Axion Providing Lead-Acid Ultracapacitor Hybrid Batteries for Demonstration Projects With the US Army and Penn State University
|Ragone plot of the PbC (e3 Supercell) and other storage technologies. Click to enlarge.|
Axion Power International, Inc. is providing “PbC Ultracapacitors” (PbC)—multi-celled asymmetrically supercapacitive lead-acid-carbon hybrid batteries—to the Applied Research Laboratory at Penn State University for testing in conjunction with the US Army’s Tank and Automobile Command (TACOM). The PbC is a hybrid device that uses the standard lead acid battery positive electrode and a supercapacitor negative electrode that is made of activated carbon. (Earlier post.)
The TACOM project will cover a broad range of established and emerging military vehicle applications including starting, lighting and ignition systems (SLI) for diesel engines that operate in extreme weather conditions; hybrid electric vehicle drive systems; silent watch systems; and other military applications where the lighter weight, higher power, and longer cycle-life of Axion’s PbC Ultracapacitors will improve the performance of existing systems and facilitate the roll out of new systems.
Testing is beginning immediately with the shipment of several different versions of PbC Ultracapacitors to the Applied Research Laboratory at Penn State.
PbC batteries use five-layer carbon assemblies to replace the simple sponge lead plate negative electrodes used by most other lead-acid battery manufacturers. The Axion assembly consists of a carbon electrode, a corrosion barrier, a current collector, a second corrosion barrier and a second carbon electrode. These electrode assemblies are then sandwiched together with conventional separators and positive electrodes to make the battery, which is filled with an acid electrolyte, sealed and connected in series to the other cells.
During charge and discharge, the positive electrode undergoes the same common half reaction based on the double sulphate theory for lead acid batteries—i.e. lead dioxide reacts with acid and sulphate ions to form lead sulphate and water.
Positive electrode: PbO2 + 4H+ + SO42- + 2e- ⇌ PbSO4 + 2H2O (+1.685 V)
However, instead of having a lead electrode that reacts with sulphate to form lead sulphate, the activated carbon electrode in the PbC is a very high surface area material that acts to adsorb and desorb protons (H+) ions in solution.
nC6x-(H+)x ⇌ nC6(x-2)-.(H+)x-2 + 2H+ + 2e- (discharged)
In conventional lead acid batteries, the concentration of acid changes from being very concentrated in the charged state to somewhat dilute in the discharged state as the acid is converted to water. In contrast, the PbC stores the protons in the negative electrode in the fully charged state which move to the positive electrode during discharge where they are neutralized to form water. The result is reduced acid concentration swings from the charged to discharged state which reduces grid corrosion on the positive electrode and leads to longer cycle life of the positive electrode, according to Axion.
The resulting device offers energy storage approaching lead-acid batteries, coupled with far longer cycle-life and power output approaching ultracapacitors. These low-cost devices recharge rapidly and use 40% less lead. During internal testing, prototypes have withstood more than 1,600 deep discharge cycles (charge-discharge every 7 hours to a 90% depth of discharge). In comparison, most lead-acid batteries designed for deep discharge applications can only survive 300 to 500 cycles under these operating conditions, according to Axion.
|Axion PbC Price Performance Characteristics|
|Chemistry||Energy Density||Useful life||Cost of ownership per kWh|
The current PbC energy densities have improved from earlier versions of the technology, which Axion called the e3 Supercell. (Earlier post.) Axion is positioning its PbC technology as offering a balance between price, energy and power that is well suited to a variety of existing and emerging energy storage applications.
Axion has been producing prototype PbC Ultracapacitors at its lead-acid battery plant in New Castle, Pennsylvania for more than a year using the same cases, positive electrodes, separators, electrolytes and manufacturing equipment used in its other lead-acid battery lines. The only notable manufacturing difference is the use of Axion’s proprietary carbon electrode assemblies instead of lead-based negative electrodes.
In addition to Axion’s efforts to develop PbC Ultracapacitor solutions to existing and emerging military applications, Axion has begun work on a complementary project to replace the nickel metal hydride battery systems in a pair of Honda Civic hybrids with a less expensive and comparably sized PbC alternative. Testing protocols are being finalized with Penn State and will include data from upwards of 100,000 miles of road tests. In the second phase of the hybrid project, Axion will add Toyota Prius and other vehicles to the test regime.