Electrolytic generation of nitrogen

Details Electrolytic generation of nitrogen Electrolytic generation of nitrogen

1999-07-14

2001-10-09

Gorgos, Kathryn (Department: 1741)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C205S780500, C205S617000

Reexamination Certificate

active

06299743

ABSTRACT:

FIELD OF THE INVENTION
The invention is in the field of methods and apparatus for electrochemical generation of nitrogen and hydrogen gases. Particularly generation of nitrogen gas from organic hydrazides (RCONHNH
2
) and hydrazino-carboxylates (RCOONHNH
2
), and amino guanidine salts, circuits for spontaneous oxidation of such nitrogen compounds to generate nitrogen gas and mechanical transducers actuated by the nitrogen gas so produced, particularly in the field of fluid dispensers.
BACKGROUND OF THE INVENTION
The controlled electrolytic generation of gases is useful to convert chemical to mechanical energy in a variety of applications. For example, a variety of lubricant or fluid delivery systems driven by the electrolytic generation of a gas are known. For example, U.S. Pat. No. 4,023,648 to Orlitzky et al. (1977) shows a lubricant applicator driven by gas generated in an electrochemical cell and provides a method for the electrochemical generation of hydrogen gas.
Fluid dispensers driven by electrochemically generated gases, and other electrochemical transducers may often be used in circumstances which give rise to special operational requirements. Typically, components of any electrolytic cell used in such an application must be stable over time and over a range of temperatures. In such devices, it is undesirable to have highly reactive gases generated, such as hydrogen or oxygen. Once the circuits are closed to initiate electrolytic gas generation, it is desirable to have relatively fast electrode reactions with low overpotential (i.e. a small difference between the electrode potential under electrolysis conditions and the thermodynamic value of the electrode potential in the absence of electrolysis), small concentration polarisation of solutes across the cell (i.e. rapid diffusion of reactants to the electrode surfaces), and small separator resistance effects (i.e. little resistance caused by solid separators within the cell. It is also desirable to produce gases from a small amount of material, i.e. to have efficient gas generation and high stoichiometric coefficients for gaseous reaction products.
Hydrogen and oxygen gases are used in a variety of known electrochemical gas generators. One disadvantage of such systems is the chemical reactivity of those gases. Another disadvantage of hydrogen in particular is that it diffuses relatively rapidly through a variety of polymeric barriers that might otherwise be used to contain the electrolytically generated gas in a mechanical transducer, such as a fluid dispenser.
Nitrogen is a relatively inert gas that may usefully be produced by electrolytic reactions to provide controlled amounts of gas. However, existing methods for the electrolytic generation of nitrogen suffer from a number of disadvantages.
U.S. Pat. No. 5,567,287 issued to Joshi et al. (1996) discloses a solid state electrochemical nitrogen gas generator for fluid dispensing applications. Nitrogen is produced in that system by the electro-oxidation of a decomposable solid material of the generic formula A
x
N
y
in a divided electrochemical cell, where “A” is an alkali metal such as sodium or lithium, “N” is nitrogen, x is 1 to 3 and y is 1 to 3. Example compounds disclosed therein include LiN
3
(lithium nitride) and NaN
3
(sodium azide). The azide half cell reaction in such a system (reaction
1
) may however be slow, in part because of the high overpotential required for the electro-oxidation of azide.
2N
3−
→3N
2
+2e
−
  (1)
To overcome the problem of the sluggish kinetics of the azide half-cell, additives such as thiocyanate may be used to catalyse the iodine mediated formation of nitrogen from azides, as in reactions 2 and 3:
2I
−
→I
2
+2e
−
  (2)
I
2
+2N
3
−
2I
−
+3N
2
  (3)
However, such systems suffer from the disadvantages that azides are toxic and the thiocyanate salt catalysts are also toxic. The presence of toxic compounds may make it difficult to dispose of a device which generates nitrogen gas from azides.
SUMMARY OF THE INVENTION
The invention provides methods and devices for the electrochemical generation of nitrogen from organic nitrogen compounds, such as hydrazides (RCONHNH
2
), the corresponding organic hydrazino-carboxylates (RCO
2
NHNH
2
) and amino-guanidine salts (e.g. aminoguanide bicarbonate H
2
NNHC(NH)NH
2
.H
2
CO
3
). A variety of organic hydrazides and hydrazino-carboxylates may be used, and empirically tested for performance. For example, in the hydrazides and hydrazino-carboxylates “R” may be selected from suitable alkyl, alkenyl, alkynyl or aryl groups, in some embodiments methyl, ethyl, or benzyl. The alkyl, alkenyl and alkynyl groups may be branched or unbranched, substituted or unsubstituted. Some such compounds may not work in all embodiments, as determined by routine functional testing. The utility of such compounds may, for example, be routinely assayed in accordance with the guidance provided herein, including the Examples set out herein in which alternative nitrogen compounds may be substituted for routine test purposes.
The present invention also provides methods and devices for the auto-electrolytic generation of nitrogen, using electrochemical cells that comprise both a nitrogen compound capable of acting as a reductant in an electrochemical reaction to produce nitrogen gas, and an electrochemical oxidant capable of driving the oxidation of the nitrogen compound.
The present invention also provides a housing for electrochemical gas generating cells. The housing acts to compress a flexible electrochemical cell to help maintain electrochemical contacts in the cell over a prolonged period of operation, during which the compositions within the cell may contract while gas is evolved from the cell. The housings of the invention may be used with a wide variety of gas-generating electrochemical cells, including hydrogen, oxygen and nitrogen generating cells.
The housings of the invention may also be adapted to enclose a plurality of cells, in which case the cells may be arranged in series to increase the potential drop across the cells. There may be advantages associated with arranging electrochemical gas generating cells in series to increase the potential of the circuit, particularly when the cells are to be used in fluid dispensers. A higher potential difference across the cells allows for the use of a larger (and in some embodiments variable) resistance in the circuit of the electrochemical cell. The larger the resistance, the less sensitive the circuit is to variations in temperature.
The sensitivity of the circuit (the electrochemical cell and the external electronic components) to temperature change generally comes about as a result of the fact that increasing temperature will generally decrease the effective resistance of the electrochemical cell and increase the current in the circuit. However, increasing temperature will normally increase the resistance of the electronic components of the circuit (i.e. the external electronic resistance) and this partially compensates for the effect of temperature on the electrochemical cell. In other words, the temperature coefficient of resistivity of the electrochemical cell, which is an ionic resistance, is negative, whereas the temperature coefficient of resistivity of the external circuit, which is an electronic resistance, is usually positive (although of a lower order of magnitude than for the cell). Providing for operation with a greater potential in the circuit allows the circuit to include a higher external electronic resistance, and thus makes the circuit less sensitive to temperature changes. In a fluid dispenser, it is generally desirable to provide a constant current that does not fluctuate substantially with temperature in order to provide a constant flow of fluid. Of course, if it is desired to make the circuit temperature sensitive, this may also be accomplished in accordance with the circuits of the invention.


REFERENCES:
patent: 1561308 (1925-11-01), Brown
patent: 18

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