Converting chicken manure into highly active N–P co-doped metal-free biocarbon electrocatalysts: effect of chemical treatment on their catalytic activity for the ORR

Abstract

In this work, the use of chicken manure as the raw material for obtaining metal-free biocarbons as electrocatalysts was studied. This abundant biomass waste was subjected to several treatments (pyrolysis, chemical activation using KOH and H₃PO₄ and solvothermal treatment with hydrazine) to modify its properties until nitrogen and phosphorous co-doped biocarbons were obtained. The effect of each treatment on the physicochemical characteristics and catalytic activity of the obtained catalysts for the oxygen reduction reaction (ORR) in alkaline media was studied by several techniques such as XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), Raman spectroscopy, nitrogen adsorption analysis, scanning electron microscopy (SEM) and rotating-ring disc electrode analysis. The most active electrocatalyst was that labeled B_K, activated with KOH (an onset potential of 0.86 V vs. RHE and a current density of −4.52 mA cm⁻²); however, its electron transfer number was n = 3.6 probably due to a combined 4 and 2e⁻ transfer pathway. Nevertheless, the catalytic activity of B_K for the ORR increased after accelerated degradation tests (3000 cycles). Even more, the electrocatalysts activated with H₃PO₄ in an air atmosphere and subjected to a solvothermal process (labeled as B_H-ST) also showed good catalytic activity (an onset potential of 0.85 V vs. RHE and a current density of −3.26 mA cm⁻²) and a 4e⁻ transfer pathway. We found that co-doping with nitrogen and phosphorous has a positive effect by improving the kinetics of these electrocatalysts. Additionally, the effect of other treatments on their catalytic activity is also discussed in this study.