Thermoelectricity Generation and Electron-Magnon Scattering in a Natural Chalcopyrite Mineral from a Deep-Sea Hydrothermal Vent

Energy-Saving Pathways for Thermoelectric Nanomaterial Synthesis: Hydrothermal/Solvothermal, Microwave-Assisted, Solution-Based, and Powder Processing

The pillars of Green Chemistry necessitate the development of new chemical methodologies and processes that can benefit chemical synthesis in terms of energy efficiency, conservation of resources, product selectivity, operational simplicity and, crucially, health, safety, and environmental impact. Implementation of green principles whenever possible can spur the growth of benign scientific technologies by considering environmental, economical, and societal sustainability in parallel. These principles seem especially important in the context of the manufacture of materials for sustainable energy and environmental applications. In this review, the production of energy conversion materials is taken as an exemplar, by examining the recent growth in the energy-efficient synthesis of thermoelectric nanomaterials for use in devices for thermal energy harvesting. Specifically, "soft chemistry" techniques such as solution-based, solvothermal, microwave-assisted, and mechanochemical (ball-milling) methods as viable and sustainable alternatives to processes performed at high temperature and/or pressure are focused. How some of these new approaches are also considered to thermoelectric materials fabrication can influence the properties and performance of the nanomaterials so-produced and the prospects of developing such techniques further.

Flexible n-Type Abundant Chalcopyrite/PEDOT:PSS/Graphene Hybrid Film for Thermoelectric Device Utilizing Low-Grade Heat

Combining inorganic thermoelectric (TE) materials with conductive polymers is one promising strategy to develop flexible thermoelectric (FTE) films and devices. As most inorganic materials tried up until now in FTE composites are composed of scarce or toxic elements, and n-type FTE materials are particularly desired, we combined the abundant, inexpensive, nontoxic Zn-doped chalcopyrite (Cu_1-xZn_xFeS₂, x = 0.01, 0.02, 0.03) with a flexible electrical network constituted by poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) and graphene for n-type FTE films. Hybrid films from the custom design of binary Cu_1-xZn_xFeS₂/PEDOT:PSS to the optimum design of ternary Cu_0.98Zn_0.02FeS₂/PEDOT:PSS/graphene are characterized. Compared with the binary film, a 4-fold enhancement in electrical conductivity was observed in the ternary film, leading to a maximum power factor of ∼ 23.7 μW m^-1 K^-2. The optimum ternary film could preserve >80% of the electrical conductivity after 2000 bending cycles, exhibiting an exceptional flexibility due to the network constructed by PEDOT:PSS and graphene. A five-leg thermoelectric prototype made of optimum films generated a voltage of 4.8 mV with a ΔT of 13 °C. Such an evolution of an inexpensive chalcopyrite-based hybrid film with outstanding flexibility exhibits the potential for cost-sensitive FTE applications.

Mechanochemistry for Energy Materials: Impact of High-Energy Milling on Chemical, Electric and Thermal Transport Properties of Chalcopyrite CuFeS2 Nanoparticles

Chalcopyrite CuFeS2 , a semiconductor with applications in chemical sector and energy conversion engineering, was synthetized in a planetary mill from elemental precursors. The synthesis is environmentally friendly, waste-free and inexpensive. The synthesized nano-powders were characterized by XRD, SEM, EDX, BET and UV/Vis techniques, tests of chemical reactivity and, namely, thermoelectric performance of sintered ceramics followed. The crystallite size of ∼13 nm and the strain of ∼17 were calculated for CuFeS2 powders milled for 60, 120, 180 and 240 min, respectively. The evolution of characteristic band gaps, Eg, and the rate constant of leaching, k, of nano-powders are corroborated by the universal evolution of the parameter SBET /X (SBET -specific surface area, X-crystallinity) introduced for complex characterization of mechanochemically activated solids in various fields such as chemical engineering and/or energy conversion. The focus on non-doped semiconducting CuFeS2 enabled to assess the role of impurities, which critically and often negatively influence the thermoelectric properties.

Manipulating the Ge Vacancies and Ge Precipitates through Cr Doping for Realizing the High-Performance GeTe Thermoelectric Material

GeTe alloy is a promising medium-temperature thermoelectric material but with highly intrinsic hole carrier concentration by thermodynamics, making this system to be intrinsically off-stoichiometric with Ge vacancies and Ge precipitations. Generally, an intentional increase of formation energy of Ge vacancy by element substitution will lead to an effective dissolution of Ge precipitates for reduction in hole concentration. Here, an opposite direction of decreasing the formation energy of Ge vacancies is demonstrated by substituting Cr at Ge site. This strategy produces more but nearly homogenously distributed Ge precipitations and Ge vacancies, which provides enhanced phonon scattering and effectively reduces the lattice thermal conductivity. Furthermore, Cr atom carries one more electron than Ge and serves as an electron donor for decreasing the hole carrier concentrations. Further optimization incorporates the effect of Bi substitution for facilitating band convergence. A maximum figure of merit (ZT) of 2.0 at 600 K with average ZT of over 1.2 is achieved in the sample of Ge_0.92 Cr_0.03 Bi_0.05 Te, making it one of the best thermoelectric materials for medium-temperature application.

Spontaneous and Widespread Electricity Generation in Natural Deep-Sea Hydrothermal Fields

Deep-sea hydrothermal vents discharge abundant reductive energy into oxidative seawater. Herein, we demonstrated that in situ measurements of redox potentials on the surfaces of active hydrothermal mineral deposits were more negative than the surrounding seawater potential, driving electrical current generation. We also demonstrated that negative potentials in the surface of minerals were widespread in the hydrothermal fields, regardless of the proximity to hydrothermal fluid discharges. Lab experiments verified that the negative potential of the mineral surface was induced by a distant electron transfer from the hydrothermal fluid through the metallic and catalytic properties of minerals. These results indicate that electric current is spontaneously and widely generated in natural mineral deposits in deep-sea hydrothermal fields. Our discovery provides important insights into the microbial communities that are supported by extracellular electron transfer and the prebiotic chemical and metabolic evolution of the ocean hydrothermal systems.