▎ 摘　　要

The simple, green, cost-effective and environmental friendly technique used by our south-Indian peoples to make their very favourite and world-famous meal i.e. "Idli" was adopted here to prepare the three-dimensional heteroatom-doped graphene foam. The graphene-Idli was first prepared in a domestic microwave oven using heteroatom-doped (boron, nitrogen, sulphur and phosphorous) graphene and rice flour and calcined to produce graphene foam which was used as a metal-free bifunctional catalyst for oxygen evolution (OER) and oxygen reduction (ORR) reactions. The performance of heteroatom-doped foams (nitrogen, sulphur, boron and phosphorous) was studied and it was found that sulphur-doped graphene foam (s-DGF) shows better performance as electrocatalyst, in terms of high BET surface area (499 m(2)/g), electrochemical surface area (0.271 cm(2)), high roughness factor (0.690) and high porosity. As a bifunctional electrocatalyst, it possess a lowest onset potential (0.29 V vs Ag/AgCl), highest current density (497 mA cm(-2)), lowest overpotential (128 mV) and lowest Tafel slope (35.71 mV/dec) for oxygen evolution reaction and an onset potential of (-0.02 V), current density (-125 mA cm(-2)) and Tafel slope (49.83 mV/dec) for oxygen reduction reaction. The obtained values are best among other reported metal-free electrocatalyst and superior to several metal-based electrocatalysts. The prepared foams were free from any methanol cross-over effect and show a good stability in all the aspect i.e. performance, storage etc. Being a bifunctional catalyst, the S-DGF was successfully implemented for the fabrication of rechargeable zinc-air battery also. This finding offers a straightforward environment-compatible route for synthesizing attractive bifunctional electrocatalyst material as promising candidates for the large-scale commercialization of regenerative fuel cells and rechargeable metal-air batteries. In addition, the proposed metal-free foam based catalyst are able to overcome the multiple disadvantages of the existing electrocatalyst like high cost, poor stability, low selectivity and detrimental environmental effects.