Conductive metal-organic frameworks for zinc-air battery application: design principles, recent trends and prospects

Summarises electronic charge carriers, band versus hopping transport, and conductivity pathways available in conductive MOFs.

Relevance: Core · 2 · 2. Conductivity in MOFs

Frames ZABs as promising aqueous energy-storage devices limited by sluggish ORR/OER kinetics, and positions conductive MOFs as multifunctional porous electrocatalyst candidates.

Relevance: Core · 1-2 · 1. Introduction

Contrasts two-probe, four-probe, van der Pauw, pellet, film and single-crystal approaches and warns that technique choice strongly affects reported conductivity.

Relevance: Core · 4 · 4. Conductivity measurement techniques in MOFs · Table 1

Traces early conductive MOFs from pdt dithiolene networks through M-CAT, BHT, HITP and HHTP systems, with emphasis on conductivity, structure and emerging energy applications.

Relevance: Core · 3 · 3. Recent progress in the development of conductive MOFs · Fig. 1

Identifies synthesis, structural characterisation, material flexibility, binder-free electrodes, anodic issues and mechanism elucidation as main improvement areas.

Relevance: Core · 10-12 · 7. Prospects or areas which need to be highlighted/explored · Fig. 7

Concludes that cMOF ZAB research is still at an initial stage and needs dedicated work to expand and improve the field.

Relevance: Supporting · 12-13 · 8. Summary and future perspectives

Reviews cMOF-based ZAB cathode reports, including Co/Ni-HITP, Co3O4/Ni-Co-HITP, Ru-doped Ni-HHTP, Fe/Ni-BTC, Mn/Fe-HIB, HCF-MOF, Co-CAT/NiFe-LDH/CNFs and CuCo-HITP.

Relevance: Core · 6-10 · 6. Conductive MOFs for ZAB application · Table 2

Defines liquid and solid-state ZAB configurations, primary versus rechargeable cathode requirements, air-cathode architecture, and ORR/OER reaction constraints.

Relevance: Core · 5 · 5. ZAB configuration and mechanism · Fig. 3

Rechargeable ZAB air-cathode architecture

The air cathode is framed as a layered interface combining catalytic, hydrophobic gas access and electronic-current-collection functions.

Categories: Bifunctional catalyst layer · Gas diffusion layer · Conductive current collector

5 · 5. ZAB configuration and mechanism · Fig. 3

Conductivity pathways in cMOFs

Conductive-MOF strategies are organised by charge delocalisation through metal-ligand bonding, extended ligand conjugation, non-covalent pi-pi contact, redox hopping or guest-assisted conduction.

Categories: Bond pathways · Prolonged conjugation · Space pathways · Redox hopping · Guest promoted pathways

2 · 2. Conductivity in MOFs

Areas for improvement in cMOF-ZAB research

Fig. 7 explicitly groups the review's outlook into six improvement areas for cMOF application in zinc-air batteries.

Categories: Synthesis · Characterization techniques · Variety or flexibility · Binder free electrode design · Anodic challenges · Mechanism elucidation

12 · 7. Prospects · Fig. 7

Pre-synthesis and post-synthesis conductivity enhancement

Fig. 1 and the associated text arrange conductivity enhancement into intrinsic design before framework formation and extrinsic incorporation after synthesis.

Categories: Pre-synthesis mixed-valence metal ions · Pre-synthesis conductive ligands · Post-synthesis metal clusters · Post-synthesis conducting polymers · Post-synthesis organic molecules · Post-synthesis small conductive species

3 · 2. Conductivity in MOFs · Fig. 1

Electronic transport modes in MOFs

The review distinguishes delocalised transport through continuous energy bands from hopping between localised neighbouring sites.

Categories: Band transport · Hopping transport

2 · 2. Conductivity in MOFs

ZAB configuration categories

The review separates ZABs by electrolyte state and by whether the air cathode is monofunctional or bifunctional.

Categories: Liquid ZABs · Solid-state ZABs · Primary ZABs · Rechargeable ZABs

5 · 5. ZAB configuration and mechanism · Fig. 3

Sulfur-rich conductive MOFs based on benzenehexathiol ligands, including Ni and Cu BHT frameworks.

Conduction: Soft sulfur donors are consistent with through-bond conductivity where metal and ligand energy levels are well matched.

Representative materials: Ni3(BHT)2 · Cu3BHT · Cu3(BHT)2 · Pd3(BHT)2

Nodes / linkers: Ni · Cu · Pd · BHT · benzenehexathiol

3 · 3. Recent progress · Table 1

Bimetallic conductive MOFs using smaller or hybrid ligands, including Fe/Ni-BTC and Co/Fe HCF-MOF cathode materials.

Conduction: The review attributes activity to high intrinsic conductivity, redox-active sites, efficient transport routes and cooperative bimetallic interactions.

Representative materials: Fe/Ni-BTC · HCF-MOF

Nodes / linkers: Fe · Ni · Co · benzene tricarboxylic acid · HCF hybrid cobalt and iron based linker set

8-10 · 6. Conductive MOFs for ZAB application · Table 2

Hexaiminobenzene-based conductive MOFs with dual metal centres such as Mn/Fe for flexible solid-state ZABs.

Conduction: The review links quaternary nitrogen to conductivity enhancement and pyridinic/graphitic nitrogen to oxygen-electrocatalysis metrics.

Representative materials: Mn/Fe-HIB-MOF · Cu3(HIB)2 · Ni3(HIB)2

Nodes / linkers: Mn · Fe · Cu · Ni · HIB · hexaiminobenzene

9 · 6. Conductive MOFs for ZAB application · Fig. 5

Conjugated 2D MOFs and coordination polymers based on hexaiminotriphenylene linkers with Ni, Co, Cu or mixed metals.

Conduction: Conduction is discussed through metal-ligand orbital overlap plus pi-pi stacking; active-site identity depends on metal and ligand electronic states.

Representative materials: Ni3(HITP)2 · Co3(HITP)2 · Co3O4/Ni-Co-HITP · CuCo-HITP

Nodes / linkers: Ni · Co · Cu · HITP · hexaiminotriphenylene

6-7 · 6. Conductive MOFs for ZAB application · Table 2

2D layered MOFs containing Ni, Co or Cu and 2,3,6,7,10,11-hexahydroxytriphenylene/catecholate-type linkers.

Conduction: The review attributes conductivity to pi-d conjugation and additional pi-pi interactions between layers.

Representative materials: Cu-CAT · Ni-CAT · Co-CAT · Co-CAT/NiFe-LDH/CNFs · Ni5.7Ru0.3(HHTP)3(H2O)x

Nodes / linkers: Cu · Ni · Co · Ru · HHTP · catecholate

3 · 3. Recent progress

Early conductive frameworks based on 2,3-pyrazinedithiolate units with Cu or mixed Cu/Ni metal sites.

Conduction: Cu(II) d9 sites are interpreted as supporting charge delocalisation better than Ni(II) substitution.

Representative materials: Cu[Cu(pdt)2] · Cu[Ni(pdt)2]

Nodes / linkers: Cu · Ni · 2,3-pyrazinedithiolate

3 · 3. Recent progress · Table 1

Binder-free cMOF electrode design

Move from powder electrocatalysts fixed with binders toward directly integrated or binder-free cMOF electrodes.

Claimed effects: Would avoid binder-induced conductivity loss, electrode weight increase, energy-density decrease and alkaline binder degradation.

Controlling variables: Electrode-substrate adhesion · Catalyst loading · Electronic contact · Alkaline stability

Representative materials: cMOF-based bifunctional electrocatalysts

Caveat: Presented as an outlook target rather than a demonstrated strategy for most reviewed cMOF ZABs.

12 · 7.4. Binder-free electrode design for the ZAB application · Fig. 7

Conductive organic ligand selection

Select soft donor or fully conjugated linkers such as HHTP, HITP, HIB and BHT to support metal-ligand orbital overlap and extended conjugation.

Claimed effects: Supports through-bond and prolonged-conjugation pathways and can maintain MOF structural features while improving electronic conductivity.

Controlling variables: Donor atom identity · Ligand conjugation length · Chelating functional group · Layer stacking

Representative materials: M-CAT · Ni3(HITP)2 · Cu3BHT · Mn/Fe-HIB-MOF

Caveat: Costly large conjugated ligands and difficult single-crystal synthesis remain explicit limitations.

2-3 · 2. Conductivity in MOFs · Fig. 1

Electrochemical deposition of cMOFs

Grow bimetallic MOFs electrochemically on conductive supports as a faster and more scalable alternative to solvothermal synthesis.

Claimed effects: The review presents cathodic electro-synthesis of Fe/Ni-BTC as faster, easier, scalable and less expensive than common hydrothermal routes.

Controlling variables: Metal salt selection · Ligand selection · Current density · pH · Substrate

Representative materials: Fe/Ni-BTC

Caveat: The review gives this as a promising example rather than a generally established synthesis standard.

8 · 6. Conductive MOFs for ZAB application

Post-synthetic guest-promoted conductivity

Introduce redox-active or conductive guests into MOF pores after framework formation to enhance electronic transport.

Claimed effects: Can tune conductivity in otherwise poorly conducting frameworks, as illustrated by TCNQ infiltration of HKUST-1.

Controlling variables: Guest molecule redox activity · Pore accessibility · Guest loading · Framework stability

Representative materials: TCNQ@Cu3(BTC)2 · HKUST-1

Caveat: The review separately warns that incorporating conductive additives may block pores and impede electrochemical applications.

2-3 · 1. Introduction; 3. Recent progress · Fig. 1

Mixed-valence or multi-metal node design

Use multiple or mixed metal centres to tune electronic states, conductivity and bifunctional catalytic active sites.

Claimed effects: Can improve active-site exposure and tune ORR/OER behaviour, but may reduce conductivity when secondary phases aggregate or disrupt ordered stacking.

Controlling variables: Metal identity · Metal oxidation/electronic state · Metal mixing ratio · Coordination geometry

Representative materials: Co3HITP2 · Ni3HITP2 · Co3O4/Ni-Co-HITP · CuCo-HITP

Caveat: Activity and conductivity are not monotonic with metal content; the review notes detrimental conductivity effects at higher Co content in Co3O4/Ni-Co-HITP.

7 · 6. Conductive MOFs for ZAB application

Supported 3D cMOF hybrid growth

Grow 2D conductive MOFs in situ on conductive or catalytic supports to create hierarchical 3D catalysts with improved electron and mass transfer.

Claimed effects: Combining conductive MOFs with NiFe-LDH/CNF is described as improving active-site exposure and forming better conducting pathways for ZAB cathodes.

Controlling variables: Support morphology · MOF loading · Interfacial contact · Porosity · Active-site exposure

Representative materials: Co-CAT/NiFe-LDH/CNFs

Caveat: Hybrid supports complicate attribution of activity to the MOF itself and may move beyond intrinsic cMOF behaviour.

10 · 6. Conductive MOFs for ZAB application · Fig. 6

Most ZAB-oriented cMOF reports are 2D, while 3D cMOFs with diverse topologies are underexplored.

Proposed direction: Develop 3D cMOFs with high surface area and abundant active sites for multifunctional electrocatalysis.

10-12 · 7.1. Synthesis; 7.2. Structural characterization

Most MOF/cMOF ZAB work targets the cathode, while Zn dendrite, passivation and hydrogen evolution issues remain critical.

Proposed direction: Develop standard protocols and conditions for Zn-anode improvement alongside cathode catalyst optimisation.

12 · 7.5. Consideration of anodic challenges · Fig. 7

Powder-form MOF electrocatalysts require binders that can reduce electrode conductivity, add weight and degrade under alkaline oxidising conditions.

Proposed direction: Design binder-free cMOF-based bifunctional electrodes with robust electronic contact and alkaline stability.

12 · 7.4. Binder-free electrode design for the ZAB application · Fig. 7

Many reported cMOFs still have low conductivity and require better understanding of electron charge transport.

Proposed direction: Use design studies that connect metal/linker selection, stacking and measured conductivity.

10 · 7.1. Synthesis

Conductive MOF synthesis often relies on tedious processes and costly ligands.

Proposed direction: Explore cost-effective ligand choices and user-friendly methods such as mechanochemical synthesis.

10 · 7.1. Synthesis · Fig. 7

Reaction mechanisms and catalyst design principles require better direct monitoring and interpretation.

Proposed direction: Use in situ Raman, X-ray absorption, infrared spectroscopy, in situ TEM and computational studies.

12 · 7.6. Mechanism elucidation · Fig. 7

The ZAB literature has focused on intrinsically conductive MOFs that are difficult to synthesise as well-defined single crystals.

Proposed direction: Explore post-synthetic guest doping and multi-metal MOFs to broaden the cMOF design space.

12 · 7.3. More flexibility in the use of conductive MOFs for ZABs

Single-crystalline structures of most conductive MOFs are not clearly understood, limiting atomic-level structure-property arguments.

Proposed direction: Develop ligands and synthesis methods that produce well-diffracting cMOF single crystals.

12 · 7.2. Structural characterization