Title of Invention	A PROCESS FOR IN SITU PREPARATION OF ELECTROLYTE FOR MEASURING HYDROGEN IN HIGH TEMPERATURE FLUIDS
Abstract	An electrolyte for measuring hydrogen in high temperature fluids which comprises a mixture of MCI2 (where M represents Ca or Sr), LiCI and M'H2 (where M' represents Ca2 or Sr) or LiH. Preferably the mixture consists of 70 (±2) mol % LiCI, 13(±2) MOL% CaHCI or SrHCI, 17(±2) moi% CaCI2 or SrCI2). Specifically the mixture consists of 70 mol% LiCI, 16 mo1% CaC12, 14 mo1% CaHCI.

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THE PATENTS ACT 70 (Act 39 of 70) COMPLETE SPECIFICATION (see Section 10) ) ANlMPROVED ELECTROLYTE USEFUL FOR MEASURING HYDROGEN IN HlGIflEMPERATURE FLUIDS Department of Atomic Energy, A department of the Govt of India having its office at Anushakthi Bhavan , Chathrapathy Shivaji Maharaj Marg, Mumbai, 400030 , Maharashtra , India The following specification particularly describes the nature of this invention and the manner in which it is to be performed This invention relates to an improved electrolyte for measuring hydrogen in high temperature fluids. The electrolyte of the present invention is particularly useful for measuring hydrogen in high temperature fluids such as inert gases, liquid metals and fused salts. The electrolyte of the present invention is highly hydride ion conducting mixtures of MCl2 (where M represents Ca or Sr), LiCl and M H2 (where M represents Ca or Sr) or LiH. The present invention also relates to a process for the preparation of the improved electrolyte. Proton conducting electrolytes such as Hydrogen Uranyl Phosphate(HUP), SrCe03 etc. require humid conditions for their conductivity. Therefore they are not compatible for use in dry environments such as gas mixtures of hydrogen and argon, nitrogen etc., fused salt mixtures of Alkali and alkaline earth halides and in liquid metals. HUP is also thermally unstable at high operating temperatures. The choice is thus restricted to hydride ion conducting electrolytes. The first electochemical measurement of hydrogen in sodium [R.J NEWCOMBE AND J.THOMPSON, Journal of Polarographic Society,14(1968)104] was unsuccessful. They used aluminosilicate glass as the electrolyte, which was attacked by sodium. [R.B.Holden and N.Fuhrman, USAEC Report, ANL-7520,Vol. 1(1968)262] employed concentration cells that used a eutectic melt of LiCl-KCl containing 0.2 to 2 mol% of either LiH or CaH2 as electrolyte. But these electrolytes had significant electronic conductivity because of the dissociation of the less stable KH in the eutectic, which increased the presence of free metal in the electrolyte. A solid electrolyte, CaCl2 mixed with CaH2, which is thermodynamically stable under the operating conditions, was therefore used for this purpose [C. A. Smith, CEGBXReport, UK, RD/B/N/2331(1972)], [T. Gnanasekaran, K.H. Mahendran, R. Sridharan, V. Ganesan, G. Periaswami and C.K. Mathews, Nuclear Technology,90(1990)408]. The CaCl.2-CaH2 pseudo-binary system is characterised by a compound CaHCI which is the hydride ion conducting phase in the biphasic electrolyte. However, the solid electrolyte did not have a good ionic conductivity and showed appreciable electronic conductivity at 450°C and at low hydrogen pressures(in the range of 0.05 Pa - 5 Pa) [R. Sridharan, K.H. Mahendran, T. Gnanasekaran, G. Periaswami, U.V. Varadaraju, C.K. Mathews, Journal of Nuclear Materials,223 (1995)72 ]. The presence of electronic conduction in the electrolyte led to internal shorting of the cell and caused the polarisation of the electrodes. So, there is need to develop an electrolyte which has improved characteristics such as (i) high hydride ion conduction at high temperatures such as in the range:400 to 500°C and (ii) good thermodynamic stability at very low hydrogen pressures (in the range of 0.05 Pa - 5 Pa). Only an electrolyte with such an electrolyte will be helpful to construct a meter that gives linear and stable output while measuring hydrogen at very low levels. The main objective of the present invention is , therefore , to provide a hydride ion conducting electrolyte that works at high temperature for measuring hydrogen concentration in non-aqueous medium such as gas mixtures of hydrogen and argon, hydrogen and nitrogen etc., and also in high temperature liquid metals and fused salts. Another objective of the present invention is to provide a hydride ion conducting electrolyte that works in moisture free environment for measuring hydrogen concentration in non-aqueous medium such as gas mixtures of hydrogen and argon, hydrogen and nitrogen etc., and also in high temperature liquid metals and fused salts. The invention is based on the identification of a ternary electrolyte, which has high hydride ion conduction in the temperature range 400° - 500°C. We found that to achieve the above desired properties, a new electrolyte, based on a ternary salt mixture of LiCl,CaCl2 and CaHCl, can be developed by considering the melting points of pure salts, the stability of hydrides and high ionic conductivity at the operating conditions. It was clear from the reports of T. Gnanasekaran, K.H. Mahendran , R Sridharan ,V Ganeasn G Periaswami and C.K.Mathews , Nuclear Technology,90 (1990) 408 and R Sridharan, K.H. Mahendran, T. Gnanasekaran, G. Periaswami, U.V. Varadaraju, C.K. Mathews, 1995, Journal of Nuclear Materials, 223 (1995) 72, that CaCl2 and CaHCl system would provide a thermodynamically stable electrolyte at high temperatures and low hydrogen pressures of our interest even though it has poor hydride ion conduction. It is also known from literature that LiCl and CaCl2 system has an eutectic temperature of approximately 490°C and LiCl and SrCl2 system has an eutectic temperature of ~ 475°C. Hence, we expected intuitively, that a mixture of LiCl, MC12 (M = Ca or Sr)and M'H2 (M' = Ca or Sr) or LiH, in proper ratio would provide an electrolyte system that would exhibit high hydride ion conduction and also would be thermodynamically stable at very low hydrogen pressures in the temperature range 400 to 500 C and thus would provide an electrolyte with the desired properties. Accordingly the present invention provides an electrolyte for measuring hydrogen in high temperature fluids which comprises a mixture of MC12 (where M represents Ca or Sr), LiCl and LiH or M H2 (where M represents Ca, or Sr). In a preferred embodiment of the present invention there is provided an electrolyte for measuring hydrogen in high temperature fluids which comprises a mixtures of 70(±2) mol % LiCl, 13(±2) mol% CaHCl or SrHCl, 17(±2) mol % CaCl2 or SrCl2). In still further embodiment of the present invention there is provided an electrolyte for measuring hydrogen in high temperature fluids which comprises a mixture of 70 mol% LiCl, 16 mol% CaCl2, 14 mol% CaHCl The electrolyte of the present invention is found to possess a liquid phase at temperatures^ >460°C. This facilitates high H" ion conduction in the electrolyte. Electrochemical cells formed with a reference hydrogen electrode and anyone of these electrolytes and a sample electrode can detect hydrogen concentrations of 50 ppb onwards in liquid metal samples and up to percentage levels in the gas samples. Preparation of Electrolyte and Electrode Materials The chemicals used, for the preparation of the electrolyte of the present invention, were purified from commercially available CaCL; and AR grade LiCl . The purified mixture of these two salts were taken along with 99.5% pure calcium shots in the electrolyte compartment inside an argon atmosphere glove box. It was placed in a vessel and sealed at the top with copper metal gasket. The calcium was hydrided in hydrogen flowing outside the electrolyte contained in a thin walled iron thimble at 723 K for 4 to 5 days to form CaH2 in situ. This process ensures the formation of CaHCl phase which is formed by the reaction between CaCl2 and CaH2 that is produced in situ. This process eliminates pick up of impurities from hydrogen gas by calcium to form oxides, nitrides and hydroxides, because only pure hydrogen can diffuse through the iron thimble and react with calcium. After raising the temperature to -823 K, another thin walled iron thimble containing the reference electrode materials namely a mixture of Li and LiH was then inserted into electrolyte. The assembly was slowly cooled to room temperature. This process thus ensures formation of the electrolyte-mixture in very high purity. An electrochemical cell that was formed by using this new molten salt based electrolyte and Li.LiH as the reference electrode was tested for its performance in gas mixtures of H2 and Ar, H2 and N2 and also at low hydrogen pressure systems (0.05 Pa - 5 Pa) such as in liquid sodium. Construction of the Meter A schematic of the electrochemical hydrogen sensor is shown in Fig. 1 given below . The sensor was constructed by the following procedure: An iron thimble of 16 mm outer dia, 0.25 mm wall thickness and 40 mm height for housing the electrolyte and another one of 8mm OD, 0.25mm wall thickness and 25mm height for holding the reference electrode-materials, were machined from a pure soft iron bar. These pieces were decarburised in sodium containing saturated amount of calcium at 973 K for 72 hours. These were then welded to stainless steel pipes as shown in Fig.l and meter was assembled. A knife-edged flange of 70 mm diameter was welded at the top end of the electrolyte compartment and a swagelok fitting was welded at the top of the pipe to hold the reference compartment. The knife-edged flange at the top of the probe helped to connect the meter to the bench-top loops containing sodium. A copper gasket pressed between the knife-edges of the flanges was used as a seal between the meter and its housing. TESTING OF THE METERS Meter Testing Assembly A mini sodium loop the schematic of which is shown in Fig. 2 was used to test the meters for their performance. The loop consists of a meter pot into which the meter is inserted, a cold-trap, an intermediate buffer heating zone between cold trap and meter pot, and connecting pipes to close the loop. Metal sheathed resistance heaters of ~ lkW capacity were used for heating. Sheathed chromel-alumel thermocouples, were brazed to the loop at various locations to measure the temperature profile in the loop. Thermowells were provided in the meter pot and cold trap to measure and control temperatures in these parts within ± 1 C. PI temperature controllers with auto reset facility were used for controlling the temperatures. 2 or 3 grams of NaOH was initially added to cold trap while loading sodium into the loop in an argon glove box. The loop had the capacity to hold 2500 ml of sodium which was circulated by an electromagnetic pump consisting of a permanent magnet (0.6 Tesla) and two electrodes through which a DC current was passed across the liquid sodium in a direction perpendicular to the magnetic field. The hydrogen level in the sodium was maintained at any particular concentration by controlling the cold trap temperature, TCT. The sensor output was measured by means of a voltmeter with an input impedance of ~ 109Ohms. Typical Sensor Performance Unlike the sensors which used the solid electrolyte CaCl2-CaHCl, [R Sridharan, K.H. Mahendran, T. Gnanasekaran, G. Periaswami, U.V. Varadaraju, C.K. Mathews, 1995, Journal of Nuclear Materials, 223 (1995) 72], this sensor does not show the problems of electronic conduction at low hydrogen pressures equivalent to hydrogen concentration of 50 to 1000 ppb in sodium. The electrolyte used in this investigation has much higher conductivity ( ~10"2 Siemens, cm"1) than CaCl2-CaHCl (~10~6 siemens.cm"1) electrolyte. log CH(/ppm) *ECHM-1 EMF(mV) **ECHM-2 EMF(mV) -1.30564 83.5 103 -1.27445 82 101.3 -1.22826 80 97.5 -1.19786 76 96 -1.15281 70.4 88.2 -1.12316 66.4 85.4 -1.0938 66.6 85.3 -1.06472 59 76 -1.05029 57.3 77.6 -1.03593 53.5 73.7 -0.90978 35.8 51.4 -0.81536 25.3 40 -0.80213 25.4 42.7 -0.5985 -6 5.5 *ECHM-1 used 70 mol% LiCl , 16 mol% CaCl2 , 14 mol% CaHCl as electrolyte. **ECHM-2 used 70 mol% LiCl , 16 mol% SrCl2 , 14 mol% SrHCl as electrolyte. The output showed a linear relationship with log CH- The expressions obtained by least square analysis is given by: - E(mV)= -76*:4-125.5 log [H]Na (ECHM-1) E(mV)= -65.7-131.8 log [H]Na (ECHM-2) Advantages of the invention The new hydride ion conducting electrolyte works at high temperature (400 to 500 °C) and in moisture free environment. Th electrolyte , therefore , can be used for measuring hydrogen concentration in high temperature fluids such as inert gases, liquid metals (such as sodium)and fused salts. WE CLAIM : 1. A process for in situ preparation of electrolyte for measuring hydrogen in high temperature fluids comprising providing a mixture of MCI2 (where M represents Ca or Sa), LiCI and M'H2 (where M' represents Ca2 or Sr) or LiH, hydriding calcium to form CaH2 which leads to formation of CaHCI phase. 2. A process as claimed in claim 1 wherein the mixture consists of 70 (±2) mol % LiCI, 13(+2) mol % CaHCI or SrHCI, 17(±2) mol % CaCI2 or SrCI2). 3. A process as claimed in claim 1 wherein the mixture consists of 70 (±2) mol % LiCI, 16 mol % CaCI2, 14 mol % CaHCI. 4. A process for in situ preparation of electrolyte for measuring hydrogen in high temperature fluids substantially as herein described. Dated this 17tn day of October 2001 11

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