Molten Salt Electrolysis Yields Cobalt Boride Electrodes That Boost Supercapacitor Power

Supercapacitors sit at the crowded intersection of batteries and conventional capacitors, promising bursts of power that lithium-ion cells cannot deliver and storage capacities that ordinary capacitors can only dream of. The bottleneck has always been the electrode: the material that actually shuttles charge at the interface with the electrolyte. Now, researchers at Istanbul Technical University report a remarkably simple route to one of the more promising classes of electrode materials, cobalt borides, using nothing more exotic than molten borax, cobalt hydroxide, a low-carbon steel substrate and an electric current. Writing in the Journal of Materials Science, Mehtap Arslan-Kaba and Guldem Kartal Sireli describe how a one-hour bath of molten salt electrolysis at 900 degrees Celsius produces composite cobalt and cobalt boride powders whose electrochemical performance shifts dramatically with a single tunable knob: the current density.

The chemistry behind the process is elegantly economical. Rather than relying on high-temperature furnaces fed with purified metal and boron precursors, or on wet-chemical routes that generate solvent waste, the team used an oxide-based electrolyte containing sodium tetraborate, Na2B4O7, and cobalt hydroxide. When heated above its melting point, this salt bath becomes an ionic liquid capable of conducting charge, and the applied current drives the co-deposition of cobalt and boron onto the steel cathode. The approach belongs to the family of molten salt electrochemical processes that have attracted growing attention as greener alternatives for producing metals and compounds directly from their oxides, sidestepping energy-intensive reductions and corrosive reagents. Because both feedstocks are inexpensive and widely available, the authors position the method as fast, economical and environmentally friendlier than conventional boride synthesis.

What makes the study particularly interesting is the level of control the researchers achieved. When they ran the electrolysis at a current density of 200 milliamperes per square centimeter, X-ray diffraction revealed that the deposited powders consisted of metallic cobalt together with a single boride phase, Co3B. Cranking the current up to 600 milliamperes per square centimeter changed everything: the higher overpotential and faster deposition kinetics pushed the reaction toward boron-rich chemistry, yielding a multiphase mixture of CoB, Co2B and Co3B alongside residual metallic cobalt. In other words, the boron content of the final composite could be tuned simply by adjusting how hard the cell was driven, without reformulating the electrolyte or changing the temperature.

Characterization went well beyond phase identification. Scanning electron microscopy and transmission electron microscopy allowed the team to trace the growth mechanism of the composite structure and examine its morphology in detail, while Brunauer-Emmett-Teller surface area measurements confirmed that the powders possessed an accessible surface area suitable for electrode-electrolyte interaction, a prerequisite for any material hoping to store charge efficiently. This combination of structural and textural analysis matters because supercapacitor performance is exquisitely sensitive to how much of the electrode surface the electrolyte can actually reach; a material with impressive intrinsic chemistry but a closed, inaccessible microstructure will squander most of its potential.

The electrochemical payoff was substantial. Fabricated into electrodes and tested by cyclic voltammetry and galvanostatic charge-discharge, the boride-rich composite produced at 600 milliamperes per square centimeter delivered an areal capacitance of 4180 millifarads per square centimeter, along with an energy density of 78 milliwatt-hours per square centimeter and a power density of 2.8 watts per square centimeter. Its counterpart grown at the lower current density, dominated by Co3B and metallic cobalt, reached 2900 millifarads per square centimeter, 48 milliwatt-hours per square centimeter and 2.3 watts per square centimeter. The comparison makes a clear case that the multiphase, boron-rich composition is not an incidental byproduct of harsher conditions but the active ingredient in the enhanced performance.

Why would a mixture of several cobalt boride phases outperform a simpler composite? Transition metal borides have emerged in recent years as serious contenders for pseudocapacitive electrodes, the family of materials that store charge through fast, reversible surface redox reactions rather than purely electrostatic ion adsorption. Cobalt borides in particular have been incorporated into composites with clays, MXenes, graphene and activated carbon in a string of recent studies, and the Istanbul group itself has previously reported nickel boride electrodes made by similar molten salt routes. Multiple coexisting phases can offer complementary redox activity, improved electronic pathways and abundant interphase interfaces where charge transfer is facilitated. The multiphase Co/CoxB powders appear to exploit exactly such synergy, with the residual metallic cobalt likely contributing conductivity while the varied boride phases supply redox-active sites.

To understand how the electrodes actually store charge, the researchers applied the Trasatti approach, a classical analysis that separates surface-controlled capacitive contributions from diffusion-controlled ones by examining how voltammetric currents scale with scan rate. The verdict: both electrodes operate through a mixed capacitive-diffusive mechanism. That hybrid behavior is generally desirable in pseudocapacitive materials, since surface-dominated storage supports high-rate operation while diffusion-controlled contributions deepen the total charge available at slower rates. It also suggests that the composites function as genuine battery-capacitor hybrid materials, combining some of the energy density of battery-type cobalt chemistry with the rate capability of capacitive storage.

The team also interrogated the electrodes after cycling, using X-ray photoelectron spectroscopy to see what the electrochemical punishment had done to the surface. The spectra revealed persistent Co-B bonding along with Co(OH)2 species on the electrode surface, a signature of the surface transformation that cobalt-based pseudocapacitive materials typically undergo in alkaline electrolytes. Documenting this surface chemistry is more than academic housekeeping; it connects the observed capacitance to specific surface species and provides a baseline for assessing how the active layer evolves over a device’s lifetime, a central concern for any electrode material hoping to leave the laboratory.

The broader significance of the work lies in how it reframes electrode manufacturing. Molten salt electrolysis has historically been the domain of extractive metallurgy, most famously in processes that reduce metal oxides directly to metals in fused salt baths. Applying it to co-deposit two elements into a functional composite, and tuning the product’s phase composition through current density alone, points toward a scalable and low-waste synthesis platform for energy materials. If boride electrodes for supercapacitors, and perhaps for electrocatalysis and hydrogen evolution, can be grown in one step from cheap oxide salts, the arithmetic of manufacturing high-performance storage devices starts to look considerably friendlier than routes that demand multiple chemical synthesis stages, sacrificial solvents and costly precursors.

Challenges remain before molten-salt-grown cobalt borides can power anything in your pocket. The 900-degree-Celsius processing temperature demands robust equipment, and the reported metrics are areal values measured on laboratory electrodes rather than the gravimetric figures and long-cycle-life data that device engineers ultimately need. Still, the study delivers a compelling demonstration that sustainability and performance need not be traded against each other. A one-hour electrochemical bath in molten borax, steered by nothing more than the current dial, produced electrodes storing roughly 44 percent more charge per unit area than their lower-current counterparts, with energy and power densities to match. As the search intensifies for electrode materials that can bridge the gap between batteries and capacitors, this multiphase cobalt boride chemistry, grown from humble ingredients in a single pot of molten salt, is a result worth watching closely.

Subject of Research: Sustainable molten salt electrolysis synthesis of multiphase Co/CoxB composite electrodes for supercapacitor applications

Article Title: Multiphase Co/CoxB composite electrodes produced via sustainable molten salt electrolysis: structural and electrochemical performance evaluation

Article References: Arslan-Kaba, M., & Kartal Sireli, G. (2026). Multiphase Co/CoxB composite electrodes produced via sustainable molten salt electrolysis: structural and electrochemical performance evaluation. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13806-5

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13806-5

Keywords: supercapacitors, cobalt boride, molten salt electrolysis, energy storage, pseudocapacitance, composite electrodes, electrochemistry, Co3B, energy density, Trasatti analysis, X-ray photoelectron spectroscopy, sustainable materials