MEASUREMENT of STIBINE and ARSINE GENERATION from the EXIDE 3100-Ah LEAD-ACID MODULE

Home , Stibine

Chemical Technology ANL-87-1 Division Chemical Technology Division Chemical Technology L/.r. ,ion Chemical Technology Di'; ..on Chemical Technology Measurement of Stibine and Division Chemical Technology Arsine Generation from the Division Exide 3100-Ah Lead-Acid Module Chemical Technology Division Chemical Technology D . ion by J. J. Marr and J. A. Smaga Chemical Technology Division Chemical Technology Division Chemical Technology Division Chemical Technology Division Chemical Technology Division Chemtcai Technology Division Chemical Technology Division Chemical Technology Division Chemical Technology Division

Argonne National Laboratory, Argonne, Illinois 60439 operated by The University of Chicago for the United States Department of Energy under Contract W-31-109-Eng-38

Chemical Technology Division Chemical Technology Division Chemical Technology Division Chemical Technology Division charged at a constant current of 512 A. After approximately four hours, the module reached a prescribed voltage (adjusted for the temperature of the module) and was held at this voltage until the current decayed to about 155 A. The current was maintained at this level until the total charge passed was equivalent to 110% of the ampere-hours discharged during the previous cycle- Every thirteenth cycle, the module typically received an equalization charge, which extended the finishing charge an additional 10%. The cycles selected for sample collection occurred at least five cycles after an equalization charge to ensure that the measured hydride levels were not influenced by any residual effects of equalization.

Each cell was sampled ten times during the charge cycle, with an addi tional sample taken during the initial portion of the next discharge cycle. The first sample interval began at a cell voltage of 2.35 V. The sampling interval was progressively shortened as the voltage approached 2.5 V, and then lengthened once this value was exceeded. The gas samples were collected con tinuously by switching between the two gas trains. Each time, before trans ferring the samples to labeled storage bottles, the gas line and the space above the absorbing solution in the bubblers were purged with helium for two to three minutes to remove any traces of residual toxic gases. Samples from both bubblers were then analyzed for stibine and arsine.

2.3 Chemical Analyses

Antimony was determined by decoloring an aliquot of absorbing solution with hypophosphite and measuring the Sbl^' concentration by spectrophotometric analysis. The concentration of antimony in the aliquot was determined by comparing the aliquot absorbance to the absorbance of solution standards con taining known quantities of antimony. The arsenic concentration was measured with an atomic absorption spectrophotometer that was equipped with a hydride generation system for the reconversion of the arsenic to arsine. The arsine was swept into the flame, and the peak height of the absorbance was measured. The amount of arsenic in the sample was determined from a working curve that related the peak height to calibration standards. The sensitivity of these methods was 10 yg SbHj and 1 pg of AsHj for each 100 mL of absorbing solution.

3.0 RESULTS AND DISCUSSION

3.1 Hydride Generation Curves

The rates of stibine and arsine generation in units of gg/min are plotted as a function of charging time in Figs. 2 through 4. Variations in both the cell voltage and the charge current are also shown in these figures. The rates of hydride generation rapidly increase after the threshold voltage of 2.4 V, reach a maximum value shortly after exceeding this voltage, and decline to lower levels during the balance of the charge cycle. For all three cells, the maxima in the arsine generation rates precede the maxima in the stibine generation rates. The former occurred within a voltage range of 2.51 to 2.52 V, while the latter occurred within a voltage range of 2.53 to 2.54 V. The maxima and subsequent decays to lower generation rates are thought to reflect (1) depletion of antimony and arsenic concentrations built up at the negative plates prior to reaching the threshold voltage for hydride evolution and (2) inadequate replenishment of these species during the balance of the charging period. t«0 1 1 500

VOLTAOI 400 t.sol— < 900 H X — III 200 1 CUKRCMT

100 Pig. 2. 0 __ Stibine and Arsine Generation Rates, Current, and Voltage vs Charging Time for Cell 1.

200 900 400 500 CHARGING TIME, mte

2 60 500

400

H 900

H 200 I CURRENT — too

Fig. 3 — 0 2 90 Stibine and Arsine Generation Rates, Current, and Voltage vs. Charging 20K 200 TiM for Cell 3. SbH, I "^ ISO I 7 12I— — 100 A«H «A — 50

0 200 300 400 500 CHARGING TIME, mm 500

—I 400

300 o: 200 c o 100

0 Fig. 4.

Stibine and Arsine Generation — 200 Rates, Current, and Voltage vs Charging Time for Cell 5. SbH — 150

AsH — 50

200 300 400 500 CHARGING TIME, mm

Figures 5 and 6 are the rate profiles for stibine and arsine, respec tively. For stibine, the maxima in the profiles are closely grouped and average 215 ±7 yg/min. As charging progresses, the rates decline from these maxima and reach values at the end of charge that are 10 to 25% lower than the peak rates. The maxima in the rate profiles for arsine average 19.7 +1.5 Mg/min. These profiles show sharper declines from the maxima, and they approach or achieve constant values by the end of charging that are 50 to 70% lower than their peak rates. Qualitative agreement exists among the profiles shown here and the rate curves reported for stibine^'^ and arsine^ evolution from different cells.

3.2 Characterization of Hydride Generation

The cumulative amounts of stibine and arsine collected from each cell are presented in Table 1. Included in these amounts are the low concentrations of hydrides that were present in the final samples; these samples were collected during the one-hour open-circuit interval that followed charging. Typically, the quantities of both arsine and stibine in the final samples were -2% of the total amounts. These low concentrations undoubtedly reflect the delays in clearing the air space within the cells of remaining hydrides and in trans porting these gases through the collection system. On average, 19 mg of stibine and 1.4 mg of arsine were generated during each charge cycle. 230

200 —

150 —

g '00 K

500 CHARGING TIME, min

Fig. 5. Rate Profiles for Stibine Generation

Table 1 also lists three additional parameters, including previously dis cussed peak rates, that characterize the degree of hydride generation. To derive average generation rates, the total quantities of stibine and arsine vere divided by the duration of the constant-current finishing charge. This duration was variable and ranged from 101 to 119 minutes for the three cycles of interest. The average value for the three stibine rates was 175 ±4 ug/min. Sinilarly, the average of the arsine rates was 12.6 ±1.0 yg/min. The third parameter, terned the "overcharge rate** in this report, was determined by dividing the total hydride weight by the ampere-hours of overcharge ('-250 Ah). The averages for the overcharge rates are 75.8 ± 9.5 and 5.46 ± 0.81 yg/Ah for stibine and arsine, respectively. 22

I 10 (A <

200 300 400 500 CHARGING TIME, min Fig. 6. Rate Profiles of Arsine Generation

The overcharge rates were calculated in order to compare results with an earlier study discussed in Section 4.16 of the Exide report.^ In that study, test cells with a rated capacity of 400 Ah were subjected to a continuous overcharge at 30 A as an accelerated method of determining grid corrosion and hydride evolution rates. One cell in the test group contained positive plates with the same composition as the six-cell module and positive grids (4% Sb-0.05% As-bal. Pb) similar in composition to the proprietary alloy used in the module. After 40 kAh of overcharge at 50 to 55**C, the average over charge rates for the accelerated tests were 52 and 2.6 yg/Ah for stibine and arsine, respectively. These values are 31% lower for stibine and 52% lower for arsine than the comparable average rates for the module cells. Table 1. Stibine and Arsine Generation for Each Cell

Cell Total Velght, Peak Rate, Average Rate, Overcharge Rate, No. mg Mg/"in pg/min Ug/Ah

Stibine

1 21.34 218 179 85.3

3 18.17 208 173 72.9

5 17.40 220 172 69.1

Arsine

1 1.50 18.3 12.6 5.98

3 1.43 21.2 13.6 5.74

5 1.17 19.7 11.6 4.65

A nuaber of experimental differences vere considered in attempting to explain the disparity between the accelerated overcharge rates and the cycled overcharge rates. Two factors, cell temperature and finishing current, can be discounted for the following reasons. During the cycles that measurements vere made on the module, the cell temperatures ranged from 42.5 to 47.2^C, or roughly 7 to 8^C lover than the average temperature for the accelerated tests; the complete Exide study^ and other studies^'^^ indicate that lover test tem peratures reduce arsine and stibine evolution. Additionally, vhen the fin ishing currents are normalized for the differences in rated capacity, the charging regime used for the accelerated tests vas more severe than the module charge, 0.075 A per Ah versus 0.050 A per Ah. The indications from other vork^'^ are that lover finishing currents, vhich also mean lover voltages, also reduce hydride evolution.

Other factors, one of vhich is specifically related to arsine evolution, may account for the variation betveen module and accelerated overcharge rates. The positive grids in the module contained somevhat more arsenic, and this vould favor greater arsine evolution. The module cells also experienced a greater degree of overcharging, as measured by the ratio of cumulative over charge to the rated capacity. The continuously overcharged cell had a value of 100 (40 kAh divided by 400 Ah) for this ratio, vhereas the module cells had a value of 151 ((80% DOD of rated capacity) times (10% overcharge per cycle) times (1890 cycles) divided by (the rated capacity)) at the time of testing. If the normalized values for cumulative overcharge are vieved as a crude mea sure of cell degradation (e.g., a greater degree of positive grid corrosion), a higher overcharge ratio vould be expected for the module cells. In fact, the Exide report^ does shov a consistent trend of increased hydride evolution vith cumulative overcharge for cells with other grid compositions. Addition ally* positive grid corrosion under the constant-current condition used for overcharge tests and under the cycling algorithm used for the module could be markedly different. 10

3.3 Effect of Small Variations in Operating Parameters

The hydride generation data in Table 1 were analyzed to determine if relatively small variations in cell operation parameters had a discernible effect on the observed values. Table 2 lists the cell-to-cell variations for the parameters that were considered. An obvious correlation exists between the duration of the constant charging at the end of a cycle and the total quantity of hydride generation. However, no sound correlations could be established between listed parameters and the peak, average, and overcharge rates. Any trends due to variations in one parameter may have been negated by opposing trends due to another parameter. In addition, inherent variations from the nominal grid composition and slight differences in the individual cell degradation rates could also mask trends related to operation parameters.

Table 2. Key Cell Parameters for the Cycles of Hydride Collection

Cell Number 1 3 5

Cycle Number 1886 1883 1893

Temperature, "C 42.5 47.2 44.8

Final Voltage, V 2.53 2.54 2.55

Finishing Current, A 155.8 157.3 150.5

Current Duration, min 119 105 101

3.4 Ventilation Requirements for Hydride Removal

The minimum airflow for safe ventilation of these toxic gases was deter mined based on exposure limits set by the National Institute for Occupational Safety and Health (NIOSH). The NIOSH threshold limit value-time weighted averages (TLV-TWAs) are 0.1 ppm for stibine and 0.05 ppm for arsine. Since the results of this study indicate that the ratio of stibine to arsine for the Exide module is typically 14 to 1, an airflow sufficient to control the level of stibine will also suffice to prevent hazardous arsine accumulation. The average stibine rate of 175 yg/min per cell is equivalent to the evolution of 3.4 X 10~^ L/min at ambient temperature. Thus, the minimum safe airflow would be 340 L/min per cell if no device is used for hydride abatement. This value provides some margin of safety since hydride evolution occurs for only two of the twelve hours needed to complete a typical duty cycle. Prudence would also dictate the use of a multiplicative safety factor. This factor should be based on the maximum charging current that can be delivered in the event of a malfunction. This consideration is based on the finding of Loutfy et al.^ that the stibine evolution was found to be roughly proportional to the finishing current.

For comparison purposes, the minimum airflow needed to dilute the hydro gen gas generated during charging was also calculated. From Faraday's law, the rate of hydrogen evolution equals the charging current multiplied by a 11 proportionality constant of 7 x 10'^ L/(min-cell-A). At 155 A, the evolution rate is about 1.1 L/min per cell. An airflow of at least tventy-five times this value, or 27.5 L/min per cell, is needed to reduce the hydrogen concen tration belov the 4% flaramability limit. This airflov is over tvelve times lover than the minimum airflov for safe hydride removal. This comparison reveals that, in providing adequate ventilation for a lead-acid battery with out a hydride abatement device, the stibine generation rate can be a far more crucial design consideration.

Some final calculations vere made to gain perspective on vhat these air flov rates mean in terms of ventilation requirements for an actual facility. The lead-acid bay of the Battery Energy Storage Test (BEST) facility is designed to house batteries of up to one megavatt-hour in size. A hypo thetical Exide 1-MVh battery vould require 28 of the 36-kWh modules (i.e., 168 cells). If the same type of duty cycle used for the module vas folloved for the battery, the minimum airflov for safe removal of stibine vould be 57.1 m^/min, or about 2020 cubic feet per minute. The design capacity of the ventilation system for the lead-acid bay is 10,000 cfm.^^ Thus, the normal airflov through the bay is nearly five times greater than the minimum airflov needed for compliance vith NIOSH standards on stibine exposure.

Any discussion of the ventilation requirements for hydride off-gases, hovever, vould be remiss vithout considering the instability of these gases. Dixon and Keoff^^ found that stibine decomposes, primarily to antimony trioxide, vhen exposed to air. They reported that 50% decomposition occurs vithin 6 to 12 minutes. Presumably, the SbOj particles are fine enough to remain suspended in the exhaust gases. These decomposition products could pose an eventual health concern to areas surrounding the utility plant. Simon's reviev^ alludes to a number of materials vhich catalyze the decom position process. An abatement device at the cell or module level that incor porated such a catalyst vould provide an improved margin of safety in tvo vays: collection of hazardous antimony and arsenic compounds vould be simpli fied to a routine maintenance procedure vithin the plant, and the level of hydroxide emissions from the plant vould be reduced.

4.0 SUMMART

The hydride generation rates vere determined for three of the six cells in the Exide 3100-Ah lead-acid module as this module approached 1900 cycles of operation. The three cells had similar rate profiles. The maximum rate of arsine generation averaged 19.7 yg/min and occurred at a cell voltage of 2.52 tO.Ol V. The maximum rate of stibine generation averaged 215 yg/min and occurred vhen the cell voltage vas 2.54 ±0.01 V. Both peaks then decayed and approached vhat appeared to be steady-state values by the end of the charge cycle. Little or no hydride vas collected during the open-circuit period immediately folloving the charge cycle.

Vhen computed on the basis of ampere-hours of overcharge, the rates of hydride evolution vere 75.8 and 5.46 yg/Ah for stibine and arsine, respec tively. These overcharge rates are roughly 1.5 to 1.8 times greater than the respective values reported by Exide for continuous charging tests. The dif ferences may reflect a greater degree of positive grid corrosion due to the greater cumulative overcharge for the module cells and/or a lover corrosion rate under constant-current operation. 12

The average rates of hydride evolution, as determined by dividing the total quantity of arsine or stibine by the duration of the finishing charge, were 12.6 yg/min for arsine and 175 yg/min for stibine. Based on the latter rate and a 0.1 ppm exposure limit for stibine, the minimum airflow requirement (exclusive of a safety factor) is 340 L/min per cell. This airflow is over 12 times greater than the airflow required for safe ventilation of the hydrogen evolved during charging. The normal ventilation capacity of the lead-acid bay in the BEST facility is nearly five times greater than the minimum required for stibine removal from a hypothetical 1-MWh battery based on this Exide module and duty cycle. This margin of safety could be improved even further through the use of catalytic devices to convert the air-unstable hydrides to their solid decomposition products.

ACRNOVLEDGMENTS

The authors would like to thank William C. Spindler, EPRI Project Manager, and James F. Miller, our colleague at Argonne, for their insightful comments and suggestions in reviewing this report.

This work was funded by the Electric Power Research Institute under contract No. RP-2216-2. 13

REFBRBNCES

!• A. C. Simon, **Stibine Generation in the Lead-Acid Battery,'* in Stibine Formation and Detection in Lead-Acid Batteries, ed. W. C. Spindler, Electric Power Research Institute Report EPRI EH-448-SR, pp. 2-1 to 2-27 (1977).

2. R. Varma and N. P. Yao, Stibine and Arsine Generation from a Lead-Acid Cell during Charging Modes under a Utility Load-Leveling Duty Cycle, Argonne National Laboratory Report ANL/OEPM-77-5 (1978).

3. R. 0. Loutfy et al., Stibine/Arsine Monitoring during EV Operation, Argonne National Laboratory Report ANL/OEPH-81-1 (1981).

4. J. L. Davson, M. I. Gillibrand, and J. Wilkinson, J. Inorg. Nucl. Chem. 32, 501 (1970).

5. J. L. Davson, M. I. Gillibrand, and J. Wilkinson, "The Chemical Role of Antimony in the Lead-Acid Battery," in Pover Sources 3, ed. D. H. Collins, Oriel Press, pp. 1-12 (1971).

6. Exide Management and Technology Co., Final Report on Research, Development and Demonstration of Advanced Lead-Acid Batteries for Utility Load-Leveling, Argonne National Laboratory Report ANL/OEPM-83-6 (1983).

7. J. F. Miller et al., "Testing and Evaluation of Advanced Lead-Acid Batteries for Utility Load-Leveling Applications," in Proc. 18th Intersociety Energy Conversion Eng. Conf., August 21-26, 1983, Vol. 4, pp. 1595-1598 (1983).

8. J. F. Miller et al., "Performance of Advanced Lead-Acid Batteries for Load-Leveling Applications," in Proc. of the Symp. on Advances in Lead- Acid Batteries, eds. K. R. Bullock and D. Pavlov, The Electrochem. Soc, Pennington, NJ, pp. 522-527 (1984).

9. R. Holland, "The Evolution of Stibine from Lead-Acid Batteries," in Proc. Int. Symp. on Batteries, Christchurch, Hants, England, October 21-23, 1958, paper (i) (1958).

10. A. M. Dasoyan, "Gas Liberation in a Lead-Acid Storage Battery," in Stibine Formation and Detection in Lead-Acid Batteries, ed. V. C. Spindler, Electric Pover Research Institute Report EPRI EN-448-SR, pp. 3-1 to 3-3 (1977).

11. Public Service Electric and Gas Co., Battery Energy Storage Test (BEST) Facility First Progress Report, Electric Pover Research Institute Report EPRI EM-1005 (1979).

12. B. F. Dixon and P. R. Kiff, J. Appl. Chem. 8, 631 (1958). Argonne National Laboratory, with facilities in the states of Illinois and Idaho, is owned by the United States government, and operated by The University of Chicago under the provisions of a contract with the Department of Energy.

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, com pleteness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific com mercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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DOE-TIC, for distribution per UC-94ca (222) Manager, Chicago Operations Office, DOE Chemical Technology Division Review Committee Members: S. Baron, Brookhaven National Laboratory, Upton, NY T. A. Milne, Solar Energy Research Institute, Golden, CO L. Newman, Brookhaven National Laboratory, Upton, NY J. B. Wagner, Arizona State University, Tempe, AZ R. Winston, U. of Chicago, Chicago, IL E. B. Yeager, Case Western Reserve University, Cleveland, OH R. G. Wymer, Oak Ridge National Laboratory, Oak Ridge, TN T. L. Brown, U. of Illinois, Urbana, IL K. F. Barber, Div. of Electric and Hybrid Propulsion, USDOE W. Bauer, KW Battery Co., Skokie, IL J. Birk, Electric Power Research Institute, Palo Alto, CA D. P. Boden, C&D Batteries, Plymouth Meeting, PA P. J. Brown, Division of Electric and Hybrid Propulsion, USDOE K. R. Bullock, Johnson Controls, Inc., Milwaukee, WI D. M. Bush, Sandia National Laboratories, Albuquerque, NM E. J. Cairns, Lawrence Berkeley Laboratory, Berkeley, CA S. H. Caulder, Naval Research Laboratory, Washington, DC A. M. Chreitzberg, Exide Corporation, Yardley, PA R. P. Clark, Sandia National Laboratories, Albuquerque, NM J. E. Clifford, Battelle Columbus Laboratory, Columbus, OH R. Corbin, Delco Remy, Anderson, IN W. J. Dippold, Div. of Electric and Hybrid Propulsion, USDOE J. B. Doe, GNB Batteries, Inc., Langhorne, PA D. L. Douglas, Bloomington, MN E. Dowgiallo, Div. of Electric and Hybrid Propulsion, USDOE C. K. Dyer, Bell Communications Research, Murray Hill, NJ J. Hardin, EG&G Idaho, Inc., Idaho Falls, ID G. L. Hunt, EG&G Idaho, Inc., Idaho Falls, ID E. A. Hyman, Public Service Electric and Gas Company, Newark, NJ J. J. Kelley, Exide Corporation, Yardley, PA R. S. Kirk, Div. of Electric and Hybrid Propulsion, USDOE A. R. Landgrebe, Energy Storage and Distribution Division, USDOE G. E. Mayer, Mellon Institute, Pittsburgh, PA F. McLarnon, Lawrence Berkeley Laboratory, Berkeley, CA L. G. O'Connell, Electric Power Research Institute, Palo Alto, CA P. G. Patil, Office of Vehicle & Engine R&D, USDOE 15

A. Pivec, Public Service Electric and Gas Company, Newark, NJ J. E. Quinn, Div. of Energy Utilization Research, USDOE G. Rodriguez, Southern California Edison Company, Rosemead, CA J. J. Rovlette, Jet Propulsion Laboratory, Pasadena, CA W. H. Shafer, Commonvealth Edison Company, Chicago, IL V. Spindler, Electric Pover Research Institute, Palo Alto, CA W. J. Stolte, Bechtel National Inc., San Francisco, CA P. C. Symons, Electrochemical Engineering Consultants, Inc., Palo Alto, CA J. Ssymborski, GNB Batteries, Inc., Langhorne, PA y. H. Tiedemann, Johnson Controls, Inc., Milvaukee, WI G. J. Walker, Office of Vehicle and Engine R&D, USDOE C. E. Weinlein, Johnson Controls, Inc., Milvaukee, WI N. P. Yao, Clarendon Hills, IL

AMMYJNf NAU-NAI I AH VVI

4444 obmm L/19 1,1 1 UU i luii v^a L^i^v*./ * Energy Storage—Electrochemical Near-Term Batteries (UC-94ca)

ANL-87-1

ARGONNE NATIONAL LABORATORY 9700 South Cass Avenue Argonne, Illinois 60439

MEASUREMENT OF STIBINE AND ARSINE GENERATION FROM THE EXIDE 3100-Ah LEAD-ACID MODULE

J. J. Marr and J. A. Smaga

Chemical Technology Division

January 1987

Prepared for the Electric Pover Research Institute, Palo Alto, California, under contract No. RP-2216-2

TABLE OP CONTENTS

Page

ABSTRACT 1

1.0 INTRODUCTION 1 2.0 BZPERIMBNTAL 2 2.1 Gas Collection Apparatus 2 2.2 Saapling Procedure 3 2.3 Cheaical Analyses 4 3.0 RESULTS AND DISCUSSION 4 3.1 Hydride Generation Curves 4 3.2 Characterization of Hydride Generation 6 3.3 Effect of Saall Variations in Operating Paraaeters 10 3.4 Ventilation Requirements for Hydride Reaoval 10 4.0 SUNMART 11

ACmOVLEDGHENTS 12

REFERENCES 13

iii

HEASUREHENT OF STIBINE AND ARSINE GENERATION FROM THE EXIDE 3100-Ah LEAD-ACID MODULE

J. J. Marr and J. A. Smaga

ABSTRACT

Stibine and arsine evolution from lead-acid cells in a 36-kWh Exide load-leveling module vas measured as this module approached 1900 cycles of operation. A specially prepared gas-collection apparatus enabled us to determine the maximum and average rates for evolution of both toxic hydrides. Hydride generation began once the cell voltage exceeded 2.4 V. The maximum rate for arsine occurred just above 2.5 V and consistently preceded the peak rate for stibine for each sampled cell. When adjusted for size effects, the degree of stibine and arsine evolution vas greater than found in a continuous overcharge study con ducted by Exide. The average rates of hydride generation vere found to be 175 yg/min for stibine and 12.6 yg/min for arsine. The former rate proved to be the critical value in determining safe ventilation requirements for cell off-gases. The minimum airflov requirement vas cal culated to be 340 L/min per cell. Projections for a hypothetical 1-MWh Exide battery vithout an abatement system indicated that the normal ventilation capacity in the Battery Energy Storage Test facility provides nearly five times the airflov needed for safe hydride removal.

1.0 INTRODUCTION

Large Exide lead-acid cells (3100-Ah rated capacity) are under develop ment for planned usage in utility load-leveling plants. Because these cells use electrode grids fabricated out of lead alloys that contain antimony and arsenic, toxic hydrides can be generated during part of the recharge cycle. The cumulative generation from arrays of these cells in such a plant could pose a health hazard unless adequate abatement and/or ventilation safeguards are in place. To determine the magnitude of this concern, the Electric Pover Research Institute (EPRI) funded this experimental effort to monitor stibine (SbHj) and arsine (AsHj) evolution from a prototypic module.

Antimony and arsenic are used in grid alloys because they impart certain beneficial properties to the grids. Antimony additives improve both the castability and the strength of electrode grids. While these advantages are obtainable vith other alloying elements, cells vith Pb-Sb grids maintain the ability to vithstand deep discharges over extended operational times. Arse nic, vhich can also be present as an impurity, is intentionally added to improve the corrosion resistance of the grids. Any reduction in grid corro sion should also reduce stibine evolution for reasons made clearer belov. Information on hydride evolution from batteries is rather limited, espe cially for arsine. Simon^ made a comprehensive review of the available liter ature in the late seventies, and, while the ability to measure hydride evolu tion rates has improved,^'^ basic understanding of the generation processes has not advanced significantly. Much of what is known about the electro chemistry involved in stibine generation is the result of work done by Dawson, Gillibrand, and Wilkinson.^'^ During charge, corrosion of the positive grid produces complexed Sb(III) and Sb(V) ions. Some Sb(III) is complexed as cations that migrate and deposit on the negative plate. The Sb(V) anions remain absorbed on the lead dioxide of the positive plate but are released during discharge. Some of the Sb(V) ions diffuse to the negative plate during this period and absorb onto the lead sulfate as it forms. These ions can be reduced to Sb(III) and deposited on the negative plate during the next charge cycle. Stibine generation begins when the cell voltage reaches a threshold value, generally between 2.4 and 2.5 V, and quickly reaches a maximum as the deposited antimony is converted to its hydride. If antimony is present in the negative grid alloy, it can also contribute to hydride evolution. Higher operating temperatures^'^ and higher Sb contents^ in the grid alloy increase the evolution rate.

The six-cell, 36-kWh module selected for these hydride measurements has been the topic of a number of reports.^"® The development and test leading to the fabrication of this module has been documented in a topical report pre pared by Exide.^ Miller et al. have discussed the design and initial perfor mance of this module,^ and they have also published a performance update.® This Exide module was placed under test at the Chemical Technology Division of Argonne National Laboratory in April of 1982 and accumulated over 1880 cycles at 40 to 50°C prior to the hydride measurements. The objective of this work was twofold; (1) to quantify the rates of stibine and arsine generation during a typical duty cycle as an aid in determining ventilation requirements, and (2) to compare this data with test results reported by Exide^ in order to assess the effect of cycle life on hydride generation.

2.0 EXPERIMENTAL

2.1 Gas Collection Apparatus

The hydride collection method developed by Varma and Yao^ was used with minor modifications for these experiments. The gas collection system is shown schematically in Fig. 1. The inlet and outlet connections to a cell were made at diametrically opposed corners at the top of the cell. Two separate gas- collection trains were used to collect alternate gas samples in a manner that allowed continuous sample collection throughout the duration of a charge cycle. Each train consisted of two gas bubblers, each containing 100 mL of absorbing solution (Holland's solution: 8% KI and 1% Ij in 3N HjSO^). The second bubbler prevented the loss of hydrides due to saturation of the solu tion in the first bubbler. Each bubbler is equipped with a gas inlet tube that ends in a porous disc element for efficient dispersion of the electroly sis gas through the absorbing solution. The empty bottle in front of the first bubbler in each train served as a safeguard against accidental back- suction of the absorbing solution into the cell. A peristaltic pump was PURGING EXHAUST GAS FLOWMETER t] r-€M-^ *>—C^ RELIEF >K^XTien VALVE t I N "- — - -" N: -H-O FOR DILUTING EXIT GASES

CARRIER EXCESS GAS /N, VENT -e- -S-On ELECTROLYSIS }i:^:;w:^r ri FLOWMETER H.0 FRITTED-OISC |PERIPERISTALTIS C PUMP (H,S04 + I,+K1) ABSORBING SOLUTION LEAD-ACID K CELL MOISTURE TRAP

Pig. 1. Schematic of Stibine and Arsine Collection System installed on the exit line from the cell to conduct the carrier gas and the electrolysis gases into the gas collection system. The pump vas necessary because the cell lacks hermetic seals around the cover and the electrical connections through the cover. Attempts to seal these areas ^ situ vere unsuccessful. The pump vas set to produce a constant flov rate of 2.0 L/min. The contribution of electrolysis gases to this flov rate vas calculated to vary from 0.0 to 1.77 L/min under the standard charging conditions used for this module. The N, carrier gas vas used to offset this variable difference in a flov rate. Surplus N^ from the supply line vas vented through an added outlet in the bottle that precedes the cell in the carrier gas stream. This design prevented the cell from being either underpressurized or overpressur- ized. The exit gas lines from the tvo collection trains vere vented through an electrically grounded, 0.48-cm-ID copper tube leading to the top of the fume hood. A second N^ gas line vith a flov rate of 5.0 L/min vas also vented through the copper tube to dilute the explosive gaseous mixture and ensure that the maximum Hj concentration did not exceed hazardous levels. The diluting gas also served to increase the exit velocity of the gases, thereby preventing a flashback should a detonation occur.

2.2 Sampling Procedure

Samples vere collected from three cells (Cells 1, 3, and 5) in the six- cell module. Bach cell vas sampled during a different cycle, but the charging regime vas the same for each of these cycles. Initially, the module vas