Electrochemical performance of a glucose/oxygen microfluidic biofuel cell

A microfluidic glucose/O₂ biofuel cell, delivering electrical power, is developed based on both laminar flow and biological enzyme strategies. The device consists of a Y-shaped microfluidic channel in which fuel and oxidant streams flow laminarly in parallel at gold electrode surfaces without convective mixing. At the anode, the glucose is oxidized by the enzyme glucose oxidase whereas at the cathode, the oxygen is reduced by the enzyme laccase, in the presence of specific redox mediators. Such cell design protects the anode from interfering parasite reaction of O₂ at the anode and works with different streams of oxidant and fuel for optimal operation of the enzymes. The dependence of the flow rate on the current is evaluated in order to determine the optimum flow that would provide little to no mixing while yielding high current densities. The maximum power density delivered by the assembled biofuel cell reaches 110 μW cm⁻² at 0.3 V with 10 mM glucose at 23 °C. This research demonstrates the feasibility of advanced microfabrication techniques to build an efficient microfluidic glucose/O₂ biofuel cell device.     

A fuel cell operating between room temperature and 250 °C based on a new phosphoric acid based composite electrolyte

A phosphoric acid based composite material with core-shell microstructure has been developed to be used as a new electrolyte for fuel cells. A fuel cell based on this electrolyte can operate at room temperature indicating leaching of H₃PO₄ with liquid water is insignificant at room temperature. This will help to improve the thermal cyclability of phosphoric acid based electrolyte to make it easier for practical use. The conductivity of this H₃PO₄-based electrolyte is stable at 250 °C with addition of the hydrophilic inorganic compound BPO₄ forming a core-shell microstructure which makes it possible to run a PAFC at a temperature above 200 °C. The core-shell microstructure retains after the fuel cell measurements. A power density of 350 mW/cm² for a H₂/O₂ fuel cell has been achieved at 200 °C. The increase in operating temperature does not have significant benefit to the performance of a H₂/O₂ fuel cell. For the first time, a composite electrolyte material for phosphoric acid fuel cells which can operate in a wide range of temperature has been evaluated but certainly further investigation is required.     

A comparative study of La0.8Sr0.2MnO3 and La0.8Sr0.2Sc0.1Mn0.9O3 as cathode materials of single-chamber SOFCs operating on a methane-air mixture

As candidates of cathode materials for single-chamber solid oxide fuel cells, La_0.8Sr_0.2MnO₃ (LSM) and La_0.8Sr_0.2Sc_0.1Mn_0.9O₃ (LSSM) were synthesized by a combined EDTA-citrate complexing sol-gel process. The solid precursors of LSM and LSSM were calcined at 1000 and 1150 °C, respectively, to obtain products with similar specific surface area. LSSM was found to have higher activity for methane oxidization than LSM due to LSSM's higher catalytic activity for oxygen reduction. Single cells with these two cathodes initialized by ex situ reduction had similar peak power densities of around 220 mW cm⁻² at 825 °C. The cell using the LSM cathode showed higher open-circuit-voltage (OCV) at corresponding temperatures due to its reduced activity for methane oxidation relative to LSSM. A negligible effect of methane and CO₂ on the cathode performance was observed for both LSM and LSSM via electrochemical impedance spectroscopy analysis. The high phase stability of LSSM under reducing atmosphere allows a more convenient in situ reduction for fuel cell initiation. The resultant cell with LSSM cathode delivered a peak power density of ∼200 mW cm⁻² at 825 °C, comparable to that from ex situ reduction.     

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.     

NaBH4/H2O2 fuel cells for air independent power systems

The performance and characteristics of direct sodium-borohydride/hydrogen-peroxide (NaBH₄/H₂O₂) fuel cells are studied in the context of potential applications for air independent propulsion for outer space and underwater. Due to the existence of ocean (sea) water as a natural heat sink, this new fuel cell technology is best suited for underwater propulsion/power systems for small scale high performance marine vehicles. The characteristics of such a power system are compared to other options, specifically for the underwater scenario. The potential of this fuel cell is demonstrated in laboratory experiments. Power density over 1.5 W cm⁻², at 65 °C and ambient pressure, have been achieved with the help of some unique treatments of the fuel cell. One such treatment is an in-situ electroplating technique, which results in electrodes with power density 20–40% higher, than that of the electrodes produced by the ordinary ex-situ electroplating method. This unique process also makes repair or reconditioning of the fuel cell possible and convenient.     

Cathode contact optimization and performance evaluation of intermediate temperature-operating solid oxide fuel cell stacks based on anode-supported planar cells with LaNi0.6Fe0.4O3 cathode

This paper reports the development of intermediate temperature-operating solid oxide fuel cell stacks using anode-supported planar cells with LaNi_0.6Fe_0.4O₃ (LNF)cathode. We developed metallic separators with radial gas flow channels and an anode seal structure. To achieve good power-generating characteristics, we propose two cathode contact methods. According to a performance evaluation at 800 °C, power density of 0.5 W cm⁻² is obtained at the current density of 1.0 A cm⁻² when operating with a sufficient fuel amount, and power conversion efficiency of over 50% LHV is obtained at the current density of more than 0.2 A cm⁻² when operating at a high fuel utilization rate.     

High-performance biofuel cell made with hydrophilic ordered mesoporous carbon as electrode material

A highly hydrophilic ordered mesoporous carbon has been synthesized by a microwave assisted method from a mixture containing glucose and poly(vinyl alcohol) and with a silica template to have high hydrophilicity, low charge transfer resistance and large specific surface area. The new carbon material is further used as an electrode material to fabricate an anode-limited glucose/O₂ biofuel cell, which gives an output power density of 110 μW cm⁻² with cell voltage of 0.72 V, a performance much higher than the reported anodes made from SWNT, bi-polymer layer and carbon black at the same or even higher glucose concentration. This work provides a universal approach to synthesize functional carbon nanomaterials with desired architectures and properties for various important applications in energy conversion systems such as fuel cells and solar cells.     

Fuel-flexible operation of a solid oxide fuel cell with Sr0.8La0.2TiO3 support

Solid oxide fuel cells with Sr_0.8La_0.2TiO₃ anode-side supports, Ni- Sm-doped ceria adhesion layer, Ni- Y₂O₃-stabilized ZrO₂ (YSZ) anode active layer, YSZ electrolyte, and La_0.8Sr_0.2MnO₃(LSM)–YSZ cathode are described. These cells are stable in simulated natural gas at current densities as low as 0.2 A cm⁻². This represents much-improved stability against coking in natural gas, compared with conventional Ni–YSZ anode-supported SOFCs which rapidly coke, even at higher current densities. Cell operation in H₂ fuel with 50–100 ppm, H₂S results in an initial decrease in cell power density, but no long-term degradation occurs and full recovery to the initial performance level is observed after dry H₂ fuel flow is restored. Degradation is not observed during or after seven redox cycles between H₂ and air.     

Pt/Y0.16Zr0.84O1.92/Pt thin film solid oxide fuel cells: Electrode microstructure and stability considerations

Symmetric, free standing thin film micro-solid oxide fuel cells of Pt/Y_0.16Zr_0.84O_1.92/Pt structure are studied in this report. The role of parameters such as electrolyte thickness and electrode porosity on fuel cell power output was investigated. A peak power density of 1037 mW cm⁻² was exhibited with an optimized structure consisting of nano-porous Pt electrodes and 100 nm thick Y_0.16Zr_0.84O_1.92 with an open circuit voltage of 0.968 V at 500 °C using H₂ as fuel and standing air as the oxidant. A twelve hour test of these devices indicates that overall performance shows extreme sensitivity to microstructural changes in the pure metallic electrodes. Results presented herein enable mechanistic routes to performance optimization, provide device stability data and are relevant to advancing micro-fuel cells for portable energy.     

Performance of Li-ion secondary batteries in low power,hybrid power supplies

Small, portable electronic devices need power supplies that have long life, high energy efficiency, high energy density, and can deliver short power bursts. Hybrid power sources that combine a high energy density fuel cell, or an energy scavenging device, with a high power secondary battery are of interest in sensors and wireless devices. However, fuel cells with low self-discharge have low power density and have a poor response to transient loads. A low capacity secondary lithium ion cell can provide short burst power needed in a hybrid fuel cell–battery power supply. This paper describes the polarization, cycling, and self-discharge of commercial lithium ion batteries as they would be used in the small, hybrid power source. The performance of 10 Li-ion variations, including organic electrolytes with Li_xV₂O₅ and Li_xMn₂O₄ cathodes and LiPON electrolyte with a LiCoO₂ cathode was evaluated. Electrochemical characterization shows that the vanadium oxide cathode cells perform better than their manganese oxide counterparts in every category. The vanadium oxide cells also show better cycling performance under shallow discharge conditions than LiPON cells at a given current. However, the LiPON cells show significantly lower energy loss due to polarization and self-discharge losses than the vanadium and manganese cells with organic electrolytes.     

Catalytic behavior of Co3O4 in electroreduction of H2O2

Spinel structure Co₃O₄ nanoparticles with an average diameter of around 17 nm were prepared and evaluated as electrocatalysts for H₂O₂ reduction. Results revealed that Co₃O₄ exhibits considerable activity and good stability for electrocatalytic reduction of H₂O₂ in 3 M NaOH solution. The reduction occurs mainly via the direct pathway when H₂O₂ concentration is lower than 0.5 M. An Al-H₂O₂ semi fuel cell using Co₃O₄ as cathode catalyst was constructed and tested at room temperature. The fuel cell displayed an open circuit voltage of 1.45 V and a peak power density of 190 mW cm⁻² at a current density of 255 mA cm⁻² operating with a catholyte containing 1.5 M H₂O₂. This study demonstrated that Co₃O₄ nanoparticles are promising cathode catalysts, in place of precious metals, for fuel cells using H₂O₂ as oxidant.     

Fabrication of multi-layered solid oxide fuel cells using a sheet joining process

Ceria-based electrolytes can be used in solid oxide fuel cells (SOFCs) that operate at intermediate-temperature due to their high ionic conductivity. However, the difficulty in fabricating a thin and dense ceria-based electrolyte on an anode support is well-known. In this study, a new sheet joining process is suggested to produce a thin and dense ceria-based electrolyte for anode-supported SOFCs. The main idea used with the sheet joining process is a combination of a sheet cell fabricated by tape casting, and an anode pellet, fabricated by pressing. The maximum power density of a single cell, fabricated by the sheet joining process, is 0.315 W cm⁻² at 600 °C in a power generation test when Pr_0.3Sr_0.7Co_0.3Fe_0.7O_3−δ was used as the cathode material. The performance durability of a single cell is tested over 1000 h of operation in which a dense electrolyte was observed to survive.     

New insights in the polarization resistance of anode-supported solid oxide fuel cells with La0.6Sr0.4Co0.2Fe0.8O3 cathodes

In this study, the polarization resistance of anode-supported solid oxide fuel cells (SOFC) with La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) cathodes was investigated by I-V sweep and electrochemical impedance spectroscopy under a series of operating voltages and cathode environments (i.e. stagnant air, flowing air, and flowing oxygen) at temperatures from 550 °C to 750 °C. In flowing oxygen, the polarization resistance of the fuel cell decreased considerably with the applied current density. A linear relationship was observed between the ohmic-free over-potential and the logarithm of the current density of the fuel cell at all the measuring temperatures. In stagnant or flowing air, an arc related to the molecular oxygen diffusion through the majority species (molecular nitrogen) present in the pores of the cathode was identified at high temperatures and high current densities. The magnitude of this arc increased linearly with the applied current density due to the decreased oxygen partial pressure at the interface of the cathode and the electrolyte. It is found that the performance of the fuel cell in air is mainly determined by the oxygen diffusion process. Elimination of this process by flowing pure oxygen to the cathode improved the cell performance significantly. At 750 °C, for a fuel cell with a laser-deposited Sm_0.2Ce_0.8O_1.9 (SDC) interlayer, an extraordinarily high power density of 2.6 W cm⁻² at 0.7 V was achieved in flowing oxygen, as a result of reduced ohmic and polarization resistance of the fuel cell, which were 0.06 Ω cm² and 0.03 Ω cm², respectively. The results indicate that microstructural optimization of the LSCF cathode or adoption of a new cell design which can mitigate the oxygen diffusion limitation in the cathode might enhance cell performance significantly.     

Performance increase of microfluidic formic acid fuel cell using Pd/MWCNTs as catalyst

This paper shows that the combination of an O₂ saturated acidic fluid setup (O₂-setup) and a composite of Pd nanoparticles supported on multiwalled-carbon nanotubes (Pd/MWCNTs) as anode catalyst material, results in the improvement of microfluidic fuel cell performance. Microfluidic fuel cells were constructed and evaluated at low HCOOH concentrations (0.1 and 0.5 M) using Pd/V XC-72 and Pd/MWCNTs as anode and Pt/V XC-72 as cathode electrode materials, respectively. The results show a higher power density (2.9 mW cm⁻²) for this cell when compared to the value reported in the literature that considers a commercial Pd/V XC-72 and 3.3 mW cm⁻² using a Pd/MWCNTs with a 50% less Pd loading than that commercial Pd/V XC-72.     

Use of bioelectrode containing DNA-wrapped single-walled carbon nanotubes for enzyme-based biofuel cell

Biofuel cells that utilize enzymes are attractive alternatives to metal catalyst-based cells because they are environmentally friendly, renewable and operate well at room temperature. Glucose oxidase (GOD)/laccase based biofuel cells have been evaluated to determine if they are useful power supplies that can be implanted in vivo. However, the usefulness of GOD/laccase systems is limited because they produce low level of electrical power. The effects of DNA-wrapped single-wall carbon nanotubes (SWNTs) on the electrical properties of a fuel cell are evaluated under ambient conditions in an attempt to increase the electrical power of an enzyme-based biofuel cell (EFC). The anode (GOD) and cathode (laccase) system in the EFC is composed of gold electrodes that are modified with DNA-wrapped SWNTs. Glucose (for anode) and O₂ (for cathode) are used as the substrates. The anodic electrical properties increase significantly with a bioelectrode that contains DNA-wrapped SWNTs as an electron-transfer mediator. Furthermore, the modified bioelectrode results in increased activities and stabilities of GOD and laccase, which enhance power production (442 μW cm⁻² at 0.46 V) compared with a basic EFC.     

A comprehensive evaluation of a Ni-Al2O3 catalyst as a functional layer of solid-oxide fuel cell anode

An inexpensive 7 wt.% Ni-Al₂O₃ composite is synthesized by a glycine-nitrate process and systematically investigated as anode catalyst layer of solid-oxide fuel cells operating on methane fuel by examining its catalytic activity towards methane partial oxidation, steam and CO₂ reforming at 600-850 °C, cell performance, mechanical performance, and carbon deposition properties. Ni-Al₂O₃ shows comparable catalytic activities to Ru-CeO₂ for the above three reactions. The cell with a Ni-Al₂O₃ catalyst layer delivers maximum peak power densities of 494 and 532 mW cm⁻² at 850 °C, operating on methane-H₂O and methane-CO₂ mixture gases, respectively, which are comparable to those operating on hydrogen. Ni-Al₂O₃ is found to have better mechanical performance than Ru-CeO₂. O₂-TPO demonstrates that Ni-Al₂O₃ does not inhibit the carbon formation under pure methane atmosphere, while the introduction of steam or CO₂ can effectively suppress coke formation. However, due to the low nickel content in the catalyst layer, the coke formation over the catalyst layer is actually not serious under real cell operation conditions. More importantly, Ni-Al₂O₃ effectively protects the anode layer by greatly suppressing carbon formation over the anode layer, especially near the anode-electrolyte interface. Ni-Al₂O₃ is highly promising as an anode functional layer for solid-oxide fuel cells.     

Silver infiltrated La0.6Sr0.4Co0.2Fe0.8O3 cathodes for intermediate temperature solid oxide fuel cells

Porous La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) electrodes on anode support cells were infiltrated with AgNO₃ solutions in citric acid and ethylene glycol. Two types of solid oxide fuel cells with the LSCF–Ag cathode, Ni–YSZ/YSZ/LSCF–Ag and Ni–Ce_0.9Gd_0.1O_1.95(GDC)/GDC/LSCF–Ag, were examined in a temperature range 530–730 °C under air oxidant and moist hydrogen fuel. The infiltration of about 18 wt.% Ag fine particles into LSCF resulted in the enhancement of the power density of about 50%. The maximum power density of Ni–YSZ/YSZ/LSCF was enhanced from 0.16 W cm⁻² to 0.25 W cm⁻² at 630 °C by infiltration of AgNO₃. No significant degradation of out-put power was observed for 150 h at 0.7 V and 700 °C. The Ni–GDC/GDC/LSCF–Ag cell showed the maximum power density of 0.415 W cm⁻² at 530 °C.     

Development of electrolyte-supported intermediate-temperature single-chamber solid oxide fuel cells using Ln0.7Sr0.3Fe0.8Co0.2O3−δ (Ln = Pr, La, Gd) cathodes

Iron-cobalt-based perovskite oxides with general formula Ln_0.7Sr_0.3Fe_0.8Co_0.2O_3−δ (where Ln = La, Pr and Gd) have been investigated for their application as intermediate-temperature cathodes in solid oxide fuel cells (SOFCs). Powdered samples of these materials were synthesized by a novel gel combustion process and then characterized by X-ray powder diffraction (XPD) and scanning electron microscopy (SEM). XPD patterns were satisfactorily indexed with an orthorhombic GdFeO₃-type structure and, for all samples, a mean particle size of less than 1 μm was estimated from the SEM data. Experimental single-chamber SOFCs using with these materials as cathodes and NiO-SDC (samaria-doped ceria) and SDC as anode and electrolyte, respectively, were evaluated at 600 °C in a methane/oxygen mixtures. Peak power densities of 65.4, 48.7 and 46.2 mW cm⁻² were obtained for Ag|Ln_0.7Sr_0.3Fe_0.8Co_0.2O_3−δ|SDC|NiO-SDC|Pt cells with Ln = Pr, La and Gd, respectively. The relatively high power density obtained for the Pr compound shows that it could be an interesting material for cathode of single-chamber SOFCs.     

Measurement of oxygen chemical potential in Gd2O3-doped ceria-Y2O3-stabilized zirconia bi-layer electrolyte, anode-supported solid oxide fuel cells

Solid oxide fuel cells (SOFC) were fabricated with gadolinia-doped ceria (GDC)-yttria stabilized zirconia (YSZ), thin bi-layer electrolytes supported on Ni + YSZ anodes. The GDC and YSZ layer thicknesses were 45 μm, and ∼5 μm, respectively. Two types of cells were made; YSZ layer between anode and GDC (GDC/YSZ) and YSZ layer between cathode and GDC (YSZ/GDC). Two platinum reference electrodes were embedded within the GDC layer. Cells were tested at 650 °C with hydrogen as fuel and air as oxidant. Electric potentials between embedded reference electrodes and anode and between cathode and anode were measured at open circuit, short circuit and under load. The electric potential was nearly constant through GDC in the cathode/YSZ/GDC/anode cells. By contrast, it varied monotonically through GDC in the cathode/GDC/YSZ/anode cells. Estimates of oxygen chemical potential, μO₂, variation through GDC were made. μO₂ within the GDC layer in the cathode/GDC/YSZ/anode cell decreased as the current was increased. By contrast, μO₂ within the GDC layer in the cathode/YSZ/GDC/anode cell increased as the current was increased. The cathode/YSZ/GDC/anode cell exhibited maximum power density of ∼0.52 W cm⁻² at 650 °C while the cathode/GDC/YSZ/anode cell exhibited maximum power density of ∼0.14 W cm⁻² for the same total electrolyte thickness.     

Lithium and lanthanum promoted Ni-Al2O3 as an active and highly coking resistant catalyst layer for solid-oxide fuel cells operating on methane

Ni-Al₂O₃ catalyst is modified with Li₂O₃, La₂O₃ and CaO promoters to improve its resistance to coking. These catalysts are used as the materials of the anode catalyst layer in solid-oxide fuel cells operating on methane. Their catalytic activity for the partial oxidation, steam reforming and CO₂ reforming of methane at 600-850 °C is investigated. Their catalytic stability and carbon deposition properties are also studied. The LiLaNi-Al₂O₃ catalyst shows a catalytic activity that is comparable to those of LaNi-Al₂O₃ and LiNi-Al₂O₃ catalysts for all three reactions. However, it displays a higher catalytic activity than those of CaLaNi-Al₂O₃ and CaNi-Al₂O₃ catalysts. Among the various catalysts, the LiLaNi-Al₂O₃ catalyst presents the highest catalytic stability. O₂-TPO profiles indicate that the modification of the Ni-Al₂O₃ catalyst with Li and La greatly reduces carbon deposition under pure methane atmosphere. The LiLaNi-Al₂O₃ catalyst is applied as the anode functional layer of a Ni + ScSZ anode-supported fuel cell. The cell is operated on methane-O₂, methane-H₂O or methane-CO₂ gas mixtures and yields peak power densities of 538, 532 and 529 mW cm⁻² at 850 °C, respectively, comparable to that of hydrogen fuel. In sum, the LiLaNi-Al₂O₃ is highly promising as a highly coking resistant catalyst layer for solid-oxide fuel cells.