Methodology And Electrocatalysts For Oxygen Reduction Reaction

Proton exchange membrane fuel cells which can directly converted chemical energy into electrical energy near room temperature, and emit almost no or just a small amount of pollutants. Therefore, since the1960s, the R&D of such fuel cells has been greatly supported worldwide. However, by now such fuel cells have been successfully commercialized on large scale. Wherein an important bottleneck is that the kinetics for oxygen cathode reaction is very slow. Understanding the mechanism of the oxygen reduction reaction (ORR) has been one of the important research topic in the fuel cell catalysis, at the best Pt based electrocatalysts available so far, the overpotential at the onset for ORR is still as high as0.25V. After60years of research, much atomic, molecular level understanding for ORR has been gained. However, no consensus on ORR mechanism and key factors which limits ORR kinetics has been reached so far.This is probably due to, on one hand, ORR is a reaction involves4electron, multiple step complex process, on the other hand, some misunderstanding exists on using thin film rotating disk electrode method, the key technique used for evaluating nanocatalysts activity for ORR. To address these problems,I have carried out the following work in my ph. D theis:1. ORR Methodology evaluation. Due to the low solubility of oxygen in the solution, the mass transport of02in both the solution phase and in the catalyst layer will affect the electrode reaction. At the same time, the internal resistance (IR) of solution will affect the estimation of the overpotential of the oxygen reduction reaction. To completely exclude the effects of the mass transfer and the uncompensated IR, is the prerequisite for deriving the intrinsic activity of nanocatalysts for ORR. After sysmtematic studies on the effects of the internal resistance of the solution on measurement of the oxygen reduction reaction, we have pointed out the method for correct IR compensation as well as misleading conclusions in the literature; By systematically varying the loading amount of the catalyst system, we have demonstrated the impact of mass transfer on evaluating catalysts activity for ORR. Some guidelines on how to rationally evaluate of nano-catalysts activity for ORR has been given.2. Even for the best Pt based electrocatalyst for ORR, the overpotential at the onset is still above0.25V. In order to figure the origin for such high overpotential, we have systematically studied the the pH of the electrolyte solution on the thermodynamics and kinetics of OHad aodsorption.desorption as well on ORR kinetics. We found that OHad+H++e(?)H2O OHad+e(?)OH-, as parallel reaction occurs during ORR in the kinetic and kinetic and mass transport limited potential region, whose thermoelectrochemistry and kinetics strongly affect ORR kinetics. As the last step for ORR, the reversible desorption of OHad, one one hand its upper potential limit for its reversibility determines the onset potential for ORR, and its coverage determines the available sites for ORR. On the other hand, the net rate beteen the forward (OHad desorption) and backward reaction (H2Oâ†'OHad+H++e or OHâ†'Had+e) determines the net rate for ORR. We have also prepared Ti supported PtxY catalysts with various pretreatment conditions and studied their ORR behavior, which confirms the above conclusions.