Porous yet densely packed metal–organic frameworks (MOFs) toward ultrastable oxygen evolution at practical current densities†

It is known that the increase of catalyst loadings usually leads to activity decay owing to the increased mass transport limitations. And most catalyst electrodes are thus restricted to small mass loadings (0.1–1 mg cm⁻²). However, there are exceptions. Here we have reported the confined growth of nickel, iron-metal–organic framework (NiFe-MOF) electrodes characteristic of porous yet densely packed architectures. The NiFe-MOF electrode has shown elevated activities for the catalyst loadings increasing from 1 to 10 mg cm⁻², and achieving excellent oxygen evolution at the practical levels of catalyst loading (∼10 mg cm⁻²). Further detailed study reveals the NiFe-MOF electrode is composed of self-assembled MOF nanoribbons in 3D honeycomb architecture on a nickel foam substrate. The electrode can afford hierarchical macro–micro-porosity that facilitates fast mass transport, in addition to high catalyst loadings for securing strong durability. Consequently, NiFe-MOF electrodes are optimized to deliver the best oxygen evolution activities ever reported for MOFs, characteristic of a low overpotential of 226 mV at 10 mA cm⁻², and a prolonged stability up to 666 h at 100 mA cm⁻² or 100 h at 500 mA cm⁻².