Keywords
Surface reconstruction, In situ Raman spectra, Oxygen evolution
reaction, Transition metal phosphides, Phase transformation
1. Introduction
In recent years, electrocatalytic water splitting has been recognized as
a reliable way to produce green hydrogen in the field of energy
conversion, which can help alleviate environmental problems caused by
fossil energy[1,2]. However, the water splitting
reaction is severely inhibited by the four-electron transfer process
corresponding to the anode oxygen evolution reaction (OER), which makes
it urgent to explore catalysts with suitable cost and activity to
optimize the overall reaction rate[3,4].
Ruthenium/iridium-based noble metal catalysts have demonstrated
excellent catalytic activity in alkaline OER, but large-scale
application strategies are limited by high cost and low
stability[5-8]. As typical representatives of
non-noble metal materials, transition metal-based oxides (hydroxides),
phosphides and selenides can optimize the adsorption/desorption behavior
and rate-limiting step free energy of oxygen intermediates, which
alleviates energy consumption issues in the green hydrogen production
process[9-13]. Importantly, the above catalytic
configuration is often accompanied by surface reconstruction behavior
under operating conditions, which leads to the transformation of the
original configuration into a hydrogen oxide phase or a superoxide
phase. Therefore, it is necessary to comprehensively understand the
intrinsic reconstruction kinetics of catalytic configurations in water
electrolysis reactions, which can help distinguish real active phases
and design more valuable structure-activity relationships.
Compared with transition metal oxides (TMOs), the quasi-covalent
bridging mode between metal and non-metal site in transition metal
phosphides (TMPs) leads to excellent conductivity and water splitting
activity, but complex reconstruction behavior is difficult to be
visually observed at the atomic scale[14-19].
Although the terminal reconstruction configuration of TMPs is also
considered to be oxyhydroxide or superoxide, but the evolution behavior
of phosphorus species during the initial reconstruction process has not
been fully elucidated, which is crucial for regulating the generation
behavior of the active phase[20-22]. In addition,
the impact of the charge redistribution effect introduced by the doping
strategy on the important reconstruction process has not been elucidated
from a theoretical level, which ignores the complex interaction between
phosphorus species and oxygen species[23-25].
Considering that the interaction between lattice phosphorus and oxygen
adsorbed species plays an important driving role in the overall
reconstruction, which urgently requires the combination of in-situ
spectroscopic characterization methods, thermodynamic theoretical
calculations, and molecular dynamics simulations to comprehensively
provide the internal connection between hybrid structure and
configuration evolution process.
In this work, CoP nanowires prepared by hydrothermal-phosphating method
were used as catalyst models, which has demonstrated excellent
water-splitting activity and long-term stability. Typical double
oxidation peaks are found in the cyclic voltammetry (CV) curve of NiCoP
nanowires, which combined with quasi in-situ X-ray photoelectron
spectroscopy (XPS) are identified as the autoxidation behavior of metal
sites, including Co2+/Co3+ and
Co3+/Co4+. Interestingly, the
typical vibrational mode of Co(OH)2 in the in situ Raman
spectrum is found at 0.7 V vs RHE, indicating that the initial
reconstruction behavior of the CoP system can be driven by a lower
non-Faradaic potential. Based on adsorption-desorption thermodynamics
and kinetics, density functional theory (DFT), ab initio molecular
dynamics (AIMD) simulations and in situ spectroscopic characterization
intuitively provide the configuration evolution process of CoP-based
catalysts from the atomic scale, which mainly includes:
OH- adsorption, spontaneous proton desorption, P-O
coordination, destroyed Co-P bond, Co-OH, Co-O, Co-OOH and Co-OO (Figure
1a). In addition, the reconstruction process of CoP can be significantly
promoted by the heteroatom Ni, which mainly reduces the formation energy
of P vacancies. Summary, the initial reconstruction behavior and phase
transition regulation mechanism of typical CoP model were studied, which
provides novel insights into the advanced reconstruction-activity
relationships of water electrolysis catalysts.
2. Results and discussion
2.1. Synthesis and Structural Characterizations
As a catalytic model, CoP-based nanowires were uniformly prepared on the
surface of carbon cloth (CC) through a hydrothermal-phosphating method
(Figure S1). X-ray diffraction (XRD) patterns exhibit an orthorhombic
phase of CoP-based nanowires with space group Pnma (62), well matched
with CoP (PDF#29-497) (Figure S2)[26,27].
Compared with CoP, the individual diffraction signals of Ni-CoP are
slightly shifted towards low Bragg diffraction angles, implying that the
heteroatom Ni is successfully incorporated into CoP (Figure S2a).
According to scanning electron microscope (SEM) and high angle annular
dark field (HAADF), the CC fibers are coated by Ni doped CoP nanowires
with an average length 2 μm, which still maintains the nanowire shape of
precursor (Figure 1b and Figure S3 and Figure S4). The high-resolution
TEM (HR-TEM) images of Ni-CoP indicate lattice fringes of 0.241 nm and
0.249 nm, corresponding to the (102) and (111) planes (Figure
1c)[28]. The constituent elements of CoP and
Ni-CoP are uniformly distributed on the nanowires, and TEM-energy
dispersive spectrometer (TEM-EDS) confirm that the constituent elements
basically meet the reagent dosage ratio in the experimental strategy
(Figure 1b and Figure S5a-c). The bright diffraction rings are exhibited
in selected area electron diffraction (SAED) patterns of CoP and Ni-CoP
nanowires, corresponding to (011), (111), (112) and (020), which is
consistent with the preferred orientation in the XRD pattern (Figure
S5d,e).
XPS and X-ray absorption spectroscopy (XAS) were used to further analyze
the elemental composition and electron population of the CoP-based
catalyst, which provided the control behavior of various heteroatoms on
the original electronic structure. The constituent elements of CoP and
Ni-CoP are observed in XPS, which is corroborated by the TEM-EDS image
(Figure S6a). In the Co 2p spectra of CoP, the double peaks appearing at
779.05 and 794.25 eV are attributed to Co-P bond (Figure 1d). The peaks
centered at 782.22 and 798.68 eV and two satellite peaks obtained at
788.21 and 804.19 eV can be attributed to Co-POxresulting from oxygen group adsorption behavior. In addition, peak
signals in the high-resolution P 2p spectrum are found at 129.29 eV and
130.17 eV, corresponding to P 2p3/2 and P
2p1/2 (Figure S6b). The peak signal provided by the P-O
bond is observed at 134.3 eV. The electronegativities Ni are 1.91, which
are higher than that of Co (1.88)[29,30]. The
heteroatom Ni in CoP lattice causes electrons to transfer from Co to Ni,
leading to the positively shift in binding energy. Hence, compared with
the typical peak signal of CoP, Co 2p and P 2p signal of Co site in
Ni-CoP is positively shifted, which is located at electron loss region.
Synchrotron radiation-based soft XAS was used to further determine the
modulation behavior of the heteroatom Ni on the charge population around
the Co site. The Co L-edge XAS spectra of CoP nanowire are divided into
L3-edge (780.33 ~ 783.68 eV) and the
L2-edge (795.88 ~ 798.19 eV) (Figure
1e). Besides, slight shoulder signals are observed near Co
L2,3-edge peaks, which are attributed to two
coordination environments around the Co site, including to Co-P and
Co-POx bond. Obvious absorption peaks are found at
136.87 ~ 139.23 eV and 146.84 eV, which correspond to
the P 2p signal in CoP (Figure S6c). The positive shift behavior of Co
and P L-edge affected by heteroatom Ni confirm the Co 2p and P 2p in
XPS. Further, the soft XAS peaks are located at approximately 854.9 eV
(L3-edge) and 873.24 eV (L2-edge),
corresponding the electron redistribution from Ni 2p3/2and 2p1/2 to 3d orbitals, which means the successful
incorporation of Ni heteroatoms into the CoP lattice (Figure S6d,e). The
initial reconstruction behavior and the rate-limiting energy barrier can
be controlled by the charge redistribution effect between the heteroatom
Ni and Co sites, which will be introduced in the theoretical
calculations.