PART 3

Stability of AEMs

Quaternary ammonium group degradation mechanisms

OH- attack to the ammonium group

Degradation Reduced IECTwo main degradation pathways:

Nucleophilic attack (Nucleophilic substitution)

B-elimination

OH- ion attack

trigonal bipyramidal TS

amine and alcohol as products

Presence of H atom in B position

Antiperiplanar conformation of TS

Alkene, amine and wateras products

Quaternary ammonium group degradation mechanisms

Adv. Sci. 2018, 5, 180065

stability studies on cationic polymers obtained from quaternatization of VBC (vinyl benzyl chloride) with different tertiary amines followed by a free radical polimerization

VBC

High pH environment makes it inevitable that QAs will be degraded over time

The synthetized compounds were submitted to a stability standard test using:

KOH 2M at 80°C for 96 hours.

N+

( )

N+

( )

N+

( )

[PVBMPL][Cl]

>

[PVBTMA][Cl]

>

N

>

[PMVBIm][Cl][PDMVBI][Cl]

N+

( )

N

>

[PVBPy][Cl]

( )

>

[PVBTMP][Cl]

( )

P+

Cl- Cl- Cl- Cl-

N+Cl- Cl-

Alkaline Stability order of QAs

Among the cationic polymers studied in this work, the [PVBMPL]+ showed the best alkaline stability.

Stability order[PVBMPL]+ > [PVBTMA]+ > [PDMVBIm]+ > [PMVBIm]+ > [PVBPy]+ > [PVBTMP]+

Polymeric backbone stability

Macromolecules 2016, 49, 9, 3361-3372

PSU (polysulfone)

PSU

SEBS (polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene)

PPO (poly(phenylene oxide) PB (trifluoromethyl-containing polybiphenylalkylene)

O

n

CF3

n

S

O

O

O O

n

x y z

PSU

PB

SEBS

PPO

Polymeric backbone stability

Macromolecules 2016, 49, 9, 3361-3372

PSU (polysulfone)

PSU

S

O

O

O O

n

PSUPSU showed the poorest chemical stability because the electron

withdrawing substituents (sulfone) present accelerated the degradation

in alkaline media

Polymeric backbone stability

Macromolecules 2016, 49, 9, 3361-3372

PSU (polysulfone)

PSU

S

O

O

O O

n

PSUPSU showed the poorest chemical stability because the electron

withdrawing substituents (sulfone) present accelerated the degradation

in alkaline media

Aryl ether bond cleavage takes place

Polymeric backbone stability

Macromolecules 2016, 49, 9, 3361-3372

SEBS (polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene)

x y z

SEBSCF3

n

PB

SEBS and PB remained stable

PB (trifluoromethyl-containing polybiphenylalkylene)

Polymeric backbone stability

Macromolecules 2016, 49, 9, 3361-3372

PPO (poly(phenylene oxide)

O

nPPO

PPO resulted to be stable as starting polymer but not when transformed

in AEM. Aryl ether bonds were cleaved in alkaline media because of

the presence of benzyl trimethylammonium cations

PART 3

Characterization of AEMs

IP and TP conductivity

• These property can be determined for ion transport within the plane of material (in-plane direction, IP) or through the thickness of the membrane (through-plane orientation, TP).

IP and TP conductivity

0

500

1000

1500

2000

2500

3000

3500

4000

500 2000 3500 5000Z' (W)

-Z'' (

W)

0

1

2

3

4

5

6

Lo

g f

(H

z)

—— -Z''(W)

▪ ▪ ▪ ▪ Log f (Hz)

(a)

0

100

200

300

400

500

600

700

800

0 100 200

Z' (mW)

-Z'' (

mW

)

0

1

2

3

4

5

6

Lo

g f

(H

z)

—— -Z''(mW)

▪ ▪ ▪ ▪ Log f (Hz)

(b)

a) 4-probe impedance spectra for an in-plane conductivity measurement, in flowing N2 degased deionized liquid water (r≥10MW·cm), of a cast AEM piece having width 1.15 cm and wet thickness 120 µm. Frequency range 100 kHz – 100 Hz, voltage amplitude 10 mV.

b) Impedance spectra for a through-plane conductivity measurement in flowing N2-degased deionized liquid water (r≥10MW·cm), of a cast AEM square having area cm2 and wet thickness 40 µm. Frequency range 100 kHz – 100 Hz, voltage amplitude 5 mV.

IEC

ACID-BASE BACKTITRATION

Sample pre-treated in 1M KOH, 50°C, 1h

Soaked in determinated V of standardized

HCl, 30 min

Excess of HCl was backtitrated with standardized KOH, potentiometric or conductimetric end-point detection.

WU

AEM pretreated in 1M KOH, 50 °C, 1h

After 30 min equilibration wet values were measured

Dry weight of the AEM piece determined after 24h equilibration, 50°C

In the electrolyzer water is reduced with OH- formation and H2 evolution in the cathode, while OH- is

oxidized to water with O2 production in the anode.

H2 Crossover

Voltammetry (Fig.1) is recorded at 60 mV/s between -0.05V and 0.8V until stability.

it is possible to estimate the hydrogen crossover under realistic cell operating conditions

The Anodic gas flow coming from a test cell, is injectedin a gascromatograph and the H2 vol.% measured.

GCAnodic gas blend gaschromatogram

Electrochemical methods (CV)

.

the anode is supplied with humidified nitrogen, while the cathode is feed with humidified hydrogen

Cathodic backpressure is controlled by a needle valve

The value of H2 vol.% must be less than 4% for safetyreasons.

Morphological Studies: Microscopy Tecniques

The surfaces of the membranes can be observed directly while the cross-sections are obtained by a simple cold (liquid N2) fracture.

Fig. 2 Bright field TEM images of acidic Nafion films (a); section magnified 4× (b) [Membranes 2013, 3(4), 424-439]

Fig. 1 SEM images of: cross-section of Nafion 117 (A) and top surface of Nafion 117 (D) [Desalination and water treatment 57(58):1-9 (2016)]

The aim is to acquire visual information of the membrane structure and porosity through magnification by scanning electron microscope (SEM) (Fig.1 )or a transmission electron microscope (TEM) (Fig.2).

Mechanical characterizations

Uniaxial tensile testing is one of the most used testing

methods to evaluate the mechanical behaviors of materials

and of polymeric membranes.

The test process involves placing the test specimen in the

testing machine and slowly extending it until it fractures.

During this process, the elongation (Δl) of the gauge

section is recorded against the applied force.

engineering strain ε:

ε = Δl/l0

The force measurement is used to calculate the engineering

stress σ, using the following equation:[ù

σ = F/A

where F is the tensile force and A is the nominal cross-

section of the specimen.

Mechanical characterizations

Davis, Joseph R. (2004). Tensile tesing (2nd ed.). ASM International.

• The machine does these calculations as the force increases, so that the data points can be graphed into a stress-strain curve.

• In addition, the area under the stress-strain curve determines the material toughness, which relates to the material energy absorption capacity.

Thank you

all for your attention!