Nanostructuring has become a key factor in controlling the electrochemical performance of materials. The development of novel nanomaterials with versatile chemical compositions and morphologies enables the manufacture of electrodes with an anisotropic structure and excellent electrical and mechanical properties.
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By exploiting the laws of fluid mechanics, microfluidics helps researchers to create nanoparticles with unique sizes and precisely controlled shapes that open possibilities for designing next-generation high-power energy storage devices.
Electricity is essential for modern society and is one of the main forms of power used globally. Efficient storage and electricity supply is an indispensable technology for many domestic and industrial applications, such as portable consumer electronics, medical devices, and electric vehicles.
Several factors underpin the ever-growing use of nanomaterials in energy storage applications. Nanostructuring is one of the key factors that can enhance the electrochemical performance of various materials. This is accomplished by exploiting various charge storage mechanisms, such as surface-based ion adsorption, pseudocapacitance, and diffusion-limited intercalation processes.
The recent development of new high-performance nanomaterials, such as redox-active transition metal carbides (MXenes), resulted in novel electrode materials with conductivity exceeding carbon and other conventional materials. In addition, creating nanocomposite hybrid architectures, such as carbon-silicon and carbon-sulfur, together with the development of versatile nanostructuring methods, can provide solutions for the realization of the next-generation high-power and long-lasting energy storage devices.
Large-Scale Synthesis of Nanostructures Is Challenging
Compared with conventional battery and supercapacitor materials, nanomaterials offer greatly improved ionic transport and electronic conductivity. These features make nanomaterial-based electrodes able to tolerate high currents and many charge-discharge cycles, thus offering a promising solution for efficient energy storage.
However, many challenges associated with using such materials in energy storage applications remain unsolved. At present, multiwall carbon-nanotube additives and carbon-coated silicon particles (used in lithium-ion battery electrodes) are the only nanomaterials used in commercial devices.
Unlike the conventional manufacturing processes, using nanomaterials to build sophisticated electrode architectures requires innovative manufacturing approaches, such as 3D printing, self-assembly from solutions, atomic layer deposition, and other advanced techniques that ensure accurate control over the manufacturing process. Such advanced approaches can also enable the development of flexible, stretchable, and wearable energy storage and harvesting solutions for consumer products.
Microfluidics for Superior Control of Nanoscale Synthesis
Microfluidics exploits the movement and mixing of fluids within microscopic channels and chambers of specific geometries, integrating sample preparation, reaction, separation, and detection in a single device. Microfluidic technology provides the means to overcome some of the most critical drawbacks of the conventional methods for nanomaterial synthesis, owing to the small dimensions of the capillaries in the microfluidic reactors and the resulting large surface-to-volume ratio.
These features permit rapid and uniform mass transfer and superior control over the characteristics of the synthesized nanomaterials. Besides, the reduced dimensions and unique geometries of such microreactors require smaller reagent volumes and enable precise control of fluid mixing, improved heat transfer, ease of automation, and greatly reduced reaction time.
The advantages of using microfluidic methods over the traditional nanoparticle synthesis approaches led to increased use of microfluidics in preparing highly stable, uniform, monodisperse nanomaterials.
Nanoparticles and Nanosheets with Exceptional Electrical Properties
By employing multi-phase microfluidic systems, researchers from the Pohang University of Science and Technology in Korea synthesized a wide range of porous nanoparticles based on conductive polymers and metal-organic framework precursors that exhibited high conductivity and large capacitance.
A similar microfluidic approach enabled a research team at Tsinghua University in Beijing to create nanosheets of reduced graphene oxide and molybdenum disulfide with a uniform size distribution, ultrathin lamellar structure, large specific surface area, and outstanding electrochemical activity. Such nanosheets are regarded as perfect electrode materials for supercapacitors and batteries owing to their tunable bandgap, low framework density, and good physicochemical properties for electron conduction and ion storage.
Microfiber- and Microfabric-Based Energy Storage Devices
By further refining the microfluidics approach for nanoscale synthesis, researchers from the Qingdao University and the Beijing Institute of Technology in China developed a microfluidic chip that enables continuous spinning of graphene-based fiber supercapacitor suitable for large-scale production.
The superior control of the reaction between reduced graphene oxide and alginate calcium–polyvinyl alcohol resulted in a nanofiber-based supercapacitor device, where the alginate calcium–polyvinyl alcohol electrolyte is intrinsically laminated by two reduced graphene oxide electrodes supplemented with carbon nanotubes directly in the microfluidic channel.
The researchers have also demonstrated that the technology can be extended to the microfluidic 3D printing method, enabling the fabrication of 2D microfabric-based supercapacitors with an ultrahigh energy density of more than 109 mWh cm−3 and incomparable mechanical stability under cyclic endurance.
These promising results demonstrate that the microfluidic technology has the potential to dominate the energy storage field in the future, facilitating the synthesis of nano and micro building blocks, 1D microfibers, and 2D microfabrics for large-scale production of fiber-based supercapacitors and batteries.
References and Further Reading
Pomerantseva, K. E., et al. (2019) Energy storage: The future enabled by nanomaterials. Science, 366, eaan8285. Available at: https://doi.org/10.1126/science.aan8285
Wu, X., et al. (2022) Review on Microfluidic Construction of Advanced Nanomaterials for High-Performance Energy Storage Applications. Energy & Fuels, 36, 4708-4727. Available at: https://doi.org/10.1021/acs.energyfuels.2c00576
Madhusudan, B. K. and Goel, S. (2020) Microfluidic devices for synthesizing nanomaterials—a review. Nano Ex., 1, 032004. Available at: https://doi.org/10.1088/2632-959X/abcca6