Figure 1. Schematic illustration of the synthesis of the CoN_x/C electrocatalysts.

(1) Mixing of carbon, nitrogen, and cobalt precursors (Co-oPD, CoTMPP, or VB12) with the silica template and pyrolyzing of the mixture; (2) NaOH etching to remove the silica template and form CoNPs/CoN_x/C composites; (3) Acid etching with H₂SO₄ to remove Co-containing nanoparticles and formation of carbon-based catalysts containing CoN_x sites. The second and third heat treatments after the NaOH and H₂SO₄ etching steps are not shown.