A novel electrochemical approach to the CO2 reduction in Alkaline Hydrothermal Vent

More than a century has passed since the hypothesis was first proposed that primordial biological molecules could have formed from non-biological material with the input of some form of energy. Great efforts have been made to test possible energy sources in various environments and to determine whether the abiogenesis of biological molecules is possible. Among all the theories, the one involving hydrothermal vents has recently captured particular attention because it is based on the idea that the reduction of CO₂ and the initiation of a proto-metabolism could have occurred by exploiting a life-like thermodynamic disequilibrium on mineral structure that shows structural and compositional similarities with some catalytic centre of enzimes.^1–3

Hydrothermal Vent are geological formation generated from the upwelling of geothermal fluids into the ocean, there are two main types of HTV: black smokers (acidic ones) and white smokers (alkaline ones). In the Archean era Alkaline Hydrothermal Vent were generated by the reaction between alkaline (pH 10-11), warm and hydrogen rich fluids with the ocean rich in CO₂ (acidic 10-11) and metal ions such as Fe, Ni, Zn, Co, Mn; here at the mixing point a mineral barrier precipitate, composed mainly of iron oxide and hydroxide, green rust and iron sulphide. Across this mineral membrane an electrochemical potential difference is generated, because of the disparity in pH and redox species between the inner and outer sides of the vent, this thermodynamic disequilibrium can be dissipated by coupling two opposite reactions: CO₂ reduction and H₂ oxidation, the two semireaction take place on the opposite sides of the same mineral structure but in two different environments: the first acidic, the second alkaline. ⁴

Electrochemistry applied to the study of the behaviour of mineral materials from hydrothermal vents is a valuable tool because it allows for a precise investigation of the reactivity of material surfaces and correlates it with their electronic structure. ⁵

A hydrothermal vent system can be modelled as a short-circuited fuel cell, with a continuous flow of reactants to the electrodes. These electrodes are made of the material that forms the barrier and are located in two different environments: the first electrode functions as a cathode for the reduction of CO₂ in an acidic environment, while the second functions as an anode for the oxidation of hydrogen (or other molecules) in an alkaline environment. An electric current is recorded between the two short-circuited electrodes. This coupling of reactions can be represented in an Evans diagram, analogous to a corrosion process.

Figure 1. Evans diagram in various condition

Figure 2. Short circuited fuel cell model of AHTV

In our laboratory, we developed a technique for synthesizing Mackinawite (FeS_m) and Violarite (FeNi₂S₄). The samples have been characterized using spectroscopic, microscopic, and electrochemical methods. Using these materials, we have prepared electrodes for testing. A series of electrolysis experiments have demonstrated that these materials can electrochemically reduce CO₂ at negative potentials as -1.2 V, producing formic acid, methanol, and carbon monoxide. The efficiency of this reaction decreases significantly when less extreme potentials are applied.

 

Figure 3. Production of formic acid and methanol during a potentiostatic electrolysis on Mackinawite or Violarite.

The behaviour of the electrodes was studied by recording Tafel plots (log(I) vs E) and creating an Evans diagram. This diagram illustrates the operational conditions of pH, catalytic material, and reaction environment under which it is possible to couple the CO₂ reduction and hydrogen oxidation reactions effectively. Subsequently, the short-circuited fuel cell was constructed, allowing for the measurement of the current flow (which is proportional to the reaction rate and indicates the cell's polarity) and the electric potential at which the coupled reactions occur.

The results indicate that once the short-circuited fuel cell is assembled, in the absence of reactants and without a pH difference between the two compartments, no current is registered, suggesting that no reaction is occurring. However, upon introducing the CO₂ and H₂ reactants into their respective compartments, a pH gradient (6.5 vs 8.8) is established. Under these conditions, a reaction current is observed, with its direction indicating reduction at the pole containing CO₂ and oxidation at the pole with H₂. The potential at which this coupling occurs, on the synthesized metal sulphides materials, is -0.03 V vs SHE (@ pH 6.5), a value too positive to promote the CO₂ reduction reaction. The limiting factor in this setup is the anodic reaction, so other conditions have been tested for improving the catalytic activity of the anode, changing electrolyte composition and pH, flux of reactant and even the composition of the electrode itself. For example, using platinum as the anode (a material known for its catalytic properties in reactions involving hydrogen), a coupling potential of -0.44 V is observed, a value within the range where the reduction reaction of CO₂ at the cathode can occur.

This approach to measurement and interpretation of Alkaline Hydrothermal Vent functioning represents, in our opinion, a groundbreaking development in the field of studies on this topic. The future challenge lies in identifying the optimal operational conditions that accurately simulate the real environment of an alkaline hydrothermal vent on the Archean ocean floor, capable of facilitating a spontaneous reduction reaction of CO₂.

References

1.Russell, M. J. Green rust: The simple organizing ‘seed’ of all life? Life vol. 8 Preprint at https://doi.org/10.3390/life8030035 (2018).

2. Branscomb, E. & Russell, M. J. Frankenstein or a Submarine Alkaline Vent: Who is Responsible for Abiogenesis?: Part 2: As life is now, so it must have been in the beginning. BioEssays vol. 40 Preprint at https://doi.org/10.1002/bies.201700182 (2018).

3. Russell, M. J., Nitschke, W. & Branscomb, E. The inevitable journey to being. Philosophical Transactions of the Royal Society B: Biological Sciences 368, (2013).

4. Hudson, R. et al. CO2 reduction driven by a pH gradient. Proc Natl Acad Sci U S A 117, 22873–22879 (2020).

5. Nitschke, W. et al. Aqueous electrochemistry: The toolbox for life’s emergence from redox disequilibria. Electrochemical Science Advances vol. 3 Preprint at https://doi.org/10.1002/elsa.202100192 (2023).