Studies of alkaline solid polymer electrolyte and mechanically alloyed polycrystalline Mg2Ni for use in nickel metal hydride batteries

Journal of Alloys and Compounds 337 (2002) 208–213

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Studies of alkaline solid polymer electrolyte and mechanically alloyed polycrystalline Mg 2 Ni for use in nickel metal hydride batteries a b c a, A.A. Mohamad , N.S. Mohamed , Y. Alias , A.K. Arof * b

a Physics Department, University of Malaya, 50603 Kuala Lumpur, Malaysia Center for Foundation Studies in Science, University of Malaya, 50603 Kuala Lumpur, Malaysia c Chemistry Department, University of Malaya, 50603 Kuala Lumpur, Malaysia

Received 21 August 2001; accepted 10 October 2001

Abstract Ni–MH cells comprising PVA-KOH solid polymer electrolyte and mechanically alloyed Mg 2 Ni as the negative electrode have been fabricated. The alkaline solid polymer electrolyte with PVA:KOH wt.% ratio of 60:40 exhibits the highest room temperature ionic conductivity of 8.5310 24 S cm 21 . This sample is mostly amorphous. The cell was charged at a constant current of 10 mA and discharged at 0.1 mA. The discharge characteristics improved upon cycling and the plateau voltage maintained above 1.2 V for |10 h.  2002 Elsevier Science B.V. All rights reserved. Keywords: Alloys; Hydrogen absorbing materials; Electrode materials; Electrochemical reaction

1. Introduction The developments of the Ni–MH battery have broadened the application of hydrogen storage alloys in the battery industry. It is well known that of all the hydrogen storage alloys, Mg 2 Ni-type alloy has a greater application potential than the commercial alloys of the LaNi 5 -type or ZrV2 -type because of its high hydrogen storage capability and low cost [1–3]. The theoretical discharge capacity of Mg 2 Ni (ca. 999 mAh g 21 ) evaluated from the amount of absorbed hydrogen is much greater than those of LaNi 5 (ca. 372 mAh g 21 ) and ZrV2 (ca. 763 mAh g 21 ) which are typical of the conventional AB 5 - and AB 2 -type hydrogen storage alloys, respectively [3–5]. The hydrogen content in Mg 2 NiH 4 is also highest, being 3.6 wt.%, but only 1.5 wt.% in LaNi 5 H 6 , 1.4 wt.% in Ti 2 NiH 2 and 1.7 wt.% in ZrMn 2 H 3.6 [6–8]. Despite its superior characteristics in these aspects, practical applications of these materials have been hampered by two major difficulties: sluggish hydriding kinetics at room temperature and easy oxidation [2,6,9–15]. Thus, this alloy was thought unsuitable for rechargeable anode *Corresponding author. Tel.: 160-3-7967-4085; fax: 160-3-79674146. E-mail address: [email protected] (A.K. Arof).

materials in an alkaline solution [1,16]. It has also been reported that it is feasible to prepare Mg-based metal hydride alloys by mechanical alloying [17] that can be hydrided / dehydrided in an alkaline solution at ambient temperature [1,16]. However, the alloys still oxidized on contact with potassium hydroxide (KOH) alkaline electrolyte solution. One way to overcome this problem is by using solid polymer electrolytes (SPE) to replace the conventional potassium hydroxide aqueous electrolyte. Some of the polymer electrolyte systems that have been tried are tetramethylammonium hydroxide penthydrate (CH 3 ) 4 NOH?5H 2 O [18], poly(ethylene oxide) (PEO) [19– 22], copolymer of epichlorohydrin and ethylene oxide P(ECH-co-EO) [23] and poly(vinyl alcohol) (PVA) [24]. In this work, solid-state Ni–MH cells utilizing PVAbased SPE containing different wt.% KOH were prepared and characterized. The polymer systems were prepared by the solution casting technique using potassium hydroxide as the doping salt and water as solvent. The system with the highest conductivity was tried for the fabrication of Ni–MH cells utilizing ball-milled Mg 2 Ni as negative electrode and Ni(OH) 2 as positive electrode.

2. Ni–MH chemistry For a Ni–MH battery using a Mg 2 Ni-type hydrogen

0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01916-8

A. A. Mohamad et al. / Journal of Alloys and Compounds 337 (2002) 208 – 213

storage alloy anode, the charge–discharge reaction can be summarized as follows [25]: at the positive electrode: charge

→ NiOOH 1 H 2 O 1 e 2 Ni(OH) 2 1 OH 2 ← discharge at the negative electrode: Mg 2 Ni 1 4H 2 O 1 4e

2

charge

→ Mg 2 NiH 4 1 4OH 2 ←

discharge

and the overall cell reaction:

209

PVA solution as binder. The two materials were mixed and pressed onto nickel current collector. The film with the highest electrical conductivity was used for cell fabrication. The cells were constructed using a prismatic design with surface area of 15 cm 2 , the assembly of which is shown in Fig. 1. The cells were prepared by assembling a square-shaped metal hydride electrode (thickness: 1.00 mm), a square-shaped nickel electrode (thickness: 0.60 mm) and solid polymer electrolyte film (thickness: 0.40 mm), which also acts to separate the anode and cathode. The assembly was hot pressed, placed in a moisture proof casing and then sealed.

charge

→ 4NiOOH 1 Mg 2 NiH 4 4Ni(OH) 2 1 Mg 2 Ni ← discharge Thus, Ni(OH) 2 is oxidized to NiOOH at the positive electrode during charging. Hydrogen which is formed at the surface of the negative electrode subsequently diffuses into the bulk of the Mg 2 Ni alloy to form Mg 2 NiH 4 . The reaction proceeds in the opposite direction during discharge. The process of charging and discharging of these batteries involves the insertion of ions into both positive and negative electrodes. In Ni–MH batteries, the hydrogen ion shuttles between the positive and the negative electrode, as lithium ions in lithium batteries [21,26,27].

3. Experimental

3.1. Material preparation PVA (Fluka) with molecular weight |67 000, KOH and deionized water were used as starting materials for the preparation of SPE. One gram PVA and a suitable amount of KOH were separately dissolved in 10 ml deionized water at room temperature. The two solutions were then mixed, stirred for several hours, poured into a petri dish and left for slow drying to form films. Further drying was done by keeping the polymer films in a desiccator for a month. Mg 2 Ni alloy powders were synthesised by mechanical alloying. Pure Mg powders (R&M Chemical, |100 mm, 99.9% purity) and Ni powders (Aldrich, |3.6 mm, 99.9% purity) were mixed to give the desired composition. These starting materials were put into a sealed stainless steel bowl under argon atmosphere with stainless steel balls (2 cm in diameter). Mechanical alloying was carried out with a ball to powder ratio of 15:1, using a Pascal 9VS ball mill at a milling speed of 200 rpm for 7 days. The negative electrode was prepared by mixing Mg 2 Ni alloy powder and graphite powder with weight ratio 5:1, using 10% PVA solution as binder. The mixture was then pressed onto a nickel current collector. The positive electrode was prepared using Ni(OH) 2 (H.C. Starck, Germany) as the active material and 10%

3.2. Material and cell characterization The electrical conductivity of PVA–KOH–H 2 O films was determined by impedance spectroscopy. Impedance spectroscopy was performed using a Hioki 3531-01 LCR bridge interfaced to a computer for data acquisition. The study was carried out in the frequency range 40–5 MHz. The conductivity of the sample, s was calculated using the expression s 5 t /(R b A) where t is the thickness of the film, R b is the bulk impedance and A is the area of the cross-section of the film. The value of the R b was confirmed by comparing with the value obtained from a complex admittance graph. The XRD studies were carried out using a Philips X’Pert diffractometer. For scanning electron microscopy (SEM) and energy depressive analysis of X-rays (EDAX) the Oxford Model 5431 equipment was used. A BAS LG-50 galvanostat electroanalytical system was

Fig. 1. Schematic cell design of Ni–MH secondary cell.

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used to perform constant-current charge and discharge of the cells. The charge–discharge life cycle measurements were carried out in a voltage range from 0 to 1.8 V. The cell was charged at 10.0 mA for 1 h and discharged at 0.1 mA.

4. Results and discussion Films of the polymer systems containing #25 wt.% KOH were transparent while white rubbery films have been obtained for the systems with higher KOH compositions. Brittle films were obtained when the KOH concentration exceeds 60 wt.% KOH. The variation of electrical conductivity, s as a function of the composition, i.e. the KOH wt.%, is as shown in Fig. 2. The conductivity of pure PVA is |10 210 S cm 21 and it increases sharply to |10 26 S cm 21 on complexing the PVA with 10 wt.% KOH. The increase in conductivity becomes slower on further addition of KOH to PVA. The highest conductivity of 8.5310 24 S cm 21 was achieved for the system containing 40 wt.% KOH. Above 60 wt.% KOH, the sample becomes mechanically unstable and it is difficult to make conductivity measurements. It is generally believed that the conductivity increases as the degree of

crystallinity decreases or, in other words, the flexibility of the polymeric backbone increases. XRD patterns of PVA–KOH complexes are shown in Fig. 3. The XRD pattern of the pure PVA based solid films shows a broad lump between 17.5 and 27.58 which can be associated with the amorphous behavior of pure PVA films. This broad peak becomes less intense as the KOH content is increased to 30 wt.%. The sample with 40 wt.% KOH is the most amorphous explaining why it exhibits the highest room temperature electrical conductivity. On further addition of KOH to 50 wt.% the conductivity begins to decrease. This is attributed to the sample beginning to become crystalline. In the films with 50 wt.% KOH the peak at 2u 530.98 is that of KOH as can be verified from the JCPDS 1995 data (15-890). These KOH peaks increase in intensity in the sample 60 wt.% KOH. The XRD pattern of the Mg 2 Ni prepared in this work is shown in Fig. 4 compares quite well with values from literature [28,29]. Table 1 below lists the 2u values of the Mg 2 Ni XRD pattern from the present work compared with that from the literature. The peaks at 32.5 and 34.58 are probably due to Mg [17]. It can be observed from the table that almost all peaks in the literature are found in the XRD pattern of the present work although the peaks may differ in intensity.

Fig. 2. Variation of conductivity with KOH weight percentage.

A. A. Mohamad et al. / Journal of Alloys and Compounds 337 (2002) 208 – 213

211

Fig. 3. XRD patterns of pure PVA and complexes of different KOH weight percentage.

Fig. 4. XRD patterns of Mg 2 Ni ball-milled for 5 and 7 days.

Table 1 XDR pattern for present work compared to that of Inoue et al. [28,29] 2u Refs. [28,29] Present work

21.0 –

– 33.5

– 35.5

37.1 37

41.0 –

45.2 45.2

– 48.5

52.0 52.5

57.0 57.5

64.0 64.0

69.0 69

– 70.5

72.0 72.5

76.5 76.5

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A. A. Mohamad et al. / Journal of Alloys and Compounds 337 (2002) 208 – 213

Fig. 5. Scanning electron micrograph at 35000 magnification, of PVA–KOH system for (a) 30, (b) 40, (c) 50 and (d) 60 KOH wt.%.

Fig. 6. Charge–discharge characteristics of Mg 2 Ni-C / PVA1KOH / Ni(OH) 2 electrochemical cells.

A. A. Mohamad et al. / Journal of Alloys and Compounds 337 (2002) 208 – 213

However, the most intense peak at 2u 545.28 agrees with literature. Only the peaks at 2u 521.0 and 41.08 in the literature are not present in the XRD pattern of our work. EDAX analyses show the elemental at percentage of Mg and Ni as 67.8 and 32.2%, respectively. The starting compositions were 66.8% Mg and 33.2% Ni. SEM studies depicted in Fig. 5 were taken at a magnification of 35000. In the micrograph of the sample with 30 wt.% KOH (Fig. 5a) there are still some crystalline dendritic structures. These microstructural features are absent in the other samples. The film with the highest electrical conductivity (i.e. the film with 40 wt.% KOH) was used for cell fabrication. The charge / discharge characteristic curves of the cells are displayed in Fig. 6. For a charging current of 10 mA the cell can be charged to |1.8 V in 1 h. Upon discharge at a constant current of 0.1 mA the discharge curve maintains a plateau voltage above 1.2 V for |10 h. The plateau time increases with the number of cycles. This suggests that activation of the alloy surface took place during the initial charge / discharge cycling [30]. The longest plateau time of 9.5 h is observed in the seventh cycle. However, a large drop in voltage is observed at the eighth cycle. This may be due to large interfacial resistance between the electrodes and the electrolyte which is aggravated during cycling [21], poor absorption / desorption kinetics and pressure–composition–temperature (PCT) properties of the hydrogen storage alloys of Mg [31,32].

5. Conclusion A PVA-based solid polymer electrolyte prepared in this study exhibits the highest room temperature ionic conductivity of the order |10 23 S cm 21 . A Ni–MH cell using this polymer electrolyte was fabricated with Mg 2 Ni and Ni(OH) 2 as negative and positive electrodes, respectively. The preliminary results obtained from this study show that solid state Ni–MH cells can be prepared with the utilization of PVA–KOH SPE. Further work is necessary to improve the performance of this type of polymer based Ni–MH cell.

Acknowledgements The authors gratefully acknowledge the grant IRPA No. 09-02-03-0806 given by the Malaysian Government to carry out this work.

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