Design Criteria, Operating Conditions, and Nickel–Iron Hydroxide Catalyst Materials for Selective Seawater Electrolysis

TLDR

Seawater is abundant and its direct electrolysis avoids competition with freshwater use, but achieving oxygen selectivity is difficult due to competing chloride oxidation reactions. This study proposes a design criterion that identifies alkaline conditions as optimal for high oxygen evolution reaction selectivity. The criterion requires catalysts that sustain the desired operating current with an overpotential below 480 mV in alkaline pH to attain 100 % oxygen/hydrogen selectivity. NiFe layered double hydroxide satisfies this criterion at pH 13, exhibits a pH‑independent Tafel slope, shows chloride‑dependent changes in nickel reduction peak and high‑current activity and stability at pH 9.2, and achieves nearly 100 % Faradaic efficiency under the predicted conditions.

Abstract

Abstract Seawater is an abundant water resource on our planet and its direct electrolysis has the advantage that it would not compete with activities demanding fresh water. Oxygen selectivity is challenging when performing seawater electrolysis owing to competing chloride oxidation reactions. In this work we propose a design criterion based on thermodynamic and kinetic considerations that identifies alkaline conditions as preferable to obtain high selectivity for the oxygen evolution reaction. The criterion states that catalysts sustaining the desired operating current with an overpotential <480 mV in alkaline pH possess the best chance to achieve 100 % oxygen/hydrogen selectivity. NiFe layered double hydroxide is shown to satisfy this criterion at pH 13 in seawater‐mimicking electrolyte. The catalyst was synthesized by a solvothermal method and the activity, surface redox chemistry, and stability were tested electrochemically in alkaline and near‐neutral conditions (borate buffer at pH 9.2) and under both fresh seawater conditions. The Tafel slope at low current densities is not influenced by pH or presence of chloride. On the other hand, the addition of chloride ions has an influence in the temporal evolution of the nickel reduction peak and on both the activity and stability at high current densities at pH 9.2. Faradaic efficiency close to 100 % under the operating conditions predicted by our design criteria was proven using in situ electrochemical mass spectrometry.