Amorphous (Fe)Ni-MOF-derived hollow (bi)metal/oxide@N-graphene polyhedron as effectively bifunctional catalysts in overall alkaline water splitting

The unique hollow nanostructures (Ni1-xFex-HP) were fabricated by the template-engaged pyrolysis from amorphous (bi)metal-organic-frameworks with tunable polyhedron-like morphology by various Fe content. Ni1-xFex-HP features as hollow polyhedron with mesoporous wall, consisting of dominant metal/alloy coupled with a small fraction of their oxides and few-layer N-graphene shell (FLNG). Thanks for the structural and compositional feature to strengthen the synergism of Ni, NiO and N-graphene on HER, as well as FeNi alloy, NiFe oxide, and N-graphene on OER, the optimal Ni-HP and Ni0.5Fe0.5-HP octahedron present good bifunctional properties for HER (overpotential of 215 mV to reach 10 mA cm−2, Tafel slope of 133 mV dec−1) and OER (overpotential of 280 mV to reach 10 mA cm−2, Tafel slope of 79 mV dec−1) on glass carbon electrode in 1.0 M alkaline solution. Moreover, the electrocatalytic performance can be further enhanced by directly coating on nickel foam (NF), where the overpotentials of Ni-HP/NF and Ni0.5Fe0.5-HP/NF is only 98 and 220 mV at current density of 10 mA cm−2 for HER and OER, respectively. The integrated asymmetrical two-electrode configuration (Ni0.5Fe0.5-HP/NF||Ni-HP/NF) displays low cell voltage of 1.57 V to reach 10 mA cm−2, accompanying with strong durability for overall alkaline water splitting. This work opens up an effectively time-saving strategy using aMOFs with well-defined structure instead of highly crystalline metal-organic frameworks (cMOFs) as precursor to design hollow and nanostructured hybrids for energy conversion.