Zero-Valent Iron Nanoparticles Supported on Si/N Codoped Carbon Materials: From Biomass to Oxygen Reduction Electrocatalysts and Supercapacitors

Biomass has a unique ability to capture atmospheric CO₂, although temporarily as its decomposition eventually releases an equivalent amount of CO₂. Converting biomass into hydrochar offers a promising way to stabilize atmospheric carbon in solid form. Herein, nitrogen, silicon, and iron codoped nanostructured hydrochar is synthesized via single-step hydrothermal carbonization of biomass. Silica is found to aid the formation of CC bonds, resulting in the formation of crystalline carbon layers. After pyrolysis treatment, the resulting material features uniformly dispersed zero-valent iron nanoparticles (Fe NPs) encapsulated in graphitic or iron oxide shells. A growth mechanism is proposed, and the materials’ structure and microstructure are characterized using complementary multi-scale techniques. The electrocatalytic activity toward oxygen reduction reaction is evaluated in a three-electrode setup and a proton exchange membrane fuel cell, achieving onset potentials and power densities as high as 0.76 V versus RHE and 14.2 mW cm⁻², respectively. Stability is assessed through accelerated stress tests and continuous load measurements. Poststability analyses reveal that electrocatalytic activity is maintained only by the graphite-encapsulated Fe NPs. Finally, the capacitive properties of the composite materials are examined, with the best sample showing specific capacitance and energy of 60 F g⁻¹ and 8.1 Wh kg⁻¹, respectively.