How Anions Shape Ni(OH)₂ Synthesis and Seawater Electrolysis Performance

A nickel hydroxide electrode designed to split seawater has delivered a striking combination of hydrogen- and oxygen-producing performance—but its activity fell sharply after only 12 hours, highlighting the formidable durability challenge facing direct seawater electrolysis. The catalyst, developed by Qiong Fu and Xiaoqiang Du, is made from anion-doped nickel hydroxide grown directly on a porous nickel-foam substrate. In laboratory electrochemical tests, the best-performing material required an overpotential of just 290 millivolts for the oxygen evolution reaction and 110 millivolts for the hydrogen evolution reaction at a current density of 10 milliamperes per square centimetre. Those figures place the material among promising candidates for bifunctional water-splitting electrodes, although the short-term stability result shows that strong initial activity is not enough to make the technology ready for real-world seawater systems.

The study focuses on a deceptively important chemical detail: the identity of negatively charged ions, or anions, present during the catalyst’s synthesis. The researchers systematically introduced fluoride, chloride and carbonate ions while preparing nickel hydroxide, Ni(OH)₂, through a one-step hydrothermal process. Hydrothermal synthesis uses a sealed, heated aqueous environment to promote the growth of crystalline or nanostructured materials under controlled conditions. Rather than producing a powder that must later be mixed with a binder and attached to an electrode, the team grew the catalyst directly on nickel foam. This self-supported arrangement can reduce electrical resistance, improve contact between the active material and the current collector, and expose more catalytic surface to the electrolyte. The resulting structures included a material described as Ni(OH)(CO₃)-Cl, in which carbonate- and chloride-related chemical environments were incorporated into or associated with the nickel hydroxide-based electrode.

Anions can influence a catalyst in several ways at once. During synthesis, they may alter how nickel-containing precursors nucleate and grow, changing particle size, porosity, thickness and the arrangement of crystal domains. They can also modify the electronic structure of nearby nickel atoms, affecting how strongly the surface binds reaction intermediates. In water electrolysis, these intermediates include adsorbed hydrogen-containing species during hydrogen evolution and oxygenated species such as hydroxyl, oxo and hydroperoxo groups during oxygen evolution. If the binding is too weak, molecules do not activate efficiently; if it is too strong, the products can become difficult to release. Anion doping is therefore being explored as a way to tune the catalyst’s “structure-performance-stability” relationship rather than treating the electrode as a chemically static material.

The researchers examined the products using several complementary techniques. Scanning electron microscopy provided information about surface morphology at the micrometre scale, revealing how the material developed across the three-dimensional nickel-foam framework. Transmission electron microscopy offered finer structural detail, including nanoscale features and crystallinity. X-ray photoelectron spectroscopy was used to probe the chemical states of elements at the surface, where electrochemical reactions actually occur. Together, these methods allowed the team to connect the choice of anion with changes in morphology, crystal structure and surface chemistry. That combination is crucial for interpreting electrocatalyst results: a lower voltage requirement may arise from a larger active surface area, faster charge transfer, altered adsorption energies, improved wetting, or several of these effects operating simultaneously.

The standout electrode was tested as a bifunctional catalyst, meaning that the same material was evaluated for both half-reactions needed to split water. At the cathode, the hydrogen evolution reaction reduces water to hydrogen, consuming electrons. In alkaline conditions, it can be represented broadly as 2H₂O + 2e⁻ → H₂ + 2OH⁻. At the anode, the oxygen evolution reaction oxidizes hydroxide or water to produce oxygen and releases electrons; in alkaline form, the overall reaction is commonly written as 4OH⁻ → O₂ + 2H₂O + 4e⁻. The two reactions proceed at different rates and involve multiple elementary steps, which is why an efficient overall electrolyzer needs catalysts capable of accelerating both. The Ni(OH)(CO₃)-Cl electrode showed particularly low overpotentials for each reaction at the reported test current.

Overpotential is the extra voltage required beyond the thermodynamic minimum to drive an electrochemical reaction at a useful rate. A lower overpotential generally indicates that less electrical energy is lost to reaction kinetics, although it does not by itself establish the total efficiency of a complete electrolyzer. The study also reported Tafel slopes of 110.41 millivolts per decade for oxygen evolution and 108.96 millivolts per decade for hydrogen evolution. A Tafel slope describes how rapidly the required potential changes as the reaction current increases on a logarithmic scale. It is often used to compare apparent reaction kinetics and infer possible rate limitations, but it depends on measurement conditions, electrode architecture and data analysis. The reported values therefore provide useful evidence of catalytic behaviour while leaving important questions about energy efficiency, gas separation, operating pressure and performance at industrial current densities unanswered.

The most consequential result emerged during the chronostatic potential stability test, in which the electrode was held under a sustained electrochemical operating condition for 12 hours. After that period, the researchers observed an obvious decline in activity. The finding matters because seawater is not simply dilute alkaline water. It contains chloride and other ions that can compete for surface sites, alter local pH and participate in unwanted side reactions. Under anodic oxygen-evolution conditions, chloride oxidation can generate chlorine-containing species, raising concerns about corrosion, selectivity and environmental safety. Nickel hydroxide may also undergo surface reconstruction during operation, changing into oxyhydroxide-like phases that can be catalytically active but structurally different from the as-synthesized material. The study did not establish the precise cause of the deterioration, but it identifies stability as a central obstacle rather than a minor engineering detail.

The work is consequently best understood as a mechanistic step toward seawater electrolysis, not as a demonstration of a finished hydrogen-production device. The authors argue that future experiments should examine the detailed mechanism of seawater splitting and the electrode’s resistance to chlorine-related corrosion in genuine seawater. Such tests will need to move beyond short laboratory measurements and include realistic salinity, impurities, flow conditions, larger current densities and extended operating times. Researchers will also need to determine whether the active surface changes during electrolysis, which anions remain present, whether nickel dissolves, and how effectively oxygen evolution can be separated from competing chloride oxidation. Even with those limitations, the study offers a potentially useful design principle: carefully selected anions can reshape nickel hydroxide during growth and produce an electrode with strong initial activity for both hydrogen and oxygen evolution. The challenge now is to preserve that performance long enough for the ocean to become a practical feedstock for renewable hydrogen.