Design Insights for Tuning the Electrocatalytic Activity of Perovskite

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Design Insights for Tuning the Electrocatalytic Activity of Perovskite Oxides For the Oxygen Evolution Reaction Souradip Malkhandi, Phong Trinh, Aswin K Manohar, Ayyakkannu Manivannan, Mahalingam Balasubramanian, G. K. Surya Prakash, and S.R. Narayanan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512722x • Publication Date (Web): 20 Mar 2015 Downloaded from http://pubs.acs.org on March 24, 2015

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The Journal of Physical Chemistry

Design Insights for Tuning the Electrocatalytic Activity of Perovskite Oxides For the Oxygen Evolution Reaction S. Malkhandi1, P. Trinh1, Aswin K. Manohar1, A. Manivannan2, M. Balasubramanian3, G. K. Surya Prakash1, S.R. Narayanan1* 1

Loker Hydrocarbon Research Institute, Department of Chemistry University of Southern California, Los Angeles, CA 90089

2

National Energy Technology Laboratory, Department of Energy Morgantown, WV 26507

3

Argonne National Laboratory, 9700 S. Cass Avenue Lemont, IL 60439

* Corresponding Author, email: [email protected], telephone: (213) 740 5965.

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Abstract Rechargeable metal-air batteries and water electrolyzers based on aqueous alkaline electrolytes hold the potential to be sustainable solutions to address the challenge of storing large amounts of electrical energy generated from solar and wind resources. For these batteries and electrolyzers to be economically viable, it is essential to have efficient, durable and inexpensive electrocatalysts for the oxygen evolution reaction. In this manuscript, we describe new insights for predicting and tuning the activity of inexpensive transition metal oxides for designing efficient and inexpensive electrocatalysts. We have focused on understanding the factors determining the electrocatalytic activity for oxygen evolution in a strong alkaline medium. To this end, we have conducted a systematic investigation of nano-phase calcium-doped lanthanum cobalt manganese oxide, an example of a mixed metal oxide that can be tuned for its electrocatalytic activity by varying the transition metal composition. Using X-ray Absorption Spectroscopy (XANES), X-ray Photoelectron Spectroscopy (XPS), electrochemical polarization experiments, and analysis of mechanisms, we have identified the key determinants of electrocatalytic activity. We have found that the Tafel slopes are determined by the oxidation states and the bond energy of the surface intermediates of Mn–OH and Co–OH bonds while the catalytic activity increased with the average d-electron-occupancy of the σ* orbital of the M-OH bond. We anticipate that such understanding will be very useful in predicting the behavior of other transition metal oxide catalysts.

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Keywords : Metal-air rechargeable batteries, transition metal oxide, electrocatalysis, oxygen evolution

Introduction The electrochemical oxygen evolution reaction (OER) occurs in rechargeable metal-air batteries, water electrolyzers, and in the electro-winning of metals. A significant fraction of the energy losses in these electrochemical systems is a consequence of the sluggish kinetics of the oxygen evolution reaction. Thus, efficient electrocatalysts to reduce the energy loss has been a topic of importance for many decades. This topic has also seen a recent resurgence due to the growing need for low-cost energy storage for the integration of intermittent electricity production from solar and wind-based generation systems, and for increasing interest in hydrogen generation by water electrolysis.1-5 Our focus has been on developing an efficient alkaline ironair rechargeable battery as a robust and inexpensive energy storage system for meeting the demands of large-scale electrical energy storage.6-9

While significant advances to the

technology of the iron electrode have been made by us recently7-10 the round-trip energy efficiency of the iron-air battery still ranges from 50-70% at even modest rates of charge and discharge. This reduced value of energy efficiency is almost entirely attributable to the voltage losses at the air electrode. Rechargeable alkaline zinc-air and non-aqueous lithium-air batteries also suffer from similar limitations.11-13

Thus, developing efficient electrocatalysts for the

oxygen evolution reaction and the oxygen reduction reaction is of immediate interest. During charging of an alkaline iron-air battery, the oxygen evolution reaction occurs at the positive electrode (Eq. 1). 3



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4OH−⇌ O2 + 2H2O + 4e-

Eo = +0.41 V

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(1)

While iridium oxide, bismuth ruthenate, and ruthenium oxide are quite active as electrocatalysts for OER, these precious metal-based materials preclude any cost-effective deployment on a large scale.14-17 Therefore, our focus has been on relatively inexpensive materials such as transition metal oxides.18

However, even with the best of such transition metal oxide electrocatalysts,

spinel nickel cobalt oxide (NiCo2O4) and lanthanum nickel oxide,19-24 the voltage losses during oxygen evolution are as high as 0.20 V even at a modest current density of 10-20 mA/cm2 . Thus, for an iron-air battery with an operating voltage of about 1.2 V, such a voltage loss of 0.2 V reduces the energy efficiency during charging by about 15%. Similar reduction of efficiency also occurs during discharge due to the overpotential losses associated with the oxygen reduction reaction. Consequently, the round-trip energy efficiency of the iron-air battery is often as low as 70% simply due to the voltage losses at the oxygen electrode.

To achieve a high round-trip

energy efficiency, rechargeable air electrodes are often operated at current densities that are well below 20 mA/cm2 that entails additional electrode, stack and system costs. Therefore, there is a dire need to improve the electrocatalytic activity of transition metal oxides for OER. Transition metal oxides of the perovskite and spinel family present an opportunity for designing new low-cost catalysts because of the variety of compositions that are conceivable.5, 21, 25-40

The perovskite oxides have the general formula ABO3, where A is a rare earth metal ion and

B is a transition metal ion. Typically, the transition metal ion at the B site is the catalytically active center for oxygen evolution.30 Since most of the transition metals form perovskite oxides, the compositions with different metals in the B-site are numerous. The variety that is possible in the composition of oxides, presents a significant opportunity for tuning and enhancing the

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electrocatalytic activity.28-30,

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In the present study, we provide new insights into the factors

controlling the electrocatalytic activity of such oxides. A popular approach to alter the electrocatalytic properties of a perovskite oxide, is to partially substitute the A and B atoms with different elements A' and B', to achieve compositions of the general formula, AyA'1-yBxB'1-xO3. This type of metal substitution has been adopted by solid-state physicists to tune the electrical and magnetic properties of such oxides.42-54 When A and A’ have different valences as in lanthanum(III) and calcium(II), an increase in the oxidation state of the B atom or an increase in the number of oxygen vacancies occurs to maintain overall charge neutrality in the lattice. As a result of these changes, the electrical, magnetic and electrocatalytic properties are usually modified. For example, lanthanum cobalt oxide (LaCoO3), lanthanum manganese oxide (LaMnO3) and lanthanum nickelate (LaNiO3) have been widely investigated, and the catalytic activity of these oxide catalysts is dependent on the type and oxidation state of the transition metal55-58.

For example, partial substitution of

lanthanum by calcium or strontium as in the compositions, La0.6Ca0.4CoO3 or La0.8Sr0.2CoO3, results in a mixture of Co2+, Co3+, Co 4+ and generation of oxygen vacancies, accompanied by an increase in electrical conductivity18,

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. Also, when the transition metal in the B-site of the

perovskite is substituted by one or more of the first-row transition metals, a variety of d-electron configurations are presented at the surface. The possibility of tuning the composition at the A and B site for electrocatalytic activity has thus evoked considerable interest. 55-58 By studying the oxygen evolution activity of substituted transition metal oxides we can gain further insights into the role of the B-site in modifying electrocatalytic activity. For this purpose, we have focused on calcium-doped lanthanum cobalt oxide perovskite (of the formula La0.6Ca0.4CoO3) with various amounts of manganese substituting for cobalt (as in the formula, 5



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La0.6Ca0.4MnxCo1-xO3). The inspiration for the choice of this perovskite arose from the attributes of manganese, namely: a) a low-cost and globally-abundant material b) environment-friendly, and c) ability to achieve a range of oxidation states of +2 to +7. Therefore, we have investigated the effect of the systematic replacement of cobalt with equivalent amounts of manganese on the electrocatalytic activity of the perovskite oxide towards oxygen evolution. Such a study was aimed at providing new insights into how the transition metal site may be modified to alter activity and also reduce the cost of the electrocatalyst.

Experimental Oxide Synthesis and Physical Characterization Calcium-doped lanthanum cobalt manganese oxides (LCCMO) of the formula, La0.6Ca0.4MnxCo1-xO3 were synthesized in-house by a modified Pechini process.4,

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Stoichiometric amounts of lanthanum nitrate, calcium nitrate, cobalt(II) nitrate and manganese (II) nitrate required for preparing a 1 gram batch of the oxide were dissolved in 50 mL of deionized water (18.2 M Ohm cm) to which 100 mL of a 1% aqueous solution of citric acid was added. This solution mixture was stirred at room temperature for approximately 2 hours at 80 oC and during this time the water gradually evaporated and about 30 mL of a viscous gel was left in the beaker. This gel was left for 18 hours at room temperature before being dried in a vacuum oven at 90 oC for 12 hours. The resulting sol-gel was heated in air in two steps. At first, the material was heated at 150 oC for 30 minutes upon which a black, high-surface-area oxide was rapidly formed, accompanied by combustion. This black powder was ground gently in a mortar and pestle for about 15 minutes, and then the powder was subjected to a second step of heat

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treatment at 700oC for 2 hours to yield the required final sample of oxide. The composition of the perovskite oxide samples that were prepared by the foregoing process are shown in Table 1.

Table 1. List of calcium-doped lanthanum cobalt manganese oxides studied. x 0.0 0.1 0.3 0.5 0.7 0.9 1.0

Formula, La0.6Ca0.4MnxCo1-xO3 La0.6Ca0.4CoO3 La0.6Ca0.4Mn0.1Co0.9O3 La0.6Ca0.4Mn0.3Co0.7O3 La0.6Ca0.4Mn0.5Co0.5O3 La0.6Ca0.4Mn0.7Co0.3O3 La0.6Ca0.4Mn0.9Co0.1O3 La0.6Ca0.4MnO3

The molecular formula in Table 1 is for the bulk of the sample. Just based on transition metal oxidation states measured by XANES (discussed here later), the bulk oxygen stoichiometry varied over the composition range studied. Based on a charge conservation calculation, the oxygen stoichiometry would change from a slight oxygen excess to a slight oxygen deficiency as we go through the series. However, in the absence of direct confirmation of the oxygen stoichiometry, we refrained from applying the “(1-δ)” designation for oxygen non-stoichiometry. The phase purity of the samples was determined by X-ray Diffraction Analysis (Rigaku Ultima IV, using Cu Kα). The crystallite size was calculated from the full-width at half maximum of the (220) reflections using the Scherrer formula after correcting for instrument broadening. The samples were studied using X-ray photoelectron spectroscopy (XPS) with a magnesium X-ray 7



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source (1253.6 eV, SPECS XPS at NETL).

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Bulk-sensitive X-ray Absorption Near Edge

Spectroscopy (XANES) studies were carried out at Sector 20-bending magnet beam-line in the Advanced Photon Source at Argonne National Laboratory. For the X-ray absorption studies, the oxide samples were ground into fine powders and an appropriate quantity was mixed thoroughly with boron nitride and cold-pressed into pellets. Care was taken to minimize distortions to the absorption spectra from thickness effects. The measurements were carried out in transmission mode using a Si (111) mono-chromator.

Harmonic contamination was minimized using a

rhodium-coated harmonic rejection mirror. Reference manganese and cobalt foils were used for energy calibration. The threshold energies (defined using the position of the first inflection point) at the manganese and cobalt K-edges were taken as 6537.7 eV and 7708.8 eV, for the respective foils60. The relative uncertainty in energy between the various samples was estimated to be ± 0.05 eV. Data reduction followed standard procedures using the Athena program in the IFEFFIT suite of software61.

Electrochemical Measurements Electrochemical testing was performed in a three-electrode polyfluoroethylene cell with a mercury/mercuric oxide (MMO) reference electrode and a platinum wire counter electrode. The working electrode consisted of a clean and polished glassy carbon rotating disk electrode (5 mm diameter, Pine Instruments, Inc.) on which the electrocatalytic oxide was coated using a “catalyst ink”. The catalyst ink was prepared by mixing 8 mg of catalyst, 2 mg of acetylene black, and 2 mL of a mixture consisting of 89.5 vol% water, 10 vol% propanol, and 0.5 vol% Nafion solution (5% solution of Nafion® 1100 EW ionomer, Sigma Aldrich). The ink was subjected to 40 minutes of ultrasonic agitation after which a uniform dispersion was obtained. A 20 µL 8


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droplet of this catalyst dispersion was placed on the clean surface of a glassy carbon electrode and dried in air at 85 °C for 15 minutes. In all the experiments, the loading of the oxide catalyst on the carbon disk was approximately 80 µg. The choice of this amount of catalyst is based on our previous studies on oxygen reduction,62 and also is consistent with the range of catalyst loadings on RDE used by others.63,64 Since these electrodes were used not only in an oxygen evolution study but also in a subsequent study of oxygen reduction, the 20% acetylene black was retained in the electrocatalyst layer. Polarization studies with and without the added acetylene black yielded similar results for oxygen evolution activity confirming that the carbon did not have any significant contribution to the current (see Figure S2 in supporting information). The potential of the MMO reference electrode (in 20% potassium hydroxide solution) was measured to be +0.90 V vs. the reversible hydrogen electrode in 1 M potassium hydroxide (without correction for liquid junction potential). High-purity water (18.2 M Ohm cm, 4 ppb total organic carbon) was used in all the experiments. A 1M solution of potassium hydroxide was used as the electrolyte. The electrolyte was continuously purged with high-purity argon (Airgas Ultra-high purity grade 99.999%) to remove dissolved air from the electrolyte and to maintain a carbon dioxide-free environment. The oxygen evolution activity was studied by steady-state potentiostatic polarization measurements.

Results and discussion Structural Characterization X-ray diffraction analysis confirmed that all the compositions in Table 1 formed a pure perovskite phase consistent with the powder diffraction file data (PDF#00-041-0496) for La0.5Ca0.5Mn0.5Co0.5O3. (Figure 1a, 1c). The XRD peaks shifted to lower 2-theta values with 9



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increasing manganese content (Figure 1b), consistent with an increase of cell dimension to be expected from the higher ionic radii of manganese ions.48 The peak width at half-maximum was independent of the cobalt and manganese fraction, and the crystallite size varied between 11 and 22 nm (Figure 1 d). (

(

(

Figure 1 (a) X-Ray diffraction pattern for La0.6Ca0.4Co1-xMnxO3 for the various compositions indicated (b) Magnified region 2θ =30° - 35°, (c) Powder diffraction file data PDF#00-041-0496 for La0.5Ca0.5Mn0.5Co0.5O3. (d) Crystallite size for various manganese fractions calculated from 2θ=33° for plane (220). Thus, the relatively low temperature of processing allowed us to successfully produce nano-crystalline oxide materials. The nano-particulates were thin flakes about 100-200 nm thick (Figure 2). This morphology was similar for all the ratios of manganese to cobalt studied here (Figure S1, Supporting Information). 10


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1 µm

Figure 2. Scanning electron micrograph of La0.6Ca0.4Co1-xMnxO3, x = 0.3 . Oxidation state of manganese and cobalt in the various oxides The binding energy values for the manganese-2p and cobalt-2p levels obtained by X-ray photoelectron spectroscopy (Figures 3a, 3b) confirmed that both these transition metals were in the oxidized state. In the case of oxygen -1s two distinct binding energy values were found, a small peak that corresponded to elemental oxygen possibly from absorbed oxygen species, and a more prominent peak at lower binding energy that indicated presence of anionic oxygen corresponding to an oxide (Figure 3c).

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Figure 3. X-ray photoelectron spectroscopy of (a) Mn-2p (b) Co-2p and (c) O-1s for the various compositions indicated by the values of atomic ratio ratio x = Mn/Co. XPS spectra were corrected using carbon spectra as a standard.

The binding energy of the oxidized states of manganese 2p3/2 spanned from 641 to 645 eV and the peaks were asymmetric suggesting that many oxidation states were present on the surface. Similarly with cobalt, the binding energy associated with the 2p3/2 peak spanned from 779 to 784 eV. To obtain further insight into the distribution of the oxidation states, we deconvoluted the Mn 2p3/2 and Co 2p3/2 peaks for each catalyst (see Figure S3, Supporting

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Information) and estimated the contributions of Mn2+, Mn3+, Mn4+, Co3+ and Co4+ (Figures 4a, 4b, 4c, and 4d).

Figure 4. (a) Variation of concentration of Mn2+, Mn3+, Mn4+ species with the manganese fraction determined by deconvolution of Mn-2p3/2 XPS peaks from (

data presented in Figure 3. (b) Average oxidation state of manganese as a function of manganese fraction, x. (c) Variation of concentration of Co3+ and Co4+ species with the manganese fraction as determined by deconvolution of Co-2p3/2 peaks in (

Figure 3.

(d) Calculated average oxidation state of cobalt as a function of

manganese fraction, x.

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The binding energy values of the individual oxidation states used for the de-convolution were Mn2+: 641.4 eV, Mn3+: 642.5 eV, Mn4+ : 644.1 eV, Co3+ : 779.6 eV, and Co4+ : 781.5 eV. These assignments are based on the values for the oxides of cobalt and manganese reported in various compounds.65 The sample without manganese shows nearly 80 % of Co3+, whereas introduction of manganese lowers the surface concentration of Co3+ to nearly 55 %

and

subsequently increased to about 70% as the manganese fraction approaches x=0.9. A large fraction of the surface manganese ions was distributed between oxidation states of +3 and +4. A significant amount of Mn4+ was present even in the samples with low manganese content, while the Mn2+ content approached 50% at x=1.0. The distribution of manganese among the lower oxidation states (as for example 2+) suggested the presence of oxygen vacancies. a)

(b)

(

Figure( 5(a). Manganese XANES for manganese fraction x = 0.1 to 1 in La0.6Ca0.4Co1-xMnxO3 and LaMnO3 (Marked as LMO). Inset has been included for clarity (b) Cobalt

XANES

for manganese fraction x = 0 to 0.9 in

La0.6Ca0.4Co1-xMnxO3.

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The XANES data corresponding to manganese is presented in Figure 5 a.

In the pre-

edge region some residual oscillations from the lanthanum L1-edge are present.

These

oscillations marginally distort the manganese XANES for the x = 0.1 sample and contributes to a slowly-varying and small background. For higher manganese fractions the distortion of the XANES spectra is greatly reduced. With respect to the pure LaMnO3, the main edge position of the x=1.0 sample is shifted to larger values by ~ 0.5 eV. This observation is consistent with the increased average oxidation state of manganese resulting from the replacement of some lanthanum with calcium. For samples containing both manganese and cobalt, the charge compensation for calcium replacing lanthanum can be accommodated by both the transition metals. From the manganese K-edge XANES it is clear that when cobalt replaces manganese (keeping the La/Ca ratio the same) the average oxidation state of manganese increases.

Specifically, as the manganese

content reduces from x=1.0 to x=0.3, the manganese edge position shifts progressively to higher energies. The edge position for the x=0.1 and x=0.3 sample is coincident. With respect to x=1.0 sample the edge position of x=0.1 sample is shifted to larger values by ~ 0.5 eV. These observations indicate the enhanced presence of Mn4+ even as the manganese content decreases. This observation is consistent with the XPS results that also indicated the presence of Mn4+ in the samples x=0.1 to 0.8. Earlier studies have explored the relationship of the edge position with the Mn oxidation state and oxygen content of samples in other manganites66-68. A shift of ~ 3.3 eV / valence unit has been reported in one study; however, a much smaller shift has also been seen in another study. These differences are attributed to differences in oxygen stoichiometry of the various sample sets. Finally, the pre-edge peaks are broad and largely similar with only minor changes 15



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in position and shape. This XANES observation reveals the absence of any significant amount of Mn2+ in the bulk of the samples. This observation is consistent with the XPS results that indicate minimal concentration of Mn2+ in almost all the samples except for x=0.9 and x=1. Vashook et al

48

have reported from XANES studies of La0.6Ca0.4Co1-xMnxO3 that the

oxidation state of manganese is dependent on fraction x. These studies indicate that changes in oxidation state of manganese can be induced by cobalt substitution. indicate that the relative amount of Mn

4+

Our XANES studies

increases with as the ratio of cobalt to manganese

increases. (Figure 5). Also, the trend of oxidation state of manganese with x (Mn fraction) is opposite to that of XPS. Analysis of the XANES data shows that the average bulk oxidation state increases slightly with increasing manganese content, whereas XPS indicates a slight decrease in surface oxidation states. This inconsistency can arise from XPS being a surfacesensitive measurement, whereas XANES is primarily sampling the bulk of the material. This difference between XANES and XPS results suggested a higher concentration of oxygen vacancies on the surface relative to the bulk leading to the lower oxidations state of manganese on the surface. Consequently, examining XPS data in conjunction with the XANES data provided insight into the difference between the bulk and surface compositions. The results of the cobalt XANES (Figure 5b) suggested that the main edge position for samples with high cobalt content (x=0-0.3) was largely coincident but as the cobalt content further diminished (x = 0.5-0.9) the edge position shifted systematically to lower energies. In prior studies, Sikora et al67 reported that addition of manganese to LaCoO3 induced a reduction of Co3+ to Co2+, with a shift in the edge position by ~ 3.2 eV/unit valence. In samples where cobalt could potentially exist as 4+, such as in La1-xSrxCoO3, very little change in edge position was seen69.

It has been argued that the higher formal valence of cobalt did not lead to a 16


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significant edge shift, in this system, presumably due to the higher covalency of the Co-O bond. Thus the Co K-edge position might not have much direct sensitivity to the presence of Co4+. A comparison of the Co XANES of x=0 and x=0.9 samples indicates a lowering of edge position by ~ 1.5 eV, which can be ascribed to the presence of Co2+ in samples as the amount of manganese increases. This increased presence of Co2+ can be interpreted from the evolution of white line (first main peak), which is also similar to that seen in the data of Sikora et al.67 In contrast to the results from the XANES studies, the results from XPS indicated a nearly constant average oxidation state close to +3.4 for cobalt in all the samples. The deconvoluted peaks of cobalt indicated "U-shaped" and “inverted-U" shaped trend for Co3+ and Co4+ variation with fraction x, respectively. These differences suggested that the surface and bulk compositions of the perovskites could vary significantly. Thus, care must be taken in attributing the observed trends in catalytic activity simply to the bulk properties. Also, the surface properties could undergo change during operation of the electrode in copious amounts of oxygen and water. The changes in surface properties due to the presence of water and oxygen can be studied using in situ techniques such as vibrational spectroscopy. Electrochemical Activity Steady-state polarization studies were conducted in the electrode potential range of 400 mV to 650 mV by holding the electrode at each value of potential for 300 seconds. The values of electrode potential were corrected for the ohmic drop. The oxidation current was not significant below 450 mV (Figure 6). The polarization plots showed a distinct Tafel region in the potential range of 550-650 mV, spanning over almost two decades in current.

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The specific activity measurements (current/mass of catalyst) at 550 mV, 615 mV and 650 mV indicated that a manganese fraction beyond 0.3 led to a significant reduction in activity (Figure 7a). The determination of the electrochemically active surface area of oxide catalysts is not readily possible for these catalysts. However, we did measure the BET surface area and normalized the activity for this area. We noticed that the trends in activity did not change (Figure S4, supplementary information). Also, the value of the Tafel slope in the potential region of 550-625 mV increased from about 55 mV/decade (or approximately, 2.3RT/F) to progressively larger values, and reached about 120 mV/decade ( or approximately 4.6 RT/F) at a manganese fraction of 1.0 (Figure 7b), where, R is the universal gas constant, T is the absolute temperature and F is the Faraday constant.

Figure 6.

Potentiostatic polarization curves for oxygen evolution on the

various catalysts (indicated by x values) in 1 M potassium hydroxide at 25oC. E

MMO

vs. RHE = +0.90 V Oxide amount on the electrode was about 80

microgram for all the samples.

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(a)

(b)(

650 mV 615 mV 550 mV

Figure 7. Effect of manganese fraction on (a) Specific activity at 550, 615 and 650 mV vs. MMO reference ( E

MMO

vs. RHE = +0.90 V),

(b) Tafel slope in the potential region 550 to 650 mV The significant change of Tafel slope with increasing manganese fraction suggests a substantial change in the mechanism of oxygen evolution upon replacing cobalt by manganese. As indicated by Bockris and Otagawa a couple of decades ago28-30 , the most plausible mechanistic pathway on perovskite oxides was the pathway consisting of elementary steps of adsorption of hydroxide ions at the B-site (Eq. 2) followed by a rate-determining electrochemical desorption step to generate adsorbed oxygen (Eq. 3). The next step of re-combination of the oxygen atoms to desorb as molecular oxygen was considered to be relatively fast compared to step 2 (Eq. 4). Adsorption of OH- :

Mz+ + OH─ ⇌ Mz+─ OH + e─

(2) 19



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Page 20 of 32

Electrochemical desorption(rate-determining) : Mz+─ OH + OH─ → Mz+ ─ O + H2O + e─ (3) Desorption : 2 Mz+ ─ O ⇌ Mz+ + O2

(4)

In the above chemical equations, Mz+ is the transition metal ion with valence state z+ at the catalyst surface. In our case, Mz+ is a mixture of cobalt and manganese, and z+ is the oxidation state of the metal ions on the surface. An alternate rate-determining electrochemical desorption step was also proposed by Bockris and Otagawa that involved formation of peroxo species (Eq.5) followed by release of oxygen (Eq. 6). Mz+ ─ OH + OH─ → Mz+ —H2O2 + e─

(5)

Mz+—H2O2 ad + Mz+ —HO2- → H2O + OH- +O2

(6)

Bockris and Otagawa28-30 reported that the Tafel slope at an overpotential of 0.3 V ( or 0.6 V vs. MMO) for the un-doped cobalt perovskite (LaCoO3) was 2.3 RT/F, while that for the un-doped manganese perovskite (LaMnO3) was 4.6 RT/F. In our measurements of calciumdoped perovskites (La0.6Ca0.4CoO3 and La0.6Ca0.4MnO3) we found that the Tafel slope values followed the same trend as reported by Bockris and Otagawa for the un-doped perovskites. As we substitute the cobalt progressively with manganese, the Tafel slope increased from 2.3 RT/F to 4.6 RT/F (Figure 6b). Based on the analysis provided by Bockris and Otagawa28-30, we suggest that the difference in Tafel slope values arises from the difference in coverage of the surface by the OHads species on the two oxides. When the heat of adsorption for the formation of Mz+ ─ OH is low, the surface coverage of oxygen atoms will be low (with θ values ranging from 0.2 to 0.8) with adsorbate-adsorbate interactions limiting the equilibrium coverage. Under these conditions the 20


Page 21 of 32

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application of the Temkin isotherm is appropriate and this leads to a Tafel slope value of 2.3 RT/F. When the Mz+ ─ OH bond is very strong, the coverage is extensive and the application of the Langmuir isotherm leads to a Tafel slope of 4.6 RT/F

28

.

Thus, the experimentally-

determined value of Tafel slope correlated with the strength of the Mz+ ─ OH interaction. The average bond energy for a Co3+—OH and Mn3+—OH are 544 kJ/mole and 627 kJ/mole, respectively30. This difference in bond strength of about 83 kJ/mole is large enough to result in a difference in adsorption energy and surface coverage of the OH species and could explain the observed change in Tafel slopes with the composition of the oxide. More generally, for transition metal ions with a larger number of d-electrons, the occupancy of the anti-bonding orbitals in the Mz+—OH bond increases and hence the bond is expected to be much weaker. Consequently, the higher oxidation states of any particular transition metal will have fewer d-electrons and this will lead to stronger Mz+—OH and thereby a higher value of Tafel slope. The spin state of the metal ion will also have to be considered in such bond strength estimates.70 Thus, the bond strength estimated from the electron occupancy of the molecular orbital scaffold for the M3+—OH (see Figure S5, Supporting Information), will be lower for cobalt compared to manganese. In the case of mixed perovskites with both cobalt and manganese, we can expect a smooth increase in the values of Tafel slopes as we move from surface compositions rich in Co3+ to those richer in Mn3+ and Mn 4+. For the calcium-doped mixed metal perovskites studied here, the XPS results indicated that the surface consisted of various oxidation states of manganese and cobalt and possibly oxygen vacancies (Figure 4). As per our XPS results, we have a distribution of surface oxidation states for the various catalysts. Thus, knowing the distribution of surface oxidation states we have calculated the average value of electron occupancy of the anti-bonding orbitals for each of 21



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the catalysts studied.

Page 22 of 32

This average value of electron occupancy for each of the oxide

compositions of Table 2 is the weighted average of the oxidation state values derived from XPS data after deconvolution (Figure 4).

Table 2. Number of d electrons for Mz+ and occupancy of antibonding orbital of Mz+—OH bond Metal ion Mn2+ Mn3+ Mn4+ Co3+ (h.s.) Co3+ (l.s.) Co4+

d electron configuration d5 d4 d3 d6 d6 d5

Electron-occupancy of anti-bonding orbital of Mz+— OH bond 3 2 1 4 3 3

For Co3+ on the surface with MO5 co-ordination only low spin t2g6 and high spin t2g5eg1are favorable. Since the common high spin state t2g4eg2 for octahedral co-ordination is not feasible, we use the intermediate spin case (t2g5eg1 ) but denote it as high-spin (h.s.) .71 This situation has been explained in the supplementary information following Figure S5. We expect that with increasing electron occupancy in the anti-bonding orbital the Mz+-OH bond weakens, and OER becomes more facile. Consistent with this expectation, we find a direct correlation between the observed activity and the calculated average value for electron occupancy (Figure 8).The molecular orbital diagrams used for the calculation of the average value of d-electron occupancy are provided as supporting material.

22


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Figure 8. Oxygen evolution activity and average number of electrons occupying in the antibonding orbitals of Mz+-OH at 650 mV vs. MMO.

Conclusions Electrochemical

studies

show

that

nanocrystalline

oxides

of

the

general

formula

La0.6Ca0.4MnxCo1-xO3 are of the perovskite structure and are active electrocatalysts for the oxygen evolution reaction. Our work demonstrates how catalysts with multiple type of B sites in ABO3 type perovskite oxides can be tuned systematically for OER activity, extending the understanding from previous studies. XPS and XANES studies indicated that the oxidation state of cobalt in the cobalt-rich compositions was largely in the 3+ state, while a more reduced cobalt surface was seen in the manganese-rich composition.

The manganese ions in all the samples

were distributed between oxidation states of +2, +3 and +4 on the surface as per XPS, while the bulk of the samples contained mainly manganese in the 4+ state even at low manganese content. While manganese is relatively inexpensive, substitution of cobalt by manganese beyond x=0.3 led to an order of magnitude decrease in electrochemical activity and increase of Tafel slope. Analysis of the kinetics based on surface coverage by Mz+-OH species suggested that the relative 23



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Page 24 of 32

strength of the cobalt-oxygen and manganese-oxygen bonds determined the surface activity; higher bond strengths of the manganese (III)-OH and manganese (IV)-OH most likely led to extensive coverage of the surface by hydroxide in the manganese rich samples, reducing activity, and increasing the Tafel slope value of 2.3RT/F to 4.6RT/F.

The average electron occupancy

of the σ* anti-bonding orbital of the Mz+—OH bond correlated with the increase in bond energy and reduction of activity for OER confirming that the higher oxidation state of the manganese site is not desirable for increased activity.

This insight into the role of the surface oxidation

states of the transition metal ions, bond energies of the Mz+-OH bonds, and the population of the σ* anti-bonding orbitals of the Mz+—OH can be used to predict and tune the activity of the catalysts for OER.

Acknowledgement The research reported here was supported by the U.S. Department of Energy ARPA-E (GRIDS program, DE-AR0000136), the Loker Hydrocarbon Research Institute, and the University of Southern California.

Supporting Information Figure S1: Scanning electron micrographs of La0.6Ca0.4Co1-xMnxO3; (a) x = 0 (b) x = 0.1; (c) x =0.5 (d) x = 0.7 (e) x = 0.9 (f) x= 1.0. Figure S2.

Polarization curve for oxygen evolution in 1M potassium hydroxide using

La0.6Ca0.4CoO3 with and without 20% acetylene black. E MMO vs. RHE = +0.90 V Figure S3. De-convolution of Mn 2p3/2 and Co 2p3/2 XPS peaks for various compositions Figure S4. Specific activity normalize for BET surface area at 650 mV vs. MMO reference, E MMO vs.

RHE = +0.90 V. 24


Page 25 of 32

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Figure S5: Molecular Orbital ordering schematic for Mz-OH bonds for cobalt and manganese. This information is available free of charge via the Internet at http://pubs.acs.org

25



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Page 26 of 32

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