Accelerated anode and cathode reaction due to direct electron uptake and consumption by manganese dioxide and...

The movement towards the clinical application of iron (Fe) has been hindered by the slow degradation rate in physiological environments. Herein, manganese dioxide (MnO₂) particles were compounded with titanium dioxide (TiO₂) particles by mechanical ball milling, and then the mixed powders were incorporated into Fe and fabricated into an implant using selective laser melting. On the one hand, MnO₂ had a higher work function (5.21 eV) than Fe (4.48 eV), which inclined electrons to transfer from Fe to MnO₂ to accelerate the anode reaction. On the other hand, MnO₂ catalysed the oxygen reduction reaction (ORR) through a four-step proton-electron-coupled reaction, which caused more oxygen to flow into the sample to improve the cathode performance. Besides, anatase TiO₂ with high conductivity was compounded with MnO₂ to construct a composite cathode, which facilitated electron transport from the cathode to the electrolyte, further consuming electrons and promoting cathode reaction. Results showed that Fe-MnO₂-TiO₂ had a high limiting current density of 5.32 mA·cm^-2 and a large half-wave potential of -767.4 mV, indicating an enhanced ORR activity. More significantly, Fe-MnO₂-TiO₂ had a higher average electron transfer number (2.9) than Fe-MnO₂ (2.5), demonstrating a faster electronic consumption reaction and higher cathode performance. In addition, the Fe-MnO₂-TiO₂ also exhibited fast instantaneous and long-term degradation rates (0.33 ± 0.03 and 0.19 ± 0.02 mm/year), suggesting a high anode dissolution rate. In conclusion, introducing the cathode with high work function and ORR activity provides novel pathways for accelerating the degradation rate of Fe-based implants.