S1 Elucidating Surface Restructuring-Induced Catalytic Reactivity of Cobalt Phosphide Nanoparticles under Electrochemical Conditions Zishan Wu, †,‡ Quan Gan, †,‡,§ Xiaolin Li, †,‡,⁑ Yiren Zhong, †,‡ Hailiang Wang *,†,‡ † Department of Chemistry, Yale University, New Haven, Connecticut 06511, United States ‡ Energy Sciences Institute, Yale University, West Haven, Connecticut 06516, United States § Department of Chemistry, Nankai University, Tianjin 300071, The People’s Republic of China ⁑ College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, The People’s Republic of China * email: hailiang.wang@yale.edu