Supporting Information

Ideal nanoporous gold based supercapacitors

with theoretical capacitance and high energy/power density

Sun-I Kim, Sung-Wook Kim, Kyoungok Jung, Jin-Baek Kim, and Ji-Hyun Jang*

School of Energy and Chemical Engineering, 689-798, UNIST, Republic of Korea

Department of Chemistry, 305-701, KAIST, Republic of Korea

Calculation of supercapacitor capacitance.

(1) Volumetric capacitance

The volume of the electrode was calculated after measuring each area and height. The

volumetric capacitance was calculated by considering both the volume of the NPG

electrode and that of Ni(OH)2. We assumed that the active material Ni(OH)2 occupied the

whole volume of the inner pores of NPG when the area of electrode was fixed as 1 cm X

1 cm = 1 cm2. The height of the electrode was measured from cross-sectional SEM

images of the samples.

Total volumeof electrode=Active area X Height of electrode

In the case of the Ni(OH)2/NPG electrode with a total height of 1.140 um, 270 nm

Ni(OH)2 and 870 nm NPG, reacting for 10 min.

Total volumeof electrode=1 cm×1 cm × (1.140 ×10−4 ) cm

¿1.14 ×10−4 cm3

a. Cyclic voltammograms1

C= 12∙ v ∙∆ V ∙ v∫ idV C specific capacitance

v volume of electrode (cm-3)

∆V window range

V scan rate (V/s)

ʃ Idv area of CV curve

C= 12 × (1.14 × 10-4 cm3 ) × (0.65-0.1 )× (1 × 10-3 V /s )

× (0.26× 10-3 )

¿2073 F /cm3

b. Galvanostatic discharge curves

C s=i

v ¿¿ C specific capacitance

M mass of active materials

∆V window range (remove IR loss)

∆t discharge time

i current density

C= (0.4 × 10-3 A )

(1.14 × 10-4 cm3 ) × (0.65 -0.1-0.05 (IR loss) ) / (316.8 s )

¿2223 F /cm3

(2) Gravimetric capacitance.

1) Considering the mass of Ni(OH)2 only for the mass of active materials:

The weight of loaded active materials was calculated by using the mass of Ni(OH)2

2

only, since the mass of NPG is 6~7 times higher than the mass of Ni(OH)2. The

weight was carefully measured using a micro-balance (Cubis_micro balance

MSE3.6P, Sartorius) at least three times in order not to underestimate the exact mass

of the active materials.

Weight of active material=Total weight−Weight of Au electrode

The capacitance was calculated by cyclic voltammograms and galvanostatic discharge

curves:

In the case of the Ni(OH)2/NPG electrode with the total height of 1.140 um (270 nm

Ni(OH)2 and 870 nm NPG),

Total weight of active materials: 382.933 mg - 382.853 mg = 0.08mg

a. Cyclic voltammograms

C= 12∙ m∙ ∆ V ∙ v∫ i dV C specific capacitance

M mass of active materials

∆V window range

V scan rate (V/s)

ʃ Idv area of CV curve

C= 12 × (0.08 × 10-3 g ) × (0.65-0.1 )× (1 × 10-3 V /s )

× (0.26× 10-3 )

¿2954 F /g

a. Galvanostatic discharge curves

C s=i

m¿¿ C specific capacitance

M mass of active materials

∆V window range (remove IR loss)

3

∆t discharge time

i current density

C= ( 0.4 ×10 -3 A )

(0.08 × 10-3 g ) × (0.65 -0.1-0.05 (IR loss) ) / (316.8 s )

= 5 A/g(0.65 -0.1-0.05 (IR loss) ) / (316.8 s )

¿3168 F/ g

2) Including the weight of NPG as the active materials

When the mass of 870 nm height of NPG (0.5mg) was taken into account for the total

weight, the mass of active materials increased.

Weight of active material=Total weight−Weight of Si wafer

In the case of Ni(OH)2/NPG electrode with total height 1.140 um, 270 nm Ni(OH)2 and

870 nm NPG, reacting for 10 min.

Total weight of active materials: 382.933 mg - 382.353 mg = 0.58 mg

(Deposited Ni(OH)2 weight : 0.08 mg, Deposited NPG weight : 0.5 mg)

a. Cyclic voltammograms

C= 12∙ m∙ ∆ V ∙v∫ i dV C specific capacitance

M mass of active materials

∆V window range

4

V scan rate (V/s)

ʃ Idv area of CV curve

C= 12 × (0.58 × 10-3 g ) × (0.65-0.1 )× (1 × 10-3 V /s )

× (0.26× 10-3 )

¿407 F / g

b. Galvanostatic discharge curves

C s=i

m¿¿ C specific capacitance

M mass of active materials

∆V window range (remove IR loss)

∆t discharge time

i current density

C= ( 0.4 ×10 -3 A )

(0.58 × 10-3 g ) × (0.65 -0.1-0.05 (IR loss) ) / (316.8 s )

¿437 F / g

Activematerials Capacitance Cycle retention Reference

ppy/NPG, MnO2/NPG 195 F/g 85% (2000 cycle) [16] J. Mater. Chem A (2014) 2, 10910

MnO2/NPG 160 F/g 81 % (1000 cycle) [17] J. Mater. Chem A (2013) 1, 9202

MnO2/NPG 1145 F/g,1160 F/cm3 ~80 % (500 cycle) [18] Nature Nanotechnology (2011) 6, 232

5

SnO2/NPG 75 F/cm3 ~ 8% variation(30,000 cycle)

[19] Adv. Energy Mater. (2014) 4, 1301809

RuO2/NPG, Co(OH)2/NPG

1300 F/g,1800 F/g(350 F/g)

~78% (3000 cycle) [20] J. Mater. Chem A (2014) 2, 8448

RuO2/NPG 1450 F/g no measure [21] Adv. Energy Mater. (2013) 3, 851

Pani/NPG 1500 F/cm3 no measure [22] J. power sources (2012) 197, 325

Ni(OH)2/NPG 3168 F/g2223 F/cm3 ~90 % (30000 cycle) Our research

Table 1. Comparison of the capacitance and cycle retention with other previously reported supercapacitor using metal oxide/NPG electrode

Ohmic contacts between metal and semiconductor are formed when the charge induced in the

semiconductor in aligning the Fermi levels is provided by majority carriers.

6

Figure S1. Ohmic metal-semiconductor contacts: the equilibrium band diagram for the junction of (a) n-type semiconductor, and (b) p-type semiconductor

In the case of n-type semiconductor, work function of semiconductor is larger than that of

metal.

Φm < Φs

Similarly, in the case of p-type semiconductor, work function of semiconductor is smaller

than that of metal.

Φm > Φs

We estimated the work function of the semiconductor as the value between Fermi level and

conduction band (n-type) or valance band (p-type).

Table 2. Work function of gold and that of metal oxides.

N typeor

P typemetal oxide

Bandgap

Electron affinity

Work function of

semiconductor

Work functionof Gold

P-type (Ni(OH)2) 3.5 eV 1.47 eV 2.58~4.97 5.1 eVP-type (Co(OH)2) 2.7 eV ~2 eV 3.35~4.7 5.1 eVP-type (RuO2) 2.2 eV 4.87 eV 5.97~7.07 5.1 eVP-type (PANI) 2.7 eV 4.4 eV 5.75~7.1 5.1 eVn-type (MnO2) 2.5 eV 2.9 eV 2.9~4.15 5.1 eVn-type (SnO2) 2.7 eV 4.32 eV 4.32~5.67 5.1 eV

7

Figure S2. Ohmic contact between the NPG electrode and Ni(OH)2 semiconductor: schematic image of measurement of Current-Voltage characteristics (Top) and the measured Current–Voltage curve (Bottom).

8

Figure S3. Scheme of fabricating the NPG electrode

Figure S4. XRD curve of the Ni(OH)2/NPG electrode

9

It was extremely difficult to detect Ni(OH)2 in the XRD data (Figure S1) of the Ni(OH)2/Au

composite due to the poor crystalline nature of α-type Ni(OH)2 with lower intensity. After

depositing Ni(OH)2, new peaks near 11o and 19 o, which corresponded to the (300) and (001)

plane of Ni(OH)2, respectively, appeared as shown in the magnified XRD pattern, implying

that α-type Ni(OH)2 was well synthesized on the NPG electrode without any impurity.

10

Figure S5. XPS curve of bare NPG (black line) and Ni(OH)2/NPG (red line)

In order to further confirm the existence of Ni(OH)2 on the NPG electrode, we measured the

XPS as shown in Figure S2. While there was no distinct peak for the NPG, Ni2p peaks at the

binding energy from 850 eV to 890 eV and a strong O1s peak at the binding energy of 530.9

eV appeared in Ni(OH)2/NPG, which indicated the presence of Ni(OH)2 in the NPG

electrode. The small O1s peak in the NPG electrode (black line) may be due to adsorbed

water or oxygen molecules from ambient air conditions, and there were no other peaks

except the main peaks of Ni(OH)2.

11

Figure S6. Schematic illustration, SEM image, and TEM image of boundary of two regions

Figure S7. SEM images of the morphology changes of Ni(OH)2/NPG with various 12

lengths of hydrothermal reactions during the deposition of Ni(OH)2.

By increasing the hydrothermal reaction time, the thickness of the active materials, Ni(OH)2

on NPG, increased. In fact, the actual thickness of Ni(OH)2 dramatically increased when the

reaction time was over 30 mins, due to lack of control during long deposition times, and this

caused the creation of many lumps of Ni(OH)2 on the evenly deposited Ni(OH)2 layer. The

actual height of the electrode in region A thus varied from 240 nm, 480 nm, ~6.3 μm to ~11.8

μm on a fixed height of 870 nm NPG when the reaction time was increased from 10 to 40

min. The volumetric capacitance value roughly considering lumps of Ni(OH)2 that were

residually deposited on the uniform Ni(OH)2 layer at longer hydrothermal reaction times is

1,810 F/cm3, 2,663 F/cm3, 742 F/cm3, 478 F/cm3.

13

S8. SEM images of Ni(OH)2/NPG after Ni(OH)2 depositing time for 30min and 40 min.

14

Figure S9. TEM images of Ni(OH)2/NPG after depositing Ni(OH)2 for 10, 20, and 30 min. The inner pores were largely blocked by Ni(OH)2 for more than 20 min.

15

Figure S10. Cross-sectional SEM morphologies of Ni(OH)2(270nm)/NPG with different heights of NPG prepared by Au/Ag electrodeposition times of 1min 15s, and 2min 30s, and 5min. (a, c, and e) Cross-sectional SEM morphologies of the as-prepared NPG with the different heights. (b, d and f) Cross-sectional SEM morphologies of Ni(OH)2/NPG with the different heights of NPG with 270 nm of Ni(OH)2.

16

Figure S11. EIS plot of Ni(OH)2/NPG with various heights

The intercept at the high frequency region in Nyquist plot, Rs, implies contact resistance and

the internal resistance of the electrode. The diameter of the semicircle at the middle

frequency region represents the interfacial charge transfer resistances, Rct. The Nyquist

spectra at the low frequency region provides information on the diffusion of ions in the

electrode. Therefore, by increasing reaction time, the height of Ni(OH)2 on NPG increased,

and naturally, the ohmic resistance and the internal resistance of the electrode increased.

Furthermore, the slope at the low frequency region of 270 nm-Ni(OH)2 samples becomes

more vertical than other electrodes, implying ideal supercapacitor properties.

17

Figure S12. EIS plot of Ni(OH)2(270nm)/NPG with different heights of NPG.

By increasing the height of the NPG electrode, the portion of the Ni(OH)2 in region B (which

is in direct contact with NPG) increased and the electrode showed a lower Rs value as

compared with other electrodes. Furthermore, the larger area of good Ohmic contact bewteen

Ni(OH)2 and NPG in the 1700 nm NPG samples provides better charge transport behavior,

showing low Rct (diameter of the semicircle), and good kinetic properties.

18

Figure S13. Cyclic voltammograms and galvanostatic discharge curves of Ni(OH)2-NPG//MnO2-NPG two electrode supercapacitor

19