Cosputtered Calcium Manganese Oxide Electrodes for Water Oxidation

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Cosputtered Calcium Manganese Oxide Electrodes for Water Oxidation Hamed Simchi,† Kayla A. Cooley,† Jonas Ohms,‡ Lingqin Huang,† Philipp Kurz,*,‡ and Suzanne E. Mohney*,† †

Department of Materials Science and Engineering and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ Institut für Anorganische und Analytische Chemie and Freiburger Materialforschungszentrum (FMF), Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany ABSTRACT: Calcium manganese oxide films were prepared by cosputter deposition from Mn and CaMnO3 targets and evaluated for their suitability as catalysts for the oxygen evolution reaction (OER). Scanning electron microscopy (SEM) revealed a compact morphology for the as-deposited films and the formation of nanorodlike features on the surfaces after annealing at 600 °C. X-ray-photoelectron-spectroscopy analysis showed that the surface oxidation state is close to +III (as in Mn2O3) for the as-deposited films and increases slightly to a mixture of III and IV after annealing occurs in dry air at 400−600 °C. Glancing-incidence X-ray diffraction (GIXRD) suggested that the CaMnxOy films are amorphous even when heated to 600 °C. However, transmission electron microscopy (TEM) showed that there is actually a polycrystalline component of the film, which best matches Mn3O4 (hausmannite with the average Mn oxidation state of ∼+2.7) but may have a slightly expanded unit cell because of the incorporation of Ca. Electrochemical analyses revealed that the as-deposited CaMnxOy films were OER-inactive. In contrast, annealing at 400 or 600 °C resulted in an increase of ∼15-fold in the current densities, which reached j ≅ 1.5 mA· cm−2 at OER overpotentials of η ≈ 550 mV in cyclic voltammetry (CV) sweeps. For the same η, annealed CaMnxOy electrodes also showed good electrochemical stabilities during 2 h of electrolysis, as rather constant steady-state current densities of j ≅ 0.4−0.5 mA·cm−2 were observed. The thicknesses and surface morphologies of the CaMnxOy films did not change during the electrochemical measurements, indicating that corrosion was negligible. In comparison with a previous study in which Ca-free thin layers of MnOx were evaluated, the results demonstrate that Ca2+ incorporation can enhance the OER activity of MnOx electrocatalysts prepared by sputter deposition. This work provides guidance for designing new electrodes for water oxidation on the basis of the abundant and nontoxic elements manganese and calcium.

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replaced.8,18 For synthetic manganese oxides with Ce4+ as the sacrificial oxidant, the best catalytic rates per mass or per Mn ion have been reported for amorphous, layered oxides with a Ca:Mn ratio of ∼1:5.19 Different approaches have also been employed to prepare CaMnxOy on conductive surfaces to obtain anodes for electrochemical water-oxidation catalysis, including the thermal reduction of pristine perovskite microspheres and nanoparticles synthesized from the thermal decomposition of carbonate precursors,20 solid-state reactions of CaCO3 and MnO2 powders,21 citrate processes,21 calcination of solidsolution carbonate precursors,11 screen printing of presynthesized CaMnxOy,22 and electrodeposition from Mn- and Cacontaining solutions.1 Nevertheless, poor mechanical properties and lack of control over the composition and morphology of the oxides often limit the performances of the electrodes prepared by these routes.

INTRODUCTION Manganese oxide (MnOx)-based catalysts for the oxidation of water have attracted a lot of attention because of their low cost, robustness, environmental friendliness, and encouraging catalytic properties for the OER (based on many prior studies).1−8 However, their use in OER electrocatalysis is currently still limited by their catalytic rates, which are lower than those found for the oxides of Ru, Ir, or Ni and which might in part be caused by the poor electrical conductivities of manganese oxide compounds.9 In addition, there is still a debate concerning the most favorable structure and route for synthesis of these catalysts, as a large number of stable and metastable forms of manganese oxides are known.10−15 Some of us discovered in recent years that the activity of water oxidation by manganese oxide compounds can be enhanced significantly by calcium incorporation.16,17 This situation resembles the biological water-oxidation process that takes place within the enzyme complex photosystem II, in which the catalytic site is a manganese calcium (μ-oxidoMn4Ca) cluster, which shows significantly reduced (even completely lost in most cases) OER activity when Ca2+ is © XXXX American Chemical Society

Received: October 26, 2017

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DOI: 10.1021/acs.inorgchem.7b02717 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. SEM images of 150 nm thick (measured by profilometer) CaMnxOy films deposited on fluorine-doped tin oxide coated glass substrates. (a) As-deposited, (b) annealed at 400 °C, and (c) annealed at 600 °C. In addition, it could also be speculated that hydroxides might form in water vapor, changing catalytic behaviors. The surface microstructure of the films was studied using a Leo 1530 (operated at 5 kV) field-emission scanning electron microscope (SEM). X-ray photoelectron spectra were acquired using a Physical Electronics VersaProbe II equipped with dual-beam charge neutralization. Measurements were performed with monochromatic Al Kα Xray excitation (1486.6 eV) under an applied voltage of 20 kV with a takeoff angle of 45° relative to the sample surface. The X-ray-beam spot size was 200 μm in diameter. The analysis was performed with 0.1 eV steps sized at a pass energy of 23.5 eV. The error bar for the peak position is 1.5 mA·cm−2 at pH 7 for F


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Cross-sectional microscopy, provided in Figure 7, shows that the film thickness of the CaMnOx and FTO layers combined (excluding the nanorods and derived from at least five measurements for each sample) also did not change significantly from its initial value of 750 nm as a result of the electrochemical measurements. A TEM micrograph was used to measure the initial film thickness (Figure 7a), and SEM was used for postelectrochemistry cross-sectional imaging of a sample annealed at 600 °C and cleaned in deionized water under sonication. The contrasts in these images are different because of the use of two different microscopy techniques, yet the information we sought (the preservation of film thickness) is still readily available.

The Penn State authors thank the Penn State Institutes of Energy and the Environment for financial support. Research activities in Freiburg were financially support by the Federal Ministry of Education and Research (BMBF cluster project MANGAN) and the German Research Foundation (DFG priority program SPP1613, grant KU2885/2-2). Notes

The authors declare no competing financial interest.

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CONCLUSION Calcium manganese oxide films were sputtered on FTO substrates and annealed at 400−600 °C in dry air. GIXRD analysis suggested that the as-deposited films were amorphous and did not reveal crystallization after the annealing step. However, a TEM analysis of films heated to 600 °C showed that portions of the film had crystallized into fine grains that could be indexed as Mn3O4 and might have slightly enlarged lattices due to Ca doping. XPS analysis revealed that the heat treatment in air at 400−600 °C led to a pronounced increase in the average Mn oxidation state at the surfaces of the materials from +2.7 (a mixture of Mn2+ and Mn3+) for the asprepared films to ∼+3.1 (a mixture of Mn3+ and Mn4+). A consecutive investigation of the electrocatalytic properties by cyclic voltammetry indicated that this increase of the Mn valency was linked to water-oxidation activity: only the CaMnxOy films annealed at 400 or 600 °C were found to be active electrocatalysts. In addition, the time-dependent behaviors of the CaMnxOy electrodes were investigated for a duration of 2 h by chronoamperometry. Although the asdeposited films yielded current densities of only j ≅ 0.03 mA· cm−2, they could be greatly increased to stable water-oxidation current densities of j ≅ 0.4 mA·cm−2 and 0.5 mA·cm−2 for the electrodes annealed in dry air at 400 and 600 °C, respectively. The sputtered and annealed CaMnxOy films are thus 3−5 times more active in OER electrocatalysis than the MnOx films previously prepared by us via a Mn-evaporation route. This increase does not come at the cost of lower stability. The results thus show once again that the incorporation of calcium ions into manganese oxide is beneficial for OER performance, and the presented procedure offers a new method for the deposition of rather thin (∼150 nm), dense calcium− manganese oxide layers well connected to an underlying conductive support (FTO in our case). Overall, these results point to the great potential of sputtering methods in the preparation of large-area, stable CaMnxOy electrodes to be used in water-splitting electrolyzers.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.E.M.). *E-mail: [email protected] (P.K.). ORCID

Hamed Simchi: 0000-0003-4004-1360 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. G


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Cosputtered Calcium Manganese Oxide Electrodes for Water Oxidation

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