Post on 19-Mar-2021
transcript
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!