A high-performance ammonia-fueled SOFC based on a YSZ thin-film electrolyte

Ammonia is a possible candidate as the fuel for solid oxide fuel cells (SOFCs). In this work, an anode-supported SOFC based on yttrium-stabled zircite (YSZ) thin-film electrolyte was fabricated by a simple dry-pressing process. Directly fueled by commercial liquefied ammonia, the single cell was tested at temperatures from 650 to 850 °C. The maximum power densities were 299 and 526 mW cm⁻² at 750 and 850 °C, respectively, only slightly lower than that fueled by hydrogen. Analysis of open current voltages (OCVs) of the cell indicated the oxidation of ammonia within a SOFC is a two-stage process. Impedance spectra showed the cell fueled by ammonia had the same electrolyte resistances as that fueled by hydrogen, but a little larger interfacial polarization resistances. Further, the performances of the cell were essentially determined by the interfacial resistances under 750 °C.     

Comparative study on the performance of a SDC-based SOFC fueled by ammonia and hydrogen

A nickel-based anode-supported solid oxide fuel cell (SOFC) was assembled with a 10 μm thick Ce_0.8Sm_0.2O_2−δ (SDC) electrolyte and a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathode. The cell performance was investigated with hydrogen and ammonia gas evaporated from liquefied ammonia as fuel. Fueled by hydrogen the maximum power densities were 1872, 1357, and 748 mW cm⁻² at 650, 600, and 550 °C, respectively. While with ammonia as fuel, the cell showed the maximum power densities of 1190, 434, and 167 mW cm⁻², correspondingly. The power densities lower than that predicted, particularly at the lower operating temperatures for ammonia fuel cell, compared to hydrogen fuel cell, could be attributed to actual lower temperature than thermocouple display due to endothermic reaction of ammonia decomposition as well as the rather larger inlet ammonia flow rate. The results demonstrated that the ammonia was a right convenient liquid fuel for SOFCs as long as it was keeping the decomposition completion of ammonia in the cell or before entering the cell.     

Externally reformed solid oxide fuel cell–micro-gas turbine (SOFC–MGT) hybrid systems fueled by methanol and di-methyl-ether (DME)

Solid oxide fuel cell–micro-gas turbine (SOFC–MGT) hybrid power plants integrate a solid oxide fuel cell and a micro-gas turbine and can achieve efficiencies of over 60% even for small power outputs (200–500 kW). The SOFC–MGT systems currently developed are fueled with natural gas, which is reformed inside the same stack, but the use of alternative fuels can be an interesting option. In particular, as the reforming temperature of methanol and di-methyl-ether (DME) (200–350 °C) is significantly lower than that of natural gas (700–900 °C), the reformer can be sited outside the stack. External reforming in SOFC–MGT plants fueled by methanol and DME enhances efficiency due to improved exhaust heat recovery and higher voltage produced by the greater hydrogen partial pressure at the anode inlet. The study carried out in this paper shows that the main operating parameters of the fuel reforming section (temperature and steam-to-carbon ratio (SCR)) must be carefully chosen to optimise the hybrid plant performance. For the stoichiometric SCR values, the optimum reforming temperature for the methanol fueled hybrid plant is approximately 240 °C, giving efficiencies of about 67–68% with a SOFC temperature of 900 °C (the efficiency is about 72–73% at 1000 °C). Similarly, for DME the optimum reforming temperature is approximately 280 °C with efficiencies of 65% at 900 °C (69% at 1000 °C). Higher SCRs impair stack performance. As too small SCRs can lead to carbon formation, practical SCR values are around one for methanol and 1.5–2 for DME.     

Carbon doped MO–SDC material as an SOFC anode

Oxide mixtures MO–SDC, M = Cu, Ni, Co, SDC = Ce_0.9Sm_0.1O_1.95 were synthesized by employing a citrate/nitrate combustion technique. Two kinds of Carbon materials, activated carbon (AC) and vapor grown carbon fiber (VGCF) were homogeneously dispersed into the MO–SDC. The materials can be used as anodes to fabricate single cells using a uniaxial die-press method. The sintering temperature was studied to optimize cell performance. Experimental results showed that cells sintered at 700 °C had better performance. When the temperature was above 750 °C, the cells were severely distorted, and cannot be tested. Compared with the basic MO–SDC anode, AC and VGCF improve the solid oxide fuel cell (SOFC) anode properties, due to a change of the microstructures of the anode materials which enhance their electron conductivity. Single cell performances were evaluated by I–V measurements, and when 1.25 wt.%VGCF was introduced into the MO–SDC by ball-milling, termed: 1.25 wt.%VGCF–MO–SDC, the 1.25 wt.%VGCF–MO–SDC anode material could achieve the highest power density of up to 0.326 W cm⁻² with H₂ as fuel. The calcination temperature of the MO–SDC dry gel also strongly influenced the electrochemical performance of the 1.25 wt.%VGCF–MO–SDC material. XRD spectra for each calcined temperature and the I–V measurement both suggest that calcinations at 550 °C for 1 h are suitable. 1.0 wt.%AC–MO–SDC and 1.25 wt.%VGCF–MO–SDC have similar performance when the cell was fed in methanol/3%H₂O, and the corresponding power density was up to 0.253 W cm⁻². Traces of carbon were found in the off-gases.     

Mixed transport properties of Ce1−xSmxO2−x/2 system under fuel cell operating conditions

Ceria-samaria compositions were prepared by freeze-drying, yielding very homogeneous and fluorite single phase powders at temperatures as low as 375 °C. These powders were used to obtain dense ceramics by sintering at 1600 °C for 10 h. The mixed transport properties of these materials were characterized by impedance spectroscopy and the Hebb–Wagner ion-blocking method, at temperatures in the range 700–950 °C. The onset of n-type electronic conductivity was expressed as a function of oxygen partial pressure and temperature, and was found dependent on the Sm-content. Mixed transport properties were also analyzed for conditions in which the samples are exposed to fuels in a SOFC, either as electrolyte or component of cermet anodes. The degree of fuel conversion and corresponding changes in gas composition produce changes in the mixed transport properties of SOFC electrolytes, with emphasis on the fuel side. The mixed transport properties were also used to estimate open cell voltage as a function of fuel conversion.     

Proton-conducting fuel cells operating on hydrogen,ammonia and hydrazine at intermediate temperatures

Anode-supported proton-conducting fuel cell with BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) electrolyte and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathode was fabricated. Peak power densities of ∼420 and 135 mW/cm² were achieved, respectively, at 700 and 450 °C for a cell with 35 μm thick electrolyte operating on hydrogen fuel. The endothermic nature of the ammonia decomposition reaction, however, resulted in cell temperature 30–65 °C lower than the furnace when operating on ammonia. Accounting the cooling effect, comparable power density was achieved for the cell operating on ammonia and hydrogen at high temperature. At reduced temperature, the cell demonstrated worse performance when operating on ammonia than on hydrogen due to the poor activity of the anode towards NH₃ catalytic decomposition. By applying on-line catalytic decomposition products of N₂H₄ as the fuel, similar cell performance to that with NH₃ fuel was also observed.     

Low-temperature ceria-electrolyte solid oxide fuel cells for efficient methanol oxidation

Low temperature anode-supported solid oxide fuel cells with thin films of samarium-doped ceria (SDC) as electrolytes, graded porous Ni-SDC anodes and composite La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF)-SDC cathodes are fabricated and tested with both hydrogen and methanol fuels. Power densities achieved with hydrogen are between 0.56 W cm⁻² at 500 °C and 1.09 W cm⁻² at 600 °C, and with methanol between 0.26 W cm⁻² at 500 °C and 0.82 W cm⁻² at 600 °C. The difference in the cell performance can be attributed to variation in the interfacial polarization resistance due to different fuel oxidation kinetics, e.g., 0.21 Ω cm² for methanol versus 0.10 Ω cm² for hydrogen at 600 °C. Further analysis suggests that the leakage current densities as high as 0.80 A cm⁻² at 600 °C and 0.11 A cm⁻² at 500 °C, resulting from the mixed electronic and ionic conductivity in the SDC electrolyte and thus reducing the fuel efficiency, can nonetheless help remove any carbon deposit and thereby ensure stable and coking-free operation of low temperature SOFCs in methanol fuels.     

Design of compact methanol reformer for hydrogen with low CO for the fuel cell power generation

The effect of the heat transfer area and the thermal conductivity of the reactor materials are evaluated with three identical structured reactors having multiple columned-catalyst bed and using three different reactor materials, aluminum alloy, brass and stainless steel. A series of compact methanol reformers are then designed and fabricated with the use of large reactor surface area in catalyst beds and high heat transfer constant to produce hydrogen fuel with 2–4 ppm of CO for the fuel cell (FC) power generation. The same design principle is successfully used for easy scale up of the reactor capacity from 250 L/h to 10,000 L/h. This low CO hydrogen (68–70%) used as the fuel for the fuel cell power generation provides a very competitive cost of hydrogen and electric power, $0.20–0.23/m³ of H₂ and $0.196/KWh, respectively.     

Experimental study on effect of compaction pressure on performance of SOFC anodes

NiO/yttria-stabilized zirconia (YSZ) anode substrates were fabricated at two compaction pressures of 200 and 1000 MPa, the particle size distributions of NiO and YSZ were investigated with powders treated under different conditions using a laser scattering technique (Mastersizer 2000, Malvern Instruments) and the effect of compaction pressure on the performance of solid oxide fuel cell (SOFC) anodes was investigated by studying the effect of compaction pressure on compaction density, sintered density, sintering shrinkage behavior, electronic and ionic conductivities. The results of investigation indicated that the SOFC with the anode compacted at a higher pressure exhibited a superior output performance, for example, a single cell with hydrogen as fuel and oxygen as oxidant exhibited excellent maximum power densities of 2.77 and 0.90 W cm⁻² at 800 and 650 °C, respectively, which suggested the development of an intermediate temperature SOFC through optimization of anode fabrication parameters.     

Direct alkaline fuel cell for multiple liquid fuels: Anode electrode studies

A direct alkaline fuel cell with a liquid potassium hydroxide solution as an electrolyte is developed for the direct use of methanol, ethanol or sodium borohydride as fuel. Three different catalysts, e.g., Pt-black or Pt/Ru (40 wt.%:20 wt.%)/C or Pt/C (40 wt.%), with varying loads at the anode against a MnO₂ cathode are studied. The electrodes are prepared by spreading the catalyst slurry on a carbon paper substrate. Nickel mesh is used as a current-collector. The Pt–Ru/C produces the best cell performance for methanol, ethanol and sodium borohydride fuels. The performance improves with increase in anode catalyst loading, but beyond 1 mg cm⁻² does not change appreciably except in case of ethanol for which there is a slight improvement when using Pt–Ru/C at 1.5 mA cm⁻². The power density achieved with the Pt–Ru catalyst at 1 mg cm⁻² is 15.8 mW cm⁻² at 26.5 mA cm⁻² for methanol and 16 mW cm⁻² at 26 mA cm⁻² for ethanol. The power density achieved for NaBH₄ is 20 mW cm⁻² at 30 mA cm⁻² using Pt-black.     

Synergistic integration of a gas turbine and solid oxide fuel cell for improved transient capability

A theoretical solid oxide fuel cell–gas turbine hybrid system has been designed using a Capstone 60 kW micro-gas turbine. Through simulation it is demonstrated that the hybrid system can be controlled to achieve transient capability greater than the Capstone 60 kW recuperated gas turbine alone. The Capstone 60 kW gas turbine transient capability is limited because in order to maintain combustor, turbine and heat exchangers temperatures within operating requirements, the Capstone combustor fuel-to-air ratio must be maintained. Potentially fast fuel flow rate changes, must be limited to the slower, inertia limited, turbo machinery air response. This limits a 60 kW recuperated gas turbine to transient response rates of approximately 1 kW s⁻¹. However, in the SOFC/GT hybrid system, the combustor temperature can be controlled, by manipulating the fuel cell current, to regulate the amount of fuel sent to the combustor. By using such control pairing, the fuel flow rate does not have to be constrained by the air flow in SOFC/GT hybrid systems. This makes it possible to use the rotational inertia of the gas turbine, to buffer the fuel cell power response, during fuel cell fuel flow transients that otherwise limit fuel cell system transient capability. Such synergistic integration improves the transient response capability of the integrated SOFC gas turbine hybrid system. Through simulation it has been demonstrated that SOFC/GT hybrid system can be developed to have excellent transient capability.     

Fabrication of micro-tubular solid oxide fuel cells with a single-grain-thick yttria stabilized zirconia electrolyte

This study discusses the fabrication and electrochemical performance of micro-tubular solid oxide fuel cells (SOFCs) with an electrolyte consisting a single-grain-thick yttria stabilized zirconia (YSZ) layer. It is found that a uniform coating of an electrolyte slurry and controlled shrinkage of the supported tube leads to a dense, crack-free, single-grain-thick (less than 1 μm) electrolyte on a porous anode tube. The SOFC has a power density of 0.39 W cm⁻² at an operating temperature as low as 600 °C, with YSZ and nickel/YSZ for the electrolyte and anode, respectively. An examination is made of the effect of hydrogen fuel flow rate and shown that a higher flow rate leads to better cell performance. Hence a YSZ cell can be used for low-temperature SOFC systems below 600 °C, simply by optimizing the cell structure and operating conditions.     

Novel copper-based anodes for solid oxide fuel cells with samaria-doped ceria electrolyte

A novel copper-based anode for low-temperature solid oxide fuel cells was prepared through the conventional ceramic technology and using CuO and SDC (Ce_0.8Sm_0.2O_1.9) powders with controlled particle size. The new Cu–SDC anode also contained highly dispersed CeO₂ and Ni particles to increase its surface area and fuel cell performance. The specific surface area of the Cu–SDC bare anode, CeO₂ and Ni-dispersed phases were estimated to be 1.53, 39.4 and 86.4 m² g⁻¹, respectively. Solid oxide fuel cells having the new anode were tested for both humid hydrogen and methane. Power densities of ca. 250 mW cm⁻² were achieved in H₂ at 600 °C and in CH₄ 700 °C, even if the SDC–electrolyte supporting membrane was 250-μm thick. Short term stability tests (maximum 64 h) showed an initial impairment, but not dramatic, of the new anode performance and the formation of carbon deposits. The addition of MoO_x to the new anode did not prevent the formation of carbon deposits.     

Electrochemical impedance studies on carbon supported PtRuNi and PtRu anode catalysts in acid medium for direct methanol fuel cell

The anodic Pt–Ru–Ni/C and the Pt–Ru/C catalysts for potential application in direct methanol fuel cell (DMFC) were prepared by chemical reduction method. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were carried out by using a glassy carbon working electrode covered with the catalyst powder in a solution of 0.5 mol L⁻¹ CH₃OH and 0.5 mol L⁻¹ H₂SO₄ at 25 °C. EIS information discloses that the methanol electrooxidation on the Pt–Ru–Ni/C catalyst at various potentials shows different impedance behaviors. The mechanism and the rate-determining step of methanol electrooxidation are changed with increasing potential. Its rate-determining steps are the methanol dehydrogenation and the oxidation reaction of adsorbed intermediate CO_ads and OH_ads in low (400–500 mV) and high (600–800 mV) potentials, respectively. The catalytic activity of the Pt–Ru–Ni/C catalyst is higher for methanol electrooxidation than that of the Pt–Ru/C catalyst. Its tolerance performance to CO formed as one of the intermediates of methanol dehydrogenation is also better than that of the Pt–Ru/C catalyst.     

Orthogonal flow membraneless fuel cell

This communication reports the development and performance of a membraneless fuel cell that utilizes a convective electrolyte flow orthogonal to the plane of the cell electrodes. The orthogonal flow acts to convect reactants to the electrodes and as a pseudo-membrane between anode and cathode, preventing mixed potentials caused by back diffusion of oxidant onto the anode. The membraneless design allows the cell parameters to be defined by the fuel rather than the membrane. The convective electrolyte flow eliminates the mass transport limited current regime and allows the cell to run under semi-mixed conditions. Power densities as high as 46 mW cm⁻² were achieved with both hydrogen and methanol as fuels. Methanol had a fuel utilization of 42%.     

Dynamic evaluation of low-temperature metal-supported solid oxide fuel cell oriented to auxiliary power units

A metal-supported solid oxide fuel cell (SOFC) composed of a Ni–Ce_0.8Sm_0.2O_2−δ (Ni–SDC) cermet anode and an SDC electrolyte was fabricated by suspension plasma spraying on a Hastelloy X substrate. The cathode, an Sm_0.5Sr_0.5CoO₃ (SSCo)–SDC composite, was screen-printed and fired in situ. The dynamic behaviour of the cell was measured while subjected to complete fuel shutoff and rapid start-up cycles, as typically encountered in auxiliary power units (APU) applications. A promising performance – with a maximum power density (MPD) of 0.176 W cm⁻² at 600 °C – was achieved using humidified hydrogen as fuel and air as the oxidant. The cell also showed excellent resistance to oxidation at 600 °C during fuel shutoff, with only a slight drop in performance after reintroduction of the fuel. The Cr and Mn species in the Hastelloy X alloy appeared to be preferentially oxidized while the oxidation of nickel in the metallic substrate was temporarily alleviated. In rapid start-up cycles with a heating rate of 60 °C min⁻¹, noticeable performance deterioration took place in the first two thermal cycles, and then continued at a much slower rate in subsequent cycles. A postmortem analysis of the cell suggested that the degradation was mainly due to the mismatch of the thermal expansion coefficient across the cathode/electrolyte interface.     

Anode-supported micro tubular SOFCs for advanced ceramic reactor system

In this study, micro tubular SOFCs under 1 mm diameter have been fabricated and investigated at 450–550 °C operating temperature with H₂ fuel. The performance of the 0.8 mm diameter tubular SOFC was 110–350 mW cm⁻² at 450–550 °C operating temperatures. To maximize the performance of the cell as well as to optimize the geometry of tubular cells, a current collecting method used in the experiment was examined. A model was proposed to estimate the loss of performance for single cell due to the current collecting method as functions of anode tube length and thickness. The results showed that the losses of performance were calculated to be 0.8, 2.0, and 4.6% at 450, 500, and 550 °C operating temperatures, respectively, for the 0.8 mm diameter tubular SOFC with the length of 1.2 cm.     

Improvement of output performance of solid oxide fuel cell by optimizing Ni/samaria-doped ceria anode functional layer

Anode functional layers (AFLs) were fabricated using slurry spin coating method on anode substrates to improve the performance of cells based on samaria-doped ceria (SDC) films. The effects of the chemical compositions of AFL and AFL thickness on the performance of solid oxide fuel cell anodes were investigated by studying their effect on the ohmic loss, electrode overpotential, and output performance of cells in different atmospheres. With humidified hydrogen used as fuel and oxygen as oxidant, the cell with an 8-μm-thick AFL (NiO:SDC = 6:4) exhibited excellent maximum power densities of 3.41, 2.89, 1.46 and 0.80 W cm⁻² at 650, 600, 550 and 500 °C, respectively.     

A high-performance no-chamber fuel cell operated on ethanol flame

A no-chamber solid-oxide fuel cell operated on a fuel-rich ethanol flame was reported. Heat produced from the combustion of ethanol thermally sustained the fuel cell at a temperature of 500–830 °C. Considerable amounts of hydrogen and carbon monoxide were also produced during the fuel-rich combustion which provided the direct fuels for the fuel cell. The location of the fuel cell with respect to the flame was found to have a significant effect on the fuel cell temperature and performance. The highest power density was achieved when the anode was exposed to the inner flame. By modifying the Ni + Sm_0.2Ce_0.8O_1.9 (SDC) anode with a thin Ru/SDC catalytic layer, the fuel cell envisaged not only an increase of the peak power density to ∼200 mW cm⁻² but also a significant improvement of the anodic coking resistance.     

Advanced tubular solid oxide fuel cells with high efficiency for internal reforming of hydrocarbon fuels

Solid oxide fuel cells (SOFCs) constitute an attractive power-generation technology that converts chemical energy directly into electricity while causing little pollution. NanoDynamics Energy (NDE) Inc. has developed micro-tubular SOFC-based portable power generation systems that run on both gaseous and liquid fuels. In this paper, we present our next generation solid oxide fuel cells that exhibit total efficiencies in excess of 60% running on hydrogen fuel and 40+% running on readily available gaseous hydrocarbon fuels such as propane, butane etc. The advanced fuel cell design enables power generation at very high power densities and efficiencies (lower heating value-based) while reforming different hydrocarbon fuels directly inside the tubular SOFC without the aid of fuel pre-processing/reforming. The integrated catalytic layered SOFC demonstrated stable performance for >1000 h at high efficiency while running on propane fuel at sub-stoichiometric oxygen-to-fuel ratios. This technology will facilitate the introduction of highly efficient, reliable, fuel flexible, and lightweight portable power generation systems.