A powder electrocatalyst characterized by nitrogen-coordinated single Ni atoms embedded in graphene layers (referred to henceforth as "NiNC" catalyst) was synthesized by pyrolysis of a Ni-imidazolate precursors (Figure ). This catalyst will henceforth be referred to as "NiNC-IMI". A benchmark polyanilin-derived NiNC electrocatalyst, referred to as NiNC-PANI served as reference catalyst. To characterizes the chemical state, composition, morphology, porosity, and microstructure of the two catalysts, a number of analytical techniques (Supplementary Figure to 3), involving powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and N2 physisorption analysis were performed.