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Pd single site-anchored perovskite cathode for CO2 electrolysis in solid oxide electrolysis cells
Nano Energy 71 (2020) 104598
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Communication
Pd single site-anchored perovskite cathode for CO2 electrolysis in solid oxide electrolysis cells Yingjie Zhou a, b, 1, Le Lin b, c, 1, Yuefeng Song b, d, Xiaomin Zhang b, Houfu Lv b, d, Qingxue Liu b, d, Zhiwen Zhou b, d, Na Ta b, Guoxiong Wang b, **, Xinhe Bao b, * a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, 201620, China State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China c School of Physical Science and Technology, ShanghaiTech University, Shanghai, 201210, China d University of Chinese Academy of Sciences, Beijing, 100039, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon dioxide electrolysis Solid oxide electrolysis cell Perovskite Single site catalysis Palladium
Pd single site-anchored La0.5Sr0.5FeO3-δ-Ce0.8Sm0.2O2-x (Pd-LSF(SDC)) cathode is constructed for CO2 electrolysis in solid oxide electrolysis cells. The decoration of Pd species on LSF greatly improves CO2 electrolysis perfor mance with 65.7% enhancement in current density at 1.6 V compared with the LSF counterpart. Electrochemical and density functional theory calculation results suggest that the Pd species are atomically anchored on the LSF phase. The coordinatively unsaturated Pd with oxygen vacancies facilitates the CO2 dissociative adsorption, electron transfer and mass transport, thus greatly decreases the polarization resistance of the cathode and pro motes the CO2 electrolysis performance. This work provides an effective strategy to design the supported single site catalyst for high-temperature CO2 electrolysis.
1. Introduction CO2 electrolysis via solid oxide electrolysis cells (SOECs) has attracted increasing attention due to low overpotential, high current density and energy efficiency with CO as the major product, which could €psch (Fbe further converted to chemicals and liquid fuels by Fischer-Tro T) synthesis [1,2]. The state-of-the-art SOECs usually consist of the yttria-stabilized zirconia (YSZ) electrolyte, strontium-doped lanthanum manganese (LSM) anode and Ni-YSZ cathode. However, Ni-YSZ cathode suffers from irreversible microstructure damages and consequent elec trolysis performance loss due to the nickel coarsening during long-term operation at high temperatures [3]. Nowadays, perovskite oxides such as La0.8Sr0.2Cr0.5Mn0.5O3 [4], Sr2Fe1.5Mo0.5O6-x [5,6], Sr0.94Ti0.9Nb0.1O3 [7], La0.2Sr0.8TiO3þδ [8] and La0.6Sr0.4Fe0.8Ni0.2O3-δ [9] have been explored as alternative cathodes to Ni-YSZ. These cathodes possess excellent mixed ionic and electronic conductivities (MIECs) and redox stability, however, show electrolysis performance inferior to the Ni-YSZ cathode. Therefore, developing efficient perovskite oxide cathodes
remains a great challenge for boosting CO2 electrolysis in SOECs. Recently, catalysts with atomically dispersed active centers have demonstrated distinctive activity for various catalytic reactions [10,11]. The unique electronic properties of single metal active sites [12,13] provide a great potential to promote CO2 electrolysis performance in SOECs. Moreover, the atomically isolated catalytic active centers on the electrodes can augment the electron transport channels without block ing the porous configuration, leading to the extended triple phase boundaries (TPBs) and faster electrode kinetics. However, preparation of stable single site catalysts remains a grand challenge because single sites with high surface energy are easy to diffuse and aggregate at high temperatures. Manipulating the interactions between the single sites and candidate hosts can avoid sintering and make for high stability and activity in single site catalysis [14]. Several studies have shown that Fe-based perovskite oxides could serve as hosts of highly dispersed palladium (Pd), because the Pd species could reversibly migrate into and out of B site of perovskite, which suppresses the agglomeration of Pd nanoparticles [15,16]. The La0.5Sr0.5FeO3-δ (LSF) as a cathode material
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Wang), [email protected] (X. Bao). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.nanoen.2020.104598 Received 25 November 2019; Received in revised form 16 January 2020; Accepted 8 February 2020 Available online 11 February 2020 2211-2855/© 2020 Elsevier Ltd. All rights reserved.
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exhibits an excellent electrocatalytic activity for CO2 electrolysis in SOECs, and the doping at the Fe site greatly promotes its electrolysis performance [17]. Consequently, the single Pd site supported on LSF as a cathode material for SOECs is expected to achieve performance improvement in CO2 electrolysis at SOECs. Herein, Pd-decorated LSF is prepared by ball-milling and subsequent firing, in which the Pd species are atomically anchored on the LSF phase with oxygen surroundings. The decoration of Pd species on LSF greatly improves CO2 electrolysis performance with 65.7% enhancement in current density at 1.6 V compared with the LSF counterpart. Electro chemical and density functional theory (DFT) calculation results suggest that the coordinatively unsaturated Pd atoms, together with the neigh boring oxygen vacancies, serve as active centers to dissociatively adsorb and activate CO2, as well as facilitate electron transfer and mass transport.
LSF. Moreover, no aggregated nanoparticles could be observed for the Pd-LSF(SDC) with low Pd concentrations except that sub-nano PdO2 particles appeared on the LSF phase for 1% Pd-LSF(SDC) in HRTEM images. To further assess Pd dispersion in the nanocomposite, larger selected areas in 0.8% Pd-LSF(SDC) were probed by scanning trans mission electron microscopy (STEM) (Fig. 2 and Fig. S3), and it is observed that most Pd species were atomically dispersed within the nanocomposite. In order to further understand the doping effect of Pd atoms into LSF lattice, DFT calculations of the substitution energies were performed to compare the ability of Pd doped into Fe sites, and the detailed calcula tion process were illustrated in the Supporting Information (SI). As listed in Table S1, the substitution energies gradually decrease with lessening the concentration of Pd. Pd atom is thermodynamically capable of being doped into LSF lattice by substituting Fe atom at low concentrations, but saturated in bulk lattice and thus segregated on the LSF surface when the concentration of Pd is up to a certain extent. In addition, the lattice expansion from (1 � 1) LSF (354.85 Å3) to (1 � 1) Pd-LSF (359.13 Å3) could be observed, and this may benefit from a bond distance of Pd–O (1.99 Å) in Pd-LSF longer than that of Fe–O (1.96 Å) in LSF. Pd K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) in Fig. 1(b) and (c) reveal that Pd species in the LSF(SDC) are in a higher oxidation state than Pd2þ, and the Pd oxidation degree first decreases and then in creases with increasing Pd concentration. The Fourier transformed K3weighted EXAFS results for all Pd-LSF(SDC) in Fig. 1(c) exhibit similar Pd coordination environment, and no Pd–Pd coordination is observed compared with the Pd foil reference. In combination with the TEM, STEM and DFT calculation results, it could be deduced that most Pd species are atomically anchored on the LSF phase with oxygen surroundings. X-ray photoelectron spectroscopy (XPS) measurements reveal that
2. Results and discussion X-ray diffraction (XRD) patterns of the Pd-LSF(SDC) in Fig. 1(a) exhibit the mixture of rhombohedral (R3c) LSF [18] and cubic SDC. No characteristic diffraction peaks ascribed to Pd species could be observed, indicating the absence of crystalline Pd or PdOx nanoparticles. Scanning electron microscopy (SEM) images in Fig. S1 present that all nano particles are bonded with each other to form porous networks, and the Pd concentration barely affects the morphology. High-resolution trans mission electron microscopy (HRTEM) images in Figs. S2(a)–(d) show that the LSF closely attaches with the SDC phase. Besides, the intro duction of Pd slightly increases the lattice space of the LSF phase, probably due to the doping of the larger Pd ions into the Fe sites. As verified by the energy-dispersive X-ray (EDX) spectroscopy, the Pd species are majorly trapped on the LSF phase instead of the SDC phase, suggesting that the Pd species have a strong chemical affinity toward
Fig. 1. (a) XRD patterns of LSF-SDC with various Pd loadings, (b) XANES spectra and (c) Fourier transformed K3-weighted EXAFS spectra of Pd in the nano composites, (d) XPS spectra of Pd 3d in the nanocomposites. 2
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increases and then decreases with increasing Pd concentration. Overall, the 0.8% Pd-LSF(SDC) possesses the best CO2 adsorption capacity and the highest oxygen vacancy concentration at 800 � C. The electrochemical performances of the Pd-LSF(SDC) were assessed in SOEC for CO2 electrolysis with YSZ electrolyte and LSM/YSZ anode. The current-voltage curves and potentiostatic tests are presented in Fig. 3 and Fig. S6. Current densities increase steadily as the applied voltages rise. The introduction of Pd can effectively enhance the CO2 electrolysis performance. The changes in current densities followed the same sequence as the CO2 adsorption and dissociation ability as well as the oxygen vacancy concentrations at 800 � C (Fig. S5), that is, the cur rent densities first increased and then decreased with increasing Pd concentration. The 0.8% Pd-LSF(SDC) with the best CO2 adsorption capacity and the highest oxygen vacancy concentration shows the highest average current density of 0.58 A cm 2 at 1.6 V, which is about 65.7% higher than that of LSF(SDC). Faradaic efficiencies are around 100% over all the cathodes, and the average CO production rate in creases with applied voltages. Moreover, the CO production rate in creases from 2.60 to 4.01 mL min 1 cm 2 at 1.6 V as the Pd concentration increases to 0.8%. To gain kinetic insights for enhanced performance in CO2 electrolysis over the Pd-LSF(SDC), AC electrochemical impedance spectroscopy (EIS) at different applied voltages (Fig. S7) and analysis in the distri bution of relaxation times (DRT) (Fig. S8) were carried out. The cell polarization resistance (Rp) dramatically decreases as the electrolysis voltage increases, and the introduction of Pd greatly decreases the Rp at a specific given voltage. This is probably due to not only the enhanced CO2 adsorption/dissociation ability, but also the extra Pd4þ/Pd2þ ion pairs and oxygen vacancies in the cathode at the reaction temperature, which facilitates the electron-ion transportation. The 0.8% Pd-LSF(SDC) with the highest concentration of Pd4þ/Pd2þ ion pair and oxygen va cancies as well as the best CO2 adsorption capacity exerts the lowest Rp of ca. 0.256 Ω cm2 at 1.6 V and the highest CO2 electrolysis performance. DRT curves and the breakdown of the EIS at 1.4 V based on the equivalent circuit model were presented in Fig. S9. Commonly, each (RC) element corresponds to a DRT peak, and the peak area is propor tional to the contribution of a specific sub-process [25]. P1 and P2 barely change with the applied voltages and cathode materials, thus they probably are related to the inherent oxygen ions transportation through the electrolyte or across the LSM/YSZ interfaces since the SOECs in this work employed the same YSZ electrolyte and LSM/YSZ anode. On the contrary, P3–P6 are strongly dependent on the applied voltages and cathode materials. The higher applied voltages, the higher frequency regions of P3–P6, indicating quicker responses [25–27]. These are due to that the high applied voltages could create more oxygen vacancies at the cathode, thus facilitate the reaction kinetics in the electrodes [28]. As it is reported, P3 represents the charge transferring process in the elec trodes [25,27], and P4 is related to the exchange and diffusion of oxygen species at the TPBs [25,26], while P5 and P6 are associated with the mass transfer process of reactants [27,29]. The introduction of Pd de creases all the resistances (Table S4), and the decrease in R3, R5 and R6 are more noticeable. This is due to that the decoration of Pd not only introduces extra Pd4þ/Pd2þ ion pairs, favoring the charge transfer in the cathode, but also benefits the CO2 dissociative adsorption as well as the surface diffusion and conversion of active reactant species. The stability of CO2 electrolysis over all the cathodes was recorded at 1.2 V as shown in Fig. S10. The current densities slightly decreased with time on stream. The degradation rate of the current density for 0.8% PdLSF(SDC) is about 0.003 A cm 2 h 1. Both RΩ and Rp of the SOEC after the stability test increase, and the changes in R1, R2 and R4 are more obvious, suggesting that the O2- transportation and diffusion decrease with time on stream. This is probably due to the diminished TPBs during the stability test at high temperatures. In order to figure out the cell degradation factors, XRD, cross-sectional SEM and element maps mea surements of the SOEC after the stability test were performed. As shown in Fig. S11, the cathode coated with Au paste firmly clings on the YSZ
Fig. 2. STEM images and corresponding element maps of the 0.8% PdLSF(SDC).
the loading of Pd species hardly changes the surface electronic states of La 3d and Ce 3d while changes that of Fe 2p (Fig. S4). The Fe 2p spectra were fitted with two components of Fe3þ and Fe4þ [19], while the Pd 3d spectra in Fig. 1(d) could be deconvoluted into Pd4þ and Pd2þ, respec tively [20]. The introduction of Pd leads to a slight decrease in the relative atomic ratio of Fe3þ/Fe4þ (Table S2). As the concentration of Pd increases, the surface atomic ratio of Fe3þ/Fe4þ first increases and then decreases, in contrast to that of Pd4þ/Pd2þ. The 0.8% Pd-LSF(SDC) shows the highest amount of Pd4þ/Pd2þ ionic pairs. This result is consistent with the EXAFS analysis of Pd. For the O 1s XPS shown in Fig. S4(d) and listed in Table S3, the surface O species exist as the adsorbed oxygen in the form of molecular water (OH) [21,22] and ox ygen or hydroxyl groups (O/OOH) [21–23], highly oxidative species 2(O-/O22 ), and lattice oxygen (O ) within the LSF and SDC, respectively. The addition of Pd greatly enhances the relative atomic ratio of O2-, and the relative content of O-/O22 species barely changes. These results suggest that partial Pd species might anchor on the LSF(SDC) surface in oxidation state. Temperature-programmed desorption-mass spectrometry (TPD-MS) measurements of CO2 over Pd-LSF(SDC) were performed to take a deep insight into their surface chemical properties (Fig. S5). Both CO (m/z ¼ 28) and O2 (m/z ¼ 32) could be detected besides CO2 (m/z ¼ 44). Three CO2 desorption peaks could be observed, corresponding to the release of the physisorbed or linear CO2, the carbonates, and the tridentate configuration (see the following calculations and models shown in Figure CS3), respectively. The addition of Pd greatly suppresses the formation of foregoing carbonates but avails the formation of tridentate configuration. Moreover, the former gradually increases and the latter first increases and then decreases with increasing Pd concentration. CO desorption in the high temperature range is consistent with that of CO2 but differs from O2. These results suggest that the desorbed O2 is majorly associated with the evolution of non-stoichiometric lattice oxygen [24]. All samples maintain almost the identical O2 desorption temperature, but the Pd-LSF(SDC) shows a slightly higher desorption amount than LSF(SDC). These results indicate that the Pd in the LSF(SDC) can enhance the mobility of lattice oxygen at high temperatures, verified by the DFT calculation results of substituted Pd exerting less oxygen coor dination and longer Pd–O bond. The O2 desorption amount first 3
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Fig. 3. (a) I–V curves of SOEC for CO2 electrolysis, (b) CO production rate and Faradaic efficiencies of SOECs with the LSF(SDC) and 0.8% Pd-LSF(SDC) cathode versus applied voltage (error bars represent the standard deviations).
electrolyte, and a new broad XRD diffraction peak located around 15.8� appears and the characteristic peak of LSF at around 22.4� becomes broader, which is mainly attributed to the deposition of carbon on the cathode. Compared with the SEM image of 0.8% Pd-LSF(SDC) in Fig. S1, white particles are noticeable after the stability test. The backscattering of the image associated with the EDX result shows that the white
particles are mainly composed of Sr, C, Fe, O. The segregation of Fe species suggests that the atomically dispersed Pd species on the LSF surface might migrate into the LSF lattice at the Fe sites during the re action. Meanwhile, the surface Sr segregation could be obviously observed. In order to further evaluate the carbon coking, TEM images of all the cathodes after stability test were shown in Fig. S12, and apparent Fig. 4. Theoretical analysis of the CO2 electro reduction to CO by DFT calculations. (a) Potential energy landscape of the CO2 electrolysis process over the three sites: Fe–CN4 in LSF(110), SS-Pd-CN3 and SS-Pd-CN2 in Pd-LSF(110) at 800 � C and 1.6 V. * (int.) and *(Vo) denote the site without and with one oxygen vacancy. (b) The sum of up- and down-spin dorbital projected DOS of active Fe and Pd atoms for the corresponding adsorption sites. The dashed cyan line marks the Fermi level. (c) Optimized atomic configurations of the CO2 electroreduction to CO process over SS-Pd-CN2 (side view), which corre sponds to the red line in (a). The dashed pink circle denotes the oxygen vacancy. (d) Amplified local atomic geometry truncated by CO2*(Vo) and CO* (int.) of (c). (e) 2D contour plots of the charge den sity differences for CO2*(Vo) and CO*(int.) over SSPd-CN2. Red and blue regions denote increased and decreased electron densities, respectively.
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carbon deposition could be observed on the catalyst surface in the cathodes. Therefore, the carbon deposition, Fe and Sr segregation might be the major reasons for the decrease in the TPBs, thus lead to cell degradation during stability test. These results in turn verify that the surface Pd sites play a much more dominant role in CO2 electrolysis compared with the surface Fe sites. In order to clarify the difference in CO2 electrolysis performance, DFT calculations were performed to obtain the potential energy land scapes of CO2 electrolysis on the Fe and Pd centers (Fig. 4(a)), and the corresponding intermediates were shown in Fig. 4(c) and S13. Firstly, the two coequal surfaces with six oxygen vacancies are compared, where the oxygen coordination number (CN) of Fe site and Pd site (SS–Pd) are 4 and 3, respectively. More details can be seen in SI. Throughout the whole cathodic process at 800 � C and 1.6 V [30], the CO2 dissociation and CO desorption steps are always downhill on Fe–CN4 (orange line) and SS-Pd-CN3 (blue line), whereas the CO2 adsorption is endergonic with energies of 1.40 and 0.30 eV and depends intensely on the active centers. As the experimental results shown, Pd-doped LSF exhibits an enhanced CO2 adsorption and electroreduction activity, also implying a strong correlation between CO2 adsorption and electrolysis perfor mance. Therefore, we suggest that the CO2 adsorption solely serves as the rate-limiting step in the intrinsic reactivity variation, which was ever reported by Lu et al. [31]. Based on this, CO2 electrolysis on SS-Pd-CN2 is further studied as the red line in Fig. 4(a) and the structures in Fig. 4 (c), where CO2 forms an activated tridentate configuration (also called a bent COδ2 species [32]) with an adsorption energy of 0.48 eV and then can dissociate into one CO* adsorbed on Pd center and one O* filled up the oxygen vacancy in Fig. 4(d). With the confined coordination envi ronment considered, SS-Pd-CN3 is bound to three oxygen atoms and thus shows a worse adsorption of both CO2 and CO, compared to SS-Pd-CN2. Overall, the Pd single site (especially SS-Pd-CN2) plays a crucial role in the CO2 electroreduction to CO over the LSF system. The coordinatively unsaturated Pd atom, together with the neighboring oxygen vacancy, makes up the active center to adsorb and activate CO2. Furthermore, the electronic structures were expected to understand the dissociation process and the nature of active Pd centers. As electron density differences map depicted in Fig. 4(e) and Fig. S14, CO2 is adsorbed on SS-Pd-CN2 through a firm C–Pd bond (seen from red areas), a mild O1–Pd bond, and an O2-vacancy coalition. Later, the dissociated groups of CO2 are anchored onto SS-Pd-CN2 in modes of that O2 filled up the vacancy and that CO species attached on the Pd. Hereto, the dissociation step is accomplished. Through the d-orbital projected density of states (DOS) in Fig. 4(b), it can be seen that the DOS of SS-PdCN2 around the Fermi level is more abundant than that of Fe and also that of SS-Pd-CN3. Therefore, SS-Pd-CN2 facilitates the CO2 adsorption and activation, in synergism with a surface oxygen defect.
Acknowledgment We gratefully acknowledge financial support from the National Key R&D Program of China (Grant 2017YFA0700102), the National Natural Science Foundation of China (Grants 21573222 and 91545202), Dalian National Laboratory for Clean Energy (DNL180404), Dalian Institute of Chemical Physics (Grant DICP DMTO201702), Dalian Outstanding Young Scientist Foundation (Grant No. 2017RJ03), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB17020200). G.W. thanks the financial support from CAS Youth Innovation Promotion (Grant No. 2015145). Y.Z. received financial support from the 14th Six Talents Peak Project of Jiangsu Province (XNYQC-016) and State Key Laboratory of Catalysis (N-18-09). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.nanoen.2020.104598. References [1] D.U. Nielsen, X.-M. Hu, K. Daasbjerg, T. Skrydstrup, Chemically and electrochemically catalysed conversion of CO2 to CO with follow-up utilization to value-added chemicals, Nat. Catal. 1 (2018) 244–254. [2] H. Shen, Z. Gu, G. Zheng, Pushing the activity of CO2 electroreduction by system engineering, Sci. Bull. 64 (2019) 1805–1816. [3] Y. Song, Z. Zhou, X. Zhang, Y. Zhou, H. Gong, H. Lv, Q. Liu, G. Wang, X. Bao, Pure CO2 electrolysis over an Ni/YSZ cathode in a solid oxide electrolysis cell, J. Mater. Chem. A. 28 (2018) 13661–13667. [4] F. Bidrawn, G. Kim, G. Corre, J.T.S. Irvine, J.M. Vohs, R.J. Gorte, Efficient reduction of CO2 in a solid oxide electrolyzer, Electrochem. Solid State Lett. 11 (2008) B167–B170. [5] Q. Liu, C. Yang, X. Dong, F. Chen, Perovskite Sr2Fe1.5Mo0.5O6 δ as electrode materials for symmetrical solid oxide electrolysis cells, Int. J. Hydrogen Energy 35 (2010) 10039–10044. [6] Q. Liu, X. Dong, G. Xiao, F. Zhao, F. Chen, A novel electrode material for symmetrical SOFCs, Adv. Mater. 22 (2010) 5478–5482. [7] L. Yang, K. Xie, S. Xu, T. Wu, Q. Zhou, T. Xie, Y. Wu, Redox-reversible niobiumdoped strontium titanate decorated with in situ grown nickel nanocatalyst for hightemperature direct steam electrolysis, Dalton Trans. 43 (2014) 14147–14157. [8] S. Li, Y. Li, Y. Gan, K. Xie, G. Meng, Electrolysis of H2O and CO2 in an oxygen-ion conducting solid oxide electrolyzer with a La0.2Sr0.8TiO3þδ composite cathode, J. Power Sources 218 (2012) 244–249. [9] Y. Tian, L. Zhang, L. Jia, X. Wang, J. Yang, B. Chi, J. Pu, J. Li, Novel quasisymmetrical solid oxide electrolysis cells with in-situ exsolved cathode for CO2 electrolysis, J. CO2 Util. 31 (2019) 43–50. [10] A. Wang, J. Li, T. Zhang, Heterogeneous single-atom catalysis, Nat. Rev. Chem. 2 (2018) 65–81. [11] X. Zhao, K. Wu, W. Liao, Y. Wang, X. Hou, M. Jin, Z. Suo, H. Ge, Improvement of low temperature activity and stability of Ni catalysts with addition of Pt for hydrogen production via steam reforming of ethylene glycol, Green Energy Environ. 4 (2019) 300–310. [12] C. Yan, H. Li, Y. Ye, H. Wu, F. Cai, R. Si, J. Xiao, S. Miao, S. Xie, F. Yang, Y. Li, G. Wang, X. Bao, Coordinatively unsaturated nickel–nitrogen sites towards selective and high-rate CO2 electroreduction, Energy Environ. Sci. 11 (2018) 1204–1210. [13] B. Qiao, A. Wang, X. Yang, L.F. Allard, Z. Jiang, Y. Cui, J. Liu, J. Li, T. Zhang, Single-atom catalysis of CO oxidation using Pt1/FeOx, Nat. Chem. 3 (2011) 634–641. [14] Z. Zhang, Y. Zhu, H. Asakura, B. Zhang, J. Zhang, M. Zhou, Y. Han, T. Tanaka, A. Wang, T. Zhang, N. Yan, Thermally stable single atom Pt/m-Al2O3 for selective hydrogenation and CO oxidation, Nat. Commun. 8 (2017) 16100. [15] Y. Nishihata, J. Mizuki, T. Akao, H. Tanaka, M. Uenishi, M. Kimura, T. Okamoto, N. Hamada, Self-regeneration of a Pd-perovskite catalyst for automotive emissions control, Nature 418 (2002) 164–167. [16] M.B. Katz, G.W. Graham, Y. Duan, H. Liu, C. Adamo, D.G. Schlom, X. Pan, SelfRegeneration of Pd–LaFeO3 catalysts: new insight from atomic-resolution electron microscopy, J. Am. Chem. Soc. 133 (2011) 18090–18093. [17] Y. Zhou, Z. Zhou, Y. Song, X. Zhang, F. Guan, H. Lv, Q. Liu, S. Miao, G. Wang, X. Bao, Enhancing CO2 electrolysis performance with vanadium-doped perovskite cathode in solid oxide electrolysis cell, Nano Energy 50 (2018) 43–51. [18] X.N. Ying, L. Zhang, Phase transitions in La0.5Sr0.5FeO3 δ investigated by mechanical spectrum, Solid State Commun. 152 (2012) 1252–1255. [19] K. Zhu, H. Liu, X. Li, Q. Li, J. Wang, X. Zhu, W. Yang, Oxygen evolution reaction over Fe site of BaZrxFe1-xO3-δ perovskite oxides, Electrochim. Acta 241 (2017) 433–439. [20] Y. Zheng, L. Kovarik, M.H. Engelhard, Y. Wang, Y. Wang, F. Gao, J. Szanyi, Lowtemperature Pd/Zeolite passive NOx adsorbers: structure, performance, and adsorption chemistry, J. Phys. Chem. C 121 (2017) 15793–15803.
3. Conclusion In summary, LSF(SDC) decorated by Pd species was constructed as cathode materials for CO2 electrolysis in SOECs. The Pd species were atomically anchored on the LSF phase with oxygen surroundings. The decoration of Pd species on LSF(SDC) showed a 65.7% higher current density than LSF (SDC) at 1.6 V with ~100% Faradaic efficiency. Elec trochemical and DFT calculation results verify that the performance enhancement is mainly attributed to the facilitated electrode kinetics, such as the CO2 dissociative adsorption, electron transfer and mass transport. This work provides insights and a new avenue for construct ing single site-anchored cathode in SOECs for CO2 electrolysis. Declaration of competing interest The authors declare no competing financial interest.
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Nano Energy 71 (2020) 104598 Yuefeng Song is a Ph.D. candidate at DICP, CAS. He received his B.S. in chemistry from Jilin University in 2014. His research interests are electrochemical energy storage and conversion, particularly focusing on the electrochemical reduction of CO2/ H2O in solid oxide electrolysis cells (SOECs).
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Xiaomin Zhang is an assistant professor at State Key Labora tory of Catalysis, DICP, CAS. She received her Ph.D. in chemical engineering from DICP, CAS in 2015. Her research interests are in the areas of highly efficient electrode materials and processes for electrochemical energy conversion and storage, especially high temperature electrolysis of CO2/H2O in SOECs.
Houfu Lv is a Ph.D. candidate at DICP, CAS. He received his M. S. in Zhengzhou University in 2015. His research interests are electrochemical energy storage and conversion, particularly focusing on the electrochemical reduction of CO2/H2O in SOECs.
Yingjie Zhou received her Ph.D. (2014) in materials physics and chemistry from Sun Yat-sen University. She was also a joint training Ph.D. at Texas State University and University of Connecticut from 2012 to 2014. She joined Nanjing University of Information Science & Technology from 2014 to 2019. After working as a postdoctoral researcher at State Key Laboratory of Catalysis at Dalian Institute of Chemical Physics (DICP), Chi nese Academy of Sciences (CAS) from 2016 to 2018, she joined Donghua University as an associate professor since 2019. Her research interests are developing highly efficient hybrid nano composites for electrochemical energy conversion and storage.
Qingxue Liu is a Ph.D. candidate at DICP, CAS. She received her B.S. degree in Sichuan University in 2016. Her research interests are the high-temperature electrochemical reduction of CO2/H2O in SOECs.
Le Lin is a Ph.D. candidate in School of Physical Science and Technology, Shanghaitech University. He received his B.S. in marine chemistry (major) and international economy and trade (second major) from Qingdao University of Science & Tech nology (2015). His research interest is mainly the relationship between structure and performance in ultrathin oxide-film catalysts via Density Functional Theory (DFT) calculations, particularly providing mechanisms for heterogeneous catalytic reactions concerning energy and environment.
Zhiwen Zhou is a Ph.D. candidate at DICP, CAS. He received his B.S. in Chemistry from Nanjing University (2014). His research interest is describing and screening oxide catalysts through Density Functional Theory (DFT) calculations providing reasonable mechanisms for various catalytic re actions concerning energy and environment.
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Nano Energy 71 (2020) 104598 Na Ta is an Associate Professor at DICP, CAS. Her major research interests focus on the structural analysis of catalysis materials and their dynamic behavior under working state by electron microscope.
Xinhe Bao received his Ph.D. in Physical Chemistry from Fudan University in 1987. He held an Alexander von Humboldt Research Fellow position in Fritz-Haber Institute between 1989 and 1995, hosted by Prof. Gerhard Ertl. Following that, he joined DICP as a full Professor. He became a member of the CAS in 2009. His research interest is nano and interfacial catalysis, focusing on the fundamental understanding of heterogeneous catalysis, including development of new catalysts and novel catalytic processes related to energy conversion and storage.
Guoxiong Wang received his B.S. from Wuhan University in 2000 and Ph.D. in physical chemistry from DICP, CAS in 2006. After working at Catalysis Research Center, Hokkaido Univer sity, Japan from 2007 to 2010 as postdoctoral researcher, he joined State Key Laboratory of Catalysis, DICP as an associate professor and was promoted as full professor in 2015. His research interests include highly efficient catalytic materials and processes for electrochemical energy conversion and storage.
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Contents lists available at ScienceDirect
Nano Energy journal homepage: http://www.elsevier.com/locate/nanoen
Communication
Pd single site-anchored perovskite cathode for CO2 electrolysis in solid oxide electrolysis cells Yingjie Zhou a, b, 1, Le Lin b, c, 1, Yuefeng Song b, d, Xiaomin Zhang b, Houfu Lv b, d, Qingxue Liu b, d, Zhiwen Zhou b, d, Na Ta b, Guoxiong Wang b, **, Xinhe Bao b, * a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, 201620, China State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China c School of Physical Science and Technology, ShanghaiTech University, Shanghai, 201210, China d University of Chinese Academy of Sciences, Beijing, 100039, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon dioxide electrolysis Solid oxide electrolysis cell Perovskite Single site catalysis Palladium
Pd single site-anchored La0.5Sr0.5FeO3-δ-Ce0.8Sm0.2O2-x (Pd-LSF(SDC)) cathode is constructed for CO2 electrolysis in solid oxide electrolysis cells. The decoration of Pd species on LSF greatly improves CO2 electrolysis perfor mance with 65.7% enhancement in current density at 1.6 V compared with the LSF counterpart. Electrochemical and density functional theory calculation results suggest that the Pd species are atomically anchored on the LSF phase. The coordinatively unsaturated Pd with oxygen vacancies facilitates the CO2 dissociative adsorption, electron transfer and mass transport, thus greatly decreases the polarization resistance of the cathode and pro motes the CO2 electrolysis performance. This work provides an effective strategy to design the supported single site catalyst for high-temperature CO2 electrolysis.
1. Introduction CO2 electrolysis via solid oxide electrolysis cells (SOECs) has attracted increasing attention due to low overpotential, high current density and energy efficiency with CO as the major product, which could €psch (Fbe further converted to chemicals and liquid fuels by Fischer-Tro T) synthesis [1,2]. The state-of-the-art SOECs usually consist of the yttria-stabilized zirconia (YSZ) electrolyte, strontium-doped lanthanum manganese (LSM) anode and Ni-YSZ cathode. However, Ni-YSZ cathode suffers from irreversible microstructure damages and consequent elec trolysis performance loss due to the nickel coarsening during long-term operation at high temperatures [3]. Nowadays, perovskite oxides such as La0.8Sr0.2Cr0.5Mn0.5O3 [4], Sr2Fe1.5Mo0.5O6-x [5,6], Sr0.94Ti0.9Nb0.1O3 [7], La0.2Sr0.8TiO3þδ [8] and La0.6Sr0.4Fe0.8Ni0.2O3-δ [9] have been explored as alternative cathodes to Ni-YSZ. These cathodes possess excellent mixed ionic and electronic conductivities (MIECs) and redox stability, however, show electrolysis performance inferior to the Ni-YSZ cathode. Therefore, developing efficient perovskite oxide cathodes
remains a great challenge for boosting CO2 electrolysis in SOECs. Recently, catalysts with atomically dispersed active centers have demonstrated distinctive activity for various catalytic reactions [10,11]. The unique electronic properties of single metal active sites [12,13] provide a great potential to promote CO2 electrolysis performance in SOECs. Moreover, the atomically isolated catalytic active centers on the electrodes can augment the electron transport channels without block ing the porous configuration, leading to the extended triple phase boundaries (TPBs) and faster electrode kinetics. However, preparation of stable single site catalysts remains a grand challenge because single sites with high surface energy are easy to diffuse and aggregate at high temperatures. Manipulating the interactions between the single sites and candidate hosts can avoid sintering and make for high stability and activity in single site catalysis [14]. Several studies have shown that Fe-based perovskite oxides could serve as hosts of highly dispersed palladium (Pd), because the Pd species could reversibly migrate into and out of B site of perovskite, which suppresses the agglomeration of Pd nanoparticles [15,16]. The La0.5Sr0.5FeO3-δ (LSF) as a cathode material
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Wang), [email protected] (X. Bao). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.nanoen.2020.104598 Received 25 November 2019; Received in revised form 16 January 2020; Accepted 8 February 2020 Available online 11 February 2020 2211-2855/© 2020 Elsevier Ltd. All rights reserved.
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exhibits an excellent electrocatalytic activity for CO2 electrolysis in SOECs, and the doping at the Fe site greatly promotes its electrolysis performance [17]. Consequently, the single Pd site supported on LSF as a cathode material for SOECs is expected to achieve performance improvement in CO2 electrolysis at SOECs. Herein, Pd-decorated LSF is prepared by ball-milling and subsequent firing, in which the Pd species are atomically anchored on the LSF phase with oxygen surroundings. The decoration of Pd species on LSF greatly improves CO2 electrolysis performance with 65.7% enhancement in current density at 1.6 V compared with the LSF counterpart. Electro chemical and density functional theory (DFT) calculation results suggest that the coordinatively unsaturated Pd atoms, together with the neigh boring oxygen vacancies, serve as active centers to dissociatively adsorb and activate CO2, as well as facilitate electron transfer and mass transport.
LSF. Moreover, no aggregated nanoparticles could be observed for the Pd-LSF(SDC) with low Pd concentrations except that sub-nano PdO2 particles appeared on the LSF phase for 1% Pd-LSF(SDC) in HRTEM images. To further assess Pd dispersion in the nanocomposite, larger selected areas in 0.8% Pd-LSF(SDC) were probed by scanning trans mission electron microscopy (STEM) (Fig. 2 and Fig. S3), and it is observed that most Pd species were atomically dispersed within the nanocomposite. In order to further understand the doping effect of Pd atoms into LSF lattice, DFT calculations of the substitution energies were performed to compare the ability of Pd doped into Fe sites, and the detailed calcula tion process were illustrated in the Supporting Information (SI). As listed in Table S1, the substitution energies gradually decrease with lessening the concentration of Pd. Pd atom is thermodynamically capable of being doped into LSF lattice by substituting Fe atom at low concentrations, but saturated in bulk lattice and thus segregated on the LSF surface when the concentration of Pd is up to a certain extent. In addition, the lattice expansion from (1 � 1) LSF (354.85 Å3) to (1 � 1) Pd-LSF (359.13 Å3) could be observed, and this may benefit from a bond distance of Pd–O (1.99 Å) in Pd-LSF longer than that of Fe–O (1.96 Å) in LSF. Pd K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) in Fig. 1(b) and (c) reveal that Pd species in the LSF(SDC) are in a higher oxidation state than Pd2þ, and the Pd oxidation degree first decreases and then in creases with increasing Pd concentration. The Fourier transformed K3weighted EXAFS results for all Pd-LSF(SDC) in Fig. 1(c) exhibit similar Pd coordination environment, and no Pd–Pd coordination is observed compared with the Pd foil reference. In combination with the TEM, STEM and DFT calculation results, it could be deduced that most Pd species are atomically anchored on the LSF phase with oxygen surroundings. X-ray photoelectron spectroscopy (XPS) measurements reveal that
2. Results and discussion X-ray diffraction (XRD) patterns of the Pd-LSF(SDC) in Fig. 1(a) exhibit the mixture of rhombohedral (R3c) LSF [18] and cubic SDC. No characteristic diffraction peaks ascribed to Pd species could be observed, indicating the absence of crystalline Pd or PdOx nanoparticles. Scanning electron microscopy (SEM) images in Fig. S1 present that all nano particles are bonded with each other to form porous networks, and the Pd concentration barely affects the morphology. High-resolution trans mission electron microscopy (HRTEM) images in Figs. S2(a)–(d) show that the LSF closely attaches with the SDC phase. Besides, the intro duction of Pd slightly increases the lattice space of the LSF phase, probably due to the doping of the larger Pd ions into the Fe sites. As verified by the energy-dispersive X-ray (EDX) spectroscopy, the Pd species are majorly trapped on the LSF phase instead of the SDC phase, suggesting that the Pd species have a strong chemical affinity toward
Fig. 1. (a) XRD patterns of LSF-SDC with various Pd loadings, (b) XANES spectra and (c) Fourier transformed K3-weighted EXAFS spectra of Pd in the nano composites, (d) XPS spectra of Pd 3d in the nanocomposites. 2
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increases and then decreases with increasing Pd concentration. Overall, the 0.8% Pd-LSF(SDC) possesses the best CO2 adsorption capacity and the highest oxygen vacancy concentration at 800 � C. The electrochemical performances of the Pd-LSF(SDC) were assessed in SOEC for CO2 electrolysis with YSZ electrolyte and LSM/YSZ anode. The current-voltage curves and potentiostatic tests are presented in Fig. 3 and Fig. S6. Current densities increase steadily as the applied voltages rise. The introduction of Pd can effectively enhance the CO2 electrolysis performance. The changes in current densities followed the same sequence as the CO2 adsorption and dissociation ability as well as the oxygen vacancy concentrations at 800 � C (Fig. S5), that is, the cur rent densities first increased and then decreased with increasing Pd concentration. The 0.8% Pd-LSF(SDC) with the best CO2 adsorption capacity and the highest oxygen vacancy concentration shows the highest average current density of 0.58 A cm 2 at 1.6 V, which is about 65.7% higher than that of LSF(SDC). Faradaic efficiencies are around 100% over all the cathodes, and the average CO production rate in creases with applied voltages. Moreover, the CO production rate in creases from 2.60 to 4.01 mL min 1 cm 2 at 1.6 V as the Pd concentration increases to 0.8%. To gain kinetic insights for enhanced performance in CO2 electrolysis over the Pd-LSF(SDC), AC electrochemical impedance spectroscopy (EIS) at different applied voltages (Fig. S7) and analysis in the distri bution of relaxation times (DRT) (Fig. S8) were carried out. The cell polarization resistance (Rp) dramatically decreases as the electrolysis voltage increases, and the introduction of Pd greatly decreases the Rp at a specific given voltage. This is probably due to not only the enhanced CO2 adsorption/dissociation ability, but also the extra Pd4þ/Pd2þ ion pairs and oxygen vacancies in the cathode at the reaction temperature, which facilitates the electron-ion transportation. The 0.8% Pd-LSF(SDC) with the highest concentration of Pd4þ/Pd2þ ion pair and oxygen va cancies as well as the best CO2 adsorption capacity exerts the lowest Rp of ca. 0.256 Ω cm2 at 1.6 V and the highest CO2 electrolysis performance. DRT curves and the breakdown of the EIS at 1.4 V based on the equivalent circuit model were presented in Fig. S9. Commonly, each (RC) element corresponds to a DRT peak, and the peak area is propor tional to the contribution of a specific sub-process [25]. P1 and P2 barely change with the applied voltages and cathode materials, thus they probably are related to the inherent oxygen ions transportation through the electrolyte or across the LSM/YSZ interfaces since the SOECs in this work employed the same YSZ electrolyte and LSM/YSZ anode. On the contrary, P3–P6 are strongly dependent on the applied voltages and cathode materials. The higher applied voltages, the higher frequency regions of P3–P6, indicating quicker responses [25–27]. These are due to that the high applied voltages could create more oxygen vacancies at the cathode, thus facilitate the reaction kinetics in the electrodes [28]. As it is reported, P3 represents the charge transferring process in the elec trodes [25,27], and P4 is related to the exchange and diffusion of oxygen species at the TPBs [25,26], while P5 and P6 are associated with the mass transfer process of reactants [27,29]. The introduction of Pd de creases all the resistances (Table S4), and the decrease in R3, R5 and R6 are more noticeable. This is due to that the decoration of Pd not only introduces extra Pd4þ/Pd2þ ion pairs, favoring the charge transfer in the cathode, but also benefits the CO2 dissociative adsorption as well as the surface diffusion and conversion of active reactant species. The stability of CO2 electrolysis over all the cathodes was recorded at 1.2 V as shown in Fig. S10. The current densities slightly decreased with time on stream. The degradation rate of the current density for 0.8% PdLSF(SDC) is about 0.003 A cm 2 h 1. Both RΩ and Rp of the SOEC after the stability test increase, and the changes in R1, R2 and R4 are more obvious, suggesting that the O2- transportation and diffusion decrease with time on stream. This is probably due to the diminished TPBs during the stability test at high temperatures. In order to figure out the cell degradation factors, XRD, cross-sectional SEM and element maps mea surements of the SOEC after the stability test were performed. As shown in Fig. S11, the cathode coated with Au paste firmly clings on the YSZ
Fig. 2. STEM images and corresponding element maps of the 0.8% PdLSF(SDC).
the loading of Pd species hardly changes the surface electronic states of La 3d and Ce 3d while changes that of Fe 2p (Fig. S4). The Fe 2p spectra were fitted with two components of Fe3þ and Fe4þ [19], while the Pd 3d spectra in Fig. 1(d) could be deconvoluted into Pd4þ and Pd2þ, respec tively [20]. The introduction of Pd leads to a slight decrease in the relative atomic ratio of Fe3þ/Fe4þ (Table S2). As the concentration of Pd increases, the surface atomic ratio of Fe3þ/Fe4þ first increases and then decreases, in contrast to that of Pd4þ/Pd2þ. The 0.8% Pd-LSF(SDC) shows the highest amount of Pd4þ/Pd2þ ionic pairs. This result is consistent with the EXAFS analysis of Pd. For the O 1s XPS shown in Fig. S4(d) and listed in Table S3, the surface O species exist as the adsorbed oxygen in the form of molecular water (OH) [21,22] and ox ygen or hydroxyl groups (O/OOH) [21–23], highly oxidative species 2(O-/O22 ), and lattice oxygen (O ) within the LSF and SDC, respectively. The addition of Pd greatly enhances the relative atomic ratio of O2-, and the relative content of O-/O22 species barely changes. These results suggest that partial Pd species might anchor on the LSF(SDC) surface in oxidation state. Temperature-programmed desorption-mass spectrometry (TPD-MS) measurements of CO2 over Pd-LSF(SDC) were performed to take a deep insight into their surface chemical properties (Fig. S5). Both CO (m/z ¼ 28) and O2 (m/z ¼ 32) could be detected besides CO2 (m/z ¼ 44). Three CO2 desorption peaks could be observed, corresponding to the release of the physisorbed or linear CO2, the carbonates, and the tridentate configuration (see the following calculations and models shown in Figure CS3), respectively. The addition of Pd greatly suppresses the formation of foregoing carbonates but avails the formation of tridentate configuration. Moreover, the former gradually increases and the latter first increases and then decreases with increasing Pd concentration. CO desorption in the high temperature range is consistent with that of CO2 but differs from O2. These results suggest that the desorbed O2 is majorly associated with the evolution of non-stoichiometric lattice oxygen [24]. All samples maintain almost the identical O2 desorption temperature, but the Pd-LSF(SDC) shows a slightly higher desorption amount than LSF(SDC). These results indicate that the Pd in the LSF(SDC) can enhance the mobility of lattice oxygen at high temperatures, verified by the DFT calculation results of substituted Pd exerting less oxygen coor dination and longer Pd–O bond. The O2 desorption amount first 3
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Fig. 3. (a) I–V curves of SOEC for CO2 electrolysis, (b) CO production rate and Faradaic efficiencies of SOECs with the LSF(SDC) and 0.8% Pd-LSF(SDC) cathode versus applied voltage (error bars represent the standard deviations).
electrolyte, and a new broad XRD diffraction peak located around 15.8� appears and the characteristic peak of LSF at around 22.4� becomes broader, which is mainly attributed to the deposition of carbon on the cathode. Compared with the SEM image of 0.8% Pd-LSF(SDC) in Fig. S1, white particles are noticeable after the stability test. The backscattering of the image associated with the EDX result shows that the white
particles are mainly composed of Sr, C, Fe, O. The segregation of Fe species suggests that the atomically dispersed Pd species on the LSF surface might migrate into the LSF lattice at the Fe sites during the re action. Meanwhile, the surface Sr segregation could be obviously observed. In order to further evaluate the carbon coking, TEM images of all the cathodes after stability test were shown in Fig. S12, and apparent Fig. 4. Theoretical analysis of the CO2 electro reduction to CO by DFT calculations. (a) Potential energy landscape of the CO2 electrolysis process over the three sites: Fe–CN4 in LSF(110), SS-Pd-CN3 and SS-Pd-CN2 in Pd-LSF(110) at 800 � C and 1.6 V. * (int.) and *(Vo) denote the site without and with one oxygen vacancy. (b) The sum of up- and down-spin dorbital projected DOS of active Fe and Pd atoms for the corresponding adsorption sites. The dashed cyan line marks the Fermi level. (c) Optimized atomic configurations of the CO2 electroreduction to CO process over SS-Pd-CN2 (side view), which corre sponds to the red line in (a). The dashed pink circle denotes the oxygen vacancy. (d) Amplified local atomic geometry truncated by CO2*(Vo) and CO* (int.) of (c). (e) 2D contour plots of the charge den sity differences for CO2*(Vo) and CO*(int.) over SSPd-CN2. Red and blue regions denote increased and decreased electron densities, respectively.
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carbon deposition could be observed on the catalyst surface in the cathodes. Therefore, the carbon deposition, Fe and Sr segregation might be the major reasons for the decrease in the TPBs, thus lead to cell degradation during stability test. These results in turn verify that the surface Pd sites play a much more dominant role in CO2 electrolysis compared with the surface Fe sites. In order to clarify the difference in CO2 electrolysis performance, DFT calculations were performed to obtain the potential energy land scapes of CO2 electrolysis on the Fe and Pd centers (Fig. 4(a)), and the corresponding intermediates were shown in Fig. 4(c) and S13. Firstly, the two coequal surfaces with six oxygen vacancies are compared, where the oxygen coordination number (CN) of Fe site and Pd site (SS–Pd) are 4 and 3, respectively. More details can be seen in SI. Throughout the whole cathodic process at 800 � C and 1.6 V [30], the CO2 dissociation and CO desorption steps are always downhill on Fe–CN4 (orange line) and SS-Pd-CN3 (blue line), whereas the CO2 adsorption is endergonic with energies of 1.40 and 0.30 eV and depends intensely on the active centers. As the experimental results shown, Pd-doped LSF exhibits an enhanced CO2 adsorption and electroreduction activity, also implying a strong correlation between CO2 adsorption and electrolysis perfor mance. Therefore, we suggest that the CO2 adsorption solely serves as the rate-limiting step in the intrinsic reactivity variation, which was ever reported by Lu et al. [31]. Based on this, CO2 electrolysis on SS-Pd-CN2 is further studied as the red line in Fig. 4(a) and the structures in Fig. 4 (c), where CO2 forms an activated tridentate configuration (also called a bent COδ2 species [32]) with an adsorption energy of 0.48 eV and then can dissociate into one CO* adsorbed on Pd center and one O* filled up the oxygen vacancy in Fig. 4(d). With the confined coordination envi ronment considered, SS-Pd-CN3 is bound to three oxygen atoms and thus shows a worse adsorption of both CO2 and CO, compared to SS-Pd-CN2. Overall, the Pd single site (especially SS-Pd-CN2) plays a crucial role in the CO2 electroreduction to CO over the LSF system. The coordinatively unsaturated Pd atom, together with the neighboring oxygen vacancy, makes up the active center to adsorb and activate CO2. Furthermore, the electronic structures were expected to understand the dissociation process and the nature of active Pd centers. As electron density differences map depicted in Fig. 4(e) and Fig. S14, CO2 is adsorbed on SS-Pd-CN2 through a firm C–Pd bond (seen from red areas), a mild O1–Pd bond, and an O2-vacancy coalition. Later, the dissociated groups of CO2 are anchored onto SS-Pd-CN2 in modes of that O2 filled up the vacancy and that CO species attached on the Pd. Hereto, the dissociation step is accomplished. Through the d-orbital projected density of states (DOS) in Fig. 4(b), it can be seen that the DOS of SS-PdCN2 around the Fermi level is more abundant than that of Fe and also that of SS-Pd-CN3. Therefore, SS-Pd-CN2 facilitates the CO2 adsorption and activation, in synergism with a surface oxygen defect.
Acknowledgment We gratefully acknowledge financial support from the National Key R&D Program of China (Grant 2017YFA0700102), the National Natural Science Foundation of China (Grants 21573222 and 91545202), Dalian National Laboratory for Clean Energy (DNL180404), Dalian Institute of Chemical Physics (Grant DICP DMTO201702), Dalian Outstanding Young Scientist Foundation (Grant No. 2017RJ03), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB17020200). G.W. thanks the financial support from CAS Youth Innovation Promotion (Grant No. 2015145). Y.Z. received financial support from the 14th Six Talents Peak Project of Jiangsu Province (XNYQC-016) and State Key Laboratory of Catalysis (N-18-09). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.nanoen.2020.104598. References [1] D.U. Nielsen, X.-M. Hu, K. Daasbjerg, T. Skrydstrup, Chemically and electrochemically catalysed conversion of CO2 to CO with follow-up utilization to value-added chemicals, Nat. Catal. 1 (2018) 244–254. [2] H. Shen, Z. Gu, G. Zheng, Pushing the activity of CO2 electroreduction by system engineering, Sci. Bull. 64 (2019) 1805–1816. [3] Y. Song, Z. Zhou, X. Zhang, Y. Zhou, H. Gong, H. Lv, Q. Liu, G. Wang, X. Bao, Pure CO2 electrolysis over an Ni/YSZ cathode in a solid oxide electrolysis cell, J. Mater. Chem. A. 28 (2018) 13661–13667. [4] F. Bidrawn, G. Kim, G. Corre, J.T.S. Irvine, J.M. Vohs, R.J. Gorte, Efficient reduction of CO2 in a solid oxide electrolyzer, Electrochem. Solid State Lett. 11 (2008) B167–B170. [5] Q. Liu, C. Yang, X. Dong, F. Chen, Perovskite Sr2Fe1.5Mo0.5O6 δ as electrode materials for symmetrical solid oxide electrolysis cells, Int. J. Hydrogen Energy 35 (2010) 10039–10044. [6] Q. Liu, X. Dong, G. Xiao, F. Zhao, F. Chen, A novel electrode material for symmetrical SOFCs, Adv. Mater. 22 (2010) 5478–5482. [7] L. Yang, K. Xie, S. Xu, T. Wu, Q. Zhou, T. Xie, Y. Wu, Redox-reversible niobiumdoped strontium titanate decorated with in situ grown nickel nanocatalyst for hightemperature direct steam electrolysis, Dalton Trans. 43 (2014) 14147–14157. [8] S. Li, Y. Li, Y. Gan, K. Xie, G. Meng, Electrolysis of H2O and CO2 in an oxygen-ion conducting solid oxide electrolyzer with a La0.2Sr0.8TiO3þδ composite cathode, J. Power Sources 218 (2012) 244–249. [9] Y. Tian, L. Zhang, L. Jia, X. Wang, J. Yang, B. Chi, J. Pu, J. Li, Novel quasisymmetrical solid oxide electrolysis cells with in-situ exsolved cathode for CO2 electrolysis, J. CO2 Util. 31 (2019) 43–50. [10] A. Wang, J. Li, T. Zhang, Heterogeneous single-atom catalysis, Nat. Rev. Chem. 2 (2018) 65–81. [11] X. Zhao, K. Wu, W. Liao, Y. Wang, X. Hou, M. Jin, Z. Suo, H. Ge, Improvement of low temperature activity and stability of Ni catalysts with addition of Pt for hydrogen production via steam reforming of ethylene glycol, Green Energy Environ. 4 (2019) 300–310. [12] C. Yan, H. Li, Y. Ye, H. Wu, F. Cai, R. Si, J. Xiao, S. Miao, S. Xie, F. Yang, Y. Li, G. Wang, X. Bao, Coordinatively unsaturated nickel–nitrogen sites towards selective and high-rate CO2 electroreduction, Energy Environ. Sci. 11 (2018) 1204–1210. [13] B. Qiao, A. Wang, X. Yang, L.F. Allard, Z. Jiang, Y. Cui, J. Liu, J. Li, T. Zhang, Single-atom catalysis of CO oxidation using Pt1/FeOx, Nat. Chem. 3 (2011) 634–641. [14] Z. Zhang, Y. Zhu, H. Asakura, B. Zhang, J. Zhang, M. Zhou, Y. Han, T. Tanaka, A. Wang, T. Zhang, N. Yan, Thermally stable single atom Pt/m-Al2O3 for selective hydrogenation and CO oxidation, Nat. Commun. 8 (2017) 16100. [15] Y. Nishihata, J. Mizuki, T. Akao, H. Tanaka, M. Uenishi, M. Kimura, T. Okamoto, N. Hamada, Self-regeneration of a Pd-perovskite catalyst for automotive emissions control, Nature 418 (2002) 164–167. [16] M.B. Katz, G.W. Graham, Y. Duan, H. Liu, C. Adamo, D.G. Schlom, X. Pan, SelfRegeneration of Pd–LaFeO3 catalysts: new insight from atomic-resolution electron microscopy, J. Am. Chem. Soc. 133 (2011) 18090–18093. [17] Y. Zhou, Z. Zhou, Y. Song, X. Zhang, F. Guan, H. Lv, Q. Liu, S. Miao, G. Wang, X. Bao, Enhancing CO2 electrolysis performance with vanadium-doped perovskite cathode in solid oxide electrolysis cell, Nano Energy 50 (2018) 43–51. [18] X.N. Ying, L. Zhang, Phase transitions in La0.5Sr0.5FeO3 δ investigated by mechanical spectrum, Solid State Commun. 152 (2012) 1252–1255. [19] K. Zhu, H. Liu, X. Li, Q. Li, J. Wang, X. Zhu, W. Yang, Oxygen evolution reaction over Fe site of BaZrxFe1-xO3-δ perovskite oxides, Electrochim. Acta 241 (2017) 433–439. [20] Y. Zheng, L. Kovarik, M.H. Engelhard, Y. Wang, Y. Wang, F. Gao, J. Szanyi, Lowtemperature Pd/Zeolite passive NOx adsorbers: structure, performance, and adsorption chemistry, J. Phys. Chem. C 121 (2017) 15793–15803.
3. Conclusion In summary, LSF(SDC) decorated by Pd species was constructed as cathode materials for CO2 electrolysis in SOECs. The Pd species were atomically anchored on the LSF phase with oxygen surroundings. The decoration of Pd species on LSF(SDC) showed a 65.7% higher current density than LSF (SDC) at 1.6 V with ~100% Faradaic efficiency. Elec trochemical and DFT calculation results verify that the performance enhancement is mainly attributed to the facilitated electrode kinetics, such as the CO2 dissociative adsorption, electron transfer and mass transport. This work provides insights and a new avenue for construct ing single site-anchored cathode in SOECs for CO2 electrolysis. Declaration of competing interest The authors declare no competing financial interest.
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Nano Energy 71 (2020) 104598 Yuefeng Song is a Ph.D. candidate at DICP, CAS. He received his B.S. in chemistry from Jilin University in 2014. His research interests are electrochemical energy storage and conversion, particularly focusing on the electrochemical reduction of CO2/ H2O in solid oxide electrolysis cells (SOECs).
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Xiaomin Zhang is an assistant professor at State Key Labora tory of Catalysis, DICP, CAS. She received her Ph.D. in chemical engineering from DICP, CAS in 2015. Her research interests are in the areas of highly efficient electrode materials and processes for electrochemical energy conversion and storage, especially high temperature electrolysis of CO2/H2O in SOECs.
Houfu Lv is a Ph.D. candidate at DICP, CAS. He received his M. S. in Zhengzhou University in 2015. His research interests are electrochemical energy storage and conversion, particularly focusing on the electrochemical reduction of CO2/H2O in SOECs.
Yingjie Zhou received her Ph.D. (2014) in materials physics and chemistry from Sun Yat-sen University. She was also a joint training Ph.D. at Texas State University and University of Connecticut from 2012 to 2014. She joined Nanjing University of Information Science & Technology from 2014 to 2019. After working as a postdoctoral researcher at State Key Laboratory of Catalysis at Dalian Institute of Chemical Physics (DICP), Chi nese Academy of Sciences (CAS) from 2016 to 2018, she joined Donghua University as an associate professor since 2019. Her research interests are developing highly efficient hybrid nano composites for electrochemical energy conversion and storage.
Qingxue Liu is a Ph.D. candidate at DICP, CAS. She received her B.S. degree in Sichuan University in 2016. Her research interests are the high-temperature electrochemical reduction of CO2/H2O in SOECs.
Le Lin is a Ph.D. candidate in School of Physical Science and Technology, Shanghaitech University. He received his B.S. in marine chemistry (major) and international economy and trade (second major) from Qingdao University of Science & Tech nology (2015). His research interest is mainly the relationship between structure and performance in ultrathin oxide-film catalysts via Density Functional Theory (DFT) calculations, particularly providing mechanisms for heterogeneous catalytic reactions concerning energy and environment.
Zhiwen Zhou is a Ph.D. candidate at DICP, CAS. He received his B.S. in Chemistry from Nanjing University (2014). His research interest is describing and screening oxide catalysts through Density Functional Theory (DFT) calculations providing reasonable mechanisms for various catalytic re actions concerning energy and environment.
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Nano Energy 71 (2020) 104598 Na Ta is an Associate Professor at DICP, CAS. Her major research interests focus on the structural analysis of catalysis materials and their dynamic behavior under working state by electron microscope.
Xinhe Bao received his Ph.D. in Physical Chemistry from Fudan University in 1987. He held an Alexander von Humboldt Research Fellow position in Fritz-Haber Institute between 1989 and 1995, hosted by Prof. Gerhard Ertl. Following that, he joined DICP as a full Professor. He became a member of the CAS in 2009. His research interest is nano and interfacial catalysis, focusing on the fundamental understanding of heterogeneous catalysis, including development of new catalysts and novel catalytic processes related to energy conversion and storage.
Guoxiong Wang received his B.S. from Wuhan University in 2000 and Ph.D. in physical chemistry from DICP, CAS in 2006. After working at Catalysis Research Center, Hokkaido Univer sity, Japan from 2007 to 2010 as postdoctoral researcher, he joined State Key Laboratory of Catalysis, DICP as an associate professor and was promoted as full professor in 2015. His research interests include highly efficient catalytic materials and processes for electrochemical energy conversion and storage.
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