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The steady-state hydrogen content of catalytically active noble metal cathodes during hydrogen bubble evolution at high current densities
SHORT COMMUNICATION THE STEADY-STATE HYDROGEN CONTENT OF CATALYTICALLY ACTIVE NOBLE METAL CATHODES DURING HYDROGEN BUBBLE EVOLUTION AT HIGH CURRENT DENSITIES M. C.
WITHERSPOON,
Chemistry
R. C. JotINsToN and F. A. Lr:wa
Dept., Queen’s
University.
It has been shown in previous studies[1&6] involving cathodes of palladium and palladium alloys. with surfaces of high catalytic activity for hydrogen dissociation. that at cd of - 100 5OOmA cm-l. their steady-state hydrogen contents during electrolysis can attain upper limitmg values which are considerably less than their ‘saturation’ hydrogen contents. These hydrogen contents arc appropriate to reversible equilibrium conditions having been established between the elcctrodc surface and the saturation of hydrogen molecules dissolved in the interfacial layer of catholyte, under conditions where hydrogen is evolving from the cathode by a combination of bubble formation and diffusive molecular transport. The hydrogen content of the cathodes has been determined from measurements of electrode potential after interrupting electrolysls, and of electrical resistance made both during and after electrolysis. In experiments with pure palladium cathodes indications have also been obtamed[l, 71 from the form of plots against time of the change of open circuit potential following interruption of electrolysis that at relatively high cd (>2amp cmz2) there might be a reduction in the steadystate upper limiting value of the hydrogen content of the cathode. It seemed useful to see if more definite conclusions could be obtained by means of electrical resistance measurements. The effect of electrolytically evolved gas bubbles on mass transfer processes at electrode interfaces is of some general current interestrX].
Belfast. N. Ireland
The reliability of clcctrical rcsistancc measurements is hmlted[46.9] by (i) co-conductlon of the measuring current through the electrolyte, (ii) the scnsltivity of clcctrical resistance changes to changes of hydrogen content and (iii) reversibility or absence of hysteresis of the relationships between hydrogen content and electrical resistance durmg absorption and desorption of hydrogen. Problems concerning factors (Ii) and (iii) are particularly significant when specimens are very short lengths of wire used in trying to extend measurements to very high cd. Indications were available[4-7.9-l I] that certain compositions within the palladium/platinum series of alloys might. however, he sultable in regard to the above criteria. Figure 1 illustrates alterations with changing cd of the steady-state relative electrical resistance (R/Ro) of a 19’;; Pt-X10/, Pd alloy cathode (coated with palladium black) in well-stirred 0.01 N HClO, saturated with hydrogen gas (1 atm) at 25-C. The electrolytic ce11[5,7] was immersed in a water bath thermostated to +O.l”C. Also plotted in Fig. I are changes with time of R/Ro and electrode potetltial (wrt Pt/H,) following interruption of electrolysis. In the absence of any maxima[S, I l] in the post-electrolysis plots of R/Ro against time, it can be taken that if strictly isothermal conditions applied, then hydrogen content should increase with increasing R/Ro over the whole range of values recorded in Fig 1. On this basis the decreases of the steady-state values of R/Ro with an increase of current density from 830 mA cm ’ to 4470 mh cm. ’
5
Fig. 1. Plots of relative electrical resistance (R/Ro) and electrode potential (w.r.t. Pt/H,) in hydrogensaturated (1 atm pressure) 01 N HClO, following interruption of catholysis of a palladium-black coated 81% Pd-19”/U Pt alloy following electrolysis at A, IO; Cl. 42; A. 120; 0, 313; 0, 830; X. 1780; M. 4470 mA cm-*. Symbols with horizontal and vertical bars at time t = 0 are steady-state values of R/Ro during catholysis. Broken horzontal line in Fig. la at R/Ro - 1.05 corresponds to value of R/Ro in equllihrlum with 1 atm Hz. 519
SHORT C~MMUNICATWNS
520
I
10
I
20 Staody-5tot.Y
I
30 ,emLls*“m
I
1
40 Of cottdyte
50
.C
Fig. 2. Steady-state values of catholyte temperature and R/Ro of a palladium-black coated 81% Pd-19% Pt alloy during electrolysis at 0, 314; 0, 830; x , 1780 and M. 4COO mA cm-* in 0.01 N HClO, at 5” intervats ot thermostated bath temperature.
should indicate a decrease of hydrogen content under the higher cd conditions. The possible effects of current/resistive heating of the electrolyte must also be considered. However, since alloys of thd class under study have been found to retain a positive (metallic) temperature coefficient of resistance at all hydrogen contents, any increase of electrode temperature over the range of cd between 830 mA cm-‘-4380 mA cm-’ without change of hydrogen content should be reflected by an increase of electrical resistance rather than the decrease observed experimentally. It is therefore clear from these considerations of the resistance measurements that the steady-state hydrogen contents during electrolysis had attained an upper limit within the range 313 mA cm-* to 1790 mA cm-’ and then gradually decreased under conditions of increasingly vigorous bubble evolution. The question next arises as to whether this decrease of the hydrogen content of the electrode at the highest cd reflects a significant decrease of the effective pressure (concentration) of dissolved hydrogen in the solution side of the interface due to a mechanistic change of conditions, or whether it is merely a secondary consequence of the local heating caused by electrolysis unaccompanied by a lowering of the effective hydrogen pressure. This proves to be a difficult problem to resolve exactly, since the contributions of the possible consequences of an increase of local electrode and electrolyte heating on the electrode resistance tend to be mutually cancelling. A study has been made of the steady-state values of R/Ro at 5°C intervals of thermostated bath temperature over the range 1QSO”C at a series of cd where bubble evolution is occurring increasingly generally over the whole electrode surface[4. 71. Steady-state values of R/Ro and electrolyte temperature close to the electrode are shown in Fig. 2. Any additional localised specimen heating due to a surface resistance drop was shown to be relatively negligible by separate experiments with a @inurn wire electrode, which in view of its extremely small absorption of hydrogen in this temperature/equivalent pressure range[ 121 could be employed as a resistance thermometer. R/R0 in Fig. 2 has been everywhere corrected for temperature variations of the initial hydrogen-free resistance. Ro. and specimen leads resistance which were determined in separate experiments. The solubility of hydrogen in palladium alloys of the type under study decreases with temperatureC9, 121 and this will be reflected by a decrease of R/Ro in the present case. Exact relationships between hydrogen pressure and either hydrogen solubitity or the monitoring function R/Ro over the whole range of pressure and temperature condi-
tions relevant to Fig. 2 are not yet available for the 19s; Pt-8 1% Pd alloy. However for a ‘notional pressure’ of dissolved hydrogen during electrolysis of approximately IOatm (suggested from consideration of the open circuit potential measurements) which is assumed to remain constant over the whole temperature range, the decreases of R/Ro with temperature shown in Fig. 2 are consistent with the changes of R/Ro with temperature at a constant hydrogen pressure of I atm[9] for Pd alloys containing approximately I2 and 15% Pt, which recent R/Ro, pressure relationships[l3] suggest have pressure, solubility relationships of a closely similar form to the 197; Pt alloy. Figure 2 shows there is the same measure of change of R/Ro at each 5°C interval of measurement. A minor difference from the experiment illustrated in Fig. la is that small increases of R/Ro (and so hydrogen solubilily) occur between 314 and 83OmA cm although there is a negligible increase of electrolyte temperature up to the latter cd. However, correction for the approx 0.5’C increases of ternperature associated with eleclrolysis at 1780 mA cm- ’ still indicates a decrease of R/KU and so an accompanying dccrease of hydrogen content compared with 830 rnA cm ‘. Moreover this decrease and the more marked further reduction of R/Ro accompanying electrolysis at 4ooO mA cm-> are both shown to exceed the decrease which could have been expected to occur merely as a result of an increase of the temperature of the electrolyte at an unchanged pressure. These findings therefore suggest some mechanistic reduction of the concentration Iprcssurc) of dissolved hydrogen molecules adjacent to the electrode under conditions of very dense bubble evolution. Clear evidence exists that the upper ltmlting value of hydrogen pressure durmg bubble evolution is dcprndcnt on the hydrogen ion concentration in the rlectralyte[7]. In the case of UOI N HC104 calculations by the relation 2FE = RT In p following extrapolation of the open circuit potential, E. in Fig. l(b) back to the time of current interruption[l-77 yield values of hydrogen pressure, p, of approx IOatm (EC 29mV). The subsequent reduction of effective pressure of dissolved molecules, p, during electrolysis at 4OOOmA cm-’ would seem to be in the order of 2 or 3 atm although it is difficult to obtain precise values of hydrogen pressure by this means at such high cd because of possible residual effects of other contributions to hydrogen over-potential at short times after current interruption following electrolysis. Any reduction in the interfacial concentration (pressure) of dissolved hydrogen could be expected to reflect a more effective and rapid transfer of the dissolved molecules away from the electrode surface into the bulk of the electrolyte as a consequence of more vigorous stirring of the diffusion layer by the evolving gas bubbles. This effect would be in keeping with other evidence for a reduction of the effective thickness of the diffusion layer under such conditionsr81.
REFI;.KENC:ES I. R. Clamroth and C. A. Knorr. Z. Elrktrocizrj~. 57. 3Y9 (1953). 2. J. C. Barton and F. A. Lewis. Nature 192. 54Y (1961) i. J. C. Barton and F. A. Lewis, Z. phq’s. Chrrn. Neuc* Folge 33. 99 (1962). 4. J. A. S. Green and F. A. Lewis. 7?trr1s. f+‘cl~n&~~Sot. 6u. 2234 (1964). 5. J. A. S. Green and F. A. Lewis J. r/~c~orhrm Sur 113. 95 (1966). 6. F. A. Lewis, Recent Prugr~w rn Sur/;lc~~ LSor~wr. Academic Press, New York, Vol. 3, p. 72 (1970). 7. R. C. Johnston, Ph.D. Thesis Quem’s Urrw. Brlfust I Y 7 I and results to be published. 8. L. J. J. Janssen and J. G. Hoogland, Elecrrocit~m. Actu 18. 543 (1973). 9. A. W. Carson, T. B. Flanagan and F. A. Lewis. liatis. Faraday Sot. 56. 363, 1311. 1332 (1960).
SHORT
IO. P
Vignet,
Crntre
R. Gabillg.
SEru&
J. Lutz
Nurl~krrrt~
Rpt,t.
C~MMLINICATI~NS
and R. ZermiLoglou. DPC.JS.SC I /63-3461
PVDG ( 1963). 1 I. F A. Lewis, W. D. McCall and ‘r. C. Wltherspoon. 2. ph~vs. Chcm. !vcdlrc, ~olye 84, 3 I ( 1973).
521
12. F. A. Lewis, The Palladiurn/Hydrog~rI Systm. Academic Press, New York (1967). 13. B. Baranowskl F. A. Lewis, S. Majchrzak and R. WiSniewski. J. Chern. Sot. Furudq Truns. I 68. 824 (1972).
WITHERSPOON,
Chemistry
R. C. JotINsToN and F. A. Lr:wa
Dept., Queen’s
University.
It has been shown in previous studies[1&6] involving cathodes of palladium and palladium alloys. with surfaces of high catalytic activity for hydrogen dissociation. that at cd of - 100 5OOmA cm-l. their steady-state hydrogen contents during electrolysis can attain upper limitmg values which are considerably less than their ‘saturation’ hydrogen contents. These hydrogen contents arc appropriate to reversible equilibrium conditions having been established between the elcctrodc surface and the saturation of hydrogen molecules dissolved in the interfacial layer of catholyte, under conditions where hydrogen is evolving from the cathode by a combination of bubble formation and diffusive molecular transport. The hydrogen content of the cathodes has been determined from measurements of electrode potential after interrupting electrolysls, and of electrical resistance made both during and after electrolysis. In experiments with pure palladium cathodes indications have also been obtamed[l, 71 from the form of plots against time of the change of open circuit potential following interruption of electrolysis that at relatively high cd (>2amp cmz2) there might be a reduction in the steadystate upper limiting value of the hydrogen content of the cathode. It seemed useful to see if more definite conclusions could be obtained by means of electrical resistance measurements. The effect of electrolytically evolved gas bubbles on mass transfer processes at electrode interfaces is of some general current interestrX].
Belfast. N. Ireland
The reliability of clcctrical rcsistancc measurements is hmlted[46.9] by (i) co-conductlon of the measuring current through the electrolyte, (ii) the scnsltivity of clcctrical resistance changes to changes of hydrogen content and (iii) reversibility or absence of hysteresis of the relationships between hydrogen content and electrical resistance durmg absorption and desorption of hydrogen. Problems concerning factors (Ii) and (iii) are particularly significant when specimens are very short lengths of wire used in trying to extend measurements to very high cd. Indications were available[4-7.9-l I] that certain compositions within the palladium/platinum series of alloys might. however, he sultable in regard to the above criteria. Figure 1 illustrates alterations with changing cd of the steady-state relative electrical resistance (R/Ro) of a 19’;; Pt-X10/, Pd alloy cathode (coated with palladium black) in well-stirred 0.01 N HClO, saturated with hydrogen gas (1 atm) at 25-C. The electrolytic ce11[5,7] was immersed in a water bath thermostated to +O.l”C. Also plotted in Fig. I are changes with time of R/Ro and electrode potetltial (wrt Pt/H,) following interruption of electrolysis. In the absence of any maxima[S, I l] in the post-electrolysis plots of R/Ro against time, it can be taken that if strictly isothermal conditions applied, then hydrogen content should increase with increasing R/Ro over the whole range of values recorded in Fig 1. On this basis the decreases of the steady-state values of R/Ro with an increase of current density from 830 mA cm ’ to 4470 mh cm. ’
5
Fig. 1. Plots of relative electrical resistance (R/Ro) and electrode potential (w.r.t. Pt/H,) in hydrogensaturated (1 atm pressure) 01 N HClO, following interruption of catholysis of a palladium-black coated 81% Pd-19”/U Pt alloy following electrolysis at A, IO; Cl. 42; A. 120; 0, 313; 0, 830; X. 1780; M. 4470 mA cm-*. Symbols with horizontal and vertical bars at time t = 0 are steady-state values of R/Ro during catholysis. Broken horzontal line in Fig. la at R/Ro - 1.05 corresponds to value of R/Ro in equllihrlum with 1 atm Hz. 519
SHORT C~MMUNICATWNS
520
I
10
I
20 Staody-5tot.Y
I
30 ,emLls*“m
I
1
40 Of cottdyte
50
.C
Fig. 2. Steady-state values of catholyte temperature and R/Ro of a palladium-black coated 81% Pd-19% Pt alloy during electrolysis at 0, 314; 0, 830; x , 1780 and M. 4COO mA cm-* in 0.01 N HClO, at 5” intervats ot thermostated bath temperature.
should indicate a decrease of hydrogen content under the higher cd conditions. The possible effects of current/resistive heating of the electrolyte must also be considered. However, since alloys of thd class under study have been found to retain a positive (metallic) temperature coefficient of resistance at all hydrogen contents, any increase of electrode temperature over the range of cd between 830 mA cm-‘-4380 mA cm-’ without change of hydrogen content should be reflected by an increase of electrical resistance rather than the decrease observed experimentally. It is therefore clear from these considerations of the resistance measurements that the steady-state hydrogen contents during electrolysis had attained an upper limit within the range 313 mA cm-* to 1790 mA cm-’ and then gradually decreased under conditions of increasingly vigorous bubble evolution. The question next arises as to whether this decrease of the hydrogen content of the electrode at the highest cd reflects a significant decrease of the effective pressure (concentration) of dissolved hydrogen in the solution side of the interface due to a mechanistic change of conditions, or whether it is merely a secondary consequence of the local heating caused by electrolysis unaccompanied by a lowering of the effective hydrogen pressure. This proves to be a difficult problem to resolve exactly, since the contributions of the possible consequences of an increase of local electrode and electrolyte heating on the electrode resistance tend to be mutually cancelling. A study has been made of the steady-state values of R/Ro at 5°C intervals of thermostated bath temperature over the range 1QSO”C at a series of cd where bubble evolution is occurring increasingly generally over the whole electrode surface[4. 71. Steady-state values of R/Ro and electrolyte temperature close to the electrode are shown in Fig. 2. Any additional localised specimen heating due to a surface resistance drop was shown to be relatively negligible by separate experiments with a @inurn wire electrode, which in view of its extremely small absorption of hydrogen in this temperature/equivalent pressure range[ 121 could be employed as a resistance thermometer. R/R0 in Fig. 2 has been everywhere corrected for temperature variations of the initial hydrogen-free resistance. Ro. and specimen leads resistance which were determined in separate experiments. The solubility of hydrogen in palladium alloys of the type under study decreases with temperatureC9, 121 and this will be reflected by a decrease of R/Ro in the present case. Exact relationships between hydrogen pressure and either hydrogen solubitity or the monitoring function R/Ro over the whole range of pressure and temperature condi-
tions relevant to Fig. 2 are not yet available for the 19s; Pt-8 1% Pd alloy. However for a ‘notional pressure’ of dissolved hydrogen during electrolysis of approximately IOatm (suggested from consideration of the open circuit potential measurements) which is assumed to remain constant over the whole temperature range, the decreases of R/Ro with temperature shown in Fig. 2 are consistent with the changes of R/Ro with temperature at a constant hydrogen pressure of I atm[9] for Pd alloys containing approximately I2 and 15% Pt, which recent R/Ro, pressure relationships[l3] suggest have pressure, solubility relationships of a closely similar form to the 197; Pt alloy. Figure 2 shows there is the same measure of change of R/Ro at each 5°C interval of measurement. A minor difference from the experiment illustrated in Fig. la is that small increases of R/Ro (and so hydrogen solubilily) occur between 314 and 83OmA cm although there is a negligible increase of electrolyte temperature up to the latter cd. However, correction for the approx 0.5’C increases of ternperature associated with eleclrolysis at 1780 mA cm- ’ still indicates a decrease of R/KU and so an accompanying dccrease of hydrogen content compared with 830 rnA cm ‘. Moreover this decrease and the more marked further reduction of R/Ro accompanying electrolysis at 4ooO mA cm-> are both shown to exceed the decrease which could have been expected to occur merely as a result of an increase of the temperature of the electrolyte at an unchanged pressure. These findings therefore suggest some mechanistic reduction of the concentration Iprcssurc) of dissolved hydrogen molecules adjacent to the electrode under conditions of very dense bubble evolution. Clear evidence exists that the upper ltmlting value of hydrogen pressure durmg bubble evolution is dcprndcnt on the hydrogen ion concentration in the rlectralyte[7]. In the case of UOI N HC104 calculations by the relation 2FE = RT In p following extrapolation of the open circuit potential, E. in Fig. l(b) back to the time of current interruption[l-77 yield values of hydrogen pressure, p, of approx IOatm (EC 29mV). The subsequent reduction of effective pressure of dissolved molecules, p, during electrolysis at 4OOOmA cm-’ would seem to be in the order of 2 or 3 atm although it is difficult to obtain precise values of hydrogen pressure by this means at such high cd because of possible residual effects of other contributions to hydrogen over-potential at short times after current interruption following electrolysis. Any reduction in the interfacial concentration (pressure) of dissolved hydrogen could be expected to reflect a more effective and rapid transfer of the dissolved molecules away from the electrode surface into the bulk of the electrolyte as a consequence of more vigorous stirring of the diffusion layer by the evolving gas bubbles. This effect would be in keeping with other evidence for a reduction of the effective thickness of the diffusion layer under such conditionsr81.
REFI;.KENC:ES I. R. Clamroth and C. A. Knorr. Z. Elrktrocizrj~. 57. 3Y9 (1953). 2. J. C. Barton and F. A. Lewis. Nature 192. 54Y (1961) i. J. C. Barton and F. A. Lewis, Z. phq’s. Chrrn. Neuc* Folge 33. 99 (1962). 4. J. A. S. Green and F. A. Lewis. 7?trr1s. f+‘cl~n&~~Sot. 6u. 2234 (1964). 5. J. A. S. Green and F. A. Lewis J. r/~c~orhrm Sur 113. 95 (1966). 6. F. A. Lewis, Recent Prugr~w rn Sur/;lc~~ LSor~wr. Academic Press, New York, Vol. 3, p. 72 (1970). 7. R. C. Johnston, Ph.D. Thesis Quem’s Urrw. Brlfust I Y 7 I and results to be published. 8. L. J. J. Janssen and J. G. Hoogland, Elecrrocit~m. Actu 18. 543 (1973). 9. A. W. Carson, T. B. Flanagan and F. A. Lewis. liatis. Faraday Sot. 56. 363, 1311. 1332 (1960).
SHORT
IO. P
Vignet,
Crntre
R. Gabillg.
SEru&
J. Lutz
Nurl~krrrt~
Rpt,t.
C~MMLINICATI~NS
and R. ZermiLoglou. DPC.JS.SC I /63-3461
PVDG ( 1963). 1 I. F A. Lewis, W. D. McCall and ‘r. C. Wltherspoon. 2. ph~vs. Chcm. !vcdlrc, ~olye 84, 3 I ( 1973).
521
12. F. A. Lewis, The Palladiurn/Hydrog~rI Systm. Academic Press, New York (1967). 13. B. Baranowskl F. A. Lewis, S. Majchrzak and R. WiSniewski. J. Chern. Sot. Furudq Truns. I 68. 824 (1972).