Sustainability of hydrogen manufacturing: a review

Abstract

Hydrogen is a highly versatile energy vector, and most importantly, its oxidation, which releases energy, is a green process with no associated emissions. Hence, it is considered a green alternative that can supply energy and simultaneously reduce global warming. This gas, however, does not occur naturally in sufficient quantities and needs to be synthesized using different resources. The two most feasible methods for producing H₂ are steam methane reforming and water splitting via electrolysis. Therefore, these two processes were reviewed first, and subsequently, a complete sustainability analysis was performed using currently available data. It has been found that input raw materials such as methane and water will be required in ‘gigatonne’ quantity every year. Although availability of water does not pose supply risk, methane production falls far short of the requirement and becomes a supply risk. The conversion of these into H₂ requires energy and results in the production of ‘Gt’ of CO₂. For example, the production of 1 Gt of H₂ using the steam methane reforming process requires ∼3.6 EJ of energy and releases ∼10 Gt of CO₂. In contrast, water splitting electrolysis requires ∼198 EJ of energy and releases anywhere from 102 Gt to 220 Gt of CO₂, depending on the electricity generation mix. Additionally, they have ecological impacts in the form of acidification, marine toxicity, particulate emissions and so on, which affect all life forms on the earth. This analysis clearly shows that complete transitioning to H₂-based energy supply is unsustainable and only a fraction of the energy needs can be supplemented.