Effects of Particle Size on Mg2+ Ion Intercalation into λ-MnO2 Cathode Materials.

An emergent theme in mono- and multivalent ion batteries is to utilize nanoparticles (NPs) as electrode materials based on the phenomenological observations that their short ion diffusion length and large electrode-electrolyte interface can lead to improved ion insertion kinetics compared to their bulk counterparts. However, the understanding of how the NP size fundamentally relates to their electrochemical behaviors (e.g., charge storage mechanism, phase transition associated with ion insertion) is still primitive. Here, we employ spinel λ-MnO2 particles as a model cathode material, which have effective Mg2+ ion intercalation but with their size effect poorly understood to investigate their operating mechanism via a suite of electrochemical and structural characterizations. We prepare two differently sized samples, the small nanoscopic λ-MnO2 particles (81 ± 25 nm) and big micron-sized ones (814 ± 207 nm) via postsynthesis size-selection. Analysis of the charge storage mechanisms shows that the stored charge from Mg2+ ion intercalation dominates in both systems and is ∼10 times higher in small particles than that in the big ones. From both X-ray diffraction and atomic-resolution scanning transmission electron microscopy imaging, we reveal a fundamental difference in phase transition of the differently sized particles during Mg2+ ion intercalation: the small NPs undergo a solid-solution-like phase transition which minimizes lattice mismatch and energy penalty for accommodating new phases, whereas the big particles follow conventional multiphase transformation. We show that this pathway difference is related to the improved electrochemical performance (e.g., rate capability, cycling performance) of small particles over the big ones which provides important insights in encoding within the particle dimension, that is, the single-phase transition pathway in high-performance electrode materials for multivalent ion batteries.

A Porous Pyrochlore Y2 [Ru1.6 Y0.4 ]O7-δ Electrocatalyst for Enhanced Performance towards the Oxygen Evolution Reaction in Acidic Media.

A robust porous structure is often needed for practical applications in electrochemical devices, such as fuel cells, batteries, and electrolyzers. While templating approach is useful for the preparation of porous materials in general, it is not effective for the synthesis of oxide-based electrocatalysts owing to the chemical instability of disordered porous materials thus created. Now the synthesis of phase-pure porous yttrium ruthenate pyrochlore oxide using an unconventional porogen of perchloric acid is presented. The lattice oxygen defects are formed by the mixed-valence state of Ru4+/5+ through the partial substitution of Ru4+ with Y3+ cations, leading to the formation of mixed B-site Y2 [Ru1.6 Y0.4 ]O7-δ . This porous Y2 [Ru1.6 Y0.4 ]O7-δ electrocatalyst exhibits a turnover frequency (TOF) of 560 s-1 (at 1.5 V versus RHE) for the oxygen evolution reaction, which is two orders of magnitude higher than that of the RuO2 reference catalyst (5.41 s-1 ).

High-Performance Pyrochlore-Type Yttrium Ruthenate Electrocatalyst for Oxygen Evolution Reaction in Acidic Media.

Development of acid-stable electrocatalysts with low overpotential for oxygen evolution reaction (OER) is a major challenge to produce hydrogen directly from water. We report in this paper a pyrochlore yttrium ruthenate (Y2Ru2O7-δ) electrocatalyst that has significantly enhanced performance toward OER in acid media over the best-known catalysts, with an onset overpotential of 190 mV and high stability in 0.1 M perchloric acid solution. X-ray absorption near-edge structure (XANES) indicates Y2Ru2O7-δ electrocatalyst had a low valence state that favors the high OER activity. Density functional theory (DFT) calculation shows this pyrochlore has lower band center energy for the overlap between Ru 4d and O 2p orbitals and is therefore more stable Ru-O bond than RuO2, highlighting the effect of yttrium on the enhancement in stability. The Y2Ru2O7-δ pyrochlore is also free of expensive iridium metal and thus is a cost-effective candidate for practical applications.

Cobalt-Based Nonprecious Metal Catalysts Derived from Metal-Organic Frameworks for High-Rate Hydrogenation of Carbon Dioxide.

The development of cost-effective catalysts with both high activity and selectivity for carbon-oxygen bond activation is a major challenge and has important ramifications for making value-added chemicals from carbon dioxide (CO2). Herein, we present a one-step pyrolysis of metal organic frameworks that yields highly dispersed cobalt nanoparticles embedded in a carbon matrix which shows exceptional catalytic activity in the reverse water gas shift reaction. Incorporation of nitrogen into the carbon-based supports resulted in increased reaction activity and selectivity toward carbon monoxide (CO), likely because of the formation of a Mott-Schottky interface. At 300 °C and a high space velocity of 300 000 mL g-1 h-1, the catalyst exhibited a CO2 conversion rate of 122 µmolCO2 g-1 s-1, eight times higher than that of a reference Cu/ZnO/Al2O3 catalyst. Our experimental and computational results suggest that nitrogen-doping lowers the energy barrier for the formation of formate intermediates (CO2* + H* → COOH* + *), in addition to the redox mechanism (CO2* + * → CO* + O*). This enhancement is attributed to the efficient electron transfer at the cobalt-support interface, leading to higher hydrogenation activity and opening new avenues for the development of CO2 conversion technology.

Phenothiazinedioxide-conjugated sensitizers and a dual-TEMPO/iodide redox mediator for dye-sensitized solar cells.

Metal-free dyes containing a phenothiazinedioxide entity in the spacer were synthesized. The best conversion efficiency (7.47%) of the dye-sensitized solar cell (DSSC) by using new sensitizers with chenodeoxycholic acid as a co-adsorbent and the I(-) /I3 (-) electrolyte reached over 90% of that of the standard N719-based cell (8.10%). A new type of ionic liquid containing the nitroxide radical (N-O(.) ) and iodide was successfully synthesized and applied to the DSSCs. If the I(-) /I3 (-) electrolyte was replaced with a dual redox electrolyte, that is, a TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) derivative with a dangling imidazolium iodide entity, the cell exhibited a high open-circuit voltage of 0.85 V and a cell efficiency of 8.36%.

Ionic liquid with a dual-redox couple for efficient dye-sensitized solar cells.

A new type of ionic liquid that contains a nitroxide radical (N-O(.)) and iodide as two redox couples, JC-IL, has been successfully synthesized for high-performance dye-sensitized solar cells (DSSCs). Both of the redox couples exhibit distinct redox potentials and attractive electrochemical characteristics. The UV/Vis absorption spectra of JC-IL shows a low-intensity peak compared to the strong absorption of I2 in the wavelength region of 350-500 nm. The high open-circuit voltage of DSSCs with JC-IL is over 850 mV, which is approximately 150 mV higher than that of the DSSCs with a standard iodide electrolyte. The dramatic increase in the standard heterogeneous electron-transfer rate constant leads to an increase in the short-circuit current for JC-IL compared to that of 2,2,6,6-tetramethylpiperidin-N-oxyl (TEMPO). DSSCs with the JC-IL electrolyte show promising cell efficiencies if coupled with dyes CR147 (8.12%) or D149 (6.76%). The efficiencies of the DSSCs based on the JC-IL electrolyte are higher than those of DSSCs based on either TEMPO electrolyte or standard iodide electrolyte alone.