TIMELESS QUESTIONS AND MODERN CHALLENGES: A JOURNEY FROM THE ORIGIN OF LIFE IN HYDROTHERMAL VENTS TO ELECTROCHEMICAL CO2 REDUCTION
The objective of the work presented here is to investigate the functioning of the alkaline hydrothermal vent from an electrochemical point of view in order to inspire new technologies for CO2 reduction. Oceanic hydrothermal vents (HTVs) are geological structures that played a key role 4 billion years ago in the formation of the very first organic molecules on our planet and, eventually, life itself. The minerals composing the vent structure, mainly iron (-nickel) sulphides and oxyhydroxide, separate two environments with opposite characteristics: on the ocean side an acidic environment containing CO2 and on the hydrothermal fluid side an alkaline environment containing H2. Across this mineral barrier an electrochemical potential difference is generated; the dissipation of this energy takes place by coupling the CO2 reduction (CO2R) and the hydrogen oxidation (HOR), thus generating the very first organic molecule on Earth. The behaviour of HTVs can be modelled as a “short-circuited fuel cell”: in this system, H2 and CO2 in the respective environment serve as fuels and the two electrodes, composed of mineral materials, are short-circuited. The coupling between CO2R and HOR is rendered thermodynamically favourable by the pH difference between the two environments. The reduction potentials of the involved redox couples shift as a function of pH; consequently, a reaction that would be non-spontaneous if both half-reactions occurred at the same pH, can become spontaneous (ΔG < 0) when the two processes take place under distinct pH conditions. First, two model materials, Mackinawite [FeSm] and Violarite [(Fe,Ni)3S4] have been prepared by homogeneous precipitation method, obtaining the materials in powder form. The samples have been widely characterized in terms of crystalline structure, particle dimensions, porosity, and surface oxidation state. A series of “mineral-based electrode” have been prepared, by depositing the powder on carbon substrate, and tested for CO2 reduction, showing the ability of these materials to convert CO2 into formic acid (FeSm at E < - 0.56 V vs SHE, (Fe,Ni)3S4 at E = -1.2 V), methanol (FeSm at E = -1.2 V vs SHE) and carbon monoxide ((Fe,Ni)3S4 at E = -1.2 V), although limited faradic efficiencies (between 0.02 and 0.05 %) These electrodes have been used in the short-circuited fuel cell configuration, however, due to surface oxidation occurring during the preparation stages, the electrochemical measurements of these materials have only limited significance. For this reason, a new synthesis method for iron sulphide was developed, based on electrochemically assisted precipitation. This approach enables the rapid and reproducible formation of a thin FeSm film within a few minutes, which can be tested immediately after synthesis in order to minimize surface alteration. The resulting material was characterized by Raman spectroscopy and X-ray absorption spectroscopy (XAS), both ex-situ and under operando conditions during CO2 reduction, confirming the nature of the deposit produced and showing that mackinawite can be converted into metallic iron at potential < - 0.7 V vs SHE. The open circuit potential of FeSm electrode have been measured in solutions with different composition, containing CO2 or H2 and at different pH; in this way a measurement of cell potential (Uc = 0.05 ± 0.01 V for ΔpH = 2.3 and 0.09 ± 0.02 V for ΔpH = 4.5) can be obtained, representing the electrochemical potential generated on the two surfaces of the mineral barrier in the real HTV system. Than the FeSm have been used for building an Evans diagram, containing the current-potential response for the oxidation in presence of H2 and the reduction in presence of CO2, the crossing point between the two curves indicates the mixing potential (Em) - i.e. the potential of the mineral barrier itself while performing the two reactions - and the current of reaction (Ir ) – i.e. the flux of electrons across the mineral barrier itself. The same parameters can also be measured by building a real short-circuited fuel cell by placing two FeSm electrodes in an H-type cell containing the respective solutions and short-circuiting them using a zero-resistance ammeter circuit. The results show that: the Em value is in a potential window where CO2R can occur (around -0.62/-0.65 V vs SHE) and that the current Ir correctly flows from the H2 side to the CO2 side (μA range); notably this is the first time that the spontaneous generation of electron flux is recorded across a simulated HTV mineral barrier. The particular interest of this investigation is in understanding the role of the mineral barrier itself, so using a Y-shape microfluidics system, three different mineral barriers have been precipitated: iron sulphide, iron hydroxide and magnesium hydroxide. The growth process of these structures has been observed by microscopy, showing a layered self-assembly morphology emerging from the interaction between the oceanic and hydrothermal simulant fluids. Using a specially designed microfluidics-microelectrochemistry device, it was possible to measure the potential of the micrometer-thick mineral barrier itself (called intrinsic potential Ei) while the structure precipitate. At the same time, electrochemical characterization of the precipitated material can be performed using cyclic voltammetry. A comprehensive investigation of the electrochemical behaviour of hydrothermal vent systems—particularly the role of redox potentials at the surface of the mineral barrier—extends beyond fundamental geochemical inquiry into the origin of life some four billion years ago. It opens the possibility of reinterpreting these natural systems as functional energy-conversion devices, in which geochemical gradients are harnessed to sustain chemically productive states far from equilibrium. The method developed for the preparation of metal sulphide thin films shows promise for applications in electronics and optics. Furthermore, these mineral materials, composed of Earth-abundant elements, may serve as cost-effective electrocatalysts for CO2 reduction. More broadly, the HTV-inspired configuration—where a single structure, the mineral barrier, simultaneously functions as an electronic conductor, ionic conductor, and electrocatalyst, and exploits a pH gradient to drive the coupling of CO2 reduction (CO2R) and hydrogen oxidation (HOR)—offers a conceptual framework that could be translated into industrial processes, such as the reverse water–gas shift reaction and Fischer–Tropsch synthesis.
electrochemistry, origin of life, fuel cell, CO2 reduction, green chemistry, emergence of life, hydrothermal vent, Settore CHEM-02/A - Chimica fisica
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