ALBERT

Description: The traditional electrochemical storage concepts are difficult to translate into high power, high voltage multikilowatt storage systems. The increased use of electronics, and the use of electrochemical couples that minimize the difficulties associated with the corrective measures to reduce the cell to cell capacity dispersion were adopted by battery technology. Actively cooled bipolar concepts are described which represent some attractive alternative system concepts. They are projected to have higher energy densities lower volumes than current concepts. They should be easier to scale from one capacity to another and have a closer cell to cell capacity balance. These newer storage system concepts are easier to manage since they are designed to be a fully integrated battery. These ideas are referred to as system level electrochemistry. The hydrogen-oxygen regenerative fuel cells (RFC) is probably the best example of the integrated use of these principles.

Description: A somewhat analogous situation among groupings of alkaline fuel cells is described where the stochastic aspects were much more accurately documented and then it was illustrated how this problem was eliminated using straight forward principles of pore size engineering. This is followed by a suggested method of adapting these same design principles to nickel-cadmium cells. It must be kept in mind that when cells are cycled to typically twenty percent depth of discharge that eighty percent of the weight of the cell is simply dead weight. Some of this dead weight might be put to better use by trading it for a scheme that would increase the time during which the cell would be working more closely to its optimum set of operating parameters.

Description: Nickel hydrogen cells, and more recently, bipolar batteries have been built by a variety of organizations. The design principles that have been used by the technology group at the Lewis Research Center draw upon their extensive background in separator technology, alkaline fuel cell technology, and several alkaline cell technology areas. These design principles have been incorporated into both the more contemporary individual pressure vessel (IPV) designs that were pioneered by other groups, as well as the more recent bipolar battery designs using active cooling that are being developed at LeRC and their contractors. These principles are rather straightforward applications of capillary force formalisms, coupled with the slowly developing data base resulting from careful post test analyses. The objective of this overall effort is directed towards the low Earth orbit (LEO) application where the cycle life requirements are much more severe than the geosynchronous orbit (GEO) application. Nickel hydrogen cells have already been successfully flown in an increasing number of GEO missions.

Description: In the early 1980's the NASA Lewis group addressed the topic of designing nickel hydrogen cells for LEO applications. As published in 1984, the design addressed the topics of gas management, liquid management, plate expansion, and the recombination of oxygen during overcharge. This design effort followed principles set forth in an earlier Lewis paper that addressed the topic of pore size engineering. At about that same time, the beneficial effect on cycle life of lower electrolyte concentrations was verified by Hughes Aircraft as part of a Lewis funded study. A succession of life cycle tests of these concepts have been carried out that essentially verified all of this earlier work. During these past two decades, some of the mysteries involved in the active material of the nickel electrode have been resolved by careful research efforts carried out at several laboratories. At The Aerospace Corporation, Dr. Zimmerman has been developing a sophisticated model of an operating nickel hydrogen cell which will be used to model certain mechanisms that have contributed to premature failures in nickel hydrogen and nickel cadmium cells. During the course of trying to understand and model abnormal nickel hydrogen cell behaviors, we have noted that not enough attention has been paid to the potassium ion content in these cells, and more recently batteries. Several of these phenomenon have been well known in the area of alkaline fuel cells, but only recently have they been examined as they might impact alkaline cell designs. This paper will review three general areas where the potassium ion content can impact the performance and life of nickel hydrogen and nickel cadmium devices, Once these phenomenon are understood conceptually, the impact of potassium content on a potential cell design can be evaluated with the aid of an accurate model of an operating cell or battery. All three of these areas are directly related to the volume tolerance and pore size engineering aspects of the components used in the cell or battery design: (1) The gamma phase uptake of potassium ion can result in a lowering of the electrolyte concentration. This leads to a higher electrolyte resistance as well as electrolyte diffusional limitations on the discharge rate. This phenomenon will also impact the response of the cell to a reconditioning cycle. (2) The impact of low level shunt currents in multi-cell con figurations will result in the movement of potassium ion from one part of the battery to another. This will impact the electrolyte volume/vapor pressure relationships within the cell or battery. (3) The transport of water vapor from place to place under the driving force of a tempetature gradient has already impacted cells for the case where water vapor is condensed on a colder cell wall. The paper will explore the convective and diffusive movement of gases saturated with water vapor from a warmer plate pack to a cooler one - both with and without liquid communication.

Description: The destructive physical analysis (DPA) of electrochemical devices is an important part of the overall test. Specific tests were developed to investigate the degradation mode or the failure mechanism that surfaces during the course of a cell being assembled, acceptance tested, and life-cycle tested. The tests that have been developed are peculiar to the cell chemistry under investigation. Tests are often developed by an individual or group of researchers as a result of their particular interest in an unresolved failure mechanism or degradation mode. A series of production, operational, and storage issues that were addressed by the Electrochemistry Group at The Aerospace Corporation are addressed. As a result of these investigations, as well as associated research studies carried out to develop a clearer understanding of the nickel oxyhydroxide electrode, a series of unique and useful specialized tests were developed. Some of these special tests were assembled to describe the methods that were found to be particularly useful in resolving a wide spectrum of manufacturing, operational, and storage issues related to nickel-hydrogen cells. The general methodology of these tests is given here with references listed to provide the reader with a more detailed understanding of the tests. The tests are classified according to the sequencing, starting with the impregnation of the nickel plaque material and culminating with the storage of completed cells. The details of the wet chemical procedures that were found to be useful because of their accuracy and reproducibility are given. The equations used to make the appropriate calculations are listed.