Controlling the Thermoelectric Behavior of La-Doped SrTiO₃ through Processing and Addition of Graphene Oxide

The addition of graphene has been reported as a potential route to enhance the thermoelectric performance of SrTiO3. However, the interplay between processing parameters and graphene addition complicates understanding this enhancement. Herein, we examine the effects of processing parameters and graphene addition on the thermoelectric performance of La-doped SrTiO3 (LSTO). Briefly, two types of graphene oxide (GO) at different oxidation degrees were used, while the LSTO pellets were densified under two conditions with different reducing strengths (with/without using oxygen-scavenging carbon powder bed muffling). Raman imaging of the LSTO green body and sintered pellets suggests that the added GO sacrificially reacts with the lattice oxygen, which creates more oxygen vacancies and improves electrical conductivity regardless of the processing conditions. The addition of mildly oxidized electrochemical GO (EGO) yields better performance than the conventional heavily oxidized chemical GO (CGO). Moreover, we found that muffling the green body with an oxygen-scavenging carbon powder bed during sintering is vital to achieving a single-crystal-like temperature dependence of electrical conductivity, implying that a highly reducing environment is critical for eliminating the grain boundary barriers. Combining 1.0 wt % EGO addition with a highly reducing environment leads to the highest electrical conductivity of 2395 S cm–1 and power factor of 2525μW m–1 K–2 at 300 K, with an improved average zT value across the operating temperature range of 300–867 K. STEM-EELS maps of the optimized sample show a pronounced depletion of Sr and evident deficiency of O and La at the grain boundary region. Theoretical modeling using a two-phase model implies that the addition of GO can effectively improve carrier mobility in the grain boundary phase. This work provides guidance for the development of high-performance thermoelectric ceramic oxides

Facile Co-Electrodeposition Method for High-Performance Supercapacitor Based on Reduced Graphene Oxide/Polypyrrole Composite Film

A facile co-electrodeposition method has been developed to fabricate reduced graphene oxide/polypyrrole (rGO/PPy) composite films, with sodium dodecyl benzene sulfonate as both a surfactant and supporting electrolyte in the precursor solution. The introduction of rGO into the PPy films forms porous structure and enhances the conductivity across the film, leading to superior electrochemical performance. By controlling the deposition time and rGO concentration, the highest area capacitance can reach 411 mF/cm² (0.2 mA/cm²) for rGO/PPy films, whereas optimized specific capacitance is as high as 361 F/g (0.2 mA/cm²). All of the composite films exhibit excellent rate capability (at least 175 F/g at the current density of 12 mA/cm²) compared with pure PPy film (only 12 F/g at the current density of 12 mA/cm²). The rGO/PPy composite exhibits excellent cycling stability that maintains 104% of its initial capacitance after cycling for 2000 cycles and 80% for 5000 cycles. The two-electrode solid-state supercapacitor (SC) based on rGO/PPy composite electrodes demonstrates good rate performance, excellent cycling stability, as well as a high area capacitance of 222 mF/cm². The solid-state planar SC based on the rGO/PPy composite exhibits an area capacitance of 9.4 mF/cm², demonstrating great potential for fabrication of microsupercapacitors

Supercapacitor Electrodes from the in Situ Reaction between Two-Dimensional Sheets of Black Phosphorus and Graphene Oxide

Two-dimensional materials show considerable promise as high surface area electrodes for energy-storage applications such as supercapacitors. A single sheet of graphene possesses a large specific surface area because of its atomically thin thickness. However, to package this area efficiently in a device, it must be confined within a finite three-dimensional volume without restacking of the sheet faces. Herein, we present a method of maintaining the high surface area through the use of a hybrid thin film in which few-layer-exfoliated black phosphorus (BP) reduces graphene oxide (GO) flakes. When the film is exposed to moisture, a redox reaction between the BP and the GO forms an interpenetrating network of reduced GO (RGO) and a liquid electrolyte of intermediate phosphorus acids H_<i>x</i>PO_<i>y</i>. The presence of the liquid H_<i>x</i>PO_<i>y</i> electrolyte in the RGO/H_<i>x</i>PO_<i>y</i> film stabilizes and preserves an open-channel structure enabling rapid ion diffusion, leading to an excellent charging rate capability (up to 500 mV s^–1 and retaining 62.3% of initial capacitance at a large current density of 50 A g^–1) when used as electrodes in supercapacitors

Two-Step Electrochemical Intercalation and Oxidation of Graphite for the Mass Production of Graphene Oxide

Conventional chemical oxidation routes for the production of graphene oxide (GO), such as the Hummers’ method, suffer from environmental and safety issues due to their use of hazardous and explosive chemicals. These issues are addressed by electrochemical oxidation methods, but such approaches typically have a low yield due to inhomogeneous oxidation. Herein we report a two-step electrochemical intercalation and oxidation approach to produce GO on the large laboratory scale (tens of grams) comprising (1) forming a stage 1 graphite intercalation compound (GIC) in concentrated sulfuric acid and (2) oxidizing and exfoliating the stage 1 GIC in an aqueous solution of 0.1 M ammonium sulfate. This two-step approach leads to GO with a high yield (>70 wt %), good quality (>90%, monolayer), and reasonable oxygen content (17.7 at. %). Moreover, the as-produced GO can be subsequently deeply reduced (3.2 at. % oxygen; C/O ratio 30.2) to yield highly conductive (54 600 S m^–1) reduced GO. Electrochemical capacitors based on the reduced GO showed an ultrahigh rate capability of up to 10 V s^–1 due to this high conductivity

Synergistic Effects of Surface Chemistry and Topologic Structure from Modified Microarc Oxidation Coatings on Ti Implants for Improving Osseointegration

Microarc oxidation (MAO) coating containing Ca, P, Si, and Na elements on a titanium (Ti) implant has been steam-hydrothermally treated and further mediated by post-heat treatment to overcome the compromised bone-implant integration. The bone regeneration, bone-implant contact, and biomechanical push-out force of the modified Ti implants are discussed thoroughly in this work. The best <i>in vivo</i> performances for the steam-hydrothermally treated one is attributed to the synergistic effects of surface chemistry and topologic structure. Through post-heat treatment, we can decouple the effects of surface chemistry and the nanoscale topologic structure easily. Attributed to the excellent <i>in vivo</i> performance of the surface-modified Ti implant, the steam-hydrothermal treatment could be a promising strategy to improve the osseointegration of the MAO coating covered Ti implant