Electronic and Morphological Dual Modulation of Cobalt Carbonate

May 23, 2017 - Electronic and Morphological Dual Modulation of Cobalt Carbonate Hydroxides by Mn Doping toward Highly Efficient and Stable Bifunctiona...
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Electronic and Morphological Dual Modulation of Cobalt Carbonate Hydroxides by Mn Doping toward Highly Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting Tang Tang,†,§,▽ Wen-Jie Jiang,†,∥,▽ Shuai Niu,† Ning Liu,‡,∥ Hao Luo,† Yu-Yun Chen,† Shi-Feng Jin,‡,∥ Feng Gao,*,§ Li-Jun Wan,†,∥ and Jin-Song Hu*,†,∥ †

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China § Laboratory of Functionalized Molecular Solids, Ministry of Education, Anhui Key Laboratory of Chemo/Biosensing, Laboratory of Optical Probes and Bioelectrocatalysis, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China ‡ Research & Development Center for Functional Crystals, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ∥ University of the Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Developing bifunctional efficient and durable non-noble electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is highly desirable and challenging for overall water splitting. Herein, Co−Mn carbonate hydroxide (CoMnCH) nanosheet arrays with controllable morphology and composition were developed on nickel foam (NF) as such a bifunctional electrocatalyst. It is discovered that Mn doping in CoCH can simultaneously modulate the nanosheet morphology to significantly increase the electrochemical active surface area for exposing more accessible active sites and tune the electronic structure of Co center to effectively boost its intrinsic activity. As a result, the optimized Co1Mn1CH/NF electrode exhibits unprecedented OER activity with an ultralow overpotential of 294 mV at 30 mA cm−2, compared with all reported metal carbonate hydroxides. Benefited from 3D open nanosheet array topographic structure with tight contact between nanosheets and NF, it is able to deliver a high and stable current density of 1000 mA cm−2 at only an overpotential of 462 mV with no interference from high-flux oxygen evolution. Despite no reports about effective HER on metal carbonate hydroxides yet, the small overpotential of 180 mV at 10 mA cm−2 for HER can be also achieved on Co1Mn1CH/NF by the dual modulation of Mn doping. This offers a twoelectrode electrolyzer using bifunctional Co1Mn1CH/NF as both anode and cathode to perform stable overall water splitting with a cell voltage of only 1.68 V at 10 mA cm−2. These findings may open up opportunities to explore other multimetal carbonate hydroxides as practical bifunctional electrocatalysts for scale-up water electrolysis.

1. INTRODUCTION

utilization. To overcome these disadvantages, design of costeffective, earth-abundant catalysts for water electrolysis is highly desirable. Transition-metal (Fe, Co, Ni, Mn, Mo, and W) based catalysts have attracted considerable attention as their promising electrocatalytic performance, such as metal carbides/nitride,11−13 metal alloys,14,15 metal chalcogenides16−23 for HER; and metal phosphate, 6,24 metal oxides,25−28 hydroxides 29−35 for OER. Differently, metal carbonate hydroxides (MCHs) received less attention. MCHs are expressed by general formula of Ax(OH)2(x‑1)CO3·nH2O,

Regarding the increasing concerns on severe environmental pollution and rapid fossil fuel depletion, it is necessary to develop renewable and sustainable energy sources. Electrolytic water splitting into oxygen and hydrogen is one of the most promising and competitive solutions due to its great advantage of carbon-free emissions.1−3 However, the large overpotential of both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) greatly limited the practical applications of overall water splitting.4−6 It is thus imperative to develop highly active electrocatalysts. At present, the stateof-the-art electrocatalysts for HER are Pt-based materials and for OER are Ir- or Ru-based oxides.7−10 But the high cost, scarcity and unsatisfactory durability limit their practical © 2017 American Chemical Society

Received: April 7, 2017 Published: May 23, 2017 8320

DOI: 10.1021/jacs.7b03507 J. Am. Chem. Soc. 2017, 139, 8320−8328

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Journal of the American Chemical Society

2. EXPERIMENTAL SECTION

where A is typically bivalent metal ions. A few reports demonstrated the OER activity of MCHs but with a relatively large overpotential.36−38 For example, cobalt carbonate hydroxide on carbon cloth required an overpotential of 509 mV;37 and hierarchical cobalt carbonate hydroxide needed 466 mV for OER to achieve 10 mA cm−2.38 Therefore, the OER catalytic activity of MCHs needed to be significantly improved to compete with other candidates. The electrocatalytic performance improvement can be achieved in two aspects. 1) Increase the quantity of catalytically active sites. This can be done by the morphology control and the suitable design of electrode structure. Currently, many transition metal-based OER catalysts were developed in the form of powders and usually evaluated by coating them onto a conductive substrate (glass carbon, Ni mesh, Ni foam (NF), carbon cloth, carbon paper, etc.) with polymeric binders, which is unfavorable for exposing the active sites. Such glued OER catalysts were also prone to peel off from substrate during the vigorous gas evolution under high current density, greatly deteriorating their durability. Considering two-dimensional (2D) materials can expose more active sites and facilitate mass diffusions during the reactions,39−42 direct growth of 2D active materials on three-dimensional (3D) porous and conductive substrates could be an effective way to solve the above issues and make highly active and durable electrodes for water electrolysis.43 2) Enhance the intrinsic activity of active site. The electronic modulation of the electrocatalytically active center has been proved to effectively affect the activity of electrocatalysts.44−47 Besides, it would be preferable that an electrolytic electrode could concurrently catalyze both OER and HER in consideration of minimizing the production cost,48 although it is still challenging due to the intrinsic incapability of most materials. To our knowledge, the HER activity of MCHs has never been reported to date. It would be a great advance if MCHs can be developed as a state-of-the-art electrocatalyst for overall water splitting. Motivated by the above discussion, we herein developed a facile one-step hydrothermal method to directly grow 2D cobalt carbonate hydroxide (CoCH) nanosheet array onto 3D porous and conductive Ni foam (NF). It was discovered that Mn doping can simultaneously modulate the nanosheet morphology to significantly increase the electrochemical active surface area for exposing more accessible active sites and the electronic structure of Co center in CoCH to effectively boost the intrinsic activity of active site. As a result, the developed CoMnCH/NF electrode exhibited superior OER activity with ultrasmall overpotential of 294 and 322 mV at 30 and 50 mA cm−2, respectively, much lower than the values for all reported metal carbonate hydroxides.36−38 Notably, the high current density of 1000 mA cm−2 could be achieved at the overpotential of 462 mV without severe interference by oxygen bubble. Interestingly, CoMnCH/NF electrode also demonstrated an excellent HER activity with a low overpotential of 180 mV to reach 10 mA cm−2, which is the first report about the HER activity of MCHs with such low overpotential to our best knowledge. A further step is taken toward an alkaline electrolyzer that can output a current density of 10 mA cm−2 at 1.68 V over a long-term operation. Such superior catalytic performance enables the potential application of novel CoMnCH/NF catalyst in practical larger-scale electrochemical water splitting.

2.1. Chemicals and Materials. Cobalt(II) acetate tetrahydrate (C4H6CoO4·4H2O), manganese(II) acetate tetrahydrate (C4H6MnO4· 4H2O) and hexamethylenetetramine (C6H12N4) were purchased from Alfa Aesar. Ethanol, ethylene glycol, hydrochloric acid and potassium hydroxide were all analytic grade (AR) and purchased from Beijing Chemical Work. All these chemical reagents were used as received without any further purification. Milli-Q ultrapure water (resistance of 18.2 MΩ·cm at 25 °C) were used for all experiments. Nickel foams (NFs) (thickness: 1.8 mm, area: 2 cm × 4 cm) were used as the substrate. In order to remove the oxidation layer on surface, Ni foams were ultrasonically washed with a mixture solution of ethanol and acetone (1:1 in volume) for 30 min, followed by sonicating in HCl solution (3 M) for 15 min, washing with water and ethanol two times each, and drying in air for use. 2.2. Synthesis of Co1Mn1CH/NF. In a typical procedure, a mixture of 2 mmol of cobalt(II) acetate tetrahydrate, 2 mmol of manganese(II) acetate tetrahydrate, and 250 mg of hexamethylenetetramine were dissolved in ethylene glycol (13 mL) and ethanol (14 mL) with continuously stirring for 30 min to obtain a clear solution, then transferred into a 50 mL Teflon-lined stainless-steel autoclave with a piece of cleaned NF (2 cm × 4 cm) in it. The autoclave was locked tightly and maintained in an electric oven at 170 °C for 8 h, then naturally cooled down to room temperature. The resulting brownish material on NF was washed with ethanol three times and dried naturally in air overnight. The loading amount of the final catalyst on NF was determined to be about 5.6 mg cm−2. 2.3. Synthesis of CoxMn yCH/NF. CoCH, Co1.5Mn0.5CH, Co0.5Mn1.5CH, and MnCO3 were prepared as control samples using the same method as for Co1Mn1CH/NF except for changing the feeding ratio of cobalt precursor to manganese precursor. Specifically, the feeding ratio of Co to Mn is 2:0 for CoCH preparation, 1.5:0.5 for Co1.5Mn0.5CH, 0.5:1.5 for Co0.5Mn1.5CH, and 0:2 for MnCO3. The total amount of metal precursors (Co + Mn) was kept constant as 4 mmol. The elemental ratios of metals in CoxMnyCH were determined by the average results of EDS measurements, which are similar to the feeding ratio. The products were accordingly designated with the feeding ratios. 2.4. Synthesis of Co1Mn1CH Powder. In order to demonstrate the role of NF substrate in catalytic performance, the Co1Mn1CH powder catalyst was also prepared using the same method as for Co1Mn1CH/NF except for no addition of NF in the autoclave. 2.5. Synthesis of CoMnCH-ce. A piece of as-prepared CoCH/NF was thoroughly washed with water and then soaked in Mn(NO3)2 aqueous solutions (1 mM, 50 mL) for cation exchange reaction. After 8 h, the pink color of CoCH changed into a dark-brown color. The product was rinsed with water and ethanol three times and dried overnight for use as the control sample CoMnCH-ce. 2.6. Characterizations. The crystalline structures of products were identified by powder X-ray diffraction (XRD) on Rigaku D/max 2500 with a Cu Kα1 radiation source (λ = 1.54056 Å) at a scanning rate of 2° min−1. The morphologies were inspected using a scanning electron microscope (Hitachi SU-8020) at an accelerating voltage of 10 kV and a transmission electron microscope (JEOL, JEM-2100F) with an acceleration voltage of 200 kV. Energy-dispersive X-ray spectra (EDS) were collected on an Oxford Materials Analysis EDX equipped on the SEM and TEM. The thickness of 2D nanosheets was determined by atomic force microscope (AFM) on Multimode 8 (Bruker) at room temperature. X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the chemical bonding states using the Thermo Scientific ESCALab 250Xi with 300 W monochromatic Al Kα radiation. The base pressure in the analysis chamber was about 3 × 10−10 mbar. The hydrocarbon C 1s line at 284.8 eV from adventitious carbon was used for binding energy referencing. FTIR spectra were measured on TENSOR-27 (Bruker). 2.7. Electrochemical Measurements. All electrochemical measurements were performed on Autolab PGSTAT302N (Metrohm, Netherlands) electrochemical workstation at room temperature (25 °C). The catalysts were evaluated in 1 M KOH aqueous solution using 8321

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Journal of the American Chemical Society a conventional three-electrode configuration, in which CoxMnyCH/ NF or CoxMnyCH powders on glassy carbon were used as the working electrode; platinum wire as counter electrode and Hg/HgO (1 M NaOH) electrode as reference electrode. Cyclic voltammograms (CVs) were conducted at a scan rate of 50 mV/s. Linear sweep voltammetry (LSV) polarization curves were recorded at a scan rate of 5 mV/s with 90% iR-compensation unless specifically indicated. Electrochemical impedance spectroscopy (EIS) measurements were recorded at the open-circuit potential in the frequency range of 100 kHz to 10 mHz. Chronopotentiometry (CP) curves were recorded at a constant potential without iR-compensation. The electrochemical surface area (ECSA) was evaluated by the capacitance measurements in the potential range without Faradaic process at various scan rates including 16, 14, 12, 10, 8, 6, and 4 mV s−1. The overall water splitting measurements were performed using Co1Mn1CH/NF as both anode and cathode in a two-electrode configuration. The active geometric area of both electrodes is 0.5 cm × 0.5 cm. All potentials in this work were reported versus the reversible hydrogen electrode (RHE), which were converted according to the following equation

Figure 1. XRD patterns of CoxMnyCH samples. The peaks marked by pentacle, asterisk, and hash symbols are indexed to NF, MCHs, and manganese carbonate, respectively. The reference XRD patterns of manganese carbonate (JCPDS No. 44−1472) and cobalt carbonate hydroxide (JCPDS No.48−0084) are provided for comparison.

E RHE = E Hg/HgO + 0.098 + 0.059 × pH where EHg/HgO is the measured potential against the reference electrode. 2.8. Density of States (DOS) Calculations. The first-principles calculations were performed with the CASTEP program code with the plane-wave pseudopotential method.49 We adopted the generalized gradient approximation (GGA) in the form of the Perdew−Burke− Ernzerhof for the exchange-correlation potentials.50 Spin-polarized and LDA+U calculations were made to correctly deal with our system. The ultrasoft pseudopotential with a plane-wave energy cutoff of 380 eV. The first Brillouin zone was sampled with grid spacing of 0.024 Å−1. The self-consistent field was set as 5 × 10−7 eV/atom. Based on the experimental lattice parameters, all independent internal atomic coordinates were optimized with the convergence standard given as follows: energy change less than 5 × 10−6 eV/atom, residual force less than 0.01 eV/Å.

at 3360 cm−1 could be indexed to the stretching mode of hydrogen-bonded hydroxyl groups from both the hydroxide layers and interlayer water52 or the stretching mode of N−H bond.53 Two peaks at 2932 and 2858 cm−1 are attributed to the stretching mode of C−H bond52 and the peak at 1574 cm−1 is attributed to the scissoring mode of NH2.54 These features could be attributed to the intermediate species of the reactant hexamethylenetetramine (HMTA) adsorbed on the surfaces of CoxMnyCH samples. Besides, the presence of CO32− was evidenced by its fingerprint peaks of D3h symmetry at 1412, 1075, 887, and 760 cm−1, which can be assigned to the vibrational modes of ν3(E′), ν1(A′1), ν2(A″1), and ν4(E″), respectively.55 The spectra at the low wavenumber (500−1000 cm−1) should originate from the metal−oxygen vibrations, as shown in Figure S3b.56 The peaks at 667 and 617 cm−1 could be assigned to the vibration of Co−O bond. These two peaks are getting weaker with the increase of Mn feeding ratio and eventually were replaced by two new peaks at 646 and 571 cm−1 in MnCO3 sample, which could be well assigned to Mn− O vibration. Furthermore, X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface chemical states on Co1Mn1CH. As shown in Figure S4a, the Co 2p3/2 XPS spectrum for Co1Mn1CH shows an obvious shakeup peak at 785.0 eV, indicating the presence of Co2+ state.57 In Figure S4b, the Mn 2p3/2 XPS spectrum for Co1Mn1CH at 641.5 eV is ascribed to Mn2+.58,59 Moreover, Mn 3s spectrum was also measured in view of its high sensitivity to Mn valence. As displayed in Figure S4c, the difference of two peaks (ΔE) in Mn 3s spectrum for Co1Mn1CH is 6.1 eV, which is the typical gap (about 6.0 eV) for Mn2+ (Figure S5). O 1s spectrum shows a peak at 531.1 eV (Figure S4d), suggesting the formation of Co/Mn−OH chemical bond.60 These results corroborate the formation of Mn-doped cobalt carbonate hydroxides. The morphologies of all Co x Mn y CH samples were investigated by scanning electron microscope (SEM). With no addition of Mn source, flower-like CoCH nanostructure in an average size of about 2 μm were uniformly grown on Ni foam (Figure S6a,b), which were assembled by irregular nanosheets (Figure 2a and S6c). After the introduction of Mn, Co1.5Mn0.5CH exhibited a similar structure to CoCH but in a slightly larger size of about 2.6 μm (Figure S6d,e). While

3. RESULTS AND DISCUSSION A series of samples were prepared by adjusting the feeding ratio of Co/Mn ranging from 2:0 to 0:2 (details seen in the Experimental Section), noted as CoxMnyCH, where x and y stand for the feeding amount of Co and Mn, respectively. X-ray diffraction (XRD) technique was first applied to examine the effect of Co/Mn ratio on the crystalline phase and composition. As shown in Figure 1, except for the peaks indexed to NF (marked by pentacles), two broad peaks at 33.4° and 59.6° (marked by asterisks) for all CoxMnyCH are well attributed to the diffractions of MCHs,51 indicative of the formation of MCHs when cobalt was present in the synthetic system. It should be noted that the strongest diffraction peak at 33.4° for pure CoCH showed an obvious positive shift compared with that for CoxMnyCH (Figure S1), indicating Mn doping into CoCH. However, the intensities of XRD signals for CoxMnyCH samples decreased with the increase of Mn content. The low intensity of XRD signals for CoxMnyCH samples implied their low crystallinity. The photograph image (Figure S2a) showed that the pink color of CoCH gradually fades away (Figure S2b− d) with the incremental introduction of Mn, implying the formation of Co−Mn bimetallic carbonate hydroxides. When only Mn was introduced, XRD pattern of the product was well indexed to the rhodochrosite MnCO3 (Figure 1, Joint Committee on Powder Diffraction Standards (JCPDS), No. 44−1472), as supported by the black color (Figure S2e). Fourier transform infrared spectroscopy (FTIR) was further used to disclose the chemical compositions and bonding states of CoxMnyCH samples. As shown in Figure S3a, the broad peak 8322

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Figure 2. SEM images with a magnification of 25 000× for (a) CoCH, (b) Co1.5Mn0.5CH, (c) Co1Mn1CH, (d) Co0.5Mn1.5CH, and (e) MnCO3. (f) TEM image of Co1Mn1CH. The inset in panel f is high-resolution TEM image of selected area in f marked by red line.

Figure 3. (a) Influence of Co/Mn ratio on Cdl and thus the surface area for CoxMnyCH samples. (b) OER and (c) HER polarization curves of CoxMnyCH samples and bare NF with iR-compensation in 1 M KOH at a scan rate of 5 mV s−1. (d) Chronopotentiometric curves of Co1Mn1CH at constant current density of 50 mA cm−2 for OER and 10 mA/cm2 for HER. The potentials are given without iR-compensation.

keeping increase the amount of Mn, Co1Mn1CH showed smoother surface with no obvious flower-like protrusion and the size of nanosheet was getting slightly smaller but much denser (Figure S6g−h). When Mn source dominated in the feeding materials, the cracks started to show up in Co0.5Mn1.5CH (Figure S6j,k), implying that Co0.5Mn1.5CH nanosheet may be easily peeled off during the electrocatalytic tests. The lateral size of Co0.5Mn1.5CH nanosheets became larger compared with Co1Mn1CH (Figures 2d and S6l). When

only Mn source was introduced, the size of MnCO3 nanosheets kept increasing with cracks on the nanosheet film on Ni foam (Figures 2e and S6m). These results clearly indicated that the introduction of Mn can effectively modulate the morphology, size and contact of CoxMnyCH nanosheets on Ni foam substrate. It seems that Co1Mn1CH nanosheets have smallest size and thus largest surface area as well as the tight contact with substrate as confirmed below, which are favorable for the electrochemical applications.48,61 Furthermore, the growth 8323

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nanosheets delivered a high and stable current density of 1000 mA cm−2 at an extremely low overpotential of 462 mV without appreciable interference by the produced O2 bubbles, indicating that the Co1Mn1CH electrode greatly favored the fast release of O2 bubble which is critical for practical application. Such outstanding performance of Co1Mn1CH could be attributed to the architecture of 2D highly active nanosheet arrays directly grown on 3D conductive substrate with open framework. This unique structure not only promised a larger surface area for more accessible active sites as well as the 3D open conductive network for efficient mass and electron transfer and gas release, but also the tight contact between Co1Mn1CH nanosheets and NF for ensuring the direct electron transfer and enduring the interference of a tremendous number of gas bubbles at large current density.67 To get further insight into this benefit, Co1Mn1CH powder was also prepared in parallel and loaded on glass carbon electrode with the aid of Nafion binder to evaluate the OER performance. The significant increased overpotential of 436 mV (vs 322 mV) was needed to get 50 mA cm−2 (Figure S15a), which could be aroused by large resistance as shown in Figure S15b. Moreover, it was impossible to achieve the stable current density over 300 mA cm−2 due to the deadly influence of tremendous gas bubbles Figure S15a. This result supports the importance of low-resistance and intimate interface between catalysts and substrate. The catalytic activity for hydrogen evolution also showed the similar trend on CoxMnyCH samples. It is the first report about the metal carbonate hydroxide for HER to our knowledge. Polarization curves with and without iR-compensation are shown in Figures 3c and S16, respectively. Co1Mn1CH and Co1.5Mn0.5CH exhibited the similar overpotentials of 180 and 328 mV to deliver 10 and 100 mA cm−2, respectively, which are much smaller than those for bare NF (390 mV at 10 mA cm−2, Figure S14b). However, at the large current density, Co1Mn1CH exhibited the best performance, suggesting the more facile mass transport and faster reaction kinetics in Co1Mn1CH. The large current density of 500 mA cm−2 could be obtained at the overpotential of 453 mV. Such performance is comparable to the recently reported metal oxides,48,62,63 MoS268 (Table S3). CoCH, Co0.5Mn1.5CH, and MnCO3 required the similar overpotentials of 218 mV to achieve 10 mA cm−2 but needed 356, 349, and 367 mV to reach 100 mA cm−2, larger than that for Co1Mn1CH. The difference became more obvious at large current density. These results indicated that the modulation by Mn doping also significantly affected the HER performance of CoxMnyCH catalysts. The long-term electrochemical durability of the best catalyst Co1Mn1CH were further evaluated at a static current density of 50 and 10 mA cm−2 for OER and HER, respectively. As shown in Figure 3d, the overpotential exhibited the negligible increase (only 10 mV) after continuous OER test for 18 h and HER test (only 6 mV) for 10 h, suggesting its outstanding durability. The morphology and elemental distribution of Co1Mn1CH were well-maintained after long-term durability tests as indicated in the SEM image and the corresponding EDS mapping images (Figure S17). On the contrary, MnCO3 sample on NF surface showed the much worse durability with a large overpotential loss of 42 mV after only 5 h test, probably due to the peel-off of the catalyst as implied by the cracks on the electrode surface (Figures S18 and S6m). Besides the influence of morphology and ECSA modulation by Mn doping on the electrochemical performance, to further understand the role of Mn doping the cation exchange protocol

process and morphology evolution of Co1Mn1CH nanosheets were systematically investigated via time-dependent experiments. As shown in Figure S7, the small discrete nanosheets appeared on NF surface at the initial stage (Figure S7a). As the reaction proceeded, the new nanosheets formed around the initial ones to cover the bare NF surface. Instead of aggregating into larger flower-like protrusion as the case of CoCH, these nanosheets preferred to spread over the surface and directly grow on the NF possibly due to the fast nucleation, which allows the firm contact with the NF substrate. The transmission electron microscope (TEM) image (Figure 2f) confirmed the nanosheet-like structure of Co1Mn1CH. The high-resolution TEM image (inset in Figure 2f) revealed that the lattice fringes with a distance of 0.195 nm, which correspond well to the dspacing of (340) planes of MCH. No continuous lattice fringes in a large scale was observed (Figure S8), corroborating the low crystallinity of Co1Mn1CH nanosheets. The EDS mapping results taken on both nanosheet film (Figure S9) and single nanosheet (Figure S10) suggested the homogeneous distribution of Co, Mn, O and C elements with a Co: Mn atomic ratio of 51:49, consistent with the feeding ratio (the rest materials were listed in Table S1). The thickness of Co1Mn1CH nanosheet was measured to be about 15 nm by atomic force microscopy (AFM, Figure S11), agreeing with the SEM results. The electrochemical surface areas (ECSAs) of the CoxMnyCH samples were evaluated by capacitance measurements via cyclic voltammograms in the double layer region (without faradaic processes) at different scan rates (Figure S12). As seen in Figure 3a, with the increase of Mn source, the Cdl increased from 92.7 mF cm−2 for CoCH to 292.5 mF cm−2 for Co1.5Mn0.5CH and climbed to the top value of 380.2 mF cm−2 for Co1Mn1CH. As the Mn amount kept increasing, the Cdl sharply decreased to 86.4 mF cm−2 for Co0.5Mn1.5CH and 56.7 mF cm−2 for MnCO3. This result indicates that the Mn doping is a feasible way to modulate the ECSAs of CoxMnyCH samples. The electrocatalytic activity of CoxMnyCH for OER were further evaluated. The polarization curves with iRcompensation were shown in Figure 3b and the corresponding curves without iR-compensation were shown in Figure S13. The bare NF and MnCO3 showed low OER activities, suggesting the manganese carbonate itself is not very active for OER. On the contrary, CoCH nanosheets exhibited the relatively good OER activity in terms of overpotentials of 337 mV, 358 mV and 387 mV at 30, 50, and 100 mA cm−2, respectively (Figure S14a). After introducing Mn, the OER activity of Co1.5Mn0.5CH apparently increased as indicated by the appreciable negative-shift of polarization curve (blue curve vs red curve). Increasing Mn amount to Co1Mn1CH achieved the highest OER activity in terms of the lowest overpotentials of 294 mV, 322 mV and 349 mV at 30, 50, and 100 mA cm−2, respectively. These overpotentials are comparable to those of other state-of-the-art OER catalysts, such as NiCo2O4,48,62,63 NiFe LDH/NF,64 Ni5P465 and Co-CoOx/CN66 (Table S2), and much smaller than those reported ones for metal carbonate hydroxides (509 mV37 and 466 mV38 at 10 mA cm−2), indicating the present Co1Mn1CH exhibited much better catalytic performance for OER than those metal carbonate hydroxides. However, further increase of Mn to Co0.5Mn1.5CH led to the sharply decrease of OER activity (382 mV at 50 mA cm−2), which should be related to the significant morphology change and thus the drastic decrease of the ECSA for Co0.5Mn1.5CH. Importantly, benefited from the morphology modulation and tight contact with NF substrate, Co1Mn1CH 8324

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Figure 4. (a) EDS spectra, (b) ECSA evaluation, (c) OER polarization curves without iR-compensation, and (d) Co 2p3/2 spectra of CoCH and CoMnCH-ce. The inset in panel a is a photograph of Co MnCH-ce. (e) DOS of Co 3d orbital and (f) corresponding number of electrons in the 3d orbital per Co atom in CoCH and CoMnCH.

release another electron. At last, adsorbed O2 desorbs from the catalyst surface (Figure S21).69 The reaction of forming adsorbed OOH species is considered as the rate-limiting step in OER.69 Moreover, it was reported that the interaction of adsorbed OOH species and 3d orbital of transition metal determined the OER activities of transition metal-based materials.46,70 The increase of 3d orbital electron density in Co-based OER catalysts was previously reported to favor the formation of adsorbed OOH species, thus enhancing the OER activity.45,47 Therefore, the density of states (DOS) of Co 3d orbital in both CoCH and CoMnCH were calculated. The intercalated carbonate anions in CoCH and CoMnCH are omitted, and the optimized structures were given in Figure S22. As presented in Figure 4e, DOS of Co 3d orbital for CoMnCH showed a negative shift compared with that for undoped CoCH, indicating that the Mn doping lowered the energy of Co atoms and enabled Co atoms to gain more electrons. To give the accurate number of electrons in 3d orbital per Co atom in these structures, DOS profiles were integrated as shown in Figure 4f. The number of electrons in the 3d orbital of Co atom in CoCH was about 7.15 (theoretical value for Co2+ is 7), which was lower than 7.30 for that in CoMnCH. This change was consistent with the lower binding energy of Co 2p3/2 for CoMnCH (Figure 4d). The greater number of electrons in Co 3d orbital for CoMnCH could benefit the formation of adsorbed OOH species and thus facilitate the OER process.45,47 Besides, it was reported that the increased DOS on Co atom could lead to the downshift of its d-band center and the optimization of the key descriptor for HER performance (ΔGH) by weakening the binding of hydrogen on Co sites, thus enhancing the catalytic activity for HER.71−73 On the basis of this analysis, it reasonable to believe that the enhanced DOS on Co center by the modulation of Mn doping in our CoMnCH would similarly improve its HER activity when compared with CoCH (Figure S20). Inspired by the excellent OER and HER performance of bifunctional Co1Mn1CH, a two-electrode electrolyzer using Co1Mn1CH as both anode and cathode (Co1Mn1CH//

(details seen in the Experimental Section) was used to deliberately introduce Mn ions into CoCH without changing the morphology and ECSA of the catalyst. The obtained sample was noted as CoMnCH-ce. After cation exchange, the color of CoCH changed from pink to dark brown (insets in Figure 4a), similar to the color of CoMnCH samples, implying the successful introduction of Mn. This was corroborated by the clear Mn signals in EDS spectrum as indicated by brown dash lines in Figure 4a. XPS spectrum displayed that the chemical states of both Co and Mn in CoMnCH-ce were very similar to those in Co1Mn1CH samples (Figure S19). Importantly, electrochemical measurements verified that CoMnCH-ce showed capacitance similar to that of CoCH (103.4 vs 92.7 mF cm−2, Figure 4b), indicating the similar ECSA and amount of exposed catalytic active sites and thus ruling out the influence of morphology and ECSA on catalytic activity. However, CoMnCH-ce exhibited an enhanced OER performance with an overpotential of 356 mV at 50 mA/cm−2 compared with 380 mV for CoCH (Figure 4c). The same phenomenon was also observed on HER performance with a decreased overpotential of 59 mV (180 vs 239 mV at 10 mA cm−2 without iR-compensation, Figure S20). By excluding the influence of morphology and ECSA, such activity enhancement should be attributed to the improvement of intrinsic activity of active sites by Mn incorporation. Thereby, XPS was applied to investigate the electronic structure modulation on Co center by Mn doping. As shown in Figure 4d, Co 2p3/2 spectra indicated that Co cation in CoMnCH-ce was bivalent Co2+ like that in CoCH, but the binding energy down-shifted by 0.70 eV (780.87 vs 781.57 eV), suggesting that Mn doping can appreciably modulate the electronic structure of Co centers. Generally, OER starts with the adsorption and discharge of OH− anion at the surface to form adsorbed OH species, followed by the reaction of OH− with the adsorbed OH species to produce H2O and adsorbed atomic O and to release an electron. The next step involves the reaction of OH− with the adsorbed atomic O to form adsorbed OOH species, which then react with additional OH− to form adsorbed O2 and H2O and 8325

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Figure 5. (a) LSV polarization curve (without iR-compensation) of bifunctional Co1Mn1CH in 1 M KOH for overall water splitting at a scan rate 5 mV s−1. The inset is an optical photograph during the measurements. (b) Long-term durability of overall water splitting at a current density of 10 mA cm−2. The inset is SEM image of Co1Mn1CH after durability test.

doping; (ii) the enhanced intrinsic activity of active sites by the electronic modulation via Mn doping; and (iii) the efficient electron and mass transfer as well as gas evolution and release from the architecture and intimate contact of 2D active nanosheets on 3D conductive substrate with open framework. These findings may inspire the design and exploration of new cost-effective bifunctional electrodes by metal center modulation for practical scaling up of water electrolysis.

Co1Mn1CH) was assembled to evaluate its performance for practical overall water splitting in 1 M KOH. The inset in Figure 5a is the photograph picture of the electrolyzer system, where the left electrode is the anode for O2 production and the right one is the cathode for H2 production, as shown in Movie S1. The polarization curve (Figure 5a) for overall water splitting displayed a cell voltage of 1.68 V to afford a current density of 10 mA cm−2, only 450 mV higher than the theoretical reversible potential of water splitting reaction (1.23 V). Such a low overpotential of the novel CoMnCH bifunctional electrocatalyst for overall water splitting is impressive and comparable to the values for recently reported state-of-the-art materials (such as 470 mV for Ni5P465and NiFe LDH/NF,64 480 mV for Co−P/NC74 and 610 mV for NiCo2O4/NF,48 as summarized in Table S4). More importantly, the present Co1Mn1CH//Co1Mn1CH electrolyzer showed excellent durability with a negligible overpotential loss after long-term operations (Figure 5b). SEM image of Co1Mn1CH electrode after durability test (inset in Figure 5b) demonstrated that the nanosheet array structure was maintained very well, consistent with its stable performance. These results promise Co1Mn1CH as a new type of electrocatalyst for practical overall water splitting in alkaline media.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b03507. XRD patterns, photographs, FTIR spectra, XPS spectra, Mn 3s spectra, SEM, TEM, and HRTEM images, EDS mappings, AFM data, ECSA evaluation, OER and HER polarization curves, chronopotentiometric curves, proposed mechanism diagram for electrochemical oxygen evolution, crystal structures, elemental ratios, OER and HER performance comparisons, water-splitting performance data, (PDF) Movie S1 showing vigorous evolution of O2 and H2 bubbles from the electrode surface at a constant potential (AVI)

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4. CONCLUSION Co−Mn carbonate hydroxide (CoMnCH) nanosheets with controllable morphology and composition were prepared on nickel foam through a facile and one-step hydrothermal method. The systematical experiments evidenced that the Mn introduction could significantly and favorably modulate the morphology, ECSA, and electronic structure of CoCH. The optimal Co1Mn1CH could serve as a new type of efficient and durable bifunctional electrocatalyst for both HER and OER. The ultrasmall overpotential of 322 mV at 50 mA cm−2 and 462 mV at 1000 mA cm−2 for OER outperformed all reported metal carbonate hydroxides. The small overpotential of 180 mV at 10 mA cm−2 for HER in 1 M KOH first suggested that such metal carbonate hydroxides could act as HER electrode as well. As a result, the two-electrode electrolyzer using Co1Mn1CH as both anode and cathode demonstrated a practical setup for efficient and stable overall water splitting with a cell voltage of only 1.68 V at 10 mA cm−2. On the basis of experimental results and theoretical analysis, such impressive catalytic performance of Co1Mn1CH for overall water splitting could be attributed to several features: (i) a large quantity of accessible active sites due to vertical and unaggregated nanosheet array structure from Mn

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Wen-Jie Jiang: 0000-0003-1852-0429 Feng Gao: 0000-0003-3173-1650 Li-Jun Wan: 0000-0002-0656-0936 Jin-Song Hu: 0000-0002-6268-0959 Author Contributions ▽

T.T. and W.-J.J. contributed equally to the creation of this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Key Project on Basic Research (2015CB932302), the National Key Research and Development Program of China (2016YFB0101202), the National Natural Science Foundation of China (91645123, 21575004, and 21573249), the Strategic 8326

DOI: 10.1021/jacs.7b03507 J. Am. Chem. Soc. 2017, 139, 8320−8328

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Journal of the American Chemical Society

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Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020100) Program for New Century Excellent Talents in University (NCET-12-0599), National Postdoctoral Program for Innovative Talents (BX201700250), the project sponsored by SRF for ROCS, SEM, and the Foundation for Innovation Team of Bioanalytical Chemistry of Anhui Province. We also thank the following staff scientists at the Center for Analysis and Testing, ICCAS for their help: Dr. Zhi-Juan Zhao and Xiao-Yu Zhang for XPS analysis; Yang Sun for XRD analysis; and Dr. Bo Guan, Ji-Ling Yue, and Li-Rong Liang for SEM and TEM supports.

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