Synthetic approaches based on deep eutectic solvents allow the preparation of phophate-functionalized carbon monoliths with a hierarchical structure. Both functionalization and hierarchy—combining macropores and high surface areas while preserving high monolith densities—are critical for the ahievement of remarkable energy densities in supercapacitor cells. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.