Ceria Cermet Research Articles

Electrochemical behaviors of copper/manganese-doped ceria cermet as a fuel electrode for high-temperature solid oxide cells

Electrochemical behaviors of copper/manganese-doped ceria cermet as a fuel electrode for high-temperature solid oxide cells

Polarization Resistance of Ceria-Containing Fuel Electrodes in Solid Oxide Cells Studied By Impedance and DRT Analysis

Solid oxide electrolysis cells (SOECs) can be used to perform steam electrolysis in a process which is the reverse of power generation using a solid oxide fuel cell (SOFCs). They are capable of highly efficient hydrogen production. In this study, various model fuel electrode materials were compared and evaluated over a wide range of current densities in both SOFC and SOEC modes. Here, we prepared three types of cells: (i) with a conventional Ni-scandia-stabilized-zirconia (Ni-ScSZ) cermet fuel electrode, (ii) with a Ni-gadolinia-doped ceria (Gd0.1Ce0.9O2, Ni-GDC) cermet fuel electrode, and (iii) with a Ni-GDC co-impregnated fuel electrode fabricated by the co-impregnation method. The electrode reactions are characterized through measurements of electrochemical impedance spectra (EIS) and subsequent analysis of distribution of relaxation times (DRT). The results suggest that the use of mixed ionic and electronic conductor GDC as a fuel electrode material is advantageous especially in SOEC operation mode.

Pt-SDC alloy anode for methanol fueled low temperature solid oxide fuel cell

Direct methanol-fueled low-temperature solid oxide fuel cells (DM LT-SOFCs) operating temperature ≤ 500 ℃ are promising candidates as portable power sources owing to the high energy density and portability of methanol. Herein, we have systematically studied the Pt-samaria-doped ceria (SDC) cermet alloy anodes with varying Pt: SDC ratios to achieve high methanol oxidation reaction activity (MOR) and resistance to CO poisoning. It is observed that the optimal composition of the Pt-SDC alloy anode is Pt0.83SDC0.17, which exhibited lower activation resistance by 63 %, better tolerance to CO poisoning by 44 %, and higher thermal stability than that of pure Pt upon direct methanol operation at 450 ℃. Such improvements were ascribed to high Pt-SDC interfacial density of reaction sites with less CO poisoning due to oxygen spill over from SDC, while thermally stable SDC also helped preserve the morphology of the Pt anode at elevated temperatures.

루테늄과 사마리아 도핑된 세리아의 서멧 구조 스퍼터 박막의 조성 비율 변화에 따른 스크래치 특성

Interest in the use of thin film of Ruthenium-Samaria doped ceria cermet (Ru-SDC) as anode in solid oxide fuel cells is increasing due to its high oxygen storage capacity and high chemical and thermal stability. To have enough structural integrity between sputtered Ru-SDC films and underlying substrates, good adhesion property is required. In this work, scratch resistance and failure mode for Ru-SDC films with various SDC composition were investigated using a scratch test method employing linearly increasing load from 1 to 50 N using a 200 μm radius Rockwell C indenter. Scratched surfaces were examined with a field emission scanning electron microscope. Chemical compositions in scratch tracks were analyzed by energy dispersive X-Ray spectroscopy. Critical loads for films with different SDC ratios were assessed and associated failure modes were identified. The highest scratch resistance among tested film compositions was the one that contained 50% of SDC. Failure modes of tested films regardless of the ratio of SDC were identified to be the initiation of tensile cracks with rapid increase of friction coefficient followed by chipping, and eventually the generation of a severe crack network.

How the surface state of nickel/gadolinium-doped ceria cathodes influences the electrochemical performance in direct CO2 electrolysis

The effect of the surface state on the electrochemical performance of nickel gadolinium-doped ceria (NiGDC) cermet electrodes in direct CO2 electrolysis was studied by operando near ambient pressure X-ray photoelectron spectroscopy combined with on-line gas phase and electrical measurements. The CO2 electrolysis was limited at overpotentials below the carbon deposition threshold to avoid irreversible cathode deactivation. The results revealed the dynamic evolution of the NiGDC electrode surface and disclose the side reactions associated to electrode activation in CO2 electrolysis. Comparison of reduced and oxidized electrodes shows that metallic Ni is a prerequisite for CO2 electrolysis, at least at low potentials, suggesting that CO2 electoreduction occurs primarily at the three phase boundaries between gas, metallic nickel and partially reduced ceria. We also provide evidences of in situ reduction of NiO upon polarization in CO2, implying that addition of reductive gases to CO2 is not indispensable to maintain the cermet electrode in the reduced state. Inspired by this observation, we use a conventional button cell setup to demonstrate improved i-V characteristics of NiGDC electrodes in direct CO2 electrolysis as compared to CO2/H2 fuel conditions and we rationalize this behavior based on NAP-XPS results.

Effect of open pore and pore interconnectivity in the Ni-SDC cermet anode microstructure on the performance of solid oxide fuel cells

In nickel-samarium-doped ceria (Ni-SDC) cermet anode layers, the open pores and interconnected pores in the microstructure are the main factors that affect the mechanical and electrical properties. In this work, porous Ni-SDC cermet anode layers are fabricated using various quantities of potato starch (0 to 25 wt.%) as a pore forming in the anode powders. The properties of the Ni-SDC cermet anode layers were characterised by FESEM-BSE microscopy, Archimedes method for density measurement, Vickers hardness, flexural strength, and DC four-point electrical conductivity. The findings revealed that the different content of potato starch greatly affected the percentage of porosity and pore interconnectivity in the microstructure and consequently altered the mechanical and electrical properties of the Ni-SDC cermet anode. The degree of shrinkage, relative density, mechanical strength and electrical conductivity of the Ni-SDC cermet anodes decreased as their pore former content increased. Furthermore, the research shows that the large porosity (> 40%) in the Ni-SDC cermet anode microstructure affected the continuity of Ni-Ni, SDC and Ni-SDC phases and thereby affected the mechanical and electrical properties. The Ni-SDC cermet anode with 10 wt.% exhibited sufficient porosity, Vickers hardness, flexural strength and electrical conductivity of 34%, 48 MPa, 72 MPa and 2028 S/cm (at 800 °C), respectively. Therefore, optimisation of porosity in the Ni-SDC cermet anode microstructure strongly contributes to the well-connected pore channels for the rapid diffusion of hydrogen for oxidation and mechanical strength.

Depth-direction analysis of nickel depletion in a Ni–gadolinia-doped ceria hydrogen electrode after steam electrolysis operation

We have examined the initial-stage degradation of a Ni–gadolinia-doped ceria (GDC) cermet hydrogen electrode prepared on a GDC buffer layer of a stabilized-zirconia electrolyte supported cell during steam electrolysis operation at 800 °C. With use of an air reference electrode, the IR-free electrode potential E and ohmic resistance of the hydrogen electrode side (RH2-side) were recorded. After a steam electrolysis operation at −1.0 A cm−2 for 211 h, the RH2-side increased appreciably, by 36 %, while the value of E was nearly unchanged. It was found via a depth-direction analysis with use of focused ion beam-scanning ion microscopy that the remaining percentage of Ni decreased to ca. 60 % in the layer between 1 and 3 µm from the top of the GDC buffer layer, followed by a further decrease to 42 % at 0.5 µm. Since this suggests a significant cathodic polarization within the thin reaction zone, an enlargement of the reaction zone, together with a stabilization of Ni contacting with GDC, could be essential to mitigate such a degradation.

Hydrogen Oxidation Pathway Over Ni-Ceria Electrode: Combined Study of DFT and Experiment.

Ni–ceria cermets are potential anodes for intermediate-temperature solid oxide fuel cells, thanks to the catalytic activity and mixed conductivities of ceria-based materials associated with the variable valence states of cerium. However, the anodic reaction mechanism in the Ni–ceria systems needs to be further revealed. Via density functional theory with strong correlated correction method, this work gains insight into reaction pathways of hydrogen oxidation on a model system of Ni10-CeO2(111). The calculation shows that electrons tend to be transferred from Ni10 cluster to cerium surface, creating surface oxygen vacancies. Six pathways are proposed considering different adsorption sites, and the interface pathway proceeding with hydrogen spillover is found to be the prevailing process, which includes a high adsorption energy of −1.859 eV and an energy barrier of 0.885 eV. The density functional theory (DFT) calculation results are verified through experimental measurements including electrical conductivity relaxation and temperature programmed desorption. The contribution of interface reaction to the total hydrogen oxidation reaction reaches up to 98%, and the formation of Ni–ceria interface by infiltrating Ni to porous ceria improves the electrochemical activity by 72% at 800°C.

(Invited) Ruthenium/Samaria-Doped Ceria Cermet Anode through Plasma Enhanced Atomic Layer Deposition

Direct methane-fueled solid oxide fuel cells (DM-SOFCs) operating at low temperature range (<450 C) are promising next-generation energy conversion devices, in which, however, anode reactions should be facilitated for high performance even at low temperatures. Obtaining catalyst with high activity, thermal stability and carbon coking resistance is essential for high-performance SOFC operating with hydrocarbon fuel. Noble metal catalyst, e.g., Ru, can be effective for direct methane oxidation in DM-SOFC anode. However, their high costs prevent them from being extensively applied to such devices as SOFCs, and, therefore, significant efforts have been made to lower the noble metal loading in designing fuel cell electrodes.Herein, we report the design and fabrication of heterogeneous catalyst with ultra-low-loading Ru nanostructures via plasma enhanced atomic layer deposition (PEALD) on SDC backbone anode. We confirmed the presence of highly dispersed PEALD Ru particles over the SDC backbone compared to sputtered Ru deposited on only porous sputtered SDC surface. We found out that PEALD Ru of 100 ALD cycles, or approximately 5nm in thickness with Ru loading of only 0.004 mg/cm2, was sufficient for the use of anode catalyst with methane fuel. Furthermore, compared to the cell with 100 nm-thick sputtered Ru/sputtered SDC anode, the cell with PEALD Ru/sputtered SDC anode showed 45 % higher peak power density with only 1/20 of Ru loading. Moreover, after 4 hours operation with dry methane at 450 C, the activation resistance of the cell with the sputtered Ru composite anode increased by 7 times, while that of the cell with the PEALD Ru composite anode increased by only 2 times. This result suggests that, using PEALD, one can simultaneously reduce Ru loading, thereby reducing cost, and improve the performance and the thermal stability of the anode for DM-SOFC.

Ultrathin sputtered platinum–gadolinium doped ceria cathodic interlayer for enhanced performance of low temperature solid oxide fuel cells

Improvement of the sluggish oxygen reduction reaction (ORR) is the key to lower the operating temperature of conventional solid oxide fuel cells (SOFCs). Developing a novel nanostructure in the cathodic catalyst layer is one of the efficient ways to reduce the operating temperature while improving fuel cell performance. In this paper, all components of low-temperature SOFCs were prepared on a nanoporous substrate by the sputtering method. For the performance enhancement at low temperature, an ultrathin Pt-Gadolinium Doped Ceria (GDC) cermet interlayer was deposited on the cathode side of electrolyte and demonstrated. A significant improvement in electrochemical performance was observed in the Pt-GDC interlayer fuel cell compared to a reference cell. The peak power density of Pt-GDC cathodic cermet interlayer cell was 334 mW/cm2 at 500 °C, which was 42.7% higher than that of the reference cell. Through additional analysis, we confirmed that the observed performance enhancement was attributed to the ultrathin cathodic cermet interlayer, which increases the triple phase boundary and enhances the reaction kinetics for ORR.

Improving fuel cell performance via optimal parameters identification through fuzzy logic based-modeling and optimization

Improving the performance of solid oxide fuel cell via maximizing its available peak power density is a key requirement of research in the field of renewable energy. This could be achieved through identifying the optimal controlling parameters such as, the deposition instrument power (P), the temperature (T), and the electrolyte thickness (Thick). Nickel–Gadolinium Doped Ceria cermet anode films are deposited on one side of the Zirconia electrolyte by radio frequency sputtering. The sputtering plasma power is varied at 50, 100, 150, and 200 W. Lanthanum Strontium Manganite cathodes were screen-printed on the other side of the electrolyte supports to complete the configuration. Cells were electrochemically tested at various intermediate solid oxide fuel cell temperatures of 600, 700 and 800 °C using different electrolyte thicknesses. The cell’s current density, I (A/cm2) and voltage (V) and hence the power density (W/cm2) are recorded in each case. Based on the obtained experimental results, a fuzzy model is built using different control parameters. Then, the particle swarm optimization technique is used for obtaining the best parameters of the cell that maximizes its power density. The results show that by utilizing the optimized conditions, the power density can be increased to 0.39 (W/cm2), which is almost two times higher than the maximum power density obtained experimentally.

The Relation of Microstructure, Materials Properties and Impedance of SOFC Electrodes: A Case Study of Ni/GDC Anodes

Detailed insight into electrochemical reaction mechanisms and rate limiting steps is crucial for targeted optimization of solid oxide fuel cell (SOFC) electrodes, especially for new materials and processing techniques, such as Ni/Gd-doped ceria (GDC) cermet anodes in metal-supported cells. Here, we present a comprehensive model that describes the impedance of porous cermet electrodes according to a transmission line circuit. We exemplify the validity of the model on electrolyte-supported symmetrical model cells with two equal Ni/Ce0.9Gd0.1O1.95-δ anodes. These anodes exhibit a remarkably low polarization resistance of less than 0.1 Ωcm2 at 750 °C and OCV, and metal-supported cells with equally prepared anodes achieve excellent power density of &gt;2 W/cm2 at 700 °C. With the transmission line impedance model, it is possible to separate and quantify the individual contributions to the polarization resistance, such as oxygen ion transport across the YSZ-GDC interface, ionic conductivity within the porous anode, oxygen exchange at the GDC surface and gas phase diffusion. Furthermore, we show that the fitted parameters consistently scale with variation of electrode geometry, temperature and atmosphere. Since the fitted parameters are representative for materials properties, we can also relate our results to model studies on the ion conductivity, oxygen stoichiometry and surface catalytic properties of Gd-doped ceria and obtain very good quantitative agreement. With this detailed insight into reaction mechanisms, we can explain the excellent performance of the anode as a combination of materials properties of GDC and the unusual microstructure that is a consequence of the reductive sintering procedure, which is required for anodes in metal-supported cells.

Steam effect on Gerischer impedance response of a Ni/GDC|YSZ|LSM fuel cell / anode

The electrochemical impedance response of an electrolyte supported Solid Oxide Fuel Cell (SOFC) was studied for different compositions of feed in the anode compartment. The cell was composed of Yttria Stabilized Zirconia (YSZ), as electrolyte, and commercial Nickel/Gadolinium doped Ceria (Ni/GDC) cermet and Lanthanum Strontium Manganite (LSM), as anode and cathode electrodes, respectively. The impedance responses of the anode and cathode were deconvoluted using the analysis method of differences in the impedance spectra (ADIS). This allowed focusing on the electrochemical processes, which occur at the anode electrode and analyzing them adequately via equivalent electric circuit modelling. The main finding is that the Ni/GDC anode electrode exhibits a Gerischer impedance response, characteristic of chemical and electrochemical competing processes, which was associated with the formation of OHad− on GDC oxygen vacancies as the prevailing intermediate species for the electro-oxidation of H2 on Ni/GDC anode. It is shown that H2O plays a dominant role for the formation of the OHad− species on the GDC surface through its reaction with O2−, thus contributing autocatalytically on the promotion of H2 electro-oxidation on Ni/GDC anode.

Enhancement of anode activity and stability by Cr addition at Ni/Sm-doped CeO2 cermet anodes in NH3-fueled solid oxide fuel cells

Chromium was added to nickel/Sm2O3-doped ceria (Ni/SDC) cermets by a CrCl3 impregnation method, and the activity of the resulting Ni-Cr/SDC cermets was evaluated as anodes in ammonia-fueled solid oxide fuel cells (SOFCs) in the temperature range of 773 to 1173 K. It has been revealed that the addition of Cr was effective for the catalytic activity for NH3 reforming, followed by hydrogen oxidation. The optimum amount of Cr was 3 at.% of the total metal in the Ni-Cr/SDC cermets. In addition, we demonstrated the improved stability of the Ni97-Cr3/SDC cermets in NH3 atmosphere at 873 K. The addition of Cr suppressed the agglomeration of Ni, and promoted the anode durability. XRD analysis suggested the formation of a nitride CrN during operation in the Ni97–Cr3/SDC anode, and the role of the nitride was discussed in detail.

Simultaneous Function of Gd2O3 As Solid Dopant and Sintering Aid for Anode Support Type Solid Oxide Fuel Cells

For anode support-type solid oxide fuel cell (SOFC), the mechanical strength of anode layer is important factor for commercialization and scale-up. The high sinterability of anode support increase the strength of the cell. NiO/Gd-doped ceria(GDC) cermet is commonly used material for anode support. However, in this study NiO/undoped-ceria(CeO2) was employed as starting material due to the purpose of adopting Gd2O3 as a sintering aid and a dopant. Gd2O3 was uniformly formed on the surface of NiO/CeO2 anode particles. During the sintering procedure, the Gd-ion migrate from Gd2O3 which was formed on the pre-sintered anode support to CeO2 due to similar atomic radius between Gd and Ce. In addition, sinterability of anode support was improved since Gd2O3 functioned as sintering aids. Finally, open-circuit voltage (OCV) was enhanced by less electrical loss and gas separation in soild electrolyte.

Evaluation of the Influence of Gadolinium Doped Ceria Particle Size on the Electrochemical Performance and Microstructure of Nickel-Gadolinium Doped Ceria Anodes

Last years, composite anodes consisted of nickel-gadolinium doped ceria (Ni-GDC) cermet gained a lot of attention as they have potential to be a beneficial solution for the intermediate temperature solid oxide fuel cells. In the range of temperatures below 700oC, a GDC cermet is characterized by a higher ionic conductivity compared to conventionally used yttria-stabilized zirconia (YSZ) cermet. Moreover, with the use of GDC as the mixed ionic-electronic conductivities (MIEC) material, electrochemical reaction sites are not restricted to triple phase boundaries (TPBs), but also extended to the double phase boundaries (DPBs) which consist of GDC/pore interfaces. Therefore, microstructure of electrodes has the crucial effect for the performance of the SOFCs and the evaluation of its changes are required to predict the deterioration of cell performances. Moreover, the precise mechanism behind observed microstructural reaction kinetics has not been fully revealed yet. For the further understanding of the electrochemical reaction kinetics, evaluation of the influence of TPB and DPB reaction for the overall electrochemical performance of MIEC anodes is required. In this paper, the influence of the initial microstructure of Ni-GDC anode on the electrochemical performance and its deterioration after 100-h operation are investigated using electrolyte supported cells. The initial microstructure of anode is determined by the different aspect ratios and sizes of GDC powder used in the fabrication of cells. Four types of SOFC anodes were fabricated with the initial composition of NiO:GDC=60:40vol%. The microstructural parameters of the fabricated anodes have been investigated by the focused-ion-bean – scanning electron microscopy tomography and 3D reconstruction of the representative structures. The parameters such as TPB length, surface area, porosity, tortuosity factor and connectivity have been quantitatively evaluated. From the observed microstructural changes and cell performances, contribution of the Ni-GDC microstructure and electrochemical reaction kinetics on anode performance are discussed.

Evaluation of the Influence of Gadolinium Doped Ceria Particle Size on the Electrochemical Performance and Microstructure of Nickel-Gadolinium Doped Ceria Anodes

Composite anodes consisted of nickel-gadolinium doped ceria (Ni-GDC) cermet increasingly gain a lot of attention as they have potential to be a beneficial solution for the intermediate temperature solid oxide fuel cells. In this paper, the influence of the initial microstructure of Ni-GDC anode on the electrochemical performance and its deterioration in 100-h operation are investigated. The initial microstructure of investigated anodes is determined by the different aspect ratios and sizes of GDC powder used in the fabrication, but all anodes had the same initial chemical compositions of NiO:GDC=60:40 vol%. The microstructure of the fabricated anodes has been investigated by focused ion beam - scanning electron microscopy tomography (FIB-SEM) and quantitatively evaluated to deliver the factors expressing the microstructural characteristics, especially important ones for the electrochemical performance: Triple-Phase-Boundary (TPB) density, Double-Phase-Boundary (DPB) density and tortuosity factor. From the experimentation, contribution of the Ni-GDC microstructure to the anode electrochemical performance is discussed.

Effect of Infiltrated Transition Metals in Ni/Sm-Doped CeO2 Cermets Anode in Direct NH3-Fueled SOFCs

The activity of transition metals (Cr and V) infiltrated Ni/Sm-doped ceria (SDC) cermets anodes for NH3 oxidation was investigated at temperature of 973 to 1173 K. The 3 at.% addition of Cr and V with respect to the total metal composition improved the anode activity for NH3 oxidation. The relationship between the anode activity of the Ni-based binary anode and the metal-nitrogen binding energy of the added transition metal, which is a key to realize the highly active anode for direct NH3-fueled SOFCs, is discussed.

Comparative study on ammonia oxidation over Ni-based cermet anodes for solid oxide fuel cells

In the current work, we investigate the performance of solid oxide fuel cells (SOFCs) with Ni‒yttria-stabilized zirconia (Ni–YSZ) and Ni‒gadolinia-dope ceria (Ni–GDC) cermet anodes fueled with H2 or NH3 in terms of the catalytic activity of ammonia decomposition. The cermet of Ni–GDC shows higher catalytic activity for ammonia decomposition than Ni–YSZ. In response to this, the performance of direct NH3–fueled SOFC improved by using Ni–GDC anode. Moreover, we observe further enhancement in the cell performance and the catalytic activity for ammonia decomposition with applying Ni−GDC anode synthesised by the glycine–nitrate combustion process. These results reveal that the high performance of Ni–GDC anode for the direct NH3–fueled SOFC results from its mixed ionic–electronic conductivity as well as high catalytic activity for ammonia decomposition.

Enhancement of anode activity at Ni/Sm-doped CeO2 cermet anodes by Mo addition in NH3-fueled solid oxide fuel cells

Molybdenum was added to nickel/Sm2O3-doped ceria (Ni/SDC) cermets by a MoCl5 impregnation method, and the activity of the resulting Ni–Mo/SDC cermets was evaluated as anodes in ammonia-fueled solid oxide fuel cells (SOFCs) in the temperature range of 973–1173K. It has been revealed that the addition of Mo improved remarkably the catalytic activity for NH3 reforming and enhanced the cell performance. The Ni–Mo/SDC anode with a Mo content of ~3 atomic % of the total metal showed the highest catalytic activity for NH3 reforming. XRD analysis of a NiO–MoO3 powder mixture annealed in NH3 revealed the formation of a nitride NixMo1–xN during operation of Ni–Mo/SDC anodes, and the role of the nitride in NH3 reforming was discussed in detail.