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• 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 (E 1/2) of 0.78 V in ORR. Single-step phase transformation from amorphous to crystalline material avoiding drastic reaction condition is a novel approach in the synthetic and applied materials chemistry research. Both conjugated microporous polymers (CMPs) and graphene/graphene analogues are very demanding materials in the energy research. Depending upon the nature of the graphene material like amorphous, crystalline, functionalized, single-layered and multi-layered, its property and potential application can be largely tuned. In this perspective, 2D carbon allotropes like graphene and its analogues have attracted a huge attention due to their unique electrical, thermal, mechanical, and physico-chemical properties. On the other hand, CMPs are another class of porous nanomaterial synthesized through the polycondensation/coupling of reactive monomeric building units via bottom-up chemistry approach, where extended π-conjugation prevails in the entire organic framework. To the best of our knowledge, herein we first demonstrated a novel methodology for amorphous CMP to crystalline graphene analogue through single step chemical transformation avoiding drastic reaction conditions. For this purpose, we have synthesized a new pyrene-naphthalene based spherical amorphous conjugated microporous polymer, designated as pyrene-naphthalene CMP (PNC), which upon chemical treatment (oxidative C- C coupling reaction) transforms into a highly porous crystalline multilayer porous nanographene sheet-like material pyrene-naphthalene-graphene (PNG). The BET surface area has been drastically enhanced from 600 m2/g for the CMP to 1414 m2/g for this graphene analogue PNG. As analysed by scanning tunnelling spectroscopy (STS), the PNG material has showed atomic level graphitic images with almost zero bandgap, which suggested highly conductive nature for this crystalline graphene analogue. This material has been further explored in the electrochemical oxygen reduction reaction (ORR) by grafting silver nanoparticles (AgNPs) onto the interlayer nanosheets of PNG. The ORR results displayed an admirable half-wave potential (E 1/2) and onset potential of 0.78 V and 0.984 V, respectively. The electrocatalyst achieved a very high current density of −0.53 mA cm−2 under the basic electrolyte medium (0.1 M KOH). An efficient ORR catalyst is very demanding towards the fabrication of metal-air batteries as well as fuel cells, which are promising for clean and green energy production. [ABSTRACT FROM AUTHOR]