The following specification particularly describes the invention and the manner
in which it is to be performed
Field of Invention
The present invention relates to an electrolyzer having exfoliated graphite
separator as reactant flow field plate for hydrogen generation from depolarized
water. It includes also a process of generating hydrogen using exfoliated
graphite plate as reactant flow field plate in depolarizer assisted PEM water
electrolyzer under pulse DC operation.
Background of the invention
In a future energy scenario based on renewable energy, hydrogen is an
attractive energy carrier and advanced water electrolysis will be the most
efficient and practical production process of hydrogen. Hydrogen is known to
have many applications other than energy sector ranging from analytical
chemistry (equipment for gas chromatography, hydrogen supply for laboratory
needs), hydrogen welding, metallurgy of special-purity metals and alloys,
production of pure substances for electronic industry, hydrogenation in various
industrial processes and spacecrafts. A promising option for hydrogen
production from renewable resources is electrolysis, in which electricity is used
to dissociate water into hydrogen and oxygen. It is very important to reduce
production and capital costs of water electrolysis by using cost effective
materials for cell construction and improving the specific performance of
electrode catalysts, in order to increase specific production rate at lowered
specific electric power consumption. Polymer Electrolyte Membrane (PEM)
based electrolyzer possesses certain advantages compared with the classical
alkaline process like: increased energy efficiency and specific production
capacity at a low temperature. The theoretical energy consumption for water
electrolysis is 3.54 kwhr/Nm3 and it would be over 4.5 kwhr/Nm3 even in the
most efficient electrolyzer. The process for an economic method to produce
hydrogen with direct electrolysis of aq.methanol by using PEM has been
described in the Patent application no. 331 3lDELl2012.
PEM based electrolyzers typically employ a Membrane Electrode Assembly
(MEA) consisting of a Solid Polymer Electrolyte, or ion exchange membrane
disposed between two electrodelcatalyst layers (GDE). The MEA is typically
interposed between two gas flow bipolar plates, which are substantially
impermeable to the reactant fluid streams, to form an electrolytic cell
assembly. The separator plates act as a support for the adjacent electrodes,
act as current collectors, and contain reactant distribution channels for
supplying reactants to the MEA. In order to ensure effective electrical contact
between the separator plates and the electrodes, they are compressed
between two end plates made of suitable materials such as aluminum and the
like. Effective sealing is provided at the edges of the plates to prevent
reactants leakage and cross over. In the case of conventional PEM water
electrolysis process, separator plates with some form of titanium, or a coated
stainless steel is employed. Titanium offers excellent strength, low initial
resistivity, high initial thermal conductivity and low permeability, however,
especially on the oxygen (anode) side; titanium corrodes and develops a
passive oxide layer that greatly increases the contact resistance. To help to
protect the titanium separator plate, precious metal coatings and alloys have
been used. This drastically reduces the corrosion rate. However negates any
cost savings and adds an extra processing step in the form of an expensive
coating material to the already expensive titanium base.
The process for an economic method to produce hydrogen with direct I electrolysis of Aq.-methanol by using PEM requires only Ca. 0.4 V as cell
voltage and only consumes 113~o~f electricity compared to PEM water
electrolysis. In addition to this, the corrosion in the PEM methanol electrolysis
is less severe than that in the PEM water electrolysis. Carbon-based and
stainless steel materials, therefore, can be used in the PEM methanol
electrolysis even though these materials which are vulnerable to corrosion at
high operating voltage making them unsuitable in the anode section of the
PEM water electrolysis. Moreover, milling process is used to form the desired
flow pattern in the plates, which is energy and time intensive process.
Expanded graphite, also known as exfoliated graphite, also known as flexible
graphite is a material that is suitable in the manufacture of reactant flow plates
(Prior art US patent 6777086 B2, 2004, US patent 6706400 B2, 2004, US
patent 6756027 B2, 2004). This is a compressible material, in sheet form, and
by an embossing process, such as roller embossing /reciprocal embossing the
reactant fluid channels can be formed. Embossing is a process in which
preferred pattern can be made on the preferred surface of the plate with the
help of roller or by stamping. . It is observed that in the prior art processes,
there are many intermediate steps like sheet forming, pasting, cutting to the
size etc., which make the process complicated and time consuming. Further it
is also not possible to get plates which are impervious to gasses and coolants.
An improved process for the preparation of exfoliated graphite separator
plates for use in PEM fuel cells which avoids many intermediate steps like
sheet forming, pasting, cutting to the size etc has been described in Indian
patent Application 1206lDEU2006. Further the improved gas flow field plate
design using the above prepared exfoliated graphite separator plate for PEM
fuel cell application has also been described in Indian Patent Application
2339lDEU2008. The present invention describes the use of the said exfoliated
graphite separator plate as reactant flow field plate in depolarizer assisted
PEM water electrolyzer for hydrogen generation process.
Objectives of the present invention
I
Main objective of the invention is to use exfoliated graphite separator plate as
reactant flow field plate in depolarizer assisted PEM water electrolyzer for
hydrogen generation process, thereby reducing the cost and mass of the
electrolyzer stack.
Another objective of the invention is to compare the performance obtained by
the use of exfoliated graphite separator plate as reactant flow field plate in
depolarizer assisted PEM water electrolyzer to that of conventional graphite
bipolar plates.
Yet another object of this invention is the use of exfoliated graphite separator
plate as reactant flow field plate in depolarizer assisted PEM water
electrolysis, which is useful for mass production.
Another object of this invention is the use of exfoliated graphite separator plate
as reactant flow field plate in depolarizer assisted PEM water electrolysis,
under pulsed DC operation
Another object of this invention is to develop exfoliated graphite flow field plate
to be used in depolarizer assisted PEM water electrolysis process that is
impervious to the fluids and gases flowing in its channels.
Summary of the Invention
Accordingly, the present invention provides a process for the use of exfoliated
graphite separator plate in depolarizer assisted PEM water electrolysis
process as cathode and anode flow field plate. Another aspect of the present
invention is the design of exfoliated graphite sheet having desired flow
patterns for the passage of the reactant and'to evacuate the produced gas in
the electrolytic cell. The present invention also discloses that the exfoliated
graphite separator plate is impervious to the fluids and gases flowing in its
channels during electrolyzer operation.
Another aspect of the invention is the construction of the electrolytic celllstack
using exfoliated graphite separator plate for hydrogen generation through PEM
based depolarizer assisted water electrolysis. According to yet another
embodiment of the present invention the performance of electrolyzer celllstack
is provided incorporating the exfoliated graphite platejas separator plate under
DC supply and under pulsed DC supply.
Brief Description of the drawingslfig ures
These and other features, aspects, and advantages of the present invention
will become better understood when the following detailed description is read
with reference to the accompanying drawings
Fig 1. Schematic drawing of an electrolyzer cell module.
Fig 2. A schematic of electrolytic cell assemblies combined electrically in
series to form a multi cell electrolyzer stack
Fig.3 Schematic drawing of exfoliated graphite separatorl anode flow field
plate according to an aspects of an embodiment of the invention
Fig 4. Schematic drawing of exfoliated graphite separatorl cathode flow field
plate according to another aspects of an embodiment of the invention.
Fig 5 .A schematic of PEM electrolyzer stack operation.
Fig.6. Performance of depolarizer assisted PEM based water electrolyzer as a
Function of applied current.
Wherein, 1,MEA; 2a Anode inlet; 2c. Cathode inlet; 3.Flow field plate; 4.
Current collector;5a. Anode outlet; 5c.Cathode outlet; 6.End plate; 7.Gasketl
insulator plates; 8 Monopolar plate; 9 Bi-polar plate; 10. Reservoir
(depolarized water tank); 1 1 .Anode; 12. Cathode; 13.Electrolyser stack; 14.Gas
I liquid separator; 15.Pump; 16Gas flow meter; 17.Carbon dioxide exhaust;
18.Hydrogen collecting port; 19.Trap; 20. Digital voltmeter; 21. Digital ammeter
and 22. DC Power supply
Detailed Description of the Invention
The PEM based electrolyzer cell module includes an anode electrode and a
cathode electrode. An electrolyte membrane is arranged between the anode
electrode and the cathode electrode and it forms a Membrane Electrode
Assembly (MEA)(I). The electrolyzer cell module also includes a first catalyst
layer between the anode electrode and the electrolyte membrane and a
second catalyst layer between the cathode electrode and the electrolyte
membrane. The fabricated MEA is placed in-between anode and cathode flow
field plates (3). These electrolyzer cell components are assembled together
with anode & cathode end plates (6) using tie rods. Nuts and washers or other
fastening means are used, for tightening the whole assembly. A DC power
supply is connected between the anode and cathode terminal. In operation,
a
aqueous methanol is introduced into the anode compartment via liquid input
port and it is dissociated electrochemically according to the following reaction.
I The protons produced at the anode compartment pass through the electrolyte
membrane to the cathode and electrochemically reduced to hydrogen
molecules according to the reaction given below. The produced carbon-dioxide
is drawn out through liquidlgas output port along with unreacted
aqueous methanol.
Referring now to Fig 1, the electrolyzer cell module includes only one
electrolyzer cell, however an electrolyzer cell stack will usually include a
number of electrolyzer cell stacked together. The components of electrolyzer
cell are enclosed by supporting components of the electrolyzer cell, which
includes an anode insulator plate1 gasket (7), an anode current collector plate
(5c), a cathode current collector plate (5a) and a cathode insulator plate (7).
The main components that make up each electrochemical cell are
I
I approximately repeated in sequence using bi polar flow field plate to provide
an electrolyzer stack that produces the desired hydrogen output. (Fig 2). In
order to hold the electrolyzer cell module together, tie rods are used. Nuts and
washers or other fastening means are also used for tightening the whole
assembly and to ensure that the various elements of the individual
electrochemical cells are held together tightly.
The separator plate used for this invention is made up of exfoliated graphite
material and it was prepared by embossing method as described in the Indian
patent Application 1206/DEU2006. The exfoliated graphite powder of known
volume is compressed with embossing die consisting of desired flow field
pattern using a hydraulic equipment and finished separator plate having a
density of 0.9- 1 .O g/cm3.
The plates prepared by the processes described above are subjected to
permeability tests for both gas and reactants. This was done by passing gas 1
reactant (aqueous methanol) from one side of the plate and observed the
permeability of gaslcoolant on the other side of the plate. It was observed that
there was no permeability,,which indicated that the plates are impervious for
both gas and reactant.
Conventionally anode flow field plates usually have a different flow field
pattern as compared to cathode flow field plates due to the different
stoichiometries of process gaseslreactants associated with each flow field
plate. The different stoichiometries often require different amount of each
process gaslliquid to be accommodated on each respective flow field plate,
which in turn requires the flow field channels on each respective plate to
support more or less volume than a corresponding flow field plate on the other
side of the electrolyte. Nevertheless, the flow field plate structures and MEA
used thus far are fairly complex structures that require highly skilled workers
for the assembly of electrochemical cell stacks. For example, the different
versions of flow field plates (anode or cathode) have to be chosen in a proper
sequence and placed in a correct orientation.
The anode flow field plate having serpentine flow field pattern as shown in
Fig.3 was used in various embodiments of the present invention. Flow field
plate typically includes a number of manifolds that each serve as a portion of a
corresponding distribution channel for fluid reactants. The flow field plate also
consists of inlet and outlet manifold for reactant circulation in the electrolyzer
celllstack. Generally, it is possible to have multiple inlet and outlet manifolds
on a flow field plate for reactant fluid depending on the electrolyzer cell design.
The cathode flow field plates also have serpentinelpin type flow field design as
shown in Fig 4 and it includes inlet and outlet manifold for hydrogen gas
collection. In some embodiments, the cathode of an electrolyzer cell need not
be supplied with an input process gaslfluid and only hydrogen gas and Aq.
methanol need to be evacuated. In such electrolyzer cells a flow field plate
does not require an input manifold for the cathode but does require an output
8
manifold. In some cases, the anode liquid manifold apertures have
substantially the same areas as the cathode manifold apertures. It should be
noted that the relative sizing of the manifold apertures with respect to one
another is not essential and that each may be a different size depending upon
the requirement. However, in some cases, making all of the manifold
apertures of the same size does simplify the design of a flow field plate and
possibly reduces associated manufacturing and assembly costs. A sealing
surface is also provided around the flow field, the various manifold openings
and the through holes to accommodate a seal that is employed to prevent
leaking and mixing of process liquidlgases. The exfoliated graphite material
based flow field plates can be of any shape suitable for a particular design of
an electrolyzer cell stack.
In practice, number of electrolyzer cells, all of one type can be arranged in
stacks having common features such as fluidlgas feeds, drainage, electrical
connections and regulation devices. That is, an electrolyzer cell module is
typically made up of a number of singular electrolyzer cells connected in series
to form an electrolyzer cell stack.
As shown in Fig.5 deploarised water is pumped from the liquid tank(l0) into
the electrolyser stack(l3) by the use of low power pump (1 5). C02 is produced
at the anode (1 1) of the electrolizer ( 13) and the C02 produced along with
unutilized solution is sent to a gas /liquid separator and C02 is separated and
unutilzed solution is sent back to the reservoir ( liquid tank). The electrolysis is
conducted to generate hydrogen at the cathode (12), by applying current
across the two terminals of the electrolyser using a DC power supply unit(22)
having constant current and constant voltage mode provisions. Hydrogen
produced in the electrolyser can be suitably collected. Hydrogen production
rate can also be calculated from the cell current and cross examined by gas
volume measurement.
The fig 6 shows the I-V characteristics of PEM electrolyser stack for hydrogen
generation. The electrolyzer performance was evaluated at various current by
9
measuring the stack voltage and hydrogen production rate. As can be seen in
the fig. the stack current increases with increase in operating voltage and the
stack was able to generate hydrogen gas about 1000 Ilh at 77 A, 15.9 V with
the average cell voltage of about 0.5 V. and the corresponding energy
consumption is 1.40 k w h r l ~ m ~ .
The following typical examples are given to illustrate the invention and should
not be construed to limit the scope of this invention
Example 1
An electrolyzer cell having exfoliated graphite material based flow field plate
was used for depolarizer assisted water electrolysis. The anode and cathode
flow field plates are having identical straight parallel type grooves with the area
of 55 x 55 mm and the corresponding flow field plate area was aprx 72x 72
mm. The above assembled electrolyzer cell was tested for hydrogen
production and the observed current density was aprx. 220 mA/cm2 at the
operating voltage of 0.8V.
Example 2
A two cell electrolyzer stacks having exfoliated graphite as flow field plate of
dimensions 150 xlOO mm was used for electrochemical cell operation. The
stack was assembled based on bipolar plate configuration. The anode and
cathode plates are having similar serpentine type flow channels with the area
of 100 x 80 mm. The performance of electrolyzer stack was tested at 60°C and
the corresponding observed current density was aprx. 200mA/cm2 @ 0.8
Vlce l I.
Example 3
The electrolyzer stack made from 15 cells of 200 X50I m m electrode area was
fabricated using graphite material as flow field plate. The stack was assembled
based on bipolar plate configuration. The anode and cathode plates are
having horizontal and vertical type serpentine type flow channels respectively
10
with the area of 130 x 120 mm. The performance of electrolyzer stack was
tested at 60°C and the corresponding observed hydrogen production rate was
100 Ilhr at average cell voltage of aprx 0.520V.
Example 4
A two cell stacks having exfoliated graphite as flow field plate of dimensions
430 x 330 mm was used for electrolyzer cell operation. The stack was
assembled based on bipolar plate configuration. The anode and cathode
plates are having serpentine and pin type flow channels with the area of 296 x
250 mm. The performance of electrolyzer stack was tested at 60°C and the
corresponding observed hydrogen production rate was 701lhr at average cell
voltage of aprx 0.590V.
Example 5
A thirty two cell stacks having exfoliated graphite as flow field plate of
dimensions 430 x 330 mm was used for electrolyzer cell operation. The stack
was assembled based on bipolar plate configuration. The anode and cathode
plates are having serpentine and pin type flow channels with the area of 296 x
250 mm. The performance of electrolyzer stack was tested at 60°C and the
corresponding observed hydrogen production rate was 10001lhr at average
cell voltage of aprx 0.550V.
The present invention thus describes the fabrication of an electrolyzer with low
cost exfoliated graphite material as flow field material instead of coated
metallic flow field plates traditionally used for hydrogen generation.
The process is easily scalable to different dimensions, different densities and
to different patterns and amenable for mass production. The electrolyser
incorporating the EFG plates is economical and efficient for hydrogen
generation. The present invention has been described in detail with few
embodiments enabling a person in the art to understand and visualize our
invention. It is also to be understood that the invention is not limited in its
application to the details set forth in the above description or illustrated in the
11
drawings. Although the invention has been described in considerable detail
with particular reference to certain preferred embodiments thereof, variations
and modifications can be effected within the spirit and scope of the invention
as described herein above and as defined in the appended claims.

We Claim,
1. An electrolyzer having exfoliated graphite separator as reactant flow
field plate for hydrogen generation from water comprising a plurality
of fabricated Membrane Electrode Assembly (MEA), having an
electrolyte membrane arranged between the anode electrode and the
cathode electrode and further provided with a first catalyst layer
between the anode electrode and the electrolyte membrane and a
second catalyst layer between the cathode electrode and the
electrolyte membrane, placed in-between anode and cathode
separator, current collector separated by gaskets1 insulator plates ,
and these cell components are assembled together with anode &
cathode end plates using tie rods with Nuts and washers or other
fastening means for tightening the whole assembly characterized in
that the separator plates act as flow field plates made up of
embossed exfoliated graphite material and when depolarized water
is fed as reactant and a DC power supply connected between the
anode and cathode terminal, it is capable of generating hydrogen.
2. An electrolyzer for hydrogen generation as claimed in claiml,
wherein the electrolyte is water containing depolarizers selected from
alcohols, carbohydrates and amines.
3. An electrolyzer for hydrogen generation as claimed in claiml,
wherein flow field plates are formed by compressing the particles of
exfoliated and expanded natural graphite.
4. An electrolyzer for hydrogen generation as claimed in claiml,
wherein said flow field plate is having serpentine (Fig.3) I pin type
(Fig.4) flow field pattern and includes a number of manifolds so that
each serves as a portion of a corresponding distribution channel for
fluid reactants.
5. An electrolyzer for hydrogen generation as claimed in claim 4,
wherein said flow field plate also consists of inlet and outlet manifold
for reactant circulation in the celllstack.
6. An electrolyzer for hydrogen generation as claimed in claim 4,
wherein said flow field plate is having multiple inlet and outlet
manifolds on a flow field plate for reactant fluid depending on the cell
design.
7. An electrolyzer for hydrogen generation as claimed in claim 4,
wherein said flow field plate is having inlet and outlet manifold for
hydrogen gas collection.
8. An electrolyzer for hydrogen generation as claimed in claim 4,
wherein the relative sizing of the manifold apertures with respect to
one another is not essential and that each may be a different size
depending upon the requirement.
9. An electrolyzer for hydrogen generation as claimed in claim 4,
wherein a sealing surface is provided around the flow field and
various manifold openings and the through holes provided is to
accommodate a seal that is employed to prevent leaking and mixing
of process liquidlgases.
10. An electrolyzer for hydrogen generation as claimed in claiml,
wherein flow field plates have uniform conductivity and is not
anisotropic.
11. An electrolyzer for hydrogen generation as claimed in claiml,
wherein flow field plates are thermally stable in the operating
temperatures of the electrolytic cell.
12. An electrolyzer for hydrogen generation as claimed in claiml,
wherein flow field plates are chemically stable and non corrosive to
the fluid and the gases flowing in its channels.
13. An electrolyzer for hydrogen generation as claimed in claiml,
wherein flow field plates is impervious to the fluids and gases flowing
in its channels
14. An electrolyzer for hydrogen generation as claimed in claiml,
wherein flow field plates is free of void and non porous and has a
density of 0.9- 1.5 g/cm3.
15. A process of generating hydrogen by the electrolyzer having
exfoliated graphite separator as reactant flow field plate as claimed in
claim1 , comprising the step of:
a) pumping the depolarized water from the liquid tank(l0) into the
electrolyser stack (1 3) by a pump (1 5),
b) applying current across the two terminals of the electrolyzer using
a DC power supply unit(22) having constant current and constant
voltage mode provisions. I
c) generation of Hydrogen at cathode (12) and C02 at the anode
(11) of the electrolizer ( 13) and
d) separation of Con from the unutilized solution coming out of the
electrolyser in a gas /liquid separator (14) and sending back the
unutilised solution to the reservoir ( liquid tank) for recirculation to
said electrolyser while collecting the C02 as by product.