The present disclosure relates to a production of non-pyrophoric iron
active material for alkaline iron accumulators. More particularly, the present
disclosure relates to a non- pyrophoric iron active material for alkaline iron
accumulators that renders a substantial amount of metallic iron (α-Fe) and has
substantially reduced oxide content (Fe3O4) of iron. The present disclosure also
relates to a process of producing the non-pyrophoric iron active material.
Background of the invention
Iron is fourth most abundant element on the earth. It is cost effective, has large
theoretical specific capacity and is non-toxic. Accordingly, there is an increased
interest in developing iron-based accumulators. Iron-based accumulators are
both mechanically and electrically rugged. Besides, iron electrodes are
environmentally benign unlike other battery electrode materials such as
cadmium, lead, nickel and zinc.
The charge-discharge reactions of an alkaline iron electrode are given below.
During the first charge-discharge step Fe (II) is reduced to Fe and vice versa as
shown in reaction (1).
Fe + 2OH¯ ⇆ Fe (OH)2 + 2e− E° = - 0.88 V vs. SHE (1)
The theoretical specific discharge capacity for the first step happens to be 961
mAh/g. This step is followed by oxidation of Fe(II) to Fe(III) as depicted in
reaction (2).
Fe (OH)2+ OH¯ ⇆ Fe OOH + H2O + e− E° = - 0.56 V vs. SHE (2)
In reactions (1) and (2), E° is a standard reduction potential for the
respective reactions and SHE stands for the standard hydrogen electrode. The
open-circuit potential of a charged alkaline iron electrode (E° = - 0.88 V vs.
SHE) is always more negative than the hydrogen evolution reaction in the same
solution. Consequently, iron is thermodynamically unstable and suffers
3
corrosion through concomitant evolution of hydrogen according to the reaction
(3).
2H2O + 2e−⇆ H2 + 2OH¯ E° = - 0.83 V vs. SHE (3)
Dissolved oxygen in alkaline solution could also lead to the following reduction
reaction during the corrosion of the iron electrode as depicted by reaction (4).
O2 + 2 H2O + 4e− ⇆ 4 OH¯ E° = 0.41 V vs. SHE (4)

Reference is made to US2011/0236747A1 which discloses an iron active
material derived from ferrous oxalate dihydrate precursor to yield 15 w/w % αFe and 85 w/w % Fe3O4 with a specific discharge capacity of about 220 mAh/g
at the current density of 40 mA/g. The low specific discharge capacity of the
iron electrode is apparently due to the lower content of α-Fe.
PCT/US2012/042750 discloses an iron electrode derived from carbonyl iron
powder with specific discharge capacity of about 300 mAh/g, at about C/10
rate.
US4250236 discloses an iron electrode derived from carbonyl iron powder with
zinc sulphide additive that exhibits a specific discharge capacity of 370
mAh/g.
A publication titled, “Self-Assembled Monolayers of n-Alkanethiols suppress
Hydrogen Evolution and Increase the Efficiency of Rechargeable Iron Battery
Electrodes”, by Souradip Malkhandi et al., in Journal of American
Chemical
Further, reference is made to Society 2013, 135, 347−353, which discloses the
effect of n-Alkanethiols in suppressing hydrogen evolution on Iron electrode
and reports a specific discharge capacity of 300 mAh/g.
A publication titled “Understanding the Factors Affecting the
Formation of Carbonyl Iron Electrodes in Rechargeable Alkaline Iron
Batteries”, by Aswin K. Manohar et al., in the Journal of The Electrochemical
4
Society 159 (12) A2148-A2155 (2012), discloses the effect of wetting agent,
pore former and sulphide additives on the formation of carbonyl iron electrodes.
A publication titled, “An ultrafast nickel–iron battery from strongly
coupled inorganic nanoparticle/nanocarbon hybrid materials”, by Hailiang
Wang et al., in Nature Communications 3:917 doi: 10.1038/ncomms1921
(2012), discloses the ultra-fast Ni-Fe battery with FeOx nanoparticles grown on
reduced graphene oxide and Ni(OH)2 nanoplates grown on oxidized MWNT
(Multi Wall Carbon Nanotubes) that exhibit a specific discharge capacity of 102
mAh/g at 37 A/g.
The effect of various polymeric binders on the electrochemical performance
and stability of the iron electrode are disclosed in a publication titled, “Effect of
binder materials on cycling performance of Fe2O3 electrodes in alkaline
solution”, by Hiroki Kitamura et al., in the Journal of Power Sources 208
(2012) 391–396.
A publication titled “The role of FeS and (NH4)2CO3 additives on the pressed
type Fe electrode”, by Caldas et al., in the Journal of Power Sources 74 (1998)
108–112, discloses the effect of sulphide additive and pore forming agent on the
performance of the iron electrode.
In a publication titled, “The Role of Sulfide Additives in Achieving Long
Cycle Life Rechargeable Iron Electrodes in Alkaline Batteries,” by by S.R.
Narayanan et al., in Journal of Electrochemical Society 162(9) A1864-A1872,
describes iron electrodes containing iron (II) sulfide and bismuth oxide
additives that do not exhibit any noticeable capacity loss even after 1200 cycles
was achieved by adding sulfide ions to the electrolyte.
A publication titled, “Stannate Increases Hydrogen Evolution
Overpotential on Rechargeable Alkaline Iron Electrodes”, by Mylad Chamoun
5
et al., in Journal of Electrochemical Society 164 (6) A1251-A1257, presents the
effect of the K2SnO3 to the alkaline Fe electrode with significant capacity
retention with high efficiency by suppressing the hydrogen evolution.
A publication titled, “An in situ carbon-grafted alkaline iron electrode
for iron-based accumulators”, by A. Sundar Rajan et al., in journal of Energy
and Environmental Science 2014, 7, 1110-1116, describes an in situ carbongrafted alkaline iron electrode prepared by decomposing the iron oxalate with
polyvinyl alcohol (PVA) composite at 6000C in a vacuum that gave a specific
discharge capacity of 400 mAh/g.
Furthermore, a patent application WO2016110862A1, discloses the
accumulator based on iron. However, this prior art does not disclose the nonpyrophoric iron. This accumulator has the problem of pyrophoricity i.e. being
ignited.
All of the above said prior arts though gave desired capacity but cycle life and
pyrophoricity of iron active material is a major concern in their production.
Therefore, there is a need to develop an alkaline iron electrode comprising an
active non-pyrophoric material which has substantial amount of metallic iron
and a reduced amount of corresponding iron oxides. Besides, the alkaline iron
electrode should effectively impart an increased specific discharge capacity, a
reduced hydrogen evolution and increased charge-discharge cycle life.
Although the prior arts disclose various accumulators, but enhancement in
cycle life and pyrophoricity of iron active material has not been disclosed for
accumulator. In view of this, the inventors of the present disclosure felt a need
to develop an accumulator which overcomes all the problems of the prior arts
and is cost effective. Particularly there is a need for enhancement in cycle life
and pyrophoricity. A non-pyrophoric iron based material is proposed by the
inventors of the present disclosure to improve the cycle life and pyrophoricity
of iron.
6
The primary object of the present disclosure is to provide a active
material for an alkaline iron electrode to be used in iron-based accumulators,
such as nickel-iron and iron-air, by incorporating a substantial amount of
metallic iron which is non-pyrophoric and has high electrochemically active
surface area with substantially reduced amount of iron oxide.
Another object of the present disclosure is to provide a grafted alkaline
iron electrode a graphitic-carbon coated material with an optimum porosity.
Yet another object of the present disclosure is to provide a alkaline iron
electrode in which the parasitic reaction resulting from hydrogen evolution is
mitigated.
Yet another object of the present disclosure is to provide a alkaline iron
electrode for an accumulator with an increased specific discharge capacity.
Still another object of the present disclosure is to provide a alkaline
iron electrode for an accumulator that imparts an increased number of chargedischarge cycles.
It is also an object of the present disclosure to provide a method for the
preparation of alkaline iron electrode for an accumulator, in which graphite is
grafted in situ into the active material (a mixture of α-Fe and Fe3O4), while
decomposing a metal oxalate precursor, in the presence of an organic carbon
source.
These and the other objects and the appurtenant advantages of the
embodiments herein will be understood by studying the following specification
with the accompanying drawings.
7
Summary of the invention
The present disclosure relates to a production of non-pyrophoric iron active
material for alkaline iron accumulators. More particularly the present disclosure
relates to a non-pyrophoric Iron active material for alkaline iron accumulators
that renders a substantial amount of metallic iron (α-Fe) and has substantially
reduced oxide content (Fe3O4) of iron. The present disclosure also relates to a
process of producing the non-pyrophoric iron active material.
The present disclosure provides a process for the production of non-pyrophoric
graphitic-carbon-grafted active material for an alkaline iron electrode which
will have an increased amount of metallic iron (α-Fe) as well as reduced amount
of oxide of iron, namely Fe3O4. The active material in the iron electrode is
grafted with graphite through an organic carbon source, preferably polyvinyl
alcohol. The iron electrode also includes an additive to inhibit hydrogen
evolution, a conductive carbon having a high surface-area-to-volume ratio and a
metal salt. The non-pyrophoric active material in the alkaline iron electrode of
the present disclosure is provided with graphitic-carbon coated particles that
impart an increased specific discharge capacity along with a faradaic efficiency
of about 90%. The present disclosure also provides a method to prepare
graphite-grafted alkaline iron electrode for an accumulator in which graphite is
grafted in situ into the active material, while decomposing a metal oxalate
precursor along with an organic carbon source.
Brief description of the drawings
FIG.1 represents a flow chart depicting process steps for the preparation of αFeC2O4.2H2O (precursor for non-pyrophoric graphitic-carbon-grafted active
material).
FIG.2 represents a flow chart depicting process steps for the preparation of
non-pyrophoric graphitic-carbon-grafted active material.
8
FIG. 3 shows a powder X-ray diffractogram for non-pyrophoric
graphitic-carbon-grafted active material (NPAM) obtained by decomposing αFeC2O4.2H2O and 10 w/w % Polyvinyl alcohol (PVA) composite processed at
7500C. International Centre for Diffraction Data (ICDD) number for α-Fe and
Fe3O4 are 00-006-0696 and 00-019-0629, respectively.
FIG.4 shows galvanostatic charge and discharge data for the alkaline
iron electrode comprising non-pyrophoric iron active material (NPAM) at
charge and discharge current density of 100mA/g.
Detailed description of the invention
While the disclosure is susceptible to various modifications and alternative forms,
an embodiment thereof has been shown by way of example and the drawings and
will be described here below. It should be understood, however that it is not
intended to limit the disclosure to the particular forms disclosed, but on the
contrary, the disclosure is to cover all modifications, equivalents, and alternative
falling within the spirit and the scope of the disclosure.
The terms “comprises”, “comprising”, or any other variations thereof, are
intended to cover a non-exclusive inclusion, such that a setup, structure or method
that comprises a list of components or steps does not include only those
components or steps but may include other components or steps not expressly
listed or inherent to such setup or structure or method.
For the better understanding of this disclosure, reference would now be made to
the embodiment illustrated in the accompanying figures and description here
below.
The present disclosure relates to a non-pyrophoric graphitic-carbon grafted iron
active material for alkaline electrodes for iron based accumulator, such as nickeliron and iron-air with substantial amount of metallic iron(α-Fe) and reduced
amount of corresponding oxide content of iron (Fe3O4) comprising:
80-95 w/w % of non-pyrophoric iron (α-Fe), 5-20 w/w % of oxide content
(Fe3O4), about 2-3 w/w % of graphitic carbon about 10-20 w/w % of
9
conductive carbon about 1-2 w/w % of faradaic efficiency agent about 1-2 w/w
% of metal salt and about 6-15 w/w% of at least a polymeric binder.
In one embodiment of the disclosure the organic carbon source is selected but
not limited to from the group consisting of polyvinyl alcohol, resorcinolformaldehyde resin, sucrose and starch, preferably polyvinyl alcohol.
In one embodiment of the disclosure the conductive carbon is preferably carbon
black.
In one embodiment of the disclosure the faradaic efficiency agent is selected
but not limited to the group consisting of bismuth sulphide (Bi2S3), bismuth
oxide (Bi2O3), lead sulphide and ferrous sulphide, sodium sulphide, preferably
bismuth sulphide.
In one embodiment of the disclosure the binder is selected but not limited to
the group consisting of such as polytetrafluoroethylene (PTFE), polyvinylidene
fluoride (PVDF), styrene-butadiene-rubber (SBR), acrylic resin,
hydroxypropyl-methyl-cellulose (HPMC), neoprene latex, polyethylene (PE),
poly tetrafluoroethylene-co-vinylidene fluoride (P(TFE-VDF)) or a mixture
thereof, preferably PTFE.
In one embodiment of the disclosure the electrode has a specific discharge
capacity of about 350 mAh/g (at C/4 rate or 100 mA/g) with a faradaic
efficiency of about 90%.
In one embodiment of the disclosure the graphitic-carbon-grafted active
material is provided with an enhanced surface area in the range of 10-15 m2
/g.
In one embodiment of the disclosure the material is graphitic-carbon grafted.
Yet another embodiment of the disclosure involves a process for preparing a
non-pyrophoric alkaline iron active material for an iron accumulator, the
method comprising the steps of:
10
-preparing non-pyrophoric ferrous oxalate dihydrate (α-FeC2O4.2H2O)
from FeSO4.7H2O and oxalic acid dihydrate;
-grafting of graphite by decomposing ferrous oxalate dihydrate in the
presence of an organic carbon source;
-adding faradaic efficiency agent and carbon black to the graphiticcarbon-grafted active material; and
-compacting and curing the material in the presence of a binder and an
inert atmosphere to obtain an alkaline iron electrode having a
substantial amount of metallic iron and a substantially reduced amount
of oxide of iron.
In one embodiment of the disclosure the metallic iron is 80-95 w/w % and
oxides of iron is 5-20 w/w %.
In one embodiment of the disclosure the organic carbon source is conductive
carbon of 10-20 w/w %.
In one embodiment of the disclosure the organic carbon source is selected
from the group consisting of polyvinyl alcohol, resorcinol-formaldehyde resin,
sucrose and starch, preferably polyvinyl alcohol.
In one embodiment of the disclosure the conductive carbon is carbon black.
In one embodiment of the disclosure the faradaic efficiency agent is selected
from the group consisting of bismuth sulphide (Bi2S3), bismuth oxide (Bi2O3),
lead sulphide, ferrous sulphide, sodium sulphide, preferably bismuth sulphide.
In one embodiment of the disclosure the binder is selected from the group
consisting of such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride
(PVDF), styrene-butadiene-rubber (SBR), acrylic resin, hydroxypropylmethyl-cellulose (HPMC), neoprene latex, polyethylene (PE), poly
11
tetrafluoroethylene-co-vinylidene fluoride (P(TFE-VDF)) or a mixture thereof,
preferably PTFE.
In one embodiment of the disclosure the step of grafting is conducted at a
temperature in the range of 250-750°C in vacuum.
In one embodiment of the disclosure the step of compacting and curing is
carried out at a temperature of about 300 -350°C.
In one embodiment of the disclosure the material is graphitic-carbon grafted.
Yet another embodiment of the disclosure involves an electrode comprising the
non-pyrophoric graphitic-carbon grafted iron active material as claimed in one
of claims 1 to 8.
The following is a detailed description of non-pyrophoric graphitic-carbongrafted iron active material for an accumulator according to the present
disclosure.
The invention is characterized by a non-pyrophoric graphitic-carbon-grafted
iron active material for an accumulator comprising as its principal ingredient, a
substantial amount of metallic iron with an enhanced surface area, an optimum
porosity and with a substantially reduced amount of corresponding oxide
content, to impart an increased specific discharge capacity and an improved
faradaic efficiency.
The non-pyrophoric graphitic-carbon grafted iron active material, of the
present disclosure comprises approximately 80-95 w/w % of metallic iron (αFe), 5-20 w/w % of oxide content (Fe3O4). The graphitic-carbon-grafted active
material is provided with an enhanced surface area in the range of 10-15 m2
/g.
12
The non-pyrophoric graphitic-carbon-grafted iron active material of the present
disclosure includes graphitic carbon in the range of about 2-3 w/w %. The
graphitic carbon is formed from the organic carbon source, selected from
polyvinyl alcohol (PVA). Polyvinyl alcohol acts not only as a reducing agent
but also as a graphitic carbon source.
In another aspect of the present disclosure, non-pyrophoric graphiticcarbon-grafted iron active material is also used to prepare the alkaline iron
electrode which contains conductive carbon with high surface-area-to-volume
ratio, preferably carbon black incorporated in desired quantities for imparting
an enhanced electrical conductivity. In this disclosure, advantageously, carbon
black is used in the range of about 10-20 w/w %.
In yet another aspect of the present disclosure, the graphitic-carbon-grafted
alkaline iron electrode prepared from non-pyrophoric graphitic-carbon-grafted
alkaline iron active material of the present disclosure includes a faradaic
efficiency agent. The faradaic efficiency agent increases hydrogen overpotential
and hence the charge acceptance of the iron electrode. In a preferred
embodiment bismuth sulphide (Bi2S3) is used as faradaic efficiency agent in the
range of about 1-2 w/w %. The other suitable faradaic efficiency agents, such as
bismuth sulphide (Bi2S3), Bismuth oxide (Bi2O3), lead sulphide and ferrous
sulphide, sodium sulphide, preferably bismuth sulphide and zinc sulfide, can
also be suitably used. The faradaic efficiency agent is used to improve
coulombic efficiency and to inhibit kinetics of hydrogen evolution.
The graphite-grafted alkaline iron electrode prepared from non-pyrophoric
graphitic-carbon-grafted alkaline iron active material of the present disclosure
further includes a metal salt such as nickel sulphate, in the range of about 1-2
w/w %.
In another preferred aspect of the present disclosure the graphite-grafted
alkaline iron electrode prepared from non-pyrophoric graphitic-carbon-grafted
alkaline iron active material of the present disclosure further comprises at least
13
one binder such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride
(PVDF), styrene-butadiene-rubber (SBR), acrylic resin, hydroxypropyl-methylcellulose (HPMC), neoprene latex, polyethylene (PE), poly tetrafluoroethyleneco-vinylidene fluoride (P(TFE-VDF)) or a mixture thereof.
In yet another aspect of the present disclosure alkaline iron electrode prepared
from non-pyrophoric graphitic-carbon-grafted alkaline iron active material of
the present disclosure used polytetrafluoroethylene (PTFE), as a polymer
binder, in the range of about 6-15 w/w%.
The alkaline iron electrode made from non-pyrophoric graphitic-carbongrafted alkaline iron active material of the present disclosure were found to
deliver a discharge capacity of 350 mAh/g with a coulombic efficiency of about
90%, at current density of 100 mA/g.
Now specifically referring to FIG. 1 & 2, of the accompanied drawings that
represent the process for the preparation of alkaline iron electrode from nonpyrophoric graphitic-carbon-grafted iron active material of the present
disclosure for an iron-based accumulator and includes the steps of preparing
ferrous oxalate dihydrate (α-FeC2O4.2H2O) from FeSO4.7H2O and oxalic acid
dihydrate, which is followed by grafting of graphite in situ, by decomposing
ferrous oxalate dihydrate, in the temperature range of 250-750°C in vacuum and
in the presence of an effective amount of organic carbon source. A faradaic
efficiency agent and carbon black added to the graphic-carbon-grafted active
material. This material is compacted and cured in the presence of a binder at a
temperature of about 350°C, in an inert atmosphere, to obtain alkaline iron
electrode having a substantial amount of metallic iron and a substantially
reduced amount of oxide of iron.
Various steps of the process for the preparation of non-pyrophoric graphiticcarbon-grafted iron active material of the present disclosure for an alkaline iron
accumulator are now described in detail, by referring to flow-drawings as
14
shown in FIGs. 1 & 2. The present disclosure provides a process for the
production of non-pyrophoric graphitic-carbon-grafted active material for an
alkaline iron electrode which will have an increased amount of metallic iron (αFe) as well as reduced amount of oxide of iron, namely Fe3O4. The active
material in the iron electrode is grafted with graphite through an organic carbon
source, preferably polyvinyl alcohol. The iron electrode also includes an
additive to inhibit hydrogen evolution, a conductive carbon having a high
surface-area-to-volume ratio and a metal salt. The non-pyrophoric active
material in the alkaline iron electrode of the present disclosure is provided with
graphitic-carbon coated particles that impart an increased specific discharge
capacity along with a faradaic efficiency of about 90%. The present disclosure
also provides a method to prepare graphite-grafted alkaline iron electrode for an
accumulator in which graphite is grafted in situ into the active material, while
decomposing a metal oxalate precursor along with an organic carbon source.
Referring to FIG. 3 of the accompanied drawings that depict XRD pattern for nonpyrophoric graphitic-carbon-grafted active material (NPAM) processed at 7500C.
The diffraction lines corresponding to α-Fe and Fe3O4 are indicated in the XRD
patterns as for α-Fe and for Fe3O4. Following steps occur while preparing the
graphite-grafted active material:(i) α-FeC2O4.2H2O–PVA composite to α-FeC2O4–
PVA composite i.e., dihydrate to anhydrous compound, (ii) thermal decomposition
of α-FeC2O4 and PVA, (iii) carbonization of polymer (PVA), (iv) reduction of
Fe3O4 (magnetite) to α-Fe (metallic iron) by carbonaceous products and (v) high
temperature sintering to get non-pyrophoric compound of the present disclosure.
Besides, about 2– 3 w/w % of graphitic carbon is obtained as a conductive coating
on the surface of the iron and magnetite particles. Accordingly, PVA acts both as a
reducing agent and graphitic-carbon source.
Referring to FIG.4, which is a plot depicting galvanostatic charge and discharge
profiles for alkaline iron electrode prepared from non-pyrophoric graphite-grafted
active material (NPAM) of the present disclosure. It is seen from FIG. 4, that
alkaline iron electrode with Bi2S3 additive, exhibits a specific discharge capacity of
about 350 mAh/g (at C/4 rate or 100 mA/g) with a faradaic efficiency of about
15
90%. In this exemplary embodiment, the current density value for both charge and
discharge processes are maintained at 100 mA/g. The charge potential profiles for
iron electrode follow a S-shaped curve with two plateaus at -1.1 V vs.
mercury/mercuric oxide (MMO) and -1.15 V vs. MMO.
Preparation of α-FeC2O4.2H2O (precursor for active material)
A solution of an iron source is prepared by dissolving the commercially
available iron source in hot and de-ionized water at a temperature in the range
of 60-80°C, preferably at about 70°C. In the process of the present disclosure,
the iron source that is advantageously used is commercial grade ferrous
sulphate heptahydrate (FeSO4.7H2O). It is understood here that other iron
sources such as hydrated ferrous ammonium sulphate ((NH4)2Fe(SO4)2.6H2O)
and hydrated iron chloride (FeCl2.2H2O) can also be suitably used as precursors
for the process. Thus, a solution of ferrous sulphate heptahydrate
(FeSO4.7H2O) is prepared. A suitable dicarboxylic acid is selected and a
solution of the same is also prepared. The solution of dicarboxylic acid is
prepared by dissolving the selected dicarboxylic acid in de-ionized water at 60-
80°C, preferably at about 70°C. Advantageously, in the present disclosure, as an
exemplary embodiment, commercially available oxalic acid dihydrate
(H2C2O4.2H2O) is used for preparing the desired dicarboxylic acid solution.
The other dicarboxylic acids that can be suitably used are malonic acid, succinic
acid and adipic acid. The oxalic acid dihydrate (H2C2O4.2H2O), solution thus
prepared is slowly added into ferrous sulphate solution under continuous
stirring at a temperature in the range of about 60 to 70°C to precipitateα-ferrous
oxalate dihydrate (α-FeC2O4.2H2O). The precipitated α-FeC2O4.2H2O is
allowed for ageing along with the supernatant liquid for about two hours,
allowing the precipitate to settle down, followed by filtration. The filtered
product is dried in air oven at a temperature in the range of 60-70°C.
16
Preparation of graphitic-carbon-grafted active material
A composite is prepared by mixing an organic carbon source, such as
Polyvinyl Alcohol (PVA), of desired molecular weight with α-Ferrous oxalate
dihydrate (α-FeC2O4.2H2O). In this process PVA is advantageously used as the
organic carbon source. However, other organic carbon sources such as
resorcinol-formaldehyde resin, sucrose and starch can also be suitably used. The
composite is initially heated to about 250°C and maintained at that temperature
for about one hour, for converting α-FeC2O4.2H2O-PVA composite to αFeC2O4-PVA composite, which is an anhydrous compound. Subsequently, αFeC2O4-PVA composite is heated to about 500°C and maintained at that
temperature for about two hours. Finally, the temperature is increased to about
750°C and maintained for about two to three hours to obtain non-pyrophoric
graphite-grafted active material (NPAM), which is a mixture of graphitic
carbon coated α-Fe and Fe3O4. The carbonaceous material, which gets formed
while decomposing PVA, reduces Fe3O4 to α-Fe (metallic iron). The graphitegrafted active material thus obtained comprises porous cuboidal particles of αFe and Fe3O4, where the particle size is of the material is in the range of 5-10
microns (µm).
Example
In the first step, 111 g of commercial grade ferrous sulphate heptahydrate
(FeSO4.7H2O) is dissolved in 400 mL of hot de-ionized water (about 70°C) and
then 56.5 g of commercial grade oxalic acid dihydrate (H2C2O4.2H2O) is
dissolved in 400 mL of hot de-ionized water (about 70°C). In order to avoid
oxidation of ferrous ions in the solution by dissolved oxygen, nitrogen gas is
bubbled for 10-15 minutes into the de-ionized water prior to the experiments.
Then oxalic acid solution is slowly added into ferrous sulphate solution with
continuous stirring at about 70°C and the α-ferrous oxalate dihydrate (α-
17
FeC2O4.2H2O) gets precipitated. The precipitated α-FeC2O4.2H2O is allowed
for ageing along with the supernatant liquid for about 2 h at 70°C, filtered and
dried in air oven at 60-70°C.
In the second step, 135 g of α-FeC2O4.2H2O and 15 g of Polyvinyl
Alcohol (PVA, Mw = 1, 25,000) are extensively mixed and the resultant
composite is first heated up to 250°C and maintained at that temperature for 1h
for converting α-FeC2O4.2H2O-PVA composite to α-FeC2O4-PVA composite.
Subsequently α-FeC2O4-PVA composite is heated to 500°C and maintained at
that temperature for 2h. Finally, the temperature is raised to 750°C and
maintained for 2-3 h to obtain non-pyrophoric graphitic-carbon-grafted active
material (NPAM), which is a mixture of graphitic carbon coated α-Fe and
Fe3O4. The carbonaceous material, which gets formed while decomposing
PVA, reduces a part of Fe3O4 to α-Fe (metallic iron). Here the decomposition is
done under vacuum. The mixture comprises porous cuboidal particles of about
5-10 microns.
Referring to FIG.4, which is a plot depicting galvanostatic charge and
discharge profiles for alkaline iron electrode prepared from non-pyrophoric
graphite-grafted active material (NPAM) of the example. It is seen from FIG.
4, that alkaline iron electrode with Bi2S3 additive, exhibits a specific discharge
capacity of about 350 mAh/g (at C/4 rate or 100 mA/g) with a faradaic
efficiency of about 90%. In this exemplary embodiment, the current density
value for both charge and discharge processes are maintained at 100 mA/g. The
charge potential profiles for iron electrode follow a S-shaped curve with two
plateaus at -1.1 V vs. mercury/mercuric oxide (MMO) and -1.15 V vs. MMO.
The alkaline iron electrode renders a compelling specific discharge capacity
and faradaic efficiency.
18
Comparative example:
The electrode preparation of the above example is repeated except the nonpyrophoric iron is used in 60 % and oxide content of 30% of the active material.
The comparative example is a carbonyl iron component. The results obtained
are shown below.
Inventive example Comparative example (SM grade
Carbonyl Iron from BASF, Germany)
specific discharge capacity = 350
mAh/g
specific discharge capacity =300mAh/g
Non-Pyrophoric Non- Pyrophoric
As seen from the table above, the specific discharge capacity (350 mAh/g) of
the inventive example is far superior on comparison to the specific discharge
capacity (300 mAh/g) of the comparative example. This leads to the enhanced
battery performance and enhanced power output. The electrode of the inventive
example is clearly superior to the comparative example.
Although the embodiments herein are described with various specific
examples and process parameters, it will be obvious for a person skilled in the
art to practice the embodiments herein with modifications and these
modifications are deemed to be within the scope of the appended claims.
Advantages of the present invention
The present disclosure provides a cost-effective non-pyrophoric
graphitic-carbon-grafted iron active material of the present disclosure from
commercially available ferrous sulphate heptahydrate (FeSO4.7H2O) and oxalic
acid dihydrate (H2C2O4.2H2O) and alkaline iron electrode prepared from that is
environmentally benign and which can be employed in any nickel-iron and
19
iron-air accumulators. The alkaline iron electrode renders a compelling specific
discharge capacity and faradaic efficiency.
The graphitic-carbon-grafted iron active material of the present disclosure has
enhanced cycle life and the has non-pyrophoric properties.

We Claim:
1. A non-pyrophoric iron active material for an alkaline electrode for iron based
accumulator, such as nickel-iron and iron-air, with substantial amount of metallic
iron(α-Fe) and reduced amount of corresponding oxide content of iron (Fe3O4)
comprising:
80-95 w/w % of non-pyrophoric iron (α-Fe),
5-20 w/w % of oxide content (Fe3O4),
about 2-3 w/w % of graphitic carbon,
about 10-20 w/w % of conductive carbon
optionally about 1-2 w/w % of faradaic efficiency agent
about 1-2 w/w % of metal salt and
about 6-15 w/w% of at least a polymeric binder.
2. The non-pyrophoric iron active material as claimed in claim 1, wherein the
organic carbon source is selected from the group consisting of polyvinyl alcohol,
resorcinol-formaldehyde resin, sucrose and starch, preferably polyvinyl alcohol.
3. The non-pyrophoric iron active material as claimed in claim 1, wherein the
conductive carbon is preferably carbon black.
4. The non-pyrophoric iron active material as claimed in claim 1, wherein the
faradaic efficiency agent is selected from the group consisting of bismuth sulphide
(Bi2S3), bismuth oxide (Bi2O3), lead sulphide and ferrous sulphide, sodium
sulphide, preferably bismuth sulphide.
5. The non-pyrophoric iron active material as claimed in claim 1, wherein the
binder is selected from the group consisting of such as polytetrafluoroethylene
(PTFE), polyvinylidene fluoride (PVDF), styrene-butadiene-rubber (SBR), acrylic
resin, hydroxypropyl-methyl-cellulose (HPMC), neoprene latex, polyethylene (PE),
poly tetrafluoroethylene-co-vinylidene fluoride (P(TFE-VDF)) or a mixture
thereof, preferably PTFE.
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6. The non-pyrophoric iron active material as claimed in claim 1, wherein the
electrode has a specific discharge capacity of about 350 mAh/g (at C/4 rate or 100
mA/g) with a faradaic efficiency of about 90%.
7. The non-pyrophoric iron active material as claimed in claim 1, wherein the
graphitic-carbon-grafted active material is provided with an enhanced surface area
in the range of 10-15 m2
/g.
8. The non-pyrophoric iron active material as claimed in claim 1, wherein the
material is graphitic-carbon grafted.
9. A process for preparing a non-pyrophoric alkaline iron active material for an
iron accumulator, the method comprising the steps of:
-preparing non-pyrophoric ferrous oxalate dihydrate (α-FeC2O4.2H2O) from
FeSO4.7H2O and oxalic acid dihydrate;
- grafting of graphite by decomposing ferrous oxalate dihydrate in the presence
of an organic carbon source;
- adding faradaic efficiency agent and carbon black to the graphitic-carbongrafted active material; and
- compacting and curing the material in the presence of a binder and an inert
atmosphere to obtain an alkaline iron electrode having a substantial amount of
metallic iron and a substantially reduced amount of oxide of iron.
10. The process as claimed in claim 9, wherein the metallic iron is 80-95 w/w %
and oxides of iron is 5-20 w/w %.
11. The process as claimed in claim 9, wherein the organic carbon source is
conductive carbon of 10-20 w/w %.
12. The process as claimed in claim 9, wherein the organic carbon source is
selected from the group consisting of polyvinyl alcohol, resorcinol-formaldehyde
resin, sucrose and starch, preferably polyvinyl alcohol.
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13. The process as claimed in claim 9, wherein the conductive carbon is carbon black.
14. The process as claimed in claim 9, wherein the faradaic efficiency agent is
selected from the group consisting of bismuth sulphide (Bi2S3), bismuth oxide
(Bi2O3), lead sulphide, ferrous sulphide, sodium sulphide, preferably bismuth
sulphide.
15. The process as claimed in claim 9, wherein the binder is selected from the
group consisting of such as polytetrafluoroethylene (PTFE), polyvinylidene
fluoride (PVDF), styrene-butadiene-rubber (SBR), acrylic resin, hydroxypropylmethyl-cellulose (HPMC), neoprene latex, polyethylene (PE), poly
tetrafluoroethylene-co-vinylidene fluoride (P(TFE-VDF)) or a mixture thereof,
preferably PTFE.
16. The process as claimed in one of claims 9 to 14, wherein the step of grafting is
conducted at a temperature in the range of 250-750°C in vacuum.
17. The process as claimed in one of claims 9 to 14, wherein the step of
compacting and curing is carried out at a temperature of about 300 -350°C.
18. The process as claimed in one of claim 9, wherein the material is graphitic
carbon grafted.
19. An electrode comprising the non-pyrophoric graphitic-carbon grafted iron
active material as claimed in one of claims 1 to 8.