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 (H 2 O 2 ) 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 Ni 3 HAB 2 (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 Ni 3 HAB 2 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 O 2 -saturated conditions, the electronic and physical structures of Ni 3 HAB 2 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 , H 2 O 2 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 H 2 O 2 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.