The projected life of RFB systems is longer than that of conventional batteries, although it should be still verified in field tests. Combined with higher usable storage capacity of RFBs, it results in their lower levelized cost of electricity even when the capital cost is twice as much.RFBs are the most economical when the discharge time is >3 h (C-rate < 1/3) and the system cost decreases with the duration of an application.they have less manufacturability issues due to the uniformity of electrolytes and cells. independence of power and energy in rfbs allows for using the same chemistry in different applications. however, both fundamental and applied research is needed for industry acceptance and wide implementation of this technology in energy-storage systems.

Specific needs in chemistry, materials science, and engineering are as follows:

(1) The search for novel electrochemical couples based on inexpensive components, with high solubility and redox potential, and exhibiting fast electrode kinetics. Recently the design space for redox couples was substantially expanded due to introduction of organic active materials and redox-active ligands for metal complexes. Employing multielectron transfer may increase an energy density without an increase in solubility, which in many cases is close to the practical limit due to high viscosity.

(2) The development of electrocatalysts with high activity, which could be incorporated into an electrode structure with high surface area. In many cases electrode kinetics for promising energy-dense materials is too slow for practical use but can be improved using electrocatalysts or electron-transfer mediators.

(3) A fundamental understanding of complex electrochemical processes in electrolytes, especially for multielectron active materials. Many processes in electrolytes initiated by an electron transfer are coupled with chemical reactions in solution or on the electrode surface. Understanding of these processes will help in selection of electrode materials, electrocatalysts, and additives for dendrite-free electrodeposition.

(4) The development of ion-selective, cost-effective membranes with high conductivity and long lifetime. This is a critical component of RFBs that defines efficiency and lifetime of the whole system. Although 100% ion selectivity may be not achievable, the use of charged ionic materials may radically reduce the crossover.

(5) Advanced cell/stack design and electrochemical engineering. Bipolar electrodes represent a big part of the RFB cost structure and may be an important resource for the cost reduction. In addition, an advanced design of flow fields could make in impact on RFB power density and, therefore, on the cost. Issues of materials degradation, water management, cell impedance, and balance-of-plant losses should be also addressed.

(6) The development of detailed computational models for steady-state and transient operations. Computational models could save resources and time in scaling up and accurately predict the state of health and lifetime of an RFB for different applications. There are a few results on thorough testing data of RFB stacks published in the literature.

(7) The evaluation of RFB environmental safety in long-term operations. Although intrinsically safer than conventional batteries, an industrial-scale RFB is essentially a miniature chemical plant containing toxic and/or corrosive high energy substances. Therefore, all hazards and environmental and safety issues should be adequately addressed during a scale-up including development of preventative and protective measures.

Research in these directions requires new breakthroughs and inventions but if successful it will enable an “ideal” RFB, which can cover the wide spectrum of stationary energy-storage applications using the same chemistry. The lower energy density of RFBs compared to that for Li-ion batteries makes their use in mobile applications unlikely. Worth noting, however, is the recent announcement by NanoFlowcell AG (http://www.nanoflowcell.com) of the Quant car powered by an RFB with a claimed energy density of 200 Wh/L. Is it possible to predict the “ideal” RFB or, at least, point to chemistries having the greatest potential? It is a difficult question. Sheer volume of needed energy storage measured in GWh demands millions of tons of active materials. A few redox-active metals (Fe, Cu, Zn, and Pb) and basic inorganic (e.g., sulfuric and hydrochloric acids, sodium hydroxide, and chlorine) and organic (e.g., methanol, acetic acid, and phenol) chemicals are produced worldwide at such a scale. Therefore, based on scale and availability, the “ideal” flow battery for large-scale deployment should be aqueous and use one (or more) of the most abundant metals (Fe, Cu, Zn, or Pb) or organic active materials and basic inorganic salts as electrolytes. On the basis of the cost criteria, in addition to inexpensive electrolyte materials, RFB electrode reactions should be fast and electrolyte conductivity should be high to provide high power and thus reduce the stack size, which again points to aqueous electrolytes. The importance of RFB efficiency criteria will grow with time, so minimization of the active species crossover will be achieved via development of high selectivity membranes or adjusting the size and charge of active materials. To minimize the system size and footprint, it is necessary to use highly soluble, multielectron active materials, inorganic or possibly organic. Summarizing, the “ideal” RFB most probably will employ a single, high energy density electrolyte based on inexpensive materials and an advanced electrode design and materials to maximize power density.