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).