液流氧化还原电池综述2011

ORIGINAL PAPER Redox flow batteries: a review Adam Z. Weber • Matthew M. Mench • Jeremy P. Meyers • Philip N. Ross • Jeffrey T. Gostick • Qinghua Liu Received: 12 July 2011 / Accepted: 16 August 2011 � The Author(s) 2011. This article is published with open access at Springerlink.com Abstract Redox flow batteries (RFBs) are enjoying a renaissance due to their ability to store large amounts of electrical energy relatively cheaply and efficiently. In this review, we examine the components of RFBs with a focus on understanding the underlying physical processes. The various transport and kinetic phenomena are discussed along with the most common redox couples. Keywords Flow battery � Redox � Regenerative fuel cell � Flow cell � Vanadium List of symbols ak,p Interfacial surface area between phases k and p per unit volume (cm-1) ci Concentration of species (mol/cm 3) df Fiber diameter (cm) Di Fickian diffusion coefficient of species i in a mixture (cm2/s) E0 Standard cell potential (V) Eeq Equilibrium cell potential (V) F Faraday’s constant, 96487 C/equiv i Superficial current density (A/cm2) i0 Exchange current density (A/cm 2) ih,k-p Transfer current density of reaction h per interfacial area between phases k and p (A/cm2) k Permeability (m2) k0 Standard rate constant, varies m Valence state n Valence state or number of electrons transferred in a reaction Ni Superficial flux density of species i (mol/cm 2 s) p Pressure (Pa) rl,k-p Rate of reaction l per unit of interfacial area between phases k and p (mol/cm2 s) R Ideal-gas constant, 8.3143 J/mol K Rg,k Rate of homogenous reaction g in phase k (mol/ cm3 s) Ri,j Resistance of resistor i, j in Fig. 10 where ct stands for charge-transfer (X cm2) si,k,l Stoichiometric coefficient of species i in phase k participating in reaction l t Time (s) T Absolute temperature (K) ui Mobility of species i (cm 2 mol/J s) v Superficial velocity (cm/s) x Stoichiometric coefficient y Stoichiometric coefficient zi Valence or charge number of species i Greek a Transfer coefficient ai Transport coefficient of species i (mol 2/J cm s) e Porosity A. Z. Weber (&) � P. N. Ross Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA e-mail: azweber@lbl.gov M. M. Mench � Q. Liu Department of Mechanical, Aerospace and Biomedical Engineering, University of Tennessee, Knoxville, TN, USA M. M. Mench Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA J. P. Meyers Department of Mechanical Engineering, The University of Texas, Austin, TX, USA J. T. Gostick Department of Chemical Engineering, McGill University, Montreal, QC, Canada 123 J Appl Electrochem DOI 10.1007/s10800-011-0348-2 e0 Permittivity (F/cm) n Electroosmotic coefficient q Density (g/cm3) qc Charge density (C/cm 3) r Conductivity of the electronically conducting phase (S/cm) g Overpotential (V) j Conductivity of the ionically conducting phase (S/cm) l Viscosity (Pa s) li (Electro)chemical potential of species i (J/mol) Uk Potential in phase k (V) wi Permeation coefficient of species i (mol/s cm bar) Super/subscripts * Reference state 0 Solvent 1 Electronically conducting phase 2 Ionically conducting phase O Oxidant R Reductant 1 Introduction Renewable-energy sources, such as solar and wind, are being deployed in larger numbers than ever before, but these sources are intermittent and often unpredictable. These characteristics limit the degree to which utilities can rely upon them, and, as such, renewables currently comprise a small percentage of the primary power sources on the US electrical grid. Analysis suggests that an electric grid could become destabilized if non-dispatchable renewable energy exceeds 20% of the energy-generation capacity without energy storage [1]. However, many utilities are mandating renewable portfolios approaching this level of deployment, thus there is a pressing need for storage technologies to complement and enable renewable standards. Other than capacitors, however, there is no way to store electrical energy as such. Instead, if electricity is to be stored, it must first be converted to some other form of energy. There are some technologies that enable practical storage of energy at their current levels of deployment, but only a very small fraction of North American power plants employ such technology [2]. To ensure that renewable energy succeeds in delivering reliable power to US consumers, the nation needs cost effective and reliable storage at the grid scale. Conventional rechargeable batteries offer a simple and efficient way to store electricity, but development to date has largely focused on transportation systems and smaller systems for portable power or intermittent backup power; metrics relating to size and volume are far less critical for grid storage than in portable or transportation applications. It therefore stands to reason that optimizing battery per- formance over a different set of variables might result in an implementation that delivers superior performance for reduced cost. Batteries for large-scale grid storage require durability for large numbers of charge/discharge cycles as well as calendar life, high round-trip efficiency, an ability to respond rapidly to changes in load or input, and rea- sonable capital costs [3]. Redox flow batteries (RFBs) or redox flow cells (RFCs), shown schematically in Fig. 1, promise to meet many of these requirements [4]. As shown in Fig. 1, a key component of RFBs is the ability to separate power and energy. The power is con- trolled by the stack while the energy is stored within the separated reactants. Thus, one can optimize over a greater range of variables and storage can be increased with relatively ease and minimal cost compared to the stack, which is typically the most expensive system component. To examine the technologies that are under development to meet the cost requirements of the marketplace and enable wide-scale storage, we consider the existing port- folio of RFB storage technologies and the possibilities of each. To that end, we introduce the various technologies and discuss in more depth the general attributes and con- cerns facing RFBs. The overall purpose of this review is to examine systemic issues for the field of RFBs, and not just examine a specific chemistry or the various proposed RFBs. Excellent reviews of these latter issues and energy storage for the grid in general can be found in the literature [5–8]. The structure of this paper is as follows. After an introduction and short overview of the various major RFBs, the kinetic and transport issues are examined in turn. Next, some overall electrode/cell modeling and designs are reviewed. Finally, some comments about future research needs are made. It should be noted that this review is focused on cell-level issues and RFB chemistries, therefore issues of system integration and components are not examined in depth, although they can be critically important for system commercialization. Before discussing the various RFB chemistries, it is worthwhile to examine their current major applications. 1.1 Grid-storage needs The present electric grid constitutes an enormous physical infrastructure, with a near-instantaneous transmission of value from primary power sources and generation assets to end users and with almost no storage capability. Because of this dearth of storage, the existing grid must conform to fluctuations in customer demand, resulting in the con- struction of power plants that may only operate for 100 h a year or less and can account for up to 30 MWh in capacity [9]. These generators are dispatched to respond to small oscillations in demand over very short time scales of J Appl Electrochem 123 less than 1 h. They are also turned on and sped up to meet increasing load during the peak time of the day, and, at the other extreme of wastefulness, brought on by the lack of storage. For example, wind energy is wasted because of the inability to dispatch wind power at night when wind gen- eration is at a maximum but customer demand is at a minimum; thus, there is a significant value added by the incorporation of storage [10]. Similarly, photovoltaics and solar-energy implementation will also require arbitrage since although the solar irradiation received terrestrially in about 1 h is sufficient to meet worldwide energy require- ments for a year, the sun does set daily. Storage is a vital tool that would uncouple customer demand from the gen- eration side of the grid, thereby allowing vital flexibility in control and maintenance of the electric grid. To date, however, energy storage comprises only about 2% of the installed generation capacity in the U.S. Because of dif- ferences in government policy and more favorable eco- nomics, storage plays a larger role in Europe and Japan, at 10 and 15%, respectively [11]. The current worldwide electric generation capacity was estimated to be about 20 trillion kilowatt hours in 2007 [12]. More than two-thirds of the current mix is from some form of fossil fuel, with most of the balance coming from nuclear and hydroelectric power generation; at present, only about 3% comes from renewable-energy technologies. Further- more, developing economies and electrification of the transportation sector both point to strong year-over-year growth in terms of electrical demand. While coal is already the primary source of power in the US electricity sector, there are concerns that it will become a larger portion of electricity production as increased global demand competes for cleaner resources like natural gas. Coal is, of course, the most carbon-intensive resource used in this sector; how- ever, while debate continues about how to address anthropogenic global warming gas emissions from a policy standpoint, coal plants are less capable of handling transient loads than the ‘‘peaker’’ plants that largely sit idle and which are deployed only to handle the peak loads. Growing demand implies not only an increase in the base load, which might be handled by coal if government and the energy sector choose not to prioritize carbon-emissions reductions, but also to larger peak loads, which will either require more intermittent generation assets or storage. In addition to improvements in resiliency that can enable increased renewable-energy generation, integration of storage into the smart grid also promises to enable greater system efficiency, even with existing generation assets. The Electric Power Research Institute has com- pleted a study that suggests that the widespread adoption of smart grid technologies could yield a 4% reduction in energy use by 2030 [13], roughly equivalent to eliminating the emissions of 50 million cars. Beyond the emissions impact, that savings translates to more than $20 billion annually for utility customers nationwide. With a more robust and efficient system, and more data about demand patterns, it will be easier for utilities to manage the inte- gration of intermittent renewable-energy sources. Energy storage can also support requirements for reserve genera- tion in place of fossil-fuel-based facilities, yielding zero emissions and lowered operating costs. It seems apparent that being able to harvest energy from more diverse sources, and being able to deploy this energy to the end user when it is demanded, should lower oper- ating costs and promote the robustness and quality of power on the grid. Why then, is the penetration of storage onto the grid so small? The answer is primarily cost. There are multiple costs associated with the installation and operation of a RFB system: one must consider the opera- tion and maintenance costs, as well as up-front capital costs Fig. 1 A schematic diagram of a redox flow battery with electron transport in the circuit, ion transport in the electrolyte and across the membrane, active species crossover, and mass transport in the electrolyte J Appl Electrochem 123 and life-cycle costs. Because of the decoupling of energy and power in RFB configurations, we can consider both cost per unit of power generation/storage capability ($/kW) and the cost per unit of energy-storage capacity ($/kWh). We note that the cost per unit energy storage is not the incremental cost of producing or storing that energy as would be expected in a utility bill, but the cost per unit of energy-storage capacity. In addition to costs, robust system lifetimes of *10 years, high efficiency, and cyclic dura- bility are necessary for grid-level storage. Different applications have different acceptable costs, and the total power and total duration of storage provided will differ from application to application. As such, it is difficult to target a single metric that can concisely address the ultimate cost target for grid-based storage. Table 1 below, from a report prepared by the Nexight Group based upon a workshop convened by Sandia, PNNL, and the Minerals, Metals, and Materials Society (TMS) for the US Department of Energy, suggests the following cost per- formance targets for key utility applications, and identify cost targets for flow batteries of $250/kWh in capital costs in 2015, decreasing to $100/kWh by 2030 [14]. Current estimates of costs for conventional batteries and flow bat- teries are significantly higher than the required targets: a 2008 estimate of RFB costs suggested nearly $2500/kW, albeit without specification of duration or sizing [15]. Regardless of detail, however, significant cost reduction must be achieved: technological improvements, material development, and economies of scale must be achieved to ensure success in the marketplace. 2 Redox-flow-battery overview Redox flow batteries can be classified by active species or solvent (aqueous and non-aqueous, respectively). Figure 1 shows a generic RFB system. In the discharge mode, an anolyte solution flows through a porous electrode and reacts to generate electrons, which flow through the external circuit. The charge-carrying species are then transported to a separator (typically an ion-exchange membrane (IEM)), which serves to separate the anolyte and catholyte solutions. The general reactions can be written as Anþ þ xe� �!charge Aðn�xÞþ and Aðn�xÞþ �!discharge Anþ þ xe� n [ xð Þ ð1Þ and Table 1 Key performance targets for grid-storage applications, from Ref. [14] Application Purpose Key performance targets Area and frequency regulation (short duration) Reconciles momentary differences between supply and demand within a given area Service cost: $20/MW Roundtrip efficiency: 85–90% System lifetime: 10 years Discharge duration: 15 min–2 h Response time: milliseconds Renewables grid integration (short duration) Offsets fluctuations of short-duration variation of renewables generation output Accommodates renewables generation at times of high grid congestion Roundtrip efficiency: 90% Cycle life: 10 years Capacity: 1–20 MW Response time: 1–2 s Transmission and distribution upgrade deferral (long duration) Delays or avoids the need to upgrade transmission and/or distribution infrastructure Reduces loading on existing equipment to extend equipment life Cost: $500/kWh Discharge duration: 2–4 h Capacity: 1–100 MW Reliability: 99.9% System life: 10 years Load following (long duration) Changes power output in response to the changing balance between energy supply and demand Operates at partial load (i.e., increased output) without compromising performance or increasing emissions Capital cost: $1,500/kW or $500/kWh Operations and maintenance cost: $500/kWh Discharge duration: 2–6 h Electric energy time shift (long duration) Stores inexpensive energy during low demand periods and discharges the energy during times of high demand (often referred to as arbitrage) Capital cost: $1,500/kW or $500/kWh Operations and maintenance cost: $250–$500/kWh Discharge duration: 2–6 h Efficiency: 70–80% Response time: 5–30 min J Appl Electrochem 123 Bmþ � ye� �!charge BðmþyÞþ and BðmþyÞþ �!discharge Bmþ � ye� ð2Þ for the anode (negative electrode) and cathode (positive electrode), respectively. The key transport mechanisms are shown in Fig. 1 for this generic system. The dominant losses in these systems, other than charge-transfer reaction kinetics, are related to the charge and mass transport in the electrolyte and sepa- rator, which are each discussed in turn in later sections of this review. Additionally, a key factor in many of these systems is crossover of species through the separator, which is dependent on current and membrane permeability. A sample RFB cell performance is shown in Fig. 2, where the charge and discharge are at different rates or current densities. One can see that similar to a fuel-cell polari- zation curve, there can be ohmic, mass-transport, and/or kinetic losses. The first part of the curves is dominated by kinetic overpotential, especially on charge. The middle part of the curves is dominated by ohmic or ionic-conduction losses, and the last part of the curves is typically a signature of reactant mass-transport limitations. The reactor in Fig. 1 consists of a stack of individual cells, where each cell contains the sites where electro- chemical charge-transfer reactions occur as electrolyte flows through them, as well as a separator (either an electrolyte-filled gap or a selective membrane) to force the electrons through the external circuit. The arrangement of a typical cell is shown in Fig. 3, and individual cells can be arranged in series to increase the overall stack voltage. Generally, stacks are arranged in a bipolar fashion so that current flows in series from one cell to the next. One of the key attributes of RFBs that suggests signif- icant promise for stationary applications is the fact that, for many configurations, there is no physical transfer of material across the electrode/electrolyte interface. While there are some configurations that can be categorized as flow batteries only in the sense that the active material flows from outside of the cell to the electrode surface, most flow-battery systems under development utilize reversible solution-phase electrochemical couples on two electrodes to store chemical energy. Instead of storing the electro- chemical reactants within the electrode itself, as with metal/metal alloy or intercalation electrodes, the reactants are dissolved in electrolytic solutions and stored in external tanks. Both the oxidized and reduced form of each reactant are soluble in the electrolyte, so they can be carried to/from the electrode surface in the same phase. Only the relative concentrations of oxidized and reduced forms change in each stream over the course of charge and discharge. The electrodes in most RFB configurations are not required to undergo physical changes such as phase change or insertion/deinsertion during operation because the changes are occurring in the dissolved reactants in the solution phase adjacent to the solid-electrode surfaces. Though there are exceptions to this formulation, as men- tioned in the next section, this feature generally affords the opportunity to simplify the electrode design considerably. As a consequence of the charge-transfer characteristics, the cycle life of a RFB is not directly influenced by depth-of- discharge or number of cycles the way that conventional rechargeable batteries are. Side reactions can, of course, complicate design and operation, but if the reactions pro- ceed as intended, degradation of the electrode surface need not proceed as a matter of course. The decoupling of