The anodic electrochemical synthesis of a Cu₃(HHTP)₂ thin film and its thermoelectric properties are reported for the first time.

In this work, a switch from n-type to p-type conductivity in electrodeposited Cu 3 (2,3,6,7,10,11-hexahydroxytriphenylene) 2 [Cu 3 (HHTP 2 )] has been observed, which is most likely due to oxygen molecular doping. The synthesis of electrically conductive 2D metal–organic frameworks (MOFs) has been achieved through the introduction of highly conjugated organic linkers coordinated to their constituent metal-ion centers. However, the porous structure and unsaturated metal sites in MOFs make them susceptible to ambient adsorbates, which can affect their charge transport properties. This phenomenon has been experimentally investigated by GIXRD, Hall effect and Seebeck measurements, and X-ray photoelectron spectroscopy.

We report the first result of a study in which molecular iodine has been incorporated via incipient wetness impregnation into the two-dimensional semiconducting metal−organic framework (MOF) Cu 3 (2,3,6,7,10,11-hexahydroxytriphenylene) 2 Cu 3 (HHTP) 2 to enhance its thermoelectric properties. A power factor of 0.757 μW m −1 K −2 for this MOF was obtained which demonstrates that this provides an effective route for the preparation of moderate-performance thermoelectric MOFs.

The electrical conductivity and porosity of the 2-dimensional metal-organic framework Cu₃(2,3,6,7,10,11-hexahydroxytriphenylene)₂ [Cu₃(HHTP)₂] make it a promising candidate for thermoelectric applications. In this work, we report the electrochemical synthesis of Cu₃(HHTP)₂ films by an anodization approach and an evaluation of its thermoelectric properties. The electrochemically synthesised Cu₃(HHTP)₂ thin films were transferred using a wet chemical method in order to perform electrical measurements. We are reporting the first thermoelectric measurements of this framework both in bulk and thin film form which resulted in Seebeck coefficients of -7.24 µV/K and -121.4 µV/K with a power factor of 3.15x10^-3 µW m^-1 for the film respectively. The negative Seebeck coefficients suggest that Cu₃(HHTP)₂ behaves as an n-type semiconductor.

In recent years, there has been special interest in developing devices capable to harvest and store energy from natural resources without the generation of pollution. Thermoelectric generator (TEG) is an emergent technology to harvest energy, especially in those environments in which heat waste is involved. These solid-state devices are capable of generating an output voltage as a function of a temperature difference. The conversion of thermal energy into electrical energy in these devices is attributed to the Seebeck effect. The efficiency of a TEG is evaluated through the dimensionless figure of merit, Z. In order to achieve a competitive the figure of merit, a material with a high Seebeck coefficient, electrical conductivity and low thermal conductivity is desirable. Conductive metal organic frameworks (c-MOFs) are hybrid materials composed of inorganic and organic building blocks, in which metal nodes are coordinated to highly conjugated organic linkers.1,2 The overlap between the metal and ligand frontier orbitals facilitates the charge transport in these materials. Porosity and heterogeneity in atomic species and linkers are features that have led to a predictably low thermal conductivity3, a key aspect to optimize Z, making MOFs potential candidates for TEG. To implement their practical use, the synthesis and study of ultrathin c-MOFs nanosheets have recently been reported4; however, the processing at large scale of these materials is still a challenge. In this work we present an electrochemical approach to the growth of conducting thin films of the 2D c-MOF Cu3(HHTP)2 (where HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene). Bulk Cu3(HHTP)2 was synthesized solvothermally according to the literature5 and we have subsequently fabricated thin films of this important framework by anodic electrochemical synthesis.6 We report the first thermoelectric measurements of this framework both in bulk and thin film form which resulted in Seebeck coefficients of -7.24 μV K-1 and -121.4 μV K-1 with a power factor of 3.15x10-3 μW m-1 for the film respectively. The study of conducting MOFs and their performance as TEG is expected to expand and offer alternatives to non-toxic, scalable and high-efficiency novel TEG materials. [1] P. Q. Liao, J. Q. Shen, J. P. Zhang, Coord. Chem. Rev. 373, 22, 2018 [2] L. Sun, M. G. Campbell, M. Dincă, Angew. Chem. Int. 55, 3566, 2016 [3] Huang , A. McGaughey , M. Kaviany , Int. J. Heat Mass Transf. 50, 393, 2007 [4] W. Zhaoa, et al., Coord. Chem. Rev. 377, 44, 2018 [5] M. Hmadeh, et al., Chem. Mater. 24, 3511, 2012 [6] Ameloot, R. et al., Chem. Mater. 21, 2580–2582, 2009