Surface Acidity as Descriptor of Catalytic Activity for Oxygen Evolution

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Surface Acidity as Descriptor of Catalytic Activity for Oxygen Evolution Reaction in Li-O2 Battery Jinzhen Zhu, Fan Wang, Beizhou Wang, Youwei Wang, Jianjun Liu, Wenqing Zhang, and Zhaoyin Wen J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b07792 • Publication Date (Web): 05 Oct 2015 Downloaded from http://pubs.acs.org on October 6, 2015

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Journal of the American Chemical Society

Surface Acidity as Descriptor of Catalytic Activity for Oxygen Evolution Reaction in Li-O2 Battery Jinzhen Zhu1,† , Fan Wang2,†, Beizhou Wang1, Youwei Wang, Jianjun Liu1,* , Wenqing Zhang1,3,*, Zhaoyin Wen2,* 1

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, 2CAS Key laboratory of Materials for Energy Conversion, Shanghai institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China; 3Materilas Genome Institute, Shanghai University, 99 Shangda Road, Shanghai 200444, China † Co-first authors * Corresponding authors; E-mail: [email protected]; [email protected]; [email protected]

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Abstract Unraveling the descriptor of catalytic activity which is related to physical properties of catalysts is a major objective of catalysis research. In the present study, the first-principles calculations based on interfacial model were performed to study the oxygen evolution reaction mechanism of Li2O2 supported on active surfaces of transition metal compounds (TMC: oxides, carbides, and nitrides). Our studies indicate that the O2 evolution and Li+ desorption energies show linear and volcano relationship with surface acidity of catalysts, respectively. Therefore, the charging voltage and desorption energies of Li+ and O2 over TMC could correlate with their corresponding surface acidity. It is found that certain materials with an appropriate surface acidity can achieve the high catalytic activity in reducing charging voltage and activation barrier of rate-determinant step. According to this correlation, CoO should have as active catalysis as Co3O4 in reducing charging overpotential, which is further confirmed by our comparative experimental studies. Co3O4, Mo2C, TiC, and TiN are predicted to have a relatively high catalytic activity, which is consistent with the previous experiments. The present study enables the rational design of catalysts with greater activity for charging reactions of Li-O2 battery.

Key words: Li-O2 Battery, Oxygen Evolution Reaction, Catalytic Mechanism, Surface Acidity, First-Principles Calculations

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1. Introduction There has been a tremendous interest in the field of catalysis to establish the descriptor of the catalytic activity and the selectivity correlated with physical properties of catalysts. An effective descriptor can be intensively applied to design a novel and highly active catalyst or structural optimization to improve catalytic activity for specific materials. For example, Hammer and Nørskov have developed a theoretical model of “d-band center” to describe the relationship of molecular adsorption strength on metal surface with the energy of the d-band center in the metal.1 Such a model and extended models have proved useful to predict the catalytic activity of metal and alloys for many molecular reactions such as CO oxidation, O2 reduction, and oxygen evolution reaction from water.2-8 Inspired by the success of the d-band center model, many molecular reactions catalyzed by metal alloys and metal oxides were studied to identify the descriptors of catalytic activity.2,5,9 However, extending the model of catalytic molecular reaction to catalytic reaction in solid-solid interface, developing an effective model to describe catalytic activity of solid-solid interfacial reaction remains challenging due to complexity,10 but highly desired for screening a novel catalyst. Rechargeable lithium-oxygen batteries are considered as promising next-generation devices for energy storage and conversion because of their high theoretical specific energy (>3500 Whkg-1), which is 3-5 times larger than that of current lithium-ion batteries.10-16 However, they suffer from several issues such as high overpotential (0.5~1.2V),12,17,18 poor cycle performance,18-20 and limited rate capability.11,21,22 These critical challenges which limit its practical application is mainly ascribed to slightly slow oxygen reduction reaction (ORR: 2Li++O2+2e-→Li2O2) and significantly sluggish oxygen evolution reaction (OER, 3



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Li2O2→2Li++O2+2e-). In the past several years, a great deal of research efforts were made to develop highly active catalysts to improve ORR and OER kinetics, such as transition metal oxides(TMO),8,9,15-20 noble metals,14,23,24 carbonaceous materials, transition metal carbide, and nitride, and perovskites.25-27 Shao-Horn et al performed systematic studies for some noble metals as ORR catalysts of Li-O2 battery via rotating disk electrode measurements.23 They established volcano relationship of discharging voltages with oxygen adsorption energies. Later, Xu et al performed extensive first-principles calculations for these catalytic mechanisms of noble metals and revealed consistent trend of catalytic activity.28,29 The highest active catalyst should have an appropriate O adsorption strength close to those of well-established Pt and Pd catalysts. In contrast, there are not many systematic experimental and theoretical studies to reveal underlying essence of catalytic activity for OER in Li-O2 battery although OER catalysts are more efficient than ORR catalysts in reducing charging-discharging overpotential and a large number of OER catalysts have been explored independently. However, identifying activity descriptors of OER in electrocatalytic water splitting has been extensively studied. Suntivich and Vojvodic et al have determined surface oxygen binding energy, eg occupancy and 3d electron number of transition metal ions as the activity descriptors of OER in an electrocatalytic water splitting, respectively.30,31 Unfortunately, these activity descriptors identified in molecule-surface heterogeneous catalysis are not directly transferrable to apply for solid-solid interfacial catalysis although some OER mechanisms are relevant. Based on first-principles calculations, Kim et al. recently revealed electron-withdrawing surface structures of PtTM(TM=Co, Ti) to have high activity and identified adsorption energies of Li 4


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and LiO2 as catalytic descriptor.32 However, it is not straightforward to predict that inexpensive compounds with high OER activity by this proposed descriptor. Very recently, we reported for the first time facet-dependent OER catalytic activity of Co3O4 by using first-principles calculations and found the Co3O4 octahedron with exposed (111) plane has the higher catalytic activity in reducing charging voltage than (110) and (001) planes.33 These calculations are in good agreement with experimental measurements.34-36 Interfacial electrocatalysis has become an attractive point in a rechargable Li-O2 battery research in order to imporve cyclic performance and reduce overpotential in OER. Many transition metal compounds (TMC: oxide, carbide, and nitride) have been studied as OER catalysts.17,25,26 Based on these experimental reports and as an extension for our previous Co3O4 computational studies, some TMC-catalyzed OER mechanisms are studied in this work in order to identify a descriptor of catalytic activity for OER in Li-O2 battery. These studies provide a fundamental base for designing a novel and highly active catalyst of OER in future.

2. Computational and Experimental Details 2.1 Computational Methods First-principles calculations in this work were conducted within the formalism of density functional theory (DFT) and the generalized gradient approximation (GGA) of the exchange-correlation function as formulated by Perdew, Burke, and Ernzerhof (PBE). The valence electron-ion interaction was modeled by the projector augmented wave (PAW) potential implemented in the Vienna ab initio simulation package (VASP).37,38 The plane wave basis set with a cut-off energy of 450eV was used. Electron correlation within the d 5



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states significantly affects the electronic structure and energetic properties of transition metal compounds. Therefore, the on-site Coulomb correlation effects for 3d orbitals of some transition metals were included in our calculations. According to the previous publications, the effective interaction strengths (Ueff = U – J) of different 3d orbitals were set as 2.5=2.5-0.0 for Ti atom in TiO, TiCl3, TiN, and TiC,39 4.0=5.0-1.0 for Mn atom in MnO and Mn2O3,40 6.4=7.4-1.0 for Ni atom in NiO,40 3.3=3.8-0.5 for Co atom in CoO,40 2.0=2.0-0.0 for Co atom in Co3O4,33 3.6=4.5-0.9 for Fe atom in Fe3O4,41 and 4.0=5.0-1.0 for Zn atom in ZnO,42 in which U is on-site Hubbard repulsion and J is Hund’s exchange interaction. The calculated total energies are insensitive to J when Ueff is fixed. Plain PBE (Ueff=0.0) was applied in Mo2C and MoS2 systems based on the previous publications.43,44 Our plain PBE and PBE+U test calculations based on these parameters were in fair agreement with experimental band gaps and bulk lattice parameters. The surface Brillouin zone was sample with the K-points generated by the Monkhorst-Pack scheme with a space less than 0.05 Å-1. The structures have been relaxed until a maximum force of

Surface Acidity as Descriptor of Catalytic Activity for Oxygen Evolution

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