Direct ammonia solid oxide fuel cell based on thin proton-conducting electrolyte

Thin proton-conducting electrolyte with composition BaCe_0.8Gd_0.2O_3−δ (BCGO) was prepared over substrates composed of Ce_0.8Gd_0.2O_1.9 (CGO)-Ni by the dry-pressing method. Solid oxide fuel cells (SOFCs) were fabricated with the structure Ni-CGO/BCGO/Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO)-CGO. The performance of a single cell was tested at 600 and 650 °C, with ammonia directly used as fuel. The open circuit voltages (OCVs) were 1.12 and 1.1 V at 600 and 650 °C, respectively. The higher OCV may be due to both the compaction of the BCGO electrolyte (no porosity) and complete decomposition of ammonia. The maximum power density was 147 mW cm⁻² at 600 °C. Comparisons of the cell with hydrogen as fuel indicate that ammonia can be treated as a substitute liquid fuel for SOFCs based on a proton-conducting solid electrolyte.     

Electrochemical modeling of ammonia-fed solid oxide fuel cells based on proton conducting electrolyte

An electrochemical model was developed to study the NH₃-fed and H₂-fed solid oxide fuel cells based on proton conducting electrolyte (SOFC-H). The modeling results were consistent with experimental data in literature. It is found that there is little difference in working voltage and power density between the NH₃-fed and the H₂-fed SOFC-H with an electrolyte-support configuration due to an extremely high ohmic overpotential in the SOFC-H. With an anode-supported configuration, especially when a thin film electrolyte is used, the H₂-fed SOFC-H shows significantly higher voltage and power density than the NH₃-fed SOFC-H due to the significant difference in concentration overpotentials. The anode concentration overpotential of the NH₃-fed SOFC-H is found much higher than the H₂-fed SOFC-H, as the presence of N₂ gas dilutes the H₂ concentration and slows down the transport of H₂. More importantly, the cathode concentration overpotential is found very significant despite of the thin cathode used in the anode-supported configuration. In the SOFC-H, H₂O is produced in the cathode, which enables complete fuel utilization on one hand, but dilutes the concentration of O₂ and impedes the diffusion of O₂ to the reaction sites on the other hand. Thus, the cathode concentration overpotential is the limiting factor for the H₂-fed SOFC-H and an important voltage loss in the NH₃-fed SOFC-H. How to reduce the concentration overpotentials at both electrodes is identified crucial to develop high performance SOFC-H.     

Mathematical modeling of ammonia-fed solid oxide fuel cells with different electrolytes

An electrochemical model was developed to study the ammonia (NH₃)-fed solid oxide fuel cells with proton-conducting electrolyte (SOFC-H) and oxygen ion-conducting electrolyte (SOFC-O). Different from previous thermodynamic analysis, the present study reveals that the actual performance of the NH₃-fed SOFC-H is considerably lower than the SOFC-O, mainly due to higher ohmic overpotential of the SOFC-H electrolyte. More analyses have been performed to study the separate overpotentials of the NH₃-fed SOFC-H and SOFC-O. Compared with the NH₃-fed SOFC-H, the SOFC-O has higher anode concentration overpotential and lower cathode concentration overpotential. The effects of temperature and electrode porosity on concentration overpotentials have also been studied in order to identify possible methods for improvement of SOFC performance. This study reveals that the use of different electrolytes not only causes different ion conduction characteristics at the electrolyte, but also significantly influences the concentration overpotentials at the electrodes. The model developed in this article can be extended to 2D and 3D models for further design optimization.     

Modeling of methane fed solid oxide fuel cells: Comparison between proton conducting electrolyte and oxygen ion conducting electrolyte

An electrochemical model was developed to study the methane (CH₄) fed solid oxide fuel cell (SOFC) using proton conducting electrolyte (SOFC-H) and oxygen ion conducting electrolyte (SOFC-O). Both the internal methane steam reforming (MSR) and water gas shift (WGS) reactions are considered in the model. Previous study has shown that the CH₄ fed SOFC-H had significantly better performance than the SOFC-O. However, the present study reveals that the actual performance of the CH₄ fed SOFC-H is considerably lower than the SOFC-O, partly due to higher ohmic overpotential of SOFC-H. It is also found that the CH₄ fed SOFC-H has considerably higher cathode concentration overpotential and lower anode concentration overpotential than the SOFC-O. The anode concentration overpotentials of the CH₄ fed SOFC-H and SOFC-O are found to decrease with increasing temperature, which is different from previous analyses on the H₂ fed SOFC. Therefore, high temperature is desirable for increasing the potential of the CH₄ fed SOFC. It is also found that there exist optimal electrode porosities that minimize the electrode total overpotentials. The analyses provided in this paper signify the difference between the CH₄ fed SOFC-H and SOFC-O. The model developed in this paper can be extended to 2D or 3D models to study the performance of practical SOFC systems.     

An improved electrochemical model for the NH3 fed proton conducting solid oxide fuel cells at intermediate temperatures

An improved electrochemical model is developed to study the ammonia fed solid oxide fuel cell based on proton conducting electrolyte (SOFC-H). Including the chemical reaction kinetics of NH₃ catalytic thermal decomposition, the present model can be used to predict the performance of the NH₃ fed SOFC-H at an intermediate temperature (i.e. 773 K). Comparison between the simulation results using the present model and experimental data from literature validates the accuracy of this model. Parametrical analyses reveal that at a high operating temperature (i.e. 1073 K), the NH₃ fuel is completely decomposed to H₂ and N₂ within a very thin layer (30 μm) near the anode surface of an SOFC-H. It is also found that operating the NH₃ fed SOFC-H at an intermediate temperature of 773 K is feasible due to sufficiently high rate of NH₃ decomposition. However, further decreasing the temperature to 673 K is not recommended as less than 10% NH₃ fuel can be decomposed to H₂ and N₂ in the SOFC-H. The effects of current density and electrode microstructure on the performance of the NH₃ fed SOFC-H are also studied. It is found that increasing electrode porosity and pore size is beneficial to increase the partial pressure of H₂ at the anode–electrolyte interface. The model developed in this paper can be extended to 2D or 3D models to study practical tubular or planar SOFCs.     

Theoretical analysis of reversible solid oxide fuel cell based on proton-conducting electrolyte

A single reversible solid oxide fuel cell (RSOFC) can accomplish two functions: (1) as a solid oxide steam electrolyzer (SOSE) for hydrogen production and (2) as a solid oxide fuel cell (SOFC) for power generation. An electrochemical model was developed to study the performance of an RSOFC based on a proton-conducting electrolyte (RSOFC-H). In both SOSE and SOFC modes, the hydrogen electrode-supported configuration was identified as the most favorable design to achieve high energy conversion efficiency of RSOFC-H. For comparison, in a previous study on conventional RSOFC based on an oxygen ion-conducting electrolyte (RSOFC-O), the hydrogen electrode-supported configuration was found to be favorable in the SOFC mode but such configuration would cause high concentration overpotential in the SOSE mode. Thus, the oxygen electrode-supported configuration was desirable for RSOFC-O operating in the SOSE mode. The results obtained in this study show that RSOFC-H has a natural advantage over RSOFC-O in terms of structural design. The modeling study signifies the difference between RSOFC-H and RSOFC-O and can serve as a useful tool for further design optimization.     

A novel electrolyte for intermediate solid oxide fuel cells

Scheelite-type, La_xCa_1−xMoO_4+δ electrolyte powders, are prepared by the sol-gel process. The crystal structure of the samples is determined by employing the technique of X-ray diffraction (XRD). According to XRD analysis, the continuous series of La_xCa_1−xMoO_4+δ (0 ≤ x ≤ 0.3) solid solutions have the structure of tetragonal scheelite. Their lattice parameters are greater than that of the original sample, and increase with increasing values of x in the La-substituted system. Results of sinterability and electrochemical testing reveal that the performances of La-doped calcium molybdate are superior to that of pure CaMoO₄. La_xCa_1−xMoO_4+δ ceramics demonstrate higher sinterability. The La_0.2Ca_0.8MoO_4+δ sample that achieved 96.5% of the theoretical density was obtained after being sintered at 1250 °C for 4 h. The conductivity increases with increasing lanthanum content, and a total conductivity of 7.3 × 10⁻³ S cm⁻¹ at 800 °C could be obtained in the La_0.2Ca_0.8MoO_4+δ compound sintered at 1250 °C for 4 h.     

The effect of electrolyte type on performance of solid oxide fuel cells running on hydrocarbon fuels

A two-dimensional model is developed to simulate the performance of methane fueled solid oxide fuel cells (SOFCs), focusing on the effect of electrolyte type on SOFC performance. The model considers the heat and mass transfer, direct internal reforming (DIR) reaction, water gas shift reaction (WGSR), and electrochemical reactions in SOFCs. The electrochemical oxidation of CO in oxygen ion-conducting SOFC (O-SOFC) is considered. The present study reveals that the performance of H-SOFC is lower than that of O-SOFC at a high temperature or at a low operating potential, as electrochemical oxidation of CO in O-SOFC contributes to power generation. This finding is contrary to our common understanding that proton conducting SOFC (H-SOFC) always performs better than O-SOFC. However, at a high operating potential of 0.8 V or at a lower temperature, H-SOFC does exhibit better performance than O-SOFC due to its higher Nernst potential and higher ionic conductivity of the electrolyte. This indicates that the proton conductors can be good choices for SOFCs at intermediate temperature, even with hydrocarbons fuels. The results provide better understanding on how the electrolyte type influences the performance of SOFCs running on hydrocarbon fuels.     

Study on GDC-LSGM composite electrolytes for intermediate-temperature solid oxide fuel cells

The electrolyte materials Ce_0.9Gd_0.1O_1.9 (GDC) and La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSGM) were synthesized by means of glycine-nitrate processes, respectively, then GDC-LSGM composite electrolytes were prepared by mixing GDC and LSGM. The GDC and LSGM powders were mixed in the weight ratio of 95:5, 90:10 and 85:15 and named as GL9505, GL9010 and GL8515. Their structures and ionic conductivities were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and AC impedance spectroscopy. The grain sizes of GDC-LSGM composites could be increased distinctly and the grain boundary resistance could be significantly decreased by small addition of LSGM. The experimental results show that the GDC-LSGM composites exhibit excellent ionic conductivity and could significantly enhance the fuel cell performances. The open circuit voltages are higher in the cell with composite electrolytes than in the cell with single GDC as electrolyte at the working temperature. Among these electrolytes, GL9505 has the highest ionic conductivity and the maximum power density.     

Entropy generation analysis in a monolithic-type solid oxide fuel cell (SOFC)

The aim of the paper is to investigate possible improvements in the geometry design of a monolithic solid oxide fuel cells (SOFCs) through analysis of the entropy generation terms. The different contributions to the local rate of entropy generation are calculated using a computational fluid dynamic (CFD) model of the fuel cell, accounting for energy transfer, fluid dynamics, current transfer, chemical reactions and electrochemistry. The fuel cell geometry is then modified to reduce the main sources of irreversibility and increase its efficiency.     

Transient modeling of anode-supported solid oxide fuel cells

An isothermal 2-D transient model is developed for an anode-supported solid oxide fuel cell. The model takes into account the transient effects of both charge migration and species transport in PEN assembly. Due to the lack of transient experimental data, the transient model, under steady state operating conditions, is validated using experimental results from open literature. Numerical results show that the cell can obtain very quick transient current response when subjected to a step voltage change, followed by a slow current transient period due to species diffusion effects within porous electrodes. It is also found that the transient response of the cell current is sensitive to oxygen concentration change at cathode/channel interface, whereas the current response is slow when step change of hydrogen concentration is applied at anode/channel interface. The cell transient performance can be improved by increasing porosity or decreasing tortuosity of electrodes.     

Electrochemical testing of suspension plasma sprayed solid oxide fuel cell electrolytes

Electrochemical performance of metal-supported plasma sprayed (PS) solid oxide fuel cells (SOFCs) was tested for three nominal electrolyte thicknesses and three electrolyte fabrication conditions to determine the effects of electrolyte thickness and microstructure on open circuit voltage (OCV) and series resistance (R_s). The measured OCV values were approximately 90% of the Nernst voltages, and electrolyte area specific resistances below 0.1 Ω cm² were obtained at 750 °C for electrolyte thicknesses below 20 μm. Least-squares fitting was used to estimate the contributions to R_s of the YSZ bulk material, its microstructure, and the contact resistance between the current collectors and the cells. It was found that the 96% dense electrolyte layers produced from high plasma gas flow rate conditions had the lowest permeation rates, the highest OCV values, and the smallest electrolyte-related voltage losses. Optimal electrolyte thicknesses were determined for each electrolyte microstructure that would result in the lowest combination of OCV loss and voltage loss due to series resistance for operating voltages of 0.8 V and 0.7 V.     

Crack severity in relation to non-homogeneous Ni oxidation in anode-supported solid oxide fuel cells

The full oxidation of Ni-YSZ anode-supported cells at high temperatures (>700 °C) is shown here to lead to much more severe degradation (larger quantity and wider cracks in the electrolyte) than at lower temperatures. This correlates with the linear mass gain/time profile observed in TGA experiments at high temperatures, indicative of diffusion controlled Ni oxidation and thus the presence of O₂ (and Ni/NiO) concentration gradients into the depth of the anode layer. At low partial pressures of O₂, the severity of cracking also increases. SEM studies of partially oxidized anode layers confirmed that Ni oxidation is non-homogeneous when carried out at either high temperatures or low pO₂, in which case the outer regions of the anode (near the anode/air interface) become almost fully oxidized, while the inner regions (near the electrolyte) remain metallic. Under these conditions, the continued volume expansion associated with NiO formation can then only occur towards the electrolyte, increasing the compressive stress inside the anode as the Ni continues to be oxidized, leading to electrolyte cracking and warping (convex to the electrolyte). To prevent severe degradation to the cell, efforts should therefore be made to avoid gradients in NiO/Ni content during oxygen exposure of Ni-YSZ anode-supported cells at high temperatures.     

Performance of anode-supported solid oxide fuel cell using novel ceria electrolyte

The potential of a novel co-doped ceria material Sm_0.075Nd_0.075Ce_0.85O_2−δ as an electrolyte was investigated under fuel cell operating conditions. Conventional colloidal processing was used to deposit a dense layer of Sm_0.075Nd_0.075Ce_0.85O_2−δ (thickness 10 μm) over a porous Ni-gadolinia doped ceria anode. The current-voltage performance of the cell was measured at intermediate temperatures with 90 cm³ min⁻¹ of air and wet hydrogen flowing on cathode and anode sides, respectively. At 650 °C, the maximum power density of the cell reached an exceptionally high value of 1.43 W cm⁻², with an area specific resistance of 0.105 Ω cm². Impedance measurements show that the power density decrease with decrease in temperature is mainly due to the increase in electrode resistance. The results confirm that Sm_0.075Nd_0.075Ce_0.85O_2−δ is a promising alternative electrolyte for intermediate temperature solid oxide fuel cells.     

CeO2 coated NaFeO2 proton-conducting electrolyte for solid oxide fuel cell

Nowadays, the low-temperature operation has become an inevitable trend for the development of SOFCs. Transition metal layered oxides are considered as promising electrolyte materials for low-temperature solid oxide fuel cells (LT-SOFCs). In this work, we report the CeO₂ coated NaFeO₂ as an electrolyte material for LT-SOFC. The study results revealed that the piling of CeO₂ significantly influenced the open-circuit voltage (OCV) as well as the power output of the fuel cells. In comparison with pure NaFeO₂, the denser structure of CeO₂ coated NaFeO₂ leads to higher OCV (1.06 V, 550 °C). The electrochemical impedance spectrum (EIS) fitted results showed that NaFeO₂–CeO₂ composites possessed higher ionic boundary conductivity. This is because that the hetero-interfaces between NaFeO₂ and CeO₂ provide fast ion conducting path. The high ionic conductivity of CeO₂ coated NaFeO₂ lead to admirable fuel cell power output of 727 mW cm^?2 at 550 °C.     

Scientometric review of proton-conducting solid oxide fuel cells

Proton-conducting solid oxide fuel cells (P–SOFCs) are promising energy conversion devices that convert chemical energy directly to electrical energy. P–SOFCs have attracted significant attention in the past few years because of their superiority over the oxygen-ion-conducting solid oxide fuel cells (O–SOFCs) in terms of better feasibility of efficient operation at lower temperatures, non-dilution of fuel at the anode, and higher theoretical efficiency. This review focuses on the scientometric analysis of 1008 quality articles retrieved from the Scopus database. The historical trends and progress in P–SOFCs are presented starting from the inception of the demonstration of the concept of proton conductivity in solid oxide fuel cells from 1986 to 2021. Furthermore, the notable achievements in the material development of various components of P–SOFC are expounded. The scientometric analysis reveals that only 28% of the countries in the world are involved in P–SOFC research and the National Natural Science Foundation of China is the top featured funding sponsor for many research studies related to P–SOFC development. This article can serve as an easy guide for P–SOFC research enthusiasts to navigate through the overview of this research area and identify potential collaborators, funding sponsors, most impactful researchers, countries, and articles.     

Fabrication of hydrogen electrode supported cell for utilized regenerative solid oxide fuel cell application

A utilized regenerative solid oxide fuel cell (URSOFC) provides the dual function of performing energy storage and power generation, all in one unit. When functioning as an energy storage device, the URSOFC acts like a solid oxide electrolyzer cell (SOEC) in water electrolysis mode; whereby the electric energy is stored as (electrolyzied) hydrogen and oxygen gases. While hydrogen is useful as a transportation fuel and in other industrial applications, the URSOFC also acts as a solid oxide fuel cell (SOFC) in power generation mode to produce electricity when needed. The URSOFC would be a competitive technology in the upcoming hydrogen economy on the basis of its low cost, simple structure, and high efficiency. This paper reports on the design and manufacturing of its anode support cell using commercially available materials. Also reported are the resulting performance, both in electrolysis and fuel cell modes, as a function of its operating parameters such as temperature and current density. We found that the URSOFC performance improved with increasing temperature and its fuel cell mode had a better performance than its electrolysis mode due to a limited humidity inlet causing concentration polarization. In addition, there were great improvements in performance for both the SOFC and SOEC modes after the first test and could be attributed to an increase in porosity within the oxygen electrode, which was beneficial for the oxygen reaction.     

Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells

A comparison of three solid oxide electrolyte fabrication processes, namely dip coating, screen printing and tape casting, for planar anode supported solid oxide fuel cells (SOFCs) is presented in this study. The effect of sintering temperature (1325–1400 °C) is also examined. The anode and cathode layers of the anode-supported cells, on the other hand, are fabricated by tape casting and screen printing, respectively. The quality of the electrolytes is evaluated via performance measurements, impedance analyses and microstructural investigations of the cells. It is found that the density of the electrolyte increases with the sintering temperatures for all fabrication methods studied. The results also show that with the process and fabrication parameters considered in this study, both dip coating and screen printing do not yield a desired dense electrolyte structure as proven by open circuit potentials measured and SEM photos. The cells with tape cast electrolytes, on the other hand, provide the highest performances regardless of the electrolyte sintering and cell operating temperatures. The best peak performance of 0.924 W/cm² is obtained from the cell with tape cast electrolyte sintered at 1400 °C. SEM investigations and measured open circuit potentials reveal that almost fully dense electrolyte layer can be obtained with a tape cast electrolyte particularly sintered at temperatures higher than 1350 °C. Impedance analyses indicate that the main reason behind the significantly higher performances is due to not only increased electrolyte density but a decrease in the interface resistance of the anode functional and electrolyte layer is also responsible. This can be explained by the load applied during the lamination step in the fabrication of the tape cast electrolyte, providing better powder compaction and adhesion.     

A review of numerical modeling of solid oxide fuel cells

The solid oxide fuel cell (SOFC) is one of the most promising fuel cells for direct conversion of chemical energy to electrical energy with the possibility of its use in co-generation systems because of the high temperature waste heat. Various mathematical models have been developed for three geometric configurations (tubular, planar, and monolithic) to solve transport equations coupled with electrochemical processes to describe the reaction kinetics including internal reforming chemistry in SOFCs. In recent years, considerable progress has been made in modeling to improve the design and performance of this type of fuel cells. The numbers of the contributions on this important type of fuels have been increasing rapidly. The objective of this paper is to summarize the present status of the SOFC modeling efforts so that unresolved problems can be identified by the researchers.