The study, published in Advanced Science, centers on zeolitic imidazolate framework-67, or ZIF-67, a cobalt-based metal-organic framework grown directly on nickel foam. Metal-organic frameworks are attractive starting points for catalysts because their metal sites are arranged with atomic regularity and their porosity is built in by design. Yet in their native form they make poor oxygen-evolving electrodes, because the insulating organic ligands that hold the framework together strangle electrical conductivity, and the high-temperature treatments usually used to convert them into active oxides tend to collapse the very pores that make them useful.

The Korean team, led by Wonyoung Lee with Seungwoo Han and Yongbeen Kim as co-first authors, sidestepped both problems with a cation exchange strategy. They immersed the ZIF-67 electrode in a vanadium precursor solution at room temperature. The incoming high-valence vanadium ions displaced the framework’s chemistry in a cascade: the organic ligands were stripped out, the cobalt ions reorganized into cobalt hydroxide, Co(OH)2, and the vanadium lodged itself into the new lattice. Thermogravimetric analysis confirmed the ligand removal was essentially complete, with less than 0.1 percent mass loss in the temperature range where ZIF-67’s organic components normally decompose.

The reconstruction also transformed the material’s texture. Electron microscopy showed that the plate-like morphology of the parent framework survived the exchange, but with partially peeled and concave surfaces that betrayed the internal rearrangement. Diffraction patterns and lattice fringes confirmed the emergence of crystalline Co(OH)2, while elemental mapping revealed cobalt, vanadium, and oxygen distributed uniformly throughout, with no vanadium-enriched domains or separate vanadium oxide phases. Most strikingly, the optimized electrode, dubbed V20-Co(OH)2, achieved a specific surface area of 6.87 square meters per gram, a 350 percent increase over the pristine ZIF-67’s 1.52 square meters per gram, with clear mesoporous characteristics opened up by the departing ligands.

The real magic, however, happened at the level of electrons and defects. X-ray photoelectron and X-ray absorption spectroscopy showed that vanadium acted as an electron acceptor, pushing cobalt into higher oxidation states as the vanadium content rose. Because the vanadium-oxygen and cobalt-oxygen bonds have different lengths, their coexistence distorted the local lattice and encouraged the formation of oxygen vacancies, atomic-scale holes in the oxygen sublattice. The V20 electrode showed the highest fraction of defect-associated oxygen in its spectra. Notably, vanadium itself stayed mostly in the +4 and +5 states across all compositions, suggesting it served as an electronic modulator and vacancy-forming agent rather than as an active redox center in its own right.

When the team tested the electrodes for oxygen evolution in 1.0 M potassium hydroxide, the results were dramatic. Pristine ZIF-67 needed overpotentials of 337 and 364 millivolts to reach current densities of 50 and 100 milliamperes per square centimeter. The V20-Co(OH)2 electrode delivered the same currents at just 268 and 293 millivolts, and its Tafel slope dropped from 135.9 to 73.3 millivolts per decade, indicating far faster intrinsic reaction kinetics. Electrochemical impedance measurements showed the lowest charge-transfer resistance of any sample, at 0.48 ohm square centimeters, and the electrode ran stably at 100 milliamperes per square centimeter for 150 hours with negligible degradation.

One of the study’s most interesting findings concerns what actually drives the improvement. The team compared the physical surface area measured by gas adsorption with the electrochemically accessible area measured through double-layer capacitance. While the physical surface areas of the vanadium-exchanged electrodes stayed roughly similar, the capacitive response climbed steadily with vanadium content. In other words, the gain in active sites was not a matter of more surface but of better surface: defect- and electronic-structure changes made the existing area more accessible and reactive under operating conditions. Electrodes with more oxygen vacancies also showed higher Co3+ fractions and faster charge transfer, tying the defect chemistry directly to performance.

The researchers then dug into the reaction mechanism itself. Oxygen evolution can proceed through the conventional adsorbate evolution mechanism, in which surface-bound intermediates carry the reaction, or through the lattice oxygen mechanism, in which oxygen atoms from the catalyst’s own lattice participate directly, bypassing a notoriously difficult step. Using tetramethylammonium cations as chemical probes that suppress lattice-oxygen-related intermediates, the team found that the V20 electrode’s current fell by 85.5 percent in the probe electrolyte, compared with only 49.4 percent for pristine ZIF-67, a strong sign of lattice oxygen involvement. Distributed relaxation time analysis and pH-dependent measurements reinforced the picture, with a proton reaction order of 0.82 for the optimized electrode versus 0.63 for the parent framework. In situ Raman spectroscopy captured the electrode transforming into a CoOOH-like active surface during operation, while post-stability analysis showed the oxygen-vacancy-rich environment survived the harsh conditions.

Crucially, the design principle proved portable. In supercapacitor tests, the V20 electrode delivered a specific capacitance of 1227.31 millifarads per square centimeter, more than four times the 275.62 millifarads per square centimeter of pristine ZIF-67, along with a fourfold gain in energy density. As the air electrode in a zinc-air battery, it pushed the maximum power density to 205.7 milliwatts per square centimeter, well above the 89.7 milliwatts per square centimeter of the ZIF-67-based cell, and achieved a specific capacity of 908 milliampere-hours per gram versus 733 for the parent material, all while cycling stably for 50 hours.

The broader lesson is that a single, mild, room-temperature synthesis step can accomplish what previously required separate, often harsh treatments: rebuilding a framework’s structure, tuning its electronic configuration, and seeding the defects that unlock faster reaction pathways. Because the approach avoids high-temperature processing and preserves the electrode’s macroscopic integrity on conductive nickel foam, the authors argue it offers a scalable platform not just for better oxygen catalysts but for a whole family of energy storage and conversion electrodes. If green hydrogen is to become cheap, breakthroughs like this one, which squeeze more performance out of earth-abundant cobalt and vanadium rather than precious metals, will be an essential part of the recipe.

Subject of Research: Vanadium cation exchange reconstruction of MOF-derived cobalt hydroxide electrodes for oxygen evolution electrocatalysis and energy storage

Article Title: Vanadium Cation Exchange‐Driven Reconstruction of MOF‐Derived Cobalt Hydroxide Electrodes for Electrocatalysis and Energy Storage

Article References: Kim, Y., Han, S., & Lee, W. (2026). Vanadium Cation Exchange‐Driven Reconstruction of MOF‐Derived Cobalt Hydroxide Electrodes for Electrocatalysis and Energy Storage. Advanced Science, 13(56), Article e76610. https://doi.org/10.1002/advs.76610

Image Credits: AI Generated

DOI: 10.1002/advs.76610

Keywords: oxygen evolution reaction, electrocatalysis, metal-organic frameworks, ZIF-67, cobalt hydroxide, vanadium doping, oxygen vacancies, lattice oxygen mechanism, water splitting, supercapacitors, zinc-air batteries, defect engineering

The thermodynamics of water splitting look deceptively simple. Whether the electrolyte is acidic or alkaline, the minimum cell voltage needed to drive the reaction is 1.23 volts. In practice, electrolyzers demand considerably more, because the OER is a four-electron, multi-step process riddled with energy barriers, whereas the hydrogen evolution reaction at the cathode is a comparatively easy two-electron affair. That extra voltage, the overpotential, is pure waste heat and lost efficiency. In acidic systems, proton exchange membranes deliver low resistance and high gas purity, but the corrosive environment forces the use of scarce, expensive iridium and ruthenium oxides at the anode. Alkaline electrolysis, by contrast, opens the door to first-row transition metals that cost a fraction as much and can match, or even exceed, the performance of precious metal systems.

Understanding why the OER is so sluggish begins with its mechanism. The dominant framework, the adsorbate evolution mechanism, describes a sequence in which hydroxide species adsorb onto metal sites, convert into oxygen intermediates, and ultimately release molecular oxygen. Each step carries a Gibbs free energy cost, and the largest one sets the rate-determining step and thus the overpotential. The problem is that the adsorption energies of the intermediates are not independent. Because the *OOH and *OH species bind to the surface through a single oxygen atom, their energies scale linearly across virtually all oxide surfaces. The difference between the oxygen and hydroxide binding energies, roughly 3.2 electron volts on average, cannot be compressed below about 1.6 electron volts, while an ideal catalyst would need only 1.23. That mismatch imposes a theoretical overpotential floor of roughly 0.37 electron volts, and it turns catalyst design into a game of climbing a volcano-shaped activity plot, where the sweet spot lies at neither too strong nor too weak oxygen binding.

Some catalysts, however, refuse to obey the volcano. Their activity exceeds what the scaling relationship permits, and their kinetics shift with electrolyte pH in ways the adsorbate mechanism cannot explain. The resolution is the lattice oxygen mechanism, in which oxygen atoms belonging to the catalyst’s own crystal lattice participate directly in forming the O–O bond, bypassing the hydroperoxide intermediate entirely. The evidence is compelling: when researchers label lattice oxygen with the isotope oxygen-18 and run the reaction in an oxygen-16 electrolyte, differential electrochemical mass spectrometry detects mixed isotopic oxygen molecules, proof that lattice oxygen is being released as product gas. The mechanism is favored when the metal–oxygen bond is highly covalent, which is governed by the position of the oxygen 2p band relative to the metal 3d band. Raising the oxygen p-band center toward the Fermi level activates oxygen redox chemistry, promotes vacancy formation, and unlocks the lattice oxygen pathway, offering a route around the scaling-relation ceiling.

Armed with these mechanistic insights, catalyst designers have developed a toolkit of activity descriptors that predict performance without simulating every reaction step. Beyond the oxygen binding energy difference, the d-band center theory links adsorbate binding strength to the energy of the metal’s d electrons, while the occupancy of e_g orbitals in octahedrally coordinated transition metals provides a remarkably simple rule of thumb. Shao-Horn and colleagues showed that perovskite oxides perform best when e_g filling is near unity, with the perovskite Ba0.5Sr0.8Co0.2Fe0.2O3−δ sitting at the volcano apex. The oxygen p-band center itself has emerged as a descriptor that captures both activity and the instability that sometimes accompanies it. Perhaps the most striking demonstration of descriptor-guided design came from Sargent’s group, which gelled iron, cobalt, and tungsten into an atomically homogeneous oxyhydroxide. Iron binds oxygen too strongly, cobalt and tungsten too weakly, but interpolation among the three achieved near-optimal binding, delivering a record-low overpotential of 191 millivolts at 10 milliamperes per square centimeter and 500 hours of stable operation.

One of the field’s most consequential realizations is that many catalysts are not what they appear to be. Sulfides, selenides, nitrides, and phosphides that show impressive alkaline OER activity are frequently oxidized in situ under the high anodic potentials of operation, transforming into metal (oxy)hydroxides that are the true active phases. The as-synthesized materials are, in effect, precatalysts. Operando and in situ characterization techniques have confirmed that most transition metal cations undergo surface reconstruction during the OER, typically involving rising oxidation states and the leaching of specific cations or anions. Rather than fighting this transformation, researchers have learned to choreograph it. Nickel substitution in zinc cobaltate spinels pushes the oxygen p-band above the metal d-band, triggering reconstruction into an active oxyhydroxide surface and a sevenfold activity boost that surpasses the iridium oxide benchmark. Iron substitution in otherwise inert cobalt aluminate initiates reconstruction that then self-terminates, preserving the underlying structure.

Crystallinity matters too. Amorphous materials, with their abundant defects and open structures, reconstruct far more readily than their crystalline counterparts. An amorphous NiFeMo oxide prepared by fast co-precipitation rebuilt itself into an oxygen-vacancy-rich oxyhydroxide layer within moments of the reaction starting, requiring only 280 millivolts of overpotential, while the crystalline version needed 358 millivolts because molybdenum ions diffuse sluggishly through the ordered lattice. Deliberate leaching has become a design strategy in its own right. Activating an inactive cobalt chromite spinel at high potential oxidizes chromium from the 4+ to the 6+ state, driving it out of the lattice and leaving behind defects and vacancies that let cobalt reconstruct into active oxyhydroxide, an activity gain of more than an order of magnitude. In another approach, molybdenum sulfide nanosheets coated with nickel-iron layered double hydroxide served as sacrificial templates; five cycles of voltammetry stripped the molybdenum into the growing oxyhydroxide, yielding a catalyst that needed just 242 millivolts at 10 milliamperes per square centimeter.

The review’s survey of recent multi-metal catalysts reveals how these principles converge in practice. Tensile lattice strain engineered into NiFe2O4 spinel ferrites flattened the electronic bands near the Fermi level and tuned e_g occupancy at iron sites, cutting the overpotential to 180 millivolts. Co-doping ruthenium and zinc into Co3O4 selectively activated lattice oxygen while preventing over-oxidation of ruthenium, achieving 172 millivolts and 100 hours of stability at a punishing 500 milliamperes per square centimeter. Vanadium leaching from Co2Fe0.25V0.75O4 left behind a stable vanadium oxide underlayer that compensated charge and prevented cobalt dissolution, sustaining 600 hours at 500 milliamperes per square centimeter. High-valence dopants such as molybdenum, chromium, niobium, and tungsten stabilize low-valence iron and nickel species, and the resulting Fe2+ fraction correlates directly with activity; NiFeMo oxyhydroxide reached 180 millivolts. Anchoring single molybdenum atoms onto NiFe oxyhydroxide nanoarrays strengthened metal–oxygen covalency and activated lattice oxygen, delivering 193 millivolts. Non-oxide systems joined the race as well: cobalt-nickel phosphide nanosheets hit 209 millivolts, vacancy-rich NiFe sulfide reached 213 millivolts and performed at 235 millivolts under quasi-industrial conditions of 6 molar potassium hydroxide at 60 degrees Celsius, and a hierarchical CoMnP/Ni2P array worked as a bifunctional catalyst for both half-reactions, splitting water at just 1.54 volts.

The newest frontier is high-entropy materials, in which five or more elements mix in near-equal proportions to form single-phase solid solutions. The compositional disorder stabilizes the phase, and the so-called cocktail effect of synergistic elemental interactions tunes adsorption energies toward the Sabatier optimum. A high-entropy sulfide of chromium, manganese, iron, cobalt, and nickel outperformed every unary through quaternary sulfide comparison, reaching 100 milliamperes per square centimeter at 295 millivolts, with charge migration from manganese and iron optimizing the cobalt active sites. High-entropy perovskites and spinels followed suit, and in the spinel (CrFeCoNiMo)3O4 the high-valence cobalt and molybdenum shifted the reaction pathway toward the lattice oxygen mechanism, verified by isotope labeling. An amorphous NiFeCoMnAl oxide, dealloyed into a nanoporous architecture, posted 190 millivolts. The authors of the review are candid about what remains: the synergistic interactions among multiple elements are still incompletely understood, stability at industrial current densities is not guaranteed, and bridging from laboratory electrodes to scalable electrolyzer architectures is the decisive next step. But the trajectory is unmistakable. By combining electronic structure engineering, dynamic surface reconstruction, and compositional complexity, researchers are turning the OER from the weakest link in electrochemistry into a solvable design problem, and with it, the prospect of hydrogen made cleanly, cheaply, and at scale moves measurably closer.

Subject of Research: Design strategies and recent advances in 3d transition metal-based multi-metal electrocatalysts for the alkaline oxygen evolution reaction

Article Title: An overview of design strategies and recent advancements in complex 3d transition metal-based electrocatalysts for alkaline oxygen evolution reaction

Article References: Lee, S., Shin, Y., Yeom, K., Shim, J., & Sung, Y.-E. (2025). An overview of design strategies and recent advancements in complex 3d transition metal-based electrocatalysts for alkaline oxygen evolution reaction. Advances in Industrial and Engineering Chemistry, 1(1), Article 2. https://doi.org/10.1007/s44405-025-00001-4

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00001-4

Keywords: oxygen evolution reaction, electrocatalysis, water electrolysis, green hydrogen, transition metal catalysts, surface reconstruction, lattice oxygen mechanism, activity descriptors, high-entropy materials, layered double hydroxides, alkaline media, multi-metal catalysts