Background:
Breakthroughs in the atomic elementality of the catalyst surface can build a synergistic center of activity, which is helpful for complex multi-step catalytic reactions. Xie Shuifen's team from Huaqiao University reported a defect-derived strategy to create phosphorus vacancies (p-vacancies) on the surface of nanoscale Rh2P electrocatalysts, thereby significantly improving the electrocatalytic performance of alkaline hydroxide reactions (HOR). DFT computation provides an atomic-level theoretical explanation for the mechanism of this strategy.
Calculation method: DFT calculations are done using the CaStep module in the Materials Studio software, using Generalized Gradient Approximation (GGA) and Perdew-Burke-Ernzerh (PBE) functionals to describe the exchange and correlation interactions. The interaction between valence electrons and ion nuclei is described by ultrasoft pseudopotential. In this study, a four-layer of Rh2P(200) sheets with a vacuum gap of 15 3 supercells was constructed to simulate P-Rh2P, while removing the outermost half of the Rh and P atoms to simulate D-Rh2P, which is rich in surface defects. The electron wave function unfolds on the basis of a plane wave with a truncated energy of 400 ev.
For geometric optimization, the k-point of the 2 2 1 monkhorst-pack mesh is used for calculation, and the convergence thresholds for energy and force are set to 1., respectively0 6 ev and 003 ev/å。The adsorption free energies of each intermediate (including HAD, OHAD, HAD+OHAD and H2OAD at different coverages) are determined by the formula δg = δe + ZPE tδs, where δe, δZPE and δs represent the binding energy, zero energy change and entropy change of the adsorption process, respectively.
Results & Discussion.
Previous studies by the team have confirmed that the P-capping ends on the Rh2P(200) facet exhibit the best hydrogen-electrocatalytic performance, thanks to their relatively low thermoneutral ΔGH values. However, considering the bifunctional mechanism of basic HOR or the interfacial water recombination theory, the presence of surface hydroxyl groups may also be a key factor for excellent catalytic performance, and further research on the mechanism of this process is needed. Since there is only one type of P site on a highly symmetrical Rh2P surface, there may be competing adsorption behavior between HAD and OHAD (and potentially H2O molecules), which in turn hinders the overall reaction kinetics. Thus, the disruption of the atomic singleness catalytic surface of the p-cap contributes to the generation of additional active centers to decouple the competitive adsorption behavior. 
figure 1.Structural Models and Theoretical Calculations. Top and side views of the structural models of (a, b) (a) p-rh2p and (b) d-rh2p, where the blue-green, purple, red, and white spheres represent rh, p, o, and h atoms, respectively. (c) Adsorption behavior of HAD, OHAD, and H2OAD on two catalyst surface models. The coordination number is indicated by a number in square brackets on the horizontal axis. (D, e) Structural models of D-RH2P and P-RH2P with different HAD and OHAD coverage. (f) OHBE and HBE at different coverage levels (OHAD + HAD) on P-RH2P and D-RH2P
This strategy is achieved through defect engineering. To investigate this strategy, density functional theory (DFT) calculations were first performed to check the differences between P-RH2P and D-RH2P (see Figures 1A, B). In contrast to P-Rh2P, which exhibits only one type of active site (P-terminal atoms) on the surface, four different P-sites with different unsaturated coordination numbers are exposed on D-Rh2P, and there is one open Rh site. Figure 1c illustrates the adsorption behavior of HAD, OHAD, and H2OAD on two surface models. On P-RH2P, the advantage of appropriate HBE values is offset by stronger OHBE because a single type of reactive P-site elicits competitive adsorption behavior. Thus, on P-RH2P, most of the P sites are occupied by OH. On the D-RH2P surface, similar competitive adsorption behavior was observed at the P site (P1 P3). However, exposed Rh atoms with thermally neutral Hbe values can serve as specialized H adsorption sites. The presence of OHAD and H2OAD at the P site does not affect the adsorption of H by Rh atoms exposed at these sites.
These theoretical calculations show that the Rh2P catalyst exhibits abundant defects and or surface P vacancies, which can be used for alkaline HOR electrocatalysis. Previous studies [32,34] have confirmed the advantages of one-dimensional ultrathin structures, such as zigzag nanowires, with abundant grain boundaries, stepped atomic surfaces, and reactive unsaturated sites. These zigzag nanowires are considered to be ideal platform electrocatalysts for the preparation of defective Rh2P. Therefore, in this study, sawtooth RH nanowires rich in grain boundaries were prepared by the polyol method with an average thickness of only 19 nm (see Figure S1). Subsequently, D-RH2P NWS was prepared by a heat-induced (240°C) method using a phosphate treatment of prefabricated defective ultrathin RH. The treatment is performed in oleamine and tri-n-octyl phosphine (TOP) is used to provide pH3 as a phosphorus source (see Figure 2A). High-angle annular darkfield scanning transmission electron microscopy images (HAADF-STEM) confirmed that the prepared D-RH2P NWS (see Figures 2b and c) retained the wavy morphology of the original RH NW. According to statistics, the average diameter of phosphorylated D-Rh2P ranges from 19 nm increased to 28 nm (see Figure S4), which can be attributed to the larger unit cell parameters of Rh2P (5.)5021 å）
With RH (33804) compared to D-RH2P NW (55021) STEM-EDS profiles (see Figure 2D) clearly show the coexistence of P and Rh elements in D-RH2P NW. More importantly, the high-resolution HAADF-STEM images show abundant grain boundaries and unsaturated coordinated steps that are the atomic structure of the catalyst surface (see Figure 2E-G). The lattice fringe spacing is 0., respectively275 and 0198 nm, corresponding to the (200) and (220) planes of Rh2P. The corresponding Fast Fourier Transform (FFT) mode (see Figure 2F) further confirms the crystalline phase formation of Rh2P. The atomic projection shows the highlights of D-RH2P NW (inset), and the intensity shows the irregularities and abundant P-vacancies of the defect-derived surface atoms. In contrast, the prepared P-RH2P NC, with a flat exposed surface, was conformal phosphating by implementing the synthesis method (see Figures S5 and S6). Detailed structural characterization (see Figure S7) verifies that the resulting P-RH2P NC has a flat surface at the atomic scale, indicating the presence of a homogeneous P-terminated surface.
Figure 2: (a) Schematic diagram of the synthesis procedure for obtaining D-RH2P nanowires;(B, C) Haadf-stem images of D-Rh2P nanowires;(d) STEM images of D-RH2P nanowires and corresponding EDS element mappings;(e, f) Distortion-corrected HAADF-STEM image and corresponding fast Fourier transform pattern, taken from the yellow square (inset in f);(g) Atomic projection with bright spot contrast intensity (inset), showing the atomic irregularities on the surface of the D-Rh2P nanowires and the arrangement of Rh and P atoms.
In this study, the prepared catalyst was loaded on a carbon support (VULCAN XC-72) with a metal load of 20 Wt%, and its electrochemical performance was studied by ICP-MS. The performance of D-RH2P NWS C, P-RH2P NCS C, and commercial PT C catalysts was compared under the same conditions. As shown in Figure 4A, D-RH2P NWS C outperforms the other reference catalysts in terms of HOR activity, exhibiting a higher anode current density as the applied current increases. The polarization curve of D-RH2P NWS C was recorded as 625 to 2500 rpm in the rotational speed range (Figure 4B). The plateau current increases with increasing rotational speed, suggesting that this part of the reaction process is controlled by H2 mass transfer. Using these platform currents, a koutecky levich (k-l) diagram was constructed along with the corresponding rotational speeds. In these plots, the slopes of D-RH2P NWS C, P-RH2P NCS C, and PT C are respectively. 86 and 480 cm2 MA1S12 (Figure S8). These values correspond to the reported theoretical values of the two-electron hor pathway (487 cm2 ma 1 s 1 2) are highly consistent.
The electrochemically active surface area (ECSAS) of the detected sample was estimated by CO peeling (Figure S9). Due to its ultra-thin one-dimensional morphology and defective surface, the ECSA value of D-RH2P NWS C is larger than that of P-RH2P NCS C and PT C. The exchange current density (J0) of these three catalysts was determined using the Butler-Volmer equation by nonlinear fitting of their Tafel plots. As shown in Figure 4C, D-RH2P NWS C shows the highest J0 value (317 mA2), almost pt c(101 mA2), which is three times that of P-RH2P ncs C(169 m2). The kinetic energy current density (JK) of the catalysts was extracted from their polarization curves by the K-L equation to evaluate the charge transfer rate at their surface.
Figure 4D and Table S4 show the MA(JK,M) and SA(JK,S) obtained by normalizing the JK values using PGM mass loading and ECSAS at = 50 MV, respectively. D-RH2P NWS C exhibits extremely high MA (785 A mgpgm 1) and sa (114 mA cmecsa2), these values are higher than those of P-RH2P ncs C(1.).36 A mgpgm 1 and 249 mA CMECSA2) as well as commercial PT C (055 A mgpgm 1 and 116 mA cmecsa2) is 57/4.5 and 143/9.8 times. The superior electrocatalytic performance of Dh2P NWS C is significantly better than that of other recently reported basic HOR catalysts (Figure 4F and Table S5). In addition, the Rh2P nanomaterials prepared in this study showed significantly improved activity compared with monometal PT, especially after the construction of abundant defect surface sites.
Figure 4: (A) Ir-corrected D-RH2P NWS C, P-RH2P NCS C, and commercial PT C catalysts at room temperature at 0HOR polarization curve in 1 M koh;(b) The HOR polarization curves of D-RH2P NWS C at different rotational speeds and their corresponding K-L plots at = 150 mV(c) Tafel diagram of the catalyst obtained by Butler-Volmer fitting;(d) Histograms of Sa and MA at = 50 mV;(e) Timing current response at = 50 mV;(f) Comparison of MA of D-RH2P NWS C with other recently reported catalysts under alkaline conditions. (Note: Figure 3 is not interpreted, Figure 4 is the original ** serial number).
Conclusions and prospects.
The surface phosphorus vacancies of the defects can break the singleness and atomic flatness of the metal phosphide surface, and significantly improve the electrocatalytic performance of alkaline HOR. Density functional theory calculations show that exposing Rh atoms on defective surfaces can decouple the competing adsorption behaviors of HAD and OHAD on P-capped P-Rh2P(200) surfaces. The presence of P vacancies exposes the subsurface RH site, which becomes a unique H adsorption site, which cooperates with OHAD at the peripheral P site to effectively accelerate the alkaline HOR.
In addition, this study proves by the calculated work function that the inhomogeneous defect surface is more conducive to causing confusion of interfacial water molecules, resulting in the presence of more "H-up" structured H2O with mismatched geometries. These theoretical results were confirmed by experiments. Defect-rich D-RH2P nanowires (NWS) and elemental P-RH2P nanocubes (NCS) were synthesized through a thermally induced consistent phosphating process to represent the catalytic surfaces of D-RH2P and P-RH2P experimentally.
The experimental results are consistent with the expected results of DFT theory, and the defect-rich D-RH2P NWS catalyst has better basic HOR activity and stability. In this study, the monotony of atoms on the surface of metal phosphide was broken through theoretical calculation and experimental verification, and the extraordinary electrocatalytic performance of alkaline HOR was realized.
Bibliographic information. huang, hongpu, liu, kai, yang, fulin, cai, junlin, wang, shupeng, chen, weizhen, wang, qiuxiang, fu, luhong, xie, zhaoxiong, xie, shuifen. breaking surface atomic monogeneity of rh2p nanocatalysts by defect-derived phosphorus vacancies for efficient alkaline hydrogen oxidation. angew. chem. int. ed. 2023, e202315752.
doi: 10.1002/anie.202315752