Fabrication And Characterizations Of MnO₂/C Electrode As A Catalyst For Oxygen Evolution

Hydrogen is generally acknowledged as clean and low-carbon energy, and it is concerned by people more than ever. One of the most used technologies of hydrogen production is alkaline water electrolysis. The high anodic overpotential leads to the low efficiency and high consumption of the electrolysis process, so to develop new kinds of anode materials with higher performance and lower overpotential of O2evolution is essential to reduce the consumption.In this paper, the thin coating of MnO2was prepared on graphite electrodes in0.1M Na2SO4+0.1M Mn(CH3COO)2solution by cyclic voltammetric and potentiostatic electrodeposition techniques. The structure of coating was characterized by scanning electron microscopy (SEM) and atomic force microscope (AFM). The results show that:the thin coating of homogeneous honeycomb-like MnO2can be obtained on graphite by potentiostatic electrodeposition. MnO2/C electrode has the better O2evolution properties when the deposition amount of MnO2is94.5μg·cm-2in weight. Furthermore, MnO2/C electrode has lower CO2evolution rate due to its protective effect for graphite during the constant potential electrolysis.Steady state polarization and impedance spectroscopy measurements were used to investigate the O2evolution process of graphite electrode and MnO2/C electrode in pH=13NaOH electrolyte. The results show that O2evolutions on both kinds of electrodes are controlled by charge transfer process. The kinetic parameters of O2evolution show that the values of Tafel slope for graphite are27mV·dec-1at low overpotential and89mV·dec-1at high overpotential. The values of Tafel slope for MnO2/C electrode are27mV·dec-1at low overpotential and79mV·dec-1at high overpotential, which represents lower polarization resistance. Besides the value of charge transfer resistence Rct,0for graphite electrode is2.8×104Ω·cm2, and the the value of Rct,0for MnO2/C electrode is2.1×103Ω·cm2; the value of exchange current density i0for graphite electrode is2.9×10-6A·cm-2, and the value of i0for C electrode is5.6×10-5A·cm-2. The lower charge transfer resistance Rct and higher exchange current density i0for MnO2/C electrode indicate that modified graphite electrode with MnO2coating can effectively reduce the resistance of oxygen evolution and catalyze O2evolution of the graphite electrode.