3.2 g of urea was dissolved in 10 mL of water, then slowly added to the Hb dispersion under stirring. The obtained mixture was subjected to ultrasonication for 30 min and then evaporated to dryness on a hot plate at 80 °C for 1 h. The solid residue was then pyrolyzed by heating in air at 250 °C for 2 h (2.5 °C/min) and then 900 °C for 3 h under an argon atmosphere. After cooling to room temperature, the sample was treated with 1 M H2SO4 at 80 °C overnight to remove any unstable or loosely bound Fe species from the electrocatalyst's surface. After the acid treatment, the samples were centrifuged, washed thoroughly with water and ethanol, then finally vacuumdried at 40 °C. The obtained sample, prepared using 16 wt.% Hb powder in the precursor mixture, is denoted herein as FeSN-3/C-3, or simply FeSN/C (Table ). Further samples were obtained by a similar method with no Hb (SN/C-3) and ca. 9 wt.% Hb (FeSN-2/C-3), respectively. Samples with no urea and only AL were labelled FeS-1/C-3 and FeSN-1/C-2, respectively. A physical mixture of Hb and carbon nanopowder in 1:3 weight ratio was labelled Fe/C-1.