âCujuâ-Structured Iron Diselenide-Derived Oxide: A Highly Efficient

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“Cuju”-Structured Iron Diselenide-Derived Oxide: a Highly Efficient Electrocatalyst for Water Oxidation Zhaoxi Yang, Jun-Ye Zhang, Zaiyong Liu, Zhishan Li, Lin Lv, Xiang Ao, Yifan Tian, Yi Zhang, Jianjun Jiang, and Chundong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14072 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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ACS Applied Materials & Interfaces

“Cuju”-Structured Iron Diselenide-Derived Oxide: a Highly Efficient Electrocatalyst for Water Oxidation †+

Zhaoxi Yang,

†+

Jun-Ye Zhang, Zhang

*, ‡

†

†

†

†

†

Zaiyong Liu, Zhishan Li, Lin Lv, Xiang Ao, Yifan Tian, Yi †

, Jianjun Jiang and Chundong Wang

*,†

†

School of Optical and Electronic Information, Huazhong University of Science and Technology,

Wuhan 430074, P.R. China ‡

School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan, 430073,

P.R. China

KEYWORDS: diselenides-derived oxide, iron-based material, electrocatalyst, oxygen evolution, lattice distortion, vacancy defects

ABSTRACT: Electrocatalysts with outstanding performance have been highly desired towards exploration of the new energy storage and conversion devices/systems as well as making an efficient and eco-friendly utilization of the green energy nowadays. In this study, we have composed an iron-based binary diselenide-derived Oxide (Fe-SDO) with a facile one-step hydrothermal method to utilize the earth abundant iron and the probably prosperous catalytic performance of metal-selenides compounds. The catalyst exhibits an overpotential of 226 mV at a current density of 10 mA/cm2, a Tafel slope of 41 mV dec-1 and a robust durability after catalyzing vigorous OER for 36 hours constantly. Through several analytical methods conducted before and after the oxygen evolution reaction activation on FeSe2, it was discovered that such catalyst possessed a morphology as “Cuju”-like balls with porosity inside, in which we explored the [+] The authors contributed equally to this work.

ACS Paragon Plus Environment


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vacancy defects and lattice distortion that play significant roles in generating high electrocatalytic performance of our proposed catalyst by inducing remarkable electron conductivity in the porous “Cuju” balls (a Chinese traditional football). Throughout our work, the superb electrocatalyst performance of the iron-based compounds was demonstrated and sequentially the behind reason for such electrocatalyst performance was addressed, which may push boundaries for the exploration of iron-based compounds as the OER catalyst and large-scale commercial application of such compounds in the future.

1. INTRODUCTION The wild utilization of the conventional energy resources has induced several severe environmental problems, which at present raise great awareness of the quest for new forms of energy with zero carbon-dioxide and air-pollutant emission.1-10 Out of all the endeavors, sprout the requirements for efficient energy conversing process and storage technology, which has paved attention to regenerative hydrogen fuel cells.11-13 To produce hydrogen for fuel cell, electrochemical water-splitting reaction is universally the most desired process ((H2O(l) → H2(g) + 1/2O2(g): ∆G° = +237 200 J/mol, ∆E° = 1.23 V).14-19 Commonly, the proposed module designs of this water-splitting process are composed with two most significant proportions, which is the cathode part generating hydrogen evolution reaction (HER) and the anode part involving oxygen evolution reaction (OER).20-23 However, these reactions, especially OER, demand a certain amount of overpotential to overcome the kinetic barrier for them to occur because OER including four proton-coupled electron transfer and oxygen-oxygen bond formation is not relatively kinetically efficient, hence it is in requirement for catalysts to expedite this reaction.24-25 Therefore, electrocatalysis for OER are


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popular among experts who are in search of high efficient electrocatalysts to generate such reaction and numerous studies have been carried out along with various catalysts designed and invented

to

enhance

electrode

kinetics

and

stability

meeting

different

electrolytes

environments.17-18, 20 Currently, the Ir- and Ru- based compounds are convinced to exhibit great electrocatalytic performance with low overpotential and small Tafel slope, however, noble metals as they are can not be utilized in massive industrious production owing to their uneconomical expense, which arouses concentration on studying compounds composed of transition metals including iron (Fe),26-32 cobalt (Co),25, 30, 32 and nickel (Ni),28, 33 in replace of Ir and Ru, due to their practically metallic natures as well as splendid electrical conductivity.33-35 Among these three elements mentioned above, there has been long-term comparison between Ni and Fe over their electrocatalytic performance, and Ni-based compounds are commonly recognized to possess better performance than Fe-based compounds does, leading much popularity to the former while iron-based compounds remains less valued although Fe is the most earth-abundant transition metal.36 Positively, up-to-date researches have been conducted on Febased compounds but acquired inefficient catalytic performance, approximately for the intrinsicly low electrical conductivity of many iron-based materials (FeS2, Fe2O3, etc.) which produces Schottky barriers at both catalyst−electrolyte and catalyst−support electrode interfaces,37-39 hence the electrocatalytic property becomes less ideal. Nevertheless noticeably, selenides of transition metals are acknowledged to possess relatively superb electrocatalytic ability in comparison with Sulfide and Phosphide catalysts owing to its relatively rapider delivery of dioxygen molecule inherited from the negative charge localized on Se sites and the cumulative 3d-2p repulsion



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between the metal d-band center and Se d-band center, while majority of the concentration is on Cobalt-based selenides.40 Besides, as we all know, some of chalcogenides could be hydroxylated and then transferred into oxides or hydroxides during OER process, not to mention that selenide-derived-oxide (SDO) has been reported to be an extraordinary OER catalyst, which may inspire an innovative approach to explore iron-based catalyst with high electrocatalytic efficiency.41 Accordingly,

to

contradistinguish

the

nature

of

iron-based

catalyst

with

other

transition-metal-based catalysts and to demonstrate its identically splendid catalytic property for OER, we managed to synthesize iron diselenide-derived oxide (Fe-SDO) through a hydrothermal reduction route on

Ni foam as well as electrochemical activation process, which displays

relatively high catalytic performance for oxygen evolution reaction in comparison with nickel diselenide-derived oxide (Ni-SDO) and manganese selenide-derived oxide (Mn-SDO) fabricated through the same procedures. The iron-diselenide-derived oxide electrocatalyst has shown a remarkably low overpotential of 226 mV to achieve the current density at 10 mA/cm2, and the morphology of the as-prepared iron diselenide as well as its SDO is well examined to analyze the indwelling account for the excellent electrocatalytic property of iron diselenide derived oxide, which will inspire the attention of whether the compounds based on Fe, largely existing in nature, can perform efficiently in catalizing OER or possess stronger catalytic ability, and may as well push further research and application of iron-based electrocatalysts in a significant scale.42

2. EXPRIMENTAL SECTION All chemical reagents were directly used without any further purification. 2.1 Synthesis of Fe-SDO: The Fe-SDO was prepared through two steps:


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Fisrt, the one-pot hydrothermal process was utilized to synthesize the FeSe2 precursor and all reagents in application for this experiment are at analytical grade requiring no further purification in advance. Before the synthesis, a piece of rectangular Ni foam at size of 1 cm* 2cm was washed sequentially with 6M HCL, deionized water, absolute ethanol, and then desiccated in the vacuum oven till dehydrated completely. To achieve the synthesis, 2mmol FeSO4·7H2O and 4mmol Se powder were dissolved into 20 mL deionized water. Afterwards, 3 mL hydrazine and 6 mL ammonia water were added into the mixture gently. Then the as-prepared Ni foam was completely immersed into the solution. Continuously, the solution was heated at 120°C for 12 hours after contained in a Teflon-lined stainless steel autoclave. Eventually, the Ni foam grown with the proposal FeSe2 was taken out and washed in turn with deionized water and absolute ethanol for three or four times. Second, the FeSe2 was activated through CV scanning at a scan rate of 0.1 V s-1 during 40 circles (the scan voltage window of which is between 1.0V(vs.RHE) and 1.7V(vs. RHE)) in the 1M KOH solution as electrolyte and with Ag/AgCl as the reference electrode, Pt as the counter electrode, to obtain the Fe-SDO proposed. The CV curves after 5, 20 and 60 circles have been included in the Figure S8, which appears that CV curve has been stabilized after 20 cirlces indicating that 40 cycles is sufficient to deplete most of Se in the precursor. 2.2 Electrochemical measurements The standardized three-electrode configuration applying a CHI 760E electrochemistry workstation is where all electrolysis experiments were conducted. Ag/AgCl was utilized as the reference electrode and Pt as the counter electrode during the examination on the electrochemical potential of all samples. The mass loading is 2.6mg cm-2. On account of the reversible hydrogen electrode



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(RHE), the converting equation is demonstrated as follow: = + 0.197V + 0.059pH. In addition, linear sweep voltammetry (LSV) was tested at a scan rate of 5 mV s-1 in order to obtain the polarization curves. Chronopotentiometry was carried out to test the stability of Fe-SDO under a steady current density of 10 mA cm-2. Electrochemical impedance spectroscopy (EIS) was taken out at a frequency between 0.01 Hz and 100 KHz. The potential window was 1.0-1.1 V versus RHE and the scan rates were at 20, 40, 60, 80, 100 mV s-1 during the electrochemically active surface area (ECSA). Faradaic efficiency is determined through rotating ring-disk electrode (RRDE) composed of a disk electrode and a ring electrode with Ag/AgCl as reference electrode, Pt as the counter electrode and 0.1 M KOH as electrolyte. The Fe-SDO powder was coated on the RRDE and a rotating rate of 1500 rpm was applied. The ring potential was firstly commenced as 1.50 V vs. RHE to reduce the probable impact from HO2- intermediates, and decreased to 0.20 V vs. RHE to examine the O2 from Fe-SDO powder attached to the RRDE. The efficiency is calculated by: = ⁄( ) Where Id is the disk current, Ir the ring current, and N is the current collection efficiency of RRDE, which is equal to 0.2. The turnover frequency is determined according to the equation by A.T.Bell as follow: 43 = ∗ ⁄(4"#$%& ) I represents the current obtained at 700 mV, NA is the Avogadro number (6.023 ×1023mol−1), Natoms is the number of atoms on the surface and F stands for the Faraday Constant. TOFmax was estimated from the surface area of film and the number of Fe atoms on the surface utilizing a value of 6.4 × 1014 Fe atoms per cm2 area.


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Mass transport is determined by altering the scan rate from 1 to 5 mV s-1 to obtain the LSV polarization curves from high voltage to low voltage. 2.3 Materials and characterization The phase of the attained iron SDO were characterized by X-ray powder diffraction (XRD) patterns on Empyrean (PANalytical B.V. with Cu-Kα radiation). The morphology of the as-prepared material is identified through the high-resolution transmission electron microscopy (HRTEM), together with corresponding elemental mapping carried out on a TecnaiG2 20 (Philips) at an accelerating voltage of 200 kV. The X-ray photoelectron spectroscopy (XPS) data were analyzed on a Kratos AXIS Ultra DLD-600W XPS system with a monochromatic Al Kα (1486.6 eV) X-ray source.

3

RESULTS AND DISCUSSION It is mentionable that the iron diselenide precursor was fabricated through the facile one-pot

hydrothermal process. The process is illustrated in Figure 1, while more details of fabrication and activation procedure can be referred from the experimental section. In an attempt to observe the morphology and microstructure of the as-synthesized FeSe2 precursor to better comprehend its insight situation, scanning electron microscopy (SEM) was conducted in a scale of 10µm (Figure 2a) and 1µm (Figure 2b), of which Figure 2a has identified that the FeSe2 organized like microspheres piling up on the Ni foam with porosities among them, which probably can result into the limited surface contact among the microspheres, nevertheless, generate a large number of active sites for FeSe2 contacting with electrolyte as we speculated. To take closer observation of these microspheres, Figure 2b further illustrates that the shape of microspheres composed of iron diselenide is “Cuju”-like rattan balls to a certain extent (“Cuju” is



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a Chinese traditional football with void inside) and implies that such balls may partially contain lacuna inside. Hence, in order to further demonstrate such topography, the transmission electron microscope (TEM) in a scale of 1µm is carried out to assist analyze the structure to a higher degree in Figure 2c. Notably, such microsphere with porous structure resulted from the fabrication process is convenient to expose more active sites concealed in sphere core as well as transport reactants.44 Additionally, not only inner area, but also outer surface is exhibited in the extra-magnified SEM image (Figure 2d) in a scale of 500nm. Such image delivers a closer observation on the sample and recognizes that the micro-ball is the consequence of iron diselenide nanoparticles converging together, which derives the coarse surface of the rattan ball, and as well may lead to a large surface contact between the electrocatalyst and electrolyte. To identify the structural information, the as-prepared samples were examined by X-ray diffraction (XRD). Apparently, the XRD patterns in Figure 2 reveals vivid peaks at 30.939°, 34.631°, 38.034°, 57.363°, 59.769°, 64.383° and 75.644° which are respectively correspondent with the faces of (200), (210), (211), (230), (321), (400) and (421) of cubic phase of FeSe2 (JCPDS Card no.:00-048-1881; space group: 205; a=5.776, b=5.776, c=5.776, β=90.00). Hence, the XRD analysis stated above firmly demonstrates that FeSe2 is constituted in our work. In an agreement with XRD graph, from Figure 2e, the lattice fringe was observed and it is correspondent with (210) plane, which is clarified to possess inter-planar distance of 0.254nm, and the (210) plane happens to exhibit a prominent peak in Figure 2f, which manifests the cubic structure of FeSe2 as elucidated in Figure 2g. While to sequentially testify the element distribution of these nanoparticles, EDS mapping was utilized and thus Figure 2h was achieved, revealing that Fe and Se are homogenously and majorly distributed, clarifying our successful fabrication of


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FeSe2. Moreover, the EDS mapping of oxygen was as well acquired, which however realized that O exist but under considerably low proportion in the rattan ball as shown in the Figure 2h, and its existence could probably be the consequence of the oxidation on the surface of such “Cuju”-like microspheres which is echoed by X-ray photoelectron spectroscopy measurement stated later. Therefore, from all morphological analysis mentioned above, it is apparent that the as-prepared iron diselenide in cubic lattice structure comes into existence as such porous Cuju-like microspheres composed of numerous nanosparticles on the surface, which might make large active space for the catalyst and thus produce high electrocatalytic efficiency for Fe-SDO, and such speculation is further validated by the electrochemical analysis mentioned later. As some selenides can be transferred into oxides during catalytic process,41 to reveal the real active substance, corroborate the stability of morphology and take a further insight of the catalyst after the OER stability test (which is the active form of the catalyst) at a morphological level, the SEM and TEM were applied to the as-synthesized iron diselenide-derived oxide after the oxygen evolution reaction stability test. As shown in Figure 3a, 3b and 3c, the porous structure and the general spherical morphology of as-mentioned “Cuju” balls which is convenient to expose active sites and transport reactants, remains still and directly prove this stable morphology. Interestingly, the surface of the microspheres is smoothened as the nanoparticles attached to the surface vanish in comparison with Figure 2a, 2b and 2c, which probably indicates the compositional variation from iron diselenide to iron SDO and consequently generates the larger exposure of porosities within the balls (vividly exhibited in Figure 3d). Notably, the detection of vacancy spots (the white spots marked with circled in the figure) appears in magnified TEM image as shown in Figure 3e , notifying the the occurrence of vacancy defects in the structure, which can be ascribed



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to the hydroxylation of iron diselenide to iron SDO and the valence change of Fe during the electrocatalytic process. Additionally, the lattice distortion was surprisingly discovered, reconfirming the insight alteration of the as-obtained catalyst during the OER and such lattice distortion proves the occurrence of vacancy defects which could make contribution to generate a high electron conductivity during the OER process. To solidly verify the compositional alteration from FeSe2 to iron SDO, the contrast EDS examinations before and after the catalytic process were conducted as shown in Figure 3f, and the peak representing Se diminished and the peak of Fe intensified after the OER, which is another validation of compositional change as well as the transformation from FeSe2 to Fe SDO during the electrochemical process. The EDS mapping after the OER was as well attained exhibited by Figure 3g, from which the constitution of oxygen and the depletion of Se in the catalyst were identified in comparison with the former EDS mapping before the OER, again revealing the compositional change and the transformation from the precursor to the catalyst when functioning. To supply more profound information of surface chemical state and unveil this transformation of as prepared Fe-SDO, as-obtained iron diselenide as well as iron SDO was further detected by XPS, in which Fe, Se and O are well identified in Figure 4. From the Figure 4a which displays the situation of Fe 2p before OER, it can be spotted that there are two deconvolved peaks for Fe 2p3/2 at 706.76 eV, 711.82 eV and one for Fe 2p1/2 at 725.27 eV, of which the peak at 706.76 eV indicates the presence Fe2+ and the latter two peaks as well as the satellite peaks for Fe 2p3/2 at 715.84 eV and 719.82 eV in the graph overall suggest the existence of Fe3+ which could be the contribution from the proportional surface oxidation of iron ion.45-47 The XPS graphs of Se 3d before the OER activity is presented in Figure 4b, in which four peaks fitting of Se 3d is identified.


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Peaks at 54.0 and 54.6 eV are correspond to Se 3d5/2 , indicating the existence of Fe-Se bonding while peaks at 55.29 and 55.80 eV are inherently related to Se 3d3/2, identifying the living of Se.41 Two peaks at 58.69 and 59.46 eV ascribed to Se-O bonding structure suggests the oxidation of Se on the surface.45, 48 The state of O before the chronopotentiometry test is as well appeared in the graph (Figure 4c), where the existence of binding energy of O 1s peaks at 530.95, 531.56, and 533.22 eV in correspondence with O 1s can be spotted. Peaks at 530.95 eV marked as OI probably related to oxygen in -OH groups due to the physical absorption of -OH during the hydrothermal process fabricating the catalyst,45 and working in concert with the Se 3d spectra, the peak at 531.56 eV responsible for OII indicates the SeO2 constituting on the surface of FeSe2 before the test while the peak located at 533.2 eV assigned to OIII is suggested to be the water absorbed chemically and physically at or near the surface.49 In regard to iron SDO, variation of the surface characteristics has been observed. From what is appeared in Figure 4d, there remains one prominent peak for Fe 2p3/2 at binding energy of 712.0 eV and the other one for Fe 2p1/2 at 725.0 eV as well as the one satellite peak for 718.26 eV, which notifies the disappearance of Fe2+ and mere presence of Fe3+,41, 45 revealing the oxidation of Fe2+ and the compositional alteration during the oxygen evolution reaction. Interestingly, in an answer to our speculation before, the loss of Se 3d is identified as the peak fades away after OER exhibited in Figure 4e, reaffirming the inherent transition from iron diselenide to iron SDO during OER process. The presence of the prominent O 1s peaks referring to Figure 4f after OER indicates more insights into the OER activity as the peak at 531.0 eV suggests the existence of oxygen in -OH groups, which implies the hydroxylation of the catalyst and explain the disappearance of peak attributed from Se 3d.41 Such process has a splendid agreement with the result of EDS signal



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and mapping graphs. Peak situating at the binding energy of 531.7 eV could be the consequence from the surface oxidation of the catalyst during OER activity.49 Therefore, according to the investigation of morphology and surface chemical state, the compositional transition process from selenide to oxide, in which occurred the hydroxylation of iron diselenide, along with the reduction of Se proportion, and the valence state alteration of Fe from Fe2+ to Fe3+, induced the production of vacancy defects and lattice distortion, which may sequentially improve the electron conductivity. To further confirm the hypothesis stated before about conductivity and demonstrate the splendid catalytic performance for oxygen evolution reaction of the as-prepared iron SDO, there are several electrochemical examinations conducted via a standard three-electrode system in O2-saturated 1 M KOH solution. Notably, Mn-SDO and Ni-SDO were fabricated and examined for contrast with the same facilities, under the identical condition and through the uniform process, to testify whether this as-obtained iron-SDO catalyst performs as parallelly well as or even superior than other transition-metal-compounds do. Moreover, to guarantee that the catalytic performance is not from the nickel foam serving as the scaffold where the iron-SDO germinates on, the electrochemical performance of the Ni foam was as well examined. Apart from all above, it is mentionable that every SDO was electrochemically cycled to achieve a stable status for the validation of all electrochemical analysis (further details are stated in the experimental section). Sequentially, to obtain the overpotential correspondent with a certain current density, the linear sweep voltammeter (LSV) was performed, mentionably, if the scanning was conducted from low voltage to high voltage, the strong redox peak during such process will appear and consequently lower the overpotential of all examined samples. Hence, to eliminate such side


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effects and obtain more authentic data, the scanning commenced from high voltage to low voltage at a rate of 5 mV s-1 and the polarization curves were obtained. Referring to Figure 5a, the iron SDO catalyst exhibits superior catalytic nature than the support and other SDOs do with relatively lower threshold potential and higher current density. More specifically, to reach a current density of 10 mA cm-2 which is the universal criterion for analyzing the overpotential of all catalysts, the as-prepared iron SDO displays an overpotential of 226 mV, which is rather lower than the overpontential of manganese SDO as 339 mV and that of nickel SDO, which is 338 mV (overpotentials with other catalysts reported are included in Table S1 for contrast). Also apparently, the overpotential of the support (Ni foam) at current density of 10 mA cm-2 appears to be 390 mV which is way more higher than that of iron SDO, indicating that the electrocatalytic property of such scaffold has no significant impacts on the catalytic performance of the as-attained iron SDO. In addition, such overpotential is exceedingly better than many of the Ni-based compounds like sulfides,50 phosphides,50-51 nitrides39, 52-53 and other iron-based compounds, which are included in the Table S1. Hence, it is well signified that the iron SDO prepared is a relatively and significantly outstanding catalyst for oxygen evolution reaction. Apart from overpotential, the Tafel slope is another kinetic parameter to evaluate the performance of electrocatalysts and to determine, owing to the fact that a relatively smaller Tafel slope represents a more rapid increase in current density achieved by a smaller overpotential, which is believed to be related to the rate-determining step and electron transfer reaction correspondent with the mechanism of the reaction and very helpful for understanding the essential behavior between the reactant and the electrocatalyst.54 To be specific, most of electrochemical systems involve a series of consecutive reaction steps, which can be electron transfer steps or



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chemical steps, one of which can be the rate-determining step. A smaller Tafel slope symbolizes that the rate-determining step (RDS) is the reaction occurring later in the whole reaction systems, which is commonly recognized as a good character of electrocatalysts (the relationship of Tafel slope and RDS is related by the transfer coefficient). For example, a Tafel slope of 120 mV dec-1 reveals that the RDS is the first electron transfer reaction, the Tafel slope of 60 mV dec-1 relates to the chemical reaction after one-electron transfer reaction and the Tafel slope of 40 mV dec-1 is ascribed to the second electron transfer step.55 Therefore, to testify such electrochemical property, the Tafel slope of various catalysts and Ni foam were determined, which is vividly shown in the Figure 5b that the Tafel slope of iron SDO catalyst appears to be quite small as 41 mV dec-1 and the smallest Tafel slope among all the samples examined for contrast in our work, of which the nickel SDO exhibits a Tafel slope of 86 mV dec-1, manganese SDO possesses a Tafel slope of 117 mV dec-1, and the Tafel slope of Ni foam appears to be 131 mV dec-1 . Thus, it is again corroborated that the as-obtained iron SDO possess superior catalytic properties, as the rate determining step is the third step of the reaction, which is the second electron transfer step. Additionally, the charge transport kinetics of iron SDO is as well more active than that of other selenides and the scaffold, Ni foam, which is well corroborated by the electrochemical impedance spectroscopy (EIS). Figure 5c exhibits the result of such measurement that the Fe-SDO displays superior charge transport capability as its charge transfer resistance, which is ascribed to the diameter of the semi-circle in the frequency range,56 is smaller than that of the other SDOs and Ni foam. Therefore, the EIS spectroscopy also confirmed our speculation that the formation of vacancy defects and lattice distortion actually make contribution to facilitating electron conducting.


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Electrochemical surface area (ECSA) was then performed to normalize the current in order to reflect the intrinsic catalytic nature by detecting the active sites on the catalyst. To evaluate the ECSA, which is associated with the electrochemical double layer capacitance (Cdl), the Cdls of nickel SDO and iron SDO were confirmed and shown in Figure 5d (the LSV curves collected to confirm the Cdl is included in Figure S6), from which it can be observed that the as-proposed iron SDO possesses a relatively larger Cdl value (7 mF cm-2) than the nickel SDO (2.6 mF cm-2) does, thus revealing that the number of the active sites of iron SDO are relatively larger than that of nickel SDO, further demonstrating the superior electrochemical properties of the iron SDO catalyst. Therefore, such superior nature of the as-attained iron SDO catalyst can be well elucidated for that the as-acquired iron SDO catalyst enjoys a quite remarkable ability in charge transport correspondent with its large numbers of active sites demonstrated by ECSA, which accordingly boosts its catalytic capability in OER, and generates the superb overpotential of iron SDO stated above. While the certain origin inducing such splendid charge transport ability of iron SDO catalyst remains undiscovered. The scan rate dependence of the electrocatalyst was as well examined to recognize whether the mass transport is rapid enough for OER acitivity. As the scan rate altered from 1 to 5 mV s-1 (Figure S5), the LSV curves of iron SDO exhibits no evident change, indicating that the mass transport was adequately rapid. Turnover frequency was meanwhile calculated as 495 s-1 by current density at 707 mV (Figure 6a) and the equation for calculation is within the supporting information.57 Faradaic efficiency is to determine the efficiency of the electrocatalyst in transferring the electrons generated by the external circuit and an OER catalyst with strong redox



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peaks within the potential window will possibly lose such efficiency,

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40

therefore RRDE was

conducted to identify the Faradaic efficiency, showing that the as-prepared catalyst has an efficiency of ~95.6% according to Figure 6b. To be a promising electrocatalyst for water-splitting reaction, the stability of the catalyst after its catalysis for OER is as well a significant parameter. Thus, the durability of the iron SDO catalyst was investigated with chronopotentiometry measurement at a current density of 10 mA cm-2 in 1M KOH for 36 hours constantly, and according to Figure 6c the overpotential of as-prepared iron diselenide stayed firmly invariable, exhibiting no obvious vibration within the 36-hour vigorous oxygen evolution reaction, and also, from what is shown in Figure 6d, the polarization curve after the stability test stayed in identical tune with the formal curve of iron SDO before the stability test, guaranteeing the durability of the as-prepared iron SDO catalyst.

4

CONCLUSION In this work, we managed to fabricate FeSe2 on Ni foam by the facile selenizing route based

on hydrothermal method and obtain the iron SDO for oxygen evolution reaction through electrochemical activation. Remarkably, such “Cuju”-like Fe-SDO catalyst possesses a splendid OER activity with a relatively low overpotential of 226 mV at a current density of 10 mA cm-2, small Tafel slope of 41 mV decade-1, and superb durability in alkaline medium at pH 14. To the best of our knowledge, the performance of such catalyst composed with the most abundant transition metal, iron, is comparably as good as or even superior than most of the non-noble metal catalysts. Such excellent electrocatalytic performance of iron-based SDO is ascribed to the high conductivity resulted from the occurrence of the vacancy due to the hydroxylation of FeSe2 and the valence change of Fe (Fe2+→Fe3+) during the OER, which is validated by several


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electrochemical analysis conducted before and after the OER. We suppose that our work would push boundaries for further development and application of iron-based and selenium-derived catalysts, to search for catalysts of these sorts with higher electrocatalytic performance.

ASSOCIATED CONTENT *Supporting Information Additional SEM images of FeSe2 precursor and Fe-SDO (Figure S1), TEM images of FeSe2 precursor and Fe-SDO (Figure S2), LSV polarization curves of Fe-SDO, NiSe2, MnSe, and Ni foam scanning from low voltage to high voltage and Cyclic voltammograms (CV) polarization curves of Fe-SDO, Ni-SDO, Mn-SDO, and Ni foam.(Figure S4), Linear sweep voltammetric curves of Fe-SDO at increasing scan rates from 1 to 5 mV s-1 in 1 M KOH solution. (Figure S5), ECSA curves with different scan rates Fe-SDO and Ni-SDO. (Figure S6), Standardized three-electrode electrochemically examining system for stability test (Figure S7), The CV curves after 5, 20 and 60 cycles (Figure S8), XRD patterns of NiSe2 and MnSe (Figure S9), SEM images of NiSe2, Ni-SDO, MnSe and Mn-SDO (Figure S10), OER activity of the transition-metal based catalysts (Table S1).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.



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ACKNOWLEDGEMENTS We thank the financial supporting from the National Natural Science Foundation of China (Grants 51502099 and 51571096), Natural Science Foundation of Hubei Province (No. 2016CFB129), and “the Fundamental Research Funds for the Central Universities”, HUST: 2016YXMS211. C.D.W. acknowledges the Hubei “Chu-Tian Young Scholar” program. Y.Z. thanks the Scientific Research Foundation of Wuhan Institute of Technology (No. K201761). The authors appreciate the technical support from the Analytical and Testing Center of Huazhong University of Science and Technology.

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Figures

Figure 1. Schematic illustration of the synthesis process of FeSe2 Cuju-balls on Ni foam.


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Figure 2. (a-b) low-magnification SEM images of FeSe2 grown on the surface of nickel foam. (c) low-magnification TEM images. (d) high-magnification SEM images of FeSe2. (e) high-magnification TEM images of FeSe2. (f) XRD pattern of the FeSe2. (g) crystal atom model of the FeSe2. (h) EDS mapping of FeSe2.



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Figure 3. (a-b) low-magnification SEM images of Fe-SDO grown on the surface of nickel foam after the stability test. (c) low-magnification TEM images after the stability test. (d) high-magnification SEM images of Fe-SDO after the stability test. (e) high-magnification TEM images of Fe-SDO after the stability test. (f) contradictory EDS pattern of the FeSe2 before and after the activation. (g) EDS mapping of Fe-SDO after the stability test.


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Figure 4. (a) Fe 2p (b) Se 3d (c) O 1s core-level XPS spectra of FeSe2 before chronopotentiometry measurement. (d) Fe 2p (e) Se 3d (f) O 1s core-level XPS spectra of Fe-SDO after chronopotentiometry examination.



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Figure 5. OER catalytic performances of Fe-SDO. (a) LSV polarization curves of Fe-SDO, Mn-SDO, Ni-SDO samples and the Ni foam scaffold, and (b) the corresponding Tafel slope; (c) EIS of Fe-SDO, Mn-SDO, Ni-SDO samples and the Ni foam scaffold. (d) Cdl of Fe-SDO and Ni-SDO samples that derived from current density differences vs scan rate. The inset of (c) is the measured EIS in high frequency region.


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Figure 6. (a) curves indicating turnover frequency of Fe-SDO catalyst; (b) Faradaic efficiency measurement: curves collected by rotating ring-disk electrode (RRDE); (c) durability test of the catalyst; (d) LSV curves of as-prepared catalyst before and after the chronopotentiometry measurement.



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Table of Content Graphic

The “Cuju”-structured Fe-SDO with relatively good OER electrocatalytic performance provides a prosperous future for economical Fe-based OER electrocatalyst.

TOC


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âCujuâ-Structured Iron Diselenide-Derived Oxide: A Highly Efficient

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