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Electrocatalyst performance in industrial water electrolysers
lnt J. Hydrogen Energy, Vol. 10. No. 1. pp, 11-19. 1985.
036{~3199/85 $3.(1(I ~ 0,00 Pergamon Press Ltd © 1985 International Association lot Hydrogen Energy
Printed in Great Britain.
ELECTROCATALYST P E R F O R M A N C E IN INDUSTRIAL WATER ELECTROLYSERS M. B. I. JANJUA and R. L. LE RoY Electrolyser, Inc. at 122 The West Mall, Etobicoke, Ontario, M9C 1B9 Canada and Noranda Research Centre, 240 Hymus Boulevard, Pointe Claire, Ou~bec, H9R 1G5 Canada
Abstraet--Electrocatalyst materials used in industrial water electrolysis equipment must meet stringent requirements for long-term stability. Low electrode overvoltages must be sustained over prolonged periods of normal operation, including power interruptions. This paper presents an overview of the catalyst systems currently favoured for use in alkaline electrolyte. Performance data covering test periods exceeding 30 000 h are presented for representative commercial electrocatalysts. Results obtained in 100 000-A unipolar cells are correlated in detail with expectations based on measurements in laboratory and pilot-plant equipment. Particular attention is given to the effects of open-circuit conditions on electrode stability. An accelerated reverse-potential cycling test is described which allows identification of materials expected to withstand industrial operating conditions. It is found that the better materials which have been identified can be used with confidence. at least in electrolvsers of the unipolar design in which potential variations encountered during current interruptions are modest.
I N D U S T R I A L E L E C T R O C A T A L Y S T SYSTEMS Successful electrode activation is an essential requirement for all advanced alkaline electrolyser technologies. Early efforts were made by some groups to achieve high energy conversion efficiencies through operation at high temperatures alone, without electrode treatment [1-2]. These attempts have now been largely abandoned for reasons related to economics, materials stability and thermodynamics [3-5]. General recognition in the mid 1970s of the central importance of electrocatalysis led to intensive and broadly based development efforts by many groups. Several excellent reviews are available [6-10]. A n o d e activation systems given particular attention have included porous nickel sinters [I 1], nickel cobalt spinels [12-17], cobalt oxides and lithiated cobalt oxides [16, 18, 19], nickel hydroxides [20-22], various perovskite oxides [14, 23-25] and plasma-sprayed alloys of nickel and stainless steel [26,27]. A m o n g the more successful cathode treatments studied are iron and nickel molybdates [8, 28], nickel borides [29-31], the traditional nickel sulphides [32], various Raney iron or nickel materials [25, 33--38], and nickel--cobalt thiospinel s or mixed sulfides [7, 39]. In the past few years, there has been a remarkable convergence in understanding of which electrode treatment systems are effective and economic for use in alkaline water electrolysers [40]. Groups working
* Electrolyser Inc. is a joint venture of The Electrolyser Corporation and Noranda. 1l
towards development of industrial equipment have focused their attention on anodes based on nickel and cobalt oxides prepared on nickel substrates, while preferred cathodes are largely limited to Raney nickel and cobalt, and sulphided nickel. The emphasis in most programs has now shifted from identification of new materials to demonstration of long-term stability under industrial operating conditions.
LONG-TERM STABILITY AND THE T R A N S I T I O N TO C O M M E R C I A L ELECTROLYSERS As high performance anode and cathode electrocatalysts have been identified, increasing attention has been directed to demonstrating stability over prolonged operating periods, and to scaling up of the electrode treatment procedures to meet the requirements of commercial gas-generating systems. A series of publications has recorded the progression in evaluation of electrode systems developed by,Eiectrolyser*, from 150 m A test cells to 100 000-A commercial cells [4, 41,42]. Figure 1 records representative results obtained in a series of 1-kA experimental unipolar electrolysers of the Stuart-cell design [42]. Current density was held at the traditional operating level of 1 3 4 m A cm -2 for the first 4Y2r, and was subsequently increased to 215 m A cm- . Electrolyte in every case was 25% K O H , thermostated at 70°C. The cells differed from one another in the electrode materials used. One electrolyser was operated for a year with unactivated mildsteel cathodes and nickel-plated steel anodes, for tom-
12
M. B. I. JANJUA AND R. L. LEROY
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13
ELECTROCATALYST PERFORMANCE IN INDUSTRIAL WATER ELECTROLYSERS the test electrode by two Teflon disks. The design leaves a gap between the two stainless steel plates to allow free passage of electrolyte to the test electrode. A Luggin probe is positioned less than I mm above the test electrode, while the counterelectrode is mounted several centimeters further away. The entire assembly is sufficiently small that it can be mounted into a oneliter polypropylene bottle. Representative results obtained with this microcell are presented in Fig. 3, for one of Electrolyser's proprietary cathode materials (NE-C-500). The data have been analysed by the statistical procedure which is detailed in reference [31], yielding the indicated values of the exchange current density/o, the Tafel slope b, and the resistance factor R. Representative voltages calculated from these parameters are indicated in Fig. 3. Similar statistical analyses have been made on the electrodes of the three 100-kA cells which are installed in the experimental plant which has been built by Electrolyser in collaboration with Hydro-Quebec at Varennes, south east of Montreal [42]. Each of these cells has four Luggin probes distributed over each of its anode and cathode surfaces. Values of the electrochemical parameters are determined in periodic voltage-current density scans. Results obtained after the first 1000 h of operation of these cells are summarized in Table 1, in comparison with data obtained for the same electrode systems in the microcell. Electrode
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M. B. I. JANJUA AND R. L. LEROY
overvoltages and Tafel parameters for the 100-kA cells are in each case averages of values determined with four different Luggin probes. Table 1 includes two distinct comparisons. First, the Tafel slope b measured in the microcell for each electrode material is compared with the value measured for the same material in the 100-kA cells. The total Tafel parameter for each cell is then compared with the value determined from the 100-kA cell by statistical analysis. In each case, the agreement is very satisfactory. Secondly, electrode overvoltages estimated from the laboratory data are compared with measurements in the commercial cells. An equivalent comparison is then made between the cell voltages projected from the microcell data and the actual measured cell voltages. These voltage estimates have been made using a resistante factor for each cell which was determined by statistical analysis. These values have been reported previously [27]. In each case, the agreement obtained suggests that projections from laboratory data are a reliable indicator of the voltage performance which can be anticipated in commercial electrolysers, at least for cells of Electrolyser Inc.'s unipolar Generation I design. OPEN-CIRCUIT STABILITY Vandenborre and his colleagues [7] have summarized succinctly the technical requirements for a good electrocatalyst system. It must demonstrate • Good electrocatalytic properties. • Resistance to mechanical and electrical wear. and, in the case of oxide surfaces on anodes • Good electrical conductivity. Experience gained in long-term testing of electrocatalyst materials suggests that an additional criterion must now be added to this list. This is that electrocatalysts must exhibit • Good stability under open-circuit conditions typical of those experienced in commercial electrolyser equipment. Appleby and Cr6py [20] published the first reference to destruction of active electrocatalysts under open-
circuit conditions. Their cobalt molybdate cathode material was found to dissolve visibly during current interruptions, and this led to identification of other molybdate compounds which were apparently more stable. More recently, Mahmood and his colleagues [45] have given detailed attention to the stability of related cathode catalysts under repeated current interruptions, with variable results. Data reported by Teledyne for similar nickel molybdate cathodes [46] are also suggestive of instability under open-circuit conditions. Other electrode activation systems seem sensitive to exposure in the electrolyte when gas is not being evolved. Wendt and his colleagues [13] have observed that two types of effective anode catalysts experience serious corrosion when not anodically polarized. These are the nickel-cobalt mixed oxides, and lanthanumstrontium--cobalt perovskites. Tseung [47] has reported the first quantitative study of electrode stability in caustic electrolyte at open-circuit potentials. He found significant corrosion rates for both his nickel cobalt thiospinel material [39] and for unmodified nickel cathodes. Thus, evaluation of electrocatalysts at open-circuit potentials must be an integral part of any program developing industrial electrodes. In fact, it is normal that current interruptions are experienced in a longterm evaluation program, because of planned stops for installation of new cells, accidental trips of safety systems, and unplanned interruption of the power supply. Several such interruptions are apparent from the data of Fig. 1, and these are detailed explicitly in Table 2. The long-term voltage results show no evidence of instability after periodic open-circuit exposure. A more detailed view of the performance of two 1kA pilot ceils during power interruptions encountered over a 1-y test period is presented in Fig. 4. One cell had unactivated electrodes, while the electrodes of the second were treated with an early activation system. Both cells showed some benefit from depolarization occurring during an interruption, although this effect was much more pronounced in the case of unactivated electrodes. The phenomena responsible are believed related to oxidation/reduction reactions in the oxide coating on the anodes [21] and dehydriding effects on
Table 2. Current Interruptions Experienced in the 1-kA Pilot Cells* Current interruptions Catalyst system anode/cathode NE-A-300/NE-C-200 NE-A-300/NE-C-303 NE-A-320/~E-C-220 NE-A-220/NE-C-220 NE-A-3(~/NE-C-401 NE-A-220/NE-C-220 NE-A-320/NE-C-220
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the cathodes [48]. These effects do not appear to persist for prolonged periods after power is restored; cell voltages regain the values which they had before power interruption within a few days. The only trend which can be discerned in this example is a gradual reduction in voltage of the activated cell. This downward trend reflected reduction of the oxygen overvoltage, and it has been found to be characteristic of a number of anode activation systems. The mechanism is believed to be related to conditioning of the surface oxide, and it may be enhanced by the power interruptions which the electrode experiences.
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The stability of an electrocatalyst under open-circuit >O conditions depends on two principal factors: the character of the electrocatalyst itself; and the potential ~ 1.0 experienced by the electrodes in the particular electrolyser design which is used. This latter element can depend strongly on the electrolyser type, with electrodes in unipolar cells behaving quite differently from those 0.5 in bipolar cells. Figure 5 illustrates some of these effects. The upper frame of this figure follows the voltages of eight 1-kA pilot electrolysers after current had been turned off. O The cells, which are described in reference 42, were connected electrically in series with a 20V, 1000A I I I rectifier. All of the 1-kA cells contained different elec0 5 IO 15 20 trode activation systems under evaluation in the Electrolyser program. DEPOLARIZATION TIME, h When the power was shut off, cell voltage dropped quickly to about 100 mV above the reversible potential, Fig, 5. Depolarization of a bank of eight 1-kA unipolar elecreflecting the fuel-cell reactions taking place on the trolyser cells with (a) no short circuiting resistor and (b) a lelectrodes: oxygen reduction at the positive electrode g2 shunt resistor connected across the cell bank.
16
M. B. I. JANJUA AND R. L. LEROY
and hydrogen oxidation at the negative electrode. The cell voltage initially exceeded its theoretical value, likely because of the high effective pressure of hydrogen absorbed in the cathode [48]. The fuel cell reactions drove a current against the external resistance of the circuit, in this case the resistance of the rectifier diodes against reverse flow of current. As this resistance is high, the depolarization current is typically extremely low in a gas generating system based on unipolar cells. Nevertheless, two of the eight cells did show a sharp voltage decrease after several hours with the current turned off. The lower frame of Fig. 5 illustrates the phenomena for the same line of cells under conditions which may be closer to those experienced in an electrolyser of the bipolar design. The line of electrolysers was short circuited through a 1.0 Q shunt resistor immediately after the power was turned off. This allowed a maximum depolarization current of 10 A to flow, equivalent to a maximum current density of approximately 2.5 m A cm -2. Such current levels can be representative of the depolarization currents which would flow in a bipolar electrolyser through the channels used for electrolyte feed and gas removal. In this case, depolarization of individual cells occurred much more rapidly, and to substantially lower levels. In fact, the cell voltage actually reversed on one electrolyser after 6 h of depolarization (cell No. 9). This suggests that the remaining cells in the line, acting as fuel cells, are driving corrosion reactions in this particular cell. Variation of the potentials experienced by the anodes and cathodes of an electrolyser are considered more closely in the example of Fig. 6. These data were obtained on a pilot electrolyser which had been operating with an applied current of 6.4 kA, equivalent to 500 mA cm -2. In this experiment, the power supply was turned off, and the anode and cathode were connected through a 0.22 Q-current-limiting resistor at the times indicated. The lower frame of the figure shows the depolarization current measured with the shunt closed, while the upper frame monitors the corresponding cell voltage. The central section of Fig. 6 tracks the potential of the anode and cathode electrodes with respect to mercury/mercury oxide reference electrodes. The reversible potentials for hydrogen and oxygen evolution are indicated for reference. Two principal observations can be made. First, depolarization of the hydrogen electrode is much less marked than that of the oxygen electrode; decrease in the positive potential is largely responsible for the observed decrease in cell voltage. The observed stability of the negative electrode may be related to the anodic oxidation of hydrogen [48] being sustained by a large reserve of absorbed hydrogen. The second observation is that, in the absence of a short-circuiting resistor, reduction of the potential of the oxygen electrode appears to be limited to values around 500 mV negative with respect to the Hg/HgO reference. At this level, the trivalent transition metal
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oxides can be expected to be relatively stable [49]. In contrast, when an external depolarization path is provided the potential decreases much more sharply, to values where oxide reduction can be anticipated. R E V E R S E - P O T E N T I A L - C Y C L I N G TESTS An accelerated test has been devised for assessment of the stability of different electrode treatments under open-circuit conditions. Initial attention has been focused on the cathodes, because anode materials do not appear to enter a dangerous potential region in unipolar electrolyser equipment under depolarization conditions likely to be encountered in industrial practice. Representative evaluations of two of Electrolyser's proprietary cathode materials are summarized in Fig. 7. The test is carried out as follows. An electrode sample is mounted in a polarization test cell, typically the microcell of Fig. 2, and is operated for some time at normal cathodic current densities (e.g. 250 mA cm-2). The electrode is then scanned under potentiostatic control to 400 mV positive with respect to the reversible
ELECTROCATALYST PERFORMANCE IN INDUSTRIAL WATER ELECTROLYSERS
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hydrogen potential, at a rate of 1 mV s -1. The scan is then reversed and potential is decreased at the same rate to 50 mV negative with respect to the reversible hydrogen potential. This cycling process is continued until significant deterioration is observed to have occurred. The two materials represented in Fig. 7 behaved similarly in the location of oxidation peaks, while the magnitude of the oxidation currents were quite different. It is noted that the height of the oxidation peak decreases slowly on successive scans, until, for some materials, .there is a rapid decrease which is directly associated with loss of electrode activity. While no detailed mechanistic analysis has been
attempted, it is likely that one of the anodic peaks corresponds to hydrogen oxidation, while the second peak is associated with oxidation of the active electrode material or oxidation of hydride structures. Insofar as these oxidation peaks remain stable, the oxidation/ reduction process is relatively reversible, and activity towards hydrogen evolution is maintained. The rapid decrease in the oxidation current is, then, associated with irreversible oxidation of the electrocatalyst. The reverse-potential-cycling (RPC) test is based on a logarithmic plot of the maximum oxidation current on each cycle, vs the number of cycles. Representative results are ploted in Fig. 8 for five different classes of proprietary cathode materials. The results in general
18
M. B. I. JANJUA AND R. L. LEROY ZOO
2. M. H. Miles, G. Kissel, P. W. T. Lu and S. Srinivasan, Effect of temperature on electrode kinetic parameters for hydrogen and oxygen evolution reactions on nickel elecI00 trodes in alkaline solutions. J. Electrochem. Soc. 123,332 (1976). 3. R. L. LeRoy, Industrial water electrolysis: present and 50 future, Int. J. Hydrogen Enes~y 8. 401 ('1983). 4. R. L. LeRoy. Hydrogen production by the electrolysis of E water: the kinetic and thermodynamic framework. J. Electrochem. Soc. 130, 2158 (1983). I--f- 20 5. P. Combrade, General and localized corrosion of nickel t.9 in contact with caustic anolytes at lo0 and 200°C. In m "IHydrogen as an Energy Carrier (G. Imarisio and A. S. to Strub, eds), p. 183. D. Reide!. Dordrecht !983. P. Combrade, Behaviour of Nickel and some r,,icke! base 2llovs W Q. in contact with caustic anolvte at 16~~ and 2i)9°C. in Hydrogen Energy Progress IV. (T. N. Veziro~lu, W. D. 0.2 Van Vorst andJ. H. Kelley, eds). Vol. 1, p. 3557Pergamon ,. N E - C - 6 0 0 Press, Oxford (1982). 6. L. Martin, J. Diette, M. Prigent, M. Bernard, J. Demarsy ,,I I t 1 I OJ and C. Sellier, Mise au pointe de nouveaux o ,5 IO 15 20 25 61ectrocatalyseurs pour 61ectrolyse avancde, Commission N U M B E R OF CYCLES of the European Communities Report EUR 7068 FR. Brussels (1981). Fig. 8. Decay of the anodic oxidation peak on continued cycling 7. H. Vandenborre, R. Leysen and Ph. Vermeiren, Active in the RPC test, for representative cathode catalyst systems. electrodes to be used in advanced alkaline-water electrolysis, Centre d'6tude d'fnergie nucleare Report DE83 900365, Mol, Belgium, September (1982). show a broad plateau with, in the case of three of the 8. D. E. Brown, M. N. Mahmood, A. K. Turner, S. M. Hall materials, a rapid decrease in the peak oxidation current and P. O, Fogarty, Low overvottage electrocatalysts for after a n u m b e r of cycles ranging from 2 to 12. hydrogen evolving electrodes. In Hydrogen Energy ProIt must be e m p h a s i z e d that this is a most severe test. gress, (T, N. Veziroglu, R. Fueki and T. Ohta, eds), Vol As the central frame of Fig. 6 indicates, it is unlikely 1, p. 151. Pergamon Press, Oxford (1980). that a cathode in a unipolar electrolyser would e v e r 9. D. E. Hall. Oxygen anodes for water electrolvsi,~. In experience a potential as high as 400 m V positive with Extended Abstracts, Vol. 83--2, p. 642. The Electrochemical Society, Princeton (1983). respect to the reversible hydrogen potential. E l e c t r o d e 10. J.-M. Gras, Etude bibliographique de l'61ectrocatalyse par materials such as N E - C - 2 0 0 can be e x p e c t e d to last les oxydes mixtes en milieu alcalin, Electricit6 de France essentially indefinitely u n d e r industrial operating conReport P/539/77/20, May (1977). Reprinted as U.S. ditions. T h e N E - C - 3 0 0 electrocatalysts, h o w e v e r , must National Technical Information Service Report N79-be considered suspect, e v e n though they have per27284, (1979). formed satisfactorily in pilot electrolysers for m a n y 11. R. D. Giles, Evaluation of the factors affecting the peryears. T h e NE-C-500 and NE-C-600 materials are formance of porous nickel in alkaline electrotyser environexpected to be absolutely resistant to activity loss on ments. In Hydrogen as an Energy Carrier, (G. Imarisio p o w e r interruption. and A. S. Strub, eds), p. 171. D. Reidel, Dordrecht (1983). 12. W. J. King and A. C. C. Tseung, The reduction of oxygen on nickel--cobalt oxides, Electrochimica Acta 19. 485, a93 Acknowledgements--We are pleased to acknowledge the (1974). energetic contributions of Mr B. F. Henshaw in construction 13. J. Fischer, H. Hofmann, G. Luft and H. Wendt, Fundaand operation of the pilot electrolysers in which much of the mental investigations and electrochemical engineering work reported here was carried out. Also, discussions with Mr aspects concerning an advanced concept for alkaline water H. J. Davis and Mr C. T. Bowen led to the understanding of electrolysis, AIChE J. 26, 794 (1980). open-circuit effects which is presented. The review at the 14. H. Wendt, H. Hofmann, H. Berg, V. Plzak and J. Fischer, beginning of this paper is based in part on a manuscript Alkaline water electrolysis at enhanced temperatures (120 ° prepared by Mr Bowen under a Canadian government contract to 160°C): basic and material studies, engineering and for the International Energy Agency Hydrogen Program. Pareconomics. In Hydrogen as an Energy Carrier, (G. Imarisio tial financial support of the electrocatalysis program by the and A. S. Strub, eds), p. 267. D. Reidel, Dordrecht (1983). Government of Canada, through its Program for Advancement 15. H. Vandenborre, R. Leysen, H. Nackaerts, Ph. Van of Industrial Technology and through National Energy Plan Asbroeck and J. Piepers, Advanced alkaline water elecfunds administered by the National Research Council of Cantrolysis using IME technology, ibid, p. 139. ada, is gratefully acknowledged. 16. P. Rasiyah and A. C. C. Tseung, Oxygen evolution on cobalt and nickel oxides at elevated temperatures, ibid, p. 110; P. Rasiyah and A. C. C. Tseung, Mechanism of REFERENCES oxygen evolution on semiconducting oxides in alkaline 1. G. Imarisio, Progess in water electrolysis at the conclusion solutions. In Hydrogen Energy Progress IV, (T. N. Vezirof the first hydrogen programme of the European comoglu, W. D. Van Vorst and J. H. Kelley, eds), Vol. 1, p. munities, Int. J. Hydrogen Energy 6, 153 (1981). 383. Pergamon Press, Oxford (1982).
~
ELECTROCATALYST PERFORMANCE IN INDUSTRIAL WATER ELECTROLYSERS 17. C. R. Davidson. G. Kissel and S. Srinivasan, Electrode kinetics of the oxygen evolution reaction at NiCo2Oa from 30% KOH: dependence on temperature. J. Electroanalyt. Chem. 132, 129 (1982). 18. P. Rasiyah and A. C. C. Tseung, A mechanistic study of oxygen evolution on Li-doped Co304, J. Electrochem. Soc. 130, 365 (1983). 19. H. Wendt and V. Plzak, Electrocatalytic and thermal activation of anodic oxygen- and cathodic hydrogenevolution in alkaline water electrolysis, Electrochimiea Acta 28, 27 (1983). 20. A. J. Appleby and G. Cr6py, New developments in alkaline electrolysis technology, In Industrial Water Electrolysis, (S. Srinivasan, F. J. Salzano and A. R. Landgrebe, eds), p. 150. Electrochemical Society, Princeton (1978). 21. P. W. Lu and S. Srinivasan, Electrochemical ellipsometric studies of oxide films formed on nickel during oxygen evolution. In Electrode Materials and Processes for Energy Conversion and Storage. (L D. E. Mclntyre, S. Srinivasan and F. G. Will. eds), p. 396. Electrochemical Society, Princeton (1977). 22. D. E. Hall, Ni(OH):-Impregnated anodes for alkaline water electrolysis. J. Electrochem. Soc. 130, 317 (1983). 23. A. C. C. Tseung and H. L. Bevan, Improvements in and relating to catalvtic processes, U.K. Patent 1,352.995, 15 May (1974). 24. G. Fiori and C. M. Marl, Scaling up and optimization of electrodes for oxygen evolution from alkaline solutions. In Hydrogen as an Energy Carrier, (G. Imarisio and A. S. Strub, eds), D. Reidel, Dordrecht (1983); G. Fiori and C. M. Marl, The use of superficial techniques of analysis in the evaluation of electrocatalytic materials in the anodic evolution of oxygen. In Hydrogen Energy Progress-IV (T. N. Veziroglu, W. D. Van Vorst and J. H. Kelley, eds). Vol. 1, p. 291. Pergamon Press, Oxtord, (1982). 25. M. Prigent. T. Nenner, L. Martin and M. Roux, The development of new electrocatalysts for advanced water electrolysis. In Hydrogen as an Energy Carrier, (G. Imarisio and A. S. Strub, eds), p. 256. D. Reidel, Dordrecht (1983). 26. A. Nidola, P. M. Spazianti and L. Giuffre, Electrolytic production of hydrogen from alkaline solutions--active electrodes for KOH electrolysis. In Industrial Water Electrolysis (S. Srinivasan, F. J. Salzano and A. R. Landgrebe, eds). p. 102. The Electrochemical Society, Princeton (1978). 27. C. T. Bowen. H. J. Davis, B. F. Henshaw, R. Lachance, R. L. LeRoy and R. Renaud, Developments in advanced alkaline water electrolysis, Int. J. Hydrogen Energy 9.59 (1984). 28. A. J. Appleby and G. Cr6py, Advanced electrolysis in alkaline solutions. In Electrode Materials and Processes tbr Energy Conversion and Storage (J. D. E. Mclntyre, S. Srinivasan and F. G. Will. eds), p. 382. The Electrochemical Society Princeton (1977). 29. G. Benczur-Urmossy. Catalysts for the cathodic evolution of hydrogen. U.S. Patent 3.926,844 (1975). 30. G. Kissel, F. Kulesa, C. R. Davidson, and S. Srinivasan, Selection and evaluation of materials for advanced alkaline water electrolysers. In Inaustrial Water Electrolysi~ (S. Srinivasan. F. J. Salzano and A. R. Landgrebe, eds). p. 218. The Electrochemical Society, Princeton (1978). 31. R. L. LeRoy, M. B. I. Janjua, R. Renaud and U. Leuenberger, Analysis of time-variation effects in water electrolyzers J. Electrochem. Soc. 126, 1674 (1979).
19
32. M. Prigent and L. J. Mas, Applications of electrocatalysis to the electrolysis of water at high temperature and high current density. In Seminar on Hydrogen as an Energy Vector: Its Production, Use and Transportation, EEC Report EUR 6085, p. 181. Brussels, October (1978). 33. H. Hofmann, V. Plzak and H. Wendt, Anode and cathode activation, diaphragm construction and electrolyzer configuration in advanced alkaline water electrolysis. In Hydrogen Energy Progress-IV (T. N. Veziroglu, W. D. Van Vorst and J. H. Kelley, eds). Vol. 1, p. 407. Pergamon Press, Oxford (1982). 34. M. Prigent and T. Nenner, Anodic and Cathodic Catalysts for High Temperature. High Current Density Alkaline Water Electrolysis, ibid, p. 299. 35. I. Abe, T. Fujimaki and M. Matsubara, Hydrogen production by high temperature, high pressure water electrolysis, results of test plant operation, ibid, p. 167. 36. T. J. Gray, Raney alloy coated cathode for chlor-alkali cells, U.S. Patent 4,394,228, 19 July, (1983). 37. J. Divisek, J. Mergel and H. Schmitz, Improvements of water electrolysis in alkaline media at intermediate temperatures, Int. J. Hydrogen Energy' 7,695 (1982). 38. J. Divisek and J. Mergel, Porose Nickelelektrode fiir Alkalische Elektrolysen, Verfahren zur Herstellung Derselben und Deren Verwendung, German Patent DE 2914094 C2, 10 February, (1983). 39. A. C. C. Tseung and M. C. M. Man, Method of catalysing the evolution of gaseous hydrogen, US Patent 4,279,713, 21 July, (1981). 40. H. Wendt. Seven years of research and development of advanced water electrolysers. In Hydrogen as an Energy Carrier (G. Imarisio and A. S. Strub, eds), p. 91. D Reidel, Dordrecht (1983). 41. R. L. LeRoy and A. K. Stuart, Advanced unipolar electrolysis, Int. J. Hydrogen Energy, 6,589 (1981). 42. R. L. LeRoy and A. F. Hufnagl, Progress in industrial demonstration of advanced unipolar electrolysis, Int. J. Hydrogen Energy 8, 581 (1983). 43. J. N. Murray and D. E. Hall, O,. Evolution electrode properties and the influence of the electrode separator, In Extended Abstracts, Vol, 82-1, p. 754. The Electrochemical Society, Princeton. May (1982). 44. R. L. LeRoy, C. T. Bowen and D. J. LeRoy. The thermodynamics of aqueous water electrolysis, J. Electrochem. Soc. 127, 1954 (1980). 45. M. N. Mahmood, A. K. Turner, M. C. M. Man and P. O. Fogarty, Hydrogen as a byproduct of chlorine/caustic production. Paper presented to the Society of Chemical Industry, London, 11 October (1983). 46. J. N. Murray, Advanced alkaline electrolysis cell development: contract summary report, Brookhaven National Laboratory Report BNL 51433. May (1981). 47. P. Rasiyah and A. C. C. Tseung, Corrosion of Ni and NiCo2S4 at open circuit potential. Paper presented at the International Energy Agency Annex IV Technical Meeting, Ispra, Italy, 1 June (1983). 48. B. E. Conway. H. Angerstein-Kozlowska and M. A. Sattar, Study of a decomposing hydride phase at nickel cathodes by measurement of open-circuit potential decay, J. Electrochem. Soc. 130, 1825 (1983). 49. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, Oxford (1966).
036{~3199/85 $3.(1(I ~ 0,00 Pergamon Press Ltd © 1985 International Association lot Hydrogen Energy
Printed in Great Britain.
ELECTROCATALYST P E R F O R M A N C E IN INDUSTRIAL WATER ELECTROLYSERS M. B. I. JANJUA and R. L. LE RoY Electrolyser, Inc. at 122 The West Mall, Etobicoke, Ontario, M9C 1B9 Canada and Noranda Research Centre, 240 Hymus Boulevard, Pointe Claire, Ou~bec, H9R 1G5 Canada
Abstraet--Electrocatalyst materials used in industrial water electrolysis equipment must meet stringent requirements for long-term stability. Low electrode overvoltages must be sustained over prolonged periods of normal operation, including power interruptions. This paper presents an overview of the catalyst systems currently favoured for use in alkaline electrolyte. Performance data covering test periods exceeding 30 000 h are presented for representative commercial electrocatalysts. Results obtained in 100 000-A unipolar cells are correlated in detail with expectations based on measurements in laboratory and pilot-plant equipment. Particular attention is given to the effects of open-circuit conditions on electrode stability. An accelerated reverse-potential cycling test is described which allows identification of materials expected to withstand industrial operating conditions. It is found that the better materials which have been identified can be used with confidence. at least in electrolvsers of the unipolar design in which potential variations encountered during current interruptions are modest.
I N D U S T R I A L E L E C T R O C A T A L Y S T SYSTEMS Successful electrode activation is an essential requirement for all advanced alkaline electrolyser technologies. Early efforts were made by some groups to achieve high energy conversion efficiencies through operation at high temperatures alone, without electrode treatment [1-2]. These attempts have now been largely abandoned for reasons related to economics, materials stability and thermodynamics [3-5]. General recognition in the mid 1970s of the central importance of electrocatalysis led to intensive and broadly based development efforts by many groups. Several excellent reviews are available [6-10]. A n o d e activation systems given particular attention have included porous nickel sinters [I 1], nickel cobalt spinels [12-17], cobalt oxides and lithiated cobalt oxides [16, 18, 19], nickel hydroxides [20-22], various perovskite oxides [14, 23-25] and plasma-sprayed alloys of nickel and stainless steel [26,27]. A m o n g the more successful cathode treatments studied are iron and nickel molybdates [8, 28], nickel borides [29-31], the traditional nickel sulphides [32], various Raney iron or nickel materials [25, 33--38], and nickel--cobalt thiospinel s or mixed sulfides [7, 39]. In the past few years, there has been a remarkable convergence in understanding of which electrode treatment systems are effective and economic for use in alkaline water electrolysers [40]. Groups working
* Electrolyser Inc. is a joint venture of The Electrolyser Corporation and Noranda. 1l
towards development of industrial equipment have focused their attention on anodes based on nickel and cobalt oxides prepared on nickel substrates, while preferred cathodes are largely limited to Raney nickel and cobalt, and sulphided nickel. The emphasis in most programs has now shifted from identification of new materials to demonstration of long-term stability under industrial operating conditions.
LONG-TERM STABILITY AND THE T R A N S I T I O N TO C O M M E R C I A L ELECTROLYSERS As high performance anode and cathode electrocatalysts have been identified, increasing attention has been directed to demonstrating stability over prolonged operating periods, and to scaling up of the electrode treatment procedures to meet the requirements of commercial gas-generating systems. A series of publications has recorded the progression in evaluation of electrode systems developed by,Eiectrolyser*, from 150 m A test cells to 100 000-A commercial cells [4, 41,42]. Figure 1 records representative results obtained in a series of 1-kA experimental unipolar electrolysers of the Stuart-cell design [42]. Current density was held at the traditional operating level of 1 3 4 m A cm -2 for the first 4Y2r, and was subsequently increased to 215 m A cm- . Electrolyte in every case was 25% K O H , thermostated at 70°C. The cells differed from one another in the electrode materials used. One electrolyser was operated for a year with unactivated mildsteel cathodes and nickel-plated steel anodes, for tom-
12
M. B. I. JANJUA AND R. L. LEROY
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13
ELECTROCATALYST PERFORMANCE IN INDUSTRIAL WATER ELECTROLYSERS the test electrode by two Teflon disks. The design leaves a gap between the two stainless steel plates to allow free passage of electrolyte to the test electrode. A Luggin probe is positioned less than I mm above the test electrode, while the counterelectrode is mounted several centimeters further away. The entire assembly is sufficiently small that it can be mounted into a oneliter polypropylene bottle. Representative results obtained with this microcell are presented in Fig. 3, for one of Electrolyser's proprietary cathode materials (NE-C-500). The data have been analysed by the statistical procedure which is detailed in reference [31], yielding the indicated values of the exchange current density/o, the Tafel slope b, and the resistance factor R. Representative voltages calculated from these parameters are indicated in Fig. 3. Similar statistical analyses have been made on the electrodes of the three 100-kA cells which are installed in the experimental plant which has been built by Electrolyser in collaboration with Hydro-Quebec at Varennes, south east of Montreal [42]. Each of these cells has four Luggin probes distributed over each of its anode and cathode surfaces. Values of the electrochemical parameters are determined in periodic voltage-current density scans. Results obtained after the first 1000 h of operation of these cells are summarized in Table 1, in comparison with data obtained for the same electrode systems in the microcell. Electrode
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M. B. I. JANJUA AND R. L. LEROY
overvoltages and Tafel parameters for the 100-kA cells are in each case averages of values determined with four different Luggin probes. Table 1 includes two distinct comparisons. First, the Tafel slope b measured in the microcell for each electrode material is compared with the value measured for the same material in the 100-kA cells. The total Tafel parameter for each cell is then compared with the value determined from the 100-kA cell by statistical analysis. In each case, the agreement is very satisfactory. Secondly, electrode overvoltages estimated from the laboratory data are compared with measurements in the commercial cells. An equivalent comparison is then made between the cell voltages projected from the microcell data and the actual measured cell voltages. These voltage estimates have been made using a resistante factor for each cell which was determined by statistical analysis. These values have been reported previously [27]. In each case, the agreement obtained suggests that projections from laboratory data are a reliable indicator of the voltage performance which can be anticipated in commercial electrolysers, at least for cells of Electrolyser Inc.'s unipolar Generation I design. OPEN-CIRCUIT STABILITY Vandenborre and his colleagues [7] have summarized succinctly the technical requirements for a good electrocatalyst system. It must demonstrate • Good electrocatalytic properties. • Resistance to mechanical and electrical wear. and, in the case of oxide surfaces on anodes • Good electrical conductivity. Experience gained in long-term testing of electrocatalyst materials suggests that an additional criterion must now be added to this list. This is that electrocatalysts must exhibit • Good stability under open-circuit conditions typical of those experienced in commercial electrolyser equipment. Appleby and Cr6py [20] published the first reference to destruction of active electrocatalysts under open-
circuit conditions. Their cobalt molybdate cathode material was found to dissolve visibly during current interruptions, and this led to identification of other molybdate compounds which were apparently more stable. More recently, Mahmood and his colleagues [45] have given detailed attention to the stability of related cathode catalysts under repeated current interruptions, with variable results. Data reported by Teledyne for similar nickel molybdate cathodes [46] are also suggestive of instability under open-circuit conditions. Other electrode activation systems seem sensitive to exposure in the electrolyte when gas is not being evolved. Wendt and his colleagues [13] have observed that two types of effective anode catalysts experience serious corrosion when not anodically polarized. These are the nickel-cobalt mixed oxides, and lanthanumstrontium--cobalt perovskites. Tseung [47] has reported the first quantitative study of electrode stability in caustic electrolyte at open-circuit potentials. He found significant corrosion rates for both his nickel cobalt thiospinel material [39] and for unmodified nickel cathodes. Thus, evaluation of electrocatalysts at open-circuit potentials must be an integral part of any program developing industrial electrodes. In fact, it is normal that current interruptions are experienced in a longterm evaluation program, because of planned stops for installation of new cells, accidental trips of safety systems, and unplanned interruption of the power supply. Several such interruptions are apparent from the data of Fig. 1, and these are detailed explicitly in Table 2. The long-term voltage results show no evidence of instability after periodic open-circuit exposure. A more detailed view of the performance of two 1kA pilot ceils during power interruptions encountered over a 1-y test period is presented in Fig. 4. One cell had unactivated electrodes, while the electrodes of the second were treated with an early activation system. Both cells showed some benefit from depolarization occurring during an interruption, although this effect was much more pronounced in the case of unactivated electrodes. The phenomena responsible are believed related to oxidation/reduction reactions in the oxide coating on the anodes [21] and dehydriding effects on
Table 2. Current Interruptions Experienced in the 1-kA Pilot Cells* Current interruptions Catalyst system anode/cathode NE-A-300/NE-C-200 NE-A-300/NE-C-303 NE-A-320/~E-C-220 NE-A-220/NE-C-220 NE-A-3(~/NE-C-401 NE-A-220/NE-C-220 NE-A-320/NE-C-220
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the cathodes [48]. These effects do not appear to persist for prolonged periods after power is restored; cell voltages regain the values which they had before power interruption within a few days. The only trend which can be discerned in this example is a gradual reduction in voltage of the activated cell. This downward trend reflected reduction of the oxygen overvoltage, and it has been found to be characteristic of a number of anode activation systems. The mechanism is believed to be related to conditioning of the surface oxide, and it may be enhanced by the power interruptions which the electrode experiences.
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The stability of an electrocatalyst under open-circuit >O conditions depends on two principal factors: the character of the electrocatalyst itself; and the potential ~ 1.0 experienced by the electrodes in the particular electrolyser design which is used. This latter element can depend strongly on the electrolyser type, with electrodes in unipolar cells behaving quite differently from those 0.5 in bipolar cells. Figure 5 illustrates some of these effects. The upper frame of this figure follows the voltages of eight 1-kA pilot electrolysers after current had been turned off. O The cells, which are described in reference 42, were connected electrically in series with a 20V, 1000A I I I rectifier. All of the 1-kA cells contained different elec0 5 IO 15 20 trode activation systems under evaluation in the Electrolyser program. DEPOLARIZATION TIME, h When the power was shut off, cell voltage dropped quickly to about 100 mV above the reversible potential, Fig, 5. Depolarization of a bank of eight 1-kA unipolar elecreflecting the fuel-cell reactions taking place on the trolyser cells with (a) no short circuiting resistor and (b) a lelectrodes: oxygen reduction at the positive electrode g2 shunt resistor connected across the cell bank.
16
M. B. I. JANJUA AND R. L. LEROY
and hydrogen oxidation at the negative electrode. The cell voltage initially exceeded its theoretical value, likely because of the high effective pressure of hydrogen absorbed in the cathode [48]. The fuel cell reactions drove a current against the external resistance of the circuit, in this case the resistance of the rectifier diodes against reverse flow of current. As this resistance is high, the depolarization current is typically extremely low in a gas generating system based on unipolar cells. Nevertheless, two of the eight cells did show a sharp voltage decrease after several hours with the current turned off. The lower frame of Fig. 5 illustrates the phenomena for the same line of cells under conditions which may be closer to those experienced in an electrolyser of the bipolar design. The line of electrolysers was short circuited through a 1.0 Q shunt resistor immediately after the power was turned off. This allowed a maximum depolarization current of 10 A to flow, equivalent to a maximum current density of approximately 2.5 m A cm -2. Such current levels can be representative of the depolarization currents which would flow in a bipolar electrolyser through the channels used for electrolyte feed and gas removal. In this case, depolarization of individual cells occurred much more rapidly, and to substantially lower levels. In fact, the cell voltage actually reversed on one electrolyser after 6 h of depolarization (cell No. 9). This suggests that the remaining cells in the line, acting as fuel cells, are driving corrosion reactions in this particular cell. Variation of the potentials experienced by the anodes and cathodes of an electrolyser are considered more closely in the example of Fig. 6. These data were obtained on a pilot electrolyser which had been operating with an applied current of 6.4 kA, equivalent to 500 mA cm -2. In this experiment, the power supply was turned off, and the anode and cathode were connected through a 0.22 Q-current-limiting resistor at the times indicated. The lower frame of the figure shows the depolarization current measured with the shunt closed, while the upper frame monitors the corresponding cell voltage. The central section of Fig. 6 tracks the potential of the anode and cathode electrodes with respect to mercury/mercury oxide reference electrodes. The reversible potentials for hydrogen and oxygen evolution are indicated for reference. Two principal observations can be made. First, depolarization of the hydrogen electrode is much less marked than that of the oxygen electrode; decrease in the positive potential is largely responsible for the observed decrease in cell voltage. The observed stability of the negative electrode may be related to the anodic oxidation of hydrogen [48] being sustained by a large reserve of absorbed hydrogen. The second observation is that, in the absence of a short-circuiting resistor, reduction of the potential of the oxygen electrode appears to be limited to values around 500 mV negative with respect to the Hg/HgO reference. At this level, the trivalent transition metal
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oxides can be expected to be relatively stable [49]. In contrast, when an external depolarization path is provided the potential decreases much more sharply, to values where oxide reduction can be anticipated. R E V E R S E - P O T E N T I A L - C Y C L I N G TESTS An accelerated test has been devised for assessment of the stability of different electrode treatments under open-circuit conditions. Initial attention has been focused on the cathodes, because anode materials do not appear to enter a dangerous potential region in unipolar electrolyser equipment under depolarization conditions likely to be encountered in industrial practice. Representative evaluations of two of Electrolyser's proprietary cathode materials are summarized in Fig. 7. The test is carried out as follows. An electrode sample is mounted in a polarization test cell, typically the microcell of Fig. 2, and is operated for some time at normal cathodic current densities (e.g. 250 mA cm-2). The electrode is then scanned under potentiostatic control to 400 mV positive with respect to the reversible
ELECTROCATALYST PERFORMANCE IN INDUSTRIAL WATER ELECTROLYSERS
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POTENTIAL VS RHE, mV Fig. 7. Representative cyclic voltamagrams obtained with proprietary cathode electrocatalysts on potential cycling from -50 to + 400 mV with respect to the reversible hydrogen potential. Measurements were carried out in the microeell of Fig. 2.
hydrogen potential, at a rate of 1 mV s -1. The scan is then reversed and potential is decreased at the same rate to 50 mV negative with respect to the reversible hydrogen potential. This cycling process is continued until significant deterioration is observed to have occurred. The two materials represented in Fig. 7 behaved similarly in the location of oxidation peaks, while the magnitude of the oxidation currents were quite different. It is noted that the height of the oxidation peak decreases slowly on successive scans, until, for some materials, .there is a rapid decrease which is directly associated with loss of electrode activity. While no detailed mechanistic analysis has been
attempted, it is likely that one of the anodic peaks corresponds to hydrogen oxidation, while the second peak is associated with oxidation of the active electrode material or oxidation of hydride structures. Insofar as these oxidation peaks remain stable, the oxidation/ reduction process is relatively reversible, and activity towards hydrogen evolution is maintained. The rapid decrease in the oxidation current is, then, associated with irreversible oxidation of the electrocatalyst. The reverse-potential-cycling (RPC) test is based on a logarithmic plot of the maximum oxidation current on each cycle, vs the number of cycles. Representative results are ploted in Fig. 8 for five different classes of proprietary cathode materials. The results in general
18
M. B. I. JANJUA AND R. L. LEROY ZOO
2. M. H. Miles, G. Kissel, P. W. T. Lu and S. Srinivasan, Effect of temperature on electrode kinetic parameters for hydrogen and oxygen evolution reactions on nickel elecI00 trodes in alkaline solutions. J. Electrochem. Soc. 123,332 (1976). 3. R. L. LeRoy, Industrial water electrolysis: present and 50 future, Int. J. Hydrogen Enes~y 8. 401 ('1983). 4. R. L. LeRoy. Hydrogen production by the electrolysis of E water: the kinetic and thermodynamic framework. J. Electrochem. Soc. 130, 2158 (1983). I--f- 20 5. P. Combrade, General and localized corrosion of nickel t.9 in contact with caustic anolytes at lo0 and 200°C. In m "IHydrogen as an Energy Carrier (G. Imarisio and A. S. to Strub, eds), p. 183. D. Reide!. Dordrecht !983. P. Combrade, Behaviour of Nickel and some r,,icke! base 2llovs W Q. in contact with caustic anolvte at 16~~ and 2i)9°C. in Hydrogen Energy Progress IV. (T. N. Veziro~lu, W. D. 0.2 Van Vorst andJ. H. Kelley, eds). Vol. 1, p. 3557Pergamon ,. N E - C - 6 0 0 Press, Oxford (1982). 6. L. Martin, J. Diette, M. Prigent, M. Bernard, J. Demarsy ,,I I t 1 I OJ and C. Sellier, Mise au pointe de nouveaux o ,5 IO 15 20 25 61ectrocatalyseurs pour 61ectrolyse avancde, Commission N U M B E R OF CYCLES of the European Communities Report EUR 7068 FR. Brussels (1981). Fig. 8. Decay of the anodic oxidation peak on continued cycling 7. H. Vandenborre, R. Leysen and Ph. Vermeiren, Active in the RPC test, for representative cathode catalyst systems. electrodes to be used in advanced alkaline-water electrolysis, Centre d'6tude d'fnergie nucleare Report DE83 900365, Mol, Belgium, September (1982). show a broad plateau with, in the case of three of the 8. D. E. Brown, M. N. Mahmood, A. K. Turner, S. M. Hall materials, a rapid decrease in the peak oxidation current and P. O, Fogarty, Low overvottage electrocatalysts for after a n u m b e r of cycles ranging from 2 to 12. hydrogen evolving electrodes. In Hydrogen Energy ProIt must be e m p h a s i z e d that this is a most severe test. gress, (T, N. Veziroglu, R. Fueki and T. Ohta, eds), Vol As the central frame of Fig. 6 indicates, it is unlikely 1, p. 151. Pergamon Press, Oxford (1980). that a cathode in a unipolar electrolyser would e v e r 9. D. E. Hall. Oxygen anodes for water electrolvsi,~. In experience a potential as high as 400 m V positive with Extended Abstracts, Vol. 83--2, p. 642. The Electrochemical Society, Princeton (1983). respect to the reversible hydrogen potential. E l e c t r o d e 10. J.-M. Gras, Etude bibliographique de l'61ectrocatalyse par materials such as N E - C - 2 0 0 can be e x p e c t e d to last les oxydes mixtes en milieu alcalin, Electricit6 de France essentially indefinitely u n d e r industrial operating conReport P/539/77/20, May (1977). Reprinted as U.S. ditions. T h e N E - C - 3 0 0 electrocatalysts, h o w e v e r , must National Technical Information Service Report N79-be considered suspect, e v e n though they have per27284, (1979). formed satisfactorily in pilot electrolysers for m a n y 11. R. D. Giles, Evaluation of the factors affecting the peryears. T h e NE-C-500 and NE-C-600 materials are formance of porous nickel in alkaline electrotyser environexpected to be absolutely resistant to activity loss on ments. In Hydrogen as an Energy Carrier, (G. Imarisio p o w e r interruption. and A. S. Strub, eds), p. 171. D. Reidel, Dordrecht (1983). 12. W. J. King and A. C. C. Tseung, The reduction of oxygen on nickel--cobalt oxides, Electrochimica Acta 19. 485, a93 Acknowledgements--We are pleased to acknowledge the (1974). energetic contributions of Mr B. F. Henshaw in construction 13. J. Fischer, H. Hofmann, G. Luft and H. Wendt, Fundaand operation of the pilot electrolysers in which much of the mental investigations and electrochemical engineering work reported here was carried out. Also, discussions with Mr aspects concerning an advanced concept for alkaline water H. J. Davis and Mr C. T. Bowen led to the understanding of electrolysis, AIChE J. 26, 794 (1980). open-circuit effects which is presented. The review at the 14. H. Wendt, H. Hofmann, H. Berg, V. Plzak and J. Fischer, beginning of this paper is based in part on a manuscript Alkaline water electrolysis at enhanced temperatures (120 ° prepared by Mr Bowen under a Canadian government contract to 160°C): basic and material studies, engineering and for the International Energy Agency Hydrogen Program. Pareconomics. In Hydrogen as an Energy Carrier, (G. Imarisio tial financial support of the electrocatalysis program by the and A. S. Strub, eds), p. 267. D. Reidel, Dordrecht (1983). Government of Canada, through its Program for Advancement 15. H. 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