Polarography is the measurement of the current that flows in solution as a function of an applied voltage. The actual form of the observed polarographic current depends upon the manner in which the voltage is applied and on the characteristics of the working electrode. The new gas polarographic H2 sensor shows a current level increment with concentration of the gaseous H2 similar to those relating to metal ions in liquid electrolytes in well-known polarography. This phenomenon is caused by the fact that the diffusion of the gaseous H2 through a gas diffusion hole built in the sensor is a rate-determining step in the gaseous-hydrogen sensing mechanism. The diffusion hole artificially limits the diffusion of the gaseous H2 toward the electrode located at the sensor cavity. This gas polarographic H2 sensor. is actually an electrochemical-pumping cell since the gaseous H2 is in fact pumped via the electrochemical driving force generated between the electrodes. Gaseous H2 enters the diffusion hole and reaches the first electrode (anode) located in the sensor cavity to be transformed into an H+ ions or protons; H+ ions pass through the electrolyte and reach the second electrode (cathode) to be reformed to gaseous H2. Gas polarographic 02 sensors are commercially available; a gas polarographic 02 sensor was used to prove the feasibility of building a new gas polarographic H2 sensor

Barile, Ron

Dominguez, Jesus A.

NASA Technical Reports Server

NEW GAS POLAROGRAPHIC HYDROGEN SENSOR 
Jestls A. Dominguez, Ron Barile 
ASRC Aerospace Corporation 
Kennedy Space Center, Mail Stop ASRC-28. 
Kennedy Space Center, Florida 32899 
Abstract 
Polarography is the measurement of the current that 
flows in solution as a function of an applied voltage. The 
actual form of the observed polarographic current 
depends upon the manner in which the voltage is applied 
and on the characteristics of the working electrode. The 
new gas polarographic H2 sensor shows a current level 
increment with concentration of the gaseous H2 similar to 
those relating to metal ions in liquid electrolytes in well-
known polarography. This phenomenon is caused by the 
fact that the diffusion of the gaseous H2 through a gas 
diffusion hole built in the sensor is a rate-determining 
step in the gaseous-hydrogen sensing mechanism. The 
diffusion hole artificially limits the diffusion of the 
gaseous H2 toward the electrode located at the sensor 
cavity. 
This gas polarographic H2 sensor. is actually an 
electrochemical-pumping cell since the gaseous H2 is in 
fact pumped via the electrochemical driving force 
generated between the electrodes. Gaseous H2 enters the 
diffusion hole and reaches the first electrode (anode) 
located in the sensor cavity to be transformed into an H+ 
ions or protons; H+ ions pass through the electrolyte and 
reach the second electrode (cathode) to be reformed to 
gaseous H2. 
Gas polarographic 02 sensors are commercially 
available; a gas polarographic 02 sensor was used to 
prove the feasibility of building a new gas polarographic 
H2 sensor. 
Background. 
Gas polarographic 02 sensors are commercially available; a 
gas polarographic 02 sensor was used to prove the 
feasibility of building a new gas polarographic H2 sensor. 
Figure 1 shows a schematic diagram of the commercial gas 
polarographic 02 sensor. The sensor consists of an 
electrochemical 02-pumping cell and a cap with a 
cylindrical gas diffusion hole for the artificial diffusion 
control of the gaseous 02. A tiny heater is mounted on the 
cap of the sensor to keep the sensor temperature at 400-450 
°C. The 02-pumping cell has the form of a thick disk of 
450-500-pm thickness and 60-70-pm diameter and is made 
of yttria-stabilized zirconia (YSZ); porous platinum
electrodes are attached to both sides of the electrolyte disk. 
The voltage is applied so that the electrode in the sensor 
cavity is negative (cathode) to control the diffusion of 
gaseous 02 and reduce it to negative 02 ions. These 
negative ions pass through the YSZ electrolyte and reach the 
positive electrode (anode), where they subsequently reform 
to gaseous 02. 
In existing and commercially available gas polarographic 
02 sensors, the electrode in the sensor cavity has to be 
negative (cathode) as gaseous 02 is reduced to negative ions 
(02-) at the cathode. In contrast, a gas polarographic H2 
sensor should have the positive electrode (anode) in the 
sensor cavity as gaseous H2 needs to release electrons and 
generate H^ ions or protons. These H+ ions (or protons) 
should pass through the YSZ electrolyte to the negative 
electrode (cathode), where they subsequently reform to 
gaseous H2. Figure 2 shows a schematic diagram of the 
new gas polarographic H2 sensor. This gas polarographic 
hydrogen sensor is actually an electrochemical-pumping cell 
since the gaseous H2 is in fact pumped via the 
electrochemical driving force generated between electrodes. 
YSZ is used as electrolyte in most of the commercial gas 
polarographic 02 sensors as well as in Solid Oxide Fuel 
Cells (SOFC) because of its excellent oxygen ion (02) 
conductivity. YSZ should be also a protonic (H') conductor 
to use it as electrolyte in the new gas polarographic H2 
sensor. The conduction of protons through solid oxide 
electrolytes has been subject of some discussion, and it is 
generally believed to occur through a "hopping' mechanism. 
In this the conduction occurs by the proton associating with 
an oxygen atom, forming a transient hydroxyl specie that is 
"transported" through the material by the migration of the 
hydrogen ion from one oxygen atom to a neighboring 
oxygen atom. Since the exact details remain unclear, 
dynamic quantum mechanical modeling would be needed tO 
understand the mechanism of proton conduction. Recent 
research has reported electrochemical properties of different 
ceramic protonic conductors and the feasibility of H2 
pumping for the purpose of tritium extraction in fusion fuel 
systems. 
Sensing Mechanism. 
Having a gas binary system and assuming steady-state 
conditions, no reaction, stagnant background gas, and mass 
transfer only in one direction and rate-limited at the diffusion
https://ntrs.nasa.gov/search.jsp?R=20120003305 2019-08-30T19:24:45+00:00Z
20'— O,+4e 
101 
Pie? iru rn electrodes 
hole, the limiting output current is predicted to be a linear 
function of the logarithm of the background gas 
concentration. See enclosed appendix A for detailed 
derivation of this function. This linear relation between the 
output limiting current and the logarithm of the background 
molar fraction has a slope that includes the area and the 
length of the gas diffusion hole, the gas pressure, the gas 
temperature, the diffusion coefficient of the gaseous specie in 
the binary gas mixture, the Faraday constant, and the number 
of electrons per mol.e of the gaseous specie transferred 
between electrodes (this number is 4 for 02 and 2 for H2 as 
indicated in figures 1 and 2 respectively). 
B/ifuaion hole d/e rreter: 60-70 pm	 Healer 
Figure 1. Schematic diagram of commercially available gas 
polarographic 0 2 sensor. 
	
Offus/on hole dierneler 60-70 pm	 Healer 
—
600pm. 
	
Anc/o	 H, - 2H	 + 2E	 2e 4-
	
P/at/mire electrodes	 Y2 electrolyte	 J H' 
N__________ ___ 
Cathode
2H + 2e - H, 
Figure 2. Schematic diagram of new gas polarographic H2 
sensor. 
It is generally known that YSZ exhibits purely oxygen ionic 
conduction (with no electronic conduction) over a wide range
of oxygen partial pressures. Researchers have corroborated 
that YSZ is also a protonic conductor. Research work is 
currently oriented to build a ceramic electrolyte that can 
perform proton conduction at temperatures lower than those 
needed with YSZ. Further research might lead to the 
feasibility of building ceramic materials with hydrogen 
pumping capabilities for the purpose of tritium extraction in 
fusion fuel systems. 
Results. 
Plot 1 displays the voltage-current characteristics of the 
commercial gas polarographic 02 sensor. Plot 2 shows the 
voltage-current characteristics of the gas polarographic H2 
sensor in H2-N2 gas mixtures (from 0-3% H2) when voltage 
is applied so that the electrode in the sensor cavity is positive 
(see figure 2) in order to control the diffusion of gaseous H2. 
As expected, limiting current proved to be dependent on H2 
composition and increased with increasing H2 concentration 
in the ambient gas mixture. However, plot 2 does not show 
defined limiting-current plateaus similar to those shown in 
plot 1. In both cases we used the same sensor with a 
difference only on the electrode polarity. The size of the 
diffusion hole (60-70-pm diameter) was actually designed by 
the sensor manufacturer to limit diffusion of 02-size 
molecules. A gas polarographic H2 sensor with a smaller 
diffusion hole (20-30-pm diameter) should lead to defined 
limiting-current plateaus. 
Plots 3 and 4 show output current generated by the gas 
polarographic H2 sensor at different ambient H2 
concentrations (from 0.25-2.0% H2) using 0.3 and 0.6 volts 
respectively. As in plot 2, plots 3 and 4 show results 
generated by a gas polarographic H2 sensor having the 
positive electrode (anode) in the sensor cavity (see figure 2). 
As expected, the output limiting current proved to be 
dependent on H2 composition; the output limiting current is 
increased as the gaseous . H2 concentration increases. As in 
the case of the gas polarographic 02 sensing, a linear relation 
between the output limiting current and the logarithm of the 
mole fraction of the background gas was also observed for 
the gas polarographic H2 sensing as shown in plot 5; further 
test runs and precise measurements of the diameter and the 
length of the diffusion hole are required to corroborate this 
linear relation (see appendix A). The output-current response 
peaks shown in plots 3 and 4 should be attenuated with a 
smaller diffusion hole; 20-30 pm might be a suitable 
diffusion hole diameter for a gas polarographic H2 sensor. 
As mentioned above, the size of the diffusion hole (60-70-pm 
diameter) was actually designed to limit diffusion of 02-size 
molecules. 
Plot 6 shows output current generated by the above sensor 
but having the negative electrode (cathode) in the sensor 
cavity as set up in the gas polarographic 02 sensor. The 
output limiting current shows no dependency on H2 
concentration proving that limiting-current mechanism on a
H2 pumping cell can be possible only if the positive 
electrode (anode) is in the sensor cavity. 
The above results show that if the diffusion of gaseous H2 is 
artificially controlled and a protonic conductor is used as 
electrolyte, a gaseous H2 sensor might be built based on a 
gaseous H2-sensing mechanism attained on the analogy of 
the widely known 02-sensing mechanism. 
Based on the results shown in this H2 sensing approach along 
with what is already known from the gas polarographic 02 
sensor, the concept of polarography should be reconsidered 
and redefined to be classified in two types—gas 
polarography and the liquid polarography—on the basis of 
the phase of the measured substance, the diffusion of which 
is a rate-determining step. For example, the concept of 
chromatography is well defined and clearly divided into the 
gas chromatography and liquid chromatography because of 
the kind of mobile phase. Thus, taking analogous 
consideration, this proposal is clearly reasonable. 
Research and development on solid materials that can 
perform proton conducting at intermediate temperature have 
dramatically increased due to the renewed interest in the 
industry for solid-electrolyte fuel cells. Commercially 
available Proton Exchange Membranes (PEM) might be 
utilized as solid-electrolyte in the gas polarographic H2 
sensor. PEM performs proton conducting at relatively low 
temperature (5-120 °C). Research on new zirconia-based 
electrolyte materials are currently focus on obtaining 
materials that can perform proton conducting at intermediate 
temperature; YSZ, the electrolyte currently used in SOFC 
and commercial gas polarographic 02 sensor, requires 
temperatures higher than 400 °C. Figure 3 shows the proton 
conductivity of BCY and SCZT85 (two new zirconia-based 
materials) along with YSZ proton conductivityU]. 
200	 200	 0Q	 4(0	 0cc	 000	 0(0	
4iCt.JV2 12CC 20X. :420 
Figure 3. Proton Conductivity of BCY, SCZT85 and YSZ 
with Temperature'
Conclusion. 
The engineering and manufacturing requirements of this new 
gas polarographic H2 sensor would be basically the same as 
those already in place for the commercially available gas 
polarographic 02 sensors. The gas polarographic H2 sensor 
would need a smaller diffusion hole and the anode in the 
sensor cavity. Further research is needed to determine the 
proper size of the diffusion hole and evaluate different types 
of solid electrolytes including electrolytes with more H+ ion 
and less 0-2 ion conduction selectivity and capable of 
conducting protons at lower temperatures. 
An array of two gas polarographic sensors with different 
diffusion hole sizes and voltage polarities might be able to 
detect three components-02, H2, and humidity. A smart 
sensing system equipped with the proper algorithm might be 
able to extract mutual cross-sensitivity interference of H2 and 
humidity on the 02 sensor and 02 and humidity on the H2 
sensor. 
References. 
[1] D.Browning, M. Weston, J.B. Lakeman, P. Jones, M. 
Cherry, J. T. S. Irvine, D. J. D. Corcoran; "Proton 
Conducting Ceramics for Use in Intermediate 
Temperature Proton Conducting Fuel Cells", Journal of 
New Material for Electrochemical Systems, 5, page 28 
(2002). 
[2] K. Lundstrom, M. Shivaraman, and C. Svenson, J. 
Apply. Phys., 46, 3976 (1975). 
[3]Y. Tan, T. C. Tan; "Sensing Behavior of an 
Amperometric Hydrogen Sensor", J. Electrochem. Soc., 
Vol 142, No 6, June 1995. 
[4] T. Usui, A. Asada, K. Ishibashi, M. Nakazaka, 
"Humidity Sensing Characteristics in a wet air of a gas 
polarographic Oxygen Sensor Using Zirconia 
Electrolyte", Electroshem. Soc. 138 (1991) 585-588. 
[5] J. Gopaul, W. C. Maskell, K. E. Pitt, "Planar Oxygen 
Sensor: Part I. Effect of Crazing of Zirconia Thick Film 
on an Alumina Substratem J. Appl. Electrochem., 29 
(1999) 93-100. 2: 
0
.4.
'Lot(1 - YH2) 
Experintentaf - Predicted! 
Plot 5. Experimental vs Predicted results. 
0.00018 
0.000 16 
0.00014 
cc 
0.00012 
0.00010 
0.00008 
0
0.00006 
0.00004 
0.00002
0
Co,?ca 5%-O2.rnge Sensor: Limiting Current Profile. 	 Gas Potarograptric H2 Sensor 0.6 Volts. Anode in Sensor Cavity. 
Ges: 02 in N2. 27C, 760 Torr, 5000 seem. Aerit 8,2002 	 Gas: 112 in NZ 27C. 768 Tori: 6000 seem. May 22. 2002 
0.00	 0.25	 0.50	 0.75	 1.00	 1.25	 1.50	 1.75	 2.00	 2.25
Voltage (It) 
0%02(N2i.1.0%O2-2.5%02I 
Plot 1. 02 Limiting Current Profile 
Gas Polarugraphic 112 Sensor via YSZ Electrol4e: Limftlng Current Profrie. 
Gas:H2 in N2, 27C, 768 Ton'. 6000 seem. May20. 2002
500 
450 
400 p 
350 
300 P 
250 
200 
150 
100 
50 
0.2	 0.3	 0.4	 0.5	 0.6	 0.7	 0.8	 0.9 
lime (fir) 
RCA Inlet: ppm 112 - Output Current (nticroAmp) I 
Plot 4. H2 measurement profile at 0.6 V with anode in 
sensor cavity. 
Gas Potarorjrapltic H2 Sensor' Predicted vs. Expel intenlat Perforritance 
Gas: H2 in N2. 27 C, 768 Tort: 6000 seem. Moy 2002. 
19900 
17400 
14900 
12400 
9900 
7400 (0 
cc 4900 
2400 
-100
0.1 
2000 
1750 
1500 
1250 
1000 
o 750 
.500 
0
250
0 4-" I	 I	 I	 I	 U	 T""Y	 ' 
0.0	 0.!	 0.2	 0.3	 0.4	 0.5	 0.6	 0.7	 0.8	 0.9	 1.0	 1.1	 1.2
Voltage (it) 
Ho%H2 -- 1 %112 --2%H2 -.--3%H21 
Plot 2. H2 Limiting Current Profile.
250 
225 
200 
175 
iso P S 
125 
100 
75 
5° 
25 
•0 
0.9
Gas Potarographie 112 Sensor 0.3 Volts. Anode in Sensor Cavity. 
Gas: 112 in N2, 27C. 768 Ton'. 6000 sccm. May20. 2002 
19900 
17400 - 
14900
________ 
--EJ
	
j} 
kLJLL$hL
: 
moe (In) 
RGA lintel): ppm 112 	 Output Current (nticroAnlp) I
Gas Potarographlc H2 Sensor 0.6 Volts. Cathode in Sensor Cavity: 
Gas: H2 in N2. 27C. 768 Ton'. 6000 sccm. May 20. 2002 
19900 
17400 
14900 
5 12400
' 9900
7400 
cc 4900 
2400 
-100
0.00	 0.10	 0.20	 0.30	 0.40	 0.50	 0.60	 0.70 
Tinte (hi) 
	
RCA (input): ppm H2	 Output Current (rnicmAmpt I
120 
110 
100 
90 
80 
70 
60 
40 
30 
20 
10 
0 
Plot 3.. H2 measurement profile at 0.3 V with anode in	 Plot 6. H2 measurement profile at 0.3 V with cathode in 
sensor cavity, 	 sensor cavity. 
Assuming steady state conditions, no reaction, and mass 
transfer only in one direction (x direction) equation (2) 
becomes: 
aNAX/ax = O	 (3) 
Assuming a binary system denoting A as the component 
being measured and B as the background gas, N can be 
expressed as: 
NA = -CDAB 8YAII3X + YA(NA + NB)	 (4) 
Where: 
NA Molar flux of specie A. 
NB Molar flux of specie B. 
C Molar concentration. 
D Diffusion coefficient of A into B. 
YA Molar fraction of specie A.
Assuming specie B (background gas) is stagnant (NB = 0), 
equation 4 becomes: 
NA = -CDAB 3YA/3X + YANA	 (5) 
Rearranging equation (5): 
NA = -[CDAB/( l - YA)] 3YA/8x	 (6) 
Substituting equation (6) into equation (3): 
a {[CDAB/( l - YA)J 3yA/3x}/8x = 0	 (7) 
The general equation for molar flux of specie A ( NA) can be 
expressed as: [11(1- YA)JaYA/8X = k 1 (8) 
RA	 Reaction of specie A. 
APPENDIX A 
Gas Polarographic Sensor:
Theoretical Mechanism 
Diffs10 8Grner
x=o 
X=L 
Assuming a binary mixture (A and B where B is the 
background gas). The limiting current is expressed as: 
1L —IFSN A .)	 (1) 
Where
Limiting current. 
H Number of electrons per mole of specie A 
transferred from the anode to the cathode. 
H = 4 if A specie is 02 (02 + 4e - 202). 
H = 2 if A specie is H 2 (2H + 2& - H2). 
F	 Faraday constant (electrical charge of a mole of 
electrons). 
S	 Cross area of diffusion barrier. 
NA(x=L) Molar flux of specie A at the electrode.
Integrating (7) once: 
aCAIat + ( aN,1a aNAyI5Y + aN/az) = RA	 (2) 
Where 
CA	 Molar concentration of specie A.
N, NAN, and NAZ Molar flux of specie A in x, y, and
z directions.
After separating variables and integrating again: 
-Ln(1- YA) = K 1 x ^k2	 (9) 
Using the boundary conditions YA = YA(amb) at x=0 and A = 
YA(XL) at x=L we find constant K 1 and K2. YA(b) is the 
The reference limiting-current value ( IL) is often 
measured using a calibration gas containing a known 
composition value (YA(amb.) (Ref.)) that is equal to the upper 
composition value of the sensor's range. 
molar fraction of specie A in the ambient and A(x=L) is the 
molar fraction of specie A at the bottom of the diffusion hole 
and the top of the electrode. 
K 1 = ( 1IL)Ln[(1 - YA(b))/( i - YA(=L))]	 (10) 
K2 = -Ln(1 - YA(amb))	 (11) 
Substituting (10) into (8): 
aYA/aX = ( 1 IL) Ln[( I - YA(amb))/( 1 - YA( X=L))] (12) 
Substituting (12) into (6): 
NA = -(CDAB/L)Ln[(1 - YA(b))/(l - YA(L))} (13) 
Assuming: 
YA(X L) << A(ainbient) and Ideal gas (C = PIRT) equation (13) 
becOmes: 
NA =- [(PDAB)/(RTL)]Ln[1 - A(amb.)I
	
(14) 
Substituting (14) into (1): 
= -[(J4FSPDAB)/(RTL )IlLn [ 1 - YA(b) ]	 (15) 
Equation (15) states that the limiting current can be used to 
measure the composition of specie A since there is a linear 
relation between 'L (limiting current) and Ln [YA(bjent) - 11. 
The slope of this linear relation is (FTFSPD AB)/(RTL) leading 
to a limiting current dependence on the geometric of the 
diffusion hole (L = length and S = Cross area), pressure (P) 
temperature (1), specie A itself (H = number of electrons per 
mole of specie A transferred from the anode to the cathode), 
and DAB = diffusion coefficient of A into B. To avoid the 
need of using these parameters and assuming that T, P, and 
D remain unchanged during the measurement, equation 
(15) may be reduced to: 
-	 (Ref.)ILn[y	 (Ref.) -
	 I Ln [YA(amb) - 1]	 (16) 1L {IL 
Where
Limiting current at 
Mole fraction of specie A. 
Reference limiting current at y	 (Ref.) A(ainb.) 
y	 (Ref.) A(amb.)	 Reference mole fraction of specie A.


New Gas Polarographic Hydrogen Sensor

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