Vanadium Swap Turns MOF Into Defect-Rich Cobalt Hydroxide for Water Splitting

Splitting water into clean hydrogen has long been held back by one stubborn half of the reaction: the oxygen evolution reaction, or OER, which demands a large extra voltage before oxygen gas will bubble out of an electrode. Noble metal oxides such as ruthenium and iridium oxides can drive the reaction efficiently, but their scarcity and cost make them poor candidates for the gigawatt-scale electrolyzers a hydrogen economy would require. A team of researchers in South Korea now reports a surprisingly simple fix that works at room temperature: swapping vanadium cations into a metal-organic framework to rebuild it into a defect-rich cobalt hydroxide electrode that outperforms its parent material by a wide margin.

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