High-Entropy Layered Hydroxides Boost Water Splitting by Restructuring Reaction Pathways

Electrochemical water splitting holds immense potential for the scalable generation of green hydrogen, yet its overall efficiency remains fundamentally bottlenecked by the sluggish kinetics of the anodic oxygen evolution reaction. Traditional electrocatalysts engineered for this proton-coupled, four-electron transfer process are severely constrained by linear-scaling relationships among reaction intermediates, where the binding energies of hydroxyl, atomic oxygen, and hydroperoxyl species are mutually interdependent. Conventional bimetallic layered double hydroxides composed of positively charged brucite-like host layers and charge-balancing interlayer anions offer attractive chemical tunability and low material costs as alternatives to precious noble metal oxides like iridium and ruthenium dioxides. However, standard bimetallic layered hydroxides frequently suffer from poor intrinsic electrical conductivity, an insufficient density of active sites, and rapid operational degradation, including metal cation dissolution and phase segregation under harsh anodic oxidation potentials.

To overcome these structural and kinetic shortcomings, researchers have strategically integrated high configurational entropy into two-dimensional layered double hydroxide host matrices by uniformly incorporating multiple principal transition metals into a single-phase crystal lattice. Typically involving five or more distinct metallic species, high-entropy layered double hydroxides leverage high configurational entropy to stabilize thermodynamically metastable solid-solution phases that would otherwise undergo phase separation. These structural modifications drastically reconstruct the local coordination geometry and electronic band alignments. In particular, the synergistic multi-element environment systematically tunes the d-band center relative to the Fermi level, allowing independent modulation of the adsorption free energies of key oxygen-containing intermediates and accelerating interfacial charge-transfer dynamics across the electrode surface.

A critical mechanistic transformation highlighted in the survey is the deliberate transition from the conventional adsorbate evolution mechanism to the kinetically faster lattice oxygen mechanism. In the traditional adsorbate evolution pathway, the catalytic sequence proceeds across four consecutive proton-coupled electron-transfer steps at surface-confined metal centers, where intermediate adsorption energies scale linearly and constrain minimum overpotentials. By contrast, rationally designed high-entropy layered double hydroxides drastically amplify metal-oxygen orbital hybridization and covalency, especially when combined with structural defect engineering or heterostructure construction. The resulting electronic rearrangement promotes the direct chemical participation of lattice oxygen anions in oxygen-oxygen bond formation. As demonstrated by on-site Coulomb interaction modeling and projected density of states calculations, elevating the oxidation states of late transition metals expands the Hubbard energy splitting between lower and upper Hubbard bands, driving the lower Hubbard band downward and generating oxygen non-bonding states. Electrons can subsequently be extracted directly from these unhybridized oxygen non-bonding states rather than rigid metal-oxygen bonding states, bypassing conventional thermodynamic scaling constraints and substantially accelerating reaction kinetics.

Material synthesis for these multicomponent architectures requires specialized processing routes to ensure homogeneous compositional distribution and prevent premature phase segregation caused by disparate metal hydroxide solubility products. Synthetic protocols highlighted in the literature include multi-metal co-precipitation, hydrothermal crystallization directly on conductive substrates, electrodeposition, room-temperature chemical reduction with subsequent alkaline etching, and gentle ion-exchange routes. To prevent non-uniform precipitation during classic wet chemical syntheses, researchers have utilized zeolitic imidazolate frameworks as continuous, mild basic buffers, successfully synthesizing atomically thin monolayers of zinc-cobalt-iron-magnesium-nickel double hydroxides without creating excessive hydroxide concentration gradients.

Systematic experimental investigations illustrate the tangible catalytic enhancements achievable through entropy-driven multi-metal coordination. When evaluating a quinary iron-cobalt-nickel-manganese-aluminum layered hydroxide against its simpler trimetallic and quaternary analogues in one molar potassium hydroxide electrolyte, the high-entropy configuration delivered an overpotential of merely 242 millivolts at a current density of 100 milliamperes per square centimeter, accompanied by a small Tafel slope of 31.4 millivolts per decade. In another systematic study, constructing hollow polyhedral frameworks ranging from binary cobalt-nickel to quinary iron-cobalt-nickel-manganese-molybdenum compositions revealed that catalytic overpotentials at 10 milliamperes per square centimeter steadily decreased from 310 millivolts down to 203 millivolts, accompanied by a Tafel slope reduction to 53 millivolts per decade. Spectroscopic characterization and density functional theory demonstrated that iron and molybdenum withdrew electron density from catalytic nickel centers through cation-oxygen bridging interactions, while manganese functioned as an electron buffer against excessive depletion, thereby optimizing the formal oxidation state of nickel as a universal activity descriptor.

Environmental remediation has also been successfully coupled with catalyst engineering through the isomorphic substitution of calcium ions in calcium-iron layered precursors with wastewater-derived cobalt, nickel, copper, and zinc. High-resolution transmission electron microscopy and geometric phase analysis revealed significant localized lattice strain and reduced interplanar spacing along the crystallographic (024) plane due to the smaller ionic radii of the incorporated transition metals. Extended X-ray absorption fine structure and crystal orbital Hamilton population calculations showed that multi-metal substitution generated abundant metal vacancies, reduced metal-oxygen antibonding orbital occupation, and strengthened metal-oxygen covalency, with interaction values reaching 1.28 electron-volts for cobalt-oxygen, 1.10 electron-volts for nickel-oxygen, and 0.83 electron-volts for iron-oxygen pairs. Consequently, this synthesized calcium-cobalt-nickel-copper-zinc-iron catalyst lowered the operational overpotential to 310.7 millivolts at 10 milliamperes per square centimeter compared to 767.8 millivolts for the parent calcium-iron material, reduced the Tafel slope from 158.2 to 62.1 millivolts per decade, and maintained continuous stability for 250 hours.

Advanced hybrid configurations and heterostructures further suppress active metal dissolution during prolonged exposure to oxidative potentials. Incorporating atomically dispersed gold single atoms alongside oxygen vacancies into manganese-iron-cobalt-nickel-copper layered double hydroxides elevated the oxygen 2p band, weakened metal-oxygen bonds, and yielded a remarkably low overpotential of 213 millivolts at 10 milliamperes per square centimeter alongside an outstanding mass activity of 732.925 amperes per gram at 250 millivolts overpotential, operating stably beyond 700 hours at 100 milliamperes per square centimeter. Similarly, growing nickel-iron-cobalt-manganese-magnesium layers on manganese-based metal-organic frameworks intercalated organic benzene-1,3,5-tricarboxylate linkers into the hydroxide galleries, mitigating aggressive surface reconstruction and achieving 211 millivolts at 10 milliamperes per square centimeter with 100 hours of continuous durability.

Validating catalyst durability and operational kinetics under realistic industrial environments represents a crucial development milestone, particularly within anion exchange membrane water electrolyzers. When integrated into an anion exchange membrane water electrolyzer at 60 degrees Celsius, a defective nickel-chromium-cobalt-zinc-iron hydroxide on nickel foam achieved a cell voltage of 1.81 volts at one ampere per square centimeter, maintaining stable operation for more than 300 hours.

Elucidating these transient catalytic phenomena has required comprehensive operando and in situ characterization tools capable of monitoring live potential-driven surface reconstructions. In situ ambient-pressure X-ray photoelectron spectroscopy and X-ray absorption near-edge structure measurements confirm that nickel centers undergo dynamic oxidation from divalent to high-valent trivalent and tetravalent states, identifying nickel as the primary redox-active hub during electrocatalysis, while complementary iron-oxygen bond compression preserves overall crystalline integrity. In situ Raman spectroscopy reliably captures fingerprint metal-oxygen vibrational modes, identifying characteristic active oxyhydroxide phases at 476 and 558 reciprocal centimeters at noticeably lower onset potentials when surface oxygen vacancies are present. Direct experimental evidence for the lattice oxygen mechanism has been verified through operando differential electrochemical mass spectrometry utilizing isotopic oxygen-18 labeling. Detecting gaseous mixed-isotope oxygen molecules confirms that lattice oxygen atoms directly participate in oxygen-oxygen coupling. Simultaneously, attenuated total reflectance surface-enhanced infrared absorption spectroscopy tracking heterojunctions synthesized using supercritical carbon dioxide reveals distinct absorption bands at 1151 and 3439 reciprocal centimeters corresponding to surface-bound active oxygen intermediates and hydroxyl stretching vibrations.

Despite these encouraging performance metrics, navigating the vast compositional design space of high-entropy materials remains computationally and experimentally challenging. To expedite compositional screening without exhaustive trial-and-error synthesis, data-driven machine learning algorithms have been trained on multi-metal experimental datasets. Applying extreme gradient boosting regression modeling to experimental records from 70 five-metal compositions allowed researchers to evaluate 10,626 hypothetical candidate formulations with a coefficient of determination of 0.84 and a root-mean-square error of 9.95 millivolts. The algorithm successfully identified an optimal iron-cobalt-chromium-manganese-copper stoichiometry exhibiting a predicted overpotential of 261.3 millivolts, deviating by only 3 percent from the experimentally measured 270 millivolts while reducing screening expenditure by 99.3 percent. The review authors emphasize that future research must combine high-throughput machine learning, Hubbard-corrected electronic calculations, and operando spectroscopy to probe dynamic leaching mechanisms, optimize membrane electrode assemblies, and translate atomic-scale entropy engineering into scalable industrial electrolyzers.

Subject of Research: Electrocatalytic oxygen evolution reaction mechanisms and performance of high-entropy layered double hydroxides

Article Title: Recent progress in high-entropy layered double hydroxides for the oxygen evolution reaction

Article References: Jeon, H., Lee, J., Noh, W. J., Jeon, C., Lim, D., Park, D. H., Kwon, S., Piao, H., Lim, J., & Jin, X. (2026). Recent progress in high-entropy layered double hydroxides for the oxygen evolution reaction. Advances in Industrial and Engineering Chemistry, 2(1), Article 16. https://doi.org/10.1007/s44405-026-00057-w

Image Credits: AI Generated

DOI: 10.1007/s44405-026-00057-w

Keywords: Layered Double Hydroxides, Oxygen Evolution Reaction, High-Entropy Catalysts, Electrochemical Water Splitting, Lattice Oxygen Mechanism, Adsorbate Evolution Mechanism, Lattice Strain, d-Band Center, In Situ Spectroscopy, Operando Characterization, Anion Exchange Membrane Electrolyzers, Metal Leaching, Density Functional Theory, Electrochemical Reconstruction, Active Oxyhydroxide Phases, Supercritical Carbon Dioxide, Machine Learning Screening