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Preparation and use of Raney-Ni activated cathodes for large scale hydrogen production
PREPARATION CATHODES
AND USE OF RANEY-Ni ACTIVATED FOR LARGE SCALE HYDROGEN PRODUCTION K. LOHRBERG and
P. KOHL
Lurgi GmbH, Gervinusstr. 17-19, 6000 Frankfurt, F.R.G. {Received
IS May 1984)
Abstract-Raney-Ni catalysts, which are operated in technical electrolysis plants for cathodic hydrogen evolutionand which are appliedto the electrode structure by different means, were tested with respect lo their
catalytic activity under industrial conditions in an undivided cell. It was shown that in contrast to other catalysts an additional overvoltage occurred with rolled Raney-nickel due to smaller bubble radii (some pm). Concerning the design of technical electrodes one can conclude that louvered electrodes exhibiting heights of the single stripe up to 15 cm are equivalent to the normally used expanded mesh-or perforated electrodes with respect to their performance and cell resistance. With respect to construction and fabrication engineering they exhibit distinct advantages, which lead to sign&ant cost reductions for electrolysisplants.
(2) Coating
I. INTRODUCTION
Raney-Ni is one of the most active electrocatalysts for the hydrogen electrode; it is used anodically in fuel. cells as well as cathodically for electrolytic hydrogen production. Up to now, most of the electrochemitally produced hydrogen is delivered by chloralkaliplants; therefore, this paper refers to experiences with cathodes in industrial membrane cells. Of course, the results of activity measurements as well as the principles of industrial electrodedesign may be transferred directly to water electrolysers. The activity of RaneyNi is believed to be related to its hexagonal structure, which is unstable and tends to form the normal cubic lattice of bulk nickel[l]. This process limits the use of Raney-Ni to temperatures lower than 10@12o”C. It is the aim of this paper to compare Raney-Ni-activations from different origins under identical test conditions,
II.
GENERAL
PROPERTIES CATALYSTS
OF
thickness
Several investigators report that up to 100 pm the activity increases strongly with increasing thickness of the coating, a further advantage is observed up to 150 pm thickness, then the activity remains constant. This is explained by the maximum penetration depth of the current into the coating. (3) Amount
of unleached
Al
Besides the formation of NiAl, which cannot be leached with caustic, aluminium is present in the crystal lattice of the Raney-Ni and is hampering its recrvstallization. Small: additional amounts of Co, Ti, MO: V or W are acting in the same way. According to Justi up to 15% of the aluminium remain in the catalyst after leaching for 40 days at room temperature. A subsequent rise of the temperature had no significant effect. (4) Temperature
RANEY-Ni
According to the work of Justi et al.[23 the activity of the electrode is mainly determined by: (1) the of Raney-Ni within the coating, %oncentration” (2) the coating thickness, (3) the amount of u&ached aluminium and (4) the temperature during the coating application. (1) Concentrntion ofRaney-Ni
If the coating is heated above 65O”C, the b-phase (NiAIB) forms Ni2A13 -t Al; the aluminium attacks the matrix nickel forming NiAI. This reaction can occur either if a Ni/AI alloy is plasma- or flamesprayed, or, as discussed by Justi and confirmed by measurements with rolled coatings, if the annealing temperature exceeds 700°C. If a significant amount of unleachable NiAl was formed during the coating application, the electrode will exhibit a potential increase of m lO& 150mV at 3 kAmm2.
The activity of the catalyst, expressed as apparent current density at constant overvoltage, increases linearly from a pure Ni-matrix to the pure Raney-Ni as shown by Justi. On the other hand, the pure catalyst is mechanically unstable. The optimum results are achieved at the ratio Ni/Al-alloy to matrix-M = I : 1. 1557
III. PREPARATION (1) Electrodeposition
OF THE ELECTRODES of Ni/Zn-alloys
A Ni/Zn-alloy is precipitated from an acidic plating bath, the composition of the alloy can becontrolled by
1558
K. LOHRBERGAND
the current density and by the electrolyte composition as well. Our test was prepared according to a pro-
cedure patented by KfA Jiilich[3]. The structure of the support was roughened Ni, the roughness depends on the coating thickness which can only be varied up to a maximum value. Thicker coatings exhibit poor adherence and are not stable under electrolysis conditions. KfA Jiilich has published lifetime test results which show stable operation at 4 kAm_” over 10,000 h. Other methods to stabilize the powdery catalyst are: plasma spraying of a nickel-sublayer to improve the roughness of the electrode structure, or the addition of metallic nickel to the plating bath, which is precipitated together with the Ni/Zn-alloy, or the variation of the coating composition, starting with pure Ni, then afterwards increasing the Zn-content of the alloy up to the desired amount. (2) P losma spraying
of Ni/Al-alloys
A mixture of dendritic Mond-Ni and Al is sprayed onto the sandblasted electrode structure. Due to the high temperatures of the plasma jet the aluminium will be dissolved during the activation of the coating with caustic, leaving a porous structure of matrix-nickel, which is coated with Raney-Ni. This electrode was delivered by Heraeus Elektroden for our test. The activity can be influenced by the grain size, the addition of leachable inert salts and the conditions of the plasma jet[4]. These electrodes are tested in diaphragm cells, they are reported to withstand the baking temperatures of the diaphragm furnace without significant losses of their activity. (3) Rolling
of Ni/Al-alloys
This application method was developed by Metallgesellschaft and is based on the research work of Justi and co-workers[5]. Ni/Al-alloy and Mond-Ni are thoroughly mixed and suspended in an alcoholic solution of methylcellulose. The suspension is applied to the sandblasted nickelplate and dried. The plates are then rolled, whereby the thickness of the nickel plate is considerably decreased (up to 50%) to ensure the desired adherence of the coating. The rolling program (pressure, speed, number of cycles} depends on the electrode material. The rolled electrode plates are annealed at temperatures c 700°C in reducing atmosphere. After this heat treatment the adherence of the coating is good enough, so that the plates may be flattened and cut without any deterioration of the coating. The tests were carried out with samples, taken from the electrode production for quality tests. IV. CURRENT-VOLTAGE
P.KwL
Figure I shows the Tafel plots of the three differently catalyzed nickel electrodes. In addition, the results for a mechanically roughened Ni-electrode are shown. At 3 kAm_’ the voltage savings are 25&350 mV. The results from the rolled coating are based on a large number of coating batches, whereas the curves for the elcctrodeposited and the plasma sprayed coating represent only one singular sample under investigation. These current-voltage curves are uncertain by some 10 mV. Besides the different activities for different coatings significant differences cannot be detected. But if we try to apply a theoretical model of the hydrogen evolution kinetics, the rolled coating exhibits some peculiarities: at current densities below 1 kA me2 the currentvoltage curve becomes linear, but the straight lines do not pass through the origin. Figure 2 shows this effect at various temperatures. If the apparent overvoltage is corrected by this constant contribution to the total polarization, the whole current-voltage curve may be described by a pure charge-transfer controlled process: the exchange current densities derived from the charge-transfer resistances and those derived from the Tafel plots are identical for all samples under investigation. This means that the coverage degree of adsorbed hydrogen is not significantly changed over the whole current density range. V. INTERPRETATION VOLTAGE
OF CURRENTCURVES
Justi supposed that the overvoltage at i = 0 kA me2 is caused by increased Hr activity due to the small diameter of the gas bubbles formed inside the pores of the coating[2]. For this model the average size of the lg III 1!
I kA/m’) L32
1
05
o-
CURVES
Out of the various preparation methods for RaneyNi coatings samples of 40mm diameter were tested under standard test conditions, ie by taking stationary current-voltage curve in 30%-NaOH at 80°C us a Hg/HgO electrode. Under these conditions all cathodes exhibit a rest potential of -930 mV. In addition, the temperature behaviour of the rolled coating was investigated.
-17 0
-100
-200
-300
-l&O
-SOD
q(d)
Fig. 1. Tafel plot for the cathodic hydrogen evolution at different Ni-electrodes: (1) non-activated roughened Ni, (2) Raney-Ni, plasma sprayed, (3) Raney-Ni, &ctrodeposited, (4) Raney-Ni, rolled. 30% caustic, T = 80°C.
Raney-Ni activated cathodes 1 kAim21
I
1.
1559
rlpm)
1.00
0.75
> 0.5 -I
25. o,
/ I 4
LO
20
Ofs Fig. 2. Current-voltage relationship for hydrogen evolution at rolled Raney-Ni cathodes in 30% caustic: o, 23°C. A, 41°C; n , 50°C; l, 64°C; A, 70°C; o , 77°C; V, 82°C.
G=aO=z
0
r
80
100 TIOCI
Fig. 3. Temperature dependence for the radius of hydrogen bubbles evolved at rolled Raney-Ni.
bubbles can be estimated from the equation 3v
60
b [mVI 220
=
2Fv (i= 0).
(1)
For precise evaluation of Equation (1) the vapour content of the evolved hydrogen has to be considered. Figure 3 shows the temperature dependence of the bubble radius r calculated from 4 (i = 0) according to Equation (1). The surface tension of the caustic was estimated from the simplified McL.eod equation using the well-known data for the specific gravity of caustic[6]: 0 = 61.4+ 10.79~~
(2)
where 0 and p are expressed in cgs-units. Presuming a catalyst model with cyclindrical pores of radius r Justi et nl. found that the radius of a single pore within the coating layer (not within the Raney-Ni) is - 2 pm in agreement with the results given above for 40°C. At 3 kA m-’ we can calculate for this model the absolute current per pore to be 2.3 @. The initial current is decreased by the factor of e = 2.73 after 24 pm penetration depth of the pore. We can estimate the maximum penetration of the current to be five times this figure, about 120 q. On the other hand, we can estimate the total number of actually working pores to be 1.3 x lo5 per cm*, which corresponds to an actually working surface ofabout 1.5 o/0of the nominal electrode area. These figures are nearly the same as measured by Justi et al. The difference is caused by a closer screening of the catalyst grains, whereas we are using all grains below a maximum size. These results do not allow us to determine the pore distribution across the area.
i,
200
180
160
1LO
120
100 2b
rio
io
io T (“Cl
Fig. 4. Temperature dependence of the Tafei factor for the hydrogen evolution reaction for rolled Raney-Ni-cathodes.
The Tafel factor b exhibits a temperature dependence (Fig. 4) different from theory: it should increase linearly with increasing temperature, if the reaction
K. LOHRBERG AND P. KOHL
I.560
mechanism remains constant, whereas it is lowered from 215 mV at 23°C to 133 mV at temperatures higher than 75°C. At higher temperatures the Tafel factor becomes constant and corresponds to a chargetransfer coefficient ot = 0.46 measured, as well, for nonactivated Ni-electrodes. The temperature dependence of both the average bubble radius as well as the Tafel factor seem to be related to the structure of the coating: blending the coating material with Na2C03 as filler yields in an electrocatalyst, which exhibits no overvoltage at i = 0 and constant charge-transfer coefficients close to 0.5. If the interpretation of the residual overvoltage at i = 0 is correct, ie the hydrogen evolution is a purely charge transfer controlled process, we can calculate from the temperature dependence of the exchange cut-rent density the activation energy to be E = 20.4 kJ. The frequency factor is k” = 0.15cm s- ‘. This figure if compared with the value k” - 1O’cm s- ’ expected for adiabatic charge transfer processes[7] yields an effective roughness factor off - 10m5. This may be compared with direct measurements made by Schmidt[S], who obtained a surface area of - IOOm g-’ Raney-nickel, which meansf> 105, by determination of the absorption capacity. Obviously, only an extremely small fraction of the total Raney-Ni-surface is needed to take the whole current. It is assumed that the expected rise of the concentration polarization inside the catalyst grains is mostly compensated by an increase of the number of active sites, when the current is increased. If we try to apply this model to the electrodeposited Raney-Ni, we get strange results: the concentration polarization is nearly the same, but the exchange current density is in the range of 0.32 kA m2. To fit the experiments to a purely charge transfer controlled process the charge transfer coefficient must be chosen smaller than 0.2. We must conclude, therefore, that in this case the subsequent reaction-either the Tafel or the Heyrovsky reaction-are influencing the currentvoltage curve. That means the degree of surface coverage 8, becomes potential dependent, most likely because of a lower number of active sites compared with the rolled coating. The plasma sprayed coating acts as an extremely roughened Ni-electrode, which may also be interpreted by a lower number of active sites. Table I lists the kinetic data of the different Raney-Ni coatings.
VI. LARGE
SCALE
ELECTRODES
The question which arises is in which way these activated electrodes may be scaled up to industrial sizes. Due to the high deformation of the structure material during the rolling process only plate type electrodes can be produced, whereas electrodeposition and plasma spraying can be applied to any electrode structure. The following comparison shows that two electrode typeeplates and perforated electrodesare advantageous with respect to several aspects of electrode design and fabrication.
Table 1. Kinetic parameters of the hydrogen evolution reaction at differently applied Raney-nickel coatings Coating application
i0 (kAm_‘)
b (mv)
0.202 0.134 1.267 0.001
86 106 133 121
Electrodeposited Plasma sprayed Rolled Non-activated T = SOT, 30 wt. “/, NaOH
(1) Material problems and cell design We are presuming a corrosion resistant structure material since no coated iron or copper structures have proven an adequate lifetime in membrane cells. It is quite clear that the electrode gap should be as small as possible. For a plate electrode, however, there is a limitation: obviously, the cell cannot work if the plate is placed directly at the separator (membrane or diaphragm). Perforated electrodes, however, can be placed at zero distance; in this case the area of the holes and of the backside of the structure are working. To get advantage of this electrode configuration, the structure should be thin compared to the conventional electrode gap of max. 3 mm. In this case one has to install a current distribution system and the number of contacts can easily be calculated from the size and the specific resistivity of the electrode structure. This is explained by the following example: a model cell which is 0.6 m wide and 1.2 m high shall be operated at 3.5 kA m- ‘. One cathode frame operating at both sides will carry 5 kA, which have to be distributed evenly over the electrode area. A voltage drop of SOmV across the support, which due to the ten-fold higher voltage drop across the separator, does not cause a serious change of the current distribution, seems to be tolerable. For this current density and allowed Ohmic potential drop a nickel plate of 1 mm thickness can transport the current over a length of 46.5 cm, the minimum thickness of the plate in our model cell should be 1.3 mm. A perforated plate of 70% void area should have a minimum thickness of 1.9 mm, ignoring the somewhat longer way to pass the for caustic whole cell width. The same calculation resistant stainless steels, because of its about ten-fold lower conductivity, yields in a minimum thickness of 19 and 28 mm respectively. Thus steel electrodes can only be used in bi-polar arrangements without current distribution in the electrode plane. In monopolar cells, a plate electrode without current collectors is by far less expensive. This is mainly caused by the reduction of the necessary welding work, but also by the possible reduction of the minimum frame thickness from about 10 cm for frames with current collectors to 3 cm for plate type cells. (2) &Txi of bubble accumulation The following calculations describe the behaviour of a plate electrode for the a.m. cell under standard conditions 180°C. 30 % NaOHI. The current densitv at the lower kdge ‘of ice elect&de is assumed to’ be 2.5 kAm_‘, the electrode gap is 3 mm. Due to the accumulation of gas bubbles the specific conductivity
Raney-Ni activated cathodes
of the electrolyte will decrease Bruggemann equation: K =
K,,(l
-
according
to the
,
1561
a(kA/m2)
la)
2.5
v /Y)“‘. g
(3)
degosiflcal~on
efhuency
ierr = 2.20
:
kAl
50%
m7
resulting current distribution is shown in the upper curve of Fig. 5. According to Tobias[9] we can describe the curve by: The
i,Ji, = &
Wx + 2)’ 0.5
where i. denotes the average current density, ix the current density at the reduced height x = h/H and k is a constant factor for a given cell geometry and gas velocity. The average current density is 2.3 kAm_‘. IF one considers the dependence of the bubble velocity on the gas concentration according to Kozo Koido et al.[lO] u = uJO.27 + 0.73. (1 - V,/ Y)=]
II
0
0.5
--
reduced
Ii
I (kA/m*l
Fig. 6. Current density distribution across a louvered cathode same conditions as Fig. 5, but 10% void area. (a) Degasification efficiency 50 %. (b) degasification eficiency 100%.
means 50% of the gas reaching the upper edge of a stripe are released to the rear) the maximum difference of the current densities is less than 5%. The lower average current density of 2.2 kAm_’ is due to the definition of the technical current density, which is related to the cross-section of the cell. For degasification efficiencies near 100 % the current density remains constant even within 0.5 %. Considering the decrease of the current efficiency of membrane cells at higher caustic concentrations, we would expect a slight increase of the current efficiency compared to the zero gap cell configuration due to the remarkably decreased local caustic concentration at the membrane surface. Further tests in industrial cells shall prove the validity of the electrode performance. Up to now only uncoated louvered cathodes areinstalled in membrane cells which are being successfully operated.
REFERENCES
1.3kAlmZ
b
0
*
1.0 hsght hi
(5)
one gets the lower curve with an effective current density of only 1.3 kAmm2. Considering the fact that the Bruggemann equation becomes invalid at gas void fractions higher than 40 %, the curve is expected to be even steeper and no even current density distribution seems to be possible, unless an electrolyte motion due to possible gas lift effects is superimposed. In fact, chlorate cells of the same height and the same electrode gap are operating without any dif%ulties. To improve the situation one can divide the plate; for instance in nine horizontal stripes, each of which is 12cm high and separated by gaps which are 1.5cm wide. Figure 6 shows the effect on the current density distribution: at 50% degasification efficiency (that
‘_(
reduced
height
1 hl
H
Fig. 5. Current density distribution across a vertical eiectrade according to the Bruggcmann equation (a) and considering gasconcentration dependence of the gas velocity (b). Presumed height of the electrode H = 120 cm, initial cd. at the lower edge 2.5 kA m-‘.
1. Y. S. Terminasow and M. S. Beletzki, Dokl. Akad. Nauk SSSR 63,411 (1948). 2. E. Justi, M. Pillcuhn, W. Scheibe and A. Winsel, Ahi. lMssenschaftten Lirerarur Abh. d. mrh.-norurw. Klasse 8 (1959). 3. German patent application P 29 14 094.1. 4. German patent application P 32 18 429.8. 5. DOS 28 29 901. 6. G. M. Barrow, Physikaliscfw Chemie, 2nd edn, Part III, p. 58. Bohmann, Heidelberg (1972). 7. R. A. Marcus., A. Rev. phys. Chm. 15, 155 (1964). 8. A. Schmidt, Thesis, TU Braunschweig (1973). 9. C. W. Tobias, J. electrochem. Sot. 106, 833 (1959). 10. Kozo K&de, T. Hirahara and H. Kubota, C/tern. Engng, Tokyo, 5. 38 (1967).
AND USE OF RANEY-Ni ACTIVATED FOR LARGE SCALE HYDROGEN PRODUCTION K. LOHRBERG and
P. KOHL
Lurgi GmbH, Gervinusstr. 17-19, 6000 Frankfurt, F.R.G. {Received
IS May 1984)
Abstract-Raney-Ni catalysts, which are operated in technical electrolysis plants for cathodic hydrogen evolutionand which are appliedto the electrode structure by different means, were tested with respect lo their
catalytic activity under industrial conditions in an undivided cell. It was shown that in contrast to other catalysts an additional overvoltage occurred with rolled Raney-nickel due to smaller bubble radii (some pm). Concerning the design of technical electrodes one can conclude that louvered electrodes exhibiting heights of the single stripe up to 15 cm are equivalent to the normally used expanded mesh-or perforated electrodes with respect to their performance and cell resistance. With respect to construction and fabrication engineering they exhibit distinct advantages, which lead to sign&ant cost reductions for electrolysisplants.
(2) Coating
I. INTRODUCTION
Raney-Ni is one of the most active electrocatalysts for the hydrogen electrode; it is used anodically in fuel. cells as well as cathodically for electrolytic hydrogen production. Up to now, most of the electrochemitally produced hydrogen is delivered by chloralkaliplants; therefore, this paper refers to experiences with cathodes in industrial membrane cells. Of course, the results of activity measurements as well as the principles of industrial electrodedesign may be transferred directly to water electrolysers. The activity of RaneyNi is believed to be related to its hexagonal structure, which is unstable and tends to form the normal cubic lattice of bulk nickel[l]. This process limits the use of Raney-Ni to temperatures lower than 10@12o”C. It is the aim of this paper to compare Raney-Ni-activations from different origins under identical test conditions,
II.
GENERAL
PROPERTIES CATALYSTS
OF
thickness
Several investigators report that up to 100 pm the activity increases strongly with increasing thickness of the coating, a further advantage is observed up to 150 pm thickness, then the activity remains constant. This is explained by the maximum penetration depth of the current into the coating. (3) Amount
of unleached
Al
Besides the formation of NiAl, which cannot be leached with caustic, aluminium is present in the crystal lattice of the Raney-Ni and is hampering its recrvstallization. Small: additional amounts of Co, Ti, MO: V or W are acting in the same way. According to Justi up to 15% of the aluminium remain in the catalyst after leaching for 40 days at room temperature. A subsequent rise of the temperature had no significant effect. (4) Temperature
RANEY-Ni
According to the work of Justi et al.[23 the activity of the electrode is mainly determined by: (1) the of Raney-Ni within the coating, %oncentration” (2) the coating thickness, (3) the amount of u&ached aluminium and (4) the temperature during the coating application. (1) Concentrntion ofRaney-Ni
If the coating is heated above 65O”C, the b-phase (NiAIB) forms Ni2A13 -t Al; the aluminium attacks the matrix nickel forming NiAI. This reaction can occur either if a Ni/AI alloy is plasma- or flamesprayed, or, as discussed by Justi and confirmed by measurements with rolled coatings, if the annealing temperature exceeds 700°C. If a significant amount of unleachable NiAl was formed during the coating application, the electrode will exhibit a potential increase of m lO& 150mV at 3 kAmm2.
The activity of the catalyst, expressed as apparent current density at constant overvoltage, increases linearly from a pure Ni-matrix to the pure Raney-Ni as shown by Justi. On the other hand, the pure catalyst is mechanically unstable. The optimum results are achieved at the ratio Ni/Al-alloy to matrix-M = I : 1. 1557
III. PREPARATION (1) Electrodeposition
OF THE ELECTRODES of Ni/Zn-alloys
A Ni/Zn-alloy is precipitated from an acidic plating bath, the composition of the alloy can becontrolled by
1558
K. LOHRBERGAND
the current density and by the electrolyte composition as well. Our test was prepared according to a pro-
cedure patented by KfA Jiilich[3]. The structure of the support was roughened Ni, the roughness depends on the coating thickness which can only be varied up to a maximum value. Thicker coatings exhibit poor adherence and are not stable under electrolysis conditions. KfA Jiilich has published lifetime test results which show stable operation at 4 kAm_” over 10,000 h. Other methods to stabilize the powdery catalyst are: plasma spraying of a nickel-sublayer to improve the roughness of the electrode structure, or the addition of metallic nickel to the plating bath, which is precipitated together with the Ni/Zn-alloy, or the variation of the coating composition, starting with pure Ni, then afterwards increasing the Zn-content of the alloy up to the desired amount. (2) P losma spraying
of Ni/Al-alloys
A mixture of dendritic Mond-Ni and Al is sprayed onto the sandblasted electrode structure. Due to the high temperatures of the plasma jet the aluminium will be dissolved during the activation of the coating with caustic, leaving a porous structure of matrix-nickel, which is coated with Raney-Ni. This electrode was delivered by Heraeus Elektroden for our test. The activity can be influenced by the grain size, the addition of leachable inert salts and the conditions of the plasma jet[4]. These electrodes are tested in diaphragm cells, they are reported to withstand the baking temperatures of the diaphragm furnace without significant losses of their activity. (3) Rolling
of Ni/Al-alloys
This application method was developed by Metallgesellschaft and is based on the research work of Justi and co-workers[5]. Ni/Al-alloy and Mond-Ni are thoroughly mixed and suspended in an alcoholic solution of methylcellulose. The suspension is applied to the sandblasted nickelplate and dried. The plates are then rolled, whereby the thickness of the nickel plate is considerably decreased (up to 50%) to ensure the desired adherence of the coating. The rolling program (pressure, speed, number of cycles} depends on the electrode material. The rolled electrode plates are annealed at temperatures c 700°C in reducing atmosphere. After this heat treatment the adherence of the coating is good enough, so that the plates may be flattened and cut without any deterioration of the coating. The tests were carried out with samples, taken from the electrode production for quality tests. IV. CURRENT-VOLTAGE
P.KwL
Figure I shows the Tafel plots of the three differently catalyzed nickel electrodes. In addition, the results for a mechanically roughened Ni-electrode are shown. At 3 kAm_’ the voltage savings are 25&350 mV. The results from the rolled coating are based on a large number of coating batches, whereas the curves for the elcctrodeposited and the plasma sprayed coating represent only one singular sample under investigation. These current-voltage curves are uncertain by some 10 mV. Besides the different activities for different coatings significant differences cannot be detected. But if we try to apply a theoretical model of the hydrogen evolution kinetics, the rolled coating exhibits some peculiarities: at current densities below 1 kA me2 the currentvoltage curve becomes linear, but the straight lines do not pass through the origin. Figure 2 shows this effect at various temperatures. If the apparent overvoltage is corrected by this constant contribution to the total polarization, the whole current-voltage curve may be described by a pure charge-transfer controlled process: the exchange current densities derived from the charge-transfer resistances and those derived from the Tafel plots are identical for all samples under investigation. This means that the coverage degree of adsorbed hydrogen is not significantly changed over the whole current density range. V. INTERPRETATION VOLTAGE
OF CURRENTCURVES
Justi supposed that the overvoltage at i = 0 kA me2 is caused by increased Hr activity due to the small diameter of the gas bubbles formed inside the pores of the coating[2]. For this model the average size of the lg III 1!
I kA/m’) L32
1
05
o-
CURVES
Out of the various preparation methods for RaneyNi coatings samples of 40mm diameter were tested under standard test conditions, ie by taking stationary current-voltage curve in 30%-NaOH at 80°C us a Hg/HgO electrode. Under these conditions all cathodes exhibit a rest potential of -930 mV. In addition, the temperature behaviour of the rolled coating was investigated.
-17 0
-100
-200
-300
-l&O
-SOD
q(d)
Fig. 1. Tafel plot for the cathodic hydrogen evolution at different Ni-electrodes: (1) non-activated roughened Ni, (2) Raney-Ni, plasma sprayed, (3) Raney-Ni, &ctrodeposited, (4) Raney-Ni, rolled. 30% caustic, T = 80°C.
Raney-Ni activated cathodes 1 kAim21
I
1.
1559
rlpm)
1.00
0.75
> 0.5 -I
25. o,
/ I 4
LO
20
Ofs Fig. 2. Current-voltage relationship for hydrogen evolution at rolled Raney-Ni cathodes in 30% caustic: o, 23°C. A, 41°C; n , 50°C; l, 64°C; A, 70°C; o , 77°C; V, 82°C.
G=aO=z
0
r
80
100 TIOCI
Fig. 3. Temperature dependence for the radius of hydrogen bubbles evolved at rolled Raney-Ni.
bubbles can be estimated from the equation 3v
60
b [mVI 220
=
2Fv (i= 0).
(1)
For precise evaluation of Equation (1) the vapour content of the evolved hydrogen has to be considered. Figure 3 shows the temperature dependence of the bubble radius r calculated from 4 (i = 0) according to Equation (1). The surface tension of the caustic was estimated from the simplified McL.eod equation using the well-known data for the specific gravity of caustic[6]: 0 = 61.4+ 10.79~~
(2)
where 0 and p are expressed in cgs-units. Presuming a catalyst model with cyclindrical pores of radius r Justi et nl. found that the radius of a single pore within the coating layer (not within the Raney-Ni) is - 2 pm in agreement with the results given above for 40°C. At 3 kA m-’ we can calculate for this model the absolute current per pore to be 2.3 @. The initial current is decreased by the factor of e = 2.73 after 24 pm penetration depth of the pore. We can estimate the maximum penetration of the current to be five times this figure, about 120 q. On the other hand, we can estimate the total number of actually working pores to be 1.3 x lo5 per cm*, which corresponds to an actually working surface ofabout 1.5 o/0of the nominal electrode area. These figures are nearly the same as measured by Justi et al. The difference is caused by a closer screening of the catalyst grains, whereas we are using all grains below a maximum size. These results do not allow us to determine the pore distribution across the area.
i,
200
180
160
1LO
120
100 2b
rio
io
io T (“Cl
Fig. 4. Temperature dependence of the Tafei factor for the hydrogen evolution reaction for rolled Raney-Ni-cathodes.
The Tafel factor b exhibits a temperature dependence (Fig. 4) different from theory: it should increase linearly with increasing temperature, if the reaction
K. LOHRBERG AND P. KOHL
I.560
mechanism remains constant, whereas it is lowered from 215 mV at 23°C to 133 mV at temperatures higher than 75°C. At higher temperatures the Tafel factor becomes constant and corresponds to a chargetransfer coefficient ot = 0.46 measured, as well, for nonactivated Ni-electrodes. The temperature dependence of both the average bubble radius as well as the Tafel factor seem to be related to the structure of the coating: blending the coating material with Na2C03 as filler yields in an electrocatalyst, which exhibits no overvoltage at i = 0 and constant charge-transfer coefficients close to 0.5. If the interpretation of the residual overvoltage at i = 0 is correct, ie the hydrogen evolution is a purely charge transfer controlled process, we can calculate from the temperature dependence of the exchange cut-rent density the activation energy to be E = 20.4 kJ. The frequency factor is k” = 0.15cm s- ‘. This figure if compared with the value k” - 1O’cm s- ’ expected for adiabatic charge transfer processes[7] yields an effective roughness factor off - 10m5. This may be compared with direct measurements made by Schmidt[S], who obtained a surface area of - IOOm g-’ Raney-nickel, which meansf> 105, by determination of the absorption capacity. Obviously, only an extremely small fraction of the total Raney-Ni-surface is needed to take the whole current. It is assumed that the expected rise of the concentration polarization inside the catalyst grains is mostly compensated by an increase of the number of active sites, when the current is increased. If we try to apply this model to the electrodeposited Raney-Ni, we get strange results: the concentration polarization is nearly the same, but the exchange current density is in the range of 0.32 kA m2. To fit the experiments to a purely charge transfer controlled process the charge transfer coefficient must be chosen smaller than 0.2. We must conclude, therefore, that in this case the subsequent reaction-either the Tafel or the Heyrovsky reaction-are influencing the currentvoltage curve. That means the degree of surface coverage 8, becomes potential dependent, most likely because of a lower number of active sites compared with the rolled coating. The plasma sprayed coating acts as an extremely roughened Ni-electrode, which may also be interpreted by a lower number of active sites. Table I lists the kinetic data of the different Raney-Ni coatings.
VI. LARGE
SCALE
ELECTRODES
The question which arises is in which way these activated electrodes may be scaled up to industrial sizes. Due to the high deformation of the structure material during the rolling process only plate type electrodes can be produced, whereas electrodeposition and plasma spraying can be applied to any electrode structure. The following comparison shows that two electrode typeeplates and perforated electrodesare advantageous with respect to several aspects of electrode design and fabrication.
Table 1. Kinetic parameters of the hydrogen evolution reaction at differently applied Raney-nickel coatings Coating application
i0 (kAm_‘)
b (mv)
0.202 0.134 1.267 0.001
86 106 133 121
Electrodeposited Plasma sprayed Rolled Non-activated T = SOT, 30 wt. “/, NaOH
(1) Material problems and cell design We are presuming a corrosion resistant structure material since no coated iron or copper structures have proven an adequate lifetime in membrane cells. It is quite clear that the electrode gap should be as small as possible. For a plate electrode, however, there is a limitation: obviously, the cell cannot work if the plate is placed directly at the separator (membrane or diaphragm). Perforated electrodes, however, can be placed at zero distance; in this case the area of the holes and of the backside of the structure are working. To get advantage of this electrode configuration, the structure should be thin compared to the conventional electrode gap of max. 3 mm. In this case one has to install a current distribution system and the number of contacts can easily be calculated from the size and the specific resistivity of the electrode structure. This is explained by the following example: a model cell which is 0.6 m wide and 1.2 m high shall be operated at 3.5 kA m- ‘. One cathode frame operating at both sides will carry 5 kA, which have to be distributed evenly over the electrode area. A voltage drop of SOmV across the support, which due to the ten-fold higher voltage drop across the separator, does not cause a serious change of the current distribution, seems to be tolerable. For this current density and allowed Ohmic potential drop a nickel plate of 1 mm thickness can transport the current over a length of 46.5 cm, the minimum thickness of the plate in our model cell should be 1.3 mm. A perforated plate of 70% void area should have a minimum thickness of 1.9 mm, ignoring the somewhat longer way to pass the for caustic whole cell width. The same calculation resistant stainless steels, because of its about ten-fold lower conductivity, yields in a minimum thickness of 19 and 28 mm respectively. Thus steel electrodes can only be used in bi-polar arrangements without current distribution in the electrode plane. In monopolar cells, a plate electrode without current collectors is by far less expensive. This is mainly caused by the reduction of the necessary welding work, but also by the possible reduction of the minimum frame thickness from about 10 cm for frames with current collectors to 3 cm for plate type cells. (2) &Txi of bubble accumulation The following calculations describe the behaviour of a plate electrode for the a.m. cell under standard conditions 180°C. 30 % NaOHI. The current densitv at the lower kdge ‘of ice elect&de is assumed to’ be 2.5 kAm_‘, the electrode gap is 3 mm. Due to the accumulation of gas bubbles the specific conductivity
Raney-Ni activated cathodes
of the electrolyte will decrease Bruggemann equation: K =
K,,(l
-
according
to the
,
1561
a(kA/m2)
la)
2.5
v /Y)“‘. g
(3)
degosiflcal~on
efhuency
ierr = 2.20
:
kAl
50%
m7
resulting current distribution is shown in the upper curve of Fig. 5. According to Tobias[9] we can describe the curve by: The
i,Ji, = &
Wx + 2)’ 0.5
where i. denotes the average current density, ix the current density at the reduced height x = h/H and k is a constant factor for a given cell geometry and gas velocity. The average current density is 2.3 kAm_‘. IF one considers the dependence of the bubble velocity on the gas concentration according to Kozo Koido et al.[lO] u = uJO.27 + 0.73. (1 - V,/ Y)=]
II
0
0.5
--
reduced
Ii
I (kA/m*l
Fig. 6. Current density distribution across a louvered cathode same conditions as Fig. 5, but 10% void area. (a) Degasification efficiency 50 %. (b) degasification eficiency 100%.
means 50% of the gas reaching the upper edge of a stripe are released to the rear) the maximum difference of the current densities is less than 5%. The lower average current density of 2.2 kAm_’ is due to the definition of the technical current density, which is related to the cross-section of the cell. For degasification efficiencies near 100 % the current density remains constant even within 0.5 %. Considering the decrease of the current efficiency of membrane cells at higher caustic concentrations, we would expect a slight increase of the current efficiency compared to the zero gap cell configuration due to the remarkably decreased local caustic concentration at the membrane surface. Further tests in industrial cells shall prove the validity of the electrode performance. Up to now only uncoated louvered cathodes areinstalled in membrane cells which are being successfully operated.
REFERENCES
1.3kAlmZ
b
0
*
1.0 hsght hi
(5)
one gets the lower curve with an effective current density of only 1.3 kAmm2. Considering the fact that the Bruggemann equation becomes invalid at gas void fractions higher than 40 %, the curve is expected to be even steeper and no even current density distribution seems to be possible, unless an electrolyte motion due to possible gas lift effects is superimposed. In fact, chlorate cells of the same height and the same electrode gap are operating without any dif%ulties. To improve the situation one can divide the plate; for instance in nine horizontal stripes, each of which is 12cm high and separated by gaps which are 1.5cm wide. Figure 6 shows the effect on the current density distribution: at 50% degasification efficiency (that
‘_(
reduced
height
1 hl
H
Fig. 5. Current density distribution across a vertical eiectrade according to the Bruggcmann equation (a) and considering gasconcentration dependence of the gas velocity (b). Presumed height of the electrode H = 120 cm, initial cd. at the lower edge 2.5 kA m-‘.
1. Y. S. Terminasow and M. S. Beletzki, Dokl. Akad. Nauk SSSR 63,411 (1948). 2. E. Justi, M. Pillcuhn, W. Scheibe and A. Winsel, Ahi. lMssenschaftten Lirerarur Abh. d. mrh.-norurw. Klasse 8 (1959). 3. German patent application P 29 14 094.1. 4. German patent application P 32 18 429.8. 5. DOS 28 29 901. 6. G. M. Barrow, Physikaliscfw Chemie, 2nd edn, Part III, p. 58. Bohmann, Heidelberg (1972). 7. R. A. Marcus., A. Rev. phys. Chm. 15, 155 (1964). 8. A. Schmidt, Thesis, TU Braunschweig (1973). 9. C. W. Tobias, J. electrochem. Sot. 106, 833 (1959). 10. Kozo K&de, T. Hirahara and H. Kubota, C/tern. Engng, Tokyo, 5. 38 (1967).