Cracking reactions play essential roles in pore development of activated carbon (AC) during activation of a biomass feedstock with K₂C₂O₄. Strengthening activity of activating agents for cracking might increase capability for creating pores. This was verified herein by conducting activation of poplar wood and its derived biochar by K₂C₂O₄ with Ni presence for enhancing activity for cracking. The results indicated that nickel species promoted cracking, decreasing substantially yield of AC by 41.4?% and abundance of organics in resulting bio-oils. Nonetheless, nickel presence remarkably enlarged pore volume (1.0 versus 0.7?cm³/g in control experiment without Ni) and average pore diameter (1.7 versus 1.0?nm), creating more mesopores (84.7?% versus 96.8?%) and macropores. Activation of poplar-derived biochar enhanced specific surface area (1167.4 versus 1033.4?m²/g) and also formation of mesopores (81.0?% versus 98.1?%) with nickel presence. Additionally, nickel also promoted dehydrogenation and deoxygenation, rendering AC of higher aromatic degree. However, some nickel species inserting skeleton of AC were difficult to be removed via acid washing, which blocked some pores. Nickel presence also led to more fragmented structure of AC in either activation of PW or the biochar, generating more larger pores with minimized steric hindrance for adsorption of large molecules like tetracycline.