• Energy Research
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Retaining optimum acid-contents in membranes and electrodes is critical to maintaining the performance and durability of acid-doped high-temperature (HT) polymer-electrolyte-membrane fuel cells (PEMFCs). Since the distribution of acids is influenced by the operating and compression conditions of the stack, there is great demand for understanding the behavior of individual membrane-electrode-assemblies (MEAs) while operating the cells in a stack. In this study, an in-situ diagnosis method using electrochemical impedance spectroscopy (EIS) is implemented during the durability test of an HT-PEMFC stack. Adopting a lumped equivalent-circuit model, the specific parameters are obtained from EIS results, and the changes of the values are compared with the performance loss of individual MEA. From this analysis it can be concluded that the main cause of performance degradation of the stack is due to the loss of electrolytes in the cathode, which leads to an increase in the proton transport resistance of cathode catalyst layers. In addition to the proton transport loss in the cathode, the charge transfer resistance of the oxygen reduction reaction has contributed to the performance decay of the stack. The causes of the increase in the cathode charge transfer resistance for each cell of the stack are discussed.

The anodic performance enhancement of solid oxide fuel cells (SOFCs) by introducing penetrating electrolyte structures was investigated using a random resistor network model considering the transport of electrons and ions, and the electrochemical reaction in composite anodes. The composite anode was modeled as a mixture of ionic and electronic particles, randomly distributed at simple cubic lattice points. The dependence of the anodic polarization resistances on the volume fraction of the electronic phase, the thickness of the anode, and the insertion of various penetrating electrolyte structures were explored to obtain design criteria for best performing composite anodes. The network simulation showed that the penetrating electrolyte structures are advantageous over flat electrolytes by enabling more efficient use of electrochemical reaction sites, and thereby reducing the polarization resistances.

Abstract Regarding the reliability of the polymer electrolyte membrane (PEM) fuel cell, the advancement of fuel, thermal and water management techniques of the system have been critically investigated. In this study, a 1 kW class PEM fuel cell stack is built with an Aciplex-S™ membrane, which is integrated to be automatically controlled in a hydrogen-fueled power generation system of a 80 cm ×70 cm ×120 cm single unit with a dc/ac inverter which produces 220 VAC power. Intensive and systematic care should be taken especially with the longer cell stack which is being operated under repeated current load change. The automatically controlled fuel-feed and thermal management system achieved in this study can markedly enhance the fuel efficiency and the reliability of the cell stack. The devices in the sub-systems are all electrically controlled versions to be manipulated on a touch screen via a PLC unit. The thermal and fuel-feed control logic are pre-built-in the CPU of the PLC unit based on an early study of cell stack evaluation. In addition, the power inverting and dummy load unit is coupled to the power generation system, and an additional data acquisition system has been constructed.

Abstract Sodium borohydride (NaBH 4 ) in the presence of sodium hydroxide as a stabilizer is a hydrogen generation source with high hydrogen storage efficiency and stability. It generates hydrogen by self-hydrolysis in aqueous solution. In this work, a Co–B catalyst is prepared on a porous nickel foam support and a system is assembled that can uniformly supply hydrogen at >6.5 L min −1 for 120 min for driving 400-W polymer electrolyte membrane fuel cells (PEMFCs). For optimization of the system, several experimental conditions were changed and their effect investigated. If the concentration of NaBH 4 in aqueous solution is increased, the hydrogen generation rate increases, but a high concentration of NaBH 4 causes the hydrogen generation rate to decrease because of increased solution viscosity. The hydrogen generation rate is also enhanced when the flow rate of the solution is increased. An integrated system is used to supply hydrogen to a PEMFCs stack, and about 465 W power is produced at a constant loading of 30 A.

Abstract Local compression distribution in the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell (PEMFC) and the associated effect on electrical material resistance are examined. For this purpose a macroscopic structural material model is developed based on the assumption of orthotropic mechanical material behaviour for the fibrous paper and non-woven GDLs. The required structural material parameters are measured using depicted measurement methods. The influence of GDL compression on electrical properties and contact effects is also determined using specially developed testing tools. All material properties are used for a coupled 2D finite element simulation approach, capturing structural as well as electrical simulation in combination. The ohmic voltage losses are evaluated assuming constant current density at the catalyst layer and results are compared to cell polarisation measurements for different materials. The results show that the largest part of the polarisation difference found between roll-good and batch type materials with wide channel flowfields is well captured by the simulation and is due to additional electrical losses in the locally low compressed GDL. Thus, for the first time a broader understanding of the significant performance impact of diffusion layer mechanical properties is generated. However, at higher loads an interaction of compression with electrical and additional heat and mass transport effects occurs, which will be included in the next part of the study. This part is limited to structural mechanics and coupled electrical transport effects.

Abstract During cycling, mechanical stresses can occur in the composite electrode, inside the active material, but also in the solid electrolyte interphase layer. A mechanical model is proposed based on a system made of a spherical graphite particle surrounded by the solid electrolyte interphase layer. During lithium intercalation or de-intercalation, stresses in the graphite are produced, governed by the diffusion induced stress phenomena and in the solid electrolyte interphase, driven by the graphite expansion. The stresses in both materials were simulated and a sensitivity analysis was performed to clarify the influence of principal parameters on both processes. Finally, assuming that the solid electrolyte interphase is the weakest material and therefore more prone to fracture than graphite, the experimental capacity fade during cycling was modeled based on its break and repair effect rather than on the fracture of the active material. The mechanical model of the solid electrolyte interphase was implemented in a single particle lithium ion battery model in order to reproduce capacity fade during battery lifetime. The model results were compared against cycle life aging experimental data, reproducing accurately the influence of the depth of discharge as well as the average state of charge on the capacity fade.

Abstract Polymer electrolyte membrane fuel cell (PEMFC) stacks in a fuel cell vehicle can be inevitably exposed to harsh environments such as cold weather in winter, causing water flooding by the direct flow of condensed water to the electrodes. In this study, anode flooding was experimentally investigated with condensed water generated by cooling the anode gas line during a long-term operation (∼1600 h). The results showed that the performance of the PEMFC was considerably degraded. After the long-term experiment, the thickness of the anode decreased, and the ratio of Pt to carbon in the anode increased. Moreover, repeated fuel starvation of the half-cell severely oxidized the carbon surface due to the high induced potential (>1.5 VRHE). The cyclic voltammogram of the anode in the half-cell experiments indicated that the characteristic feature of the oxidized carbon surface was similar to that of the anode in the single cell under anode flooding conditions during the long-term experiment. Therefore, repeated fuel starvation by anode flooding caused severe carbon corrosion in the anode because the electrode potential locally increased to >1.0 VRHE. Consequently, the density of the tri-phase boundary decreased due to the corrosion of carbons supporting the Pt nanoparticles in the anode.

Abstract In this paper the losses in coulombic efficiency are investigated for a vanadium/air redox flow battery (VARFB) comprising a two-layered positive electrode. Ultraviolet/visible (UV/Vis) spectroscopy is used to monitor the concentrations c V 2 + and c V 3 + during operation. The most likely cause for the largest part of the coulombic losses is the permeation of oxygen from the positive to the negative electrode followed by an oxidation of V2+ to V3+. The total vanadium crossover is followed by inductively coupled plasma mass spectroscopy (ICP-MS) analysis of the positive electrolyte after one VARFB cycle. During one cycle 6% of the vanadium species initially present in the negative electrolyte are transferred to the positive electrolyte, which can account at most for 20% of the coulombic losses. The diffusion coefficients of V2+ and V3+ through Nafion® 117 are determined as D V 2 + , N 117 = 9.05 · 10 − 6 cm2 min−1 and D V 3 + , N 117 = 4.35 · 10 − 6 cm2 min−1 and are used to calculate vanadium crossover due to diffusion which allows differentiation between vanadium crossover due to diffusion and migration/electroosmotic convection. In order to optimize coulombic efficiency of VARFB, membranes need to be designed with reduced oxygen permeation and vanadium crossover.

Using the finite element method, lateral currents in two different configurations for current distribution mapping in polymer electrolyte fuel cells (PEFCs) were simulated and the impact on the accuracy of measurement was analysed. The measurement techniques were of a conventional and a newly developed type, based on a segmented bipolar plate (BPP), respectively, a non-segmented bipolar plate combined with a printed circuit board (PCB). In both cases, neither the membrane electrode assemblies (MEAs) nor the gas diffusion layers (GDLs) were segmented and local currents were detected passively. The resistance of the measurement circuit and the current density gradient between neighbouring segments were found to be the major parameters causing current spreading. As expected, a significantly higher uncertainty of measurement could be observed and experimentally verified for the non-segmented bipolar plate. However, the accuracy is increasing with increasing homogeneity of the current density distribution. Achieving a uniform utilisation of the active area is a major task in fuel cell development and approaching this objective also improves the quality of measurement. Consequently, the application of non-segmented bipolar plates as highly flexible and practical measurement technique is a suitable option for current distribution mapping in technically relevant single cells and fuel cell stacks.

Abstract A cyclic open circuit voltage (COCV) accelerated stress test (AST) is designed to screen the simultaneous effect of chemical and mechanical membrane degradation in polymer electrolyte fuel cells. The AST consists of a steady state OCV phase to accelerate chemical degradation and periodic wet/dry cycles to provide mechanical degradation. The membrane degradation process induced by COCV AST operation is analyzed using a standard MEA with PFSA ionomer membrane. The OCV shows an initially mild decay rate followed by a higher decay rate in the later stages of the experiment. Membrane failure, defined by a threshold convective hydrogen leak rate, is obtained after 160 h of operation. Uniform membrane thinning is observed with pinhole formation being the primary cause of failure. Mechanical tensile tests reveal that the membrane becomes stiffer and more brittle during AST operation, which contributes to mechanical failure upon cyclic humidity induced stress. Solid state 19 F NMR spectroscopy and fluoride emission measurements demonstrate fluorine loss from both side chain and main chain upon membrane exposure to high temperature and low humidity OCV condition.