Cobalt Sulfide Meets Rare Earth Oxide in New Supercapacitor Built for Trains

Every time a metro train brakes, it dumps a burst of electricity back into the grid, and every time it accelerates, it snatches that power right back again. Those split-second swings are brutal on batteries, which is why engineers have long dreamed of supercapacitors rugged enough to sit between the rails and the power supply. A new study from researchers at Heilongjiang Communications Polytechnic in Qiqihar, China, published in the journal Ionics, reports a promising material pairing built precisely for that kind of punishing, pulse-heavy duty: a composite of cobalt disulfide and neodymium oxide on the positive side, and sulfur-treated carbon nanotubes on the negative side, assembled into an aqueous asymmetric supercapacitor.

The choice of cobalt disulfide, or CoS2, is no accident. Transition metal sulfides have become darlings of the pseudocapacitance world because their sulfur atoms help shuttle electrons quickly through the crystal lattice while the metal centers host fast, reversible surface redox reactions. CoS2 in particular offers metallic-like conductivity compared with many oxide alternatives, which means charge can move in and out of the active material without the crippling internal resistance that plagues poorer conductors. The catch is that sulfide electrodes often degrade over thousands of cycles, so the team paired CoS2 with neodymium oxide, a rare earth sesquioxide known for chemical stability and a knack for improving charge-transfer kinetics when incorporated into composite electrodes.

The synthesis was tuned with care. The researchers settled on a confirmed cobalt-to-neodymium molar feed ratio of 2:1, a proportion that produced a composite dominated by mesopores, pores in the two-to-fifty-nanometer range that are large enough for electrolyte ions to penetrate rapidly yet numerous enough to expose enormous internal surface area. Gas adsorption measurements using the Brunauer-Emmett-Teller method put that surface area at 213.21 square meters per gram, a figure comparable to many dedicated activated carbons but achieved here in a redox-active composite. That combination of open pore architecture and electrochemically active components is what allows an electrode to deliver charge both quickly and in quantity.

When tested in a three-electrode configuration, the CoS2/Nd2O3 composite delivered a specific capacitance of 1961 farads per gram at a modest current density of 1 ampere per gram. More impressive still is what happened under stress: at a punishing 20 amperes per gram, twenty times the current, the electrode still managed 1401 farads per gram. That rate capability, retaining roughly 71 percent of its low-rate capacitance, is the signature of an electrode where ions do not get bottlenecked in tortuous pores and electrons do not get stranded in resistive regions. After 10,000 charge-discharge cycles, the material retained 87.6 percent of its capacitance, evidence that the neodymium oxide component and the composite structure helped stabilize the sulfide against the mechanical and chemical wear that typically erodes such electrodes.

On the other side of the cell, the team turned to carbon nanotubes, the cylindrical graphitic filaments prized for their continuous one-dimensional conductivity. Pristine nanotubes store charge mostly through the electric double-layer mechanism, physically separating ions at their surface, which is fast but modest in capacity. The researchers treated the nanotubes with sulfur, creating sulfur-related surface species while preserving the continuous conductive network. The result was an S-CNT negative electrode delivering 431 farads per gram at 1 ampere per gram, 28.7 percent higher than untreated nanotubes measured under identical conditions. Even at 20 amperes per gram the S-CNTs held onto 81.0 percent of their capacitance, confirming that the sulfur modification added pseudocapacitive character without sacrificing the speed that makes carbon electrodes attractive.

Putting the two together yielded an aqueous asymmetric supercapacitor, a device architecture that exploits the different operating voltage windows of the two electrodes to push the total cell voltage well beyond what either electrode could sustain alone. Water-based electrolytes are inherently safer and cheaper than the organic solvents used in many commercial supercapacitors, though they traditionally cap the voltage window near 1.6 volts before water begins to split. The asymmetric design squeezes maximum benefit from that constraint by balancing a high-capacity battery-like positive electrode against a fast, stable carbon negative electrode.

The full two-electrode device held 73.3 percent of its capacitance when the current density was raised from 1 to 15 amperes per gram, and it survived 10,000 cycles with approximately 95.0 percent retention, a durability figure that stands out even among laboratory-scale aqueous devices. The authors are candid about the engineering subtleties involved. A retrospective first-order charge-balance analysis, which examines whether the charges stored on the positive and negative electrodes are properly matched, indicated that further optimization of the electrode mass ratio would be beneficial. In asymmetric supercapacitors, mismatched electrode masses leave one electrode underutilized, wasting capacity and shortening cycle life, so this kind of self-critical analysis points directly at the next round of improvement.

The motivation behind the work is refreshingly concrete. Short-duration and repetitive power fluctuations from train acceleration, braking, and regenerative-energy recovery demand storage devices with rapid response, high-rate capability, and stable cycling, and the authors frame their material pair as a laboratory-scale platform for exactly that kind of short-duration power buffering in rail transit. Regenerative braking energy is notoriously difficult to capture because it arrives in sharp spikes rather than smooth flows; batteries absorb it slowly and degrade quickly, while conventional double-layer capacitors lack the energy density to make a dent in a metro line’s power budget. A pseudocapacitive composite that can swallow thousands of farads per gram and release them at 20 amperes per gram, cycle after cycle, fits the profile of what wayside and on-board storage systems need.

The team is equally clear about what remains to be done before such a device could ride the rails. They note that composition optimization, complete-device impedance characterization, and application-specific pulse validation would further strengthen the assessment for rail-transit use. Impedance characterization, in particular, matters because real transit duty cycles involve not just repeated pulses but temperature swings, vibration, and years of continuous service, all of which show up first as changes in a device’s internal resistance. The authors also report that no datasets were generated or analyzed beyond the study itself, and the work was supported by the Key Project of the 2025 Educational Science Program of Heilongjiang Province under the 14th Five-Year Plan.

Still, the numbers tell a compelling story. A positive electrode approaching 2000 farads per gram, a sulfur-doped carbon negative electrode beating its untreated counterpart by nearly 29 percent, a full cell that keeps 95 percent of its capacitance over ten thousand cycles, and an honest roadmap for refinement together sketch a credible path from bench chemistry to power electronics. If subsequent work tightens the mass balance and validates the device under realistic pulse profiles, the humble pairing of a cobalt sulfide, a rare earth oxide, and a dash of sulfur on carbon nanotubes could find itself quietly smoothing the electricity of the world’s metro systems, one braking train at a time.

Subject of Research: CoS2/Nd2O3 composite and sulfur-doped carbon nanotube electrodes for asymmetric supercapacitors aimed at rail-transit power buffering

Article Title: Preparation of CoS2/Nd2O3 composites and S-CNTs and their electrochemical performance in asymmetric supercapacitors

Article References: Zhang, W., Lan, Y., Zheng, X., Zhang, Y., & Shen, W. (2026). Preparation of CoS2/Nd2O3 composites and S-CNTs and their electrochemical performance in asymmetric supercapacitors. Ionics. https://doi.org/10.1007/s11581-026-07486-z

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07486-z

Keywords: asymmetric supercapacitor, CoS2/Nd2O3 composite, sulfur-doped carbon nanotubes, pseudocapacitance, electrode materials, regenerative braking, rail transit energy storage, mesoporous materials, aqueous electrolyte, cycle stability, rate capability, electrochemistry