Selectivity of Electrochemical Ion Insertion into Manganese Dioxide Polymorphs.
ACS applied materials & interfaces
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 alpha-MnO2, and Li+ in lambda-MnO2), with all other ion insertion being metastable. We find Al3+ may insert into the delta, R, and lambda 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 alpha-MnO2 to achieve greater coordination by oxygen, while solvation energies and kinetic effects dictate water coinsertion in delta-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.
View details for DOI 10.1021/acsami.2c16589
View details for PubMedID 36546548
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.
View details for DOI 10.1039/d0nr07960j
View details for PubMedID 33367382
Air-Stability and Carrier Type in Conductive M3(Hexaaminobenzene)2, (M = Co, Ni, Cu).
Journal of the American Chemical Society
Herein, we investigate the effects of changing the metal ions in the M-HAB system, with HAB = hexaaminobenzene ligands and M = Co, Ni, Cu. The phyiscal characteristics of this MOF family are insensitive to changes in the metal cation, which enables systematic evaluation of the effect of metal cation identity on electrical transport properties. We observe that the metal ion profoundly influences the electrical conductivity and dominant carrier type in the resulting MOF and the air-stability thereof. Cu-HAB and Co-HAB are determined to exhibit n-type conduction under both ambient and nitrogen conditions; Ni-HAB is found to be ambipolar, with its dominant carrier type dramatically affected by the environment. We examine these results through calculation of the band structure, the partial density of states, and charge transfer analysis. Unlike traditional conductive organic materials, we find that the air-stability is not well predicted by the LUMO level of these n-type MOFs but instead is additionally dependent on the occupancy and orientation of the metal ion's d-orbitals and the resulting interaction between the metal ion and ligand. This study provides fundamental insights for rational design of air-stable, electronically conductive MOFs.
View details for DOI 10.1021/jacs.0c03500
View details for PubMedID 32475120
Colloidal synthesis of powder catalysts with well-defined nanoparticle ensembles
AMER CHEMICAL SOC. 2019
View details for Web of Science ID 000478860502221