Enabling efficient decoupled alkaline water electrolysis using a low-cost sodium manganate solid-state redox mediator

The environmental pollution and energy crisis caused by the excessive consumption of fossil fuels have prompted a more urgent demand for the development of clean energy. As a carbon-free clean energy carrier, the sustainable production of hydrogen (H2) is a key step in the future decarbonization of the planet. Among many methods of hydrogen production, water electrolysis using renewable energy sources (e.g., solar, wind) can enable the generation of high-purity H2 in a low-carbon economy, which favors the alleviation of environmental and energy problems. In the past decades, water electrolysis has been dominated by alkaline water electrolysis, however, the oxidation and reduction reactions of water in conventional water electrolysis are closely coupled in space and time, which introduces challenges for the separation of gas products and the utilization of renewable energy. Herein, low-cost Na0.44MnO2 was chosen as solid-state redox mediator to decouple the evolution of H2 and O2 in alkaline electrolyte and obtain high-purity H2 without using any separator (membrane) or purification step. As an intermediate carrier for charge storage, the energy efficiency and decoupling efficiency can reach 98.7% and 97%, respectively, at constant current of 1.3 mA/cm2. Furthermore, we achieved flexible production of high-purity H2 by directly driving decoupled water electrolysis using intermittent renewable energy sources, confirming the potentially practical application of Na0.44MnO2.