Proton-conductive metal-organic frameworks: Recent advances and perspectives

States the review's dual emphasis on proton-transport characterisation methods and strategies for tuning conductivity in representative proton-conductive MOFs.

Relevance: Core · p001 · Abstract

Covers dielectric permittivity and loss as probes of dipole relaxation, guest-water dynamics and functional-group-dependent local motion in MOFs.

Relevance: Supporting · p002 · Broadband dielectric spectroscopy · Fig. 5; Fig. 6

Shows how acidic/hydrophilic functional groups, acid impregnation and direct proton introduction can raise conductivity but require hydrolytic stability.

Relevance: Core · p006 · Bronsted acidity and functionalized group · Table 1

Summarises DFT, AIMD and empirical force-field simulations as complementary approaches to assign proton topology and simulate confined proton/water dynamics.

Relevance: Core · p003 · Theoretical/computational methodologies · Fig. 10-Fig. 15

Synthesises the review into a design theme: modulate hydrogen-bond networks while solving stability, powder-measurement, interface and force-field-transferability challenges.

Relevance: Core · p008 · Conclusions and perspectives

Explains EIS measurement, equivalent-circuit interpretation, humid versus anhydrous operating regimes, blocking electrodes, anisotropic single-crystal measurements and Arrhenius activation-energy interpretation.

Relevance: Core · p002 · Electrochemical impedance spectroscopy · Fig. 1; Fig. 2; Fig. 3; Fig. 4

Reviews topology, metal identity, guest molecules, counterions and sulfonate/phosphonate groups as ways to control hydrogen-bond pathways and proton-carrier availability.

Relevance: Core · p005 · Framework and guest molecules/ions · Fig. 16-Fig. 25; Table 1

Introduces MOFs as tailorable proton conductors, frames hydrogen bonding and guest media as central design elements, and flags measurement challenges.

Relevance: Core · p001 · Introduction

Covers SC-SC transformations, pressure responses, ligand defects, missing-linker defects and order-disorder/glass transitions as routes to alter hydrogen-bond topology and proton mobility.

Relevance: Core · p007 · Structural control via phase transition, defects, and amorphization · Fig. 26-Fig. 29; Table 1

Positions QENS as a hydrogen-sensitive probe for proton and water dynamics, including rotating water, ammonium motion and confined free-diffusion mechanisms.

Relevance: Core · p003 · Quasi-elastic neutron scattering · Fig. 7; Fig. 8

Describes solid-state NMR uses for locating hydrogen-bond interactions, protonated species dynamics and guest-framework contacts where XRD cannot localise H atoms reliably.

Relevance: Core · p003 · Solid-state nuclear magnetic resonance · Fig. 9

Activation-energy heuristic for proton mechanism

The authors present an activation-energy threshold as a common interpretive rule for MOF proton conduction.

Categories: Ea below 0.4 eV attributed to Grotthuss mechanism · Ea above 0.4 eV attributed to vehicular mechanism

p002 · Electrochemical impedance spectroscopy

Computational method hierarchy

The review distinguishes electronic-structure, first-principles dynamics and lower-cost force-field approaches for proton-transport questions.

Categories: DFT · AIMD · empirical force-field molecular simulation · modified force fields such as MS-EVB

p003 · Theoretical/computational methodologies

Blocking and non-blocking EIS interfaces

The review treats interface character as central to impedance interpretation, especially the presence or absence of low-frequency tails.

Categories: blocking interface · non-blocking interface

p002 · Electrochemical impedance spectroscopy · Fig. 1

Transport-characterisation toolkit

The review maps different techniques to conductivity measurement, anisotropy, dynamic processes and microscopic pathway elucidation.

Categories: EIS · BDS · QENS · solid-state NMR · diffraction · computational simulation

p001 · Introduction

Humid versus anhydrous proton conduction

EIS conditions are separated by PEMFC-relevant hydrous and intermediate-temperature anhydrous regimes.

Categories: humid water-assisted conduction at 293-353 K · anhydrous conduction at 373-523 K through heteroaromatics or oxoacids

p002 · Electrochemical impedance spectroscopy

Two-part review scope

The authors explicitly delimit the review to method/tool coverage and structure-property strategies for tuning MOF proton conductivity.

Categories: characterisation and computational methods · factors and strategies for tuning conductivity

p001 · Introduction

Structural transformation controls

The review groups transformation and defect phenomena by how they change hydrogen-bond topology, pore space or mobile carrier populations.

Categories: single-crystal to single-crystal phase transition · pressure response · ligand defects and vacancies · order-to-disorder transformation · crystal-to-glass transition

p007 · Structural control via phase transition, defects, and amorphization · Fig. 26-Fig. 29

Three tuning routes

The tuning section is explicitly divided into structural/guest, acidity/functionalisation and transformation/defect routes.

Categories: framework and guest molecules/ions · functional groups and Bronsted acidity · phase transition, defects and amorphisation

p005 · Tuning the proton conductivity of MOFs · Table 1

MOFs or coordination polymers using imidazole, triazole or imidazolium-like species as anhydrous proton media.

Conduction: Anhydrous conduction is associated with guest alignment, imidazolium mobility and order-disorder transformations.

Representative materials: triazole-loaded beta-PCMOF2 · [Zn(HPO4)(H2PO4)2](ImH2)2 · [ImH2][Cu(H2PO4)2Cl]H2O · Im@MIL-53(Al)

Nodes / linkers: Zn · Cu · Al · phosphate · terephthalate · azole guests

p001 · Introduction

Water-stable chiral bioMOFs with ordered water in large one-dimensional pores.

Conduction: Experimental measurements and AIMD support a Grotthuss-type process involving O-H bond cleavage/formation and water reorientation.

Representative materials: CaCu alamox bioMOF

Nodes / linkers: Ca · Cu · bis(amino acid)oxalamide

p005 · Framework and guest molecules/ions · Fig. 19

Coordination polymers where defects, mobile proton carriers or disorder generate enhanced anhydrous proton conduction.

Conduction: Defect sites or disorder provide mobile carriers and expanded hopping paths.

Representative materials: [Zn(H2PO4)2HTz2]n · [ImH2][Cu(H2PO4)2Cl]H2O · [Cd(H2PO4)2(TzH)2]

Nodes / linkers: Zn · Cu · Cd · phosphate · triazole

p007 · Structural control via phase transition, defects, and amorphization · Fig. 29; Table 1

Porous crystalline frameworks composed of inorganic metal centres and organic ligands that can host proton carriers or functional groups for solid-state proton conduction.

Conduction: Transport is mediated by hydrogen-bond networks, confined water or heteroaromatic/oxoacid proton media.

Representative materials: MOFs · porous coordination polymers

Nodes / linkers: inorganic metal centers · organic ligands

p001 · Introduction

MOFs where metal substitution and aqua/oxalate arrays control network topology and proton pathway dimensionality.

Conduction: Infinite channel networks and ordered aqua/oxalate hydrogen-bond arrays are linked to more efficient proton motion.

Representative materials: LaCr(ox)3.10H2O · LaCo(ox)3.10H2O · LaRu(ox)3.10H2O · [Eu2(CO3)(ox)2(H2O)2].4H2O

Nodes / linkers: La · Cr · Co · Ru · Eu · oxalate · carbonate

p005 · Framework and guest molecules/ions · Fig. 16; Fig. 18

3D extended metal catecholates or iron-rod MOFs where bound sulfate ligands and DMA guests line pores and support proton transport.

Conduction: DMA and sulfate ions are interpreted as contributors to high proton conductivity beyond simple water uptake.

Representative materials: Fe-CAT-5 · Ti-CAT-5 · VNU-15

Nodes / linkers: Fe · Ti · triphenylene hexakis(olate) · BDC · NDC

p006 · Framework and guest molecules/ions · Fig. 24; Table 1

MOF single crystals and nanofilms configured to resolve directional or surface-mediated proton conduction.

Conduction: In-plane versus out-of-plane measurements and oriented films identify preferential paths and surface dangling-group contributions.

Representative materials: [Zn(H2PO4)2(TzH)2]n · Cu-TCPP nanosheet film

Nodes / linkers: Zn · Cu · TzH · TCPP

p002 · Electrochemical impedance spectroscopy · Fig. 2; Fig. 3

MOFs containing sulfonate or phosphonate groups that provide acidic sites, hydrated channels or anhydrous guest-loading environments.

Conduction: Acidic oxygens, lattice water and guest triazole/methanol can create connected proton pathways.

Representative materials: beta-PCMOF2 · PCMOF2(1/2) · PCMOF3 · PCMOF5 · PCMOF10

Nodes / linkers: Na · Zn · La · Mg · benzenetrisulfonate · benzenetriphosphonate · tetrakisphosphonomethylbenzene · dicarboxy-benzenediphosphonic acid

p005 · Framework and guest molecules/ions · Fig. 21; Fig. 22; Table 1

3D polyoxometalate open frameworks with large helical channels and guest solvent molecules.

Conduction: Humidity and temperature alter proton conductivity, and low activation energy is interpreted as Grotthuss-like.

Representative materials: POM-OF

Nodes / linkers: Zn · Mo · P · polyoxometalate clusters

p006 · Framework and guest molecules/ions · Fig. 25; Table 1

Zr6-node frameworks used to study proton topology, functional-group acidity, water networks and missing-linker defects.

Conduction: Functional groups and defects alter water hydrogen bonding, proton topology and carrier mobility.

Representative materials: UiO-66-SO3H · UiO-66-2COOH · UiO-66-(CO2H)2 · NU-1000 · defective UiO-66

Nodes / linkers: Zr6 clusters · BDC derivatives · sulfonated BDC · carboxylated BDC

p006 · Bronsted acidity and functionalized group · Table 1

Introduce defects or controlled disorder

Create ligand-substitution defects, vacancies, mobile-carrier defects, order-disorder transitions or glassy states to increase pore volume and carrier mobility.

Claimed effects: Defects can increase charge-carrier concentration and mobility, but can also trap protons at strong sites.

Controlling variables: defect concentration · missing-linker chemistry · pore volume · secondary acid treatment · degree of disorder

Representative materials: defective UiO-66 · sulfonated defective UiO-66 · [Zn(H2PO4)2HTz2]n · [ImH2][Cu(H2PO4)2Cl]H2O

Caveat: The review explicitly notes cases where conductivity remains lower than other MOFs because protons are trapped at defect clusters.

p007 · Structural control via phase transition, defects, and amorphization · Fig. 28; Fig. 29

Introduce external Bronsted acids or protons

Impregnate acid-stable MOFs with strong acids or protonate channels by acid treatment to raise mobile proton concentration.

Claimed effects: Acid loading can produce very high proton conductivity, but the review emphasises hydrolytic stability and possible free-ion contributions.

Controlling variables: acid identity · pH · loading reversibility · water stability · leaching

Representative materials: H2SO4@MIL-101 · H3PO4@MIL-101 · H+@Ni2(dobdc)

Caveat: Swelling, degradation, carrier loss and contributions from free ions must be assessed.

p006 · Bronsted acidity and functionalized group · Table 1

Modify ligand functional groups

Introduce acidic and hydrophilic ligand substituents such as SO3H and COOH to raise proton activity and water hydrogen-bond density.

Claimed effects: Functional groups tune conductivity and activation energy through cooperative acidity and hydrophilicity.

Controlling variables: functional-group pKa · hydrophilicity · hydrogen-bond donor/acceptor geometry · framework robustness

Representative materials: UiO-66-SO3H · UiO-66-2COOH · MIL-53(Fe)-(COOH)2

Caveat: High acidity is not sufficient without water stability and retained framework integrity.

p006 · Bronsted acidity and functionalized group · Table 1

Introduce guest molecules or ions as proton media

Place water, azoles, oxoacids, DMA, sulfate or other guests in pores so they form hydrogen-bonded pathways or mobile proton-carrier networks.

Claimed effects: Can enable hydrous or anhydrous conduction, but proton-carrier loss and solvent leakage are practical concerns.

Controlling variables: guest identity · guest loading · pore confinement · humidity · guest-framework interaction

Representative materials: beta-PCMOF2 with triazole · Fe-CAT-5 · VNU-15 · POM-OF

Caveat: The review cautions that water concentration alone does not determine conductivity; guest and counterion identity also matter.

p006 · Framework and guest molecules/ions · Fig. 24; Fig. 25

Isomorphous ligand replacement

Partially replace sulfonate ligands with structurally compatible phosphonate ligands to create mixed-linker proton pathways.

Claimed effects: The review reports a large conductivity increase for PCMOF2(1/2) versus beta-PCMOF2 under the same humid condition.

Controlling variables: replacement fraction · ligand compatibility · hydration state

Representative materials: PCMOF2(1/2) · beta-PCMOF2

Caveat: The review does not generalise this strategy beyond the PCMOF example set.

p005 · Framework and guest molecules/ions · Table 1

Change metal nodes or counterions to alter topology

Use different metals or counterions to change framework dimensionality, channel connectivity and lattice-water organisation.

Claimed effects: Network topology and metal identity can shift conductivity by orders of magnitude by changing whether continuous hydrogen-bond pathways exist.

Controlling variables: metal cation · counterion · framework topology · lattice water content

Representative materials: LaCr/LaCo versus LaRu/LaLa oxalate frameworks · Li/Na/K/Cs-HPAA · Co/Mn anionic BTC frameworks

Caveat: The review frames these as structure-property correlations, not proof that metal identity alone controls transport.

p005 · Framework and guest molecules/ions · Fig. 16; Fig. 17

Use single crystals and oriented nanofilms for pathway-resolved measurements

Prepare electrode-contacted crystals or oriented nanosheet films to separate anisotropic and surface-mediated proton pathways from powder averages.

Claimed effects: Improves interpretability of preferential transport paths and grain-boundary effects relative to powder pellets.

Controlling variables: crystal orientation · electrode geometry · film orientation · surface dangling groups

Representative materials: [Zn(H2PO4)2(TzH)2]n · Cu-TCPP nanofilm

Caveat: Single crystals are sometimes difficult to obtain, and most reported tests remain powder-based.

p002 · Electrochemical impedance spectroscopy · Fig. 2; Fig. 3

Exploit phase transitions and SC-SC transformations

Use humidity, temperature or pressure to reversibly reorganise hydrogen-bond networks and release protons into conductive pathways.

Claimed effects: SC-SC transformations can shift conductivity by one to five orders of magnitude by connecting or interrupting hydrogen-bond networks.

Controlling variables: relative humidity · temperature · pressure · lattice water · hydrogen-bond topology

Representative materials: CoLa-II/CoLa-III · CoCa.4H2O/CoCa.2H2O · ZAG family

Caveat: Transformation pathways and stability under cycling need case-specific verification.

p007 · Structural control via phase transition, defects, and amorphization · Fig. 26; Fig. 27

Use sulfonate and phosphonate linkers

Build frameworks from acidic sulfonate or phosphonate groups to provide proton sources, hydrated channels and robust hydrogen-bond arrays.

Claimed effects: Phosphonate and sulfonate motifs can combine high conductivity with stability and controllable hydrous/anhydrous pathways.

Controlling variables: acidic group type · free versus coordinated acid groups · water site connectivity · framework stability

Representative materials: PCMOF5 · PCMOF10 · PCMOF3 · beta-PCMOF2

Caveat: Conductivity can be negligible at low humidity and depends on water-site continuity.

p005 · Framework and guest molecules/ions · Fig. 21; Fig. 22

Conductivity changes after phase transition, defects or amorphisation require identification of the changing hydrogen-bond network.

Proposed direction: Track structural transformations in situ with spectroscopy, diffraction and dynamics simulations.

p001 · Introduction

The growing number of proton-conductive MOFs calls for force fields with higher accuracy and transferability.

Proposed direction: Develop transferable reactive or proton-hopping-capable force fields to compare simulations and experiments more efficiently.

p008 · Conclusions and perspectives

For MOF/polymer mixed membranes, the review highlights the unresolved question of how protons migrate at MOF-polymer interfaces.

Proposed direction: Develop interfacial models and experiments that distinguish framework, polymer and interfacial proton pathways.

p008 · Conclusions and perspectives

Explicit proton-transport pathways are difficult to identify because proton-carrier orientations and pore solvent molecules can be highly disordered.

Proposed direction: Combine crystallography, spectroscopy, single-crystal measurements and simulation to resolve hydrogen-bond topology during operation.

p001 · Introduction

Electroosmosis, solvent leakage, carrier loss, swelling and framework degradation can undermine high short-term conductivity values.

Proposed direction: Assess conductivity together with long-term chemical, thermal and hydrolytic stability under realistic PEMFC conditions.

p008 · Conclusions and perspectives

Most experimental tests are performed on powders, while grain size dispersion, orientation and boundary effects are less studied.

Proposed direction: Increase single-crystal, oriented-film and grain-aware impedance studies before translating powder values to PEMFC claims.

p008 · Conclusions and perspectives