Nanoporous carbons derived from metal-conjugated phosphoprotein/silica: Efficient electrocatalysts for oxygen reduction and hydrazine oxidation reactions

Efficient metal-free or nonprecious metal-based electrocatalysts are currently needed for many renewable energy conversion and storage devices. In this work, transition metal-conjugated casein-derived nanoporous heteroatom-doped carbon electrocatalysts for the oxygen reduction reaction (ORR) and the hydrazine oxidation reaction (HzOR) in fuel cells are reported. The electrocatalysts are synthesized from casein (or dairy phosphoprotein) that is derived from milk past its shelf life. In the synthesis, casein is mixed with a small amount of non-noble transition metal ions (Cu²⁺ or Ni²⁺) and silica, then pyrolyzed and finally treated with basic and acidic solutions to remove the silica and metallic species in it. The metal ions in the precursor are found to be instrumental in the formation of electrocatalytically active sites in the nanoporous carbon materials (as the control material prepared without including metals in the precursor shows a much weaker electrocatalytic activity for both reactions). Interestingly, the carbon materials obtained after removal of the metals exhibit greater electrocatalytic activities for both reactions. Specifically, the one obtained after the removal of Cu from the corresponding carbonized product catalyzes both reactions with lower overpotentials and higher current densities than the other materials studied herein as well as many other notable related materials reported before. These mean, having silica and Cu²⁺ ions with casein creates more electrocatalytically active sites in the carbon material derived from casein via pyrolysis.