Thomas A. Trabold

Water Transport Mechanisms in PEMFC Gas Diffusion Layers

Understanding how water produced in the cathode catalyst layer is removed during proton exchange membrane fuel cell (PEMFC) operation is critical for optimization of materials and model development. The present work combines in situ and ex situ experiments designed to elucidate the dominant water discharge mechanism when considering capillary and vapor transport at normal PEMFC operating conditions. The flux of water vapor driven by the thermal gradient in the cathode diffusion layer can alone be sufficient to remove product water at high current densities even with saturated gas in the delivery channels. The role of an intermediate microporous layer and its impact in vapor vs liquid transport is also considered. We propose that the primary role of the microporous layer is to prevent condensed water from accumulating on and blocking oxygen access to the cathode catalyst layer. .

Minichannels in Polymer Electrolyte Membrane Fuel Cells

Abstract This paper provides an overview of the application of minichannels, typically on the order of a 1 mm hydraulic diameter, in the design of polymer electrolyte membrane (PEM) fuel cells. In these electrochemical devices, minichannels deliver reactant hydrogen and oxygen to the anode and cathode electrodes, respectively, while transporting product water out of the cell. The channels must be designed for low pressure drop to avoid excessive parasitic power losses from gas handling equipment. However, the channels also need to operate in a flow regime in which the overall water balance in the fuel cell can be maintained. The various aspects of minichannel design, including size and cross-sectional shape, are discussed, with a particular emphasis on fuel cell water management. In addition to reviewing these fundamental aspects of minichannel design, examples are given of new experimental tools currently under development that are applied to relate channel water transport and accumulation to fuel cell performance.

Anaerobic co-digestion of commercial food waste and dairy manure: characterizing biochemical parameters and synergistic effects

Anaerobic digestion of commercial food waste is a promising alternative to landfilling commercial food waste. This study characterized 11 types of commercial food wastes and 12 co-digestion blends. Bio-methane potential, biodegradable fraction, and apparent first-order hydrolysis rate coefficients were reported based upon biochemical methane potential (BMP) assays. Food waste bio-methane potentials ranged from 165 to 496 mL CH4/g VS. Substrates high in lipids or readily degradable carbohydrates showed the highest methane production. Average bio-methane potential observed for co-digested substrates was -5% to +20% that of the bio-methane potential of the individual substrates weighted by VS content. Apparent hydrolysis rate coefficients ranged from 0.19d(-1) to 0.65d(-1). Co-digested substrates showed an accelerated apparent hydrolysis rate relative to the weighted average of individual substrate rates. These results provide a database of key bio-digestion parameters to advance modeling and utilization of commercial food waste in anaerobic digestion.

Accumulation and Removal of Liquid Water in Proton Exchange Membrane Fuel Cells

Removal of liquid water from proton exchange membrane fuel cells is critical for efficient performance, low temperature operation, and robustness for start-up under freezing conditions. The present work investigates the three-dimensional location of liquid water during steady-state operation and the governing mechanisms that control its accumulation and removal. A combination of ex situ and in situ methods with neutron imaging diagnostics is used to investigate water transport in the bulk materials and flow field channels. These results provide experimental evidence of the mechanisms by which water vapor condenses in the anode during operation and the transport resistance for its removal during shutdown purge. Based on these results, a one-dimensional model is proposed that can be applied to calculate the effectiveness of cathode purge starting from a wide range of fuel cell shutdown conditions.

Life cycle assessment integrated with thermodynamic analysis of bio-fuel options for solid oxide fuel cells

A methodology that integrates life cycle assessment (LCA) with thermodynamic analysis is developed and applied to evaluate the environmental impacts of producing biofuels from waste biomass, including biodiesel from waste cooking oil, ethanol from corn stover, and compressed natural gas from municipal solid wastes. Solid oxide fuel cell-based auxiliary power units using bio-fuel as the hydrogen precursor enable generation of auxiliary electricity for idling heavy-duty trucks. Thermodynamic analysis is applied to evaluate the fuel conversion efficiency and determine the amount of fuel feedstock needed to generate a unit of electrical power. These inputs feed into an LCA that compares energy consumption and greenhouse gas emissions of different fuel pathways. Results show that compressed natural gas from municipal solid wastes is an optimal bio-fuel option for SOFC-APU applications in New York State. However, this methodology can be regionalized within the U.S. or internationally to account for different fuel feedstock options.

Lifecycle Greenhouse Gas Analysis of an Anaerobic Codigestion Facility Processing Dairy Manure and Industrial Food Waste

Anaerobic codigestion (AcoD) can address food waste disposal and manure management issues while delivering clean, renewable energy. Quantifying greenhouse gas (GHG) emissions due to implementation of AcoD is important to achieve this goal. A lifecycle analysis was performed on the basis of data from an on-farm AcoD in New York, resulting in a 71% reduction in GHG, or net reduction of 37.5 kg CO2e/t influent relative to conventional treatment of manure and food waste. Displacement of grid electricity provided the largest reduction, followed by avoidance of alternative food waste disposal options and reduced impacts associated with storage of digestate vs undigested manure. These reductions offset digester emissions and the net increase in emissions associated with land application in the AcoD case relative to the reference case. Sensitivity analysis showed that using feedstock diverted from high impact disposal pathways, control of digester emissions, and managing digestate storage emissions were opportunities to improve the AcoD GHG benefits. Regional and parametrized emissions factors for the storage emissions and land application phases would reduce uncertainty.

Through-Plane Water Transport Visualization in a PEMFC by Visible and Infrared Imaging

In this study, water transport and thermal profile in the through-plane direction of a proton exchange membrane fuel cell (PEMFC) gas diffusion layer (GDL) are reported. Direct optical and infrared access to both cathode and anode GDLs are provided in a typical 50 cm test section. Dynamic visualization (pixel resolution of 0.6 lm at 28 Hz) of liquid water transport and emergence in the gas distribution channels and diffusion layers are reported and the underlying transport processes are discussed. The temperature distributions across the anode and cathode GDL are also measured with a high resolution infrared camera with a pixel resolution of 5 lm at 30 Hz. VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3560163] All rights reserved.

WATER TRANSPORT VISUALIZATION AND TWO-PHASE PRESSURE DROP MEASUREMENTS IN A SIMULATED PEMFC CATHODE MINICHANNEL

Two-phase flow and water transport in a 1.08 mm hydraulic diameter by 25-cm long gas-transport minichannel are investigated. High-speed side-view images are obtained of water droplets moving through gas diffusion media (GDM) and into a gas channel. This system simulates water transport and the flow of air and water in a polymer electrolyte membrane fuel cell (PEMFC) cathode gas channel. Advancing and receding contact angles and departure droplet diameters are measured with respect to superficial gas velocity for two GDM samples. Pressure drop is measured and compared to two-phase pressure drop correlations for three different water flow and five different airflow rates, and channel-water and GDM-water interactions are described.

Assessment of bio-fuel options for solid oxide fuel cell-based auxiliary power units

It is now illegal in many states to idle heavy-duty trucks to supply the cabin electrical needs of the driver. An alternative approach is the solid oxide fuel cell-based auxiliary power unit (APU), which can produce electrical power with much higher efficiency and lower greenhouse gas emissions. The “standard” APU system includes a fuel reformer, which converts on-board diesel fuel into a hydrogen-rich effluent that is delivered to the anode side of the fuel cell stack. As renewable bio-fuels begin to realize greater market penetration, there is a need to determine which bio-fuel option provides the optimal pathway to economic value and emissions reduction when converted for use in a fuel cell APU system. The results of this study indicate that biodiesel from waste cooking oil may be the optimal bio-fuel option because of its relatively low energy use and greenhouse gas (GHG) emissions, as well as chemical and physical properties that are similar to those of petroleum-based diesel fuel.

Stack Compression of PEM Fuel Cells

The effect of dimensional changes of fuel cell components from temperature and hydration cycles on the stack compression is investigated in this paper. Using a simple spring model including the membrane electrode assembly (MEA), gas diffusion layers (GDL), bipolar plates, seal gaskets, current collectors, insulation plates, end plates, and side plates, we find significant compression changes from 30% over-compression to 23% compression loss from both temperature and humidity changes. The wide range of variation in stack compression is attributed to the swelling behavior of polymer electrolyte membranes, the compression behavior of gas diffusion layers, and the design of stack assembly. This paper also reports the use of finite element method to investigate the compression of MEA and GDL over the channel area where MEA buckling from membrane swelling can result in separation of MEA and GDL. It is suggested that the compression over channels can be improved with higher transverse shear modulus in the GDL in addition to the use of narrower channels. In this paper, we will also discuss the challenges facing the fuel cell manufacturers and component suppliers on the needs for new materials with improved mechanical properties and better testing/modeling techniques to help achieving stable compression and better fuel cell stack designs.© 2004 ASME