• Energy Research
• 3. Good health close
• CA close
• Journal of Power Sources close

Abstract Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite, Li[Ni 0.5 Mn 0.3 Co 0.2 ]O 2 /graphite and Li[Ni 0.6 Mn 0.2 Co 0.2 O 2 ]/graphite pouch cells were examined with and without electrolyte additives using the ultra high precision charger at Dalhousie University, electrochemical impedance spectroscopy, gas evolution measurements and “cycle-store” tests. The electrolyte additives tested were vinylene carbonate (VC), prop-1-ene-1,3-sultone (PES), pyridine-boron trifluoride (PBF), 2% PES + 1% methylene methanedisulfonate (MMDS) + 1% tris(trimethylsilyl) phosphite (TTSPi) and 0.5% pyrazine di-boron trifluoride (PRZ) + 1% MMDS. The charge end-point capacity slippage, capacity fade, coulombic efficiency, impedance change during cycling, gas evolution and voltage drop during “cycle-store” testing were compared to gain an understanding of the effects of these promising electrolyte additives or additive combinations on the different types of pouch cells. It is hoped that this report can be used as a guide or reference for the wise choice of electrolyte additives in Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite, Li[Ni 0.5 Mn 0.3 Co 0.2 ]O 2 /graphite and Li[Ni 0.6 Mn 0.2 Co 0.2 O 2 ]/graphite pouch cells and also to show the shortcomings of particular positive electrode compositions.

A two-dimensional model of a flowing-electrolyte direct methanol fuel cell has been developed to predict the performance of the cell under various operating conditions. Governing equations including the proton and electron transport, continuity, momentum, species transport for methanol, water, and oxygen, and the auxiliary equations are coupled to determine the output parameters. These parameters are the concentration distribution of the species, cell voltage, power density, and the electrical efficiency of the cell. After validation with the experimental data, several simulations are carried out to study the effects of the fluid velocity at the fuel, air, and flowing electrolyte channel inlets on the output parameters. In addition, the effect of recirculating the methanol at the flowing electrolyte channel outlet is assessed. The results show that higher fluid velocities at the fuel, air, and flowing electrolyte channel inlets are needed to obtain higher power densities. However, an increase in the fluid velocity at the fuel channel inlet causes a decrease in the electrical efficiency of the cell. Iris also found that the electrical efficiency of the FE-DMFC can be further increased if the methanol leaving the flowing electrolyte channel is recirculated into the methanol storage tank. (C) 2012 Elsevier B.V. All rights reserved.

Abstract In this first of a series of two papers, we report an in-depth analysis of the impact of the gas diffusion layer (GDL) structure on the polymer electrolyte membrane (PEM) fuel cell performance through the use of custom GDLs fabricated in-house. Hydrophobic electrospun nanofibrous gas diffusion layers (eGDLs) are fabricated with controlled fibre diameter and alignment. The eGDLs are rendered hydrophobic through direct surface functionalization, and this molecular grafting is achieved in the absence of structural alteration. The fibre diameter, chemical composition, and electrical conductivity of the eGDL are characterized, and the impact of eGDL structure on fuel cell performance is analysed. We observe that the eGDL facilitates higher fuel cell power densities compared to a commercial GDL (Toray TGP-H-60) at highly humidified operating conditions. The ohmic resistance of the fuel cell is found to significantly increase with increasing inter-fiber distance. It is also observed that the addition of a hydrophobic treatment enhances membrane hydration, and fibres perpendicularly aligned to the channel direction may enhance the contact area between the catalyst layer and the GDL.

In this study, the performance characteristics of a flowing electrolyte-direct methanol fuel cell (FE-DMFC) and a direct methanol fuel cell (DMFC) are evaluated by computer simulations; and results are compared to experimental data found in the literature. Simulations are carried out to assess the effects of the operating parameters on the output parameters; namely, methanol concentration distribution, cell voltage, power density, and electrical efficiency of the cell. The operating parameters studied include the electrolyte flow rate, flowing electrolyte channel thickness, and methanol concentration at the feed stream. In addition, the effect of the circulation of the flowing electrolyte channel outlet stream on the performance is discussed. The results show that the maximum power densities that could be achieved do not significantly differ between these two fuel cells; however the electrical efficiency could be increased by 57% when FE-DMFC is used instead of DMFC.

This paper presents an overview of polymer electrolyte membrane fuel cell (PEMFC) stack testing. Stack testing is critical for evaluating and demonstrating the viability and durability required for commercial applications. Single cell performance cannot be employed alone to fully derive the expected performance of PEMFC stacks, due to the non-uniformity in potential, temperature, and reactant and product flow distributions observed in stacks. In this paper, we provide a comprehensive review of the state-of-the art in PEMFC testing. We discuss the main topics of investigation, including single cell vs. stack-level performance, cell voltage uniformity, influence of operating conditions, durability and degradation, dynamic operation, and stack demonstrations. We also present opportunities for future work, including the need to verify the impact of stack size and cell voltage uniformity on performance, determine operating conditions for achieving a balance between electrical efficiency and flooding/dry-out, meet lifetime requirements through endurance testing, and develop a stronger understanding of degradation.

Abstract For the first time, pore network modelling is applied to polymer electrolyte membrane (PEM) electrolyzers. Realistic sintered titanium powder-based porous transport layers (PTLs) are generated via stochastic modelling, and we determine the influence of PTL-catalyst coated membrane (CCM) contact, pore and throat sizes, and porosity on two-phase transport. The PTL-CCM interfaces exhibit large open pore spaces, which are catastrophic for effective reactant transport due to the localized preferential accumulation of oxygen gas. As expected, increasing the sizes of the pores and throats results in higher oxygen gas saturations; however, we concurrently observe a surprisingly dominant mass transport effect whereby liquid water permeability is enhanced. The numerically generated PTLs are further tailored for enhanced mass transport by imposing a porosity gradient. By increasing the porosity from the PTL-CCM interface to the PTL-flow field interface, lower gas saturations and higher liquid water permeabilities are observed. With the opposite porosity gradient, the majority of pores adjacent to the high porosity CCM region become invaded with oxygen gas. We recommend a backing layer (microporous layer) at the CCM-PTL interface that improves the contact and permeation of product oxygen away from the CCM, thereby enhancing the performance of the PEM electrolyzer.

Abstract The role of triallyl phosphate as an electrolyte additive in Li(Ni0.42Mn0.42Co0.16)O2/graphite pouch cells was studied using ex-situ gas measurements, ultra high precision coulometry, automated storage experiments, electrochemical impedance spectroscopy, long-term cycling and X-ray photoelectron spectroscopy. Cells containing triallyl phosphate produced less gas during formation, cycling and storage than control cells. The use of triallyl phosphate led to higher coulombic efficiency and smaller charge endpoint capacity slippage during ultra high precision charger testing. Cells containing triallyl phosphate showed smaller potential drop during 500 h storage at 40 °C and 60 °C and the voltage drop decreased as the triallyl phosphate content in the electrolyte increased. However, large amounts of triallyl phosphate (>3% by weight in the electrolyte) led to large impedance after cycling and storage. Symmetric cell studies showed large amounts of triallyl phosphate (5% or more) led to significant impedance increase at both negative and positive electrodes. X-ray photoelectron spectroscopy studies suggested that the high impedance came from the polymerization of triallyl phosphate molecules which formed thick solid electrolyte interphase films at the surfaces of both negative and positive electrodes. An optimal amount of 2%–3% triallyl phosphate led to better capacity retention during long term cycling.

Abstract Proton-conducting solid electrolytes composed of gadolinium-doped barium cerate (BCG) or gadolinium and praseodymium-doped barium cerate (BCGP) were tested in an intermediate-temperature fuel cell in which hydrogen or ammonia was directly fed. At 700 °C, BCG electrolytes with porous platinum electrodes showed essentially no loss in performance in pure hydrogen. Under direct ammonia at 700 °C, power densities were only slightly lower compared to pure hydrogen feed, yielding an optimal value of 25 mW cm −2 at a current density of 50 mA cm −2 . This marginal difference can be attributed to a lower partial pressure of hydrogen caused by the production of nitrogen when ammonia is decomposed at the anode. A comparative test using BCGP electrolyte showed that the doubly doped barium cerate electrolyte performed better than BCG electrolyte. Overall fuel cell performance characteristics were enhanced by approximately 40% under either hydrogen or ammonia fuels using BCGP electrolyte. At 700 °C using direct ammonia feed, power density reached 35 mW cm −2 at a current density of approximately 75 mA cm −2 . Minimal loss of performance was noted over approximately 100 h on-stream in alternating hydrogen/ammonia fuels.

Abstract Degradation mechanism of the metal-supported SOFCs with NiO–Ce0.8Sm0.2O2−δ(NiO–SDC) as anode, SDC as electrolyte, Sm0.5Sr0.5CoO3 (SSCo)–SDC composite as cathode was addressed with an emphasis on metal oxidation and thermal expansion mismatch. The diagnosis was based on an improved equivalent circuit model combined with impedance, microstructure and composition analysis of the cell and cell components. The impedance diagnosis indicates that the high contact resistance is a prominent factor impeding the performance of metal-supported SOFCs at 450–600 °C. The observed oxide scale at the interface between metallic substrate and anode, and, the weak bonding between the electrolyte and the cathode may be responsible for the high contact resistances. Energy-dispersive X-ray spectroscopy (EDX) was applied to analyze the change of surface composition of metallic substrate joined with anode in order to elucidate the formed oxide scale. In addition, based on the improved equivalent circuit model, internal shorting current of the cell due to electronic conduction was evaluated.