Core–Shell Electrode Design Pushes Asymmetric Supercapacitors Toward Higher Energy Storage

Supercapacitors have long promised a middle ground between the instant power of conventional capacitors and the sustained energy delivery of batteries, but realizing that promise in a practical device has required chemists to solve a stubborn set of materials problems. A new study published in the journal Ionics by Longyuan Xie, Ziqiang Wang and Wenyu Cui of Harbin University of Commerce, with Xie also affiliated with Nanjing University of Information Science and Technology, reports a carefully engineered pair of electrodes that together deliver an asymmetric supercapacitor with a stable 1.6-volt operating window and a maximum energy density of approximately 43.4 watt-hours per kilogram, with power density reaching the order of ten thousand watts per kilogram. The work targets three chronic weaknesses of nickel molybdate electrodes: insufficient electrical conductivity, limited utilization of the redox-active sites buried inside the material, and the gradual loss of capacitance that comes with repeated charging and discharging.

The positive electrode, or cathode, at the heart of the study is a composite built around nickel molybdate nanowires. Nickel molybdate, usually written NiMoO4, is a well-known battery-type electrode material for aqueous supercapacitors because nickel and molybdenum can both participate in reversible redox chemistry, storing charge through faradaic reactions rather than simple electrostatic adsorption. Its Achilles heel, however, is that the material is a relatively poor electrical conductor, so many of the active sites deep within the nanowires never see an electron when the device is cycled quickly. The research team addressed this by using the NiMoO4 nanowires as a structural framework and growing a shell derived from an iron–cobalt layered double hydroxide, abbreviated FeCo-LDH, on their surface. Layered double hydroxides are a class of two-dimensional, brucite-like materials whose layers carry a positive charge and are balanced by interlayer anions; they are prized in supercapacitor research for their high density of accessible hydroxyl redox sites and their open, sheet-like morphology.

The crucial twist in the new work is what the authors call co-regulation. Rather than simply wrapping the nanowires in a generic FeCo-LDH shell, the researchers introduced a small additional amount of cobalt precursor during synthesis to tune the interfacial composition between the core and the shell. The amount matters: the optimized sample contained 0.7 percent additional cobalt regulation. This trace-level compositional adjustment modifies the heterointerface where the nickel molybdate core meets the iron–cobalt hydroxide shell, and the team’s electrochemical data suggest that this interface, combined with the cobalt regulation, improves charge transfer across the boundary between the two materials. In effect, the design turns a structural junction into an electrochemical asset, giving electrons a smoother path from the conductive shell into the redox-active core.

Characterization confirmed that the synthesis produced the intended architecture. Scanning electron microscopy and transmission electron microscopy showed that the optimized composite maintained a coupled wire–sheet core–shell structure, with nanowires sheathed in hydroxide nanosheets. X-ray diffraction identified the crystalline phases, while X-ray photoelectron spectroscopy probed the chemical states of the elements at the surface. Nitrogen adsorption–desorption measurements revealed a porous texture dominated by mesopores, pores in the range of roughly 2 to 50 nanometers that are large enough for electrolyte ions to penetrate quickly yet numerous enough to provide extensive surface area. Elemental mapping showed a uniform distribution of nickel, molybdenum, iron, cobalt and oxygen throughout the composite, indicating that the shell had grown evenly rather than clustering into islands. Together, these features create multilevel ion-transport channels and a dense array of multimetallic redox active sites, exactly the combination needed to make every gram of active material count.

The electrochemical performance of the cathode was striking. In a three-electrode test configuration, the standard way to evaluate a single electrode material in isolation, the 0.7 percent cobalt-regulated NiMoO4@FeCo-LDH-derived composite delivered a specific capacitance of 2178 farads per gram at a current density of 1 ampere per gram. That figure places the composite among the higher-performing nickel molybdate-based electrodes reported in the literature, and the authors attribute it to the combined effect of the conductive, redox-rich shell and the improved interfacial charge transfer. Just as important for real-world use is durability: after cycling tests conducted at a demanding current density of 5 amperes per gram, the cathode retained 96.7 percent of its capacitance. Cycling stability has historically been the weak point of hydroxide-based electrodes, which can swell, dissolve or mechanically degrade over thousands of charge–discharge cycles, so a retention figure this high suggests that the nanowire framework does more than carry charge; it also anchors the shell and buffers the volume changes that accompany repeated redox reactions.

A high-performance cathode alone does not make a device, and the second half of the study concerns the negative electrode, or anode. In an asymmetric supercapacitor, the positive and negative electrodes are deliberately mismatched in their charge-storage mechanisms and potential ranges so that, when combined, the cell voltage window is wider than either electrode could support alone. The team prepared a carbon nanotube/palladium oxide anode using a two-step impregnation–reduction followed by low-temperature oxidation route. Carbon nanotubes provide a highly conductive, entangled network that acts as both a current collector and a scaffold, while palladium oxide nanoparticles contribute pseudocapacitive charge storage through their own redox chemistry. The pairing is chemically sensible: the anode operates in a potential range complementary to the nickel-based cathode, and the carbon nanotube network compensates for the modest conductivity of the oxide. The CNT/PdO anode achieved a specific capacitance of 776 farads per gram at 2 amperes per gram, a respectable value for a negative electrode material in an aqueous system.

With both electrodes characterized individually, the researchers matched their potential windows and assembled a full asymmetric supercapacitor pairing the cobalt-regulated cathode with the CNT/PdO anode. The device operated stably at a cell voltage of 1.6 volts, which is a meaningful achievement for an aqueous device, since water itself begins to break down beyond roughly 1.23 volts in ideal conditions and practical aqueous cells must be engineered to suppress gas evolution and electrode degradation at the extremes of the window. Widening the voltage window matters enormously for energy density, because the energy stored in a capacitor scales with the square of the voltage; a modest increase in voltage yields a disproportionately large increase in stored energy. Referenced to the total mass of active material in the device, the assembled cell delivered a maximum energy density of approximately 43.4 watt-hours per kilogram, with power density reaching the order of 10,000 watts per kilogram. That combination of energy and power is precisely the profile that makes supercapacitors attractive for applications such as regenerative braking, grid frequency regulation and burst-power delivery in portable electronics.

The study’s broader significance lies less in any single number than in the design philosophy it demonstrates. Nickel molybdate electrodes have been modified in many ways before, including selenidation, silver decoration, coupling with reduced graphene oxide and growth of nickel cobalt oxide shells, and layered double hydroxides have themselves been engineered extensively for supercapacitor use. What this work adds is a demonstration that a very small, precisely controlled addition of cobalt precursor can regulate the interfacial composition of a core–shell heterostructure and measurably improve both capacitance and cycling stability. It is a reminder that in composite electrode design, the interface is not merely a boundary but an active chemical region whose composition can be tuned like any bulk phase. The authors conclude that the combined effect of appropriate cobalt regulation, the core–shell heterointerface and the conductive network of the CNT/PdO anode is what drives the improved charge transfer and energy storage performance of the nickel-based composite electrodes.

There remain, of course, the usual caveats that separate laboratory electrochemistry from commercial hardware. The reported energy density is referenced to the mass of active electrode material rather than to a complete packaged cell, and aqueous devices must still prove their longevity over the tens of thousands of cycles that grid and automotive applications demand. The three-electrode figures for individual electrodes, however impressive, do not translate directly into device performance. Yet the trajectory of the field is clear, and studies like this one show how rational structural and compositional engineering, applied simultaneously to both electrodes of a device, can push aqueous asymmetric supercapacitors toward energy densities that begin to overlap with battery technologies while retaining the power delivery and cycle life that only capacitors can offer. For a technology increasingly seen as a complement to batteries in a renewable-energy world, that combination of attributes is exactly what the market is waiting for.

Subject of Research: Core–shell nickel molybdate composite electrodes for high-performance asymmetric supercapacitors

Article Title: Co-regulated NiMoO₄@FeCo-LDH-derived core–shell cathode and CNT/PdO anode for asymmetric supercapacitors

Article References: Xie, L., Wang, Z., & Cui, W. (2026). Co-regulated NiMoO₄@FeCo-LDH-derived core–shell cathode and CNT/PdO anode for asymmetric supercapacitors. Ionics. https://doi.org/10.1007/s11581-026-07529-5

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07529-5

Keywords: supercapacitors, NiMoO4, FeCo-LDH, core–shell structure, asymmetric supercapacitor, CNT/PdO anode, cobalt regulation, energy density, pseudocapacitance, electrode materials, aqueous electrolyte, energy storage