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Jun 25, 2019 - Efficient electrocatalysts for oxygen reduction reaction (ORR) are critical to energy conversion and storage devices, such as fuel cell...
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Research Article Cite This: ACS Catal. 2019, 9, 7137−7142

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Effective Promotion of Oxygen Reduction Reaction by in Situ Formation of Nanostructured Catalyst Yu Chen,† Seonyoung Yoo,† Weilin Zhang,† Jun Hyuk Kim,† Yucun Zhou,† Kai Pei,† Nicholas Kane,† Bote Zhao,† Ryan Murphy,† YongMan Choi,*,§ and Meilin Liu*,† †

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States College of Photonics, National Chiao Tung University, Tainan 71150, Taiwan

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ABSTRACT: Efficient electrocatalysts for oxygen reduction reaction (ORR) are critical to high-performance energy conversion and storage devices. As an important family of functional materials, alkaline earth metal oxides are generally considered inert toward electrochemical reactions. Here we report the remarkable enhancement of ORR activity and durability of electrodes enabled by surface modification with a precursor of barium nitrate. During cell start-up process, an ORR active cobaltite catalyst was in situ formed on the electrode surface, as confirmed by scanning transmission electron microscopy (STEM) analysis. A combination of in situ/operando Raman and electrochemical impedance study suggests that the transition from nitrate to cobaltite may occur at ∼565 °C. The peak power density of a single cell at 750 °C is increased from ∼0.85 to ∼1.15 W cm−2 (35% increase) by the surface modification, demonstrating the remarkable enhancement of electrocatalytic activity under realistic operating conditions. KEYWORDS: oxygen reduction reaction, perovskite, barium cobaltite, fuel cells, nanoparticles

1. INTRODUCTION Oxygen reduction reaction (ORR) is one of the most important reactions in energy conversion and storage devices, such as solid oxide fuel cells and metal−air batteries.1−5 It is well accepted that ORR is a complex and notoriously sluggish process; its rate is limited mainly by one or more elementary steps associated with adsorption and dissociation of oxygen and diffusion of oxygen species on the surface to the active site for reduction as well as the bulk diffusion process.6 In light of that, significant efforts have been implemented to develop high-performing cathodes or surface coatings for facile ORR.1,7 A great deal of effort has recently been devoted to nanostructured materials due to their unusual electrical, electrochemical, and/or catalytic properties.4,8,9 Surface modifications with isolated nanoparticles (on the order of tens of nanometers) and/or thin conformal coatings (∼5−20 nm thick) can effectively enhance the activity and/or durability of cathodes.9,10 Surface decoration by electrocatalytically active materials is an effective method to engineer the cathode surface properties to facilitate the reaction.9,11−14 It has been demonstrated in our previous report that the surface coating of a highly active electrocatalyst may dramatically enhance the kinetics of the surface oxygen exchange processes, while a conformal surface coating may suppress cation segregation, thus enhancing the robustness of the cathodes against contaminants.15 For example, discrete nanoparticles of Sm0.5Sr0.5CoO3, nanoparticles of doped ceria or stabilized zirconia, conformal coatings of perovskite materials such as Sr© XXXX American Chemical Society

doped LaMnO3, or hybrid catalysts with multiphase on cathode surface have demonstrated excellent activity and durability.16−18 In spite of the different morphologies, one fundamental requirement of the catalysts is the use of transition metal oxides (TMO), taking advantage of their unique oxygen exchange capability.12,15 The TMO-containing catalysts are either oxygen ion conductors or mixed ionic and electronic conductors, enabling facile electron or ion transfer, which is of great significance to ORR. However, several transition metals are rare and expensive. Here, we report a cathode surface, engineered initially by a simple, cheap “BaO” with very limited electronic or ionic conductivity, which eventually demonstrates excellent ORR activity or durability after cell start-up. We further unraveled the detailed mechanism via careful TEM analysis and unique in situ Raman spectroscopy. It is concluded that the in situ formation of barium cobaltite on the surface of the electrode is the main reason for the performance improvement.

2. EXPERIMENTAL SECTION 2.1. Preparation of PBCC Powder and Solution for Surface Modification. The PBCC (PrBa0.8Ca0.2Co2O5+δ) powder was prepared by the glycine−nitrate solution combustion method.19 A nitrate mixture of praseodymium Received: April 28, 2019 Revised: June 20, 2019 Published: June 25, 2019 7137

DOI: 10.1021/acscatal.9b01738 ACS Catal. 2019, 9, 7137−7142

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ACS Catalysis

Figure 1. Electrochemical performance of the two electrodes: bare PBCC and “BaO”-coated PBCC. (a) SEM image of bare PBCC and “BaO”coated PBCC. (b) Electrochemical impedance spectra of the two electrodes at 600 °C under open-circuit voltage (OCV) condition. (c) Temperature dependence of the interfacial polarization resistance of the two electrodes. (d) Durability of the two electrodes at 750 °C under OCV condition. (e) Typical I−V−P curves. (f) EIS of single cells with a porous PBCC (bare or coated) as cathode.

containing alloy was in direct contact with the cathode with an Ag mesh. The anode-supported button cells were mounted on an alumina tube for full cell performance evaluation. The cells were tested at 750 °C with humidified hydrogen (3 vol % H2O) as the fuel and ambient air as the oxidant. 2.5. In Situ Raman Study. PBCC pellets with a relative density of ∼98% were fabricated by uniaxially pressing PBCC powders followed by sintering at 1200 °C for 5 h. The experimental settings and conditions for the in situ Raman study can be found in our previous study.21 Dense PBCC pellets with dried surface coatings were then put in the Raman chamber for the in situ Raman experiment. 2.6. Computational Method. Periodic density functional theory (DFT) calculations were carried out using the Vienna ab initio simulation package (VASP)22,23 with the projectoraugmented-wave (PAW) method24 and the spin-polarization method. The generalized gradient approximation (GGA) used the Perdew−Burke−Ernzerhof (PBE)25 exchange-correlation functional for BaO (Ba 32 O 32 ; Fm3̅ m ) and BaCoO 3 (Ba4Co4O12; Pm-3m). While all structures were constructed with a kinetic energy cutoff for a plane wave basis set of 415 eV, Monkhorst−Pack meshes26 with (3 × 3 × 3) and (3 × 3 × 1) were applied for bulk and surface calculations, respectively. For the 2-D surface calculations, slabs were separated by a vacuum space of 15 Å to avoid any interactions between slabs. The four and eight bottom layers of BaO and BaCoO3 surfaces, respectively, were fixed to their bulk structure. The adsorption energy (Ead) of oxygen on a surface was calculated from Ead = E[O-surface] − (E[bare surface] + E[O]), where E[O-surface], E[bare surface], and E[O] are the predicted electronic energies for an adsorbed O species on a surface, a bare surface, and an atomic O species, respectively.

nitrate, barium nitrate, calcium nitrate, and cobalt nitrate (all commercially available from Alfa Aesar) was mixed in DI water to form a clear solution with a concentration of 0.1 mol/L. Glycine was added in the cathode nitrate solution with a mole ratio of glycine:NO3− = 1:2, functioning as the complexing agent and fuel for the following combustion. The ash was then fired at 800 °C for 1 h. To create the surface modification solution, barium nitrate hydrate was dissolved in DI water to form a 0.1 M Ba(NO 3 ) 2 solution. A surfactant of polyvinylpyrrolidone (PVP) solution with a concentration of 5 wt % was added to the catalyst solution. 2.2. Fabrication of Symmetrical Cells. SDC pellets were fabricated by uniaxially pressing commercial SDC powder (from Fuelcell Materials, US). Dense SDC pellets with a relative density of ∼98% were accomplished by sintering at 1450 °C for 5 h. The PBCC green tapes and symmetrical cells are fabricated using our standard procedures; more details can be found in our previous report.20 The symmetrical cells (with an effective area of 0.316 cm2) were then fired at 1080 °C for 2 h to form porous PBCC electrodes. Ba(NO3)2 solution with an amount of 5 or 10 μL was coated on the porous PBCC backbone surface (Figure S1), which were fired at 900 °C for 2 h. 2.3. Fabrication of Anode-Supported Cells. The details for fabrication of NiO−YSZ anode-supported cells with YSZ electrolyte and SDC buffer layer can be found in our previous study.20 An SDC buffer layer with a thickness of ∼2−4 μm was deposited on the YSZ electrolyte surface by a drop-coating process, followed by firing at 1200 °C for 2 h. The PBCC cathode was then brush painted onto the electrolyte surface and fired at 900 °C. Ba(NO3)2 solution was deposited on the porous PBCC cathode surface using the same method as for the symmetrical cells. 2.4. Electrochemical Measurements. The area specific resistance (ASR) of symmetrical cells with PBCC cathodes (coated with in situ formed catalyst coating) was measured at 600−750 °C in air. The details of cell configuration, current collector, and impedance spectra measurement can be found in our earlier study.20 To evaluate the Cr poisoning effect, a Cr-

3. RESULTS AND DISCUSSION Shown in Figure 1a (left) is a typical SEM image of the bare PrBa0.8Ca0.2Co2O6‑δ (PBCC) cathode backbone, showing smooth surface and clear grains. Shown in Figure 1a (right) is an image of the PBCC cathode covered with discrete “BaO” nanoparticles (NPs). The actual composition of these NPs will 7138

DOI: 10.1021/acscatal.9b01738 ACS Catal. 2019, 9, 7137−7142

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Figure 2. Microstructure of PBCC grain with surface modification of in situ-formed Co-containing nanoparticles. (a) Bright-field TEM image of coated PBCC. (b) High resolution TEM of a catalyst particle. (c) STEM of coated PBCC. (d) EDX spectrum of point 1 in c (inset is the 2D EDX spectra along the line marked in c). (e) EELS spectra along the line marked in c.

enhancement or contaminant tolerance is still not clear. Both BaO and BaCO3 are insulating and inert to ORR. The formation of some compounds during fabrication or testing is likely the reason for the enhanced ORR kinetics. In order to understand the microscopic features of the electrode surface, we performed microstructure analyses of the PBCC cathode coated with nanoparticles, as shown in Figure 2a. Ba(NO3)2 solution was first infiltrated into the porous cathode backbone and then fired at 900 °C for 2 h. The surface of PBCC grains was covered by nanoparticles (Figure 2a) with a possible composition of Ba and Co (Figure 2b), as evidenced by energy-dispersive X-ray (EDX) spectroscopy at point 1 and the scan along the red line, as marked in Figure 2c and 2d. The inset of Figure 2d is the 2D energy-dispersive X-ray (EDX) spectrum of the linear scanning of the red line in Figure 2c. Co is almost everywhere along the line from the particles surface to PBCC grain, even if the particles were derived from the combustion of Ba(NO3)2. It suggested that the Co likely diffused from PBCC grain and reacted with “BaO” to form barium cobaltite, which is consistent with the electron energy loss spectroscopy (EELS) results shown in Figure 2e. The slight shift of the Co/Ba peak indicated a gradient of Co distribution with less Co on the outer surface (Figure S5). To understand the formation process of barium cobaltite on the surface, we fabricated a dense PBCC pellet with surface modification (Figure 3a−c) and applied in situ Raman spectroscopy (Figure 3d) to probe the incipient surface species/phases under operating conditions. Raman spectroscopy is a powerful tool to probe the chemical and physical properties of materials or species from their vibrational modes, providing critical insights into structural evolution of materials as a function of external parameters. The catalyst was introduced by dropping Ba(NO3)3 solution, followed by firing at 900 °C for 2 h, consistent with the symmetrical electrode fabrication process. In the in situ Raman experiment, a 2 μL amount of a solution of 0.1 M Ba(NO3)2 was first dropped on the dense PBCC pellet. After drying at 70 °C for 2 h, the pellet with barium nitrate salt was moved to a Raman chamber. Since PBCC has a tetragonal structure, its symmetry is barely Raman

be elaborated on later. The electrochemical activity of coated PBCC cathodes was evaluated by measuring the electrochemical impedance spectra (EIS) of symmetrical cells using a configuration of cathode/electrolyte/cathode. The ORR activity of Ba species-coated PBCC cathode has been improved significantly (Figure 1b and 1c, Figure S1). Further analysis of impedance by distribution of relaxation time (DRT, shown in Figure S2) indicated that the coating facilitated a process in the middle frequency range (∼1−100 Hz), which is likely associated with the surface exchange process. 6 More importantly, the BaO-based oxide-coated PBCC also shows very stable performance during the ∼140 h operation (Figure 1d) and good tolerance against Cr poisoning (see Figure S3), implying that there is potential to achieve long-term durability, although testing for a longer period of time would be necessary to validate the durability. In order to evaluate the ORR activity improvement on an actual fuel cell, we constructed a single cell using a Ni−YSZ as anode support, a YSZ thin film as electrolyte, a SDC as buffer layer, and a porous PBCC layer as cathode. The cathode was coated with nanoparticles, similar to what we did on symmetrical cells. Shown in Figure 1e and 1f are some typical I-V-P curves and impedance spectra, respectively, of the single cells (Figure S4) tested at 750 °C with humidified H2 (with ∼3% H2O) as a fuel and air as an oxidant. At 750 °C, a peak power density of ∼1.15 W cm−2 (35% increase) was achieved, which is higher than that of the single cells with a bare cathode (∼0.85 W cm−2), demonstrating the remarkable electrocatalytic activity of the PBCC cathode under realistic operating conditions. It has been reported by Xia et al. that the barium carbonate nanoparticles functioned as an ORR catalyst and facilitated the surface exchange coefficient by a factor of 8.27 Jiang et al. reported that BaO nanoparticles can significantly enhance the contaminant tolerant of cathode via the formation of BaCrO4 instead of SrCrO4 on the electrode surface. With the help of the BaO coating, Sr segregation from La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathode was suppressed, thus enhancing the Cr tolerance of the cathode.28 However, the mechanism of ORR 7139

DOI: 10.1021/acscatal.9b01738 ACS Catal. 2019, 9, 7137−7142

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ACS Catalysis

down, blue triangles). Between these two sets of measurements we annealed the cell at 800 °C for 2 h to complete the formation of cobaltite. As expected, the Rp values at temperature below 600 °C in these two ways (heating up and cooling down) are significantly different, indicating that the species on the electrode surface are different. For instance, as shown in Figure 4b, at 500 °C, the electrode shows a Rp of approximately 6.5 Ω cm2 in heating up (higher than that of bare PBCC), which is much higher than that of 0.9 Ω cm2 in cooling down (lower than that of bare PBCC). Combined with earlier Raman results, it can be safely concluded that during heating up the surface species below 600 °C are mainly inert barium carbonate, while above 600 °C they are mainly barium cobaltite. The Rp values at temperatures above 600 °C under either way of measurement are almost identical, indicating that the surface species completely becomes barium cobaltite. When the temperature is approximately above 565 °C (see Figure 4a), the surface catalysts (might be a mixture of carbonate and cobaltite) started to facile the electrode ORR activity. This electrochemical measurement further confirms that the dominating phase is likely cobaltite after being annealed at temperatures higher than 565 °C. The formation of barium cobaltite is further confirmed by the cross-sectional STEM analyses of PBCC pellets coated with nanoparticles. The results shown in Figure 5a indicate that the nanoparticles on the surface of the PBCC pellet are cobaltite, most likely BaCoO3−x (BCO). Similar to our previous studies,6 we applied periodic density functional theory (DFT) calculations to computationally support the experimental finding that BCO may be the key nanoparticles formed on the PBCC surface for the ORR enhancement, not because of Ba-containing intermediate materials, such as BaO. As a descriptor, the adsorption energy of oxygen (Ead)32 was calculated. On the basis of the optimized bulk structures of BaO and BaCoO3 (Figure S7), the BaO(100)33 and BaCoO(110) surfaces were constructed (Figure S8). Similar to our previous study,6 a BaCoO-terminated (110) surface was used for the adsorption energy calculations. Our systematic study confirms that no adsorption of atomic oxygen species may occur on the BaO surface and at the Ba ion on the BaCoO surface as their optimized O bond distances are 2.37 and 2.41 Å, respectively. However, as shown in Figure 5b, the adsorption of oxygen on the BaCoO3 surface takes place at the Co ion with an adsorption energy of −2.6 eV and a Co−O bond distance of 1.65 Å. This adsorption energy calculation clearly manifests that the nanoparticles formed on PBCC are barium cobaltite rather than barium oxide since BaO cannot augment the ORR activity. Furthermore, as we demonstrated in the previous study6 using a nanostructured BaCoOx model on CoO-terminated PBCC (010), the proposed elementary electrochemical processes associated with the dramatically enhanced ORR activity on the cathode can be schematically illustrated in Figure 5c. As the oxygen vacancy formation energy on BCO is much lower than that on PBCC (0.55 versus 1.18 eV), dissociative oxygen adsorption may first take place at the nanostructured BCO surface formed on the PBCC cathode, such as at oxygen-vacancy-rich moieties, resulting in the incorporation of reduced oxygen species into BCO lattices. Then dissociated oxygen species may be combined with oxygen vacancies of PBCC via a surface diffusion, followed by a fast bulk diffusion through pore channels.34 Then its bulk diffusion occurs until it reaches the electrolyte material.

Figure 3. Surface chemistry of model electrode of dense PBCC pellets with isolated Ba-related nanoparticles. (a) Schematics of in situ Raman setup. Surface SEM of PBCC pellets without (b) and with (c) NPs. (d) In situ Raman spectra of PBCC pellet with NPs from room temperature to 600 and 900 °C.

active. However, Ba(NO3)2 has very strong bands near 732 and 1049 cm−1,29 although the intensities diminish with temperature. For example, the intensity of the peaks corresponding to −NO3 is vanishingly small at ∼400 °C. It is found that at this temperature the band corresponding to −CO3 appeared at 1056 cm−1.30 The −CO3 is likely originated from the glycine−nitrate reaction, which was generated in air during solution combustion. The band of −CO3 was still observable at 600 °C. Since the temperature of the in situ Raman chamber is unable to reach over 600 °C, the sample was taken out and fired in air at 900 °C. After the firing the band of −CO3 (1056 cm−1) disappeared and the Raman spectra were dominated by the features of the brownmillerlitelike phase of BaCoO3−δ.31 To confirm this we first measured the EIS of symmetrical cells of PBCC electrodes with a coating of 5 μL of 0.1 M Ba(NO3)2 solution (without firing at 900 °C for 2 h prior to testing) in two different ways: from 450 to 800 °C (heating up, red squares in Figure 4a) and from 800 to 450 °C (cooling

Figure 4. (a) Interfacial polarization resistance of symmetrical cells with PBCC electrodes coated with 5 μL of Ba(NO3)2 solution measured from 450 to 800 °C (red squares) and from 800 to 450 °C (blue triangle). (b) Typical EIS of bare electrode (in black) and electrodes with coatings of carbonate (in red) or cobaltite (in blue) at 500 °C under open-circuit voltage conditions in ambient air. 7140

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Figure 5. (a) Cross-sectional TEM image of PBCC pellets with NPs, and mapping of Ca, Pr, Ba and Co. Nanoparticle is likely BaCoO2.8 (pdf no. 10-0245). (b) Geometrical illustration of oxygen adsorption at the Co ion on a BaCoO-terminated BaCoO3(110) surface. Its adsorption energy of oxygen is −2.6 eV, while the Co−O bond length is 1.65 Å. (c) Schematic of electrode reaction processes by forming in situ nanostructured BaCoO3 on PBCC. V represents an oxygen vacancy.

4. CONCLUSIONS In summary, we confirmed that the in situ-formed catalyst can dramatically enhance the ORR kinetics. A series of analyses including in situ/operando Raman spectroscopy, STEM, electrochemical impedance study, and DFT simulations clearly indicate that the active sites of ORR are actually nanoparticles of barium cobaltite, not other Ba-containing materials (i.e., BaO), which are mainly generated after 565 °C during cell start-up process. The in situ formation of such a highly active nanostructured barium cobaltite is critical to the ORR activity and durability, providing a new avenue for rational design of next-generation ORR electrocatalysts.

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surface models of BaO-terminated BaO (100) and BaCoO-terminated BaCoO3 (PDF)

AUTHOR INFORMATION

Corresponding Authors

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

Bote Zhao: 0000-0003-1236-6862 Meilin Liu: 0000-0002-6188-2372 Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT

ACKNOWLEDGMENTS This work was supported by the US Department of Energy SECA Core Technology Program (under award numbers FC FE0026106 and FC-FE0009652). We are grateful to the National Center for High-Performance Computing for computer time and facilities. This work was supported by the Higher Education Sprout Project of the National Chiao Tung University and Ministry of Education (MOE), Taiwan.

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b01738. Interfacial polarization resistance of the electrodes as a function of temperature; distribution of relaxation time (DRT) analysis of the impedance spectra of cathode; short-term durability of cathode against contaminates; SEM image of a single cell with the catalyst coated cathode; 2D EELS spectra of cathode grains; Raman spectra of model cell surface and BCO powder at room temperature; bulk structures of BaO and BaCoO3; 2D

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DOI: 10.1021/acscatal.9b01738 ACS Catal. 2019, 9, 7137−7142