From wood to flexible Zn-air Battery: Fe3O4 nanoparticles synergistic single iron atoms on N-doped carbon nanosheets electrocatalyst and Lignosulfonate-Functionalized gel electrolyte

High-efficiency and low-cost electrocatalysts for oxygen reduction reaction (ORR), as well as gel electrolytes with high water retention, mechanical adaptability and zinc dendrites inhibition are required for flexible Zn-air batteries. Herein, abundant cellulose extracted from wood was employed to prepare ORR catalysts consisting of single iron atoms and Fe3O4 nanoparticles embedded within nitrogen-doped carbon nanosheets (SA-FeCNS). The optimized SA-FeCNS-800 demonstrated remarkable ORR catalytic activity, showcasing a half-wave potential of 0.85 V vs. RHE and a diffusion-limiting current of 5.54 mA cm−2 at 0.3 V vs. RHE. Its efficacy was further demonstrated in Zn-air battery technology, where it exhibited superior discharging and charging capabilities, allowing for 618 h of continuous operation at 5 mA cm−2, along with enduring performance at high current densities (10 mA cm−2 and 20 % depth of discharge). Furthermore, a solid-state electrolyte named SLS@PVA-SSE was fabricated utilizing wood's sodium lignosulfonate-functionalized polyvinyl alcohol hydrogel. This innovation effectively curbed the formation of zinc dendrites and enhanced alkaline resistance, owing to the incorporation of positively charged sulfonate groups. The flexible Zn-air batteries assembled with SA-FeCNS-800 and SLS@PVA-SSE exhibit long cycling life of for 85 h at 2 mA cm−2 and 37 h at 5 mA cm−2. This study offers a sustainable and effective way to make high-performance catalysts and solid electrolytes for advanced flexible energy materials using renewable biomass.