Influence of Natural Seawater Electrolyte on CO2 Electroreduction on Transition Metal Phosphate Electrocatalysts and Suppression of Cathode Scaling with Buffering Action

Abstract

Abstract Carbon dioxide electroreduction (CO2 ER) provides a promising approach to produce renewable fuels; however, achieving selective formation of higher-value C1+ products is still a challenge. In this work, we explored whether untreated natural seawater can modify the electrochemical environment at the electrode–electrolyte interface and influence the product distribution, compared to the conventional synthetic electrolyte, on transition metal phosphate electrocatalysts. Metal phosphate catalysts (MnPO4·H2O, Co3(PO4)2, and Co3(PO4)2·8H2O) exhibit electrolyte-dependent reactivity, with NaHCO3-based electrolyte favoring C1 product (CH3OH) generation, while seawater-based electrolyte favors C–C coupling, leading to C2 product (C2H5OH) generation. Using literature-reported computational insights, it is proposed that seawater’s intrinsic metal ions promote C–C coupling by creating a local hydrophobic environment at the electrode interface. In situ Raman spectroscopy and kinetic analyses for HER support the formation of a local hydrophobic environment at the electrode interface. Additionally, the cathode-electrode scaling challenge in the seawater electrolyte is overcome by buffering with NaH2PO4, which suppresses precipitation. Interestingly, a change in seawater electrolyte’s pH from alkaline to acidic, by buffering action, does not change the C2 product selectivity. Furthermore, catalyst’s stability tests were also carried out. Moreover, HRTEM and SAED analyses of the spent catalyst demonstrated that the electrocatalysts underwent surface reconstruction. This work demonstrates electrolyte-driven selectivity control and establishes abundant natural seawater as a scalable, ion-rich medium for advancing CO2 conversion to C2 renewable fuels.