Old Phone Batteries Turned Into Cheap Catalysts That Split Water for Hydrogen

Every year, hundreds of millions of mobile phones are retired, and inside each of them sits a small lithium-ion cell packed with cobalt, one of the most strategically valuable and environmentally contested metals on the planet. Researchers in Brazil have now shown that the cobalt locked inside a discarded Nokia BL-5C battery can be transformed, with almost no loss, into a working electrode that helps split water into hydrogen and oxygen. The study, published in the journal Ionics, describes a recycling route so simple that it relies on little more than sulfuric acid, hydrogen peroxide, potassium hydroxide, graphite powder and paraffin wax, yet it produces an oxygen-evolving anode whose performance rivals catalysts made through far more elaborate synthetic procedures.

The team, led by Eric M. Garcia at the Federal University of São João del-Rei, focused on the oxygen evolution reaction, or OER, the half of water electrolysis that has long been the technology’s Achilles heel. While hydrogen gas bubbles off the cathode with relative ease, the anode must tear four electrons away from two water molecules to release one molecule of oxygen, and that multi-step process is kinetically sluggish. Electrolyzers must therefore push the anode several hundred millivolts beyond the thermodynamic requirement, an excess called overpotential that is paid for directly in electricity. Because roughly a million tons of lithium-ion batteries are expected to reach end-of-life by 2030, the researchers saw an opportunity to attack two problems at once: recovering cobalt from a growing mountain of electronic waste and using it to lower the cost of green hydrogen production.

The recycling chemistry begins with a battery dismantled by hand, its cathode material gently scraped from the aluminum foil current collector. After a thermal treatment at 400 degrees Celsius and a thorough washing to remove residual electrolyte, 5.3 grams of recovered active material were leached in 3.0 molar sulfuric acid at 100 degrees Celsius in the presence of 1.0 molar hydrogen peroxide. The peroxide plays a crucial chemical role: cobalt in lithium cobalt oxide sits in the +3 oxidation state, which dissolves poorly, but the reducing agent converts it to the far more soluble Co2+ species. Atomic absorption spectroscopy of the resulting leachate showed a cobalt concentration of 0.054 moles per liter, corresponding to about 3.18 grams of cobalt recovered from a theoretical content of roughly 3.19 grams, an apparent leaching efficiency of approximately 99.7 percent.

With the cobalt in solution, the researchers then raised the pH to 10 by slowly adding potassium hydroxide, precipitating cobalt hydroxide with a gravimetrically determined yield of about 98 percent. X-ray diffraction identified the product as predominantly the hexagonal brucite-like beta phase of Co(OH)2, in which charge-neutral layers of octahedrally coordinated Co2+ ions are stacked through hydrogen bonding. Applying the Scherrer equation to five diffraction peaks gave apparent crystallite sizes between roughly 5 and 10 nanometers, averaging 7.5 nanometers, a small domain size that typically favors catalytic activity by maximizing exposed surface. Infrared spectroscopy added nuance: broad hydroxyl stretching bands and the absence of the sharp 3645 per centimeter feature characteristic of well-ordered beta-phase material pointed to local structural disorder and hydration, likely influenced by residual sulfate species carried over from the acid leaching step, which energy-dispersive X-ray spectroscopy confirmed as trace sulfur alongside potassium from the precipitant.

Rather than coating this powder onto a conventional substrate, the team pressed it directly into the body of an electrode. Graphite powder was mixed with the recycled cobalt hydroxide at loadings ranging from 5 to 80 percent by weight, dispersed in ethanol, ultrasonicated for two hours and dried. The powder was then blended into molten paraffin wax at 80 degrees Celsius and compacted under ten tons of uniaxial pressure into a self-supported cylindrical pellet of two square centimeters. The paraffin serves as a hydrophobic binder that gives the pellet mechanical cohesion, while the graphite provides electrical percolation throughout the bulk. The approach deliberately avoids polymer binders such as Nafion and skips film-deposition steps entirely, meaning the catalyst is distributed through the electrode volume rather than confined to a thin surface layer.

Cyclic voltammetry screening revealed a clear optimum at 40 percent cobalt hydroxide, where the current density at 0.6 volts versus Ag/AgCl peaked before declining at higher loadings. The researchers attribute this maximum to a trade-off between catalytic content and conductivity: too much insulating hydroxide disrupts the graphite network through which electrons must travel, and the hydrophobic paraffin matrix already partially restricts electrolyte penetration to particles buried deep in the composite. The voltammograms also displayed a distinctive anodic process beginning near 0.2 volts, matched by a cathodic peak on the reverse scan, corresponding to the reversible oxidation of Co(OH)2 to cobalt oxyhydroxide, CoOOH. This transformation is widely regarded as the activation step in cobalt-based oxygen evolution catalysis, because the oxyhydroxide phase is more conductive and hosts the active sites where hydroxide ions are adsorbed and oxidized.

Quantitatively, the optimized electrode delivered a current density of 10 milliamperes per square centimeter at an overpotential of approximately 310 millivolts in 1.0 molar potassium hydroxide, with a Tafel slope of 75 millivolts per decade. Both figures compare favorably with literature benchmarks: the overpotential is lower than values reported for nickel phosphide on glassy carbon at 390 millivolts, carbon-supported NiCo nanospheres at 330 millivolts, and cobalt hydroxide on glassy carbon electrodes at 414 millivolts, though it does not match the 236 millivolts achieved by a sophisticated NiFe alloy nanotube catalyst. For an electrode whose active ingredient was pulled from a dead mobile phone and bound in candle wax, the performance is striking. The researchers also measured a reaction order of 0.87 with respect to hydroxide ions by varying the KOH concentration from 0.1 to 6.0 molar, a near-unity value indicating that hydroxide adsorption participates directly in the rate-determining step, consistent with the classical Bockris-Otagawa mechanism in which an adsorbed hydroxyl intermediate reacts with a second hydroxide ion to form a peroxide-like species before oxygen release.

Interface diagnostics reinforced the picture. Double-layer capacitance, a proxy for the electrochemically accessible surface area, rose from 1.69 to 14.2 microfarads per square centimeter upon incorporating the recycled hydroxide, an 8.4-fold increase. Polarization resistance, measured near the equilibrium potential, fell from about 11.4 to 3.15 kiloohm square centimeters, a 72 percent reduction. Most importantly for any practical device, the electrode held a current density close to 10 milliamperes per square centimeter for 180 hours of continuous operation at the 310 millivolt overpotential, with a slight upward drift in current that the authors interpret as progressive electrochemical activation of the cobalt surface under anodic polarization.

The authors are careful about the limits of their evidence. They did not measure Faradaic efficiency for oxygen, perform post-mortem microscopy or spectroscopy, or run electrochemical impedance spectroscopy, so the long-term test demonstrates operational stability of the current rather than proving that the electrode’s structure survived intact. They likewise note that definitive identification of the CoOOH-like active phase would require operando spectroscopy, since cobalt surfaces are known to reconstruct substantially under oxygen-evolving conditions, changing oxidation state and coordination as they work. Trace impurities below the detection limits of their analytical methods also cannot be excluded.

Even with those caveats, the study offers a compelling template for a circular hydrogen economy. It demonstrates that a battery destined for the shredder can pass through a single acid leach and a single precipitation step and emerge as a competitive water-oxidation anode, without phase engineering, nanostructure synthesis or noble metals. As electrolyzer deployments scale alongside renewable power, and as the first great wave of lithium-ion batteries reaches retirement, routes like this one suggest that yesterday’s phones could help supply the electrodes that split tomorrow’s water.

Subject of Research: Recycling cobalt from spent lithium-ion battery cathodes into low-cost electrodes for the oxygen evolution reaction in alkaline water electrolysis

Article Title: Recycled Co(OH)₂ from spent LiCoO₂ cathodes as a low-cost graphite-based composite electrode for the oxygen evolution reaction

Article References: Garcia, E. M., Taroco, H. A., Melo, J. O. F., & Taroco, C. G. (2026). Recycled Co(OH)₂ from spent LiCoO₂ cathodes as a low-cost graphite-based composite electrode for the oxygen evolution reaction. Ionics. https://doi.org/10.1007/s11581-026-07540-w

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07540-w

Keywords: lithium-ion battery recycling, cobalt hydroxide, oxygen evolution reaction, electrocatalysis, water electrolysis, green hydrogen, LiCoO2 cathode, graphite composite electrode, cobalt recovery, Tafel analysis, alkaline medium, circular economy