Hierarchical Cobalt-Doped MolybdenumâNickel Nitride Nanowires as

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Hierarchical Cobalt-Doped Molybdenum-Nickel Nitride Nanowires as Multifunctional Electrocatalysts Zhuoxun Yin, Yue Sun, Yongjie Jiang, Feng Yan, Chunling Zhu, and Yujin Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06543 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Applied Materials & Interfaces

Hierarchical Cobalt-Doped Molybdenum-Nickel Nitride

Nanowires

as

Multifunctional

Electrocatalysts Zhuoxun Yin, † , ‡Yue Sun, ‡ Yongjie Jiang, † Feng Yan, †Chunling Zhu,*, † Yujin Chen*,† , ‡ †Laboratory of Superlight Materials and Surface Technology, Ministry of Education, and College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China ‡ College of Science, Harbin Engineering University, Harbin 150001, China

KEYWORDS: Multifunctional catalysts, water electrolysis, zinc-air battery, Co doping, metal nitride nanowire, hierarchically porous structure

ACS Paragon Plus Environment

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ABSTRACT: Herein we demonstrate hierarchically porous Co-doped MoNi nitride nanowires for

multifunctional electrocatalysts. After the Co incorporation for water electrolysis and zinc-air systems, the active surface area is enhanced, while and the charge-transfer and mass-transfer resistance is reduced significantly. Due to the dual modulation in the electric conductivity and active surface area induced by the Co doping, the hierarchically porous trimetal nitrides shows high activity and good stability for hydrogen evolution (HER), oxygen evolution (OER) and oxygen reduction reactions (ORR). The two-electrode electrolyser assembled by the bifunctional electrocatalysts can deliver 10 mA cm-2 at a voltage of merely 1.57 V, among the best reported electrocatalysts. In the meanwhile, two all-solid-state zinc-air batteries in series can power more than 50 red Light-emitting diodes and the two-electrode electrolyser catalysed by the multifunctional electrocatalysts with excellent operation stability.


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1. INTRODUCTION Practical application of renewable energy is a highly efficient approach to solve increasing excessive consumption of petrochemical energy. Most renewable energy sources including solar and wind energies are intermittently supplied. Thus, high-efficiency energy storage and conversion systems are highly required for the usage of the renewable and clean energy sources.1-3 Electrochemical water electrolysis and metal-air batteries are considered as potential systems for the utilization of the renewable energies. Electrochemical water splitting involves hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), while metal-air batteries such as zinc-air battery involves OER and oxygen reduction reaction (ORR). To realize high conversion efficiency of electrochemical water splitting and the ability of energy storage of metal-air batteries, electrocatalysts are highly needed for catalyzing HER, OER and ORR. To date, precious metals such as Pt are the best electrocatalysts for HER and ORR, while RuO2 and IrO2 exhibit high activities for OER.

4–6

However, the scarce reserves restrict their practical

applications. Over the last few decades, inexpensive transition-metal compounds have been widely exploited to replace the precious electrocatlysts for HER, OER and ORR.4–10 For example, transition-metal sulfides or selenides, phosphides and carbides for HER,7–14 transition-metal oxides and hydroxides/(oxy) oxides for OER,

15–18

and transition-metals and oxides for ORR19–36 have been

developed, respectively. Although some bifunctional electrocatalysts for HER/OER,37-42 and OER/ORR19-21,

25, 26

have fabricated so far, multifunctional electrocatalysts that can activate

HER, OER and ORR have been achieved rarely.43,44 Therefore, the development of highperformance multifunctional electrocatalysts for water splitting device and metal-air batteries remains a challenge.


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Recently, various strategies have been explored to increase activities of electrocatalysts. Reducing size

7, 16, 23,24,37

and designation of porous or hollow the electrocatalysts

20,31, 45-47

to

make more exposed active sites are usually adopted for the enhancement of the activity of electrocatalyst. Besides, improving the conductivities of the electrocatalysts can significantly enhance the kinetics characteristics, and thereby increase the electrochemical performance. For example, the activities of electrocatalysts after coupling with various carbonaceous scaffolds were remarkably improved compared to the bare electrocatlysts.10,11,

25-30

Furthermore,

introducing heteroatom into the electrocatalysts can tune the electronic structure and thus affect the catalytic activity.48 Typically, Ni-Fe films showed significantly enhanced OER activities in comparison to Ni and Fe parent film.29 In addition, establishing the defects in the electrocatalysts is an efficient approach for improving the catalytic performance.9,

18, 49

For example, more

defects existed in the MoS2 nanosheets, more active edge sites would be exposed for HER.6 Based on the reports above, integration of the strategies mentioned above for reasonable design of multifunctional electrocatalysts is necessary. In this work, we demonstrate a facile method to synthesize hierarchically porous Co-doped NiMo nitride nanowires composed of nanosheets as multifunctional electrocatalysts for overall water splitting and zinc-air battery. The hierarchically porous nanowires containing defects have porous feature and high conductivity, exhibiting excellent HER, ORR and OER activities. Twoelectrode alkaline electrolyzer assembled by the hierarchically porous nanowires can drive 10 mA cm–2 at a low voltage of 1.57 V, superior to the counterpart without Co doping and the most reported HER/OER electrocatalysts. The maximal power density of primary zinc-air battery with hierarchically porous Co-doped NiMo nitride nanowires as air cathode is 130 mW cm–2 at 230 mA cm–2, favorably compared to commercial Pt/C electrocatalysts. The rechargeable zinc-air


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battery with hierarchically porous Co-doped NiMo nitride nanowires as air cathode shows robust stability. Moreover, the hierarchically porous nanowires-based water splitting device can be driven by zinc-air battery assembled with hierarchically porous nanowires. In addition, two zincair batteries assembled with hierarchically porous nanowires can power more than 50 red lightemitting diodes (LEDs).

2. EXPERIMETNAL SECTION The fabrication process and structural characterizations of samples, and the electrochemical measurements see in Supporting Information.

3. RESULTS AND DISCUSSION The hierarchical porous Co-doped NiMo nitride nanowires (Co-NiMoN NRs) were through the following procedures. Ultralong MoO3 NRs with orthorhombic crystalline structure were first synthesized by a solvothermal method ( the detailed synthesis processes see Supporting Information). Figure S1a shows the X-ray diffraction (XRD) patterns of the as-fabricated ultrolong MoO3 nanowires, in which the diffraction peaks can be indexed to orthorhombic MoO3 (JCPDs card number, 05-0508). The length and diameter of MoO3 NRs are above 200 μm and approximately 600 nm, respectively (Figure S1b-c). Selected area electron pattern (SAED) and high-resolution transmission electron microscopy (HRTEM) image confirm the crystal nature of the ultralong MoO3 NRs (The inset in Figure S1c and Figure S1d). The labeled interplanar spacings are 1.845 and 3.94 nm, corresponding to the (002) and (100) lattice planes of orthorhombic MoO3, respectively. Therefore, the ultralong MoO3 NRs with a high crystallinity can be synthesized through the solvothermal method. Then the precursors for CoNiMoN NRs were fabricated through a weak-alkaline-etching method using the ultralong MoO3


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NRs as staring materials in water-ethanol (volume ratio = 1 : 1) solution containing nickel acetate and cobalt chloride.50 The precursors exhibit hierarchical structures with a similar length to that of ultralong MoO3 NRs (Figure S2a), and are composed of MoO3 core and NiCo hydroxide shell (Figure S2b).50 The atomic ratio of Ni/Co is estimated to be around 7: 1, determined by energy dispersive X-ray spectrometry (EDX) and inductively coupled plasma optical emission spectrometry (ICP-OES) (Table S1). After heating the precursors at the different temperature for 2 h under a NH3/Ar flow, the hierarchical porous Co-NiMoN NRs were obtained. For convenience, the samples synthesized at 300, 400 and 500oC were denoted as CoNiMoN-300, Co-NiMoN-400 and Co-NiMoN-500, respectively. In addition, the sample without Co-doping was also prepared under similar conditions except that the cobalt chloride was not added to the reaction system and the heating temperature was set at 400oC, denoted as NiMoN400.


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Figure 1. a) XRD patterns of Co-NiMoN-400 and NiMoN-400 NRs. b) Survey XPS, c) highresolution Ni 2p XPS, d) high-resolution Mo 3d XPS, e) high-resolution Co 2p XPS, and f) highresolution N 1s XPS spectra of the Co-NiMoN-400 NRs. Figure 1a displays X-ray diffraction (XRD) patterns of Co-NiMoN-400 and NiMoN-400 NRs. The NiMoN-400 NRs are composed of hexagonal Ni0.2Mo0.8N (JCPDS no.29-0931), and hexagonal Ni3N (JCPDS no.10-0280), similar to our previous report.51 After Co incorporation, the diffraction peaks from hexagonal Ni3N disappeared, leaving the hexagonal Ni0.2Mo0.8N and cubic Ni/Co or CoNi alloy (Ni, JCPDS no.04-0850; Co, JCPDS no.15-0806) behind in the CoNiMoN-400 NRs. Careful analyses of the XRD pattern demonstrates that the diffraction peaks are located between the standard positions of cubic Co and cubic Ni (Figure S3), which indicates that CoNi alloys are formed after the heating treatment.52,53 In order to determine if Co is doped into Ni0.2Mo0.8N, we immersed the Co-NiMoN-400 in 0.5 M HCl solution for 1 h to remove the CoNi alloys. The resultant sample was denoted as Co-Ni0.2Mo0.8N NRs. Figure S4a reveals that the Ni0.2Mo0.8N phase is remained, while the diffractions from CoNi alloys disappear. However, EDS pattern indicates that Co elements are existed in the Co-Ni0.2Mo0.8N NRs, suggesting the Co doping into Ni0.2Mo0.8N phase (Figure S4b). The EDX elemental mappings for the CoNi0.2Mo0.8N NRs reveal that Ni, Co, Mo and N distribute uniformly, further confirming the Co doping into Ni0.2Mo0.8N (Figure S4c). The little difference of the positions of the diffraction peaks between Ni0.2Mo0.8N and Co-doped Ni0.2Mo0.8N is due to similar ion radius of Co to Ni. The survey X-ray photoelectron spectroscopy (XPS) spectra (Figure 1b) indicate that Ni, Co, Mo and N elements exist in the Co-NiMoN-400 NRs, confirming the successful Co doping. There are six deconvoluted peaks in Ni 2p spectra (Figure 1c), in which the ones at 852.3 and 869.4 eV correspond to metallic nickel or nickel nitrides, while the others at 855.2 and 872.7 eV


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with satellite peaks at 861.1 and 879.6 eV are attributed to NiOx and/or Ni(OH)2. 54 The highresolution Mo 3d XPS spectra (Figure 1d) can be deconvoluted to three peaks at 229.1, 229.9 and 232.1 eV, corresponding to Mo2/3+, Mo3/4+ and Mo6+ species, respectively.55,56 The highresolution Co 2p XPS spectra (Figure 1e) can be deconvoluted into six peaks at 777.9, 780.6, 786.0, 794.8, 796.0 and 801.6 eV, respectively. The peaks at 777.9 and 794.8 eV correspond to metal cobalt or cobalt nitride, while the ones at 780.6 and 796.0 as well as satellite peaks at 786.0 and 801.6 can be assigned to CoOx and/or Co(OH)2.9,18,55 In N 1s spectra (Figure 1f), the binding energies at 397.4 and 399.2 eV are associated with metal nitride species and NH group, respectively, while the one at 395.1 eV corresponds to Mo 3p species.

Figure 2. Structural characterization of the Co-NiMoN-400 NRs. a-c) SEM images, d) lowmagnification TEM image, e) HRTEM images, and f) EDX elemental mapping images of Ni, Mo, Co and N.


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Structural information of the hierarchical nitrides was further analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM image indicates that the length Co-NiMoN-400 NRs is several micrometers (Figure 2a). The hierarchical Co-NiMoN-400 NRs consist of interconnected nanosheets with a thickness of approximately 15 nm (Figure 2b, 2c). TEM image confirms hierarchical and porous features of the Co-NiMoN-400 NRs (Figure 2d). Figure 2e displays high-resolution TEM (HRTEM) image of Co-NiMoN-400 NRs taken at the edge of the nanosheets. Lattice fringes are clearly identified in the image, revealing crystal nature of Co-NiMoN-400 NRs. The interplanar spacing of the marked lattice fringes is 0.276 nm, corresponding to (001) plane of Co-doped Ni0.2Mo0.8N, while these ones with lattice fringe distances of 0.202 and 0.175 nm with an angle of 54.7o correspond to (111) and (200) lattice planes of CoNi alloy, respectively. Notably, there are many defects such as lattice distortions and amorphous/crystalline interfaces existed in the Co-NiMoN-400 NRs in terms of HRTEM observations (Figure S5). The defects can generate more active sites for improving the catalytic activity of electrocatalysts. 9, 18, 49 The EDX elemental mappings for one NR reveal that Ni, Co, Mo and N distribute uniformly except the porous regions, further confirming the Co doping and porous feature of the hierarchical nitride NRs (Figure 2f). The porous feature is evidenced by the nitrogen adsorption−desorption isotherms (Figure S6). Bruanuer−Emmett−Teller (BET) surface area of the hierarchical Co-NiMoN-400 NRs is calculated to be 45.6 m2 g−1. The pores are centered at 8 nm and 120 nm, and the corresponding pore volume is 0.29 cm3 g−1. The porous feature facilitates the contact of active sites with the electrolyte for enhancing catalytic performance of the hierarchical Co-NiMoN-400 NRs. Similar to the Co-NiMoN-400 NRs, the NiMoN-400 NRs, Co-NiMoN-300 NRs and Co-NiMoN-500 NRs have hierarchical characteristic, as shown in Figure S7-9. Hexagonal Ni3N existed in the NiMoN-400 NRs and the


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Co-NiMoN-300 NRs; however, not in the Co-NiMoN-400 NRs and the Co-NiMoN-500 NRs, as shown in Figure S10. Furthermore, the degree of crystallinity of the Co-doped NRs is increased with increasing heating temperature (Figure S10). This result indicates that the higher temperature is favorable for the formation of Co-doped Ni0.2Mo0.8N and CoNi alloys. The Codoped NRs have almost the same Ni/Co ratio as that of the precursor determined by EDS and ICP-OES (Table S2), little affected by the heating temperature.

Figure 3. a) The polarization curves and b) Tafel slopes of Co-NiMoN, NiMoN, NF and IrO2 toward OER. c) Stability test for the Co-NiMoN-400 NRs by CV scanning for 1000 cycles at a scan rate of 50 mV s-1. d) The long-term stability of Co-NiMoN-400 NRs at 317 mV toward OER. The OER activities of the hierarchical Co-doped NRs and the NiMoN-400 NRs loaded on the nickel foam (NF) was first evaluated in 1.0 M KOH using three-electrode setup. The electrocatalytic performance was measured by the linear sweep voltammetry (LSV). The OER


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performances of the commercial IrO2 and NF were also measured under the same conditions as reference. The NF shows a poor OER activity with an OER current density of 10 mA cm−2 at overpotential (ηOER@10) of 365 mV (Figure 3a). To drive 10 mA cm−2, NiMoN-400 NRs need an overpotential of 326 mV, inferior to that of the Co-NiMoN-300 (311 mV), the Co-NiMoN-400 NRs (294 mV) and the Co-NiMoN-500 (323 mV) NRs (Figure 3a and Figure S11a). Thus, the OER activities of these catalysts decrease in the order Co-NiMoN-400 NRs > Co-NiMoN-300 NRs > Co-NiMoN-500 NRs > NiMoN-400 NRs. The results above demonstrate that the Co doping can remarkably improve the OER activity of the hierarchical nitride NRs. Furthermore, the Co-NiMoN-400 NRs exhibit comparable OER activity to the commercial IrO2 (ηOER@10 = 282 mV) at low overpotential region, while superior OER activity to the commercial IrO2 as an overpotential exceeds 349 mV. For example, to deliver 100 mA cm−2, the commercial IrO2 requires 376 mV, while the Co-NiMoN-400 NRs need merely 367 mV. In addition, compared to the best reported OER electrocatalysts, the Co-NiMoN-400 NRs also show comparable or superior OER activity (see Table S3 for details). The results demonstrate that the Co-NiMoN400 NRs are promising inexpensive OER catalysts for practical applications. To further investigate the catalytic performance of the catalyst, Tafel slope values were determined by Tafel equation (η = a + b log | j |, where η, a, and b represent overpotential, a constant, the Tafel slope, respectively). As shown in Figure 3b, the Tafel slope of the Co-NiMoN-400 NRs is 73.0 mV dec−1, equal to that of the commercial IrO2 (73.0 mV dec−1), but significantly smaller than that of NF (93.7 mV dec−1) and the NiMoN-400 NRs (85.6 mV dec−1). The small Tafel slope indicates the favourable kinetics of the Co-NiMoN-400 NRs for OER. The Tafel slope near to 60 mV dec−1, suggesting that the OER mechanism of the Co-NiMoN-400 NRs is in connection with a rate-limiting chemical step following the first electron transfer.57-60


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To analyze the reasons for the enhancement of the Co-doping nitride NRs in OER performance, the double-layer capacitances (Cdl) were calculated by cyclic voltammetry curves recorded in potential region of 1.016 – 1.036 V at different scan rates (Figure S12). The Cdl value for the Co-NiMoN-400 NRs is 14.86 mF cm−2, larger than those of the Co-NiMoN-300 NRs (12.34 mF cm−2), Co-NiMoN-500 NRs (11.24 mF cm−2) and NiMoN-400 NRs (10.46 mF cm−2). The Cdl data reveals that the electrochemically active surface area (ECSA) of our electrocatalysts decreases in the order Co-NiMoN-400 > Co-NiMoN-300 > Co-NiMoN-500 > NiMoN-400, in line with the trend of the catalytic activities. Thus, the increased ECSA value is responsible for the improvement of OER activities of the Co-doped nitride NRs. Besides, the charge transfer resistance (Rct) is another important factor for the OER performance of the electrocatalyst. To obtain Rct data, we measured the electrochemical impedance spectroscopy (EIS) of our electrocatalysts. Figure S11a displays the Nyquist plots of our electrocatalysts at ηOER of 300 mV. Co-NiMoN-400 NRs has the smallest Rct value (1.71 Ω cm−2) among the measured electrocatalysts (Figure S13a). The reduced Rct value facilitates the OER kinetics and thereby OER performance. In addition, the oxidation peaks before OER for the Co-doped nitride NRs shift toward high potential slightly (Figure 3a), indicating that the Ni oxidation before OER becomes more difficult in comparison with the NiMoN-400 NRs. Such behaviors have been found in the Fe-doped Ni and Fe-doped Co electrocatalysts, and the shift can boost the OER activities because the electronic structure of Ni can be tuned by Fe doping.29,35 For example, Bell et al. demonstrated that the introduction Fe promoted the transformation of partial Ni(OH)2 to NiOOH, and the modulation of the electronic structure (the transformation Ni2+ to Ni3+) improved OER significantly. 61, 62


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The durability of Co-NiMoN-400 NRs for OER was tested by CV measurements at a high scan rate of 50 mV s−1. After 1000 CV the current density keeps near same values at the same overpotential, indicating good stability of Co-NiMoN-400 NRs for OER (Figure 3c). The stability of Co-NiMoN-400 NRs was further evaluated carried out using chronopotentiometric measurements at ηOER of 317 mV. After continuous 10 h OER process, 91.5% of the initial current density can be kept, further demonstrating the robust durability of the Co-NiMoN-400 NRs. After the OER stability test, the structure of the Co-NiMoN-400 NRs was characterized again. SEM and TEM images indicate that the hierarchical and porous features of the CoNiMoN-400 NRs keep almost unchanged (Figure S14a,b). Although amorphous layer appears in the outmost surface, the well-resolved lattice fringes are still observed clearly (Figure S14c). The labeled the interplanar spacings with 0.27, 0.175and 0.204 nm can be assigned to (001) plane of the Co-doped Ni0.2Mo0.8N, (200) and (111) planes of the CoNi alloy, respectively. The EDX elemental mappings indicate that Ni, Co, Mo and N still distribute throughout the NRs (Figure S14d). The above analyses reveal excellent structural stability of the Co-NiMoN-400 NRs for OER. The Ni 2p XPS spectra show that the Ni-N species disappear, while the binding energies corresponding to NiOx/Ni(OH)2 are increased by approximately 0.1 eV after the OER process (Figure S15a,b). As previously reported, Ni species with the increased valence state are real active phases. 9, 10, 57-60


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Figure 4. a) The polarization curves and b) Tafel slopes of Co-NiMoN NRs, NiMoN NRs, NF and Pt toward HER. c) Stability test for the Co-NiMoN-400 NRs by CV scanning for 1000 cycles in at a scan rate of 50 mV s-1. d) The long-term stability of Co-NiMoN-400 NRs at 77 mV toward HER. The HER properties of hierarchical Co-doped NRs and the NiMoN-400 NRs loaded on NF were measured using three-electrode setup. For comparison, the HER performances of the Pt foil and the NF substrate were also assessed. The NF substrate shows a negligible HER activity with a current density of 10 mA cm−2 at an overpotential (ηHER@10) of 181 mV (Figure 4a). To deliver a HER current density of 10 mA cm−2, the NiMoN-400 NRs need ηHER of 90 mV, smallerr than that of the Co-NiMoN-300 (143 mV), but larger than that of the Co-NiMoN-400 (45 mV) and the Co-NiMoN-500 (86 mV) NRs (Figure 4a and Figure S11b). Thus, the HER activities of these catalysts decrease in the order Co-NiMoN-400 NRs > Co-NiMoN-500 NRs> NiMoN-400 NRs > Co-NiMoN-300 NRs. Notably, compared to the best reported HER electrocatalysts, the CoNiMoN-400 NRs also have comparable or superior HER activity (see Table S4 for details).


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Therefore, the Co-NiMoN-400 NRs are promising inexpensive HER catalysts for practical applications. To clarify the kinetics of the HER electrocatalysts, the Tafel plots were given in terms of polarization curves. The Tafel slop of the NF substrate is as large as 111.4 mV dec−1, further suggesting its negligible HER activity (Figure 4b). The Tafel slope of Co-NiMoN-400 NRs is 72.2 mV dec−1, larger than that of the Pt foil (54.7 mV dec−1), but smaller than that of the NiMoN-400 NRs (86.5 mV dec−1). The Tafel slope near to 60 mV dec−1, suggesting that HER occurring on the Co-NiMoN-400 NRs may follow the Volmer–Heyrovsky mechanism. Based on the ESI measurements, the Co-NiMoN-400 NRs have the smallest Rct value (2.87Ω cm−2) among those electrocatalysts (Figure S13b). In addition, the To explain the reasons for the HER activity enhancement of the Co-doping nitride NRs, the Co-NiMoN-400 NRs have the largest Cdl values at non-fradaic potential range (Figure S12). Therefore, the enhancement of the Co-NiMoN-400 NRs in HER performance can be attributed to their larger ECSA and smaller Rct values. The durability of the Co-NiMoN-400 NRs toward HER was first tested by CV measurements at a high scan rate of 50 mV s−1. After 1000 CV cycling, the polarization curve is almost overlapped with the one obtained at the first cycle, indicating good stability of the Co-NiMoN400 NRs toward HER at the high scan rate (Figure 4c). The stability of the Co-NiMoN-400 NRs was further evaluated carried out using chronopotentiometric measurements at ηHER = 77 mV. At the beginning stage the current density is about 25.2 mA cm−2, and changes to 22.7 mA cm−2 after HER process for 22 h, further demonstrating good stability of the Co-NiMoN-400 NRs toward HER (Figure 4d). After the HER stability test, the structure of the Co-NiMoN-400 NRs was analyzed again. SEM and TEM images show that the hierarchical and porous features of the Co-NiMoN-400 NRs keep almost unchanged after the HER stability test (Figure S16a,b). The well-resolved lattice fringes can be observed clearly (Figure S16c), and the labeled interplanar


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spacings with 0.246 and 0.205 nm can be indexed to (100) plane of hexagonal Ni0.2Mo0.8N and (111) planes of the cubic Ni, respectively. The EDX elemental mappings for the post-OER NRs (Figure S16d) indicate that Ni, Co, Mo and N still distribute throughout the NRs uniformly. The above SEM and TEM measurements reveal that the Co-NiMoN-400 NRs toward HER have good structural stability. High-resolution Ni 2p XPS spectra (Figure S15a, c) shows that the binding energies corresponding to NiOx/Ni(OH)2 are almost unchanged after the HER process. The amorphous NiOx/Ni(OH)2 layer at the outmost surface of the Co-NiMoN-400 NRs can be observed in the HRTEM image (Figure S16c), marked by frames. The amorphous NiOx/Ni(OH)2 layer may be real active sites it is directly contact with the electrolyte, while the inner highly conductive nitrides ensure low charge transfer resistance. In fact, various transition-metal hydroxides on conductive substrates exhibited excellent HER activities.63-66 After the HER stability test, these hydroxides had little change in composition and valence states, which indicated that the hydroxides were active sites for HER.65, 66

Figure 5. a) Polarization curves of the Co-NiMoN-400 | Co-NiMoN-400 and IrO2|Pt couples. b) The long-term stability of Co-NiMoN-400 | Co-NiMoN-400 couples at 1.60 V.


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Based on excellent HER and OER properties, the Co-NiMoN-400 NRs can be served as bifunctional electrocatalysts for overall water splitting. The electrochemical properties of the electrolyers were assessed using NF coated with bifunctional electrocatalysts as both cathode and anode. For comparison, the NF|NF, NiMoN-400|NiMoN-400 and the IrO2|Pt couples were also assessed under the same conditions. To drive 10 mA cm−2, the NF|NF, NiMoN-400|NiMoN-400 couples require cell voltages of 1.82 and 1.65 V, respectively, inferior to IrO2|Pt couple (1.57 V) (Figure 5a). In contrast, the electrolyser assembled with the Co-NiMoN-400 NRs exhibit comparable electrochemical property to that of the IrO2|Pt couple. Typically, to deliver 10 mA cm−2 the electrolyser assembled with the Co-NiMoN-400 NRs requires the same voltage as that of the IrO2|Pt couple. Even at a high voltage of 1.70 V, the current density driven by the CoNiMoN-400 NR-based electrolyzer is 49.6 mA cm−2, still comparable to that driven by IrO2|Pt couple (55.3 mA cm−2). Furthermore, the electrolyser assembled with the Co-NiMoN-400 NRs exhibit favorably comparable electrochemical property to the ones based on some reported bifunctional electrocatalysts (see Table S5 for detail). Figure 5b displays the durability of the CoNiMoN-400|Co-NiMoN-400 couple at the cell voltage of 1.60 V over 20 h electrolysis of water. After the 20 h operation, the current density can remain 99.0% of the initial value, suggesting the excellent durability of the Co-NiMoN-400 NRs for overall water splitting. The highly catalytic activity as well as good stability demonstrates that Co-NiMoN-400 NRs can be served as bifunctional catalyst for H2 and O2 generation.


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Figure 6 a) CV curves of the Co-NiMoN-400 NRs in N2- / O2-saturated KOH solution. b) LSV curves of NiMoN-400 NRs, Co-NiMoN-400 NRs and Pt/C for ORR in O2-saturated 0.1 M KOH solution at 1600 rpm. c) LSVs of Co-NiMoN-400 NRs at different rotating speeds. d) K-L plots of the Co-NiMoN-400 NRs. e) Long-time discharge curves and f) discharge polarization and power density curves of the primary zinc-air batteries with the Co-NiMoN-400 NRs and the commercial Pt/C air cathodes. g) Galvanostatic discharge-charge cycling curves of rechargeable zinc-air batteries with the Co-NiMoN-400 NRs and Pt/C air cathodes at 10 mA cm-2 (the insets show the discharge and charge curves of the rechargeable zinc-air batteries with the Co-NiMoN400 NRs at the initial and final stages). h) Photograph of 55 red LED powered by two zinc-air batteries in series. i) Photograph of the two-electrode electrolyser based on the Co-NiMoN-400 NRs powered by two batteries in series.


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The ORR properties of NiMoN-400 and Co-doped nitride NRs were evaluated in 0.1 M KOH. The ORR activities of our electrocatalysts decreased in the order Co-NiMoN-500 > Co-NiMoN400 > Co-NiMoN-300 > NiMoN-400 (Figure S17). Considering the best HER and OER properties of the Co-NiMoN-400 NRs, we mainly focused on the comparison of the ORR performance of the Co-NiMoN-400 NRs, the NiMoN-400 NRs and 20% Pt/C. The ORR property of Co-NiMoN-400 NRs was first assessed by the CV measurements in a N2-or O2saturated 0.1 M KOH. An sharp oxygen reduction peak appears in the CV in O2-saturated KOH, while only electrochemical double layer capacitive current is observed in N2-saturated KOH, demonstrating a good intrinsic ORR activity of Co-NiMoN-400 NRs (Figure 6a). LSV curves were measured on rotating disk electrode (RDE) at a rotation speed of 1600 rpm to further evaluate the ORR performances of the electrocatalysts. The onset potential (E0) and the corresponding half-wave potential (E1/2) of the Co-NiMoN-400 NRs are 0.890 V and 0.730V, respectively (Figure 6b). These potentials are lower than those of 20% Pt/C (E0 =1.03 V, E1/2 =0.849 V), but more positive than those of NiMoN-400 NRs (E0 =0.874V, E1/2 =0.726 V). Furthermore, the limited current density (Jl) of Co-NiMoN-400 NRs is 4.3 mA cm−2, greatly larger than that of NiMoN-400 NRs (Jl =3.7 mA cm−2). Therefore, the ORR performance of the hierarchical porous nitride NRs is enhanced by the Co-doping. To study ORR mechanism of CoNiMoN-400 NRs, the LSVs were measured at various rotation speeds (Figure 6c). The Koutecky-Levich (K-L) plots for Co-NiMoN-400 NRs show good linearity at potentials of 0.4 – 0.7 V, implying the first-order reaction kinetics toward dissolved oxygen.6 The average number of electron transferred during ORR is about 3.98, indicating that the ORR on Co-NiMoN-400 NRs follows a direct four-electron oxygen reduction process (Figure 6d). Besides, the CoNiMoN-400 NRs show good OER activity in 0.1 M KOH with a ηOER of 400 mV at 10 mA


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cm−2, 377 mV smaller than that of Pt/C catalyst (Figure S18). ΔE values were used to evaluate the efficiency of oxygen electrode, ΔE = EOER,10 – E1/2 where EOER,10 is the potential at 10 mA cm-2 driven by the catalyst for OER. ΔE value for CoNiMoN-400 NRs is 0.90 V, significantly smaller than that of the Pt/C (1.16 V), suggesting that the Co-NiMoN-400 NRs can be used as highly active oxygen catalysts. Considering good ORR and OER activities, we investigated the feasibility of the practical applications of the Co-NiMoN-400 NRs in zinc-air battery. As shown in Figure 6e, the maximal power density of one primary zinc-air battery with Co-NiMoN-400 NRs as oxygen catalysts is 130 mW cm-2. The value is larger than that of primary zinc-air battery with Pt/C air-cathode (110 mW cm-2) and ones using other reported oxygen bifunctional catalysts as air-cathode (see Table S6 for detail). At the discharge current density of 10 mA cm-2, the primary battery with CoNiMoN-400 NRs as oxygen catalysts exhibits a stable potential plateaus of 1.30 V with specific capacity of 736 mAh g-1, while the Pt/C-based battery show a voltage plateaus of 1.35 V with a specific capacity of 648 mAh g-1. The higher specific capacities reveal the potential application of the Co-NiMoN-400 NRs in the high-performance energy storage system. We further assemble rechargeable zinc-air battery with a zinc plate, the Co-NiMoN-400 NRs-loaded carbon paper with a gas diffusion layer and 6.0 M KOH solution containing 0.2 M zinc acetate as the anode, the air-cathode and the electrolyte, respectively. Figure 6g shows the galvanostatic dischargecharge cycling curves of the rechargeable zinc-air batteries at 10 mA cm-2. The initial voltage gap for Co-NiMoN-400 NR-based zinc-air battery is 1.11 V, lower than that of Pt/C-based zincair battery (1.67 V). After 24 h cycling measurements, the voltage gap for Co-NiMoN-400 NRbased zinc-air battery is decreased by merely 93 mV; however, it is sharply increased by 520 mV


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for Pt/C-based zinc-air battery, suggesting high activity and robust cycling durability of CoNiMoN-400 NRs for rechargeable zinc-air battery. Finally, we fabricated all-solid-state zinc-air battery using a zinc plate, Co-NiMoN-400 NRs-loaded carbon fiber cloth and poly (vinyl alcohol) / KOH (PVA/KOH) gel as anode, air-cathode and solid-state electrolyte, respectively. The open circuit voltage (OCV) of one zinc-air battery is about 1.337 V (Figure S19). The allsolid-state zinc-air battery exhibits robust cycling stability with a stable discharge and charge voltage gap for 10 h at 1 and 10 mA cm-2 (Figure S19, 20). Two zinc-air batteries in series can drive 55 red LEDs (Figure 6h). Furthermore, the Co-NiMoN-400 NRs|Co-NiMoN-400 eletrolyzer can be self-powered powered by two zinc-air batteries (Figure 6i).

4. CONCLUSIONS In summary, we fabricate hierarchically porous Co-doped NiMo nitride nanowires through a facile method. We find that the Co doping greatly enhances HER, OER and ORR performances. In 1.0 M KOH solution, the Co-doped nanowires exhibit high activities for HER and OER with OER and HER overpotentials of 294 and 45 mV at 10 mA cm-2, respectively. The electrolyser assembled by Co-doped nanowires can deliver 10 mA cm-2 at 1.57 V, comparable to IrO2|Pt couple. Furthermore, the hierarchically porous nanowires have good ORR catalytic activity in 0.1M KOH. One all-solid-state zinc-air battery with the hierarchically porous nanowires aircathode has an OCV of 1.337 V. Moreover, two zinc-air batteries can drive more than 50 red LEDs and the two-electrode electrolyser catalysed by the hierarchically porous nanowires. Supporting Information Figure S1–Figure S19 and Table S1–Table S6. The synthesis processes, characterization techniques and the measurement of the catalytic performance of the catalysts, the preparation of Zn-air battery, structural characterization of ultralong MoO3 NRs, the precursor for Co-NiMoN


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NRs, catalytic activities of Co-NiMoN-300, Co-NiMoN-400 and Co-NiMoN-500, CV curves and Nyquist plots of Co-NiMoN-300, Co-NiMoN-400 and Co-NiMoN-500, OCV and cycling stability of solid-state Zn-air battery, and comparison of the catalytic activities of Co-NiMoN400 with those of the catalysts reported previously. The Supporting Information is available free of charge on the ACS Publications website at DOI: AUTHOR INFORMATION Corresponding Authors C. Z. e-mail: [email protected]. *Y.C. e-mail: [email protected]. ORCID Yujin Chen: 0000-0002-6794-2276. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 51572051), the Natural Science Foundation of Heilongjiang Province (E2016023), and also the Fundamental Research Funds for the Central Universities (HEUCF201708). The Solubility and Diffusion Coefficient of Oxygen in Potassium Hydroxide Solutions REFERENCES


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(56) Cao, B. F.; Veith, G. M.; Neuefeind, J. C.; Adzic R. R.; Khalifah, P. G. Mixed ClosePacked Cobalt Molybdenum Nitrides as Non-noble Metal Electrocatalysts for The Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 19186-19192. (57) Chen, W. F.; Sasaki, K.; Ma, C.; Frenkel, A. I.; Marinkovic, N.; Muckerman, J. T.; Zhu, Y. M.; Adzic, R. R.; Hydrogen-Evolution Catalysts Based on Non-Noble Metal NickelMolybdenum Nitride Nanosheets. Angew. Chem., Int. Ed. 2012, 51, 6131–6135. (58) Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W.; Cobalt-Iron (Oxy)hydroxide Oxygen Evolution Electrocatalysts: The Role of Structure and Composition on Activity, Stability, and Mechanism. J. Am. Chem. Soc. 2015, 137, 3638−3648. (59) Surendranath, Y.; Kanan, M. W.; Nocera, D. G. Nocera Mechanistic Studies of The Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH. J. Am. Chem. Soc. 2010, 132, 16501-16509. (60) Xu, K.; Chen, P. Z.; Li, X. L.; Tong, Y.; Ding, H.; Wu, X. J.; Chu, W. S.; Peng, Z. M.; Wu, C. Z.; Xie, Y. Metallic Nickel Nitride Nanosheets Realizing Enhanced Electrochemical Water Oxidation. J. Am. Chem. Soc. 2015, 137, 4119−4125. (61) Louie, M. W.; Bell, A. T. An Investigation of Thin-Film Ni–Fe Oxide Catalysts for the Electrochemical Evolution of Oxygen. J. Am. Chem. Soc 2013, 135, 33, 12329-12337 (62) Zhu, K. Y.; Zhu, X. F.; Yang, W. S. Application of In Situ Tcheniques for the Characterization of NiFe-Based Oxygen Evolution Reaction (OER) Electrocatalysts. Angew. Chem., Int. Ed. 2018, 57, 2 -16. (63) Tang, T.; Jiang, W. –J.; Niu, S.; Liu, N.; Luo, H.; Zhang, Q.; Wen, W.; Chen, Y. -Y.; Huang, L. B.; Gao, F.; Hu, J. S. Kinetically Controlled Coprecipitation for General Fast


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