High temperature-induced myoglobin-mimic catalytic structure having high axial ligand content for one-compartment hydrogen peroxide fuel cells

A facile and inexpensive method of fabricating a myoglobin-mimic nanostructure is introduced by evaluating the influence of temperature conditions on the axial coordination between the Fe core of hemin and amine of polyethyleneimine (PEI). Through the high-temperature (100°C) synthesis condition, more hemin molecules are strongly attached to the carbon nanotube and PEI composite owing to the amide bond formation, whereas the energy distribution of hemin is deformed, and the electrical connection is improved by the coordination of axial ligands when the catalyst is synthesized on a lower temperature (25°C). Benefiting from the high concentration of axial ligands, the onset potential is positively shifted by 0.258 V, and the highest current density (155.43 μA cm⁻²) is observed with 10 mM H₂O₂ under physiological conditions. These phenomena occur because of the different hydrogen peroxide reduction reaction (HPRR) mechanisms and the overpotential stemming from the effect of the axial ligand, which induces the lowest catalytic and charge transfer resistance for HPRR at 51 and 820 Ω cm⁻², respectively. In the polarization curves measured using a 3D printed membraneless flow-type fuel cell, the maximum power density reaches 129.0 μW cm⁻² with 0.340 V of open-circuit voltage, respectively, which offers the best performance among the reported studies for the membraneless hydrogen peroxide fuel cells driving under physiological conditions so far.