Baoguo Wang

Droplet Microfluidics for Fabrication of Non-Spherical Particles

We describe new developments for controlled fabrication of monodisperse non-spherical particles using droplet microfluidics. The high degree of control afforded by microfluidic technologies enables generation of single and multiple emulsion droplets. We show that these droplets can be transformed to non-spherical particles through further simple, spontaneous processing steps, including arrested coalescence, asymmetric polymer solidification, polymerization in microfluidic flow, and evaporation-driven clustering. These versatile and scalable microfluidic approaches can be used for producing large quantities of non-spherical particles that are monodisperse in both size and shape; these have great potential for commercial applications.

Fabrication of Monodisperse Toroidal Particles by Polymer Solidification in Microfluidics

Microdoughnuts: Polymer toroidal particles such as the one shown in the left picture have been prepared by a capillary microfluidic technique. Droplets of polymer solution undergo non-uniform solidification to form the anisotropic polymer particles. By incorporating functional materials inside the polymer network, functional toroidal particles (center and right images) can be tailor-made for specific applications such as magnetic actuation.

A bifunctional electrocatalyst α-MnO2-LaNiO3/carbon nanotube composite for rechargeable zinc–air batteries

Development of highly active, non-precious, electrochemical catalysts is needed to optimize rechargeable zinc–air battery electrodes. Herein, we fabricate a bifunctional electrocatalyst α-MnO2-LaNiO3/carbon nanotube (CNTs) composite to improve the activity of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). CNTs are used to support the bifunctional catalysts in one integrated material. The chemical and physical characterization of the bifunctional catalyst is performed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The electrochemical properties of the bifunctional catalyst tested by a rotating disc electrode system revealed enhanced catalytic activity toward both the ORR and OER. A four-electron reduction pathway contributes to the ORR process at different rotation speeds indicating an effective catalytic activity. A rechargeable zinc–air battery using the bifunctional catalyst achieved a maximum power density of 55.1 mW cm−2, and its voltage polarization showed a 1.4% decrease in discharge and a 4.8% increase in charge after 75 charge–discharge (C–D) cycles. The charge transfer resistance for the ORR and OER catalyzed by the bifunctional catalyst increased from 1.24 to 4.68 Ω after 75 C–D cycles. The bifunctional electrocatalysts show satisfactory performance in an electrically rechargeable zinc–air battery.

Adsorption and Diffusion of VO2+ and VO2 + across Cation Membrane for All‐Vanadium Redox Flow Battery

A method based on a selectivity coefficient and the Nernst‐Planck equation is proposed to determine diffusion coefficients of vanadium ions across a cation exchange membrane in VO2+/H+ and VO2 +/H+ systems. This simplified method can be applied to high concentrations of vanadium ions. Three cation exchange membranes were studied. The logarithmic value of the selectivity coefficient was linearly dependent on the molar fraction of vanadium ions in solution. The diffusion coefficient of vanadium ions decreased with decreasing water content. The membrane with the lowest diffusion coefficient was selected as a battery separator and showed the lowest capacity loss of the studied membranes.

A membrane-less Na ion battery-based CAPMIX cell for energy extraction using water salinity gradients

“Salinity energy” stored as the salinity difference between sea water and fresh water is a large scale renewable resource that can be harvested and converted to electricity directly. In this study, a high performance membrane-less Na ion battery-based “capacitive mixing” cell (CAPMIX) was fabricated to extract the entropy energy from the water salinity gradient. The CAPMIX used copper hexacyanoferrate and silver as the electrodes in an aqueous electrolyte. A maximum working voltage of 0.65 V and a maximum power output of 2798 μW g−1 was obtained, which is a significant improvement over previous reported results. This energy harvesting process does not seem to play any significant role in the natural process and no deleterious substances were added to the output stream; it is a completely environmentally friendly power source. The entropy energy extracting battery represents a novel electrochemical device and is also a promising technology in the development of renewable energy production.

Hierarchical porous nitrogen doped reduced graphene oxide prepared by surface decoration–thermal treatment method as high-activity oxygen reduction reaction catalyst and high-performance supercapacitor electrodes

Nitrogen doped (N-doped) porous reduced graphene oxide (rGO) is successfully obtained by a two-step method, which includes a surface finishing of graphene oxide (GO) followed by thermal treatment. Distinct from many other methods, such as heat treatment, chemical vapor deposition or plasma treatment, a special atmosphere (NH3) and special equipment is avoided using this method. The nitrogen atoms are introduced into the reduced graphene oxide by this method according to the X-ray photoelectron spectroscopy (XPS) results. Based on the high specific surface area, high porosity, and doped heteroatoms, the as-prepared N-doped reduced graphene oxide shows excellent electrocatalytic activity towards the oxygen reduction reaction (ORR) and high capacitance, compared with a pristine reduced graphene oxide sample. Therefore, this work demonstrates a good example of both electrocatalytic activity and capacitance properties of N-doped reduced graphene oxide prepared by the surface decoration of GO followed by heat treatment method, which opens the door for creating functional graphene materials with highly promising applications in high-performance renewable energy conversion and storage devices.

Tunable Morphology of Monodisperse Polymer Particles With Microfluidics

In this work, we propose a novel method to fabricate polymer particles with controlled morphology in microchannel by solidification of polymer solutions and study the transition in the shape of these particles from spheres, to cups, and to donuts. The non-spherical geometries are achieved through nonuniform diffusion of the solvent, leading to non-uniform solidification. By adjusting the flow rates of both continuous and dispersed phases, the spatial heterogeneity in the solvent diffusion rate and thus the morphology of the final polymer particles can be tuned. This approach offers a one-step continuous process for the fabrication of non-spherical polymer particles. The technique is applicable to different polymers under appropriate polymer-solvent combinations. The ability to tune the shapes of polymer particles easily will create new opportunities for applications that require anisotropic functional particles, such as in biomedical engineering.Copyright