SEGMENTATION OF PROTON EXCHANGE MEMBRANE FUEL CELL IN THE LAND-CHANNEL DIRECTION

In a proton exchange membrane fuel cell (PEMFC) mass is transported in three directions- 1) through-plane 2) land-channel, and 3) flow-channel. In all these directions, because of the non-uniform distribution of oxygen and hydrogen the current density distribution is non-uniform which translates to uneven water generation and ohmic resistance distribution. These non-uniformities affect the overall power density and efficiency of the fuel cell. In the recent decade, segmentation of PEMFC in the flow-channel direction became an indispensable diagnosis tool to study mass transport losses but are limited to flow-channel direction. So the objective of this Ph.D. work is to develop an experimental technique to the segment PEMFC in the land-channel direction and study the mass transport losses in the land-channel direction. To achieve this target PEMFC is segmented in the land-channel direction and parameters such as current density, electrical resistance, electrochemical surface area, and ohmic resistance distribution were measured at 350 μm resolution of 1 mm wide land and channel configuration flow field. Three novel designs of segmented PEMFC were examined to measure these parameters: 1) First generation plug-in segmented anode 2) Second generation plug-in segmented anode and 3) μ segmented anode flow field. First and second generation plug-in segmented are designed to insert in large scale fuel cell to study local mass transport losses and μ segmented anode flow field is introduced to further simplify experimental set-up. Later two designs demonstrated success and were validated by conducting series of in-situ and ex-situ tests. Second generation plug-in flow field and μ segmented anode flow field applied to measure current density distribution in land-channel direction in wet, extremely dry, and moderate conditions. In addition, to these designs an experimental set-up to measure ohmic resistance distribution in conjunction with current density distribution is demonstrated. A methodology is described to use current density and ohmic resistance distribution data to analyze the effect of land-channel geometry on estimating oxygen concentration, membrane water content, and oxygen transport resistance distribution in moderate and dry conditions, and in dry condition cathode electrode resistance and membrane water content. This technique further applied to interdigitated flow field

Current density distribution in the lateral direction of conventional and interdigitated flow field

© The Electrochemical Society. The design of flow field significantly affects the mass transport in an operating PEMFC. One example of such design is interdigitated flow field. In this kind of flow field depending on the location both convective and diffusive mass transfer occur in the land-channel direction. These two modes of mass transfer influence the local current and water generation in the land-channel direction. To understand the difference between these two mass transport mechanism the comparison of local current density and ohmic resistance distribution in the land-channel direction of conventional and interdigitated flow fields is imperative. This work is focused on measurement of current density and ohmic resistance distribution in the land-channel direction of a conventional and an interdigitated flow fields. A customized catalyst coated membrane of active area 9 mm2 was prepared and a segmented anode was employed to measure the parameters mentioned at 350 μm resolution in the land-channel direction of an interdigitated flow field. At 60 °C and 90 % RH current density and ohmic resistance distribution were measured for both kind of flow fields, and discussion is presented

Sources of current density distribution in the land-channel direction of a PEMFC

© The Author(s) 2016. Published by ECS. A proton exchange membrane fuel cell (PEMFC) was segmented to measure local current density, electrochemical surface area, and high frequency resistance (HFR) distribution in the land-channel direction at resolution of 350 μm. An in-house catalyst coated membrane of 3mm×3mmactive areawas prepared to represent a small area in a larger scale cell with 1mmland and channelwidths. This design was employed to measure current density and HFR distribution at 60°C with several different operating conditions. Local electrical resistance was also measured separately so that local protonic resistances can be discerned from local HFR. To analyze the effect of the land-channel geometry a method was developed to quantify the sources of current distribution, such as distributions of oxygen concentration at the electrode, oxygen transport resistance, cathode catalyst layer resistance, and membrane water content. Current density distribution is strongly correlated with the distribution of membrane water content and electrode resistance in dry condition, and oxygen concentration distribution in wet condition, while in moderate condition both oxygen concentration and water content in membrane are critical to the local current density distribution. The results imply the limitation of uniform condition assumption used in a differential cell study

In-plane distribution analysis in the land-channel direction of a proton exchange membrane fuel cell (PEMFC)

© The Author(s) 2015. Published by ECS. A novel design was implemented to segment a proton exchange membrane fuel cell (PEMFC) in the land-channel direction to measure the current density distribution at sub-millimeter resolution of 340 μm. This design was validated by performing an ex-situ electrical resistance test, in-situ hydrogen pumping test, and by comparing performances between non-segmented and segmented cells at two different conditions. Further, this design was utilized to measure the current density distribution in wet conditions, in extremely dry condition, and in moderate condition. In wet conditions current densities are found to be higher in the channel areas than in the land areas. To differentiate between the effects of transport distances from channel to the catalyst layer and non-uniform water saturation distribution in gas diffusion medium, oxygen concentration ratios were estimated in wet conditions. Predicted oxygen ratio distribution showed that the effect of water accumulation under the land in gas diffusion medium is more significant than the effect of transport distance which can be well supported by in-situ microscopy study published in literature. However, in dry condition, current density was higher in the land areas than in the channel while in moderate condition current density was found to be uniform

Segmentation of proton exchange membrane fuel cell (PEMFC) in the land-channel direction

© 2014 The Electrochemical Society. All rights reserved. The land-channel geometry in a typical proton exchange membrane fuel cell (PEMFC) causes the non-uniform current generation in the land-channel direction. In order to measure the current density distribution in land-channel direction at sub-millimeter resolution, a new design and fabrication method are proposed. Ex-situ electronic resistance measurements were conducted to validate the design, and in-situ measurements with H2 and air were performed to obtain the current distribution in the land-channel direction in dry and wet conditions

Current density and ohmic resistance distribution in the land-channel direction of a proton exchange membrane fuel cell

© 2015 Elsevier B.V. All rights reserved. A highly instrumented segmented cell is designed to measure current density and ohmic resistance distribution in the land-channel direction of a proton exchange membrane fuel cell at resolution of 350 μm. A customized catalyst coated membrane with an active area of 9 mm \u3c sup\u3e 2 is prepared, and a printed-circuit board technique is introduced to ease fabrication of segmented anode and to adapt design to any flow arrangement. Design of segmented cell is validated by electrochemical pumping of hydrogen from anode to cathode. Current density and ohmic resistance distribution are measured in two wet conditions (at 40 °C and 60 °C) and a dry condition at 60 °C. In all cases a strong correlation between current generation and ohmic resistance distribution is observed. Outcomes from these experiments revealed that the water distribution has a strong effect on the local current generation and ohmic resistance. In wet condition ohmic resistance is uniform but current generation found to be non-uniform because of the non-uniform liquid water distribution. In dry condition, on the other hand, non-uniform water generation resulted in both uneven current generation and ohmic resistance

Experimental study of oxygen transport mechanisms in PEMFC interdigitated flow field

Proton exchange membrane fuel cells for automotive application need to provide high performance at higher current density region where the oxygen transport is the limiting factor. Because of convective mass transport mechanism, interdigitated flow field has a potential to improve the oxygen transport limitation. In this paper, the mass transport mechanisms in an interdigitated flow field are experimentally studied using a segmented cell with 350 μm resolution in land-channel direction. At high current density operation, in general, the region under the land has higher values of local current density than regions under the channel which is opposite to a typical profile for conventional flow field. The local oxygen transport resistances measured at limiting current density conditions show the nearly constant values at lower oxygen concentration operation and then sudden increase at higher concentration. This indicates the presence of liquid water in the oxygen transport path when operated with high oxygen concentration. Convective-dominant and diffusive-dominant oxygen transport regions in gas diffusion layer (GDL) are identified from the oxygen transport resistance distribution, and the change of effective oxygen diffusivity in GDL due to liquid water is also estimated

A novel interdigitated electrode design and methodology for the measurement of ionic conductivity of ionomer thin film on carbon substrate

In this work, we introduce a new design of the interdigitated electrode (IDE) for the measurement of conductivity of ionomer thin film on the planar carbon surface, which is relevant to many electrochemical systems including fuel cells. The design is an advancement over commonly used SiO2 substrate based IDE, previously employed for Nafion thin film conductivities. Electrochemical impedance spectroscopy technique was applied to measure the conductivity of 55 nm thick Nafion film on carbon surface. Through complementary modeling work using equivalent circuit, we identify that the competition between the double layer capacitances at ionomer/carbon and ionomer/electrode interfaces can mask the conventionally observed high-to-medium impedance arc observed for Nafion films on SiO2 substrate. This problem can be circumvented by using electrochemically active Pt electrode in hydrogen environment. Hence, this systematic study describes a method to determine the thin film conductivity on carbon substrate

Cross-Correlated Humidity Dependent Structural Evolution of Nafion Thin Film Confined on Platinum Substrate

Nanometer thin films of Nafion ionomer interfacing with platinum form the functional electrode in many electrochemical devices including fuel cells and electrolyzers. To impart facile proton conduction in Nafion ionomer, sufficient hydration of Nafion ionomer is necessary to create a percolating network of water-filled nanometer sized hydrophilic domains that manifests as macroscopic swelling. This hydration behavior of ionomer thin films is poorly understood especially for films confined on electrochemically relevant Pt substrates. In this work, we present the evolution of hydration-dependent microscopic hydrophilic domains and macroscopic expansion of 55 nm thin Nafion film on a Pt substrate. The cross-correlation among the film macro-expansion from ellipsometry, the micro-expansion from GISAXS, and the water distribution from neutron reflectometry (NR), explains the observed non-affine behavior of the film which can be attributed to the randomly and spatially non-uniform distribution of water domains. A correlation between macroscopic factor (ε/τ) for protonic conductivity, and the domain size and swelling is presented for the first time. In addition, interfacial water between Pt and the ionomer interface estimated at 75% and 84% RH, and increase in domain size with RH, are discussed to explain increased activity and oxygen diffusivity with RH

Transport and Electrochemical Interface Properties of Ionomers in Low-Pt Loading Catalyst Layers: Effect of Ionomer Equivalent Weight and Relative Humidity

Catalyst layer (CL) ionomers control several transport and interfacial phenomena including long-range transport of protons, local transport of oxygen to Pt catalyst, effective utilization of Pt catalyst, electrochemical reaction kinetics and double-layer capacitance. In this work, the variation of these properties, as a function of humidity, for CLs made with two ionomers differing in side-chain length and equivalent weight, Nafion-1100 and Aquivion-825, was investigated. This is the first study to examine humidity-dependent oxygen reduction reaction (ORR) kinetics in-situ for CLs with different ionomers. A significant finding is the observation of higher ORR kinetic activity (A/cm2Pt) for the Aquivion-825 CL than for the Nafion-1100 CL. This is attributed to differences in the interfacial protonic concentrations at Pt/ionomer interface in the two CLs. The differences in Pt/ionomer interface is also noted in a higher local oxygen transport resistance for Aquivion-825 CLs compared to Nafion-1100 CLs, consistent with stronger interaction between ionomer and Pt for ionomer with more acid groups. Similar dependency on Pt utilization (ratio of electrochemically active area at any relative humidity (RH) to that at 100% RH) as a function of RH is observed for the two CLs. As expected, strong influence of humidity on proton conduction is observed. Amongst the two, the CL with high equivalent weight ionomer (Nafion-1100) exhibits higher conduction