Linking Life Cycle and Integrated Assessment Modeling to Evaluate Technologies in an Evolving System Context: A...

Carbon-neutral hydrogen (H₂) can reduce emissions from hard-to-electrify sectors and contribute to a net-zero greenhouse gas economy by 2050. Power-to-hydrogen (PtH₂) technologies based on clean electricity can provide such H₂, yet their carbon intensities alone do not provide sufficient basis to judge their potential contribution to a sustainable and just energy transition. Introducing a prospective life cycle assessment framework to decipher the non-linear relationships between future technology and energy system dynamics over time, we showcase its relevance to inform research, development, demonstration, and deployment by comparing two PtH₂ technologies to steam methane reforming (SMR) across a series of environmental and resource-use metrics. We find that the system transitions in the power, cement, steel, and fuel sectors move impacts for both PtH₂ technologies to equal or lower levels by 2100 compared to 2020 per kg of H₂ except for metal depletion. The decarbonization of the United States power sector by 2035 allows PtH₂ to reach parity with SMR at 10 kg of CO_2e/kg H₂ between 2030 and 2050. Updated H₂ radiative forcing and leakage levels only marginally affect these results. Biomass carbon removal and storage power technologies enable carbon-negative H₂ after 2040 at about -15 kg of CO_2e/kg H₂. Still, both PtH₂ processes exhibit higher impacts across most other metrics, some of which are worsened by the decarbonization of the power sector. Observed increases in metal depletion and eco- and human toxicity levels can be reduced via PtH₂ energy and material use efficiency improvements, but the power sector decarbonization routes also warrant further review and cradle-to-grave assessments to show tradeoffs from a systems perspective.