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reduced graphene oxide nanohybrid as an efficient bifunctional electrocatalyst for oxygen evolution and reduction reactions
Journal of Power Sources 333 (2016) 53e60
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NiFe layered double hydroxide/reduced graphene oxide nanohybrid as an efficient bifunctional electrocatalyst for oxygen evolution and reduction reactions Tianrong Zhan a, *, Yumei Zhang a, Xiaolin Liu a, SiSi Lu a, Wanguo Hou b, ** a Key Laboratory of Sensor Analysis of Tumor Marker (Ministry of Education), State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China b Key Laboratory of Colloid & Interface Chemistry (Ministry of Education), Shandong University, Jinan 250100, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
NiFe-LDH/rGO catalyst was obtained via solvothermal and chemical reduction method. OER and ORR activities of NiFe-LDH/ rGO in alkaline media were firstly studied. NiFe-LDH/rGO shows better OER activity with a lower onset overpotential of 240 mV. NiFe-LDH/rGO gives a 0.796 V ORR onset potential mainly involving the 4e pathway.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 July 2016 Received in revised form 12 September 2016 Accepted 26 September 2016
Highly active and low-cost bifunctional electrocatalysts for oxygen evolution and reduction reactions (OER and ORR) hold a heart position for the renewable energy technologies such as metal-air batteries and fuel cells. Here, we reported the synthesis of NiFe layered double hydroxide/reduced graphene oxide (NiFe-LDH/rGO) nanohybrid via the facile solvothermal method followed by chemical reduction. The template role of surfactant and the hybridization of rGO supplied the NiFe-LDH/rGO catalyst with a porous nanostructure and an enhanced conductivity, favoring both mass transport and charge communication of electrocatalytic reactions. The NiFe-LDH/rGO composite not only displayed highly efficient OER activity in alkaline solution with a low onset overpotential of 240 mV, but also only needed an overpotential of 250 mV to reach the 10 mA cm2 current density. The NiFe-LDH/rGO nanohybrid also offered excellent ORR catalytic activity with onset potential at 0.796 V in alkaline media. The rotatingdisk and rotating-ring-disk electrodes both revealed that the ORR on NiFe-LDH/rGO mainly involved a direct four-electron reaction pathways accompanying part of the two-electron process. The excellent bifunctional activity of the NiFe-LDH/rGO nanohybrid could be attributed to the synergistic effects of rGO and NiFe-LDH components due to the strongly coupled interactions. © 2016 Elsevier B.V. All rights reserved.
Keywords: NiFe layered double hydroxide Reduced graphene oxide Oxygen evolution reaction Oxygen reduction reaction
1. Introduction * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T. Zhan), [email protected] (W. Hou). http://dx.doi.org/10.1016/j.jpowsour.2016.09.152 0378-7753/© 2016 Elsevier B.V. All rights reserved.
The growing energy demand and the worsening global environment have inspired a substantial and intensive development of
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alternative clean and renewable energy to fossil fuel [1,2]. Electrocatalytic oxygen evolution and reduction reactions (OER and ORR) are unambiguously the core processes of the many sustainable energy technologies, including rechargeable metal-air batteries and unitized regenerative fuel cells [3e5]. The OER on the anode is a charging process for energy storage in solar fuel synthesis and water splitting, whilst the ORR on the cathode is a discharging process in fuel cells and metaleair batteries [6,7]. However, the sluggish kinetics of OER and ORR generally result in the wide charging-discharging potential gap, consequently impairing the efficiency of fuel cells and metal-air batteries [6e8]. Hence, it is the challenging work to develop the bifunctional electrocatalysts with low overpotentials for both the OER and ORR [1,9]. Some precious metals such as Pt, Ir, and Ru based materials have been potentially used as bifunctional catalysts to expedite the low oxygen reactions, however their high cost and scarcity hamper their long-term and wide commercial applications [1,10]. Therefore, many efforts have been focused on the development of inexpensive bifunctional electrocatalysts with robust efficiency. Transition metal hydroxides [11], oxides [10], chalcogenides [12], phosphate [13], metal carbides [14], perovskites [6] and their composite [15] have drawn much interest as low-cost alternatives to noble-metal catalysts for both the ORR and OER. However, these bifunctional electrocatalysts still experience poor catalytic efficiency owing to their low conductivity, instability, and small surface area. Layered double hydroxides (LDH), also named lamellar anionic clay, present a class of hydrotalcite-like compounds with a xþ n generic chemical formula of [MII1-xMIII x (OH)2] [(A )x/n$mH2O] [16]. They are composed of brucite-like layers with forever positive charges owing to the isomorphic substitution of MII by MIII, and the charge balancing anions in interlayer regions. The major advantages of these 2D materials are that they can be constructed by the broad-spectrum metal cations and the exchangeable interlayer intercalators [17,18]. Additionally considering the intrinsic catalytic activity, different types of transition metal-based LDH and their composites have been prepared for the potential application in electrocatalysis. For example, NiFe-LDH [19e21], ZnCo-LDH [22], NiCo-LDH [23] and CoMn-LDH [24] have been testified as excellent OER catalysts in alkaline solution. Although the original LDH lamellar materials have been hardly used as ORR catalysts, some oxides derived from them have displayed the outstanding catalytic performance toward ORR [10,25]. However, the positively charged LDH surface is desirable for oxygen adsorption and concomitant ORR [26], which can endow LDH and their composites with the reasonably expectable ORR activity. Given that the 3d metals, such as Ni, Co and Fe, in the oxide state are the electroactive species of OER and ORR [11,27], the 3d metal based hydroxide and their composite can be a class of promising bifunctional catalysts. On the other hand, due to the poor conductivity of LDH catalysts, their catalytic performance can be further enhanced by combining with nano-carbon such as reduced grapene oxide (rGO) or carbon nanotubes (CNT) [1,27]. Therein, high conductivity, better stability and large surface area make graphene as an ideal conductive substrate for LDH. FeNi-LDH/graphene [20] and NiFeLDH/rGO [28] have been fabricated as the superior OER electrocatalysts. However, to the best of our knowledge, the NiFe-LDH/rGO composite has been not yet investigated as a bifunctional catalyst for both of OER and ORR so far. In view of the high electro-active peculiarity and the strong hybridizing ability of Ni and Fe elements [29], we presently describe the preparation of NiFe-LDH/rGO nanohybrid as an efficient bifunctional electrocatalyst toward OER and ORR. Through a one-pot solvothermal method in presence of surfactant and the following chemically reductive treatment, we have afforded the porous NiFe-LDH/rGO composite. The intimate hybridization of
NiFe-LDH nanoplatelets with rGO nanoparticles can offer the enhanced conductivity and catalytic activity. The NiFe-LDH/rGO nanohybrid displays the excellently bifunctional catalytic performance for both OER and ORR in alkaline medium. Furthermore, the NiFe-LDH/rGO catalyst exhibits the better bifunctional catalytic activity than NiFe-LDH and NiFe-LDH/GO counterparts, which can be ascribed to the facilitated charge transportation and the good synergistic effects of rGO and NiFe-LDH in the electro-catalyst. 2. Experimental 2.1. Chemicals Graphite powder (average particle size 30 mm) was obtained from Colloid Chemical Co. (Shanghai, China). Ethylene glycol (EG), sodium dodecyl sulfonate (SDS) and hydrazine monohydrate were purchased from Aladdin Chemistry Co. (Shanghai, China) as analytical pure. Nafion solution (5%) was obtained from DuPont Co. (USA). Polytetrafluoroethylene (PTFE: 60%, 0.20 mm) was supplied by Shenzhen Dechengwang S&T Ltd. All of the reagents were used as received without further purification. 2.2. Preparation of catalyst Firstly, graphene oxide (GO) was synthesized from nature graphite powder by a modified Hummer's method [30]. The resultant GO powder was ultrasonically dispersed in EG with concentration of 0.5 wt%. NiFe-LDH/rGO composite was prepared by a solvothermal method. Typically, 0.67 mmol of FeCl3$6H2O and 2.00 mmol of NiCl2$6H2O were dissolved in 20 mL above EG solution (containing 0.5 wt% GO). Then 0.6 g SDS was added and stirred for 1 h at 40 C. Then 10 mL EG and 0.16 g NaOH were added into the reactor with additional 30 min magnetic stirring. The mixtures were transferred to the Teflon lined stainless steel autoclave and thermally treated at 160 C for 24 h. After centrifuging and washing with excess water and ethanol orderly, the black NiFe-LDH/GO powders were obtained. In order to acquire the completely reductive products, NiFe-LDH/GO powders were re-dispersed in 100 mL water with the final concentration as 1 mg/mL 50 mL hydrazine hydrate (35 wt%) and 350 mL ammonia water (27 wt%) were further added into solution for reduction. After 5 min stirring, the mixed solution was transferred to Teflon lined stainless steel reactor for solvothermal reaction at 90 C for 1 h. With similar purification, the NiFe-LDH/rGO composited catalyst was afforded. For the comparison, the pure FeNi-LDH sample was also prepared by the same procedure without GO and reductive step. 2.3. Characterization of catalyst X-ray diffraction patterns (XRD) were characterized on a Rigaku powder diffractometer equipped with Ni-filters Cu Ka radiation (l ¼ 1.54050 Å, 40 kV and 100 mA). The morphology was observed with a JSM-6700F scanning electron microscope (SEM, Japan Electron Co.) and a JEOL JEM-2000EX transmission electron microscope (TEM, Japan Electron Co.). The elemental analysis of the samples were performed on an energy-dispersive X-ray spectroscopy (EDS, Oxford instruments X-Max). By the BET method, the specific surface areas are obtained from N2 adsorption isotherms using an apparatus (Quantachrome-Autosorb-1C; Quantachrome Instr., USA). XPS was conducted by using an HP 5950A ESCA spectrometer with an MgKa source. 2.4. Electrochemical measurements Electrochemical measurements were performed on CHI 660D
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workstation with a standard three-electrode system. The saturated Ag/AgCl and platinum flake (1 2 cm2) were respectively used as reference electrode and counter electrode. The reference potential was converted to the reversible hydrogen electrode (RHE) via the Nerst equation [31]:
ERHE ¼ EAg=AgCl þ 0:059pH þ 0:1976
(1)
OER test: 10 mg active material, 400 mL ethanol and 20 mL PTFE were ultrasonically mixed to form a homogeneous ink. The working electrode was fabricated by pipetting out some slurry on Ni foam electrode (1 1 cm2) with the final loading of 0.5 mg/cm2 1.0 M KOH solution was acted as electrolyte and saturated with N2 before each test. Linear sweep voltammetry (LSV) was carried out at a scan rate of 10 mV s1 for polarization and the Tafel plots were transformed from LSV. ORR test: 10 mg active material, 400 mL ethanol and 15 mL Nafion (1.0%) were ultrasonically mixed by ultrasonic to form a homogeneous ink. The working electrode was prepared by casting some slurry on Rotating disk electrode (RDE, 0.1256 cm2) with the final loading of 0.18 mg/cm2 0.1 M KOH solution was acted as electrolyte and saturated with O2 before each test. CV was performed at a scan rate of 50 mV s1, LSVs was recorded at 10 mV s1 with various rotate speeds (400e2025 rpm). The exact kinetic parameters for the ORR activities, including electron transfer number (n) and kinetic current density (Jk), were investigated based on the KouteckyeLevich (K-L) equation [1]:
1 1 1 1 1 ¼ þ ¼ þ J JK JL JK B$u1=2 2=3
B ¼ 0:62$n$F$C0 $D0 $n1=6
(2)
(3)
where J is the measured current density, Jk is the kinetic current density, u is the electrode rotate speed, n represents the number of electron transferred per oxygen molecule, F is the Faraday constant (96485 C/mol), D0 is the diffusion coefficient of O2 in 0.1 M KOH (1.90 105 cm2/s), v is the kinematic viscosity of the electrolyte (0.01 cm2/s), and C0 is the bulk concentration of O2 (1.2 106 mol/ cm3) [1]. The constant 0.2 is used when the rotate speed is expressed in rpm. For the Rotating ring disk electrode (RRDE) measurements, the %HO 2 and the electron transfer number n were determined by the following equations [15]:
%HO 2 ¼
200$Id N$Id þ Ir
(4)
n ¼ 4Id
Ir Id þ N
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(5)
where Id is the current of disk, Ir is the current of ring, and N is the current collection efficiency of the Pt ring. N was determined to be 0.40 from the reduction of K3Fe(CN)6, well consisting with the manufacturer's value.
3. Results and discussion 3.1. Structural characterization of catalyst The synthetic route of the NiFe-LDH/rGO is schematically presented in Scheme 1. Firstly, the hydrophilic GO nanosheets enter the inner space of SDS reverse micelles, which adsorbs the mixed nickel and iron hydrochlorides on the hydrophilic end of SDS and the surface of GO by electrostatic interaction. Then addition of alkali sources induces the in situ nucleation of LDH crystallite in micelle inside. After solvothermal treatment and reductive process of hydrazine, crystallization of NiFe-LDH and reduction of GO to rGO give rise to a hierarchical NiFe-LDH/rGO nanostructure. Fig. 1a shows the SEM image of NiFe-LDH/rGO. It can be obviously seen that NiFeLDH shows the plate-like morphology with sizes of ca. 40e100 nm. These building blocks are aggregated to form many imperfect microspheres due to the roles of reverse micelles. The broken GO nanoplatelets are anchored on LDH surface and cross-link these nanoparticles together through oxygen-containing groups. Therefore a rough surface is clearly observed for the NiFe-LDH/rGO composite. The detailed morphology is further characterized by TEM and HRTEM in Fig. 1b and c. In the irregular microsphere, the ultrathin NiFe-LDH sheets are intertwined with rGO nanoparticles due to the electrostatic attraction of GO toward the metal ions and the strong interactions between NiFe-LDH nanosheets and rGO nanoparticles. The corresponding lattice fringes of 0.26 and 0.34 nm are indexed for the d(012) of NiFe and d(002) of rGO, respectively [32,33]. The EDS spectrum (Fig. S1) of the given area in Fig. 2b distinctly demonstrates that the NiFe-LDH/rGO composite is comprised of Ni, Fe, S, C, O and Cl elements, in which C and S signals are derived from SDS and the carbon-supported grid. The typical powder XRD patterns of NiFe-LDH and NiFe-LDH/ rGO hybrids are shown in Fig. S2. For NiFe-LDH phase, the two strong diffraction peaks for (003) and (006) are shifted to the lower 2q values due to the preparation in SDS reverse micelles. The corresponding d003 value is 2.62 nm, similar to the previous reports [34,35]. After assembled with rGO nanoparticles, the (003) and (006) peaks are retained, demonstrating that NiFe-LDH keeps its hydrotalcite structure in NiFe-LDH/rGO composite. However, their
Scheme 1. Schematic illustration for the synthesis of the NiFe-LDH/rGO composite.
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Fig. 1. SEM (a) and TEM (b) images of the aseprepared NiFe-LDH/rGO composites; the corresponding high-resolution TEM image (c).
Fig. 2. (A) XPS of NiFe-LDH/GO (blue) and NiFe-LDH/rGO (black) nanocomposites; (B) high resolution of C 1s spectra derived from XPS of NiFe-LDH/GO (blue) and NiFe-LDH/rGO (black) nanocomposites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(003) and (006) peaks shift to the larger 2q positions compared to those for NiFe-LDH. The GR (002) peak cannot be observed in the XRD patterns of the NiFe-LDH/rGO nanocomposite, suggesting that the strong interactions between rGO and LDH nanoplatelets effectively hinder the former from aggregating [17]. NiFe-LDH/rGO gives the smaller basal spacing (1.131 nm) than NiFe-LDH, manifesting that many SDS anions have been exchanged by rGO counterparts. XPS is also employed to further investigate the chemical and valence states of catalysts. The XPS survey spectrum (Fig. 2A) exhibits classic information for the Ni, Fe, S, C, O and Cl elements, highly identical to the EDS results. Fig. 2B displays the C1s core level spectrums for NiFe-LDH/GO and NiFe-LDH/rGO. For NiFe-LDH/GO, carbon species are found mostly to be the non-oxygenated ring C (CeC) at 284.6 eV accompanying a noticeable oxygenated carbon species at 289.9 eV (OeC]O) [36]. After complete reduction by hydrazine, NiFe-LDH/rGO almost reveals only the non-oxygenated C species, indicating that GO is transformed into rGO in the materials. The Ni 2p spectrum (Fig. S3A) shows that Ni element is mostly in the Ni2þ state at binding energies of 856.1 and 873.7 eV [23]. It can be seen from Fig. S3B that the Fe 2p doublet (Fe 2p 3/2 and 2p 1/ 2) is positioned at 712.3 and 725.8 eV, implying the þ3 oxidation state in composite [18]. The surface area and porosity analysis of NiFe-LDH/rGO was determined by its N2 adsorptionedesorption curves. As shown in Fig. 3, the isotherms obey type-IV isotherm in the IUPAC classification and the hysteresis loop is a classical H3-type (P/P0 > 0.4), indicating that the NiFe-LDH/rGO catalyst contains slit-type pore according to the general plate-like nanostructure. Its specific
surface area and average pore size were respectively obtained as 91.64 m2/g and 5.10 nm. The specific surface area value is greatly higher than those for LDH obtained basing on other methods [16]. Both the higher specific surface area and the unique 3D porous nanostructure can endow the NiFe-LDH/rGO nanohybrid with a superior catalytic activity, which is resulted from the higher active
Fig. 3. BET and pore size distribution (inset) curves of NiFe-LDH/rGO hybrid catalyst.
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site density on catalyst surface and a lower mass transport resistance [1]. 3.2. Electrochemical OER and ORR The electrocatalytic performance toward OER is evaluated in a typical three-electrode system in 1.0 M KOH solution. Ni foam uniformly covered with the NiFe-LDH/rGO hybrid is directly used as the working electrode. For comparison, three samples including GO, NiFe-LDH and NiFe-LDH/GO are also determined under the same conditions. Fig. 4A shows the polarization curves of Ni foam loading various catalysts at a scan rate of 10 mV s1. Bare Ni foam exhibits the very poor OER catalytic activity. Coating of GO slightly enhances the OER activity. Noticeably, the NiFe-LDH nanosheets perform the higher activity toward the OER with a lower overpotential of ca. 307 mV, which is obviously smaller than those for Ni foam and GO (372 and 359 mV, respectively). As for NiFe-LDH/GO, a lower overpotential (272 mV) and a much higher current density are gotten, revealing a further improved activity for OER (Fig. 4B, blue column). It is noteworthy that NiFe-LDH/rGO far outperforms the other catalysts in current density over the full potential window with lower overpotential value. The OER onset overpotential is achieved as low as 240 mV (~1.470 V vs. RHE), which is far smaller than the previous value for IrO2 in 1.0 M KOH (290 mV, ~1.52 V vs. RHE) [28]. Such value is also much lower than that of ZnCo- [37,38], CoFe- [39], CoNi-hydroxides [40] and NiCo-LDH 420 mV [23,41], and even compare to that of the best reported NiFe-LDH [32]. Given that these results, we can conclude that NiFe-LDH/rGO has not only raised the activity of active sites, but also made more active sites exposed by the assistance of rGO and reverse micelles. Hence, the
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NiFe-LDH/rGO composite performs the excellent OER activity, which can also be verified by the vigorous release of bubbles from the electrode surface during the reaction process (see Movie S1 for oxygen evolution in Supporting information). Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.jpowsour.2016.09.152.Supp. Video S1. The required overpotential to accomplish 10 mA cm2 current density is an important threshold for an OER catalyst because it represents approximately the current density for a 10% efficient solar-to-fuel conversion device [42,43]. The NiFe-LDH/rGO nanohybrid requires the lowest overpotential of 250 mV to achieve the current density of 10 mA cm2, which is obviously smaller than those for NiFe-LDH/GO (284 mV), NiFe-LDH (310 mV) and GO (408 mV) catalysts (Fig. 4B, red curve) and other reported nonprecious metal ones (Table S1). The current density at an overpotential of 300 mV is also used to quantify the improvement of activity. Fig. 4B (black curve) displays that the current densities for NiFe-LDH/rGO (31.15 mA/cm2) at h ¼ 300 mV is enhanced by 2.75and 1.68-folds upon pure NiFe-LDH (11.34 mA/cm2) and NiFe-LDH/ GO (18.54 mA/cm2), respectively. The improved catalytic kinetics catalyst is also comparably evaluated by using tafel slopes (Fig. 4C). It can be clearly found that NiFe-LDH/rGO offers a lower tafel slope of 91 mV dec1, smaller than those of NiFe-LDH/GO (106 mV dec1) and NiFe-LDH (143 mV dec1), respectively. This smaller tafel slope value for the NiFe-LDH/rGO catalyst results in a further enhancement toward OER catalytic activity at h > 300 mV, which can be identified by the more quickly increased current density with a decrease of overpotential. In other word, the lowered tafel slope indicates that electron transportation becomes more achievable on NiFe-LDH/rGO medium compared with NiFe-LDH or NiFe-LDH/GO
Fig. 4. (A) LSVs for OER on different modified electrode. (B) Onset overpotentials of different modified electrode (blue column); Overpotentials of different modified electrode at J ¼ 10 mA/cm2 (red curve); Current densities at h ¼ 300 mV of different modified electrode (black curve). (C) Tafel plots derived from the corresponding polarization curves. (D) Chronopotentiometry curve of NiFe-LDH/GO and NiFe-LDH/rGO catalysts. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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composite. Moreover, chronoamperometric tests are also carried out to investigate the stability of NiFe-LDH/GO and NiFe-LDH/rGO. It is apparently observed that both catalysts provide the stable potential values for 9 h when biased galvanostatically at 10 mA cm2 (Fig. 4D), suggesting an acceptable stability in alkaline condition. It is well known that electrochemical catalysts based transitionmetal are bifunctional activities toward both ORR and OER. Therefore, the ORR activity of NiFe-LDH/rGO composite is firstly evaluated by CV method in both O2- or N2-saturated 0.1 M KOH. By comparison, both control catalysts of NiFe-LDH and NiFe-LDH/GO are also determined at the equal conditions and results are shown in Fig. 5A. In N2-saturated solutions, three materials exhibit the featureless voltammetric currents in the given potential range.
After 0.1 M KOH solutions are saturated with O2, the three catalysts display the well-defined peaks at ca. 0.78 V for the reduction of oxygen, manifesting their undoubtedly electrocatalytic activity toward ORR. More interestingly, NiFe-LDH/rGO performs the almost equal onset potential to NiFe-LDH/GO, but the stronger reductive current than the latter. However, their electrocatalytic activities toward ORR are distinctly superior to pristine NiFe-LDH. Their ORR electrocatalytic performances are further characterized by LSV (Fig. 5B). In line with the CV results, NiFe-LDH/rGO and NiFe-LDH/ GO give rise to the better electrocatalytic activity with the more positive onset overpotentials than pure NiFe-LDH (~0.792 V). Although the nearly same onset potential (~0.796 V), the highest limiting current density and half-wave potential for NiFe-LDH/rGO indicate the best ORR activity among three catalysts. This can be
Fig. 5. (A) CVs in N2- and O2-saturated and (B) LSV in O2-saturated 0.1 M KOH for NiFe-LDH, NiFe-LDH/GO and NiFe-LDH/rGO composites with 1600 rpm. (C) RDE voltammograms of rGO-LDH in O2-saturated 0.1 M KOH at a scan rate of 10 mV s1 at different rotating speed and (D) the corresponding K-L plot of J1 vs. u1/2. The inset shows the electron transfer number (n). (E) RRDE voltammograms of NiFe-LDH/rGO in O2-saturated 0.1 M KOH. The disk potential is scanned at 10 mV s1 and the ring potential is constant at 1.3 V. (F) The electron transfer number n and percentage of HO 2 at certain potentials based on the corresponding RRDE results.
Fig. 6. (A) ORR polarization curves of NiFe-LDH/rGO composite in O2 saturated 0.1 M KOH solution with and without 1.0 M methanol at 1600 rpm. (B) ORR polarization curves of NiFe-LDH/rGO composite before and after 5000 potential cycles in O2 saturated 0.1 M KOH solution. Scan rate is 10 mV s1.
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attributed to the more exposed active sites on NiFe-LDH/rGO catalyst. For further insight into the ORR kinetics, the polarization curves of NiFe-LDH, NiFe-LDH/GO and NiFe-LDH/rGO are recorded by using RDE at various rotating speeds at a scan rate of 10 mV s1 (Figs. S4A, S5A and Fig. 5C, respectively). For three catalysts, their limiting-current gradually increases with the rotating speed owing to the reduced diffusion layer [44]. Their corresponding KouteckyLevich plots (j1 vs. u1/2) are obtained from the LSV diffusion limiting current densities at 0.3000, 0.3875, 0.4750, 0.5625 and 0.6500 V as shown in (Figs. S4B, S5B and Fig. 5D, respectively). Consequently, the transferred electrons numbers (n) are calculated from the K-L slopes as 2.1, 3.4 and 3.7 for NiFe-LDH, NiFe-LDH/GO and NiFe-LDH/rGO, respectively. These values suggest that the whole ORR process of pure NiFe-LDH is a 2e transfer approach, while the NiFe-LDH/GO and NiFe-LDH/rGO composites experience a reduction mechanism of dominant 4e with partial 2e. It is worthily pointed that the n value of 3.7 for NiFe-LDH/rGO almost reveals a 4e reduction process. Accordingly, it can be concluded that NiFe-LDH/rGO is a more effective ORR catalyst with less HO 2 product during the ORR process than NiFe-LDH/GO. The RRDE technique is also employed for the further verification of the ORR catalytic pathway at NiFe-LDH/rGO. The yield of HO 2 and the electron transfer number calculated are accurately measured as illustrated in Fig. 5E and F. In the chosen potential range, the peroxide species yield is under 10% and the n value always approaches 3.8. This is highly accordant to the results based on RDE measurements, revealing that NiFe-LDH/rGO mainly takes place a 4e reduction process toward ORR accompanying a very small number 2e pathway in alkaline solution. It is well acknowledged that the tolerance to methanol toward ORR is of very importance to practical application in fuel cell. Generally, methanol permeation from the anode to the cathode (methanol crossover) will seriously damage the fuel cell's efficiency, which can be greatly decayed by more than 50% at low current densities [45]. Thus the ORR polarization curves of NiFeLDH/rGO catalyst in the presence and the absence of 1.0 M methanol are further offered in Fig. 6A. Remarkably, the cathodic current of catalyst decreases by 0.052% in the given potential range after the injection of methanol. The corresponding half-wave potential for the ORR polarization curve shifts to negative direction by 4 mV after adding methanol. This result unambiguously manifests that NiFe-LDH/rGO possesses a strong methanol tolerance to methanol crossover, demonstrating an excellent selectivity for ORR in alkaline electrolytes. On the other hand, durability is also an important metric for the practical application of an electrocatalyst. Consequently, the stability was also analyzed by cycling the electrode potential from 0.3 V to 0.8 V at a scan rate of 10 mV s1 in O2 saturated 0.1 M KOH solution. Fig. 6B exhibits the ORR voltammogram of NiFe-LDH/rGO at the first cycle and the 5001th potential cycle. After 5000 continuous scanning, both of its onset potential and current density undergo almost no decay, and its half wave potential experiences only a 2 mV negative shift. The outstanding electrochemical stability of the NiFe-LDH/rGO catalyst might be due to the homogeneous hybridation of rGO and LDH building blocks. After the surface degradation of the catalyst, the new active sites are evenly exposed to sustain the constant catalytic performance [46]. 4. Conclusion In conclusion, the electrocatalyst of NiFe-LDH/rGO nanohybrid has been synthesized via a two-step strategy comprising solvothermal process and a subsequent reductive treatment. The resultant NiFe-LDH/rGO catalyst presents a porous nanostructure and displays a high OER catalytic activity in 1.0 M KOH solution. The
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NiFe-LDH/rGO nanohybrid also exhibits a high ORR catalytic ability with the predominant four electron pathway in 0.1 M KOH solution. Furthermore, the NiFe-LDH/rGO composite shows the lowest overpotential for both OER and ORR, outperforming the pure NiFeLDH and the NiFe-LDH/GO catalysts. This enhanced bifunctional catalytic performance may be mainly arisen from the porous microstructure and the strong coupling between rGO and NiFeLDH, which are considerably beneficial to the fast electron transfer, the oxygen transportation and the more exposure of active sites. Hence, the NiFe-LDH/rGO nanohybrid can be employed as a potential bifunctional oxygen catalyst in future energy conversion technologies. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (21173135, 21403121 and 21573133), the Natural Science Foundation of Shandong Province, China (ZR2014JL013 and ZR2013BQ013), the open foundation from the Key Laboratory of Marine Bioactive Substance and Modern Analysis Technology, SOA (MBSMAT-2015-04, MBSMAT-2014-02, MBSMAT2013-01 and MBSMAT-2012-07). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.09.152. References [1] T. Liu, Y.F. Guo, Y.M. Yan, F. Wang, C. Deng, D. Rooney, K.N. Sun, Carbon 106 (2016) 84e92. [2] Z. Yang, J. Zhang, M.C. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Chem. Rev. 111 (2011) 3577e3613. [3] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Chem. Rev. 110 (2010) 6446e6473. [4] H. Wang, H. Dai, Chem. Soc. Rev. 42 (2013) 3088e3113. [5] J. Zhang, Z. Zhao, Z. Xia, L. Dai, Nat. Nano 10 (2015) 444e452. [6] S. Jin, K.J. May, H.A. Gasteiger, J.B. Goodenough, S.H. Yang, Science 334 (2011) 1383e1385. [7] B.S. Yeo, A.T. Bell, J. Am. Chem. Soc. 133 (2011) 5587e5593. [8] F. Cheng, J. Chen, Chem. Soc. Rev. 41 (2012) 2172e2192. [9] S. Chen, J. Duan, M. Jaroniec, S.Z. Qiao, Adv. Mater. 26 (2014) 2925e2930. [10] D. Wang, X. Chen, D.G. Evans, W. Yang, Nanoscale 5 (2013) 5312e5315. [11] L. Wang, C. Lin, D. Huang, F. Zhang, M. Wang, J. Jin, Acs Appl. Mater. Interface 6 (2014) 10172e10180. [12] M.R. Gao, Y.F. Xu, J. Jiang, S.H. Yu, Chem. Soc. Rev. 42 (2013) 2986e3017. [13] M.W. Kanan, D.G. Nocera, Science 321 (2008) 1072e1075. [14] K. Jin, J. Park, J. Lee, K.D. Yang, G.K. Pradhan, U. Sim, D. Jeong, H.L. Jang, S. Park, D. Kim, N.E. Sung, S.H. Kim, S. Han, K.T. Nam, J. Am. Chem. Soc. 136 (2014) 7435e7443. [15] W. Bian, Z. Yang, P. Strasser, R. Yang, J. Power Sources 250 (2014) 196e203. [16] T. Zhan, Y. Zhang, Q. Yang, H. Deng, J. Xu, W. Hou, Chem. Eng. J. 302 (2016) 459e465. [17] T. Zhan, X. Wang, X. Li, Y. Song, W. Hou, Sensors Actuat. B Chem. 228 (2016) 101e108. [18] L.J. Zhou, X. Huang, H. Chen, P. Jin, G.D. Li, X. Zou, Dalton 44 (2015) 11592e11600. [19] M. Gong, Y. Li, H. Wang, Y. Liang, J.Z. Wu, J. Zhou, J. Wang, T. Regier, F. Wei, H. Dai, J. Am. Chem. Soc. 135 (2013) 8452e8485. [20] X. Long, J. Li, S. Xiao, K. Yan, Z. Wang, H. Chen, S. Yang, Angew. Chem. Int. Ed. Engl. 53 (2014) 7584e7588. [21] F. Song, X. Hu, Nat. Commun. 5 (2014), 4477e4477. [22] D. Tang, Y. Han, W. Ji, S. Qiao, X. Zhou, R. Liu, X. Han, H. Huang, Y. Liu, Z. Kang, Dalton 43 (2014) 15119e15125. [23] J. Jiang, A. Zhang, L. Li, L. Ai, J. Power Sources 278 (2015) 445e451. [24] F. Song, X. Hu, J. Am. Chem. Soc. 136 (2014) 16481e16484. [25] R. Huo, W.J. Jiang, S. Xu, F. Zhang, J.S. Hu, Nanoscale 6 (2013) 203e206. [26] S. Wang, D. Yu, L. Dai, J. Am. Chem. Soc. 133 (2011), 5182e5518. [27] Y. Xu, W. Bian, W. Jiao, J.H. Tian, R. Yang, Electrochim. Acta 151 (2015) 276e283. [28] D.H. Youn, Y.B. Park, J.Y. Kim, G. Magesh, Y.J. Jang, J.S. Lee, J. Power Sources 294 (2015) 437e443. [29] R. Subbaraman, D. Tripkovic, D. Strmcnik, K.C. Chang, M. Uchimura, A.P. Paulikas, V. Stamenkovic, N.M. Markovic, Science 334 (2011) 1256e1260.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
NiFe layered double hydroxide/reduced graphene oxide nanohybrid as an efficient bifunctional electrocatalyst for oxygen evolution and reduction reactions Tianrong Zhan a, *, Yumei Zhang a, Xiaolin Liu a, SiSi Lu a, Wanguo Hou b, ** a Key Laboratory of Sensor Analysis of Tumor Marker (Ministry of Education), State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China b Key Laboratory of Colloid & Interface Chemistry (Ministry of Education), Shandong University, Jinan 250100, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
NiFe-LDH/rGO catalyst was obtained via solvothermal and chemical reduction method. OER and ORR activities of NiFe-LDH/ rGO in alkaline media were firstly studied. NiFe-LDH/rGO shows better OER activity with a lower onset overpotential of 240 mV. NiFe-LDH/rGO gives a 0.796 V ORR onset potential mainly involving the 4e pathway.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 July 2016 Received in revised form 12 September 2016 Accepted 26 September 2016
Highly active and low-cost bifunctional electrocatalysts for oxygen evolution and reduction reactions (OER and ORR) hold a heart position for the renewable energy technologies such as metal-air batteries and fuel cells. Here, we reported the synthesis of NiFe layered double hydroxide/reduced graphene oxide (NiFe-LDH/rGO) nanohybrid via the facile solvothermal method followed by chemical reduction. The template role of surfactant and the hybridization of rGO supplied the NiFe-LDH/rGO catalyst with a porous nanostructure and an enhanced conductivity, favoring both mass transport and charge communication of electrocatalytic reactions. The NiFe-LDH/rGO composite not only displayed highly efficient OER activity in alkaline solution with a low onset overpotential of 240 mV, but also only needed an overpotential of 250 mV to reach the 10 mA cm2 current density. The NiFe-LDH/rGO nanohybrid also offered excellent ORR catalytic activity with onset potential at 0.796 V in alkaline media. The rotatingdisk and rotating-ring-disk electrodes both revealed that the ORR on NiFe-LDH/rGO mainly involved a direct four-electron reaction pathways accompanying part of the two-electron process. The excellent bifunctional activity of the NiFe-LDH/rGO nanohybrid could be attributed to the synergistic effects of rGO and NiFe-LDH components due to the strongly coupled interactions. © 2016 Elsevier B.V. All rights reserved.
Keywords: NiFe layered double hydroxide Reduced graphene oxide Oxygen evolution reaction Oxygen reduction reaction
1. Introduction * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T. Zhan), [email protected] (W. Hou). http://dx.doi.org/10.1016/j.jpowsour.2016.09.152 0378-7753/© 2016 Elsevier B.V. All rights reserved.
The growing energy demand and the worsening global environment have inspired a substantial and intensive development of
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alternative clean and renewable energy to fossil fuel [1,2]. Electrocatalytic oxygen evolution and reduction reactions (OER and ORR) are unambiguously the core processes of the many sustainable energy technologies, including rechargeable metal-air batteries and unitized regenerative fuel cells [3e5]. The OER on the anode is a charging process for energy storage in solar fuel synthesis and water splitting, whilst the ORR on the cathode is a discharging process in fuel cells and metaleair batteries [6,7]. However, the sluggish kinetics of OER and ORR generally result in the wide charging-discharging potential gap, consequently impairing the efficiency of fuel cells and metal-air batteries [6e8]. Hence, it is the challenging work to develop the bifunctional electrocatalysts with low overpotentials for both the OER and ORR [1,9]. Some precious metals such as Pt, Ir, and Ru based materials have been potentially used as bifunctional catalysts to expedite the low oxygen reactions, however their high cost and scarcity hamper their long-term and wide commercial applications [1,10]. Therefore, many efforts have been focused on the development of inexpensive bifunctional electrocatalysts with robust efficiency. Transition metal hydroxides [11], oxides [10], chalcogenides [12], phosphate [13], metal carbides [14], perovskites [6] and their composite [15] have drawn much interest as low-cost alternatives to noble-metal catalysts for both the ORR and OER. However, these bifunctional electrocatalysts still experience poor catalytic efficiency owing to their low conductivity, instability, and small surface area. Layered double hydroxides (LDH), also named lamellar anionic clay, present a class of hydrotalcite-like compounds with a xþ n generic chemical formula of [MII1-xMIII x (OH)2] [(A )x/n$mH2O] [16]. They are composed of brucite-like layers with forever positive charges owing to the isomorphic substitution of MII by MIII, and the charge balancing anions in interlayer regions. The major advantages of these 2D materials are that they can be constructed by the broad-spectrum metal cations and the exchangeable interlayer intercalators [17,18]. Additionally considering the intrinsic catalytic activity, different types of transition metal-based LDH and their composites have been prepared for the potential application in electrocatalysis. For example, NiFe-LDH [19e21], ZnCo-LDH [22], NiCo-LDH [23] and CoMn-LDH [24] have been testified as excellent OER catalysts in alkaline solution. Although the original LDH lamellar materials have been hardly used as ORR catalysts, some oxides derived from them have displayed the outstanding catalytic performance toward ORR [10,25]. However, the positively charged LDH surface is desirable for oxygen adsorption and concomitant ORR [26], which can endow LDH and their composites with the reasonably expectable ORR activity. Given that the 3d metals, such as Ni, Co and Fe, in the oxide state are the electroactive species of OER and ORR [11,27], the 3d metal based hydroxide and their composite can be a class of promising bifunctional catalysts. On the other hand, due to the poor conductivity of LDH catalysts, their catalytic performance can be further enhanced by combining with nano-carbon such as reduced grapene oxide (rGO) or carbon nanotubes (CNT) [1,27]. Therein, high conductivity, better stability and large surface area make graphene as an ideal conductive substrate for LDH. FeNi-LDH/graphene [20] and NiFeLDH/rGO [28] have been fabricated as the superior OER electrocatalysts. However, to the best of our knowledge, the NiFe-LDH/rGO composite has been not yet investigated as a bifunctional catalyst for both of OER and ORR so far. In view of the high electro-active peculiarity and the strong hybridizing ability of Ni and Fe elements [29], we presently describe the preparation of NiFe-LDH/rGO nanohybrid as an efficient bifunctional electrocatalyst toward OER and ORR. Through a one-pot solvothermal method in presence of surfactant and the following chemically reductive treatment, we have afforded the porous NiFe-LDH/rGO composite. The intimate hybridization of
NiFe-LDH nanoplatelets with rGO nanoparticles can offer the enhanced conductivity and catalytic activity. The NiFe-LDH/rGO nanohybrid displays the excellently bifunctional catalytic performance for both OER and ORR in alkaline medium. Furthermore, the NiFe-LDH/rGO catalyst exhibits the better bifunctional catalytic activity than NiFe-LDH and NiFe-LDH/GO counterparts, which can be ascribed to the facilitated charge transportation and the good synergistic effects of rGO and NiFe-LDH in the electro-catalyst. 2. Experimental 2.1. Chemicals Graphite powder (average particle size 30 mm) was obtained from Colloid Chemical Co. (Shanghai, China). Ethylene glycol (EG), sodium dodecyl sulfonate (SDS) and hydrazine monohydrate were purchased from Aladdin Chemistry Co. (Shanghai, China) as analytical pure. Nafion solution (5%) was obtained from DuPont Co. (USA). Polytetrafluoroethylene (PTFE: 60%, 0.20 mm) was supplied by Shenzhen Dechengwang S&T Ltd. All of the reagents were used as received without further purification. 2.2. Preparation of catalyst Firstly, graphene oxide (GO) was synthesized from nature graphite powder by a modified Hummer's method [30]. The resultant GO powder was ultrasonically dispersed in EG with concentration of 0.5 wt%. NiFe-LDH/rGO composite was prepared by a solvothermal method. Typically, 0.67 mmol of FeCl3$6H2O and 2.00 mmol of NiCl2$6H2O were dissolved in 20 mL above EG solution (containing 0.5 wt% GO). Then 0.6 g SDS was added and stirred for 1 h at 40 C. Then 10 mL EG and 0.16 g NaOH were added into the reactor with additional 30 min magnetic stirring. The mixtures were transferred to the Teflon lined stainless steel autoclave and thermally treated at 160 C for 24 h. After centrifuging and washing with excess water and ethanol orderly, the black NiFe-LDH/GO powders were obtained. In order to acquire the completely reductive products, NiFe-LDH/GO powders were re-dispersed in 100 mL water with the final concentration as 1 mg/mL 50 mL hydrazine hydrate (35 wt%) and 350 mL ammonia water (27 wt%) were further added into solution for reduction. After 5 min stirring, the mixed solution was transferred to Teflon lined stainless steel reactor for solvothermal reaction at 90 C for 1 h. With similar purification, the NiFe-LDH/rGO composited catalyst was afforded. For the comparison, the pure FeNi-LDH sample was also prepared by the same procedure without GO and reductive step. 2.3. Characterization of catalyst X-ray diffraction patterns (XRD) were characterized on a Rigaku powder diffractometer equipped with Ni-filters Cu Ka radiation (l ¼ 1.54050 Å, 40 kV and 100 mA). The morphology was observed with a JSM-6700F scanning electron microscope (SEM, Japan Electron Co.) and a JEOL JEM-2000EX transmission electron microscope (TEM, Japan Electron Co.). The elemental analysis of the samples were performed on an energy-dispersive X-ray spectroscopy (EDS, Oxford instruments X-Max). By the BET method, the specific surface areas are obtained from N2 adsorption isotherms using an apparatus (Quantachrome-Autosorb-1C; Quantachrome Instr., USA). XPS was conducted by using an HP 5950A ESCA spectrometer with an MgKa source. 2.4. Electrochemical measurements Electrochemical measurements were performed on CHI 660D
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workstation with a standard three-electrode system. The saturated Ag/AgCl and platinum flake (1 2 cm2) were respectively used as reference electrode and counter electrode. The reference potential was converted to the reversible hydrogen electrode (RHE) via the Nerst equation [31]:
ERHE ¼ EAg=AgCl þ 0:059pH þ 0:1976
(1)
OER test: 10 mg active material, 400 mL ethanol and 20 mL PTFE were ultrasonically mixed to form a homogeneous ink. The working electrode was fabricated by pipetting out some slurry on Ni foam electrode (1 1 cm2) with the final loading of 0.5 mg/cm2 1.0 M KOH solution was acted as electrolyte and saturated with N2 before each test. Linear sweep voltammetry (LSV) was carried out at a scan rate of 10 mV s1 for polarization and the Tafel plots were transformed from LSV. ORR test: 10 mg active material, 400 mL ethanol and 15 mL Nafion (1.0%) were ultrasonically mixed by ultrasonic to form a homogeneous ink. The working electrode was prepared by casting some slurry on Rotating disk electrode (RDE, 0.1256 cm2) with the final loading of 0.18 mg/cm2 0.1 M KOH solution was acted as electrolyte and saturated with O2 before each test. CV was performed at a scan rate of 50 mV s1, LSVs was recorded at 10 mV s1 with various rotate speeds (400e2025 rpm). The exact kinetic parameters for the ORR activities, including electron transfer number (n) and kinetic current density (Jk), were investigated based on the KouteckyeLevich (K-L) equation [1]:
1 1 1 1 1 ¼ þ ¼ þ J JK JL JK B$u1=2 2=3
B ¼ 0:62$n$F$C0 $D0 $n1=6
(2)
(3)
where J is the measured current density, Jk is the kinetic current density, u is the electrode rotate speed, n represents the number of electron transferred per oxygen molecule, F is the Faraday constant (96485 C/mol), D0 is the diffusion coefficient of O2 in 0.1 M KOH (1.90 105 cm2/s), v is the kinematic viscosity of the electrolyte (0.01 cm2/s), and C0 is the bulk concentration of O2 (1.2 106 mol/ cm3) [1]. The constant 0.2 is used when the rotate speed is expressed in rpm. For the Rotating ring disk electrode (RRDE) measurements, the %HO 2 and the electron transfer number n were determined by the following equations [15]:
%HO 2 ¼
200$Id N$Id þ Ir
(4)
n ¼ 4Id
Ir Id þ N
55
(5)
where Id is the current of disk, Ir is the current of ring, and N is the current collection efficiency of the Pt ring. N was determined to be 0.40 from the reduction of K3Fe(CN)6, well consisting with the manufacturer's value.
3. Results and discussion 3.1. Structural characterization of catalyst The synthetic route of the NiFe-LDH/rGO is schematically presented in Scheme 1. Firstly, the hydrophilic GO nanosheets enter the inner space of SDS reverse micelles, which adsorbs the mixed nickel and iron hydrochlorides on the hydrophilic end of SDS and the surface of GO by electrostatic interaction. Then addition of alkali sources induces the in situ nucleation of LDH crystallite in micelle inside. After solvothermal treatment and reductive process of hydrazine, crystallization of NiFe-LDH and reduction of GO to rGO give rise to a hierarchical NiFe-LDH/rGO nanostructure. Fig. 1a shows the SEM image of NiFe-LDH/rGO. It can be obviously seen that NiFeLDH shows the plate-like morphology with sizes of ca. 40e100 nm. These building blocks are aggregated to form many imperfect microspheres due to the roles of reverse micelles. The broken GO nanoplatelets are anchored on LDH surface and cross-link these nanoparticles together through oxygen-containing groups. Therefore a rough surface is clearly observed for the NiFe-LDH/rGO composite. The detailed morphology is further characterized by TEM and HRTEM in Fig. 1b and c. In the irregular microsphere, the ultrathin NiFe-LDH sheets are intertwined with rGO nanoparticles due to the electrostatic attraction of GO toward the metal ions and the strong interactions between NiFe-LDH nanosheets and rGO nanoparticles. The corresponding lattice fringes of 0.26 and 0.34 nm are indexed for the d(012) of NiFe and d(002) of rGO, respectively [32,33]. The EDS spectrum (Fig. S1) of the given area in Fig. 2b distinctly demonstrates that the NiFe-LDH/rGO composite is comprised of Ni, Fe, S, C, O and Cl elements, in which C and S signals are derived from SDS and the carbon-supported grid. The typical powder XRD patterns of NiFe-LDH and NiFe-LDH/ rGO hybrids are shown in Fig. S2. For NiFe-LDH phase, the two strong diffraction peaks for (003) and (006) are shifted to the lower 2q values due to the preparation in SDS reverse micelles. The corresponding d003 value is 2.62 nm, similar to the previous reports [34,35]. After assembled with rGO nanoparticles, the (003) and (006) peaks are retained, demonstrating that NiFe-LDH keeps its hydrotalcite structure in NiFe-LDH/rGO composite. However, their
Scheme 1. Schematic illustration for the synthesis of the NiFe-LDH/rGO composite.
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Fig. 1. SEM (a) and TEM (b) images of the aseprepared NiFe-LDH/rGO composites; the corresponding high-resolution TEM image (c).
Fig. 2. (A) XPS of NiFe-LDH/GO (blue) and NiFe-LDH/rGO (black) nanocomposites; (B) high resolution of C 1s spectra derived from XPS of NiFe-LDH/GO (blue) and NiFe-LDH/rGO (black) nanocomposites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(003) and (006) peaks shift to the larger 2q positions compared to those for NiFe-LDH. The GR (002) peak cannot be observed in the XRD patterns of the NiFe-LDH/rGO nanocomposite, suggesting that the strong interactions between rGO and LDH nanoplatelets effectively hinder the former from aggregating [17]. NiFe-LDH/rGO gives the smaller basal spacing (1.131 nm) than NiFe-LDH, manifesting that many SDS anions have been exchanged by rGO counterparts. XPS is also employed to further investigate the chemical and valence states of catalysts. The XPS survey spectrum (Fig. 2A) exhibits classic information for the Ni, Fe, S, C, O and Cl elements, highly identical to the EDS results. Fig. 2B displays the C1s core level spectrums for NiFe-LDH/GO and NiFe-LDH/rGO. For NiFe-LDH/GO, carbon species are found mostly to be the non-oxygenated ring C (CeC) at 284.6 eV accompanying a noticeable oxygenated carbon species at 289.9 eV (OeC]O) [36]. After complete reduction by hydrazine, NiFe-LDH/rGO almost reveals only the non-oxygenated C species, indicating that GO is transformed into rGO in the materials. The Ni 2p spectrum (Fig. S3A) shows that Ni element is mostly in the Ni2þ state at binding energies of 856.1 and 873.7 eV [23]. It can be seen from Fig. S3B that the Fe 2p doublet (Fe 2p 3/2 and 2p 1/ 2) is positioned at 712.3 and 725.8 eV, implying the þ3 oxidation state in composite [18]. The surface area and porosity analysis of NiFe-LDH/rGO was determined by its N2 adsorptionedesorption curves. As shown in Fig. 3, the isotherms obey type-IV isotherm in the IUPAC classification and the hysteresis loop is a classical H3-type (P/P0 > 0.4), indicating that the NiFe-LDH/rGO catalyst contains slit-type pore according to the general plate-like nanostructure. Its specific
surface area and average pore size were respectively obtained as 91.64 m2/g and 5.10 nm. The specific surface area value is greatly higher than those for LDH obtained basing on other methods [16]. Both the higher specific surface area and the unique 3D porous nanostructure can endow the NiFe-LDH/rGO nanohybrid with a superior catalytic activity, which is resulted from the higher active
Fig. 3. BET and pore size distribution (inset) curves of NiFe-LDH/rGO hybrid catalyst.
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site density on catalyst surface and a lower mass transport resistance [1]. 3.2. Electrochemical OER and ORR The electrocatalytic performance toward OER is evaluated in a typical three-electrode system in 1.0 M KOH solution. Ni foam uniformly covered with the NiFe-LDH/rGO hybrid is directly used as the working electrode. For comparison, three samples including GO, NiFe-LDH and NiFe-LDH/GO are also determined under the same conditions. Fig. 4A shows the polarization curves of Ni foam loading various catalysts at a scan rate of 10 mV s1. Bare Ni foam exhibits the very poor OER catalytic activity. Coating of GO slightly enhances the OER activity. Noticeably, the NiFe-LDH nanosheets perform the higher activity toward the OER with a lower overpotential of ca. 307 mV, which is obviously smaller than those for Ni foam and GO (372 and 359 mV, respectively). As for NiFe-LDH/GO, a lower overpotential (272 mV) and a much higher current density are gotten, revealing a further improved activity for OER (Fig. 4B, blue column). It is noteworthy that NiFe-LDH/rGO far outperforms the other catalysts in current density over the full potential window with lower overpotential value. The OER onset overpotential is achieved as low as 240 mV (~1.470 V vs. RHE), which is far smaller than the previous value for IrO2 in 1.0 M KOH (290 mV, ~1.52 V vs. RHE) [28]. Such value is also much lower than that of ZnCo- [37,38], CoFe- [39], CoNi-hydroxides [40] and NiCo-LDH 420 mV [23,41], and even compare to that of the best reported NiFe-LDH [32]. Given that these results, we can conclude that NiFe-LDH/rGO has not only raised the activity of active sites, but also made more active sites exposed by the assistance of rGO and reverse micelles. Hence, the
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NiFe-LDH/rGO composite performs the excellent OER activity, which can also be verified by the vigorous release of bubbles from the electrode surface during the reaction process (see Movie S1 for oxygen evolution in Supporting information). Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.jpowsour.2016.09.152.Supp. Video S1. The required overpotential to accomplish 10 mA cm2 current density is an important threshold for an OER catalyst because it represents approximately the current density for a 10% efficient solar-to-fuel conversion device [42,43]. The NiFe-LDH/rGO nanohybrid requires the lowest overpotential of 250 mV to achieve the current density of 10 mA cm2, which is obviously smaller than those for NiFe-LDH/GO (284 mV), NiFe-LDH (310 mV) and GO (408 mV) catalysts (Fig. 4B, red curve) and other reported nonprecious metal ones (Table S1). The current density at an overpotential of 300 mV is also used to quantify the improvement of activity. Fig. 4B (black curve) displays that the current densities for NiFe-LDH/rGO (31.15 mA/cm2) at h ¼ 300 mV is enhanced by 2.75and 1.68-folds upon pure NiFe-LDH (11.34 mA/cm2) and NiFe-LDH/ GO (18.54 mA/cm2), respectively. The improved catalytic kinetics catalyst is also comparably evaluated by using tafel slopes (Fig. 4C). It can be clearly found that NiFe-LDH/rGO offers a lower tafel slope of 91 mV dec1, smaller than those of NiFe-LDH/GO (106 mV dec1) and NiFe-LDH (143 mV dec1), respectively. This smaller tafel slope value for the NiFe-LDH/rGO catalyst results in a further enhancement toward OER catalytic activity at h > 300 mV, which can be identified by the more quickly increased current density with a decrease of overpotential. In other word, the lowered tafel slope indicates that electron transportation becomes more achievable on NiFe-LDH/rGO medium compared with NiFe-LDH or NiFe-LDH/GO
Fig. 4. (A) LSVs for OER on different modified electrode. (B) Onset overpotentials of different modified electrode (blue column); Overpotentials of different modified electrode at J ¼ 10 mA/cm2 (red curve); Current densities at h ¼ 300 mV of different modified electrode (black curve). (C) Tafel plots derived from the corresponding polarization curves. (D) Chronopotentiometry curve of NiFe-LDH/GO and NiFe-LDH/rGO catalysts. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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composite. Moreover, chronoamperometric tests are also carried out to investigate the stability of NiFe-LDH/GO and NiFe-LDH/rGO. It is apparently observed that both catalysts provide the stable potential values for 9 h when biased galvanostatically at 10 mA cm2 (Fig. 4D), suggesting an acceptable stability in alkaline condition. It is well known that electrochemical catalysts based transitionmetal are bifunctional activities toward both ORR and OER. Therefore, the ORR activity of NiFe-LDH/rGO composite is firstly evaluated by CV method in both O2- or N2-saturated 0.1 M KOH. By comparison, both control catalysts of NiFe-LDH and NiFe-LDH/GO are also determined at the equal conditions and results are shown in Fig. 5A. In N2-saturated solutions, three materials exhibit the featureless voltammetric currents in the given potential range.
After 0.1 M KOH solutions are saturated with O2, the three catalysts display the well-defined peaks at ca. 0.78 V for the reduction of oxygen, manifesting their undoubtedly electrocatalytic activity toward ORR. More interestingly, NiFe-LDH/rGO performs the almost equal onset potential to NiFe-LDH/GO, but the stronger reductive current than the latter. However, their electrocatalytic activities toward ORR are distinctly superior to pristine NiFe-LDH. Their ORR electrocatalytic performances are further characterized by LSV (Fig. 5B). In line with the CV results, NiFe-LDH/rGO and NiFe-LDH/ GO give rise to the better electrocatalytic activity with the more positive onset overpotentials than pure NiFe-LDH (~0.792 V). Although the nearly same onset potential (~0.796 V), the highest limiting current density and half-wave potential for NiFe-LDH/rGO indicate the best ORR activity among three catalysts. This can be
Fig. 5. (A) CVs in N2- and O2-saturated and (B) LSV in O2-saturated 0.1 M KOH for NiFe-LDH, NiFe-LDH/GO and NiFe-LDH/rGO composites with 1600 rpm. (C) RDE voltammograms of rGO-LDH in O2-saturated 0.1 M KOH at a scan rate of 10 mV s1 at different rotating speed and (D) the corresponding K-L plot of J1 vs. u1/2. The inset shows the electron transfer number (n). (E) RRDE voltammograms of NiFe-LDH/rGO in O2-saturated 0.1 M KOH. The disk potential is scanned at 10 mV s1 and the ring potential is constant at 1.3 V. (F) The electron transfer number n and percentage of HO 2 at certain potentials based on the corresponding RRDE results.
Fig. 6. (A) ORR polarization curves of NiFe-LDH/rGO composite in O2 saturated 0.1 M KOH solution with and without 1.0 M methanol at 1600 rpm. (B) ORR polarization curves of NiFe-LDH/rGO composite before and after 5000 potential cycles in O2 saturated 0.1 M KOH solution. Scan rate is 10 mV s1.
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attributed to the more exposed active sites on NiFe-LDH/rGO catalyst. For further insight into the ORR kinetics, the polarization curves of NiFe-LDH, NiFe-LDH/GO and NiFe-LDH/rGO are recorded by using RDE at various rotating speeds at a scan rate of 10 mV s1 (Figs. S4A, S5A and Fig. 5C, respectively). For three catalysts, their limiting-current gradually increases with the rotating speed owing to the reduced diffusion layer [44]. Their corresponding KouteckyLevich plots (j1 vs. u1/2) are obtained from the LSV diffusion limiting current densities at 0.3000, 0.3875, 0.4750, 0.5625 and 0.6500 V as shown in (Figs. S4B, S5B and Fig. 5D, respectively). Consequently, the transferred electrons numbers (n) are calculated from the K-L slopes as 2.1, 3.4 and 3.7 for NiFe-LDH, NiFe-LDH/GO and NiFe-LDH/rGO, respectively. These values suggest that the whole ORR process of pure NiFe-LDH is a 2e transfer approach, while the NiFe-LDH/GO and NiFe-LDH/rGO composites experience a reduction mechanism of dominant 4e with partial 2e. It is worthily pointed that the n value of 3.7 for NiFe-LDH/rGO almost reveals a 4e reduction process. Accordingly, it can be concluded that NiFe-LDH/rGO is a more effective ORR catalyst with less HO 2 product during the ORR process than NiFe-LDH/GO. The RRDE technique is also employed for the further verification of the ORR catalytic pathway at NiFe-LDH/rGO. The yield of HO 2 and the electron transfer number calculated are accurately measured as illustrated in Fig. 5E and F. In the chosen potential range, the peroxide species yield is under 10% and the n value always approaches 3.8. This is highly accordant to the results based on RDE measurements, revealing that NiFe-LDH/rGO mainly takes place a 4e reduction process toward ORR accompanying a very small number 2e pathway in alkaline solution. It is well acknowledged that the tolerance to methanol toward ORR is of very importance to practical application in fuel cell. Generally, methanol permeation from the anode to the cathode (methanol crossover) will seriously damage the fuel cell's efficiency, which can be greatly decayed by more than 50% at low current densities [45]. Thus the ORR polarization curves of NiFeLDH/rGO catalyst in the presence and the absence of 1.0 M methanol are further offered in Fig. 6A. Remarkably, the cathodic current of catalyst decreases by 0.052% in the given potential range after the injection of methanol. The corresponding half-wave potential for the ORR polarization curve shifts to negative direction by 4 mV after adding methanol. This result unambiguously manifests that NiFe-LDH/rGO possesses a strong methanol tolerance to methanol crossover, demonstrating an excellent selectivity for ORR in alkaline electrolytes. On the other hand, durability is also an important metric for the practical application of an electrocatalyst. Consequently, the stability was also analyzed by cycling the electrode potential from 0.3 V to 0.8 V at a scan rate of 10 mV s1 in O2 saturated 0.1 M KOH solution. Fig. 6B exhibits the ORR voltammogram of NiFe-LDH/rGO at the first cycle and the 5001th potential cycle. After 5000 continuous scanning, both of its onset potential and current density undergo almost no decay, and its half wave potential experiences only a 2 mV negative shift. The outstanding electrochemical stability of the NiFe-LDH/rGO catalyst might be due to the homogeneous hybridation of rGO and LDH building blocks. After the surface degradation of the catalyst, the new active sites are evenly exposed to sustain the constant catalytic performance [46]. 4. Conclusion In conclusion, the electrocatalyst of NiFe-LDH/rGO nanohybrid has been synthesized via a two-step strategy comprising solvothermal process and a subsequent reductive treatment. The resultant NiFe-LDH/rGO catalyst presents a porous nanostructure and displays a high OER catalytic activity in 1.0 M KOH solution. The
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NiFe-LDH/rGO nanohybrid also exhibits a high ORR catalytic ability with the predominant four electron pathway in 0.1 M KOH solution. Furthermore, the NiFe-LDH/rGO composite shows the lowest overpotential for both OER and ORR, outperforming the pure NiFeLDH and the NiFe-LDH/GO catalysts. This enhanced bifunctional catalytic performance may be mainly arisen from the porous microstructure and the strong coupling between rGO and NiFeLDH, which are considerably beneficial to the fast electron transfer, the oxygen transportation and the more exposure of active sites. Hence, the NiFe-LDH/rGO nanohybrid can be employed as a potential bifunctional oxygen catalyst in future energy conversion technologies. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (21173135, 21403121 and 21573133), the Natural Science Foundation of Shandong Province, China (ZR2014JL013 and ZR2013BQ013), the open foundation from the Key Laboratory of Marine Bioactive Substance and Modern Analysis Technology, SOA (MBSMAT-2015-04, MBSMAT-2014-02, MBSMAT2013-01 and MBSMAT-2012-07). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.09.152. References [1] T. Liu, Y.F. Guo, Y.M. Yan, F. Wang, C. Deng, D. Rooney, K.N. Sun, Carbon 106 (2016) 84e92. [2] Z. Yang, J. Zhang, M.C. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Chem. Rev. 111 (2011) 3577e3613. [3] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Chem. Rev. 110 (2010) 6446e6473. [4] H. Wang, H. Dai, Chem. Soc. Rev. 42 (2013) 3088e3113. [5] J. Zhang, Z. Zhao, Z. Xia, L. Dai, Nat. Nano 10 (2015) 444e452. [6] S. Jin, K.J. May, H.A. Gasteiger, J.B. Goodenough, S.H. Yang, Science 334 (2011) 1383e1385. [7] B.S. Yeo, A.T. Bell, J. Am. Chem. Soc. 133 (2011) 5587e5593. [8] F. Cheng, J. Chen, Chem. Soc. Rev. 41 (2012) 2172e2192. [9] S. Chen, J. Duan, M. Jaroniec, S.Z. Qiao, Adv. Mater. 26 (2014) 2925e2930. [10] D. Wang, X. Chen, D.G. Evans, W. Yang, Nanoscale 5 (2013) 5312e5315. [11] L. Wang, C. Lin, D. Huang, F. Zhang, M. Wang, J. Jin, Acs Appl. Mater. Interface 6 (2014) 10172e10180. [12] M.R. Gao, Y.F. Xu, J. Jiang, S.H. Yu, Chem. Soc. Rev. 42 (2013) 2986e3017. [13] M.W. Kanan, D.G. Nocera, Science 321 (2008) 1072e1075. [14] K. Jin, J. Park, J. Lee, K.D. Yang, G.K. Pradhan, U. Sim, D. Jeong, H.L. Jang, S. Park, D. Kim, N.E. Sung, S.H. Kim, S. Han, K.T. Nam, J. Am. Chem. Soc. 136 (2014) 7435e7443. [15] W. Bian, Z. Yang, P. Strasser, R. Yang, J. Power Sources 250 (2014) 196e203. [16] T. Zhan, Y. Zhang, Q. Yang, H. Deng, J. Xu, W. Hou, Chem. Eng. J. 302 (2016) 459e465. [17] T. Zhan, X. Wang, X. Li, Y. Song, W. Hou, Sensors Actuat. B Chem. 228 (2016) 101e108. [18] L.J. Zhou, X. Huang, H. Chen, P. Jin, G.D. Li, X. Zou, Dalton 44 (2015) 11592e11600. [19] M. Gong, Y. Li, H. Wang, Y. Liang, J.Z. Wu, J. Zhou, J. Wang, T. Regier, F. Wei, H. Dai, J. Am. Chem. Soc. 135 (2013) 8452e8485. [20] X. Long, J. Li, S. Xiao, K. Yan, Z. Wang, H. Chen, S. Yang, Angew. Chem. Int. Ed. Engl. 53 (2014) 7584e7588. [21] F. Song, X. Hu, Nat. Commun. 5 (2014), 4477e4477. [22] D. Tang, Y. Han, W. Ji, S. Qiao, X. Zhou, R. Liu, X. Han, H. Huang, Y. Liu, Z. Kang, Dalton 43 (2014) 15119e15125. [23] J. Jiang, A. Zhang, L. Li, L. Ai, J. Power Sources 278 (2015) 445e451. [24] F. Song, X. Hu, J. Am. Chem. Soc. 136 (2014) 16481e16484. [25] R. Huo, W.J. Jiang, S. Xu, F. Zhang, J.S. Hu, Nanoscale 6 (2013) 203e206. [26] S. Wang, D. Yu, L. Dai, J. Am. Chem. Soc. 133 (2011), 5182e5518. [27] Y. Xu, W. Bian, W. Jiao, J.H. Tian, R. Yang, Electrochim. Acta 151 (2015) 276e283. [28] D.H. Youn, Y.B. Park, J.Y. Kim, G. Magesh, Y.J. Jang, J.S. Lee, J. Power Sources 294 (2015) 437e443. [29] R. Subbaraman, D. Tripkovic, D. Strmcnik, K.C. Chang, M. Uchimura, A.P. Paulikas, V. Stamenkovic, N.M. Markovic, Science 334 (2011) 1256e1260.
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