Enhanced Catalysis of the Electrochemical Oxygen Evolution

Dec 29, 2017 - However, the efficiency of this process is greatly limited by the slow kinetics of the anodic oxygen evolution reaction (OER; 4OH– â†...
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Research Article Cite This: ACS Catal. 2018, 8, 807−814

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Enhanced Catalysis of the Electrochemical Oxygen Evolution Reaction by Iron(III) Ions Adsorbed on Amorphous Cobalt Oxide Luo Gong,† Xin Yu Esther Chng,† Yonghua Du,‡ Shibo Xi,‡ and Boon Siang Yeo*,† †

Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543 Institute of Chemical and Engineering Sciences, A*STAR, 1 Pesek Road, Jurong Island, Singapore 627833

‡

S Supporting Information *

ABSTRACT: The oxygen evolution reaction (OER) is the bottleneck in the efficient production of hydrogen gas fuel via the electrochemical splitting of water. In this work, we present and elucidate the workings of an OER catalytic system which consists of cobalt oxide (CoOx) with adsorbed Fe3+ ions. The CoOx was electrodeposited onto glassy-carbon-disk electrodes, while Fe3+ was added to the 1 M KOH electrolyte. Linear sweep voltammetry and chronopotentiometry were used to assess the system’s OER activity. The addition of Fe3+ significantly lowered the average overpotential (η) required by the cobalt oxide catalyst to produce 10 mA/cm2 O2 current from 378 to 309 mV. The Tafel slope of the CoOx + Fe3+ catalyst also decreased from 59.5 (pure CoOx) to 27.6 mV/dec, and its stability lasted ∼20 h for 10 mA/cm2 O2 evolution. Cyclic voltammetry showed that oxidation of the deposited CoOx, from Co2+ to Co3+ occurred at a more positive potential when Fe3+ was added to the electrolyte. This could be attributed to interactions between the Co and Fe atoms. Comprehensive X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy were conducted. The in situ XANES spectra of Co sites in the CoOx, CoOx + Fe3+, and control Fe48Co52Ox catalysts were similar during the OER, which indicates that the improved OER performance of the CoOx + Fe3+ catalyst could not be directly correlated to changes in the Co sites. The XANES spectra of Fe indicated that Fe3+ adsorbed on CoOx did not further oxidize under OER conditions. However, Fe’s coordination number was notably reduced from 6 in pure FeOx to 3.7 when it was adsorbed on CoOx. No change in the Fe−O bond lengths/strengths was found. The nature and mechanistic role of Fe adsorbed on CoOx are discussed. We propose that Fe sites with oxygen vacancies are responsible for the improved OER activity of CoOx + Fe3+ catalyst. KEYWORDS: oxygen evolution reaction, synergistic effect, X-ray absorption near edge structure, extended X-ray absorption fine structure, iron active site, electrochemistry

1. INTRODUCTION The search for renewable energies to replace our dwindling supply of fossil fuels is a key challenge that confronts humanity in the 21st century.1,2 Solar and wind energies are highly promising alternatives, but their use is limited by their intrinsic intermittencies and geographically uneven distributions. Thus, suitable energy storage strategies are needed. One way to store these energies is to convert them into transportation fuels such as hydrogen gas via electrochemical water splitting. However, the efficiency of this process is greatly limited by the slow kinetics of the anodic oxygen evolution reaction (OER; 4OH− → O2 + 2H2O + 4e− in alkaline electrolytes).3 While Ru- and Ir-based oxides are to date the most active catalysts for the OER, overpotentials (η) in excess of 200 mV are still required for a benchmark 10 mA/cm2 O2 production.4−6 The scarcity of these metals and their instability for prolonged OER operation also make them unsuitable for industrial scale utilization.7 Great efforts have therefore been devoted to the search for more efficient and durable OER catalysts made of nonprecious metals. © XXXX American Chemical Society

From the viewpoints of OER activity, cost effectiveness, and natural abundance, cobalt-based catalysts, in particular the CoFe oxides, have been found to be highly promising as alternatives to Ru and Ir oxides.8−22 CoFe2O4 nanoparticles attached on carbon fiber papers required an overpotential of 378 mV for 10 mA/cm2 O2 production and had good stabilities for >15 h.14 Hierarchically structured CoFe2O4/biocarbon nanocomposites gave excellent OER catalytic activity with a current density of 17.7 mA/cm2 at η = 547 mV. This efficacy has been attributed to the strong coupling between the CoFe2O4 nanoparticles and biocarbon, as well as the high surface area of the catalyst.19 Spinel type Fe-Co3O4 catalysts with varied amounts of Fe have also been prepared, and a catalyst with a Co:Fe atomic ratio of 32:1 required an overpotential of 526 mV for 10 mA/cm2 O2 production.18 Received: October 14, 2017 Revised: November 20, 2017

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DOI: 10.1021/acscatal.7b03509 ACS Catal. 2018, 8, 807−814

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ACS Catalysis

Figure 1. SEM images of (a) as-deposited CoOx, (b) CoOx after OER in 1 M KOH electrolyte, and (c) CoOx after OER in 1 M KOH + 0.3 mM Fe3+ electrolyte. (d) TEM image (some capsule-shaped particles are indicated by the white dotted lines) and the SAED pattern (inset on the top right) of the as-deposited CoOx catalyst. The scale bar in the SAED image represents 2 nm−1. (e) EDS of the chosen area for TEM. The Ni signal originates from the nickel grid. (f) XRD pattern of the as-deposited and post-OER CoOx on a glassy-carbon substrate. The standard XRD patterns of Co3O4 (JCPDS 01-073-1701), Co(OH)2 (JCPDS 00-003-0913), CoOOH (JCPDS 00-007-0169), and graphite carbon (JCPDS 00-001-0646) were also included for comparison. (g) Raman spectra of (i) as-deposited CoOx and (ii) Co3O4 control.

the 1 M KOH electrolyte. We shall then show, using cyclic voltammetry (CV) and in situ X-ray absorption fine structure (XAFS) spectroscopy, that the adsorbed Fe sites are responsible for enhancing the catalytic activity of the composite catalyst.

These aforementioned studies have indicated that iron and cobalt can interact synergistically to give an overall OER activity that is higher than that of the sum of the individual components.13−22 However, few studies have been conducted to elucidate the nature of this interaction and to identify the catalytically active site present in the Co-Fe oxides. Recently, Burke et al. have reported that the Fe sites in a Co-Fe mixed oxide composite exhibited a higher turnover frequency for O2 evolution in comparison to that of the Co sites.13 They have also shown that pure FeOOH had a much lower electrical conductivity than pure CoOOH at η < 400 mV. Fe was thus hypothesized as the more catalytically active OER site, and the double-layered CoOOH only acted as a conductive host for the Fe sites. Nonetheless, direct experimental evidence is needed to elucidate how these two metals interact with each other and to identify the principal active sites of the Co-Fe catalyst for OER. Inspired by the ability of CoOx to absorb free Fe3+ ions from the electrolyte,13 we present here a CoOx + Fe3+ catalytic system, which we shall first demonstrate to be outstanding in catalyzing the O2 evolution reaction. The CoOx was prepared via electrodeposition, and the Fe3+ ion was directly spiked into

2. RESULTS AND DISCUSSION 2.1. Preparation and Characterizations of the Catalyst. CoOx catalyst optimized for OER was electrodeposited onto glassy-carbon working electrodes by CV scans in an aqueous Co2+ electrolyte (0.01 M Co(NO3)2 + 0.1 M KCl, section S1 in the Supporting Information). The mass loading of cobalt was estimated by inductively coupled plasma atomic emission spectroscopy (ICP-AES) to be 9.5 ± 0.3 μg (9.0 ± 1.3 μg after 1 h OER) per cm2 of the working electrode surface (section S2 in the Supporting Information). The catalyst was characterized by scanning and transmission electron microscopy (SEM and TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and Raman spectroscopy (section S3 in the Supporting Information). The as-deposited cobalt oxide catalysts were capsule-shaped particles with ∼20 808

DOI: 10.1021/acscatal.7b03509 ACS Catal. 2018, 8, 807−814

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ACS Catalysis nm diameters and ∼60−100 nm lengths (Figure 1a). These morphologies were largely maintained even after the catalysts had been used for O2 evolution for 1 h (chronopotentiometry at 10 mA/cm2) in either KOH or KOH spiked with Fe3+ electrolyte (Figure 1b,c). The TEM image of the as-deposited CoOx particles is shown in Figure 1d (some of the particles might have been broken during the TEM sample preparation process), and a selected area electron diffraction (SAED) showed no distinct diffraction pattern, which indicated that the catalyst was in the amorphous state. The measured EDS spectrum showed that the analyzed species was Co oxide (Figure 1e). XRD of the CoOx catalyst revealed only peaks from the glassy-carbon substrate (Figure 1f). No signal belonging to Co oxides could be discerned, which agrees with the SAED data that the catalyst is X-ray amorphous. The amorphous state of the catalyst remained unchanged after it was employed for OER in 1 M KOH + 0.3 mM Fe3+ electrolyte at 10 mA/cm2 for 1 h (Figure 1f). Ex situ Raman spectroscopy of the as-prepared catalyst revealed a broad peak centered around 599 cm−1 (Figure 1g), which we assign to the Co−O stretching vibration of amorphous CoOx. It should be noted that the Raman shift of this band cannot be reasonably assigned to any well-defined forms of Co oxide such as Co3O4, Co(OH)2, etc.23−25 X-ray photoelectron spectroscopy (XPS) was used to characterize the CoOx, both as a freshly prepared sample and after it was used as an OER catalyst in 1 M KOH electrolyte spiked with Fe3+. Prior to the use of CoOx as catalyst, the Co 2p3/2 spectrum of CoOx showed two main peaks at 781.2 and 782.6 eV and their satellites at 786.3 and 790.8 eV (from Co(OH)2), while the O 1s spectrum showed a main peak of 531.2 eV (Co−OH) and a small peak at 529.5 eV (Co−O, terminal and lattice oxygen) (Figure 2a).25−28 The amount of Co(OH)2 present was 96.5%. The other minor peaks in the Co 2p3/2 spectrum belonged to CoOOH. For the post-OER

catalyst, both CoOOH (two main peaks at 779.9 and 781.5 eV and satellites at 783.7 and 789.8 eV) and Co(OH)2 were found, and their relative ratio was 71.6 (CoOOH):28.4 (Co(OH)2).25,27 This indicates that CoOOH was the main species on the surface of the catalyst after the OER. In the O 1s XPS spectrum, the lattice and terminal oxygen atoms (529.5 eV) occupied a higher percentage (21.7% = (lattice and terminal oxygen)/[(lattice and terminal oxygen) + (hydroxyl oxygen)]) than it did in the as-deposited CoOx (1.95%) (Figure 2b). This is consistent with the formation of CoOOH, as CoOOH possesses equal amounts of Co−O and Co−OH groups, while Co(OH)2 only has Co−OH groups. Iron was detected in the post-OER CoOx by XPS (insets in Figure 2), and the Fe 3p peak was assigned to Fe3+ (56.2 eV).29 No Fe2+ species, which is expected to have a Fe 3p peak at ∼54 eV,29,30 was detected. The assignments of the peaks are summarized in section S3 (Tables S1−S4) in the Supporting Information. 2.2. Oxygen Evolution using CoOx + Fe3+ Catalyst. The O2 evolving activity of the CoOx catalyst was tested using linear sweep voltammetry in 1 M KOH electrolyte spiked with 0, 0.1, 0.3, 0.5, or 1 mM Fe3+ ions (section S4 in the Supporting Information). A H-cell with a glass frit separating the anodic and cathodic compartments was used (three-electrode configuration). It is notable that the measured OER activity increased with Fe3+ concentration and was optimized after the Fe3+ concentration was ≥0.3 mM (Figure S4 in the Supporting Information). This implies that the addition of Fe3+ ions into the electrolyte could be a more viable and easier method to prepare optimized binary Co-Fe catalyst, instead of combinatorial synthesis with various atomic ratios of Co to Fe.17,20 As the Fe3+ ions were absorbed by CoOx from the KOH electrolyte, we hypothesize that the Fe3+ ions should be adsorbed on the surface of CoOx, likely at its edges or defects.31 A concentration of 0.3 mM was taken as the optimized Fe3+ concentration and was used for the rest of the experiments. The LSVs of the CoOx catalyst measured at 1 mV/s, with and without 0.3 mM Fe3+ in 1 M KOH (30 mL) electrolyte, are presented in Figure 3a (both sets of measurements were made with freshly electrodeposited catalysts and the Fe3+ ions were added just before starting the experiment, section S4 in the Supporting Information). To achieve an O2 current density of 10 mA/cm2 (10 mA/cm2 current density is a typical value to match a 10% efficient solar water-splitting device under 1 sun illumination),32 the CoOx catalyst needed an overpotential (η) of 378 ± 7 mV, but it only needed an overpotential of 309 ± 1 mV when Fe3+ ion was added. For comparison, the LSVs of blank glassy-carbon (GC) electrodes immersed in 1 M KOH spiked with Fe3+, Co2+ or Fe3+ + Co2+ ions are shown in Figure 3a. No significant OER activities could be observed in these systems. Thus, the increase in OER activity for the CoOx catalyst after the addition of Fe3+ cannot be attributed to the interaction between the underlying glassy-carbon substrate and these metal ions. The Tafel slope of CoOx was 59.5 ± 4.0 mV/ dec in pure 1 M KOH, which is a typical value reported for cobalt oxide catalysts (Figure 3b).13,33 Remarkably, this decreased to 27.6 ± 1.5 mV/dec when Fe3+ was added to the KOH electrolyte, which indicated an acceleration of the OER reaction kinetics. Chronopotentiometry (CP) of the CoOx catalyst was conducted to check its stability (Figure 3c). In 1 M KOH, the potential required by CoOx for 10 mA/cm2 O2 evolution increased from 1.6 to 1.8 V (all potentials reported are with respect to the reversible hydrogen electrode, RHE) within 5 h.

Figure 2. XPS spectra of (a) as-prepared CoOx and (b) CoOx after OER in 1 M KOH + 0.3 mM Fe3+ electrolyte for 1 h. The insets in (a) and (b) indicate that Fe3+ is only present in the post-OER catalyst. The axes of the insets have the same meanings as in the main figure. The CO and C−O peaks come from K2CO3, which was formed by the reaction of remnants of KOH (post-OER) and CO2 from the air. 809

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Table 1. Comparison among Cobalt-Based OER Catalysts catalyst CoOxa CoOx-(a)32 CoOx-(b)32 CoO33 mesoporousCo3O434 Co3O433 CoOxa

Co1−xFexOOH13 CoFe6 CoFeOx32 CoFe2O414 CoFe2O4/ biocarbon19

Figure 3. (a) LSV of CoOx in 1 M KOH and 1 M KOH + Fe3+ electrolytes at 1 mV/s. LSV of bare glassy carbon (GC) in 1 M KOH, 1 M KOH + Fe3+, 1 M KOH + Co2+, and 1 M KOH + Co2+ + Fe3+ electrolytes are also shown for comparison. The inset is the enlarged view from 1.55 to 1.63 V. (b) Tafel plots of CoOx measured at 1 mV/s in 1 M KOH electrolyte, with and without added Fe3+. (c) Chronopotentiometry of CoOx in 1 M KOH electrolyte, both with and without added Fe3+. The inset shows an SEM image (scale bar 100 nm) of the catalyst after the OER in 1 M KOH + 0.3 mM Fe3+ for 20 h. (d) Faradaic efficiency (FE) for the production of H2, measured by GC. The hydrogen evolution reaction was conducted in 1 M KOH catholyte, and the cathodic compartment was separated from the anodic compartment (1 M KOH + 0.3 mM Fe3+ + 0.3 mM Co2+) by a salt bridge (KNO3 + agarose) to prevent the contamination of Fe3+.

a

electrolyte

η at 10 mA/cm2 (mV)

Tafel slope (mV/dec)

KOH NaOH NaOH NaOH NaOH KOH

378 ± 7 392 ± 2 390 ± 40 420 ± 20 450 ± 10 476

59.5 ± 4.0 52.2 ± 1.5

1 M NaOH 1 M KOH + 0.3 mM Fe3+ 1 M NaOH + 0.3 mM Fe3+ 1 M KOH

500 ± 10 309 ± 1

60.9 27.6 ± 1.5

313 ± 3

27.0 ± 0.6

1 M NaOH 1 M NaOH 1 M NaOH 0.1 M NaOH

350 ± 10 370 ± 20 378 ∼417

1 1 1 1 1 1

M M M M M M

39.8

62 (x = 0) 26−39 (0.33 < x < 0.79)

∼73

This work.

catholyte, and a platinum wire was used as the cathode. In order to extend the durability of the water-splitting system, Co2+ ions were also introduced into the anolyte. The addition of both Fe3+ and Co2+ into the electrolyte could greatly extend the stability of the CoOx catalyst (section S7). Half of the anolyte was exchanged every 12 h with a new batch of electrolyte. Under these conditions, we found that the system could be driven at 10 mA/cm2 (in terms of OER) for more than 2 days in a potential range of 1.60−1.71 V. The Faradaic efficiency for oxygen evolution was measured to be ∼99% (at 0.75 mA/cm2 current) by using the rotating ring disk electrode (RRDE) method43 (section S7). The yield of H2 at the cathode was also measured using gas chromatography (GC), and an average Faradaic efficiency of >97% for H2 formation was obtained (Figure 3d, section S7). Thus, the added ions in the anodic compartment did not adversely affect the H2 evolution reaction. 2.3. Interactions between Co and Fe for oxygen evolution. 2.3.1. Cyclic Voltammetry. The cyclic voltammetry of CoOx was performed in 1 M KOH and 1 M KOH + 0.3 mM Fe3+ electrolytes (Figure 4). The anodic peak corresponding to the oxidation of Co(OH)2 to CoOOH44 shifted from 1.178 V in 1 M KOH to 1.319 V in 1 M KOH + 0.3 mM Fe3+

However, in 1 M KOH + Fe3+, the potential required for 10 mA/cm2 increased from 1.54 to 1.6 V in the first 5 h and then stayed at 1.6 V until the 20th hour. No visible changes to the morphology of the catalyst could be discerned even after conducting OER for 20 h (inset in Figure 3c). After that, the deposited catalyst gradually peeled off and the potential increased to >1.8 V. Ex situ EDS analysis and imaging revealed that the CoOx catalyst had ∼4 atom % Fe after the OER (CP at 10 mA/cm2 in 1 M KOH + 0.3 mM Fe3+ electrolyte for 1 h) and those adsorbed iron atoms were uniformly distributed (Figure S7 in the Supporting Information). In comparison to many other cobalt-based OER catalysts, our CoOx + Fe3+ system required a smaller overpotential at least 30 mV lower to drive 10 mA/cm2 O2 production (Table 1).6,13,14,19,32−34 Its Tafel slope of 27.6 mV/dec is also among the lowest of those cobalt-based OER catalysts, which have Tafel slopes in the range of 26−73 mV/dec.13,14,33 We also calculated the TOF of O2 evolution exhibited by our CoOx + Fe3+ catalyst, with the conservative assumption that all the metal atoms deposited were active sites. At overpotentials of 300, 350, and 400 mV, our catalyst exhibited TOF values of 0.08, 1.6, and 16.9 s−1 respectively, which are comparable to or higher than those of many Co-based and NiFe OER catalysts (Table S5 in the Supporting Information).13,34−42 When a prolonged water splitting reaction is considered, the added Fe3+ ions in the electrolyte may gradually be deposited onto the cathode and lead to an increase in overpotential for the hydrogen evolution reaction (HER). To prevent this, a salt bridge was introduced to better separate the cathodic and anodic compartments in our system (section S7 in the Supporting Information). Pure 1 M KOH was used as the

Figure 4. Cyclic voltammetry (100 mV/s) of electrodeposited CoOx in 1 M KOH and 1 M KOH + 0.3 mM Fe3+ electrolytes. The blue arrow indicates the shift direction of the anodic peak. 810

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ACS Catalysis electrolyte. This shift indicates that the oxidation of Co2+ ions into Co3+ becomes more difficult after the addition of Fe3+ ions into the electrolyte. This could be due to electronic coupling between Co and Fe and indicates that the Fe3+ ions are chemically bonded to the CoOx surface. We note that similar results have also been reported by Burke et al. for the Co-Fe system and by Louie et al. for the Ni-Fe system, where the respective oxidation of Co2+ and Ni2+ was shown to be hindered by the presence of Fe.5,13 2.3.2. In situ X-ray Absorption Fine Structure Spectroscopy. In situ X-ray absorption fine structure (XAFS) studies were conducted to elucidate changes in the electronic and geometrical configurations of Co and Fe atoms during the OER (section S8 in the Supporting Information). The Co and Fe oxides used were deposited onto carbon cloths. Fe ions were adsorbed in advance onto CoOx (the resulting Fe atomic percentage was 4.6%), and only pure 1 M KOH electrolyte was used during the XAFS measurements. 2.3.2.1. XAFS of Co. Regardless of whether Fe3+ ions were adsorbed on it or not, when the CoOx catalyst was biased 1.6 V (the voltage was applied 5 min before commencing the XAS measurement and maintained throughout the XAS measurement), its Co K absorption edge shifted by +0.4 eV (indicated by the arrow on the XANES spectra shown in Figure 5a, taken at half-height) relative to that under open circuit potential (OCP, 0.9 V) (Figure 5a). The energy shift of +0.4 eV indicates an increase in the oxidation state of the Co atoms when an OER potential was applied.45 This is consistent with our XPS result showing the oxidation of Co(OH)2 to CoOOH after the OER (Figure 2). The XANES spectra of Co(OH)2 and CoOOH were also measured and included in Figure 5a for reference. Here, it is notable that despite a vast difference between the OER activities of CoOx and CoOx + Fe3+ systems (Figure 3a), their Co XANES spectra were the same during the OER. This indicates that the enhanced OER activity of the CoOx + Fe3+ system could not be solely explained by the changes in the electronic properties of CoOx upon Fe3+ adsorption. We also need to stress that this result does not contradict the observation of a shift in the oxidation peak of Co in its CV when Fe3+ was added to the electrolyte (Figure 4). Though this peak occurred at a more positive potential when Fe3+ was present, the OER potential which we applied here (1.6 V) was beyond the peak potentials in both cases (with or without Fe3+). Thus, it is reasonable for Co to have the same XANES shift.46 XAFS is a bulk technique, and information about the surface Co atoms could be obscured by signals originating from the bulk. We thus imaged the sample (CoOx deposited on carbon cloth) by SEM and found that they were ∼10 nm flakes (Figure S12 in the Supporting Information). The percentage of electrochemically active cobalt atoms in comparison to the total deposited cobalt atoms was also estimated using the method of Burke et al. to be 37 ± 4% (section S8 in the Supporting Information).43 The small sizes of the CoOx flakes and the high percentage of active cobalt atoms indicate that the obtained XANES spectrum should have a considerable contribution from the Co atoms that took part in the OER reaction. Furthermore, we reduced the amount of deposited CoOx to one-fourth and the XANES spectrum obtained (Figure S16 in the Supporting Information) was spectroscopically identical with that shown in Figure 5a. In order to further eliminate the possibility that signals from the catalytically active Co in the CoOx + Fe3+ was masked by the underlying bulk

Figure 5. (a) XANES spectra of Co K edge (CoOx and CoOx with Fe3+) under open circuit potential (OCP) and oxygen evolution reaction (OER, 1.6 V vs RHE) in 1 M KOH electrolyte. The inset is the magnified view from 7722 to 7724 eV, and the arrow indicates an energy shift of +0.4 eV. The spectra of Co(OH)2 and CoOOH were added for reference. (b) XANES spectra of Co in Fe48Co52Ox catalyst under OER (1.6 V). XANES spectra of CoOx and CoOx+ Fe3+ under OER were added for comparison. (c) XANES spectra of Fe K edge (Fe3++CoOx) under OCP and OER in 1 M KOH electrolyte. The spectra of electrodeposited FeOx, standard Fe2O3 and FeOOH were added for reference. (d) Fourier-transformed Fe K-edge EXAFS spectra of FeOx and Fe + CoOx. The R value was without phase-shift correction. (e) XANES spectra of Fe K edge (FeOx) under OCP and OER in 1 M KOH. (f) XANES spectra of Fe in Fe4.4Co95.6Ox catalyst. The Fe XANES spectra of CoOx+ Fe3+ were added for comparison.

CoOx, we prepared a Fe48Co52Ox (atomic ratio) catalyst by electrochemical deposition and measured the XANES spectrum of Co under OER. We found that, under OER, the XANES spectrum of Co in Fe48Co52Ox was the same as that of CoOx and CoOx + Fe3+ (Figure 5b, section S8). With these measurements, we conclude that the measured Co XANES signal was reflective of the catalyst in its operating state and was not obscured by signals from the underlying bulk CoOx. To summarize, the Co sites in the CoOx, CoOx + Fe3+, and Fe48Co52Ox catalysts showed similar XANES spectra under OER conditions. This strongly suggests that their (the Co sites) OER activities are similar. 2.3.2.2. XAFS of Fe. A pre-edge peak originating from the quadrupole 1s → 3d transition, which may also have extra contribution from the dipole 1s → hybridized 3d−4p orbital transition, was observed in the Fe K-edge spectrum of pure FeOx (Figure 5c). When Fe3+ ions were adsorbed on CoOx, this feature increased in intensity in comparison to that of pure FeOx (at OCP). Simultaneously, the intensity of the white line decreased. These observations are indicative of a lowering of the coordination number (CN) of Fe (Fe−O) when it was adsorbed on CoOx.47,48 When the CN decreases, the 4p states 811

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ACS Catalysis M−O + OH− → MOOH + e−

hybridize with 3d states due to the deviation from centrosymmetry, providing more dipole-allowed 1s → hybridized 3d−4p orbital transitions to the pre-edge peak and resulting in the increase in its intensity. Concurrently, the population of hybridized Fe 4p−O 2p states available for 1s electrons to be excited to decreases, which leads to a decrease in the intensity of the white line. The reference spectra of Fe2O3 and FeOOH in Figure 5c indicate that the valence state of the Fe is +3 in our catalyst. We could not discern any oxidation of Fe3+ to Fe4+ when an OER potential of 1.6 V was applied, as there is no shift in the XANES edge of Fe to higher energy. The decrease in the coordination environment of Fe adsorbed on CoOx was further confirmed by EXAFS (section S8 in the Supporting Information), which showed the lowering of the Fe CN from 6.0 (pure FeOx, OCP) to 3.7 (CoOx + Fe3+, OCP) (Table 2). Interestingly, we found that the Fe−O bond

MOOH + OH− → M + O2 + H 2O + e−

The electrodeposited CoOx could have provided the host structure for the adsorbed Fe3+ ions and electronically affected them to be the principal OER active sites. DFT+U simulations have been made to understand how surface orientations, surface steps, and oxygen vacancies of Fe oxides affect their ability to catalyze the OER.53 Fe sites with oxygen vacancies were found to be the most effective in lowering the overpotential of the O2 evolution reaction. Oxygen vacancies would increase the conductivity of the catalyst due to the delocalization of the electrons around the vacancies.54 Thus, electrons would be more easily transferred during the OER. Furthermore, Fe sites with oxygen vacancies would be more accessible to incoming OER reactants such as OH−, thus facilitating the O2 evolution.54,55 An ultrathin NiCo2O4 nanosheet catalyst that is rich in oxygen vacancies has also been reported to show superior OER performance in comparison to the vacancy-free samples.54 Here, we note that the reduction of the CN of Fe might strengthen its remaining Fe−O bonds. This could then adversely affect the OER (as Fe lies on the strong binding arm of the OER volcano plot).46,56 However, our data did not indicate this occurrence, as the Fe− O bond lengths were the same (2 Å) both in FeOx and CoOx + Fe3+ systems. To prepare OER catalysts with excellent activities, it is important to fully expose their active sites to the electrolytes. The way our catalyst was prepared fulfilled this objective, as the active Fe sites were directly adsorbed from the electrolyte onto the surface of the CoOx host. We predict that even more superior OER catalysts could be made by carefully selecting the appropriate host structures.

Table 2. Coordination Number of Fe Catalysts under OCP and OER

a

conditions

FeOx

OCP OER (1.6 V)

6.0 ± 1.1 5.5 ± 1.5

a

CoOx + Fe3+

Fe4.4Co95.6Ox

3.7 ± 0.8 3.8 ± 0.4

4.4 ± 0.5 4.7 ± 0.9

The error bars are from the fitting procedures.

lengths in pure FeOx and CoOx + Fe3+ were the same (2.0 Å, under OCP) (Figure 5d and Table S6 in the Supporting Information). This indicates that the Fe−O bond strength did not increase when the CN of Fe decreased under modification by CoOx. The approximate octahedral coordination environment of Fe should thus be maintained (with some oxygen vacancies to account for the decrease of the CN number). Otherwise, the Fe−O bond length would have been shortened (for example, the tetrahedral Fe3+ has an Fe−O bond length of 1.88 Å).49,50 Analogous EXAFS measurements of the samples at 1.6 V gave similar data. For comparison, we note that the XANES absorption edge of the Fe K edge spectrum of FeOx did not change significantly at OCP and at 1.6 V (Figure 5e, Table S6), which is consistent with previous measurements made by Friebel et al.46 An electrodeposited Fe4.4Co95.6Ox (atomic ratio) catalyst was also prepared. Its Fe XANES spectrum had a lower pre-edge peak and higher white-line peak, in comparison to that of the CoOx + Fe3+ catalyst (Figure 5f). This is consistent with an increase in the coordination of Fe, which we also confirmed by EXAFS (4.4 under OCP and 4.7 under OER for Fe4.4Co95.6Ox). Interestingly, the OER performance of the electrodeposited Fe4.4Co95.6Ox catalyst falls inbetween that of pure FeOx and CoOx + Fe3+ catalyst (Figure S17 in the Supporting Information). These data indicate that Fe3+ ions with oxygen vacancies (lower CN number) adsorbed on CoOx could be the main active sites responsible for the improved OER of the CoOx + Fe3+ system. 2.3.3. Role of Fe in the Catalysis of O2 Evolution. The commonly proposed mechanism for O2 evolution46,51,52 in alkaline electrolyte is

3. CONCLUSIONS We present here a CoOx + Fe3+ catalytic system which can catalyze 10 mA/cm2 oxygen evolution at an overpotential of only 309 mV. The Tafel slope was as low as 27.6 mV/dec. The stability of the CoOx + Fe3+ catalyst was also improved in comparison to that of the CoOx catalyst. The Faradaic efficiency of O2 evolution by the CoOx + Fe3+ catalyst system was measured to be ∼99%. By combining CV and XAFS characterizations, we found that strong electronic interactions happened between the adsorbed Fe atoms and the CoOx host. The Co sites in the CoOx, CoOx + Fe3+, and Fe48Co52Ox catalysts showed similar XANES spectra under OER conditions, which indicated that the Co sites in the CoOx + Fe3+ catalyst were not the principal active sites for the improved OER performance. The Fe sites in the CoOx + Fe3+ catalyst have notably lower average coordination numbers, but their coordination geometries, bond lengths, or oxidation states were not changed in comparison to that of pure FeOx. We propose that the adsorbed Fe atoms with oxygen vacancies are the principal active sites for the improved OER activity of the CoOx + Fe3+ catalysts. 4. EXPERIMENTAL SECTION Details of the experiments are given in the Supporting Information. In brief, cyclic voltammetry from −0.412 to 1.588 V (vs RHE) at 500 mV/s for 20 cycles was used to deposit the CoOx catalyst. An aqueous solution containing 0.01 M Co(NO3)2 and 0.1 M KCl was used as the electrolyte for

M + OH− → M−OH + e− (M = active site, e− = electron) M−OH + OH− → M−O + H 2O + e− 812

DOI: 10.1021/acscatal.7b03509 ACS Catal. 2018, 8, 807−814

Research Article

ACS Catalysis Notes

deposition. A three-electrode system was used. A glassy-carbondisk electrode (surface area 0.0707 cm2, CHI), a graphite rod, and a Ag/AgCl (saturated KCl, CHI) electrode were respectively employed as the working, counter, and reference electrodes. For all other electrochemical measurements, a Hg/ HgO (1 M KOH, CHI) electrode and a platinum coil (Goodfellow) were used as the reference and counter electrodes, respectively. iR compensation was applied for all the electrochemical measurements by using the current interruption mode, except for the water-splitting experiment in a two-electrode configuration (section S7 in the Supporting Information). XAFS was conducted at the XAFCA beamline of the Singapore Synchrotron Light Source (SSLS).57 A polypropylene bag was used as the electrochemical cell for XAFS measurements. A Ag/AgCl electrode (saturated KCl, CHI) and a platinum coil were respectively used as the reference and counter electrodes. CoOx, FeOx, Fe4.4Co95.6Ox and Fe48Co52Ox were deposited on carbon cloth (1 × 2 cm2, Cetech) working electrodes. The deposition methods for the last three catalysts were adapted from the literature.5 In short, a cathodic current of −10 mA was applied for 1 min. The deposition solution was prepared from Co(NO3)2 and FeSO4 in the following stoichiometries: Co(NO3)2:FeSO4 = 50:50 (mole ratio) for Fe48Co52Ox deposition and Co(NO3)2:FeSO4 = 5:95 for Fe4.4Co95.6Ox deposition; total concentration of metal ions 10 mM. The actual stoichiometries of Fe 4.4 Co 95.6 O x and Fe48Co52Ox were ascertained by EDS. XANES and EXAFS data were processed with the ATHENA software package. EXAFS data were fitted with the ARTEMIS software.58 The k weight in the background function determination was set to 2. The frequency cutoff parameter, Rbkg, was set to 1. Edge energy (E0) is defined as the energy of the first significant peak in the first-derivative spectrum. Fourier transforms of χ(k) were performed using a k range between 2 and 8 Å−1 and the Hanning window function with a sill width of Δk = 1 Å−1. The fitting was conducted using a R range between 1 and 2.1 Å. The samples were also characterized using SEM, TEM, EDS, XRD, Raman spectroscopy and ICP-AES. Details of the analyses are given in the Supporting Information.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by an academic research fund (R-143000-587-112) from the National University of Singapore. L.G. received a Ph.D. research scholarship from the Ministry of Education, Singapore. We acknowledge Professor Rong Xu from the Nanyang Technological University for providing us the reference XANES spectrum of CoOOH.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b03509. Electrodeposition of CoOx on a glassy-carbon electrode, ICP-AES measurements, details of various characterizations (SEM, TEM, XRD, Raman, XPS, EDS), electrochemical experimental details, turnover frequency calculations, water splitting setup and experiment, measurements of Faradaic efficiency of O2 and H2, and XAFS experimental details and data fitting details (PDF)

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AUTHOR INFORMATION

Corresponding Author

*B.S.Y.: e-mail, [email protected]; fax, +65 6779 1691; tel, +65 6516 2836. ORCID

Luo Gong: 0000-0002-1127-6008 Yonghua Du: 0000-0003-2655-045X Boon Siang Yeo: 0000-0003-1609-0867 813

DOI: 10.1021/acscatal.7b03509 ACS Catal. 2018, 8, 807−814

Research Article

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DOI: 10.1021/acscatal.7b03509 ACS Catal. 2018, 8, 807−814