Metal-organic frameworks for thermoelectric energy-conversion applications

Defines the Seebeck effect, figure of merit ZT, power factor and the coupled tradeoffs among S, sigma and kappa.

Relevance: Core · 877 · Introduction · Equation 1

Frames MOFs and Guest@MOFs as crystalline, porous, thermally stable alternatives to polymers whose electronic and thermal transport can be tuned through metal, ligand and guest choice.

Relevance: Core · 879 · Conducting metal-organic frameworks as thermoelectrics

Concludes that MOFs could combine inorganic-like structural order with organic-like tunability and low cost, but TCNQ@Cu3(BTC)2 remains limited by low electrical conductivity.

Relevance: Core · 881 · Summary

Reviews Cu3(BTC)2 deposition, TCNQ infiltration, measured Seebeck response, conductivity, thermal conductivity, TDTR measurement and the low power factor bottleneck.

Relevance: Core · 879 · TCNQ@Cu3(BTC)2 thermoelectric · Figures 1-2

Compares inorganic Bi2Te3 alloys, conducting polymers and coordination polymers, highlighting cost/processability benefits and n-type organic limitations.

Relevance: Core · 878 · Thermoelectric materials · Table I

Explains low lattice thermal conductivity in MOFs using phonon-glass/electron-crystal reasoning, heterogeneous bonds, soft open structures, guest effects and weak temperature dependence.

Relevance: Core · 880 · Thermal transport in MOFs · Figure 3

p-type and n-type thermoelectric elements

The review uses Seebeck sign and thermoelectric-generator architecture to separate p-type and n-type materials, then identifies stable n-type organic conductors as a key scarcity.

Categories: p-type hole conductors · n-type electron conductors

878 · Thermoelectric materials

Coordination polymers versus MOFs

The review distinguishes MOFs from broader coordination polymers by long-range crystalline order and pores that can adsorb guest molecules or nanostructures to tune transport.

Categories: nonporous coordination polymers · porous MOFs · guest-infiltrated MOFs

879 · Conducting metal-organic frameworks as thermoelectrics

Guest effects on MOF lattice thermal conductivity

The thermal-transport section classifies guests by likely effect on kappal, from little impact to rattler-driven reductions or stiffening-driven increases.

Categories: weakly interacting light guests · abundant light guests · heavy loosely bound rattlers · bond-stiffening infiltrants

881 · Thermal transport in MOFs

Thermoelectric material platforms

The article moves from conventional inorganic benchmarks to organic polymers and then to MOFs/Guest@MOFs as a hybrid platform with tunable crystalline porous structure.

Categories: Bi2Te3-based inorganic semiconductors · conducting polymers · coordination polymers · MOFs · Guest@MOFs

878 · Thermoelectric materials · Table I

Thermoelectric performance decomposition

The review organises TE performance through ZT = S2sigmaT/kappa and uses this framing to compare polymers, coordination polymers, MOFs and inorganic standards.

Categories: Seebeck coefficient S · Electrical conductivity sigma · Thermal conductivity kappa · Power factor S2sigma · Figure of merit ZT

877 · Introduction · Equation 1

Thermal conductivity components

The review separates lattice and electronic thermal conductivity and uses the Wiedemann-Franz relationship to explain why improving carrier transport can increase heat transport.

Categories: lattice thermal conductivity kappal · electronic thermal conductivity kappae

878 · Introduction

Conventional inorganic semiconductors and alloys used as high-performance TE benchmarks.

Conduction: Low-bandgap semiconductor transport with mature TE performance but limited processability for complex or low-cost large-area substrates.

Representative materials: Bi0.5Sb1.5Te3 · Bi2Te2.7Se0.3 · Bi2Te3 and alloys

Nodes / linkers: Bi · Sb · Te · Se · not applicable

878 · Thermoelectric materials · Table I

Organic conjugated polymers considered for flexible, processable, low-cost thermoelectrics.

Conduction: Transport relies on doped conjugated chains; dopants improve carrier concentration but may introduce disorder and reduce mobility.

Representative materials: PEDOT:PSS · polyacetylene · polyaniline · polypyrrole

Nodes / linkers: not applicable · conjugated polymer backbones · polystyrene sulfonate dopant

878 · Thermoelectric materials

Repeating metal-organic coordination complexes used as p-type and n-type polymeric TE comparators.

Conduction: Electronic structure is dominated by transition-metal d electrons and their interaction with organic ligands rather than only carbon-backbone pi orbitals.

Representative materials: Poly(Cux[Cu-ett]) · Poly(Kx[Ni-ett])

Nodes / linkers: Cu · Ni · K counterion · 1,1,2,2-ethenetetrathiolate

878 · Thermoelectric materials · Table I

Crystalline porous metal-organic frameworks whose electronic and geometric structure can be tuned by metal, ligand and guest selection.

Conduction: MOF crystallinity may promote mobility while pores allow guest molecules to tune electronic and thermal transport.

Representative materials: Cu3(BTC)2 · TCNQ@Cu3(BTC)2 · MOF-5 · MOF-74

Nodes / linkers: Cu · Zn and other MOF nodes · benzene-1,3,5-tricarboxylate · TCNQ guest molecules · porous organic linkers

877 · Abstract

Cu3(BTC)2 framework infiltrated with TCNQ molecules that bridge Cu(II) dimers and create an electronic conduction pathway.

Conduction: TCNQ LUMO levels near the Cu3(BTC)2 valence band shift EF toward the valence band, giving p-type Seebeck response but low conductivity.

Representative materials: TCNQ@Cu3(BTC)2 · Cu3(BTC)2 · TCNQ

Nodes / linkers: Cu(II) dimers · benzene-1,3,5-tricarboxylate · tetracyanoquinodimethane guest

879 · TCNQ@Cu3(BTC)2 thermoelectric · Figure 1

Open, mechanically soft MOFs whose complex, heterogeneous structures can reduce lattice thermal conductivity.

Conduction: Low-energy propagation velocities, mass/bond heterogeneity and phonon scattering can lower kappal, although porosity and disorder may harm electrical conductivity.

Representative materials: MOF-5 · MOF-74 · Cu3(BTC)2 · TCNQ@Cu3(BTC)2

Nodes / linkers: Zn · Cu · mixed MOF metal nodes · porous organic linkers · guest molecules

881 · Thermal transport in MOFs · Figure 3

Maintain crystallinity while reducing lattice thermal conductivity

The review argues that intentionally reducing vibrational energy-transfer length scales is preferable to introducing atomic disorder that may damage charge transport.

Claimed effects: A viable path to high ZT is to preserve electrical transport while suppressing kappal through nanoscale vibrational mechanisms.

Controlling variables: crystallinity · phonon mean free path · structural disorder · porosity

Representative materials: MOFs · Guest@MOFs

Caveat: Increasing porosity can lower heat transport but is expected to have detrimental effects on electrical conductivity and hence ZT.

880 · Thermal transport in MOFs

Minimise dopant volume and control oxidation state in conducting polymers

The review describes polymer TE optimisation as requiring dopants for conductivity while limiting dopant-induced disorder and insulating volume fraction.

Claimed effects: Precise control is associated with high PEDOT:PSS power factor, but interpretation can be confounded by measurement geometry.

Controlling variables: dopant volume fraction · oxidation state · chain-to-chain coupling · carrier concentration

Representative materials: PEDOT:PSS

Caveat: Dopants can occupy volume, introduce disorder, decrease coupling and depress mobility.

878 · Thermoelectric materials

Guest engineering for lattice thermal conductivity

Guest infiltration is proposed not only for electronic tuning but also for controlling vibrational scattering, bond heterogeneity and rattling-like acoustic modes.

Claimed effects: Heavy loosely bound guests may reduce kappal by localised modes, enhanced anharmonicity and lower group velocities, while other guests may stiffen bonds.

Controlling variables: guest mass · host-guest binding strength · bond heterogeneity · pore loading · anharmonicity

Representative materials: Guest@MOFs · TCNQ@Cu3(BTC)2 · zeolites · skutterudites · clathrates

Caveat: The review explicitly cautions that TCNQ infiltration can increase calculated lattice thermal conductivity through bond stiffening.

881 · Thermal transport in MOFs

Guest-molecule infiltration to create electronic pathways

TCNQ infiltration into Cu3(BTC)2 is reviewed as a way to bridge Cu(II) dimers, create a conduction pathway and shift electronic states near the Fermi level.

Claimed effects: TCNQ increases conductivity by many orders of magnitude and produces p-type thermopower, but the resulting power factor remains low.

Controlling variables: guest molecule electron affinity · guest loading · pore connectivity · host-guest orbital alignment

Representative materials: TCNQ@Cu3(BTC)2

Caveat: The review notes that the low electrical conductivity of TCNQ@Cu3(BTC)2 is still the main performance limitation.

879 · TCNQ@Cu3(BTC)2 thermoelectric · Figure 1

Tune MOF electronic structure by metal and ligand choice

MOFs are presented as synthetically versatile systems where metal and ligand selection can tune electronic structure for p-type or n-type ZT.

Claimed effects: Appropriate metal-ligand choices could optimise electronic structure and promote charge mobility without depressing Seebeck coefficient.

Controlling variables: metal node · organic ligand · crystalline order · band alignment

Representative materials: conducting MOFs · Guest@MOFs

Caveat: The field was described as in its infancy, with only TCNQ@Cu3(BTC)2 explored for TE applications at that time.

879 · Conducting metal-organic frameworks as thermoelectrics

Solution growth and inkjet printing of MOF thin films

The review identifies solution-based thin-film growth and inkjet printing as routes to pattern MOF layers and contacts on low-cost substrates.

Claimed effects: Inkjet printing is attractive for low-cost TE fabrication because relatively thick patterned layers and collectors can be rapidly deposited on paper or fabrics.

Controlling variables: deposition method · substrate · pattern geometry · current collector deposition

Representative materials: Cu3(BTC)2 · TCNQ@Cu3(BTC)2

Caveat: Device-relevant conductivity still depends on framework crystallinity and guest infiltration quality.

879 · TCNQ@Cu3(BTC)2 thermoelectric · Figure 1b

The review cautions that increasing MOF porosity may lower heat transport but harm electrical conductivity and ZT.

Proposed direction: Design porous frameworks that scatter phonons while preserving connected electronic pathways and crystallinity.

881 · Thermal transport in MOFs

Guest molecules can stiffen bonds or create rattling-like scattering, so their net effect on kappa and sigma needs predictive control.

Proposed direction: Engineer guest mass, binding, electron affinity and bond heterogeneity to reduce heat-carrying phonons while improving charge transport.

881 · Thermal transport in MOFs

The review flags that interpretation of a high PEDOT:PSS PF was confounded because sigma and kappa were not measured on the same specimens or geometry.

Proposed direction: Use consistent specimen geometry and co-located measurement protocols when comparing TE benchmarks across polymers and MOFs.

878 · Thermoelectric materials

At the time of the review, only one MOF system had been explored for thermoelectric applications.

Proposed direction: Evaluate additional inherently conducting porous frameworks and guest-infiltrated MOFs for TE transport.

879 · Conducting metal-organic frameworks as thermoelectrics

The review notes that a solution-based processing route to Poly(Kx[Ni-ett]) had not been demonstrated.

Proposed direction: Develop scalable solution processing for n-type coordination polymer thermoelectrics or translate their electronic-structure advantages into MOFs.

878 · Thermoelectric materials

The review identifies stable high-performance n-type organic semiconductors as a long-standing challenge.

Proposed direction: Use MOF/Guest@MOF tunability to design electron-transporting materials with suitable orbital alignment and stability.

877 · Abstract

Despite high Seebeck coefficient and low kappa, TCNQ@Cu3(BTC)2 has insufficient sigma and a low power factor.

Proposed direction: Increase framework crystallinity and conduction pathways while monitoring whether kappa rises in parallel.

880 · TCNQ@Cu3(BTC)2 thermoelectric · Figure 2