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Enhancing cathode performance for CO2 electrolysis with Ce0.9M0.1O2−δ (M=Fe, Co, Ni) catalysts in solid oxide electrolysis cell
Journal of Energy Chemistry 40 (2020) 46–51
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Journal of Energy Chemistry journal homepage: www.elsevier.com/locate/jechem
Enhancing cathode performance for CO2 electrolysis with Ce0.9 M0.1 O2 − δ (M=Fe, Co, Ni) catalysts in solid oxide electrolysis cell Zhidong Huang a,b, Zhe Zhao a, Huiying Qi a,b, Xiuling Wang a, Baofeng Tu a, Mojie Cheng a,∗ a
Division of Fuel Cells, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China b University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 19 January 2019 Revised 19 February 2019 Accepted 22 February 2019 Available online 26 February 2019 Keywords: Solid oxide electrolysis cell Carbon dioxide conversion Doped ceria Distribution of relaxation times Electroreduction
a b s t r a c t Electrochemical conversion with solid oxide electrolysis cells is a promising technology for CO2 utilization and simultaneously store renewable energy. In this work, Ce0.9 M0.1 O2− δ (CeM, M=Fe, Co, Ni) catalysts are infiltrated into La0.6 Sr0.4 Cr0.5 Fe0.5 O3 − δ –Gd0.2 Ce0.8 O2 − δ (LSCrFe-GDC) cathode to enhance the electrochemical performance for CO2 electrolysis. CeCo-LSCrFe-GDC cell obtains the best performance with a current density of 0.652 A cm−2 , followed by CeFe-LSCrFe-GDC and CeNi-LSCrFe-GDC cells with the value of 0.603 and 0.535 A cm−2 , respectively, about 2.44, 2.26 and 2.01 times higher than that of the LSCrFe-GDC cell at 1.5 V and 800 °C. Electrochemical impedance spectra combined with distributions of relaxed times analysis shows that both CO2 adsorption process and the dissociation of CO2 at triple phase boundaries are accelerated by CeM catalysts, while the latter is the key rate-determining step. © 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
1. Introduction Electroreduction of CO2 with solid oxide electrolysis cells (SOECs) powered by renewable energy sources, such as wind and solar energy, has received tremendous attention since it can recycle the greenhouse gas back into fuels and simultaneously store renewable energy [1–4]. Superior to other low-temperature electrolysis systems, SOECs can provide high electrolysis efficiency as well as fast electrode reaction kinetics owing to its high operation temperature [5,6]. Ni-Yttria-stabilized zirconia (Ni-YSZ) composite has been used as the conventional cathode material for water and/or carbon dioxide electrolysis in SOECs. However, it suffers from severe electrical conductivity loss and deactivation due to its redox-instability and low coke resistance [7–9]. Perovskite oxides Lax Sr1 −x Cry M1 −y O3 − δ (LSCrM, M=Fe, Mn) have also been highlighted as alternative cathode materials for CO2 electrochemical reduction because of their excellent coking resistance and redox stability [10–12]. Nevertheless, these electrodes exhibit extremely low catalytic activity, only 0.1 A cm−2 achieved at 1.5 V and 800 °C on a SOEC with LSCrMnbased cathode [10] and 0.39 A cm−2 on a SOEC with LSCrFe-based cathode [11]. Their electrode kinetic process is restricted by the
∗
Corresponding author. E-mail address: [email protected] (M. Cheng).
limited catalytic activity, leading to low cathode performance. Therefore, it is critical to boost the catalytic activity for electrocatalytic CO2 reduction of these cathode materials. Cerium oxide is widely used as a catalyst towards a lot of oxidation/reduction reactions because it possesses excellent oxygen storage capacity (OSC) and redox behavior [13–15], which can be related to its flexible transformation between the oxidation states Ce4+ and Ce3+ . Previous studies have suggested that doping transition metals such as Fe, Co and Ni into CeO2 can greatly enhance the reducibility of Ce4+ /Ce3+ redox cycle and oxygen storage capacity, resulting in the easier formation of surface oxygen vacancy [16,17]. The produced oxygen vacancies on Ce1 −x Mx O2 − δ (M=Fe, Co, Ni) oxides can not only enhance surface basicity for CO2 capture, but also directly participate in the binding of CO2 to form carbonate species [18,19]. These features suggest that Ce1 −x Mx O2 − δ (M=Fe, Co, Ni) oxides can be good candidate catalysts for CO2 reduction in SOEC cathodes. In this work, catalytically active Ce0.9 M0.1 O2 − δ (denoted as CeM, M=Fe, Co, Ni) oxide catalysts are impregnated into La0.6 Sr0.4 Cr0.5 Fe0.5 O3 − δ -Gd0.2 Ce0.8 O2 − δ (LSCrFe-GDC) cathode for CO2 electrolysis. In this infiltrated cathode, LSCrFe can act as the electron conductor, GDC as the oxide ion conductor while CeM oxide as the main catalyst for CO2 electrolysis. The phase structure and the catalytic activity of CeM oxide are investigated with X-ray diffraction (XRD), temperature programmed reduction (TPR) and temperature-programmed desorption (TPD) experiments,
https://doi.org/10.1016/j.jechem.2019.02.007 2095-4956/© 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
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respectively. The electrochemical performance of the new cathodes and the electrochemical processes of high temperature CO2 electrolysis are also investigated and evaluated. 2. Experimental 2.1. Powder synthesis and cell preparation Ce0.9 M0.1 O2 − δ (M=Fe, Co, Ni) precursor solutions were prepared with nitrate salts and citric acid with a molar ratio of citric acid to metal cations of 0.5:1. The concentration of the metal ions in the solution was 2 M. The CeM powders were also obtained from the precursor solutions with a combustion reaction and calcination in air at 800 °C for 2 h. La0.6 Sr0.4 Cr0.5 Fe0.5 O3 − δ (LSCrFe), Ce0.9 Gd0.2 O2− δ (GDC) and (La0.8 Sr0.2 )0.95 MnO3− δ (LSM) powders were synthesized through the same method, but LSCrFe and LSM followed by a calcination in air at 1100 °C for 2 h, and GDC sintered in air at 800 °C for 2 h. YSZ electrolyte supports with 25 mm in diameter and 150 μm in thickness were fabricated by tape casting. Then GDC was printed on the YSZ substrate and sintered in air at 10 0 0 °C for 2 h to form a buffer layer. The (La0.8 Sr0.2 )0.95 MnO3+ δ (LSM)-YSZ electrode with a weight ratio of 60:40 was prepared on the surface of YSZ substrate with an active area of 0.5 cm2 and sintered in air at 1200 °C for 2 h. Then LSCrFe-GDC (50 wt%–50 wt%) composite electrode with an area of 0.5 cm2 was obtained through depositing the slurry on the GDC buffer layer and sintering in air at 1150 °C for 2 h. The slurry for LSCrFe-GDC composite electrode was prepared by milling LSCrFe, GDC and home-made binder (6 wt% ethyl cellulose dissolved in terpineol). The CeM precursor solutions were infiltrated into the LSCrFe-GDC electrode and sintered in air at 600 °C for 1 h to form CeM-LSCrFe-GDC cathodes. The infiltration was repeated until the contents of CeM in LSCrFe-GDC backbone reached 20 wt%. Then the cells were sintered in air at 800 °C for 2 h. 2.2. Characterization of the samples and SOECs The XRD patterns of the samples were recorded on a X-ray diffractometer (RINT D/Max-2500) in 2θ range from 20° to 80° ˚ at 40 kV and 200 mA. The miusing Cu Kα radiation (λ = 1.54 A) crostructure and elemental distribution of the cathodes were examined with Scanning Electron Microscope (SEM, JSM-7800F) and Energy dispersive spectrometer (EDS). H2 -temperature-programmed reduction (H2 -TPR) experiments were carried out on a home-made apparatus. 100 mg sample was pretreated at 120 °C for 30 min in pure He (≥99.999 vol%, 50 mL min−1 ). After cooling down to room temperature (RT), 10% H2 -90% Ar was switched into the system. The H2 -TPR profile was obtained during the sample temperature ramping from RT to 900 °C with a rate of 10 °C min−1 . H2 consumption was recorded with a thermal conductivity detector. Temperature programmed desorption (TPD) experiments were performed by placing 200 mg sample in a quartz reactor and pretreated in pure O2 (O2 -TPD) or 5% CO2 -He flow (CO2 -TPD) at 800 °C for 1 h. After cooling down to RT, pure He was switched into the system and the thermal desorption was carried out by heating the sample from RT to 900 °C at a rate of 10 °C min−1 . The output gas was analyzed by mass spectrometer (Pfeiffer Vacuum, Ominstar GSD 301 O2). The electrochemical performances of the electrolyte-supported single cells were tested on home-made testing station with CO2 -CO (90-10) mixture supplied to cathode, and pure O2 supplied to anode. The flow rates of both gases were kept at 50 mL min−1 using mass flow meters. The current-voltage (I-V) polarization curves and electrochemical impedance spectra (EIS) were measured using a Solartron 1287 potentiostat and a 1260 frequency
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response analyzer. I-V curves were measured from 0.5 V to 1.8 V from 700 to 800 °C. The EIS were performed in the frequency range from 100 kHz to 0.08 Hz, with a 10 mV AC perturbation. The outlet products were analyzed by on-line gas chromatography (GC) in 10 min intervals to evaluate the CO production rate. 3. Results and discussion 3.1. XRD characterization Fig. 1(a) depicts the XRD patterns of CeM samples after calcination at 800 °C. For the CeFe sample, all diffraction peaks are assigned to CeO2 fluorite phase (PDF#43-1002), suggesting that solid solution can be formed by incorporation of Fe ions into CeO2 . For the CeCo and CeNi samples, most diffraction peaks are assigned to CeO2 fluorite phase, and minor Co3 O4 phase for CeCo and NiO phase for CeNi are detected. These results show that the dopants have been incorporated successfully into the lattice of CeO2 and CeFe exhibits a more stable structure than CeCo and CeNi. Fig. 1(b) shows the XRD patterns of CeM-LSCrFe-GDC composite cathodes. The diffraction peaks are well assigned to CeO2 fluorite phase and LSCrFe perovskite phase for all cathodes, suggesting that CeM samples have good chemical compatibility with LSCrFe-GDC composite. 3.2. H2 -TPR and TPD characterization Fig. 2(a) presents the H2 -TPR profiles of CeFe, CeCo and CeNi samples. As shown, the reduction of CeFe oxide is evidenced by four peaks at ca. 383 °C, 455 °C, 610 °C and 786 °C. The first two are associated with the reduction of Fe3+ to Fe2+ and the reduction of surface ceria, respectively, while the third one correlates with the reduction of Fe2+ to Fe [20]. The last one at 800 °C corresponds to the reduction of bulk ceria. Three reduction peaks also appear on the CeCo sample center at 300 °C, 360 °C and 790 °C, which are from the reduction of Co3+ to Co2+ , the reduction of surface ceria and the reduction of bulk ceria, respectively [21]. For CeNi sample, the one with peak temperature at 250 °C is from the reduction of Ni2+ to Ni and the other one at 335 °C is from the reduction of surface ceria [22]. The last one at high temperature is attributed to the reduction of bulk ceria. Compared with that on CeO2 [23], the lower reduction temperatures of Ce4+ to Ce3+ on the CeM oxides indicate that the interaction between transition metal ions and cerium ions in solid solutions enhances the redox activity of surface cerium ions. Fig. 2(b) depicts the O2 -TPD patterns of CeM samples. Notably, a very weak O2 desorption band in 100–250 °C can be observed on both CeCo and CeNi samples, which is associated with O2 – and/or O2 2– species adsorbed on oxygen vacancies [24]. This result indicates that the concentration of surface oxygen vacancies in CeCo and CeNi oxides is promoted with the partial substitution of Ce by Co or Ni ions, which is beneficial for the adsorption of gas molecular such as CO2 and O2 . However, no high temperature peak appears on CeNi, which should be assigned to the liberation of oxygen from the bulk of the oxides. A sharp and big desorption peak in 600–820 °C can be observed on CeCo which is larger than that on CeFe sample, suggesting that the lattice oxygen in CeCo is more easily released from the bulk than that on CeFe and CeNi, and CeCo exhibits the highest surface oxygen exchange kinetic. Apparently, the surface oxygen exchange kinetic at high temperature obtained from the O2 -TPD analysis followed the sequence: CeNi < CeFe < CeCo. These observations reflect that incorporation of Co into CeO2 can not only improve concentration of surface oxygen vacancies but also greatly enhance the surface oxygen exchange kinetic.
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Fig. 1. XRD patterns for (a) CeFe, CeCo and CeNi samples and (b) LSCrFe-GDC, CeFe-LSCrFe-GDC, CeCo-LSCrFe-GDC and CeNi-LSCrFe-GDC cathodes.
Fig. 2. (a) H2 -TPR behaviors of CeFe, CeCo and CeNi samples. TPD profiles for (b) O2 desorption, (c) CO2 desorption and (d) CO desorption on CeFe, CeCo and CeNi samples.
CO2 /CO-TPD after CO2 adsorption is conducted to investigate the surface reaction on CeM samples as shown in Fig. 2(c) and (d). Apparently, two CO2 desorption peaks are observed below 300 °C on all samples, which are associated with the physisorbed CO2 and monodentate carbonates interacting with surface transition metal ions or Ce ions [25,26]. However, no CO2 desorption peak appears at high temperature. Instead, as shown in Fig. 2(d), a strong CO desorption peak appears above 400 °C on CeFe and CeCo with peak temperature at 496 °C and 453 °C, respectively. These CO molecules are from the decomposition of the stable carbonates which are bonded to oxide surface with two oxygen ions and exhibit higher thermal stability [25]. Different from CeFe and CeCo, only one CO desorption peak appears on CeNi at 182 °C, which may be assigned to the decomposition of monodentate carbonates. Therefore, both CeCo and CeFe have higher capacity for strongly adsorbed carbonates than CeNi. The strong interaction of surface carbonates on CeCo and CeFe can be due to the weak bonding of surface
lattice oxygen to Ce3+ , Co2+ or Fe2+ ions, in well agreement with the higher reducibility of CeCo and CeFe from TPR and higher surface oxygen exchange kinetic from O2 -TPD.
3.3. Microstructure of the electrochemical cell and cathodes Fig. 3 depicts the SEM images of the four cathodes. From the cross-section view in Fig. 3(a), it can be seen that the cathode adheres to the electrolyte very well with a thickness of about 50 μm. From Fig. 3(b), LSCrFe and GDC particles sized at 20 0–30 0 nm are well connected with each other and form a three-dimensional network for gas transportation. Fig. 3(c)–(e) shows the microstructure and EDS results of the CeM-LSCrFe-GDC (M=Co, Fe, Ni) cathode. It can be seen that the infiltrated CeM particles with particles sized at 50 nm are located at the three-phase boundary (TPB) of LSCrFe, GDC and air, where CeM can accept electrons from LSCrFe
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Fig. 3. SEM micrograph of (a) the cross-section of the whole cathode/electrolyte interface and (b) LSCrFe-GDC cathode. SEM micrograph and EDS results for (c) CeCo-LSCrFeGDC, (d) CeFe-LSCrFe-GDC and (e) CeNi-LSCrFe-GDC cathodes.
and transfer oxygen ions to GDC. Obviously, the TPB is extended to LSCrFe-CeM-gas through the introduction of CeM particles. 3.4. Performance for electrocatalytic CO2 reduction I-V curves of the cells with LSCrFe-GDC, CeFe-LSCrFe-GDC, CeCo-LSCrFe-GDC and CeNi-LSCrFe-GDC cathodes operated at 800 °C are shown in Fig. 4(a). The measured open circuit voltages (OCV) for LSCrFe-GDC based cells are 0.885 V in CO2 -CO (90-10) at 800 °C, agreeing well with the theoretical values (0.885 V) predicted by Nernst equation. The current density of the LSCrFe-GDC cell is 0.266 A cm−2 at 1.5 V and 800 °C. However, improved current density is obtained in the CeM-LSCrFe-GDC cells, which can be ascribed to their improved electrocatalytic activity for CO2 reduction through synergetic effect of the CeM catalysts and LSCrFeGDC electrode at high temperatures. CeCo-LSCrFe-GDC cell obtains the best performance with a current density of 0.652 A cm−2 at
1.5 V and 800 °C, followed by CeFe-LSCrFe-GDC and CeNi-LSCrFeGDC cells with the value of 0.603 and 0.535 A cm−2 , respectively, about 2.44, 2.26 and 2.01 times higher than that of the LSCrFe-GDC cell. CO production rates of the cells with four cathodes shown in Fig. 4(b) further confirm the enhanced electrocatalytic activity for CO2 reduction by CeM catalyst. The CO production rate on LSCrFe-GDC cell is about 2.09 mL min−1 cm−2 at 1.6 V. However, the CO production rates are greatly improved after introduction of CeM catalysts. CeCo-LSCrFe-GDC cell reaches a stable and maximum CO production rate of 4.94 mL min−1 cm−2 , followed by CeFe-LSCrFe-GDC (4.31 mL min−1 cm−2 ) and CeNi-LSCrFe-GDC (2.98 mL min−1 cm−2 ) cells at 1.6 V and 800 °C. The improved current densities and CO production rates in the cells using CeMLSCrFe-GDC cathodes demonstrate the high catalytic activity of CeM for CO2 electrolysis. The electrochemical impedance spectra (EIS) and the total polarization resistance of LSCrFe-GDC cell, CeFe-LSCrFe-GDC cell,
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Fig. 4. (a) I-V curves and (b) CO production of LSCrFe-GDC cell, CeFe-LSCrFe-GDC cell, CeCo-LSCrFe-GDC cell and CeNi-LSCrFe-GDC cell operated at 800 °C.
Fig. 5. (a) Impedance spectra at 800 °C and (b) total polarization resistance of LSCrFe-GDC cell, CeFe-LSCrFe-GDC cell, CeCo-LSCrFe-GDC cell and CeNi-LSCrFe-GDC cell.
CeCo-LSCrFe-GDC cell and CeNi-LSCrFe-GDC cell at 800 °C are shown in Fig. 5. The overall ohmic resistances (Rohm ) and total polarization resistance (RP ) can be obtained from the EIS, where the high frequency intercept of impedance spectra on Z’ axis represents the Rohm and the difference between the real axes intercepts on each plot represents the RP . The Rohm values of the cells with CeM-LSCrFe-GDC (M=Co, Fe, Ni) cells are the same (0.49 cm2 ), slightly lower than that on the LSCrFe-GDC cell, suggesting that better interfacial contact between the cathode and YSZ electrolyte is formed after the introduction of CeM catalysts. Moreover, the introduction of CeM catalysts also leads to a large decrease in total polarization resistance, as depicted in Fig. 5(a) and (b). At 800 °C, CeCo-LSCrFe-GDC cell exhibits the smallest RP value of 0.64 cm2 , only 34.4% of the value on LSCrFe-GDC cell (1.90 cm2 ). The RP value of CeFe-LSCrFe-GDC cell and CeNiLSCrFe-GDC cell is 0.89 cm2 and 1.03 cm2 at 800 °C, respectively, also much lower than that of the LSCrFe-GDC cell. Since the same anode is applied in the four cells, the reduction of RP is ascribed to the decreased cathode polarization. Therefore, these results indicate that CeM catalysts take the main responsibility in catalyzing CO2 reduction reaction. The CO2 reduction reaction at SOEC cathodes may generally involve various elementary processes such as CO2 adsorption, charge transfer, dissociation and oxygen incorporation. In order to further understand the electrode processes and the contributions from different elementary electrode processes, the distributions of relaxed times (DRT) [27–29] are calculated from the impedance spectra with an extended equivalent circuit model as shown in Fig 6(a) and (b). Each DRT peak reflects an individual electrode process and the area reflects the corresponding resistance. The DRT spectra of LSCrFe-GDC cell, CeFe-LSCrFe-GDC cell, CeCo-LSCrFeGDC cell and CeNi-LSCrFe-GDC cell at 800 °C are compared in Fig. 6(c). Apparently, six rate-determining steps are depicted in
DRT plots: P1A, P2A and P3A represent three oxygen electrode processes [30,31] while P1C, P2C and P3C represent three cathode processes and each peak is an indication of a sub-step for CO2 reduction. Since the same anode is applied in the four cells, P1A, P2A and P3A remain the same in all cells. The P1C process with a peak frequency at >10 kHz is attributed to the oxygen ions transfer through the cathode/electrolyte interface and incorporation into the YSZ electrolyte [31]. Apparently, the introduction of CeM catalysts mainly affects the P2C and P3C process. It is reported that the electroreduction of CO2 on doped CeO2 precedes a formation of carbonate intermediate and an electron transfer process in accompany with an oxygen incorporation reaction [18]. The areas of P2C are about the same in CeFe-LSCrFe-GDC cell, CeCo-LSCrFe-GDC cell and CeNi-LSCrFe-GDC cell, much lower than that on LSCrFeGDC cell, which may be associated with the processes of CO2 adsorption and carbonate intermediate formation accelerated by the increased oxygen vacancy concentration and TPBs from CeM catalysts. As for the P3C process, it is assigned to the dissociation of carbonate intermediate, in accompany with incorporation of dissociated oxygen ion into oxygen vacancy. P3C exhibits the largest difference in the four cells. A larger P3C peak than P2C peak indicates that dissociation process of carbonate intermediate is the key rate-limiting step on the cells. However, the peak area of P3C is reduced after impregnation of CeM catalysts, reflecting that carbonate intermediate reduction reaction is also significantly accelerated. Notably, CeCo-LSCrFe-GDC cell has the smallest peak area of P3C followed by CeFe-LSCrFe-GDC cell and CeNi-LSCrFe-GDC cell, suggesting that CeCo has the highest catalytic activity towards CO2 reduction. These results indicate that the dissociation of carbonate intermediate at TPBs occurs much easier on CeCo. The activation energy of elementary electrode process is also shown in Fig. 6(d). The activation energies for P3C process are 96.69 kJ mol−1 , 124.21 kJ mol−1 and 144.66 kJ mol−1 on
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4. Conclusions In summary, we modified the LSCrFe-GDC electrode with Ce0.9 M0.1 O2 − δ (M=Fe, Co, Ni) by impregnation method. TPR and TPD results indicate that both redox activity and surface oxygen exchange kinetic are enhanced by the introduction of transition metal ions. All CeM samples show a CO desorption peak after CO2 adsorption, reflecting their good catalytic activity for CO2 reduction. CeCo-LSCrFe-GDC cell obtains the best performance with a current density of 0.652 A cm−2 at 1.5 V and 800 °C, followed by CeFe-LSCrFe-GDC and CeNi-LSCrFe-GDC cells with the value of 0.603 and 0.535 A cm−2 , respectively, about 2.44, 2.26 and 2.01 times higher than that of the cell with the LSCrFe-GDC cathode. EIS combined with DRT analysis shows that both CO2 adsorption and the carbonate intermediate dissociation at TPBs are accelerated by CeM catalysts, while the latter is the key rate-determining step. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 91534128, 21506208 and 21476230), the Ministry of Science and Technology of China (Grants 2016YFE0118300) and the DNL Cooperation Fund, CAS (DNL180306). References
Fig. 6. (a) Extended equivalent circuit, (b) fitting result for CeCo-LSCrFe-GDC cell, and (c) DRT spectra for all cells at 800 °C. (d) Arrhenius plots of ln(1/RP ) versus temperature on the cells for P3C process.
CeCo-LSCrFe-GDC, CeFe-LSCrFe-GDC and CeNi-LSCrFe-GDC cells, respectively, much lower than that on LSCrFe-GDC cell (152.7 kJ mol−1 ). The lowest activation energy of P3C on CeCoLSCrFe-GDC cell also suggests that CeCo exhibits the best electrochemical and catalytic properties towards CO2 electrolysis. CeM oxides with high oxygen vacancy concentration have the potential to capture CO2 molecule through chemical adsorption at these defect sites. The TPBs, involved active transition metal ions and Ce ions, takes the responsibility for CO2 reduction. Possibly for this reason, the cathode performance can be enhanced by introduction of CeM oxides. Moreover, the best performance with CeCo for CO2 reduction may be attributed to the higher reducibility and surface oxygen exchange kinetic of CeCo which can lead to a faster charge transfer reaction and oxygen incorporation. Therefore, high oxygen vacancy concentration, active transition metal ions and Ce ions as well as excellent surface oxygen exchange kinetic of CeM oxides play important roles in the enhanced cathode process of P2C and P3C, resulting in better cathode performance for CO2 electrolysis.
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Enhancing cathode performance for CO2 electrolysis with Ce0.9 M0.1 O2 − δ (M=Fe, Co, Ni) catalysts in solid oxide electrolysis cell Zhidong Huang a,b, Zhe Zhao a, Huiying Qi a,b, Xiuling Wang a, Baofeng Tu a, Mojie Cheng a,∗ a
Division of Fuel Cells, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China b University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
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Article history: Received 19 January 2019 Revised 19 February 2019 Accepted 22 February 2019 Available online 26 February 2019 Keywords: Solid oxide electrolysis cell Carbon dioxide conversion Doped ceria Distribution of relaxation times Electroreduction
a b s t r a c t Electrochemical conversion with solid oxide electrolysis cells is a promising technology for CO2 utilization and simultaneously store renewable energy. In this work, Ce0.9 M0.1 O2− δ (CeM, M=Fe, Co, Ni) catalysts are infiltrated into La0.6 Sr0.4 Cr0.5 Fe0.5 O3 − δ –Gd0.2 Ce0.8 O2 − δ (LSCrFe-GDC) cathode to enhance the electrochemical performance for CO2 electrolysis. CeCo-LSCrFe-GDC cell obtains the best performance with a current density of 0.652 A cm−2 , followed by CeFe-LSCrFe-GDC and CeNi-LSCrFe-GDC cells with the value of 0.603 and 0.535 A cm−2 , respectively, about 2.44, 2.26 and 2.01 times higher than that of the LSCrFe-GDC cell at 1.5 V and 800 °C. Electrochemical impedance spectra combined with distributions of relaxed times analysis shows that both CO2 adsorption process and the dissociation of CO2 at triple phase boundaries are accelerated by CeM catalysts, while the latter is the key rate-determining step. © 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
1. Introduction Electroreduction of CO2 with solid oxide electrolysis cells (SOECs) powered by renewable energy sources, such as wind and solar energy, has received tremendous attention since it can recycle the greenhouse gas back into fuels and simultaneously store renewable energy [1–4]. Superior to other low-temperature electrolysis systems, SOECs can provide high electrolysis efficiency as well as fast electrode reaction kinetics owing to its high operation temperature [5,6]. Ni-Yttria-stabilized zirconia (Ni-YSZ) composite has been used as the conventional cathode material for water and/or carbon dioxide electrolysis in SOECs. However, it suffers from severe electrical conductivity loss and deactivation due to its redox-instability and low coke resistance [7–9]. Perovskite oxides Lax Sr1 −x Cry M1 −y O3 − δ (LSCrM, M=Fe, Mn) have also been highlighted as alternative cathode materials for CO2 electrochemical reduction because of their excellent coking resistance and redox stability [10–12]. Nevertheless, these electrodes exhibit extremely low catalytic activity, only 0.1 A cm−2 achieved at 1.5 V and 800 °C on a SOEC with LSCrMnbased cathode [10] and 0.39 A cm−2 on a SOEC with LSCrFe-based cathode [11]. Their electrode kinetic process is restricted by the
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Corresponding author. E-mail address: [email protected] (M. Cheng).
limited catalytic activity, leading to low cathode performance. Therefore, it is critical to boost the catalytic activity for electrocatalytic CO2 reduction of these cathode materials. Cerium oxide is widely used as a catalyst towards a lot of oxidation/reduction reactions because it possesses excellent oxygen storage capacity (OSC) and redox behavior [13–15], which can be related to its flexible transformation between the oxidation states Ce4+ and Ce3+ . Previous studies have suggested that doping transition metals such as Fe, Co and Ni into CeO2 can greatly enhance the reducibility of Ce4+ /Ce3+ redox cycle and oxygen storage capacity, resulting in the easier formation of surface oxygen vacancy [16,17]. The produced oxygen vacancies on Ce1 −x Mx O2 − δ (M=Fe, Co, Ni) oxides can not only enhance surface basicity for CO2 capture, but also directly participate in the binding of CO2 to form carbonate species [18,19]. These features suggest that Ce1 −x Mx O2 − δ (M=Fe, Co, Ni) oxides can be good candidate catalysts for CO2 reduction in SOEC cathodes. In this work, catalytically active Ce0.9 M0.1 O2 − δ (denoted as CeM, M=Fe, Co, Ni) oxide catalysts are impregnated into La0.6 Sr0.4 Cr0.5 Fe0.5 O3 − δ -Gd0.2 Ce0.8 O2 − δ (LSCrFe-GDC) cathode for CO2 electrolysis. In this infiltrated cathode, LSCrFe can act as the electron conductor, GDC as the oxide ion conductor while CeM oxide as the main catalyst for CO2 electrolysis. The phase structure and the catalytic activity of CeM oxide are investigated with X-ray diffraction (XRD), temperature programmed reduction (TPR) and temperature-programmed desorption (TPD) experiments,
https://doi.org/10.1016/j.jechem.2019.02.007 2095-4956/© 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
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respectively. The electrochemical performance of the new cathodes and the electrochemical processes of high temperature CO2 electrolysis are also investigated and evaluated. 2. Experimental 2.1. Powder synthesis and cell preparation Ce0.9 M0.1 O2 − δ (M=Fe, Co, Ni) precursor solutions were prepared with nitrate salts and citric acid with a molar ratio of citric acid to metal cations of 0.5:1. The concentration of the metal ions in the solution was 2 M. The CeM powders were also obtained from the precursor solutions with a combustion reaction and calcination in air at 800 °C for 2 h. La0.6 Sr0.4 Cr0.5 Fe0.5 O3 − δ (LSCrFe), Ce0.9 Gd0.2 O2− δ (GDC) and (La0.8 Sr0.2 )0.95 MnO3− δ (LSM) powders were synthesized through the same method, but LSCrFe and LSM followed by a calcination in air at 1100 °C for 2 h, and GDC sintered in air at 800 °C for 2 h. YSZ electrolyte supports with 25 mm in diameter and 150 μm in thickness were fabricated by tape casting. Then GDC was printed on the YSZ substrate and sintered in air at 10 0 0 °C for 2 h to form a buffer layer. The (La0.8 Sr0.2 )0.95 MnO3+ δ (LSM)-YSZ electrode with a weight ratio of 60:40 was prepared on the surface of YSZ substrate with an active area of 0.5 cm2 and sintered in air at 1200 °C for 2 h. Then LSCrFe-GDC (50 wt%–50 wt%) composite electrode with an area of 0.5 cm2 was obtained through depositing the slurry on the GDC buffer layer and sintering in air at 1150 °C for 2 h. The slurry for LSCrFe-GDC composite electrode was prepared by milling LSCrFe, GDC and home-made binder (6 wt% ethyl cellulose dissolved in terpineol). The CeM precursor solutions were infiltrated into the LSCrFe-GDC electrode and sintered in air at 600 °C for 1 h to form CeM-LSCrFe-GDC cathodes. The infiltration was repeated until the contents of CeM in LSCrFe-GDC backbone reached 20 wt%. Then the cells were sintered in air at 800 °C for 2 h. 2.2. Characterization of the samples and SOECs The XRD patterns of the samples were recorded on a X-ray diffractometer (RINT D/Max-2500) in 2θ range from 20° to 80° ˚ at 40 kV and 200 mA. The miusing Cu Kα radiation (λ = 1.54 A) crostructure and elemental distribution of the cathodes were examined with Scanning Electron Microscope (SEM, JSM-7800F) and Energy dispersive spectrometer (EDS). H2 -temperature-programmed reduction (H2 -TPR) experiments were carried out on a home-made apparatus. 100 mg sample was pretreated at 120 °C for 30 min in pure He (≥99.999 vol%, 50 mL min−1 ). After cooling down to room temperature (RT), 10% H2 -90% Ar was switched into the system. The H2 -TPR profile was obtained during the sample temperature ramping from RT to 900 °C with a rate of 10 °C min−1 . H2 consumption was recorded with a thermal conductivity detector. Temperature programmed desorption (TPD) experiments were performed by placing 200 mg sample in a quartz reactor and pretreated in pure O2 (O2 -TPD) or 5% CO2 -He flow (CO2 -TPD) at 800 °C for 1 h. After cooling down to RT, pure He was switched into the system and the thermal desorption was carried out by heating the sample from RT to 900 °C at a rate of 10 °C min−1 . The output gas was analyzed by mass spectrometer (Pfeiffer Vacuum, Ominstar GSD 301 O2). The electrochemical performances of the electrolyte-supported single cells were tested on home-made testing station with CO2 -CO (90-10) mixture supplied to cathode, and pure O2 supplied to anode. The flow rates of both gases were kept at 50 mL min−1 using mass flow meters. The current-voltage (I-V) polarization curves and electrochemical impedance spectra (EIS) were measured using a Solartron 1287 potentiostat and a 1260 frequency
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response analyzer. I-V curves were measured from 0.5 V to 1.8 V from 700 to 800 °C. The EIS were performed in the frequency range from 100 kHz to 0.08 Hz, with a 10 mV AC perturbation. The outlet products were analyzed by on-line gas chromatography (GC) in 10 min intervals to evaluate the CO production rate. 3. Results and discussion 3.1. XRD characterization Fig. 1(a) depicts the XRD patterns of CeM samples after calcination at 800 °C. For the CeFe sample, all diffraction peaks are assigned to CeO2 fluorite phase (PDF#43-1002), suggesting that solid solution can be formed by incorporation of Fe ions into CeO2 . For the CeCo and CeNi samples, most diffraction peaks are assigned to CeO2 fluorite phase, and minor Co3 O4 phase for CeCo and NiO phase for CeNi are detected. These results show that the dopants have been incorporated successfully into the lattice of CeO2 and CeFe exhibits a more stable structure than CeCo and CeNi. Fig. 1(b) shows the XRD patterns of CeM-LSCrFe-GDC composite cathodes. The diffraction peaks are well assigned to CeO2 fluorite phase and LSCrFe perovskite phase for all cathodes, suggesting that CeM samples have good chemical compatibility with LSCrFe-GDC composite. 3.2. H2 -TPR and TPD characterization Fig. 2(a) presents the H2 -TPR profiles of CeFe, CeCo and CeNi samples. As shown, the reduction of CeFe oxide is evidenced by four peaks at ca. 383 °C, 455 °C, 610 °C and 786 °C. The first two are associated with the reduction of Fe3+ to Fe2+ and the reduction of surface ceria, respectively, while the third one correlates with the reduction of Fe2+ to Fe [20]. The last one at 800 °C corresponds to the reduction of bulk ceria. Three reduction peaks also appear on the CeCo sample center at 300 °C, 360 °C and 790 °C, which are from the reduction of Co3+ to Co2+ , the reduction of surface ceria and the reduction of bulk ceria, respectively [21]. For CeNi sample, the one with peak temperature at 250 °C is from the reduction of Ni2+ to Ni and the other one at 335 °C is from the reduction of surface ceria [22]. The last one at high temperature is attributed to the reduction of bulk ceria. Compared with that on CeO2 [23], the lower reduction temperatures of Ce4+ to Ce3+ on the CeM oxides indicate that the interaction between transition metal ions and cerium ions in solid solutions enhances the redox activity of surface cerium ions. Fig. 2(b) depicts the O2 -TPD patterns of CeM samples. Notably, a very weak O2 desorption band in 100–250 °C can be observed on both CeCo and CeNi samples, which is associated with O2 – and/or O2 2– species adsorbed on oxygen vacancies [24]. This result indicates that the concentration of surface oxygen vacancies in CeCo and CeNi oxides is promoted with the partial substitution of Ce by Co or Ni ions, which is beneficial for the adsorption of gas molecular such as CO2 and O2 . However, no high temperature peak appears on CeNi, which should be assigned to the liberation of oxygen from the bulk of the oxides. A sharp and big desorption peak in 600–820 °C can be observed on CeCo which is larger than that on CeFe sample, suggesting that the lattice oxygen in CeCo is more easily released from the bulk than that on CeFe and CeNi, and CeCo exhibits the highest surface oxygen exchange kinetic. Apparently, the surface oxygen exchange kinetic at high temperature obtained from the O2 -TPD analysis followed the sequence: CeNi < CeFe < CeCo. These observations reflect that incorporation of Co into CeO2 can not only improve concentration of surface oxygen vacancies but also greatly enhance the surface oxygen exchange kinetic.
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Fig. 1. XRD patterns for (a) CeFe, CeCo and CeNi samples and (b) LSCrFe-GDC, CeFe-LSCrFe-GDC, CeCo-LSCrFe-GDC and CeNi-LSCrFe-GDC cathodes.
Fig. 2. (a) H2 -TPR behaviors of CeFe, CeCo and CeNi samples. TPD profiles for (b) O2 desorption, (c) CO2 desorption and (d) CO desorption on CeFe, CeCo and CeNi samples.
CO2 /CO-TPD after CO2 adsorption is conducted to investigate the surface reaction on CeM samples as shown in Fig. 2(c) and (d). Apparently, two CO2 desorption peaks are observed below 300 °C on all samples, which are associated with the physisorbed CO2 and monodentate carbonates interacting with surface transition metal ions or Ce ions [25,26]. However, no CO2 desorption peak appears at high temperature. Instead, as shown in Fig. 2(d), a strong CO desorption peak appears above 400 °C on CeFe and CeCo with peak temperature at 496 °C and 453 °C, respectively. These CO molecules are from the decomposition of the stable carbonates which are bonded to oxide surface with two oxygen ions and exhibit higher thermal stability [25]. Different from CeFe and CeCo, only one CO desorption peak appears on CeNi at 182 °C, which may be assigned to the decomposition of monodentate carbonates. Therefore, both CeCo and CeFe have higher capacity for strongly adsorbed carbonates than CeNi. The strong interaction of surface carbonates on CeCo and CeFe can be due to the weak bonding of surface
lattice oxygen to Ce3+ , Co2+ or Fe2+ ions, in well agreement with the higher reducibility of CeCo and CeFe from TPR and higher surface oxygen exchange kinetic from O2 -TPD.
3.3. Microstructure of the electrochemical cell and cathodes Fig. 3 depicts the SEM images of the four cathodes. From the cross-section view in Fig. 3(a), it can be seen that the cathode adheres to the electrolyte very well with a thickness of about 50 μm. From Fig. 3(b), LSCrFe and GDC particles sized at 20 0–30 0 nm are well connected with each other and form a three-dimensional network for gas transportation. Fig. 3(c)–(e) shows the microstructure and EDS results of the CeM-LSCrFe-GDC (M=Co, Fe, Ni) cathode. It can be seen that the infiltrated CeM particles with particles sized at 50 nm are located at the three-phase boundary (TPB) of LSCrFe, GDC and air, where CeM can accept electrons from LSCrFe
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Fig. 3. SEM micrograph of (a) the cross-section of the whole cathode/electrolyte interface and (b) LSCrFe-GDC cathode. SEM micrograph and EDS results for (c) CeCo-LSCrFeGDC, (d) CeFe-LSCrFe-GDC and (e) CeNi-LSCrFe-GDC cathodes.
and transfer oxygen ions to GDC. Obviously, the TPB is extended to LSCrFe-CeM-gas through the introduction of CeM particles. 3.4. Performance for electrocatalytic CO2 reduction I-V curves of the cells with LSCrFe-GDC, CeFe-LSCrFe-GDC, CeCo-LSCrFe-GDC and CeNi-LSCrFe-GDC cathodes operated at 800 °C are shown in Fig. 4(a). The measured open circuit voltages (OCV) for LSCrFe-GDC based cells are 0.885 V in CO2 -CO (90-10) at 800 °C, agreeing well with the theoretical values (0.885 V) predicted by Nernst equation. The current density of the LSCrFe-GDC cell is 0.266 A cm−2 at 1.5 V and 800 °C. However, improved current density is obtained in the CeM-LSCrFe-GDC cells, which can be ascribed to their improved electrocatalytic activity for CO2 reduction through synergetic effect of the CeM catalysts and LSCrFeGDC electrode at high temperatures. CeCo-LSCrFe-GDC cell obtains the best performance with a current density of 0.652 A cm−2 at
1.5 V and 800 °C, followed by CeFe-LSCrFe-GDC and CeNi-LSCrFeGDC cells with the value of 0.603 and 0.535 A cm−2 , respectively, about 2.44, 2.26 and 2.01 times higher than that of the LSCrFe-GDC cell. CO production rates of the cells with four cathodes shown in Fig. 4(b) further confirm the enhanced electrocatalytic activity for CO2 reduction by CeM catalyst. The CO production rate on LSCrFe-GDC cell is about 2.09 mL min−1 cm−2 at 1.6 V. However, the CO production rates are greatly improved after introduction of CeM catalysts. CeCo-LSCrFe-GDC cell reaches a stable and maximum CO production rate of 4.94 mL min−1 cm−2 , followed by CeFe-LSCrFe-GDC (4.31 mL min−1 cm−2 ) and CeNi-LSCrFe-GDC (2.98 mL min−1 cm−2 ) cells at 1.6 V and 800 °C. The improved current densities and CO production rates in the cells using CeMLSCrFe-GDC cathodes demonstrate the high catalytic activity of CeM for CO2 electrolysis. The electrochemical impedance spectra (EIS) and the total polarization resistance of LSCrFe-GDC cell, CeFe-LSCrFe-GDC cell,
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Fig. 4. (a) I-V curves and (b) CO production of LSCrFe-GDC cell, CeFe-LSCrFe-GDC cell, CeCo-LSCrFe-GDC cell and CeNi-LSCrFe-GDC cell operated at 800 °C.
Fig. 5. (a) Impedance spectra at 800 °C and (b) total polarization resistance of LSCrFe-GDC cell, CeFe-LSCrFe-GDC cell, CeCo-LSCrFe-GDC cell and CeNi-LSCrFe-GDC cell.
CeCo-LSCrFe-GDC cell and CeNi-LSCrFe-GDC cell at 800 °C are shown in Fig. 5. The overall ohmic resistances (Rohm ) and total polarization resistance (RP ) can be obtained from the EIS, where the high frequency intercept of impedance spectra on Z’ axis represents the Rohm and the difference between the real axes intercepts on each plot represents the RP . The Rohm values of the cells with CeM-LSCrFe-GDC (M=Co, Fe, Ni) cells are the same (0.49 cm2 ), slightly lower than that on the LSCrFe-GDC cell, suggesting that better interfacial contact between the cathode and YSZ electrolyte is formed after the introduction of CeM catalysts. Moreover, the introduction of CeM catalysts also leads to a large decrease in total polarization resistance, as depicted in Fig. 5(a) and (b). At 800 °C, CeCo-LSCrFe-GDC cell exhibits the smallest RP value of 0.64 cm2 , only 34.4% of the value on LSCrFe-GDC cell (1.90 cm2 ). The RP value of CeFe-LSCrFe-GDC cell and CeNiLSCrFe-GDC cell is 0.89 cm2 and 1.03 cm2 at 800 °C, respectively, also much lower than that of the LSCrFe-GDC cell. Since the same anode is applied in the four cells, the reduction of RP is ascribed to the decreased cathode polarization. Therefore, these results indicate that CeM catalysts take the main responsibility in catalyzing CO2 reduction reaction. The CO2 reduction reaction at SOEC cathodes may generally involve various elementary processes such as CO2 adsorption, charge transfer, dissociation and oxygen incorporation. In order to further understand the electrode processes and the contributions from different elementary electrode processes, the distributions of relaxed times (DRT) [27–29] are calculated from the impedance spectra with an extended equivalent circuit model as shown in Fig 6(a) and (b). Each DRT peak reflects an individual electrode process and the area reflects the corresponding resistance. The DRT spectra of LSCrFe-GDC cell, CeFe-LSCrFe-GDC cell, CeCo-LSCrFeGDC cell and CeNi-LSCrFe-GDC cell at 800 °C are compared in Fig. 6(c). Apparently, six rate-determining steps are depicted in
DRT plots: P1A, P2A and P3A represent three oxygen electrode processes [30,31] while P1C, P2C and P3C represent three cathode processes and each peak is an indication of a sub-step for CO2 reduction. Since the same anode is applied in the four cells, P1A, P2A and P3A remain the same in all cells. The P1C process with a peak frequency at >10 kHz is attributed to the oxygen ions transfer through the cathode/electrolyte interface and incorporation into the YSZ electrolyte [31]. Apparently, the introduction of CeM catalysts mainly affects the P2C and P3C process. It is reported that the electroreduction of CO2 on doped CeO2 precedes a formation of carbonate intermediate and an electron transfer process in accompany with an oxygen incorporation reaction [18]. The areas of P2C are about the same in CeFe-LSCrFe-GDC cell, CeCo-LSCrFe-GDC cell and CeNi-LSCrFe-GDC cell, much lower than that on LSCrFeGDC cell, which may be associated with the processes of CO2 adsorption and carbonate intermediate formation accelerated by the increased oxygen vacancy concentration and TPBs from CeM catalysts. As for the P3C process, it is assigned to the dissociation of carbonate intermediate, in accompany with incorporation of dissociated oxygen ion into oxygen vacancy. P3C exhibits the largest difference in the four cells. A larger P3C peak than P2C peak indicates that dissociation process of carbonate intermediate is the key rate-limiting step on the cells. However, the peak area of P3C is reduced after impregnation of CeM catalysts, reflecting that carbonate intermediate reduction reaction is also significantly accelerated. Notably, CeCo-LSCrFe-GDC cell has the smallest peak area of P3C followed by CeFe-LSCrFe-GDC cell and CeNi-LSCrFe-GDC cell, suggesting that CeCo has the highest catalytic activity towards CO2 reduction. These results indicate that the dissociation of carbonate intermediate at TPBs occurs much easier on CeCo. The activation energy of elementary electrode process is also shown in Fig. 6(d). The activation energies for P3C process are 96.69 kJ mol−1 , 124.21 kJ mol−1 and 144.66 kJ mol−1 on
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4. Conclusions In summary, we modified the LSCrFe-GDC electrode with Ce0.9 M0.1 O2 − δ (M=Fe, Co, Ni) by impregnation method. TPR and TPD results indicate that both redox activity and surface oxygen exchange kinetic are enhanced by the introduction of transition metal ions. All CeM samples show a CO desorption peak after CO2 adsorption, reflecting their good catalytic activity for CO2 reduction. CeCo-LSCrFe-GDC cell obtains the best performance with a current density of 0.652 A cm−2 at 1.5 V and 800 °C, followed by CeFe-LSCrFe-GDC and CeNi-LSCrFe-GDC cells with the value of 0.603 and 0.535 A cm−2 , respectively, about 2.44, 2.26 and 2.01 times higher than that of the cell with the LSCrFe-GDC cathode. EIS combined with DRT analysis shows that both CO2 adsorption and the carbonate intermediate dissociation at TPBs are accelerated by CeM catalysts, while the latter is the key rate-determining step. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 91534128, 21506208 and 21476230), the Ministry of Science and Technology of China (Grants 2016YFE0118300) and the DNL Cooperation Fund, CAS (DNL180306). References
Fig. 6. (a) Extended equivalent circuit, (b) fitting result for CeCo-LSCrFe-GDC cell, and (c) DRT spectra for all cells at 800 °C. (d) Arrhenius plots of ln(1/RP ) versus temperature on the cells for P3C process.
CeCo-LSCrFe-GDC, CeFe-LSCrFe-GDC and CeNi-LSCrFe-GDC cells, respectively, much lower than that on LSCrFe-GDC cell (152.7 kJ mol−1 ). The lowest activation energy of P3C on CeCoLSCrFe-GDC cell also suggests that CeCo exhibits the best electrochemical and catalytic properties towards CO2 electrolysis. CeM oxides with high oxygen vacancy concentration have the potential to capture CO2 molecule through chemical adsorption at these defect sites. The TPBs, involved active transition metal ions and Ce ions, takes the responsibility for CO2 reduction. Possibly for this reason, the cathode performance can be enhanced by introduction of CeM oxides. Moreover, the best performance with CeCo for CO2 reduction may be attributed to the higher reducibility and surface oxygen exchange kinetic of CeCo which can lead to a faster charge transfer reaction and oxygen incorporation. Therefore, high oxygen vacancy concentration, active transition metal ions and Ce ions as well as excellent surface oxygen exchange kinetic of CeM oxides play important roles in the enhanced cathode process of P2C and P3C, resulting in better cathode performance for CO2 electrolysis.
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