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Sn clathrates are promising phonon glass, electron crystal materials (PGEC), in which the phonon free paths are short and the electron free paths are long. We analysed the relaxed structure of Sn clathrates using four different Density Funtional Exchange-Correlation functionals. The phonon structures were investigated as a first step in order to determine the phonon contribution to the thermal conductivity. We determined the Seebeck coefficient and electrical conductivity of the clathrate compound and the thermoelectric figure of merit. A glimpse into the dynamics of the system for the evaluation of the thermoelectric and electronic properties is presented.

Electrochemical reduction of CO2 by Cu hollow fibers to CO with high selectivity has previously been reported but selective conversion of CO2 to formic acid at high current densities, although highly desirable, is still challenging. Herein, a Cu hollow fiber with an interconnected pore structure is fabricated via a facile method and used as a stand-alone cathode for highly efficient electrochemical reduction of CO2 to formate. We obtain a high selectivity for CO2 reduction to formate with a maximum FE of 77.1% at a high current density of 34.7 mA cm−2, one of the highest FE on Cu-based materials. Our results suggest that delivering the CO2 gas into the inner space of the hollow fiber leads to a higher CO2 partial pressure in the pores due to the pressure drop across the wall of the Cu hollow fiber. As both the CO2 and H+ ions (from the electrolyte) compete for adsorption on the Cu hollow fiber active sites, the higher CO2 partial pressure makes CO2 adsorption more favorable, thereby reducing the concentration of the H+ on the active sites. This effectively suppresses the major competing reaction, hydrogen evolution reaction (HER), from 46.9% Faradaic efficiency (FE) to 15.0%. Furthermore, our studies reveals the impotency of the co-existence of Cu(100) and Cu(110) active site facets towards excellent selectivity of formate formation under high CO2 partial pressure. Additionally, the designed catalyst also exhibits out-standing long-term stability at high current density, demonstrating potential for large-scale practical applications.

Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is fundamentally different from skutterudites, clathrates as well as Na analogue. We have also tentatively studied the phonon band structure of this material by DFT lattice dynamics simulation, showing the relative contribution to phonons in the distinct layers of Ca3Co4O9.

We thank Mohamed N. Noui-Mehidi for fruitful discussions. The research reported in this publication was supported by Saudi Aramco under the grant agreement number RGC/3/3158-02. It was also supported by funding from King Abdullah University of Science and Technology (KAUST).