Enhanced hydrogen production with carbon storage by olivine alteration in CO ₂ -rich hydrothermal environments

Here we report a novel experimental study on simultaneous hydrogen (H ₂ ) production and CO ₂ storage. A high concentration of NaHCO ₃ (source of CO ₂ ) is utilized to promote H ₂ generation from olivine (Mg,Fe) ₂ SiO ₄ ) hydration, and then CO ₂ is reduced to formic acid (HCOOH) or sequestrated in magnesite (MgCO ₃ ) in a hydrothermal system. The effects of NaHCO ₃ concentration (0-1.0 mol/L) and initial pH (8-11) on H ₂ production and CO ₂ storage were experimentally lab-scale tested at 300 °C. Both reaction pathways and reaction rates changed with variation of the reaction conditions. Under CO ₂ -rich conditions, olivine consumption was promoted with the absence of Fe(II)-bearing brucite (Mg,Fe(OH) ₂ ), by which more Mg and Fe ions were released. Thus, the production of H ₂ and carbonation processes were significantly accelerated. The highest H ₂ generation rate reached 3.13 mmol/kg _olivine ·h, which is more than 15 times higher than previously reported. The HCOOH yield was 129.1 mmol/kg _olivine and magnesite ((Mg,Fe)CO ₃ ) generation reached a maximum of 19.2 wt% in 72 h. The enhancement of H ₂ production at lower pH is primarily attributed to the presence of HCO ₃ ^- , rather than the pH changes caused by NaHCO ₃ addition. The system proposed here has significant potential to be applied at the field-scale.