Preparation And Research Of High Performance And Ultra-Low Platinum Loading Cathode Catalyst Layer In PEMFC

At present,proton exchange membrane fuel cells（PEMFCs）have attracted increasing attention because they convert chemical energy into electric energy with high conversion efficiency and no pollution.However,high cost,durability and performance of PEMFCs which cannot fully meet the commercial targets,it is still in the early stage of commercial development.In terms of performance,each country has proposed future plan,and the Ministry of Science and Technology of China has proposed the performance target of Pt dosage≤0.125g/kW,power density≥1.4Wcm²in the 13th Five-Year Plan.The US Department of Energy（DOE）has similar targets.Therefore,there is an urgent need to develop low-Pt-load high-performance membrane electrode assembly to fully meet the needs of commercialization.When reducing the Pt loading,two major problems will be encountered.one is that when the catalyst layer loading drops to very low,it is not enough to provide sufficient activity in the catalyst layer for the redox reaction,so it is necessary to find a more active catalyst to prepare the membrane electrode assembly.The second is that with the decrease of the catalyst loading in the catalyst layer,the oxygen transfer resistance of the catalyst layer will gradually increase,especially when the catalyst loading decreases below 0.2 mg/cm²,the oxygen mass transfer resistance will increase at a larger rate.This paper focuses on these two issues.For the first question,in rotating disk system,we found that PtCoMn/C and PtCo/C commercial catalysts have higher redox activity relative to Pt/C catalyst at voltage of 0.9 V,but we need to find a new method for testing activity of the fuel cell at the rated operating voltage（0.6-0.8 V）.The charge mass transfer impedance(R_ct)represents the charge transport resistance between the catalyst particles to its surface material,which can be used to evaluate the activity of the catalyst at rated operating voltage.Test is measured by EIS,but the measurement results at the operating voltage is different from the results at 0.9 V voltage in the RDE system,because the experimental conditions of the RDE system are limited,the gas mass transfer resistance measured at the operating voltage is too large,and the measured resistance is the total resistance of the two,when the gas mass transfer resistance is too large,it cannot be compared directly.So it is necessary to prepare the MEA for the measurement.By adjusting the experimental conditions,the gas mass transfer resistance of the catalyst layer is minimized at the operating voltage.It can be found that the PtCoMn/C catalyst has the best activity at the working voltage,so the PtCoMn/C catalyst is chosen to replace the Pt/C catalyst.The second problem is that the oxygen mass transfer resistance of the catalytic layer will gradually increase with the decrease of the catalyst loading.The analysis found that with the decrease of catalyst particles in the catalytic layer,oxygen needs a longer path to reach the surface of the catalyst particles after entering the catalytic layer,and one catalyst particle will have more oxygen molecules to react with it.Both processes will increase the resistance of oxygen mass transfer.In response to this problem,it was decided to prepare an ultrathin catalytic layer to solve the problem of increasing the resistance of oxygen mass transfer at low Pt loading.The following conclusions are drawn through experiments:（1）The redox activity of Pt/C,PtCo/C and PtCoMn/C commercial catalysts was evaluated by charge-transfer impedance method.When the voltage was greater than0.73 V,the activity order was:PtCoMn/C>PtCo/C>Pt/C;when the voltage was between 0.68-0.73 V,the activity of PtCo/C and PtCo/C remained the best;when the voltage was lower than 0.68 V,the activity of the three catalysts differed little.（2）When the loading of the cathode catalytic layer is 0.125 mg/cm² and the thickness of the catalytic layer is about 3μm,the oxygen mass transfer resistance of the catalytic layer is the minimum.（3）PtCoMn/C catalyst was used to prepare membrane electrode assembly with a total loading of 0.147 mg/cm2 and a cathode loading of 0.124 mg/cm2.The highest power density of the membrane electrode is 1.42 W/cm2,and the amount of Pt is as low as 0.1035 g Pt/kW,which is less than 0.125 g Pt/kW,which achieves the performance index of DOE.