The active site density, intrinsic activity, and supporting substrate of cobalt phosphide catalysts are vital to their performance in alkaline water electrolysis. In this work, a CoP/Co2P loaded on cellulose nanofiber-derived carbon aerogels (CP/CCAs) bifunctional electrocatalyst with a three-dimensional network and heterostructure is illustrated through sequential facile hydrothermal, freeze-drying, and phosphorylation processes. The three-dimensional network of carbon aerogels derived from cellulose nanofibers reveals a specific surface area of 183.41 m2/g, greatly enriching the active sites and facilitating the electron and mass transportation. Besides, interactions between CoP/Co2P with a heterogeneous structure and carbon aerogels modulate the electronic structure to enhance the intrinsic activity of the catalysts. Benefiting from these advantages, CP/CCAs-2 demonstrates a superior oxygen evolution reaction activity (η10 mA cm-2 = 277 mV) over the benchmark RuO2 and a moderate hydrogen evolution reaction performance (η10 mA cm-2 = 63 mV). Density functional theory calculations further reveal that the coupling of CoP/Co2P and cellulose-derived carbon aerogels promotes the water adsorption and activates the H-O bond. As a result, the alkaline electrolyzer assembled with CP/CCAs-2 both as a cathode and an anode shows a low cell voltage of 1.59 V at a current density of 10 mA cm-2 and good stability. This work provides a strategy for phosphide electrocatalysts composited with green carbon carriers for overall water splitting.