Steam reforming on Ni-samaria-doped ceria cermet anode for practical size solid oxide fuel cell at intermediate temperatures

Direct internal and external reforming operations on Ni-samaria-doped ceria (SDC) anode with the practical size solid oxide fuel cell (SOFC) at intermediate temperatures from 600 to 750 °C are carried out to reveal the reforming activities and the electrochemical activities, being compared with the hydrogen-fueled power generation. The cell performance with direct internal and external steam reforming of methane and their limiting current densities were almost the same irrespective of the progress of reaction in the methane reformate at 700 and 750 °C. The durability test for 5.5 h at 750 °C with direct internal reforming operation confirmed that the cell performance did not deteriorate. The operation temperature of the cell controlled the reforming activities on the anode, and the large size electrode gave rise to high conversion due to the slow space velocity of the steam reforming. Direct internal steam reforming attained sufficient level of conversion for SOFC power generation with methane at 700 and 750 °C on the large Ni-SDC cermet anode.     

A new phenomenon of a fuel-free current during intermittent fuel flow over Ni-YSZ anode in direct methane SOFCs

A solid oxide fuel cell (SOFC) test unit was constructed with YSZ electrolyte as the support, and with Ni-YSZ anode (Ni:YSZ = 3:5 in weight) and Pt cathode. Direct methane SOFC operation at 800 °C with 10% CH₄ in argon was carried out. A new phenomenon of the generation of the electrical current without the fuel was observed and termed the fuel-free current. An operation of intermittent methane supply was designed to take advantage of three driving forces, i.e. methane in the gas phase, the deposited carbon at the anode surface, and a deficiency of the bulk lattice-oxygen concentration on the anode side, for the generation of the electrical current. A continuous generation of the electrical current is obtained with a methane pulse of only one-fifth of the total operation time. The operation of intermittent methane flow can reduce or even avoid SOFC deactivation by the carbon deposition; at the same time, the deposited carbon can be fully utilized for the power generation. It was also found that hydrogen from methane has been mostly evolved to the outlet gaseous product and the amount of CO formation is much higher than that of CO₂; the operation of intermittent methane flow can further increase the amount of CO over that of CO₂; these are beneficial for the co-generation of synthesis gas.     

Direct methane solid oxide fuel cell working by gradual internal steam reforming: Analysis of operation

A solid oxide fuel cell was designed to be operated with pure hydrocarbons, without additive or carrier gas, in order to bring technological simplifications, cost reductions and to extend the fuel flexibility limits. The cell was built-up from a conventional cell (LSM/YSZ/Ni-YSZ), to which was added a Ir-CeO₂ catalyst layer at the anode side and an original current collecting system. The cell was first operated with steam in gradual internal reforming (GIR) conditions (R = [H₂O]/[CH₄] < 1) with carrier gas at the anode. The optimal operating parameters were determined in terms of flow rates, cell potential, and fuel utilisation. The cell was finally operated with pure dry methane at 900 °C at 0.6 V yielding current density of about 0.1 A cm⁻² at max power for 120 h. Small but abrupt deterioration of the performances was observed, but no carbon deposition. Electrical and chemical analysis of this degradation are provided.At total, the fuel cell was operated for more than 200 h in pure dry methane, demonstrating that gradual internal reforming actually occurred efficiently in the anode compartment, which make possible operation without reforming agent such as H₂O or CO₂ for other hydrocarbon fuels.     

Catalytic steam reforming of methane,methanol, and ethanol over Ni/YSZ: The possible use of these fuels in internal reforming SOFC

This study investigated the possible use of methane, methanol, and ethanol with steam as a direct feed to Ni/YSZ anode of a direct internal reforming Solid Oxide Fuel Cell (DIR-SOFC). It was found that methane with appropriate steam content can be directly fed to Ni/YSZ anode without the problem of carbon formation, while methanol can also be introduced at a temperature as high as 1000 °C. In contrast, ethanol cannot be used as the direct fuel for DIR-SOFC operation even at high steam content and high operating temperature due to the easy degradation of Ni/YSZ by carbon deposition. From the steam reforming of ethanol over Ni/YSZ, significant amounts of ethane and ethylene were present in the product gas due to the incomplete reforming of ethanol. These formations are the major reason for the high rate of carbon formation as these components act as very strong promoters for carbon formation.     

The impact of steam and current density on carbon formation from biomass gasification tar on Ni/YSZ, and Ni/CGO solid oxide fuel cell anodes

The combination of solid oxide fuel cells (SOFCs) and biomass gasification has the potential to become an attractive technology for the production of clean renewable energy. However the impact of tars, formed during biomass gasification, on the performance and durability of SOFC anodes has not been well established experimentally. This paper reports an experimental study on the mitigation of carbon formation arising from the exposure of the commonly used Ni/YSZ (yttria stabilized zirconia) and Ni/CGO (gadolinium-doped ceria) SOFC anodes to biomass gasification tars. Carbon formation and cell degradation was reduced through means of steam reforming of the tar over the nickel anode, and partial oxidation of benzene model tar via the transport of oxygen ions to the anode while operating the fuel cell under load. Thermodynamic calculations suggest that a threshold current density of 365 mA cm⁻² was required to suppress carbon formation in dry conditions, which was consistent with the results of experiments conducted in this study. The importance of both anode microstructure and composition towards carbon deposition was seen in the comparison of Ni/YSZ and Ni/CGO anodes exposed to the biomass gasification tar. Under steam concentrations greater than the thermodynamic threshold for carbon deposition, Ni/YSZ anodes still exhibited cell degradation, as shown by increased polarization resistances, and carbon formation was seen using SEM imaging. Ni/CGO anodes were found to be more resilient to carbon formation than Ni/YSZ anodes, and displayed increased performance after each subsequent exposure to tar, likely due to continued reforming of condensed tar on the anode.     

The interaction of biomass gasification syngas components with tar in a solid oxide fuel cell and operational conditions to mitigate carbon deposition on nickel-gadolinium doped ceria anodes

The combination of biomass gasification with solid oxide fuel cells (SOFCs) is gaining increasing interest as an efficient and environmentally benign method of producing electricity and heat. However, tars in the gas stream arising from the gasification of biomass material can deposit carbon on the SOFC anode, having detrimental effects to the life cycle and operational characteristics of the fuel cell. This work examines the impact of biomass gasification syngas components combined with benzene as a model tar, on carbon formation on Ni/CGO (gadolinium-doped ceria) SOFC anodes. Thermodynamic calculations suggest that SOFCs operating at temperatures > 750 °C are not susceptible to carbon deposition from a typical biomass gasification syngas containing 15 g m⁻³ benzene.However, intermediate temperature SOFCs operating at temperatures < 650 °C require threshold current densities well above what is technologically achievable to inhibit the effects of carbon deposition. SOFC anodes have been shown to withstand tar levels of 2-15 g m⁻³ benzene at 765 °C for 3 h at a current density of 300 mA cm⁻², with negligible impact on the electrochemical performance of the anode. Furthermore, no carbon could be detected on the anode at this current density when benzene levels were <5 g m⁻³.     

Internal reforming characteristics of cermet supported solid oxide fuel cell using yttria stabilized zirconia fed with partially reformed methane

In order to investigate the internal reforming characteristics in a cermet supported solid oxide fuel cell (SOFC) using YSZ as the electrolyte, the concentration profiles of the gaseous species along the gas flow direction in the anode were measured. Partially reformed methane using a pre-reformer kept at a constant temperature is supplied to the center of the cell which is operated with a seal-less structure at the gas outlet. The anode gas is sucked in via silica capillaries to the initially evacuated gas tanks. The process is simultaneously carried out using five sampling ports. The sampled gas is analyzed by a gas chromatograph. Most of the measurements are made at the cell temperature (T_cell) of 750 °C and at various temperatures of the pre-reformer (T_ref) with various fuel utilizations (U_f) of the cell. The composition of the fuel at the inlet of the anode was confirmed to be almost the same as that theoretically calculated assuming equilibrium at the temperature of the pre-reformer. The effect of internal reforming in the anode is clearly observed as a steady decrease in the methane concentration along the flow axis. The effect of the water-gas shift reaction is also observed as a decrease in the CO₂ concentration and an increase of CO concentration around the gas inlet region, as the water-gas shift reaction inversely proceeds when T_cell is higher than T_ref. The diffusion of nitrogen from the seal-less outermost edge is observed, and the diffusion is confirmed to be more significant as U_f decreases. The observations are compared with the results obtained by the SOFC supported by lanthanum gallate electrolyte. With respect to the internal reforming performance, the cell investigated here is found to be more effective when compared to the previously reported electrolyte supported cell.     

Modeling of SOFC running on partially pre-reformed gas mixture

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.     

Elementary reaction kinetic model of an anode-supported solid oxide fuel cell fueled with syngas

In this paper, a detailed one-dimension transient elementary reaction kinetic model of an anode-supported solid oxide fuel cell (SOFC) operating with syngas based on button cell geometry is developed. The model, which incorporates anodic elementary heterogeneous reactions, electrochemical kinetics, electrodes microstructure and complex transport phenomena (momentum, mass and charge transport) in positive electrode|electrolyte|negative electrode (PEN), is validated with experimental performance for various syngas compositions at 750, 800 and 850 °C. The comparisons show that the simulation results agree reasonably well with the experimental data. Then the model is applied to analyze the effects of temperature and operation voltage on polarizations in each component of PEN, electronic current density in both electrodes and species concentrations distributions in anode. The numerical results of carbon deposition simulation indicate that higher temperature and lower operation voltage are helpful to reduce the possibility of carbon deposition on Ni surfaces by Bouduard reactions. Furthermore, a sensitivity analysis of cell performance on syngas composition is performed for the typical syngas from entrained-flow coal gasifier and natural methane thermochemical reforming processes. The cell performance increases with the increasing of effective compositions (e.g. H₂ and CO) in syngas and the large N₂ content introduced by using air as oxidant leads to significant deterioration of performance.     

Experimental investigation of temperature distribution of planar solid oxide fuel cell: Effects of gas flow,power generation,and direct internal reforming

The temperature distribution of an operating planar solid oxide fuel cell (SOFC) is experimentally investigated under direct internal reforming conditions. An in situ measurement is conducted using a cell holder and an infrared (IR) camera. The effects of the gas flow configuration, exothermic power generation reaction, and endothermic steam–methane reforming reaction are examined at a furnace temperature of 770 °C. The fuel flow and airflow are set to a coflow or counterflow configuration. The heat generation and absorption by the reactions are varied by tuning the average current density and the concentration of methane in the supplied fuel. The maximum value of the local temperature gradient along the cell tends to increase with increasing internal reforming ratio, regardless of the gas flow configuration. From the view point of a small temperature gradient, the counterflow configuration clearly shows better characteristics than that of the coflow, regardless of the internal reforming ratio.     

High-entropy alloy anode for direct internal steam reforming of methane in SOFC

High-entropy alloy (HEA) anode and reforming catalyst, supported on gadolinium-doped ceria (GDC), have been synthesized and evaluated for the steam reforming of methane under SOFC operating conditions using a conventional fixed-bed catalytic reactor. As-synthesized HEA catalysts were subjected to various characterization techniques including N₂ adsorption/desorption analysis, SEM, XRD, TPR, TPO and TPD. The catalytic performance was evaluated in a quartz tube reactor over a temperature range of 700–800 °C, pressure of 1 atm, gas hourly space velocity (GHSV) of 45,000 h^?1 and steam-to-carbon (S/C) ratio of 2. The conversion and H₂ yield were calculated and compared. HEA/GDC exhibited a lower conversion rate than those of Ni/YSZ and Ni/GDC at 700 °C, but showed superior stability without any sign of carbon deposition unlike Ni base catalyst. HEA/GDC was further evaluated as an anode in a SOFC test, which showed high electrochemical stability with a comparable current density obtained on Ni electrode. The SOFC reported low and stable electrode polarization. Post-test analysis of the cell showed the absence of carbon at and within the electrode. It is suggested that HEA/GDC exhibits inherent robustness, good carbon tolerance and stable catalytic activity,` which makes it a potential anode candidate for direct utilization of hydrocarbon fuels in SOFC applications.     

Electrolytic effect in solid oxide fuel cells running on steam/methane mixture

A two-dimensional model is developed to study the performance of a planar solid oxide fuel cell (SOFC) running on steam/methane mixture. The model considers the heat/mass transfer, electrochemical reactions, direct internal reforming of methane (CH₄), and water gas shift reaction in an SOFC. It is found that at an operating potential of 0.8 V, the upstream and downstream of SOFC work in electrolysis and fuel cell modes, respectively. At the open-circuit voltage, the electricity generated by the downstream part of SOFC is completely consumed by the upstream through electrolysis, which is contrary to our common understanding that electrochemical reactions cease under the open-circuit conditions. In order to inhibit the electrolytic effect, the SOFC can be operated at a lower potential or use partially pre-reformed CH₄ as the fuel. Increasing the inlet gas velocity from 0.5 m s⁻¹ to 5.0 m s⁻¹ does not reduce the electrolytic effect but decreases the SOFC performance.     

Microtubular SOFC anode optimisation for direct use on methane

The main problems of small-scale solid oxide fuel cell (SOFC) devices are the rapid start-up, durability and operation on available fuels such as methane. This paper describes how microtubular anode-supported SOFCs can be started rapidly and run on methane. However, the key factor was the activity of the nickel anode, especially its surface area and conductivity, which were found to depend on the reduction method and the operating fuel. Controlled reduction experiments in hydrogen at temperatures between 650 and 850 °C were performed. Reduction temperature and gas composition were altered and the resultant electrical performance and exhaust gases recorded. The conclusion was that microtubular SOFC can be successfully run on methane to outperform pure hydrogen by up to 9%.     

Thermodynamic analysis and experimental study of electrode reactions and open circuit voltages for methane-fuelled SOFC

Natural gas is one of the most important fuels for solid oxide fuel cell (SOFC). The relationships among the reactions of methane over the nickel-based anode, fuel compositions, carbon deposition, electromotive force (EMF) and open circuit voltage (OCV) of SOFC are investigated in this work. With the increase of temperature, EMF and OCV of SOFC decrease gradually when the cell uses humidified hydrogen as fuel. Reactivity of methane increases gradually with the increase of temperature, which can affect the EMF and OCV of SOFC. When the humidified mixture of nitrogen and methane is used as the fuel, the EMF and OCV of SOFC increase gradually with the increase of temperature. EMF and OCV of SOFC with humidified mixture of hydrogen and methane (M_CH4: M_H2: M_H2O = 12.2: 85.3: 2.5) as fuel decrease gradually with the increase of temperature when the temperature is lower than 873 K, which is similar to that with humidified hydrogen as fuel. While when the temperature is higher than 923 K, the EMF and OCV of SOFC with humidified mixture of hydrogen and methane as fuel increase gradually with the increase of temperature, which is similar to that with humidified mixture of nitrogen and methane as fuel. OCV of SOFC is mainly affected by thermodynamic equilibriums for methane-fuelled SOFC when the anode activity is high enough, which is close to the EMF calculated according to the thermodynamic equilibriums. While with the increase of carbon deposition, the anode activity decreases apparently and the OCV of SOFC also decreases apparently, which shows that the OCV is affected by the anode activity for methane-fuelled SOFC when the anode activity is low.     

High temperature electrolyte supported Ni-GDC/YSZ/LSM SOFC operation on two-stage Viking gasifier product gas

This paper presents the results from a 150 h test of a commercial high temperature single planar solid oxide fuel cell (SOFC) operating on wood gas from the Viking two-stage fixed-bed downdraft gasifier, which produces an almost tar-free gas, that was further cleaned for particulates, sulphur and tar traces. The chosen SOFC was electrolyte supported with a nickel/gadolinium-doped cerium oxide (Ni-GDC) anode, known for its carbon deposition resistance. Through humidification the steam to carbon ratio (S/C) was adjusted to 0.5, which results in a thermodynamically carbon free condition at the SOFC operating temperature T = 850 °C. The cell operated with a fuel utilisation factor (U_f) around 30% and a current density of 260 mA cm⁻² resulting in an average power density of 207 mW cm⁻². Throughout the duration of the test, only a minor cell overpotential increase of 10 mV was observed. Nevertheless, the V–j (voltage–current density) curves on H₂/N₂ before and after the wood gas test proved identical. Extensive SEM/EDS examination of the cell's anode showed that there was neither carbon deposition nor significant shifts in the anode microstructure or contamination when compared to an identical cell tested on H₂/N₂ only.     

Combustion characteristics of mixture of anode off gas and LNG in reformer

Fuel processing system which converts hydrocarbon fuel into hydrogen rich gas (by stream reforming, partial oxidation, auto-thermal reforming) needs high temperature environment (600-1000 °C). Generally, anode off gas or mixture of anode off gas and LNG are used as input gas for a fuel reformer. In order to constitute efficient and low emission burner system for fuel reformer, it is necessary to elucidate the combustion and emission characteristics of fuel reformer burner. In this study, lean flat flame using the ceramic porous burner was analyzed numerically and experimentally. Burning velocity of anode off gas calculated by CHEMKIN simulation was 51.8 cm, which was faster than that of LNG having 40.63 cm/s at the stoichiometric ratio because of high composition of hydrogen in anode off gas. As composition of LNG in mixture of anode off gas + LNG is increased, the burning velocity decreases and in the other hand the adiabatic temperature increases. CO, NO_x were measured below 50 ppm in operating load range of the reformer. Blue flame pattern was found as stable flame region for design of fuel reformer and anode off gas flame was maintained in blue flame pattern at equivalence ratio 0.55-0.62 under 1-5 kW power range.     

Internal reforming of hydrocarbon fuels in tubular solid oxide fuel cells

The application of heterogeneous catalysis has an important role to play in the successful commercial development of solid oxide fuel cell (SOFC) technology. In this paper, we present an SOFC that combines a catalyst layer with a conventional anode, allowing internal reforming via partial oxidation (POX) of fuels such as methane, propane, butane, biomass gas, etc., without coking and yielding stable power output. The catalyst layer is fabricated on the anode simply by catalyst support coating and reforming catalyst impregnation. The composition and microstructure of catalyst support layer as well as the catalyst composition was easily tailored to meet the demand of in situ reforming. The usage of catalyst layer as an integrated part of the traditional SOFC will provide a simple low-cost power-generating system at substantially higher fuel efficiency and faster start-ups, and may accelerate the application of SOFCs through the direct use of hydrocarbon fuels.     

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.     

Au-doped Ni/GDC as a new anode for SOFCs operating under rich CH4 internal steam reforming

In the present work we describe a Solid Oxide Fuel Cell (SOFC) that comprises a Ni/GDC cermet anode, doped with a potentially commercially viable concentration of gold nano-particles. Specifically, gold was applied prior to anode sintering via the deposition–precipitation method. This procedure resulted in a SOFC that allowed carbon tolerant operation at T = 850 °C under fuel rich internal steam reforming of methane, with a stable power density of 0.41 W cm⁻² at 810 mV for over 200 h.     

Impact of gas-phase reactions in the mixing region upstream of a diesel fuel autothermal reformer

The use of diesel fuel to power a solid oxide fuel cell (SOFC) presents several challenges. A major issue is deposit formation in either the external reformer, the anode channel, or within the SOFC anode itself. One potential cause of deposit formation under autothermal reforming conditions is the onset of gas-phase reactions upsteam of the catalyst to form ethylene, a deposit precursor. Another potential problem is improper mixing of the fuel, air, and steam streams. Incomplete mixing leads to fuel rich gas pockets in which gas phase pyrolysis chemistry might be accelerated to produce even more ethylene. We performed a combined experiment/modeling analysis to identify combinations of temperature and reaction time that might lead to deposit formation. Two alkanes, n-hexane and n-dodecane, were selected as surrogates for diesel fuel since a detailed mechanism is available for these species. This mechanism was first validated against n-hexane pyrolysis data. It was then used to predict fuel conversion and ethylene production under a variety of reforming conditions, ranging from steam reforming to catalytic partial oxidation. Assuming that the reactants are perfectly mixed at 800 K, the predictions suggest that a mixture must reach the catalyst in less than 0.1 s to avoid formation of potentially troublesome quantities of ethylene. Additional calculations using a simple model to account for improper mixing demonstrate the need for the components to be transported to the catalyst on a much shorter time scale, since both the relatively lean and relatively rich regions react faster and rapidly form ethylene.