pH Dependent Reversible Formation of a Binuclear Ni2 Metal-Center Within a Peptide Scaffold

Operando Elucidation of Electrocatalytic and Redox Mechanisms on a 2D Metal Organic Framework Catalyst for Efficient Electrosynthesis of Hydrogen Peroxide in Neutral Media

The practical electrosynthesis of hydrogen peroxide (H₂O₂) is hindered by the lack of inexpensive and efficient catalysts for the two-electron oxygen reduction reaction (2e^- ORR) in neutral electrolytes. Here, we show that Ni₃HAB₂ (HAB = hexaaminobenzene), a two-dimensional metal organic framework (MOF), is a selective and active 2e^- ORR catalyst in buffered neutral electrolytes with a linker-based redox feature that dynamically affects the ORR behaviors. Rotating ring-disk electrode measurements reveal that Ni₃HAB₂ has high selectivity for 2e^- ORR (>80% at 0.6 V vs RHE) but lower Faradaic efficiency due to this linker redox process. Operando X-ray absorption spectroscopy measurements reveal that under argon gas the charging of the organic linkers causes a dynamic Ni oxidation state, but in O₂-saturated conditions, the electronic and physical structures of Ni₃HAB₂ change little and oxygen-containing species strongly adsorb at potentials more cathodic than the reduction potential of the organic linker (E_redox ∼ 0.3 V vs RHE). We hypothesize that a primary 2e^- ORR mechanism occurs directly on the organic linkers (rather than the Ni) when E > E_redox, but when E < E_redox, H₂O₂ production can also occur through Ni-mediated linker discharge. By operating the bulk electrosynthesis at a low overpotential (0.4 V vs RHE), up to 662 ppm of H₂O₂ can be produced in a buffered neutral solution in an H-cell due to minimized strong adsorption of oxygenates. This work demonstrates the potential of conductive MOF catalysts for 2e^- ORR and the importance of understanding catalytic active sites under electrochemical operation.

Bioinorganic Chemistry of Nickel

Following the discovery of the first specific and essential role of nickel in biology in 1975 (the dinuclear active site of the enzyme urease) [...]