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Possibilities for battery application of CoxByHz colloid particles
Colloids and Surfaces A: Physicochemical and Engineering Aspects 149 (1999) 413–419
Possibilities for battery application of Co B H colloid particles x y z M. Mitov a,*, A. Popov b, I. Dragieva b a Department of Chemistry, South-West University, Blagoevgrad, Bulgaria b Central Laboratory of Electrochemical Power Sources, Bulgarian Academy of Sciences, Sofia, Bulgaria Received 26 August 1997; accepted 25 March 1998
Abstract Amorphous Co B H colloid particles, produced by borohydride reduction process for aqueous CoSO solution, x y z 4 were studied for their possible application to metal hydride rechargeable batteries. Electrodes, prepared from powders by pressing and sintering, were tested in 20% KOH solution by means of cyclic voltammetry and chronopotentiometry. The influence of pressure and sintering temperature applied on the electrode performance was elucidated. Electrodes, prepared under low pressure (20–30 MPa) and sintering temperature (200–300°C ) show the best characteristics in respect of the hydrogen electrochemical absorption–desorption. Crystallization of powders, occurring at higher temperatures, causes a considerable decrease in amount of sites available for hydrogen storage. A discharge capacity of 250 mA h g−1 was attained with electrodes possessing amorphous structure. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Borohydride reduction; Co B H colloid particles; Electrode structure; Hydrogen storage; Metal hydride x y z electrodes
1. Introduction Metal hydride (MH ) electrodes have recently attracted attention as the most competitive substitutes for toxic Cd in conventional rechargeable Ni/Cd batteries. Many materials capable of sorbing and desorbing hydrogen have been investigated for battery applications, but only some of them, mainly on the basis on AB and AB type alloys 5 2 [1–4], have been commercialized until now. In our previous papers [5,6 ], it has been demonstrated that electrodes, prepared from powders synthesized by means of borohydride reduction method, could be charged and discharged under galvanostatic conditions in strongly alkaline solu* Corresponding author. Fax: +359 73 29325; e-mail: [email protected]
tions. The reactions taking place have been assigned [6 ] to electrochemical reduction of water to hydrogen, which adsorbs, absorbs, diffuses into the bulk of electrode and probably forms hydrides with the substrate and vice versa. Some of the electrodes studied have exhibited discharge capacities comparable with those of commercially used MH electrodes. This paper presents information concerning the influence of preparation conditions on the electrochemical performance of electrodes prepared from amorphous Co B H particles. A technology conx y z sisting of two procedures was used for electrode fabrication. A pre-determined amount of material studied was pressed under definite pressure and the samples obtained were sintered at controlled temperature in nitrogen atmosphere. Both pressing and thermal treatment influence the powder prop-
0927-7757/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0 9 2 7- 7 7 5 7 ( 9 8 ) 0 03 9 8 - 7
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M. Mitov et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 413–419
Fig. 1. TEM pattern of Co B H particles. x y z
erties such as density, composition, structure, electric resistance, magnetic properties, etc. [7]. The elucidation of their influence on the electrochemical performance in respect of hydrogen absorption/desorption is the main purpose of this study.
2. Experimental 2.1. Materials Powders with composition Co B H were synx y z thesized by reduction of CoSO aqueous solution 4 with NaBH at room temperature and definite pH, 4 which was carried out using two kinds of reactors. In the first case, both solutions were mixed simultaneously in the upper part of a ‘‘Y’’-type reactor. The value of pH was adjusted to 6.7. In the second case, one of the solutions was added to the other one put in advance into the reactor. This technique is known as a ‘‘tea’’ method. The pH of the working solution was adjusted to 8.3 and mechanical stirring was applied. The concentration of CoSO (112.44 g l−1) and NaBH (46.11 g l−1) as 4 4 well as the reaction time (10.5 min) were identical in both preparational experiments. To avoid complications, in further exposure the powder synthesized in the ‘‘Y’’-type reactor is denoted powder
A and that prepared in the second reactor powder B. The so-prepared powders were separated from the solution by filtration, washed and dried under vacuum for 24 h. Vacuum drying was applied before each of the subsequent procedures, such as elemental analysis, pressing, sintering, etc. The powders have amorphous structure [7]. Despite the concentrations of the reacting solutions being identical, the titrimetrically determined boron content [8] in powder A was slightly higher (7.66 wt%) than that in powder B (7.45 wt%).
2.2. Electrode preparation Cylindrical samples with diameter 6 mm and thickness 2 mm were prepared by pressing about 0.2 g of the powders. All samples from powder A were fabricated under pressure of 100 MPa. Samples pressed under pressure of 20, 30, 50, 75 and 100 MPa were obtained from powder B. The so-prepared samples were then sintered in a quartz reactor under N atmosphere. The ther2 mal treatment of samples prepared from powder A was carried out at different temperatures in the range 200–800°C. Half of the samples from powder B were treated subsequently at 150, 250, 350 and 450°C and the rest were heated only at 250 and
M. Mitov et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 413–419
(a)
(a)
(b)
(b)
Fig. 2. Cyclic voltammograms obtained with Co B H x y z electrodes prepared from: (a) powder A sintered at 200°C (curve 1), 600°C (curve 2); (b) powder B pressed under 20 MPa (curve 1), 100 MPa (curve 2). Sweep rate 5 mV s−1; 20% KOH solution.
450°C. At each stage the sintering was carried out for 10 min. 2.3. Techniques Electrochemical experiments were performed in a three-electrode H-shaped cell with separate compartments for the working and auxiliary electrodes. The measurements were carried out in 20% KOH aqueous solution. The cylindrical samples prepared as described above were connected as working electrode and a large nickel foil was used as auxiliary electrode. The working electrode potential was measured vs. a Hg/HgO reference electrode. The experiments were performed using a PJT 35-2 potentiostat–galvanostat (Radiometer-
415
Fig. 3. Dependence of: (a) cathodic, (b) anodic peak intensity in cyclic voltammograms on the sintering temperature. Sweep rate 5 mV s−1; 30 cycles; 20% KOH solution.
Tacussel, France) with IMT 101 electrochemical interface and Volta Master 2 Software. An ohmic drop compensation was applied. Cyclic voltammetry was carried out in the potential range between −1.2 and −0.5 V vs. Hg/HgO, starting from open circuit potential, E , and shifting the potential oc towards more negative values initially. After repeated cycling (30 cycles) with a potential sweep rate of 5 mV s−1, the electrodes were charged at a constant cathodic current of 10 mA and discharged at the same anodic current up to a cut-off potential of −0.5 V vs. Hg/HgO. The morphology of the powders was examined by transmission electron microscopy ( TEM ) and the electrode surface before and after electrochemical experiments was observed by scanning electron microscopy (SEM ) (JEM-200 CX, JEOL, Japan). Electrode surface area was determined by the
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M. Mitov et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 413–419
well-known gas absorption surface area measurement (BET ) method, using gaseous N (apparatus 2 Flowsorb II 2300, Micrometrics).
3. Results and discussion Both powders subjected in this study consist of particles of spherical shape and colloidal sizes in the range 50–100 nm, as seen in Fig. 1. No shells of borates, hydrates and other oxygen-containing substances are observed on their surfaces. Patterns of cyclic voltammograms, obtained with the electrodes studied, are shown in Fig. 2. As assumed in a previous paper [6 ], the cathodic and anodic peaks observed are associated with electrochemical absorption and desorption of hydrogen taking place through a mechanism proposed for hydrogen storage materials [9]. The heights of cathodic and anodic peaks are proportional to the amounts of hydrogen sorbed and desorbed, respectively, and the relative position of peak potentials is a stability criterion for hydrogen atoms stored in different interstitial sites [10,11]. Two well-defined groups of electrodes prepared from powder A are formed: first, including those sintered at 200–300°C; and second, containing the electrodes treated at 400–800°C. The existence of broad anodic and cathodic waves in the voltammograms is typical for the electrodes from the first group, as seen from Fig. 2(a). Usually, two anodic and one cathodic well-shaped peaks are observed, but in some patterns even more smaller maxima may be distinguished. Lower single anodic and cathodic peaks are observed in the voltammograms characterizing the performance of electrodes from the second group. These peaks appear at lower overpotentials than the corresponding most intensive peaks in the voltammograms of electrodes from the first group. The dependence of peak heights on the sintering temperature is presented in Fig. 3. Increasing the temperature induces decreasing peak heights in the whole temperature range investigated. However, the peak heights obtained with the electrodes from the first group are several times higher than those of the electrodes included in the second group. The distinguished performance of electrodes
sintered under and over 400°C may be related mainly to their different structure. It was established [12] that the amorphous Co B H powders x y z crystallize at a temperature range of 400–500°C with the formation of Co B, Co B and Co phases. 3 2 Therefore, the electrodes sintered under 400°C are amorphous, while those treated over this temperature have crystalline structure. Besides this, when the temperature is elevated, the hydrogen contained in powder gradually evolves, so the electrodes sintered at higher temperatures have smaller hydrogen content after preparation. Obviously, the change in the material structure and composition results in the presence of different amounts and types of sites available for hydrogen storage. Fig. 4 presents the dependence of the roughness factor, estimated as a ratio between the BET surface area and the geometric surface area of the electrode samples, on the sintering temperature applied. The most electrochemically active electrodes possess the highest BET surface areas. Increasing the temperature leads to a decrease in the roughness factor, especially for temperatures above 400°C. The relatively high roughness factor of the electrode sintered at 400°C may be explained assuming that the sample is only partially crystallized as a result of the thermal treatment performed. Despite the similarity between the dependencies presented in Fig. 3 and Fig. 4, there is no linear correlation between the electrochemical response and thus the determined roughness factor.
Fig. 4. Dependence of the roughness factor, estimated as a ratio of BET surface area to geometric surface area, on the sintering temperature.
M. Mitov et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 413–419
This is expected, considering the presence of different types of sites available for hydrogen storage in both groups of electrode samples. The intensities and areas under the peaks in the voltammograms obtained with the electrodes from powder B, presented in Fig. 2(b), are comparable with those observed on the crystallized samples prepared from powder A. However, both anodic and cathodic peaks obtained with the former electrodes are located at potentials more typical for the highest peaks in the patterns, recorded with the electrodes sintered at 200 and 300°C. The dependence of peak heights in the voltammograms, obtained with electrodes from powder B, on the pressure applied during sample preparation is shown in Fig. 5. Increasing the pressure up to 50 MPa leads to decreasing heights of both cathodic and anodic peaks. The peak intensities change insignificantly with higher pressure applied. This performance is probably due to changes in
417
material density and accessibility in respect to penetration and occupation of available sites into the substrate. The electrodes sintered subsequently at four temperatures exhibit higher activity than those treated at two temperatures. SEM micrographs of untreated and treated by cyclic voltammetry electrode samples are presented in Fig. 6. The treated sample has a rougher surface, which corresponds to the activation of electrodes established in a previous study [6 ]. After such activation, the electrodes were charged and discharged under galvanostatic conditions. The plot of potential vs. time was recorded during discharging steps. Chronopotentiometric discharge curves obtained with some of the electrodes investigated are shown in Fig. 7. As seen, the electrode possessing amorphous structure exhibits the longest transition time. Several times shorter transition times are obtained with all other
(a)
(b)
Fig. 5. Dependence of: (a) cathodic, (b) anodic peak intensity in cyclic voltammograms on the pressure. Full circles: fourstage sintering; open circles: two-stage sintering.
Fig. 6. SEM micrographs of: (a) untreated, (b) treated by repeated cycling electrode sample.
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M. Mitov et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 413–419
(a)
Fig. 7. Chronopotentiometric discharge curves of Co B H x y z electrodes prepared from: powder A — solid line, sintered at 300°C (curve A1), 500°C (curve A2); powder B — dashed line, pressed under 20 MPA (curve B1), 100 MPa (curve B2). Fourstage sintering. Discharge current 10 mA; 20% KOH solution.
electrodes. The electrode discharge capacity was estimated from the chronopotentiograms. The highest value of about 250 mA h g−1 is obtained for the amorphous electrodes. The dependencies of discharge capacity on the sintering temperature and pressure are plotted in Fig. 8(a) and (b), respectively. They confirm the tendencies for the influence of both factors on the electrode performance, observed by means of cyclic voltammetry (see Figs. 3 and 5) and above discussed. Therefore, the cyclic voltammetry may be used as a primary express method for evaluation of the characteristic features of hydrogen storage, particularly in this type of electrode.
4. Conclusions The influence of pressure and sintering temperature applied during preparation of electrodes from amorphous Co B H particles, synthesized x y z by borohydride reduction method, on their electrochemical performance in a strongly alkaline electrolyte was examined by means of cyclic voltammetry and chronopotentiometry. The sintering procedure influences more significantly the electrode performance because of the different material structure, which may be obtained by choosing an appropriate temperature. Considering the similarity of the results obtained
(b)
Fig. 8. Dependence of discharge capacity on: (a) sintering temperature; (b) pressure. Full circles: four-stage sintering; open circles: two-stage sintering.
by both electrochemical methods, we recommend cyclic voltammetry as an express method for primary evaluation and prognosis of the electrochemical performance of the investigated materials, particularly in respect to battery applications.
Acknowledgments The authors would like to thank Dr. I. Markova from the University of Chemical Technology and Metallurgy, Sofia, Bulgaria for sample sintering.
References [1] M.A. Fetcenko, S. Venkatesan, S.R. Ovshinsky, in: D.A. Corrigan, S. Srinivasan ( Eds.), Proc. Symp. Hydrogen Storage Materials, Batteries and Electrochemistry, vol. 92-5, The Electrochemistry Society, Pennington, NJ, 1992, p. 76.
M. Mitov et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 413–419 [2] A. Anani, A. Visintin, S. Srinivasan, A.J. Appleby, in: D.A. Corrigan, S. Srinivasan (Eds.), Proc. Symp. Hydrogen Storage Materials, Batteries and Electrochemistry, vol. 92-5, The Electrochemistry Society, Pennington, NJ, 1992, p. 105. [3] A. Visintin, K. Petrov, S. Srinivasan, A.J. Appleby, J.J. Reily, J.R. Johnson, H.S. Lim, Ext. Abstr. 183rd Meet. Electrochemical Society, vol. 93-1, The Electrochemistry Society, Princeton, NJ, 1993, p. 79. [4] A. Anani, A. Visintin, S. Srinivasan, A.J. Appleby, Ext. Abstr. 183rd Meet. Electrochemical Society, vol. 93-1, The Electrochemistry Society, Princeton, NJ, 1993, p. 82. [5] M. Mitov, A. Popov, K. Petrov, I. Dragieva, Z. Stoynov, in: V. Bostanov, N. Atanasov ( Eds.), Proc. Nat. Sci. Session of Bulgarian Electrochemical Society, Sofia, 1996, p. 233.
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[6 ] M. Mitov, A. Popov, I. Dragieva, J. Appl. Electrochem, in press. [7] I. Dragieva, Amorphous metal magnetic powders obtained by borohydride reduction process, Dr. Sci. Thesis, Sofia, Bulgaria, 1992. [8] I. Dragieva, Kr. Russev, M. Stanimirova, J. Less-Common Met. 158 (1990) 295. [9] A. Lasia, D. Gregoire, J. Electrochem. Soc. 142 (1995) 3393. [10] R. Kirchheim, M. Kieninger, X.Y. Huang, S.M. Filipek, J. Rush, T. Udovic, J. Less-Common Met. 172 (1991) 880. [11] M. Ciureanu, Q.M. Yang, D.H. Ryan, J.O. Strom-Olsen, J. Electrochem. Soc. 141 (1994) 2430. [12] E. Lefterova, I. Dragieva, V. Tchanev, D. Mehandjiev, M. Mikhov, J. Magn. Magn. Mater. 140–144 (1995) 457.
Possibilities for battery application of Co B H colloid particles x y z M. Mitov a,*, A. Popov b, I. Dragieva b a Department of Chemistry, South-West University, Blagoevgrad, Bulgaria b Central Laboratory of Electrochemical Power Sources, Bulgarian Academy of Sciences, Sofia, Bulgaria Received 26 August 1997; accepted 25 March 1998
Abstract Amorphous Co B H colloid particles, produced by borohydride reduction process for aqueous CoSO solution, x y z 4 were studied for their possible application to metal hydride rechargeable batteries. Electrodes, prepared from powders by pressing and sintering, were tested in 20% KOH solution by means of cyclic voltammetry and chronopotentiometry. The influence of pressure and sintering temperature applied on the electrode performance was elucidated. Electrodes, prepared under low pressure (20–30 MPa) and sintering temperature (200–300°C ) show the best characteristics in respect of the hydrogen electrochemical absorption–desorption. Crystallization of powders, occurring at higher temperatures, causes a considerable decrease in amount of sites available for hydrogen storage. A discharge capacity of 250 mA h g−1 was attained with electrodes possessing amorphous structure. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Borohydride reduction; Co B H colloid particles; Electrode structure; Hydrogen storage; Metal hydride x y z electrodes
1. Introduction Metal hydride (MH ) electrodes have recently attracted attention as the most competitive substitutes for toxic Cd in conventional rechargeable Ni/Cd batteries. Many materials capable of sorbing and desorbing hydrogen have been investigated for battery applications, but only some of them, mainly on the basis on AB and AB type alloys 5 2 [1–4], have been commercialized until now. In our previous papers [5,6 ], it has been demonstrated that electrodes, prepared from powders synthesized by means of borohydride reduction method, could be charged and discharged under galvanostatic conditions in strongly alkaline solu* Corresponding author. Fax: +359 73 29325; e-mail: [email protected]
tions. The reactions taking place have been assigned [6 ] to electrochemical reduction of water to hydrogen, which adsorbs, absorbs, diffuses into the bulk of electrode and probably forms hydrides with the substrate and vice versa. Some of the electrodes studied have exhibited discharge capacities comparable with those of commercially used MH electrodes. This paper presents information concerning the influence of preparation conditions on the electrochemical performance of electrodes prepared from amorphous Co B H particles. A technology conx y z sisting of two procedures was used for electrode fabrication. A pre-determined amount of material studied was pressed under definite pressure and the samples obtained were sintered at controlled temperature in nitrogen atmosphere. Both pressing and thermal treatment influence the powder prop-
0927-7757/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0 9 2 7- 7 7 5 7 ( 9 8 ) 0 03 9 8 - 7
414
M. Mitov et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 413–419
Fig. 1. TEM pattern of Co B H particles. x y z
erties such as density, composition, structure, electric resistance, magnetic properties, etc. [7]. The elucidation of their influence on the electrochemical performance in respect of hydrogen absorption/desorption is the main purpose of this study.
2. Experimental 2.1. Materials Powders with composition Co B H were synx y z thesized by reduction of CoSO aqueous solution 4 with NaBH at room temperature and definite pH, 4 which was carried out using two kinds of reactors. In the first case, both solutions were mixed simultaneously in the upper part of a ‘‘Y’’-type reactor. The value of pH was adjusted to 6.7. In the second case, one of the solutions was added to the other one put in advance into the reactor. This technique is known as a ‘‘tea’’ method. The pH of the working solution was adjusted to 8.3 and mechanical stirring was applied. The concentration of CoSO (112.44 g l−1) and NaBH (46.11 g l−1) as 4 4 well as the reaction time (10.5 min) were identical in both preparational experiments. To avoid complications, in further exposure the powder synthesized in the ‘‘Y’’-type reactor is denoted powder
A and that prepared in the second reactor powder B. The so-prepared powders were separated from the solution by filtration, washed and dried under vacuum for 24 h. Vacuum drying was applied before each of the subsequent procedures, such as elemental analysis, pressing, sintering, etc. The powders have amorphous structure [7]. Despite the concentrations of the reacting solutions being identical, the titrimetrically determined boron content [8] in powder A was slightly higher (7.66 wt%) than that in powder B (7.45 wt%).
2.2. Electrode preparation Cylindrical samples with diameter 6 mm and thickness 2 mm were prepared by pressing about 0.2 g of the powders. All samples from powder A were fabricated under pressure of 100 MPa. Samples pressed under pressure of 20, 30, 50, 75 and 100 MPa were obtained from powder B. The so-prepared samples were then sintered in a quartz reactor under N atmosphere. The ther2 mal treatment of samples prepared from powder A was carried out at different temperatures in the range 200–800°C. Half of the samples from powder B were treated subsequently at 150, 250, 350 and 450°C and the rest were heated only at 250 and
M. Mitov et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 413–419
(a)
(a)
(b)
(b)
Fig. 2. Cyclic voltammograms obtained with Co B H x y z electrodes prepared from: (a) powder A sintered at 200°C (curve 1), 600°C (curve 2); (b) powder B pressed under 20 MPa (curve 1), 100 MPa (curve 2). Sweep rate 5 mV s−1; 20% KOH solution.
450°C. At each stage the sintering was carried out for 10 min. 2.3. Techniques Electrochemical experiments were performed in a three-electrode H-shaped cell with separate compartments for the working and auxiliary electrodes. The measurements were carried out in 20% KOH aqueous solution. The cylindrical samples prepared as described above were connected as working electrode and a large nickel foil was used as auxiliary electrode. The working electrode potential was measured vs. a Hg/HgO reference electrode. The experiments were performed using a PJT 35-2 potentiostat–galvanostat (Radiometer-
415
Fig. 3. Dependence of: (a) cathodic, (b) anodic peak intensity in cyclic voltammograms on the sintering temperature. Sweep rate 5 mV s−1; 30 cycles; 20% KOH solution.
Tacussel, France) with IMT 101 electrochemical interface and Volta Master 2 Software. An ohmic drop compensation was applied. Cyclic voltammetry was carried out in the potential range between −1.2 and −0.5 V vs. Hg/HgO, starting from open circuit potential, E , and shifting the potential oc towards more negative values initially. After repeated cycling (30 cycles) with a potential sweep rate of 5 mV s−1, the electrodes were charged at a constant cathodic current of 10 mA and discharged at the same anodic current up to a cut-off potential of −0.5 V vs. Hg/HgO. The morphology of the powders was examined by transmission electron microscopy ( TEM ) and the electrode surface before and after electrochemical experiments was observed by scanning electron microscopy (SEM ) (JEM-200 CX, JEOL, Japan). Electrode surface area was determined by the
416
M. Mitov et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 413–419
well-known gas absorption surface area measurement (BET ) method, using gaseous N (apparatus 2 Flowsorb II 2300, Micrometrics).
3. Results and discussion Both powders subjected in this study consist of particles of spherical shape and colloidal sizes in the range 50–100 nm, as seen in Fig. 1. No shells of borates, hydrates and other oxygen-containing substances are observed on their surfaces. Patterns of cyclic voltammograms, obtained with the electrodes studied, are shown in Fig. 2. As assumed in a previous paper [6 ], the cathodic and anodic peaks observed are associated with electrochemical absorption and desorption of hydrogen taking place through a mechanism proposed for hydrogen storage materials [9]. The heights of cathodic and anodic peaks are proportional to the amounts of hydrogen sorbed and desorbed, respectively, and the relative position of peak potentials is a stability criterion for hydrogen atoms stored in different interstitial sites [10,11]. Two well-defined groups of electrodes prepared from powder A are formed: first, including those sintered at 200–300°C; and second, containing the electrodes treated at 400–800°C. The existence of broad anodic and cathodic waves in the voltammograms is typical for the electrodes from the first group, as seen from Fig. 2(a). Usually, two anodic and one cathodic well-shaped peaks are observed, but in some patterns even more smaller maxima may be distinguished. Lower single anodic and cathodic peaks are observed in the voltammograms characterizing the performance of electrodes from the second group. These peaks appear at lower overpotentials than the corresponding most intensive peaks in the voltammograms of electrodes from the first group. The dependence of peak heights on the sintering temperature is presented in Fig. 3. Increasing the temperature induces decreasing peak heights in the whole temperature range investigated. However, the peak heights obtained with the electrodes from the first group are several times higher than those of the electrodes included in the second group. The distinguished performance of electrodes
sintered under and over 400°C may be related mainly to their different structure. It was established [12] that the amorphous Co B H powders x y z crystallize at a temperature range of 400–500°C with the formation of Co B, Co B and Co phases. 3 2 Therefore, the electrodes sintered under 400°C are amorphous, while those treated over this temperature have crystalline structure. Besides this, when the temperature is elevated, the hydrogen contained in powder gradually evolves, so the electrodes sintered at higher temperatures have smaller hydrogen content after preparation. Obviously, the change in the material structure and composition results in the presence of different amounts and types of sites available for hydrogen storage. Fig. 4 presents the dependence of the roughness factor, estimated as a ratio between the BET surface area and the geometric surface area of the electrode samples, on the sintering temperature applied. The most electrochemically active electrodes possess the highest BET surface areas. Increasing the temperature leads to a decrease in the roughness factor, especially for temperatures above 400°C. The relatively high roughness factor of the electrode sintered at 400°C may be explained assuming that the sample is only partially crystallized as a result of the thermal treatment performed. Despite the similarity between the dependencies presented in Fig. 3 and Fig. 4, there is no linear correlation between the electrochemical response and thus the determined roughness factor.
Fig. 4. Dependence of the roughness factor, estimated as a ratio of BET surface area to geometric surface area, on the sintering temperature.
M. Mitov et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 413–419
This is expected, considering the presence of different types of sites available for hydrogen storage in both groups of electrode samples. The intensities and areas under the peaks in the voltammograms obtained with the electrodes from powder B, presented in Fig. 2(b), are comparable with those observed on the crystallized samples prepared from powder A. However, both anodic and cathodic peaks obtained with the former electrodes are located at potentials more typical for the highest peaks in the patterns, recorded with the electrodes sintered at 200 and 300°C. The dependence of peak heights in the voltammograms, obtained with electrodes from powder B, on the pressure applied during sample preparation is shown in Fig. 5. Increasing the pressure up to 50 MPa leads to decreasing heights of both cathodic and anodic peaks. The peak intensities change insignificantly with higher pressure applied. This performance is probably due to changes in
417
material density and accessibility in respect to penetration and occupation of available sites into the substrate. The electrodes sintered subsequently at four temperatures exhibit higher activity than those treated at two temperatures. SEM micrographs of untreated and treated by cyclic voltammetry electrode samples are presented in Fig. 6. The treated sample has a rougher surface, which corresponds to the activation of electrodes established in a previous study [6 ]. After such activation, the electrodes were charged and discharged under galvanostatic conditions. The plot of potential vs. time was recorded during discharging steps. Chronopotentiometric discharge curves obtained with some of the electrodes investigated are shown in Fig. 7. As seen, the electrode possessing amorphous structure exhibits the longest transition time. Several times shorter transition times are obtained with all other
(a)
(b)
Fig. 5. Dependence of: (a) cathodic, (b) anodic peak intensity in cyclic voltammograms on the pressure. Full circles: fourstage sintering; open circles: two-stage sintering.
Fig. 6. SEM micrographs of: (a) untreated, (b) treated by repeated cycling electrode sample.
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M. Mitov et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 413–419
(a)
Fig. 7. Chronopotentiometric discharge curves of Co B H x y z electrodes prepared from: powder A — solid line, sintered at 300°C (curve A1), 500°C (curve A2); powder B — dashed line, pressed under 20 MPA (curve B1), 100 MPa (curve B2). Fourstage sintering. Discharge current 10 mA; 20% KOH solution.
electrodes. The electrode discharge capacity was estimated from the chronopotentiograms. The highest value of about 250 mA h g−1 is obtained for the amorphous electrodes. The dependencies of discharge capacity on the sintering temperature and pressure are plotted in Fig. 8(a) and (b), respectively. They confirm the tendencies for the influence of both factors on the electrode performance, observed by means of cyclic voltammetry (see Figs. 3 and 5) and above discussed. Therefore, the cyclic voltammetry may be used as a primary express method for evaluation of the characteristic features of hydrogen storage, particularly in this type of electrode.
4. Conclusions The influence of pressure and sintering temperature applied during preparation of electrodes from amorphous Co B H particles, synthesized x y z by borohydride reduction method, on their electrochemical performance in a strongly alkaline electrolyte was examined by means of cyclic voltammetry and chronopotentiometry. The sintering procedure influences more significantly the electrode performance because of the different material structure, which may be obtained by choosing an appropriate temperature. Considering the similarity of the results obtained
(b)
Fig. 8. Dependence of discharge capacity on: (a) sintering temperature; (b) pressure. Full circles: four-stage sintering; open circles: two-stage sintering.
by both electrochemical methods, we recommend cyclic voltammetry as an express method for primary evaluation and prognosis of the electrochemical performance of the investigated materials, particularly in respect to battery applications.
Acknowledgments The authors would like to thank Dr. I. Markova from the University of Chemical Technology and Metallurgy, Sofia, Bulgaria for sample sintering.
References [1] M.A. Fetcenko, S. Venkatesan, S.R. Ovshinsky, in: D.A. Corrigan, S. Srinivasan ( Eds.), Proc. Symp. Hydrogen Storage Materials, Batteries and Electrochemistry, vol. 92-5, The Electrochemistry Society, Pennington, NJ, 1992, p. 76.
M. Mitov et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 413–419 [2] A. Anani, A. Visintin, S. Srinivasan, A.J. Appleby, in: D.A. Corrigan, S. Srinivasan (Eds.), Proc. Symp. Hydrogen Storage Materials, Batteries and Electrochemistry, vol. 92-5, The Electrochemistry Society, Pennington, NJ, 1992, p. 105. [3] A. Visintin, K. Petrov, S. Srinivasan, A.J. Appleby, J.J. Reily, J.R. Johnson, H.S. Lim, Ext. Abstr. 183rd Meet. Electrochemical Society, vol. 93-1, The Electrochemistry Society, Princeton, NJ, 1993, p. 79. [4] A. Anani, A. Visintin, S. Srinivasan, A.J. Appleby, Ext. Abstr. 183rd Meet. Electrochemical Society, vol. 93-1, The Electrochemistry Society, Princeton, NJ, 1993, p. 82. [5] M. Mitov, A. Popov, K. Petrov, I. Dragieva, Z. Stoynov, in: V. Bostanov, N. Atanasov ( Eds.), Proc. Nat. Sci. Session of Bulgarian Electrochemical Society, Sofia, 1996, p. 233.
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[6 ] M. Mitov, A. Popov, I. Dragieva, J. Appl. Electrochem, in press. [7] I. Dragieva, Amorphous metal magnetic powders obtained by borohydride reduction process, Dr. Sci. Thesis, Sofia, Bulgaria, 1992. [8] I. Dragieva, Kr. Russev, M. Stanimirova, J. Less-Common Met. 158 (1990) 295. [9] A. Lasia, D. Gregoire, J. Electrochem. Soc. 142 (1995) 3393. [10] R. Kirchheim, M. Kieninger, X.Y. Huang, S.M. Filipek, J. Rush, T. Udovic, J. Less-Common Met. 172 (1991) 880. [11] M. Ciureanu, Q.M. Yang, D.H. Ryan, J.O. Strom-Olsen, J. Electrochem. Soc. 141 (1994) 2430. [12] E. Lefterova, I. Dragieva, V. Tchanev, D. Mehandjiev, M. Mikhov, J. Magn. Magn. Mater. 140–144 (1995) 457.