Nanostructured Nickel Cobaltite Antispinel as Bifunctional

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Nanostructured Nickel Cobaltite Anti-spinel as Bifunctional Electrocatalyst for Overall Water-Splitting Leiming Tao, Yibing Li, Man Li, Guoying Gao, Mingkui Wang, Xingxing Jiang, Xiaowei Lv, Qingwei Li, Shasha Zhang, Zhixin Zhao, Chuan Zhao, Yan Shen, and Xin Xiao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08814 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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The Journal of Physical Chemistry

Nanostructured Nickel Cobaltite Anti-spinel as Bifunctional Electrocatalyst for Overall WaterSplitting Leiming Tao†, Yibing Li‡, Man Li†, GuoyingGao§, Xin Xiao†, Mingkui Wang†*, Xingxing Jiang†, Xiaowei Lv†, Qingwei Li†, Shasha Zhang†, Zhixin Zhao†, Chuan Zhao‡ and Yan Shen† †

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and

Technology, Luoyu Road 1037, 430074, Wuhan, P. R. China ‡

School of Chemistry, University of New South Wales, New South Wales 2052, Sydney,

Australia §

Department of Physics, Huazhong University of Science and Technology, Luoyu Road 1037,

430074, Wuhan, P. R. China ABSTRACT: One major goal in the field of catalysis is to develop efficient, low cost and stable catalysts to replace noble metal-based materials. Herein, we for the first time present the nickel cobaltite (NiCo2O4) of nanosheets morphology (NCO-NSs) and nanowires morphology (NCONWs) as efficient electrocatalyst for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution, affording a current density of 10 mA cm-2 at low overpotential of 170 mV for the HER and 230 mV for the OER, respectively. With the aid of

ACS Paragon Plus Environment

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DFT calculation, we have evaluated their special catalytic activities by focusing on the position of d-band centers and adsorption energy of intermediates (H2*, H*, H2O*, OH*, O*, OOH*, and O2*) on catalyst surface. The result reveals that the rate-determining-step for HER and OER is H* and O* adsorption, respectively, with the NiTd3+ (Ni3+ occupied at the tetrahedral site) being the active site for water-splitting. A highly efficient two-electrode electrolyzer based on NCONSs||NCO-NWs configuration is designed and tested for long-term stability evolution. Furthermore, a NCO-NSs||NCO-NWs based alkaline electrolyzer can be approach 15 mA cm-2 at 1.65 V, superior to that of Pt||IrO2 couple, along with strong stability.


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1. INTRODUCTION Currently the energy crisis presents major challenges and alternative sources to natural resources should be sought to ensure sustainable development while reducing energy consumption. Hydrogen is a sustainable energy carrier that promises an environmentally friendly alternative to meet the future global energy demand. At present, hydrogen is mainly produced through steam methane reforming and coal gasification. However, those methods raise the fuel's carbon footprint despite the fact that it burns cleanly. Electrochemical technology is cleaner and can offer environmental protection through nonpolluting energy sources. Hydrogen production via water electrolysis in combination with renewable energy sources offers the possibility of increased production capacity with no greenhouse emissions. Alkaline water electrolysis is the stat of the art for industrial large scale water electrolysis systems. In order to further increase the efficiency of industrial alkaline water electrolysis, it is highly challengeable to reduce the large overpotential for both half reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) during water electrolysis. Particularly the OER is highly irreversible in aqueous electrolytes and causes substantial energy loss in the electrochemical cell. The main requirements of the electrodes for water splitting are high surface area, high electric conductivity, good electrocatalytic properties, and minimization of gas bubble adhesion and low cost. Commercial electrolyzers typically operate at a cell voltage of 1.8-2.0 V, which is much higher than the theoretical value of 1.23 V for water splitting.1 Therefore, efficient catalysts for HER and OER with long-term stability are required. Noble metals such as Pt- and Ir (Ru)-based catalysts work most efficiently for the HER and the OER, respectively, which however suffer from scarcity and high cost, thus hindering their widespread utilization.2-3 Great progress has been made over the past few years in developing low-cost catalysts from earth-abundant


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transition metals for the HER (e.g. nitrides, chalcogenides, carbides, and phosphides)4-7 and the OER (e.g. oxides, hydroxides, perovskites, nitrides, chalcogenides, and phosphates).8-13 Transition metal oxides have been proposed as efficient catalysts for chemical and electrochemical reactions due to their various valence states.3 Among transition metal oxide, spinel oxides form an interesting class of compounds with remarkable optical, electrical, magnetic and catalytic properties.14 For example, cobalt spinels oxides have received great attention due to their high catalytic activity, large availability, low cost and their good corrosion stability.15 The cobalt spinels oxides have a general chemical formula of MCo2O4 (where M=Ni, Fe, Co, Mn, Cu, Zn) that exhibits a cubic unit cell and FD-3m space group, where M represents a divalent cation (M2+), which occupies one-eighth of tetrahedral holes, where as the trivalent cation (Co3+) occupies one half of the octahedral holes in a cubic close packed array of oxide anions.16 These compounds exhibit relatively high conductivity due to electron transfer between the cations, with low activation energy due to hopping mechanism. The NiCo2O4 was found to possess high stability, good electrochemical property for the OER, and low cost.17 NiCo2O4 with different morphologies has been successfully prepared via liquid phase co-precipitation, solvothermal/hydrothermal synthesis, microwave, electrodeposition, electrospinning and selfassembly methods.18-21 Recently, hierarchical NiCo2O4 hollow microcuboids constructed by 1D nanowires has been reported as an efficient bifunctional electrocatalyst for overall watersplitting, which can deliver a cell current density of 10 and 20 mA cm-2 at 1.65 and 1.74 V, respectively.22 The relationship between the morphology and catalytic activity has been intensively investigated by controlling the size, shape, and facets of the nanocrystals.23-28 However, it still remains a challenge to rationally design morphology-specific catalysts for efficient water splitting application. The morphology-dependent overpotential of OER was


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observed for NiCo2O4 but without pointing out the reaction mechanism, such as core-ring nanoplate (0.315 V vs. SCE), aerogel (0.184 V vs. Hg/HgO), nanorod (0.31 V vs. Ag/AgCl), and nanoporous nanosheet (0.22 V vs. RHE).18, 29-31 Density functional theory (DFT) calculation can be a powerful tool to investigate surface reactivity of MCo2O4 spinels oxides, identifying corrections between activity and adsorption strength of reaction intermediates and electronic structure.17,

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In order to design a suitable bifunctional catalyst for overall water splitting

application, it is highly necessary to understand the relationship between morphology, structure, composition and electron transfer properties. In this research we have prepared NiCo2O4 of various morphologies and addressed their catalytic activity towards oxygen evolution and hydrogen evolution reactions. The aim of the present study was to develop electrode materials for hydrogen production in order to improve the efficiency and durability. Our investigation reveals the ultrathin NiCo2O4 nanosheets (NCO-NSs) and NiCo2O4 nanowires (NCO-NWs) can be responsible for the high activity of HER and OER in 1 M KOH solution, affording a current density of 10 mA cm-2 at low overpotential of 170 mV for HER and 230 mV for OER, respectively. This suggests a smart approach for engineering the catalytic performance by simply alternating the morphology structure of the martial. With the help of DFT calculation on the d-band center energy and the adsorption energy of H2*, H*, H2O*, OH*, O*, OOH*, and O2* on different shaped NiCo2O4, we have successfully identify spinel nickel cobaltite catalysts of two distinctly various morphologies with highly catalytic capacity for water splitting. We demonstrate the adsorption of H* and O* on NiCo2O4 (311) surface is the rate-determining-step of the HER and OER process in alkaline solution, respectively, in which the NiTd3+ (Ni3+ occupied at the tetrahedral site) acts as the active site for water-splitting. Furthermore, a NCO-NSs||NCO-NWs two-electrode alkaline electrolyzer approached 15 mA cm-


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at a low cell voltage of 1.65 V, which is superior to that of Pt/IrO2 couple, along with strong

stability. 2. EXPEIMENTAL SECTION Preparation of the NCO-NSs. In a typical synthesis process, 3.6 mM Ni(NO3)2·6H2O, 7.2 mM Co(NO3)2·6H2O and 15 mM urea were added into a mixed solvent of 30mL deionized (DI) water and 10 mL ethylene glycol under stirring to form a homogeneous solution. The solution was then transferred to a 50 mL Teflon-lined stainless steel autoclave and heated at 100 oC for 2 h. After retrieved by centrifugation and washed several times with distilled water and absolute ethanol, the resulting product was dried in vacuum at 80oC for 12 h. The final NCO-NSs were obtained through the heat treatment of the NiCo2 precursor at 400 oC for 2 h. Preparation of the NCO-NWs. 3.6 mM Ni(NO3)2·6H2O, 7.2 mM Co(NO3)2·6H2O and 35 mM urea were added into a mixed solvent of 30 mL DI water and 20 mL ethanol. The solution was then transferred to a Teflon-lined stainless steel autoclave followed by putting a piece of cleaned carbon fiber cloth (1 cm×2 cm). The hydrothermal reaction occurs for 8 h at 100oC. The final NCO-NWs were obtained at 400 oC for 2 h. Characterization. The morphology, chemical composition, and structure of samples were characterized X-ray diffraction (XRD) measurements were performed by X'pert PRO diffractometer (PANalytical B.V.) using a Cu Ka radiation source as the X-ray source for excitation, operated at 40.0 kV and 40.0 mA within the 2θ range from 15 to 80°, the field emission scanning electron microscopy (FE-SEM, Nova NanoSEM 450), the high resolution transmission electron microscopy (HRTEM, 300 kV Titan Probe corrected TEM, Titan G2 60300), and X-ray photoelectron spectroscopy (XPS, Thermofisher-ESCALab 250). The Brunauer-


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Emmett-Teller (BET) surface area (SBET) and pore size distribution were determined using a Micromeritics ASAP 2000 nitrogen adsorption apparatus. All the samples were degassed at 180 °C prior to BET measurements. Atomic force microscopy (AFM) measurements were obtained through Veeco Dimension 3100 NanoScope in a tapping mode measured by Bruker RTESPA300 probes. Electrochemical measurements. For the electrode preparation, 2 mg of NiCo2O4 powder was dispersed into a 1 mL mixture of water, ethanol and Nafion (5 wt. % solution in a mixture of lower aliphatic alcohols and water, Aldrich) with the volume ratio of 1:3.85:0.15 by ultrasonication for 30 min. This afforded a catalyst ink of concentration 2.0 mg mL-1. Afterward, 19.8 µL of the suspension was transferred onto the polished 5 mm in diameter glassy-carbon electrode, giving a mass loading of 0.2 mg cm-2. The cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were conducted with an electrochemical station (CHI 750D), in which the GC electrode (5 mm in diameter, Pine Instruments) was used as the working electrode, a graphite electrode as the auxiliary electrode, and a Hg/HgO as the reference electrode. The electrochemical impedance spectroscopy was measured by an Autolab PGSTAT302N at a frequency ranging from 1 mHz to 1 MHz with a potential amplitude of 10 mV. The rotating ring-disk electrode (RRDE) measurements were conducted in 1 M KOH electrolyte at room temperature using a three-electrode system (Pine Instruments and WaveDriver Workstation). All potentials were measured versus a Hg/HgO, and a carbon electrode was used as the counter electrode. Several cyclic voltammetry cycles were taken from 0 to 0.8 V (vs. Hg/HgO) to stabilize the HER and OER performance of the catalyst before polarization curves were recorded. Recently, it had been reported that the amount of Pt deposited


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on catalyst surface after multiple circular scanning improves the catalytic performance.33 Therefore, a graphite electrode was used as the counter electrode in this study. All potentials measured were calibrated to the reversible hydrogen electrode (RHE) using the following equation: ERHE = EHg/HgO+ 0.098 V + 0.059 pH. For oxygen evolution reaction OER (or HER) tests, first of all, the electrochemical accessibility of the working electrode was optimized by potential cycling between 1.1 and 1.6V (or -0 and -0.3V for HER) at 50 mVs-1 in 1M KOH until stable voltammogram curves were obtained. Then, the polarization curves and Tafel plots were recorded at scan rates of 5 mV s-1 and 0.1 mV s-1, respectively. The solution impedance (R) of 1M KOH measured was 6.9Ω at room temperature. Accurate Tafel plots of samples were obtained according to a previous method.

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RRDE was conducted in N2-saturated 1M KOH

solution seven times with altering rotation speeds (400 r.p.m., 620 r.p.m.,900 r.p.m., 1225 r.p.m., 1600 r.p.m., 2025r.p.m., 2500 r.p.m., respectively). The turnover frequency (TOF) was evaluated by the following standard equation:36

J×A TOF = 4 × F × m

(1)

Here, J is the current density (A cm-2) at an overpotential of 0.3V. A and m are the area of the electrode and the number of moles of the active materials that were deposited onto the electrode, respectively. F is the Faraday constant (96,485 C mol-1). The Faradaic efficiency (FE) was obtained according to the previous literature:37 I ring

FE= Ce × I disk

(2)


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Here, Idisk is the given current on the disk electrode. Iring is the collection current on the Pt ring electrode at a constant potential of 0.4V versus RHE. Ce is the oxygen collection coefficient (~0.2) for this type of electrode configuration. Theory. The calculation was based on density functional theory using a plane-wave pseudopotential formalism, aided by the Accelrys Materials Studio (Accelrys Inc.) graphical front-end interface. The detail information was put in the Supplementary Methods. 3.

RESULTS AND DISCUSSION

The ultrathin NiCo2O4 nanosheets (NCO-NSs) and NiCo2O4 nanowires (NCO-NWs) samples were synthesized according to previous reports with some modification.19,

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The synthesis

process of NiCo2O4 samples is given in experimental section. The morphology, chemical composition, and crystal structure of the obtained samples were characterized with field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). Figure 1a and 1d show the FE-SEM images of resulted NCO-NSs and NCO-NWs samples. The NCO-NWs are uniform and composed of nanowires (1.5 µm in length and 20 nm in diameter). The NCO-NSs are uniform and free standing with a micron-sized planar area. Atomic force microscopy image (Figure 1h) confirms the 5 nm-thick of NCO-NSs. The morphology and inter-lattice structure detail were further studied by high-resolution TEM (HRTEM, Figure 1b, 1c, 1e and 1f). Both the NCO-NSs and NCO-NWs exhibit clear lattice fringes with inter-planar spacing of d311 = 0.24 nm, which interlaces with each other at an angle of 36.7o and matches well with the spinel NiCo2O4 phase indicated by XRD patterns (Figure 1g). The selected area electron diffraction (SAED)


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characterization as shown in the inset of Figure 1c and 1f confirms polycrystalline structure with dominant exposed (311) for both samples.

Figure 1. Typical SEM images of (a) NCO-NSs and (d) NCO-NWs; TEM images of (b) NCONSs and (e) NCO-NWs; HRTEM images of (c) NCO-NSs and (f) NCO-NWs, (inset) selected area electron diffraction (SAED) acquired from the corresponding of NCO-NSs and NCO-NWs; (g) XRD of spinel NiCo2O4 (JCPDF 20-0781), NCO-NSs and NCO-NWs; (h) Atomic force


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microscopy image of NCO-NSs scanned with 3 µm × 3µm size and enlargement of partial NCONSs and the corresponding height profile of the lines shown in the inset; NCO-NSs possess uniform thickness of 5 nm from the height profile. Scale bars, 1µm (a, d), 100 nm (b, e), 5 nm (c, f), and 0.5 µm (h), respectively. It is reported that the activity of nanostructured catalysts could be partly due to their increased electrochemical surface area (ECSA) that provides a larger number of catalytically active sites. Therefore, the ECSA is the product of RFS, in which S stands for the real surface area of the smooth metal electrode, which generally equals to the geometric area of glassy carbon electrode.39 In this study, the surface area of nanostructured NCO-NSs and NCO-NWs was characterized by estimation of the roughness factors RF from the ratio of Cdl of the test sample and the Cdl (= 60 µF cm-2) of a smooth surface.40 The RF obtained with cyclic voltammetry (Figure S1) indicates the total catalytic activity area in the order of NCO-NWs (266) > RuO2 (91) > NCO-NSs (88)> Pt/C (6) (Table S1). The measurement with impendence spectroscopy further confirms this tendency (Figure S2). This suggests that the NCO-NWs could present higher reaction activity being relative to surface area compared to NCO-NSs. For the following characterization, we have corrected the activity of NCO-NSs and NCO-NWs by considering the contribution of ECSA (ECSA-corrected). The catalytic activity of the prepared NCO-NSs and NCO-NWs loaded onto glassy carbon electrodes towards oxygen evolution and hydrogen evolution reactions was studied by voltammetry in a three-electrode electrochemical cell. Figure 2a presents the HER polarization curves without iR compensation for the NCO-NSs and NCO-NWs electrodes in 1.0 M KOH solution at a scan rate of 5 mV s-1, respectively. For comparison purpose, the commercial electrodes of Pt/C and RuO2 were also characterized. The activity of an electrochemical device is


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usually expressed in terms of exchange current density or overpotential. The exchange current density (j0) is the current density of the reversible potential (for OER as an example) and the overpotential (η) measures how much the potential has departed from the reversible value to drive a reaction of a given current density. The overpotential η of various electrode for HER at a current density of 10 mA cm-2 follows the order of Pt/C (120 mV) < NCO-NSs (180 mV) < NCO-NWs (290 mV) < RuO2 (340 mV), with the NCO-NSs having the second best HER capability after Pt. It is significant that the overpotential η of NCO-NSs and NCO-NWs catalysts for HER at j = 10 mA cm-2 are smaller than the previously reported NiCo2O4 HER electrode (317 mV).1, 41 The catalytic kinetics of NCO-NSs and NCO-NWs was assessed by Tafel plot (Figure 2b) derived from Koutecky-Levich plots in an N2-satturated 1M KOH solution (Figure S5) on the rotating ring-disk electrode (RRDE) at rotation speeds of 400, 625, 900, 1225, 1600, 2025, 2500 rpm (Figure S3 and S4).34 The Tafel slope provides insight into the reaction mechanism and catalytic activity. A smaller Tafel slope indicates the overpotential of the catalytic is lower at the same dynamic current density or apparent current density.1 As shown in Figure 2b, the resultant Tafel slop of NCO-NSs (63 mV dec-1) is smaller than that of NCO-NWs (85 mV dec-1), signifying its superior HER reaction kinetics. The close values of overpotential and Tafel slope for the ultrathin NiCo2O4 nanosheets electrode to the Pt/C electrode (39 mV dec-1) indicate its superior activity to the HER. We further analyzed the catalytic dynamics in terms of turnover frequency (TOF) towards the HER. A high turnover frequency (TOFH) (0.26 s-1 calculated at an overpotential of 0.30 V) is observed for the NCO-NSs, which is significantly larger than that of NCO-NWs (0.07 s-1) (Figure S6a). Likewise the OER performance of these samples was evaluated. Figure 3c presents the OER polarization curves without iR compensation for various electrodes in 1.0 M KOH solution at a scan rate of 5 mV s-1, respectively. The overpotential η of


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various electrode for OER at a current density of 10 mA cm-2 follows the order of RuO2 (220 mV)

Nanostructured Nickel Cobaltite Antispinel as Bifunctional

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