Engineering Catalytic Active Sites on Cobalt ... - Wiley Online Library

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Engineering Catalytic Active Sites on Cobalt Oxide Surface for Enhanced Oxygen Electrocatalysis Xiaopeng Han, Guowei He, Yu He, Jinfeng Zhang, Xuerong Zheng, Lanlan Li, Cheng Zhong, Wenbin Hu, Yida Deng,* and Tian-Yi Ma* as controlling the size, structure, and surface chemistry to meet the requirement of next-generation devices.[2] As a commonly available metal oxide, spinel-type Co3O4, where Co2+ and Co3+ ions occupy the tetrahedral and octahedral sites, has versatile applications in magnetism,[3] catalysis,[2d,e] and electrochemical energy technologies including Li-ion batteries, water oxidation, and supercapacitors, etc.[4] Nanoscaled Co3O4 crystals can be designed with specifically exposed crystal planes, which not only affect the geometric shape of the final products, but also have critical influence on the surface-sensitive physicochemical properties due to different surface atomic configurations.[5] For example, Co3O4 has been extensively investigated for CO oxidation and CH4 combustion.[1d,5a] The {110} and {112} enclosed Co3O4 nanocrystals exhibit superior catalytic activity than that enclosed by {100} and {111} facets because of the substantially higher density of catalytic active Co3+ sites in the former.[5a,d,6] Note that tailoring the surface active sites of Co3O4 derived from the morphology and facet control synthesis can dramatically promote the catalytic efficiency by enabling proper bond strength between catalytic sites and reactants.[5a,6,7] However, in order to synthesize Co3O4 nanocrystals surrounded by different facets, various strategies are employed,[5d,7,8] such as pH-control hydrothermal strategy[5d,7c] and precipitation-thermal treatment route,[7a,b] which are complicated, time-consuming, and in some

Tuning the catalytic active sites plays a crucial role in developing low cost and highly durable oxygen electrode catalysts with precious metal-competitive activity. In an attempt to engineer the active sites in Co3O4 spinel for oxygen electrocatalysis in alkaline electrolyte, herein, controllable synthesis of surfacetailored Co3O4 nanocrystals including nanocube (NC), nanotruncated octahedron (NTO), and nanopolyhedron (NP) anchored on nitrogen-doped reduced graphene oxide (N-rGO), through a facile and template-free hydrothermal strategy, is provided. The as-synthesized Co3O4 NC, NTO, and NP nanostructures are predominantly enclosed by {001}, {001} + {111}, and {112} crystal planes, which expose different surface atomic configurations of Co2+ and Co3+ active sites. Electrochemical results indicate that the unusual {112} plane enclosed Co3O4 NP on rGO with abundant Co3+ sites exhibit superior bifunctional activity for oxygen reduction and evolution reactions, as well as enhanced metal–air battery performance in comparison with other counterparts. Experimental and theoretical simulation studies demonstrate that the surface atomic arrangement of Co2+/Co3+ active sites, especially the existence of octahedrally coordinated Co3+ sites, optimizes the adsorption, activation, and desorption features of oxygen species. This work paves the way to obtain highly active, durable, and cost-effective electrocatalysts for practical clean energy devices through regulating the surface atomic configuration and catalytic active sites.

1. Introduction Transition metal oxides have attracted extensive attention because of their electrical, optical, magnetic, catalytic, and electrochemical properties.[1] During the past decades, much effort has been devoted to improving their performance by materials design such Dr. X. Han, G. He, Y. He, Dr. J. Zhang, X. Zheng, Prof. C. Zhong, Prof. W. Hu, Prof. Y. Deng Tianjin Key Laboratory of Composite and Functional Materials School of Materials Science and Engineering Tianjin University Tianjin 300072, China E-mail: [email protected] Dr. X. Han, Prof. C. Zhong, Prof. W. Hu, Prof. Y. Deng Key Laboratory of Advanced Ceramics and Machining Technology (Ministry of Education) Tianjin University Tianjin 300072, China The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201702222.

Dr. L. Li School of Materials Science and Engineering Hebei University of Technology Tianjin 300130, China Dr. T.-Y. Ma Discipline of Chemistry University of Newcastle Callaghan, Newcastle, NSW 2308, Australia E-mail: [email protected] Dr. T.-Y. Ma School of Chemical Engineering University of Adelaide Adelaide, SA 5005, Australia

DOI: 10.1002/aenm.201702222

Adv. Energy Mater. 2017, 1702222

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cases also involve the use of surfactants and templates.[8a,b] Moreover, it is not easy to achieve high-Miller index planes on face-centered cubic (fcc) metal oxides because the surface energy increases accompanied with the increasing density of dangling bonds.[8c] Thus, controllable and facile synthesis of uniformly shaped Co3O4 with specific atom arrangement and active site distribution deserves great interest to expose the most advantageous planes and catalytic active sites of Co3O4, thus optimizing the overall performance of cobalt-based catalysts. Metal (e.g., Zn–, Al–, and Mg)–air batteries based on aqueous alkaline electrolyte have received particular research attention because of their prominent merits regarding low cost, high abundance, environmental compatibility, and high theoretical energy density.[2c,d,9] The catalytic oxygen reduction and evolution reactions (ORR/OER) critically determine the electrochemical performance of these promising metal–air battery technologies.[9] However, these reactions are intrinsically sluggish owning to the involved multielectron transfer and complex reaction steps.[10] In spite of the highly efficient electrocatalysts on the basis of precious metals (e.g., Pt for ORR and Ir, Ru for OER), these catalysts are prohibitively expensive and scarce in nature, inevitably constraining their widespread applications.[11] Furthermore, in electrically rechargeable metal–air battery systems, the catalysts are expected to reversibly catalyze not only ORR but also OER, serving as bifunctional electrocatalysts.[9] Unfortunately, the aforementioned precious metal catalysts are generally incapable to provide sufficient catalytic bifunctionality for dual ORR/OER. Therefore, it is extremely demanded to pursue high-performance alternatives on the basis of earthabundant elements as bifunctional oxygen electrode materials, enabling the scalable implementation of air-relevant energy technologies.[12] Among the investigated candidates, spinel-type Co3O4 compounds have been identified as potential oxygen catalysts owning to their advantages of inexpensive, high abundance, environmental green, and considerable stability.[4a,13] For instance, to reduce the charge–discharge overpotentials of Li– O2 batteries in organic media, the crystal plane-dependent catalytic activities of Co3O4 nanostructures follow an order of {111} > {112} > {110} > {100} owning to the diverse reactive sites and surface electronic structure.[5d] To the best of our knowledge, for catalyzing ORR and OER by Co3O4 nanocrystals in aqueous solution, great efforts have been devoted to addressing their inherently low electrical conductivity by dispersing them on highly conductive supports (e.g., carbon) to provide efficient charge-transport capability;[14] a few studies have been focused on revealing the correlation between the microscopic surface crystal facets and the macroscopic electrocatalytic activity. Meanwhile, the surface active sites of Co3O4 catalysts that govern the oxygen catalytic reactions in aqueous alkaline metal–air batteries have not been fully understood. Bearing in mind the above issues needed to be alleviated, we demonstrate a facile, one-pot, and template-free hydrothermal strategy to in situ grow crystallized Co3O4 nanocube (NC), nanotruncated octahedron (NTO), and nanopolyhedron (NP) with different exposed crystal facets on nitrogen-doped reduced graphene oxide (N-rGO), designated as Co3O4-NC/N-rGO, Co3O4-NTO/N-rGO and Co3O4-NP/N-rGO, respectively. The asprepared Co3O4 nanocrystals are respectively enclosed by {001}, {001} + {111}, and {112} planes, which are featured by different


concentration and distribution of Co2+/Co3+ active sites on the surface. The detailed comparison of their shape-dependent electrocatalytic properties toward ORR, OER, and metal–air batteries in alkaline media is presented. The parallel formation of different shaped Co3O4 nanostructures makes it possible to systematically elucidate the relationship between electrocatalytic behavior and active site distribution of Co3O4-based oxygen catalysts. Electrochemical results reveal that Co3O4-NP/N-rGO surrounded by the unusual {112} crystal plane manifests superior ORR/OER and Zn–air battery performance than those of Co3O4-NC/N-rGO and Co3O4-NTO/N-rGO in terms of lower overpotential, higher efficiency, and better reversibility. The optimized active site configuration, exposed Co3+ catalytic sites in octahedral interstices (Co3+Oh) on {112} facet, and the coupling interaction between Co3O4 and N-doped graphene in Co3O4-NP/N-rGO synergistically favor the adsorption–desorption and activation of oxygen species, thus facilitating the electrocatalytic ORR and OER during discharge and charge processes of metal–air batteries.

2. Results and Discussion 2.1. Material Characterization The synthesis of Co3O4 nanocrystals with different shapes supported on conductive N-doped reduced graphene oxide is illustrated in Figure 1. The morphologies of Co3O4 nanostructures anchored on 2D graphene matrix can be easily tuned from nanocube to nanotruncated octahedron and then nanopolyhedron by simply optimizing the dosage of Co(NO3)2, NH3∙H2O and corresponding GO solution while keeping other reaction parameters unchanged (Table S1, Supporting Information). NH3·H2O species act as nitrogen source during the hydrothermal reaction when GO is reduced to N-rGO, leading to the formation of Co3O4 and N-rGO composites. The N-doped graphene also provides abundant anchoring sites for the growth of metal oxide nuclei, resulting in strong interaction between metal species and graphene support.[15] The coupling effect between them can effectively prevent the recrystallization of nanocrystals into large particles and also avoid the catalyst agglomeration during the catalytic processes.[16] The crystalline structure of as-synthesized composites was examined by powder X-ray diffraction (XRD) and Raman spectra. As shown in Figure 2a, eight diffraction peaks can be observed in the XRD patterns of Co3O4-NC/N-rGO, Co3O4NTO/N-rGO, and Co3O4-NP/N-rGO, which locate at 2θ = 19.0°, 31.3°, 36.8°, 38.5°, 44.8°, 55.6°, 59.3°, and 65.2°, respectively, corresponding to the (111), (220), (311), (222), (400), (422), (511), and (440) planes of fcc spinel Co3O4 (cubic, Fd-3m, a = b = c = 8.084 Å; JCPDS Card No. 74-2120). No other peaks can be found, indicating the high purity of synthesized Co3O4based composites. As revealed by Raman results (Figure 2b), GO is reduced to rGO after the chemical reduction in the hydrothermal process. Two broad peaks at around 1350 and 1590 cm−1 are observed for all four samples, which correspond to D and G bands of disordered and graphitized carbons, respectively. The G band indicates the scattering of E2g mode of

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Figure 1. Schematic illustration of the controllable fabrication of Co3O4-NC/N-rGO, Co3O4-NTO/N-rGO, and Co3O4-NP/N-rGO nanocomposites via a facile and template-free hydrothermal strategy.

sp2-hybridized carbon atoms, while D band is a typical signal of disordered graphitic carbon. The intensity ratio of D- to G-band (ID/IG) is an indicator of the disorder degree in the matrix and gives information on the percentage of the sp2 carbon atoms. The Raman peak values of the G and D bands in the composites are similar to those observed in GO. The ID/IG ratios are fitted to be 0.82, 1.04, 1.05, and 1.06 for GO, Co3O4-NC/ rGO, Co3O4-NTO/rGO, and Co3O4-NP/rGO, respectively. This change suggests the decreased size of the in-plane sp2 domains and more disordered graphitic structure in rGO upon reducing the exfoliated GO. Additionally, three characteristic Raman peaks at 477, 520, and 684 cm−1 are attributed to the E2g, F2g, and A1g modes of Co3O4 nanocrystals,[4d] revealing the formation of Co3O4 spinels, which is corroborated by the Fourier transform infrared spectroscopy (FTIR; Figure S1, Supporting Information). Collectively, XRD, Raman, and FTIR demonstrate the successful preparation of Co3O4 nanocrystals supported on reduced graphene oxide.

X-ray photoelectron spectroscopy (XPS) was further performed to characterize the surface chemical composition, element valence state, and nitrogen doping in Co3O4/N-rGO nanocomposites (Figure 3). The XPS survey spectra of three Co3O4/N-rGO nanocomposites (Figure 3a; Figure S2, Supporting Information) show similar elemental compositions of C, Co, O, and N (Table S2, Supporting Information). The presence of the N 1s peak reveals the successful doping of nitrogen atoms into the carbon framework. The high-resolution XPS spectrum of N 1s (Figure 3b; Figure S3a, Supporting Information) shows that the incorporated nitrogen atoms exist in two forms, namely, pyridinic N at 398.2 eV and pyrrolic N at 399.7 eV, both of which locate at the graphitic edges. Note that the N edge atom configuration is widely recognized to be the catalytic active sites for ORR,[17] which implies the potential of our synthesized nanocomposites as superior oxygen electrocatalysts. Meanwhile, the comparable N content and similar N type eliminate the impact of doped N species on the electroactivity of three Co3O4-based

Figure 2. a) XRD patterns and b) Raman spectra of Co3O4-NC/N-rGO, Co3O4-NTO/N-rGO, and Co3O4-NP/N-rGO.


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Figure 3. a) XPS survey spectrum, high-resolution XPS spectra of the b) N 1s, and c) Co 2p core levels in Co3O4-NP/N-rGO, d) Co 2p3/2, and e) O 1s core levels in Co3O4-NC/N-rGO, Co3O4-NTO/N-rGO, and Co3O4-NP/N-rGO, and f) C 1s core level in Co3O4-NP/N-rGO.

hybrids. The Co 2p spectrum of Co3O4-NP/N-rGO displays two major peaks at 780 and 795 eV, ascribed to the typical Co 2p3/2 and Co 2p1/2 orbitals, respectively (Figure 3c). The deconvolution of Co 2p3/2 shows peaks at 780.1 and 782.1 eV, which can be respectively assigned to Co3+ and Co2+ cations. The other two peaks at 795.4 and 798.0 eV are ascribed to spin–orbit characteristics of Co 2p1/2. An approximate 15 eV energy variation between Co 2p3/2 and Co 2p1/2 peaks is observed, again evidencing the coexistence of Co2+ and Co3+ species in Co3O4-NP/ N-rGO.[7a,18] Further, a detailed fitting reveals that the relative area ratio of Co3+ to Co2+ in Co3O4-NP/N-rGO (4.03) is significantly higher than those in Co3O4-NC/N-rGO (1.07) and Co3O4NTO/N-rGO (1.35) (Figure 3d; Table S3, Supporting Information), indicating more exposed Co atoms are in the oxidation state of +3 on the surface of Co3O4 NP. In the O 1s spectra (Figure 3e), two peaks centered at 530.2 and 531.8 eV arise


from crystal lattice (Olat) and adsorbed oxygen (Oads), respectively. Notably, the highest ratio of surface to lattice oxygen in Co3O4-NP/N-rGO suggests the strongest interaction between the Co3O4-NP/N-rGO and surface oxygen-adsorbed species (Table S3, Supporting Information), which is expected to facilitate the oxygen absorption process for enhanced electrocatalytic reactivity.[11a] The C 1s peaks located at 284.7, 285.8, and 287.9 eV are attributed to the CC, CN, and CN of N-rGO in Co3O4-NP/N-rGO, respectively (Figure 3f; Figure S3b, Supporting Information).[19] Compared to that of GO (Figure S4, Supporting Information), the peak intensity of CO and CO in N-doped rGO is much lower, an indication that most of the oxygen-containing functional groups in GO has been removed, in consistent with Raman spectra. The nanostructure and surface morphology of the synthesized materials were characterized by transmission electron

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Figure 4. TEM, HRTEM images, and FFT patterns of a–c) Co3O4-NC/N-rGO, d–f) Co3O4-NTO/N-rGO, and g–i) Co3O4-NP/N-rGO. Insets in panels (a), (d), and (g) show the simulated models and typical shape of NC, NTO, and NP nanostructures.

microscopy (TEM) and scanning electron microscopy (SEM). As observed from the low-magnification TEM images of Co3O4-NC/N-rGO (Figure 4a; Figure S5a, Supporting Information), Co3O4 nanocubes with perfect sharp edges, corners, and well-defined faces are homogeneously anchored on N-rGO nanosheets. The average particle size of Co3O4 nanocubes is about 21.4 nm. High-resolution TEM (HRTEM) reveals the high crystallinity of the individual Co3O4 nanocube. The measured Adv. Energy Mater. 2017, 1702222

neighboring interlayer distances of 0.202 and 0.285 nm match well with the spacing between (400) and (220) planes, while an interfacial angle of 45° is observed (Figure 4b), which indicates the projected direction is [001] and the nanocubes are dominantly enclosed by the {001} planes,[7a] well agreed with the inset fast-Fourier-transform (FFT) pattern (Figure 4c). TEM image of Co3O4-NTO/N-rGO reveals the truncated octahedral Co3O4 nanocrystals with average size of 18.1 nm well

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dispersed on N-rGO nanosheets (Figure 4d; Figure S5b, Supporting Information). The corresponding high-resolution image (Figure 4e) corroborates the lattice fringes of Co3O4NTO to be (2−20), (20−2), and (0−22) planes of spinel Co3O4 with the interplanar spacing of 0.285 nm and 60° interfacial angle. This is well consistent with the FFT pattern (Figure 4f), in which all the spots can be well indexed to (2−20), (20−2), and (0−22) along the [111] zone axis, suggesting dominantly exposed facet of {111} in Co3O4-NTO.[5a,d] Actually, according to the crystallographic structure model of fcc Co3O4, Co3O4NTO is surrounded by eight {111} and six {001} planes.[4c] As for Co3O4-NP/N-rGO (Figure 4g; Figure S5c, Supporting Information), ellipsoid-like Co3O4 multifaceted nanostructures with uniform size of ≈17.2 nm are highly dispersed on N-rGO. The dominantly exposed plane of Co3O4 nanopolyhedron is {112}, which is the only plane normal to the first set of (311) planes with a lattice distance of 0.243 nm, the set of (220) with the crossing lattice space of 0.285 nm, and the set of (1−11) with the lattice space of 0.467 nm (Figure 4h, i).[5d] The nanocube, truncated nanooctahedron, and nanopolyhedron morphologies are also confirmed by SEM images (Figure S6a–c, Supporting Information). Therefore, combining the structural information provided by the HRTEM and FFT, it can be concluded that the exposed crystal planes of Co3O4 nanostructures are {001}, {001} + {111}, and {112} for Co3O4-NC/N-rGO, Co3O4-NTO/ N-rGO, and Co3O4-NP/N-rGO, respectively. On the basis of XPS characterization discussed above, more exposed Co3+ species are detected on {112} facets in Co3O4 NP, which is indicative of the successful engineering of Co2+/Co3+ active sites through different surface atomic structure fabrication. Furthermore, the mass percentages of Co3O4 in the hybrids are determined to be approximate 82 wt% for Co3O4-NC/N-rGO, 78 wt% for Co3O4-NTO/N-rGO, and 80 wt% for Co3O4-NP/N-rGO according to the weight loss below 500 °C provided by thermogravimetric analysis (Figure S7, Supporting Information). The similar compositional ratio is also confirmed by the energydispersive spectra (EDS; Figure S6b–d, Supporting Information). The specific surface areas are determined to be 44.8, 52.1, and 47.2 m g−1 for Co3O4-NC/N-rGO, Co3O4-NTO/N-rGO, and Co3O4-NP/N-rGO, respectively, obtained by N2 adsorption–desorption analysis using Brunauer–Emmett–Teller (BET) method (Figure S8, Supporting Information). The presented similar material properties including comparable elemental composition (especially the N content in N-rGO), average crystallite size, Co3O4 percentages, and specific surface areas in as-synthesized three Co3O4/N-rGO composites allow us to exclude the influence from these physical factors but to focus on illuminating the impact of exposed crystal planes and active sites on electrocatalytic behaviors. The formation mechanism of Co3O4 nanostructures with different exposed crystal planes on N-doped graphene is further clarified in details. It has been reported that the shape of face-centered cubic nanocrystals varies as a function of the ratio (R) of the growth rates along 〈001〉 direction to that along 〈111〉 direction.[4c,20] In our synthetic system, the shape evolution of synthesized Co3O4 nanocrystals varies from NC to NTO when the added amounts of Co(NO3)3·H2O and NH3·H2O are both increased though the molar ratio of the two precursors is fixed. This is attributed to the different adsorption


interactions between various concentrations of capping ions and specific crystal planes of Co3O4,[21] as depicted in Figure S9 in the Supporting Information. In fcc-phase Co3O4, the {111} facet with higher oxygen atom density has a stronger adsorption affinity toward NH4+ ions than other facets.[21a] The adsorption interaction on {111} surface is relatively weak in low NH4+ concentration, which benefits the growth rate along 〈111〉 and promotes the formation of Co3O4-NC exposed with {001} planes. With an increase of the NH4+ concentration, the moderate adsorption leads to the reduced growth rate along 〈111〉 while that along 〈001〉 is enhanced, producing Co3O4-NTO exposed with {001} + {111} planes. When the added amount of NH3·H2O is further increased, the adsorption on {111} becomes more pronounced, leading to the final generation of pure {111}-enclosed Co3O4 nanooctahedron (Co3O4-NO, see detailed characterization in Figure S10 in the Supporting Information). This is in accordance with the fact that the highrelative content of alkaline resource is favorable to the stability of {111} planes in fcc-phase crystals.[4c,22] Furthermore, when the dosage ratio of NH3·H2O to Co(NO3)3·H2O is substantially increased, another mechanism stressing the formation of Co(NH3)63+ and the intermediates of Co(OH)2/CoOOH is proposed to account for the {112} crystal plane oriented Co3O4NP (Figure S11, Supporting Information). In high-concentration ammonia, oxygen from air can oxidize Co(NH3)62+ to Co(NH3)63+ (see color change of the solution in Figure S12 in the Supporting information),[11a] leading to the generation of CoOOH instead of Co(OH)2 intermediates, which further act as precursors of the resultant Co3O4, as evidenced by the ex situ XRD analysis of collected precursors (Figure S13, Supporting Information). The phase formation of CoOOH in strong alkaline solution (e.g., ammonia, NaOH) is a well-accepted process.[23] The high Co valence in CoOOH intermediate (+3) compared to Co(OH)2 (+2) is mainly responsible for the shape speciation of {112}-enclosed Co3O4 nanopolyhedron, which is enriched by Co3+ cations on the surface. Aforementioned results demonstrate a dual role played by ammonia with different concentrations in affecting the surface exposed crystal planes of Co3O4 nanocrystals. 2.2. Electrocatalytic Performance To evaluate the ORR catalytic kinetics of Co3O4-based catalysts, catalyst-modified rotating disk electrodes with the same catalyst mass loading were tested in O2– or, for reference, N2-saturated 0.1 m KOH solution. As shown in cyclic voltammograms (CVs), compared with the featureless curves in N2 atmosphere, the obvious cathodic reduction peaks are attributed to the catalyzed oxygen reduction (Figure 5a). Particularly, Co3O4-NP/NrGO exhibits a reduction peak at 0.78 V, which is more positive than that of Co3O4-NC/N-rGO (0.70 V) and Co3O4-NTO/N-rGO (0.74 V) counterparts, highlighting the superior ORR catalytic activity of the former. This is in good accordance with the results obtained from the linear sweeping voltammograms (LSVs) at 1600 round per minute (rpm) at a scan rate of 5 mV s−1 after subtracting the background current (Figure 5b; Figure S14, Supporting Information). The potential-current response is first kinetically controlled in the potential range of 0.9–0.8 V,

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Figure 5. a) CV curves in N2-saturated (dash line) and O2-saturated (solid line) 0.1 m KOH solution, b) LSVs at 1600 rpm in O2-saturated 0.1 m KOH solution, c) K–L plots at 0.65 V, d) determined n (dash line) and HO2− yield (solid line) by RRDE data, and e) Tafel plots for Co3O4-NC/N-rGO, Co3O4NTO/N-rGO, Co3O4-NP/N-rGO, and Pt/C. f) Chronoamperometric response of Co3O4-NP/N-rGO in comparison with that of Pt/C at 0.60 V.

followed by the mixed kinetic-diffusion controlled in the potential region of 0.8–0.6 V, and finally diffusion-limiting controlled below 0.6 V with obvious current plateau observed. Since a comparable performance is afforded by mixed {001} + {111} plane enclosed Co3O4-NTO/N-rGO and pure {111} surrounded Co3O4-NO/N-rGO (Figure S15, Supporting Information), we focus on three structures (i.e., NC, NTO, and NP) herein to simplify our discussion. Among the prepared three Co3O4/N-rGO composites, Co3O4-NP/N-rGO presents the best performance including the lowest overpotential (Figure S16, Supporting Information), highest half-wave potential, and largest reduction current throughout the whole voltage window. More specifically, in the diffusion-controlled region, the diffusion-limited current densities decrease in the sequence of Co3O4-NP/NrGO (5.48 mA cm−2) > Co3O4-NC/N-rGO (4.98 mA cm−2) ≈ Co3O4-NTO/N-rGO (4.95 mA cm−2) at 1600 rpm. The favorable kinetic property of Co3O4-NP/N-rGO can also be gleaned from the determined kinetic current density at 0.75 V versus RHE (6.75 mA cm−2), much larger than that of Co3O4-NC/N-rGO Adv. Energy Mater. 2017, 1702222

(0.82 mA cm−2) and Co3O4-NTO/N-rGO (1.53 mA cm−2), revealing the superior intrinsic activity of Co3O4-NP/N-rGO. Remarkably, the onset potential and limiting current density afforded by Co3O4-NP/N-rGO are comparable to those of the state-of-the-art Pt/C (0.89 vs 0.94 V; 5.48 vs 5.37 mA cm−2; Table 1) and also rival those of the most active electrocatalysts based on precious metal-free elements reported so far (see detailed comparison in Table S4 in the Supporting Information). The different catalytic ORR activities can be ascribed to the designed shape and surface-exposed active species of Co3O4 nanocrystals. As expected, all the composites exhibit much enhanced ORR performance to that of bare Co3O4 nanoparticles (Figure S17, Supporting Information), underscoring the advantages of facetcontrolled nanocrystals and the synergistic interaction between Co3O4 and N-graphene in facilitating oxygen kinetics. The linearity of the Koutecký–Levich (K–L) plots and near parallelism of the fitting lines suggest first-order reaction kinetic toward the concentration of dissolved oxygen and the similar electron transfer number (n) during catalyzing

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Table 1. Comparison of electrocatalytic performance of the as-synthesized Co3O4-based nanocomposites and the Pt/C and RuO2 benchmarks. ORRa)

Catalyst

OERb)

Eonset [V]

Ehalf [V]

Id [mA cm−2]

Ik [mA cm−2]

Tafel slope [mV dec−1]

η [mV]

Is [mA cm−2]

Tafel slope [mV dec−1]

Rct [Ω]

Co3O4-NC/N-rGO

0.82

0.68

−4.98

0.82

90

440

18.5

124

14.8

Co3O4-NTO/N-rGO

0.84

0.71

−4.95

1.53

95

410

49

78

5.8

Co3O4-NP/N-rGO

0.89

0.76

−5.48

6.75

62

380

84.8

62

3.9

Pt/C

0.94

0.78

−5.37

18.31

69

–

–

–

–

RuO2

–

–

–

–

350

36.5

92

21.4

Ehalf, and Id represent onset potential, half-wave potential, and diffusion-limited current density of ORR. Ik is the kinetic current density at 0.75 V versus RHE; b)η is the overpotential (mV) at 10 mA cm−2, Is is the specific OER current density at 1.70 V versus RHE, and Rct is the charge-transfer resistance at 1.71 V versus RHE.

a)E onset,

ORR at different potentials (Figure S14, Supporting Information). ORR initiated on Pt-based catalysts is recognized to perform through the direct four-electron (4e) transfer pathway in aqueous solution. Thus, the similar slopes of Co3O4-NP/N-rGO and Pt/C indicate that Co3O4-NP/N-rGO also catalyzes ORR via a quasi-four-electron mechanism, whereas the larger slopes of Co3O4-NC/N-rGO and Co3O4-NTO/N-rGO mean that the apparent n values are between 2 and 4 (Figure 5c). This is also proven by the rotating ring-disk electrode (RRDE) technique (Figure S18, Supporting Information), in which the electron transfer number and the yield of peroxide species can be quantitatively determined based on the collected disk and ring currents. As illustrated in Figure 5d, the calculated n value is above 3.85 and the peroxide yield is below 7% for Co3O4-NP/N-rGO within the potential window of 0.3–0.7 V, very close to those of Pt/C, which is indicative of the desirable highly efficient ORR electrocatalysis. The superb ORR catalytic activity of Co3O4NP/N-rGO is further gleaned from Tafel plots, displaying a smaller slope and a larger kinetic current density than those of Co3O4-NC/N-rGO and Co3O4-NTO/N-rGO (Figure 5e). It should be pointed out that the Tafel slope of Co3O4-NP/N-rGO (62 mV dec−1) is even smaller than that of Pt/C (69 mV dec−1), further signaling its favorable ORR kinetics. Apart from the high activity, Co3O4-NP/N-rGO also exhibits considerable catalytic durability. The chronoamperometric ORR current maintains more than 95% of the initial value after a continuous polarization period of 25 000 s, which obviously outperforms that of commercial Pt/C (70%; Figure 5f). The salient catalytic durability is also verified by no significant activity loss after 2000 successive CV cycles between 0.60 and 1.15 V at 100 mV s−1 (Figure S19, Supporting Information). Furthermore, in order to test the possible crossover effect on the catalytic performance, the electrocatalytic selectivity of Co3O4-NP/N-rGO against electrooxidation of methanol was measured (Figure S20, Supporting Information). For the Pt/C electrode, a distinct decline (≈60%) in current density is observed when methanol is added (20 vol%) to the O2-saturated 0.1 m KOH electrolyte. On the contrary, the current density retention of Co3O4-NP/N-rGO electrode achieves ≈80% after methanol addition. The ascendant long-time stability as well as the high selectivity to ORR collectively renders Co3O4-NP/N-rGO as a promising ORR electrocatalyst for direct methanol fuel cells. In the context of exploiting ORR and OER bifunctional catalysts for regenerative fuel cells and rechargeable metal–air batteries, the OER performance of as-prepared electrocatalysts


was investigated using a standard three-electrode system in N2-saturated 1.0 m KOH aqueous solution with the catalyst mass loading of 0.15 mg cm−2. In the LSVs for OER (Figure 6a), Co3O4-NP/N-rGO requires an overpotential of 0.38 V to deliver an evolution current density of 10 mA cm−2, much lower than that of Co3O4-NTO/N-rGO (0.41 V) and Co3O4-NC/N-rGO (0.44 V; Table 1). Furthermore, the activity of Co3O4-NP/NrGO even surpasses that of the precious metal RuO2 catalyst at the current density beyond 20 mA cm−2. Upon the potential increasing to 1.70 V, Co3O4-NP/N-rGO achieves an extremely large current density of 84.8 mA cm−2, far exceeding that of the RuO2 benchmark (36.5 mA cm−2). For more quantitative analysis, the mass activities of these catalysts were also compared (Figure S21, Supporting Information). At 1.70 V, the values are 122, 327, 565, and 243 A g−1 for Co3O4-NC/N-rGO, Co3O4NTO/N-rGO, Co3O4-NP/N-rGO, and RuO2, respectively, further demonstrating the high mass performance of Co3O4-NP/NrGO for catalyzing OER. Moreover, as observed from Tafel plots in Figure 6b, the Tafel slopes of 62, 78, 124, and 92 mV dec−1 are determined for Co3O4-NP/N-rGO, Co3O4-NTO/N-rGO, Co3O4-NC/N-rGO, and RuO2, respectively, again corroborating their electroactivity following the order of Co3O4-NP/N-rGO > Co3O4-NTO/N-rGO > Co3O4-NC/N-rGO. The charge-transfer resistance (Rct) fitted from EIS profiles provides 3.9, 5.8, and 14.8 Ω for Co3O4-NP/N-rGO, Co3O4-NTO/N-rGO, and Co3O4NC/N-rGO, respectively (Figure 6c), sharing the same sequence to that of Tafel slopes, overpotentials, and mass activities. This coincidence implies that the enhanced charge transfer in Co3O4-NP/N-rGO can greatly contribute to the superior activity and reaction kinetics in OER. Further, the electrochemical active surface area was analyzed by the double-layer capacitance (Cdl), affording 0.53, 2.08, and 5.90 mF cm−2 for Co3O4NC, Co3O4-NTO, and Co3O4-NP (Figure S22, Supporting Information), respectively, which suggests that the {112} enclosed Co3O4 nanopolyhedrons can expose more active sites to the electrolyte. The RRDE technique was further employed to gain insight into the OER mechanism. A very low ring current is detected, which is at least three orders lower than the disk current (Figure 6d). This features a desirable four-electron OER pathway initiated on the surface of Co3O4-NP/N-rGO with negligible hydrogen peroxide formation, i.e., 4OH− → O2 + 2H2O + 4e−. In addition, the observed oxidation current can be fully ascribed to OER with a high Faradaic efficiency of 97.3% (Figure 6e).[13a,18b]

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Figure 6. a) LSV plots for OER, b) Tafel plots, and c) EIS curves at 1.71 V of Co3O4-NC/N-rGO, Co3O4-NTO/N-rGO, Co3O4-NP/N-rGO, and RuO2. Inset shows the equivalent circuit used to fit the experimental data (Rs: electrolyte resistance, CPE: double layer capacity, Rct: charge transfer resistance). Ring current of Co3O4-NTO/N-rGO on RRDE at 1600 rpm with the ring potential fixed at d) 1.50 V and e) 0.40 V. f) Chronopotentiometry response of Co3O4-NP/N-rGO at a constant current density of 1.0 mA cm−2 in comparison with that of RuO2.

Moreover, for long-term catalytic stability, the overpotential of Co3O4-NP/N-rGO remains unchanged, while that of RuO2 increases for about 40 mV after 25 000 s at an operating current density of 1 mA cm−2 (Figure 6f). No obvious change of morphology can be observed after the cycling test (Figure S23, Supporting Information), an indication of the robust structural stability of the composite. Aforementioned facts demonstrate that Co3O4 NP enclosed by unusual high-index facet of {112} is more ORR and OER electroactive than basic {001} and {001} + {111} faceted Co3O4 NC and NTO; Co3O4-NP/N-rGO also exhibits superior catalytic durability over the Pt/C and RuO2 benchmarks. The remarkable ORR and OER performance makes Co3O4-NP/N-rGO a promising material to serve as the bifunctional catalyst for reversible oxygen electrochemistry in air cathodes of fuel cells and metal–air batteries. In order to clarify the synergistic contribution of N-rGO in enhancing the ORR/OER performance, further control


experiments were performed in the absence of GO. Compared to Co3O4/N-rGO composites, the shape configuration and dispersion of pure Co3O4 nanocrystals are found to be poor (Figure S24, Supporting Information), demonstrating an influential role played by N-rGO in creating anchoring sites for the nucleation and dispersion of Co3O4 nanocrystals. Benefitting from the enhanced electrical conductivity and the effective coupling interaction between Co3O4 and N-rGO (as testified by shifted peaks in Co and N XPS spectra; Figure S25, Supporting Information), the hybrid exhibits much enhanced ORR/OER bifunctional performance than that of individual component (Figure S26, Supporting Information), as has been widely reported recently.[15a,16b] However, it is possible that the activity improvement may differ from each other owning to the specific interaction of diverse exposed crystal planes with N-rGO in this work. Nevertheless, our results highlight the importance of surface atomic arrangement and active sites

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of metal oxides in optimizing catalytic efficiency for oxygen electrocatalysis.

2.3. Alkaline Metal–Air Battery To elucidate the practical applicability, two electrode-based Zn– air batteries were assembled using the metallic Zn plate as the anode, Co3O4/N-rGO or Pt/C-RuO2 loaded on carbon cloth as the cathode, and mixed aqueous solution of 6.0 m KOH and 0.2 m Zn(ac)2 as the electrolyte (Figure 7a; S27, Supporting Information). The open-circuit voltage of the battery employing Co3O4-NP/N-rGO as the cathode catalyst is 1.51 V, higher than that involving Co3O4-NC/N-rGO (1.39 V) and Co3O4-NTO/NrGO (1.41 V; Figure 7b), consistent with its superior electrocatalytic ORR/OER activity as observed in three-electrode system. The polarization curves suggest the best battery performance based on Co3O4-NP/N-rGO cathode among the three composite catalysts (Figure 7c), which even competes with the battery catalyzed by Pt/C-RuO2 (e.g., 1.18 and 1.95 V at 10 mA cm−2,

1.06 and 2.03 V at 50 mA cm−2 for Co3O4-NP/N-rGO vs 1.19 and 1.95 V at 10 mA cm−2, and 1.06 and 2.05 V at 50 mA cm−2 for Pt/C-RuO2). The power density acquired from the battery based on Co3O4-NP/N-rGO peaks at 118 mW cm−2, achieveing 92.9% of that based on Pt/C catalysts (127 mW cm−2; Figure 7d). Moreover, when used in a primary battery, Co3O4-NP/N-rGO enables the large specific discharge capacity of 786 mA h g−1 with corresponding energy density of 997 Wh kg−1 (calculated based on the mass of consumed Zn; Figure S28, Supporting Information), which rivals that of Pt/C (833 mA h g−1, 1015 Wh kg−1). Significantly, the achieved extremely highenergy densities approach closely to the theoretical energy capacity of Zn–air primary batteries (1090 Wh kg−1).[24] On the other hand, the battery cycling stability with Co3O4-NP/N-rGO and Pt/C-IrO2 cathodes is further measured at a current density of 5 mA cm−2 with a duration of 400 s per cycle (Figure 7e). Impressively, the cell catalyzed by Co3O4-NP/N-rGO (geometric area: 1.0 cm2) shows superior rechargeability. Stable charging and discharging voltages can be maintained after 150 cycles (voltage gap slightly increased from 0.76 to 0.87 V), while the

Figure 7. a) The schematic configuration of the assembled Zn–air battery. b) Open-circuit plots, c) charging–discharging polarization (V–J) curves, and d) discharging polarization curves and the corresponding power densities of Zn–air batteries based on Co3O4-NC/N-rGO, Co3O4-NTO/N-rGO, Co3O4-NP/N-rGO, Pt/C, and Pt/C-RuO2 cathode catalysts. e) Cycling performance of Co3O4-NP/N-rGO and Pt/C-RuO2 electrodes with a duration of 400 s per cycle at 5 mA cm−2. f) Two-series Zn–air batteries power 40 red LEDs with “TJU” shape. g) Photograph of an upgraded Zn–air battery with enlarged electrode area of ≈11.8 cm2. h) Terminated discharge and charge voltages of the Zn–air battery using Co3O4-NP/N-rGO operated in the battery configuration described in panel (g) at a cycling rate of 5 mA cm−2 with a duration of 400 s per cycle.


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voltage gap sharply increases to 1.31 V after 75 cycles in the battery catalyzed by Pt/C-RuO2 under the same condition. As a demo, the connected two Zn–air batteries in series are capable of lighting 40 static red light-emitting diodes (LEDs, 1.6 V) for more than 15 hours (Figure 7f; S29, Supporting Information) and can also stably power dynamic 19 LEDs (Video S1, Supporting Information), emphasizing the practicability of Co3O4-NP/N-rGO in realistic energy devices. Furthermore, an updated Zn–air battery was fabricated with enlarged electrode area of ≈11.8 cm2 (Figure 7g). The ultralong stability and high reversibility can also be realized, that is, a 180 h prolonged operation lifespan with more than 1600 discharge–charge cycles after refuelling the Zn foil once (Figure 7h; S30, Supporting Information), implying the potential for large-scale application of Co3O4/N-rGO nanocomposites. The key battery parameters in this work are compared to the state-of-the-art electrocatalysts previously reported, showing the comparative primary battery performance and the top rechargeability in secondary battery form (see details in Tables S5 and S6 in the Supporting Information). As a step beyond, Co3O4-NP/N-rGO was also functioned in other alkaline metal–air system, such as

in the proof-of-concept Al–air prototype, affording comparable working voltage plateau and longer discharge period relative to that of Pt/C cathode (Figure S31, Supporting Information). These battery parameters based on Co3O4-NP/N-rGO highlight its attractive potential as an efficient air cathode catalyst to replace expensive precious metals for electrochemical metal–air energy conversion applications.

2.4. Discussion Co3O4 has the normal spinel-phase structure of Co2+Co3+2O4, in which one Co2+ ion in the unit cell occupies the tetrahedral site, while two Co3+ ions occupy the octahedral sites (Figure S32, Supporting Information). Generally speaking, different Miller-index planes of the Co3O4 nanocrystal possess different surface topographies, atomic arrangements, and electronic structures, which may result in the variation of electrochemical performance.[5d,22] The surface atomic configurations in the {001}, {111}, and {112} planes of the Co3O4 unit cell are shown in Figure 8a–c. In term of the surface arrangement of

Figure 8. a–c) The Co2+/Co3+ surface atomic configurations on (001), (111), and (112) planes. d–f) Top and side views of calculated O2-adsorption matter and OO activation bond lengths on the surface of (001), (111), and (112) facets in Co3O4. Cyan, red, and yellow atoms in panels (d), (e), and (f) represent Co, lattice O, and adsorbed O, respectively.


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metal ions, the {001} or {111} planes contain only tetrahedrally coordinated Co2+(Co2+Td) sites under normal condition without octahedrally coordinated Co3+ sites (Co3+Oh), while the {112} planes contain not only Co2+ but also Co3+ cations. This is also observed from the aforementioned XPS analysis (Figure 3d), which demonstrates that the average oxidation state of Co ions on the surface of Co3O4 NP is obviously higher than those of Co3O4 NTO and Co3O4 NC. Previous theoretical and experimental studies[5d,7b,25] evidenced that the Co3+Oh species can act as superior active sites over Co2+Td species in efficiently functioning with reactants during the adsorption, activation, and desorption processes, strongly supporting our finding herein that the {112} faceted Co3O4-NP/N-rGO with abundant Co3+Oh sites presents substantially higher catalytic activity than Co3O4NTO/N-rGO and Co3O4-NC/N-rGO. Despite the Oh site of Co3+ presents a superior activity over Td site of Co2+, to the best of our little knowledge, one can only successively improve the ratio of Co3+ to Co2+, but is unable to obtain a state-of-the-art sample with only Co3+ exposed on its surface.[4c,7b,26] An insight into the electrocatalytic mechanism can be accessed from the view of ligand field theory. Specifically, promoted electron donating ability of Co3+Oh in comparison with that of Co2+Td can largely benefit the O22−/OH− displacement during the ORR and OER processes (see the calculation of crystal field stabilization energy in Figure S33 in the Supporting Information).[27] To better understand the ORR catalytic behaviors on different faceted Co3O4 surface, the density functional theory calculations were carried out. As can be seen from Figure 8b–d, the molecular oxygen is adsorbed on {001} and {112} facets in the lateral Griffith manner (two O atoms on one Co atom) and on {111} surface in the bridge form (two O atoms on two Co atoms), both of which are beneficial to the rupture of OO bonds and the direct 4e− ORR pathway.[28] Upon adsorption, all OO double bonds are lengthened relative to that in vacuum (1.24 Å), indicating the activation and partial dissociation of O2. The extent of OO elongation can be viewed as a descriptor to assess the interaction between catalyst surface and oxygen. The bond lengths of activated OO are 1.349, 1.513, and 1.380 Å on {001}, {111}, and {112} surfaces (Figure S34, Supporting Information), respectively. Since a good ORR electrocatalyst should interact with oxygen neither too weakly nor too strongly,[27a,28] {112} faceted Co3O4 surface with a modest oxygen-binding ability provides the most favorable balance between adsorption and desorption of oxygen species, yielding the highest ORR activity. Furthermore, compared to {001} and {111}, the high-index facet of {112} has a more open structure and possesses higher density of stepped atoms.[1d,7c] Overall, the exposed crystal facets affect the oxygen electrocatalytic activities on Co3O4 surface by tuning the catalytic active sites, surface atomic configuration, and oxidation state of Co species, consequently leading to the highly active surface involving dominantly {112}.

3. Conclusion In summary, this work presents the engineering toward exposed facets and active sites of Co3O4 nanostructures anchored on N-rGO, as well as the rational tailoring of their


performance for ORR/OER and metal–air batteries. The unique {112} planes surrounded Co3O4-based nanocomposite (Co3O4NP/N-rGO) displays superior bifunctional electroactivity in catalyzing ORR/OER over other counterparts; the Zn–air batteries based on Co3O4-NP/N-rGO exhibit large power density (118 mW cm−2), low overpotential (0.76 V at 5 mA cm−2), and prolonged cycling lifetime (≈1600 cycles using a 11.8 cm2 cathode), which can compete with state-of-the-art precious and transition metal catalysts ever reported. Further investigation demonstrates that the surface atomic configuration and charge distribution of Co2+/Co3+ active sites, especially the presence of octahedrally coordinated Co3+ sites, are capable of facilitating the adsorption, dissociation, and desorption of oxygen species and thereby contributing to the overall enhanced electrochemical performance. This work provides an atomic level surface regulating strategy of nanocatalysts that are highly promising for practical and powerful energy devices.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the Joint Funds of National Natural Science Foundation of China, and Guangdong Province (Grant No. U1601216), National Natural Science Foundation of China (Grant Nos. 51602216 and 51472178), Tianjin Natural Science Foundation (Grant No. 17JCQNJC02100), Australian Research Council (ARC) Discovery Early Career Researcher Award (Grant No. DE150101306), and Linkage Project (Grant No. LP160100927).

Conflict of Interest The authors declare no conflict of interest.

Keywords Co3O4 spinel, controllable synthesis, metal–air batteries, nanocomposite, oxygen electrocatalysis Received: August 13, 2017 Revised: September 30, 2017 Published online:

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