Enhanced water splitting electrocatalysis over MnCo2O4 via

Subscriber access provided by University of Winnipeg Library

Article

Enhanced water splitting electrocatalysis over MnCo2O4 via introduction of suitable Ce content Xiubing Huang, Haiyan Zheng, Guilong Lu, Peng Wang, Liwen Xing, Jingjing Wang, and Ge Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04814 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 19, 2018

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 23 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

ACS Sustainable Chemistry & Engineering

Enhanced Water Splitting Electrocatalysis over MnCo2O4 via Introduction of Suitable Ce Content Xiubing Huang†*, Haiyan Zheng†, Guilong Lu, Peng Wang, Liwen Xing, Jingjing Wang, and Ge Wang* Beijing Advanced Innovation Center for Materials Genome Engineering, Beijing Key Laboratory of Function Materials for Molecule & Structure Construction, School of Materials Science and Engineering, University of Science and Technology Beijing, No. 30 Xueyuan Road, Haidian District, Beijing, 100083, People’s Republic of China *Corresponding Authors Emails: Dr. Xiubing Huang ([email protected]), Prof. Dr. Ge Wang ([email protected]). † These authors contributed equally to this work.

ABSTRACT The research and development of transition metal oxides based electrocatalysts with high activity and stability for both oxygen evolution reaction and hydrogen evolution reaction via a facile design strategy is of critical importance. Herein, we fulfil both significant oxygen evolution reaction and hydrogen evolution reaction improvement in activity by hierarchically nanostructured Ce-MnCo2O4 prepared by an oxalate co-precipitation method and a followed calcination process. X-ray photoelectron spectroscopy and transmission electron microscopy with energy-dispersive

ACS Paragon Plus Environment

1

ACS Sustainable Chemistry & Engineering 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 23

X-ray spectroscopy mappings analysis show that the hierarchically nanostructured Ce-MnCo2O43% sample is homogeneously modified by 1.49 wt% Ce with increased Co3+ species. We suspect that the introduction of suitable Ce content into MnCo2O4 facilitates the oxygen transfer and the formation of Co3+ species, and modifies the local chemical binding, resulting in active performance for oxygen evolution reaction (390 mV at 10 mA·cm−2 and a Tafel slope of 125 mV·dec−1) in 1.0 M KOH solution. In addition, the Ce-MnCo2O4-3% sample also exhibits hydrogen evolution activity with overpotential of 389 mV at 10 mA·cm-2 and a Tafel slope of 96 mV·dec-1, and relatively good long-time stability for 12 h.

KEYWORDS: MnCo2O4, Ce doping, Oxygen evolution reaction, Hydrogen evolution reaction, Oxygen transfer

INTRODUCTION Electrochemical water splitting has been widely considered as an efficient and promising route for converting water into renewable hydrogen (H2) and oxygen (O2).1,2 The oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) usually require large overpotentials to drive the electrochemical water splitting.3 Currently, IrO2 and RuO2 are reported to be very active for OER while Pt is considered as one of the best electrocatalysts for HER with a very low overpotential, however their severe scarcity and high cost significantly impede their widely practical applications. Accordingly, considerable efforts have been made to hunting efficient electocatalysts based on abundant and cheap 3d transition metals, such as hydroxides, oxides, borides and selendies for OER,4-7 and phosphides, carbides, sulphide and nitrides for HER.8-10 Unfortunately, the best performance for these HER electrocatalysts is usually achieved in acidic solution while


2

Page 3 of 23 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


for OER catalysts in alkaline environment. It is desirable to accomplish overall water splitting by the HER and OER electrocatalysts both working in the same electrolyte. However, the mismatch working conditions in the current prevailing approaches would require complicated processes and lead to inferior overall performance. Among the various kinds of strategies, bifunctional electrocatalysts which can efficiently work well for both OER and HER with low overpotentials in the same electrolyte, provide promising potentials, nevertheless which is still rather challenging. Spinel-type oxides (e.g., Co3O4,11,12 MnCo2O4,13,14 CoFe2O4,15 NiCo2O416) have been demonstrated to be promising OER alternatives for noble metals oxides in alkaline electrolyte solutions because of their excellent redox properties, multiple valence states, etc.4 In particular, MnCo2O4 was reported to show certain electrocatalytic performance, which can be further improved by several strategies, such as surface modification with conducting polypyrrole,17 amorphous Mn-Co-P shell,18 or supporting on conductive carbon substrates (e.g., graphene, carbon nanotubes)19,20, but their electrocatalytic performance is still far from satisfied.17,21,22 Recently, investigators have reported that CeO2-decorated or incorporated electrocatalysts could exhibit promising performance with low overpotentials and excellent stability for OER and HER, which have been attributed to the facilitated electron transfer between Ce3+/Ce4+ and active sites, improved stability in an alkaline electrolyte, high oxygen transfer to the reaction sites, high oxygen storage capacity and modified local chemical binding.23-30 Recently, Vojvodic et al. reported an improved electrochemical performance due to the modified chemical binding environment by introducing Ce into Ni oxide.31 Later, Yang et al. reported a highly efficient bifunctional electrocatalyst by composition and surface structure controlling of nickel nanoparticles decorated transition metal oxide nanosheets supported on a ceria film.27 However, the improvement was mainly achieved on the interface of CeOx-electrocatalysts with limited contact. Generally, doping


3


Page 4 of 23

suitable cerium into the structure or modifying the surface of electrocatalyst with small CeO2 nanocrystals may further maximize the active sites, shorten the electron transfer length and greatly promote the electrocatalytic water splitting performance. Herein, we reported a facile oxalate co-precipitation method to synthesize Ce-MnCo2O4 nanomaterials with homogeneous Ce distribution to enhance the oxygen and electron transfer. With optimized composition, Ce-MnCo2O4-3% sample with 1.49 wt% Ce exhibited efficient OER with overpotential of 390 mV at 10 mA·cm−2 and Tafel slope of 125 mV·dec−1 as well as high HER performance with overpotential of 389 mV at 10 mA·cm−2 and a Tafel slope of 96 mV·dec1

in 1.0 M KOH alkaline electrolyte. Significantly, the Ce-MnCo2O4-3% also exhibited excellent

stability with only a small decrease in activity after a 12 h chronopotentiometry test. EXPERIMENTAL SECTION Sample preparation Ce-MnCo2O4 samples were synthesized by an oxalate co-precipitation method according to our previous report.32 Typically, 3.0 mmol of Co(NO3)2·6H2O, 1.5 mmol of Mn(CH3COO)2·4H2O and a certain amount of Ce(NO3)3·6H2O were firstly dissolved in 25 mL of deionized water, which was then added at one time to a mixed solution containing 22.5 mmol of H2C2O4·2H2O, 25 mL ethanol and 25 mL deionized water under vigorous magnetic stirring. After 2 h, the mixtures were put in a drying oven at 50 oC for 6 h. After filtration, and drying at 80 oC for 12 h, the final products were obtained by calcination at 500 oC for 4 h with a 3 oC·min-1 increasing rate. The prepared samples were named as Ce-MnCo2O4-x, in which x is the molar ratio (1%, 3% or 7%) of Ce(NO3)3·6H2O to MnCo2O4. Pure CeO2-δ nanowires were obtained according to our previous work.33


4

Page 5 of 23 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


Characterization Powder X-ray diffraction (XRD) were measured on a M21X X-ray Reflection Diffractometer with Cu Kα irradiation (λ = 1.541 Ǻ). High resolution transmission electron microscopy (HRTEM) images were measured on a JEOL JEM-100CX Ⅱ equipped with energy-dispersive X-ray spectroscopy (EDS) at 200 kV. The morphology and microstructure of all samples were checked on a ZEISS SUPRA55 Field Scanning Electron Microscopy (FESEM) operated at 20 kV. The textural properties of as-prepared samples were determined using nitrogen sorption isotherms with a Quantachrome Autosorb-iQ2-MP gas sorption instrumentation at 77 K. The specific surface areas were determined by using the Brunauer-Emmett-Teller (BET) method while the pore volumes were determined by using the Barrett-Joyner-Halenda (BJH) method. X-ray photoelectron spectroscopy (XPS) spectra were obtained using a Thermo Scientific ESCA Lab220i-XL electron spectrometer with 300 W Al Kα radiation. Peak calibration was performed using the adventitious C 1s at 284.9 eV. The contents of Ce, Co and Mn of the samples were determined by inductivity coupled plasma-atomic emission spectrometry (ICP-AES, Agilent 7500ce). The surface oxygen activity characterization was performed by temperature-programmed reduction program with H2 (H2-TPR) on an AutoChem II 2920 (Micromeritics, USA) from 100 to 800 oC with a heating rate of 10 K·min-1 under 10%H2/90%Ar and the thermal conductivity detector (TCD) signal was recorded according to the consumption of H2 gas with temperature. Electrochemical Measurements The working electrode was prepared according to our previous work.34 6 mg of catalyst and 4 mg of acetylene black were ultrasonically dispersed in a solution of 50 μL of 5 wt% Nafion and 0.35 mL of ethanol for at least 45 min. Then 7 μL of the catalyst suspension (containing 105 μg of


5


Page 6 of 23

catalyst) was added onto a glassy carbon electrode (GCE) with a diameter of 3 mm. Electrochemical data were recorded on a PMC-1000 workstation using a three-electrode system with a Pt sheet as the counter electrode, an Ag/AgCl electrode as the reference electrode and catalyst as the working electrode. Polarization curves were collected at a potential scan rate of 5 mV·s-1. All the potentials were calculated with reference to the reversible hydrogen electrode (RHE) according to Equation 1. Evs. RHE = Evs.Ag/AgCl + Eθ Ag/AgCl + 0.059 pH

(1)

Impedance measurements were performed with frequency ranged from 0.1 Hz to 10 kHz with amplitude of 5 mV at the open-circuit potentials in 1.0 M KOH solution. Impedance plots were fitted using the Zview software. All the electrochemical measurements were IR-compensated. RESULTS AND DISCUSSION The XRD patterns of as-prepared samples are represented in Figure 1. All the XRD pattern reflections in Figure 1a could be indexed to MnCo2O4 with face-centered-cubic spinel structure (space group Fd3m (227), JCPDS no. 23-1237).35 In addition, the broadening peaks of MnCo2O4 indicate a nano-crystalline nature after calcination at 500 oC for 4 h. In Ce-MnCo2O4-1% (Figure 1b), there are no obvious diffraction peaks for CeO2, suggesting the doping of Ce species into MnCo2O4 structure or the coating of small CeO2-δ nanoclusters on the surface.36,37 In the XRD patterns of Ce-MnCo2O4-3% (Figure 1c), there are tiny diffraction peaks indexed to (111) and (200) of CeO2-δ due to the formation of CeO2-δ nanocrystals with increased Ce content, nevertheless most of Ce may be still doped into the crystal structure of MnCo2O4. Even though the ionic radii of Ce3+ (1.01 Å) and Ce4+ (0.87 Å) are larger than those of Mn and Co ions (e.g., RMn2+ = 0.83 Å, RMn3+ = 0.645 Å, RCo2+ = 0.745 Å, RCo3+ = 0.61 Å), there is no obvious XRD shift to lower angles in Ce-MnCo2O4-x samples due to the simultaneously increased molar ratio of


6

Page 7 of 23 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


Co3+/Co2+ with the increment of Ce content, as well as the stable ratio of Mn3+/Mn2+ and the low content of Ce, as determined by XPS and ICP-AES analysis in the following paragraphs. With further increasing Ce content, the diffraction peaks for CeO2-δ in Ce-MnCo2O4-7% (Figure 1d) are more obvious, implying the surface distribution of more CeO2-δ nanocrystals with bigger size on Ce-MnCo2O4-7%. The grain sizes of MnCo2O4 and Ce-MnCo2O4-x samples were estimated from peak (311) according to Scherrer equation and summarized in Table 1, showing the slightly decreased crystal size with the increasing Ce amount maybe due to the restricted growth of MnCo2O4 by Ce species and the formed lattice defects by Ce doping.38

Figure 1. XRD patterns for (a) MnCo2O4, (b) Ce-MnCo2O4-1%, (c) Ce-MnCo2O4-3%, (d) CeMnCo2O4-7%, and (e) CeO2-δ. The surface morphology of as-obtained samples was studied by FESEM and TEM, as presented in Figure 2 and Figure 3. MnCo2O4 possessed the microrod morphology with hierarchical nanopores among the small primary nanoparticles, as demonstrated by its FESEM (Figure 2a) and TEM (Figure 2b) images. When Ce(NO3)3·6H2O was added during the co-precipitation process,


7


Page 8 of 23

the morphology of obtained Ce-MnCo2O4-x composite obviously changed, exhibiting spindle-like morphology (Figure 2e, Figure 2g, Figure 3a). The TEM and HRTEM images in Figure 2f, Figure 2h and Figure 3b reveal that hierarchically nanostructured Ce-MnCo2O4-x composed of nanoparticles with size of around 10 nm and secondary nanopores among them have been successfully prepared. The HRTEM images of Ce-MnCo2O4-3% (Figure 3c and Figure 3d) reveal its poly-crystalline nature, and the distinct lattice fringes with an interplanar distance of 0.248 nm and 0.286 nm can be indexed to the (311) and (220) crystal planes of MnCo2O4, respectively. In addition, trace CeO2-δ nanocrystals with size of 2-5 nm were formed on Ce-MnCo2O4-3% (Figure 3d). From the STEM and its corresponding elemental mapping images (Figure 3e), it is demonstrated that Ce species are well distributed over the entire Ce-MnCo2O4-3%, suggesting the doping of Ce species into MnCo2O4 and the coating of CeO2-δ nanocrystals/nanoclusters with small size. Nevertheless, there are more CeO2-δ nanocrystals observed in Ce-MnCo2O4-7%, as illustrated in its HRTEM (Figure 2h) and STEM EDS elemental mapping images (Figure 2i). The coprecipitated method is facile to uniformly distribute Ce species into hierarchically nanostructured MnCo2O4, which may not only enhance the oxygen and electron transfer between Ce species and MnCo2O4 to improve the electrochemical performance, but also keep catalyst stable during the electrocatalytic reactions.33


8

Page 9 of 23 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


Figure 2. FESEM (a, c, e, g) and TEM (b, d, f, h) images for MnCo2O4 (a, b), CeO2-δ (c, d), CeMnCo2O4-1% (e, f) and Ce-MnCo2O4-7% (g, h). STEM elemental mappings (i) of Ce-MnCo2O47%.

Figure 3. SEM (a), TEM (b-d) images and STEM elemental mappings (e) of Ce-MnCo2O4-3%. The nanostructures of as-prepared MnCo2O4 and Ce-MnCo2O4-x are also demonstrated by N2 adsorption-desorption measurements at 77 K (Figure 4). These samples exhibited typical type II


9


Page 10 of 23

with a H3 hysteresis loop, which is usually characteristic of mesoporous materials by aggregates of secondary nanoparticles. As summarized in Table 1, the BET specific surface areas of MnCo2O4, Ce-MnCo2O4-1%, Ce-MnCo2O4-3%, Ce-MnCo2O4-7% and CeO2 were 71.5, 82.7, 85.8, 86.9 and 118.1 m2·g-1, and their corresponding pore volumes were 0.24, 0.31, 0.36, 0.34 and 0.38 cm3·g-1, respectively. The addition of Ce(NO3)3·6H2O would lead to larger surface areas and pore volumes due to the smaller grain size of Ce-MnCo2O4-x nanocrystals than that of MnCo2O4. It is worth mentioning that the hierarchical nanostructures could provide abundant catalytic active sites as well as maintaining morphology stability, thus possibly enhancing the electrocatalytic performance and stability.

Figure 4. N2 adsorption/desorption isothermal curves for MnCo2O4, Ce-MnCo2O4-1%, CeMnCo2O4-3% and Ce-MnCo2O4-7%. The surface chemical state, interaction and charge transfer in MnCo2O4 and Ce-MnCo2O4-x samples was checked by XPS analysis, as shown in Figure 5. The Co 2p spectra in Figure 5a can be deconvoluted into two spin–orbit doublets characteristic of Co2+ and Co3+ at binding energies of around 780.2 and 795.4 eV and two shake-up satellites at binding energies of around 786.6 and 803.1 eV.39 The Mn 2p spectra in Figure 5b displays two pairs of spin–orbit deconvoluted peaks, indicating the coexistence of Mn2+ and Mn3+ cations.32 The calculated molar ratios of Co3+/Co2+


10

Page 11 of 23 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


and Mn3+/Mn2+ (Table 1) based on the deconvoluted peak areas show that Ce-MnCo2O4-x samples possess increased Co3+/Co2+ molar ratios with the increment of Ce amount while there are no obvious changes in the molar ratios of Mn3+/Mn2+. The increased percentage for Co3+ species suggests a likely change in the electronic structure of Co in MnCo2O4 via Ce introduction. When the cobalt ions (i.e., Co2+ or Co3+) in the octahedral interstices are replaced by Ce ions, lattice distoration, imperfections and positive charges would be induced due to the heavier Ce3+/Ce4+ ions with larger ionic radii and higher valences. It has been reported that the homogeneously distributed CeO2-δ with ∼1 atom% (relative to ∼99% for Co) on the CoOx surface would lead to more proportion of highly oxidized Co in the CeOx/CoOx than the CoOx.28,40 This speculation is also supported by previous theoretical and experimental results that adjacent elements can be electronically perturbed by their interactions with Ce species with Ce3+/Ce4+.38,41-43 For Ce 3d spectra in Figure 5c, even though the relative intensities of Ce 3d XPS spectra of Ce-MnCo2O4-x samples are obviously different with that of pure CeO2-δ, the Ce 3d spectrum can be all deconvolved into eight different peaks indicative of Ce in both 3+ and 4+ states.42 Such Ce3+/Ce4+ couples exhibit rich redox properties for oxygen activation on the surface. In addition, the molar ratios of Ce3+/Ce4+ of Ce-MnCo2O4-x samples are much larger than that of pure CeO2-δ, suggesting the possible doping of Ce species into MnCo2O4 structure and/or the coating of CeO2-δ nanoclusters/nanocrystals with intimate contact and strong electron transfer on the surface of MnCo2O4. With the increase of Ce content, the molar ratios of Ce3+/Ce4+ slightly decreased maybe due to the formation of CeO2-δ nanoclusters or nanocrystals, in accordance with the XRD and HRTEM results.


11


Page 12 of 23

Figure 5. Co 2p (a), Mn 2p (b), and Ce 3d (c) XPS spectra of as-prepared samples. Table 1. The textural properties of as-prepared samples Materials MnCo2O4 Ce-MnCo2O4-1% Ce-MnCo2O4-3% Ce-MnCo2O4-7% CeO2-δ

Surface area (m2/g) [a] 71.5 82.7 85.8 86.8 118.1

Pore volume (cm3/g)[b] 0.24 0.31 0.36 0.34 0.38

Grain size (nm)[c] 11.6 9.8 9.6 9.4 6.2

Ce content (wt%)[d] 0.79 1.49 2.89 81.59

[e]

Ratio of Co3+/Co2+ 0.66 0.70 0.85 1.02 -

[e]

Ratio of Mn3+/Mn2+

[e]

Ratio of Ce3+/Ce4+

0.79 0.71 0.75 0.77 -

0.33 0.32 0.30 0.25

[a] Obtained by the BET method. [b] Calculated from N2 adsorption branch with P/P0 lower than 0.997. [c] Calculated from the XRD patterns according to Scherrer equation. [d] Obtained based on the ICP-AES results. [e] Calculated based on the deconvoluted peak areas. The electrochemical catalytic performance of the obtained samples toward OER was evaluated under strongly alkaline conditions (1.0 M KOH). Figure 6a displays the LSV curves of CeMnCo2O4-1%, Ce-MnCo2O4-3%, Ce-MnCo2O4-7%, MnCo2O4 and CeO2-δ catalysts. CeO2-δ


12

Page 13 of 23 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


displays almost no electrocatalytic activity with an onset overpotential about 630 mV. It can be seen that an overpotential (η) of 390 mV was required for Ce-MnCo2O4-3% to reach the current density of 10 mA·cm-2, which is smaller compared with those of Ce-MnCo2O4-1% (430 mV), CeMnCo2O4-7% (440 mV) and MnCo2O4 (560 mV). The Tafel slope is usually calculated to evaluate the kinetics of electrocatalysts. A smaller Tafel slope normally reveals that the catalyst needs lower overpotential to reach the required current.44 Remarkably, the fitted Tafel slope of 125 mV·dec-1 for Ce-MnCo2O4-3% is much smaller compared with those of Ce-MnCo2O4-1% (155 mV·dec-1), Ce-MnCo2O4-7% (156 mV·dec-1) and MnCo2O4 (167 mV·dec-1) for OER (Figure 6b). All these results suggest that the electrochemical activity of MnCo2O4 can be improved by Ce-indroduction, which may be due to the unique hierarchical structures, the rich redox properties for oxygen activation, the higher Co3+ species and the synergetic effect between MnCo2O4 and Ce species for electron transfer. In an oxidation environment, Ce species with Ce3+ sites are able to increase the adsorption probability of O2,45 dissociate, and then release atomic O for the OER,46 and abundant amount of O vacancies to dissociate water.47 Since highly oxidized Co species usually play a very important role in improving the OER activity,48 Ce-MnCo2O4-3% with higher Co3+ contents would form more highly oxidized Co oxide species (such as CoOOH), leading to a higher OER activity than MnCo2O4. It is interesting to note that even though the molar ratio of Co3+/Co2+ in CeMnCo2O4-7% is higher than that of Ce-MnCo2O4-3%, Ce-MnCo2O4-7% exhibited a bit higher overpotential (440 mV at 10 mA·cm-2) than Ce-MnCo2O4-3% (390 mV at 10 mA·cm-2). As demonstrated, pure CeO2-δ nanocrystals exhibited very poor OER performance (Figures 6a and 6b). As a result, the coating of too many CeO2-δ nanocrystals on the surface in Ce-MnCo2O4-7% (Figure 2h) would reduce the active sites for OER even though Ce-MnCo2O4-7% possessed the highest Co3+ content and still higher performance than MnCo2O4. Overall, Ce-MnCo2O4-3%


13


Page 14 of 23

showed good intrinsic OER activity which may be resulted from the best synergistic effects via suitable Ce species introduction.28,38,46

Figure 6. The LSV curves (a) and fitted Tafel slopes (b) of as-prepared samples toward OER; the LSV curves (c) and fitted Tafel slopes (d) of as-prepared samples toward HER; Electrochemical impedance spectroscopy obtained at the open-circuit potentials (inset: the equivalent circuit), the lines and hollow marks are for the measured and calculated data, respectively (e); stability test under the voltage of 1.56 V (vs. RHE) (f). All the tests were carried out in 1.0 M KOH solution. To further demonstrate the synergetic effects between Ce species and MnCo2O4, the electrochemical activity of electrocatalysts toward HER was further performed in 1.0 M KOH, and the LSV curves are shown in Figure 6c. It needs the overpotentials of 389 mV for Ce-MnCo2O43%, 466 mV for Ce-MnCo2O4-7%, 467 mV for Ce-MnCo2O4-1% and 477 mV for MnCo2O4 to achieve the current density of 10 mA·cm-2, respectively. The corresponding fitted Tafel slopes of Ce-MnCo2O4-3% (96 mV·dec-1) is much smaller than that of Ce-MnCo2O4-1% (137 mV·dec-1), Ce-MnCo2O4-7% (113 mV·dec-1), MnCo2O4 (116 mV·dec-1) and CeO2 (139 mV·dec-1) (Figure 6d), indicating the superior HER activity of Ce-MnCo2O4-3% among the obtained catalysts. Electrochemical impedance spectroscopy (EIS) was performed with frequency from 0.1 Hz to 10 kHz with amplitude of 5 mV at the open-circuit potentials to evaluate the electrode kinetics of the


14

Page 15 of 23 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


catalysts. The Nyquist plots are shown in Figure 6e and the inset is the equivalent circuit. The equivalent circuit of Nyquist plots is composed of solution resistance (Rs), related to the double layer capacitance (CPE), charge transfer resistance (Rct) and Warburg impedance (Ws). The lower value of Rct value means the faster electrochemical reaction rate, which can be observed from the diameter of semicircles.49-51 It is obvious that Ce-MnCo2O4-3% reveals the lowest charge transfer resistance value (Rct), suggesting that the introduction of Ce with suitable content would lead to much higher charge-transfer kinetics due to the rich redox properties and enhanced surface activity, which is highly accordant with its lowest Tafel slope and overpotential. In addition, the low frequency straight line can be ascribed to the response from the constricted diffusion of OHwithin nanopores of nanostructured electrocatalysts.52 In addition, Ce-MnCo2O4-3% exhibits an outstanding durability by the chronoamperometric technology, as shown in Figure 6f. Only a slight fluctuation in the current density can be observed which can be attributed to the release of bubbles under the voltage of 1.56 V (vs. RHE) over 12 h, implying the long-term catalytic durability in alkaline media. Besides, Ce-MnCo2O4-3% is superior or comparable in activity to some previously reported spinel-type electrocatalysts in the literature (Table 2). Table 2. Comparison of OER catalytic activity over different electrocatalysts.

Catalyst Ce-MnCo2O4-3% dp-MnCo2O4 MnCo2O4 porous dumbbell Mesoporous MnCo2O4 MnCo2O4 Co3O4 Mn-Co oxyphosphide Mn-Co oxide MnCo2O4-δ Mesoporous Co3O4

Current density j ( mA·cm -2) 10 10 10 10 100 100 10 10 10 10

η (mV) at the corresponding j

Tafel slope (mV·dec -1)

Loading of catalyst (mg·cm-2)

390  426 400 300 720 370 420 530 525

125 93 90 112.2 205.3 66 60 -

1.48 0.1 1.0  3.9 4.5 0.25 0.25 0.074 0.13

Ref. This work 21 53 54 55 55

56 56 57 58


15


CuxCoyO4

10

471

-

Page 16 of 23

59

0.12

Since the HER and OER processes are related with the surface oxygen vacancies, oxygen adsorption and oxygen transfer ability, usually lower energy required for the removal of surface oxygen indicates higher oxygen transfer ability and higher activity for OER.25 A hydrogentemperature programmed reduction (H2-TPR) was carried out with the view of surface oxygen removal ability based on the thermal conductivity detector (TCD) signal associated with the consumption of H2 gas. As displayed in Figure 7, the starting reduction temperatures for MnCo2O4 and Ce-MnCo2O4-1% are about 150 oC, while with the increasing of Ce amount the reduction temperatures gradually decreased from 120 oC for Ce-MnCo2O4-3% to 100 oC for Ce-MnCo2O47%. Ce-MnCo2O4-3% and Ce-MnCo2O4-7% also showed lower reduction temperatures than that of MnCo2O4 in the range of 100-400 oC. As a control, the reduction peak temperatures for pure CeO2-δ nanowires are determined to be 432 oC and 699 oC. Therefore, the introduction of Ce species with suitable amount (i.e., > 1.49 wt% Ce) into MnCo2O4 is demonstrated to promote the surface oxygen adsorption and transfer property which is related with the oxygen evolution activity. Interestingly, even though Ce-MnCo2O4-7% exhibited the lowest reduction temperature from the H2-TPR curves, too much Ce content (e.g., 2.89 wt% for Ce-MnCo2O4-7%) would lower the electrocatalytic performance of MnCo2O4 maybe due to the existence of too much CeO2-δ nanocrystals with poor activity for OER and HER on the surface (Figure 2h). Summarily, CeMnCo2O4-3% with relatively low reduction temperature and suitable Ce content (i.e., 1.49 wt% Ce) yields the best electrocatalytic OER and HER performance.


16

Page 17 of 23 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


Figure 7. H2-TPR curves of as-prepared samples. CONCLUSIONS A facile oxalate co-precipitation synthesis method has been adopted to prepare hierarchically nanostructured Ce-MnCo2O4 with tunable amount of introduced Ce content. After careful characterization and test, our investigations suggest that Ce-MnCo2O4-3% with 1.49 wt% Ce exhibited both promising OER (a low overpotential of 390 mV at 10 mA·cm−2 and a small Tafel slope of 125 mV·dec−1) and HER (a low overpotential of 389 mV at 10 mA·cm−2 and a Tafel slope of 96 mV·dec-1) activities in 1.0 M KOH, which was more active than those of MnCo2O4, CeMnCo2O4-1% with 0.79 wt% Ce, Ce-MnCo2O4-7% with 2.89 wt% Ce and pure CeO2-δ. CeMnCo2O4-3% also maintained excellent stability with only a small decrease in activity after a 12 h chronopotentiometry test. It is anticipated that the improved activity is attributed to the high molar ratio of Co3+/Co2+ and the optimized synergetic effect between Ce species and MnCo2O4 in Ce-MnCo2O4-3%. These results provide a meaningful strategy to the new design of spinels-based (e.g., MnCo2O4) electrocatalysts by Ce species introduction with the best synergetic effects. NOTES The authors declare no competing financial interest.


17


Page 18 of 23

ACKNOWLEDGMENT The authors are grateful for financial support from the National Natural Science Foundation of China (51802015), the National Key Research and Development Program of China (No. 2016YFB0701100), the Fundamental Research Funds for the Central Universities (FRF-BD-17013A) and the Program of Young Scholar sponsored by Beijing Organization Department (2017000020124G090). REFERENCES (1) Yan, Y.; Xia, B. Y.; Zhao, B.; Wang, X. A review on noble-metal-free bifunctional heterogeneous catalysts for overall electrochemical water splitting. J. Mater. Chem. A 2016, 4, 17587-17603, DOI 10.1039/C6TA08075H. (2) You, B.; Jiang, N.; Sheng, M.; Gul, S.; Yano, J.; Sun, Y. High-performance overall water splitting electrocatalysts derived from cobalt-based metal–organic frameworks. Chem. Mater. 2015, 27, 7636-7642, DOI 10.1021/acs.chemmater.5b02877. (3) Tahir, M.; Pan, L.; Idrees, F.; Zhang, X.; Wang, L.; Zou, J.-J.; Wang, Z. L. Electrocatalytic oxygen evolution reaction for energy conversion and storage: A comprehensive review. Nano Energy 2017, 37, 136-157, DOI 10.1016/j.nanoen.2017.05.022. (4) Zhao, Q.; Yan, Z.; Chen, C.; Chen, J. Spinels: Controlled preparation, oxygen reduction/evolution reaction application, and beyond. Chem. Rev. 2017, 117, 10121-10211, DOI 10.1021/acs.chemrev.7b00051. (5) Nsanzimana, J.M.V.; Peng, Y.; Xu, Y.Y., Thia, L.; Wang, C.; Xia, B.Y.; Wang, X. An efficient and earth-abundant oxygen-evolving electrocatalyst based on amorphous metal borides. Adv. Energy Mater. 2018, 8, 1701475, DOI 10.1002/aenm.201701475. (6) Ma, X.; Wen, J.; Zhang, S.; Yuan, H.; Li, K.; Yan, F.; Zhang, X.; Chen, Y. Crystal CoxB (x = 1–3) synthesized by a ball-milling method as high-performance electrocatalysts for the oxygen evolution reaction. ACS Sustainable Chem. Eng. 2017, 5, 10266-10274, DOI 10.1021/acssuschemeng.7b02281. (7) Waghmode, B. J.; Gaikwad, A. P.; Rode, C. V.; Sathaye, S. D.; Patil, K. R.; Malkhede, D. D. Calixarene intercalated NiCo layered double hydroxide for enhanced oxygen evolution catalysis. ACS Sustainable Chem. Eng. 2018, 6, 9649-9660, DOI 10.1021/acssuschemeng.7b04788. (8) Chang, Q.; Ma, J.; Zhu, Y.; Li, Z.; Xu, D.; Duan, X.; Peng, W.; Li, Y.; Zhang, G.; Zhang, F.; Fan, X. Controllable synthesis of ruthenium phosphides (RuP and RuP2) for pH-universal hydrogen evolution reaction. ACS Sustainable Chem. Eng. 2018, 6, 6388-6394, DOI 10.1021/acssuschemeng.8b00187. (9) Gao, W.; Shi, Y.; Zhang, Y.; Zuo, L.; Lu, H.; Huang, Y.; Fan, W.; Liu, T. Molybdenum carbide anchored on graphene nanoribbons as highly efficient all-pH hydrogen evolution reaction electrocatalyst. ACS Sustainable Chem. Eng. 2016, 4, 6313-6321, 10.1021/acssuschemeng.6b00859. (10) Hu, J.; Huang, B.; Zhang, C.; Wang, Z.; An, Y.; Zhou, D.; Lin, H.; Leung, M. K. H.; Yang, S. Engineering stepped edge surface structures of MoS2 sheet stacks to accelerate the hydrogen


18

Page 19 of 23 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


evolution reaction. Energy Environ. Sci. 2017, 10, 593-603, DOI 10.1039/C6EE03629E. (11) Cai, Z.; Bi, Y.; Hu, E.; Liu, W.; Dwarica, N.; Tian, Y.; Li, X.; Kuang, Y.; Li, Y.; Yang, X.Q.; Wang, H.; Sun, X. Single-crystalline ultrathin Co3O4 nanosheets with massive vacancy defects for enhanced electrocatalysis. Adv. Energy Mater. 2018, 8, 1701694, DOI 10.1002/aenm.201701694. (12) Yu, M.; Chan, C.K.; Tueysuez, H. Coffee-waste templating of metal ion-substituted cobalt oxides for the oxygen evolution reaction. ChemSusChem 2018, 11, 605-611, DOI 10.1002/cssc.201701877. (13) Bin, D.; Guo, Z.; Tamirat, A. G.; Ma, Y.; Wang, Y.; Xia, Y. Crab-shell induced synthesis of ordered macroporous carbon nanofiber arrays coupled with MnCo2O4 nanoparticles as bifunctional oxygen catalysts for rechargeable Zn-air batteries. Nanoscale 2017, 9, 11148-11157, DOI 10.1039/C7NR03009F. (14) Kong, X.; Zhu, T.; Cheng, F.; Zhu, M.; Cao, X.; Liang, S.; Cao, G.; Pan, A. Uniform MnCo2O4 porous dumbbells for lithium-ion batteries and oxygen evolution reactions. ACS Appl. Mater. Interfaces 2018, 10, 8730-8738, DOI 10.1021/acsami.7b19719. (15) Calvillo, L.; Carraro, F.; Vozniuk, O.; Celorrio, V.; Nodari, L.; Russell, A. E.; Debellis, D.; Fermin, D.; Cavani, F.; Agnoli, S.; Granozzi, G. Insights into the durability of Co-Fe spinel oxygen evolution electrocatalysts via operando studies of the catalyst structure. J. Mater. Chem. A 2018, 6, 7034-7041, DOI 10.1039/C7TA10892C. (16) Fang, L.; Jiang, Z.; Xu, H.; Liu, L.; Guan, Y.; Gu, X.; Wang, Y. Crystal-plane engineering of NiCo2O4 electrocatalysts towards efficient overall water splitting. J. Catal. 2018, 357, 238246, DOI 10.1016/j.jcat.2017.11.017. (17) Cao, X.; Yan, W.; Jin, C.; Tian, J.; Ke, K.; Yang, R. Surface modification of MnCo2O4 with conducting polypyrrole as a highly active bifunctional electrocatalyst for oxygen reduction and oxygen evolution reaction. Electrochim. Acta 2015, 180, 788-794, DOI 10.1016/j.electacta.2015.08.160. (18) Han, J.; Hao, S.; Liu, Z.; Asiri, A. M.; Sun, X.; Xu, Y. In situ development of amorphous Mn-Co-P shell on MnCo2O4 nanowire array for superior oxygen evolution electrocatalysis in alkaline media. Chem. Commun. 2018, 54, 1077-1080, DOI 10.1039/C7CC08895G. (19) Wang, H. L.; Yang, Y.; Liang, Y. Y.; Zheng, G. Y.; Li, Y. G.; Cui, Y.; Dai, H. J. Rechargeable Li-O2 batteries with a covalently coupled MnCo2O4-graphene hybrid as an oxygen cathode catalyst. Energy Environ. Sci. 2012, 5, 7931-7935, DOI 10.1039/C2EE21746E. (20) Yang, J.; Park, S.; Choi, K. y.; Park, H.-S.; Cho, Y.-G.; Ko, H.; Song, H.-K. Activitydurability coincidence of oxygen evolution reaction in the presence of carbon corrosion: Case study of MnCo2O4 spinel with carbon black. ACS Sustainable Chem. Eng. 2018, 6, 9566-9571, DOI 10.1021/acssuschemeng.8b01879. (21) Ge, X.; Liu, Y.; Goh, F. W. T.; Hor, T. S. A.; Zong, Y.; Xiao, P.; Zhang, Z.; Lim, S. H.; Li, B.; Wang, X.; Liu, Z. Dual-phase spinel MnCo2O4 and spinel MnCo2O4/nanocarbon hybrids for electrocatalytic oxygen reduction and evolution. ACS Appl. Mater. Interfaces 2014, 6, 1268412691, DOI 10.1021/am502675c. (22) Cao, X.; Jin, C.; Lu, F.; Yang, Z.; Shen, M.; Yang, R. Electrochemical properties of MnCo2O4 spinel bifunctional catalyst for oxygen reduction and evolution reaction. J. Electrochem. Soc. 2014, 161, H296-H300, DOI 10.1149/2.029405jes.


19


Page 20 of 23

(23) Zhao, D.; Pi, Y.; Shao, Q.; Feng, Y.; Zhang, Y.; Huang, X. Enhancing oxygen evolution electrocatalysis via the intimate hydroxide–oxide interface. ACS Nano 2018, 12, 6245-6251, DOI 10.1021/acsnano.8b03141. (24) Zhang, R.; Ren, X.; Hao, S.; Ge, R.; Liu, Z.; Asiri, A. M.; Chen, L.; Zhang, Q.; Sun, X. Selective phosphidation: an effective strategy toward CoP/CeO2 interface engineering for superior alkaline hydrogen evolution electrocatalysis. J. Mater. Chem. A 2018, 6, 1985-1990, DOI 10.1039/C7TA10237B. (25) Gao, W.; Xia, Z.; Cao, F.; Ho, J.C.; Jiang, Z.; Qu, Y. Comprehensive understanding of the spatial configurations of CeO2 in NiO for the electrocatalytic oxygen evolution reaction: Embedded or surface-loaded. Adv. Funct. Mater. 2018, 28, 1706056, DOI 10.1002/adfm.201706056. (26) Wang, Z.; Du, H.; Liu, Z.; Wang, H.; Asiri, A. M.; Sun, X. Interface engineering of a CeO2Cu3P nanoarray for efficient alkaline hydrogen evolution. Nanoscale 2018, 10, 2213-2217, DOI 10.1039/C7NR08472B. (27) Long, X.; Lin, H.; Zhou, D.; An, Y.; Yang, S. Enhancing full water-splitting performance of transition metal bifunctional electrocatalysts in alkaline solutions by tailoring CeO2–transition metal oxides–Ni nanointerfaces. ACS Energy Lett. 2018, 3, 290-296, DOI 10.1021/acsenergylett.7b01130. (28) Kim, J.-H.; Shin, K.; Kawashima, K.; Youn, D. H.; Lin, J.; Hong, T. E.; Liu, Y.; Wygant, B. R.; Wang, J.; Henkelman, G.; Mullins, C. B. Enhanced activity promoted by CeOx on a CoOx electrocatalyst for the oxygen evolution reaction. ACS Catal. 2018, 8, 4257-4265, DOI 10.1021/acscatal.8b00820. (29) Obata, K.; Takanabe, K. A permselective CeOx coating to improve the stability of oxygen evolution electrocatalysts. Angew. Chem. Int. Ed. 2018, 57, 1616-1620, DOI 10.1002/anie.201712121. (30) Feng, J.X.; Ye, S.H.; Xu, H.; Tong, Y.X.; Li, G.R. Design and synthesis of FeOOH/CeO2 heterolayered nanotube electrocatalysts for the oxygen evolution reaction. Adv. Mater. 2016, 28, 4698-4703, DOI 10.1002/adma.201600054. (31) Ng, J. W. D.; García-Melchor, M.; Bajdich, M.; Chakthranont, P.; Kirk, C.; Vojvodic, A.; Jaramillo, T. F. Gold-supported cerium-doped NiOx catalysts for water oxidation. Nat. Energy 2016, 1, 16053, DOI 10.1038/NENERGY.2016.53. (32) Huang, X.; Liu, L.; Gao, H.; Dong, W.; Yang, M.; Wang, G. Hierarchically nanostructured MnCo2O4 as active catalysts for the synthesis of N-benzylideneaniline from benzyl alcohol and aniline. Green Chem. 2017, 19, 769-777, DOI 10.1039/C6GC02065H (33) Huang, X.; Zhao, G.; Chang, Y.; Wang, G.; Irvine, J. T. S. Nanocrystalline CeO2−δ coated βMnO2 nanorods with enhanced oxygen transfer property. Appl. Surf. Sci. 2018, 440, 20-28, DOI 10.1016/j.apsusc.2017.12.197. (34) Zheng, H.; Huang, X.; Wu, Z.; Gao, H.; Dong, W.; Wang, G. Controlled synthesis of 3D flower-like Ni2P composed of mesoporous nanoplates for overall water splitting. Chem. Asian J. 2017, 12, 2956-2961, DOI 10.1002/asia.201701255. (35) Xu, Y.; Wang, X.; An, C.; Wang, Y.; Jiao, L.; Yuan, H. Facile synthesis route of porous MnCo2O4 and CoMn2O4 nanowires and their excellent electrochemical properties in supercapacitors. J. Mater. Chem. A 2014, 2, 16480-16488, DOI 10.1039/C4TA03123G (36) Jiang, S.-D.; Bai, Z.-M.; Tang, G.; Song, L.; Stec, A. A.; Hull, T. R.; Zhan, J.; Hu, Y. Fabrication of Ce-doped MnO2 decorated graphene sheets for fire safety applications of epoxy


20

Page 21 of 23 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


composites: flame retardancy, smoke suppression and mechanism. J. Mater. Chem. A 2014, 2, 17341-17351, DOI 10.1039/C4TA02882A. (37) Anandha babu, G.; Ravi, G.; Mahalingam, T.; Navaneethan, M.; Arivanandhan, M.; Hayakawa, Y. Size and surface effects of Ce-doped NiO and Co3O4 nanostructures on ferromagnetism behavior prepared by the microwave route. J. Phys. Chem. C 2014, 118, 2333523348, DOI 10.1021/jp504573c. (38) Xu, H.; Wang, B.; Shan, C.; Xi, P.; Liu, W.; Tang, Y. Ce-doped NiFe-layered double hydroxide ultrathin nanosheets/nanocarbon hierarchical nanocomposite as an efficient oxygen evolution catalyst. ACS Appl. Mater. Interfaces 2018, 10, 6336-6345, DOI 10.1021/acsami.7b17939. (39) Huang, X.; Zhao, G.; Wang, G. Sub-nano CoOx attached onto WO3 for efficient photocatalytic and photoelectrochemical water oxidation. J. Mater. Chem. A 2017, 5, 2463124635, DOI 10.1039/C7TA08734A. (40) Palmas, S.; Ferrara, F.; Vacca, A.; Mascia, M.; Polcaro, A. M. Behavior of cobalt oxide electrodes during oxidative processes in alkaline medium. Electrochim. Acta 2007, 53, 400-406, DOI 10.1016/j.electacta.2007.01.085. (41) Bruix, A.; Rodriguez, J. A.; Ramírez, P. J.; Senanayake, S. D.; Evans, J.; Park, J. B.; Stacchiola, D.; Liu, P.; Hrbek, J.; Illas, F. A New Type of Strong Metal–Support Interaction and the Production of H2 through the Transformation of Water on Pt/CeO2(111) and Pt/CeOx/TiO2(110) Catalysts. J. Am. Chem. Soc. 2012, 134, 8968-8974, DOI 10.1021/ja302070k. (42) Huang, X.; Zhao, G.; Wang, P.; Zheng, H.; Dong, W.; Wang, G. Ce1−xCrxO2−δ nanocrystals as efficient catalysts for the selective oxidation of cyclohexane to KA oil at low temperature under ambient pressure. ChemCatChem 2018, 10, 1406-1413, DOI 10.1002/cctc.201701762. (43) Huang, X.; Ni, C.; Zhao, G.; Irvine, J. T. S. Oxygen storage capacity and thermal stability of the CuMnO2-CeO2 composite system. J. Mater. Chem. A 2015, 3, 12958-12964, DOI 10.1039/C5TA01361E. (44) Zou, X.; Zhang, Y. Noble metal-free hydrogen evolution catalysts for water splitting. Chem. Soc. Rev. 2015, 44, 5148-80, DOI 10.1039/C4CS00448E. (45) Suchorski, Y.; Wrobel, R.; Becker, S.; Weiss, H. CO oxidation on a CeOx/Pt(111) inverse model catalyst surface: Catalytic promotion and tuning of kinetic phase diagrams. J. Phys. Chem. C 2008, 112, 20012-20017, DOI 10.1021/jp806033v. (46) Yang, F.; Graciani, J.; Evans, J.; Liu, P.; Hrbek, J.; Sanz, J. F.; Rodriguez, J. A. CO oxidation on inverse CeOx/Cu(111) catalysts: High catalytic activity and ceria-promoted dissociation of O2. J. Am. Chem. Soc. 2011, 133, 3444-3451, DOI 10.1021/ja1087979. (47) Burch, R. Gold catalysts for pure hydrogen production in the water–gas shift reaction: Activity, structure and reaction mechanism. Phys. Chem. Chem. Phys. 2006, 8, 5483-5500, DOI 10.1039/B607837K. (48) Favaro, M.; Yang, J.; Nappini, S.; Magnano, E.; Toma, F. M.; Crumlin, E. J.; Yano, J.; Sharp, I. D. Understanding the oxygen evolution reaction mechanism on CoOx using operando ambient-pressure X-ray photoelectron spectroscopy. J. Am. Chem. Soc. 2017, 139, 8960-8970, DOI 10.1021/jacs.7b03211. (49) Du, C.; Yang, L.; Yang, F.; Cheng, G.; Luo, W. Nest-like NiCoP for highly efficient overall water splitting. ACS Catal. 2017, 7, 4131-4137, DOI 10.1021/acscatal.7b00662. (50) Feng, Y.; Yu, X. Y.; Paik, U. Nickel cobalt phosphides quasi-hollow nanocubes as an efficient electrocatalyst for hydrogen evolution in alkaline solution. Chem. Commun. 2016, 52, 1633-1636, DOI 10.1039/C5CC08991C.


21


Page 22 of 23

(51) Li, D.; Hui, J.; Liu, S.; Ni, C.; Ni, J. Role of a topotactic electrochemical reaction in a perovskite-type Anode for lithium-ion batteries. ChemElectroChem 2017, 4, 2474-2479, DOI 10.1002/celc.201700381. (52) Herraiz-Cardona, I.; González-Buch, C.; Valero-Vidal, C.; Ortega, E.; Pérez-Herranz, V. Co-modification of Ni-based type Raney electrodeposits for hydrogen evolution reaction in alkaline media. J. Power Sources 2013, 240, 698-704, DOI 10.1016/j.jpowsour.2013.05.041. (53) Abidat, I.; Bouchenafa-Saib, N.; Habrioux, A.; Comminges, C.; Canaff, C.; Rousseau, J.; Napporn, T. W.; Dambournet, D.; Borkiewicz, O.; Kokoh, K. B. Electrochemically induced surface modifications of mesoporous spinels (Co3O4−δ, MnCo2O4−δ, NiCo2O4−δ) as the origin of the OER activity and stability in alkaline medium. J. Mater. Chem. A 2015, 3, 17433-17444, DOI 10.1039/C5TA04437E. (54) Wang, W.; Kuai, L.; Cao, W.; Huttula, M.; Ollikkala, S.; Ahopelto, T.; Honkanen, A.; Huotari, S.; Yu, M.; Geng, B. Mass-production of mesoporous MnCo2O4 spinels with manganese(IV)- and cobalt(II)-rich surfaces for superior bifunctional oxygen electrocatalysis. Angew. Chem. 2017, 129, 15173-15177, DOI 10.1002/ange.201708765. (55) Zhou, J.-J.; Han, X.; Tao, K.; Li, Q.; Li, Y.-L.; Chen, C.; Han, L. Shish-kebab type MnCo2O4@Co3O4 nanoneedle arrays derived from MnCo-LDH@ZIF-67 for high-performance supercapacitors and efficient oxygen evolution reaction. Chem. Eng. J. 2018, 354, 875-884, DOI 10.1016/j.cej.2018.08.102. (56) Guan, B. Y.; Yu, L.; Lou, X. W. General synthesis of multishell mixed-metal oxyphosphide particles with enhanced electrocatalytic activity in the oxygen evolution reaction. Angew. Chem. Int. Ed. 2017, 56, 2386-2389, DOI 10.1002/anie.201611804. (57) Abidat, I.; Bouchenafa-Saib, N.; Habrioux, A.; Comminges, C.; Canaff, C.; Rousseau, J.; Napporn, T. W.; Dambournet, D.; Borkiewicz, O.; Kokoh, K. B. Electrochemically induced surface modifications of mesoporous spinels (Co3O4-δ, MnCo2O4-δ, NiCo2O4-δ) as the origin of the OER activity and stability in alkaline medium. J. Mater. Chem. A 2015, 3, 17433-17444, DOI 10.1039/C5TA04437E. (58) Tüysüz, H.; Hwang, Y. J.; Khan, S. B.; Asiri, A. M.; Yang, P. J. N. R. Mesoporous Co3O4 as an electrocatalyst for water oxidation. Nano Res. 2013, 6, 47-54, DOI 10.1007/s12274-0120280-8. (59) Grewe, T.; Deng, X.; Weidenthaler, C.; Schüth, F.; Tüysüz, H. Design of ordered mesoporous composite materials and their electrocatalytic activities for water oxidation. Chem. Mater. 2013, 25, 4926-4935, DOI 10.1021/cm403153u.


22

Page 23 of 23 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


For Table of Contents Use Only

Synopsis: Both significant OER and HER improvements were achieved in 1.0 M KOH alkaline electrolyte by introducing suitable Ce content into MnCo2O4.


23

Enhanced water splitting electrocatalysis over MnCo2O4 via

Recommend Documents