The Palladium Generator-Separator-A Combined Electrolytic Source and Sink for Hydrogen in Closed Circuit Gas Chromatography J. E. Lovelock,l P. G . Simmonds, and G. R. Shoemake Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif. An electrolytic cell with a hollow palladium alloy cathode i s known as a source of high pressure, ultrapure hydrogen. An extension of the principles involved has been used to construct a practical device to serve as a combined source and sink for hydrogen. The cell electrolyzes water to generate hydrogen at the cathode of the cell in sufficient quantity to transport a gaseous sample through a gas chromatographic column. The hydrogen i s subsequently removed at the anode of the cell, prior to the gas chromatographic detector. When the cathode and anode are connected through a gas chromatographic column, the cell operates at 100% electrolytic efficiency for the generation and removal of hydrogen. This device provides a means of sample enrichment and facilitates special gas chromatographic techniques such as flow and pressure programming.
AN ELECTROLYTIC CELL with a hollow palladium alloy cathode is well known as a source of high pressure (up to 40 atmospheres) ultra-pure hydrogen (1, 2). There is no complete description of the physical process by which hydrogen, liberated at the cathode surface, is transferred against a pressure gradient to the inner space of the hollow cathode tube. However, there is no doubt that in practice, palladiumhydrogen generators are stable and reliable devices. Their electrolytic efficiency approaches 100 for current densities less than 0.1 A cm-2. Commercial hydrogen generators of this type are used as a source of hydrogen carrier gas for gas chromatography. The unique selectivity of hydrogen permeation through palladium has been used recently in another gas chromatographic application. With hydrogen as the carrier gas, sample components in the effluent stream of a gas chromatograph column can be obtained in pure form free of carrier gas by passing the stream into a palladium alloy tube heated in air or oxygen. This procedure was proposed as the basis of a separator between a gas chromatograph and a mass spectrometer by Lucero and Haley (3), and has been proved in practice by Simmonds et al. (4). It has also been successfully employed as a transmodulator (5) to transfer the dilute stream of sample components in the column carrier gas to a precisely controlled lesser flow of some chosen second carrier gas. By this means the sensitivity, accuracy, and flexibility of operation of a gas chromatograph can be much improved. This paper describes
x
Present address, Bowerchalke, Nr. Salisbury, Wilts., England. Correspondence should be directed to Dr. G. R. Shoemake, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, Calif. 91103. (1) A. S . Darling, Platinum Metals Rev., 7 , 126 (1963). (2) J. D. Jacobsen, ANAL.CHEM., 37, 319 (1965). (3) D. P. Lucero and F. C. Haley, J. Gas Chromatogr., 6, 477
(1968). (4) P. G. Simmonds, G. R. Shoemake, and J. E. Lovelock, ANAL. 42, 881 (1970). CHEM., ( 5 ) J. E. Lovelock, K. W. Charlton, and P. G. Simmonds, ibid., 41, 1048 (1969).
the construction, performance, and practical application of a combined source and sink for hydrogen carrier gas. It consists of a simple electrolytic cell in which both electrodes are hollow tubes of palladium alloy. The cathode is the hydrogen source and the anode is the sink for hydrogen. BASIS OF OPERATION
The palladium separator or transmodulator consists of a narrow tube made of palladium (75 x)-silver (25 %) alloy. Pure palladium has been found to have less satisfactory mechanical properties and has a lower diffusion rate for hydrogen (6). When such a palladium alloy tube is heated in air or oxygen to temperatures in the range 120-350 "C hydrogen diffuses through the metal walls and rapidly reacts with the oxygen at the outer surface to form water vapor. The potential of this chemical pump is determined by the hydrogen pressure at equilibrium in the reaction: 2 H2 0 2 = 2 HzO. In practice, Young (7) has found that residual hydrogen pressures with a palladium tube heated in oxygen are as low as loe8 Torr. Provided that the gas to be pumped is hydrogen, a heated palladium membrane with one side in contact with oxygen, is one of the simplest vacuum pumps. Not only can it reach very low pressures, but also its pumping capacity is generous. A pumping capacity of 0.1 Torr liter sec-l cmd2is generally available. The anode compartment of an electrolytic cell provides a convenient oxidizing environment. It therefore seemed worthwhile trying to combine in the same electrolytic cell, a palladium alloy cathode tube to generate hydrogen and a palladium alloy anode tube to remove hydrogen and return it to the electrolyte. Such a combination is pregnant with advantages. They are as follows: (1) The two electrodes of the cell are very similar. In a basic medium, such as a molten alkali, the electrode reactions are suspected to be 2 HtO 2 e-' = H2 2 OH-. The forward reaction occurs at the cathode and the reverse reaction at the anode. The potential of the electrochemical reaction is approximately 0.83 V. Hence, the equilibrium potential of the cell tends toward zero and will always be less than that of a conventional hydrogen generator which requires approximately 1.23 V for the electrolysis of water. It follows that the power required to cycle a given quantity of hydrogen through the electrochemical cell is less than that required to generate an equivalent quantity of hydrogen. (2) The continuous recycling of the hydrogen ensures that no change occurs in the chemical composition of the electrolyte so that, in principle at least, the cell can operate indefinitely. (3) The flow of hydrogen from the cathode is determined by Faraday's law. Thus, 96,500 coulombs would generate 11.2 liters of hydrogen at STP. A current of 1 A is therefore equivalent to 6.96 ml of hydrogen per minute at STP.
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(6) A. S. Darling, Symposium on Less Common Means of Separation, Institution of Chemical Engineers, 103, 1963. (7) J. R. Young, Rev, Sci. Instrum., 34, 374 (1963).
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Figure 1. Construction of palladium-silver electrolytic hydrogen generator-separator
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Figure 3. Removal of hydrogen from a mixed stream of hydrogen and nitrogen at different applied potentials. Parameter: Hydrogen flow rate ( A ) 2.0, ( B ) 5.4, (0 15.2 ml per minute. Nitrogen flow rate constant at 2.15 ml per minute
electrolyte. The performance of the cathode is impaired if other metals are electrolytically deposited upon its surface. Iron and nickel appear to be particularly bad in this respect. Before use, the anode and cathode tubes were cleaned as follows: 1. Washed with chloroform. 2. Dried, then heated to dull red heat for 1 hour and then heated in air at 500 "C for 10 hours. 3. Soaked for 24 hours in 10% HCI. 4. Washed with water and dried by evaporation in air.
AMPERES
Figure 2. Rate of hydrogen production and removal as a function of current EXPERIMENTAL
Apparatus. The construction of the electrolytic cell used in the experiments here reported is illustrated in Figure 1. The cathode of the cell is a hollow tube of palladium (7573silver (25x) alloy, 10 cm long, 0.125-cm external diameter and 0.0125-cm wall thickness, sealed at one end. The anode was constructed by joining in parallel three 30-cm lengths of palladium-silver alloy tubing 0.05-cm external diameter and 0.0125-cm wall thickness. The tubes were wound to form a helix 8 cm long and 0.6 cm in diameter. The anode tubes were supported symmetrically around the cathode by Teflon (Du Pont) spacers. The electrode assembly was mounted on and through a Teflon plug which fitted into the open end of the cell body, which was also made of Teflon. The cell body was 1.0-cm internal diameter and 2.0-cm external diameter. The length of the cell was 20 cm and of the inner cavity 15 cm. A helical slot 0.4 cm deep was cut into the outer surface of the cell and into this slot 26 SWG nichrome wire was loosely wound to serve as the resistive heating element. It was found that, provided that sufficient slack was left when winding the wire to permit the free expansion of the Teflon when hot, no deformation of cold flow of the Teflon occurred even after many cycles of use at temperatures up to 250 "C. The cell was insulated with about 1.0 cm thickness of glass fiber and the whole assembly encased in a thin metal tube. When filled with electrolyte (15 ml) it weighed 70 grams. External connections were made to the palladium tubes by brazing with pure silver in a hydrogen atmosphere. It was found important to ensure that no metal, other than palladium alloy or the noble metals, was in contact with the 970
Proper cleaning and surface activation are critical for efficient operation of the cell. Practical considerations in the fabrication of hydrogen diffusion electrodes have been given by Cliford (8). The electrolyte used was KOH 67.5x, LiOH lO.O%, and water 22.5x;. It was made by adding 10 grams of LiOH to 90 grams of KOH pellets containing 25% water. This mixture could be used over the temperature range 160 to 250 "C. The LiOH served to lower the melting point of the electrolyte. Before it was added to the cell, the electrolyte was pre-electrolyzed between platinum electrodes at a constant current of 0.01 A for 8 hours to remove traces of ferrous and other metals. The potential was not allowed to exceed 1.2 V to avoid possible electrodecomposition of the platinum. Measurement of Cell Performance. The capacity of the cell to generate and to remove hydrogen was first tested with each electrode taken separately. The cathode output of hydrogen was measured using a soap bubble meter. The rate of removal of hydrogen at the anode was measured, also by a soap bubble flow meter, by observing the difference between the input and the output when the anode was supplied with an excess of pure hydrogen. Flows indicating theoretical maximum output of hydrogen at the cathode were measured in this manner, therefore diffusional losses in the measurement system were minimal. Tests were made over the temperature range 160 to 250 "C and are illustrated in Figure 2. It is noted that the anode is slightly more efficient (8) J. E. Cliford, E. Kolic, and C. L. Faust, "Research on a Gravity
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
Independent Water Electrolytic Cell With a Palladium-Silver Alloy Cathode," Battelle Memorial Institute Technical Documentary Report No. AMRL-TDR-34-44, Columbus, Ohio, 1964.
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9
Figure 5. A closed circuit gas chromatograph in which the carrier gas is generated at a palladium-silver cathode ( A ) , passes through a sample inlet valve ( B ) , through the column (C), to the palladiumsilver anode where the carrier gas is removed ( D ) . The sample components are conveyed to detector ( E ) by a stream of second carrier gas introduced at point (0
1.
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Figure 4. Relationship between applied potential and current A . Hydrogen removal at the anode. B. Closed circuit operation. C. Hydrogen generation at the cathode.
for hydrogen removal than the cathode for hydrogen generation. This is attributable to the dependence of the cathode on surface activity for efficient hydrogen generation. The dependence on surface activity for hydrogen removal at the anode is not as marked. Measurements were also made of the proportion of hydrogen removed when the cell was polarized at different potentials. This mode of expression is illustrated in Figure 3. The nitrogen flow rate was maintained constant at 2.15 ml per minute. Curves A , B, and C represent hydrogen flows of 2.0, 5.4, and 15.2 ml per minute, respectively, at STP. For the particular cell geometry employed, complete hydrogen removal is limited to flows of about 5 ml per minute at STP. The cell potential must not exceed 0.8 V. Above this potential, silver is removed from the anode causing decomposition of this electrode. If flow rates in excess of 5 ml per minute are desired, the cell geometry may be altered to increase the electrode surface area. Measurement of the Electrical Efficiency of the Cell. The electrical efficiency of the cell was measured by observing the potential difference between the cell electrodes at different currents and conditions of operation. The results of these measurements are illustrated in Figure 4. Curve A shows the voltage-current relationship for hydrogen removal at the anode. Curve C shows the voltage-current relationship for hydrogen generation in the cathode. The hydrogen output from the cathode was then connected directly to the anode, thus closing the loop for hydrogen flow. Curve B shows the voltage-current relationship for this closed loop operation. The portion of the curve with a minimum slope illustrates the region where the cell is operating at its maximum efficiency and in electrochemical balance. It is obvious that this closed loop operation results in a considerable power saving. Operational Tests. It is commonplace in engineering that closing the loop brings benefits. A closed loop gas chromatograph in which the carrier gas is recycled through the electrochemical cell is no exception to this rule. The performance of the cell under operational conditions was tested by making it a part of a gas chromatograph as illustrated in Figure 5. The hydrogen output from the cathode passed through a gas sample introduction valve with a 0.1-ml sample loop to a
column filled with 80-100 mesh Molecular Sieve, 5A. The column was 75 cm long and 0.25 cm in diameter. The column outlet was connected to the anode input via a T piece into which a second carrier gas, or in some experiments additional hydrogen, could be passed. The output from the anode tube passed directly into an ionization cross-section detector. For tests at atmospheric pressure, the detector had a geometric volume of 0.12 ml. For tests at reduced pressure, Le., 5 Torr, a detector with a geometric volume of 1.0 ml was used. The additional column volume introduced by the anode tube was 0.012 ml, therefore it was insignificant. The electrical signal from the detector was fed to a potentiometric recorder via an electrometer amplifier in the usual manner. RESULTS AND DISCUSSION
The electrolytic production of hydrogen by permeation through the cathode tubing is limited, so far as efficient operation is concerned, to a current density of 0.1 A cm-*. The cell used in these tests had a cathode area of 10 cm2, so that it would be expected to produce hydrogen at the rate indicated by the electrochemical equivalent of 6.96 ml per minute per ampere at currents up to 1 A. The results illustrated in Figure 2 confirm this limitation. It is interesting to observe, however, that the fall in efficiency with increasing current is slight, and even at a current of 5 A, hydrogen is still produced at 77z of the theoretical electrochemical rate. Where the efficiency is less than loo%, the hydrogen which fails to pass into the cathode tube bubbles off in the electrolyte. Where the cell is closed it is possible that this hydrogen will combine at the anode to form water, so that although there may be a loss of electrical efficiency, there need not necessarily be a loss of electrolyte. The anode efficiency shows no such limitation, at least not up to 0.5 A crnd2. Experience with the cell in Figure 1 and others of similar design indicates that where the anode and cathode surface areas are comparable, the anode is always more efficient at hydrogen removal than the cathode at hydrogen production. So long as the inefficiency is slight, as it will be when the current density is less than 0.1 A cmU2,this is a favorable result. For the successful application of the cell anode as a separator for a gas chromatograph-mass spectrometer combination, it is essential that the carrier gas should be completely removed. The slight excess anode efficiency assures that this condition is realized. Reference to Curve B of Figure 4 reveals that the cell current is a linear function of the applied potential below the maximum hydrogen diffusion current. The hydrogen output at the cathode is a direct function of the current. Therefore, the hy-
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
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TiFlON
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Figure 6. Electrode assembly of a dual anode electrolytic cell drogen output is a direct function of the applied potential. Hence, the flow of hydrogen through a gas chromatographic column can be turned on, controlled, or stopped simply by analogous changes in the applied potential of the cell. Storage cylinders, flow and pressure regulators, gas purifiers, and other gas control devices are now unnecessary. Procedures such as flow and pressure programming can now come from externally impressed electrical analog instruction. In addition, the usual disadvantages of flow or pressure programming, namely the adverse effect upon detector performance due to large changes in carrier gas flow rate, are no longer a penalty. This is because the removal of the carrier gas at the anode tube prevents any changes in the flow rate from penetrating through to the detector. When the cell is first switched on after assembly, as much as 5 minutes may elapse before the hydrogen flow from the cathode commences. This delay is presumably connected with the time required to saturate the cathode tube with hydrogen, Patience is necessary at this stage, otherwise a perfectly good cellmay be dismantled or discarded. After this initial turn-on, the cell responds rapidly to changes in voltage and a new equilibrium current is reached within 20 seconds at temperatures in the region of 200 "C. Another measure of the efficiency of the cell is the power required to produce a given hydrogen flux. Hydrogen production by simple electrolysis, when oxygen is simultaneously liberated at the anode, requires a starting potential in excess of 1.23 V. When hydrogen is returned at the anode however, the starting potential for hydrogen flow is in the region of 70 mV. If ohmic losses are disregarded, the power required to recycle hydrogen can be as low as one twentieth of that required for its electrolytic production, i.e., 0.06 V . With the cell used in these experiments, the electrical efficiency at 1 A was 80 and as the current rose, the efficiency fell still further. From an operational viewpoint, this is not necessarily a disadvantage, since some power is needed to maintain the electrolyte at its operating temperature. Palladium-silver alloy has a comparatively high resistivity and some of the efficiency lost at high currents is undoubtedly due to the fall in potential along the electrodes themselves. The arrangement of the anode as three parallel tubes was dictated by the fact that the anode tubing used had a resistance of 0.025 ohm cm-1. In an earlier version of the cell a long, 100-cm anode tube was used which had a resistance of 2.5 ohms. Although it functioned well in removing the hydrogen, the potential required to cause 6.96 ml of hydrogen per minute was 1.2 V. This high potential not only implied a low electrical efficiency for the cell, but worse, the cell failed after about 30 hours of continuous use through the electrolytic corrosion of the anode tube. The cell illustrated in Figure 1 has been operated for a total of 600 hours at temperatures in the range 200 to 250 "C and the anode tubes still show no obvious signs of corrosion or diminution of diameter. However, with other cell de972
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Figure 7. Analysis of a gas mixture using the closed circuit gas chromatograph illustrated in Figure 5. Temperature: 23 "C; detector pressure: 5 torr; sample size: 0.1 ml; column: 5A molecular sieve A . Open loop: current off; carrier gas flow: 6.2 ml/min, STP B. Closed loop: current 0.82 A; carrier gas flow: 6.9 ml/min, STP signs and especially when operated at high applied potentials and with the anode starved of hydrogen, the rapid corrosion of the anode can occur. In the interest of high efficiency and the prevention ofelectrolytic corrosion, the inlet and outlet of the anode tube should be joined together electrically so that the effective length of the anode tube along which the electrical must flow is halved. It is advisable also, in the practical application of the generatorseparator, to supply the cell with current from a lo impedance source of constant potential, which should not excees 0.7 V. The performance of the cell under such conditions can be determined from the measurements illustrated in Figure 3. During operation in a gas chromatograph system, the hydrogen output flow from the electrochemical cell was very stable. No variation in hydrogen carried gas flow rate could be detected in time by measurement with a soap bubble flow meter. Laboratory tests indicated that hydrogen carrier gas flow could be selected within approximately 2 of the value predicted by use of operating curves similar to Figure 4B. The cell design in Figure 1 is not necessarily the best. An arrangement of coaxial tubes with a central anode tube surrounded by the electrolyte and by a concentric cathode tube is likely to be more efficient. It was not made or used in these experiments because of the difficulties of construction, particularly the containment of the electrolyte. Palladium-silver alloy has served well in these developments. However, it is possible that other alloys, such as those of palladium-rhodium or palladium-gold, may be less prone to corrosion under severe conditions of operation and also
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
possibly less active in the catalytic hydrogenation of some sample components. As stated by McKinley (9) alloys other than palladium-silver are less readily poisoned by compounds such as hydrogen sulfide. A detailed account of the changes in the chemical composition or structure of compounds passing through the palladium separator is the subject of a separate paper (4). Briefly it can be said that these changes are mild and predictable. It is possible that through the choice of tubing material and the conditions of operation, it may eventually be possible to overcome them. The cell illustrated in Figure 1 can be used without a supply of second carrier gas when the detector is the ion source of a mass spectrometer. This is because the pressure gradient toward the vacuum of the ion source prevents the stagnation of sample components within the anode tubing after the hydrogen has been removed. At the pressure usually used with gas chromatography detectors, namely from a few Torr to 1 atmosphere, some additional supply gas to convey the samples to the detector after removal of the hydrogen is essential. One of the principal benefits of the electrolytic cell as a carrier gas source, is avoidance of storage cylinders and flow regulators. This advantage would be largely offset if these were needed for the second carrier gas. This difficulty can be avoided however, if hydrogen is the second carrier gas also. At low current densities, the generation and removal of hydrogen are closely in balance. Therefore, if a small additional supply of hydrogen from a second electrochemical cell is added as the second carrier gas, it will emerge from the anode tube without loss and serve to convey the sample to the detector. The rate of flow of the second carrier gas is as easily controlled electrically as is the first. If it is thought necessary, a second palladium-silver anode can be used after the detector to return the second hydrogen stream to its electrolyte. It is not necessary to use two separate cells. The second cell can be included within the electrolyte compartment of the first. Figure 6 illustrates the electrode assembly of such a dual function cell. The cell body is identical with that in Figure 1. In this new cell the cathode is common and sup(9) D. L. McKinley, U.S. Patent No. 3,350,845 (1965).
plies both the first and second carrier gas supplies. The principal anode is the same as that in the cell of Figure 1 but an additional small anode made from a strip of platinum foil, 1 cm long by 0.3 cm width, is included and connected to a separate electrical supply. With this arrangement there will be a small net loss of electrolyte as the second hydrogen carrier gas escapes. However, the flow of second carrier gas needed is usually in the range 0.1 to 1.O ml per minute. The electrolysis of 1 gram of water would supply this for between 20 and 200 hours. An unexpected benefit of this arrangement is that the principal anode now need never become hydrogen starved and as a consequence, even with the palladium-silver alloy, corrosion is no longer a problem. The potential of the second anode will always be greater than 1.23 V. Figure 7 is a chromatogram of the separation of atmospheric gases made with the apparatus shown in Figure 5 and using the cell in Figure 1. In this experiment the detector was operated at a pressure of 5 Torr. This somewhat unusual chromatographic condition was chosen to test the ability of the complete system to function in the analysis of the Martian atmosphere and at supposed Martian surface pressure. Tests under comparable conditions, using a separate hydrogen supply and the electrolytic cell unpolarized, showed that the detectivity for trace compounds had increased approximately tenfold through the removal of hydrogen carrier gas by the anode tubing. This preliminary experiment is included here to illustrate the potentialities of the system which incorporates the electrolytic cell as part of a gas chromatograph. The apparatus of this experiment, not including the recorder and power supplies, weighed 100 grams. With either a thermal conductivity or an ionization cross-section detector, it had the capacity for accurate atmospheric gas analysis. At a signal level twice the peak system noise level, it had a detection limit of parts per 100 billion. RECEIVED for review March 23, 1970. Accepted June 4, 1970. This paper presents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract No. NAS 7-100, sponsored by the National Aeronautics and Space Administration.
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