Selectivity of Electrochemical Ion Insertion into Manganese Dioxide Polymorphs.

The ion insertion redox chemistry of manganese dioxide has diverse applications in energy storage, catalysis, and chemical separations. Unique properties derive from the assembly of Mn-O octahedra into polymorphic structures that can host protons and nonprotonic cations in interstitial sites. Despite many reports on individual ion-polymorph couples, much less is known about the selectivity of electrochemical ion insertion in MnO2. In this work, we use density functional theory to holistically compare the electrochemistry of AxMnO2 (where A = H+, Li+, Na+, K+, Mg2+, Ca2+, Zn2+, Al3+) in aqueous and nonaqueous electrolytes. We develop an efficient computational scheme demonstrating that Hubbard-U correction has a greater impact on calculating accurate redox energetics than choice of exchange-correlation functional. Using PBE+U, we find that for nonprotonic cations, ion selectivity depends on the oxygen coordination environments inside a polymorph. When H+ is present, however, the driving force to form hydroxyl bonds is usually stronger. In aqueous electrolytes, only three ion-polymorph pairs are thermodynamically stable within water's voltage stability window (Na+ and K+ in α-MnO2, and Li+ in λ-MnO2), with all other ion insertion being metastable. We find Al3+ may insert into the δ, R, and λ polymorphs across the full 2-electron redox of MnO2 at high voltage; however, electrolytes for multivalent ions must be designed to impede the formation of insoluble precipitates and facilitate cation desolvation. We also show that small ions coinsert with water in α-MnO2 to achieve greater coordination by oxygen, while solvation energies and kinetic effects dictate water coinsertion in δ-MnO2. Taken together, these findings explain reports of mixed ion insertion mechanisms in aqueous electrolytes and highlight promising design strategies for safe, high energy density electrochemical energy storage, desalination batteries, and electrocatalysts.

Size-controlled nanocrystals reveal spatial dependence and severity of nanoparticle coalescence and Ostwald ripening in sintering phenomena.

A major aim in the synthesis of nanomaterials is the development of stable materials for high-temperature applications. Although the thermal coarsening of small and active nanocrystals into less active aggregates is universal in material deactivation, the atomic mechanisms governing nanocrystal growth remain elusive. By utilizing colloidally synthesized Pd/SiO2 powder nanocomposites with controlled nanocrystal sizes and spatial arrangements, we unravel the competing contributions of particle coalescence and atomic ripening processes in nanocrystal growth. Through the study of size-controlled nanocrystals, we can uniquely identify the presence of either nanocrystal dimers or smaller nanoclusters, which indicate the relative contributions of these two processes. By controlling and tracking the nanocrystal density, we demonstrate the spatial dependence of nanocrystal coalescence and the spatial independence of Ostwald (atomic) ripening. Overall, we prove that the most significant loss of the nanocrystal surface area is due to high-temperature atomic ripening. This observation is in quantitative agreement with changes in the nanocrystal density produced by simulations of atomic exchange. Using well-defined colloidal materials, we extend our analysis to explain the unusual high-temperature stability of Au/SiO2 materials up to 800 °C.