Co3O4 Nanoparticles on Nitrogen-Doped

Aug 28, 2017 - For comparison, IrO2 and RuO2 reach 10 mA·cm–2 at an overpotential of 340 mV and 350 mV, respectively, agreeing with the reported da...
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High-Valence-State NiO/Co3O4 Nanoparticles on Nitrogen-Doped Carbon for Oxygen Evolution at Low Overpotential Muhammad Tahir,†,‡,§,∇ Lun Pan,†,‡,∥,∇ Rongrong Zhang,† Yi-Cheng Wang,∥ Guoqiang Shen,†,‡ Imran Aslam,⊥ M. A. Qadeer,†,‡ Nasir Mahmood,†,‡ Wei Xu,# Li Wang,†,‡ Xiangwen Zhang,†,‡ and Ji-Jun Zou*,†,‡ †

Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ‡ Collaborative Innovative Centre of Chemical Science and Engineering (Tianjin), Tianjin 300072, China § Department of Physics, The University of Lahore, Lahore, Pakistan ∥ School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States ⊥ National Centre for Nanoscience and Technology, Chinese Academy of Science, Beijing, China # Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, CAS, Beijing, China S Supporting Information *

ABSTRACT: The electrocatalytic oxygen evolution reaction (OER) plays a critical role in sustainable energy conversion and storage, but OER is severely hampered owing to the lack of highly efficient catalysts. Here, we report an efficient electrocatalyst, with NiO/Co3O4 nanoparticles decorated on nitrogen-doped carbon (NiO/Co3O4@ NC). Abundant high-valence Ni3+ and Co3+ species were observed on the surface of the hybrid due to the strong NC−metal oxide and NiO−Co3O4 interactions. This unique structure leads to excellent OER performance, delivering a very low overpotential of 240 mV@10 mA·cm−2 on glassy carbon and 200 mV@10 mA·cm−2 on Ni foam in KOH and having a turnover frequency (@350 mV overpotential) 6 and 16 times higher than that of IrO2 and RuO2, respectively.

of Ni to Co plays critical role for OER.26 Furthermore, decorating these catalysts on highly conductive nitrogen-doped carbon (NC) can improve the activity by enhancing the conductivity and interaction between metal oxides and carbon.12,13,27 In this work, we report coupled Co3O4 and NiO nanocrystals on nitrogen-doped carbon backbone (NiO/ Co3O4@NC), which is rich in high-valence-state species (Ni3+ and Co3+) on the surface and shows excellent OER performance (240 mV@10 mA·cm−2 on glassy electrode). NiO/Co3O4@NC was synthesized through thermal annealing of the mixture of melamine (initially HNO3 treated), NiCl2, and CoCl2 (Figure 1a). Upon annealing, g-C3N4, NiO, and Co3O4 are obtained when individual melamine, nickel, and cobalt salts are used (Figures S1 and S2), while the coexistence of metal salts and melamine produces N-doped carbon (NC)

O

xygen evolution reaction (OER) is the most imperative reaction for several energy conversion and storage devices such as water-splitting, fuel cells, and metal−air batteries.1−6 To date, RuO2 and IrO2 are commonly used for OER,6−9 but these metals are rare and expensive, and their activities are still unsatisfactory. Earthabundant first-row (3d) transition-metal-based electrocatalysts, like Co and Ni-based oxides, are considered as good alternative because they can produce O2 under mild conditions and modest overpotentials.9−19 Moreover, the performance of multimetal oxides usually outperforms that of single-metal oxide.20−22 Co3+ has been confirmed as the active catalytic sites for OER through inducing deprotonation of OOH species to produce O2 for its lower coordination number and larger H2O molecules adsorption energy.6,15,21,22 Similarly, surface Ni3+ species are regarded as the real active site for OER on nickel oxide.22−25 Although there are some reports revealing higher oxidation states of solely Ni or Co lead to good OER activity, the fabrication of higher oxidation states of both Ni and Co to get the combined benefit has not been reported. Also, the ratio © 2017 American Chemical Society

Received: August 1, 2017 Accepted: August 28, 2017 Published: August 28, 2017 2177

DOI: 10.1021/acsenergylett.7b00691 ACS Energy Lett. 2017, 2, 2177−2182

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Figure 1. (a) Synthesis process and schematic structure, (b) XRD pattern, (c) TEM image, (d) HR-TEM image, and (e−j) element mapping of NiO/Co3O4@NC.

determined by XPS analysis (Table S1). The size of pure Co3O4, NiO, and Co3O4/NiO composite is 2−10 μm, while it is decreased to 5−10 nm for NiO@NC, Co3O4@NC, and NiO/Co3O4@NC (Figure S2, S3), indicating the function of NC on inhibiting crystal growth. The X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies were carried out to characterize the oxidation state of metal atoms in the bulk. The Co pre-edge peak of NiO/Co3O4@NC shifts toward higher energy by ∼1.0 eV compared to standard Co3O4; also, the main peak moves to higher energy (Figure 2a), and the Co peak of NiO/Co3O4@NC is located between Co3O4 and Co2O3 (Figure S6), both suggesting Co atoms of the hybrid are in higher oxidation state compared with pristine Co3O4.31 In EXAFS, the Co−O bond of NiO/Co3O4@NC is shortened because of less electron density of metal ions that reduces the repulsion with O atom (Figures 2c and S6, Table S2). Similarly, the pre-edge Ni peak and main Ni peak of NiO/ Co3O4@NC shift toward higher energy (Figure 2b), and the Ni−O bond is also shortened compared to standard NiO (Figure 2d, Table S2), supporting the higher valence state of Ni in the hybrid. XPS characterization was further conducted to detect the valence state of oxide on the surface. Peak fitting shows NiO/Co3O4@NC possesses more Co3+ and Ni3+ than Co3O4 and NiO (Figure 2e,f), respectively. XPS analysis also indicates carbon in the hybrid is not in form of g-C3N4 (Figure S7a),32−34 and fitting the C 1s spectrum shows four peaks corresponding to the graphite-like sp2 C (39.6%), N-sp2C(39.5%), N-sp3-C (8.6%) along with some surface oxide carbon (12.3%) (Figure S7b), which suggests the doping of N

decorated with NiO/Co3O4 (NiO/Co3O4@NC, Figures S1 and S3), so the metal atoms enhance the carbonization of melamine and the formed carbon may influence the status of metal oxides. The X-ray diffraction (XRD) pattern reveals the existence of both Co3O4 and NiO (Figure 1b). The broad but weak peak at 2θ = 24° suggests the presence of graphitic carbon, which is further confirmed by typical D and G bands in Raman spectra (Figure S4).28−30 Thermogravimetric analysis shows the hybrid contains 10−11%wt of NC (Figure S5a), and inductively coupled plasma analysis shows the atomic ratio of Ni/Co is ca. 1, almost identical to the ratio of metal precursors used in the synthesis. The N2-adsorption isotherm measurement shows the surface area of NiO/Co3O4@NC (25.5 cm2/g) is between NiO@NC (21.1 cm2/g) and Co3O4@NC (38.5 cm2/g), but overall there is no big difference (Figure S5b). The scanning electron microscopy (SEM) image of NiO/ Co3O4@NC delineates good distribution of oxides nanocrystals on NC (Figure S3c). The transmission electron microscopy (TEM) images show the surface of graphitic sheets is almost fully decorated by nanocrystals (Figure 1c), consisting of compactly connected Co3O4 and NiO nanoparticles with the interplanar distances of d220(Co3O4) = 0.287 nm and d200(NiO) =0.209 nm (Figure 1d). Importantly, the intimate interface between well-defined Co3O4 and NiO is clearly observed (Figure 1d). Element mapping spectra (Figure 1e−j) show the distribution of Co and Ni in the hybrid, and importantly the Co-rich places are Ni-deficient and vice versa, i.e., both are present in the vicinity of each other. Moreover, the existence of doped nitrogen in the carbon backbone is confirmed by element mapping (Figure 1f,g); the ratio of N/C is 0.06% as 2178

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Figure 2. (a, b) Co and Ni K-edge XANES (inset is the magnified pre-edge adsorption), (c, d) Fourier transforms of k-space oscillations to R space of Co and Ni K-edge EXAFS, and (e, f) XPS spectra of Co 2p and Ni 2p.

in the carbon matrix.27 The existence of pyrrolic (19.7%) and pyridinic (41.9%) N 1s peaks furthermore confirms the Ndoping in carbon (Figure S7c), and importantly, the new peak at 399.5 eV (38.4%) suggests an interaction between metal oxides and NC through the N−M bonds.34 Accordingly, the XAFS and XPS results confirm NiO/ Co3O4@NC is rich in high-valence Co and Ni (i.e., Co3+ and Ni3+). Meanwhile, the species of Co3+ and Ni3+ are also found on the surface of Co3O4@NC and NiO@NC according to XPS analysis (Figure S8), indicating melamine and/or the formed NC has considerable influence on the electronic structure of oxides, with the chemical bonding between nitrogen and metal such as N−Co and N−Ni. In addition, the NiO/Co3O4@NC possesses more Ni3+ than NiO@NC but slightly less Co3+ than Co3O4@NC (Figure S8), suggesting the intimate interaction between NiO and Co3O4 in the bond of Co−O−Ni, as verified

by the intimate interparticle interface (Figure 1d), our density function theory calculation (Figure S9), and previous research that proved the electron transfer from Ni to Co.1,22 To summarize, metal oxides transfer electrons to NC through nitrogen−metal bonds, and that is why both Co and Ni have high oxidation states in NiO@NC and Co3O4@NC. However, in NiO/Co3O4@NC hybrid, Ni atoms transfer electrons to Co through the intimate interface, so Ni3+ increases but Co3+ decreases, but overall both are increased compared with normal NiO and Co3O4. The NiO/Co3O4@NC hybrid was coated on glassy carbon electrode and used for OER in 1 M KOH solution. As shown in Figure 3a, a sharply increased anodic current response starts at an onset potential of 1.4 V (the potential required to reach current density of 2 mA·cm−2) is observed. It reaches current density of 10 mA·cm−2 at 1.47 V vs RHE at a very low 2179

DOI: 10.1021/acsenergylett.7b00691 ACS Energy Lett. 2017, 2, 2177−2182

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Figure 3. (a) LSV curves; (b) TOF and (c) Tafel plots of NiO@NC, Co3O4@NC, NiO/Co3O4@NC, IrO2, RuO2, and Pt/C for OER in 1 M KOH; and (d) stability of NiO/Co3O4@NC @ 10 mA·cm−2.

(Co3+ and Ni3+), and the high conductivity of NC. Meanwhile, when the two oxides are combined in the form of NiO/Co3O4 or NiO/Co3O4@NC, the overpotential can be significantly decreased to 380 or 240 mV, respectively. Hybrids with different Ni/Co ratios of 1:2, 1:1, and 2:1 all show improved activity compared with individual oxides (Figure S10b), although the 1:1 ratio is the best. It is worth noting that the physical mixture of NiO+Co3O4 and NiO@NC+Co3O4@NC does not show any synergetic improvement in OER performance (Table S4). Therefore, it is the intimate electronic interaction, instead of physical mixture of metal oxides, that plays an important role in the improvement of OER behavior. Actually, the ECSA of NiO/Co3O4@NC (315 cm2) is much higher than that of NiO@NC (50 cm2) and Co3O4@NC (55 cm2) (Figure S15), indicating more metallic sites are activated when NiO and Co3O4 are coupled together by NC. Also, smaller semicircle of EIS for NiO/Co3O4@NC (Figure S16) indicate high ionic conductivity and low ionic residence as compared to NiO@NC, Co3O4@NC, and mixture of NiO@ NC+Co3O4@NC. Therefore, the superior performance of NiO/Co3O4@NC is mainly based on the unique surface electronic structure of oxides, especially the high valence state of metal atoms. The existence of Co and Ni in higher oxidation state improves the electrophilicity of adsorbed oxygen (Co3+-O or Ni3+-O) and therefore enables the formation of Co3+- or Ni3+-OOH through nucleophilic attack with OH−. This is thought to stimulate the OOH species deprotonation through the inductive effect of electron-withdrawing to harvest O2.21−23 Moreover, the in situ formed N-doped carbon is also very important to increase the conductivity of the hybrid,27−29 reduce the particle size of oxides, and anchor them to form intimate interparticle interface.

overpotential of 240 mV. For comparison, IrO2 and RuO2 reach 10 mA·cm−2 at an overpotential of 340 mV and 350 mV, respectively, agreeing with the reported data (Table S3). To our best knowledge, this hybrid is one of the best catalysts with the overpotential less than 250 mV@10 mA·cm−2 and is superior to any other catalysts on glassy carbon electrode (Table S3). Furthermore, it reaches a high current density of 30 mA·cm−2 and 161 mA·cm−2 at over potential of 300 and 500 mV, respectively (Figures 3a and S10a). The hybrid exhibits a turnover frequency (TOF) of 0.49 (s−1) per total metal atoms at 350 mV (Figure 3b), ca. 6 and 16 times higher than that of IrO2 and RuO2, respectively. The Tafel slope is 73 mV/dec, which suggests both water adsorption and O−O are ratedetermining;6,35 in addition, this value is comparable to that of with IrO2 and RuO2 (Figure 3c). It also possesses excellent stability without big change in the normalized current (with efficiency of ∼95% after 48 h) and CV curve after 2000 cycles (Figure 3d, S11). XPS characterization performed after OER (Figure S12) shows Ni3+ is increased greatly while Co3+ is decreased slightly, suggesting a synergy between these two species in the electrocatalysis, although the accurate mechanism is still unclear. The activity was further tested on Ni foam (Figure S13), and it reaches 10 mA·cm−2 at an extremely low overpotential of 200 mV, because the three-dimensional porous structure of Ni foam contributes to better diffusion and transfer and increased electrochemically active surface area (ESCA) (Figure S14). For pure NiO and Co3O4, the overpotentials @ 10 mA·cm−2 are 500 mV and 400 mV, respectively, while they are 310 and 270 mV for NiO@NC and Co3O4@NC, respectively (Figure S10a and Table S4). The significant improvement by NC support should result from the greatly reduced particle size (from 2−10 μm to 5−10 nm), formation of high valence state 2180

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oxygen-involving electrocatalysis. Angew. Chem., Int. Ed. 2017, 56, 8539−8543. (6) Tahir, M.; Pan, L.; Zhang, X.; Wang, L.; Zou, J.-J.; Wang, Z. L. Electrocatalytic oxygen evolution reaction for energy conversion and storage: a comprehensive review. Nano Energy 2017, 37, 136−157. (7) Spöri, C.; Kwan, J. T. H.; Bonakdarpour, A.; Wilkinson, D. P.; Strasser, P. The stability challenges of oxygen evolving catalysts: towards a common fundamental understanding and mitigation of catalyst degradation. Angew. Chem., Int. Ed. 2017, 56, 5994−6021. (8) Xu, Y.-F.; Chen, Y.; Xu, G.-L.; Zhang, X.-R.; Chen, Z.; Li, J.-T.; Huang, L.; Amine, K.; Sun, S.-G. RuO2 nanoparticles supported on MnO2 nanorods as high efficient bifunctional electrocatalyst of lithium-oxygen battery. Nano Energy 2016, 28, 63−70. (9) Zhu, Y. P.; Guo, C.; Zheng, Y.; Qiao, S. Z. Surface and interface engineering of noble-metal-free electrocatalysts for efficient energy conversion processes. Acc. Chem. Res. 2017, 50, 915−923. (10) Duan, J.; Chen, S.; Vasileff, A.; Qiao, S. Z. Anion and cation modulation in metal compounds for bifunctional overall water splitting. ACS Nano 2016, 10, 8738−8745. (11) Nardi, K. L.; Yang, N.; Dickens, C. F.; Strickler, A. L.; Bent, S. F. Creating highly active atomic layer deposited NiO electrocatalysts for the oxygen evolution reaction. Adv. Energy Mater. 2015, 5, 1500412. (12) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat. Mater. 2011, 10, 780−786. (13) Fominykh, K.; Feckl, J. M.; Sicklinger, J.; Döblinger, M.; Böcklein, S.; Ziegler, J.; Peter, L.; Rathousky, J.; Scheidt, E.-W.; Bein, T.; et al. Ultrasmall dispersible crystalline nickel oxide nanoparticles as high-performance catalysts for electrochemical water splitting. Adv. Funct. Mater. 2014, 24, 3123−3129. (14) Yeo, B. S.; Bell, A. T. Enhanced activity of gold-supported cobalt oxide for the electrochemical evolution of oxygen. J. Am. Chem. Soc. 2011, 133, 5587−5593. (15) Li, S.; Peng, S.; Huang, L.; Cui, X.; Al-Enizi, A. M.; Zheng, G. Carbon-coated Co3+-rich cobalt selenide derived from ZIF-67 for efficient electrochemical water oxidation. ACS Appl. Mater. Interfaces 2016, 8, 20534−20539. (16) Haber, J. A.; Cai, Y.; Jung, S.; Xiang, C.; Mitrovic, S.; Jin, J.; Bell, A. T.; Gregoire, J. M. Discovering Ce-rich oxygen evolution catalysts, from high throughput screening to water electrolysis. Energy Environ. Sci. 2014, 7, 682−688. (17) Rosen, J.; Hutchings, G. S.; Jiao, F. Ordered mesoporous cobalt oxide as highly efficient oxygen evolution catalyst. J. Am. Chem. Soc. 2013, 135, 4516−4521. (18) Ling, T.; Yan, D.-Y.; Jiao, Y.; Wang, H.; Zheng, Y.; Zheng, X.; Mao, J.; Du, X.-W.; Hu, Z.; Jaroniec, M.; Qiao, S. Z. Engineering surface atomic structure of single-crystal cobalt (II) oxide nanorods for superior electrocatalysis. Nat. Commun. 2016, 7, 12876. (19) Zhu, Y. P.; Ma, T. Y.; Jaroniec, M.; Qiao, S. Z. Self-templating synthesis of hollow Co3O4 microtube arrays for highly efficient water electrolysis. Angew. Chem., Int. Ed. 2017, 56, 1324−1328. (20) Yang, Y.; Fei, H.; Ruan, G.; Xiang, C.; Tour, J. M. Efficient electrocatalytic oxygen evolution on amorphous nickel−cobalt binary oxide nanoporous layers. ACS Nano 2014, 8, 9518−9523. (21) Zhang, B.; Zheng, X.; Voznyy, O.; Comin, R.; Bajdich, M.; García-Melchor, M.; Han, L.; Xu, J.; Liu, M.; Zheng, L.; et al. Homogeneously dispersed multimetal oxygen-evolving catalysts. Science 2016, 352, 333−337. (22) Zhao, S.; Wang, Y.; Dong, J.; He, C.-T.; Yin, H.; An, P.; Zhao, K.; Zhang, X.; Gao, C.; Zhang, L.; et al. Ultrathin metal−organic framework nanosheets for electrocatalytic oxygen evolution. Nat. Energy 2016, 1, 16184. (23) Friebel, D.; Louie, M. W.; Bajdich, M.; Sanwald, K. E.; Cai, Y.; Wise, A. M.; Cheng, M.-J.; Sokaras, D.; Weng, T.-C.; Alonso-Mori, R.; et al. Identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting. J. Am. Chem. Soc. 2015, 137, 1305− 1313.

In summary, we have prepared a NiO/Co3O4@NC hybrid with optimized electronic structure for OER. The in situ formed nitrogen-doped carbon backbone decreases NiO and Co3O4 particles to nanosize and anchors them to build intimate interface. Importantly, the hybrid shows unique surface electronic structure with high concentration of Co3+ and Ni3+ species. As a result, it exhibits superior activity for OER in KOH, with an extremely low overpotential of 240 and 200 mV (@10 mA cm−2) on glassy carbon electrode and Ni foam, respectively, and has a turnover frequency (@350 mV overpotential) that is 6 and 16 times higher than IrO2 and RuO2, respectively. Furthermore, the hybrid shows excellent stability and efficiency (∼95% after 48 h). Thus, the NiO/ Co3O4@NC is a very promising candidate for electrocatalytic water splitting.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00691. Detailed experimental procedures, characterization, and computation results; OER data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nasir Mahmood: 0000-0002-8340-1058 Wei Xu: 0000-0001-8006-2399 Ji-Jun Zou: 0000-0002-9126-1251 Author Contributions ∇

M.T. and L.P. contributed equally in this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the support from the National Science Foundation of China (51661145026, 21676193, 21506156), Tianjin Municipal Natural Science Foundation (15JCZDJC37300), and Pakistan Science Foundation PSF/ NSFC-Eng/P-UOL(02) . Authors also appreciate the help with XAS characterization from Beijing Synchrotron Radiation, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China.

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REFERENCES

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