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Ultrathin cobalt-manganese layered double hydroxide is an efficient oxygen evolution catalyst Fang Song, and Xile Hu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja5096733 • Publication Date (Web): 07 Nov 2014 Downloaded from http://pubs.acs.org on November 9, 2014

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Ultrathin cobalt-manganese layered double hydroxide is an efficient oxygen evolution catalyst Fang Song and Xile Hu* Laboratory of Inorganic Synthesis and Catalysis, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), EPFL-ISIC-LSCI, BCH 3305, Lausanne, CH 1015 Switzerland.

* SnO2(FTO)

*

10

20

* *

30

015 107 0012 1010 0111 110 113 0114

a

012 009

robust and earth-abundant oxygen evolution reaction (OER) catalysts. Herein, we report that ultrathin nanoplates of cobaltmanganese layered double hydroxide (CoMn LDH) are a highly active and stable oxygen evolution catalyst. The catalyst was fabricated by a one-pot co-precipitation method at room temperature and its turnover frequency (TOF) is more than 20 times higher than the TOFs of Co and Mn oxides and hydroxides, and 9 times higher than the TOF of a precious IrO2 catalyst. The activity of the catalyst was promoted by anodic conditioning which was proposed to form amorphous regions and reactive Co(IV) species on the surface. The stability of the catalyst was demonstrated by continued electrolysis.

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ABSTRACT: Cost-effective production of solar fuels requires

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Supporting Information Placeholder

Intensity (a. u.)

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*

*

50

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* CoMn LDH

70

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2(degree)

c

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5.2 nm 2.9 nm

3.2 nm

‐Hydroxide

Water splitting offers an attractive chemical method for renewable energy storage.1 The oxidative half reaction of water splitting, the oxygen evolution reaction (OER), is kinetically sluggish and leads to significant overpotential and energy loss.2 Precious-metal electrocatalysts such as IrO2 and RuO2 are good OER catalysts, but their low abundance and high cost prohibit a large-scale application. Therefore, there is tremendous interest in developing inexpensive and earth-abundant OER catalysts.3 In neutral and weakly basic solutions Co-phosphate and Ni-borate catalysts are efficient but still require about 400 mV overpotential to reach 10 mA/cm-2.4 In alkaline solutions several forms of NiFeOx and perovskite Ba0.5Sr0.5Co0.8Fe0.2O3- (BSCF) are even more active than IrO2 and RuO2, reaching 10 mA cm-2 below 300 mV overpotential.5 It was proposed that the active form of NiFeOx catalyst has a layered structure.6 In agreement with this hypothesis, NiFe layered double hydroxide (NiFe LDH) is a very active OER catalyst in basic solutions, and its activity can be further enhanced by coupling to a carbon nanotube or by exfoliation.7 Many Co and Mn oxides including Co-containing LDHs such as NiCo and CoCo LDHs were studied for OER as well;6,8 however, they are generally less active than IrO2 and RuO2. To expand the scope of non-precious OER catalysts, more active Co and Mn-based oxides are desirable. Herein we report that the one-pot synthetic ultrathin nanoplates of cobalt-manganese LDH (CoMn LDH) are a highly active and stable catalyst for OER. The CoMn LDH catalyst exhibits much higher intrinsic activity than Co and Mn oxides, CoCo and NiCo LDHs, and IrO2 nanoparticles.

4.6 nm

CoMn LDH

Figure 1. (a) PXRD pattern of CoMn LDH. Asterisks denote peaks from fluorine-doped SnO2 glasses (JCPDS card No. 00046-1088) which is the substrate for XRD measurement. (b) Structures ofCo(OH)2 and CoMn LDH. (c) TEM image of CoMn LDH nanoplates. Parallel red short dash lines mark the thickness of upstanding nanoplates. The inset shows the SAED pattern of a lying nanoplate along the [00n] axis. The CoMn LDH nanoplates were synthesized at room temperature by a simple co-precipitation method. An aqueous solution containing Co(NO3)2•6H2O, Mn(NO3)2•4H2O, NaNO3 and NH4F was purged by N2, and then H2O2 was added to oxidize Mn(II) to Mn(III). The pH was adjusted to about 10 by drop-wise addition of a N2-purged NaOH solution (see Supporting Information for details). The CoMn LDH was formed as an earthy yellow precipitate. The X-ray diffraction (XRD) pattern (Figure 1a) confirmed the LDH structure of the compound.9 Figure 1(b) compares the atomic arrangement of CoMn LDH and Co(OH)2. Replacement of some Co(II) ions in -Co(OH)2 by Mn(III) yields CoMn LDH. To compensate the extra positive charge, an equivalent amount of anion is intercalated between the edge-sharing MO6 (M = Co or Mn) octahedral layers. This results in the layered structure of CoMn LDH (Figure 1b). Estimated from the angles of the

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XRD peaks, the interlayer distance is about 0.759 nm for CoMn LDH. The energy-dispersive X-ray spectrum (EDX, Figure S1, Supporting Information) confirmed the presence of Co and Mn. The atomic ratios of Co:Mn in the as-synthesized samples were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Table S1). The Co:Mn ratios are close to the ratios of Co:Mn in the starting materials, and can be varied from 1:1 to 4:1. X-ray photoelectron spectroscopy (XPS, Figure S2) suggests that Co and Mn ions are mainly in oxidation states Co(II) and Mn(III), respectively. Transmission electron microscopy (TEM) image indicates that the CoMn LDH has a platelet-like shape, with a diameter of 50~100 nm (Figure 1c). The near transparency to the electron beams (reflected by a faint image contrast) is indicative of the ultrathin nature. The thickness distribution was examined by measuring 153 upstanding platelets using TEM (Figure S3). The average thickness is 3.6 nm, corresponding to 6 edge-sharing octahedral MO6 layers. The selected area electron diffraction (SAED) pattern (inset in Figure 1c) was measured from one lying platelet. The hexagonally arranged spots indicate the single crystal nature of the platelet.

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30 60 90 120 -1 Scan rate (mV s )

Figure 2. (a) Linear sweep voltammetric curves of various metal oxides in 1 M KOH; scan rate: 1 mV s-1; scan direction: from lower to higher potentials. The inset showed corresponding Tafel plots. (b) Overpotential required for J = 10 mA cm-2 (@J = 10 mA cm-2) and current density at = 350 mV (J@ = 350 mV). I, II, III and IV represent CoMn LDH, IrO2 nanoparticles, Co(OH)2 + Mn2O3 and spinel MnCo2O4+, respectively. (c) Charging current density differences (J = Ja - Jc) plotted against scan rates. The linear slope is equivalent to twice of the double-layer capacitance Cdl. The error bar represents the range of results from three independent measurements. The electrochemical activity of CoMn LDH (Co:Mn = 2:1) and reference samples in OER in alkaline solutions was evaluated in 1 M KOH using a standard three-electrode system. Catalysts were uniformly drop-casted on a glassy carbon (GC) electrode with a loading of 0.142 mg cm-2. After electrochemical conditioning by 30 cyclic voltammetric scans to reach a relatively stable state, the OER activity was probed by linear sweep voltammetry (LSV) at a scan rate of 1 mV s-1. The reference samples include a mechanical mixture of Co(OH)2 and Mn2O3 (see Supporting Information for synthesis, Figure S4), nonstoichiometric spinel MnCo2O4+ (0