Solid oxide fuel cells (SOFCs) can be operated in a reversed mode as electrolyzer cells for electrolysis of H2O and CO2. In this paper, a 2D thermal model is developed to study the heat/mass transfer ...and chemical/electrochemical reactions in a solid oxide electrolyzer cell (SOEC) for H2O/CO2 co-electrolysis. The model is based on 3 sub-models: a computational fluid dynamics (CFD) model describing the fluid flow and heat/mass transfer; an electrochemical model relating the current density and operating potential; and a chemical model describing the reversible water gas shift reaction (WGSR) and reversible methanation reaction. It is found that reversible methanation and reforming reactions are not favored in H2O/CO2 co-electrolysis. For comparison, the reversible WGSR can significantly influence the co-electrolysis behavior. The effects of inlet temperature and inlet gas composition on H2O/CO2 co-electrolysis are simulated and discussed.
► A 2D model is developed for co-electrolysis of H2O/CO2 for syngas production. ► Methanation and internal reforming are not favored in the co-electrolysis process. ► Reversible water gas shift reaction significantly affects H2O/CO2 co-electrolysis. ► Large variation in gas composition complicates the co-electrolysis behavior.
Co-electrolysis of CO2 and H2O in a solid oxide electrolyzer cell (SOEC) offers a promising way for syngas production. In this study, an electrochemical model is developed to simulate the performance ...of an SOEC used for CO2/H2O co-electrolysis, considering the reversible water gas shift reaction (WGSR) in the cathode. The dusty gas model (DGM) is used to characterize the multi-component mass transport in the electrodes. The modeling results are compared with experimental data from the literature and good agreement is observed. Parametric simulations are performed to analyze the distributions of WGSR and gas composition in the electrode. A new method is proposed to quantify the contribution of WGSR to CO production by comparing the CO fluxes at the cathode-electrolyte interface and at the cathode surface. It is found that the reversible WGSR could contribute to CO production at a low operating potential but consume CO at a high operating potential at an operating temperature of 1073 K and inlet gas composition (molar fraction) of H2O: 49.7%, CO2: 25%, H2: 25%, CO: 0.3%. In addition, the contribution of WGSR to CO production also depends on the operating temperature and inlet gas composition.
A heat and mass transfer model is developed for a compact reformer. Hydrogen production from methane steam reforming is simulated. Increasing temperature greatly increases the reaction rates at the ...inlet. Temperature in the downstream is increased at higher rate of heat supply. Larger permeability enhances gas flow and reaction rates in the catalyst layer.
Compact reformers (CRs) are promising devices for efficient fuel processing. In CRs, a thin solid plate is sandwiched between two catalyst layers to enable efficient heat transfer from combustion duct to the reforming duct for fuel processing. In this study, a 2D heat and mass transfer model is developed to investigate the fundamental transport phenomenon and chemical reaction kinetics in a CR for hydrogen production by methane steam reforming (MSR). Both MSR reaction and water gas shift reaction (WGSR) are considered in the numerical model. Parametric simulations are performed to examine the effects of various structural/operating parameters, such as pore size, permeability, gas velocity, temperature, and rate of heat supply on the reformer performance. It is found that the reaction rates of MSR and WGSR are the highest at the inlet but decrease significantly along the reformer. Increasing the operating temperature raises the reaction rates at the inlet but shows very small influence in the downstream. For comparison, increasing the rate of heat supply raises the reaction rates in the downstream due to increased temperature. A high gas velocity and permeability facilitates gas transport in the porous structure thus enhances reaction rates in the downstream of the reformer.
The prevalence of myopia is increasing globally. Complications of myopia are associated with huge economic and social costs. It is believed that high myopia in adulthood can be traced back to school ...age onset myopia. Therefore, it is crucial and urgent to implement effective measures of myopia control, which may include preventing myopia onset as well as retarding myopia progression in school age children. The mechanism of myopia is still poorly understood. There are some evidences to suggest excessive expansion of Bruch's membrane, possibly in response to peripheral hyperopic defocus, and it may be one of the mechanisms leading to the uncontrolled axial elongation of the globe. Atropine is currently the most effective therapy for myopia control. Recent clinical trials demonstrated low-dose atropine eye drops such as 0.01% resulted in retardation of myopia progression, with significantly less side effects compared to higher concentration preparation. However, there remain a proportion of patients who are poor responders, in whom the optimal management remains unclear. Proposed strategies include stepwise increase of atropine dosing, and a combination of low-dose atropine with increase outdoor time. This review will focus on the current understanding of epidemiology, pathophysiology in myopia and highlight recent clinical trials using atropine in the school-aged children, as well as the treatment strategy in clinical implementation in hyperopic, pre-myopic and myopic children.
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.
► O-SOFC and H-SOFC are compared with hydrocarbon fuels. ► Electrochemical oxidation of CO is considered in O-SOFC. ► O-SOFC performs better than H-SOFC at a high temperature or low potential. ► H-SOFC shows better performance at reduced temperature or high potential. ► The results deepen our understanding on SOFC electrolyte.
As a highly appealing technology for hydrogen generation, water electrolysis including oxygen evolution reaction (OER) at the anode and hydrogen evolution reaction (HER) at the cathode largely ...depends on the availability of efficient electrocatalysts. Accordingly, over the past years, much effort has been made to develop various electrocatalysts with superior performance and reduced cost. Among them, ruthenium (Ru)-based materials for OER and HER are very promising because of their prominent catalytic activity, pH-universal application, the cheapest price among the precious metal family, and so on. Herein, recent advances in this hot research field are comprehensively reviewed. A general description about water splitting is presented to understand the reaction mechanism and proposed scaling relations toward activities, and key stability issues for Ru-based materials are further given. Subsequently, various Ru-involving electrocatalysts are introduced and classified into different groups for improving or optimizing electrocatalytic properties, with a special focus on several significant bifunctional electrocatalysts along with a simulated water electrolyzer. Finally, a perspective on the existing challenges and future progress of Ru-based catalysts toward OER and HER is provided. The main aim here is to shed some light on the design and construction of emerging catalysts for energy storage and conversion technologies.
A two dimensional model is developed to study the transport and reaction processes in solid oxide fuel cells (SOFCs) fueled by partially pre-reformed gas mixture, considering the direct internal ...reforming (DIR) of methane and water gas shift (WGS) reaction in the porous anode of SOFC. Electrochemical oxidations of H2 and CO fuels are both considered. The model consists of an electrochemical, a chemical model, and a computational fluid dynamics (CFD) model. Two chemical models are compared to examine their effects on SOFC modeling results. Different from the previous studies on hydrogen fueled SOFC, higher gas velocity is found to slightly decrease the performance of SOFC running on pre-reformed gas mixture, due to suppressed gas composition variation at a higher gas velocity. The current density distribution along the gas channels at an inlet temperature of 1173K is quite different from that at 1073K, as DIR reaction is facilitated at a higher temperature. It is also found that neglecting the electrochemical oxidation of CO can considerably underestimate the total current density of SOFC running on pre-reformed hydrocarbon fuels. An alternative method is proposed to numerically determine the open-circuit potential of SOFC running on hydrocarbon fuels. Electrochemical reactions are observed at open-circuit potentials.
► Electrochemical oxidations of both H2 and CO fuel are considered in SOFC. ► Two chemical models are compared in SOFC modeling. ► Running on pre-reformed hydrocarbon fuels, the SOFC performance is found to decrease slightly at a higher gas velocity. ► An alternative method is proposed to determine the open-circuit potential of SOFC running on pre-reformed hydrocarbon fuels. ► Electrochemical reactions are observed at the open-circuit potentials of SOFC.
One challenge for the commercial development of solid oxide fuel cells as efficient energy-conversion devices is thermo-mechanical instability. Large internal-strain gradients caused by the mismatch ...in thermal expansion behaviour between different fuel cell components are the main cause of this instability, which can lead to cell degradation, delamination or fracture
. Here we demonstrate an approach to realizing full thermo-mechanical compatibility between the cathode and other cell components by introducing a thermal-expansion offset. We use reactive sintering to combine a cobalt-based perovskite with high electrochemical activity and large thermal-expansion coefficient with a negative-thermal-expansion material, thus forming a composite electrode with a thermal-expansion behaviour that is well matched to that of the electrolyte. A new interphase is formed because of the limited reaction between the two materials in the composite during the calcination process, which also creates A-site deficiencies in the perovskite. As a result, the composite shows both high activity and excellent stability. The introduction of reactive negative-thermal-expansion components may provide a general strategy for the development of fully compatible and highly active electrodes for solid oxide fuel cells.
► A 2D model is developed for solid oxide fuel cells (SOFCs). ► CH4 reforming by CO2 (MCDR) is included. ► SOFC with MCDR shows comparable performance with methane steam reforming SOFC. ► Increasing ...CO electrochemical oxidation greatly enhances the SOFC performance. ► Effects of potential and temperature on SOFC performance are also discussed.
A two-dimensional model is developed to simulate the performance of solid oxide fuel cells (SOFCs) fed with CO2 and CH4 mixture. The electrochemical oxidations of both CO and H2 are included. Important chemical reactions are considered in the model, including methane carbon dioxide reforming (MCDR), reversible water gas shift reaction (WGSR), and methane steam reforming (MSR). It’s found that at a CH4/CO2 molar ratio of 50/50, MCDR and reversible WGSR significantly influence the cell performance while MSR is negligibly small. The performance of SOFC fed with CO2/CH4 mixture is comparable to SOFC running on CH4/H2O mixtures. The electric output of SOFC can be enhanced by operating the cell at a low operating potential or at a high temperature. In addition, the development of anode catalyst with high activity towards CO electrochemical oxidation is important for SOFC performance enhancement. The model can serve as a useful tool for optimization of the SOFC system running on CH4/CO2 mixtures.
Solid oxide fuel cell （SOFC） is a high temperature （800--1000℃） power source, which can directly convert the chemical energy of a fuel into electrical energy via electrochemical reactions. As the ...energy conversion pro- cess is straightforward, the electric efficiency of SOFC （〉50 %） is considerably higher than that of conventional heat engines （〈40 %）. Compared with low temperature fuel cells, such as proton exchange membrane fuel cells （PEMFC） or alkaline fuel cells （AFC）, SOFCs are fuel flexible and can make use of various hydrocarbon fuels as internal reforming of hydrocarbon fuels can take place in the porous anode.
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