Electrocatalysis induced by surface-modification with Pd through sol–gel method for Ti33V20Cr47 alloy

international journal of hydrogen energy 35 (2010) 8088–8091

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Electrocatalysis induced by surface-modification with Pd through sol–gel method for Ti33V20Cr47 alloy Xiangcheng Kong a,b, Junlin Du a,b, Kun Wang a,b, Jun Ni a, Naixin Xu a, Zhu Wu a,* a

Energy Science and Technology Laboratory, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, PR China b Graduate School of The Chinese Academy of Sciences, Beijing 100039, PR China

article info

abstract

Article history:

A body-centered-cubic (bcc) phase Ti33V20Cr47 alloy surface-modified with Pd particles was

Received 27 September 2009

prepared through sol–gel method. The composite showed significantly improved electro-

Received in revised form

chemical hydrogen release capacities, reaching 225 mAh g1 at a discharge current of

14 January 2010

60 mA g1 at 293 K in the second cycle. Such a large capacity could be attributed to the

Accepted 14 January 2010

contribution of catalysis of Pd particles, which induced the Ti33V20Cr47 alloy to release

Available online 13 February 2010

hydrogen more easily. These results provide a new approach to wide applications of Ti–Vbased bcc phase alloys in high-energy batteries.

Keywords:

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Pd particles Electrocatalysis Surface-modification Sol–gel method

1.

Introduction

BCC type Ti–V–Cr alloys exhibit excellent effective hydrogen storage capacity of >2% (mass percent, ditto) and a proper plateau pressure in the pressure–composition (P–C) isotherm for the absorption and desorption of hydrogen, which makes them suitable as anode material in Ni–MH batteries [1–3]. However, these alloys are barely activated during charging in alkaline solution due to the formation of a dense oxide layer on their surface [4,5]. In order to induce their electrochemical activity some second phases such as TiNi phase and C14 laves phase should be introduced into the alloys [6–8], or surface-modified with LaNi5 and La–Mg–Ni alloys prepared simply by ball milling [9–11]. But all these methods reduce the V-based alloy’s own capacity significantly while improve performance of the electrode. In this paper, to improve the electrode characteristics of Ti33V20Cr47 alloy, the effect of surface- modification with Pd

particles through sol–gel method on the capacity of hydrogen storage alloys was investigated. There was only a little loss of the hydrogen storage capacity of the alloy, however it dramatically improved the performance of electrode characteristics of Ti–Cr–V alloys. It was also shown that the Ti–Cr–V alloys could be modified in solution.

2.

Experimental

Ti33V20Cr47 alloy sample was prepared by magnetic levitation melting. To ensure the homogeneity and avoid oxidation of the alloy, a 50 g ingot was turned over and remelted four times in the ambience of argon. The as-cast ingot was crushed into powder under 200 mesh for X-ray diffraction (XRD) measurement and P–C isotherm measurement. Colloidal palladium (Pd) was purchased from Shenzhen Tianli Plating Materials Port Co. Ltd. China. Powders of

* Corresponding author. Tel.: þ86 21 62511070; fax: þ86 21 32200534. E-mail address: [email protected] (Z. Wu). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.01.045

international journal of hydrogen energy 35 (2010) 8088–8091

Ti33V20Cr47 þ 4% Pd were produced by sol–gel method. The required amount of NaCl was added, stirring until complete dissolution (210 g/L). Then HCl was added (5 mL/L, the tank is topped up with water to the make-up level. Finally colloidal Pd was added (100 mg/L). All pumps and heaters were switched on and heated up to set temperature (5 min, 308 K). The Ti33V20Cr47 alloy powder was added and stirred for 5 minutes. Then H2SO4 was added (20 mL/L). The mixture was filtered, washed and dried at 353 K for more than 1 h. The crystal structure and lattice parameters of the powder were examined by X-ray diffractometer using Cu Ka radiation (Rigaku, D/Max2200/PC). The microstructures were observed by scanning electron microscopy (Hitachi S4700). P–C isotherm curves quantitatively characterizing the hydrogen desorption properties of hydrogen storage alloys were measured by a Sieverts-type gas reaction controller (GRC), a product of Advanced Materials Corporation (USA). Alloy powder sample of 3 g was weighed and sealed into the sample reaction chamber which was evacuated at 773 K for 3 h before P–C isotherm measurement. The alloy powder was mixed with Ni powder in a weight ratio of 1:4, and 1.5 g of the mixture was pressed under a pressure of 300 MPa to form a circular pellet of 15.5 mm diam. Electrochemical measurements were performed in a three-electrode system with a 6 mol/L KOH aqueous solution used as electrolyte. Ni(OH)2/NiOOH and Hg/HgO were used as the counter electrode and reference electrode respectively. The charge–discharge measurements were performed by an automatic galvanostatic system (DC-5) and each negative electrode was charged at 60 mA g1 with a potential limit of 2 V and discharged at 60 mA g1 with a cutoff voltage set at 0.5 V (vs. Hg/HgO).

3.

Results and discussion

3.1. Effect of surface-modification on the structure of alloy powders It is known that nanocrystalline materials possess ultrafine grains with a large amount of grain boundaries that may act as fast atomic diffusion channels. The grain boundaries with various kinds of nonequilibrium defects also constitute a high excess of energy that may further facilitate their chemical reactivity [12]. Fig. 1 shows the XRD of the Ti33V20Cr47 alloy before and after surface-modification. The sample before surface-modification displays one phase: Ti33V20Cr47 with bcc structure. After surface-modification, the diffraction peaks of Pd appeared while the diffraction peaks of bcc phase showed no significant change. This is the result of the Pd particles precipitated out from the colloid and deposited on the alloy surface. Fig. 2 shows the SEM image of Ti33V20Cr47 and Ti33V20Cr47 þ 4%Pd powder. It is suggested that the BCC type Ti33V20Cr47 alloy is encapsulated by the Pd particles. The Pd particles on the BCC alloy surface may provide a short cut for absorption and desorption of hydrogen, and contribute to the improvement in the surface activation of the electrode.

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Fig. 1 – XRD patterns of Ti33V20Cr47 alloy and Ti33V20Cr47 D 4%Pd alloy.

3.2.

Desorption behavior of hydrogen in the alloy

Fig. 3 shows hydrogen desorption curves of the Ti33V20Cr47 and Ti33V20Cr47 þ 4%Pd alloy powder at 353 K. The Ti33V20Cr47 alloy exhibits a maximum hydrogen storage capacity of approximately 3.45% H2 and for Ti33V20Cr47 þ 4%Pd it is 3.2% H2. There are two reasons why the addition of Pd decreases the hydrogen storage capacity of Ti33V20Cr47 alloy. Firstly, Pd possesses lower hydrogen absorption capacity (1.8%) in comparison with Ti33V20Cr47 alloy (3.45%). Another reason could be that a part of Ti33V20Cr47 alloy is eroded during preparation in acid. Compared with other approaches, there is only a little decrease of the hydrogen absorption capacity of the alloy. The hydrogen desorption performance of alloys is dependent on their maximum hydrogen storage capacity and hydrogen desorption plateau pressure both [13]. As the Pd deposited on Ti33V20Cr47 alloy surface, hydrogen storage capacity decreased 0.25%, while the desorption plateau pressure of the alloy increases, which induces the alloy to release hydrogen more easily. This is important in practical applications. From Fig. 3, we can see the desorption of Ti33V20Cr47 alloy is 2.25% and for Ti33V20Cr47 þ 4%Pd it is 2.1%, there is 0.15% decrease of desorption capacity, lower than that of absorption 0.25%.

3.3. Effect of Pd addition on the discharge performance and cycle life of alloy electrodes Fig. 4 shows the electrochemical discharge capacities at 293 K for electrodes made from Ti33V20Cr47 alloy and Ti33V20Cr47 þ 4%Pd at discharge current of 60 mA g1. The better characteristic of potential plateau demonstrated a great improvement on discharge performance when adding Pd particles to Ti33V20Cr47 alloy. The discharge capacity for Ti33V20Cr47 alloy electrode was only 12 mAh g1, while the Ti33V20Cr47 þ 4%Pd alloy showed a much higher discharge capacity up to 225 mAh g1, and 90% of total discharge capacity could be obtained below 1 V. The intrinsic discharge capacity of the Ti33V20Cr47 alloy was impulsed

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international journal of hydrogen energy 35 (2010) 8088–8091

Fig. 2 – SEM images of Ti33V20Cr47 alloy and Ti33V20Cr47 D 4%Pd alloy.

to release through Pd particles surface-modification. The Ti33V20Cr47 alloy couldn’t be activated during charging in alkaline solution due to the formation of a dense oxide layer on its surface. The Pd particles on the alloy surface can protect the alloy from oxidation. In the other way, the Pd particles not only perform better charge–discharge capacity themselves, acting as both an electrocatalyst and a microcurrent collector, but also provide rapid diffusion pathways. These might explain the improved

electrochemical kinetics of the composite and the dramatic increase in the discharge capacity. The cycle life curves of the two samples are illustrated in Fig. 5. The Ti33V20Cr47 þ 4%Pd alloy reaches its maximum discharge capacities at the second cycle. The alloy shows good cycle life characteristics. From the second cycle to the 20th, there is almost no change, and from the 20th cycle, the discharge capacity begins to reduce quickly. The inevitable dissolution of V

Fig. 3 – Hydrogen desorption curves of Ti33V20Cr47 alloy and Ti33V20Cr47 D 4%Pd alloy at 353 K.

Fig. 4 – Maximum discharge capacity curves of Ti33V20Cr47 alloy and Ti33V20Cr47 D 4%Pd alloy.

international journal of hydrogen energy 35 (2010) 8088–8091

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references

Fig. 5 – Cycle life curves of Ti33V20Cr47 alloy and Ti33V20Cr47 D 4%Pd alloy.

of the Ti33V20Cr47 into the electrolyte during discharge cycles, detected by inductively coupled plasma atomic emission spectroscopy, is the main reason for the decrease of discharge capacity. The discharge capacity of Ti33V20Cr47 alloy is only about 10 mAh g1 from the first cycle to 25th.

4.

Conclusions

In summary, Ti33V20Cr47 þ 4%Pd alloy prepared by sol–gel method showed dramatically improved electrochemical discharge capacity. In this method, the good characteristic of discharge curve plateau and cycle life can be obtained and the discharge capacity can reach high up to 225 mAh g1 at a 60 mA g1 discharge current and 293 K after the second cycle. It is thought that the high discharge capacity is due to the enhanced electrochemical-catalytic activity of the alloy surface covered with Pd particles. Our results provide new method for use of Ti–V-based bcc phase alloy as high-energy Ni–MH batteries.

Acknowledgment This research was supported by Hi-Tech Research and Development Program of China (grant no. 2007AA05Z149).

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