|Energy capacity comparison of gasoline, hydrogen and electrochemical energy sources. Shading superimposed on solid colors indicates practical capacities. Click to enlarge. Source: Licht 2008.|
A team of researchers led by Dr. Stuart Licht at the University of Massachusetts, Boston, has developed a vanadium boride (VB2)/air cell—a new renewable electrochemical energy system which stores more energy than gasoline and has an order of magnitude higher capacity than lithium-ion batteries. A report on their work is published in the 28 July issue of the journal Chemical Communications.
The energy capacity gap between gasoline and electrochemical storage systems has been a fundamental barrier to more widespread use of electric drive vehicles. Gasoline has a practical energy storage capacity of about 2.7 kWh/Liter, Li-ion about 0.5 kWh/L. Zinc-air cells, another high-capacity electrochemical system, have a practical capacity of about 1.75 Wh/L according to Licht and his colleagues. The new VB2 system has a practical capacity of 5 kWh/L, they calculate.
Practical, compared to intrinsic, energy electrochemical capacity is limited by the delivered energy and system mass, incorporates all voltage losses, air cathode size, and all other cell components. For example, the practical energy of a small, portable commercial zinc air cell exceeds 18% of the intrinsic energy capacity, and can be higher in an optimized, large fuel cell configuration. The relative practical capacity of the VB2/air cell can be estimated as similar to that of the well studied Zn/air system (electrolytes and cathodes are similar). Based on this analog, the practical vanadium boride fuel has a lower limit of 18% of its intrinsic 27 kWh L-1, for an estimated vanadium boride air practical storage capacity of 5 kWh L-1.—Licht 2008
As in a zinc air cell, the vanadium boride cell reacts oxygen brought in via the cathode with the anode to produce electricity. And also as in a zinc-air cell, the reaction is irreversible; spent anodes need to be replaced in a “refueling” operation and chemically regenerated. (Earlier post.) The vanadium boride cells combine a conventional air cathode with a zirconia-stabilized vanadium boride anode.
|Optimization of the vanadium boride air cell anode capacity as a function of the indicated anode composition, capacity, and discharge load conditions. Click to enlarge. Source: Licht 2008.|
The researchers used the zirconia coating to avoid issues such as boride corrosion, which can result in “not only a chemical loss of the electrochemical capacity, but evolved hydrogen is flammable, and the evolved gas can swell or even crack a cell.” Zirconia is highly stable and maintains effective charge transfer during boride anodic discharge.
The researchers overcame a series of impediments to the effective discharge of the vanadium boride fuel cell and showed experimentally that they could realize substantial capacity of VB2. (See plot at right.)
For regeneration of the anodes, Licht and his team proposed a solar photochemical pathway based on Mg reduction of the fuel cell discharge products.
The large volumetric capacity of the fuel cell, and the pathway for a renewable (solar) energy recharge, are positive attributes of this novel vanadium boride air cell. Systems aspects will continue to be analyzed and optimized. Liquid (higher temperature, solar driven), rather than solid, Mg, should facilitate the recharge formation of VB2...The discharge studies indicate that sub micron particle size VB2, as available following high energy ball milling, can further improve anodic kinetics and coulombic efficiency.—Licht 2008
This material was based on work supported in part by the United States National Science Foundation, with research support to Stuart Licht while working at the Foundation.
Stuart Licht, Huiming Wu, Xingwen Yu and Yufei Wang (2008) Renewable highest capacity VB2/air energy storage. Chem. Commun., 2008, 3257-3259 doi: 10.1039/b807929c