Ni3+ Induced Hole States Enhance the Oxygen Evolution Reaction

Subscriber access provided by UNIVERSITY OF SASKATCHEWAN LIBRARY

Article 3+

Ni Induced Hole States Enhance the Oxygen Evolution Reaction Activity of NiCo O Electrocatalysts x

3-x

4

Meiyan Cui, Xingyu Ding, Xiaochun Huang, Zechao Shen, Tien-Lin Lee, Freddy E. Oropeza, Jan P. Hofmann, Emiel J.M. Hensen, and Kelvin H. L. Zhang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02453 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Ni3+ Induced Hole States Enhance the Oxygen Evolution Reaction Activity of NixCo3-xO4 Electrocatalysts Meiyan Cui,† Xingyu Ding,† Xiaochun Huang,† Zechao Shen,† Tien - Lin Lee,‡ Freddy E. Oropeza,*, § Jan P. Hofmann,§ Emiel J. M. Hensen,§ and Kelvin H. L. Zhang*, † †State

Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China ‡Diamond Light Source Ltd., Harwell Science and Innovation Campus, Didcot, OX11 0DE, United Kingdom §Laboratory of Inorganic Materials and Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands

ABSTRACT: This work reports a systematical study on the relationship of electronic structure to oxygen evolution reaction (OER) activity of NixCo3-xO4 (x=0-1) mixed oxides. The specific OER activity is substantially increased by 16 times from 0.02 mA cm-2BET for pure Co3O4 to 0.32 mA cm-2BET for x=1 at an overpotential of 0.4 V and exhibits a strong correlation with the amount of Ni ions in +3 oxidation state. X-ray spectroscopic study reveals that inclusion of Ni3+ ions upshifts the occupied valence band maximum (VBM) by 0.27 eV toward the Fermi level (EF), and creates a new hole (unoccupied) state located ~1 eV above the EF. Such electronic features favour the adsorption of OH surface intermediates on NixCo3-xO4, resulting in enhanced OER. Furthermore, the emerging hole state effectively reduces the energy barrier for electron transfer from 1.19 eV to 0.39 eV, and thereby improves the kinetics for OER. The electronic structure features that lead to a higher OER in NixCo3-xO4 can be extended to other transition metal oxides for rational design of highly active catalysts.

1

ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 20

INTRODUCTION Electrochemical water splitting to produce hydrogen and oxygen is a promising pathway for storage of intermittent renewable energy in the form of chemical fuels.1-3 Water splitting consists of two half reactions: (i) hydrogen evolution reaction (HER, 2H+(aq) + 2e- H H2(g)) and (ii) the oxygen evolution reaction (OER, 2H2O(l) H O2(g) + 4H+(aq) + 4e-). The change in the Gibbs free energy of the whole process corresponds to a standard potential of 1.23 V. However, in practice an additional overpotential (0.4-0.8V) is required to drive especially the OER because of its sluggish kinetics involving a complex four-electron/proton transfer process.4,5 Therefore, a highly active OER electrocatalyst is key to improve the energy efficiency of electrolyzers. Ru and Ir oxides are regarded as the most efficient OER catalysts, but their scarcity and instability hinder large-scale applications.6-8 These problems have stimulated a wide search for earth-abundant transition-metal (TM) oxides as OER electrocatalysts, mostly focused on Mn, Fe, Co and Ni.8-11 In order to develop highly active oxide catalysts, efforts to revealing the catalytic mechanisms and identification of the structural and electronic factors determining the activity are essential. The OER performance is intrinsically related to the electronic structure of transition metal oxide catalysts, because the electronic structure determines the adsorption strength of reaction intermediates.12,13 Therefore, many researchers have attempted to correlate the electronic structure to the OER activity. The electronic structure parameters used in such correlations include the number of electrons in the eg orbitals of TM cations (with eg=1.2 displaying the highest OER activity),14 the degree of TM 3d-O 2p hybridization,15 the energy position of the O 2p band center,16 as well as related structural parameters such as the surface-oxygen adsorption energy as proposed by Nørskov et al.,17 and lattice oxygen vacancies.18 Spinel Co3O4 is one of the most studied OER electrocatalysts because of its high activity and relative earth-abundance of Co.19-21 Extensive efforts have been made to improve its OER activity by fabricating nanostructures to increase the surface area,22,23 morphology control to expose more active crystal facets,24,25 and doping to modulate its electronic structure.26-28 In the spinel structure of Co3O4, the tetrahedral (Td) sites are occupied by Co2+ with e4t23 electron configuration, whereas the octahedral (Oh) sites are Co3+ with t2g6eg0 electron configuration (see Figure 1a).29 Since Td coordinated Co2+ and Oh coordinated Co3+ exhibit different electronic properties, which site is the active site in 2


Page 3 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the OER is an open question. For example, Chen et al. claimed that Td coordinated Co2+ is the main active site for OER due to the ease of formation of an active cobalt oxyhydroxide species.30 In contrast, Choi et al. proposed that Oh coordinated Co3+ is the active site as inferred from the result that ZnCo2O4 exhibits similar OER activity with Co3O4, despite the absence of Td coordinated Co2+ in ZnCo2O4.31 On the other hand, doping of Co3O4 with other TMs, e.g., Cr, Mn, Fe and Ni, has been shown to substantially improve the OER activity, but the underlying material chemistry may be more complex.32-36 To date, increased electrical conductivity and a modified electronic structure upon doping are the cause of the activity improvement.35,37,38 However, there is a lack of understanding on how the electronic structure is modulated by these dopants, and how the electronic structure influences the energetics of adsorbates and OER activity. Such insightful information on relations of composition, electronic structure and activity is of paramount importance for rational design of highly active electrocatalysts. In this work, we report a systematic study on the effect of Ni addition on the electronic structures of NixCo3-xO4 and its influence on OER activity. The evolution of the electronic structure of NixCo3-xO4 is investigated by a combination of X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), which provide information about the occupied and unoccupied density of states (DOS) near the Fermi level (EF), respectively. The OER activity of NixCo3-xO4 exhibits a strong correlation with the amount of Ni cations in +3 oxidation state. This correlation is a result of three important features in the electronic structure induced by Ni3+ relevant to the OER: (i) an upshift of the occupied valence band maximum (VBM); (ii) creation of a new hole (unoccupied) state at ~1.0 eV above the EF, and (iii) an increased hybridization of O 2p with Ni and Co 3d orbitals. The upshift of the VBM and emergence of a hole state reduce the energy cost for the dissociative adsorption of water to form OH, which is considered the rate-determining step (RDS) during the OER.39 Additionally, the emerging hole state reduces the energy barrier for electron transfer at the interface, and thereby facilitates a faster electron transfer from reaction intermediates to the catalyst.

EXPERIMENTAL SECTION 2.1. Material Synthesis. The NixCo3-xO4 powders were synthesized using a hydrothermal method. In a typical procedure, stoichiometric amounts of Co(NO3)2 6H2O (Alfa, 99%) and 3



Page 4 of 20

Ni(NO3)2 6H2O (Alfa, 98%) were dissolved in a mixture of 5 mL ethanol and 5 mL deionized (DI) water with 4 mmol metal ions in total. 1 g polyvinylpyrrolidone (PVP, K-30, Alfa) were added and stirred at least 30 minutes until clear, then 25 mL of 0.4 M NaOH aqueous solution were dropwise added slowly followed by stirring for 4 hours. The reaction suspension was quickly transferred into a Teflon-lined autoclave, then maintained at 120 °C for 10 hours. After cooling to room temperature, the product was centrifuged and washed with DI water for several times. The precursors were dried at 60 °C in a vacuum oven for 12 hours. The final materials were obtained by annealing the precursor at 450 °C for 2 hours in a tube furnace under atmospheric environment.

2.2. Characterizations. Phase identification was done by powder X-ray diffraction (XRD, Rigaku, Japan) with Cu

radiation ( =1.5418 Å) operating at 40 kV and 40 mA emission

current. Rietveld refinement was used to determine lattice parameters and the crystal structure by the GSAS software and EXPGUI interface. Sample morphology and microstructures were examined by Sigma field emission scanning electron microscopy (SEM, ZEISS, Germany) and Tecnai F20 Field Emission transmission electron microscopy (TEM, FEI, USA). The samples were dispersed in mixture solution of water/isopropanol with volume ratio of 3:1 by ultrasonication for 30 minutes to form a homogeneous ink and then dropped on a silicon slice for SEM and Cu grid for TEM. In order to exclude the interference of Nafion on HR-TEM of the sample after OER, Nafion solution was not added in the ink when the work electrode was prepared. The element contents of metal ions were determined by energy dispersive spectroscopy (EDS). The Brunauer-Emitter-Teller (BET) specific surface areas were obtained based on N2 adsorption/desorption isotherms at 77 K using ASAP 2020 (Micromeritics company, USA). For XPS and XAS measurements, the powders were pressed into pellets. XPS were measured using a monochromatic Al

1

X-ray ( =1486.6 eV) source at normal emission

(electron take-off angle = 90o relative to the surface plane) with a SPECS PHOIBOS 150 electron energy analyzer. The total energy resolution was about 0.50 eV. The binding energy (BE) was calibrated using a polycrystalline Au foil placed in electrical contact with the samples. XAS measurements were performed at the I09 Surface and Interface Structural Analysis beamline of the Diamond Light Source (DLS). The XAS spectra were taken in total electron yield (TEY) mode, with an effective resolution of 0.2 eV. 4


Page 5 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.3. Electrochemical Measurements. All electrochemical measurements were performed in a three-electrode system connected with glassy carbon rotating disk electrode (RDE) (Pine Research Instrumentation), using a CHI 660E electrochemical workstation. A Hg/HgO (1 M KOH) electrode was used as the reference electrode, and a graphite rod was used as the counter electrode. It has been shown that a small trace amount of Fe impurity in electrolyte can significantly enhance the OER activity of the Ni-based materials. In order to exclude the interference of Fe impurity, the KOH electrolyte was purified using a purification method reported before.40 The 1 M KOH electrolyte was purged with a flow of O2 gas to ensure its continued O2 saturation. 5 mg of the samples and 20 L 5% Nafion solution were put in 1 mL mixture of water/isopropanol with volume ratio of 3:1 and dispersed by ultrasonication for 60 minutes to form a homogeneous ink. After that, 10 L of the catalyst ink was dropped onto a glassy carbon electrode (GCE) with a diameter of 5 mm and dried in air atmosphere to form a catalyst mass loading of about 0.25 mg cm-2. Linear sweep voltammetry (LSV) polarization curves were acquired at a scan rate 10 mV s-1 with the potential from -0.1 to 0.8 V (vs. Hg/HgO), after 10 cycles of Cyclic voltammetry (CV) tests. The electrochemical impedance spectroscopy (EIS) tests were measured at a potential of 0.7 V (vs. Hg/HgO) in the frequency range of 100 kHz to 1 Hz. The potentials were converted to reversible hydrogen electrode (RHE) by using ERHE=EHg/HgO + 0.059 pH + 0.098 V.

RESULTS AND DISCUSSION 3.1. Crystal Structure and Chemical States in NixCo3-xO4. A series of NixCo3-xO4 (x=0, 0.2, 0.3, 0.5, 0.6, 0.7, 0.8, and 1.0) catalysts were synthesized by a hydrothermal method followed by annealing at 450 °C in air. Powder XRD patterns shown in Figure 1b and Figure S1 confirm that all NixCo3-xO4 samples have a spinel structure. A gradual shift of the diffraction peaks to lower angles with increasing Ni content was observed, due to expansion of the lattice parameters from 8.07 Å for Co3O4 to 8.10 Å for NiCo2O4. The evolution of lattice parameters is summarized in Figure 1c and Table S1. It should be pointed out that when Ni content is large than 1, NiO secondary phase forms, which has also been observed in ZnCo2-xNixO4 system 41. Further morphology characterizations (Figure 1d and Supplementary Figure S2) indicate that all samples have a similar 5



Page 6 of 20

hexagonal plate morphology with preferentially exposed {111} crystal facets. Nitrogen physisorption using the BET method was used to measure the surface area, indicating that the NixCo3-xO4 samples have similar surface areas in the range of 30-38 m2 g-1 (isotherms in Figure S3). The similar BET surface areas and morphology suggest the slight variation of surface area has little contribution for enhancing OER activities of NixCo3-xO4 discussed in the following. Figure 1e shows the Co 2p XPS for NixCo3-xO4. The spectrum for Co3O4 (black curve) contains the characteristic features of mixed Co2+ and Co3+. Ni incorporation does not induce appreciable change to the Co 2p spectra. On the other hand, the Ni 2p3/2 spectra shown in Figure 1f consist of complex features of multiple splitting, satellites and mixed Ni2+ and Ni3+ states. The two features marked as a and b in Figure 1f result from many-electron excitation processes during photoemission, and should not be assigned to Ni2+ and Ni3+.42 To determine the oxidation state of Ni, we compared the Ni 2p3/2 lineshapes with those from reference samples, i.e., NiO representing Ni2+ and LiNiO2 representing Ni3+. Detailed comparison suggests that when the Ni content is low (x < 0.6), the majority of Ni cations are present as Ni3+, while the percentage of Ni2+ starts increasing for x > 0.6. This trend is further supported by Ni L edge XAS in Figure S4, showing an increase of a double peak feature at the Ni L2 edge (870.2 eV) for x > 0.6, which is assigned to Ni2+. As will be discussed in the following, our results reveal that the electronic states associated with Ni3+ ions play a crucial role in the enhanced OER activity of NixCo3-xO4. 3.2. OER Performance of NixCo3-xO4. The OER activities of the NixCo3-xO4 were evaluated in a standard three-electrode setup in an O2-saturated 1 M KOH solution using a rotating disk electrode. Figure 2a shows the LSV curves of NixCo3-xO4 after 10 cycles of CV measurement. The LSV current density is normalized to the BET surface area in order to exclude influence of surface area on the intrinsic activity. A substantial increase of OER activity upon introduction of Ni is readily observed. Furthermore, as shown in Figure 2b, the Tafel slope for Co3O4 is 56 mV dec-1 and decreases to 48 mV dec-1 for Ni-containing compositions. Figure 2c plots Tafel slopes and the current densities at an overpotential of 0.4 V (i.e., specific OER activity) as a function of Ni content. It shows that for x < 0.6, the specific OER activities of NixCo3-xO4 are significantly improved by Ni addition, whereas a further increase is less pronounced for higher Ni content. We also performed EIS measurements to investigate the kinetics of charge transfer at the 6


Page 7 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrode/electrolyte interface. Figure 2d displays the Nyquist plots of the EIS at a potential of +1.63 V vs. RHE, which can be fitted using an equivalent circuit (shown in the inset) composed of electrolyte resistance (Rs), a charge-transfer resistance (Rct) with an electrical double-layer capacitance (Cdl), and a resistance (Ra) with a pseudo-capacitance (Ca) associated with the adsorption of species.43 The resistance and capacitance values are obtained and tabulated in Table S2. The Rct is reduced from 45 W for Co3O4 to 17 W for Ni0.3Co2.7O4, and 7 W for NiCo2O4, suggesting a significant increase of the charge-transfer kinetics for OER upon Ni incorporation. Additionally, high-resolution TEM images (Figure 2e and 2f) indicate the NiCo2O4 catalysts keep the crystalline structure without obvious amorphous phases formed at the surface region after 10 cycles of OER measurement, ruling out the factor of amorphization on the enhanced OER performance38. 3.3. Electronic Structures of NixCo3-xO4. The incorporation of Ni in Co3O4 induced significant changes in the DOS near the EF. The VB spectra of Co3O4 shown in Figure 3a consist of Co 3d states in the region of 0-3 eV (marked as A) and O 2p in 3-6 eV (marked as B).44 The VBM is determined to be at 0.4 eV below the EF. Ni incorporation leads to two changes in the VB spectra of NixCo3-xO4: (i) the VBM is shifted toward the EF due to hole doping by Ni3+ (details of the shift are summarized in Figure S5a and S5c), and (ii) the band width of feature A becomes broader, due to the inclusion of new Ni 3d states near the EF, which enhances the hybridization of O 2p with Ni and Co 3d. Figure 3b and Figure S5b show the O K edge XAS spectra of NixCo3-xO4. The O K edge probes the electronic transition from O 1s to unoccupied states with partial O 2p character and therefore can be qualitatively related to the unoccupied density of states above the EF.45,46 The O K edge XAS for Co3O4 (black curve) agrees well with data in literature: the feature centered at 530.5 eV (marked as C) corresponds to the unoccupied Co 3d hybridized with O 2p.47 With increasing Ni content, a new hole state at ~529 eV and a shoulder at 532 eV (marked as D) gradually increase. By comparing the spectra with these of NiO and LiNiO2, the hole state at ~529 eV is associated with the unoccupied eg state of Ni3+ (3d7) and feature D to Ni2+ (3d8), both hybridized with O 2p. To quantitatively measure the fraction of hole state as function of Ni content, we normalized the integrated spectral area of the hole state by the total areas of peaks C and 7



Page 8 of 20

D. The results are plotted in Figure 3c, showing that the percentage of hole state is considerably increased when x < 0.6, while gradually saturates at x > 0.6. Figure 3d (left panel) shows a direct comparison of the measured DOS near the EF of Co3O4 with NiCo2O4 clearly manifesting the broadening of the top of VB states and appearance of hole states. The substantial change of the electronic structure of Co3O4 upon Ni incorporation can be rationalized as follows. Co3O4 has a normal spinel structure, where Co2+ is located at Td sites and Co3+ at Oh sites, whereas NiCo2O4 has an inverse spinel structure, where ideally Ni2+ occupies Oh sites and Co3+ occupies both Oh and Td sites. However, Co3+ at Td sites is thermodynamically unstable and prone to convert to a more stable Co2+ state.48 This results in an oxidation of a similar amount of Ni2+ to Ni3+ in order to maintain charge neutrality. This change leads to a hole state with Ni 3d character and a shift of the EF closer to the VB. These synergistic interactions can substantially increase the electronic conductivity, and sometimes result in a metallic state in NiCo2O4.49,50 Indeed, as shown in Figure S6, the room temperature conductivity of our NixCo3-xO4 is dramatically increased from 10-5 S cm-1 for Co3O4 to 1 S cm-1 for samples with x > 0.6. 3.4. Correlation of the OER Activity with Ni3+ Induced Electronic Structure Changes. The OER can be described in the following four elementary steps (shown in right panel in Figure 3d).11,51 * + OH-(aq) H *OH + e-

1

*OH + OH-(aq) H *O + H2O(l) + e-

2

*O + OH-(aq) H *OOH + e-

3

*OOH + OH-(aq) H * + O2 (g) + H2O(l) + e-

4

where * represents the surface active sites and *OH, *O and *OOH represent adsorbed reaction intermediates, respectively. Our above results clearly demonstrate the incorporation of Ni3+ has major effects on the electronic structure near the EF of NixCo3-xO4. The electronic states determine the binding strength between catalysts with reaction intermediates and hence the reaction energy barriers and kinetics. Therefore, the electronic structure induced by Ni3+ is the key to explaining the enhanced OER activity 8


Page 9 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in NixCo3-xO4. As mentioned above, the hole state derives from strongly hybridized states of unoccupied Ni 3d eg of Ni3+ with O 2p. The density of this hole state is proportional to the Ni3+ content. We therefore plot the specific OER activity (current density at 1.63 V normalized by BET surface area) versus the hole state content in Figure 3e. It clearly shows that the specific OER activities of NixCo3-xO4 strongly correlates to the density of hole state with varying Ni content, i.e., the amount of Ni3+. Additionally, the OER activity also shows the similar linear relation with the shift of VBM shown in Figure S5d, further supporting the strong correlation with the hole state. These can be reasonably rationalized on the basis of experimentally measured occupied and unoccupied DOS. The hole doping associated with Ni3+ leads to a stronger hybridization between O 2p and Co/Ni 3d and an upshift of VB, i.e., O p-band center, towards to the EF (details of the shift are summarized in Figure S5a and S5c ).52,53 The increase of the DOS near the EF will enhance the orbital overlapping with adsorbed oxygen species, and thus decrease the energy penalty to accept/donate electrons at the adsorbate-catalyst interface reaction according to the Gerischer–Marcus model of charge transfer.54 This is in line with recent proposal that the TM 3d-O 2p covalency and O 2p band center are effective descriptors for OER activity of perovskite oxides.15,16,55 Furthermore, the formation of the hole state reduces the energy barrier for electron transfer associated with water oxidation. The conduction band minimum (CBM) for Co3O4 is located at ~ 3.65 eV relative to the vacuum level,56 resulting in a large barrier of 1.19 eV for electron transfer, i.e., the energy difference between CBM and OER potential (see Figure 3d). On the other hand, the new hole state in NixCo3-xO4 significantly reduces the energy barrier to 0.39 eV. This is in agreement with the much smaller EIS-determined charge-transfer resistance for NiCo2O4 (7 W< than that for Co3O4 (45 W< Nørskov et al. performed mechanistic studies using DFT+U calculations and found that adsorption of OH is the RDS of Co3O4 for the OER.39 On the other hand, Fu et al. recently found that NiO has a low OER activity compared to LiNiO2, because the formation of adosrbed OH is limited by a weak binding of OH on the NiO surface, whereas Ni3+ states induced by Li doping help to stabilize the adsorption of OH, thereby 9



Page 10 of 20

improving the kinetics for OER.53 We therefore propose that the presence of Ni3+ states in Co3O4 can enhance the adsorption of OH intermediate. An experimental indication for a favorable OH adsorption is found in a systematic increase of XPS peak intensity related to surface adsorbed OH species in the O 1s core level spectra with Ni content, as shown in Figure 4. The O 1s spectra in NixCo3-xO4 can be fitted with three components located at 529.8 eV, 531.0 eV and 532.5 eV. The former is assigned to lattice oxygen, and the latter two correspond to surface absorbed hydroxyl and surface C-O.57 Figure 4b plots the percentage of adsorbed hydroxyl as a function of Ni content (more details are shown in Figure S7 and Table S3). A relatively higher proportion (29%) of the peak at ~531.0 eV is observed compared with Co3O4 (23%), indicating an enhanced OH adsorption capacity upon Ni doping.58,59 This may lead to a change of the rate-determining step from OH adsorption to OOH formation, which is consistent with the reduced Tafel slope.

CONCLUSIONS In summary, we have demonstrated that the OER activity of NixCo3-xO4 exhibits a strong correlation with the presence of Ni3+. Combined XPS VB and XAS spectra reveal important insights in the electronic structure that provide favorable energetics for OER activity. The incorporation of Ni3+ ions upshifts the occupied VBM by 0.27 eV and increases the hybridization of O 2p with Ni 3d and Co 3d orbitals. Such electronic features enhance the adsorption of OH on NixCo3-xO4 for a fast OER, because the adsorption of OH is the rate-determining step during OER for Co3O4. Furthermore, the creation of an unoccupied hole state effectively reduces the energy barriers for electron transfer from 1.19 eV to 0.39 eV, and thereby facilitates OER kinetics. The electronic features governing high OER activity for NiCo2O4 can be extended to other TM oxides. Our study highlights the importance of an in-depth understanding of the electronic structure of TM oxides for rational material engineering needed in the design of highly active, earth-abundant OER catalysts.

ASSOCIATED CONTENT Supporting Information 10


Page 11 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. XRD patterns for all samples; Lattice parameters and Ni/Co ratios; SEM images; BET measurements; Ni L2,3 edge XAS spectra; The obtained impedance parameters by fitting the EIS data; VB XPS spectra and O K edge XAS spectra for all samples; Correlation among VBM shift, Ni content and OER activities normalized by BET; room temperature conductivity; and O 1s XPS spectra.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (F. E. O.). *E-mail: [email protected] (K. H. L. Z.). ORCID Freddy E. Oropeza: 0000-0001-7222-9603 Jan P. Hofmann: 0000-0002-5765-1096 Emiel J. M. Hensen: 0000-0002-9754-2417 Kelvin H. L. Zhang: 0000-0001-9352-6236

ACKNOWLEDGEMENTS K. H. L. Z. gratefully acknowledges the funding support by the Thousand Youth Talents Program and financial support by the National Natural Science Foundation of China (Grant no. 21872116). F. E. O., J. P. H. and E. J. M. H. acknowledge support from the Netherlands Center for Multiscale Catalytic Energy Conversion (MCEC), an NWO Gravitation program funded by the Ministry of Education, Culture and Science of the government of The Netherlands.

11



Page 12 of 20

REFERENCES (1) Stamenkovic, V. R.; Strmcnik, D.; Lopes, P. P.; Markovic, N. M. Energy and Fuels from Electrochemical Interfaces. Nat. Mater. 2016, 16, 57-69. (2) Zou, X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148-5180. (3) Fabbri, E.; Habereder, A.; Waltar, K.; Kotz, R.; Schmidt, T. J. Developments and Perspectives of Oxide-Based Catalysts for the Oxygen Evolution Reaction. Catal. Sci. Technol. 2014, 4, 3800-3821. (4) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110, 6474-6502. (5) Koper, M. T. M. Thermodynamic Theory of Multi-Electron Transfer Reactions: Implications for Electrocatalysis. J. Electroanal. Chem. 2011, 660, 254-260. (6) Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H.; Miller, E.; Jaramillo, T. F. Technical and Economic Feasibility of Centralized Facilities for Solar Hydrogen Production via Photocatalysis and Photoelectrochemistry. Energy Environ. Sci. 2013, 6, 1983-2002. (7) Reier, T.; Oezaslan, M.; Strasser, P. Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials. ACS Catal. 2012, 2, 1765-1772. (8) Suen, N. T.; Hung, S. F.; Quan, Q.; Zhang, N.; Xu, Y. J.; Chen, H. M. Electrocatalysis for the Oxygen Evolution Reaction: Recent Development and Future Perspectives. Chem. Soc. Rev. 2017, 46, 337-365. (9) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of Electrocatalysts for Oxygen- and Hydrogen-Involving Energy Conversion Reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. (10) Han, L.; Dong, S.; Wang, E. Transition-Metal (Co, Ni, and Fe)-Based Electrocatalysts for the Water Oxidation Reaction. Adv. Mater. 2016, 28, 9266-9291. (11) Lu, F.; Zhou, M.; Zhou, Y.; Zeng, X. First-Row Transition Metal Based Catalysts for the Oxygen Evolution Reaction under Alkaline Conditions: Basic Principles and Recent Advances. Small 2017, 13, 1701931. (12) Hammer, B.; Nørskov, J. K. Theoretical Surface Science and Catalysis–Calculations and Concepts. Adv. Catal. 2000, 45, 71-129. (13) Nørskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Towards the Computational Design of Solid Catalysts. Nat. Chem. 2009, 1, 37-46. (14) Man, I. C.; Su, H. Y.; Calle-Vallejo, F.; Hansen, H. A.; Martinez, J. I.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Nørskov, J. K.; Rossmeisl, J. Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces. Chemcatchem 2011, 3, 1159-1165. (15) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383-1385. (16) Hong, W. T.; Stoerzinger, K. A.; Lee, Y.-L.; Giordano, L.; Grimaud, A.; Johnson, A. M.; Hwang, J.; Crumlin, E. J.; Yang, W.; Shao-Horn, Y. Charge-Transfer-Energy-Dependent Oxygen Evolution Reaction Mechanisms for Perovskite Oxides. Energy Environ. Sci. 2017, 10, 2190-2200. (17) Grimaud, A.; May, K. J.; Carlton, C. E.; Lee, Y.-L.; Risch, M.; Hong, W. T.; Zhou, J.; Shao-Horn, Y. Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution. Nat. Commun. 2013, 4, 2439. 12


Page 13 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18) Grimaud, A.; Diaz-Morales, O.; Han, B.; Hong, W. T.; Lee, Y. L.; Giordano, L.; Stoerzinger, K. A.; Koper, M. T. M.; Shao-Horn, Y. Activating Lattice Oxygen Redox Reactions in Metal Oxides to Catalyse Oxygen Evolution. Nat. Chem. 2017, 9, 457-465. (19) Li, Y.; Li, F. M.; Meng, X. Y.; Li, S. N.; Zeng, J. H.; Chen, Y. Ultrathin Co3O4 Nanomeshes for the Oxygen Evolution Reaction. Acs Catal. 2018, 8, 1913-1920. (20) Ma, L.; Hung, S. F.; Zhang, L. P.; Cai, W. Z.; Yang, H. B.; Chen, H. M.; Liu, B. High Spin State Promotes Water Oxidation Catalysis at Neutral pH in Spinel Cobalt Oxide. Ind. Eng. Chem. Res. 2018, 57, 1441-1445. (21) Xu, L.; Jiang, Q.; Xiao, Z.; Li, X.; Huo, J.; Wang, S.; Dai, L. Plasma-Engraved Co3O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction. Angew. Chem. Int. Ed. 2016, 55, 5277-5281. (22) Sidhureddy, B.; Dondapati, J. S.; Chen, A. Shape-Controlled Synthesis of Co3O4 for Enhanced Electrocatalysis of the Oxygen Evolution Reaction. Chem. Commun. 2019, 55, 3626-3629. (23) Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Metal-Organic Framework Derived Hybrid Co3O4-Carbon Porous Nanowire Arrays as Reversible Oxygen Evolution Electrodes. J. Am. Chem. Soc. 2014, 136, 13925-13931. (24) Zhou, X.; Liu, Z.; Wang, Y.; Ding, Y. Facet Effect of Co3O4 Nanocrystals on Visible-Light Driven Water Oxidation. Appl. Catal. B- Environ. 2018, 237, 74-84. (25) Liu, L.; Jiang, Z.; Fang, L.; Xu, H.; Zhang, H.; Gu, X.; Wang, Y. Probing the Crystal Plane Effect of Co3O4 for Enhanced Electrocatalytic Performance toward Efficient Overall Water Splitting. ACS Appl. Mater. Interfaces 2017, 9, 27736-27744. (26) Wang, T.; Sun, Y.; Zhou, Y.; Sun, S.; Hu, X.; Dai, Y.; Xi, S.; Du, Y.; Yang, Y.; Xu, Z. J. Identifying Influential Parameters of Octahedrally Coordinated Cations in Spinel ZnMnxCo2-xO4 Oxides for the Oxidation Reaction. ACS Catal. 2018, 8, 8568-8577. (27) Wang, Z. C.; Liu, H. L.; Ge, R. X.; Ren, X.; Ren, J.; Yang, D. J.; Zhang, L. X.; Sun, X. P. Phosphorus-Doped Co3O4 Nanowire Array: A Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting. ACS Catal. 2018, 8, 2236-2241. (28) Ramsundar, R. M.; Pillai, V. K.; Joy, P. A. Spin State Engineered ZnxCo3-xO4 as an Efficient Oxygen Evolution Electrocatalyst. Phys. Chem. Chem. Phys. 2018, 20, 29452-29461. (29) Sun, S.; Sun, Y.; Zhou, Y.; Xi, S.; Ren, X.; Huang, B.; Liao, H.; Wang, L. P.; Du, Y.; Xu, Z. J., Shifting Oxygen Charge Towards Octahedral Metal: A Way to Promote Water Oxidation on Cobalt Spinel Oxides. Angew. Chem. Int. Ed. 2019, 58, 1-7. (30) Wang, H. Y.; Hung, S. F.; Chen, H. Y.; Chan, T. S.; Chen, H. M.; Liu, B. In Operando Identification of Geometrical-Site-Dependent Water Oxidation Activity of Spinel Co3O4. J. Am. Chem. Soc. 2016, 138, 36-39. (31) Kim, T. W.; Woo, M. A.; Regis, M.; Choi, K. S. Electrochemical Synthesis of Spinel Type ZnCo2O4 Electrodes for Use as Oxygen Evolution Reaction Catalysts. J. Phys. Chem. Lett. 2014, 5, 2370-2374. (32) Lin, C.-C.; McCrory, C. C. L. Effect of Chromium Doping on Electrochemical Water Oxidation Activity by Co3-xCrxO4 Spinel Catalysts. ACS Catal. 2016, 7, 443-451. (33) Hirai, S.; Yagi, S.; Seno, A.; Fujioka, M.; Ohno, T.; Matsuda, T. Enhancement of the Oxygen Evolution Reaction in Mn3+-Based Electrocatalysts: Correlation Between Jahn–Teller Distortion and Catalytic Activity. RSC Adv. 2016, 6, 2019-2023. (34) Song, W.; Ren, Z.; Chen, S. Y.; Meng, Y.; Biswas, S.; Nandi, P.; Elsen, H. A.; Gao, P. X.; Suib, S. L. Ni-

and

Mn-Promoted

Mesoporous

Co3O4:

A

Stable

Bifunctional

Catalyst

with 13



Page 14 of 20

Surface-Structure-Dependent Activity for Oxygen Reduction Reaction and Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 2016, 8, 20802-20813. (35) Antony, R. P.; Satpati, A. K.; Bhattacharyya, K.; Jagatap, B. N. MOF Derived Nonstoichiometric NixCo/[8O=[ Nanocage for Superior Electrocatalytic Oxygen Evolution. Adv. Mater. Interfaces 2016, 3, 1600632. (36) Liu, X.; Chang, Z.; Luo, L.; Xu, T.; Lei, X.; Liu, J.; Sun, X. Hierarchical ZnxCo3–xO4 Nanoarrays with High Activity for Electrocatalytic Oxygen Evolution. Chem. Mater. 2014, 26, 1889-1895. (37) Li, Y.; Hasin, P.; Wu, Y. NixCo3-xO4 Nanowire Arrays for Electrocatalytic Oxygen Evolution. Adv. Mater. 2010, 22, 1926-1929. (38) Abidat, I.; Morais, C.; Comminges, C.; Canaff, C.; Rousseau, J.; Guignard, N.; Napporn, T. W.; Habrioux, A.; Kokoh, K. B. Three Dimensionally Ordered Mesoporous Hydroxylated NixCo/[8O4 Spinels for the Oxygen Evolution Reaction: On the Hydroxyl-Induced Surface Restructuring Effect. J. Mater. Chem. A 2017, 5, 7173-7183. (39) García-Mota, M.; Bajdich, M.; Viswanathan, V.; Vojvodic, A.; Bell, A. T.; Nørskov, J. K. Importance of Correlation in Determining Electrocatalytic Oxygen Evolution Activity on Cobalt Oxides. J. Phys. Chem. C 2012, 116, 21077-21082. (40) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. Nickel-Iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: The Role of Intentional and Incidental Iron Incorporation. J. Am. Chem. Soc. 2014, 136, 6744-6753. (41) Duan, Y.; Sun, S.; Sun, Y.; Xi, S.; Chi, X.; Zhang, Q.; Ren, X.; Wang, J.; Ong, S. J. H.; Du, Y.; Gu, L.; Grimaud, A.; Xu, Z. J. Mastering Surface Reconstruction of Metastable Spinel Oxides for Better Water Oxidation. Adv. Mater. 2019, 1807898. (42) Zhang, J. Y.; Li, W. W.; Hoye, R. L. Z.; MacManus-Driscoll, J. L.; Budde, M.; Bierwagen, O.; Wang, L.; Du, Y.; Wahila, M. J.; Piper, L. F. J.; Lee, T. L.; Edwards, H. J.; Dhanak, V. R.; Zhang, K. H. L. Electronic and Transport Properties of Li-Doped NiO Epitaxial Thin Films. J. Mater. Chem. C 2018, 6, 2275-2282. (43) Lai, Y.; Li, Y.; Jiang, L.; Xu, W.; Lv, X.; Li, J.; Liu, Y. Electrochemical Behaviors of Co-Deposited Pb/Pb–MnO2 Composite Anode in Sulfuric Acid Solution – Tafel and EIS Investigations. J. Electroanal. Chem. 2012, 671, 16-23. (44) Qiao, L.; Xiao, H. Y.; Meyer, H. M.; Sun, J. N.; Rouleau, C. M.; Puretzky, A. A.; Geohegan, D. B.; Ivanov, I. N.; Yoon, M.; Weber, W. J.; Biegalski, M. D. Nature of the Band Gap and Origin of the Electro-/Photo-Activity of Co3O4. J. Mater. Chem. C 2013, 1, 4628-4633. (45) Zhang, K. H. L.; Du, Y.; Sushko, P. V.; Bowden, M. E.; Shutthanandan, V.; Sallis, S.; Piper, L. F. J.; Chambers, S. A. Hole-Induced Insulator-to-Metal Transition in La1[8SrxCrO3 Epitaxial Films. Phys. Rev. B 2015, 91, 155129. (46) Zhang, K. H. L.; Du, Y.; Papadogianni, A.; Bierwagen, O.; Sallis, S.; Piper, L. F.; Bowden, M. E.; Shutthanandan, V.; Sushko, P. V.; Chambers, S. A. Perovskite Sr-Doped LaCrO3 as a New P-Type Transparent Conducting Oxide. Adv. Mater. 2015, 27, 5191-5195. (47) Van Elp, J.; Wieland, J. L.; Eskes, H.; Kuiper, P.; Sawatzky, G. A.; De Groot, F. M. F.; Turner, T. S. Electronic Structure of CoO, Li-Doped CoO, and LiCoO2. Phys. Rev. B 1991, 44, 6090-6103. (48) Bitla, Y.; Chin, Y. Y.; Lin, J. C.; Van, C. N.; Liu, R.; Zhu, Y.; Liu, H. J.; Zhan, Q.; Lin, H. J.; Chen, C. T.; Chu, Y. H.; He, Q. Origin of Metallic Behavior in NiCo2O4 Ferrimagnet. Sci. Rep. 2015, 5, 15201. (49) Perkins, J. D.; Paudel, T. R.; Zakutayev, A.; Ndione, P. F.; Parilla, P. A.; Young, D. L.; Lany, S.; Ginley, D. S.; Zunger, A.; Perry, N. H.; Tang, Y.; Grayson, M.; Mason, T. O.; Bettinger, J. S.; Shi, Y.; 14


Page 15 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Toney, M. F. Inverse Design Approach to Hole Doping in Ternary Oxides: Enhancing P-Type Conductivity in Cobalt Oxide Spinels. Phys. Rev. B 2011, 84, 205207. (50) Silwal, P.; Miao, L.; Stern, I.; Zhou, X.; Hu, J.; Ho Kim, D. Metal Insulator Transition with Ferrimagnetic Order in Epitaxial Thin Films of Spinel NiCo2O4. Appl. Phys. Lett. 2012, 100, 032102. (51) N. - T. Suen; S.- F. Hung; Q. Quan; N. Zhang; Y.- J. Xu; Chen, H. M. Electrocatalysis for the Oxygen Evolution Reaction: Recent Development and Future Perspectives. Chem. Soc. Rev. 2017, 46, 337-365. (52) Zhang, K. H.; Xi, K.; Blamire, M. G.; Egdell, R. G. P-Type Transparent Conducting Oxides. J. Phys.: Condens. Matter. 2016, 28, 383002. (53) Fu, G.; Wen, X.; Xi, S.; Chen, Z.; Li, W.; Zhang, J.-Y.; Tadich, A.; Wu, R.; Qi, D.-C.; Du, Y.; Cheng, J.; Zhang, K. H. L. Tuning the Electronic Structure of NiO via Li Doping for the Fast Oxygen Evolution Reaction. Chem. Mater. 2018, 31, 419-428. (54) Heller, I.; Kong, J.; Williams, K. A.; Dekker, C.; Lemay, S. G. Electrochemistry at Single-Walled Carbon Nanotubes: the Role of Band Structure and Quantum Capacitance. J. Am. Chem. Soc. 2006, 128, 7353-7359. (55) Wei, C.; Feng, Z.; Scherer, G. G.; Barber, J.; Shao-Horn, Y.; Xu, Z. J. Cations in Octahedral Sites: A Descriptor for Oxygen Electrocatalysis on Transition-Metal Spinels. Adv Mater 2017, 29, 1606800. (56) Miura, A.; Uraoka, Y.; Fuyuki, T.; Yoshii, S.; Yamashita, I. Floating Nanodot Gate Memory Fabrication with Biomineralized Nanodot as Charge Storage Node. J. Appl. Phys. 2008, 103, 074503. (57) Marco, J. F.; Gancedo, J. R.; Ortiz, J.; Gautier, J. L. Characterization of the Spinel-Related Oxides NixCo/[8O4 (x=0.3, 1.3, 1.8) Prepared by Spray Pyrolysis at 350 °C. Appl. Surf. Sci. 2004, 227, 175-186. (58) Stoerzinger, K. A.; Wang, X. R.; Hwang, J.; Rao, R. R.; Hong, W. T.; Rouleau, C. M.; Lee, D.; Yu, Y.; Crumlin, E. J.; Shao-Horn, Y. Speciation and Electronic Structure of La1[8SrxCoO/[] During Oxygen Electrolysis. Top. Catal. 2018, 61, 2161-2174. (59) Stoerzinger, K. A.; Du, Y.; Ihm, K.; Zhang, K. H. L.; Cai, J.; Diulus, J. T.; Frederick, R. T.; Herman, G. S.; Crumlin, E. J.; Chambers, S. A. Impact of Sr-Incorporation on Cr Oxidation and Water Dissociation in La(1-x )SrxCrO3. Adv. Mater. Interfaces 2018, 1701363.

15




Page 16 of 20

Page 17 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60





Page 18 of 20

Page 19 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60





Page 20 of 20

Ni3+ Induced Hole States Enhance the Oxygen Evolution Reaction

Recommend Documents