Magnesium-Doped Nickel Oxide Nanoparticles Boost Supercapacitor Power and Magnetism

A team of researchers in India has shown that a simple, industrial-scale grinding technique can transform an ordinary ceramic powder into a dual-purpose material that both stores electrical charge and exhibits magnetism at room temperature. In a study published in the journal Ionics, scientists led by Bhagaban Kisan of Jawaharlal Nehru University and Utkal University prepared magnesium-doped nickel oxide nanoparticles using high-energy ball milling and demonstrated that the doped powders deliver respectable supercapacitor performance while also showing moderate ferromagnetism, a property that could make them attractive for spintronic devices as well as energy storage. The work stands out because it relies on mechanochemistry rather than the wet-chemical routes that dominate the nanomaterials literature, offering a potentially cheaper and more scalable path to functional nickel oxide powders.

Nickel oxide is a workhorse of modern materials science. In its bulk form it is a classic antiferromagnetic insulator, meaning its magnetic moments cancel each other out and it conducts electricity poorly. Yet when shrunk to the nanoscale, nickel oxide develops a range of useful behaviors: it catalyzes electrochemical reactions, senses gases, changes color under applied voltage, and stores charge through fast surface redox reactions. These properties have made it a popular electrode material for supercapacitors, devices that bridge the gap between batteries, which store a lot of energy but deliver it slowly, and conventional capacitors, which deliver power in a flash but hold little of it. Supercapacitors that rely on fast surface reactions, known as pseudocapacitors, are particularly promising for applications that demand rapid charge and discharge cycles, such as regenerative braking, grid buffering, and portable electronics.

The Indian team set out to tune nickel oxide by substituting magnesium atoms onto nickel lattice sites across a wide composition range, from 5 to 50 atomic percent magnesium. Magnesium is an interesting dopant choice because the magnesium ion carries the same +2 charge as nickel in nickel oxide and adopts a similar size, so it slots into the crystal lattice without introducing large amounts of charge-compensating defects. At the same time, magnesium doping is known to perturb the delicate magnetic order of nickel oxide, which normally arises from superexchange interactions mediated by oxygen atoms between nickel moments. By systematically varying the dopant level, the researchers could probe how composition influences crystal structure, magnetism, and electrochemical charge storage in one coherent study.

The synthesis itself is strikingly simple. The researchers placed the starting powders in a high-energy ball mill and milled them for 30 hours, a process in which hardened steel balls repeatedly fracture, cold-weld, and re-fracture the material, driving solid-state diffusion and forcing magnesium into the nickel oxide lattice at room temperature. The milled powders were then sintered at 500 degrees Celsius for 3 hours to relieve strain and stabilize the doped structure. This mechanochemical approach avoids the solvents, precursors, and lengthy reaction steps of solution-based synthesis, and it produces gram quantities of powder directly, which matters for any realistic electrode manufacturing process.

Structural characterization confirmed that the doping worked as intended. X-ray diffraction patterns showed that all compositions retained the face-centered cubic rock-salt structure of nickel oxide, and Rietveld refinement, a technique that fits a calculated diffraction profile to the measured one to extract precise lattice parameters and phase fractions, verified that the samples were single-phase solid solutions rather than mixtures of separate nickel oxide and magnesium oxide phases. The lattice parameter evolved systematically with magnesium content, consistent with successful substitution on the nickel sublattice. Maintaining a single crystalline phase across such a wide doping range is important because grain boundaries or secondary phases can scatter charge carriers, degrade magnetic coherence, and add parasitic resistance that undermines electrode performance.

The magnetic results are perhaps the most scientifically intriguing part of the study. Bulk nickel oxide is antiferromagnetic, yet all of the doped nanoparticle samples displayed moderate ferromagnetism at room temperature, with the saturation magnetization reaching a maximum of 6 electromagnetic units per gram at the highest doping level of 50 percent magnesium. The authors attribute this ferromagnetism to the coexistence of mixed nickel valence states, specifically Ni2+ and Ni3+, within the doped samples. Mixed valence in nickel oxides is well known to create charge-carrier-mediated exchange interactions that can tip the balance from antiferromagnetic order toward ferromagnetic alignment, and nanoscale effects such as broken exchange symmetry at particle surfaces amplify the response. Room-temperature ferromagnetism in a non-magnetic host oxide is the hallmark of the dilute magnetic oxide family of materials, which are candidates for spintronics, a technology that aims to manipulate electron spin as well as charge in devices.

On the energy storage front, the doped nanoparticles performed as pseudocapacitive electrodes with capacitance that climbed steadily with magnesium content. The specific capacitance values were 142 farads per gram for 5 percent doping, 162 farads per gram for 10 percent, 210 farads per gram for 30 percent, and 245 farads per gram for 40 percent magnesium. The best electrode, the 40 percent magnesium-doped sample, delivered a power density of 660.396 watts per kilogram and an energy density of 5.63 watt-hours per kilogram at a slow scan rate of 1 millivolt per second, demonstrating good pseudocapacitive behavior. These numbers place the ball-milled powders in a competitive range for nickel oxide-based electrodes, particularly given the simplicity of the synthesis. The trend with doping suggests that magnesium substitution increases the density of electrochemically active nickel sites or improves ion and electron transport within the powder, allowing more of the material’s theoretical charge storage capacity to be accessed during cycling.

Durability is where many pseudocapacitive metal oxide electrodes stumble, because repeated insertion and removal of ions from the electrolyte gradually pulverizes or dissolves the active material. The Mg-doped nickel oxide electrodes proved remarkably resilient: every doped composition retained 97 percent of its initial capacitance after 1000 charge-discharge cycles. That level of retention over a thousand cycles indicates that the mechanochemically synthesized particles, once sintered, form a mechanically and chemically stable framework for the surface redox reactions that store charge. For practical supercapacitors, which may endure hundreds of thousands of cycles, further longevity testing will be needed, but a 97 percent retention benchmark after 1000 cycles is a strong early indicator of robustness.

The study also connects to a broader research landscape in which doped nickel oxide nanostructures are being engineered for multiple applications simultaneously. Recent work has explored zinc, manganese, cobalt, iron, tungsten, and rare-earth dopants in nickel oxide for supercapacitors, photocatalysis, gas sensing, and electrochromic devices, while magnesium-doped nickel oxide has separately found roles as a hole transport layer in quantum dot light-emitting diodes and in photo-supercapacitor heterojunctions. What distinguishes the new study is the combination of a wide doping range, a solvent-free synthesis, and a three-way characterization of structure, magnetism, and electrochemistry in the same samples. That combination allows the authors to argue credibly that a single material platform could serve both energy storage and spintronic applications, depending on how the composition is tuned.

There remain caveats and open questions. The specific capacitances reported, while respectable, are below the highest values achieved by nanostructured nickel oxide composites incorporating conductive carbon scaffolds, and the energy density of 5.63 watt-hours per kilogram reflects the intrinsic limits of pseudocapacitive oxides tested as powder electrodes. The precise microscopic mechanism linking mixed nickel valence to the observed ferromagnetism would benefit from direct spectroscopic confirmation of the Ni3+ fraction, and the cycling data, while encouraging, cover a relatively short window by commercial standards. Nevertheless, the demonstration that a 30-hour ball mill and a modest 500-degree anneal can produce phase-pure, magnetically active, electrochemically durable doped nickel oxide is a meaningful result. If the mechanochemical route can be scaled to kilogram batches without losing performance, magnesium-doped nickel oxide could find itself competing for a place in next-generation supercapacitor electrodes and, more speculatively, in the spintronic devices of the future.

Subject of Research: Magnesium-doped nickel oxide nanoparticles synthesized by ball milling for supercapacitor electrodes and room-temperature ferromagnetism

Article Title: Structural, magnetic, and supercapacitor properties of Mg-doped NiO nanoparticles prepared by the ball milling method

Article References: Kisan, B., Das, R. K., Sarangi, T. K., Dash, B. B., Bhuyan, R. K., Kumar, S., Gautam, D., Kand, L., & Naik, K. K. (2026). Structural, magnetic, and supercapacitor properties of Mg-doped NiO nanoparticles prepared by the ball milling method. Ionics. https://doi.org/10.1007/s11581-026-07577-x

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07577-x

Keywords: nickel oxide, magnesium doping, nanoparticles, ball milling, supercapacitor, pseudocapacitance, ferromagnetism, energy storage, spintronics, X-ray diffraction, Rietveld refinement, mixed valence