Ordered Mesoporous Intermetallic PtP 2 Nanoparticles with Enhanced Electrocatalytic Activity and Stability for the Hydrogen Evolution

Open AccessCCS ChemistryRESEARCH ARTICLES7 Nov 2022Ordered Mesoporous Intermetallic PtP2 Nanoparticles with Enhanced Electrocatalytic Activity and Stability for Hydrogen Evolution Yanzhi Wang, Lizhi Sun, Hao Lv, Chengbin Zheng and Ben Liu Yanzhi Wang Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064 Google Scholar More articles by this author , Lizhi Sun Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064 Google Scholar More articles by this author , Hao Lv Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064 Google Scholar More articles by this author , Chengbin Zheng *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064 Google Scholar More articles by this author and Ben Liu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202451 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Ordered mesoporous noble metals (NMs) have displayed unique catalytic and electrocatalytic performance distinctive from traditional nanoparticles. Despite great efforts, the range of mesoporous NMs is mainly limited to single metals and their metal alloys with random atomic arrangements. Herein, we report a simple solid-phase synthesis of novel mesoporous intermetallic noble metal-nonmetal (MI-PtX2) nanoparticles with hierarchical orderliness as highly efficient electrocatalysts for hydrogen evolution reaction (HER). The synthesis relies on a Pt-to-PtX2 evolution with mesoporous Pt confined in thermally stable KIT-6 (Korea Advanced Institute of Science and Technology-6) as the concurrent template. Meanwhile, the method could be extended readily to control structures and compositions of mesoporous intermetallic nanoparticles such as hollow structures and ternary MI-PtMP2. Samples featured rhombic dodecahedral morphology, ordered gyroid mesostructure, and cubic/hexagonal intermetallic phase, producing abundant undercoordinated sites and optimized surface electronic structures. These features kinetically accelerate H2O dissociation to remarkably enhance electrocatalytic HER performance. The optimum MI-PtP2 disclosed ultrahigh mass/specific activity (3.31 A mgpt−1/7.75 mA cmpt−2) and superior stability (only 15.2% of mass activity loss after an accelerated durability test for 30,000 cycles), suppressing the reported electrocatalysts. Our work opens up new opportunities for designing and synthesizing novel hierarchically ordered mesoporous electrocatalysts with targeted functions for a variety of applications. Download figure Download PowerPoint Introduction Cathodic hydrogen evolution reaction (HER) plays a more decisive role in electrochemical water splitting.1,2 It has potentially produced the greenest and carbon-free molecular H2, but the performance still falls short of expectations because of the lack of highly active and stable cathode electrocatalysts, especially in alkaline solutions.3–6 Platinum (Pt)-based nanomaterials have widely been studied for alkaline HER and still suffer from the unsatisfied activity and poor stability ascribed from the insufficient kinetics (especially in a low concentration of protons).7–10 An effective route to accelerate HER kinetics is alloying Pt with other elements, including Co, Ni, and so on.11–17 The introduction of proper secondary elements can effectively optimize the surface electronic structure of Pt and facilitate the dissociation of H2O into protons (in Volmer and Heyrovsky steps). These electrocatalysts’ design principle has recently extended to atomically ordered Pt-based intermetallics with strong interatomic (d-orbital) interaction, as well as strict stoichiometry and atomic arrangement.18–21 Compared with random alloys, intermetallic electrocatalysts are electrochemically more stable against chemical corrosion and leaching of active metals.22–26 Despite some encouraging achievements over recent years, their performance is still unsatisfactory, implying a vast room for further enhancement, including designing novel noble metal (NM)-nonmetal intermetallics.27–32 Nanostructuring Pt-based electrocatalysts represents another effective route to expose more Pt active sites and enhance their electrochemical performance.33–36 As second-generation mesoporous materials, mesoporous NMs have exhibited diversified advantages, including abundant undercoordinated metal sites, high mass/electron transfer, inherent antiaggregation ability, and so forth.37–43 Moreover, three-dimensional (3D) mesoporous NMs possess remarkably enhanced activity and stability in various catalytic and electrocatalytic reactions ( Supporting Information Figure S1).44–50 Generally, mesoporous NM-based nanomaterials are prepared under well-regulated conditions to precise engineer kinetically driven crystalline growth along mesoporous templates.39,51,52 However, they are very inconsistent with the synthesis conditions for preparing atomically ordered intermetallics that require high-temperature to control the atomic interdiffusion thermodynamically.53 Therefore, designing thermally stable mesoporous templates that facilitate the atomic interdiffusion of ordered intermetallics and simultaneously stabilize the templated synthesis of mesoporous structure is critical to prepare ordered mesoporous intermetallic nanomaterials. In spite of some progress, there are very limited examples of successful synthesis of mesoporous intermetallics.54,55 Furthermore, the preparation of novel hierarchically ordered mesoporous noble metal-nonmetal (and even noble metal–metal-nonmetal) intermetallics and further exploration of their electrochemical performance have never been reported, as far as we know. In this work, we report the first synthesis of four mesoporous intermetallic noble metal-nonmetal PtX2 (MI-PtX2) (X = P, S, Se, and Te) and three noble metal–metal-nonmetal MI-PtMP2 (M = Ni, Co, and Fe) with high macroscopic/mesoscopic and atomic orderliness. The synthesis relied on a Pt-to-PtX2 (PtM-to-PtMP2) evolution in a concurrent template [mesoporous Pt confined within thermally stable mesoporous KIT-6 (M-Pt/KIT-6) (KIT-6 = Korea Advanced Institute of Science and Technology-6)], which enabled precise nucleation growth of ordered PtX2/PtMP2 without destroying the ordered mesoporous structure (Figure 1a). Ordered MI-PtP2 nanoparticles optimize physicochemical properties and kinetically accelerate the dissociation of H2O, remarkably enhancing HER activity and stability in alkaline media. Typically, ordered MI-PtP2 electrocatalyst discloses a low overpotential of 33.6 mV at 10 mA cm−2 and high mass and specific activity of 3.31 A mgpt−1 and 7.75 mA cmpt−2 at 100 mV, surpassing the state-of-the-art electrocatalysts in the literature. Impressively, ordered MI-PtP2 maintains 84.8% of mass activity and 82.4% of electrochemically active surface area (ECSA) even after an accelerated durability test (ADT) for 30,000 cycles. Thus, our result provides new opportunities for designing a novel high-performance electrocatalyst composed of hierarchically ordered macro/mesoporous and atomic structures for various catalytic and electrocatalytic applications. Figure 1 | Synthesis strategy. (a) Schematic illustrations of synthesizing ordered MI-PtX2 nanoparticles through a solid-phase concurrent template strategy. (b) SAXS and (c) PXRD patterns and corresponding atomic structures of ordered MI-PtP2, MI-PtS2, MI-PtSe2, and MI-PtTe2 nanoparticles. Download figure Download PowerPoint Experimental Methods Synthesis of M-Pt/KIT-6 hybrids and M-Pt nanoparticles M-Pt/KIT-6 hybrids were synthesized by a classic nanocasting method. In a typical synthesis, 0.30 g of KIT-6 and 80.0 mg of K2PtCl4 were dispersed in 2.0 mL of water and dried under a vacuum to obtain a powder. Then, 2.0 mL of 0.25 M freshly prepared ascorbic acid (AA) solution was added dropwise to the powder to start the crystallization of M-Pt within mesoporous KIT-6. M-Pt/KIT-6 was obtained by further reacting at room temperature for 10 h. After that, monometallic M-Pt nanoparticles were prepared by etching with 5% hydrogen fluoride (HF), washed three times with water/ethanol and dried at 50 °C. This hybrid was used to synthesize MI-PtP2 nanoparticles. Meanwhile, M-Pt/KIT-6 hybrids with smaller M-Pt were also prepared to synthesize hollow structured MI-PtP2 and other MI-PtX2 nanoparticles. The synthesis products were the same as indicated above, but 40.0 mg of K2PtCl4 was used as the Pt source. Synthesis of ordered MI-PtX2 nanoparticles MI-PtX2 was synthesized by a concurrent template strategy. In a typical synthesis of MI-PtP2, 500 mg of NaH2PO2 and 100 mg of M-Pt/KIT-6 hybrids were first mixed and then ground. Then, the mixture was treated at 400 °C for 6 h in a tube furnace under an H2/N2 atmosphere (5:95) with a flow rate of 0.20 L min−1. MI-PtP2/KIT-6 hybrids were obtained accordingly. Finally, ordered MI-PtP2 nanoparticles were obtained by etching MI-PtP2/KIT-6 with 5% HF and washing five times with water/ethanol (3:1) and further soaking in 1.0 M HCl solution for 12 h. Ordered MI-PtS2, MI-PtSe2, and MI-PtTe2 were synthesized by a similar procedure, but different X sources were used (see Supporting Information for the details). Synthesis of hollow structured MI-PtP2 nanoparticles Typically, 500 mg of NaH2PO2 and 100 mg of M-Pt/KIT-6 hybrids were first mixed and further ground. Then, the mixture was treated at 400 °C for 6 h in a tube furnace under an H2/N2 atmosphere (5:95) with a flow rate of 0.08 L min−1. MI-PtP2/KIT-6 hybrids were obtained accordingly. Finally, ordered MI-PtP2 nanoparticles were obtained by etching MI-PtP2/KIT-6 with 5% HF and washing five times with water/ethanol/HCl. Synthesis of ordered MI-Pt1-xMxP2 nanoparticles Typically, 8.0 mg of NiCl2/CoCl2/FeCl3 and 50 mg of M-Pt/KIT-6 were mixed initially, then ground. Subsequently, the mixture was heated at 400 °C for 6 h in a tube furnace under an H2/N2 (5:95) atmosphere with a flow rate of 0.08 L min−1. M-Pt1-xMx/KIT-6 was obtained accordingly. The synthetic processes of MI-Pt1-xMxP2 nanoparticles were the same as MI-PtP2 nanoparticles. Electrocatalytic hydrogen evolution reaction All electrochemical experiments were performed at room temperature (25 °C) using a CH Instruments electrochemical workstation (CHI 660E; CH Instruments,Chenhua, China). A standard three-electrode system, consisting of a graphite rod as the counter electrode, saturated Ag/AgCl as the reference electrode, and a catalyst-loaded glassy carbon (0.07 cm2) electrode as the working electrode (5 mm × 5 mm), was used for the electrochemical experiments. The potentials corresponding to the reversible hydrogen electrode (RHE) were calculated using the following equation: ERHE = EAg/AgCl + (0.197 + 0.0591 × pH) V. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were performed in N2-saturated 1.0 M KOH, 1.0 M phosphate-buffered saline (PBS) solution (pH = 7.2), and 0.50 M H2SO4 solution (see Supporting Information for all the details). Results and Discussion The strategies for the preparation of a series of ordered MI-PtX2 nanoparticles are shown in Figure 1a. First, mesoporous KIT-6 was prepared as a rigid/thermally stable template to form M-Pt/KIT-6 hybrid by reducing in situ Pt precursor within the KIT-6 template through a traditional nanocasting template ( Supporting Information Figure S2a,b).55,56 Subsequently, a nonmetal source (NaH2PO2, Na2S2O3, selenium, or tellurium powder) was physically mixed with as-synthesized M-Pt/KIT-6 and directly pyrolyzed at an appropriate temperature under a reducing atmosphere (H2/N2, 5%/95%). In this step, nonmetals were gradually inserted into the Pt nanocrystals, resulting in a confined Pt-to-PtX2 evolution, while retaining the ordered mesoporous structure of KIT-6 through a concurrent template route ( Supporting Information Figure S2c,d). Synthetically, this step occurred in a solid-phase, with no solvents or extra toxic chemicals involved. Last, the MI-PtX2/KIT-6 intermediate was etched with HF to remove the KIT-6 template. Surface-clean, ordered MI-PtX2 nanoparticles were prepared accordingly. Here, four ordered MI-PtX2 nanoparticles, including MI-PtP2, MI-PtS2, MI-PtSe2, and MI-PtTe2, with a precise Pt-nonmetal atomic ratio of 1/2 were synthesized. In comparison, the traditional nanocasting method disabled the synthesis of ordered MI-PtX2 nanoparticles because of the inherent volume shrinkage of Pt precursor during the crystallization ( Supporting Information Figure S3). This result highlights the highly effective concurrent template strategy for synthesizing ordered mesoporous intermetallic metal-nonmetal nanoparticles. The successful synthesis of ordered MI-PtX2 nanoparticles is first characterized by small-angle X-ray scattering (SAXS) and powder X-ray diffraction (PXRD). As shown in Figure 1b, all four MI-PtX2 nanoparticles displayed a characteristic peak with a q value of ∼0.66, corresponding to an average mesoporous periodicity of 9.5 nm. The periodicity matched well with the KIT-6 template data, indicating that the MI-PtX2 nanoparticles were perfectly replicated from ordered KIT-6. Thus, the results confirmed our design principle to obtain ordered mesoporous materials. Meanwhile, PXRD patterns revealed atomic phase structures of MI-PtX2 nanoparticles (Figure 1c). All four MI-PtX2 nanoparticles showed a series of phase-pure characteristic PXRD signals corresponding to ordered PdX2 phases at an atomic level. Among them, MI-PtP2 nanoparticles had a cubic Pa3 space group, while MI-PtS2, MI-PtSe2, and MI-PtTe2 were composed of a typical hexagonal crystal phase with a P 3 ¯ m 1 space group. Hence, the results highlighted the successful synthesis of MI-PtX2 nanoparticles with ordered mesoscopic and atomic structures through a concurrent template method. Mesoscopic morphology/structure and atomic crystalline phase of ordered MI-PtX2 nanoparticles were further characterized by electron microscopy techniques. Here, MI-PtP2 nanoparticles were investigated thoroughly as an example, since they exhibited the best electrochemical HER performance ( Supporting Information Figure S4). Scanning electron microscopy (SEM) images showed that the products were highly uniform with a polyhedral morphology (Figure 2a). The diameter of MI-PtP2 nanoparticles was ∼235 nm, slightly larger than the parent M-Pt nanoparticles ( Supporting Information Figure S5, ∼205 nm), indicating a Pt-to-PtP2 evolution. Transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) images further revealed highly dispersed and homogeneous nanoparticles of MI-PtP2 (Figure 2b,c). Abundant, highly penetrated mesopores were detectable throughout the polyhedral nanoparticles. HAADF-STEM images and the corresponding Fourier transform (FT) patterns of a single nanoparticle observed from different angles displayed nearly rhombic dodecahedral morphology at the macroscopic level and highly ordered I a 3 ¯ d structure at the mesoscopic level (Figure 2d). Obviously, ultrathin nanowires frameworks were interconnected into the ordered, double/single gyroid mesostructures with a rhombic dodecahedral morphology. Moreover, high-magnification HAADF-STEM images showed both double-gyroid and single-gyroid surfaces along the sides of MI-PtP2 nanoparticles (Figure 2e,f). This is because the metal nanocrystals were cast into two/one sets of I a 3 ¯ d mesochannels of the KIT-6 template, further confirming the concurrent template synthesis of ordered MI-PtP2 nanoparticles ( Supporting Information Figure S6). Besides, we considered readily tuning the size of the parent M-Pt in M-Pt/KIT-6 and gas flow rate for the synthetic process, achieving hollow structured MI-PtP2 nanoparticles in association with a Kirkendall cavitation synthesis ( Supporting Information Figure S7).57–59 Figure 2 | Morphology/structure and crystalline characterizations. (a) Low-magnification SEM, (b) TEM, and (c) HAADF-STEM images of ordered MI-PtP2 nanoparticles. (d) HAADF-STEM images and (insets) corresponding FT patterns of single MI-PtP2 nanoparticle with different observation angles and simulated image. (e and f) High-magnification HAADF-STEM images, (g) STEM EDX mappings, and (h) high-resolution TEM image and corresponding (020) plane atomic distribution of MI-PtP2. (i) High-resolution XPS Pt 3d spectra of MI-PtP2 and M-Pt nanoparticles. Download figure Download PowerPoint The atomic crystalline orderliness of MI-PtP2 nanoparticles is further investigated. HAADF-STEM energy dispersive X-ray (EDX) mapping images revealed that the noble metal (Pt) and nonmetal (P) elements were distributed uniformly in the whole nanoparticle (Figure 2g). No phase-separated compositions matched well with the crystalline structure of ordered MI-PtP2 nanoparticles, as indicated by the PXRD pattern above. The molar ratio of Pt and P is ∼1∶2.08, which was in good agreement with the intermetallic phase of PtP2. A similar atomic ratio of 1:1.96 was further confirmed by inductively coupled plasma mass spectrometry (ICP-MS) ( Supporting Information Table S1). An ordered PtP2 phase was apparent by high-resolution TEM (Figure 2h). The observed lattice spacings of 2.8 Å along the (001) zone axis were attributed to the (020) and (200) planes of the PtP2 phase, as revealed in the crystal model. The formation of ordered MI-PtP2 nanoparticles also changed the surface electronic structure of Pt. In comparison to monometallic M-Pt, a remarkable positive shift of binding energy in X-ray photoelectron spectroscopy (XPS) Pt 4f was seen for MI-PtP2, indicating that the transport of electrons from Pt to P in PtP2 results in an electron-deficient Pt surface (Figure 2i). The two side peaks at 73.7 and 76.9 eV could be attributed to the Pt oxidation of unsaturated coordination sites caused by the mesoporous structure. In the P 2p spectrum, the main peak observed at 129.7 eV was assigned to the Pt-P bond ( Supporting Information Figure S8); it exhibits a negative shift compared to P0 (130.2 eV), further confirming a negatively charged P. These results confirmed the successful synthesis of MI-PtP2 nanoparticles with high atomic orderliness. Similarly, ordered MI-PtS2, MI-PtSe2, and MI-PtTe2 nanoparticles were characterized thoroughly by SEM, TEM, and STEM mapping images ( Supporting Information Figures S9–S11). Electrochemical HER performance of ordered MI-PtP2 nanoparticles is performed in N2-saturated 1.0 KOH and compared with monometallic M-Pt. Meanwhile, commercial Pt/C was investigated as the benchmarked catalyst in the same condition. CV curves were first acquired to calculate the electrochemical surface areas (ECSAs) of the electrocatalysts ( Supporting Information Figure S12a). The ECSAs of MI-PtP2 and M-Pt were calculated as 42.7 and 48.2 m2 g−1, respectively, smaller than that of commercial Pt/C (71.2 m2 g−1) ( Supporting Information Figure S12b). This is mainly because the size of Pt nanoparticles in Pt/C (∼3 nm) was smaller than the framework thicknesses of MI-PtP2 and M-Pt (4–6 nm). LSV curves of different electrocatalysts are recorded in Figure 3a. Strikingly, ordered MI-PtP2 nanoparticles exhibited the most positive LSV curve, indicating the highest HER activity. The overpotentials of MI-PtP2 were only 33.6 mV at 10 mA cm−2 and 101.6 mV at 100 mA cm−2, respectively. Correspondingly, M-Pt and Pt/C showed higher overpotentials at 10 and 100 mA cm−2 (42.8 and 167.6 mV (M-Pt), 50.9 and 168 mV (Pt/C)) ( Supporting Information Figure S13). Meanwhile, the mass and specific activities also demonstrated the highest HER activity of ordered MI-PtP2 nanoparticles (Figure 3b). Specifically, the mass activity of MI-PtP2 nanoparticles reached up to 3.31 A mgpt−1 at 100 mV, which was 3.2 and 6.5 times higher than those of M-Pt (1.03 mgpt−1) and Pt/C (0.51 A mgpt−1). Meanwhile, ordered MI-PtP2 nanoparticles showed the highest specific activity of 7.75 mA cmpt−2 with 3.3 and 10.5 enhancement factors (EFs) than M-Pt (2.35 mA cmpt−1) and Pt/C (0.74 mA cmpt−1). Moreover, the turnover frequency (TOF) values for different catalysts were calculated ( Supporting Information Figure S14): MI-PtP2 nanoparticles displayed the highest TOF values over the entire voltage range. Specifically, the TOF values of MI-PtP2 were as high as 4.6 s−1 at 50 mV and 21.4 s−1 at 100 mV. By sharp contrast, M-Pt and Pt/C achieved lower TOF values of 2.3 s−1/7.6 s−1 and 1.7 s−1/6.7 s−1, respectively (Figure 3c). Figure 3 | HER performance. (a) LSV curves and (b) summarized mass activities and specific activities at 100 mV, (c) TOF values of MI-PtP2, M-Pt, and Pt/C. (d) LSV curves of MI-PtP2 before and after the ADTs for 30,000 cycles. (e) Summarized ECSAs and mass activities of MI-PtP2, M-Pt, and Pt/C before and after the ADTs for 30,000 cycles. (f) Chronoamperometry curves of MI-PtP2 and Pt/C at the overpotentials of 10, 20, and 30 mV cm−2. (g) Comparisons of alkaline HER overpotentials of MI-PtP2 and MI-Pt1-xNixP2 with the state-of-the-art noble metal-based catalysts reported in the literature. All HER tests were performed in N2-saturated 1.0 M KOH. Download figure Download PowerPoint Meanwhile, we thoroughly evaluate the HER stability of the electrocatalysts through the ADT and chronoamperometry (i–t) experiments. First, we collected CV and LSV curves of MI-PtP2, M-Pt, and commercial Pt/C in 1.0 M KOH during the ADT experiments for 30,000 cycles (Figure 3d and Supporting Information Figures S15–S17). As reported, commercial Pt/C gradually decreased the ESCAs and mass activities during the ADT processes (Figure 3e). In comparisons, ordered MI-PtP2 and M-Pt nanoparticles retained their ECSAs and mass activities. Specifically, ordered MI-PtP2 and M-Pt nanoparticles maintained 82.4% and 72.5% of their initial ECSAs, while commercial Pt/C exhibited 39.3% of ECSA loss after the ADT. Moreover, the MA loss for ordered MI-PtP2 nanoparticles was only 15.2%, much less than those of M-Pt (34.6%) and Pt/C (42.7%). Second, electrochemical i–t experiments showed that ordered MI-PtP2 nanoparticles maintained their activities well at current densities of 10, 20, and 30 mA cm−2 (Figure 3f). By sharp contrast, commercial Pt/C showed a rapid loss of mass activity under the same conditions. These results further highlight the high HER stability of ordered MI-PtP2 nanoparticles. Moreover, compared with the HER performances of the state-of-the-art noble metal-based electrocatalysts in the literature, ordered MI-PtP2 nanoparticles also represent one of the most active and stable HER electrocatalysts in alkaline conditions (Figure 3g and Supporting Information Table S2). Electrocatalytic kinetics were further investigated to reveal why ordered MI-PtP2 nanoparticles could enhance HER performance. Electrochemical impedance spectroscopy (EIS) was first used to study the charge transfer kinetics during the HER electrocatalysis. Nyquist plots from the equivalent circuit fitting showed that MI-PtP2 electrocatalyst had a minimum charge transfer resistance of 48 Ω, much smaller than that of M-Pt (63 Ω) and Pt/C (80 Ω), indicating both mesoporous structure and PtP2 intermetallic phase synergistically enhanced the interfacial electron transfer capability (Figure 4a). Second, Tafel slopes were plotted to characterize electrocatalytic kinetic processes further. Obviously, Tafel slope values of MI-PtP2, M-Pt, and Pt/C were >30 mV dec−1, implying that the HER underwent a Volmer–Heyrovsky route in alkaline media (Figure 4b).60 In this process, the Volmer step involved the dissociation of H2O (H2O → OH− + H*), while the Heyrovsky step corresponded to the H2O dissociation and the subsequent conversion of H* to molecular H2 (H2O + H* → OH− + H2).7,61,62 Specifically, MI-PtP2 nanoparticles showed the smallest Tafel slope of 52.4 mV dec−1 than those of M-Pt (63.0 mV dec−1) and Pt/C (65.8 mV dec−1), suggesting that compositional and structural synergies accelerated the dissociation of H2O and enhanced the alkaline HER kinetics. Figure 4 | HER kinetics and mechanism. (a) EIS plots, (b) Tafel slopes, (c) relationship of OH adsorption ability and HER activity, and (d) CV curves of MI-PtP2, M-Pt, and Pt/C. (e) Schematic illustrations of the reaction pathways of ordered MI-PtP2 for alkaline HER electrocatalysis. Download figure Download PowerPoint The dissociation of H2O was experimentally confirmed by the *OH binding energy (OHBE) of different electrocatalysts. Considering the rapid departure of adsorbed *CO intermediates by *OH, we conducted electrochemical CO stripping experiments to evaluate the OHBEs of electrocatalysts in 1.0 M KOH ( Supporting Information Figure S18).63,64 A lowest CO striping peak potential of 0.671 V was achieved for MI-PtP2 nanoparticles, demonstrating an excellent OHBE. In comparisons, CO stripping peak potentials obtained for M-Pt and Pt/C were 0.696 V and 0.698 V, respectively. Next, we elucidate the relationship between the OHBE values and the HER activities of different electrocatalysts (Figure 4c). Apparently, the ability to dissociate H2O monotonically corresponded to the OHBE value and HER activity of the electrocatalysts, further confirming HER kinetics. Meanwhile, hydrogen binding energies (HBEs) of different electrocatalysts were measured with scanning CVs to imitate the conversion of H* in the Heyrovsky step (Figure 4d). We calculated the HBE values of active sites according to the potential of underpotential deposition hydrogen (Hupd) desorption peak (Epeak) (ΔH = −FEpeak).60,65 Ordered MI-PtP2 nanoparticles had lower HBE values than those of M-Pt and Pt/C, indicating H* adsorbed on MI-PtP2 was easily desorbed and further coupled to molecular H2. Correspondingly, the H2 temperature programmed desorption (H2-TPD) experiment simulated the desorption of molecular H2 on the catalyst surface. Obviously, the peak temperature of MI-PtP2 was lower than that of M-Pt, indicating that H2 readily desorbed from the MI-PtP2 surface ( Supporting Information Figure S19). The potential origin between ΔGH* and the electronic structure of the Pt active site has been determined by Fermi softness,66,67 which indicated that the density of states (DOS) around the Fermi energy (EF) of the Pt site significantly affected the ΔGH*. One of the main reasons is the reduced DOS of the Pt site near the EF in the electron-deficient state was not conducive to hydrogen adsorption.66,67 On the basis of the above investigations, we ascribed the high HER performance to compositional and structural synergies of ordered MI-PtP2 nanoparticles (Figure 4e). On the one hand, the electron-deficient surface of the ordered PtP2 intermetallic phase with strong d-orbital interactions downshifted the Femi level of Pt sites, which kinetically facilitated the dissociation of H2O (Volmer step and Heyrovsky step), thereby weakening the adsorption of H* (Heyrovsky step); thus, greatly accelerating H2 production through HER electrocatalysis.63,68 On the other hand, ordered mesoporous structures exposed more undercoordinated Pt sites and kinetically accelerated transport of the electrons and reactants within the electrocatalyst, contributing to enhanced HER activity.49,69 Moreover, a 3D interconnected mesoporous structure effectively inhibits the physical Ostwald ripening process, enhancing HER stability remarkably. Meanwhile, the atomically ordered PtP2 intermetallic phase endowed MI-PtP2 electrocatalyst with enthalpy-like stability to synergistically boost HER stability. We also demonstrated that ordered MI-PtP2 nanoparticles could perform well as a high-performance electrocatalyst for the HER in all pH ranges (neutral and acidic media). In 0.50 M H2SO4 solution, for example, MI-PtP2 electrocatalyst had the smallest overpotential of 27 mV at 10 mA cm−2, the lowest Tafel slope value of 26.7 mV dec−1, and the smallest impedance arc diameter of 26 Ω, all of which were remarkably better than those of M-Pt and Pt/C ( Supporting Information Figure S20).