Understanding hydrothermal vent electrochemical behaviour: a short-circuited fuel cell model

This work presents an electrochemical interpretation of Alkaline Hydrothermal Vents (HTVs), geological structures considered plausible sites for the origin of life. At these vents, a life-like thermodynamic disequilibrium is generated by the interaction of two fluids with contrasting characteristics across opposite surfaces of a mineral membrane. On the ocean-facing side, an acidic, CO₂-rich fluid prevails, while on the vent side, an alkaline, reducing hydrothermal fluid containing H₂ with minor concentrations of hydrogen sulphides (HS— > H2S) flows. The redox potential difference between these environments can drive the formation of organic molecules by coupling CO₂ reduction with H₂ oxidation at two different pHs. In this study, we validate the robustness of this model through electrochemical experiments, treating the system analogously to a short-circuited fuel cell. Using an H-type electrochemical cell, we place two electrodes in separate compartments and measure the open-circuit potential (OCP) under varying pH and solution compositions. This allows us to determine the cell potential (Uc) generated when a proton motive force—i.e., a pH gradient—is present across the membrane. Uc reflects the redox disequilibrium between the two fluids and is influenced not only by their bulk properties but also by specific interfacial interactions between the electrolytes and electrode materials. Because the mineral material is a reasonable electronic conductor, short-circuiting the electrodes equalizes their potentials (relative to an external reference electrode). The system thus transitions from a purely thermodynamic to a kinetic regime, allowing current to flow and redox reactions to proceed. An Evans diagram is a valuable tool to visualize this: the intersection of the anodic and cathodic curves defines the mixed potential (Em) and the corresponding reaction current (Ir). We use mackinawite as the model mineral, both for its electrochemical properties and its premetamorphic abundance in Archean HTVs. Two mackinawite electrodes were placed in the H-cell, with one exposed to an alkaline, H₂-rich solution and the other to an acidic, CO₂-rich solution, separated by a cation-exchange membrane. Upon short-circuiting, the resulting mixed potential (Em) was measured and found to lie within a range where CO₂ reduction is feasible on mackinawite. Moreover, spontaneous current generation was observed only under far-from-equilibrium conditions. The direction of current confirmed that reduction occurs on the acidic side and oxidation on the alkaline side, as theoretically predicted. These findings were then compared with an Evans-style diagram constructed from linear sweep voltammetry performed on individual electrodes.