Seawater nanoarchitectonics for an eco-friendly dual-function activator-catalyst producing graphene-decorated activated biochar for applications in electrochemical energy storage

Abstract

Activated biochar (AB) powder derived from sawdust was prepared using 0.6 M NaCl as a dual-function activator-catalyst at temperatures ranging from 500 to 1000 °C. The AB synthesized at 900 °C (NaCl-AB-900) shows the highest specific surface area (880.05 m² g⁻¹). Seawater was then used as the dual activator-catalyst to explore sustainable alternatives, producing Seawater-AB-900 powder with a specific surface area similar to the NaCl-AB-900 powder (890.34 m² g⁻¹). In contrast, the non-activated biochar (Non-AB-900, 782.45 m² g⁻¹) has a smaller specific surface area than both AB powders. SEM and TEM analyses reveal that both AB powders have graphene decoration and porosity, whereas the Non-AB-900 powder only exhibits porosity. Electrodes made from NaCl-AB-900 and Seawater-AB-900 powders demonstrate higher specific capacitances (81.42 and 84.45 F g⁻¹) compared to Non-AB-900 (64.39 F g⁻¹) at 0.3 A g⁻¹ in a three-electrode system using 6 M KOH. They also exhibit better rate capability (76.54% and 70.44%) than Non-AB-900 (65.24%). In a two-electrode mode, NaCl-AB-900 and Seawater-AB-900 supercapacitors outperform Non-AB-900 in capacitance (51.76 F g⁻¹, 50.68 F g⁻¹ vs. 25.96 F g⁻¹ at 0.3 A g⁻¹) and rate capability (64.50%, 65.20% vs. 27.59% at 10 A g⁻¹). In a Zn-ion battery system, NaCl-AB-900 (137.07 mAh g⁻¹) and Seawater-AB-900 (142.29 mAh g⁻¹) deliver higher specific capacities than Non-AB-900 (69.03 mAh g⁻¹) at 0.3 A g⁻¹. Both AB electrodes show excellent cycle stability, with over 70% retention after 40000 supercapacitor cycles and 90% after 7000 battery cycles. The Seawater-AB-900-powered supercapacitors and batteries successfully lit an LED for over 2 min.