Water Oxidation Catalysis: Tuning the Electrocatalytic ...

Research Article pubs.acs.org/acscatalysis

Water Oxidation Catalysis: Tuning the Electrocatalytic Properties of Amorphous Lanthanum Cobaltite through Calcium Doping Cuijuan Zhang,*,†,‡ Xinyue Zhang,‡ Katelynn Daly,† Curtis P. Berlinguette,†,§ and Simon Trudel*,† †

Department of Chemistry and Centre for Advanced Solar Materials, University of Calgary, 2500 University Drive Northwest, Calgary, Canada T2N 1N4 ‡ School of Chemical Engineering and Technology, Tianjin University, Tianjin, China 300350 § Departments of Chemistry and Chemical & Biological Engineering, The University of British Columbia, 2026 Main Mall, Vancouver, BC, Canada V6K 1Z6 S Supporting Information *

ABSTRACT: The influence of calcium doping on the electrocatalytic activity of amorphous lanthanum cobaltite aLa1−yCayCoOx with respect to the oxygen evolution reaction in 0.1 M KOH is investigated. The introduction of calcium slightly decreases the activity and does not hamper the short-term stability very much. a-La0.7Ca0.3CoOx demonstrates the highest activity among the calcium-containing materials, which is ascribed to the higher concentration of Co3+ and lower film resistance as determined from ex situ X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. KEYWORDS: water splitting, amorphous materials, calcium-doped lanthanum cobaltite, oxygen evolution reaction, photochemical thin-film deposition, solar fuels, electrocatalysis

1. INTRODUCTION Recent years have witnessed the rapid development of catalysts for the water-splitting reaction, as this reaction is heralded as a scalable solution to the problem of intermittency of sustainable energies (e.g., solar, wind, and tidal). Using renewably generated electricity in periods of abundance and low demand, water can be converted to hydrogen, a storable chemical fuel that can be used in periods of low energy supply and high demand.1,2 Despite recent efforts, there is still a long way to go before commercially viable water splitting is implemented at scale, because the oxygen evolution reaction (OER) is a kinetically challenging process with four-proton and fourelectron transfers. A high overpotential is required to achieve desirable performance, which translates in lower efficiency and higher hydrogen production costs. Therefore, highly active OER catalysts are desirable for the effort to decrease the overpotential and thus to enhance the efficiency of fuel production. Much work3−15 has been devoted to the study of factors associated with electrocatalytic activity, which can be broadly divided into geometric and electronic factors. The former relates to the specific surface area and concentration of active sites in the catalysts, while the latter pertains to the electronic structure and nature of active sites. Higher concentrations of the active site will contribute to higher activity.5,11,12,16 On the basis of their results on a series of perovskite-type metal oxides, Matsumoto et al.3 proposed that transition metal oxides with conduction bands of σ* character show a higher activity with © XXXX American Chemical Society

respect to the OER, speaking to the importance of the electronic structure of the system. Later, Bockris and Otagawa8,9 found a strong dependence of activity of perovskite-type LaMOx with respect to the OER on the number of electrons in the M dz2 orbital, the activity being higher with a larger electron population of the M dz2 orbital. They proposed that LaCoO3 would be highly electrocatalytically active with respect to the OER. Tseung and Jasem4 and Raish and Tseung7 studied the role of the metal/metal oxide or the metal oxide with a lower/higher oxidation state in determining the activity with respect to the OER and proposed that the ideal couple has a potential similar to or lower than the theoretical potential of the oxygen electrode. Thus, Co2O3/CoO2 would be an excellent candidate for the OER.4,7 Trasatti6,7 proposed a volcano-type dependence of electrocatalytic activity on the enthalpy of the lower-to-higher oxidation state transition. The electrocatalytic activity increased with electrical conductivity and the concentration of metal cations with a higher oxidation state, which is well proven in the widely studied doped Co3O414,15 and doped perovskite oxides.8,9,17−20 The activity of La1−xMxCoO3 (M = Ca or Sr) reaches a maximum value with 40% Sr or Ca, which correlates with the trend in electrical conductivity in these materials.17,21 Correspondingly, the 40% Ca-doped LaCoO3 has been widely studied as an electrocatalyst for the OER.18,19,22−24 Received: June 30, 2017 Published: August 7, 2017 6385

DOI: 10.1021/acscatal.7b02145 ACS Catal. 2017, 7, 6385−6391

Research Article

ACS Catalysis

The resultant films deposited on the FTO substrate were subjected to electrochemical tests, including cyclic voltammetry (CV), steady state measurements, and electrochemical impedance spectroscopy on a CH Instrument workstation 660D potentiostat. Details have been reported elsewhere.34−38 Films on the FTO substrate, a Pt mesh, and Ag/AgCl (saturated KCl) were the working, counter, and reference electrodes, respectively. The capacitance of the films was determined from cyclic voltammograms in the non-Faradaic reaction region [0.583−0.633 V vs a reversible hydrogen electrode (RHE)] measured at different scan rates (10−70 mV s−1). The proportionality between current density J and scan rate ν is the double-layer capacitance (Cdl) of the film (J = Cdlν).37 Impedance measurements were performed with a perturbation amplitude of 10 mV over frequencies ranging from 106 to 10−1 Hz. Before each measurement, a 2 min settling period is used to reach equilibrium. The data were fit to an appropriate equivalent circuit using the ZSimpWin software. The short-term stability of a-La1−yCayCoOx was evaluated by monitoring the required potential to maintain a constant current density of 1 mA cm−2 over a 24 h period. The electrolyte for all electrochemical tests is 0.1 M KOH. Unless otherwise specified, current density J in this work was calculated on the basis of the geometric surface area of the samples. For specific current density Jsp calculation, the roughness factor of calcium-containing samples was normalized to that of calciumfree samples, and the surface area was normalized using these relative roughness factors. X-ray photoelectron spectra (XPS) of as-prepared aLa1−yCayCoOx (y = 0, 0.1, 0.3, or 0.5) were recorded on a Physical Electronics PHI VersaProbe 5000-XPS spectrometer using a monochromatic Al Kα source (1486.6 eV, 33.7 W, 200.0 μm in diameter) at 45° to the sample surface. The binding energies are referenced to the adventitious C 1s photoelectron peak at 284.6 eV. All spectral analysis was performed with the CasaXPS software.42 The morphology of the films was investigated through topview images of the films using a field emission scanning electron microscope (FE-SEM, Zeiss Σigma VP). The compositions of a-LaCoOx and a-La0.7Ca0.3CoOx films before and after stability testing were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Varian 725-ES ICP optical emission spectrometer). The samples (films on the FTO substrate) were dissolved in diluted HNO3 prior to ICP-OES analysis.

High processing temperatures are often encountered in the synthesis of perovskite materials; this leads to annealing and a loss of electrochemically active surface area.16 For example, it was shown that for nanocrystalline La0.6Ca0.4CoO3, one must balance the improved activity of reactive sites with higher annealing temperatures (>600 °C) with the reduced surface area.24 Ideally, high activity would be achieved without detrimental high-temperature sintering. Low processing temperatures would be expected to result in poorly crystalline or even amorphous materials. While historically crystalline systems have been the focus of attention, a growing body of evidence suggests that amorphous materials can form highly active OER catalysts.25−30 Recent investigation of high-activity perovskites has shown their surface rapidly amorphizes, concomitant with an increase in activity.30−33 Therefore, directly accessing highactivity amorphous perovskite materials is of considerable interest. We recently reported a family of amorphous metal oxide catalysts (first-row transition metal oxides,34,35 iridium oxide,36 La-based binary oxides,37 Al-based ternary oxides,38 and Ba− Sr−Co−Fe quaternary oxides39), fabricated by a facile, scalable photochemical thin-film deposition process. These materials, including the perovskite-inspired compositions,37,39 showed benchmark performance. To the best of our knowledge, no other method has been reported to directly access amorphous perovskites. Our investigation of perovskite-inspired amorphous LaMOx (M = Cr, Mn, Fe, Co, or Ni)37 shows that their catalytic performance is on par with or superior to that of their crystalline counterparts of the same composition, a-LaCoOx having the highest activity among this series. This high activity is presumably due to the presence of a large number of coordinately unsaturated surface metal sites available for the reaction, and the isotropic and single-phase nature of amorphous materials.40 Herein, we take advantage of our capability to systematically prepare amorphous metal oxides of a desired composition to extend our investigation to the influence of calcium addition on the electrocatalytic activity of a-LaCoOx with respect to the OER.

2. EXPERIMENTAL SECTION Amorphous calcium-doped lanthanum cobaltite films, aLa1−yCayCoOx (y = 0−0.5), were prepared by a photochemical deposition protocol as reported elsewhere.34−38,41 The precursors lanthanum(III) 2-ethylhexanoate [10% (w/v) in hexanes], calcium(II) 2-ethylhexanoate [40% in mineral spirits], and cobalt(II) 2-ethylhexanoate [65% (w/v) in mineral spirits] were purchased from Alfa Aesar and used as received without further treatment. Fluorine-doped tin oxide (FTO)coated glass substrates (Hartford Glass Co., TEC7) were sequentially cleaned with detergent, water, acetone, and ethanol, dried under a stream of air, and then cleaned in an ozone plasma (37.5 W, 15 min) before being used. Precursor solutions of the desired metal stoichiometry were prepared in hexanes at a concentration of 15% (w/w). These solutions were then spin-coated onto FTO substrates. The films were then irradiated with ultraviolet light (λ = 185 and 254 nm) for 24 h. The evolution of ligand absorption was monitored by Fouriertransform infrared (FTIR) spectroscopy (Nicolet 470 FT-IR spectrometer). The resultant films were annealed in air at 100 °C for 1 h at a heating rate of 20 °C min−1 in air. The resultant film is strongly adhered to the FTO substrate and used as is without the need for a binder or supporting material. The typical film thickness is in the range of 100−200 nm.

3. RESULTS AND DISCUSSION The vibrational spectra of the precursor thin film used to prepare a-La0.5Ca0.5CoOx collected through the photolysis process are shown in Figure S1. The photolysis of the precursors is complete within 12 h, as evidenced by the disappearance of absorption bands associated with the 2ethylhexanoate ligand (νC−H vibrations in the 3000−2800 cm−1 window and coordinated carboxylate stretches in the 1700− 1400 cm−1 window).37,41 The new bands at 1471, 1387, 1151, and 842 cm−1 are due to the vibrations of carbonate ion, which is probably generated by the reaction of a-La2O3 with CO2 present in ambient air or produced from photolysis. Similar results were observed in our previous work on La-based binary oxides.37 Scanning electron microscopy (SEM) images of the aLa1−yCayCoOx films (Figure S2) show a good coverage on the FTO substrate. The microstructure differs with the variation in 6386


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ACS Catalysis calcium content. The calcium-free films are rough and slightly structured. There are cracks and wrinkled structures on the films with y values of 0.1 and 0.2. Finally, the films with higher Ca contents (y = 0.3−0.5) are smooth and featureless. The roughness of the surface can be evaluated by double-layer capacitance measurements. Figure 1A provides an example to show how double-layer capacitance Cdl was estimated by fitting the linear dependence

Figure 1. (A) Linear fitting of capacitance current density vs scan rate for a sample with y = 0.3 to calculate the double-layer capacitance. The cyclic voltammograms are in the non-Faradaic reaction region (0.583− 0.633 V vs RHE) in 0.1 M KOH at different scan rates. (B and C) Variation of the capacitance and corresponding roughness factor, respectively, with 60 μF cm−2 as the baseline for samples with different calcium contents.

of the current density to scan rate. As shown in Figure 1B, the capacitance of a-La1−yCayCoOx films decreases rapidly until y = 0.3 and then slightly increases with a further increase in Ca content. The roughness factor is calculated by assuming a double-layer capacitance of 60 μF cm−2 for a smooth oxide surface.43 The a-La0.7Ca0.3CoOx film exhibits the lowest roughness factor among all the samples in this work (Figure 1C). XPS was employed to examine the oxidation state of cobalt in the films. The fit XPS spectra of Co 2p are presented in Figure 2A, while the fitting results are listed in Table 1. The corresponding results for O 1s and C 1s are included for later discussion. The La 3d and Ca 2p spectra are found in Figure S3 and Table S1. The Co 2p spectra were fit with two spin−orbit split doublets with difference splitting of ∼15.2 eV. The peaks at ∼779.7/795.0 and ∼780.8/796.5 eV are ascribed to Co3+ and Co2+, respectively (Figure 2A). The concentration ratio of Co3+ to Co2+ increases with Ca content until y = 0.3 and then decreases (Figure 2B). The film with y = 0.3 delivers the highest Co3+ concentration of 58.1%. As the potential of the Co(III)/Co(IV) couple is closer to that of the OER than to that of Co(II)/Co(IV), a higher concentration of Co3+ is expected to facilitate the OER process.17,21 It can be reasonably expected that the a-La0.7Ca0.3CoOx sample will show better catalytic activity among the calcium-containing samples.

Figure 2. (A) Co 2p, (B) Co3+:Co2+ ratio, (C) O 1s, and (D) C 1s XPS spectra of fresh a-La1−yCayCoOx films.

The electrochemical behavior of a-La1−yCayCoOx was investigated with CV (Figure 3A). Very small redox peaks are found on a film with y = 0 at Ea = 1.10 V and Ec = 1.08 V (vs RHE) (Figure 3B). These peaks are at potentials similar to those seen in pure a-CoOx (Ea = 1.1, and Ec = 1.0 V), corresponding to the Co2+-to-Co3+ transformation.34 Calciumfree a-LaCoOx shows the highest redox current density. The total charge passed during the oxidation peak at 0.95−1.25 V (Figure 3C) shows a maximum value for the calcium-free sample, decreases to a minimal value for y = 0.3, and then slightly increases with an increase in calcium content. The redox peaks are essentially absent for the y = 0.3 film, indicating this sample supports the highest concentration of Co3+ ions. Overall, this trend tracks that in Co3+ content (Figure 2B). The cyclic voltammograms also reveal that the electrocatalytic activity of a-LaCoOx initially decreases significantly 6387


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ACS Catalysis Table 1. Co 2p, O 1s, and C 1s XPS Fitting Results of aLa1−yCayCoOx sample

element

binding energy (eV)

atom %

assignment

y=0

Co 2p

779.7/795.0 780.8/796.5 529.4 531.8 284.4 288.5 780.0/795.1 781.0/796.5 529.1 531.1 532.8 284.4 289.0 780.1/795.2 781.4/796.8 529.2 530.9 531.6 284.4 288.8 780.1/795.2 781.0/796.7 529.3 531.1 532.9 284.4 289.0

34.2 65.8 6.8 93.2 90.9 9.1 46.7 53.3 4.7 81.6 13.7 79.8 20.2 58.1 41.9 3.5 59.2 37.3 80.8 19.2 43.8 56.2 4.7 88.5 6.8 66.4 33.6

Co3+ Co2+ metal−O CO32−, OH− C−C, C−H CO32− Co3+ Co2+ metal−O CO32− OH− C−C, C−H CO32− Co3+ Co2+ metal−O CO32− OH− C−C, C−H CO32− Co3+ Co2+ metal−O CO32− OH− C−C, C−H CO32−

O 1s C 1s y = 0.1

Co O

C y = 0.3

Co O

C y = 0.5

Co O

C

concentration of carbonate ion at the surface of the latter. The O 1s peaks at binding energies of 531.1−531.8 eV (Figure 2C) and C 1s peaks at 288.5−289.0 eV (Figure 2D) are assigned to carbonate (CO32−) species.44 The presence of CO32− has been confirmed by infrared spectroscopy (Figure S1). The CO32− concentration is nearly doubled from 9.2% for y = 0 to 19.2% for y = 0.3. The presence of CO32− on the catalyst’s surface is generally detrimental to the catalytic activity.45,46 Steady state measurement was performed to further characterize the electrocatalytic activity of a-La1−yCayCoOx catalysts. The Tafel plots are shown in Figure 4. Three

Figure 4. Tafel plots of a-La1−yCayCoOx films on the FTO substrate. Lines are fits to the data.

parameters (Tafel slope, onset overpotential, and overpotential at 1 mA cm−2) were screened to evaluate the activity of these films with respect to the OER. As discussed above, the roughness of a-La1−yCayCoOx films varies with calcium content. To eliminate the influence of roughness on activity, the specific current density (Jsp) was calculated and the change in overpotential with calcium content at Jsp = 1 mA cm−2 is shown in Figure 5 along with the three parameters mentioned above. All three parameters for the calcium-containing samples are slightly higher than those of the calcium-free catalyst. The y = 0.3 sample exhibits the lowest Tafel slope, onset overpotential, and overpotential at 1 mA cm−2 among the calcium-containing samples. In particular, the overpotential at 1 mA cm−2 parallels that at a geometric current density of 1 mA cm−2, which infers that it is the electronic factor, rather than the geometric factor, that determines the electrocatalytic activity of a-La1−yCayCoOx. On the whole, the y = 0.3 composition exhibits the highest activity with respect to the OER among the doped samples, which is different from that (y = 0.4) for the crystalline materials.18,19,22−24 A comparison of a-La0.7Ca0.3CoOx OER activity with related cobalt-containing crystalline perovskite materials (Table 2) indicates that it exhibits a competitive lower Tafel slope and a lower overpotential at a given current density. Considering the high concentration of unsaturated surface metal sites (here, cobalt ions) at the surface of amorphous films, it is likely the surface coverage of hydroxide ion is high.39

Figure 3. (A) Cyclic voltammograms of a-La1−yCayCoOx films at a scan rate of 10 mV s−1. (B) Enlargement of cyclic voltammograms in the potential range of 0.9−1.3 V vs RHE. (C) Total charge passed through the oxidation peak in the range of 0.95−1.25 V shown in panel B.

with introduction of calcium, then increases with calcium content until y = 0.3, and finally decreases slightly with further calcium content. The calcium-free a-LaCoOx shows a catalytic activity higher than that of the y = 0.3 film, despite a lower concentration of Co3+, which may be attributed to the higher 6388


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ACS Catalysis

because of the highest concentration of Co4+ originating from the charge transfer to the more highly electronegative gold. Electrochemical impedance spectroscopy is a good technique for detecting the processes during the OER. The impedance spectra of a-La0.7Ca0.3CoOx at different applied potentials (Figure 6A,B) and a-La1−yCayCoOx at a potential of 1.593 V

Figure 5. OER kinetic parameters for a-La1−yCayCoOx: (A) Tafel slopes and (B) overpotential at the onset of catalysis and the overpotential required to achieve geometric and specific current densities of 1 mA cm−2.

Figure 6. (A and B) Impedance spectra of a-La0.7Ca0.3CoOx at different potentials vs the RHE. (C) Impedance spectra of aLa1−yCayCoOx at a potential of 1.593 V vs the RHE. (D) Equivalent circuit used to fit the impedance spectra. Lines in panels A−C are fits to the data.

Table 2. Comparison of the OER Activity of aLa0.7Ca0.3CoO3−x with the Reported Crystalline Perovskite Analogues composition

Tafel slope (mV decade−1)

η (V) @ J (mA cm−2)

a-La0.7Ca0.3CoO3−x

48.6 ± 0.5

0.33 @ 1

La0.6Ca0.4CoO3 La0.6Ca0.4CoO3 La0.6Sr0.4CoO3 La0.9Sr0.1CoO3 La0.8Sr0.2CoO3 Pr0.5Ba0.5CoO3−δ Ba0.5Sr0.5Co0.8Fe0.2O3−δ

− 55−60 63 ± 1 66 64 60 50

0.6 @ 5 − − 0.3 @ 0.0025 0.3 @ 0.01 − 0.25 @ 0.05

versus the RHE (Figure 6C) are given in Figure 6. All spectra were fit using the equivalent circuit shown in Figure 6D. Two semicircles are observed with capacitances of 10−7−10−6 and 10−3−10−2 F cm−2 from high to low frequencies, which are related with the dielectric film and the double-layer capacitance at the electrode−electrolyte interface, respectively.49 The film (Rfilm) and charge transfer (Rct) resistances of all the films are shown in Figure 7. Rfilm remains almost constant, while Rct decreases exponentially over the potential range tested. Rfilm is higher than Rct at high potentials, becoming the dominant resistance of this system. Accordingly, decreasing the film resistance will undoubtedly enhance the electrocatalytic activity

ref this work 17 24 19 9 9 33 13

The Tafel slope of the materials in this work is around 50 mV decade−1 (Figure 5A and Table S2) and exhibits a strong dependence on the concentration of Co3+; i.e., the higher the concentration of Co3+, the lower the Tafel slope. Correspondingly, we can propose that the transformation from metal hydroxide to metal oxide with a higher oxidation state is the possible rate-determining step according to either Bockris’s electrochemical path or Kobussen’s path.8 It further emphasizes the importance of the high concentration of active metal ions with high oxidation state to promote the OER process. Similar trends hold for crystalline catalysts. It is generally accepted that a metal catalytic active site with a higher oxidation state is required for high OER activity.4,7 The higher the oxidation state of a surface site, the greater its ability to dissociate water. Malkhandi et al.47 prepared La0.6Ca0.4CoO3 by the sol−gel method and found that the binding energy of cobalt increases with annealing temperature from 600 to 750 °C (i.e., the content of Co3+ increases), which correlated with the specific activity with respect to OER. Similarly, Yeo and Bell48 found the 0.4 monolayer of cobalt oxide deposited on gold shows the highest OER activity among several metal substrates

Figure 7. Film and charge transfer resistances of a-La1−yCayCoOx determined by impedance spectroscopy in 0.1 M KOH. 6389


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ACS Catalysis of the films, especially at high potentials. Rfilm is minimal for y = 0.3. The conductivity enhancement of crystalline LaCoO3 with Ca doping has been widely reported.17,21 The improved film conductivity is beneficial for the electrocatalytic activity, but it is not the determining factor. The change in Rct with calcium content is in agreement with the activity evaluated by CV and Tafel (Figures 3−5); the y = 0.3 film shows the lowest Rct among the doped samples, only slightly higher than that of aLaCoOx. The y = 0.3 film possesses the lowest Rfilm and Rct and thus the highest activity among the calcium-containing samples. Finally, the influence of the addition of calcium on the shortterm stability is evaluated by chronopotentiometry at 1 mA cm−2 in 0.1 M KOH. As shown in Figure 8, the Ca-free sample

materials seen to apply to amorphous material, aiding in the design of future catalysts that can be easily accessed via photochemical deposition.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b02145. IR spectra, SEM images before and after the stability test, XPS results of La 3d and Ca 2p, and OER activity parameters (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Curtis P. Berlinguette: 0000-0001-6875-849X Simon Trudel: 0000-0001-5757-2219 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Canadian Research Chairs, NSERC of Canada (Discovery Grant), and MITACS for operating funds. This research used facilities funded by the University of Calgary and the Canadian Foundation for Innovation (John R. Evans Leaders Fund).

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Figure 8. Short-term stability of a-La1−yCayCoOx films tested by a chronopotentiometric technique at a constant current density of 1 mA cm−2 in 0.1 M KOH.

REFERENCES

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shows degradation with time. The introduction of Ca does not hamper the stability very much, especially at high doping levels (y = 0.3−0.5). Among the Ca-doped samples, the y = 0.3 sample shows the highest OER activity and high stability. SEM imaging of y = 0 films post-testing reveals several cracks on the film; meanwhile, the microstructure is essentially unchanged for y = 0.3 (Figure S4). Lanthanum, calcium, and cobalt were detected by ICP-OES from the electrolyte solution after testing, suggesting some dissolution of these elements during the stability test. This seems to be a common degradation pathway in lanthanum cobaltites, as similar leaching of cobalt in La1−xCaxCoO350 and La1−xSrxCoO320 and lanthanum and cobalt in La0.6Ca0.4CoO322 has also been observed.

4. CONCLUSIONS We used a photochemical thin-film deposition method to directly access calcium-doped amorphous lanthanum cobaltite. The influence of the addition of calcium on the electrocatalytic activity of amorphous La1−yCayCoOx with respect to the OER is studied. We find the activity of the calcium-doped materials is generally decreased when compared to that of undoped aLaCoOx. Of these materials, a-La0.7Ca0.3CoOx demonstrates the highest activity among the calcium-containing materials. The introduction of calcium does not significantly hamper the OER catalytic stability in 24 h tests. We attribute this higher activity of a-La0.7Ca0.3CoOx to the combination of a lowered film resistance and a higher concentration of Co3+, both of which are expected to facilitate the OER process. Importantly, this study highlights that the activity trends expected for crystalline 6390


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