Hydrogen storage in porous media: Understanding and mitigating microbial risks for a sustainable future

World is presently facing major challenges in energy industry to reduce or replace greenhouse gases (CO2, CH4, NOx etc.) emissions to net zero in the coming decades while decreasing the reliance on traditional fossil fuels. In the current scenario, hydrogen (H2) appears to be an excellent alternative energy source to meet the criteria. Storage of H2 over a long period of time in large quantities poses a significant challenge for its widespread uses. In order to overcome this issue, the idea of using large-scale depleted underground hydrocarbon reservoirs, salt caverns, underground aquifers, etc. is considered to be a suitable option for underground hydrogen storage. However, underground storage of H2 for a longer time period comes up with a potentially high microbial activity where H2 can be utilized in the microbial metabolism and converted into undesirable gases like CH4 and H2S which in turn, damage the porous medium due to clogging, acid formation, and corrosion activity. This review paper explores the various scientific challenges during large-scale H2 storage in porous medium to facilitate the global H2 economy. Moreover, a detailed description of the effect of subsurface environment conditions such as pressure, temperature, pH, and salinity on microbes is included in this study. In addition to this, steps to counter microbial activity in stored H2 monitoring and mitigation approaches are also discussed in this article.