Radiochemical measurements of tritium during heavy water electrolysis at palladium cathodes in closed cells

175

J. Electroanal. Chem., 312 (1991) 175-184 Elsevier Sequoia S.A., Lausanne

Radiochemical measurements of tritium during heavy water electrolysis at palladium cathodes in closed cells Dennis

A. Corrigan,

Brian K. Schwemmin

General Motors Research Laboratories,

and Eric W. Schneider

Warren, MI 48090 (USA)

(Received 2 May 1991)

Abstract

Closed cell electrolysis of heavy water at palladium cathodes was maintained for 100 days by the catalytic recombination of evolved gases. Closed cell operation eliminated several experimental complications including electrolytic isotope separation effects. This allowed for the detection of tritium accumulation at a level two orders of magnitude below that cited by Fleischmann, Pons, and Hawkins as evidence for cold fusion. Closed cell electrolysis of alkaline heavy water was performed in six cells with several variations in palladium cathode pretreatment, anode type, cell vessel material, electrolyte preparation, and current density. In each case, light water control cells were operated under otherwise identical conditions. Under conditions close to those under which cold fusion phenomena have been reported, radiochemical measurements have indicated no accumulation of tritium.

INTRODUCTION

The report of Fleischmann, Pons, and Hawkins (FP&H) [l] on electrochemically induced nuclear fusion of deuterium has stimulated a variety of calorimetric and nuclear measurements in search of “cold fusion.” Evidence cited by FP&H included the generation of excess heat, neutrons, and tritium. The present paper focuses on the possible generation of tritium. There have been persistent claims that tritium is generated during the electrolysis of heavy water at palladium cathodes [l-7]. However, there are several complications to experiments involving the open cells used in the cited work. First, there have been concerns about contamination. Open cells operated over extended periods require electrolyte replenishment through periodic additions of heavy water solvent which could lead to inadvertent tritium contamination. Second, tritium accumulation can occur through electrolytic separation effects [8-lo]. Third, tritium generated in an open system could escape measurement if evolved as a gas. Our present work includes radiochemical tritium measurements on closed cells where complications due to separation effects and inadvertent contamination are 0022-0728/91/$03.50

0 1991 - Elsevier Sequoia S.A.

TEFLON

TOP

RECOMBINATION CATALYST

ELECTFKXYTE

Pt

or Ni ANODE

Pd

CATHODE

CELL

CONTAINER

Fig. 1. Diagram of closed electrolysis cells showing platinum) anodes and recombination catalysts.

arrangement

of palladium

cathode,

nickel

(or

minimized, and the loss of gas phase tritium is prevented. Experimental conditions were chosen to be similar to those reporting anomalous tritium generation [l-7] and other cold fusion phenomena [ll-131. We took care to operate an otherwise identical control cell with light water for each test cell with heavy water. This provided information on the statistical or systematic deviations which could occur in selected cells. EXPERIMENTAL

Twelve closed electrolysis cells were constructed with palladium cathodes and nickel or platinum anodes. The cells were constructed from a cylindrical vessel sealed with rubber sealant to a PTFE cap through which electrode leads were inserted. Recombination of evolved gases was achieved on high surface area platinum materials positioned above the electrolyte level. These catalytic materials were cut from hydrogen fuel cell electrodes obtained from Hughes Aircraft Company. They were comprised of high surface area platinum on carbon bonded in a hydrophobic matrix. Several pieces of the electrode materials were arranged around the upper circumference of the cell vessel as diagrammed in Fig. 1. In each cell, the palladium cathode, 2 cm long by 1 mm in diameter (99.9% Pd Johnson and Matthey Lot 19622) was spot-welded to a platinum lead wire, which was shielded by PTFE shrink-tubing. The nickel or platinum anode surrounded the

177

TABLE

1

Cell variables a Cells

Pd Cathode

Anode

Vessel

Electrolyte b

Tl, T3, T5, T7, T9, Tll,

annealed palladized annealed annealed as received etched

Ni Ni Ni Ni Pt Pt

Ni Ni ss ss plastic plastic

Alfa Li *O Alfa Li,O Foote Li Foote Li; added T Cerac Li ,O Cerac Li ,O

T2 T4 T6 T8 TlO T12

a See text for details. b Source of Li in electrolyte. Extra tritium was added to the electrolytes of Cells ‘I7 and T8.

cathode in a tightly wound (3 mm spacing) cylindrical coil 1 cm in diameter by 2.5 cm long. Each cell contained 25 ml of electrolyte. The heavy water electrolytes were prepared from Cambridge Isotope Laboratories DLM4 D,O enriched to contain 99.8% D with an impurity tritium activity of about 15 disintegrations min-’ ml-’ (dpm/ml). The light water electrolytes were prepared from MilliQ-grade water. The twelve cells consisted of 6 pairs of heavy water and light water (control) cells. Variables included the palladium cathode (annealed, palladized, as-received, or acid etched), the counter electrode (nickel or platinum), the cell vessel (nickel, stainless steel, or polystyrene plastic), and the electrolyte (lithium source and tritium activity). In each pair of heavy water and light water cells, these variables were held constant. The variables for each cell are summarized in Table 1 and detailed below. In Cells Tl and T2, the palladium cathodes were annealed in vacuum at 900°C for 10 h, cooled in vacuum, exposed to argon gas upon recompression, and stored in heavy water (Cell T2) or light water (Cell Tl) solvent prior to use (within several days). The anodes were fashioned from Marz grade Ni wire (0.5 mm diameter) from Materials Research Corp. The cell vessels were constructed from Ni 270 (99.8% Ni). The 0.10 M LiOD (Cell T2) and 0.10 M LiOH (Cell Tl) electrolytes were prepared by dissolution of Alfa Products Li,O (95%) into the heavy water and light water solvents, respectively. Variables for Cells T3 and T4 were the same as in Cells Tl and T2, respectively, except for the palladium cathodes. Instead of annealing, the palladium cathodes were palladized; a high surface area (dendritic) palladium surface was obtained by cathodic polarization at 200 mA/cm’ for 20 s in a solution of 2% PdCl, in 0.1 M HCl. The palladized Pd cathodes were then rinsed and stored in heavy water (Cell T4) or light water (Cell T3) solvents prior to use. In Cells T5 and T6, the palladium cathodes were annealed and the anodes were made from Ni wire as in Cells Tl and T2. The cell vessels were constructed from stainless steel 316. The 0.10 M LiOD (Cell T6) and 0.10 M LiOH (Cell T5) electrolytes were prepared by reaction of Foote Mineral Company lithium metal (98%) with the heavy water and light water solvents, respectively. Variables in Cells T7 and T8 were the same as in Cells T5 and T6, respectively, except for the tritium activity in the solvents. The tritium activity of the LiOD + D,O

178

electrolyte in Cell T8 was increased to about 1000 dpm/ml by the addition of tritiated D,O. The tritium activity of the LiOH + H,O electrolyte in Cell T7 was increased to about 1000 dpm/ml by the addition of tritiated I-&O. In Cells T9 and TlO, the palladium cathodes were used as received from Johnson Matthey. (Cathodes were wiped with tissue paper.) The anodes were fashioned from platinum wire 0.5 mm in diameter. The cell vessels were constructed from polystyrene plastic. The 0.10 M LiOD (Cell TlO) and 0.10 M LiOH (Cell T9) electrolytes were prepared by dissolution of Cerac Li,O (99.9%) into the heavy water and light water solvents, respectively. Variables for Cells Tll and T12 were the same as in Cells T9 and TlO, respectively, except for the palladium cathodes. The palladium cathodes were etched in 1 M HCl for 15 min and then rinsed with MilliQ water. Electrolysis of cells in each pair was performed simultaneously in series with the same power supply, which controlled the current to an accuracy of 5%. Electrolysis was carried out at room temperature, about 22°C Cells T5-T72 were weighed before and after electrolysis. Tritium analyses were performed upon three 1.0 ml aliquots of electrolyte samples after electrolysis in each of the 12 closed cells. Simultaneously, tritium analyses were performed upon three 1.0 ml aliquots of the original electrolytes as prepared prior to electrolysis. The electrolyte samples were introduced into 15 ml of Packard Insta-Gel XF scintillation solution. Subsequently, scintillation counting was performed using a Packard Tri-Carb 2200 CA liquid scintillation spectrometer. The detection efficiency of this instrument for tritium was 67% as determined by use of a certified standard. Quench corrections were made to the counting efficiency of each sample by use of an external gamma-ray source. The effect of chemiluminescence in the samples was rendered insignificant by waiting 24 h after preparation of the scintillation cocktails before starting counting measurements. Each aliquot was counted three times, with each counting interval 60 minutes in duration. All counts were corrected for a background of 32.1 f 0.3 dpm, which was obtained by counting the scintillation solution without any added electrolyte. RESULTS

Operation of Cells Tl, T2, T3, and T4 at 40 mA (60 mA/cm2) yielded stable cell voltages at 3.3, 3.6, 2.0, and 3.1 V, respectively, during operation over 10 days. Subsequent electrolysis at 400 mA (600 mA/cm2) increased the cell voltages to 7.8, 8.8, 8.6, and 8.5 V, respectively. After 24 h, the cell voltages declined to 7.0, 7.8, 7.7, and 7.1 V, respectively, as the cells increased somewhat in temperature. In total, Cells Tl-T4 were subjected to 19 Ah of electrolysis over 11 days. The electrolysis was then discontinued. The hydrogen, deute~um, and tritium species were electrochemically desorbed from the palladium cathodes by anodic polarization at 10 mA (15 mA/cm2) for 1 h. The tritium activity measurements, given in Table 2, show no significant increase in tritium activity during electrolysis of Cells Tl-T4. The higher

179 TABLE 2 Electrolyte tritium measurements Sample a

TI Sl T2 s2 T3 S3(=Sl) T4 S4(=S2) T5 SS T6 S6 n s7 T8 S8 T9 $9 T10 SlO Tll Sll (= S9) T12 s12 ( = SlO)

3T activity ’ dpm ml-’

Std. dev. dpm ml-’

4 4 16 15 5 4 15 15 3 2 14 17 998 990 911 917 4 2 13 14 3 2 13 14

0.8 0.6 0.3 0.4 0.7 0.6 1.0 0.4 0.8 0.7 1.7 4.0 7.9 2.6 3.6 1.7 1.1 0.7 0.6 0.4 0.7 0.7 0.4 0.4

3T activity increase Measured ’ dpm ml- ’

Projected d dpm ml-’

o+

3

0

1+

2

14

lzk 3

0

o+

3

14

1+

3

0

-3+12

126 0

8+24

126

-6+12 4

0

-1+

2

88

-1+

3

0

-1*

2

88

2+

a Electrolyte samples Tl-T12 from cells after electrolysis compared to samples Sl-S12 electrolysis. Odd and even numbered samples contain light and heavy water, respectively. Average of 3 measurements each on 3 atiquots. ’ Uncertainty given as 3 times standard deviation. d Projected from tritium generation rates reported by FP&H [I].

prior to

activity in Cells T2 and T4 was due to tritiated impurities in the heavy water solvent. During electrolysis of Cells T5, Td, T7, and T8 at 40 mA (60 mA/cm2) for 100 days, the cell voltages averaged 2.3, 2.7, 2.4, and 2.8 V, respectively. The cell voltages dropped an average of 0.2 V during the electrolysis period. During 96 Ah of electrolysis over 100 days, Cells T5, T6, T7, and T8 lost 0.12, 0.17, 0.18, 0.26 g of weight, respectively. The electrolysis was then discontinued. The hydrogen, deuterium, and tritium species were electrochemically desorbed from the palladium cathodes by anodic polarization at 10 mA (15 mA/cm*) for 1 h. The tritium activity measurements, given in Table 2, show no significant increase in tritium activity during electrolysis of Cells T5-T8. The much higher activity in Cells T7 and T8

180

resulted from the spiking of these electrolytes with tritiated solvents prior to electrolysis. Operation of Cells T9, TlO, Tll, and T12 at 40 mA (60 mA/cm2) yielded stable cell voltages at 3.t, 3.5, 2.8, and 3.4 V, respectively, over a 70 day electrolysis period. During 67 Ah of electrolysis, Cells T9, TlO, Tll, and T12 lost 1.16, 0.68, 0.75, and 0.69 g of weight, respectively. The electrolysis was then discontinued and hydrogen, deuterium, and tritium species were el~tr~he~c~ly desorbed from the palladium cathodes by anodic polarization at 10 mA (15 mA/cm2) for 1 h. The tritium activity measurements, given in Table 2, show no significant increase in tritium activity during electrolysis of Cells T9-T12. All observed standard deviations (see Table 2) are consistent with those expected from statistical randomness of radioactive decay in the samples and backgro~d. DISCUSSION

It is the operation of closed electrolysis cells that provided for the simplicity of our interpretation and the accuracy of our results. The closed cell operation required effective catalytic recombination to return gases evolved during electrolysis to the electrolyte solution. The efficiency of recombination was determined by comparison of the volume of heavy and light water solvents consumed during electrolysis to the measured weight loss in each cell. From Faraday’s law, Cells T5-T8 consumed over 32 g of solvents during electrolysis. From the measured weight loss, only about 0.2 g of evolved gases escaped from Cells T5-T8. With less than 1% escaping, the recombination efficiency was over 99%. Similarly, the recombination efficiency in Cells T9-T12 was about 97%. Thus, recombination in our cells was highly effective. The catalytic recombination of evolved gases removed several potential experimental complications concerning the measurements of tritium. First, it eliminated the need for periodic additions of solvent to replace that consumed during electrolysis. From Faraday’s law, Cells Tl-T4 consumed 6 ml, Cells T5-T8 consumed 32 ml, and Cells T9-T12 consumed 22 ml of solvents. Without recombination, the electrolyte volume in Cells T5-T8 would have been completely depleted. With 99% recombination, it was actually depleted by less than 1%. Without recombination, the electrolyte volume in Cells T9-T12 would have been 90% depleted. With 97% recombination, it was depleted by only about 3%. Similarly, the depletion of electrolyte in Cells Tl-T4 was probably a small fraction of the 24% consumed during electrolysis. ~eple~shment of the electrolyte through periodic additions of heavy water solvent could easily lead to inadvertent t~tium conta~nation. The tritium activity in various sources of heavy water varies by orders of mag~tude. We found 2100 dpm/ml tritium activity in Baker D,O (99.75% D) vs. 15 dpm/ml in the DLM4 DzO obtained from Cambridge Isotope Labs. Individual batches from the same supplier can also differ. In our experience, frequent heavy water additions led, in one instance, to inadvertent tritium contamination. This possibility is eliminated when recombination obviates the need for solvent additions.

181

Second, catalytic recombination eliminates complications [8-lo] from isotope separation effects. Heavy water contains significant tritium impurities [10,14]. The tritium concentration increases during electrolysis as D, gas is evolved in preference to the heavier DT and ‘IT gases. Without replenishment of consumed electrolyte, the tritium concentration can become arbitrarily high according to [lo]: [T] = [T]; . ( Vi/V)(l-l’s)

(1)

where [T]; is the initial tritium concentration, V is the electrolyte volume, Vi is the initial electrolyte volume, and S is the separation factor So., If the electrolyte volume is maintained in an open cell by continuous addition of heavy water solvent, the tritium concentration increases according to [4,9]: [T]=[T];+S-[T]i*(S-1).

exp(--Rl/l/)

(2)

where R is the electrolysis rate, t is the time, and [Tli is the concentration of tritium in the feedstock as well as that in the starting electrolyte. The tritium concentration can thus eventually rise to a multiple of that in the heavy water feedstock equal to the separation factor: [T] = [T], . S

(3)

The separation factor S,,r has been determined previously [9,10] to be about 2. Thus, a doubling of the tritium activity would be expected with the electrolyte volume maintained. However, the separation factor is somewhat dependent upon experimental conditions [9] such as current density, temperature, and hydrogen species content in the palladium. Further, it could be strongly dependent on the presence of metallic impurities, plated out on the palladium surface, which could increase the separation factor significantly. Thus, reliable corrections for separation effects may be impractical. Third, in open cells, it is possible that tritium generated in or on the palladium (via cold fusion) could escape from the cell entirely as DT gas without passing into the electrolyte. With recombination, any DT gas would be converted into DTO liquid whose activity would be detected in our measurements. To desorb tritium species bound in the palladium metal, each electrolysis period was followed by an anodic stripping step. Recombination of gases evolved during the desorption step served to locate all tritium species in the electrolyte solution where they could be detected. The electrolysis of heavy water at palladium cathodes is conceptually a simple experiment. However, a wide variety of experimental variables could impact on the occurrence of any electrochemically induced nuclear fusion including metallurgical and surface properties of the palladium cathode, the current density, the electrolysis time, the materials for the anode and cell container, and the electrolyte impurities. Investigators are not in agreement on what conditions lead to positive reports on cold fusion phenomena [l-7,11-13]. We have chosen the experimental conditions in Table 1 based on some of these reports of positive results. Tritium accumulation as described by FP&H [l] was associated with electrolysis of 1 mm diameter palladium rods, obtained from Johnson Matthey, cathodically

182

polarized at 64 mA/cm2 for 14 days in 0.1 M LiOD heavy water electrolyte. Bockris and coworkers [2] found tritium accumulation associated with electrolysis of 1 mm diameter palladium rods cathodically polarized in 0.1 M LiOD electrolyte for about 2 weeks. In this latter study, tritium accumulation followed an increase in current density from 50 to 500 mA/cm 2. In our experiments, we used 1 mm diameter palladium rods, obtained from Johnson Matthey, cathodically polarized for 11 to 100 days. The current density was generally 60 mA/cm2 with two pairs of cells also subjected to one day of higher current density electrolysis at 600 mA/cm2. Tritium accumulation has been associated with palladium cathodes both with and without annealing [l-7]. Our experiments included cathodes with and without annealing. Annealing was recommended [15] to remove hydrogen from the palladium to allow more complete loading with deuterium. Accordingly, we took care to cool annealed palladium samples in vacuum and back-fill with argon gas to avoid hydrogen contamination. In one case [2], tritium accumulation has been associated with palladium cathodes etched in acid. Thus, one of our sets of palladium cathodes was etched according to the procedure of Bockris and coworkers [2]. There has also been considerable discussion about the importance of surface dendrites in promoting cold fusion phenomena [12,16]. We therefore included in our study one set of palladized palladium electrodes. Recently, we learned [17] that FP&H used palladium electrodes as received from Johnson Matthey. A set of such electrodes was included in our study. Tritium accumulation has been associated with both platinum [l] and nickel [2] anodes. Both were used in our study. We used three types of containers: nickel, steel, and plastic. The plastic container is most resistant to chemical attack, although both nickel and steel passivate in basic solution. The plastic container was least likely to introduce unwanted impurities. The nickel container is less likely to introduce impurities than the nickel anode. Stainless steel containers have been used in experiments where anomalous heat generation was observed [ll] Further, it has been speculated that the presence of iron impurities is associated with cold fusion phenomena [16]. We did not use glass containers. This is possibly significant since both FP&H [l] and Bockris [2] used glass containers in previous observations of tritium. Electrolytic impurities depend in part on details of preparation of the LiOD electrolytes. It is not clear how FP&H [l] prepared their electrolyte. Bockris and coworkers [2] prepared 0.1 M LiOD by the reaction of Li metal with D,O. Electrolytes for our Cells T6 and T8 were prepared by reaction of Li with D,O. We also prepared electrolytes from Li,O. Cells T9-T12 were prepared from the same Li,O as used by Yeager and coworkers [4] in their observations of tritium accumulation. Finally, the tritium content in the heavy water electrolyte could have an effect on cold fusion phenomena according to some proposed mechanisms [18]. For this reason, Cells T7 and T8 were spiked with tritium, increasing the tritium concentration by two orders of magnitude. Cell T2 in particular was operated under conditions very similar to those under

183

which Bockris and coworkers [2] observed increases in tritium activity of over lo6 dpm/ml. In our closed cell experiment, the tritium activity remained constant to within +2 dpm/ml. Cell T10 was operated under conditions very similar to those of FP&H [l] who observed an increase in tritium activity of about 100 dpm/ml. Under these conditions, we observed no increase within error limits of f 2 dpm/ml. In contrast with the results of Bockris [2], Storms [6], or Iyengar [3], we clearly did not observe massive accumulations of tritium to 1000 dpm/ml or more in any of our cells. Nor did we observe the more modest accumulations reported by FP&H [l] and Yeager [4]. In Table 2, the observed change in tritium concentration is compared to that projected from the tritium generation rates reported by FP&H [l]. (Differences in electrode size, electrolyte volume, and electrolysis time are accounted for.) These projected tritium generation rates were up to 44 times the error limits in our experiments. Under a variety of conditions close to those under which cold fusion phenomena have been reported, we have observed no accumulation of tritium.

ACKNOWLEDGEMENTS

We thank H.H. Rogers (Hughes Aircraft) for providing the hydrogen electrode materials, R.Y. Ying for providing cell vessels, G. Meisner for assistance in annealing palladium samples, and E.B. Yeager (Case Western Reserve University) for providing lithium oxide. We also thank H.H. Rogers, R.Y. Ying, R.R. Witherspoon, E.B. Yeager, J.O’M. Bockris, N. Packham, E. Storms, U. Landau, and K.L. Wolf for helpful discussions.

REFERENCES

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184 13 R.C. Kainthla, 0. Velev, L. Kaba, G.H. Lin, N.J.C. Packham, M. Szklarczyk, J. Wass and J.O’M. Bockris, Electrochim. Acta, 34 (1989) 1315. 14 G. Kreysa, G. Marx and W. Ptieth, J. Electroanal. Chem., 266 (1989) 437. 15 R.A. Huggins, DOE Workshop on Cold Fusion Phenomena, Sante Fe, NM, May 1989. 16 G.H. Lin, R.C. Kainthla, N.J.C. Packham and J.O’M. Bockris, J. Electroanal. Chem., 280 (1990) 207. 17 M. Hawkins, National Cold Fusion Institute, Salt Lake City, UT, personal communication, 1990. 18 Y.E. Kim, Purdue Nuclear Theory Group, Department of Physics, Purdue University, Report PNTG-89-5, June 1989.