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Hexagonal β-Ni(OH)2 nanoplates with oxygen vacancies as efficient catalysts for the oxygen evolution reaction
Electrochimica Acta 324 (2019) 134868
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Hexagonal b-Ni(OH)2 nanoplates with oxygen vacancies as efficient catalysts for the oxygen evolution reaction Namil Kim 1, Dongwook Lim 1, Yeji Choi, 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
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 March 2019 Received in revised form 31 July 2019 Accepted 10 September 2019 Available online 11 September 2019
Water splitting system by transition metal-based catalysts has drawn much attention in renewable energy storage and conversion system. In this study, b-Ni(OH)2 nanoplates with abundant oxygen vacancies were synthesized via the hydrothermal method followed by a partial reduction reaction in a mild hydrogen atmosphere, and applied to the oxygen evolution reaction (OER) in an alkaline solution. The bNi(OH)2 nanoplates prepared by partial reduction in a hydrogen atmosphere at 100 C exhibited a current density of 10 mA cm2 at a low overpotential of 340 mV in 1 M KOH; the overpotential was much lower than those required for pristine b-Ni(OH)2 nanoplates (391 mV) and air-treated b-Ni(OH)2 nanoplates (369 mV). Furthermore, the H2-treated b-Ni(OH)2 nanoplates also displayed outstanding long-term stability even after 1000 cyclic voltammetry cycles. This excellent OER activity and stability could be ascribed to the abundant oxygen vacancies and a well-defined hexagonal structure. It is worth noting that this method will offer a facile synthesis of metal hydroxides for generating oxygen vacancy, improving the electrochemical performance of energy storage applications. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Oxygen evolution reaction Nickel hydroxide Oxygen vacancy Water splitting
1. Introduction Electrochemical water splitting has been considered as a promising strategy for renewable energy storage and conversion applications [1e4]. Water electrolysis, in which water is split into highly pure hydrogen (via hydrogen evolution reaction (HER)) and oxygen (via oxygen evolution reaction (OER)), is an eco-friendly and efficient method to produce clean hydrogen fuel [5e9]. In this process, the OER is regarded as rate-determining step due to its sluggish kinetics of OeH bond breaking, OeO bond formation and multi electron-involved reaction [8,10e12]. To address this issue, vigorous research efforts have been dedicated to developing highly efficient catalysts for the OER to meet the practical requirements. Generally, the most widely used noble-metal oxide such as RuO2 and IrO2 showed outstanding performance in the OER [13e19]. However, their poor resources, high price, and degradation of performance make them impossible to be commercialized for large-scale applications [20]. Ni-based oxides [7,21,22], hydroxides [12,23e27], oxyhydroxides [28], layered double hydroxides [29,30],
* Corresponding author. E-mail address: [email protected] (S.-H. Baeck). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.electacta.2019.134868 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
and alloys [31e33] have received great attention because of their excellent electrochemical oxygen evolution activities and longterm stabilities. Among the Ni-based materials, nickel hydroxide has been widely considered a promising OER catalyst because of its well-defined electrochemical redox property, various valence states, and high OER performance [34]. For instance, Zhou et al. reported that ultrathin b-Ni(OH)2 nanoplates (NPs) and their composites with multi-walled carbon nanotubes generated a current density of 10 mA cm2 at an overpotential of 474 mV in 0.1 M KOH [12]. Stern and co-workers investigated high-OERperformance Ni(OH)2 NPs with different morphologies and sizes [21]. The OER performance of electrocatalysts can be significantly improved by increasing the number of active sites [30,35,36]. Among the strategies for increasing the intrinsic reactivity of active sites, introduction of oxygen vacancies can efficiently alter the coordination numbers and electronic properties, which in turn can lower the adsorption energy of H2O, and finally, enhance the OER performances [37e45]. There are various methods to introduce oxygen vacancies into the catalyst surfaces such as reduction by chemicals (e.g., using NaBH4) [44], Ar plasma process [37], water plasma technique [38], and heat treatment in a hydrogen atmosphere. For instance, Xu et al. generated oxygen vacancies on the
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surface of Co3O4 through the Ar-plasma engraving method, which resulted in a lower coordination number and enhanced OER activity [37]. In addition, Wang et al. improved the OER performance of NiCo2O4 by introducing oxygen vacancies through NaBH4 reduction [44]. However, these methods require high-temperature treatment and sophisticated equipment, which render them unsuitable for large-scale applications. In comparison, hydrogen treatment can introduce oxygen vacancies under relatively mild conditions (thermal treatment below 200 C), which ultimately can be used for large-scale applications. In this study, the role of oxygen vacancy in nickel hydroxide was investigated. b-Ni(OH)2 NPs with abundant oxygen vacancies were successfully synthesized via the hydrothermal method, followed by a partial reduction reaction in a mild hydrogen atmosphere, and the NPs were used as a catalyst for the OER in an alkaline solution. The oxygen-deficient b-Ni(OH)2 NPs synthesized by partial reduction in a hydrogen atmosphere at 100 C required a low OER overpotential of 340 mV vs. reversible hydrogen electrode (RHE) to produce a current density of 10 mA cm2 in 1 M KOH, and the sample exhibited outstanding stability even after 1000 cyclic voltammetry (CV) cycles. The outstanding activity and durability were attributed to a well-defined hexagonal structure, the abundant oxygen vacancies, and reversible redox properties, increasing the active sites, and facilitating the formation of active species (Scheme 1).
2. Experimental 2.1. Synthesis of b-Ni(OH)2 NPs Typically, NiCl2$4H2O (0.9 g) was dissolved in 60 mL of deionized (DI) water (resistivity of 18.2 MU cm). The resulting solution was continuously stirred for 30 min. Then, 15 mL of 1 M NaOH was added to the solution and stirred for 30 min. Subsequently, the mixture was transferred to a Teflon-lined steel autoclave and heated at 200 C for 12 h. After the autoclave was naturally cooled down to room temperature, the obtained precipitate was collected, washed with DI water several times, and then dried at 80 C overnight. The synthesized catalysts were denoted as b-Ni(OH)2 NPs.
2.2. Synthesis of b-Ni(OH)2 NPs-H2 and b-Ni(OH)2 NPs-Air The prepared powders were first heated to 100 C at a rate of 10 C min1 in a horizontal quartz tube furnace under Ar at 80 sccm (standard cubic centimeters per minute), and then heat-treated in a reaction tube for 5 h in either 10%/90% mixture of H2/Ar or air condition at 100 C and ambient pressure. The synthesized bNi(OH)2 catalysts were denoted as b-Ni(OH)2 NPs-H2 and b-Ni(OH)2 NPs-Air according to the heat-treatment condition.
2.3. Physical characterization of the prepared b-Ni(OH)2 NPs The crystallinity and crystal structures of the synthesized catalysts were investigated by X-ray diffraction (XRD; RIGAKU, D/MAX 2200 V/PC) using the Cu Ka radiation (l ¼ 0.154 nm). The surface morphology of the catalysts was characterized by scanning electron microscopy (SEM; Hitachi, S-4300SE) and high-resolution transmission electron microscopy (HRTEM; JEOL, JEM2100F). X-ray photoelectron spectroscopy (XPS; Thermo scientific, K-Alpha) was used for the surface chemical analysis. The specific surface area of the catalysts was evaluated using Brunauer-Emmett-Teller (BET) analysis (ASAP 2020). Before the analysis, all catalysts were degassed at 373 K under vacuum for 6 h. 2.4. Electrochemical characterization of the prepared b-Ni(OH)2 NPs 2 mg of the as-prepared catalyst was dispersed in 100 mL of isopropanol and 384 mL of DI water; subsequently, 20 mL of 5 wt% Nafion solution was added, and the mixture was dispersed by ultrasonication for 30 min to obtain a homogeneous suspension. Next, 5 mL of the catalyst ink was dropped on the surface of a polished glassy carbon electrode (GCE, 3 mm in diameter) and dried at room temperature. The working electrode was immersed in a glass cell containing 1 M KOH aqueous electrolyte. A platinum plate and Ag/AgCl/3 M KCl served as the counter and reference electrodes, respectively. Before measurements, the electrolyte solution was purged with N2 for 30 min. The OER measurements were performed in the potential range 0.05e0.8 V vs. RHE with a rotation speed of 1600 rpm at a sweep rate of 5 mV s1. All polarization curves were corrected with iR-compensation (85%). In addition, all current densities were normalized to the geometrical area of the GCE. All potentials were converted from the Ag/AgCl scale to the RHE scale using equation E (RHE) ¼ E (Ag/AgCl) þ 0.0592 pH þ E (Ag/AgCl) in 1 M KOH. The OER activity was surveyed in 1 M KOH aqueous solution by linear sweep voltammetry (LSV) at a scan rate of 5 mV s1. The OER stability was evaluated by comparison of CVs of 1st cycle and 1000th cycles between 1.2 V and 1.6 V vs. RHE with a scan rate of 100 mV s1. The charge-transfer resistances of the catalysts were determined by electrochemical impedance spectroscopy (EIS) in the frequency range from 100 mHz to 100 kHz at 1.65 V vs. RHE. The capacitance (Cdl) was estimated by CV, where no apparent Faradaic processes occurred, in the region from 0.9 V to 1 V vs. RHE with different scan rates (10, 30, 50, 70, and 100 mV s1). The turnover frequency (TOF) was calculated according to the equation:
TOF ¼
j*A 4*F*m
where j is the current density (A cm2) obtained at the overpotential of 420 mV, A is the surface area of the GCE, F is the Faraday
Scheme 1. Schematic illustration of oxygen evolution in the presence of oxygen deficient b-Ni(OH)2 NPs.
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constant (96485C mol1), and m is the number of moles of the active material loaded on the GCE. All measurements were carried out at room temperature (~25 C). In order to ensure measurement reproducibility, at least three samples were tested under the same conditions.
3. Results and discussion 3.1. Physical characterization of the prepared b-Ni(OH)2 NPs XRD analysis was carried out to confirm the crystal structures of the b-Ni(OH)2 NP samples (Fig. 1a). All the characteristic peaks matched well with that of b-Ni(OH)2 (JCPDS No. 14e0117); in addition, no noticeable impurity peaks (due to Ni, NiO, and etc.) were detected [46], which indicates that well-defined crystalline structure of b-Ni(OH)2 were successfully synthesized in all the prepared samples. Interestingly, a slight peak shift to higher angles (38e39 ) was observed for b-Ni(OH)2 NPs-H2 and b-Ni(OH)2 NPsAir (Fig. 1b), suggesting a decrease in the lattice parameters due to the heat treatment. In general, introduction of defects such as oxygen vacancies can compress the crystal lattice due to slight destruction of the structure, resulting in lattice contraction and peak shift to higher angle in XRD analysis [47]. The peak shifts to higher angle were also observed in both 32e34 and 57.5e60.5 peaks (data not shown), which further confirms the lattice contraction. The change in XRD can be an evidence of the successful introduction of oxygen vacancies into the b-Ni(OH)2 NPs, thus expecting high redox properties. The surface morphologies of the Ni(OH)2 NP samples were examined by TEM. As seen in the TEM images (Fig. 2), the shape and size of the b-Ni(OH)2 NPs were retained after partial reduction in a hydrogen atmosphere, thus confirming that the heat treatment did not significantly affect the structure of the NPs. All the prepared catalysts exhibited well-defined hexagonal shapes with an average size of about 50 nm; in addition, the NPs were nearly transparent to the electron beam, indicating an ultrathin NP structure. A characteristic lattice fringe with an interplanar spacing of 0.217 nm (Fig. 2e) was confirmed in the high-resolution TEM analysis and can be corresponding to the (001) plane of b-Ni(OH)2. Notably, the lattice fringes of b-Ni(OH)2 NPs-H2 appeared more rough and
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irregular (Fig. 2e) than that of the other two catalysts, indicating that the partial reduction reaction in a hydrogen atmosphere caused lattice distortion [30]. These changes could be ascribed to the introduction of numerous oxygen vacancies, which caused lattice destruction. To obtain further insight into the chemical states and oxygen vacancies of the samples, we performed XPS measurements, and the results are shown in Fig. 3. The survey spectrum of b-Ni(OH)2 NPs-H2 (Fig. S3) confirmed the presence of Ni and O atoms on the surface of the catalyst. The peaks at binding energies around 854.7 eV and 872.4 eV (see Fig. 3a) were assigned to Ni 2p3/2 and Ni 2p1/2, respectively; in addition, the shake-up satellites (denoted as “sat.” in Fig. 3a) revealed the presence of both Ni2þ and Ni3þ [40,48]. Among the prepared catalysts, b-Ni(OH)2 NPs-H2 displayed the highest proportion of Ni2þ/Ni3þ. Commonly, b-NiOOH can be simply formed from the Ni2þ species in Ni-based materials during oxidation of water [25e27]. The in situ generated b-NiOOH species on the surface of b-Ni(OH)2 NPs-H2 are known as OER active sites. The O 1s spectrum was investigated to further confirm the enrichment of oxygen vacancies on the surface of b-Ni(OH)2 NPsH2. Fig. 3b shows the O 1s spectra of the as-synthesized catalysts. The O 1s peak was deconvoluted into four peaks, which were assigned to the metal-oxygen bonds (O1, ~529.8 eV), oxygen atoms of the hydroxyl groups (O2, ~530.9 eV), low-coordinated oxygen ions at the surface and defect sites (O3, ~531.8 eV), and physisorbed or chemisorbed water (O4, ~532.4 eV) [39,40]. From the O 1s spectrum, the O3 peak area ratio was calculated to be 20.2%, 22.5%, and 26.7% for b-Ni(OH)2 NPs, b-Ni(OH)2 NPs-Air, and b-Ni(OH)2 NPs-H2, respectively (Table S1). The b-Ni(OH)2 NPs-H2 showed the largest peak area ratio among the prepared samples, indicating that the hydrogen-treated sample processed abundant oxygen vacancies. And, although both b-Ni(OH)2 NPs-Air and b-Ni(OH)2 NPsH2 exhibited peak shift in XRD analysis, b-Ni(OH)2 NPs-H2 sample showed higher O3 ratio than b-Ni(OH)2 NPs-Air. Oxygen vacancies play a significant role in the enhancement of the OER process. The vacancies facilitate the adsorption of OH and other intermediate products, dramatically enhancing the OER performance. The XPS results indicated that the introduction of oxygen vacancies affected the oxidation state of Ni and number of low-coordinated oxygen ions and defect sites, which possibly led to improvement in the OER
Fig. 1. (a) XRD patterns of b-Ni(OH)2 NPs, b-Ni(OH)2 NPs-H2, and b-Ni(OH)2 NPs-Air. (b) Magnified XRD patterns corresponding to the region 2 theta ¼ 36.5e40.5 .
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Fig. 2. Low-magnification TEM images of (a) b-Ni(OH)2 NPs, (b) b-Ni(OH)2 NPs-H2, and (c) b-Ni(OH)2 NPs-Air, and high-magnification HRTEM images of (d) b-Ni(OH)2 NPs, (e) bNi(OH)2 NPs-H2, and (f) b-Ni(OH)2 NPs-Air.
Fig. 3. High-resolution XPS (a) Ni 2p and (b) O 1s profiles of b-Ni(OH)2 NPs, b-Ni(OH)2 NPs-H2, and b-Ni(OH)2 NPs-Air.
activity. To determine the specific surface areas of the samples and confirm their porous structures, we performed the N2 adsorption/ desorption analysis (see Fig. S5). The nitrogen sorption isotherms of all the prepared catalysts were type IV curves with hysteresis loops, suggesting their mesoporous nature. As shown in Fig. S5 and Table 1, b-Ni(OH)2 NPs-H2 showed a high surface area of 48.76 m2 g1 and pore volume of 0.303 cm3 g1 with an average pore diameter of 21.91 nm, indicating the mesoporous structure of the catalyst; the surface area of the catalyst was larger than that of
b-Ni(OH)2 NPs and b-Ni(OH)2 NPs-Air. Unquestionably, such a mesoporous structure with large specific surface area will be helpful to appropriately expose electrocatalytic active sites with enhanced accessibility of electrolyte, which is crucial for the electrochemical reaction. The XRD, TEM, and XPS results revealed the successful preparation of hexagonal b-Ni(OH)2 NPs with abundant oxygen vacancies and high Ni2þ/Ni3þ ratio via the hydrothermal method and subsequent heat-treatment in hydrogen.
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Table 1 Physical and electrochemical properties of the samples. Samples
h (mV) J ¼ 10 mA cm2
Tafel slope (mV dec1)
Rcta (U)
Cdl (mF cm2)
TOFb (s1)
SBET (m2 g1)
b-Ni(OH)2 NPs b-Ni(OH)2 NPs-H2 b-Ni(OH)2 NPs-Air
391 340 369
186.9 69 85
821 45 316
4.9 10 8
0.041 0.182 0.099
39.31 48.76 45.39
a b
Electrochemical impedance spectroscopy (EIS) at 1.65 V vs. RHE. Turnover frequency (TOF) at overpotential of 0.4 V.
3.2. Electrochemical characterization of the prepared b-Ni(OH)2 NPs To further determine the affirmative effect of oxygen vacancies on the OER performance, we performed electrochemical measurements on b-Ni(OH)2 NPs, b-Ni(OH)2 NPs-H2, and b-Ni(OH)2 NPs-Air after drop casting them onto polished GCEs with a loading density of 0.2 mg cm2. All measurements were conducted in 1 M KOH at room temperature using a standard three-electrode system. The LSV curves recorded for the prepared catalysts are shown in Fig. 4a. As observed, b-Ni(OH)2 NPs-H2 showed the highest OER activity and achieved a current density of 10 mA cm2 at the low overpotential of 340 mV, outperforming the state-of-the-art RuO2 (Table S2). On the other hand, b-Ni(OH)2 NPs and b-Ni(OH)2 NPsAir required overpotentials of 391 and 369 mV, respectively, to deliver the same current density. The performance of b-Ni(OH)2 NPs-H2 was found to be superior to that of the OER electrocatalysts previously reported (Table S2). The Tafel slopes (see Fig. 4c) were calculated according to the Tafel equation, h ¼ b log(j/j0), where h is the overpotential, b is the Tafel slope, j is the current density, and j0 is the exchange current density [49]. As shown in Fig. 4c, the Tafel slope of b-Ni(OH)2 NPs-H2 (69 mV decade1) was lower than those of b-Ni(OH)2 NPs (187 mV decade1) and b-Ni(OH)2 NPs-Air (85 mV
decade1), and this value is comparable to that of benchmark RuO2 (92 mV decade1) [26], indicating that the kinetics of the OER was more facile on the surface of b-Ni(OH)2 NPs-H2. This showed that bNi(OH)2 NPs-H2 has more efficient kinetics of OER than b-Ni(OH)2 NPs and b-Ni(OH)2 NPs-Air. The high electrochemical activity of bNi(OH)2 NPs-H2 could be attributed to its hexagonal structure with mesopore and proper quantity of oxygen vacancies, which facilitated OH adsorption and improved the charge transport. The effects of oxygen vacancies on the electrocatalytic activity of metal hydroxide materials were previously examined in several previous studies, in which theoretical calculations were based on densityfunctional theory (DFT) and projected density of states (PDOS) [39,50,51]. The DFT calculations demonstrated that the formation of oxygen vacancies can lower the adsorption energy of OH on bridge site of oxygen-deficient hydroxide, thus leading to a more kinetically favorable mechanism and improved OER performance [39]. In addition, the PDOS analysis revealed that low valence states of metal in metal hydroxide derived from introduction of the oxygen vacancies resulted in a narrow band gap, expecting the enhancement of electronic conductivity in oxygen-deficient metal hydroxide, which was consistent with enhanced electrochemical activity for the OER [50,51]. Consequently, the experimental and previously reported calculation results further indicated that introduction of
Fig. 4. Electrochemical characterization results: (a) Polarization curves of b-Ni(OH)2 NPs, b-Ni(OH)2 NPs-H2, and b-Ni(OH)2 NPs-Air in N2-saturated 1 M KOH at a scan rate of 5 mV s1 and 1600 rpm. (b) Nyquist plots of the as-prepared electrocatalysts in 1 M KOH at 1.65 V vs. RHE. (c) Tafel plots corresponding to the data presented in (a). (d) Correlation between oxidation charge and TOF for the as-prepared b-Ni(OH)2 NPs.
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oxygen vacancies is efficient for enhancing electrochemical activity toward the OER. To further investigate the electrocatalytic OER kinetics, we measured EIS at a constant potential of 1.65 V vs. RHE between 100 mHz and 100 kHz. As shown in Fig. 4b, the Nyquist plots of all the prepared catalysts contained semicircles in the high-frequency region corresponding to the charge transfer resistance (Rct). Rct of bNi(OH)2 NPs-H2 was significantly lower than that of b-Ni(OH)2 NPs and b-Ni(OH)2 NPs-Air, suggesting the superior charge transport characteristics and higher electrical conductivity of b-Ni(OH)2 NPsH2 during the OER process. To determine the electrochemical active surface area (ECSA) of the prepared catalysts, we estimated the electric double-layer capacitance (Cdl) by CV with various scan rates from 10 mV s1 to 100 mV s1 (Figs. S6 and S7). Cdl has been known to be linearly proportional to the ECSA, which was derived from the slope of the current density vs. scan rate curve (linear relationship). As expected, among the prepared catalysts, b-Ni(OH)2 NPs-H2 (10 mF cm2) showed higher the Cdl value than either b-Ni(OH)2 NPs (4.9 mF cm2) or b-Ni(OH)2 NPs-Air (8 mF cm2), implying high ECSA of the hydrogen-treated sample. This result was consistent with the BET surface area measurement results. The high ECSA of bNi(OH)2 NPs-H2 was attributed to the presence of oxygen vacancies, which exposed more active sites and accelerated the ion exchange between the electrolyte and active sites. To further elucidate the role of oxygen vacancies in b-Ni(OH)2 NPs-H2, the LSV curves of prepared catalysts were normalized by the ECSA, as shown in Fig. S8. The b-Ni(OH)2 NPs-H2 displays higher specific current density normalized by the ECSA than the b-Ni(OH)2 NPs and bNi(OH)2 NPs-Air. This result also indicated that the introduction of
oxygen vacancies can improve the intrinsic catalytic activity for OER. The intrinsic characteristics of the prepared catalysts were determined from the TOF values assuming that the Faraday efficiency was 100% and nickel atoms acted as active sites for the OER. As listed in Table 1, the TOF of b-Ni(OH)2 NPs-H2 (0.182 s1) at the overpotential of 420 mV was much higher than that of either bNi(OH)2 NPs (0.041 s1) or b-Ni(OH)2 NPs-Air (0.099 s1). This result indicated that the introduction of oxygen vacancies into bNi(OH)2 NPs led to significant improvement in the intrinsic properties of the catalysts, which led to favorable OER kinetics. To obtain further insight into the correlation between oxygen vacancies and the superior performance of b-Ni(OH)2 NPs-H2, we analyzed the oxidation peak in the polarization curve and calculated the oxidation charge for NiOOH formation, as shown in Fig. 4d. The in situ formed NiOOH oxidation peak area was used to estimate the intrinsic active sites for the OER. The oxidation charge of b-Ni(OH)2 NPs-H2 was found to be 2.12 mC cm2, which was five times higher than that of b-Ni(OH)2 NPs (0.43 mC cm2) and two times higher than that of b-Ni(OH)2 NPs-Air (1.03 mC cm2). The high oxidation charge caused by the transformation of Ni(OH)2 to NiOOH was consistent with the Ni2þ/Ni3þ ratio determined from the XPS analysis and led to higher TOF compared to those of the other samples (Fig. 4d). Therefore, it is suggested that, via an electronic property engineering, the intrinsic activity of the bNi(OH)2 NPs-H2 could be well adjusted and optimized by proper incorporation of oxygen vacancies into the b-Ni(OH)2 material. Long-term stability is one of the important factors for electrochemical OER catalysts for practical applications. Fig. 5aec shows the LSV curves corresponding to the 1st and 1000th cycle. As shown
Fig. 5. Polarization curves of (a) b-Ni(OH)2 NPs, (b) b-Ni(OH)2 NPs-H2, and (c) b-Ni(OH)2 NPs-Air before and after 1000 CV cycles. (d) Ni2þ/Ni3þ ratio of b-Ni(OH)2 NPs, b-Ni(OH)2 NPs-H2, and b-Ni(OH)2 NPs-Air before and after stability tests, as determined by XPS.
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in Fig. 5b, the OER performance of b-Ni(OH)2 NPs-H2 was nearly the same even after 1000 cycles with negligible activity loss, demonstrating the outstanding stability of the OER electrode. In contrast, b-Ni(OH)2 NPs and b-Ni(OH)2 NPs-Air exhibited apparent degradation in the OER performance after 1000 cycles (Fig. 5a and c), and in cases of the two samples, after 1000 cycles the anodic peak was shifted to higher anodic potential. The long-term durability of bNi(OH)2 NPs-H2 was attributed to its enhanced redox property resulting from the introduction of oxygen vacancies. As illustrated in Fig. 5d, no significant change in the Ni2þ/Ni3þ ratio was observed for b-Ni(OH)2 NPs-H2 after the stability test. However, the proportion of Ni2þ in b-Ni(OH)2 NPs and b-Ni(OH)2 NPs-Air considerably decreased after 1000 CV cycles. These results indicated that the introduction of oxygen vacancies substantially improved the intrinsic redox property by tuning the electronic structure, consequently enhancing the electrochemical stability for the OER. The excellent electrochemical performance and outstanding long-term stability of b-Ni(OH)2 NPs-H2 served as the catalyst for OER can be attributed to the hexagonal Ni(OH)2 nanoplates architecture and the appropriate introduction of oxygen defects in the catalyst surface. First, the unique structure of the Ni(OH)2-based material achieved via a simple hydrothermal process provided several surface active Ni atoms, such as easily accessible corner and edge Ni atoms with a low coordination number. Second, the incorporation of oxygen vacancies into Ni(OH)2 augmented the catalytic activity, and finally, led to an improvement in the sample durability by modifying its electronic properties, surface oxidation state, and physical properties. Third, the high Ni2þ/Ni3þ ratio derived from the hydrogen treatment significantly facilitated the formation of nickel oxyhydroxide (NiOOH), a key factor for the enhancement of the OER performance. The synergistic effects of hexagonal structure, the numerous oxygen vacancies, and high Ni2þ population of the b-Ni(OH)2 NPs-H2 lead to a promising electrocatalyst for OER and provides a way to design and construct the electrode material with high performance for energy storage applications. 4. Conclusions In summary, hexagonal b-Ni(OH)2 NPs with oxygen vacancies were successfully synthesized by the hydrothermal method and a partial reduction reaction in a hydrogen atmosphere. Because of the hexagonal structure, numerous oxygen vacancies, and high Ni2þ ratio, the as-prepared b-Ni(OH)2 NPs-H2 sample displayed enhanced electrocatalytic OER performance. The sample exhibited a current density of 10 mA cm2 at an overpotential of ~340 mV and displayed a Tafel slope as small as 69 mV decade1 and charge transfer resistance of 45.44 U at 1.65 V vs. RHE. In addition, the H2treated b-Ni(OH)2 sample also displayed substantial long-term stability even after 1000 CV cycles. The introduction of oxygen vacancies effectively increased the number of exposed active sites and facilitated the formation of active NiOOH species for the OER. Together with the promising high performance and long-term stability, it is likely that such inexpensive transition metal hydroxide electrocatalysts can be easily applied to renewable energy storage and conversion systems. Acknowledgements 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 & Energy(MOTIE) of the Republic of Korea (No. 20194030202340).
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Meyer, Catalytic oxidation of water by an oxobridged ruthenium dimer, J. Am. Chem. Soc. 104 (1982) 4029e4030. [16] V. Petrykin, K. Macounova, O.A. Shlyakhtin, P. Krtil, Tailoring the selectivity for electrocatalytic oxygen evolution on ruthenium oxides by zinc substitution, Angew. Chem. Int. Ed. 49 (2010) 4813e4815. [17] L. Trotochaud, J.K. Ranney, K.N. Williams, S.W. Boettcher, Solution-cast metal oxide thin film electrocatalysts for oxygen evolution, J. Am. Chem. Soc. 134 (2012) 17253e17261. [18] Y. Lee, J. Suntivich, K.J. May, E.E. Perry, Y. Shao-Horn, Synthesis and activities of rutile IrO2 and RuO2 nanoparticles for oxygen evolution in acid and alkaline solutions, J. Phys. Chem. Lett. 3 (2012) 399e404. [19] J. Rossmeisl, Z.W. Qu, H. Zhu, G.J. Kroes, J.K. Nørskov, Electrolysis of water on oxide surfaces, J. Electroanal. Chem. 607 (2007) 83e89. [20] S. Song, H. Zhang, X. Ma, Z. Shao, R.T. Baker, B. Yi, Electrochemical investigation of electrocatalysts for the oxygen evolution reaction in PEM water electrolyzers, Int. J. Hydrogen Energy 33 (2008) 4955e4961. [21] L.A. Stern, X. Hu, Enhanced oxygen evolution activity by NiOx and Ni(OH)2 nanoparticles, Faraday Discuss 176 (2014) 363e379. € blinger, S. Bo €cklein, J. Ziegler, [22] K. Fominykh, J.M. Feckl, J. Sicklinger, M. Do L. Peter, J. Rathousky, E.W. Scheidt, T. Bein, D. Fattakhova-Rohlfing, Ultrasmall dispersible crystalline nickel oxide nanoparticles as high-performance catalysts for electrochemical water splitting, Adv. Funct. Mater. 24 (2014) 3123e3129. [23] S. Klaus, Y. Cai, M.W. Louie, L. Trotochaud, A.T. Bell, Effects of Fe electrolyte impurities on Ni(OH)2/NiOOH structure and oxygen evolution activity, J. Phys. Chem. C 119 (2015) 7243e7254. [24] M. Gao, W. Sheng, Z. Zhuang, Q. Fang, S. Gu, J. Jiang, Y. Yan, Efficient water oxidation using nanostructured a-nickel-hydroxide as an electrocatalyst, J. Am. Chem. Soc. 136 (2014) 7077e7084. [25] L. Wang, H. Chen, Q. Daniel, L. Duan, B. Philippe, Y. Yang, H. Rensmo, L. Sun, Promoting the water oxidation catalysis by synergistic interactions between Ni(OH)2 and carbon nanotubes, Adv. Energy Mater. 6 (2016) 1600516. [26] S. Anantharaj, P.E. Karthik, S. Kundu, Petal-like hierarchical array of ultrathin Ni(OH)2 nanosheets decorated with Ni(OH)2 nanoburls: a highly efficient OER electrocatalyst, Catal. Sci. Technol. 7 (2017) 882e893.
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[27] X. Kong, C. Zhang, S.Y. Hwang, Q. Chen, Z. Peng, Free-standing holey Ni(OH)2 nanosheets with enhanced activity for water oxidation, Small 13 (2017) 1700334. [28] L. Trotochaud, S.L. Young, J.K. Ranney, S.W. Boettcher, Nickel-Iron oxyhydroxide oxygen-evolution electrocatalysts: the role of intentional and incidental iron incorporation, J. Am. Chem. Soc. 136 (2014) 6744e6753. [29] M. Gong, H. Dai, A mini review of NiFe-based materials as highly active oxygen evolution reaction electrocatalysts, Nano Res. 8 (2014) 23e39. [30] Y. Wang, Y. Zhang, Z. Liu, C. Xie, S. Feng, D. Liu, M. Shao, S. Wang, Layered double hydroxide nanosheets with multiple vacancies obtained by dry exfoliation as highly efficient oxygen evolution electrocatalysts, Angew. Chem. Int. Ed. 56 (2017) 5867e5871. [31] T. Naresh Kumar, S. Sivabalan, N. Chandrasekaran, K.L. Phani, Synergism between polyurethane and polydopamine in the synthesis of Ni-Fe alloy monoliths, Chem. Commun. 51 (2015) 1922e1925. [32] J. Ahmed, S. Sharma, K.V. Ramanujachary, S.E. Lofland, A.K. Ganguli, Microemulsion-mediated synthesis of cobalt (pure fcc and hexagonal phases) and cobalt-nickel alloy nanoparticles, J. Colloid Interface Sci. 336 (2009) 814e819. [33] S. Barison, S. Cattarin, S. Daolio, M. Musiani, A. Tuissi, Characterisation of surface oxidation of nickel-titanium alloy by ion-beam and electrochemical techniques, Electrochim. Acta 50 (2004) 11e18. [34] D.S. Hall, D.J. Lockwood, C. Bock, B.R. MacDougall, Nickel hydroxides and related materials: a review of their structures, synthesis and properties, Proc. R. Soc. A 471 (2014) 20140792. [35] Y. Zhao, X. Jia, G. Chen, L. Shang, G.I.N. Waterhouse, L.Z. Wu, C.H. Tung, D. Ohare, T. Zhang, Ultrafine NiO nanosheets stabilized by TiO2 from monolayer NiTi-ldh precursors: an active water oxidation electrocatalyst, J. Am. Chem. Soc. 138 (2016) 6517e6524. [36] Y. Sun, S. Gao, F. Lei, Y. Xie, Atomically-thin two-dimensional sheets for understanding active sites in catalysis, Chem. Soc. Rev. 44 (2015) 623e636. [37] L. Xu, Q. Jiang, Z. Xiao, X. Li, J. Huo, S. Wang, L. Dai, Plasma-engraved Co3O4 nanosheets with oxygen vacancies and high surface area for the oxygen evolution reaction, Angew. Chem. Int. Ed. 55 (2016) 5277e5281. [38] R. Liu, Y. Wang, D. Liu, Y. Zou, S. Wang, Water-plasma-enabled exfoliation of ultrathin layered double hydroxide nanosheets with multivacancies for water oxidation, Adv. Mater. 29 (2017) 1701546. [39] D. Zhou, X. Xiong, Z. Cai, N. Han, Y. Jia, Q. Xie, X. Duan, T. Xie, X. Zheng, X. Sun, X. Duan, Flame-engraved nickel-iron layered double hydroxide nanosheets for boosting oxygen evolution reactivity, Small Methods 2 (2018) 1800083. [40] Q. He, Y. Wan, H. Jiang, Z. Pan, C. Wu, M. Wang, X. Wu, B. Ye, P.M. Ajayan, L. Song, Nickel vacancies boost reconstruction in nickel hydroxide
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Hexagonal b-Ni(OH)2 nanoplates with oxygen vacancies as efficient catalysts for the oxygen evolution reaction Namil Kim 1, Dongwook Lim 1, Yeji Choi, 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
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 March 2019 Received in revised form 31 July 2019 Accepted 10 September 2019 Available online 11 September 2019
Water splitting system by transition metal-based catalysts has drawn much attention in renewable energy storage and conversion system. In this study, b-Ni(OH)2 nanoplates with abundant oxygen vacancies were synthesized via the hydrothermal method followed by a partial reduction reaction in a mild hydrogen atmosphere, and applied to the oxygen evolution reaction (OER) in an alkaline solution. The bNi(OH)2 nanoplates prepared by partial reduction in a hydrogen atmosphere at 100 C exhibited a current density of 10 mA cm2 at a low overpotential of 340 mV in 1 M KOH; the overpotential was much lower than those required for pristine b-Ni(OH)2 nanoplates (391 mV) and air-treated b-Ni(OH)2 nanoplates (369 mV). Furthermore, the H2-treated b-Ni(OH)2 nanoplates also displayed outstanding long-term stability even after 1000 cyclic voltammetry cycles. This excellent OER activity and stability could be ascribed to the abundant oxygen vacancies and a well-defined hexagonal structure. It is worth noting that this method will offer a facile synthesis of metal hydroxides for generating oxygen vacancy, improving the electrochemical performance of energy storage applications. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Oxygen evolution reaction Nickel hydroxide Oxygen vacancy Water splitting
1. Introduction Electrochemical water splitting has been considered as a promising strategy for renewable energy storage and conversion applications [1e4]. Water electrolysis, in which water is split into highly pure hydrogen (via hydrogen evolution reaction (HER)) and oxygen (via oxygen evolution reaction (OER)), is an eco-friendly and efficient method to produce clean hydrogen fuel [5e9]. In this process, the OER is regarded as rate-determining step due to its sluggish kinetics of OeH bond breaking, OeO bond formation and multi electron-involved reaction [8,10e12]. To address this issue, vigorous research efforts have been dedicated to developing highly efficient catalysts for the OER to meet the practical requirements. Generally, the most widely used noble-metal oxide such as RuO2 and IrO2 showed outstanding performance in the OER [13e19]. However, their poor resources, high price, and degradation of performance make them impossible to be commercialized for large-scale applications [20]. Ni-based oxides [7,21,22], hydroxides [12,23e27], oxyhydroxides [28], layered double hydroxides [29,30],
* Corresponding author. E-mail address: [email protected] (S.-H. Baeck). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.electacta.2019.134868 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
and alloys [31e33] have received great attention because of their excellent electrochemical oxygen evolution activities and longterm stabilities. Among the Ni-based materials, nickel hydroxide has been widely considered a promising OER catalyst because of its well-defined electrochemical redox property, various valence states, and high OER performance [34]. For instance, Zhou et al. reported that ultrathin b-Ni(OH)2 nanoplates (NPs) and their composites with multi-walled carbon nanotubes generated a current density of 10 mA cm2 at an overpotential of 474 mV in 0.1 M KOH [12]. Stern and co-workers investigated high-OERperformance Ni(OH)2 NPs with different morphologies and sizes [21]. The OER performance of electrocatalysts can be significantly improved by increasing the number of active sites [30,35,36]. Among the strategies for increasing the intrinsic reactivity of active sites, introduction of oxygen vacancies can efficiently alter the coordination numbers and electronic properties, which in turn can lower the adsorption energy of H2O, and finally, enhance the OER performances [37e45]. There are various methods to introduce oxygen vacancies into the catalyst surfaces such as reduction by chemicals (e.g., using NaBH4) [44], Ar plasma process [37], water plasma technique [38], and heat treatment in a hydrogen atmosphere. For instance, Xu et al. generated oxygen vacancies on the
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surface of Co3O4 through the Ar-plasma engraving method, which resulted in a lower coordination number and enhanced OER activity [37]. In addition, Wang et al. improved the OER performance of NiCo2O4 by introducing oxygen vacancies through NaBH4 reduction [44]. However, these methods require high-temperature treatment and sophisticated equipment, which render them unsuitable for large-scale applications. In comparison, hydrogen treatment can introduce oxygen vacancies under relatively mild conditions (thermal treatment below 200 C), which ultimately can be used for large-scale applications. In this study, the role of oxygen vacancy in nickel hydroxide was investigated. b-Ni(OH)2 NPs with abundant oxygen vacancies were successfully synthesized via the hydrothermal method, followed by a partial reduction reaction in a mild hydrogen atmosphere, and the NPs were used as a catalyst for the OER in an alkaline solution. The oxygen-deficient b-Ni(OH)2 NPs synthesized by partial reduction in a hydrogen atmosphere at 100 C required a low OER overpotential of 340 mV vs. reversible hydrogen electrode (RHE) to produce a current density of 10 mA cm2 in 1 M KOH, and the sample exhibited outstanding stability even after 1000 cyclic voltammetry (CV) cycles. The outstanding activity and durability were attributed to a well-defined hexagonal structure, the abundant oxygen vacancies, and reversible redox properties, increasing the active sites, and facilitating the formation of active species (Scheme 1).
2. Experimental 2.1. Synthesis of b-Ni(OH)2 NPs Typically, NiCl2$4H2O (0.9 g) was dissolved in 60 mL of deionized (DI) water (resistivity of 18.2 MU cm). The resulting solution was continuously stirred for 30 min. Then, 15 mL of 1 M NaOH was added to the solution and stirred for 30 min. Subsequently, the mixture was transferred to a Teflon-lined steel autoclave and heated at 200 C for 12 h. After the autoclave was naturally cooled down to room temperature, the obtained precipitate was collected, washed with DI water several times, and then dried at 80 C overnight. The synthesized catalysts were denoted as b-Ni(OH)2 NPs.
2.2. Synthesis of b-Ni(OH)2 NPs-H2 and b-Ni(OH)2 NPs-Air The prepared powders were first heated to 100 C at a rate of 10 C min1 in a horizontal quartz tube furnace under Ar at 80 sccm (standard cubic centimeters per minute), and then heat-treated in a reaction tube for 5 h in either 10%/90% mixture of H2/Ar or air condition at 100 C and ambient pressure. The synthesized bNi(OH)2 catalysts were denoted as b-Ni(OH)2 NPs-H2 and b-Ni(OH)2 NPs-Air according to the heat-treatment condition.
2.3. Physical characterization of the prepared b-Ni(OH)2 NPs The crystallinity and crystal structures of the synthesized catalysts were investigated by X-ray diffraction (XRD; RIGAKU, D/MAX 2200 V/PC) using the Cu Ka radiation (l ¼ 0.154 nm). The surface morphology of the catalysts was characterized by scanning electron microscopy (SEM; Hitachi, S-4300SE) and high-resolution transmission electron microscopy (HRTEM; JEOL, JEM2100F). X-ray photoelectron spectroscopy (XPS; Thermo scientific, K-Alpha) was used for the surface chemical analysis. The specific surface area of the catalysts was evaluated using Brunauer-Emmett-Teller (BET) analysis (ASAP 2020). Before the analysis, all catalysts were degassed at 373 K under vacuum for 6 h. 2.4. Electrochemical characterization of the prepared b-Ni(OH)2 NPs 2 mg of the as-prepared catalyst was dispersed in 100 mL of isopropanol and 384 mL of DI water; subsequently, 20 mL of 5 wt% Nafion solution was added, and the mixture was dispersed by ultrasonication for 30 min to obtain a homogeneous suspension. Next, 5 mL of the catalyst ink was dropped on the surface of a polished glassy carbon electrode (GCE, 3 mm in diameter) and dried at room temperature. The working electrode was immersed in a glass cell containing 1 M KOH aqueous electrolyte. A platinum plate and Ag/AgCl/3 M KCl served as the counter and reference electrodes, respectively. Before measurements, the electrolyte solution was purged with N2 for 30 min. The OER measurements were performed in the potential range 0.05e0.8 V vs. RHE with a rotation speed of 1600 rpm at a sweep rate of 5 mV s1. All polarization curves were corrected with iR-compensation (85%). In addition, all current densities were normalized to the geometrical area of the GCE. All potentials were converted from the Ag/AgCl scale to the RHE scale using equation E (RHE) ¼ E (Ag/AgCl) þ 0.0592 pH þ E (Ag/AgCl) in 1 M KOH. The OER activity was surveyed in 1 M KOH aqueous solution by linear sweep voltammetry (LSV) at a scan rate of 5 mV s1. The OER stability was evaluated by comparison of CVs of 1st cycle and 1000th cycles between 1.2 V and 1.6 V vs. RHE with a scan rate of 100 mV s1. The charge-transfer resistances of the catalysts were determined by electrochemical impedance spectroscopy (EIS) in the frequency range from 100 mHz to 100 kHz at 1.65 V vs. RHE. The capacitance (Cdl) was estimated by CV, where no apparent Faradaic processes occurred, in the region from 0.9 V to 1 V vs. RHE with different scan rates (10, 30, 50, 70, and 100 mV s1). The turnover frequency (TOF) was calculated according to the equation:
TOF ¼
j*A 4*F*m
where j is the current density (A cm2) obtained at the overpotential of 420 mV, A is the surface area of the GCE, F is the Faraday
Scheme 1. Schematic illustration of oxygen evolution in the presence of oxygen deficient b-Ni(OH)2 NPs.
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constant (96485C mol1), and m is the number of moles of the active material loaded on the GCE. All measurements were carried out at room temperature (~25 C). In order to ensure measurement reproducibility, at least three samples were tested under the same conditions.
3. Results and discussion 3.1. Physical characterization of the prepared b-Ni(OH)2 NPs XRD analysis was carried out to confirm the crystal structures of the b-Ni(OH)2 NP samples (Fig. 1a). All the characteristic peaks matched well with that of b-Ni(OH)2 (JCPDS No. 14e0117); in addition, no noticeable impurity peaks (due to Ni, NiO, and etc.) were detected [46], which indicates that well-defined crystalline structure of b-Ni(OH)2 were successfully synthesized in all the prepared samples. Interestingly, a slight peak shift to higher angles (38e39 ) was observed for b-Ni(OH)2 NPs-H2 and b-Ni(OH)2 NPsAir (Fig. 1b), suggesting a decrease in the lattice parameters due to the heat treatment. In general, introduction of defects such as oxygen vacancies can compress the crystal lattice due to slight destruction of the structure, resulting in lattice contraction and peak shift to higher angle in XRD analysis [47]. The peak shifts to higher angle were also observed in both 32e34 and 57.5e60.5 peaks (data not shown), which further confirms the lattice contraction. The change in XRD can be an evidence of the successful introduction of oxygen vacancies into the b-Ni(OH)2 NPs, thus expecting high redox properties. The surface morphologies of the Ni(OH)2 NP samples were examined by TEM. As seen in the TEM images (Fig. 2), the shape and size of the b-Ni(OH)2 NPs were retained after partial reduction in a hydrogen atmosphere, thus confirming that the heat treatment did not significantly affect the structure of the NPs. All the prepared catalysts exhibited well-defined hexagonal shapes with an average size of about 50 nm; in addition, the NPs were nearly transparent to the electron beam, indicating an ultrathin NP structure. A characteristic lattice fringe with an interplanar spacing of 0.217 nm (Fig. 2e) was confirmed in the high-resolution TEM analysis and can be corresponding to the (001) plane of b-Ni(OH)2. Notably, the lattice fringes of b-Ni(OH)2 NPs-H2 appeared more rough and
3
irregular (Fig. 2e) than that of the other two catalysts, indicating that the partial reduction reaction in a hydrogen atmosphere caused lattice distortion [30]. These changes could be ascribed to the introduction of numerous oxygen vacancies, which caused lattice destruction. To obtain further insight into the chemical states and oxygen vacancies of the samples, we performed XPS measurements, and the results are shown in Fig. 3. The survey spectrum of b-Ni(OH)2 NPs-H2 (Fig. S3) confirmed the presence of Ni and O atoms on the surface of the catalyst. The peaks at binding energies around 854.7 eV and 872.4 eV (see Fig. 3a) were assigned to Ni 2p3/2 and Ni 2p1/2, respectively; in addition, the shake-up satellites (denoted as “sat.” in Fig. 3a) revealed the presence of both Ni2þ and Ni3þ [40,48]. Among the prepared catalysts, b-Ni(OH)2 NPs-H2 displayed the highest proportion of Ni2þ/Ni3þ. Commonly, b-NiOOH can be simply formed from the Ni2þ species in Ni-based materials during oxidation of water [25e27]. The in situ generated b-NiOOH species on the surface of b-Ni(OH)2 NPs-H2 are known as OER active sites. The O 1s spectrum was investigated to further confirm the enrichment of oxygen vacancies on the surface of b-Ni(OH)2 NPsH2. Fig. 3b shows the O 1s spectra of the as-synthesized catalysts. The O 1s peak was deconvoluted into four peaks, which were assigned to the metal-oxygen bonds (O1, ~529.8 eV), oxygen atoms of the hydroxyl groups (O2, ~530.9 eV), low-coordinated oxygen ions at the surface and defect sites (O3, ~531.8 eV), and physisorbed or chemisorbed water (O4, ~532.4 eV) [39,40]. From the O 1s spectrum, the O3 peak area ratio was calculated to be 20.2%, 22.5%, and 26.7% for b-Ni(OH)2 NPs, b-Ni(OH)2 NPs-Air, and b-Ni(OH)2 NPs-H2, respectively (Table S1). The b-Ni(OH)2 NPs-H2 showed the largest peak area ratio among the prepared samples, indicating that the hydrogen-treated sample processed abundant oxygen vacancies. And, although both b-Ni(OH)2 NPs-Air and b-Ni(OH)2 NPsH2 exhibited peak shift in XRD analysis, b-Ni(OH)2 NPs-H2 sample showed higher O3 ratio than b-Ni(OH)2 NPs-Air. Oxygen vacancies play a significant role in the enhancement of the OER process. The vacancies facilitate the adsorption of OH and other intermediate products, dramatically enhancing the OER performance. The XPS results indicated that the introduction of oxygen vacancies affected the oxidation state of Ni and number of low-coordinated oxygen ions and defect sites, which possibly led to improvement in the OER
Fig. 1. (a) XRD patterns of b-Ni(OH)2 NPs, b-Ni(OH)2 NPs-H2, and b-Ni(OH)2 NPs-Air. (b) Magnified XRD patterns corresponding to the region 2 theta ¼ 36.5e40.5 .
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Fig. 2. Low-magnification TEM images of (a) b-Ni(OH)2 NPs, (b) b-Ni(OH)2 NPs-H2, and (c) b-Ni(OH)2 NPs-Air, and high-magnification HRTEM images of (d) b-Ni(OH)2 NPs, (e) bNi(OH)2 NPs-H2, and (f) b-Ni(OH)2 NPs-Air.
Fig. 3. High-resolution XPS (a) Ni 2p and (b) O 1s profiles of b-Ni(OH)2 NPs, b-Ni(OH)2 NPs-H2, and b-Ni(OH)2 NPs-Air.
activity. To determine the specific surface areas of the samples and confirm their porous structures, we performed the N2 adsorption/ desorption analysis (see Fig. S5). The nitrogen sorption isotherms of all the prepared catalysts were type IV curves with hysteresis loops, suggesting their mesoporous nature. As shown in Fig. S5 and Table 1, b-Ni(OH)2 NPs-H2 showed a high surface area of 48.76 m2 g1 and pore volume of 0.303 cm3 g1 with an average pore diameter of 21.91 nm, indicating the mesoporous structure of the catalyst; the surface area of the catalyst was larger than that of
b-Ni(OH)2 NPs and b-Ni(OH)2 NPs-Air. Unquestionably, such a mesoporous structure with large specific surface area will be helpful to appropriately expose electrocatalytic active sites with enhanced accessibility of electrolyte, which is crucial for the electrochemical reaction. The XRD, TEM, and XPS results revealed the successful preparation of hexagonal b-Ni(OH)2 NPs with abundant oxygen vacancies and high Ni2þ/Ni3þ ratio via the hydrothermal method and subsequent heat-treatment in hydrogen.
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5
Table 1 Physical and electrochemical properties of the samples. Samples
h (mV) J ¼ 10 mA cm2
Tafel slope (mV dec1)
Rcta (U)
Cdl (mF cm2)
TOFb (s1)
SBET (m2 g1)
b-Ni(OH)2 NPs b-Ni(OH)2 NPs-H2 b-Ni(OH)2 NPs-Air
391 340 369
186.9 69 85
821 45 316
4.9 10 8
0.041 0.182 0.099
39.31 48.76 45.39
a b
Electrochemical impedance spectroscopy (EIS) at 1.65 V vs. RHE. Turnover frequency (TOF) at overpotential of 0.4 V.
3.2. Electrochemical characterization of the prepared b-Ni(OH)2 NPs To further determine the affirmative effect of oxygen vacancies on the OER performance, we performed electrochemical measurements on b-Ni(OH)2 NPs, b-Ni(OH)2 NPs-H2, and b-Ni(OH)2 NPs-Air after drop casting them onto polished GCEs with a loading density of 0.2 mg cm2. All measurements were conducted in 1 M KOH at room temperature using a standard three-electrode system. The LSV curves recorded for the prepared catalysts are shown in Fig. 4a. As observed, b-Ni(OH)2 NPs-H2 showed the highest OER activity and achieved a current density of 10 mA cm2 at the low overpotential of 340 mV, outperforming the state-of-the-art RuO2 (Table S2). On the other hand, b-Ni(OH)2 NPs and b-Ni(OH)2 NPsAir required overpotentials of 391 and 369 mV, respectively, to deliver the same current density. The performance of b-Ni(OH)2 NPs-H2 was found to be superior to that of the OER electrocatalysts previously reported (Table S2). The Tafel slopes (see Fig. 4c) were calculated according to the Tafel equation, h ¼ b log(j/j0), where h is the overpotential, b is the Tafel slope, j is the current density, and j0 is the exchange current density [49]. As shown in Fig. 4c, the Tafel slope of b-Ni(OH)2 NPs-H2 (69 mV decade1) was lower than those of b-Ni(OH)2 NPs (187 mV decade1) and b-Ni(OH)2 NPs-Air (85 mV
decade1), and this value is comparable to that of benchmark RuO2 (92 mV decade1) [26], indicating that the kinetics of the OER was more facile on the surface of b-Ni(OH)2 NPs-H2. This showed that bNi(OH)2 NPs-H2 has more efficient kinetics of OER than b-Ni(OH)2 NPs and b-Ni(OH)2 NPs-Air. The high electrochemical activity of bNi(OH)2 NPs-H2 could be attributed to its hexagonal structure with mesopore and proper quantity of oxygen vacancies, which facilitated OH adsorption and improved the charge transport. The effects of oxygen vacancies on the electrocatalytic activity of metal hydroxide materials were previously examined in several previous studies, in which theoretical calculations were based on densityfunctional theory (DFT) and projected density of states (PDOS) [39,50,51]. The DFT calculations demonstrated that the formation of oxygen vacancies can lower the adsorption energy of OH on bridge site of oxygen-deficient hydroxide, thus leading to a more kinetically favorable mechanism and improved OER performance [39]. In addition, the PDOS analysis revealed that low valence states of metal in metal hydroxide derived from introduction of the oxygen vacancies resulted in a narrow band gap, expecting the enhancement of electronic conductivity in oxygen-deficient metal hydroxide, which was consistent with enhanced electrochemical activity for the OER [50,51]. Consequently, the experimental and previously reported calculation results further indicated that introduction of
Fig. 4. Electrochemical characterization results: (a) Polarization curves of b-Ni(OH)2 NPs, b-Ni(OH)2 NPs-H2, and b-Ni(OH)2 NPs-Air in N2-saturated 1 M KOH at a scan rate of 5 mV s1 and 1600 rpm. (b) Nyquist plots of the as-prepared electrocatalysts in 1 M KOH at 1.65 V vs. RHE. (c) Tafel plots corresponding to the data presented in (a). (d) Correlation between oxidation charge and TOF for the as-prepared b-Ni(OH)2 NPs.
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oxygen vacancies is efficient for enhancing electrochemical activity toward the OER. To further investigate the electrocatalytic OER kinetics, we measured EIS at a constant potential of 1.65 V vs. RHE between 100 mHz and 100 kHz. As shown in Fig. 4b, the Nyquist plots of all the prepared catalysts contained semicircles in the high-frequency region corresponding to the charge transfer resistance (Rct). Rct of bNi(OH)2 NPs-H2 was significantly lower than that of b-Ni(OH)2 NPs and b-Ni(OH)2 NPs-Air, suggesting the superior charge transport characteristics and higher electrical conductivity of b-Ni(OH)2 NPsH2 during the OER process. To determine the electrochemical active surface area (ECSA) of the prepared catalysts, we estimated the electric double-layer capacitance (Cdl) by CV with various scan rates from 10 mV s1 to 100 mV s1 (Figs. S6 and S7). Cdl has been known to be linearly proportional to the ECSA, which was derived from the slope of the current density vs. scan rate curve (linear relationship). As expected, among the prepared catalysts, b-Ni(OH)2 NPs-H2 (10 mF cm2) showed higher the Cdl value than either b-Ni(OH)2 NPs (4.9 mF cm2) or b-Ni(OH)2 NPs-Air (8 mF cm2), implying high ECSA of the hydrogen-treated sample. This result was consistent with the BET surface area measurement results. The high ECSA of bNi(OH)2 NPs-H2 was attributed to the presence of oxygen vacancies, which exposed more active sites and accelerated the ion exchange between the electrolyte and active sites. To further elucidate the role of oxygen vacancies in b-Ni(OH)2 NPs-H2, the LSV curves of prepared catalysts were normalized by the ECSA, as shown in Fig. S8. The b-Ni(OH)2 NPs-H2 displays higher specific current density normalized by the ECSA than the b-Ni(OH)2 NPs and bNi(OH)2 NPs-Air. This result also indicated that the introduction of
oxygen vacancies can improve the intrinsic catalytic activity for OER. The intrinsic characteristics of the prepared catalysts were determined from the TOF values assuming that the Faraday efficiency was 100% and nickel atoms acted as active sites for the OER. As listed in Table 1, the TOF of b-Ni(OH)2 NPs-H2 (0.182 s1) at the overpotential of 420 mV was much higher than that of either bNi(OH)2 NPs (0.041 s1) or b-Ni(OH)2 NPs-Air (0.099 s1). This result indicated that the introduction of oxygen vacancies into bNi(OH)2 NPs led to significant improvement in the intrinsic properties of the catalysts, which led to favorable OER kinetics. To obtain further insight into the correlation between oxygen vacancies and the superior performance of b-Ni(OH)2 NPs-H2, we analyzed the oxidation peak in the polarization curve and calculated the oxidation charge for NiOOH formation, as shown in Fig. 4d. The in situ formed NiOOH oxidation peak area was used to estimate the intrinsic active sites for the OER. The oxidation charge of b-Ni(OH)2 NPs-H2 was found to be 2.12 mC cm2, which was five times higher than that of b-Ni(OH)2 NPs (0.43 mC cm2) and two times higher than that of b-Ni(OH)2 NPs-Air (1.03 mC cm2). The high oxidation charge caused by the transformation of Ni(OH)2 to NiOOH was consistent with the Ni2þ/Ni3þ ratio determined from the XPS analysis and led to higher TOF compared to those of the other samples (Fig. 4d). Therefore, it is suggested that, via an electronic property engineering, the intrinsic activity of the bNi(OH)2 NPs-H2 could be well adjusted and optimized by proper incorporation of oxygen vacancies into the b-Ni(OH)2 material. Long-term stability is one of the important factors for electrochemical OER catalysts for practical applications. Fig. 5aec shows the LSV curves corresponding to the 1st and 1000th cycle. As shown
Fig. 5. Polarization curves of (a) b-Ni(OH)2 NPs, (b) b-Ni(OH)2 NPs-H2, and (c) b-Ni(OH)2 NPs-Air before and after 1000 CV cycles. (d) Ni2þ/Ni3þ ratio of b-Ni(OH)2 NPs, b-Ni(OH)2 NPs-H2, and b-Ni(OH)2 NPs-Air before and after stability tests, as determined by XPS.
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in Fig. 5b, the OER performance of b-Ni(OH)2 NPs-H2 was nearly the same even after 1000 cycles with negligible activity loss, demonstrating the outstanding stability of the OER electrode. In contrast, b-Ni(OH)2 NPs and b-Ni(OH)2 NPs-Air exhibited apparent degradation in the OER performance after 1000 cycles (Fig. 5a and c), and in cases of the two samples, after 1000 cycles the anodic peak was shifted to higher anodic potential. The long-term durability of bNi(OH)2 NPs-H2 was attributed to its enhanced redox property resulting from the introduction of oxygen vacancies. As illustrated in Fig. 5d, no significant change in the Ni2þ/Ni3þ ratio was observed for b-Ni(OH)2 NPs-H2 after the stability test. However, the proportion of Ni2þ in b-Ni(OH)2 NPs and b-Ni(OH)2 NPs-Air considerably decreased after 1000 CV cycles. These results indicated that the introduction of oxygen vacancies substantially improved the intrinsic redox property by tuning the electronic structure, consequently enhancing the electrochemical stability for the OER. The excellent electrochemical performance and outstanding long-term stability of b-Ni(OH)2 NPs-H2 served as the catalyst for OER can be attributed to the hexagonal Ni(OH)2 nanoplates architecture and the appropriate introduction of oxygen defects in the catalyst surface. First, the unique structure of the Ni(OH)2-based material achieved via a simple hydrothermal process provided several surface active Ni atoms, such as easily accessible corner and edge Ni atoms with a low coordination number. Second, the incorporation of oxygen vacancies into Ni(OH)2 augmented the catalytic activity, and finally, led to an improvement in the sample durability by modifying its electronic properties, surface oxidation state, and physical properties. Third, the high Ni2þ/Ni3þ ratio derived from the hydrogen treatment significantly facilitated the formation of nickel oxyhydroxide (NiOOH), a key factor for the enhancement of the OER performance. The synergistic effects of hexagonal structure, the numerous oxygen vacancies, and high Ni2þ population of the b-Ni(OH)2 NPs-H2 lead to a promising electrocatalyst for OER and provides a way to design and construct the electrode material with high performance for energy storage applications. 4. Conclusions In summary, hexagonal b-Ni(OH)2 NPs with oxygen vacancies were successfully synthesized by the hydrothermal method and a partial reduction reaction in a hydrogen atmosphere. Because of the hexagonal structure, numerous oxygen vacancies, and high Ni2þ ratio, the as-prepared b-Ni(OH)2 NPs-H2 sample displayed enhanced electrocatalytic OER performance. The sample exhibited a current density of 10 mA cm2 at an overpotential of ~340 mV and displayed a Tafel slope as small as 69 mV decade1 and charge transfer resistance of 45.44 U at 1.65 V vs. RHE. In addition, the H2treated b-Ni(OH)2 sample also displayed substantial long-term stability even after 1000 CV cycles. The introduction of oxygen vacancies effectively increased the number of exposed active sites and facilitated the formation of active NiOOH species for the OER. Together with the promising high performance and long-term stability, it is likely that such inexpensive transition metal hydroxide electrocatalysts can be easily applied to renewable energy storage and conversion systems. Acknowledgements 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 & Energy(MOTIE) of the Republic of Korea (No. 20194030202340).
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