FeCoPxOy(OH)z as Bifunctional Electrodeposited-Film

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FeCo/FeCoPxOy(OH)z as Bifunctional ElectrodepositedFilm Electrodes for Overall Water Splitting Fu-Te Tsai, Hsuan-Chi Wang, Chun-Hung Ke, and Wen-Feng Liaw ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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ACS Applied Energy Materials

FeCo/FeCoPxOy(OH)z as Bifunctional Electrodeposited-Film Electrodes for Overall Water Splitting

Fu-Te Tsai,* Hsuan-Chi Wang, Chun-Hung Ke, and Wen-Feng Liaw*

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan

1

ACS Paragon Plus Environment

ACS Applied Energy Materials 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

Abstract Development of the efficient/robust/economical electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is emerging as a grand challenge. In consideration of OER activity of iron/cobalt oxides and the superior electro-conductivity/propitious H atom binding energy of metallic iron/cobalt, bifunctional electrodeposited-film electrode CFeCoP composed of CoFe and CoFePxOy(OH)z was prepared from irreversible cathodic deposition of FeSO4 and CoSO4 on the surface of graphite plate in 1 M phosphate buffer (pH = 7). The as-prepared CFeCoP electrode exhibits excellent HER activity (specific activity (js) = 0.169 mA/cm2) with low charge transfer resistance (4.5 Ω) and overpotential of 57 mV achieving current density of 10 mA/cm2, and also shows OER activity (js = 1.316 mA/cm2) with low charge transfer resistance (7.8 Ω) and 282 mV overpotential approaching a current density of 10 mA/cm2 in 1 M NaOH aqueous solution. In addition, CFeCoP electrode displays long-term stability (139 h) for both HER and OER activity with stable current density. In the whole cell, CFeCoP-CFeCoP electrode-pair setting achieves current density of 10 mA/cm2 at voltage 1.56 V (Tafel slope of 51 mV/dec), close to those (10 mA/cm2 at voltage of 1.58 V (Tafel slope of 68 mV/dec)) of Pt-IrO2 electrode-pair device in 1 M NaOH aqueous solution. On the basis of XPS, SEM-EDX and pXRD, the fabric of metallic iron/cobalt buried in metal-oxide matrix and iron/cobalt (oxy)hydroxides embedded in metal-oxide matrix as films on the graphite surface for CFeCoP electrode implicates that the electrodeposition-derived film majorly made up of FeCo and FeCoPxOy(OH)z may mediate the kinetics occurred at the interface of catalyst-electrolyte and display stability/intrinsic catalytic activity for overall water splitting. KEYWORDS: FeCo-film electrode, electrocatalyst, alkaline hydrogen evolution reaction, alkaline water splitting, low overpotential, high stability 2


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Introduction Driven by global energy markets becoming more carbon-constrained, a growing awareness of the issues about anthropogenic climate change and an increase in global energy demand have promoted the research for storing the clean/renewable energy. Electro- and photo-catalytic methods for generating hydrogen and oxygen from water have been explored as promising and cost-effective ways for the conversion and storage of renewable energy sources.1-5 In order to increase kinetic rates and lower applied potential of electrocatalytic water splitting, efforts have been devoted to develop economical/robust/efficient catalysts efficiently performing hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).6-10 Since platinum and iridium are limited by its scarcity and high cost, the design and synthesis of state-of-the-art electrocatalysts composed of non-noble metal and even metal-free materials remains a grand challenge. Innovative breakthroughs based on molecular catalysts consisting of first-row transition metals were reported in the past decades for homogeneous electrocatalytic HER and OER.11-15 The heterogeneous catalysts comprising

both

transition-metal

chalcogenides/pnictides/carbides/borides

and

non-metal materials showing catalytic HER in aqueous solution were reported.16 Also, heterogeneous

OER

electrocatalysts

composed

of

Co/Ni/Mn/Fe/Cu

oxides/hydroxides/oxyhydroxides with remarkable OER activities were demonstrated in alkaline aqueous solution.17 Practical implementations of these materials as catalytic electrodes in an integrated electrolyzer are often hampered by the disparity in electrolytes although both HER and OER catalysts remain the high activities and longevity. Presumably, integration of HER and OER to construct the efficiently bifunctional electrocatalysts that possess binding affinity to hydrogen- and oxygen-containing intermediates is of great benefit for improving the efficiency of overall water splitting. In pursuit of economical catalysts displaying low working 3



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overpotential, long-term stability, tolerance for wide pH-range and compatibility with diverse water source, design and synthesis of the bifunctional electrocatalysts active for overall water splitting in the same electrolyte is most desirable.3-4, 18 The promising electrocatalytic activity and stability of Ni-based bifunctional electrocatalysts, including NiFe LDH, NiSe, Ni4Mo, Ni/Mo2C/porous C, NiFeOx, Ni/N-doped graphene, Cu@NiFe LDH, Ni3FeN, Ni11(HPO3)8(OH)6 and so on, are well-documented.10,19-26

Although

good

OER

performance

of

CoFe-based

electrocatalysts was demonstrated, the investigation of CoFe-based films used as electrocatalysts for HER and overall water splitting is limited.27-29 Recently, it was reported that cobalt iron hydroxide on nickel foam required the overpotential of 110 mV and the cell voltage of 1.64 V to achieve current density of 10 mA/cm2 for alkaline HER and water splitting, respectively.30 In an effort to develop CoFe-based bifunctional electrocatalysts for HER and overall water splitting with low overpotential and rationalize catalytic activity through the electronic modification by alloying iron with cobalt, an economic and facile method based on cathodic deposition of FeSO4 and CoSO4 was utilized for the preparations of Fe-based, Co-based and CoFe-based electrodes. The reductive deposition of FeSO4 and CoSO4 generate catalytically active material as film on the graphite plate surface in neutral phosphate buffer that acts as active HER and OER electrodes (CFeP and CCoP electrodes). In particular, iron-cobalt-alloying film electrode CFeCoP derived from FeSO4 (2 mM) and CoSO4 (2 mM) cathodically deposited on graphite plate in neutral phosphate buffer shows enhanced activity for HER and OER. Integration of the electrodeposited-film

electrodes

into

a

single

electrode-pair

device

(CFeCoP-CFeCoP electrode-pair setting) for electrocatalytic water splitting exhibits overpotential of 330 mV (410 mV) achieving a current density 10 mA/cm2 (20 mA/cm2) and Tafel slope of 51 mV/dec in alkaline electrolyte (Tables 1 and 2). 4


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Results and Discussion Electrochemical Study. In consideration of non-innocent property of nitric oxide and delocalized covalent character of Fe-NO bond to regulate the electron density of [Fe(NO)2] motif for chemical reactions in two-electron regime, it is presumed that dimeric {Fe(NO)2}10 DNIC, [(N2,N2-BADI)(Fe(NO)2)2] (1) (N2,N2-BADI = 2-N, 3-N-bis[2-(dimethylamino)ethylbutane-2,3-diimine)] (Figure S1), may serve as molecular electrocatalyst to trigger hydrogen evolution from water, relying on the reversible redox couple between {Fe(NO)2}9 DNIC and {Fe(NO)2}10 DNIC, the flexible coordination number and the adjustable [Fe(NO)2]-[Fe(NO)2] distance.14 However, the shapes of CV curves showing crossing character and a “S-shape (sigmoidal)” catalytic curve not achieved reflect that both homogeneous and heterogeneous catalytic processes may run in parallel at early stage of water reduction, and the electrode introduces the nucleation sites for electrodeposition of metal-containing species as catalytically active materials on the electrode surface under phosphate buffer aqueous electrolyte. On the basis of this observation, the bifunctional electrocatalyst CFeP electrode prepared from facile cathodic deposition of the economic FeSO4 in 1 M neutral phosphate buffer was demonstrated (Figure S2). The electrodeposition of FeSO4 was conducted by CV scans of a graphite working electrode in 1M neutral phosphate buffer. As shown in Figure S2a, HER catalytic current densities increase with the repeated CV scans of FeSO4-containing 1 M neutral phosphate buffer employing graphite plate as working electrode, which is a typical feature of electrodeposition of active species as heterogeneous catalyst on the graphite surface.6-7,10 To investigate the active species cathodically deposited on the graphite surface, controlled potential electrolysis was performed at -1.0 V (vs RHE) with 2 mM FeSO4 dissolved in phosphate buffer. The graphite plate was used as auxiliary electrode to eliminate Pt contamination. The current density increases from 5



53 mA/cm2 to 168 mA/cm2 over the course of 2.5 h (Figure S2b). The sustained current density is accompanied by the formation of a dark electrodeposited film on the surface of graphite electrode, CFeP electrode, along with H2 effervescence. The modified electrode CFeP was then transferred to a FeSO4-free, freshly-prepared 1 M NaOH electrolyte to assess its electrocatalytic properties. As shown in Figures 1a and 1b, linear sweep voltammetry (LSV) curves indicate that CFeP achieves current density of 10 mA/cm2 with overpotential of 193 mV and Tafel slope of 52 mV/dec for HER electrocatalysis in 1 M NaOH electrolyte. Since metal cobalt has been calculated to have low energy barrier for H atom absorption, cobalt-alloying composite may be a promising material to increase HER catalytic activity.8 CCoP and CFeCoP electrodes were prepared from extended electrolysis with applied potential -1.0 V (vs RHE) in 1 M neutral phosphate buffer containing CoSO4 (2 mM) and FeSO4/CoSO4 (2 mM/2 mM), respectively. The current density reaches an asymptotic limit of 275 and 420 mA/cm2 over the course of 2.5 h (Figures S3-S4), and the sustained current is accompanied by the formation of CCoP and CFeCoP films on the graphite surface, respectively, along with H2 gas release. Impressively, the electrodeposited iron-cobalt-alloying-film electrode CFeCoP displays excellent HER activity with overpotential of 57 mV to achieve current density of 10 mA/cm2 and Tafel slope of 40 mV/dec, as compared with those (180 mV overpotential reaching 10 mA/cm2 and Tafel slope of 57 mV/dec) of CCoP electrode in 1 M NaOH aqueous solution (Figures 1a-1b). Presumably, Tafel slopes of 52, 57 and 40 mV/dec for CFeP, CCoP and CFeCoP electrodes, respectively, may suggest that HER catalytic process involves Volmer-Heyrovsky mechanism, indicating electrochemical desorption of hydrogen as rate-determining step.31-33 As collected in Table S1, these results demonstrate that CFeCoP electrode is a highly efficient HER electrocatalyst, better than other reported nonprecious HER electrocatalysts deposited 6


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on carbon-based and metal-based support in alkaline condition. The comparable polarized curves of CFeP, CCoP and CFeCoP electrodes before and after 5000 cycles suggest that CFeP, CCoP and CFeCoP are stable HER electrocatalysts in alkaline condition (Figure S5). During the long-term CPE in 1 M NaOH aqueous solution employing CFeP and CFeCoP as working electrodes at applied overpotential of 200 mV for 139 h, the stable current density of 11 and 100 mA/cm2 with total charge accumulation of 883 and 8045 Coulomb were observed, respectively (Figure 1c). The potential cycling and amperometric studies demonstrate that CFeCoP could act as a robust and efficient film-electrode for HER in alkaline condition. To inspect the possibility of CFeP, CCoP and CFeCoP electrodes for overall water splitting, evaluation of OER catalytic activity in alkaline condition was conducted. As shown in Figure 1d, LSV investigation on OER activity reveals that CFeP and CCoP electrodes demanded the overpotential of 400 and 330 mV to reach the current density of 10 mA/cm2 (Tafel slopes of 73 and 75 mV/dec) in 1 M NaOH aqueous solution, respectively. The overpotential of 282 mV is required to achieve current density of 10 mA/cm2 (Tafel slope of 44 mV/dec) for CFeCoP electrode, comparable to that (10 mA/cm2 at overpotential of 320 mV (Tafel slope of 50 mV/dec)) of IrO2 catalyst in alkaline condition (Figures 1d and 1e). For OER longevity test (Figure 1f), CFeP and CFeCoP electrodes exhibit good stability with current density of 1.6 and 100 mA/cm2 and total charge accumulation of 130 and 8042 Coulomb for 139 h at applied overpotential of 335 mV, respectively, in aqueous NaOH electrolyte. As summarized in Table S2, CFeCoP electrode acts as a new/efficient/robust OER electrocatalyst in the noble-metal-free, carbon- and metal-based-support family in alkaline condition. Because the long-term stability test is a dynamic process, the slight variation in current density observed in Figures 1c and 1f may be ascribed to the factors including bubble adsorption and desorption on the 7



surface of the electrodes. Double-layer capacitance (Cdl), proportional to electrochemically active surface area (ECSA), was obtained by CV measurements to rationalize the better performance of CFeCoP electrode in catalytic HER and OER. Figures 2a and 2d describe the current densities as function of scan rates obtained from the corresponding CV curves (Figure S6) to calculate Cdl, ECSA, roughness factor (RF) and specific activity (js) for CFeP, CCoP and CFeCoP electrodes.4, 34-35 As shown in Figure 2a and Table 1, the Cdl of 5.27 mF/cm2 displayed by CFeCoP electrode is higher than those of CFeP (3.85 mF/cm2) and CCoP (3.56 mF/cm2) electrodes. It is presumed that the enhanced HER activity for CFeCoP electrode is due to the larger ECSA which makes great exposure of active sites to water molecules and is beneficial to the efficiency of mass transfer and gas diffusion. The specific activity and RF-normalized current density of CFeCoP electrode (js = 0.169 mA/cm2) which are significantly larger than those of CFeP (js = 0.015 mA/cm2) and CCoP (js = 0.023 mA/cm2) electrodes implicate that electrode kinetics may play a significant role in catalytic HER process (Figure 2b). Electrochemical impedance spectroscopy (EIS) is utilized to probe film resistance and the kinetics of electrocatalytic reaction that occurs at the interface between electrode and electrolyte.36-38 In contrast to the significant film resistance (Rf) and charge transfer resistance (Rct) displayed by CFeP (Rf = 2.1 Ω, Rct = 16.2 Ω) and CCoP (Rf = 1.4 Ω, Rct = 13.4 Ω) electrodes, CFeCoP electrode exhibits the smaller Rf (0.4 Ω) and Rct (4.5 Ω) at overpotential of 190 mV (Figure 2c). Compared to CFeP and CCoP electrodes, the CFeCoP electrode enjoins the favorable electron transport in HER process, leading to the smaller Tafel slope. In the aspect of OER electrocatalysis, CV study shows that Cdl value of CFeCoP electrode (4.60 mF/cm2) is higher than those of CFeP (3.65 mF/cm2) and CCoP electrodes (4.13 mF/cm2) (Figure 2d), indicative of the larger ECSA and RF values of CFeCoP electrode (Table 1). It is 8


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noticed that the specific activity and RF-normalized current density of CFeCoP electrode (js = 1.316 mA/cm2) is significantly larger than those of CFeP (js = 0.024 mA/cm2) and CCoP (js = 0.171 mA/cm2) electrodes (Figures 2e). The smaller Rf (1.1 Ω) and Rct (7.8 Ω) of CFeCoP electrode rationalize the larger specific activity and the enhanced RF-normalized current density, in comparison with CFeP (Rf = 5.0 Ω, Rct = 53.3 Ω) and CCoP (Rf = 2.0 Ω, Rct = 17.2 Ω) electrodes at overpotential of 350 mV (Figures 2f). Furthermore, the intrinsic activity of CFeP and CFeCoP electrodes were evaluated by determining the mass activities and turnover frequencies (TOF) by assuming that every metal ion deposited on the surface of graphite plate is electrocatalytically active (details shown in Experimental Section). As shown in Table S4, CFeCoP electrode exhibits higher TOF of 0.0024 s-1 per mole of metal (Fe + Co) and mass activity of 5.83 A/g for HER, as compared with those (TOF = 0.0003 s-1, mass activity = 0.77 A/g) of CFeP electrode. Also, the measured TOF (Fe + Co) and mass activity of CFeCoP electrode are 0.0091 s-1 and 41.37 A/g for OER, respectively, outperforming CFeP electrode (TOF = 0.0005 s-1, mass activity = 1.60 A/g). The study of intrinsic activity (js, TOF and mass activity) suggests that CFeCoP electrode is superior to CFeP electrode for HER and OER in alkaline condition. Characterization of Electrodeposited Films. SEM-EDX, FT-IR, pXRD and XPS were adopted to probe the morphology and composition of CFeP and CFeCoP electrodes. The scanning electron microscopic (SEM) images of CFeP and CFeCoP electrodes show nearly complete coverage of the particles distributed on the graphite surface. Cross-sectional SEM image shows that film thickness of CFeP and CFeCoP electrodes is 45.3 µm and 86.2 µm, respectively (Figure S7). In parallel with the observation of a broad peak at 44o in powder X-ray diffraction (pXRD) patterns of CFeP electrode for HER and CFeP/CFeCoP electrodes for OER, CFeCoP electrode for HER exhibits a strong diffraction peak at 44.6o ascribed to (110) plane of 9



crystalline α-iron (ICSD PDF No. 01-071-4648 6-696) and (111) plane of metallic cobalt (Figures S8-S11),8 consistent with Fe 2p and Co 2p XPS results (Figures 3b-3c). Fourier-transform infrared spectroscopy (FT-IR spectrum) was used to disclose the possible chemical compositions and bonding states of CFeCoP electrode. As shown in Figure S12, the broad peak around 3360 cm-1 could be indexed to the stretching mode of hydrogen-bonded hydroxyl groups from hydroxide layers and interlayer water.39 The presence of phosphate (PO43-) is evidenced by its fingerprint peaks at 1015 and 1660 cm-1.40 The spectrum at the high wavenumber (3450-3700 cm-1) may correspond to the O-H bond vibration modes of metal-hydroxides species.41 Also, the peaks observed in the range from 450 to 900 cm-1 may originate from the vibrational modes of metal-oxygen (M-O) and metal-hydroxide (M-OH) bonds. The peaks at 521 cm-1 is assigned to the vibration of Fe-O bond. The stretching frequency of 744 cm-1 is attributed to Co-OH bond, and two peaks at 620 and 683 cm-1 are ascribed to Co-O vibrational modes.39.42 The energy-dispersive X-ray (EDX) spectra (SEM-EDX mapping) of CFeP and CFeCoP electrodes show no signal arising from noble-metal elements. As opposed to EDX spectroscopy probing deeply into the cathodically deposited films, XPS probes the chemical composition on the material surface.7 The X-ray photoelectron spectra (XPS) of the freshly electrodeposited CFeP and CFeCoP electrodes show the presence of iron, cobalt, oxygen and phosphorus on the surface. In addition to phosphate (PO43-), absorbed water and sodium KLL auger showing the peaks located at 531.8, 533.2 and 535.4 eV, respectively, the deconvolution of O 1s XPS spectra reveals additional two peaks at 529.9 and 530.9 eV, corresponding to metal oxide and metal hydroxide, respectively (Figure S13).45-47 The P 2p regions of CFeP and CFeCoP electrodes exhibit two peaks with 133.4 and 134.4 eV binding energies (ratio of 2 : 1) attributed to 2p3/2 and 2p1/2 core levels of phosphorus in phosphate species (Figure S14).7 XPS spectra of 10


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CFeP electrode show the Fe/P atomic ratios of [1 :1.1] and [1 : 0.3] for HER and OER, respectively, while, the EDX spectra display the Fe/P atomic ratios of [1 : 0.2] and [1 : 0]. For CFeCoP electrode, the Fe/Co/P atomic ratios of [1 : 1.6 : 2.0] and [1 : 1.8 : 1.4] in XPS spectra and ratios of [1 : 1.4 : 0] and [1 : 1.2 : 0.1] in EDX spectra were observed for HER and OER, respectively (Tables S5 and S6). These results demonstrate that the compositions of CFeP and CFeCoP films are not homogeneous between bulk and surface. The high-resolution XPS spectra of Fe 2p and Co 2p of the freshly electrodeposited CFeP and CFeCoP electrodes are described in Figure 3. In addition to the peaks at 709.5 eV for 2p3/2 and 723.1 eV for 2p1/2 with shake-up satellites assigned to ferrous species, the deconvolution of Fe 2p XPS spectrum of CFeP electrode reveals the presence of ferric species showing two signals corresponding to Fe 2p3/2 (711.5 eV) and Fe 2p1/2 (725.2 eV) with the satellite peaks. Interestingly, upon conducting OER electrocatalysis for 10 min in 1 M NaOH solution, the Fe 2p XPS spectrum of CFeP indicates the presence of ferric state (Figures 3a and 3d). For CFeCoP electrode in HER, the two major peaks located at 706.8 eV (2p3/2) and 720.1 eV (2p1/2) indicate the presence of metallic iron. Two core-level peaks of Fe centered at 709.5 and 723.1 eV are ascribed to Fe2+ 2p3/2 and Fe2+ 2p1/2, respectively, accompanied with two satellite peaks (Figure 3b). As shown in Figure 3c, in addition to the peaks of 778.3 and 793.4 eV binding energies assigned to Co 2p3/2 and Co 2p1/2 of metallic cobalt, respectively, two signals corresponding to Co 2p3/2 (781.1 eV) and Co 2p1/2 (797.0 eV) with the satellite peaks suggest the presence of Co2+ species.43-47 Presumably, metallic iron/cobalt which typically offer rapid electron transportation may serve as active sites for HER catalysis, consistent with small Rf (0.4 Ω) and Rct (4.5 Ω) observed in EIS. After OER electrocatalysis for 10 min in 1 M NaOH solution, it is noticed that 11



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no signal assigned to metallic iron/cobalt was observed in XPS spectra of CFeCoP (Figures 3e-3f). CFeCoP retains the nature of Co2+ along with the other two peaks at 779.7 (2p3/2) and 795.6 eV (2p1/2), suggesting the presence of Co3+ state.44-45,47 In addition, ferrous species were observed with newly emerged binding peaks at 711.5 (2p3/2) and 725.2 eV (2p1/2), indicating the presence of Fe3+ state.43-44,46-47 Presumably, the formation of iron/cobalt (oxy)hydroxides under anodic potential acts as catalytic sites for OER in alkaline solution,45-46 while iron/cobalt oxides may serve as the conductive and chemically stabilizing host. In contrast to CFeP showing large Rf (5.0 Ω) and Rct (53.3Ω) in alkaline OER catalysis, the smaller Rf (1.1 Ω) and Rct (7.8 Ω) of CFeCoP electrode may be attributed to the synergistic cooperation between iron/cobalt (oxy)hydroxides and iron/cobalt oxides (Figure 2f and Table 1). Conclusively, CFeCoP electrode composed of metallic iron/cobalt (FeCo) and phosphate-containing iron/cobalt (oxy)hydroxides (FeCoPxOy(OH)z) serving as a bifunctional catalytic material for overall water splitting may be suggested. Electrocatalytic Water Splitting. Compared to CFeP-CFeP and CCoP-CCoP electrode-pair devices achieving the current density of 10 mA/cm2 (20 mA/cm2) at operation voltages 1.75 V (1.81 V) and 1.71 V (1.76 V), respectively, the applied voltages of 1.56 V and 1.64 V are required for CFeCoP-CFeCoP electrode-pair setting reaching current density of 10 mA/cm2 and 20 mA/cm2, respectively, which are close to those (10 and 20 mA/cm2 at voltage 1.58 V and 1.67 V, respectively) of Pt-IrO2

electrode-pair

setting

in

1

M

NaOH

aqueous

solution.

Also,

CFeCoP-CFeCoP electrode-pair setting exhibits the smaller Tafel slope (51 mV/dec) than those of CFeP-CFeP (77 mV/dec), CCoP-CCoP (67 mV/dec) and Pt-IrO2 (68 mV/dec) electrode-pair settings (Figures 4a-4b and Table 2), implicating that film derived from alloying iron with cobalt improves kinetic activity for alkaline water splitting. In comparison with CFeP-CFeP electrode-pair setting exhibiting stable 12


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current density of 25 mA/cm2 with charge accumulation of 723 C for 50 h, CFeCoP-CFeCoP and CCoP-CCoP electrode-pair devices maintain the steady current density of 100 and 40 mA/cm2 with nearly linear charge accumulation of 8016 C and 3249 C for 139 h electrolysis, respectively, at applied overpotential of 600 mV in 1 M NaOH aqueous solution (Figure 4c). Gas chromatography (GC) measurements were conducted to detect H2 and O2 produced from overall water splitting employing CFeP-CFeP, CCoP-CCoP and CFeCoP-CFeCoP electrode-pair settings. Faradaic efficiency, calculated from the quantitation of the theoretical and experimental hydrogen/oxygen (ratio of H2:O2 = 2:1) generated during water splitting, is approximate ~100% (Figure S15). As summarized in Table S3, the efficiency of CFeCoP-CFeCoP electrode-pair setting for alkaline water splitting is comparable to the reported non-noble metal bifunctional electrocatalysts deposited on either carbon-based

or

metal-based

support.

Notably,

a

two-electrode

system

CFeCoP-CFeCoP electrode-pair device which operates at a battery voltage of 1.5 V drives overall water splitting with observable gas bubble release (Figure S16, see video in Supporting Information). These results highlight the advantages of CFeCoP electrode as a promising catalytic-active electrodeposited-film electrode for water splitting in the same alkaline electrolyte. The morphology and chemical composition of CFeCoP electrocatalyst on the cathode and anode after longevity test were examined. Figures S17 and S18 show SEM and SEM-EDX elemental mapping images of CFeCoP electrodes after alkaline water splitting for 139 h. Compared with size redistribution of the agglomerated particles (diameter ~ 10 µm) to small-scale particles (diameter ~ 1 µm) for CFeCoP cathode, CFeCoP anode consists of platelets (1~ 5 mm in length) that tend to be vertically oriented and randomly distributed on the electrode surface, implying that phase transformation may occur during long-term water electrolysis in 1 M NaOH 13



aqueous solution. In consideration of EDX elemental mapping showing the significant increase in sodium and oxygen content inside the film, the Fe/Co/P atomic ratios of [1 : 1.2 : 0] and [1 : 1.3 : 0.2] for CFeCoP cathode and anode, respectively, are observed in EDX spectra. Also, XPS spectra indicate the Fe/Co/P atomic ratios are [1 : 1.5 : 0] and [1 : 1.4 : 0] (Tables S7). No phosphate was observed on CFeCoP electrode surface. Also, the analysis of O 1s XPS spectra of CFeCoP cathode and anode indicates four types of different species, including absorbed water, sodium cation, metal oxide and metal hydroxide (Figure S19). The deconvolution of Fe 2p and Co 2p XPS spectra for CFeCoP cathode reveals the presence of metallic iron/cobalt, ferrous/ferric and Co2+ species (Figures S20a-S20b). The low intensity of metallic iron/cobalt and the presence of ferric species may be ascribed to surface oxidation caused by oxygen from air and OER catalyzed by CFeCoP anode. As shown in Figures S20c and S20d, Fe 2p and Co 2p XPS spectra for CFeCoP anode could be deconvoluted into Fe2+/Fe3+ and Co2+/Co3+ species, which is similar to as-prepared CFeCoP electrode for OER. The XPS and SEM-EDX analysis suggest no significant change in the composition of iron and cobalt during longevity test, implicating that the electrode composed of FeCo and FeCoPxOy(OH)z is highly stable and efficient bifunctional electrocatalyst toward overall water splitting in alkaline condition.

Conclusion The electrodeposited-film CFeCoP electrode displays superior bifunctional catalytic efficiency for HER and OER in alkaline condition. The catalytic activity of CFeCoP electrode demands overpotentials 57 mV and 282 mV for HER and OER to achieve current density of 10 mA/cm2 (Tafel slopes of 40 and 44 mV/dec for HER and OER, respectively). The excellent HER catalytic ability of CFeCoP electrode 14


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could be attributed to (a) the high ECSA (large Cdl and RF) that enables great exposure of active sites and effective release of the produced gases, (b) the electric conductivity of metallic iron/cobalt (low Rct and Rf) that is favorable to electron transfer, and (c) the metallic iron/cobalt embedded in metal oxides matrix that serve as active sites for HER catalysis. Meanwhile, both iron/cobalt (oxy)hydroxides as active OER catalysts and iron/cobalt oxides as conductive/chemically-stabilizing hosts, which are constructed as film on CFeCoP electrode surface, work in concert to rationalize the outstanding OER. That is, FeCo/FeCoPxOy(OH)z composition derived from electrodeposition of [FeSO4 + CoSO4] phosphate buffer aqueous solution may mediate the kinetics occurred at the interface of catalyst and electrolyte. In the whole cell, an alkaline electrolyzer that is built up by CFeCoP-CFeCoP electrode-pair setting achieves current density of 10 and 20 mA/cm2 at voltages 1.56 V and 1.64 V, respectively, close to those (10 and 20 mA/cm2 at voltages 1.58 V and 1.67 V, respectively) of Pt-IrO2 electrode-pair setting in 1 M NaOH aqueous solution. Remarkably, the CFeCoP-CFeCoP electrode-pair setting exhibits better kinetic efficiency (Tafel slope 51 mV/dec) than that (Tafel slope 68 mV/dec) of Pt-IrO2 electrode-pair setting. Presumably, the modification of composition/architecture/ electronic structure by alloying iron with cobalt optimizes the hydrogen adsorption free energy for HER activity and increases the chemical/structural stability against corrosion during OER process.

Experimental Section Manipulations, reactions, and transfers were conducted under nitrogen according to Schlenk techniques or in a glovebox (nitrogen gas). Solvents were distilled under nitrogen from appropriate drying agents (methylene chloride from CaH2; hexane, diethyl ether and tetrahydrofuran (THF) from sodium benzophenone) and stored in 15



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dried, N2-filled flasks over 4 Å molecular sieves. Nitrogen was purged through these solvents before use. Solvent was transferred to the reaction vessel via stainless cannula under positive N2 pressure. FeSO4．6H2O, CoSO4．6H2O (Alfa Aesar), NaNO2, NaNO3 (Aldrich), nitronium tetrafluoroborate (Lancaster), iron pentacarbonyl (Strem), 99.9% IrO2 powder, Nafion® perfluorinated resin solution (5 wt. % in lower aliphatic alcohols and water, contains 15-20% water) (Sigma-Aldrich), platinum disk electrode (surface area = 0.0707 cm2) (ALS), glassy carbon disk electrode (surface area = 0.0707 cm2, CH Instruments) and graphite plate (R8340, 50×4×1 mm, Great Carbon Co.,

Ltd.,

Taiwan)

were

used

as

received.

Compounds

[Na-18-crown-6-ether][Fe(NO)(CO)3] and [Fe(CO)2(NO)2] were synthesized and characterized by published procedures.48 Infrared spectra of NO stretching frequencies

were

recorded

on

a

PerkinElmer

model

spectrum

one

B

spectrophotometer with sealed solution cells (0.1 mm, CaF2 windows) or KBr solid. UV–vis spectra were recorded on Agilent 8453 equipped with Unisoku cryostat. 1H NMR spectrum was obtained on a Varian Unity-500 spectrometer. Electrochemical measurement was performed with CHI model 621b (CH Instrument) potentiostat instrumentation. Analyses of carbon, hydrogen, and nitrogen were obtained with a CHN analyzer (Heraeus). ICP-MS measurements were performed on a Agilent 7500ce instrument. Samples were dissolved in concentrated aqua regia. Electrochemical Measurements. Electrochemical measurements were carried out using a CHI model 621b (CH Instrument) potentiostat instrumentation with a standard three-electrode system. The graphite plates (surface area = 0.16 cm2) were used as working electrode and auxilary electrode, respectively. SCE (saturated calomel electrode, pH = 7) and Hg/HgO (pH =14) were used as reference electrodes. The working electrodes, CFeP, CCoP and CFeCoP, were prepared from electrochemical deposition of FeSO4 (2 mM)/NaNO3 (5 mM), CoSO4 (2 mM)/NaNO3 (5 mM) and 16


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FeSO4 (2 mM)/CoSO4 (2 mM)/NaNO3 (10 mM), respectively, in 1 M phosphate buffer (pH = 7) under N2 atmosphere onto the graphite plates using a gas-tight, two-compartment cell with iR compensation. The cathodic deposition process is based on the reduction of nitrate to ammonium cation along with the generation of hydroxide that could facilitate the depositon of metal oxide on the electrode surface.38 Electrodeposition was accomplished by applying -1.0 V at 40 oC until the current density reaches an asymptotic limit (about 2.5 h). Films characterized ex situ prior to further electrochemical measurements are referred to “as-prepared” electrodes. Polarization curves of “as-prepared” electrodes in 1 M NaOH electrolyte were recorded by sweeping the potential from 0.00 V to -0.80 V and from 1.20 V to 2.00 V for HER and OER, respectively, with scan rate of 1 mV/s. The performance of overall water splitting was assessed in 1 M NaOH aqueous solution using a two-electrode configuration, and the LSV measurement was conducted at a scan rate of 1 mV/s. The longevity of HER, OER and alkaline water electrolysis were evaluated using amperometry at a constant overpotentials of 200, 335 and 600 mV, respectively, in 1 M NaOH electrolytes. All electrochemical measurements were performed at ambient temperature. All of the potentials were calibrated to a reversible hydrogen electrode (RHE) according to Nernst equation (ERHE = ESCE + 0.05916pH + 0.244; ERHE = EHg/HgO + 0.05916pH + 0.098).8,46 The equilibrium potential (Eo) for HER and OER is 0.00 V and 1.23 V vs RHE, respectively. Preparation of platinized Pt electrode. Before platinization, the platinum surface was cleaned by immersing Pt disc electrode in aqua regia (50% solution, i.e., 3 mL of 12 M HCl, 1 mL of 16 M HNO3 and 4 mL of water). Platinization was conducted at a current density of 5 mA/cm2 in aqueous solution containing 10 mM of chloroplatinic acid for up to 30 minutes. The process evolved chlorine at the anode, but the interaction of the chlorine with the cathode was prevented by employing a 17



gas-tight, two-compartment cell with a suitable separation (e.g. permeable membrane, 0.2 µm). After platinization, the electrode should be rinsed and stored in N2-filled distilled water. Preparation of IrO2 electrode on glassy carbon. To prepare IrO2 electrode, 55 mg of IrO2, 82 µL of Nafion, 750 µL of ethanol and 550 µL of deionized water were ultrasonicated for 30 min to obtain a homogeneous dispersion. 6 µL of dispersion ink was coated onto a cleaned glassy carbon using micropipette, and then was dried in air at ambient temperature. The loading amount of IrO2 catalyst on glassy carbon was estimated as 3.0 mg/cm2. Electrochemical Capacitance Measurements. The electrochemically active surface area (ECSA), roughness factor (RF) and specific activity (js) of CFeP, CCoP and CFeCoP electrodes were estimated by electrochemical double-layer capacitance (Cdl) determined by cyclic voltammetry (CV) in 1 M NaOH aqueous solution.34-35 The specific activity was measured at overpotentials of 100 and 350 mV for HER and OER, respectively. The potential range where there is a non-Faradaic current response was typically a 100 mV potential window centered on open-circuit potential (OCP) of the system. CV measurements were conducted in static solution by sweeping the potential across the non-Faradaic region from positive to negative potential and back at different scan rates. The working electrode was held at each potential vertex for 10 s before initiating the next sweep. The electrochemically active surface area (ECSA) of a film-electrode sample is calculated from the measured double-layer capacitance (Cdl, mF) and specific capacitance (Cs, mF/cm2). ECSA = Cdl / Cs Specific capacitance is the capacitance of an atomically smooth planar surface of the material per unit area under identical electrolyte condition. It has been reported that 18


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general specific capacitance is estimated as 0.04 mF/cm2 in 1 M NaOH aqueous solution.34-35 The roughness factor (RF) is then calculated by dividing ECSA by 0.16 cm2, the geometric area of the electrode. Electrical Impedance Spectroscopy (EIS). EIS measurements were carried out using a Zahner Zennium ε galvanostatic instrumentation with a standard three-electrode system. After LSV polarization curves were recorded, AC impedance measurements were conducted in the frequency range between 100 kHz and 100 mHz with AC modulation of 10 mV amplitude at overpotential of 190 and 350 mV for HER and OER, respectively, in 1 M NaOH aqueous solution. Voigt circuit was proposed to illustrate HER and OER impedance spectra.36-38 The high frequency resistive response, Re, represents the ohmic loss from electrolyte resistance. In this study, Re value of 1 M NaOH electrolyte is in the range of 2.4 and 2.7 Ω. Film resistance (Rf) is related to the ohmic drop caused by the film resistivity and electrolyte resistance drop due to porous morphology of the film. Charge transfer resistance (Rct) is connected to the kinetics of the interfacial charge transfer reaction. The complex nonlinear least-squares (CNLS) fitting of the impedance data was performed with Zview 3.0 software package. Determinations of Mass Activity, TOF and Faradaic Efficiency. The amounts of CFeP and CFeCoP catalysts cathodically deposited on the surface of graphite plate, determined by the weight differences of graphite plate before and after material deposition, is approximately 3.4 and 3.7 mg/cm2, respectively. Upon conducting OER in 1 M NaOH for 10 min, the amounts of CFeP and CFeCoP catalysts on the surface of graphite plate were estimated as 3.0 and 3.5 mg/cm2, respectively. The value of mass activity (A/g) was calculated from the catalyst loading (m, mg/cm2) and the measured current density (j, mA/cm2) at overpotentials of 100 and 350 mV for HER and OER, respectively. 19



mass activity = j / m Values of the lower TOF (s-1) limits were calculated by assuming that every metal atom is involved in the catalysis. TOF = (j x S) / (z x F x n) Here, j is the measured current density at overpotentials of 100 and 350 mV for HER and OER, respectively. S (0.16 cm2) is geometric surface area, and F is Faraday’s constant (96485.3 C/mol). The value of z means the number of electrons involved in the formation of per mole of hydrogen (z = 2) and oxygen (z = 4). The value of n is the moles of deposited metal atoms on the electrode surface, which was determined by ICP-MS measurements. A calibration curve was built by gas chromatography (SRI 8610C, molecular sieves (MS-13x) column and helium ionization detector (HID)) analysis via injection of the known amount of pure hydrogen and oxygen. The amount of hydrogen and oxygen dissolved in water was corrected by Henry’s law (KH = 7.8 × 10-4mol/atm·L for H2 and 1.3×10−3mol/atm·L for O2). The detection of hydrogen and oxygen was performed in two-compartment CV cell equipped with CFeP-CFeP, CCoP-CCoP or CFeCoP-CFeCoP electrode-pair setting in 1 M NaOH aqueous solution. Before the detection of the gas product, the cell is firmly sealed and subsequently purged with nitrogen for 30 min. Upon conducting the electrolysis at a current density of 100 mA/cm2, the gas products were analyzed to determine Faradaic efficiency. Material Characterization. SEM images and EDX spectra were collected by a field emission scanning electron microscopy (JSM-6330F, JEOL Co. Ltd., Japan) operating at an accelerating voltage of 10 kV equipped with energy dispersive X-ray spectroscopy (Oxford). Powder X-ray diffraction (pXRD) data were obtained using a Bruker D8 X-ray Powder Diffractometer with a Cu K-α radiation source in the range 2θ = 10-70°. X-ray photoelectron spectroscopy (XPS) analyses were performed with a 20


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Phi Quantera SXMTM (Ulvac-Phi. Inc) using a high-resolution monochromatic Al Kα line X-ray source (1486.6 eV) and 200 µm spot size on the surface of the sample. The X-ray source was directed 45o with respect to the sample surface. The analyzer is located perpendicular to the sample surface. The spectra were registered at a base pressure of

FeCoPxOy(OH)z as Bifunctional Electrodeposited-Film

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