Nanoneedle-forest NiCo2O4@nitrogen-doped Carbon-Mo2C/CC as Double Functional Electrocatalysts for Water Hydrolysis and Zn-air Batteries

China Abstract The construction of efficient and stable non-precious metal electrocatalysts for water hydrolysis and metal-air batteries is a key to tackle the energy shortage and environmental pollution. Herein, a composite consisting of needle-forest nickel cobalt oxide and nitrogen-doped carbon (NDC)-molybdenum carbide loaded on carbon cloth (NiCo 2 O 4 @ NDC - Mo 2 C/CC ) is synthesized from the hydrothermal method. The introduction of nitrogen-doped carbon and carbon cloth improves the electrical conductivity of the composite, the existence of NDC - Mo 2 C on CC makes NiCo 2 O 4 produce generous oxygen vacancies. Therefore, NiCo 2 O 4 @ NDC - Mo 2 C/CC shows the low overpotentials of HER (120 mV) and OER (220 mV) @10 mA cm -2 and excellent performances for overall water electrolysis in alkaline medium. Also, NiCo 2 O 4 @ NDC - Mo 2 C/CC exhibits robust oxygen reduction reaction (ORR) performance and a large specific capacity of 778 mAh g − 1 and energy density of mW cm − 2 as air cathode in Zn-air batteries. This work enriches knowledge to explore multifunctional electrocatalysts to replace noble metallic catalysts. between NiCo 2 O 4 @NCD - Mo 2 C/CC and Pt/C+IrO 2 the ideal reversible property of the as - prepared bifunctional electrode. These results indicate that the NiCo 2 O 4 @NCD - Mo 2 C/CC has high ORR activity potential as ideal electrocatalysts for Zn - air batteries. 2 - driven Zn - air battery showed a decreased efficiency from 56.02% to 52.74% discharged for just 220 cycles. These results prove that NiCo 2 O 4 @NCD - Mo 2 C/CC has promising practical applications in many fields.


Introduction
Water splitting devices, fuel cells, and rechargeable metal-air batteries have been regarded as the promising ways to solve energy shortage and environmental pollution [1,2]. However, exploiting high-active and efficient electrocatalysts with low overpotentials in hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is still a big challenge [3].
Currently, platinum (Pt), palladium (Pd), ruthenium oxide (RuO2) and iridium dioxide (IrO2) based noble metallic catalysts have been commercially used as the ideal electrocatalysts for HER, OER and ORR due to their low overpotential.
However, limited by the high cost and low abundance, they have not yet been widely applied in industrial applications [4,5].
Notably, most of the as-synthesized electrocatalysts possess high activity for only single reaction in one electrolyte. In the case two groups of catalysts need being coupled, the incompatible cooperation generally causes reduction in the water hydrolysis efficiency [15,16]. In addition, active ORR catalysts usually show poor OER performances and vice versa. Therefore, design of multifunctional catalysts possessing high activity towards HER, OER and ORR are highly necessary [17].
The Co3O4 [18] and Co-based transition metal oxides [19] have attracted considerable attention and are investigated as a group of high-performance OER electrocatalyst because of their high electrochemical activity, rich oxidation state as well as good durability. Among them, NiCo2O4 has drawn much research interest [20,21], since the Ni ion partially replaces the octahedral position of trivalent Co ion in spinel structure, which improves the electric conductivity and promotes OER kinetics. Recently, due to the multiple-step redox transformation (Co 3+ /Co 2+ , Ni 3+ /Ni 2+ ) researchers found that NiCo2O4 exhibit high catalytic activity for ORR [22]. Xiao et al. [23] prepared NiCo2O4/CNTs which exhibits a superior OER and ORR catalytic activity.
The intrinsic activity and the quantity of active sites are regarded as two crucial factors affecting catalytic activity [24], which can be improved by regulating the chemical composition, physical size, micro-scale morphology and the overall crystallinity of electrocatalysts. Among them, increase of the surface area of electrocatalyst by designing three-dimensional (3D) nanostructure has been an efficient and simply way to enhance the number of active sites for improvement of electrochemical activities [25].
In this paper, in order to form 3D NiCo2O4 nano-structure and increase the surface area, a conductive carbon cloth (CC) was used as the support for growth of NiCo2O4 nanocatalyst. Meanwhile, to enhance the HER catalytic performance of NiCo2O4, the CC was pre-coated by a layer of molybdenum carbide (Mo2C) [26,27]. Of note, the Mo2C easily tends to disproportionate growth and aggregation during synthesis, which generally greatly weakens its catalytic activity. Thus, the Mo2C was composited with polyaniline (PANI) nanofibers before in-situ anchored on CC.
Under calcination of the NiCo2O4@PANI-Mo2C/CC-based electrode at high temperature, PANI was decomposed into nitrogen-doped carbon (NDC), and the NiCo2O4@nitrogen-doped carbon-Mo2C/CC (NiCo2O4@NDC-Mo2C/CC) catalyst was finally obtained. The coating of NDC-Mo2C between CC and NiCo2O4 makes NiCo2O4 nanoneedles grow thinner, resulting in more oxygen vacancies formed in the nanoneedle-forest NiCo2O4. The electric conductivity of Mo2C is also enhanced by compositing with NDC, which significantly promote the OER and HER catalysis of the system under the cooperation of NiCo2O4 and NDC-Mo2C. In addition, the as-prepared catalyst shows obvious ORR performance capable of being used in both water hydrolysis and Zn-air batteries (Scheme 1). Scheme 1 Schematic illustration of nanoneedle-forest NiCo2O4@NDC-Mo2C/CC for water hydrolysis and Zn-air batteries.

Preparation of PANI
Polyaniline was prepared by water/organic interface method. Firstly, aniline is distilled under vacuum. Then, 0.1 ml of aniline was cast in 30 ml of toluene solvent, and the solution A was obtained by stirring vigorously for 2 hours at room temperature. Afterwards, take a beaker containing 30 ml of 1.0 M HCl solution, and add 0.25 mmol (0.05705 g) of ammonium persulfate to it. Stirred for 10 minutes, then an inorganic phase of solution B was obtained. Mixing of solution A and B in a 100 ml beaker gives rise to formation of an interface system, which was reacted for 24 hours at 5 °C. The created precipitates are filtered and washed several times by deionized water, methanol and toluene, and then dried in a vacuum oven at 50 °C for 48 hours. The as-obtained PANI product shows nanofiber morphology observed by SEM image (Fig. S5a), and the diameter is around 55 nm confirmed by TEM image shown in Fig. S5b.

Synthesis of NDC-Mo2C/CC
Typically, 3 mmol of (NH4)6Mo7O24·4H2O was dissolved in 35 ml of distilled water and then 1.76 gram of polyaniline was added under magnetic stirring for 5 minutes. Meanwhile, 4.494 mmol of C16H33(CH3)3NBr was dissolved in 180 ml distilled water. After 10 minutes the above two solutions were mixed by stirring and then put aside for 12 hours at room temperature. Afterwards, the viscous emulsion at the bottom was transferred to a 50 ml autoclave. At the same time, a piece of cleaned 4 cm × 5 cm carbon cloth was immersed into the solution and maintained at 200 °C for 12 hours. The obtained PANI/Mo2C/CC was washed with distilled water and dried at 50 °C for 2 days. Then, the dry PANI/Mo2C/CC was placed into a tube furnace, annealed at 1000 °C for 6 hours in N2. In the high temperature calcination, the PANI was decomposed to nitrogen-doped carbon (NDC) segments, which were composited on the lattice of Mo2C. After cooled naturally, NDC-Mo2C/CC composites were obtained.
Then, the mixture was placed in a Teflon-lined autoclave (100 ml) in which a sheet of NDC-Mo2C/CC was immersed, and then reacted at 120 °C for 2 hours. After cooled to room temperature, the NiCo2O4@NDC-Mo2C/CC sheet was taken out and washed by using ethanol and distilled water. Upon drying at 60 °C for 24 hours, the NiCo2O4@NDC-Mo2C/CC was annealed at 600 °C in N2 atmosphere for 3 hours, and then cooled naturally. For comparison, the NiCo2O4/CC and NDC-Mo2C/CC composites were also synthesized according to a similar procedure.

Material characterization
Surface morphology of the samples was monitored by using a JEOL S4800 scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDX) detector operated at 200 kV. The transmission electron microscope (TEM) images were obtained on a JEM-2100 plus machine. The crystallinity was examined by using X-ray diffraction (XRD) on a Rigaku D/MAX-2400 powder diffractometer employing nickel-filtered copper radiation (Cu Kα, λ=1.5406 Å) as a monochromatic detector in a 2θ range of 10-80 o . Raman measurements were carried out on an ALMEGA Dispersive Raman spectrometer equipped with a laser emitting at 532 nm. X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB 250 spectrometer with an Al Kα X-ray source (1486.6 eV). All binding energies were calibrated based on C 1s signal for carbon at 284.5 eV. The electrochemical analysis was conducted employing a CHI660E workstation (Shanghai Chenhua). Electrochemical impendence spectroscopy (EIS) of the samples was investigated in the frequency range from 100000 Hz to 0.1 Hz at open circuit potential with an AC perturbation of 5 mV. The electrical conductivity of the samples was measured by using the standard four-probe approach at room temperature. Five measurements were generally carried out for each sample to supply the averaged value. The surface area of samples was detected by a Micromeritics TriStar 3020 machine.

Electrochemical measurement
In this paper, the electrochemical properties of the as-prepared NiCo2O4@NDC-Mo2C/CC were used for the anode, electrolyte and the air electrode, respectively. Battery testing and cycling experiments were performed on CT3008W battery testing system (Neware, China).

Results and Discussion
X-ray diffraction (XRD) experiment was carried out to find the crystalline character of the prepared electrocatalysts, as shown in Fig. 1a. From Fig. 1a, the broad feature at around 2θ = 26° corresponds to the c-characteristic peak [28]   can be fitted with two peaks, and the peak at binding energy of 529.7 eV is associated with metal oxygen bond (O1) [36], while the other peak at 531.5 eV is attributed to the chemisorbed oxygen at the oxygen vacancy (O2) [37−39]. The peak area ratio of O2 to O1 is calculated as 1.9:1, indication of large number of oxygen vacancies presented at the NiCo2O4 crystals. The high resolution XPS spectrum of Ni 2p is shown in Fig. 3f. The peaks located at 855.6 and 873.5 eV and two satellite features at 861.7 and 879.8 eV are attributed to Ni 2p3/2 and Ni 2p1/2, respectively [40], which indicates the presence of Ni 2+ /Ni 3+ in the samples. For the high-resolution Co 2p feature (Fig. 3g), the Co 2p3/2 and Co 2p1/2 XPS spectra can be divided into four peaks at binding energies of 780.5, 796.5, 786.9 and 802.8 eV corresponding to Co 2+ /Co 3+ [41] and their satellites peaks, respectively. All these findings confirm the surface chemistry of NiCo2O4@NDC-Mo2C/CC. As illustrated in  forests are observed on the obtained NiCo2O4@NDC-Mo2C/CC (Fig. 3d-f). The EDS spectra (Fig. 3h) of NiCo2O4@NDC-Mo2C/CC suggest that the existence of Ni, Co and O elements in the samples, which confirms the needle-forests are composed of NiCo2O4. The diameter of needles on NiCo2O4@NDC-Mo2C/CC is ~60 nm (Fig. 3f), which is thinner than that on NiCo2O4/CC (80 nm, Fig. S2). This is probably because   NiCo2O4/CC has a pair of distinct redox peaks located around 0.41/0.34 V (Fig. 4a), corresponding to the reversible Co 2+ /Co 3+ and Ni 2+ /Ni 3+ transitions in alkaline medium [42]. However, for NiCo2O4@NDC-Mo2C/CC electrode, the integral area of the CV curve is enhanced. This shows that the composite has excellent conductivity, which leads to higher peak current of NiCo2O4@NDC-Mo2C/CC than the other samples. The CV curves of NiCo2O4@NDC-Mo2C/CC composite at different scanning speeds in the range 10−100 mV s −1 are illustrated in Fig. 4b. As can be seen from the graphs, the shape of the CV curves for NiCo2O4@NDC-Mo2C/CC remains basically unchanged with the increase of the scanning rate, indicating that the composite can be rapidly redox transformed. In addition, with the increase of scanning rate, the oxidation peak and the reduction peak move to the positive and negative directions respectively, and the potential difference between the oxidation peak and the reduction peak improves. This is because with the increment of scanning rate, the resistance of ion diffusion within the material increases too.   [45] and Mo2C@C [46], and near to that of 2D Mo2C-C [47] and N,P-doped Mo2C [48], as summarized in Table 1.
In addition, the Tafel slope is another essential parameter reflecting the performance of hydrogen evolution, which shows the hindrance of the electrode reaction. The HER kinetics of the catalysts were investigated by analyzing Tafel plots ( Fig. 5b), which reveal that the Tafel slope of the NiCo2O4/CC and NDC-Mo2C/CC were 117 and 357 mV dec −1 , respectively. In comparison, the NiCo2O4@NDC-Mo2C/CC catalyst shows the smallest Tafel slope (92 mV dec −1 ), which indicates that it has the closest hydrogen evolution efficiency to Pt/C (80 mV dec −1 ). To estimate the potential application, the HER stability of NiCo2O4@NDC-Mo2C/CC was then assessed by long-duration chronoamperometry (Fig. 5c) and the LSV curves before and after the cycled test (Fig. 5d). Not only the i-t curve of the NiCo2O4@NDC-Mo2C/CC catalyst at −1.3, −1.4 and −1.5 V exhibits a negligible current decay (Fig. 5c), but also its polarization curve remains almost unchanged before and after i-t test (Fig. 5d). The durability results indicate clearly that NiCo2O4@NDC-Mo2C/CC catalysts exhibit a superior long-term stability owing to the enhanced conductivity from NDC and the strong mechanical support from CC. As a bifunctional electrocatalyst, it must have excellent HER and OER catalytic activity in the same electrolyte. Thus, the OER performance of the prepared electrocatalysts has been studied in the potential range of 1.0~1.8 V (RHE) by means similar to HER, and compared with iridium oxide (IrO2). From Fig. 6a [53] and NiCo2O4-G-NCD [54]. It is just relatively higher than that of NiCo2O4 rosettes-like hierarchical spinel (155 mV) [55], as shown in Table 1.
In Fig. 6b, the result of Tafel slope also corresponds to the above conclusion.  Table S1, O2 turn over frequencies (TOFs) prove the high electrocatalytic activity of NiCo2O4@NDC-Mo2C/CC electrode, which are obviously larger than that of NiCo2O4/CC and NDC-Mo2C/CC at the same overpotential. Moreover, the TOF value is increased with the increase of overpotential.
This indicates that the quantity of O2 induced by the current flow was proportional to the amount of electricity passed through the samples.   (Fig. S3e), respectively. Therefore, the NiCo2O4@NDC-Mo2C/CC presents a largest ECSA value.
These results support the NiCo2O4@NDC-Mo2C/CC has the best electrocatalytic activity among the as-derived catalysts.  Because NiCo2O4@NCD-Mo2C/CC electrodes have demonstrated excellent electrocatalytic activity towards both OER and HER, therefore, the overall water splitting was investigated by using NiCo2O4@NCD-Mo2C/CC as both the anode and cathode in 1 M KOH in a two-electron configuration (inset in Fig. 7a). From the polarization curve in Fig. 7a, a low external bias of 1.64 V @10 mA cm −2 and 1.87 V @50 mA cm −2 was observed, with hydrogen and oxygen gas bubbles continuously released (inset of Fig. 7a). Furthermore, NiCo2O4@NCD-Mo2C/CC electrodes in this work show an external bias voltage close to the commercial Pt/CǁIrO2 pair (Fig. 8a), and also lower than the reported NiFe-NiCoO2 [56], NFP@NG [57], Ni(OH)2/NiCo2O4 [58] and Co5Mo1.0O NSs@NF [59] @10 mA cm −2 shown in Table S2. This is attributed to that the massive oxygen vacancy sites in the nanoneedle arrays of NiCo2O4 not only allow easy adsorption of water into the electrode but also facilitate the water splitting to quickly generate gas bubbles.
In addition, the regulated electron transport of Mo2C by nitrogen doping carbon also leads to lowered overvoltage. These results highlight the importance of the chemical or structure regulation in design of low-cost and high-performance electrocatalysts. Moreover, compared with Pt/CǁIrO2 pair electrode, NiCo2O4@NCD-Mo2C/CCǁNiCo2O4@NCD-Mo2C/CC electrodes demonstrate the constant current density @10 mA cm -2 during the 100-h cycling (Fig. 7b).
Additionally, the XRD features of NiCo2O4@NCD-Mo2C/CC show the unchanged shape after cycled (Fig. 7c), with the original needle-forest morphology maintained ( Fig. 7d), which indicates high stability in the potential applications. Interestingly, except the efficient overall water splitting performance demonstrated above, the NiCo2O4@NCD-Mo2C/CC also exhibits good ORR performance. The cyclic voltammetry curve of NiCo2O4@NCD-Mo2C/CC in O2-saturated solution illustrates a pronounced cathodic peak, while no peaks could be found in N2-saturated solution (Fig. 8a). As shown in Fig. 8b, the onset potential of NiCo2O4@NCD-Mo2C/CC is nearly the same as Pt/C but showing a far more positive half-wave potential (0.79 V). Moreover, for NiCo2O4@NCD-Mo2C/CC, no obvious decline was found before and after 1000 CV cycles (Fig. 8c), which indicated the excellent electrocatalytic durability of the sample. In the whole region of OER and ORR (Fig. 8d), the NiCo2O4@NCD-Mo2C/CC exhibits a ΔE value of 0.66 V, which is smaller than that of the precious Pt/C+IrO2 (0.69 V). Compared by the difference between OER and ORR (ΔE = Ej=10-E1/2), the small difference value (0.03 V) between NiCo2O4@NCD-Mo2C/CC and Pt/C+IrO2 indicates the ideal reversible property of the as-prepared bifunctional electrode. These results indicate that the NiCo2O4@NCD-Mo2C/CC has high ORR activity potential as ideal electrocatalysts for Zn-air batteries. As a proof-of-concept, a rechargeable Zn-air battery device was made using NiCo2O4@NCD-Mo2C/CC as the air cathode, as illustrated in Fig. 9a. A mixture of Pt/C+IrO2 (1:1 wt %) was also tested for comparison. The open-circuit voltage of NiCo2O4@NCD-Mo2C/CC driven Zn-air battery is 1.393 V (Fig. S4). From the charging/discharging polarization curves in Fig.  9a, the NiCo2O4@NCD-Mo2C/CC-based battery exhibited a smaller polarization voltage relative to the noble metal-based battery. Importantly, the peak power density of NiCo2O4@NCD-Mo2C/CC is 104 mW cm −2 , larger than that of Pt/C+IrO2 (79 mW cm −2 ; Fig. 9b). From the continuous discharge profiles in Fig. 9c, NiCo2O4@NCD-Mo2C/CC battery showed stable discharging ability. Furthermore, the battery based on NiCo2O4@NCD-Mo2C/CC delivered a higher specific capacity of 778 mAh g −1 than Pt/C+IrO2 (657 mAh g −1 ; Fig. 9d).

Declaration
The authors declare that they have no conflict of interest that could have appeared to influence the work reported in this paper.
• The catalyst shows more oxygen vacancies and enhanced conductivity.
• The multifunctional catalyst presents improved OER, HER and ORR performance.