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AB5-catalyzed hydrogen evolution cathodes
Int. J. Hydrogen Energy, Vol. 9, No. 12, pp. 1005-1009, 1984 Printed in Great Britain.
0360-3199/84 $3.00 + 0.00 Pergamon Press Ltd. International Association for Hydrogen Energy.
ABs-CATALYZED HYDROGEN EVOLUTION CATHODES D. E. HALL* and V. R. SHZPARD, J r t Inco Alloy Products, Research Center, Sterling Forest, Suffern, NY 10901, U.S.A. (Received 5 May 1984)
Abstract--The AB5 metal compounds are highly efficient hydrogen evolution electrocatalystsin alkaline electrolyte. Three types of ABs-catalyzed cathode structures were made, using the hydride-forming AB5 compounds in particulate form. Plastic-bonded cathodes containing >90 w/o AB5 (finished-weight basis) were the most efficient, giving hydrogen evolution overpotentials (r/us) of about 0.05 V at 200 mA cm -2. However, they tended to swell and shed material during electrolysis. Pressed, sintered cathodes containing 40-70 w/o catalyst in a nickel binder gave r/H2- 0.08 V; catalyst retention was excellent. Porous, sintered cathode coatings were made with 30-70 w/o AB5 catalyst loadings. Their overpotentials were similar to those of the pressed, sintered cathodes. However, at catalyst loadings below about 40 w/o, high overpotentials characteristic of the nickel binder were observed. The structural and electrochemical properties of the three ABs-catalyzed cathodes are discussed.
INTRODUCTION Development of more energy-efficient hydrogen evolution cathodes is an important goal in the alkaline electrolysis industry. During the past few years, numerous patents and publications have described new cathode materials and fabrication techniques. The AB5 metal compounds, found to be excellent hydride formers by Zijlstra et al. [1], are interesting candidates for hydrogen evolution electrocatalysts, although results reported to date have not included practical cathodes operating at high current density. Using cyclic volammetry at a potential sweep rate of 2 V min -~, Miles [2] studied the hydrogen evolution characteristics of several metallic elements and compounds, including LaNis, in massive form. In Miles' study, restricted to low current density, LaNi5 was among the better electrocatalysts in 30 w/o K O H at 80°C. Later, Kitamura et al. [3] showed that both LaNi5 and MNi5 (M = mischmetal) had electrocatalytic activities comparable to Pt or Pd. The objective of the present work was to develop ABs-catalyzed cathodes with high electrocatalytic activity and the stability required for practical industrial electrolysis. The problem was approached by starting with the AB5 compounds in finely-divided form. Three types of cathodes made with powdered AB5 catalysts are described in this paper. EXPERIMENTAL
discharge cycles were needed for particle size reduction. In a few instances, powders were made by mechanical crushing and subsequent grinding. The powder comminution method had no apparent effect on electrochemical behavior. All AB5 powders were sieved through a 325 mesh screen before cathode preparation. The sieved powders consisted of sharp-edged, smooth-faced, angular particles. The particles ranged from about 5 to 40/~m across, with most in the range 20--40 gin. Plastic-bonded, ABs-catalyzed cathodes were made using the two mixtures in Table 1. Each cathode mix was ground in a motorized mortar and pestle to fibrillate the PTFE. The mixture was then worked under pressures of up to 4.6 x 106 Pa at 120°C until a homogeneous sheet was obtained. Cathode blanks about 2 cm square were cut from the sheet. Nickel screen current distributors were embedded in the cathode blanks by pressing at 2.2 x 107 Pa at 120°C. The polyethylene oxide was then leached from the cathodes by soaking them in water for 24h. After leaching, the cathodes were pressed between two fine metal screen platens at pressures ranging from 7.6 × 105 to 1.5 x 106 Pa. The final pressing temperatures were 177°C for mix A and 120°C for the polyethylene-containing mix B.
Table 1. Plastic-bonded AB5 cathode mix formulations
The AB5 metal compounds were generally converted to powder form by alternately charging and discharging them with hydrogen gas. No more than 2--4 charge/ * Present address: American Cyanamid Company, Chemical Division Research Laboratories, Stamford, CT 06904, U.S.A. i- Present address: Duracell, Duracell Products Technology, Tarrytown, NY 10591, U.S.A. 1005
Component Metal hydride powder* PTFE Carbon black Polyethylene Polyethylene oxide * See Table 2.
Mix A (wt%)
Mix B (wt%)
86 2 2 0 10
86 1 3 2 8
1006
D.E. HALL AND V. R. SHEPARD, JR
Pressed, sintered cathodes were made with blended mixtures of AB5 powder (40-70 weight percent) and INCO* type 123 nickel powder ('Ni 123' below). Ni 123 powder particles are about 5/~m in diameter, have a spiky, high surface morphology, and sinter readily into a porous structure. The mixed powders were pressed at 3.4--3.9 × 109 Pa, and were then sintered in a hydrogen atmosphere at 900°C for 10 rain. ABs-catalyzed cathode coatings were made using essentially the same method reported earlier for porous, sintered Ni 123 anode coatings [4]. The coatings were applied as polysilicate paints containing Ni 123 and AB5 (30-70 w/o) powders. The cathode substrates were mild steel screens. Application of a Watts nickel flash ( - 2 . 5 / z m ) to the steel before coating resulted in more uniform paint retention on the screens. The coated cathodes were air dried and then sintered under hydrogen for 10 min at 800-900°C. Electrochemical measurements were made galvanostatically. All cathodes were operated at a current density of 200 m A cm -2 in 30 w/o K O H electrolyte at 80°C. Most cathodes were tested for 6 h; some, as reported below, were tested for longer periods, Periodically, a series of current density vs potential measurements was made from 600 to I m A c m -2, in descending order. For 6 h tests, the final data, recorded just before the conclusion of each test, are reported here. Raw current vs potential data were corrected for the effects of uncompensated resistance using a cornpurer method [5] which we have used with success in earlier studies [4]. Because of the very low overpotentials at high current densities in the present work, coefficients of determination for the fitted Tafel lines were lower (generally ~0.93) t h a n i n earlier work (generally >10.99). With some cathodes, computer iR corrections were compared with those obtained by the current interruption method. Good agreement was found between the two methods. The structures and surface morphologies of the cathodes were examined by optical and scanning electron microscopy (SEM), both before and after electrochemical testing. For SEM studies, small amounts of carbon were vapor deposited on the cathodes to prevent surface charging,
RESULTS Plastic-bonded cathodes Plastic-bonded cathodes made with the hydride-forming compounds LaNi5 and its substituted derivatives were highly efficient. In particulate form, the AB5 cornpounds exhibited significantly lower overpotentials than reported elsewhere [2, 6] for solid AB5 cathodes. All of the AB5 compounds produced comparable~/~ values of about 0.05 V at a current density of 200 m A c m -2. Thus, the hydride plateau pressure, over the five-fold * Trademark, The International Nickel Company.
Table 2. Plateau pressures for formation of fl-ABsHx for selected compounds at 80°C* AB5 LaNi3Co~ LaNi~TAl0.~ LaNi4Cu LaNi~
P,b~ 2.8 5.3 -5.6 14.9
Pd¢s 2.3 4.9 -4.6 11.1
* Pressures from P. D. Goodell, Inco Alloy Products (×10~Pa). range for these compounds (Table 2), had no observable effect on ~H2. The hydrogen evolution overpotentials of the plastic-bonded AB5 cathodes were generally insensitive to changes in process variables such as the fabrication pressures. Several cathodes were pre-conditioned before electrolysis by electrochemical charge/discharge cycling. These cathodes usually produced lower overpotentials than similar, unconditioned cathodes during the first two or three hours of electrolysis. This advanrage was temporary, as 0n2 at unconditioned cathodes declined during electrolysis. When plastic-bonded cathodes were tested for 100 h, the final overpotentials were essentially the same as those measured after 6 h, indicating no loss of catalytic activity. However, the AB5 catalysts displayed an unusual degree of change in catalyric activity as a function of operating temperature. The overpotentials of LaNi5 and LaNi, TAl03 cathodes increased to 0.16-0.18 V at 200 m A cm -2 when operated at 25°C. Scanning electron microscopy showed that the plastic-bonded cathodes were well-fibrillated, with the AB5 catalyst particles bound inside a web of fine polymer strands (Fig. 1). This structure remained intact after 6 h of electrolysis. However, the cathodes swelled considerably due to entrapment of gas and electrolyte. Mix B cathodes, with their higher residual polymer content, were more resistant to swelling than mix A cathodes. After 100 h of electrolysis, visible catalyst loss was noted frequently with mix A cathodes and occasionally with mix B cathodes.
Pressed sintered cathodes Pressed, sintered cathodes were made with LaNL.TA10ffNi123 mixtures containing 40-70 w/o of the AB5 catalyst powder. A t 70 w/o catalyst loading, there was sufficient nickel binder to form a strong sintered matrix around the catalyst particles. The AB5 catalysts do not sinter in the temperature range used to sinter nickel binder. Raising the temperature to sinter the catalysts as well as the nickel would have introduced the possibility of catalyst composition changes arising from Ni-AB5 interdiffusion, a subject which was not pursued in the present work. Optical and scanning electron microscopy showed that the pressed, sintered
ABs-CATALYZED HYDROGEN EVOLUTION CATHODES
1007
Fig. 1. Cross-section of plastic-bonded LaNhTAl03 cathode, mix A. Magnification 1000 ×.
cathodes contained well-dispersed LaNi4.TAl0.3particles, mostly 10--20/~m in diameter, The pressed, sintered cathodes, with their lower catalyst contents, produced overpotentials - 3 0 mV higher than plastic-bonded cathodes, as shown by the data in Table 3. Given the normal scatter in rpa2 data and the difficulties in correcting low overpotential, high current density data for resistance losses, Table 3 shows that none of the cathode compositions lost catalytic activity between 6 and up to 100 h of elect/olysis,
Because the AB5 catalysts are more expensive than the nickel binder and do not contribute to coating strength, it is desirable to use the lowest AB5 loading needed for minimum overpotential. The data in Table 3 demonstrated that r/H2 was independent of AB5 loading over the range 40-70 w/o. This subject is addressed again in the following section. Overpotentials measured within minutes after the beginning of electrolysis were as high as 0.2 V. It is not known whether the subsequent r/n2 decrease resulted from an increase in effective cathode surface area, or from a process such as removal of a surface oxide film Table 3. Hydrogen evolution overpotential of pressed, sintered or surface hydride formation. However, some ABs-cata" cathodes lyzed cathodes suspected to have higher than usual oxide contents showed an unusually slow overpotential Wt. Pct. LaNi~.TAl0.3 Time (h) H V (at 200 mA cm-2) * decrease. 40 50 60 70
6 47 95 6 27 95 6 48 72 6 48 51
0.07 0.09 0.07 0.06 0.08--0.09 0.06 0.07-0.08 0.07 0.07-0.09 0.07-0.08 0.07-0.08 0.07-0.09
• Resistance-corrected overpotentials in 30 w/o KOH electrolyte at 80°C. Coefficients of determination >0.93.
Sintered ABs/Ni 123 cathode coatings Sintered cathode coatings, applied using polysilicate-based paints containing Ni 123 powder and from 30 to 70 w/o LaNi4.TA10.3 powder, were sintered onto nickel-flashed, woven steel screens. The hydrogen evolution overpotentials of the coated cathodes were essentially the same as those of pressed, sintered cathodes of similar composition. Reducing the LaNi4.rAl0.3 content of the catalyzed coatings to as low as 30 w/o, however, revealed a sharp transition in the relationship between ~/H:and catalyst content. This behavior is shown clearly in Fig. 2 for two series of cathode coatings sintered at 800 and 900°C.
1008
D . E . HALL AND V. R. SHEPARD, JR 04
o
0.35 0.3
\~ ~q~ \ \
>- 0.25 "- o.z o.,s o o., -
a--.o aoo*c c--~ ~ o * c
_ _ ~ V '
"
....
0.05 I t I h { 50 55 6o 65 70 75 pm-cent.o~e AS5 Fig. 2. Hydrogen evolution overpotentials of cathodes with sintered Ni123/LaNi4.TA103 coatings. Coefficients of determination for computer calculation of/R-free overpotentials were ~>0.946. q
I ao
t 35
I I 40 45
The minimum catalyst loading required for maximum catalytic activity was ~40 w/o. Increasing the catalyst loading further produced no additional r/H2reduction, Figure 2 also shows that the sintering temperature had no effect on the catalytic activities of cathode coatings containing >~50w/o catalyst. In contrast, raising the sintering temperature and/or time produced substantial ,/r~2 increases on sintered cathode (or anode) coatings containing only nickel powder [4]. In the uncatalyzed coatings where nickel itself was the active
material, increasing the degree of sintering reduced the effective electrode surface area, resulting in a higher effective current density and thus a higher overpotential. In the ABs-catalyzed cathode coatings, the sintered nickel matrix contributes very little to the cathode efficiency. The morphology of the AB5 catalyst, shown in Fig. 3, is unaffected by the heat treatment required to sinter the nickel into a matrix around it. The figure also shows that the LaNi4.TA10.3particles do not appear to be metallurgically bonded to the surrounding nickel, but are instead physically confined in the porous nickel matrix. Two cathodes with sintered coatings containing 50w/o LaNi4.TAl0.3were operated for more than 3000h at 200 m A cm -2. After an initial break-in period, both cathodes assumed stable hydrogen evolution overpotentials of 0.09-0.10 V. (Coefficients of determination for t h e / R - f r e e overpotentials over the last 1000 h of operation were all >0.95, and were frequently >0.99.) A t the conclusion of the 3000h tests, both cathodes were examined by scanning electron microscopy. There was no loss of overall coating integrity. The coating morphology was unchanged except for some cracking of the LaNL.TA10.3particles. The cracked material was retained in the coating. In addition to stability under cathodic polarization, the catalyst showed excellent open-circuit corrosion resistance. During one long-term electrolysis experiment, in 30 w/o K O H electrolyte at 80°C, current was switched off for over 90 h. When electrolysis was resumed, the cathode potential regained its former
Fig. 3. Surface of 50 w/o LaNh7Al03-coated cathode sintered at 900°C. Angular catalyst particles are surrounded by a matrix of sintered nickel.
ABs-CATALYZED HYDROGEN EVOLUTION CATHODES value after a break-in period, and continued to operate in a stable manner. There was no apparent physical damage to the cathode.
DISCUSSION The AB5 compounds examined in this study are highly efficient hydrogen evolution electrocatalysts. Under cathodic polarization, these compounds can also take up hydrogen to form bulk metal hydrides. However, the ability of an intermetallic compound to store hydrogen is not a sufficient condition for efficient hydrogen evolution. This was demonstrated by preparing plastic-bonded cathodes, containing AB and A2B intermetallic compounds known to be good hydrogen storage materials, and testing them at 80°C in 30 w/o KOH. The AB compounds (Fe0.9Mn0.~)Ti and (Fe0.sNi0.2)Ti produced overpotentials of 0.16-0.22V at a current density of 200 mA cm -2, while the A 2 B compound MgzNi was still less efficient (r/H2 - 0.36 V). Thus, the AB5 compounds appear to be unique among the three classes in providing excellent hydrogen evolution catalysis at high electrolyte temperatures characteristic of commerical electrolytic hydrogen production, and bulk hydrogen storage at low electrolyte temperatures characteristic of battery operation, In the present study, three types of cathode structures were investigated. The plastic-bonded cathodes are a type well-known in battery research. Those used in this study have the advantage of avery high catalyst loading, so that the ultimate efficiency of the catalyst is approached. However, plastic-bonded cathodes with low polymer content are not optimized for rapid, sustained gas evolution. As a result, they swell and shed catalyst during electrolysis. It is doubtful that these cathodes, in the form evaluated in this work, would be suitable in any application in which they were unconstrained. The plastic-bonded sheet cathode is thicker than necessary for high efficiency, and thus, catalyst loading per apparent surface area is high. Catalyst use could be reduced by blending nickel powder into the mix, but then the unique advantage of this cathode, relative to the other two types examined, is lost. These disadvantages might be eliminated if the plastic-bonded sheet
1009
were made very thin (e.g. 100/~m) and pressure-laminated as a coating onto a robust cathode support. The pressed, sintered cathodes, while much less porous, are similar to sintered nickel battery plaques. At a catalyst loading of 40%, they still use a considerable amount of catalyst per unit area if they are made thick enough to be self-supporting. Despite r/H2 values about 30 mV higher than with plastic-bonded cathodes, the ABs/Ni 123 sintered type is still highly efficient. This difference in performance might not be observed in large commercial cells. The electrical resistivity of the AB5 compounds is much higher than that of nickel. For example, the resistivity of LaNi5 is 7.2-7.6 x 10-4 f~cm, depending on its state of charge [7], while the resistivity of nickel is only - 7 x 10 -6 ~-cm. The internal resistance of either type of sintered cathode should thus be considerably lower than the plastic-bonded type, from the standpoint of materials as well as the continuity of the sintered nickel structure. The catalytic sintered coating applied to a suitable substrate is, in many respects, the most practical of the three cathodes studied. As efficient as the pressed, sintered sheet, it uses much less catalyst per unit area than either of the other cathodes when applied thinly. The coating is also readily applied to a variety of metallic substrates, including those presently used in the electrolytic industry. Acknowledgements--The authors thank W. D. K. Clark, J.
T. Arms, G. D. Sandrock, F. J. Mulligan and especially P. D. Goodell for valuable discussions during the course of this work. Thanks are also due to A. M. Rapun, C. Knipple and G. Hawks for experimental assistance. REFERENCES 1. H. Zijlstra et al., Solid State Commun. 7, 857 (1969). 2. M. H. Miles, Electroanalyt. Chem. Interracial Electrochem. 60, 89 (1975). 3. T. Kitamura, C. Iwakura and H. Tamura, Chem. Lett. 1981, 965. 4. D. E. Hall, J. Electrochem. Soc. 128, 740 (1981). 5. R. L. LeRoy, M. B. I. Janjua, R. Renaud and U. Leuenberger, J. Electrochem. Soc. 126, 1674 (1979). 6. T. Kitamura, C. Iwakura and H. Tamura, Chem. Lett. 1981, 1755. 7. G. Adachi, K. Niki and J. Shiokawa, J. Less Common Metals 81,345 (1981).
0360-3199/84 $3.00 + 0.00 Pergamon Press Ltd. International Association for Hydrogen Energy.
ABs-CATALYZED HYDROGEN EVOLUTION CATHODES D. E. HALL* and V. R. SHZPARD, J r t Inco Alloy Products, Research Center, Sterling Forest, Suffern, NY 10901, U.S.A. (Received 5 May 1984)
Abstract--The AB5 metal compounds are highly efficient hydrogen evolution electrocatalystsin alkaline electrolyte. Three types of ABs-catalyzed cathode structures were made, using the hydride-forming AB5 compounds in particulate form. Plastic-bonded cathodes containing >90 w/o AB5 (finished-weight basis) were the most efficient, giving hydrogen evolution overpotentials (r/us) of about 0.05 V at 200 mA cm -2. However, they tended to swell and shed material during electrolysis. Pressed, sintered cathodes containing 40-70 w/o catalyst in a nickel binder gave r/H2- 0.08 V; catalyst retention was excellent. Porous, sintered cathode coatings were made with 30-70 w/o AB5 catalyst loadings. Their overpotentials were similar to those of the pressed, sintered cathodes. However, at catalyst loadings below about 40 w/o, high overpotentials characteristic of the nickel binder were observed. The structural and electrochemical properties of the three ABs-catalyzed cathodes are discussed.
INTRODUCTION Development of more energy-efficient hydrogen evolution cathodes is an important goal in the alkaline electrolysis industry. During the past few years, numerous patents and publications have described new cathode materials and fabrication techniques. The AB5 metal compounds, found to be excellent hydride formers by Zijlstra et al. [1], are interesting candidates for hydrogen evolution electrocatalysts, although results reported to date have not included practical cathodes operating at high current density. Using cyclic volammetry at a potential sweep rate of 2 V min -~, Miles [2] studied the hydrogen evolution characteristics of several metallic elements and compounds, including LaNis, in massive form. In Miles' study, restricted to low current density, LaNi5 was among the better electrocatalysts in 30 w/o K O H at 80°C. Later, Kitamura et al. [3] showed that both LaNi5 and MNi5 (M = mischmetal) had electrocatalytic activities comparable to Pt or Pd. The objective of the present work was to develop ABs-catalyzed cathodes with high electrocatalytic activity and the stability required for practical industrial electrolysis. The problem was approached by starting with the AB5 compounds in finely-divided form. Three types of cathodes made with powdered AB5 catalysts are described in this paper. EXPERIMENTAL
discharge cycles were needed for particle size reduction. In a few instances, powders were made by mechanical crushing and subsequent grinding. The powder comminution method had no apparent effect on electrochemical behavior. All AB5 powders were sieved through a 325 mesh screen before cathode preparation. The sieved powders consisted of sharp-edged, smooth-faced, angular particles. The particles ranged from about 5 to 40/~m across, with most in the range 20--40 gin. Plastic-bonded, ABs-catalyzed cathodes were made using the two mixtures in Table 1. Each cathode mix was ground in a motorized mortar and pestle to fibrillate the PTFE. The mixture was then worked under pressures of up to 4.6 x 106 Pa at 120°C until a homogeneous sheet was obtained. Cathode blanks about 2 cm square were cut from the sheet. Nickel screen current distributors were embedded in the cathode blanks by pressing at 2.2 x 107 Pa at 120°C. The polyethylene oxide was then leached from the cathodes by soaking them in water for 24h. After leaching, the cathodes were pressed between two fine metal screen platens at pressures ranging from 7.6 × 105 to 1.5 x 106 Pa. The final pressing temperatures were 177°C for mix A and 120°C for the polyethylene-containing mix B.
Table 1. Plastic-bonded AB5 cathode mix formulations
The AB5 metal compounds were generally converted to powder form by alternately charging and discharging them with hydrogen gas. No more than 2--4 charge/ * Present address: American Cyanamid Company, Chemical Division Research Laboratories, Stamford, CT 06904, U.S.A. i- Present address: Duracell, Duracell Products Technology, Tarrytown, NY 10591, U.S.A. 1005
Component Metal hydride powder* PTFE Carbon black Polyethylene Polyethylene oxide * See Table 2.
Mix A (wt%)
Mix B (wt%)
86 2 2 0 10
86 1 3 2 8
1006
D.E. HALL AND V. R. SHEPARD, JR
Pressed, sintered cathodes were made with blended mixtures of AB5 powder (40-70 weight percent) and INCO* type 123 nickel powder ('Ni 123' below). Ni 123 powder particles are about 5/~m in diameter, have a spiky, high surface morphology, and sinter readily into a porous structure. The mixed powders were pressed at 3.4--3.9 × 109 Pa, and were then sintered in a hydrogen atmosphere at 900°C for 10 rain. ABs-catalyzed cathode coatings were made using essentially the same method reported earlier for porous, sintered Ni 123 anode coatings [4]. The coatings were applied as polysilicate paints containing Ni 123 and AB5 (30-70 w/o) powders. The cathode substrates were mild steel screens. Application of a Watts nickel flash ( - 2 . 5 / z m ) to the steel before coating resulted in more uniform paint retention on the screens. The coated cathodes were air dried and then sintered under hydrogen for 10 min at 800-900°C. Electrochemical measurements were made galvanostatically. All cathodes were operated at a current density of 200 m A cm -2 in 30 w/o K O H electrolyte at 80°C. Most cathodes were tested for 6 h; some, as reported below, were tested for longer periods, Periodically, a series of current density vs potential measurements was made from 600 to I m A c m -2, in descending order. For 6 h tests, the final data, recorded just before the conclusion of each test, are reported here. Raw current vs potential data were corrected for the effects of uncompensated resistance using a cornpurer method [5] which we have used with success in earlier studies [4]. Because of the very low overpotentials at high current densities in the present work, coefficients of determination for the fitted Tafel lines were lower (generally ~0.93) t h a n i n earlier work (generally >10.99). With some cathodes, computer iR corrections were compared with those obtained by the current interruption method. Good agreement was found between the two methods. The structures and surface morphologies of the cathodes were examined by optical and scanning electron microscopy (SEM), both before and after electrochemical testing. For SEM studies, small amounts of carbon were vapor deposited on the cathodes to prevent surface charging,
RESULTS Plastic-bonded cathodes Plastic-bonded cathodes made with the hydride-forming compounds LaNi5 and its substituted derivatives were highly efficient. In particulate form, the AB5 cornpounds exhibited significantly lower overpotentials than reported elsewhere [2, 6] for solid AB5 cathodes. All of the AB5 compounds produced comparable~/~ values of about 0.05 V at a current density of 200 m A c m -2. Thus, the hydride plateau pressure, over the five-fold * Trademark, The International Nickel Company.
Table 2. Plateau pressures for formation of fl-ABsHx for selected compounds at 80°C* AB5 LaNi3Co~ LaNi~TAl0.~ LaNi4Cu LaNi~
P,b~ 2.8 5.3 -5.6 14.9
Pd¢s 2.3 4.9 -4.6 11.1
* Pressures from P. D. Goodell, Inco Alloy Products (×10~Pa). range for these compounds (Table 2), had no observable effect on ~H2. The hydrogen evolution overpotentials of the plastic-bonded AB5 cathodes were generally insensitive to changes in process variables such as the fabrication pressures. Several cathodes were pre-conditioned before electrolysis by electrochemical charge/discharge cycling. These cathodes usually produced lower overpotentials than similar, unconditioned cathodes during the first two or three hours of electrolysis. This advanrage was temporary, as 0n2 at unconditioned cathodes declined during electrolysis. When plastic-bonded cathodes were tested for 100 h, the final overpotentials were essentially the same as those measured after 6 h, indicating no loss of catalytic activity. However, the AB5 catalysts displayed an unusual degree of change in catalyric activity as a function of operating temperature. The overpotentials of LaNi5 and LaNi, TAl03 cathodes increased to 0.16-0.18 V at 200 m A cm -2 when operated at 25°C. Scanning electron microscopy showed that the plastic-bonded cathodes were well-fibrillated, with the AB5 catalyst particles bound inside a web of fine polymer strands (Fig. 1). This structure remained intact after 6 h of electrolysis. However, the cathodes swelled considerably due to entrapment of gas and electrolyte. Mix B cathodes, with their higher residual polymer content, were more resistant to swelling than mix A cathodes. After 100 h of electrolysis, visible catalyst loss was noted frequently with mix A cathodes and occasionally with mix B cathodes.
Pressed sintered cathodes Pressed, sintered cathodes were made with LaNL.TA10ffNi123 mixtures containing 40-70 w/o of the AB5 catalyst powder. A t 70 w/o catalyst loading, there was sufficient nickel binder to form a strong sintered matrix around the catalyst particles. The AB5 catalysts do not sinter in the temperature range used to sinter nickel binder. Raising the temperature to sinter the catalysts as well as the nickel would have introduced the possibility of catalyst composition changes arising from Ni-AB5 interdiffusion, a subject which was not pursued in the present work. Optical and scanning electron microscopy showed that the pressed, sintered
ABs-CATALYZED HYDROGEN EVOLUTION CATHODES
1007
Fig. 1. Cross-section of plastic-bonded LaNhTAl03 cathode, mix A. Magnification 1000 ×.
cathodes contained well-dispersed LaNi4.TAl0.3particles, mostly 10--20/~m in diameter, The pressed, sintered cathodes, with their lower catalyst contents, produced overpotentials - 3 0 mV higher than plastic-bonded cathodes, as shown by the data in Table 3. Given the normal scatter in rpa2 data and the difficulties in correcting low overpotential, high current density data for resistance losses, Table 3 shows that none of the cathode compositions lost catalytic activity between 6 and up to 100 h of elect/olysis,
Because the AB5 catalysts are more expensive than the nickel binder and do not contribute to coating strength, it is desirable to use the lowest AB5 loading needed for minimum overpotential. The data in Table 3 demonstrated that r/H2 was independent of AB5 loading over the range 40-70 w/o. This subject is addressed again in the following section. Overpotentials measured within minutes after the beginning of electrolysis were as high as 0.2 V. It is not known whether the subsequent r/n2 decrease resulted from an increase in effective cathode surface area, or from a process such as removal of a surface oxide film Table 3. Hydrogen evolution overpotential of pressed, sintered or surface hydride formation. However, some ABs-cata" cathodes lyzed cathodes suspected to have higher than usual oxide contents showed an unusually slow overpotential Wt. Pct. LaNi~.TAl0.3 Time (h) H V (at 200 mA cm-2) * decrease. 40 50 60 70
6 47 95 6 27 95 6 48 72 6 48 51
0.07 0.09 0.07 0.06 0.08--0.09 0.06 0.07-0.08 0.07 0.07-0.09 0.07-0.08 0.07-0.08 0.07-0.09
• Resistance-corrected overpotentials in 30 w/o KOH electrolyte at 80°C. Coefficients of determination >0.93.
Sintered ABs/Ni 123 cathode coatings Sintered cathode coatings, applied using polysilicate-based paints containing Ni 123 powder and from 30 to 70 w/o LaNi4.TA10.3 powder, were sintered onto nickel-flashed, woven steel screens. The hydrogen evolution overpotentials of the coated cathodes were essentially the same as those of pressed, sintered cathodes of similar composition. Reducing the LaNi4.rAl0.3 content of the catalyzed coatings to as low as 30 w/o, however, revealed a sharp transition in the relationship between ~/H:and catalyst content. This behavior is shown clearly in Fig. 2 for two series of cathode coatings sintered at 800 and 900°C.
1008
D . E . HALL AND V. R. SHEPARD, JR 04
o
0.35 0.3
\~ ~q~ \ \
>- 0.25 "- o.z o.,s o o., -
a--.o aoo*c c--~ ~ o * c
_ _ ~ V '
"
....
0.05 I t I h { 50 55 6o 65 70 75 pm-cent.o~e AS5 Fig. 2. Hydrogen evolution overpotentials of cathodes with sintered Ni123/LaNi4.TA103 coatings. Coefficients of determination for computer calculation of/R-free overpotentials were ~>0.946. q
I ao
t 35
I I 40 45
The minimum catalyst loading required for maximum catalytic activity was ~40 w/o. Increasing the catalyst loading further produced no additional r/H2reduction, Figure 2 also shows that the sintering temperature had no effect on the catalytic activities of cathode coatings containing >~50w/o catalyst. In contrast, raising the sintering temperature and/or time produced substantial ,/r~2 increases on sintered cathode (or anode) coatings containing only nickel powder [4]. In the uncatalyzed coatings where nickel itself was the active
material, increasing the degree of sintering reduced the effective electrode surface area, resulting in a higher effective current density and thus a higher overpotential. In the ABs-catalyzed cathode coatings, the sintered nickel matrix contributes very little to the cathode efficiency. The morphology of the AB5 catalyst, shown in Fig. 3, is unaffected by the heat treatment required to sinter the nickel into a matrix around it. The figure also shows that the LaNi4.TA10.3particles do not appear to be metallurgically bonded to the surrounding nickel, but are instead physically confined in the porous nickel matrix. Two cathodes with sintered coatings containing 50w/o LaNi4.TAl0.3were operated for more than 3000h at 200 m A cm -2. After an initial break-in period, both cathodes assumed stable hydrogen evolution overpotentials of 0.09-0.10 V. (Coefficients of determination for t h e / R - f r e e overpotentials over the last 1000 h of operation were all >0.95, and were frequently >0.99.) A t the conclusion of the 3000h tests, both cathodes were examined by scanning electron microscopy. There was no loss of overall coating integrity. The coating morphology was unchanged except for some cracking of the LaNL.TA10.3particles. The cracked material was retained in the coating. In addition to stability under cathodic polarization, the catalyst showed excellent open-circuit corrosion resistance. During one long-term electrolysis experiment, in 30 w/o K O H electrolyte at 80°C, current was switched off for over 90 h. When electrolysis was resumed, the cathode potential regained its former
Fig. 3. Surface of 50 w/o LaNh7Al03-coated cathode sintered at 900°C. Angular catalyst particles are surrounded by a matrix of sintered nickel.
ABs-CATALYZED HYDROGEN EVOLUTION CATHODES value after a break-in period, and continued to operate in a stable manner. There was no apparent physical damage to the cathode.
DISCUSSION The AB5 compounds examined in this study are highly efficient hydrogen evolution electrocatalysts. Under cathodic polarization, these compounds can also take up hydrogen to form bulk metal hydrides. However, the ability of an intermetallic compound to store hydrogen is not a sufficient condition for efficient hydrogen evolution. This was demonstrated by preparing plastic-bonded cathodes, containing AB and A2B intermetallic compounds known to be good hydrogen storage materials, and testing them at 80°C in 30 w/o KOH. The AB compounds (Fe0.9Mn0.~)Ti and (Fe0.sNi0.2)Ti produced overpotentials of 0.16-0.22V at a current density of 200 mA cm -2, while the A 2 B compound MgzNi was still less efficient (r/H2 - 0.36 V). Thus, the AB5 compounds appear to be unique among the three classes in providing excellent hydrogen evolution catalysis at high electrolyte temperatures characteristic of commerical electrolytic hydrogen production, and bulk hydrogen storage at low electrolyte temperatures characteristic of battery operation, In the present study, three types of cathode structures were investigated. The plastic-bonded cathodes are a type well-known in battery research. Those used in this study have the advantage of avery high catalyst loading, so that the ultimate efficiency of the catalyst is approached. However, plastic-bonded cathodes with low polymer content are not optimized for rapid, sustained gas evolution. As a result, they swell and shed catalyst during electrolysis. It is doubtful that these cathodes, in the form evaluated in this work, would be suitable in any application in which they were unconstrained. The plastic-bonded sheet cathode is thicker than necessary for high efficiency, and thus, catalyst loading per apparent surface area is high. Catalyst use could be reduced by blending nickel powder into the mix, but then the unique advantage of this cathode, relative to the other two types examined, is lost. These disadvantages might be eliminated if the plastic-bonded sheet
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were made very thin (e.g. 100/~m) and pressure-laminated as a coating onto a robust cathode support. The pressed, sintered cathodes, while much less porous, are similar to sintered nickel battery plaques. At a catalyst loading of 40%, they still use a considerable amount of catalyst per unit area if they are made thick enough to be self-supporting. Despite r/H2 values about 30 mV higher than with plastic-bonded cathodes, the ABs/Ni 123 sintered type is still highly efficient. This difference in performance might not be observed in large commercial cells. The electrical resistivity of the AB5 compounds is much higher than that of nickel. For example, the resistivity of LaNi5 is 7.2-7.6 x 10-4 f~cm, depending on its state of charge [7], while the resistivity of nickel is only - 7 x 10 -6 ~-cm. The internal resistance of either type of sintered cathode should thus be considerably lower than the plastic-bonded type, from the standpoint of materials as well as the continuity of the sintered nickel structure. The catalytic sintered coating applied to a suitable substrate is, in many respects, the most practical of the three cathodes studied. As efficient as the pressed, sintered sheet, it uses much less catalyst per unit area than either of the other cathodes when applied thinly. The coating is also readily applied to a variety of metallic substrates, including those presently used in the electrolytic industry. Acknowledgements--The authors thank W. D. K. Clark, J.
T. Arms, G. D. Sandrock, F. J. Mulligan and especially P. D. Goodell for valuable discussions during the course of this work. Thanks are also due to A. M. Rapun, C. Knipple and G. Hawks for experimental assistance. REFERENCES 1. H. Zijlstra et al., Solid State Commun. 7, 857 (1969). 2. M. H. Miles, Electroanalyt. Chem. Interracial Electrochem. 60, 89 (1975). 3. T. Kitamura, C. Iwakura and H. Tamura, Chem. Lett. 1981, 965. 4. D. E. Hall, J. Electrochem. Soc. 128, 740 (1981). 5. R. L. LeRoy, M. B. I. Janjua, R. Renaud and U. Leuenberger, J. Electrochem. Soc. 126, 1674 (1979). 6. T. Kitamura, C. Iwakura and H. Tamura, Chem. Lett. 1981, 1755. 7. G. Adachi, K. Niki and J. Shiokawa, J. Less Common Metals 81,345 (1981).