Researchers develop a porous nickel-modified graphene fiber that enhances energy density, conductivity, and durability in flexible supercapacitor devices.
(Nanowerk Spotlight) As electronics become smaller, lighter, and more integrated into clothing, sensors, and mobile systems, the demand for energy storage devices that can operate under flexible, high-frequency conditions has grown sharply. Conventional batteries, while energy-dense, are often too rigid, slow to charge, and prone to degradation over repeated use.
These limitations pose particular challenges for emerging technologies like wearable electronics, autonomous drones, and soft robots, which require power sources that are not only compact but mechanically robust and capable of delivering bursts of energy on demand.
Supercapacitors offer an alternative. They store energy through charge separation or fast surface redox reactions rather than through slower chemical transformations, allowing for rapid charging, long cycle life, and better reliability under repeated mechanical stress. However, their lower energy density—compared to batteries—has remained a critical obstacle, especially for applications where space and weight are constrained. Improving this trade-off between energy density and mechanical adaptability has become a central research goal.
Graphene-based fiber electrodes have drawn interest for their high conductivity, large surface area, and compatibility with flexible substrates. Yet their tendency to stack tightly due to attractive forces between layers reduces available surface area and impedes ion transport. Attempts to overcome these issues by integrating metal oxides or other active materials have been hindered by uneven material distribution, reduced structural strength, and fabrication processes that are difficult to scale.
By integrating synthesis, assembly, and structuring into a continuous process, this method aims to overcome the persistent limitations of previous designs—offering a pathway toward supercapacitors that are not only high-performing, but practical for real-world use.
Developed by a team at Nanjing Tech University and Nanjing University of Science and Technology, the method is called microfluidic-spinning-chemistry (MSC). It enables the in situ growth of nickel molybdate (NiMoO₄), a redox-active metal oxide, onto porous graphene-based fibers. Unlike traditional batch methods, this technique allows real-time control over fluid flow, reaction conditions, and fiber structure, ensuring more uniform material distribution and scalable production.
Schematic of synthesis of NiMoO4/PGCF by MSC method. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
Nickel molybdate was chosen for its strong redox activity and compatibility with graphene. The researchers prepared two solutions: one containing graphene oxide (GO) and nickel ions, and another with molybdate ions and urea. These were injected into a Y-shaped microchannel, where they mixed and reacted under ultrasonic agitation. The confined space of the microchannel promoted uniform growth of NiMoO₄ on the GO sheets. Simultaneously, the decomposition of urea released gases that created pores in the structure, resulting in a fiber with both high surface area and a network of channels that allow ions to move easily.
After thermal treatment to stabilize the structure and improve conductivity, the resulting NiMoO₄/porous graphene carbonene fiber (PGCF) demonstrated strong electrochemical performance. Its surface area reached 433.77 square meters per gram—more than double that of fibers made without NiMoO₄. The porous structure had a wide range of pore sizes, including micropores that store charge and mesopores that allow rapid ion movement. The material also maintained good mechanical strength and electrical conductivity, both essential for wearable or flexible applications.
Performance was evaluated using two types of systems: a traditional three-electrode setup with aqueous potassium hydroxide, and a solid-state configuration with a polymer gel electrolyte based on EMIMBF₄, a thermally stable ionic liquid. In the aqueous system, the fibers showed an areal capacitance of 3597.7 millifarads per square centimeter at a current density of 1 milliamp per square centimeter—an exceptionally high value compared to other fiber-based supercapacitors. Even at higher current densities, the material retained strong performance, showing efficient charge storage and fast electron transfer.
The solid-state version, using a flexible gel electrolyte, allowed the device to operate at voltages up to 2.5 volts. This is a notable improvement over aqueous systems, which are typically limited by water’s electrochemical window. In this setup, the fibers achieved a capacitance of 1006.8 millifarads per square centimeter and an energy density of 218.5 microwatt-hours per square centimeter—more than three times that of the graphene-only control. The device also retained over 90% of its capacitance after 20,000 charge-discharge cycles, and maintained stability under a range of bending, twisting, and temperature conditions.
The combination of performance metrics and durability suggests these fibers are suitable for practical use in compact energy systems. To demonstrate real-world utility, the researchers integrated the supercapacitor into two test applications. First, they connected multiple fibers in parallel to form a circular chip-type capacitor, which was charged by a solar cell and then discharged to power a small toy windmill. This system was able to store solar energy and convert it into mechanical energy, rotating the windmill continuously for over half a minute.
In a second demonstration, the fiber-based supercapacitor was used to control the takeoff of a small unmanned glider. The device was integrated with a simple transistor-based circuit: once the supercapacitor charged above a threshold voltage of 0.7 volts, it triggered the transistor to activate a motor, allowing the aircraft to take off. As the voltage increased, the aircraft reached higher altitudes, with flight heights rising from zero to over one meter depending on the stored energy. When the voltage dropped below the threshold, the motor switched off, and the aircraft returned to the ground. This simple controller used no additional power source, highlighting the capacitor’s ability to manage short bursts of energy in an efficient, self-contained system.
These application tests illustrate how high-performance supercapacitor fibers might be used in low-power, lightweight systems that require quick response times and minimal hardware. Potential applications include soft robotics, unmanned vehicles, and smart fabrics. Because the MSC process is continuous and modular, it offers a route to scale production by adding parallel microfluidic channels without altering the chemistry. Compared to existing batch methods, MSC also reduces energy use, material waste, and post-processing requirements.
The process’s flexibility also enables fine-tuning of material composition. For example, the team found that increasing the NiMoO₄ content up to 8.4% by weight improved both electrical conductivity and capacitance. But higher concentrations caused the material to clump, blocking ion pathways and reducing performance. This balance between conductivity, porosity, and structural integrity is difficult to achieve with conventional coating or blending techniques but can be controlled more precisely in the MSC setup.
The MSC strategy combines several process innovations: it shifts fabrication from batch to continuous operation, integrates material synthesis and spinning into one step, and leverages precise flow control and ultrasound to create uniform hybrid structures. The microchannel environment enables fine control over reaction dynamics, leading to consistent fiber morphology and distribution of active components. The result is a material that stores more energy, operates more reliably, and can be produced more efficiently than previous graphene-based fiber designs.
This study provides a strong foundation for future development of supercapacitor-based systems in flexible electronics. It also highlights how combining microfluidics with advanced material chemistry can solve long-standing challenges in electrochemical device fabrication. By demonstrating both performance and practical usability, the work outlines a realistic pathway toward integrating high-capacity, mechanically resilient supercapacitors into the next generation of autonomous and adaptive technologies.
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