Comprehensive Review of Serpentinization and the Experimental Generation of Hydrogen

Abstract In the recent years, hydrogen energy has emerged as a compelling new source of energy. However, most hydrogen that is currently being produced is through steam-methane reforming, which is both energy- and carbon-intensive. Contrastingly, hydrogen that is generated naturally through geologic processes is considered to have a low carbon footprint while also being less expensive compared to other production methods. Although natural hydrogen occurrences have been reported around the world, little is understood of the mechanisms involved in its generation and accumulation in the subsurface. There are numerous recognized processes that can generate hydrogen naturally in the subsurface, including radiolysis of water, serpentinization, primordial formation in the Earth's mantle, thermal cracking of organic matter under extreme heat, and bacteria or magma degassing. In this study, we focus on the present approaches to experimentally generate hydrogen through serpentinization and understand the ideal conditions needed for natural hydrogen generation in the subsurface. Serpentinization is a promising mechanism for hydrogen generation as it occurs under favorable timescales and can be replicated in the lab. In this process, ultramafic rocks are oxidized by water into serpentinite, resulting in the reduction of water to hydrogen. Fifteen experimental studies were reviewed, and the resultant hydrogen generated was compared. Various types of apparatuses (e.g., autoclaves, gold tubes, and flow-through reactors) were assessed to understand serpentinization and quantify hydrogen generation from different rock types. Published experimental studies show that hydrogen generation is dependent on many factors including temperature, water-to-rock ratios, pH, salinity, and original mineral compositions of the rock. Therefore, conducting extensive serpentinization experiments can help predict the maximum generation potential for hydrogen in natural settings as well as understand the benefits and limitations to hydrogen generation from various rock types.