Rational Design of Cobalt-Iron Selenides for Highly Efficient

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Rational Design of Cobalt-Iron Selenides for Highly Efficient Electrochemical Water Oxidation Junye Zhang, Lin Lv, Yifan Tian, Zhishan Li, Xiang Ao, Yucheng Lan, Jianjun Jiang, and Chundong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08917 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on August 30, 2017

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ACS Applied Materials & Interfaces

Rational Design of Cobalt-Iron Selenides for Highly Efficient Electrochemical Water Oxidation Jun-Ye Zhang,

†+

†+

Lin Lv,

and Chundong Wang

†

†

†

‡

Yifan Tian, Zhishan Li, Xiang Ao, Yucheng Lan, Jianjun Jiang*,

†

*,†

†

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

Wuhan 430074, P.R. China ‡

Department of Physics and Engineering Physics, Morgan State University, Baltimore, MD

21254, USA

ABSTRACT: Exploring active, stable, earth-abundant, low-cost and high-efficiency electrocatalysts is highly desired for large-scale industrial applications towards the low-carbon economy. In this study, we apply a versatile selenizing technology to synthesize Se-enriched Co1-xFexSe2 catalysts on nickel foams for oxygen evolution reactions (OER), and disclose the relationships between the electronic structures of Co1-xFexSe2 (via regulating the atom ratio of Co/Fe) and the OER performance. Owing to the fact that the electron configuration of the Co1-xFexSe2 compounds can be tuned by the incorporated Fe species (electron transfer and lattice distortion), the catalytic activity can be adjusted according to the Co/Fe ratios in the catalyst. Moreover, the morphology of Co1-xFexSe2 is verified to be also strongly depend on the Co/Fe ratios, and the thinner Co0.4Fe0.6Se2 nanosheets are obtained upon selenizing treatment, in which it allows more active sites to be exposed to the electrolyte, in turns promoting the OER performance. The Co0.4Fe0.6Se2 nanosheets not only exhibit superior OER performance with a low overpotential of 217 mV at 10 mA cm-2 and a small Tafel slope of 41 mV dec-1 but also possess ultrahigh durability with a dinky degeneration of 4.4% even after 72 h fierce water oxidation test in alkaline solution, which outperforms the commercial RuO2 catalyst. As expected, the Co0.4Fe0.6Se2 nanosheets have shown the great prospects for practical applications toward water oxidation.

[+] The authors contributed equally to this work.

ACS Paragon Plus Environment


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KEYWORDS: cobalt-iron selenides, electrocatalyst, oxygen evolution, Se-rich effect, lattice distortion

1. INTRODUCTION Energy crisis is highly concerned world-widely because of the world’s unprecedented increase demand for the limited fossil fuels.1-3 Thus enormous efforts have been devoted to find renewable energies, such as solar and wind energy. In contribution for this, electrochemical water splitting (H2O(l) → H2(g) + 1/2O2(g): ∆G° = +237 200 J/mol, ∆E° = 1.23 V vs reversible hydrogen electrode (RHE)) is another particular promising solution towards energy storage, and recommended being a potential alternative to the conventional fossil fuels (coal, petroleum, natural gas, et al.).4-6 The efficiency of water oxidation reaction (or even CO2 reduction reactions) is mainly determined by an oxygen evolution reaction (OER; 4OH- (aq) → O2(g) + 2H2O (aq) + 4e-).7-8 Owing to O-H bond breaking and attendant O-O bond formation, together with multiple proton-coupled electron transfers being involved in the reaction process, OER is basically kinetically sluggish, hence of which efficient electrocatalysts are highly desired.9-10 Currently, the state-of-the-art OER catalysts are noble-metal-based oxides such as IrO2 and RuO2. However, the scarcity and high cost of Ru and Ir make it impractical for large-scale applications.11-12 In recent years, extensive efforts have been made to seek high efficient, stable, and low-cost alternative electrocatalysts, particularly earth-abundant 3d transition metal oxide based electrocatalysts.13 Among the various VIII group 3d metal (Fe, Co, and Ni) based compounds, such as metal oxides,14 sulfides,15-16 layered double hydroxides (LDH),17-18 selenides,19 phosphides,20 carbides, 21

and nitrides,22-23 have been intensively investigated, exhibiting remarkable OER catalytic

behaviors. Basically, the OER kinetics of group VIII 3d metals are much faster in alkaline media than in acidic media compared with the noble metal based catalyst (IrO2 and RuO2),24 of which the correlated mechanism could be referred to the Krasil’shchkov, Bockris, Yeager,25 Wade and Hackerman26 pathways besides the general electrochemical oxide pathway. Under alkaline conditions, the electrochemical reaction proceeds via the essential steps of hydroxide coordination to the active sites.25 Of note, the sum of kinetic barriers related to the elementary steps is the overall overpotential (η) that OER required, among which the step with the most sluggish kinetics is defined as rate-determining step (RDS).27-28 However, under high oxidative potentials in


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alkaline media, group VIII 3d metals-based compounds generally exhibit limited stability. Thus, we may now interrogate what the true nature of the active species is, and how to effectively improve it. Previously, it has been disclosed that Ni2P and CoP were in situ transformed into oxides or hydroxides partially upon catalyzing,29-31 benefiting the OER process and responsible for the remarkable catalytic activity. For metal selenides, which has a similar chemical reactivity to the counterpart sulfides, ultra-stable OER performance was observed in NiSe and CoSe2.32-33 In these cases, NiSe/CoSe2 were completely transformed into nickel/cobalt hydroxides during OER processes. This observation indicates that metal oxides or hydroxides are the active chemical forms of metal selenides in OER. Moreover, high electrical conductivity is suggested to be another main contributor on the enhancements of OER activity as Zhao et al. demonstrated that engineering Ag-doped CoSe2 could effectively boost oxygen evolutions.32 In view of its intrinsic metallicity of CoSe2 (t2g6eg1 electronic configuration), it promotes us that a rational design of an unknown cobalt iron diselenide (Co1-xFexSe2) acting as templating precursors to highly active metal oxide/hydroxide OER catalysts is a possible scheme. Herein, Co1-xFexSe2 nanoflakes (termed as CFS) were synthesized on nickel foam (NF) by a facile hydrothermal method. By introducing a different amount of Fe into the bimetallic selenide, various morphologies and different OER catalytic activities were exhibited. In particular, when x=0.6 (i.e. Co1-xFexSe2-0.6; CFS-0.6), outstanding catalytic activities and robust stabilities were achieved in alkaline medium (pH 14), that is, at a current density of 10 mA cm-2 with 217 mV overpotential and shows a dinky degradation of 4.4% after 72 h testing. Notably, compared with Ar-plasma treatments of CoFe-LDH,34 the OER performances of CFS nanoflakes impart us that selenides could be a kind of super advanced electrocatalysts because of their merits such as remarkable kinetic properties, and high conductivity nature. Moreover, in the cobalt-iron bimetallic compounds, by a selenization process, the RDS could be varied from the second step to the third step in a four-step OER process, resulting in low Tafel slopes as well as boosting OER kinetics.

2. EXPRIMENTAL SECTION All chemical reagents were directly used without any further purification. 2.1 Synthesis of Co-Fe precursors: CoFe-LDH nanosheets supported on nickel foams were



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fabricated by a hydrothermal method. In details, 1 mmol CoSO4·7H2O, 1.5 mmol FeSO4·7H2O, 5 mmol NH4F, and 5 mmol CO(NH2)2 were dissolved in 60 mL deionized (DI) water under stirring for 5 minutes. The mixture was transferred to a 100 mL Teflon-lined stainless autoclave, and then a piece of treated nickel foam (2cm × 4cm) was added. Then the autoclave was sealed and subjected to an oven at 160 oC for 12 h. After cooling down naturally, the as-prepared product was washed with DI water and ethanol for three times respectively and dried at 60 oC for overnight. The precursor was obtained and was defined as CoFe-precursor. For comparison, the metal sources were put in with other ratios of Co/Fe (5:0, 4:1, 3:2, 1:4, and 0:5, total molar weight: 2.5 mmol) under other conditions unchanged. 2.2 Synthesis of Co-Fe-Se: In a typical procedure, the as-prepared precursors, together with 50 mL dimethyl formamide (DMF), 1 mL hydrazine (85 wt%), and 3.75 mmol Se, was put in a 100 mL Teflon-lined stainless autoclave and maintained at 200 oC for 6 h. After cooling down naturally, the product was washed with DI water and ethanol for three times respectively and dried at 60 oC for overnight. The final obtained product was nominated as Co1-xFexSe2 with different ratios of Co/Fe. 2.3 Material characterizations The phase of the obtained compounds were identified by X-ray powder diffraction (XRD) patterns on Empyrean (PANalytical B.V. with Cu-Kα radiation). The morphology of the synthesized materials were characterized by field-emission scanning electron microscopy (FESEM) performed with a JEOL JSM-7100F. High-resolution transmission electron microscopy (HRTEM), together with corresponding elemental mapping were carried out on a TecnaiG2 20 (Philips) at an accelerating voltage of 200 kV. X-Ray fluorescence (XRF) spectrum was measured on an EAGLE III. The X-ray photoelectron spectroscopy (XPS) data were collected on a Kratos AXIS Ultra DLD-600W XPS system using a monochromatic Al Kα (1486.6 eV) X-ray source. 2.4 Electrochemical measurements All electrolysis experiments were carried out in a standard three-electrode configuration using a CHI 760E electrochemistry workstation. The loading mass is ~2.1 mg cm-2. The electrochemical potential was tested using Ag/AgCl as reference electrode, Pt as the counter electrode, and given with respect to the reversible hydrogen electrode (RHE), which is converted through the equation,


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‫ܧ‬ோுா = ‫ܧ‬஺௚஼௟ + 0.197V + 0.059pH. Linear sweep voltammetry (LSV) was tested 5 mV s-1 for the polarization curves. Chronopotentiometry was collected under a constant current density of 10 mA cm-2. Electrochemical impedance spectroscopy (EIS) was measured at a frequency between 0.01 Hz and 100 KHz. For electrochemically active surface area (ECSA), the potential window was 1.0-1.1 V versus RHE and the scan rates were 20, 40, 60, 80, 100 mV s-1.

3. RESULTS AND DISCUSSION For preparation of the CFS-0.6 nanoflake arrays on Ni foams, two sequential hydrothermal methods were executed as shown in Figure 1. To unveil the crystal structure of the as-prepared CFS and the corresponding precursors, XRD characterization was carried out. Figure 2a shows the XRD pattern of one sample of CFS (x=0.6; i.e. CFS-0.6). Well-identified peaks at 33.237°, 一

44.786°, 51.494°, and 63.194° were observed, indexed to (112), (114), (310), (222) planes of the monoclinic phase of CoFe2Se4 (JCPDS card no.65-2337; space group: I2/m). The crystal structure of the corresponding precursor is illustrated in Figure S1. For comparison, XRD patterns of CFS with different ratios of Co/Fe (x=0, 0.2, 0.4, 0.8 and 1) were all collected (shown in Figure S2). It shows that a slight peak shift was observed with x increasing, while the change of peak intensity was well identified. When x was increased to 1, that is, FeSe2 was formed with a cubic phase (JCPDS no.48-1881), of which it could be confirmed from XRD. The intensity of the main diffraction peaks of the as-prepared Co1-xFexSe2 changed obviously with the iron content. It is suspicious that the morphology of the as-obtained products may also be different, which in due results in different electrocatalysis performances (will be talked hereinafter). As presented in Figure 2b, the FESEM images of CFS-0.6 indicated that the standing nanosheets were densely, vertically, and uniformly grown on the Ni foams. To gain an insight into more details of the nanosheets, high-magnification FESEM images were recorded and shown as the inset of Figure 2b. The FESEM images revealed that the thickness of the nanosheets was around 27.3 nm. Such an ultrathin structure may allow make more active sites being exposed to electrolytes and provide fast transportation pathways for electrons/ions, giving rise to a superior performance in oxygen evolution. Actually, it is noteworthy that CFS-0.6 nanosheets inherit the morphology of its precursor as shown in Figure 2c, which is also vertically interacted



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free-standing CoFe layered double hydroxide (LDH) nanoflakes. Interestingly, the thickness of CFS-0.6 nanosheets is decreased from 63.9 nm to 27.3 nm upon selenizing, which may well be attributed to the collapse of the layered structure of the CoFe carbonate hydroxide precursor possessing a interlayered distance of ~0.85 nm calculated from the precursor XRD pattern (Figure S1).35 Interestingly, through adjusting the Co/Fe ratio, different morphologies could be obtained (Figure S3), which could be associated to the fact that the crystal structure are impacted by the interaction between Co and Fe element. Figure 2d shows a typical TEM image of CFS-0.6 scraped from the as-prepared sample. It is consistent with the FESEM observations that the CFS-0.6 is the nanoflake structure with clean surfaces and edges. Nonetheless, some very small particles (less than 5 nm; highlighted by the dash circles in Figure 2f) were found imbedded in the flakes, suggesting being the active sites for electrocatalyst reactions.36 The lattice fringes of 0.268 nm and 一

0.264 nm were determined from Figure 2g, which can be assigned to (202) and (112) planes of CoFe2Se4, respectively. EDX mapping in Figure 2e discloses that selenium element are distributed uniformly as that of cobalt and ion elements. However, regarding to the synthesized CFS-0.6 sample, there should also have some slight oxidation on the surfaces due to its long-term exposure in air. Oxygen element was detected in the EDX spectrum, for which oxygen element was collected and given in Figure S4. Another thing required to point out that the synthesized CFS-0.6 nanoflakes are selenium-enriched as validated by XRF (Figure S5) and EDX-TEM (Figure S6), which are also reported by Wang et al. 37 X-ray photoelectron spectroscopy (XPS) was further employed to explore the chemical composition of the CFS-0.6 sample. The survey spectrum discloses the presence of Co, Fe, and Se (see Figure S7). Figure 3a shows the core-level spectrum of Co 2p, in which Co3+ 2p1/2, Co2+ 2p3/2, Co3+ 2p1/2, and Co2+ 2p3/2 were identified at 779.6, 783.0, 794.3, and 797.7 eV, respectively, indicating the coexistence of Co2+ and Co3+ species in CFS nanoflakes.38 In Fe 2p XPS spectrum (Figure 3b), the binding energy can be deconvolved to 707.6 eV (Fe 2p3/2) and 720.8 eV (Fe 2p1/2), disclosing the metallic state of Fe; and the broad peak at 712.9 eV could be assigned to Fe3+ 2p3/2.39-40 In addition, the Se 3d core-level spectrum was collected and shown in Figure 3c, where four peaks at 54.9 eV, 55.7 eV, 59.2 eV, and 60.1 eV were identified, among which the peaks at


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54.9 eV and 55.7 eV are associated to elemental selenium.37 As such, XRF and EDX also corroborate the selenium-enriched nature of the as-prepared Co1-xFexSe2, of which it could refer to Figure S5 and Figure S6. Besides, two more deconvolved peaks at 59.2 eV and 60.1 eV were also observed, which are related to Se-O bonding structures and evidences of the surface oxidation of

Se species (SeO2).36 It is noteworthy that this observation is consistent with TEM elemental mappings, where oxygen element distribution was recorded and displayed (see Figure S6). Figure 3d shows the core-level spectrum of O 1s, where three deconvolved peaks are discerned, referring to OI (low binding energy) to OIII (high binding energy).29 The OI contribution (531.0 eV) is related to the oxygen in –OH groups, signifying that the surfaces of CFS-0.6 were partially hydroxylated.29 The OII contribution (531.4 eV) could be assigned to the surface oxidation of Se species,29 being consistent with Se 3d spectra. Finally, the weak as well as broad OIII peak (532.3 eV) should be attributed to the physi/chemi-sorbed water at/near the surfaces of the selenide because of its exposure to air.29 The electrocatalytic OER activities of CFS samples were evaluated in 1 M KOH electrolyte using a standard three-electrode system. For comparison, similar measurements for the CoFe-LDH precursor were also performed. Of note, all catalysts were electrochemically cycled to reach a stable status (more details can refer to the experimental section). Then, the OER catalytic performance was accessed by linear sweep voltammetry (LSV). However, due to the existing strong redox peak, the overpotential is generally less than the real one when scanning was performed from low voltage to high voltage. Given the cathodic peak of CFS-0.6 is the most cuspate one compared with other samples, it imparts that a possible OER catalyst could be rather than being a pseudocapacitive supercapacitor, which is consist with the corresponding cyclic voltammograms (CV) curves (Figure S8). In view of this reason, the polarization curves were collected from high voltage to low voltage at a scan rate of 5 mV s-1 as shown in Figure 4a. Remarkably, CFS-0.6 shows an overpotential of 217 mV at the current density of 10 mA cm2, which is the lowest one among the CFS catalysts, and also much lower than that of the CoFe-LDH precursor (277 mV), bare Ni foam (376 mV) and RuO2 (319 mV). To more vividly exhibit the difference of the electrocatalytic performance of all the catalysts, the overpotentials at a fixed current density of 10 mA cm2 were summarized and depicted in Figure 4b. CoSe2 has the highest



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overpotential, even much larger than that of CoFe-LDH precursor. Upon increasing the Fe percentage in CFS compounds, the overpotential decrease effectively; nonetheless, when x is over 0.6, the overpotential increase appreciably towards increasing the Fe percentage in CFS compounds. To the best of our knowledge, the electrocatalytic performance of the catalyst (CFS-0.6) is comparable or superior to most of the Co-, Fe-, or Ni-based compounds, of which the comparison could refer to Table S1. Although CoSe2 has been reported to be an advanced catalyst for water oxidation,41 a higher overpotential was obtained in our case, in which a larger oxidation peak was seen, which suggests being assigned to different synthesis approach and different used precursor. Towards changing Fe ratio in CFS compounds, two variations were identified: 1) Overpotential dramatically decreased from 316 mV (CoSe2) to 260 mV (CFS-0.2) when x was increased from 0 to 0.2, which would further decrease until x=0.6; 2) Anodic cobalt redox peaks were shifted to high voltage (Figure 4a, S8). In light of these findings, we are concerned what the main contributor to the remarkable electrocatalytic activity really is. As it was noticed that OER performance was effectively improved after introduction of trace amount of Fe to NiOOH and that phenomenon was possibly ascribed to the formation of highly valence nickel (Ni4+) active site.42-43 Nonetheless, what is the real role/function of Fe for water oxidation, there is no distinct conclusion reported yet.44-45 Considering the remarkable electrochemical behaviors, together with the aforementioned two points and previous literature,46 we do believe that the introduced Fe may vary the electron configuration of CFS, which is resulted from subtle lattice distortion, and it may well be the main contributor for the remarkable electrocatalytic performance in our case.47 In addition, engineering morphology could be another possible path for tuning the catalytic activity because it is quite close related to the exposed active sites. The OER kinetics of the catalysts were also evaluated by Tafel plots (Figure 4c). Again, among all the catalysts, the Tafel slope of CFS-0.6 is the smallest one (41 mV decade-1). It is much smaller than the values of 66 mV decade-1 for CoSe2 and 82 mV decade-1 for CoFe-LDH precursor. Noteworthy, the Tafel slope is comparable or even smaller than most of previous reported Co-based OER catalysts, indicating the advanced OER kinetics of the constructed CFS-0.6 composites. It is well known that Tafel slope is closely related to rate-determining steps (RDS) and electron transfer reactions.28 As reported by Wang et al.,34 the Tafel slopes of 120, 60,


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and 40 mV decade-1 corresponds to the first step (one electron transfer), second step (two electron transfer), and third step (three electron transfer) of OER if this was the RDS. Thus, it is generally accepted that a high-efficiency electrocatalyst has a small Tafel slope, particularly for the case that the Tafel slope is close to or less than 40 mV decade-1, the RDS is the third step with a three-electron transfer. Since the Tafel slope of CFS-0.6 (41 mV decade-1) is much smaller than that of the corresponding precursor (82 mV decade-1), it suggests that the RDS would be altered from the second step (two electron transfer, the corresponding Tafel slope is 80 mV decade-1 ) to the third step (three electron transfer, the corresponding Tafel slope is 40 mV decade-1) after the selenization. The dynamic process of the OER are given in Figure 4d. Besides the Tafel slope, mass transport may also be involved in the OER kinetics.17 In order to confirm whether the mass transport makes contribution to OER activity, the scan rate dependence of the activity of CFS-0.6 was tested. When increasing the scan rate from 2 to 5 mV s-1, no obvious change in activity was observed, evidencing that the mass transport was fast enough (shown in Figure S9). Next, we consider another key factor for the responsible improved catalytic activity, that is, electrochemically active surface area (ECSA).48 To explore the reason for the significant difference in electrocatalytic behaviors of CFS-0.6 and CoFe-LDH precursor, the ECSA was evaluated from the electrochemical double-layer capacitance (Cdl). As presented in Figure 5a, the area of cyclic voltammetry at 20 mV s-1 for CFS-0.6 is enlarged compared with that of the precursor, indicating the electrochemical activity for CFS-0.6 is drastically enhanced after selenization treatment. In particular, it was found that the calculated Cdl of CFS-0.6 is 3.1 mF cm-2 (Figure 5b), which is 2-times higher than that of its counterpart precursor (more details could refer to Figure S10), verifying that the number of active sites for OER are increased effectively. To further understand the OER kinetics of CFS, electrochemical impedance spectroscopy (EIS) analysis was performed. Figure 5c exhibits the Nyquist plots of CFS-0.6 and its precursor. It is well known that the diameter of semi-circles in the high-frequency range corresponds to the charge transfer resistance (Rct).49 Since Rct of CFS-0.6 (0.54 Ω) is much smaller than that of the counterpart LDH precursor (0.9 Ω), the enhanced charge transfer kinetics of our synthesized selenide is validated, facilitating the charge transfer process and in turn lead to remarkable electrocatalytic activity, which is also in concert with the common sense that selenide has high



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conductivity compared with hydroxides and oxides. 50 In light of the high electrocatalytic activity of CFS-0.6 for water oxidation, we further investigated the durability of the electrode by using chronoamperometry measurements (j ~ t). Figure 5d exhibits the stability of catalyst measured at a constant current density of 10 mA cm-2 in 1 M KOH. After testing for 72 h, a dinky stability decay of 4.4% was observed. Moreover, the steadily vigorous effervescence was maintained even after 72 h (inset of Figure 5d; the enlarged image in Figure S11), indicating the robust stability of the synthesized CFS-0.6 under harsh water oxidation

conditions.

To

further

identify

the

OER

process

on

CFS-0.6

was

a

four-electron-dominated reaction pathway, the RRDE testing was carried out, where the faradaic efficiency was calculated to be ~96.5% (more details could refer to Supporting Information, Figure S12). To shed light on the microstructure variations and chemical oxidation state changes of the selenide in the electrochemical reaction, FESEM, TEM and XPS characterization were carried out after the stability evaluation. As shown in Figure S13, even after vigorous electrochemical reactions, the vertical interacted standing construction was still preserved. Nonetheless, in stark contrast to the fresh and clean nanoflakes (Figure 2b and Figure 3d), the surface of the nanoflake has been severely roughening. Closer observation can be further identified from selected enlarged region in SEM images (dashed circle), where it shows that the nanosheet has evolved to “bubble sheets”, of which the “bubble” is composed of aggregated ultrafine nanoparticles with several nanometers in size. As shown in TEM images of the cycled sample in Figure S14, “bubble sheets” structure has apparently formed, more nanoparticle can be observed compared with the pristine CFS-0.6, indicating the occurrence of phase change during electrolysis. Actually, these observation/change should be associated with the Se-rich effect, because after element Se was oxidized and dissolved in electrolyte, more active sites would be exposed, which was also suggested by Xu et al.36 XPS spectrum (Figure S15) discloses that the signal peaks of Co 2p and Fe 2p maintain unchanged, the Se-O bonding intensity increased while the peaks associated to elemental selenium decreased. Together with the raised OII contribution in O core-level spectrum, it confirms more Se species was oxidized after the measurements. In other words, during the initial electrochemical reaction, CFS was oxidized to Co-Fe-Se based oxide/hydroxide, which can then endure the long-term fierce electrochemical reaction, leading to


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the robustness of the catalyst.

4. CONCLUSION In summary, we have successfully fabricated CFS nanoflakes on Ni foams by a facile selenizing route based on hydrothermal method. It shows outstanding OER activity with an ultra-low overpotential of 217 mV at a current density of 10 mA cm-2, small Tafel slope of 41 mV decade-1, and robust stability in alkaline medium (pH~14). To the best of our knowledge, this performance is comparable or superior to most of the non-noble metal catalysts, particularly metallic or bimetallic selenides. The impressive electrocatalytic performance Co1-xFexSe2 is attributed to different rate-determining step (RDS) mechanism, involved mass transport, high conductivity, and remarkable OER kinetics. Moreover, due to the excess metallic selenide nature of the as-prepared CFS, more active sites will be exposed, allowing OH- to be absorbed and facilitating oxygen bubbles being released from the catalyst surface. We believe that this work paves pathways for constructing high efficient selenide-based OER catalyst based on the modification of electron configuration by introduction of other transition-metals.

ASSOCIATED CONTENT *Supporting Information XRD pattern of the CoFe-LDH precursor and Co1-xFexSe2 (Figure S1, S2), SEM images of CoFe-LDH precursor and Co1-xFexSe2 (Figure S3), EDX spectrum of CFS-0.6 (Figure S4), XRF spectrum of CFS-0.6 (Figure S5), Elemental mapping of oxygen (Figure S6), XPS Survey spectrums of CFS-0.6 (Figure S7), LSV polarization curves (Figure S8, S9), ECSA curves (Figure S10), three electrodes configuration (Figure S11), Faradaic efficiency measurement (Figure S12), SEM images after cycling (Figure S13), TEM images after cycling (Figure S14), XPS spectrum after cycling (Figure S15), OER activities of catalysts (Table S1).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]



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Notes The authors declare no competing financial interest.

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. 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 CFS-0.6 nanoflakes on Ni foams.

Figure 2. (a) XRD pattern of the CFS-0.6 nanoflakes. (b) low-magnification SEM image of CFS-0.6 grown on the surface of a nickel foam. Inset: high-magnification SEM image. (c) SEM image of CoFe-LDH precursor. Inset: high-magnification SEM image. (d) low-magnification TEM image. (e) EDS mapping of CFS-0.6. (f-g) high-magnification TEM images.



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Figure 3. (a) Co 2p, (b) Fe 2p, (c) Se 3d, and (d) O 1s core-level XPS spectra of CFS-0.6.

Figure 4. OER catalytic performances of CFS. (a) LSV polarization curves of CFS samples, the CoFe-LDH precursor, Ni foam and RuO2 and (b) the corresponding overpotentials (required to reach 10 mA cm-2). (c) Tafel plots for CFS samples and the CoFe-LDH precursor. (d) OER process with emphasized RDS (before and after selenization).


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Figure 5. (a) Cyclic voltammograms of the CoFe LDH precursor and CFS-0.6. (b) Cdl of CFS-0.6 and CoFe-LDH precursor that derived from current density differences vs scan rate. (c) EIS of CFS-0.6 and CoFe-LDH precursor. The inset of (c) is the equivalent circuit diagram based on the Nyquist plots. (d) Chronopotentiometry curves of CFS-0.6 tested at a constant current density of 10 mA cm-2 for 72 h. The inset of (d) is a photograph of CFS-0.6 acting as a working electrode for OER, from which O2 bubbles were generated steadily vigorous on the electrode. All tests were carried out in O2-saturated 1 M KOH (pH~14).



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Figure captions Figure 1. Schematic illustration of the synthesis process of CFS-0.6 nanoflakes on Ni foams.

Figure 2. (a) XRD pattern of the CFS-0.6 nanoflakes. (b) low-magnification SEM image of CFS-0.6 grown on the surface of a nick foam. Inset: high-magnification SEM image. (c) SEM image of CoFe-LDH precursor. Inset: high-magnification SEM image. (d) low-magnification TEM image. (e) EDS mapping of CFS-0.6. (f-g) high-magnification TEM images.

Figure 3. (a) Co 2p, (b) Fe 2p, (c) Se 3d, and (d) O 1s core-level XPS spectra of CFS-0.6.

Figure 4. OER catalytic performances of CFS. (a) LSV polarization curves of CFS samples and the CoFe-LDH precursor, and (b) the corresponding overpotentials (required to reach 10 mA cm-2). (c) Tafel plots for CFS samples and the CoFe-LDH precursor. (d) OER process with emphasized RDS (before and after selenization).

Figure 5. (a) Cyclic voltammograms of the CoFe LDH precursor and CFS-0.6. (b) Cdl of CFS-0.6 and CoFe-LDH precursor that derived from current density differences vs scan rate. (c) EIS of CFS-0.6 and CoFe-LDH precursor. The inset of (c) is the equivalent circuit diagram based on the Nyquist plots. (d) Chronopotentiometry curves of CFS-0.6 at a constant current density of 10 mA cm-2 for 72 h. The inset of (d) is a photograph of CFS-0.6 acting as a working electrode for OER, from which O2 bubbles were generated steadily vigorous on the electrode. All tests were carried out in O2-saturated 1 M KOH (pH~14).


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Graphical abstract 82x55mm (150 x 150 DPI)


Rational Design of Cobalt-Iron Selenides for Highly Efficient

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