Modulating Oxygen Evolution Reactivity in MnO2 Through

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Modulating Oxygen Evolution Reactivity in MnO Through Polymorphic Engineering 2

Prashant Kumar Gupta, Arihant Bhandari, Sulay Saha, Jishnu Bhattacharya, and Raj Ganesh S. Pala J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05823 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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The Journal of Physical Chemistry

Modulating Oxygen Evolution Reactivity in MnO2 Through Polymorphic Engineering Prashant Kumar Gupta,1# Arihant Bhandari,2# Sulay Saha,1# Jishnu Bhattacharya,2 and Raj Ganesh S. Pala1,3* 1Department 2Department

of Chemical Engineering, Indian Institute of Technology, Kanpur, 208016, India

of Mechanical Engineering, Indian Institute of Technology, Kanpur, 208016, India

3Materials

Science Programme, Indian Institute of Technology, Kanpur, 208016, India *Corresponding #Equally

authors: [email protected]

contributed to the work

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Abstract: Enhanced electrocatalytic activity of Oxygen Evolution Reaction (OER) can be achieved through modulation of electronic structure of the electrocatalytic active sites. This modulation can also be achieved through stabilization of metastable or ‘non-native’ polymorphs of the electrocatalyst. Non-native (NN) crystal structures differ in their discrete translational symmetry from the bulk native (N) crystal. The variable oxygen evolution reactivity in a basic medium of different polymorphs are demonstrated by synthesizing β/N-, γ/N-NN1-, r/NN1-, α/NN2-, and δ/NN3MnO2 polymorphs of MnO2 which show different active site densities on the surface, XPS derived oxidation state of Mn and bulk electronic conductivity. The specific OER activity [activity per electrochemical surface area (ECSA)] of MnO2 is co-dependent on the oxidation state of Mn and electronic conductivity. A volcano based relationship is observed for the specific OER activity of MnO2 polymorphs with the universal descriptor, ∆GO*-∆GHO*, computed from Density Functional Theory (DFT). Both δ/NN3-MnO2 and α/NN2-MnO2 lie closer to the volcano peak but on opposite legs of the volcano. δ/NN3-MnO2 shows higher specific activity due to its low oxidation state (+3.5) of Mn which is confirmed through the calculated average oxidation states (AOS) from XPS and Bader charge from DFT studies. α/NN2-MnO2 shows higher specific activity due to higher electronic conductivity which is correlated with its low oxygen vacancy formation energy. Further, the electronic origin of high OER activity in two of the most non-native polymorphs (δ/NN3-MnO2 and α/NN2-MnO2) is ascribed to a shift in the valence Mn-d band closer to the Fermi level leading to stronger O adsorption. The present study demonstrates the efficacy of utilizing non-native polymorphs for OER and unfolds the correlation between OER and variable oxidation states and electronic conductivities, thereby providing directions towards the generalization of these effects to other polymorphic compounds.


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Introduction: Solar driven electrocatalytic water oxidation is intensely explored for the generation of carbonfree fuels (i.e. hydrogen) whose demand is ever increasing.1 However, the wide-scale industrial application of electrolysis of water is hindered by the lack of efficient, robust, and inexpensive catalysts,2 where still Ru- and Ir-based expensive electrocatalysts are being used.3-5 The usage of earth-abundant inexpensive oxide water oxidation electrocatalysts is limited by their low electrocatalytic activities and stabilities.5-7 However, modulation of strain and charge densities (or oxidation state) on active sites of the catalyst leads to manifold increase in catalytic activity.8-13 The modulation of oxidation state and strain of active sites are generally achieved through the introduction of dopants in catalyst or support.9-14 A similar change in nature of active sites can be achieved through stabilization of “non-native” structures,15-17 which are all the other crystalline and gel-like amorphous states, that differ from the “native” phase (or ground-state) in terms of discrete translational symmetry in the sub-surface region. The “native” structure refers to the most stable crystalline phase that can be obtained at a given set of thermodynamic conditions when there are no surface effects (i.e. in sufficiently large crystals).15-17 While for a given composition, there is a unique native state for a given set of thermodynamic condition while, there can be many nonnative structures having different bond-angles, bond-distances and surface atom densities from the native phase, leading to different electrocatalytic properties.18-25 Kitchin et al. had computationally shown that ‘non-native’ phase of columbite IrO2 would act better for water oxidation than traditionally used ‘native’ phase of rutile IrO2 electrocatalyst.26 In this context, polymorphic engineering of stabilizing ‘non-native phase’ offers a potential approach for improving oxygen evolution electrocatalysts and its activity. It is to be noted that polymorphic engineering has led to better design for other electrochemical applications like the cathodes of Lithium-ion batteries, wherein choosing the optimal material is a trade-off between structural stability and the maximum amount of lithium storage, which is often obtained as a mixture of native and non-native structure27, 28 or in photoelectrochemical (PEC) systems where iso-material heterostructures of native and non-

native structure promote electron-hole separation.29 However, stabilization of non-native polymorphs has remained a challenge, and crucially depends on synthesis conditions and stabilizing ligands. Traditionally, a variation of temperature or pressure during synthesis has led to the crystallization of different metastable phases.30-32 However, new strategies for stabilization of non-native polymorphic structures have been found 3 ACS Paragon Plus Environment


to be effective with the advent of nanotechnology and ligand stabilized synthesis through lowering of surface energy,.17,

33

Thin nano-films of metastable polymorphs can be stabilized through

support-facet induced effect.34, 35 The general strategy of stabilization of non-native phase includes the increase in the surface to the bulk ratio, which can stabilize the non-native phase.15-17, 36 In the present study, the electrochemical water oxidation activities of different polymorphs are demonstrated through MnO2, which is an earth-abundant inexpensive metal oxide having more than ten polymorphic structures. The thermodynamic stability of different polymorphs of MnO2 varies as β- (most stable with 1×1 channel) > γ- (composite structure of 1×1 and 2×1 channels) > r- (2×1 channel) > α- (2×2 channel) > δ- (layered structure) as observed experimentally and reported computationally using density functional theory (DFT).37 The stability order may change for experimentally obtained different MnO2 structures with particle size, potassium-ion concentration and pH.38 To simplify the discussion on native (N) and non-native (NN) structure in MnO2 polymorphs, we adopt the following nomenclature along with the conventional nomenclature β/N-MnO2, γ/N-NN1-MnO2, r/NN1-MnO2, α/NN2-MnO2, δ/NN3-MnO2 throughout our work to emphasize the trends in bulk thermodynamic stability of different polymorphs. Numerous studies have been performed on the electrochemical and photo-electrochemical (PEC) water oxidation activities of MnO2 polymorphs.10, 21-25, 39-50 Only α/NN2- and β/N-MnO2 have been found to show modest stability in the acidic and neutral medium in electrochemical oxidative conditions while other electrocatalysts have been found to dissolve rapidly. Comparative study of electrochemical OER activity per unit geometric surface area in different MnO2 polymorphs shows that α/NN2-MnO2 displays the highest activity (per unit geometric surface area) and stability.23, 25 However, operando spectroscopy results suggest that a disordered structure which depends upon the synthesis conditions of δ/NN3-MnO2 is the most active phase for OER.43 It has also been seen that the high spin electron configuration of Mn3+ weakens the metal-oxo bonds while retaining the structure and in the process improving the kinetics of OER reaction. The amount of Mn3+ in a material depends upon the synthesis and the post-processing conditions.43, 51, 52 Recently a new active site for water oxidation has been revealed on electrochemically induced δ-MnO2.53 Using DFT computations, it has been shown that a special edge site with neighboring Mn vacancy provides the best OER activity with an overpotential of 0.59 V, which is 0.19 V lower than that of pristine MnO2.53 The presence of the Mn vacancy near the active site enhances the adsorption of OH intermediate in OER. Hence, a similar edge site on (216) surface of δ-MnO2 has been 4 ACS Paragon Plus Environment

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considered in this study to model the OER on δ-MnO2. Although, the activities of Mn-oxide polymorphs have been previously compared, the specific activity and the active site densities have not been decoupled. Also, most studies dealt with few polymorphs of MnO2, and PEC water oxidation reactions were widely performed to characterize the activities.10, 21-25, 39-50, 54 Herein, we first synthesize the different polymorphs β/N-, γ/N-NN1-, α/NN2-MnO2, δ/NN3-MnO2 by hydrothermal method and r/NN1-MnO2 by a slow-acid-digestion method. The so synthesized MnO2 phases are utilized for deconvoluting specific OER activity [i.e. OER activity per electrochemical surface area (ECSA)] in terms of average Mn oxidation state (AOS) and electronic conductivity using both experiments and DFT-based computations. In general, a low oxidation state of Mn and high electronic conductivity has been found to promote specific OER activity. A volcano relationship with both δ/NN3-MnO2 and α/NN2-MnO2 lying towards the volcano peak but on opposite legs of the volcano is observed for the specific OER activity of MnO2 polymorphs with the universal descriptor, ∆GO*-∆GHO*, computed from DFT. The lowering of the overpotential of OER activity in the most non-native polymorphs, δ/NN3-MnO2 and α/NN2MnO2, is ascribed to the shift in the valence Mn-d band closer to the Fermi level leading to stronger O adsorption and lowering of ∆GO* values. A decrease in the oxidation state of near-surface Mnsites in different polymorphs lead to increase in Mn-O bond length and decrease in the strain on Mn-active sites which leads to an increase in specific OER activity with stronger Mn-O bonds between different polymorphs. The trend of the average Mn oxidation state obtained from the XPS matches with that obtained Bader charge analysis, while the electronic conductivity is correlated with the ease of oxygen vacancy formation energy. δ/NN3-MnO2 is been found to show maximum specific OER activity due to the lowest oxidation state of Mn (+3.5) among considered MnO2 polymorphs. α/NN2-MnO2 shows high specific activity due to its high electronic conductivity which reduces the barrier for electron-transport during electro-oxidation, despite possessing the maximum oxidation state of Mn (+3.74) among MnO2 polymorphs. Also, α/NN2-MnO2 exhibits the highest apparent OER activity [activity per geometrical surface area (GSA)], due to its high active site density w.r.t. GSA. It is hoped such a deconvolution between oxidation stateconductivity-specific activity relationship among polymorphs of MnO2 can be generalized to other systems to facilitate a more comprehensive design strategy in search of better OER electrocatalyst.

1. Experimental Methods: 5 ACS Paragon Plus Environment


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1.1. Chemicals: All chemicals are used as purchased without further purification. MnSO4•H2O, KMnO4, (NH4)2S2O8 and H2SO4 are supplied by Fisher scientific, and LiMn2O4 is supplied by SigmaAldrich, India. 1.2. β/N-, γ/N-NN1- α/NN2-and δ/NN3-MnO2 Synthesis: The polymorphs of MnO2 β/N-, γ/N-NN1-, α/NN2-, δ/NN3- are synthesized by hydrothermal method.55-57 The reactants are dissolved in 40 ml deionized (DI) water in weight ratio as given in Table 1 and stirred for 30 minutes. The solution is transferred into a Teflon-lined stainless-steel autoclave (50 ml) and heated at different temperature for various polymorphs as given in Table 1. After heating, the autoclave is allowed to cool to room temperature and the solution is centrifuged at 8000 rpm followed by several times washing with DI water to remove any unreacted reagent. The powder is dried at 100 oC overnight. Table 1: Precursor ratio and reaction conditions for the synthesis of different MnO2 crystal structures (β/N-MnO2, γ/N-NN1-MnO2, α/NN2-MnO2, and δ/NN3-MnO2). MnO2 Polymorph

Precursor ratio (weight ratio)

Reaction conditions [ temperature (Duration)]

β/N-

KMnO4: MnSO4.H2O (2:3)

160 oC (12 h)

γ/N-NN1-

MnSO4.H2O:(NH4)2S2O8 (3:4)

90 oC (24 h)

α/NN2-


160 oC (24 h)

δ/NN3-


200 oC (24 h)

1.3. r/NN1-MnO2: Highly crystalline r/NN1-MnO2 is synthesized by the reaction of spinel LiMn2O4 in 2.6 M H2SO4 at 95 oC for 48 h. under stirring condition as suggested in our previous work.27 The resultant solution is centrifuged at 8000 rpm and washed with DI water several times to remove unreacted acid. The synthesized particles are dried at 100 oC overnight. 1.4. Electrode Fabrication:


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The carbon paper from Thermax is used as a support. Carbon paper is cut into electrodes of size 2×1 cm and then sonicated in acetone and methanol for 30 min in each to clean the surface from any impurities. To fabricate electrode, 5 mg of MnO2 powder is dispersed in 5 ml of isopropanol and 20 µl of Nafion (5%) solution is added to the solution and sonicated for 30 min. The above solution is added on carbon paper while heating the carbon paper at 65 oC. 1.5. Material Characterization: X-ray powder diffraction data are collected over the 2θ range 20–70° with Cu-Kα (40 kV, 40 mA) radiation on a PANalytical diffractometer. Surface morphology is determined by scanning electron micrographs by using an instrument of FESEM - SUPRA 40VP Gemini, Zeiss. The surface elemental composition and chemical state of the components are analysed by XPS studies using PHI Versa Probe II Scanning XPS Microprobe. The high-resolution XPS spectra are analysed and fitted using CasaXPS software (version 2.3.12). The C-1s photoelectron line at 284.6 eV is used as a reference for correction of XPS spectra. 1.6. Electrochemical Characterization: All the electrochemical characterizations are performed in 1 M NaOH solution. All potentials reported are measured against Ag/AgCl (saturated KCl) as reference electrode and Pt mesh as the counter electrode. All the potentials are with respect to Ag/AgCl if otherwise not mentioned. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) of the electrodes are performed in a single-compartment cell using potentiostat (Autolab PGSTAT302N). The electrochemical characterizations are carried out in the potential window of 0-0.7 V (vs Ag/AgCl). The electrochemical surface area (ECSA) is calculated through electrochemical methods of our previous reports9.

2. Computational Methodology: The OER involves 4 electron transfer and can be broken into four steps, each with a single electron transfer:58 𝐻2𝑂(𝑙) + ∗ ⇌𝐻𝑂 ∗ + 𝐻 + + 𝑒 ―

(1)

𝐻𝑂 ∗ ⇌𝑂 ∗ + 𝐻 + + 𝑒 ―

(2)

𝐻2𝑂(𝑙) + 𝑂 ∗ ⇌𝐻𝑂𝑂 ∗ + 𝐻 + + 𝑒 ―

(3)

𝐻𝑂𝑂 ∗ ⇌ ∗ + 𝑂2 + 𝐻 + + 𝑒 ―

(4)



where,

∗

is a surface site. It has been observed that the difference in Gibbs Free Energies ∆GO*-

∆GHO* is a unique universal descriptor for the specific OER activity which can predict OER activity with high-degree of accuracy through a volcano-like relationship.58 We compute the ∆GO*∆GHO* (relative to the native (β/N-MnO2)) for different polymorphs of MnO2 using Density Functional Theory as implemented in VASP.59-61 Projector augmented wave (PAW) pseudopotentials are employed for the electronic core states.62 We use the PBEsol functional.63 An Energy cut-off value of 520 eV and an optimized Gamma-centered k-point mesh is used with a reciprocal space discretization of 0.25 A˚−1 for all polymorphs of MnO2. All calculations are spin-polarized. Antiferromagnetic ordering for Mn as shown in Ref.[64] is used to make the bulk ground state structures for all the polymorphs. The surfaces for OER on different polymorphs are chosen based on previous studies: β/N-(110),,65, 66 α/NN2-(100),67 r/NN1-(201) and δ/NN3-(21 6).53 The surface oxidation state of the Mn atoms obtained from XPS is correlated with that obtained from the Bader charge analysis.68 The experimental electronic conductivity is correlated with oxygen vacancy formation energy. A Hubbard correction for Mn, corresponding to the effective U-J=3.9 eV,69, 70 is used to predict the density of states accurately.64

3. Result and Discussions: 3.1. Synthesis of different polymorphs of MnO2 and their structure determination: MnO2 possesses different polymorphic structures and they are stabilized through variation in synthesis conditions (e.g. precursors, precursor ratio, reaction temperature and duration) which are documented in detail in the previous section and are characterized by XRD as shown in Figure 1. MnO2 has diverse crystal structures which differ on account of their alignment of [MnO6] units. Among these β/N-, γ/N-NN1-, r/NN1- and α/NN2-MnO2 have tunnel structures with tunnel sizes being (1×1), (2×1 and 1×1), (2×1) and (2×2) respectively.71-73 The α/NN2-MnO2 structure (JCPDS No. 00-044-0141) is assigned to a tetragonal type MnO2 (space group-87), having large tunnels. The diffraction pattern of tetragonal β/N-MnO2 with small tunnels matches with standard diffraction (JCPDS No. 00-024-0735) of rutile structure (space group-136) with highly crystalline character. XRD data of γ/N-NN1-MnO2 shows broad peaks which match with Nasutite structure (JCPDS No- 00-044-0142) which is reported to have a mixture of intergrowth of tunnel structure characteristics of β/N-MnO2 and r/NN1-MnO2 with De Wolff defects and micro-twinning planes.74 In that respect, r/NN1-MnO2 orthorhombic (space group-62) shows a crystalline characteristic in their XRD (JCPDS No. 00-007-0222). The less sharp peaks for the r/NN1-MnO2 suggests that 8 ACS Paragon Plus Environment

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there are some De Wolff defects and micro-twinning planes. XRD data of δ/NN3-MnO2 showed a more amorphous character with the diffraction pattern corresponding to Birnessite phase (JCPDS No. 01-080-1098) of MnO2 (space group-12). δ/NN3-MnO2 has been reported to have layered structure which has MnO6-shared edges in each layer with alkaline metals or water molecules occupying the space between the layers.

Figure 1: XRD pattern of /N-MnO2, /N-NN1-MnO2, r/NN1-MnO2, /NN2-MnO2 and /NN3MnO2 polymorphs. The computed lattice parameters from DFT and the crystal structures of different polymorphs of MnO2 are shown in Table S1 (in supporting information) and Figure 2 respectively. The lattice parameters are in agreement with the previous computational literature.64 The order of the relative formation energy of polymorphs relative to the native β/N-MnO2 is β/N-MnO2 (0 meV)< r/NN1MnO2 (13.5 meV/f.u.) < α/NN2-MnO2 (63.6 meV/f.u.) < δ/NN3-MnO2 (233.7 meV/f.u.) which is consistent with the experimentally observed trend of thermodynamic stability and that reported in 9 ACS Paragon Plus Environment


previous computational literature.64 The /N-NN1-MnO2 is referred to the intergrowth structures of /N-MnO2 and r/NN1-MnO2 with stability lying in between both the individual phases, depending upon the relative percentage of each phase.

Figure 2. Crystal structures of MnO2 polymorphs: (a) β/N-MnO2 (b) r/NN1-MnO2 (c) α/NN2MnO2 (d) δ/NN3-MnO2. Antiferromagnetic ordering of Mn is shown with purple for spin up and orange for spin down. 3.2. Morphology and surface structure of polymorphs of MnO2: The morphology of different polymorphs of MnO2 is investigated through SEM as given in Figure 3. On visual inspection of Figure 3, the polymorphs are observed to have different types of morphologies which would expose an unequal number of active sites. α/NN2-, γ/N-NN1- and δ/NN3-MnO2 form spherical flower shaped particles of different diameters with spike-like overgrowths. β/N-MnO2 and r/NN1-MnO2 have different morphologies, the former polymorph (Figure 3a) resembling nanowires of high aspect ratios and the latter (Figure 3c) constitutes of nanorods having lower aspect ratio than β/N-MnO2. 10 ACS Paragon Plus Environment

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As different polymorphs form distinct morphologies with various exposed facets, the distribution of the active sites would be different. Hence, the roughness factor (RF), i.e. ECSA per GSA, exposed by different polymorphs are quantified using electrochemical means7, 9 and compared in Figure 3f. In order to calculate ECSA, the double layer capacitance for different MnO2 polymorphs are calculated by correlating the change in current density with the scan rate in Non-Faradaic region (Figure S1 in supporting information).9 It is observed that δ/NN3-MnO2 (ECSA=1.837 m2/g) has the lowest surface roughness factor due to micron size diameters of the particles with the pin-like surface overgrowths (Figure 3e). Similar micron-size particles with pin-like surface overgrowths are also observed for α/NN2-MnO2 (ECSA=18.39 m2/g) (Figure 3d) and γ/N-NN1MnO2 (ECSA= 21.17 m2/g) (Figure 3b) but show higher surface roughness (RF~700-900). The reason for high RF can be two-fold, (1) the pin-like overgrowths have high (L/D) ratio, thus offering greater surface area and (2) the active site per unit GSA is higher leading to higher ECSA. β/N-MnO2 (ECSA= 31.75 m2/g) exhibit the maximum RF with a value of 1270. This may be due to its nanowire like morphology. The synthesis methodology of β/N-MnO2 followed in this work provides a way to modulate morphology which helps to enhance RF as opposed to spherical particles synthesized by other methods18. The r/NN1-MnO2 is having the lowest ECSA (9.64 m2/g) among the polymorphs. The above observation shows that morphology tuning can lead to higher apparent electrocatalytic activity (i.e. activity w.r.t. GSA) through the exposure of greater number of active sites.



Figure 3: Morphological analysis through scanning electron microscopy (SEM) of (a) /NMnO2, (b) /N-NN1-MnO2, (c) r/NN1-MnO2, (d) /NN2-MnO2 and (e) /NN3-MnO2. The experimentally measured roughness factor is shown in (f). 3.3. OER activity of polymorphs of MnO2: The electrochemical OER activities of different polymorphs /N-MnO2, /N-NN1-MnO2, r/NN1MnO2, /NN2-MnO2 and /NN3MnO2 were evaluated in 1.0 M NaOH solution. The electrocatalytic OER study was restricted to only alkaline medium, as except /NN2-MnO2 and β/N-MnO2, rest of the MnO2 polymorphs are reported to be unstable in the acidic medium for long duration.25 The OER activities of MnO2 polymorphs are benchmarked at a condition of the current 12 ACS Paragon Plus Environment

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density at an overpotential of 0.5 V in the 5th electrochemical cycle [see Table 2 & Figure S2 (in supporting information)].9 Using these benchmarking conditions, /NN2-MnO2 is found to be far more superior OER electrocatalyst in comparison to other polymorphs which is concurrent with previous literature reports.23, 25 The OER activity (apparent, i.e. w.r.t. GSA) is found to be in the order of α/NN2-MnO2 > β/N-MnO2 > γ/N-NN1-MnO2 > r/NN1-MnO2 > δ/NN3-MnO2. However, apparent OER activityis not a reliable tool for benchmarking OER activities as supportelectrocatalyst interaction

23, 75

or variation of active site density per GSA76 could yield higher

current densities (JE=0.7 V mA/cm2GSA) despite having lower specific activity (JE=0.7 V mA/cm2ECSA). For this reason, the specific activity (JE=0.7 V mA/cm2ECSA) of each MnO2 polymorphs is calculated by taking account of ECSA and the results are tabulated in Table 2. The order of specific OER activity is δ/NN3-MnO2 > α/NN2-MnO2 > /N-NN1-MnO2 > β/N-MnO2 > r/NN1-MnO2 in 1 M NaOH alkaline medium (Table 2). The specific OER activity of ‘non-native’ δ/NN3-MnO2 is almost four times higher than ‘native’ β/N- MnO2. Further, to understand the performance of electrocatalysts, Tafel analysis (Figure S4) is performed. The order of the lower region Tafel slope is found to be δ/NN3-MnO2 < α/NN2-MnO2 < γ/N-NN1-MnO2 < β/N-MnO2 < r/NN1-MnO2 which is consistent with the understanding that lowering of Tafel slope would yield a better electrocatalyst. Table 2: Electrochemical OER activity (current density (JE=0.7 V) @ 0.7 V vs Ag/AgCl of the different polymorphs in 1.0 M NaOH solution and their specific activity. Materials

JE=0.7 V(mA/cm2GSA)

JE=0.7 V (µA/cm2ECSA)

Tafel Slope (mV/dec)

/N-MnO2

2.457

1.75 ± 0.001

134

/N-NN1-MnO2

1.916

2.26 ± 0.001

130

r/NN1-MnO2

0.630

1.65 ± 0.001

186

/NN2-MnO2

5.096

6.43 ± 0.01

125

/NN3-MnO2

0.488

6.54 ± 0.02

118

The experimentally observed specific OER activity is plotted with the computed value of ∆GO*∆GHO* (relative to the native β/N-MnO2) for different polymorphs of MnO2 in Figure 4 which 13 ACS Paragon Plus Environment


shows a volcano based plot. We see that both δ/NN3-MnO2 and α/NN2-MnO2 lie closer tothe volcano peak but on opposite legs of the volcano. The calculation thus rationalizes the observance of high OER activity in both δ/NN3-MnO2 and α/NN2-MnO2 reported by different experimental studies.41, 51 We further note that r/NN1-MnO2 and β/N-MnO2 lie on different legs of the volcano, and the intergrowth of both these polymorphs, the γ/N-NN1-MnO2, shows better specific activity than both of them (see Table 2). We further speculate, that an intergrowth of δ/NN3-MnO2 and α/NN2-MnO2, from two different legs of the volcano, can be an improved catalyst for the OER.

Figure 4: The volcano based relationship for the specific OER activity of MnO2 polymorphs with the computed universal descriptor ∆GO*-∆GHO* (eV) (relative to the native phase β/NMnO2). The specific OER activity of different polymorphs can be with trends in the band structure (Figure 5). The coverage and strength of oxygen adsorption on the MnO2 surface play a key role in determining the OER activity.77 The adsorbed oxygen accepts electrons from the valence d band of the coordinatively unsaturated Mn atom at the surface, which leads to the formation of bonding and anti-bonding states in the electronic space.77 The bonding states are fully occupied, while the occupancy of antibonding states depends upon the level of the valence Mn-d band. The higher the position of the valence Mn-d band, the lesser the occupancy of antibonding states and the stronger the O adsorption. As the thermodynamic stability of the polymorph decreases and the polymorph becomes more non-native, the position of the valence Mn-d band center shifts upwards and closer 14 ACS Paragon Plus Environment

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to the Fermi level (see Figure 5). The upward shift in the valence Mn-d band leads to a stronger O adsorption at the coordinatively unsaturated surface Mn-sites in the more non-native structure leading to a higher specific OER activity. Hence, the upward shift in the valence Mn-d band center explains the position of δ/NN3-MnO2 and α/NN2-MnO2 at the top of the volcano in specific OER activity landscape of MnO2 (Figure 4).

Figure 5: The density of states plot for different polymorphs of MnO2. The experimental band gap is taken from ref.78, 79 The valence Mn-d band center shifts upwards and closer to the Fermi level as we move towards more non-native polymorphs. The stabilities of the polymorphs of MnO2 are investigated by performing chrono-amperometry at an electrochemical potential of 0.7 V vs Ag/AgCl for 10 h (Figure 6). The reasons for the loss of stability (e.g., corrosion, surface passivation or many other reasons) cannot be decoupled from each other7. Formation of permanganate (MnO4-) in anodic potential (MnO2 + 2H2O  MnO4- + 4H+ + 3e-) results in the dissolution of active sites and the major cause of the loss of activity.80 The polymorphs undergo major dissolution within their first hour under electrochemical potential when they undergo surface reconstruction. α/NN2-MnO2 is the most stable polymorph among all polymorphs and retains ~30% activity even after 10 h. Though δ/NN3-MnO2 exhibits the maximum specific OER activity, it has the lowest stability among the investigated MnO2 15 ACS Paragon Plus Environment


polymorphs and loses ~90% activity in the very first hour. β/N-MnO2 loses its 80% OER activity by the second hour, however, the reconstructed surface is quite stable and more active with an increase in activity (Jt=10hrs/ Jt=2hrs =1.03). This may be due to the fact that the under co-ordinated Mn sites in stable bridge surface shows higher OER overpotential than under co-ordinated Mn sites in step surface which are generated due to surface reconstruction phenomenon during dissolution.76, 80 However, other polymorphs undergo continuous dissolution with time, though the rate of loss of activity decreases over time.

Figure 6: Stability analysis (descriptor= Jt=t hrs/ Jt=0 hrs) through chrono-amperometry for /NMnO2, γ/N-NN1-MnO2, r/NN1-MnO2, α/NN2-MnO2 and δ/NN3-MnO2 under 1 M NaOH medium in 0.7 V Ag/AgCl electrode potential. 3.4. The chemical state of polymorphs of MnO2: Proper understanding of the correlation between the electronic structure and surface structure of the electrocatalyst is essential towards comprehending the origins of OER activity. The polymorphs of MnO2 possess mixed oxidation states (i.e. Mn3+ and Mn4+) at different sites that are inhomogeneously distributed. To investigate different oxidation states of polymorphs, XPS is 16 ACS Paragon Plus Environment

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performed and their oxidation states are computed through calculating the spatial difference between deconvoluted Mn-3s peaks (ΔE3s) and average oxidation states (AOS) as suggested in previous studies81 (Table 3 and Figure S5). ΔE3s values of MnO2 polymorphs differ in the range between 4.6 - 4.8 eV. A higher difference of ΔE3s suggests a shift towards lower oxidation of Mn.81 Both α/NN2-MnO2 and r/NN1-MnO2 have the lowest ΔE3s values of 4.61 eV with a corresponding oxidation state of +3.74 while the β/N-MnO2 and γ/N-NN1-MnO2 has similar ΔE3s values at ~ 4.75 eV with an oxidation state of ~ +3.58. δ/NN3-MnO2 has the highest ΔE3s values and consequently having the lowest oxidation state of +3.50. The lowest oxidation state of δ/NN3-MnO2 is correlated to the presence of K+/H3O+ ions present in the interspaces of the layered structure which provide the structural stability.82 The as computed oxidation states (Table 3) of MnO2 polymorphs may vary depending upon the synthesis methodology and formation of oxygen vacancies which may also lower AOS. Table 3. XPS calculated △E3s splitting and AOS from binding energies (BE) computed from △E3s splitting. Materials

ΔE3s

AOS

β/N-MnO2

4.76

3.57

γ/N-NN1-MnO2

4.75

3.58

r/NN1-MnO2

4.61

3.74

α/NN2-MnO2

4.61

3.74

δ/NN3-MnO2

4.82

3.50

The Mn-2p XPS spectra are shown in Figure 7, after deconvolution to two peaks Mn3+ and Mn4+ we can see the change in the ratio of both the ions in different polymorphs of MnO2. The order of increase in Mn3+ is δ/NN3-MnO2 > α/NN2-MnO2 > /N-NN1-MnO2 > β/N-MnO2 > r/NN1-MnO2, which is consistent with the specific activity of different polymorphs. The δ/NN3-MnO2 shows lowest Mn-oxidation state (AOS) among MnO2 polymorphs while giving the highest specific OER activity. 17 ACS Paragon Plus Environment


Figure 7: XPS of Mn 2p3/2 spectra of /N-MnO2, γ/N-NN1-MnO2, r/NN1-MnO2, α/NN2-MnO2 and δ/NN3-MnO2.

We also find that the AOS of surface Mn atoms can be correlated with the oxidation state obtained from the Bader partitioning of charge density68 calculated via DFT. The oxidation state of active surface Mn sites obtained from Bader charges analysis are shown in Figure 8 for different polymorphs of MnO2 (in electronic units). We can see that the trend of oxidation state as obtained from Bader charge analysis is in agreement with that obtained experimentally from XPS. The lowest oxidation state of δ/NN3-MnO2 is clearly the reason behind its high OER activity. The experimentally measured specific activity is also plotted with respect to calculated Bader charge (Figure S6 in supplymenty information).


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Figure 8: The oxidation state of active surface Mn sites, observed from XPS and from Bader partitioning of charge density computed from DFT. Further, it is possible to differentiate the extent of crystal oxygen, hydroxylated oxygen and structural water present in different polymorphs of MnO2 from O1s spectra. In general, three types of peaks are found in O1s spectra shown in Figure 9, viz. (1) non-hydrogenated O bonded to Mnatoms at ~ 529.2 – 529. 5 eV, (2) OH-bonded to Mn atoms at ~ 531 - 532 eV and (3) O of structural water at ~ 535 -536 eV. However, no structural or crystal water is observed in any of MnO2 crystal structures which indicate that despite having large interspaces present in δ/NN3-MnO2 or tunnels in α/NN2-MnO2, they are not conducive for the diffusion of H2O molecules. This suggests that the nature of crystal motifs (e.g. tunnel or layered structure) are not responsible for OER activities and no general rules can be made unlike other electrochemical properties (e.g. capacitive property or intercalation of Li-ions).18, 27, 83 The larger space present in δ/NN3-MnO2 and α/NN2-MnO2, we have seen the presence of K+ ion from the XPS which provides the structural stability and changes the local environment of Mn. This change in the local environment changes the oxidation state and increase the activity of the electrode. On comparison of the other two deconvoluted peaks of O1s, we conclude that the extent of hydroxylation is maximum in β/N-MnO2 while lowest in δ/NN3MnO2. However, we could not correlate this ratio with OER activity as hydroxylation may arise 19 ACS Paragon Plus Environment


from surface hydroxylation of active sites or hydroxylation in bulk structures which does not contribute to OER activities. Also, we have compared the peak position of O-1s of crystal oxygen which is found to be in the order of δ/NN3-MnO2> γ/N-NN1-MnO2 > α/NN2-MnO2 ~ r/NN1MnO2 > β/N-MnO2 which is in sync with AOS of Mn in MnO2. The above observation indeed proves that it is possible to change the oxidation state of active site atoms from ‘native’ structure and in the process obtain higher OER activity.

Figure 9: XPS of O 1s spectra of /N-MnO2, γ/N-NN1-MnO2, r/NN1-MnO2, α/NN2-MnO2 and δ/NN3-MnO2. 3.5. Dependence of specific activity of MnO2 on electronic conductivity and AOS: The electrocatalytic properties (in terms of specific activity) of electrocatalyst is determined by (1) energetics towards creating the active sites and (2) electronic conductivities of bulkelectrocatalyst. The electronic conductivity of different polymorphs is found to be in the order of α/NN2-MnO2 > r/NN1-MnO2 > γ/N-NN1-MnO2 > δ/NN3-MnO2 > β/N-MnO2 as measured in the 20 ACS Paragon Plus Environment

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present study (as shown in Figure 10) and also as reported in the previous experimental literature.84 The trend of electronic conductivity can be correlated to the formation energy of bulk oxygen vacancies as computed from DFT (Figure S7). As the oxygen vacancy formation energy decreases, the electronic conductivity increases. Though electronic conductivity is enhanced by the presence of oxygen vacancies whose concentration is dependent on the lattice structure, no general correlation with Mn-oxidation state can be observed as electronic conductivity is a bulk property whereas the XPS measured oxidation state is a surface property. In Figure 10, we correlate the specific activity against chemical oxidation states of Mn and bulk electronic conductivities. Figure 10 suggests that there are two ways to get high specific activity: 1) Low oxidation state of Mn and 2) High conductivity. Computational studies suggest that presence of Mn3+ oxidation states would enhance OER rate which is experimentally exemplified by higher OER activity of Mn2O3 than MnO2.45, 85 The order of increase in Mn3+ is δ/NN3-MnO2 > α/NN2-MnO2 > /N-NN1-MnO2 > β/N-MnO2 > r/NN1-MnO2 (Figure 7), which is consistent with the specific activity of different polymorphs. The δ/NN3-MnO2 shows lowest Mn-oxidation state (AOS) among MnO2 polymorphs while giving the highest specific OER activity. Here we note that δ/NN3-MnO2 and β/N-MnO2 which have the lowest AOS lie on one leg of the volcano (in Figure 4) while α/NN2-MnO2 and r/NN1-MnO2 which have the highest electronic conductivity lie on the other leg of the volcano (in Figure 4). We further note /N-NN1-MnO2, which is an intergrowth of β/N-MnO2 and r/NN1-MnO2 shows better activity than both the individual components. The activity of intergrowth structure of /N-NN1-MnO2 can be understood in terms of individual components, where β/N-MnO2 provides the lower AOS and r/NN1-MnO2 provides the higher electronic conductivity. Speculating further, an intergrowth structure of δ/NN3-MnO2 (which shows the lowest AOS) and α/NN2-MnO2 (which shows the highest electronic conductivity) can be a novel improved electrocatalyst for OER. These results suggest that both conductivity and oxidation states of active sites in the polymorphic structure play a predominant role in determining the electrocatalytic OER activities and can be engineered to design better OER catalysts.



Figure 10: Specific OER activities of different polymorphs MnO2 [β/N-, γ/N-NN1-, r/NN1-, α/NN2- and δ/NN3-] with their oxidation state of Mn (AOS) and bulk electronic conductivities. The blue and green dots represent projection in specific activity-electronic conductivity and specific activity-AOS planes respectively. 3.6. Polymorphic engineering and the design of OER electrocatalyst: By polymorphic engineering, we are referring to the controlled modulation of physicochemical properties via stabilization of various polymorphs. In the context of OER, it is anticipated that the apparent activity [activity per geometrical surface area (GSA)] is dependent on the active site densities on the surface. The active site density can be modulated through tuning morphologies to yield higher surface roughness. The different channel size of MnO2 plays no significant role in determining specific OER activity unlike in capacitors, as the size of the channels are not big enough to allow for continuous diffusion of H2O molecules. This absence of “bulk” water within the channel is supported by the XPS studies on MnO2 polymorphs. Hence, the nature of bulk tunnel structure does not have much ramification in deciding either the specific 22 ACS Paragon Plus Environment

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activity or stability during OER as OER is a surface phenomenon. In case of applications like pseudo-capacitor involving changes in the near-surface structures, it has been observed that nonnative structures, although having high pseudo-capacitance, may show greater capacity fading with cycling.18 In contrast to capacitor and OER applications which are mainly determined by surface physiochemical processes, some applications like Li-Ion-Batteries (LIB) involve ion-intercalation in the bulk. In such cases, the more open non-native phases are more suitable for Li-storage/Lidiffusion despite the fact that they transform to the native phase which has higher stability but poor Li-storage property.27 In fact, in these LIB systems, it may be anticipated that the optimal material might be a composite material involving non-native structures stabilized by a native structure. Unlike as mentioned applications, in case of OER, the activity-stability relations between nonnative and native structures are not yet established in the literature as there are no apparent bulk contributions nor visible structural changes. In the case of OER, the specific activity of different polymorphs can be understood from a universal descriptor ∆GO*-∆GHO* (Figure 4) and the band structure (Figure 5). As we move towards more non-native polymorphs, the valence Mn d band shifts upwards (closer to the Fermi level), causing stronger O adsorption (lowering ∆GO*) leading to a higher specific activity. Two of the most non-native polymorphs (δ/NN3-MnO2 and α/NN2-MnO2) are found to be on the top of the activity volcano and on opposite legs. The specific OER activity can be further decoupled into contributions from the surface and the bulk. A low surface oxidation state of Mn and a higher bulk electronic conductivity of the material is found to give higher OER activity. The δ/NN3-MnO2 shows the maximum specific activity due to lowest Mn-AOS confirmed from XPS and Bader partitioning of the charge density obtained from DFT calculations. The electronic conductivity increases with the formation of oxygen vacancies. The formation of oxygen vacancies in /NMnO2 is difficult in the native structure and becomes progressively easier in non-native structure. The Formation of oxygen vacancies is much easier in /NN2-MnO2 due to its non-native characteristics resulting in higher electronic conductivity. Further, the intergrowth structure of /N-NN1-MnO2 is found to show better OER activity than the individual components: /N-MnO2 and r/NN1-MnO2. The higher activity of the intergrowth structure can be rationalized from the location of the individual components on the opposite legs of the volcano: where /N-MnO2 has lower Mn-AOS and r/NN1-MnO2 has higher electronic conductivity. Speculating further, an intergrowth of δ/NN3-MnO2 (which has the lowest Mn-AOS) and /NN2-MnO2 (which has the 23 ACS Paragon Plus Environment


highest electronic conductivity) from two different legs of the volcano can be an improved electrocatalyst for the OER. This indicates that both surface and bulk phenomenon are responsible for deciding the electrocatalytic activity, as in the case of pseudo-capacitance applications. Thus, engineering the optimal polymorph or composite polymorph can lead to the design of better OER electrocatalysts. Conclusion: The present study contrasts the specific OER activity of different polymorphs of MnO2 to decouple the role of the oxidation state of Mn in the crystal structure and its bulk electron transport barrier (i.e. electronic conductivity). In general, a lower oxidation state of Mn in combination with higher electronic conductivity yields a higher specific OER activity. A volcano based relationship is observed for the specific OER activity of MnO2 polymorphs with the universal descriptor, ∆GO*∆GHO*, computed from DFT. The non-native polymorphs δ/NN3-MnO2 and α/NN2-MnO2 lie near the top of the volcano but on opposite legs. The electronic origin of highest OER activity in the aforementioned polymorphs is because of the upward shift in the valence Mn d band closer to the Fermi level, which leads to stronger O adsorption thereby lowering the activation barrier of OER. δ/NN3-MnO2 has been found to show maximum specific OER activity due to the lowest oxidation of state of Mn (+3.5) as observed from XPS as well as from Bader partitioning of charge density computed from DFT. α/NN2-MnO2 shows higher specific activity due to its high electronic conductivity which is correlated with its low oxygen vacancy formation energy. Also, α/NN2MnO2 exhibits the highest apparent OER activity [activity per geometrical surface area (GSA)] despite having lower specific OER activity, due to its high active site density w.r.t. GSA. These results suggest that both conductivity and oxidation states of active sites in the polymorphic structure play a predominant role in determining the electrocatalytic OER activities and can be engineered to design better OER catalysts. The present study underlines the importance of stabilization of non-native structures which has the potential to give higher electrocatalytic activity thus providing greater options in search of better OER electrocatalysts. Supporting Information: The supporting information contains lattice parameters and formation energies of different MnO2 polymorphs, current density vs scan rate curve, LSV, chrono-amperometric curves, Tafel slopes 24 ACS Paragon Plus Environment

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in alkaline medium, Mn-3s XPS spectra, plot for the experimentally obtained specific activity vs DFT calculated Bader charge, and correlation curve of experimentally obtained electronic conductivity and relative bulk oxygen vacancy formation energy of different polymorphs of MnO2 with details of AOS calculation, OER computational methodology and oxygen vacancy formation energies. Author contributions: The manuscript was written through the contributions of all authors. P.K.G., A.B. and S.S. contributed equally in the prepration of manuscript. J.B. and R.G.S.P. conceived the original idea and supervised the project. Acknowledgements: We thank the Center for Nanoscience at IIT Kanpur for providing the experimental facilities. Funding: Authors are grateful to the Science and Engineering Research Board (SERB), Department of Science and Technology, Government of India for supporting this work via Grant No. SERB/F/11147/2017-2018.



References: 1. Chu, S.; Majumdar, A., Opportunities and Challenges for a Sustainable Energy Future. Nature 2012, 488, 294-303. 2. Galán-Mascarós, J. R., Water Oxidation at Electrodes Modified With Earth-Abundant TransitionMetal Catalysts. ChemElectroChem 2015, 2, 37-50. 3. Stoerzinger, K. A.; Qiao, L.; Biegalski, M. D.; Shao-Horn, Y., Orientation-Dependent Oxygen Evolution Activities of Rutile IrO2 and RuO2. J. Phys. Chem. Lett. 2014, 5, 1636-1641. 4. Kishor , K.; Saha , S.; Gupta , M. K.; Bajpai, A.; Chatterjee, M.; Sivakumar , S.; Pala , R. G. S., Roughened Zn-Doped Ru–Ti Oxide Water Oxidation Electrocatalysts by Blending Active and Activated Passive Components. ChemElectroChem 2015, 2, 1839-1846. 5. Reier, T.; Nong, H. N.; Teschner, D.; Schlögl, R.; Strasser, P., Electrocatalytic Oxygen Evolution Reaction in Acidic Environments–Reaction Mechanisms and Catalysts. Adv. Energy Mater. 2017, 7, 1601275. 6. Spoeri, C.; Kwan, J. T. H.; Bonakdarpour, A.; Wilkinson, D. P.; Strasser, P., The Stability Challenges of Oxygen Evolving Catalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation. Angew Chem Int Edit 2017, 56, 5994-6021. 7. Jung, S.; McCrory, C. C. L.; Ferrer, I. M.; Peters, J. C.; Jaramillo, T. F., Benchmarking Nanoparticulate Metal Oxide Electrocatalysts for the Alkaline Water Oxidation Reaction. J. Mater. Chem. A 2016, 4, 3068-3076. 8. McFarland, E. W.; Metiu, H., Catalysis by Doped Oxides. Chem. Rev. 2013, 113, 4391-4427. 9. Kishor, K.; Saha, S.; Sivakumar, S.; Pala, R. G. S., Enhanced Water Oxidation Activity of the Cobalt(II,III) Oxide Electrocatalyst on an Earth-Abundant-Metal-Interlayered Hybrid Porous Carbon Support. ChemElectroChem 2016, 3, 1899-1907. 10. Seitz, L. C.; Hersbach, T. J.; Nordlund, D.; Jaramillo, T. F., Enhancement Effect of Noble Metals on Manganese Oxide for the Oxygen Evolution Reaction. J. Phys. Chem. Lett 2015, 6, (20), 4178-4183. 11. Saha, S.; Kishor, K.; Pala, R. G. S., Dissolution Induced Self-Selective Zn-and Ru-Doped TiO2 Structure for Electrochemical Generation of KClO3. Cat. Sci. Tech. 2018, 8, 878-886. 12. Busch, M.; Wang, R. B.; Hellman, A.; Rossmeisl, J.; Grönbeck, H., The Influence of Inert Ions on the Reactivity of Manganese Oxides. J. Phys. Chem. C 2017, 122, 216-226. 13. Frydendal, R.; Busch, M.; Halck, N. B.; Paoli, E. A.; Krtil, P.; Chorkendorff, I.; Rossmeisl, J., Enhancing Activity for the Oxygen Evolution Reaction: The Beneficial Interaction of Gold With Manganese and Cobalt Oxides. ChemCatChem 2015, 7, 149-154. 14. Halck, N. B.; Petrykin, V.; Krtil, P.; Rossmeisl, J., Beyond the Volcano Limitations in Electrocatalysis - Oxygen Evolution Reaction. Phys. Chem. Chem. Phys. 2014, 16, 13682-13688. 15. Navrotsky, A., Nanoscale Effects on Thermodynamics and Phase Equilibria in Oxide Systems. ChemPhysChem 2011, 12, 2207-2215. 16. Pandey, M.; Pala, R. G. S., Stabilization and Growth of Non-Native Nanocrystals at Low and Atmospheric Pressures. J. Chem. Phys. 2012, 136, 044703. 17. Nam, K. M.; Seo, W. S.; Song, H.; Park, J. T., Non-Native Transition Metal Monoxide Nanostructures: Unique Physicochemical Properties and Phase Transformations of CoO, MnO and ZnO. NPG Asia Mater. 2017, 9, e364. 18. Devaraj, S.; Munichandraiah, N., Effect of Crystallographic Structure of MnO2 on Its Electrochemical Capacitance Properties. J. Phys. Chem. C 2008, 112, 4406-4417. 19. Kang, Q.; Vernisse, L.; Remsing, R. C.; Thenuwara, A. C.; Shumlas, S. L.; McKendry, I. G.; Klein, M. L.; Borguet, E.; Zdilla, M. J.; Strongin, D. R., Effect of Interlayer Spacing on the Activity of Layered Manganese Oxide Bilayer Catalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2017, 139, 1863-1870.


Page 26 of 31

Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20. Bediako, D. K.; Lassalle-Kaiser, B.; Surendranath, Y.; Yano, J.; Yachandra, V. K.; Nocera, D. G., Structure–Activity Correlations in a Nickel–Borate Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2012, 134, 6801-6809. 21. Bergmann, A.; Zaharieva, I.; Dau, H.; Strasser, P., Electrochemical Water Splitting by Layered and 3D Cross-Linked Manganese Oxides: Correlating Structural Motifs and Catalytic Activity. Energy Environ. Sci. 2013, 6, 2745-2755. 22. Robinson, D. M.; Go, Y. B.; Mui, M.; Gardner, G.; Zhang, Z.; Mastrogiovanni, D.; Garfunkel, E.; Li, J.; Greenblatt, M.; Dismukes, G. C., Photochemical Water Oxidation by Crystalline Polymorphs of Manganese Oxides: Structural Requirements for Catalysis. J. Am. Chem. Soc. 2013, 135, 3494-3501. 23. Meng, Y.; Song, W.; Huang, H.; Ren, Z.; Chen, S.-Y.; Suib, S. L., Structure–Property Relationship of Bifunctional MnO2 Nanostructures: Highly Efficient, Ultra-Stable Electrochemical Water Oxidation and Oxygen Reduction Reaction Catalysts Identified in Alkaline Media. J. Am. Chem. Soc. 2014, 136, 1145211464. 24. Najafpour, M. M.; Renger, G.; Hołyńska, M.; Moghaddam, A. N.; Aro, E.-M.; Carpentier, R.; Nishihara, H.; Eaton-Rye, J. J.; Shen, J.-R.; Allakhverdiev, S. I., Manganese Compounds as WaterOxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures. Chem. Rev 2016, 116, 2886-2936. 25. Pokhrel, R.; Goetz, M. K.; Shaner, S. E.; Wu, X.; Stahl, S. S., The “Best Catalyst” for Water Oxidation Depends on the Oxidation Method Employed: A Case Study of Manganese Oxides. J. Am. Chem. Soc. 2015, 137, 8384-8387. 26. Xu, Z.; Kitchin, J. R., Tuning Oxide Activity Through Modification of the Crystal and Electronic Structure: From Strain to Potential Polymorphs. Phys. Chem. Chem. Phys. 2015, 17, 28943-28949. 27. Gupta, P. K.; Bhandari, A.; Bhattacharya, J.; Pala, R. G. S., Correlating Voltage Profile to Molecular Transformations in Ramsdellite MnO2 and Its Implication for Polymorph Engineering of Lithium Ion Battery Cathodes. J. Phys. Chem. C 2018, 122, 11689-11700. 28. Juran, T. R.; Young, J.; Smeu, M., Density Functional Theory Modeling of MnO2 Polymorphs as Cathodes for Multivalent Ion Batteries. J. Phys. Chem. C 2018, 122, 8788-8795. 29. Behara, D. K.; Sharma, G. P.; Upadhyay, A. P.; Gyanprakash, M.; Pala, R. G. S.; Sivakumar, S., Synchronization of Charge Carrier Separation by Tailoring the Interface of Si–Au–TiO2 Heterostructures via Click Chemistry for PEC Water Splitting. Chem. Engg. Sci. 2016, 154, 150-169. 30. Rastogi, C. K.; Sharma, S. K.; Patel, A.; Parthasarathy, G.; Pala, R. G. S.; Kumar, J.; Sivakumar, S., Dopant Induced Stabilization of Metastable Zircon-Type Tetragonal LaVO4. J. Phys. Chem. C 2017, 121, 16501-16512. 31. Ovsyannikov, S. V.; Abakumov, A. M.; Tsirlin, A. A.; Schnelle, W.; Egoavil, R.; Verbeeck, J.; Van Tendeloo, G.; Glazyrin, K. V.; Hanfland, M.; Dubrovinsky, L., Perovskite-Like Mn2O3: A Path to New Manganites. Angew. Chem. 2013, 52, 1494-1498. 32. Lee, S.; Xu, H., Size-Dependent Phase Map and Phase Transformation Kinetics for Nanometric Iron (III) Oxides (γ→ ε→ α pathway). J. Phys. Chem. C 2016, 120, 13316-13322. 33. Saha, S.; Pala, R. G. S.; Sivakumar, S., Catalyzing Cubic-to-Hexagonal Phase Transition in NaYF4 via Ligand Enhanced Surface Ordering. Cryst Growth Des 2018, 18, (9), 5080-5088. 34. Pandey, M.; Pala, R. G. S., Stabilization of Rocksalt CdSe at Atmospheric Pressures via Pseudomorphic Growth. J. Phys. Chem. C 2013, 117, 7643-7647. 35. Xu, Z.; Salvador, P.; Kitchin, J. R., First-Principles Investigation of the Epitaxial Stabilization of Oxide Polymorphs: TiO2 on (Sr,Ba)TiO3. ACS Appl. Mater. Interfaces 2017, 9, (4), 4106-4118. 36. Huang, J. L.; Li, Z.; Duan, H. H.; Cheng, Z. Y.; Li, Y. D.; Zhu, J.; Yu, R., Formation of HexagonalClose Packed (HCP) Rhodium as a Size Effect. J. Am. Chem. Soc. 2017, 139, 575-578. 37. Kitchaev, D. A.; Dacek, S. T.; Sun, W.; Ceder, G., Thermodynamics of Phase Selection in MnO2 Framework Structures Through Alkali Intercalation and Hydration. J. Am. Chem. Soc. 2017, 139, 26722681.



38. Chen, B.-R.; Sun, W.; Kitchaev, D. A.; Mangum, J. S.; Thampy, V.; Garten, L. M.; Ginley, D. S.; Gorman, B. P.; Stone, K. H.; Ceder, G., Understanding Crystallization Pathways Leading to Manganese Oxide Polymorph Formation. Nat. Commun. 2018, 9, (1), 2553. 39. Najafpour, M. M.; Allakhverdiev, S. I., Manganese Compounds as Water Oxidizing Catalysts for Hydrogen Production via Water Splitting: From Manganese Complexes to Nano-Sized Manganese Oxides. Int. J. Hydrogen Energ. 2012, 37, 8753-8764. 40. Suib, S. L., Structure, Porosity, and Redox in Porous Manganese Oxide Octahedral Layer and Molecular Sieve Materials. J. Mater. Chem. 2008, 18, 1623-1631. 41. Gorlin, Y.; Jaramillo, T. F., A Bifunctional Nonprecious Metal Catalyst for Oxygen Reduction and Water Oxidation. J. Am. Chem. Soc. 2010, 132, 13612-13614. 42. Smith, P. F.; Deibert, B. J.; Kaushik, S.; Gardner, G.; Hwang, S.; Wang, H.; Al-Sharab, J. F.; Garfunkel, E.; Fabris, L.; Li, J., Coordination Geometry and Oxidation State Requirements of CornerSharing MnO6 Octahedra for Water Oxidation Catalysis: An Investigation of Manganite (γ-MnOOH). ACS Catal. 2016, 6, 2089-2099. 43. Huynh, M.; Shi, C.; Billinge, S. J. L.; Nocera, D. G., Nature of Activated Manganese Oxide for Oxygen Evolution. J. Am. Chem. Soc. 2015, 137, 14887-14904. 44. Lian, S.; Browne, M. P.; Dominguez, C.; Stamatin, S. N.; Nolan, H.; Duesberg, G. S.; Lyons, M. E. G.; Fonda, E.; Colavita, P. E., Template-Free Synthesis of Mesoporous Manganese Oxides With Catalytic Activity in the Oxygen Evolution Reaction. Sustainable Energ. Fuels 2017, 1, 780-788. 45. Gorlin, Y.; Lassalle-Kaiser, B.; Benck, J. D.; Gul, S.; Webb, S. M.; Yachandra, V. K.; Yano, J.; Jaramillo, T. F., In Situ X-ray Absorption Spectroscopy Investigation of a Bifunctional Manganese Oxide Catalyst With High Activity for Electrochemical Water Oxidation and Oxygen Reduction. J. Am. Chem. Soc. 2013, 135, 8525-8534. 46. Fekete, M.; Hocking, R. K.; Chang, S. L.; Italiano, C.; Patti, A. F.; Arena, F.; Spiccia, L., Highly Active Screen-Printed Electrocatalysts for Water Oxidation Based on β-Manganese Oxide. Energ. Env. Sci. 2013, 6, 2222-2232. 47. Indra, A.; Menezes, P. W.; Zaharieva, I.; Baktash, E.; Pfrommer, J.; Schwarze, M.; Dau, H.; Driess, M., Active Mixed‐Valent MnOx Water Oxidation Catalysts through Partial Oxidation (Corrosion) of Nanostructured MnO Particles. Angew. Chem. 2013, 52, 13206-13210. 48. Boppana, V. B. R.; Jiao, F., Nanostructured MnO2: An Efficient and Robust Water Oxidation Catalyst. Chem. Comm. 2011, 47, 8973-8975. 49. Browne, M. P.; Nolan, H.; Duesberg, G. S.; Colavita, P. E.; Lyons, M. E. G., Low-Overpotential High-Activity Mixed Manganese and Ruthenium Oxide Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media. ACS Catal. 2016, 6, 2408-2415. 50. Mette, K.; Bergmann, A.; Tessonnier, J. P.; Hävecker, M.; Yao, L.; Ressler, T.; Schlögl, R.; Strasser, P.; Behrens, M., Nanostructured Manganese Oxide Supported on Carbon Nanotubes for Electrocatalytic Water Splitting. ChemCatChem 2012, 4, 851-862. 51. Zhang, B.; Li, Y.; Valvo, M.; Fan, L.; Daniel, Q.; Zhang, P.; Wang, L.; Sun, L., Electrocatalytic Water Oxidation Promoted by 3D Nanoarchitectured Turbostratic δ‐MnOx on Carbon Nanotubes. ChemSusChem 2017, 10, (22), 4472-4478. 52. Tesch, M. F.; Bonke, S. A.; Jones, T. E.; Shaker, M. N.; Xiao, J.; Skorupska, K.; Mom, R.; Melder, J.; Kurz, P.; Knop‐Gericke, A., Evolution of Oxygen–Metal Electron Transfer and Metal Electronic States During Manganese Oxide Catalyzed Water Oxidation Revealed with In Situ Soft X‐Ray Spectroscopy. Angew. Chem. 2019, 131, (11), 3464-3470. 53. Li, Y.-F.; Liu, Z.-P., Active Site Revealed for Water Oxidation on Electrochemically Induced δMnO2: Role of Spinel-to-Layer Phase Transition. J. Am. Chem. Soc. 2018, 140, (5), 1783-1792. 54. Li, A.; Ooka, H.; Bonnet, N.; Hayashi, T.; Sun, Y.; Jiang, Q.; Li, C.; Han, H.; Nakamura, R., Stable Potential Windows for Long‐Term Electrocatalysis by Manganese Oxides Under Acidic Conditions. Angew. Chem. 2019, 131, (15), 5108-5112. 55. Wang, X.; Li, Y., Synthesis and Formation Mechanism of Manganese Dioxide Nanowires/Nanorods. Chem. Eur. J. 2003, 9, 300-306. 28 ACS Paragon Plus Environment

Page 28 of 31

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


56. Liang, S.; Teng, F.; Bulgan, G.; Zong, R.; Zhu, Y., Effect of Phase Structure of MnO2 Nanorod Catalyst on the Activity for CO Oxidation. J. Phys. Chem. C 2008, 112, 5307-5315. 57. Cheng, F.; Zhao, J.; Song, W.; Li, C.; Ma, H.; Chen, J.; Shen, P., Facile Controlled Synthesis of MnO2 Nanostructures of Novel Shapes and Their Application in Batteries. Inorg. Chem. 2006, 45, 20382044. 58. Man, I. C.; Su, H. Y.; Calle‐Vallejo, F.; Hansen, H. A.; Martínez, J. I.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Nørskov, J. K.; Rossmeisl, J., Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces. ChemCatChem 2011, 3, (7), 1159-1165. 59. Kresse, G.; Hafner, J., Ab Initio Molecular Dynamics for Liquid Metals. Phys Rev B 1993, 47, (1), 558. 60. Kresse, G.; Furthmüller, J., Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors using a Plane-Wave Basis Set. Comp Mater Sci 1996, 6, (1), 15-50. 61. Kresse, G.; Hafner, J., Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal–AmorphousSemiconductor Transition in Germanium. Phys Rev B 1994, 49, (20), 14251. 62. Blöchl, P. E., Projector Augmented-Wave Method. Phys Rev B 1994, 50, (24), 17953. 63. Perdew, J. P.; Ruzsinszky, A.; Csonka, G. I.; Vydrov, O. A.; Scuseria, G. E.; Constantin, L. A.; Zhou, X.; Burke, K., Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces. Phys. Rev. Lett. 2008, 100, (13), 136406. 64. Kitchaev, D. A.; Peng, H.; Liu, Y.; Sun, J.; Perdew, J. P.; Ceder, G., Energetics of MnO2 Polymorphs in Density Functional Theory. Phys Rev B 2016, 93, (4), 045132. 65. Tompsett, D. A.; Parker, S. C.; Islam, M. S., Rutile (β-) MnO2 Surfaces and Vacancy Formation for High Electrochemical and Catalytic Performance. J. Am. Chem. Soc. 2014, 136, (4), 1418-1426. 66. Su, H.-Y.; Gorlin, Y.; Man, I. C.; Calle-Vallejo, F.; Nørskov, J. K.; Jaramillo, T. F.; Rossmeisl, J., Identifying Active Surface Phases for Metal Oxide Electrocatalysts: A Study of Manganese Oxide BiFunctional Catalysts for Oxygen Reduction and Water Oxidation Catalysis. Phys. Chem. Chem. Phys. 2012, 14, (40), 14010-14022. 67. Tompsett, D. A.; Parker, S. C.; Islam, M. S., Surface Properties of α-MnO2: Relevance to Catalytic and Supercapacitor Behaviour. J. Mater. Chem. A 2014, 2, (37), 15509-15518. 68. Henkelman, G.; Arnaldsson, A.; Jónsson, H., A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comp Mater Sci 2006, 36, (3), 354-360. 69. Dudarev, S.; Botton, G.; Savrasov, S.; Humphreys, C.; Sutton, A., Electron-Energy-Loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+ U Study. Phys Rev B 1998, 57, (3), 1505. 70. Wang, L.; Maxisch, T.; Ceder, G., Oxidation Energies of Transition Metal Oxides Within the GGA+ U Framework. Phys Rev B 2006, 73, (19), 195107. 71. Suib, S. L., Porous Manganese Oxide Octahedral Molecular Sieves and Octahedral Layered Materials. Accounts Chem Res 2008, 41, 479-487. 72. Huang, H.; Chen, C.-H.; Xu, L.; Genuino, H.; Garcia-Martinez, J.; Garces, H. F.; Jin, L.; Kithongo, C. K. o.; Suib, S. L., Single-Step Synthesis of Manganese Oxide Octahedral Molecular Sieves With Large Pore Sizes. Chem. Comm. 2010, 46, 5945-5947. 73. Duan, X.; Yang, J.; Gao, H.; Ma, J.; Jiao, L.; Zheng, W., Controllable Hydrothermal Synthesis of Manganese Dioxide Nanostructures: Shape Evolution, Growth Mechanism and Electrochemical Properties. CrystEngComm 2012, 14, 4196-4204. 74. Chabre, Y.; Pannetier, J., Structural and Electrochemical Properties of the Proton / γ-MnO2 System. Progress Solid State Chem. 1995, 23, 1-130. 75. Gorlin, Y.; Chung, C.-J.; Benck, J. D.; Nordlund, D.; Seitz, L.; Weng, T.-C.; Sokaras, D.; Clemens, B. M.; Jaramillo, T. F., Understanding Interactions Between Manganese Oxide and Gold That Lead to Enhanced Activity for Electrocatalytic Water Oxidation. J. Am. Chem. Soc. 2014, 136, 4920-4926.



76. Stoerzinger, K. A.; Diaz-Morales, O.; Kolb, M.; Rao, R. R.; Frydendal, R.; Qiao, L.; Wang, X. R.; Halck, N. B.; Rossmeisl, J.; Hansen, H. A., Orientation-Dependent Oxygen Evolution on RuO2 without Lattice Exchange. ACS Energy Lett. 2017, 2, 876-881. 77. Tao, H. B.; Fang, L.; Chen, J.; Yang, H. B.; Gao, J.; Miao, J.; Chen, S.; Liu, B., Identification of Surface Reactivity Descriptor for Transition Metal Oxides in Oxygen Evolution Reaction. J. Am. Chem. Soc. 2016, 138, (31), 9978-9985. 78. Druilhe, R.; Suchet, J., Electron Transport in CrO2 and MnxCr1−xO2. Czech. J'. Phys. B 1967, 17, (4), 337-346. 79. ul Islam, A. F.; Islam, R.; Khan, K., Studies on the Thermoelectric Effect in Semiconducting MnO2 Thin Films. J. Mater. Sci.: Mater. Electron. 2005, 16, (4), 203-207. 80. Frydendal, R.; Paoli, E. A.; Chorkendorff, I.; Rossmeisl, J.; Stephens, I. E., Toward an Active and Stable Catalyst for Oxygen Evolution in Acidic Media: Ti‐Stabilized MnO2. Adv. Energy Mater. 2015, 5, 1500991. 81. Galakhov, V.; Demeter, M.; Bartkowski, S.; Neumann, M.; Ovechkina, N.; Kurmaev, E.; Lobachevskaya, N.; Mukovskii, Y. M.; Mitchell, J.; Ederer, D., Mn 3s Exchange Splitting in MixedValence Manganites. Phys. Rev. B 2002, 65, 113102. 82. Chen, R.; Zavalij, P.; Whittingham, M. S., Hydrothermal Synthesis and Characterization of KxMnO2·yH2O. Chem. Mater. 1996, 8, 1275-1280. 83. Thackeray, M. M., Manganese Oxides for Lithium Batteries. Progress Solid State Chem. 1997, 25, 1-71. 84. Ghodbane, O.; Pascal, J.-L.; Favier, F., Microstructural Effects on Charge-Storage Properties in MnO2-Based Electrochemical Supercapacitors. ACS Appl. Mater. Interfaces 2009, 1, 1130-1139. 85. Morgan Chan, Z.; Kitchaev, D. A.; Nelson Weker, J.; Schnedermann, C.; Lim, K.; Ceder, G.; Tumas, W.; Toney, M. F.; Nocera, D. G., Electrochemical Trapping of Metastable Mn3+ Ions for Activation of MnO2 Oxygen Evolution Catalysts. Proc. Natl. Acad. Sci. 2018, 115, (23), E5261–E5268.


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Modulating Oxygen Evolution Reactivity in MnO2 Through

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