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Superior Oxygen Electrocatalysis on Nickel Indium Thiospinels for Rechargeable Zn−Air Batteries Gengtao Fu,†,# Yu Wang,‡,# Yawen Tang,Δ Kun Zhou,*,‡,§ John B. Goodenough,⊥ and Jong-Min Lee*,† Downloaded via 31.40.210.83 on August 13, 2019 at 11:58:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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School of Chemical and Biomedical Engineering, Nanyang Technological University 637459, Singapore Environmental Process Modelling Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University 637141, Singapore § School of Mechanical and Aerospace Engineering, Nanyang Technological University 639798, Singapore Δ Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China ⊥ Materials Science and Engineering Program & Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States ‡
S Supporting Information *
ABSTRACT: Developing active bifunctional oxygen catalysts to eliminate/ reduce the reliance on precious-metal-based ones in metal−air batteries is nowadays of great importance. Here, we report the synthesis of nickel indium thiospinel nanosheets supported on carbon nanofibers (denoted as NiIn2S4/CNFs) via a facile in situ solvothermal growth process and, for the first time, demonstrate its superior bifunctional oxygen electrocatalytic performances as a precatalyst. Electrocatalytic experiments show that NiIn2S4/CNFs not only exhibits a positive half-wave potential of 0.81 V for the oxygen reduction reaction (ORR) but also delivers a lower overpotential of 0.39 V for the oxygen evolution reaction (OER) at 10 mA cm−2, outperforming those of monometallic Ni or In sulfides. Theoretical calculations confirm that the (220) and (111) plane in NiIn2S4 are selectively active to the ORR and OER, respectively. Apart from the intrinsic nature of NiIn2S4, the superior OER activity of NiIn2S4/CNFs is also related to the oxide/hydroxide species in-situ-formed on NiIn2S4 surface under the OER condition. Moreover, the developed NiIn2S4/ CNFs pre-catalyst is demonstrated to be an efficient air-cathode for Zn−air batteries. This work brings a new perspective for designing various ternary thiospinels for energy conversion and storage.
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Recently, bimetallic sulfides have been in the spotlight as one class of efficient materials for the electrocatalysis.27−30 Among them, ternary thiospinels AB2S4 (A2+ and B3+ on behalf of two types of metal cations) have attracted more attention as bifunctional oxygen electrocatalysts.31−36 Such a spinel structure can offer not only abundant oxygen adsorption and activation centers,33 but also has low energy electron hopping between different cations that endow AB2S4 with improved electrical conductivity.33,35 One notable example is by Zhang et al., who for the first time confirmed the excellent bifunctional activities of NiCo2S4 towards OER and ORR, with N, S-
echargeable Zn−air batteries with low-cost, high-safety, environmental friendliness, and high theoretical energy density (1086 Wh kg−1) promote their development with the aim of substituting for Li-ion batteries.1−5 Because of complicated multi-electron transfer steps, two half-reactions of oxygen reduction, and oxygen evolution (ORR/OER) involved in air-cathodes greatly suffer from sluggish kinetics,6−12 resulting in needing bifunctional electrocatalysts to facilitate these processes.13,14 This challenge has been unmet because expensive and scarce precious-metal materials (e.g., Pt/C and IrO2/RuO2) are still the best known electrocatalysts to reduce the overpotentials of the ORR and OER.15−20 Therefore, there is currently a dire need to explore cost-efficient alternatives with desired bifunctional properties for both half-reactions.21−26 © 2019 American Chemical Society
Received: April 4, 2019 Accepted: May 24, 2019 Published: May 29, 2019 123
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Figure 1. (a) Schematic illustration of the formation of NiIn2S4/CNFs. (b) XRD patterns of NiIn2S4/CNFs, Ni3S4/CNFs, and In2S3/CNFs. (c) Crystal structure of spinel NiIn2S4.
graphene as an electronic conductor.32 Since then, some similar results were also demonstrated in other CoNi2S4 and CuCo2S4 ternary thiospinels.34−36 Recent research discovered that indium (In)-doped transition metals, such as In-doped MnOx and Co3InC0.75/C, could greatly improve oxygen electrocatalytic activities,37,38 especially for the ORR. More recently, our group also demonstrated that the incorporation of In into CoSx could promote the reversible oxygen electrocatalysis by using a combination of experiment and theory.39 However, it is still in the infancy for the investigation of the bifunctional properties of ternary In-based thiospinels. It also remains unclear where exactly on In-based thiospinels surfaces are the active-sites that promote the ORR or OER. Therefore, it is an interesting topic to dig deeper into the bifunctional properties of In-based ternary thiospinels, which is significantly important to make them as efficient air-cathodes in rechargeable Zn-air batteries. Herein, a new and promising electrocatalyst, ternary NiIn2S4 decorated carbon nanofibers (NiIn2S4/CNFs), is demonstrated to be efficient for catalyzing the ORR and OER. Prepared by a facile solvothermal method, sheet-like NiIn2S4 nanocrystals can be grown directly on the surface of carbon nanofibers. Compared with monometallic Ni- or In-sulfides, the obtained NiIn2S4/CNFs present impressive electrocatalytic performance towards ORR and OER, which are comparable to conventional precious-metal catalysts. Density functional theory (DFT)
studies reveal that the outstanding bifunctional properties of NiIn2S4 should be ascribed to the active (220) and (111) surfaces, which are prone to ORR and OER, respectively. Furthermore, NiIn2S4 nanosheets assembled on the surface of CNFs provide more active sites for electrocatalysis while CNFs serve as an electronic conduction network that facilitates electron transport. With NiIn2S4/CNFs as an air-cathode, assembled Zn−air batteries deliver an extraordinary charge− discharge performance with large power/energy densities and long cycle life, which outperforms that based on the combination of Pt/C+RuO2 catalysts. The fabrication process of NiIn2S4/CNFs is schematically illustrated in Figure 1a. The CNFs were first treated by reflux with nitric acid (HNO3) to introduce carboxylate ions (−COO−) onto the CNFs surface,40,41 that can serve as anchor sites of metal ions (Ni2+ and In3+) via electrostatic binding. Through a simple in situ growth method, a uniform coating of sheet-like NiIn2S4 was produced on the skeleton of CNFs with high density, during which thioacetamide (TAA) acts as the sulfur source. The formation of NiIn2S4 nanocrystals was confirmed by X-ray diffraction (XRD, Figure 1a). The diffraction peaks at (111), (220), (311), (222), (400), (511), (440), (620), and (533) peaks pertain to the diffraction planes of NiIn2S4 phase (JCPDS card no. 70-2900). The structural drawing of NiIn2S4 was presented in Figure 1c. The NiIn2S4 crystallizes into a typical spinel structure with an Fd3m space 124
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Figure 2. (a) TEM images of pure CNFs. (b and c) TEM images of NiIn2S4/CNFs. (d) SEM image of NiIn2S4/CNFs. (e) EDX element mappings of NiIn2S4/CNFs. (f) N2 adsorption−desorption isotherm of NiIn2S4/CNFs and CNFs. High-resolution (g) In 3d, (h) Ni 2p, and (i) S 2p XPS spectra.
induced from the interconnection between nanosheets. The scanning electron microscopy (SEM) images further demonstrate an open and porous architecture for the NiIn2S4/CNFs (Figure 2d). The pure NiIn2S4 sample also reveals a hierarchically porous architecture consisting of numerous nanosheets (Figure S3), implying that the growth and selfassembly of NiIn2S4 could be tuned on CNFs surface. When CNFs without undergo carboxylation, mixtures of CNFs and irregularly assembled sheet-like nanocrystals were obtained (Figure S4), demonstrating that acid treatment is critical for the successful growth of NiIn2S4 nanosheets on the CNFs. This novel carbon supported hierarchical structure can provide multiple benefits for electrocatalysis, such as large specific surface area, abundant active sites for redox reactions, and more accessible pathways for rapid ion/electron transport. The high-resolution (HRTEM) images shown in Figure S5 indicate that the obtained NiIn2S4 nanosheets mainly expose the (111), (220), and (311) planes. The EDX element mappings (Figure 2e and Figure S6) show the uniform distribution of Ni, In, and S elements over the CNFs surface, further verifying the homogeneous growth of NiIn2S4 nanosheets on the CNFs. It is verified that NiIn2S4 nanosheets grown on CNFs endow
group with each unit cell consisting of [InS6] octahedron and [NiS4] tetrahedron clusters and metal cations located in the centre. For comparison, the CNFs supported monometallic nickel sulfide and indium sulfide were also prepared through the similar solvothermal procedure. The crystal structure of nickel sulfide and indium sulfide is consistent with the spinel Ni3S4 (JCPDS no. 76-1813) and In2S3 (JCPDS no. 65-0459), respectively. Transmission electron microscopy (TEM) images of functionalized CNFs show a typical nanobundle structure consisting of several self-assembled nanofiber-like secondary units (Figure S1). The length of nanofibers can be up to micrometer-scale and the diameter of nanofibers is approximately 5.0−7.0 mm (Figure 2a). With the functionalized CNFs as templates, the sheet-like NiIn2S4 nanocrystals were in situ grown on their surface after the hydrothermal reaction, as verified by TEM images (Figure 2c and Figure S2). No bare CNFs were observed, indicating a high-yield of homogenous deposition of NiIn2S4 nanosheets on the CNFs surface. From the width of wire-like dark region (Figure 2c), the thickness of nanosheets was determined to be approximately 5.0 nm. Moreover, some pores could be clearly observed that were 125
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Figure 3. (a) ORR polarization curves in O2-saturated 0.1 M KOH (rotation rate = 1600 rmp; sweep rate = 5 mV s−1). (b) Bar plots of the half-wave potential (E1/2). (c) Rotation-rate-dependent current−potential curves for NiIn2S4/CNFs. Inset shows the corresponding Koutecky−Levich plots at different potentials. (d) OER polarization curves in O2-saturated 0.1 M KOH (rotation rate = 1600 rmp; sweep rate = 5 mV s−1). (e) Overpotentials at a chosen current density of 10 mA cm−1. (f) Bar plots of the value of ΔE for catalysts (ΔE = EJ10 − E1/2).
NiIn2S4/CNFs with larger BET specific surface area (196.3 m2 g−1) than those of bare CNFs (124.7 m2 g−1), as shown in Figure 2f. The atomic Ni/In/S ratio determined by EDX analysis was approximately 13.0:27.6:59.4 (Figure S7), as expected for NiIn2S4. The full X-ray photoelectron spectroscopy (XPS) spectrum of NiIn2S4/CNFs (Figure S8) reveals the presence of Ni, In, and S with desired stoichiometric ratio (Ni/In/S = 14.6:29.1:56.3), in consistent with the EDX analysis. For the In 3d spectrum (Figure 2g), two peaks at 445.1 and 452.6 eV correspond to the 3d 5/2 and 3d 3/2 core levels of In3+.39 In the Ni 2p spectrum (Figure 2h), the peaks located at around 856.6 and 874.3 eV are assigned to the spin− orbit characteristic of Ni3+ oxidation state, whereas the strong satellite peaks at around 862.5 and 880.5 eV correspond to the Ni2+ oxidation state, demonstrating the coexistence of Ni3+ and Ni2+. Figure 2i shows the high-resolution S 2p spectrum. Both 2p3/2 and 2p1/2 core levels at 161.6 and 162.8 eV are assigned to the M−S bonds (M = Ni and In). Moreover, the C−S−C bond can be observed clearly at 163.5 eV, indicating that S atoms are successfully doped into the carbon materials.42 The peak at 169.1 eV belongs to the S−O species, owing to the partial oxidation of S.43,44 All XPS results confirmed that the NiIn2S4/CNFs have a composition of Ni3+, Ni2+, and In3+ cations, M−S bonds and S2− anions, which further confirm the formation of NiIn2S4 phase. It is accepted that transition metals with mixed valences can offer donor−acceptor chemisorption sites for oxygen, and provide high electrical conductivity for electron hopping between different cations.31−36 Meanwhile, 1D carbon nanofibers as the electronic conduction networks can facilitate
electron transport from semi-conductive NiIn2S4 to the external circuit. Therefore, we expect that the mixed-valent NiIn2S4 coupled with CNFs gives potentially high electrocatalytic performance to NiIn2S4/CNFs hybrids. Half-cell electrochemical tests were employed to evaluate the ORR and OER activities of NiIn2S4/CNFs. Activity comparisons were made with monometallic Ni3S4/CNFs, In2S3/CNFs and precious-metal catalysts. Outstanding ORR activity was demonstrated by the NiIn2S4/CNFs in Figure 3a, where the half-wave potential (E1/2) of about 0.81 V and current density at 0.4 V of about 5.58 mA cm−2 are very similar to the commercial Pt/C (0.85 V and 5.73 mA cm−2). In comparison to Ni3S4/CNFs and In2S3/CNFs (Figure 3b), the NiIn2S4/ CNFs exhibits a 60 mV and 170 mV improvement in half-wave potential, respectively. The results indicate that the synergy of In and Ni within the sulfides endows NiIn2S4/CNFs with better electrocatalytic activity than sulfide with one of them alone for the ORR. The high E1/2 of NiIn2S4/CNFs renders itself among the most effective non-Pt electrocatalysts reported recently (Table S2). The introduction of indium may play a pivotal role in such excellent ORR activity of NiIn2S4/CNFs, as previously highlighted by some important studies.37−39 Figure 3c shows the current−potential curves of NiIn2S4/CNFs at different rotation-rates. The current increase at high rotation rate originates from the shortened diffusion layer.45,46 The ORR kinetics can be theoretically analyzed using the Koutecky−Levich (K−L) equation.47 According to the values at different potentials, n was estimated to be approximate 4.0, which is closed to that of Pt/C (Figure S9). To further determine the reaction pathway on NiIn2S4/CNFs, we 126
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Figure 4. Free energy diagrams of (a) ORR and (d) OER pathway on (220), (111), and (311) planes of NiIn2S4. The potential-limiting steps are highlighted by the dash lines. The free energy changes (ΔG) of potential-limiting steps are given in eV. (c) Free energy diagrams of ORR pathway on NiIn2S4 (110) plane at U = 0 and 0.90 V. Geometric structures of intermediates OOH*, O*, and OH* on (b) (220) and (e) (111) planes. Azure, brown, yellow, red, and green balls represent Ni, In, S, O, and H atoms, respectively.
conductivity,49 thus facilitating electron transfer to the substrate. The open and porous hierarchical architecture of NiIn2S4 nanosheets with high specific surface area provides the efficient transport pathways for reactant species and expose more catalytically active sites during reversible ORR and OER.50,51 The synergistic effect of NiIn2S4 and CNFs should be another important contribution to the high activity of NiIn2S4/CNFs, as NiIn2S4 or S-doped CNTs (S-CNTs) exhibits the inferior ORR and OER activities (Figure S11). The introduction of CNFs can promote the electrical conductivity of NiIn2S4, which contributes to the rapid electron transport from the electrode to the synthesized NiIn2S4 nanosheets.52,53 The superior ion and charge transport capability of CoIn2S4/S-rGO electrode was confirmed by the lower the charge transfer resistance (Rct) than that of pure CoIn2S4 (Figure S12). Moreover, the tight contact between CNFs and NiIn2S4 nanoshheets also endows the as-prepared catalysts with excellent stability, as assessed through i−t chronoamperometric measurement at specific ORR (0.70 V) and OER (1.60 V) potentials. As indicated in Figure S13, the NiIn2S4/CNFs exhibit better electrochemical stability with a current attenuation of only 21.8% and 39.4% for the ORR and OER, respectively, than state-of-the-art Pt/C (current attenuation of 32% for the ORR) and RuO2 (current attenuation of 62.6% for the OER) after running 5000 s. Therefore, the NiIn2S4/CNFs possess not only outstanding
performed rotating ring-disk electrode (RRDE) test to monitor the peroxide yield. As indicated in Figure S10, the ORR on NiIn2S4/CNFs exhibits a low HO2− yield (3.88) at the potential range of 0.35−0.85 V, comparable to that on Pt/C catalyst, demonstrating that the ORR occurred on NiIn2S4/CNFs by an efficient four-electron transfer pathway. Aside from excellent ORR activity, attractive OER activity is considered as another vital characteristic of bifunctional oxygen electrocatalysts. Figure 3d displays the polarization curves for the OER. The Tafel slope value of NiIn2S4/CNFs was about 91 mV dec−1 (close to 120 mV dec−1), suggesting that the first discharge step controls the reaction kinetics of OER.48 At a current density of 10 mA cm−2 (Figure 3e), the NiIn2S4/CNFs delivered a lower overpotential of about 0.39 V than those of Ni3S4/CNFs (0.41 V) and In2S3/CNFs (0.59 V), which can be comparable with RuO2 (0.38 V) and even smaller than that of many non-precious metal catalysts reported (Table S3). The remarkable OER activity of NiIn2S4/CNFs was also evidenced by its current density being much higher than that of monometallic sulfides over the whole potentials. These results outlined here highlight the importance of the synergy of In and Ni to improve the OER activity of NiIn2S4/CNFs. The In3+ cations have a strong octahedral-site preference energy that can effective catalyze ORR and OER at a relative low activation potential,39 while the Ni tetrahedral sites give good electronic 127
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Figure 5. (a) Open-circuit plots of Zn−air cells. (b) Discharge polarization curves and the corresponding power density plots of Zn−air cells. (c) Galvanostatic discharge curves at 5 mA cm−2. (d) Discharge profiles at different current densities (open-circuit voltage (OCV), 10, 20, and 50 mA cm−2, and recovery to OCV). (e) Long-term cycling performance at 10 mA cm−2 with NiIn2S4/CNFs and Pt/C+RuO2 catalyst as the air-cathodes, respectively. (f) Photograph showing a stopwatch running by Zn−air cell cell using NiIn2S4/CNFs as the cathode.
(CHE) approach, which is a powerful tool in predicting the potential-limiting step, theoretical limiting-potential (Ulim) and overpotential (η) of the electrochemical process on a given catalytic system (for details, see Supporting Information). The free-energy diagrams of the reaction pathway and the structures of some critical intermediates are shown in Figure 4. From the free energy diagram of ORR (Figure 4a), the potential-limiting step of the (220) plane is OOH* formation of which the free energy (ΔG) is −0.90 eV, while that of both the (311) and (111) planes is the reduction of OH* to OH−. Compared with the (220) plane, the ΔG of potential-limiting step of the (311) and (111) planes are identified to be more positive with −0.70 and −0.50 eV, respectively, which is unfavorable for ORR. Hence, among the three surfaces, the (220) plane is most active to ORR with a Ulim of 0.90 V, followed by the (311) plane (0.70 V) and (111) plane (0.50 V). Note that the theoretical Ulim of (220) plane is consistent with the measured onset-potential of NiIn2S4 (0.91 V). Besides superior activity, (220) plane also possesses good 4e pathway selectivity, which is quite beneficial to efficient O2 reduction. As shown in Figure 4c, the reduction of OOH* to OOH− (2e
bifunctional activities, but also significantly enhanced stability, which make them promising as a bifunctional electrocatalyst. The outstanding bifunctional properties of NiIn2S4/CNFs were further corroborated through overall oxygen electrode activities (Figure S14), determined by the potential difference (ΔE) between the OER potential at 10 mA cm−2 (EJ10) and the E1/2 of the ORR. Compared with monometallic Ni3S4/ CNFs (ΔE = 0.89 V) and In2S3/CNFs (ΔE = 1.18 V), the NiIn2S4/CNFs manifests the smaller ΔE value of 0.81 V (Figure 3f), while it is comparable to a mixed Pt/C+RuO2 catalyst (ΔE = 0.76) and numerous nonprecious metal bifunctional catalysts reported before (Table S4). To gain fundamental insight into the bifunctional behavior of the as-prepared NiIn2S4 for ORR and OER, we conducted spin unrestricted density functional theory (DFT) calculations with focus on the reaction intermediates (OOH*, O*, and OH*) and the catalytic pathways over the active surfaces of NiIn2S4. On the basis of the HRTEM analysis, three typical NiIn2S4 slabs, including (220), (111), and (311) planes, were constructed accordingly. Here, the catalytic activity of NiIn2S4 slabs were studied by using computational hydrogen electrode 128
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NiIn2S4/CNFs-based cell, demonstrating an outstanding rate capability and a fast dynamic response of NiIn2S4/CNFs-based cell. The cycling stability of the NiIn2S4/CNFs cathode was further evaluated under continuous galvanostatic discharge and charge at 10 mA cm−2 with each cycle being 20 min. As observed in Figure 5e, the cell loaded with NiIn2S4/CNFs manifests relatively stable operation for approximately 4000 min, which is longer than the Pt/C+RuO2 based cell (∼720 min). After continuous 200 cycles (4000 min), the voltage gap only increased by 0.10 V (the voltage gap changes from 0.70 V at the 1st cycle to 0.80 V at the 200th cycle), corresponding to only 4.2% decrease of round-trip efficiency (discharge end voltage divided by charge end voltage). For a conventional Pt/ C+RuO2-based cell, its discharge voltage was observed to degrade to
