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Transition metal ions regulated oxygen evolution reaction performance of Ni-based hydroxides hierarchical nanoarrays

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ABSTRACT

Nickel-based hydroxide hierarchical nanoarrays (NiyM(OH)x HNAs M = Fe or Zn) are doped with non-noble transition metals to create nanostructures and regulate their activities for the oxygen evolution reaction. Catalytic performance in these materials depends on their chemical composition and the presence of nanostructures. These novel hierarchical nanostructures contain small secondary nanosheets that are grown on the primary nanowire arrays, providing a higher surface area and more efficient mass transport for electrochemical reactions. The activities of the NiyM(OH)x HNAs for the oxygen evolution reaction (OER) followed the order of Ni2.2Fe(OH)x > Ni(OH)2 > Ni2.1Zn(OH)x, and these trends are supported by density functional theory (DFT) calculations. The Fe-doped nickel hydroxide hierarchical nanoarrays (Ni2.2Fe(OH)x HNAs), which had an appropriate elemental composition and hierarchical nanostructures, achieve the lowest onset overpotential of 234 mV and the smallest Tafel slope of 64.3 mV dec−1. The specific activity, which is normalized to the Brunauer–Emmett–Teller (BET) surface area of the catalyst, of the Ni2.2Fe(OH)x HNAs is 1.15 mA cm−2BET at an overpotential of 350 mV. This is ~4-times higher than that of Ni(OH)2. These values are also superior to those of a commercial IrOx electrocatalyst.

No MeSH data available.


Low- and high-magnification (inset) SEM images of the (a) Ni(OH)2, (b) Ni2.2Fe(OH)x, and (c) Ni2.1Zn(OH)x HNAs.
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f2: Low- and high-magnification (inset) SEM images of the (a) Ni(OH)2, (b) Ni2.2Fe(OH)x, and (c) Ni2.1Zn(OH)x HNAs.

Mentions: NiyM(OH)x HNAs, where M = Fe and Zn, were fabricated by dipping Cu foam substrates coated with one-dimensional (1D) Cu2O nanowire arrays into an aqueous solution containing metal chloride salts and sodium hyposulfite using a solution-phase cation exchange method at room temperature. During the cation exchange process, the Cu2O nanowires were etched by S2O32−, releasing OH−. During this process NiyM(OH)x HNAs precipitated, these new NiyM(OH)x HNAs structures inherited the geometry of the Cu2O template. Secondary NiyM(OH)x HNAs nanostructures also formed depending on the solubility of the products and the pH of the reaction system. As illustrated in Fig. 1, the secondary nanostructures of the NiyM(OH)x HNAs were regulated during this process. Low magnification SEM images of the NiyM(OH)x HNAs revealed that the surface of the Cu foam substrate was completely covered with vertically aligned nanoarrays (Fig. 2). The inset to Fig. 2a showed the morphology of the Ni(OH)2 HNAs, which inherited the shape of the 1D Cu2O nanowire arrays (Fig. S1) along the axial direction. After doping Ni(OH)2 with transition metals, the surfaces of the nanowires became rougher, and their morphologies markedly changed into hierarchical structures with secondary nanosheets grown on the primary nanowire arrays (see insets to Fig. 2b,c). The degree of surface roughness on the NiyM(OH)x HNAs followed the order of Ni(OH)2 < Ni2.1Zn(OH)x < Ni2.2Fe(OH)x, indicating a marked increasement in surface area when the appropriate elements were used as dopants. Transmission electron microscopy (TEM) images of the NiyM(OH)x HNAs further revealed the presence of secondary nanosheets (Figs 3 and S2). The Ni2.2Fe(OH)x HNAs had the most irregularly shaped nanosheet coating. As shown in Fig. S3, the Ni(OH)2, Ni2.1Zn(OH)x, and Ni2.2Fe(OH)x HNAs had diameters of 30.5 nm, 51 nm, and 103.4 nm, respectively. Cross-sectional SEM images of the NiyM(OH)x HNAs (Fig. S4) revealed that the three NiyM(OH)x HNAs had similar lengths of 2 μm. Figure 3c,d show high-magnification and high-resolution TEM (HRTEM) images of the Ni2.2Fe(OH)x HNAs. Ultra-thin (~2.1 nm) character was clearly observed on one edge curled nanosheet of the Ni2.2Fe(OH)x HNAs, indicating more exposure of low coordinated surface atoms and thus abundant catalytically active sites. HRTEM images indicated that the Ni(OH)2 HNAs were predominantly crystalline, while the Ni2.2Fe(OH)x and Ni2.1Zn(OH)x HNAs were amorphous (Figs S2b, 3d and S2e). Fast Fourier transform (FFT) images were in agreement with the HRTEM images. SEM and TEM results indicated that the proposed method effectively regulated the growth of nanostructures on the Ni(OH)2 HNAs using Fe and Zn as dopants. When doping with the transition metals Fe and Zn, the surfaces of the HNAs become rougher and more highly amorphous. Figure S5 shows X-ray diffraction (XRD) patterns for the Cu2O nanowire arrays and the CoyFe1-y(OH)x HNAs. The diffraction patterns for the Cu2O nanowire arrays indicated the presence of Cu2O phases (PDF#65-3288) and Cu (PDF# 65-9743). The diffraction patterns for the Ni(OH)2, Ni2.1Zn(OH)x, and Ni2.2Fe(OH)x HNAs did not contain any characteristic peaks for Ni, Fe, or Zn compounds. Only Cu and a small amount of Cu2O were present (Fig. S5), revealing that the three samples had amorphous structures. It should be noted that the amorphous nature of the Ni(OH)2 HNAs observed by XRD did not conflict with the crystal structures obtained from HRTEM, because the faint crystal lattice and weak FFT pattern of the Ni(OH)2 HNAs indicated a low crystallinity3233.


Transition metal ions regulated oxygen evolution reaction performance of Ni-based hydroxides hierarchical nanoarrays
Low- and high-magnification (inset) SEM images of the (a) Ni(OH)2, (b) Ni2.2Fe(OH)x, and (c) Ni2.1Zn(OH)x HNAs.
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f2: Low- and high-magnification (inset) SEM images of the (a) Ni(OH)2, (b) Ni2.2Fe(OH)x, and (c) Ni2.1Zn(OH)x HNAs.
Mentions: NiyM(OH)x HNAs, where M = Fe and Zn, were fabricated by dipping Cu foam substrates coated with one-dimensional (1D) Cu2O nanowire arrays into an aqueous solution containing metal chloride salts and sodium hyposulfite using a solution-phase cation exchange method at room temperature. During the cation exchange process, the Cu2O nanowires were etched by S2O32−, releasing OH−. During this process NiyM(OH)x HNAs precipitated, these new NiyM(OH)x HNAs structures inherited the geometry of the Cu2O template. Secondary NiyM(OH)x HNAs nanostructures also formed depending on the solubility of the products and the pH of the reaction system. As illustrated in Fig. 1, the secondary nanostructures of the NiyM(OH)x HNAs were regulated during this process. Low magnification SEM images of the NiyM(OH)x HNAs revealed that the surface of the Cu foam substrate was completely covered with vertically aligned nanoarrays (Fig. 2). The inset to Fig. 2a showed the morphology of the Ni(OH)2 HNAs, which inherited the shape of the 1D Cu2O nanowire arrays (Fig. S1) along the axial direction. After doping Ni(OH)2 with transition metals, the surfaces of the nanowires became rougher, and their morphologies markedly changed into hierarchical structures with secondary nanosheets grown on the primary nanowire arrays (see insets to Fig. 2b,c). The degree of surface roughness on the NiyM(OH)x HNAs followed the order of Ni(OH)2 < Ni2.1Zn(OH)x < Ni2.2Fe(OH)x, indicating a marked increasement in surface area when the appropriate elements were used as dopants. Transmission electron microscopy (TEM) images of the NiyM(OH)x HNAs further revealed the presence of secondary nanosheets (Figs 3 and S2). The Ni2.2Fe(OH)x HNAs had the most irregularly shaped nanosheet coating. As shown in Fig. S3, the Ni(OH)2, Ni2.1Zn(OH)x, and Ni2.2Fe(OH)x HNAs had diameters of 30.5 nm, 51 nm, and 103.4 nm, respectively. Cross-sectional SEM images of the NiyM(OH)x HNAs (Fig. S4) revealed that the three NiyM(OH)x HNAs had similar lengths of 2 μm. Figure 3c,d show high-magnification and high-resolution TEM (HRTEM) images of the Ni2.2Fe(OH)x HNAs. Ultra-thin (~2.1 nm) character was clearly observed on one edge curled nanosheet of the Ni2.2Fe(OH)x HNAs, indicating more exposure of low coordinated surface atoms and thus abundant catalytically active sites. HRTEM images indicated that the Ni(OH)2 HNAs were predominantly crystalline, while the Ni2.2Fe(OH)x and Ni2.1Zn(OH)x HNAs were amorphous (Figs S2b, 3d and S2e). Fast Fourier transform (FFT) images were in agreement with the HRTEM images. SEM and TEM results indicated that the proposed method effectively regulated the growth of nanostructures on the Ni(OH)2 HNAs using Fe and Zn as dopants. When doping with the transition metals Fe and Zn, the surfaces of the HNAs become rougher and more highly amorphous. Figure S5 shows X-ray diffraction (XRD) patterns for the Cu2O nanowire arrays and the CoyFe1-y(OH)x HNAs. The diffraction patterns for the Cu2O nanowire arrays indicated the presence of Cu2O phases (PDF#65-3288) and Cu (PDF# 65-9743). The diffraction patterns for the Ni(OH)2, Ni2.1Zn(OH)x, and Ni2.2Fe(OH)x HNAs did not contain any characteristic peaks for Ni, Fe, or Zn compounds. Only Cu and a small amount of Cu2O were present (Fig. S5), revealing that the three samples had amorphous structures. It should be noted that the amorphous nature of the Ni(OH)2 HNAs observed by XRD did not conflict with the crystal structures obtained from HRTEM, because the faint crystal lattice and weak FFT pattern of the Ni(OH)2 HNAs indicated a low crystallinity3233.

View Article: PubMed Central - PubMed

ABSTRACT

Nickel-based hydroxide hierarchical nanoarrays (NiyM(OH)x HNAs M&thinsp;=&thinsp;Fe or Zn) are doped with non-noble transition metals to create nanostructures and regulate their activities for the oxygen evolution reaction. Catalytic performance in these materials depends on their chemical composition and the presence of nanostructures. These novel hierarchical nanostructures contain small secondary nanosheets that are grown on the primary nanowire arrays, providing a higher surface area and more efficient mass transport for electrochemical reactions. The activities of the NiyM(OH)x HNAs for the oxygen evolution reaction (OER) followed the order of Ni2.2Fe(OH)x&thinsp;&gt;&thinsp;Ni(OH)2&thinsp;&gt;&thinsp;Ni2.1Zn(OH)x, and these trends are supported by density functional theory (DFT) calculations. The Fe-doped nickel hydroxide hierarchical nanoarrays (Ni2.2Fe(OH)x HNAs), which had an appropriate elemental composition and hierarchical nanostructures, achieve the lowest onset overpotential of 234&thinsp;mV and the smallest Tafel slope of 64.3&thinsp;mV dec&minus;1. The specific activity, which is normalized to the Brunauer&ndash;Emmett&ndash;Teller (BET) surface area of the catalyst, of the Ni2.2Fe(OH)x HNAs is 1.15&thinsp;mA&thinsp;cm&minus;2BET at an overpotential of 350&thinsp;mV. This is ~4-times higher than that of Ni(OH)2. These values are also superior to those of a commercial IrOx electrocatalyst.

No MeSH data available.