Journal of New Materials for Electrochemical Systems 7, 257-268 (2004)
c J. New. Mat. Electrochem. Systems
Plating of High Quality Electrolytic Manganese Dioxide at 120 - 125 Degrees C. and 6X
Normal Current Density
Stuart M. Davis
The Gillette Company
Gillette Advanced Technology Center - US, 37 A Street, Needham, MA, 02492, USA
( Received November 27, 2003 ; received in revised form August 27, 2004 )
Abstract: A pressurized cell was constructed to plate EMD from a MnSO4 -H2 SO4 bath at temperatures up to 150 deg. C and 5 atm. gauge
pressure. The cell was designed to plate 1.5 Kg. of EMD on a Ti anode in 10 days at a CD of 6.25 A.ft−2 . Construction details for the 16 l cell are
shown. More than 30 trials were completed at temperatures of 120 - 125 deg. C and reaching current densities as high as 37.5 A.ft−2 . Variables
investigated were: Bath composition (MnSO4 , H2 SO4 ), temperature, current density, Ti doping and grinding methods. Two exceptional, high power
EMD samples were prepared under different plating conditions. The possibility of producing excellent quality EMD at current densities 4X as high
as in conventional (unpressurized) baths was demonstrated. Good quality EMD was even plated at 6X normal current density (37.5 A.ft−2 ) with
no evidence of Ti passivation. Some conclusions of this work are: EMD plated on Ti anodes at elevated temperature tends to show a higher level
of "Ruetschi protons", higher SO=
4 , higher x in MnOx and higher OCV than conventional EMD. Since the throughput of a 120 deg. C pressurized
cell can be up to 6X that of a conventional cell (probably greater at even higher temperatures) there could be capital and operational cost savings
if practical commercial sized pressure cells can be developed.
Key words :
1.
INTRODUCTION
and E. Preisler, "Problems Involved in the Technical Preparation.of Top Quality Electrolytic Manganese Dioxide" [3].
Electrolytic Manganese Dioxide (EMD) has been utilized as a
battery active material since commercial production was begun
in Washizu, Japan by Tokyo Shibaura Electric Co. in 1944 [1].
Current world production is around 250,000 metric tons / year.
The greater portion of this is consumed in production of primary
Zn-carbon, alkaline Zn-MnO2 and Li-MnO2 batteries. Smaller
quantities are employed as catalysts (e.g. Zn-air alkaline batteries, O3 destruction), as an oxidant in chemical syntheses, in soft
ferrite production and in H2 O purification.
Commercial EMD is plated from a mixed MnSO4 / H2 SO4 bath
at elevated temperature according to the following reactions:
1) Anodic Reaction Mn++ + 2H2 O –> MnO2 + 4H+ + 2e−
2) Cathodic Reaction 2H+ + 2e− –> H2
3) Overall Reaction: Mn++ + 2H2 O –> MnO2 + H2 + 2H+
Initially graphite or Pb anodes were employed with graphite, Pb
or stainless steel cathodes. Today, particularly for production of
the highest quality EMD, Ti anodes are used along with graphite
or Cu cathodes. Each producer employs unique technology and
operating conditions. Typical conditions for commercial EMD
plating are shown in Table 1.
Excellent reviews of EMD science and manufacturing technology are found in the articles by T. Andersen, "Effect of Some
EMD Structural Features on Alkaline Discharge Capacity" [2],
∗ To
whom correspondence should be addressed:
257
258
Stuart M. Davis et al./ J. New Mat. Electrochem. Systems 7, 257-268 (2004)
Table 1: Typical Conditions for Commercial EMD Plating
Temperature
(Deg. C)
MnSO4
H2 SO4
(M.l−1 )
(M.l−1 )
{g Mn.l−1 } {g H2 SO4 .l−1 }
Ordinary Alkaline Grade EMD
94 - 98
0.5 - 1.2
0.4 - 0.8
{28 - 66}
{39 - 79}
"High Power" Alkaline EMD
95 - 98
0.09 - 0.55
0.2 - 0.6
Kerr McGee US Pat. 6,527,941
{5.0 - 30}
{20 - 60}
Re: US Pat 6,527,941, an additional condition is: 2 < ( [H2 SO4 ] / [MnSO4 ]) < 4
where [ ] indicates concentration.
It has been recognized for some time that the highest quality
EMD (defined here as EMD which shows the best alkaline battery performance on heavy drains), can be produced only at the
highest possible plating temperatures. Since the usual MnSO4 /
H2 SO4 bath compositions boil at 103 deg. C and since commercial EMD plating cells are unpressurized, operation has been
limited to temperatures below the electrolyte boiling point, as
shown in Table 1.
R. Williams et al conducted a series of EMD plating trials in an
open (unpressurized) cell at three temperatures (90, 95 and 100
deg. C), approaching the b.p. of commercial electrolyte [4]. A
40 l cell with Ti anode was employed. In addition to studying
the effects of temperature, bath composition and current density
(CD) were also investigated. Focusing attention on the temperature and CD effects only, one may conclude from William’s
study that:
a) BET area, total pore volume, % combined H2 O and Mn4+
cation vacancies all decreased with increasing temperature and increased with increasing CD.
b) % Mn4+ (i.e. average Mn oxidation state) increased with
increasing temperature and decreased slightly with increasing CD.
c) Principal XRD peaks were shifted to higher angles (smaller
d spacing) with increasing temperature.
d) When operating at higher CD, in order to maintain the
desired "epsilon like structure" as opposed to "gamma"
structure, higher temperature is required.
William’s team presented no data for mAh capacity or discharge
performance of their EMD samples in practical galvanic cells.
M. Mauthoor et. al were first to publish on EMD deposition in
a pressurized cell, operating above the normal boiling point of
conventional bath compositions [5]. A 10 l, Teflon lined cell
with Ti anode and Ag cathode was employed. Temperatures of
90 to 108 deg. C were investigated at a constant bath composition of 0.62 M.l−1 MnSO4 , 0.33 M.l−1 H2 SO4 and a constant
CD of 6.0 A.ft−2 {= 65 A.m−2 }. Mauthoor’s team concluded:
Current Density
(A.ft−2 )
{A.m−2 }
5.5 - 8.0
{59 - 86}
2.5 - 6.0
{27 - 65}
a) BET area declined as deposition pressure increased (presumably as deposition temperature increased since the
two were co-variant in these trials).
b) % MnO2 and mAh capacity increased as temperature increased to 105 deg. C, then declined at 108 deg. C. (mAh
capacity was measured by forced discharge in a flooded
cell with KOH electrolyte.) The maximum mAh capacity
(180 mAh/g) was obtained for a deposition temperature
of 104 deg. C
c) "Q" ratio (22 deg. XRD peak / 37 XRD deg. peak, Cu
Kα ) increased regularly from 0.6 to 1.0 over the interval
96 to 108 deg. C.
The most recent paper relating to EMD deposition in pressurized cells, of which we are aware, is that of L. Hill et al who
investigated EMD plating in the temperature range 80 deg. C to
180 deg. C [6]. A small cell plating 500-600 mg of EMD on a
Au anode was employed.
Electrolytes contained more Mn++ than normal, i.e. 1.0 M.l−1
or 3.0M.l−1 MnSO4 and less acid than normal, 0.1M.l−1 H2 SO4 .
Two CDs were employed: 0.35 mA/cm2 {= 0.33 A.ft−2 or
3.5 A.m−2 } and 1.75 mA.cm−2 {=1.62 A.ft−2 or 17.5 A.m−2 }.
These CDs fall well below the usual range for commercial EMD
production, as presented in Table 1. In some trials a mixed
MnSO4 / Li2 SO4 electrolyte was employed. Hill’s team concluded:
a) The hydrothermal method (i.e. a pressurized cell) can
produce α , β or γ MnO2 , depending upon conditions.
b)
MnO2 can be prepared without K+ and this material
shows good cyclability (in Li cells)
α
c) When γ MnO2 was produced it was more crystalline than
normal.
d) When cycled in flooded cells against a Li anode, the γ
MnO2 intercalated 0.67 Li/Mn and the α MnO2 intercalated 0.52 Li/Mn
Plating of High Quality Electrolytic Manganese Dioxide at 120 - 125 Degrees C. . / J. New Mat. Electrochem. Systems 7, 257-268 (2004)
259
No test results for EMD samples discharged in alkaline electrolytes were reported.
Based on the general observation that highest quality (alkaline
grade) EMD has been produced only at the highest deposition
temperatures and also in view of the encouraging results presented in Mauthoor’s paper, we decided to undertake a more
comprehensive study of EMD deposition in a pressurized cell at
higher than normal temperatures and to determine the alkaline
discharge performance of EMD samples produced under these
conditions.
2.
EXPERIMENTAL
A 16 liter cell was constructed of Teflon lined pipe spools and
flanges as shown schematically in Fig. 1 and in photographs in
Figs. 2 and 3.
Figure 3: Cell Cover Assembly Showing Kel-F Cover, 2
Graphite Cathodes, 1 Ti Anode, 2 PFA Thermowells and PTFE
Liner in Main Pipe Spool.
Figure 1: Gillette’s 16 l Pressurized EMD Electrolysis Cell Schematic
The main vessel consisted of a Resistoflex PTFE lined 10" steel
pipe spool with PTFE faced bottom flange assembly. The top
assembly consisted of a 2" thick Kel-F cover with 12 drilled
and tapped feed-throughs, backed by a matching steel flange.
Total internal volume was 16.1 l and the working volume, with
the liquid maintained 2" below the cover, was 11.6 l.
The vessel was designed to operate up to 5 atm. gauge pressure
and 150 deg. C.
Heating was accomplished by two electric heating tapes
(1.3 KW, 0.3 KW) which were coiled around the exterior of
the pipe spool. The 1.3 KW tape was controlled by a Type T
Cu-Constantan thermocouple positioned on the OUTER wall
of the pipe spool and a Digisense temperature controller (Cole
Parmer, model A890000-05) set to maintain a temperature a few
degrees below the desired operating temperature of the cell. By
positioning the thermocouple close to the heater tape, a quick
response was assured and a very steady temperature could be
maintained on the exterior surface of the vessel, providing a sort
of "virtual insulation".
Figure 2: 16 l Pressurized Cell, Support Structure, Heating
Jacket, Power Supplies and Instrumentation.
The 0.3 KW tape was controlled by a thermocouple positioned
in a PFA thermowell, in the bath, next to one of the graphite
cathodes. This smaller heater provided the final regulation of
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Stuart M. Davis et al./ J. New Mat. Electrochem. Systems 7, 257-268 (2004)
the bath at the desired set-point. Although the thermal response
was slow, the large mass of the vessel and its contents and the
small size of the heater assured a steady temperature, normally
within +/- 0.5 degrees of the set-point.
A 2nd identical thermocouple was positioned near the other
cathode to provide an independent readout of the temperature.
A bimetallic dial thermometer stem was also positioned nearby
in a 3rd PFA thermo-well for an additional readout of the temperature. A small amount of mineral oil was included in each
thermo-well to improve thermal conductivity.
The cell was fitted with one electrolyte inlet tube (1/4" PFA
heavy-wall tubing) and two, electrolyte / gas outlet tubes (1/4"
PFA). The inlet tube terminated at a level coincident with the
bottom edge of the cathodes. The two outlet tubes terminated
about 2" below the cover, at the desired liquid level. The outlet tubes allowed excess electrolyte to exit when the liquid level
was closer than 2" to the cover and also allowed excess gas pressure (H2 and steam) to be relieved, when the pressure rose above
the desired set-point.
Pressure relief valves (Griffco 1/4" PTFE body, PTFE coated
Hypalon diaphragm) were attached to each outlet tube and these
could be set to the desired operating pressure by means of a
screw adjustment. Cracking pressure was set about 20 psig
above the equilibrium vapor pressure of the electrolyte composition (at the cell operating temperature). After passing through
the relief valves, the exit lines were directed to a liquid / gas separator column consisting of a 6" OD CPVC pipe packed with 1"
diameter PP balls. This allowed the H2 gas to exit from a small
hole in the top of the column while spent electrolyte could drain
from a small hole in the bottom to a receiver, where the accumulated volume was noted.
Fresh electrolyte was fed by a FMI positive displacement piston
pump with PTFE cylinder and ceramic piston, rated at 100 psi
maximum outlet pressure. A third Griffco valve, functioning as
a check-valve, was positioned between the pump and the pressurized cell to prevent any possible back-flow, in case of a catastrophic failure of the pump (this never occurred). This check
valve was set about 20 psi above the cracking pressure of the
two relief valves on the outlet tubes.
The Griffco valves exhibit as much as 15 psi hysteresis, hence
a differential of 20 psi in cracking pressure assures that no unwanted overlap in the pressure ranges will occur. This is especially important to avoid a condition where the vapor pressure
of the electrolyte might temporarily exceed the closing pressure
of an outflow check valve, which could result in boiling away a
substantial quantity of electrolyte.
Pressure was monitored by a 0 - 60 psig strain gauge pressure
transmitter (Cole-Parmer model # A-07356-02) and a 0 - 60 psig
SS Bourdon type dial gauge (Cole Parmer model # A68022-03).
Both of these were isolated from the working fluid by a Gauge
Protector with Kel-F body and Teflon isolation diaphragm (Cole
Parmer model # A-07359-08).
At the bottom of the vessel there were two PTFE lined reducing
flanges, to effect a diameter reduction from 10" pipe to 1" pipe,
connecting to a 1" PTFE coated graphite rupture disc, (Zook Enterprises, model # 1DG2X1X85X70). The disc was calibrated
at 85 psig for room temperature (the actual relief pressure would
be slightly lower at the operating temperature of the cell.). The
rupture disc flange was fitted with a short length of 3/8" OD
copper tubing which descended into a receiving tank half filled
with about 60 liters of water. A preliminary test of the system
in which a deliberate overpressure condition was created in the
cell showed that venting occurred gently and reliably at the calibration pressure.
Ti anodes were purchased from Timetal, grade 50A Ti plate,
5.5" X 7.0" X 1/4" = 83.25 in2 . (Exception: Shakedown trial
ER-1 was conducted with miniature electrodes. In this trial the
anode was Timetal 50A Ti plate 1.75" x 3.5" X 1/4" = 14.9 in2 .)
Anodes were sandblasted before initial use and periodically
throughout the course of this study, but not after every plating
trial.
Graphite cathodes were from SGL Carbon, 6.0" x 8.0" x 1/2".
(Exception: Shakedown trial ER-1 was conducted with miniature cathodes, SGL Carbon, 2.0" x 4.0" x 3/8".) Cathodes were
rinsed between trials. There was no other cleaning or surface
preparation required, as very little surface discoloration or filming was observed throughout the trials.
Both the anode and two cathodes were suspended on 1/4" dia. Ti
hanger rods, threaded at the bottom for apx, 1/2" (10-24 straight
machine thread) to match a threaded hole in the top edge of
Ti or graphite electrodes. The hanger rods were sheathed in
3/8" OD PFA tubing, with just the threaded end portion exposed. The rods were screwed into the respective electrodes
so that the end of PFA sheath was tightly pressed against the top
edge of the electrode. This joint was then covered by a layer of
fluoro-silicone rubber which was cured in boiling H2 O, prior to
mounting the electrodes in the cell. Following each plating trial
the joint on the Ti anode was disassembled, cleaned and then
reassembled with a fresh application of fluoro-silicone rubber.
(The cathodes were left undisturbed.)
Extra thermocouples were positioned at critical points on the
cell exterior to furnish supplementary temperature readings; e.g.
contacting the underside of the Kynar cover, the outer wall of
the pipe spool and the flange holding the rupture disc. The entire
cell was insulated with multiple wraps of woven glass tape and
this was further enclosed with a glass wool insulating blanket,
except for the top cover flange which was left bare.
Plating of High Quality Electrolytic Manganese Dioxide at 120 - 125 Degrees C. . / J. New Mat. Electrochem. Systems 7, 257-268 (2004)
Temperature, pressure and operating Voltage were recorded continuously by a YEW (Yokogawa) strip chart recorder,
model µR100.
261
ple, so the experimental procedure is described in detail. "ER"
stands for "extended range")
Target conditions for this trial were as follows:
Electrolyte solutions were prepared with reagent MnSO4 ·H2 O
(Spectrum, ACS, M1115), reagent H2 SO4 (Fisher, ACS, A300212) and deionized H2 O. Final pH was adjusted, as needed, by
small additions of reagent MnCO3 (Spectrum, ACS, M1100)
or H2 SO4 . After pH adjustment the solutions were clarified by
addition of small quantities of reagent 30% H2 O2 (Alfa Aesar,
ACS, stock # 33323).
With the exception of the very first trial, where miniature electrodes were employed in an excess of static electrolyte, the cell
was run with a constant feed of fresh electrolyte to replenish the
Mn++ plated from solution and to eliminate the excess H2 SO4
generated, according to reaction 3) above. This is the same procedure practiced for industrial plating of EMD. Assuming that
pump speed and CD are well controlled, and that plating efficiency is around 100%, the composition of the plating bath will
be invariant.
The cell itself was charged with the particular electrolyte composition chosen for the trial, e.g. 0.79M MnSO4 + 0.5 M H2 SO4 .
Based on the desired CD, the desired Mn++ stripping ratio (that
is, the fraction of dissolved Mn plated out as MnO2 on each pass
of the electrolyte through the cell, e.g. 33%) and an imposed
practical lower limit of 0.05M H2 SO4 in the feed solution (to
avoid Mn(OH)2 precipitation) the starting MnSO4 concentration in the feed solution and the required pumping speed were
calculated with the help of an EXCEL spreadsheet.
Current was supplied by a 50 A / 6V Kepco constant current DC
supply, model ATE6-50M. (Exception: Trials with miniature
electrodes employed a similar but smaller Kepco DC supply for
more precise regulation of current.) Our longest plating trial
was 257 hours and the current variation was observed to be less
than +/- 1%.
At the conclusion of a trial, the spent electrolyte was regenerated by reacting with MnCO3 to neutralize excess H2 SO4 and
to increase the MnSO4 content to its original or desired value.
Composition was checked by measuring pH (Fisher Accumet
pH meter, model 15), density (hydrometer) and conductivity
(GLI electrodeless conductivity meter, model 23). Any two of
the 3 variables are sufficient to define a unique composition of
MnSO4 -H2 SO4 -H2 O. We found that pH was the least reliable
of the 3 measurements, hence the final adjustment was made
via density and conductivity, using pH as a confirming check.
Trial ER-1: (The intent of this trial was to check the functioning of the electrolysis cell using miniature electrodes operating
in an excess of static electrolyte, hence a nearly constant composition. This would avoid the need to pump fresh cell electrolyte,
on the initial run. This trial led to a particularly interesting sam-
Temp.
120 C
Curr. Density
6.19 A.ft−2
[H2 SO4 ] [MnSO4 ]
0.63 M
0.88 M
(avg)
(avg)
R: 14.88 in2 anode, static electrolyte
Other
R
The cell was heated from ambient to the operating temperature
over a period of about 5 hours, during which time no current
was passed and the cell Voltage was not monitored. Upon reaching 120 deg. C, the power supply was connected and a current
of 0.64A was applied (6.19A.ft−2 ). It was immediately noted
that the cell exhibited reverse polarity with the Ti anode running negative to the graphite cathode. This situation persisted
for 1 or 2 minutes until we realized that, most likely, the Ti was
acting as a galvanically active anode, discharging occluded H2
or TiH2 resulting from corrosion experienced during the heatup phase. (Subsequent analysis of this EMD sample showed >
2,000 ppm Ti in the product, confirming that Ti corrosion had
indeed occurred.)
We increased the current from 0.64 to 1.5A and immediately
normal cell polarity was restored, the Ti anode now running positive to the cathode. Current was reduced to 0.64A and normal
polarity was maintained for the remainder of the trial.
Plating was continued for 257 hrs. during which time the average cell Voltage was 1.94V. Temperature was maintained at
120.0 +/- 0.25 deg. C. At the end of the trial the EMD plate
was removed from the anode by impact and ground for about 2
minutes in a SPEX "Shatterbox" between a hardened steel ring
and puck. The ground "chip" was screened through a 120 mesh
(125 micro-meter) SS screen and the oversize material returned
to the "Shatterbox". This procedure was repeated as many times
as necessary to convert the majority of chip to powder.
The ground chip was then washed several times with DI H2 O
until a pH of 2.0 was attained. The powder was suspended in
H2 O with vigorous stirring and 5% NaOH solution was then
added slowly and incrementally to raise the pH of the slurry
to 6.2 (target pH range for all samples in this study was 5.5
to 6.5). After assuring that a stable pH had been reached, the
neutralized EMD powder was separated by decantation, spread
out in a glass tray (1/2" layer) and air dried at 60 deg. C for 16
hrs.
A similar procedure was followed for all the remaining samples prepared in this study with the exception that most of these
were plated on full size electrodes with constantly refreshed
electrolyte. Drying times were somewhat longer for the larger
batches. Where conditions or procedures are materially different than those cited above for ER-1, these will be noted by exception.
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Stuart M. Davis et al./ J. New Mat. Electrochem. Systems 7, 257-268 (2004)
Trials ER-2 and ER-3
These were the first 2 trials with pumped electrolyte and full size
electrodes. Electrode dimensions were: Ti anode: 5.5" x 7.0"
x 1/4" = 83.25 in2 (0.578 ft2 or 0.0537 m2 ), Graphite cathodes:
6.0" x 8.0" x 1/2" = 110 in2 (0.764 ft2 or 0.0710 m2 ).
These trials failed due to shorting because of difficulty in maintaining parallel electrode alignment. To remedy the problem we
drilled and threaded two holes at the bottom corners of the Ti
anode and fitted these with PP screws extending apx. 1" on both
sides of the anode. These insulating "standoffs" provided positive positioning for all 3 electrodes and the shorting problem
was solved.
Trials ER-4 and ER-6 to ER-9
Five trials were run at 120 deg. C and progressively increasing
current densities in a bath of constant composition. Particular
conditions were as follows:
Temp.
C
120
Curr. Densities. [H2 SO4 ] [MnSO4 ] Other
A.ft−2
M
M
6.25, 12.5,
0.5
0.9
R
18.75, 25.0,
37.5
R: 83.25 in2 anode constantly refreshed electrolyte
As the cell was heated to operating temperature, we applied a
polarizing current of 50 mA (Ti positive to Graphite) to maintain the Ti anode in a passivated condition. Once at temperature, the current was increased incrementally, doubling once
/ minute, until the final target value was achieved. With this
procedure there was no indication of Ti corrosion, as evidenced
by normal polarity on startup and low residual Ti in the product. The polarization procedure was employed in all subsequent
plating trials.
Plating was continued for 236 hrs. at 6.25 A.ft−2 (ER-5) and for
proportionately shorter periods of time as the CD was increased,
so as to plate a more or less constant quantity of EMD (apx.
1.5 Kg). In these trials the feed solution was 1.18M MnSO4 and
0.05M H2 SO4 , calculated to give the bath composition shown
above, based on indicated CD and pumping speed. (Pumping
speed was adjusted in proportion to the CD, e.g. 155 ml.hr−1
at 6.25 A.ft−2 , 310 ml.hr−1 at 12.50 A.ft−2 etc.)
The chip was finished as described for ER-1 except that grinding of the larger mass of chip was done in a rubber lined steel
mill jar with 2" diameter high density Al2 O3 media. This grinding method was pursued for trials ER-5 to ER-14. Later on,
we discovered that the rubber liner was causing some chemical
reduction of the EMD and we abandoned the mill jar in favor
of multiple batch grindings in the "Shatterbox" for trials ER-15
and beyond.
All samples were subjected to physical, chemical and electrochemical analyses. Selected samples were also evaluated for
electrochemical performance in AA alkaline MnO2 cells, constructed according to current Duracell designs. Controls for cell
performance evaluation were built with a 50/50 blend of commercial "high power" EMD and standard alkaline grade EMD.
Trial ER-5
In a private communication, R. Williams of Delta EMD had suggested to us that EMD plate deposited on the electrode edges
might exhibit properties different than that deposited on the electrode faces. While not a concern for commercial production, it
might seriously affect laboratory trials with small electrodes,
where the edges accounted for a significant fraction of the total
surface. We decided to investigate this possibility.
Trial ER-5 was a repeat of ER-4 (6.25 A.ft−2 ) except that we
separated the chip recovered from the edges of the anode and
that recovered from the faces. The two portions were finished
separately. Unfortunately, the question of edges vs. faces became confounded by two different grinding procedures- the larger
quantity of face material having been ground in a rubber lined
mill jar and the smaller quantity of edge material in the Shatterbox. Concerning the porosimetry of the two samples, no difference could be seen. But OCV for the jar mill ground face material was significantly lower than that for the Shatterbox ground
edges.
Ultimately we came to recognize that the rubber lining of the
mill was reducing the EMD and we abandoned the use of the
mill jar, starting with trial ER-15, in favor of the Shatterbox.
Trials ER-10 to ER15
As mentioned above, sample ER-1 had shown unusual physical,
chemical and electrochemical properties. We were particularly
impressed by its heavy drain performance in AA cells, which
significantly exceeded that of a 50/50 blend of standard EMD
with "high power" EMD. Trials ER-5 to ER-9 yielded several
samples showing cell performance equivalent to the blend, but
none equal to ER-1.
The next 17 trials were aimed at reproducing the superior high
power performance of ER-1.
Recognizing that ER-1 had a high Ti content, trials ER-10 to
ER-15 were carried out with deliberate Ti doping of the electrolyte. Various levels (0.304 to 0.82 g.l−1 ) of TiOSO4 ·2H2 SO4
(Alfa Aesar, stock # 39591) were added to the cell electrolyte
and feed solution.
Trial ER-10 with 0.82 g.l−1 of TiOSO4 ·2H2 SO4 in the feed solution failed, due to plugging of the inlet line, presumably by
precipitated TiO2 . (Laboratory trials later disclosed that
263
Plating of High Quality Electrolytic Manganese Dioxide at 120 - 125 Degrees C. . / J. New Mat. Electrochem. Systems 7, 257-268 (2004)
TiOSO4 ·2H2 SO4 exhibits a negative temperature coefficient of
solubility in our cell electrolyte. Upon entering the cell and
warming, there is a danger of precipitation in the feed tube,
as the solution descends towards the bottom of the cell.) The
remaining 5 trials (0.304 to 0.405 g.l−1 TiOSO4 ·2H2 SO4 ) ran
satisfactorily and produced EMD with elevated levels of Ti, as
intended.
Key operating parameters for these trials were as follows:
Trial
Temp.
(deg. C)
Curr.
Dens.
(A.ft−2 )
[H2 SO4 ]
(M)
ER-11
120
6.25
0.49
ER-12
121
12.5
0.54
ER-13
121
18.75
0.48
ER-14
120
12.5
0.53
ER-15
120
6.25
0.48
R: 83.25 in2 anode refreshed electrolyte
[MnSO4 ]
(M)
TiOSO4
.2H2 SO4
(g.l−1 )
Other
0.73
0.68
0.75
0.70
0.75
0.304
0.304
0.304
0.405
0.405
R
R
R
R
R
Physical, chemical and electrochemical properties of these samples failed to match those of ER-1 and we concluded that Ti
doping probably was not the main reason for the exceptional
performance of ER-1.
Trials ER-16 and ER-17
If Ti doping were not the key to duplicating ER-1, then perhaps the use of miniature electrodes in a static bath might be
involved? Trials ER-16 and ER-17 sought to reproduce the
original conditions of ER-1, including the prolonged heat-up
time with no applied passivation current (see description of ER1 conditions above).
In trial ER-16 we observed no reversal of potential during startup and the resulting EMD product was unremarkable.
In trial ER-17 we deliberately applied a reverse potential (anode
negative) of 1.0 V to the electrodes as the cell was heated from
30 deg. C to 120 deg C over a period of 4.3 hours. The initial
corrosion current was 8.5 mA, but this declined to 1 mA after
1 hr. and remained more or less at this level for the remainder
of heat-up. Thus, 10 mAh of corrosion charge was passed. As
usual, the electrolysis began with a current of 50 mA which was
maintained for 2.5 minutes after which we began to double the
current, once/min. A reverse Voltage was noted up to the first
doubling of the current at 2.5 min., i.e. up to the passage of
2mAh.
At the conclusion of trial ER-17 we allowed the cell to cool
undisturbed, overnight. The cell was opened and the cover slowly
raised to avoid disturbing the solution. A plastic tube was inserted and samples of the electrolyte were drawn from both the
bottom and top of the cell.
Chemical analysis as well as paired measurements of density
and pH disclosed an unexpectedly high concentration of MnSO4
at the bottom of the cell (below the electrodes) of 2.25 M. At
the top of the cell, MnSO4 was measured as 0.83M. The average
MnSO4 level for this trial is 0.88M, based on a controlled starting composition of 1.0M and a final calculated value of 0.76M
(from Faraday’s laws). Chemical analyses for H2 SO4 were inconclusive, due to the presence of concentrated MnSO4 .
The rupture disc at the very bottom of the cell normally runs
"cold" ( 45 deg. C). We speculated that a thermal gradient had
been established along the vertical axis of the cell. This, combined with the relatively quiescent electrolyte, due to the small
electrodes, large volume of solution and lack of vigorous H2
bubbling, might have provoked a thermal diffusion effect (Soret
effect) which materially disturbed the electrolyte composition
in the vicinity of the electrodes (these were positioned at mid
height). The unexpected bath composition confounded with the
effects of extra Ti doping, appeared to us the most likely explanation for the exceptional properties of ER-1 and the difficulties
we had encountered in reproducing this sample.
If this explanation were correct, it made no sense to continue
trials aimed at duplicating the exact conditions of ER-1, but to
proceed with a broader investigation of the combined effects of
CD, temperature and bath composition under better controlled
conditions, i.e. with larger electrodes, vigorous H2 bubbling
and constantly refreshed electrolyte.
Trials ER-18 to ER-26
Trial
Temp.
Curr. Density [H2 SO4 ]
(deg. C)
(A.ft−2 )
(M)
ER-18
120
9.38
0.30
ER-19
120
18.77
0.30
ER-20
120
18.77
0.31
ER-21
120
31.72
0.30
ER-22
120
18.75
0.47
ER-23
120
18.75
0.77
ER-24
120
18.75
1.22
ER-25
120
18.75
1.0
ER-26
121
18.75
0.86
R: 83.25 in2 anode refreshed electrolyte
X: Bad anode connection. Stop trial at 20 hrs.
[MnSO4 ]
(M)
0.19
0.19
1.20
0.19
0.81
0.77
0.57
0.79
0.93
Other
R
R
R
R
R
R
R
X
R
This series of trials produced varying results, but none of the
samples approached the properties or performance of ER-1.
With regard to the problems encountered in trial ER-25, upon
disassembly of the cell it was seen that the threaded connection
between the Ti hanger rod and the Ti anode had corroded (black
corrosion products) and the threads no longer made a tight connection. This, despite the overlying layer of fluorosilicone rubber. To increase the reliability of this connection we adopted the
following improvements for future trials:
1. A 0.1" x 1.0" x 0.002" piece of Au foil was wrapped
around the threads of the hanger rod, before screwing it
into the Ti anode.
264
Stuart M. Davis et al./ J. New Mat. Electrochem. Systems 7, 257-268 (2004)
2. A Ti jamb nut, previously threaded onto the hanger rod,
was screwed down tightly against the Ti anode, increasing
pressure of the threads against the mating threads of the
anode.
As before, a layer of fluorosilicone rubber was applied over the
jamb nut and the joint.
Results for ER-1 and E-27 were particularly interesting and
these are presented in more detail in Table 4 along with typical values for commercial EMD.
An SEM photo of ER-1 chip (prior to grinding) is presented in
Fig. 4. The X-ray powder diffraction pattern for ER-1 (compared to commercial EMD) is shown in Fig. 5.
Trial ER-27
Trial
Temp.
Curr. Density
[H2 SO4 ]
ER-27
120
9.38
1.04
R: 83.25 in2 anode refreshed electrolyte
[MnSO4 ]
0.75
Other
R
This trial returned to more modest conditions of CD and MnSO4
concentration but with higher acid than usual in an attempt to
increase Mn4+ vacancies (sites occupied by the "Ruetschi protons" thought to be responsible for high OCV and high rate capability) while maintaining a reasonable BET surface area. It
will be seen in the RESULTS section, Table 2, that sample ER27 closely approximates ER-1 in terms of physical / chemical
properties and discharge performance.
Figure 4: SEM Image of ER-1 Chip Sample
3.
RESULTS AND DISCUSSION
The following properties were measured or calculated in accord with our normal characterization protocol: Electrochemical plating efficiency, yield of finished powder from chip, meq
of Na/g. EMD required to neutralize, pH, x in MnOx , %Mn,
%MnO2 , %MnOOH, H2 O from MnOOH, H2 O 110C, H2 O 400C,
"Ruetschi H2 O" = (H2 O 400C - H2 O MnOOH), OCV vs. Zn
in KOH electrolyte, Stepped Potential Electrochemical Spectroscopy Coefficient of Performance (SPECS Coefficient) [7],
Density in He, BET area, Micropore Area, Desorption Pore Volume, Micropore Volume, Modal Pore Diameter, SO=
4 , Al, Na,
Ti, AA cell performance. (There were occasional omissions for
some parameters or samples.)
Spot checks were done on certain samples for the following: Q
ratio by XRD, Acid Insolubles, Ca, Cr, Co, Cu, Fe, Mo, Ni, K.
Due to the sheer volume of data, it is not practical to present
all of the results. Nor is it judged necessary as in some cases
the data are redundant (e.g. "Ruetschi H2 O" = 400 C H2 O less
H2 O coming from MnOOH). The following key properties are
presented in Table 2 for every sample: x in MnOx , Ruetschi
H2 O, SO=
4 , Ti, BET area, total pore volume, OCV, and mAh
Capacity to 0.9V in AA cells (where available).
For the remaining properties, Table 3 gives the average value for
each item based on a majority of the 120 deg. C trials. In calculating average values, trials ER-1 and ER-11 to 15 are excluded
in order to avoid skewing results on Ti levels or other properties
which might be affected by deliberate Ti doping.
Figure 5: XRD Pattern for Sample ER-1 Compared to Commercial EMD
Battery discharge curves for ER-1 and commercial EMD are
shown in Fig. 6.
Battery performance data for ER-1, ER-27 and Commercial
EMD controls are summarized in Table 5. It is seen that ER1 and ER-27 exhibit similar performance advantages compared
to their respective controls. Differences in performance for the
two control groups constructed with commercial EMD are attributed to small changes in materials, components and tooling (the two series of tests were performed about 20 months
apart) and possible variations in discharge temperature within
Plating of High Quality Electrolytic Manganese Dioxide at 120 - 125 Degrees C. . / J. New Mat. Electrochem. Systems 7, 257-268 (2004)
Table 2: Selected Properties for 24 EMD Samples Plated at 120 Deg. C
Sample
ER-#
x in
MnOx
Ruetschi
H2 O
(%)
SO=
4
(%)
Ti
(ppm)
BET
Area
(m2. g−1 )
Total
Pore
Volume
(cm3 .g−1 )
0.0372
0.0569
0.0337
0.0563
0.0499
0.0507
0.0419
0.0515
0.0511
0.0605
0.0498
0.0490
0.0394
0.0321
0.0279
0.0333
0.0470
0.0404
0.1260
0.0666
0.0399
0.0362
0.0398
0.0383
0.0380
0.030.05
OCV vs.
Zn 9N
KOH (V)
Capacity
to 1.0V at
1A cont.
(mAh)
1.031
278
NA
NA
670
594
567
558
NA
587
750
687
NA
NA
NA
NA
NA
NA
NA
NA
634
683
657
684
813
680890
1
1.975
3.236
1.58
2160
26.7
1.691
4
1.963
2.15
1.45
33.3
20.21
1.532
5 edge
NA
NA
NA
NA
18.3
1.609
5 face
NA
NA
NA
NA
21.0
1.558
6
1.977
2.864
1.48
206
30.7
1.613
7
1.961
3.311
1.72
<180
32.2
1.638
8
1.972
2.655
1.65
124
36.7
1.627
9
NA
NA
1.66
62
47.4
1.622
10
1.979
3.020
1.79
1750
28.5
1.670
11
1.959
2.673
1.28
1100
23.6
1.598
12
1.967
3.508
1.30
770
39.9
1.650
13
1.974
3.412
1.70
1240
52.8
1.650
14
1.958
3.127
1.44
1360
36.0
1.660
15
1.963
2.516
1.39
460
20.2
1.605
16
1.963
2.301
NA
NA
16.8
1.577
17
1.970
2.282
NA
NA
17.6
1.590
18
1.969
2.617
1.21
<32
10.6
1.668
19
1.971
3.537
1.31
<26
21.6
1.600
20
1.958
2.681
1.37
<34
32.5
1.560
21
1.966
3.523
1.59
<42
37.3
1.643
22
1.96
2.767
1.60
<48
28.8
1.622
23
1.967
3.726
1.59
<47
34.0
1.650
24
1.974
4.102
1.80
<44
47.4
1.683
26
1.965
3.490
1.87
71
40.0
1.652
27
1.973
3.514
1.81
64
27.7
1.685
Various
1.94 3.11.0 10201.63Commercial
1.96
3.4
1.0
15
40
1.65
Samples
Note: Trials ER-2,3,and 25 were incomplete and no sample was generated. NA = Not Available.
Comments
Mini-electrodes static electr.
6.25 A.ft−2
Shatterbox
Ball Mill
12.5 A.ft−2
18.8 A.ft−2
25.0A.ft−2
37.5 A.ft−2
Ti doped
Ti doped
Ti doped
Ti doped
Ti doped
Ti doped
Repeat ER1
Repeat ER1
Standard and
High Power
Alkaline EMD Included
265
266
Stuart M. Davis et al./ J. New Mat. Electrochem. Systems 7, 257-268 (2004)
Table 3: Average Properties for 18 EMD Samples Plated at 120 Deg. C (Excepting Those Samples with Ti Doping)
Electrochemical
Plating Efficiency
97.93%
%MnOOH
(dry basis)
6.22%
He Density
Yield of Powder
from Chip
92.20%
H2 O from
MnOOH
0.28%
BET Area
meq Na/g of EMD
Final pH
x in MnOx
Mn (dry basis)
MnO2 (dry basis)
1.967
"Ruetschi
H2 O"
3.03%
Micropore Volume
(DeBoer t method)
0.00321
cm3 .g−1
Q Ratio
59.61%
OCV vs. Zn
(9N KOH)
1.618V
Modal Pore
Diameter
18.48
Angstroms
Acid Insols.
3,004ppm
apx. 50ppm
5.84
H2 O
400 deg. C
3.32%
Desorption
Pore Volume
0.04856
cm3 .g−1
AA Cell 1A
Cont. to 1.0V
614 mAh
91.23%
SPECS Coefficient
2.67
SO=
4
28.93
m2 .g−1
Na
0.24 meq.g−1
H2 O C
110 deg.
0.70%
Micropore Area
(DeBoer t method)
6.43
m2 .g−1
Ti
4.416
g.cm−3
Al
Shatterbox:
<50ppm Ball Mill
2,000-6,000 ppm
Cr
7.5 ppm
0.7
59 ppm
Co
<5 ppm
Cu
<3 ppm
Fe
566 ppm
Mo
<3 ppm
Shatterbox:
apx. 0.2% Ball Mill:
Apx. 1.0%
Ni
<3 ppm
1.58%
Ca
K
<50 ppm
Comparing results in Tables 4 and 5 for the two high performance samples ER-1 and ER-27, to those for commercial EMD
(same tables) we can conclude that high performance EMD plated
at 120 deg. C exhibits: higher peroxidation, higher "Ruetschi
H2 O", significantly higher SO=
4 , higher Ti, about the same BET
area, about the same desorption pore volume, and significantly
higher OCV. "Q: ratio also appears to be significantly higher
and density in He slightly lower than the typical range for commercial EMD.
Table 4: Physical and Chemical Properties for Samples ER-1
and ER-27 Compared to Commercial EMD
Figure 6: Discharge Curves for EMD ER-1 vs. Commercial
EMD in AA MnO2 /Zn Cells
the AINSI prescribed limits of 21.0 +/- 1.1deg.C. In these trials, controls were discharged at the same time as experimental
cells, hence environmental temperatures were equal. Control
performance on the 1A continuous drain fell within the normal
expected range for fresh commercial AA cells.
Comparing results in Table 3 for 18 samples plated at 120 deg.C
(does not include samples with deliberate Ti doping) to those for
commercial EMD (see Table 2, last line) we can conclude that
EMD plated at 120 deg C exhibits on average: slightly higher
peroxidation (higher "x" in MnOx ), slightly lower "Ruetschi
H2 O", significantly higher SO=
4 , significantly higher Ti, about
the same BET area, about the same desorption pore volume and
slightly lower OCV.
Sample –>
ER-1
ER-27
Commercial
EMD*
1.96
1.60 - 1.65
10 - 15
1.0 - 1.2
4.4 - 4.6
3.2 - 3.6
3.0 - 3.1
0.6 - 0.7
20 - 35
x in MnOx
1.98
1.97
OCV vs. Zn (V)
1.69
1.69
Ti (ppm)
2,160
64
SO=
1.58
1.81
4 (%)
He Density (g.cm−3 )
4.34
4.39
400 C H2 O (%)
3.45
3.75
Ruetschi H2 O (%)
3.23
3.51
"Q" Ratio
1.26
1.88
BET Area m2 .g−1
26.7
27.7
*Includes alkaline grade EMDs from
various commercial sources and also high power EMD
Considering those samples plated from a conventional electrolyte
composition (0.9M MnSO4 , 0.5M H2 SO4 ) at progressively increasing CDs (trials ER-4 and ER6-9), Fig. 7 summarizes AA
cell performance as a function of CD for 3 different discharge
Plating of High Quality Electrolytic Manganese Dioxide at 120 - 125 Degrees C. . / J. New Mat. Electrochem. Systems 7, 257-268 (2004)
267
Table 5: AA Cell 1 A Continuous Discharge Performance for
ER-1, ER-27 Compared to Commercial EMD Controls*
Sample
ER-1**
Com. EMD
(ER-1 Ctrl)**
ER-27***
A-h to
1.1V
0.686
(+35%)
0.508
A-h to
1.0V
1.028
(+15%)
0.894
A-h to
0.9V
1.322
(+14%)
1.159
A-h to
0.8V
1.508
(+17%)
1.284
0.521
(+23%)
0.423
0.813
(+16%)
0.699
1.044
(+14%)
0.913
1.211
(+14%)
1.060
Com. EMD
(ER-27 Ctrl)***
* Controls = 50/50 blend of Kerr-McGee Trona D and
Kerr-McGee High Power EMD
** ER-1–>2 cells discharged. Control–> 4 cells
discharged. See Fig. 6 for reproducibility.
*** ER-27–> 6 cells discharged. Control–> 6 cells discharged.
ER-27 CV = 7.8%, 2.9%, 3.7%, 5.0%
CTRL CV = 4.1%, 2.1%, 3.0%, 4.4%
(to 1.1, 1.0, 0.9, 0.8V)
regimes and makes a comparison to commercial EMD (square
markers on vertical axis).
From these graphs, it can be concluded that when plating at
120 deg. C, high quality EMD can be produced at 2X to 4X
normal CD; i.e. at CDs of 12.5 to 25.0 A.ft2 . An EMD closely
approaching high power commercial EMD can even be plated
at a CD as high as 6X normal or 37.5 A/ft2 . It should be noted
that despite the very high CDs and moderately high acid levels employed in these trials, there was no indication of anode
passivation, as would normally be expected for Ti anodes in a
conventional (non-pressurized) bath under these conditions.
Considering all of the above, and particularly the difficulty that
was initially encountered in duplicating the properties and performance of sample ER-1, the reader may naturally ask, what
was it about ER-1 (and ER-27) that was responsible for their
unique properties? What is or are the key characteristics which
define a superior high power EMD and explain its discharge
performance?
Progress has been made in answering this question and a patent
that defines the physico-chemical properties of superior high
power EMD is currently pending. Once opened for examination, it is our intent to write a continuation of this paper which
will further explain our understanding of the structure and discharge mechanisms of high power EMD. At the present time we
can only offer the opinion, that the most practical and economical route to the production of high power EMD will ultimately
prove to be in a pressurized cell, operating above the normal
Figure 7: AA Cell Performance of EMD Samples Plated 120
Deg. C and Various Current Densities Compared to Commercial EMD
boiling point of conventional plating baths, as we have reported
here.
4.
CONCLUSION
High quality, high power EMD may be plated at 2X to 4X normal CD, possibly even at 6X normal CD, when carried out in a
pressurized cell at temperatures above the normal boiling point
of conventional bath compositions e.g. at 120 deg. C or higher.
High OCV, high peroxidation (x in MnOx ), high SO=
4 , and high
"Ruetschi H2 O" are necessary, but not sufficient conditions to
define a high power EMD.
Ti doping is not a necessary condition for producing high power
EMD. Ti doping (as is true for doping by other ions) tends to
disrupt the regular crystalline growth and gives rise to higher
surface area EMD. Combinations of Ti doping and high tem-
268
Stuart M. Davis et al./ J. New Mat. Electrochem. Systems 7, 257-268 (2004)
perature deposition do exist which can lead to high power EMD
(as for ER-1).
Plating EMD on small electrodes in a large volume of quiescent
solution may lead to "stratification" of the solution; i.e. concentration gradients, due to the "Soret" effect.
Unpolarized Ti anodes may corrode in typical EMD bath compositions when heated above 100 deg. C. This problem is avoided
when traces of MnO2 (from a previous plating trial) are present
or a positive polarity is applied to the Ti anode during heat-up.
Ti passivation may be avoided when plating at higher temperatures. Even when plating at 18.8 A.ft−2 in a bath containing
0.95M H2 SO4 at 125.5 deg. C, no passivation was observed.
5.
ACKNOWLEDGMENTS
The author gratefully acknowledges the invaluable assistance of
Mr. Jonas Krisciunas who assembled the pressurized cell and
Mr. Gary Miller who operated the cell to produce the samples
reported in this paper. Thanks also to Dr. Bill Bowden, Dr. Rick
Moses and Dr. Rima Sirotina for their helpful advice and assistance in characterizing many of the prepared EMD samples and
to the Gillette Company which supported this work and kindly
allowed the author to publish this manuscript.
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