Balancing the Electrical Double Layer Capacitance

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Journal Name ARTICLE Balancing the Electrical Double Layer Capacitance Pseudocapacitance of Hetero-atom Doped Carbon Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/

a,b

Zi-Hang Huang,†

b

Tian-Yu Liu,† Yu Song,

a,b

b

and

a

Yat Li* and Xiao-Xia Liu*

Heteroatom-doped carbonaceous materials derived from polymers are emerging as a new class of promising supercapacitor electrodes. These electrodes have both electrical double layer capacitance (from carbon matrix) and pseudo-capacitance (from hetero-atoms). Balancing the electrical double layer capacitance and pseudo-capacitance is a key to achieve large capacitance at ultrafast current densities. Here we investigate the influence of pyrolysis temperature on capacitive performance of hetero-atom (oxygen and nitrogen) doped carbons derived from polypyrrole nanowire arrays. Structural and electrochemical characterizations reveal that the concentration of hetero-atoms as well as the ratio of electrical double layer capacitance and pseudo-capacitance can be tuned by varying the pyrolysis temperature. In fact the hetero-atom doped carbon sample obtained at a relatively lower pyrolysis temperature (500 oC) exhibits the optimal capacitive performance. It yields an outstanding areal capacitance of 324 mF/cm2 at 1 mA/cm2 (141 F/g @ 0.43 A/g), and more importantly, retains areal capacitance of 184.7 mF/cm2 (80.3 F/g @ 43.5 A/g) at an ultrahigh current density of 100 mA/cm2. An asymmetric supercapacitor consists of the hetero-atom doped carbon as anode delivers a maximum volumetric energy density of 1.7 mWh/cm3 at a volumetric power density of 0.014 W/cm3, which is among the best values reported for asymmetric supercapacitors.

Introduction Supercapacitors are electrical energy storage devices that target at applications where high power uptake and output are needed. Typical applications include hybrid electrical vehicles, 1-6 portable electronics and power backups. To meet the increasing energy demand of these devices, enhancing energy density at ultrafast charging rates (high power density) is critical. Since energy density of supercapacitors is directly proportional to the device capacitance and the square of voltage window, it is desirable to enlarge both factors simultaneously. The voltage window can be readily broadened by using non-aqueous electrolytes with intrinsic large potential 7 windows (e.g., organic electrolytes or ionic liquids). Alternatively, asymmetric supercapacitors can achieve a boarder voltage windows via assembly of different anode and 8 cathode materials with different potential windows. However, achieving high capacitance at ultrafast charging rates is challenging. Carbon-based materials have excellent electrical conductivity but suffer from intrinsic low capacitance. Pseudocapacitive materials typically have outstanding capacitance but most of them are electrically insulating. Their poor electrical a.

Department of Chemistry, Northeastern University, Shenyang, 110819, China. Department of Chemistry and Biochemistry, University of California, 1156 High Street, Santa Cruz, California, 95064, United States. † These authors contributed equally to this work. Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x b.

conductivity hinders electron transfer and leads to limited capacitance achieved at large current densities (poor rate capability). One strategy to circumvent the aforementioned limitation is to synthesize materials that combine the strengths of carbon and pseudo-capacitive materials. Hetero-atom doped carbon materials derived from polymers (e.g., polyaniline, polypyrrole, and chitosan etc.) is one of the promising candidates. Heteroatom-doped carbonaceous materials are composed of carbon matrices doped with heteroatom-atoms, e.g., oxygen, nitrogen, sulfur atoms etc. The carbon matrices provide excellent electrical conductivity and electrical double layer capacitance (EDLC), while the surface functionalities containing the heteroatoms contribute pseudo-capacitance (PC). To date, a number of nanostructured hetero-atom doped 9 10 carbons, including nanofiber arrays, nanotube arrays 11-12 13 14 nanowires, nanospheres, nanocages, and nanocoil 15 arrays have been prepared by carbonization of polymers in different morphologies. Among various synthetic parameters, the pyrolysis temperature is a key factor that determines the electrochemical performance of the carbonaceous products. High temperature annealing would completely carbonize polymers and yield carbon material with little amount of hetero-atoms, and hence, their capacitance is relatively small due to limited number of pseudo-capacitive active sites. Low temperature annealing favors the formation of products with considerable amount of hetero-atoms. Yet low degree of carbonization would lead to poor electrical conductivity and resulting in poor rate capability. Although annealing

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temperature is an important factor, the low pyrolysis temperature of conducting polymers to fabricate carbon materials has not been seriously studied, especially for o temperatures below 600 C. Most previous works have been primarily focused on carbon materials obtained at o temperatures beyond 600 C, as people generally believed that polymer derived carbons obtained at higher annealing 9-10,16 temperature have better capacitive performance. Therefore, the capacitive performance of polymer-derived carbons still has room to be further optimized and improved. Here we investigate the effect of annealing temperatures on electrochemical performance of the nitrogen and oxygen co-doped carbon nanowire arrays (NOC NWAs) prepared via pyrolysis of polypyrrole (PPy). Significantly, our study shows that the atomic percentage of heteroatoms, the electrical conductivity, the ratio of electrical double layer capacitance to pseudocapacitance and the capacitances at various current densities can be well tailored by varying the annealing temperature. The NOC NWAs obtained at the optimal o annealing temperature of 500 C exhibits the highest areal 2 2 capacitance of 184.7 mF/cm at current density of 100 mA/cm An asymmetric supercapacitor consists of a NOC electrode and a manganese dioxide (MnO2) nanosheets electrode has a large voltage window of 2.1 V and achieves a remarkable volumetric 3 energy density of 1.7 mWh/cm at a power density of 0.014 3 W/cm , which is comparable with performance achieved by most state-of-the-art asymmetric supercapacitors.

Experimental Section Materials All chemicals were purchased from the Sinopharm Chemical Reagent Co., Ltd. and used as received, except pyrrole which o was distilled (80 kPa, 110 C) for purification prior to use. Cellulose separators were purchased from Nippon Kodoshi Corporation (Japan). Carbon cloths were purchased from Fuel Cell Earth (United States) and graphite foils were from SGL group (Germany). Synthesis of nitrogen- and oxygen-doped carbon nanowire arrays (NOC NWAs) PPy nanowire arrays (PPy NWAs), were first deposited on 17 carbon cloth via an electro-polymerization method. The polymerization was performed in a three-electrode electrolytic cell with a piece of carbon cloth, a saturated calomel electrode (SCE) and a piece of graphite foil as the working, reference and counter electrode, respectively. The electrolyte is a phosphate buffer solution (pH = 6.86) containing 0.02 M p-toluene sulfonate acid (TsOH) and 0.145 M pyrrole. To initiate 2 polymerization of pyrrole, a constant current of 1.5 mA/cm were applied for ca. 80 min. A uniform black film on the carbon cloth was obtained after the polymerization. The PPydeposited carbon cloth was rinsed thoroughly with distilled water and then dried in vacuum at 60 C for 24 h. The NOC NWAs were obtained by annealing the PPy NWAs in N2 atmosphere at various temperatures including 400 C, 500 C, or 600 C for 30 min. The as-prepared samples were denoted

as NOC-x, where x=400, 500 or 600, representing the annealing temperature. The active material mass loadings of 2 all these electrodes were controlled to be 2.3 mg/cm by adjusting the electro-deposition time. Areal capacitance is 2 normalized to the geometric working area (i.e., 1.0 cm ) of the electrode and gravimetric capacitance is normalized to the mass loading of NOC NWAs. Here we chose to use the geometric area for evaluation of areal capacitance because it is application-oriented and more practical than using the actual surface area for calculations. Electro-deposition of manganese dioxide nanosheet arrays (MnO2 NSAs) MnO2 NSAs were electrochemically deposited on carbon cloth in a solution containing 0.01 M manganese acetate and 0.02 M 2 ammonium acetate with a constant current of 0.4 mA/cm for 80 min. Then the product was washed thoroughly with distilled water and dried in vacuum at 60 C for 24 h. The mass loading 2 of MnO2 NSAs is 1.0 mg/cm . Assembly of the asymmetric supercapacitors (ASC) The ASC consists of NOC NWAs-500 as positive electrode and MnO2 NSAs as negative electrode was assembled by sandwiching a piece of cellulose separator between the two electrodes (the device is denoted as NOC//MnO2 ASC). Both electrodes were soaked in 5 M LiCl aqueous solution for 3 h to absorb electrolyte prior to assembly. The total mass loading of 2 2 the active material are 3.3 mg/cm , which 2.3 mg/cm for NOC 2 NWAs-500 and 1.0 mg/cm for MnO2 NSAs. The working area of positive and negative electrode was kept the same (1 cm×1 cm) for charge balance (see Calculation, Supporting 3 Information). The total volume of the device was 0.0735 cm [1 cm (L) × 1 cm (W) × 0.0735 cm (H)], including the volume of two electrodes, electrolyte and separator. The thicknesses of the assembled supercapacitors were measured by a digital screw micrometer (Hengliang, 0.001 mm sensitivity, Figure S1, ESI†). Characterizations Scanning electron microscopy (SEM) images and elemental dispersive X-ray spectroscopy (EDS) were acquired by a SEM microscope (Ultra Plus, Carl Zeiss, Germany). Transmission electron microscopy (TEM, Tecnai G2 20) was performed by a TEM microscope. Samples for TEM characterization were prepared by ultra-sonicating a piece of active materialsdeposited carbon cloth in ethanol for 20 min. The mass loadings of electrodes were determined by the weight difference between the electrode and a piece of bare carbon cloth (with the same area), using a Sartorius BT 25S semimicrobalance with a sensitivity of 0.01 mg. X-ray photoelectron spectroscopy (XPS) was performed by a XPS spectrometer (ESCALAB 250Xi, Thermo Scientific Escalab, USA). All core level XPS spectra were calibrated using the C 1s photoelectron peak centered at 284.6 eV as reference. XPS peak de-convolution was performed by using the XPSPEAK

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software. The electrical conductivities were measured by a four-point probe device (ST2253, Jingge Electronic Co., Ltd.,

Figure 1 (a) Schematic illustration of the fabrication process of NOC NWAs. SEM images of (b) PPy NWAs and (c) NOC-500 NWAs. Inset shows the magnified SEM image of NWAs. (d) SEM image of NOC-500 and corresponding elemental mapping images of C Kα1, N Kα1 and O Kα1.

China). Cyclic voltammetry, galvanostatic charge-discharge experiments and electrochemical impedance spectroscopy were performed with a multichannel electrochemical workstation (VMP3, Bio-Logic-Science Instruments, France) in a three-electrode electrolytic cell filled with 5 M LiCl aqueous electrolyte. A SCE and a piece of graphite foil served as the reference and the counter electrode, respectively. The electrodes were also analyzed by electrochemical impedance spectroscopy (EIS) at open-circuit potential in a frequency range from 0.05 Hz to 40 kHz with a perturbation of 10 mV. Electrochemical performance of the ASC was evaluated by a two-electrode testing system using the same electrochemical workstation.

Results and discussions Morphology and Chemical Composition Figure 1a illustrates a two-step strategy to synthesize NOC NWAs. First, PPy NWAs are electro-deposited on a piece of carbon cloth. A black film was uniformly covered on the carbon cloth substrate after electro-deposition. The scanning electron microscopy (SEM) image shows that each carbon fiber is uniformly coated with PPy NWAs (Figure 1b). Subsequent pyrolysis of the PPy NWAs at different temperatures yields the NOC-x NWAs (where x=400, 500 or 600 representing the annealing temperature). The carbonization process did not alter the NWAs morphology (Figure 1c and Figure S2, ESI†).

The NWs have smooth surface and an average diameter of ca. 100 nm (Figure 1c inset). Elemental dispersive X-ray spectroscopy (EDS) studies revealed that carbon, oxygen and nitrogen elements are uniformly distributed in the NOC NWAs (Figure 1d). The oxygen and nitrogen are believed to originate from the dopants (i.e., p-toluene sulfonate acid) in PPy and the nitrogen functionalities of PPy, respectively. Thermal gravimetric (TG) analysis, Raman spectra and Xray photoelectron spectroscopy (XPS) were performed to further characterize the effect of annealing temperature on the chemical composition of NOC NWAs. As shown in Figure 2a, the initial 1% weight loss is believed to be due to water 18 o evaporation. PPy NWAs starts to decompose at ca. 300 C, as the weight begins to gradually decrease beyond this o temperature. The continuous mass reduction beyond 600 C also suggests that all NOC samples are only partially carbonized. The integrity of electrode obtained at all temperatures is well preserved (Figure 2b). Figure S3 (ESI†) shows the Raman spectra and the major peaks of PPy NWAs, and all NOC samples. The Raman spectra of all NOC samples suggest that they have been transformed into carbonaceous materials after annealing temperature. Figure S4 (ESI†) shows the XPS survey spectra of PPy NWAs, and all NOC samples. It is noteworthy that PPy NWAs contain C, O, N and S (from dopants), while S is absent in all NOC samples. The disappearance of S signal indicates that the dopants are completely decomposed during the thermal annealing. The

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total atomic percentage of the heteroatoms (N and O) for PPy NWAs, NOC-400, NOC-500 and NOC-600 are calculated to be 26.37%, 21.9%, 19.24% and 11.95% (based on peak area) respectively (Table S1, Supporting Information), which is consistent with the previous report that high temperature 15 annealing reduces the concentration of heteroatoms. As the amount of heteroatoms is closely related to electrical 9, 19-20 conductivity and pseudo-capacitance contribution, the chemical nature of each sample is expected to play an important role in determining the electrochemical performance. Further analysis of the high- resolution N and O signals disclose some important information. Figure 2b and 2c display the N 1s core level and O 1s core level XPS spectra of the NOC-500 sample as an example, respectively. N 1s core level XPS spectra and O 1s core level XPS spectra collected for PPy, NOC-400 and NOC-600 are presented in Figure S5 (ESI†) and Figure S7 (ESI†), respectively in the Supporting Information. The N 1s peak of PPy NWAs can be de-convoluted into two synthetic peaks centered at binding energies of 400.1 eV and 398.5 eV. These binding energies are consistent with the reported values for pyrrolic nitrogen (N-5) and quinonoid 21 imine groups (-N=). For NOC samples, the N 1s peaks can be de-convoluted into four synthetic peaks which can be assigned to N-5, pyridinic nitrogen (N-6, 398.7 eV), quaternary nitrogen 22-23 (N-Q, 400.8 eV) and oxidized nitrogen (N-O, 402.5 eV). A schematic illustration of all the nitrogen functionalities is depicted in Figure S6 (ESI†). The atomic percentage of these nitrogen atoms in different samples are summarized in Table 1. It is noticeable that the amount of N-5 and N-6 decreases with increasing pyrolysis temperature. It is in agreement with the previous report that N-5 and N-6 are instable at elevated 24 temperatures. Such the reduction could modify the electrochemical performance, because pseudo-capacitive reactions mainly take place at N-5 and N-6, whilst N-Q and N-O 20, 25 have no capacitive activity. The O 1s XPS spectra of all samples can be de-convoluted into four different types of oxygen functionalities: quinone group, hydroxyl group (C-OH, 532.1 eV), ether-type oxygen atoms in esters and anhydrides (C-O-C, 533.3 eV), and 26-27 carboxylic group [C(=O)-O-, 534.4 eV]. The atomic percentage of these oxygen atoms in different samples are summarized in Table 1. Oxygen functionalities, especially hydroxyl groups and carboxyl groups have been found to be effective in improving carbon wettability and beneficial for electrolyte infiltration of the electrode. These two factors can 28-30 improve the electrode capacitance and rate capability. Electrochemical Performance of Single Electrodes All electrochemical characterizations of single electrodes are conducted in a three-electrode electrolytic cell containing 5 M LiCl aqueous electrolyte (Figure S8, ESI†). Figure 3a compares the cyclic voltammograms (CVs) collected for PPy NWAs and all NOC samples at a high scan rate of 200 mV/s. The CV curve of PPy NWAs and NOC-400 are distorted from rectangular shape, indicating their non-ideal capacitive performance at such a large scan rate. On the contrary, the

Figure 2 (a) The TG curve collected for the PPy NWAs. Sample weight (including the mass of carbon cloth substrate) is 8.8 mg. (b) A digital photograph of PPy, NOC-400, NOC-500 and NOC-600 electrodes. Samples used for XPS measurements are obtained at stages highlighted by the arrows. (c) High resolution N 1s XPS peak and O 1s XPS peak of NOC-500. The solid black curves are experimental data. The dashed lines are synthetic peaks and the solid red lines are the sums of all synthetic peaks.

quasi-rectangular CV curve of NOC-500 and NOC-600 are clear evidence of their excellent capacitive performance and small electrical resistance. The CV curve of the bare carbon cloth was negligible, which exhibit the carbon cloth itself contributes little capacitance. Likewise, the galvanostatic charge-discharge (GCD) profile of NOC-500 and NOC-600 also exhibit smaller IR drop than that of PPy NWAs and NOC-400 (Figure 3b). These results suggest the electrical conductivity of NOC increases with increase of carbonization degree (annealing temperature). This is indeed supported by electrochemical impedance spectroscopy (EIS) studies. In the Nyquist plots (Figure 3d), NOC samples present considerably smaller diameter of the semi-circle (correlated to charge transfer resistance, Rct) and much steeper slope (correlated to diffusion Warburg resistance, Wd) in middle- and low-frequency domains than that of PPy NWAs. The magnitude of Z’-intercept correlated to equivalent series resistance, Rs. The Rs includes the electrical resistance of electrode and decreases with increasing carbonization temperature (PPy: 1.39, NOC-400: 1.36, NOC-500: 0.96, NOC600: 0.91), as more electrically conductive carbon can be obtained at higher temperatures. Consistently, four probe measurements also show that electrical conductivity increases with increasing annealing temperature (PPy: 47 S/m; NOC-400: 257 S/m; NOC-500: 379 S/m; NOC-600: 705 S/m). It should be noted that the slightly larger IR drop of the NOC-600 NWAs than that of NOC-500 NWAs is not contradictory to the EIS and the four-probe test. It is because the IR drop does not solely depend on the electrical conductivity of electrode material. It reflects the overall resistance of the entire testing system, consisting of the contact resistance between electrode materials and current collectors (i.e., the carbon cloth in our

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Table 1. Atomic percentage (unit in at.%) of different nitrogen and oxygen functionalities Sample

N-6

N-5

N-Q

N-O

=N-

N Total

O-1

O-2

O-3

O-4

O Total

PPy

-*

10.38

-

-

1.59

11.97

2.20

8.55

2.72

0.93

14.40

NOC-400

3.53

2.72

3.22

0.4

-

9.87

4.57

3.75

2.61

1.10

12.03

NOC-500

2.25

1.57

3.24

0.89

-

7.95

3.61

3.06

3.27

1.35

11.29

NOC-600

1.41

0.97

2.36

0.76

-

5.50

2.01

1.64

2.00

0.80

6.45

* “-” means not detected. Notes: N-6: pyridinic nitrogen; N-5: pyrrolic nitrogen; N-Q: quaternary nitrogen; N-O: oxidized nitrogen; O-1: quinone group; O-2: hydroxyl group (C-OH); O-3: ether group (C-O-C); O-4: carboxylic group [C(=O)-O-]

work), the resistance of electrode materials, the ion diffusion or infiltration resistance and the Rct. While for the four-probe measurement, it merely gauges the resistance of electrode material without the influence from electrolytes. Actually, as shown in the electrochemical impedance spectroscopy (Figure 3d or below), NOC-600 NWAs exhibits slightly smaller combined series resistance (the x-axis intercept) than the NOC-500 NWAs, which is consistent with the result revealed by the four-probe tests. But NOC-600 NWAs has a linear tail with smaller slope than NOC-500 NWAs, meaning that NOC NWAs-600 experienced a larger ion diffusion resistance than the latter one. The enhanced ion diffusion resistance could be attributed to reduced hetero-atom content that retards electrolyte infiltration of the electrodes (because hetero31-32 atoms render wettability of carbon materials). Although NOC-600 is the most conductive electrode, it does not achieve the highest capacitance at ultrahigh current 2 densities (e.g., 100 mA/cm ) due to its limited pseudocapacitance from insufficient amount of electrochemically active heteroatoms. Among all the electrodes, NOC-500 electrode achieves the highest capacitance at large scan rates/current densities. Figure 3c shows the areal capacitance of PPy and NOC samples as a function of current density. As expected, the areal capacitances of less conductive PPy and NOC-400 samples decrease drastically with the increase of current density, 2 2 retaining only 0.5 % (2.63 mF/cm ) and 24.9 % (108.1 mF/cm ) of areal capacitance when the current density increased from 2 1 to 100 mA/cm . On the contrary, NOC-500 and NOC-600 exhibits excellent rate capability performance (NOC-500: 57%, NOC-600: 60.7%). Their corresponding gravimetric and areal specific capacitance, along with N and O contents and combined series resistance are calculated and summarized in Table S1 (Supporting Information). NOC-500 achieves an 2 2 outstanding areal capacitance of 324 mF/cm at 1 mA/cm (equivalent to 141 F/g @ 0.43 A/g). And more importantly, it 2 2 still retains a capacitance of 184.7 mF/cm at 100 mA/cm (equivalent to 80.3 F/g @ 43.5 A/g, Figure S9, ESI†). The large 2 mass loading of the NOC-500 NWAs (2.3 mg/cm ) prevents the gravimetric capacitance from reaching ultrahigh values but nevertheless, these values are substantially higher than capacitances obtained from other porous carbons or graphene-based electrodes in neutral electrolytes, including

graphene-based porous electrodes (98 F/g @ 0.4 A/g, and 46.2 33 F/g@ 0.8 A/g), nitrogen-doped activated carbon (104 F/g @ 2 34 mV/s, and 73 F/g @ 100 mV/s), silver nanoparticles 35 decorated graphene foam (110 F/g @ 25 mV/s), porous 36 carbon (80 F/g @ 50 A/g).

Figure 3 Electrochemical performances of PPy NWAs and NOC samples collected in 5 M LiCl aqueous electrolyte. (a) CV curves measured at a scan rate of 200 mV/s. (b) GCD profiles measured at 30 mA/cm2. (c) A plot of areal capacitance of all samples as a function of current density. (d) Nyquist plots collected at open circuit potential with a perturbation of 10 mV. Inset shows the high frequency domain of the Nyquist plots. (e) Percentage of capacitance contribution evaluated for all electrodes based on Trasatti analysis.

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To check the capacitance contributed from the substrate, i.e., carbon cloths, we annealed bare carbon cloth substrates o o o at 400 C, 500 C, and 600 C in nitrogen atmosphere (the same temperatures used to obtain NOCs) and measured their capacitive performance. Figure S10a (ESI†) compares the cyclic voltammograms (CVs) collected for the carbon cloth samples at a scan rate of 200 mV/s. It is clear that the annealing temperature has little effect on the capacitance of the carbon cloth, as expected. Furthermore, areal capacitance evaluated using the galvanostatic charge-discharge (GCD) profiles of all carbon cloth samples are negligible compared to the NOCs (Figure S10b and S10c, ESI†), again proving that the carbon cloth substrate contributes little capacitance. PPy is a conventional pseudocapacitive material, while carbonaceous materials are often EDLC materials. It is important to understand the capacitive behaviour of NOC samples, which is expected to have both PC and EDLC. EDLC would be mainly contributed from the carbon matrices whilst PC comes from the surface functionalities containing the heteroatoms (O and N). Trasatti method analysis (Figure 3e, Figure S11, ESI†) show that the capacitance of PPy is mainly from PC. Upon pyrolysis, the contribution from EDLC increases drastically due to the conversion of PPy to carbon. For NOC400, the EDLC accounts for around two-third of its total capacitance. The EDLC contribution in NOC-500 and NOC-600 can reach around 90% of the total capacitance. The Dunn method analysis (Figure S12, ESI†) revealed the identical conclusions. The PPy electrode is a pseudocapacitive material judging from the pronounced capacitance contribution from pseudocapacitance at all selected scan rates. While the electrical double layer capacitance dominates the capacitance of all NOC samples. Pseudo-capacitive reactions typically have relatively slow kinetics than the formation of electrical double 37 layer by ion adsorption at the electrode/electrolyte surface, therefore the more contribution from PC, the more capacitance will be lost at ultrafast current densities. This trend exactly matches with the rate capability performance shown in Figure 3c. On the other hand, although the large amount of EDLC ensures good rate capability performance, it limits the total capacitance, as the case of NOC-600. Among the NOC samples, NOC-500 displays an optimal performance at ultrafast current densities by balancing the contribution from EDLC and PC. Significantly, unlike most conducting polymers that typically cannot survive from frequent charge 38 and discharge cycles, NOC-500 exhibits outstanding cycling stability with ca. 95% capacitance after 60000 cycles (Figure S13, ESI†), possibly due to its partial conversion to carbon, a more stable form than conducting polymers. Taken together, these results clearly prove that the annealing temperature is a critical factor to optimize the capacitive performance of polymer-derived carbons at ultrafast charging/discharging rates.

Section). The ASC is denoted as NOC-500//MnO2. MnO2 has been widely used as cathode material to enlarge operating voltage for supercapacitor because of its high charge storage 39 potential limit. The ultrathin nanosheet structure is favorable for ion diffusion and supplying large electrolyte-accessible area for charge storage (Figure S14 and S15, ESI†). The MnO2 NSAs show superior electrochemical performance within a potential window of 1.0 V (Figure S16, ESI†). Figure 4a shows the CVs collected for the NOC500//MnO2 ASC at different voltage windows with a scan rate of 100 mV/s. The ASC shows a nearly rectangular CV at a maximum voltage window of 2.1 V, without obvious oxygen or hydrogen evolution on electrodes. The achievement of the relatively wide voltage window in neutral electrolytes could be 40 + ascribed to the following reasons: i) the limited number of H and OH impede evolution of H2 and O2, respectively; ii) Strong + solvation of alkaline ions, e.g., Li in our case, impedes water + decomposition and enhances its over-potential. The Li bonded water molecules need to be de-solvated before being split. The de-solvation energy must be given by the electric energy and therefore lead to the need of higher applied voltage than the theoretical value (~1.23 V); iii) Intrinsic electrical resistances of ion-absorption processes and work functions of electrode materials also contribute to the over41 potential of water splitting. The three factors altogether could broaden potential windows. Some asymmetric supercapacitors reported previously have achieved potential windows equal to or wider than 2.0 V in aqueous 42-44 electrolytes. The isosceles triangular-shaped GCD profiles are indicators of the near-ideal capacitive behavior of NOC500//MnO2 ASC (Figure S17, ESI†). Based on the GCD profile 2 obtained at 1.0 mA/cm , the ASC displays a volumetric 3 capacitance of 2.77 F/cm (equivalent to 61.7 F/g,

Electrochemical Performance of Asymmetric Supercapacitor

Figure 4 (a) CVs of NOC-500//MnO2 ASC collected at 100 mV/s within different voltage windows. (b) A plot of volumetric capacitance and capacitive retention as a function of current density. (c) Ragone plot and (d) cycling stability of the NOC-500//MnO2 ASC. Inset of (c) shows

An asymmetric supercapacitor (ASC) was fabricated via the assembly of a NOC-500 anode, a manganese oxide nanosheet arrayed cathode and a piece of separator (the Experimental

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the structure of the ASC and inset of (d) compares the GCD profiles collected at the first and last charge and discharge cycle at a current density of 30 mA/cm 2. Table 2. Electrochemical performance of ASC devices. ASC Device*

Highest Energy Density

Highest Power Density

Cycling Stability

Fe2O3 nanorods//MnO246

0.4 mWh/cm3

0.1 W/cm3

81.6% (5 000 cycles)

H-TiO2@C//H-TiO2@MnO245

0.30 mWh/cm3

0.23 W/cm3

91.2% (5 000 cycles)

Ti-Fe2O3@PEDOT//MnO256

0.89 mWh/cm3

0.44 W/cm3

85.4% (6 000 cycles)

MnO2/ZnO core-shell nanorods//reduced graphene oxide57

0.234 mWh/cm3

0.133 W/cm3

98.5% (5 000 cycles)

CNT/PPy//CNT/MnO258

0.31 mWh/cm3

0.06 W/cm3

85% (5 000 cycles)

TRGO//RGO@MnO259

1.23 mWh/cm3

0.55 W/cm3

91% (10 000 cycles)

Fe2O3@PANI NWAs//PANI@carbon cloth60

0.35 mWh/cm3

0.3 W/cm3

95.7% (10 000 cycles)

NOC//MnO2 (This work)

1.7 mWh/cm3

0.43 W/cm3

91% (10 000 cycles); 90% (20 000 cycles)

*Acronyms: Ti-Fe2O3: titanium-doped hematite; PEDOT: poly(3,4-ethylenedioxythiophene); CNT: carbon nanotube; TRGO: thick reduced graphene oxide; (20 000 cycles) PANI: polyaniline

based on the total mass of active materials), which is substantially higher than other ASCs assembled with 45-47 nanoarray structured electrodes, such as hydrogen-treated 3 titania (H-TiO2)@C NWs//H-TiO2@MnO2 NWs (0.71 F/cm at 45 3 10 mV/s), Fe2O3 nanorods//MnO2 ASC (1.21 F/cm at 0.5 2 46 mA/cm ), reduced graphene oxide (RGO)//MnO2 nanorods 3 47 ASC (0.72 F/cm at 10 mV/s). Significantly, as the current 2 density increases 20 times from 1 to 20 mA/cm , the ASCs device still maintains 72.4% of the volumetric capacitance, 3 reaching 2.01 F/cm (Figure 4b). Figure 4c shows the Ragone plot of NOC-500//MnO2 ASC. It yields an outstanding volumetric energy density of of 1.7 3 3 mWh/cm (at a power density of 0.014 W/cm ), and still 3 3 retains 1.1 mWh/cm at power density of 0.429 W/cm . These values are among the best values reported for ASCs (Table 2). We have calculated the gravimetric energy density and power density normalized to the total mass loadings of active 2 materials on two electrodes (3.3 mg/cm ) and added them in Figure S18 (Supporting Information). Our device achieved a high specific energy density of 38 Wh/kg at the power density of 0.32 kW/kg and an energy density of 28 Wh/kg at a high power density of 6.4 kW/kg, which is higher than or comparable to other ASCs assembled with nanostructured electrodes, such as CNT/PPy//CNT/MnO2 (22.8 Wh/kg at 0.86 48 kW/kg, and 6.2 Wh/kg at 2.7 kW/kg), vanadium nitride (VN) 49 nanowire//VOx nanowire (2.1 Wh/kg at 0.003 kW/kg), 50 PPy//MnO2 (27.2 Wh/kg at 0.85 kW/kg), rGO (reduced graphene oxide)/carbon cloth//SnO2/carbon cloth (22.8 Wh/kg 51 at 0.85 kW/kg, and 4.84 Wh/kg at 5.6 kW/kg), VN//MnO2 (38

52

Wh/kg at 0.0073 kW/kg), activated carbon nanofiber//MnO2/carbon nanofiber (30.6 Wh/kg at 0.2 kW/kg, 53 and 20.8 Wh/kg at 8.7 kW/kg). Moreover, the ASC device is very durable, with around 90% of capacitance maintained after 20000 consecutive charge-discharge cycles (Figure 4d, actual time spent: 120 hours) in a voltage window of 2.1 V at 30 2 mA/cm . The long-term stability is better than most of other ASC devices (Table 2). The coulombic efficiency is almost 100% at all time. We attribute the outstanding energy density and power density of this ASC device to these aspects: 1) the wide voltage window of 2.1 V that drastically increase the energy density; 2) the highly conductive carbon fibers and seamless anchoring of active materials without the need of binders result in ultrasmall internal resistance of the ASC, which is supported by the negligible IR drop in the GCD profiles collected at all current densities (Figure S17, ESI†). Such a small internal resistance can greatly reduce energy dissipation caused by the internal resistance and improve energy density. 3) The morphology of ordered NWAs and ultrathin NSAs allows efficient ion diffusion with reduced diffusion length for ions as well as reduces “dead 54-55 volume”.

Conclusions In summary, we have demonstrated that the pyrolysis temperature is an important factor determining the capacitive performance of NOC. The concentration of hetero-atoms (N and O), the electrical conductivity and ultimately, the EDLC/PC contribution of NOC can be well tailored by varying the

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pyrolysis temperature. We found that high temperature pyrolysis is not necessarily the best strategy for improving the capacitive performance of polymer derived carbons. Balancing the EDLC/PC contribution through tuning of electrical conductivity and concentration of hetero-atoms is the key to achieve optimal performance. Indeed the NOC electrode o prepared at relatively low pyrolysis temperature (500 C in this case) achieved the best rate capability and the largest capacitance at high current density. We believe this work should exert broad and significant impact for the energy storage community, as nanostructured polymers are one of the most commonly selected carbon precursors for electrode material preparation. Additionally, the electrochemical properties of other polymer derived carbonaceous materials can also possibly be improved by tuning the pyrolysis temperature, which will diversify the toolbox of carbonaceous materials and enable tailor-made electrochemical properties suitable for a variety of electronic devices beyond supercapacitors, including lithium ion batteries and electrochemical sensors.

Acknowledgements We gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. 21673035, 21273029). Tianyu Liu thanks the financial support from the Chancellor Dissertation Year Fellowship awarded by University of California, Santa Cruz.

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Supporting Information Balancing the Electrical Double Layer Capacitance and Pseudocapacitance of Hetero-atom Doped Carbon Zi-Hang Huang a,b‡, Tianyu Liub‡, Yu Songa,b, Yat Lib* and Xiao-Xia Liua*

a

Department of Chemistry, Northeastern University, Shenyang, 110819, China

b

Department of Chemistry and Biochemistry, University of California, 1156 High Street, Santa Cruz, California, 95064, United States

* Corresponding authors: Xiao-Xia Liu, [email protected]; Yat Li, [email protected]

‡ These authors contributed equally to this work.

-1-

1. Calculations 1.1 Capacitances of single electrode The areal and gravimetric capacitance of a single electrode can be calculated based on galvanostatic charge-discharge experiments according to Equation S1 and S2: 𝐼×𝑡

𝐶𝑠 = ∆𝑈×𝑆

(Equation S1)

𝐼×𝑡

𝐶𝑚 = ∆𝑈×𝑚

(Equation S2)

Where Cs and Cm (mF/cm2 or F/g) are the areal and gravimetric capacitance, I the discharge current (mA), t the time (s), ΔU the potential window (V), S the working area of electrode (cm2), m is the mass of active material (NOC or MnO2). The mass loadings of all NOC electrodes were controlled to be 2.3 mg/cm2 by adjusting the electro-deposition time. The mass loading of MnO2 NSAs is 1.0 mg/cm2. 1.2 Capacitances of NOC//MnO2 Devices The volumetric capacitance (Cv, unit in F/cm3) of NOC//MnO2 can be calculated based on galvanostatic charge-discharge experiments according to Equation S3: 𝐼×𝑡

𝐶𝑉 = ∆𝑈×𝑉

(Equation S3)

Where I is the charge-discharge current (A), t the discharge time (s), ΔU the potential window (V) and V the total volume (cm3) of the whole device stack including two electrodes, electrolyte-soaked separator and packages. The working area and thickness of the NOC//MnO2 device are ca.1 cm2 and 0.0735 cm, respectively. The whole volume of the NOC//MnO2 device are about 0.0735 cm3 [1 cm (L) × 1 cm (W) ×0.0735 cm (H)]. The gravimetric capacitance of ASC device can be calculated based on galvanostatic charge-discharge experiments according to Equation S4: 𝐶𝑚 =

𝐼×𝑡

(Equation S4)

∆𝑈×𝑚

where Cm (F/g) is the gravimetric capacitance. I is the discharge current (mA). t is the discharge time (s). ΔU is the potential window (V). m is the mass of active material (mg). 1.3 Energy density and power density of NOC//MnO2 Devices Volumetric energy density (E, Wh/cm3) and volumetric power density (P, W/cm3) are calculated using the following two equations: 𝐸𝑉 =

𝐶𝑉 ×∆𝑈 2

(Equation S5)

2×3600

-2-

𝐸𝑉 =

3600×𝐸

(Equation S6)

𝑡

Where Cv (F/cm3) is the specific capacitance, ΔU the potential window (V) and t the discharge time (s). Gravimetric energy density (E, Wh/kg) and power density (P, W/kg) are calculated using the following two equations: 𝐸𝑚 =

1000×𝐶𝑚 ×∆𝑈 2

(Equation S7)

2×3600

𝑃𝑚 =

3600×𝐸𝑚

(Equation S8)

𝑡

where Cm (F/g) is the specific capacitance of ASC device, ΔU is the potential window (V) and t is the discharge time (s). 1.4 Charge balance for NOC//MnO2 Devices To achieve the maximum and stable performance of the ASC device, the capacity (Q) of negative and positive electrode should be balanced, i.e., 𝑄+ = 𝑄−

(Equation S9)

The capacity is associated with areal capacitance (Cs), potential window (ΔU) and working area of electrode (S), as shown in Equation S10: 𝑄 = 𝐶𝑠 × ∆𝑈 × 𝑆

(Equation S10)

Combining Equation S9 and S10, the areal ratio of negative electrode to positive electrode should satisfy Equation S11: 𝑆− 𝑆+

(𝐶 ,+ )×(∆𝑈+ )

= (𝐶𝑠

𝑠,− )×(∆𝑈− )

(Equation S11)

Substitute Cs,+=356.4 mF/cm2 (@ 1 mA/cm2), Cs,-=324 mF/cm2 (@ 1 mA/cm2), ΔU-= 1.1 V and ΔU+= 1.0 V, the areal ratio is about 1. In our work, we fixed the working area of both electrodes to be 1.0 cm2.

-3-

2. Supplementary Figures and Tables

Figure S1. Digital photographs of ASC devices’ components include NOC-500 anode, MnO2 NSAs cathode, a piece of cellulose separator and parafilm (a). Photograph of thickness measurements process (b, c).

Figure S2. SEM images of (a) NOC-400, (b) NOC-500 and (c) NOC-600.

Figure S3. Raman spectra of PPy NWAs, and all NOC samples (a). The tables show the assignments for the major peaks in the spectra. -4-

Figure S4. XPS survey spectra collected for (a) PPy NWAs, (b) NOC-400, (c) NOC-500 and (d) NOC-600.

Table S1 Summary of the atomic percentage and electrochemical performances of prepared samples Sample Name

Element composition (at.%) N

O

Combined series resistance (ohm)

Capacitance @ 1 mA/cm2

@ 100 mA/cm2 2.63 mF/cm2 (1.14 F/g @43.5A/g)

PPy

11.97

14.40

1.39

526 mF/cm2 (228 F/g @0.43A/g)

NOC-400

9.87

12.03

1.36

434 mF/cm2 (188 F/g @0.43A/g)

108 mF/cm2 (47 F/g @43.5A/g) 184.7 mF/cm2 (80.3 F/g @43.5A/g) 139 mF/cm2 (60.5 F/g @43.5A/g)

NOC-500

7.95

11.29

0.96

324 mF/cm2 (141 F/g @0.43A/g)

NOC-600

5.50

6.45

0.91

229 mF/cm2 (99.6 F/g @0.43A/g) -5-

Figure S5. High resolution N 1s XPS peak of (a) PPy NWAs, (b) NOC-400 and (c) NOC-600. The solid black curves are experimental data. The dashed lines are synthetic peaks and the solid red lines are the sums of all synthetic peaks.

Figure S6. Schematic illustration showing different N-functionalities: pyrrolic nitrogen (N-5), pyridinic nitrogen (N-6), quaternary nitrogen (N-Q), and oxidized nitrogen (N-O).

Figure S7. High resolution O 1s XPS peak of (a) PPy NWAs, (b) NOC-400 and (c) NOC-600. The solid black curves are experimental data. The dashed lines are synthetic peaks and the solid red lines are the sums of all synthetic peaks. -6-

Figure S8. A digital picture of the three-electrode electrolytic cell used for electrochemical tests.

Figure S9. Gravimetric capacitance of all NOC and PPy electrodes collected at different scan rates ranging from 0.43 A/g to 43.5 A/g. Among all the NOC electrodes and the PPy electrode, NOC-500 exhibited the highest gravimetric capacitance of 80.3 F/g at 43.5 A/g with a rate capability of 57% (from 0.43 A/g to 43.5 A/g). This trend is in agreement with the areal capacitance.

-7-

Figure S10. Electrochemical performance of carbon cloth substrates in 5 M LiCl aqueous electrolyte. (a) Cyclic voltammograms collected for carbon cloth samples treated at different temperatures (scan rate: 200 mV/s). (b) Comparison of the GCD profile of NOC-500 (1 mA/cm2) and the GCD profiles of various carbon substrates (0.1 mA/cm2). (c) GCD profiles of various carbon cloth samples.

Trasatti Method Analysis Trasatti method is used to evaluate capacitive contribution from electrical double layer and pseudo-capacitive reactions. The main steps involving the analysis are as follows: 1. Data collection Collect cyclic voltammograms at different scan rates and evaluate corresponding areal capacitances based on the following equation: 𝑆 （Equation S12） 2 ∙ ∆𝑈 ∙ 𝑣 where C is the areal capacitance (in mF/cm2), ΔU the potential window (in V), S the area 𝐶=

enclosed by corresponding cyclic voltammograms (in mA·V/cm2) and v the scan rate (in V/s). 2. Evaluation of maximum capacitance (CT) Assuming ion diffusion follows a 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 areal capacitance (C-1) and the square root of scan rates (v1/2) should be observed3: 𝐶 −1 = constant ∙ 𝑣 1/2 + 𝐶𝑇 −1

(Equation S13)

where C, v and CT represent calculated areal capacitance, scan rate and maximum areal capacitance, respectively. The “maximum capacitance (CT)” is treated as the sum of electrical double layer capacitance and pseudo-capacitance.4 CT equals the reciprocal of the y-intercept of the -8-

C-1 vs. v1/2 plot (Figure S10, left columns). 3. Evaluation of maximum electrical double layer capacitance and maximum pseudo-capacitance Plotting the calculated areal capacitances (C) against the reciprocal of square root of scan rates (v-1/2) should also give a linear correlation described by the following equation (assuming a semi-infinite diffusion pattern)5: (Figure S10, right columns) 𝐶 = constant ∙ 𝑣 −1/2 + 𝐶𝐸𝐷𝐿

(Equation S14)

where C, v and CEDL is the calculated areal capacitance, scan rate and maximum electrical double layer capacitance, respectively. Linear fit the plot and extrapolate the fitting line to y-axis gives the CEDL.4 Subtraction of CEDL from CT yields the maximum pseudo-capacitance (CPS). 4. Evaluation of the percentage of capacitance contribution The capacitance contribution can be evaluated based on the following equation: 𝐶𝐸𝐷𝐿 % = 𝐶𝑃𝑆 % =

𝐶𝐸𝐷𝐿 𝐶𝑇

𝐶𝑃𝑆 𝐶𝑇

× 100%

× 100%

(Equation S15) (Equation S16)

where CEDL% and CPS% stand for capacitance percentage of electrical double layer capacitance and pseudo-capacitance.

-9-

Figure S11. Left column: plots of reciprocal of areal capacitance (C-1) vs. square root of scan rate (v1/2). Right column: plots of gravimetric capacitance (C) vs. reciprocal of square root of scan rate (v-1/2). The solid lines are linear fitting lines of data points. The algebraic equations of the fitting -10-

lines are shown in the inset. (a) PPy NWAs, (b) NOC-400, (c) NOC-500, and (d) NOC-600.

Figure S12. Percentage of capacitance contribution evaluated for PPy and all NOC samples under different scan rates. The details about the capacitance differentiation can be found in the previously reported work.5

Figure S13. Cycling stability of NOC-500 evaluated by cyclic voltammetry at 50 mV/s in 5 M LiCl aqueous electrolyte. Inset compares the CVs collected at the first and last cycle and the electrode before and after the cycling stability test.

-11-

Characterizations of manganese dioxide nanosheet arrays (MnO2 NSAs) Figure S14a-c show the morphology of the as-prepared MnO2 NSAs. It can be seen that MnO2 clusters were uniformly deposited on carbon fibers. Close examination reveals that each cluster is consisted of a number of vertically aligned MnO2 nanosheets interconnected with each other (Figure S14c). The highly open and porous 3D architecture is beneficial for fast ion diffusion. Figure S14d shows a representative TEM image of several MnO2 nanosheets. The thickness of nanosheet is around 10-15 nm and the selected area electron diffraction (SAED) pattern (Figure S14d inset) confirms that the armorphous nature of the as-deposited MnO2 nanosheets. To determine the chemical composition of the deposited NSAs, XPS spectra was collected. The survey spectrum (Figure S15a) contains signals from Mn, C and O elements in the electrode which indicates manganese oxide is successfully deposited on carbon fibers. To further investigate the valence state of Mn, the high resolution Mn 2p peak and Mn 3s peak are de-convoluted. In the Mn 2p spectrum (Figure S15b), there are two synthetic peaks located at 654.3 and 642.4 eV, which can be assigned to Mn 2p1/2 and Mn 2p3/2 spin-orbit peaks of MnO2, respectively.6 The energy separation of the two peaks from Mn 3s (Figure S15c) is measured to be 4.57 eV, a characteristic value of Mn4+ ions.7-9 Taking together, we are confident that the chemical composition of the deposited NSAs is MnO2.

-12-

Figure S14. (a-c) The SEM images and (d) the TEM image of MnO2 NSAs. Inset of (d) shows the SAED image.

Figure S15. (a) XPS survey spectrum of MnO2 NSAs. High resolution (b) Mn 2p and (c) Mn 3s XPS peak. Peak separations are highlighted and labelled.

Figure S16. Electrochemical performance of the MnO2 NSAs tested in 5 M LiCl aqueous electrolyte. (a) GCD profiles collected at various current densities. (b) CVs measured at different scan rates in a three-electrode cell. (c) Plot of areal capacitance vs. various current densities. Capacitances are calculated based on GCD profiles. (d) Cycling stability with the inset shows the -13-

GCD profiles collected in the first and the last cycle.

Figure S17. Electrochemical performance of the NOC-500//MnO2 ASC. (a) CVs measured at various large scan rates and (b) GCD profiles collected at various current densities.

Figure S18. The gravimetric energy density and power density of the NOC-500//MnO2 ASC.

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