Promotion of the Electrocatalytic Oxygen Evolution Reaction by

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Promotion of the Electrocatalytic Oxygen Evolution Reaction by Chemical Coupling of CoOOH Particles to 3D Branched γ‑MnOOH Rods Meilin Cui, Huihui Zhao, Xiaoping Dai,* Yang Yang, Xin Zhang, Xuebin Luan, Fei Nie, Ziteng Ren, Yin Dong, Yao Wang, Juntao Yang, and Xingliang Huang

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State Key Laboratory of Heavy Oil Processing, China University of PetroleumBeijing, 18 Fuxue Road, Changping, Beijing 102249, P. R. China S Supporting Information *

ABSTRACT: The chemical-coupled three-dimensional (3D) γ-MnOOH with CoOOH particles (γ-MnOOH/CoOOH) was fabricated through a two-step strategy involving a hydrothermal and reduction−oxidation process. The strong solid−solid interfacial interactions between γ-MnOOH rods and CoOOH particles feature the Mn−O−Co bond on the interface and a high valence state of cobalt in CoOOH. The branched γ-MnOOH/CoOOH composites demonstrate a significantly strengthened synergy effect for the oxygen evolution reaction (OER). The optimal B-MCO-0.1 (γ-MnOOH/CoOOH-0.1) shows an excellent OER performance, which requires an overpotential of 313 mV to reach the current density of 10 mA cm−2 with a lower Tafel slope (87 mV dec−1) and the strong durability in alkaline electrolyte. Specially, the intrinsic OER activity (versus electrochemical active surface area, ECSA) on B-MCO-0.1 at an overpotential of 420 mV is 16.3 and 8.9 times higher than those of γ-MnOOH and CoOOH, respectively. The results demonstrate that OER activity originates mainly from the interfacial coupling of γ-MnOOH and CoOOH. Our work highlights the vital function of the interfacial interactions in modulating the electronic structure of the active sites and suggests an effective avenue to rationally design highly active electrocatalysts. KEYWORDS: Branched 3D γ-MnOOH/CoOOH, Interfacial coupling, Synergistic effect, Oxygen evolution reaction

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INTRODUCTION The excessive consumption of fossil fuel and the worsening environmental problems make the development of sustainable energy urgent.1−4 Hydrogen has been proposed as one of the most promising candidates for the future.5 As an attractive and competitive technology, electrochemical water splitting has the advantage of an acknowledged ecofriendly process for the production of highly pure hydrogen. However, the half reaction on the anode (oxygen evolution reaction, OER) shows slow kinetics because of the transfer mechanism of a complex four-electron (4e) pathway and further results in the high overpotential of the process and the poor efficiency of the hydrogen generation.6−9 So far, stainless steel is widely used in the field of industrial electrolysis of water in alkaline electrolyte, which usually is an alloy containing iron, nickel, and chromium.10 The revisit of NiFe-based OER electrocatalysts has focused on the higher activity and stability by rational design of NiFe oxides, layered double hydroxides, and their derivatives. Beyond that, there is a great interest in fabricating new nanostructured materials with cost-effective OER catalysts with high activity and stability. Manganese is the 10th most abundant element on earth, which has highly various oxides with multiple valence. Inspired by nature’s oxygen-evolving CaMn4O5 catalyst photosystem,11 © XXXX American Chemical Society

some attention has been focused on Mn-based oxides and hydr(oxy)oxides as efficient catalysts in water oxidation.3,12,13 Among those Mn-based electrocatalysts, γ-MnOOH is considered as a more active oxide for OER, but its activity is still lower than those of Fe, Co, and Ni hydr(oxy)oxides because of the greater strength of the OH-Mn2+δ energetics.3,14 Notably, the OER activity for Mn-based oxides can be enhanced by synergistic effects with noble metals.15,16 Xu et al.17 successfully prepared a gold-particle-decorated 2D δMnO2 nanosheet through the electrostatic interaction between gold and δ-MnO2 and showed good OER activity by the green laser irradiation with low power. Recently, Qiao’s group18 engineered the high-energy interface of high-index facet Mn3O4 nano-octahedrons with CoO nanoclusters to achieve higher OER activity in alkaline electrolyte. The strong interactions between CoO nanoclusters and Mn3O4 octahedrons resulted in the formation of a high-energy interfacial Mn−O−Co bond with the high valence state of cobalt. Wang et al.19 presented a Co doping strategy by two-step hydrothermal method to fabricate CoMn2O4−MnOOH nanoReceived: April 15, 2019 Revised: June 24, 2019 Published: June 28, 2019 A

DOI: 10.1021/acssuschemeng.9b02106 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration for the synthesis of γ-MnOOH/CoOOH. Synthesis of the Branched γ-MnOOH/CoOOH. The branched γ-MnOOH was used to prepare the hybrid materials of γ-MnOOH/ CoOOH (B-MCO-x) by a low-temperature reduction−oxidation method, where x is the mole ratio of Co to Mn. Typically, 16.53 mg of Co(NO3)2·6H2O and 50 mg of γ-MnOOH were dissolved in 100 mL of DI water with ultrasound for 30 min. Then, the homogeneous solution was kept at 75 °C under stirring for 1 h. A solution (10 mL) containing 26.2 mg of NaBH4 was added dropwise under vigorous stirring at 75 °C. After that, the product was collected, washed, and dried at 90 °C for 12 h under vacuum conditions to obtain B-MCO0.1. The samples with various Co/Mn ratios were prepared by the same procedure, which were named as B-MCO-x (x = 0.05, 0.2, and 0.4), respectively. The CoOOH was also prepared with the same process as B-MCO-x without branched γ-MnOOH. Material Characterization. The crystalline phase of the samples was analyzed on a Bruker AXS D8 Advance instrument with a Cu Kα radiation (λ = 0.154 03 nm). The morphology was revealed by fieldemission scanning electron microscopy (FESEM, FEI, Quanta 200F) and transmission electron microscopy (TEM, JEM 2100 LaB6, and FEI Tecnai G2 F20). The composition and element distribution were characterized by SEM-EDS (energy-dispersive X-ray spectroscopy). X-ray photoelectron spectroscopy (XPS) was undertaken on a Thermo escalab 250Xi with calibration of C 1s (284.6 eV). Raman spectra were acquired by the Renishaw Micro-Raman System 2000 spectrometer. N2 adsorption/desorption were measured on a Micromeritics JW-BK222. Electrochemical Measurements. The conventional threeelectrode system was used to test the OER performance on the CHI 760E electrochemical workstation in 1.0 M KOH solution, where a glassy carbon was used as the working electrode (GCE, diameter: 3 mm), platinum wire (Pt) as the counter electrode, and standard Hg/HgO as the reference electrode, respectively. The mixture of 2 mg of as-prepared catalyst, 1 mg of carbon black, and a solution containing 800 μL of water, 200 μL of ethanol, and 40 μL of Nafion (5%) was sonicated for 30 min for complete dispersion to form homogeneous ink. A 5 μL portion of ink was pipetted and dropped onto the GCE and dried at room temperature. The potentials can be calibrated to a reversible hydrogen electrode (RHE) according to ERHE = EHg/HgO + 0.9024 V. Before the linear sweep voltammetry (LSV, 50 mV s−1), O2 was pumped into the electrolyte solution for 30 min, and the cyclic voltammetry (CV) measurement (0−0.65 V versus Hg/HgO) for 40 cycles was necessary to activate the catalysts with the scan rate of 50 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements were conducted at 0.65 V (versus Hg/HgO) from 105 to 0.01 Hz. The CVs with various scan rates were tested to obtain double-layer charging curves and further investigate the electrical double-layer specific capacity (Cdl). The durability of γ-MnOOH/CoOOH was also investigated through chronoamperometry. Turnover frequency, mass

rods (NRs) with an uneven surface, which exhibited the significantly improved OER activity. Sun’s group20 demonstrated that the electronic structure of Ni−Fe LDH can be modulated by the strong interfacial interactions with FeOOH particles. The solid−solid interfacial chemistry of binary metal oxides in the catalyst is a crucial factor because these interfaces play important roles in electron pathways.21 Many previous reports show the relationship between Mn3+ content and OER activity on Mn-based oxides.14,22,23 Mn 3+ is of great importance to achieve high activity.24 Suib’s group 25 modulated the concentration of Mn3+ in mesoporous Mn2O3 by various calcination temperatures, which further result in the enhanced catalytic OER activity. Robinson et al.26 investigated eight manganese oxides containing Mn3+ and Mn4+ and found that only Mn2O3, Mn3O4, and λ-MnO2 with Mn3+-O bands between edge-sharing MnO6 octahedra have the catalytic activity. For MnOOH, the stability of Mn3+ is of great importance to improve OER activity because Mn4+ and Mn2+ species are inactive.14 Co-(oxy)hydroxide (CoOOH) with excellent stability under general alkaline conditions generally serves as active sites to promote the OER process.27,28 The CoOOH can also act as a chemically stable host to enhance the electrical conductivity and electrocatalytic activity of Fe-based OER catalysts.29,30 Engineering the interface of MnOOH with CoOOH by strong chemical coupling at the mesoscale should be highly desirable to improve the OER performance. Herein, we report that the interfacial coupling between multiple branched 3D γ-MnOOH and CoOOH particles could greatly boost the OER performance. Benefiting from the steady backbone for the charge transporting, the highly accessible surface to adsorb ions, and the effectively interfacial synergism between MnOOH and CoOOH, the optimal B-MCO-0.1 shows high activity and excellent durability in alkaline electrolyte.

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EXPERIMENTAL SECTION

Synthesis of the Branched γ-MnOOH. Typically, 2 mmol of KMnO4 (0.316 g) and 1 mmol of (NH4)2C2O4 (0.142 g) were dissolved into 30 mL of deionized (DI) water. Then, 2 mL of ethyl acetate was pipetted and added into the solution mentioned above under vigorous stirring to form a homogeneous solution, which was transferred into a closed 50 mL autoclave to maintain at 180 °C for 24 h. After that, the gray−black precipitate was centrifuged, washed 3 times with a mixture of water and absolute ethanol (1:1, v/v), and dried at 90 °C for 12 h. B


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Figure 2. SEM images of the morphological evolution of branched γ-MnOOH with different times: (a) 0.5 h, (b) 1 h, (c) 8 h, (d)12 h, and (e) 24 h. activity, and Faradaic efficiency (FE) were obtained according to a previous reference.7

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RESULTS AND DISCUSSION Material Preparation and Characterization. Figure 1 presents the schematic illustration for the fabrication of the γMnOOH/CoOOH, where the branched γ-MnOOH was first synthesized by reduction of KMnO4 with (NH4)2C2O4 under hydrothermal conditions and was followed by the reduction− oxidation process of Co2+ to couple CoOOH particles on the surface of γ-MnOOH. During this process, the addition of ethyl acetate can modulate the growth direction of the γMnOOH crystal. To investigate the formation process of the branched γ-MnOOH, time-dependent experiments were conducted with the constant amount of ethyl acetate (2 mL), and the results show the homogeneous ultrathinnanosheet stacking structure within 0.5 h in Figure 2a. As the time increases to 1 h, a small amount of nanowires begin to appear, which further grow to a branched structure (Figure 2b). For a longer reaction time, the multiple branched and surface-smooth micro-nanorods are ultimately formed (Figure 2c−e). The morphologies of γ-MnOOH were also investigated by various amounts of ethyl acetate (0, 1, 2, 5, 10 mL). Obviously, the uniform rod-like structure is obtained without adding ethyl acetate, while with the increasing amount of ethyl acetate, a branched γ-MnOOH structure is produced (Figure S1). Next, Co2+ adsorbed on γ-MnOOH is reduced with NaBH4 and mildly oxidized with air in solution to form hybrids of γ-MnOOH/CoOOH. The resultant γ-MnOOH/CoOOH hybrids with various ratios of Co to Mn are named as B-MCOx (x = 0.05, 0.1, 0.2, and 0.4), respectively. Notably, the smooth surface of γ-MnOOH becomes rough after being coupled with CoOOH (Figure 3a,b and Figure S2). The CoOOH particles with tens of nanometers can be clearly seen in Figure 3c. The morphological evolution of γ-MnOOH/ CoOOH with various ratios of Co to Mn is shown in Figure S2. At the beginning, irregular particles are formed on the surface of γ-MnOOH (Figure S2c,d). With the increasing ratio of Co to Mn, some uniform particles with a diameter of several tens of nanometers are observed (Figure 3a,b), and further increasing the ratio of Co to Mn, the CoOOH nanosheets are gradually formed on γ-MnOOH (Figure S2e−h). The corresponding elements mapping for B-MCO-0.1 in Figure

Figure 3. (a) SEM image of B-MCO-0.1. (b) Magnified SEM image of B-MCO-0.1. (c−g) TEM images of B-MCO-0.1 and the corresponding element mappings.

3c−g shows the existence and uniform distribution of Mn, Co, and O elements. The X-ray diffraction (XRD) characterizations of γ-MnOOH and B-MCO-x were then performed to investigate the effect of ethyl acetate and the ratio of Co to Mn on the crystalline phase. All characteristic peaks for γ-MnOOH and B-MCO-x match well with the monoclinic γ-MnOOH (P21/c, JCPDS 41-1379) in Figure S3a,b. The sharp peaks centered at 2θ = 26.1°, 33.9°, 37.1°, 40.4°, 51.2°, 53.8°, 54.9°, and 65.2° can be assigned to the (111̅), (020), (002), (202̅), (022), (222̅), (311̅), and (313̅) plane, suggesting good crystallinity of γMnOOH.31,32 There is no obvious peak for Co oxide on CoOOH and B-MCO-x, indicating the existence of amorphous Co oxide on γ-MnOOH.33 Figure S3c shows that the XRD patterns of γ-MnOOH with different crystallization times display the increasing intensity for γ-MnOOH, indicating the higher crystallinity. The B-MCO-x and CoOOH are further investigated by the Raman spectrum in Figure 4a and Figure S4. The spectrum of CoOOH (Figure S4a) presents four vibrational modes as 688, 615, 518, and 477 cm−1, well consistent with the previous report for CoOOH.34 The Raman spectrum of γ-MnOOH (Figure S4b) shows the characteristic C


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Figure 4. (a) Raman spectrum of B-MCO-x and CoOOH. High-resolution XPS of (b) Mn 2p, (c) Co 2p, and (d) O 1s of γ-MnOOH, CoOOH, and B-MCO-0.1.

peaks at 145, 262, 283, 354, 383, 528, 554, and 624 cm−1, which also proves the previous results on γ-MnOOH, and the peaks at 528, 554, and 624 cm−1 are assigned to be the stretching modes of Mn−O in MnO6 octahedra.35 The BMCO-x shows the mixed peaks for γ-MnOOH and CoOOH (Figure 4a), but the significantly positive shifts for γ-MnOOH and negative shifts for CoOOH imply the strong interaction between γ-MnOOH and CoOOH. Notably, the presence of a new and wide peak at 650 cm−1 on B-MCO-0.1 and right shift with the increasing ratio of Co to Mn should be relevant with strong interactions between CoOOH and MnOOH to form the Mo−O−Co bond. The intensity of the shoulder at 688 cm−1 slightly increases with the increasing ratio of Co to Mn. Nitrogen adsorption−desorption isotherms indicate the porous structure of the samples in Figure S5, which displays a typical IV isotherm with type H1 (CoOOH) and H2 (γMnOOH and B-MCO-0.1).36 The BET surface area is 65 m2 g−1 for B-MCO-0.1, bigger than those of γ-MnOOH (48.61 m2 g−1) and B-MCO-0.1 (39.45 m2 g−1). The large BET surface area and appropriate pore size are favorable to expose more active sites in the OER process.37 To gain further insight into the surface structure and the oxidation state, X-ray photoelectron spectroscopy (XPS) was carried out, which displays the atomic concentration of 24.1 atom % for Co on CoOOH nanosheet (TEM, Figure S6), 25.7

atom % for Mn on γ-MnOOH, as well as 12.2 atom % for Co and 11.6 atom % for Mn on B-MCO-0.1 (Figure S7a). The high-resolution spectra of Mn 2p (Figure 4b) show the binding energies for Mn 2p3/2 (641.4 eV) and Mn 2p1/2 (652.7 eV) with two satellite peaks at 642.8 and 654.2 eV on γ-MnOOH and B-MCO-0.1.38−40 The multiple splitting Mn 3s spectra of γ-MnOOH (Figure S7b) display two peaks located at 82.7 and 88.35 eV.41,42 Figure 4c shows the Co 2p spectra of CoOOH and B-MCO-0.1, which are fitted into the characteristic peak of Co3+ and its corresponding shake-up satellites, respectively. The Co 2p3/2 and Co 2p1/2 peaks at 780.92 and 796.83 eV are assigned to be the CoIII of CoOOH.43,44 Compared with CoOOH and MnOOH, the shift of peak position for Co 2p3/2 (∼0.49 eV), Co 2p1/2 (∼0.57 eV), and Mn 3s (∼0.41 eV) in BMCO-0.1 implies the strong electronic interactions between γMnOOH and CoOOH.45,46 Three peaks for O 1s located at 529.6, 530.8, and 532.0 cm−1 are observed for γ-MnOOH, CoOOH, and B-MCO-0.1 in Figure 4d, which correspond to the M−O−M bond (M = Mn, Co), Mn (or Co)−O−H bond, and H−O−H bond in physisorbed H2O,39 respectively. Notably, the binding energy for the metal−oxygen bond on B-MCO-0.1 is slightly different compared to those of the Mn− O bond for γ-MnOOH and Co−O bond for CoOOH, which could be attributed to the formation of the Mn−O−Co bond.18 Those results indicate that CoOOH acts as an electron D


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Figure 5. (a) LSV curves. (b) Tafel slopes. (c) Current density at 1.2524 V (vs RHE) as a function of scan rate fitted to a linear regression allowing for the estimation of Cdl. (d) Nyquist slopes. (e) TOFs and the mass activity by assuming that all Mn and Co atoms are catalytically active. (f) Chronoamperometry measurement of B-MCO-0.1 at a value of 313 mV (vs RHE) and durability tests for γ-MnOOH/CoOOH-0.1 after 5000 potential cycles at a rate of 100 mV s−1.

donor that provides electrons to γ-MnOOH via the interfacial Mn−O−Co bond.18 Electrocatalytic Performance toward the OER. The OER activity of B-MCO-x and controlling catalysts was investigated in 1.0 M KOH solution by LSV. As shown in Figure 5a and Figure S8, the B-MCO-0.1 presents a much lower overpotential (η10 = 313 mV), which is lower than those of control samples, such as γ-MnOOH (494 mV), B-MCO0.05 (355 mV), B-MCO-0.2 (376 mV), B-MCO-0.4 (383 mV), and CoOOH nanosheet (420 mV), and is also significantly lower among most reported OER catalysts, such as CoMn LDH (324 mV),47 CoO/hi-Mn3O4 (378 mV),18

Co3O4−x-Carbon@Fe2−yCoyO3 (350 mV),48 MnO2-0.5IL nanowire (394 mV),49 and Co/MnO@GC-700 (358 mV).50 The Tafel slope (Figure 5b) (87 mV dec−1) of B-MCO-0.1 is smaller than those of γ-MnOOH (122 mV dec−1), B-MCO0.05 (88 mV dec−1), B-MCO-0.2 (106 mV dec−1), B-MCO0.4 (113 mV dec−1), and pure CoOOH (140 mV dec−1), indicating the fast OER kinetics process of B-MCO-0.1. Compared with B-MCO-0.1, the physical mixture of γMnOOH and CoOOH has a much lower OER activity with a larger overpotential of 392 mV at 10 mA cm−2 and higher Tafel slope of 182 mV dec−1 (Figure S14) as compared to that of B-MCO-0.1, suggesting that the strong solid−solid E


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As mentioned above, the lower Rads of B-MCO-0.1 is favorable to stabilize the surface species and then leads to the smaller adsorption energy. Furthermore, compared with the γMnOOH and CoOOH, the enhanced OER activity of BMCO-0.1 can be attributed to the promoted conductivity and electron transfer ability on the chemical-coupling γ-MnOOH/ CoOOH and the strong solid−solid interfacial interactions between γ-MnOOH and CoOOH. The long-term stability is also an important indicator for OER electrocatalysts. Figure 5f shows a chronoamperometry curve with the overpotential of 313 mV, which only has a negligible loss of the current density after 18 h for the B-MCO0.1. The chronoamperometry curve under high current density (100 mA cm−2) by loading the catalyst into Ni foam with the mass loading of 3 mg cm−2 as working electrode also exhibits excellent stability with as high a retention rate as 95.5% in 16 h (Figure S13). The inset of Figure 5f indicates the LSV curves measured before and after the 5000 cycles with a scan rate of 100 mV s−1. Compared with the initial one, the B-MCO-0.1 shows a small increase of the potential at 10 mA cm−2, further verifying the strong stability of B-MCO-0.1. The XPS spectra of B-MCO-0.1 after reaction in Figure S7 are very similar to the fresh sample, but the shift of peak position suggests more strong interactions between CoOOH and MoOOH after reaction.

interfacial interaction is a key factor on the improvement of the catalytic activity in the OER process. The double-layer capacitances (Cdl) were obtained by estimating the linear slope of the curve of current density at various scan rates (Figure S9). The B-MCO-0.1 shows the biggest Cdl value (11.5 mF cm2) among the control samples (Figure 5c and Figure S10a). The electrochemical active surface area (ECSA) could be calculated according to ECSA = Cdl/Cs × A, where Cs = 0.04 mF cm−2 in 1.0 M KOH, and A is the geometric area of the GCE (0.070 65 cm−2). The B-MCO0.1 also shows the largest electrochemical active surface area (ECSA, Figure S10b), which has the same trend as the BET surface of B-MCO-0.1, indicating more accessible active sites in the OER. A simplified Randles circuit was obtained by fitting the Nyquist plots (Figure S11). Rs represents the uncompensated contract and solution resistance, and Rads affects the ease of formation of surface intermediates; Rct is the electron transfer resistance, and it can control the charge transfer rates of the reaction steps, including the ratedetermining step.51,52 The Rads is relevant with the stability of surface species, where low Rads is favorable to stabilize the surface species.53 The Rads value on B-MCO-x is significantly smaller than that on γ-MnOOH (Table S1). Further, smaller Rct and Rs on B-MCO-0.1 demonstrate the tremendous contributions from the interfacial coupling with CoOOH particles for fast electron transfer by the conductive CoOOH matrix.29,30 Mass activity and TOF of B-MCO-0.1 present the highest values among γ-MnOOH, CoOOH, and B-MCO-x (Figure 5e). TOF and mass activity on B-MCO-0.1 at the overpotential of 320 mV are about 50/50.3 and 4.3/6.6 times higher than those of γ-MnOOH and CoOOH (Table S2), respectively. The results suggest that the interfacial interactions between γ-MnOOH and CoOOH can significantly improve the intrinsic activity in the OER. The OER current density can also be normalized with the corresponding ECSA to eliminate the effect of ECSA (Figure S12). The current density of BMCO-0.1 is 5.71 mA cm−2 at 420 mV, which is about 16.3 and 8.9 times higher than those of γ-MnOOH and CoOOH, respectively, demonstrating the crucial role of interfacial coupling between γ-MnOOH and CoOOH. The insignificant difference between ring current density and the disk current density in Figure S15 indicates the negligible hydrogen peroxide formation. The electron transfer number (inset of Figure S15a) is ∼3.9 at 1.65−1.8 V (versus RHE), demonstrating a four-electron pathway in the OER (4OH− → O2 + H2O + 4e−). The Faradaic efficiency was tested with a ring potential of 0.4 V (versus RHE), which is in the oxygen reduction reaction (ORR) region and can make the O2 formed on the disk reduced on the ring electrode.45,54 With a constant current (∼206 μA) of the disk electrode, a ring current of about 76 μA (collection efficiency 0.37) can be identified, and a high Faradaic efficiency which is close to 98% (Figure S15b) can be detected. The OER mechanism in alkaline media is usually considered as the following processes: (1) M + OH → M−OH + e−; (2) M−OH + OH → M−O + e− + H2O; (3) M−O + OH → M−OOH + e−; (4) M−OOH + OH → M− O2 + H2O + e−; (5) M−O2 → M + O2; M in the equation refers to the active sites.53,55 There are going to be various intermediates in the OER process, such as MOH, M−O, M− OOH, and M−O2. Finally, the O2 + M (gas) are obtained, and the true activity sites in the reaction are metal oxide/ hydr(oxy)oxides.56 The OER kinetics is related to the adsorption energy of OH− on the surface of the catalyst.57

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CONCLUSIONS In summary, engineering the interface of 3D branched γMnOOH with CoOOH particles is proposed by a chemicalcoupling strategy, where the strong solid−solid interfacial interactions feature the Mn−O−Co bond on the interface and high valence state of cobalt in CoOOH. The optimized BMCO-0.1 (γ-MnOOH/CoOOH-0.1) exhibits the largest effective surface area (ECSA) and smaller electron transfer resistance (Rct), and as a result, it shows an excellent OER performance with an overpotential of 313 mV at 10 mA cm−2, a lower Tafel slope of 87 mV dec−1, and the strong durability in the alkaline electrolyte. The intrinsic OER activity (versus ECSA) on B-MCO-0.1 at an overpotential of 420 mV is 16.3 and 8.9 times higher than those of γ-MnOOH and CoOOH, respectively. The results indicate that OER activity originates mainly from the interfacial coupling of γ-MnOOH and CoOOH, demonstrating the vital role of solid−solid interfacial interactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b02106. SEM images; TEM images; XRD patterns; Raman spectrum; nitrogen adsorption−desorption isotherms; XPS survey spectra; high-resolution Mn 3s, Mn 2p, and Co 2p spectra; overpotential at 10 mA cm−2; CV scans for double-layer capacitances (C dl ); C dl ; ECSA; simplified Randles circuit and results; comparison of OER activity; LSV polarization curves based on ECSA; chronoamperometry measurement at 100 mA cm−2; and Faradaic efficiency (PDF) F


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(13) Huynh, M.; Bediako, D. K.; Nocera, D. G. A functionally stable manganese oxide oxygen evolution catalyst in acid. J. Am. Chem. Soc. 2014, 136, 6002−6010. (14) Smith, P. F.; Deibert, B. J.; Kaushik, S.; Gardner, G.; Hwang, S.; Wang, H.; Al-Sharab, J. F.; Garfunkel, E.; Fabris, L.; Li, J.; Dismukes, G. C. Coordination geometry and oxidation state requirements of corner-sharing MnO6 octahedra for water oxidation catalysis: an investigation of Manganite (γ-MnOOH). ACS Catal. 2016, 6, 2089− 2099. (15) Gao, Q.; Ranjan, C.; Pavlovic, Z.; Blume, R.; Schlögl, R. Enhancement of stability and activity of Mnox/Au electrocatalysts for oxygen evolution through adequate electrolyte composition. ACS Catal. 2015, 5, 7265−7275. (16) Seitz, L. C.; Hersbach, T. J.; Nordlund, D.; Jaramillo, T. F. Enhancement effect of noble metals on manganese oxide for the oxygen evolution reaction. J. Phys. Chem. Lett. 2015, 6, 4178−4183. (17) Xu, J.; Gu, P.; Birch, D. J. S.; Chen, Y. Plasmon-promoted electrochemical oxygen evolution catalysis from gold decorated MnO2 nanosheets under green light. Adv. Funct. Mater. 2018, 28, 1801573. (18) Guo, C.; Zheng, Y.; Ran, J.; Xie, F.; Jaroniec, M.; Qiao, S. Z. Engineering high-energy interfacial structures for high-performance oxygen-involving electrocatalysis. Angew. Chem., Int. Ed. 2017, 56, 8539−8543. (19) Wang, Y.; Hu, T. J.; Liu, Q.; Zhang, L. M. CoMn2O4 embedded in MnOOH nanorods as a bifunctional catalyst for oxygen reduction and oxygen evolution reactions. Chem. Commun. 2018, 54, 4005− 4008. (20) Chen, J. D.; Zheng, F.; Zhang, S.-J.; Fisher, A.; Zhou, Y.; Wang, Z. Y.; Li, Y. Y.; Xu, B.-B.; Li, J.-T.; Sun, S.-G. Interfacial interaction between FeOOH and Ni−Fe LDH to modulate the local electronic structure for enhanced OER electrocatalysis. ACS Catal. 2018, 8, 11342−11351. (21) Kim, G. P.; Lim, D.; Park, I.; Park, H.; Shim, S. E.; Baeck, S. H. RuO2 nanoparticles decorated MnOOH/C as effective bifunctional electrocatalysts for lithium-air battery cathodes with long-cycling stability. J. Power Sources 2016, 324, 687−693. (22) Mattioli, G.; Zaharieva, I.; Dau, H.; Guidoni, L. Atomistic texture of amorphous manganese oxides for electrochemical water splitting revealed by ab initio calculations combined with X-Ray spectroscopy. J. Am. Chem. Soc. 2015, 137, 10254−10267. (23) McKendry, I. G.; Kondaveeti, S. K.; Shumlas, S. L.; Strongin, D. R.; Zdilla, M. J. Decoration of the layered manganese oxide birnessite with Mn(II/III) gives a new water oxidation catalyst with fifty-fold turnover number enhancement. Dalton. Trans. 2015, 44, 12981− 12984. (24) Takashima, T.; Hashimoto, K.; Nakamura, R. Mechanisms of pH-dependent activity for water oxidation to molecular oxygen by MnO2 electrocatalysts. J. Am. Chem. Soc. 2012, 134, 1519−1527. (25) Kuo, C.-H.; Mosa, I. M.; Poyraz, A. S.; Biswas, S.; El-Sawy, A. M.; Song, W. Q.; Luo, Z.; Chen, S.-Y.; Rusling, J. F.; He, J.; Suib, S. L. Robust mesoporous manganese oxide catalysts for water oxidation. ACS Catal. 2015, 5, 1693−1699. (26) Robinson, D. M.; Go, Y. B.; Mui, M.; Gardner, G.; Zhang, Z.; Mastrogiovanni, D.; Garfunkel, E.; Li, J.; Greenblatt, M.; Dismukes, G. C. Photochemical water oxidation by crystalline polymorphs of manganese oxides: structural requirements for catalysis. J. Am. Chem. Soc. 2013, 135, 3494−3501. (27) Bajdich, M.; Garcia-Mota, M.; Vojvodic, A.; Norskov, J. K.; Bell, A. T. Theoretical investigation of the activity of cobalt oxides for the electrochemical oxidation of water. J. Am. Chem. Soc. 2013, 135, 13521−13530. (28) Doyle, R. L.; Godwin, I. J.; Brandon, M. P.; Lyons, M. E. Redox and electrochemical water splitting catalytic properties of hydrated metal oxide modified electrodes. Phys. Chem. Chem. Phys. 2013, 15, 13737−13783. (29) Burke, M. S.; Zou, S.; Enman, L. J.; Kellon, J. E.; Gabor, C. A.; Pledger, E.; Boettcher, S. W. Revised oxygen evolution reaction activity trends for first-row transition-metal (oxy)hydroxides in alkaline media. J. Phys. Chem. Lett. 2015, 6, 3737−63742.

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 10 89734979. Fax: +86 10 89734979. E-mail: [email protected]. ORCID

Xiaoping Dai: 0000-0003-2289-8133 Yao Wang: 0000-0001-9578-9128 Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (U1662104 and 21576288). REFERENCES

(1) Zhang, Y.; Zhang, H.; Fang, L.; Deng, J.; Wang, Y. Facile synthesis of nickel manganese composite oxide nanomesh for efficient oxygen evolution reaction and supercapacitors. Electrochim. Acta 2017, 245, 32−40. (2) Turner, J. A. Sustainable hydrogen production. Science 2004, 305, 972−974. (3) Subbaraman, R.; Tripkovic, D.; Chang, K.-C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in activity for the water electrolyser reactions on 3d M(Ni, Co, Fe, Mn) hydr(oxy)oxide catalysts. Nat. Mater. 2012, 11, 550−557. (4) Wang, Y.; Yu, Y.; Jia, R.; Zhang, C.; Zhang, B. Electrochemical synthesis of nitric acid from air and ammonia through aaste utilization. Natl. Sci. Rev.2019, in press. DOI: 10.1093/nsr/nwz019 (5) Wang, J.; Cui, W.; Liu, Q.; Xing, Z.; Asiri, A. M.; Sun, X. Recent progress in cobalt-based heterogeneous catalysts for electrochemical water splitting. Adv. Mater. 2016, 28, 215−230. (6) Liang, Y.; Yu, Y.; Huang, Y.; Shi, Y.; Zhang, B. Adjusting the electronic structure by Ni incorporation: a generalized in situ electrochemical strategy to enhance water oxidation activity of oxyhydroxides. J. Mater. Chem. A 2017, 5, 13336−13340. (7) Zhao, H. H.; Yang, Y.; Dai, X. P.; Qiao, H. Y.; Yong, J. X.; Luan, X. B.; Yu, L.; Luan, C. L.; Wang, Y.; Zhang, X. NiCo-DH nanodots anchored on amorphous nico-sulfide sheets as efficient electrocatalysts for oxygen evolution reaction. Electrochim. Acta 2019, 295, 1085−1092. (8) Qiao, H. Y.; Yong, J. X.; Dai, X. P.; Zhang, X.; Ma, Y. D.; Liu, M. Z.; Luan, X. B.; Cai, J. C.; Yang, Y.; Zhao, H. H.; Huang, X. L. Hollow FeNi-based hybrid polyhedron derived from unique sulfur-modulating coordinated transition bimetal complexes for efficient oxygen evolution reactions. J. Mater. Chem. A 2017, 5, 21320−21327. (9) Stevens, M. B.; Trang, C. D. M.; Enman, L. J.; Deng, J.; Boettcher, S. W. Reactive Fe-sites in Ni/Fe (oxy)hydroxide are responsible for exceptional oxygen electrocatalysis activity. J. Am. Chem. Soc. 2017, 139, 11361−11364. (10) Gong, M.; Dai, H. A mini review of NiFe-based materials as highly active oxygen evolution reaction electrocatalysts. Nano Res. 2015, 8, 23−39. (11) Jeong, D.; Jin, K.; Jerng, S. E.; Seo, H.; Kim, D.; Nahm, S. H.; Kim, S. H.; Nam, K. T. Mn5O8 nanoparticles as efficient water oxidation catalysts at neutral pH. ACS Catal. 2015, 5, 4624−4628. (12) He, J.; Wang, M.; Wang, W.; Miao, R.; Zhong, W.; Chen, S. Y.; Poges, S.; Jafari, T.; Song, W.; Liu, J.; Suib, S. L. Hierarchical mesoporous NiO/MnO2@PANI core-shell microspheres, highly efficient and stable bifunctional electrocatalysts for oxygen evolution and reduction reactions. ACS Appl. Mater. Interfaces 2017, 9, 42676− 42687. G


Research Article

ACS Sustainable Chemistry & Engineering (30) Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W. Cobalt-iron (oxy)hydroxide oxygen evolution electrocatalysts: the role of structure and composition on activity, stability, and mechanism. J. Am. Chem. Soc. 2015, 137, 3638−3648. (31) Zhang, L.; Zhang, X.; Wang, Z.; Xu, J.; Xu, D.; Wang, L. High aspect ratio γ-MnOOH nanowires for high performance rechargeable nonaqueous lithium-oxygen batteries. Chem. Commun. 2012, 48, 7598−7600. (32) Zheng, D. S.; Yin, Z. L.; Zhang, W.; Tan, X. J.; Sun, S. X. Novel branched γ-MnOOH and β-MnO2 multipod nanostructures. Cryst. Growth Des. 2006, 6, 1733−1735. (33) Lin, C. Characterization of sol-gel-derived cobalt oxide xerogels as electrochemical capacitors. J. Electrochem. Soc. 1998, 145, 4097− 4103. (34) Feng, X. M.; Xing, W. Y.; Song, L.; Hu, Y. In situ synthesis of a MoS2/CoOOH hybrid by a facile wet chemical method and the catalytic oxidation of CO in epoxy resin during decomposition. J. Mater. Chem. A 2014, 2, 13299. (35) Gao, T.; Fjellvag, H.; Norby, P. A comparison study on Raman scattering properties of α- and β-MnO2. Anal. Chim. Acta 2009, 648, 235−239. (36) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl. Chem. 1985, 57, 603−619. (37) Lu, X. F.; Gu, L. F.; Wang, J. W.; Wu, J. X.; Liao, P. Q.; Li, G. R. Bimetal-organic framework derived CoFe2O4/C porous hybrid nanorod arrays as high-performance electrocatalysts for oxygen evolution reaction. Adv. Mater. 2017, 29, 1604437. (38) Ardizzone, S.; Bianchi, C. L.; Tirelli, D. Mn3O4 and γ-MnOOH powders, preparation, phase composition and XPS characterisation. Colloids Surf., A 1998, 134, 305−312. (39) Gao, T.; Krumeich, F.; Nesper, R.; Fjellvåg, H.; Norby, P. Microstructures, surface properties, and topotactic transitions of Manganite nanorods. Inorg. Chem. 2009, 48, 6242−6250. (40) Sun, W.; Hsu, A.; Chen, R. Carbon-supported tetragonal MnOOH catalysts for oxygen reduction reaction in alkaline media. J. Power Sources 2011, 196, 627−635. (41) Beyreuther, E.; Grafström, S.; Eng, L. M.; Thiele, C.; Dörr, K. XPS investigation of Mn valence in lanthanum Manganite thin films under variation of oxygen content. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73 (15), 155425. (42) Murray, J. W.; Dillard, J. G.; Giovanoli, R.; Moers, H.; Stumm, W. Oxidation of Mn (II): initial mineralogy, oxidation state and ageing. Geochim. Cosmochim. Acta 1985, 49, 463−470. (43) Zhang, D.; Kong, X.; Zhao, Y.; Jiang, M.; Lei, X. CoOOH ultrathin nanoflake arrays aligned on nickel foam: fabrication and use in high-performance supercapacitor devices. J. Mater. Chem. A 2016, 4, 12833−12840. (44) Xu, Y.-Z.; Yuan, C.-Z.; Chen, X.-P. A facile strategy for the synthesis of NiSe@CoOOH core-shell nanowires on nickel foam with high surface area as efficient electrocatalyst for oxygen evolution reaction. Appl. Surf. Sci. 2017, 426, 688−693. (45) Li, X. Z.; Fang, Y. Y.; Lin, X. Q.; Tian, M.; An, X. C.; Fu, Y.; Li, R.; Jin, J.; Ma, J. T. MOF derived Co3O4 nanoparticles embedded in N-doped mesoporous carbon layer/MWCNT hybrids: extraordinary bi-functional electrocatalysts for OER and ORR. J. Mater. Chem. A 2015, 3, 17392−17402. (46) Wu, J.; Xue, Y.; Yan, X.; Yan, W. S.; Cheng, Q. M.; Xie, Y. Co3O4 nanocrystals on single-walled carbon nanotubes as a highly efficient oxygen-evolving catalyst. Nano Res. 2012, 5, 521−530. (47) Song, F.; Hu, X. Ultrathin cobalt-manganese layered double hydroxide is an efficient oxygen evolution catalyst. J. Am. Chem. Soc. 2014, 136, 16481−16484. (48) Xu, W. W.; Xie, W. W.; Wang, Y. Co3O4−x-Carbon@ Fe2−yCoyO3 heterostructural hollow polyhedrons for the oxygen evolution reaction. ACS Appl. Mater. Interfaces 2017, 9, 28642− 28649.

(49) Yan, G. B.; Lian, Y. B.; Gu, Y. D.; Yang, C.; Sun, H.; Mu, Q. Q.; Li, Q.; Zhu, W.; Zheng, X. S.; Chen, M.; Zhu, J.; Deng, Z.; Peng, Y. Phase and morphology transformation of MnO2 induced by ionic liquids toward efficient water oxidation. ACS Catal. 2018, 8, 10137− 10147. (50) Xu, J.; Zhang, H.; Xu, P.; Wang, R.; Tong, Y.; Lu, Q.; Gao, F. In situ construction of hierarchical Co/MnO@graphite carbon composites for highly supercapacitive and OER electrocatalytic performances. Nanoscale 2018, 10, 13702−13712. (51) Zhang, J.; Li, X.; Liu, Y.; Zeng, Z.; Cheng, X.; Wang, Y.; Tu, W.; Pan, M. Bi-metallic boride electrocatalysts with enhanced activity for the oxygen evolution reaction. Nanoscale 2018, 10, 11997−12002. (52) Harrington, D. A.; Conway, B. E. ac Impedance of faradaic reactions involving electrosorbed intermediatesI. Kinetic theory. Electrochim. Acta 1987, 32, 1703−1712. (53) Chen, Z.; Kronawitter, C. X.; Yeh, Y.-W.; Yang, X. F.; Zhao, P.; Yao, N.; Koel, B. E. Activity of pure and transition metal-modified CoOOH for the oxygen evolution reaction in an alkaline medium. J. Mater. Chem. A 2017, 5, 842−850. (54) Al-Mamun, M.; Wang, Y.; Liu, P.; Zhong, Y. L.; Yin, H.; Su, X.; Zhang, H.; Yang, H.; Wang, D.; Tang, Z.; Zhao, H. One-step solid phase synthesis of a highly efficient and robust cobalt pentlandite electrocatalyst for the oxygen evolution reaction. J. Mater. Chem. A 2016, 4, 18314−18321. (55) Feng, J. X.; Xu, H.; Dong, Y. T.; Ye, S. H.; Tong, Y. X.; Li, G. R. FeOOH/Co/FeOOH hybrid nanotube arrays as high-performance electrocatalysts for the oxygen evolution reaction. Angew. Chem., Int. Ed. 2016, 55, 3694−8. (56) Yang, Y.; Luan, X.; Dai, X.; Zhang, X.; Qiao, H.; Zhao, H.; Yong, J.; Yu, L.; Han, J.; Zhang, J. Partially sulfurated ultrathin nickeliron carbonate hydroxides nanosheet boosting the oxygen evolution reaction. Electrochim. Acta 2019, 309, 57−64. (57) Shao, Y.; Zheng, M.; Cai, M.; He, L.; Xu, C. Improved electrocatalytic performance of core-shell NiCo/NiCoO with amorphous FeOOH for oxygen-evolution reaction. Electrochim. Acta 2017, 257, 1−8.

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Promotion of the Electrocatalytic Oxygen Evolution Reaction by

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