High performance of direct methane-fuelled solid oxide fuel cell with samarium modified nickel-based anode

Natural gas is one of the most attractive fuels for solid oxide fuel cell (SOFC), while the anode activity for methane fuel has a great influence on the performance and stability of SOFC. Samarium is a good catalyst promoter for methane reforming. In this work, samarium is used to modify nickel catalyst, which results in small nickel oxide particles. The SmNi-YSZ (yttria-stabilized zirconia) anode has smaller particles and better interfacial contact between nickel and YSZ compared with conventional Ni-YSZ anode. The fine structure of SmNi-YSZ anode results in high activity for electrochemical oxidation of hydrogen and low polarization resistance of the cell. The performance of SmNi-YSZ anode cell with humidified methane as fuel is greatly improved, which is similar to that with hydrogen as fuel. The maximum power densities of SmNi-YSZ anode cell are 1.56 W cm⁻² for humidified hydrogen fuel and 1.54 W cm⁻² for humidified methane fuel at 800 °C. The maximum power density is increased by 221% when samarium is used to modify Ni-YSZ anode for humidified methane fuel at 650 °C. High cell performance results in good stability of SmNi-YSZ anode cell and the cell runs stably for more than 600 min for humidified methane fuel.     

Fine interface contact and high activity of nickel-based anode modified by gadolinium for hydrogen and methane fuel

In this work, gadolinium is used to modify nickel catalyst, which can improve the properties of nickel oxide particle and inhibit its sintering and grain growth. Interface contact between nickel catalyst and YSZ is significantly improved and fine anode microstructure can be obtained when gadolinium is used to modify Ni-YSZ anode. Fine interface contact of GdNi-YSZ anode can accelerate charge transfer process and steam formation process, which leads to high activity for electrochemical oxidation of hydrogen and low impedance resistance. The remarkable characteristic of GdNi-YSZ anode cell is that the cell performance for humidified methane fuel is greatly improved due to the high anode activity for methane reforming and electrochemical oxidation of hydrogen. The maximum power density of GdNi-YSZ anode cell with humidified methane as fuel can reach 1.59 W/cm² at 800 °C and 0.46 W/cm² at 650 °C. High performance of GdNi-YSZ anode cell with humidified methane as fuel leads to much H₂O produced during the electrochemical oxidation process, which can depress carbon deposition and improve the cell stability for humidified methane fuel.     

Mini CHP based on the electrochemical generator and impeded fluidized bed reactor for methane steam reforming

The paper presents a configuration of mini CHP with the methane reformer and planar solid oxide fuel cell (SOFC) stacks. This mini CHP may produce electricity and superheated steam as well as preheat air and methane for the reformer along with cathode air used in the SOFC stack as an oxidant. Moreover, the mathematical model for this power plant has been created. The thermochemical reactor with impeded fluidized bed for autothermal steam reforming of methane (reformer) considered as the basis for the synthesis gas (syngas) production to fuel SOFC stacks has been studied experimentally as well. A fraction of conversion products has been oxidized by the air fed to the upper region of the impeded fluidized bed in order to carry out the endothermic methane steam reforming in a 1:3 ratio as well as to preheat products of these reactions. Studies have shown that syngas containing 55% of hydrogen could be produced by this reactor. Basic dimensions of the reactor as well as flow rates of air, water and methane for the conversion of methane have been adjusted through mathematical modelling.The paper provides heat balances for the reformer, SOFC stack and waste heat boiler (WHB) intended for generating superheated water steam along with preheating air and methane for the reformer as well as the preheated cathode air. The balances have formed the basis for calculating the following values: the useful product fraction in the reformer; fraction of hydrogen oxidized at SOFC anode; gross electric efficiency; anode temperature; exothermic effect of syngas hydrogen oxidation by air oxygen; excess entropy along with the Gibbs free energy change at standard conditions; electromotive force (EMF) of the fuel cell; specific flow rate of the equivalent fuel for producing electric and heat energy. Calculations have shown that the temperature of hydrogen oxidation products at SOFC anode is 850 °C; gross electric efficiency is 61.0%; EMF of one fuel cell is 0.985 V; fraction of hydrogen oxidized at SOFC anode is 64.6%; specific flow rate of the equivalent fuel for producing electric energy is 0.16 kg of eq.f./(kW·h) while that for heat generation amounts to 44.7 kg of eq.f./(GJ). All specific parameters are in agreement with the results of other studies.     

Simulation of SOFC performance using a modified exchange current density for pre-reformed methane-based fuels

Numerical simulations can be used to visualize and better understand various distributions such as gas concentration and temperature in solid oxide fuel cells (SOFCs) under realistic operating conditions. However, pre-existing models generally utilize an anode exchange current density equation which is valid for humidified hydrogen fuels – an unrealistic case for SOFCs, which are generally fueled by hydrocarbons. Here, we focus on developing a new, modified exchange current density equation, leading to an improved numerical analysis model for SOFC anode kinetics. As such, we experimentally determine the exchange current density of SOFC anodes fueled by fully pre-reformed methane. The results are used to derive a new phenomenological anode exchange current density equation. This modified equation is then combined with computational fluid dynamics (CFD) to simulate the performance parameters of a three-dimensional electrolyte-supported SOFC. The new modified exchange current density equation for methane-based fuels reproduces the I–V characteristics and temperature distribution significantly better than the previous models using humidified hydrogen fuel. Better simulations of SOFC performance under realistic operating conditions are crucial for the prediction and prevention of e.g. fuel starvation and thermal stresses.     

Lignite as a fuel for direct carbon fuel cell system

Lignite, also known as brown coal, and char derived from lignite by pyrolysis were investigated as fuels for direct carbon solid oxide fuel cells (DC-SOFC). Experiments were carried out with 16 cm² active area, electrolyte supported solid oxide fuel cell (SOFC), using pulverized solid fuel directly fed to DC-SOFC anode compartment in a batch mode, fixed bed configuration. The maximum power density of 143 mW/cm² was observed with a char derived from lignite, much higher than 93 mW/cm² when operating on a lignite fuel. The cell was operating under electric load until fuel supply was almost completely exhausted. Reloading fixed lignite bed during a thermal cycle resulted in a similar initial cell performance, pointing to feasibility of fuel cell operation in a continuous fuel supply mode. The additional series of experiments were carried out in SOFC cell, in the absence of solid fuels, with (a) simulated CO/CO₂ gas mixtures in a wide range of compositions and (b) humidified hydrogen as a reference fuel composition for all cases considered. The solid oxide fuel cell, operated with 92%CO + 8%CO₂ gas mixture, generated the maximum power density of 342 mW/cm². The fuel cell performance has increased in the following order: lignite (DC-SOFC) < char derived from lignite (DC-SOFC) < CO + CO₂ gas mixture (SOFC) < humidified hydrogen (SOFC).     

Sensitivity of nickel cermet anodes to reduction conditions

The direct use of methane as fuel for solid oxide fuel cell (SOFC) without pre-reforming would reduce running costs and enable higher efficiencies. But methane generally causes carbon deposition on the nickel anode and subsequent power degradation. This paper shows that carbon deposition from methane is very sensitive to anode reduction conditions. The effect of direct methane on microtubular SOFC reduced at two different conditions was studied at temperatures above 800 °C. Reducing the cells at high temperature gave good performance on hydrogen but the current degraded quickly on methane, suggesting that carbon was blocking the nickel surfaces. This was not recoverable by bringing in hydrogen to replace the methane. Cells reduced under low temperature conditions gave higher current on methane than on hydrogen, showing that carbon deposited from the methane improved nickel anode conductivity in this case. These cells also did not degrade on methane under certain conditions but lasted for a long period. Extracting the carbon by feeding the cell with hydrogen interrupted this newly formed linkage between the nickel particles, reducing the electrical conductivity, which could be recovered by reintroducing methane. The conclusion was that nickel cermet anodes are very sensitive to reduction conditions, with low temperature reduction being preferred if methane is to be used as the chosen fuel.     

Characterization of a 100 W SOFC stack fed by carbon monoxide rich fuels

This paper presents the evaluation of the performance of a 100 W Solid Oxide Fuel Cell (SOFC) stack with CO rich fuels as anode gas. The study aims at measuring the Open Circuit Voltage (OCV) and the Area Specific Resistance (ASR) when the amount of CO in the anode flow varies from 0 to 40% in volume. At the same time, the FCTestQA procedures were applied and evaluated as methodology for Solid Oxide Fuel Cell testing. The theoretical OCV was measured considering both H₂ and CO oxidation and the water gas shift reaction. The OCV values, as a function of CO concentration, resulted closely related to theoretical ones and the ASR value, calculated for different mixtures of fuel, did not change with anode gas composition and it seemed to be a function of the temperature and the degradation of the materials only.     

Use of hydrogen produced by the air conversion of motor diesel fuel in an electrochemical generator

At present, the production of electric energy consumer remote from agro-based centralized networks is done using diesel-generator technology with limited service life of the engine and the extremely low efficiency in the use of expensive fuel. In this paper, is considered an innovative technology of combined production of electricity and heat using a preliminary conversion of diesel fuel in the synthesis gas and then serving it at high temperature electrochemical generator. Synthetic gas to operate the generator air conversion is made of an electrochemical engine diesel fuels in catalytic reactor-burner. On the basis of heat balances of the torch, battery power and boiler are calculated: battery electric efficiency SOFC, chemical efficiency burner, SOFC anode temperature, EMF Planar element, the proportion of hydrogen, oxidized anode SOFC, unit cost of diesel fuel for the production of electricity and thermal energy. Specific consumption of diesel fuel for the production of electrical energy 114 g/kWh (162 g. w. t./kWh), and thermal 31.7 kg/Gj (45.1 kg/GJ, w. t. 189 kg standard fuel/Gcal).     

Performance of solid oxide fuel cell with chemical looping gasification products as fuel

Chemical looping gasification (CLG) can achieve the utilization of solid fuels for syngas production. The CLG system integrated with solid oxide fuel cell (SOFC) is a promising energy conversion way. In this work, an integration system of CLG and SOFC is evaluated via the implementation of a multi-field coupling modelling, where the products from the CLG are directly transported into the SOFC as the fuel and the coke deposition effect on the cell performance is evaluated. The results reveal that SOFC temperature using pure hydrogen as fuel has an increase of around 4 K compared to that with gas mixture as fuel owing to the inhibition of carbon deposition. It is found that the arrangement of anode and cathode in the countercurrent mode can promote the overall uniformity of current density compared to that in the cocurrent flow. Moreover, the impact of operating parameter of the CLG system on the SOFC performance is also examined. The results demonstrate that the increase of fuel reactor (FR) temperature and H₂O/C molar ratio in the CLG system is beneficial to the inhibition of carbon deposition and the enhancement of the SOFC performance.     

Ceria-based electrolyte reinforced by sol–gel technique for intermediate-temperature solid oxide fuel cells

High performance solid oxide fuel cells (SOFCs) based on gadolinia-doped ceria (GDC) electrolyte are demonstrated for intermediate temperature operation. The inherent technical limitations of the GDC electrolyte in sinterability and mechanical properties are overcome by applying sol–gel coating technique to the screen-printed film. When the quality of the electrolyte film is enhanced by the additional sol–gel coating, the OCV and maximum power density increase from 0.73 to 0.90 V and from 0.55 to 0.95 W cm⁻², respectively, at 650 °C with humidified hydrogen (3% H₂O) as fuel and air as oxidant. The impedance analysis reveals that the reinforcement of the thin electrolyte with sol–gel coating significantly reduces the polarization resistance. Elementary reaction steps for the anode and cathode are analyzed based on the systematic impedance study, and the relation between the structural integrity of the electrolyte and the electrode polarization is discussed in detail.     

Operation of solid oxide fuel cells on glycerol fuel: A thermodynamic analysis using the Gibbs free energy minimization approach

Solid oxide fuel cells (SOFCs) are very flexible, unlike other fuel cells. In principle, SOFCs can operate on almost any fuel. Currently much effort is invested in the development of SOFCs for portable applications operating directly on liquid fuels such as methanol and ethanol rather than hydrogen. However, there are very few publications dealing with the direct use of glycerol in SOFCs for portable systems. A recently published study shows that the performance achieved for an SOFC fueled by pure glycerol is quite interesting even when there is a thick electrolyte membrane, indicating that glycerol is a promising fuel for portable applications. For this reason a thermodynamic analysis for SOFCs operating directly on glycerol fuel is performed in the present study. The Gibbs energy minimization method computes the equilibrium compositions of the anode gas mixture, carbon deposition boundaries and electromotive forces (EMFs) as a function of fuel utilization and temperature. Moreover, the minimum amounts of H₂O, CO₂ (direct internal reforming case) and air (partial oxidation case) to be added to glycerol in the feedstock to avoid carbon deposition at the open circuit voltage (OCV) are calculated. Finally, a thermodynamic analysis is performed, taking into account the experimental conditions employed in a previous study. Experimental observations concerning carbon deposition in an SOFC operating on glycerol can be explained by the theoretical analysis developed in the present study. Additionally, the effect of mixed electronic-ionic conduction of the electrolyte on carbon deposition at the anode is discussed based on the thermodynamic analysis of the C-O system.     

An additional layer in an anode support for internal reforming of methane for solid oxide fuel cells

Anode supported solid oxide fuel cells (SOFCs) have been extensively investigated for their ease of fabrication, robustness, and high electrochemical performance. SOFCs offer a greater flexibility in fuel choice, such as methane, ethanol or hydrocarbon fuels, which may be supplied directly on the anode. In this study, SOFCs with an additional Ni–Fe layer on a Ni–YSZ support are fabricated with process variables and characterized for a methane fuel application. The addition of Ni–Fe onto the anode supports exhibits an increase in performance when methane fuel is supplied. SOFC with a Ni–Fe layer, sintered at 1000 °C and fabricated using a 20 wt% pore former, exhibits the highest value of 0.94 A cm⁻² and 0.85 A cm⁻² at 0.8 V with hydrogen and methane fuel, respectively. An impedance analysis reveals that SOFCs with an additional Ni–Fe layer has a lower charge transfer resistance than SOFCs without Ni–Fe layer. To obtain the higher fuel cell performance with methane fuel, the porosity and sintering temperature of an additional Ni–Fe layer need to be optimized.     

Performance analysis of hollow fibre-based micro-tubular solid oxide fuel cell utilising methane fuel

Solid oxide fuel cell (SOFC) has been studied as one of the most amazing development in energy production that could work directly with hydrocarbon fuel without reforming procedure. This study was conducted to analyse the micro-tubular solid oxide fuel cell (MT-SOFC) in terms of its performance by utilising methane as the fuel, subsequently compared with hydrogen. MT-SOFC that was investigated in this work consisted of thin cathode layer, coated onto co-extruded anode/electrolyte dual-layer hollow fibre (HF); in which its anode was made of nickel (Ni), coupled with cerium-gadolinium oxide (CGO) as an electrolyte, whereas the cathode was lanthanum strontium cobalt ferrite (LSCF) and CGO. The physical analyses carried out were three-point bending test and scanning electron microscopy (SEM). X-ray diffraction (XRD) analysis was further conducted to examine the carbon deposition in HFs. In evaluating the performance of HFs, current-voltage (IV) measurement, as well as impedance analysis of various temperatures range from 500 °C to 700 °C were performed. Based on the results, the OCV, maximum power density and ohmic ASR of MT-SOFC exposed to methane fuel, were at 0.79 V, 0.22 W cm⁻² and 0.31 Ω cm²; compared to the other that was exposed to hydrogen fuel, recorded at 0.89 V, 0.67 W cm⁻² and 0.19 Ω cm² respectively. This indicates that there was a significant reduction in cell performance when methane was used as the fuel, due to the carbon deposition as proven by SEM, three-point bending and XRD.     

Performance of Ni/ScSZ cermet anode modified by coating with Gd0.2Ce0.8O2 for an SOFC running on methane fuel

A Ni/scandia-stabilized zirconia (ScSZ) cermet anode was modified by coating with nano-sized gadolinium-doped ceria (GDC, Gd_0.2Ce_0.8O₂) prepared using a simple combustion process within the pores of the anode for a solid oxide fuel cell (SOFC) running on methane fuel. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed in the anode characterizations. Then, the short-term stability for the cells with the Ni/ScSZ and 2.0 wt.%GDC-coated Ni/ScSZ anodes in 97%CH₄/3%H₂O at 700 °C was checked over a relative long period of operation. Open circuit voltages (OCVs) increased from 1.098 to 1.179 V, and power densities increased from 224 to 848 mW cm⁻², as the operating temperature of an SOFC with 2.0 wt.%GDC-coated Ni/ScSZ anode was increased from 700 to 850 °C in humidified methane. The coating of nano-sized Gd_0.2Ce_0.8O₂ particle within the pores of the porous Ni/ScSZ anode significantly improved the performance of anode supported cells. Electrochemical impedance spectra (EIS) illustrated that the cell with Ni/ScSZ anode exhibited far greater impedances than the cell with 2.0 wt.%GDC-coated Ni/ScSZ anode. Introduction of nano-sized GDC particles into the pores of porous Ni/ScSZ anode will result in a substantial increase in the ionic conductivity of the anode and increase the triple phase boundary region expanding the number of sites available for electrochemical activity. No significant degradation in performance has been observed after 84 h of cell testing when 2.0 wt.%GDC-coated Ni/ScSZ anode was exposed to 97%CH₄/3%H₂O at 700 °C. Very little carbon was detected on the anodes, suggesting that carbon deposition was limited during cell operation. Consequently, the GDC coating on the pores of anode made it possible to have good stability for long-term operation due to low carbon deposition.     

Is steam addition necessary for the landfill gas fueled solid oxide fuel cells?

Landfill gas in Hong Kong – a mixture of about 50% (by volume) CH₄ and 50% CO₂ – can be utilized for power generation in a solid oxide fuel cell (SOFC). Conventional way of utilizing CH₄ in a SOFC is by adding H₂O to CH₄ to initiate methane steam reforming (MSR) and water gas shift reaction (WGSR). As the methane carbon dioxide reforming (MCDR: CH₄ + CO₂ ↔ 2CO + 2H₂) is feasible in the SOFC anode, it is unknown whether H₂O is needed or not for landfill gas fueled SOFC. In this study, a numerical model is developed to investigate the characteristics of SOFC running on landfill gas. Parametric simulations show that H₂O addition may decrease the performance of short SOFC at typical operating conditions as H₂O dilute the fuel concentration. However, it is interesting to find that H₂O addition is needed at reduced operating temperature, lower operating potential, or in SOFC with longer gas channel, mainly due to less temperature reduction in the downstream and easier oxidation of H₂ than CO. This preliminary study could help identify strategies for converting landfill gas into electrical power in Hong Kong.     

Thermodynamic performance comparison of SOFC-MGT-CCHP systems coupled with two different solar methane steam reforming processes

Multi-energy complementary distributed energy system integrated with renewable energy is at the forefront of energy sustainable development and is an important way to achieve energy conservation and emission reduction. A comparative analysis of solid oxide fuel cell (SOFC)-micro gas turbine (MGT)-combined cooling, heating and power (CCHP) systems coupled with two solar methane steam reforming processes is presented in terms of energy, exergy, environmental and economic performances in this paper. The first is to couple with the traditional solar methane steam reforming process. Then the produced hydrogen-rich syngas is directly sent into the SOFC anode to produce electricity. The second is to couple with the medium-temperature solar methane membrane separation and reforming process. The produced pure hydrogen enters the SOFC anode to generate electricity, and the remaining small amount of fuel gas enters the afterburner to increase the exhaust gas enthalpy. Both systems transfer the low-grade solar energy to high-grade hydrogen, and then orderly release energy in the systems. The research results show that the solar thermochemical efficiency, energy efficiency and exergy efficiency of the second system reach 52.20%, 77.97% and 57.29%, respectively, 19.05%, 7.51% and 3.63% higher than those of the first system, respectively. Exergy analysis results indicate that both the solar heat collection process and the SOFC electrochemical process have larger exergy destruction. The levelized cost of products of the first system is about 0.0735$/h that is lower than that of the second system. And these two new systems have less environmental impact, with specific CO₂ emissions of 236.98 g/kWh and 249.89 g/kWh, respectively.     

Oxidation-induced degradation and performance fluctuation of solid oxide fuel cell Ni anodes under simulated high fuel utilization conditions

High fuel utilization (Uf) conditions in a small-scale electrolyte-supported solid oxide fuel cell (SOFC) with an Ni-ScSZ anode were approximated by adjusting the gas composition to correspond to that in the downstream region of an SOFC stack. At Uf = 80%, and with a cell voltage of 0.5 V, the ohmic resistance fluctuated slightly from the early stages of operation, and became much more significant after 80 h. High current density and large polarization were found to promote Ni agglomeration, leading to insufficient connectivity of the Ni nanoparticles. At Uf = 95%, and with a cell voltage of 0.6 V, fluctuations in the polarization were observed at a much earlier stage, which are attributed to the highly humidified fuel. In particular, significant degradation was observed when the compensated anode potential (which incorporates the anode ohmic losses) approached the Ni oxidation potential. Ohmic losses in the anode are considered to influence Ni oxidation by exposing Ni near the electrolyte to a more oxidizing atmosphere with the increase in oxygen ion transport. Stable operation is therefore possible under conditions in which the compensated anode potential does not approach the Ni oxidation potential, assuming a stable interconnected Ni network.     

Importance of pressure gradient in solid oxide fuel cell electrodes for modeling study

The pressure gradients in the electrodes of a solid oxide fuel cell (SOFC) are frequently neglected without any justification in calculating the concentration overpotentials of the SOFC electrodes in modeling studies. In this short communication, a comparative study has been conducted to study the effect of pressure gradients on mass transfer and the resulting concentration overpotentials of an SOFC running on methane (CH₄) fuel. It is found that the pressure gradients in both anode and cathode are significant in the fuel cell electrochemical activities. Neglecting the anode pressure gradient in the calculation can lead to underestimation of the concentration overpotential by about 20% at a typical current density of 5000 A m⁻² and at a temperature of 1073 K. The deviation can be even larger at a higher temperature. At the cathode, neglecting the pressure gradient can result in overestimation of the concentration overpotential by about 10% under typical working conditions.     

Integration of solid oxide fuel cell and palladium membrane reactor: Technical and economic analysis

This paper presents a technical and economic analysis of a solid oxide fuel cell system equipped with a palladium membrane reactor (PMR–SOFC) with the aim of determining the benefits of such an integrated unit over the conventional reformer module (CON-SOFC). The performance of both SOFC systems under the conditions for energetically self-sustaining operation (Q_NET = 0) was achieved by varying the fuel utilization for each operating voltage. Two types of fuels, i.e., methane and desulphurized biogas, are considered. The simulation results show that the maximum power density of the CON-SOFC fuelled by methane (0.423 W/cm²) is higher than that of the CON-SOFC fuelled by biogas (0.399 W/cm²) due to the presence of CO₂ in biogas. For the PMR–SOFC, it is found that the operation at a higher permeation pressure offers higher power density because lower fuel utilization is required when operating the SOFC at the energy self-sustained condition. When the membrane reactor is operated at the permeation pressure of 1 bar, the methane-fuelled and biogas-fuelled PMR–SOFCs can achieve the maximum power density of 0.4398 and 0.4213 W/cm², respectively. Although the PMR–SOFC can offer higher power density, compared with the CON-SOFC, the capital costs of supporting units, i.e., palladium membrane reactor, high-pressure compressor, and vacuum pump, for PMR–SOFC need to be taken into account. The economic analysis shows that the PMR–SOFC is not a good choice from an economic viewpoint because of the requirement of a large high-pressure compressor for feeding gas to the membrane reactor.     

Stabilisation of composite LSFCO–CGO based anodes for methane oxidation in solid oxide fuel cells

A La_0.6Sr_0.4Fe_0.8Co_0.2O₃–Ce_0.8Gd_0.2O_1.9 (LSFCO–CGO) composite anode material was investigated for the direct electrochemical oxidation of methane in intermediate temperature solid oxide fuel cells (IT-SOFCs). A maximum power density of 0.17 W cm⁻² at 800 °C was obtained with a methane-fed ceria electrolyte-supported SOFC. A progressive increase of performance was recorded during 140 h operation with dry methane. The anode did not show any structure degradation after the electrochemical testing. Furthermore, no formation of carbon deposits was detected by electron microscopy and elemental analysis. Alternatively, this perovskite material showed significant chemical and structural modifications after high temperature treatment in a dry methane stream in a packed-bed reactor. It is derived that the continuous supply of mobile oxygen anions from the electrolyte to the LSFCO anode, promoted by the mixed conductivity of CGO electrolyte at 800 °C, stabilises the perovskite structure near the surface under SOFC operation and open circuit conditions.