The utilization of bio-mass waste as valuable materials is of strategic importance to our future. Here, a simple and inexpensive metal-free catalyst was synthesised by employing the bio-waste reed as the single precursor for both carbon (C) and silicon (Si). Interestingly, after the treatment with NH3, the Si element in reed became very active for oxygen reduction reaction (ORR) in alkaline media. The degree of NH3-treatment has a big effect on its ORR performance. Impressively, the onset potential of the optimized sample reaches up to ~1.00 V (vs. reversible hydrogen electrode), the half-wave potential is 0.89 V, and it also shows longer term stability for ORR as compared to commercial Pt/C. Moreover, this reed waste derived Si-N-C metal-free catalyst exhibits excellent performance as an air cathode for a Zn-air battery device. The coexistence of Si and N is essential for the high ORR activity. DFT calculation corroborated this conclusion by indicating that the Si-N structure is essential for the high activity. These new findings will not only open an avenue for the rational design of highly active ORR electrocatalysts, but also symbolize an exciting area for sustainable and low-cost materials for advanced clean energy applications.