Rational design of two-dimensional hybrid Co/N-doped carbon nanosheet arrays for efficient bi-functional electrocatalysis

Transition metals with desirable valence states have been proposed as efficient non-noble-metal electrocatalytic systems for selective water splitting. In this work, two-dimensional (2D) nitrogen-doped leaflike carbon matrix arrays functionalized with multi-valence-state transition metal (cobalt) hybrids were successfully prepared by in situ calcination of the corresponding bimetallic leaf-like zeolitic imidazolate framework (ZIF-L) in an inert atmosphere. The 2D morphology of the matrix along with the particle size and surface valence state of the anchored particles has been successfully controlled via precisely adjusting the Co2+/(Zn2+ + Co2+) ratio in the bimetallic ZIF-L precursor. Electrochemical measurements show that the kinetics of and stability towards the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are highly dependent on the particle size as well as the valence states. It was found that the particles with sizes less than 60 nm exhibit mixed Co0/Co2+ valence states, and therefore demonstrate bi-functional electrocatalytic performance. The theoretical simulations revealed that the bi-functional electrocatalytic activity should be attributed to the synergetic effect of mixed Co0/Co2+ and electronic coupling in the hybrids, which are of benefit to the catalytic kinetics and dynamics. High HER and OER activities of the hybrids have been verified, in which overpotentials of 171 and 280 mV to deliver a current density of 10 mA cm-2 for the HER and OER, respectively, were achieved. The obtained correlation between non-noble-metalbased carbon composites and HER/OER activities may be exploited as a rational guideline in the design and engineering of electrocatalysts. Disciplines Engineering | Physical Sciences and Mathematics Publication Details Cheng, N., Ren, L., Casillas, G., Zhou, S., Zhuang, J., Wang, L., Xu, X., Dou, S. Xue. & Du, Y. (2019). Rational design of two-dimensional hybrid Co/N-doped carbon nanosheet arrays for efficient bi-functional electrocatalysis. Sustainable Energy and Fuels, 3 (7), 1757-1763. Authors Ningyan Cheng, Long Ren, Gilberto Casillas, Si Zhou, Jincheng Zhuang, Li Wang, Xun Xu, Shi Xue Dou, and Yi Du This journal article is available at Research Online: https://ro.uow.edu.au/aiimpapers/3706 ARTICLE Please do not adjust margins Please do not adjust margins Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x Rational Design of Two-Dimensional Hybrid Co/N-Doped Carbon Nanosheet Arrays for Efficient Bi-Functional Electrocatalysis Ningyan Cheng, Long Ren, Gilberto Casillas, Si Zhou, Jincheng Zhuang, Li Wang, Xun Xu,* Shi Xue Dou and Yi Du* Transition metals with desirable valence states have been proposed as efficient non-noble-metal electrocatalytic systems for selective water splitting. In this work, two-dimensional (2D) nitrogen-doped leaf-like carbon matrix arrays functionalized with multi-valence-state transition metal (cobalt) hybrids were successfully prepared by in-situ calcination of the corresponding bimetallic leaf-like zeolitic imidazolate framework (ZIF-L) in inert atmosphere. The 2D morphology of the matrix along with the particle size and surface valence state of the anchored particles has been successfully controlled via precise adjusting the Co/(Zn + Co) ratio in the bimetallic ZIF-L precursor. Electrochemical measurements show that the kinetics and stability towards the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are highly dependent on the particle size as well as their valence states. It was found that the particles with sizes less than 60 nm exhibit mixed Co/Co valence states, which demonstrate a bi-functional electrocatalytic performance. The theoretical simulations revealed that the bi-functional electrocatalytic activity should be attributed to the synergic effect of mixed Co/Co and electronic coupling in the hybrids, which are of benefit to the catalytic kinetics and dynamics. High HER and OER activities of the hybrids have been verified, in which an overpotential of 171 and 280 mV to deliver the current density of 10 mA/cm for the HER and OER, respectively, were achieved. The obtained correlation between non-noble-metal-based carbon composites and HER/OER activities may be exploited as a rational guideline in the design and engineering of


Introduction
Electrocatalytic water splitting, which has huge potential for economical green energy conversion, suffers from unaffordable costs and the limited supply of noble metals, which possess high activities toward the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). 1,2 Great efforts have been devoted to develop effective and applicable non-noble-metal-based electrocatalysts. [1][2][3][4][5] Taking advantages of their high intrinsic electrocatalytic activities and earth abundance, 3d transition-metals (TMs, e.g., Ni, Co, and Fe) are regarded as the most promising substitutes for noble metals in the HER and/or OER. [6][7][8][9][10][11][12] Unlike the noble metals, most 3d TMs possess various valence states arising from their d-orbitals, which are not fully occupied. This feature enables the 3d TMs to be used as electrocatalysts for both the HER and the OER, which is determined by the valence states. For example, the existence of Ni components with mixed valence states (Ni 0 and Ni +2 ) enables Ni/NiO-nanofibers (NF) to possess bi-functional electrocatalytic activity towards water splitting. 13 In Ni/NiO-NF, Ni 0 is the HER active site while Ni +2 is the source of the OER active site. Besides variable valence states, the distribution of TM-based particles on conductive supports also significantly affects their electrocatalytic activities. 10,14,15 An even distribution has been proven to effectively increase the surface active sites. The conductive supports can enhance charge transfer in electrocatalytic systems. Therefore, it is highly desirable to develop the 3d TM-based electrocatalysts, which possess variable valence states and exhibit even dispersions on their conductive supports. Using an annealing process is the general approach to achieve 3d TMbased electrocatalysts with variable valence states. By precisely controlling the annealing conditions, mixed valence states can be modulated accurately within the 3d TM-based electrocatalysts, which, in turn, facilitates the synthesis of multifunctional TM-based electrocatalysts. Nevertheless, it is hard to avoid particle aggregation in annealing, which decreases the density of active surface sites. [15][16][17] Wet-chemical routes, such as the hydrothermal method, on the other hand, can suppress the aggregation by controlling particle dispersion, but face difficulties in modulating valence states. Thus, it is still extremely challenging to synthesize well-distributed TM particles with controllable mixed valence states on conductive supports for electrocatalysis, which is very much in demand for the development of multi-functional TM-based electrocatalysts. In this work, we designed and successfully synthesized valence-state controllable Co based nanoparticles in 2D nitrogen-doped leaf-like carbon matrix arrays on conductive carbon cloth (CoNC-x/CC, where x is the ratio of Co 2+ /(Zn 2+ + Co 2+ ) in the precursor), which was derived from bimetallic leaf-like zeolitic imidazolate framework nanosheet arrays (BMZIF-x/CC, where x is the ratio of Co 2+ /(Zn 2+ + Co 2+ ) in the precursor). The distance between adjacent Co 2+ ions in BMZIF-x/CC, which determines the valence states and distribution of the Co based particles in CoNC-x/CC, is mainly controlled by the value of x. Meanwhile, the 2D-type ZIF matrix maximizes exposure of the functional particles on the surface after pyrolysis, which helps to fully exploit the catalytic activity of the Co with a desirable valence state. It was found that the obtained 2D CoNC-x/CC (x = 0.5, 0.625, 0.75, and 0.875) with uniformly dispersed cobalt-based particles and possessing mixed valence states of Co 0 and Co +2 as the surface active sites, has high bi-functional electrocatalytic performance. The typical CoNC-0.75/CC only needs overpotentials (η) of 171 and 280 mV to deliver 10 mA/cm 2 for the HER and OER in 1.0 M KOH, respectively, and this high catalytic activity can be maintained for at least 40 hours. Theoretical calculations were also performed, and the results further explained and confirmed these experimental observations.

Experimental
Co(NO3)2·6H2O, Zn(NO3)2·H2O, 2-methylimidazole, KOH, and Nafion (5 wt%) were purchased from Sigma-Aldrich. All chemicals were used as received without any further purification. Carbon cloth (CC) was provided by Hongshan District, Wuhan Instrument Surgical Instruments. The water used throughout all the experiments was purified through a Millipore system.

Preparation of BMZIF-x/CC
In a typical synthesis, 0.4 × x (x = 0, 0.25, 0.5, 0.625, 0.75, 0.875, 1) mmol Co(NO3)2·6H2O and 0.4 × (1-x) mmol Zn(NO3)2·H2O were dissolved in 4 mL of deionized water to form the clear solution A. Similarly, 0.13 g of 2-methylimidazole was dissolved in 4 ml deionized water to form solution B. Then, solution A was quickly poured into solution B, and then a piece of CC, which was sequentially cleaned by sonication in acetone, water, and ethanol for 30 min each, was immediately immersed in the above mixed solution. After 2 hours of reaction, the CC was taken out and washed with deionized water and dried at 60 o C.

Preparation of CoNC-x/CC
The BMZIF-x/CC was placed in a ceramic crucible with a cover and then calcined at 800 o C for 1 h at a ramp rate of 5 o C/min under N2 atmosphere. The obtained materials were used without any further treatment. Characterization Scanning electron microscopy (SEM) images were collected on a JEOL JSM-7500FA scanning electron microscope. Transmission electron microscope (TEM) images were collected on a JEOL JEM-2010 electron microscope operated at 200 kV. High resolution scanning TEM (STEM) images, and STEM-energy dispersive X-ray spectroscopy (EDX) images were obtained using a JEOL ARM-200CF operated at 200 kV with an X-ray collection angle of ~1 sr. The EDX elemental mapping was obtained using NSS 3.3.94 Noran System Seven software, in which principal component analysis can be performed to identify regions with a similar composition within a map. Powder X-ray diffraction (XRD) patterns were collected using MAC Science. Electrochemical measurements Electrochemical measurements were performed with a Bio Logic Science Instruments VSP-300 electrochemistry workstation. All the electrochemical measurements were conducted in a typical threeelectrode setup with an electrolyte solution of 1.0 M KOH using CC and CoNC-x/CC directly as the working electrode, a graphite rod as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode. In all measurements, the SCE reference electrode was calibrated with respect to reversible hydrogen electrode (RHE). Linear sweep voltammetry (LSV) measurements were conducted at scan rates of 2 mV/s and 5 mV/s for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively. All the potentials reported in our work are vs. SCE in 1.0 M KOH. E (RHE) = E (SCE) + 0.059 pH + 0.242. For the OER, the overpotential (η) = E (RHE) -1.23 V. The calculation of current density was based on the geometrical area. All the data are presented without iR compensation.

Computational methods
Spin-polarized density functional theory (DFT) calculations were performed with the Vienna ab initio simulation package (VASP), 18 using the planewave basis with an energy cutoff of 500 eV, projector augmented wave pseudopotentials, 19 and the generalized gradient approximation parameterized by Perdew, Burke and Ernzerhof (GGA-PBE) for the exchange-correlation functional. 20 Solids of Co in the face-centered cubic (fcc) phase, CoO, RuO2 (110), and Pt (111) were modeled by four-layer slabs, with 2 × 2 unit cells for the lateral dimensions of the supercells. Both (111) and (100) surfaces were considered for Co and CoO. The NG@Co heterostructure was modeled by a supercell consisting of 4 × 4 unit cells for graphene on a Co (111) surface. The in-plane lattice of Co was slightly compressed to fit that of graphene, giving a lattice mismatch of 1.99%. Then, one N atom was substituted into the graphene sheet, corresponding to a doping concentration of 3.12 at.%. For all the models, a vacuum space 16 Å in thickness was applied in the vertical direction. The Brillouin zones of the supercells were sampled by a uniform k point mesh with density higher than 0.03 Å −1 . With fixed cell parameters, the model structures were fully optimized using the convergence criteria of 10 −4 eV for the electronic energy and 10 −2 eV/Å for the forces on each atom. The bottom-layer atoms were fixed during structure relaxation to mimic semi-infinite solids. Grimme's semi-empirical DFT-D3 scheme for dispersion correction was adopted to describe the van der Waals (vdW) interactions in layered materials. 21 The climbing-image nudged elastic band (CI-NEB) method was employed to determine the kinetic barriers and transition states for the water dissociation process. 22 Five images were used to mimic the reaction path. The intermediate images were relaxed until the perpendicular forces were less than 10 -2 eV/ Å.

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Please do not adjust margins spectra (Fig. S4), and Raman spectra (Fig. S5) are consistent with the reported data on the bimetallic leaf-like zeolitic imidazolate framework precursor (ZIF-L). [23][24][25][26] This confirms that the as-prepared BMZIF-x/CC can be assigned to ZIF-L, regardless of the value of x. A distinct morphological evolution was observed during the calcination process. After calcination, the 2D leaf-like nanosheet arrays of CoNCx/CC (x = 0.5, 0.625, 0.75, 0.875, and 1) retained their 2D structures with nanoparticles attached on the surface (Fig. S6). The particle size increases along with the value of x. In contrast, the BMZIF-x/CC samples with x = 0 and 0.25 lose their 2D features and fully collapse after calcination, as shown in Fig. S6. We believe that the collapse gives rise to the carbonization of the ZIF framework as well as evaporation of Zn at high calcining temperatures. 27 As shown in   Fig. S8. Moreover, increasing x results in an increase in the particle size. Interestingly, the valence state of cobalt also varies, depending on the particle size. The highangle annular dark-field STEM (HAADF-STEM) image and corresponded energy dispersive X-ray (EDX) spectrum and elemental maps of a 2D CoNC-0.75 nanosheet (Fig. 2b-2f and Fig. S9), which was scratched from a typical CoNC-x/CC (x = 0.75) sample (Fig. 2a), reveals the existence and uniform dispersion of C and N. The HAADF-STEM images of a selected small particle ~7 nm in size ( Fig. 2g and  2h, and Fig. S10) show that it is a pure metallic Co particle covered by a graphitic layer, which is believed to prevent the metallic Co from being oxidized during annealing. In a large particle (~70 nm), the covering layer of Co is distinct from the graphitic layers of the small particles. The phase mapping (Fig. S11) reveals that this is a CoO shell, which is also confirmed by the semi-coherence between Co and CoO down the [011] zone axis in interface of the HAADF-STEM images (Fig. 2i). The particles 50 nm in size show the both features of both small and large particles, that is, the Co core is covered by a shell consisting of graphitized carbon and CoO, as shown in Fig. 2j-l. Thus, the valence states of cobalt in CoNC-x/CC can be tuned by the sizes of the nanoparticles, which is determined by the ratio x of Co 2+ /(Zn 2+ + Co 2+ ) in the precursor. as terminated surface sites may offer CoNC-x bi-functional elecrocatalytic activity. To verify this hypothesis, electrocatalytic HER and OER activities over CoNC-x/CC were evaluated in 1.0 M KOH aqueous solution by using a standard three-electrode system. For comparison, the electrocatalytic activity of CC was also examined. The polarization curves in Fig. 3a show that blank CC exhibits negligible HER activity. Though possessing a similar collapsed structure, to reach the current density of 10 mA/cm 2 , the overpotential required by CoNC-0.25/CC (256 mV) is notably lower than that of cobalt metal-free CoNC-0/CC (444 mV). Meanwhile, the overpotentials required for CoNC-0.5/CC, CoNC-0.625/CC, CoNC-0.75/CC, CoNC-0.875/CC, and CoNC-1/CC with retained 2D leaf-like structure are only 194, 188, 171, 168, and 195 mV, respectively. To reach the high current density of 100 mA/cm 2 , CoNC-0.5/CC, CoNC-0.625/CC, CoNC-0.75/CC, and CoNC-0.875/CC need 626, 589, 577, 606, and 657 mV, respectively. Thus, it is clear that CoNC-x/CC (x = 0.5, 0.625, 0.75, and 0.875), which contain mixed valence state Co based particles, exhibit very similar catalytic activity, but it is obviously higher than that of CoNC-x/CC (x = 0, 0.25, and 1). The catalytic activity of the as-prepared catalysts also compares favorably with the behavior of most recently reported cobalt and cobalt oxide based electrocatalysts, as summarized in Table S1. Tafel slop is commonly used to determine the reaction kinetics and ratedetermining step in HER. It is established that the Tafel slopes are about 30, 40, 120 mV/dec for the Tafel, Heyrovsky, and Volmer reaction mechanism in the HER, respectively. [29][30][31][32] Since the Tafel slop of the CoNC-x/CC is higher than 110 mV/dec (Fig. S12a), therefore, it suggests that the rate-determining step here is the electrochemical desorption, and the Volmer-Heyrovsky mechanism applies for towards HER. Since long-term stability is another critical parameter in determining the practical capability of an electrocatalyst, a stability test of the typical CoNC-0.75/CC was also performed. The long stability of CoNC-0.75/CC was confirmed by the negligible difference between the polarization curves obtained before and after continuous cyclic voltammetry (CV) scanning for 1000 cycles, and the time-dependent current density curve under the static overpotential of 170 mV (Fig. 3b). Moreover, after the stability test, SEM images show that CoNC-0.75/CC still retains its 2D nanosheet arrays without collapse ( Fig.  S13a and S13b). The XRD pattern (Fig. S13c) collected after the test is also similar with the one obtained before test, and only peaks attributed to graphitic carbon and metallic Co could be found. Therefore, all the data show there is no obvious change on the morphology and composition of the sample after the stability test, which further confirms the high stability of the sample. The electrocatalytic OER activity of CC and the series of CoNC-x/CC samples was also evaluated under the same test conditions. Similar to the case of the HER, Fig. 3c shows that CC possesses negligible catalytic activity towards the OER. To achieve current density of 10 mA/cm 2 , the overpotential required for CoNC-0/CC and CoNC-0.25/CC was 567 and 434 mV, respectively, which are much higher than those of CoNC-0.5/CC (307 mV), CoNC-0.625/CC (265 mV), CoNC-0.75/CC (280 mV), CoNC-0.875/CC (279 mV), and CoNC-1/CC (392 mV). The samples (CoNC-x/CC, x = 0.5, 0.625, 0.75, and 0.875) which retain the 2D structure and contain mixed valence-state type Co-based particles, exhibit very similar OER catalytic activity and Tafel slopes (Fig. S12b), which demonstrate identical catalytic mechanism. However, their catalytic activity is superior to that of CoNC-x/CC (x = 0, 0.25, and 1). As summarized in Table S2, the OER activity of the typical CoNC-0.75/CC is comparable to those of many outstanding cobalt-based electrocatalysts. The catalytic OER stability of CoNC-0.75/CC was also evaluated by comparing the polarization curves before and after continuous CV scanning for 1000 cycles (as shown in Fig. 3d), and no obvious difference was observed. The timedependent current density curve under a static overpotential of 340 mV for 40 h further demonstrates that CoNC-0.75/CC possesses high electrocatalytic stability. As shown in Fig. S14, the high stability of the sample was also confirmed by the fact that there is no obvious change in the morphology and composition before and after the long-time stability test. To gain insight on the atomic level into the origins of the bi-functional activity of the CoNC-x composites, density functional theory calculations were performed to examine the electrocatalytic properties of different components in the composites. As shown in Fig. 4a and Fig. S15, we consider the Co (111) surface covered by an N-doped graphene monolayer to mimic the sample region where Co nanoparticles are embedded in N-doped graphitic carbon shells (denoted by NG@Co). We use bared Co and CoO solids to model the regions where graphitic sheets rupture and Co nanoparticles are exposed and (partially) oxidized into CoO. Besides the (111) surface, which is the dominant exposed surface of Co based nanoparticles according to the experimental transmission electron microscope (TEM) and STEM images, the (100) surface was also examined. The OER overpotentials (η OER ) of these model structures were calculated by the standard hydrogen electrode method, considering the four electron pathway in alkaline medium. 33 The HER activity was characterized by the Gibbs free energy for H* adsorption (ΔGH*), 34 and the kinetic barrier for water to dissociate into H + and OH − ions. The details of the computational method can be found elsewhere. [33][34][35][36] In general, an ideal bi-functional OER and HER catalyst has η OER and ΔGH* equal to zero, requiring that the catalyst can provide moderate binding strength for the reaction intermediates (O*, OH*, OOH* and H* species) to minimize the potential barriers between consecutive reaction steps. 37 Catalysts for the HER in an alkaline medium are also expected to facilitate water dissociation. Fig. 4b, 4c displays the free energy diagrams of the OER and HER for different model structures. Overall, the N-doped graphene supported by the Co (111) surface exhibits high electrocatalytic activity towards both the OER and the HER. The OER overpotential required is as low as 0.44 V, comparable to that of RuO2 (0.40 V for the (110) surface). The most active sites come from the C atoms in the para-position with respect to the N dopant and on the hollow sites of the Co (111) surface. The bare CoO (100) surface also shows activity towards the OER with Co serving as the reaction sites and giving η OER = 0.50 V. For both model structures, the formation of OOH* intermediate from O* and OH − species is the rate-limiting step. For the HER, the C atoms next to the N dopant in the Co supported graphitic sheet have ΔGH* = 0.13 eV, which is most competitive to that of Pt (-0.09 eV for the (111) surface). The bare Co (111) and (100) surfaces provide too strong and too weak H* binding for HER catalysis, with ΔGH* = -0.48 and 0.24 eV, respectively. In comparison, free-standing N-doped graphene (denoted by NG) has much weaker binding capability than the graphene supported by Co metal, and consequently, it has lower OER activity and is ineligible for HER catalysis. The detailed catalytic performances of various model structures are given by Table S3. The synergic effects of heterostructures of graphitic carbon materials and transition metals is attributed to the interfacial electronic coupling, which has been well elucidated by previously reported theoretical work. 35,36  Moreover, the dissociation of water molecules is facile on N-doped graphene supported by the Co (111) surface, involving a barrier of 1.50 eV, comparable to that of Pt (0.89 eV), 38 and much lower than the value for free-standing N-doped graphene (2.71 eV). Exposure of the CoO (100) surface, which is hydrophilic, can further promote water dissociation, with a barrier of only 1.10 eV, as shown by Fig. 4d. Our calculations reveal that the synergic effects between Co nanoparticles, N-doped graphitic sheets, and the oxidized Co surfaces, endow the hybrid material with bi-functional activities towards the OER and HER, with different reaction sites for the two processes, as illustrated by Fig. 4a. Partial exposure of the CoO (100) surface can provide more reaction sites for the OER and water dissociation, and thus enhance the bi-catalytic performance. These theoretical results are well consistent with the experimental observations that the CoNC-x composites with the coexistence of both Co 0 and Co +2 states have the best bi-functional performance, better than either pure Co 0 or pure Co +2 .

Conclusions
In conclusion, Co-based CoNC-x/CC nanosheet arrays with controllable valence states of Co, tunable particle size, and precise nanoparticle distributions have been successfully synthesized. The valence states and the distribution of the Co based particles in CoNCx/CC were determined by the distance between adjacent Co 2+ ions in BMZIF-x/CC, which can be controlled by precisely adjusting the ratio of Co 2+ /(Zn 2+ + Co 2+ ) in the precursor. It was found that the particles with sizes less than 60 nm exhibited mixed Co 0 /Co +2 valence states, so that they demonstrate a bi-functional electrocatalytic performance. The theoretical simulations confirm that bi-functional electrocatalytic activity is attributable to the synergic effects of mixed Co 0 /Co +2 and the electronic coupling in the hybrids, which are of benefit to the catalytic kinetics and dynamics. High HER and OER activities of the hybrids have been demonstrated. The obtained correlation between non-noble-metal-based carbon composites and HER/OER activities offers a simple route to design and engineer hybrid systems for bi-functional electrocatalysis.

Conflicts of interest
There are no conflicts to declare.