Interlayer Effect in NiCo Layered Double Hydroxide for Promoted

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Interlayer Effect in NiCo Layered Double Hydroxide for Promoted Electrocatalytic Urea Oxidation Min Zeng, Jinghua Wu, Zhiyun Li, Haihong Wu, Jinling Wang, Hualin Wang, Lin He, and Xuejing Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04953 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 2, 2019

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ACS Sustainable Chemistry & Engineering

Interlayer Effect in NiCo Layered Double Hydroxide for Promoted Electrocatalytic Urea Oxidation Min Zeng,†a Jinghua Wu,†b Zhiyun Li,†c Haihong Wu,a Jinling Wang,d Hualin Wang,d Lin He*a and Xuejing Yang*d a

State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of

Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou, 730000, China b

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences,

Ningbo 315201, China c

Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-

Bionics, Chinese Academy of Sciences, Suzhou 215123, China d

National Engineering Laboratory for Industrial Wastewater Treatment, East China University

of Science and Technology, Shanghai 200237, China. †The

three authors contribute equally to this paper

*Correspondence

authors: Lin He and Xuejing Yang

Email: [email protected]; [email protected]

ACS Paragon Plus Environment

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KEYWORDS: NiCo Layerd Double Hydroxide (NiCo LDH), Urea Electrooxidation, Interlayer Effect ABSTRACT. Urea electrolysis is a promising route to utilize urea-rich wastewater as an energy source to produce hydrogen on the cathode or generate electricity through direct urea fuel cell, which offers great potential for simultaneous water remediation and energy recovery. Here, we report a scalable synthetic strategy to prepare NiCo layer double hydroxide (NiCo LDH) as an efficient catalyst for urea electrooxidation. NiCo LDH with NO3− intercalant exhibited the best electro-catalytic performance and selectivity toward urea oxidation with a low onset potential, high Faradaic efficiency and high durability. The interlayer spacing in the LDH structure was found to play a pivotal role in the urea oxidation electrocatalysis with higher activity/selectivity under larger spacings. Further analysis of the urea oxidation product could potentially enable selective urine treatment into environmental-friendly products.


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INTRODUCTION. Human urine is one of the most important anthropogenic nutrient solutions covering 1% in volume enriches about 75%~85% of the nitrogen and 50% of the phosphorous to the domestic wastewater.1, 2 Urea is an energy-enriched compound, which is advantageous for its high aqueous solubility (1079 g/L @20℃), low volatilization, non-toxicity, ideal energy density (16.9 MJ/L and hydrogen content of 6.71 wt%) as well as a low theoretical voltage of 0.084 V for oxidation.3,

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The energy recovery from urea is a sustainable pathway toward both pollutant

control and energy harvesting. Recently, urea electrolysis has been considered as a potential route of utilizing urea-rich wastewater as an energy source to produce hydrogen on the cathode or generate electricity through direct urea fuel cell.5,

6, 7, 8, 9

It offers an appealing method to alleviate water

contamination eco-friendly and provide alternative energy sources. It also brought positive net effect to the down-steam urban infrastructure by reducing the energy consumption on nutrient removal in wastewater treatment plant.10 Compared to precious metals, nickel-based catalysts exhibit higher current densities and lower oxidation overpotential for urea electro-oxidation. During the intrinsically sluggish 6e− transfer process (CO(NH2)2+ 6OH− → N2 + 5H2O+ CO2 + 6e), nickel-based species can be oxidized to NiOOH (Ni3+) as the active sites for urea oxidation.3, 4, 11, 12

However, for the nickel-based catalysts, the overpotential of urea electro-oxidation is still

high (higher than 0.9 V).3, 11 In addition, Ni-based catalysts can also catalyze oxygen evolution reaction,

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which might serve as a competitive side-reaction to lower the chemoselectivity

toward urea oxidation. It poses a tremendous opportunity to develop efficient, robust, and


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inexpensive electro-catalysts to reduce the overpotential and improve the efficiency for urea electro-oxidation. Low dimensional nanomaterials have been applied to decrease the overpotential and promote the current density of electro-oxidation reactions.

13, 14, 15

Layered double hydroxides

(LDHs) are a class of two-dimensional (2D) clays that can be expressed as [M2+1xM

3+

+(An−)

x(OH)2]x

x/n

mH2O, and the exchangeable interlayer anions existed between the

positively charged brucite-like host layers keep the charge-balancing.16,

17

In such brucite-like

layers, a portion of divalent metal ions coordinated by hydroxyl groups in the octahedral structure are substituted by trivalent metal ions with positive charge. The interlayer charge compensating anion A can be different inorganic (such as NO3, CO32−, Cl−, SO42−, etc.) or various organic anions.18 Except for the anion intercalation, the vacancy in LDHs can also induce a reversible intercalation of cation, which develops a general method for earth-abundant transition-metal based materials in the application of electrochemical energy storage and conversion.19 Through incorporating various valence transition metal ions into the layered structure, the flexibility of LDH structure deliver the massive possibility to develop highperformance catalysts. Changing the nature of metal cations, the molar ratio of M2+/M3+, the type of interlayer anions and so on are recognized as the effective strategies to regulate the physical and chemical properties of LDHs, which satisfy the specific requirements in the different practical applications.16, 20 Within this context, we try to synthesize two-dimensional NiCo LDH nano-sheets as electrocatalysts for urea electroxidation, with the intention to decrease the overpotential and enhance the current efficiency for urea electrolysis. To better understand the mechanism of


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NiOOH (Ni3+) as the active sites, the rotating ring disc electrode (RRDE) and the online gas chromatography technique were applied to study the kinetic process and the oxidation product. Besides, we demonstrated the interlayer effect of anion confined in NiCo LDHs for enhancing urea oxidation performance. EXPERIMENTAL SECTION. Preparation of NiCo hydroxide and NiCo LDH. In a typical synthesis,21,

22

we dissolved

NiCl2·6H2O (0.594 g), CoCl2·6H2O (1.189 g), and hexamethylenetetramine (HMT) (5.6 g) in 1000 mL of de-ionized water and transferred into a 1000 mL round-bottom flask. The reaction solution was refluxed at 110 oC under N2 protection for 8 h to form light pink precipitate (NiCo hydroxide). The formed precipitate was filtered, then washed with ethanol and deionized water for at least 5 times. The pink precipitate was dried at 60 °C for 2 h in vacuum oven. Subsequently, NiCo hydroxide (500 mg) was dispersed in 500 mL of acetonitrile in an airtight flask with adding 3 mL of Br2 as oxidant. It is worth nothing that the airtight flask should be covered with tin foil to prevent from light. After magnetically stirring at room temperature for 48 h, the remaining mixture was collected through centrifugation, then ethanol washing process was repeated at least five times to obtain the yellowish green solid (NiCo LDH-Br). To prepare NiCo LDH-CO3 or NiCo LDH-NO3, NiCo LDH-Br (300 mg) was re-dispersed in an ethanol/water (1:1 v/v) solution saturated with Na2CO3 or NaNO3. This ion-exchange process with CO32− or NO3− was performed through the agitation at room temperature for 48 h by a mechanical shaker. Eventually, NiCo LDH-CO3 or NiCo LDH-NO3 was obtained through filtration followed by ethanol washing, then dried at 60 °C for 2 h in vacuum oven.


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Characterizations. The morphological characteristics for the synthesized NiCo LDH were analyzed by Scanning electron microscopy (SEM) using Supera 55 Zeiss scanning electron microscope. Transmission electron microscopy (TEM) images were performed on an FEI Tecnai F20 transmission electron microscope at 200 kV. The crystalline strutures were determined on X-ray Diffraction (XRD) through PANalytical X-ray diffractometer. The surface states of the samples were examined by X-ray Photoelectron Spectroscopy (XPS) spectra (SSI S-Probe XPS Spectrometer). The inter-planar distance and crystallite dimension were calculated from XRD through Bragg equation and Scherrer equation, respectively (Table S2). Bragg equation: 2dsinθ=nλ

(eq.1)

Scherrer equation: D=Kλ/Bcosθ

(eq.2)

Where d is the inter-planar spacing (nm), θ is the Braggs' angle (deg.), λ is the X-ray wavelength in eq. 1; D is the mean size of crystallites (nm), K is crystallite shape factor (K≈0.89), B is the full width at half the maximum (FWHM) of the X-ray diffraction peak in eq. 2. Electrochemical measurements. For urea electrolysis experiments, 1 mg of NiCo hydroxide, NiCo LDH-NO3 or NiCo LDH-CO3 powder and 0.5 mg of Ketjen black were dispersed into 250 μl ethanol with the addition of 10 μl of 5 wt% Nafion® solution. The homogeneous ink was obtained with the assistance of sonication for at least 40 min in cold water. Then 20 μl ink was dropcasted onto the rotation ring disk electrode (RRDE) (diameter 5.6 mm) to obtain a catalyst mass loading of 0.3 mg cm−2.


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RRDE voltammetry and polarization curves were performed on CHI 760 bipotentiostat in 1 M KOH solution with the scan rate of 50 mV s−1. Here a Hg/HgO electrode was applied as the reference electrode while a graphite rod was used as the counter electrode. The Faradaic efficiency ε for the competing OER reaction for urea electrolysis was calculated using the following equation: Ir

ε = Id ∗ S

(eq.3)

Where Ir is the collected ring current when the disk current Id was set on 200 μA. Here S is the standard current collection efficiency (Ir=42.6 μA, S=21.3 %) for OER which was calibrated by IrO2 catalyst film electrode.23, 24 To test the gas product during the urea electrolysis, the electrochemical measurements were carried out in an airtight two-compartment electrochemical cell which was separated by Selemion anion exchange membrane (Scheme S1). Hg/HgO electrode and Pt sheet (1 cm*2 cm) were applied as the reference electrode and counter electrode, respectively. To detect the faradic efficiency and current efficiency, the prepared catalyst ink was dropcast onto 1×1 cm2 Teflontreated carbon fiber paper (TGP-H-090, TORAY) to get a catalyst mass loading of 1 mg cm−2. During the electrocatalytic urea oxidation measurements, the electrolyte in the cathodic compartment was stirred at a speed of 300 rpm. Ar gas was introduced at ca. 20 ml min-1 using the mass flow controller (S48 300/HMT). The applied cell voltages were varied from 1.35 to 1.60 V. The generated gas bubbles were directly transferred into the 1 mL gas sampling loop in a gas chromatograph (Agilent 7890B) for the further quantitative analysis. Oxidation gas products were analyzed by a thermal conductivity detector (TCD) for the dectection of N2 concentration


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and then examined by a flame ionization detector (FID) with a methanizer for the quantitative test of CO2 and CO. The concentration of gaseous products was quantified with the calibration of standard gas. The faradic efficiency was calculated through the following equation: 1 mL Y NF 22.4L / mol FE(%)  100%  1 mL Q totalch arg e i  60 s / min  Q production

(eq.4)

where Y is the measured concentration of product gas based on the calibration with a standard gas, N is the number of electrons transferred in the reaction for the product gas, F is the Faraday constant (96500 C mol−1), i is the recorded current, and ν is the flow rate of Ar (ν= 20 sccm). RESULTS AND DISCUSSIONS Figure 1a schematically outlines our synthetic strategy toward NiCo LDH. In a typical synthesis, high-quality Ni1/3Co2/3(OH)2 nanoplates (denoting as NiCo hydroxide) were first synthesized from the controlled slow co-hydrolysis of Co2+ and Ni2+ salts with the assistance of hexamethylenetetramine (HMT).21,

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A brucite structure was demonstrated with an interlayer

spacing of 0.47 nm for NiCo hydroxide through the XRD pattern (Figure 1e and Table S2), while the SEM image revealed a morphology of hexagonal nanoplate with smooth surfaces and clean edges (Figure 1b). The lateral size of each crystallite was about 1~4 μm. Then these nanoplates were further oxidized by excessive Br2 for the replace of Co2+ with Co3+ in NiCo hydroxide, while the interlayer was expanded to 0.80 nm (Figure S1). NiCo LDH-Br was obtained during this Br- intercalation process. The product was further subject to an ion exchange with NaNO3 solution to obtain our desired product (denoted as NiCo LDH-NO3) with a further expanded interlayer spacing of 0.86 nm (Figure 1e and Table S2). Here the original hexagonal shape and lateral size was well maintained in NiCo LDH- NO3 nanoplates which was indicated by SEM


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(Figure 1c) and TEM (Figure 1d). The selected area electron diffraction (SAED) pattern exhibited a single set of diffraction spots along the [001] zone axis in NiCo LDH structure (Inset in Figure 1d). Based on the Scherrer equation, the thickness for each crystallite in NiCo LDHNO3 and NiCo hydroxide was calculated to be ~13.4 nm and ~31.9 nm, suggesting the presence of a delamination process during the intercalation and ion exchange steps. XPS analysis was further performed to investigate the surface chemical oxidation states of Ni and Co, which were consistent with the reported Co 2p and Ni 2p features in NiCo LDH-based materials (Figure S2 and S3).25, 26 We first investigated the electrochemical behavior of the NiCo LDH-NO3 catalyst in 1 M KOH with and without the presence of urea. The cyclic voltammetry curve of the NiCo LDHNO3 catalyst under no urea addition showed a pair of well-shaped redox peaks within the potential range from 0.2 to 0.6 V vs. Hg/HgO (Figure 2a), which corresponds to the Ni redox reactions between Ni2+ and Niδ+(δ≥3).27,

28, 29, 30

Further current increment at more positive

potentials is related to the electrocatalytic oxygen evolution reaction (OER). In strong contrast, the presence of 0.33 M urea brought a strong current increase at a potential where Ni2+ is oxidized, in well agreement with a traditional CV profile of an EC’ mechanism.5, 31 It suggests that the oxidized Ni species is highly active for catalytic urea electrooxidation. The slightly lower peak current density during the backward scan than the forward scan could possibly be resulted from the inefficient desorption of the urea transformation products on the electrode. The existence of the current rise at relatively large overpotentials is due to OER, implying the competition between urea electrooxidation and OER. The smaller Tafel slope (91 mV/decade) in the presence of 0.33 M urea demonstrated its faster kinetics compared to OER (Figure S4). Further, the onset potential of NiCo LDH-NO3 was 0.37 V vs. Hg/HgO, which is lower than


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most of the reported literature results (Table S1). The ideal urea electrooxidation reaction to form N2 can be expressed as CO(NH2)2+ 6OH− → N2 + 5H2O + CO2 + 6e with a total of six electron transfers.3, 12 Subsequently, we investigated the role of the urea and pH in our system. Upon the addition of 0.1 M urea, the catalyst electrode exhibited the characteristic oxidation peaks in both forward and backward scans. Increasing the urea concentration to 0.2 M and 0.33 M could lead to an obvious current increase, while further increasing the urea concentration above 0.5 M only showed slight current increase at a much lower rate than the concentration increase, indicating the depletion of urea molecules in the vicinity of the electrode (Figure 2c). It suggests that the electrocatalytic urea oxidation is a kinetically-controlled process at relatively low urea concentrations but a diffusion-controlled process at high urea concentrations. Therefore, a concentration of 0.33 M urea is used in the following study to maximize the current density in the kinetically-controlled region as well as to mimic the urea concentration in human urines. Furthermore, the electrochemical urea oxidation reactivity is highly dependent on the pH of the system. At a pH above 13, obvious urea oxidation peaks and Ni reduction peaks can be clearly resolved. However, at less alkaline conditions, both features disappeared with only negligible current rise at relatively large overpotentials (Figure 2b). To exclude the effect of solution resistance, we also measured the activity with the addition of supporting electrolyte to maintain the ionic strength and still observed lower reactivity toward urea oxidation. This is caused by the participation of hydroxide ion in the Ni hydroxide oxidation into Niδ+(δ≥3) species that is highly active for urea oxidation.3, 11 Durability is another important parameter to govern the long-term practical application for the electrocatalysts. The current density of NiCo LDH-NO3 maintained 45% after 40000 s durability test at a potential of 0.41 V vs. Hg/HgO, which is more stable than most of the


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reported urea oxidation electrocatalysts. As shown in the insets of Figure 2d, 95% of the peak current density in the CV curve maintained compared to the initial scan before 40000 s. After the electrolyte was changed to fresh 1 M KOH+0.33 M urea electrolyte, the peak current density was recovered to ~99%, illustrating that the activity loss was resulted from the consumption of urea. Since a combination of urea oxidation and oxygen evolution reaction could be observed at positive potentials, the chemoselectivity of the electrocatalyst was further studied in details. A high Faradaic efficiency toward urea oxidation is desired, because the catalytic active intermediates during oxygen evolution may bring deleterious side reactions in real applications (e.g. urine oxidation). We first carried out the voltage step analyses of NiCo LDH-NO3 catalyst electrodes versus a Pt cathode with and without the addition of urea, which aims to quantify the amount of current density which is generated from urea oxidation or water oxidation at different cell voltages (Figure 3a). A current efficiency is derived from the fraction of current density from urea oxidation through subtracting the current density in absence of urea from that in presence of urea at a given potential. The selectivity toward urea oxidation can reach >54 % at all potentials studied and a maximal current efficiency of ca. 88% can be obtained at a cell voltage of 1.50 V. Further increasing the cell voltage beyond 1.50 V can slightly suppress the current efficiency resulted from the competitive oxygen evolution reaction (Figure 3b). In addition to the current efficiency, we further explored possible ways of quantifying the O2 side products in order to obtain the Faradaic efficiency toward urea oxidation. Here rotating ringdisk electrode (RRDE) technique was applied to detect the Faradaic efficiency. The ring potential was set at 0.40 V (vs RHE) which was negative engouh to reduce the generated O2 from the center catalyst film, resulting to a continuous OER (disk electrode) → ORR (ring electrode) process.32 In Figure S5, negligible ring current was observed under no disk


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current, and the current of the ring is about 40 uA when we performed 200 μA the disk current in 1 M KOH. High current efficient toward oxygen evolution was obtained through the equation: 39.8 μA/21.3%/200 μA=93.4% (Here 21.3% is the current collection efficiency calibrated with the standard IrO2). As shown in Figure 3c, the constant disk current at 200 μA sustained a ring current of 7.6 μA in 1 M KOH + 0.33 M urea, indicating a current efficiency of ~20% (7.6 μA/21.3%/200 μA=17.8%) toward oxygen evolution and consequently ~80% toward urea oxidation. This is in excellent

agreement with the voltage step analyses. Further, we could quantify the amount of oxygen generated through the online gas chromatography (GC) and calculate the Faradaic efficiency toward OER. Assuming that the rest of the current was used to oxidize urea, we could obtain consistently high Faradaic efficiencies of 80-90% toward urea oxidation (Figure 3d). Interestingly, N2 and CO were detected during the GC analyses, and further efforts will be taken on analyzing the transformation product in the liquid phase for fully characterizing the detailed pathway of urea oxidation (Figure S6). The formation of CO2 in aqueous was confirmed by adding of Ba(OH)2 (as shown in Figure S7), which could format BaCO3 precipitation when CO2 reacting with hydroxide anions into CO32-. As reported in the previous studies,33, 34 urea may be possibly oxidized by the hydroxyl radicals formed on the anode, then nitrites or nitrates may be the product of this oxidation process. The detailed analysis of the reaction products is currently undergoing, and will be presented in our future study. One of the advantages of using NiCo LDH-NO3 catalyst is its well-defined layered structure with tunable intercalants for affecting catalysis. Thus, we further glean into the structural effects


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on the catalytic activity of urea oxidation, which might shine light on the catalyst design for activity improvement. NiCo hydroxide without the LDH structure and NiCo LDH with CO32− intercalant were utilized as control groups to compare the activity. The XRD patterns illustrate that NiCo hydroxide adopts the ordered brucite structure and exhibits lattice expansion along zaxis upon NO3− and CO32− intercalation (Figure 4a). The catalyst with NO3− intercalant shows slightly larger interlayer spacing than that with CO32− intercalant owing its larger anion size. Figure 4b shows the CV curve comparison of NiCo hydroxide, NiCo LDH-CO3 and NiCo LDHNO3 for urea electrooxidation and the activity followed the order: NiCo LDH-NO3 > NiCo LDHCO3 > NiCo hydroxide, which correlates well with the trend in the interlayer spacings (Table S2). It possibly suggests that the urea electrooxidation may partly take place in the interlayer region. The large electrochemical active surface area (ECSA) of NiCo LDH, calculated through the electrochemical double-layer capacitance (Cdl), may also account for their urea electrooxidation performance. As shown in Figure S8, the Cdl of NiCo LDH-NO3 was determined to be 3.1 mF cm─2, which is almost two times higher than that of NiCo hydroxide (Cdl= 1.6 mF cm−2) and larger than that of NiCo LDH-CO3 (Cdl= 2.4 mF cm−2). Previous reports suggested that large ECSAs could facilitate the transfer of electrolyte into the catalytically active sites on the surfaces.35 Accordingly, the high ECSA of NiCo LDH-NO3 further suggested its favorable reaction kinetics. The NiCo LDH-NO3 further demonstrated the lowest charge transfer resistance (Rct) among all three catalysts according to the electrochemical impedance spectroscopy (EIS), which may also contribute to the larger current rise for urea oxidation (Figure 4d). Additionally, different intercalants or interlayer spacings can pose effects on the chemoselectivity of the urea oxidation. Through voltage step analyses, NiCo hydroxide was found to exhibit low current efficiency


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Interlayer Effect in NiCo Layered Double Hydroxide for Promoted

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