Enhancing Bi-functional Electrocatalytic Activity of Perovskite by

Aug 21, 2013 - Enhancing Bi-functional Electrocatalytic Activity of Perovskite by Temperature Shock: A Case Study of LaNiO3−δ. Wei Zhou*† and Jak...
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Enhancing Bi-functional Electrocatalytic Activity of Perovskite by Temperature Shock: A Case Study of LaNiO3−δ Wei Zhou*,† and Jaka Sunarso‡ †

School of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia Institute for Frontier Materials, Deakin University, 221 Burwood Highway, Burwood, Victoria 3125, Australia

‡

S Supporting Information *

ABSTRACT: Perovskite oxide offers an attractive alternative to precious metal electrocatalysts given its low cost and high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity. The results obtained in this work suggest a correlation of crystal structure with ORR and OER activity for LaNiO3−δ. LaNiO3−δ perovskites with different crystal structure were obtained by heating at different temperatures, e.g., 400, 600, and 800 °C followed by quenching into room temperature. Cubic structure (relative to rhombohedral) leads to higher ORR and OER activity as well as enhanced bifunctional electrocatalytic activity, e.g., lower difference in potential between the ORR at −3 mA cm−2 and OER at 5 mA cm−2 (ΔE). Therefore, this work shows the possibility to adjust bi-functional activity through a simple process. This correlation may also extend to other perovskite oxide systems. SECTION: Energy Conversion and Storage; Energy and Charge Transport

E

metal selection and stoichiometry and thus, may also be tuned.21−24 Shao-Horn and co-workers25,26 report that perovskites containing transition-metals with an eg = 1 electron configuration showed a high ORR and OER activity in alkaline solution. In particular, LaMnO3 and LaNiO3 exhibit promising ORR and OER activity comparable to Pt/C or IrO2. The drawback of these compounds probably lies in their substantially lower mass activities due to their large particle size.27 Nanostructuring perovskite is later on devised and demonstrated as an efficient way to improve the mass activity of perovskite electrocatalysts.28−31 Herein, we report a simple strategy that led to the electrocatalytic activity enhancement of perovskite (represented here by LaNiO3‑δ) by tuning the crystallographic structure without incorporating any other additives. This work highlights the crystal structure-electrocatalytic properties relationship in perovskite, which has been well-demonstrated for solid oxide fuel cell and oxygen ionic transport membrane applications but rather overlooked for its (ORR and OER) electrocatalyst applications in alkaline solution.32−35 LaNiO3−δ perovskite powders were synthesized by combined EDTA−citrate complexing process as described in detail elsewhere.36 Briefly, stoichiometric La(NO3)3 and Ni(NO3)2 were mixed in de-ionized water with EDTA and citric acid as the complexing agents. The solution pH was adjusted to 6 by adding NH3 aqueous solution and well stirring. The solution was then heated and dried to form a solid precursor, which was

lectrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) lie at the center of renewable-energy technologies such as metal-air batteries, fuel cells, and water splitting.1−5 While primary metal-air batteries and fuel cells rely on ORR in its cathode to function, hydrogen and oxygen production from water splitting may be accomplished using OER. A major step forward from these more conventional technologies is to develop catalyst materials that can perform both ORR and OER with very low overpotential between them, e.g., the bi-functional catalyst. Such materials would enable advanced energy conversion and storage devices, e.g., rechargeable metal-air batteries and regenerative fuel cells1,6,7 So far, the sluggish, strong irreversible nature of the oxygen electrochemical kinetics in conjunction with the distinct potential and conditions necessary for ORR and OER has made finding single bi-functional material a difficult task. It is also well-known that metal such as Pt performs very well for ORR (and performs poorly for OER), while metal oxide such as RuO2 and IrO2 performs very well for OER (and performs poorly for ORR).8−10 Moreover, high cost, limited supply, and poor durability of these precious metals (and metal oxides) hinder their large-scale applications even as a single ORR or OER catalyst.11−13 Transition metal oxides are a more economical alternative to these metals, especially in alkaline solution (electrolytes).14−16 Several classic works report correlation between the electrocatalytic activity of these oxides and d-electron density of the transition metal cations.17−20 Likewise, the electrocatalytic properties of perovskite oxides which normally consist of several different transition metal cations vary depending on the © 2013 American Chemical Society

Received: June 7, 2013 Accepted: August 3, 2013 Published: August 21, 2013 2982

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Figure 1. (a) XRD patterns of LN-RT, LN-400, LN-600, and LN-800. Inset is the magnified diffraction peak(s) at around 32° for the LaNiO3 perovskites with various crystal structure, which merged into a single peak at 600 °C and above, indicating phase transition from rhombohedral (R3̅c) to cubic (Pm3̅m). (b) Crystal structure of rhombohedral (R3̅c) and cubic (Pm3̅m) LaNiO3 perovskites.

Table 1. Structure Parameters for LN-RT, LN-400, LN-600, and LN-800 crystal structure lattice parameters (Å)

LN-RT

LN-400

R3c̅ a = b = 5.462(2) c = 13.19(0)

R3c̅ a = b = 5.483(4) c = 13.30(7)

La Ni O

x 0 0 0.496(5)

bond length Ni−O (Å) bond angle Ni−O−Ni (deg) Rp Rwp χ2

1.922(2) 178.845(1) 0.0309 0.1174 3.80

y 0 0 0

z 0.25 0 0.25

x 0 0 0.496(9) 1.932(8) 178.991(5) 0.0310 0.1228 3.96

y 0 0 0

LN-600

z 0.25 0 0.25

LN-800

Pm3m ̅ a = b = c = 3.878(4)

Pm3m ̅ a = b = c = 3.888(2)

x 0.5 0 0.5

x 0.5 0 0.5

1.939(2) 180 0.0310 0.1335 4.30

y 0.5 0 0

z 0.5 0 0

y 0.5 0 0

z 0.5 0 0

1.944(1) 180 0.0310 0.1274 4.10

600, and 800 °C (Figure S1). A minor weight gain (0.14 wt %) was observed during heating. The oxygen concentration in LNRT and LN-600 was determined by iodometric titration. The resultant stoichiometric formula of LN-RT and LN-600 are LaNiO2.98 and LaNiO3. This implies that the weight gain is due to Ni oxidation, which results in the phase transition. While LaNiO3 normally exhibits a cubic structure, a 0.05 wt % weight loss may cause cubic to rhombohedral phase transition.37 Particle size may affect the ORR and OER activity of the perovskite catalyst. Since similar specific surface area (e.g., 3.8 ± 0.5 m2 g−1; obtained from N2-adsorption measurements) and crystallite sizes (e.g., 25 ± 5 nm; calculated by Scherrer equation from XRD patterns) were obtained for all LaNiO3‑δ samples, it is valid to assume that relatively short treatment (15 min) at different temperature did not contribute substantially toward particle size variation, thus excluding its effect to electrocatalytic activity. The ORR and OER activities of different LaNiO3‑δ were probed using rotating ring-disk electrode (RRDE) measurements. LaNiO3−δ powders (10 mg) were sonicated in mixture with carbon black (2 mg), tetrahydrofuran (THF, 1 mL) and 5 wt % Nafion solution (100 μL). A 10 μL drop of this solution was deposited into glassy carbon electrode and dried at room temperature; leading to a total catalyst (LaNiO3‑δ and carbon) loading of 870 μg cm−2. Figure 2a shows the typical ORR voltammograms for LN-RT, LN-400, LN-600 and LN-800 thin film electrocatalyst in O2-saturated 0.1 M KOH at a potential

then ground into powder and calcined in air at 800 °C for 5 h to obtain LaNiO3‑δprecursor powders (noted as LN-RT, RT for room temperature). LaNiO3‑δperovskites with different crystal structure were obtained by heating LaNiO3‑δprecursor powders at 400, 600, or 800 °C in air for 15 min and then quenching (e.g., fast cooling of less than 5 s) to room temperature (see Experimental section in Supporting Information), which are noted as LN-400, LN-600 and LN-800. Quenching preserved the high temperature structure of LaNiO3−δ. X-ray diffraction patterns (in combination with Rietveld refinements) (Figure 1a) show the formation of rhombohedral structure (in R3̅c (#167) space group) for LNRT and LN-400 and the formation of cubic structure (in Pm3̅m (#221) space group) for LN-600 and LN-800, respectively. The phase transition is reflected in the significant change of the bond structure, e.g., Ni−O−Ni bonds were previously distorted in rhombohedral structure (Figure 1b). Table 1 lists the lattice parameters, bond lengths of O−Ni, and bond angles of Ni−O− Ni obtained from Rietveld refinements. The Rietveld refinement yielded satisfactory results, based on the consideration of lower profile Rp, weighted-profile Rwp, and goodness of fit χ2 values, as shown in Table 1. O−Ni bond stretching (e.g., elongated bond) occurred (and was enhanced) with temperature rise. This was also accompanied by the increase of bond angle of Ni−O−Ni into 180 o at 600 °C. Thermal gravimetric analysis (TGA) was performed on LNRT from 30 to 800 °C in air with 15 min dwelling time at 400, 2983

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Figure 2. ORR current densities of glassy carbon (GC)-supported thin film LN-RT, LN-400, LN-600, and LN-800 electrocatalysts at 1600 rpm in O2-saturated 0.1 M KOH at 5 mV s−1. Background ORR activity of thin-film Nafion-bonded carbon thin-film electrode is shown for reference (a). The onset potentials of ORR curves (b) and ORR mass activities (im) (c) for thin film LN-RT, LN-400, LN-600 and LN-800 electrodes. The im value is the current after correction for capacitive effect and O2-diffusion limitation and division by the total mass of catalysts (LaNiO3‑δ+carbon).

Figure 3. Typical ORR polarization curves on glassy carbon (GC)-supported thin film LN-600 electrocatalyst at different rotation rates. (a) K-L plots for GC-supported thin film LN-RT, LN-400, LN-600, LN-800, and Pt/C (40 wt %) electrocatalysts with identical electrocatalyst loading. (b) Peroxide yield (X, percentage of HO2− relative to total products) during ORR using the thin film LN-RT, LN-400, LN-600, and LN-800 electrocatalysts.

scan rate of 5 mV s−1 and rotation speed of 1600 rpm between −0.5−0 V versus Ag|AgCl (3 M NaCl) reference electrode. All

potential values are iR-corrected during each cyclic voltammogram (CV) run to compensate the effect of solution resistance. 2984

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Figure 4. (a) OER current densities of GC-supported thin film LN-RT, LN-400, LN-600, and LN-800 electrocatalysts at 1600 rpm in O2-saturated 0.1 M KOH at 5 mV s−1. (b) OER mass activities for thin film LN-RT, LN-400, LN-600, and LN-800 electrocatalysts. The current im is the total mass of catalysts (LaNiO3−δ and carbon).

ORR kinetics was then assessed using Koutecky−Levich (KL) plots constructed from rotation rate dependent current density (Figure 3a). K-L plots for various LaNiO3‑δ electrocatalysts are displayed in Figure 3b; that of Pt/C (40 wt % Pt on high surface area carbon) electrocatalyst is also shown as reference. For LN-600 and LN-800, the mass-transport limited current density is close to 5 mA cm−2, indicating four-electron transfer ORR mechanism.27 However, the current densities are 3.74 and 3.95 mA cm−2 for LN-RT and LN-400, which indicates less than four-electron transfer ORR mechanism. We cross-checked this using K-L analyses and RRDE test. The slopes of K-L curves of LN-600 and LN-800 are close to that of 40 wt % Pt/C, while those of LN-RT and LN-400 are larger. This supports less than four-electron transfer on LN-RT and LN-400 electrocatalysts. ORR kinetics was also examined using RRDE. The peroxide yield (X, percentage of HO2− relative to total products) from the ring/disk currents (IR/ID) can be calculated through the following equation:

The ORR activity of the carbon added to oxide electrodes is also shown in Figure 2a, indicating that background contribution from the carbon is relatively low down to −0.5 V vs Ag|AgCl (3 M NaCl); consistent with another report.27 Carbon inclusion in relatively low amount here (e.g., ∼16.6%) is mainly intended as a conductivity additive without ruling out possible carbon catalyst role as reported elsewhere.38,39 CV curves for LN-600 and LN-800 exhibit ∼50 mV and ∼70 mV more positive value of half-wave potential relative to LN-RT and LN-400. Figure 2b compares the ORR onset potential of these four compounds. LN-800 demonstrates the most positive onset potential (e.g., −0.165 V vs Ag|AgCl (3 M NaCl)) followed by LN-600 (−0.203 V vs Ag|AgCl (3 M NaCl)), LN400 (−0.217 V vs Ag|AgCl (3 M NaCl)) and LN-RT (−0.228 V vs Ag|AgCl (3 M NaCl)). The onset potential (Vonset) is defined as the potential at which the ORR current is 5% of that measured at the diffusion-limited current.40 Figure 2c displays mass activity polarization curves of the electrodes. The Tafel slope values lie around 100 ± 3 mV dec−1. The slope trend suggests slight value reduction with increasing quenching temperature. The reported Tafel slopes of LaNiO3‑δ perovskites varied in literatures (e.g., 45, 60, and 120 mV dec−1) due to different preparation details and electrode composition.20,27,41,42 The ORR profile of a 40 wt % Pt/C was also probed (the preparation and ink composition of which is identical to LaNiO3‑δ thin-film electrode). The 40 wt % Pt/C electrode shows a Tafel slope of 80 mV dec−1 (Figure S2), which is close to the 75 mV dec−1 reported by Dodelet’s group43 yet higher than 60 mV dec−1 reported by Shao-horn’s group.27 This is likely to be related to the Pt/C loading difference, e.g., 10.7 μg cm−2 in Shao-horn’s work and 800 μg cm−2 in Dodelet’s work of which the latter value is quite similar to the Pt/C and LN/carbon loading of this work (e.g., 870 μg cm−2). The high LN loading may correlate with the larger Tafel slopes observed. As a trend, the mass activity increases with increasing quenching temperature. Quite low values (e.g., ∼0.89 and 1.07 A gtotal−1 at −0.3 V vs Ag|AgCl (3 M NaCl)) were obtained for LaNiO3 perovskites with rhombohedral structure, which were enhanced ∼3−7 times for those with cubic structure. Moreover, the mass activity of LN-RT is much lower than the activity (32 A g−1 at 0.693 V vs HRE) of LaNiO3‑δ supported on high specific surface area carbon (1080 m2 g−1) reported by Hardin et al.,28 probably due to the lower specific surface area of LN-RT (3.8 m2 g−1) and carbon (60 m2 g−1) used in this work.

X = 200IR /(NID + IR )

(1)

where N is the current collection efficiency of RRDE. As shown in Figure 3c, the H2O2 formation during ORR are below 9% for LN-600 and LN-800, which is analogous to that of 40 wt % Pt/ C electrode (Figure S3). On the other hand, H2O2 formation reaches 28−45% between −0.6 and −0.35 V vs Ag|AgCl (3 M NaCl)) for LN-RT and LN-400. These results confirmed fourelectron transfer mechanism in ORR for LN-600 and LN-800, which is not the case for LN-RT and LN-400. The electrontransfer number on LaNiO 3‑δ perovskite varies in the literature.27,28 Suntivich et al. claimed a four-electron-transfer process for ORR on LaNiO3‑δ.27 Hardin et al. reported that the LaNiO3−δ supported on high specific surface area carbon shows a diffusion limited current density of 3.5 mA cm−2, indicating a less than four electron-transfer process for ORR.28 This discrepancy may originate from different loading and/or reaction mechanism as is reported elsewhere.44 Accordingly, the effect of different crystal structure of LaNiO3‑δ perovskites on OER was determined in O2-saturated 0.1 M KOH at 1600 rpm (Figure 4a). The Tafel curves of OER on the LaNiO3−δ perovskites are shown in Figure 4b. The previous Figure 4b is deleted. The Tafel slopes decrease from 108 to 80 mV dec−1, suggesting the improvement of the OER activity on LaNiO3−δ perovskites as the phase changes from rhombohedral to cubic structure. The Tafel slopes of OER for LaNiO3−δ electrocatalyst vary between 43 and 130 mV dec−1 2985

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Table 2. Assessment of Catalyst Bi-functionality for LaNiO3−δ Perovskites and Other LN-Based Electrocatalysts electrocatalysts

ORR potential (V) @ −3 mA cm−2

OER potential (V) @ 5 mA cm−2

LN-RT LN-400 LN-600 LN-800 nsLaNiO3/NC28 LaNiO3/NC28

−0.40 vs Ag|AgCl −0.39 vs Ag|AgCl −0.34 vs Ag|AgCl −0.32 vs Ag|AgCl 0.64 vs RHE 0.56 vs RHE

0.76 vs Ag|AgCl 0.74 vs Ag|AgCl 0.72 vs Ag|AgCl 0.69 vs Ag|AgCl 1.58 vs RHE 1.62 vs RHE

depending on the preparation methods.45−48 For example, Bockris and Otagawa45 synthesized LaNiO3−δ by two different methods: high-temperature ceramic and coprecipitation followed by decomposition at 800 °C. The former work led to the slope value of 65−130 mV dec−1, while the latter led to the slope value of 43 mV dec−1. Like ORR case, it is highly probable that the discrepancy of Tafel slopes for OER is related to the different electrocatalyst loading. The mass activity of OER at 0.8 V vs Ag|AgCl (3 M NaCl) is 19.8, 29.8, 46.0, and 135 A g−1 (obtained by extrapolation of Tafel curve to higher potential). The mass activity is improved by 6.7 times for LN800 relative to LN-RT. To measure the bi-functional activity of the catalyst, the potential difference when ORR current reaches −3 mA cm−2 and OER current reaches 5 mA cm−2 (ΔE) is calculated and listed in Table 2. The ΔE decreases with increasing quenching temperature and reaches its minimum value of 1.01 V for LN800 electrode; the value of which is comparable to those based on high specific surface area LaNiO3‑δ (6 or 11 m2 g−1) and carbon support (1080 m2 g−1).28 To this end, we have demonstrated the influence of crystal structure into ORR and OER activity of LaNiO3−δ perovskite electrocatalysts in alkaline solution. Figure 5 plots ORR and

electrolyte 0.1 0.1 0.1 0.1 0.1 0.1

M M M M M M

KOH KOH KOH KOH KOH KOH

ΔE (V) 1.16 1.13 1.06 1.01 0.94 1.06

of electrons from adsorbent bonding levels within the conduction band of the solid to its Fermi surface is the rate determining step in liquid phase heterogeneous catalysis.51 Here, the large Fermi surface in cubic LaNiO3 perovskite may facilitate electrons transfer during ORR/OER in alkaline solution. Moreover, the bond elongation in rhombohedral LaNiO3‑δ has less apparent effect toward improving the activity as compared to the case in cubic structure. Hyodo et al. found that the ORR activity of LnMnO3 (Ln = lanthanoids or Y) perovskites increases with increasing ionic radius of the Ln3+.52 The ORR and OER reaction is claimed to take place on the transition-metal ion (B-site) in perovskites19,53 such that the positive effect of increasing Ln3+ can be explained by increasing Mn−O bond length. Suntivich et al. claimed that the s*-orbital (eg) occupation and oxygen covalency of transition metal are directly related to the ORR and OER activity.27 Ruling out the likelihood of substantial change in the valence state of Ni for all four LaNiO3−δ perovskites samples, the effect of eg oxygen occupation on the activity may be least expected. Therefore, the change of bond length would be the next crucial factor affecting the oxygen covalency of Ni−O in LaNiO3−δ perovskites. For catalysts that bind oxygen strongly, the rate is limited by the removal of surface oxide, while for catalysts that bind oxygen weakly the rate is limited by the transfer of electrons and protons to adsorbed O2. Here, the slight elongation of the Ni− O bond may adjust the binding ability with the oxygen to mediate the removal and adsorption of the oxygen to a particular condition, the details of which requires a more specific study. In summary, we have demonstrated that the crystal structure of LaNiO3‑δ perovskites influences the ORR and OER activity dramatically in the alkaline aqueous electrolyte. LaNiO3−δ perovskites with different crystal structures were obtained by quenching. Cubic LaNiO3 perovskites display higher ORR and OER activity (and bi-functional activity) relative to the rhombohedral ones. Elongated Ni−O bond length enhances the ORR and OER activity (and bi-functional activity) only in cubic perovskite case. Our observation here opens an interesting question into how far the bond length can be tweaked to improve the properties and whether this is applicable also to other perovskites.

Figure 5. Correlation of the ORR/OER activity of LaNiO3−δ perovskites with their crystallographic structure.

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OER activity against bond length of Ni−O. It is likely that the improvement of ORR and OER activity is originated from two factors, i.e., bond length and crystal structure. The phase transition from rhombohedral to cubic led to dramatic improvement in ORR and OER activity. This dramatic improvement may be related to the special Fermi surface in the cubic LaNiO3 perovskites. It was reported that the cubic LaNiO3 perovskite structure has a large Fermi surface with a hole inside, while in the rhombohedral one, the large Fermi surface collapses into small surfaces with electrons and holes.49 The large Fermi surfaces play important roles in the superconductivity, which may also correlate with the high ORR/OER activity.50 It has been reported that the promotion

ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental methods, TGA, ORR polarization curve, and peroxide yield during ORR using the thin film 40 wt % Pt/ C electrocatalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +61 733653687; fax: +61 733654199. E-mail address: [email protected]. 2986

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Wei Zhou acknowledges the support of an Australian Research Council (ARC) Discovery Project grant.

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