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Electrocatalytic properties of new electrocatalysts for hydrogen evolution in alkaline water electrolysis
International Journal of Hydrogen Energy 25 (2000) 111±118
Electrocatalytic properties of new electrocatalysts for hydrogen evolution in alkaline water electrolysis Weikang Hu* Department of Structural Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91, Stockholm, Sweden
Abstract New electrocatalysts consisting of hydrogen storage alloys, such as MmNi3.6Co0.75Mn0.4Al0.27 alloy (Mm 0 misch metal), LaNi4.9Si0.1 alloy and Ti2Ni alloy and nickel-molybdenum coatings were prepared. Their electrocatalytic activity for hydrogen evolution and their time stability were investigated in 30 wt% KOH at 708C. The surface morphology and chemical composition of these electrodes were also examined before and after electrolysis. The results show that the activity for hydrogen evolution increases with increase in molybdenum content and is dominated by electrode surface composition. The hydrogen storage alloys inside the electrodes mainly serve the functions of enhancing corrosion-resistance against power interruptions through electrochemically releasing absorbed hydrogen. These electrocatalysts not only have a low hydrogen overpotential, but also excellent time stability under both conditions of continuous and intermittent electrolysis. # 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Hydrogen evolution; Alkaline water electrolysis; Electrocatalysts; Hydrogen storage alloys
1. Introduction Hydrogen is considered to be an ideal future energy carrier. One of the most promising methods for production of hydrogen is water electrolysis using a variety of energy sources such as solar, geothermal, hydroelectric, nuclear or fusion energy. In order to make water electrolysis more ecient and economical, reduction of the cell voltage is vital. Many catalytic materials for anodes and cathodes have been proposed and examined. Raney nickel electrodes were found to have low oxygen overpotential and their activity remained unchanged for more than 13,000 h [1,2]. Raney nickel and nickel-molybdenum alloys as hydrogen cathodes have also been examined extensively and
* Tel.: +46-8-162-383; fax: +46-8-152-187. E-mail address: [email protected] (W. Hu).
proved to be very eective for hydrogen evolution in alkaline solution [3±25]. The composite-coated Raney nickel and thermally deposited nickel-molybdenum coating electrodes showed a low hydrogen overpotential and very good time stability under continuous electrolysis [6,8,18,19]. However, the main drawback of the two kinds of electrocatalysts is that activities for hydrogen evolution reaction (HER) are easily lost after intermittent operation, especially after a long period [24,26,27]. The reasons for degradation are mainly ascribed to the oxidized dissolution of the catalyst components such as Al, Zn for Raney nickel alloys and Mo for nickel-molybdenum alloys [24,29,30]. In order to overcome the drawback, we have recently developed a new electrocatalyst for hydrogen evolution in alkaline solution [31±33]. The new electrocatalyst mainly consists of hydrogen storage alloys and nickelmolybdenum coatings. The hydrogen storage alloys absorb hydrogen in electrolysis process and release the
0360-3199/00/$20.00 # 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 9 9 ) 0 0 0 2 4 - 5
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W. Hu / International Journal of Hydrogen Energy 25 (2000) 111±118
absorbed hydrogen during intermittent operation through a series of electrochemical reactions. The new electrocatalyst shows excellent stability under intermittent operation as well as continuous electrolysis. The purpose of this paper is to compare the performance and electrochemical properties of these catalysts with dierent hydrogen storage alloys. 2. Experiment 2.1. Electrode preparation and characterization The electrocatalyst consists of hydrogen storage alloys, a nickel-molybdenum coating and a nickel foam substrate. The hydrogen storage alloys used in this work are MmNi3.6Co0.75Mn0.4Al0.27 alloy, LaNi4.9Si0.1 alloy and Ti2Ni alloy, respectively. These alloys were prepared by arc melting under Ar atmosphere. The alloy samples obtained were crushed into powders and then screened to a size of 075 mm by passing through sieves. Prior to electrode preparation, a small amount of hydrogen storage alloy (070 mg) was ®rst mixed with carbonyl Ni powder under air atmosphere in the weight ratio of 5:1. They were then further mixed with a small amount of polyvinylalcohol (PVA) solution (2.5 wt%). The mixture was then poured into a nickel foam. After the electrode was dried in air, the electrode surface was modi®ed using a nickel-molybdenum alloy coating by means of electrodeposition. A more detailed process has been described in our previous publications and a patent [32±34]. The total geometric area of each sample was about 2.358 cm2. The surface morphology of the electrode was observed by scanning electron microscopy (SEM). The average chemical composition on the electrode surface was measured by energy dispersive analysis of X-ray (EDAX). 2.2. Electrochemical activity measurements The electrocatalytic activity of the electrode was examined using a steady-state galvanostatic method in a three-compartment conventional double walled thermostated glass cell. The electrolyte was a 30 wt% KOH solution prepared from KOH (G. R.) and deionized water. An Hg/HgO electrode (30 wt% KOH) was used as the reference electrode which was linked to the main compartment by a Luggin capillary. The distance between the Luggin capillary and the working electrode was about 1 mm in order to minimize IR drop. A platinum gauze of a large area was used as a counter-electrode. The temperature was maintained at 708C by means of external heating. The polarization curve was measured in the range of 300 mA/cm2 to 50 mA/cm2 with decreasing the current density. The cur-
rent density is expressed with respect to the geometric area of the electrode. 2.3. Long-term stability test During long-term electrolysis tests, the electrode was polarized with a constant current density of 200 mA/ cm2. A large Ni (99.9%) sheet was used as the anode. The hydrogen overpotential of the electrode was measured after certain intervals. During intermittent electrolysis with power interruption, the electrode was ®rst polarized as above for a few days, then the power was shuto. The temperature of electrolyte was cooled from 708C to room temperature during the power interruption. After a certain period of power interruption, the electrolysis was resumed under condition of 200 mA/cm2 at 708C.
3. Results and discussion 3.1. Electrode morphology and surface composition Fig. 1 shows SEM micrographs of three electrodes of dierent hydrogen storage alloys. The electrodes have almost the same morphology with a coarsely porous structure. This is similar to the appearance of nickel-molybdenum coating deposited on a nickel sheet substrate as shown in Fig. 2(a). In order to estimate the thickness of the nickel-molybdenum coating, the coating deposited on a nickel sheet substrate, which was pre-polished using a series of ®ne emery papers, was made under the same experimental conditions. The SEM image of the cross-section of the nickel-molybdenum coating is presented in Fig. 2(b). The thickness is in the range of 17±20 mm. The nickelmolybdenum deposit loading expressed as the amount of deposit on an apparent substrate surface, is in the range of 60±80 g/m2. X-ray diraction showed an amorphous structure of the nickel-molybdenum coating. The average compositions for these electrodes are listed in Table 1. Under the same electrodeposition conditions, the chemical composition of Ni-Mo coating was relative to the surface state of electrode and electrode substrate. As seen from Table 1, the molybdenum concentration in the Ti2Ni alloy-containing electrode is obviously higher than that in the other two electrodes. In addition, a small amount of La and Ti was found on the LaNi4.9Si0.1 alloy-containing and the Ti2Ni alloy-containing electrode surfaces, respectively. This indicates that only the MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode is completely covered by the nickel-molybdenum coating.
W. Hu / International Journal of Hydrogen Energy 25 (2000) 111±118
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Fig. 2. SEM images of (a) Ni-Mo coating on a nickel sheet, (b) the cross-section of Ni-Mo coating.
3.2. HER polarization
Fig. 1. SEM micrographs of (a) Ti2Ni alloy-containing electrode, (b) MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode, (c) LaNi4.9Si0.1 alloy-containing electrode.
Fig. 3 shows the polarization curves of three electrodes with dierent hydrogen storage alloys. The hydrogen overpotential at 200 mA/cm2 at 708C in 30 wt% KOH is summarized in Table 1. The catalytic activity for hydrogen evolution increases with the following order: MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode
2.11 60
17.35 80.54
nickel-molybdenum alloys. This is partly ascribed to the good synergetic eects between nickel and molybdenum. In our cases, we also found that the catalytic activity was mainly dominated by surface composition, namely the activity increases with increase in molybdenum content as seen from Table 1.
7.23
2.46 90.31
Fig. 3. Polarization curves of three electrodes at 708C in 30 wt% KOH.
Z: hydrogen overpotential at 200 mA/cm2. a
2.58
84 88
9.23 90.77
2.30 93.05 4.65
12.31 85.09
3.3. Electrode stability
Mo Ni Mm La Ti Z/mVa
After intermittent electrolysis (at%) Before electrolysis (at%) Before electrolysis (at%)
After intermittent electrolysis (at%)
LaNi4.9Si0.1 alloy-containing electrode MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode Element
Table 1 The average chemical composition on the surface before and after electrolysis (intermittent), and hydrogen overpotential
Before electrolysis (at%)
W. Hu / International Journal of Hydrogen Energy 25 (2000) 111±118 Ti2Ni alloy-containing electrode
114
Besides the catalytic activity, the time stability of activity is also required. The dependence of electrolysis time on HER overpotential at 200 mA/cm2 and 708C was examined. The overpotential after more than 3500 h is about 63, 86 and 90 mV for the Ti2Ni alloycontaining electrode, LaNi4.9Si0.1 alloy-containing electrode and MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode, respectively. During the electrolysis, the main drawback of hydrogen storage alloys is that it is easily pulverized due to absorbing amounts of hydrogen and lattice expansion [35,36]. In our cases, however, the phenomenon can be prevented by predepositing a layer of thin nickel coating on these alloy surfaces. As a result, no eect of inside hydrogen storage alloys on electrode stability was observed. These results demonstrated that these electrodes have excellent time stability and durability under condition of continuous electrolysis. Electrodes for practical use in industrial electrolysis are required not only to maintain their activities over prolonged periods of operations, but also be able to withstand intermittent power interruptions during cell shutdowns. The intermittent electrolysis with power interruptions lasting various durations (456 h for the Ti2Ni alloy-containing electrode, 552 h for the LaNi4.9Si0.1 alloy-containing electrode and 504 h for the MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode) was examined. No obvious increase in overpotential for these electrodes after intermittent electrolysis was observed. For nickel-molybdenum elec-
W. Hu / International Journal of Hydrogen Energy 25 (2000) 111±118
trocatalysts, however, the HER overpotential increased signi®cantly after power interruptions of 2 weeks or more [19,21,26]. It is well known that the electrode is usually in a depolarized state due to a reverse current during the period of power interruptions. If the power interruption lasts for a long period, the electrode potential usually migrates in a positive direction. Therefore, the electrode materials are susceptible to corrosion, and electrode composition and structure are possibly changed severely. Divisek and his co-workers [27±29] examined and analyzed the possible reasons for the loss of catalytic activity of Raney nickel and nickel-molybdenum electrocatalysts after intermittent operation. They found that the HER overpotential depended strongly on the amount of residual Al in Raney nickel alloys. The reasons for deactivation were primarily ascribed to the dissolution of catalyst components such as Al in Raney nickel and Mo in nickelmolybdenum alloys. In addition, they also observed that the deactivation of such electrodes was usually accompanied by a visible destruction of the surface structure of the electrodes. In our cases, the electrode contains hydrogen storage alloys inside. The hydrogen storage alloys (such as AB5 or AB2-type alloys) are well known to be able to absorb and release large amounts of hydrogen at convenient temperature and pressures and therefore have been extensively studied as negative electrodes of rechargeable Ni/metal hydride batteries [37,38]. Due to the special properties of hydrogen storage alloys, these electrocatalysts for HER are expected to be able to resist a long period of power interruption. Fig. 4 presents the potential changes of two electrodes during the power interruptions. The residual potential of MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode was more negative than that of the Ti2Ni alloy-containing electrode because the MmNi3.6Co0.75Mn0.4Al0.27 alloy has larger hydrogen
Fig. 4. Potential change of electrodes during power interruptions.
115
Fig. 5. SEM micrographs of electrodes after intermittent electrolysis, (a) MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode, (b) LaNi4.9Si0.1 alloy-containing electrode.
storage capacity than the Ti2Ni alloy in this work. The SEM micrographs of electrodes after intermittent electrolysis with power interruptions are shown in Fig. 5. As seen from Fig. 5, no visible destruction of the electrode surface structure is observed. The electrode surface structure after intermittent electrolysis is similar to that before electrolysis. The average compositions on the electrode surface after intermittent electrolysis are summarized in Table 1 after EDAX analysis. Although the compositions had changed as compared with those before electrolysis, no obvious in¯uence of these changes on the hydrogen overpotential was found. This indicated that these electrocatalysts exhibited excellent time stability under intermittent conditions. The reasons why these electrocatalysts show excellent time stability may be explained by considering the following reactions.
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W. Hu / International Journal of Hydrogen Energy 25 (2000) 111±118
During continuous electrolysis: ÿ
ÿ
1
MH ad 4 Diffusion 4 Transfer M 0 H ad
2
MH ad H2 O eÿ M H2 " OHÿ
3
M H2 O e MH ad OH
or 2MH ad 2M H2 "
4
M 0 H ad M 0 H ab Hydrides
5
During power interruption shutdown: M 0 H ab Hydrides 4 Decomposition 4 Diffusion MH ad
6
MH ad OHÿ M H2 O eÿ
7
Where M and M' represent the Ni-Mo alloy and the hydrogen storage alloy, respectively. The MH(ad) and M'H(ab) denote the electrochemical adsorption of hydrogen on electrode surface and the hydrogen absorbed by hydrogen storage alloys, respectively. During continuous electrolysis, there are 5 important reactions occurring on the electrode surface and in the bulk. They are Volmer reaction (reaction 1), hydrogen diusion reaction (reaction 2), Heyrovsky reaction (reaction 3), Tafel reaction (reaction 4) and metal hydride forming reaction (reaction 5), respectively. Most of the hydrogen produced on the surface is evolved. The path-way of the hydrogen evolution reaction includes the reactions 1, 3 or 4. At the same time, hydrogen adsorbed on the electrode surface is partially diused into the bulk from the surface and then is absorbed by the hydrogen storage alloys and the metal hydride compound is formed (reactions 2 and 5). However, no further hydrogen diusion (reaction 2) and hydride forming (reaction 5) take place after a sat-
uration is obtained. When power interruption shutdown occurs, the hydrogen absorbed by hydrogen storage alloys will be released and diused to electrode surface (reaction 6), then consumes the reverse current caused by power interruption shutdown through the electrochemical reaction (reaction 7). Therefore, it was thought that the excellent time stability during intermittent electrolysis is due to the internal hydrogen storage alloys which can absorb and release amounts of hydrogen through a series of electrochemical reactions and eectively protect Ni-Mo electrocatalyst on the electrode surface from oxidative dissolution. 3.4. Stability test after oxidation treatments Oxidation treatments could give the more severe eect on the electrocatalytic activity. In this case, the Ti2Ni alloy-containing electrode was subsequently oxidized with a constant anode current density of 1 mA/ cm2 for 5 and 10 h, respectively. After anode oxidation, the electrolysis was resumed and results are presented in ®gure 6. The number in parentheses in Fig. 6 denotes the electrode residual potentials at the end of oxidation. It could be seen that the activity of the electrode remained almost unchanged after the ®rst 5 h of oxidation. However, the hydrogen overpotential increased noticeably after another 10 h oxidation. The SEM micrograph after oxidation treatments is presented in Fig. 7. As may be seen, the electrode surface was severely destructured as compared with the surface before electrolysis as shown in Fig. 1(a). The composition after EDAX analysis is listed in Table 2. Almost no Mo on the surface was observed. Similar observations were reported by Divisek et al. [27]. This was due to the fact that Mo is easily leached out of the electrode in strong alkaline solution, especially in the case of anode oxidation. The signi®cant increase of Ti content and poisonous elements such as Fe, Ca on the surface was also found, as shown in Table 2. The leaching of Mo and deposition of poisonous elements on the electrode surface could jointly result in the loss
Fig. 6. Stability against anode oxidation for Ti2Ni alloy-containing electrode (no IR compensation).
W. Hu / International Journal of Hydrogen Energy 25 (2000) 111±118
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results of the MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode after anode oxidation. These results imply that the time stability of the electrode is strongly dependent on the electrode composition, as well as the residual potential after power interruptions or oxidation.
4. Conclusions New electrocatalysts composed of dierent hydrogen storage alloys and nickel-molybdenum coatings have been made. Their electrocatalytic activity for HER, time stability, electrode structure and chemical compositions were investigated. The conclusions are summarized as follows: Fig. 7. SEM micrograph of Ti2Ni alloy-containing electrode after oxidation treatments.
of electrocatalytic activity. In addition, the Ti2Ni alloy has lower hydrogen storage capacity than the MmNi3.6Co0.75Mn0.4Al0.27 alloy. As a matter of fact, the depolarization ability was improved using the MmNi3.6Co0.75Mn0.4Al0.27 alloy instead of the Ti2Ni alloy because the electrode potential during oxidation is dependent on what kind of hydrogen storage alloy rather than the amount of alloy used. Fig. 8 gives the
1. The electrocatalytic activity for HER is improved with the following order: MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode
Table 2 Average chemical composition of Ti2Ni alloy-containing electrode before and after oxidation treatments Element
Original composition (at%)
Composition after oxidation treatments (at%)
Mo Ni Ti Fe Ca
17.35 80.54 2.11
0.00 68.13 22.18 6.08 3.60
Fig. 8. Stability against anode oxidation for MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode (no IR compensation).
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W. Hu / International Journal of Hydrogen Energy 25 (2000) 111±118
release, and consuming the reverse current through electrochemical discharge reactions during intermittent periods. 3. The time stability of the electrode, especially in intermittent electrolysis, depends on the kind of hydrogen storage alloy used, the amount of hydrogen storage alloy, the reverse current and oxidation time. The MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode has more resistant ability against anode oxidation than the Ti2Ni alloy-containing electrode. 4. The mechanism of good time stability has been also discussed according to the series of electrochemical reactions occurring on the electrode surface and in the bulk. These electrocatalysts show a high potential as electrode materials for alkaline water electrolysis.
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
References
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[1] Divisek J, Schmitz H, Mergel J. Chem-Int-Tech 1980;52:465. [2] Divisek J, Malinowski P, Mergel J, Schmitz H in IEA Annex IV Worksop `Electrolytic Hydrogen Production', ISPRA 1983. [3] Rami A, Lasia A. J Appl Electrochem 1992;22:376. [4] Raj IA, Vasu KI. J Appl Electrochem 1990;20:32. [5] Los P, Rami A, Lasia A. J Appl Electrochem 1993;23:135. [6] Endoh E, Otouma H, Morimoto T, Oda Y. Int J Hydrogen Energy 1987;12:473. [7] Endoh E, Otouma H, Morimoto T. Int J Hydrogen Energy 1988;13:207. [8] Choquette Y, Menard H, Brossard L. Int J Hydrogen Energy 1989;14:637. [9] Choquette Y, Menard H, Brossard L. Int J Hydrogen Energy 1990;15:21. [10] De Giz MJ, Silva JCP, Ferreira M, Machado SAS, Ticianelli EA, Avaca LA, Gonzalez ER. Int J Hydrogen Energy 1992;17:725. [11] De Giz MJ, Machado SAS, Avaca LA, Gonzalez ER. J Appl Electrochem 1992;22:973.
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Schiller G, Borck V. Int J Hydrogen Energy 1992;17:261. Rausch S, Wendt H. J Appl Electrochem 1992;22:1025. Chen L, Lasia A. J Electrochem Soc 1992;139:1058. Chen L, Lasia A. J Electrochem Soc 1992;139:3214. Cheong AK, Lasia A, Lessard J. J Electrochem Soc 1993;140:2721. Miousse D, Lassia A, Borck V. J Appl Electrochem 1995;25:592. Brown DE, Mahmood MN, Turner AK, Hall SM, Fogarty PO. Int J Hydrogen Energy 1982;7:405. Brown DE, Mahmood MN, Man MCM, Turner AK. Electrochim Acta 1984;29:1551. Yurdean ML, Huot JY, Schulz R. Appl Phys Lett 1991;58:2764. Huot J-Y, Yurdean ML, Schulz R. J Electrochem Soc 1991;138:1316. Chen L, Lasia A. J Electrochem Soc 1992;139:3458. Raj IA. J Mater Sci 1993;28:4375. Fan C, Piron DL, Sleb A, Paradis P. J Electrochem Soc 1994;141:382. Simpraga R, Bai L, Conway BE. J Appl Electrochem 1995;25:628. Mahmood MN, Turner AK, Man MCM, Fogarty PO. Chemistry Industry (London) 1984;2:50. Divisek J, Schmitz H, Balej J. J Appl Electrochem 1989;19:519. Divisek J, Mergel J, Schmitz H. Int J Hydrogen Energy 1990;15:105. Divisek J, Schmitz H, Steen B. Electrochim Acta 1994;39:1723. Divisek J, Steen B, Schmitz H. Int J Hydrogen Energy 1994;19:579. Liu B, Hu W. J Applied Electrochem 1998;28:120. Hu W, Cao X, Wang F, Zhang Y. Int J Hydrogen Energy 1997;22:441. Hu W, Lee JY. Int J Hydrogen Energy 1998;23(4):253. Hu W. Chinese Patent Appl. 95116198.9 1995. Schlapbach L, Seiler A, Stucki F, Siebmann HC. J LessCommon Met 1980;73:145. Willems JJG. Philips J Res Suppl 1984;39(1):1. Fetcenko MA, Venkatesan S, Ovshinsky SR Proceedings of the Symposium on Hydrogen Storage Materials, Batteries and Electrochemistry, Electrochemical Society, Pennington, NJ, 1992. p. 141. Sakai T, Miyamura H, Kuriyama N, Ishikawa H, Uehara I. Z Phys Chem 1994;183:333.
Electrocatalytic properties of new electrocatalysts for hydrogen evolution in alkaline water electrolysis Weikang Hu* Department of Structural Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91, Stockholm, Sweden
Abstract New electrocatalysts consisting of hydrogen storage alloys, such as MmNi3.6Co0.75Mn0.4Al0.27 alloy (Mm 0 misch metal), LaNi4.9Si0.1 alloy and Ti2Ni alloy and nickel-molybdenum coatings were prepared. Their electrocatalytic activity for hydrogen evolution and their time stability were investigated in 30 wt% KOH at 708C. The surface morphology and chemical composition of these electrodes were also examined before and after electrolysis. The results show that the activity for hydrogen evolution increases with increase in molybdenum content and is dominated by electrode surface composition. The hydrogen storage alloys inside the electrodes mainly serve the functions of enhancing corrosion-resistance against power interruptions through electrochemically releasing absorbed hydrogen. These electrocatalysts not only have a low hydrogen overpotential, but also excellent time stability under both conditions of continuous and intermittent electrolysis. # 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Hydrogen evolution; Alkaline water electrolysis; Electrocatalysts; Hydrogen storage alloys
1. Introduction Hydrogen is considered to be an ideal future energy carrier. One of the most promising methods for production of hydrogen is water electrolysis using a variety of energy sources such as solar, geothermal, hydroelectric, nuclear or fusion energy. In order to make water electrolysis more ecient and economical, reduction of the cell voltage is vital. Many catalytic materials for anodes and cathodes have been proposed and examined. Raney nickel electrodes were found to have low oxygen overpotential and their activity remained unchanged for more than 13,000 h [1,2]. Raney nickel and nickel-molybdenum alloys as hydrogen cathodes have also been examined extensively and
* Tel.: +46-8-162-383; fax: +46-8-152-187. E-mail address: [email protected] (W. Hu).
proved to be very eective for hydrogen evolution in alkaline solution [3±25]. The composite-coated Raney nickel and thermally deposited nickel-molybdenum coating electrodes showed a low hydrogen overpotential and very good time stability under continuous electrolysis [6,8,18,19]. However, the main drawback of the two kinds of electrocatalysts is that activities for hydrogen evolution reaction (HER) are easily lost after intermittent operation, especially after a long period [24,26,27]. The reasons for degradation are mainly ascribed to the oxidized dissolution of the catalyst components such as Al, Zn for Raney nickel alloys and Mo for nickel-molybdenum alloys [24,29,30]. In order to overcome the drawback, we have recently developed a new electrocatalyst for hydrogen evolution in alkaline solution [31±33]. The new electrocatalyst mainly consists of hydrogen storage alloys and nickelmolybdenum coatings. The hydrogen storage alloys absorb hydrogen in electrolysis process and release the
0360-3199/00/$20.00 # 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 9 9 ) 0 0 0 2 4 - 5
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W. Hu / International Journal of Hydrogen Energy 25 (2000) 111±118
absorbed hydrogen during intermittent operation through a series of electrochemical reactions. The new electrocatalyst shows excellent stability under intermittent operation as well as continuous electrolysis. The purpose of this paper is to compare the performance and electrochemical properties of these catalysts with dierent hydrogen storage alloys. 2. Experiment 2.1. Electrode preparation and characterization The electrocatalyst consists of hydrogen storage alloys, a nickel-molybdenum coating and a nickel foam substrate. The hydrogen storage alloys used in this work are MmNi3.6Co0.75Mn0.4Al0.27 alloy, LaNi4.9Si0.1 alloy and Ti2Ni alloy, respectively. These alloys were prepared by arc melting under Ar atmosphere. The alloy samples obtained were crushed into powders and then screened to a size of 075 mm by passing through sieves. Prior to electrode preparation, a small amount of hydrogen storage alloy (070 mg) was ®rst mixed with carbonyl Ni powder under air atmosphere in the weight ratio of 5:1. They were then further mixed with a small amount of polyvinylalcohol (PVA) solution (2.5 wt%). The mixture was then poured into a nickel foam. After the electrode was dried in air, the electrode surface was modi®ed using a nickel-molybdenum alloy coating by means of electrodeposition. A more detailed process has been described in our previous publications and a patent [32±34]. The total geometric area of each sample was about 2.358 cm2. The surface morphology of the electrode was observed by scanning electron microscopy (SEM). The average chemical composition on the electrode surface was measured by energy dispersive analysis of X-ray (EDAX). 2.2. Electrochemical activity measurements The electrocatalytic activity of the electrode was examined using a steady-state galvanostatic method in a three-compartment conventional double walled thermostated glass cell. The electrolyte was a 30 wt% KOH solution prepared from KOH (G. R.) and deionized water. An Hg/HgO electrode (30 wt% KOH) was used as the reference electrode which was linked to the main compartment by a Luggin capillary. The distance between the Luggin capillary and the working electrode was about 1 mm in order to minimize IR drop. A platinum gauze of a large area was used as a counter-electrode. The temperature was maintained at 708C by means of external heating. The polarization curve was measured in the range of 300 mA/cm2 to 50 mA/cm2 with decreasing the current density. The cur-
rent density is expressed with respect to the geometric area of the electrode. 2.3. Long-term stability test During long-term electrolysis tests, the electrode was polarized with a constant current density of 200 mA/ cm2. A large Ni (99.9%) sheet was used as the anode. The hydrogen overpotential of the electrode was measured after certain intervals. During intermittent electrolysis with power interruption, the electrode was ®rst polarized as above for a few days, then the power was shuto. The temperature of electrolyte was cooled from 708C to room temperature during the power interruption. After a certain period of power interruption, the electrolysis was resumed under condition of 200 mA/cm2 at 708C.
3. Results and discussion 3.1. Electrode morphology and surface composition Fig. 1 shows SEM micrographs of three electrodes of dierent hydrogen storage alloys. The electrodes have almost the same morphology with a coarsely porous structure. This is similar to the appearance of nickel-molybdenum coating deposited on a nickel sheet substrate as shown in Fig. 2(a). In order to estimate the thickness of the nickel-molybdenum coating, the coating deposited on a nickel sheet substrate, which was pre-polished using a series of ®ne emery papers, was made under the same experimental conditions. The SEM image of the cross-section of the nickel-molybdenum coating is presented in Fig. 2(b). The thickness is in the range of 17±20 mm. The nickelmolybdenum deposit loading expressed as the amount of deposit on an apparent substrate surface, is in the range of 60±80 g/m2. X-ray diraction showed an amorphous structure of the nickel-molybdenum coating. The average compositions for these electrodes are listed in Table 1. Under the same electrodeposition conditions, the chemical composition of Ni-Mo coating was relative to the surface state of electrode and electrode substrate. As seen from Table 1, the molybdenum concentration in the Ti2Ni alloy-containing electrode is obviously higher than that in the other two electrodes. In addition, a small amount of La and Ti was found on the LaNi4.9Si0.1 alloy-containing and the Ti2Ni alloy-containing electrode surfaces, respectively. This indicates that only the MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode is completely covered by the nickel-molybdenum coating.
W. Hu / International Journal of Hydrogen Energy 25 (2000) 111±118
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Fig. 2. SEM images of (a) Ni-Mo coating on a nickel sheet, (b) the cross-section of Ni-Mo coating.
3.2. HER polarization
Fig. 1. SEM micrographs of (a) Ti2Ni alloy-containing electrode, (b) MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode, (c) LaNi4.9Si0.1 alloy-containing electrode.
Fig. 3 shows the polarization curves of three electrodes with dierent hydrogen storage alloys. The hydrogen overpotential at 200 mA/cm2 at 708C in 30 wt% KOH is summarized in Table 1. The catalytic activity for hydrogen evolution increases with the following order: MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode
2.11 60
17.35 80.54
nickel-molybdenum alloys. This is partly ascribed to the good synergetic eects between nickel and molybdenum. In our cases, we also found that the catalytic activity was mainly dominated by surface composition, namely the activity increases with increase in molybdenum content as seen from Table 1.
7.23
2.46 90.31
Fig. 3. Polarization curves of three electrodes at 708C in 30 wt% KOH.
Z: hydrogen overpotential at 200 mA/cm2. a
2.58
84 88
9.23 90.77
2.30 93.05 4.65
12.31 85.09
3.3. Electrode stability
Mo Ni Mm La Ti Z/mVa
After intermittent electrolysis (at%) Before electrolysis (at%) Before electrolysis (at%)
After intermittent electrolysis (at%)
LaNi4.9Si0.1 alloy-containing electrode MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode Element
Table 1 The average chemical composition on the surface before and after electrolysis (intermittent), and hydrogen overpotential
Before electrolysis (at%)
W. Hu / International Journal of Hydrogen Energy 25 (2000) 111±118 Ti2Ni alloy-containing electrode
114
Besides the catalytic activity, the time stability of activity is also required. The dependence of electrolysis time on HER overpotential at 200 mA/cm2 and 708C was examined. The overpotential after more than 3500 h is about 63, 86 and 90 mV for the Ti2Ni alloycontaining electrode, LaNi4.9Si0.1 alloy-containing electrode and MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode, respectively. During the electrolysis, the main drawback of hydrogen storage alloys is that it is easily pulverized due to absorbing amounts of hydrogen and lattice expansion [35,36]. In our cases, however, the phenomenon can be prevented by predepositing a layer of thin nickel coating on these alloy surfaces. As a result, no eect of inside hydrogen storage alloys on electrode stability was observed. These results demonstrated that these electrodes have excellent time stability and durability under condition of continuous electrolysis. Electrodes for practical use in industrial electrolysis are required not only to maintain their activities over prolonged periods of operations, but also be able to withstand intermittent power interruptions during cell shutdowns. The intermittent electrolysis with power interruptions lasting various durations (456 h for the Ti2Ni alloy-containing electrode, 552 h for the LaNi4.9Si0.1 alloy-containing electrode and 504 h for the MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode) was examined. No obvious increase in overpotential for these electrodes after intermittent electrolysis was observed. For nickel-molybdenum elec-
W. Hu / International Journal of Hydrogen Energy 25 (2000) 111±118
trocatalysts, however, the HER overpotential increased signi®cantly after power interruptions of 2 weeks or more [19,21,26]. It is well known that the electrode is usually in a depolarized state due to a reverse current during the period of power interruptions. If the power interruption lasts for a long period, the electrode potential usually migrates in a positive direction. Therefore, the electrode materials are susceptible to corrosion, and electrode composition and structure are possibly changed severely. Divisek and his co-workers [27±29] examined and analyzed the possible reasons for the loss of catalytic activity of Raney nickel and nickel-molybdenum electrocatalysts after intermittent operation. They found that the HER overpotential depended strongly on the amount of residual Al in Raney nickel alloys. The reasons for deactivation were primarily ascribed to the dissolution of catalyst components such as Al in Raney nickel and Mo in nickelmolybdenum alloys. In addition, they also observed that the deactivation of such electrodes was usually accompanied by a visible destruction of the surface structure of the electrodes. In our cases, the electrode contains hydrogen storage alloys inside. The hydrogen storage alloys (such as AB5 or AB2-type alloys) are well known to be able to absorb and release large amounts of hydrogen at convenient temperature and pressures and therefore have been extensively studied as negative electrodes of rechargeable Ni/metal hydride batteries [37,38]. Due to the special properties of hydrogen storage alloys, these electrocatalysts for HER are expected to be able to resist a long period of power interruption. Fig. 4 presents the potential changes of two electrodes during the power interruptions. The residual potential of MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode was more negative than that of the Ti2Ni alloy-containing electrode because the MmNi3.6Co0.75Mn0.4Al0.27 alloy has larger hydrogen
Fig. 4. Potential change of electrodes during power interruptions.
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Fig. 5. SEM micrographs of electrodes after intermittent electrolysis, (a) MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode, (b) LaNi4.9Si0.1 alloy-containing electrode.
storage capacity than the Ti2Ni alloy in this work. The SEM micrographs of electrodes after intermittent electrolysis with power interruptions are shown in Fig. 5. As seen from Fig. 5, no visible destruction of the electrode surface structure is observed. The electrode surface structure after intermittent electrolysis is similar to that before electrolysis. The average compositions on the electrode surface after intermittent electrolysis are summarized in Table 1 after EDAX analysis. Although the compositions had changed as compared with those before electrolysis, no obvious in¯uence of these changes on the hydrogen overpotential was found. This indicated that these electrocatalysts exhibited excellent time stability under intermittent conditions. The reasons why these electrocatalysts show excellent time stability may be explained by considering the following reactions.
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During continuous electrolysis: ÿ
ÿ
1
MH ad 4 Diffusion 4 Transfer M 0 H ad
2
MH ad H2 O eÿ M H2 " OHÿ
3
M H2 O e MH ad OH
or 2MH ad 2M H2 "
4
M 0 H ad M 0 H ab Hydrides
5
During power interruption shutdown: M 0 H ab Hydrides 4 Decomposition 4 Diffusion MH ad
6
MH ad OHÿ M H2 O eÿ
7
Where M and M' represent the Ni-Mo alloy and the hydrogen storage alloy, respectively. The MH(ad) and M'H(ab) denote the electrochemical adsorption of hydrogen on electrode surface and the hydrogen absorbed by hydrogen storage alloys, respectively. During continuous electrolysis, there are 5 important reactions occurring on the electrode surface and in the bulk. They are Volmer reaction (reaction 1), hydrogen diusion reaction (reaction 2), Heyrovsky reaction (reaction 3), Tafel reaction (reaction 4) and metal hydride forming reaction (reaction 5), respectively. Most of the hydrogen produced on the surface is evolved. The path-way of the hydrogen evolution reaction includes the reactions 1, 3 or 4. At the same time, hydrogen adsorbed on the electrode surface is partially diused into the bulk from the surface and then is absorbed by the hydrogen storage alloys and the metal hydride compound is formed (reactions 2 and 5). However, no further hydrogen diusion (reaction 2) and hydride forming (reaction 5) take place after a sat-
uration is obtained. When power interruption shutdown occurs, the hydrogen absorbed by hydrogen storage alloys will be released and diused to electrode surface (reaction 6), then consumes the reverse current caused by power interruption shutdown through the electrochemical reaction (reaction 7). Therefore, it was thought that the excellent time stability during intermittent electrolysis is due to the internal hydrogen storage alloys which can absorb and release amounts of hydrogen through a series of electrochemical reactions and eectively protect Ni-Mo electrocatalyst on the electrode surface from oxidative dissolution. 3.4. Stability test after oxidation treatments Oxidation treatments could give the more severe eect on the electrocatalytic activity. In this case, the Ti2Ni alloy-containing electrode was subsequently oxidized with a constant anode current density of 1 mA/ cm2 for 5 and 10 h, respectively. After anode oxidation, the electrolysis was resumed and results are presented in ®gure 6. The number in parentheses in Fig. 6 denotes the electrode residual potentials at the end of oxidation. It could be seen that the activity of the electrode remained almost unchanged after the ®rst 5 h of oxidation. However, the hydrogen overpotential increased noticeably after another 10 h oxidation. The SEM micrograph after oxidation treatments is presented in Fig. 7. As may be seen, the electrode surface was severely destructured as compared with the surface before electrolysis as shown in Fig. 1(a). The composition after EDAX analysis is listed in Table 2. Almost no Mo on the surface was observed. Similar observations were reported by Divisek et al. [27]. This was due to the fact that Mo is easily leached out of the electrode in strong alkaline solution, especially in the case of anode oxidation. The signi®cant increase of Ti content and poisonous elements such as Fe, Ca on the surface was also found, as shown in Table 2. The leaching of Mo and deposition of poisonous elements on the electrode surface could jointly result in the loss
Fig. 6. Stability against anode oxidation for Ti2Ni alloy-containing electrode (no IR compensation).
W. Hu / International Journal of Hydrogen Energy 25 (2000) 111±118
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results of the MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode after anode oxidation. These results imply that the time stability of the electrode is strongly dependent on the electrode composition, as well as the residual potential after power interruptions or oxidation.
4. Conclusions New electrocatalysts composed of dierent hydrogen storage alloys and nickel-molybdenum coatings have been made. Their electrocatalytic activity for HER, time stability, electrode structure and chemical compositions were investigated. The conclusions are summarized as follows: Fig. 7. SEM micrograph of Ti2Ni alloy-containing electrode after oxidation treatments.
of electrocatalytic activity. In addition, the Ti2Ni alloy has lower hydrogen storage capacity than the MmNi3.6Co0.75Mn0.4Al0.27 alloy. As a matter of fact, the depolarization ability was improved using the MmNi3.6Co0.75Mn0.4Al0.27 alloy instead of the Ti2Ni alloy because the electrode potential during oxidation is dependent on what kind of hydrogen storage alloy rather than the amount of alloy used. Fig. 8 gives the
1. The electrocatalytic activity for HER is improved with the following order: MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode
Table 2 Average chemical composition of Ti2Ni alloy-containing electrode before and after oxidation treatments Element
Original composition (at%)
Composition after oxidation treatments (at%)
Mo Ni Ti Fe Ca
17.35 80.54 2.11
0.00 68.13 22.18 6.08 3.60
Fig. 8. Stability against anode oxidation for MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode (no IR compensation).
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W. Hu / International Journal of Hydrogen Energy 25 (2000) 111±118
release, and consuming the reverse current through electrochemical discharge reactions during intermittent periods. 3. The time stability of the electrode, especially in intermittent electrolysis, depends on the kind of hydrogen storage alloy used, the amount of hydrogen storage alloy, the reverse current and oxidation time. The MmNi3.6Co0.75Mn0.4Al0.27 alloy-containing electrode has more resistant ability against anode oxidation than the Ti2Ni alloy-containing electrode. 4. The mechanism of good time stability has been also discussed according to the series of electrochemical reactions occurring on the electrode surface and in the bulk. These electrocatalysts show a high potential as electrode materials for alkaline water electrolysis.
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