Ultrathin Metal–Organic Framework Nanosheets-Derived Yolk–Shell Ni 0.85 Se@NC with Rich Se-Vacancies for Enhanced Water Electrolysis
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-11-20 |
| Journal | CCS Chemistry |
| Authors | Zhaodi Huang, Chao Feng, Jianpeng Sun, Ben Xu, Tianxiang Huang |
| Institutions | China University of Petroleum, East China |
| Citations | 37 |
Abstract
Section titled “Abstract”Open AccessCCS ChemistryRESEARCH ARTICLE1 Nov 2021Ultrathin Metal-Organic Framework Nanosheets-Derived Yolk-Shell Ni0.85[email protected] with Rich Se-Vacancies for Enhanced Water Electrolysis Zhao-Di Huang, Chao Feng, Jian-Peng Sun, Ben Xu, Tian-Xiang Huang, Xiao-Kang Wang, Fang-Na Dai and Dao-Feng Sun Zhao-Di Huang College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 , Chao Feng State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao, Shandong 266580 , Jian-Peng Sun College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 , Ben Xu College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 , Tian-Xiang Huang State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao, Shandong 266580 , Xiao-Kang Wang College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 , Fang-Na Dai Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 and Dao-Feng Sun Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Science, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580 State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao, Shandong 266580 https://doi.org/10.31635/ccschem.020.202000537 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail We present a controlled fabrication of selective ultrathin metal-organic framework (MOF) nanosheets as preassembling platforms, yolk-shell structured with a few-layered N-doped carbon (NC) shell-encapsulated Ni0.85Se core (denoted as Ni0.85[email protected]) via an oriented phase modulation (OPM) strategy. The ultrathin nature of the MOF nanosheets gave rise to the modification of structure at the electronic level with abundant Se-vacancies and effective electronic coupling via an Ni-Nx coordination at the interface between the Ni0.85Se core and NC shell. The Ni0.85[email protected] obtained exhibited low overpotentials for both oxygen evolution reaction (OER; 300 mV) and hydrogen evolution reaction (HER; 157 mV) at 10 mA·cm−2 under an alkaline condition, outperforming their corresponding bulk MOF-derived counterparts. By exploiting Ni0.85[email protected] as anode and cathode catalysts, a low cell voltage of 1.61 V was achieved by performing alkaline water electrolysis. Remarkably, it also reached a high activity in natural seawater (pH = 7.98) and simulated seawater (pH = 7.86) electrolytes, even surpassing integrated Pt/C-RuO2/CC electrodes. Density functional theory (DFT) studies illustrated that abundant Se-vacancies effectively regulated the electronic structure of Ni0.85[email protected] by accelerating electron transfer from Ni to N atoms at the interface, and thus, enabling the Ni0.85[email protected] to attain a near-optimal electronic configuration that stimulated ideal adsorption-free energy toward key reaction intermediates. Download figure Download PowerPoint Introduction Electrochemical water splitting, including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), holds great potential as a promising technology for sustainable storage, conversion, and transportation of hydrogen energy, and it strongly depends on the thermodynamics, along with kinetic parameters of its two half-reactions.1-3 Considering the sustainability of H2 production, seawater electrolysis is an eminently desirable path owing to its earth-abundant reserves, accounting for 96.5% of the world’s total water resources. Although Pt- and Ru/Ir-based electrocatalysts are recognized as reducing reaction energy barriers and boosting seawater splitting efficiency, their limited earth-abundance and expensive market-price hamper their extensive applications enormously.4,5 Accordingly, tremendous endeavors have been undertaken to search for advanced alternatives to precious metal electrocatalysts. Transition metal-based nanomaterials with the feature of earth-abundant, low cost, and high efficiency have been widely explored as promising overall seawater splitting electrocatalysts.6,7 Among them, Ni-Se nanomaterials (e.g., Ni0.85Se, NiSe, Ni3Se2, and NiSe2) stand out as they bear the advantages of large anionic size, accelerated electron mobility, and suitable bandgaps, which could accelerate electron transfer and enlarge their inherent catalytic sites.8-11 However, most Ni-Se particles easily agglomerate in seawater electrolysis and have difficulty in holding durative catalytic active sites.12,13 Therefore, the development of effective bifunctional electrocatalysts for water splitting, and especially show good catalytic activity, as well as durability in seawater, are demonstrated to be exceedingly desirable. By targeting the improvement in stimulating intrinsic activity and bearing better tolerance, heteroatom-doped carbon matrix (HCM)-encapsulated transition metal (TM)-based core-shell nanomaterials ([email protected]) would provide abundant electrocatalytic active sites, high electrical conductivity, and suitable carbon protective layers.14,15 Using metal-organic frameworks (MOFs) as superior precursors to controllably fabricate [email protected] nanomaterials could partially achieve the above-mentioned goals.16-19 Compared with bulk MOF, a thinner MOF tends to acquire more accessible active sites with enhanced catalytic activity.20-23 For example, Li et al.24 selected a thick-layered MOF (∼30 nm) as a template to construct Co3O4/CBDC nanosheet arrays and used it as an OER electrocatalyst. However, ultrathin MOF nanosheets with higher aspect ratio and ultrathin characteristics than bulk MOFs and thick-layered MOFs are supposed to be ideal preassembling platforms: First, the ultrathin nature of MOF nanosheets enables it to curl naturally during a calcination process, such that the carbon skeleton from the MOF ligands form a stable, protective layer, thereby avoiding a corrosion. Meanwhile, it builds an inimitable functional interface, rendering it convenient for systematic investigation of its different components’ interaction mechanisms. Second, the ultrathin characteristics of MOF nanosheets enable them to form a few-layers, not a multilayer, and an HCM shell after pyrolysis, leading to a well-defined core-shell/yolk-shell structure. Such a particular architecture could provide a high percentage of exposed active centers to enhance electron-transfer ability, aiming to optimize the intrinsic adsorption free energy. Third, under pyrolysis conditions, the ultrathin MOF nanosheets could effectively trigger the creation of anion-vacancies or defects, which are conductive based on the modification of intrinsic catalytic activity at the electronic level, thereby improving the seawater-splitting performance. Herein, inspired by the above-mentioned expected merits, we chose a well-defined ultrathin MOF nanosheets (∼5 nm, [Ni(HBTC)(DABCO)·3DMF], HBTC = trimesic acid, DABCO = 1,4-diazabicyclo [2.2.2] octane, DMF = N,N-dimethylmethanamide) as preassembling platforms. Favorably, it was possible to synthesize the core-shell NiSe2@NC nanomaterials through pyrolysis, followed by selenization of ultrathin MOF nanosheets. The oriented structural and compositional transformations from orthorhombic NiSe2@NC to hexagonal Ni0.85[email protected] were revealed, resulting in the formation of yolk-shell Ni0.85[email protected] nanomaterials. The ultrathin nature of MOF nanosheets contributed to the formation of a few-layer N-doped carbon (NC) shell and abundant Se-vacancies for effective electronic coupling, thereby effectively tuning the electronic structure of the Ni0.85[email protected] nanomaterial. As expected, the Ni0.85[email protected] obtained exhibited low overpotentials in the overall-water-splitting to produce O2 and H2 with a low cell voltage of 1.61 V at 10 mA·cm−2 in an alkaline medium, outperforming the corresponding bulk MOF-derived counterparts. Remarkably, the synthesized Ni0.85[email protected] also demonstrated excellent performance in natural seawater (pH = 7.98) and simulated seawater (pH = 7.86), even outperforming integrated Pt/C-RuO2/CC electrodes. Density functional theory (DFT) studies illustrated that the C atoms next to the pyrrolic N could serve as the most active OER sites. In contrast, the C atoms in the para-position of pyrrolic N facilitated HER. Meanwhile, the abundant Se-vacancies enhanced the binding strength of O-containing intermediates, resulting in a change of the rate-determining step (RDS) from O → OOH to O* → OH*, which was conducive to the optimized OER kinetics. Also, it further elevated the charge-transfer efficiency significantly in the coupling interface via the chemical bond Ni−Nx, which synergistically favored the H- and O-containing intermediates’ adsorption and activation. Experimental Section Preparation of ultrathin MOF nanosheets The ultrathin MOF nanosheets were synthesized according to a method described previously,25 with a slight modification. First, 0.145 g (0.5 mmol) of Ni(NO3)2·6H2O, 0.056 g (0.5 mmol) of DABCO, 0.053 g (0.25 mmol) of 1,3,5-Benzenetricarboxylic acid (H3BTC), and 1 g of polyvinylpyrrolidone (PVP) were dissolved in DMF (10 mL) solution. The mixture was stirred at room temperature for 30 min at 600 rpm. Then it was transferred to a 25 mL reaction vessel, packaged, and transferred to an oven at 120 °C for 24 h. After cooling naturally to room temperature, the product was separated by centrifugation (8000 rpm for 30 min) three times with DMF and methanol. Then a final pale green powder was obtained as ultrathin MOF nanosheets, which were activated by drying in vacuum at 80 °C for 12 h. Synthesis of ultrathin MOF nanosheets-derived [email protected], NiSe2@NC, and Ni0.85[email protected] For the synthesis of [email protected] nanomaterials, activated MOF nanosheets were transferred to a square porcelain boat, followed by placing in a furnace, and heating to 600 °C for 2 h with a heating rate of 5 °C·min−1 under an argon atmosphere. The NiSe2@NC nanomaterials were synthesized by mixing the as-obtained [email protected] powder with selenium (Se) powder evenly in the mortar at a mass ratio of 1∶2, and then the mixture transferred to a furnace. The furnace was heated at 350 °C for 2 h to obtain NiSe2@NC. Subsequently, the Ni0.85[email protected] was synthesized by further annealing the as-prepared NiSe2@NC at 850 °C for 2 h. Preparation of bulk MOF The preparation of bulk MOF was similar to that of ultrathin MOF nanosheets, except that no PVP was used. Synthesis of bulk MOF-derived [email protected], B-NiSe2@NC, and B-Ni0.85[email protected] nanomaterials The preparations of [email protected], B-NiSe2@NC, and B-Ni0.85[email protected] nanomaterials were similar to those of ultrathin MOF nanosheets-derived nanomaterials, except that the precursor for the reaction was the bulk MOF. Characterization of the synthesized nanomaterials Powder X-ray diffraction (XRD) was performed on a Bruker AXS D8 (Cu Kα radiation, λ = 1.5406 Å) under an operating voltage of 40 kV and a current of 30 mA. The morphology was characterized by scanning electron microscopy (SEM; JEOL JSM-6330, Beijing, China) and high-resolution transmission electron microscopy (HRTEM; Hitachi JEM-2100F, Beijing, China). Thermogravimetric analysis (TGA) curves were recorded on a Mettler Toledo instrument (Shanghai, China) at a heating rate of 10 °C·min−1 in the range of 40-800 °C under an O2 atmosphere. Nitrogen physisorption isotherms were recorded at 77 K using an Autosorb-iQ nitrogen volumetric adsorption instrument (Quantachrome Instruments, Shanghai, China). Before measurement, the samples were degassed at 100 °C for 12 h. X-ray photoelectron spectra (XPS) analysis was performed on an ESCALAB 250 (Thermo Electron Corporation, Shanghai, China) with Al Kα radiation (1486.6 eV). The in situ electron paramagnetic resonance (EPR) measurement was performed using an ENDOR spectrometer (JEOL ES-ED3X, Beijing, China) at a liquid nitrogen temperature of 77 K. The g factor was obtained using manganese (Mn) signal as an internal standard. Preparation for electrochemical measurements Working electrodes The prepared nanomaterial samples (5 mg) and Nafion solution (50 μL, 5 wt %) were dispersed in a mixture of deionized (DI) water (450 μL) and ethanol (500 μL). The mixture was sonicated for 30 min to form a homogeneous ink. Then, 5 μL of the ink was drop-dried on a polished glassy carbon electrode (GCE) with the requirement of a diameter of 3 mm (loading: 0.35 mg·cm−2). Before taking measurements, the GCE loaded with active materials was allowed to dry at room temperature for 1 h. As a comparison, 5.0 mg of commercial Pt/C and RuO2 powders were also dispersed on a polished GCE using the same approach. Electrochemical measurements The electrocatalytic activity was measured on a Gamry Potentiostat Reference 3000 electrochemical workstation (Ningbo Gamry Optical Instrument Co. Ltd., Shanghai, China) with a standard three-electrode system. An Ag/AgCl (KCl saturated) electrode and a carbon rod (3 mm in diameter) were used as reference and counter electrode, respectively. All current densities were normalized to the geometrical surface area, and all potentials were converted to the reversible hydrogen electrode (RHE), according to the equation ( E RHE = E Ag / AgCl + 0.22 + 0.059 pH ) . OER measurements For the OER test, 1.0 M KOH solution was used as an aqueous electrolyte and bubbled with O2 for at least 30 min before taking the electrochemical measurement. The linear sweep voltammetry (LSV) curves were obtained by sweeping the potential from −1 to −1.6 V (vs RHE) at a scan rate of 5 mV·s−1. The electrochemical surface areas (ECSAs) were investigated by measuring the double-layer capacitance (Cdl) via cyclic voltammograms (CVs), according to the equation (ECSA = Cdl/Cs, where Cs is the specific capacitance and taken as 0.85 mF·cm−2).26 The CV curves were measured at various scan rates (40-200 mV·s−1) in the potential range from 0.25 to 0.35 V (vs RHE). The electrochemical impedance spectroscopy (EIS) was performed on the Gamry potentiostat, evaluating a frequency range from 105-10−2 Hz at −1.5 V (vs RHE) using a 10 mV amplitude. The stability measurements were carried out using CV between −1.05 and −1.35 V (vs RHE) for 2000 cycles and long-term chronoamperometry under a fixed voltage of 1.57 V. HER measurements The process of HER measurements was similar to that of OER, but a flow of N2 was employed instead of a flow of O2 to ensure an H2/H2O equilibrium. Overall water splitting Typically, 5 mg active samples were dispersed in a water/ethanol solution (500 μL, 3:1 v/v) with 25 μL of Nafion solution by sonicating for 2 h to form a homogeneous ink. Then, 200 μL of the uniform ink was drop-casted on carbon cloth (CC; 1.0 × 1.0 cm2) and dried at room temperature (mass loading: 1.0 mg·cm−2). A piece of CC was carefully pretreated by sonication in 6.0 M HCl, DI water, and ethanol for 15 min, respectively, to remove surface oxide. In this study, CC was chosen as the conductive substrate because it is porous and has a negligible catalytic or noncatalytic activity in the investigated potential region. Overall seawater splitting For overall seawater splitting measurements, the typical process was similar to the overall water splitting, except for electrolyte differences. Natural seawater (pH = 7.98) electrolyte was collected from Golden Beach (Qingdao, Shandong Province) and filtered to remove visible impurities before use. Simulated seawater (pH = 7.86) electrolyte was prepared by mixing 6.683 g of NaCl, 0.565 g MgCl2, 0.813 g MgSO4, 0.280 g CaCl2, 0.048 g NaHCO3, 0.87 g Na2SO4, and 0.180 g KCl in 250 mL of DI water (18 MΩ), according to a previous report.27 Computational details Spin-polarized DFT calculations were performed using the Vienna Ab initio Simulation Packages (VASP; the Chinese Academy of Sciences, Beijing) and employed using the generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional to describe the exchange and correlation energy in all calculations.28 The projector augmented wave (PAW) method was used to represent the interactions between valence electrons and ionic cores. The plane wave cutoff energy was fixed at 400 eV. To model Ni0.85[email protected] nanohybrids, an N-doped graphene layer adsorbed on Ni-terminated Ni0.85Se slab with exposed (1 1 1) surface was used, resulting in a model with the lowest lattice mismatch for the following calculation. The supercell consists of 2 × 2 unit cells for Ni0.85Se slab and 4 × 4 unit cells for the N-doped graphene layer with a 15 Å vacuum region to simulate the adsorption. For the Ni0.85[email protected] model, the top two layers together with the adsorbates were fully relaxed in all dimensions until the maximum force on a single atom was smaller than 0.02 eV·Å−1, and the energy and force convergence was set to 1 × 10−4 Ha. The Brillouin zone was sampled by the Monkhorst-Pack method with a 2 × 2 × 1 k-point mesh. For the HER, the Gibbs free energy (ΔGH*) is calculated as follows29: Δ G H * = Δ E H + Δ E ZPH − T Δ S H ΔEH, ΔEZPE, and ΔSH are the adsorption energy of hydrogen, the zero-point energy difference, and the entropy difference, respectively. In standard conditions, ΔEZPE − TΔSH is ∼0.24 eV. Hence, ΔGH* is calculated using the equation ΔEH + 0.24. The OER estimation follows four elementary steps. The free energies of the intermediates at 298.15 K were obtained by: Δ G = Δ E + Δ E ZPE − T Δ S − e U where ΔS and U are the zero-point energy changes, entropy changes, and applied potentials. ΔE is the binding energy of adsorption species HO*, O*, and HOO*, with defined as follows: Δ E = E substrate + adsorbate − E substrate − E adsorbate Finally, the theoretical overpotential η is determined by the potential limiting step: η = max [ Δ G HO * , Δ G O * , Δ G HOO * , Δ G O 2 ] / e − 1.23 [ V ] Results and Discussion Ultrathin MOF nanosheets [Ni(HBTC)(DABCO)·3DMF] were synthesized and characterized initially, as described in the Experimental Section. The XRD pattern, Brunauer-Emmett-Teller (BET) measurements, and Flourier transformation infrared (FT-IR) spectroscopy spectra of the MOF framework agreed well with previous reports, suggesting their high-phase purity ( Supporting Information Figures S1 and S2).25,30 Further, combined characterizations of TEM, coupled with atomic force microscopy (AFM) ( Supporting Information Figure S3a), unveiled the MOF possessed loosely packed nanosheet morphology with a 5 nm thickness ( Supporting Information Figure the theoretical of the ultrathin MOF nanosheets was calculated to be nm ( Supporting Information Figures and it could be that the layer of the as-obtained ultrathin MOF nanosheets is Using the expected ultrathin MOF nanosheets as a the yolk-shell Ni0.85[email protected] with Se-vacancies was by an oriented phase modulation (OPM) as illustrated in Figure and described in the following three as and the ultrathin MOF nanosheets were at 600 °C under through which the ligands and were and [email protected] core-shell nanomaterials. The nanosheet as in the with Ni ( Supporting Information Figure The XRD diffraction at and Ni species Supporting Information Figure Subsequently, the prepared [email protected] nanomaterials were further converted NiSe2@NC nanomaterials through a selenization reaction at 350 Finally, the Ni0.85[email protected] nanomaterials with Se-vacancies were synthesized by the NiSe2@NC nanomaterials at 850 Figure 1 The process of core-shell NiSe2@NC yolk-shell Ni0.85[email protected] from ultrathin MOF nanosheets. Download figure Download PowerPoint spectra of Ni0.85[email protected] and NiSe2@NC the electrons with the g of the of Se-vacancies Also, it demonstrated that strongly were in Ni0.85[email protected], with NiSe2@NC, the of an in stimulating abundant We also performed scanning measurements of NiSe2@NC in the N2 to the compositional transformation As in Figure three temperature of mass are corresponding to the of ligands the of and the oriented structure transformation from orthorhombic NiSe2@NC to hexagonal Ni0.85[email protected] respectively. The corresponding that the three were by with and In contrast, the Ni0.85[email protected] nanomaterials obtained exhibited excellent mass was in the temperature range from 40 to 850 °C ( Supporting Information Figure In previous similar phase transformations from to [email protected] nanomaterials were and the fabrication of was achieved through selenization of However, the could not be applied to MOF precursors for the fabrication of NiSe2@NC nanomaterials. the was used for further selenization and phase is that the phase transformation from NiSe2@NC to Ni0.85[email protected] nanomaterials was by evolution from the core-shell to the The formation of the yolk-shell structure was to the by the different rates of metal atoms during the Se-vacancies creation As characterized by and it was that and Ni0.85Se with an diameter of nm were on the surface of an carbon shell and Supporting Information Figure The in Figures and show that the lattice in the were and 0.35 nm, corresponding to and Ni0.85Se respectively. The lattice of the carbon shell with the nm to C the of internal effectively the evolution from the core-shell to Such a yolk-shell structure could the active species from and the charge-transfer and the The ( Supporting Information Figure and spectra ( Supporting Information Figure of Ni0.85[email protected] nanomaterials the and homogeneous of and on the atomic ratio of was calculated to be the formation of Ni0.85[email protected] nanomaterials. The selected electron diffraction of Ni0.85[email protected] nanomaterials, in Supporting Information Figure a of well-defined The with were to the and of Ni0.85Se, a green was to the plane of with the Figure 2 spectra of NiSe2@NC and Ni0.85[email protected] nanomaterials. curves of NiSe2@NC nanomaterials performed in N2 atmosphere. and and and of NiSe2@NC and Ni0.85[email protected] nanomaterials. Download figure Download PowerPoint The powder XRD studies further illustrated the phase transformation from NiSe2@NC to Ni0.85[email protected] As in Figure the XRD of NiSe2@NC nanomaterials could be easily to in with lattice parameters of a = = = = = = In contrast, the XRD of Ni0.85[email protected] could be to Ni0.85Se in with lattice parameters of a = = = = = = 120 coupled further the measured atomic ratio of Ni to from to The strongly the phase transformation from NiSe2@NC to Ni0.85[email protected] that because of the higher of Ni-Se than that of NC no diffraction could be for carbon in the based on the spectra in Figure the of and the of and the ratio of from NiSe2@NC to Ni0.85[email protected] suggesting that more and were present in the Ni0.85[email protected] nanomaterials. Nitrogen isotherms and curves of Ni0.85[email protected] nanomaterials are in the Supporting Information Figure Accordingly, the surface is and the were in the range of The and large surface areas could accelerate the further a exposed active for H2 and O2 To the surface electronic and of during the oriented structural transformation process, was toward the Ni0.85[email protected] and NiSe2@NC nanomaterials. The spectra the of including C and and O from ( Supporting Information Figure For the NiSe2@NC nanomaterials, the could be two at the binding energies of and corresponding to the and After to Ni0.85[email protected], the at valence at and and was to and Meanwhile, the of was higher than that of the above-mentioned that Ni0.85[email protected] a of and and was the valence which the creation of The Ni spectra for NiSe2@NC and Ni0.85[email protected] nanomaterials the same as illustrated in Figure For Ni two at and was to Ni-Se and binding energy at and the of The corresponding at and eV). the C spectra could be two at and to the and respectively. on the N spectra three were with binding energies of and corresponding to pyrrolic and respectively. Ni0.85[email protected], a at was