Electrical Properties of Hollandite-Type Ba1.33Ga2.67Ti5.33O16, K1.33Ga1.33Ti6.67O16, and K1.54Mg0.77Ti7.23O16

In Situ Generation of Pseudorutile Oxide as the Cathode for Direct Electrolysis of CO2

The conversion of CO2 into CO in high-temperature solid oxide electrolysis cells (SOECs) is an attractive route for the CO2 utilization using the intermittent renewables. The low-cost and highly catalytic cathode is important for the direct electrolysis of pure CO2. In this study, non-perovskite Fe0.5Mg0.25+0.5xTi0.25-0.5xNb1-xMoxO4 oxides (denoted as Mo-x when x is equal to 0, 0.1, and 0.2) are evaluated as the cathode of an SOEC for the direct electrolysis of CO2. Mo6+ doping converted the wolframite Mo-0 into an α-PbO2-type with cation disordering, while further doping to Mo-0.2 showed a wolframite with cation ordering again. The SOEC with Mo-0.2 as the cathode exhibits the best electrochemical performance for the direct electrolysis of CO2 as a large portion of the oxide converted into oxygen-deficient pseudorutile-type oxide with a nominal formula of M5O9 (M = cation). The pseudorutile, a crystallographic shear phase of rutile, can be obtained after 60 h of direct electrolysis in CO2 at a 1.3 V bias rather than a reduction under 5% H2. The SOEC with Mo-0.2 as the cathode imparted a stable current density of 0.45 A cm-2, which could be related to the production of pseudorutile decorated with nanoparticles of MoO2. These results show that molybdenum doping is an effective strategy for developing oxygen-deficient rutile (pseudorutile) for the electrolysis of CO2.

Flux Growth of Single-Crystalline Hollandite-Type Potassium Ferrotitanate Microrods From KCl Flux

Hollandite-type crystals have unique and interesting physical and chemical properties. Here, we report the flux growth of hollandite-type single-crystalline potassium ferrotitanate (KFTO) with faceted surface features from a KCl flux. We varied the flux growth conditions, including the kind of flux, holding temperature, and solute concentration for growing faceted crystallites. KCl was found to be the best flux to grow the single-crystalline KFTO particles, while heating at or above 900°C was needed to yield the KFTO single crystals. The crystal growth was only weakly dependent on the solute concentration. Next, we characterized the grown single crystals and discussed the manner of their growth from the KCl flux. TEM images with clear electron diffraction spots indicated that the KFTO crystals grew along the <001> direction to form microrods ~10 μm in size. DFT calculation results indicated that the surface energy of the (100) face is lower than that of the (001) face. Based on these characterization results, we proposed a possible growth mechanism of the KFTO crystals.