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A novel cathode for alkaline water electrolysis
ht. J. Hydrogen Energ.v,Vol. 22, No. 6, pp. 621-623, 1997 (0 1997 International
Pergamon
for Hydrogen Energy Elsevier Science Ltd All rights reserved. Printed in Great Britain 0360-3199/97 $17.00+0.00
PII: SO360-3199(96)00191-7
Association
SHORT COMMUNICATION: A NOVEL
CATHODE
FOR ALKALINE
WATER
ELECTROLYSIS
WEIKANG HU, XUEJUN CAO, FUPENG WANG and YUNSHI ZHANG Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071,China
(Received for publication
18 July 1996)
novel cathode with a multilayer structure was developed. Mm-based hydride alloys served as the hydrogen absorbing alloy of the novel cathode. A Ni-Mo alloy coating was used as the electrocatalyst of the cathode. The experimental results showed that the novel cathode not only had highly catalytic activity for HER, but also exhibited excellent durability and stability under the conditions of continuous electrolysis and intermittent electrolysis with short circuiting for more than 2 weeks. 0 1997International Association for Hydrogen Energy Abstract-A
1. INTRODUCTION In order to lower the energy consumption of alkaline water electrolysis, many catalytic materials for cathodes have been proposed and examined. Among these materials, Raney nickel and Ni-Mo alloys as catalysts for hydrogen evolution in alkaline solution have been widely investigated [l-24]. Various preparation methods for Raney nickel and Ni-Mo alloys have also been developed. The composite-coated Raney nickel and thermally deposited Ni-Mo coating electrodes have proved to be very effective hydrogen cathodes [4,6, 16, 171.They exhibited not only high electrocatalytic activity, but also good time stability under the condition of continuous electrolysis (for example, nickel substrate coated with the NimMo,,O electrocatalyst was found to exhibit overpotential between 60 and 80 mV at 0.5 A/cm2 for 10,000 h in 30 wt% KOH at 70°C [17]. However, the activities on such electrodes were deactivated during intermittent electrolysis with the short-circuiting shutdown for 2 weeks [25, 261. The reasons for the deterioration of the activity for HER were ascribed to the dissolution of the catalyst components [23, 271. In order to overcome the disadvantage of the deterioration of the catalyst activity, we design a new kind of hydrogen cathode which features a multilayer structure with a Ni skeleton/metal hydride alloys/a catalyst. The purpose of the study is to prepare the novel cathode, and then to examine the catalytic activity of the novel cathode and time stabilities under the conditions of continuous and intermittent electrolysis in 30 wt.% KOH at 70°C.
2. EXPERIMENT 2.1. Cathode preparation A porous nickel substrate with a size of 0.980 x 0.950 x 0.184 cm was used as the cathode frame. MmNi, 6C00,75Mn042Al,,27 alloy powders, which were screened to 300 mesh by passing through sieves, served as the hydrogen absorbing alloy of cathode; a Ni-Mo alloy coating as the electrocatalyst of the cathode. The amount of the metal hydride alloy was determined by the time of short circuiting and the reverse current. The thickness of Ni-Mo electrocatalyst was about 12-l 5 pm. The geometric area of the cathode obtained was 2.572 cm’. The detailed procedure of the cathode preparation was presented in a patent [28]. 2.2. Electrochemical property The hydrogen overpotential was measured by a steadystate galvanostatic method using a three-compartment conventional double-walled thermostated glass cell. The electrolyte was a 30 wt.% KOH solution prepared from KOH (analytic purity) and distilled water. The electrolytic temperature is 70°C. A Hg/HgO (30 wt.% KOH) electrode served as the reference electrode and was linked to the main compartment by a Luggin capillary. The distance between the Luggin capillary and the cathode was about 1 mm in order to minimize errors due to IR drop in electrolytes. The counterelectrode was a platinum gauze with a large area. During long-term electrolysis
WEIKANG HU et al.
622
Fig. 1. Durability test of the novel cathode during continuous electrolysis in 30 wt% KOH at 200 mA/cm2 and 7O”C, (without IR compensation).
g;;
0
2000
1000
3000
t 09 Fig. 2. Stability test cf the cathode during intermittent electrolysis with short circuiting for three weeks, (without lK compensation). The dotted line denotes the period of the short circuiting. experiments, the cathode was polarized with a constant current density of 200 mA/cm* with respect to the geometric area of the cathode at 70°C and at certain intervals the hydrogen overpotential of the cathode was measured. The loss of water during electrolysis was compensated by adding distilled water. The electrolyte, after being used for 2 months, was replaced by a fresh one. 3. RESULTS
AND
DISCUSSION
The novel cathode obtained consists mainly of the following materials: a Ni skeleton, MmNi&o,,, Mn0,42A10.27 alloys and a Ni-Mo alloy coating. The NiMO alloy coating was placed on the top surface of the cathode since the electrocatalytic activity of the electrode depends mainly on the surface concentration (not bulk concentration) and a Ni-Mo alloy is considered as one of the most active catalysts for HER [17, 191. After electrochemical measurement, the hydrogen overpotential of the cathode at 200 mA/cm* and 70°C is about 88 mV. Figure 1 shows the durability of the cathode under the condition of continuous electrolysis at 200 mA/cm’ and 70°C. The hydrogen overpotential has almost been kept constant over 4300 h electrolysis. In addition, no hydrogen embrittlement of the Mm-based hydride alloy is observed after long-term electrolysis. This indicates that the cathode has good time stability and durability under continuous electrolysis. Cathodes for industrial use should not only have good durability under continuous electrolysis, but also excellent resistance against cell shutdowns, especially for a long time, which result from maintenance or unforeseen problems. So, the short-circuiting test of the novel cathode was also conducted. The time of short-circuiting was about three weeks. During the period of short-circuiting, the temperature of the electrolyte was naturally cooled from 70°C to room temperature (20°C). Figure 2 shows
the result of the short-circuiting lasting three weeks. The dotted line in Fig. 2 denotes the period of short-circuiting of the cathode. It may be seen that the activity of the cathode does not change much after 3000 h intermittent electrolysis, especially after short-circuiting for three weeks. It indicates that the cathode has excellent stability against short-circuiting for a long time. In contrast, conventional nickel-based electrocatalysts such as Raney nickel and Ni-Mo alloy cathodes are usually oxidized and deactivated after short-circuiting for two weeks [25271. The reason for the excellent stability against the short-circuiting shutdown of the novel cathode can be explained as follows. During electrolysis, the internal MmNi,,Co,.,,Mn~.4zAl,,, alloys can absorb an amount of hydrogen which is diffused from the cathode surface. When the cell short-circuiting occurs, the metal hydrides will desorb the hydrogen and make the hydrogen arrive on the surface by diffusion, which will consume the reverse current caused by the short-circuiting. Therefore, the internal metal hydride alloys can effectively prevent the surface Ni-Mo electrocatalyst from deteriorating during short-circuiting. The novel cathode, exhibiting high activity for HER and excellent stability against short-circuiting for a long time, is an ideal electrode material for alkaline water electrolysis.
REFERENCES 1. Rami A. and Lasia, A., Journal of Applied Electrochemistry, 1992, 22, 376. 2. Raj, I. A. and Vasu, K. I., Journal of Applied Electrochemistry, 1990,20, 32. 3. Los, P., Rami, A. and Lasia, A., Journal of Applied Electrochemistry, 1993, 23, 135. 4. Endoh, E., Otouma, H., Morimoto, T. and Oda, Y., International Journal of Hydrogen Energy, 1981, 12,473.
A NOVEL
CATHODE
FOR ALKALINE
Endoh, E., Otouma, H. and Morimoto, T., International Journal of Hydrogen Energy, 1988, 13,207. Choquette, Y., Menard, H. and Brossard, L., International Journal of Hydrogen Energy, 1989, 14, 631. Choquette, Y., Menard, H. and Brossard, L., International Journal of Hydrogen Energy, 1990,15, 2 1. De Giz, M. J., Silva, J. C. P., Ferreira, M., Machado, S. A. S., Ticianelli, E. A., Avaca, L. A. and Gonzalez, E. R., International Journal of Hydrogen Energy, 1992, 11.125. 9. De Giz, M. J., Machado, S. A. S., Avaca, L. A. and Gonzalez, E. R., Journal of Applied Electrochemistry, 1992,22,913. 10 Schiller, G. and Borck, V., International Journalof‘Hydrogen Energy, 1992, 17, 261. 11 trochemistry, Rausch. S. and Elec1992,Wendt. 22, 1025.H.. Journal of” Aoulied _. 12 Chen, L. and Lasia, A., Journal of the Electrochemical Society, 1992, 139, 1058. 13 Chen, L. and Lasia, A., Journal of the Electrochemical Society, 1992, 139. 3214. 14 Cheong, A. K., Lasia, A. and Lessard, J., Journal qf the Electrochemical Societv. 1993. 140. 2121. 15 Miousse, D., Lasia, A.‘and Borck, V., Journal of Applied Electrochemistry, 1995, 25, 592. 16 Brown, D. E., Mahmood, M. N., Turner, A. K., Hall, S.
17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 21. 28.
WATER
ELECTROLYSIS
623
M. and Fogarty, P. O., International Journal of Hydrogen Energy, 1982,7,405. Brown, D. E., Mahmood, N. M., Man, M. C. M. and Turner, A. K., Electrochimica Acta, 1984, 29, 1551. Turdean, M. L., Huot, J. Y. and Schulz, R., Applied Physics-Letters, 1991, 58, 2164. Raij. I. A. and Vasu, K. I., Journal of Applied Electrochemistry, 1990, 20, 132. Huot, J.-Y., Trudeau, M. L.. and Schulz, R., Journal afthe Electrochemical Society, 1991, 138, 13 16. Chen, L. and Lasia, A., .Journal of the Electrochemical Society, 1992, 139, 3458. Raj, I. A. and Mater, J., Science, 1993, 28,4375. Fan, C., Piron, D. L., Sleb, A. and Paradis, P., Journal af the Electrochemical Society, 1994, 141, 382. Simpraga, R., Bai, L. and Conway, B. E., Journalof Applied Electrochemistry, 1995, 25, 628. Mahmood, M. N., Turner, A. K., Man, M. C. M. and Fogarty, P. O., Chem. Ind., (London), 1984, 50. Divisek, J., Schmitz, H. and Balej, J.. Journal qf Applied Electrochemistry, 1989, 19, 519. Divisek, J., Schmitz, H. and Steffen, B., Electrochimica Acta, 1994,39, 1723. Weikang, H., Chinese Patent Appl. 95116198.9 (1995).
Pergamon
for Hydrogen Energy Elsevier Science Ltd All rights reserved. Printed in Great Britain 0360-3199/97 $17.00+0.00
PII: SO360-3199(96)00191-7
Association
SHORT COMMUNICATION: A NOVEL
CATHODE
FOR ALKALINE
WATER
ELECTROLYSIS
WEIKANG HU, XUEJUN CAO, FUPENG WANG and YUNSHI ZHANG Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071,China
(Received for publication
18 July 1996)
novel cathode with a multilayer structure was developed. Mm-based hydride alloys served as the hydrogen absorbing alloy of the novel cathode. A Ni-Mo alloy coating was used as the electrocatalyst of the cathode. The experimental results showed that the novel cathode not only had highly catalytic activity for HER, but also exhibited excellent durability and stability under the conditions of continuous electrolysis and intermittent electrolysis with short circuiting for more than 2 weeks. 0 1997International Association for Hydrogen Energy Abstract-A
1. INTRODUCTION In order to lower the energy consumption of alkaline water electrolysis, many catalytic materials for cathodes have been proposed and examined. Among these materials, Raney nickel and Ni-Mo alloys as catalysts for hydrogen evolution in alkaline solution have been widely investigated [l-24]. Various preparation methods for Raney nickel and Ni-Mo alloys have also been developed. The composite-coated Raney nickel and thermally deposited Ni-Mo coating electrodes have proved to be very effective hydrogen cathodes [4,6, 16, 171.They exhibited not only high electrocatalytic activity, but also good time stability under the condition of continuous electrolysis (for example, nickel substrate coated with the NimMo,,O electrocatalyst was found to exhibit overpotential between 60 and 80 mV at 0.5 A/cm2 for 10,000 h in 30 wt% KOH at 70°C [17]. However, the activities on such electrodes were deactivated during intermittent electrolysis with the short-circuiting shutdown for 2 weeks [25, 261. The reasons for the deterioration of the activity for HER were ascribed to the dissolution of the catalyst components [23, 271. In order to overcome the disadvantage of the deterioration of the catalyst activity, we design a new kind of hydrogen cathode which features a multilayer structure with a Ni skeleton/metal hydride alloys/a catalyst. The purpose of the study is to prepare the novel cathode, and then to examine the catalytic activity of the novel cathode and time stabilities under the conditions of continuous and intermittent electrolysis in 30 wt.% KOH at 70°C.
2. EXPERIMENT 2.1. Cathode preparation A porous nickel substrate with a size of 0.980 x 0.950 x 0.184 cm was used as the cathode frame. MmNi, 6C00,75Mn042Al,,27 alloy powders, which were screened to 300 mesh by passing through sieves, served as the hydrogen absorbing alloy of cathode; a Ni-Mo alloy coating as the electrocatalyst of the cathode. The amount of the metal hydride alloy was determined by the time of short circuiting and the reverse current. The thickness of Ni-Mo electrocatalyst was about 12-l 5 pm. The geometric area of the cathode obtained was 2.572 cm’. The detailed procedure of the cathode preparation was presented in a patent [28]. 2.2. Electrochemical property The hydrogen overpotential was measured by a steadystate galvanostatic method using a three-compartment conventional double-walled thermostated glass cell. The electrolyte was a 30 wt.% KOH solution prepared from KOH (analytic purity) and distilled water. The electrolytic temperature is 70°C. A Hg/HgO (30 wt.% KOH) electrode served as the reference electrode and was linked to the main compartment by a Luggin capillary. The distance between the Luggin capillary and the cathode was about 1 mm in order to minimize errors due to IR drop in electrolytes. The counterelectrode was a platinum gauze with a large area. During long-term electrolysis
WEIKANG HU et al.
622
Fig. 1. Durability test of the novel cathode during continuous electrolysis in 30 wt% KOH at 200 mA/cm2 and 7O”C, (without IR compensation).
g;;
0
2000
1000
3000
t 09 Fig. 2. Stability test cf the cathode during intermittent electrolysis with short circuiting for three weeks, (without lK compensation). The dotted line denotes the period of the short circuiting. experiments, the cathode was polarized with a constant current density of 200 mA/cm* with respect to the geometric area of the cathode at 70°C and at certain intervals the hydrogen overpotential of the cathode was measured. The loss of water during electrolysis was compensated by adding distilled water. The electrolyte, after being used for 2 months, was replaced by a fresh one. 3. RESULTS
AND
DISCUSSION
The novel cathode obtained consists mainly of the following materials: a Ni skeleton, MmNi&o,,, Mn0,42A10.27 alloys and a Ni-Mo alloy coating. The NiMO alloy coating was placed on the top surface of the cathode since the electrocatalytic activity of the electrode depends mainly on the surface concentration (not bulk concentration) and a Ni-Mo alloy is considered as one of the most active catalysts for HER [17, 191. After electrochemical measurement, the hydrogen overpotential of the cathode at 200 mA/cm* and 70°C is about 88 mV. Figure 1 shows the durability of the cathode under the condition of continuous electrolysis at 200 mA/cm’ and 70°C. The hydrogen overpotential has almost been kept constant over 4300 h electrolysis. In addition, no hydrogen embrittlement of the Mm-based hydride alloy is observed after long-term electrolysis. This indicates that the cathode has good time stability and durability under continuous electrolysis. Cathodes for industrial use should not only have good durability under continuous electrolysis, but also excellent resistance against cell shutdowns, especially for a long time, which result from maintenance or unforeseen problems. So, the short-circuiting test of the novel cathode was also conducted. The time of short-circuiting was about three weeks. During the period of short-circuiting, the temperature of the electrolyte was naturally cooled from 70°C to room temperature (20°C). Figure 2 shows
the result of the short-circuiting lasting three weeks. The dotted line in Fig. 2 denotes the period of short-circuiting of the cathode. It may be seen that the activity of the cathode does not change much after 3000 h intermittent electrolysis, especially after short-circuiting for three weeks. It indicates that the cathode has excellent stability against short-circuiting for a long time. In contrast, conventional nickel-based electrocatalysts such as Raney nickel and Ni-Mo alloy cathodes are usually oxidized and deactivated after short-circuiting for two weeks [25271. The reason for the excellent stability against the short-circuiting shutdown of the novel cathode can be explained as follows. During electrolysis, the internal MmNi,,Co,.,,Mn~.4zAl,,, alloys can absorb an amount of hydrogen which is diffused from the cathode surface. When the cell short-circuiting occurs, the metal hydrides will desorb the hydrogen and make the hydrogen arrive on the surface by diffusion, which will consume the reverse current caused by the short-circuiting. Therefore, the internal metal hydride alloys can effectively prevent the surface Ni-Mo electrocatalyst from deteriorating during short-circuiting. The novel cathode, exhibiting high activity for HER and excellent stability against short-circuiting for a long time, is an ideal electrode material for alkaline water electrolysis.
REFERENCES 1. Rami A. and Lasia, A., Journal of Applied Electrochemistry, 1992, 22, 376. 2. Raj, I. A. and Vasu, K. I., Journal of Applied Electrochemistry, 1990,20, 32. 3. Los, P., Rami, A. and Lasia, A., Journal of Applied Electrochemistry, 1993, 23, 135. 4. Endoh, E., Otouma, H., Morimoto, T. and Oda, Y., International Journal of Hydrogen Energy, 1981, 12,473.
A NOVEL
CATHODE
FOR ALKALINE
Endoh, E., Otouma, H. and Morimoto, T., International Journal of Hydrogen Energy, 1988, 13,207. Choquette, Y., Menard, H. and Brossard, L., International Journal of Hydrogen Energy, 1989, 14, 631. Choquette, Y., Menard, H. and Brossard, L., International Journal of Hydrogen Energy, 1990,15, 2 1. De Giz, M. J., Silva, J. C. P., Ferreira, M., Machado, S. A. S., Ticianelli, E. A., Avaca, L. A. and Gonzalez, E. R., International Journal of Hydrogen Energy, 1992, 11.125. 9. De Giz, M. J., Machado, S. A. S., Avaca, L. A. and Gonzalez, E. R., Journal of Applied Electrochemistry, 1992,22,913. 10 Schiller, G. and Borck, V., International Journalof‘Hydrogen Energy, 1992, 17, 261. 11 trochemistry, Rausch. S. and Elec1992,Wendt. 22, 1025.H.. Journal of” Aoulied _. 12 Chen, L. and Lasia, A., Journal of the Electrochemical Society, 1992, 139, 1058. 13 Chen, L. and Lasia, A., Journal of the Electrochemical Society, 1992, 139. 3214. 14 Cheong, A. K., Lasia, A. and Lessard, J., Journal qf the Electrochemical Societv. 1993. 140. 2121. 15 Miousse, D., Lasia, A.‘and Borck, V., Journal of Applied Electrochemistry, 1995, 25, 592. 16 Brown, D. E., Mahmood, M. N., Turner, A. K., Hall, S.
17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 21. 28.
WATER
ELECTROLYSIS
623
M. and Fogarty, P. O., International Journal of Hydrogen Energy, 1982,7,405. Brown, D. E., Mahmood, N. M., Man, M. C. M. and Turner, A. K., Electrochimica Acta, 1984, 29, 1551. Turdean, M. L., Huot, J. Y. and Schulz, R., Applied Physics-Letters, 1991, 58, 2164. Raij. I. A. and Vasu, K. I., Journal of Applied Electrochemistry, 1990, 20, 132. Huot, J.-Y., Trudeau, M. L.. and Schulz, R., Journal afthe Electrochemical Society, 1991, 138, 13 16. Chen, L. and Lasia, A., .Journal of the Electrochemical Society, 1992, 139, 3458. Raj, I. A. and Mater, J., Science, 1993, 28,4375. Fan, C., Piron, D. L., Sleb, A. and Paradis, P., Journal af the Electrochemical Society, 1994, 141, 382. Simpraga, R., Bai, L. and Conway, B. E., Journalof Applied Electrochemistry, 1995, 25, 628. Mahmood, M. N., Turner, A. K., Man, M. C. M. and Fogarty, P. O., Chem. Ind., (London), 1984, 50. Divisek, J., Schmitz, H. and Balej, J.. Journal qf Applied Electrochemistry, 1989, 19, 519. Divisek, J., Schmitz, H. and Steffen, B., Electrochimica Acta, 1994,39, 1723. Weikang, H., Chinese Patent Appl. 95116198.9 (1995).