Manganese and Cobalt in Olivine-Group Silicates Offer Potential for Natural H2 Generation

Abstract

As demand for hydrogen (H₂) rises to achieve decarbonization and sustainable energy goals, the need to explore natural H₂ sources is pressing. The potential for geologic H₂ generation via serpentinization has focused on fayalite (Fe^II₂SiO₄) and other iron-rich ultramafic minerals, but there are compelling reasons to think beyond iron. Here, we consider tephroite (Mn^II₂SiO₄) and Co-olivine (Co^II₂SiO₄), olivine-group minerals that contain redox-active transition metals, manganese, and cobalt. We detail mineral alteration reactions with respect to productive and nonproductive pathways for H₂ generation and discuss relevant thermodynamic and kinetic factors. The ligand-stabilization energy of Mn^II is low, leading to tephroite having rapid dissolution kinetics. This suggests the potential for more rapid H₂ generation compared to fayalite in conditions where Mn^II oxidation is thermodynamically favored. H₂ generation is maximized when geochemical conditions such as temperature, pH, and silica activity promote precipitation of secondary minerals ((hydr)oxides and silicates) that stabilize the metals in their oxidized forms. Tephroite serpentinization can generate many Mn^III-stabilizing secondary minerals and minerals that stabilize Mn^II have a significant likelihood of dissolving and remobilizing the reduced metal, unlike the serpentinization products of iron-rich olivine. For Mn and Co, more research is needed to understand the mineral transformation sequences and conditions that maximize H₂ generation, from both pure end members as well as solid solutions, as well as to address data gaps that limit predictions from reactive transport simulators. Thinking beyond iron may lead to favorable prospects for siting geologic H₂ generation operations, as Mn and Co are less abundant than Fe but are geographically ubiquitous.