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.

Colloidal nanocrystals allow investigating sintering phenomena in supported catalysts.

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 A x MnO2 (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.

The ion insertion redox chemistry of manganese oxides 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 non-protonic cations in interstitial sites. Despite many experimental reports targeting specific applications, a comprehensive understanding of ion insertion in Mn oxides remains elusive. In this work, we use density functional theory to study the electrochemistry of AxMnO2 (where A = H+, Li+, Na+, K+, Mg2+, Ca2+, Zn2+ & Al3+) in aqueous and non-aqueous 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 non-protonic cation insertion into MnO2 depends on the oxygen coordination environments inside a polymorph but that when H+ is present, the driving force to form hydroxyl bonds is generally stronger. 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 aqueous ion insertion relying on metastability. Al3+ insertion into the 𝛿, R, and λ polymorphs may enable the full 2-electron redox of MnO2 at high voltage, but electrolytes must be designed to impede formation of insoluble precipitates and facilitate ion desolvation. We also show that water co-insertion stabilizes small ions in 𝛼-MnO2, while solvation energies and kinetic effects dictate water insertion in 𝛿-MnO2. Taken together, these findings rationalize experimental reports of mixed ion insertion mechanisms in aqueous batteries and highlight promising design strategies for safe, high energy density electrochemical energy storage.

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 non-protonic 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 non-aqueous 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 non-protonic 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 multi-valent ions must be designed to impede formation of insoluble precipitates and facilitate cation desolvation. We also show that small ions co-insert with water in 𝛼-MnO2 to achieve greater coordination by oxygen, while solvation energies and kinetic effects dictate water co-insertion 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.