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Product analysis from D2O electrolysis with Pd and Ti cathodes
Elecrrochimica Acta, Vol. Printed in Great Britain.
37, No.
2, pp. 215-219,
1992 0
PRODUCT
ANALYSIS FROM D,O ELECTROLYSIS Pd AND Ti CATHODES
0013-4686/92 $5.00 + 0.00 1991. krgamon Press pk.
WITH
E. BIULLAS,*J. E~TEVE,~G. SARDIN,~J. CASADO,$X. D~I+&NECH$and J. A. S;~NCHEZ-CABEZA$ *Department de Quimica Fisica, Facultat de Quimica, Universitat de Barcelona, Marti i Franqubs 1, 08028-Barcelona, Spain de Fisica Aplicada i Electronica, Facultat de Fisica, Universitat de Barcelona, Marti i Franques 1, 08028-Barcelona, Spain tDepartament de Quimica, Universitat Autonoma de Barcelona, 08193Bellaterra (Barcelona), Spain #Servei de Fisica de les Radiations, Departament de Fisica, Universitat Autonoma de Barcelona, 08193-Bellaterra (Barcelona), Spain tkpartament
(Received 27 March 1991; in revisedform 21 May 1991)
Abstract--The enrichment of tritium in the electrolyte and incorporation of T, Li and Pt in cathodes during the electrolysis of 0.1 M LiOD solutions with Pd and Ti cathodes in open cells have been studied. All electrolytes show an increase in their tritium activity which is explained by considering values for the T-D separation factor of all cathodes lower than 1. Accumulation of small amounts of T in the Pd bulk, proceeding from the absorption of the species pre-existing in the electrolyte, has been detected by electrolytic transfer of accumulated tritium to a 0.1 M LiOH solution, as well as by extraction of gases absorbed in the cathode, which were identified by mass spectrometry. Small quantities of Li and Pt are also incorporated in Pd and Ti cathodes, which increase by raising the current density. SIMS analysis of both cathodes show a preferential accumulation of Li and H in their surface layers and confirms the absence of T in Ti. Key words: tritium analysis, D,O electrolysis, Pd cathode, Ti cathode, SIMS.
INTRODUCTION After the recent papers of Fleischmann et al.[l] and Jones et a1.[2] dealing with the existence of D-D fusion reactions during the electrolysis of deuterated solutions with Pd and Ti cathodes at room temperature, several research groups[3-121 have questioned the species generated as reaction products. For Pd open cells containing 0.1 M LiOD, Fleischmann et al.[l] observed a small generation of tritium in the electrolyte, considering that such species proceeds from a D-D fusion reaction taking place near to the electrode surface. A similar tritium production has been reported by several authorsp, 4, lo], but it has been rejected by Williams et a1.[5], who consider that during the electrolysis only the tritium pre-existing in solution is concentrated due to the different discharge rate of D and T on Pd. Precise measurements of the tritium content of solution and an adequate interpretation of the expected tritium enrichment of the electrolyte are then required to check the possible generation of this species by cathodic discharge of deuterium on Pd and Ti. During the electrolysis, large amounts of deuterium are absorbed by Pd and Ti cathodes to yield the respective fl-Pd + D and y -Ti + D phases [S-7, 121. However, less is known about other species either incorporated or possibly generated in cathodes. In this way, ChiZne and Brass[l l] reported a small tritium production into Pd during the electrolysis of 0.1 M LiOD in open cells at high current densities. Several authors[7,8] have proposed a slow diffusion of Li+ ions into the bulk of Pd, although Williams
et a[.[51 reported that lithium is only present in its surface layer (about 1 pm of depth). In this paper, we present results of a study on the electrolysis products of 0.1 M LiOD solutions with Pd and Ti cathodes in open cells carried out to gain a better understanding of species either incorporated to cathodes or produced in them, such as tritium, lithium and platinum, as well as to investigate the tritium enrichment of the electrolyte. Similar experiments using a Pt cathode, where no nuclear events are expected, have also been performed. EXPERIMENTAL Electrochemical cells
0.1 M LiOD solutions were prepared by addition of lithium metal to 99.95% D,O and 0.1 M LiOH solutions by adding lithium hydroxide to twice distilled light water. All the chemicals were Merck, p.a. A 0.1 M LiOD solution with a tritium content cu three times higher than that of pure D,O was prepared by addition of standardized tritium labelled water, supplied by Amersham. Pd and Pt sheets of 99.9% purity (SEMP, Madrid) with respective dimensions of 0.1 x 1.5 x 3 cm and 0.25 mm x 1.5 cm x 3 cm, and 1.4 cm diameter x 3 cm Ti rods of 99.9% purity (Inagasa, Barcelona), were employed as cathodes in the electrolysis at low current density. In the experiments at high current densities, Pd and electrolytic Ti (99.9% purity) sheet cathodes of 0.1 cm x 0.5 cm x 0.5 cm were used. Electrical connections were made using a wire 215
216
E. BRILLASetal.
of the same pure metal spot-welded to the corresponding cathode. Pt wires (99.9% purity) of 0.5 mm diameter x 20 cm were used as anodes. D,O electrolyses were carried out either at a low current density of 5 mA cmm2 for periods up to 30 days or at high current densities of 100 and 300mA cmm2 for 15 to 16 days, whereas H,O electrolyses were run galvanostatically at 5 mA cmm2 for 30 days. Experiments were conducted in thermostatted cylindrical glass cells containing 20 ml of electrolyte. The cathodes were placed in the centre of cells, being surrounded by spiral Pt anodes of 4cm diameter x 4 cm height. The volume of the electrolyte was maintained constant by addition of controlled amounts of solution after sampling for tritium analysis. The temperature was always kept constant at 250°C. Gases evolved during electrolyses were vented through a glass rod placed in the top of the cells. Tritium determination
Tritium specific activity on samples of the electrolyte was determined using a Wallac Quantulus 1220 liquid scintillation detector. A 1 ml aliquot was periodically withdrawn from the cell and thoroughly mixed with 9ml of a high efficiency water soluble scintillation cocktail (Optiphase Hisafe 3, LKB) in a 20 ml polyethylene vial. Samples were left to stabilize in the detector chamber for at least 24 h and counted for a 60 min period. The tritium efficiency of the system was found to be 0.38. No chemiluminescence or significant quenching was observed in any of the samples. Analysis of products accumulated in the cathodes
The products accumulated near the surface of cathodes used in D20 and H,O electrolysis experiments were analysed from SIMS spectra obtained with an Atomika A-DIDA 3000-30 secondary ion microscope, using 0: with 6 keV energy as primary ion beam. A Jeol JSM 840 scanning electron microscope (SEM) with 20 keV accelerating voltage was used to determine the depth of the 0: attack by SIMS. The overall Li and Pt contents for Pd and Ti sheets of 0.25 cm2 area and for Ti rods of 0.5 cm height, before and after electrolysis, were respectively obtained with a Varian 875 atomic absorption spectroscope and a Jobin Yvon JY38VHR inductively coupled argon plasma spectroscope, after dissolving the metals with HCl acid. Identification of gases absorbed in Pd cathodes during electrolysis
Open cylindrical teflon cells, sealed with Pd foils at the bottom, were used in these experiments. Pd foils (99.9% purity) of 1 cm2 x 0.025 mm thick were supplied by Alfa. Cells contained 3 ml of 0.1 M LiOD solution and a Pt anode. Electrolyses were run galvanostatically at 7&lOOmA cmm2 for periods of 3&60 min, at room temperature. Gases absorbed by the cathodes during the electrolysis were extracted through their sides by means of a vacuum pump and further detected with a Quadruvac 100 quadrupolar mass spectrometer, which allowed simultaneous tracking of the evolution of the partial pressure of ions formed from desorbed species with m/e ratios
from 2 to 6 against the electrolysis real-time. Results were interpreted taking into account those obtained in the electrolyses of 50% (u/v) D,G-H,O mixtures containing 0.1 M LiOH. RESULTS
AND DISCUSSION
Tritium analysis for electrolysed D,O solutions andfor cathodes
A mean tritium specific activity of 0.077 f 0.008 Bq ml-’ was obtained for all H20 solutions before and during their electrolyses with Pd, Ti and Pt cathodes at 5 mA cme2. The mean tritium specific activities, aor for the two 0.1 M LiOD solutions studied were 280 f 5 and 817 f 12 (after addition of tritium) Bq ml-‘. In all D20 electrolyses carried out with Pd, Ti and Pt cathodes, at low and high current densities, a gradual increase of the tritium specific activity of the electrolyte at constant volume, a,,“, was always found. The a,, value (in Bq ml-‘) for the ith determination during a given electrolysis was calculated as ai,”= $ k Vi + a,( VO- Vi)], 0
(1)
where Vi is the volume of the electrolyte with tritium specific activity ai (in Bq ml-‘), and V,,the volume of the initial D,O solution with tritium specific activity aO. The loss of volume during electrolysis was practically wholly due to D,O decomposition (within a 6% error). The increase in a,,+with time for all cathodes was explained from the expected tritium enrichment in open cell, according to the equation[5]
, (2)
where S&r denotes the observed tritiumdeuterium separation factor of the cathode, r the electrolysis rate, t the electrolysis time and ND the number of atoms of deuterium present in the solution. Using equation (2) Sobsvalues of 0.4-0.6 for Pd and Ti and of 0.55-0.75 for Pt were obtained to fit the experimental increase of the a,.,/a, ratio with t. The change of the total net tritium activity of the electrolyte, Ai (ie the balance between the total observed tritium and the total blank activity) with time was also studied. For the ith determination in a given electrolysis, Ai (in Bq) was calculated as ,ml Ai = a, V, - a0 VO + c (a,Uj.allq - aoq 1, (3) ,=I where vj,.,iqis the volume of the aliquot of electrolyte sampled to measure the tritium specific activity a, in the jth determination, and vj the corresponding volume of fresh D,O added to keep the electrolyte volume constant at VO. Figure 1 shows that Ai for D,O electrolyses carried out with Pd, Ti and Pt cathodes at 5 mA cmm2 always decreases linearly with time after a few days from their beginning. Similar Ai us t plots were obtained for cells running at high current densities. The linear decrease of Ai indicates that tritium is discharged at a constant rate during electrolysis, as assumed in equation (2), excluding a tritium production in the electrolyte. The higher tritium release rate (the slope
D,O electrolysis with Pd and Ti cathodes
217
activity, indicating the presence of a small amount of T into Pd cathodes. This can be explained if the tritium adsorbed on its surface, proceeding from the cathodic discharge of the species pre-existing in the electrolyte, diffuses slowly to its bulk. Li and R contents of cathodes
-3000
I 20
I 10
0
Table 1 summarizes the overall Li and Pt contents obtained for Pd and Ti cathodes used in the different electrolysis experiments. From comparison with blank determinations, it can be established that small quantities of Li are incorporated into both cathodes after prolonged electrolyses. It is also found that a higher amount of Li is accumulated into Ti than into Pd under the same experimental conditions, whereas the Li content of both cathodes increases as long as the current density raises. These findings can be explained if Li electrodeposited on metal surfaces from the reduction of Li+ cations present in solution is slowly incorporated in the bulk of metals by diffusion, increasing its electrodeposition rate by raising the current density. Small quantities of Pt proceeding from anode are electrodeposited on Pd, as well as on electrolytic Ti, with a rate which is increased by raising the current density (Table 1). Surprisingly, Pt is not electrodeposited on Ti rods at low current densities.
I 30
time/days
Fig. 1. Plot of the total net tritium activity of the electrolyte [given by equation (3)] us time for electrolysis of a 0.1 M LiOD solution with an initial tritium activity of 280 f 5 Bq ml-‘, at cathodes of (0) Pd, (A) Ti rod, (W) Pt; at 5 mA cm-’ and at 25.0%
of the Ai us t plot) for Pt with respect to that of Pd and Ti (see Fig. 1) is consistent with the higher S,, value found for Pt from equation (2). Several Pd and Ti cathodes previously charged either at low or high current densities were thoroughly rinsed with bidistilled water and further introduced in cells containing 20 ml of 0.1 M LiOH, which were then run at 100 mA cmW2for 24 h. After this, all species (D and T) previously contained in cathodes were completely displaced by hydrogen and transferred to the electrolyte, as confirmed by SIMS. The tritium activity of these electrolytes was determined and compared with that of the initial 0.1 M LiOH solution. Electrolytes proceeding from Ti cells did not show any increase in tritium activity, as expected if this species is not accumulated into Ti during D,O electrolysis. Electrolytes of Pd cells, however, showed increases of 40 to 60 Bq in tritium
SIMS analysis of species accumulated near the surface of cathodes SIMS depth profiles in the form intensity (in counts s-i) us sputter time (directly proportional to the depth) were simultaneously recorded for the positive or negative secondary ions generated until a m/e ratio .of 7 from the 0: attack on Pd and Ti cathodes was reached. Figure 2 shows typical depth profiles for positive secondary ions detected in a Pd cathode. Further examination of all cathodes by SEM allowed the determination of the penetration depth as about 1 pm after 30min of 0: attack by SIMS. Positive and negative secondary ions with a m/e ratio of 1, corresponding to H+ and H- ions, were always detected. In addition, positive secondary ions
Table 1. Lithium and platinum content obtained for Pd and Ti cathodes before and after the electrolysis experiments
Sample l$ 2 3 4 5 6 73 8 9 lO$ 11 12
Cathode Pd Pd Pd Pd Pd Pd Ti rod Ti rod Ti rod Ti sheet Ti sheet Ti sheet
Electrolyte 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M
Current density (mA cm-*)
Duration (days)
Li content* (ppm)
30 30 15 16 15 30 30 15 15
1.6 1.9 2.2 3.4 3.6 4.4 13.5 16.5 17.3 21.6 28.3 30.3
LiOH LiOD$ LiOD# LiODy LiOD$ LiOH LiOD$ LiOq LiOD$
: 100 100 300 5 5 100 300
*Determined by atomic absorption analysis. TDetermined by inductively coupled argon plasma analysis. SBefore electrolysis. $Initial tritium specific activity of 280 + 5 Bq ml-‘. BInitial tritium specific activity of 817 f 12 Bq ml-‘.
Pt content?
hvm) 82 101 104 286 302 360 <4 7 8 <4 405 603
218
E. BRILLASet
I
0
I
10
I
20
sputter
tlmr
30
al.
I 20 ’
40
60
I min
Fig. 2. SIMS depth profiles for positive secondary ions with lower m/e ratio (number in each curve) detected in a Pd cathode after 15 days of electrolysis of a 0.1 M LiOD solution with an initial tritium specific activity of 280 f 5 Bq ml-‘, at lOOmA cm-* and at 250°C.
with m/e values of 6 and 7, associated to 6Li+ and ‘Li+ ions, respectively, were also detected. Spectra for ions with m/e = 7 always showed an intensity cu 12.5 times greater than that of ions with m/e = 6 (see Fig. 2) in accordance with the ratio of natural abundances between ’ Li and 6Li. Table 2 summarizes relative intensities (with respect to that of H+ and Hions) of secondary ions recorded for Pd and Ti cathodes after a sputter time of 20min. Table 2 shows that after H,O electrolysis, the intensities of spectra of H + and H - ions obtained for Pd and Ti cathodes were ca 20 times higher than those of their virgin electrodes, as expected if hydrogen is absorbed during electrolysis. High increases of ca 20 and 500 times in intensity with respect to that of virgin electrodes were found for the spectra of Li+ ions recorded for Pd and Ti cathodes, respectively, indicating a preferential accumulation of Li near to
Fig. 3. Evolution of the partial pressure for ions with m/e values of 2, 3 and 4 (number in each curve) detected from species desorbed by a Pd cathode used in the electrolysis of a 0.1 M LiOD solution with a tritium specific activity of 280 f 5 Bq ml-’ at 70 mA cm-*. Solid lines are recorded during the electrolytic charge of Pd and dashed lines during its discharge after stopping the electrolysis.
the electrode surface. Positive ions with m/e values of 2 and 3, ascribed to Ht and Ht ions, respectively, were also detected for Pd cathodes employed in H,O electrolysis. All cathodes used in D20 electrolysis showed SIMS spectra for positive and negative secondary ions with m/e ratios of 2, which can be respectively ascribed to D+ and D- ions proceeding from deuterium absorbed during electrolysis. The relative intensity of D+ ions for cathodes charged at high current densities is much higher than those charged at 5 mA cm-2 (see Table 2), as expected if a larger amount of deuterium is absorbed by Pd and Ti to yield the respective j-phase and y-phase. The H/D ratio near to the electrode surface was found to be between 3 and 0.3, a value much higher than 0.02-
Table 2. Relative intensities* for positive and negative secondary ions with lower m/e ratio detected by SIMS for Pd and Ti cathodes before and after the electrolysis experiments m/e (positive ions)f Sample? 1 2 3 4 5 6 7 8 9 10 11 12
2 1 x lo-* 3 x IO-’ 3 1 2 5 x 10-I 3x 10-i 2 x 10-l
3 3x10-3 7 x 10-r 4 x IO-2 2 x 10-2 3 x IO-2 -
4
5 6 2 4
x IO-’ x lo-* X 10-2 x 10-2
1 -
I
tIme/min
-
m/e (negative ions) 5
1 3 x IO-’ 6 x lo-’ 4 X 10-r 5 x lo-’ -
6 9 x 10-I 1 3 1 1 2 6 x IO-* 3 5 4 x 10-r 2 2
2 5 x 10-r 1 5 x 10-l 1 1 x IO-’ 9 x IO-2 8 x IO-*
*Relative intensity with respect to that of m/e = 1 (corresponding to H+ for positive secondary ions and to H- for negative secondary ions) from SIMS analysis after a sputter time of 20 min. tExperimenta1 conditions described in Table 1. SPositive secondary ions of m/e = 7 are observed in all cases (see Fig. 2), with an intensity close to 12.5 times greater than that of m/e = 6.
DzO electrolysis with Pd and Ti cathodes
0.04 determined by Williams et a/.[51 for the overall H/D ratio in Pd after many weeks of D,O electrolysis, indicating that H is preferentially absorbed in the surface layer of Pd and Ti cathodes during electrolysis. The results in Table 2 are also indicative of a preferential accumulation of Li near to the surface of all Ti cathodes used in D,O electrolysis. For Pd cathodes, however, the intensities of depth profiles for Li+ ions were only slightly higher than those of the virgin electrode, as expected if the incorporated lithium is uniformly distributed in them. Inspection of Table 2 confirms that tritium is not accumulated into Ti, because positive ions with a m/e ratio of 3, which could be ascribed to T+ ions, are not detected. All Pd cathodes after D20 electrolysis showed positive ions with m/e values of 3, 4 and 5 (see Fig. 2). Since tritium is absorbed by Pd and H2+ and H: ions are detected for Pd cathodes saturated with hydrogen, T+ and DH+ ions can be ascribed to m/e = 3, TH+, D2f and DHZ ions to m/e = 4, and TD+ , D, H + and TH: ions to m/e = 5. Data reported in Table 2 for samples 3-6 show that an increase of 10 times in the amount of absorbed deuterium causes increases of 6, 10 and 2 times in the relative intensity of ions with respective m/e values of 3,4 and 5. This indicates a high generation probability of ions with m/e values of 3 and 4 from D absorbed in Pd, suggesting that DH+ and D: ions are their respective predominant species. The lower increase in relative intensity for m/e = 5 can be explained if the generation of this type of ions is limited by the amount of T absorbed in Pd, DT+ ion being the predominant species. Pd and Ti cathodes previously charged with deuterium and further electrolysed in 0.1 M LiOH solutions at 100 mA cm-’ for 24 h showed the same SIMS spectra obtained for cathodes only used in H20 electrolysis, as is expected if all species previously contained in them are displaced by hydrogen and transferred to the electrolyte. Identification sis real-time
of gases absorbed
in
Pd during electroly-
Ions with m/e values of 2, 3 and 4 were always detected by mass spectrometry from gases desorbed by a Pd cathode during the electrolysis of DzO and 50% (u/v) DrO-H,O mixtures. Typical evolutions of the partial pressure of these three species against time during the charge of Pd by DzO electrolysis are shown as solid lines in Fig. 3. As can be seen, after 15-25 min of electrolysis, the partial pressure of all ions remains practically constant, indicating that desorption of species has reached saturation. After stopping the electrolysis, the partial pressure for all ions decreases rapidly with time (see dashed lines in Fig. 3), indicating a fast discharge of the cathode. Ions with m/e values of 2, 3 and 4 can be respectively
219
ascribed to Hz, DH+ and D: ions, which are formed by ionization of the corresponding Hz, DH and D, molecules, generated by combination of H and D atoms extracted from the electrode. In the D,O electrolysis experiments, moreover, a value of ca 1.5 x lo-l4 bar was always determined for the saturation partial pressure of ions with m/e = 5, which is at least four orders of magnitude lower than that obtained for D; ions. Since triatomic species of the type DrH+ (m/e = 5) have not been observed in the electrolysis of 50% (U/Y) D,O-H,O mixtures, ions of m/e = 5 detected in D,O electrolysis should be preferentially ascribed to TD+ ions proceeding from desorbed T and D. The small saturation partial pressure found for TD+ ions is indicative of small amounts of tritium into Pd, in agreement with results obtained from tritium analysis of this cathode. Acknowledgements-The
authors are grateful to DGICYT
(Ministerio Educacibn y Ciencia, Spain) for financial support (grant APC-28/89), to F. Lopez (Dept. Fisica Aplicada i Electronica, Universitat de Barcelona) for his help in the interpretation of SIMS results. to Prof. P. Salvador (CSIC. Madrid) for the preparation of electrolytic Ti sheets’and to the Serveis Cientific-Ttcnics de la Universitat de Barcelona for assistance in the analysis of Li and Pt contents of cathodes.
REFERENCES 1. M. Fleischmann, S. Pons and M. Hawkins, J. electrounal. Chem. 261, 301 (1989): 263. 187 (1989). 2. S. E. Jones, J. B. Czirr, ‘D. L: Decker, d. L. Jensen, J. M. Thorne, S. F. Taylor and J. Rafelski, Nature 338, 737 (1989). 3. A. J. Appleby and S. Srinivisan, Proc. of the Workshop on Cold Fusion Phenomena,
Santa Fe, NM, May 25-28,
1989. 4. N. J. C. Packham, K. L. Wolf, J. C. Wass, R. C. Kainthla and J. G’M. Bockris, J. electroanal. Chem. 270, 451 (1989). 5. D. E. Williams. D. J. S. Findlav. D. H. Craston, M. R. Send, M. Bailey, S. Croft, B. W. Hooton, C. P. Jones, A. R. J. Kucemak, J. A. Mason and R. I. Taylor, Nature 342, 375 (1989).
6. M. Keddam, Electrochim. Acta 34, 995 (1989). 7. J. W. Schultze, U. Konig, A. Hochfeld, C. Van Calker and W. Kies, Electrochim. Acta 34, 1289 (1989). 8. F. Dalard, M. Ulmann, J. Augustynski and P. Selvam, J. elecrroanal. Chem. 270, 445 (1989). 9. R. D. Armstrong, E. A. Charles, I. Fells, L. Molyneux and M. Todd, Electrochim. Acta 34, 1319 (1989). 10. R. D. Armstrong, E. A. Charles, I. Fells, L. Molyneux and M. Todd, J. eleciroanal. Chem. 272, 293 (1989). 11. J. Ch&ne and A. M. Brass. J. electroanal. Chem. 280,199 (1990). 12. P. A. Rock, W. H. Fink, D. A. McQuarrie, D. H. Volman and Y. F. Hung. J. electroanal. Chem. 293, 261 (1990).
37, No.
2, pp. 215-219,
1992 0
PRODUCT
ANALYSIS FROM D,O ELECTROLYSIS Pd AND Ti CATHODES
0013-4686/92 $5.00 + 0.00 1991. krgamon Press pk.
WITH
E. BIULLAS,*J. E~TEVE,~G. SARDIN,~J. CASADO,$X. D~I+&NECH$and J. A. S;~NCHEZ-CABEZA$ *Department de Quimica Fisica, Facultat de Quimica, Universitat de Barcelona, Marti i Franqubs 1, 08028-Barcelona, Spain de Fisica Aplicada i Electronica, Facultat de Fisica, Universitat de Barcelona, Marti i Franques 1, 08028-Barcelona, Spain tDepartament de Quimica, Universitat Autonoma de Barcelona, 08193Bellaterra (Barcelona), Spain #Servei de Fisica de les Radiations, Departament de Fisica, Universitat Autonoma de Barcelona, 08193-Bellaterra (Barcelona), Spain tkpartament
(Received 27 March 1991; in revisedform 21 May 1991)
Abstract--The enrichment of tritium in the electrolyte and incorporation of T, Li and Pt in cathodes during the electrolysis of 0.1 M LiOD solutions with Pd and Ti cathodes in open cells have been studied. All electrolytes show an increase in their tritium activity which is explained by considering values for the T-D separation factor of all cathodes lower than 1. Accumulation of small amounts of T in the Pd bulk, proceeding from the absorption of the species pre-existing in the electrolyte, has been detected by electrolytic transfer of accumulated tritium to a 0.1 M LiOH solution, as well as by extraction of gases absorbed in the cathode, which were identified by mass spectrometry. Small quantities of Li and Pt are also incorporated in Pd and Ti cathodes, which increase by raising the current density. SIMS analysis of both cathodes show a preferential accumulation of Li and H in their surface layers and confirms the absence of T in Ti. Key words: tritium analysis, D,O electrolysis, Pd cathode, Ti cathode, SIMS.
INTRODUCTION After the recent papers of Fleischmann et al.[l] and Jones et a1.[2] dealing with the existence of D-D fusion reactions during the electrolysis of deuterated solutions with Pd and Ti cathodes at room temperature, several research groups[3-121 have questioned the species generated as reaction products. For Pd open cells containing 0.1 M LiOD, Fleischmann et al.[l] observed a small generation of tritium in the electrolyte, considering that such species proceeds from a D-D fusion reaction taking place near to the electrode surface. A similar tritium production has been reported by several authorsp, 4, lo], but it has been rejected by Williams et a1.[5], who consider that during the electrolysis only the tritium pre-existing in solution is concentrated due to the different discharge rate of D and T on Pd. Precise measurements of the tritium content of solution and an adequate interpretation of the expected tritium enrichment of the electrolyte are then required to check the possible generation of this species by cathodic discharge of deuterium on Pd and Ti. During the electrolysis, large amounts of deuterium are absorbed by Pd and Ti cathodes to yield the respective fl-Pd + D and y -Ti + D phases [S-7, 121. However, less is known about other species either incorporated or possibly generated in cathodes. In this way, ChiZne and Brass[l l] reported a small tritium production into Pd during the electrolysis of 0.1 M LiOD in open cells at high current densities. Several authors[7,8] have proposed a slow diffusion of Li+ ions into the bulk of Pd, although Williams
et a[.[51 reported that lithium is only present in its surface layer (about 1 pm of depth). In this paper, we present results of a study on the electrolysis products of 0.1 M LiOD solutions with Pd and Ti cathodes in open cells carried out to gain a better understanding of species either incorporated to cathodes or produced in them, such as tritium, lithium and platinum, as well as to investigate the tritium enrichment of the electrolyte. Similar experiments using a Pt cathode, where no nuclear events are expected, have also been performed. EXPERIMENTAL Electrochemical cells
0.1 M LiOD solutions were prepared by addition of lithium metal to 99.95% D,O and 0.1 M LiOH solutions by adding lithium hydroxide to twice distilled light water. All the chemicals were Merck, p.a. A 0.1 M LiOD solution with a tritium content cu three times higher than that of pure D,O was prepared by addition of standardized tritium labelled water, supplied by Amersham. Pd and Pt sheets of 99.9% purity (SEMP, Madrid) with respective dimensions of 0.1 x 1.5 x 3 cm and 0.25 mm x 1.5 cm x 3 cm, and 1.4 cm diameter x 3 cm Ti rods of 99.9% purity (Inagasa, Barcelona), were employed as cathodes in the electrolysis at low current density. In the experiments at high current densities, Pd and electrolytic Ti (99.9% purity) sheet cathodes of 0.1 cm x 0.5 cm x 0.5 cm were used. Electrical connections were made using a wire 215
216
E. BRILLASetal.
of the same pure metal spot-welded to the corresponding cathode. Pt wires (99.9% purity) of 0.5 mm diameter x 20 cm were used as anodes. D,O electrolyses were carried out either at a low current density of 5 mA cmm2 for periods up to 30 days or at high current densities of 100 and 300mA cmm2 for 15 to 16 days, whereas H,O electrolyses were run galvanostatically at 5 mA cmm2 for 30 days. Experiments were conducted in thermostatted cylindrical glass cells containing 20 ml of electrolyte. The cathodes were placed in the centre of cells, being surrounded by spiral Pt anodes of 4cm diameter x 4 cm height. The volume of the electrolyte was maintained constant by addition of controlled amounts of solution after sampling for tritium analysis. The temperature was always kept constant at 250°C. Gases evolved during electrolyses were vented through a glass rod placed in the top of the cells. Tritium determination
Tritium specific activity on samples of the electrolyte was determined using a Wallac Quantulus 1220 liquid scintillation detector. A 1 ml aliquot was periodically withdrawn from the cell and thoroughly mixed with 9ml of a high efficiency water soluble scintillation cocktail (Optiphase Hisafe 3, LKB) in a 20 ml polyethylene vial. Samples were left to stabilize in the detector chamber for at least 24 h and counted for a 60 min period. The tritium efficiency of the system was found to be 0.38. No chemiluminescence or significant quenching was observed in any of the samples. Analysis of products accumulated in the cathodes
The products accumulated near the surface of cathodes used in D20 and H,O electrolysis experiments were analysed from SIMS spectra obtained with an Atomika A-DIDA 3000-30 secondary ion microscope, using 0: with 6 keV energy as primary ion beam. A Jeol JSM 840 scanning electron microscope (SEM) with 20 keV accelerating voltage was used to determine the depth of the 0: attack by SIMS. The overall Li and Pt contents for Pd and Ti sheets of 0.25 cm2 area and for Ti rods of 0.5 cm height, before and after electrolysis, were respectively obtained with a Varian 875 atomic absorption spectroscope and a Jobin Yvon JY38VHR inductively coupled argon plasma spectroscope, after dissolving the metals with HCl acid. Identification of gases absorbed in Pd cathodes during electrolysis
Open cylindrical teflon cells, sealed with Pd foils at the bottom, were used in these experiments. Pd foils (99.9% purity) of 1 cm2 x 0.025 mm thick were supplied by Alfa. Cells contained 3 ml of 0.1 M LiOD solution and a Pt anode. Electrolyses were run galvanostatically at 7&lOOmA cmm2 for periods of 3&60 min, at room temperature. Gases absorbed by the cathodes during the electrolysis were extracted through their sides by means of a vacuum pump and further detected with a Quadruvac 100 quadrupolar mass spectrometer, which allowed simultaneous tracking of the evolution of the partial pressure of ions formed from desorbed species with m/e ratios
from 2 to 6 against the electrolysis real-time. Results were interpreted taking into account those obtained in the electrolyses of 50% (u/v) D,G-H,O mixtures containing 0.1 M LiOH. RESULTS
AND DISCUSSION
Tritium analysis for electrolysed D,O solutions andfor cathodes
A mean tritium specific activity of 0.077 f 0.008 Bq ml-’ was obtained for all H20 solutions before and during their electrolyses with Pd, Ti and Pt cathodes at 5 mA cme2. The mean tritium specific activities, aor for the two 0.1 M LiOD solutions studied were 280 f 5 and 817 f 12 (after addition of tritium) Bq ml-‘. In all D20 electrolyses carried out with Pd, Ti and Pt cathodes, at low and high current densities, a gradual increase of the tritium specific activity of the electrolyte at constant volume, a,,“, was always found. The a,, value (in Bq ml-‘) for the ith determination during a given electrolysis was calculated as ai,”= $ k Vi + a,( VO- Vi)], 0
(1)
where Vi is the volume of the electrolyte with tritium specific activity ai (in Bq ml-‘), and V,,the volume of the initial D,O solution with tritium specific activity aO. The loss of volume during electrolysis was practically wholly due to D,O decomposition (within a 6% error). The increase in a,,+with time for all cathodes was explained from the expected tritium enrichment in open cell, according to the equation[5]
, (2)
where S&r denotes the observed tritiumdeuterium separation factor of the cathode, r the electrolysis rate, t the electrolysis time and ND the number of atoms of deuterium present in the solution. Using equation (2) Sobsvalues of 0.4-0.6 for Pd and Ti and of 0.55-0.75 for Pt were obtained to fit the experimental increase of the a,.,/a, ratio with t. The change of the total net tritium activity of the electrolyte, Ai (ie the balance between the total observed tritium and the total blank activity) with time was also studied. For the ith determination in a given electrolysis, Ai (in Bq) was calculated as ,ml Ai = a, V, - a0 VO + c (a,Uj.allq - aoq 1, (3) ,=I where vj,.,iqis the volume of the aliquot of electrolyte sampled to measure the tritium specific activity a, in the jth determination, and vj the corresponding volume of fresh D,O added to keep the electrolyte volume constant at VO. Figure 1 shows that Ai for D,O electrolyses carried out with Pd, Ti and Pt cathodes at 5 mA cmm2 always decreases linearly with time after a few days from their beginning. Similar Ai us t plots were obtained for cells running at high current densities. The linear decrease of Ai indicates that tritium is discharged at a constant rate during electrolysis, as assumed in equation (2), excluding a tritium production in the electrolyte. The higher tritium release rate (the slope
D,O electrolysis with Pd and Ti cathodes
217
activity, indicating the presence of a small amount of T into Pd cathodes. This can be explained if the tritium adsorbed on its surface, proceeding from the cathodic discharge of the species pre-existing in the electrolyte, diffuses slowly to its bulk. Li and R contents of cathodes
-3000
I 20
I 10
0
Table 1 summarizes the overall Li and Pt contents obtained for Pd and Ti cathodes used in the different electrolysis experiments. From comparison with blank determinations, it can be established that small quantities of Li are incorporated into both cathodes after prolonged electrolyses. It is also found that a higher amount of Li is accumulated into Ti than into Pd under the same experimental conditions, whereas the Li content of both cathodes increases as long as the current density raises. These findings can be explained if Li electrodeposited on metal surfaces from the reduction of Li+ cations present in solution is slowly incorporated in the bulk of metals by diffusion, increasing its electrodeposition rate by raising the current density. Small quantities of Pt proceeding from anode are electrodeposited on Pd, as well as on electrolytic Ti, with a rate which is increased by raising the current density (Table 1). Surprisingly, Pt is not electrodeposited on Ti rods at low current densities.
I 30
time/days
Fig. 1. Plot of the total net tritium activity of the electrolyte [given by equation (3)] us time for electrolysis of a 0.1 M LiOD solution with an initial tritium activity of 280 f 5 Bq ml-‘, at cathodes of (0) Pd, (A) Ti rod, (W) Pt; at 5 mA cm-’ and at 25.0%
of the Ai us t plot) for Pt with respect to that of Pd and Ti (see Fig. 1) is consistent with the higher S,, value found for Pt from equation (2). Several Pd and Ti cathodes previously charged either at low or high current densities were thoroughly rinsed with bidistilled water and further introduced in cells containing 20 ml of 0.1 M LiOH, which were then run at 100 mA cmW2for 24 h. After this, all species (D and T) previously contained in cathodes were completely displaced by hydrogen and transferred to the electrolyte, as confirmed by SIMS. The tritium activity of these electrolytes was determined and compared with that of the initial 0.1 M LiOH solution. Electrolytes proceeding from Ti cells did not show any increase in tritium activity, as expected if this species is not accumulated into Ti during D,O electrolysis. Electrolytes of Pd cells, however, showed increases of 40 to 60 Bq in tritium
SIMS analysis of species accumulated near the surface of cathodes SIMS depth profiles in the form intensity (in counts s-i) us sputter time (directly proportional to the depth) were simultaneously recorded for the positive or negative secondary ions generated until a m/e ratio .of 7 from the 0: attack on Pd and Ti cathodes was reached. Figure 2 shows typical depth profiles for positive secondary ions detected in a Pd cathode. Further examination of all cathodes by SEM allowed the determination of the penetration depth as about 1 pm after 30min of 0: attack by SIMS. Positive and negative secondary ions with a m/e ratio of 1, corresponding to H+ and H- ions, were always detected. In addition, positive secondary ions
Table 1. Lithium and platinum content obtained for Pd and Ti cathodes before and after the electrolysis experiments
Sample l$ 2 3 4 5 6 73 8 9 lO$ 11 12
Cathode Pd Pd Pd Pd Pd Pd Ti rod Ti rod Ti rod Ti sheet Ti sheet Ti sheet
Electrolyte 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M
Current density (mA cm-*)
Duration (days)
Li content* (ppm)
30 30 15 16 15 30 30 15 15
1.6 1.9 2.2 3.4 3.6 4.4 13.5 16.5 17.3 21.6 28.3 30.3
LiOH LiOD$ LiOD# LiODy LiOD$ LiOH LiOD$ LiOq LiOD$
: 100 100 300 5 5 100 300
*Determined by atomic absorption analysis. TDetermined by inductively coupled argon plasma analysis. SBefore electrolysis. $Initial tritium specific activity of 280 + 5 Bq ml-‘. BInitial tritium specific activity of 817 f 12 Bq ml-‘.
Pt content?
hvm) 82 101 104 286 302 360 <4 7 8 <4 405 603
218
E. BRILLASet
I
0
I
10
I
20
sputter
tlmr
30
al.
I 20 ’
40
60
I min
Fig. 2. SIMS depth profiles for positive secondary ions with lower m/e ratio (number in each curve) detected in a Pd cathode after 15 days of electrolysis of a 0.1 M LiOD solution with an initial tritium specific activity of 280 f 5 Bq ml-‘, at lOOmA cm-* and at 250°C.
with m/e values of 6 and 7, associated to 6Li+ and ‘Li+ ions, respectively, were also detected. Spectra for ions with m/e = 7 always showed an intensity cu 12.5 times greater than that of ions with m/e = 6 (see Fig. 2) in accordance with the ratio of natural abundances between ’ Li and 6Li. Table 2 summarizes relative intensities (with respect to that of H+ and Hions) of secondary ions recorded for Pd and Ti cathodes after a sputter time of 20min. Table 2 shows that after H,O electrolysis, the intensities of spectra of H + and H - ions obtained for Pd and Ti cathodes were ca 20 times higher than those of their virgin electrodes, as expected if hydrogen is absorbed during electrolysis. High increases of ca 20 and 500 times in intensity with respect to that of virgin electrodes were found for the spectra of Li+ ions recorded for Pd and Ti cathodes, respectively, indicating a preferential accumulation of Li near to
Fig. 3. Evolution of the partial pressure for ions with m/e values of 2, 3 and 4 (number in each curve) detected from species desorbed by a Pd cathode used in the electrolysis of a 0.1 M LiOD solution with a tritium specific activity of 280 f 5 Bq ml-’ at 70 mA cm-*. Solid lines are recorded during the electrolytic charge of Pd and dashed lines during its discharge after stopping the electrolysis.
the electrode surface. Positive ions with m/e values of 2 and 3, ascribed to Ht and Ht ions, respectively, were also detected for Pd cathodes employed in H,O electrolysis. All cathodes used in D20 electrolysis showed SIMS spectra for positive and negative secondary ions with m/e ratios of 2, which can be respectively ascribed to D+ and D- ions proceeding from deuterium absorbed during electrolysis. The relative intensity of D+ ions for cathodes charged at high current densities is much higher than those charged at 5 mA cm-2 (see Table 2), as expected if a larger amount of deuterium is absorbed by Pd and Ti to yield the respective j-phase and y-phase. The H/D ratio near to the electrode surface was found to be between 3 and 0.3, a value much higher than 0.02-
Table 2. Relative intensities* for positive and negative secondary ions with lower m/e ratio detected by SIMS for Pd and Ti cathodes before and after the electrolysis experiments m/e (positive ions)f Sample? 1 2 3 4 5 6 7 8 9 10 11 12
2 1 x lo-* 3 x IO-’ 3 1 2 5 x 10-I 3x 10-i 2 x 10-l
3 3x10-3 7 x 10-r 4 x IO-2 2 x 10-2 3 x IO-2 -
4
5 6 2 4
x IO-’ x lo-* X 10-2 x 10-2
1 -
I
tIme/min
-
m/e (negative ions) 5
1 3 x IO-’ 6 x lo-’ 4 X 10-r 5 x lo-’ -
6 9 x 10-I 1 3 1 1 2 6 x IO-* 3 5 4 x 10-r 2 2
2 5 x 10-r 1 5 x 10-l 1 1 x IO-’ 9 x IO-2 8 x IO-*
*Relative intensity with respect to that of m/e = 1 (corresponding to H+ for positive secondary ions and to H- for negative secondary ions) from SIMS analysis after a sputter time of 20 min. tExperimenta1 conditions described in Table 1. SPositive secondary ions of m/e = 7 are observed in all cases (see Fig. 2), with an intensity close to 12.5 times greater than that of m/e = 6.
DzO electrolysis with Pd and Ti cathodes
0.04 determined by Williams et a/.[51 for the overall H/D ratio in Pd after many weeks of D,O electrolysis, indicating that H is preferentially absorbed in the surface layer of Pd and Ti cathodes during electrolysis. The results in Table 2 are also indicative of a preferential accumulation of Li near to the surface of all Ti cathodes used in D,O electrolysis. For Pd cathodes, however, the intensities of depth profiles for Li+ ions were only slightly higher than those of the virgin electrode, as expected if the incorporated lithium is uniformly distributed in them. Inspection of Table 2 confirms that tritium is not accumulated into Ti, because positive ions with a m/e ratio of 3, which could be ascribed to T+ ions, are not detected. All Pd cathodes after D20 electrolysis showed positive ions with m/e values of 3, 4 and 5 (see Fig. 2). Since tritium is absorbed by Pd and H2+ and H: ions are detected for Pd cathodes saturated with hydrogen, T+ and DH+ ions can be ascribed to m/e = 3, TH+, D2f and DHZ ions to m/e = 4, and TD+ , D, H + and TH: ions to m/e = 5. Data reported in Table 2 for samples 3-6 show that an increase of 10 times in the amount of absorbed deuterium causes increases of 6, 10 and 2 times in the relative intensity of ions with respective m/e values of 3,4 and 5. This indicates a high generation probability of ions with m/e values of 3 and 4 from D absorbed in Pd, suggesting that DH+ and D: ions are their respective predominant species. The lower increase in relative intensity for m/e = 5 can be explained if the generation of this type of ions is limited by the amount of T absorbed in Pd, DT+ ion being the predominant species. Pd and Ti cathodes previously charged with deuterium and further electrolysed in 0.1 M LiOH solutions at 100 mA cm-’ for 24 h showed the same SIMS spectra obtained for cathodes only used in H20 electrolysis, as is expected if all species previously contained in them are displaced by hydrogen and transferred to the electrolyte. Identification sis real-time
of gases absorbed
in
Pd during electroly-
Ions with m/e values of 2, 3 and 4 were always detected by mass spectrometry from gases desorbed by a Pd cathode during the electrolysis of DzO and 50% (u/v) DrO-H,O mixtures. Typical evolutions of the partial pressure of these three species against time during the charge of Pd by DzO electrolysis are shown as solid lines in Fig. 3. As can be seen, after 15-25 min of electrolysis, the partial pressure of all ions remains practically constant, indicating that desorption of species has reached saturation. After stopping the electrolysis, the partial pressure for all ions decreases rapidly with time (see dashed lines in Fig. 3), indicating a fast discharge of the cathode. Ions with m/e values of 2, 3 and 4 can be respectively
219
ascribed to Hz, DH+ and D: ions, which are formed by ionization of the corresponding Hz, DH and D, molecules, generated by combination of H and D atoms extracted from the electrode. In the D,O electrolysis experiments, moreover, a value of ca 1.5 x lo-l4 bar was always determined for the saturation partial pressure of ions with m/e = 5, which is at least four orders of magnitude lower than that obtained for D; ions. Since triatomic species of the type DrH+ (m/e = 5) have not been observed in the electrolysis of 50% (U/Y) D,O-H,O mixtures, ions of m/e = 5 detected in D,O electrolysis should be preferentially ascribed to TD+ ions proceeding from desorbed T and D. The small saturation partial pressure found for TD+ ions is indicative of small amounts of tritium into Pd, in agreement with results obtained from tritium analysis of this cathode. Acknowledgements-The
authors are grateful to DGICYT
(Ministerio Educacibn y Ciencia, Spain) for financial support (grant APC-28/89), to F. Lopez (Dept. Fisica Aplicada i Electronica, Universitat de Barcelona) for his help in the interpretation of SIMS results. to Prof. P. Salvador (CSIC. Madrid) for the preparation of electrolytic Ti sheets’and to the Serveis Cientific-Ttcnics de la Universitat de Barcelona for assistance in the analysis of Li and Pt contents of cathodes.
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