89
conductivity by the framework itself, whereas extrinsic conductivity is due to incorporated guests or post-synthetic modifications of the framework [5].
Ionic conductivity (σ i ) in solids is measured in terms of Ohm
−1
cm
−1
or S cm
−1
(Note that S stands for Siemens, which is the inverse of resistance, having the unit
of Ohm
−1
). When σ i is on the order of 10
−4
to 10
−1
S cm
−1
at 300 K, a material is
classified as a superionic conductor, and when both σ i > 0.1 S cm
−1
at 300 K and the
activation energy for ion transport is small (about 0.1 eV), materials are classified as
advanced superionic conductors. The most famous example of an advanced superionic conductor-solid electrolyte is RbAg 4 I 5 where σ i > 0.25 S cm
−1
and the electrical conductivity, σ e , is very small ~10
−9
S cm
−1
at 300 K. Another term for a solid
electrolyte is a “fast ion conductor,” which is considered to be useful when the
conductivity is at or above 1 × 10
−4
S cm
−1
for lithium ions [6], and 1 × 10
−2
S cm
−1
for protons [7].
The bulk ionic conductivity of a material reflects transport of both anions and
cations as the sum of all mobile ion conductivities. Often, for practical devices the
effective conductivity of a specific ion in the material is of paramount interest. This
is defined as the transference number for the specific ion, formulated as the fraction
of the ionic current carried by this ion divided by the total ion current. The closer
this transference number is to unity, the more specificity for movement of that particular ion in the material (approaching 1 for single-ion conducting materials). As
protons are usually able to move quicker than other species, transference numbers
are not typically reported for proton conduction and are assumed to be very close to
1. For other ions, however, they can be very important, in particular for electrolytes
Fig. 2 The vehicle mechanism is described by the movement of an ion (blue) that is transported
with another carrier (the vehicle) and requires higher activation energy (0.50–0.90 eV) than the
Grotthuss mechanism (0.10–0.40 eV), whereby the ion (specifically a proton) hops from one stationary site to another. The counter diffusion of unprotonated vehicles (e.g. H 2 O) results in the net
transport of protons, so this type of conductivity is a function of the medium (Reprinted with permission from ref. [4])
Polymer Nanocomposites for Ion Transport
conductivity by the framework itself, whereas extrinsic conductivity is due to incorporated guests or post-synthetic modifications of the framework [5].
Ionic conductivity (σ i ) in solids is measured in terms of Ohm
−1
cm
−1
or S cm
−1
(Note that S stands for Siemens, which is the inverse of resistance, having the unit
of Ohm
−1
). When σ i is on the order of 10
−4
to 10
−1
S cm
−1
at 300 K, a material is
classified as a superionic conductor, and when both σ i > 0.1 S cm
−1
at 300 K and the
activation energy for ion transport is small (about 0.1 eV), materials are classified as
advanced superionic conductors. The most famous example of an advanced superionic conductor-solid electrolyte is RbAg 4 I 5 where σ i > 0.25 S cm
−1
and the electrical conductivity, σ e , is very small ~10
−9
S cm
−1
at 300 K. Another term for a solid
electrolyte is a “fast ion conductor,” which is considered to be useful when the
conductivity is at or above 1 × 10
−4
S cm
−1
for lithium ions [6], and 1 × 10
−2
S cm
−1
for protons [7].
The bulk ionic conductivity of a material reflects transport of both anions and
cations as the sum of all mobile ion conductivities. Often, for practical devices the
effective conductivity of a specific ion in the material is of paramount interest. This
is defined as the transference number for the specific ion, formulated as the fraction
of the ionic current carried by this ion divided by the total ion current. The closer
this transference number is to unity, the more specificity for movement of that particular ion in the material (approaching 1 for single-ion conducting materials). As
protons are usually able to move quicker than other species, transference numbers
are not typically reported for proton conduction and are assumed to be very close to
1. For other ions, however, they can be very important, in particular for electrolytes
Fig. 2 The vehicle mechanism is described by the movement of an ion (blue) that is transported
with another carrier (the vehicle) and requires higher activation energy (0.50–0.90 eV) than the
Grotthuss mechanism (0.10–0.40 eV), whereby the ion (specifically a proton) hops from one stationary site to another. The counter diffusion of unprotonated vehicles (e.g. H 2 O) results in the net
transport of protons, so this type of conductivity is a function of the medium (Reprinted with permission from ref. [4])
Polymer Nanocomposites for Ion Transport