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reduced graphene oxide composite as an efficient electrocatalyst for electrochemical and photoelectrochemical water oxidation
Journal of Power Sources 294 (2015) 437e443
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
One-pot synthesis of NiFe layered double hydroxide/reduced graphene oxide composite as an efficient electrocatalyst for electrochemical and photoelectrochemical water oxidation Duck Hyun Youn a, 1, Yoon Bin Park b, 1, Jae Young Kim a, Ganesan Magesh a, Youn Jeong Jang b, Jae Sung Lee a, * a b
School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, South Korea Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea
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 is prepared by a simple solvothermal method in one-pot. NiFe LDH/RGO was used as an efficient catalyst for oxygen evolution reaction (OER). It shows excellent OER performance with a low benchmark h10 value of 0.245 V. NiFe LDH/RGO works as OER cocatalyst for a hematite photoanode.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 March 2015 Received in revised form 2 June 2015 Accepted 16 June 2015 Available online xxx
As an efficient non-precious metal catalyst for oxygen evolution reaction (OER) in electrochemical and photoelectrochemical water splitting, NiFe layered double hydroxide (LDH)/reduced graphene oxide (NiFe/RGO) composite is synthesized by a simple solvothermal method in one-pot. NiFe LDHs are uniformly deposited on RGO layers of high electrical conductivity and large surface area. In electrochemical water splitting, NiFe/RGO shows superior OER performance compared to bare NiFe and reference IrO2 with a lower benchmark h10 value (required overpotential to drive 10 mA cm2) of 0.245 V. Furthermore, NiFe/RGO substantially increases the performance of a hematite photoanode in photoelectrochemical water oxidation, demonstrating its potential as an OER co-catalyst for photoelectrodes. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nickel iron layered double hydroxide Reduced graphene oxide Oxygen evolution reaction Water splitting Electrocatalysts
1. Introduction Electrochemical and photoelectrochemical water splitting powered by solar or wind energy is a clean technology to produce
* Corresponding author. E-mail address: [email protected] (J.S. Lee). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2015.06.098 0378-7753/© 2015 Elsevier B.V. All rights reserved.
hydrogen as a promising energy carrier [1,2]. The four-electron involved oxygen evolution reaction (OER) is kinetically slower than the two-electron hydrogen evolution reaction (HER) [3], and thus development of efficient OER electrocatalysts is required for large-scale hydrogen production. At present, the typical electrocatalysts for OER are based on precious metals like Ir or Ru [4], but their high cost and limited supply impose barrier to the practical use. Thus far, various low-cost alternatives have been investigated to catalyze OER including metal oxides [5,6], hydroxides [7,8],
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chalcogenides [9,10], phosphate [11,12], and perovskites [13]. However, further improvement is needed in terms of activity, stability, and cost. Layered double hydroxides (LDHs) are hydrotalcite-like ionic lamellar compounds represented by a chemical formula of [M2þ1xM3þx(OH)2]xþ[Ax/n$H2O]n [14]. They are composed of positively charged metal hydroxide layers with an interlayer space containing charge compensating anions. Owing to the tunable composition and flexible ion exchangeability, LDHs have attracted a broad range of interest in energy applications encompassing batteries [15,16], supercapacitors [17,18], fuel cells [19], and photocatalysts [20,21]. Only recently, abundant 3d metal-based LDHs including NiFe [22], CoMn [23], and ZnCo [24] LDHs are recognized as efficient catalysts for OER in alkaline media. For example, Song et al. reported superior OER activity of exfoliated NiFe LDH nanosheets (NiFe-NS) compared to the NiCo-NS, CoCo-NS, and bulk form of NiFe LDH [22]. ZnCo LDH grown on Ni metal by an electrodeposition method exhibited higher activity than that of CoeOH [24]. The performances of LDHs for OER could be further improved by combining with nanocarbons such as carbon nanotube and graphene. In general, nanocarbons provide electrically conductive pathways to the loaded LDHs as well as a large surface area. Also, it could control the aggregation of the LDH crystals. NiFe LDH/CNT hybrid showed higher activity than reference Ir/C catalyst with greatly reduced onset potential of 1.45 V vs RHE [25]. And, ZnCo/ reduce graphene oxide (RGO) composite exhibited much higher OER performance compared to bare ZnCo [26]. Here, we report an efficient OER electrocatalyst composed of NiFe LDH and RGO (NiFe/RGO). By a simple solvothermal method in one-pot, we could fabricate uniformly deposited NiFe LDHs on RGO. Intimate contact between NiFe LDHs and RGO layers markedly increased the conductivity of the catalyst. Thus, the NiFe/RGO catalyst exhibits a remarkably higher activity compared to the bare NiFe LDH (NiFe) and commercial IrO2 catalyst with excellent stability. The faradaic efficiency for oxygen evolution was ca. 97% in a gas evolution measurement suggesting its potential in practical electrochemical water splitting. Furthermore, we have demonstrated the effective role of NiFe/RGO as a co-catalyst for photoanode (hematite on FTO glass) in a photoelectrochemical (PEC) water oxidation for the first time, where the photocurrent of hematite measured at 1.23 V vs RHE increased more than two times after junction with NiFe/RGO. 2. Experimental section
structures were investigated using X-ray diffraction (XRD, PANalytical pw 3040/60 X'pert diffractometer) and chemical states of the catalyst were elucidated with Raman spectroscopy (Horiba, LabRam Aramis spectrometer) and X-ray photoelectron spectroscopy (XPS, ESCALAB, 250Xi). Surface areas of the catalysts were characterized by N2-sorption isotherms measured at 77 K (Mirae scientific instruments, Nanoporosity-XQ). Conductivity of the catalysts was measured by the four-point probe method (Keithley 2400) using a pelletized sample (thin disk type) without any additives. 2.3. Electrochemical measurement Electrochemical measurements including linear sweep voltammetry (LSV) and stability tests were carried out in a conventional three electrode cell with O2 purged aqueous solution of 1 M KOH using a potentiostat (Ivium technologies) equipped with a rotating disk electrode setup (RDE, PAR Model 636 RDE). The Ag/ AgCl (3 M NaCl) electrode and a Pt wire were used as reference and counter electrodes, respectively. All the potentials were referred to the reversible hydrogen electrode (RHE) without specification. The working electrodes were prepared by dispersing 20 mg of catalyst in 1.6 ml of deionized water, 0.4 ml of ethanol, and 40 ml of 5% Nafion solution and pipetting out 20 ml of slurry onto a glassy carbon electrode (0.19635 cm2). The LSV tests were performed at a scan rate of 5 mV s1 with 900 rpm. The stability tests were carried out by repeating the potential scan from 1.10 V to 1.85 V with 1000 cycles. In the identical cell setup, electrochemical impedance spectroscopy (EIS) was carried out. The frequency range was from 100 kHz to 1 mHz with a modulation amplitude of 10 mV at 1.55 V bias voltage. The EIS spectra were fitted by the Z-view software. 2.4. Gas evolution measurements The produced oxygen on the NiFe/RGO and hydrogen at Pt wire counter electrode were detected by a gas chromatography (Agilent, 6890) connected to a sealed three cell electrode cell. The working electrode was fabricated by dropping the catalyst slurry onto the carbon paper (1 1 cm) with a catalyst loading of 1 mg/cm2 and attaching copper wire to the carbon paper. Other conditions were identical to the electrochemical measurements. Before experiment, the electrolyte was degassed by argon purging to remove the dissolved oxygen for 30 min. Static overpotential of 0.30 V was applied to the electrode and the produced gases were monitored in-situ by GC every 15 min for 180 min.
2.1. Catalyst preparation 2.5. Photoelectrochemical measurements NiFe/RGO composite was synthesized by a solvothermal method [25]. GO was prepared by Hummer's method [27] and ultrasonically dispersed in water (2 mg/ml). 20 ml of 0.2 M Ni(OAc)2$4H2O and 4 ml of 0.2 M Fe(NO3)3$9H2O aqueous solutions were mixed under stirring followed by addition of 24 ml of water, 12 ml of GO solution, and 30 ml of N,N-dimethylformamide (DMF). 20 ml of 65% hydrazine was further added to the solution as a reducing agent, which was transferred to a Teflon lined stainless steel reactor and treated solvothermally at 120 C for 18 h and at 160 C for 2 h. After filtering and washing with excess amount of water, we could obtain NiFe/ RGO powder. As a control experiment bare NiFe was prepared by carrying out the procedure without GO and hydrazine.
10 mg of NiFe/RGO was dispersed ultrasonically in 2 ml of ethanol. 5, 10, and 20 ml of the solution were dropped onto the hematite photoanodes prepared by previous literatures [28,29], which were used as working electrodes. And, the photocurrents were measured in a conventional three electrode cell under simulated solar light generated by a solar simulator (91160, Oriel) with an air mass 1.5 G filter. Light intensity of the solar simulator was calibrated to 1 sun (100 mW/cm2) using a reference cell certified by the National Renewable Energy Laboratories, U.S. The Ag/AgCl (3 M NaCl) electrode, a Pt wire, and 1 M NaOH were used as reference electrode, counter electrode, and electrolyte, respectively.
2.2. Catalyst characterization
3. Results and discussions
Structural details of the prepared catalysts were analyzed by high-resolution transmission electron microscopy (TEM, JEOL, JEM2100F) and energy dispersive spectroscopy (EDS). Crystalline
3.1. Physical properties of NiFe/RGO Scheme 1 displays our synthetic method and schematic model
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Scheme 1. Schematic illustration of the synthetic method and formed NiFe LDH on RGO support for oxygen evolution.
of the NiFe/RGO composite. Aqueous solution containing nickel acetate and iron acetate was mixed with graphene oxide (GO) solution, followed by addition of N,N-dimethylformamide (DMF) and hydrazine to the solution. Solvothermal treatment at 120 C for 18 h and 160 C for 2 h gave NiFe/RGO powder. During the solvothermal treatment, crystallization of NiFe and reduction of GO to RGO were achieved simultaneously. Furthermore, plate-shaped NiFe crystals were selectively deposited onto RGO layer due to the oxygen functional groups in GO. Structural details of the prepared catalysts were investigated by transmission electron microscopy (TEM) in Fig. 1 and Fig. S1. For bare NiFe, plate-like morphology with sizes of several tens of hundreds nanometers are clearly observed in Fig. 1a (bottom inset). The bare NiFe nanoplates were stacked each other showing a lattice spacing of 0.25 nm corresponding to d(012) of NiFe [25]. In contrast, NiFe layers are well dispersed on the RGO layers (Fig. 1b). Energy dispersive X-ray spectroscopy (EDS) of TEM provides further morphological information for NiFe/RGO. Elemental mapping images of nickel, iron, and oxygen are perfectly consistent with that of carbon in Fig. S2, which reveals that NiFe nanoplates are uniformly deposited onto RGO layers. Oxygen-containing functional groups on GO attract the metal precursors [30], and thus crystallization of NiFe occurs on RGO layer selectively. In inset of Fig. 1b, lattice fringes of 0.25 and 0.34 nm correspond to the d(012) of NiFe and d(002) of RGO, respectively [25,31]. For both of the catalysts, small and dark FeOx nanoparticles (~5 nm) are observed onto the NiFe
layers. The existence of FeOx particles could be verified by TEM-EDS measurements in Fig. S3. The XRD patterns of bare NiFe and NiFe/RGO composite are shown in Fig. 2a, which is consistent with their reference XRD patterns (JCPDS No. 00-038-0715). No other phases were detected and hence NiFe LDH is the dominant phase. Their similar XRD patterns suggest that crystalline structure of NiFe was preserved after combined with RGO. The mean basal plane distance was calculated to be 0.78 nm. Fig. 2b compares the Raman spectrum of NiFe/RGO with pristine GO. The numbers in the figure denote the intensity ratios between D and G peaks (ID/IG ratios). ID/IG ratio of NiFe/RGO was measured to be 1.12, which was higher than that of GO. The increased ID/IG ratio relative to the value of GO implies restoration of sp2 carbon and a decrease in average size of the sp2 domains upon reduction of GO [32,33]. Thus, it is concluded that GO is clearly reduced to RGO during the solvothermal process, which is further verified by XPS measurements below. Chemical states of the prepared samples are investigated by XPS. XPS spectra of bare NiFe and NiFe/RGO for Ni 2p in Fig. 2c are similar. It indicates that the state of Ni in NiFe does not change after combined with RGO. Ni 2p1/2 and 2p3/2 peaks appear at 873.8 and 855.5 eV, respectively, indicating that oxidation state is mainly þ2. In Fig. 2d, Fe species in both bare NiFe and NiFe/RGO are mostly in the þ3 oxidation state with Fe 2p1/2 peak at 724 and 2p3/2 peak at 713.7 eV [25,34]. The C1s spectrum of GO in Fig. S4a shows various peaks at 284.8, 286.2, 287.8, and 289.0 eV due to CeC, CeO, C]O,
Fig. 1. TEM images and the corresponding high-resolution (upper insets) TEM images of a) NiFe and b) NiFe/RGO. The bottom inset in a denotes the low-magnification TEM image showing the entire morphology of NiFe. (See Fig. S1 for enlarged inset images.)
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Fig. 2. a) XRD patterns of the prepared catalysts with a structural model. b) Raman spectra of NiFe/RGO and GO. XPS spectra of the prepared catalysts for c) Ni 2p and d) Fe 2p.
and C(O)O, respectively. Typical GO contains several oxygencontaining functional groups such as hydroxyl, carboxyl, and epoxy, and thus it exhibits broad peaks in 280e290 eV range [30]. However, NiFe/RGO exhibits significantly decreased intensity of those peaks (Fig. S4b) suggesting reduction of GO to RGO. Thus, considering the increased ID/IG ratio in Raman spectrum and the decreased intensity of the XPS peaks for oxygen-containing functional groups, we could confirm that GO is effectively reduced to RGO during the solvothermal treatment. Textural properties of the synthesized catalysts were revealed by N2-soprtion isotherms in Fig. S5. NiFe and NiFe/RGO exhibit type Ⅳ isotherms suggesting the existence of meso pores. The BET surface area of NiFe/RGO (80.04 m2 g1) is nearly two times higher than that of bare NiFe (44.74 m2 g1). Large surface area of RGO renders more NiFe nanoplates to be exposed to contact with electrolyte, which could enhance the electrocatalytic activity compared to bare NiFe. Conductivities of the prepared catalysts were measured by the four point probe method, and the results are summarized in Table 1. Bare NiFe is not a good electrical conductor with sheet resistance of 1.72 106 U ,1. The NiFe/RGO shows a markedly reduced sheet resistance of 2.72 103 U ,1. Thus, the conductivity was ~103 times higher than that of bare NiFe. A good conductivity is an important requirement for a high activity in electrocatalysis, which
Table 1 Electrical conductivities of bare NiFe and NiFe/RGO with various compositions. Catalysts
Sheet resistance (U ,1)
NiFe NiFe/RGO (5:1) NiFe/RGO (4:1) NiFe/RGO (6:1)
1.72 2.72 2.86 4.93
(±0.17) (±0.10) (±0.09) (±0.14)
106 103 103 103
Conductivity (S m1) 3.97 103 1.84 1.75 1.01
was easily achieved here by dispersing NiFe nanoplates onto RGO layers. Thus, by combining NiFe nanoplates with RGO, following two advantages are expected: i) Large surface area of RGO could enhance the contact area between electrolyte and NiFe nanoplates, which contributes to increase the number of active sites. ii) RGO could provide conductive electron pathways for the loaded NiFe resulting in the enhanced activity of each active site. 3.2. Electrochemical and photoelectrochemical OER Before the electrochemical measurements, the compositions between nickel and iron in NiFe/RGO samples were optimized. The NiFe/RGO samples with different molar ratios are denoted as NixFey/RGO, where x and y represent a molar ratio of nickel and iron, respectively. The energy dispersive spectroscopy (EDS) results in Fig. S6 exhibit that their compositions are close to the nominal compositions. In the polarization results (Fig. S7), Ni5Fe1/RGO exhibits the highest current density in the whole potential region. Further, the onset overpotential (h0) of Ni5Fe1/RGO was measured to be 0.230 V, which was lower than those of the other samples (0.232 V for Ni4Fe1/RGO and 0.240 V for Ni6Fe1/RGO sample). In addition, required overpotentials to drive current density of 10 mAcm2 (h10) are 0.245, 0.253, and 0.280 V for Ni5Fe1/RGO, Ni4Fe1/RGO, and Ni6Fe1/RGO samples, respectively. These activity results are generally proportional to their electrical conductivities. As indicated in Table 1, Ni5Fe1/RGO catalyst exhibits higher conductivity of 1.84 S m1 than those of Ni4Fe1/RGO (1.75 S m1) and Ni6Fe1/RGO (1.01 S m1) samples. Thus, we use Ni5Fe1/RGO as an optimized catalyst for further electrochemical measurements without subscripts. Fig. 3a displays the polarization curves of the prepared catalysts and commercial IrO2 catalyst in 1 M KOH solution. The observed peak around 1.43 V of NiFe/RGO is originated from the Ni(II)/Ni(III
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Fig. 3. Electrochemical characterizations of the prepared catalysts. a) Polarization curves, b) required overpotentials to drive 10 and 20 mA cm2, c) stability measurements (empty circles and rectangles denote the current densities of each catalyst after 1000 repeated potential cyclings), d) Nyquist plots with an equivalent circuit.
or IV) redox process [25]. The NiFe/RGO far outperforms the other catalysts in current density over the whole potential range with lower h0 value. The h0 values are 0.23, 0.27, and 0.29 V for NiFe/ RGO, NiFe, and IrO2, respectively. These results lead us to conclude that NiFe/RGO has enhanced activity of each active site and increased number of active sites by the aid of RGO. Furthermore, h10 value of NiFe/RGO (0.245 V) is much lower than that of bare NiFe (0.353 V) and IrO2 (0.422 V) in Fig. 3b. Commonly, h10 value is used as a benchmark indicator for comparing the activities of OER catalysts because a solar light-coupled hydrogen production apparatus usually runs at 10e20 mA cm2 [4,35]. In this respect, the performance of our NiFe/RGO catalyst is one of the best among recently reported LDH-based materials and other non-precious metal catalysts (see comparative table in Table S1) [7,9,22e26,34,36,37]. Stability of OER catalyst is a critical issue due to the highly corrosive reaction conditions. The NiFe/RGO catalyst exhibits a very good electrochemical stability with negligible activity loss after thousand potential-cycling tests between 1.10 and 1.85 V in Fig. 3c. However, commercial IrO2 shows an activity loss as evidenced by increased h10 value from 0.422 to 0.457 V. Such a good stability and high activity of the NiFe/RGO catalyst verify the effectiveness of our strategy to fabricate practical eletrocatalysts for OER. To check the chemical stability of the catalyst, we have measured XPS spectra of NiFe/RGO sample before and after the durability test. As depicted in Fig. S8, no distinct difference is observed before and after the test in spectra of Ni 2p and Fe 2p. Thus, it is concluded that the chemical states of Ni and Fe in NiFe/RGO remains unchanged. Electrochemical impedance spectroscopy (EIS) was conducted for further characterization of the prepared catalysts. The obtained Nyquist plots are presented in Fig. 3d. The data were fitted to an equivalent circuit illustrated in the inset of Fig. 3d, and the resultant fitting parameters are summarized in Table S2. A semicircle in the Nyquist plots is originated from the charge-transfer resistance (Rct) and the corresponding capacitance, which describes the chargetransfer process at the interface between electrocatalyst and
electrolyte. In general, Rct value is inversely proportional to the electrocatalytic activity. The recorded Rct value of NiFe/RGO (12.1 U) is much lower than those of bare NiFe (43.0 U) and IrO2 (129.3 U), which represents the high electrocatalytic activity of NiFe/RGO for OER owing to the highly conductive RGO by enhancing the charge transfer characteristics of NiFe nanoplates. Even though bare NiFe exhibited lower Rct value compared to IrO2, its low conductivity limits its OER activity. In addition to the low Rct value, NiFe/RGO shows a significantly higher capacitance value of 526 mF than those of bare NiFe (220.5 mF) and IrO2 (170.0 mF). The capacitance is proportional to the contact area between the catalysts and electrolyte. Thus, high capacitance value is advantageous in electrocatalysis. The capacitance value of NiFe/RGO was 2.4 times higher than that of NiFe, which is in good agreement with their BET surface areas in Fig. S5. By combining NiFe nanoplates with RGO, contact area of NiFe/RGO is substantially increased, which could contribute to the high OER activity. Fig. 4 shows the oxygen evolution on NiFe/RGO electrode at static overpotential of 0.30 V monitored by a gas-chromatography (GC). Produced hydrogen gas at counter electrode is also recorded. Stable oxygen evolution was observed for 180 min. Notably, its faradaic efficiency for oxygen evolution was ca. 97% at 180 min, indicating measured current density of NiFe/RGO is mostly originated from water oxidation reaction. The molar ratio between produced hydrogen and oxygen is 2:1. Thus, it is concluded that our NiFe/RGO obviously induces the perfect electrochemical water splitting. Apart from the electrochemical water splitting, our NiFe/RGO was employed as a co-catalyst for the photoanode in PEC system for the first time. Here, hematite (a-Fe2O3) was selected and fabricated as a model photoanode due to its excellent stability under alkaline condition, small bandgap of 2.1 eV, and low price [28,29]. The NiFe/ RGO was dispersed in ethanol ultrasonically and three different amounts of the solution (5, 10, and 20 ml) were dropped to the bare hematite photoanodes. Fig. 5 shows water oxidation photocurrent
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Fig. 4. Gas evolution of the NiFe/RGO electrode monitored by a gas chromatography.
RGO layers indicating the effectiveness of our strategy to fabricate OER catalysts. The electrochemical characterizations verified the superior activity and stability of NiFe/RGO compared to bare NiFe and reference IrO2. Especially, its h10 value of 0.245 V represents one of the best performance among the recently reported LDHbased materials as well as other non-precious metal catalysts. Here, RGO layers played an important role to enhance the activity of the catalyst by providing a good electrical pathway and a high surface area to the loaded NiFe nanoplates. Furthermore, NiFe/RGO recorded high faradaic efficiency of 97% for oxygen evolution with produced hydrogen to oxygen molar ratio of 2:1, indicating perfect electrochemical water splitting was induced. In addition, the effective role of NiFe/RGO as a co-catalyst for photoanode in PEC system was demonstrated for the first time. The photocurrent of NiFe/RGO modified hematite photoanode was increased more than two times with negatively shifted onset potential. Thus, the NiFe/ RGO could be a practical electrocatalyst for OER owing to its high activity, stability, and low cost. Acknowledgments This work was supported by Brain Korea Plus Program of Ministry of Education, and Basic Science Research Program (No. 2012017247) and Korean Center for Artificial Photosynthesis (NRF-2011C1AAA0001-2011-0030278) funded by MISIP and Project No. 10050509 funded by MOTIE of Republic of Korea. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.06.098. References
Fig. 5. Water oxidation photocurrents measured under simulated solar light irradiation (1 sun, AM 1.5).
response for the prepared hematite photoanodes under 1 sun and AM 1.5 simulated solar light irradiation. The hematite photoanode modified with 10 ml of NiFe/RGO solution (simply presented as 10 ml NiFe/RGO in Fig. 4) marks a much higher photocurrent of 0.95 mA cm2 at 1.23 V compared to the bare hematite (0.45 mA cm2) and the other modified photoanodes (5 ml NiFe/ RGO for 0.63 and 20 ml NiFe/RGO for 0.46 mA cm2). Notably, the onset potentials of the modified photoanodes are negatively shifted ca. 150 mV than that of bare hematite, revealing the effectiveness of NiFe/RGO as a co-catalyst to enhance the photoelectrochemical activity of hematite photoanode for water oxidation [28,38]. Further optimization to fabricate hematite photoanodes with thin layer of NiFe/RGO and a detailed study to reveal the co-catalytic role of NiFe/RGO are under progress.
4. Conclusions In summary, we have successfully developed NiFe/RGO composite for OER electrocatalyst by a simple solvothermal method in one-pot. The NiFe/RGO exhibited synergistic effect between active NiFe nanoplates and RGO layers possessing high electrical conductivity and large surface area. As revealed by physicochemical characterizations, NiFe nanoplates were uniformly deposited on
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
One-pot synthesis of NiFe layered double hydroxide/reduced graphene oxide composite as an efficient electrocatalyst for electrochemical and photoelectrochemical water oxidation Duck Hyun Youn a, 1, Yoon Bin Park b, 1, Jae Young Kim a, Ganesan Magesh a, Youn Jeong Jang b, Jae Sung Lee a, * a b
School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, South Korea Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea
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 is prepared by a simple solvothermal method in one-pot. NiFe LDH/RGO was used as an efficient catalyst for oxygen evolution reaction (OER). It shows excellent OER performance with a low benchmark h10 value of 0.245 V. NiFe LDH/RGO works as OER cocatalyst for a hematite photoanode.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 March 2015 Received in revised form 2 June 2015 Accepted 16 June 2015 Available online xxx
As an efficient non-precious metal catalyst for oxygen evolution reaction (OER) in electrochemical and photoelectrochemical water splitting, NiFe layered double hydroxide (LDH)/reduced graphene oxide (NiFe/RGO) composite is synthesized by a simple solvothermal method in one-pot. NiFe LDHs are uniformly deposited on RGO layers of high electrical conductivity and large surface area. In electrochemical water splitting, NiFe/RGO shows superior OER performance compared to bare NiFe and reference IrO2 with a lower benchmark h10 value (required overpotential to drive 10 mA cm2) of 0.245 V. Furthermore, NiFe/RGO substantially increases the performance of a hematite photoanode in photoelectrochemical water oxidation, demonstrating its potential as an OER co-catalyst for photoelectrodes. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nickel iron layered double hydroxide Reduced graphene oxide Oxygen evolution reaction Water splitting Electrocatalysts
1. Introduction Electrochemical and photoelectrochemical water splitting powered by solar or wind energy is a clean technology to produce
* Corresponding author. E-mail address: [email protected] (J.S. Lee). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2015.06.098 0378-7753/© 2015 Elsevier B.V. All rights reserved.
hydrogen as a promising energy carrier [1,2]. The four-electron involved oxygen evolution reaction (OER) is kinetically slower than the two-electron hydrogen evolution reaction (HER) [3], and thus development of efficient OER electrocatalysts is required for large-scale hydrogen production. At present, the typical electrocatalysts for OER are based on precious metals like Ir or Ru [4], but their high cost and limited supply impose barrier to the practical use. Thus far, various low-cost alternatives have been investigated to catalyze OER including metal oxides [5,6], hydroxides [7,8],
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chalcogenides [9,10], phosphate [11,12], and perovskites [13]. However, further improvement is needed in terms of activity, stability, and cost. Layered double hydroxides (LDHs) are hydrotalcite-like ionic lamellar compounds represented by a chemical formula of [M2þ1xM3þx(OH)2]xþ[Ax/n$H2O]n [14]. They are composed of positively charged metal hydroxide layers with an interlayer space containing charge compensating anions. Owing to the tunable composition and flexible ion exchangeability, LDHs have attracted a broad range of interest in energy applications encompassing batteries [15,16], supercapacitors [17,18], fuel cells [19], and photocatalysts [20,21]. Only recently, abundant 3d metal-based LDHs including NiFe [22], CoMn [23], and ZnCo [24] LDHs are recognized as efficient catalysts for OER in alkaline media. For example, Song et al. reported superior OER activity of exfoliated NiFe LDH nanosheets (NiFe-NS) compared to the NiCo-NS, CoCo-NS, and bulk form of NiFe LDH [22]. ZnCo LDH grown on Ni metal by an electrodeposition method exhibited higher activity than that of CoeOH [24]. The performances of LDHs for OER could be further improved by combining with nanocarbons such as carbon nanotube and graphene. In general, nanocarbons provide electrically conductive pathways to the loaded LDHs as well as a large surface area. Also, it could control the aggregation of the LDH crystals. NiFe LDH/CNT hybrid showed higher activity than reference Ir/C catalyst with greatly reduced onset potential of 1.45 V vs RHE [25]. And, ZnCo/ reduce graphene oxide (RGO) composite exhibited much higher OER performance compared to bare ZnCo [26]. Here, we report an efficient OER electrocatalyst composed of NiFe LDH and RGO (NiFe/RGO). By a simple solvothermal method in one-pot, we could fabricate uniformly deposited NiFe LDHs on RGO. Intimate contact between NiFe LDHs and RGO layers markedly increased the conductivity of the catalyst. Thus, the NiFe/RGO catalyst exhibits a remarkably higher activity compared to the bare NiFe LDH (NiFe) and commercial IrO2 catalyst with excellent stability. The faradaic efficiency for oxygen evolution was ca. 97% in a gas evolution measurement suggesting its potential in practical electrochemical water splitting. Furthermore, we have demonstrated the effective role of NiFe/RGO as a co-catalyst for photoanode (hematite on FTO glass) in a photoelectrochemical (PEC) water oxidation for the first time, where the photocurrent of hematite measured at 1.23 V vs RHE increased more than two times after junction with NiFe/RGO. 2. Experimental section
structures were investigated using X-ray diffraction (XRD, PANalytical pw 3040/60 X'pert diffractometer) and chemical states of the catalyst were elucidated with Raman spectroscopy (Horiba, LabRam Aramis spectrometer) and X-ray photoelectron spectroscopy (XPS, ESCALAB, 250Xi). Surface areas of the catalysts were characterized by N2-sorption isotherms measured at 77 K (Mirae scientific instruments, Nanoporosity-XQ). Conductivity of the catalysts was measured by the four-point probe method (Keithley 2400) using a pelletized sample (thin disk type) without any additives. 2.3. Electrochemical measurement Electrochemical measurements including linear sweep voltammetry (LSV) and stability tests were carried out in a conventional three electrode cell with O2 purged aqueous solution of 1 M KOH using a potentiostat (Ivium technologies) equipped with a rotating disk electrode setup (RDE, PAR Model 636 RDE). The Ag/ AgCl (3 M NaCl) electrode and a Pt wire were used as reference and counter electrodes, respectively. All the potentials were referred to the reversible hydrogen electrode (RHE) without specification. The working electrodes were prepared by dispersing 20 mg of catalyst in 1.6 ml of deionized water, 0.4 ml of ethanol, and 40 ml of 5% Nafion solution and pipetting out 20 ml of slurry onto a glassy carbon electrode (0.19635 cm2). The LSV tests were performed at a scan rate of 5 mV s1 with 900 rpm. The stability tests were carried out by repeating the potential scan from 1.10 V to 1.85 V with 1000 cycles. In the identical cell setup, electrochemical impedance spectroscopy (EIS) was carried out. The frequency range was from 100 kHz to 1 mHz with a modulation amplitude of 10 mV at 1.55 V bias voltage. The EIS spectra were fitted by the Z-view software. 2.4. Gas evolution measurements The produced oxygen on the NiFe/RGO and hydrogen at Pt wire counter electrode were detected by a gas chromatography (Agilent, 6890) connected to a sealed three cell electrode cell. The working electrode was fabricated by dropping the catalyst slurry onto the carbon paper (1 1 cm) with a catalyst loading of 1 mg/cm2 and attaching copper wire to the carbon paper. Other conditions were identical to the electrochemical measurements. Before experiment, the electrolyte was degassed by argon purging to remove the dissolved oxygen for 30 min. Static overpotential of 0.30 V was applied to the electrode and the produced gases were monitored in-situ by GC every 15 min for 180 min.
2.1. Catalyst preparation 2.5. Photoelectrochemical measurements NiFe/RGO composite was synthesized by a solvothermal method [25]. GO was prepared by Hummer's method [27] and ultrasonically dispersed in water (2 mg/ml). 20 ml of 0.2 M Ni(OAc)2$4H2O and 4 ml of 0.2 M Fe(NO3)3$9H2O aqueous solutions were mixed under stirring followed by addition of 24 ml of water, 12 ml of GO solution, and 30 ml of N,N-dimethylformamide (DMF). 20 ml of 65% hydrazine was further added to the solution as a reducing agent, which was transferred to a Teflon lined stainless steel reactor and treated solvothermally at 120 C for 18 h and at 160 C for 2 h. After filtering and washing with excess amount of water, we could obtain NiFe/ RGO powder. As a control experiment bare NiFe was prepared by carrying out the procedure without GO and hydrazine.
10 mg of NiFe/RGO was dispersed ultrasonically in 2 ml of ethanol. 5, 10, and 20 ml of the solution were dropped onto the hematite photoanodes prepared by previous literatures [28,29], which were used as working electrodes. And, the photocurrents were measured in a conventional three electrode cell under simulated solar light generated by a solar simulator (91160, Oriel) with an air mass 1.5 G filter. Light intensity of the solar simulator was calibrated to 1 sun (100 mW/cm2) using a reference cell certified by the National Renewable Energy Laboratories, U.S. The Ag/AgCl (3 M NaCl) electrode, a Pt wire, and 1 M NaOH were used as reference electrode, counter electrode, and electrolyte, respectively.
2.2. Catalyst characterization
3. Results and discussions
Structural details of the prepared catalysts were analyzed by high-resolution transmission electron microscopy (TEM, JEOL, JEM2100F) and energy dispersive spectroscopy (EDS). Crystalline
3.1. Physical properties of NiFe/RGO Scheme 1 displays our synthetic method and schematic model
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Scheme 1. Schematic illustration of the synthetic method and formed NiFe LDH on RGO support for oxygen evolution.
of the NiFe/RGO composite. Aqueous solution containing nickel acetate and iron acetate was mixed with graphene oxide (GO) solution, followed by addition of N,N-dimethylformamide (DMF) and hydrazine to the solution. Solvothermal treatment at 120 C for 18 h and 160 C for 2 h gave NiFe/RGO powder. During the solvothermal treatment, crystallization of NiFe and reduction of GO to RGO were achieved simultaneously. Furthermore, plate-shaped NiFe crystals were selectively deposited onto RGO layer due to the oxygen functional groups in GO. Structural details of the prepared catalysts were investigated by transmission electron microscopy (TEM) in Fig. 1 and Fig. S1. For bare NiFe, plate-like morphology with sizes of several tens of hundreds nanometers are clearly observed in Fig. 1a (bottom inset). The bare NiFe nanoplates were stacked each other showing a lattice spacing of 0.25 nm corresponding to d(012) of NiFe [25]. In contrast, NiFe layers are well dispersed on the RGO layers (Fig. 1b). Energy dispersive X-ray spectroscopy (EDS) of TEM provides further morphological information for NiFe/RGO. Elemental mapping images of nickel, iron, and oxygen are perfectly consistent with that of carbon in Fig. S2, which reveals that NiFe nanoplates are uniformly deposited onto RGO layers. Oxygen-containing functional groups on GO attract the metal precursors [30], and thus crystallization of NiFe occurs on RGO layer selectively. In inset of Fig. 1b, lattice fringes of 0.25 and 0.34 nm correspond to the d(012) of NiFe and d(002) of RGO, respectively [25,31]. For both of the catalysts, small and dark FeOx nanoparticles (~5 nm) are observed onto the NiFe
layers. The existence of FeOx particles could be verified by TEM-EDS measurements in Fig. S3. The XRD patterns of bare NiFe and NiFe/RGO composite are shown in Fig. 2a, which is consistent with their reference XRD patterns (JCPDS No. 00-038-0715). No other phases were detected and hence NiFe LDH is the dominant phase. Their similar XRD patterns suggest that crystalline structure of NiFe was preserved after combined with RGO. The mean basal plane distance was calculated to be 0.78 nm. Fig. 2b compares the Raman spectrum of NiFe/RGO with pristine GO. The numbers in the figure denote the intensity ratios between D and G peaks (ID/IG ratios). ID/IG ratio of NiFe/RGO was measured to be 1.12, which was higher than that of GO. The increased ID/IG ratio relative to the value of GO implies restoration of sp2 carbon and a decrease in average size of the sp2 domains upon reduction of GO [32,33]. Thus, it is concluded that GO is clearly reduced to RGO during the solvothermal process, which is further verified by XPS measurements below. Chemical states of the prepared samples are investigated by XPS. XPS spectra of bare NiFe and NiFe/RGO for Ni 2p in Fig. 2c are similar. It indicates that the state of Ni in NiFe does not change after combined with RGO. Ni 2p1/2 and 2p3/2 peaks appear at 873.8 and 855.5 eV, respectively, indicating that oxidation state is mainly þ2. In Fig. 2d, Fe species in both bare NiFe and NiFe/RGO are mostly in the þ3 oxidation state with Fe 2p1/2 peak at 724 and 2p3/2 peak at 713.7 eV [25,34]. The C1s spectrum of GO in Fig. S4a shows various peaks at 284.8, 286.2, 287.8, and 289.0 eV due to CeC, CeO, C]O,
Fig. 1. TEM images and the corresponding high-resolution (upper insets) TEM images of a) NiFe and b) NiFe/RGO. The bottom inset in a denotes the low-magnification TEM image showing the entire morphology of NiFe. (See Fig. S1 for enlarged inset images.)
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Fig. 2. a) XRD patterns of the prepared catalysts with a structural model. b) Raman spectra of NiFe/RGO and GO. XPS spectra of the prepared catalysts for c) Ni 2p and d) Fe 2p.
and C(O)O, respectively. Typical GO contains several oxygencontaining functional groups such as hydroxyl, carboxyl, and epoxy, and thus it exhibits broad peaks in 280e290 eV range [30]. However, NiFe/RGO exhibits significantly decreased intensity of those peaks (Fig. S4b) suggesting reduction of GO to RGO. Thus, considering the increased ID/IG ratio in Raman spectrum and the decreased intensity of the XPS peaks for oxygen-containing functional groups, we could confirm that GO is effectively reduced to RGO during the solvothermal treatment. Textural properties of the synthesized catalysts were revealed by N2-soprtion isotherms in Fig. S5. NiFe and NiFe/RGO exhibit type Ⅳ isotherms suggesting the existence of meso pores. The BET surface area of NiFe/RGO (80.04 m2 g1) is nearly two times higher than that of bare NiFe (44.74 m2 g1). Large surface area of RGO renders more NiFe nanoplates to be exposed to contact with electrolyte, which could enhance the electrocatalytic activity compared to bare NiFe. Conductivities of the prepared catalysts were measured by the four point probe method, and the results are summarized in Table 1. Bare NiFe is not a good electrical conductor with sheet resistance of 1.72 106 U ,1. The NiFe/RGO shows a markedly reduced sheet resistance of 2.72 103 U ,1. Thus, the conductivity was ~103 times higher than that of bare NiFe. A good conductivity is an important requirement for a high activity in electrocatalysis, which
Table 1 Electrical conductivities of bare NiFe and NiFe/RGO with various compositions. Catalysts
Sheet resistance (U ,1)
NiFe NiFe/RGO (5:1) NiFe/RGO (4:1) NiFe/RGO (6:1)
1.72 2.72 2.86 4.93
(±0.17) (±0.10) (±0.09) (±0.14)
106 103 103 103
Conductivity (S m1) 3.97 103 1.84 1.75 1.01
was easily achieved here by dispersing NiFe nanoplates onto RGO layers. Thus, by combining NiFe nanoplates with RGO, following two advantages are expected: i) Large surface area of RGO could enhance the contact area between electrolyte and NiFe nanoplates, which contributes to increase the number of active sites. ii) RGO could provide conductive electron pathways for the loaded NiFe resulting in the enhanced activity of each active site. 3.2. Electrochemical and photoelectrochemical OER Before the electrochemical measurements, the compositions between nickel and iron in NiFe/RGO samples were optimized. The NiFe/RGO samples with different molar ratios are denoted as NixFey/RGO, where x and y represent a molar ratio of nickel and iron, respectively. The energy dispersive spectroscopy (EDS) results in Fig. S6 exhibit that their compositions are close to the nominal compositions. In the polarization results (Fig. S7), Ni5Fe1/RGO exhibits the highest current density in the whole potential region. Further, the onset overpotential (h0) of Ni5Fe1/RGO was measured to be 0.230 V, which was lower than those of the other samples (0.232 V for Ni4Fe1/RGO and 0.240 V for Ni6Fe1/RGO sample). In addition, required overpotentials to drive current density of 10 mAcm2 (h10) are 0.245, 0.253, and 0.280 V for Ni5Fe1/RGO, Ni4Fe1/RGO, and Ni6Fe1/RGO samples, respectively. These activity results are generally proportional to their electrical conductivities. As indicated in Table 1, Ni5Fe1/RGO catalyst exhibits higher conductivity of 1.84 S m1 than those of Ni4Fe1/RGO (1.75 S m1) and Ni6Fe1/RGO (1.01 S m1) samples. Thus, we use Ni5Fe1/RGO as an optimized catalyst for further electrochemical measurements without subscripts. Fig. 3a displays the polarization curves of the prepared catalysts and commercial IrO2 catalyst in 1 M KOH solution. The observed peak around 1.43 V of NiFe/RGO is originated from the Ni(II)/Ni(III
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Fig. 3. Electrochemical characterizations of the prepared catalysts. a) Polarization curves, b) required overpotentials to drive 10 and 20 mA cm2, c) stability measurements (empty circles and rectangles denote the current densities of each catalyst after 1000 repeated potential cyclings), d) Nyquist plots with an equivalent circuit.
or IV) redox process [25]. The NiFe/RGO far outperforms the other catalysts in current density over the whole potential range with lower h0 value. The h0 values are 0.23, 0.27, and 0.29 V for NiFe/ RGO, NiFe, and IrO2, respectively. These results lead us to conclude that NiFe/RGO has enhanced activity of each active site and increased number of active sites by the aid of RGO. Furthermore, h10 value of NiFe/RGO (0.245 V) is much lower than that of bare NiFe (0.353 V) and IrO2 (0.422 V) in Fig. 3b. Commonly, h10 value is used as a benchmark indicator for comparing the activities of OER catalysts because a solar light-coupled hydrogen production apparatus usually runs at 10e20 mA cm2 [4,35]. In this respect, the performance of our NiFe/RGO catalyst is one of the best among recently reported LDH-based materials and other non-precious metal catalysts (see comparative table in Table S1) [7,9,22e26,34,36,37]. Stability of OER catalyst is a critical issue due to the highly corrosive reaction conditions. The NiFe/RGO catalyst exhibits a very good electrochemical stability with negligible activity loss after thousand potential-cycling tests between 1.10 and 1.85 V in Fig. 3c. However, commercial IrO2 shows an activity loss as evidenced by increased h10 value from 0.422 to 0.457 V. Such a good stability and high activity of the NiFe/RGO catalyst verify the effectiveness of our strategy to fabricate practical eletrocatalysts for OER. To check the chemical stability of the catalyst, we have measured XPS spectra of NiFe/RGO sample before and after the durability test. As depicted in Fig. S8, no distinct difference is observed before and after the test in spectra of Ni 2p and Fe 2p. Thus, it is concluded that the chemical states of Ni and Fe in NiFe/RGO remains unchanged. Electrochemical impedance spectroscopy (EIS) was conducted for further characterization of the prepared catalysts. The obtained Nyquist plots are presented in Fig. 3d. The data were fitted to an equivalent circuit illustrated in the inset of Fig. 3d, and the resultant fitting parameters are summarized in Table S2. A semicircle in the Nyquist plots is originated from the charge-transfer resistance (Rct) and the corresponding capacitance, which describes the chargetransfer process at the interface between electrocatalyst and
electrolyte. In general, Rct value is inversely proportional to the electrocatalytic activity. The recorded Rct value of NiFe/RGO (12.1 U) is much lower than those of bare NiFe (43.0 U) and IrO2 (129.3 U), which represents the high electrocatalytic activity of NiFe/RGO for OER owing to the highly conductive RGO by enhancing the charge transfer characteristics of NiFe nanoplates. Even though bare NiFe exhibited lower Rct value compared to IrO2, its low conductivity limits its OER activity. In addition to the low Rct value, NiFe/RGO shows a significantly higher capacitance value of 526 mF than those of bare NiFe (220.5 mF) and IrO2 (170.0 mF). The capacitance is proportional to the contact area between the catalysts and electrolyte. Thus, high capacitance value is advantageous in electrocatalysis. The capacitance value of NiFe/RGO was 2.4 times higher than that of NiFe, which is in good agreement with their BET surface areas in Fig. S5. By combining NiFe nanoplates with RGO, contact area of NiFe/RGO is substantially increased, which could contribute to the high OER activity. Fig. 4 shows the oxygen evolution on NiFe/RGO electrode at static overpotential of 0.30 V monitored by a gas-chromatography (GC). Produced hydrogen gas at counter electrode is also recorded. Stable oxygen evolution was observed for 180 min. Notably, its faradaic efficiency for oxygen evolution was ca. 97% at 180 min, indicating measured current density of NiFe/RGO is mostly originated from water oxidation reaction. The molar ratio between produced hydrogen and oxygen is 2:1. Thus, it is concluded that our NiFe/RGO obviously induces the perfect electrochemical water splitting. Apart from the electrochemical water splitting, our NiFe/RGO was employed as a co-catalyst for the photoanode in PEC system for the first time. Here, hematite (a-Fe2O3) was selected and fabricated as a model photoanode due to its excellent stability under alkaline condition, small bandgap of 2.1 eV, and low price [28,29]. The NiFe/ RGO was dispersed in ethanol ultrasonically and three different amounts of the solution (5, 10, and 20 ml) were dropped to the bare hematite photoanodes. Fig. 5 shows water oxidation photocurrent
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Fig. 4. Gas evolution of the NiFe/RGO electrode monitored by a gas chromatography.
RGO layers indicating the effectiveness of our strategy to fabricate OER catalysts. The electrochemical characterizations verified the superior activity and stability of NiFe/RGO compared to bare NiFe and reference IrO2. Especially, its h10 value of 0.245 V represents one of the best performance among the recently reported LDHbased materials as well as other non-precious metal catalysts. Here, RGO layers played an important role to enhance the activity of the catalyst by providing a good electrical pathway and a high surface area to the loaded NiFe nanoplates. Furthermore, NiFe/RGO recorded high faradaic efficiency of 97% for oxygen evolution with produced hydrogen to oxygen molar ratio of 2:1, indicating perfect electrochemical water splitting was induced. In addition, the effective role of NiFe/RGO as a co-catalyst for photoanode in PEC system was demonstrated for the first time. The photocurrent of NiFe/RGO modified hematite photoanode was increased more than two times with negatively shifted onset potential. Thus, the NiFe/ RGO could be a practical electrocatalyst for OER owing to its high activity, stability, and low cost. Acknowledgments This work was supported by Brain Korea Plus Program of Ministry of Education, and Basic Science Research Program (No. 2012017247) and Korean Center for Artificial Photosynthesis (NRF-2011C1AAA0001-2011-0030278) funded by MISIP and Project No. 10050509 funded by MOTIE of Republic of Korea. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.06.098. References
Fig. 5. Water oxidation photocurrents measured under simulated solar light irradiation (1 sun, AM 1.5).
response for the prepared hematite photoanodes under 1 sun and AM 1.5 simulated solar light irradiation. The hematite photoanode modified with 10 ml of NiFe/RGO solution (simply presented as 10 ml NiFe/RGO in Fig. 4) marks a much higher photocurrent of 0.95 mA cm2 at 1.23 V compared to the bare hematite (0.45 mA cm2) and the other modified photoanodes (5 ml NiFe/ RGO for 0.63 and 20 ml NiFe/RGO for 0.46 mA cm2). Notably, the onset potentials of the modified photoanodes are negatively shifted ca. 150 mV than that of bare hematite, revealing the effectiveness of NiFe/RGO as a co-catalyst to enhance the photoelectrochemical activity of hematite photoanode for water oxidation [28,38]. Further optimization to fabricate hematite photoanodes with thin layer of NiFe/RGO and a detailed study to reveal the co-catalytic role of NiFe/RGO are under progress.
4. Conclusions In summary, we have successfully developed NiFe/RGO composite for OER electrocatalyst by a simple solvothermal method in one-pot. The NiFe/RGO exhibited synergistic effect between active NiFe nanoplates and RGO layers possessing high electrical conductivity and large surface area. As revealed by physicochemical characterizations, NiFe nanoplates were uniformly deposited on
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