Hemp Stem Derived Platinum Metal-Free Electrocatalysts for Oxygen Reduction Reaction in Alkaline Electrolyte

This study focuses on the synthesis of low-cost platinum group metal-free (PGM-free) electrocatalysts for the oxygen reduction reaction (ORR) in alkaline media. With that purpose, hemp stem derived products referred to as fibers (HF) and shives (HS), commonly waste by-products of the textile industry, are used as readily available carbon sources that are transformed into conductive char through pyrolysis. Three pyrolysis temperatures are investigated (400, 600 and 800°C). Subsequently, the char is chemically activated and functionalized with iron azamacrocyles to obtain Fe-Nx-C defect-rich porous structures with the desired distribution of active moieties in a porous architecture. The influence of the raw material on the physicochemical properties and electrochemical performance of the electrocatalysts is thoroughly examined. 129Xe NMR indicates carbonaceous porous structures in the mesoporous range, mainly created in the activation process. XRD reveals amorphous carbon signals and iron oxide-related peaks in the synthesized materials, while Raman spectroscopy shows defect-rich architectures that favor ORR. Additionally, XPS confirms the coexistence of various nitrogen and iron-containing active moieties and SEM-EDX images show homogeneously distributed iron particles with little formation of clusters. All electrocatalysts demonstrate significant ORR activity under alkaline conditions; the highest performance is achieved by the sample HF600 in terms of Eon and E1/2, with values of 0.97 and 0.92 V vs RHE, respectively. This research presents a promising route towards a cost-effective production of PGM-free ORR electrocatalysts with an upgrading of waste biomass for environmentally friendly energy applications.