Dual function of phosphate buffer in untreated seawater electrolysis: boosting oxygen evolution reaction efficiency and inhibiting cathode scaling

Abstract

The transition to carbon-neutral energy systems has intensified interest in sustainable hydrogen production pathways. Among them, seawater electrolysis has emerged as a promising alternative to conventional freshwater-based electrolysis because of its abundant availability. However, the inherent chemical complexity of seawater, arising from the presence of chloride ions and alkaline-earth cations (Mg²⁺ and Ca²⁺), introduces severe challenges such as competing chlorine evolution and precipitation of insoluble hydroxides, which compromise the efficiency of the electrolysis. In this study, we present an electrolyte design strategy based on buffering in natural seawater, and investigate the effect of an H₂PO₄⁻ additive on the faradaic efficiency of the OER (FE_OER) and on cathodic scaling. The phosphate buffering medium effectively regulates the pH, shifts the reaction activity towards the OER with a decrease in FE of the chlorine evolution reaction (CER) and also mitigates precipitation at the cathode. Notably, this additive is also effective in enhancing OER activity even in a chloride ion free electrolyte, attributed to control of the local pH. Notably, the buffered electrolyte enhances the FE of the OER to that of the CER in natural seawater electrolyte (Bay of Bengal, Chennai) at both low (10 mA cm⁻²) and high (200 mA cm⁻²) current densities. Different concentrations of NaH₂PO₄ were examined from 0 to 3 M, and an FE_OER greater than 80% was observed at 2 M on the benchmark Ir/C catalyst. It is proposed that the combined effects of pH buffering and electrostatic repulsion between the negatively charged additive’s phosphate ions and the chloride ions can lead to a low FE of the CER. This work highlights the role of buffering in simultaneously addressing two main challenges in untreated seawater electrolysis, promoting efficient O₂ generation and suppressing scaling on the cathode, and underscores the potential of buffered seawater electrolysis as an efficient platform for green hydrogen production.