*2.2. Electrochemical Fabrication of Pd Particles and Pd–Pt Bimetallic Catalysts*

Firstly, prior to the electrodeposition of Pd particles, the carbon paper was ultrasonically cleaned with anhydrous ethanol, acetone, and deionized water for 30 min separately. Secondly, Pd particles was electrochemically deposited on the surface of the carbon paper at −0.15 mA cm–2 for 45 min in an aqueous solution of 5 mM PdCl2 and 0.5 M H2SO4, using a conventional electrochemical cell with a three-electrode system (Ivium Stat, Ivium Technologies, Eindhoven, Netherlands). The cleaned carbon paper served as the working electrode. A platinum plate electrode and a mercurous sulfate electrode (MSE) were used as the counter electrode and the reference electrode, respectively. At last, based on the obtained pure Pd particles, a Pd–Pt bimetallic catalysts/carbon paper electrode was finally synthesized in a two-electrode system by PSWP (NF BP4610, NF Corporation, Yokohama Japan) with different frequencies (10 Hz, 50 Hz, and 90 Hz) and modification times (1 h, 2 h, and 4 h) in an aqueous solution of 0.1 mM H2PtCl6 and 1 M H2SO4. Correspondingly, the obtained Pd–Pt samples were denoted as PdPts–10 Hz, PdPts–50 Hz (4 H), PdPts–90 Hz, PdPts–1 H, and PdPts–2 H, respectively. The upper and lower limit potentials of the PSWP were 0.6 V and −3.2 V, respectively.
