hierarchically porous carbon foams for electric double ...10.1007... · 2 department of chemistry...

Nano Res.

Electronic Supplementary Material

Hierarchically porous carbon foams for electric doublelayer capacitors

Feng Zhang1,2,§, Tianyu Liu2,§, Guihua Hou1, Tianyi Kou2, Lu Yue1, Rongfeng Guan1, and Yat Li1 ()

1 Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province, Yancheng Institute of Technology, Yancheng

224051, China 2 Department of Chemistry and Biochemistry, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA § These authors contributed equally to this work.

Supporting information to DOI 10.1007/s12274-016-1173-z

Calculation

Gravimetric/specific capacitances were calculated from GCD profiles according to Eq. (S1)

C = ImΔt/ΔV (S1)

Where C (F/g) is the gravimetric/specific capacitance, Im (A/g) is the current density, Δt (s) is the discharge time,

and ΔV (V) is the potential window.

Volumetric capacitances were evaluated from GCD profiles according to Eq. (S2)

Cv = Csm/Velec (S2)

where Cv stands for the volumetric capacitance, Cs is the gravimetric (or specific) capacitance, m is the total mass

of the PCF used (0.7 mg), and Velec represents total volume of the electrode (0.5 cm (L) × 0.4 cm (W) × 0.2 mm (H),

0.004 cm3 which includes the volume of PCF and nickel foam substrate). For the symmetric device, the total

mass of the PCF is 1.4 mg and the total volume of the device is 0.022 cm3 (0.6 cm (L) × 0.8 cm (W) × 0.45 mm (H),

including the volume of PCF, two pieces of nickel foam substrates, electrolyte and separator).

Energy density (E, Wh/kg) and power density (P, W/kg) were evaluated by Eqs. (S3) and (S4), respectively

E = CcellΔV2/(2 × 3.6) (S3)

P = 3600E/Δt (S4)

Here Ccell represents the specific capacitance of the symmetric supercapacitor calculated by Eq. (S1).

The imaginary capacitances (C′′) were evaluated using the EIS data via the following equation

C′′ = Z′(ω)/[ω|Z(ω)|2] (S5)

Address correspondence to [email protected]

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Here the electrochemical impedance Z(ω) is defined as Z(ω) = Z′(ω) + jZ′′(ω), where Z′(ω) and Z′′(ω) are the real

part and the imaginary part (in Ω), respectively. ω is the frequency (in Hz) and j is the imaginary number.

|Z(ω)|2 = Z′(ω)2 + Z′′(ω)2

Supplementary figures

Figure S1 (a) Simplified illustration of the cross-link reaction between chitosan and glutaraldehyde. The active functional groups involved in the reaction are labelled in red (amine groups) and blue (aldehyde groups). Digital pictures of (b1) chitosan solution and (b2) chitosan hydrogel obtained after the cross-link reaction. (c) FT-IR spectrum collected for crosslinked chitosan and bare chitosan. Characteristic peaks of O–H, C–H, N–H and C=O are highlighted by dashed lines and the characteristic C=N stretching peak at 1,574 cm–1 is labeled. Peaks around 2,400 cm–1 are due to presence of carbon dioxide in air.

The intermolecular cross-link is the major reaction between chitosan and glutaraldehyde based on (1) the

experimental observation and (2) the new FTIR spectroscopic data:

(1) Hydrogel was obtained by mixing chitosan solution with the cross-linker, glutaraldehyde, (Fig. S1(b)). In

fact, the increase of solution viscosity is a direct proof of intermolecular cross-link. Intermolecular crosslink

binds different chitosan chains together, thus fixes polymer chains and increases viscosity. On the contrary,

intramolecular cross-link folds polymers and typically leads to a decrease in viscosity due to volume contraction

of the polymer coils [S1]. Therefore, we believe chitosan and glutaraldehyde are mainly inter-molecularly cross-

linked.

(2) Besides, we have performed FT-IR spectroscopy to prove the cross-link reaction. Figure S1(c) compares

the FT-IR spectrum of chitosan aerogel and glutaraldehyde-crosslinked chitosan aerogel. Besides a number of

characteristic peaks that can be assigned to O–H and N–H stretching (3,420 cm–1) [S2], C–H stretching (2,920

and 2,850 cm–1) [S3], and C=O stretching (from residual acetyl groups, 1,650 cm–1) [S4], a unique peak at

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1,574 cm–1 was observed in the spectrum of the cross-linked chitosan. This peak can be ascribed to stretching of

C=N (imine bond) [S5]. Imine bonds are formed after the cross-link reaction, as shown in Fig. S1(a). For the bare

chitosan aerogel, only a broad peak spanning from 1,650 to 1,520 cm–1 is detected. This peak is associated with

N–H bending of amine groups [S4].

Figure S2 Low and high magnification SEM images of ((a) and (b)) K2CO3-embedded chitosan aerogel and ((c) and (d)) plain chitosan aerogel.

Formation mechanism of chitosan aerogel composed of chitosan sheet network

To investigate the role of freeze-drying, we have prepared chitosan xerogels without freeze-drying (i.e., natural

dried in room temperature) as the control sample. SEM shows that the chitosan xerogels are thick films with

smooth and flat surface (Fig. S3). Besides, chitosan monoliths obtained after freeze-drying in the absence of

glutaraldehyde shows similar 3D sheet network structure (Fig. S8(a)) as glutaraldehyde cross-linked chitosan

aerogels. Therefore, it is clear that freeze-drying is the necessary step to achieve sheet-like network. Based on the

aforementioned observations, we have proposed the following mechanism:

1) In diluted chitosan solution, many liquid water “islands” are randomly dispersed among chitosan polymer

chains, due to the limited interaction between chitosan polymers and water [S6]. Addition of glutaraldehyde

cross-links chitosan chains and forms chitosan “cages” that hold water inside (Fig. S4(a)).

2) During freezing, water “islands” expand and squeeze the as-formed chitosan “cage” walls to thin chitosan

sheets (Fig. S4(b)).

3) During freeze-drying process, ice “islands” sublimate and leaves the porous chitosan network composed

of chitosan sheets behind, as observed in the SEM images (Fig. S4(c)).

TG curve (Fig. S5) shows two stages of decomposition. The first stage with ca. 10% weight loss between 75

and 220 °C can be ascribed to evaporation of physically absorbed water [S7]. The second stage with a weight

loss of 47% is attributed to the decomposition of chitosan. The decomposition temperature (i.e., the starting

temperature of stage 2) is ca. 220 °C, which is similar as other cross-linked chitosan aerogels [S7]. When


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Figure S3 SEM image of chitosan xerogel obtained without freeze-drying.

Figure S4 Schematic illustration of the formation mechanism of porous chitosan aerogels composed of chitosan sheets. (a) Chitosan solution. (b) Frozen chitosan solution. (c) Freeze-dried chitosan aerogel. SEM image shows the porous structure created by ice sublimation. Step A: freezing. The expansion of liquid water squeezes the chitosan “cages” along the directions indicated by red arrows. Step B: freeze-drying. Ice “islands” sublimates and leaves the porous chitosan aerogel.

Figure S5 TG curve of glutaraldehyde cross-linked chitosan aerogel with a scan rate of 20 K/min. Insets: digital pictures of the sample before and after carbonization.


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temperature is above 450 °C, the mass loss becomes slow and eventually reaches a plateau at around 750 °C

where the carbonization process is complete. The carbonization rate is evaluated via the following equation

temperature

carbonization

%m A vv

T

Where vcarbonization, vtemperature, m, A% and ΔT represent the carbonization rate, scan rate of temperature (= 20 K/min),

mass of sample (5.3 mg), percentage of weight loss in stage 2 (= 47%), and temperature window of stage 2 (= 260 K),

respectively. The carbonization rate is determined to be 0.2 mg/min.

Figure S6 SEM images collected from the carbon sheet edge of (a) PCF and (b) CF structures showing the sheet thickness.

Figure S7 Three-dimensional scanned topography of the PCF carbon sheet.

As shown in the 3D topography image (Fig. S7), no phase boundary is observed, indicating PCF is in a

continuous phase. The carbon sheet surface has relatively low roughness with a Ra value (average roughness,

the arithmetic average of the vertical distance from the mean line) of 6.6 nm. The surface roughness could be due

to the existence of micro-pores and meso-pores.

As shown in Fig. S10(b), two regions, the graphitized region and the micro-porous amorphous region can be


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clearly distinguished. The closed loop lattice fringe in the graphitized region is characteristic of graphitized

carbons [S8], which is in agreement with the Raman results.

Figure S8 (a) SEM image of the carbon monolith prepared without glutaraldehyde. Inset shows the digital picture of a piece of carbon foam. (b) SEM image of a carbon layer showing the lateral thickness.

Figure S9 Pore size distribution of PCF and CF. The pore size distribution of macro-/meso-pores and micro-pores are obtained by Barrett–Joyner–Halenda (BJH) and Horváth–Kawazoe (HK) model, respectively.

Figure S10 (a) The low magnification and (b) high magnification TEM images of a piece of PCF carbon sheet. The dashed lines mark the boundary between graphitized region and micro-porous amorphous region.


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Figure S11 Potential vs. current plots collected from the PCF and other commonly used current collectors. The distance between two probes is 1 cm. (b) shows the zoomed-in part highlighted by the dashed box of (a).

In a potential vs. current plot, the electrical resistance is directly proportional to the slope of the straight line.

As shown in Fig. S11, the electrical resistance of the PCF electrode is comparable to the highly conductive

nickel foam, and is much smaller than that of stainless steel plate, the graphite paper and the carbon cloth. The

small electrical resistance of PCF allows fast electron transport in the PCF electrode, which is consistent with

the small Rs and Rct values obtained in the EIS experiment.

Trasatti method is used to evaluate capacitive contribution from electrical double layer and pseudo-capacitive

reactions. First, cyclic voltammograms of PCF at different scan rates (20, 40, 60, 80, 100 and 200 mV/s) were

collected and corresponding gravimetric capacitances were evaluated based on the following equation

2

SC

U v

where C is the gravimetric capacitance (in F/g), ΔU the potential window (in V), S the area enclosed by

corresponding cyclic voltammograms (in A·V/g) and v the scan rate (in V/s).

Figure S12 The XPS survey spectrum of PCF.


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Assuming ion diffusion follows the semi-infinite diffusion pattern (i.e., ions unrestrictedly diffuse to electrode/

electrolyte interface from bulk electrolyte), a linear correlation between the reciprocal of the calculated gravimetric

capacitance (C–1) and the square root of scan rates (v1/2) should be observed (Fig. S14(a)) [S9].

1 1/ 2TconstantC v C

where C, v and CT represent experimental gravimetric capacitance, scan rate and total capacitance, respectively.

Data points collected at larger scan rates deviated from this linear relationship due to intrinsic resistance of the

electrode and deviation from semi-infinite ion diffusion [S10]. These deviated data points were masked during

linear fitting. The “total capacitance” equals the sum of electrical double layer capacitance and pseudo-

capacitance [S11].

Similarly, plotting the calculated gravimetric capacitances (C) against the reciprocal of square root of scan

rates (v1/2) should also give a linear correlation described by the following equation (if assuming a semi-infinite

diffusion of ions) (Fig. S14(b)) [S10]

1/2

EDLconstantC v C

where C, v and CEDL is experimental gravimetric capacitance, scan rate and electrical double capacitance,

respectively. Linear fit the plot and extrapolate the fitting line to y-axis gives the maximum electrical double

layer capacitance [S11]. Subtraction of CEDL from CT yields the maximum pseudo-capacitance (Cps). The CT, CEDL,

and Cps are calculated to be 241.5, 201.7 and 39.8 F/g, respectively.

Figure S13 (a) CV curves collected for PCF and CF at a scan rate of 100 mV/s in 3 M KOH aqueous electrolyte; (b) CV curves of CF. (c) GCD profiles of CF. (d) Nyquist plot collected at open-circuit potential. Open dots and solid lines are experimental data and fitting curves, respectively. Inset shows the normalized C’’ vs. frequency plot.


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Figure S14 (a) Plots of reciprocal of gravimetric capacitance (C–1) vs. square root of scan rate (v1/2). (b) Plots of gravimetric capacitance (C) vs. reciprocal of square root of scan rate (v–1/2). The red lines are linear fitting lines of data points. The algebraic equations of the fitting lines are shown in the inset. Data points in grey are masked during linear fitting.

Figure S15 Equivalent electric circuit used for EIS data fitting. Parameters: Rs—combined series resistance; Rct—charge-transfer resistance; W—Warburg element; Cdl—electrical-double-layer capacitance; Csf —surface capacitance (related to pseudocapacitance) ; Rsf —surface resistance.

Table S1 Parameters of equivalent electric circuit elements

Sample Rs (Ω) Rct (Ω) W (Ω/s0.5) Cdl (F) Csf (F) Rsf (Ω) τ0 (s)

CF 0.2553 330.6 2.34 0.024 0.00233 1.729 >10

PCF 0.1788 1.071 0.0462 0.264 0.00304 0.957 2.3

Figure S16 (a) Cycling stability of PCF in 3 M KOH at a scan rate of 200 mV/s; (b) the CV curves collected before and after 10,000 scans.


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The capacitance of the PCF//PCF symmetric supercapacitor remains almost the same under different working

conditions (flat or bend), suggesting the outstanding mechanical flexibility of the device (Figs. S17(a) and S17(b)

inset). The device also showed excellent bending stability. As shown in Fig. S17(b), no obvious capacitance

degradation is observed after 200 repeated bending times.

Figure S17 (a) Digital picture showing a PCF film pressed on a piece of nickel foam under bending condition. (b) Bending stability of the PCF//PCF symmetric supercapacitor. Inset shows the cyclic voltammograms collected for PCF//PCF symmetric supercapacitor at a scan rate of 100 mV/s under flat (black curve) and bending condition (red curve).

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