Figure 2. Electrochemical properties of bare CC, CoP, Ni-CoP and RuO2: a) LSV curves for OER, b) Tafel slope for OER, c) LSV curves for HER. d) Polarization curves of overall water splitting. e) Chronopotentiometry curve of Ni-CoP-0.1 mmol || Ni-CoP-0.1 mmol with a constant current density of 10 and 100 mA cm-2 for 100 h.
2.3. Self-reconstruction process and theoretical simulation
The complex reconstruction process of transition metal-based catalysts in alkaline OER processes has always been considered an important issue in the field of water electrolysis. The actual catalytic active phase and configuration evolution mechanism are difficult to identify by traditional spectroscopic characterization, especially the atomic-level evolution kinetics of non-metallic sites in catalyst systems such as phosphide and sulfide. In previous reports, the active phase of transition metal-based sulfide and phosphide in alkaline OER is usually considered to be oxyhydroxide, which fails to intuitively propose a complete evolution path, key thermodynamic steps and regulation method. Here, the CoP-based catalytic configuration with excellent water splitting activity is used as a typical model. The configuration evolution mechanism is systematically studied based on electrochemical characteristic curve coupled in-situ spectroscopy characterization, DFT calculations and AIMD simulations. Double redox peaks were observed in the CV curve of Ni-CoP catalyst, which represents the typical autoxidation behavior of M2+/M3+ and M3+/M4+ at metal sites, approximately corresponding to 1.1 V and 1.3 V vs RHE (Figure 3a). The valence evolution behavior of the Co site was confirmed in the quasi in-situ high resolution Co 2p spectrum, which gradually changes from Co-POX bonding mode to Co2+, Co2+/Co3+ and Co3+/Co4+ contributed by Co-O bonding mode (Figure 3b). Interestingly, in situ Raman spectroscopy shows that CoP-based catalysts exhibit vibration signals of hydroxide species at non-Faradic interval, corresponding to the Co-OH configuration, which is obviously different from the reconstruction behavior of traditional oxides (Figure 3c). DFT calculations were used to elucidate the interaction between the lattice P sites and adsorbed oxygen species during the initial reconstruction process from the atomic scale, which demonstrates the lower adsorption (0.068 eV) and proton desorption energy barrier (-0.438 eV) of -OH at the P site (Figure 3e). The above calculation results provide the basic path of the initial reconstruction process from a static thermodynamic perspective, which is mainly the -OH adsorption at the P site followed by the spontaneous proton desorption to form a single oxygen (-O) group. AIMD provides an intuitive initial reconstruction process from the perspective of dynamic simulation, which obviously confirms the results of in-situ Raman spectroscopy and DFT calculations. Interestingly, abundant -O groups are formed by the -OH adsorption process at active sites and spontaneous proton desorption behavior, which is accompanied by P vacancies caused by POX coordination and delattice process (Figure 3d). Furthermore, the connection between different surface coverage of -O groups and surface reconstruction is further discussed through AIMD. When the -O group coverage occupies half of the active sites, although P-O bonds are formed, but Co-P bonds are maintained in the original configuration (Figure S17a). Considering that the configured surface is exposed to a richer -OH environment at the anodic potential, the coverage of the -O group is further adjusted from half coverage to full coverage (Figure S17b). The results show that POXcoordination and the breaking behavior of Co-P bonds are positively correlated with the coverage of -O groups, which is accompanied by the generation of P vacancies (Figure 3f and Figure S17c). Therefore, P vacancy is an important intermediate configuration in the initial reconstruction process of TMPs-based catalytic systems in OER. The broken Co-P bond is accompanied by the formation of abundant unsaturated coordination metal sites, which promotes the formation process of the Co-OH intermediate state. Based on reduced Co-P peak area in quasi in-situ high-resolution P 2p spectrum and the stretched Co-O bond provided by AIMD, it can be clearly observed that the interaction between Co and P sites is weakened during the reaction process (Figure S18).