Future sustainable energy generation technologies such as photovoltaic and wind farms require advanced energy storage systems on a massive scale to make the alternate (green) energy options practical. The daunting requirements of such large-scale energy systems — such as long operating and cycle life, safety, and low cost — are not adequately met by state-of-the-art energy storage technologies such as vanadium flow cells, lead-acid, and zinc-bromine batteries. Much attention is being paid to redox batteries — specifically to the vanadium redox battery (VRB) — due to their simplicity, low cost, and good life characteristics compared to other related battery technologies.
NASA is currently seeking high-specific-energy and long-cycle-life rechargeable batteries in the 10-to-100-kW range to support future human exploration missions, such as planetary habitats, human rovers, etc. The flow batteries described above are excellent candidates for these applications, as well as other applications that propose to use regenerative fuel cells.
A new flow cell technology is proposed based on coupling two novel electrodes in the form of solvated electron systems (SES) between an alkali (or alkaline earth) metal and poly aromatic hydrocarbons (PAH), separated by an ionically conducting separator. The cell reaction involves the formation of such SES with a PAH of high voltage in the cathode, while the alkali (or alkaline earth metal) is reduced from such an MPAH complex in the anode half-cell. During recharge, the reactions are reversed in both electrodes. In other words, the alkali (alkaline earth) metal ion simply shuttles from one M-PAH complex (SES) to another, which are separated by a metal-ion conducting solid or polymer electrolyte separator.
As an example, the concept was demonstrated with Li-naphthalene//Li –DDQ (DDQ is 2,3-Dichloro-5,6-dicyano- 1,4-benzoquinone) separated by lithium super ion conductor, either ceramic or polymer (solid polymer or gel polymer) electrolytes. The reactants are Li-naphthalene dissolved in tetrahydrofuran (THF) with a lithium salt of 1M LiBF4 (lithium tetra fluoroborate) in the anode compartment, and DDQ again dissolved in THF and also containing 1M LiBF4 salt in the cathode half-cell. The solid electrolyte separator used in the first set of experiments is a ceramic solid electrolyte, available from a commercial source. The open circuit voltage of the cells is close to 3.0 V, as expected from the individual half-cell voltages of Linaphthalene and Li-DDQ.
Upon discharge, the cell shows steady discharge voltage of –2.7 V, which confirms that the electrochemical processes do involve lithium ion shuttling from the anodic compartment to the cathode halfcell. The reversibility or rechargeability is demonstrated by charging the partially discharged cells (i.e., with lithium present in the DDQ half). Once again, a steady voltage close to 3.0 V was observed during charge, indicating that the system is quite reversible. In the subsequent concept- demonstration studies, the ceramic electrolyte has been replaced with a gel polymer electrolyte, e.g., PVDF-HFP (poly vinylene difluoride–hexafluoropropene) gel, which has several advantages such as high ionic conductivity (almost comparable to liquid electrolyte and about 2 orders of magnitude better than the ceramic equivalent), lower cost, and possibly higher chemical stability at the anode. In addition, it can be bonded to the electrode by thermal fusion to form membrane electrode assemblies (MEAs), as is done in fuel cells.
Though the initial experiments were performed with Pt electrodes, subsequent tests with porous carbon electrodes showed better kinetics, yielding higher discharge currents. Combining the polymer electrolytes with carbon substrates, flow-cell stacks with membrane electrode assemblies (MEAs) may be configured much like with fuel cells with suitable flow-fields in biplates for an all-liquid rechargeable flow-battery.
There are several unique attributes of this flow cell, which is amongst the highest voltage flow batteries, with cell voltages higher than the prior non-aqueous 1.7 V vanadium acetylacetone redox flow battery. (1) The reaction involves the shuttling of lithium ions from the anolyte to catholyte, much like with traditional Li-ion cells; (2) The reactions involved at both electrodes are mostly chemical, with the oxidized or reduced lithium reacting with the liquid active materials; (3) Both the anolyte and catholyte are electronically conducting with some lithium, thus negating the need for ionic conduction through a lithium salt solution; and (4) The electrodes are only for current collection purposes, which precludes any morphological or interfacial changes at the electrode. All these features will, in principle, contribute to a long cycle life, calendar life, safety, and low self-discharge rates.
This work was done by Ratnakumar V. Bugga, William C. West, Andrew Kindler, and Marshall C. Smart of Caltech for NASA’s Jet Propulsion Laboratory.
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