Tuning ORR electrocatalytic functionalities in CGFO-GDC composite cathode for low-temperature solid oxide fuel cells

Mixed ionic and electronic conduction (MIEC) in the composite cathode can alter oxygen stoichiometry and other physiochemical properties, eventually promoting the electrocatalytic functionalities for oxygen reduction reaction (ORR) at low operational temperatures (<650 °C). Here, we demonstrate a composite cathode of CoGd_0.8Fe_1.80O₄ /Gd_0.10Ce_0.9O_2?δ (CGFO-GDC), which delivers low electrode polarization resistance of 0.60 Ω cm² at 550 °C. The best-performing sample CGFO-GDC exhbits the peak power density (PPD) of 611-343 mW cm^?2 at 550-470 °C under a fuel cell conditions. Moreover, durability measurement verifies CGFO-GDC as a chemically stable cathode with improved ORR catalytic functionality. Additionally, first principle calculations using density function theory (DFT) were also conducted to analyze the ion diffusion mechanism of fabricated CGFO-GDC cathode. Our findings certify that introducing ionic conducting GDC into CGFO sample improves the catalytic functionalities. As a result, the composite CGFO-GDC based SOFC delivers minimum electrode polarization resistance with improved power output owing to its enhanced oxygen vacancies and fast catalytic reactions at 550 °C.     

Polarization effects in electrolyte/electrode-supported solid oxide fuel cells

The effects of activation, ohmic and concentration polarization on the overall polarization in solid oxide fuel cells are presented. A complete analysis was conducted based on thermodynamic principles for the calculation of cell voltage. Treating the fuel cell as a control volume, the irreversibility term in a steady flow thermodynamic system was related to the overall polarization. The entropy production was calculated and related to the lost work of the fuel cell, while the heat loss from the cell was determined from the entropy balance. To generalize the cell voltage–current density expression, the Butler–Volmer model was used in the calculation of activation polarization and both ordinary and Knudsen diffusions were considered in the calculation of concentration polarization. The overall cell resistance was deduced from the generalized cell voltage–current density expression. The concentration resistance at the anode can be minimized by humidifying the hydrogen with an appropriate amount of water, depending on the thickness of the anode used. Comparison of polarization effects on the cell performance between the electrolyte-supported and anode-supported cells showed that the latter would give a better cell performance.     

A medium entropy cathode with enhanced chromium resistance for solid oxide fuel cells

Perovskite type SrCo_0.9Ta_0.1O_3-δ (SCT91) cathode exhibits high activity for oxygen reduction reactions, but the instability in Cr-containing atmosphere restricts its application in intermediate temperature solid oxide fuel cells (IT-SOFCs). In this study, a B-site medium-entropy SrCo_0.5Fe_0.2Ti_0.1Ta_0.1Nb_0.1O_3-δ (SCFTTN52111) cathode is proposed and investigated as a potential Cr-tolerance cathode. The electrochemical activity of pristine SCT91 cathode degrades rapidly in the presence of volatile chromium species. In contrast, SCFTTN52111 performs very stable. Chromium vapors prefer to react with segregated SrO species rather than Co₃O₄ precipitates_. Significant secondary phases of SrCrO₄ and Co₃O₄ are detected on SCT91 electrode, while only trace by-products are found on medium-entropy cathode. The better Cr tolerance is closely related to the enhanced structural stability by medium-entropy engineering and reduced surface Sr segregations. This work sheds light on the development of robust cathodes for IT-SOFCs through rational design of configuration entropy.     

Effect of cathode fabrication method on characteristics of anode-supported tubular solid oxide fuel cells

We have adopted three different methods: dip-coating, brush pen painting, and ion-impregnating, to fabricate cathodes for anode-supported tubular solid oxide fuel cells; and studied the performances of the cells using cathodes fabricated by these three different methods. The cell with ion-impregnated cathode presented the best electrochemical performances in these three cells, and it generated a maximum power density of 446 mW cm⁻² at 850 °C, when operating with humidified hydrogen. The cells with dip-coated cathode and brush pen painted cathode produced acceptable electrochemical performances; they generated maximum power densities of 403 and 405 mW cm⁻² at 850 °C, respectively, when running on humidified hydrogen; also, they represented more stable, much easier processes and lower cost.     

Optimization of laser-patterned YSZ-LSM composite cathode-electrolyte interfaces for solid oxide fuel cells

Patterned cathode/electrolyte interfaces formed by a hexagonal array of ～22 μm deep wells with 24 μm lattice parameter have been prepared by pulsed laser machining to enlarge the contact surface and, consequently, to reduce the cathode polarization of Solid Oxide Fuel Cells. These new interfaces have been tested in YSZ-LSM/YSZ/YSZ-LSM symmetrical cells, where the cathode is deposited by dip-coating. Appropriate ceramic suspensions have been formulated to penetrate into deep wells without presenting interfacial delamination after sintering. We analyse their applicability by comparing their rheology with the microstructure and electrochemical performance of the cells. The activation component of the polarization resistance is reduced by ～50% using ethanol-based suspensions with 20 wt% solids loading, although the gas diffusion component increases due to excessive densification. Alternative ceramic suspensions with 17.5 wt% solids loading provide optimum electrode gas diffusion but lower activation components, resulting in an overall decrease of ～20% in polarization resistance.     

Development of LSM/YSZ composite cathode for anode-supported solid oxide fuel cells

This paper presents the effect of (La,Sr)MnO₃ (LSM) stoichiometry on the polarization behaviour of LSM/Y₂O₃-ZrO₂ (YSZ) composite cathodes. The composite cathode made of A-site deficient (La_0.85Sr_0.15)_0.9MnO₃ (LSM-B) showed much lower electrode interfacial resistance and overpotential losses than that made of stoichiometric (La_0.85Sr_0.15)_1.0MnO₃ (LSM-A). The much poorer performance of the latter is believed to be due to the formation of resistive substances such as La₂Zr₂O₇/SrZrO₃ between LSM and YSZ phases in the composite electrode. A slight A-site deficiency (∼0.1) was effective in inhibiting the formation of these resistive substances. A power density of ∼1 W cm⁻² at 800 °C was achieved with an anode-supported cell using an LSM-B/YSZ composite cathode. In addition, the effects of cathodic current treatment and electrolyte surface grinding on the performance of composite cathodes were also studied.     

Nano-structured composite cathodes for intermediate-temperature solid oxide fuel cells via an infiltration/impregnation technique

Solid oxide fuel cells (SOFCs) are high temperature energy conversion devices working efficiently and environmental friendly. SOFC requires a functional cathode with high electrocatalytic activity for the electrochemical reduction of oxygen. The electrode is often fabricated at high temperature to achieve good bonding between the electrode and electrolyte. The high temperature not only limits material choice but also results in coarse particles with low electrocatalytic activity. Nano-structured electrodes fabricated at low temperature by an infiltration/impregnation technique have shown many advantages including superior activity and wider range of material choices. The impregnation technique involves depositing nanoparticle catalysts into a pre-sintered electrode backbone. Two basic types of nano-structures are developed since the electrode is usually a composite consists of an electrolyte and an electrocatalyst. One is infiltrating electronically conducting nano-catalyst into a single phase ionic conducting backbone, while the other is infiltrating ionically conducting nanoparticles into a single phase electronically conducting backbone. In addition, nanoparticles of the electrocatalyst, electrolyte and other oxides have also been infiltrated into mixed conducting backbones. These nano-structured cathodes are reviewed here regarding the preparation methods, their electrochemical performance, and stability upon thermal cycling.     

Sr and Fe co-doped Ba2In2O5 as a new proton-conductor-derived cathode for proton-conducting solid oxide fuel cells

BaSrInFeO₅ (BSIF), a new cathode material for proton-conducting solid oxide fuel cells (SOFCs), is designed based on the modification of the Ba₂In₂O₅ proton conductor with Sr and Fe cations. Compared with the Ba₂In₂O₅ proton conductor tailored with only Fe cations (Ba₂InFeO₅, BIF), doping Sr can improve the chemical stability and also benefit the formation of oxygen vacancies. The proton mobility is also improved with Sr-doping, which is confirmed by first-principles calculations and experimental studies. An H-SOFC using the BSIF cathode generates a relatively high peak power density of 1192 mW cm^-2 at 700 ^oC, which is superior to many cells in previous reports. First-principles calculations find that the cathode oxygen reduction reaction (ORR) energy barrier for BSIF is significantly lower than that for BIF. Although Ba₂In₂O₅ is less studied, the derived cathode materials can still present decent performance, probably offering new material selections for H-SOFCs.     

Electrospun composite nanofibers for intermediate-temperature solid oxide fuel cell electrodes

Considering the dominant loss associated with surface oxygen exchange reactions among other complex electrochemical processes, the design of an electrode structure for the reasonable operation of solid oxide fuel cells is challenging. The surface oxygen exchange reaction can be considerably facilitated using composite nanofibers containing the electrode, electrolyte, and catalyst. The composite nanofiber electrodes containing Pd show the smallest polarization resistance of 0.031 Ωcm² and the maximum power density of 0.7 W/cm² at 650 °C, which are 39.2% and 12.5% improved values compare to the catalyst-free composite nanofiber electrodes, respectively. These results provide an facile fabrication strategy for developing high-performance electrodes for use in solid oxide fuel cells.     

Novel CO2-tolerant Co-based double perovskite cathode for intermediate temperature solid oxide fuel cells

For the commercial application of solid oxide fuel cells (SOFCs), CO₂-tolerant cathode materials with high electrochemical activity are required. Here, we discuss the performance of double perovskite Pr_0.2Sr_1.8CoTiO_6?δ (P02STC) as a potential cathode material for SOFCs. P02STC has a cubic structure and keeps lattice structure stable in the CO₂ atmosphere. The average thermal expansion coefficient is 17.8 × 10^–6 K^–1 at 30–900 °C in air. The P02STC cathode exhibits good electrochemical performance with a low polarization resistance of 0.080 Ωcm² at 700 °C. The P02STC cathode shows good structure stability, electrochemical performance stability, and excellent tolerance to CO₂ poisoning for the symmetrical cells based on the 350 h stability test in air and the 150 h stability test in O₂ containing 5%CO₂ at 700 °C. The electrolyte-supported single cell with a P02STC cathode shows a maximum power density of 677 mW cm^? 2 at 800 °C. The single cell operates stably for 250 h at a constant current of 0.3 A/cm² without obvious degrading performance. According to all of the experimental results, the P02STC sample might be a promising candidate cathode for SOFCs.     

Effect of NiO/YSZ cathode support pore structure on CO2 electrolysis via solid oxide electrolysis cells

Gas diffusion within supporting cathodes of solid oxide electrolysis cells (SOECs) plays an important role in CO₂ electrolysis process. This study has investigated the effect of cathode pore structure on gas diffusion during CO₂ electrolysis. The cathode pore structure was adjusted by applying the different amounts of pore former during cathode preparation. The more pore former added produced the higher porosity of cathode and the higher limiting current density. High limiting current densities are beneficial to diminish or even eliminate gas diffusion limitation in practical applications, where the electrolysis is expected to be operated at low CO₂ concentrations to increase CO₂ conversion. An advanced impedance spectroscopy study is performed to confirm the limiting current density measured according to current-voltage curves. It was revealed that CO₂ electrolysis performance is greatly affected by gas diffusion, which is determined by the employed cathode pore structure.     

Investigation on cathode degradation of direct methanol fuel cell

The cathode degradation of a direct methanol fuel cell (DMFC) was investigated after a 240 h discontinuous galvostatic operation at 80 °C. The catalyst coated membrane (CCM) and the cathode diffusion layer were not combined so as to isolate electrochemical and mass transport processes. It was indicated by the EDS and SEM tests that the loss of the cathode electrochemical surface area (ESA) was associated with the decays of the Pt/C catalyst and the interfacial contact. Furthermore, Ru crossover and higher methanol crossover resulting from the anode failure aggravated the degradation of the cathode. On the other hand, the change of the pore structure led to a higher wettability of the cathode microporous layer. Therefore, the oxygen transport was suppressed due to the decrease of hydrophobic passages.     

Crack formation in ceramic films used in solid oxide fuel cells

The manufacture of solid oxide fuel cells (SOFCs) involves fabrication of a multilayer ceramic structure, for which constrained sintering is a key processing step in many cases. Defects are often observed in the sintered structure, but their formation during sintering is not well understood. In this work, various ceramic films were fabricated by screen printing and a variety of defects observed. Some films showed “mud-cracking” defects, whereas others presented distributed large pores. “Mud cracking” defects were found to originate from a network of fine cracks present in the green film and formed during drying and binder burn-out. Control of these early stages is essential for producing crack-free films. In order to investigate how defects evolve during sintering, artificial cracks were introduced in the green films using indentation. It was observed that crack opening always increased during constrained sintering. In contrast, similar initial cracks could be closed and healed during co-sintering.     

Numerical modeling of solid oxide fuel cells

This paper presents a numerical model for a planar solid oxide fuel cell (SOFC) with mixed ionic-electronic conducting electrodes. Transport of positive or negative charges, which takes place in the direction of down- or up-gradient electric potential, respectively, within the composite electrodes and through the electrolyte membrane, is mimicked by making use of an algorithm for Fickian diffusion in the commercial software. The output cell voltage, which is the potential difference between the two current collectors, is fixed at a given value. The coupled equations describing the conservation of mass, momentum and energy and the chemical and electrochemical processes are solved using the commercial package Star-CD, augmented with subroutines developed in-house. Results for the concentration of chemical species and the distributions of temperature and current density in an anode-supported SOFC with direct internal reforming are presented and discussed. The potential for using this model as a general numerical tool to study the impact of the detailed processes taking place in SOFCs is discussed.     

Micro‐modeling of porous composite anodes for solid oxide fuel cells

A new anode micromodel for solid oxide fuel cells to predict the electrochemical performance of hydrocarbon‐fuelled porous composite anodes with various microstructures is developed. In this model, the random packing sphere method is used to estimate the anode microstructural properties, and the complex interdependency among the multicomponent mass transport, electron and ion transports, and electrochemical and chemical reactions is taken into account. As a case study, a porous Ni–YSZ composite anode operated with biogas fuel is simulated numerically and distributions of the current density, polarization, and mole fraction and rate of flux of the fuel components along the thickness of the anode are determined. The effect of the anode microstructural variables including the porosity, thickness, particle‐size ratio, and particle size and volume fraction of Ni particles on the anode electrochemical performance is also studied. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1893–1906, 2012     

BSCF/GDC as a refined cathode to the single-chamber solid oxide fuel cell based on a LAMOX electrolyte

In an effort to build the solid oxide fuel cell for intermediate temperature operations, the oxide ion conductor member of LAMOX family appears to be an ideal candidate for electrolyte since its parent crystal La₂Mo₂O₉ shows a monoclinic-cubic phase transition around 580 °C. Nonetheless, members of the LAMOX family are much less refractory than the conventional electrode compositions which are targeted to coordinate with the electrolyte of yttrium stabilized zirconia. In this work, we study the viability of a cathode composite of Ba_0.5Sr₀₅Co_0.8Fe_0.2O₃ (BSCF) and gadolinium doped ceria (GDC) to match the electrolyte La_1.8Dy_0.2Mo₂O₉ (LDM). Severe interfacial reactions between BSCF and LDM require a ceria-based diffusion barrier between them. The iron-doped GDC barrier of high sinterability is a convenient choice to block the unwanted reactions, allowing us to devise a BSCF/GDC composite cathode of gradient GDC content to relieve thermal stresses. The cell, operated in a mixed reactant chamber with flowing methane/air, functions properly at operation temperature 625–700 °C. Its maximum power output is recorded at 675 °C, since the BSCF crystal begins to degrade at 700 °C under the methane/air atmosphere.     

Enhancing the performance of traditional La2NiO4+x cathode for proton-conducting solid oxide fuel cells with Zn-doping

A Zn-doping strategy was employed to modify the Ruddlesden-Popper (R–P) structure oxide La₂NiO_4+x to improve hydration and proton diffusion ability. First-principles calculations indicated the formation of interstitial oxygen instead of oxygen vacancy is favorable for Zn-modified and Zn-free La₂NiO_4+x. The doping of Zn significantly lowered the hydration energy for La₂NiO_4+x and decreased the proton migration barrier. The electrical conductivity relaxation confirmed that the Zn-modified La₂NiO_4+x sample had a higher proton diffusion rate than the Zn-free sample. Furthermore, the Zn-doping strategy did not alter the thermal expansion behavior of the material, and both Zn-modified and Zn-free La₂NiO_4+x samples showed a similar thermal expansion coefficient value, which was also close with the electrolyte materials. As a result, the Zn-modified La₂NiO_4+x exhibited suitability as the cathode for proton-conducting solid oxide fuel cells (H–SOFCs). An H–SOFC using the Zn-modified La₂NiO_4+x showed a relatively high peak power density of 1070 mW cm^-2 at 700 °C, significantly larger than that La₂NiO_4+x-based H–SOFCs reported previously.     

Investigation of the active thickness of solid oxide fuel cell electrodes using a 3D microstructure model

A 3D microstructure model is used to investigate the effect of the thickness of the solid oxide fuel cell (SOFC) electrode on its performance. The 3D microstructure model, which is based on 3D Monte Carlo packing of spherical particles of different types, can be used to handle different particle sizes and generate a heterogeneous network of the composite materials from which a range of microstructural properties can be calculated, including phase volume fraction, percolation and three phase boundary (TPB) length. The electrode model can also be used to perform transport and electrochemical modelling such that the performance of the synthetic electrode can be predicted. The dependence of the active electrode thickness, i.e. the region of the anode, which is electrochemically active, on operating over-potential, electrode composition and particle size is observed. Operating the electrode at an over-potential of above 200 mV results in a decrease in the active thickness with increasing over-potential. Reducing the particle size dramatically enhances the percolating TPB density and thus the performance of the electrode at smaller thicknesses; a smaller active thickness is found with electrodes made of smaller particles. Distributions of local current generation throughout the electrode reveal the heterogeneity of the 3D microstructure at the electrode/electrolyte interface and the dominant current generation in the vicinity of this interface. The active electrode thickness predicted using the model ranges from 5 μm to 15 μm, which corresponds well to many experimental observations, supporting the use of our 3D microstructure model for the investigation of SOFC electrode related phenomena.     

Recent advances and prospects of symmetrical solid oxide fuel cells

Solid oxide fuel cells (SOFCs) with symmetrical electrodes have been investigated extensively because of their potential significant advantages compared to the traditional configurations, regarding manufacturing, thermomechanical compatibility with cell components, operation stability, anting sulfur poisoning and carbon deposition. Many electrodes with novel structure and properties are currently being developed and studied in recent years. In this review, we summarized the recent advances of symmetrical SOFCs on their electrode materials, applications and prospects. The electrode materials include single phased perovskite, double perovskite, perovskite derived structures and composite electrodes. The relationships between the electrode materials and relevant properties are discussed. The applications and perspectives are highlighted, providing critical and useful directions for researchers to prepare and design electrode materials rationally.     

Novel and high-performance (La,Sr)MnO3 based composite cathodes for intermediate-temperature solid oxide fuel cells

The rapid decrease of the electrocatalytic activity at low temperature (<700 ℃) limits the popularization and application of the classical cathode material LSM (Sr doped LaMnO₃) in SOFC. Herein, we report that the introduction of CBO (CuBi₂O₄) oxide could not only reduce the sintering temperature of LSM-based cathode, but also significantly improves its electrochemical performance at intermediate temperature range of 500–700 ℃. The polarization resistance (R_p) of LSM-CBO20 (including 20 wt. % CBO) composite cathode on GDC electrolyte is only 0.13 Ω cm² at 700 ℃, which is significantly lower than the LSM cathode. The study found that the quite promoted oxygen surface exchange kinetics and catalytic activity of CBO, and the much reduced sintering temperature of the composite cathode contribute to the dramatic decrease of R_p. In addition, when Gd_0.1Ce_0.9O_1.95 (GDC) is introduced, the polarization resistance is further reduced to 0.11 Ω cm² at 700 ℃. The maximum power density of the single cell with LSMGDC-CBO20 triadic phase cathode reaches to 1460 mW cm^-2 at 700 ℃. The present study demonstrates that introducing CBO is an effective and promising approach to improve the electrochemical performance of conventional LSM-based cathode at reduced temperatures.