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Origin of the Overpotential for the Oxygen Evolution Reaction on a Well-defined Graphene Electrode Probed by in situ Sum Frequency Generation Vibrational Spectroscopy Qiling Peng, Jiafeng Chen, Hengxing Ji, Akihiro Morita, and Shen Ye J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08285 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 6, 2018
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Journal of the American Chemical Society
Origin of the Overpotential for the Oxygen Evolution Reaction on a Well-defined Graphene Electrode Probed by in situ Sum Frequency Generation Vibrational Spectroscopy Qiling Peng,† Jiafeng Chen,‡ Hengxing Ji,‡* Akihiro Morita,§‖ and Shen Ye§‖* †Institute
for Catalysis, Hokkaido University, Sapporo 001-0021, Japan
‡Department
of Materials Science & Engineering, University of Science and Technology of China, Hefei 230026, China
§Department
of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
‖Elements
Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8520, Japan Supporting Information Placeholder dimensional model system for the carbon materials and is receiving wide attention due to its unique electronic, optical, and mechanical properties.7-10 Recently, a technique for preparing the graphene monolayer with a dimension up to several hundred cm2 has been established, which facilitated studies of electrochemical reactions on the well-defined graphene surfaces.11-14 However, these previous studies were mainly based on the electrochemical characterizations, providing only limited information about the reaction mechanism on the graphene. To obtain further insight into the interface structure and reaction mechanism on the graphene, a collaboration with spectroscopic measurement is quite helpful.
ABSTRACT: To develop an efficient material for the cathode of the lithium-oxygen (Li-O2) secondary battery, the oxygen reduction and evolution reactions (ORR and OER) on a welldefined graphene monolayer have been investigated in a typical organic solvent, dimethyl sulfoxide (DMSO). The adsorption and desorption behaviors of the solvents on the graphene electrode surface were evaluated by an intrinsically surface-selective vibrational spectroscopy of sum frequency generation (SFG) during the ORR and OER. After the initial ORR depositing lithium peroxide (Li2O2) on the graphene electrode surface in a LiClO4/DMSO solution, the SFG spectroscopy revealed that the subsequent OER oxidizing the Li2O2 preferentially proceeds at the interface between the Li2O2 and graphene rather than that between the Li2O2 and bulk solution. Therefore, the OER tends to reduce the electric conductivity between the Li2O2 and graphene by decreasing their contact area before a large part of the deposited Li2O2 was oxidized, which elucidates the origin of the high overpotential for the OER.
In the present study, for the first time, to the best of our knowledge, we investigated the electrochemical ORR/OER on a welldefined graphene monolayer electrode in aprotic electrolyte solutions by the combination of electrochemistry and sum frequency generation (SFG) vibrational spectroscopy. The latter is intrinsically selective and sensitive to the molecular structures on a surface and/or interface.15-18 The adsorption and desorption behaviors of the solvents on the graphene electrode surface were probed in situ by the SFG spectroscopy to clarify the mechanism of the ORR and OER. The graphene monolayer with a large dimension (25×25 mm2) was prepared by chemical vapor deposition19 and transferred onto the flat surface of a calcium fluoride (CaF2) prism by the PMMA-assisted method (see SI).20 The DMSO-based electrolyte solutions containing 0.5 M tetra-n-butylammonium perchlorate (TBAClO4) or 0.5 M lithium perchlorate (LiClO4) were purged by O2 or Ar for 30 min before each experiment.
The Li-O2 secondary battery is attracting significant attention due to its high specific energy in comparison to that of the Li-ion battery and is considered as a candidate power source for electric vehicles.1-3 However, its development is accompanied by many practical problems such as a high charging overpotential and poor cyclability.1-6 These are largely because of the sluggish kinetics in the charge and discharge processes on the cathode of the Li-O2 battery, i.e., oxygen evolution reaction (OER) and oxygen reduction reactions (ORR), respectively. Many experimental efforts have been devoted to developing the cathode materials for the Li-O2 battery in aprotic solutions while details about the ORR/OER mechanisms are still unknown at the molecular level, which make the development difficult. The porous carbon materials are widely used as the cathode materials due to their low cost, high conductivity, large surface area and three-dimensional structure. The porous structure, however, is hard to be characterized on the molecular level, which makes it difficult to evaluate the reaction mechanism and kinetics on the carbon surface in detail. On the other hand, graphene is a welldefined monolayer of sp2-carbon. It is considered as a two-
The electrochemical cell is combined with the SFG measurement using the internal reflection geometry from the graphene electrode side. In contrast to the conventional electrochemical SFG setup where the external reflection geometry from a thin-layer solution is employed,21-23 the present setup avoids optical loss of an infrared beam by the bulk solvent absorption, and also significantly improves the electrochemical conditions such as mass transfer and potential control.24 Because of these advantages, this method will be widely adopted by the in situ spectroscopic investigation of the transparent electrode surface. A broadband SFG system used in the study was described elsewhere.25-27 All the SFG spectra were recorded with polarization combinations of ssp (i.e., s-polarized SFG, s-polarized visible, and p-polarized infrared, same as below) and sps. All the potentials in the paper are referred to Li+/Li. Figure 1a shows a cyclic voltammogram (CV) measured on a 1
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4.5 V. The CV (Fig. 2a) is very different from that in the Li-ion free solution (Fig. 1a). Two cathodic peaks were found at 2.47 V and 2.35 V in the negative-going sweep. The peak at a less positive potential (2.35 V) was not observed in the Li-free solution (Fig. 1a). In the positive-going sweep, wide anodic waves with several small peaks were observed from ca. 2.8 V to a potential as high as 4.5V (Fig. 2a). As previously discussed, the Li-ion is involved in the electrochemical ORR/OER in the nonaqueous solutions as shown in Equation (2).28-30 O2 is electrochemically reduced to Li2O2 in two steps. Since Li2O2 hardly dissolves in DMSO, it deposits on the graphene surface. The thickness of the electrochemically grown Li2O2 layer on the carbon or gold electrodes is known to be approximately several nanometers.
graphene in an O2-saturated DMSO with 0.5 M TBAClO4 (solid trace). The potential was first swept from its open circuit potential (OCP) around 3.1V in the negative direction in the potential region between 1.8V and 3.6V. A couple of redox peaks were found at 2.53 V and 2.70 V. No Faradaic current was observed in the same solution saturated by Ar (Fig. A5, SI), which allows us to assign the redox peaks to the one electrode reduction of O2 to the superoxide (O2–) and the reoxidation of O2– on the graphene electrode (Eq. 1).28-30
Figure 1b shows a series of ssp-SFG spectra (2600~3400 cm–1) simultaneously acquired on the graphene surface during the CV measurement (Fig. 1a). A peak was observed at 2918 cm–1 at the OCP. This peak is assiged to the symmetric C-H stretching mode of the methyl of DMSO on the graphene surface. The peak is located in the higher frequency region than that on the DMSO/air interface (2900 cm–1),31 suggesting some interactions are present between the DMSO and graphene surface. A similar peak was also observed in the spsSFG spectra but with a much weaker intensity (Fig. A7, SI). The structure of the DMSO on the graphene surface is being investigated by molecular dynamics simulations and will be discussed elsewhere. In this paper, the square root of the SFG peak amplitude (shortly denoted as “SFG peak intensity” below ) is semi-quantitatively employed as an indicator for coverage of the DMSO adsorbed on the graphene surface, based on the basic theory of SFG.15 The SFG peak intensity is plotted as a function of the electrode potential (block symbols in Fig. 1a) in which the colors of the symbols correspond to the different stages in the CV. In the negative-going sweep (OCP1.8V, blue), the SFG peak intensity decreased as soon as the cathodic current started to flow and became almost constant. As the potential was swept back to the positive direction (1.8V3.7V, red), the SFG peak intensity increased as the anodic current flowed and recovered to the initial value. A high correlation was found between the electrochemical currents and SFG peak intensities. On the other hand, no change in the SFG peak intensity was observed during the potential sweep in the same solution but saturated by Ar (Fig. A8, SI). Thus, the decrease (increase) in the SFG peak intensity (i.e., DMSO coverage) on the graphene surface reversibly occurs with the ORR (OER). A previous surface enhanced Raman scattering (SERS) study30 reported that the adsorption and desorption of O2– on a gold surface during the ORR and OER showed a similar potential dependence as that of the DMSO coverage on the graphene surface in Fig. 1a. The present reversible SFG spectra of the C-H band of DMSO strongly imply the reversible replacement between the DMSO and O2– species on the graphene surface during the ORR and OER (Fig. 1c). The O2– species generated in the ORR are adsorbed on the graphene surface by replacing the adsorbed DMSO (Fig. 1c2) and will be oxidized to O2 and diffuse into the bulk solution during the OER (Fig. 1c3).
Figure 2b shows a series of ssp-SFG spectra (2600~3400 cm–1) during the CV in Li-ion containing DMSO (Fig. 2a). At the OCP around 3.1V, a peak was observed at 2918 cm–1, similar to that observed in the Li-ion free solution (Fig. 1b). Figure 2a also displays the SFG peak intensity for the DMSO (block symbols) as a function of the potential. The SFG peak intensity quickly decreased as the cathodic current started to flow in the negative-going sweep and became unchanged as the potential becomes more negative than 2.5V (blue blocks). As the potential was swept back to the positive direction, the SFG peak intensity for DMSO started to increase as the small oxidation current flows around 2.7V, and recovered to its initial value around 3.2V. No change in the SFG intensity was detected in the potential region more positive than 3.2V (red and black blocks, Fig.2a) while the anodic current still flowed in a wide region as high as 4.5V. We noted in Fig. 2a that the potential dependences of the SFG peak intensity are different in the ORR and OER. In the negative-going sweep for the ORR region, the SFG peak intensities well correspond the cathodic current around 2.6V (blue blocks, Fig. 2a), which is understood that part of the DMSO at the interface is replaced by the deposition of Li2O2. In the positive-going sweep for the OER, the recovery of the SFG peak intensity starts around 2.7 V and finishes around 3.2 V (red blocks, Fig. 2a) where the wide anodic current wave (3.0 V ~ 4.5 V) just starts to flow (Fig. 2a). This behavior indicates that the SFG peak intensity of DMSO at the interface is recovered to the original level, irrespective of a large part of the remaining Li2O2 at the interface. On the other hand, no change in the SFG peak intensity was observed during the potential sweep in the same solution but saturated by Ar (Fig. A9, SI). These behaviors are also different from those in the Li-free DMSO solution (Fig. 1). We summarize the above observations of the DMSO solution with Li-ion in the following. The amount of deposited Li2O2 on the graphene varies during the ORR and OER stages, while the present in situ SFG signal of C-H band detects the amount of DMSO on the graphene surface. The combination of CV and SFG indicates that the
Figure 2a shows a CV on the graphene in an O2-saturated DMSO with 0.5 M LiClO4. The potential was first swept from its OCP (ca. 3.1 V) to the negative direction in the potential region between 1.7 V and
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Journal of the American Chemical Society amounts of Li2O2 and DMSO on the graphene surface do not appear to be mutually exclusive in the positive-going sweep. Normally, one may conceive that the maximum coverage of DMSO on the graphene surface would be realized when the Li2O2 layer deposited on the graphene is fully oxidized. This intuition is clearly contradictory to the present observation, implying an alternative reaction mechanism of OER during the positive-going sweep. The OER preferentially takes place from the interface between the Li2O2 and graphene32-35 and initially oxidizes Li2O2 in contact with graphene. Consequently, the DMSO can diffuse into the gap space between the Li2O2 and graphene, which increases the SFG signal for DMSO on the graphene surface (Fig.2c3). It is known that the electric conductivity of Li2O2 is low,1 which probably favors the initial oxidation on the graphene/Li2O2 interface. This mechanism of initial oxidation also elucidates the origin of the high overpotential for the OER. The initial oxidation worsens the electric conductivity between the remaining Li2O2 and graphene due to the decrease in the contact area of the two materials. The further oxidation of the leftover Li2O2 species probably occurs together with the partial decomposition of the DMSO solvent at a potential as high as 4.5V.
(6) Ganapathy, S.; Adams, B. D.; Stenou, G.; Anastasaki, M. S.; Goubitz, K.; Miao, X.-F.; Nazar, L. F.; Wagemaker, M. J. Am. Chem. Soc. 2014, 136, 16335. (7) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (8) Novoselov, K. S.; Falko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. Nature 2012, 490, 192. (9) Allen, M. J.; Tung, V. C.; Kaner, R. B. Chem. Rev. 2010, 110, 132. (10) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Ri Kim, H.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.; Ozyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S. Nat. Nanotech. 2010, 5, 574. (11) Valota, A. T.; Kinloch, I. A.; Novoselov, K. S.; Casiraghi, C.; Eckmann, A.; Hill, E. W.; Dryfe, R. A. W. ACS Nano 2011, 5, 8809. (12) Valota, A. T.; Toth, P. S.; Kim, Y.-J.; Hong, B. H.; Kinloch, I. A.; Novoselov, K. S.; Hill, E. W.; Dryfe, R. A. W. Electrochimica Acta 2013, 110, 9. (13) Brownson, D. A. C.; Varey, S. A.; Hussain, F.; Haigh, S. J.; Banks, C. E. Nanoscale 2014, 6, 1607. (14) Güell, A. G.; Cuharuc, A. S.; Kim, Y.-R.; Zhang, G.; Tan, S.-y.; Ebejer, N.; Unwin, P. R. ACS Nano 2015, 9, 3558. (15) Shen, Y. R. The Principles of Nonlinear Optics; John Wiley & Sons, Inc.: New York, 1984. (16) Holman, J.; Davies, P. B.; Nishida, T.; Ye, S.; Neivandt, D. J. J. Phys. Chem. B (Feature Article) 2005, 109, 18723 (17) Nihonyanagi, S.; Mondal, J. A.; Yamaguchi, S.; Tahara, T. Annu. Rev. Phys. Chem. 2013, 64, 579. (18) Ye, S.; Tong, Y.; Ge, A.; Qiao, L.; Davies, P. B. The Chemical Record 2014, 14, 791. (19) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Science 2009, 324, 1312. (20) Reina, A.; Son, H.; Jiao, L.; Fan, B.; Dresselhaus, M. S.; Liu, Z.; Kong, J. J. Phys. Chem. C 2008, 112, 17741. (21) Guyot-Sionnest, P.; Tadjeddine, A. Chem. Phys. Lett. 1990, 172, 341. (22) Bain, C. D. J. Chem. Soc. Faraday Trans. 1995, 91, 1281. (23) Xu, S.; Xing, S.; Pei, S.-S.; Ivaništšev, V.; Lynden-Bell, R.; Baldelli, S. J. Phys. Chem. C 2015, 119, 26009. (24) Tong, Y.; Zhao, Y.; Li, N.; Ma, Y.; Osawa, M.; Davies, P. B.; Ye, S. J. Chem. Phys. 2010, 133, 034705. (25) Liu, H.; Tong, Y.; Kuwata, N.; Osawa, M.; Kawamura, J.; Ye, S. J. Phys. Chem. C (Letter) 2009, 113, 20531. (26) Yu, L.; Liu, H.; Wang, Y.; Kuwata, N.; Osawa, M.; Kawamura, J.; Ye, S. Angew. Chem. Int. Ed. 2013, 52, 5753. (27) Peng, Q.; Liu, H.; Ye, S. J. Electroanal. Chem. 2017, 800, 134. (28) Peng, Z. Q.; Freunberger, S. A.; Hardwick, L. J.; Chen, Y. H.; Giordani, V.; Barde, F.; Novak, P.; Graham, D.; Tarascon, J. M.; Bruce, P. G. Angew. Chem. Int. Ed. 2011, 50, 6351. (29) Trahan, M. J.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M. J. Electrochem. Soc. 2013, 160, A259. (30) Qiao, Y.; Ye, S. J. Phys. Chem. C 2015, 119, 12236. (31) Ding, F.; Zhong, Q.; Brindza, M. R.; Fourkas, J. T.; Walker, R. A. Optics Express 2009, 17, 14665. (32) Zhong, L.; Mitchell, R. R.; Liu, Y.; Gallant, B. M.; Thompson, C. V.; Huang, J. Y.; Mao, S. X.; Shao-Horn, Y. Nano Letters 2013, 13, 2209. (33) Kushima, A.; Koido, T.; Fujiwara, Y.; Kuriyama, N.; Kusumi, N.; Li, J. Nano Letters 2015, 15, 8260. (34) Liu, C.; Ye, S. J. Phys. Chem. C 2016, 120, 25246. (35) Wang, J.; Zhang, Y.; Guo, L.; Wang, E.; Peng, Z. Angew. Chem. Int. Ed. 2016, 55, 5201. (36) Chen, Y. H.; Freunberger, S. A.; Peng, Z. Q.; Fontaine, O.; Bruce, P. G. Nat. Chem. 2013, 5, 489. (37) Qiao, Y.; Ye, S. J. Phys. Chem. C 2016, 120, 15830. (38) Gao, X.; Chen, Y.; Johnson, L. R.; Jovanov, Z. P.; Bruce, P. G. Nature Energy 2017, 2, 17118. (39) Park, J. B.; Lee Seon, H.; Jung, H. G.; Aurbach, D.; Sun, Y. K. Advanced Materials 2017, 30, 1704162. (40) Chen, J.; Han, Y.; Kong, X.; Deng, X.; Park, H. J.; Guo, Y.; Jin, S.; Qi, Z.; Lee, Z.; Qiao, Z.; Ruoff, R. S.; Ji, H. Angew. Chem. Int. Ed. 2016, 55, 13822.
To overcome the high OER overpotential in the Li-O2 battery, new strategies including using redox mediator have been proposed to homogeneously oxidize the Li2O2 layer on the graphene surface with a low overpotential.36-39 On the other hand, the chemical modification of the graphene surface40 is also considered to improve the reactivity for the ORR and OER. In conclusion, we have evaluated the ORR and OER behaviors on the well-defined graphene electrode by a combination of electrochemistry and in situ SFG vibration spectroscopy for the first time. We revealed that the OER preferentially takes place at the interface between the Li2O2 and graphene. We attribute this finding as a key factor for the high OER overpotential on the carbon cathode of the Li-O2 battery. The present study demonstrates that the in situ SFG spectroscopy is powerful to reveal detailed mechanisms of electrochemical reactions on the graphene electrode surface on a molecular level. We believe that the present combination of electrochemistry and SFG using graphene will play important roles in the fundamental research and application of carbon materials. ACKNOWLEDGMENT This study was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA), specially promoted research for innovative next generation batteries (SPRING) from the Japan Science and Technology Agency (JST). HXJ thank funding support from the Natural Science Foundation of China (21373197), Fundamental Research Funds for the Central Universities (WK2060140003), and iChEM. SUPPORTING INFORMATION Supporting Information Available: Detailed experimental method and supporting figures including Raman spectra, in situ SFG cell setup, CVs, SFG spectra, SFG spectra in DMSO saturated by O2 or Ar.
REFERENCES (1) Luntz, A. C.; McCloskey, B. D. Chem. Rev. 2014, 114, 11721. (2) Lu, J.; Li, L.; Park, J. B.; Sun, Y. K.; Wu, F.; Amine, K. Chem. Rev. 2014, 114, 5611. (3) Aurbach, D.; McCloskey, B. D.; Nazar, L. F.; Bruce, P. G. Nature Energy 2016, 1, 16128. (4) Albertus, P.; Girishkumar, G.; McCloskey, B.; Sánchez-Carrera, R. S.; Kozinsky, B.; Christensen, J.; Luntz, A. C. J. Electrochem. Soc. 2011, 158, A343. (5) Johnson, L.; Li, C.; Liu, Z.; Chen, Y.; Freunberger, S. A.; Ashok, P. C.; Praveen, B. B.; Dholakia, K.; Tarascon, J.-M.; Bruce, P. G. Nat. Chem. 2014, 6, 1091.
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Figure 2. ORR/OER on a graphene electrode in 0.5M LiClO4/DMSO saturated by O2 characterized by (a) CV (1mV/s, 1.7V ~ 4.5V) starting from 3.1V (OCP) and (b) in situ SFG spectra continuously recorded with the CV. Each SFG spectrum was accumulated for 80s. The SFG peak intensity (in square root) was also plotted as a function of the potential in (a). The color of the SFG spectra and intensities correspond to the different periods in the CV, i.e., OCP ~ 1.7V (blue), 1.7V ~ 4.5V (red) and 4.5V ~ 2.7V (black). (c) A schematic model for the changes on the graphene surface at (1) OCP, (2) end of ORR (3) middle and (4) end stage of OER. The relative coverages of DMSO (green) and Li O (blue) on the graphene surface are represented by the area while the light green region corresponds to the bulk DMSO solution. ACS Paragon Plus Environment
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