AU2002317082A1

AU2002317082A1 - Method and product for improving performance of batteries/fuel cells

Info

Publication number: AU2002317082A1
Application number: AU2002317082A
Authority: AU
Inventors: Elod Lajos Gyenge; Joey Chung-Yen Jung; Klaus Heinrich Oehr; Colin W. Oloman; Steven Splinter
Original assignee: MagPower Systems Inc
Current assignee: MagPower Systems Inc
Priority date: 2001-08-01
Filing date: 2002-06-25
Publication date: 2003-05-29
Anticipated expiration: 2022-06-25

Description

METHOD AND PRODUCTS FOR IMPROVING PERFORMANCE OF BATTERIES/FUEL CELLS

FIELD The present invention relates to methods and products

for improving the performance of magnesium containing metal

air battery/fuel cells in one or more ways including:

increasing anode utilization efficiency (suppressing

hydrogen evolution) , increasing energy density, increasing

power density or increasing cell voltage.

BACKGROUND

It is well known in the prior art that certain battery

electrodes, especially those used in metal-air

batteries/fuel cells, suffer from undesirable hydrogen

evolution during their "discharge" in which they generate

electrical power or when they are stored, due to corrosion

and/or moderate energy density i.e. watt-hours/litre output

and/or low cell voltage. These electrodes include those

containing magnesium and aluminum and/or zinc alone or in

combination, as examples. The production of hydrogen is

described by commercial fuel cell (battery) suppliers (e.g. www.greenvolt.com/fuel cells.htm) . This producer portrays

this as a safety issue. However it also represents a waste of metal fuel . It is well documented that magnesium suffers from

parasitic hydrogen evolution in inorganic electrolytes .

For example Antonyraj (Antonyraj , A. and CO. Augustin,

5 1998, "Anomalous Behaviour of Magnesium Anodes in Different

Electrolytes at High Concentrations", Corrosion Reviews,

16(1-2): 127-138) states "when magnesium metal comes in

contact with aqueous electrolytes, self-dissolution of the

metal and the evolution of hydrogen take place

0 simultaneously" (see pg 131) . Song et al. (Song, G. et

al., 1997, "The Electrochemical Corrosion of Pure Magnesium

in IN NaCl" , Corrosion Science, 39(5): 855-875) indicate

that "under free corrosion conditions, magnesium corrosion

can be considered to occur by the interaction of local

15 anodes and cathodes" (see pg 871). Song et al. suggest

that magnesium can be converted to hydride by the following

electrochemical reaction (see pg 858) :

Mg + 2H⁺ + 2e^" = MgH₂ (1)

>0

MgH₂ + H₂0 = Mg^2* + 20H^" + 2H₂ (2)

Proof of this suggested mechanism is given by Nazarov

!5 et al. (Nazarov, A. P. et al . , 1989, "Formation of MgH₂ on Electrochemical Dissolution of Magnesium in Aqueous Electrolytes, Zashchita Metallov, 25(5): 760-765).

United States Patent No. 5,024,904, issued to Curiel,

describes the use of metal anodes, preferably made of

magnesium, aluminum or magnesium-aluminum alloy, in combination with salt containing electrolytes and air

cathodes for purposes of producing portable, direct current

electrical power. Testing of the Curiel prototype by the

current inventors has revealed the following major weakness: magnesium utilization efficiency as low as 30%

due to parasitic hydrogen evolution.

United States Patent No. 4,908,281, issued to

O'Callaghan describes the undesirable production of hydrogen on aluminum electrodes in aluminum air cells (pg 1

lines 63+) . "As with other batteries this hydrogen can

easily reach explosive concentrations." (page 2 lines 10 to

12) . One of the purposes of the O'Callaghan invention is

to create a system designed to properly vent hydrogen to

help prevent explosions. The electrolyte is designed to

flow upwards and over a weir to discharge aluminum hydroxide product into an electrolyte reservoir. Air is

used to dilute hydrogen below explosive limits. Tuck (Tuck, Clive D.S., Modern Battery Technology, 489-490) also

describes parasitic, gaseous hydrogen evolution on aluminum

contained in aqueous electrolytes.

5 Quraishi et al . (Quraishi, M.A. et al . , 1999,

"Dithiobiurets : A Novel Class of Acid Corrosion Inhibitors for Mild Steel, Journal of Applied Electrochemistry) have

described the inhibition of corrosion/hydrogen evolution on

steel, in strongly acidic environments using dithiobiurets

[0 with the following structure:

H H H

I I I R-N-C-N-C-N-R'

II II

15 S S where R and R' are aryl substituted functional groups such

as phenyl, tolyl and so on.

10 United States Patent No. 5,004,654 issued to Hunter

et al. describes the benefits of a source of tin e.g. tin containing ions such as stannate ions, on undesirable

hydrogen evolution in aluminum/air cells.

.5 United States Patent No. 3,594,235 issued to Moran

describes the use of quaternary ammonium salt containing

electrolyte in combination with metal/air batteries (fuel cells) containing cadmium or magnesium electrodes. The use of quaternary ammonium salt as the sole electrolyte

component other than water, especially at an excessively

high concentration of 10% by weight, makes the Moran

invention prohibitively expensive for non-military

applications.

The prior art related to batteries, especially

metal/air batteries (fuel cells) such as those including

magnesium and aluminum and/or zinc, alone or in combination

e.g. as alloys, has failed to incorporate knowledge in the use of hydrogen evolution inhibitors derived for steel,

especially in highly acidic environments. Attempts to

minimize deleterious evolution of hydrogen have been

generally restricted to the use of exotic and/or expensive

metal alloys.

Finally, the prior art related to magnesium/air

batteries and fuel cells has failed to incorporate

knowledge derived by the aluminum industry related to corrosion inhibition by tin containing electrolytes .

Accordingly, it is an object of the current invention

to provide improved methods for inhibition of hydrogen evolution (improved anode utilization efficiency) and/or

energy density and/or cell voltage and/or power density

improvement in batteries, especially metal/air batteries

(fuel cells) , especially those containing magnesium, magnesium and aluminum, magnesium and zinc.

SUMMARY OF THE INVENTION

The invention relates to a method of improving the

performance of magnesium containing electrodes used in

metal/air batteries (fuel cells) , comprising the addition

of one or more additives to the electrolyte or electrode

surface. More specifically it relates to performance

improvement due to any one of the following factors alone or in combination: the inhibition of hydrogen evolution

(improvement of electrode utilization) , improvement of

energy density, improvement of power density and/or

increase in cell voltage. The additives are selected from

any of the following groups: dithiobiuret, tin, and tin

plus a quaternary ammonium salt.

Advantageously, dithiobiuret additives may be used,

which have the following structure: H H H

I I I R-N-C-N-C-N-R'

II II S S in which either or both of the R and or R' function groups

contain an aryl group (aromatic ring structure) , for

example, in which R is a tolyl group -C₆H₅-CH₃ and R' is a

phenyl group C₆H₅-.

Tin containing additives may be used either in the

electrolyte or on the electrode surface, for example, in

the form of stannate salts such as sodium stannate .

Tin containing additives may also be used either in

the electrolyte or on the electrode surface, for example,

in the form of stannate salts such as sodium stannate, in

combination with a quaternary ammonium salt such as

tricaprylmethylammonium chloride (e.g. Aliquate 336) .

The invention also includes improved metal/air fuel

cells and batteries based on the above methods.

DETAILED DESCRIPTION

The following non-limiting examples show the flexibility of

the invention as applied to magnesium/air battery/fuel

cells: Example 1

Magnesium AM60 alloy sheet anode (94% magnesium and 6%

aluminum content by weight) was submerged together with an air cathode in a seawater electrolyte with and without the

addition of 0.0001 molar dithiobiuret containing p-tolyl

and phenyl R and R' functional groups, respectively. The cell was operated at a discharge current of 5 amperes (32

mamp/cm² starting anode current density) without

replenishment of the electrolyte until the cell voltage

dropped to zero due to dissolution of magnesium plus

aluminum. The electrolyte was initially at room

temperature. The average cell voltage, power density (watts per liter, w/L) energy density (watt hours per liter, Wh/L) and average anode utilization efficiency (100%

- hydrogen production efficiency) for a single-cell system

are summarized below:

Example 2

Magnesium AM60 alloy sheet anode was submerged

together with an air cathode in a 13% by weight sodium

chloride electrolyte with and without the addition of

0.0001 molar dithiobiuret containing p-tolyl and phenyl R

and R' functional groups respectively. The cell was

operated at a discharge current of 5 amperes (32 mamp/cm²

starting anode current density) without replenishment of

the electrolyte until the cell voltage dropped to zero due

to dissolution of magnesium plus aluminum. The electrolyte

was initially at room temperature. The average cell

voltage, power density (watts per liter, W/L) energy

density (watt hours per liter, wh/L) and average anode utilization efficiency (100% - hydrogen production efficiency) for a single-cell system are summarized below:

Example 3

Magnesium AM60 alloy sheet anode was submerged

together with an air cathode in a 24% sodium citrate, 12%

sodium sulphate, 1% sodium chloride (all % by weight)

electrolyte with and without the addition of 0.003 molar

sodium stannate (Na₂Sn0₃) . The cell was operated at a

discharge current of 5 amperes (32 mamp/cm² starting anode

current density) without replenishment of the electrolyte

until the cell voltage dropped to zero due to dissolution

of magnesium plus aluminum. The electrolyte was initially

at room temperature. The average cell voltage, power density (watts per liter, W/L) energy density (watt hours

per liter, Wh/L) and average anode utilization efficiency

(100% - hydrogen production efficiency) for a single-cell

system are summarized below:

Example 4

The experiment in Example 3 above was repeated with the further addition of a quaternary ammonium salt,

tricaprylmethylammonium chloride ( [CH₃ (CH₂) ₇] ₃CH₃N⁺C1^_,

®

Aliquat 336) to the electrolyte at 0.0001 molar

concentration. The average cell voltage, power density

(watts per liter) , energy density (watt hours per liter, Wh/L) and average anode utilization efficiency (100%

hydrogen production efficiency) are summarized below:

This experiment clearly shows the beneficial interaction between tin and quaternary ammonium salt

additives in improvement of the metal/air battery

performance with anodes containing magnesium or its alloys

Example 5

The experiment in Example 4 above was repeated with

the removal of the tin additive (i.e. stannate) from the

electrolyte, while retaining the quaternary ammonium salt

additive Aliquat 336. The average cell voltage, power

density (watts per liter, W/L) , energy density (watt hours

per liter Wh/L) and average anode utilization efficiency (100% - hydrogen production efficiency) are summarized

below:

Although the addition of the quaternary ammonium salt

additive improved the cell performance, the combination of

tin containing additives with the quaternary ammonium salt

and magnesium containing anodes, was clearly superior to

that of a quaternary ammonium salt alone, as shown by the

energy density and anode utilization efficiency comparison

with Example 4. The combination of a tin additive and a

quaternary ammonium salt suppressed hydrogen evolution on a

magnesium containing anode to a greater extent than either

additive used alone. Example 6

In order to investigate the effect of the additives in

conjunction with zinc-containing magnesium alloys

5 experiments were performed using AZ31 alloy sheet anode submerged with an air cathode in an electrolyte mixture

composed of 24% by weight sodium citrate, 12% by weight

sodium sulfate and 1% by weight sodium chloride.

Experiments were performed with and without additives

10 present in the electrolyte. The additives were either

0.0001 molar Aliquat 336 or a combination of 0.0001 molar

Aliquat 336 and 0.003 molar sodium stannate. A discharge

current per cell of 5 A was applied (anode current density

at start of 35 mamp/cm²) and the experiment was continued

15 until the cell voltage dropped to 0.8 V. The electrolyte

was initially at room temperature and it was used without

replenishment. The average cell voltage, power density

(watt per liter, W/L) , energy density (watt hours per

liter, Wh/L) and anode utilization efficiency (100% -

-0 hydrogen production efficiency) per single cell are

summarized below:

The above example shows that using the combination

additive (i.e. quaternary ammonium salt Aliquat 336 and

stannate) in conjunction with the AZ31 alloy, improved all

4 performance factors of the magnesium-air fuel cell containing a magnesium-aluminum-zinc alloy. Accordingly, while this invention has been described

with reference to illustrative embodiments, this

description is not intended to be construed in a limiting

sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon

reference to this description. It is therefore contemplated that the appended claims will cover any such

modifications or embodiments as fall within the scope of the invention.

Claims

WE CLAIM :

1. A method of improving performance of magnesium containing electrodes used in metal/air batteries or fuel

cells, comprising:

(a) adding one or more additives to an electrolyte or

an electrode surface, said additives selected from the group consisting of: dithiobiuret, tin,

and tin plus a quaternary ammonium salt .

2. The method according to claim 1, wherein said tin is

added to said electrolyte as a stannate salt.

3. The method according to claim 1, wherein said tin is

added to said electrode surface as tin metal, for example as a magnesium/tin alloy.

4. The method according to claim 2, wherein said stannate

salt is sodium stannate.

5. The method according to claim 1, wherein said

dithiobiuret has the structure:

H H H

I I I R-N-C-N-C-N-R'

II II s s

6. The method according to claim 5 , wherein one or both

of the R and R" function groups contain an aryl group with

an aromatic ring structure.

7. The method according to claim 6 , wherein said R group

is a tolyl group (-C₆H₅-CH₃) and said R' group is a phenyl

group (-C₆H_S-) .

8. A method of inhibiting hydrogen formation on a metal

containing magnesium, comprising: allowing a dithiobiuret

additive to contact a surface of said metal.

9. The method according to claim 8, wherein said allowing

step is achieved by adding said dithiobiuret additive to a

liquid which is in contact with said metal.

10. The method according to claim 9, wherein said metal is

dipped in a dithiobiuret containing liquid and then allowed to dry.

11. The method according to claim 10, wherein said liquid

is allowed to dry by evaporation.

12. The method according to claim 1, wherein said

quaternary ammonium salt is tricaprylmethyammonium

chloride .

5 13. The method according to claim 1, wherein said

magnesium containing electrode also contains aluminum

and/or tin and/or zinc.

14. An improved metal/air fuel cell or battery,

0 comprising:

(a) a magnesium-containing electrode;

(b) an air electrode,-

(c) an electrolyte; and

(d) one or more additives, said one or more additives

5 being added to said magnesium-containing

electrode or said electrolyte, said one or more

additives being selected from the group

consisting of: dithiobiuret, tin, and tin plus a

quaternary ammonium salt,

:0 wherein said one or more additives results in a performance

improvement of said metal/air batteries or fuel cells.

15. The improved fuel cell/battery according to claim 14,

wherein said tin is added to said electrolyte as a stannate

salt.

5 16. The improved fuel cell/battery according to claim 14,

wherein said tin is added to said electrode surface as tin

metal, for example as a magnesium/tin alloy.

17. The improved fuel cell/battery according to claim 15,

0 wherein said stannate salt is sodium stannate.

18. The improved fuel cell/battery according to claim 14 ,

wherein said dithiobiuret has the structure:

H H H

II II s s

!0

19. The improved fuel cell/battery according to claim 18, wherein one or both of the R and R" function groups contain

an aryl group with an aromatic ring structure.

20. The improved fuel cell/battery according to claim 19,

:5 wherein said R group is a tolyl group (-C₆H_S-CH₃) and said

R' group is a phenyl group (-C₆H_S-) .

21. An improved fuel cell/battery comprising: (a) a magnesium-containing electrode;

(b) an air electrode;

(c) an electrolyte; and

(d) a dithiobiuret additive contacting a surface of

said magnesium-containing electrode,

wherein said dithiobiuret additive inhibits hydrogen

formation at said magnesium-containing electrode.

22. The improved fuel cell/battery according to claim 21,

wherein said contact is achieved by the adding said

dithiobiuret additive to a liquid which is in contact with

said metal .

23. The improved fuel cell/battery according to claim 21,

wherein said metal is dipped in a dithiobiuret containing

liquid and then allowed to dry.

24. The improved fuel cell/battery according to claim 23,

wherein said liquid can evaporate.

25. The improved fuel cell/battery according to claim 14,

wherein said quaternary ammonium salt is

tricaprylmethyammonium chloride.

26. The improved fuel cell/battery according to claim 14,

wherein said magnesium containing electrode also contains aluminum and/or tin and/or zinc .