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).