Oxygen Reduction Activity Indicator For Fuel Cell Catalysts At Rated Voltage

High cost is a major bottleneck hindering the commercial development of proton exchange membrane fuel cells(PEMFCs),and the use of Pt-based catalysts has greatly increased the cost of fuel cells.Therefore,the development of high-performance membrane electrode assemblies(MEAs)with low Pt loading has become a trend.As the cathode Pt loading decreases,the activation overpotential increases sharply,and a catalyst with higher catalytic activity is urgently needed.However,the existing indicators of oxygen reduction activity for catalysts are characterized at high potentials,while fuel cells generally operate at 0.8 V-0.6 V.How to evaluate the oxygen reduction activity of catalysts under actual conditions is the key issue for the development and screening of the new catalysts.In our study,charge-transfer resistance(Rct)was proposed to evaluate the oxygen reduction activity of catalysts in the rated potential range.The complete mathematical expression of charge-transfer resistance is systematically derived.Rct of the commercial Pt/C catalysts in the potential range of 0.68 V-0.92 V was obtained,then the Tafel slopes were calculated based on mathematical derivation,which verified the rationality and feasibility of Rct as an indicator of oxygen reduction activity in the high potential range and the rated potential range.In addition,the oxygen reduction activity of commercial PtxCoy/C alloy catalysts was reevaluated and compared with commercial Pt/C catalysts.Furthermore,the variation of Rct for commercial Pt/C catalysts during the durability test was monitored to evaluate the decline in catalyst activity.Conclusions can be reached as follow:1.Theoretically,the charge-transfer resistance can be related to the exchange current density and the Tafel slope,and it is reasonable to characterize the oxygen reduction activity.2.The Rct of commercial Pt/C catalysts in the potential range of 0.68 V-0.92 V was tested,and the Tafel slope was calculated by it.It was found that in the high potential region(potential>0.8V),the Tafel slope is basically a fixed value(80mV/dec.),belonging to the plateau area;While in the lower potential range(potential<0.8V),the Tafel slope gradually increased with the decrease of potential.3.The oxygen reduction activity of the Pt_x Co_y/C alloy catalyst at different potential range was reevaluated and compared with the Pt/C catalyst.The study has found that:(1)The oxygen reduction activity of the Pt_xCo_y/C alloy catalyst is indeed better than that of the Pt/C catalyst,but its advantage is not significant in the rated potential range of fuel cell;(2)The activity difference between the Pt_x Co_y/C alloy catalyst and the Pt/C catalyst decreases as the potential decreased.And the difference between the two values of Rct for Pt/C and Pt_xCo_y/C catalysts is weakened from 37.77?·cm~2(@0.9 V)to 0.72?·cm~2(@0.7 V)as potential decreased.4.In the durability experiment,it was found that the charge-transfer resistance of commercial Pt/C catalysts increased significantly with time,which increased by nearly 125.6%after 900 hours.While the reduction of ECSA accounts for approximately only 1/5 of the increment of Rct.Other factors contributing to the increment of Rct remain further study.