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Highly efficient CO2 electrolysis on cathodes with exsolved alkaline earth oxide nanostructures Lingting Ye, Changchang Pan, Minyi Zhang, Chunsen Li, Fanglin Chen, Lizhen Gan, and Kui Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07039 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 9, 2017
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ACS Applied Materials & Interfaces
Highly Efficient CO2 Electrolysis on Cathodes with Exsolved Alkaline Earth Oxide Nanostructures Lingting Ye1, Changchang Pan1, Minyi Zhang2, Chunsen Li2, Fanglin Chen3, Lizhen Gan1,4,*, Kui Xie1,* 1
Key Lab of Design & Assembly of Functional Nanostructure, Fujian Institute of Research on the Structure of
Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. 2
State Key Lab of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy
of Sciences, Fuzhou, Fujian 350002, China. 3
Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208,
USA. 4
School of Transportation and Civil Engineering, Fujian Agriculture and Forestry University, No.15 Shangxiadian
Road, Fuzhou, Fujian 350002, China. *
Corresponding author:
[email protected] (L Gan);
[email protected] (K Xie)
Abstract Solid oxide CO2 electrolyser has the potential to provide storage solutions for intermittent renewable energy sources as well as reducing greenhouse gas emissions. One of the key challenges remains poor adsorption and activity towards CO2 reduction on the electrolyser cathode at typical operating conditions. Here we show a novel approach in tailoring a perovskite titanate (La,Sr)TiO3+δ cathode surface, by in situ growing SrO nano-islands from the host material through control of perovskite non-stoichiometry. These nano-islands provide much enhanced CO2 adsorption and activation, stable up to 800°C, which is shown to be in a form intermediate between carbonate ions and molecular CO2. The activation of adsorbed CO2 molecules results from the interaction by exsolved SrO nano-islands and defected titanate surface as revealed by DFT calculations. These cathode surface modifications result in exceptionally high direct CO2 electrolysis performance with current efficiencies near 100%. Keywords: Alkaline earth oxide; Perovskite; Solid oxide electrolyser; Carbon dioxide; Electrolysis 1. Introduction Solid oxide electrolysers (SOEs) have tremendous potential to convert CO2 into fuels. Their high operating temperatures mean favourable thermodynamics and electrode kinetics, thereby reducing electrical load. Especially when such devices utilise waste heat and surplus electricity from renewable sources, they could play a significant role in storing energy, whilst additionally reducing atmospheric CO2 levels1-3. Ceramic cathodes can offer promising performances in direct CO2 electrolysis, with the added benefits of providing stability in both oxidising and reducing conditions and resistance against coking. Especially materials exhibiting n-type conducting properties are expected to be highly suitable, due to the strongly reducing conditions at the cathode during electrolysis operation. Perovskite type doped strontium titanates, (La,Sr)TiO3+δ (LSTO), are such materials, due to the reducibility of Ti4+ to Ti3+, and have attracted a significant amount of interest in both solid oxide electrolysis and solid oxide fuel cell electrode research4,5. Cathodes based on La0.2Sr0.8TiO3.1 have recently been shown to be suitable for direct CO2 electrolysis6, but their performance is still 1
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limited by insufficient electro-catalytic activity and weak high temperature chemical adsorption of reactants7. The poor adsorption at high temperatures is caused by CO2 molecules lacking polarity due to their linearity. This in turn is believed to cause local starvation of the reactants at the cathode under electrolysis operation8,9. Currently, the main method of promoting preferential chemical adsorption of CO2 on solid oxide materials is by grafting solid amines, which produces an alkaline surface with affinity for CO2 molecule. However, the desorption temperature for such surfaces is generally below 500°C which is too low for typical electrolyser operating temperatures, i.e. 600 – 800°C10,11. Alternatively, we could consider using alkali and/or alkaline earth oxides, such as SrO and BaO, which have been used in hydrocarbon reforming processes to adsorb and activate CO212-15. They are also used as additives in heterogeneous catalysis of CO2 reduction by hydrogen to tailor product selectivity16,17. Their presence on the catalyst’s surface promotes CO2 reactivity, facilitating its hydrogenation reaction. It is therefore expected that depositing alkaline earth oxide nanoparticles onto SOE cathodes may be beneficial for CO2 adsorption and activation, whilst additionally elevating the chemical desorption to electrolyser relevant temperatures, creating a highly active surface for CO2 reduction. In this work we aim to grow SrO nano-islands in situ on LSTO perovskite surfaces, through control of perovskite non-stoichiometry. It is expected that by synthesising A-site excess perovskites, SrO nano-islands may reversibly be exsolved from the host LSTO lattice under reducing conditions, creating a nano-structured cathode surface with enhanced activity towards CO2 reduction. The cathode performance is assessed by investigating chemical adsorption/desorption of CO2 and through electrochemical methods. Theoretical calculations are performed to provide mechanistic insights. 2. Experimental section 2.1 Method Ceramic oxides including (La0.2Sr0.8)Sr0Ti1.0O3+δ (LSS0T), (La0.2Sr0.8)Sr0.02Ti1.0O3+δ (LSS0.02T), (La0.2Sr0.8)Sr0.05Ti1.0O3+δ (LSS0.05T), (La0.2Sr0.8)Sr0.08Ti1.0O3+δ (LSS0.08T), (La0.2Sr0.8)Sr0.1Ti1.0O3+δ (LSS0.1T), (La0.8Sr0.2)0.95MnO3-δ (LSM) and Ce0.8Sm0.2O2-δ (SDC) powders were synthesised using a solid-state reaction method performed in air5. Phase formations were confirmed by using X-ray diffraction (XRD, Cu Kα, Miniflex 600, Rigaku Corporation, Japan). Scanning electron microscopy (SEM, SU-8010, JEOL Ltd, Japan) and high-resolution transmission electron microscopy (HRTEM, Tecnai F20, FEI Ltd, USA) were employed to investigate the segregation of nanoparticles. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo, USA) with mono chromatised Al Kα at hv = 1486.6 eV was utilised to analyse elemental oxidation states. Thermogravimetric analysis (TGA) was conducted on a Netzsch STA449F3 to determine the amount of CO2 adsorption. All infrared spectra were collected using Fourier transform infrared spectrometer (VERTEX 70, Bruker). The powder samples were first reduced in 5% H2/Ar at 800°C for 20 h and then treated in CO2 at 450°C for 1 h. And the in situ IR test was subsequently performed. Temperature programmed desorption (TPD) of CO2 was recorded with a Micromeritics-Hiden Autochem II 2920-QIC20. 2
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ACS Applied Materials & Interfaces
A group of 0.5 mm thick YSZ electrolyte supports was prepared by dry-pressing YSZ powders into green disks with a diameter of 20 mm and then fired at 1550°C for 20 h in air. The two surfaces of the obtained YSZ electrolyte support were mechanically polished and then ultrasonically cleaned in distilled water. The prepared LSSxT-SDC powders and LSM-SDC powders (at a ratio 65 : 35 wt%) were mixed with alpha-Terpineol and appropriate amounts of cellulose additives to form a slurry and then printed onto the two surfaces of YSZ discs in an area of 1.0 cm2 to assemble single solid oxide electrolyser cells, respectively. The electrodes were then treated at 1200°C for 3 h in air. The current collector was made with silver paste (SS-8060, Xinluyi, China) and treated at 550°C for 30 min in air. Electrochemical measurements were performed using an electrochemical station (IM6, Zahner, Germany). The frequency range was 4 MHz to 100 mHz, and the voltage perturbation was 10 mV. The gas flow was controlled with mass flow meters (D08-3F, Sevenstar, China). The electrolysis experiment was conducted by initially exposing the cathode to 5% H2/Ar (at the flow rate of 50 mL•min-1) for 1.5 h, followed by pure hydrogen (at the flow rate of 50 mL•min-1) for 2 h at operating temperature, to ensure sufficient reduction and activation of the cathode. CO2 electrolysis was subsequently performed. Online gas chromatography (GC2014, Shimazu, Japan) on the output gas from the electrolyser cells was used to analyse the CO production at the CO2 flow rate of 50 mL•min-1 at 800°C. 2.2 First-principle calculations Density functional theory (DFT)18,19 calculations were performed using the plane wave basis set Vienna Ab-initio Simulation Package (VASP) code20,21. Within the projector augmented wave (PAW) framework, the plane wave cut-off was set to 500 eV used for total energy calculations. The generalised gradient approach (GGA) with Perdew-Burke-Ernzerhof (PBE) functional22 was used to describe exchange and correlation. The energies and residual forces were converged to 10– 6 eV and 0.02 eV/Å, respectively. In order to simplify and facilitate the calculations, we used unmodified SrTiO3 (STO). The STO crystal is a perovskite structure (Pm-3m). The lattice parameter of SrTiO3 (STO) optimised with a 6×6×6 k-point grid23 was a=b=c=3.949Å, which is in good agreement with our experimental values24. The periodic slab model was used to simulate the (100) surface of STO with Ti-O terminations. For this slab model, two Ti-O layers and two Sr-O layers were included in the slab. The two bottom layers were fixed to its bulk geometry during optimisation and other atoms were fully relaxed. The vacuum region is 15 Å thick, which is large enough to avoid spurious interactions between the slabs and ensures that the electrostatic potential is flat in the vacuum region for each result. We then constructed a (100) SrO/STO surface containing a small SrO chain25 (consisting of three Sr and O atoms each) over a four-layer p (4×4) STO substrate with Ti-O terminations. A 2×2×1 k-point grid was used for Brillouinzone sampling of (100) SrO/STO surface system. 3. Results and discussion Previous work has shown that an effective way to create nano-sized catalysts on perovskite surfaces is to incorporate them into the host lattice during synthesis in air and subsequently exsolving them under reducing conditions26. Here we aim to achieve SrO exsolution by introducing excess Sr in La0.2Sr0.8TiO3+δ, i.e. (La,Sr)SrxTiO3+δ, or LSSxT. Figure 1a shows the X-ray diffraction (XRD) patterns of oxidized powder samples. Single phase materials with cubic symmetry, space group Pm-3m, are obtained under oxidising conditions for x≤ 0.1; larger excess 3
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of Sr results in SrCO3 formation. After being reduced in 5 % H2/Ar (800°C, 20 h) in Figure 1b, the LSSxT (0
