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Design of Hierarchical NiCo2O4 Nanocages with Excellent Electrocatalytic Dynamic for Enhanced Methanol Oxidation

Although sheet-like materials have good electrochemical properties, they still suffer from agglomeration problems during the electrocatalytic process. Integrating two-dimensional building blocks into a hollow cage-like structure is considered as an effective way to prevent agglomeration. In this work, the hierarchical NiCo2O4 nanocages were successfully synthesized via coordinated etching and precipitation method combined with a post-annealing process. The nanocages are constructed through the interaction of two-dimensional NiCo2O4 nanosheets, forming a three-dimensional hollow hierarchical architecture. The three-dimensional supporting cavity effectively prevents the aggregation of NiCo2O4 nanosheets and the hollow porous feature provides amounts of channels for mass transport and electron transfer. As an electrocatalytic electrode for methanol, the NiCo2O4 nanocages-modified glassy carbon electrode exhibits a lower overpotential of 0.29 V than those of NiO nanocages (0.38 V) and Co3O4 nanocages (0.34 V) modified glassy carbon electrodes. The low overpotential is attributed to the prominent electrocatalytic dynamic issued from the three-dimensional hollow porous architecture and two-dimensional hierarchical feature of NiCo2O4 building blocks. Furthermore, the hollow porous structure provides sufficient interspace for accommodation of structural strain and volume change, leading to improved cycling stability. The NiCo2O4 nanocages-modified glassy carbon electrode still maintains 80% of its original value after 1000 consecutive cycles. The results demonstrate that the NiCo2O4 nanocages could have potential applications in the field of direct methanol fuel cells due to the synergy between two-dimensional hierarchical feature and three-dimensional hollow structure.


Introduction
The ever-worsening energy and global warming issues have triggered significant research efforts in the design and development of advanced energy devices. Direct methanol fuel cells (DMFCs) have exhibited great commercialization potential credited to high energy density, low cost, easy storage and low pollutant emissions [1,2]. Generally, the performance of DMFCs was mainly related to the activity of methanol electrocatalyst [3]. Traditionally, platinum group precious metals (Pt, Ru, and Pd, etc.) were always employed as electrocatalysts for methanol. Although high electrocatalytic activity was achieved, the precious metals still suffer from high cost and low working stability [4][5][6]. In this regard, the design of Pt-free catalysts was considered as the best alternative to solve the problems.
Transition metal oxides (TMOs) were recognized as ideal substitutions for noble metals due to their high-active redox sites, low cost and high physicochemical stability. Over the last decade, significant efforts on TMOs have been made to obtain high-performance Pt-free electrocatalysts for methanol [7]. Generally, the nanomaterials in conventional forms of aggregated particles generally have no significant advantages in electrocatalysis. Inspired by kinetics, quantities of TMOs with different microstructures were constructed to improve electrocatalytic kinetics and high electrocatalytic activity was obtained. Thereinto, two-dimensional (2D) nanosheets were demonstrated as ideal structure in electrocatalysis due to the unique physicochemical properties issued from high structural and morphologic anisotropies [8]. However, agglomeration of 2D nanosheets was easy to occur in electrocatalytic reactions because of the large lateral specific surface areas, leading to the decrease of active sites and diffusion channels.
Integrating amounts of 2D nanosheets into three-dimensional (3D) hierarchical nanocages provided an efficient way to obtain highly active structures. The hierarchical nanocages effectively prevented the aggregation of 2D building blocks and afforded large specific surface areas, which provided sufficient active sites for electrooxidation of methanol [9]. Meanwhile, the pores formed by the interaction of nanosheets not only provided diffusion channels for methanol and intermediate products, but also relieved the volume change and structural strain during electrocatalysis, resulting in excellent stability [10]. Further, the 2D feature of building blocks and porous thin shell of hierarchical nanocages accelerated both the collected and transfer efficiency of catalytic electrons during electrocatalysis, leading to high electrocatalytic activity. Therefore, highly active and stable methanol electrocatalysts can be acquired through the design of hierarchical porous hollow nanocages. NiCo 2 O 4 possesses bimetallic active sites (Co 2+ /Co 3+ and Ni 2+ /Ni 3+ ) and excellent conductivity, exhibiting potential applications in the field of methanol oxidization [11]. In this report, NiCo 2 O 4 nanocages (NCs) were prepared by coordinated etching and precipitation (CEP) method combined with a post-annealing process. As an electrode for methanol electrooxidation, NiCo 2 O 4 NCs-modified glassy carbon electrode (GCE) exhibited low overpotential, high current density and excellent stability.

Preparation of NiCo 2 O 4 NCs
Cu 2 O templates were firstly prepared according to our previous work [12]. Simply, 10 mL NaOH solution (2 M) was added into 100 mL of CuCl 2 ·2H 2 O (0.01 M) and stirred at 55 • C for 30 min. Then, 10 mL of AA (0.6 M) was added. After 3 h of reaction, Cu 2 O cubes were collected and dried in vacuum.
A total of 10 mg cubic Cu 2 O, 1mg NiCl 2 ·6H 2 O and 2 mg CoCl 2 ·6H 2 O were dispersed into 10 mL ethanol/water (1:1), and then, 0.33 g PVP was added and stirred for 30 min. Afterwards, 4 mL Na 2 S 2 O 3 ·5H 2 O solution (1 M) was slowly dropped at room temperature. After 3 h, hydroxide precursors were collected and dried in vacuum. Finally, the precursors were calcined using a tube furnace at 400 • C in air for 2 h with a heating rate of 1 • C min −1 . Co 3 O 4 NCs and NiO NCs were respectively prepared as contrast samples using CoCl 2 ·6H 2 O and NiCl 2 ·6H 2 O only in the CEP process.

Electrochemical Measurements
Cyclic voltammetry (CV), chronoamperometry and electrochemical impedance spectroscopy (EIS) were performed in 1 M KOH solution on CH1760E A191018 electrochemical workstation at room temperature. A three-electrode system was used with Ag/AgCl Nanomaterials 2021, 11, 2667 3 of 10 (saturated with KCl) and platinum disk (Φ = 2 mm) as the reference and counter electrodes, respectively. The Co 3 O 4 NCs, NiO NCs and NiCo 2 O 4 NCs-modified glassy carbon electrodes (GCE, Φ = 3 mm) were applied as working electrodes. Typically, GCE was carefully polished with 3 µm, 0.5 µm and 0.05 µm alumina powders, respectively. Then, 5 µL of the prepared sample suspension (1 mg mL −1 in 0.1% Nafion solution) was measured with a pipette and dropped onto the surface of GCE, and then dried naturally.

Materials Characterization
The microstructures and morphologies of the samples were observed by field emission electron microscope (FESEM, SU8020) and high-resolution transmission electron microscope (HRTEM, FEI F20). The crystal structure and elemental composition were recorded by X-ray powder diffractometer (XRD, Rigaku D/Max-2400 using Cu-Ka radiation λ = 1.54 Å). The chemical state was determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) using a 500 µm X-ray spot (energy resolution 0.4 eV). The Brunauer-Emmett-Teller (BET, Belsort-max) was applied to analyze the specific surface area and pore structure.

Characterization
As shown in Figure 1a, Co 2+ (1)). The part-hydrolyzation of S 2 O 3 2− also facilitated the supply of OH − (reaction (2)) [14]. Reactions (1) and (2) concurrently pushed reaction (3) forward, facilitating the formation of Ni-Co hydroxide precursor. The diffusion of S 2 O 3 2− from the surface into the interior of the shell directly affected the rate of Cu 2 O etching, while the transport of OH − from internal to external sites promoted the growth of Ni-Co hydroxide precursor [15]. The two reaction processes coordinated together to achieve dynamic balance to promote the formation of hollow structure. In order to confirm the formation mechanism, the precipitate prepared at 0, 10, 20, 30, and 180 min was collected and observed by TEM (Figure 1b). With the introduction of S 2 O 3 2− , Cu 2 O was gradually etched into the polyhedral structure due to the higher diffusion intensity of ions at the corners [15,16]. After the reaction lasted for 3 h, Cu 2 O completely disappeared and hierarchical porous nanocages were obtained ( Figure S1). Finally, NiCo 2 O 4 NCs were obtained through the annealing of Ni-Co hydroxide precursor (reaction (4)). As observed in Figure 1c, the color of the reaction system gradually became shallow and the light green precipitates generated at the same time. The fading was attributed to the etching of Cu 2 O, while the green precipitates were correlated to the formation of Ni-Co hydroxide precursor.
As shown in Figure 2a, the strong peaks from the templates at 30 • , 37 • , 42 • , 62 • , 74 • and 78 • matched well with PDF#77-0199 of cubic Cu 2 O. As observed in Figure 2b, no significant diffraction peaks were observed in the precursor, revealing poor crystallinity of Ni-Co hydroxide precursor. After calcination, the crystallinity of materials was obviously improved and the diffraction peaks at 36 • , 43 • , 64 • , 75 • and 77 • were well indexed to the (111), (200), (220), (311) and (400) crystal planes of face-centered cubic NiCo 2 O 4 . XRD results clearly demonstrated the formation of high purity NiCo 2 O 4 product. Furthermore, XPS measurements were performed to obtain detailed information on the elements and oxidation state of prepared NiCo 2 O 4 . As shown in Figure 2c, the survey spectrum displayed a series of strong peaks related to Ni, Co, O and C species, indicating the main chemical elements of the NiCo 2 O 4 . In Figure 2d, two states of Co 2+ and Co 3+ were clearly observed according to Gaussian fitting. Specifically, the fitting peaks at 779.3 eV and 794.3 eV were ascribed to Co 3+ . Another two fitting peaks at 781.0 eV and 795.8 eV were ascribed to Co 2+ [17]. Analogously, the Ni 2p spectra included two kinds of nickel species of Ni 2+ and Ni 3+ in Figure 2e. The fitting peaks at 854.0 eV and 871.7 eV were ascribed to Ni 2+ , while the fitting peaks at 855.9 eV and 873.9 eV were related to Ni 3+ [18]. As shown in Figure 2f, the fine spectrum of O 1s displayed three peaks originated from M-O-M, C-O=C and O=C. The fitting peak of M-O-M at 528.5 eV was the typical metal-oxygen bond [19]. C-O=C at a binding energy of 530.5 eV corresponded to the high number of defect sites containing low oxygen coordination [20]. O=C at a binding energy of 531.7 eV could be ascribed to the multiplicity of physisorbed water at and within the surface [17,21]. The results of XPS demonstrated a mixed valence containing Co 2+ , Co 3+ , Ni 2+ and Ni 3+ , which was consistent with previous reports [22]. The complex electronic states of Ni 2+ /Ni 3+ and Co 2+ /Co 3+ could afford enough active sites for methanol oxidation, which may be one of the important factors contributing to the high electrocatalytic performance.
As shown in Figure 2a, the strong peaks from the templates at 30°, 37°, 42°, 62°, 74° and 78° matched well with PDF#77-0199 of cubic Cu2O. As observed in Figure 2b, no significant diffraction peaks were observed in the precursor, revealing poor crystallinity of Ni-Co hydroxide precursor. After calcination, the crystallinity of materials was obviously improved and the diffraction peaks at 36°, 43°, 64°, 75° and 77° were well indexed to the (111), (200), (220), (311) and (400) crystal planes of face-centered cubic NiCo2O4. XRD results clearly demonstrated the formation of high purity NiCo2O4 product. Furthermore, XPS measurements were performed to obtain detailed information on the elements and oxidation state of prepared NiCo2O4. As shown in Figure 2c, the survey spectrum displayed a series of strong peaks related to Ni, Co, O and C species, indicating the main The surface morphologies of the CuO 2 templates, NiO NCs and Co 3 O 4 NCs were examined and displayed in Figure 3. As shown in Figure 3a, the Cu 2 O templates were uniformly dispersed, which was conducive to the adsorption of Ni 2+ and Co 2+ . The surface of Cu 2 O was smooth and the edge size was about 500 nm ( Figure 3b). As observed in Figure 3c, the NiO cube was composed of a large number of stacked nanoparticles, and the NiO cube had nearly the same size compared to Cu 2 O. As shown in Figure 3d, the NiO cube displayed a hollow structure with a wall thickness of about 80 nm. Similarly, the Co 3 O 4 cube also displayed a hollow cubic feature (Figure 3e). However, the Co 3 O 4 NCs mainly consisted of a large number of stacked nanosheets, forming a network structure (Figure 3f). was consistent with previous reports [22]. The complex electronic states of Ni 2+ /Ni 3+ and Co 2+ /Co 3+ could afford enough active sites for methanol oxidation, which may be one of the important factors contributing to the high electrocatalytic performance. The surface morphologies of the CuO2 templates, NiO NCs and Co3O4 NCs were examined and displayed in Figure 3. As shown in Figure 3a, the Cu2O templates were uniformly dispersed, which was conducive to the adsorption of Ni 2+ and Co 2+ . The surface of Cu2O was smooth and the edge size was about 500 nm ( Figure 3b). As observed in Figure  3c, the NiO cube was composed of a large number of stacked nanoparticles, and the NiO cube had nearly the same size compared to Cu2O. As shown in Figure 3d, the NiO cube displayed a hollow structure with a wall thickness of about 80 nm. Similarly, the Co3O4 cube also displayed a hollow cubic feature (Figure 3e). However, the Co3O4 NCs mainly consisted of a large number of stacked nanosheets, forming a network structure ( Figure  3f).
As observed in Figure 4a, the uniformly distributed Ni-Co hydroxide precursor accurately replicated the cubic structure of Cu2O templates and had an edge size of 500 nm. As shown in Figure 4b, the surface of Ni-Co hydroxide precursor was composed of a large number of interacted nanosheets and formed a network structure. In addition, the precursor displayed a cage-like structure and the shell thickness was about 200 nm. After calcination, the nanosheets on the surface became thicker, more compact and the thickness of the shell reduced to about 100 nm (Figure 4c). Notably, the crinkly nanosheets structure was clearly investigated in Figure 4d  (400) planes of spinel NiCo2O4, respectively. The results were consistent with the XRD analysis and a previous report [22]. On the basis of the above discussion, NiCo2O4 NCs were constructed by the combination of the CEP method and post calcination. The highly porous structure provided sufficient active sites and mass transport channels, which are beneficial for electrocatalytic kinetics, leading to high electrocatalytic activity [23,24].   As observed in Figure 4a, the uniformly distributed Ni-Co hydroxide precursor accurately replicated the cubic structure of Cu 2 O templates and had an edge size of 500 nm. As shown in Figure 4b, the surface of Ni-Co hydroxide precursor was composed of a large number of interacted nanosheets and formed a network structure. In addition, the precursor displayed a cage-like structure and the shell thickness was about 200 nm. After calcination, the nanosheets on the surface became thicker, more compact and the thickness of the shell reduced to about 100 nm (Figure 4c). Notably, the crinkly nanosheets structure was clearly investigated in Figure 4d  The results were consistent with the XRD analysis and a previous report [22]. On the basis of the above discussion, NiCo 2 O 4 NCs were constructed by the combination of the CEP method and post calcination. The highly porous structure provided sufficient active sites and mass transport channels, which are beneficial for electrocatalytic kinetics, leading to high electrocatalytic activity [23,24].

Electrocatalytic Activity of NiCo2O4 NCs/GCE Towards Methanol
The electrocatalytic activity of NiCo2O4 NCs/GCE and the contrast samples was detailly evaluated by CV and EIS. Figure 5a shows the CV curves of the three electrodes in 1 M KOH in the absence of methanol. The distinct pairs of redox peaks were observed in all the three CV curves. The redox peaks of NiO NCs/GCE corresponded to the reversible transition of Ni ions, such as Ni 2+ /Ni 3+ [25]. Similarly, the redox peaks of Co3O4 NCs/GCE were attributed to the transition between Co 2+ /Co 3+ or Co 3+ /Co 4+ [26]. The CV curve of NiCo2O4 NCs/GCE exhibited a much larger enclosed area than those of the Co3O4 NCs/GCE and NiO NCs/GCE. This may be due to the fact that NiCo2O4 was generally regarded as a binary TMO, which has more complicated redox electrical pairs [27]. As displayed in Figure 5b, the electrocatalytic current towards methanol on the NiCo2O4 NCs/GCE, Co3O4 NCs/GCE and NiO NCs/GCE can be clearly observed compared to

Electrocatalytic Activity of NiCo 2 O 4 NCs/GCE towards Methanol
The electrocatalytic activity of NiCo 2 O 4 NCs/GCE and the contrast samples was detailly evaluated by CV and EIS. Figure 5a shows the CV curves of the three electrodes in 1 M KOH in the absence of methanol. The distinct pairs of redox peaks were observed in all the three CV curves. The redox peaks of NiO NCs/GCE corresponded to the reversible transition of Ni ions, such as Ni 2+ /Ni 3+ [25]. Similarly, the redox peaks of Co 3 O 4 NCs/GCE were attributed to the transition between Co 2+ /Co 3+ or Co 3+ /Co 4+ [26]. The CV curve of NiCo 2 O 4 NCs/GCE exhibited a much larger enclosed area than those of the Co 3 O 4 NCs/GCE and NiO NCs/GCE. This may be due to the fact that NiCo 2 O 4 was generally regarded as a binary TMO, which has more complicated redox electrical pairs [27]. As displayed in Figure 5b, the electrocatalytic current towards methanol on the NiCo 2 O 4 NCs/GCE, Co 3 O 4 NCs/GCE and NiO NCs/GCE can be clearly observed compared to Figure 5a, demonstrating that all the three electrodes showed catalytic activity towards methanol. Notably, the NiCo 2 O 4 NCs/GCE presented a larger catalytic current than the other two electrodes. With the potential rising to 0.45 V, the current of NiCo 2 O 4 NCs/GCE was 3.16 and 9.11 times that of Co 3 O 4 NCs/GCE and NiO NCs/GCE, respectively. In addition, the onset potential towards methanol oxidation on the NiCo 2 O 4 NCs/GCE was about 0.29 V ( Figure S2, which was lower than those of Co 3 O 4 NCs/GCE (0.34 V, Figure S3) and NiO NCs/GCE (0.38 V, Figure S4), revealing higher electrocatalytic activity. As shown in Figure 5c, the EIS was carried out in 1 M KOH containing 0.5 M methanol and the equivalent circuit is displayed in the insert. In the circuit, R s , C, R ct and Z w were the internal resistance, redox capacitance, charge transfer resistance and Warburg resistance, respectively [28,29]. Notably, the R ct value of NiCo 2 O 4 NCs/GCE (4.4 KΩ) was obviously lower than those of Co 3 O 4 NCs/GCE (10.8 KΩ and NiO NCs/GCE (18.3 KΩ), indicating fast electron transfer rate within the electrode or at the electrode/electrolyte interface. The lower charge transfer resistance was related to the anisotropic feature of building blocks and relatively high conductivity of NiCo 2 O 4 . At low frequencies, the NiCo 2 O 4 NCs/GCE displayed larger Z w than Co 3 O 4 NCs/GCE and NiO NCs/GCE, revealing lower ion diffusion resistance. The lower ion diffusion resistance might be attributed to ample diffusion channels afforded by the interacted NiCo 2 O 4 nanosheets. In order to support the kinetics analysis of EIS, the surfaces area and porosity of NiCo 2 O 4 NCs were tested Nanomaterials 2021, 11, 2667 7 of 10 by BET. In Figure 5d, the curve presents a H 3 -type hysteric loop in the range of 0.45-1.0, indicating a typical mesoporous characteristic [30,31]. The mean pore size of NiCo 2 O 4 NCs/GCE was around 9 nm, which was ideal for the diffusion of methanol [32]. Moreover, the specific surface area and pore volume were 38.3 m 2 g −1 and 0.2 cm 3 g −1 , respectively, which were both higher than those of the precursor (30.0 m 2 g −1 , 0.1 cm 3 g −1 , Figure S5). The large specific surface area provided abundant active sites for methanol catalysis, and the appropriate pore volume provided ordered diffusion channels for rapid transport [33]. In short, NiCo 2 O 4 NCs/GCE exhibited rich redox active sites and transmission channels, leading to excellent electrocatalytic activity.
tron transfer rate within the electrode or at the electrode/electrolyte interface. The lower charge transfer resistance was related to the anisotropic feature of building blocks and relatively high conductivity of NiCo2O4. At low frequencies, the NiCo2O4 NCs/GCE displayed larger Zw than Co3O4 NCs/GCE and NiO NCs/GCE, revealing lower ion diffusion resistance. The lower ion diffusion resistance might be attributed to ample diffusion channels afforded by the interacted NiCo2O4 nanosheets. In order to support the kinetics analysis of EIS, the surfaces area and porosity of NiCo2O4 NCs were tested by BET. In Figure  5d, the curve presents a H3-type hysteric loop in the range of 0.45-1.0, indicating a typical mesoporous characteristic [30,31]. The mean pore size of NiCo2O4 NCs/GCE was around 9 nm, which was ideal for the diffusion of methanol [32]. Moreover, the specific surface area and pore volume were 38.3 m 2 g −1 and 0.2 cm 3 g −1 , respectively, which were both higher than those of the precursor (30.0 m 2 g −1 , 0.1 cm 3 g −1 , Figure S5). The large specific surface area provided abundant active sites for methanol catalysis, and the appropriate pore volume provided ordered diffusion channels for rapid transport [33]. In short, NiCo2O4 NCs/GCE exhibited rich redox active sites and transmission channels, leading to excellent electrocatalytic activity. The chronoamperometry is an effective tool to investigate electrochemical stability of the electrocatalyst. As shown in Figure 6a, the electrochemical stability of the NiCo 2 O 4 NCs/GCE, Co 3 O 4 NCs/GCE and NiO NCs/GCE for methanol oxidation at 0.45 V was investigated. Notably, the NiCo 2 O 4 NCs/GCE displayed largest electrocatalytic current towards 0.5 M methanol. The current of NiCo 2 O 4 NCs/GCE displayed a decrease at the initial stage due to poisoning of the intermediates, and then kept a relatively steady value until 1100 s [34,35]. The final current still maintained 85% of its original value, which was three times of the Co 3 O 4 NCs/GCE and thirteen times of the NiO NCs/GCE. The CV tests were carried out for 1000 cycles to further investigate the stability of NiCo 2 O 4 NCs/GCE. The maximum current density presented an 8% decrease at the 500th cycle, and maintained 80% of the initial value after 1000 cycles. The hierarchical porous structure provided sufficient interspaces for accommodation of volume change and structural strain during electrocatalysis, resulting in excellent long-term stability towards methanol. three times of the Co3O4 NCs/GCE and thirteen times of the NiO NCs/GCE. The CV tests were carried out for 1000 cycles to further investigate the stability of NiCo2O4 NCs/GCE. The maximum current density presented an 8% decrease at the 500th cycle, and maintained 80% of the initial value after 1000 cycles. The hierarchical porous structure provided sufficient interspaces for accommodation of volume change and structural strain during electrocatalysis, resulting in excellent long-term stability towards methanol.

Conclusions
In summary, the NiCo2O4 NCs were successfully synthesized through the CEP method combined with a post-annealing process. The designed NiCo2O4 NCs were constructed through the interaction between NiCo2O4 NSs and formed a hierarchical cagelike structure. As a catalytic electrode for methanol oxidation, the NiCo2O4 NCs/GCE exhibited high electrocatalytic activity in terms of low onset potential (0.29 V) and excellent long-term stability (80% after 1000 cycles). It is demonstrated that the NiCo2O4 NCs/GCE was an ideal electrode for DMFCs and the design of hollow hierarchical structure was an effective method to obtain highly active 2D electrocatalysts.

Supplementary Materials:
The Supporting Information is available free of charge on the MDPI Publications website at www.mdpi.com/xxx/s1, Figure S1: XPS survey of NiCo2O4, Figure S2: CV curves of NiCo2O4 NCs/GCE in 1 M KOH without methanol and with 0.5 M methanol at 50 mV s −1 , Figure  S3: CV curves of Co3O4 NCs/GCE in 1 M KOH without methanol and with 0.5 M methanol at 50 mV s −1 , Figure S4: CV curves of NiO NCs/GCE in 1 M KOH without methanol and with 0.5 M methanol at 50 mV s −1 , Figure S5: N2 adsorption-desorption isotherms of the Ni-Co hydroxide precursors.

Conclusions
In summary, the NiCo 2 O 4 NCs were successfully synthesized through the CEP method combined with a post-annealing process. The designed NiCo 2 O 4 NCs were constructed through the interaction between NiCo 2 O 4 NSs and formed a hierarchical cage-like structure. As a catalytic electrode for methanol oxidation, the NiCo 2 O 4 NCs/GCE exhibited high electrocatalytic activity in terms of low onset potential (0.29 V) and excellent long-term stability (80% after 1000 cycles). It is demonstrated that the NiCo 2 O 4 NCs/GCE was an ideal electrode for DMFCs and the design of hollow hierarchical structure was an effective method to obtain highly active 2D electrocatalysts.  Figure S5: N 2 adsorption-desorption isotherms of the Ni-Co hydroxide precursors.