Manganese and Cobalt in Olivine-Group Silicates Offer Potential for Natural H 2 Generation

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 (FeII2SiO4) and other iron-rich ultramafic minerals, but there are compelling reasons to think beyond iron. Here, we consider tephroite (MnII2SiO4) and Co-olivine (CoII2SiO4), 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 MnII is low, leading to tephroite having rapid dissolution kinetics. This suggests the potential for more rapid H2 generation compared to fayalite in conditions where MnII 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 MnIII-stabilizing secondary minerals and minerals that stabilize MnII 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.