Water Oxidation Catalysis: Tuning the Electrocatalytic Properties of

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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 ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02145 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 7, 2017

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ACS Catalysis

Water oxi oxidation catalysis: Tuning the electrocatalytic electrocatalytic properties of amorphous lanthanum cobaltite cobaltite through calcium doping Cuijuan Zhang*a,b, Xinyue Zhangb, Katelynn Dalya, Curtis P. Berlinguettea,c and Simon Trudel *a a

Department of Chemistry and Centre for Advanced Solar Materials, University of Calgary, 2500 University Drive N. W., Calgary, Canada T2N 1N4 b

School of Chemical Engineering and Technology, Tianjin University, Tianjin, China 300350

c

Departments of Chemistry and Chemical & Biological Engineering, The University of British Columbia, 2026 Main Mall, Vancouver, BC Canada V6K 1Z6

ABSTRACT: The influence of calcium doping on the electrocatalytic activity of amorphous lanthanum cobaltite a-La1yCayCoOx towards the oxygen-evolution reaction (OER) 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 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 intermittency of sustainable energies (e.g., solar, wind, 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 four-electron 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 to decrease the overpotential and thus to enhance the efficiency of fuel production. A lot of work3-15 has been devoted to the study of factors associated with the electrocatalytic activity, which can be broadly divided into geometric and electronic factors. The former relate to the specific surface area and concentration of active sites in of the catalysts, while the latter pertains to the electronic structure and nature of active sites. Higher concentration of active site will contribute to higher activity.5, 11, 12, 16 Based on their results on a series of perovskite-type metal oxides, Matsumoto et al. proposed that transition metal oxides which conduction bands of the σ* character show a higher activity towards the OER, speaking to the importance of the electronic

structure of the system.3 Later, Bockris and Otagawa found strong dependence of activity of perovskite-type LaMOx towards the OER on the number of electron in the M dz2 orbital; the activity being higher with higher electron population of the M dz2 orbital. They proposed that LaCoO3 would be highly electrocatalytically active towards the OER.8, 9 Tseung and Jasem,4 and Raish and Tseung7 studied the role of the metal /metal oxide or the metal oxide of lower/higher oxidation state in determining the activity towards the OER, and proposed that the ideal couple has a potential similar to or lower than the theoretical potential of oxygen electrode. Thus, Co2O3/CoO2 would be an excellent candidate for the OER.4, 7 Trasati proposed a volcano-type dependence of electrocatalytic activity on the enthalpy of the lower-tohigher oxidation state transition.6, 7 The electrocatalytic activity increased with electrical conductivity and concentration of metal cations with higher oxidation state, which is well proven in the widely studied doped Co3O4 14, 15 and doped-perovskite oxides.8, 9, 17-20 The activity of La1xMxCoO3 (M = Ca, 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 electrocatalyst for the OER.18, 19, 22-24 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 (above

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600 oC) 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, increasing 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 high-activity amorphous perovskite materials is of considerable interest. We recently reported a family of amorphous metaloxide 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 perovskiteinspired 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, and Ni)37 shows that their catalytic performance is on-par or superior to their crystalline counterparts of the same composition, aLaCoOx 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 calciumaddition on the electrocatalytic activity of a-LaCoOx towards the OER. 2.

Experimental

Amorphous calcium-doped lanthanum cobaltite films, a-La1-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 Company, 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 use. Precursor solutions of the desired metal stoichiometry were prepared in hexanes at a 15 w/w% concentration. These solutions were then spin-coated onto FTO substrates. The films were then irradiated with UV light (λ = 185 and 254 nm) for 24 h. The evolution of ligand absorption was monitored by Fourier-transform infrared (FTIR) spectroscopy (Nicolet 470 FT-IR spectrometer). The resultant films were annealed in air at 100 oC for 1 h at a heating rate of 20 oC 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 100-200 nm range.

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The resultant films deposited on FTO substrate were subjected to electrochemical tests including cyclic voltammetry (CVs), 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 CVs in the non-Faradaic reaction region (0.583-0.633 V vs. RHE) measured at different scan rates (10-70 mV s-1). The proportionality between the current density J and the scan rate ν is the double-layer capacitance Cdl of the film, J = Cdlν.37 Impedance measurements were performed with a pertubation 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, the current density J in this work was calculated based on 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 calcium-free samples, and the surface area normalized using these relative roughness factors. X-ray photoelectron spectra of as-prepared a-La1(y = 0, 0.1, 0.3, and 0.5) were collected 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 45o to the sample surface. The binding energies are referenced to the advanticious C1s photoelectron peak at 284.6 eV. All spectra analysis was performed with the CasaXPS software.42 yCayCoOx

The morphology of the films was investigated through top-view 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 FTO substrate) were dissolved in dilute HNO3 prior to ICP-OES analysis. 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 Fig. 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, 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 pho-


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tolysis. Similar results were observed in our previous work on La-based binary oxides.37 SEM images of the a-La1-yCayCoOx films (Fig. S2) show a good coverage on the FTO substrate. The microstructure differs with varying calcium content. The calciumfree films are rough and slightly structured. There are cracks and wrinkled structures on the films with y = 0.1 and 0.2. Finally, the films with higher Ca content (y = 0.30.5) are smooth and featureless. The roughness of the surface can be evaluated by double layer capacitance measurements.

(Fig. 2B). The film with y= 0.3 delivers the highest Co3+ concentration of 58.1%. As the potential of Co(III)/Co(IV) couple is closer to OER than 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 aLa0.7Ca0.3CoOx sample will show better catalytic activity among the calcium-containing samples.

Figure 1A provides an example to show how the double layer capacitance Cdl was estimated by fitting the linear dependence of the current density on scan rate. As shown in Fig. 1B, the capacitance of a-La1-yCayCoOx films decreases rapidly until y = 0.3 and then slightly increased with further increasing Ca content. The roughness factor is calculated by assuming a double layer capacitance of 60 µF cm-2 for smooth oxide surface.43 The a-La0.7Ca0.3CoOx film exhibited the lowest roughness factor among all the samples in the present work (Fig. 1C).

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

XPS was employed to examine the oxidation state of cobalt in the films. The fit XPS spectra of the Co 2p are presented in Fig. 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 Fig. S3 and Table S1. The Co 2p spectra was fit with two spin-orbit split doublets with difference splitting of around 15.2 eV. The peaks at ~779.7/795.0 and ~780.8/796.5 eV are ascribed to Co3+ and Co2+, respectively (Fig. 2A). The concentration ratio of Co3+ to Co2+ increases with Ca content until y = 0.3 and then decreases

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


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centration of carbonate ion at the surface of the latter. The O 1s peaks at binding energies of 531.1-531.8 eV (Fig. 2C) and C 1s peaks at 288.5-289.0 eV (Fig. 2D) are assigned to carbonate (CO32-) species.44 The presence of CO32- has been confirmed by IR (Fig. 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 Table 1. Co 2p, O 1s, C 1s XPS fitting results of a-La1-yCayCoOx

Sample y=0

Element Co 2p O 1s C 1s

y = 0.1

Co O

C y = 0.3

Co O

C y = 0.5

Co O

C

BE (eV)

Atom%

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

Assignment 3+ Co 2+ Co Metal-O 2CO3 ,OH C-C, C-H 2CO3 3+ Co 2+ Co Metal-O 2CO3 OH C-C, C-H 2CO3 3+ Co 2+ Co Metal-O 2CO3 OH C-C, C-H 2CO3 3+ Co 2+ Co Metal-O 2CO3 OH C-C, C-H 2CO3

The electrochemical behavior of a-La1-yCayCoOx was investigated with CV (Fig. 3A). Very small redox peaks are found on film with y = 0 at Ea = 1.10 V and Ec = 1.08 V (vs. RHE) (Fig. 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+-Co3+ transformation.34 Calcium-free a-LaCoOx shows the highest redox current density. The total charge passed during the oxidation peak at 0.95-1.25 V (Fig. 3C), shows a maximum value for the calcium-free sample, decreases to a minimal value for y = 0.3, and then slightly increases with increasing 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 (Fig. 2B).

Figure 3. (A) CVs of a-La1-yCayCoOx films at a scan rate of 10 mV s-1. (B) Enlargement of CVs 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 B.

Steady-state measurement was performed to further characterize the electrocatalytic activity of a-La1-yCayCoOx catalysts. The Tafel plots are shown in Fig. 4. Three parameters: (i) Tafel slope, (ii) onset overpotential, (iii) overpotential at 1 mA cm-2, were screened to evaluate the activity of these films towards the OER. As discussed above, the roughness of a-La1-yCayCoOx films varies with calcium content. To eliminate the influence of roughness on the activity, the specific current density (Jsp) was calculated and the change of overpotential with calcium content at Jsp = 1 mA cm-2 is shown in Fig. 5 along with the three parameters mentioned above.

The CVs also reveal that the electrocatalytic activity of a-LaCoOx initially decreases significantly with introduction of calcium, then increases with calcium content until y = 0.3, to finally decrease slightly with further calcium content. The calcium free a-LaCoOx, shows a higher catalytic activity than y = 0.3, despite a lower concentration of Co3+, which may be attributed to the higher con-


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ACS Catalysis samples, which is different from that (y = 0.4) for the crystalline materials.18, 19, 22-24 A comparison of aLa0.7Ca0.3CoOx OER activity with related cobalt-containing crystalline perovskite materials (Table 2) indicates that it exhibits competitive lower Tafel slope and lower overpotential at a given current density.

Table 2. Comparison of OER activity of a-La0.7Ca0.3CoO3-x with the reported crystalline perovskite analogues. Composition

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

η (V) @ -2 J (mA cm )

Ref.

a-La0.7Ca0.3CoO3-x

Tafel slope -1 (mV dec ) 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 [email protected] [email protected] [email protected]

this work 17 24 19 9 9 33 13

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 The Tafel slope of the materials in this work is around 50 mV dec-1 (Fig. 5A and Table S2) and it exhibits strong dependence on the concentration of Co3+, i.e., the higher concentration of Co3+, the lower the Tafel slope. Correspondingly, we can propose that the transformation from metal hydroxide to metal oxide of 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 high concentration of active metal ions with high oxidation state to promote the OER process.

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.

All three parameters for the calcium-containing samples are slightly higher than those of the calcium-free catalyst. The sample with y = 0.3 exhibits the lowest Tafel slope, onset overpotential and overpotential @ 1 mA cm-2 among the calcium-containing samples. Especially, the overpotential at specific 1 mA cm-2 parallels with that at 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-La1yCayCoOx. On the whole, the y = 0.3 composition exhibits the highest activity towards the OER among the doped

Similar trends hold for crystalline catalysts. It is generally accepted that a metal catalytic active site with 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. 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 oC (i.e., the content of Co3+ increases), which correlated with the specific activity towards OER.47 Similarly, Yeo and Bell found the 0.4 monolayer of cobalt oxide deposited on gold shows the highest OER activity among several metal substrates owing to the highest concentration of Co4+ originating from the charge transfer to the more highly electronegative gold.48 Electrochemical impedance spectroscopy is a good technique to detect the processes during OER. The impedance spectra of a-La0.7Ca0.3CoOx at different applied potentials (Fig. 6A-B) and a-La1-yCayCoOx at a potential of 1.593 V vs. RHE (Fig. 6C) are given in Fig. 6. All spectra were fit using the equivalent circuit shown in Fig. 6D. Two semi-circles are observed with capacitance of 10-7-10-6


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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

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

Figure 6. (A,B) Impedance spectra of a-La0.7Ca0.3CoOx at different potentials vs. RHE; (C) Impedance spectra of a-La1-yCayCoOx at a potential of 1.593 V vs. RHE; (D) Equivalent circuit used to fitting the impedance spectra. Lines in (A-C) is the fitting to data.

The film (Rfilm) and charge transfer (Rct) resistances of all the films are shown in Fig. 7. Rfilm remains almost constant whilst Rct decreases exponentially over the potential range tested. Rfilm is higher than Rct at high potential, becoming the dominant resistance of this system. Accordingly, to reduce the film resistance will undoubtedly enhance the electrocatalytic activity 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 determinant factor. The change of Rct with calcium content is in agreement with the activity evaluated by CV and Tafel (Figs. 3-5); the film with y = 0.3 shows the lowest Rct among the doped samples, only slightly higher than a-LaCoOx. The film with y = 0.3 possesses the lowest Rfilm and Rct and thus highest activity among the calciumcontaining samples.

Finally, the influence of calcium addition on the shortterm stability is evaluated by chronopotentiometry at 1 mA cm-2 in 0.1 M KOH. As shown in Fig. 8, the Ca-free sample shows degradation with time. The introduction of Ca does not hamper the stability very much, especially at high doping level (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 posttesting reveals several cracks on the film; meanwhile the microstructure is essentially unchanged found for y = 0.3 (Fig. S4). Lanthanum, calcium and cobalt were detected by ICP-OES from the electrolyte solution after testing, suggesting some dissolution of these elements during stability test. This seems to be a common degradation pathway in lanthanum cobaltites, as similar leaching of cobalt in La1-xCaxCoO350 and La1-xSrxCoO351 and lanthanum and cobalt in La0.6Ca0.4CoO322 has also been observed.

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

4.

Conclusions

We used a photochemical thin-film deposition method to directly access calcium-doped amorphous lanthanum


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cobalite. The influence of calcium addition on the electrocatalytic activity of amorphous La1-yCayCoOx towards the OER is studied. We find the activity of the calciumdoped materials is generally lowered when compared to the undoped a-LaCoOx. Of these materials, aLa0.7Ca0.3CoOx demonstrates the highest activity among the calcium-containing materials. The introduction of calcium does not significantly hamper the OER catalytic stabilitythe stability in 24-hour tests. We attribute this higher activity of a-La0.7Ca0.3CoOx to a combined lowered film resistance and 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 materials seen to apply to amorphous material, aiding in the design of future catalysts that can be easily accessed via photochemical deposition.

ASSOCIATED CONTENT Supporting Information. IR spectra, SEM images before and after stability test, XPS results of La 3d and Ca 2p, and OER activity parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]; [email protected]

ACKNOWLEDGMENT 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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Water Oxidation Catalysis: Tuning the Electrocatalytic Properties of

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