*2.1. Reagents and Materials*

Palladium (II) chloride (PdCl2; Adamas Reagent Co., Ltd., Shanghai, China), chloroplatinic acid hexahydrate (H2PtCl6·6H2O; Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China), sulfuric acid (H2SO4; Jiangtian Chemical Technology Co., Ltd., Tianjin, China), formic acid, and anhydrous ethanol (Yuanli Chemical Technology Co., Ltd., Tianjin, China) were of analytical reagent grade. A carbon paper (TGP-H-060) and commercial 10 wt % Pd/C catalysts were purchased from Toray (Toray Industries, Tokyo, Japan) and Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China), respectively. The deionized water was produced with a Millipore Milli-Q system (18.2 MΩ cm).
