Highly Conductive Garnet-Type Electrolytes: Access to Li6.5La3Zr1.5Ta0.5O12 Prepared by Molten Salt and Solid-State Methods.

Tantalum-doped garnet (Li6.5La3Zr1.5Ta0.5O12, LLZTO) is a promising candidate to act as a solid electrolyte in all-solid-state batteries owing to both its high Li+ conductivity and its relatively high robustness against the Li metal. Synthesizing LLZTO using conventional solid-state reaction (SSR) requires, however, high calcination temperature (>1000 °C) and long milling steps, thereby increasing the processing time. Here, we report on a facile synthesis route to prepare LLZTO using a molten salt method (MSS) at lower reaction temperatures and shorter durations (900 °C, 5 h). Additionally, a thorough analysis on the properties, i.e., morphology, phase purity, and particle size distribution of the LLZTO powders, is presented. LLZTO pellets, either prepared by the MSS or the SSR method, that were sintered in a Pt crucible showed Li+ ion conductivities of up to 0.6 and 0.5 mS cm-1, respectively. The corresponding activation energy values are 0.37 and 0.38 eV, respectively. The relative densities of the samples reached values of approximately 96%. For comparison, LLZTO pellets sintered in alumina crucibles or with γ-Al2O3 as sintering aid revealed lower ionic conductivities and relative densities with abnormal grain growth. We attribute these observations to the formation of Al-rich phases near the grain boundary regions and to a lower Li content in the final garnet phase. The MSS method seems to be a highly attractive and an alternative synthetic approach to SSR route for the preparation of highly conducting LLZTO-type ceramics.

Highly Porous MIL-100(Fe) for the Hydrogen Evolution Reaction (HER) in Acidic and Basic Media.

The present study reports the synthesis of a porous Fe-based MOF named MIL-100(Fe) by a modified hydrothermal method without the HF process. The synthesis gave a high surface area with the specific surface area calculated to be 2551 m2 g-1 and a pore volume of 1.407 cm3 g-1 with an average pore size of 1.103 nm. The synthesized electrocatalyst having a high surface area is demonstrated as an excellent electrocatalyst for the hydrogen evolution reaction investigated in both acidic and alkaline media. As desired, the electrochemical results showed low Tafel slopes (53.59 and 56.65 mV dec-1), high exchange current densities (76.44 and 72.75 mA cm-2), low overpotentials (148.29 and 150.57 mV), and long-term stability in both media, respectively. The high activity is ascribed to the large surface area of the synthesized Fe-based metal-organic framework with porous nature.

Photoelectrochemical Solar Water Splitting: The Role of the Carbon Nanomaterials in Bismuth Vanadate Composite Photoanodes toward Efficient Charge Separation and Transport.

Photoelectrochemical performance of bismuth vanadate (BiVO4) photoanode is limited by poor charge separation and transport properties. The roles of carbon nanotube, reduced graphene oxide, or graphitic carbon nitride in BiVO4 composite photoanode were investigated toward enhancing light absorption and reducing overall impedance during photoelectrochemical water oxidation process. X-ray diffraction and Tauc analysis showed that BiVO4 retains its monoclinic phase, n-type semiconductor nature, and band gap in all carbon nanomaterials-incorporated composite photoanodes. It was observed that the carbon nanomaterials incorporation in BiVO4 film increases its surface porosity, ultimately leading to enhanced light absorption. The BiVO4 photoanode with reduced graphene oxide and graphitic carbon nitride showed same bulk charge separation efficiency, whereas the latter showed better charge transfer. It was found that the graphitic carbon nitride formed composite with BiVO4 to provide enhanced light absorption efficiency, i.e., 89% in 350-505 nm range. The BiVO4 with graphitic carbon nitride photoanode showed the best performance with a photocurrent of 2.2 mA cm-2, charge separation efficiency of 67%, and photocurrent of 4.0 mA cm-2 with cobalt-phosphate surface catalyst at 1.23 VRHE for water oxidation under 1 sun illumination. The Mott-Schottky and impedance measurements confirmed the shift of conduction band position toward hydrogen reduction potential and reduction in film resistance, respectively, with carbon nanomaterials addition, and the shift was most significant for graphitic carbon nitride. It is concluded that by concomitant formation of junction during photoanode fabrication between carbon nanomaterials, BiVO4, and fluorine-doped tin oxide glass substrate, better charge separation, transport, and light absorption can be achieved.

Enhanced Photoelectrochemical Water Splitting with Er- and W-Codoped Bismuth Vanadate with WO3 Heterojunction-Based Two-Dimensional Photoelectrode.

A novel two-dimensional (2D) heterojunction photoelectrode composed of WO3 and (Er,W):BiVO4 is proposed for water oxidation with efficient photoinduced charge carrier separation and transfer. Er stoichiometric along with W nonstoichiometric codoping was introduced to simultaneously manage vacancy creation during substitutional doping, enhance light absorption, and reduce overall impedance. It was found that Er3+ is substituted at the Bi3+ sites in the BiVO4 lattice to provide expanded light absorption from 400 to 680 nm. The fabricated WO3/(Er,W):BiVO4 electrode shows photocurrent densities of 4.1 and 7.2 mA cm-2 at 1.23 and 2.3 V (vs reversible hydrogen electrode, RHE), respectively, under a 1 sun illumination in K2HPO4 electrolyte. This electrode has shown remarkably high charge separation efficiency of 93% at 1.23 V (vs RHE). With the addition of a standard surface catalyst (i.e., Co-Pi), the WO3/(Er,W):BiVO4/Co-Pi electrode exhibits the highest photocurrent of 5.6 ± 0.3 mA cm-2 at 1.23 V (vs RHE), nearing the theoretical limit (i.e., 7.5 mA cm-2) while retaining 98% of the photoelectrochemical cell performance after 3 h. By concomitantly doping the Bi3+ and V5+ sites to enhance absorption, this study demonstrates for the first time a planar WO3/BiVO4 heterojunction that reaches 88% of the record-high performance of its nanostructured counterpart. Through a detailed characterization of the electrodes, it is concluded that the stoichiometric Er and nonstoichiometric W codoping extend light absorption region and improve charge separation efficiency by reducing bulk resistance. The photoactive materials with 2D morphology were synthesized using a facile ultrasonic spray-coating technique without any complex process steps and thus it can be scaled for commercial development.