The secret to cobalt’s versatility lies in its electronic structure. The element sits in the middle of the transition-metal series, where its 3d orbitals are neither too full nor too empty, giving it access to multiple oxidation states, most commonly Co2+ and Co3+, and sometimes Co4+. That multivalent redox flexibility means cobalt atoms can readily exchange electrons with their surroundings, a property that underpins nearly every application the review describes. In a supercapacitor electrode, cobalt centers shuttle between oxidation states to store charge reversibly at the surface. In an electrocatalyst, they mediate the multi-electron transfers required to split water into hydrogen and oxygen. In a photocatalyst, they help capture and steer the charge carriers generated when light strikes a semiconductor. Add in cobalt’s strong magnetic behavior, its two accessible crystal phases, hexagonal close-packed and face-centered cubic, and its good electrical conductivity, and the result is a material platform with an unusually wide design space.

The review devotes considerable attention to how cobalt-based materials are actually made, because synthesis route often determines performance as much as composition does. Solid-state reactions, in which powders are ground and heated to high temperatures, remain the workhorse for producing robust thermoelectric ceramics such as the layered cobalt oxide Ca3Co4O9 and the skutterudite CoSb3. Hydrothermal and solvothermal methods, which crystallize materials from hot pressurized solutions, offer finer control over particle size and morphology, yielding everything from mesoporous Co3O4 nanorods for lithium-ion battery anodes to cobalt sulfide nanoflakes grown directly on carbon cloth for flexible supercapacitors. Electrodeposition provides a fast, room-temperature route to films such as CoSe2 counter electrodes for dye-sensitized solar cells, while template-assisted and sol–gel approaches, sometimes performed in ionic liquids or assisted by microwaves, allow chemists to sculpt porosity and doping profiles with precision. The authors emphasize that compositional engineering, defect modulation, and heterostructure design are the three levers that transform these syntheses from routine chemistry into performance tuning.

Thermoelectric materials, which convert temperature gradients directly into electricity, form one of the review’s central pillars. Cobalt-based compounds excel here because of an unusual combination of high electrical conductivity and complex crystal structures that scatter heat-carrying phonons. Layered cobalt oxides such as Ca3Co4O9 have attracted interest for high-temperature applications, and recent work shows that doping the calcium site with gadolinium enhances thermoelectric transport through spin entropy and size effects. Meanwhile, cobalt antimonide skutterudites, CoSb3, have become one of the most intensively studied thermoelectric families in the world. Filling the empty cages of the skutterudite crystal with guest atoms such as ytterbium, substituting nickel or aluminum onto the framework, and introducing nano-inclusions all serve to suppress lattice thermal conductivity while preserving or even boosting the power factor. The review also highlights half-Heusler alloys such as ZrCoBi, which have achieved high conversion efficiencies, and cobalt telluride–selenide solid solutions and thin films that push performance in the mid-temperature range.

On the energy storage front, cobalt compounds have become staples of supercapacitor research, where they operate through fast surface redox reactions known as pseudocapacitance. Spinel cobalt oxides such as Co3O4, binary metal oxides like NiCo2O4, and cobalt sulfides, selenides, and tellurides all offer abundant electroactive sites and favorable charge-transport characteristics. The review traces how researchers have boosted capacitance by pairing cobalt with nickel, copper, manganese, or zinc in bimetallic and even ternary oxides, and by growing hierarchical nanostructures that expose more surface area to the electrolyte. Cobalt-based metal–organic frameworks, porous crystalline solids in which cobalt ions are linked by organic struts, have emerged as a particularly exciting electrode class, with layered cobalt MOFs demonstrating ultrahigh capacities and MOF-derived composites, sometimes made by rapid cold plasma synthesis on reduced graphene oxide, delivering both high areal capacitance and operational stability.

Electrocatalysis for water splitting is another arena where cobalt shines, largely because it is far more abundant and cheaper than the noble metals that traditionally dominated the field. The review surveys cobalt-based catalysts for both the hydrogen evolution reaction and the oxygen evolution reaction, the sluggish half-reaction that limits overall water-splitting efficiency. Bimetallic iron–cobalt sulfides show synergistic activity, cobalt phosphide nanoflowers have been adapted for zinc-ion batteries, and cobalt-containing bimetallic zeolitic imidazolate frameworks and their derivatives serve as efficient oxygen evolution catalysts. Spin states of the metal centers, the review notes, play a decisive role in determining how strongly reaction intermediates bind to the surface, making spin engineering a frontier topic. Core–shell bimetallic catalysts are even being applied to carbon dioxide hydrogenation for light-hydrocarbon synthesis, extending cobalt’s reach into carbon utilization.

Perhaps the most environmentally resonant section of the review concerns photocatalysis, the use of light-activated materials to break down organic pollutants in water. Cobalt oxides, cobalt sulfides, and cobalt MOFs have all been shown to degrade dyes such as methylene blue, rhodamine B, methyl orange, crystal violet, and malachite green under ultraviolet or visible light. Strategies for improving performance include doping, for example tin-doped Co3O4 or neodymium-doped Co3O4 for enhanced visible-light activity, constructing heterojunctions such as Co3O4/Bi2O3 and CoO/ZnO that separate photogenerated electrons and holes more effectively, and building Z-scheme and S-scheme architectures that combine strong oxidation and reduction power without sacrificing light absorption. Cerium–cobalt bimetallic MOFs with mixed ligands, cobalt telluride nanosheets, and Co-MOF composites with bacterial cellulose or graphitic carbon nitride illustrate how far heterostructure design has progressed. The review links these gains to improved charge separation, broader light harvesting, and faster surface reaction kinetics.

None of this progress is without caveats, and the authors are candid about the obstacles standing between laboratory breakthroughs and real-world deployment. Long-term stability remains a concern for many cobalt catalysts and electrodes, which can degrade, dissolve, or restructure under operating conditions. Cobalt compounds carry known toxicity risks, and the element’s environmental and biological footprint, from mining impacts to potential leaching, demands careful management. Large-scale synthesis is another bottleneck: hydrothermal autoclaves and template methods that work beautifully at the gram scale are not trivially translated to the ton scale, and scaling considerations for water-splitting catalysts have been flagged as a broader challenge in the field. Cost and supply-chain concentration add further pressure, since cobalt is a strategically sensitive raw material.

The review closes with a forward-looking agenda that reads as a roadmap for the next decade of cobalt research. Interface engineering, deliberately designing the boundaries between cobalt phases and their supports to accelerate charge transfer, tops the list, followed by defect regulation, in which vacancies, dopants, and interstitial atoms are used as atomic-scale control knobs for electronic and catalytic properties. Sustainable synthesis routes, including plant-extract and biopolymer-assisted methods that replace harsh reagents, are gaining momentum, and advanced hybrid architectures that couple cobalt compounds with graphene, carbon nitride, conductive polymers, or biodegradable chitosan matrices promise multifunctional devices that simultaneously store energy, generate power from heat, and clean water. For a metal that already powers the batteries in electric vehicles, the review makes a persuasive case that cobalt’s scientific story is only beginning, and that rational design at the level of crystal phase, defect, and interface will determine how much of its potential the world ultimately captures.

Subject of Research: Cobalt-based functional materials for thermoelectric, electrochemical energy storage, and photocatalytic applications

Article Title: Recent advances in elemental cobalt and cobalt-based compounds for thermoelectric, electrochemical, and photocatalytic applications: a comprehensive review

Article References: Keerthana, M., Sahana, B. V., Swetha, R., Kruthika, S., & Kumari, L. (2026). Recent advances in elemental cobalt and cobalt-based compounds for thermoelectric, electrochemical, and photocatalytic applications: a comprehensive review. Ionics. https://doi.org/10.1007/s11581-026-07508-w

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07508-w

Keywords: cobalt, thermoelectric materials, supercapacitors, electrocatalysis, photocatalysis, cobalt oxides, cobalt chalcogenides, metal-organic frameworks, water splitting, energy storage, heterostructures, wastewater treatment