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Thermoelectric generators (TEGs) convert temperature differences into electrical power and are attractive among energy harvesting devices due to their autonomous and silent operation. While thermoelectric materials have undergone substantial improvements in material properties, a reliable and cost-effective fabrication method suitable for microgravity and space applications remains a challenge, particularly as commercial space flight and extended crewed space missions increase in frequency. This paper demonstrates the use of plasma-jet printing (PJP), a gravity-independent, electromagnetic field-assisted printing technology, to deposit colloidal thermoelectric nanoflakes with engineered nanopores onto flexible substrates at room temperature. We observe substantial improvements in material adhesion and flexibility with less than 2% and 11% variation in performance after 10 000 bending cycles over 25 mm and 8 mm radii of curvature, respectively, as compared to previously reported TE films. Our printed films demonstrate electrical conductivity of 2.5 × 10 3 S m −1 and a power factor of 70 μW m −1 K −2 at room temperature. To our knowledge, these are the first reported values of plasma-jet printed thermoelectric nanomaterial films. This advancement in plasma jet printing significantly promotes the development of nanoengineered 2D and layered materials not only for energy harvesting but also for the development of large-scale flexible electronics and sensors for both space and commercial applications. 

Thermoelectric generators are being used as a successful power sources for space applications since 1960's in radioisotope-thermoelectric generators (RTGs) to supply power to space systems in deep space. RTG’s are capable of directly converting heat energy to uninterrupted electric power with no moving parts involved. The ability of thermoelectric materials to convert heat energy to electrical energy is defined by a dimensionless value known as the thermoelectric figure of merit (ZT) 1. This value quantifies the maximum thermoelectric efficiency of a thermoelectric generator (TEG) and is calculated by ZT= S2σT/κ, where S, σ, T, and κ represent Seebeck coefficient, electrical conductivity, temperature, and thermal conductivity, respectively. Among all of the thermoelectric materials, Bi2Te3 and its alloys have been reported to have high ZT values for low temperature energy harvesting and are highly suitable for powering wearables and self-powering sensors2, 3. 

In this paper, ordered TiO 2 nanotubes were grown on a Ti substrate via electrochemical anodization and subsequently annealed at 450 °C for 4 h under various atmospheres to create different point defects. Oxygen-deficient environments such as Ar and N 2 were used to develop oxygen vacancies, while a water vapor (WV) atmosphere was used to generate titanium vacancies. Computational models by density functional theory predicted that the presence of oxygen vacancies would cause electronic conductivity to increase, while the presence of Ti vacancies could lead to decreased conductivity. The predictions were confirmed by two-point electrical conductivity measurements and Mott–Schottky analysis. Raman spectroscopy was also conducted to confirm the presence of defects. The annealed samples were then evaluated as anodes in lithium-ion batteries. The oxygen-deficient samples had an improvement in capacity by 10% and 25% for Ar- and N 2 -treated samples, respectively, while the WV-treated sample displayed a capacity increase of 24% compared to the stoichiometric control sample (annealed in O 2 ). Electrochemical impedance spectroscopy studies revealed that the WV-treated sample's increased capacity was a consequence of its higher Li diffusivity. The results suggest that balanced electrical and ionic conductivity in nanostructured metal oxide anodes can be tuned through defect generation using heat treatments in various atmospheres for improved electrochemical properties.