Prussian blue analogue-derived Mn–Fe oxide nanocubes with controllable crystal structure and crystallinity as highly efficient OER electrocatalysts

Journal Pre-proof Prussian blue analogue-derived Mn–Fe oxide nanocubes with controllable crystal structure and crystallinity as highly efficient OER electrocatalysts Quanyin Ma, Rui Dong, Heng Liu, Anquan Zhu, Lulu Qiao, Yongjin Ma, Juan Wang, Jianping Xie, Jun Pan PII:

S0925-8388(19)34684-5

DOI:

https://doi.org/10.1016/j.jallcom.2019.153438

Reference:

JALCOM 153438

To appear in:

Journal of Alloys and Compounds

Received Date: 3 June 2019 Revised Date:

17 September 2019

Accepted Date: 17 December 2019

Please cite this article as: Q. Ma, R. Dong, H. Liu, A. Zhu, L. Qiao, Y. Ma, J. Wang, J. Xie, J. Pan, Prussian blue analogue-derived Mn–Fe oxide nanocubes with controllable crystal structure and crystallinity as highly efficient OER electrocatalysts, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153438. 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. © 2019 Published by Elsevier B.V.

Mn-Fe PBA were used as precursor to obtain Mn-Fe ternary oxides with controllable crystal structure and crystallinity for oxygen evolution

Prussian Blue Analogue-Derived Mn-Fe Oxide Nanocubes with Controllable Crystal Structure and Crystallinity as Highly Efficient OER Electrocatalysts

Quanyin Ma a, Rui Dong a, Heng Liu a, Anquan Zhu a, Lulu Qiao a, Yongjin Ma a, Juan Wang a, Jianping Xie*, b, and Jun Pan*, a

a

State Key Laboratory for Powder Metallurgy, Central South University,

Lushan South Road 932, Changsha 410083, P. R. China b

School of Minerals Processing and Bioengineering, Central South

University, Lushan South Road 932, Changsha 410083, P. R. China

*To whom correspondence should be addressed: [email protected]

ABSTRACT A series of ternary manganese iron oxides with different crystal structures, oxidation states and crystallinities were successfully fabricated by modulating the calcination conditions of Mn-Fe Prussian blue analogue (PBA) precursor (Mn3[Fe(CN)6]2·nH2O). The obtained Mn-Fe oxides retained the nanocubic morphology of the PBA precursor, and a mesoporous structure was acquired as a result of gas molecule release during the pyrolysis process. Electrochemical oxygen evolution reaction (OER) activity of the as-prepared catalysts was tested, and among the bimetallic oxides, the catalyst that had a crystal structure similar to cubic bixbyite Mn1.2Fe0.8O3 (space group: Ia-3) with low crystallinity exhibited the most advanced OER activity. An overpotential of only 245 mV was required to achieve a current density of 10 mA cm-2, and the Tafel slope value was only 38 mV dec-1. The excellent OER activity is likely due to the hollow porous morphology of the samples, the synergistic effect of Mn and Fe, the defect-rich low crystallinity of the catalyst, and the cubic Mn1.2Fe0.8O3 structure (space group: Ia-3), which has an intrinsic activity superior to that of spinel Mn1.8Fe1.2O4.

Keywords: Prussian blue analogous; Oxygen evolution reaction; Electrocatalyst; Mn-Fe oxides; Water splitting

1. Introduction With the increasing consumption of non-renewable fossil fuels and the resulting deterioration of the environment, it is of vital importance to develop new energy sources. Electrocatalysis is considered to be one of the most promising options for providing clean and renewable energy and addressing global environmental pollution and energy issues [1-4]. As one of the half reactions of electrochemical water splitting process, the oxygen evolution reaction (OER) is more energy intensive than the hydrogen evolution reaction (HER) due to the intrinsically slow kinetics of multistep proton-coupled electron transfer [5-7]. Although noble metal catalysts such as RuO2 and IrO2 exhibit high OER activity, their limited availability and high cost hinder their use in broad applications. Therefore, exploring potential oxygen evolution reaction catalysts with excellent catalytic activity and low cost is urgently required. Transition metal-based electrocatalysts, which have attracted extensive attention in recent years, provide a benefit due to their natural abundance, low cost and high catalytic activity [2, 8-14]. Many studies have focused on different aspects of transition metal-based electrocatalysts, including preparation methods and their chemical composition, crystal structure, and valence state [8, 10, 15-19]. Robinson et al. [20] systematically synthesized eight types of manganese oxides and compared their water oxidation activity. The results showed that Mn(III)-containing materials exhibited superior catalytic performance, and they ascribed the intrinsic high activity to the weaker, more flexible Mn(III)-O bonds in the edge sharing a MnO6 octahedral that arises from the occupation of the antibonding eg orbital of Mn(III). Moreover,

compared with monometallic catalysts, bimetallic transition metal compounds are considered to have higher catalytic activity due to the synergistic effect between different transition metal elements [21-24]. Li et al. used operando differential electrochemical mass spectrometry (DEMS) and X-ray absorption spectroscopy (XAS) to investigate the atomic structure and oxidation state of (Ni, Fe)OOH during the OER process. Their experimental results indicated that high valence nickel (Ni4+) is the active site of OER, while doping with iron promotes the formation of Ni4+ [24]. Recently, an increasing number of studies have demonstrated that amorphous and low-crystallinity catalysts can exhibit higher electrocatalytic activities compared with their high crystalline counterparts [14, 25-29]. The improved catalytic performance can be attributed to the rich surface defects that serve as active catalytic sites and to high-energy unsaturated bonds, which promote the adsorption of intermediates on the catalyst surface. Indra et al. applied a facile solvothermal method to synthesize cobalt-iron mixed metal oxides and obtained both amorphous and crystalline catalysts by changing the solvent and reaction time [28]. The photochemical catalysis and electrochemical catalysis results indicated that the amorphous catalyst had superior catalytic performance compared with the crystalline catalyst. Jiang et al. developed a facile aqueous reaction to obtain OER catalysts with nickel boride (Ni3B) cores and nickel borate (Ni-Bi) shells; the corresponding OER tests showed that the electrochemical activity was highly relevant to the crystallinity of the Ni-Bi shells [30]. The catalysts with partially crystalline Ni-Bi shells presented much better activity than the amorphous or crystalline analogues. Kuang et al. synthesized

Fe-Mn-O

hybrid

nanosheets

through

a

reflux

reaction

and

subsequent

low-temperature calcination process and attributed the superior OER performance to the defect-rich low-crystallinity nanosheet structure that exposed more catalytically active sites [14]. These studies indicate that electrocatalysts with amorphous or low-crystalline structures represent a great developmental potential and that the modulation of crystallinity could be an attractive way to enhance intrinsic electrochemical activity and to investigate the highly reactive sites of the catalysts. Prussian blue analogues (PBAs) are a type of metal-organic framework (MOF) and are composed of transition mental ion nodes connected by cyano group ligands. Recently, PBAs have attracted much attention due to their broad application prospects in the fields of catalysis, energy storage and medicine [31-40]. PBAs can be produced by facile preparation methods and used as precursors and templates to obtain products with desired morphologies and properties [33, 35, 36, 41]. Wang et al. [42] and Guo et al. [43] respectively used Mg-Fe and Co-Fe Prussian blue analogs as precursors and obtained corresponding spinel ternary transition metal oxides via pyrolysis in air for use in lithium-ion battery (LIB) anodes. Zou et al. prepared Ni-Fe PBA cubes and converted them into hollow (Ni0.62Fe0.38)2P nanocubes by heating in a furnace, with NaH2PO4·2H2O as the phosphorus source [36]. The obtained Ni-Fe bimetallic phosphide nanocubes displayed a much better OER performance than binary phosphide Ni-P or Fe-P. Herein, we report a facile solution precipitation method to prepare Mn-Fe PBA precursor nanocubes and then convert them into Mn-Fe ternary oxides by thermal

decomposition in air. The as-prepared Mn-Fe oxides maintain the nanocubic morphology, with a hollow and mesoporous structure formed during the pyrolysis process that can facilitate OER activity. Moreover, by adjusting the calcination conditions, Mn-Fe oxides with different crystal structures and crystallinities were obtained. The electrochemical OER activity of a series of resulting samples was tested, and the results showed that the Mn1.2Fe0.8O3 sample with a low crystallinity had the highest OER activity. Several factors act synergistically to give rise to the high electrochemical efficiency. We expect that this work will help to develop a more profound understanding of fabricating PBA-derived transition metal base catalysts.

2. Experimental section 2.1 Material preparation All chemicals employed were commercially available and were used without any further purification. Preparation of Mn-Fe PBA precursor nanocubes: Mn-Fe PBA nanocubes were prepared using a facile solution method. First, MnSO4·H2O (243.39 mg) was dissolved into a mixed solution of deionized water/ethanol (40/40 ml) under stirring to obtain a transparent solution (solution A). Second, K3[Fe(CN)6]·H2O (158.04 mg) was also dissolved in deionized water/ethanol (40/40 ml) and stirred to give a transparent solution. Next, PVP (K30, 2.5 g) was added into this solution and stirred until a homogeneous and transparent solution was formed (solution B). Solution A was then slowly dripped into solution B under vigorous stirring, and after stirring for another 10 min the obtained mixture was incubated for 24 h at room temperature without further manipulation. Finally, the resulting brown precipitate was centrifuged and washed with absolute ethanol several times and dried overnight at 60 . Preparation of Mn-Fe bimetallic oxide nanocubes: Various Mn-Fe ternary oxides were prepared by thermal decomposition of the as-synthesized Mn-Fe PBA nanocubes under different calcination conditions. On the one hand, the PBA precursor was calcined directly at 450 , 500 increasing gradient of 2

and 550

in air for 2 h respectively, with an

min-1. The resulting oxides were denoted as MFO-T, where

T indicates the calcination temperature (T= 450, 500, 550). The MFO-450 sample was also annealed under N2 atmosphere in a furnace tube at 350

for 2 h and named as

MFO-450-annealing. Furthermore, a two-stage pyrolysis was carried out. The precursor was first calcined at 450

for 2 h with an increasing gradient of 2

and the temperature was then quickly increased to 550

min-1,

at a heating rate of 10

min-1 and maintained for either 5 min, 20 min, 30 min or 45 min. The corresponding oxides were denoted as MFO-TS-t, where t indicates the heating time at 550

(t= 5,

20, 30, 45). 2.2 Material characterization The phase analysis of the samples was performed by powder X-ray diffraction (XRD, Rigaku Corporation, D/max 2550, Cu-Kα radiation, λ = 0.15405 nm). The surface morphology and element mapping was observed by using a scanning electron microscope (SEM, Helios Nanolab G3 UC, FEI Co., Ltd). Transmission electron microscopy and high-resolution transmission electron microscopy images were obtained by a transmission electron microscope (TEM, Tecnai G2 F20, FEI Co., Ltd, USA). X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi Thermo Fisher-VG Scientific, Al-Kα radiation, Pass Energy 30.0) was used to analyze the surface chemical composition of the samples. The Brunauer−Emmett−Teller (BET) nitrogen adsorption data were measured at 77 K using an ASAP 2020 surface area analyzer to obtain the surface area and pore distribution of the samples. 2.3 Electrochemical measurements Electrochemical measurements were performed on an electrochemical workstation (CHI 660E) with a conventional three-electrode system; the platinum plate, saturated Ag/AgCl electrode (EθAg/AgCl=0.1976 V, at 25 ) and Ni foam loading with catalyst

served as the counter, reference, and working electrodes, respectively. To prepare the working electrode, 5 mg of catalyst was added into a solvent composed of 20 µL of Nafion (5 wt %) and 480 µL of ethanol (for a total of 500 µL). The catalyst was then dispersed by 1 h of ultrasonication and formed a homogeneous ink. Finally, 100 µL of ink was dropped onto a 1×1 cm Ni foam (loading density 1 mg cm-2) and dried for 2 h in a vacuum at 60 . The OER test was performed in O2-saturated 1 M KOH electrolyte at room temperature. First, cyclic voltammetry (CV) from -0.1 to 0.1 V (versus Ag/AgCl electrode) at a scan rate of 50 mV s-1 for 40 cycles was conducted to activate the working electrode. Next, CV curves (-0.1 to 0.1 V vs. Ag/AgCl) at various sweep rates (20, 40, 60, 80, 100 mV s−1) were tested by calculating the difference between the two current densities (∆j=(ja-jc)/2) at 0.05 V vs. Ag/AgCl; this was used it to draw the plots of ∆j against the scan rate. In this way, the double-layer capacitance (Cdl) was obtained. To investigate the OER activity of the catalysts, polarization curves (linear sweep voltammetry, LSV) with 90% iR compensation were measured at a scan rate of 5 mV s−1. Furthermore, electrochemical impedance spectroscopy (EIS) measurements were conducted over the frequency range of 0.01 Hz to 100 kHz at 0.5 V (vs. Ag/AgCl) to acquire the Nyquist plots. Turnover frequency (TOF) is calculated according to the equation: TOF=jS/zFn, where j is the current density (mA cm-2), S is the surface area of the working electrode (1 cm-2), z=4 for the OER process, F= 96483.3 C mol-1, and n is the number of moles of metal atoms on the electrode. Moreover, due to the fact that Cl ion in a Ag/AgCl electrode may release to the

electrolyte in an alkaline electrolyte and affect the test accuracy, we also used an Hg/HgO electrode (EθHg/HgO=0.1976 V, at 25 ) as the reference electrode to test the LVS, amperometric i-t curve and multi-current density process of the best-performing sample. All potentials were calibrated to a reversible hydrogen electrode (RHE) following the Nernst equation.

3. Results and discussion A schematic diagram of the preparation process is illustrated in Fig. 1. In our experiment, first of all, we prepared Mn-Fe Prussian blue analogue nanocubes via a facile solution method. As shown in Fig. 2a, the X-ray diffraction (XRD) pattern of the Mn-Fe PBA precursor can be indexed to Mn3[Fe(CN)6]2·nH2O (ICSD: 240929), and no other peaks indicating impurities were detected. To characterize the morphology of Mn-Fe PBA we used scanning electron microscopy (SEM) to obtained SEM images, as shown in Fig. 2b. The as-synthesized Mn-Fe precursor had a uniform nanocubic morphology with an average size of approximately 200 nm. A series of Mn-Fe ternary transition metal oxides were obtained by calcining the precursor at various experimental parameters in air. To investigate the influence of calcination temperature, the PBA precursor was calcined at 450 , 500 air for 2 h respectively, with a heating rate of 2

and 550

in

min-1. The obtained samples were

named as MFO-T (T=450, 500, 550). As shown in Fig. S1, the SEM images indicated that all of these samples retained the nanocubic structure of the Mn-Fe PBA precursor, while the size of the cubes shrank slightly. To further investigate the crystal structure of these oxides, X-ray diffraction (XRD) analysis was conducted. According to the XRD patterns shown in Fig. 3a, the samples that underwent direct calcining at 450 (MFO-450) and 500

(MFO-500) have diffraction peaks that can be indexed to the

standard diffraction pattern of spinel structure Mn1.8Fe1.2O4 (PDF #75-0035, space group: Fd3m). The weak and broad diffraction peaks indicate that the crystallinity of these samples is low, and the deviation from the standard pattern may be due to the

broadening effect of the small crystalline size [34]. Furthermore, according to Fig. S2, the XRD pattern of MFO-450 after annealing (MFO-450-annealing) showed a pattern similar to MFO-450, but it had higher and sharper diffraction peaks and a smaller deviation from the Mn1.8Fe1.2O4 standard, indicating increased crystallinity and crystal size after annealing. Nevertheless, when the heating temperature was further increased to 550 , the corresponding XRD pattern (Fig. 3a) demonstrated that the crystal structure was transformed to a bixbyite cubic structure, Mn1.2Fe0.8O3 (PDF #75-0894, space group: Ia-3), and the crystallinity was high, which can be deduced from the strong and narrow diffraction peaks. Further raising the calcining temperature to 600

or more resulted in the damage of the nanocubic structure (Fig. S3a), which

hinders electrochemical performance. The corresponding XRD pattern (Fig. S3b) shows that the basis component was still MnxFe2-xO3, while the newly emerging peaks in XRD could be index to MnO2 (PDF #81-2261); MnO2 is considered to be an inferior OER catalyst compared with Mn2O3 [19, 44], which may have further decreased the OER activity compared with the samples obtained at lower temperature. In summary, by adjusting the calcination temperature, Mn-Fe ternary oxides with different crystal structures can be obtained, and the crystalline transformation from spinel Mn1.8Fe1.2O4 to bixbyite Mn1.2Fe0.8O3 occurs at 550 . To further modulate the crystal structure and crystallinity we applied a two-stage pyrolysis method. The Mn-Fe PBA precursor was first heated at 2

min-1 to 450

and maintained there for 2 h to thoroughly complete the thermal decomposition process, it was then rapidly warmed at 10

min-1 to reach 550

and held there for 5,

20, 30 or 45 min; these samples are denoted as MFO-TS-t (t= 5, 20, 30, 45). The SEM images of the MFO-TS-t samples are shown in Fig. 4a-d. Similar to the MFO-T samples, the nanocubic structure had mostly remained, and the surface of these samples became rough and porous, which was caused by the release of small gas molecules (NOx and CO2 etc.) during the thermal decomposition. The porous morphology with high specific area increased the exposure of active sites and facilitated the mass transport during OER. Additionally, the elemental mapping of MFO-TS-30 nanocubes shown in Fig. 4g indicates the homogeneous distribution of Fe, Mn, and O elements. The MFO-TS-t sample XRD results are shown in Fig. 3b and Fig. S4. The diffraction pattern of MFO-TS-5 still match to spinel Mn1.8Fe1.2O4, which means that the majority of the crystal remained unchanged after such a short time at 550 . When the time at 550

was increased to 20 min, the spinel Mn1.8Fe1.2O4 diffraction peaks

basically disappeared, and no other newly generated peaks could be observed. This finding indicates that the MFO-TS-20 sample may be in an intermediate state of crystalline transition. The peaks that represent cubic Mn1.2Fe0.8O3 emerged when the soaking time was further prolonged to 30 min or more, and the crystallinity increased during the heat preservation time; the weak diffraction peaks of the MFO-TS-30 sample changed into stronger and narrower peaks of MFO-TS-45 and MFO-550 (Fig. S4b). Notably, the valence state of Mn and Fe in Mn1.2Fe0.8O3 is +3, which is higher than the valence state of Mn and Fe in Mn1.8Fe1.2O4 (+2, +3, +3). Trivalent manganese and iron are considered to be more catalytically active according to the descriptors for

OER (eg occupancy, covalency etc.) and previous reports [10, 16, 17, 19, 20, 44-47], potentially endowing Mn1.2Fe0.8O3 samples with a higher intrinsic OER activity. It is well known that the transition metal oxides have broad application prospects in the field of energy conversion due to their excellent activity and low cost. Nevertheless, among the widely investigated first-row transition metal (Mn, Fe, Co, Ni, Cu, Zn etc.) ternary oxides that are derived from corresponding PBAs, according to previous reports, the pyrolysis products are basically spinel AB2O4 structures, while only Mn-Fe Prussian blue analogue precursors could be converted to ternary oxides with cubic MnxFe2-xO3 structures [23, 34, 38, 43, 48-53]. The probable reason for this may be that both manganese and iron can form monometallic oxides with crystal structures that analogous to MnxFe2-xO3. As shown in Fig. S5, the diffraction peaks of β-Fe2O3 (PDF #39-0238), α-Mn2O3 (PDF #41-1442), and Mn1.2Fe0.8O3 (PDF #75-0894) are almost exactly the same. The similar crystal structure and crystal plane spacing enable the formation of Mn-Fe oxides with a MnxFe2-xO3 structure, which can be explained by the solid solution theory. Therefore, the crystalline nature of manganese iron oxides facilitates the formation of Mn1.2Fe0.8O3 during the calcination process at 550

and makes it peculiar compared with other ternary

first-row transition metal oxides. Therefore, besides the more active +3 oxidation state of Mn and Fe in Mn1.2Fe0.8O3, Mn3+ and Fe3+ in bixbyite structures are octahedrally coordinated by oxygen anions. These [MO6] (M=Mn or Fe) octahedra share corners and edges and exhibit a wide range of different M-M and M-O bond distances due to the Jahn−Teller effect [54]. The more flexible structure could benefit the OER

performance, which has been considered to be the key factor enabling the highest water oxidation activity of α-Mn2O3 among the manganese monometallic oxides, shown in many previous studies [10, 20, 55, 56]. Nevertheless, to the best of our knowledge, bixbyite structure Mn-Fe bimetallic oxides have rarely been investigated in the field of oxygen evolution. Transmission electron microscopy (TEM) was employed to further characterize the MFO-TS-30 sample, as shown in Fig. 4e. The relatively compact shell and loose center verify the hollow structure, which can be explained by the Kirkendall effect; meanwhile, the foam-like inner architecture indicates the presence of numerous pores. These results suggest that the atomic-scale dispersion of Mn and Fe in the Mn3[Fe(CN)6]2·nH2O precursor facilitates the uniform nucleation of Mn1.2Fe0.8O3 nanoparticles during calcination, and the thermal decomposition of cyan linkers (-CN) into the released gases prevents aggregation of the generated nanoparticles, resulting in the porous morphology of the nanocubes. The high-resolution TEM (HR-TEM) and selected area electron diffraction (SAED) patterns were also obtained to further explore the crystal structure and crystallinity. The HR-TEM image shown in Fig. 4f reveals the partially crystalline structure of the MFO-TS-30 sample; the lattice spacing of 0.201 nm in the crystalline regions can be assigned to the (332) plane of Mn1.2Fe0.8O3. The partially crystallized Mn1.2Fe0.8O3 can possess abundant grain boundaries, defect sites, and dangling bonds, which are generally considered to be the catalytically active sites, therefore leading to the enhanced OER performance. Moreover, the diffraction rings in the SAED pattern (inset in Fig. 4f) further indicate

the polycrystalline nature of MFO-TS-30. These results are consistent with the XRD analysis, demonstrating the defect-rich and low-crystallinity Mn1.2Fe0.8O3 structure of the MFO-TS-30 sample. The Brunauer−Emmett−Teller (BET) surface area of MFO-TS-30 was also tested and confirmed the large surface of the MFO-TS-30 sample (80.66 m2/g). To further verify the porous structure of various Mn-Fe oxide nanocubes, the BET nitrogen adsorption analysis was applied to MFO-450 and MFO-TS-30. As shown in Fig. 5, the N2 adsorption-desorption isotherms of both samples exhibit a typical type IV isotherm with an H3-type hysteresis loop, indicating the existence of a large number of mesopores. Moreover, MFO-450 showed a higher surface area (100.8092 m²/g) and a smaller average pore diameter (inset Fig. 5) compared with MFO-TS-30, implying that the oxide nanoparticles may suffer slight aggregation under prolonged calcination. Nevertheless, the BET results indicate that the PBA-derived Mn-Fe nanocubic oxides have a large mesoporous surface area resulting from the gas released during thermal decomposition. This high specific surface area would promote electrocatalytic OER activity. The surface valence state and chemical composition of the as-prepared catalysts were analyzed by X-ray photoelectron spectroscopy (XPS). As shown in Fig. S6a, the C 1s, O 1s, Mn 2p, and Fe 2p peaks in the survey spectrum further confirm the presence of Mn, Fe, and O elements in the MFO-TS-30 sample. Figure 6a displays the high-resolution Fe 2p profile. Deconvolution of the Fe 2p energy region was achieved according to previous reports [17, 57-59]. Due to the complex multiplet splitting

arising from unpaired electrons, the Fe 2p3/2 peak was well fitted by five multiplets belonging to Fe3+. The peak splitting between Fe 2p3/2 (710.5 eV) and the satellite (718.6 eV) was 8.1 eV, in accordance with the Fe3+ oxidation state (approximately 5 eV for Fe2+) [17]. Moreover, the binding energy interpolation of Fe 2p3/2 and Fe 2p1/2 was 13.6 eV, further confirming the dominance of Fe3+ in MFO-TS-30 [14]. For the Mn 2p spectra, because the Mn 2p peaks for the Mn oxides also have many multiplet-split components, deconvolution was applied with the multiplet structure, as shown in Fig. 6b. After peak deconvolution, the Mn 2p3/2 peak could be fitted appropriately by the characteristic multiplet structure of Mn3+ [57, 59, 60], and no satellite peak between the Mn 2p3/2 and Mn 2p1/2 peaks was observed, indicating the absence of Mn2+. Therefore, the Mn 2p spectra validates that the manganese present in MFO-TS-30 is in the form of Mn3+. Previous investigations have demonstrated that Mn3+ sites are the catalytically active species, rather than Mn2+ or Mn4+. This means that samples with an Mn1.2Fe0.8O3 structure could have superior OER activity compared with Mn1.8Fe1.2O4 oxides [10, 19, 20, 44]. To compare the oxidation state of Mn and Fe in different crystal structures, we further compared the Fe 2p and Mn 2p spectra of MFO-450, MFO-TS-20 and MFO-TS-30. As shown in Fig. S7, MFO-TS-20 has XPS peak features and a binding energy similar to MFO-TS-30, indicating that the oxidation of Mn and Fe in MFO-TS-20 is also +3. Meanwhile, the lower binding energy of Mn and Fe in MFO-450 implies a lower average oxidation state, which is in accordance with the Mn1.8Fe1.2O4 crystal structure. The O 1s spectra in Fig. S6b was also resolved. The three peaks at 529.6, 531.1 and 532.1 eV could be

respectively ascribed to the lattice oxygen of Mn1.2Fe0.8O3, the oxygen of the hydroxide ion, and adsorbed molecular water. To investigate the OER activity of different manganese iron oxides we conducted linear scan voltammetry (LSV) in an alkaline solution. Polarization curves for different electrodes are shown in Fig. 7a and Fig. S8a. Among all of the Mn-Fe-O samples, MFO-TS-30 exhibited the best electrocatalytic OER performance. Figure 7b displays the required overpotential at 10 mA cm−2 for all samples. As can be seen from the histogram, for the best performing sample (MFO-TS-30), only 245 mV overpotential was required to reach a 10 mA cm−2 current density, even outperforming the state-of-the-art RuO2 electrode shown in Fig. S9 (282 mV at 10 mA cm−2). The LSV curves of the MFO-TS-t samples were transformed to mass activity (Fig. S10), and the low-crystallinity Mn1.2Fe0.8O3 samples could produce an activity of 81.5 A mg-1 under 280 mV of overpotential. A high mass activity was demonstrated for MFO-TS-30. Meanwhile, the Tafel slopes of various MFO-TS-t catalysts were obtained to analyze the electrochemical oxygen evolution kinetics. As can be seen from Fig. 7c, the MFO-TS-30 sample had the smallest Tafel slope (38 mV dec−1), superior to those of MFO-TS-5 (49 mV dec−1), MFO-TS-20 (43 mV dec−1), MFO-TS-45 (42 mV dec−1) and Ni foam (126 mV dec−1). A smaller Tafel slope reflects superior catalytic kinetics during OER, enabling a larger current density at the same overpotential. The TOF for OER at η = 280 mV for various samples is shown in Table S1. MFO-TS-30 exhibited the largest TOF (0.01597 s-1), further confirming its outstanding intrinsic activity. Mn and Fe are regarded as the active sites

when calculating the TOF; however, the detailed mechanism for OER on Mn1.2Fe0.8O3 interfaces could be more complicated. It has been speculated that during electrocatalytic water oxidation, electronic excitation could create oxygen radicals or peroxo intermediates on Mn and Fe elements in highly active corner-shared [M3+O6] octahedrons, releasing O2 from these sites [10, 20, 61, 62]. The stability of an electrocatalyst is a vital indicator of its potential for use in large-scale energy conversion systems. Therefore, the amperometric i-t curve was tested at a current density of 10 mA cm-2 for the MFO-TS-30 sample. After 20 h of electrochemical OER the current density (shown in Fig. 7d) presented no evident attenuation, and the current density retention rate (Ir) of the MFO-TS-30 catalyst after the long durability test was 94.1%, demonstrating the high stability of the low-crystallinity Mn1.2Fe0.8O3 oxides derived from Mn-Fe PBAs. The electrocatalytically active surface area (ECSA) is considered as a key factor to evaluate the electrocatalytic performance [63, 64, 65]; generally, the double-layer capacitance (Cdl) is applied to estimate the ECSA. Therefore, we conducted cyclic voltammetry (CV) measurements at various sweep rates from -0.1 to 0.1 V (vs. Ag/AgCl), where the current response is relevant only with charging of the double layer to calculate Cdl [64]. The double-layer capacitance (Cdl) is equal to the curve slope of ∆j vs. the scan rate (Fig. 8a), which can be obtained by linear fitting. The obtained Cdl values of the MFO-TS-t samples are depicted in Fig. 8a. MFO-TS-30 displayed the largest Cdl (4.10 mF cm-2), which means it could provide more accessible electrochemically active sites during the oxygen evolution process.

Electrochemical impedance spectrum (EIS) measurement was also conducted to further analyze the OER performance by exploring the charge transfer ability. The obtained Nyquist plots are shown in Fig. 8b and Fig. S8b. The charge transfer resistance (Rct) was assessed based on the equivalent circuit. According to the fitting results, the Rct of various samples showed a performance difference that was basically identical to the LSV results, where the MFO-TS-30 sample presented the smallest Rct (1.95 Ω), implying an accelerated charge transfer ability and superior catalytic reaction kinetics. Due to the fact that Cl ion in the Ag/AgCl electrode may be released in an alkaline electrolyte and affect the test accuracy, we also used Hg/HgO as a reference electrode to test the LVS, amperometric i-t curve and multi-current density process of the best performing MFO-TS-30 sample. The LSV result show a similar polarization curve (Fig. S11a) compared with the Ag/AgCl electrode, indicating that the Ag/AgCl electrode may not have an evident impact on the LSV test. The amperometric i-t curve is shown in Fig. S11b. The catalyst remained at 95.4% of the original current density after 20 h, and the abnormal variation of the current density was likely due to the fluctuating room temperature. Figure 9 presents a multi-step chronopotentiometric curve for the MFO-TS-30 electrode. The potential instantaneously leveled off at 20 mA cm-2 and remained constant for the next 300 s; the other steps presented similar results up to 250 mA cm-2, demonstrating the outstanding mass transport properties and mechanical robustness of the MFO-TS-30 electrode.

Based on the above analysis, the MFO-TS-30 catalyst with cubic bixbyite structure and low crystallinity showed the best OER performance among the various as-prepared catalysts. Furthermore, compared with the Mn/Fe monometallic oxides and PBA-derived ternary spinel structure oxides from previous reports, the MFO-TS-30 sample exhibited higher water oxidation activity (Table 1). Several factors work synergistically to enable this excellent performance: (1) The hollow mesoporous nanocubic morphology can expose more active surface area for oxygen reaction evolution and facilitates mass transport. (2) The peculiar cubic Mn1.2Fe0.8O3 structure contains a higher catalytically active Mn3+ and Fe3+ oxidation state and [MO6] (M=Mn or Fe) octahedra with more flexible M-M and M-O bonds. (3) The atomic-scale synergistic effect of Mn and Fe elements in the ternary oxides. The introduction of Fe could modulate the band structure and energy level of Mn1.2Fe0.8O3, thereby improving the inherent poor conductivity of manganese oxide. Though the essential reason for the synergistic effect during OER is still vague and controversial, we think that the atomic-scale interaction of Mn and Fe may cause electronic structural changes at the active sites and further facilitates the adsorption and reaction of the intermediates [14, 21, 69-71]. Additionally, the abundant Mn could also prevent crystal structure transitions and ensure the stability of the high-activity cubic bixbyite structure [72]. (4) The low crystallinity of the catalyst can offer rich surface defects and high-energy dangling bonds to increase the catalytically active sites and promote the adsorption of intermediates on the catalyst surface.

4. Conclusion In summary, manganese iron Prussian blue analogue (PBA) precursor was obtained via a facile preparation strategy and was then converted into mesoporous Mn-Fe ternary oxide nanocubes with different crystal structures and crystallinities by modulating the calcination conditions. The MFO-TS-30 sample that was obtained via a two-stage pyrolysis process exhibited the best electrochemical OER activity in 1 M KOH among all of the samples. It produced a small Tafel slope value of 38 mV dec-1 and only required 245 mV overpotential at a current density of 10 mA cm-2. The excellent catalytic activity of MFO-TS-30 could be attributed to its porous nanocubic morphology with a bixbyite cubic crystal structure and a low degree of crystallinity, which offered a large number of active sites. This work clearly introduces the influence of the calcining conditions on the PBA-derived Mn-Fe ternary oxides and sheds some light on the fabrication of facile, cost-effective and low-crystallinity transition metal oxide catalysts.

Acknowledgements We greatly acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 11674398, 51871250).

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Figures section

Fig. 1. Schematic diagram of the preparation process.

Fig. 2. (a) XRD pattern of Mn-Fe Prussian blue analogue precursor. (b) SEM image of the Mn-Fe PBA nanocubes.

Fig. 3. (a) XRD patterns of MFO-T (T=450, 500, 550). (b) XRD patterns of MFO-TS-t (t=5, 20, 30, 45).

Fig. 4. (a-d) SEM images of the Mn-Fe ternary transition metal oxides MFO-TS-5, MFO-TS-20, MFO-TS-30, and MFO-TS-45, respectively. (e) TEM image of MFO-TS-30 nanocubes. (f) HRTEM image of MFO-TS-30 nanocubes. The inset image shows the SAED pattern. (g) EDX mapping of Fe, Mn and O in MFO-TS-30 nanocubes.

Fig. 5. N2 adsorption-desorption isotherms and pore size distributions (inset) of MFO-450 (a) and MFO-TS-30 (b).

Fig. 6. XPS spectra of MFO-TS-30. (a) Fe 2p spectrum (b) Mn 2p spectrum.

Fig. 7. (a) Polarization curves for different MFO-TS-t electrodes (iR corrected) in 1 M KOH at 10 mV s-1. (b) Corresponding overpotential at 10 mA cm-2 for various samples. (c) Tafel slopes of MFO-TS-t samples and Ni foam. (d) Stability test of MFO-TS-30 in a 1 M KOH solution at a constant current density of 10 mA cm-2 for 20 h.

Fig. 8. (a) The capacitive current densities of MFO-TS-t samples at 1.073 V versus

RHE as a function of scan rate. (b) Nyquist plots of MFO-TS-t samples at an overpotential of 1.523 V versus RHE; inset is the corresponding equivalent circuit.

Fig. 9. Multistep chronopotentiometric curve of MFO-TS-30 without iR compensation.

Catalyst Low-crystallinity Mn1.2Fe0.8O3 nanocubes

Tafel slope ηI=10 mA (mV) (mV dec-1)

Electrolyte

References

245

38

1 M KOH

This work

α-Fe2O3

317

58.5

1 M NaOH

[6]

Porous α-Mn2O3 layers

340

50

1 M KOH

[55]

N-doped carbon nanotube-supported Ni-NiM2O4 (M=Fe/Mn, respectively)

250/300

51/89

1 M KOH

[66]

Low-crystallinity Fe-Mn-O hybrid nanosheet

273

63.9

1 M KOH

[14]

High-index (012) facet

PBA-derived Co3O4 microframes

370

53

1 M KOH

[67]

NiO/NiFe2O4 hollow NCs

303

58.5

1 M KOH

[23]

PBA-derived 2D ultrathin CoFe2O4 nanosheets

275

42.1

1 M KOH

[68]

PBA-derived mesoporous

Table 1. Comparison of OER performance of Mn/Fe-based and PBA-derived electrocatalysts.

Highlights Mn-Fe ternary oxide nanocubes were obtained via thermal decomposition of Mn-Fe Prussian blue analogue precursor. Catalysts with different crystal structure and crystallinity could be obtained through modulating of calcination condition. Mn-Fe Prussian blue analogue precursor could be transformed to bimetallic oxides with cubic bixbyite structure, which is peculiar among Prussian blue analogue-derived first-row transition metal oxides. Cubic bixbyite Mn1.2Fe0.8O3 samples showed superior intrinsic oxygen evolution reaction activity than samples with spinel Mn1.8Fe1.2O4 structure. Bimetallic Mn1.2Fe0.8O3 catalyst with low crystallinity shows the best electrocatalytic activity towards water oxidation.