Water Vapor Exchange Flow Channels to Enhance the Performance of Polymer Electrolyte Fuel Cells Without Cathode Humidification

The membrane hydration of the polymer electrolyte fuel cell (PEFC) is required to maintain the efficient and stable operation. In general, the inlet gases are humidified for this purpose. However, from the viewpoint of the total efficiency of a PEFC system and the cost, the humidifier should be omitted. In particular, the cathode side is feasible due to the product water. We have thus developed a flow field design having flow channels for water exchange between the cathode and anode to circulate the product water in a cell. The anode GDL used at the active area was a commercial carbon paper (SGL SIGRACET® 24BA) without the MPL, which was treated by 5 wt% polytetrafluoroethylene (PTFE) loading to impart hydrophobicity. The cathode GDL used at the active area was a 24BA GDL coated with a hydrophobic MPL. The GDLs used at the water vapor exchange part were a 24BA GDL, a hydrophobic MPL coated GDL and a hydrophilic MPL coated GDL. The hydrophilic MPL consisting of polyvinyl alcohol (PVA) and carbon black was coated on the 24BA GDL with a PVA content of 20 wt%. This result indicates that the water vapor exchange part with the hydrophilic MPL coated GDL at the cathode is effective for the recovery of the water contained in the cathode downstream gas, which promotes water transport from the cathode to the anode dry gas in the upstream. This is effective to prevent the membrane dehydration, which reduces the ohmic loss, enhancing PEFC performance, as shown in Acknowledgments This work was supported by Grant-in-Aid for Scientific Research (S) 23226015 from Japan Society for the Promotion of Science (JSPS). References 1. T. Kitahara, H. Nakajima, and K. Mori, Journal of Power Sources, 199, 29 (2012). Abstract #1623, 224th ECS Meeting

Comparison of Humidified Hydrogen and Partly Pre-Reformed Natural Gas as Fuel for Solid Oxide Fuel Cells applying Computational Fluid Dynamics

A three-dimensional computational fluid dynamics (CFD) approach based on the finite element method (FEM) is used to investigate a solid oxide fuel cell (SOFC). Governing equations for heat, gas-phase species, electron, ion and momentum transport are implemented and coupled to kinetics describing electrochemical as well as internal reforming reactions. The model cell design is based on a cell from Ningbo Institute of Material Technology and Engineering in China and the electrochemical area-to-volume ratios are based on experimental work performed at Kyushu University in Japan. A parameter study is performed focusing on the inlet fuel composition, where humidified hydrogen, 30 % pre-reformed natural gas (as defined by IEA) and 50 % pre-reformed natural gas (as defined by Kyushu University) are compared. It is found that when 30 % pre-reformed natural gas is supplied as fuel the air mass flow rate is halved, compared to the case with humidified hydrogen, keeping the inlet and outlet temperatures constant. The current density is decreased but the fuel utilization is kept at 80 %. It is found that the cathode support layer has a significant oxygen gas-phase resistance in the direction normal to the cathode/electrolyte interface (at positions under the interconnect ribs), as well as an electron resistance inside the cathode (at positions under the air channel) in the same direction. The methane steam reforming reaction is shown, both according to the experiments and to the models, to proceed along the main flow direction throughout the cell

Observation of micropores in hard-carbon using Xe-129 NMR porosimetry

The existence of micropores and the change of surface structure in pitch-based hard-carbon in xenon atmosphere were demonstrated using Xe-129 NMR. For high-pressure (4.0 MPa) Xe-129 NMR measurements, the hard-carbon samples in Xe gas showed three peaks at 27, 34 and 210 ppm. The last was attributed to the xenon in micropores (<1 nm) in hard-carbon particles. The NMR spectrum of a sample evacuated at 773 K and exposed to 0.1 MPa Xe gas at 773 K for 24 h showed two peaks at 29 and 128 ppm, which were attributed, respectively, to the xenon atoms adsorbed in the large pores (probably mesopores) and micropores of hard-carbon. With increasing annealing time in Xe gas at 773 K, both peaks shifted and merged into one peak at 50 ppm. The diffusion of adsorbed xenon atoms is very slow, probably because the transfer of molecules or atoms among micropores in hard-carbon does not occur readily. Many micropores are isolated from the outer surface. For that reason, xenon atoms are thought to be adsorbed only by micropores near the surface, which are easily accessible from the surrounding space.</p

Molecular Orientations Change Reaction Kinetics and Mechanism: A Review on Catalytic Alcohol Oxidation in Gas Phase and Liquid Phase on Size-Controlled Pt Nanoparticles

Catalytic oxidation of alcohols is an essential process for energy conversion, production of fine chemicals and pharmaceutical intermediates. Although it has been broadly utilized in industry, the basic understanding for catalytic alcohol oxidations at a molecular level, especially under both gas and liquid phases, is still lacking. In this paper, we systematically summarized our work on catalytic alcohol oxidation over size-controlled Pt nanoparticles. The studied alcohols included methanol, ethanol, 1-propanol, 2-propanol, and 2-butanol. The turnover rates of different alcohols on Pt nanoparticles and also the apparent activation energy in gas and liquid phase reactions were compared. The Pt nanoparticle size dependence of reaction rates and product selectivity was also carefully examined. Water showed very distinct effects for gas and liquid phase alcohol oxidations, either as an inhibitor or as a promoter depending on alcohol type and reaction phase. A deep understanding of different alcohol molecular orientations on Pt surface in gas and liquid phase reactions was established using sum-frequency generation spectroscopy analysis for in situ alcohol oxidations, as well as density functional theory calculation. This approach can not only explain the entirely different behaviors of alcohol oxidations in gas and liquid phases, but can also provide guidance for future catalyst/process design

Alcohol Oxidation at Platinum-Gas and Platinum-Liquid Interfaces: The Effect of Platinum Nanoparticle Size, Water Coadsorption and Alcohol Concentration

Alcohol oxidation reaction over platinum nanoparticles with size ranging from 2 to 8 nm deposited on mesoporous silica MCF-17 was studied in the gas and liquid phases. Among methanol, ethanol, 2- propanol, and 2-butanol oxidations, the turnover frequency increased as the nanoparticle size became large in both reaction phases. The activation energy in the gas phase was higher than that in the liquid phase. Water coadsorption decreased the turnover rate of all the gas and liquid phase oxidations except for the gas-phase 2-butanol case, while a certain amount of water promoted 2-propanol oxidation in the liquid phase. Sum frequency generation vibrational spectroscopy study and DFT calculation revealed that the alcohol molecules pack horizontally on the metal surface in low concentration and stand up in high concentration, which affects the dissociation of β-hydrogen of the alcohol as the critical step in alcohol oxidation

Molecular orientations change reaction kinetics and mechanism: A review on catalytic alcohol oxidation in gas phase and liquid phase on size-controlled Pt nanoparticles

Catalytic oxidation of alcohols is an essential process for energy conversion, production of fine chemicals and pharmaceutical intermediates. Although it has been broadly utilized in industry, the basic understanding for catalytic alcohol oxidations at a molecular level, especially under both gas and liquid phases, is still lacking. In this paper, we systematically summarized our work on catalytic alcohol oxidation over size-controlled Pt nanoparticles. The studied alcohols included methanol, ethanol, 1-propanol, 2-propanol, and 2-butanol. The turnover rates of different alcohols on Pt nanoparticles and also the apparent activation energy in gas and liquid phase reactions were compared. The Pt nanoparticle size dependence of reaction rates and product selectivity was also carefully examined. Water showed very distinct effects for gas and liquid phase alcohol oxidations, either as an inhibitor or as a promoter depending on alcohol type and reaction phase. A deep understanding of different alcohol molecular orientations on Pt surface in gas and liquid phase reactions was established using sum-frequency generation spectroscopy analysis for in situ alcohol oxidations, as well as density functional theory calculation. This approach can not only explain the entirely different behaviors of alcohol oxidations in gas and liquid phases, but can also provide guidance for future catalyst/process design