Redox-Driven Route for Widening Voltage Window in Asymmetric Supercapacitor , Republic of Korea. Korea. *Corresponding author

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1 Redox-Driven Route for Widening Voltage Window in Asymmetric Supercapacitor Ramkrishna Sahoo a,b, Duy Tho Pham a,b, Tae Hoon Lee a,b, Thi Hoai Thuong Luu a,b, Jinbong Seok a,b, Young Hee Lee a, b * a Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Suwon 16419, Republic of Korea. b Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea. *Corresponding author address: leeyoung@skku.edu (Y. H. Lee). S1

2 a b c C 1s O 1s Mn 2p ev C=C ev C -OH ev C -O-C ev C=O ev O-C =O Binding enrgy (ev) d M n C O Figure S1: Physical characterization of rgo@mn 3 O 4. (a) Wide range XPS. (b) C 1s XPS. (c) FESEM of pure Mn 3 O 4 nanoparticles. (d) EDX and elemental area mapping. a b c Weight (%) Temparature ( o C) 86 % 43 % 23 % N 2 Volume (cc/g) 8 Specific Surface Area = 6.26 m 2/g Relative pressure(p/p ) DV(r) dv [cc/g] Pore volume =.17 cc/g Average pore diameter (nm) Figure S2: Physical characterization of rgo@mn 3 O 4. (a) TGA graphs of different composites. (b) Nitrogen adsorption/desorption isotherms. (c) Pore size distributions curves. S2

3 Potential (V) a Specific capactance (F/g) d Scan rate (mv/s) Potential (V) e 3 f mg/cm 2 Energy efficiency (%) Capacitance (%) g Specific current (A/g) Number of cycle Coulombic efficiency (%) Coulombic efficiency (%) b h Z // imaginary (ohm) Specific current (A/g) Specific capactance (F/g) rgo rgo@mn 3 O 4 at 5 mv/s Scan rate (mv/s) Z // imaginar y (ohm) Initial after 5 cycles after cycles Z / r eal (ohm) 1.5 mg/cm mg/cm 2 3 Z / (ohm) real Specific current (A/g) OER i Potential (V) Specific capactance (F/g) c RS (ohm) RCT (ohm) ESR (ohm) Initial After 5 cycles After cycles rgo@mn 3 O 4 pure Mn 3 O 4 Time (s) 86% Mn 3 O 4 43% Mn 3 O 4 23% Mn 3 O 4 Scan rate (mv/s) Figure S3: Electrochemical performance of rgo@mn 3 O 4. (a) Specific capacitance values at different scan rates. (b) Comparative CV curve of pure rgo and rgo@mn 3 O 4 at 5 mv/s (inset CV curve of pure rgo). (c) Comparative CD curves of pure Mn 3 O 4 NPs and rgo@mn 3 O 4 composite. (d) Plot of energy efficiency and Coulombic efficiency as a function of specific currents of the rgo@mn 3 O 4 composite. (e, f) Plot of specific capacitance values as a function of scan rates at a different mass loading of rgo@mn 3 O 4 composites and at different Mn 3 O 4 loading on different composites. (g) Cycling performance up to GCD cycles. (h) Plot of electrochemical impedance spectra. (i) Corresponding impedance parameters at different cyclic stages. S3

4 a b c 2p 3/2 Mn 2+ (641.3 ev) Mn 3+ (642.9 ev) Initial satellite peak ( ev) Mn 2+ (6.96 ev) Mn 3+ ( ev) Mn 4+ After 1st charge satellite peak ( ev) ( ev) d e f Mn 2+ ( ev) After 1 st charge -discharge Mn 3+ (643.1 ev) satellite peak (644.6 ev) Mn 2+ (6.79 ev) Mn 3+ ( ev) After activation Mn 4+ ( ev) satellite peak ( ev) No satellite peak; no Mn 2+ Mn 3+ (642.5 ev) After cyclic performance Mn4 + (643.5 ev) Figure S4: Physical characterization of rgo@mn 3 O 4 after the performance. (a) FESEM after CD cycles. Deconvoluted Mn 2p 3/2 XPS spectrum (b) before the CD performance, (c) after 1 st charge, (d) after 1 st charge-discharge, (e) after the activation process and (f) after CD cycles. S4

5 Contribution % c a 1 Diffusion control 11 Peak 1 Cpacitive control Scan rate (mv/s) Contribution % 1 11 d b Diffusion control Peak 2 Cpacitive control Scan rate (mv/s) Specific current (ma/cm 2 ) 2 1 initial after 1 st charge after 1 st ch-dch after 1 th cyles Potential (V) Specific current (ma/cm 2 ) 4 2 On Ni foam, On set potential 1.12 V vs Ag/Agcl and over potential.499 V On CC, On set potential 1.3 V vs Ag/Agcl and over potential.679 V Potential (V) Figure S5: Percentage contribution of capacitive and diffusion-control reaction to charge storage for rgo@mn 3 O 4 ; (a) Peak 1; for Mn 2+/3+ redox couple. (b) Peak 2; for Mn 3+/4+ redox couple. (c) Comparative LSV curves at different reaction conditions on carbon cloth current collector. (d) Comparative LSV curves after electrochemical activation using Carbon cloth and Ni foam as a current collector. Onset potentials for OER on Ni foam and Carbon cloth are 1.12 V and 1.3 V vs Ag/AgCl. a b c C 1s O 1s V 2p ev (C=C) ev (C-C) ev (C-O) C 1s ev(c=o) ev(o=c-oh) ev (V-O) O 1s ev (-C=O) ev (-C-O) ev (-C-OH) Figure S6: XPS of rgo@vo 2. (a) Wide range spectrum. (b, c) C 1s and O 1s core-level spectra. The wide scan of the composite reveals the presence of C, O, and V (Figure S6a). The deconvoluted core-level spectra of C 1s revels that the sp 2 carbon is positioned at ev and the others peaks are mainly due to the oxygen functional groups of rgo (Figure S6 b). The O 1s S5

6 spectrum is deconvoluted into four peaks (Figure S6c). The most intense peak at ev signifies the V-O bond. S1 Other peaks are indicated different carbon-oxygen bonds from rgo. a b c d Figure S7: Morphological characterization of rgo@vo 2. (a, b, c) FESEM images V 2 O 5 powder, as-prepared pure VO 2, and as-prepared rgo@vo 2 composite. (d) TEM image of the rgo@vo 2 composite. S6

7 C O V Figure S8: Elemental Characterization of rgo@vo 2. EDX and elemental area mapping of the rgo@vo 2 composite. a b N2 Volume (cc/g) 1 Surface Area 44.5 m 2 /g Relative pressure(p/p ) DV(r) dv [cc/g].4.2. Pore volume.1769 cc/g Average pore diameter (nm) Figure S9: BET study of rgo@vo 2. (a) Nitrogen adsorption/desorption isotherms. (b) pore size distributions curves of the rgo@vo 2 composite. S7

8 Energy efficiency (%) a Specific capactance (F/g) Scan rate (mv/s) Specific current (A/g) 1 Coulombic efficiency (%) Specific capactance (F/g) Specific capactance (F/g) Scan rate (mv/s) Specific current (A/g) c d e b 3% VO 2 5% VO 2 66% VO 2 Specific capactance (F/g) Scan rate (mv/s) 1.1 mg/cm mg/cm mg/cm 2 Figure S1: Electrochemical performance of rgo@vo 2. (a, b) Plot of specific capacitance values as a function of scan rates and specific currents for. (c) Plot of Coulombic efficiency and energy efficiency with respect to specific current. (d, e) Plot of specific capacitance values as a function of scan rates at different VO 2 loading on different composites and at a different mass loading of rgo@vo 2 composites. Figures S1a,b show the specific capacitance values as a function of scan rates and specific currents of the rgo@vo 2 composite. The data clearly exhibit the high rate of the composite. Figure S1c displays the energy efficiency and Coulombic efficiency at different specific currents. At low current Coulombic efficiency and energy efficiency are low and it increases with the specific currents. At 4 A/g, the Coulombic efficiency of the composite is ~ % which clearly suggest the excellent kinetic reversibility of the composite. Figure S1d demonstrates the specific capacitance values as a function of scan rates for different V 2 O 5 loadings. Here, it is observed that for the 5 % of VO 2 loading on rgo, the composite exhibits best S8

9 pseudocapacitance activity. Figure S1e exhibits the electrochemical performance at different mass loadings on the current collector. a b c % capacitance Number of cycle Coulombic efficiency Z // imaginary (ohm) 15 5 Z // (o hm) imagina ry Initial Z / real (ohm) after 5 cycles after cycles Z / real(o hm) R S (ohm) R CT (ohm) ESR (ohm) Initial After 5 cycles After cycles Figure S11: Electrochemical performance of rgo@vo 2. (a) Plot of percentage capacitance retention and Coulombic efficiency up to cycles. (b) Plot of electrochemical impedance spectra. (c) Corresponding impedance spectra at cyclic stages of the rgo@vo 2 composite. To study the electrochemical stability of the composite, we performed the CD test of the composite at 12.5 A/g for 1, cycles (Figure S11a). The composite maintained 95% of its initial capacitance value, suggesting the excellent stability of the composite. Figure S11a also suggests that the sample maintains a high Coulombic efficiency throughout the 1, cycles indicating strong kinetic reversibility at the high current value. To support the high cyclic stability of the material, we performed the EIS analysis of the composite before and after the cyclic test (Figure S11b). Figure S11c displays the impedance parameters at different cyclic states. Before and after 1, cycles, the change in ESR is negligible, suggesting the superior cyclic stability of the composite. S9

10 a 2p 3/2 V ev b 1st charge 2p 3/ 2 Initial ev V 3+ V ev ev V ev V ev V 5+ 1st charge-discharge ev V 3+ V ev V p 3/ ev c d e 2p 3/2 After activation ev V 3+ V ev V ev 2p 3/2 After cyclic performance ev V ev V ev V Figure S12: XPS of XPS of rgo@vo 2 after the performance. (a-e) Deconvoluted V 2p 3/2 XPS spectrum before the CD performance, after 1 st charge, after 1 st charge-discharge, after the activation process and after CD cycles. S1

11 peak 2 peak a log current (log i) d Specific current (ma/cm 2 ) peak 1; b=.75 peak 2; b = On cc after activation, on-set potential -1.1 V vs Ag/Agcl and over potential.1 V On cc before activation, on-set potential -.99 V vs Ag/Agcl and over potential.381 V Specific current (A/g) Potential (V) Potentia l(v) log sweep rate (log ν4 b Contribution % e Specific current (ma/cm 2 ) Diffusion control Cpacitive control initial after 1 st charge after 1 st ch-dch after 1 th cycles Peak Scan rate (mv/s) Potential (V) c f Specific current (ma/cm 2 ) On Ni foam, on-set potential V vs Ag/Agcl and over potential.353 V On cc, on-set potential -1.1 V vs Ag/Agcl and over potential.1 V Potential (V) Figure S13: Kinetic and physical characterization of rgo@vo 2. (a) Plot of rate law (log scan rate vs log peak current). (b) Ratio of capacitive contribution and diffusive contribution. (c) FESEM after CD cycles. (d) Comparative LSV curves on the carbon cloth before and after activation. (e) Comparative LSV curves on the carbon cloth at different reaction conditions. (f) Comparative LSV curves of the rgo@vo 2 composite on the carbon cloth and on the Ni foam. Figure S13a exhibits the b values of peak 1 (reaction from V 4+ to V 3+ ) and peak 2 (reaction between V 2+ and V 3+ ), which are.75 and.7, respectively. The results indicate that for both reactions, the contribution by the diffusion control reaction dominates. To quantitatively study the energy storage mechanism of the composite, we separated the capacitive current from the total current. The capacitive contribution increases with the increase in the scan rate (Figure S13b). We performed LSV experiments and calculated the onset potential and overpotential for HER of the rgo@vo 2. Figure S13d shows the LSV curves for the HER of the composite on the carbon cloth current collector before and after the activation process. Before and after the activation, the onset potentials were -.99 V and -1.1 V (vs Ag/AgCl) with overpotential values S11

12 Pe ak 1 Pea k of.381 V and.1 V, respectively. This result supports the stable working potential of -1 V. HER study at different reaction conditions reveals that after the first charge onwards HER starts above -1 V (Figure S13e) which clearly confirms the stable potential window of the rgo@vo 2 composite up to -1 V. We also studied the HER activity using Ni foam as the current collector. The onset potential and overpotential for HER on the Ni foam current collector were V and.353 V, respectively (Figure S13f). This result indicates the advantage of using carbon cloth as the current collector over the Ni foam collector. a Specific current (A/g) Total capacitance (F/g) st cycle 2 nd cycles 3 1 rd cycles 4 th cycles 5 th cycles 1 th cycles 8 15 th cycles th cycles d g Contribution % 5 mv/s Voltage (V) Scan rate (mv/s) Specific current (A/g) Energy efficiency (%) b e 1 Diffusion control 11 Cpacitive control Scan rate (mv/s) V 2. V 1.8 V Voltage (V) Specific current (A/g) h Current (A) mv/s c Coulombic efficiency (%) Voltage (V) Logcurrent (log i) 44 % capacitive f Voltage (V) Time (s) Specific current (A/g) Total current Capacitive current Voltage (V) 2.2 V 2. V 1.8 V peak 1; b =.73 peak 2; b = Log sweep rate (log ν) Figure S14: Electrochemical characterization of the AAS, rgo@mn 3 O 4 //rgo@vo 2. (a) CV curves up to first cycles (b, c) CV curves (at 5 mv/s) and GCD curves (at.6 A/g) at 1.8 V, 2 V and 2.2 V. (d) Plot of specific capacitance value as a function of scan rate. (e) Plot of energy efficiency and Coulombic efficiency at different specific currents. (f) Rate law curve. (g) Plot of the percentage contribution of capacitive contribution and diffusive contribution at the different scan rate. (h) Plot of capacitive contribution to the total current at 5 mv/s scan rate. S12

13 Figure S14f exhibits the rate law for the AAS. For peak 1, b =.73 and for peak 2, b =.86, which suggest that the energy storage mechanism of the AAS is dominated by both capacitive contribution and diffusion control reactions. Figure S14g demonstrates the ratio of capacitive contribution and diffusive contribution for each scan rate. Figure 14h shows that 44% of the total current is capacitive for the AAS at 5 mv/s scan rate. a b Z // imaginary (ohm) Initial 35 after cycles Z // imaginary (ohm) 5 1 Z / real (ohm) -5 3 Z / real (ohm) R S (ohm) R CT (ohm) ESR (ohm) Initial After cycles Figure S15: Electrochemical performance of the AAS. (a) Plot of electrochemical impedance spectra. (b) Corresponding impedance spectra at cyclic stages of the rgo@vo 2 composite. S13

14 Voltage (V) experimental fitting curve for (V=V -mt 1/2 int ) fitting curve for (V=V e -t/τ int +V ) fitting curve for (V=V 1 e -t/τ 1 +V 2 e -t/τ 2 +V 3 e -t/τ 3 + V ) 5 15 Time (s) Figure S16: Self-discharge curve for the AAS. Figure S16 exhibits the self-discharge curve (SD) of the as-fabricated AAS when it was charged at.4 A/g. The SD curve can be divided into three parts, (i) divided potential driven (DPD) (red line); Here we found three time constants at three different potential area (τ 1 =23.69 s, τ 2 = s and τ 3 =162 s). In that area, SD was very fast. (ii) Single potential driven (SPD) (blue line): Here we found only one time constant (τ=69899 s). This step is relatively slow compared to the first step. (iii) Diffusion driven (cyan line): This part is slowest sep where the driving force is faradic diffusion reaction. S14

15 Table S1: Comparison of electrochemical performances of Mn 3 O 4 based pseudocapacitors. Materials Electrolyte Potential window Specific capacitance Rate capability References Porous Mn 3 O 4 nanoplate 6 M KOH -.1 to.55 V 246.6(5 mv/s) 43.2 ( mv/s) S2 Mesoporous Mn 3 O 4 1 M Na 2 SO 4 to.9 V 155(.3 A/g) 9.3 (.6 A/g) S3 Mn 3 O 4 nanosheet 1 M Na 2 SO to.9 V 278(5 mv/s) 62.9 ( mv/s) S4 3D mesoporous Mn 3 O 4.5 M Na 2 SO 4 to 1 V 13(5 mv/s) 5 ( mv/s) S5 Mn 3 O 4 nanosheet.5 M Na 2 SO 4 to.9 V ~222(2 A/g) ~9.1 (4 A/g) S6 Porous Mn 3 O 4 microsphere 1 M Na 2 SO 4 to 1 V 236.8(1 mv/s) 48.8 ( 5 mv/s) S7 3 D sponge like Mn 3 O 4 1 M Na 2 SO 4 to 1 V 274(.5 A/g) 52.7 (3 A/g) S8 rgo@mn 3 O 4 NPs 1 M Na 2 SO to 1.2 V 288 F/g(.7 A/g).4 (24 A/g) This work 284 F/g (2 mv/s) 51.4 ( mv/s) S15

16 Table S2: Binding energy of Mn 2p 3/2 core-level XPS for 3 O 4 at different stages of the electrochemical reaction. Mn 2p 3/2 Mn 2p 3/2 Mn 2p 3/2 Mn 2+ Mn 3+ Mn 4+ Satellite peak+ Mn 7+ Different stages Binding energy % area HWFM Binding energy % area HWFM Binding energy % area HWFM Binding energy % area HWFM 1. Initial After 1 st charge After 1 st ch-dch After activation After cycles S16

17 Table S3: Comparison of electrochemical performances of VO 2 based pseudocapacitors. Materials Electrolyte Potential window Specific capacitance References Graphene /VO 2 nanoflower.5 M K 2 SO V to.8 V 225 F/g at.25 A/g S1 VO 2.nH 2 O NF@3DG.5 M K 2 SO V to.6 V 57 F/g at 3 ma/cm 2 S9 VO 2 nanobelt@3d graphene.5 M K2SO V to.6 V 426 F/g at 1 A/g S1 VO M Na 2 SO V to.8 V 246 F/g at.5 A/g S11 G/VO X nanotube 1 M Na 2 SO to.7 V 21 F/g at 5 mv/s S12 rgo@vo 2 nanoribbon composite 1 M Na 2 SO 4-1 V to V 349 F/g at 2 mv/s 326 F/g at.85 A/g This work S17

18 Table S4: Binding energy of V 2p 3/2 core-level XPS for rgo@vo 2 at different stages of the electrochemical reaction. V 2p 3/2 V 2p 3/2 V 2p 3/2 V 3+ V 4+ V 5+ Different stages Binding energy % area HWFM Binding energy % area HWFM Binding energy % area HWFM 1. Initial After 1 st charge After 1 st ch-dch After activation After cycles S18

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