Gas Transfer Resistance Research Articles

Structural tailoring of the current collector/anode dual-layer hollow fibers to enhance the performance of micro-tubular protonic ceramic fuel cells

Micro-tubular protonic ceramic fuel cells (MT-PCFCs) offer significant advantages in energy utilization and storage, including high stability/durability and intermediate working temperature. Inefficient intraluminal current collection and costly fabrication processes are challenges for high-performance MT-PCFCs. Herein, hierarchically structured Ni/Ni–BaCe0.7Zr0.1Y0.2O3-δ dual-layer hollow fibers (DLHFs) have been fabricated by the phase-inversion assisted co-spinning/co-sintering technique. By adjusting the viscosity of anode suspension, the DLHFs' microstructure is tailored and optimized for porosity, pore size, and thickness of collector layer and spongy region. After assembling MT-PCFCs, effects of DLHFs’ microstructure on fuel cell performance are investigated, revealing that a thinner spongy region reduces gas transfer resistance, lowers polarization impedance, and enhances fuel cell performance. A maximum power density of 687.1 mWcm−2 for the optimum MT-PCFC is reached at 700 °C. The innovative DLHF design enhances current collecting efficiency for MT-PCFCs and exhibits potential in other micro-tubular applications, such as hydrogen pumps and electrochemical reactors.

Alleviating anode flooding by mesoporous carbon supports for alkaline polymer electrolyte fuel cells

Water management is one of the critical issues affecting the performance and stability of alkaline polymer electrolyte fuel cells (APEFCs). This study focuses on the effect of carbon supports on the water management of APEFCs. A series of carbon-supported Ru catalysts (Ru/MCP-x) were prepared using mesoporous carbon powder (MCP-x) with three-dimensional through-nanopore structures as supports and compared with Ru/XC72. The results show that although the catalysts’ hydrogen oxidation reaction (HOR) catalytic activities are similar in KOH solutions, the APEFCs performance can be severely different. APEFCs assembled with Ru/MCP-x and Ru/XC72 (noted as Ru/MCP-x cell and Ru/XC72 cell) have little performance difference at high inlet H2 flow (1000 mL/min), but have a noticeable difference at low inlet H2 flow (200 mL/min). By combining the electrochemical AC impedance and distribution of relaxation time (DRT) method, we quantitatively identify that severe gas transfer resistance occurred in the anode catalyst layer of the Ru/XC72 cell under low inlet H2 flow, which is more in line with the actual APEFCs operating conditions, led to considerable degradation of the Ru/XC72 cell performance. The high gas transfer resistance is later ascribed to the anode flooding by combining the voltammetry curves and DRT plots. In contrast, for Ru/MCP-x cells, increasing the carbon supports’ mesopore diameter dramatically reduced the gas transfer resistance and mitigated the anode flooding. This study shows a quantitative comparison of the impacts of different carbon supports on APEFCs performance and gas transfer resistance, indicating that mesoporous carbon materials have the potential to be supports for APEFCs anode catalysts to alleviate the anode flooding.

Study on the 4-channel micro-monolithic design with geometry control for reversible solid oxide cell

Reversible solid oxide cell (R-SOC) has been attracting considerable attention as a technology capable of power generation and CO2 electrolysis. In addition to new material development, innovation in structural design is also a decisive factor. In this study, a 4-channel micro-monolithic design, in the form of a tear-drop inner channel structure, was successfully developed. The micro-monolith obtained includes plurality of micro-channels growing from multiple directions and spongy active regions near the exterior surface. Uniquely, the irregular tear-drop inner channels further increase the proportion of the electrochemically active region to the overall circumference, achieving more efficient utilization of the geometric surface area. Such micro-structured cells with Ni-YSZ/YSZ/YSZ-LSM materials exhibited effective performance in the reversible operation of R-SOC. A superior performance of 1.20 W·cm−2 at 800 ℃ for H2 fuel cell was demonstrated. Similarly, during the electrolysis of CO2, current density recorded by the cells reached 1.20 A·cm−2 at 1.5 V and 800 ℃, which is competitive compared with the values of the previous design investigations. Relatively low diffusion polarization shown in electrochemical impedance spectroscopy (EIS) suggests that this is due to the very gas transfer resistance in the fuel electrode. This novel multi-channel micro-monolithic structure shows a potential to substitute the conventional tubular counterpart.

Monolithic Ni‐Mo‐B Bifunctional Electrode for Large Current Water Splitting

AbstractScreening and developing highly efficient electrodes is key to large‐scale water electrolysis. The practical industrial electrode should fulfill several criteria of high activity, structural stability, and fast bubble evolution at a large current density. In this study, a novel monolithic 3D hollow foam electrode that can achieve the requirements of large current density water electrolysis is developed and fabricated through a simple electroless plating‐calcination strategy. This strong 3D Ni‐Mo‐B hollow foam electrode can withstand a pressure of 2.37 MPa and exhibits high electrochemical surface area, high conductivity, and low gas transfer resistance, drastically boosting its catalytic performance. It affords 50 mA cm–2 at overpotentials of only 68 mV for hydrogen evolution reaction and 293 mV for oxygen evolution reaction and can survive at a large current density of 5 A cm–2 while maintaining its structure and performance in 1.0 m KOH. The advantages of facile preparation, high mechanical strength, high gas mass transfer ability, and excellent performance enable this structure to be a potential electrode, active substrate, or 3D catalyst in many fields.

Nanofiber Fuel Cell MEAs with a PtCo/C Cathode

PtCo/C and Pt/C catalyst powders were incorporated into electrospun nanofiber and conventional sprayed cathode membrane-electrode-assemblies (MEAs) at a fixed electrode loading of 0.1 mgPt/cm2. The binder for PtCo/C nanofiber cathodes and Pt/C nanofiber anodes was a mixture of Nafion and poly(acrylic acid) (PAA), whereas the sprayed electrode MEAs utilized a neat Nafion binder. The structure of electrospun fibers was analyzed by scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS), which showed that the fibers were ∼30% porous with a uniform distribution of catalyst and binder in the axial and radial fiber directions. The initial performance of nanofiber MEAs at 80°C was 20% better than the sprayed electrode MEA (a maximum power density of 1,045 mW/cm2 vs. 869 mW/cm2). The benefit of the nanofiber electrode morphology was most evident at end-of-test (after a metal dissolution accelerated stress test), where power densities dropped by only 8%, after 30,000 square wave voltage cycles (0.6 V to 0.95 V), as compared to a 35% drop in the maximum power for the sprayed electrode MEA. The use of a recovery protocol improved the initial performance of a nanofiber MEA by ∼13%, to 1,070 mW/cm2 at 0.65 V, and increased the power after a metal dissolution stress test by 5–10% (e.g. 840 mW/cm2 at 0.65 V after 30,000 voltage cycles). At rated power, the nanofiber MEA generated more than 1,000 mW/cm2 at 99°C and a pressure of 250 kPaabs. The high performance and durability of PtCo/C nanofiber cathode MEAs is due to the combined effects of a highly active cathode catalyst and the unique nanofiber electrode morphology, where there is a uniform distribution of catalyst and binder (no agglomeration) and short transport pathways across the submicron diameter fibers (which lowers gas transfer resistance and facilitates water removal from the cathode).

Optimization of perfluorosulfonic acid ionomer loadings in catalyst layers of proton exchange membrane fuel cells

The effect of perfluorosulfonic acid ionomer loadings on electrochemical surface area, gas transfer resistance and output voltage performance was systematically studied by both electrochemical and fuel cell experiments. Differently from the previous reports, a high Pt loading catalyst (60 wt.% Pt/C) was used to evaluate the effect in this work. The optimum loading of perfluorosulfonic acid ionomers in catalyst layers was determined. It was found that the ionomer anode content slightly influenced single ell performance and the optimum ionomer anode loading was 30 wt.%. Based on reasonable gas transfer resistance and fuel cell performance, the optimum ionomer cathode loading was 25–30 wt.%. Significantly, the optimum loading of the ionomer showed a nondependence relationship with Pt loadings in catalyst layers.

Development of an unsteady‐state model for a biological system in miniaturized bioreactors

In the present study, the special shake flasks, so-called ventilation flasks, are equipped with oxygen sensors and then an unsteady-state gas transfer model for shake flasks was developed and experimentally investigated for a wide range of gas transfer resistances (kplug). For the validation of our unsteady-state model to simulate the gas transfer in a biological system in the ventilation flasks, a strain of Corenobacterium glutamicum DM1730 was used as a model organism. For further easy processing, the resulting total mass-transfer resistance (kplug) is described as a function of the mass flow through the sterile plug (OTRplug) by an empirical equation. This equation is introduced into a simulation model that calculates the gas partial pressures in the headspace of the flask. Additionally, the gas transfer rates through the sterile closure and gas/liquid interface inside the flask are provided. This unsteady-state model would be a very useful method for scaling up from a shake flask to a fermentor; comparing the results of the gas concentration in the gas phase, there is good agreement between the introduced unsteady-state model and experimental results for the biological system.

PBI-based composite membranes for polymer fuel cells

In the present study poly(2,2-(2,6-pyridin)-5,5-bibenzimidazole) was used for the preparation of novel MEAs for high-temperature polymer fuel cells (HT-PEMFCs). We prepared hybrid materials with two types of silica fillers in order to increase the MEA performances using this polymer. The membranes were characterized in terms of their microstructure and thermal stability. Cell operation tests and Electrochemical Impedance Spectroscopy were used for the characterization of the MEAs. A maximum power density of about 80 mW cm −2 was obtained at 300 mA cm −2 by using an imidazole-modified silica filler. The EIS technique showed that the fillers chiefly help to reduce the charge transfer resistance of the cathodic side. The gas transfer resistance may be neglected with respect to R ct, at least at low current densities.

Electrochemical characterization of a polybenzimidazole-based high temperature proton exchange membrane unit cell

This work constitutes detailed EIS (Electrochemical Impedance Spectroscopy) measurements on a PBI-based HT-PEM unit cell. By means of EIS the fuel cell is characterized in several modes of operation by varying the current density, temperature and the stoichiometry of the reactant gases. Using Equivalent Circuit (EC) modeling key parameters, such as the membrane resistance, charge transfer resistance and gas transfer resistance are identified, however the physical interpretation of the parameters derived from EC’s are doubtful as discussed in this paper. The EC model proposed, which is a modified Randles circuit, provides a reasonably good fit at all the conditions tested. The measurements reveal that the cell temperature is an important parameter, which influences the cell performance significantly, especially the charge transfer resistance proved to be very temperature dependent. The transport of oxygen to the Oxygen Reduction Reaction (ORR) likewise has a substantial effect on the impedance spectra, results showed that the gas transfer resistance has an exponential-like dependency on the air stoichiometry. Based on the present results and results found in recent publications it is still not clear what exactly causes the distinctive low frequency loop occurring at oxygen starvation. Contrary to the oxygen transport, the transport of hydrogen to the Hydrogen Oxidation Reaction (HOR), in the stoichiometry range investigated in this study, shows no measurable change in the impedance data. Generally, this work is expected to provide a basis for future development of impedance-based fuel cell diagnostic systems for HT-PEM fuel cell.

PRODUCTION OF ACETIC ACID FROM HYDROGEN AND CARBON DIOXIDE BY CLOSTRIDIUM SPECIES ATCC 29797

Abstract Clostridium sp. ATCC 29797 converts H2 and CO2 to acetic acid in stationary phase in almost 100% yield. The resting cells are stable for up to 300 h and produce acetate to a final product concentration of 221 mM. The specific productivity of acetate during the stationary phase appears limited by the specific activity of hydrogenase. A high pressure fermentor was constructed to investigate the use of pressure, as a general strategy, for increasing the rates of microbial syngas conversions. Fermentations were conducted at pressures up to 1000 psig using various mixtures of H2 and CO2. The use of high pressure proved to be an effective method for eliminating gas transfer resistances and increasing the volumetric productivity. Hydrostatic pressure had little effect on the specific productivity of acetate, however, high partial pressures of hydrogen caused the specific productivity and total acetate accumulated to decrease. High pressures of hydrogen also caused a decrease in the specific activity of ...

Ozone Sensitivity of Leaves: Relationship to Leaf Water Content, Gas Transfer Resistance, and Anatomical Characteristics

Relative water content, resistance to gas transfer, stomatal spacing, and other characteristics of primary bean leaves were studied in relation to ozone sensitivity and injury. Cells of primary bean leaves are maximally sensitive to ozone exposure 9–10 days after germination under our experimental conditions. The stage of maximum sensitivity was not correlated with changes in stomatal number or resistance on either adaxial or abaxial leaf surfaces. It was deduced that bean leaf sensitivity was a function of more internal circumstances, and gas exchange was never the limiting factor through the developmental period studied. Changes in resistance were not significantly altered by ozone levels that produced no visible injury. After exposure to high ozone doses, a decrease in adaxial resistance occurred apparently as a result of palisade and epidermal cell lysis. Normally most gas exchange occurs through the adaxial surface. A 10 % decrease in relative water content accompanying a 60-min ozone exposure of 0.55 ppm could not be explained physiologically on the basis of cell injury as no visible leaf injury occurred.

Seasonal Patterns of Acid Metabolism and Gas Exchange in Opuntia basilaris

Acid metabolism and gas exchange studies were conducted in situ on the cactus Opuntia basilaris Engelm. and Bigel. A pattern of significant seasonal variation was evident. The pattern was controlled by rainfall, which significantly influenced plant water potentials, total gas transfer resistances, and nocturnal organic acid synthesis. In winter and early spring, when plant water stress was mild, stomatal and mesophyll resistances remained low, permitting enhanced nocturnal assimilation of (14)CO(2). The day/night accumulation of acidity was large during these seasons. In summer and fall, plant water stress was moderate, although soil water stress was severe. The nocturnal assimilation of (14)CO(2) was very low during these seasons, even in stems with open stomata, indicating large mesophyll resistances restricting exogenous gas incorporation. The day/night accumulation of acidity was reduced, and a low level of acid metabolism persisted throughout this period. The rapid response to a midsummer rainfall emphasizes the importance of plant water potential as a parameter controlling over-all metabolic activity. The seasonal variations of acid metabolism and gas exchange significantly influenced the efficiency of water use and carbon dioxide assimilation. Periods of maximal efficiency followed rainfall throughout the course of the year.

Inhomogeneity effects on O 2 and CO pulmonary diffusing capacity estimates by steady-state methods. Theory

The effects of pulmonary functional inhomogeneities (uneven distributions of a, D and ) on steady-state O 2 and CO diffusing capacity estimates were analysed in terms of gas transfer resistances for O 2 and for CO. To this end, a mathematical treatment was developed which is applicable to the transfer of any gas in non-homogeneous lungs. Quantitative comparisons of diffusing capacity, D, with its estimates, “D”, (as obtained by classical steady-state equations) were established on multi-compartmental lung models similar to those used by West (1969). Results show that, for inhomogeneous lungs, diffusing capacity estimates are lower than diffusing capacity. This divergence, which increases with the amount of inhomogeneity, is larger for D O 2 than for D CO. In contrast, it was also found that, in the particular case where D CO estimates are computed according to Filley, these can be higher than D CO. These findings can be adequately apprehended in terms of network analysis. They provide an explanation for the well-known discrepancy between morphometric and functional D estimates, as well as for the fact that D CO values higher than D O 2 values are encountered in the literature.