Trimetallic Molybdate Nanobelts as Active and Stable Electrocatalysts

3 days ago - The research on high-efficient oxygen evolution reaction (OER) electrocatalysts based on non-precious metal materials is extremely urgent...
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Trimetallic Molybdate Nanobelts as Active and Stable Electrocatalysts for Oxygen Evolution Reaction Xiaoling Luo, Qi Shao, Yecan Pi, and Xiaoqing Huang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04521 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 21, 2018

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Trimetallic Molybdate Nanobelts as Active and Stable Electrocatalysts for Oxygen Evolution Reaction Xiaoling Luo, Qi Shao,* Yecan Pi and Xiaoqing Huang* College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123, China.

ABSTRACT: The research on high-efficient oxygen evolution reaction (OER) electrocatalysts based on non-precious metal materials is extremely urgent for water dissociation, but still challenging. Herein, a unique class of molybdate nanobelts (NBs) with high surface area, fast electron transportation and fast diffusion of electrolyte, were constructed for enhanced OER via the electronic engineering. We discovered that introducing the third metal modifies the electronic environment of molybdate NBs, leading to the favourable intermediate adsorption on the catalysts. The optimized Mo51Ni40Fe9 NBs exhibit an overpotential of 257 mV to obtain 10 mA cm-2 and a Tafel slope of 51 mV dec-1. By combining with commercial Pt/C as the cathodic catalyst, the water electrolyzer only requires a potential of 1.55 V to achieve 10 mA cm -2 and exhibits superior stability with small current density decay after 18 h. Based on X-ray photoelectron spectroscopy and surface valence band spectra, the enhanced OER performance is tightly connected with the appropriate binding energy between reactive intermediates and active sites. This work proposes an open avenue of electronic engineering strategy for designing efficient electrocatalysts. KEYWORDS: Nanobelt, Electronic regulation, Trimetallic, Overall water splitting, Oxygen evolution electrocatalysis

1. INTRODUCTION The development of renewable and clear hydrogen-based fuel to replace the limited and environmental unfriendly fossil fuels is vital for energy conversions and storage.[1-3] To this end, hydrogen and oxygen evolution from water splitting have been deeply studied by the virtue of earth-abundant water resource, renewable electricity, nontoxicity and easier transportation of H2, encouraging the design of catalysts with outstanding performances.[4-8] However, it is still a great challenging for OER to reduce a large overpotential and energy consumption, due to the sluggish kinetics. Iridium and ruthenium-based oxides exhibit satisfied performances for OER,[9-15] while their applications on a large-scale are largely subject to the unacceptable cost and the shortage of resources. Hence, it is extremely urgent to design alternative nonprecious metal materials with outstanding performances for watersplitting reaction. In recent years, many researches on nonprecious metal materials for OER, especially Fe,[16-18] Co,[19-21] Ni,[22-24] Mo,[25-27] have been reported, while most of them suffer large overpotentials to achieve a desirable output yet. In general, the design of highly efficient non-precious metal nanocatalysts is fundamentally based on two key issues: the quantity of the catalytic sites[28] and the catalytic efficiency of the single active site.[29] In order to improve the quantity of catalytic sites, generating novel structures with high surface area ratios is necessary. Among various structures, nanobelts (NBs) exhibit the unique merits of highly accessible active sites, fast electron transportation and diffusion of electrolyte, ensuring the intimate contact between the catalyst and electrolyte during electrolysis.[30-32] For another issue, improving the intrinsic activity of the active site is another important strategy for catalytic improvement via modulating the catalyst’s electronic structure, which can be achieved by introducing the additional component.[33-35] Inspired by these guidelines, it is of great significance to combine both the NB structure and electronic engineering strategy to improve the OER efficiency of non-precious metal-based catalysts. Herein, we report the electronic engineering of molybdate NBs via introducing additional component to fulfill the highly efficient OER catalysts. This approach substantially improves the OER

performance via modulating the favorable intermediate absorption on the catalysts. Consequently, the best Mo51Ni40Fe9 NBs deliver an overpotential of 257 mV to achieve 10 mA cm-2 and the Tafel slope of 51 mV dec-1, far better than those of their binary counterparts and commercial IrO2. By combining with commercial Pt/C, the couple requires 1.55 V to obtain 10 mA cm-2 and reaches favourable stability with small activity change after more than 18 h.

Scheme 1. Illustration of fabricating Mo51Ni40Fe9 NBs for OER.

2. EXPERIMENTAL SECTION 2.1. Chemical Reagents: MoCl5 (99.6%, Alfa Aesar), NiCl2·6H2O (≥98%, Alfa Aesar), ethanol and methylamine (≥99.7%, Sinopharm Chemical Reagent Co. Ltd.), Fe(acac)3 (97%, Sigma-Aldrich) were used for catalyst prepatration. All the chemicals were utilised without further purification. The Pt/C (20 wt%, 2-5 nm Pt nanoparticles, Johnson Matthey Corporation) was used for electrocatalytic measurement. The ultra-pure grade water was used for all experiments. 2.2. Syntheses of Mo77Fe23 nanobelts (NBs) and Mo55Ni45 NBs: In a typical preparation of Mo77Fe23 NBs, MoCl5 (27 mg), Fe(acac)3 (9mg), 9 mL ethanol and 0.5 mL methylamine were placed in a Teflon-lined stainless-steel autoclave (20 mL). The mixture was magnetically stirred under 1000 r s-1 for half an hour, then heated to 160 °C and kept it for 10 h. The Mo77Fe23 NBs was collected by centrifugation and washed with an ethanol/acetone mixture. Mo55Ni45 NBs were synthesized using a similar procedure to that of Mo77Fe23 NBs, except for the use of nickel (II)

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chloridehexahydrate (24 mg) as the nickel metal source. 2.3. Synthesis of MoNiFe NBs: MoNiFe NBs were synthesized using a similar procedure to that of Mo77Fe23 NBs, except for the use of nickel (II) chloridehexahydrate (24 mg) as the nickel metal source and different amounts of Fe(acac)3 (1, 3, 5, 7mg). 2.4. Characterization: We used Bruker Dimension Icon to collect Atomic force microscopy (AFM) images. Other characterizations were investigated by the same instruments mentioned in our previous studies. [13, 35] 2.5. Electrochemical Analysis: A typical three-electrode system was used by a CHI 660E electrochemistry workstation. The saturated calomel electrode (SCE) and a graphite rod were used as the reference and counter electrodes. Firstly, 2 mg sample was dispersed in 0.5 mL isopropyl alcohol, and then 10 μL Nafion (5 wt%) was injected into it. To obtain a super-distribution ink, the mixture was continually sonicated for 30 min. 30 μL obtained ink was then dropped on the a glassy-carbon electrode (GCE, diameter: 5 mm, area: 0.196 cm2). Linear sweep voltammetry (LSV) was conducted at 5 mV s-1. The Tafel slopes were obtained from LSV collected. Unless otherwise mentioned, all the potentials are carried out by iR corrected. The electrochemical double-layer capacitance is utilized to calculate the electrochemically active surface area (ECSA) according to the previous report.[35] The formula, E (RHE) = E (SCE) + 0.241V + 0.0591pH, was used to the potential calibration. 2.6. Turnover frequency (TOF) calculation: The TOF value is collected by the following formula: TOF = I / 4nF where I (A) is the current derived from the LSV measurements. F represents the faraday constant (96485 C/mol). n denotes the amount of moles of active sites.

3. RESULTS AND DISCUSSION

Figure 1. (a) HAADF-STEM and (b) TEM images of the Mo51Ni40Fe9 NBs. (c) EDS, (d) STEM-EDS elemental line-scan and (e) STEM-EDS mappings of the Mo51Ni40Fe9 NBs.

The OER electrocatalysts were fabricated via a one-pot, chemical reaction (Scheme 1, see experimental section). We first prepared the bimetallic molybdate NBs, naming Mo55Ni45 NBs (Figure S1a,b), as characterized by the X-ray diffraction (XRD) (Figure S1d). To fabricate the trimetallic molybdate, the third metal

precursor was introduced. The obtained Mo51Ni40Fe9 NBs keep a typical NB shape with a diameter of ~100 nm, confirmed by the high-angle annular dark-field scanning TEM (STEM) (Figure 1a) and Transmission electron microscopy (TEM) images (Figure 1b). The Mo51Ni40Fe9 NBs were also characterized by AFM (Figure S2), where the thickness was analyzed to be ~11.5 nm. EDS spectrum confirms the existences of Mo, Ni and Fe and the atomic ratio is close to the feeding ratio of reactants (Figure 1c). The XRD peaks can be assigned to the combination of NiMoO4 and NiMo3O10∙9H2O (Figure S4d). The line scans (Figure 1d) and EDS mappings (Figure 1e) demonstrate that Mo, Ni, and Fe are distributed uniformly, suggesting the successful fabrication of trimetallic molybdate. We also prepared other trimetallic MoNiFe NBs with different atomic ratios (Figure S3). EDS analyses suggest the molar ratios of the NBs are close to the feeding ratios of the reactants (Figure S4a-c). XRD patterns (Figure S4d) display that all the Mo-Ni-Fe NBs have the same phases as Mo55Ni45 NBs. Binary Mo77Fe23 NBs were also fabricated for comparison (Figure S5). Before starting the measurement, all the catalysts were loaded on carbon black. For comparison, the performances of bimetallic molybdates (Mo77Fe23 NBs and Mo55Ni45 NBs) and commercial IrO2 were also tested. Figure 2a and Figure S6a show the LSV curves of different catalysts. Obviously, the trimetallic molybdates display better OER activity than the bimetallic molybdates and IrO2, in which Mo51Ni40Fe9 NBs exhibit the best OER activity. To achieve the current density of 10 mA cm-2, Mo51Ni40Fe9 NBs just need an overpotential of 257 mV, far lower than those of commercial IrO2 (330 mV), Mo77Fe23 NBs (455 mV), Mo55Ni45 NBs (353 mV), Mo51Ni48Fe1 NBs (300 mV), Mo51Ni45Fe4 NBs (270 mV) and Mo52Ni38Fe10 NBs (265 mV). Such a low overpotential (η10 = 257 mV) is very admirable for a non-precious metal OER catalyst in basic solution and even better than most reported catalysts (Table S1). Mo51Ni40Fe9 NBs also represent the smallest Tafel value (51 mV·dec-1) compared to IrO2 (55 mV·dec-1), Mo77Fe23 NBs (58 mV·dec-1), Mo55Ni45 NBs (105 mV·dec-1), Mo51Ni48Fe1 NBs (59 mV dec-1), Mo51Ni45Fe4 NBs (57 mV dec-1) and Mo52Ni38Fe10 NBs (46 mV dec-1) (Figure 2c and Figure S6b), indicating the fast OER kinetics. As shown in Figure 2d and Figure S6c, electrochemical impedance spectroscopy (EIS) depicts Mo51Ni40Fe9 NBs possess smaller charge transfer impedance than bimetallic NBs (Mo77Fe23 NBs, Mo55Ni45 NBs) and other Mo-Ni-Fe NBs. In addition, the calculated TOFs of different catalysts demonstrated that the Mo51Ni40Fe9 NBs show the highest TOF, as confirmed by Figure S7. All these results suggest that the introduced third metal is of great significance in terms of the electrochemical performance. The OER stability was then assessed by 1000 CVs (Figure 2e) and long-duration chronoamperometry (Figure 2f). The OER LSV curves of Mo51Ni40Fe9 NBs before and after 1000 CVs ranging from 1.25 V to 1.75 V vs. RHE are coincided with each other, while rapid decay happened to the LSV curve of commercial IrO2 after 1000 CVs. Additionally, the current density of Mo51Ni40Fe9 NBs can be largely maintained up to 40 h, as confirmed by the current-time curve, while great loss of current density occurs on commercial IrO2. This excellent stability can be further reflected by the largely maintained structure and negligible change of molar ratio of Mo51Ni40Fe9 NBs after longduration stability test (Figure S8). OER has been considered to be the bottleneck reaction of overall water splitting due to its sluggish kinetics.[36,37] Therefore, it is significant to figure out the reason for the promoted OER activity of the trimetallic NBs. The electrochemical surface areas (ECSA)

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Figure 2. (a) LSV of different catalysts. (b) Overpotentials at different current densities of different catalysts. (c) Tafel plots of different catalysts. (d) Nyquist plots of different catalysts at 1.55 V (vs. RHE). (e) Polarization curves of Mo51Ni40Fe9 NBs and commercial IrO2 before and after 1000 CVs. (f) The i-t curve of Mo51Ni40Fe9 NBs and commercial IrO2 at 1.49 V (vs. RHE).

of all samples were first measured (Figure S9), in which the ECSA is out of proportion to the OER performance, indicating that the ECSA is not the key factor for performance enhancement. Since OER is a typical surface-sensitive reaction, the electronic structures of the catalytic sites on the catalysts were then investigated by X-ray photoelectron spectroscopy (XPS). Let us make a first glance of the OER mechanism. A proper OER pathway for electrocatalysts in basic solution is profiled in Figure 3a, where the active site is denoted as “M”. There are four reaction steps for OER process. The first step describes the formation of M-OH by oxidizing hydroxide anion and adsorbing hydroxyl radical on the active site. Afterwards, the deprotonation of OH* to O* results in the formation of M-O. Subsequently, a hydroxyl radical is adsorbed on the M-O intermediate to produce the M-OOH intermediate. Then a further deprotonation and electron transfer from M-OOH contributes to the generation of O2 and the recovery of the pristine active site. Therefore, the activity of OER is largely dependent on the absorption of O*, OH* and OOH*.[38] Although Ni provides the main active sites in the present catalyst system, it has a too strong absorption to O*, which largely prevents the further reaction.[39] Consequently, we analysed the Ni XPS to understand the enhanced OER activity by Mo 51Ni40Fe9 NBs. As summarized in Figure 3b, the Ni2+/Ni3+ ratio increases first but then drops with increasing Fe. Moreover, the OER overpotentials of catalysts decline with increasing the molar ratio of

Figure 3. (a) A generalized OER mechanism. (b) Ni2+/Ni3+ molar ratios and overpotential of all catalysts. (c) Ni 2p XPS spectra of of Mo55Ni45 NBs and Mo51Ni40Fe9 NBs. (d) Surface valence band photoemission spectra of Mo51Ni40Fe9 NBs and Mo55Ni45 NBs. All the spectra are background corrected. The vertical lines indicate the d-band centres of samples relative to Fermi level.

Ni2+/Ni3+, indicating Ni2+ plays a vital role in the OER process. As shown in Figure 3c, one pair of peaks assigned to Ni 2p3/2 and Ni 2p1/2, can be discovered in the Ni 2p XPS spectra. The peaks of Ni 2p3/2 of Mo55Ni45 NBs and Mo51Ni40Fe9 NBs were located at 855.71 eV and 855.58 eV, respectively, in which a negative shift of 0.13 eV was observed. The result indicates the transformation of the electron from Fe to Ni, giving the increase of lower valence state Ni2+. We further compare the XPS Ni peaks of different trimetallic molybdates. As shown in Figure S10, all MoNiFe NBs show negative shift compared with bimetallic Mo55Ni45 NBs, in which Mo51Ni40Fe9 NBs exhibit the maximum negative shift of 0.13 eV. According to the OER results, Mo51Ni40Fe9 NBs show the best OER performance, indicating the existence of the best composition. The combination of XPS results and OER results demonstrates the appropriate content of Fe can optimize the electronic states of catalyst, resulting in the largely subdued adsorption of surface O* on Ni and improving the OER activity. Such tendency was also evidenced by the surface valence band photoemission spectra of Mo55Ni45 NBs and Mo51Ni40Fe9 NBs (Figure 3d). The d-band locations of Mo55Ni45 NBs and Mo51Ni40Fe9 NBs are -5.18 eV and -5.32 eV, respectively. The shift to the lower energy level relative to Mo55Ni45 NBs indicates the decrease of the binding strengths of OH*, O* and OOH*, consisting with the XPS results.[40,41] Therefore we can conclude that the binding energy of the intermediate to Mo51Ni40Fe9 NBs was located in an optimal position in comparison to other catalysts, resulting in the best OER performance. All in all, the enhanced activity is attributed to the modification of electronic properties of trimetallic NBs, which modulates the reaction pathway.

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The authors declare no competing financial interests.

ACKNOWLEDGMENT X. H. acknowledges the National Natural Science Foundation of China (21571135), Ministry of Science and Technology (2016YFA0204100, 2017YFA0208200), Jiangsu Province Natural Science Fund for Distinguished Young Scholars (BK20170003) and Young Thousand Talented Program. Q. S. acknowledges Natural Science Foundation of Jiangsu Higher Education Institutions (17KJB150032). REFERENCES

Figure 4. (a) Schematic diagram and (b) polarization curves for overall water splitting. (c) Chronoamperometric measurement of the overall water splitting at the potential of 1.60 V of different catalysts.

To improve the practical application of trimetallic molybdates for overall water splitting, a two-electrode system was installed in alkaline solution using Mo51Ni40Fe9 NBs as anode and commercial Pt/C as cathode (Figure 4a and Figure S11). For comparison, a commercial IrO2 || Pt/C couple was also investigated. Figure 4b shows that the cell voltage of Mo51Ni40Fe9 NBs || Pt/C couple for overall water splitting at 10 mA cm-2 is 1.55 V, which is 70 mV less than that of commercial IrO2 || Pt/C couple. Remarkably, the activity of the Mo51Ni40Fe9 NBs || Pt/C couple is superior to most of reported catalysts (Table S2). Stability test was also conducted by applying a cell voltage of 1.60 V. The commercial IrO2 || Pt/C couple showed obvious current density loss, while more than 85% of the original current density can be maintained for Mo 51Ni40Fe9 NBs || Pt/C after 18 h overall water splitting, showing the enhanced activity and endurance of Mo51Ni40Fe9 NBs || Pt/C (Figure 4c). 4. CONCLUSIONS We have designed a class of highly efficient trimetallic MoNiFe NBs for water-splitting reaction. The optimized Mo51Ni40Fe9 NBs show excellent OER activity with a lowest overpotential (257 mV) at 10 mA cm-2, far better than those of their binary counterparts and commercial IrO2. The enhanced OER activity is put down to the electronic effect and the downshift of d-band center. For water-splitting reaction, only a potential of 1.55V is needed for the Mo51Ni40Fe9 NBs || Pt/C couple to obtain 10 mA cm-2. It also exhibits superior stability with small activity decrease after 18 h. The present work sheds light on the electronic engineering strategy for pursuing efficient catalysts.

ASSOCIATED CONTENT Supporting Information The experimental data is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author [email protected], [email protected]

Notes

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