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Re-assessment of ‘overstepping’ phenomena at cathodes of palladium and palladium alloys
Elcftrochimiea ~*a. 1966. Vol. 11. pp. 931 to 934. parllamon PWS Ltd. P&&d in Notthem b&nd
SHORT
COMMUNICATIONS
A RE-ASSESSMENT OF ‘OVERSTEPPING’ PHENOMENA AT CATHODES OF PALLADIUM AND PALLADIUM ALLOYS* J. C. BARTON, J. A. S. GREEN and F. A. LEWIS Chemistry Department, The Queen’s University of Belfast, Northern Ireland IN A review of changes of physical and physicochemical parameters of palladium cathodes, Smith1 interpreted what he described as ‘overstepping’ phenomena, in terms of the presence of hydrogen in an expanding and contracting system of voids or ‘rifts’ within the solid. Smith defined overstepping to have occurred when a plot against time of the parameter under study passes through a maximum or a minimum after alteration or cessation of the electrolytic current. Smith and collaborators2*3 reported such oversteppings in studies of changes of electrode potential, electrical resistance and specimen length. No attempts have been made subsequently to define the dimensions of the rifts or to correlate them with different types of dislocation networks. Moreover, more recently other experimental results4 concerning electrical resistance, which had been interpreted by Smith’ as due to the behaviour of hydrogen in rifts, have in fact been conGrmed6~8 to be due to parallel conductance of the “bridge” or “measuring” current through the catholyte. The present note primarily is concerned with reassessing the earlier evidence of overstepping in the light of more recent experimental studies with palladium, and of the various sources of error encountered. The most pronounced oversteppings were recorded during the measurements of electrode potential of Smith and Martin 2: however this was an early study of this parameter, and it is only recently that disputes concerning the results of several subsequent studies have been resolved.’ It now has been established both for pure palladium and also for several palladium alloys that provided true steady state conditions-with respect to the gaseous environment of the cathode and of its content of hydrogen-have been established at a particular current density, no oversteppings are recorded on alteration or cessation of current.3-10 It has however been illustrated* that an apparent overstepping of the electrode potential, E, of a palladium cathode could occur if the cathode had not been saturated homogeneously with hydrogen before electrolysis was interrupted. The subsequent overstepping of E was a reflexion of a continued diffusion of hydrogen from the surface so rapidly into the interior that the hydrogen content of the surface temporarily decreased below both the surface composition during electrolysis, and the composition in equilibrium with the concentration of hydrogen dissolved in the catholyte. * Manuscript received 31 August 1965. 931
communications
Short
932
Figure 1 illustrates how overstepping of both the electrode potential and also the relative electrical resistance, R/R,,, of a palladium cathode also have been found to occur when oxidizing species formed around the (platinum) anode are not prevented from diffusing into the cathode compartment. Hydrogen gas was bubbled through the catholyte in all experiments. When (oxidizing) products forming at the anode were swept continuously from the anode compartment by a stream of bubbles of argon, both E and R/R,returned smoothly without oversteppings to values indicating equilibration with the hydrogen dissolved in solution. However, I
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20
30
40
Time,
-
L______J 45
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1.74
80
min
1. Time dependenceof R/R,, (filledsymbols)and electrode potential E (hydrogen scale) ( x and open symbols)for palladizedpalladium wire cathodes followinginterruption of electrolysisin hydrogen-saturatedO-01 N HCI. (a) precautionstaken to prevent anodic products diffusingto the cathode; A and A, O-0122 cm dia. wire; x ,0.0274 cm. (b) 0,. and q indicate measurements with catholytes discoloured by anodic products. 0 denotes value of R/R0prior to cathodization. A and B denote points where the rate of hydrogen-stirring was greatly increased. FIG.
when there was no flow of argon through the anode compartment, reddish-coloured products (probably oxides of chlorine) began to accumulate around the anode. After lengthy electrolysis these gradually permeated into the cathode compartment; and then, on interruption of electrolysis, oversteppings of both E and R/R,, were recorded as illustrated in Fig. 1. These oversteppings can be explained as being due to the reddish compounds (or other uncoloured oxidants produced in conjunction) having been in high enough concentration in the catholyte so as temporarily to reduce the hydrogen content of the cathode below the value in equilibrium with hydrogen gas dissolved in solution. However, the concentration.of anodic products in the cathode compartment gradually decreased after electrolysis since they continued to be swept from the catholyte by the flow of hydrogen gas. During the same time hydrogen was reabsorbed until E and R/R,, assumed values appropriate to equilibrium solely with the concentration of dissolved hydrogen. The experimental procedures in the papers of Smith and Martin2 and of Harding
Short commuoieatio~~
933
and Smiths were restudied to see whether overstepping phenomena reported in them could have been due to either of the alternative reasons discussed above: namely incomplete permeation of the lattice by hydrogen, or the diffusion of oxidizing species from the anode compartment. In general the evidence cannot be regarded as very strongly in favour of either of these explanations, since there seems to have been a long diffusion path from the anode to the cathode, and electrolysis seems to have been continued for very long times at the same current density before it was interrupted or altered in value. However, the re-examination also revealed that in the work of Smith and Martin2 the oversteppings of electrode potential were rather erratic and irreproducible and, indeed, virtually disappeared after a series of cycles of interruption and recommencement of electrolysis. Moreover the oversteppings of electrical resistance and of specimen length in the work of Harding and Smiths were not marked; and the exact changes of current density, and the times at which measurements were made after such changes, are not easy to correlate with the symbols in the plots of results. In all, therefore, the results in these two papers now seem to constitute only meagre evidence in support of the speculative and otherwise unsubstantiated hypotheses that significant changes in the parameters can be due to expansion and contraction of voids or ‘rifts’ and/or to hydrogen contained in them. The measurements of Smith and his collaborators were carried out without a detailed appreciation of the thermodynamic correlation between electrode potential measurements and the pressure/composition relationships for the palladium/hydrogen and related systems,7-r0 or a knowledge of the sensitivity of measurements of electrode potential to the catalytic activity of the surfaces.8J There also was insufficient appreciation at that time, of the importance of considering the equilibrium set up between the cathode and the hydrogen molecules dissolved in the surrounding electrolyte.s-10 More recent investigations also have increasingly revealed that the state of activation of the surface is a vital consideration both with regard to this equilibrium, and also with regard to the extent to which co-conduction of the measuring current through the solution can complicate measurements.5~6 All these other factors also have been taken into consideration during the course of a prolonged series of measurements of electrode potential and electrical resistance which we have made, prior to, during, and subsequent to the cathodization of palladium and of series of palladium/nickel and palladium/rhodium al10ys.~~~ Few changes of these parameters have been found which could not be ascribed to changes in the hydrogen contents of the bulk lattices of the electrodes. Furthermore (apart from the other causes discussed earlier in this note-incomplete permeation of hydrogen through the solid, and oxidizing impurities in the catholyte) all these other changes have been traced to mundane causes, such as the development of poor electrical contact between the specimen and its leads,la a gradual (and irreversible) change of external dimensions following cycles of absorption and desorption of hydrogen,lJ3 and (particularly in the case of alloys with higher contents of Ni and Rh) the irreversible development of substantial intercrystalline cracks.11J4 (With certain of the Pd - Ni and Pd - Rh alloys some effects which could be loosely identified with overstepping, also could arise from the existence of maximall in relationships between hydrogen content and R/R,, combined with an associated hysteresis in the ‘absorption’ and ‘desorption’ relationships-however these rather represent an extra case, which 1s
934
Short communications
in any event also seems capable to interpretation in terms of changes of the bulk lattices). In compiling this survey it has been found di5cult to avoid ambiguity in using the term ‘overstepping’. Because of this, and also because of its association with interpretations centred round the presence of hydrogen in rifts or voids, there seems little to commend any extension of its usage. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14.
D. P. D. P. E. A. G. A. C. A.
SMTII, Hydrogen in Metals, Chicago Univ. Press (1947). SMITHand F. H. MARTIN,J. Am. them. Sot. 38,2577 (1916). HARDINGand D. P. Swrrr, J. Am. them. Sot. 40,1508 (1918). MOORE, Trans. electrochem. Sot. 75,237 (1939). KNORRand E. SCHWARTZ,2. Elektrochem. 40,37 (1934). J. C. BARTONand F. A. LEWIS,Trans. Faraday Sot. 58, lb3 (i962). P. C. ABEN and W. G. BUROERS.l?ans. Faradav Sot. 58.1989 (1962). T. B. FLANAGANand F. A. LEGS, Trans. Fara&y Sot. %,lk (1959). J. C. BARTONand F. A. LEWIS,Z. phys. Chem. N. F. 33,99 (1962). J. A. S. GREENand F. A. LEWIS, Trans. Faraaby Sot. 60,2234 (1964). J, C. BARTON,J. A. S. GREENand F. A. LEWIS,Trans. Faraday Sot. 62,961(1966); J. A. S. GREEN and F. A. LEWIS,Trans. Faraday Sot. 62,971 (1966). J. C. BARTON,W. F. N. Lsrrcrr and F. A. LEWIS, Trans. Far&y Sot. 59,1208 (1963). W. KRAUSEand L. KAHLENBERG, Trans. electrochem. Sot. 68,449 (1935). J. J. VAN Losr, Phil@ Res. Rep. l&71 (1963).
Electrochimica Acta,1966.Vol. 11,pp. 934to 939.Persamon Pr6s.s Ltd.Printed in Northern Ireland
CURRENT DISTRIBUTION IN POROUS ELECTRODES OPERATING UNDER FORCED FLOW* S. SZPAK, J. D. ELWIN and T. KATAN Materials Sciences Laboratory, Lockheed Palo Alto Research Laboratory, Lockheed Missiles t Space Company, Palo Alto, Calif., U.S.A. PRACTICAL
considerations in the construction of electrochemical energy conversion devices, operating with a continuous supply of reactant and removal of products, necessitate high current densities at low overpotentials, Hence, the current distribution within the porous electrode structure and the effects of geometry and the mode of operation on the current distribution are of considerable interest. The usual procedure in the determination of various distribution functions, eg current, potential and concentration of reactant(s) and product(s), invokes the laws of conservation of mass and charge. 1 Thus, the distribution functions depend on the manner in which the transport of reactant to the metal/solution interface takes place and on the type of overpotential/current relationship governing the charge transfer process. Several cases, usually involving restrictive assumptions, have been treated in the past years.= This communication considers the case where the reactant is passed through the porous structure by forced flow and offers the experimentally observed current distributions for the reduction of ferricyanide ion, oxidation of hydrazine and ethylene in potassium hydroxide solution. * Manuscript received 22
October 1965.
SHORT
COMMUNICATIONS
A RE-ASSESSMENT OF ‘OVERSTEPPING’ PHENOMENA AT CATHODES OF PALLADIUM AND PALLADIUM ALLOYS* J. C. BARTON, J. A. S. GREEN and F. A. LEWIS Chemistry Department, The Queen’s University of Belfast, Northern Ireland IN A review of changes of physical and physicochemical parameters of palladium cathodes, Smith1 interpreted what he described as ‘overstepping’ phenomena, in terms of the presence of hydrogen in an expanding and contracting system of voids or ‘rifts’ within the solid. Smith defined overstepping to have occurred when a plot against time of the parameter under study passes through a maximum or a minimum after alteration or cessation of the electrolytic current. Smith and collaborators2*3 reported such oversteppings in studies of changes of electrode potential, electrical resistance and specimen length. No attempts have been made subsequently to define the dimensions of the rifts or to correlate them with different types of dislocation networks. Moreover, more recently other experimental results4 concerning electrical resistance, which had been interpreted by Smith’ as due to the behaviour of hydrogen in rifts, have in fact been conGrmed6~8 to be due to parallel conductance of the “bridge” or “measuring” current through the catholyte. The present note primarily is concerned with reassessing the earlier evidence of overstepping in the light of more recent experimental studies with palladium, and of the various sources of error encountered. The most pronounced oversteppings were recorded during the measurements of electrode potential of Smith and Martin 2: however this was an early study of this parameter, and it is only recently that disputes concerning the results of several subsequent studies have been resolved.’ It now has been established both for pure palladium and also for several palladium alloys that provided true steady state conditions-with respect to the gaseous environment of the cathode and of its content of hydrogen-have been established at a particular current density, no oversteppings are recorded on alteration or cessation of current.3-10 It has however been illustrated* that an apparent overstepping of the electrode potential, E, of a palladium cathode could occur if the cathode had not been saturated homogeneously with hydrogen before electrolysis was interrupted. The subsequent overstepping of E was a reflexion of a continued diffusion of hydrogen from the surface so rapidly into the interior that the hydrogen content of the surface temporarily decreased below both the surface composition during electrolysis, and the composition in equilibrium with the concentration of hydrogen dissolved in the catholyte. * Manuscript received 31 August 1965. 931
communications
Short
932
Figure 1 illustrates how overstepping of both the electrode potential and also the relative electrical resistance, R/R,,, of a palladium cathode also have been found to occur when oxidizing species formed around the (platinum) anode are not prevented from diffusing into the cathode compartment. Hydrogen gas was bubbled through the catholyte in all experiments. When (oxidizing) products forming at the anode were swept continuously from the anode compartment by a stream of bubbles of argon, both E and R/R,returned smoothly without oversteppings to values indicating equilibration with the hydrogen dissolved in solution. However, I
I
’ -----7
I
\
‘,7a
I I \\
/IL 17’ ‘I I’
4
/
I
-
I.76 g
!’ i I
I
I
I
I
IO
20
30
40
Time,
-
L______J 45
I.75
1.74
80
min
1. Time dependenceof R/R,, (filledsymbols)and electrode potential E (hydrogen scale) ( x and open symbols)for palladizedpalladium wire cathodes followinginterruption of electrolysisin hydrogen-saturatedO-01 N HCI. (a) precautionstaken to prevent anodic products diffusingto the cathode; A and A, O-0122 cm dia. wire; x ,0.0274 cm. (b) 0,. and q indicate measurements with catholytes discoloured by anodic products. 0 denotes value of R/R0prior to cathodization. A and B denote points where the rate of hydrogen-stirring was greatly increased. FIG.
when there was no flow of argon through the anode compartment, reddish-coloured products (probably oxides of chlorine) began to accumulate around the anode. After lengthy electrolysis these gradually permeated into the cathode compartment; and then, on interruption of electrolysis, oversteppings of both E and R/R,, were recorded as illustrated in Fig. 1. These oversteppings can be explained as being due to the reddish compounds (or other uncoloured oxidants produced in conjunction) having been in high enough concentration in the catholyte so as temporarily to reduce the hydrogen content of the cathode below the value in equilibrium with hydrogen gas dissolved in solution. However, the concentration.of anodic products in the cathode compartment gradually decreased after electrolysis since they continued to be swept from the catholyte by the flow of hydrogen gas. During the same time hydrogen was reabsorbed until E and R/R,, assumed values appropriate to equilibrium solely with the concentration of dissolved hydrogen. The experimental procedures in the papers of Smith and Martin2 and of Harding
Short commuoieatio~~
933
and Smiths were restudied to see whether overstepping phenomena reported in them could have been due to either of the alternative reasons discussed above: namely incomplete permeation of the lattice by hydrogen, or the diffusion of oxidizing species from the anode compartment. In general the evidence cannot be regarded as very strongly in favour of either of these explanations, since there seems to have been a long diffusion path from the anode to the cathode, and electrolysis seems to have been continued for very long times at the same current density before it was interrupted or altered in value. However, the re-examination also revealed that in the work of Smith and Martin2 the oversteppings of electrode potential were rather erratic and irreproducible and, indeed, virtually disappeared after a series of cycles of interruption and recommencement of electrolysis. Moreover the oversteppings of electrical resistance and of specimen length in the work of Harding and Smiths were not marked; and the exact changes of current density, and the times at which measurements were made after such changes, are not easy to correlate with the symbols in the plots of results. In all, therefore, the results in these two papers now seem to constitute only meagre evidence in support of the speculative and otherwise unsubstantiated hypotheses that significant changes in the parameters can be due to expansion and contraction of voids or ‘rifts’ and/or to hydrogen contained in them. The measurements of Smith and his collaborators were carried out without a detailed appreciation of the thermodynamic correlation between electrode potential measurements and the pressure/composition relationships for the palladium/hydrogen and related systems,7-r0 or a knowledge of the sensitivity of measurements of electrode potential to the catalytic activity of the surfaces.8J There also was insufficient appreciation at that time, of the importance of considering the equilibrium set up between the cathode and the hydrogen molecules dissolved in the surrounding electrolyte.s-10 More recent investigations also have increasingly revealed that the state of activation of the surface is a vital consideration both with regard to this equilibrium, and also with regard to the extent to which co-conduction of the measuring current through the solution can complicate measurements.5~6 All these other factors also have been taken into consideration during the course of a prolonged series of measurements of electrode potential and electrical resistance which we have made, prior to, during, and subsequent to the cathodization of palladium and of series of palladium/nickel and palladium/rhodium al10ys.~~~ Few changes of these parameters have been found which could not be ascribed to changes in the hydrogen contents of the bulk lattices of the electrodes. Furthermore (apart from the other causes discussed earlier in this note-incomplete permeation of hydrogen through the solid, and oxidizing impurities in the catholyte) all these other changes have been traced to mundane causes, such as the development of poor electrical contact between the specimen and its leads,la a gradual (and irreversible) change of external dimensions following cycles of absorption and desorption of hydrogen,lJ3 and (particularly in the case of alloys with higher contents of Ni and Rh) the irreversible development of substantial intercrystalline cracks.11J4 (With certain of the Pd - Ni and Pd - Rh alloys some effects which could be loosely identified with overstepping, also could arise from the existence of maximall in relationships between hydrogen content and R/R,, combined with an associated hysteresis in the ‘absorption’ and ‘desorption’ relationships-however these rather represent an extra case, which 1s
934
Short communications
in any event also seems capable to interpretation in terms of changes of the bulk lattices). In compiling this survey it has been found di5cult to avoid ambiguity in using the term ‘overstepping’. Because of this, and also because of its association with interpretations centred round the presence of hydrogen in rifts or voids, there seems little to commend any extension of its usage. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14.
D. P. D. P. E. A. G. A. C. A.
SMTII, Hydrogen in Metals, Chicago Univ. Press (1947). SMITHand F. H. MARTIN,J. Am. them. Sot. 38,2577 (1916). HARDINGand D. P. Swrrr, J. Am. them. Sot. 40,1508 (1918). MOORE, Trans. electrochem. Sot. 75,237 (1939). KNORRand E. SCHWARTZ,2. Elektrochem. 40,37 (1934). J. C. BARTONand F. A. LEWIS,Trans. Faraday Sot. 58, lb3 (i962). P. C. ABEN and W. G. BUROERS.l?ans. Faradav Sot. 58.1989 (1962). T. B. FLANAGANand F. A. LEGS, Trans. Fara&y Sot. %,lk (1959). J. C. BARTONand F. A. LEWIS,Z. phys. Chem. N. F. 33,99 (1962). J. A. S. GREENand F. A. LEWIS, Trans. Faraaby Sot. 60,2234 (1964). J, C. BARTON,J. A. S. GREENand F. A. LEWIS,Trans. Faraday Sot. 62,961(1966); J. A. S. GREEN and F. A. LEWIS,Trans. Faraday Sot. 62,971 (1966). J. C. BARTON,W. F. N. Lsrrcrr and F. A. LEWIS, Trans. Far&y Sot. 59,1208 (1963). W. KRAUSEand L. KAHLENBERG, Trans. electrochem. Sot. 68,449 (1935). J. J. VAN Losr, Phil@ Res. Rep. l&71 (1963).
Electrochimica Acta,1966.Vol. 11,pp. 934to 939.Persamon Pr6s.s Ltd.Printed in Northern Ireland
CURRENT DISTRIBUTION IN POROUS ELECTRODES OPERATING UNDER FORCED FLOW* S. SZPAK, J. D. ELWIN and T. KATAN Materials Sciences Laboratory, Lockheed Palo Alto Research Laboratory, Lockheed Missiles t Space Company, Palo Alto, Calif., U.S.A. PRACTICAL
considerations in the construction of electrochemical energy conversion devices, operating with a continuous supply of reactant and removal of products, necessitate high current densities at low overpotentials, Hence, the current distribution within the porous electrode structure and the effects of geometry and the mode of operation on the current distribution are of considerable interest. The usual procedure in the determination of various distribution functions, eg current, potential and concentration of reactant(s) and product(s), invokes the laws of conservation of mass and charge. 1 Thus, the distribution functions depend on the manner in which the transport of reactant to the metal/solution interface takes place and on the type of overpotential/current relationship governing the charge transfer process. Several cases, usually involving restrictive assumptions, have been treated in the past years.= This communication considers the case where the reactant is passed through the porous structure by forced flow and offers the experimentally observed current distributions for the reduction of ferricyanide ion, oxidation of hydrazine and ethylene in potassium hydroxide solution. * Manuscript received 22
October 1965.