Hydrogen is a promising substitute fuel to petrol and ethanol, especially for transport industry. Indeed, H2 fuel cells and generators are technologies producing energy with zero carbon emission, the sub-product being water. Emerging energy-based hydrogen systems have not received investment by governments and entrepreneurs because of the high financial and environmental cost of synthesizing hydrogen, polluting when the precursors are methane or ethanol. However, with the recent discoveries of natural hydrogen sources, the humankind can use hydrogen as carbon-free renewable energy with a drastically lower cost to exploit it. The community in the village of Bourakebougou in Mali [1] is the first to produce natural hydrogen though the PETROMA company.
However, the origin and formation of natural hydrogen on Earth remain poorly understood since its discovery. Recent researches indicated that there are natural hydrogen sources in Brazil [2] and it becomes crucial to better understand the formation of hydrogen and its storage in order to invest in green technologies based on hydrogen. Some controversial hypotheses have been formulated to explain the natural hydrogen formation, as primitive hydrogen in the centre of Earth, in mantle rocks [3,4] and/or in the terrestrial crust.
In this study, we will review the possible routes for H2 formation and we propose new chemical reactions based on simulation and geological data observations. The calculations are made using ab initio methods in VASP package [5], including chemical potential approaches to predict the direction of the reactions as a function of temperature and partial pressures. Among the reactions, we show the favourable reactions from iron oxide precursors:
α-Fe2O3 + 4 H2S → 2 FeS2 + 3 H2O + 2 H2 (1)
Fe3O4 + 6 H2S → 3 FeS2 + 4 H2O + 2 H2 (2)
From all the hypotheses, we highlight a clear link with the iron cycle, H2S, water and pyrite FeS2 formation. H2 synthesis involves oxido-reduction reactions and it can be also linked to the life cycle from its origin. Other gases have been identified both in ophiolitic deposits and continental crust such as CH4, N2, He. Therefore, knowing the geology of the crust and the chemical reactions is a useful indicator for the search of hydrogen.
A key ecological issue is to use natural hydrogen for fuel cell vehicles which will become in addition highly competitive in the transportation market.
[1] http://africa-me.com/hydrogen-power-in-mali/
[2] Alain Prinzhofer and Eric Deville. Hydrogène naturel. La prochaine révolution énergétique ?, Ed. Belin, 2015.
[3] Deville, E.; Prinzhofer, A. Chemical Geology 2016, 440, 139.
[4] Vacquand, C.; Prinzhofer, A.; Deville, E. Geochimica Et Cosmochimica Acta 2010, 74, A1068.
[5] G. Kresse and J. Furthmüller. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B, 54:11169, 1996.
Acknowledgment: The author would like to thank Hervé Toulhoat for fruitful discussions. CA thanks CNPq for post-doctoral fellowship at the UFRJ.
Keywords: renewable energy, zero carbon emission, pyrite, green chemistry
4b. Affordable and clean energy
