106
of methanol’s poor proton conductivity and was reasoned to have origins in the
interaction between methanol and the framework phosphate groups [5]. Structural
defects were utilized in a nonporous coordination polymer to incorporate mobile,
uncoordinated H 3 PO 4 , H 2 PO 4
−
, and H 2 O. EDX element mappings for Zn and P show
homogeneous distributions of the elements, confirming the homogeneous distribution of H 3 PO 4 throughout the defects. Embedding the phosphoric acid significantly
lowers activation energy, as shown in Fig. 5 (right) [73]. Cheetham et al. found that
a dense MOF will undergo an insulator-to-proton conductor transition upon hydration. Exposure to water results in a huge increase in conductivity at 17 °C to
3.9 × 10
−5
S cm
−1
. The large associated activation energy (0.64 eV), as well as the
small humidity dependence in the RH range 67–33%, suggests that protons do not
transport through free water molecules adsorbed on the surface but through the
framework and/or micropores. Furthermore, as the conductivity depends on humidity, the mobile ions are judged to be protons, not other ions such as Li
+
[77].
There is some concern as to the role of the outer vs. inner surface of MOF crystals and the influence of grain boundaries between micro and nanosized crystals on
the conductivity. Control of MOF morphology is an ongoing theme for many applications as well as increased understanding of the role of defects in MOF properties
including conduction. To explore this, Cu MOF-based thin films were made. This
format reduced the overall resistance of the electrolyte membrane and increased the
efficiency of the device. Proton conductivities of the MOF nanofilm at RH 95%
were measured, and the activation energy of proton conductivity was calculated to
be 0.28 eV. The highest recorded conductivity value for the nanofilm was
3.9 × 10
−3
S cm
−1
at 98% RH [45]. This demonstrated that the external surface of the
crystals plays a measurable role. Shimizu also broached this topic. Synthesis of a
series of Lanthanide MOFs, [Ln(H 5 L)(H 2 O)n](H 2 O) (L = 1,2,4,5-tetrakis(phosphon
omethyl)benzene, Ln = La, Ce, Pr, Nd, Sm, Eu, Gd) resulted in MOFs with variable
crystal sizes. The La and Pr complexes, which yield the biggest crystals within this
series, are the best conductors of the group. This result contradicts previous concerns over degradation of MOFs at grain boundaries, which may result in small
particles that enable proton conduction via extrinsic pathways. If this were the case,
the ball milled samples would in fact exhibit the highest conductivity values [59].
Controlled hydrophilicity is also important as proton conductivity increases with
hydrophilicity, but it is more difficult to retain high humidities during operation of
a device. Authors report that a hydrophilic MOF, {NMe 3 (CH 2 COOH)}
[FeCr(oxalate) 3 ]·nH2O, strongly absorbed a large number of water molecules to
result in high proton conductivities of ~10
−4
S cm
−1
, even at a low RH of 65% [54].
Also, an anionic layered structure based on sulfonated indium, incorporating
hydrogen- bonded dimethylammonium cations and water molecules, was shown to
exhibit conductivity over 10
−3
S cm
−1
at 25 °C and 40% RH, a very high proton
conduction value for low humidity and moderate temperature along two axes of the
crystals [56]. A Eu MOF exhibits temperature-dependent but humidity independent
conductivity that is highest at 150 °C when associated water molecules are activated, and drops after loss of water. However, it can be rehydrated and reused [52].
C. A. Bauer
of methanol’s poor proton conductivity and was reasoned to have origins in the
interaction between methanol and the framework phosphate groups [5]. Structural
defects were utilized in a nonporous coordination polymer to incorporate mobile,
uncoordinated H 3 PO 4 , H 2 PO 4
−
, and H 2 O. EDX element mappings for Zn and P show
homogeneous distributions of the elements, confirming the homogeneous distribution of H 3 PO 4 throughout the defects. Embedding the phosphoric acid significantly
lowers activation energy, as shown in Fig. 5 (right) [73]. Cheetham et al. found that
a dense MOF will undergo an insulator-to-proton conductor transition upon hydration. Exposure to water results in a huge increase in conductivity at 17 °C to
3.9 × 10
−5
S cm
−1
. The large associated activation energy (0.64 eV), as well as the
small humidity dependence in the RH range 67–33%, suggests that protons do not
transport through free water molecules adsorbed on the surface but through the
framework and/or micropores. Furthermore, as the conductivity depends on humidity, the mobile ions are judged to be protons, not other ions such as Li
+
[77].
There is some concern as to the role of the outer vs. inner surface of MOF crystals and the influence of grain boundaries between micro and nanosized crystals on
the conductivity. Control of MOF morphology is an ongoing theme for many applications as well as increased understanding of the role of defects in MOF properties
including conduction. To explore this, Cu MOF-based thin films were made. This
format reduced the overall resistance of the electrolyte membrane and increased the
efficiency of the device. Proton conductivities of the MOF nanofilm at RH 95%
were measured, and the activation energy of proton conductivity was calculated to
be 0.28 eV. The highest recorded conductivity value for the nanofilm was
3.9 × 10
−3
S cm
−1
at 98% RH [45]. This demonstrated that the external surface of the
crystals plays a measurable role. Shimizu also broached this topic. Synthesis of a
series of Lanthanide MOFs, [Ln(H 5 L)(H 2 O)n](H 2 O) (L = 1,2,4,5-tetrakis(phosphon
omethyl)benzene, Ln = La, Ce, Pr, Nd, Sm, Eu, Gd) resulted in MOFs with variable
crystal sizes. The La and Pr complexes, which yield the biggest crystals within this
series, are the best conductors of the group. This result contradicts previous concerns over degradation of MOFs at grain boundaries, which may result in small
particles that enable proton conduction via extrinsic pathways. If this were the case,
the ball milled samples would in fact exhibit the highest conductivity values [59].
Controlled hydrophilicity is also important as proton conductivity increases with
hydrophilicity, but it is more difficult to retain high humidities during operation of
a device. Authors report that a hydrophilic MOF, {NMe 3 (CH 2 COOH)}
[FeCr(oxalate) 3 ]·nH2O, strongly absorbed a large number of water molecules to
result in high proton conductivities of ~10
−4
S cm
−1
, even at a low RH of 65% [54].
Also, an anionic layered structure based on sulfonated indium, incorporating
hydrogen- bonded dimethylammonium cations and water molecules, was shown to
exhibit conductivity over 10
−3
S cm
−1
at 25 °C and 40% RH, a very high proton
conduction value for low humidity and moderate temperature along two axes of the
crystals [56]. A Eu MOF exhibits temperature-dependent but humidity independent
conductivity that is highest at 150 °C when associated water molecules are activated, and drops after loss of water. However, it can be rehydrated and reused [52].
C. A. Bauer