Enhanced energy of water-based

electrochemical capacitor

Elzbieta Frąckowiak, Krzysztof Fic

AABC, Electrochemical Capacitors EC

New EC Capacitor Products, 30 January, 2017

Institute of Chemistry and Technical Electrochemistry

Poznan University of Technology, Poland

2

Nitrogen sorption isotherms of activated carbons

2364 m2/g

2136 m2/g

2097 m2/g

Kynol tissue ACC - 2364 m2/g (0.99 nm)

C = εS / d

E= 0.5 CU2

Halide (I-1, Br-1) and

pseudohalide (SCN-1) aq. solutions

as redox active electrolytes

Electrochemical activity of iodide

at carbon electrode/electrolyte interface

3

3 I-1 I3-1 + 2e-

2 I-1 I2 + 2e-

3 I3-1 3 I2 + 2e-

I2 + 6 H2O IO3- + 12 H+ + 10e-

Pourbaix diagram for iodine @ 25˚C

H3I

I-

I3-

I2IO3

-

H3IO62-

IO4-

HIO3

H5IO6

IO3-

HIO

4

5

Cyclic voltammograms (5 mV/s) for AC electrodes

Negative electrode: 6M KOH Positive electrode: 1M KI

Potential limit: -1.5V vs NHE Potential limit: +0.45 V vs NHE

Capacitance: 56 F/g, Max voltage: 1.5 V, Energy density: 8.9 Wh/kg

K. Fic et al., J. Electrochem. Soc. 162 (2015) 5140-5147

6

Ragone plot for capacitors operating in hybrid electrolytes

Electrochemical activity of bromine

at carbon electrode/electrolyte interface

Br - Braq + e- (1)

2Br - Br2aq + 2e- (2)

2 Br3- 3Br2aq + 2e- (3)

Br2aq + 2H2O 2BrO- + 4H+ +2e-

Br - + 2OH- 2BrO- + 4H2O +2e-

(4)

(5)

Br2 + 2OH- Br- + BrO- + H2O

BrO- + 4OH- BrO3- + 2H2O + 4e-

BrO3- + 5Br- + 6H+ 3Br2 + 3H2O

(6)

(7)

(8)

E. Frackowiak, K. Fic, submitted

7

Pourbaix diagram for bromine @ 25˚C

H3Br

Br5- Br2

BrO3-

BrO4-

BrOO

Br3-

Br -

Bromine solubility: 0.21 mol/L

Iodine solubility: 0.0013 mol/L

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Cyclic voltammograms (5 mV/s) for AC electrode

1 mol/L KBr

10

Cyclic voltammogram for AC electrodes

1 mol/L KBr + 0.05 mol/L KBrO3

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Galvanostatic loading (1 A/g) for AC/AC capacitor

1 mol/L KBr + 0.05 mol/L KBrO3

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Raman spectra for AC electrode in KBr/KBrO3

Charge transfer reaction on carbon surface

Bromide/bromine activity

1 mol/L KBr + 0.05 mol/L KBrO3

Comparison of various capacitor systems

Investigations done in two-electrode Swagelok cells

Capacitance determined at 1 A/g current load

Specific capacitance and energy refers to active mass of the electrodes only

13

Properties of SCN- anion (neat and solvated)

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Linear structure with 2.9 A length

Major charge distributed around S atom

Bent-structure with 7.2 A diameter

ca. 7 water molecules weakly solvated

Great solubity (up to 9 mol/L in water) preserving good conductivity of electrolyte

High mobility, high ‚oxidation’ potentials and weak hydration

Cation and concentration affect the electrolyte conductivity

15

Conductivity trend confirmed by EIS technique

ion association

Frequency response for systems

with various KSCN concentrations

16

Maximum voltage determination

17

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-200

-100

0

100

200

300

KX SS YP50F 20 mV s-1

C (

F g

-1)

U (V)

Conclusions (1)

Activity of halides and thiocyanates provides enourmous

capacitance and enhances the energy density (26 Wh/kg)

Redox activity depends on the current collector

Carbon corrosion has been investigated by Raman spectroscopy

Thiocyanates may serve as overcharging protectors

1.8 V of max. operating voltage of capacitor with

7 mol/L of KSCN solution was achieved

Charge propagation is aggravated by diffusion of reacting species

Cyclability of the redox active systems reaches 100 000 cycles

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On-line Electrochemical Mass Spectrometry

(OEMS)

as powerful technique for carbon/electrolyte

interface analysis

M. He, K. Fic, E. Frackowiak, P. Novak, E. J. Berg

Energy & Environmental Science 9 (2016) 623-633

M. He, K. Fic, E. Frackowiak, P. Novák, E.J. Berg,

Energy Storage Materials 5 (2016) 111–115

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On-line Electrochemical Mass Spectrometry (OEMS)

M. He, K. Fic, E. Frackowiak, P. Novak, E. J. Berg

Energy and Environmental Science 9 (2016) 623-633

21

Cyclic voltammetry of carbon based capacitor

at 1 mV/s in 1M Li2SO4

22

Mass signal intensities (OEMS analysis) at 0.1 mV/s

23

Mass signal intensities (OEMS analysis) at 0.1 mV/s

24

OEMS analysis at potentiostatic polarization extension (0.2V to 2V)

25

(RCO)2O – 4e-→ 2R + CO ↑+ CO2 ↑R1COCOR2 – 4e-→ R1+ R2 + 2CO ↑

R1COR2 + 2e- + 2H+→ R1CHOHR2R1COCOR2 + 4e- + 4H+→ R1(CHOH)2R2

C + O/O2 → new surface groups → CO/CO2 ↑

C + OH- - e- → C-OHads

C – OHads. + OH- - e-→ CO/CO2↑ + H2O

C + 2 H2O - 4 e-→ CO/CO2↑ + 4 H+

3C + 16 OH- - 12 e-→ 2 HCO3- + CO3

2- + 7 H2O

M. He, K. Fic, E. Frackowiak, P. Novak, E. J. Berg

Energy and Environmental Science 9 (2016) 623-633

Conclusions (2)

26

▪ On-line mass spectrometry was successfully utilized for analysis of

CO, CO2, H2 gases during capacitor operation in neutral medium

▪ Carbon corrosion was observed at higher voltage but without

oxygen evolution

▪ High current densities and fast scan rates are less harmful for

electrode degradation than soft regimes

Thank you for your attention !

The financial support from Polish-Swiss project INGEC

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Minglong He, Erik J. Berg, Petr Novak

Paul Scherrer Institute, Switzerland