A novel chemical approach for synthesizing highly porous graphene analogue and its composite with Ag nanoparticles for efficient electrochemical oxygen reduction

•Novel synthesis of porous nanographene via chemical transformation of amorphous to crystalline phase.•Conjugated microporous polymers (CMPs) to high surface area graphene analogue via simple chemical treatment.•Porous nanographene displayed exceptionally high BET surface area of 1414 m2/g and zero bandgap.•Grafting silver nanoparticles (AgNPs) has been made in the interlayer nanosheets of PNG.•AgNPs-PNG displayed an admirable half wave potential (E1/2) of 0.78 V in ORR.