Hierarchical Porous Double-Shelled Electrocatalyst with Tailored

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Hierarchical Porous Double-Shelled Electrocatalyst with Tailored Lattice Alkalinity towards Bifunctional Oxygen Reactions for Metal-air Battery Ya-Ping Deng, Yi Jiang, Dan Luo, Jing Fu, Ruilin Liang, Shaobo Cheng, Zhengyu Bai, Yangshuai Liu, Wen Lei, Lin Yang, Jing Zhu, and Zhongwei Chen ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00989 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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ACS Energy Letters

Hierarchical Porous Double-Shelled Electrocatalyst with Tailored Lattice Alkalinity towards Bifunctional Oxygen Reactions for Metal-air Battery Ya-Ping Deng1, Yi Jiang1, Dan Luo1, Jing Fu1, Ruilin Liang1, Shaobo Cheng2, Zhengyu Bai3, *, Yangshuai Liu1, Wen Lei1, Lin Yang3, Jing Zhu2, Zhongwei Chen1, * 1

Department of Chemical Engineering, Waterloo Institute for Nanotechnology, Waterloo Institute

for Sustainable Energy, University of Waterloo, ON N2L 3G1, Canada. 2

Beijing National Center for Electron Microscopy, School of Materials Science and Engineering,

The State Key laboratory of New Ceramics and Fine Processing, Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, China. 3

School of Chemistry and Chemical Engineering, Key Laboratory of Green Chemical Media and

Reactions, Ministry of Education, Collaborative Innovation Center of Henan Province for Fine Chemicals Green Manufacturing, Henan Normal University, Xinxiang 453007, China. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

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ABSTRACT

Increasing both catalytic site accessibility and intrinsic activity of multi-shelled materials is critical to boost its efficiency for bifunctional oxygen electrocatalysis and initiate its application in metal-air batteries. Herein, a hierarchical porous double-shelled (Mg, Co)3O4 electrocatalyst with controllable Mg-substitution is synthesized via a proposed in-situ coordinating process. The hierarchical porosity optimizes the availability of active sites and broadens diffusion channels for oxygen species. Meanwhile, Mg-substitution greatly increases electrical conductivity, and realizes a novel insight of tailoring lattice alkalinity. Specifically, Mg-substitution positively influences oxygen electrocatalysis by inducing lattice buffer zones that promote hydroxyl detachment from nearby catalytic sites. As a result, the final hybrid of (Mg, Co)3O4 encapsulated in N-doped graphitized carbon affords superior bifunctional oxygen electrocatalytic activities, outperforming state-of-the-art noble-metal benchmarks. When served as air electrode in Zn-air batteries, a low charge-discharge gap and long-term cyclability for over 200 hours are realized in ambient air.

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Rechargeable metal-air batteries have received considerable amount of research efforts recently as Li-ion batteries based on intercalation chemistry struggles to meet the increasing energy density demands of electric vehicles and other devices.1-3 Nevertheless, their practical applications are still hindered by sluggish oxygen electrocatalytic kinetics and poor rechargeability.4,5 Therefore, a highly efficient and durable bifunctional oxygen electrocatalyst is vital for advanced rechargeable metal-air batteries. Oxygen-involved heterogeneous electrocatalysis, including oxygen reduction (ORR) and evolution (OER) reaction, is the core of rechargeable metal-air batteries, which requires fast kinetics of both reactions to achieve high efficiency and cyclability.6-8 Although noble-metal catalysts, i.e., Pt, Ir, and Ru, have been considered as benchmarks for respective ORR and OER, their widespread application is prohibited by low reserves and high cost.9,10 More importantly, precious catalyst with single element cannot provide ideal ORR and OER activities simultaneously.11,12 Lately, hybrid catalysts of spinel oxides composited with carbon-based materials have drawn intensive attentions as alternative air electrode for metal-air batteries.4 Among the hybrids, the spinel oxides, especially cobalt-based oxides, are explored to be decent OER electrocatalysts in spite of their inefficient ORR activities.13-15 On the other hand, excellent ORR activities have been achieved on carbon materials via heteroatom doping, such as N, S, P etc.16-18 Owing to the synergic effects of the above two components, hybrid catalysts with reasonable bifunctionality and durability have been demonstrated.4,19,20 Morphological tuning on electrocatalysts to expand porosity and active sites availability is the most effective strategy to optimize oxygen electrocatalysis. In recent decades, multi-shelled microstructured metal oxides with unique hollow architecture and large surface area have been developed for different applications.21-25 Theoretically, those hollow materials can enhance the


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accessibility of electrolyte and reactants. However, they are never applied as bifunctional oxygen electrocatalyst for metal-air batteries, since their limited porosity hinders efficient ORR and OER. In other words, if hierarchical porosity is achieved on multi-shelled materials, their electrocatalytic sites abundancy could be improved drastically. Such open system is also beneficial for mass transfer of oxygen species, i.e. OH−, O2, and H2O.26 The hard-template method is the most commonly adopted to synthesize multi-shelled metal oxides. It usually contains tedious steps, including carbonaceous template preparation, repeated cation adsorption, and template removal. During the repeated cation-adsorption process, the interconnection between the metal cations and the negative charged template surface is mainly driven by electrostatic attraction, while the empty d orbits of transition metal ions can also be occupied by p electrons of oxygen-involved functional groups on hydrophilic surface of carbonaceous microspheres.21 Since so, the metal cations densely gather on the hydrophilic surface, and rarely penetrate into the carbonized core of template (Figure S1a, b).27 Upon gradual removal of the carbonaceous template, metal oxides will form into shells with very limited porosity. In contrast, if the metal ions are uniformly distributed within carbonaceous template, pore-rich multi-shelled metal oxides could be produced. Therefore, a general and feasible blueprint to synthesize multi-shelled materials with hierarchical porosity is highly desirable for the development of exceptionally efficient electrocatalysts for metal-air batteries. Another strategy of improving performance focuses on modifying the intrinsic electrocatalytic activity of spinel cobalt-based components in hybrids. The incorporation of transition-metal ions, e.g. Zn, Mn, and Ni, with tunable chemical state is commonly adopted.9,14,28 Alternatively, with the aim to expedite their poor intrinsic ORR activity, the controlled substitution of metal ions with low hydroxyl coordination capability, such as Mg2+, should also be an effective approach. Since


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Mg-substitution would create buffer zones with weakened lattice alkalinity, and hence facilitate the extraction of ORR resultants (OH−) away from lattice. However, previous attempts in increasing oxygen electrocatalysis of Co3O4 via Mg-substitution have not been successful in spite of the boosted electrical conductivity.13,29 The root cause of the failures are the excessive occupation of bivalence Mg that are inactive for oxygen electrocatalysis.13 Hence, controllable occupation of Mg2+ at relatively low amount is the key factor to realize the insight of tailoring lattice alkalinity. With the aim to realize the above hypotheses, a novel in-situ coordinating process is proposed to synthesize double-shelled spinel (Mg, Co)3O4 with hierarchical pores (HP-DS) and controlled Mg2+ doping. The positive effects of Mg-modulation on spinel structure and electrocatalytic performance are studied systemically. With the addition of a uniform layer of N-doped graphitized carbon (NGC) onto (Mg, Co)3O4, the final hybrid catalyst, signified as (Mg, Co)3O4@NGC, demonstrates superior bifunctional oxygen electrocatalytic activities. As a practical demonstration, Zn-air batteries fabricated using (Mg, Co)3O4@NGC as air electrode exhibit narrow chargedischarge voltage gaps and long-term rechargeability in ambient air. A one-step hydrothermal synthesis is presented here as illustrated in Figure 1. Firstly, a homogeneous solution of metal nitrates and glucose is sealed and heated in a Teflon reactor. During the gradual condensation of glucose under hydrothermal condition, the metal cations (Co2+ and Mg2+) are adsorbed via an in-situ coordinating process that includes intermolecular condensation of glucose and simultaneous coordination with metal ions. These metal ions can then act as “bridges” to interconnect glucose, leading to the incorporation of metal cations within the carbonaceous microspherical frameworks (Figure S1c, d). Moreover, there is a competition for free sites between Co2+ and Mg2+ throughout the in-situ coordinating process. Due to the filled 2p


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orbits of Mg2+, its interaction with the functional group is much weaker than Co2+ with empty d orbits, and hence the Mg2+ amount is controlled at low level. Afterwards, the carbonaceous frameworks are removed to obtain HP-DS (Mg, Co)3O4. Finally, self-polymerization of dopamine and subsequent cobalt-catalyzed graphitization in pyrolysis are conducted for the encapsulation of metal oxides within a uniform layer of NGC to attain targeted hybrid.30

Figure 1. Schematic of the synthesis routes of HP-DS (Mg, Co)3O4 and (Mg, Co)3O4@NGC. It is worth to mention that, based on the proposed in-situ coordinating mechanism, the morphology of spinel oxides can be rationally tuned by using metal salts with various anions. As an example, when metal acetates are adopted as raw materials with the same stoichiometric amount, quadruple-shelled hollow microspheres with limited porosity (LP-QS) are synthesized.


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The different acidity of anions is considered to be the main reason responsible for the morphology transformation.

Figure 2. SEM images for (a-c) HP-DS (Mg, Co)3O4 and (d-f) (Mg, Co)3O4@NGC. The insets are the corresponding schematic images. Through the in-situ coordinating process, nanoparticle-aggregated microspheres with diameter of ca. 1.5 µm, uniform morphology and high purity are obtained (Figure 2a). The mesh-like interconnected porous shells of HP-DS (Mg, Co)3O4 are clearly revealed by scanning electron microscopy (SEM, Figure 2b). In the SEM image of a broken microsphere (Figure 2c), the hollow interior and the porous inner shell can be easily witnessed. Furthermore, the NGC coating appears to be uniform and well-defined in (Mg, Co)3O4@NGC, and the coating process did not alternate the original morphology (Figure 2d-f). The architecture visualization of HP-DS is enhanced by three-dimensional (3D) tomography in Figure S2a, b, and the corresponding reconstructed 3D animation is provided in Video S1. The porous structures on both shells can be clearly observed at different rotational degrees and representative slices through the 3D volume of single


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microsphere at different depths. The limited porosity and quadruple-shelled structure of acetatebased LP-QS (Mg, Co)3O4 is also unveiled in Figure S3.

Figure 3. TEM images, HAADF-STEM images, corresponding EDS mappings and overlaid images of Co, Mg and C for (a, b) HP-DS (Mg, Co)3O4 and (d, e) (Mg, Co)3O4@NGC; (c, f) HRTEM images and inserted corresponding FFT patterns. Transmission electron microscopy (TEM) images and associated elemental mapping of HP-DS (Mg, Co)3O4 and (Mg, Co)3O4@NGC are provided in Figure 3. The primary particles of HP-DS (Mg, Co)3O4 are identified with average particle size of 20.1 nm (Figure S4). The energydispersive spectroscopy (EDS) results verify the uniform distribution of Co, Mg, and O. The homogenous dispersion of C and N illustrates the successful coating of NGC (Figure 3e, S5). The high-resolution TEM (HRTEM) images in Figures 3c, f suggest high crystallinity of the primary particles for both (Mg, Co)3O4 and (Mg, Co)3O4@NGC, in which the typical cubic spinel patterns are defined by the lattice fringes and fast Fourier transform (FFT). Also, the intimate contact between (Mg, Co)3O4 particle and the graphitized carbon layer is revealed with characteristic d-


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spacings (Figure 3f). These results coincide with the polycrystalline selected area electron diffractions (SAED) patterns and Raman spectrum (Figure S6, S7). The highly graphitized degree of NGC is confirmed by the low intensity ratio of disorder (D) and graphic (G) peaks (ID/IG is 0.96) in Raman spectra.19

Figure 4. (a) N2 adsorption/desorption isotherms and corresponding pore size distribution for (Mg, Co)3O4@NGC. (b) high-resolution XPS of N 1s for (Mg, Co)3O4@NGC. (c) XRD patterns and (d) crystal lattice model for Mg-substituted Fd3̅m spinel (Mg, Co)3O4. The surface area and pore size distribution of the catalysts are examined by N2 adsorption/desorption analysis. Although LP-QS consists of two additional inner shells, its estimated surface area of 32 m2 g−1 is much lower than 84 m2 g−1 of HP-DS that exhibits an obvious hierarchical pore distribution (Figure S8, Table S1). Additionally, the surface area and porosity


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are greatly increased by the NGC coating on (Mg, Co)3O4. A typical type-IV isotherm is observed for (Mg, Co)3O4@NGC with surface area as high as 194 m2 g−1 (Figure 4a, Table S1), demonstrating its apparent hierarchical porosity.4 The pore distribution indicates the well-defined coexistence of micropores with diameter less than 2 nm, mesopores distributed around 16 nm, and macropores with average size of 75 nm. X-ray photoelectron spectroscopy (XPS) is performed to analyze the effects of polydopamine coating on (Mg, Co)3O4 (Figure 4b, S9). The result signifies that the chemical valence of Co and Mg are maintained, and a newly observed peak attributed to N 1s is marked in (Mg, Co)3O4@NGC. The high-resolution N 1s spectrum demonstrates four types of N species at 398.3, 399.6, 400.9, and 403.4 eV, representing pyridinic, quaternary, pyrrolic, and oxidized N, respectively.18 Among these species, the quaternary N is responsible for the ORR activity because it induces O2 adsorption on the adjacent carbons; the OER activity is provided by the pyridinic N that expedite adsorption of the oxidation intermediates, such as OH− and OOH−.31 The X-ray diffraction (XRD) patterns in Figure 4c exhibit a typical Fd3̅m spinel structure for (Mg, Co)3O4 and (Mg, Co)3O4@NGC. Additionally, a broad peak at about 25°attributed to (002) graphitic facet is observed in (Mg, Co)[email protected] When compared to XRD pattern of Co3O4 with the same morphology (Figures S10, S11), a significant negative shift of (Mg, Co)3O4 suggests a larger lattice volume that is beneficial for the transportation of oxygen species within lattice.28,33 Meanwhile, a sharp increase in electrical conductivity is also realized via Mg-substitution in (Mg, Co)3O4 (Figure S11c, d), which coincides with previous reports regarding Mg2+ doping.29 As a result, an enhanced mass- and charge-transfer proficiency is expected. It is critical to confirm the amount of bivalent Mg2+, since it is inactive for oxygen electrocatalysis. According to inductively coupled plasma atomic emission spectroscopy (ICP-AES) result in Table S2, the atomic ratio of


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Co/Mg in (Mg, Co)3O4 is controlled as 18.2. Hence, the Mg2+ occupation in lattice is illustrated in Figure 4d. The low Mg substitution is attributed to the competition for free sites between Co2+ and Mg2+ during the in-situ coordination process.

Figure 5. (a) Bifunctional oxygen electrocatalytic activities and (b) the specific overpotential of various electrocatalysts at RDE (1600 r.p.m) in 0.1 M KOH electrolyte. LSV is adopted at a scan rate of 10 mV s−1. (c) Proposed four-electron ORR mechanism on octahedral Co sites of spinel


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oxides. (d) The polarization curves and (e) cycling performance of Zn-air batteries assembled using (Mg, Co)3O4/NGC or Pt/C + Ir/C in ambient air. Bifunctional ORR and OER activities are measured by linear sweep voltammetry (LSV) with 0.1 M KOH electrolyte as shown in Figure 5a, b. The influence of hierarchical porosity on oxygen electrocatalysis is investigated. HP-DS (Mg, Co)3O4 exhibits superior performance, i.e. better bifunctional activity of 0.889 V (vs. reversible hydrogen electrode, RHE) defined on the gap (ΔE) between OER potential at 10 mA cm−2 (EJ-10) and ORR half-wave potential (Ehalf-wave), to LP-QS of 1.046 V. Its lower Tafel slopes than LP-QS also unveil its improved bifunctional electrocatalytic kinetics (Figure S12c, d). The high surface area, nanosized primary particles and hierarchical porosity of HP-DS are the decisive factors for its excellent bifunctional activities. The hierarchical porosity is of great significance as the micropores provide abundant exposed active sites for oxygen electrocatalysis, while the meso/macropores encourage efficient transportation of oxygen species toward and away from electroactive sites.26 Besides, the effects of Mg-modulation on intrinsic electrocatalytic activity of (Mg, Co)3O4 are discussed in detail. HP-DS Co3O4 with the same morphology is also tested to eliminate any geometric influences. In the ORR segment (Figure 5a, b), HP-DS Co3O4 offers a Ehalf-wave of 0.713 V and a limiting current density (jl) of 4.35 mA cm−2, which are noticeably inferior to HP-DS (Mg, Co)3O4 with Ehalf-wave of 0.747 V and jl of 4.45 mA cm−2. To explain the improved ORR kinetics of (Mg, Co)3O4, a Mg-assisted mechanism based on coordination chemistry is described here for the first time. As illustrated in Figure 5c, the ORR process at octahedral Co sites involves fourstep proton/electron coupled mechanism, in which the steps 1 and 4 are the rate-limiting steps due to the instable Cooct−OO2− bond (step 1) and the strong Cooct−O2− bond (step 4).14 An effective strategy to accelerate ORR kinetics is to expedite the removal of resultants (OH−) from lattice.


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Since Mg2+ ions form far weaker coordination bonds with OH− than Co2+ with empty 3d orbits, they are expected to greatly lower the OH− concentration in nearby space. This can create buffer zones that provide thermodynamically favorable environment for the detachment of OH−, and thus accelerates the intermediate steps. Meanwhile, enhanced OER kinetics is also achieved by (Mg, Co)3O4, including lower onset potential and overpotential of 0.406 V at 10 mA cm−2 when compared to Co3O4 (0.424 V). The optimized bifunctional electrocatalytic kinetics is contributed by the boosted mass- and electron-transfer properties through Mg-modulation, which is supported by the lower ORR/OER Tafel slopes of (Mg, Co)3O4 in comparison to Co3O4 (Figure S12 c, d). By combining the merits of HP-DS (Mg, Co)3O4 and highly conductive NGC, (Mg, Co)3O4@NGC offers remarkable bifunctional performance. As a comparison, start-of-the-art Pt/C and Ir/C are measured as ORR or OER benchmarks, respectively. In the ORR branch, (Mg, Co)3O4@NGC provides an onset potential (Eonset) of 0.925 V and a Ehalf-wave of 0.842 V, which are superior to those of Pt/C (Eonset = 0.920 V, Ehalf-wave = 0.830 V). A first-order four-electron transfer ORR pathway for (Mg, Co)3O4@NGC is identified by Koutecky-Levich (K-L) equation based on ORR curves at various rotation rates (Figure S12a, b). As for the OER branch, despite a slightly higher onset potential, (Mg, Co)3O4@NGC affords a steeper polarization curve and reaches a current density of 10 mA cm−2 with lower overpotential of 0.346 V in comparison to Ir/C (0.373 V). The bifunctional activity (ΔE) of 0.734 V demonstrated by (Mg, Co)3O4@NGC establishes it to be one of top-tier bifunctional oxygen electrocatalysts (Table S3). The excellent bifunctional catalytic kinetics of (Mg, Co)3O4@NGC are further confirmed by its low Tafel slopes (Figure S12c, d). The synergistic effects of the two components and the optimized hierarchical porosity are the keys for such promising bifunctional performance.


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Rechargeable Zn-air batteries are fabricated using (Mg, Co)3O4@NGC as air electrode to explore its potential for practical application in ambient air. Pt/C and Ir/C (1:1 weight ratio) mixture of same mass loading is also used to fabricate the reference Zn-air batteries. The galvanodynamic charge and discharge curves are presented in Figure 5d. Both Zn-air batteries deliver a similar open circuit voltage at ca. 1.45 V and comparable charge performance. However, the Zn-air battery using (Mg, Co)3O4@NGC as air electrode outperforms the reference in the discharge segment, especially at relatively high current density; its power density (125 mW cm−2) is much higher than the noble-metal reference (90 mW cm−2; Figure S13). Notably, excellent durability is achieved by (Mg, Co)3O4@NGC as indicated in Figure 5e, S14. When cycling at a constant current density of 10 mA cm−2 with a charge-discharge period of 2 hours, the (Mg, Co)3O4@NGC assembled Zn-air battery demonstrates a charge-discharge voltage gap as low as 0.80 V and long-term cycling stability without discernible voltage degradation for over 200 hours, surpassing significantly noble-metal cathode. Even at a high current density of 20 mA cm−2, a narrow charge-discharge voltage gap of 0.90 V and durable cyclability over 100 hours are also delivered by (Mg, Co)3O4@NGC, while the noble-metal catalyst offers very limited durability less than 40 hours. In summary, benefiting from the proposed in-situ coordinating strategy, double-shelled (Mg, Co)3O4@NGC microspheres with hierarchical porosity is designed as promising bifunctional oxygen electrocatalyst with following merits. Firstly, the double-shelled architecture with hierarchical porosity greatly enhances ORR/OER kinetics. Secondly, the Mg modulation positively influences electrocatalytic activity by facilitating reactants transportation and improving electrical conductivity. Finally, a novel Mg-assisted mechanism based on coordination chemistry for tailoring lattice alkalinity is proposed to modify oxygen electrocatalysis. It is believed that


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tailoring of lattice alkalinity via controllable occupation of specific metal ions is a feasible and general strategy to alter oxygen electrocatalysis. These results open an inspiring insight into rational design of efficient oxygen electrocatalysts for practical energy conversion systems.


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ASSOCIATED CONTENT Supporting Information. Experimental details, additional morphological, structural and electrochemical characterization (PDF); the 3D reconstructed animation (Video). AUTHOR INFORMATION *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Y.D. thanks the support of China Scholarship Council (CSC). The authors acknowledge the support provided by Natural Sciences and Engineering Research Council of Canada (NSERC), University of Waterloo and Waterloo Institute for Nanotechnology. This work is financially supported by the 111 Project (No. D17007).


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Hierarchical Porous Double-Shelled Electrocatalyst with Tailored

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