Tip-generation/substrate-collection mode (TG-SC) of scanning electrochemical microscopy (SECM) was used to study oxygen reduction reaction (ORR) at nanoporous Pt films depending on their pore characteristics. Nanoporous Pt films were electrodeposited in solutions containing Pt precursor, Triton X-100, and lead acetate. The varied composition of electroplating solutions could control the porosities of the deposited Pt films with three different degrees (denoted as npPt-I, npPt-II, and npPt-III): npPt-I had only nanoscale pores but npPt-II and npPt-III had both nanoscale and microscale pores; and npPt-III possessed additional microscale pores. Recessed Pt tips deposited with three different npPts were used to observe their ORR activities in TG-SC mode of SECM, considering its advantage of ∼100% collection efficiency for H2O2 (2-e transfer product of ORR). The SECM results indicate that npPt-I is beneficial for the kinetic controlled potential region via the O2 molecule confinement within npPt-I, while npPt-III is more advantageous in the mass-transport controlled region due to the more promoted O2 influx through the microscale pores leading to the more efficient utilization of the electrode surface for ORR.
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