Effect of Thermal Treatment on the Electrochemical Hydrogen Absorption of ZrCrNi Alloy. F. C. Ruiz1, *H. A. Peretti1, A. Visintin2, S. G. Real2, E. B. Castro2, H. L. ...
Journal of New Materials for Electrochemical Systems 10, 249-254 (2007) © J. New Mat. Electrochem. Systems
Effect of Thermal Treatment on the Electrochemical Hydrogen Absorption of ZrCrNi Alloy F. C. Ruiz1, *H. A. Peretti1, A. Visintin2, S. G. Real2, E. B. Castro2, H. L. Corso1 and W.E. Triaca2 1
Centro Atómico Bariloche, Comisión Nacional de Energía Atómica (CAB-CNEA), 8400 S.C. de Bariloche – Argentina. 2 Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, C.C. 16, Suc. 4 – 1900 La Plata - Argentina Received: October 27, 2006, Accepted: March 26, 2007
Abstract: The catalytic effect of secondary phases contained in the alloy ZrCrNi on its electrochemical properties was studied. Samples of the as-melted alloy were subjected to annealing treatments at T = 1250 K during different times in order to vary the amounts of remaining microsegregated secondary phases. The electrochemical hydrogen absorption of these samples was studied by charge-discharge cycling, high rate dischargeability (HRD) and electrochemical impedance spectroscopy (EIS). The correlation between the measured properties and the secondary phases present in the alloy is discussed. Keywords: Laves phases, anneling effect, catalytic effect, hydrogen absorption, electrochemical characterization, hydrides
components (purity better than 99,9 %) using a cooled copper crucible with tungsten electrode under high purity argon atmosphere. Four pieces of the alloy in different conditions were used for this study. One piece was kept in the as-melted condition and labeled sample F, while other three pieces were previously wrapped in tantalum foil, sealed in evacuated silica capsules, and given annealing treatments at 1250 K for 10, 20 and 30 days. These samples were labeled F1, F2 and F3 respectively (see Table 1). The phase structure, morphology and composition were studied by means of x-ray diffractometry (XRD) using Cu Ka radiation, scanning electron microscopy (SEM), and energy dispersive spectroscopy microanalysis (EDS). The working electrodes were prepared by mixing 75 mg of sieved alloy powder (0.03
0.045
0.25
>10
0.1
-5
⎤ ⎥ ⎥ ⎥ ⎥⎦
(1)
ψ a = ra
1/ 2
Zi
−1
= Z di
−1
+ ZF
−1
(2)
ZF aa
Cdl being the double layer capacitance per unit interfacial area (F cm-2), and ae the interfacial area per unit volume (cm-1), and ω=2πf (f, frequency of the perturbing signal), and
Z dl =
1 i ω C dl a e
where Zf is the faradaic impedance per unit interfacial area(Ω/cm2) and aa is the active area per unit volume (cm-1). It is interesting to note that the faradaic process takes place only at the surface of alloy particles whereas the double layer charging involves the whole interfacial area of the electrode, ae, which also includes the Teflon-C particles. Zf was derived according to the model as:
Zf =
ψ h = rh
iω D
1/ 3
where
ZF =
iω D
and
−1 / 2
where L is the thickness of the electrode , the parameters κ, σ correspond to the conductivities of the liquid and solid phases multiplied by the associated volume fraction. Zi corresponds to the impedance of the solid/liquid interface per electrode unit volume (Ω/cm3). As usual, the interfacial impedance implies the double layer capacitance impedance (Zdl) linked in parallel with the faradaic reaction impedance (ZF), i.e.
Zi
the molar concentration of interstitial sites available for hydrogen and Cα the molar concentration of H in the α phase. In general the parameter ψ is a function of the radius r, the frequency ω and the diffusion coefficient of H in the α phase (D).
RT ra RT − jo F F 2 X α (1 − X α )Co D (ψ a coth(ψ h −ψ a ) + 1)
(3)
where jo is the exchange current density and
X α = C α /C o Xα being the H molar concentration quotient in the α phase, Co
⎛ SOD Cβ ⎞ ⎟ rh = ⎜1 − ⎜ (C − C ) ⎟ β α ⎠ ⎝
(4)
ra
Cβ is the H concentration in the β phase and SOD correspond to the state of discharge of the electrode. Figure 8 depicts complex impedance plots corresponding to the experimental and fitted results for constant state of SOD (30% in this case) of the metal hydride electrodes and different annealing times. A good accordance between experimental and fitted data in terms of the present model was observed in the whole frequency range and for both SOD (30% and 60%). In Reference. [6] a thorough description of the fitting procedure is presented, briefly ; the fitting algorithm is based on the NelderMeade simplex search included in the Matlab package, the objective function to be minimized is the cost function, Jp :
Jp =
1 1 Z (ω ) − Z ( p, ω ) 2 ∑ Ie( p,ωk )I = K ∑k I e k Z (ωP ) k I K k e k
2
Where K is the number of experimental frequencies (ω) and Ze and ZP the experimental and theoretical impedance values corresponding to the frequency ωk. This algorithm returns a parameter vector [p] which is a local minimizer of Jp, near the starting vector [po], so the whole fitting procedure is highly dependent on the initial values given to the parameters in [po]. The fitting was considered acceptable when Jp< 5e-3 Although the expression of the total impedance Zp includes 12 parameters, many of them have values that are either reported in the literature; Cdl, Cα , Co, κ, σ, or determined by other experimental techniques [6] : ae, Cβ, L, ra . This leaves only 3 free parameters aa, D, jo, whose inicial values are unknown. The parameters, derived from the fitting procedure, used in the calculation of Zp are depicted in Tables 4 a) and b). Table 4 c) includes parameters whose initial values were estimated from experimental determinations, i.e., initial values for ra were estimated
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from SEM images, ae was measured by the BET technique and Cβ was calculated from the rate capability Crt [6]. The analysis of impedance data in terms of the proposed physicochemical model indicates a decrease of the diffusion coefficient of H in the alloy, D, and of the exchange current density, jo, with annealing time. These parameters determine the quick activation and the better HRD performance of as-melted samples. 4. CONCLUSIONS
1. The annealing treatment of the ZrCrNi alloy results in a remarkable decrease of both the discharge capacity and the rate of activation process. This can be ascribed to the dissolution and/or modification of the microsegregated secondary phases present in the as-melted alloy. 2. The effects of the annealing are the dissolution of Zr7Ni10 and Zr9Ni11 and the decrease of the Ni/Cr ratio of the Zr(Ni(1x)Crx)3 and/or Zr8(Ni(1-y)Cry)21-type phases. 3. The presence of the secondary phases in the alloy favors the hydrogen absorption process, possibly due to the catalytic effect on the hydrogen electrode reaction, taking place in the metal alloy surface/electrolyte interface. 5. ACKNOWLEDGEMENTS
This work was supported by the following Argentinean organizations: Consejo Nacional de Investigaciones Científicas y Técnicas, and the Agencia Nacional de Promoción Científica y Tecnológica. REFERENCES
[1] D. Linden, “Handbook of batteries”, McGRAW-HILL, INC. New York, USA, 1994. [2] M. Joubert, M. Latroche, A. Percheron-Guégan, J. Bouet, J. Alloys and Compounds, 231, 494 (1995). [3] M. Joubert, M. Latroche, A. Percheron-Guégan, J. Bouet, J. Alloys and Compounds, 240, 219 (1996). [4] L. V. Mogni, H. A. Peretti, A. Visintin y W. E. Triaca in “Proceedings of the CONAMET/SAM – SIMPOSIO MATERIA 2002”, Ed. Aquiles Sepúlveda, Santiago de Chile November 12-15, Vol. II, 665 (2002). Also published in Revista Matéria, ISSN 1517-7076, Ed. Paulo Emílio Valadão de Miranda, Vol 8, No. 3, 3rd. Quarter, 228 (2003). http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10242/index.htm.
[5] F. Ruiz, H. A. Peretti, E. B. Castro, S. G. Real, A. Visintin, HYFUSEN 2005. Actas del Primer Congreso nacional: Hidrógeno y fuentes sustentables de energía. 8-10 junio 2005. Centro Atómico Bariloche-Instituto Balseiro. San Carlos de Bariloche, Provincia de Río Negro, Incluido en las Bases de Datos del INIS (International Nuclear Information System). ISBN: 98720975-0-X. http://www.cab.cnea.gov.ar/hyfusen [6] E. B. Castro, S. G. Real, A. Bonesi, A. Visintin and W. E. Triaca, Electrochim. Acta, 49, 3879 (2004). [7] A. Visintin , E. B. Castro, S. G. Real, W. E. Triaca, C. Wang and M. P. Soriaga, Electrochim. Acta, 51, 3658 (2006).
