Electrocatalytic stability (i-t curve) of (a) MNPs, (b) Mb, (c) HPG/Mb-MNPs1mg, (d) HPG/Mb-MNPs5mg, and (e) HPG/Mb-MNPs10mg in buffer pH 7.4 PBS under saturated atmospheric air conditions at an applied potential of −0.5 V vs. Ag/AgCl, 150 rpm rotation rate, 25 °C.