Toward a Distinct Classification for Engineered Above-Ground Geochemical Hydrogen Systems Proposing the Term "Cerulean Hydrogen"

Hydrogen production pathways are widely described using colour-based terminology, including green, grey, blue, white, and orange hydrogen. Although these designations lack formal international standardisation, they are broadly employed across policy, industry, and academic discourse to distinguish production methods according to feedstock origin, carbon management approach, and primary process mechanism. This paper identifies a taxonomic gap within the existing framework: the absence of a distinct classification for engineered, above-ground geochemical hydrogen systems operating through water–rock redox reactions within contained reactors, without fossil hydrocarbons as the hydrogen-bearing feedstock and without subsurface hydrogen extraction or in situ geologic stimulation. To address this omission, the term Cerulean Hydrogen is proposed as a colour designation for this pathway. A multi-axis comparative analysis indicates that such systems occupy a structurally distinct position relative to existing categories when evaluated across reaction mechanism, reaction domain, system location, subsurface hydrogen extraction or in situ stimulation, and fossil feedstock dependence. The proposed classification is structural and technology-neutral. It is independent of any specific implementation and makes no claim as to the performance, efficiency, environmental profile, or technical maturity of any system within the class.