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Dual Tuning of Composition and Nanostructure of Hierarchical Hollow Nanopolyhedra Assembled by NiCo-Layered Double Hydroxide Nanosheets for Efficient Electrocatalytic Oxygen Evolution Shanfu Sun, Chade Lv, Weizhao Hong, Xin Zhou, Fugui Wu, and Gang Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01318 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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ACS Applied Energy Materials

Dual Tuning of Composition and Nanostructure of Hierarchical Hollow Nanopolyhedra Assembled by NiCo-Layered Double Hydroxide Nanosheets for Efficient Electrocatalytic Oxygen Evolution Shanfu Sun, Chade Lv, Weizhao Hong, Xin Zhou,* Fugui Wu and Gang Chen* MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China *Corresponding author: Xin Zhou; E-mail: [email protected]; Fax: (+86)-451-86413753. Gang Chen; E-mail: [email protected]; Fax: (+86)-451-86413753.

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ABSTRACT:

NiCo-layered double hydroxides (LDH) have recently emerged as hopeful oxygen evolution reaction (OER) catalysts in alkaline solutions. However, their preferred active sites have been ill-defined up until now. In this work, hierarchical hollow nanopolyhedra assembled by NiCo-LDH nanosheets with adjustable composition and nanostructure are prepared via a facile self-templated strategy. DFT calculations indicate that Co3+ hollow sites are the preferred adsorption and active sites. The resultant hierarchical NiCo-LDH hollow nanopolyhedra realize superior OER activity by optimizing the composition of NiCo-LDH, which achieves a 10 mA·cm-2 current density at an overpotential of 314 mV. This work not only presents deeper insights into the intrinsic OER electrocatalytic activity for NiCo-LDH but also elucidates on the further optimization of the OER properties of other bimetallic electrocatalysts.


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KEYWORDS: NiCo-LDH, hierarchical hollow structure, DFT calculation, electrocatalysis, oxygen evolution reaction

1. INTRODUCTION Electrochemical splitting of water into hydrogen and oxygen is one of the most appealing strategies for producing clean fuels to alleviate ever-increasing energy demands and environmental problems.1-7 However, the half oxygen evolution reaction (OER) dramatically impedes the overall efficiency of water splitting because of its intrinsically complex fourelectron transfer process.8-14 Presently, precious metal-based electrocatalysts RuO2 and IrO2 are proven as state-of-the-art OER catalysts, yet prohibitive cost and relative scarcity limit their


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applications.15,16 To this end, up-and-coming alternatives based on first-row transition metal compounds, such as simple and mixed oxides, hydroxides, and their derivatives, have captured tremendous scientific attention for their low cost and high activity.17-21 Particularly, nickel-based layered double hydroxides (NiM-LDH; M = Co, Fe and V) have been confirmed as a promising class of highly effective OER catalysts.18,22-26 The typical two-dimensional layered structure of LDH could provide an “electron hopping” path and endow the catalysts with excellent charge transport properties. Due to M3+ incorporation into the brucite-like host layer of Ni(OH)2, extra anions could migrate to the inside of interlayers to compensate for the excess positive charges. As a result, the OH- migration can efficiently elevate the OER process in alkaline environments. Notably, the anions of interlayer are replaced by large groups could further enhance their catalytic activity.27 Additionally, bimetallic catalysts with Ni and M in NiM-LDH could give rise to synergistic interactions, which have been shown to remarkably improve catalytic performance compared to single Ni and M components.27-32 For instance, Sun’s group found that NiV-LDH and NiFe-LDH had different effects on the OER process, and their rate-limiting steps would change from the formation of *OOH to *O.31 Duan and coworkers found that the incorporation of Co2+/Co3+ modulates the electronic structure of NiFe-LDH.32 Nai et al. investigated the adsorption energy of oxygen radical (O*) in NiCo-LDH with diverse Ni/Co ratios, and confirmed that the Co sites exhibited a stronger ability to bind with the O* than Ni sites.23 Much work has demonstrated that NiCo-LDH has markedly high OER activity,18,22,29-33 but its synergistic interactions and preferred active sites are still unclear. Thus, identifying the preferred active sites between Ni and Co in NiCo-LDH and attempting to expose them by engineering functional nanostructures are extremely desirable goals toward further boosting their OER activity.


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In this work, hierarchical hollow nanopolyhedra assembled by NiCo-LDH nanosheets are prepared through a facile self-templated method. Notably, the composition and nanostructure of the hierarchical NiCo-LDH hollow nanopolyhedra could be dually tuned by simply adjusting the mole ratios of the starting materials. Furthermore, extensive electrochemical measurements combined with DFT calculation are performed to acquire an in-depth understanding of the OER mechanism for NiCo-LDH, including the preferred adsorption and active sites, and the correlation between composition and OER activity. The results show that Co3+ hollow sites have the lowest free energy for water oxidation reaction, indicating the highly active Co3+ hollow sites serve as the preferred adsorption and active sites. The exposure of more Co3+ active sites by optimizing the compositions of Ni and Co in NiCo-LDH enables the hierarchical NiCo-LDH hollow nanopolyhedra catalysts to exhibit superior OER activity, obtaining a 10 mA·cm-2 current density at an overpotential of 314 mV. This work sheds light on further boosting the OER activity of other LDH electrocatalysts.

2. EXPERIMENTAL 2.1 Preparation of hierarchical NiCo-LDH hollow nanopolyhedra The fabrication of Co5(OH)2(CH3COO)8·2H2O nanopolyhedra (Co template) is similar to that in the previous literature (details are presented in Supporting Information).34 To prepare the hierarchical NiCo-LDH hollow nanopolyhedra, 0.3 mmol as-obtained Co template and 2.25 mmol Ni(NO3)2·6H2O were dispersed in 50 ml of ethanol to form solutions A and B, respectively. Of note, here the mole ratio of two raw materials was 7.50. Next, solutions A and B were poured into a 300 ml round-bottom flask and heated to 85 °C under refluxing conditions


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using an oil bath. After reacting for 6 h, the hierarchical NiCo-LDH hollow nanopolyhedra were achieved via centrifugation, rinsed with ethanol three times and fully dried at 60 °C. Several different compositions and nanostructures of hierarchical NiCo-LDH hollow nanopolyhedra catalysts were synthesized by simply adjusting the mole ratio of the starting materials. The mole ratios of Ni(NO3)2·6H2O to Co template used were 2.50, 3.75, 5.00, 6.25, 7.50, and 10.0, and the corresponding products were denoted as NiCo-NP-2.50, NiCo-NP-3.75, NiCo-NP-5.00, NiCoNP-6.25, NiCo-NP-7.50, and NiCo-NP-10.0, respectively. In addition, bulk NiCo-LDH, Co(OH)2 nanosheets, and Ni(OH)2 nanoflowers were synthesized via a coprecipitation approach as the control samples (see the Supporting Information for details). 2.2 Structure characterization X-ray diffraction (XRD) was performed on a Rigaku D/max-2000 diffractometer with Cu Kα radiation. The field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images of the catalysts obtained were taken on an FEI HELIOS Nanolab 600i and an FEI Tecnai G2 S-Twin, respectively. Fourier transform infrared spectroscopy (FTIR) was carried out on a Shimadzu instrument from 400-4000 cm-1using KBr pellets. Brunauer-EmmettTeller (BET) specific surface was calculated with a Micromeritics ASAP 2020. Desorption isotherm was applied to determine the pore size distribution. X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo Scientific ESCALAB 250Xi with a pass energy of 20 eV and an Al Kα radiation sources. 2.3 Electrochemical measurements Unless noted otherwise, all the electrochemical tests were carried out in a three-electrode system on an Autolab electrochemical workstation composed of counter electrode (Pt net, 1 cm×1 cm),


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reference electrode (Ag/AgCl) and working electrode (glass carbon (GC) electrodes with various catalysts loading). The applied potentials were converted to the reversible hydrogen electrode (RHE) based on the formula ERHE = EAg/AgCl + E0Ag/AgCl vs RHE + 0.0592 pH, where E0Ag/AgCl vs RHE is 0.1976 V at 25 ℃ (the electrolyte was 1 M KOH). The overpotential was calculated according to the formula η = ERHE-1.23 V. Linear sweep voltammetry (LSV) curves were obtained from 0.2 to 0.8 V at a scan rate of 5 mV·s-1. Tafel slopes were gained by plotting log (j) against overpotential η from LSV curves. Electrochemical impedance spectroscopy (EIS) measurements were performed on the potential of open circuit potential and 0.56 V (vs. Ag/AgCl) over a frequency range from 100 KHz to 0.1 Hz, respectively. The chronopotentiometry measurements at fixed current density (j) (10 and 100 mA·cm-2) on GC and Ni foam electrodes were conducted to test the catalyst durability. Electrochemical surface area (ECSA) were calculated by CV at the potential range 0.30-0.40 V (vs. Ag/AgCl) with various scan rates (10 ~ 60 mV·s-1; performed on the electrochemical workstation of CHI 660E). By plotting the △j = (janodic - jcathodic) at 0.35 V (vs. Ag/AgCl) against the scan rates, the linear slope that is twice that of the double layer capacitance is used to represent ECSA. None of the measurements were corrected by iR compensation. 2.4 Density Functional Theory (DFT) calculations All the calculations were conducted basis for the spin-polarized DFT calculations as applied by the Vienna ab initio simulation package (VASP).35,36 Projector-augmented wave (PAW) potentials37 were used to describe nuclei electron interactions. Electronic exchange and correlation effects were described within the generalized gradient approximation (GGA) as given by Perdew, Burke, and Ernzerhof.38 A kinetic energy cutoff of 400 eV was used with a plane-


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wave basis set. Based on the previous work,39 (003) facet is the more exposed surface for LDH materials and was chosen as the computational model.40 The integration of the Brillouin zone was conducted using a 3 × 3 × 1 Monkhorst−Pack grid for Ni2Co-NO3--LDH and 4 × 4 × 1 for Ni3Co-NO3--LDH, respectively. 15 Å vacuum regions were used in c axis to avoid the possible interaction between two images. Energy minimizations were stopped once total energies were converged to 1.0 × 10−5 eV/atom and the forces on each atom were less than 0.01 eV/Å. A simple standard hydrogen electrode (SHE) model proposed by Norskov et al.41,42 was used to calculate the free energy of the intermediates of the electrochemical reactions. Detailed description of this model can be seen from ref. 37. In this work, the effects of pH and electron field to the reaction energy were negligible and not considered. Moreover, the influence of the different coverage of radicals (*OH, *O, *OOH) and water were also not taken into account because the contribution of them to the free energy is not significant.32

3. RESULTS AND DISCUSSION The hierarchical NiCo-LDH hollow nanopolyhedra with adjustable composition and nanostructure were prepared through a self-templated approach (see Figure S1). Briefly, solid Co5(OH)2(CH3COO)8·2H2O nanopolyhedra were selected as the simultaneous template and Co source. The nitrates come from the addition of nickel nitrate used to oxidize and etch the Co template. At the same time, the dissolved Ni2+ and released Co3+ participate in a coprecipitation process, resulting in the formation of the NiCo-LDH shell around the nanopolyhedra core. Finally, the inner core is dissolved after a long processing time to generate the hierarchical NiCoLDH hollow nanopolyhedra. The crystal structure and morphology of the NiCo-NP-7.50 are characterized by XRD, FESEM and TEM. The TEM image in Figure 1a shows that the NiCoNP-7.50 are topologically synthesized through the nitrate oxidization and etching nanopolyhedra


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Co templates, and the NiCo-LDH nanosheets are uniformly anchored on the surface of the nanopolyhedra to create the hierarchical structure. As confirmed by XRD (Figure 1e), the crystal phase of NiCo-NP-7.50 can be assigned to NiCo-LDH.24-27,43 Notably, the XRD pattern of NiCoNP-7.50 displays broad peaks, indicating low crystallization and good dispersion of the NiCoLDH nanosheets on the nanopolyhedra shell.44 Several diffraction peaks at 2 θ between 15-20 can be ascribed to the trace residual of the Co5(OH)2(CH3COO)8·2H2O template. The FTIR spectrum indicates that the extra positive charges resulting from the Co3+ in the NiCo-LDH host layer are neutralized by the interlayer nitrate ions (Figure S5). The magnified TEM image (Figure S3c) shows that NiCo-LDH nanopolyhedra possess a hollow characteristic, and have an average shell thickness of 70 nm. The distinct lattice spacing of 0.26 nm corresponds to the (012) planes of the LDH structure (Figure 1b), while a diffuse set of concentric rings revealed in selected area electron diffraction (SAED) (Figure 1c) implies that the prepared NiCo-LDH have a polycrystalline structure. Benefitting from this unique hierarchical structure, the NiCo-LDH7.50 catalyst has a high BET specific surface area of 19.6 m2 g-1 (Figure S4), suggesting the hierarchical structure might expose more active sites in favor of catalytic reactions. The XPS survey spectrum confirms the coexistence of Ni, Co, O, N and C elements in the NiCo-LDH7.50 (Figure S6). Of note, the high-resolution XPS spectra of Co 2p and Ni 2p exhibit the typical spin-orbit splitting signals between 2p1/2 and 2p3/2, certifying that the Co and Ni are in the Co2+/Co3+ and Ni2+ oxidation state,18,23,45 respectively (Figure 1f-g). Moreover, as confirmed by the energy dispersive X-ray spectroscopy (EDX) mapping analysis, Ni and Co elements are distributed uniformly in the nanopolyhedra (Figure 1d). The above analyses, verify that the hierarchical hollow nanopolyhedra assembled by NiCo-LDH nanosheets are successfully prepared.


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Figure 1. The structure and morphology characterization of the samples obtained. a) TEM image, b) HRTEM image, c) SAED pattern, d) Elemental mapping, e) XRD pattern, f) Co 2p, and g) Ni 2p of NiCo-NP-7.50 catalysts.

It is worth mentioning that the amount of Ni2+ salt addition in the mixed solvent plays a pivotal role in controlling the growth and composition of the hierarchical NiCo-LDH hollow nanopolyhedra during the hydrolysis process (Figure S7). A low concentration of nitrates in the mixed solvent precursors can not fully oxidize and etch the Co template to generate sufficient Co3+. As a result, the generation of NiCo-LDH nanosheets around the template surface is limited (Figure S7a). Then the NiCo-LDH nanosheets are formed when the ratio of Ni(NO3)2·6H2O addition reaches 3.75 (Figure S7b). After the ratio of nitrates increases from 5.0 to 7.50, more Co3+ is oxidized and generated from the Co template, and the well-defined hierarchical NiCoLDH hollow nanopolyhedra are formed (Figure S7c-e). When an excessive amount of nitrates are added, however, the morphology of NiCo-NP-10.0 is not obviously changed. This result


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implies that the hierarchical hollow nanostructure is stable (Figure S7f). Furthermore, according to the EDX spectra (Figure S8), the atomic ratios of Ni/Co is increased from 1.81 to 2.59, suggesting that the composition of the NiCo-NP-x (x = 2.50, 3.75, 5.00, 6.25, 7.50 and 10.0) can be adjusted simultaneously with the change of nanopolyhedra. Compared with NiCo-NP-7.5, the NiCo-NP-10.0 shows similar XRD diffraction peaks, which confirm the well-defined LDH structure is maintained in NiCo-NP-10.0 (Figure S9). Electrochemical measurements are carried out to evaluate the oxygen evolution performance of the as-prepared hierarchical NiCo-LDH hollow nanopolyhedra catalysts (Figure 2). As shown in the LSV curves (Figure 2a), the nitrate content plays an essential role in the OER activity. When the mole ratio of Ni(NO3)2·6H2O to Co5(OH)2(CH3COO)8·2H2O is 7.50, the NiCo-NP7.50 sample achieves the lowest overpotential of 314 mV at the j of 10 mA·cm-2. Both a lower and higher mole ratio of Ni(NO3)2·6H2O to Co5(OH)2(CH3COO)8·2H2O will decrease the OER activities of the hierarchical NiCo-LDH hollow nanopolyhedra catalysts. The overpotentials of the NiCo-NP-x (x = 2.50, 3.75, 5.00, 6.25, and 10.0) at j = 10 mA·cm-2 are displayed in Figure S10, namely, 437, 430, 384, 331, and 403 mV, respectively. Notably, the OER activity of NiCoNP-x is superior to the Co precursor, bulk NiCo-LDH, Co(OH)2 nanosheets and Ni(OH)2 nanoflowers (Figure S11). The high electrocatalytic activity of NiCo-NP-7.5, although not better than the state-of-the-art RuO2 (Figure S12), is comparable to many high-performance NiCoLDH-based catalysts (Table S1). Additionally, the mass activity of an electrocatalyst is a crucial consideration in commercial utilization.46 As seen in Figure S13, the NiCo-NP-7.50 accomplish a j of 22.5 A·g-1 at the overpotential of 300 mV, which is similar to the benchmark IrO2 electrocatalyst (27.5 A·g-1),41 signifying its high chance of replacing noble metal-based OER catalysts in commercial utilization.


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Figure 2. The electrocatalytic activity of the as-prepared hierarchical NiCo-LDH hollow nanopolyhedra. a) LSV curves, b) Tafel plots, c) charging current density differences △j plotted against scan rates, the linear slope is used to represent electrochemical surface area (ECSA), and d) ECSA and current density at 400 mV overpotential of NiCo-NP-x (x = 2.50, 3.75, 5.00, 6.25, 7.50 and 10.0) samples.

To study the reason behind the high OER activity of hierarchical NiCo-LDH hollow nanopolyhedra, electrochemical surface areas (ECSA) of all the NiCo-NP-x (x = 2.50, 3.75, 5.00, 6.25, 7.50, and 10.0) catalysts are obtained from the typical CV curves with different scan rates (10-60 mV·s-1) (Figure S14), because an increase in ECSA often exposes more electroactive sites and leads to enhancement of catalytic properties.31 As shown in Figure 2c, the ECSA of NiCoNP-x electrodes (x = 2.50, 3.75, 5.00, 6.25, 7.50 and 10.0) are gradually increased. As seen from


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the relationships of current density (j) at 400 mV overpotential with the ECSA in Figure 2d, in the early stage, the current densities of NiCo-NP-x increase as the ECSA increases, further corroborating the crucial role of ECSA for the enhancement of OER activity. However, the NiCo-NP-10.0 electrode with the largest ECSA of 75.77 mF·cm-2 only obtains at current density of 9.7 mA·cm-2, which is far smaller than the 43.1 mA·cm-2 of NiCo-NP-7.50 (59.82 mF·cm-2). Though the NiCo-NP-10.0 has nearly identical nanopolyhedra structure as that of NiCo-NP-7.50, a remarkably lower catalytic activity is obtained. This result exemplifies that the difference in compositions between Ni2+ and Co3+ may also influence their catalytic activity. To evaluate the intrinsic catalytic activity of NiCo-NP-x catalysts, LSV curves normalized by the ECSA are employed (Figure S15).47 They show an obvious difference between the catalysts, especially the NiCo-NP-7.50, which obtains the highest normalized current density (jECSA) of 0.716 mA·cm-2 at the overpotential of 400 mV. It is almost six times higher than the NiCo-NP10.0 (0.130 mA·cm-2), implying the NiCo-NP-x catalysts possess different intrinsic catalytic activity. Additionally, the Tafel slope and onset potential could also be applied to reflect the intrinsic catalytic activity of electrocatalysts.31,48,49 Specifically, the Tafel slope is only affected by the kinetics of the reaction, that is, the type of active sites rather than their quantity, surface area, or geometric structure.14,35 Hence, the lower Tafel slope of NiCo-NP-7.50 (77 mV·dec-1) than that of other NiCo-NP-x catalysts (x = 2.50, 3.75, 5.00, 6.25, and 10.0 with their corresponding Tafel slopes of 91, 86, 92, 78, and 123 mV·dec-1, respectively) signifies the higher electrocatalytic property and superior type of the active sites in NiCo-NP-7.50 (Figure 2b). Moreover, as confirmed by onsetpotential (Figure S16), the NiCo-NP-7.50 shows the lowest onsetpotential (265 mV), far less than the NiCo-NP-10.0 (338 mV), indicating its facile kinetics for OER. These results show that the intrinsic catalytic activity is influenced by the difference of


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composition of the NiCo-NP-x catalysts.

Figure 3. DFT calculation results of OER potential of Ni3Co-LDH and Ni2Co-LDH catalysts. ab) the free-energy landscape of hollow site of Co3+ and Ni2+, and c) the optimized structures of the OER intermediates on the hollow site of Co3+ for Ni2Co-LDH and Ni3Co-LDH, respectively. To further obtain mechanism insight into the OER on the surface of the bimetallic compound of NiCo-LDH, DFT methods were used to calculate the free energy of the intermediates of the electrochemical reactions. The reaction mechanism proposed by Norskov et al.41,42 for water oxidation is listed as follows:


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H2O + * → *OH + H+ + e-

(1)

*OH

(2)

*O

→ *O + H+ + e-

+ H2O → *OOH + H+ + e-

*OOH

→

*

(3)

+ O2 + H+ + e-

(4)

Here, the symbol “*” represents the adsorption site on the surface of the NiCo-LDH catalysts. “*OH”, “*O”, and “*OOH” represent the corresponding adsorbed oxygen-containing intermediate species. Free energy changes for 1-4 (denoted as △G1-△G4) are calculated and △G4 is defined as 4.92 - △G1 - △G2 - △G3 to avoid calculating the energy of O2. The overpotential (η) of OER in this mechanism is defined as η = max {(△G1, △G2, △G3, △G4)/e}-1.23 V. Based on the compositions of NiCo-NP-7.50 (Ni/Co=1.81) and NiCo-NP-10.0 (Ni/Co=2.59), models of Ni2Co-LDH and Ni3Co-LDH are built to simplify the following calculations. (003) facet is chosen as the computational model because it is the dominant exposed surface.40 Four types of adsorption sites (fcc, bridge, top and hollow sites) are taken into account. It is found that step (3) (formation *OOH from *O) is the potential-determining step for both Ni3Co-LDH and Ni2CoLDH (Figure 3, Figure S17 and Figure S18), and their minima △G3 (2.250 and 2.111 eV) at the hollow site of Co3+ corresponds to the overpotential at standard conditions (Table S2). This finding can be well understood as the *OOH has only one unbound electron, which is more suitable for the hollow sites. Furthermore, the independence of Ni2+ and Co3+ active sites for the OER process are calculated on the hollow adsorption sites of the NiCo-LDH surface. Standard free energy diagrams in Figure 3a-b show that Co3+ has a lower determining potential than Ni2+ for both Ni3Co-LDH and Ni2Co-LDH, confirming hollow Co3+ is the preferred active site. As shown in the free-energy landscape of Co3+ site (Figure 3c), an obviously lower overpotential (η = 0.88 V) for Ni2Co-LDH than Ni3Co-LDH (η = 1.02 V) is observed. This result is consistent


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with the electrocatalytic measurements showing that, NiCo-NP-7.50 exhibits a higher elecrocatalytic activity than NiCo-NP-10.0. This result may be ascribed to the relatively greater quantity of Co3+ active sites, which would promote synergistic interactions and lower the ratelimiting step potential (△G3). These results further support the conclusion that ascertaining the preferred Co3+ active sites and further exposing them by engineering functional hierarchical hollow nanostructures, remarkably boosts the OER activity of NiCo-LDH catalysts.

Figure 4. EIS measurements and Voltage-time curve of the as-prepared NiCo-NP-x (x = 2.50, 3.75, 5.00, 6.25, 7.50 and 10.0). a) Nyquist plots at the potential of 0.56 V (vs. Ag/AgCl) over a frequency range from 100 KHz to 0.1 Hz at room temperature. Inset: the corresponding equivalent circuit applies to fit the impedance date. b) voltage-time curve at j = 10 mA·cm-2.

The superior OER activity of catalysts could have also resulted from their higher conductivity.31 Therefore, EIS measurements are performed to investigate the resistance properties of the electrodes (Figure 4a, Figure S19). The semicircle in the high frequency range of NiCo-NP-x electrodes represent charge transfer resistance (Rct), which is related to the resistance to chemical reaction reflected in the intrinsic activity of electrocatalysts. A smaller diameter of the semicircle signifies at lower Rct. The EIS results are fitted to the equivalent


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circuit (Figure 4b), where Rs is the solution resistance and, Rf is the resistance of the catalyst layer which represents the electrical conductivity of the catalyst. The fitting results are displayed in Table S3, revealing that the solution resistance is approximately 10 ~ 11 Ω. The NiCo-NP7.50 electrode has the lowest Rct (35.6 Ω) and Rs (10.0 Ω), which favors its electrocatalytic activity.50 Similar results could also be confirmed by the EIS Nyqusit plots at open circuit potential (Figure S19). The electrochemical stability of catalysts is another crucial evaluation standard for their applications.50 As shown in Figure 4c, the orders of stability at j = 10 mA·cm-2 for NiCo-NP-x (x = 2.50, 3.75, 5.00, 6.25, 7.50 and 10.0) on GC electrodes correspond to their catalytic activity (Figure S10). The NiCo-NP-7.50 displays the best durability, remaining stable over 8 h with 1.48 % loss of potential. The stability of NiCo-NP-7.50 far surpasses the RuO2 (Figure S20). This result can be ascribed to the excellent nanostructure of hierarchical hollow nanopolyhedra, which would be in favor of gas diffusion and decrease the stress caused by the electrode surfaces being blocked by bubbles during OER process. Additionally, the NiCo-NP7.50 catalyst is transferred onto Ni foam to avoid potential physical losses during galvanostatic testing. The OER contribution of Ni foam can be neglected due to its lower activity (Figure S21). The NiCo-NP-7.50 catalyst exhibits a superior stability, which could even stay over 24 h at j = 100 mA.cm-2 (Figure S22). The FESEM image and EDX spectrum (Figure S23) reveal that the overall hierarchical hollow polyhedral nanostructure and Ni/Co ratio are largely retained after the stability test, confirming the excellent stability toward a long-term oxidizing reaction.

4. CONCLUSIONS In summary, hierarchical hollow nanopolyhedra assembled by Ni-Co layered double hydroxide nanosheets are synthesized via a facile self-templated method. The composition and nanostructure of the hierarchical NiCo-LDH hollow nanopolyhedra catalysts could be dually


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tuned by simply adjusting mole ratios of the starting materials. The electrochemical results, combined with DFT calculations, reveal that the Co3+ hollow sites are the preferred adsorption and active sites. Exposing more Co3+ active sites by optimizing the compositions of Ni and Co in NiCo-LDH, enables the NiCo-NP-7.50 catalyst to achieve a 10 mA·cm-2 current density at an overpotential of 314 mV. This work could pave a new path for further understanding of the OER mechanism on the surface of NiCo-LDH.

ASSOCIATED CONTENT


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Supporting information for publication

Formation mechanism of hierarchical NiCo-LDH hollow nanopolyhedra (Figure S1). XRD pattern, FESEM images, and TEM image of the Co5(OH)2(CH3COO)8·2H2O nanopolyhedra (Figure S2). FESEM images, TEM image, and HAADF-STEM image (Figure S3), BET measurement (Figure 4), FTIR spectrum (Figure S5), and XPS spectra (Figure S6) of the NiCo-NP-7.50 samples. FESEM images of NiCo-NP-x (x = 2.50, 3.75, 5.00, 6.25, 7.50 and 10.0) (Figure S7). EDX spectra of NiCo-NP-7.50 and NiCo-NP-10.0 (Figure S8). XRD pattern of the NiCo-NP-10 sample (Figure S9). Overpotential at j = 10 mA·cm-2 of the NiCo-NP-x (x = 2.50, 3.75, 5.00, 6.25, 7.50 and 10.0) samples (Figure S10). FESEM images and LSV curves of bulk NiCo-LDH, Co(OH)2 and Ni(OH)2 (Figure S11). LSV curves of RuO2 and NiCo-NP-7.50 (Figure S12). Mass activity of the NiCo-NP-7.50 (Figure S13). Cyclic voltammetry (CV) curves with different scan rates (Figure S14), the normalized electrocatalytic activity (Figure S15), and onsetpotential (Figure S16) of NiCo-NP-x (x = 2.50, 3.75, 5.00, 6.25, 7.50 and 10.0) samples. The DFT calculation results of Ni2Co-LDH (Figure S17) and Ni3Co-LDH


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(Figure S18). EIS Nyqusit plots of the NiCo-NP-x catalysts at open circuit potential (Figure S19). Voltage-time curves of the RuO2 and NiCo-NP-7.50 catalyst at j = 10 mA.cm-2 on GC electrodes (Figure S20). LSV curve of the pure Ni foam (Figure S21). Voltage-time curves of the as-prepared NiCo-NP-7.50 catalyst at j = 10 mA.cm-2 and j = 100 mA.cm-2 on Ni foam (Figure S22). FESEM and EDX spectrum of the NiCo-NP-7.50 after OER stability tests (Figure S23). Comparison of OER activity of recent reported NiCo-LDH electrocatalysts (Table S1). The calculated reaction free energies of ratelimiting step (△G*OOH) on various sites (Top, Bridge, Fcc and Hollow sites) of Ni3Co-LDH and Ni2Co-LDH (Table S2). The fitting results of EIS (Table S3). These materials are available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *Corresponding author: Xin Zhou, E-mail: [email protected]; Fax: (+86)-451-86413753. Gang Chen, E-mail: [email protected]; Fax: (+86)-451-86413753. ORCID: 0000-0003-1502-0330

Notes

The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21471040, 21303030 and 21871066). Authors Shanfu Sun, Chade Lv, Weizhao Hong, Fugui Wu and Gang Chen received funding from National Natural Science Foundation Grant 21471040. Authors Shanfu Sun, Xin Zhou and Gang Chen received funding from National Natural Science Foundation Grant 21303030 and 21871066.


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