• Energy Research
• 3. Good health close
• DE 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.

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.

Abstract Proper water management plays an essential role in the performance and durability of Polymer Electrolyte Fuel Cells (PEFCs), but it is challenged by the variety of water transport phenomena that take place in these devices. Previous experimental work has shown the existence of fluctuations between low and high current density levels in PEFCs operated with wet hydrogen and dry air feed. The alternation between both performance states is accompanied by strong changes in the high frequency resistance, suggesting a cyclic hydration and dehydration of the membrane. This peculiar scenario is examined here considering liquid water distributions from neutron imaging and predictions from a 3D two-phase non-isothermal model. The results show that the hydration-dehydration cycles are triggered by the periodic condensation and shedding of liquid water at the anode inlet. The input of liquid water humidifies the anode channel and offsets the membrane dry-out induced by the dry air stream, thus leading to the high-performance state. When liquid water is flushed out of the anode channel, the dehydration process takes over, and the cell comes back to the low-performance state. The predicted amplitude of the current oscillations grows with decreasing hydrogen and increasing air flow rates, in agreement with previous experimental data.

Abstract Proper water management plays an essential role in the performance and durability of Polymer Electrolyte Fuel Cells (PEFCs), but it is challenged by the variety of water transport phenomena that take place in these devices. Previous experimental work has shown the existence of fluctuations between low and high current density levels in PEFCs operated with wet hydrogen and dry air feed. The alternation between both performance states is accompanied by strong changes in the high frequency resistance, suggesting a cyclic hydration and dehydration of the membrane. This peculiar scenario is examined here considering liquid water distributions from neutron imaging and predictions from a 3D two-phase non-isothermal model. The results show that the hydration-dehydration cycles are triggered by the periodic condensation and shedding of liquid water at the anode inlet. The input of liquid water humidifies the anode channel and offsets the membrane dry-out induced by the dry air stream, thus leading to the high-performance state. When liquid water is flushed out of the anode channel, the dehydration process takes over, and the cell comes back to the low-performance state. The predicted amplitude of the current oscillations grows with decreasing hydrogen and increasing air flow rates, in agreement with previous experimental data.

The flow distribution in fuel cells has an important influence on both the power density and efficiency of fuel cell systems. In order to effectively utilize the area, flow distribution should be as homogeneous as possible. In addition, pressure losses should be minimized with regard to the power demand of auxiliary components as pumps and compressors. In polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells (DMFCs) the flow field is in direct contact with the diffusion layer. The main task of the diffusion layer is to distribute the reactants from the flow field towards the catalyst layer. To prevent diffusion overvoltages, the diffusion layer is in general highly porous and provides high fluxes of the reactants. Consequently, the flow distribution in the flow field can be superpositioned by a flow in the diffusion layer. In this paper, we discuss the interaction between the diffusion layer and the flow field. Experimentally, we characterized different diffusion layers with regard to their diffusion properties as well as different flow fields. Additional simulation studies help to understand the processes and to determine suitable combinations of flow fields and diffusion layers.

The flow distribution in fuel cells has an important influence on both the power density and efficiency of fuel cell systems. In order to effectively utilize the area, flow distribution should be as homogeneous as possible. In addition, pressure losses should be minimized with regard to the power demand of auxiliary components as pumps and compressors. In polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells (DMFCs) the flow field is in direct contact with the diffusion layer. The main task of the diffusion layer is to distribute the reactants from the flow field towards the catalyst layer. To prevent diffusion overvoltages, the diffusion layer is in general highly porous and provides high fluxes of the reactants. Consequently, the flow distribution in the flow field can be superpositioned by a flow in the diffusion layer. In this paper, we discuss the interaction between the diffusion layer and the flow field. Experimentally, we characterized different diffusion layers with regard to their diffusion properties as well as different flow fields. Additional simulation studies help to understand the processes and to determine suitable combinations of flow fields and diffusion layers.

Abstract The durability of a polymer electrolyte membrane (PEM) water electrolysis single cell, assembled with regular porous transport layers (PTLs) is investigated for just over 1000 h. We observe a significant degradation rate of 194 μV h−1 and conclude that 78% of the detectable degradation can be explained by an increase in ohmic resistance, arising from the anodic Ti-PTL. Analysis of the polarization curves also indicates a decrease in the anodic exchange current density, j0, that results from the over-time contamination of the anode with Ti species. Furthermore, the average Pt-cathode particle size increases during the test, but we do not believe this phenomenon makes a significant contribution to increased cell voltages. To validate the anode Ti-PTL as a crucial source of increasing resistance, a second cell is assembled using Pt-coated Ti-PTLs. This yields a substantially reduced degradation rate of only 12 μV h−1, indicating that a non-corroding anode PTL is vital for PEM electrolyzers. It is our hope that forthcoming tailored PTLs will not only contribute to fast progress on cost-efficient stacks, but also to its long-term application of PEM electrolyzers involved in industrial processes.

Abstract The durability of a polymer electrolyte membrane (PEM) water electrolysis single cell, assembled with regular porous transport layers (PTLs) is investigated for just over 1000 h. We observe a significant degradation rate of 194 μV h−1 and conclude that 78% of the detectable degradation can be explained by an increase in ohmic resistance, arising from the anodic Ti-PTL. Analysis of the polarization curves also indicates a decrease in the anodic exchange current density, j0, that results from the over-time contamination of the anode with Ti species. Furthermore, the average Pt-cathode particle size increases during the test, but we do not believe this phenomenon makes a significant contribution to increased cell voltages. To validate the anode Ti-PTL as a crucial source of increasing resistance, a second cell is assembled using Pt-coated Ti-PTLs. This yields a substantially reduced degradation rate of only 12 μV h−1, indicating that a non-corroding anode PTL is vital for PEM electrolyzers. It is our hope that forthcoming tailored PTLs will not only contribute to fast progress on cost-efficient stacks, but also to its long-term application of PEM electrolyzers involved in industrial processes.

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.