Subsurface H₂ generation from banded iron formation iron oxides during CO₂ injection: H₂S Co-production and gas purity implications

Hydrogen is receiving increasing attention as a low-carbon energy carrier; however, conventional production routes are energy-intensive, motivating evaluation of alternative subsurface-relevant pathways. Here, we investigate hydrogen generation from BIF-relevant iron oxides (hematite and magnetite) exposed to CO₂-charged water under elevated temperature and pressure conditions representative of subsurface reservoirs. Batch experiments systematically varied temperature (25 – 120 °C), CO₂ pressure (subcritical to near-/supercritical), and solid-liquid ratio to quantify gas yield and speciation. Results show that CO₂-acidified fluids can activate Fe redox cycling and generate measurable H₂, but sulfur impurities can divert H₂ to H₂S, creating operational risks for gas purity and flow assurance. The experiments also indicate that carbonate mineralization did not co-occur detectably with gas generation over the tested time scale (no siderite detected by XRD), suggesting that CO₂ primarily functions as an acidifying/reactive medium under these conditions. Overall, the study identifies subsurface operating windows where Fe-oxide systems may contribute to stimulated hydrogen production while highlighting the need for sulfur screening and H₂S management for field deployment.