MnO₂-Pt/C Composite Electrode For Restraining Voltage Reversal Effect Of Polymer Electrolyte Membrane Fuel Cell

Water is always produced at the cathode of polymer electrolyte membrane fuel cells (PEMFC).If water was not removed timely; it would accumulate at the cathode of PEMFC and further flood the cathode.With water accumulation and the cathode flooded,oxygen starvation,and even oxygen depletion would occur at the cathode. In this case,protons H⁺ reduction reaction (PRR) carries on at the cathode of PEMFC rather than oxygen reduction reaction (ORR).The potential of PRR is 1.23V less than that of ORR;it would cause a remarkable decline of cathode potential as ORR was replaced by PRR.The output voltage of a single cell with oxygen starvation would be likely reversed.This phenomenon was defined as voltage reversal effect in this paper. Aiming at solving the so-called voltage reversal effect, a MnO₂-Pt/C composite electrode was proposed to replace the conventional Pt/C electrode. The electrochemical reduction of MnO₂ in the composite electrode, which has almost the same Nernst potential as ORR, would be served as a substitute for ORR in the case of oxygen starvation. Therefore, the voltage reversal effect caused by PRR could be avoided at the cathode. The compostion of MnO₂-Pt/C composite electrode was first optimized and checked by electrochemical methods in comparsion with the conventional Pt/C electrode. The following points have been concluded in the case of oxygen starvation, the current density of Pt/C electrode is almost close to 0 above the potential of PRR .However,the current on MnO₂-Pt/C composite electrode is as high as 12mA/cm² due to the reduction reaction of MnO₂. In the case of rich oxygen, the current density on MnO₂-Pt/C electrode and Pt/C electrode are 40 and 30mA/mgPt, respectively.The MnO2 in MnO₂-Pt/C composite electrode plays a role of assistant catalysts for ORR and thus enhances the catalytic ability of the composite electrode to ORR. The study of recovery behavior of MnO₂-Pt/C composite electrode has shownthat the composite electrode of MnO₂-Pt/C can be recovered in off-current .the potential recovery is primary ascribed to the readjustion of electrode/electrotyte interface of the porous composite electrode.The proton diffusion in MnOOH,the reductant product of MnO₂ is the second contribution to the potential recovery of the composite electrode. We also found the electrode potential recovery in O₂ is almost the same as that in N₂; in O₂, the contribution of O₂ oxidizing MnOOH to the potential recovery of the composite electrode is very limited. The AC impedance analysis of MnO2-Pt/C composite electrode has disclosed: in the case of oxygen starvation with polarization potential of 0.3V vs.Ag/AgCl, the reaction resistances of Pt/C electrode and MnO2-Pt/C electrode is 196? and 3.04?, respectively. In the case of rich oxygen with polarization potential of 0.3V vs.Ag/AgCl, the reaction resistances of Pt/C electrode and MnO2-Pt/C electrode is 2.85? and 2.54?, respectively. Above two results further confirm that MnO2 of MnO2-Pt/C composite electrode can not only restrain the so-caller "voltage reversal effects"in the case of oxygen starvation, but also enhance the ORR in the case of rich oxygen.