General anion-exchange reaction derived amorphous mixed-metal oxides hollow nanoprisms for highly efficient water oxidation electrocatalysis

Journal Pre-proof General Anion-Exchange Reaction Derived Amorphous Mixed-Metal Oxides Hollow Nanoprisms for Highly Efficient Water Oxidation Electrocatalysis Qizhu Qian, Yapeng Li, Yi Liu, Genqiang Zhang

PII:

S0926-3373(20)30057-6

DOI:

https://doi.org/10.1016/j.apcatb.2020.118642

Reference:

APCATB 118642

To appear in:

Applied Catalysis B: Environmental

Received Date:

24 October 2019

Revised Date:

24 December 2019

Accepted Date:

13 January 2020

Please cite this article as: Qian Q, Li Y, Liu Y, Zhang G, General Anion-Exchange Reaction Derived Amorphous Mixed-Metal Oxides Hollow Nanoprisms for Highly Efficient Water Oxidation Electrocatalysis, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118642

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.

General Anion-Exchange Reaction Derived Amorphous Mixed-Metal Oxides Hollow Nanoprisms for Highly Efficient Water Oxidation Electrocatalysis

Qizhu Qian,a Yapeng Li,a Yi Liu,a and Genqiang Zhanga*

a

Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key

Laboratory of Materials for Energy Conversion, Department of Materials Science and

ro of

Engineering, University of Science and Technology of China, Hefei, Anhui 230026 China ⃰ Corresponding Author:

-p

G. Q. Zhang, E-mail: [email protected]

Jo

ur

na

lP

re

Graphical abstract

A series of amorphous mixed metal oxide hollow nanoprisms were fabricated by a versatile anion-exchange strategy under mild solution phase method, which can act as 1

highly efficient OER electrocatalyst in aklaline condition benefiting from the simutaneous modification on the morphology and composition. The in-situ Raman spectroscopy measurement further revealed that the electrochemically transformed

ro of

metal hydroxide/oxyhydroxide could be the real active species.

Highlights 

A series of amorphous mixed metal oxide hollow nanoprisms were fabricated by

Amorphous FeCoSeOx hollow nanoprisms show highly efficient OER

re



-p

a versatile anion-exchange strategy under mild solution phase method.

The in-situ and ex-situ measurements further revealed that the electrochemically

na



lP

performance (η10=297 mV) than the most of transition metal oxides.

Jo

ur

transformed metal hydroxide/oxyhydroxide could be the real active species.

2

Abstract High cost precious metal oxides (RuO2 and IrO2) electrocatalysts are required to overcome the sluggish kinetics of the oxygen evolution reaction (OER), which severely restricts the scalable application. Herein, we present a versatile anion-exchange based strategy to fabricate various amorphous transition metal oxide hollow nanoprisms with FeCo-MIL-88B template, which exhibit highly efficient OER

ro of

catalysis in aklaline media. Impressively, a low overpotential of 294 mV at 10 mA cm-2 with an ultrasmall Tafel slope of 45.1 mV dec-1 and decent durability can be

achieved. More importantly, the underlying origin is unraveled using the in-situ

-p

Raman spectroscopy measurement combined with the ex-situ X-ray photoelectron

re

spectroscopy anlysis, where the electrochemically transformed metal hydroxide and oxyhydroxide species could be the real active species. This work provides a universal

lP

strategy to construct transition metal oxide hollow nanostructures with enhanced OER

water splitting.

na

activity, which could push forward the advance of noble metal free electrocatalytic

ur

Keywords: amorphous mixed metal oxide; anion-exchange reaction; hollow

Jo

nanoprisms; oxygen evolution reaction; in-situ Raman spectroscopy

3

1.

Introduction With the increasing demand on energy resources and growing environmental

concerns upon over-consumption of fossil fuels, the production and utilization of the clean and renewable energy sources have become the most urgent issue for sustainable development. Developing a feasible technique capable of hydrogen (H2) production is a hot topic, due to the unique features of ultrahigh gravimetric energy

ro of

densities and zero carbon emission.[1-3] Electrocatalytic water-splitting has been

considered as one of the most promising strategy for H2 generation among various clean energy technologies.[4] Unfortunately, compared to the hydrogen evolution

-p

reaction (HER) of cathode, the sluggish kinetics of the oxygen evolution reaction

re

(OER) process of anode severely restricts the overall reaction rate due to the involved four electron transfer process associated high energy barrier for O-H bond breaking

lP

and O-O bond formation.[5-7] Currently, precious metal oxides (RuO2 and IrO2) are

na

demonstrated as the benchmarking OER electrocatalysts, while their scarcity and expensiveness undoubtedly limit their large-scale application in reality. Therefore, it

ur

is urgent yet meaningful to explore an efficient and low-cost noble metal-free OER electrocatalysts applicable for highly efficient water splitting.[8-13]

Jo

Hollow nanostructures, as an intensive focus of research on energy-related

applications, could provide more exposed active sites and shorten the mass and charge transport pathways compared to the corresponding architectures with solid interiors.[14-16] During the past decade, the researchers have made great efforts to design and synthesize various hollow nanostructure based on different reaction 4

mechanisms, aiming to further enhance the electrocatalytic OER performance.[17-24] On the other hand, compositional modulation has also been demonstrated as an efficient tool to promote the catalytic efficiency through the synergistic effect of mixed metal ions by electronic interaction.[25-27] For example, Lou group reported the controlled synthesis of Co3O4/Co-Fe oxide double-shelled nanoboxes via a facile anion-exchange reaction between ZIF-67 nanocubes and [Fe(CN)6]3- ions followed by

ro of

thermal annealing treatment for enhanced oxygen evolution reaction in alkaline solution.[17] Tang and co-workers constructed CoS hollow polyhedron decorated

with CeOx that can boost the OER activity with modified Co2+/Co3+ ratio and induced

-p

defects.[28] Although these inspiring progress, there are still remained chanllenges

re

regarding the nanostructured OER electrocatalyst. Firstly, the simultaneous manipulation of the morphology and composition in a unified system is difficult yet

lP

meaningful to unravel the possible strategy that can enhance the OER activity.

na

Secondly, most of the previous research focused on the investigation of the OER performance of crystalline nanostructured materials, while the amorphous

ur

counterparts have rarely been reported. Moreover, it is always desired to develop generalized strategies for the construction of nanostructures with identical

Jo

morphology while various compositions, which could provide a reliable platform to exploit the structure-composition relationship.[27] Herein, we reported a versatile strategy based on anion-exchange reaction with FeCo-MIL-88B template to prepare a series of amorphous mixed metal oxide hollow nanoprisms using solution phase method under mild conditions, including FeCoWOx, 5

FeCoVOx, FeCoPOx, FeCoBOx and FeCoSeOx (denoted as A-FeCoWOx-HoNPrs as an example). Benefiting from the simutaneous modification on the morphology and composition, the optimal OER activity can be achieved in A-FeCoSeOx-HoNPrs, where a lower overpotential of 294 mV at 10 mA cm-2 with a much smaller Tafel slope of 45.1 mV dec-1 than that of pristine FeCo-MIL-88B solid nanoprisms can be reached. Importantly, the possible origin of the enhanced activity are further analyzed

ro of

based on the in-situ Raman spectrocopy measurement combined with the ex-situ X-ray photoelectron spectrocopy analysis, which unravels that the electrochemically transformed metal hydroxide and oxyhydroxide species could be the active species

-p

while the role of the incorporated W6+ could be the electron modulator for Fe3+/Co2+.

re

This work not only provide an interesting strategy to constructe oxide hollow structures, but more importantly shed new light on the feasbile way for OER

Experimental section

na

2.

lP

electrocatalytic performance enhancement.

2.1. Preparation of samples

ur

All the chemicals were of analytical grade purity and used as received without further purification. In a typical synthesis of FeCo-MIL-88B nanoprisms precursor, 1

Jo

mmol FeCl3·6H2O (270.3 mg), 1 mmol Co(NO3)2·6H2O (291.03 mg) and 2 mmol p-phthalicacid were dissolved in 20 mL of DMF under stirring. Then, 4 mL of 0.2 mol L-1 NaOH solution was added under magnetic stirring for 30 min at room temperature. Finally, the mixture was transferred to a 50 mL Teflon-lined stainless stell autoclave and kept at 100°C for 15 hours. After cooled to room temperature naturally, the 6

products were collected by centrifugation, washed several times with DMF and ethanol and finally dried at 60°C for 12h. The A-FeCoWOx-y HoNPrs (y represents mass amount of anionic salts) were synthesized by the reflux method. 50 mg of FeCo-MIL-88B nanoprisms and 100 mg of Na2WO4·2H2O were dispersed in 30 mL distilled water by ultrasonication for 10 min. Then, the solution is refluxed at 90°C in an oil bath to react for 2 h. After cooling down, the precipitate was centrifuged and

ro of

washed with ethanol and DI water for three times, respectively. For comparison, A-FeCoWOx-y (y=25, 50, 150) HoNPrs with added Na2WO4·2H2O mass amount of

25 mg, 50 mg and 150 mg were also prepared, the all other experiment condition

-p

remained unchanged. The other amorphous FeCoVOx, FeCoPOx, FeCoBOx and

re

FeCoSeOx hollow nanoprisms were synthesized in parallel by the same method as that for A-FeCoWOx-y HoNPrs, except for using the corresponding mass amount of

lP

NH4VO3, NaH2PO4, NH4B5O8·4H2O and Na2SeO3 instead of Na2WO4·2H2O.

na

2.2. Materials Characterization

The morphology and structure of the products were characterized using

ur

field-emission scanning electron microscopy (FESEM, JSM-6700F), transmission electron microscopy (TEM, JEOL, JEM-2010), powder X-ray diffractor (XRD,

Jo

TTR-III,) Raman spectrometer (LabRamHR) with a 532 nm excitation laser, micropore and chemisorption analyzer (Micrometritics, ASAP 2020). Energy dispersive spectroscopy (EDS) mapping images were collected using a Talox F200X (Thermo Fisher Scientific, America) transmission electron microscope operating at 200 kV. The chemical state of the sample was performed on X-ray photoelectron 7

spectroscopy (XPS, ESCALAB 250, UK) with a Al Kα as the excitation source, the Fourier transform infrared (FTIR) spectra were recorded with a Nicolet 8700 FT-IR spectrometer (Thermo Nicolet Corporation, America). The element composition was analysed using a PerkinElmer: Optima 7300 DV ICP atomic emission spectrometer. 2.3. In-situ Raman Measurements A custom-made electrochemical cell was used for in-situ Raman spectroscopy

ro of

experiments. In-situ Raman Spectroscopy was performed with a confocal Raman

microscope (LabRam HR). The excitation source was a HeNe laser (532 nm) and at a

power of 0.5 mW at the objective. In a typical experiment, the A-FeCoWOx-100

-p

HoNPrs film deposited over a 5 mm electrochemically roughened Au disc sheathed in

re

Teflon served as the working electrode. A platinum wire and a Hg/HgO electrode served as the counter and reference electrodes, respectively. A potentiostat (CH

lP

Instruments 760E) was used to apply potentials to the working electrode while Raman

na

spectra were being acquired. The applied potential range 1.23-1.83 V vs. RHE in 0.1 M KOH and the every intended potential was held for 10 min before recording each

ur

spectrum to reach steady state conditions. 2.4. Electrochemical measurements

Jo

All electrochemical measurements were carried out with a conventional

three-electrode system on an electrochemical workstation (CH Instruments 760E) at room temperature. A glassy carbon (GC) electrode with diameter of 3 mm was used as the working electrode. Hg/HgO (1.0 M KOH) and graphite rod were used as the reference and counter electrodes in alkaline aqueous solution, respectively. All the 8

reference electrodes were calibrated by measuring the reversible hydrogen electrode (RHE) potential using a Pt electrode under the high purity hydrogen saturated electrolyte. All potentials in this study were converted to RHE reference scale according to the equation of ERHE=EHg/HgO+0.923 V in 1.0 M KOH. The overpotential (η) was calculated according to the following formula: η= ERHE - 1.23 V. The current density indicated in this paper was the apparent current density based on the

ro of

geometric area of the electrode. Before use, the GC electrodes were carefully polished

with 500 and 50 nm Al2O3 powders. The catalyst ink was prepared according to the

following procedures: 2 mg of the as-synthesized catalysts were dispersed in 0.5 mL

-p

of water/ethanol mixed solution with a volume ratio of 1:1 followed by the addition of

re

20 µL of 5 wt.% Nafion solution (Sigma-Aldrich) to form a homogeneous ink by sonication. Then, 5 µL of the above catalyst suspension was dropped onto the surface

lP

of the polished GC electrode, which was then dried overnight at room temperature

na

before OER test. The mass loading is calculated to be 0.27 mg cm-2 for all samples. All the electrochemical tests were executed in the O2-saturated 1.0 M KOH (pH=14)

ur

aqueous solution. The linear sweep voltammetry (LSV) curves were collected at a scan rate of 5 mV s-1. The Tafel slope was obtained by fitting the linear portion of the

Jo

Tafel plots to the Tafel equation [η = b log(j)+a]. The durability test is performed in O2-saturated 1.0 M KOH at room temperature for 2000 cycles with a scan rate of 100 mV s-1 and the polarization curve is recorded with a sweep rate of 5 mV s-1 at an interval of every 500 cycles. The electrochemical active surface areas (ECSAs) were estimated by calculating the double-layer capacitances (Cdl) at the solid-liquid 9

interface from cyclic voltammograms (CVs) method at the scan rates of 10-100 mV s-1, respectively. The current density differences (Δj= ja-jc) were plotted against scan rates, and the linear slope is twice the double-layer capacitance (Cdl). The long-term stability of A-FeCoSeOx-100 HoNPrs and A-FeCoWOx-100 HoNPrs was tested by chronoamperometry at overpotential of 297 mV and 314 mV (no iR-correction), respectively. Electrochemical impedance spectroscopy (EIS) was investigated in an

ro of

O2-saturated electrolyte with a 5 mV AC potential from 10 kHz to 0.01 Hz. All polarization curves were corrected by the iR drop compensation.

The turnover frequency (TOF) was calculated according to the formula: TOF =

-p

JA/4Fn, where the “J” is the current density under certain overpotential, the “A” is the

re

area of the electrode (0.07065 cm-2), the “4” means the mole of electrons consumed for evolving one mole of O2 from water, the “F” is Faraday's constant (96485 C mol-1)

lP

and the “n” is the mole number of the active metal sites for the catalysts that are

na

deposited on the electrode. The Co and Fe content in each sample were confirmed by inductively coupled plasma atomic emission spectrometry (ICP-AES) measurement. Results and discussion

ur

3.

The formation process of the amorphous hollow nanoprisms can be

Jo

schematically depicted in Fig. 1, which can be generally described as follows. Firstly, the crystalline FeCo-MIL-88B nanoprisms are prepared as precursor template by hydrothermal method according to previous literature with minor modifications.[29] In order to obtain different mixed metal oxide hollow nanoprims, the corresponding metal sources with different anions, such as Na2WO4·2H2O, NH4VO3, NaH2PO4, 10

NH4B5O8·4H2O and Na2SeO3, are dissolved into the aqueous solution with well dispersed FeCo-MIL-88B precursor, which is then refluxed at 90 oC for 2 h to prompt the anion-exchange reaction and the subsequent formation of amorphous hollow mixed metal oxide nanoprisms. The detailed conditions are provided in the Experimental section. The morphological evolution process from FeCo-MIL-88B to amorphous mixed

ro of

metal oxide hollow nanoprisms is firstly demonstrated by field-emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM)

observations. As shown in Fig. 2A-C, the FeCo-MIL-88B precursor exhibits uniform

-p

one-dimensional solid prism-like morphology with an average length of about 900 nm

re

and mean diameter of about 150 nm. The powder X-ray diffraction (XRD) pattern (Fig. S1) confirms the consistent crystal structure with the simulated Fe-MIL-88B

lP

pattern.[30] The elemental mapping results display the uniform distribution of Fe, Co,

na

C and O throughout the individual nanoprism (Fig. S2). Using the synthesis of A-FeCoWOx-HoNPrs as an example to demonstrate the formation process. After

ur

anion-exchange reaction of FeCo-MIL-88B with 100 mg of Na2WO4·2H2O, the 1D prism-like morphology can be well maintained when observed from the FESEM

Jo

images (Fig. 2D, E) while the TEM image (Fig. 2F) provides the direct evidence for the formation of hollow interior. The elemental mapping results (Fig. 2G) indicate the existence of W element besides Fe, Co, C and O, implying the replacement of organic ligand in the MOF structure with the inorganic anion of WO42-. The comparison of the Fourier transform infrared (FT-IR) spectra (Fig. S3) confirms the structural variation 11

after the reaction. Specifically, the characteristic peaks for the MOF structure located at 1581, 1389 and 751 cm-1 ascribed to the asymmetric and symmetric vibrations of the carboxyl groups and C-H bending vibrations of the benzene rings completely disappear while the new absorption peaks are presented at 934 and 861 cm-1 associated with the formation of W-O bond.[31-33] Moreover, the thermogravimetric analysis (TGA) results (Fig. S4) could provide a visualized perception for the anion

ro of

exchange induced structural evolution, where a much lower weight loss of 15% for the hollow nanoprisms can be observed compared to that of pristine MOF structure

(80% weight loss).[34] The XRD patterns of the product after anion exchange

-p

reaction demonstrate the typical amorphous character (Fig. S5). The influence of the

re

amounts of anion source and the reaction time for the morphology and crystal structure of the products were further investigated. The morphologies of the products

lP

with the addition of 25, 50 and 150 mg of Na2WO4·2H2O are first investigated. As

na

can be seen, with a small amount anion sources, most of the products exhibit open hollow prism-like shape (Fig. S6). This phenomenon could imply that the anion

ur

exchange could preferentially happen at the center part of the MOF prism precursor and is incomplete, which could be verified by the comparison of the XRD patterns

Jo

with the major peaks of the MOF structure retained after anion exchange reaction (Fig. S7). The well-defined hollow nanoprism can be obtained when the amount of Na2WO4·2H2O increases to 50 mg (Fig. S8A, B), while it will further evolve to porous nanoprism if the amount of Na2WO4·2H2O further increases to 150 mg which may be attributed to sufficiently high concentration of reactants enabling more outer 12

anionic groups to diffuse into the inner of FeCo-MIL-88B (Figure S8C, D), where similar amorphous structure can be observed (Fig. S9). Subsequently, we investigated the influence of the reaction times (30 min, 60 min and 90 min) on the morphology and structure of the products (Fig. S10, S11). As indicated, the prolonged reaction time will lead to the continuous faster outward diffusion velocity of Fe3+/Co2+, which induces the gradual formation of hollow interior of the product, as schematically

ro of

illustrated in Fig. S12. In addition, the evolution of morphology for A-FeCoSeOx-y (y=25, 50, 150) HoNPrs is similar with the A-FeCoWOx-y (y=25, 50, 150) HoNPrs

(Fig. S13), which demonstrates the similar formation process for different hollow

-p

structure.

re

In order to verify the generality of this anion-exchange reaction strategy for fabricating amorphous mixed metal oxide hollow nanoprisms, the possibility of

lP

several anionic systems including NH4VO3, NaH2PO4, NH4B5O8·4H2O and Na2SeO3

na

is further investigated with the identical conditions to the synthesis of A-FeCoWOx-100 HoNPrs. As shown in Fig. 3, the morphologies of the A-FeCoVOx-100,

A-FeCoPOx-100,

A-FeCoBOx-100

and

ur

as-synthesized

A-FeCoSeOx-100 HoNPrs also exhibit typical hollow prism-like shape, while their

Jo

corresponding elemental mapping results clearly indicate the existence of different elements derived from different anionic sources. Their XRD patterns (Fig. S14) suggest that all the anion-exchange induced mixed metal oxide hollow nanoprisms are amorphous, while their FT-IR spectra (Fig. S15) also confirm the structural change of MOF after anion-exchange reaction. 13

The electronic structure intensively affects the catalytic activity because of its great influence on the binding energy of intermediate reactants.[35-37] Therefore, the variation of the surface chemical state of A-FeCoWOx-100 HoNPrs is studied employing X-ray photoelectron spectroscopy (XPS). The comparison of the survey spectra for FeCo-MIL-88B and A-FeCoWOx-100 HoNPrs (Fig. 4A) clearly indicates the peak of W element besides Fe, Co, C and O after anion exchange reaction. The

ro of

high resolution Fe 2p spectrum (Fig. 4B) demonstrates that there are two main Fe

2p3/2 and 2p1/2 peaks located at 711.9 and 725.6 eV in the pristine MOF structure, respectively, indicating that the Fe species are in the +3 oxidation state.[38, 39]

-p

Notably, the binding energy of Fe 2p in the A-FeCoWOx-100 HoNPrs (Fe 2p3/2 at

re

711.5 eV and Fe 2p1/2 at 725.2 eV) is about 0.4 eV lower. The Co 2p spectrum of the FeCo-MIL-88B shown in Fig. 4C can be deconvoluted into two spin-orbit doublets

lP

(Co 2p3/2 at 781.7 eV and Co 2p1/2 at 797.6 eV) with spin-energy seperation of 15.6

na

eV, corresponding to the Co2+ valence state.[40] Similarly, the Co 2p also exhibits lower-energy shift of about 0.5 eV in A-FeCoWOx-100 HoNPrs. These results

ur

suggest that conversion of the MOF structure to oxide could increase the electron density of Fe, Co sites, which could well regulate OER performance.[38] Combined

Jo

with the successive evolution of morphology and structure during the reaction process, the distinct variation of the peak position of the O 1s (Fig. 4D) could further confirm the deconstruction of the MOF structure and the formation of metal oxides. Specifically, the peaks of O 1s in FeCo-MIL-88B can be deconvoluted into three peaks located 532.8, 531.8 and 530.4 eV, respectively, which are readily assigned to 14

the lattice oxygen (M-O), O=C-O and O-H.[25, 41] After the anion exchange reaction, the organic ligands could be replaced by WO42- that causes the change of the dominant peak of O 1s, where the only two peaks located at 531.54 and 530.57 eV can be observed, corresponding to the formation of O-H/M-O.[42, 43] Fig. 4E illustrates the presentence of W6+ in the as-prepared A-FeCoWOx-100 HoNPrs.[44] The XPS results for other amorphous mixed metal oxide hollow nanoprisms are also

ro of

investigated (Fig. S16, S17), where the identical phenomenon can be observed regarding the peak shift discipline of the Fe 2p and Co 2p spectra, implying the universal electronic modulation effect upon the structural evolution from MOF to

-p

amorphous oxides. Due to the formation of the hollow nanostructure, these

re

amorphous hollow nanoprisms show a higher Brunauer-Emmett-Teller (BET) specific surface area (SSA) compared to solid nanoprisms (Fig. S18, Table S1) that is

lP

favorable for improving OER catalytic performance.

na

The OER performance of various mixed metal oxide hollow nanoprisms are then systematically evaluated under a standard three-electrode system in 1.0 M KOH

ur

electrolyte on glassy carbon electrode (GCE) using graphite rod as the counter electrode with a typical mass loading of 0.27 mg cm-2 (Detailed information is

Jo

provided in Experimental section). All electrode potentials reported in this study were calibrated with respect to reversible hydrogen electrode (Fig. S19). The optimal composition for each mixed metal oxide hollow nanoprisms is firstly screened, where it can be observed that the products obtained with the addition of 100 mg of anionic salt resource exhibit the best OER activity (Fig. S20-S24). For comparison, the linear 15

sweep voltammetry (LSV) curves for the FeCo-MIL-88B, commercial RuO2 and various amorphous mixed metal oxide hollow nanoprisms including A-FeCoWOx-100, A-FeCoVOx-100, A-FeCoPOx-100, A-FeCoBOx-100 and A-FeCoSeOx-100 HoNPrs are presented in Fig. 5A, which could visually indicate the higher OER activity of the amorphous mixed metal oxide hollow nanoprisms than that of pristine FeCo-MIL-88B and commercial RuO2. Specifically, the smallest overpotential for

ro of

A-FeCoSeOx-100 HoNPrs to achieve a current density of 10 mA cm-2 is only 294 mV (Fig. 5B), which notably outperforms that of pristine MOF (322 mV@10 mA cm-2) and commercial RuO2 (346 mV@10 mA cm-2) and is also outstanding among

-p

transition metal oxide based electrocatalysts in recent literatures (Table S2). More

re

impressively, the A-FeCoSeOx-100 HoNPrs electrocatalyst exhibits an ultra-small Tafel slope of 45.1 mV dec-1 (Fig. 5C), which is much lower than that of

lP

FeCo-MIL-88B (73.8 mV dec-1) and commercial RuO2 (87.3 mV dec-1), implying the

na

greatly promoted electrochemical water oxidation kinetics.[45] In order to further investigate their intrinsic electrocatalytic activities, the electrochemical double-layer

ur

capacitance (Cdl) values are calculated and compared (Fig. 5D) based on the CV curves under different scan rates in the potential range of 0.993-1.093 V vs. RHE

Jo

without redox process (Fig. S25). As indicated, the A-FeCoSeOx-100 HoNPrs exhibit the highest Cdl value of 1.18 mF cm-2, confirming the higher catalytic activity. Electrochemical impedance spectroscopy (EIS) measurements were performed to obtain more information on the kinetics of the OER process. The A-FeCoSeOx-100 HoNPrs has the smallest semicircle radius in the Nyquist plots, suggesting a much 16

lower charge transfer resistance (Rct) and thus a higher charge-transfer rate and more favorable catalytic kinetics (Fig. S26).[46] The turnover frequency (TOF) of each catalyst is further calculated on the basis of assumption that all the metal ions are catalytically active (Fig. S27), among which the A-FeCoSeOx-100 HoNPrs possess the highest value of 0.18 s-1 at an overpotential of 350 mV . Therefore, it can be confirmed that the excellent OER activity of A-FeCoSeOx-100 HoNPrs is from the

ro of

improved intrinsic activity of the active sites. As one of the important factor that

determines the practical applications, the durability of the hollow nanoprisms were also evaluated. Fig. 5E shows the LSV curves of the A-FeCoSeOx-100 HoNPrs after

-p

successive cyclic voltammetry (CV) test at a scan rate of 10 mV s-1 ranging from 1.3

re

to 1.57 V (vs. RHE). After 2000 CV cycles, the overpotential at 10 mA cm-2 is 295 mV, which exhibits negligible change compared to initial curve (294 mV@10 mA

lP

cm-2). Moreover, the chronoamperometry measurement (i-t) for A-FeCoSeOx-100

na

HoNPrs at the overpotential of 297 mV (no iR-correction) shows that no obvious activity decay can be observed for at least 40 h continuous test (Fig. 5F). These

ur

results demonstrate that the A-FeCoSeOx-100 HoNPrs exhibit outstanding electrocatalytic activity and excellent stability for OER, which holds great potential as

Jo

anode candidates for noble-metal free electrocatalytic water splitting. By comparison, the stability test of FeCo-MIL-88B shows much poorer stability than that of A-FeCoSeOx-100 HoNPrs (Fig. S28). The ICP-AES analysis is used to check the possible change in chemical composition after OER stability test (Table S3). After the long-term chronoamperometry test in 1.0 M KOH, the content of Se element 17

decreases obviously in A-FeCoSeOx-100 HoNPrs, while the ~1.7 mg L-1 Se can be detected in the electrolyte which could be from the Se4+ diffusion from electrocatalysts. In contrast, the trace amount of Fe (~0.012 mg L-1) and Co (~0.006 mg L-1) were detected in the electrolyte after OER. Similarly, the Fe, Co and Se content displays the same variation trend after 2000 CV cycles. In addition, the comparison of the XPS spectra before and after OER testing of A-FeCoSeOx-100

ro of

HoNPrs (Fig. S29) also indicates the much lower intensity of Se after prolonged OER

test. As a result, the A-FeCoSeOx-100 HoNPrs could undergo a surface reconstruction to generate metal hydroxide and oxyhydroxide species accompanying

-p

Se dissolution, which has also been observed in recent literatures.[47, 48] In order to

the

LSV

curves

and

re

examine the durability of other amorphous mixed metal oxide hollow nanoprims, both chronoamperometry

test

were

also

conducted

for

lP

A-FeCoWOx-100 HoNPrs (Fig. S30), where robust catalytic stability can also be

na

observed. At last, the stability of A-FeCoWOx-100 HoNPrs and A-FeCoSeOx-100 HoNPrs catalysts were also tested by the chronopotentiometry at current density of 10

ur

mA cm-2 (Fig. S31), and the overpotential only increase 18 and 8 mV after 20 h, respectively, further confirming the outstanding durability of the catalyst.

Jo

The possible active species for the hollow nanoprism electroacatalysts are

unraveled

through

the

in-situ

Raman

spectroscopy

characterization

on

A-FeCoWOx-100 HoNPrs recorded in the potential range of 1.23-1.83 V vs. RHE under the 0.1 M KOH electrolyte, as shown in Fig. 6A. The electrocatalyst is held for 10 min at the intended potential to reach steady state conditions before recording each 18

spectrum. At the open circuit condition, there are three obvious peaks located at 710, 806 and 948 cm-1 corresponding to the O-W-O and W=O vibrations,[49, 50] respectively, which is consistent with the Raman spectra taken from the powder sample (Fig. S32), implying the reliability of the in-situ measurement. As the program controlled potential increase, those peaks remain constant which indicates that the catalyst of A-FeCoWOx-HoNPrs is relatively robust in the alkaline electrolyte.

ro of

Notably, with the applied potential of 1.23 V vs. RHE, two new broad peaks centered

at around 464 and 523 cm-1 are emerged, which can be attributed to the generation of

transition metal hydroxide and oxyhydroxide species.[25, 51, 52] With the increasing

-p

applied potentials, the intensity of the two peaks turns becomes slightly stronger,

re

which could originate from the increased portion of oxyhydroxide.[53, 54] The change of in-situ Raman spectra suggests the conversion of the mixed-metal oxides

lP

surface to the (oxy)hydroxides, which could act as the actual active species for OER.

na

The comparing study of the XPS results between the pristine nanoprisms and products after OER test is further performed, as shown in Fig. 6B-D and Fig. S33. The high Fe 2p3/2 (Fig. 6B) clearly indicates that the peak position

ur

resolution spectrum of

shift to the higher binding energy by about 0.4 eV,[55, 56] while that of Co 2p3/2 (Fig.

Jo

6C) is negatively shifted by 0.5 eV compared with pristine A-FeCoWOx-HoNPrs (781.4 eV),[57-59] which could be ascribed to higher oxidation states after the formation of metal oxyhydroxide species. The dominant peak of O 1s before and after the OER test (Fig. 6D) can all be deconvoluted into lattice oxygen (M-O) and hydroxyl groups (O-H). Specifically, the position of the M-O and O-H peaks 19

distinctly shift to lower binding energy after OER test, confirming the possible conversion of the A-FeCoWOx-100 HoNPrs into metal (oxy)hydroxide species during the OER process, which is consistent with previous studies.[25, 55, 58] Moreover, the intensity of W 4f spectra obviously reduces after OER test (Fig. S34). These results demonstrate that the generated metal hydroxide/oxyhydroxide species could be the electrocatalytic active species for oxygen evolution reaction in the amorphous hollow

ro of

nanoprims.[60, 61] After OER tests, the structural stability of A-FeCoWOx-100 HoNPrs were further characterized by TEM, XRD and FT-IR (Fig. S35), where it can be observed the hollow nanoprism morphology and amorphous feature are well

-p

reserved.

re

In conclusion, we have developed a versatile anion-exchange based strategy to fabricate a series of amorphous transition metal oxide hollow nanoprisms using MOF

OER

electrocatalyst

in

alkaline

condition.

Impressively,

the

na

efficient

lP

nanostructure as template under mild solution phase method, which can act as highly

A-FeCoSeOx-HoNPrs can deliver a low overpotential of 294 mV to reach the current

ur

density of 10 mA cm-2 with an ultrasmall Tafel slope of 45.1 mV dec-1 and exhibits excellent durability with negligible activity decay after 40 h continuous test. More

Jo

importantly, the underlying origination of the high OER activity for the amorphous hollow nanoprisms is unraveled using the in-situ Raman spectroscopy measurement, where it can be found that the electrochemically transformed metal hydroxide and oxyhydroxide species could be the real active species. This work provides a universal strategy to construct transition metal oxide hollow nanostructures with enhanced OER 20

activity, which could push forward the advance of noble metal free electrocatalytic water splitting.

Credit Author Statement G. Q. Z. conceived the idea and supervised this project. Q. Z. Q conducted the project. Y. P. L. and Y. L. helped in materials synthesis and electrochemical test. G. Q. Z and Y. L. co-wrote the manuscript.

ro of

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

-p

Acknowledgements

re

We acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21601174), the Recruitment Program of Global

Jo

ur

na

(WK2060190081).

lP

Experts and the Fundamental Research Funds for the Central Universities

21

References [1] J.A. Turner, Sustainable hydrogen production, Science 305 (2004) 972-974. [2] A. Zuttel, A. Remhof, A. Borgschulte, O. Friedrichs, Hydrogen: the future energy carrier, Philos. Trans. A, Math. Phys. Eng. Sci. 368 (2010) 3329-3342. [3]

L. Schlapbach, Technology: Hydrogen-fuelled vehicles, Nature 460 (2009)

ro of

809-811.

[4] C.G. Morales-Guio, L.-A. Stern, X. Hu, Nanostructured hydrotreating catalysts

for electrochemical hydrogen evolution, Chem. Soc. Rev. 43 (2014)

-p

6555-6569.

re

[5] M. Gao, W. Sheng, Z. Zhuang, Q. Fang, S. Gu, J. Jiang, Y. Yan, Efficient water oxidation using nanostructured alpha-nickel-hydroxide as an electrocatalyst, J.

lP

Am. Chem. Soc. 136 (2014) 7077-7084.

na

[6] X.-Y. Yu, Y. Feng, B. Guan, X.W. Lou, U. Paik, Carbon coated porous nickel phosphides nanoplates for highly efficient oxygen evolution reaction, Energy

ur

Environ. Sci. 9 (2016) 1246-1250. [7] H. Wang, J. Wang, Y. Pi, Q. Shao, Y. Tan, X. Huang, Double perovskite LaFex

Jo

Ni1-xO3 nanorods enable efficient oxygen evolution electrocatalysis, Angew. Chem. Int. Ed. 58 (2019) 2316-2320.

[8] B. Zhang, X. Zheng, O. Voznyy, R. Comin, M. Bajdich, M. Garcia-Melchor, L. Han, J. Xu, M. Liu, L. Zheng, F.P. Garcia de Arquer, C.T. Dinh, F. Fan, M. Yuan, E. Yassitepe, N. Chen, T. Regier, P. Liu, Y. Li, P. De Luna, A. 22

Janmohamed, H.L. Xin, H. Yang, A. Vojvodic, E.H. Sargent, Homogeneously dispersed multimetal oxygen-evolving catalysts, Science 352 (2016) 333-337. [9] J. Li, Y. Wang, T. Zhou, H. Zhang, X. Sun, J. Tang, L. Zhang, A.M. Al-Enizi, Z. Yang, G. Zheng, Nanoparticle superlattices as efficient bifunctional electrocatalysts for water splitting, J. Am. Chem. Soc. 137 (2015) 14305-14312.

ro of

[10] X.R. Wang, J.Y. Liu, Z.W. Liu, W.C. Wang, J. Luo, X.P. Han, X.W. Du, S.Z.

Qiao, J. Yang, Identifying the key role of Pyridinic-N-Co bonding in synergistic electrocatalysis for reversible ORR/OER, Adv. Mater. 30 (2018)

K. Chakrapani, G. Bendt, H. Hajiyani, T. Lunkenbein, M.T. Greiner, L.

re

[11]

-p

1800005.

Masliuk, S. Salamon, J. Landers, R. Schlögl, H. Wende, R. Pentcheva, S.

lP

Schulz, M. Behrens, The role of composition of uniform and highly dispersed

na

cobalt vanadium iron spinel nanocrystals for oxygen electrocatalysis, ACS Catal. 8 (2018) 1259-1267.

ur

[12] Y. Liu, Y. Ying, L. Fei, Y. Liu, Q. Hu, G. Zhang, S.Y. Pang, W. Lu, C.L. Mak, X. Luo, L. Zhou, M. Wei, H. Huang, Valence engineering via selective atomic

Jo

substitution on tetrahedral sites in spinel oxide for highly enhanced oxygen evolution catalysis, J. Am. Chem. Soc. 141 (2019) 8136-8145.

[13] S. Sun, Y. Sun, Y. Zhou, S. Xi, X. Ren, B. Huang, H. Liao, L.P. Wang, Y. Du, Z.J. Xu, Shifting oxygen charge towards octahedral metal: a way to promote water oxidation on cobalt spinel oxides, Angew. Chem. Int. Ed. 58 (2019) 23

6042-6047. [14] J. Qi, X. Lai, J. Wang, H. Tang, H. Ren, Y. Yang, Q. Jin, L. Zhang, R. Yu, G. Ma, Z. Su, H. Zhao, D. Wang, Multi-shelled hollow micro-/nanostructures, Chem. Soc. Rev. 44 (2015) 6749-6773. [15] X.W. Lou, L.A. Archer, Z. Yang, Hollow micro-/nanostructures: synthesis and applications, Adv. Mater. 20 (2008) 3987-4019.

ro of

[16] G. Zhang, B.Y. Xia, C. Xiao, L. Yu, X. Wang, Y. Xie, X.W. Lou, General formation of complex tubular nanostructures of metal oxides for the oxygen

reduction reaction and lithium-ion batteries, Angew. Chem. Int. Ed. 52 (2013)

-p

8643-8647.

re

[17] X. Wang, L. Yu, B.Y. Guan, S. Song, X.W.D. Lou, Metal-Organic Framework hybrid-assisted formation of Co3O4/Co-Fe oxide double-shelled nanoboxes for

lP

enhanced oxygen evolution, Adv. Mater. 30 (2018) 1801211.

na

[18] X. Gao, H. Zhang, Q. Li, X. Yu, Z. Hong, X. Zhang, C. Liang, Z. Lin, Hierarchical NiCo2O4 hollow microcuboids as bifunctional electrocatalysts for

ur

overall water-splitting, Angew. Chem. Int. Ed. 55 (2016) 6290-6294. [19] H. Zhu, D. Yu, S. Zhang, J. Chen, W. Wu, M. Wan, L. Wang, M. Zhang, M.

Jo

Du, Morphology and structure engineering in nanofiber reactor: tubular hierarchical integrated networks composed of dual phase octahedral CoMn2 O4/Carbon nanofibers for water oxidation, Small 13 (2017) 1700468.

[20] J. Pei, J. Mao, X. Liang, C. Chen, Q. Peng, D. Wang, Y. Li, Ir-Cu nanoframes: one-pot synthesis and efficient electrocatalysts for oxygen evolution reaction, 24

Chem. Commun. (Camb) 52 (2016) 3793-3796. [21] M.H. Oh, T. Yu, S.H. Yu, B. Lim, K.T. Ko, M.G. Willinger, D.H. Seo, B.H. Kim, M.G. Cho, J.H. Park, K. Kang, Y.E. Sung, N. Pinna, T. Hyeon, Galvanic replacement reactions in metal oxide nanocrystals, Science 340 (2013) 964-968. [22] S. Peng, L. Li, Y. Hu, M. Srinivasan, F. Cheng, J. Chen, S. Ramakrishna,

ro of

Fabrication of spinel one-dimensional architectures by single-spinneret electrospinning for energy storage applications, ACS Nano 9 (2015) 1945-1954.

-p

[23] H. Li, Z. Bian, J. Zhu, D. Zhang, G. Li, Y. Huo, H. Li, Y. Lu, Mesoporous

re

titania spheres with tunable chamber stucture and enhanced photocatalytic activity, J. Am. Chem. Soc. 129 (2007) 8406-8407.

lP

[24] A. Pan, H.B. Wu, L. Yu, X.W. Lou, Template-free synthesis of VO2 hollow

na

microspheres with various interiors and their conversion into V2O5 for lithium-ion batteries, Angew. Chem. Int. Ed. 52 (2013) 2226-2230.

ur

[25] Q. Qian, Y. Li, Y. Liu, L. Yu, G. Zhang, Ambient fast synthesis and active Sites deciphering

of

hierarchical

foam-like

trimetal-organic

framework

Jo

nanostructures as a platform for highly efficient oxygen evolution electrocatalysis, Adv. Mater. 31 (2019) 1901139.

[26] Y. Yang, Z. Lin, S. Gao, J. Su, Z. Lun, G. Xia, J. Chen, R. Zhang, Q. Chen, Tuning electronic structures of nonprecious ternary alloys encapsulated in graphene layers for optimizing overall water splitting activity, ACS Catal. 7 25

(2016) 469-479. [27] B. Zhang, X. Zheng, O. Voznyy, R. Comin, M. Bajdich, M. Garcia-Melchor, L. Han, J. Xu, M. Liu, L. Zheng, F.P. Garcia de Arquer, C.T. Dinh, F. Fan, M. Yuan, E. Yassitepe, N. Chen, T. Regier, P. Liu, Y. Li, P. De Luna, A. Janmohamed, H.L. Xin, H. Yang, A. Vojvodic, E.H. Sargent, Homogeneously dispersed multimetal oxygen-evolving catalysts, Science 352 (2016) 333-337.

ro of

[28] H. Xu, J. Cao, C. Shan, B. Wang, P. Xi, W. Liu, Y. Tang, MOF-derived hollow CoS decorated with CeOx nanoparticles for boosting oxygen evolution reaction electrocatalysis, Angew. Chem. Int. Ed. 57 (2018) 8654-8658.

G. Huang, F. Zhang, L. Zhang, X. Du, J. Wang, L. Wang, Hierarchical

-p

[29]

re

NiFe2O4/Fe2O3 nanotubes derived from metal organic frameworks for superior lithium ion battery anodes, J. Mater. Chem. A, 2 (2014) 8048-8053.

lP

[30] F. Millange, N. Guillou, M.E. Medina, G.r. Férey, A. Carlin-Sinclair, K.M.

na

Golden, R.I. Walton, Selective sorption of organic molecules by the flexible porous hybrid metal−organic framework MIL-53(Fe) controlled by various

ur

host−guest interactions, Chem. Mater. 22 (2010) 4237-4245. [31] J. Hanuza, M. Mączka, J.H. van der Maas, Polarized IR and Raman spectra of

Jo

tetragonal NaBi(WO4)2, NaBi(MoO4)2 and LiBi(MoO4)2 single crystals with scheelite structure, J. Mol. Struct. 348 (1995) 349-352.

[32] A. Yordanova, R. Iordanova, I. Koseva, V. Nikolov, R. Kukeva, Spectroscopic investigations of nanosized Cr3+ doped Sc2-xInx(WO4)3 : A new promising laser material, Luminescence 33 (2018) 1185-1193. 26

[33] F.-L. Li, Q. Shao, X. Huang, J.-P. Lang, Nanoscale trimetallic metal-organic frameworks enable efficient oxygen evolution electrocatalysis, Angew. Chem. Int. Ed. 57 (2018) 1888-1892. [34] J. Lee, S.-Y. Kwak, Tubular Superstructures Composed of α-Fe2O3 nanoparticles from pyrolysis of metal–organic frameworks in a confined space: effect on morphology, particle size, and magnetic properties, Cryst. Growth

ro of

Des. 17 (2017) 4496-4500.

[35] T. Wang, Y. Sun, Y. Zhou, S. Sun, X. Hu, Y. Dai, S. Xi, Y. Du, Y. Yang, Z.J.

Xu, Identifying influential parameters of octahedrally coordinated cations in

-p

spinel ZnMnxCo2–xO4 oxides for the oxidation reaction, ACS Catal. 8 (2018)

re

8568-8577.

[36] S. Zhao, Y. Wang, J. Dong, C.-T. He, H. Yin, P. An, K. Zhao, X. Zhang, C.

lP

Gao, L. Zhang, J. Lv, J. Wang, J. Zhang, A.M. Khattak, N.A. Khan, Z. Wei, J.

na

Zhang, S. Liu, H. Zhao, Z. Tang, Ultrathin metal–organic framework nanosheets for electrocatalytic oxygen evolution, Nat. Energy, 1 (2016) 16184.

ur

[37] P. Chen, Y. Tong, C. Wu, Y. Xie, Surface/interfacial engineering of inorganic low-dimensional electrode materials for electrocatalysis, Acc. Chem. Res. 51

Jo

(2018) 2857-2866.

[38] Z. Xue, Y. Li, Y. Zhang, W. Geng, B. Jia, J. Tang, S. Bao, H.-P. Wang, Y. Fan, Z.-w. Wei, Z. Zhang, Z. Ke, G. Li, C.-Y. Su, Modulating electronic structure of metal-organic framework for efficient electrocatalytic oxygen evolution, Adv. Energy Mater. 8 (2018) 1801564.. 27

[39] R. Liang, L. Shen, F. Jing, N. Qin, L. Wu, Preparation of MIL-53(Fe)-reduced graphene oxide nanocomposites by a simple self-assembly strategy for increasing interfacial contact: efficient visible-light photocatalysts, ACS Appl. Mater. Interfaces 7 (2015) 9507-9515. [40] P.W. Menezes, A. Indra, V. Gutkin, M. Driess, Boosting electrochemical water oxidation through replacement of Oh Co sites in cobalt oxide spinel with

ro of

manganese, Chem. Commun. (Camb) 53 (2017) 8018-8021.

[41] K. Rui, G. Zhao, Y. Chen, Y. Lin, Q. Zhou, J. Chen, J. Zhu, W. Sun, W. Huang, S.X. Dou, Hybrid 2D dual-metal-organic frameworks for enhanced water

-p

oxidation catalysis, Adv. Funct. Mater. 28 (2018) 1801554.

re

[42] Y. Zhang, H. Chen, C. Guan, Y. Wu, C. Yang, Z. Shen, Q. Zou, Energy-saving synthesis of MOF-derived hierarchical and hollow Co(VO3)2-Co(OH)2

lP

composite leaf arrays for supercapacitor electrode materials, ACS Appl. Mater.

na

Interfaces 10 (2018) 18440-18444.

[43] H. Chen, Z. Shen, Z. Pan, Z. Kou, X. Liu, H. Zhang, Q. Gu, C. Guan, J. Wang,

ur

Hierarchical micro-nano sheet arrays of nickel-cobalt double hydroxides for high-rate Ni-Zn batteries, Adv. Sci. (Weinh) 6 (2019) 1802002.

Jo

[44] F.Y. Xie, L. Gong, X. Liu, Y.T. Tao, W.H. Zhang, S.H. Chen, H. Meng, J. Chen, XPS studies on surface reduction of tungsten oxide nanowire film by Ar+ bombardment, J. Electron. Spectros. Relat. Phenomena. 185 (2012) 112-118. [45] T. Shinagawa, A.T. Garcia-Esparza, K. Takanabe, Insight on Tafel slopes from 28

a microkinetic analysis of aqueous electrocatalysis for energy conversion, Sci. Rep. 5 (2015) 13801. [46] S. Nandi, S.K. Singh, D. Mullangi, R. Illathvalappil, L. George, C.P. Vinod, S. Kurungot, R. Vaidhyanathan, Low band gap benzimidazole COF supported Ni3N as highly active OER catalyst, Adv. Energy Mater. 6 (2016) 1601189. [47] Y. Duan, Z.Y. Yu, S.J. Hu, X.S. Zheng, C.T. Zhang, H.H. Ding, B.C. Hu, Q.Q.

ro of

Fu, Z.L. Yu, X. Zheng, J.F. Zhu, M.R. Gao, S.H. Yu, Scaled-up synthesis of amorphous NiFeMo oxides and their rapid surface reconstruction for superior oxygen evolution catalysis, Angew. Chem. Int. Ed. 58 (2019) 15772-15777.

-p

[48] E. Fabbri, M. Nachtegaal, T. Binninger, X. Cheng, B.J. Kim, J. Durst, F.

re

Bozza, T. Graule, R. Schaublin, L. Wiles, M. Pertoso, N. Danilovic, K.E. Ayers, T.J. Schmidt, Dynamic surface self-reconstruction is the key of highly active

lP

perovskite nano-electrocatalysts for water splitting, Nat. Mater. 16 (2017)

[49]

na

925-931.

H.C. Chen, D.J. Jan, Y.S. Luo, K.T. Huang, Electrochromic and optical

ur

properties of tungsten oxide films deposited with DC sputtering by introducing hydrogen, Appl. Opt. 53 (2014) A321-329.

Jo

[50] J. Ma, Y. Ren, X. Zhou, L. Liu, Y. Zhu, X. Cheng, P. Xu, X. Li, Y. Deng, D. Zhao, Pt nanoparticles sensitized ordered mesoporous WO3 semiconductor: gas sensing performance and mechanism study, Adv. Funct. Mater. 28 (2018) 1705268. [51] L. Ma, Y. Hu, R. Chen, G. Zhu, T. Chen, H. Lv, Y. Wang, J. Liang, H. Liu, C. 29

Yan, H. Zhu, Z. Tie, Z. Jin, J. Liu, Self-assembled ultrathin NiCo2S4 nanoflakes grown on Ni foam as high-performance flexible electrodes for hydrogen evolution reaction in alkaline solution, Nano Energy 24 (2016) 139-147. [52] Z. Lu, W. Xu, W. Zhu, Q. Yang, X. Lei, J. Liu, Y. Li, X. Sun, X. Duan, Three-dimensional NiFe layered double hydroxide film for high-efficiency oxygen evolution reaction, Chem. Commun. (Camb) 50 (2014) 6479-6482. O. Diaz-Morales, D. Ferrus-Suspedra, M.T.M. Koper, The importance of

ro of

[53]

nickel oxyhydroxide deprotonation on its activity towards electrochemical water oxidation, Chem. Sci. 7 (2016) 2639-2645.

-p

[54] B.S. Yeo, A.T. Bell, In Situ Raman study of nickel oxide and gold-supported

Chem. C 116 (2012) 8394-8400.

re

nickel oxide catalysts for the electrochemical evolution of oxygen, J. Phy.

lP

[55] P.F. Liu, X. Li, S. Yang, M.Y. Zu, P. Liu, B. Zhang, L.R. Zheng, H. Zhao, H.G.

na

Yang, Ni2P(O)/Fe2P(O) interface can boost oxygen evolution electrocatalysis, ACS Energy Lett. 2 (2017) 2257-2263.

ur

[56] Q. He, H. Xie, Z.u. Rehman, C. Wang, P. Wan, H. Jiang, W. Chu, L. Song, Highly defective Fe-based oxyhydroxides from electrochemical reconstruction

Jo

for efficient oxygen evolution catalysis, ACS Energy Lett. 3 (2018) 861-868.

[57] T. Tian, J. Jiang, L. Ai, In situ electrochemically generated composite-type CoOx/WOx in self-activated cobalt tungstate nanostructures: implication for highly enhanced electrocatalytic oxygen evolution, Electrochim. Acta. 224 (2017) 551-560. 30

[58] H. Fang, T. Huang, D. Liang, M. Qiu, Y. Sun, S. Yao, J. Yu, M.M. Dinesh, Z. Guo, Y. Xia, S. Mao, Prussian blue analog-derived 2D ultrathin CoFe2O4 nanosheets as high-activity electrocatalysts for the oxygen evolution reaction in alkaline and neutral media, J. Mater. Chem. A 7 (2019) 7328-7332. [59]

H.J. Song, H. Yoon, B. Ju, G.-H. Lee, D.-W. Kim, 3D architectures of quaternary Co-Ni-S-P/Graphene hybrids as highly active and stable

ro of

bifunctional electrocatalysts for overall water splitting, Adv. Energy Mater. 8 (2018) 1802319.

[60] J. Hu, C. Zhang, L. Jiang, H. Lin, Y. An, D. Zhou, M.K.H. Leung, S. Yang,

-p

Nanohybridization of MoS2 with layered double hydroxides efficiently

[61]

re

synergizes the hydrogen evolution in alkaline media, Joule 1 (2017) 383-393. B. Zhang, K. Jiang, H. Wang, S. Hu, Fluoride-induced dynamic surface

lP

self-reconstruction produces unexpectedly efficient oxygen-evolution catalyst,

Jo

ur

na

Nano Lett. 19 (2019) 530-537.

31

Figure captions Fig. 1. The schematically illustrated general formation process of amorphous mixed metal oxide hollow nanoprisms based on anion exchange reaction with FeCo-MIL-88B template. Fig. 2. Morphological evolution from FeCo-MIL-88B to amorphous mixed metal

ro of

oxide hollow nanoprisms: (A, B) FESEM images and (C) TEM images of FeCo-MIL-88B solid nanoprisms; (D, E) FESEM, (F) TEM images and (G) corresponding elemental mapping results of A-FeCoWOx-HoNPrs.

images

and

corresponding

elemental

mapping

results

for

(A1-A4)

re

TEM

-p

Fig. 3. The verification of the generality of the anion-exchange strategy: FESEM,

A-FeCoVOx-HoNPrs; (B1-B4) A-FeCoPOx-HoNPrs; (C1-C4) A-FeCoBOx-HoNPrs

4.

The

chemical

state

analysis

between

the

FeCo-MIL-88B

and

na

Fig.

lP

and (D1-D4) A-FeCoSeOx-HoNPrs, respectively.

A-FeCoWOx-HoNPrs through the comparison of their XPS characterization: (A)

ur

Survey spectra; High resolution spectra of (B) Fe 2p; (C) Co 2p; (D) O 1s and (E) W 4f, respectively.

Jo

Fig. 5. Electrocatalytic OER performance in 1.0 M KOH electrolyte: (A) The polarization curves at a scan rate of 5 mV s-1 after iR-correction, (B) the comparing overpotential at the current density of 10 and 100 mA cm-2;

(C) the corresponding

Tafel plots; (D) capacitive current density as a function of scan rate for the FeCo-MIL-88B, commercial RuO2 and various amorphous mixed metal oxide hollow 32

nanoprisms

including

A-FeCoWOx-100,

A-FeCoVOx-100,

A-FeCoPOx-100,

A-FeCoBOx-100, A-FeCoSeOx-100 HoPrs; (E) the polarization curves after successive CV cycles and (F) chronoamperometry measurement at an overpotential of 297 mV (no iR-correction) for A-FeCoSeOx-100 HoNPrs. Fig. 6. The investigation on the underlying origination of the OER activity: (A) In-situ Raman spectroscopy measurement of the A-FeCoWOx-HoNPrs under

ro of

programmed applied potentials in 0.1 M KOH; High-resolution XPS spectra for (B)

Jo

ur

na

lP

re

-p

Fe 2p, (C) Co 2p and (D) O 1s after OER test.

33

ro of -p re lP na ur

Fig. 1. The schematically illustrated general formation process of amorphous mixed

Jo

metal oxide hollow nanoprisms based on anion exchange reaction with FeCo-MIL-88B template.

34

ro of -p

Fig. 2. Morphological evolution from FeCo-MIL-88B to amorphous mixed metal

re

oxide hollow nanoprisms: (A, B) FESEM images and (C) TEM images of

lP

FeCo-MIL-88B solid nanoprisms; (D, E) FESEM, (F) TEM images and (G)

Jo

ur

na

corresponding elemental mapping results of A-FeCoWOx-HoNPrs.

35

ro of

images

and

corresponding

elemental

mapping

re

TEM

-p

Fig. 3. The verification of the generality of the anion-exchange strategy: FESEM, results

for

(A1-A4)

A-FeCoVOx-HoNPrs; (B1-B4) A-FeCoPOx-HoNPrs; (C1-C4) A-FeCoBOx-HoNPrs

Jo

ur

na

lP

and (D1-D4) A-FeCoSeOx-HoNPrs, respectively.

36

ro of -p The

chemical

state

re

4.

analysis

lP

Fig.

between

the

FeCo-MIL-88B

and

A-FeCoWOx-HoNPrs through the comparison of their XPS characterization: (A)

na

Survey spectra; High resolution spectra of (B) Fe 2p; (C) Co 2p; (D) O 1s and (E) W

Jo

ur

4f, respectively.

37

ro of

Fig. 5. Electrocatalytic OER performance in 1.0 M KOH electrolyte: (A) The

polarization curves at a scan rate of 5 mV s-1 after iR-correction, (B) the comparing (C) the corresponding

-p

overpotential at the current density of 10 and 100 mA cm-2;

re

Tafel plots; (D) capacitive current density as a function of scan rate for the FeCo-MIL-88B, commercial RuO2 and various amorphous mixed metal oxide hollow including

A-FeCoWOx-100,

lP

nanoprisms

A-FeCoVOx-100,

A-FeCoPOx-100,

na

A-FeCoBOx-100, A-FeCoSeOx-100 HoPrs; (E) the polarization curves after successive CV cycles and (F) chronoamperometry measurement at an overpotential of

Jo

ur

297 mV (no iR-correction) for A-FeCoSeOx-100 HoNPrs.

38

ro of -p

re

Fig. 6. The investigation on the underlying origination of the OER activity: (A)

lP

In-situ Raman spectroscopy measurement of the A-FeCoWOx-HoNPrs under programmed applied potentials in 0.1 M KOH; High-resolution XPS spectra for (B)

Jo

ur

na

Fe 2p, (C) Co 2p and (D) O 1s after OER test.

39