ABSTRACT
Understanding the mechanisms of proton conduction at the interface of materials enables the development of a new generation of protonic ceramic conductors at low temperatures (<150 °C) through water absorption and proton transport on the surface and grain boundaries. Conductivity measurements under Ar-3% H2O and Ar-3% D2O revealed a σ(H2O)/σ(D2O) ratio of approximately 2, indicating a hopping-based mechanism for proton conduction at the interface. In situ Raman spectroscopy was performed on water-saturated, porous, and nanostructured TiO2 membranes to directly observe the isotope exchange reactions over the temperature range of 25 to 175 °C. The behavior of the isotope exchange reactions suggested a Grotthuss-type proton transport and faster isotope exchange reactions at 175 °C than that at 25 °C with a corresponding activation energy of 9 kJ mol-1. The quantitative and mechanistic kinetic description of the isotope exchange process via in situ Raman spectroscopy represents a significant advance toward understanding proton transport mechanisms and aids in the development of high-performance proton conductors with rapid surface exchange coefficients of importance to contemporary energy conversion and storage material development. In addition, new material systems are proposed, which combine interface and bulk effects at low temperatures (<150 °C), resulting in enhanced proton transport through interfacial engineering at the nanoscale.
ABSTRACT
For decades, the V2VI3 compounds, specifically p-type Bi2-xSbxTe3 and n-type Bi2Te3-xSex, have remained the cornerstone of commercial thermoelectric solid-state cooling and power generation near room temperature. However, a long-standing problem in V2VI3 thermoelectrics is that n-type Bi2Te3-xSex is inferior in performance to p-type Bi2-xSbxTe3 near room temperature, restricting the device efficiency. In this work, we developed high-performance n-type Bi2-xSbxTe3, a composition long thought to only make good p-type thermoelectrics, to replace the mainstream n-type Bi2Te3-xSex. The success arises from the synergy of the following mechanisms: (i) the donorlike effect, which produces excessive conduction electrons in Bi2Te3, is compensated by the antisite defects regulated by Sb alloying; (ii) the conduction band degeneracy increases from 2 for Bi2Te3 and Bi2Te3-xSex to 6 for Bi2-xSbxTe3, favoring high Seebeck coefficients; and (iii) the larger mass fluctuation yet smaller electronegativity difference and smaller atomic radius difference between Bi and Sb effectively suppresses the lattice thermal conductivity and retains decent carrier mobility. A state-of-the-art zT of 1.0 near room temperature was attained in hot deformed Bi1.5Sb0.5Te3, which is higher than those for most known n-type thermoelectric materials, including commercial Bi2Te3-xSex ingots and the popular Mg3Sb2. Technically, building both the n-leg and p-leg of a thermoelectric module using similar chemical compositions has key advantages in the mechanical strength and the durability of devices. These results attested to the promise of n-type Bi2-xSbxTe3 as a replacement of the mainstream n-type Bi2Te3-xSex near room temperature.
