Graphene nanosheets are synthesized via the carbonization of brown-rice husks followed by a one-stage KOH-activation process for the design of a sustainable electrochemical energy-storage electrode. The graphene nanosheets exhibit an ultra-thin crumpled-silk-veil-wave, sheet-like structure with a high surface area of ∼1225 m² g⁻¹ and a high porosity. The graphene-nanosheet electrode shows a specific capacitance of 115 F g⁻¹ at 0.5 mA cm⁻² and a high energy density of 36.8 W h kg⁻¹ at a power density of 323 W kg⁻¹, with an excellent cyclic stability of 88% over 2000 cycles. The observed good electrochemical energy-storage performance of the biomaterial-derived graphene-nanosheet electrode is due to the synergistic effect of the intrinsically large electrochemically active surface area, an enhanced ion diffusion, and an improved electrical conductivity.

A sustainable and efficient electrocatalyst for the oxygen evolution reaction (OER) is vital to realize green and clean hydrogen production technology. Herein, we synthesized the nanocomposites of activated carbon-anchored nickel oxide (AC-NiO) via fully green routes, and characterized their excellent OER performances. The AC-NiO nanocomposites were prepared by the facile sonication method using sonochemically prepared NiO nanoparticles and biomass-derived AC nanosponges. The nanocomposites exhibited an aggregated structure of the AC-NiO nanotablets with an average size of 40 nm. When using the nanotablets as an OER catalyst in 1 M KOH, the sample displayed superb electrocatalytic performances, i.e., a substantially low value of overpotential (320 mV at 10 mA/cm²), a significantly small Tafel slope (49 mV/dec), and a good OER stability (4% decrease of overpotential after 10 h). These outstanding OER characteristics are considered as attributing to the synergetic effects from both the ample surface area of the electrochemically active NiO nanoparticles and the high electrical conductivity of the AC nanosponges. The results pronounce that the fully ecofriendly synthesized AC-NiO nanotablets can play a splendid role as high-performance electrocatalysts for future green energy technology.

• The AC-CeO₂ nanocomposites were prepared by the facile ultrasonication method. • AC-CeO₂ showed an aggregated morphology with AC nanoflakes and CeO₂ nanoparticles. • AC-CeO₂ hybridization led to the increased charge separation of the photocarriers. • These lead to the highly-efficient photocatalytic CV dye-degradation activity. • AC-CeO₂ can be of good use as an excellent photocatalyst for dye degradation.