We performed in-situ attenuated total reflection-surface enhanced infrared adsorption spectroscopy (ATR-SEIRAS) to probe the behavior of interfacial water (Supplementary Figure ). We focused on the O-H stretching mode (νOH) at 3000 cm -1 to 3610 cm - . Here, the measured signals were deconvoluted into three vibration modes with peaks centered at ~3620 cm - 1 , 3480 cm -1 , and 3250 cm -1 , corresponding to free or weakly H-bonded H2O, two H-bonded H2O and strongly H-bonded H2O ("ice-like" H2O), respectively (Supplementary Figure ) . Shown in Fig. , Pt/C has a negligible Stark tuning effect, in agreement with literature reports, indicative of a weak water binding strength . In contrast, Ru/C has high stark tuning rates, indicative of a strong water binding strength. Compared to Ru/C and Pt/C, both Ru/pN-C and Pt/pN-C have higher stark tuning rates. Thus, according to HBEapp theory, the improved HOR activities of pN-C support samples might be explained by the enhance water binding strength 40 .