Figure 3. a) CV curves of Ni-CoP at scan rate 20 mV s-1 in 1 M KOH. b) Quasi-in situ XPS of Co 2p3/2 at different applied potentials. c) In-situ Raman spectra for Ni-CoP and CoP collected under multi-potential steps. d) Initial surface reconstruction process of CoP-based catalysts supplied by AIMD. e) The free energy of -OH adsorption and proton desorption at the P site in Ni-CoP. f) Dynamic bond lengths resulting from molecular dynamics processes.
According to in-situ Raman spectroscopy and quasi in-situ high resolution Co 2p/O 2p spectrum, CoP-based catalysts exhibit similar surface reconstruction behavior to TMOs at higher potentials, which corresponds to Co(III)-OOH and Co(IV)-OO species (Figure 4a and Figure S18)[34-38]. Considering the stability of the intermediate state generated by the reconstruction during the OER process, in-situ Raman spectra from high potential (1.5 V vs RHE) to low potential (OCP) were further collected. The vibrational state of superoxide is still can be observed, which means that the intermediate configuration can be maintained during the electrochemical process (Figure S19a). Based on the above discussion, the surface reconstruction pathway of CoP-based catalysts is summarized by the following equation:
Total process: (1)
Co site: (2)
P site (step 1): (3)
P site (step 2): (4)
Based on AIMD and DFT calculations, P vacancies were identified as the key evolution state of TMPs catalysts in the early stages of reconstruction. It is necessary to understand the specific role of doping strategies in the dissolution process of non-metal sites, which can help to controllably adjust the catalyst reconstruction efficiency and reaction kinetics. Compared with the original CoP configuration, in situ Raman spectrum shows that the reconstructed intermediate state (hydroxide, oxyhydroxide and superoxide species) of the Ni-CoP configuration can be rapidly formed at a lower anode potential (Figure 3c and Figure 4a). In addition, the superoxide phase of the Ni-CoP configuration in time-resolved in-situ Raman spectroscopy can be found at 1.3 V vs RHE, which is 0.2 V lower than the original CoP configuration (Figure S19b). Here, the regulatory effect of heteroatom Ni on the surface reconstruction process of CoP configuration is further analyzed through DFT calculations. It can be clearly observed that the heteroatom Ni can reduce the P vacancy formation energy of the original CoP during the reconstruction process by about 0.2 eV, which means that the Ni-CoP system has efficient initial reconstruction kinetics and metal-oxygen active configuration evolution process (Figure 4b).
Subsequently, DFT calculation are used to further study the regulation effect of heteroatom Ni on the HER/OER performance of CoP configuration. Considering the reconstruction behavior of the CoP-based catalytic model in the anode reaction, the oxyhydroxide model were used to discuss the impact of heteroatom Ni on the OER performance of the original configuration. DFT simulations show that the heteroatom Ni reduces formation energy barrier of key intermediate *OOH by 0.44 eV, which promotes the OER activity of the catalytic configuration by optimizing the rate-limiting step (Figure 4c). For HER, the configuration evolution behavior of CoP and Ni-CoP in 1 M KOH was studied by in situ Raman spectrum, and the vibration signals showed that the CoP-based catalytic surface can still be maintained in alkaline HER (Figure S20). Therefore, the initial CoP and Ni-CoP configurations were used as theoretical models, which further analyzed the influence of heteroatom Ni on the hydrogen adsorption energy barrier on Co sites. Compared with pristine CoP, the hydrogen adsorption energy barrier of Co sites in Ni-CoP is reduced by 0.41 eV, which means that heteroatom Ni can promote HER kinetics by adjusting the key step energy barrier (Figure 4d).