HYBRID MATERIALS FOR ENERGY STORAGE

Marilena RE, M. Federica De Riccardis, Daniela Carbone ENEA- SSPT PROMAS MATAS

NANOITALY 2015

Rome, September 21-24, 2015

Conducting Polymers (CPs)

Polymers → Molecules that form long chains, repeating themselves

Usually an organic polymer, either semiconductor or insulant, has a low conductivity (10-10–10-5 S/cm). In order to become electrically conductive, it should have delocalized and only partially filled molecular orbitals for a free movement of electrons :

1- CPs consist of alternating single and double bonds, called conjugated double bonds [The bonds between the carbon atoms are alternately single and double. Every double bond contains a localized σ bond, a strong chemical bond, and also a weaker π bond delocalized along the backbone of

the polymer] 2- The polymer needs to be “doped” by removing electrons from (oxidation, p-doping) or inserting them into (reduction, n-doping) the material

Conjugated polymers, when doped,

become conductors (1 – 104 S/cm)

Conducting Polymers (CPs)

Polypyrrole (Ppy) Polythiophene (PT) Polyfuran (PFFU) Polyaniline (PANI) Polyethylene dioxythiophene (PEDOT)

Applications

Transistors Light Emitting Diodes (LEDs) Lasers used in flat televisions Solar cells Corrosion Inhibitors Compact Capacitors …….

Only some of the CPs

Electropolymerisation

CPs can be produced through an electrochemical process which induces not only the polymerization of monomers but also polymer doping. In this way a conductive polymer coating can be deposited on an electrode.

Some advantages of this polymerization process respect to chemical process are: ― reaction at room temperature ― a careful control of the reaction rate and of

oxidation state

Why PANI? Among all CPS Polyaniline (PANI) has been one of the most studied for its characteristics:

easy synthesis electrochromic polymer

low cost good supercapacitive behaviour

environmental stability good anticorrosion properties

different domains of conductivity good electrical conductivity

Depending on the oxidation state and the

degree of protonation, PANI can be either

an insulator or a conductor with different

conductivity. Each oxidation state can exist

in the form of its base or its protonated

form (salt) by treatment of the base with an

acid. y= 1 → leucoemeraldine

y= 0.5 → emeraldine

y= 0 → pernigraniline

y 1-y

POLIANILINE

But its mechanical stability is not so much good

• M. F. De Riccardis, et al, Advances in Science and Technology Vol. 79 (2013), 81

• Martina, V., M.F. De Riccardis, et al., Journal of Nanoparticle Research Vol. 13 (2011),

:6035

HYBRID CONDUCTING NANOCOMPOSITES

HYBRID CONDUCTING NANOCOMPOSITES are produced by the combination of conducting polymers and nanoparticles

Applications: sensors, actuators, touch screens, electrode for supercapacitors, etc..

Conventional synthesis method: electrochemical synthesis starting with the aniline monomer

+

NEW MATERIAL WITH ENHANCED PROPERTIES

Here some experimental results are reported on different methods used to produce a film of PANI+CNT with interesting properties

Electrosynthesis techniques

ELECTROLYTIC DEPOSITION ELECTROPHORETIC DEPOSITION

To form advanced materials: functional and structural ceramic coatings, composite and porous materials, functionally graded materials, thin films, and nanostructured materials

Applications in several sectors: Nanotechnology

Energy

Electronic

Biomedical Optical Catalytic

Advantages: versatility to be used with different materials and combinations of materials, cost-effectiveness (simple equipments), high potential for scaling up to large product volumes and variety of product shapes.

DC power supply

ELD consists in applying an opportune potential to a working electrode immersed in an electrolyte solution. The electrodeposited film is obtained by electrode reaction from a salt solution.

+ _

EPD is achieved by (i) the migration of charged particles suspended in a liquid medium under the effect of an applied electric field, and (ii) the particles deposition on the opposite charged electrode.

ELECTROLYTIC DEPOSITION (ELD) ELECTROPHORETIC DEPOSITION (EPD)

Electrosynthesis techniques

Deposition methods su

bst

rate

CNT

aniline

PANI+CNT composite

film ELD

1° METHOD: ELD

PANI-CNTs films were electropolymerised by ELD starting with an Aniline solution containing CNTs

2° METHOD: EPD

sub

stra

te

CNT

polyaniline

PANI+CNT composite

film

EPD

PANI (in the polymerised form) and CNTs were co-deposited by EPD

1) CNTs were deposited by EPD, then 2) PANI was electropolymerised by ELD on CNTs

sub

stra

te

CNT CNT film

EPD

3° METHOD: EPD + ELD

ELD

PANI+CNT composite

film

aniline

sub

stra

te

CNT

aniline

PANI+CNT composite

film ELD

Deposition method-1: ELD

PANI-CNTs nanocomposites film is electrochemical co-deposited starting with an Aniline solution containing CNTs: it is formed during the electropolymerisation – deposition of the polymer just near the electrode surface

CNTs are functionalised by PANI in the solution through the formation of donor-acceptor complexes. In fact CNTs act as good electron acceptors, while PANI is a good electron donor .

Deposition method-1: ELD

The formation and deposition of PANI-CNTs films on carbon paper were obtained by means of 3 consecutive CV scans from 0.0 V to 1.2 V (scan rate of 20 mV/s).

0,0 0,2 0,4 0,6 0,8 1,0 1,2

-0,02

-0,01

0,00

0,01

0,02

0,03

0,04

(b)

Cu

rre

nt (A

)

Potential (V vs Ag/AgCl)

ELD PANI+0.3% v/v CNT

By comparing CV with and without CNTs in ANI, the curves for ELD PANI-CNTs exhibit a considerable increase of current intensities at the monomer oxidation region and a peak shift at less anodic potential, meaning that the presence of CNTs accelerates the growth rate of PANI. 0,0 0,2 0,4 0,6 0,8 1,0 1,2

-0,010

-0,005

0,000

0,005

0,010

0,015

0,020

0,025

(a)

Cu

rren

t (A

)

Potential (V vs Ag/AgCl)

ELD PANI

Electrochemical characterization

-0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2

-0,004

-0,003

-0,002

-0,001

0,000

0,001

0,002

0,003

0,004

C'

B'

A'

C

BA

i (A

/cm

2) V (V vs Ag/AgCl)

A typical CV in 0.5 M H2SO4 of PANI-CNTs

Peak A (0.25 V) represents the transition leucoemeraldine → protonated emeraldine

Peak C (0.8 V) corresponds to the oxidation emeraldine → pernigraniline

Peak B corresponds to the generation of radical cations.

Morphological characterization

The length of the fibrils in the composite films are longer than that of PANI fibrils, reminding a shape similar to CNTs

PANI PANI+CNTs

SEM images

TEM images

PANI+CNT composite film

sub

stra

te

CNT

polyaniline

EPD

PANI in the polymerised form and CNTs were co-deposited by EPD

Deposition method-2: EPD

PANI and CNT are suspended together and then deposited on the electrode A fundamental step for EPD deposition is the production of the suitable and very stable suspension with particles well dispersed in the liquid medium

Preparation of EPD suspension

Liquid medium = CHCl3/EtOH DBSA (dodecylbenzensulphonic acid) donates H+ to PANI making it protonated, whereas DBSA remains negatively charged

m-cresol as a co-solvent straightens the PANI chains and is therefore responsible for increasing the mean molecular area per PANI unit

Possible interactions between PANI and CNTs that do not neutralize the total positive charge of PANI chains

+ DBSA

+ m - cresol

+ +

+

+

+

+

+

+

+

-

-

-

-

-

- -

+ +

+

+

+

+ +

+

+ CNT +

-

-

-

+

+

+

+ +

PANI

OH

O -

O

NH

p-stacking

hydrogen bonding

O -

O

NH +

electrostatic interaction

25

35

45

55

65

75

0 0,1 0,2 0,3 0,4

z (m

V)

CNT content (%v/v)

PANI is effectively positively charged. After adding CNTs to PANI suspension, the zeta potential increased in positive correlation with the CNTs content.

High zeta potential means a good stability of suspension and an efficient deposition process.

Deposition method-2: EPD

Electrochemical characterization

-1,0 -0,5 0,0 0,5 1,0 1,5

-0,004

-0,003

-0,002

-0,001

0,000

0,001

0,002

0,003

0,004

C'

B'

A'

C

BA

Cu

rren

t (A

)

Potential (V vs Ag/AgCl)

EPD PANI

EPD PANI+0.1% v/v CNT

EPD PANI+0.2% v/v CNT

EPD PANI+0.3% v/v CNT

A typical CV in 0.5 M H2SO4 of PANI-CNTs obtained by EPD with 3 different contents of CNTs

A typical CV in 0.5 M H2SO4 of PANI-CNTs obtained by ELD (0.3% CNTs)

Morphological characterization

PANI

CNT

10 nm

PANI

CNT

Deposition method-3: EPD+ELD

sub

stra

te

CNT CNT film

EPD

1st step

ELD

PANI+CNT composite

film

aniline

2nd step

1st step: CNTs were deposited by EPD

2nd step: PANI was electropolymerized by ELD on CNTs

Morphological characterization

The surface morphology is porous and with many nanofibrils

By looking in an internal part of the nanocomposite film different morphological features can be seen

Electrochemical characterization

CVs in 0.5 M H2SO4 obtained for the different nanocomposite systems: • 1st scan of PANI ELD

on CNTs (CV01) • 2nd scan of PANI ELD

on CNTs (CV02) • 3rd scan of PANI ELD

on CNTs (CV03) • PANI ELD (CV05) • CNT ELD (CV04)

Electrochemical characterisation

0,2 0,3 0,4 0,5 0,6 0,7 0,8

-0,04

-0,03

-0,02

-0,01

0,00

0,01

0,02

0,03

0,04

i (A

/cm

2)

V (V vs Ag/AgCl)

ELD PANI on EPD CNTs

ELD PANI+CNTs

ELD PANI

0,2 0,3 0,4 0,5 0,6 0,7 0,8

-0,006

-0,004

-0,002

0,000

0,002

0,004

0,006

0,008

I (A

/cm

2)

V (V vs Ag/AgCl)

EPD PANI

EPD PANI+CNTs

For comparison the voltammograms acquired on pure PANI (both ELD and EPD) are reported.

CVs areas [VA/cm2] measured on PANI and PANI-CNTs films obtained by different techniques

The presence of CNTs in the polymer did not substantially change the electrochemical properties of the polymer, but the closed area of the voltammograms, related to power density provided by the composite film, was significantly changed.

ELD EPD+ELD EPD

PANI 0.00852 0.00852 0.00198

PANI+CNTs 0.01509 0.01242 0.00242

Δ% 77.1 45.8 22.2

Conclusions

I. The hybrid conducting polymer nanocomposite CNT + PANI, obtained by three electrosynthesis methods, shows better performance of the electrochemical properties respect to pure PANI (an increased power density).

II. PANI-CNTs films co-electropolymerised by ELD are easily deposited, by one-step

process that allows to control the oxidation state of PANI. The process is reproducible but not applicable on large areas.

III. PANI-CNTs films co-deposited by EPD are easily deposited, by one-step process,

applicable to large areas, but a higher number of chemicals than in ELD are used.

IV. PANI films electropolymerised on CNTs previously deposited by EPD, need a two-step process, where both the used electrodeposition techniques, EPD of CNTs and ELD of PANI, are simple and easy to control.

Work in progress….

Measurements of specific conductivity of the PANI+CNTs nanocomposites obtained by ELD, by EPD and by the combined processes

More experiments in order to optimize process parameters…..

• Variation of process parameters…. • Evaluations of the electrochemical properties ……….. • Morphological characterizations……..

New and more complex structures of hybrid conducting polymer nanocomposite can be properly tailored for energy applications

Thank you very much!