Evaluation of the H2 Evolving Activity of Benzenehexathiolate Coordination Frameworks and the Effect of Film Thickness on H2 Production.

The design of earth-abundant catalysts for the electrochemical production of H2 from water is important for the realization of a sustainable energy future. Incorporation of molecular catalysts into extended frameworks has emerged as a viable strategy for improving catalytic performance and durability while maintaining a high degree of control over the structure and properties of the catalytic active site. Here, we investigate benzenehexathiolate (BHT) coordination frameworks as electrocatalysts for the hydrogen evolution reaction (HER) in pH 1.3 aqueous solutions. The electrocatalytic HER activity of BHT-based coordination frameworks follows the order of CoBHT > NiBHT > FeBHT. CoBHT operates at an overpotential of 185 mV, the lowest observed overpotential of the reported metal dithiolene-based metal organic frameworks and coordination polymers to date. To further understand the properties that dictate electrocatalytic activity, the effect of film thickness on the HER performance of CoBHT, a parameter that has not been extensively explored for electrocatalytic coordination frameworks, was examined. As the thickness was increased to ∼1 µm, charge and proton transfer through CoBHT was hindered, the number of electrochemically accessible active sites decreased, and the mechanical robustness of the modified electrode was diminished. The observed thickness-dependent HER activity of CoBHT highlights the importance of practical electrode construction and offers insight into how to optimize proton and electron transfer properties and active site densities within coordination frameworks without reducing the mechanical robustness of the immobilized catalysts.

Metallic Conductivity in a Two-Dimensional Cobalt Dithiolene Metal-Organic Framework.

Two-dimensional (2D) metal-organic frameworks (MOFs) have received a great deal of attention due to their relatively high charge carrier mobility and low resistivity. Here we report on the temperature-dependent charge transport properties of a 2D cobalt 2,3,6,7,10,11-triphenylenehexathiolate framework. Variable temperature resistivity studies reveal a transition from a semiconducting to a metallic phase with decreasing temperature, which is unprecedented in MOFs. We find this transition to be highly dependent on the film thickness and the amount of solvent trapped in the pores, with density functional theory calculations of the electronic-structure supporting the complex metallic conductivity of the material. These results identify the first experimentally observed MOF that exhibits band-like metallic conductivity.

H2 evolution by a cobalt selenolate electrocatalyst and related mechanistic studies.

[Co(bds)2][nBu4N] (where bds = 1,2-benzenediselenolate) was identified as an electrocatalyst for the hydrogen evolution reaction. Mechanistic studies indicated that a black precipitate, which formed upon treating [Co(bds)2]- with acid, as well as the one-electron reduced species, [Co(bds)2]2-, were viable catalytic intermediates. We propose two kinetically-competent pathways for H2 evolution: EC and CE (E = electrochemical, C = chemical step).

Two-dimensional metal-organic surfaces for efficient hydrogen evolution from water.

Hydrogen production through the reduction of water has emerged as an important strategy for the storage of renewable energy in chemical bonds. One attractive scenario for the construction of efficient devices for electrochemical splitting of water requires the attachment of stable and active hydrogen evolving catalysts to electrode surfaces, which remains a significant challenge. We demonstrate here the successful integration of cobalt dithiolene catalysts into a metal-organic surface to generate very active electrocatalytic cathode materials for hydrogen generation from water. These surfaces display high catalyst loadings and remarkable stability even under very acidic aqueous solutions.