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Mesoporous Nanostructured CoFe-Se-P Composite Derived from a Prussian Blue Analogue as a Superior Electrocatalyst for Efficient Overall Water Splitting Linghao He, Bingbing Cui, Bin Hu, Jiameng Liu, Kuan Tian, Minghua Wang, Yingpan Song, Shaoming Fang, Zhihong Zhang, and Qiaojuan Jia ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00663 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Applied Energy Materials

Mesoporous Nanostructured CoFe-Se-P Composite Derived from a Prussian Blue Analogue as a Superior Electrocatalyst for Efficient Overall Water Splitting Linghao Hea, b, Bingbing Cui a‡, Bin Hu a‡, Jiameng Liua, Kuan Tiana, b, Minghua Wanga, b, Yingpan Songa, Shaoming Fang, Zhihong Zhang a, b*, Qiaojuan Jiaa

a

Henan Provincial Key Laboratory of Surface and Interface Science, Zhengzhou

University of Light Industry, No. 136, Science Avenue, Zhengzhou 450001, China.

b

Henan Collaborative Innovation Center of Environmental Pollution Control and

Ecological Restoration, School of Materials and Chemical Engineering, Zhengzhou University of Light Industry, No. 136, Science Avenue, Zhengzhou 450001, China.

Corresponding author: Zhihong Zhang *

E-mail addresses: [email protected]

‡ These authors contributed equally

1

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Abstract: Electrocatalytic water splitting requires the development of attractive and earth-abundant catalysts. In this work, a novel quarterly electrocatalyst of mesoporous cobalt/iron phosphorous-selenide nanocomposites (CoFe-Se-P) was derived from hollow CoFe Prussian blue analogues (CoFe-PBA) and combined by phosphatization and selenylation. This novel electrocatalyst displays an efficient and durable bifunctional catalysis performance for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In view of strong electron interaction among Co, Fe, Se, and P, this quarterly electrocatalyst exhibits a lower onset potential of −35.9 mV and an overpotential of −172.5 mV at a current density of 10 mA·cm-2 in acidic solution for HER. In alkaline solution, the onset potential and the overpotential are maintained at approximately −33 and −183.1 mV. Moreover, the proposed CoFe-Se-P catalyst possesses superior OER performance with a lower onset potential of 1.07 V and overpotential of 1.44 V at a current density of 10 mA·cm-2 in 0.1 M KOH solution. The versatile electrocatalyst can catalyze water splitting in a two-electrode system at a potential of 1.59 V. The proposed strategy offers insight into the rational design and development of promising electrode materials with distinct architectures for electrochemical energy storage and conversion. KEYWORDS: Prussian blue analogue, CoFe-Se-P, Hydrogen evolution reaction, Oxygen evolution reaction, Overall water splitting

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INTRODUCTION Recent studies have investigated and developed renewable energy to address the urgent need for rising global energy depletion and environmental deterioration. Hydrogen and oxygen generation via electrocatalytic water splitting is an environmentally friendly strategy for obtaining clean fuels from renewable energy sources that has attracted widespread attention.1,2 Electrocatalytic water splitting allows for the efficient large-scale production of hydrogen with high purity, with hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) occurring at the cathode and the anode, respectively.3,4 Various catalysts have been developed to accelerate the reaction rate and decrease the overpotential for HER and OER. At present, the most efficient HER and OER catalysts are noble metals and noble metal oxides, such as Pt, Pd, RuO2, and IrO2.5,6 However, these catalysts generally cannot exhibit good OER and HER performances at the same time. Moreover, the scarcity of noble metals hampers their application and development as bifunctional catalysts.7,8 Thus, a cost-effective catalyst that can drive both HER and OER at a high exchange current density and a low overpotential must be developed to realize efficient energyharvesting devices. Many previous efforts have explored efficient nonprecious metal-based electrocatalysts, such as various oxides, hydroxides, and phosphates9 for HER and chalcogenides,10 carbides,11 and phosphides12 for OER. These diverse electrocatalysts are cost-effective and can efficiently catalyze OER and HER conversion processes at the same time. Among these electrocatalysts, transition metal phosphides (TMPs) and 3



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transitional metal diselenides have attracted considerable interest due to their extremely low cost and expected long-term stability in both acidic and alkaline operating environments.13-16 To date, several TMP electrocatalysts, such as CoP,17 Ni2P,18 FeP,19 MoP,20 and WP,21 have been investigated for catalyzing water splitting. CoSe2 is regarded as an alternative catalyst because of its t2g6eg1 electronic configuration, which is close to the optimal eg filling for OER. For example, Co-MOF was successfully converted into CoSe2 microspheres after a thermally induced selenylation process under an argon atmosphere at different temperatures and was therefore applied as electrocatalysts for HER22 and OER23. New strategies, such as creating electronic interactions, constructing hybrid structures, and increasing the number of active sites, have been proposed to optimize the electrocatalytic performance of CoSe2. A binary transition metal composite material that provides two electron-donating active sites can increase HER and OER activities. Previous studies have shown that electrocatalytic activities could be improved by the introduction of other metal elements. For example, NiFe-based (oxy)hydroxides demonstrated a more efficient OER performance than that of Ni(OH) or Fe(OH)3. The synergism between Ni and Fe could tune the affinity to oxygen on catalyst surface sites toward the optimal value as well as facilitate O−O bond generation and oxygen evolution.24 Doping Mn atoms into Co phosphide or Fe phosphide effectively enhanced their OER catalytic activities. The incorporation of Mn atoms reduced the energy barrier for proton-coupled electron-transfer processes as well as promoted O−O bond generation and oxygen 4


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evolution.25

The

Co3O4@Fe−Co(OH)2

microfibers

exhibited

a

remarkable

electrocatalytic activity toward OER in 0.1 M KOH, attaining an overpotential of 0.408 and 0.386 V for the cases without and with Fe doping.26 The NiFe-Se/C composite nanorod is an efficient non-precious-metal electrochemical catalyst derived from the direct selenylation of a mixed Ni/Fe metal–organic framework. The asobtained catalyst required a low overpotential to drive 10 mA cm-2 for HER (160 mV) and OER (240 mV) in 1.0 M KOH.27 These findings suggest that using binary transition metal hybrid materials is an efficient strategy for improving electrocatalytic performance. Studies in recent decades have demonstrated that porous nanostructured electrocatalysts have excellent electrochemical properties for water splitting. Therefore, Prussian blue analogues (PBAs) with uniform sizes, various compositions, diverse morphologies, and different architectures can be utilized as ideal precursors for preparing hollow and porous electrocatalysts. Nanostructured PBAs are generally converted into functional metal oxides, sulfides, and phosphides, all of which can retain their original nanostructures and find promising applications in electrochemical storage and conversion.28 For example, Ni–Co Prussian blue analogue nanocubes were used as sacrificial templates to synthesize unique Ni–Co phosphide quasi-hollow nanocubes.29 Ni–Co phosphide quasi-hollow nanocubes synthetized at 300 °C (NiCo-P-300) exhibited better electrochemical performance for HER in alkaline solution than that of cobalt phosphides microcubes, nickel phosphides nanoplates, and Ni–Co phosphide stacked nanoplates. Porous ternary nickel-iron-phosphide (Ni-Fe-P) 5



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nanocubes were synthesized through a one-step phosphating of a Ni-Fe-based Prussian blue analogue.30 Owing to its unique porous nanocubes and chemical composition, the Ni-Fe-P nanocubes exhibited excellent HER and OER activities in alkaline medium, requiring a low overpotential of 182 and 271 mV to deliver a current density of 10 mA·cm−2. Moreover, Ni−Fe−P nanocubes displayed outstanding stability for sustained water splitting in the two-electrode alkaline electrolyzer. (Ni, Co)Se2-GA was prepared with penroseite (Ni, Co)Se2 nanocages derived from NiCo PBA anchored onto a graphene aerogel. Inheriting both the high catalytic activity of (Ni, Co)Se2 and the high conductivity of graphene sheets, this composite exhibits superior outstanding performance toward water splitting electrolysis.28 Diselenide nanocages were first obtained by treating PBA nanocube precursors with a siteselective ammonia etchant. The resultant Ni–Fe mixed diselenide nanocages served as an outstanding OER electrocatalyst.31 Although CoxFe1−xP nanocubes with different Co and Fe ratios were synthesized through a phosphating process, only the electrocatalytic activity toward HER was investigated.32 To date, few reports have explored the hollow CoFe PBA as a precursor of binary transition metal-related phosphide and selenide catalysts for water splitting. Herein, we reported a facile templated-engaged strategy for synthesizing binary transition metal phosphides/selenides (CoxFe1-x-Se-P) by using Co-Fe PBA nanocubes as self-sacrificed precursors via a one-step phosphidation and selenidation (Scheme 1). Given their unique structure and chemical composition, the resultant CoFe-Se-P composites exhibited outstanding HER performance in 0.5 M H2SO4 solution. The 6


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overpotential is approximately −172.5 mV at a current density of 10 mA cm-2, giving a Tafel slope of 58 mV dec-1. The composite also exhibit excellent OER electrocatalytic activity in 0.1 M KOH solution, yielding a very low overpotential of 1.44 V at a current density of 10 mA cm-2 and a Tafel slope of 108 mV·dec-1. An alkaline water electrolyzer was constructed using Ni foam-supported CoFe-Se-P as both the anode and the cathode with a cell voltage of 1.59 V to achieve 10 mA cm-2, which is superior to that of other analogue catalysts. Scheme 1 Schematic diagram of the preparation of the HCl-treated porous CoFe-Se-P nanocomposite and its application as bifunctional electrocatalyst for water splitting.

EXPERIMENTAL SECTION Materials. Polyvinypyrrolidone (K30, PVP, MW ≈ 40 000), K4Fe(CN)6·3H2O, Concentrated hydrochloric acid, sodium citrate, KOH, NaH2PO2·H2O, CoCl2, Selenium powder were purchased from Aladdin Industrial Corporation. Synthesis of hollow CoFe PBA. In order to create a hollow structure, the obtained 7



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FeFe@CoFe nanocubes (20 mg) was dissolved into a 2.0 M HCl solution (20 mL) under magnetic stirring. After 1 h, the solution was transferred into a stainless autoclave (50 mL) and heated at 120 °C for 3 h in an electric oven. The precipitates were collected by centrifugation and washed with distilled water and ethanol several times after the autoclave cooled down to room temperature before drying at 60 °C overnight. FeFe@CoFe nanoboxes were obtained.

Synthesis of porous CoFe-Se-P composite. For the synthesis of hollow CoFe-SeP nanocomposites, the as-prepared hollow CoFe PBA nanoboxes, Se powders and NaH2PO2 with the different mass ratio of 1:2:5 was placed in different positions in the tube furnace with NaH2PO2·H2O and Se powder at the upstream side. And then the products were heated at a temperature of 300 °C at a heating rate of 5 °C·min-1 and kept at this temperature for 3 h under N2 atmosphere. Then, the obtained different mass ratio products were dissolved into a 1.0 M HCl solution overnight, and finally dried for further use.

Characterizations. Fourier transform infrared spectra (FT-IR) were recorded on a Bruker TENSOR 27 spectrometer (32 scans at 4 cm−1 resolution). Powder X-ray diffraction (PXRD) measurements were characterized by a Rigaku D/Max-2500 Xray diffractmeter using Cu Kα as a radiation. Powder samples were prepared by crushing single crystals. The corresponding intensity data were collected in the stepscan mode with a scan rate of 5° min-1 and a step size of 0.02°. The surface morphologies of all samples were characterized by Field emission scanning electron microscopy (FE-SEM) (JSM-6490LV scanning electron microscope) and high8


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resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100) with a field emission gun of 200 kV, respectively. X-ray photoelectron spectroscopy (XPS) analysis was conducted using a Thermo Fisher ESCALAB 250Xi spectrometer equipped with an Al anode (Al-Kα 1486.6 eV). Determination specific surface area of porous CoFe-Se-P nanocomposite was carried out by Brunauer-Emmett-Teller (BET) using a Micromeritics ASAP2022 instrument at the temperature of liquid nitrogen. Prior to measuring, all of the samples were degassed at 573 K for 8 h.

Electrochemical measurements. Electrochemical measurements were performed using a CHI660E electrochemical workstation (Chenhua Instrument Company, Shanghai, China). All electrochemical measurements were conducted via a conventional three-electrode system, a glassy carbon electrode (GCE) or modified GCE severed as working electrode, a Pt wire was used as auxiliary electrode and a Ag/AgCl (in 3.0 M KCl solution) was used as the counter electrode and reference electrode, respectively. To prepare the working electrode, 1.0 mg catalyst and 100 µL of Nafion (DuPont, 5 wt %) were dispersed in 900 µL of water by sonication, thereby forming a homogeneous catalyst ink. Afterwards, 10 µL of the suspension was cast onto the GCE followed by drying at room temperature in air. The working electrode was cycled at a scanning rate of 50 mV·s−1 at least 10 times prior to the data collection. All potentials were recorded with respect to reversible hydrogen electrode (RHE) scale. To evaluate the HER activity of the samples, linear sweep voltammetry (LSV) was obtained at the potential ranging from 0 to - 0.8 V with scanning rate of 5 mV·s−1 9



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in H2SO4 solution (0.5 M). Electrolyte was degassed by bubbling N2 for 30 min prior to the start of each experiment. The OER catalytic activity of composites was determined in O2-saturated KOH solution (0.1 M) at a sweep rate of 10 mV s−1 with a speed rate of 1600 rpm using rotating disk electrode (RDE, 3 mm in diameter). Prior to its use, the RDE was polished using aqueous alumina suspensions on felt polishing pads and washed with HNO3: water (v = 1:1) and ethanol: water (v = 1:1) about 15 min, respectively.

Crystal and chemical structure of all samples. Powder X-ray diffraction (PXRD) experiments were conducted to investigate the crystal structure of the as-prepared materials. As shown in Figure S1a, the diffraction peaks of the hollow CoFe PBA at 17.3°, 24.6°, 35.9°, and 40.0° can be indexed to the (200), (220), (400), and (420) crystal planes, respectively, of the Co3[Fe(CN)6]2 with a face-centered-cubic structure33. For the porous CoFe-Se composite, the peaks observed at 23.5°, 30.7°, 34.5°, and 43.7° correspond to the (110), (101), (111), and (121) planes of CoSe2 (JCPDS No. 53-0449), and the peaks at 29.6° and 35.9° are attributed to the (101) and (012) planes of FeSe2 (JCPDS No. 79-1892), respectively. This result indicats that Se atoms were successfully introduced into the hollow CoFe PBA. The diffraction pattern of CoFe-P reveals that the two main peaks at 17.2° and 31.4° originated from CoFe PBA and assign to the (011) crystal plane of CoP (JCPDS No. 29-0497). The PXRD pattern shows three weak peaks at 30.4°, 34.8°, and 44.6°, which correspond to the (002), (200), and (210) planes of FeP (JCPDS No. 78-1443). As shown by curve iv in Figure S1a, only four peaks at 30.7°, 34.5°, 35.9°, and 48.0°, which are 10


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attributed to (Co, Fe)Se2, are observed for the porous CoFe-Se-P composite. Figure S1b shows the PXRD pattern spectra of the solid CoFe PBA and the CoFe-Se-P, which are similar to those of the hollow CoFe-PBA and the porous CoFe-Se-P composite, indicating that they have the same crystal structure. The on sign of the phosphide is probably due to the formation of more selenides on the surface during the simultaneous selenization and phosphating. The chemical and crystal structures were further characterized by XPS and TEM, respectively. To investigate the variations of the chemical structure and the components during the phosphatization and selenylation procedure of the bulk solid CoFe-Se-P and porous CoFe-Se-P catalysts, XPS characterizations were obtained (Figure S2). The XPS survey spectra of the solid CoFe PBA and the CoFe-Se-P are summarized in Figure S2a. As shown, Co 2p (781.3 eV), Fe 2p (714.2 eV), C 1s (284.2 eV), N 1s (399.2 eV), and O 1s (532.1 eV) are observed in the CoFe PBA (curve i), whereas additional signals of Se 3d (54.2 eV) and P 2p (133.2 eV) are present in the CoFe-SeP (curve ii), indicating that CoFe PBA was successfully phosphated and selenized. Figure S2b shows the XPS survey scan spectra of the hollow CoFe PBA, porous CoFe-Se, CoFe-P, and CoFe-Se-P nanocomposites. The same signals of Co 2p, Fe 2p, C 1s, N 1s, and O 1s are observed in these samples, whereas an additional Se 3d signal and P 2p signal are appeared in the porous CoFe-Se (curve ii) and the porous CoFe-P (curve iii), respectively. In addition to Co 2p, Fe 2p, C 1s, N 1s, and O 1s, both Se 3d and P 2p coexist in the porous CoFe-Se-P nanocomposite (curve iv). In all samples, the Co 2p and Fe 2p signals are caused by the transition metals contained in 11



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the PBA nanobox, whereas C 1s and N 1s are due to the presence of CN groups. XPS spectroscopy suggests that the presence of O 1s in the CoFe PBA and the hollow CoFe PBA are possibly caused by the adsorption of O2 from the air, even though Orelated components are present in the PBA. Table S1 shows the atomic% of each element of all samples. The content variation of each element is deduced during the phosphatization–selenylation procedure. As for the solid, CoFe PBA, and CoFe-Se-P nanocomposites, the simultaneous phosphatization and selenylation decrease the atomic% of C 1s and N 1s from 52.95% to 26.14% and from 29.15% to 6.31%, respectively, because of the thermal decomposition of the C and N elements. By contrast, the atomic% of O 1s increases from 5.36% to 33.82%, indicating that metal oxides are formed during the pyrolysis of the solid CoFe PBA. The atomic% of Co 2p and Fe 2p decrease from 6.19% to 4.54% and from 6.36% to 2.57%, respectively, for the solid CoFe PBA and CoFe-Se-P nanocomposites. In addition, 4.43% Se 3d and 22.19% P 2p simultaneously coexist in the CoFe-Se-P. A similar trend is observed for the variations of all elements for the hollow CoFe PBA during the phosphatization– selenylation procedure. The sole selenylation for the hollow CoFe PBA, i.e., the porous CoFe-Se, results in a very high Se 3d intensity of 21.28%, whereas 11.05% P 2p is found in the porous CoFe-P. In comparison, 9.37% Se 3d and 28.78% P 2p coexist in the porous CoFe-Se-P, which can enhance the active species of the catalysts.34 Detailed analysis of the chemical environment of each element is crucial to understanding the catalytic mechanism evolution. High-resolution analysis of all the elements contained in all samples was performed using the XPSPEAK41 software. 12


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As shown in Figure S3, the Co 2p, Fe 2p, C 1s, N 1s, and O 1s core-level XPS spectra of the nanocube CoFe PBA were deconvoluted and analyzed. For the Co 2p core-level XPS spectrum (Figure S3a), the two doublets of Co 2p are assigned to the Co element of different oxidation states. The peaks at 780.9 and 796.8 eV are assigned to Co3+ 2p3/2 and Co3+ 2p1/2, respectively. The peaks at 782.9 and 798.0 eV, as well as two satellite peaks at 787.3 and 803.9 eV, are due to Co2+ 2p3/2 and Co2+ 2p1/2, respectively. The Fe 2p core-level XPS spectrum of CoFe PBA is also analyzed (Figure S3b), of which the peaks at 709.25 and 723.01 eV are due to Fe2+ 2p3/2 and Fe2+ 2p1/2, and those at 712.5 and 726.1 eV are assigned to Fe3+ 2p3/2 and Fe3+ 2p1/2, respectively. Binding energies are observed at 707.9 and 720.72 eV, which correspond to the metal state Fe, indicating that the ionic Fe is reduced to its metal state during the preparation procedure.35 The peaks at 715.6, 718.08, and 734.9 eV are the satellite peaks of Fe 2p3/2 and Fe 2p1/2. As illustrated in Figure S3c, two distinctive peaks, namely, C–C (284.2 eV) and C–N (285.2 eV), are identified in the XPS spectrum of CoFe PBA, whereas two weak peaks at 287 and 288 eV are attributed to C–OH and O=C-N, respectively. As displayed in Figure S3d, the N 1s core-level XPS spectrum is fitted into four peaks, namely, 397.2, 397.9, 399.3, and 401.8 eV, which are assigned to the groups of pyridine N, the cyanide bonds in the PBA lattice, graphitic N, and oxide N, respectively. The peaks at 531.2 and 532.5 eV are deconvoluted into the O 1s core-level XPS spectrum of the CoFe PBA (Figure S3e), which correspond to the O vacancies and C-O groups, respectively.36 After the phosphatization and selenylation of CoFe PBA, i.e., the CoFe-Se-P 13



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nanocomposite, the XPS high-resolution spectra of all elements are supplied, as shown in Figure S4. Compared with the Co 2p core-level XPS spectrum of CoFe PBA (Figure S4a), in addition to the Co 2p3/2 and Co 2p1/2 of Co2+ and Co3+, additional peaks at 777.9 and 792.9 eV are found, which clearly originate from the Co 2p3/2 and Co 2p1/2 of the metal-state Co, respectively, suggesting that the Co2+ species are reduced to Co0 during the phosphatization procedure.37 The Co 2p3/2 peak at 780.4 eV of Co3+ and the Co 2p1/2 peak at 782.1 eV of Co2+ show a slightly negative shift compared with those of CoFe PBA, suggesting the presence of a higher electron density.38 For the Fe 2p core-level XPS spectrum of the CoFe-Se-P nanocomposite (Figure S4b), the same peaks are observed with those of the CoFe PBA. However, as compared with CoFe PBA, these peaks of Fe 2p in the CoFe-Se-P nanocomposite exhibit a positive shift, indicating the enhanced charge transfer from Fe to Co in the CoFe-Se-P nanocomposite.39 The binding energy shifts of Co 2p and Fe 2p enhance the electrocatalytic activity for water splitting. After phosphatization and selenylation, the peak belonging to cyanide at 397.9 eV in the N 1s core-level XPS spectrum (Figure S4c) is replaced by a new peak at 398.5 eV, which originates from pyridinic– N. Moreover, aromatic N (pyrrolic–N (399.8 eV), graphitic–N (401.3 eV)), and oxidized N (403.1 eV) peaks are observed, suggesting the formation of N functionalities in the CoFe-Se-P nanocomposite. Compared with the high-resolution XPS spectrum of O 1s in CoFe PBA, a new peak at 530.5 eV, which originates from the O in metal oxides, appeared in the O 1s core-level XPS spectrum of CoFe-Se-P (Figure S4d), suggesting the formation of CoOx and FeOy when the CoFe PBA was 14


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paralyzed.40 Meanwhile, the peak at 532.7 eV is assigned with the C=O group. As illustrated in Figure S4e, the high-resolution P 2p XPS spectrum of CoFe-P displays two peak regions at 129.3 eV, which is assigned to the P bonded with Co or Fe, and at 134.3 eV (133.4 and 135.0 eV), which corresponds to the oxidized P species that is likely formed upon the exposure of the catalyst to air. The formation of P–Co bonds in CoFe-Se-P is verified by analyzing the Co 2p core-level XPS spectrum (Figure S4a). As shown in Figure S4f, the Se 3d core-level XPS spectrum of the CoFe-Se composite was deconvoluted and analyzed. The two peaks centered at 53.7 and 54.6 eV (Se 3d5/2 and Se 3d3/2) are attributed to the selenide species, whereas the peak at 55.6 eV corresponds to the metal-state Se.41 The small peak at 53.4 eV corresponds to Fe 3p, and the peaks at 58.4 and 60 eV originate from Co 3p3/2 and Co 3p1/2, respectively, suggesting the presence of Co–Se and Fe–Se binding.42 The high-resolution XPS spectra of Co 2p, Fe 2p, Se 3d, P 2p, N 1s, and O 1s containing in the hollow CoFe PBA, porous CoFe-Se, porous CoFe-P, and porous CoFe-Se-P composites were analyzed, and the results are summarized in Figures 1, 2, and S5. As shown in Figure 1, the Co 2p and Fe 2p high-resolution XPS spectra of these composites were fitted and summarized. The binding energy positions of the Co- and Fe-related species are similar to those of the solid CoFe PBA but have different densities of the various valence states of Co 2p and Fe 2p species. For the Co 2p core-level XPS spectra of the porous CoFe-Se and CoFe-Se-P composites (Figures 1b1 and 1d1), a substantially sharp peak, as well as a peak at 792.7 eV, appears at 777.8 eV, which are assigned to the Co 2p3/2 and Co 2p1/2 of the metallic state of Co, 15



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i.e., Co0, indicating that some of the Co2+ or Co3+ can be reduced to Co0 during the selenylation of the hollow CoFe PBA. For the porous CoFe-P composite, Co0 has a very low intensity (Figure 1c1) mainly due to the extremely low signal intensity of Co 2p. However, after selenylation, the intensity of Fe 2p becomes very weak (Figure 1b2), leading to the disappearance of the Fe-related species. In particular, no Fe0 specie is fitted out for the porous CoFe-P and CoFe-Se-P composites (Figures 1c2 and 1d2). Additionally, the Se 3d and P 2p core-level XPS spectra of porous CoFe-Se, porous CoFe-P, and porous CoFe-Se-P nanocomposites were analyzed and shown in Figure 2. For the core-level XPS spectra of the Se 3d-containing porous CoFe-Se (Figure 2a) and porous CoFe-Se-P composites (Figure 2c), similar peak deconvolutions are observed to those of CoFe-Se-P, and no SeOx peak appears for the porous CoFe-Se composite. Moreover, the components of the core-level XPS spectra of the P 2p-containing porous CoFe-P (Figure 2b) and porous CoFe-Se-P composites (Figure 2d) are similar to those of the CoFe-Se-P composite. By contrast, the contents of the P–O peak in the porous CoFe-P composite are considerably high, indicating that most of the P are easily oxidized by high-temperature calcination. The N 1s and O 1s core-level XPS spectra of the hollow CoFe PBA, porous CoFe-Se, porous CoFe-P, and porous CoFe-Se-P nanocomposites were analyzed, and the results are shown in Figure S5. The N 1s high-resolution XPS of the hollow CoFe PBA is as same as that of the solid CoFe PBA. However, substantially different O 1s analysis results are obtained. Two additional peaks appear at 529.5 and 532.6 eV in the O 1s core-level 16


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XPS spectrum of the hollow CoFe PBA (Figure S5a2). These peaks are respectively assigned to the metal oxides (M-O) and the adsorbed oxygen species FeFe@CoFe into hollow CoFe PBA.43 Moreover, in view of the high-temperature pyrolysis, the pyridine N group is decomposed for the porous CoFe-Se (Figure S5b1) and the CoFe-Se-P (Figure S5d1), whereas the peak of pyrrolic N is not observed for

(a1) Co 2p

(a2) Fe 2p

Co 2p3/2 Co 2p1/2

Sat. Sat.

Fe 2p3/2

Fe 2p3/2 2+

Co

2+

2+

Co

Fe

Sat. Sat. 3+

Co

0

Fe

Fe

Co

2+

Fe 3+

Fe

2+

3+

(b1) Co 2p

0

Fe

(b2) Fe 2p 0

Co 0

Co

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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Page 18 of 50

Figure 1. Co 2p and Fe 2p core-level XPS spectra of (a) hollow CoFe PBA, (b) porous CoFe-Se, (c) porous CoFe-P, and (d) porous CoFe-Se-P nanocomposites. the porous CoFe-P composite (Figure S5c1). For the O 1s core-level XPS spectra of the porous CoFe-Se, CoFe-P, and CoFe-Se-P composites (Figures S5b2–S5d2), no peak appears at 529.5 eV, suggesting that no metal oxide is present in these composites. However, new peaks are deconvoluted at 531.0 and 531.9 eV, which are attributed to the P=O and P-O-P groups, respectively, suggesting the successful phosphatization of the hollow CoFe PBA. Furthermore, according to the ratio of the O vacancy peak area (the purple color) to the sum peak area of the O 1s core-level XPS spectrum, the O vacancies content of each sample can be deduced44. It demonstrates that the peak density of the O vacancy containing in the porous CoFe-Se-P composite is the largest among four samples, 74.35%. As discussed in other literature, the Ovacancies in the oxide semiconductors can serve as active sites for improving the carrier separation efficiency, ultimately improving the water splitting efficiency.45,46 XPS characterization can semi-quantitatively reveal the actual molar ratio of various elements on the sample surface with the following empirical formula: Ci= Ai/Si∑Ai/Si where Ci is the actual atomic molar ratio (i= Co, Fe) in the measured film, Ai is the absorption peak area of each element in the XPS spectrum, and Si is the sensitive factor of various elements.47 The ratios of Fe3+/Fe2+ and Co3+/Co2+ are calculated by fitting the XPS spectra of Fe and Co shown in Figure 1 and then used to calculate the valence states of Cox+ and Fey+. The changes of the Fe and Co valence states can be 18


Page 19 of 50

attributed to the solid-state redox reactions during the oxidative decomposition treatment48. As shown in Table S2, the Co3+/Co2+ and Fe3+/Fe2+ ratios in the porous CoFe-Se-P nanocomposites reach the maximum values of 1.31 and 1.74, respectively, indicating excellent catalytic performance.49 The percentages of the O-vacancies of all samples are calculated based on ratios of the O-vacancies peak area to the O 1s peak sum area, and the results are summarized in Table S2. As shown, the intensities of the O-vacancies in the CoFe-Se-P and porous CoFe-Se-P composites are very high, reaching up to 57.9% and 45.3%, respectively.

(b) P 2p

(a) Se 3d Se 3d3/2

Co 3p1/2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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(d) P 2p

(c) Se 3d

SeOx

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144

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Binding energy (eV)

Figure 2. (a) Se 3d core-level XPS spectra of porous CoFe-Se, (b) P 2p of core-level XPS spectra of porous CoFe-P, and (c) Se 3d and (d) P 2p of core-level XPS spectra of porous CoFe-Se-P nanocomposites.

Surface morphology of all samples. FE-SEM and TEM were performed to determine 19



Page 20 of 50

the microstructure and surface morphologies of all samples. As shown in Figures S6a and S6b, uniform FeFe PB nanocubes with an average size of 200 nm are obtained through typical hydrothermal method. As shown in Figures. S6c and S6d, the nanocube structure is well retained for the FeFe@CoFe PB core shell nanocomposite and displays an average size of 300 nm. Moreover, the surface of as-synthesized FeFe@CoFe PB becomes rougher than that of the FeFe PB nanocubes, suggesting that a CoFe layer is formed on the surface of FeFe PB. After the successive treatment of FeFe@CoFe with 0.1 M NaOH and HCl solution (Figures. 3a and 3b), the interior FeFe PBA core is removed, leading to the hollow CoFe PBA nanoboxes. However, after the phosphatization and selenylation of the hollow CoFe nanostructure, the nanobox collapses into a 3D hierarchical porous structure, which not only provides a favorable microenvironment and a large surface area but also promotes direct electron transfer.50 In addition, the selenylation and phosphatization of the hollow CoFe PBA were individually prepared via the same method. Their corresponding SEM images are illustrated in Figure S7. For the porous CoFe-Se and CoFe-P nanocomposites, the original hollow CoFe PBA nanoboxes completely collapse to form a porous accumulated nanostructure. The surface morphology of the porous CoFe-Se-P is similar to that of the CoFe-P nanocomposite (Figures 3c and 3d). For comparison, the solid CoFe PBA and CoFe-Se-P nanocomposites were also prepared and applied as electrocatalysts. The SEM images (Figures S8a and S8b) reveal that the nanobox structure of the solid CoFe PB, is similar to that of the FeFe@CoFe PB. After the simultaneous phosphatization and selenylation of the solid CoFe nanocubes (Figures 20


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S8c and S8d), the nanocube surface becomes considerably rough as a hierarchical porous structure is observed, indicating that selenides and phosphides are formed.

(b)

(a)

1 μm 1µm

(c)

1 µm

100 nm

(d)

100 nm

Figure 3. SEM images of (a, b) hollow CoFe PBA and (c, d) porous CoFe-Se-P nanocomposite. Moreover, the nanostructure of the hollow CoFe PBA, porous CoFe-Se, porous CoFe-P, and porous CoFe-Se-P composites was further confirmed by TEM and highresolution TEM (HR-TEM). As shown in Figure 4, the hollow CoFe PBA presents a regular hollow cubic structure with a width of approximately 200 nm (Figures 4a and 4b). The HR-TEM images reveal that the interplanar spacing of the lattice fringes is approximately 0.201 nm, which corresponds to the (110) plane of the CoFe alloy(JCPDS No.49-1567). Figure S9 shows the TEM and HR-TEM images of the porous CoFe-Se and CoFe-P composites, in which no regular cubes can be observed. 21



Page 22 of 50

Figure S9c shows the lattice spacing of 0.369 and 0.258 nm, which are assigned to

Figure 4. TEM and HR-TEM images of (a, b, c) hollow CoFe PBA and (d, e, f) porous CoFe-Se-P nanocomposite. the (Co, Fe) Se2 (110) and (111) planes (JCPDS No. 79-1892 and No. 53-0449), respectively. Figure S9f shows that the particle spacing was 0.246 and 0.253 nm, which are assigned to the (111) and (200) planes of (Co, Fe) P (JCPDS No. 78-1443 and No. 29-0497), respectively. The TEM images of the porous CoFe-Se-P nanocomposite (Figures 4d, 4e, and 4f) reveal the lattice spacing of 0.471 nm, which corresponds to the Co3O4 and/or Fe3O4 (111) plane (JCPDS No. 74-1657 and No. 261136), in addition to the lattice fringes of the (Co, Fe)Se2 (110) and (Co, Fe)P (111) planes. These results confirm the coexistence of (Co, Fe)Se2, (Co, Fe)P and (Co, Fe)3O4 nanoparticles in CoFe-Se-P nanocomposite, which is consistent with the XPS measurement results.

Comparison of the HER performances of CoFe PBA and its derivatives. The 22


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ultimate catalytic performances of transitional metal compounds are determined not only by their chemical components but also by their nano-sized shapes.51 Herein, the electrocatalytic abilities of the bulk solid CoFe PBA and its phosphor-selenide derivative (CoFe-Se-P), the hollow CoFe PBA, and the series of phosphor-selenide derivatives were investigated. As shown in Figure 5a, the polarization curves for the HER electrocatalytic activities of Pt/C (20%), hollow CoFe-PBA, porous CoFe-Se, CoFe-P, and CoFe-Se-P were determined in a 0.5 M H2SO4 solution with a typical three-electrode system. The commercial Pt/C (20%) electrocatalyst demonstrates the best electrocatalytic activity with an onset potential of nearly 0 V (η0) and required a low overpotential of 12.5 mV at a current density of 10 mA·cm-1 (η10). Thus, the porous CoFe-Se-P nanocomposite shows a small onset potential of 35.9 mV, beyond which the cathodic current rapidly increases at more negative potentials. This onset potential is substantially smaller than those of hollow CoFe PBA (76.4 mV), porous CoFe-Se (119.1 mV), and CoFe-P (121.4 mV). It is clear for that the HER activity of the proposed porous CoFe-Se-P catalyst outperforms many nanostrucutred Co- and Fe-related catalysts previously reported (Tables S3 and S4). Furthermore, the operating potential required for different electrocatalysts to deliver a current density of 10 mA·cm-2 (which is a metric related to solar fuel synthesis) was determined. The results reveal that the porous CoFe-Se-P nanocomposite can reach a current density of 10 mA·cm-2 at 172.5 mV, whereas the hollow CoFe PBA, porous CoFe-Se, and porous CoFe-P composites require an overpotential of 201.8, 202.2, and 198.7 mV, respectively, to drive the same current density. And this value is changed to 178.6 mV 23



after iR-corrected for the porous CoFe-Se-P nanocomposites. Meanwhile, a CV test in 0 to -0.2 V was carried out inorder to eliminate the influence of faradaic pseudocapacitance. The result shows that the

(b)

(a) Overpotential (V vs. RHE)

0 -40 -2

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ii

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-2

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Page 24 of 50

-80

-120

20 0 -20 -40

-160 -0.3

0

10

20 30 Time (h)

-0.2 -0.1 0.0 Potential(V vs. RHE)

40

0.1

Figure 5. HER performance of different samples. (a) LSVs obtained in N2-saturated 0.5 M H2SO4, (b) the corresponding Tafel slopes of (i) Pt/C, (ii) hollow CoFe, (iii) porous CoFe-Se, (iv) porous CoFe-P, and (v) porous CoFe-Se-P nanocomposites. (c) LSVs before and after 1000 CV cycles in N2-saturated 0.5 M H2SO4 solution of the porous CoFe-Se-P. (the Inset: Chronoamperometric technology at a constant overpotential of 70 mV of porous CoFe-Se-P for 40 h in N2-saturated H2SO4 solution. (0.5 M). overpotential is changed to 179.2 mV, and consistent with the iR-corrected. It demonstrates that the overpotential of the porous CoFe-Se-P composite is 24


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considerably lower than those of the previously reported non-noble metal catalysts (Table S4). As compared with other Co-based catalysts, the as-prepared porous CoFeSe-P nanocomposite usage is extremely low, only 0.14 mg·cm-2, which was calculated from the usage of the catalyst and the electrode area. It also shows the smaller Ƞ0 and Ƞ10 values and Tafel slope than those of Co@NG52 and exhibits comparable Ƞ10 and Tafel slope and obviously smaller Ƞ0 value than that of Ni-Co-MoS253. The Tafel plots of all samples, which were calculated using the formula mentioned in the Experimental section, are depicted in Figure 5b. The porous CoFeSe-P nanocomposite has a low Tafel slope of 58 mV·dec-1, which ias considerably lower than those of the hollow CoFe PBA (108 mV·dec-1) and close to those of the porous CoFe-Se (62 mV·dec-1) and CoFe-P (61 mV·dec-1) composites. Given that the hollow CoFe PBA has a low onset potential of 76.4 mV, the highest Tafel slope is obtained, implying the slow kinetics of the electrocatalytic activity. Nevertheless, the Tafel slope of the porous CoFe-Se-P is only slightly smaller than those of the porous CoFe-Se and CoFe-P nanocomposites, but it has a considerably low onset potential and overpotential to deliver, suggesting that synergistic effects could occur among these components. Phosphatization and selenylation treatment can enhance the electrocatalytic activity of transitional metal compounds. For instance, Xiao et al. reported that the P filling into the oxygen vacancies of Co3O4 may improve the electronic conductivity and consequently modify the electronic properties and create more active sites, thereby enhancing the electrocatalytic activity for HER and OER.54 Xiao et al. demonstrated that the electronic structure of the active sites is significantly 25



Page 26 of 50

modulated by introducing Se, which facilitates the interaction between metal ions and O-containing species in the oxygen electrocatalytic process55. For comparison, the HER performances of the solid CoFe PBA and CoFe-Se-P composites were also investigated (Figure S10), and Table S5 lists their onset potential and overpotential at a current density of 10 mA cm-2 as well as and their Tafel slopes. As shown by the polarization curves for the HER electrocatalytic activities (Figure S10a), the solid CoFe PBA exhibits very poor catalytic activity with a very high onset potential of 200.1 mV and requires a substantially large overpotential at a current density of 10 mA cm-2. However, after phosphatization and selenylation, the as-obtained CoFe-Se-P possesses a low onset potential (108.2 mV) and a small overpotential at a current density of 10 mA cm-2 (193.4 mV), but these values are greater than those of the porous CoFe-Se-P (η0=35.9 mV, η10=172.5 mV). As shown in Figure S10b, the solid CoFe-Se-P (61 mV·dec-1) has a very small Tafel slope, which izs very close to that of the porous CoFe-Se-P catalyst. This phenomenon suggests that the porous nanostructure of the catalyst facilitates the formation of additional active sites, further enhancing the electrocatalytic activity of the catalyst56. Compared with other transitional metal catalysts, such as porous structured Ni-Fe-P nanocubes, the as-prepared porous CoFe-Se-P also exhibits superior HER performance (Table S4) because of its hierarchical nanostructure, rich oxygen vacancies, and the synergism effect between the Se- and P-doped CoFecomposites. The stability of the porous CoFe-Se-P nanocomposite was evaluated by 26


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repeatedly conducting linear potential sweeps on the electrode at a scan rate of 5 mV·s-1 for 1000 cycles (Figure 5c). Only negligible decay is observed based on the polarization curves before and after 1000 cycles, indicating the excellent durability of the porous CoFe-Se-P electrocatalyst. Moreover, the current–time curve validated the long-term stability of the porous CoFe-Se-P nanocomposite for HER (the inset of Figure 5c). All of these results reveal that the porous CoFe-Se-P nanocomposite exhibits superior HER activity and excellent stability. The electrocatalytic activities of the series of porous nanocomposites, such as porous CoFe-Se, CoFe-P, and CoFe-Se-P for HER, were also determined in an alkaline solution of 0.1 M KOH. Table S6 shows their onset potential, overpotential at a current density of 10 mA cm-2, and Tafel slopes. As shown in Figure S11a, the porous CoFe-Se-P nanocomposite exhibits a high catalytic activity toward HER in 0.1 M KOH solution, requiring a low onset potential of 33 mV and an overpotential of 183.1 mV to achieve the current density of 10 mA cm-2. These values are smaller than those of the porous CoFe-P (η0 = −44 mV, η10 = −313.4 mV) and porous CoFe-P (η0 = −40 mV, η10 = −457.8 mV) composites. The porous CoFe-Se-P has the smallest Tafel slope of 181 mV dec-1, further confirming that the porous CoFe-Se-P nanocomposite is a promising HER electrocatalyst in alkaline electrolyte. To understand the improved HER performance of the porous CoFe-Se-P nanocomposite, the charge-transfer resistance (Rct) for HER was investigated through electrochemical impedance spectroscopy (EIS) at a HER overpotential of 250 mV (i.e., at a bias of −0.25 vs RHE). As shown in Figure S12, the Nyquist plots demonstrate 27



Page 28 of 50

that the porous CoFe-Se-P nanocomposite exhibits the smallest Rct value (106.8 ohm) among the different samples, including the solid CoFe-PBA (948.2 ohm), hollow CoFe PBA (765.1 ohm), porous CoFe-Se (317 ohm) and CoFe-P (202.4 ohm) nanocomposites. These findings suggest that the as-prepared porous CoFe-Se-P nanocomposite exhibits the fastest Faradaic process and the best HER kinetics. These characteristics could be attributed to the following aspects: (i) the amorphous structure contained oxygen vacancies, which can boost the charge transfer57; (ii) electron transfer can occur from metal species to Se and P in selenide and phosphide, further promoting electron transfer58; and (iii) the formed carbon matrix could serve as a buffer to release the structural changes during the electrocatalysis.59 Moreover, to evaluate the actual origin of the enhanced electrocatalytic performance, the electrochemical surface areas (ECSA) of the porous CoFe-Se-P nanocomposite and the other samples were predicted by measuring the double-layer capacitance (Cdl), because Cdl is proportional to the ECSA, which were derived from the CV curves of the hollow CoFe PBA, porous CoFe-Se, porous CoFe-P, and porous CoFe-Se-P composites (Figure S13)60,61. As shown in Figure S13e, the porous CoFe-Se-P nanocomposite possesses a considerably higher Cdl (8.23 mF·cm-2) than those of the hollow CoFe (4.9 mF·cm-2), porous CoFe-Se (4.585 mF·cm-2), porous CoFe-P (5.767 mF·cm-2) composites, implying high exposure of the active edge sites in the porous CoFe-Se-P nanostructure. Accordingly, the ECSA of the porous CoFe-Se-P nanocomposite (14.40 cm2) is the largest among four samples. As shown in Fig. S14, the N2 adsorption-desorption isotherm of the porous CoFe-Se-P was measured, form 28


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which the specific surface area was evaluated to be 24.84 m2 g−1. Combining the EIS, CV and N2 adsorption-desorption isotherm results, the porous CoFe-Se-P catalyst not only exhibits excellent electrochemical activity but also displays large catalytically active area.

Comparison of the OER performances of CoFe PBA and its derivatives. The OER activities of the as-prepared catalysts, including the RuO2, hollow CoFe-PBA, porous CoFe-Se, porous CoFe-P, and porous CoFe-Se-P nanocomposites, were assessed. The results are shown in Figure 6a, which shows the LSV curves of OER in O2-saturated KOH at 1600 rpm. Their onset potential, overpotential at a current density of 10 mA cm-2, and Tafel slopes are summarized in Table S7, which shows that that the four kinds of catalysts does not vary significantly. However, the results illustrate that the porous CoFe-Se-P exhibits a low onset potential of 1.07 V and requires a small overpotential of 1.44 V to achieve 10 mA·cm-2. And this value is changed to 1.45 V after iR-corrected for the porous CoFe-Se-P nanocomposites. Meanwhile, a CV test in 1 to 1.4 V was carried out inorder to eliminate the influence of faradaic pseudocapacitance. The result shows that the overpotential is changed to 1.45 V, consistent with the iR-corrected. These values are comparable to those of the commercial RuO2 catalyst (η0 = 1.03 V and η10 =1.38 V) and porous CoFe-P (η0 = 1.01 V and η10 = 1.47 V) and smaller those of the hollow CoFe PBA and porous CoFe-Se nanocomposites. In addition, the onset potential and overpotential are substantial smaller than those of recently reported high-performance Co- or Fe- based OER catalysts (Table S8) at the extremely catalyst usage. Also, it has a comparable 29



Tafel slope comparing with FeCo-N-doped CNT62 and NiCo/PFC aerogels63. This result indicate that the OER catalytic activity of the porous CoFe-Se-P is mainly attributed (a)

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Page 30 of 50

0

10

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40

1st cycle Time (h) 1000th cycles

0 1.0

1.2 1.4 1.6 Potential(V vs. RHE)

Figure 6. OER performances of the various catalysts. (a) LSVs of the different catalysts in 0.1 M KOH at room temperature at the scan rate of 10 mV·s-1 and (b) OER Tafel plots for the catalysts calculated from the LSVs of (i) RuO2, (ii) hollow CoFe PBA, (iii) hollow CoFe-Se, (iv) hollow CoFe-P, and (v) porous CoFe-Se-P nanocomposites; (c) The durability test and time-dependent current density (I–t) curve (the inset) of porous CoFe-Se-P in O2-saturated 0.1 M KOH solution. to the phosphatization processes.64 The corresponding Tafel slopes of all samples are calculated and shown in Figure 6b. The Tafel slope for the porous CoFe-Se-P is 108 30


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mV·dec-1, which is smaller than those of the hollow PBA (145 mV·dec-1), porous CoFe-Se (135 mV·dec-1), and CoFe-P (122 mV·dec-1) composites and very close to that of the RuO2 electrode (98 mV·dec-1). As mentioned, although the onset potential and overpotential at a current density of 10 mA cm-2 of the porous CoFe-Se-P is close to those of the porous CoFe-P, its Tafel slope is still smaller than that of the CoFe-P. It, indicates a favorable OER reaction kinetics, whichis promoted by the additional selenylation. For comparison, the OER performances of the solid CoFe PBA and its derivatives are also assessed. Their onset potential, overpotential at a current density of 10 mA cm-2, and Tafel slopes are summarized in Table S9. As shown in Figure S15a, the two samples have very small onset potentials but require a very large overpotential at a current density of 10 mA cm-2 because of their slow electron transfer. However, the solid CoFe PBA and CoFe-Se-P catalysts have very small Tafel slopes of 117 and 112 mV·dec-1, respectively, further confirming the fast kinetics of the OER process of these catalysts. This phenomenon is possibly due to the formation of the porous CoFe-Se-P nanostructure during the pyrolysis procedure, which can improve the OER catalytic activity. In addition, the OER performance of CoFe-Se-P nanocomposite is only slightly enhanced by the etching treatment of the CoFe PBA, which is in contrast to the result of its HER investigation. In addition to the electrocatalytic activity, the durability is essential for OER catalysts because of the harsh oxidative working environment. The durability of the porous CoFe-Se-P nanocomposite was examined by fixed-potential electrolysis at an overpotential of 1.44 V for 40 h. Along with time, the current is no significant change 31



Page 32 of 50

under this potential, implying that the porous CoFe-Se-P nanocomposite is stable over a long period (Figure 6c). The slight fluctuation in the current density is due to the adsorption of the as-formed oxygen bubbles on the electrode (the inset of Figure 6c). Furthermore, the PXRD, XPS, and SEM measurements were also carried out for the porous CoFe-Se-P nanocomposite toward OER after the 40 h test in 0.1 M KOH, as shown in Figures S18, S19, and S16, respectively. It shows that that no substantial change in the PXRD patterns is observed before and after the oxygen reaction for 40 h (Fig. S18). As can be seen from Fig. S19, the similar deconvulated XPS peaks for Co 2p, Fe 2p, Se 3d, and P 2p are observed for the used catalyst with the fresh one. Also, the SEM image of the used porous CoFe-Se-P nanocomposite does not show any change before and after used. It indicates that the proposed catalyst is very stable and exhibits high durability during the OER procedure.

Water splitting activity of the as-prepared porous CoFe-Se-P catalyst. Considering that the porous CoFe-Se-P nanocomposite displays good HER and OER activities in alkaline solution, we investigated its performance as a bifunctional catalyst for water slitting. As shown in Figure 7a, the electrode shows an excellent HER catalytic activity when a cathodic potential is applied (i.e., E < E0 (H2|H2O)), whereas the porous CoFe-Se-P electrode exhibits superior OER activity when an anodic potential above 1.23 V (the thermodynamic threshold for O2 evolution) is applied. The voltage (∆E) required to yield 10 mA cm−2 for both HER and OER (∆E = EOER, j=10 − EHER, j=10) is utilized to assess the water splitting performance. A smaller ∆E value corresponded to a better catalytic activity in the water electrolysis process.65 32


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The voltage of the porous CoFe-Se-P nanocomposite is 1.66 V, which is considerably lower than those of the porous CoFe-Se (1.89 V), CoFe-P (1.83 V), and RuO2 (2.10 V) and close to that of the Pt/C (20%) (1.63 V) composites, demonstrating the outstanding bifunctional activities of the porous CoFe-Se-P nanocomposite. Moreover, compared with other Co- and Fe-related catalysts for water splitting, such as Ni−Fe−P nanocubes (1.68 V)56 and Co/N-carbon (1.69 V)66, the as-obtained porous CoFe-Se-P derived from the hollow CoFe- PBA exhibits excellent catalytic ability. Therefore, the porous CoFe-Se-P nanocomposite loaded onto the nickel foam was applied as both the cathode and the anode to assess its potential application in water electrolysis devices. Figure 7b presents the LSV curves obtained by a two-electrode water electrolyzer for the porous CoFe-Se-P nanocomposite, which yields 10 mA cm-2 at a cell voltage of approximately 1.59 V. This value is considerably smaller than that of the Ni foam and the porous CoFe-Se nanocomposite but is close to those of RuO2, Pt/C (20%), and porous CoFe-P nanocomposite (1.61 V), indicating that the CoFe-SeP nanocomposite possesses excellent water splitting performance. Furthermore, the CoFe-Se-P nanocomposite is very stable, as evidenced by its chronopotentiometric curve at a constant current density of 10 mA cm-2 (Figure 7c). This anodic peak shifts to 1.38 V for the CoFe-Se-P/NF electrode and to 1.45 V for the Pt/C/NF electrode, further confirming the good catalytic activity of the as-prepared CoFe-Se-P catalyst. The porous CoFe-Se-P electrolyzer delivers a current of 10 mA cm-2 at a cell voltage of 1.59 V in 1.0 M KOH, which exceeds those of the porous CoFe-Se and CoFe-P composites and is close to those of RuO2 and the benchmark Pt/C electrode. The 33



activity of the porous CoFe-Se-P nanocomposite is much higher than those of many other recently reported transitional metal oxides, phosphides, or selenides (Table S11) when the porous CoFe-Se-P catalyst usage is only 0.01 mg·cm-2. Similar to other bihybridized, phosphorized, and selenylated compounds, the present porous CoFe-SeP nanocomposite exhibits an outstanding water splitting performance. For example, Feng et al reported that although the EG/H-Co0.85Se-P nanosheets exhibited excellent HER

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1.2 1.4 1.6 Potential (V)

1.8

(c)

20

-2

J (mA•cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

-20 -40 0

10

20 Time (h)

30

40

Figure 7. (a) Polarization curves of (i) RuO2, (ii) Pt/C, (iii) porous CoFe-Se, (iv) CoFe-P, and (v) CoFe-Se-P nanocomposites for overall water splitting performance in the three-electrode setup at the scan rate of 5 mV·s−1. (b) Polarization curves of (i) RuO2, (ii) Pt/C, (iii) porous CoFe-Se, (iv) CoFe-P, (v) CoFe-Se-P nanocomposites, 34


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and (vi) Ni foam in the two-electrode electrolyzers at the scan rate of 5 mV·s−1. (c) Long-term durability test of the CoFe-Se-P electrolyzer at 10 mA·cm−2 and the photograph during the overall water splitting (the inset).

performance in 1.0 M KOH, it required an overpotential of 1.64 V at a current density of 10 mA cm-2 in the water splitting.67 This value was higher than that of the present porous CoFe-Se-P nanocomposite. In addition, the excellent low potential of the porous CoFe-Se-P nanocomposite was comparable to or lower than the values for recently reported catalysts, such as Co–P films (1.64 V),68 NiSe nanowire film/NF (1.63 V),69 CoP nanorods (1.62 V),70 and Ni3Se2 nanoforest/NF (1.61 V).71 Furthermore, as shown in the inset of Figure 7c, gas bubbles were released at the respective electrodes at a cell voltage of 1.5 V. The proposed porous CoFe-Se-P-based electrolyzer can be stably operated for 40 h while attaining 10 mA cm-2 at −1.59 V.

Possible mechanism for the enhanced electrocatalytic activity of the porous CoFe-Se-P nanocomposite. As discussed in the section on chemical, structural and compositional characterizations, the porous CoFe-Se-P nanocomposite displays outstanding electrocatalytic activity, which can be attributed to the following reasons: (i) the synergistic effect among Co/Fe, CoP, Fe2P, CoSe2, Co3Se4, FeSe2, and Fe3Se4. The co-existence of these compounds would produce more grain boundaries at the interfaces, thereby producing additional active sites. In the as-obtained hybrid, P and Se act as proton-acceptor centers, whereas Co and Fe serve as hybrid-acceptor centers, thereby promoting the formation of Co-hydride or Fe-hydride for subsequent HER via electrochemical desorption.72,73 However, for OER, the positively charged Co and Fe 35



Page 36 of 50

species and the negatively charged Se centers could serve as hydroxyl acceptors and boost hydroxyl adsorption in the Co or Fe centers, further promoting the oxygen evolution through discharge and desorption;74,75 Moreover, during the oxidation decomposition of CoFe PBA, the redox reaction between Fe3+ and Co2+ results in the formation of Co3+, which is considered an active site for OER. The abundant Fe3+ in CoFe bimetallic catalysts can substantially improve the oxidation of Co2+ to Co3+, which facilitate the formation of surface adsorbed OH* and O* intermediates, thereby further improving the catalytic activities of OER. (ii) More edge active sites are exposed and a more fluent electron-transfer access is achieved due to the porous nanostructures.76 (iii) The integration of phosphatization and selenylation for the porous CoFe-Se-P77 and the abundant oxygen vacancies facilitate good conductivity.78 (iv) The amorphous carbon matrix could act as a buffer to release structural changes during electrocatalysis. a (v) The presence of Co-Nx and Fe-Nx in the porous CoFeSe-P result in electron modulation by changing the charge distribution and electronic performances.79

CONCLUSION In summary, we synthesized a novel hollow nanostructured CoFe PBA nanobox by using the FeFe PB as the template. After pyrolysis through the combined phosphatization and selenylation, the as-obtained quarterly porous CoFe-Se-P nanoparticle was exploited as an efficient novel bifunctional catalyst for HER and OER. Compared with other derivatives from the bulk solid CoFe PBA or hollow CoFe PBA, such as CoFe-Se-P and porous CoFe-Se or CoFe-P, the porous CoFe-Se-P 36


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exhibits efficient electrocatalytic performance with low overpotentials of 172.5 and 1.44 V to reach a current density of 10 mA·cm-2 and Tafel slopes of 58 and 108 mV dec-1 for the HER and OER, respectively. The results of this study confirm that the electrocatalytic activities of the porous CoFe-Se-P are markedly superior to those obtained for the Co- or Fe-related electrocatalysts. In addition, the porous CoFe-Se-P offers excellent durability and methanol tolerance for HER and OER. Our findings expand the design of electrodes made of transition metal phosphides or selenides for HER, OER, Li-ion batteries, and other applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is is available free of charge on the ACS Publications website at DOI: Preparation of the FeFe@CoFe PBA; PXRD patterns, XPS spectra, HER performance, EIS spectra, CV curves of hollow CoFe PBA, porous CoFe-Se, porous CoFe-P and porous CoFe-Se-P nanocomposites; SEM and TEM images of porous CoFe-Se and CoFe-P nanocomposites; Tables of parameter for HER and OER performances. (Word).

AUTHOR INFORMATION Corresponding Author Tel.: +86-37186609676. E-mail: [email protected] (Z.Z.). ORCID Zhihong Zhang: 0000-0002-5888-4107 37



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Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by Programs for the National Natural Science Foundation of China (NSFC: Account Nos. U1604127, U1704256 and 21601161), Backbone Teacher Project (2017GGJS091) and Innovative Technology Team of Henan Province (CXTD2014042).

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17288-17298.


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Graphical Abstract

A

novel

quarterly

electrocatalyst

of

mesoporous

cobalt/iron

phosphorous-selenide nanocomposites (CoFe-Se-P) was derived from hollow CoFe Prussian blue analogues (CoFe-PBA) and combined by phosphatization and selenylation and employed as an efficient and durable bifunctional catalysis performance for hydrogen evolution reaction and oxygen evolution reaction.


Mesoporous Nanostructured CoFeâSeâP Composite Derived from a

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