Passed exhaled air through electrochemical cell containing alkali metal carbonate aqueous solution, and utilizes platinized electrodes causing reaction of oxygen at cathode with water in electrolyte, producing hydroxyl ions which react with carbon dioxide to form carbonate ions

Huebscher, R. G.

Williams, C. F.

English

NASA Technical Reports Server

May 1972	 B72-10194 
NASA TECH BRIEF 
Ames Research Center	 -4r 
NASA Tech Briefs announce now technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Carbon Dioxide Concentrator 
The problem: 
To remove the carbon dioxide from the air exhaled 
by a pilot so that it may be recycled in a closed sys-
tem breathing loop.
02(CO2Depleted) 
02 , CO2 
P1	 Cathode (-)	 fl 
cJ	 Electrolyte	 Anode (+)	 U 
CO 2 Supported 
H20, in Matrix	 H2 
H2 
The solution: 
Pass the exhaled air through an electrochemical 
cell which contains an aqueous solution of an alkali 
metal carbonate and utilizes platinized electrodes to 
cause the reaction of oxygen at the cathode with the 
waterin the electrolyte. to produce hydroxyl ions 
which react with carbon dioxide and form carbonate 
ions. The reaction of hydrogen at the anode with 
hydroxyl ions produces water and depletes the by-
droxyl ions so that carbon dioxide is given off, thus 
completing the transfer of carbon dioxide from the 
oxygen atmosphere to the hydrogen atmosphere 
(which is vented).
How it's done: 
A single cell of the carbon dioxide concentrator 
(hydrogen-depolarized) is shown in the figure. Potas-
sium carbonate electrolyte is held in a porous asbestos 
matrix (0.076 cm) between two, type AB-6, platinized 
electrodes with active areas approximately 233 cm2 
(10.7 cm x 21.8 cm). Expanded silver metal (0.15-cm) 
is used as a spacer in both the anode and cathode 
gas cavities. The metal spacers are in contact with the 
electrodes and the respective silver current-collector 
plates (0.05 cm). The spacers transmit current from 
the electrodes to the current-collector plates, aid in 
heat removal, and provide physical support for the 
matrix. 
The results of parametric studies using the potas-
sium carbonate electrolyte indicate that cell or mod-
ule performance (voltage) increases with hydrogen 
flow rate and operating temperature but is indepen-
dent of oxygen flow rate changes over the range 
evaluated. Carbon dioxide transfer increases with 
increasing cathode carbon dioxide concentration and 
is relatively unaffected by operating temperature and 
oxygen flow rate over the ranges evaluated. 
In a hydrogen-depolarized, carbon dioxide-concen-
trator, an equilibrium exists between carbonate and 
bicarbonate ions; this equilibrium is dependent on 
carbon dioxide concentration, operating temperature, 
and electrolyte concentration. When potassium car-
bonate is used, potassium carbonate is in equilibrium 
with potassium bicarbonate which has a limited solu-
bility. If the electrolyte becomes too concentrated as 
a result of water imbalance, a precipitate can form at 
the anode of the cell and reduce cell performance. 
Cesium carbonate has also been examined as an 
electrolyte; in this instance, bipolar plates of titanium-
(continued overleaf) 
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clad copper can be substituted for the silver current-
collector plates. Platinum or platinum screen elec-
trodes can be substituted for anodes; expanded 
titanium can be used for gas cavity spacers. 
The module employing cesium carbonate as elec-
trolyte was not as susceptible to moisture imbalance 
problems as that with potassium carbonate; as the 
module operating temperature increased (37.8° to 
54.4°C), stack voltage increased as expected. Because 
cesium carbonate and cesium bicarbonate have higher 
solubilities than the corresponding potassium salts, 
they provide a wider tolerance to electrolyte water im-
balance. Thus, cesium carbonate electrolyte appears 
to be much more desirable in the carbon dioxide-
concentrator application. 
Notes: 
1. The following documentation may be obtained 
from: 
National Technical Information Service 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.95)
Reference: 
NASA CR-73397 (N71-11204), 
Aircrew Oxygen System Development-
Carbon Dioxide Concentrator Subsystem 
Report 
2. The system has application for aircraft, spacecraft, 
and submarines. 
3. Requests for additional information may be di-
rected to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP 7240194 
Patent status: 
No patent action is contemplated by NASA. 
Source: Charles F. Williams and

Richard G. Huebscher of

TRW, Inc.

under contract to

Ames Research Center 
(ARC-10245) 
B72-10194	 Category 05


Carbon dioxide concentrator

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