The combustion of fossil fuels for power production over the last 200 years has had a severe negative influence on the environment, air and water quality and human health. As human population continues to grow, a transition towards clean and renewable power sources is vital to curb the adverse effects of pollution and greenhouse gases. Due to the intermittent nature of renewable power sources, this important transition relies upon the ability to store excess energy when it is available and deliver it when it is not. Hydrogen production by water electrolysis (water splitting) is one of the most promising routes to renewable energy storage. Hydrogen is an energy-dense fuel that can be directly converted back to electricity via fuel cells, with water as the only by-product.

This research aims to identify and study routes and suitable materials for decoupled hydrogen production by water splitting. First, decoupled water splitting is demonstrated by integrating nickel (oxy)hydroxide electrodes that serve as redox mediators between the hydrogen and oxygen cells. In the hydrogen cell, hydroxide (OH^-) ions that are produced by the hydrogen evolution reaction are carried through the electrolytic medium to a nickel hydroxide electrode where they are consumed in the nickel hydroxide oxidation (charging) reaction whereupon it is converted to nickel oxyhydroxide. The electrons that are released in this oxidation reaction flow through a conductive wire to a nickel oxyhydroxide electrode in the oxygen cell, reducing (discharging) it to nickel hydroxide and releasing OH^- ions. These ions are then carried through the electrolyte in the second cell to the anode, where they are consumed in the oxygen evolution reaction. Electrons flow from the oxygen evolving anode to the hydrogen evolving cathode via an external circuit (under bias), thus closing the electric circuit. This separate-cell decoupled water splitting is shown to be cyclable, stable, and enables the production of pure hydrogen and oxygen without membranes.

Next, a prototype device for solar-driven photoelectrochemical water splitting with large (100 cm²) hematite photoanodes and separate hydrogen and oxygen cells is demonstrated. The design criteria, material selection and operation scheme for this benchtop prototype device are discussed, aiming towards large-scale application.

Finally, high-efficiency membrane-free water electrolysis is demonstrated with a nickel hydroxide anode in an electrochemical-chemical cycle. In the first (electrochemical) step of the cycle, hydrogen evolves at the cathode while the nickel hydroxide anode charges. Subsequently, in the second step, the charged anode undergoes controlled chemical self-discharge in a hot electrolytic solution, evolving oxygen and reverting to its original state. The low charging potential of the nickel hydroxide anode relative to the oxygen evolution reaction potential enables low-voltage and high-efficiency overall water splitting, while the temporal separation of the reactions enables membrane-free operation.

Further research is required to optimize the nickel hydroxide electrodes, to adapt their properties (e.g., surface area, electron and proton transport, stability, microstructure and composition) to these new applications. This would lead to improved efficiencies and material utilization, thus lowering the cost of the produced hydrogen and promoting the transformation towards renewable energies.