Experimental assessment of H2 generation from Central Texas alkali basalts under CO2-coupled and high pH conditions

Igneous rocks are gaining increasing attention as valuable natural resources in the energy transition. Among them, ultramafic and mafic lithologies are attractive because of their carbon mineralization potential. Another emerging aspect is abiotic hydrogen (H₂) generation via the oxidation of reduced iron in the rock-forming minerals, a well-documented process for ultramafic systems known as serpentinization. However, the reaction pathway(s) for hydrogen generation from mafic rocks remain poorly understood. Compared to ultramafic rocks, mafic rocks have a more diverse mineralogy that may include Al- and alkali-bearing silicates, which may drive H₂ production in different reaction pathways. This study evaluates the H₂ generation potential of Late Cretaceous silica undersaturated alkali basalt from the Balcones Igneous Province in Central Texas under different pressurized gas conditions (CO₂ and Ar).

Static batch experiments were conducted to study rock-water-gas interactions and to assess the H₂ generation potential of the basalt. We placed millimeter-sized rock fragments in teflon-lined Hastelloy reactors at elevated pressure (12-17 bar) and temperature (90°C), using both Ar- and CO₂-saturated water. The effect of NiCl₂, a potential soluble reaction catalyst, was also tested. Mineralogical and chemical changes resulting from rock-water-gas interactions were analyzed using optical microscopy, X-ray powder diffraction, and scanning electron microscopy. Headspace gas composition was measured with gas chromatography, and pH, conductivity, and solution chemistry were monitored throughout the experiment.

After 133 reaction days, the highest H₂ yield was observed in the experiment with CO₂-rich fluids containing added NiCl₂. Comparable H₂ production occurred in the Ar experiment, while lower H₂ yield was observed in the experiment using CO₂ alone. The results indicate that the addition of NiCl₂ to CO₂-rich fluids enhances the H₂ generation. In addition to H₂ generation, carbonate mineral precipitation was observed in CO₂ experiments, further demonstrating concurrent carbon mineralization. The solution chemistry also reflects differences between settings: the CO₂ experiments exhibited lower pH and elevated dissolved elemental concentrations, whereas the Ar experiment maintained higher pH and resulted in less dissolution of the original rock matrix.

Collectively, these findings demonstrate that silica undersaturated mafic rocks, such as the abundant intrusive bodies of the Balcones Igneous Province, have significant potential for both geologic H₂ production and carbon sequestration.