Composition-dependent electrocatalytic activities of NiFe-based selenides for the oxygen evolution reaction

Electrochimica Acta 291 (2018) 64e72

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Composition-dependent electrocatalytic activities of NiFe-based selenides for the oxygen evolution reaction Cuijuan Xuan a, 1, Kedong Xia a, 1, Wen Lei a, Weiwei Xia b, c, Weiping Xiao a, Lingxuan Chen a, Huolin L. Xin b, Deli Wang a, * a

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, China Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA c SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of the Ministry of Education, Southeast University, Nanjing 210096, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 April 2018 Received in revised form 25 July 2018 Accepted 15 August 2018 Available online 21 September 2018

Exploring low-cost and high-efficient electrocatalysts for oxygen evolution reaction (OER) is still an ongoing challenge to substitute noble metal materials. In this work, NiFe-based selenide (Ni-Fe-Se) is synthesized by hydrothermal treatment of metal-organic frameworks (MOFs) and Se powder. The effects of hydrothermal temperature and mass ratio of precursors on the decomposition and selenization of MOFs were investigated. Compared with NiFe based oxide, Ni-Fe-Se is favorable for improving the electronic conductivity and reduce the charge transfer resistance, promoting the formation of the higher oxidative valence of Ni species during OER process. Benefiting from the desirable advantages with unique composition, the Ni-Fe-Se material prepared under optimal conditions shows excellent electrocatalytic activities toward OER with the lower overpotential of 216 mV at a current density of 10 mA cm
2. Besides, the resulting Ni-Fe-Se catalyst exhibits excellent durability when used as air cathode in Zn-air battery. © 2018 Elsevier Ltd. All rights reserved.

Keywords: NiFe-Based selenides Metal-organic frameworks Oxygen evolution reaction Zn-air batteries

1. Introduction Developing sustainable and environment-friendly energy conversion and storage technologies is attracting a great deal of attention with the increasingly prominent environmental and energy issues [1]. Oxygen evolution reaction (OER) serves as the vital part particularly in the water-splitting systems and rechargeable Zn-air batteries [2,3]. The critical issue of these sustainable technologies is to explore high-efficient and stable electrocatalysts to address the sluggish kinetics of OER [4]. Presently, the benchmarking Ir- and Ru-based materials have been regarded as the most active catalysts for OER [5]. However, the large-scale application of these noble metal is not economically viable considering the cost and scarcity of raw materials [6,7]. Thus, it is appealing to develop high-efficient and cost-effective non-precious electrocatalysts for OER based on the earth-abundant elements to substitute the noble metals. Toward this end, numerous electrocatalysts have been explored

* Corresponding author. E-mail address: [email protected] (D. Wang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.electacta.2018.08.106 0013-4686/© 2018 Elsevier Ltd. All rights reserved.

including transition metal materials (such as chalcogenides [8e10], phosphides [11,12], borides [13e15], metal oxides/hydroxides [16e19]) and metal-free catalysts [20,21]. Particularly, nickel and iron based oxides/hydroxides have been reported as efficient electrocatalysts for OER in alkaline electrolyte and attracted extensive research interest [22,23]. Many reports have demonstrated that the presence of Fe can modify the electronic structure and redox potential of nickel based materials, favorable for improving the electrocatalytic activities toward OER [23,24]. However, the poor electronic conductivity of nickel and iron based oxides/hydroxides greatly restricts the further enhancement of OER activities [25]. Developing Ni and Fe based chalcogenides [26] and phosphides [27] has been recently investigated as a mean to enhance their poor conductivity and improve the OER performance [25,28]. Transition metal based selenides have attracted widespread concern as the promising candidate for OER owing to their higher electronic conductivity than the corresponding metal oxide counterparts and the fascinating electronic structure and physical properties [6,8,29e33]. Typically, the rich valance states of metal selenides would lead to different phases, possibly with unique electron structures, thus favorable for the synthesis of highefficient OER catalysts. Although some progress has been made,

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the studies of Ni-Fe based selenides as OER electrocatalysts is still in its infancy and the OER activities need to be further improved, thus, it remains appealing and will be an ongoing challenge to explore high-efficient Ni-Fe based selenides for OER based on earthabundant elements and facile synthesis strategies. Most recently, metal-organic frameworks (MOFs) have gained widespread attention in the application of energy conversion and storage devices due to their distinctively well-defined architecture and facile functionalization advantages [34e37]. Particularly, homogeneous multimetallic MOFs could be easily synthesized through utilizing various metal ions and function as the promising precursor for the exploration of multimetallic catalysts. In this work, nickel and iron Prussian blue analogue (NiFe-PBA) was employed as MOF precursors to synthesize Ni and Fe based bimetallic selenide (Ni-Fe-Se) via hydrothermal method. Benefiting from the selenization and unique composition with NiSe2 and NiFe2Se4 phases, the as-prepared Ni-Fe-Se1:1-180 exhibits better electrocatalytic activities with lower overpotentials at 10 mA cm
2 and 20 mA cm
2 than Ni-Fe based oxides and outstanding long-term stability for Zn-air battery. 2. Experimental methods 2.1. Synthesis of Ni-Fe-Se NiFe-PBA was prepared by co-precipitation of nickel chloride and potassium ferricyanide based on the literature [38] with some modification. Typically, 0.6 mmol of nickel chloride hexahydrate and 0.9 mmol of tri-sodium citrate dihydrate were dissolved in 20 mL of deionized water under stirring to form solution A; 0.4 mmol of potassium ferricyanide was dissolved in 20 mL of deionized water to form solution B. Then solution A and solution B were mixed together under continuous stirring for several minutes, and the resulting mixed solution was aged at room temperature for 10 h. After that, centrifugation was utilized to collect the precipitates followed by washing with water and ethanol for several times and drying at 60  C overnight to obtain NiFe-PBA. The synthesized NiFe-PBA was then utilized as precursor to prepare NiFebased selenides by hydrothermal strategy. In a specific procedure, 100 mg of NiFe-PBA was added to the mixed solution of water (6 mL) and hydrazine hydrate (4 mL) with sonication to form homogeneous dispersion. 100 mg of Se powder was then added to the above dispersion under sonication condition. After that, the obtained solution was poured into Teflon-lined stainless steel autoclave with a capacity of 50 mL and heated at 180  C for 5 h in an air oven. The precipitate was collected through centrifugation after the oven cooling naturally to room temperature. And then the precipitate was washed with water and ethanol and dried at 60  C under vacuum for overnight. The obtained sample was placed in tube furnace and heated up to 300  C and maintained for 1 h in N2 atmosphere. The final product was denoted as Ni-Fe-Sex-T, where x represents the mass ratio of NiFe-PBA and Se powder, and T represents the hydrothermal temperature. For comparison, Ni-Fe-O was prepared by the same method without adding Se powder. 2.2. Material characterization Field emission scanning electron microscopy (FESEM) images were acquired by using Sirion200. Transmission electron microscopy (TEM), dark field scanning transmission electron microscopy (DF-STEM), and elemental mapping were all conducted via a Gatan Tridiem spectrometer. The crystalline structure of the catalysts was characterized by performing Powder X-ray diffraction (XRD) on an X'Pert PRO diffractometer at a scan rate of 10 min
1. X-ray photoelectron spectroscopy (XPS) data were collected using an

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AXIS-ULTRA DLD-600W Instrument. X-ray Fluorescence (XRF) determination was performed on an X-ray fluorescence spectrometer (EAGLE III). Fourier transform infrared spectroscopy (FTIR) was recorded via a FTIR spectrometer (VERTEX 70, BRUKER Inc.). Brunauer-Emmett-Teller (BET) surface area was measured by using an ASAP2420-4MP analyzer. Thermogravimetric (TG) measurement was conducted in air from room temperature to 800  C at a heating rate of 10  C min
1 using a TA Q500 apparatus. 2.3. Electrochemical measurement An Autolab PG302N electrochemical workstation with highspeed rotators from Pine Instrument was utilized to perform all the electrochemical measurements. The three-electrode was established in which a modified glassy carbon electrode (GCE) with a diameter of 5 mm, the reversible hydrogen electrode (RHE) and a carbon rod serve as working electrode, reference electrode as well as counter electrode, respectively. The equation (h ¼ E(RHE)-1.23) was used to calculate the overpotential (h) [39e41]. The working electrode loaded with catalyst was prepared according to the following procedure. 5 mg of catalyst and 2 mg of Vulcan X-72 were mixed with 1 mL of Nafion/isopropyl alcohol solution and then sonicated to form the homogeneous ink dispersion; 16.5 mL of the obtained ink was dropped onto GCE and dried naturally at room temperature as the working electrode. The mass loading amount of all the catalysts was 0.42 mg cm
2. The electrocatalytic performance of catalysts for OER was conducted in O2-saturated 1.0 M KOH electrolyte. Linear sweep voltammetry (LSV) curves were collected at the potential from 0.8 to 1.8 V at the scan rate of 5 mV s
1. Chronopotentiometry test was used to evaluate the stability of catalysts at a current density of 10 mA cm
2 for 10 h with a rotation speed of 1600 rpm. Cyclic voltammetry (CV) measurement was carried out at potential ranging from 1.4 V to 1.8 V for 3000 cycles at the scan rate of 100 mV s
1 to assess the durability. Electrochemical impedance spectroscopy (EIS) detection was performed at 1.5 V in the frequency range from 100 kHz to 0.01 Hz with an ac perturbation of 5 mV. A home-made electrochemical cell was used to assess the catalytic performance of the catalysts for rechargeable Zn-air battery. In the established two-electrode system, 6 M KOH media containing 0.2 M of Zn(CH3COO)2 was employed as the electrolyte, the catalyst loaded on the gas diffusion layer as air cathode, and the pre-polished Zn foil as anode. For preparation of gas diffusion layer, 400 mg of Vulcan XC-72, 100 mg of PTFE and appropriate amount of isopropanol were ground fully in an agate mortar to form homogeneous slurry which was then pressed via a twin roller to obtain a composite sheet. The obtained sheet was dried under vacuum for overnight, and then cut into suitable size as the gas-diffusion electrode. The catalysts were mixed with Nafion/isopropyl alcohol solution to get the ink dispersion based on the above procedure, and then loaded onto the prepared gas-diffusion layer. After drying naturally at room temperature, catalyst loaded gas-diffusion layer was used directly as air cathode in Zn-air battery. A LAND-CT2001A testing device was utilized to test the discharge/charge performance. Discharging/charging cycles was realized at the current density of 10 mA cm
2 to examine the stability. 3. Results and discussion Ni-Fe-Se materials were synthesized via a facile hydrothermal approach with NiFe-PBA as the precursor. During the synthesis process, NiFe-PBA was first obtained by coprecipitating nickel chloride and potassium ferricyanide. FESEM and TEM images in Fig. 1 display that the resulting NiFe-PBA exhibited nanocube morphology with an average size ranging from 100 to 150 nm. XRD

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Fig. 1. SEM (a, b) and TEM (c, d) images of NiFe-PBA.

pattern in Fig. 2a depicts that the obtained NiFe-PBA shows the characteristic diffractions of Ni3[Fe(CN)6]2$10H2O with face-centercubic structure (JCPDS card 46-0906) and no other peaks were observed, confirming the high purity of the as-prepared NiFe-PBA. FT-IR spectrum was then performed to further illustrate the chemical structure of NiFe-PBA. As depicted in Fig. S1, NiFe-PBA exhibits typical peaks at ~3408 cm
1 and ~1611 cm
1 assigned to the crystal water, and peaks at ~2158 cm
1 and ~2113 cm
1 corresponding to the CN
group [42,43]. The resulting NiFe-PBA was then converted to Ni-Fe-Se materials by selenization treatment. During the hydrothermal process, hydrazine hydrate served as reducing agent and provided a basic

environment. Se powder was reduced to Se2
, and reacted with Ni and Fe ions dissolved from NiFe-PBA in basic solution to form Ni and Fe based selenides. The composition of Ni-Fe-Se1:1-180 materials prepared at the hydrothermal temperature of 180  C with the mass ratio of NiFe-PBA and Se powder of 1:1 is shown in Fig. 2b. NiFe-Se1:1-180 exhibits characteristic diffraction peaks of NiSe2 (JPCDS Card No. 089-7161) and NiFe2Se4 (JPCDS Card No. 0652338), in which the NiSe2 occupies the major part of the NiFe selenide with a small fraction of NiFe2Se4. For comparison, Ni-Fe-O materials were prepared under the same synthesis method without adding Se powder to illustration the effect of selenization on the composition. It can be seen from Fig. 2b that Ni-Fe-O exhibit typical

Fig. 2. (a) XRD pattern of NiFe-PBA. (b) XRD patterns of Ni-Fe-Se1:1-180 and Ni-Fe-O.

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diffraction peaks indexed to NiFe2O4 (JCPDS card 74-2081), suggesting the easy transformation of NiFe-PBA to metal oxides in the absence of Se powder during the synthesis process. XRD and FT-IR characterizations were then performed to study the effects of hydrothermal temperature and the mass ratio of NiFePBA and Se powder on the decomposition and selenization of NiFePBA. Fig. S2a depicts the XRD patterns of the as-prepared NiFe selenides under different hydrothermal temperature. Clearly, NiFe based selenides exhibit similar XRD patterns with characteristic diffraction peaks of NiSe2 and NiFe2Se4. Besides, under the lower temperature of 140  C and 160  C, some weak peaks indexed to Ni3[Fe(CN)6]$10H2O were observed, implying the incomplete decomposition of NiFe-PBA precursor under relatively low temperature, while no additional peaks of impurities were detected on the Ni-Fe-Se1:1-180 and Ni-Fe-Se1:1-200, indicating the high purity of the products can be obtained at 180  C and 200  C. Additionally, with the temperature increasing from 140  C to 200  C, the intensities of diffraction peaks assigned to NiFe2Se4 became gradually pronounced, revealing the easy formation of NiFe2Se4 crystal structure as the temperature rises. FT-IR was performed to further examine the chemical structure of the materials prepared at different hydrothermal temperature. Fig. S3a displays that after selenization treatment of MOFs, the obtained Ni-Fe-Se1:1-140 and Ni-Fe-Se1:1-160 still show some weak peaks corresponding to residual CN
group, implying the incomplete decomposition of NiFePBA under relatively low hydrothermal temperatures of 140  C and 160  C. In contrast, the typical peaks for NiFe-PBA were not observed in the spectra of Ni-Fe-Se1:1-180 and Ni-Fe-Se1:1-200, further demonstrating the easy decomposition and selenization of NiFe-PBA when the hydrothermal temperature is relatively high. Fig. S2b displays XRD patterns of NiFe selenides prepared by adding different amount of Se powder during the hydrothermal treatment. It can be seen that NiFe selenides obtained at different mass ratio of NiFe-PBA and Se powder exhibit different XRD diffraction peaks. When the mass ratio of NiFe-PBA to Se powder is 3:1, the product exhibits the diffraction peaks of NiSe (JPCDS Card No. 075-0610) with some peaks ascribed to NiFe-PBA, demonstrating the incomplete transformation of NiFe-PBA precursor to selenide when adding the small amount of Se powder. When the mass ratio of NiFe-PBA to Se powder increases to 1:1, the diffraction peaks of the obtained Ni-Fe-Se can be indexed to NiSe2 and NiFe2Se4 without additional peaks of impurities observed, and NiSe2 and FeSe2 (JPCDS Card No. 065-1455) phases are formed with further rising the mass ratio to 1:3, suggesting the gradual generation of selenium-enriched structure through increasing the proportion of Se powder. FT-IR spectra in Fig. S3b show that compared with NiFe-Se1:1-180 and Ni-Fe-Se1:3-180, Ni-Fe-Se3:1-180 exhibits some weak peaks attributed to residual CN
group, further revealing the incomplete decomposition and selenization of NiFe-PBA precursors. Typical for Ni-Fe-Se1:1-180, XRF patterns in Fig. S4 illustrate that the elemental ratio of Ni, Fe and Se was determined to be 31.51:20.91:73.79 and the ratio of Ni and Fe is consistent with that of NiFe-PBA precursor. Moreover, the large size of Se2
anion and the high electron density of Ni-Fe-Se1:1-180 with Se-enriched compositions would lead to suitable band gap and cumulative the 3d-2p repulsion between the metal d-band center and Se d-band center, thus facilitating the oxygen evolution during the OER process [33,44]. The morphology of the obtained Ni-Fe-Se1:1-180 material was then examined by SEM and TEM measurements. As presented in Fig. 3aec, the cube structure of MOFs precursors is not observed for Ni-Fe-Se1:1-180, which may be due to the destruction and collapse of the cube structure resulting from the decomposition of NiFe-PBA during selenization process. SEM images in Fig. S5 demonstrate Ni, Fe, and Se elements are uniformly distributed

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across the whole structure. Furthermore, the elemental mapping images in Fig. 3def also confirm the homogeneous distribution of Ni, Fe and Se on Ni-Fe-Se architecture. The HRTEM images shown in Fig. 3h and i reveal the inter-planar spacing of 0.30 nm and 0.20 nm, corresponding to the (200) and (221) planes of cubic NiSe2, while the inter-planar distance of 0.34 nm and 0.23 nm can be assigned to the (011) and (211) plane of NiFe2Se4, consistent with XRD patterns. The chemical composition and elemental valence state of Ni-FeSe1:1-180 were analyzed through XPS. The presence of Ni, Fe and Se is clearly observed from the XPS survey spectrum of Ni-Fe-Se1:1180 in Fig. 4a, consistent with the elemental mapping results of SEM and TEM. As displayed in Fig. 4b, the high-resolution Ni 2p spectrum is deconvoluted into six peaks, in which peaks at 854.5 eV and 872.3 eV are ascribed to Ni2þ in selenides, and peaks at 856.8 eV and 874.8 eV are attributed to oxidized Ni species due to the surface oxidation [45,46]. In the high-resolution Fe 2p spectrum (Fig. 4c), the peaks at 708 eV and 720 eV can be assigned to Fe2þ, while the peaks located at 712.2 eV and 725.5 eV verify the presence of Fe3þ. And the fitting peak area corresponding to Fe3þ is higher than that of Fe2þ, implying the iron mainly exist in the form of Fe3þ [47]. The high-resolution Se 3d spectrum in Fig. 4d shows two peaks at 54.9 eV and 55.7 eV corresponding to 3d2/5 and 3d2/3 for metal-selenium bonds. The other peaks at higher binding energy are ascribed to Se-O bonding due to the surface oxidation of Se species in air [45,48]. The above characterizations and analysis demonstrate that the prepared Ni-Fe-Se1:1-180 exhibit mixed phases of NiSe2 and NiFe2Se4 with Se-enriched compositions and uniform distribution of Ni, Fe and Se elements. Previous studies have suggested that the transition metal chalcogenides with different phases and the synergism between Ni and Fe possibly exhibited unique electron structures, favorable for OER performance [23,32]. And the nanojunction of different selenides phases contributes to easier electronic transfer and offer more active sites because of their unique valence electronic structure and unsaturated Se atoms [49,50], which would be beneficial to the improvement of electrocatalytic activities. Besides, metal selenides usually show higher electrical conductivity than their corresponding metal oxide counterparts because of the smaller electronegativity of elemental selenium (2.55) than that of elemental oxygen (3.44) [29]. Moreover, the negative charge around Se sites, as well as the cumulative 3d-2p repulsion between the metal dband center and Se d-band center may be favorable for promoting the faster delivery of the dioxygen molecule during OER process [33]. Thus, it is reasonable to expect excellent OER catalytic activities on Ni-Fe-Se1:1-180 catalysts. The electrocatalytic performances of the as-synthesized materials for OER were examined in 1 M KOH electrolyte by utilizing a three-electrode setup. The obtained electrochemical data have been corrected by IR drop compensation. Cyclic voltammogram (CV) determination was performed to demonstrate the effect of selenization on OER performance. As presented in Fig. 5a, Ni-FeSe1:1-180 and Ni-Fe-O show obvious redox peak, suggesting the oxidation and reduction of Ni (II). The generation of NiOOH species due to the anodic oxidation has been reported to act as active sites for OER [23,32]. Besides, the current density of redox peak for NiFe-Se1:1-180 is higher than that for Ni-Fe-O, implying the easier oxidation of metal selenides at the higher potential than the Ni(II) oxidation wave, which is favorable for facilitating the electron transport during OER process and also illustrates the important role of selenization. Linear sweep voltammetry (LSV) curves were then collected to assess the electrocatalytic activities of varied samples loaded on the glassy carbon electrodes. Fig. 5b and c shows that NiFe-Se1:1-180 exhibits excellent OER electrocatalytic activities with lowest overpotential of 216 mV and 244 mV for delivering current densities of 10 mA cm
2 (h10) and 20 mA cm
2 (h20), respectively,

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Fig. 3. SEM (a, b) and DF-STEM images (c) of Ni-Fe-Se1:1-180, and the corresponding elemental mapping of Ni (d), Fe (e), and Se (f). Enlarged TEM image (g) and HRTEM images (h, i) of Ni-Fe-Se1:1-180.

much lower than Ni-Fe-O (259 mV and 273 mV), Ni-Fe-PBA (409 mV and 479 mV), and even superior to Ir/C (20 wt% Ir). To further illustrate the better electrocatalytic activities of Ni-Fe-Se1:1180 than Ir/C, the total mass percentage of Ni, Fe and Se elements in Ni-Fe-Se1:1-180 was calculated to be 89.8 wt% according to the weight residues in TG curve (Fig. S6) and the XRF result. The mass current density of two materials at the potential of 1.45 V and 1.50 V was calculated to be 35 and 266 mA mg
1 for Ni-Fe-Se1:1-180 and 7 and 78 mA mg
1 for Ir/C, respectively, also revealing the superior OER activities of Ni-Fe-Se1:1-180 to Ir/C. Furthermore, the electrocatalytic activities of Ni-Fe-Se1:1-180 are comparable to or even better than most of selenide-based catalysts and Ir or Ru-based materials reported in the literature (Tables S1 and S2), further illustrating the excellent activities of Ni-Fe-Se1:1-180 for OER. In order to illustrate the charger transfer ability for OER, Tafel slope was then acquired based on the Tafel equation (h ¼ blogj þ a, where j and b are the current density and the Tafel slope, respectively) [51]. Clearly, Ni-Fe-Se1:1-180 exhibits smallest Tafel slope, implying the faster charger transfer rate for OER. Electrochemical impedance spectroscopy (EIS) was performed to further manifest charger transfer rate during OER process. The radius of semicircles in Fig. S7 is usually utilized to illustrate the charge transfer resistance and the smaller radius reflects the lower charge transfer resistance. Obviously, Ni-Fe-Se1:1-180 possesses much lower charge transfer resistance compared with Ni-Fe-O, suggesting the accelerated charger transfer rate during OER process. Nitrogen adsorptiondesorption isotherms in Fig. S8 show the existence of porous structure in Ni-Fe-Se1:1-180, Ni-Fe-O and Ir/C. Besides, the BET surface area of Ni-Fe-Se1:1-180, Ni-Fe-O, and Ir/C was determined to

be 4.6 m2 g
1, 6.7 m2 g
1, and 149.2 m2 g
1, respectively, suggesting that the better electrocatalytic activities of Ni-Fe-Se1:1-180 than NiFe-O and Ir/C are mainly attributed to the unique composition and the selenization plays an important role in enhancing the catalytic activities and accelerating charge transfer rate for OER. The effect of hydrothermal temperature on OER electrocatalytic activities was then investigated as displayed in Fig. S9. It can be seen that the Ni-Fe-Se catalysts prepared at different hydrothermal temperature all enable excellent oxygen evolution during OER process. The Ni-Fe-Se1:1-180 material achieves optimal OER catalytic activities with lower h10 and h20 of 216 mV and 244 mV at 10 mA cm
2 and 20 mA cm
2, respectively. When the reaction temperature is lower to 140  C or 160  C, higher overpotentials are needed on Ni-Fe-Se1:1-140 and Ni-Fe-Se1:1-160 catalysts to obtain the same densities mainly due to the incomplete conversion of NiFe-PBA precursor. When rising the temperature to 200  C, the obtained Ni-Fe-Se1:1-200 exhibits remarkable catalytic activities with low overpotentials of 225 mV and 246 mV at 10 mA cm
2 and 20 mA cm
2 due to the similar phase composition between Ni-FeSe1:1-180 and Ni-Fe-Se1:1-200. XRD patterns present that different mass ratio of NiFe-PBA to Se powder can lead to different crystalline structure of products which may influence the electrocatalytic activities. LSV measurements were then conducted to examine the OER catalytic activities of Ni-Fe-Se synthesized under different mass ratio of NiFe-PBA and Se powder. Fig. S10 displays that Ni-FeSe1:1-180 sample exhibits excellent electrocatalytic activities with lower overpotentials at 10 mA cm
2 and 20 mA cm
2 than Ni-FeSe3:1-180 and Ni-Fe-Se1:3-180, which may be due to the complete decomposition and selenization of MOFs precursor and the

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Fig. 4. (a) XPS survey spectrum of Ni-Fe-Se1:1-180. (b) High-resolution spectra of Ni 2p (b), Fe 2p (c), and Se 3d (d) for Ni-Fe-Se1:1-180, respectively.

resulting unique composition when the mass ratio is 1:1. According to the above electrochemical determination, Ni-Fe-Se1:1-180 material exhibits the optimal OER electrocatalytic activities under hydrothermal temperature of 180  C and the mass ratio of 1:1 benefiting from the unique compositions. The electrocatalytic activities and the stability of catalysts are both vital parameters to evaluate the OER performance. Thus, the stability of Ni-Fe-Se1:1-180 was examined via chronopotentiometry and CV determination. Fig. 5e displays that Ni-Fe-Se1:1-180 delivers outstanding stability for 12 h at current densities of 10 mA cm
2 and 20 mA cm
2, respectively, with slight variation compared with the initial potential. Besides, LSV curves were collected before and after 3000 CV cycles in the potential ranging from 1.4 V to 1.8 V. Fig. 5f shows the tiny changes with negligible activity loss, further suggesting good stability of Ni-Fe-Se1:1-180 for OER. CV curves in Fig. 5a reveal the surface oxidation of catalysts during OER process, which may lead to the generation of an oxide or hydroxide layer covered on the bulk phase. In order to further illustrate the importance of selenides for OER and the stability of material composition after CV cycling test, XRD and XPS characterizations were performed to obtain the crystalline structure and surface information of the catalysts after stability test. Fig. S11a depicts the XRD patterns of Ni-Fe-Se1:1-180 catalyst loaded on carbon paper after the durability test. It can be seen that XRD patterns before and after stability test show almost unchanged, suggesting that no obvious bulk phase change and further demonstrating the good

stability of the catalyst. Fig. S11bed shows the XPS results of the fresh catalyst and corresponding post-OER catalyst. Obviously, after OER stability test, the catalyst exhibits the near disappearance of the peak at around 55 eV in the high-resolution Se 3d XPS spectrum, as well as the obvious intensity decrease at both ~854.5 eV in the high-resolution Ni 2p XPS spectrum and ~708 eV in the highresolution Fe 2p XPS spectrum, further implying surface oxidation of Ni-Fe-Se1:1-180 after OER test. The high resolution XPS spectra of Ni, Fe, and Se elements were deconvoluted to better illustrate the change of elemental valence state after stability test. As displayed in Fig. S12, the high resolution Se 3d spectrum is fitted to one peak at 59.1 eV, corresponding to Se-O bonding [45]. Four peaks are observed in the Ni 2p spectrum, in which peaks at 857.1 eV and 874.9 eV are assigned to Ni (III) species in nickel (oxy) hydroxide with two satellite peaks [52,53]. For the high resolution Fe 2p spectrum, peaks at 713 eV and 725.5 eV correspond to Fe (III) species in iron (oxy)hydroxide, with the satellite peak at 718.5 eV [52,53]. The change of elemental valence state after OER stability test further manifests that Ni-Fe-Se1:1-180 underwent surface oxidation during OER process. The combination of XRD and XPS results suggests the formation of metal selenide/(oxy)hydroxide core-shell structure during OER process. The in-situ generated NiFe oxy/hydroxide on the surface of the catalyst has been reported as the active sites for OER [54,55], while metal selenides can improve the electronic conductivity, promoting the electronic transfer between the surface oxy/hydroxide and the adsorbed

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Fig. 5. (a) CV Curves of Ni-Fe-Se1:1-180 and Ni-Fe-O at the scan rate of 50 mV s
1. LSV curves (b) of Ni-Fe-Se1:1-180, Ni-Fe-O, NiFe-PBA, and Ir/C at the scan rate of 5 mV s
1, and (c) the corresponding overpotential for acquiring the current densities of 10 mA cm
2 and 20 mA cm
2. (d) Tafel slopes of Ni-Fe-Se1:1-180, Ni-Fe-O, NiFe-PBA, and Ir/C. (e) Chronopotentiometry curves of Ni-Fe-Se1:1-180 at a constant current density of 10 mA cm
2 and 20 mA cm
2 for 12 h. (f) OER polarization curves before and after 3000 CV cycles at potential ranging from 1.4 V to 1.8 V with the scan rate of 100 mV s
1.

inter-mediate confirmed by Tafel slope and EIS data in Fig. 5d and Fig. S5. Thus, benefiting from the no obvious change of bulk phase and in-situ formed oxy/hydroxide after stability, the Ni-Fe-Se1:1180 catalyst still exhibit excellent electrocatalytic activity. According to the aforementioned OER performance with excellent electrocatalytic activities and outstanding stability, the potential application of Ni-Fe-Se1:1-180 in Zn-air battery was investigated by using a home-made device. Considering that the rechargeable Zn-air battery involves both oxygen reduction reaction and oxygen evolution reaction, the oxygen electrode was obtained by mixing Ni-Fe-Se1:1-180 and the previously reported N,SCNT with excellent ORR performance [56] and loading the mixture

onto the gas-diffusion layer. Fig. 6b displays the polarization curve and power density of Pt/CþIr/C or Ni-Fe-Se1:1-180 þ N,S-CNT based Zn-air battery. The peak power density of Ni-Fe-Se1:1-180 þ N,SCNT was 148.6 mW cm
2, higher than that of Pt/CþIr/C. Fig. S13 shows the discharge-charge curves of Ni-Fe-Se1:1-180, N,S-CNT, and Ni-Fe-Se1:1-180 þ N,S-CNT based Zn-air batteries at a current density of 10 mA cm
2. Clearly, Ni-Fe-Se1:1-180 þ N,S-CNT exhibits higher discharge potential than Ni-Fe-Se1:1-180, and lower charge potential than N,S-CNT. Besides, the charge and discharge potential difference of Ni-Fe-Se1:1-180 þ N,S-CNT is lower than that of N,SCNT and Ni-Fe-Se1:1-180, revealing that Ni-Fe-Se1:1-180 þ N,SCNT exhibits better performance when used in Zn-air battery and

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Fig. 6. (a) Schematic diagram of zinc-air battery device. (b) Polarization and power density curves of Pt/CþIr/C or Ni-Fe-Se1:1-180 þ N,S-CNT based Zn-air batteries. (C) Discharge/ charge cycling curves of Zn-air batteries using Pt/CþIr/C or Ni-Fe-Se1:1-180 þ N,S-CNT as air electrode catalysts at a current density of 10 mA cm
2.

combining Ni-Fe-Se1:1-180 and N,S-CNT materials can effectively improve the Zn-air battery performance. Long-term stability of samples was then measured through discharge-charge cycling test at a constant current density of 10 mA cm
2. As displayed in Fig. 6c, Pt/CþIr/C underwent a remarkable voltage losses after only 60 cycles, on the contrary, the voltage of Ni-Fe-Se1:1-180 changed slightly even after 300 cycles, demonstrating its outstanding longterm stability and the promising prospect in Zn-air battery.

Innovation Research Funds of Huazhong University of Science and Technology (2017KFYXJJ164). The authors thank the Analytical and Testing Center of HUST for allowing use its facilities. This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

4. Conclusions

Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2018.08.106.

In summary, NiFe-based selenides were synthesized through the facile selenization treatment of NiFe-based MOF precursor. The Ni-Fe-Se1:1-180 material obtained under hydrothermal temperature of 180  C and suitable precursor mass ratio shows mixed phases of NiSe2 and NiFe2Se4. The unique composition and selenization are favorable for improving the electronic conductivity, reducing charge transfer resistance, leading to accelerated the OER kinetics rate and the formation of the higher oxidative valence of Ni species during OER process. Benefiting from the unique composition and selenization, Ni-Fe-Se1:1-180 exhibits excellent electrocatalytic activities for OER with much lower overpotentials at 10 mA cm
2 and 20 mA cm
2 than Ni-Fe based oxides, and also shows excellent durability when employed as the air cathode in Znair battery, possessing a great potential in energy storage and conversion applications. Acknowledgment This work was supported by the National Natural Science Foundation (21573083), 1000 Young Talent (to Deli Wang), and the

Appendix A. Supplementary data

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