Porous N-doped carbon encapsulated CoNi alloy nanoparticles

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Porous N-doped carbon encapsulated CoNi alloy nanoparticles derived from MOFs as efficient bifunctional oxygen electrocatalysts Honghui Ning, Guoqiang Li, Yu Chen, Kaikai Zhang, Zhuang Gong, Renfeng Nie, Wei Hu, and Qinghua Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13290 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 21, 2018

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ACS Applied Materials & Interfaces

Porous N-doped carbon encapsulated CoNi alloy nanoparticles derived from MOFs as efficient bifunctional oxygen electrocatalysts Honghui Ning, Guoqiang Li, Yu Chen, Kaikai Zhang, Zhuang Gong, Renfeng Nie, Wei Hu*, Qinghua Xia Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education Key Laboratory for the Synthesis and Applications of Organic Functional Molecules, Hubei University, Wuhan 430062, China. KEYWORDS. Bifunctional oxygen electrocatalysts, bimetallic metal-organic framework composite, oxygen evolution reaction, alkaline electrolyte, CoNi alloy, oxygen reduction reaction.

ABSTRACT. A porous N-doped carbon encapsulated CoNi alloy nanoparticle composite (CoNi@N-C) was prepared using the bimetallic MOFs composite as the precursor. The optimal prepared Co1Ni1@N-C material at 800 oC exhibited well-defined porosities, uniform CoNi alloy nanoparticle dispersion, a highly doped-N level, and scattered CoNi-Nx active sites, therefore afforded excellent oxygen catalytic activities towards the reduction and evolution process of oxygen. The ORR onset potential (Eonset) on Co1Ni1@N-C was 0.91 V and the half-wave 1 ACS Paragon Plus Environment

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potential (E1/2) was 0.82 V, very close to the parameters recorded on the Pt/C (20wt Pt%) benchmark. Moreover, it was worth noting that the ORR stability of Co1Ni1@N-C was prominent higher than that of Pt/C. Under OER condition, Co1Ni1@N-C generated the maximum current density at the potential of 1.7 V (8.60 mA cm-2), and the earliest Eonset (1.35 V) among all CoxNiy@N-C hybrids. Co1Ni1@N-C catalyst exhibited the smallest ΔE value, confirming the superior bifunctional activity. The high surface area and porosity, and CoNi-Nx active sites in carbon surface including the proper interactions between the N-doped C shell and CoNi nanoparticles were attributed as the main contributors to the outstanding oxygen electrocatalytic property and good stability.

1. Introduction Many clean and sustainable energy sources or emerging energy storage devices are strongly dependent on the conversion process between water and oxygen1,2, that is the oxygen reduction (ORR) and evolution reaction (OER). Nevertheless because above two processes have slow kinetics which leads to high overpotentials on electrodes, energy device efficiency has been severely limited. So far, compounds based on the precious Pt are still recognized as the best satisfactory catalysts towards ORR, but their OER catalytic activity is much poor. Other noble metals, such as Ir and/or Ru oxides exhibit the superior OER activities, while their ORR activities are unsatisfactory. In addition, high prices, scarce reserves as well as declining activities in operation3,4 make above mentioned catalysts unqualified for industrialization. Therefore, development of cheap and efficient bifunctional materials for oxygen electrocatalysis deserves being highly concerned. In recent years, various cheap materials, including transition metal oxides5,6, carbides/nitrides and their composites7,8, and heteroatoms (N/S or N/S-metal 2 ACS Paragon Plus Environment

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etc.) doped carbon9-12 have been developed as the alternative catalysts toward the oxygen electrochemical processes. It is believed that the heteroatoms-doping into carbon will modify the electronic structure and provide carbon defects, resulting in the enhanced performance for ORR. Nevertheless, the facile preparation and rational design of structures still remain the great challenges for exploiting cost-efficient bifunctional catalysts. Lately, some transition metal composites (i.e. Co, Mn, Fe or Cu) with heteroatom-doped carbons as the supports or shells represent a kind of promising alternatives for oxygen electrocatalysis, attributed to low costs and unique electronic properties13-17. Thanks to the high surface area, defined porous structures and ordered frames, metal-organic frameworks (MOFs) are generally applied as attractive templates or precursors toward porous carbon encapsulated metal (M@C) materials, and the MOFs-derived catalysts obtained by the subsequent pyrolysis keep these similar structure advantages18-22. Moreover, the abundant organic ligands in MOFs with N, S or P heteroatom will be pyrolyzed to hetroatom-doped carbons, which improve the conductivity and oxygen electrocatalytic activity23. Meanwhile, the metal precursors around by organic ligands can be in situ reduced to single metal or bi-metal alloy cores encapsulated in a heteroatom-doped C frame after pyrolysis under the inert gas atmosphere, which would modulate the electronic structure of active centers resulting in the acceleration of the catalytic reaction kinetics. To date, a various of MOFs-based Co, Fe and other transition metal nanocatalysts were reported as high efficient oxygen electrocatalysts, such as the Co-MOFs derived Co@N-doped C24,25, a hybrid named Fe3C@NCNT/NPC annealed from the Fe-MOFs26, a N-doping C@graphitic C carbonized from ZIF-8@ZIF-67 with Co doping and high nitrogen content18. Xu and coworkers have summarized that the preparation strategies or regulation means can effectively control the morphology, size, chemical composition as well as the structure of the

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MOFs-based nanomaterials, and also pointed out that it is very beneficial for improving performance by engineering these properties of MOFs-derived materials27,28. However, traditional MOFs-derived materials are mainly based on simple MOFs crystals as a single precursor, which thus mainly exhibit relatively simple configurations. Furthermore, compare to mono-metallic catalyst or active site, it is well documented that catalysts with bimetallic or multi-metallic active sites exhibit more attractive characters and higher catalytic activity because of the unique electronic effect and extra synergistic effect29,30. Unfortunately, integrating two or more metals into a single MOFs module is of difficulty due to the mononuclear metal center of many simple MOFs. Therefore, it is necessary to search a technically feasible solution to prepare MOFs composites by taking dual or many different MOFs and/or other nanomaterials with different functional species as co-precursors. For examples, Wu and Lou et al. developed a novel synthetic approach for preparing the FeCo alloy enclosed by porous N-C cage via pyrolysis of the multiple precursors including the Zn-MOF kernel and Co-MOF shell wrapped with plenty of FeOOH nanorods31, and they also prepared a multi-metal oxyphosphide nanoparticles with complex multilayers by thermolysis of Mn-Co polymer precursors32, as well as Co/N-C hollow particles with a single-hole taking PS@ZIF-67 hybrid as coprecursors33. Inspired by these strategies, rational design and controlled synthesis of various energy-related functional materials based on MOFs become more feasible. In addition to elegant design of different MOFs-based precursors with interesting structures, composition regulation is highly beneficial for controlling bifunctional activity of these MOFsderived carbon materials. It was reported ORR/OER activity of Fe was higher than that of Co and Ni34, and synergism between various metal phases will enhance the electrocatalytic activities34. Despite the better activity for ORR/OER given by Fe, relative lower conductivity 4 ACS Paragon Plus Environment

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and poor stability restricted its application35. It was recently demonstrated that the bimetallic alloy formed by Co and Ni was very favorable for various catalytic reactions, especially the ORR and OER12. Thus, we demonstrate a coprecursor strategy based on a bimetallic MOF hybrid, that is Co- and Ni-zeolitic imidazolate framework (Co- and Ni-ZIF) to fabricate a porous N-doped carbon encapsulated CoNi alloy nanoparticle (CoxNiy@N-C) as catalysts for oxygen electrocatalysis. And we consider that the thermolysis temperature and metal molar ratio are two crucial factors for the regulation of morphology and chemical composition of active centers in the resulted catalysts. Different from earlier reports using Co-ZIF or other monometallic MOFs as a precursor, the present approach realizes the one-step coordination of two metals with 2methylimidazole ligands to obtain a bimetallic ZIF composite including Co-ZIF and Ni-ZIF. This bimetallic hybrid is then developed to CoNi alloy particles wrapped by the porous N-doping carbon cages after a pyrolysis process, and the optimal CoNi(1:1)@N-C(800) sample possesses the anticipated activity for ORR and OER. 2. Experimental Section All reagents in this experiment were all analytical purity (AR) and used directly. In a typical synthesis, two metallic nitrates and ligands, i.e. 2-methylimidazole (2-MeIM) were dissolved in methanol separately. Then, the Ni- and Co-containing solution was dropped into MeIMcontaining solution slowly with continuous stirring. One hour later, the obtained mixture was diverted into an autoclave (50 ml), and stayed at 100 °C for 12 hours. Then, some bright purple precipitates (denoted as CoxNiy-ZIF) were obtained after cooling, centrifugation, washing with methanol, and vacuum drying at 50~60 oC for 12h. For comparisons, mono-metallic Co-ZIF or Ni-ZIF was also synthesized following similar protocols except no addition of the other metal nitrate. Moreover, the molar ratio of 2-MeIM to the total amount of metal ions was controlled to



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be 4, and a series of CoxNiy-ZIF hybrids were prepared by adjusting the molar ratio between Co and Ni nitrate (x/y=9:1, 4:1, 2:1, 1:1 and 1:2). The as-prepared ZIF powders were ground and calcined at the target temperature for 3 h (T was 700, 800 or 900 °C) in N2 (5 °C/min) to obtain black CoxNiy@N-C or Co@N-C powders. 3. Results and Discussion 3.1 Pyrolysis Temperature Effected Catalytic Activity The synthesis of CoxNiy@N-C was accomplished by pyrolysis of the parental bimetallic CoxNiyZIF hybrid, which was generated by coordination between the bimetallic ions and 2-MeIM (Figure 1). XRD pattern of as-prepared Co-ZIF and Ni-ZIF (Figure S1) showed that both Ni2+ and Co2+ ions can coordinate with 2-methylimidazole ligands to form Co-ZIF36,37 and Ni-ZIF38 respectively, which had different crystalline structures. While in XRD pattern for CoxNiy-ZIF (x/y=1:1), the similar peaks with Co-ZIF and the weak peaks of Ni-ZIF (“*” mark, Figure S1) were observed, suggesting a bimetallic MOF composite of Co-ZIF and Ni-ZIF was successfully synthesized. Figure 2a gave the comparison of crystalline nature examined by XRD for the derived CoNi@N-C(T) samples (Here, T was the pyrolysis temperature), exhibiting the similar patterns of CoNi@N-C. The feature peaks at 44.4o, 51.6o and 76.0o matched well with the (111), (200) and (220) faces of CoNi alloy39, respectively. The relative broad peak at 26.0o was attributed to the graphitic carbon (002) plane40; no other phases presented in three CoNi@N-C (T) samples. The locally magnified patterns from 43.0o to 45.0o clearly displayed that the peak positions for CoNi(111) planes were shift to a little higher 2θ values than that of Co(JCPDS No. 15-0806), which was resulted from the lattice contraction of Ni alloying with Co, further proving the formation of the CoNi alloy (Figure 2b). The crystallite sizes estimated from the CoNi(111) planes according to the Bragg's Formula were about 11 nm for CoNi@N-C(700), 18 nm for


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CoNi@N-C(800) and 21 nm for CoNi@N-C(900), signifying the pyrolysis temperature should be considered as one of the important preparation parameters deciding the size of CoNi alloy nanoparticles.

Figure 1 The preparation process diagram of the parental CoxNiy-ZIF and the derived CoxNiy@N-C.

Figure 2 XRD patterns of MOFs-derived CoNi@N-C samples obtained at different temperatures (a), and the corresponding locally magnified XRD patterns (b). The pyrolysis temperature effect on the morphology of as-synthesized CoNi@N-C samples was also investigated by TEM (Figure 3). Though all three samples contained carbon lamella and metal nanoparticles, the significant differences were found in particle sizes and morphologies. With the pyrolysis temperature increased, the size of the metal or alloy nanoparticles grew up, and the incomplete carbonization of organic ligands in CoNi@N-C(700) (Figure 3a), the clear 7 ACS Paragon Plus Environment


borderline in CoNi@N-C(800) (Figure 3b), and possible structural collapse in CoNi@N-C (900) (Figure 3c) were seen clearly. Moreover the metal nanoparticles in CoNi@N-C (900) agglomerated severely resulting in the particle size of 42.5±13.8 nm, which should be one of the reasons for the relative poorer catalytic activity on CoNi@N-C (900).

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Figure 3 TEM images for the CoNi@N-C samples pyrolyzed at 700 oC(a), 800 oC(b) and 900oC (c) respectively. The insets are the corresponding particle size distributions. Catalytic performances of three CoNi@N-C(T) materials reflected the pyrolysis temperature effect. CoNi@N-C(700) and CoNi@N-C(800) gave much higher current densities (j) for ORR than that of CoNi@N-C(900), and their onset (Eonset) and half-wave potential (E1/2) were close to the results on Pt/C (20wt %, commercial) (seen in Figure 4a). Furthermore, the j for OER in Figure 4b on both CoNi@N-C(700) and CoNi@N-C(800) were better than the results on CoNi@N-C(900) and Pt/C catalysts, showing a good bifunctional catalytic activity. In addition, it was found that CoNi@N-C (800) was superior to CoNi@N-C (700) by comparing the j at 0.2 V for ORR and 1.7 V for OER. The poor catalytic activity on CoNi@N-C (900) might be because of the agglomeration of the CoNi alloy nanoparticles at excessive pyrolysis temperature observed by the previous TEM characterization, which was known as an important influence factor for the electrocatalytic activity41. Moreover, it was already reported that N atoms were not stable at high temperature, so the N-doping amount decreases with increasing pyrolysis 8 ACS Paragon Plus Environment

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temperature, and N-doping would provide catalytically active sites through inducing the electronic interaction with nearby carbon/metal atoms and produce structural defects in N-carbon materials to form the O2 adsorption site42. However, CoNi@N-C(700) had the relative lower activity compared to CoNi@N-C(800), which might due to the incomplete thermolysis of ZIFs43. Therefore, it was considered that a suitable N-doping level and metal nanoparticle size, the highly accessible surface and nanoporous structure in CoNi@N-C(800) explained its excellent ORR/OER electrocatalytic activity.

Figure 4 The activity comparison of ORR (a) and OER (b) of CoNi@N-C pyrolyzed at 700, 800 and 900oC respectively, and Pt/C (20wt%, commercial). 3.2 Co/Ni Molar Ratio Effected Catalytic Activity To optimize the synthesis protocol for CoxNiy@N-C catalysts, the Co/Ni molar ratio as another influence factor on the catalytic activity was focused on in this work. Figure 5 provided the typical XRD pattern comparison of CoxNiy@N-C with different Co/Ni molar ratios prepared at 800 oC. Similar to the previous XRD results in Figure 2, the characteristic peaks of all CoxNiy@N-C samples in Figure 5a corresponded to graphitic carbon (002), and CoNi alloy (111), (200) and (220) planes in turn. Compared to the Co@N-C and Co (JCPDS#15-0806), the diffraction peak positions of (111) planes slightly shifted to the right observed from the locally



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magnified patterns from 43.0o to 45.0o in Figure 5b, verifying the formation of CoNi alloy in the obtained bimetallic CoxNiy@N-C samples. Further, Raman spectrum (Figure 5c) of three samples all displayed two clear diffraction peaks at ~1585 cm-1 for G-band and ~1340 cm-1 for D-band, respectively originating from the E2g vibration of the sp2-bonded C atoms and disordered graphitic C 44. These were the typical features of graphitic-type materials. ID/IG (I was the peak intensity), which was usually regarded as a carbon disorder measure, were 1.01 for Co1Ni1@N-C and Co2Ni1@N-C, and 1.05 for Co@N-C respectively, indicating the higher crystallinity of carbon due to the introduction of Ni, which was consistent with the following high resolution TEM results in Figure 9b and e. There also existed plenty of disordered C in these samples, meaning defects in carbon structure, possibly creating active sites for efficient electrocatalysis45.

Figure 5 XRD patterns (a) and the locally magnified XRD patterns (b) for the MOFs-derived CoxNiy@N-C (x/y=1:0, 9:1, 4:1, 2:1, 1:1 and 1:2) samples, and Raman spectrum (c) of the homemade CoxNiy@N-C (x/y=1:0, 1:1 and 2:1). The chemical composition was investigated by XPS and XRF (Table S1). For the homemade CoxNiy@N-C samples, XRF measurement gave a close Co/Ni molar ratio to the feeding composition in the precursors, while the XPS provided a relatively higher near-surface Co/Ni molar ratio. Furthermore, the surface C content of all tested samples was dominant meaning they


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might possess N-doped carbon encapsulated CoNi alloy structure. Besides, the N-doping level fluctuated slightly within the range of 3.43~4.09 at%, reflecting no connection with Ni-addition. Taking CoxNiy@N-C (x/y=1:0, 1:1 and 2:1) as the representative samples, the electronic state of the surface elements was analyzed using XPS technique. The Co 2p high-resolution spectra for three samples involved three components, Co0, Co2+, and the satellites peaks (shown in Figure 6a)40,46,47. The dominant existence of Co2+ species indicated that the external Co nanoparticles in CoxNiy@N-C composite were severely oxidized. It was attributed to the fact that Co exposed to air was easily oxidized and formed the thin CoO layer48. Moreover, the Co0 2p3/2 peaks of Co1Ni1@N-C and Co2Ni1@N-C moved to lower binding energy by about 1.1 eV and 1.2 eV respectively comparing to Co@N-C, suggesting the effect from the second metal Ni on the Co electronic structure. The deconvolution Ni 2p spectra in Figure 6b presented 4 peaks, corresponding with Ni2+ species and their satellite peaks. Such results reflected that there existed an electronic structure modification between the near-surface Co and Ni atoms. It also suggested the formation of CoNi alloy and presence of bimetallic synergy49. And in CoNi alloy, the Co atom could get d-electrons from nearby Ni atom50, therefore increasing the electron donating feature of the hybrids, which should be related to the enhanced electrochemical activity. The C 1s high-resolution spectra in Figure 6c for three samples were deconvoluted into 3 peaks, displaying three C species: 51.3% C-C, 37.9% C=N, 10.8% -O-C=O for the Co@N-C sample, 40.4% C-C, 45.4% C=N, 14.2% -O-C=O for the Co1Ni1@N-C sample, and 45.8% C-C, 37.5% C=N, 16.7% -O-C=O for the Co2Ni1@N-C sample respectively51,52. Additionally, a little higher percentage of C=N species for Co1Ni1@N-C comparing to other two samples would be the reasonable explanation for its higher ORR/OER activity. The de-convoluted N 1s spectrum (Figure 6d) revealed five different N species coexisting on the surface of the three samples:



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pyridinic N, pyrrolic N, graphitic N, oxidized N, and non-ignorable CoNx or CoNiNx53,54. Except the oxidized-N specie, the other four types of N groups played a crucial effect on HER (hydrogen evolution reaction), ORR, and OER.

Figure 6 The XPS spectrum for Co 2p(a), Ni 2p(b), C 1s(c) and N 1s(d) of CoxNiy@N-C (x/y=1:0, 1:1 and 2:1). N2 sorption isotherms of the CoxNiy@N-C (x/y=1:0, 1:1 and 2:1) samples showed distinct hysteresis loops which match the type-IV isotherm characteristics, indicating the presence of mesoporous carbon structures resulted from the pyrolysis of ZIF (Figure 7a). Moreover, a relative narrow mesopore distribution for Co@N-C and the broad pore size distribution for Co1Ni1@N-C and Co2Ni1@N-C were noted from the desorption branch in BJH plots (Figure 7b), and the corresponding pore size was ∼3.8 nm, ∼3.8 nm and ∼3.4 nm respectively. The


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approximated BET surface area decreased in the order: Co2Ni1@N-C (256.7 m2 g−1 )>Co1Ni1@N-C (215.3 m2 g−1)> Co@N-C (173.9 m2 g−1), and the corresponding pore volume was 0.2225 cm3 g-1, 0.2386 cm3 g-1 and 0.1760 cm3 g-1 respectively. It indicated Ni-addition was beneficial to increasing the surface area and pore volume of electrocatalysts, which would favor the electrocatalytic ORR/OER processes by improving mass transfer and active component loading, electrical conductivity, and the contact between reactants, catalytic sites and electrolyte.

Figure 7 N2 isotherms (a) and BJH pore size distribution (b) of CoxNiy@N-C (x/y=1:0, 1:1 and 2:1). The surface morphology of CoxNiy@N-C (x/y=1:0, 1:1 and 2:1) hybrids was investigated by FESEM. As seen in Figure 8a and Figure S2a, the mononuclear Co-ZIF-derived Co@N-C showed a polyhedron shape with tiny protrusions on the surface and a slight surface contraction due to the high temperature carbonization of its parent mono-metallic MOFs. While the Co-ZIF and Ni-ZIF composite-derived Co1Ni1@N-C exhibited significant morphology differences shown in Figure 8b and Figure S2b. Besides the similar polyhedron shape with those of the Co@N-C was seen in Co1Ni1@N-C, there existed many elongated and curly nanofibers on the



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polyhedron surface. Co2Ni1@N-C nanoparticles also had a clear polyhedron shape, but the surface grown nanofibers became scarce and short (seen in Figure 8c and Figure S2c).

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Figure 8 FESEM images of Co@N-C (a) and Co1Ni1@N-C (b) and Co2Ni1@N-C particles (c). Insets were the corresponding regional magnified images. TEM images also showed a polyhedral shape of Co@N-C and encapsulated Co nanoparticles with size of 18.7±6.0 nm (Figure 9a and inset). It can be clearly observed by HRTEM (Figure 9b) that the C shell thickness was 4~5 nm with the wrapped Co particle. The elemental mapping images in Figure 9c showed the evenly dispersion of C and N, but relatively concentrated distribution at kernel in an individual Co@N-C particle, confirming the formation of an N-doped carbon encapsulated Co structure consistent with the XPS results. For the bimetallic Co1Ni1@N-C, besides a polyhedral shape and the 18.7±5.7 nm CoNi particle size, many slender and curly carbon nanotubes could be figured out from the HRTEM images (Figure 9d). HRTEM image also revealed the presence of CoNi alloy nanoparticle surrounded by a fewlayered carbon shell whose inter-planar spacing was about 0.35 nm matching with the C (002) plane (Figure 9e). HAADF-STEM image of a single Co1Ni1@N-C nanoparticle was shown in Figure 9f to further prove the alloyed structure of CoNi nanoparticles enclosed by the graphitic carbon shells. Moreover, the elemental mapping images implied that two elements, Co and Ni dispersed relatively densely in the center of the particle comparing with the evenly distributed C


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and N elements, also illustrating the existence of CoNi alloy core and the graphitic carbon shell. Figure 9g and h displayed the polyhedral shape of Co2Ni1@N-C which was similar with Co@NC and Co1Ni1@N-C, and some tiny nanotubes on the carbon surface.

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Figure 9 TEM (a), HRTEM images (b), and the elemental mappings of C, N and Co (c) for Co@N-C; TEM (d), HRTEM (e) and the elemental mappings of C, N, Co and Ni (f) for Co1Ni1@N-C; TEM images of Co2Ni1@N-C (g and h). Inset was the size distribution of Co nanoparticles in Co@N-C. Catalytic performance toward ORR/OER of homemade CoxNiy@N-C catalysts was examined in details. Figure 10a provided the ORR polarization curve comparison, showing that the CoxNiy@N-C catalysts with different Co/Ni molar ratios had different activities. Except for the Co1Ni2@N-C, those CoxNiy@N-C materials possessed a very close onset potencial (Eonset) of 15 ACS Paragon Plus Environment


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about 0.91 V (< 0.95 V on Pt/C). For details, two ORR parameters, that is E1/2 and jk (the kinetic current density, calculated from Equation S155) were listed in Figure 10b. LSV results of Co@NC and Co9Ni1@N-C exhibited that the addition of trace Ni had almost no effect on ORR activity. While the Co/Ni molar ratio was 4:1, 2:1 and 1:1, E1/2 and jk of the corresponding catalyst increased obviously and became relative closer to the data of Pt(20wt%, commercial)/C, in which the Co1Ni1@N-C exhibited the highest jk values (1.851 mA cm-2). Figure 10c gave the CVs comparison recorded in Ar- and O2-saturated KOH electrolyte respectively, exhibiting pronounced peaks in O2-saturated electrolyte with the maximum current at about 0.845 V, while no such prominent cathode peak was observed in Ar-saturated solution. Similar results were also observed on other CoxNiy@N-C catalysts (Figure 10d). It implied that the CoxNiy@N-C materials possessed the real electrochemical activity for ORR. RRDE measurement results were shown in Figure 10e and f, and the calculated transfer electron numbers (n) of Co1Ni1@N-C and Co@N-C were ∼3.8 and ~ 3.6 respectively, revealing that the ORR on those catalysts mainly follow the four-electron (4e) process. And the measured HO2- yield was decreased from 22.2% for Co@N-C to 13.4% for Co1Ni1@N-C, meaning CoNi alloy active sites significantly improved the selectivity of catalysts. The n was also determined by recording the LSV results at different rotating speeds (ω) in Figure S3. It was seen that j in LSV curves steadily increased with the ω increasing, indicating mass transport occurred on electrode surfaces. Insets were the K-L plots, showing that the data ran linearity and were in great parallel with each other. The measured n increased from ca. 3.3 for Co@N-C to ca. 4.0 for Co1Ni1@N-C, being consistent with RRDE results. Tafel slopes of all the CoxNiy@N-C samples ranged from 56.9 to 69.1 mV dec-1, less than the slope of Pt/C (87.8 mV dec-1) (see Figure 10g and inset). Apparently, the ORR mechanism on CoxNiy@N-C differed to that on Pt/C, which meant that CoNx or CoNiNx species


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on the N-C surface and different doped N species except the oxidized-N were the ORR active sites, and it also could be related with the Co or CoNi surface/structural modification of the N-C shell53. According to the some reported ORR electrochemical data for Co-containing materials (Table S2), it was proved that Co1Ni1@N-C gave a comparable ORR activity. Additionally, the ORR stability was executed by chronoamperometric measurements at 0.69 V in O2-bubbling KOH media. The current after 14400 s remained 95.6% on Co1Ni1@N-C, 81.0% on Co@N-C and 80.8% on Pt/C respectively (Figure 10h). Moreover, after 1000 potential cycles, the E1/2 on Co1Ni1@N-C and Co@N-C exhibited almost no obviously negative shift (~1mV), while Pt/C moved negatively about 26 mV (Figure S4), demonstrating a superior ORR stability of our catalytic materials.



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Figure 10 Electrochemical oxygen reduction on CoxNiy@N-C and the Pt(20wt%, commercial)/C. (a) LSV curves on CoxNiy@N-C and Pt/C in O2-bubbling KOH media. (b) E1/2 and jk comparison. (c) CV profiles of Co@N-C, Co1Ni1@N-C, Co2Ni1@N-C and (d) CV profiles of Co4Ni1@N-C and Co9Ni1@N-C (dash and solid lines indicate CV curves recorded in Ar- and O2-saturated KOH, respectively). (e) RRDE measurement in O2-saturated KOH. (f) The electron transfer number and peroxide yield. (g) Tafel plots for CoxNiy@N-C and Pt(20wt%, commercial)/C. (h) Stability testing at 0.69 V and 1600 rpm. The OER activity was also been estimated and compared by recording the polarization curve from 1.0 V to 1.75 V. In Figure 11a, from 1.3 V to 1.75 V, the Co1Ni1@N-C catalyst gave much higher j than that of other CoxNiy@N-C competitors, but a lower j than that of IrO2, which was recognized as the best OER catalyst at a high potential. For details, the geometric area-based j at


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1.70 V (vs. RHE) were listed in Figure 11b, indicating that the addition of the second metal, Ni, enhanced the OER activity and the Co1Ni1@N-C marked the maximum j of 8.60 mA cm-2, which compared favorably to other reported ZIF-derived carbon catalysts40, but was still lower than the homemade IrO2 (10.86 mA cm-2). Of course, as the non-noble metal catalyst, CoNi@N-C exhibited less OER activity than that of IrO2 benchmark. What we can do is to close the gap between the CoNi-based materials and Ir-based catalysts. For Pt(20wt%)/C, the j at the same potential was only 0.73 mA cm−2, clearly inferior to CoxNiy@N-C catalysts. Moreover, Co1Ni1@N-C gave a small potential (1.73V) at 10 mA cm−2, lower than those of other CoxNiy@N-C catalysts, the reported N/Co-doped PCP (1.75 V) and NRGO (>2.0 V)40, but slightly higher than 1.68 V given by the rutile IrO256. Meanwhile, the obtained Eonset on Co1Ni1@N-C was 1.35 V, which was remarkable earlier compared to Co@N-C (1.41 V), Co2Ni1@N-C (1.38 V) and other CoxNiy@N-C catalysts (1.40~1.52 V), as-synthesized IrO2 (1.52 V) and Pt(20wt%)/C (1.72 V), further confirming the superior OER performance given by Co1Ni1@N-C. These results implied this excellent OER activity resulted from the synergy between N-doped C shell with a bit metal oxides and the embedded CoNi alloy nanoparticles. It was well know that Co and Ni metal itself was the active center for OER respectively, when alloying with each other, the synergistic effect of Ni and Co on OER made CoNi nanoparticle as the better active center for OER. What’s more, Zhang and coworkers34 had pointed out by DFT calculation that after Ni atom immobilizing into the carbon substrate, two N-atoms in CoNi bimetallic alloy became more negative. This trend indicated that Ni-N bonds had distinct covalent feature which was beneficial for enhancing electronic activity of N-atom. Thus it is expected that the Ni and Co co-existing N-C system may offer superb OER performance. More detailed comparison of OER parameters had been summarized in Table S3. Besides, Ni-addition



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reduced the resistance at the catalyst/solution interface (Rct) associating with the electrochemical process proved by EIS plots (Figure 11c). Figure 11d compared the potential difference (ΔE = [email protected] mA cm-2 - [email protected] mA cm-2), showing the bifunctional activity of CoxNiy@N-C catalysts decreased in order of Co1Ni1@N-C > Co2Ni1@N-C > Co4Ni1@N-C > Co9Ni1@N-C ≈ Co@N-C > Co1Ni2@N-C. What’s more, for comparing the bifunctional activity of the as-prepared Co1Ni1@N-C with some reported catalysts, details were listed in Table S4, in which E at 10 mA cm-2 was used for evaluating OER activities, and E1/2 was applied to assess ORR activities. It was found that Co1Ni1@N-C exhibited very lowΔE between ORR and OER of 0.91 V, very close to that of the Co@Co3O4/NC-1 and Co@Co3O4/NC-2 bifunctional electrocatalysts (0.85 V and 0.90 V, respectively), and lower than 1.00 V on Pt/C57. The above results illustrated that bimetallic MOFs-derived CoxNiy@N-C (typically, Co1Ni1@N-C) possesses superior catalytic activity for both ORR and OER. Previous experimental and theoretical results had exhibited that the doped N in carbon structures or matrix could reduce the electrode overpotential effectively, which would promote the application of N-doped C materials as bifunctional electrocatalysts44,58.


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Figure 11 Electrochemical oxygen evolution on CoxNiy@N-C catalysts, the Pt(20wt%, commercial)/C and homemade IrO2. (a) LSV curves at 1600 rpm. (b) Current density at 1.7 V and the Eonset. (c) Nyquist plots of CoxNiy@N-C (x/y=1:0, 1:1 and 2:1). (d) Potential differences ([email protected] [email protected] cm-2) on CoxNiy@N-C. Repeated potential cycling was fulfilled for assessing the OER stability of Co1Ni1@N-C (Figure S5). Taking Co@N-C as a reference sample, the OER polarization curve on Co1Ni1@NC after 1000 cycles showed a relative smaller [email protected] than that of the first cycle, and the overpotential improved about 9 mV. But after 1000 potential cycles, Co@N-C gave a little higher j than that of the first cycle of LSV, which might be due to the anodic oxidation of Co metal to form Co oxide that was also active for OER. These results indicated the good OER stability of this type of MOFs-derived materials (i.e. Co or CoNi nanoparticles encapsulated in N-doped C). However the effect of oxidation on the OER activity still requires the detailed research, which will be our next work. 4. Conclusions We successfully prepared a novel CoNi alloy nanoparticles@N-doped carbon composites pyrolyzed from a bimetallic CoxNiy-ZIF hybrid by adjusting two key synthesis parameters (the pyrolysis temperature and Co/Ni molar ratio) to tailor the textural features, morphologies and compositions. With appropriate temperature (800oC) and Co/Ni ratio (1:1), the optimal 21 ACS Paragon Plus Environment


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CoNi@N-C afforded excellent activity for oxygen eletrocatalysis with the small overpotential, the prominent kinetic parameters, and the outstanding ORR stability, which benefitted from the following features: (1) the high active surface and porous volume with the relatively uniform and highly dispersed CoNi nanoparticles; (2) CoNi surface/structural modification by N-doped carbon layers with CoNiNx active sites, related to the most suitable synergistic effect between the CoNi alloy and carbon shell; (3) the plenty of carbon nanotubes generated in situ on C surface; and (4) the high N-doping and metal-Nx content. This work demonstrated a feasible strategy for design a cost-efficient and bifunctional electrocatalyst based on MOFs-derived non-precious metal/heteroatom-doped carbon hybrids. ASSOCIATED CONTENT Supporting Information. Details for physical and electrochemical characterization, XRD patterns, SEM images, XRF and XPS chemical composition, K-L analysis equations and plots, RRDE measurement, LSV comparison before and after 1000 cycles, and ORR/OER electrochemical data are provided (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Wei Hu. E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Funding Sources Author Wei Hu and Renfeng Nie received funding from National Natural Science Foundation of China (NSFC No. 21606075 and NSFC No. 21603066), respectively. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (NSFC No. 21606075

and

21603066).

The

authors

thank

the

Shiyanjia

Lab

for

HRTEM

observation(www.shiyanjia.com).

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Table of Contents (TOC)

CoNi@N-C(800oC)

Frequency Distribution(%)

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 32 of 32

32

24

18.7  5.7 nm 16

8

0 10

20

30

40

Partical Size(nm)


Porous N-doped carbon encapsulated CoNi alloy nanoparticles

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