Hydrogen and deuterium in palladium

Int. J. Hydrogen Energy, Vol. 16, No. 7, pp. 491-497, 1991. Printed in Great Britain.

HYDROGEN

0360-3199/91 $3.00 + 0.00 Pergamon Press pie. © 1991 International Association for Hydrogen Energy.

AND DEUTERIUM

IN P A L L A D I U M

C. P. CHANG,* J. K. W u , t Y. D. YAO,~ C. W. WANG~ and E. K. LIN3~ *Department of Materials Engineering, Tatung Institute of Technology, Taipei, Taiwan 10451, R.O.C. tDepartment of Marine Engineering, National Taiwan Ocean University, Keelung, Taiwan 20224, R.O.C. :~Institute of Physics, Academia Sinica, Taipei, Taiwan 11529, R.O.C.

(Received for publication 13 March 1991) Abstraet--The electrochemical permeation technique has been used to measure the diffusivity and permeability of hydrogen and deuterium in palladium. A discharge technique was also used to measure the solubility of hydrogen and deuterium in palladium. The permeation results showed an Arrhenius temperature dependence of diffusivityand permeability between 298 and 340 K. The solubility values of hydrogen and deuterium in palladium from measurements show an exothemic reaction. Cathodic charging on palladium electrodes was also observed, fusion spectra was not found in our results. The surface morphology and deuterium charged specimen was examined by scanning electron microscope.

NOMENCLATURE A

INTRODUCTION

pre-exponential permeation constant, mol H m -

S-I D Do F ip i~ l J~ J~L L m n

diffusivity, m 2 s-1 pre-exponential diffusion constant, m 2 sFaraday's constant, 96500 coulomb equivalent anodic measured current, mA cm -2 steady state current, m A c m -2 current, mA steady state flux, mol H m -2 s -1 permeation rate, mol H m -1 s thickness of specimen, m mole number of hydrogen atom, mol H the number of electrons involved in a balanced electrochemical reaction equation, equivalent mol Qd diffusion activation energy, kJ m o l l Qp permeation activation energy, kJ mo1-1 S apparent solubility, tool H m 3 SO pre-exponential solubility constant, mol H m -3 tL lag time, s t time, s T temperature, K V volume of specimen, m 3 Vo volume of electrolyte, cm 3 AH heat of solution, kJ mol 1 AT temperature change, °C potential, V(SCE) ~ applied potential, V(SCE) .... rest potential, V(SCE) Cp specific heat, J cm 3

Palladium is a face-centre cubic structure. The absorption ability of hydrogen or deuterium for palladium is excellent. The mobility of hydrogen and deuterium in the fcc Pd lattice provides some useful information for the storage of energy as hydrogen storage alloys. Hydrogen storage alloys can be prepared easily by loading either from gas phase or electrolytically. Many experimental techniques have been developed to measure the diffusivity, permeability and solubility of hydrogen in metals. However, most common methods are hydrogen gas phase partial pressure measurement and electrochemical permeation technique. The transport of hydrogen in palladium has been a subject of considerable interest [1-5]. Cold fusion of deuterium in palladium has been reported recently [6-7]. The purpose of this paper is to report the diffusivity, permeability of hydrogen and deuterium through palladium by the electrochemical permeation technique. A simple electrochemical hydrogen solubility measurement [8] was also used to determine the solubility of hydrogen and deuterium in palladium. In this study, palladium sheets were also cathodic-charged in heavy and normal water to examine the fusion products.

APPLICABLE EQUATIONS

Permeation For this study, the flux of hydrogen through the specimen was measured in terms of current density, ip,

491

492

C.P. CHANG et al. (a) z5

(b)

70

T = 318 K Uncharged palladium

T = 318 K

60 ~Oeuterium charged paLLadium

20

50 15

40i

E ~

PO

30

2O

IO

k

I , 40

I

I

I

60

80

100

i O

i

I

J

I

I

I

l

I T-~.-_~

10 20 30 40 50 C::O 70 80 90 IOOI I0 [L~0 I~XD

Time (rain)

Time (rain)

Fig. 1. Typical discharging curve: (a) uncharged specimen; (b) deuterium charged specimen. and converted to hydrogen permeation flux according to equation (1). i~ J~' = ~nF' (1)

The solubility can be also expressed as -AH S = S0 e x p ( - - R - ~ ) . Excess heat

The permeation rate is defined by (2)

J~L = p L. nF

The excess heat of a constant charging current on palladium specimen can be calculated by equations (9)-(1 l)

The temperature dependence of the permeation is given by J~,L=A

(8)

exp(-~TP ) .

Input power = l(~bv -

(9)

~rest)t

Output heat = Ct, Ve A T

(3)

(10)

Excess heat = Output heat - Input power

Diffusion

For diffusion control, D is related to lag time by [9] L2 D = 6t--~' (4)

=

.

-- I(~b,, - ~r~st)t

VeAT

(1

l)

where (~b,- Or,st)t can be obtained by integrating the shaded area in Fig. 2.

The coefficient of diffusion is also temperature dependent and is given in the form of an Arrhenius equation: S = D 0exp - - ~

Ca

3 / Speamen size, 0.Sx 25 x 25 mm Charging time= I0 min

(5)

J

g5 rest

Solubility

The observed apparent solubility can be also directly determined from the simple solubility measurement curve. The amount of dissolved hydrogen or deuterium, It (mA s), in palladium is approximately the integration area of discharging curve with cathodic charging in Fig. 1, and also can be converted to mole number, m, according to equation (6): m

It nF"

The apparent solubility is then defined by m S = -~.

(6)

--~

-3

4v -5

-7

0

I

2

J

4

1

6

I

8

I

tO

I

12

Time (min}

(7)

Fig. 2. Rest and applied potential versus time for palladium in 0.1 M KHSO4 + D20.

HYDROGEN AND DEUTERIUM IN PALLADIUM

Solubility measurement

EXPERIMENTAL

Material Ninety-nine per cent palladium sheets (0.5mm x 2.5 cm × 2.5 cm) were used in this study. The specimens were used in an as-received condition. Prior to insertion in the test cell, each specimen was ground with CarbimetSiC grinding paper down to 1200 grit and rinsed with acetone. All electrolytes used in this study were prepared from reagent grade chemicals.

Electrochemical permeation The instrumentation and procedure were similar to those described elsewhere [2]. The cathodic side was galvanostically polarized at a constant charging current (100mAcm -2) in 1N HzSO4+H20 or D20 with 1 g 1- J of thiourea added as a hydrogen recombination poison. The anodic side was potentiostatted at 0 mV (SCE) in 0.1 N NaOH + H20 or D20. The potentiostatic current, i, gives a direct measure of the hydrogen or deuterium flow rate. The cell assembly was immersed in a constant temperature bath and maintained at +0.5 K. Both sides were purged by nitrogen gas. Permeation transients were measured on a strip chart recorder between 298 and 340 K. Preliminary experiments indicated good reproducibility after an initial charging run was made on each sample. Similar observations, reported by Hirth [10], are thought to result from initial filling of deep traps.

GaLvanostot

/ N2In~t

I N H 2 SO4

(A)

493

The electrochemical cells used for hydrogen and deuterium solubility measurements in a palladium sheet are shown diagrammatically in Fig. 3. It consisted of two set test cells. The counter electrode (platinum sheet), Luggin probe with saturated calomel electrode (SCE) and nitrogen gas dispersion was inserted in each test cell with prepared electrolyte. Initially the specimen was cathodic charged with a constant current (100 m A c m 2) in 1 N H2SO4 + H20 or DEO with 1 g 1 ~ of thiourea for 4 h to ensure hydrogen or deuterium saturation in the thin fiat specimen. Then, the specimen was removed from the cell, rinsed with distilled water several times and immediately immersed into a second test cell for anodic constant potential discharge measurement. The specimen in the second cell was applied with a constant anodic potential 0 mV (SCE) in 0.1 N NaOH, the system was ready to record the hydrogen discharge until a zero current was obtained. Solubility measurement was performed between 288 and 340 K.

Cold fusion experiment A galvanostat was used to provide a constant cathodic charging current density ( 5 0 m A c m 2) on palladium sheet in 0.1 M KHSO 4 + D20 and 0.1 M KHSO4 + H20 solutions respectively. A 100ml teflon beaker was used to measure evolved heat from this system as shown in Fig. 4. The 24 h spectrum accumulation of 7-rays (2.224MeV) and neutrons (2.45 MeV) emitted from the test cell due to the fusion reactions was determined using a sodium iodide crystal scintillation detector, a 30 MeV spectrum analyser and a neutron dose equivalent rate monitor. Tritium and 3He in charged specimens were determined by SNICS (source of negative ions by cesium sputtering) and RBS (Rotherford back

1

Potentiostot N z InLet

X-T

(B)

0. I N HoOH

Fig. 3. Hydrogen (deuterium) solubility measurement system: (A) cathodic constant current charging apparatus; (B) anodic constant potential discharging apparatus.

I. TefLonbeaker 2. Counter electrode 3. Workingelectrode 4. Thermometer 5. Saturated calomel,electrode 6. Mognetic bar Fig. 4. Schematic diagram of experimental apparatus for calorimetric measurement.

494

C.P. CHANG et al.

sca~;tering) with a particles and protons using National Electrostatics 9SDH-2 Pelletron AMS (accelerator mass spectrometry). Surface morphology A scanning electron microscope was used to analyse the cathodic (deuterium) charged specimens. This analysis can help to understand the hydrogen (deuterium) damages on the charged specimens. An X-ray diffraction technique was used to determine the lattice parameters of uncharged and charged specimens.

EXPERIMENTAL RESULTS Permeation rate and diffusion coefficients defined by equations (1)-(5) are presented in Figs 5 and 6. The permeation rate and diffusion of deuterium are higher than these values of hydrogen in palladium at a constant charging current density (100 mAcm-2). As at higher temperatures, thermally activated Arrhenius behavior is indicated. The solubility values from discharging techniques are shown in Fig. 7. A palladium sheet (0.5 mm × 2.5cm × 2.5cm) was cathodic charged in 0.1 M KHSO4 + D20 to examine the fusion products. However, no fusion products were detected. The temperature rise of 100 ml electrolyte from 26°C in a teflon

T lOC) 70

60

50

40

I

I

I

30

T (°C)

I 108 70

---*------+----

60

50

40

30

20

I0

I

r

I

I

I

I

I

DeuLeriu m H~lrogen

rO 7 -

rd 6

---*---

Deuterium

---+----

Hydrogen

[

i@ 1-

05

'E 110-7

E (.0

+....--~+~

03

Id

29

I

I

3.0

i

3 I

32

IO00/T

I

0 2 --

I

33

34 0

( K -i )

2.

I

I

I

I

J

I

3.0

31

32

33

34

35

Fig. 5. Permeation rate of hydrogen and deuterium in palladium against inverse temperature.

70

10 9

60

50

40

30

I

I

I

J

---*----

Deuterium

--+---

Hydrogen

id '°

50

I

o.. I--

30

<1

-

~

Specimen: 40

'to

%

(K q )

Fig. 7. Solubility of hydrogen and deuterium against inverse temperature by discharging technique.

T (°C) jd e

IOO0/T

I 36

20 +

i0 II

0.5x 25 x 25mm

/~

M KHSO4 +

D20

' ~ - M KHSO. + H ~ O

I0

Jd 29

I

r

I

I

30

3 I

3.2

3.3

I 3.4

I O O O / T ( K -I )

Fig. 6. Diffusivity of hydrogen and deuterium in palladium against inverse temperature.

0

I

I00

I

200

I

300 Time

t

400

I

500

I

600

(rain)

Fig. 8. Temperature rise of electrolyte with constant cathodic charging current (625 mA) for 9 h.

HYDROGEN AND DEUTERIUM IN PALLADIUM beaker during gaivanostatical charging was recorded and shown in Fig. 8.

Cold fusion products measurement 3He, neutrons, tritium and y-rays must be emitted from heavy water bath if the fusion reaction occurs:

DISCUSSION

Electrochemical permeation The experimental results shows the permeation rate and diffusivity of deuterium are higher than those of hydrogen in palladium. The permeation rate and diffusivity of hydrogen in palladium at constant charged current density (100 mA cm- 2) follow the Arrhenius relationships

J~L

=(7.51

T )exp[,_{-(-23-6]kJRm°l-J)

x •t o^- 4,

( m o l H m -l s -l) • ^ 6,)exp[[- - (29.4_R__TkJ mol - 1 ) D = (3.55 x lod

L_

(m2 s-l). The data for deuterium in palladium also follow the Arrhenius relationships J~ L =(3.36 x 10-4)exp[ --(20"3 kJ mo1-1)1

j

k_

(mol D m -l s -l) D =(7.32 x 10-7)exp

[ ( - 2 3 - 5 kJ m o l - l ) ]

RT

(m2 S-I). It was found that permeation rate and diffusivity of deuterium were higher than those of hydrogen in palladium. Both permeation and diffusion activation energies of deuterium are lower than those of hydrogen in palladium. These results of diffusivity are in good agreement with the value obtained by gas phase data [1].

Solubility measurement The solubility measurement results from discharging technique also show Arrhenius relationships [185kJmol 1 SM=(2.04)exp~- " ~ y )molHm S o = (724)exp

3

9.0 kJ mol- l \ ~ ) mol D m -3.

From the discharging technique, deuterium solubility were found to be greater than hydrogen in palladium at a constant charging current density (100 mA cm 2) for 4 h. The results from the discharging technique show that the dissolution for hydrogen and deuterium processes into palladium are exothermic reactions. The solubility of hydrogen and deuterium in palladium decreases with increasing temperature that holds true in this study. The heat of solution was reported as - 9 . 3 kJ mol -l for hydride formation [11] which is quite close to the value of our deuteride formation ( - 9.0 kJ mol- 1).

495

D + D--. 3He(0.82 MeV) + n(2.45 MeV)

(12)

D + D ~ T(1.01 MeV) + p (3.02 MeV)

( 13)

and p + n (2.45 MeV) ---,D + y (2.224 MeV).

(14)

To date, no fusion products were found in the present measurement. The temperature rise of electrolyte in a 100 ml teflon beaker from room temperature (26°C) with a higher charging current density ( 5 0 m A c m -2) was recorded in Fig. 8. The temperature rise in either 0.1 M KHSO 4 + D20 or 0.1 M KHSOa+H20 solution reaches a constant temperature after 5 or 6 h, and probably is due to the heat loss and cannot increase the temperature in electrolytes any more. The electrolyte of 0.1 M KHSO4+ D20 can reach higher temperatures than 0.1M KHSO4+H20. Both electrolytes evolved approximately 30% excess heat from equations (9)-(10) over the first 20 min. Actually the excess heat is much more than this calculated value, since the heat transfer to the teflon beaker and the heat loss in the electrolyte system were not measured. The calculated excess heat was underestimated in this study. The generation of excess heat was reported by many investigators [12-13]. Several scientists [14] have suggested the heat is created from deuterium and oxygen combination and deuteride formation, or is arising from a Peltier junction effect.

Surface morphology When the pure palladium (~ phase) becomes saturated with hydrogen or deuterium, a fl phase is formed [15]. The atomic ratio of D/Pd is approximately 0.1 from our solubility measurements. During the hydrogen or deuterium charging process, these tiny atoms first diffuse into palladium to form an interstitial solid solution. Later, partial PdHx or PdDy will be formed as a mixture in palladium. The surface morphology of charged palladium were examined by scanning electron microscopy as in Fig. 9. The specimen can be expanded during the cathodic charging process and will shrink after releasing the dissolved hydrogen. This causes a destructive area widespread over the charged palladium surface. The lattice parameters of pure palladium and the charged specimens were determined by the X-ray diffraction method. Our measured lattice parameter of pure palladium is 3.88/~, which is close to the standard value (3.89 A) [16]. The linear lattice expansion of tested specimens for both hydrogen and deuterium permeation and solubility measurement is about 0.5%. CONCLUSIONS The following conclusions pertain to hydrogen and deuterium transport in the palladium range from 298 to 340 K for constant values of cathodic charging current.

496

C.P. CHANG et al.

Fig. 9. Surface morphology: (a) (b) hydrogen charged specimen; (c) (d) deuterium charged specimen. (1) The permeation rate, diffusivity and solubility of deuterium are higher than hydrogen in palladium. These results are in good agreement with values obtained by gas phase data. Hydrogen and deuterium dissolved in palladium show an exothermic reaction. (2) Cold fusion experiments were performed in this study, considerable excess heat was found, but no spectra can be fitted to nuclear fusion reaction. (3) Widespread cracks and pits formed over the surface of charged palladium. Acknowledgements--The authors are grateful for the support of this research by the National Science Council, Republic of China, under Contract No. NSC 79-0405-E019-01.

REFERENCES 1. F. A. Lewis, The Palladium Hydrogen System. Academic Press, New York (1967). 2. M. A. V. Devanathan and Z. Stachurski, Proc. R. Soe. A 270, 90-I02 (1962). 3. T. lshikawa and R. B. McLellan, Acta Metall, 34, 1825-1832 (1986). 4. T. E. Ford, P. C. Searson, T. Harris and R. Mitchell, J. Eleetrochem. Soc. 137, 1175-1179 (1990). 5. H. Ziichner, Z. Naturforsch 25a, 1490-1496 (1970). 6. M. Fleischmann and S. Pons, J. Electroanal. Chem. 261+ 301-308 (1989). 7. S. E. Jones, E. P. Palmer, J. B. Czirr, D. L. Decker, G. L. Jensen, J. M. Thorne. S. F. Taylor and J. Rafelski, Nature 338, 737-740 (1989).

HYDROGEN AND DEUTERIUM IN PALLADIUM 8. C. H. Tseng, W. Y. Wei and J. K. Wu, Mater. Sci Technol. 1236 1239 (1989). 9. J. Crank, The Mathematics of Diffusion, pp. 44-49. Oxford Univ. Press, London (1979). 10. S. X. Xie and J. P. Hirth, Corrosion 38, 486-493 (1982). 11. R. Speiser, in R. W. Staehel, J. Hochman, R. D. McCright and J. E. Slater (eds) Stress Corrosion Cracking and Hydrogen Embrittlement of the Iron Base Alloys, pp. 226 243. NACE, Houston (1973).

497

12. J. O'M. Bockris, G. H. Lin and N. J. C. Packham, A Review of the Investigations of the Fleischmann Pons Phenomena. Department of Chemistry, Texas A & M University (1990). 13. Y. D. Yao, C. W. Wang, E. K. Lin andJ. K. Wu, J. Mater. Sci. Lett. 9, 228 (1990). 14. R. Pool, Science 2,44, 284 (1989). 15. F. A. Shunk, Constitution of Binary Alloys, Second Supplement. McGraw-Hill, New York (1969). 16. W. M. Mueller, J. P. Blackledge and G. G. Libowitz, Metal Hydrides. Academic Press, New York 11968).