Collective Effect of Fe and Se To Improve the Thermoelectric Performance of Unfilled p‑Type CoSb 3 Skutterudites Ruchi Bhardwaj, †,‡ Bhasker Gahtori,* ,†,‡ Kishor Kumar Johari, †,‡ Sivaiah Bathula, †,‡ Nagendra S. Chauhan, †,‡ Avinash Vishwakarma, †,‡ S. R. Dhakate, †,‡ Sushil Auluck, † and Ajay Dhar* ,‡ † CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India ‡ Academy of Scientific & Innovative Research (AcSIR), CSIR-NPL Campus, New Delhi 110012, India * S Supporting Information ABSTRACT: Filled skutterudites constitute an important class of efficient and stable thermoelectric materials for power generation; however, their commercialization has been hampered due to the usage of expensive rare-earth elements as “fillers” and the nonavailability of the efficient and compatible p-type counterpart. In view of this, we report a state-of-the-art thermoelectric figure of merit (ZT) in rare- earth-free p-type unfilled CoSb 3 skutterudite co-doped with Fe and Se, synthesized using a facile process of arc-melting and spark plasma sintering, which is both fast and scalable. The doping of Fe and Se have been chosen in accordance with the first-principles-based density functional theory (DFT) calculations which suggested that Fe leads to p-type conduction in CoSb 3 , while Se strengthens the thermoelectric properties. The experimental results also suggest that the optimized partial substitutional doping of Fe at the Co-site and Se at the Sb-site in CoSb 3 leads to a favorable tuning of the electrical and thermal transport properties, which resulted in a high ZT ∼ 0.7 at 870 K in an optimized skutterudite composition of Fe 0.25 Co 0.75 Sb 2.965 Se 0.035 , which is the highest value reported thus far for unfilled CoSb 3 -based p-type skutterudites. The resulting ZT of Fe 0.25 Co 0.75 Sb 2.965 Se 0.035 is higher by 2 orders of magnitude than that for its pristine counterpart. In addition, the theoretically estimated transport properties of pristine and doped CoSb 3 , calculated employing the density functional theory (DFT) and Boltzmann transport equations, were found to be in good qualitative agreement with those measured experimentally. KEYWORDS: thermoelectric, CoSb 3 , skutterudites, figure of merit, density functional theory, band structure, transport properties ■ INTRODUCTION Thermoelectric (TE) technology is a convenient means for the generation of clean energy as it enables solid-state interconversion of heat and electricity, based on the Seebeck effect. Among the various existing green energy generation technologies, power generation using thermoelectric technol- ogy offers high reliability with the advantages of high power density, wide scalability and motionless operability. 1 The conversion efficiency of a typical thermoelectric material is quantified in terms of its dimensionless figure of merit, ZT S T ZT / 2 σ κ = where S, σ, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. Thus, the performance of a TE device is strongly influenced by the thermal and electronic transport properties of the materials used in the device fabrication. Currently, the research efforts are aimed toward enhancing the ZT of the TE materials for achieving higher thermoelectric conversion efficiency and simultaneously reducing the material and processing costs, so that this technology can compete with other existing sources of clean energy. Several TE materials, such as tellurides, selenides, half- Heuslers, and other multi-elemental alloys, including LAST, TAGS, etc., 2-8 have been explored for harnessing the waste- heat in the mid-temperature regime. However, the majority of these TE materials, despite possessing a high ZT in the mid- temperature range, are either unstable at the operating temperature or consist of elements that are toxic or lack a compatible n/p-type counterpart, required for efficient thermoelectric power generation. In this context, skutterudites are promising TE materials owing to their high mobility and good Seebeck coefficient 9 along with high thermal stability and relative abundance of their constituent elements. 10 Its crystal structure was identified and explained by Oftedal: 11 it has a cubic structure, and there are 32 atoms and two voids in each unit cell, with eight cubes of Co occupying the 8c sites (1/4, 1/ Received: September 25, 2018 Accepted: January 8, 2019 Published: January 8, 2019 Article www.acsaem.org Cite This: ACS Appl. Energy Mater. 2019, 2, 1067-1076 © 2019 American Chemical Society 1067 DOI: 10.1021/acsaem.8b01609 ACS Appl. Energy Mater. 2019, 2, 1067-1076 Downloaded via COLUMBIA UNIV on October 22, 2019 at 06:15:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.