PdCo, oxygen reduction reaction, oxidation treatment, degree of surface oxidation, temperature-programmed reduction, PdO
One of the key issues for polymer electrolyte membrane fuel cells (PEMFCs) is to reduce or replace expensive Pt cathode catalysts. Although the development of non-Pt, such as Pd-Co catalysts for the oxygen reduction reaction (ORR) attracts growing attention, their catalytic activity is needed to be further improved so that they may complete with Pt catalysts. Herein, the enhancement of ORR activity for Pd-Co/C by oxidation treatment is investigated. Based on the XPS characterization, a new term, degree of surface oxidation (DSO), is proposed to illustrate their relationship between activity and surface oxidation extent. Temperature-programmed reduction (TPR) characterization provides the evolution of surface species on the topmost region of the catalysts. It is obviously found the oxidation at 520 K results in the formation of well-constructed PdO structures along with 100 % of DSO value, thereby leading to the superior performance of PdCo/C when compared with the as-prepared one. However, a deteriorated ORR activity is observed after severe thermal treatment due to the sintering of Pd-Co nanoparticles. Besides, a proposed model is demonstrated to illustrate the oxidation effect of PdCo/C catalysts on their ORR performance.