AbstractWe designed a metal‐free synthesis of carbon nanofiber based on ketene chemistry using phosphorus pentoxide (P₂O₅) and vegetable oil. Based on the characterization of intermediates, P₂O₅‐oil reaction yielded most possibly alkylketenes, which polymerized into poly(ketene) with abundant enol groups. The enol groups further reacted with P₂O₅, forcing the poly(ketene) to assemble into a nano‐sized preassembly structure. Moderate heating transforms these structures into carbonaceaus nanofibers. This approach is applicable to other chemicals with similar structure to vegetable oil. The carbon nanofibers with P‐O‐C functionalization show relatively high graphitization degree and promising textural properties. The C‐O‐P environment accounts for 66 at % of the total P and creates a superior thermal stability. As a model application, a CDI system built of a carbon‐nanofiber‐based electrode countered by an activated carbon‐based electrode exhibited exceptional performance.