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Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction
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Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction Dongwook Lim, Hyungseok Kong, Chaewon Lim, Namil Kim, Sang Eun Shim, Sung-Hyeon Baeck* Department of Chemistry and Chemical Engineering, Center for Design and Applications of Molecular Catalysts, Inha University, Incheon, 22212, Republic of Korea
highlights
graphical abstract
NiCo2O4 nanoparticles with oxygen vacancies were synthesized by a hydrothermal method followed by mild H2 reduction. The H2-treated NiCo2O4 exhibited a low overpotential and substantial long-term stability for oxygen reduction reaction. This
outstanding
ORR
perfor-
mance and stability can be attributed
to
the
improved
physicochemical properties by H2 treatment.
article info
abstract
Article history:
Spinel-type nickel cobaltite with numerous oxygen vacancies is successfully synthesized
Received 11 May 2019
by hydrothermal and thermal reduction using hydrogen. The effects of oxygen vacancies
Received in revised form
on the electrochemical activity and stability for the oxygen reduction reaction are inves-
8 July 2019
tigated. The prepared catalyst displays significantly enhanced oxygen reduction reaction
Accepted 11 July 2019
(ORR) catalytic performance under alkaline conditions, which is comparable to that of
Available online xxx
commercial Pt/C. The oxygen-deficient NiCo2O4 exhibits a very high limiting current
Keywords:
reversible hydrogen electrode (RHE), respectively. Additionally, it shows excellent dura-
Oxygen vacancy
bility and resistance to methanol. The enhanced ORR activity and stability of the catalyst
NiCo2O4
can be ascribed to the synergistic effects of the relatively large electrochemical surface
Spinel structure
area, more exposed active sites, and good electrical conductivity derived from abundant
Oxygen reduction reaction
oxygen vacancies.
density of 5.44 mA cm2 with onset and half-wave potentials of 0.93 and 0.78 V versus the
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (S.-H. Baeck). https://doi.org/10.1016/j.ijhydene.2019.07.091 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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Introduction The oxygen reduction reaction (ORR) is one of the most important reactions in electrochemical energy storage and conversion systems, such as fuel cells and metaleair batteries [1e4]. Highly electroactive catalysts are required for the ORR to reduce energy consumption during storage and conversion. Currently, platinum-based materials are used in state-of-theart catalysts, and these show high performance for the ORR [5e9]. However, there are various disadvantages of Pt-based material such as scarcity, high cost, and vulnerability to catalyst poisoning [10,11]. Thus, a lot of studies have been carried out to identify ways to replace conventional noble metal-based catalysts with transition metals such as Ni, Co, Fe, and Cu, which are low cost, abundant, and environmentally friendly [12e17]. In particular, first-row transition mixed metal oxides with spinel structures have been actively studied for energy storage and conversion systems because of their good electrochemical activities and stabilities in alkali solution [18e22]. The basic formula of these materials is AB2O4, where A is a divalent metal ion and B is a trivalent metal ion. Mixed balanced transition metal oxides usually exhibit high electrochemical activity because of their excellent redox properties and abundant active sites [23,24]. Alexey et al. [25] synthesized CuCo2O4 mixed valence spinel oxides by various methods such as pore-forming, sol-gel, spray-pyrolysis, and the sacrificial support method. The prepared oxides showed bi-functional electrocatalytic activity toward the oxygen evolution reaction (OER) and ORR. Kim et al. [26] prepared a hybrid composite system of MnCo2O4 nanowires on reduced graphene oxide as a catalyst for a LieO2 battery, and Wenning et al. [27] fabricated nitrogen and sulfur dual-doped CoFe2O4 nanoparticles to produce a large number of defect active sites. Among the non-noble transition metal oxides, NiCo2O4 is a promising electrocatalytic material for the ORR. Nevertheless, the intrinsic catalytic activities of nickel and cobalt-based oxides are still poor because of low electron transfer ability and conductivity [28]. To address and improve the electrochemical performance, the introduction of defects (e.g., oxygen vacancies) can be an effective way of tuning the electronic properties of the catalysts, often leading to a significantly enhanced reactant adsorption capability and increased electrochemical performance [29e31]. For instance, Jian et al. [32] prepared spinel NiCo2O4 nanosheets with oxygen deficiencies, which resulted in an increase in the reactivity and number of active sites for the OER. Cheng and co-workers [33] successfully prepared MnO2 with oxygen vacancies that enhanced the catalytic activity for ORR. Therefore, the introduction of oxygen vacancies into catalysts is regarded as a promising approach for improving the catalytic performance, increasing the number of exposed active sites, and enhancing the reaction rates. These previous studies make us to consider the correlation between the defect sites induced by oxygen vacancies and catalyst activity of metal oxide toward the ORR. Herein, we report the development of spinel-type NiCo2O4 nanoparticles with numerous structural defects for use as advanced electrode materials for the ORR. The oxygen deficient NiCo2O4 nanoparticles were obtained through thermal
treatment of NiCo2O4 in a hydrogen environment at different temperatures (50e300 C), and the influence of temperature on catalytic activity for ORR was investigated. The oxygen deficient NiCo2O4 nanoparticles thermally treated at 100 C under hydrogen flow provide a current density of 5.44 mA cm2 at 0.5 V versus the reversible hydrogen electrode (RHE). In addition, the catalysts show outstanding longterm durability with higher than 95% retention even after 10000 s, outperforming the conventional Pt/C catalyst (75%). The enhancements in electrochemical activity and stability are attributed to the increased number of active sites resulting from the introduction of abundant oxygen vacancies.
Experimental Materials The 20 wt% Pt/C commercial catalyst was purchased from AlfaeAesar (UK). Nafion (5 wt%), cobalt chloride (CoCl3), and nickel chloride (NiCl2) were purchased from SigmaeAldrich (USA). Isopropyl alcohol (2-propanol, C3H7OH, 99.8%), which was used as a solvent to disperse the electrode ink, and sodium hydroxide (NaOH) were supplied by Samchun Chemical (Republic of Korea).
Preparation of NiCo2O4 The pristine NiCo2O4 nanoparticles were prepared by a hydrothermal method combined with thermal treatment at 450 C. First, 1.5 g of NiCl2 and 3 g of CoCl3 were dissolved in 80 mL deionized (DI) water and mixed to a homogeneous solution by vigorous magnetic stirring for 30 min at 60 C, and 1 M NaOH was then added. The obtained solution was transferred to a Teflon-lined stainless-steel autoclave. Then, the autoclave was sealed and heated to 200 C for 18 h. Subsequently, the products were washed several times with DI water and dried in air at 80 C for 12 h. Finally, the dried powder put into a furnace and heat treatment was carried out at 450 C in air for 3 h to form NiCo2O4. The synthesized NiCo2O4 samples are denoted as NiCo2O4.
Preparation of oxygen-deficient NiCo2O4 After cooling to room temperature, the collected powders were heated to 50, 100, 200, and 300 C at a rate of 10 C min1 in a horizontal quartz tube furnace under a flow of argon. Then, the samples were further heated for 5 h at ambient pressure, under flowing a mixture of 90% Ar and 10% H2. The hydrogen-treated NiCo2O4 samples are denoted as NiCo2O4_X, where X represents hydrogen treatment temperature.
Physical characterizations of oxygen-deficient NiCo2O4 nanoparticles The microstructure and morphology of the synthesized NiCo2O4 powders were examined by transmission electron microscopy (TEM, JEOL, TEM-2100F) and field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4300 SE). The crystallinity was characterized by X-ray diffraction (XRD,
Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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Rigaku, MAN 2200 V). X-ray photoelectron spectroscopy (XPS, Thermo Scientific, K-Alpha) was used to analyze the composition and oxidation states.
Electrochemical tests of oxygen-deficient NiCo2O4 nanoparticles For the investigation of electrochemical properties, 2 mg of the synthesized catalyst were dispersed in a solution of 100 mL isopropanol and 384 mL of deionized (DI) water, and 20 mL of 5 wt% Nafion solution was added. The resulting mixture was dispersed by ultrasonication for 30 min to obtain a homogeneous suspension. Four microliters of the catalyst ink were dropped onto the surface of a glassy carbon electrode (GCE, 3 mm in diameter), and the electrode was dried at room temperature. The working electrode was immersed in a glass cell containing 0.1 M KOH aqueous electrolyte. A platinum plate and Ag/AgCl/3 M KCl were used as a counter and a reference electrode, respectively. Before each experiment, the electrolyte solution was purged with O2 for 30 min. For the ORR measurements, the potential range was 1.3 to 0 V versus the reversible hydrogen electrode (RHE) at a sweep rate of 5 mV s1 with a rotation speed of 1600 rpm. All polarization curves were corrected with iR-compensation (85%), and current densities were normalized to the geometrical area of the GCE. All potentials reported in this work were converted from Ag/AgCl to the RHE scale using E (RHE) ¼ E (Ag/AgCl) þ 0.0592 pH þ E (Ag/AgCl) in 0.1 M KOH. The durability and methanol crossover with respect to the ORR activity were determined by chronoamperometry (CA) at 0.6 V vs. RHE. All electrochemical experiments were carried out at room temperature.
Results and discussion The oxygen-deficient NiCo2O4 nanoparticles were synthesized via a three-step procedure. The mixed metal hydroxide nanoparticles were first prepared by a hydrothermal method. To obtain the NiCo2O4 nanoparticles, the as-prepared sample was annealed at 450 C for 3 h in an air atmosphere. Finally, heat treatment at 50, 100, 200, and 300 C for 5 h in a mixture of hydrogen and argon with a volume ratio of 1:9 resulted in the formation of oxygen-deficient NiCo2O4 nanoparticles. The phase transition from b-Ni(OH)2 and b-Co(OH)2 to spinel-type NiCo2O4 was confirmed by XRD, as shown in Fig. 1. Before heat treatment, all the peaks (black line) are almost similar to the b-Ni(OH)2 phase (JCPDS card no. 03-0177) and b-Co(OH)2 phase (JCPDS card no. 30-0443). Considering the high phase similarity of b-Ni(OH)2 and b-Co(OH)2, there is co-existence of two metal hydroxides [34]. After the heat treatment at 450 C in air, the nickel and cobalt hydroxide was converted to NiCo2O4 with a spinel structure (blue line), and the diffraction pattern is consistent with JCPDS card no. 20-0781. The XRD results confirm that the mixture of b-Ni(OH)2 and b-Co(OH)2 transforms into spinel-type NiCo2O4 through dehydration by thermal treatment at 450 C for 3 h in air. To examine the change in crystallinity and structure of the spinel-type NiCo2O4 catalysts with respect to further heat treatment at 50, 100, 200, and 300 C under flowing a mixture
Fig. 1 e XRD patterns of a mixture of nickel and cobalt hydroxide (Black Line), and spinel NiCo2O4 (Red Line). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
of 90% Ar/10% H2, their XRD patterns were compared in Fig. 2. All the peaks correspond to the characteristic reflections of spinel-type NiCo2O4 (JCPDS card no. 20-0781) except the sample treated at 300 C, in which spinel-type oxide was completely reduced to NiCo alloy phase. This diffraction pattern of NiCo2O4_300 is very close to those of either facecentered cubic (fcc) Ni (JCPDS card no. 04-0850) or fcc Co (JCPDS card no. 15-0806) with a slight shift of the peak position due to a formation of alloy. Similar diffraction peaks have been also confirmed by previously reported data of NiCo alloys [35,36]. Based on the previous results, these diffraction patterns can be matched with the (111), (200), and (220) planes of a fcc NiCo alloy [37]. In the sample treated at 200 C (NiCo2O4_200), small amount of NiO was detected with cubic NiCo2O4 from the XRD analysis. Co-existence of NiO and NiCo2O4 may be due to decomposition of spinel NiCo2O4 into NiO by hydrogen at elevated temperature [38]. In addition, Co element from the decomposition of NiCo2O4 may be Co3O4. However, the diffraction patterns of Co3O4 were overlapped with NiCo2O4 due to the high phase similarity of NiCo2O4 and Co3O4, as shown in Fig. S1. Interestingly, in the XRD pattern of NiCo2O4_100, a slight peak shift to lower angles relative to that of pristine NiCo2O4 was detected between 34 and 39 (Fig. 2b). The peak shift can be attributed to the formation of structural defects such as oxygen vacancies. The hydrogen treatment at elevated temperature results in the breakage of the metalemetal or metaleoxygen bonds. The increased ionic radius and relaxed metaleoxygen bonds derived from the removal of oxygen result in lattice expansion, leading to slight XRD peak shift to lower angles [33,39e41]. The surface morphology and microstructure of the synthesized NiCo2O4 samples were investigated by SEM and TEM. Fig. 3aef shows the morphologies of pristine NiCo2O4 and NiCo2O4_100, and no obvious differences in the surface morphology were observed after thermal treatment under hydrogen flowing, indicating that the heat treatment does not
Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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Fig. 2 e (a) XRD patterns of NiCo2O4, NiCo2O4_50, NiCo2O4_100, NiCo2O4_200, and NiCo2O4_300 and (b) a magnification of the same XRD patterns from 34 to 39 of 2q.
affect particle shape and size of the prepared samples. A characteristic lattice fringe with an interplanar spacing of 0.244 nm (Fig. 3c) was identified in the high-resolution (HR)TEM image, which is consistent with the (311) plane of NiCo2O4, which was also confirmed by the XRD patterns in Fig. 1. As shown in Fig. 3f, some lattice distortions were observed in the HR-TEM image of hydrogen-treated NiCo2O4 compared to that of pristine NiCo2O4, as indicated by red dotted circles, demonstrating the existence of structural defects [42]. To investigate the porous structure and specific surface area of the synthesized catalysts, nitrogen adsorptionedesorption isotherms were obtained, as shown in Fig. 4. All the prepared samples display type-IV isotherm curves with hysteresis in the relatively high-pressure range, suggesting their mesoporous nature. The BrunauereEmmetteTeller (BET) specific surface
area for NiCo2O4_100 was calculated to be 40 m2 g1, which is larger than that of NiCo2O4 (37 m2 g1). The unique architecture of NiCo2O4_100 and the large BET specific surface area significantly may increase the number of exposed active sites. Remarkably, the dominant pore size distribution of the NiCo2O4 was broadened and shifted to lager pore size after thermal treatment in a hydrogen atmosphere (Fig. 4b). The pore size distribution of NiCo2O4_100 covers a wide range from 24 to 132 nm. The broad pore distribution can provide an effective three-phase reaction zone and facilitate efficient mass and electron transport during the catalytic process, consequently expecting the enhanced ORR performance. The compositions and oxidation states of the obtained samples were investigated by XPS analysis, as shown in Fig. 5 and Fig. S1. All the peaks were aligned with reference to the C 1s peak at 285 eV. Fig. 5 and Fig. S1 shows the Ni 2p, Co 2p, and
Fig. 3 e SEM images of (a, b) NiCo2O4 and (d, e) NiCo2O4_100. HR-TEM images of (c) NiCo2O4 and (f) NiCo2O4_100. Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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Fig. 4 e Nitrogen adsorption and desorption isotherms of (a) NiCo2O4 and (b) NiCo2O4_100 (inset: the corresponding pore size distribution curve).
O 1s core-level XPS spectra. The Ni 2p3/2 (Fig. S1a) region for the NiCo2O4_100 displays two main peaks at 855.7 and 861.2 eV corresponding to the oxidized Ni2þ/Ni3þ in NiCo2O4 and satellite peak, respectively [43]. Fig. S1b shows the Co 2p core level XPS spectra of the prepared samples. The Co 2p3/2 and 2p1/2 peaks of all samples are positioned at binding energies of 779 and 795 eV, which correspond to the typical binding energies for cobalt in NiCo2O4 [44]. Especially, the atomic ratio of Co2þ/Co3þ of the prepared samples is increased with respect to an increase of heat treatment temperature, suggesting gradual reduction of Co species (Co3þ) in NiCo2O4 (Fig. S3 and Table S1). Also, the ratio of Ni2þ/Ni3þ in Ni 2p spectra of the catalysts shows same trend with an increase in treatment temperature from 50 to 200 C, as shown in Fig. S4 and Table S1. To address the formation of oxygen vacancies further,
the O 1s core level spectra were deconvoluted into four regions (Fig. 5aed). The four deconvoluted peaks are denoted as O1 (529.33 eV), O2 (530.6 eV), O3 (531.6 eV), and O4 (533 eV), which represent the metaleoxygen bonds, oxygen atoms in the hydroxyl groups, defect sites with low oxygen coordination, and hydroxy species of physically adsorbed water molecules, respectively [45]. Of the samples, the calculated area of the peak at 531.6 eV (O3) is highest for the hydrogen-treated NiCo2O4_100 (Table S1). The highest portion of O3 can be attributed to the formation of oxygen vacancies in spinel-type NiCo2O4, which is consistent with the XRD results in Fig. 2. The oxygen vacancies are produced due to the heat treatment under oxygen-deficient environment, resulting in the gradual collapse of spinel structure with increase of temperature and development of porous structure, as confirmed from N2
Fig. 5 e High-resolution O 1s XPS spectra of (a) NiCo2O4, (b) NiCo2O4_50, (c) NiCo2O4_100, and (d) NiCo2O4_200. Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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isotherm. The increase of oxygen vacancies in NiCo2O4 results in phase transformation as revealed in Fig. 2. Finally, the structure of NiCo2O4 is collapsed completely at 300 C and transformation to alloy is observed. The abundant lowcoordinated oxygen and defect sites in NiCo2O4 would promote the adsorption and diffusion of OH and O2, thus expecting enhanced ORR catalytic activity. To assess the effects of the oxygen vacancies on the electrocatalytic activity and stability of the catalyst, the electrochemical ORR performance of the prepared NiCo2O4 nanoparticles were investigated by CV, linear scan voltammetry (LSV), and CA measurements using a standard threeelectrode system in a 0.1 M KOH aqueous solution. The ORR catalytic performance of the samples was first evaluated by CV tests in the voltage range of 0.2e1.2 V vs. RHE (Fig. 6a). In a N2-saturated electrolyte, the CV curves have a featureless rectangular shape for the cathodic current, although welldefined cathodic peaks centered at 0.66 V vs. RHE were detected in the O2-saturated electrolyte, indicating the ORR reactivity of the partial reduced NiCo2O4 nanoparticles (NiCo2O4_100). To investigate the ORR activities of these catalysts further, LSV polarization curves were measured using a rotating disk electrode (RDE), as shown in Fig. 6b. The ORR performance of NiCo2O4 treated at different temperatures in hydrogen and argon atmospheres was measured (Fig. 6b). Initially, the ORR performance was enhanced with the increase in temperature up to 100 C, as shown in Fig. 6b. However, the higher temperature can trigger a collapse of the NiCo2O4 structure and phase transition as mentioned before, resulting in lower activity for the ORR. Finally, the sample thermally treated at 300 C was fully reduced to binary NiCo alloy phase (Fig. 2). From the results, the optimal temperature for heat treatment was determined to be 100 C. The NiCo2O4_100 catalyst exhibited an onset potential of 0.93 V and a half-wave potential of 0.78 V, which are comparable to those of the commercial Pt/C catalyst (an onset potential of 0.96 V and a half-wave potential of 0.83 V). The significantly enhanced catalytic performance of NiCo2O4_100 was also verified from the mass-transport corrected Tafel slopes. The slopes of the Tafel plot in the low-overpotential region were calculated to evaluate the kinetic mechanism of the catalysts. As shown in Fig. S5, the NiCo2O4_100 exhibits the smallest Tefel slope (71.3 mV dec1) among the prepared samples, indicating the fastest ORR kinetics. To investigate the kinetic parameters for the electron transfer numbers (n), RDE measurements from 400 to 2025 rpm were conducted. The electron transfer numbers (n) for the ORR were calculated based on the Koutecky-Levich (K-L) equation: 1 1 1 ¼ þ J JK JD
(1)
JD ¼ 0:62nFD2=3 v1=6 u1=2 CO2
(2)
where J is the measured current density, JK is the kinetic current density, and JD is the diffusion limiting current density, n is the number of transferred electrons, F is the Faraday constant (96485 C mol1), D is the diffusion coefficient of O2 in 0.1 M KOH (1.93 105 cm2 s1), v is the kinematic viscosity of the electrolyte (1.09 102 cm2 s1), w is the angular frequency of rotation (u ¼ 2pf =60, f is the rotating speed of RDE),
and CO2 is the oxygen concentration in 0.1 M KOH (1.26 106 mol cm3). The n values of NiCo2O4_100, which can be estimated from the slope of the linear relationship between the inverse measured current and the inverse square root of the rotation rate, were calculated to be 3.85e3.98 at potentials from 0.2 to 0.7 V vs. RHE (Fig. 6c), which outperforms that of NiCo2O4 (3.5, Fig. S2, Supporting Information) and is comparable to that of state-of-the-art Pt/ C catalyst (n z 4.0), indicating that the ORR process is dominated by a favorable four-electron pathway with almost no formation of H2O2. In order to further explain the ORR performance with oxygen vacancy, the relationship between O3 ratio in O 1s characterized by XPS and electrocatalytic activity at 0.5 V vs. RHE is plotted with hydrogen-treated temperature in Fig. 6d. The results in Fig. 6d also indicate the ORR catalytic activity of NiCo2O4_100 outperforms pristine NiCo2O4. Furthermore, a clear correlation is confirmed between the O3 ratio and the ORR catalytic performance of hydrogen-treated NiCo2O4. When pristine NiCo2O4 was thermally treated by hydrogen up to 100 C, the O3 ratio of NiCo2O4 is augmented with an increase in the limiting current density. Treated by hydrogen at a temperature higher than 100 C, the O3 ratio of NiCo2O4 is decreased with a gradual decline of the current density. With moderate hydrogen treatment temperature (up to 100 C), the increasing concentration of oxygen vacancies and defects would lead to increasing electrochemical active sites, further boosting the ORR catalytic performance of NiCo2O4. However, with increased treatment temperature above 100 C, the amounts of oxygen vacancies and defects diminish due to the collapse of the structure (Fig. S2) and the formation of binary metal alloy, leading to poor ORR electrocatalytic activity. This observed correlation between the O3 ratio and the current density for ORR demonstrates that proper oxygen vacancies can enhance the electrocatalytic ORR performance of hydrogen-treated NiCo2O4. To further understand the effects of oxygen vacancies on the electrocatalytic activity, theoretical calculations based on density functional theory (DFT) have been performed in several previous studies [32,46,47]. The theoretical calculations indicate that oxygen vacancies can decrease work function of the NiCo2O4, thus enabling electrons to be more easily moved from the catalyst surface to reactants, which is demonstrated by enhanced electrochemical performance toward the ORR [47]. Moreover, the oxygen vacancies offer extra level in conduction or valence bands, leading to a narrow band gap and enhanced conductivity of NiCo2O4 [32,46]. Consequently, the experimental and previously reported computational results reveal that generation of structural defect such as oxygen vacancies can dramatically improve electrochemical and catalytic performance for the ORR. The electrochemically active surface areas (ECSA) of the prepared NiCo2O4 samples were estimated via the measurement of the electrochemical double-layer capacitance (Cdl) to confirm the influence of hydrogen treatment (Fig. S3). Doublelayer capacitance was calculated from cyclic voltammetry (CV) measurement in the region from 0.9 to 1 V vs. RHE at different scan rates (10e100 mV s1), and no apparent Faradaic processes occurred. The Cdl value of NiCo2O4_100 was calculated to be 8.73 mF cm2, which is much higher than that
Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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Fig. 6 e Electrochemical characterizations of the prepared samples. (a) CV curves of NiCo2O4_100 in O2-(red line) and N2saturated (black line) 0.1 M KOH electrolyte. (b) LSV curves of the prepared samples in O2-saturated 0.1 M KOH at 1600 rpm and a sweep rate of 5 mV s¡1. (c) Rotating disk voltammograms of NiCo2O4_100 at rotation speeds from 400 to 2025 rpm. The inset in (c) shows the corresponding K-L plots at different potentials. (d) O3 ratio in O 1s and current density at 0.5 V vs. RHE as a function of hydrogen treatment temperature. (e) Methanol crossover tests performed by adding methanol to the electrolyte at 200 s at an applied voltage of 0.6 V vs. RHE. (f) Currentetime chronoamperometric responses of NiCo2O4_100 and commercial Pt/C on a glassy carbon electrode in O2-saturated 0.1 M KOH electrolyte at 0.6 V vs. RHE. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
of pristine NiCo2O4 (6.43 mF cm2). The relatively higher double layer capacitance suggests the exposure of numerous electrocatalytic active surface area, as well as favorable mass transfer of the reactants (O2), which is consistent with BET surface analysis in Fig. 4. Also, resistance to methanol crossover and long-term stability are crucial for practical applications, such as fuel cells. Resistance to methanol crossover of NiCo2O4_100 with respect to ORR was investigated by chronoamperometry (CA) at 0.6 V vs. RHE, as compared with commercial Pt/C in Fig. 6e. The current density of the commercial Pt/C catalyst was drastically decreased on the introduction of 10 mL methanol (3 M) into the O2-saturated 0.1 M KOH electrolyte, whereas there was no obvious change in that of NiCo2O4_100, indicating its
high resistance to methanol poisoning. The durability of the NiCo2O4_100 and commercial Pt/C catalysts was evaluated through CA measurements at 0.6 V in an O2-saturated 0.1 M KOH electrolyte for 10000 s, as shown in Fig. 6f. After the durability test for 10000 s, about 95% of the initial current density was preserved compared to 75% for Pt/C, demonstrating the outstanding stability of NiCo2O4_100 under alkaline conditions. The superior performance and excellent durability of the NiCo2O4_100 catalyst prepared by hydrothermal and hydrogen treatment can be explained by several advantages. First, the introduction of suitable oxygen vacancies into the catalysts adjusts the electronic properties and provides efficient electrocatalytic active sites, facilitating the adsorption of O2 and
Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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leading to a significantly enhanced catalytic performance and stability for the ORR. In addition, a broader pore size distribution of NiCo2O4_100 with increased BET surface area and electrochemically active surface area contributed to efficient mass transport and offered more accessible electrochemically active sites. Moreover, the spinel-type NiCo2O4_100 was synthesized by thermal treatment at a low temperature in an oxygen-deficient atmosphere boosted the electrocatalytic activity for the ORR with no obvious transformation of the phase and morphology. It can be concluded that the relatively large surface and more accessible electrocatalytic active sites of the optimized catalyst increased the electrocatalytic activity and stability for the ORR.
Conclusions In summary, we have successfully synthesized oxygendeficient NiCo2O4 materials by a simple hydrothermal method with subsequent thermal treatment in a hydrogen atmosphere. The limiting current density of the optimized NiCo2O4_100 is close to that of commercial 20% Pt/C, indicating its outstanding ORR performance. The four-electron pathway for the ORR was confirmed using the K-L equation and RDE experiments, further indicating efficient electrocatalysis corresponding to the direct reduction of O2 to OH. Additionally, NiCo2O4_100 shows much higher stability and tolerance to methanol than a state-of-the-art Pt-based catalyst. The significantly excellent ORR performance is mainly attributed to the incorporation of oxygen vacancies in the catalyst. It can be concluded that the creation of oxygen vacancies in spinel-type oxide electrocatalysts plays an important role in increasing the number of ORR active sites, exposing abundant electrochemical accessible active sites to reactants, and enhancing the electrochemical properties.
Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No. 2015R1A4A1042434) and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 20194030202340).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.07.091.
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Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction Dongwook Lim, Hyungseok Kong, Chaewon Lim, Namil Kim, Sang Eun Shim, Sung-Hyeon Baeck* Department of Chemistry and Chemical Engineering, Center for Design and Applications of Molecular Catalysts, Inha University, Incheon, 22212, Republic of Korea
highlights
graphical abstract
NiCo2O4 nanoparticles with oxygen vacancies were synthesized by a hydrothermal method followed by mild H2 reduction. The H2-treated NiCo2O4 exhibited a low overpotential and substantial long-term stability for oxygen reduction reaction. This
outstanding
ORR
perfor-
mance and stability can be attributed
to
the
improved
physicochemical properties by H2 treatment.
article info
abstract
Article history:
Spinel-type nickel cobaltite with numerous oxygen vacancies is successfully synthesized
Received 11 May 2019
by hydrothermal and thermal reduction using hydrogen. The effects of oxygen vacancies
Received in revised form
on the electrochemical activity and stability for the oxygen reduction reaction are inves-
8 July 2019
tigated. The prepared catalyst displays significantly enhanced oxygen reduction reaction
Accepted 11 July 2019
(ORR) catalytic performance under alkaline conditions, which is comparable to that of
Available online xxx
commercial Pt/C. The oxygen-deficient NiCo2O4 exhibits a very high limiting current
Keywords:
reversible hydrogen electrode (RHE), respectively. Additionally, it shows excellent dura-
Oxygen vacancy
bility and resistance to methanol. The enhanced ORR activity and stability of the catalyst
NiCo2O4
can be ascribed to the synergistic effects of the relatively large electrochemical surface
Spinel structure
area, more exposed active sites, and good electrical conductivity derived from abundant
Oxygen reduction reaction
oxygen vacancies.
density of 5.44 mA cm2 with onset and half-wave potentials of 0.93 and 0.78 V versus the
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (S.-H. Baeck). https://doi.org/10.1016/j.ijhydene.2019.07.091 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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Introduction The oxygen reduction reaction (ORR) is one of the most important reactions in electrochemical energy storage and conversion systems, such as fuel cells and metaleair batteries [1e4]. Highly electroactive catalysts are required for the ORR to reduce energy consumption during storage and conversion. Currently, platinum-based materials are used in state-of-theart catalysts, and these show high performance for the ORR [5e9]. However, there are various disadvantages of Pt-based material such as scarcity, high cost, and vulnerability to catalyst poisoning [10,11]. Thus, a lot of studies have been carried out to identify ways to replace conventional noble metal-based catalysts with transition metals such as Ni, Co, Fe, and Cu, which are low cost, abundant, and environmentally friendly [12e17]. In particular, first-row transition mixed metal oxides with spinel structures have been actively studied for energy storage and conversion systems because of their good electrochemical activities and stabilities in alkali solution [18e22]. The basic formula of these materials is AB2O4, where A is a divalent metal ion and B is a trivalent metal ion. Mixed balanced transition metal oxides usually exhibit high electrochemical activity because of their excellent redox properties and abundant active sites [23,24]. Alexey et al. [25] synthesized CuCo2O4 mixed valence spinel oxides by various methods such as pore-forming, sol-gel, spray-pyrolysis, and the sacrificial support method. The prepared oxides showed bi-functional electrocatalytic activity toward the oxygen evolution reaction (OER) and ORR. Kim et al. [26] prepared a hybrid composite system of MnCo2O4 nanowires on reduced graphene oxide as a catalyst for a LieO2 battery, and Wenning et al. [27] fabricated nitrogen and sulfur dual-doped CoFe2O4 nanoparticles to produce a large number of defect active sites. Among the non-noble transition metal oxides, NiCo2O4 is a promising electrocatalytic material for the ORR. Nevertheless, the intrinsic catalytic activities of nickel and cobalt-based oxides are still poor because of low electron transfer ability and conductivity [28]. To address and improve the electrochemical performance, the introduction of defects (e.g., oxygen vacancies) can be an effective way of tuning the electronic properties of the catalysts, often leading to a significantly enhanced reactant adsorption capability and increased electrochemical performance [29e31]. For instance, Jian et al. [32] prepared spinel NiCo2O4 nanosheets with oxygen deficiencies, which resulted in an increase in the reactivity and number of active sites for the OER. Cheng and co-workers [33] successfully prepared MnO2 with oxygen vacancies that enhanced the catalytic activity for ORR. Therefore, the introduction of oxygen vacancies into catalysts is regarded as a promising approach for improving the catalytic performance, increasing the number of exposed active sites, and enhancing the reaction rates. These previous studies make us to consider the correlation between the defect sites induced by oxygen vacancies and catalyst activity of metal oxide toward the ORR. Herein, we report the development of spinel-type NiCo2O4 nanoparticles with numerous structural defects for use as advanced electrode materials for the ORR. The oxygen deficient NiCo2O4 nanoparticles were obtained through thermal
treatment of NiCo2O4 in a hydrogen environment at different temperatures (50e300 C), and the influence of temperature on catalytic activity for ORR was investigated. The oxygen deficient NiCo2O4 nanoparticles thermally treated at 100 C under hydrogen flow provide a current density of 5.44 mA cm2 at 0.5 V versus the reversible hydrogen electrode (RHE). In addition, the catalysts show outstanding longterm durability with higher than 95% retention even after 10000 s, outperforming the conventional Pt/C catalyst (75%). The enhancements in electrochemical activity and stability are attributed to the increased number of active sites resulting from the introduction of abundant oxygen vacancies.
Experimental Materials The 20 wt% Pt/C commercial catalyst was purchased from AlfaeAesar (UK). Nafion (5 wt%), cobalt chloride (CoCl3), and nickel chloride (NiCl2) were purchased from SigmaeAldrich (USA). Isopropyl alcohol (2-propanol, C3H7OH, 99.8%), which was used as a solvent to disperse the electrode ink, and sodium hydroxide (NaOH) were supplied by Samchun Chemical (Republic of Korea).
Preparation of NiCo2O4 The pristine NiCo2O4 nanoparticles were prepared by a hydrothermal method combined with thermal treatment at 450 C. First, 1.5 g of NiCl2 and 3 g of CoCl3 were dissolved in 80 mL deionized (DI) water and mixed to a homogeneous solution by vigorous magnetic stirring for 30 min at 60 C, and 1 M NaOH was then added. The obtained solution was transferred to a Teflon-lined stainless-steel autoclave. Then, the autoclave was sealed and heated to 200 C for 18 h. Subsequently, the products were washed several times with DI water and dried in air at 80 C for 12 h. Finally, the dried powder put into a furnace and heat treatment was carried out at 450 C in air for 3 h to form NiCo2O4. The synthesized NiCo2O4 samples are denoted as NiCo2O4.
Preparation of oxygen-deficient NiCo2O4 After cooling to room temperature, the collected powders were heated to 50, 100, 200, and 300 C at a rate of 10 C min1 in a horizontal quartz tube furnace under a flow of argon. Then, the samples were further heated for 5 h at ambient pressure, under flowing a mixture of 90% Ar and 10% H2. The hydrogen-treated NiCo2O4 samples are denoted as NiCo2O4_X, where X represents hydrogen treatment temperature.
Physical characterizations of oxygen-deficient NiCo2O4 nanoparticles The microstructure and morphology of the synthesized NiCo2O4 powders were examined by transmission electron microscopy (TEM, JEOL, TEM-2100F) and field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4300 SE). The crystallinity was characterized by X-ray diffraction (XRD,
Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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Rigaku, MAN 2200 V). X-ray photoelectron spectroscopy (XPS, Thermo Scientific, K-Alpha) was used to analyze the composition and oxidation states.
Electrochemical tests of oxygen-deficient NiCo2O4 nanoparticles For the investigation of electrochemical properties, 2 mg of the synthesized catalyst were dispersed in a solution of 100 mL isopropanol and 384 mL of deionized (DI) water, and 20 mL of 5 wt% Nafion solution was added. The resulting mixture was dispersed by ultrasonication for 30 min to obtain a homogeneous suspension. Four microliters of the catalyst ink were dropped onto the surface of a glassy carbon electrode (GCE, 3 mm in diameter), and the electrode was dried at room temperature. The working electrode was immersed in a glass cell containing 0.1 M KOH aqueous electrolyte. A platinum plate and Ag/AgCl/3 M KCl were used as a counter and a reference electrode, respectively. Before each experiment, the electrolyte solution was purged with O2 for 30 min. For the ORR measurements, the potential range was 1.3 to 0 V versus the reversible hydrogen electrode (RHE) at a sweep rate of 5 mV s1 with a rotation speed of 1600 rpm. All polarization curves were corrected with iR-compensation (85%), and current densities were normalized to the geometrical area of the GCE. All potentials reported in this work were converted from Ag/AgCl to the RHE scale using E (RHE) ¼ E (Ag/AgCl) þ 0.0592 pH þ E (Ag/AgCl) in 0.1 M KOH. The durability and methanol crossover with respect to the ORR activity were determined by chronoamperometry (CA) at 0.6 V vs. RHE. All electrochemical experiments were carried out at room temperature.
Results and discussion The oxygen-deficient NiCo2O4 nanoparticles were synthesized via a three-step procedure. The mixed metal hydroxide nanoparticles were first prepared by a hydrothermal method. To obtain the NiCo2O4 nanoparticles, the as-prepared sample was annealed at 450 C for 3 h in an air atmosphere. Finally, heat treatment at 50, 100, 200, and 300 C for 5 h in a mixture of hydrogen and argon with a volume ratio of 1:9 resulted in the formation of oxygen-deficient NiCo2O4 nanoparticles. The phase transition from b-Ni(OH)2 and b-Co(OH)2 to spinel-type NiCo2O4 was confirmed by XRD, as shown in Fig. 1. Before heat treatment, all the peaks (black line) are almost similar to the b-Ni(OH)2 phase (JCPDS card no. 03-0177) and b-Co(OH)2 phase (JCPDS card no. 30-0443). Considering the high phase similarity of b-Ni(OH)2 and b-Co(OH)2, there is co-existence of two metal hydroxides [34]. After the heat treatment at 450 C in air, the nickel and cobalt hydroxide was converted to NiCo2O4 with a spinel structure (blue line), and the diffraction pattern is consistent with JCPDS card no. 20-0781. The XRD results confirm that the mixture of b-Ni(OH)2 and b-Co(OH)2 transforms into spinel-type NiCo2O4 through dehydration by thermal treatment at 450 C for 3 h in air. To examine the change in crystallinity and structure of the spinel-type NiCo2O4 catalysts with respect to further heat treatment at 50, 100, 200, and 300 C under flowing a mixture
Fig. 1 e XRD patterns of a mixture of nickel and cobalt hydroxide (Black Line), and spinel NiCo2O4 (Red Line). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
of 90% Ar/10% H2, their XRD patterns were compared in Fig. 2. All the peaks correspond to the characteristic reflections of spinel-type NiCo2O4 (JCPDS card no. 20-0781) except the sample treated at 300 C, in which spinel-type oxide was completely reduced to NiCo alloy phase. This diffraction pattern of NiCo2O4_300 is very close to those of either facecentered cubic (fcc) Ni (JCPDS card no. 04-0850) or fcc Co (JCPDS card no. 15-0806) with a slight shift of the peak position due to a formation of alloy. Similar diffraction peaks have been also confirmed by previously reported data of NiCo alloys [35,36]. Based on the previous results, these diffraction patterns can be matched with the (111), (200), and (220) planes of a fcc NiCo alloy [37]. In the sample treated at 200 C (NiCo2O4_200), small amount of NiO was detected with cubic NiCo2O4 from the XRD analysis. Co-existence of NiO and NiCo2O4 may be due to decomposition of spinel NiCo2O4 into NiO by hydrogen at elevated temperature [38]. In addition, Co element from the decomposition of NiCo2O4 may be Co3O4. However, the diffraction patterns of Co3O4 were overlapped with NiCo2O4 due to the high phase similarity of NiCo2O4 and Co3O4, as shown in Fig. S1. Interestingly, in the XRD pattern of NiCo2O4_100, a slight peak shift to lower angles relative to that of pristine NiCo2O4 was detected between 34 and 39 (Fig. 2b). The peak shift can be attributed to the formation of structural defects such as oxygen vacancies. The hydrogen treatment at elevated temperature results in the breakage of the metalemetal or metaleoxygen bonds. The increased ionic radius and relaxed metaleoxygen bonds derived from the removal of oxygen result in lattice expansion, leading to slight XRD peak shift to lower angles [33,39e41]. The surface morphology and microstructure of the synthesized NiCo2O4 samples were investigated by SEM and TEM. Fig. 3aef shows the morphologies of pristine NiCo2O4 and NiCo2O4_100, and no obvious differences in the surface morphology were observed after thermal treatment under hydrogen flowing, indicating that the heat treatment does not
Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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Fig. 2 e (a) XRD patterns of NiCo2O4, NiCo2O4_50, NiCo2O4_100, NiCo2O4_200, and NiCo2O4_300 and (b) a magnification of the same XRD patterns from 34 to 39 of 2q.
affect particle shape and size of the prepared samples. A characteristic lattice fringe with an interplanar spacing of 0.244 nm (Fig. 3c) was identified in the high-resolution (HR)TEM image, which is consistent with the (311) plane of NiCo2O4, which was also confirmed by the XRD patterns in Fig. 1. As shown in Fig. 3f, some lattice distortions were observed in the HR-TEM image of hydrogen-treated NiCo2O4 compared to that of pristine NiCo2O4, as indicated by red dotted circles, demonstrating the existence of structural defects [42]. To investigate the porous structure and specific surface area of the synthesized catalysts, nitrogen adsorptionedesorption isotherms were obtained, as shown in Fig. 4. All the prepared samples display type-IV isotherm curves with hysteresis in the relatively high-pressure range, suggesting their mesoporous nature. The BrunauereEmmetteTeller (BET) specific surface
area for NiCo2O4_100 was calculated to be 40 m2 g1, which is larger than that of NiCo2O4 (37 m2 g1). The unique architecture of NiCo2O4_100 and the large BET specific surface area significantly may increase the number of exposed active sites. Remarkably, the dominant pore size distribution of the NiCo2O4 was broadened and shifted to lager pore size after thermal treatment in a hydrogen atmosphere (Fig. 4b). The pore size distribution of NiCo2O4_100 covers a wide range from 24 to 132 nm. The broad pore distribution can provide an effective three-phase reaction zone and facilitate efficient mass and electron transport during the catalytic process, consequently expecting the enhanced ORR performance. The compositions and oxidation states of the obtained samples were investigated by XPS analysis, as shown in Fig. 5 and Fig. S1. All the peaks were aligned with reference to the C 1s peak at 285 eV. Fig. 5 and Fig. S1 shows the Ni 2p, Co 2p, and
Fig. 3 e SEM images of (a, b) NiCo2O4 and (d, e) NiCo2O4_100. HR-TEM images of (c) NiCo2O4 and (f) NiCo2O4_100. Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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Fig. 4 e Nitrogen adsorption and desorption isotherms of (a) NiCo2O4 and (b) NiCo2O4_100 (inset: the corresponding pore size distribution curve).
O 1s core-level XPS spectra. The Ni 2p3/2 (Fig. S1a) region for the NiCo2O4_100 displays two main peaks at 855.7 and 861.2 eV corresponding to the oxidized Ni2þ/Ni3þ in NiCo2O4 and satellite peak, respectively [43]. Fig. S1b shows the Co 2p core level XPS spectra of the prepared samples. The Co 2p3/2 and 2p1/2 peaks of all samples are positioned at binding energies of 779 and 795 eV, which correspond to the typical binding energies for cobalt in NiCo2O4 [44]. Especially, the atomic ratio of Co2þ/Co3þ of the prepared samples is increased with respect to an increase of heat treatment temperature, suggesting gradual reduction of Co species (Co3þ) in NiCo2O4 (Fig. S3 and Table S1). Also, the ratio of Ni2þ/Ni3þ in Ni 2p spectra of the catalysts shows same trend with an increase in treatment temperature from 50 to 200 C, as shown in Fig. S4 and Table S1. To address the formation of oxygen vacancies further,
the O 1s core level spectra were deconvoluted into four regions (Fig. 5aed). The four deconvoluted peaks are denoted as O1 (529.33 eV), O2 (530.6 eV), O3 (531.6 eV), and O4 (533 eV), which represent the metaleoxygen bonds, oxygen atoms in the hydroxyl groups, defect sites with low oxygen coordination, and hydroxy species of physically adsorbed water molecules, respectively [45]. Of the samples, the calculated area of the peak at 531.6 eV (O3) is highest for the hydrogen-treated NiCo2O4_100 (Table S1). The highest portion of O3 can be attributed to the formation of oxygen vacancies in spinel-type NiCo2O4, which is consistent with the XRD results in Fig. 2. The oxygen vacancies are produced due to the heat treatment under oxygen-deficient environment, resulting in the gradual collapse of spinel structure with increase of temperature and development of porous structure, as confirmed from N2
Fig. 5 e High-resolution O 1s XPS spectra of (a) NiCo2O4, (b) NiCo2O4_50, (c) NiCo2O4_100, and (d) NiCo2O4_200. Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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isotherm. The increase of oxygen vacancies in NiCo2O4 results in phase transformation as revealed in Fig. 2. Finally, the structure of NiCo2O4 is collapsed completely at 300 C and transformation to alloy is observed. The abundant lowcoordinated oxygen and defect sites in NiCo2O4 would promote the adsorption and diffusion of OH and O2, thus expecting enhanced ORR catalytic activity. To assess the effects of the oxygen vacancies on the electrocatalytic activity and stability of the catalyst, the electrochemical ORR performance of the prepared NiCo2O4 nanoparticles were investigated by CV, linear scan voltammetry (LSV), and CA measurements using a standard threeelectrode system in a 0.1 M KOH aqueous solution. The ORR catalytic performance of the samples was first evaluated by CV tests in the voltage range of 0.2e1.2 V vs. RHE (Fig. 6a). In a N2-saturated electrolyte, the CV curves have a featureless rectangular shape for the cathodic current, although welldefined cathodic peaks centered at 0.66 V vs. RHE were detected in the O2-saturated electrolyte, indicating the ORR reactivity of the partial reduced NiCo2O4 nanoparticles (NiCo2O4_100). To investigate the ORR activities of these catalysts further, LSV polarization curves were measured using a rotating disk electrode (RDE), as shown in Fig. 6b. The ORR performance of NiCo2O4 treated at different temperatures in hydrogen and argon atmospheres was measured (Fig. 6b). Initially, the ORR performance was enhanced with the increase in temperature up to 100 C, as shown in Fig. 6b. However, the higher temperature can trigger a collapse of the NiCo2O4 structure and phase transition as mentioned before, resulting in lower activity for the ORR. Finally, the sample thermally treated at 300 C was fully reduced to binary NiCo alloy phase (Fig. 2). From the results, the optimal temperature for heat treatment was determined to be 100 C. The NiCo2O4_100 catalyst exhibited an onset potential of 0.93 V and a half-wave potential of 0.78 V, which are comparable to those of the commercial Pt/C catalyst (an onset potential of 0.96 V and a half-wave potential of 0.83 V). The significantly enhanced catalytic performance of NiCo2O4_100 was also verified from the mass-transport corrected Tafel slopes. The slopes of the Tafel plot in the low-overpotential region were calculated to evaluate the kinetic mechanism of the catalysts. As shown in Fig. S5, the NiCo2O4_100 exhibits the smallest Tefel slope (71.3 mV dec1) among the prepared samples, indicating the fastest ORR kinetics. To investigate the kinetic parameters for the electron transfer numbers (n), RDE measurements from 400 to 2025 rpm were conducted. The electron transfer numbers (n) for the ORR were calculated based on the Koutecky-Levich (K-L) equation: 1 1 1 ¼ þ J JK JD
(1)
JD ¼ 0:62nFD2=3 v1=6 u1=2 CO2
(2)
where J is the measured current density, JK is the kinetic current density, and JD is the diffusion limiting current density, n is the number of transferred electrons, F is the Faraday constant (96485 C mol1), D is the diffusion coefficient of O2 in 0.1 M KOH (1.93 105 cm2 s1), v is the kinematic viscosity of the electrolyte (1.09 102 cm2 s1), w is the angular frequency of rotation (u ¼ 2pf =60, f is the rotating speed of RDE),
and CO2 is the oxygen concentration in 0.1 M KOH (1.26 106 mol cm3). The n values of NiCo2O4_100, which can be estimated from the slope of the linear relationship between the inverse measured current and the inverse square root of the rotation rate, were calculated to be 3.85e3.98 at potentials from 0.2 to 0.7 V vs. RHE (Fig. 6c), which outperforms that of NiCo2O4 (3.5, Fig. S2, Supporting Information) and is comparable to that of state-of-the-art Pt/ C catalyst (n z 4.0), indicating that the ORR process is dominated by a favorable four-electron pathway with almost no formation of H2O2. In order to further explain the ORR performance with oxygen vacancy, the relationship between O3 ratio in O 1s characterized by XPS and electrocatalytic activity at 0.5 V vs. RHE is plotted with hydrogen-treated temperature in Fig. 6d. The results in Fig. 6d also indicate the ORR catalytic activity of NiCo2O4_100 outperforms pristine NiCo2O4. Furthermore, a clear correlation is confirmed between the O3 ratio and the ORR catalytic performance of hydrogen-treated NiCo2O4. When pristine NiCo2O4 was thermally treated by hydrogen up to 100 C, the O3 ratio of NiCo2O4 is augmented with an increase in the limiting current density. Treated by hydrogen at a temperature higher than 100 C, the O3 ratio of NiCo2O4 is decreased with a gradual decline of the current density. With moderate hydrogen treatment temperature (up to 100 C), the increasing concentration of oxygen vacancies and defects would lead to increasing electrochemical active sites, further boosting the ORR catalytic performance of NiCo2O4. However, with increased treatment temperature above 100 C, the amounts of oxygen vacancies and defects diminish due to the collapse of the structure (Fig. S2) and the formation of binary metal alloy, leading to poor ORR electrocatalytic activity. This observed correlation between the O3 ratio and the current density for ORR demonstrates that proper oxygen vacancies can enhance the electrocatalytic ORR performance of hydrogen-treated NiCo2O4. To further understand the effects of oxygen vacancies on the electrocatalytic activity, theoretical calculations based on density functional theory (DFT) have been performed in several previous studies [32,46,47]. The theoretical calculations indicate that oxygen vacancies can decrease work function of the NiCo2O4, thus enabling electrons to be more easily moved from the catalyst surface to reactants, which is demonstrated by enhanced electrochemical performance toward the ORR [47]. Moreover, the oxygen vacancies offer extra level in conduction or valence bands, leading to a narrow band gap and enhanced conductivity of NiCo2O4 [32,46]. Consequently, the experimental and previously reported computational results reveal that generation of structural defect such as oxygen vacancies can dramatically improve electrochemical and catalytic performance for the ORR. The electrochemically active surface areas (ECSA) of the prepared NiCo2O4 samples were estimated via the measurement of the electrochemical double-layer capacitance (Cdl) to confirm the influence of hydrogen treatment (Fig. S3). Doublelayer capacitance was calculated from cyclic voltammetry (CV) measurement in the region from 0.9 to 1 V vs. RHE at different scan rates (10e100 mV s1), and no apparent Faradaic processes occurred. The Cdl value of NiCo2O4_100 was calculated to be 8.73 mF cm2, which is much higher than that
Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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Fig. 6 e Electrochemical characterizations of the prepared samples. (a) CV curves of NiCo2O4_100 in O2-(red line) and N2saturated (black line) 0.1 M KOH electrolyte. (b) LSV curves of the prepared samples in O2-saturated 0.1 M KOH at 1600 rpm and a sweep rate of 5 mV s¡1. (c) Rotating disk voltammograms of NiCo2O4_100 at rotation speeds from 400 to 2025 rpm. The inset in (c) shows the corresponding K-L plots at different potentials. (d) O3 ratio in O 1s and current density at 0.5 V vs. RHE as a function of hydrogen treatment temperature. (e) Methanol crossover tests performed by adding methanol to the electrolyte at 200 s at an applied voltage of 0.6 V vs. RHE. (f) Currentetime chronoamperometric responses of NiCo2O4_100 and commercial Pt/C on a glassy carbon electrode in O2-saturated 0.1 M KOH electrolyte at 0.6 V vs. RHE. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
of pristine NiCo2O4 (6.43 mF cm2). The relatively higher double layer capacitance suggests the exposure of numerous electrocatalytic active surface area, as well as favorable mass transfer of the reactants (O2), which is consistent with BET surface analysis in Fig. 4. Also, resistance to methanol crossover and long-term stability are crucial for practical applications, such as fuel cells. Resistance to methanol crossover of NiCo2O4_100 with respect to ORR was investigated by chronoamperometry (CA) at 0.6 V vs. RHE, as compared with commercial Pt/C in Fig. 6e. The current density of the commercial Pt/C catalyst was drastically decreased on the introduction of 10 mL methanol (3 M) into the O2-saturated 0.1 M KOH electrolyte, whereas there was no obvious change in that of NiCo2O4_100, indicating its
high resistance to methanol poisoning. The durability of the NiCo2O4_100 and commercial Pt/C catalysts was evaluated through CA measurements at 0.6 V in an O2-saturated 0.1 M KOH electrolyte for 10000 s, as shown in Fig. 6f. After the durability test for 10000 s, about 95% of the initial current density was preserved compared to 75% for Pt/C, demonstrating the outstanding stability of NiCo2O4_100 under alkaline conditions. The superior performance and excellent durability of the NiCo2O4_100 catalyst prepared by hydrothermal and hydrogen treatment can be explained by several advantages. First, the introduction of suitable oxygen vacancies into the catalysts adjusts the electronic properties and provides efficient electrocatalytic active sites, facilitating the adsorption of O2 and
Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091
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leading to a significantly enhanced catalytic performance and stability for the ORR. In addition, a broader pore size distribution of NiCo2O4_100 with increased BET surface area and electrochemically active surface area contributed to efficient mass transport and offered more accessible electrochemically active sites. Moreover, the spinel-type NiCo2O4_100 was synthesized by thermal treatment at a low temperature in an oxygen-deficient atmosphere boosted the electrocatalytic activity for the ORR with no obvious transformation of the phase and morphology. It can be concluded that the relatively large surface and more accessible electrocatalytic active sites of the optimized catalyst increased the electrocatalytic activity and stability for the ORR.
Conclusions In summary, we have successfully synthesized oxygendeficient NiCo2O4 materials by a simple hydrothermal method with subsequent thermal treatment in a hydrogen atmosphere. The limiting current density of the optimized NiCo2O4_100 is close to that of commercial 20% Pt/C, indicating its outstanding ORR performance. The four-electron pathway for the ORR was confirmed using the K-L equation and RDE experiments, further indicating efficient electrocatalysis corresponding to the direct reduction of O2 to OH. Additionally, NiCo2O4_100 shows much higher stability and tolerance to methanol than a state-of-the-art Pt-based catalyst. The significantly excellent ORR performance is mainly attributed to the incorporation of oxygen vacancies in the catalyst. It can be concluded that the creation of oxygen vacancies in spinel-type oxide electrocatalysts plays an important role in increasing the number of ORR active sites, exposing abundant electrochemical accessible active sites to reactants, and enhancing the electrochemical properties.
Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No. 2015R1A4A1042434) and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 20194030202340).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.07.091.
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Please cite this article as: Lim D et al., Spinel-type NiCo2O4 with abundant oxygen vacancies as a high-performance catalyst for the oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.07.091