perforations, or pinhole blisters (of the CEM) will also lead to
membrane degradation (Taniguchi et al. 2004). When perforations or pinholes occur in the membranes, the reactant gases
cross and react on the surface of the catalyst. The heat of this
reaction may cause the membrane to soften or even melt,
thereby increasing gas crossover and destructive cycling,
reducing fuel efficiency and thermodynamic efficiency
(Laconti et al. 2003; Xie et al. 2005).
4.2.3 Changes of pH and Temperature Caused
by Overlimiting Current
The generation of overlimiting current can also cause
changes in pH and temperature in the chamber, possibly
resulting into aging and degradation of the ion-exchange
membranes.
Under the overlimiting current (Region III in Fig. 4, the
water splitting often occurs at the membrane interface (Choi
and Moon 2003), and such a phenomenon is explained by
the proposed mechanism as catalytic theory and electric field
theory (Simons 1984). The former mechanism suggests that
OH
− and H
+ ions may be produced in charge transfer
reactions between charged groups and water, occurring
around a thin layer on the surface of the IEM due to the
reversible protonation of weakly basic groups such as tertiary amines (Simons 1984). The electric field theory (Li
et al. 1998) suggests that K d (dissociation constant) can be
increased as much as 10
7 times because of a Second Wien
effect due to the strong electric field (10
8 V m
−1 ) at the
membrane–solution interface under the deep concentration
polarization.
Rubinstein et.al. published a series of papers (Rubinstein
1984; Rubinstein and Shtilman 1979; Rubinstein and
Maletzki 1991; Rubinstein and Segel 1979; Rubinstein et al.
1988) in which the occurrence of the overlimiting current
region was explained by another mechanism called electroconvection. Near the limiting current density the salt concentration at the membrane surface is very low and the basic
assumption of electroneutrality does not hold anymore. The
electric double layer would be drastically distorted and a
weak space charge is build up near the membrane surface.
Due to a nonuniform ion conductance through the membrane, the electric field is not uniform. The interaction of
space charges with the electric field gives rise to a spatially
inhomogeneous bulk force that is bound to set the fluid in
the depletion diffusion layer in motion, which is called
electroconvection. It was thought that the significant
Table 6 Possible membrane degradation mechanisms under extreme pH
Degradation mechanism
Reaction mechanism
References
Rearrangement Sommelet-Hauser
rearrangement
Ghigo et al.
(2010)
Stevens
rearrangement
Ghigo et al.
(2010)
Elimination
Hofmann
elimination
Merle et al.
(2011),
Norcross
(1993)
E1 elimination
Cope and
Mehta
(1963)
Nucleophilic
substitution
SN2
Merle et al.
(2011)
Reproduced with permission
34
L. Han
membrane degradation (Taniguchi et al. 2004). When perforations or pinholes occur in the membranes, the reactant gases
cross and react on the surface of the catalyst. The heat of this
reaction may cause the membrane to soften or even melt,
thereby increasing gas crossover and destructive cycling,
reducing fuel efficiency and thermodynamic efficiency
(Laconti et al. 2003; Xie et al. 2005).
4.2.3 Changes of pH and Temperature Caused
by Overlimiting Current
The generation of overlimiting current can also cause
changes in pH and temperature in the chamber, possibly
resulting into aging and degradation of the ion-exchange
membranes.
Under the overlimiting current (Region III in Fig. 4, the
water splitting often occurs at the membrane interface (Choi
and Moon 2003), and such a phenomenon is explained by
the proposed mechanism as catalytic theory and electric field
theory (Simons 1984). The former mechanism suggests that
OH
− and H
+ ions may be produced in charge transfer
reactions between charged groups and water, occurring
around a thin layer on the surface of the IEM due to the
reversible protonation of weakly basic groups such as tertiary amines (Simons 1984). The electric field theory (Li
et al. 1998) suggests that K d (dissociation constant) can be
increased as much as 10
7 times because of a Second Wien
effect due to the strong electric field (10
8 V m
−1 ) at the
membrane–solution interface under the deep concentration
polarization.
Rubinstein et.al. published a series of papers (Rubinstein
1984; Rubinstein and Shtilman 1979; Rubinstein and
Maletzki 1991; Rubinstein and Segel 1979; Rubinstein et al.
1988) in which the occurrence of the overlimiting current
region was explained by another mechanism called electroconvection. Near the limiting current density the salt concentration at the membrane surface is very low and the basic
assumption of electroneutrality does not hold anymore. The
electric double layer would be drastically distorted and a
weak space charge is build up near the membrane surface.
Due to a nonuniform ion conductance through the membrane, the electric field is not uniform. The interaction of
space charges with the electric field gives rise to a spatially
inhomogeneous bulk force that is bound to set the fluid in
the depletion diffusion layer in motion, which is called
electroconvection. It was thought that the significant
Table 6 Possible membrane degradation mechanisms under extreme pH
Degradation mechanism
Reaction mechanism
References
Rearrangement Sommelet-Hauser
rearrangement
Ghigo et al.
(2010)
Stevens
rearrangement
Ghigo et al.
(2010)
Elimination
Hofmann
elimination
Merle et al.
(2011),
Norcross
(1993)
E1 elimination
Cope and
Mehta
(1963)
Nucleophilic
substitution
SN2
Merle et al.
(2011)
Reproduced with permission
34
L. Han