An official website of the United States government
Abstract Sub-Neptune exoplanets may have thick hydrogen envelopes and therefore develop a high-pressure interface between hydrogen and the underlying silicates/metals. Some sub-Neptunes may convert to super-Earths via massive gas loss. If hydrogen chemically reacts with oxides and metals at high pressures and temperatures (P−T), it could impact the structure and composition of the cores and atmospheres of sub-Neptunes and super-Earths. While H2gas is a strong reducing agent at low pressures, the behavior of hydrogen is unknown at theP−Texpected for sub-Neptunes’ interiors, where hydrogen is a dense supercritical fluid. Here we report experimental results of reactions between ferrous/ferric oxides and hydrogen at 20–40 GPa and 1000–4000 K utilizing the pulsed laser-heated diamond-anvil cell combined with synchrotron X-ray diffraction. Under these conditions, hydrogen spontaneously strips iron off the oxides, forming Fe-H alloys and releasing oxygen to the hydrogen medium. In a planetary context where this reaction may occur, the Fe-H alloy may sink to the metallic part of the core, while released oxygen may stabilize as water in the silicate layer, providing a mechanism to ingas hydrogen to the deep interiors of sub-Neptunes. Water produced from the redox reaction can also partition to the atmosphere of sub-Neptunes, which has important implications for understanding the composition of their atmospheres. In addition, super-Earths converted from sub-Neptunes may contain a large amount of hydrogen and water in their interiors (at least a few wt% H2O). This is distinct from smaller rocky planets, which were formed relatively dry (likely a few hundredths wt% H2O).
more »
« less
Superstoichiometric Alloying of H and Close‐Packed Fe‐Ni Metal Under High Pressures: Implications for Hydrogen Storage in Planetary Core
Abstract Although high pressure enables alloying between hydrogen and iron, hydrogen‐to‐iron molar ratio (H/Fe) so far found in experiments is mostly limited to 1 in the close‐packed iron metal under high pressure. We report a H/(Fe + Ni) ratio of 1.8 ± 0.1 from (Fe,Ni)Hx(orx ≥ 1.8) quenched from liquid, exceeding the amounts so far reported for densely packed Fe alloys. From the metastable behavior of the frozen (Fe,Ni)Hxliquid during decompression, we infer that the amount is a lower bound and therefore even a greater amount of H can be dissolved in the liquid part of Fe‐rich cores of planets. The significant H storage capacity of liquid Fe‐Ni alloy is important to consider for potential storage of H in the interiors of low‐density planets as well as rocky planets.
more »
« less
- PAR ID:
- 10400462
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 50
- Issue:
- 5
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Alkaline iron (Fe) batteries are attractive due to the high abundance, low cost, and multiple valent states of Fe but show limited columbic efficiency and storage capacity when forming electrochemically inert Fe3O4on discharging and parasitic H2on charging. Herein, sodium silicate is found to promote Fe(OH)2/FeOOH against Fe(OH)2/Fe3O4conversions. Electrochemical experiments,operandoX‐ray characterization, and atomistic simulations reveal that improved Fe(OH)2/FeOOH conversion originates from (i) strong interaction between sodium silicate and iron oxide and (ii) silicate‐induced strengthening of hydrogen‐bond networks in electrolytes that inhibits water transport. Furthermore, the silicate additive suppresses hydrogen evolution by impairing energetics of water dissociation and hydroxyl de‐sorption on iron surfaces. This new silicate‐assisted redox chemistry mitigates H2and Fe3O4formation, improving storage capacity (199 mAh g−1in half‐cells) and coulombic efficiency (94 % after 400 full‐cell cycles), paving a path to realizing green battery systems built from earth‐abundant materials.more » « less
-
Density of Fe‐Ni‐C Liquids at High Pressures and Implications for Liquid Cores of Earth and the MoonAbstract The presence of light elements in the metallic cores of the Earth, the Moon, and other rocky planetary bodies has been widely proposed. Carbon is among the top candidates in light of its high cosmic abundance, siderophile nature, and ubiquity in iron meteorites. It is, however, still controversial whether carbon‐rich core compositional models can account for the seismic velocity observations within the Earth and lunar cores. Here, we report the density and elasticity of Fe90Ni10‐3 wt.% C and Fe90Ni10‐5 wt.% C liquid alloys using synchrotron‐based X‐ray absorption experiments and first‐principles molecular dynamics simulations. Our results show that alloying of 3 wt.% and 5 wt.% C lowers the density of Fe90Ni10liquid by ∼2.9–3.1% at 2 GPa, and ∼3.4–3.6% at 9 GPa. More intriguingly, our experiments and simulations both demonstrate that the bulk moduli of the Fe‐Ni‐C liquids are similar to or slightly higher than those of Fe‐Ni liquids. Thus, the calculated compressional velocities (vp) of Fe‐Ni‐C liquids are higher than that of pure Fe‐Ni alloy, promoting carbon as a possible candidate to explain the elevatedvpin the Earth's outer core. However, the values and slopes of both density andvpof the studied two Fe‐Ni‐C liquids do not match the outer core seismic models, suggesting that carbon may not be the sole principal light element in Earth's outer core. The highvpof Fe‐Ni‐C liquids does not match the presumptivevpof the lunar outer core well, indicating that carbon is less likely to be its dominant light element.more » « less
-
Abstract Long duration energy storage (LDES) is an economically attractive approach to accelerating clean renewable energy deployment. The newly emerged solid oxide iron–air battery (SOIAB) is intrinsically suited for LDES applications due to its excellent low‐rate performance (high‐capacity with high efficiency) and use of low‐cost and sustainable materials. However, rechargeability and durability of SOIAB are critically limited by the slow kinetics in iron/iron‐oxide redox couples. Here the use of combined proton‐conducting BaZr0.4Ce0.4Y0.1Yb0.1O3(BZC4YYb) and reduction‐promoting catalyst Ir to address the kinetic issues, is reported. It is shown that, benefiting from the facilitated H+diffusion and boosted FeOx‐reduction kinetics, the battery operated under 550 °C, 50% Fe‐utilization and 0.2 C, exhibits a discharge specific energy density of 601.9 Wh kg–1‐Fe with a round‐trip efficiency (RTE) of 82.9% for 250 h of a cycle duration of 2.5 h. Under 500 °C, 50% Fe‐utilization and 0.2 C, the same battery exhibits 520 Wh kg–1‐Fe discharge energy density with an RTE of 61.8% for 500 h. This level of energy storage performance promises that SOIAB is a strong candidate for LDES applications.more » « less
-
Abstract The catalytic hydrothermal liquefaction of biomass under a hydrogen atmosphere is a promising technology to produce stable biocrude oil as a sustainable alternative to petroleum crude. A series of iron‐based non‐noble mix metal‐oxide‐on‐silica catalysts were evaluated to mimic the natural transformation that may have led to the conversion of terrestrial biomass to fossilized fuels. Switchgrass powder was liquefied to a stable bio‐oil with a 71.2% yield by using FeOx/SiO2catalyst in ethanol under a 5.5 MPa hydrogen atmosphere at 210 °C. The use of Fe‐MOx/SiO2(M = V, Mn, Co, Ni, Cu and Mo) type bimetallic oxide catalysts instead of FeOx/SiO2can produce improvements in liquefaction yields by using Mn, Co, Ni, and Cu as the second metal. The highest liquefaction yield of 78.8% was observed with the Fe‐CuOx/SiO2catalyst. Liquefaction oils were formed that were composed of complex mixtures of C6‐C12 alcohols, esters, aldehydes, and phenols. The lignin products:holocellulose products ratio changed in the range 0.35 to 0.15 and the composition of oils changed significantly with the use of mixed metal oxides in place of single metal FeOx/SiO2The most effective catalyst, Fe‐CuOx/SiO2could be reused in five cycles with a small loss in liquefaction yield from 78.8% to 70.0% after four reuse cycles and after regeneration of the catalyst at 500 °C for 3 h in air.more » « less
