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Graphene foams with three-dimensional (3D) network structure, high porosity, and ultralow density have been regarded as lightweight microwave absorption materials. Herein, nitrogen-doped reduced graphene oxide/multi-walled carbon nanotube composite foams were prepared through a two-step strategy of hydrothermal self-assembly and subsequent high-temperature calcination. Morphology analysis indicated that the 3D networks were composed of overlapped flaky reduced graphene oxide. In addition, the influences of nitrogen doping, calcination temperature, and filler ratios on microwave absorption of composite foams were explored. Results manifested that the microwave absorption of composite foams was remarkably improved with the calcination temperature increased. Dramatically, it was noteworthy that the composite foam obtained under 600 °C calcination (bulk density of ∼10.8 mg/cm3) with an 8 wt % mass filler ratio presented the strongest microwave absorption of −69.6 dB at 12.5 GHz and broadest absorption bandwidth achieved 4.3 GHz (13.2–17.5 GHz) at an extremely low matching thickness equal to 1.5 mm. Moreover, the microwave absorption performance could be conveniently adjusted through modifying the thicknesses, filler ratios, and calcination temperature. The excellent microwave absorption performance of as-prepared composite foams was greatly derived from a well-constructed 3D network structure, significant nitrogen doping, enhanced polarization relaxation, and improved conduction loss. This work proposed a new strategy for fabricating graphene-based composites with a 3D network structure as high-efficiency microwave absorbers.

A simple sonoelectrochemical method was used to realize the morphology-controlled synthesis of palladium nanostructures at room temperature. The palladium spherical nanoparticles, multitwinned particles, and spherical spongelike particles (SSPs) were successfully prepared in the presence of different surfactants or polymers. It was found that the size and shape of the Pd nanostructures could be controlled by varying current density and pH value of the precursor solution. The Pd nanostructures were characterized by transmission electron microscopy, high-resolution transmission electron microscopy, field emission scanning electron microscopy, energy-dispersive X-ray, and X-ray diffraction. The possible formation mechanism was discussed. In addition, the electrocatalytic properties of the Pd nanostructures for direct alcohol oxidation in alkaline media were systematically investigated. The results showed that SSPs had a higher electrochemical active surface and result in more stable and better electrocatalytic properties than other Pd nanostructures for the ethanol electrooxidation.

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