Application of Neutron Imaging in PEFC Research

Boillat, Pierre; Oberholzer, Pierre; Perego, R.C.; Siegrist, R.; Kaestner, Anders; Lehmann, Eberhard; Scherer, Günther G.; Wokaun, Alexander

doi:10.1149/1.3635541

Cited by 5 publications

(3 citation statements)

References 35 publications

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“…section "variation of gas humidity" below) suggest a negligible impact of bulk diffusion losses in helox operation. Concerning the artifacts introduced by helox operation sometimes discussed in the literature, 34,38,39 previous experiments with neutron imaging 45 demonstrated, besides the effect on resistivity, a change of water content in the gas diffusion layers. The time scale of water content change was observed to be 1-2 minutes.…”

Section: Methods Development Resultsmentioning

confidence: 98%

Impact of Water on PEFC Performance Evaluated by Neutron Imaging Combined with Pulsed Helox Operation

Boillat

Oberholzer

Kaestner

et al. 2012

J. Electrochem. Soc.

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A novel experimental method for the measurement of mass transport losses in a polymer electrolyte fuel cell (PEFC) was developed, based on the comparison of cell voltage during operation with air, helox, (79% He, 21% O 2 ) or pure O 2 . Using only short periods (2 seconds) of helox or pure O 2 operation, the disturbing artifacts (dry out or change of catalyst surface oxidation state) associated with continuous operation with these gases were avoided. High resolution neutron imaging was conducted simultaneously in order to study the relation between mass transport loss and liquid water accumulation. For the studied design, comparisons in steady state and dynamic experiments indicated a larger impact of water accumulation in the gas diffusion layers (GDL) under the flow channels, or in the portion of the channels near the GDL surface. Increased diffusion losses were observed, as expected, under high humidity conditions, but also at very low humidity conditions in absence of any liquid water. The latter was explained by a redistribution of current toward the rib region, having longer diffusion paths.Polymer Electrolyte Fuel Cells (PEFCs) are energy converters, bearing attractive characteristics for automotive applications in terms of efficiency and absence of pollutant emissions. The dimension of an automobile fuel cell system is primarily determined by the required peak power. In consequence, reaching the highest possible power density allows a reduction of the system size, weight, and cost. At high current density operation, losses introduced by the limitations in the diffusive transport of oxygen through the gas diffusion layers (GDLs) can become significant. The presence of liquid water in the gas diffusion layers reduces the effective diffusivity of such media and results in increased losses.The visualization of liquid water in operating PEFCs using different methods has been increasingly reported. A recent review on in situ visualization methods can be found in a publication by Tsushima et al. 1 Besides its high contrast for liquid water, neutron imaging 2-20 offers the advantages of good transparency of usual fuel cell materials (including metals), and of a negligible impact of the non-ionizing radiation on cell operation, making it an excellent non-invasive method. Water visualization in fuel cells has been a strong driving force for addressing the issue of the limited spatial resolution of neutron imaging. [19][20][21][22][23] The approach used at the Paul Scherrer Institut, based on optimized optical setup and scintillator screens, 24 combined with specific improvements for fuel cell imaging, 20,21 allowed reaching the combination of an effective spatial resolution of 20 μm with exposure times of 10 seconds. Thus, the measurement of liquid water content in different layers of a fuel cell is possible, not only in steady state, but also in transient operation. In order to assess the impact of liquid water on cell performance, a method for directly measuring diffusive transport losses is of high interest. For...

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Section: Methods Development Resultsmentioning

confidence: 98%

Impact of Water on PEFC Performance Evaluated by Neutron Imaging Combined with Pulsed Helox Operation

Boillat

Oberholzer

Kaestner

et al. 2012

J. Electrochem. Soc.

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“…Though an increasingly detailed understanding is obtained, a crucial problem is the lack of robust experimental validation. Methods which have been reported are the use of neutron radiography [6,7], synchrotron radiography [8], and soft X-ray radiography [9,10]. Although these techniques have provided useful information about liquid water distribution/behavior within an operation PEFC, they only provide macroscopic and 2 dimensional information, and it is difficult to obtain local, pore-scale information about water configurations, such as the prevalence of dead end clusters, depth of water penetration, lateral spreading or residual phase distributions.…”

Section: Introductionmentioning

confidence: 99%

Investigation on Effect of PTFE Treatment on GDL Micro-structure by High-resolution X-ray CT

Sasabe¹,

Inoue

Tsushima

et al. 2013

ECS Trans.

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To remove liquid water from a gas diffusion layer (GDL) effectively and minimize an increase of transport resistance, hydrophobic treatment with a polytetrafluoroethylene (PTFE) is widely used. Because the GDL micro-structure has strong impact on transport properties, detailed understanding of fiber and porous network and PTFE distribution is strongly requested. To investigate correlation between the GDL micro-structure, PTFE treatment, and transport properties, micro-structure of the GDL was visualized by high-resolution X-ray computed tomography (X-ray CT). Newly designed sample holder allowed to regulate compression pressure. Visualization results with spatial resolution of 1.0 um made it possible to distinguish carbon fibers, binder material, PTFE, and voids (air), and non-uniform distribution of PTFE was observed. Because non-uniform porosity and contact angle distribution in the direction of thickness change the liquid water morphology within the GDL, understanding and controlling the GDL micro-structure is important and manufacturing process of GDL should be scientifically developed.

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“…In the work presented here, the effect of the current density and of the relative humidity was studied at a local scale by using differential cells [21][22][23][25][26][27]30,[33][34][35][40][41][42][43][44][45] in a systematic fashion with two different gas diffusion layer (GDL) materials. These cells can be considered as "building blocks" over which a technical cell is constructed, and studying their characteristics is highly useful for understanding the operation of the whole cell.…”

mentioning

confidence: 99%

Local Characterization of PEFCs by Differential Cells: Systematic Variations of Current and Asymmetric Relative Humidity

Oberholzer

Boillat

2013

J. Electrochem. Soc.

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Systematic variations of current density and asymmetric relative humidity are realized on differential Polymer Electrolyte Fuel Cells (PEFCs). The water distribution is visualized and quantified by high resolution in-plane imaging, and its effect on performance is discussed. Two cells are investigated: one using paper type gas diffusion layers (GDLs) and one with cloth type GDLs. The novel output of this work is an extensive database of local measurements obtained by the differential cell approach and an asymmetric variation of relative humidity. This should be particularly valuable for the validation of modeling studies. The negative impact of water accumulation on the performance is clearly stronger for the paper type GDLs than for the cloth type GDLs. On the contrary, the performance of the cell with cloth GDLs is low in dry conditions. The accumulation of water in the channel region of the cathode GDL has a crucial impact on performance. The anode side is observed to play an important role for water removal. The presence of a maximal liquid water saturation in the GDL for a wide range of current densities is observed and discussed. In-plane water distribution profiles are presented for the channel and rib regions for all conditions.

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Application of Neutron Imaging in PEFC Research

Cited by 5 publications

References 35 publications

Impact of Water on PEFC Performance Evaluated by Neutron Imaging Combined with Pulsed Helox Operation

Impact of Water on PEFC Performance Evaluated by Neutron Imaging Combined with Pulsed Helox Operation

Investigation on Effect of PTFE Treatment on GDL Micro-structure by High-resolution X-ray CT

Local Characterization of PEFCs by Differential Cells: Systematic Variations of Current and Asymmetric Relative Humidity

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