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.