Surface Modification of Mgni by Perylene

Materials Transactions, Vol. 43, No. 11 (2002) pp. 2711 to 2716 Special Issue on Protium New Function in Materials c 2002 The Japan Institute of Metals

Surface Modiﬁcation of MgNi by Perylene

Tiejun Ma, Yuji Hatano, Takayuki Abe and Kuniaki Watanabe

Hydrogen Isotope Research Center, Toyama University, Toyama 930-8555, Japan

Amorphous MgNi was modified by ball milling with perylene and its effects on the charge/discharge capacity were examined by using a conventional two-electrode cell. It was found that both the ball milling time and perylene/MgNi ratio had great influence on the discharge capacity and cycle life of MgNi. Three types of effects were identified, depending on ball-milling conditions. One of them was the increase in the discharge capacity at the first cycle, the second type was the deceleration of the degradation of the discharge capacity with charge/discharge cycle, and the last type was the reduction in the charge/discharge capacity. Chemical states of modified surfaces were analyzed by Auger electron spectroscopy (AES) as well as ab-inito calculation. Both AES and ab-initio calculations indicated that carbon atoms can form bonding with both magnesium and nickel atoms, but bonding with magnesium atoms is most preferable. The change in the charge/discharge capacity is attributed to such kind of reactions, and the three distinct effects are ascribed to the presence of different MgNi-perylene composites, formed during the ball milling on the surface, resulting in the retardation or acceleration of Mg(OH)2 formation on the electrode.

(Received May 20, 2002; Accepted July 16, 2002) Keywords: magnesium, nickel, aromatic compound, surface modiﬁcation, battery, electrode

1. Introduction characteristics of MgNi modified by aromatic compounds such as benzene (C6H6), phenanthrene (C14H10), naphtacene Properties of amorphous MgNi alloys have been exten- (C18H12), perylene (C20H12) and corannulene (C20H8). These sively studied because of their high potential as anode ma- compounds gave qualitatively similar results, while MgNi terial of rechargeable Ni/MH batteries.1Ð6) However, owing to modified by perylene showed better characteristics than oth- 7) the formation of Mg(OH)2 during charge/discharge cycles, ers. Therefore, this paper describes the effects of MgNi modi- the discharge capacity decreases rapidly with the cycles.1Ð7) fication by perylene on the electrode properties and discusses The cycle life of amorphous MgNi is considerably shorter plausible mechanisms of the effects of surface modification than other hydrogen storage materials such as AB5 and AB2 of MgNi on the charge/discharge properties. type alloys. The present authors have found that the formation of Mg(OH)2 is not necessarily due to charge/discharge cycles, 2. Experimental but it is simply formed by chemical reactions with electrolyte 8) solution. In addition, the degradation of charge/discharge Mg2Ni (below 200 mesh) and Ni (about 3Ð7 µm) powders properties is attributed to retardation of electron-transfer re- were mixed at molar ratio of 1:1. Two grams of the mix- 3 actions by Mg(OH)2 resulting in the reduction of hydrogen ture powder were put into a chromium steel vessel of 12 cm absorption by the electrode.8) All hydrogen absorbed could together with eighteen balls of chromium steel (5.5 mm diam- be released from MgNi under conventional discharge condi- eter) in argon atmosphere by using a glove box. A planetary tions.8) The diffusion of hydrogen in MgNi-powder was not ball mill apparatus (Fritsch P5) was used to grind mechani- the rate control step for discharge9) and the diffusion con- cally the mixture powder at a rotation speed of 350 rpm. After stant of hydrogen in MgNi did not change with the charge- 80 h of ball milling, the MgNi powder was taken out from the discharge cycles.10) These results suggest that the properties vessel and kept in argon. of MgNi can be improved by surface modification if the for- The prepared MgNi powder was mixed with perylene mation of Mg(OH)2 can be inhibited. (C20H12) under argon atmosphere; the content of perylene Iwakura et al.11Ð13) reported that ball milling MgNi with was 0.2, 1, 2 and 10 mass%. Each MgNi-perylene mixture, graphite for short time increased the discharge capacity and 1 g in total mass, was ball-milled under the above-mentioned cycle life of MgNi effectively. This improvement was as- conditions for 1, 4 and 10 h. cribed to the formation of some compounds by carbon and Each of ball-milled MgNi-perylene powder, as well as MgNi, which enhanced the hydrogen absorption ability.13) MgNi powder, was mixed with electrolytic copper powder The present authors have reported that similar enhancement (2Ð3 µm) in a mass ratio of 1:3, and 1 g of such mixture was observed by vacuum evaporation of carbon.14) was compressed at 590 MPa for 10 min into a disc with These results indicate that carbon in various crystal struc- 10 mm in diameter in argon. The disc was sandwiched in tures have capabilities to improve the electrochemical proper- two nickel meshes and fixed by spot-welding. The charge- ties of MgNi. The influence of aromatic compounds and other discharge curves were measured in 6 M KOH solution con- carbon containing materials, however, has not been studied, trolled at 25◦C by water bath. Two-electrode system was although it has been reported that the hydrogen absorption used, where the sandwiched sample disc acted as the nega- of Mg-graphite composites prepared by mechanical grinding tive electrode and sintered Ni(OH)2/NiOOH the counter elec- was strongly affected by the presence of benzene, cyclohex- trode. For each charge-discharge cycle, the sample was ane and tetrahydrofuran during grinding.15Ð18) The present charged at 100 mA·g−1 for 10 h, rested for 5 min, and then authors have started a systematic study on electrochemical discharged at 20 mA·g−1 to a cut off voltage of 1.0 V. 2712 T. Ma, Y. Hatano, T. Abe and K. Watanabe

3. Results and Discussion higher cell voltage.8) Accordingly, the modification by perylene is effective to enhance hydrogen acquisition by keeping Figure 1(a) shows the effects of ball milling time on the the lower cell voltage. These observations confirmed that the discharge capacity at the first cycle for MgNiÐ0.2 mass% charge/discharge capacity can be improved by surface modi- perylene. After ball milling for 1 h, the discharge capacity fication of amorphous MgNi by aromatic carbon compounds reached the value of 460 mAh·g−1, about 27% higher than such as perylene, and both the perylene ratio and ball-milling that of bare MgNi, 362 mAh·g−1. Further ball milling, how- time are crucial for the properties of perylene modified MgNi. ever, gave smaller discharge capacities as shown in the fig- Figure 3 shows changes in the discharge properties with ure. This implies that there is an optimal ball milling time charge/discharge cycles. According to these figures, the for MgNiÐ0.2 mass% perylene to achieve high discharge ca- effects of modification could be classified into three dis- pacity. For other perylene ratios, similar results were observed. At a given ball milling time, the perylene ratio also affected the discharge capability. The result of 1 h ball milling was shown in Fig. 1(b). There was also an optimal perylene 1.5 amount, 1 mass%, for MgNi to have high discharge capacity, / V V MgNi 497 mAh·g−1 in the first cycle. Such changes in the discharge capacity were attributed to 1.4 the change in the hydrogen charging process. Figure 2 com- pares the charge curves of MgNiÐ0.2 mass% perylen-4 h (in- MgNi-0.2mass perylene-4h dicating ball milling 0.2 mass% perylene with MgNi for 4 h, Cell Voltage, similar indication is used hereafter) and bare MgNi at the 1.3 first cycle. For both the bare MgNi and MgNiÐ0.2 mass% perylene-4 h, the cell voltage began to increase on charg- 0246810 ing, but the cell voltage for MgNiÐ0.2 mass% perylene-4 h Time, t / h increased slower than bare one. It was observed that the hydrogen absorption for MgNi takes place at the cell voltage Fig. 2 Charge curves for MgNiÐ0.2 mass% perylene-4 h and bare MgNi at around 1.35 V under the present charging conditions, and the first charge/discharge cycle. evolution of H2 gas predominates over hydrogen ingress at 500 1 -1h (a) Type A

-1 480 400 0.2 -4h (a) 300 Bare MgNi / mAhg 440 200 DC

100 400

Bare MgNi -1 500 (b) Type B

360 mAhg / 400 Bare MgNi DC 2 -10h

Discharge Capacity, 300 0246810 1 -4h Ball Milling Time, t / h 200

-1 600 (b) 100 500 0 / mAhg 400 DC Bare MgNi Discharge Capacity, (c) 400 300 Bare MgNi Type C

300 200 10 -1 h

200 100 10 -10h Discharge Capacity, 0.1 0.2 0.4 0.6 0.8 1 2 4 0 Perylene Ratio (mass ) 02468

Fig. 1 Typical effects of ball milling time and perylene ratio on the dis- Cycle (n) charge capacity of MgNi: (a) the effects of ball milling time on the discharge capacity of MgNiÐ0.2 mass% perylene at first charge/discharge Fig. 3 Effects of perylene modification on the charge/discharge properties cycle; (b) the effects of perylene ratio on the discharge capacity at first of MgNi. charge/discharge cycle after 1 h ball milling time. Surface Modification of MgNi by Perylene 2713 tinct types, A, B and C. Type A is the increase in the Figure 4 shows X-ray diffraction patterns of bare and discharge capacity at the first cycle (Fig. 3(a)). For ex- perylene-modified MgNi powders. The broad peak around ample, the discharge capacity for MgNiÐ1 mass%Ð1 h and 41.5◦ was assigned to amorphous MgNi and sharp peaks at MgNiÐ0.2 mass% perylene-4 h at the first cycle were 497 and 44.5, 51.8 and 76.5◦ were due to Ni remaining in a crystalline 460 mAh·g−1, respectively; about 37% and 27% higher than state. No significant difference was observed between the that of bare MgNi (362 mAh·g−1), respectively. But the dis- diffraction patterns of bare MgNi and of perylene-modified charge capacity still decreased with cycles as bare MgNi or powders. This observation suggests that the bulk of the pow- even faster. This type of effect appeared, in general, for der did not change markedly by ball-milling with perylene. ball milling with small amount of perylene addition (less Namely, the changes in electrochemical properties observed than 1 mass%) for short time (shorter than 4 h). Type B are ascribed to the modification of surface states. is the improvement in the capacity degradation with cycles Changes in the diffraction patterns with immersion in 6 M as seen in Fig. 3(b). Namely, the degradation rate of the KOH solution for 120 h are also shown in Fig. 5. Very discharge capacity was slower than that of bare MgNi, al- strong Mg(OH)2 peaks can be seen for MgNiÐ10 mass% though there appeared no noticeable increase in the capacity perylene-10 h specimen showing type C effect; the intensi- at the first cycle. For example, MgNiÐ1 mass% perylene-4 h ties of Mg(OH)2 peaks are significantly larger than those for and MgNiÐ2 mass% perylene-10 h showed discharge capac- bare MgNi. This observation indicates that a larger amount −1 ity about 200 mAh·g at the 8th cycle, almost twice of that of Mg(OH)2 was formed during the immersion for MgNiÐ of bare MgNi. This kind of effect was observed for samples 10 mass% perylene-10 h specimen than for bare one. The of much larger perylene addition (1 and 2 mass%) and longer specimen ball-milled with 1 mass% perylene for 1 h (type ball milling than type A. A) also showed higher intensities of Mg(OH)2 peaks than In the case of larger amounts of perylene addition, e.g. bare MgNi. On the other hand, MgNiÐ1 mass% perylene-4 h 10 mass%, both the discharge capacity and cycle life were re- specimen, which showed type B effect, gave the Mg(OH)2 duced as seen in Fig. 3(c). This kind of negative effect was peaks significantly weaker intensities in comparison with classified as type C. In this case, it was observed that a longer those showing type A and C effects; the comparison with ball milling time resulted in the lower discharge capacity and the bare specimen was difficult because the intensities of shorter lifetime. Type C effect was also observed in some ex- Mg(OH)2 peaks were so small for these specimens. It is, how- tent for a long ball milling time and a small amount of pery- ever, clear that the rate of degradation in discharge capacity lene addition. increased with the increasing extent of Mg(OH)2 formation According to our previous studies,8Ð10,14) it was concluded for the specimens ball-milled with perylene. that 1) the discharge capacity of amorphous MgNi is deter- In order to examine changes in the surface states of mined by the amount of hydrogen absorbed during charge specimens with perylene-modification, Auger electron spec- process, and almost all hydrogen absorbed can be released in discharge process under the present discharge conditions, 2) Mg(OH) the formation of Mg(OH)2 on the surface of MgNi retards the (d) 2 electron transfer and consequently decreases the hydrogen ab- Ni sorption by MgNi, and 3) carbon evaporated onto MgNi surface suppresses the retardation of electron transfer and consequently improve the cycle life. On account of these observations, the changes in the electrode properties presently (c) observed by modification with perylene are ascribed to the improvement and/or debasement of the hydrogen absorption and/or the Mg(OH)2 formation.

(b) Ni Intensity (arb. unit)

(b) (a)

(a) Intensity (arb. unit)

20 40 60 80 20 40 60 80 Angle, 2 Angle, 2 Fig. 5 XRD patterns by Cu Kα radiation for some tested powders Fig. 4 XRD patterns by Cu Kα radiation for (a) bare MgNi and (b) after immersion in 6 M KOH solution for 120 h: (a) MgNi, (b) MgNiÐ10 mass% perylene-10 h. MgNiÐ10 mass% perylene-10 h (Type C), (c) MgNiÐ1 mass%-4 h (Type B) and (d) MgNiÐ1 mass% perylene-1 h (Type A). 2714 T. Ma, Y. Hatano, T. Abe and K. Watanabe

Carbon deposited specimen Mg KLL

O KLL Ni LMM C KLL Bare specimen

3 dN / dE Intensity (arb. unit) Intensity (arb. Carbon deposited specimen Electron

Bare specimen 50 55 60 65 70 75 80 85 90 Auger Kinetic Energy, E / eV 0 100 200 300 400 500 600 700 800 900 1100 1200 1300

Kinetic Energy, E / eV Fig. 8 Detailed Auger electron spectra of Ni MVV transition for bare and carbon deposited Mg2Ni specimens. Fig. 6 Auger electron spectra of bare and carbon deposited Mg2Ni specimens. selectively with Mg. By use of X-ray photoelectron spec- Carbon deposited specimen troscopy, Iwakura et al.13) have observed similar phenomena Bare specimen that the Mg 2s peak is caused to shift toward the lower energy side by ball-milling of MgNi with graphite. Both of these XPS and AES observations suggest that a part of magnesium existed on the surface had been oxidized and it was reduced partially by forming MgÐC bonds. Consequently the electron density around a magnesium atom increased, resulting in the shifts of AES and XPS peaks as mentioned above. Sim-

Auger Electron Intensity (arb. unit) ilar change in the surface state can be expected for perylene- 1160 1170 1180 1190 1200 modified MgNi. Therefore, it is plausible that the selective re- Kinetic Energy, E / eV action between carbon and Mg takes place during ball milling Fig. 7 Detailed Auger electron spectra of Mg KLL transition for bare and of MgNi with perylene. carbon deposited Mg2Ni specimens. To examine mechanisms underlying the change in electrochemical properties by the surface modification, the electron redistribution caused by interactions between MgNi and pery- troscopy was applied for a Mg2Ni plate covered with a carbon lene was calculated for a model cluster by means of Gaussian- evaporated film, because a plate type specimen is more suit- 98.21) In this calculation, amorphous MgNi is assumed to able for accurate surface analysis than powder. The surface consist of Mg2Ni2 tetrahedral cluster units as suggested by state of this specimen is not exactly the same as that of amor- Orimo et al.22) The calculations were carried out for combi- phous MgNi modified by perylene. Nevertheless, this speci- nations with perylene and benzene by keeping the Mg2Ni2 men can be considered as a model to understand the changes tetrahedral structure, because the modification by these com- in surface states, because the influences of carbon evapora- pounds gave similar charge/discharge curves and capacity tion observed in the previous study14) were similar to present degradations with cycles as mentioned in Section 1. Since type A and B effects. Figure 6 shows the typical Auger elec- the model calculations also gave a similar trend, the results tron spectra of bare and carbon evaporated Mg2Ni specimens. are shown for Mg2Ni2-benzene clusters for simplicity. Three In addition to Mg and Ni, carbon and oxygen were detected different combinations were assumed between carbon atoms from both specimens. It should be emphasized that the carbon of benzene (or perylene) and a Mg2Ni2 cluster as shown in evaporation resulted in the increase in the peak intensity of Fig. 9, which shows isosurfaces of electron densities and par- carbon as well as Mg-peak. The detailed spectra of Mg KLL tial atomic charges obtained by Gaussian-98 calculation. The transition are shown in Fig. 7. The peak took the maximum calculation showed that a compound by two MgÐC bonds at 1182.5 eV in the case of the bare specimen. According to (denoted as Mg2ÐB, Fig. 9(b)) is most stable, and the order the literature,19) metallic Mg shows a peak in a range from of compound stability was Mg2ÐB > NiMgÐB (Fig. 9(c)) 1183.0 to 1186.5 eV and it is shifted by oxidation to the lower > Ni2ÐB (Fig. 9(d)). The same order of stability was obtained energy side about −5.9to−6.2 eV. Hence, the broad peak for perylene compounds, suggesting that compounds like the observed at 1182.5 eV can be ascribed to coexisting metal- Mg2-perylene type is most likely formed by ball-milling of lic and oxidized Mg. The carbon evaporation resulted in the MgNi with perylene. shift of this peak to higher energy side as shown in this fig- As for the Mg2-perylene compound like Fig. 9(b), MgÐ ure. On the other hand, no significant change was observed in C bonding is accompanied by partial charge separation, by Ni-peak by carbon evaporation as seen in Ni MVV transitions which Mg atoms are charged much more positively and Ni shown in Fig. 8 as differential spectra. The peaks appearing more negatively than bare Mg2Ni2 (Fig. 9(a)). On forming around 64 eV for both the bare and carbon evaporated sam- the MgÐC bond, nickel atoms on the surface are kept free ples indicate that Ni was present mainly in a metallic state.20) from carbon and then are expected to play a role as active sites Hence, it can be concluded that evaporated carbon reacted for electrochemical reaction. This implies that hydrogen ab- Surface Modification of MgNi by Perylene 2715

Fig. 9 Calculation results for three different combinations between carbon atoms of benzene and a Mg2Ni2 cluster by Gaussian-98. The isosurfaces of electron densities and partial atomic charges are indicated. (a) Mg2Ni2 cluster, Mg atom (red) with positive charge and Ni (green) with negative charge, (b) carbon atoms combine with Mg atoms, Mg2ÐB, (c) carbon atoms combine with Mg atom and Ni atom, NiMgÐB, and (d) carbon atoms combine with Ni atoms, Ni2ÐB. These results were graphed by MOLEKEL 4.2.23)

sorption by the MgNi-electrode can be improved. In addition, of Mg(OH)2 formation by MgÐC bonding, although the dis- the formation of Mg(OH)2 is expected to be retarded in this tinction between these positive effects is open to further stud- case, because Mg atoms bound to C will be blocked from the ies. Similar mechanism can be expected for the improvement − attack by OH ions or H2O molecules in the electrolyte solu- in the discharge capacity and cycle life observed in the previ- 14) tion. However, the complete retardation of Mg(OH)2 forma- ous study by carbon evaporation. Conversely, type C effect tion cannot be expected because of the large positive charge can be attributed to the formation of Ni2-perylene composite, and low electron density of Mg atoms. As for Ni2-perylene where non-active Mg atoms are left on the surface, leading to compound as Fig. 9(d), there appeared no appreciable change the Mg oxidation to form Mg(OH)2. in atomic charges from the bare MgNi. In this case, how- It should be also mentioned here that Gaussian 98 calcu- ever, no improvement in electrochemical properties can be lations resulted in rupture of the MgNi tetrahedron and for- expected, because magnesium is not active for the reaction. In mation of MgÐC and NiÐC compounds in the case of no re- addition, Mg atoms can be oxidized easily to form Mg(OH)2, striction of keeping the tetrahedral structure. This suggests resulting in the poor discharge capacity and cycle life. As for that rigorous ball milling of MgNi with perylene and/or other MgNi-perylene like Fig. 9(c), it will be active in some extent carbon compounds causes destruction of the tetrahedral func- but less than Mg2-perylene. tional structure of MgNi, leading to the loss of electrochem- Although the mechanism underlying the reaction between ical and hydrogen absorbing properties. This appears to be MgNi and perylene during ball milling has not been fully un- one of the reasons of type C effect, the reduction of discharge derstood, the type A and B effects can be most likely ascribed capacity and cycle life by excessive ball milling with carbon to the formation of Mg2-perylene composite. This is because compounds. the formation of free Ni atoms on the surface and retardation Concerning AES as well as XPS analyses, a Mg AES peak 2716 T. Ma, Y. Hatano, T. Abe and K. Watanabe should shift toward lower kinetic energy side and a Mg XPS analyses. peak to higher side if bare MgNi surface is modified by carbon compounds as perylene and graphite, because the atomic REFERENCES charge of Mg becomes much positive by the modification. The present AES spectra and XPS observations by Iwakura 1) Y. Lei, Y. Wu, Q. Yang, J. Wu and Q. Wang: Z. Phys. Chem. Bd. 183 13) (1994) 379Ð384. et al., however, indicate that a part of Mg on the surface 2) L. Sun, P. Yan, H. K. Liu, D. H. 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