Results and Discussion 3-1 Preparation of Co(OH) 2 /Cu

Using Cu foil as the working electrode for the electrodeposition of Co(OH)2 at the current density (cd) of 0.5 mA cm^-2, the cathodic potential sharply increased from -1.1 V to -1.3 V in the initial stage of the first run, and then the cathodic potential was decreased slowly to -1.1 V as sown in Fig. 1. The electrochemical reactions on the Cu foil were proposed to be the reduction of NO3

- [48] and H2O

NO3

- + H2O + 2e^- → 2OH^- + NO2 - (1)

2H2O + 2e^- → H2 + 2OH

-(2)

Hence the significant increase in the cathodic potential from -1.1 to -1.3 V was mainly caused by establishing the diffusion boundary layer on the cathodic surface. The hydroxide ion (OH^-) produced in equations (1) and (2) was deposited with Co²⁺ in the solution onto the Cu foil. At the same time the increase in the concentration of H⁺ in the solution was due to the anodic

oxidation of H2O on the anode, and resulted in the decrease in the pH of solution from 5.40 to 3.30 in the first run of electrodeposition (Table 1).

The increase in the concentration of H⁺ induced the cathodic reduction of H⁺ on the cathode, and resulted in the decrease in the cathodic potential from -1.3 to -1.1 V (Fig. 1).

2H⁺ + 2e^- → H2

(3)

In the first run of electrodeposition of Co(OH)₂ a non-uniform film was obtained due to the higher Co(OH)₂ deposition rate caused by the fast OH -generation rate based on equations (1) and (2).

The cathodic reaction of H2O to produce OH^- (equation (2)) was generally replaced by the evolution of H2 (equation (3)) for the run number greater than 2 due to the decrease in pH of the solution (Table 1). For the run number greater than 2, the uniform Co(OH)2 deposits on Cu foil were obtained due to the slower electrodeposition rate caused by the less OH -generation rate for compared with the first run. Furthermore the pH and the

value for the sixth run. Increasing the run number from 2 to 6 the weight of Co(OH)2 deposit decreased from 0.913 to 0.750 mg due to the slower deposition rate (Table 1).

3.1.1 Characterization of Co(OH)2

As indicated in Table 1, the pH of solution for depositing Co(OH)₂ was less than 3.30 for the run number greater than 2. Hence a part of Co(OH)2 deposit would reaction with H⁺ in the solution to form crystal water

Co(OH)2 + xH⁺ → [Co(OH)2-x⋅(H2O)x]^x+

(4)

At the same time NO3

- was inserted into the structure to compensate the positive charge of the deposit to form α-type Co(OH)2 [49-50],

[Co(OH)2-x⋅(H2O)x]^x+ + xNO3

- →

Co(OH)2-x(NO3)x⋅y(H2O) (5)

When the as-deposited Co(OH)2 was dried in 70 ^oC vacuum oven and then

analyzed by FTIR, the wavenumbers of 630 and 1384 cm^-1 were found due to the Co-O and N-O bonds (curve (a) of Fig. 2). The broad wavenumber of 3480 cm^-1 was deduced to be the O-H stretching with the hydrogen bond.

The experimental results revealed that the electrolytic deposit was the α-Co(OH)2. The α-type Co(OH)2 was also synthesized and reported in the literatures [49-50].

The as-deposited α-Co(OH)2 immersed in 1.0 M KOH aqueous solution for various times were washed with DI water for several times, and then dries in an 70 ^oC vacuum oven for 24 h. The products were analyzed by FTIR as illustrated in curves (b) ~ (h) of Fig. 2. The strength of wavenumber of 1384 cm^-1 caused by the N-O stretching was decreased with the time for immersing in 1.0 KOH solution, and disappeared finally for the immersing time greater than 10 min. On the other hand, the peak strength of 3630 cm^-1 due to the O-H stretching without hydrogen bond increased with the immersing time as shown in curves (b) ~ (h) of Fig. 2.

The experimental results indicated that the generally replacing NO3

-inserted in the α-Co(OH)2 by OH^- resulted in the decrease in the peak strength of 1384 and 3480 cm^-1, and the increase in the 3630 cm^-1 peak

min. Decreasing the pH value for electrodeposition the amount of NO3

-inserted into the α-Co(OH)2 structure increased, and then the time for replacing NO3

- by OH^- in the immersion process increased. The time for completely converting α-Co(OH)2 prepared in pH 2.82~2.71 to its β-type was experimentally found to be 20 min.

3-1-2 Effect of pH

The compositions of the cobalt hydroxide electrosynthesized in pH of 3.30~3.14 were similar to that prepared in pH 2.82~2.71 as shown in the results of EDS (Energy Dispersive Spectrometer) analysis (Fig. 3(a) and (b)). The Au peak was caused by the preparation of sample with Au sputtering, and the Si signal was found in the samples prepared in different pH values. A part of the silicon rubber used to fix the area of substrate (Cu foil) for preparing Co(OH)2 deposit was dissolved and resulted in the co-deposition with Co(OH)2. This deduction was demonstrated by the absence of Si signal for Co(OH)2 deposited on the Cu foil without using silicon rubber to fix the area of substrate.

The nano-fibrillar structures were found from the SEM images of α-Co(OH)2 electrosynthesized in the various pH as shown in Fig. 4. The anisotropic growth of the deposit along with one crystal orientation resulted

in the nano-fibrillar structure. The nanorod structure of cobalt-hydroxide-carbonate was also reported in the literature [51]. A relative rougher nano-fibrillar structure with diameter of 30~40 nm was obtained for α-Co(OH)₂ prepared at a higher pH (3.30~3.14) (Fig. 4(a)).

Decreasing the pH of solution the electrodeposition rate of α-Co(OH)2

decreased due to the lower generation rate of OH^- from the cathode, and resulted in the decrease in the diameter of the nanofiber (Fig. 4(b) and (c)).

When α-Co(OH)2 prepared at pH of 3.30~3.14 and 2.82~2.71 were immersed in 1.0 M KOH aqueous solution for 10 and 20 min, respectively, to convert α-Co(OH)2 to be its β form, a part of nano-fibrillar structure of α-Co(OH)2 were collapsed due to the replacement of NO3

- by OH^- (Fig. 5).

3-2 Preparation and Characterization of CoO/Cu

The CoO could be prepared by the calcination of cobalt hydroxide in a high purity of N2 atmosphere. However, the doping ion NO3

-existed in the α-Co(OH)₂ played a role of oxidant to oxidize the CoO to be Co3O4 for the temperature of 300 ^oC [50]. The pure Co3O4 phase found

α-Co(OH)2 structure for the temperature greater than 350 ^oC [50]. When the calcination temperature was increased to 400 ^oC, a part of Co3O4 was decomposed to CoO by releasing oxygen, and hence formed a mixing phases of Co3O4 and CoO as illustrated in the curve (c) of Fig. 6. The pure phase of CoO was obtained for the calcination temperature greater than 500

oC.

When the α-Co(OH)2 precursors prepared in the various pH were calcinated in 500 ^oC in 99.995% N2 for 1 h, the pure CoO phase was obtained and demonstrated by the XRD spectra (curves (a) ~ (c) of Fig. 7).

The average grain size was calculated based on the Sherrer equation at 2θ of 36.5 and 42.4^o corresponded with the crystal orientation planes of (1 1 1) and (2 0 0), respectively. The grain size of CoO based on (1 1 1) orientation plane decreased from 12.88 to 6.98 nm with decreasing the pH for preparing its precursor (α-type Co(OH)2) from 3.30~3.14 to 2.82~2.71 (Table 2). Decreasing the pH of solution for preparing α-Co(OH)₂ the NO₃ -inserted into the structure of the deposit increased, and the oxygen evolved in the decomposition of Co3O4 to CoO in the calcinations process increased.

Hence the grain size of CoO decreased with the pH for preparing the precursor of CoO (α-Co(OH)2).

The nano-fibrillar structure of CoO prepared by the calcination of

α-Co(OH)2 was found from the SEM images in Fig. 8(a), (b) and (c), which

were similar with its precursors (α-type) as indicated in Fig. 4. The results revealed that the main structure of α-Co(OH)2 was not affected in the calcining procedure. The porosity of the nano-fibrillar structure of CoO decreased with the pH of the solution for preparing α-type precursor.

3.3 Charge/discharge characteristics of CoO/Cu

The charge/discharge rate of Li/CoO battery was calculated based on the theoretical capacity of CoO multiplied by the actual weight of CoO thin film on the Cu foil substrate. The theoretical discharge capacity of CoO was obtained to be 715.4 mAh g^-1 based on the following charge/discharge process [23]

CoO + 2Li⁺ + 2e^- ⇌ Li₂O + Co (6)

Using CoO/Cu prepared at pH of 3.30~3.14 as cathode, the discharge potential of CoO/Li coin cell sharply decreased from the OCV (2.704 V) to the potential of 0.86 V at 0.1 C rate corresponded to the

-1

the CoO surface, and the valence of Co was unchanged [28]. The discharge plateau in the range of 0.86 ~ 0.72 V in the first cycle corresponded with the capacity of 715 mAh g^-1 was deduced to be the reduction of CoO to Co as indicated in equation (6), which was accompanied with the decomposition of CoO grain size from 12.88 nm (Table 2) to about 1~2 nm [23]. The discharge potential decreased from0.72 to 0.02 V corresponding to the capacity of 433.1 mAh g^-1 was inferred as the formation polymer/gel-like film [26-27]. Cobalt was re-oxidized to CoO in the first charge cycle, but the particle size of CoO was remained in 1~2 nm. The discharge plateau for the reduction of CoO (equation (6)) was increased to 2.0 ~ 1.0 V in the second discharge cycle. Compared with the first discharge cycle the significant increase in the discharge plateau potential was due to the significant decrease in the particle size of CoO. Furthermore the formation of solid electrolyte interface in the first discharge cycle was not found in the second discharge cycle. The discharge capacity decreased from 1348.1 to 1089.2 mAh g^-1 by increasing the cycle number from 1 to 2 was mainly caused by the absence of the formation of solid electrolyte interface, and partially caused by the decreased in the reduction of CoO (equation (6)) as shown in Fig. 9.

As shown in Fig. 10, when the precursor of CoO (α-Co(OH)2)

was prepared at pH 3.30~3.14, the discharge capacity of Li/CoO coin cell increased from 1089.2 mAh g^-1 to a maximum value of 1589.4 mAh g^-1 with the increase in the cycle number from 2 to 25, which was defined as the activation period. As indicated in the discharge curve with cycle number of 30 in Fig. 9, compared with the results in the cycle number of 2, the increase in the discharge capacity was mainly contributed by increasing the capacity for forming polymer/gel-like film. The discharge capacity of polymer/gel-like film increased from 418.2 to 752.4 mAh g^-1 by increasing the cycle number from 2 to 30 (Fig. 9). The results indicated that the irreversible capacity in the first cycle could be completely recovered in the activation period. The stable discharge capacity could be obtained for the cycle number between 25 and 40. However, the discharge capacity decreased from 1555.1 to 774.7 mAh g^-1 with the increase in the cycle number from 40 to 70 (Fig. 10). The decrease in the discharge capacity for the cycle number greater than 40 was found to be caused by the decrease in the capacity based on the reduction of CoO (equation (6)) as indicated in the discharge cure with cycle number of 70 in Fig. 9. The results might be due to the increase in the diffusion resistance of Li⁺ through the

2.92~2.82 and 2.82~2.71, the charge/discharge behaviors of Li/CoO coin cells were similar to that the precursor prepared at pH 3.30~3.14. The activation periods were also found for the precursor of CoO prepared at pH 2.92~2.82 and 2.82~2.71 (Fig. 10). The results in Fig. 10 also revealed that the fading in the discharge capacity with the cycle number for the precursor of CoO prepared at pH 2.92~2.82 and 2.82~2.71 was significantly less than that of prepared at pH 3.30~3.14. As illustrated in Table 1 decreasing the pH for preparing α-Co(OH)2 the weight of α-Co(OH)2 decreased, and therefore the weight and thickness of the CoO thin film decreased.

Compared with the precursor prepared at pH 3.30~3.14 the insignificant fading in the discharge capacity for precursor prepared in the pH 2.92~2.81 and 2.82~2.71 might be caused by the thinner CoO film on Cu.

4. Conclusions

The characteristics of CoO thin film and its precursor (Co(OH)2), which was electrodeposited on Cu foil by the constant current method at the various pH, were analyzed by XRD, FTIR and SEM, respectively. The pH of solution for electrodepositing cobalt hydroxide decreased from 5.40 to 2.66 by increasing the run number from 1 to 6. The uniform Co(OH)2

deposits obtained for the run number greater than 2 were demonstrated to be α-type (Co(OH)_2-x(NO₃^-)_x⋅y(H2O)) by the FTIR spectra. The pure Co₃O₄, mixing Co3O4 and CoO, and pure CoO phases were obtained for the calcining temperatures of 300 ^oC, 400 ^oC and greater than 500 ^oC, respectively, at a high purity N2 atmosphere. Increasing the run number for preparing α-Co(OH)2 from 2 to 6 the weight of α-Co(OH)2 and CoO decreased from 0.913 and 0.700 mg to 0.750 and 0.525 mg due to the decrease in the pH of electrolyte for electrodepositing α-Co(OH)₂. The grain size of CoO decreased from 12.88 to 6.98 nm with the decrease in the pH for preparing α-Co(OH)2 from 3.30~3.14 to 2.82~2.71. The morphology of α-Co(OH)2 changed slightly in the calcination procedure,

α

Li/CoO coin cell was obtained to be 1589.4 mAh g^-1, which was contributed by the formation of solid electrolyte interface, the reduction of CoO and the formation of polymer/gel-like film, respectively. The irreversible discharge capacity of the Li/CoO coin cells at the first cycle could be recovered in the following activation cycles.

Acknowledgment

The financial support of the National Science Council Republic of China (Project number: NSC 96-2120-M-011-001) and Tunghai University is acknowledged.

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List of Tables

Table 1 Effect of the run number on the preparation of Co(OH)2. Working electrode: Cu foil (4×4 cm²), counter electrode: Au plate (5×5 cm²) × 2, reference electrode: Ag/AgCl/3 M NaCl aqueous solution, cd = 0.5 mA cm^-2, T = 5 ^oC, electrolysis time = 40 min, electrolyte: 0.175 M Co(NO3)2, 0.075 M NaNO3 aqueous and ethyl alcohol solution (v/v = 1), volume of electrolyte = 250 ml.

Table 2 Effect of pH for preparing α-Co(OH)2 on the grain size of CoO

Conditions for preparing α-Co(OH)2: working electrode: Cu foil (4×4 cm²), counter electrode: Au plate (5×5 cm²) × 2, reference electrode:

Ag/AgCl/3 M NaCl aqueous solution, cd = 0.5 mA cm^-2, T = 5 ^oC, electrolyte: 0.175 M Co(NO3)2, 0.075 M NaNO3 aqueous and ethyl alcohol solution (v/v = 1), volume of electrolyte = 250 ml, t = 40min.

Conditions for calcining Co(OH)2: T = 500 ^oC, temperature increasing and decreasing rate = 5 ^oC min^-1, t = 1 h, 99.995% N2.

Table 1

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

⎯⎯⎯

Run no. pH Residual Co^2+@

Weight^#/mg

%

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

α-Co(OH)2 CoO

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

⎯⎯⎯

1 5.40→3.30 99.5

---- ---

2 3.30→3.14 99.0

0.913 0.700

0.800 0.575

5 2.82→2.71 97.5

0.763 0.538

6 2.71→2.66 97.0

0.750 0.525

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

⎯⎯⎯

@: Residual Co²⁺ in the solution(%) = ([Co²⁺]/[Co²⁺]i)×100%, where [Co²⁺] and [Co²⁺]i were the concentration of Co²⁺ in the present time and the initial state, respectively.

#: The average weights were measured based on the 8 pieces of the samples with area of 1.327 cm².

Table 2

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

pH for preparing Grain size of CoO/nm precursor

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

(1 1 1)

(2 0 0)

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

2.82~2.71 6.98

7.61

2.94~2.81 8.68

7.10

3.30~3.14 12.88

12.18

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

List of Figures

Fig. 1 Effect of electrolysis time on the voltage for electrodeposition of Co(OH)2.

Working electrode: Cu foil (4×4 cm²), counter electrode: Au plate (5×5 cm²) × 2, reference electrode: Ag/AgCl/3 M NaCl aqueous solution, cd = 0.5 mA cm^-2, T = 5 ^oC, electrolyte: 0.175 M Co(NO3)2, 0.075 M NaNO3 aqueous and ethyl alcohol solution (v/v = 1), volume of electrolyte = 250 ml.

Fig. 2 FTIR spectra of Co(OH)2 electrosynthesized in pH 3.30 ~ 3.14 immersed in 1.0 M KOH(aq) with (a) 0, (b) 0.5, (c) 1.0, (d) 1.5, (e) 2.0, (f) 5.0, (g) 10.0, and (h) 15.0 min.

Fig. 3 EDS of α-Co(OH)2 prepared in pH of (a) 3.30~3.14, (b) 2.82~2.71 in the presence of silicon rubber, and in pH of (c) 3.30~3.14 in the absence of silicon rubber.

Fig. 4 SEM images of α-Co(OH)2/Cu prepared at pH of (a) 3.30~3.14, (b) 2.94~2.82, (c) 2.71~2.66.

Working electrode: Cu foil (4×4 cm²), counter electrode: Au plate (5×5 cm²) × 2, reference electrode: Ag/AgCl/3 M NaCl aqueous solution, cd = 0.5 mA cm^-2, T = 5 ^oC, electrolyte: 0.175 M Co(NO3)2, 0.075 M NaNO3 aqueous and ethyl alcohol solution (v/v = 1), volume

of electrolyte = 250 ml, electrolysis time = 40 min.

Fig. 5 SEM images of β-Co(OH)2/Cu prepared at pH of (a) 3.30~3.14 and immersed in 1.0 M KOH(aq) for 10 min, (b) 2.82~2.71 and immersed in 1.0 M KOH(aq) for 20 min.

Working electrode: Cu foil (4×4 cm²), counter electrode: Au plate (5×5 cm²) × 2, reference electrode: Ag/AgCl/3 M NaCl aqueous solution, cd = 0.5 mA cm^-2, T = 5 ^oC, electrolyte: 0.175 M Co(NO3)2, 0.075 M NaNO3 aqueous and ethyl alcohol solution (v/v = 1), volume of electrolyte = 250 ml, electrolysis time = 40 min.

Fig. 6 XRD spectra of cobalt oxides prepared by the calcination of α-Co(OH)2/Cu in the various temperatures.

Conditions for preparing α-Co(OH)2: working electrode: Cu foil (4×4

cm²), counter electrode: Au plate (5×5 cm²) × 2, reference electrode:

Ag/AgCl/3 M NaCl aqueous solution, cd = 0.5 mA cm^-2, T = 5 ^oC, electrolyte: 0.175 M Co(NO3)2, 0.075 M NaNO3 aqueous and ethyl alcohol solution (v/v = 1), volume of electrolyte = 250 ml, pH 3.30~3.14, t = 40min.

Conditions for calcinating α-Co(OH)

(b) 300, (c) 400, (d) 500 and (e) 600 ^oC.

Fig. 7 XRD spectra of cobalt oxides prepared by the calcination of α-Co(OH)2/Cu prepared at pH of (a) 2.82~2.71, (b) 2.94~2.84, (c) 3.30~3.14.

Conditions for preparing α-Co(OH)2: working electrode: Cu foil (4×4 cm²), counter electrode: Au plate (5×5 cm²) × 2, reference electrode:

Ag/AgCl/3 M NaCl aqueous solution, cd = 0.5 mA cm^-2, T = 5 ^oC, electrolyte: 0.175 M Co(NO3)2, 0.075 M NaNO3 aqueous and ethyl alcohol solution (v/v = 1), volume of electrolyte = 250 ml, t = 40min.

Conditions for calcining Co(OH)2: T = 500 ^oC, temperature increasing and decreasing rate = 5 ^oC min^-1, t = 1 h, 99.995% N2.

z: JCPDS of CoO

Fig. 8 SEM images of CoO/Cu prepared by calcinating α-Co(OH)2

electrodeposited at pH of (a) 3.30~3.14, (b) 2.92~2.82, (c) 2.82~2.71.

Fig. 9 Charge/discharge curves of Li/CoO coin cell

Conditions for preparing α-Co(OH)2: cd = 0.5 mA cm^-2, pH 3.30~3.14, t = 40 min. Calcination conditions for preparing CoO: T

= 500 ^oC, t = 1 h, 99.995% N2. Conditions for coin cell: cathode:

CoO/Cu, anode: Li foil, 1.0 M LiPF6 in EC-DEC (1:1).

Charge/discharge conditions: rate = 0.1 C, first discharge cycle:

OCV to 0.02 V, other cycles: 0.02 ~ 3.0 V, T = 30 ^oC.

Fig. 10 Effect of charge/discharge cycle number on the capacity of Li/CoO coin cell

Conditions for preparing α-Co(OH)2: cd = 0.5 mA cm^-2, t = 40 min.

Calcination conditions for preparing CoO: T = 500 ^oC, t = 1 h, 99.995% N2. Conditions for coin cell: cathode: CoO/Cu, anode: Li foil, 1.0 M LiPF6 in EC-DEC (1:1). Charge/discharge conditions:

rate = 0.1 C, first discharge cycle: OCV to 0.02 V, other cycles:

0.02 ~ 3.0 V, T = 30 ^oC.

Fig. 1

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Cycle No.

10 30 50 70 90 110

0 20 40 60 80 100 120

Capacity / mAh g-1

0 200 400 600 800 1000 1200 1400 1600

3.30~3.14(Discharge) 3.30~3.14(Charge) 2.92~2.82(Discharge) 2.92~2.82(Charge) 2.82~2.71(Discharge) 2.82~2.71(Charge)

Fig. 10

pH for preparing precursor

在文檔中 直接甲醇燃料電池奈米結構材料之研究(III) (頁 32-68)