Single atom catalysts (SACs) maximize utilization of noble metal whereas nanoparticle catalysts leave behind inner noble metal molecules intact. In this study, palladium and platinum are supported on $C@C_3N_4$ to synthesize $C@C_3N_4$ – $Pd_1$ and $C@C_3N_4$ - $Pt_1$ catalysts. Single atomic structure is confirmed by high angle annular dark field scanning tunneling electron microscopy (HAADF-STEM) image, and extended X-ray absorption fine structure (EXAFS) analysis. Electrochemical tests including cyclic voltammetry (CV) in the range of 0.05 – 1.1 V (vs. RHE) and oxygen reduction reaction (ORR) are conducted. Lack of ensemble sites in SACs results in two-electron pathway for ORR and shows high $H_2O_2$ selectivity of up to 93% for $C@C_3N_4$ – 0.5%Pd. DFT calculation is adopted to verify the source for high $H_2O_2$ selectivity. Herein, electrochemical performance and contribution of SACs in hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), formic acid oxidation reaction (FAOR) and methanol oxidation reaction (MOR) are also reported. This comprehensive study is expected to give insight into single atomic nature and foster development of single atom electrocatalysts.