Dae-Woon Lim and Hiroshi Kitagawa · Chemical Society Reviews · 2021
Review rational design strategies for highly proton-conductive MOFs, organised by framework components and illustrated with recent representative examples.
The review’s argument is preserved as a navigable set of section summaries.
Synthesises design requirements, benchmarks and remaining barriers including stability, membranes, grain boundaries, crossover and operating temperature.
Relevance: Core · 6364 · 4. Conclusions and perspectives · Table 1
Separates hydrated and anhydrous operation, using selected examples to connect water uptake, H-bond networks, phase transformation and proton mobility.
Relevance: Core · 6351 · 2. Humidity and proton conductivity in MOFs
Frames proton-conductive MOFs as solid-state proton conductors for electrochemical devices, introduces advantages, challenges, mechanisms and measurement caveats.
Relevance: Core · 6350 · Introduction · Fig. 1
Covers predesigned ligand functionalisation, mixed ligands, ligand defects and post-synthetic ligand functionalisation, especially acid groups and dangling/mobile proton species.
Relevance: Core · 6360 · 3.2 Strategies for manipulating the organic ligand · Fig. 15
Reviews metal-centre exchange, open-metal-site coordinative insertion and metal-centre defects as routes to stability, guest interaction and proton transport.
Relevance: Core · 6357 · 3.1 Strategies for manipulating the metal component · Fig. 10
Moves beyond simple guest loading to pore design, highlighting hydrophilic and hydrophobic pore environments that regulate H-bonding, confinement and anisotropic transport.
Relevance: Core · 6362 · 3.3 Strategies for manipulating the pore space
Classification systems are attributed to this review and are not treated as a global material registry.
The review's organising taxonomy treats each MOF component as a design lever for mobile proton concentration and conducting pathways.
Categories: metal centres · organic linkers · pore space
6349 · Abstract · Fig. 1
The humidity section divides examples by whether water mediates H-bond pathways or high-boiling/nonvolatile media and structural transformations enable hotter anhydrous transport.
Categories: proton conductivity under hydration · proton conductivity under anhydrous conditions
6351 · 2. Humidity and proton conductivity in MOFs
The ligand section treats acid-group introduction, mixed ligands, ligand defects and post-synthetic conversion as distinct ways to build protic sites and guest interactions.
Categories: predesigned functionalisation · mixed ligand system · ligand missing or missing connection · post-synthetic functional group modification
6360 · 3.2 Strategies for manipulating the organic ligand · Fig. 15
Within the metal-component section, the review classifies routes by how the inorganic node or its vacant coordination sites are altered.
Categories: metal ion replacement · coordinative insertion of functional organic molecules in OMSs · metal-centre missing defect site
6357 · 3.1 Strategies for manipulating the metal component · Fig. 10
Strategies are framed as either built into the MOF synthesis or introduced after framework formation.
Categories: judicious predesign strategy · post-synthetic modification
6349 · Abstract
The review uses the conventional mechanism distinction but warns that activation energy alone can be ambiguous in cooperative MOF environments.
Categories: Grotthuss hopping through H-bond reorientation · vehicle migration of protonic species
6351 · Introduction
Review-defined families retain their representative materials and conduction descriptions.
MOFs whose linkers carry carboxyl, hydroxyl, sulfonic or phosphonate functions to tune acidity and water interactions.
Conduction: Uncoordinated or partially coordinated acid groups provide proton donors and H-bonding sites, but their coordination state and framework stability matter.
Representative materials: MIL-53(Fe)-(COOH)2 · UiO-66(SO3H)2 · PCMOF10 · BUT-8(Cr)A
Nodes / linkers: Al · Fe · Zr · Mg · Cr · BDC derivatives · sulfonated BDC · phosphonate linkers
6360 · 3.2 Strategies for manipulating the organic ligand
Frameworks with missing metal centres or ligand defects that alter local acidity, trap protons or host mobile acid species.
Conduction: Defects can either buffer/trap protons or generate new hopping paths when mobile acid species occupy defect sites.
Representative materials: Zr6O4(OH)8L4.2.xH2O · defective [Zn(H2PO4)HTz2]n
Nodes / linkers: Zr clusters · Zn phosphate chains · sulfoterephthalate · triazole/phosphate
6359 · 3.1 Strategies for manipulating the metal component · Fig. 14
Frameworks bearing sulfonic acid decorated ligands whose flexible structural response preserves H-bond networks.
Conduction: Water-induced structural adaptation can maintain continuous H-bond networks and support low activation energies.
Representative materials: BUT-8(Cr)A · BUT-8(Cr) · MIL-101-SO3H
Nodes / linkers: Cr · Al · disulfonated naphthalenedicarboxylate
6353 · 2.1 Proton conductivity under hydration · Fig. 4
MOFs loaded with amphoteric, acidic or ionic guests such as imidazole, triazole, H3PO4 or H2SO4.
Conduction: Performance depends not only on carrier loading but also on guest mobility, host-guest interactions and continuity of H-bond networks.
Representative materials: Im@NENU · Im@Fe-MOF · H3PO4@MIL-101 · H2SO4@MIL-101-SO3H
Nodes / linkers: Cu polyoxometalate MOF · Fe MOF · Cr MIL-101 · btc · BDC · sulfonated linkers
6362 · 3.3 Strategies for manipulating the pore space
MOFs where removal or replacement of coordinated solvent at open metal sites introduces protic or amphoteric molecules.
Conduction: Coordinated guests alter acidity, H-bond formation, pore size and proton-carrier mobility.
Representative materials: HKUST-1-H2O · Cr-MIL-88B-PSA · MOF-74(Mg)-urea · Im-Fe-MOF
Nodes / linkers: Cu paddle-wheel · Cr trimer · Mg/Ni MOF-74 · Fe oxo cluster · btc · bdc · dobdc · terphenyl tricarboxylate
6358 · 3.1 Strategies for manipulating the metal component · Fig. 12
Oxalate/adipate or oxalate frameworks with ammonium/counterion and water networks that change with hydration.
Conduction: Humidity changes H-bond continuity and water/counterion arrangement; hydrophilicity improves low-humidity conductivity.
Representative materials: (NH4)2(adp)[Zn2(ox)3].nH2O · NMe3(CH2COOH)[FeCr(ox)3] · ferrous oxalate dihydrate
Nodes / linkers: Zn oxalate · FeCr oxalate · Fe oxalate · oxalate · adipate · carboxylate counter-cations
6352 · 2.1 Proton conductivity under hydration · Fig. 2 and Fig. 3
Frameworks where pore surface chemistry and geometry, rather than only acid loading, shape water cluster structure and proton diffusion.
Conduction: Hydrophilic small pores favour early water uptake and Grotthuss transport, while hydrophobic channels can confine water clusters and produce anisotropic diffusion.
Representative materials: Mg2H6(H3O)(TTFTB)3 · [Pt(dach)(bpy)Br]4(SO4)4.32H2O
Nodes / linkers: Mg · Pt · TTFTB · bipyridine nanotube
6363 · 3.3 Strategies for manipulating the pore space · Fig. 20
Review-level synthesis principles remain separate from primary-study recipes.
Use predesigned or post-synthetic ligand chemistry to install -COOH, -SO3H or phosphonate groups that act as proton donors and H-bonding sites.
Claimed effects: Acidic struts and uncoordinated groups can increase proton concentration and provide regular H-bond sites.
Controlling variables: functional group acidity · coordination state · water sorption · framework stability
Representative materials: MIL-53(Fe)-(COOH)2 · UiO-66(SO3H)2 · BUT-8(Cr)A
Caveat: Acid groups often coordinate to metals during synthesis; post-synthetic chemistry requires a stable host.
6361 · Post-synthetic ligand functionalization · Fig. 18
Use missing metal centres, missing connections or ligand defects to alter local acidity and host mobile proton carriers.
Claimed effects: Defects can create conduction passages or host mobile species, but can also trap protons and suppress mobile concentration.
Controlling variables: defect type · acid additive · mobile acid species · local pKa
Representative materials: Zr6O4(OH)8L4.2.xH2O · defective [Zn(H2PO4)HTz2]n
Caveat: Defects require structural and spectroscopic confirmation; PXRD alone may miss important defect chemistry.
6361 · Defect of the ligand in MOFs · Fig. 17
Design frameworks that respond to water adsorption by changing pore geometry while maintaining H-bond continuity.
Claimed effects: Humidity-induced structural changes can maintain high conductivity and low activation energy through Grotthuss-type diffusion.
Controlling variables: framework flexibility · humidity · sulfonic acid site density · structural transformation
Representative materials: BUT-8(Cr)A · BUT-8(Cr)
Caveat: The review presents this as promising but selected; broader predictive rules are not yet established.
6353 · 2.1 Proton conductivity under hydration · Fig. 4
Compare included versus coordinated guests to distinguish carrier concentration from mobility and H-bond continuity.
Claimed effects: Free, mobile imidazole in connected pores can outperform immobilised or strongly bound imidazole despite similar guest chemistry.
Controlling variables: host-guest interaction strength · guest location · pore interconnection · distance between adjacent guests
Representative materials: Im@NENU-3 · Im-Cu@(NENU-3a) · Im@Fe-MOF · Im-Fe-MOF
Caveat: Vehicle-like activation energies and reduced water uptake complicate simple comparisons.
6359 · 3.1 Strategies for manipulating the metal component
Adjust counter-cation size, functional groups or pore chemistry to increase water affinity and retain continuous H-bond networks at lower RH.
Claimed effects: Higher water uptake and hydrophilicity coincide with higher low-humidity proton conductivity.
Controlling variables: alkyl chain length · water uptake · interlayer void space · hydrophilic functional groups
Representative materials: Me-FeCr · Et-MnCr · Bu-FeCr
Caveat: High humidity may cause structural instability; relatively high conductivity under ambient conditions is emphasised as more practical.
6352 · 2.1 Proton conductivity under hydration · Fig. 3
Generate OMSs by activation and replace coordinated solvent with water, imidazole, urea or sulfonic-acid-bearing molecules.
Claimed effects: Coordinated molecules can tune acidity, stabilise H-bonds, decorate pore surfaces and increase proton conductivity.
Controlling variables: metal identity · coordinated molecule acidity · OMS density · pore size
Representative materials: HKUST-1-H2O · MOF-74(Mg)-urea · Cr-MIL-88B-PSA
Caveat: Bound guests can also reduce water uptake or mobility; placement and mobility must be characterised.
6357 · 3.1 Strategies for manipulating the metal component
Use hydrophilic small pores, large cavities, or hydrophobic channels to control water cluster structure and diffusion pathways.
Claimed effects: Pore surface and size can alter mechanism, diffusivity and anisotropy without relying only on strong acid groups.
Controlling variables: pore size · pore surface hydrophilicity · water cluster confinement · channel anisotropy
Representative materials: Mg2H6(H3O)(TTFTB)3 · [Pt(dach)(bpy)Br]4(SO4)4.32H2O
Caveat: Large-pore mechanisms may be hard to assign from activation energy alone.
6363 · 3.3 Strategies for manipulating the pore space
These are the review authors’ synthesis, not newly measured results.
For MIL-53 derivatives in the review, conductivity tracks framework acidity more strongly than total water uptake.
Evidence basis: single_reference
Caveat: This is a family-specific example and should not be generalised without primary comparison.
6361 · Predesigned ligand functionalization · Fig. 16
Anhydrous MOF proton conductors reviewed here generally remain around 10^-3 S cm^-1, below the strongest water-mediated examples.
Evidence basis: multi_reference
Caveat: The authors state this is not a fundamental maximum.
6357 · 2.2 Proton conductivity under anhydrous conditions
Some reviewed MOFs reach conductivities above 10^-1 S cm^-1, but the review warns that practical use remains limited by stability, membrane fabrication, grain boundaries, crossover and operating temperature.
Evidence basis: review_reasoning
Caveat: Do not use review-table values as primary evidence for device readiness.
6364 · 4. Conclusions and perspectives
Defects are dual-use design elements: they can produce conduction paths or mobile-species sites, but can also trap protons and reduce mobile concentration.
Evidence basis: multi_reference
Caveat: Defect identity should be verified by more than bulk diffraction.
6360 · Defective metal cluster in MOFs · Fig. 14
The review concludes that high-conductivity MOFs require robust frameworks, ordered protic sites for regular H-bonding and high carrier density.
Evidence basis: review_reasoning
Caveat: This is an interpretive synthesis across reports.
6364 · 4. Conclusions and perspectives
Activation energy thresholds are useful for assigning Grotthuss versus vehicle behaviour, but are too general when mechanisms are cooperative or borderline.
Evidence basis: multi_reference
Caveat: Mechanism assignments need complementary structural and spectroscopic evidence.
6351 · Introduction
Flexible MOFs that self-adapt to humidity can maintain more continuous H-bond networks than rigid frameworks.
Evidence basis: single_reference
Caveat: The review presents representative examples, not a universal rule.
6353 · 2.1 Proton conductivity under hydration · Fig. 4
Hydration can raise mobile proton concentration and construct H-bond pathways, but conductivity depends on the actual water-framework arrangement.
Evidence basis: multi_reference
Caveat: Humidity can also threaten stability or alter framework phases.
6352 · 2.1 Proton conductivity under hydration · Fig. 2
Pelletised two-probe conductivity averages randomly oriented microcrystals, whereas single-crystal measurements can reveal anisotropic, grain-boundary-free transport.
Evidence basis: review_reasoning
Caveat: Benchmark comparisons should distinguish pellet and single-crystal values.
6351 · Introduction
MOFs are framed as attractive proton conductors because porosity, crystallinity and structural tunability allow proton carriers and pathways to be designed and visualised.
Evidence basis: review_reasoning
Caveat: The claim is a review-level synthesis, not a primary measurement.
6350 · Introduction
Open metal sites allow coordinated molecules to tune acidity and proton donation, but guest coordination can either help or hinder transport depending on mobility and H-bond continuity.
Evidence basis: multi_reference
Caveat: Coordinated imidazole is not always superior to free imidazole in pores.
6358 · 3.1 Strategies for manipulating the metal component
Pore surface and confinement can control proton diffusion independently of strong acid moieties.
Evidence basis: multi_reference
Caveat: The hydrophobic-channel example is a metal-organic nanotube and uses single-crystal anisotropic measurement.
6363 · 3.3 Strategies for manipulating the pore space
Practical application of proton-conductive MOFs is limited by chemical/thermal stability, fuel crossover, grain boundaries in membranes and humidity dependence.
Evidence basis: review_reasoning
Caveat: Barrier list is conceptual and should be supported by primary device studies when used quantitatively.
6350 · Introduction
Post-synthetic ligand functionalisation is useful for exposing acid sites that would otherwise coordinate during synthesis, but requires a chemically stable host framework.
Evidence basis: single_reference
Caveat: The review's example is UiO-66 chemistry; transfer to less stable MOFs is uncertain.
6361 · Post-synthetic ligand functionalization · Fig. 18
In urea-coordinated MOF-74, urea primarily assists proton diffusion by stabilising H-bonds between guest water molecules rather than directly transferring protons.
Evidence basis: single_reference
Caveat: Mechanistic interpretation is drawn from the review's summary of NMR and humidity-dependent activation energies.
6359 · 3.1 Strategies for manipulating the metal component · Fig. 13
Every row remains visibly secondary and links to a primary dossier only where the mapping is verified.
| Material | Property | Reported value | Context and quality | Primary evidence | Review source |
|---|---|---|---|---|---|
| SecondaryBUT-8-(Cr)A | proton conductivity | 1.27 x 10^-1 S cm^-1 (Ea 0.11 eV) | 80 deg C, 100% RH; pelletised sample Table · Exact Reported | No verified corpus mapping | 6364 · 4. Conclusions and perspectives · Table 1 |
| SecondaryCr-MIL-88B-PESE | proton conductivity | 4.50 x 10^-2 S cm^-1 (Ea 0.34 eV) | 85% RH, 100 deg C; pelletised sample Table · Exact Reported | No verified corpus mapping | 6364 · 4. Conclusions and perspectives · Table 1 |
| SecondaryCr-MIL-88B-PSA | proton conductivity | 1.58 x 10^-1 S cm^-1 (Ea 0.38 eV) | 85% RH, 100 deg C; pelletised sample Table · Exact Reported | No verified corpus mapping | 6364 · 4. Conclusions and perspectives · Table 1 |
| SecondaryDefective [Zn(H2PO4)HTz2]n | proton conductivity | 3.16 x 10^-3 S cm^-1 | anhydrous, 150 deg C; pelletised sample Table · Exact Reported | No verified corpus mapping | 6364 · 4. Conclusions and perspectives · Table 1 |
| SecondaryIm-Fe-MOF | proton conductivity | 4.50 x 10^-2 S cm^-1 (Ea 0.44 eV) | 98% RH, 60 deg C; pelletised sample Table · Exact Reported | No verified corpus mapping | 6364 · 4. Conclusions and perspectives · Table 1 |
| SecondaryIm@NENU | proton conductivity | 1.82 x 10^-2 S cm^-1 (Ea 0.57 eV) | 98% RH, 70 deg C; pelletised sample Table · Exact Reported | No verified corpus mapping | 6364 · 4. Conclusions and perspectives · Table 1 |
| SecondaryMIL-53(Fe)-COOH2 | proton conductivity | 7 x 10^-6 S cm^-1 (Ea 0.21 eV) | 95% RH, 80 deg C; pelletised sample Table · Exact Reported | No verified corpus mapping | 6364 · 4. Conclusions and perspectives · Table 1 |
| SecondaryMg2H6(H3O)(TTFTB)3 | proton conductivity | 5.1 x 10^-4 S cm^-1 (Ea 0.4 eV) | 95% RH, 75 deg C; pelletised sample Table · Exact Reported | No verified corpus mapping | 6364 · 4. Conclusions and perspectives · Table 1 |
| Secondary{[(Me2NH2)3(SO4)]2[Zn2(ox)3]}n | proton conductivity | 4.2 x 10^-2 S cm^-1 | 98% RH, room temperature; H2O-mediated conditions Text · Exact Reported | No verified corpus mapping | 6356 · 2.2 Proton conductivity under anhydrous conditions |
| Secondary[Pt(dach)(bpy)Br]4(SO4)4.32H2O | proton conductivity | 1.7 x 10^-2 S cm^-1 (Ea 0.22 eV) | 95% RH, 55 deg C; single crystal Table · Exact Reported | No verified corpus mapping | 6364 · 4. Conclusions and perspectives · Table 1 |
| SecondaryUiO-66(SO3H)2 | proton conductivity | 8.4 x 10^-2 S cm^-1 (Ea 0.32 eV) | 90% RH, 80 deg C; pelletised sample Table · Exact Reported | No verified corpus mapping | 6364 · 4. Conclusions and perspectives · Table 1 |
| SecondaryUrea-MOF-74 (Mg) | proton conductivity | 3.69 x 10^-2 S cm^-1 (Ea 0.14 eV) | 95% RH, 55 deg C; pelletised sample Table · Exact Reported | No verified corpus mapping | 6364 · 4. Conclusions and perspectives · Table 1 |
Open questions are presented as review-author priorities, not conclusions from the primary database.
Mining reported structural databases is identified as a future route to discover proton-conductive MOF candidates.
Proposed direction: Use big-data searches over known structures to identify latent proton-conduction motifs.
6365 · 4. Conclusions and perspectives
High proton conductivity at sub-zero temperature and anhydrous conductivity comparable to water-mediated systems remain open targets.
Proposed direction: Design conducting media and frameworks that preserve mobility without liquid water over wider temperature ranges.
6365 · 4. Conclusions and perspectives
Designing flexible structures that reveal or create conducting pathways under external stimuli remains a target.
Proposed direction: Develop humidity- or temperature-responsive MOFs that maintain strong H-bond networks under low humidity.
6365 · 4. Conclusions and perspectives
Mechanistic assignments need comprehensive, multi-tool characterisation rather than reliance on activation energy.
Proposed direction: Combine impedance, single-crystal structure, SS-NMR, QENS and computation to resolve pathways.
6365 · 4. Conclusions and perspectives
Mechanical strength and PEM fabrication remain barriers for MOF proton conductors.
Proposed direction: Study hybrid MOF-polymer materials for improved proton exchange membranes.
6365 · 4. Conclusions and perspectives
Directional proton diffusion through designed pKa gradients is proposed but underdeveloped.
Proposed direction: Build frameworks with spatially organised acid-base gradients for directional transport.
6365 · 4. Conclusions and perspectives
Pore apertures and cavities need to retain conducting media while limiting fuel crossover.
Proposed direction: Combine small apertures with large cavities to encapsulate conductive media and reduce escape/crossover.
6365 · 4. Conclusions and perspectives
Structural stability, membrane fabrication, grain-boundary effects, fuel crossover and limited operating temperature still restrict practical use.
Proposed direction: Evaluate MOF proton conductors under device-relevant membrane and fuel-cell conditions, not only powder/pellet tests.
6364 · 4. Conclusions and perspectives
Mappings show which printed review references have a verified counterpart in the frozen primary corpus.
| Reference | Study | Role and context | Corpus mapping |
|---|---|---|---|
| Ref. 152014 | Title unavailable | hydration_structure · transport_benchmarkUsed for direct visualisation of humidity-dependent H-bond network changes and proton conductivity in an oxalate-based MOF. | research_0324 |
| Ref. 762017 | Title unavailable | flexible_framework · transport_benchmarkRepresentative flexible sulfonated MOF where humidity-induced adaptation preserves proton-conducting H-bond networks. | Unmapped |
| Ref. 772012 | Title unavailable | hydrophilicity · transport_benchmarkUsed for hydrophilicity control in bimetallic oxalate layered compounds through counter-cation alkyl chain length. | Unmapped |
| Ref. 782011 | Title unavailable | ligand_functionalisation · transport_benchmarkUsed to support the ligand acidity/conductivity relationship in MIL-53 derivatives. | Unmapped |
| Ref. 802020 | Title unavailable | oms_urea · transport_benchmarkRepresentative solvent-free coordinative urea insertion into MOF-74, used for OMS modification and H-bond stabilisation. | Unmapped |
| Ref. 812017 | Title unavailable | guest_inclusion · transport_benchmarkUsed to compare free versus coordinated imidazole in a Cu/polyoxometalate MOF. | Unmapped |
| Ref. 822012 | Title unavailable | open_metal_sites · measurement_contextUsed for solvent coordination at HKUST-1 OMSs and acidity-driven proton conductivity under MeOH vapour. | Unmapped |
| Ref. 832015 | Title unavailable | post_synthetic_modification · transport_benchmarkPost-synthetic oxidation of UiO-66-(SH)2 to UiO-66(SO3H)2, used as a stable-host acid-site example. | Unmapped |
| Ref. 1072015 | Title unavailable | defects · structure_propertyUsed for regular Zr-MOF defects that trap protons and can be tuned with acid species. | Unmapped |
| Ref. 1082016 | Title unavailable | ligand_defect · transport_benchmarkUsed for ligand-defect design where encapsulated phosphoric acid species provide new low-Ea hopping paths. | Unmapped |
| Ref. 1101995 | Title unavailable | mechanism_definitionCited for the Grotthuss mechanism definition used in the review's mechanistic framing. | Unmapped |
| Ref. 1111982 | Title unavailable | mechanism_definitionCited for vehicle-type proton migration in the review's mechanism framing. | Unmapped |
| Ref. 1162012 | Title unavailable | anhydrous_conduction · plastic_crystalUsed for anhydrous proton conductivity in a phosphate-type coordination polymer with plastic-crystal-like ion motion. | Unmapped |
| Ref. 1182014 | Title unavailable | hydrated_and_anhydrous · transport_benchmarkUsed for a rigid oxalate/sulfate supramolecular network that conducts under both hydration and anhydrous conditions. | Unmapped |
| Ref. 1192020 | Title unavailable | anhydrous_conduction · structural_transformationUsed as a non-MOF but MOF-relevant anhydrous strategy based on structural transformation and SCN/H2O carrier pathways. | Unmapped |
| Ref. 1262020 | Title unavailable | metal_centre_exchange · transport_benchmarkUsed for Cr substitution and sulfonic-acid coordinative insertion in MIL-88B to improve stability and conductivity. | Unmapped |
| Ref. 1282017 | Title unavailable | imidazole_coordination · transport_benchmarkUsed for comparing guest-included and coordinated imidazole in Fe-based MOFs. | Unmapped |
| Ref. 1382018 | Title unavailable | hydrophilic_pore · transport_benchmarkUsed for selective vapour pressure-dependent proton transport in hydrophilic and less-hydrophilic pores. | Unmapped |
| Ref. 1462020 | Title unavailable | hydrophobic_pore · single_crystal_benchmarkUsed for hydrophobic channel design, confined water clusters and anisotropic single-crystal proton conductivity. | Unmapped |