Operando x-ray absorption & inelastic neutron scattering studies of novel thermoelectric metal-organic-frameworks Rob Clarke 1 , Dr. Iris Nandhakumar 1 , Dr. Darren Bradshaw 1 , Dr. Luke Keenan 2 , Dr. Svemir Rudic 3 1 School of Chemistry, University of Southampton, UK, 2 Diamond Light Source, Harwell Research and Innovation Campus, Oxfordshire, UK, 3 ISIS Neutron and Muon Source, Didcot, UK Thermoelectrics (TEs) are an important class of materials that can directly convert thermal waste heat into useful electrical energy and have great potential to contribute to the sustainability agenda. Despite such possibilities, barriers exist to their widespread adoption due to the low efficiency of current materials. TE power generation has the potential of becoming a transformative technology for renewable energy generation, provided that the low efficiency of current TE materials can be addressed by developing novel materials. Metal-organic frameworks (MOFs) are a class of solid-state materials which are comprised of a self-assembled lattice structure of positive metal ions or clusters coordinated to negative anionic ‘linkers’.MOFs are attractive candidates for novel TE materials given their relatively low thermal conductivity and tunable electrical conductivity [1]. Although most MOFs tend to be insulators, significant progress has been made over the last five years exploiting both ‘through-bond’ and ‘through-space’ charge transport pathways in these frameworks[2]. Cu 3 (HHTP) 2 (Figure 1.) and Ni 3 (HITP) 2 are semiconductive 2D layered MOFs which demonstrate promising TE capabilities. Their honeycomb framework allows electron transport between the π-π stacking of the organic linker, allowing ‘through-space’ charge transport. [3]We have reported a high electrical conductivity (15.4 S m -1 ) in addition to an impressive Seebeck coefficient(+419.5 µV K -1 ) for these materials. We have also demonstrated how doping Cu 3 (HHTP) 2 thin films by incipient wetness impregnation with molecular iodine significantlyimproves the electrical conductivity and power factor. However, the exact mechanism of doping or what species are responsible for charge transport after doping is still unknown and is yet to be systematically investigated. Preliminary HERFD XANES studies have suggested doping causes a change in oxidation state of the Cu 2+ centre which may contribute to the improve electrical conductivity. Moreover, further studies will be performed on both Cu 3 (HHTP) 2 and Ni 3 (HITP) 2 frameworks, at beamline I20 and Diamond Light Source and Tosca at ISIS, to elucidate the structure-property relationship in these systems.
Figure 1: Structural representation of Cu 3 (HHTP) 2 metal-organic-framework Code: Copper (orange), carbon (grey) and oxygen (red). References 1. S. Ma and H. C. Zhou, Chemical Communications , 2010, (46), 44–53. 2. M. De Lourdes Gonzalez-Juarez, C. Morales, J. I. Flege, E. Flores, M. Martin-Gonzalez, I. Nandhakumar and D. Bradshaw, ACS Appl Mater Interfaces , 2022, (14), 12404–12411. 3. Winkler, C. & Zojer, E. Nanomaterials , 2020, (10), 1–21.
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