H Anode: Zn Zn + 2e- 4+ 3+ (both sides) Advantages and Disadvantages Cathode: 2Ce + 2e- 2Ce *Polymer Electrolyte Membrane With the electrolyte and electro- PEM* active materials stored externally, true flow batteries have many advantages, one of which is the separation of the power and energy requirements. the size of the engine and the energy current vanadium prices, or from 50 The electrodes, not being part of the density is determined by the size of the to 100 percent of the aforementioned electrochemical fuel, can be designed fuel tank. cost target of $100-200/kWh. From to have optimal power acceptance In a flow battery there is inherent safety this standpoint, identifying low-cost and delivery properties (e.g., catalytic, of storing the active materials separately redox couples with high solubility is electrical, and transport) without the from the reactive point source. Other critical to meeting the ultimate market need to also maximize energy storage advantages are quick response times requirements. density. Furthermore, the electrodes (common to all battery systems), high The other key cost driver is the do not undergo physical and chemical electricity-to-electricity conversion construction of the electrochemical cell changes during operation (because efficiency, no cell-to-cell equalization itself. The construction costs of the cell they do not contain active materials), requirement, simple state-of-charge scale with the total power requirement thus leading to more stable and durable indication (based on electro-active of the application, but these costs are performance. Therefore, engineered concentrations), low maintenance, directly rated to the specific power microstructures developed to optimize tolerance to overcharge and over- of the device itself–how effectively performance can be maintained over discharge, and perhaps most importantly, the materials are utilized. While flow the lifetime of the device: with longer the ability for deep discharges without batteries ought to be able to operate lifetimes, the capital costs of the battery affecting cycle life. The hybrid systems at relatively high current densities, as system can be amortized over a longer like those involving zinc plating do not convection can be employed to deliver period, and with a wider state-of-charge offer all these advantages, but still have reactants to the electrode surface, flow operating window, the quantity of active many of the desirable features of a true batteries have typically been operated material required to deliver power over flow battery. The main disadvantage of at ~50 mA/cm2, a current density the entire required duration of discharge flow batteries is their more complicated consistent with conventional batteries can be minimized.