Enhanced OER Activity of Co Ions Isolated in the Interlayer Space of

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Enhanced OER Activity of Co Ions Isolated in the Interlayer Space of Buserite MnO 2

Kotaro Fujimoto, Takuya Okada, and Masaharu Nakayama J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01238 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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The Journal of Physical Chemistry

Enhanced OER Activity of Co Ions Isolated in the Interlayer Space of Buserite MnO2 Kotaro Fujimoto,† Takuya Okada,† and Masaharu Nakayama*,† †

Department of Applied Chemistry, Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan

ABSTRACT

We have fabricated a thin film of layered manganese dioxide (MnO2) that accommodates cobalt ions in its interlayer space, constructing so-called buserite structure, via electrodeposition and the subsequent ion-exchange. The MnO2 layers could isolate Co2+ ions to provide an environment beneficial for oxygen evolution reaction (OER) in alkaline electrolyte, where fast electron transfer and high utilization efficiency were achieved. The catalyst with isolated Co2+ ions exhibited a mass activity as high as 63.5 A/gCo at an overpotential (η) of 0.4 V, which was much larger than those of Co ions bound in the oxide network. Moreover, it also exhibited excellent stability for long-term OER operation. Namely, the potential needed to generate a current density of 10 mA/cm2 increased only 0.073 V during 100 h operation, and no significant change was seen after 100 consecutive potential cycles between +1.0 and +2.0 V vs RHE.

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INTRODUCTION Converting renewable energy such as sunlight and wind to storable forms, including electrical and chemical energy, have drawn much attention due to the continuous depletion of fossil fuels and rising environmental concerns. Among various energy storage options, hydrogen is regarded as the most promising energy carrier because it never discharges carbon dioxide as a combustion product. Water electrolysis using renewable energy is one of the most promising approaches for hydrogen production. However, the sluggish kinetics of the oxygen evolution reaction (OER, 2H2O → 4H+ + O2 + 4e–) that occurs at the counter electrode lowers the efficiency of the whole water electrolysis system and therefore requires a high overpotential to afford sufficient current flow. To date, the most efficient electrocatalysts for OER are noble metal oxides such as RuO2 and IrO2.1–3 However, the high cost and toxicity of noble metals have severely hindered their widespread applications. In recent years, great efforts have been devoted to overcome this problem by using transition metals such as Co, Ni, Fe and Mn to replace noble metal catalysts. Over the past years, cobalt-based materials have been widely investigated as promising nonnoble catalysts for OER.4–7 The papers published so far proposed the design of catalysts to improve their performance; i.e., large current density at low overpotential and small Tafel slope. For example, doping of nitrogen into Co3O4 reduced the overpotential by 49 mV to reach a current of 10 mA/cm2.7 In this study, we will present a new concept of the catalyst design, which is associated with the existing state of cobalt species. Specifically, Co ions isolated in the interstitial site of a host matrix is featured, in comparison with those bound in the oxide network. The particle size effects in Co (oxide)-based catalysts were explored in a number of important reactions, including Fischer-Tropsch synthesis and CO2 hydrogenation.8,9 However, little has been reported on the


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size dependency for OER.10 On the other hand, single ion catalysts of Au or Pt supported on the surface of oxides such as CeO2 and ZrO2 provided enhanced utilization efficiency and selectivity.11,12 The supported single-atom catalysts can not only reduce the use of noble metals but also have a high utilization efficiency comparable to that of homogeneous catalysts. These previous works motivated us to isolate Co ions in order to enhance their catalytic activity toward OER. Various host materials such as smectite clays, zeolites, zirconium phosphates, and layered metal oxides have been investigated extensively for their versatile intercalation chemistry.13–15 However, the majority of those are redox-inactive and are in powder form since they are synthesized chemically. On the other hand, layered double hydroxides such as Ni/Fe and Co/Fe hydroxides are a unique class of two-dimensional materials, which can efficiently work as electrocatalysts for OER.16–18 We herein employed a redox-active host material, layered manganese dioxide (MnO2), in order to realize a high-degree of dispersion of Co ions. Layered MnO2 consists of a pile of MnO2 layers composed of edge-shared MnO6 octahedra with mainly Mn4+ cations. Some of the Mn4+ ions are replaced with Mn3+ ions, giving a net negative charge to be balanced electrically by intercalation of guest cations, usually hydrated alkaline metals. In 2004, we presented an electrochemical route to produce layered MnO2 intercalated with alkaline metals and alkylammonium ions as a thin film,19,20 in which aqueous Mn2+ ions were potentiostatically oxidized in the presence of the corresponding guest cations. Herein, we fabricated a binder-free OER catalyst having Co2+ ions isolated in the interlayer space of layered MnO2. Specifically, a thin film of layered MnO2 with bulky tetra-alkylammonium ions was first electrodeposited, followed by immersion in an aqueous solution containing CoSO4.21 For comparison, thin films containing Co ions with different existing states were made electrochemically; i.e., (i) α-Co(OH)2 with poor crystallinity and (ii) K+-intercalated layered


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MnO2 whose framework was doped with Co. The Co ions isolated in the interlayer space resulted in larger utilization efficiency and much better stability for long-term catalysis.

EXPERIMENTAL Materials. All chemicals were of reagent quality and used without further purification. Manganese(II) sulfate pentahydrate (99.9%), cobalt(II) sulfate heptahydrate (99.0%), cobalt(II) nitrate hexahydrate (99.5%), potassium sulfate (99.0%), potassium chloride (99.0%), and potassium hydroxide (85.0%) were obtained from Wako Pure Chemicals. Tetrabutylammonium chloride (TBA+Cl–, 98.0%) was obtained from Tokyo Chemical Industry. Electrolytes were prepared with doubly distilled water and, unless otherwise notified, were deoxygenated by bubbling with purified nitrogen gas for at least 20 min prior to use. Preparation of thin film catalysts. All electrochemical experiments were carried out using a potentiostat/galvanostat (SP-300, Bio Logic Science Instruments) in a standard three-electrode system. A platinum mesh and a standard Ag/AgCl electrode (in saturated KCl) served as the counter and reference electrodes, respectively. A fluorine-doped tin oxide (FTO)-coated glass slide (R = 10 Ω cm) was used as the working electrode to fabricate the films on it. The geometric area of the FTO slide exposed to the electrolyte was 5 × 5 mm2. Prior to electrodeposition, the electrode surface was ultrasonically cleaned in a mixed solution of ethanol and water and then rinsed thoroughly with distilled water. Electrodeposition was carried out in an aqueous solution of 2 mM MnSO4 and 50 mM TBACl. A constant potential of +1.0 V was applied to the working electrode, while a fixed electrical charge of 200 mC/cm2 was delivered. These conditions had been optimized to form a highly crystallized MnO2 film intercalated with TBA cations. The resulting electrode was immersed for 24 h in an aqueous solution of 0.5 M CoSO4 in order to


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replace the initially incorporated TBA+ ions with Co2+ in solution. After electrodeposition, the resulting film on an electrode was rinsed thoroughly with water, dried under vacuum in a desiccator, and then subjected to spectroscopic and electrochemical measurements. For comparison purposes, the TBA-incorporated MnO2 film was immersed in a 0.25 M K2SO4 solution, forming K+-incorporated MnO2. A thin film of layered MnO2 doped with Co into its framework, not interlayer space, was synthesized from a solution containing 2 mM MnSO4, 50 mM KCl, and 1mM CoSO4 heated at 70 °C by applying a constant potential of +1.0 V.22 A thin film of cobalt hydroxide (Co(OH)2) was prepared according to Ref. 20 from 0.1 M Co(NO3)2 solution at room temperature, where a constant potential of −1.044 V was applied.23 Structural characterization. X-ray diffraction (XRD) patterns were recorded on a Rigaku Ultima IV diffractometer, using Cu Kα radiation (λ = 0.154051 nm). The data were collected over the 2θ range from 1 to 40° at a scan rate of 1°/min, applying a beam voltage of 40 kV and a beam current of 40 mA. Field emission scanning electron microscopy (FE-SEM) data were obtained using a Hitachi S-4700Y microscope operating at 10 kV. Samples were sputter coated with platinum in an ion coater. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Scientific K-Alpha spectrometer with an Al Kα (1486.6 eV) monochromatic source (1805 V, 3 mA). Wide- and narrow-range spectra were acquired with a pass energy of 50 eV and channel widths of 1.0 and 0.1 eV, respectively. The binding energy scale was calibrated with respect to the C 1s signal at 284.8 eV. The amounts of Mn and Co were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) using an SII Nano Technology SPS-3500. All samples were dissolved with 2 mM HCl to make an aqueous solution of 0.005 mol/L HNO3.


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Electrochemical measurements. Electrochemical tests were carried out in an electrolyte solution of 1.0 M KOH saturated with O2. A Pt mesh was used as the counter electrode, and a standard Ag/AgCl electrode (in saturated KCl) as the reference electrode. All the potentials were calibrated to the reversible hydrogen electrode (RHE); i. e., E(RHE) = E(Ag/AgCl) + 0.059 pH + 0.195. Linear sweep voltammetry (LSV) was performed at a scan rate of 1 mV/s. Tafel plots were obtained from the rising part of LSV curves. The onset potential for OER was estimated from the beginning of linear portion in the Tafel plot. Cyclic voltammetry (CV) was made at scan rates of 2 to 200 mV/s. Chronoamperometry and chronopotentiometry were employed to further evaluate the electrocatalysts in terms of turnover frequency (TOF) and stability, respectively.

RESULTS Preparation of catalysts. The as-deposited MnO2 film with TBA provided evenly spaced three peaks at 7.03°, 14.02°, and 21.27°, as shown in Figure 1a. These peaks are diagnostic of a layered structure and indexable to the 001 plane and its second (002)- and third (003)-order diffractions from the layered MnO2 intercalated with TBA cations.24 d-spacing of the 001 peak (=d001) corresponds to the interlayer distance and was calculated to be 1.26 nm using Bragg’s equation. From the full width at half-maximum (fwhm in radians) of the 001 peak, the crystallite size was estimated to be 5.76 nm along the 001 direction (c axis) according to the modified Scherrer equation (crystallite size = 0.89λ/fwhm·cosθ, where λ is the X-ray wavelength (0.154051 nm) and θ is the Bragg angle).25 This as-deposited film is hereafter denoted as TBA/MnO2. After immersion in solutions containing metal ions (Figs. 1, b and c), the diffraction peaks in Fig. 1a shifted higher angles, correspondingly smaller d-spacings. The interlayer


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distance was estimated to be 0.97 and 0.73 nm for the films treated in CoSO4 and K2SO4 solutions, which were assigned to buserite- and birnessite-type layered MnO2, respectively.26

Figure 1. XRD patterns of the layered MnO2 films (a) before and (b and c) after immersion in (b) 0.5 M CoSO4 and (c) 0.25 M K2SO4 solutions, along with those of (d) Co(OH)2 and (e) Codoped layered MnO2 prepared according to Refs. 23 and 22, respectively. Buserite and birnessite accommodate two layers and a single layer of water molecules, respectively, as well as cations for charge compensation toward the negative charges on MnO2 layers. In most cases, divalent cations can construct the buserite structure. Clearly, the bulky TBA cations intercalated initially were successfully replaced with Co2+ or K+ ions in solution. The layered MnO2 films obtained after immersion in Co- and K-containing solutions are hereafter denoted as Co/MnO2 and K/MnO2, respectively. The morphology of Co/MnO2 film


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was examined using SEM and is shown in Figure S1. The film thickness was measured to be ~600 nm, corresponding to a delivered electrical charge of 200 mC/cm2. The electrode surface was entirely covered with a thin and uniform film. The higher magnification image presented wrinkled and crumpled sheet-like morphology, typical of the layered MnO2 made by electrodeposition.24 In Fig. 1d, a very broad diffraction peaked around 10° was observed, which is due to α-type Co(OH)2 with poor crystallinity. The similar XRD pattern was reported by Zhao et al. for α-Co(OH)2 with poor crystallinity.27 Since α-Co(OH)2 is generally considered as an active OER catalyst, compared to other Co-based materials such as α- and β-Co3O4, it is often employed as the typical bulk catalyst.28 Fig. 1e was obtained for a thin film of layered MnO2 whose framework was doped with Co. The set of two peaks at 12.23° and 24.71° is characteristic of birnessite, which is similar to the film of K/MnO2 (c). This reflects the accommodation of K ions between the MnO2 layers doped with Co,22,29 which is quite different from the Co/MnO2 film described above. The thus-prepared thin film is hereafter denoted as K/Co-MnO2.

Figure 2. XPS spectra of the layered MnO2 film electrodeposited with TBA, taken (a) before and (b) after immersion in a 0.5 M CoSO4 solution.


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Figure 2 displays XPS spectra of Mn 3s, N 1s, Co 2p, and O 1s regions of TBA/MnO2 (a) and Co/MnO2 (b) thin films. The energy separation (∆E) of the doublet peaks in the Mn 3s core level spectra is sensitive to the oxidation state of Mn in the oxide.30 The ∆E value was estimated to be 4.70 eV for both TBA/MnO2 and Co/MnO2 films, which corresponds to an average oxidation state of 3.7 according to a linear relationship between the oxidation state of Mn and ∆E reported in Ref. 31. This confirms no chemical reaction between MnO2 layers and Co ions during the immersion step. The N 1s signal peaked at 401.8 eV evidences inclusion of TBA cations.32 A minor fraction was detected at 399.6 eV, which is likely attributed to neutral nitrogen. This may be resulted from charge transfer from anionic MnO2 layer to cationic TBA. These peaks disappeared after immersion in CoSO4 solution, which indicates that the immersion time of 24 h is long enough to complete the ion-exchange and can be controlled in order to tune the incorporated amount of Co ions.

Figure 3. (a) LSVs of Co/MnO2 and K/MnO2 electrodes taken in a 1.0 M KOH solution at a scan rate of 1 mV/s, and (b) the corresponding Tafel plots.


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Instead, new peaks appeared in the Co 2p region which had been absent in the as-deposited state. The Co 2p spectrum consisted of two peaks at 782.8 and 798.3 eV, attributable to Co2+, and its shake-up satellites (786.3 and 802.9 eV).33,34 The feature in the O 1s region of TBA/MnO2 is quite typical of MnO2, involving the main peak at 529.7 eV assigned to the lattice oxygen (O2–) in metal oxides. We found an obvious increase in intensity at 531.5 eV after immersion in Co2SO4 solution. This peak is generally attributed to the O–H bond in metal hydroxides and oxyhydroxides.30 In this case, however, they were unlikely produced according to the XRD and XPS results. Rather, it is reasonable to assume that OH– ions were trapped from solution into the interlayer space of buserite together with Co2+ ions. The same O 1s signal due to O–H was reported for the Co2+-intercalated birnessite synthesized chemically by Thenuwara et al., although the detailed explanation was not given.28 OER activity of Co/MnO2 and K/MnO2. Figure 3a depicts LSVs of the FTO-supported Co/MnO2 and K/MnO2 films, recorded in a 1 M KOH solution at a scan rate of 1 mV/s. The corresponding Tafel plots are shown in Fig. 3b. K/MnO2 exhibited no significant current in the potential region examined, which demonstrates that pure MnO2 layers themselves are inactive toward OER, at least in comparison with Co/MnO2. This observation is consistent with the results reported in the previous papers by us21 and other groups.28,35 The layered MnO2 film intercalated with Co2+ ions (Co/MnO2) presented much larger anodic current. This clear contrast indicates OER activity of the Co ions existed in the interlayer of buserite-type layered MnO2. In addition to the intrinsic catalytic activity of Co ion itself, a difference in crystalline structure may be a factor for the enhanced OER activity. As already shown in Fig. 1, the Co/MnO2 film with buserite structure has a larger interlayer distance (0.97 nm) than K/MnO2 (0.74 nm) with birnessite. The larger interlayer space can be advantageous with respect to the accessibility of


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electrolyte and the release of produced oxygen. Kang et al. prepared a series of birnessite-type MnO2 materials intercalated with different alkaline metals and found a decrease in η with increasing interlayer distance, where diffusion of water can be facilitated by the increased interlayer space.36 Considering the Co 2p XPS spectrum in Fig. 2, it is reasonable to think that the OH– ions trapped inside the large interlayer of Co/MnO2 are consumed for the OER process (4OH– → 2H2O + O2 + 4e–), as will be discussed later. Effects of the existing state of cobalt. We revealed the OER activity of Co/MnO2 in the above section, which resulted from the Co ions accommodated between MnO2 layers. In this section, the Co/MnO2 catalyst will be compared with the catalyst films with Co ions in different existing states. At all electrochemical tests, the deposited amount of matrix materials (Co(OH)2 and MnO2) was fixed to 1.02 µmol per unit area of the electrode by controlling the electrical charge delivered for electrodeposition. LSVs of the Co-containing catalysts are shown in Figure 4, where the measured current was normalized to the mass loading of Co (imass in A/gCo) in the catalyst since pure MnO2 layers themselves have no significant activity toward OER, as shown in Fig. 3a. The amount of Co was determined based on the ICP-AES analysis. The imass at η = 0.4 V calculated from the LSV curves increased in the order: K/Co-MnO2 (11.8 A/gCo) < α-Co(OH)2 (33.1 A/gCo) < Co/MnO2 (63.5 A/gCo). Clearly, the Co ions intercalated in the layered MnO2 possess much higher catalytic activity than those in the other states. This indicates that a high utilization efficiency of Co ions is realized by isolating them, where the state of Co ions is quite different from those bound in the oxide network and is similar to homogeneous catalysts.12 The Tafel slope of Co/MnO2 was smaller than that of α-Co(OH)2, reflecting better electron transfer kinetics. The smallest Tafel slope was observed for K/Co-MnO2, due probably to the improved


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electrical conductivity as a result of Co-doping into MnO2.37 However, the utilization efficiency of the Co sites in this catalyst should not be as high as those in Co/MnO2.

Figure 4. (a) LSVs of Co/MnO2, α-Co(OH)2, and K/Co-MnO2 electrodes taken in a 1.0 M KOH solution at a scan rate of 1.0 mV/s, and (b) the corresponding Tafel plots.

Chronoamperometry was employed to further examine the catalytic properties of Co/MnO2, αCo(OH)2, and K/Co-MnO2, where the applied overpotential was increased by 0.1 V increments. The results are shown in Figure 5a, where the observed current density was normalized to the mass loading of Co and denoted as imass (in A/gCo), similarly to the above. At all potential steps examined, the Co/MnO2-coated FTO electrode exhibited the highest imass values. In addition, the current of Co/MnO2 remained constant at each potential step, whereas K/Co-MnO2 exhibited a significant current decrease at larger overpotentials than 0.7 V. Fig. 5b shows turnover frequencies (TOF) of the catalysts at each potential, calculated from the results of Fig. 5a. TOF is


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generally defined as the number of produced molecules per mole of the catalyst per unit time, which can be expressed by the following equation:

TOF

(1)

Figure 5. (a) Chronoamperometric curves for the indicated catalyst films under a series of applied potentials in 1.0 M KOH solution, and (b) the TOF values as a function of overpotential.

where F is the Faraday constant, and Q is the integration of current density in a period of t (360 s). n is the mole number of the loaded active metal (Co) per unit area of electrode. The TOF of Co/MnO2 linearly increased from η = 400 mV. The value measured at η = 400 mV increased in


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the order: α-Co(OH)2 (0.0046 s–1) ≈ K/Co-MnO2 (0.0048 s–1)

Enhanced OER Activity of Co Ions Isolated in the Interlayer Space of

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