Full text introduction
The development of low-cost, efficient and stable alkaline hydrogen evolution reaction electrocatalysts is a key topic in the research of water electrolysis. In this paper, we report an interface engineering strategy for Mo2N quantum dots (denoted AlO@Mo2N-NrGO) modified on conductive N-doped graphene by co-doping of monatomic Al and O, which can simultaneously regulate nanoscale structure, electronic structure and interface structure. The transition of Anderson polyoxometalate anion clusters ([AlMo6O24H6]3−, denoted AlMo6) to Mo2N quantum dots not only produces more exposed active sites, but also optimizes the electronic structure of Mo2N by co-doping atomically dispersed Al and O in situ. The study also found that the surface reconstruction of Al-OH hydrates in AlO@Mo2N quantum dots plays a crucial role in enhancing hydrophilicity and reducing the energy barrier of water dissociation and hydrogen desorption, resulting in significant basic HER performance, even better than the commercial 20% Pt/C. In addition, the strong interfacial interaction (Mo N bond) between AlO@Mo2N and N-doped graphene can significantly improve the electron transfer efficiency and interfacial stability. The results show that under the condition of current density greater than 100 mA cm−2, the catalyst has excellent stability within 300 hours and has great practical application potential.
Results and discussion
Figure 1 Schematic diagram of the synthesis of Al2O@Mo2N-NrGO quantum dots. The structure of activated AlO@Mo2N-NrGO (denoted as A-AlO@Mo2N-NrGO) is also presented, in which Al-O species are adsorbed from AlO@Mo2N-NrGO self-optimizing by electrochemical activation in an alkaline electrolyte.
POMs anion clusters ((NH4)3[AlMo6O24H6] (denoted AlMo6) are anchored to the surface of protonated polyaniline/graphene oxide (PANI/GO) nanosheets by electrostatic and H-bond interactions (Figure 1), which helps avoid agglomeration of Mo2N nanoparticles during nitriding. In addition, Al and O sources obtained from AlMo6 clusters can lead to in situ chemical doping in the AlO@Mo2N at the atomic level, which can adjust the electronic structure and expose more active sites. In addition, the strong interface between AlO@Mo2N and n-doped graphene Mo-N bonds can significantly improve the efficiency and stability of electron transfer.
Figure 2 a,e) LRTEM image, b,f) SAED pattern, c,g) HRTEM image, and d,h) hysteresis loops of NiFe and NiFe/NiFeOOH core/shell NPs at 300 K. The illustration for (a,e) is a histogram of the size distribution of NiFe and NiFe/NiFeOOH core/shell NPs.
Compared with pure AlMo6, the NH4+ peak at 1405 cm−1 in AlMo6-PANI/GO was significantly reduced, indicating that some NH4+ cations were replaced by protonated PANI/GO (Figure 2c). In AlMo6-PANI/GO, the characteristic peaks of the AlMo6 cluster at 923 cm−1 (Mo-O), 879 cm−1 (Mo-O-Mo), and 635 cm−1 (Mo-O-Mo) are well preserved (Figure 2c). In addition, the uniform distribution of Al, Mo, C, N, and O elements in AlMo6-PANI/GO is obtained by mapping the energy dispersive X-ray (EDX) elements, as shown in Figure S2 (supporting information). These results show that AlMo6 clusters are evenly distributed on the surface of PANI/GO. A more detailed microstructural analysis of Al2O@Mo2N-NrGO by TEM imaging (Figure 2e) revealed the presence of many ultra-small quantum dots embedded on NrGO nanosheets with an average diameter of 2.5 nm (inset in Figure 2e). This result confirms that the uniform dispersion of AlMo6 clusters on PANI/GO nanosheets helps avoid agglomeration of AlO@Mo2N quantum dots on N-doped graphene substrates (NrGOs).
Performance testing
Figure 3 HER performance of electrocatalysts. a) AlO@Mo2N the current density of NrGO at -0.3 V after different CV cycles at a scan rate of 5 mV s-1 in a three-electrode configuration. b,c) Pt/C, AlO@Mo2N-NrGO, Mo2N-NrGO, and NrGO polarization curves b) No and c) IR compensation. d) Tafir Plot. e) The CV of Al2O@Mo2N-NrGO has different rates from 20 to 100 mV s−1. Illustration: Capacitor current at 0.15 V as a function of different scan rates of Al2O@Mo2N-NrGO. f) Long-term durability testing at η = 300 mV for 300 hours. g) HER performance comparison of AlO@Mo2N-NrGO with other molybdenum-based electrocatalysts at 1.0 m KOH.
The self-optimizing HER activity of the Al2O@Mo2N-NrGO electrocatalyst is noted in cyclic voltammetry (CV) cycles (see Figure 3a). The current density relative to the reversible hydrogen electrode (RHE) of -0.3 V increased from 54.12 to 109.10 mA cm-1 after 430 CV cycles and remained stable after 430 to 1710 CV cycles. As shown in Figure 3b, the activated Al2O@Mo2N-NrGO electrocatalyst exhibits better HER activity with an overpotential of η10 = 111 mV (no IR compensation) compared to Mo2N-NrGO (η10 = 141 mV). For clarity, the following discussion of Al2O@Mo2N-NrGO HER performance is based on CV-activated samples. The HER activity of AlO@Mo2N-NrGO is even better than the activity of commercial 20% Pt/C at superpotential (η > 203 mV). After IR compensation, Al2O@Mo2N-NrGO still performs optimally, outperforming Pt/C at an overpotential of η > 203 mV (see Figure 3c). In addition, a small overpotential of only 285 mV yields a high current density of 400 mA cm-2, which is superior to most reported high current density electrocatalysts. In addition, Al2O@Mo2N-NrGO has approximately twice the mass activity of Mo2N-NrGO.
Fig. 4 Self-optimization mechanism of AlO@Mo2N-NrGO electrocatalyst. a) The content of Al element and the corresponding overpotential at a current density of 50 mA cm−2 (η50) as the number of CV cycles increases. b) The corresponding Cdl value as the number of CV cycles increases. c) Hydrophilicity increases with increasing number of CV cycles. d) Raman spectra of AlO@Mo2N-NrGO after different CV cycles. Add MoO3 and Na[Al(OH)4] as references. e) Aberration corrected high-angle annular darkfield scanning TEM (haadf-stem) image of AlO@Mo2N-NrGO with atomic resolution without CV activation, illustrated showing a schematic diagram of the top view structure of AlO@Mo2N-NrGO. f) Intensity distribution along the blue dotted box indicated in image e). g) Schematic diagram of AlO@Mo2N-NrGO without CVs activation. h) A-AlO@Mo2N-NrGO aberration-corrected atomic resolution HAADF-STEM image after CV activation, illustrated showing a schematic diagram of the top view structure of A-AlO@Mo2N-NrGO. i) Intensity distribution along the green and red dotted boxes indicated in image i). j) Schematic diagram of A-AlO@Mo2N-NrGO after CV activation.
As shown in Figure 5a, after 80 CV cycles, the percentage of Al in AlO@Mo2N-NrGO decreased from 3.62% to 3.35%, indicating that some Al atoms may have leached into the 1.0 mKOH electrolyte. Here, we assume that the Al atoms in AlO@Mo2N-NrGO will react with OH- to form Al-OH hydrates, which will be adsorbed to the surface of AlO@Mo2N-NrGO to enhance the hydrophilicity of the catalyst. It has been observed that after 430 CV cycles, the contact angle of AlO@Mo2N-NrGO decreases from 115° (hydrophobic surface) to 20° (hydrophilic surface) (Figure 5c). Interestingly, the Al content remained around 3.33% even after 430 CV cycles, while HER activity increased with the decrease in overpotential (η50) from 290 to 203 mV after 430 CV cycles (Figure 5a), indicating that the dissolution of Al and the adsorption of -OH hydrates on the surface of AlO@Mo2N-NrGO may be close to saturation during electrochemical activation. The above evidence suggests that the self-optimizing HER performance of AlO@Mo2N-NrGO can be attributed to the reconstruction of Al-OH hydrates on the surface of AlO@Mo2N-NrGO during electrochemical activation, which increases ECSA and hydrophilicity.
Raman spectroscopy was further utilized to reveal changes in the surface chemical environment of the Al2O@Mo2N-NrGO electrocatalyst (Figure 5d). Initially, AlO@Mo2N-NrGO showed two distinct bands at ≈820 and 993 cm−1 (red curve in Figure 5d), similar to the band of reference MoO3 (black curve in Figure 5d), which can be attributed to the characteristic Mo-O-Mo and Mo-O vibrations of MoO3, indicating that the surface of AlO@Mo2N-NrGO is oxidized and covered with amorphous MoO3 material, consistent with XPS and XAS results (Figure 3a, e). After 20 CV cycles, the Mo-O peak disappeared and the Mo-O-to-Mo-N transition peak appeared at 948 cm-1, indicating that the amorphous MoO3 on the surface of the AlO@Mo2N-NrGO was completely dissolved. In addition, after 40 CV cycles, characteristic Mo-N peaks begin to appear at ≈817 and 849 cm-1. At the same time, after 280 CV cycles, another visible band of Al-O vibrations belonging to aluminate (Na[Al(OH)4]) appeared around 1059 cm−1, indicating that Al dopings in the Mo2N lattice can dissolve to form aluminates, such as Al(OH)(H2O)32+, and adsorb on the surface of AlO@Mo2N-NrGO, resulting in increased ECSA and hydrophilicity.
In order to visually understand the self-optimization process of Al2O@Mo2N-NrGO electrocatalysts, a high-angle annular darkfield scanning TEM (HAADF-STEM) analysis was further performed. As shown in Figure 5e, AlO@Mo2N-NrGO exhibits significant changes in Al and Mo atomic strength, as confirmed by the intensity distribution of Figure 5f, indicating that Al atoms successfully doped into the Mo2N lattice in the original AlO@Mo2N-NrGO (Figure 5g), may be attacked by OH− during CV activation in the alkaline electrolyte, and gradually dissolve in the alkaline electrolyte to form Al-OH species, resulting in Al vacancies. This hypothesis is supported by the HAADF-STEM results in Figure 5h, where Al2O@Mo2N-NrGO shows atomic vacancies after aluminum atoms are dissolved. Furthermore, dissolved aluminate may be adsorbed on the Mo2N surface by Mo-O-Al bonds (Figure 5i), as confirmed by EDX element mapping (Figure S19, supporting information) and intensity distribution in Figure 5j, which indicates enhanced bright spots around the Al vacancy location. In summary, the self-optimizing HER activity of AlO@Mo2N-NrGO stems from two reasons: first, the amorphous Mo oxide initially formed on the surface of AlO@Mo2N-NrGO is dissolved, exposing more AlO@Mo2N active sites; Secondly, the Al dopant dissolves and reacts with the electrolyte to form an Al-OH substance, which can be adsorbed on the surface of AlO@Mo2N-NrGO during CV activation, thereby improving HER performance.
conclusion
In summary, we report an atomic interface engineering strategy for simultaneously modulating nanostructures, electronic structures, and interfaces on Al2O@Mo2N-NrGO. Uniformly anchored POMs anion clusters on PANI/GO through strong electrostatic interactions and hydrogen bonds can effectively prevent the aggregation of Mo2N quantum dots. In addition, with the AlMo6 anion cluster as the precursor, the in-situ co-doping of Al and O in Mo2N quantum dots can be realized, and the electronic structure of Mo2N can be effectively regulated. Leaching and reconstruction of AlO@Mo2N-NrGO surface AlOH hydrate promotes exposure to more active sites and increases surface hydrophilicity, resulting in self-optimizing HER activity. The optimized Al2O@Mo2N-NrGO electrocatalyst exhibits significant basic HER performance, with an overpotential of 82 mV relative to RHE at 10 mA cm-2, better than commercial 20% Pt/C, and an overpotential relative to RHE greater than 203 mV. In addition, excellent stability of more than 300 hours at a high current density of 114 mA cm-2 can be achieved due to the strong interface interaction between the AlO@Mo2N quantum dots and NrGO. Experimental and theoretical results show that the reconstitution of Al-OH hydrate on the AlO@Mo2N surface can effectively reduce the reaction energy barrier required for the Heyrovsky step and promote the desorption of H2 on the catalyst surface. Given the abundance and structural diversity of POMs on Earth, the molecular design of more POMs with different heteroatoms will allow for independent chemical tuning and optimization of electrocatalysts. This research can serve as a blueprint for low-cost, high-rate, and stable renewable hydrogen electrocatalysts.
bibliography
Huang, Y., Zhou, W., Kong, W., Chen, L., Lu, X., Cai, H., Yuan, Y., Zhao, L., Jiang, Y., Li, H., Wang, L., Wang, L., Wang, H., Zhang, J., Gu, J., Fan, Z., Atomically Interfacial Engineering on Molybdenum Nitride Quantum Dots Decorated N-doped Graphene for High-Rate and Stable Alkaline Hydrogen Production. Adv. Sci. 2022, 9, 2204949.