An Overlooked Natural Hydrogen Evolution Pathway: Ni²⁺ Boosting H₂O Reduction by Fe(OH)₂ Oxidation during Low‐Temperature Serpentinization

Abstract

Natural hydrogen (H 2 ) has gained considerable attentions as a renewable energy resource to mitigate the globally increasing environmental concerns. Low-temperature serpentinization (< 200 8C) as a typical water-rock reaction is a major source of the natural H 2 . However, the reaction mechanism and the controlling step to product H 2 remained unclear, which hinders the further utilization of natural H 2 . Herein, we demonstrated that the H 2 production rate could be determined by the Fe(OH) 2 oxidation during low-temperature serpentinization. Moreover, the co-existence of Ni 2+ could largely enhance the H 2 production kinetics. With the addition of only 1 % Ni 2+ , the H 2 production rate was remarkably enhanced by about two orders of magnitude at 90 8C. D 2 O isotopic experiment and theoretical calculations revealed that the enhanced H 2 production kinetics could be attributed to the catalytic role of Ni 2+ to promote the reduction of H 2 O.

Water reduction on the facets of Fe(OH)2: an experimental and DFT study

“…In our previous study, we found that pure Fe(OH) 2 suspensions can indeed produce H 2 during heating (50-90 °C), but the yield is extremely limited. 18 At the same time, we also found that coprecipitated Ni 2+ can significantly improve the capacity of Fe(OH) 2 suspensions to produce H 2 , when the OH − content in the solution was inadequate, with the generation of magnetite from 50 °C to 90 °C. Then, by means of isotope experiments and the H 2 O-Fe(OH) 2 surface reaction according to eqn (4), we reasonably explained the promotion role of coprecipitated Ni 2+ and inhibition role of OH − on H 2 O reduction at the Fe(OH) 2 surface.…”

“…Fe(OH) 2 was crafted according to our previous work. 18 5 mL of 0.04 M FeCl 2 solution and 4.6 mL of deoxygenated water were added to a 50 mL quartz reactor, and then the reactor was immediately sealed. After air extraction and ventilation with argon (99.9999%) repeatedly 3 times, 0.4 mL 1 M NaOH was injected into the reactor using a micro syringe.…”

“…Although there are many contrary experimental results on whether Fe(OH) 2 can spontaneously convert into magnetite with the generation of H 2 , the discussion on excessive Fe 2+ facilitating this conversion has always attracted extensive attention. 4,[16][17][18] Because this is one of two important premises for Schikorr to observe the Fe(OH) 2 transformation. 19 However, it is still unclear why excessive Fe 2+ has the potential to promote the conversion of Fe(OH) 2 .…”

“…[11][12][13][14][15] However, ever since the experimental results discovered by Schikorr in 1929, whether Fe(OH) 2 can spontaneously transform into magnetite and hydrogen (eqn (1)) in the presence of excess ferrous ions at room temperature or higher temperature is unclear to date. 4,[16][17][18][19] The debates on the Schikorr reaction mainly focus on two aspects: i) whether this reaction can proceed rapidly; ii) if it could, what is the mechanism and the rate-controlling factors of this reaction?…”

“…[20][21][22] Recently, increasing attention has been attracted on this kind of natural H 2 because it can be used as a source of clean hydrogen energy to alleviate the environmental problems caused by the largescale use of fossil fuels. 18,23,24 Therefore, the debate on this aspect needs a clear conclusion for all the related fields. As for the second aspect, it is generally believed that the transformation of Fe(OH) 2 to H 2 and magnetite is a disproportionation process through eqn (2) and (3).…”