Solid-state hydrogen sensors based on SrCl2 electrolyte

Solid Sta,2elonics 28-30 (1988) 1688-1692 North-Holland, Amsterdam

SOLID-STATE HYDROGEN

SF.aNSORS B A S E D

ON SrCI2 E L E C T R O L Y T E

R.V. KUMAR and D.J. FRAY Department of MaterialsScience and Metallurgy, Universityof Cambridge, PembrokeStreet, CambridgeCB2 3QZ, UK Received 28 July 1987; in revised version 17 November 1987

A solid-state sensor for measuring the partial pressure of hydrogen has been developed using a SrCl2 electrolyte, with either a Ag/AgCl or a Ni/HiCl, reference electrode for application in the temperature ranges of 600-700 K and 700-900 K, resl~tively. Although it is a chloride ion conductor, SrCh can be applied in the detection of hydrogen by using a thin layer of SrHCI/SrCI2as ar~auxiliary phase on the working surface of the electrolyte.

1. Intredection

Quantitative detection oi hydrogen is of considerable importance in a variety of industrial situations, such as the synthesis of ammonia, methanol and other chemicals, petroleum refining, the manufacture of semiconductors, chemical reduction, corrosion control and general metallurgical purposes [ 13]. In metal refining, for example, hydrogen easily dissolves, as a result o:? interaction with moisture in the atmosphere, in the liquid alloys of iron, aluminium, copper, etc. When the metal solidifies during casting, the hdyrogen solubility decreases dramaticaUy leading to the formation of porosity, which adversely affects the mechanical properties of the alloy. Once formed, the porosity is difficult to eliminate, and therefore the detection and control of hydrogen levels prior to the casting of special alloys is sometimes crucial [4 ]. In nuclear reactors, controlling the levels of hydrogen in liquid sodium coolant is important in order to avoid the formation both of sodium hydride, which can produce blockage or impair heat transfer, and of sodium hydroxide which can aid the corrosion of structural and cladding m__at.erials [ 5 ]. There is, therefore, a great demand for the development of robust, reliable, convenient and inexpensive hydrogen sensors, as the conventional analytical techniques are not entirely adequate in the detection and monitoring of hydrogen. Gas sensors based on solid-state galvanic cells have a remarkable potemial

for meeting the very demanding criteria of ruggedness, reliab~ty, simplicity o f operation, quick and selective response, low energy consumption, low cost

and compatibility with microelectronics. The initial step in the development of solid-state electrochemical sensors is the selection of a statable solid electrolyte and a reference mater~al. Application of these sensors for quantita:i,:.- measurement of chemical species has mainly been confined to those species which are ionically transferred in the dec= trolyte. Using hydrogen ion conducting solid electrolytes such as HUP, a limited number of hydrogen sensors have already been developed for room temperature applications [ 6-11 ]. This approach, how.. ever, has been restrictive in the development of a high temperature hydrogen sensor as the hydrogen ion conducting electrolytes are unstable at high temperatures (e.g. hydrogen uranyl phosphate) [ 12 ]; or have very low conductivity (e.g. hydrogen [3-alumina) [ 13 ]; or are extremely hygroscopic (e.g. Call2) [14]. To circumvent this problem, a new approach has been ~.~nployed. Thi~ method, which is rapidly gaining m~:nentum, is based upon the use of an auxi!iary pha~e nn the wnrking ~urfaea nflho, e.lae.trnl~ in order that there is a chemical coupling between the species being measured and the species ionically mobile in the electrolyte [ 15-18 ].

0 167-2738/88/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

R. V. Kumar, D.J. Fray~Solid-state hydrogen sensors based on SrCl~

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2. High temperature hydrogen sensors

In the development of solid-state hydrogen sensors for application at high temperatures, SrC12 has been used as an electrolyte. It is a typical chloride ion conductor with fluorite structure, consisting of a simple cubic array of anions with every other centred position occupied by a cation [ 19-22]. Each chloride ion is located at a site of tetrahedral symmetry in relation to the cations which are'placed on a fcc lattice. The ionic conductivity of SrCl2 ranges from ~ l 0 - 4 r - , c m - ~ at 600 K to ~ l 0 - 2 r - ~ cm- ~at 900 K. It is possible to enhance the conductivity of this electrolyte by an addition of up to 20 mole% A1203 [ 22 ]. Some of the electrolytes used in this work were based upon such a composite electrolyte of the composition SrC12-10 mol% .~2Oa. The performance of these composite electrolytes, however, was identical to that of the pure SrCl2 electrolytes, and therefore, the conductivity of pure SrCl2 electrolyte in the temperature range 600-900 K is considered adequate for their application in sensors. It has been possible to apply SrCl2, a chloride ion conductor, in the detection of hydrogen, by using a thin layer of SrHCI/SrCI2 as an auxiliary phase to facilitate the coupling between hydrogen gas and chloride ion. SrHCI is a stable compound with a m.p. of 1113 K and it is easily formed by reacting SrC12 and SrH2 in an hydrogen atmosphere at temperatures of around 1200 K. SrHCI resembles mica in appearance and is stable at high tempe:~atures in non-oxidising atmospheres. As can be seen from the SrCI2SrH2 phase diagram (fig. 1), SrCI2 and SrHCI co-exist in equilibrium up to the eutectic temperature of 953 K[23]. The availability of convenient, stable and reversible solid electrodes is one of the crucial factors in the development of solid-state :msors and the following reference electrodes were evaluated for use with the SrCl2 electrolyte: Sr/SrCI2, C/SrCffSrC12; Ag/ AgCI, and Nb'NiCi2. Only Ag/AgCi and Ni/NiCi2 references proved to be suitable with respect to stability and reproducibility. The EMFs based on Sr/SrCt2 and C/SrCJSrCI2 references, were low and irreproducible; presumably because of the onset of electronic conduction at the low partial pressures of chlorine prevailing in the reference systems. No investigation was carried out to ascertain the electrolytic domain

Mol % 900

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20

40

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t

80

60

/ #

i

800

800

oo Q. =E I--

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I SrCi 2

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1

SrHCl

S---rH2

Fig. 1. S r C l 2 / S r H 2 p h a s e d i a g r a m .

of the SrCl2 electrolyte. As demonstrated by Wagner [24], similar problems of onset of electronic conductivity are encountered on using a Ca/CaF2 reference with a CaF2 electrolyte.

3. E=eetl~hemie=d principle The hydrogen sensor can be represented by the foil.owing galvanic cell: H.~/Ar

( - -gas mix, ....

stainless

reference Ni o r A g (

st~ le~d I S r H C I , SrCI2 I'~ 712 ~lcc,rode lead

, + )"

The EMF (E) across the cell is determined by the difference in the partial pressures of chlorine at the two electrodes according ~o Nems~'s law:

£=(2.303RT/2F)[logPclrf-logPa2(gas)],

(1)

where PCl~fis the known partial pressure of the reference electrode [25], Pa2(gas) is the partial pressure of chlorine on the auxiliary phase as determined by the test gas, R is the universal gas constant, F is

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R.V. Kumar, D.J. Fray/Solid-state hydrogen sensors based on SrCi2

Faraday's constant, and T is the experimental temperature. The partial pressure of chlorine on the auxiliary phase (Pcl:(gas)) is related to the partial pressure of hydrogen of the test gas (Pro(gas)) and is determined by the following chemical equilibria:

Reference

StainlessSteel - Lead

Lead

Silica Wool

2SrCl2(s) + H2(g) ~2SrHCI(s) + Cl2(g), and

can be derived

StainlessSteel

as:

Cap

log Pcu (gas) =log PH2(gas) -- (AG°/2.303RT),

Quartz Tube

(2) where AG ° is the standard-free energy change for the above chemical reaction. On the substitution of the log Po2 value in eq. (1), the relationship between the measured EMF and the partial pressure of hydrogen of the test gas can be calculated as: log PH.,(gas) = log Pcl~f + (AG ° 12.303RT) -, ( 2 F E / 2 . 3 0 3 R T ) .

Reference Electrode

rCl 2 Pellet SrCl, SrCI2/SrHcl

[[

_ ~ -- QuartzTube __

StainlessSteel

Tube

Fig. 2. A schematic diagram of the hydrogen sensor.

(3)

The AG ° values were experimentally determined by measuring the EMF of the cell subjected to a test gas of PH2= 0.1. Data for the standard free energy of formation of SrHCI are not available in the literature for comparisons with the present calculations. However, it may be pointed out the AG ° values, as experimentally determined for calibration purposes do include the thermal EMF associated with the stainless steel lead/Ni or Ag lead.

4. Experimental A schematic diagram of the hydrogen sensor is shown in fig. 2. A very thin layer of SrCI2/SrH2 (6:1 molar ratio) is placed at the bottom of a stainless steel tube and pressure is applied to this layer using a stainlesssteel rod and a stainless steelcap fitted to the tube. The entire assembly is heated to 1200 K hi an H2 atmosphere in order to form a thin layer of the auxiliary phase SrC12SrHC1 at the bottom end, inside the stainless steel tube. After forming this layer. a tight fitting quartz tube open at both ends is inserted into the stainless steel tube. A sintered SrC12 pellet is placed on top of the auxiliary phase just inside the quartz tube and reference electrode powder is rammed on top of the electrolyte. A second quartz

tube protects the lead wire from the reference, while firmly pressing against the electrode system. A tight stainless s*.~! car is used not o,Ay to maintain good contact between the electrolyte and the auxiliary phase, but also to provide the lead wire from the working electrode. The sensor is then placed inside an alumina tube located in a resistance furnace and subjected to the following gas mixtures: (i) 10% H2/Ar, (ii) 1% H2/ Ar, (ih) 1000 ppm H2/Ar, (iv) 500 ppm H:/Ar, (v) 100 ppm H2/Ar and (vi) pure argon. The cell EMFs were measured with a Keithley digital electrometer with an input impedance of 10 ~4 ~ .

~. Results and discussion Fig. 3 presents the results of the tests at 673 K with a Ag/AgCI reference and at 873 K with a Ni/NiCI2 referenc~ The slopes of the EMF versus log Pn2 gave a response of 65 mY/decade at 673 K and 85 mV/ decade at 873 K which is iri good agreement with the values of 68 and 87 given by the Nernst equation, and the EMFs were found ~o be independent of the flow rate of the test gas. The response time was in the order of ~ 1-10 min at higher temperateres and higher concentrations of hydrogen, but it took longer

R. E Kumar, D.J. Fray~Solid-state hydrogen sensors based on SrCl2

~

AgCI or NiCI2. Reliable results were not obtained when these sensors were subjected to pure H2 gas, presumably because AgCl and NiC12 can react wi~h H2 at 1 atm, to produce HCI gas at high partial pressures (table 1). Alternative reference systems are being evaluated in order both to extend the life and to increase the operating temperature of such sensors.

' NilNiCl I ref. '

'

1691

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u~ 0.4

6. Conclusion

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High temperature solid-state hydrogen seasors have been developed using SrCl2 as the solid electrolyte and SrCI2/SrHCI as the hydrogen sensitive auxiliary phase, while Ag/AgCI or Ni/NiCI2 served as the reference electrode. Further work is needed in order to extend the life and to increase the operating temperatures of such sensors.

-2

log P..~ l

!

l

|

i

,

,

Ag/AgCl ref. 673 K

0.7

References

0 >

0.5

0.3 t

I

I

t

-6

I

I

-4

I

-2

log PH2 Results

Fig. 3. Plot of potential versus log PH2.

( ~ 15-45 min) at lower temperatures and lower concentrations of hydrogen. After several hours of testing, the EMFs of these cells began to drift to lower values, which could be caused by a chemical reaction between SrCl2 and Table 1 Reaction between H2 and AgCl or NiCI2. Chemical

Temperature

PH~

P,o

AgCl

700

1 10 -2 10-4

5.55 0.55 0.05

NiC12

800

1 10-2 10-4

2.84 0.28 0.03

[ l ] R.M. Dell, in: Solid state protonic conductors for fuel cells and sensors, Part II, eds. J.B. Goodenough, J..Jel~s~, ~,d M. Kleitz (Odense University Press, Odense, l ~ ;) p. 13. [21 P.D. Hess and G.K. Tumbull, in: Hydrogen in Metals, Proc. Conf., eds. I.M. Bemstein and A.W. Thompson (ASM, 1974). [3] S.B. Lyon and D.J. Fray, Br. Corros. j. 19 (t984) 23. [4] C.E. Ransley and H. Neufeld, J. Inst. Metals. 74 (1948) 599. [5] M. Hobdell, P. Simm and C.A. Smith, CEGB report (Nov., 1979) p. 15. [6] J3. Lundsgaard, J. Malling and M.L.S. Birchall, Solid S~ate Ionics 7 (1982) 53. [7] J. Schoonman, D.R. Franceschetti and J.W. Hanneken, Ber. Bunsenges. Physik. Chem. 86 (1982) 701. [8] S.B. Lyon and D.J. Fray, Solid State Ionics 9/10 (1983) 1295. [9] G. Hultquist, Corros. Sci. 26 (1986) 173. [1o] R.V. Kumar and D.J. Fray, in: Chemical Sensors, Proc. 2nd Intern. Meeting, Bordeaux, France, 1986, eds. J.-L. Aucoutuder, J.-S. Cauhape, M. Destiiau, P. Hagenmullcr, C. Lucat, F. Menil, J. Partier and J. Salardenne (University Bordeaux, 1986) p. 306. [11] J. Schoonman, J.R. De Roo, C.W. De Kreuk and A. Mackor, in: Chemical Sensors, Proc. 2nd Intern. Meeting, Cauhape, M. Destfiau, P. Hagenmuller, C. LucaL F. Meni!, 3. Partier and J. Salardenne (University Bordeaux, 1986) p. 319. [12] M.G. Shillon and A.T. Howe, Mater. Res. Bull. 12 (1977) 701. [13] T.A. Wheat, A. Ahmad, A.K. Kuriakose and .I.D. Canaday, Div. Report, Energy Research Prog. (CANMET, Canada, 1985).

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R.V. Kumar, D.2 ~ ~',z~,lSo!id-state hydrogen se~sors based on SrCI:

[14] R. Gee and D.J. Fray, Met. Trans. B9 (1978) 427. [15] K.T. Jacob, D.B. Rao and H.G. Nelson, J. Electrochem. Soc. 125 (1978) 758. [16] M. Itoh and Z. Kozuka, Trans. Japan Inst. Metals. 26 (1985) 17. [171 K.T. Jacob, M. lwase and Y. Waseda, Adv. Ceram. Mat. 1 (1986) 264. [181 G. H~tzel and W. Weppner, Solid State Ionics 18/19 (1986) 1223. [191 A.S. Dworkin and M.A. Bredig, J. Phys. Chem. 72 (1968) 1277.

[201 M. Dixon and M.J. Gillan, in: Fast ion transport in solids, eds. P. Vashishta, J.N. Mundy and G.K. Shenoy (NorthHohand, Amsterdam, 1579) p. 701. [21] P.J. Bendall, C.R.A. Cadow and B.E.F. Fender, J. Phys. Cl 7 (1984) 794. [221 S. Fujitsu, H. Kobayashi, IC Koumoto and H. Yanagida, J. Electrochem. SOc. 133 (1986) 1497. [23] P. Ehrlich, B. Air and L. Gentsch, Z. Anor~. Allg. Chemic 283 (1956) 65. [241 C. Wagner, J. Electrochem. SOc. 115 (1968) 933. [251 O. Kubaschewski and C.B. A|cock~ in: Metallurgical thermochemistry (Pergamon, New York, 1977).