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J. Electrochem. En. Conv. Stor.. 2018;15(3):031001-031001-15. doi:10.1115/1.4038601.

The state-of-the-art conventional technology for postcombustion capture of CO2 from fossil-fueled power plants is based on chemical solvents, which requires substantial energy consumption for regeneration. A promising alternative, available in the near future, is the application of molten carbonate fuel cells (MCFC) for CO2 separation from postcombustion flue gases. Previous studies related to this technology showed both high efficiency and high carbon capture rates, especially when the fuel cell is thermally integrated in the flue gas path of a natural gas-fired combined cycle or an integrated gasification combined cycle plant. This work compares the application of MCFC-based CO2 separation process to pulverized coal fired steam cycles (PCC) and natural gas combined cycles (NGCC) as a “retrofit” to the original power plant. Mass and energy balances are calculated through detailed models for both power plants, with fuel cell behavior simulated using a 0D model calibrated against manufacturers' specifications and based on experimental measurements, specifically carried out to support this study. The resulting analysis includes a comparison of the energy efficiency and CO2 separation efficiency as well as an economic comparison of the cost of CO2 avoided (CCA) under several economic scenarios. The proposed configurations reveal promising performance, exhibiting very competitive efficiency and economic metrics in comparison with conventional CO2 capture technologies. Application as a MCFC retrofit yields a very limited (<3%) decrease in efficiency for both power plants (PCC and NGCC), a strong reduction (>80%) in CO2 emission and a competitive cost for CO2 avoided (25–40 €/ton).

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;15(3):031002-031002-10. doi:10.1115/1.4039141.

Proton exchange membrane fuel cells (PEMFCs) require mechanical compression to ensure structural integrity, prevent leakage, and to minimize the electrical contact resistance. The mechanical properties and dimensions of the fuel cell vary during assembly due to manufacturing tolerances and during operation due to both temperature and humidity. Variation in stack compression affects the interfacial contact pressures between components and hence fuel cell performance. This paper presents a one-dimensional equivalent stiffness model of a PEMFC stack capable of predicting independent membrane and gasket contact pressures for an applied external load. The model accounts for nonlinear component compression behavior, thickness variation due to manufacturing tolerances, thermal expansion, membrane expansion due to water uptake, and stack dimensional change due to clamping mechanism stiffness. The equivalent stiffness model is compared to a three-dimensional (3D) finite element model, showing good agreement for multicell stacks. Results demonstrate that the correct specification of gasket thickness and stiffness is essential in ensuring a predictable membrane contact pressure, adequate sealing, and avoiding excessive stresses in the bi-polar plate (BPP). Increase in membrane contact pressure due to membrane water uptake is shown to be significantly greater than the increase due to component thermal expansion in the PEMFC operating range. The predicted increase in membrane contact pressure due to thermal and hydration effects is 18% for a stack containing fully hydrated Nafion® 117 membranes at 80 °C, 90% relative humidity (RH) using an eight bolt clamping design and a nominal 1.2 MPa assembly pressure.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;15(3):031003-031003-11. doi:10.1115/1.4039298.

Water management is a critical issue for a direct methanol fuel cell (DMFC). This study focuses primarily on the use of a super-hydrophilic or super-hydrophobic cathode porous flow field to improve the water management of a passive air-breathing DMFC. The flow field layer was made of an in-house copper-fiber sintered felt (CFSF) which owns good stability and conductivity. Results indicate that the super-hydrophilic flow field performs better at a lower methanol concentration since it facilitates water removal when the water balance coefficient (WBC) is high. In the case of high-concentration operation, the use of a super-hydrophobic pattern is more able to reduce methanol crossover (MCO) and increase fuel efficiency since it helps maintain a lower WBC due to its ability in enhancing water back flow from the cathode to the anode. The effects of methanol concentration and the porosity of the CFSF are also discussed in this work. The cell based on the super-hydrophobic pattern with a porosity of 60% attains the best performance with a maximum power density of 18.4 mW cm−2 and a maximum limiting current density of 140 mA cm−2 at 4 M.

Commentary by Dr. Valentin Fuster

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