Carbon-Free Energy Carriers and Circular Feedstocks: Bridging CO₂ Valorization with Hydrogen and Ammonia Economies

Direct electrification cannot reach shipping, aviation, high-temperature industry, or long-duration storage due to physical, not economic, constraints. Consequently, molecular energy carriers and circular carbon feedstocks are central to deep decarbonization strategies. The field, however, suffers from inconsistent terminology, often conflating carbon-free, carbon-neutral, and carbon-circulating systems. We address this through a strict four-class framework. Class I comprises carbon-free energy carriers (H₂, NH₃, H₂O₂, NaBH₄, NH₃BH₃, N₂H₄·H₂O). Class II includes carbon-neutral vectors—carbon-containing molecules derived from biogenic, atmospheric, or captured CO₂ under net-zero life-cycle conditions (CH₃OH, HCOOH, DME, OME₀–₅). Class III covers circular carbon feedstocks, including biomass, waste streams, and captured CO₂ as carbon inputs. Class IV is operational, describing end-use applications of any vector. Within this framework, H₂- and NH₃-based carriers are benchmarked against electrification and carbon-based pathways using thermodynamic, techno-economic, and life-cycle metrics. Ammonia emerges as a key hydrogen vector due to large-scale production, though deployment is limited by NO_x formation, cracking losses, and fuel-cell durability. Hybrid vectors (H₂/NH₃, H₂/NH₃/H₂O₂ blends) and carbon-neutral oxygenates (e-methanol, DME, OME) are transitional. TRL, MRL, and SIRL further differentiate unit feasibility from system deployability, enabling consistent comparison of carriers, clarifying efficiency, infrastructure, and life-cycle trade-offs, and supporting prioritization of scalable decarbonization pathways.