“…The overpotentials at 10 mA/cm 2 increase in the order of Co-P-0.3 (143.85 mV) < Co-P-0.2 (167.11 mV) < Co-P-0.4 (171.59 mV) < Co-P-0.1 (227.38 mV) < Co-P-0.5 (280.89 mV) < Co-P-0.6 (345.98 mV) < Co (346.27 mV), exhibiting a volcano-like trend and showing the best HER performance for Co-P-0.3, that is, Co 94 P 6 . When compared with commercial Pt-based catalysts and others, the lowest overpotential of 143.85 mV in the case of Co-P-0.3 was still higher than those of Pt/C on Ti (14 mV), 32 Pt/C on CP (10 mV), 77 NiS 2 -Ni(OH) 2 (90 mV), 78 and MoS x /Ni-metalorganic framework-74 (114 mV) 79 but comparable to other non-PGM catalysts such as CoP (130 mV), 80 FeMoS (140 mV), 81 and N, P-dual-doped MoS 2 (147 mV) 82 and even better than [Mo 3 S 13 ] 2À (188 mV), 50 MoS 2 (190 mV), 83 NbS 2 (207 mV), 84 CoCu (342 mV), 27 and VS 2 (350 mV). 85 In addition, the Tafel slopes of each catalyst at the fourth cycle (Figure 5H) exhibit the same volcano-like trend, showing the lowest slope of 60.10 mV/dec for Co-P-0.3 (Co 94 P 6 ), which is comparable to results obtained previously for Co-P, that is, Tafel slopes from 45 to 82 mV/dec.…”