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The Energy Level Regulation of CoMo Carbonate Hydroxide for the Enhanced Oxygen Evolution Reaction Activity Minmin Cai, Xiaoying Lu, Zehua Zou, KaiLu Guo, Pinxian Xi, and Cailing Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06360 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Sustainable Chemistry & Engineering

The Energy Level Regulation of CoMo Carbonate Hydroxide for the Enhanced Oxygen Evolution Reaction Activity Minmin Cai, Xiaoying Lu, Zehua Zou, Kailu Guo, Pinxian Xi* and Cailing Xu*

State Key Laboratory of Applied Organic Chemistry, Laboratory of Special Function Materials and Structure Design of the Ministry of Education, College of Chemistry and Chemical Engineering, Lanzhou University, No. 222, Tianshui South Road, Lanzhou 730000, China

Tel.: +86-931-891-2589, FAX: +86-931-891-2582 Email: [email protected], [email protected]

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ABSTRACT: Oxygen evolution reaction (OER) accompanied by multistep proton-coupled electron transfer is the decisive step of electrochemical water splitting due to the sluggish kinetics process. Enhancing the efficiency of water splitting indispensably requires stable and high-efficiency electrocatalysts for OER. The OER activity of electrocatalysts can be largely heightened by well adjusting their energy level and active sites. Herein, the amorphous iron cobalt molybdenum carbonate hydroxide core-shell microspheres (FeCoMo/CoMo) offer significant opportunities to improve the OER activity in both thermodynamics and kinetics due to the appropriate matching of energy level with the equilibrium potential of OER and the abundant active sites.The well-designed Fe0.25-CoMoCH/NF sample exhibits the prominent activity toward OER with an overpotential as low as 232 mV to deliver a current density of 10 mA cm-2, a small Tafel slope of 46 mV dec-1 and excellent stability in alkaline solution. Mechanistic studies using RRDE confirms the four-electron pathway with high faradaic efficiency (97.7%) toward OER. This research provides a model system so as to tune the inherent catalytic activity of electrocatalysts.

KEYWORDS: CoMo carbonate hydroxides; amorphous; energy level; oxygen evolution reaction.

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INTRODUCTION Developing green and renewable energy is extremely urgent to meet the ever-increasing energy demands, owing to the increasing depletion of fossil fuels and severe environmental issues.1-3 Specifically, electrolytic water splitting into oxygen and hydrogen has intrigued extensive research interest because water splitting is an eco-friendly and energy-efficient candidate to obtain clean fuels in terms of its brilliant advantage of carbon-free emissions.4,5 However, the four-electron oxygen evolution reaction (OER) at the anode is the critical step of water splitting due to its intrinsically sluggish kinetics.6-8 Inorganic nanomaterials have been widely used as efficient catalysts for accelerating the kinetics of OER. To date, the state-of-the-art OER catalysts are noble metal based-catalysts (IrO2 and RuO2) for their eminent electrochemical activity.9-11 Unfortunately, the widely commercial application is extremely restricted by their scarcity and high cost.12-14 Therefore, earth-abundant and highly efficient electrocatalysts for OER are highly desirable to overcome such deficiencies. Mixed transition metal oxides have been extensively explored as the promising electrode materials considering their reversible surface redox reactions, high natural abundance, and low environmental effect.15-17 Among them, Co-Mo oxides, especially cobalt molybdates (CoMoO4), have been reported to show the decent OER activity in view of their high redox activity and convenient synthesis.18,19 Unfortunately, the poor electron conductivity and high overpotential for OER severely hamper their wide-scale application. More recently, transition metal carbonate hydroxides (TMCH)

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have received some attentions due to the interesting layered structure, which has rich redox sites and high accessibility of electrolyte.20,21 For instance, Hu and co-workers reported the Co-Mn carbonate hydroxide (CoMnCH) nanosheet arrays with the excellent activity for HER and OER.22 Li’ group synthesized iron-cobalt carbonate hydroxide nanosheets and ultrathin graphdiyne-wrapped iron carbonate hydroxide nanosheets on 3D conductive Ni foam, which displayed highly catalytic activity for HER and OER.23,24 However, up to now, Co-Mo carbonate hydroxides (CoMoCH), as an important class of mixed Co-Mo oxide materials, have not received any attention in electrocatalytic application. Therefore, it would be a great advance engineering CoMoCH with excellent electrocatalytic activity and stability. Generally, the electrochemical performance of catalysts can be optimized via several approaches. Thereinto, creating more active sites has been demonstrated to be an available method toward the improvement of catalytic activity.25,26 This can be well done through synthesizing an amorphous catalysts because the amorphous structure possesses local disordered structure and surface defects, which can expose more active sites, allow rapid mass transport and electron transfer to ensure high OER activity.27-29 More importantly, in order to decrease the overpotential for OER, the energy level of catalysts should be closer to the equilibrium potential, 1.23 V, because of the more favorable exchange of electron between electrocatalyst and reactant, which can thermodynamically activate the reaction.30 The energy level of electrocatalyst can be modulated by fine tailoring the electronic density of electrocatalyst to in turn impact the redox potential due to the diminished barriers of

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reaction process.31 In our previous work, we found that the mixed-node metal-organic frameworks (MOFs) with the small ionization potentials, corresponding to the low energy level of valence bands, is advantageous for OER because the conductivity of MOFs would be well tuned and the electron transfer between catalysts and reactant can easily take place.32 According to the above considerations, it would be preferable for amorphous CoMoCH frameworks with appropriate energy level to enhance the activity for OER. Herein, the distinctive FeCoMo/CoMo carbonate hydroxide microspheres are designed, in which Fe element only locates in the center of microspheres and enable the formation of core-shell structure. The Mott-Schottky (MS) plots and solid-state UV-vis spectroscopy demonstrate that Fe0.25-CoMoCH microspheres possess higher valence band position than CoMoCH microspheres due to the existence of Fe in the center of microspheres, which is closer to the equilibrium potential of 1.23 V. The electrochemical results show that Fe0.25-CoMoCH microspheres display the remarkable activity for OER with the low overpotential (232 mV at 10 mA cm-2), small Tafel slope (46 mV/dec) and high faradaic efficiency (97.7%). Therefore, we convince that the well energy level matching between Fe0.25-CoMoCH and OER as well as the amorphous structure are responsible for the enhanced electrocatalytic activity. It is very different from the previous reports, which claimed that Fe element can optimize the adsorption energies of OER intermediates as an active site or restrict the geometrical site of active metal ions.23,33,34 This discovery could provide a guiding thought for the design of high-efficient electrocatalysts of OER.

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EXPERIMENTAL SECTION Materials.

Cobaltous

nitrate

hexahydrate

(Co(NO3)2·6H2O),

molybdenylacetylacetonate (C10H14MoO6), Iron (III) acetylacetonate (C15H21FeO6), glycerol, isopropanol, ethanol and potassium hydroxide (KOH) are all of analytical grade (AR) and used as received without further purification. Ni foam with a thickness of 1.6 mm and a pore density of 110 ppiis purchased from Changsha Keliyuan. Molybdenylacetylacetonate (C10H14MoO6) is obtained from Beijing bailingwei technology Co., Ltd. Iron (III) acetylacetonate (C15H21FeO6) is provided by Shanghai McLean Biochemical Technology Co., Ltd. Iridium (IV) dioxide (IrO2) is purchased from Alfa Aesar. Nafionis purchased from Sigma-Aldrich. All aqueous solutions are prepared by the ultrapure water (>18 MΩ cm) obtained from a Millipore system. Preparation of amorphous FeCoMo/CoMo carbonate hydroxide (denoted as Fex-CoMoCH) microspheres. Typically, 0.15 mmol Co(NO3)2·6H2O, 0.15 mmol molybdenylacetylacetonate and 0.0375 mmol Iron(III) acetylacetonate were dispersed in a mixture solvent of

glycerol-isopropanol (21 mL, 1:6 (v/v)) under continuously

magnetic stirring to form a rufous solution. The obtained mixture was transferred into Teflon-lined stainless-steel autoclave and reacted at 180 ºC for 12 h. After cooling to room temperature, the brown precipitate was separated by centrifugation, washed with ethanol for several times, and dried at 60 ˚C for 12 h. For comparison, some Fex-CoMoCH samples with different Fe content were obtained through changing the feeding moles of Iron (III) acetylacetonate, such as 0 mmol, 0.01875 mmol, 0.075

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mmol and 0.15 mmol, respectively. According to the feeding radio of Fe and Co, the obtained samples were denoted as CoMoCH, Fe0.125-CoMoCH, Fe0.25-CoMoCH, Fe0.5-CoMoCH and Fe1.0-CoMoCH, respectively. Preparation of CoMo-500 and Fe0.25-CoMo-500 microspheres. The CoMo-500 and Fe0.25-CoMo-500 microspheres were harvested by annealing the amorphous CoMoCH and Fe0.25-CoMoCH precursors at 500℃ for 2 hours under air atmosphere. Preparation of Fe0.25-CoMoCH/NF sample. In order to in-situ grow Fe0.25-CoMoCH on Ni foam, the similar process with the preparation of Fe0.25-CoMoCH powders was applied when a piece of Ni foam is placed in Teflon-lined stainless-steel autoclave. The obtained sample is denoted as Fe0.25-CoMoCH/NF. Characterizations. Powder X-ray diffraction (XRD) patterns were obtained by a Rigaku D/M ax-2400 diffractometer with Cu Kα irradiation to characterize the crystal structure of the samples. The morphology of samples was investigated by field emission scanning electron microscopy (FESEM, JEOLJSM-S4800) and transmission electron microscopy (TEM, TecnaiTM G2F30) with energy dispersive spectroscopy (EDX). X-ray photoelectron spectroscopy (XPS) was performed on a PHI-5702 instrument with a Mg Kα excitation source (1253.6 eV). Binding energies (BE) were determined using the C 1s peak at284.6 eV as a charge reference. Infrared spectra were recorded by a Bruker VERTEX 70v FT-IR spectrometer in the range of 500-4000/cm. An Agilent Cary 5000 spectrophotometer was utilized to run solid-state UV-Vis spectroscopy. Electrode preparation and electrochemical measurements. The electrocatalytic

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performance of different power catalysts was measured at room temperature by using rotating disk electrode (RDE) with a rotate speed of 1500 rpm in order to remove the generated O2 bubbles. A series of linear sweep voltammetry (LSV) were acquired at a scanning rate of 5 mV/s in 1.0 M KOH with a standard three-electrode setup, where a Pt wire was used as counter electrode, a Hg/HgO electrode as reference electrode, and the catalysts decorated RDE as working electrode (surface area = 0.0707 cm2). 5 mg as-synthesized powders and 20 L Nafion (5 wt%) solutions were dispersed in deionized water (1 mL) to form an evenly catalyst ink for electrochemical experiments. 6 µL as-prepared ink was dropped onto the surface of polished RDE and then dried under an infrared lamp for a few minutes. The mass loading is about 0.424 mg/cm2. Electrochemical impedance spectroscopy (EIS) was measured in the frequency range from 0.01 to 105 Hz at a potential of 1.49 V vs. RHE.The electrochemical active surface area (ECSA) of catalysts is estimated from the double-layer capacitance (Cdl) according to the equation: 𝐸𝐶𝑆𝐴 =

𝐶𝑑𝑙

𝐶𝑠(1)

where Cs is the specific capacitance of the sample and reported to be between 0.02 to 0.04 mF/cm2. In this work, the value of Cs is estimated as 0.04 mF/cm2. In order to get the Cdl, cyclic voltammetry (CV) measurements were performed at different scan rates in the potential windows from 0.80 and 0.90 V. By plotting the difference of current density between the anodic and cathodic current density at 0.85 V against the scan rates, the resulting linear slope is twice of the geometric double layer capacitance (Cdl).

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The obtained polarization curves have been iR-corrected to minimize the influence of ohmic resistance. Unless specifically stated, the potentials in this work were calibrated to a reversible hydrogen electrode (RHE) scale according to the Nernst equation: 0 ERHE  EHg / HgO  E Hg / HgO  0.059  pH ( pH  13.6)

(2)

The overpotential (η) was calculated according to the following formula: η(V) = ERHE ―1.23 V (3) The Faradaic efficiency (FE) was obtained according to the previous literature.35 The as-synthesized Fe0.25-CoMoCH dispersion was uniformly loaded onto rotating ring-disk electrode (RRDE) with Nafion as the binder, which composes of a glassy carbon disk electrode and a Pt ring electrode. For RRDE test, a scan rate of 2 mV s-1 and a rotation rate of 1500 rmp were applied. The FE was calculated asfollows:

FE 

I ring Ce  I disk

(4)

Here, Idisk is the given disk current, Iring is the collection current on the Pt ring electrode and Ce is the oxygen collection coefficient (∼0.21) for this type of electrode configuration.The electron transfer number (N) can be calculated from the ring current (Ir) and disk current (Id) of RRDE.

N

4 Id I Id  r Ce

(5)

Mott-Schottky curves of CoMoCH and Fe0.25-CoMoCH were measured to obtain flat-band potentials (Vfb). The Vfb values can be acquired from the X intercept by prolonging the linear part of Mott-Schottky curves to the potential axis.36 9



The band gap energy (Eg) was obtained from solid-state UV-Vis spectroscopy.37 The band energy (Eg) can be estimated by the following formula:

( hv) n  k (hv  Eg )

(6)

where α is the absorption coefficient, h is the Planck's constant, v is the light frequency and Eg is the band gap energy (eV), n = 2 for a direct band gap semiconductor or n = 1/2 for an indirect band gap semiconductor. The band gap energies of CoMoCH and Fe0.25-CoMoCH are estimated by extending the plots of (αhv)2 versus hv to (αhv)2 = 0.

RESULTS AND DISCUSSION The crystal structure of CoMoCH and Fex-CoMoCH samples with different x values (x = 0.125, 0.25, 0.5, 1.0) were investigated by X-ray diffraction measurement (XRD). As shown in Figure 1a and S1, no obvious diffraction peaks are collected for these samples, demonstrating the amorphous structure is formed. To further determine their composition, CoMoCH and Fe0.25-CoMoCH (taking it as an example) were annealed at 500℃ for 2 h under air. As shown in Figure 1b, the characteristic diffraction peaks at 11.8°, 20.9°, 23.7°, 24.5°, 25.4°, 30.17° and 32.9° can be indexed to the (001), (021), (002), (-112), (-311), (-222) and (400) planes of CoMoO4 (JCPDS No. 21-0868), respectively. Moreover, Fourier transform infrared spectroscopies (FT-IR) of CoMoCH and Fe0.25-CoMoCH reveal the stretching vibration of O-H group at about 3345 cm-1, the fingerprint peaks of CO32- at 1457, 1092, 860, and 749 cm-1, as well as the metal-oxygen vibrations at low wave number (500-1000 cm-1), respectively (Figure 1c).38,39 Consequently, the CoMoCH and Fe0.25-CoMoCH 10


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samples can be regarded as amorphous Co-Mo or Fe-Co-Mo carbonate hydroxides. The amorphous structure generally owns the properties of local disordered structure, surface defects and under-coordinated metal atoms, to a large extent, which is reliable for the rapid binding of adsorbate and bring down the activation barrier of catalytic reactions.28,40-42 The morphologies, microstructure and composition of Fe0.25-CoMoCH and CoMoCH were further characterized by SEM,TEM and STEM (Figure 1d-k and Figure S2). The uniform sphere-sharped morphology with a diameter of about 400 nm was

Figure 1. (a) XRD patterns of CoMoCH and Fe0.25-CoMoCH, (b) XRD patterns of CoMo-500 and Fe0.25-CoMo-500, (c) FT-IR of CoMoCH and Fe0.25-CoMoCH, (d) SEM, (e) TEM, (f) STEM and (g-k) the corresponding elemental mapping images of Fe0.25-CoMoCH. 11



observed for CoMoCH (Figure S2a-c). The corresponding elemental mapping images demonstrate the uniform distribution of Co, Mo, C, and O elements in CoMoCH microsphere (Figure S2d-g). Similarly, the Fe-decorated CoMoCH microspheres were also synthesized via the same method by introducing Iron (III) acetylacetonate as Fe source. As shown in Figure 1d-f, the Fe0.25-CoMoCH microspheres have the enlarged diameter of about 1μm and some microspheres grow together like twins. However, the corresponding elemental mapping images only exhibit the homogeneous existence of Co, Mo, C, O elements in the whole microsphere. The Fe element mainly distributes in the center of microspheres, suggesting the distinctive core-shell structure of FeCoMoCH/CoMoCH microspheres (Figure1g-k). The corresponding EDS line-scan elemental distribution profiles further confirms the concentrated distribution of Fe element in the center of microspheres (Figure S3).

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Figure 2. High resolution XPS spectra of (a) Co 2p, (b) Mo 3d, (c) C 1s and (d) O 1s for CoMoCH and Fe0.25-CoMoCH, (e) UV-Vis diffuse reflectance spectra of CoMoCH and Fe0.25-CoMoCH ( Inset is (αhν)2 versus hν curves of CoMoCH and Fe0.25-CoMoCH.) (f) The band gap structure comparison of CoMoCH and Fe0.25-CoMoCH.

The surface chemical valance state and composition of CoMoCH and Fe0.25-CoMoCH were studied by X-ray photoelectron spectroscopy (XPS) to verify

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the relationship of structure-performance. Co, Mo, O, and C elements are all detected in CoMoCH and Fe0.25-CoMoCH samples (Figure S4). However, the Fe element cannot be found in Fe0.25-CoMoCH, which further validates the main distribution of Fe in the center of microspheres, which remains in accordance with above elemental mapping images. The Co 2p spectra of CoMoCH and Fe0.25-CoMoCH display the Co 2p3/2, Co 2p1/2 peaks and two shakeup satellite peaks, respectively (Figure 2a). The Co 2p3/2 of CoMoCH can be deconvoluted into two peaks at 780.5 and 781.8 eV, respectively, which demonstrates the coexistence of Co3+ and Co2+ in CoMoCH.43 According to the peak area of Co2+ and Co3+, the Co2+/Co3+ atomic ratio can be calculated as ~ 2.26. For Fe0.25-CoMoCH, the peaks of Co 2p3/2 and Co 2p1/2 are negatively shifted compared with the primitive CoMoCH and the Co2+/Co3+ atomic ratio also increases from 2.26 to 2.30. The increased Co2+ content caused by introducing Fe into CoMoCH could play a great role in promoting the formation of cobalt oxyhydroxide (CoOOH) as active sites for OER.43 The Mo 3d spectrum of CoMoCH presents the typical peaks of Mo 3d5/2 and Mo 3d3/2 at 231.4 and 234.5 eV, respectively, and their splitting width of Mo 3d5/2 and Mo 3d3/2 is about 3.1 eV, proving that Mo is VI oxidation state (Figure 2b).44 In contrast, the Mo 3d3/2 and 3d5/2 peaks of Fe0.25-CoMoCH shifted about 0.4 eV to the low binding energy. The C 1s spectrum of CoMoCH (Figure 2c) can be deconvoluted into three peaks. The peak at 288.9 eV is attributed to the carbon element in carbonate ions.36 The peaks at 284.6 and 285.9 eV are ascribed to the surface-adsorbed hydrocarbons and their oxidative forms (e.g., C-OH). For O 1s spectra of CoMoCH (Figure 2d), two peaks located at

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about 531 and 532 eV are indexed to the CO32- and adsorbed OH-, respectively.36 Similarly, both the peaks of C 1s and O 1s are found the obvious shift to the negative direction for Fe0.25-CoMoCH. The negative shift of XPS peaks suggested that the Fe element in the center of microspheres can apparently modulate the electronic structure of CoMoCH, confirming the increased electron density on the surface of Fe0.25-CoMoCH, which will in turn influence the energy level of Fe0.25-CoMoCH.22,31 To further explore the energy level of Fe0.25-CoMoCH, the ﬂat-band potential and the band gap energy were calculated according to Mott-Schottky (MS) plots and solid-state UV-Vis spectroscopy (Figure S5 and 2e). As shown in Figure S5, the slopes of the Mott–Schottky plots of CoMoCH and Fe0.25-CoMoCH are positive, indicating their nature of n-type semiconductors. And the ﬂat band potential can be calculated as 1.05 and 1.07 V, respectively, which is close to the bottom edge of conduction band (CB).45 Figure 2e shows that the band gap energies (Eg) of CoMoCH and Fe0.25-CoMoCH are 1.45 and 1.31 eV, respectively, demonstrating the superior conductivity of Fe0.25-CoMoCH. Furthermore, the valence band (VB) of CoMoCH and Fe0.25-CoMoCH can be determined as 2.50 V and 2.38 V, respectively. Figure 2f shows the band edges of CoMoCH and Fe0.25-CoMoCH with respect to the equilibrium electrode potential of OER. In the case of Fe0.25-CoMoCH, the valence band is upward shifted, which presents the more proper matching of energy level between Fe0.25-CoMoCH and the oxidation potential of H2O, enabling the OER to occur at low overpotential.30,31

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Figure 3. (a) Polarization curves and (b) the corresponding Tafel plots of CoMoCH, Fe0.25-CoMoCH, and IrO2 loaded on RDEs, (c) Electrochemical impedance spectroscopy (EIS) of CoMoCH and Fe0.25-CoMoCH, (d) Chronopotentiometry measurement of Fe0.25-CoMoCH at a constant current density of 10 mA cm-2, (e) RRDE measurement of Fe0.25-CoMoCH on a RRDE (1500 rpm) in O2-saturated 1.0 M KOH with the ring potential of 1.50 V and (f) Ring current of Fe0.25-CoMoCH on a RRDE (1500 rpm) in N2-saturated 1.0 M KOH solution with the ring potential of 0.40 16


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V. To confirm the structure predictions, the OER activity of CoMoCH, Fex-CoMoCH and IrO2 normalized by the geometry surface of RDEs are evaluated in 1.0 M KOH solution. CoMoCH exhibits an overpotential of 345 mV at 10 mA cm-2, which is higher than that of IrO2 (306 mV) (Figure 3a), suggesting its poor activity for OER. The overpotential can be obviously decreased by the introduction of Fe (Figure S6). Especially, Fe0.25-CoMoCH exhibits the lowest overpotential of 263 mV at 10 mA cm-2 compared with Fe0.125-CoMoCH (280 mV), Fe0.5-CoMoCH (274 mV) and Fe1.0-CoMoCH (290mV)(Figure S6a). Such outstanding activity of Fe0.25-CoMoCH for OER is even superior to that of most noble-free electrocatalysts (Table S1, Supporting Information). Besides, Fe0.25-CoMoCH exhibits the lower Tafel slope (56 mV dec-1) than CoMoCH (72 mV dec-1), Fe0.125-CoMoCH (71 mV dec-1), Fe0.5-CoMoCH (68 mV dec-1), Fe1.0-CoMoCH (70 mV dec-1), IrO2 (118 mV dec-1) (Figure 3b and S6b) and those reported highly-active OER catalysts, disclosing the prominent reaction kinetics of Fe0.25-CoMoCH.46,47 Compared with the OER property of CoMo-500(373 mV) and Fe0.25-CoMo-500 (346 mV), Fe0.25-CoMoCH also manifests the enhanced electrocatalytic activity (Figure S7), confirming the amorphous nature is responsible for the enhanced OER performance in view of its lower activation barrier and plentiful active sites.27 The electrochemical impedance spectroscopies (EIS) (Figure 3c and S6c) present the lowest charge-transfer resistance (Rct) for Fe0.25-CoMoCH, indicating the high charge transport efficiency and more favorable electrode kinetics. Furthermore, the effective electrochemical surface area

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(ECSA), which is proportional to the electrochemical double-layer capacitance (Cdl) of these samples, can be evaluated by the cyclic voltammograms in non-faraday interval (0.80-0.90 V vs. RHE) at various scan rates from 10 to 50 mV s-1. In Figure S8, the calculated Cdl of Fe0.25-CoMoCH is 4.2 mF cm-2, which is larger than that of Fe0.125-CoMoCH (3.7 mF cm-2), Fe0.5-CoMoCH (3.2 mF cm-2), Fe1.0-CoMoCH (2.6 mF cm-2) and CoMoCH (3.4 mF cm-2), suggesting that Fe0.25-CoMoCH could provide more accessible active sites at the solid-liquid interface for OER. By the way, the intrinsic OER activity of these samples normalized by the ECSA further confirms the key roles of Fe elements (Figure S9). The number of transferred electrons and Faradaic efficiency during OER process are evaluated by using rotating ring-disk electrode (RRDE). A desirable four-electron pathway (4OH- → O2 + 2H2O + 4e-) is explored on the surface of Fe0.25-CoMoCH according to the negligible current density on the ring electrode (Figure 3e). What's more, a ring current of about 22.5 µA can be obtained when a constant disk current (100 µA) is applied for O2 generation, describing a 97.7% Faradaic efficiency (Figure 3f). Finally, the applied potential at 10 mA cm-2 for OER in Figure 3d shows the negligible change in a period of 24 h, demonstrating the excellent stability of Fe0.25-CoMoCH.

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Figure 4. (a) Polarization curves and (b) the corresponding Tafel plots of CoMoCH/NF,

Fe0.25-CoMoCH/NF

and

IrO2/NF,

(c)

Chronopotentiometry

measurement of Fe0.25-CoMoCH/NF at a constant current density of 10 mA cm-2 and (d) polarization curves of Fe0.25-CoMoCH/NF after 50 h chronopotentiometry test.

Notably, integrating conducting substrate, like carbon cloth and Ni foam, with electrocatalysts can largely boost the electrochemical performance of active materials due to the faster electron transport and special synergistic effects.46-49 In this regard, we fabricated the 3D electrodes by directly growing the Fe0.25-CoMoCH on Ni foam (Fe0.25-CoMoCH/NF) to further enhance its electrochemical performance. As expected, Fe0.25-CoMoCH/NF receives much improved OER performances with an

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overpotential of 232 mV at 10 mA cm-2, a small Tafel slope of 46 mV/dec and maintains its remarkable catalytic performance for 50 h (Figure 4).

Figure 5. (a) SEM, (b) TEM, (c) HRTEM and (d-f) the corresponding elemental mapping images of Fe0.25-CoMoCH after 50h OER. The morphology of Fe0.25-CoMoCH after 50h OER were shown in Figure 5. After the long-time OER process, the structural reconstruction took place, the original solid microspheres almost destroyed and turned into hollow spheres (Figure 5a and b). Meanwhile, the amorphous structure transferred into crystalline materials during OER, as shown in Figure 5c. The corresponding elemental mapping images displayed that Fe, Co and O elements distributed uniformly in the hollow spheres. However, Mo element hardly can be found owing to the instability of Mo in alkaline solution.

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Figure 6. (a) XRD pattern, XPS spectra of (b) Co 2p, (c) Fe 2p and (d) Mo 3d for Fe0.25-CoMoCH/NF after 50h stability test.

The evolution of crystal structure and chemical states were further studied by XRD and XPS measurements. As shown in Figure 6a, the amorphous Fe0.25-CoMoCH transforms into the crystalline Co/Fe oxo/hydroxides after the OER process. The intensity of Co3+ and Fe3+ peaks significantly increases due to the appearance of Co/Fe oxo/hydroxides (Figure 6b-c). On the contrary, Mo 3d peak nearly fades away by reason of the dissolution of Mo in alkaline solution (Figure 6d),50 which is well in accordance with SEM and TEM results.

CONCLUSION

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In summary, the amorphous FeCoMoCH/CoMoCH core-shell microspheres with remarkable catalytic activity for OER were synthesized by the facile solvothermal method. The fact was found that Fe0.25-CoMoCH exhibited much smaller overpotential, lower Tafel slope and better stability in alkaline media compared to CoMoCH, CoMo-500 and Fe0.25-CoMo-500. Mechanistic studies confirmed the four-electron pathway with superior faradaic efficiency for OER. The high OER activity of Fe0.25-CoMoCH could be attributed to the abundant active sites, fast charge transfer kinetics and large electrochemical active surface area induced by the amorphous structure. Above all, the XPS and band gap energy analysis reveal that the electronic effect between Co, Mo and Fe centers, where the Fe element concentrated in the center of microspheres, leads to the rich electron environment on the surface of CoMoCH. That exerts an influence on the energy level of electrocatalysts and make the energy level of valence band lie near the equilibrium potential of OER in turn, which enables OER occur at low overpotential.

ASSOCIATED CONTENT Supporting Information. The synthesis of CoMoCH and Fex-CoMoCH, standard characterization, IR spectra, XRD, X-ray absorption spectra, and UV-Vis diffuse reflectance spectra.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]. ORCID 22


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Cai-Ling Xu: 0000-0002-8784-4454 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by grants from Natural Science Foundation of China (NNSFC Nos. 21673105, 21571089). We thank the Electron Microscopy Centre of Lanzhou University for the TEM/SEM measurements and structural analysis.

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TOC GRAPHICS

The amorphous FeCoMo/CoMo core-shell microspheres are designed by a simple one-step solvothermal method, exhibiting remarkable activity toward OER.

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The Energy Level Regulation of CoMo Carbonate ... - ACS Publications

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