ZIF-Derived Carbon Nanoarchitecture as a Bifunctional pH-Universal

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ZIF-derived Carbon Nanoarchitecture as a Bi-functional pHuniversal Electrocatalyst for Energy-Efficient Hydrogen Evolution Lin Wang, Junhui Cao, Xiaodi Cheng, Chaojun Lei, Qizhou Dai, Bin Yang, Zhongjian Li, M. Adnan Younis, Lecheng Lei, Yang Hou, and Kostya (Ken) Ostrikov ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01315 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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ACS Sustainable Chemistry & Engineering

ZIF-derived Carbon Nanoarchitecture as a Bi-functional pH-universal Electrocatalyst for Energy-Efficient Hydrogen Evolution

Lin Wanga,b, Junhui Caob, Xiaodi Chengb, Chaojun Leib, Qizhou Daia*, Bin Yangb, Zhongjian Lib, M. Adnan Younisb, Lecheng Leib, Yang Houb*, Kostya (Ken) Ostrikovc a

Institute of Environmental Biology and Catalysis, College of Environment, Zhejiang University of Technology, 18 Chaowang Road, Hangzhou, Zhejiang 310014, China b

Key Laboratory of Biomass Chemical Engineering of Ministry of Education,

College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, Zhejiang 310027, China c

School of Chemistry, Physics, and Mechanical Engineering Queensland University of Technology, Brisbane, QLD 4000, Australia

Corresponding authors: Prof. Dr. Yang Hou, E-mail: [email protected] Prof. Dr. Qizhou Dai, E-mail: [email protected]

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Abstract Development of non-precious metal-based electrocatalysts supporting hydrogen evolution reaction (HER) in the entier pH range gain significant importance for harvesting green and renewable energy. Herein, we developed a novel electrocatalyst based on 3D carbon nanoarchitecture hybrid, which consists of CoP nanoparticles (CoP NPs) embedded into N-doped carbon nanotubes (NCNT), grafted on carbon polyhedron (CoP/NCNT-CP) that was prepared by carbonization and low-temperature phosphatization treatment of cobalt-based zeolite imidazole framework (ZIF). Benefiting from strong synergistic effect and unique 3D structure, the CoP/NCNT-CP hybrid loaded on Ni foam exhibited excellent electrocatalytic HER performance in base with a low overpotential of 165 mV at a current density of 10 mA cm-2, which is competitive with the previously reported Co-based hybrid electrocatalysts. Furthermore, the CoP/NCNT-CP also demonstrated high HER electrocatalytic activites in both neutral and acidic conditions with the overpotentials of 203 mV and 305 mV at the current density of 10 mA cm-2. Additionally, the bifunctional CoP/NCNT-CP electrode simultaneously acted as an anode for hydrazine oxidation reaction (HzOR) and a cathode for HER. Excellent catalytic performance was demonstrated in base conditions with a low cell potential of 0.89 V at 10 mA cm-2, which was much lower than the voltage of overall water splitting (1.91 V) at the same current density. Keywords: CoP nanoparticles, N-doped carbon nanotubes, carbon polyhedron, pH-universal electrocatalyst, energy-efficient hydrogen evolution 2


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Introduction With ever-escalating energy crisis and environmental problems, seeking sustainable energy is becoming an urgent issue.1-2 Hydrogen, as a green and renewable energy, is accepted as a highly-promising alternative to conventional fuels.3-4 Electrochemical water splitting is a viable and environmentally friendly approach to produce hydrogen.5-7 Nowadays, the Pt-based materials are state-of-the-art electrocatalysts for hydrogen evolution reaction (HER), however their scarcity and cost limit their practical application.8-11 Therefore, developing cost-effective and highly efficient non-precious metal based electrocatalysts for HER is a significant challenge.12-14 Recently, scientific community have put tremendous research efforts to develop high performance and earth-abundant electrocatalysts, offering the improved HER activity, such as transition metal phosphides,15 nitrides,16 selenides,17 sulfides,18 carbides,19 and borides,20 etc. Among the earth-abundant electrocatalysts, the transition metal phosphides (for example, CoP,21 MoP,22 FeP,23 and NiP24) have been reported as efficient electrocatalysts for HER. However, most of the recently reported transition metal phosphides only exhibited the high HER catalytic activity under either acid or basic conditions.25 Considering the practical applications of hydrogen production, an ideal electrocatalyst is expected to display excellent HER activity over a wide pH range. In recent years, nanocarbon materials have shown utility as catalyst support matrices due to their adjustable molecular structure, excellent electrical conductivity, developed pore structure, and high resistance to chemical attack.26 Construction of 3



transition metal phosphide/nanocarbon based composites can not only overcome the disadvantages of insufficient electric conductivity, but also increase the dispersion of active phases. Moreover, in order to further decrease the energy cost of overall water splitting, replacing oxygen evolution reaction (OER) by a more reactive anodic reaction, such as hydrazine oxidation27, methanol oxidation,28 and urea oxidation29, is a feasible way. Herein, we reported a novel hybrid electrocatalyst based on 3D carbon nanoarchitecture hybrid, consisting of CoP nanoparticles (CoP NPs) encapsulated within N-doped carbon nanotubes (NCNT) grown on carbon polyhedron (CoP/NCNT-CP). The hybrid catalyst was prepared by carbonization of ZIF-67 precursor under continuous flow of H2, followed by a low-temperature phosphatization treatment with NaH2PO2. The NCNT with an average diameter of ~20 nm embedded with CoP NPs were grafted on the surface of carbon polyhedron (average diameter of ~500 nm). Owing to the unique 3D structure and strong synergistic effect, the 3D CoP/NCNT-CP loaded on Ni foam displayed an impressive HER activity in basic solution and achieved a current density of 10 mA cm-2 with a small overpotential of 165 mV. Furthermore, the overpotential of CoP/NCNT-CP was comparable to the well-established Co-based hybrid electrocatalyst. Further, the CoP/NCNT-CP electrocatalyst required low overpotentials to achieve the current density of 10 mA cm-2 for HER, namely 203 mV in neutral solution and 305 mV in acidic solution. In addition, the CoP/NCNT-CP in a two-electrode system could act as both cathode for HER and anode for hydrazine oxidation reaction (HzOR). This 4


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unique combination only required a cell potential of 0.89 V to attain 10 mA cm-2, which was much smaller than the voltage (1.91 V) needed for overall electrocatalytic water splitting. 2. Experimental section Synthesis of ZIF-67 For the synthesis of ZIF-67, solution A was prepared by the dissolving 2-methylimidazole (9.85 g) into a mixed solution of methanol and ethanol (100 mL each). For solution B, the Co(NO3)2·6H2O (8.73 g) was dissolved into the mixture solution of methanol and ethanol (100 mL each). Then, the solution A was subsequently poured into solution B, and the final solution was kept at room temperature for 24 h. After that, the centrifugation was used to collect purple precipitate and washed with ethanol multiple times and dried at 60 oC. Synthesis of Co/NCNT-CP The ZIF-67 was heated in a tube furnace at 350 oC for 1.5 h. The temperature was further increased to 900 oC and maintained for 3.5 h. All reaction was at an increasing temperature rate of 2 oC min-1, under N2/H2 flow (95%/5%). Afterwards, the tube furnace was set to cooled down to room temperature naturally, and the obtain product was black powder of Co/NCNT-CP. Synthesis of CoP/NCNT-CP The as-obtained Co/NCNT-CP was put into a quartz boat, and then the quartz boat was heated into a quartz tube with 500 mg of sodium hypophosphite at a low

5



temperature (300 oC, 400 oC, and 500 °C) for 2 h. The reaction was in continuous N2 flow with a temperature ramp of 5 oC min-1. Synthesis of NCNT-CP The as-obtained Co/NCNT-CP sample was treated with 2.0 M H2SO4 with vigorously stirring for 10 h. Afterwards, the centrifugation was used to collect the product and washed by deionized water several times, finally dried at 60 oC. Synthesis of CoP The as-obtained Co/NCNT-CP sample was put into a quartz boat, and heated to 400 oC

for 2 h with increasing temperature rate of 5 °C min-1 under air. The obtained

Co3O4 product was further heated to 400 oC under N2 flow with 500 mg of sodium hypophosphite for 2 h. Preparation of CoP/NCNT-CP/NF The mixture of 5.0 mg of CoP/NCNT-CP, 450 μL of ethanol, and 50 μL of Nafion (5%) was ultrasonicated for 2 h, and then stiring for 24 h to obtain a uniform dispersion. After the CoP/NCNT-CP dispersal was dipped onto the above treated Ni foam, the CoP/NCNT-CP was gradually dried in room templeture. The loading amount of CoP/NCNT-CP on the Ni substrate was 0.28 mg cm-2 Characterization The field-emission scanning electron microscope (FESEM, Hitachi SU-8010) was used to examine the morphologies of all prepared materials. The elements composition was further analyzed by the energy-dispersive X-ray spectroscopy (EDX) analyzer (Oxford X-max80). The high-resolution transmission electron microscopy 6


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(HRTEM) and transmission electron microscope (TEM) results were characterized on JEM-2100. Powder X-ray diffraction (XRD) test was analyzed at X-pert Powder, PANalytical B.V. Raman spectra were measured with LabRAM HR Evolution. X-ray photoelectron spectroscopy (XPS) tests were carried out on (Thermo Fisher Scientific, Escalab 250Xi) with AI Ka radiation. Thermogravimetric analysis (TGA Q500) was analyzed in N2 with increasing rate of 10 °C min-1 up to 1000°C. The specific surface areas, pore volume, and pore size distribution of the products were analyzed by using the Brunauer–Emmett–Teller (BET) with N2 desorption and adsorption isotherms on a Surface Area and Porosity Analyzer (Micromeritics ASAP 2020) Electrochemical measurements All the electrochemical measurements were conducted on an electrochemical analyzer (CHI 760E) in 0.5 M H2SO4, 1.0 M KOH, and 1.0 M PBS electrolytes, and an Ag/AgCl and a graphite rod were used as reference electrode and counter electrode, respectively. A glassy carbon electrode (GCE, 3 mm in diameter) covered by catalysts was used as the working electrode. Generally, the catalyst ink was prepared by dispersing 5.0 mg of sample in a mixture of ethanol (450 μL) and 0.5 wt.% Nafion solution (50 μL), followed by sonication. 2 μL of ink was loaded on GCE to achieve a loading amount of 0.28 mg cm−2 and dried in ambient temperature. All the potentials were converted to reversible hydrogen electrode (RHE) via the Nernst equation (ERHE = EAg/Ag/Cl + 0.0591 × pH + 0.197). First, the cyclic voltammetry (CV) was used to stabilize the working electrode and scanned until the curves almost coincided. To get the polarization curve, linear sweep voltammetry with the scan rate of 5 mV s-1 was 7



performed.

All

polarization

curves

were

calibrated

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with

iR

correction.

Electrochemical stability tests were carried out by performing 1,000 CV cycles. The chronoamperometric method was used for durability test. The test range of electrochemical impedance spectroscopy (EIS) was from 100 K to 0.01 Hz and the AC amplitude was 10 mV. The electrochemically active surface area (ECSA) was measured by applying CV from 0.124 to 0.224 V (vs. RHE) at sweep rates of 20-120 mV s-1 in 1.0 M KOH. Results and discussion Characterization of CoP/NCNT-CP The synthesis method of CoP/NCNT-CP was presented in Figure 1. The CoP/NCNT-CP was fabricated by carbonization of ZIF-67 precursor under H2 atmosphere at 900 oC with a subsequent low temperature phosphatization treatment, in the presence of NaH2PO2 at 400 oC under N2 atmosphere. In this process, the metallic Co was quickly formed under H2, acted as a catalytic to promote the formation of NCNT.30 Furthermore, the Co NPs transformed into CoP NPs in the presence of PH3 that was generated by thermal decomposition of NaH2PO2.31 We systematically explored the influence of different phosphatization reaction time (1, 2, and 3 h) and temperature (300, 400, and 500 oC) (Figure S1-S5), the optimal phosphatization temperature and reaction time for CoP/NCNT-CP (400 oC and 2h) were ensured throughout this work. The FESEM image of Co/NCNT-CP features a rhombic dodecahedral structure with an average size of around 500 nm (Figure 2a). After the phosphatization treatment, 8


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the more obvious surface wrinkles of the rhombic dodecahedral structure could be observed for CoP/NCNT-CP. A closer observation reveals that CoP NPs embedded into the NCNT are grafted on the rough surface of carbon polyhedron (Figure 2b-2c). The presence of P, N, Co, and C elements in the CoP/NCNT-CP was confirmed by using elemental mapping and by EDX (Figure S6). The microstructure of CoP/NCNT-CP was identified by using TEM (Figure 2d). The TEM image of CoP/NCNT-CP reveals that the edges of carbon polyhedron surface are surrounded by NCNT with the diameter of ~20 nm. The magnified TEM image demonstrates that CoP NPs were successfully embedded in the NCNT (Figure 2e). The selected area electron diffraction (SAED) pattern of CoP/NCNT-CP shows several rings and scattered dots, which are assigned to the crystal planes of CoP and graphite carbon (inset of Figure 1d).32 The HRTEM images of CoP/NCNT-CP show that the CoP NPs were covered with carbon layers. A close interfacial contact was formed between the inner area with the lattice fringe spacings of 0.19, 0.24, and 0.28 nm corresponding to the (211), (112) and (011) planes of CoP, respectively,33 and outer area with the lattice fringe spacing of 0.34 nm attributed to (002) plane of carbon (Figure 2e-2f).34 XRD results of CoP/NCNT-CP showed that the peak at 26.2 o associated to the (002) plane of carbon,35 and the other clear peaks at 23.6o, 31.6 o, 35.3 o, 36.7 o, 46.2 o, 48.1 o, 52.3 o, and 56.8

o

are associated to the (011), (200), (102), (112), (211), and (301)

planes of CoP (JCPDS 29-0497),36-37 confirming the formation of CoP/NCNT-CP hybrid (Figure 3a). Raman spectrum of CoP/NCNT-CP displayed three notable peaks of D, G, and 2D bands at 1,356, 1,583, and 2,689 cm-1, respectively (Figure 3b).38 The 9



ID/IG value of 0.91 for CoP/NCNT-CP was much higher than that of Co/NCNT-CP (0.88), indicating that a more disordered carbon structure was involved in CoP/NCNT-CP after the phosphatization treatment. TGA results of CoP/NCNT-CP showed that the molar percentage of CoP was 20.6 % in the CoP/NCNT-CP based on the CoP conversion to Co3O4 (Figure S7). XPS survey spectrum of CoP/NCNT-CP showed the presence of P, N, Co, O, and C elements (Figure S8), while the O element appeared due to air exposure.39 The deconvolution of high-resolution C 1s spectra of CoP/NCNT-CP (Figure 3c) yielded three peaks centered at 285.9, 284.9, and 284.4 eV attributed to C-O, C-N, and C-C bonds, respectively.40 For the high-resolution N 1s spectra (Figure 3d), there are four different kinds of sub-peaks deconvoluted at 398.1, 400.2, 401.3, and 406.0 eV associated to pyridinic N, pyrrolic N, graphitic N, and oxidized N, respectively, indicating that the N atom was successfully embedded into the CoP/NCNT-CP.41 The content of N element was about 1.81 at.% (Table S1). In high-resolution Co 2p spectrum, there are four peaks at 799.0, 797.6, 783.0, and 781.6 eV, corresponding to Co2+ 2p1/2, Co3+ 2p1/2, Co2+ 2p3/2, and Co3+ 2p3/2, respectively (Figure 3e).42 Compared with the Co/NCNT-CP (Figure S9), the Co0 peaks disappeared, further indicating the formation of CoP. Similarly, in high-resolution P 2p spectrum (Figure 3f), two obvious peaks of Co-P and P-O bonds were observed at 129.5 and 134.3 eV, respectively.43 Electrochemical performance of CoP/NCNT-CP The electrocatalytic activity of CoP/NCNT-CP for HER in a broad pH range was examined by using a conventional three-electrode system. In contrast, the 10


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Co/NCNT-CP and commercial 20 wt.% Pt/C were tested with the loading amounts of 0.28 mg cm-2. The Co/NCNT-CP without phosphatization treatment displayed a poor HER activity in basic solution and afford the current density of 10 mA cm-2 with overpotential of 369 mV (Figure 4a). After phosphatization treatment, the CoP/NCNT-CP exhibited higher catalytic performance and showed up overpotential of 271 mV to attain the current density of 10 mA cm-2. For comparison, when tested separately, both NCNT-CP and bare CoP exhibited much higher overpotentials of 583 mV and 465 mV at the 10 mA cm-2 than the CoP/NCNT-CP. This result reveals that a synergistic effect between CoP and NCNT-CP plays a key role for better HER performance. In order to further enhance the HER performance, the CoP/NCNT-CP loaded on the Ni foam exhibited a much lower overpotential of 165 mV at 10 mA cm-2, which is very competitive with the previously reported Co-based hybrid electrocatalysts and commercial Pt/C (38 mV) (Table S2). The high HER performance was further supported by the Tafel slope. It is observed that a Tafel slope of 96 mV dec-1 for CoP/NCNT-CP was much smaller than that of Co/NCNT-CP (155 mV dec-1) (Figure 4b). The small value of Tafel slope indicates a favorable HER kinetics for CoP/NCNT-CP. The electrochemical impedance spectroscopy (EIS) (Figure 4c) showed that the CoP/NCNT-CP possessed a much smaller radius of semicircle than the Co/NCNT-CP (Table S3), suggesting a much lower charge-transfer resistance. Figure 4d displays the measurement of multi-step chronopotentiometric curve of CoP/NCNT-CP with the increasing value of current density from 10 to 60 mA cm-2. It is observed that the potential immediately started at 11



-0.27 V and remained constant for 100 s, proclaiming the excellent mechanical robustness and mass transport property of CoP/NCNT-CP.44 We also investigated the stability of CoP/NCNT-CP during 1,000 cyclic voltammetry (CV) cycles (Figure 4e). The polarization curve of CoP/NCNT-CP exhibited no significant increase to the driving potential. Also, the current density of CoP/NCNT-CP remained almost stable for 10 h of continuous operation, suggesting excellent catalyst stability (inset of Figure 4e). To investigate the difference in HER activity between Co/NCNT-CP and CoP/NCNT-CP, the ECSA measurements was conducted by a CV method in the non-HER region (Figure S10).45 The electrochemical double-layer capacitance (Cdl) of Co/NCNT-CP and CoP/NCNT-CP were calculated to be 6.7 and 11.9 mF cm−2 in 1.0 M KOH (Figure 4f), respectively, suggesting a higher density of active catalytic sites incorporated in the CoP/NCNT-CP. Although the CoP/NCNT-CP catalyst has a smaller BET surface area (23 m2 g-1) than the Co/NCNT-CP (211 m2 g-1) (Figure S11), the former has a much higher HER activity than the later, further confirming the high catalytic performance of the CoP/NCNT-CP in HER.46 Besides the efficient operation in basic media, the electrocatalytic activities of CoP/NCNT-CP for HER were also studied in acid and neutral media. In Figure 5a, the CoP/NCNT-CP showed a much lower overpotential of 203 mV at the current density of 10 mA cm-2 in 0.5 M H2SO4, compared with the Co/NCNT-CP (260 mV at 10 mA cm-2). Additionally, the CoP/NCNT-CP had a smaller Tafel slope of 56 mV dec-1 than Co/NCNT-CP (93 mV dec-1) (Figure 5b). Similarly, the polarization curve of 12


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CoP/NCNT-CP does not show any significant increase in 0.5 M H2SO4 to drive the potential after 1,000 CV cycles (Figure 5c), thus demonstrating remarkable stability. Also, the chronopotrntiometry curve confirmed that CoP/NCNT-CP is stable in acidic solution at 10 mA cm-2 (inset of Figure 5c). Furthermore, the CoP/NCNT-CP (Figure 5d) displayed a lower Tafel slope of 100 mV dec-1 and a much smaller overpotential of 305 mV at the current density of 10 mA cm-2, compared with the Co/NCNT-CP (249 mV dec-1, and 542 mV at 10 mA cm-2) in 1.0 M PBS (Figure 5e). The polarization curve of CoP/NCNT-CP in 1.0 M PBS exhibited no obvious change to drive the potential after 1,000 CV cycles with a stable current density for 10 h, which suggested the good stability for CoP/NCNT-CP in neutral media (Figure 5f). Capability of energy-saving hydrogen evolution To improve the energy efficiency of overall water splitting process, the OER was replaced by HzOR. As shown in Figure 6a, the CoP/NCNT-CP achieved the current density of 10 mA cm-2 at an overpotential of 0.65 V in 1.0 M KOH, which was even smaller than that of the theoretical voltage value for OER. To prove the influence of hydrazine in water splitting, the sensing performance of CoP/NCNT-CP was studied at different potentials with successive addition of 0.2 mM hydrazine. The current density of CoP/NCNT-CP was significantly increased after adding hydrazine at applied potentials of 0.8, 1.0, and 1.2 V (Figure S12). Considering that a relatively low potential is beneficial to reduce the interference of background current, the potential of 1.0 V was selected as the working potential for amperometric detection of hydrazine.47 A typical amperometric response of CoP/NCNT-CP was examined by 13



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successive introduction of hydrazine with different amounts into 1.0 M KOH (Figure 6b). The CoP/NCNT-CP exhibited a step-wise increase in current density at 1.0 V, in response to addition of hydrazine, indicating that the CoP/NCNT-CP responded sensitively and rapidly for HzOR. Figure 6c quantifies the performance two-electrode system via using CoP/NCNT-CP as both cathode and anode (CoP/NCNT-CP || CoP/NCNT-CP) in 1.0 M KOH with and without 0.5 M N2H4. The bifunctional CoP/NCNT-CP || CoP/NCNT-CP electrocatalytic system showed superior catalytic activity with a potential of 0.89 V at 10 mA cm-2 in 1.0 M KOH with 0.5 M N2H4, which was much lower than the voltage of overall water splitting (1.91 V at 10 mA cm-2) in 1.0 M KOH. These results demonstrated that replacing OER with HZOR is effective for energy-saving electrolytic

HER.

CoP/NCNT-CP

Additionally,

electrocatalyst

the

was

outstanding confirmed

by

stability the

of

bifunctional

chronopotentiometry

experiments at a current density of 10 mA cm-2 for 10 h of continues reaction in 1.0 M KOH with 0.5 M N2H4 (Figure 6d). Conclusion In summary, a novel 3D CoP/NCNT-CP hybrid was developed via carbonization treatment of ZIF-67 precursor and subsequent phosphatization treatment. The carbon polyhedron with the average size of ~500 nm was surrounded by the CoP NPs embedded in NCNT with a diameter of ~20 nm. The CoP/NCNT-CP loaded on Ni foam, possessing strong synergistic effect and unique 3D structure, showed significantly improved HER performance in basic solution with a low overpotential of 14


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165 mV at 10 mA cm-2, which is competitive with the previously reported Co-based hybrid

electrocatalysts.

The

CoP/NCNT-CP

also

delivered

good

catalytic

performance in acid and neutral media, with overpotentials of 203 mV and 305 mV at the current density of 10 mA cm-2, respectively. Additionally, using CoP/NCNT-CP both as a cathode for HER and anode for HzOR, the required potential was reduced to merely 0.89 V to achieve the current density of 10 mA cm-2 for energy-efficient HER. This potential was much lower than that of for overall water splitting (1.91 V at 10 mA cm-2), proving the capability of energy-saving electrolytic HER. This carbon nanoarchitecture hybrid with synergistic effect has strong potential for further development of metal phosphate-carbon hybrid electrocatalysts for other important electrochemical applications, such as CO2 reduction reactions, nitrogen reduction reactions, and ammonia oxidation reactions. Acknowledgements Y. Hou thanks the support of National Natural Science Foundation of China (51702284, 21878270), Zhejiang Provincial Natural Science Foundation of China (LR19B060002), and the Startup Foundation for Hundred-Talent Program of Zhejiang University (112100-193820101/001/022). Q. Dai thanks the Basic Public Projects of Zhejiang Province (LGJ18E080001). K.O. thanks the Australian Research Council for partial support. Supporting Information The supporting information is available free of charge on ACS Publication website. FESEM, EDX, XRD, Raman, BET, XPS, TGA, features of CoP/NCNT, 15



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electrochemical HER measurements, digital picture of HzOR/HER cell (Figures S1-S16), comparison of HER activity of CoP/NCNT-CP/NF with recently reported catalysts (Tables S1-S3). Notes The authors declare no competing ﬁnancial interest Corresponding authors: Prof. Dr. Yang Hou, E-mail: [email protected] Prof. Dr. Qizhou Dai, E-mail: [email protected] References 1.

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Li, J.; Mao, S.; Hou, Y.; Lei, L.; Yuan, C., 3D Edge-Enriched Fe3 C@C

Nanocrystals with a Core-Shell Structure Grown on Reduced Graphene Oxide Networks for Efficient Oxygen Reduction Reaction. ChemSusChem 2018, 11 (18), 3292-3298. 17



Page 18 of 31

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Ruthenium

Nanoparticles

as

an

Efficient

Electrocatalyst for Hydrogen Production in Alkaline Media. Adv. Mater. 2018, 30 (31), 1800676. 14. Xu, K.; Ding, H.; Zhang, M.; Chen, M.; Hao, Z.; Zhang, L.; Wu, C.; Xie, Y., Regulating Water-Reduction Kinetics in Cobalt Phosphide for Enhancing HER Catalytic Activity in Alkaline Solution. Adv. Mater. 2017, 29 (28), 1606980. 15. Hou, Y.; Qiu, M.; Zhang, T.; Zhuang, X.; Kim, C. S.; Yuan, C.; Feng, X., Ternary Porous Cobalt Phosphoselenide Nanosheets: An Efficient Electrocatalyst for

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Electrocatalytic and Photoelectrochemical Water Splitting. Adv. Mater. 2017, 29 (35), 1701589. 16. Chen, P.; Xu, K.; Fang, Z.; Tong, Y.; Wu, J.; Lu, X.; Peng, X.; Ding, H.; Wu, C.; Xie, Y., Metallic Co4N Porous Nanowire Arrays Activated by Surface Oxidation as Electrocatalysts for the Oxygen Evolution Reaction. Angew. Chem. Int. Ed. 2015, 54 (49), 14710-14714. 17. Tang, C.; Cheng, N.; Pu, Z.; Xing, W.; Sun, X., NiSe Nanowire Film Supported on Nickel Foam: An Efficient and Stable 3D Bifunctional Electrode for Full Water Splitting. Angew. Chem. Int. Ed. 2015, 54 (32), 9351-9355. 18. Cummins, D. R.; Martinez, U.; Sherehiy, A.; Kappera, R.; Martinez-Garcia, A.; Schulze, R. K.; Jasinski, J.; Zhang, J.; Gupta, R. K.; Lou, J.; Chhowalla, M.; Sumanasekera, G.; Mohite, A. D.; Sunkara, M. K.; Gupta, G., Efficient hydrogen evolution in transition metal dichalcogenides via a simple one-step hydrazine reaction. Nat. Commun. 2016, 7, 11857. 19. Chen, W. F.; Muckerman, J. T.; Fujita, E., Recent developments in transition metal carbides and nitrides as hydrogen evolution electrocatalysts. Chem. Commun. 2013, 49 (79), 8896-8909. 20. Vrubel, H.; Hu, X., Molybdenum boride and carbide catalyze hydrogen evolution in both acidic and basic solutions. Angew. Chem. Int. Ed. 2012, 51 (51), 12703-12706. 21. Zhou, Q.; Shen, Z.; Zhu, C.; Li, J.; Ding, Z.; Wang, P.; Pan, F.; Zhang, Z.; Ma, H.; Wang, S.; Zhang, H., Nitrogen-Doped CoP Electrocatalysts for Coupled Hydrogen

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Evolution and Sulfur Generation with Low Energy Consumption. Adv. Mater. 2018, 30 (27), 1800140. 22. Ma, Y. Y.; Wu, C. X.; Feng, X. J.; Tan, H. Q.; Yan, L. K.; Liu, Y.; Kang, Z. H.; Wang, E. B.; Li, Y. G., Highly efficient hydrogen evolution from seawater by a low-cost and stable CoMoP@C electrocatalyst superior to Pt/C. Energy Environ. Sci. 2017, 10 (3), 788-798. 23. Yan, Y.; Shi, X. R.; Miao, M.; He, T.; Dong, Z. H.; Zhan, K.; Yang, J. H.; Zhao, B.; Xia, B. Y., Bio-inspired design of hierarchical FeP nanostructure arrays for the hydrogen evolution reaction. Nano Research 2018, 11 (7), 3537-3547. 24. Feng, Y.; Yu, X. Y.; Paik, U., Nickel cobalt phosphides quasi-hollow nanocubes as an efficient electrocatalyst for hydrogen evolution in alkaline solution. Chem. Commun. 2016, 52 (8), 1633-1636. 25. Hou, Y.; Qiu, M.; Zhang, T.; Ma, J.; Liu, S.; Zhuang, X.; Yuan, C.; Feng, X., Efficient Electrochemical and Photoelectrochemical Water Splitting by a 3D Nanostructured Carbon Supported on Flexible Exfoliated Graphene Foil. Adv. Mater. 2017, 29 (3), 1604480. 26. He, Y.; Zhuang, X.; Lei, C.; Lei, L.; Hou, Y.; Mai, Y.; Feng, X., Porous carbon nanosheets: Synthetic strategies and electrochemical energy related applications. Nano Today 2019, 24, 103-119. 27. Zhang, J. Y.; Wang, H.; Tian, Y.; Yan, Y.; Xue, Q.; He, T.; Liu, H.; Wang, C.; Chen, Y.; Xia, B. Y., Anodic Hydrazine Oxidation Assists Energy-Efficient Hydrogen

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Evolution over a Bifunctional Cobalt Perselenide Nanosheet Electrode. Angew. Chem. Int. Ed. 2018, 57 (26), 7649-7653. 28. Luo, Q.; Peng, M. Y.; Sun, X. P.; Asiri, A. M., Hierarchical nickel oxide nanosheet@nanowire arrays on nickel foam: an efficient 3D electrode for methanol electro-oxidation. Catal. Sci. Technol. 2016, 6 (4), 1157-1161. 29. Liu, T. T.; Liu, D. N.; Qu, F. L.; Wang, D. X.; Zhang, L.; Ge, R. X.; Hao, S.; Ma, Y. J.; Du, G.; Asiri, A. M.; Chen, L.; Sun, X. P., Enhanced Electrocatalysis for Energy-Efficient Hydrogen Production over CoP Catalyst with Nonelectroactive Zn as a Promoter. Adv. Energy Mater. 2017, 7 (15), 1700020. 30. Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W.; Wang, X., A metal–organic framework-derived bifunctional oxygen electrocatalyst. Nature Energy 2016, 1 (1), 15006. 31. Yang, X. L.; Lu, A. Y.; Zhu, Y. H.; Hedhili, M. N.; Min, S. X.; Huang, K. W.; Han, Y.; Lin, L. J., CoP nanosheet assembly grown on carbon cloth: A highly efficient electrocatalyst for hydrogen generation. Nano Energy 2015, 15, 634-641. 32. Mishra, I. K.; Zhou, H. Q.; Sun, J. Y.; Qin, F.; Dahal, K.; Bao, J. M.; Chen, S.; Ren, Z. F., Hierarchical CoP/Ni5P4/CoP microsheet arrays as a robust pH-universal electrocatalyst for efficient hydrogen generation. Energy Environ. Sci. 2018, 11 (8), 2246-2252. 33. Popczun, E. J.; Roske, C. W.; Read, C. G.; Crompton, J. C.; McEnaney, J. M.; Callejas, J. F.; Lewis, N. S.; Schaak, R. E., Highly branched cobalt phosphide

21



nanostructures for hydrogen-evolution electrocatalysis. J. Mater. Chem. A. 2015, 3 (10), 5420-5425. 34. Liu, M. R.; Hong, Q. L.; Li, Q. H.; Du, Y. H.; Zhang, H. X.; Chen, S. M.; Zhou, T. H.; Zhang, J., Cobalt Boron Imidazolate Framework Derived Cobalt Nanoparticles Encapsulated in B/N Codoped Nanocarbon as Efficient Bifunctional Electrocatalysts for Overall Water Splitting. Adv. Funct. Mater. 2018, 28 (26), 1801136. 35. Sivanantham, A.; Ganesan, P.; Estevez, L.; McGrail, B. P.; Motkuri, R. K.; Shanmugam, S., A Stable Graphitic, Nanocarbon-Encapsulated, Cobalt-Rich Core-Shell Electrocatalyst as an Oxygen Electrode in a Water Electrolyzer. Adv. Energy Mater. 2018, 8 (14), 1702838. 36. Zhang, H.; Ma, Z.; Duan, J.; Liu, H.; Liu, G.; Wang, T.; Chang, K.; Li, M.; Shi, L.; Meng, X.; Wu, K.; Ye, J., Active Sites Implanted Carbon Cages in Core-Shell Architecture: Highly Active and Durable Electrocatalyst for Hydrogen Evolution Reaction. ACS Nano 2016, 10 (1), 684-694. 37. Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Norskov, J. K.; Jaramillo, T. F., Combining theory and experiment in electrocatalysis: Insights into materials design. Science 2017, 355, 146. 38. Ciesielski, A.; Samori, P., Graphene via sonication assisted liquid-phase exfoliation. Chem. Soc. Rev. 2014, 43 (1), 381-398. 39. Chang, J. F.; Xiao, Y.; Xiao, M. L.; Ge, J. J.; Liu, C. P.; Xing, W., Surface Oxidized Cobalt-Phosphide Nanorods As an Advanced Oxygen Evolution Catalyst in Alkaline Solution. ACS Catal. 2015, 5 (11), 6874-6878. 22


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40. Li, J. S.; Du, B.; Lu, Z. H.; Meng, Q. T.; Sha, J. Q., In situ-generated Co@nitrogen-doped carbon nanotubes derived from MOFs for efficient hydrogen evolution under both alkaline and acidic conditions. New J. Chem. 2017, 41 (19), 10966-10971. 41. Hou, Y.; Qiu, M.; Kim, M. G.; Liu, P.; Nam, G.; Zhang, T.; Zhuang, X.; Yang, B.; Cho, J.; Chen, M.; Yuan, C.; Lei, L.; Feng, X., Atomically dispersed nickel-nitrogen-sulfur species anchored on porous carbon nanosheets for efficient water oxidation. Nat. Commun. 2019, 10 (1), 1392. 42. Qiu, B. C.; Cai, L. J.; Wang, Y.; Lin, Z. Y.; Zuo, Y. P.; Wang, M. Y.; Chai, Y., Fabrication of Nickel-Cobalt Bimetal Phosphide Nanocages for Enhanced Oxygen Evolution Catalysis. Adv. Funct. Mater. 2018, 28 (17), 1706008. 43. Li, F.; Bu, Y.; Lv, Z.; Mahmood, J.; Han, G. F.; Ahmad, I.; Kim, G.; Zhong, Q.; Baek, J. B., Porous Cobalt Phosphide Polyhedrons with Iron Doping as an Efficient Bifunctional Electrocatalyst. Small 2017, 13 (40), 1701167. 44. Xie, L.; Zhang, R.; Cui, L.; Liu, D.; Hao, S.; Ma, Y.; Du, G.; Asiri, A. M.; Sun, X., High-Performance Electrolytic Oxygen Evolution in Neutral Media Catalyzed by a Cobalt Phosphate Nanoarray. Angew. Chem. Int. Ed. 2017, 56 (4), 1064-1068. 45. Zhao, W.; Lu, X.; Selvaraj, M.; Wei, W.; Jiang, Z.; Ullah, N.; Liu, J.; Xie, J., MXP(M = Co/Ni)@carbon core-shell nanoparticles embedded in 3D cross-linked graphene aerogel derived from seaweed biomass for hydrogen evolution reaction. Nanoscale 2018, 10 (20), 9698-9706.

23



46. Mao, S.; Wen, Z.; Huang, T.; Hou, Y.; Chen, J., High-performance bi-functional electrocatalysts of 3D crumpled graphene–cobalt oxide nanohybrids for oxygen reduction and evolution reactions. Energy Environ. Sci. 2014, 7 (2), 609-616. 47. Zhang, E.; Xie, Y.; Ci, S.; Jia, J.; Cai, P.; Yi, L.; Wen, Z., Multifunctional high-activity and robust electrocatalyst derived from metal–organic frameworks. . Mater. Chem. A. 2016, 4 (44), 17288-17298.

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Page 24 of 31

Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Schematic illustration for the synthesis process of CoP/NCNT-CP.

25



Figure 2. FESEM images of Co/NCNT-CP (a) and CoP/NCNT-CP (b-c), (d) TEM and (e-f) HRTEM images of CoP/NCNT-CP. Inset: SAED pattern of CoP/NCNT-CP.

26


Page 26 of 31

(011) (200)

(112) (211) (102) (301)

G

D

ID / IG = 0.88

30

40

2θ (degree)

50

60

1200

N 1s Graphitic N

Pyrrolic N

Oxidized N

408

404

400

Binding energy (eV)

396

2400

Co3+

Co3+

Co3+ sat.

2+

Co sat. 810

2+

Co sat.

Co2+

C-C (284.5 eV)

C-N (284.8 eV) C-O (285.9 eV)

292

3000

288

284

Binding energy (eV)

280

(f)

Co 2p

Co2+

Pyridinic N

412

1800

Raman shift (cm-1)

(e) Intensity (a.u.)

(d)

2D

ID / IG = 0.91

Co/NCNT-CP CoP/NCNT-CP

20

Intensity (a.u.)

(200)

C (002)

C 1s

(c)


(b)

(111)

Intensity (a.u.)

Intensity (a.u.)

(a)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity (a.u.)

Page 27 of 31

P 2p

133.9 eV

134.6 eV

129.5 eV

Co3+ sat.

800

790

780

Binding energy (eV)

770 140

135

130

Binding energy (eV)

125

Figure 3. (a-b) XRD patterns and Raman spectra of Co/NCNT-CP and CoP/NCNT-CP, high-resolution XPS spectra for (c) C 1s, (d) N 1s, (e) Co 2p, and (f) P 2p of CoP/NCNT-CP.

27



CoP NCNT-CP CoP/NCNT-CP

-30

-40 -0.6

-0.4

-0.2

0.0

Potential (V vs. RHE)

Co/NCNT-CP CoP/NCNT-CP CoP/NCNT-CP/NF Pt/C

-60

-80 -1.2

-0.8

-0.9

-0.6

-0.3

(d)

-0.6

-2

-2

40 mA cm -2

20 mA cm

-0.4

60 mA cm -2

50 mA cm

-2

30 mA cm

-2

10 mA cm

-0.2

0.0

0

Figure

150

4.

300

Time (s)

(a)

450

600

(c)

mV 155

0.4

dec

-1

c 96 mV de

CPE

Rs


75 -1

Rct

50

25

0.2 -1

dec 36 mV 0.0 0.0

0.0


0.6

-Z'' (ohm)

-20

Co/NCNT-CP CoP/NCNT-CP Pt/C

0 -10

0.4

0.8

-2

1.2

Log [J (mA cm )]

-0.6

Potential (V vs.RHE)

-10

100

(b)

1.6

(e)

-0.2

-20

0.0

0

2

4 6 Time (h)

8

10

-30 -40

initial after 1000 cycles

-50 -1.0

-0.8

0

1.5

10 mA cm-2

-0.4

△ J (mA cm-2)


-40

0

Current density (mA cm-2)

-20

0.8

(a) Current density (mA cm-2)


0

Potential ( V vs RHE )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

0

50

100

Z' (ohm)

150

200

(f) Co/NCNT-CP CoP/NCNT-CP

1.2

-2

0.9

. 11

F 9m

0.6

cm

c mF 6.7

-2

m

0.3

Polarization

-0.6

-0.4

-0.2


curves

of

0.0

20

Co/NCNT-CP,

40

60

80

100

Scan rate (mV s-1)

120

CoP/NCNT-CP,

CoP/NCNT-CP/NF and Pt/C. Inset: Polarization curves of NCNT-CP, CoP, and CoP/NCNT-CP. (b) Tafel slopes of Co/NCNT-CP, CoP/NCNT-CP, and Pt/C. (c) EIS Nyquist plots of Co/NCNT-CP and CoP/NCNT-CP. (d) Multi-potential step curve of CoP/NCNT-CP. (e) Polarization curves of CoP/NCNT-CP before and after 1000 cycles. Inset: Chonopotentiometry at a constant current density of 10 mA cm−2 for CoP/NCNT-CP. (f) ECSAs of Co/NCNT-CP and CoP/NCNT-CP. Electrolyte (a-f): 1.0 M KOH. The Ni foam acted as working electrode in the CoP/NCNT-CP/NF, and the glassy carbon electrode acted as working electrode in other samples.

28


-40

-60

-80 -0.6

-0.2

1.0 M PBS

-10

-20


-30

-40 -0.8

0.2

-0.6

-0.4

-0.2


0.0

0.8

1.2

1.6

(e) 1.0 M PBS

0.6

0.2

0.0 0.0

-1

ec Vd 9m 24

0.4

-1

0.8

2

0

4

6

1.2

1.6

Log [J (mA cm )]

8

Time (h)

10


-0.6

-0.4

-0.2

0.0

Potential (V vs. RHE) -0.6

(f)

1.0 M PBS 10 mA cm-2

-0.4

-0.2

0.0

0

2

4

6

Time (h)

-30

-2

(c)

10 mA cm-2

initial after 1000 cycles

-20

V dec -1 100 m dec 57 mV 0.4

0

-80 -0.8

-10


0.0

-60

-1

34 mV dec

Log [J (mA cm-2)]

0.5 M H2SO4

-0.2

-40

-1

0.4

-0.6

-0.4

-20

dec 56 mV

0.1

0.8

(d)

-1

c V de 93 m


0.0 0.0

0.0


0

-0.4


0.3

0.5 M H2SO4

0




0.5 M H2SO4

(b)


-20

0.4

(a)


0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 29 of 31

-40 -1.0

8

10

initial after 1000 cycles -0.8

-0.6

-0.4

-0.2

0.0


Figure 5. (a) Polarization curves of Co/NCNT-CP, CoP/NCNT-CP, and Pt/C. (b) Tafel slopes of Co/NCNT-CP, CoP/NCNT-CP, and Pt/C. (c) Chonopotentiometry curve at a constant current density of 10 mA cm−2 for CoP/NCNT-CP. (d) Polarization curves of Co/NCNT-CP, CoP/NCNT-CP, and Pt/C. (e) Tafel slopes of Co/NCNT-CP, CoP/NCNT-CP, and Pt/C. (f) Chonopotentiometry curve at a constant current density of 10 mA cm−2 for CoP/NCNT-CP. Insets: Chonopotentiometry curves at a constant current density of 10 mA cm−2 for CoP/NCNT-CP. Electrolyte (a-c): 0.5 M H2SO4; Electrolyke (d-f): 1.0 M PBS. The Ni foam acted as working electrode in the CoP/NCNT-CP/NF, and the glassy carbon electrode acted as working electrode in other samples.

29


1.0 M KOH 1.0 M KOH + 0.5 M N2H4

25

0 0.4 60

0.8

1.4

1.6


10

1.8

1.0 M KOH + 0.5 M N2H4 1.0 M KOH

20

0.6

1.2

Potential (V)

200

400

Time (s)

10 mM

5.0 mM

600

800

(d) 1.0 M KOH + 0.5 M N2H4

3

1.0 M KOH

2

1

1.02 V

0 0.0

0

4

(c)

40

0

2.5 mM

20

1.0 mM

OER

HzOR

30

0.5 mM

50

Page 30 of 31

(b)

0.2 mM

75

40

0.1 mM

(a)

Potential (V)


100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Current density ( mA cm-2 )


1.8

2.4

0

0

2

4

6

Time (h)

8

10

Figure 6. (a) Polarization curves of CoP/NCNT-CP in 1.0 KOH with and without 0.5 M N2H4. (b) Typical amperometric response of CoP/NCNT-CP at 1.0 V with successive addition of N2H4 into 1.0 M KOH. (c) Polarization curves of CoP/NCNT-CP || CoP/NCNT-CP couple in 1.0 M KOH with and without 0.5 M N2H4. (d) Chronopotentiometric curves of CoP/NCNT-CP || CoP/NCNT-CP couple in 1.0 M KOH with and without 0.5 M N2H4. The Ni foam acted as working electrode in the CoP/NCNT-CP/NF, and the glassy carbon electrode acted as working electrode in other samples.

30


Page 31 of 31

Table of Content Current density (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 0.5 M H2SO4 1.0 M PBS 1.0 M KOH

-20

-40

-60

-80

-0.8

-0.6

-0.4

-0.2


0.0

H2 H2O

OHN2

N2H4

H2O

OH-

A novel 3D carbon nanoarchitecture hybrid electrocatalyst is developed for energy-saving hydrogen evolution in a wide pH range.

31


ZIF-Derived Carbon Nanoarchitecture as a Bifunctional pH-Universal

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