international journal of hydrogen energy 33 (2008) 5636–5640
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Optical switching properties of RCo2-type alloy thin films by electrochemical hydrogenation G. Srinivas, V. Sankaranarayanan, S. Ramaprabhu* Alternative Energy Technology Laboratory and Low Temperature Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India
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abstract
Article history:
The switchable optical properties of Pd-protected RCo2-type Ho0.6Mm0.4Co2 alloy thin films
Received 23 October 2007
have been investigated in a KOH electrolyte. The reversible optical switching has been
Accepted 3 May 2008
carried out simultaneously by measuring transmitted light through the thin film during
Available online 17 September 2008
electrochemical charging–discharging of hydrogen. The dependence of switching speed and cyclic durability of the film on the charging and discharging current density as well as
Keywords:
concentration of KOH electrolyte has been studied. In addition, cyclic voltammetric
RCo2-type thin film
measurements have been performed to examine the hydride formation and decomposition
Optical switching
reactions.
Cyclic durability
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
Galvanostatic charging–discharging
reserved.
Cyclic voltammetry
1.
Introduction
Switchable mirrors based on Y and La hydride thin films were discovered in 1996 [1,2]. These thin films show a reversible metal-to-insulator transition on going from the dihydride to the trihydride state. The dihydrides are excellent shiny metals while the trihydrides are large gap semiconductors and transparent in the visible part of the optical spectrum [3–5]. Pronounced electron–electron interactions have been posited to lead to the opening of the large optical gap [6–8]. Thereafter it has been demonstrated that all rare earth metals, their alloys with Mg, Mg2Ni–Hy and Mg–TM-hydrides (TM ¼ Co, Fe, Mn) switch reversibly from their initial reflecting state to visually transparent state upon exposure to hydrogen gas or on cathodic polarization in alkaline electrolyte [2,9–13]. It was shown that not only Y, most R–, R–Mg and Mg-based alloy hydrides but also the cubic Laves phases CeFe2–H4.4 [14] and HoCo2–Hy [15] exhibit such a metal–semiconductor/insulator transition upon hydrogenation. Recently, the hydriding–
dehydriding, structural and transport properties of RCo2-type Ho1xMmxCo2–hydrogen system have been investigated [16–18]. Mm (mischmetal) is a natural mixture of the light rare earth metals that contains 50 wt% Ce, 35 wt% La, 8 wt% Pr, 5 wt% Nd and 1.5 wt% other rare earth elements and 0.5 wt% Fe. Mm has been used mainly for economical reasons [19]. The electrical resistivity of the hydrides of Ho1xMmxCo2 alloys in the temperature range 25–300 K exhibits gradual disappearance of magnetic transitions and semiconducting like behaviour with increasing hydrogen concentration [18]. This behaviour strongly depends on the Mm concentration and is more pronounced in Ho0.6Mm0.4Co2–Hy system. The semiconducting behaviour at higher hydrogen concentrations is mainly attributed to the strong electron–electron correlations. This has stimulated us to investigate the optical properties of Ho0.6Mm0.4Co2 thin films. In this study, the switchable optical properties of Pd-protected Ho0.6Mm0.4Co2 alloy thin film have been investigated in a KOH electrolyte. The dependence of switching speed and cyclic durability of the film on the charging and discharging current
* Corresponding author. Tel.: þ91 44 22574862; fax: þ91 44 22570509. E-mail address:
[email protected] (S. Ramaprabhu). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.05.111
international journal of hydrogen energy 33 (2008) 5636–5640
density as well as concentration of KOH electrolyte has been studied. In addition, cyclic voltammetric measurements have been performed to examine the hydride formation and decomposition reactions in more detail.
2.
Experimental
Ho0.6Mm0.4Co2 films of w200 nm thickness were deposited by e-gun evaporation in an ultrahigh vacuum chamber from an Ho0.6Mm0.4Co2 alloy ingot target onto glass substrates at 300 C. A Pd capping thin layer w15 nm was added in situ to protect the film against oxidation and to catalyze the hydrogen uptake. The X-ray measurements of the as-deposited Ho0.6Mm0.4Co2/ Pd films show weak reflections due to Pd over layer and are therefore either amorphous or nanocrystalline. The surface morphology of Ho0.6Mm0.4Co2/Pd films was tested by scanning electron microscopy (SEM) and atomic force microscopy (AFM), which reveal the uniform surface morphology of Ho0.6Mm0.4Co2/Pd film (Fig. 1a and b). The electrochemical experiments were performed using Autolab 30 potentiostat/ galvanostat in a 1–2 M KOH solution at room temperature. All potentials were given with respect to Ag/AgCl reference electrode. The optical transmission of the film was investigated in situ during hydrogen charging–discharging by illuminating the electrode and detecting the light intensity using a white light source and a spectrometer.
3.
Results and discussion
The switchable optical properties of Ho0.6Mm0.4Co2/Pd thin film during galvanostatic electrochemical charging–discharging in a 1 M KOH electrolytic solution are shown in Fig. 2a. The electrode is charged and discharged with a constant current density of 1.5 mA/cm2 for 45 s and þ0.25 mA/cm2 for 100 s, respectively. Fig. 2b shows the galvanostatic charging and simultaneously measured optical signal at 630 nm of an Ho0.6Mm0.4Co2/ Pd film. During hydrogen charging, the electrode potential exhibits single plateau at 1.3 V, after which the potential drop
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and leveling represent the hydrogen evolution. In the potential plateau region, the transmittance of the electrode starts rising sharply from its initial level of 9% and reaches to 27% at the end of the plateau. This process is completed within 28 s. The thick palladium layer on this sample limits the maximum transmittance achievable. Fig. 2c shows the galvanostatic discharging and transmitted optical signal at 630 nm of Ho0.6Mm0.4Co2/Pd film immediately after the charging cycle. During discharging, electrode shows a constant potential at 0.72 V about 40 s due to the hydrogen desorption, then slowly increases toward more positive values further 20 s and changes more abruptly after 0.7 V at the end of the hydrogen desorption process. At the starting discharge plateau region, optical transmittance of the film starts decreasing and attains its original level at the end of the discharging state. In the discharged state, the electrode transparency is same as initial level and this cycle can be repeated. The results show that the optical transmittance switching can be accomplished within about 28 s during hydrogenation process, whereas reverse switching takes about 65 s. Thus the dehydriding speed is slower than the hydriding speed and likely depending on the applied electrode current density. For this reason, the reversible optical switching properties of Ho0.6Mm0.4Co2/Pd film further tested at equal charging and discharging constant current density of 4 mA/cm2 for 50 s each in a 2 M KOH electrolyte (Fig. 3(a)). In which, the transmittance signal reaches its maximum within w10 s and in the reverse process, the minimum transmittance of the electrode was attained within w15 s. The switching speeds are very fast compared to switching speeds carried out at lower electrode current densities in a 1 M KOH. Thus the applied electrode current density and concentration of alkali in electrolyte directly reflect on the switching time. This indicates that the rate of the electrochemical surface reaction plays an important role. High concentration of alkali in electrolyte makes them quite aggressive and the concentration below 1 M KOH level failed to obtain switching results. Fig. 3b shows the switching durability of the Ho0.6Mm0.4Co2/Pd film in a 2 M KOH electrolyte. The initial 5–25 cycle durability, the film shows a constant transmitting minimum and maximum states. Though optical switching occurs essentially with same speed, the gradual increase in the
Fig. 1 – Surface morphology of a Pd-protected Ho0.6Mm0.4Co2 thin film: (a) SEM image and (b) AFM image.
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50
5
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I =-1.5 mA/cm2, for 45 s charging
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I = +0.25 mA/cm2, for 100 s discharging
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Potential (-V vs. Ag/AgCl)
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25 1.3 20 15
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10 1.1
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Time (s)
Fig. 2 – (a) The change in optical transmittance during charging–discharging of Ho0.6Mm0.4Co2/Pd thin film in a 1 M KOH electrolyte with constant charging current of L1.5 mA/cm2 for 45 s and discharging current density of D0.25 mA/cm2 for 100 s. (b) The electrode potential and optical transmission as a function of time during charging. (c) The electrode potential and optical transmission as a function of time during discharging.
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I = -4 mA/cm2; for 50 s, charging I = +4 mA/cm2; for 50 s, discharging
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80 60 40 20 0
0
20
40
60
80
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120
Number of switching cycles
Fig. 3 – (a) Change in optical transmittance during charging–discharging and (b) cyclic dependence of maximum and minimum transmittance of Ho0.6Mm0.4Co2/Pd thin film in a 2 M KOH electrolyte with constant charging–discharging current density of ±4 mA/cm2 for 50 s each.
international journal of hydrogen energy 33 (2008) 5636–5640
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and Pd layer loses its adherence to the underlying Ho0.6Mm0.4Co2 material [20]. The electrode properties have been investigated further by dynamic current–potential measurements. Fig. 5 shows the cyclic voltammograms of the Ho0.6Mm0.4Co2/Pd film, the electrode potential is scanned from 0.2 V, toward more negative values upto 1.25 V at different scanning rates. The reduction feature beginning at 0.8 V, accompanied by an increase in transmittance, is observed before the onset of hydrogen evolution at around 1.2 V. The increase in the cathodic current density represents the formation of hydride. Reversing the scanning direction leads to oxidation of the previously formed hydride. The voltammogram scanned at 0.5 mV/s given in inset of Fig. 5 clearly shows cathodic and the anodic current peaks at 1.1 V and 0.84 V representing the hydrogen absorption and desorption process, respectively.
Fig. 4 – SEM image of the surface of Ho0.6Mm0.4Co2/Pd thin film cathode after 90 cycles of hydrogenation/ de-hydrogenation in 2 M KOH solution.
maximum and minimum transmittance levels can be seen between 25 and 95 cycles. The switching cycles above 95 and subsequent cycles lead to the complete damage of the film and show drastic increase in the maximum as well as minimum transmittance levels. Fig. 4 shows the SEM image with cracked Ho0.6Mm0.4Co2/Pd layer and Pd film is pealed off from the surface after 90 cycles of hydrogenation/de-hydrogenation. The effect is assumingly provoked by the mechanical strain effects; and top Pd layer suffers from stress generation due to continuous expansion/contraction of Ho0.6Mm0.4Co2/Pd film during hydrogenation and hydrogen release processes. The switching from Ho0.6Mm0.4Co2 to transparent Ho0.6Mm0.4Co2H3.6 involves a huge volume increase of w25% [16, 18]. Thus the electrode is breathing
8
Current Density (mA/cm2)
6 4 2
Scan rate 0.0005 V/s 0.005 V/s 0.01 V/s 0.05 V/s 0.1 V/s 0.5 V/s
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Potential (V vs. Ag/AgCl) Fig. 5 – The cyclic voltammetry of Ho0.6Mm0.4Co2/Pd thin film in a 2 M KOH electrolyte at different scan rates.
4.
Conclusion
The optical properties of Ho0.6Mm0.4Co2/Pd thin film are continuously and reversibly tunable by simply changing the polarity of electrode current density. The transition speed and cyclic stability of the film largely depend on the charging– discharging current density and concentration of electrolyte. The cyclic voltammetric curves show the formation of single step hydriding and dehydriding process.
Acknowledgement The authors thank the IITM and DST, India for financial support.
references
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