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Defective and “c-Disordered” Hortensia-like Layered MnOx as Efficient Electrocatalyst for Water Oxidation at Neutral pH Biaobiao Zhang, Hong Chen, Quentin Daniel, Bertrand Philippe, Fengshou Yu, Mario Valvo, Yuanyuan Li, Ram B. Ambre, Peili Zhang, Fei Li, Håkan Rensmo, and Licheng Sun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00420 • Publication Date (Web): 04 Aug 2017 Downloaded from http://pubs.acs.org on August 4, 2017
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Defective and “c-Disordered” Hortensia-Like Layered MnOx as Efficient Electrocatalyst for Water Oxidation at Neutral pH Biaobiao Zhang,1 Hong Chen,1 Quentin Daniel,1 Bertrand Philippe,2 Fengshou Yu,3 Mario Valvo,4 Yuanyuan Li,5 Ram B. Ambre,1 Peili Zhang,1 Fei Li,3 Håkan Rensmo,2 Licheng Sun1,3,* 1
Department of Chemistry, KTH Royal Institute of Technology, 10044 Stockholm, Sweden
2
Department of Physics and Astronomy, Uppsala University, Box 516, SE-75120, Uppsala,
Sweden 3
State Key Laboratory of Fine Chemicals, Institute of Artificial Photosynthesis, DUT-KTH Joint
Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), 116024 Dalian, China 4
Department of Chemistry - Ångström Laboratory, Uppsala University, Box 538, SE-75121
Uppsala, Sweden 5
Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, 10044
Stockholm, Sweden
ABSTRACT: The development of a highly active manganese-based water oxidation catalyst in the design of an ideal artificial photosynthetic device operating under neutral pH conditions remains a great challenge, due to the instability of pivotal Mn3+ intermediates. We report here defective and “c-disordered” layered manganese oxides (MnOx-300) formed on a fluorine-doped tin oxide electrode by constant anodic potential deposition and subsequent annealing, with a catalytic onset (0.25 mA/cm2) at an overpotential (η) of 280 mV and a benchmark catalytic current density of 1.0 mA/cm2 at an overpotential (η) of 330 mV under neutral pH (1 M potassium phosphate). Steady current density above 8.2 mA/cm2 was obtained during the electrolysis at 1.4 V versus the normal hydrogen electrode for 20 h. Insightful studies showed that the main contributing factors for the observed high activity of MnOx-300 are: i) a defective and randomly stacked layered structure, ii) an increased degree of Jahn-Teller distorted Mn3+ in the MnO6 octahedral sheets, iii) effective stabilization of Mn3+, iv) a high surface area and v) improved electrical conductivity. These results demonstrate that manganese oxides as structural and functional models of an oxygen-evolving complex (OEC) in Photosystem II are a promising catalyst for water oxidation in addition to Ni/Co based oxides/hydroxides. 1 ACS Paragon Plus Environment
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Keywords: artificial photosynthesis, electrocatalyst, layered manganese oxide, oxygen evolution, solar fuel, water oxidation
1. INTRODUCTION The concept of artificial photosynthesis introduced by the Italian chemist Giacomo Ciamician in 1912 has been considered one of the most promising approaches of generating clean and renewable fuels by the conversion of solar energy.1,2 Water oxidation is crucial for natural and artificial photosynthesis as it provides electrons and protons that are essential to the generation of hydrogen or carbon-based fuels.3,4 Recently, various materials have been reported as efficient catalysts for electrochemical water oxidation, particularly in strong alkaline solutions.5 However, there is still an urgent need to develop catalysts for the design of ideal artificial photosynthetic devices operating under benign conditions.6-9 Manganese oxides are nowadays attracting great attention because manganese is the third most abundant transition metal on Earth,10 and consequently has a low price.6 Moreover, manganese has only low known toxicity and has existed naturally for billions of years in the form of Mn4CaO5 clusters in the water oxidation center of Photosystem II (PSII) (Figure S1).7,11 Over the last two decades, manganese oxides (e.g. MnO, MnO2, Mn2O3, Mn3O4 and CaMnOx) have been reported as efficient electrocatalysts in strong alkaline solutions, whereas manganese oxides generally have low catalytic activities under neutral pH conditions with an increase in overpotentials ranging from 500 to 700 mV.12-14 This is mainly due to the disproportionation of Mn3+ under conditions of pH6.0 eV, ~5.4–5.7 eV and ~4.1–4.5 eV, respectively.42,43 Figure 1 presents the Mn3s and O1s core level peaks of MnOx-300 (middle panel), MnOx-as (top) and MnOx-300 before and after electrolysis (bottom). These spectra will be further commented on later in this paper. The Mn3s core level of MnOx-300 is broad and the detected ∆E3s is about 4.8 eV, which reveals the mixed-valence state of Mn3+ and Mn4+ in MnOx-300. A curve fitting is proposed in Figure 1 showing the contribution of Mn4+ (∆E3s = 4.4 eV) and Mn3+ (∆E3s = 5.7 eV). The O1s core level is divided into three peaks. The 5 ACS Paragon Plus Environment
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peak at ~529.9 eV is assigned to the Mn-O-Mn bond, while the peak at 531.7 eV may be attributed to not only the –OH radical (i.e. the Mn–OH bond for a hydrated trivalent oxide) but also absorbed oxygen (–CO2) as observed on the C1s core level presented in Figure S5. The peak at higher binding energy (533.4 eV) similarly can be assigned to either absorbed water at the surface44 or surface contamination (–CO), as well as the combination of the two. The surface character of these last two contributions is confirmed by the O1s core level spectrum recorded at 2500 eV (i.e., a more bulk-sensitive measurement) presented in Figure S6, where it can be seen that the intensity of the peaks at a higher binding energy is much lower than that of the peak related to MnOx.
Figure 2. a) SEM image showing the uniformed film surface and the inset showing the cross section image of the MnOx-300 film; b) Magnified SEM image of the MnOx-300 film compared with a picture of a Hortensia flower in inset; c) TEM overview image of MnOx-300; d) EDX spectrum and element mapping (Mn, O and Na) of MnOx-300 film; e) AFM image of the MnOx300 film; f) Electrochemical capacitance measurements for determination of ECSA of MnOx-300 film. The inset presents the measured capacitive currents plotted as a function of scan rate.
To reveal the morphology, texture and roughness of MnOx-300 film, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic-force microscopy (AFM) measurements were carried out. The SEM images presented in Figure 2a and 2b clearly 6 ACS Paragon Plus Environment
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show the formation of a MnOx-300 film on FTO with an average thickness of 4 µm and a uniform coverage of Hortensia-like nanoflowers, each composed of ultrathin nanoflakes. A series of ultrathin nanoflakes is seen more clearly in the TEM overview image of the MnOx-300 sample (Figure 2c). The MnOx-300 film has a high porosity and a large surface area mainly owing to the accumulation of nanoflowers, which are themselves a three-dimensional stacking of nanoflakes, resulting in an overall three-dimensional hierarchical nanostructure. The homogeneous distribution of Mn, O and Na in the MnOx-300 film was revealed by energy dispersive X-ray (EDX) elemental maps (Figure 2d). Except for Sn, which arises from the FTO substrate, no other metallic impurities, such as Fe and Ni, were detected. A large root-mean-square roughness of 110 nm is also observed in the AFM image; this roughness can be attributed to the accumulation of three-dimensional nanoparticles, demonstrating a highly rough surface for the MnOx-300 film (Figure 2e). A non-faradaic capacitive current associated with electrochemical double-layer charging upon repeated potential cycling was measured to determine the electrochemically active surface area (ECSA) (Figure 2f, see Supporting Information for details).26 A high Rf value of 1000 was measured; i.e., 1000 cm2 ECSA per 1.0 cm2 geometric surface area of the MnOx-300 film.
2.2. Water oxidation catalytic behavior of MnOx-300 film 2.2.1. Catalytic activity The water oxidation properties of MnOx-300 and MnOx-as samples were evaluated by linear sweep voltammetry (LSV) in a 1.0 M pH 7.0 KPi buffer solution. The potentials are reported vs. NHE. The LSV curve of MnOx-300 without iR correction in Figure 3a shows a catalytic onset (0.25 mA/cm2) at an overpotential (η) of 280 mV and a benchmark catalytic current density of 1.0 mA/cm2 at an overpotential of η = 330 mV (where it is noted that the level of an efficient catalyst depends on the envisioned application, in alkaline water electrolysis, the catalytic current density reached often exceeds 100 mA/cm2).7,28,45,46 No obvious Mn redox peak shows up before the onset potential, indicating strong interaction between the Mn sites.7 In contrast to MnOx-300, MnOx-as does not show any activity under the same conditions. Layered manganese oxide is also known as a good material for a supercapacitor. To confirm the catalytic onset potential of MnOx300 obtained from the LSV curve, a multi-potential steps measuement was carried out. The increase in current density along with increasing potential is negligible before 1.05 V, and the 7 ACS Paragon Plus Environment
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sharp increase in the current density starts between 1.05 and 1.1 V (Figure S7). It is therefore reasonable to take 1.1 V as the onset potential for MnOx-300.
Figure 3. a) LSV curves of the MnOx-300 and MnOx-as without iR correction; b) Controlledpotential electrolysis of MnOx-300 at 1.3 V, 1.4 V and 1.5 V for 1.0 h; c) Controlled-potential electrolysis of MnOx-300 and MnOx-as at 1.4 V for 20 h; d) LSV curve of MnOx-300 with 95% iR correction; e) Mass activity and TOF of MnOx-300. The LSV scan rate is 10 mV/s. All the experiments were carried out in a 1.0 M pH 7.0 KPi solution.
The stability of MnOx-300 was studied by bulk electrolysis at a constant potential. Relatively steady current densities of 4.0, 9.7 and 15.1 mA/cm2 were obtained during 1.0 h of electrolysis at 1.3, 1.4 and 1.5 V, respectively (Figure 3b). These experiments were conducted to demonstrate that the fast decrease in the current density during the former 1000 s is due to reasons (e.g., the capacitor charging process, bubble coverage and pH value drop) other than the decomposition of the MnOx-300 catalysts. The LSV curve of MnOx-300 after electrolysis for 1.0 h at 1.4 V shows no major decrease relative to that for the MnOx-300 before electrolysis (Figure S8). Moreover, a reiteration of the electrolysis process at 1.3 V for 20 min has the same curves, instead of an obvious drop in the current density (Figure S9). These results show the catalytic durability of MnOx-300 under various potentials and current densities. To further study the long-term stability of MnOx-300, a bulk electrolysis test was run at 1.4 V for 20 h (Figure 3c). A catalytic current 8 ACS Paragon Plus Environment
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density exceeding 8.2 mA/cm2 was therefore obtained during 20 h of electrolysis. In a separate experiment, the oxygen evolved from electrolysis at 1.4 V was determined in a gas-tight cell by gas chromatography (Figure S10a). A high Faradaic efficiency of 94% was obtained. To verify that the current density at a low overpotential originates from water oxidation, the Faradaic efficiency was also measured at 1.16 V, as 92% (Figure S10b).
Table 1. Comparison of manganese oxide catalysts for water oxidation under neutral conditions.
a
Catalysts
Cbuffer mol L-1
ηa mV
Tafel slopeb mV dec-1
jc mA cm-2
Steady test
Reference
MnOx-300
1.0
330
92
8.1, 19b
480 mV, 4.0 mA cm-2, 1 h 580 mV, 8.2 mA cm-2, 20 h
This work
MnCat
0.1
590b
80
0.15b
N/A
7
Ca-birnessite
0.066
480
N/A
2.0
480 mV, 1.0 mA cm-2, 2 h
47
β-MnO2
0.5
440
93
< 1.2
600 mV, 3.0 mA cm-2, 1 h
48
Mn5O8
0.3
550b
78.7
< 0.2b
690 mV, 4.0b mA cm-2, 1.4 h
49
0.34 V). 11 ACS Paragon Plus Environment
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As a result of the above study, the electrochemical rate law for oxygen evolution on the MnOx300 in the Tafel region can be expressed as ( ). ( )( . . ) exp $
% & '(
),
(2)
where j is the catalytic current, k0 is a potential-independent constant that is proportional to the exchange current density, β is the symmetry factor for the irreversible electron transfer step, F is the Faraday constant, R is the universal gas constant and, T is the thermodynamic temperature. From this rate law, the Tafel slope can be deduced as
*
.
(3)
+, 1 . (234 ) -../0
Substituting = 1.0 M and
= 92 mV/dec into equation 3 leads to β = 1.5. The
obtained value of β is not an integer, owing to the strong interactions between the oxidation sites. Finally, the electrochemical rate law in the Tafel region can be described by ( ). ( )( . . ) exp $
&
'(
).
(4)
The above rate law can be verified by comparing the deduced Tafel slope equation and the Tafel slope equation obtained by fitting the experimental Tafel slope for different Cbuffer solutions. The Tafel slope equation derived from equation 4 is
*
., 1 . (234 ) -∗-../0
*
. (234 )1**.
.
(5)
The experimental Tafel slopes obtained for different Cbuffer solutions are shown in Figure S15. After fitting the plot of Cbuffer and the Tafel slope (Figure 4d), an equation for the Tafel plot was obtained as
Tafel slope
*
. (2
.
(6)
34 )1**.
The high consistency of the deduced Tafel slope equation and its experimental counterpart confirm the obtained electrochemical rate law in a reliable way.
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Figure 5. KIE study: a) LSV curves and b) Controlled-potential electrolysis at 1.4 V of the MnOx-300 in 1.0 M KPi H2O and D2O solutions. The inset represents the KIE values calculated from the current density ratio in H2O and D2O solutions. The isotope experiments in D2O were employed to gain insight into the role of proton transfer during the O–O bond formation in the catalytic rate determine step (RDS). The LSV curve of MnOx-300 in a 1.0 M KPi D2O solution shows much lower current density than that of MnOx300 in a 1.0 M KPi H2O counterpart, indicating an isotope effect in the catalytic reaction (Figure 5a). The bulk electrolysis at 1.4 V was conducted in both H2O and D2O media. Current densities of 9.8 and 5.9 mA/cm2 were obtained for the electrolysis processes in H2O and D2O solutions, respectively (Figure 5b). As soon as the current becomes steady, which indicates that the dominant reaction is the oxidation of H2O or D2O, the kinetic isotope effect (KIE = I(H2O)/I(D2O)) comes to a constant value of 1.64, suggesting a primary isotope effect due to a water oxidation reaction. (Figure 5b inset). The primary isotope effect, which indicates that the RDS of water oxidation involves a cleavage of the O–H bonds, might reflect a mechanism of the water nucleophilic attack pathway of O–O bond formation.56,58
2.3. Origin of high electrochemical activity To understand the effect of annealing on MnOx activation, further characterization of MnOx-as and MnOx-300 by TG/DTG analysis, XPS, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), Raman spectroscopy, ultraviolet–visible (UV-Vis) spectroscopy, SEM, XRD and TEM were carried out to shed light on changes before and after thermal treatment. TG/DTG curves of MnOx-as show three distinct losses of weight during the annealing processes, which are labeled I–III in Figure 6a. The two losses at low-temperature around 70 and 170°C are assigned to the release of physisorbed and interlayer water in MnOx, whereas the loss 13 ACS Paragon Plus Environment
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proceeding at 220°C corresponds to the removal of oxygen atoms from the octahedral layer in relation to the partial reduction of Mn4+ to Mn3+.36,39 The average oxidation state of Mn in MnOxas was determined to be 3.9, which is higher than that of MnOx-300. The decrease in the average valence after annealing was also suggested by the open-circuit potentials (OCP) of the MnOx-300 and MnOx-as films (Figure S16). The OCP of MnOx-300 is 0.68 V, which is lower than the value of 0.78 V for MnOx-as, indicating the formation of reduced Mn sites in the structure after annealing.33 The structural changes due to the loss of H2O and oxygen atoms were verified by XPS, ATR-FTIR and Raman spectroscopy. The Mn3s XPS spectrum of MnOx-as is presented in Figure 1 and shows that with a ∆E3s of 4.4 eV, this film is mainly composed of Mn4+. A shoulder is observed towards low binding energy and is assigned to a small amount of Mn2+ that mainly originated from residual species of the deposition solution at the surface of the MnOx-as film. The O1s spectrum of MnOx-as has the same peaks as the spectrum of MnOx-300; however, the peak at 531.7eV is more intense. This increase is mainly due to a Mn–OH bond rather than a –CO bond, as no modification is seen in the C1s spectrum before and after annealing (Figure S5). The annealing seems to promote the formation of Mn3+ and the formation of more Mn–OH bonds in the structure. This structural change can reasonably be explained by the loss of O atoms in the MnO6 layer framework, resulting in a breakage of the di-µ-oxo bridging and the formation of more Mn–OH bonds together with more mono-µ-oxo bridging. IR and Raman spectroscopies are potent tools that can be used to reveal the structural differences of amorphous or poorly crystalline phases of these manganese oxides.18,59 The ATR-FTIR of MnOx-as clearly shows two different kinds of H2O in the structure; i.e., there are two peaks at 3366 and 1635 cm−1 assigned to O–H stretching and bending of absorbed H2O and two other peaks at 3220 and 1436 cm−1 due to the O–H stretching and bending of hydration H2O, respectively (Figure 6b).60 Both signals from absorbed H2O and hydration H2O sharply decreased after annealing in the related spectrum, and the signature of hydration H2O almost totally disappeared. Simultaneously, a broad absorption appeared at around 1030 cm−1, which is attributed to the formation of more MnIII–OH.61 Instead of sharp peaks, a broad band is seen at 500–820 cm−1, which is typical of the Mn–O vibration of a less-ordered Mn oxide (e.g., birnessite, vernadite and todorokite). Furthermore, the shoulder peak observed in Figure 6b at 740 cm−1 has been reported as a typical peak for distinguishing the layered structure from its tunnel analogue 14 ACS Paragon Plus Environment
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(at 762 cm−1).59 The weakened absorption of MnOx-300 at 740 cm−1 indicates that the layered structure was disordered after heat treatment at 300°C. The Raman analysis demonstrated that MnOx-as and MnOx-300 share a δ-MnO2 structure having different degrees of crystalline disorder and variations in their respective manganese oxidation states. The MnOx-as sample has four defined peaks centered around 387, 500, 575 and 650 cm−1 together with broader features around 292, 610 and 735 cm−1 (Figure 6c). The characteristic peaks and associated positions agree well with those reported in previous studies on birnessite manganese dioxides.62,63 The differences in the spectra of the MnOx-300 are evident from a comparison with the spectrum obtained for the MnOx-as. In the spectra of MnOx-300, the small peak at 387 cm−1 becomes a nearly flat band while that at 500 cm−1 appears less distinct and is slightly shifted towards high wavenumbers, thus more overlapping the main characteristic feature around 575 cm-1. The latter is typically ascribed to ν3(Mn–O) stretching vibration in the basal plane of MnO6 sheets, whereas the other major peak around 650 cm−1 can be assigned to symmetric stretching vibration ν2(Mn–O) of MnO6 groups.62 The relative intensities and/or shapes of the peaks centered at 575 and 650 cm−1 are clearly affected by the annealing processes. The relative intensities of the peaks around 575 and 650 cm−1 are maintained for the spectra of MnOx-as and MnOx-300, with both retaining the most intense feature at 650 cm−1. Nevertheless, the latter is much broader for MnOx-300 displaying a clear shoulder at lower wavenumbers, which is ascribed to the increasing degree of Jahn-Teller distortion upon annealing generating vacancies in MnO6 sheets. These phenomena break down the crystal symmetry and consequently lower the symmetry of the associated spectroscopic features.64,65 Overall, the characteristics observed for the spectra of MnOx-as and MnOx-300 are in line with results of previous structural and spectroscopic analyses, confirming that the release of water from the electrodeposited specimen upon annealing plays a crucial role in producing the crystalline disorder of the resulting defective structure and its subsequent electrochemical activity.
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Figure 6. a) TG/DTG analysis of the MnOx-as; b) IR spectra of the MnOx-as and the MnOx-300; c) Normalized Raman spectra of the MnOx-as, MnOx-300 and MnOx-300 after electrolysis; d) XRD patterns of the MnOx-as, MnOx-300 and MnOx-300 after electrolysis compared with the characteristic diffractions reported in JCPDS No. 43-1456; e) TEM images of the MnOx-300, the inset is the SAED image; f) Schematic diagram of the structural change before and after the annealing.
The distortion of the layer structure and the O vacancies in MnO6 sheets resulting from annealing at 300°C are also directly evidenced by XRD and TEM results. In the XRD patterns, four substantial peaks of low intensity appearing at d-spacings of 7.2 Å (12.3°), 3.6 Å (24.8°), 2.4 Å (37.1°) and 1.4 Å (66.5°) were observed for MnOx-as (Figure 6d), and these peaks are in good agreement with the reflections of birnessite (JCPDS 43-1456). The peaks at 2.4 and 1.4 Å arise from regular distances within layers consisting of edge-sharing MnO6 octahedra; i.e., (100) and (110) diffraction bands.23,66 The other two peaks at 7.2 and 3.6 Å respectively arise from (001) and (002) reflections; i.e., the ordered stacking of octahedral sheets. For birnessite, as soon as the stacking of the sheets becomes more random (i.e., turbostratic), sharpness and the dimension of these XRD peaks decrease, and poorly resolved patterns of low amplitude appear.23,67 In the XRD patterns of MnOx-300 (Figure 6d), only three peaks at 2.4, 1.4 and 2.2 Å (42.3°) are observed. The intensity of the peak at 1.4 Å is much less than that of MnOx-as, indicating an increased 16 ACS Paragon Plus Environment
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presence of defects and a reduced size of MnO6 octahedral sheets in MnOx-300.66 The absence of (001) and (002) reflections, identified as a diagnostic feature distinguishing vernadite from birnessite,10,67-69 also indicates MnO6 octahedral sheets randomly stacked in per diffracting particle.69 Moreover, reflections having d-spacings of 2.4, 1.4 and 2.2 Å have been reported as characteristic XRD blueprints of vernadite.67,68 Therefore, after annealing, MnOx-as transformed from birnessite into a “c-disordered” layered manganese oxide like vernadite, which is also composed of extremely small, thin and randomly-stacked MnO6 octahedral sheets.70 Similarly, Nocera et al. reported that a disordered birnessite-like phase was identified for an active manganese oxide activated employing an electrochemical method.71 Furthermore, high-resolution transmission electron microscopy (HRTEM) allowed the measurement of the lattice constant of structures related to MnOx-as and MnOx-300. The lamellar structure of MnOx-as and its lattice fringes corresponding to respective d-spacings of 2.4 and 1.4 Å are shown in Figure S17 together with the selected area electron diffraction (SAED). Similarly, for MnOx-300, lattice fringes arising from the interatomic planes located at 2.4 and 2.2 Å were detected and confirmed by the related SAED (see Figure 6e and insert). These observations confirm the structural changes of MnOx-300, which were previously inferred from XRD analysis. Nevertheless, more details about the structure of this “c-disordered” layered manganese oxide can not be figured out using the present characterizations. We note that with annealing at low temperature (i.e.
