Distorted Inverse Spinel Nickel Cobaltite Grown on MoS2 Plate for

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Distorted Inverse Spinel Nickel Cobaltite Grown on MoS2 Plate for Significantly Improved Water Splitting Activity Jiangtian Li, Deryn Chu, David R. Baker, Hong Dong, Rongzhong Jiang, and Dat T. Tran Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02397 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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Chemistry of Materials

Distorted Inverse Spinel Nickel Cobaltite Grown on MoS2 Plate for Significantly Improved Water Splitting Activity Jiangtian Li,* Deryn Chu,* David R. Baker, Hong Dong, Rongzhong Jiang, and Dat T. Tran a

Sensors and Electron Devices Directorate, US Army Research Laboratory, 2800 Powder Mill Road, Adelphi 20783, Maryland * Corresponding Authors. Tel: 301-394-0308; Fax: 301-394-0273.

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Abstract Sluggish water dissociation kinetics on nonprecious metal oxide electrocatalysts are considered as the rate-limiting step for the development of hydrogen evolution in alkaline media. A unique heterostructure, a distorted inverse spinel Ni-Co-O layer on MoS2 plate, was developed in this paper. The formation of Mo-O-Co covalent bonds at the interface allows for the inherent electron transfer from MoS2 to adjacent surface spinel, and forces the nucleation and creation of disordered inverse spinel. Such a crystal-distorted structure enables the substantially improved activity for hydrogen evolution reaction (HER) by more than 40 times, and oxygen evolution reaction (OER) by 2.5 times compared to the regular spinel nickel cobaltite. Through a comprehensive kinetics study, the results reveal that this inverse spinel layer promotes the water dissociation process and the intrinsic HER activity by decreasing the apparent activation energy barrier Ea, increasing the exchange current density j0, and improving the charge transfer rate. Meanwhile, the oxygen vacancy mediated Ni3+ active center and the facilitated charge transfer are responsible for OER activity improvement. Such a unique heterostructure demonstrates robust stability for HER, OER and overall water splitting in alkaline electrolytes. Our work provides a novel concept to design effective metal oxide based HER and OER catalysts in alkaline electrolyte.


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Introduction Tremendous research efforts have been devoted to explore efficient catalysts for hydrogen and oxygen evolution by splitting water.[1-10] Fundamentally, hydrogen evolution in alkaline media undergoes two steps: the first step is the Volmer step for an initial formation of hydrogen intermediate (H*), where the catalyst accepts one electron and cleaves the adsorbed H2O molecule into a hydroxyl ion (OH-) and an adsorbed hydrogen atom (H*), i.e., H2O + e- + * → H* + OH- (* denotes a site on the electrode surface); then the hydrogen intermediates detach to generate H2 via either the electrochemical interaction between H* and H2O molecule with the assistance of one electron (Heyrovsky step, H2O + e- + H* → H2 + OH-) or the chemical recombination reaction of two H* (Tafel step, H* + H* → H2). [1-5] As a result, HER in alkaline starts with the break of H-O-H bond to generate hydrogen intermediates, which requires extra energy and thus leads to the sluggish kinetics when compared to that in acidic media. Promoting water dissociation process is crucial to boost HER catalytic activity of electrocatalysts in alkaline media. Heterostructured catalysts, which are composed of two or more active components, exhibit improved activity for HER, OER, and even overall water splitting compared to their unitary components.[3,10-20] It was widely accepted that the newly created interface between each component offers more functional sites for the expected reactions owing to the modified electronic and/or crystal structure on the vicinity of the interface.[10,11,19] A spontaneous electron transfer occurring at the heterojunction has been observed to enhance the water splitting catalytic activity by enhancing the intrinsic conductivity and increasing the active center.[20] Although numerous successful heterostructured catalysts demonstrating the enhanced performances for water splitting, challenges are remaining from the perspectives of design principles, electronic structure, and reaction mechanisms and kinetics.[3] Regularly, heterostructured catalysts still display limited catalytic performance improvement as their active sites are merely localized along the exposed interfaces of the two components. [3,10,11,19] It would be more desirable to have a heterostructure that can exhibit enhanced activity over the full exposed surface. More importantly, the key contribution to a promotion from which component in a heterostructure has not been identified so far, therefore design principles on this class of electrocatalysts have not been established. It is worthwhile to have a systematic study to comprehensively understand the reaction kinetics over a heterostructured catalyst.



In this paper, we integrated NiCo2O4 and MoS2 into a heterostructure, and fabricated a unique distorted inverse spinel Ni-Co-O layer on MoS2 plate. NiCo2O4 has been identified as a potential OER electrocatalyst due to its good electrical conductivity, low overpotential and high corrosion resistivity in alkaline solutions. [21-25] MoS2 was selected because it is well known as effective HER catalyst. [14,19,26-28] Both of them are earth abundant, and cost effective, which is of great importance from the practical perspective.[2,7,20] Figure 1 depicts the fabrication process of the heterostructure proposed in this study. Carbon fiber paper was used as the substrate to provide good electrical conductivity and high surface area for the potential growth of active electrocatalysts. The fabrication was carried out with hydrothermal processes; please see experimental details in the supporting information Figure S1. In the resultant heterostructure, an intimate interaction between MoS2 and Ni-Co-O was created by the in-situ formation of Mo-O-Co covalent bonds. The inherent electron shift from MoS2 to adjacent Ni-Co-O results in the nucleation of the distorted inverse spinel Ni-Co-O layer, which grew and finally fully covered the MoS2 plate substrate. Such a unique distorted inverse spinel Ni-Co-O layer, activated over the full basal plane, displays the bifunctionality for the improved activity on both HER and OER, and demonstrated overall water splitting. A reactivity increase as high as of 40 times achieved for HER activity when compared to the regular spinel structure in alkaline electrolyte. This study offers opportunities to design effective metal oxide based electrocatalysts, especially HER, in alkaline electrolyte.


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Figure 1. Scheme for the fabrication of the heterostructured NiCo2O4@MoS2 electrocatalysts. (a) shows the synthesis process with hydrothermal methodology. MoS2 was first fabricated on carbon paper and then employed as substrate for spinel NiCo2O4 growth. (b) illustrates the microstructure and crystal structure at the interface and surface of this heterostructure. A distorted spinel Ni-CoO created on MoS2 plate with covalent bonds. Please note the branched NiCo2O4 structure was omitted in this scheme to simplify the illustration. Experimental Fabrication of MoS2 on Carbon paper. MoS2 was deposited on carbon paper with a hydrothermal process. [21] Regularly, 1.5 mmol of Na2MoO4·2H2O and 4.8mmol of CH4N2S were dissolved in 90 mL of DI water as the MoS2 precursor solution, and was poured into a Teflon liner. A piece of carbon paper (3cm×6cm) was then immersed in. The autoclave was sealed and maintained at 200oC overnight. After the Teflon liner cooled down to the room temperature, the carbon paper with MoS2 was repeatedly washed with DI water and ethanol. Fabrication of NiCo2O4@MoS2 on Carbon paper. The NiCo2O4 electrocatalysts were prepared with a modified hydrothermal process. [21,24] NiCo2O4 precursor solution was prepared by dissolving 0.28 g of Ni(NO3)2·6H2O, 0.62 g of Co(NO3)2·6H2O, and 0.28 g of Urea in 90 mL of DI water to yield a pink solution. A hydrothermal process was then employed to grow NiCo2O4 on Carbon paper or as-prepared MoS2. The growth temperature was controlled at 120oC for 2 hours. The as-obtained samples were annealed in Ar for 2 hours at 350oC. Characterizations. The microstructure and crystal structures were determined with X-ray diffraction (XRD) and Scanning Electronic Microscopy (SEM). XRD was conducted on a diffractometer (Rigaku Flex600) from 10 to 70° with a scan rate of 20° per minute and a step interval of 0.02° upon Cu Kα radiation (λ=1.54 Å). SEM for observing microstructures was acquired on a Zeiss Auriga 60 Field Emission Scanning Microscope with an accelerating voltage of 10 KeV. Transmission electron microscope (TEM) and scanning transmission electron microscope (STEM) images were obtained with a high-resolution JEOL 2010 FE instrument at 200 KeV equipped with an EDAX X-ray detector and a high angle annular dark field (HAADF) detector. XPS measurements were carried out on a Physical Electronics VersaProbe III with a monochromatic Al-Kα source energy of 1486.6 eV. The analyzer pass energy was set to 224 eV for survey scans and 55 eV for high-resolution scans, respectively. MultiPak V9 software with functions of both Shirley background and Gaussian peak fits was employed for peak fitting of high-resolution spectra. All spectra were calibrated to the adventitious C 1s peak to 284.8 eV. 5 ACS Paragon Plus Environment


Electrochemistry for hydrogen/oxygen evolution. All electrochemical testing was performed on a Gamry Potentiostat Reference 3000. Regularly, three-electrode configuration cell was employed with catalysts as the working electrode, Ag/AgCl as the reference electrode, and Pt wire as the counter electrode. N2 purged 1 M NaOH aqueous solution was used as the electrolyte. Before hydrogen evolution evaluation, the electrode was maintained at -1.5 V vs Ag/AgCl for 30mins to clean the surface and reach a steady current density. Whereas for oxygen evolution testing, cyclic voltammetry was performed for 30 cycles within a potential window 0V~0.6 V vs Ag/AgCl and scanning rate of 20 mV/s. The measured potential vs Ag/AgCl (EAg/AgCl) was converted to reverse hydrogen electrode potential (ERHE) based on the Nernst equation, ERHE=EAg/AgCl + 0.059pH + E0, where E0=0.197 V at 25oC. Fabrication of Pt/C electrodes. 40% Pt/C particle was dispersed in propanol and water with a volume ratio 1:4, and 5% Nafion was added. The ink was sonicated for 30 mins, and then dropped onto the cleaned carbon paper with a loading level at 50 µg/cm2. The HER test was same as other catalysts. Overall Water Splitting. The overall water splitting was tested with a two-electrode configuration employing heterostructured NiCo2O4@MoS2 catalysts as both the cathode and the anode in 1M NaOH aqueous electrolyte. Results and Discussions Microstructures of NiCo2O4@MoS2 Heterostructure. As shown in Figs. 1 and S1, unitary MoS2 and NiCo2O4 were separately fabricated on carbon paper with a hydrothermal methodology, whereas the NiCo2O4@MoS2 heterostructure was prepared by growing NiCo2O4 on the asprepared MoS2/Carbon paper with the same conditions. Ni-Co hydroxides formed during hydrothermal process, and then transferred to Ni-Co-O during the subsequent annealing at 350oC for 2 hours. [21] As demonstrated on XRD patterns (Fig. 2), diffraction peaks located at 31.1o, 36.7o, 44.6o, 59.1o, and 65o in NiCo2O4 are assigned to be a spinel crystal structure, corresponding to JCPDS# 20-0781; while MoS2 can be identified with peaks at 14.4o, 32.7o, 39.5o, and 58.3o, corresponding to JCPDS# 37-1492. Feature peaks for MoS2 clearly appear in NiCo2O4@MoS2, but with very weak peaks for NiCo2O4 being detectable. This will be further discussed in TEM and XPS sections. Except for diffraction peaks due to the carbon fiber there are no peaks for impurities in this heterostructured catalyst. 6 ACS Paragon Plus Environment

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Figure 2. X-ray diffraction patterns for MoS2, NiCo2O4 and NiCo2O4@MoS2 samples referred to carbon paper. The strong diffraction peak between 20-30o coming from carbon paper was deducted with a break. The strips represent the standard patterns for JCPDS# 20-0781 (NiCo2O4) and JCPDS# 37-1492 (MoS2), respectively. Scanning electron microscopy was used to acquire morphologies of the as-obtained samples, as shown in Figs. 3 and S1. Nanowire-like NiCo2O4 aligned radially on carbon fiber with a diameter of around 200 nm (Fig.3 a and d). For MoS2 sample (Fig.3 b and e), microballs composed of MoS2 nanoplates uniformly and densely covered on carbon fibers. When the as-prepared MoS2@carbon paper was used as substrate for the further growth of NiCo2O4, NiCo2O4 branches uniformly grew from the inner ball to yield the NiCo2O4@MoS2 heterostructure (Fig.3 c, f and Fig. S1c,d).



Figure 3. SEM images for NiCo2O4 (a, d), MoS2 (b, e) and NiCo2O4@MoS2 (c, f) samples with different magnifications, respectively. The insets in (d-f) represent the schemed geometry of the corresponding samples. Transmission electron microscopy (TEM) was used to further elucidate the detailed microstructures (Figs. 4 and S2). Different to nanowire-like NiCo2O4 on carbon fiber, NiCo2O4 branches in the heterostructure apparently exhibit a hollow tube structure. A close observation (b and c) on one branch shows that the thickness of the NiCo2O4 tube wall is around 10 nm. This hollow structure offers high surface area and diffusion paths for electrolyte, which will be beneficial to the electrochemical catalytic activity. High resolution TEM (HRTEM) images (Figs. 4d, e and S2) reveal that the walls of these nanotubes are highly crystalline, and the lattice fringes of 0.28nm correspond to the (220) plane of spinel NiCo2O4. Element distribution mapping was applied to determine the distribution of each element, as shown in Fig. 4f. As expected, Mo and S are only dispersed at the core area, and branches are dominated with uniformly distributed Ni, Co and O. It is noted that apparent conformal overlap of Ni, Co and O with Mo and S occur on the core area. This implies that a NiCo2O4 layer might also form on MoS2 plate, then act as nucleation spots for hollow nanotube branches.


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Figure 4. Microstructure for NiCo2O4@MoS2 heterostructures. (a and b) TEM images for NiCo2O4@MoS2 heterostructure ball with different magnifications. (c and d) TEM images for hollow NiCo2O4 tube. (e) is the HRTEM image for the NiCo2O4 tube wall. (f) shows the element mapping for NiCo2O4@MoS2 heterostructures. (g) is the HRTEM of layered MoS2 plate on the MoS2 ball. (h) is the HRTEM image for the heterojunction between NiCo2O4 and MoS2. (i) illustrates the structure where NiCo2O4 grew and covered on MoS2 plates. Please note hollow tube NiCo2O4 structure was omitted in this scheme to simplify the illustration.



To confirm this hypothesis, a zoomed area for both surface and interface was investigated, as shown in Fig. 4g-i. One apparent difference is that the TEM electronic beam transparency is much poorer in heterostructured NiCo2O4@MoS2, compared to original MoS2 ball (as shown in Fig. S2a). MoS2 plate has a distinguished layered structure (Fig. 4g) with a layer distance of around 0.79 nm. This agrees well with the published literature. [29-32] In heterostructured NiCo2O4@MoS2 (Fig. 4h, S2d), MoS2 plate was surrounded by highly crystallized NiCo2O4 particles. The distance between MoS2 layers was determined to be 0.78nm, which is almost unchanged compared to the original MoS2. This agrees well with the XRD pattern, where the diffraction peak (002) of MoS2 remain unshifted after coating NiCo2O4. Please note it is difficult to isolate the layered structure of MoS2 plate in heterostructure, which mainly is due to a full coverage of MoS2 by the NiCo2O4 layer. EDS linear scanning was also performed as shown in Figure S2e, where Mo, S, Ni and Co all could be detected, but MoS2 is definitely dominant over Ni-Co-O. This is partially due to the fact that the spinel layer on MoS2 is very thin as shown in Figure 4h, and also probably the deep space within the MoS2 ball is not fully available for the growth of Ni-Co-O spinel layer. To sum up, when MoS2 plates were used as templates (Fig. 4i), NiCo2O4 nucleated on the surface to form a layer that fully covered the MoS2 plate and then subsequently promoted the formation and growth of hollow tube structure. The surface chemical compositions and chemical bonding states were then evaluated by X-ray photoelectron spectroscopy (XPS). From the survey spectra (Fig. S3), Mo, S and C show up in MoS2 sample, Ni, Co, O and C dominate in NiCo2O4 nanowires, and all features for both NiCo2O4 and MoS2 can be observed in NiCo2O4@MoS2 heterostructures. The high-resolution scanning for each element were shown in Fig. 5, where the spectra on bottom represent the heterostructure and the top traces correspond to the unitary counterparts MoS2 and NiCo2O4, respectively. Peak positions were listed in Table S1. There exist significant changes after the formation of this heterostructure. For NiCo2O4 nanowires alone (Fig. 5a, b), both Ni 2p3/2 and Co 2p3/2 consist of two spin-orbit doublets characteristics of +2 and +3. The O1s spectra are commonly deconvoluted into three contributions (as demonstrated in Fig. 5d), denoted as OI, OII, and OIII, which are associate with the typical lattice metal-oxygen bond (shaded with brown), low coordinated oxygen (shaded with green), and the physi-/chemisorbed water on the surface (shaded with purple), respectively. [22, 33-35] These results are consistent with the reported NiCo2O4 structures, and reveals that NiCo2O4 surface contains Co2+, Co3+, Ni2+ and Ni3+ species, which enables multiple 10 ACS Paragon Plus Environment

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redox couples and affords rich electrochemical active sites for catalytic reactions. [21, 36-38] For the original MoS2 plate, three peaks were distinguished for Mo (Fig. 5c). The two main peaks located at 228.99 eV and 229.73 eV correspond to doublet peaks of Mo4+ 3d5/2 and Mo4+ 3d3/2, whereas the peak at around 226eV (shaded with yellow in Fig. 5c) comes from the S 2s transition. [14,27,29] In Fig. 5e, the doublet peaks at 161.7 eV and 162.9 eV with a separate gap of 1.2 eV are ascribed to S 2p orbitals.

Figure 5. XPS spectra for (a) Ni 2p3, (b) Co 2p3, (c) Mo 3d, (d) O 2s, and (e) S 2p. The bottom spectra correspond to heterostructured NiCo2O4@MoS2, whereas the top traces correspond to its 11 ACS Paragon Plus Environment


unitary counterpart NiCo2O4 or MoS2, respectively. (f) show the normal spinel Ni-Co-O crystal structure, (g) illustrates the intimate interface between NiCo2O4 and MoS2 via the formation of Mo-O-Co bond. After the formation of NiCo2O4@MoS2 heterostructure, significant changes occur on both the surface and the interface. First, the signals from Ni2+/Ni3+, and Co2+/Co3+ remain for Ni and Co (Fig.5a, b), but the ratio changes dramatically. Ni3+ and Co2+ become dominant in heterostructure, which implies that the crystal structure of NiCo2O4 altered significantly. Second, a new peak located at 236 eV in Mo 3d XPS spectra (shaded as red in Fig. 5c) appears, which is ascribed to the formation of the Mo-O bond. [39,40] The appearance of the Mo-O bond may stem from either the generation of MoO3 or the covalent interaction between Mo termination in MoS2 and O termination in NiCo2O4. The first option, generation of MoO3, was excluded since neither XRD diffraction peaks nor HRTEM fringes of MoO3 were detected. Therefore, the intimate heterojunction interface is believed to be responsible for this bond. Based on the valence change of Ni and Co, the other end of the Mo-O bond must be Co since the larger electronegativity of Mo enables the inherent electron shift to Co, and leads to the decrease of Co oxidation state from 3+ to 2+. [41-46] Subsequently, on the vicinity of the interface, such a change in Co valence results in a crystal structure change. Considering the change of Ni from 2+ to 3+ in coordination, it is concluded that a distorted inverse spinel Ni-Co-O layer created on MoS2 plate, where a subset of Co atoms were no longer octahedrally coordinated with six adjacent oxygen atoms, instead tetrahedrally coordinated with four (Fig. 5f). This process occurs inversely to the interfacial Ni. The crystal structure distortion initiates the formation of oxygen defects and results in a corresponding increase of OII peak intensity in XPS spectra (Fig. 5d), where OII becomes dominant, whereas OI takes up less than 1%. Through the intermediate oxygen, a strong charge transfer from MoS2 to NiCo2O4 occurred and was further confirmed with both large parallel positive shift of S 2p doublet peaks in Fig. 5e and the positive shift of Mo4+ 3d peaks in Fig. 5c. To sum up, a distorted inverse spinel Ni-Co-O layer was created on the MoS2 plates caused by the covalent interaction between MoS2 and NiCo2O4 via Mo-O-Co covalent bonds at the interface (Fig. 5g). This interaction allows for the strong inherent electron transfer from MoS2 to NiCo2O4, which can be exploited for synergetic electrocatalytic activity. [20] Coupling with the analysis in HRTEM (Figure 4), the very thin distorted Ni-Co-O layer on the MoS2 plates contributes to the disappearance of diffraction peaks of original NiCo2O4 in heterostructure (XRD patterns in Figure 2). 12 ACS Paragon Plus Environment

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Hydrogen evolution reactivity. HER activity of as-prepared NiCo2O4@MoS2 heterostructure was first evaluated with polarization curves (linear sweep voltammetry) in a 1M NaOH aqueous electrolyte with a three-electrode configuration cell, as shown in Fig. 6a, and the HER performance parameters were summarized in Table 1. To achieve a current density of 10 mA/cm2 for HER, NiCo2O4 and MoS2 electrodes required overpotentials of 342 mV and 260 mV, respectively. In contrast, the corresponding HER overpotential for heterostructured NiCo2O4@MoS2 was only 180 mV, a substantial decrease by 162 mV and 80 mV compared to its unitary counterparts NiCo2O4 and MoS2. Carbon paper, as a reference, displays very poor activity (η=550 mV) for HER. Comparison of current densities at η=300mV for these three electrodes gave -144 mA/cm2, -26.5 mA/cm2, and -3.8 mA/cm2, respectively, for NiCo2O4@MoS2, MoS2, and NiCo2O4 (Fig. 6b and Table 1). The heterostructure exhibits pronounced improvement in HER activity, as determined by current density, by nearly 5.5 and 40 times compared to bulk MoS2 and NiCo2O4, respectively (Fig. 6b). This enhancement possibly comes from three aspects: (i) increased active interface of heterojunction, (ii) exposed MoS2 edges, and (iii) distorted inverse spinel Ni-Co-O structure. It has been accepted that the increased interface area is beneficial for the catalytic activity in a heterojunction system with a synergistic effect. [11,13,17,19] In this study, it was ruled out due to the almost full coverage of the MoS2 plate by distorted inverse spinel Ni-Co-O, that is, only very limited interface was exposed to the electrolyte. As stated in TEM characterization, we cannot preclude the exposure of uncovered MoS2 edges to the electrolyte, which has been considered as the active center for HER reaction, [27,28,47] and may contribute to the improved HER. Therefore, a MoS2 reference sample which was annealed under the identical conditions as the NiCo2O4@MoS2 heterostructure at 350oC, was also evaluated as shown in Fig. S5 and Fig. 6b. HER performance greatly lags in terms of current density, overpotential and also charge transfer ability. An increase of more than 20 times was observed (Fig. 6b) in heterostructured NiCo2O4@MoS2 when compared to the annealed MoS2. As a result, such an excellent HER activity improvement for the heterostructure originates primarily from the distorted inverse spinel Ni-CoO structure. To gain insight into the high HER activity for such a distorted inverse spinel Ni-Co-O structure, the Tafel slopes were calculated as shown in Fig. 6c. NiCo2O4@MoS2 electrode exhibits a Tafel slope of 88.2 mV/dec, smaller than those of NiCo2O4 electrode (105.6 mV/dec) and carbon paper (159 mV/dec), and close to that of MoS2 (88.8 mV/dec). As a reference, Pt/C displays a Tafel slope 13 ACS Paragon Plus Environment


of 56.7 mV/dec, comparable to the reported values under the same conditions. [48,49] Based on the Tafel slope, the HER pathway could be predicted. When the Volmer step, i.e., the water adsorption and dissociation, is the rate-determining step, a theoretical Tafel slope of around 120 mV/dec is given; when the Heyrovsky step is dominant, a Tafel value of around 40 mV/dec is predicted; when the Tafel slope is as small as 30 mV/dec, the HER performs mainly through Tafel step.[2,50,51] The Tafel slopes for all three samples lie within 40 mV/dec and 120 mV/dec, but the value for original NiCo2O4 electrode (105.6 mV/dec) is close to 120 mV/dec, indicating that the HER on normal NiCo2O4 in 1M NaOH is determined by the water dissociation on the surface. Whereas on the distorted inverse Ni-Co-O layer, the Tafel slope of around 88 mV/dev allows for a much faster reaction kinetics with the water dissociation reaction. According to the Tafel equation η=a+blogj, where b is the Tafel slope, j is the current density. The exchange current density j0 therefore could be obtained from Tafel slope intersection with the abscissa, as shown in Fig. S6 and Table 1. MoS2 and NiCo2O4 display an exchange current density of 1.04×10-2 mA/cm2 and 6.3×10-3 mA/cm2, respectively. The heterostructured NiCo2O4@MoS2 has a pronouncedly increased exchange current density j0 of 8.12×10-2 mA/cm2, nearly 8 times and 13 times of those for MoS2 and NiCo2O4. This further demonstrates the superiorly fast HER kinetics on the distorted inverse spinel Ni-Co-O structure in heterostructured NiCo2O4@MoS2 electrode.


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Figure 6. HER performances for MoS2, NiCo2O4 and NiCo2O4@MoS2 samples. (a) Polarization curves in 1 M NaOH at room temperature referred to Pt/C and carbon paper, and (b) current density with an overpotential η=0.3 V vs RHE (column bars), and the current density increase factor calculated by comparing jNiCo O @MoS /jsample; (c) the corresponding Tafel plots; (d) Arrhenius plot, logarithm of the exchange current density log(j0) versus reciprocal of temperature (1/T); (e) comparison of the EIS spectra for three catalysts measured at room temperature with an overpotential η=0.3 V vs RHE, and (f) Dependence of charge transfer Rct on the overpotential η. 2

4

2

To obtain more precise quantitative kinetic information, and understand the origin of HER enhancement by the distorted inverse Ni-Co-O structure, the EIS (electrochemical impedance spectroscopy) studies were performed, as shown in Figs. 6e and S7. The EIS experimental data were approximated with two CPE model equivalent circuits (Fig. S7g), and charge transfer resistance Rct of the HER was extracted in Table 1. Heterostructured NiCo2O4@MoS2 electrodes demonstrate the smallest charge transfer resistance of 4.74 Ω for HER reaction, apparently smaller than those for MoS2 and NiCo2O4 of 5.99 Ω and 58.34 Ω, respectively. This implies that the formation of heterostructure facilitates the charge transfer to the surface inverse spinel Ni-Co-O structure for HER reaction. Temperature and overpotential dependent EIS spectra were also performed at temperature window 30~70oC and potential window -0.3 ~ -0.15V vs RHE, as shown in Fig. S7. An excellent linear relationship between charge transfer resistance Rct versus both temperature and overpotential was observed (Figs. 6f and S7h, i). This implies that HER proceeds here mainly through Volmer-Heyrovsky pathway, [52,53] which is consistent with the conclusion from the Tafel slope prediction. Table 1. Summary of HER activity parameters for heterostructured NiCo2O4@MoS2 electrodes compared to NiCo2O4, and MoS2. Electrode Catalysts

NiCo2O4

MoS2

NiCo2O4@MoS2

Overpotential at 10mA/cm2 (mV)

342

260

180

Current Density at η =0.3V (mA/cm2)

-3.8

-26.5

-144

Tafel Slope (mV/dec)

105.6

88.8

88.2

Exchange Current Density j0 (mA/cm2)

6.3x10-3

1.04x10-2

8.12x10-2

Apparent Activation Energy Ea (kJ/mol)

54.1

38.4

23.5

Charge transfer Resistance Rct (ohm)a

58.34±0.24

5.99±0.25

4.74±0.06

a. the error bar read from the Fitting results using software ZView with the equivalent circuit.



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To reveal the reaction kinetics, temperature-dependent polarization curves were acquired within the range of room temperature to 70oC as shown in Fig. S8. As expected, the HER activity accelerates with the temperature increasing given the apparent increase in the current density. The variation of the polarization curves leads to the changes in the exchange current density j0 (Fig. S8). According to the Arrhenius equation below, ∂𝑙𝑜𝑔(𝑗0) 1 ∂ 𝑇

=―

𝐸𝑎 2.302𝑅

where Ea is the apparent electrochemical activation energy (kJ mol-1), j0 is the exchange current density, T is the temperature (K), and R is the gas constant, [48,54,55] the apparent electrochemical activation energy Ea could be determined by the plots of current density versus applied potential as a function of temperature as shown in Fig. 6d. An Arrhenius plot, logarithm of the exchange current density log(j0) versus reciprocal of temperature (1/T), revealed a very good linear function, and give the activation energy for NiCo2O4, MoS2, and heterostructured NiCo2O4@MoS2 of 54.1, 38.4 and 23.5 kJ mol-1, respectively. The significant decrease in the activation energy barrier occurs for the heterostructured NiCo2O4@MoS2 catalyst, which reveals a low energy barrier and fast reaction kinetics for water dissociation on the distorted inverse spinel Ni-Co-O structure and thus the increased inherently catalytic activity by the formation of such a unique structure. Based on the results aforementioned, it concludes that the exceptional HER activity in this proposed heterostructure primarily originates from the distorted inverse spinel Ni-Co-O layer. On the one hand, the intimate interaction between the capped Ni-Co-O and the MoS2 plate facilitates charge transfer. On the other hand, the distorted spinel structure creates crystal and oxygen defects on the surface, which significantly lowers the activation energy barrier, and allows for a fast reaction kinetics of the water dissociation over the electrocatalysts. Recent experimental and computational studies predicted that a synergetic interaction between oxygen vacancies and the O 2p ligand holes from an octahedral metal clusters can produce a near-ideal HER reaction path to adsorb H2O and release H2.[56-58] In the distorted Ni-Co-O spinel on MoS2 plate, abundant oxygen vacancies were created, which interact with adjunct octahedral metal clusters to accelerate water dissociation process and H* detachment. In addition, Ni3+ formed in the distorted Ni-Co-O layer could strengthen

the

adsorption

of

OH-

in

alkaline

electrolyte

due

to

the

enhanced

electrophilicity.[20,59,60] These synergistic effects ultimately improve the HER performance in 16 ACS Paragon Plus Environment

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distorted Ni-Co-O spinel layer in alkaline media. To the best our knowledge, this is the first time to report an effective strategy engineering nickel cobaltite spinel to achieve such an exceptional improvement of HER activity in alkaline electrolyte. Oxygen Evolution Reactivity. NiCo2O4 is an excellent OER and ORR catalyst due to the existence of multiple redox couples. [21-25] To verify whether the heterostructured catalyst could perform a highly active OER, the OER performance on various catalysts was examined in a 1 M NaOH solution as shown in Fig. 7a. As expected, MoS2 itself shows very weak activity for oxygen evolution with a potential larger than 750 mV vs Ag/AgCl to achieve a current density of 10 mA/cm2. NiCo2O4 demonstrates good OER activity with an overpotential of 355 mV to reach the current density of 10 mA/cm2, which is comparable to the activity of most reported NiCo2O4 based OER catalysts. [21-25, 33, 43, 61-73] The heterostructure exhibited a lower overpotential with a positive shift by 50 mV compared to the NiCo2O4 electrode. Comparison of the current density at η=400 mV, as shown in the inset of Fig. 7a, saw the heterostructure’s activity rise over NiCo2O4 and MoS2 by 2.5 and 53 times, respectively. The Tafel slopes were calculated as shown in Fig. 7b. MoS2 has a very large slope up to 191.6 mV/dec, whereas NiCo2O4 and the heterostructured NiCo2O4@MoS2 have the nearly close slope value, which indicates the same reaction mechanism and OER rate determining step for both of them. As evidenced from XPS and TEM, the distorted inverse Ni-Co-O layer on MoS2 plate switches the valence of Co and Ni on the vicinity of the interface, where Ni3+ becomes dominant. Through cyclic voltammetry as shown in Fig. 7c, a very broad redox pair appear in normal NiCo2O4, which is attributed to the co-existence of Co2+/Co3+ and Ni2+/Ni3+.[65-68] Whereas, in the distorted inverse spinel Ni-Co-O structure, only a sharp redox pair exist. This is attributed to the dominant role of Ni3+ in this unique structure.[68,74,75] More, the separate distance between this redox pair becomes smaller, indicating the fast charge transfer process in OER on distorted inverse spinel Ni-Co-O structure. As demonstrated in O1s XPS spectra, large amount of the defected oxygen (low-coordinated oxygen) exist in this distorted structure. Oxygen vacancies have been identified to play a key role in improving the OER performance by lowering the adsorption energy of water molecules. It was reported that two electrons neighboring an oxygen vacancy would become delocalized around the nearby cation atoms, and render the low-coordinated cation active center more activity toward the adsorption of H2O. [68-71] In this study, the metal active center would be Ni3+ as demonstrated in XPS and CV curves. Therefore, a Ni3+ center, which is octahedrally coordinated in distorted inverse spinel 17 ACS Paragon Plus Environment


structure, is more active towards the adsorption of water molecules near an oxygen vacancy. [22,33,76] Fig.7d illustrates the possible OER cycle on such a distorted inverse spinel Ni-Co-O, where Ni and Co cations distribute between interstices of two oxygen layers with oxygen vacancies. It is thermodynamically favorable for oxygen vacancy mediated Ni3+ center to adsorb and dissociate molecules to produce a surface adsorbed hydroperoxide. When surface Ni3+ is oxidized to Ni4+, it can adsorb nucleophilic species OH- to form OOH- intermediates due to its improved electrophilicity. Meanwhile, this process will result in a lattice vacancy that but can be subsequently replenished from the bulk electrolyte. [33,68,74,75] The stability of the electrode catalyst is one of the important parameters. The heterostructure NiCo2O4@MoS2 catalyst was tested for 20 hours with a constant current density of 10 mA/cm2 for both HER and OER, as shown in Fig. S9a. Robust stability could be observed with very steady applied potential. Moreover, from the polarization curves after continuously running 20 hours (Figs. S9b and c), little degradation occurred in both the current density and overpotential. Determined from the half cell (Fig. S9d), heterostructured NiCo2O4@MoS2 catalyst demonstrated the capability for both HER and OER activity, and the potential slot to reach a current density of 10 mA/cm2 for HER and OER is around 1.72 V. To further demonstrate such an activity capability, two-electrode configuration cell using heterostructured NiCo2O4@MoS2 catalyst as both anode and cathode was constructed and evaluated as shown in Fig. S10. The current density increases with an increase in the applied potential, and an applied potential of 1.75 V is required to achieve a current density of 10 mA/cm2 (Fig. S10a). This value is comparable to other bifunctional catalysts for overall water splitting. [77,78] Chronopotentiometry scanning at current density at 10 mA/cm2 reveals that this heterostructured NiCo2O4@MoS2 catalyst is stable as a bifunctional catalyst of overall water splitting (Fig. S10b).


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Figure 7. OER performance and Stability of heterostructured NiCo2O4@MoS2 electrocatalyst. (a) polarization curves for OER, and (b) the corresponding Tafel plots; (c) comparison of cyclic voltammetry scanning for NiCo2O4@MoS2 and NiCo2O4. (d) the proposed OER cycles over the NiCo2O4@MoS2, where the formation of OH-, O2-, and OOH- intermediates are reconciled with oxygen vacancy mediated Ni3+ center. The spinel structure with metal ions between interstices of two oxygen layers is used in this carton. Conclusions A unique heterostructure NiCo2O4@MoS2 was fabricated, where a distorted inverse spinel NiCo-O layer formed on MoS2 plates due to covalent Mo-O-Co bonds at the interface. The electron density shift from Mo to Co through the intermediate oxygen forces the valence changes of both Co and Ni, thus leads to the creation of distorted crystal-structure spinel Ni-Co-O. Compared to the unitary components, the heterostructured catalysts demonstrated significantly improved activity for both HER and OER, and also the capability for overall water splitting. The distorted inverse spinel Ni-Co-O layer substantially decreases the apparent activation energy barrier by 2550%, increases the exchange current density by one order of magnitude compared to the normal spinel Ni-Co-O, and thus demonstrates the superiorly fast HER kinetics on water dissociation reaction. All these effects contribute to a HER activity improvement in alkaline solution by around 19 ACS Paragon Plus Environment


40 time compared to normal spinel NiCo2O4. Moreover, the synergistic effect of heterojunction formation facilitates charge transfer during both HER and OER processes. OER performance was also significantly improved on this distorted inverse Ni-Co-O layer, where the oxygen vacancy mediated Ni3+ acts as active center for oxygen evolution. Such a unique heterostructure demonstrates robust stability for HER, OER and overall water splitting in alkaline electrolyte. This research provides new concept to design effective metal oxides based HER and OER catalysts in alkaline electrolyte. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. More SEM and TEM images, EDS Linear Scanning, XPS Surface Survey, XPS peak positions, spinel crystal structure, HER performance for annealed MoS2, Exchange current density j0, OER and HER stability, Overall water splitting performance. Author Information Corresponding Authors. *(J. L.) Email: [email protected]. *(D. C.) Email: [email protected]. Conflict of Interest. The authors declare no competing financial interest. Acknowledgement The authors gratefully acknowledge support from the US Department of the Army and US Army Future Command. J. Li also acknowledges support from the US Army Research Laboratory Senior Research Fellowship Program, which is administered by the Oak Ridge Associated Universities. References 1. Stamenkovic, V. R.; Strmcnik, D.; Lopes, P. P.; Markovic, N. M. Energy and fuels from electrochemical interfaces. Nature Mater. 2017, 16, 57-69. 2. Mahmood, N.; Yao, Y.; Zhang, J.; Pan, L.; Zhang, X.; Zou, J. Electrochemical for hydrogen evolution in alkaline electrolytes: Mechanisms, challenges, and prospective solutions. Adv. Sci. 2018, 5, 1700464. 20 ACS Paragon Plus Environment

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Distorted Inverse Spinel Nickel Cobaltite Grown on MoS2 Plate for

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