Excess renewable energy can be utilized to produce hydrogen to overcome the intermittency of wind and solar power. However, a lack of hydrogen storage solutions continues to be a barrier to widescale implementation of a renewable energy-to-hydrogen conversion. Salt caverns are feasible geostorage solutions but are geographically restricted. There is, however, abundant pore space in sedimentary basins across all fifty states that can serve as storage targets for hydrogen, but the fundamental science needed to quantify storage potential, geochemical reactivity, and hydrogen transport is missing. This work helps develop a fundamental understanding of hydrogen reactivity, specifically at subsurface conditions with pyrite, and provides a first step to enable diversifying the suite of possible hydrogen geo-storage sites across the US.

In this paper, we investigate the computational modeling of hydrogen and pyrite reactions. The calculations are set at standard temperatures of 25C, and we explore a range of pressures relevant to underground storage. We first use density functional theory (DFT) to optimize the ground state of the reactants and reaction products. With the known ground-states, we use Quantum-mechanical (QM) methods to describe the chemical reactions and other processes, such as charge transfer. By progressively repeating the simulation runs at various pressures, we were able to develop the reaction envelop for hydrogen-pyrite interactions.

The modeling results provide a molecular picture of hydrogen-pyrite interactions. The dissociation of the hydrogen molecule in the presence of Fe atoms leads to the formation of H2-FeS2 transition state and subsequent formation of pyrrhotite along with HS- and H2S byproducts. More importantly, we can provide the reaction conditions (pressure and temperature envelope) under which the hydrogen-pyrite reactions occur. This study helps develop a fundamental understanding of hydrogen reactivity with natural geo-materials like pyrite and can therefore provide suitable conditions under which hydrogen leakage can occur because of pyrite interacting with pumped hydrogen.

The topic of carbon dioxide interactions in subsurface environments has garnered decades of attention, but these results cannot be directly applied to hydrogen, given its smaller diameter and high reactivity. This work provides a fundamental first-look at operational parameters (such as pressure and temperature) that govern hydrogen-pyrite reactions. The workflow described here can easily be generalized to more complex systems to provide screening criteria for geostorage target selection.

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