To investigate the relationship between the electronic structure of active site and water dissociation, we built the Mo/Co and N-Mo/Co SAAs slab model, and calculated the charge density difference, the Bader charge and the energy barrier of water dissociation on catalyst surface (Fig. 1, Supplementary Fig. 1). The charge density difference and Bader charge analyses results show that 0.64 electrons transfer from single atom Mo to substrate in N-Mo/Co, which suggests that an asymmetric charge localization is formed on Mo (Fig. 1a).9,30,31 The energy barriers of water dissociation on the Co(001), Mo/Co SAA and N-Mo/Co were 0.62, 0.44 and 0.35 eV (Fig. 1b, Supplementary Figs. 1 and 2) respectively, indicating that the asymmetric charge localization of Mo in the N-Mo/Co SAA can improve the water dissociation activity.32–36
The water transition state Bader charge analyses performed to prove charge localization Moδ+ can effectively boost water dissociation (Fig. 1c). From the adsorbed water DOS on Mo/Co and N-Mo/Co, we found that the N-Mo/Co leads to the downshift of Mo DOS, which benefit to the activation of water molecule by delocalizing the orbital electron density of the water molecule (Fig. 1d). We also considered the HER activity on Co(101) surface and obtained the same results, proving the charge localization Moδ+ in N-Mo/Co is the key center for water dissociation to promote the kinetics of HER (Supplementary Figs. 3, 4 and 5).
Based on these results, we synthesized Mo/Co SAA and N-Mo/Co by electrochemical reduction precursor in alkaline (Supplementary Figs. 6, 7, 8, 9 and 10). X-ray diffraction (XRD) patterns clearly showed that the Mo/Co and N-Mo/Co are the hexagonal close-packed cobalt structure (Fig. 2a, Supplementary Figs. 11 and 12). From the HRTEM image (Fig. 2b), the interplanar spacing of the main surface is 0.22 nm, which belongs to the Co(001) surface. Spherical aberration corrected high angle annular dark-field scanning TEM (SAC-HAADF-STEM) image (Fig. 2c) showed that the Mo, circled in red, is single-atomic dispersion in N-Mo/Co. In addition, the intensity profiles along X-Y in the HAADF-STEM confirmed the atomically dispersion of Mo in N-Mo/Co (Fig. 2b and 2d). The energy dispersive spectrometer (EDS) mapping images (Fig. 2e) and the line-scan profiles (Supplementary Fig. 13) reveal the Co, Mo and N elements are homogenously distributed in N-Mo/Co, reveal N has been uniformly doped into the Mo/Co SAA. The electron energy loss spectroscopy (EELS) displays that there is no obvious oxygen in N-Mo/Co (Supplementary Fig. 14). Inductively coupled plasma optical emission spectrometer (ICP-OES) measurement reveals that the concentration of Mo is about 2% doped in N-Mo/Co (Table 1).
To study the electronic structure and local structure of N-Mo/Co, XAS and X-ray photoelectron spectroscopy (XPS) are performed. The K-edges energy of Mo in Mo/Co and N-Mo/Co located between Mo foil and MoO3 (Fig. 3a), proving the charge localization states of Mo contained in N-Mo/Co. The local structures of Mo in Mo/Co and N-Mo/Co are obtained by extended X-ray absorption fine structure (EXAFS) spectrometer. In the Fig. 3b, Mo/Co SAA shows a coordination peak at 2.18 Å corresponding to the crystal structure of Co. After the N doped into the Mo/Co SAA, a new peak located at 1.56 Å arises, indicating the N-Mo bond formed.37
Figure 3c shows the Co L-edge spectra of Co, Mo/Co and N-Mo/Co, the two peaks located at 778 eV and 795 eV corresponds to the electron transition from Co 2P3/2 and 2P1/2 to 3d, respectively. Compared with Co, the binding energy of Co in N-Mo/Co displays a positive shift of about 0.2 eV to higher energy region due to the N doping.38 Meanwhile, similar shift of Co was also found in the Co K-edge of N-Mo/Co (Supplementary Fig. 15). For the sample of N-Mo/Co, the peaks at 398.3 and 399.9 eV can be attribute to Mo 3p3/2 and N (Fig. 3d),26 respectively, confirming that the nitrogen was doped in the Mo/Co SAA surface to form the N-Mo/Co (Supplementary Fig. 16).
The ultraviolet photoelectron spectrometer (UPS) (Supplementary Fig. 17) indicates that the d band center of N-Mo/Co shifts away from the Fermi level after N doped in Mo/Co, suggesting that the interaction of Mo and N makes a significant contribution to valence band structure of N-Mo/Co, which is consistent with DOS analysis (Fig. 1d). Overall, all these results demonstrate that the atomically dispersed charge localization site of Moδ+ has been built in N-Mo/Co SAA.
The AP-XPS was performed to confirm the active sites of water dissociation on N-Mo/Co.2 Fig. 4a and 4b show the curve-fitting of the O 1 s XPS spectra for N-Mo/Co under ultrahigh vacuum (UHV) and water pressure of 0.15 torr. The obvious OH and chemisorbed H2O on the surface are observed under water pressure of 0.15 torr, suggesting efficient water dissociation on the sample of N-Mo/Co. Figure 4c and 4d shows the Co 2p and Mo 3d XPS spectra of N-Mo/Co under UHV and 0.15 torr water pressure. The binding energies of Co 2p3/2 and Co 2p1/2 have little change in water condition. However, the binding energies of Mo 3d5/2 and Mo 3d3/2 shows an obvious positive shift of about 0.3 eV in water condition. These results further confirm that atomically dispersed asymmetry charge localization site of Moδ+ is the active site of water dissociation.
To verify the catalytic activity of Mo sites in N-Mo/Co, the electrocatalytic performance for HER is evaluated by using a three-electrode system in 1.0 M KOH solution, with Hg/HgO as the reference electrode and graphite rod as the counter electrode, respectively. The linear sweep voltammetry (LSV) curves of Co, Mo/Co, N-Mo/Co and commercial Pt/C clearly shows that N-Mo/Co exhibits the best performance among all samples (Fig. 5a). The overpotential to achieve the current density of 10 mA cm− 2 is only 12 mV, significantly better than those of Co (209 mV), Mo/Co (82 mV) and even the commercial 20 wt% Pt/C (32 mV). In Fig. 5b, the derived Tafel slopes of Co and Mo/Co are 129 and 89 mV dec− 1, indicating that the water dissociation is the rate-determining step on Co and Mo/Co. However, the Tafel slope of N-Mo/Co decreases to 31 mV dec− 1, which is even better than Pt/C (38 mV dec− 1), suggesting that the atomically dispersed charge localization of Moδ+ site can improve the intrinsic water dissociation activity of N-Mo/Co for alkaline HER. The N-Mo/Co catalyst exhibits record activity in alkaline media (Fig. 5c). The turnover frequency (TOF) values of N-Mo/Co is also obviously larger than Mo/Co (Fig. 5d, Supplementary Fig. 18), revealing the superior intrinsic HER activity of N-Mo/Co. The electrochemical impedance spectroscopy (EIS) results shows that the charge-transfer resistance (Rct) of N-Mo/Co is only 1.7 Ω, much lower than those of Co (95.6 Ω) and Mo/Co (6.5 Ω), suggesting that the N doped in Mo/Co SAA can promote charge transport kinetics (Supplementary Fig. 19).
To assess the energy barriers of HER, we studied the effect of temperature on the performance of the catalysts and found the rate constants to follow the Arrhenius relationship. The Arrhenius plots for the Co, Mo/Co and N-Mo/Co catalysts allowed us to extract electrochemical activation energies for hydrogen production (Fig. 5e, Supplementary Fig. 20). It shows that N-Mo/Co exhibits an apparent barrier value of 10.8 kJ mol− 1, which is significantly lower than those of Co (30.0 kJ mol− 1) and Mo/Co SAA (18.2 kJ mol− 1) catalyst, indicating the obviously accelerated kinetic process of HER on N-Mo/Co catalyst. Furthermore, the durability test of N-Mo/Co electrode at potential of 12 mV was performed to evaluate the stability of the N-Mo/Co (Fig. 5f). The catalyst exhibits an initial current density of 10 mA cm− 2, and there is no obvious decrease after 200 h test, indicating its excellent electrochemical stability for long-term operation. The Faradaic efficiency remained about 100% in the HER process.
Inspired by such excellent HER activity, we also assemble a water-alkali electrolyzer with N-Mo/Co as cathode and anode simultaneously to evaluate its potential for overall water splitting. The cell voltage needed only 1.5 V to give a current density of 10 mA cm− 2 and it presented a negligible loss after a long time test of about 45 h (Supplementary Fig. 21). These results fully demonstrated that the N-induced atomically dispersed charge localization site of Moδ+ on N-Mo/Co has high water dissociation activity and stability.