2 Highlights First description of in-plane water distribution using neutron imaging in an aircooled, open-cathode fuel cell. High water content identified under cathode land area, whereas anode water content is relatively homogeneous. Water distribution in anode GDL directly linked to dispersion of PTFE. Anode GDL composition shown to affect water content and distribution in cathode. Combined X-ray computed tomography, SEM/EDS and TGA used to characterise GDL structure and composition. AbstractIn-situ diagnostic techniques provide a means of understanding the internal workings of fuel cells under normal operating conditions so that improved designs and operating regimes can be identified. Here, an approach is used which combines exsitu characterisation of two anode gas diffusion / microporous layers (GDL-A and GDL-B) with X-ray computed tomography and in-situ analysis using neutron imaging of operating fuel cells. The combination of TGA, SEM and X-ray computed tomography reveals that GDL-A has a thin microporous layer with 26 % PTFE covering a thick diffusion layer composed of 'spaghetti' shaped fibres. GDL-B is covered by two microporous media (29 % and 6.5 % PTFE) penetrating deep within the linear fibre network. The neutron imaging reveals two pathways for water management underneath the cooling channel, either diffusing through the cathode GDL to the active channels, or diffusing through the membrane and towards the 3 anode. Here, these two behaviours are directly affected by the anode gas diffusion PTFE content and porosity. KeywordsGas diffusion layer; air-cooled open-cathode; X-ray computed tomography; neutron imaging; water management. IntroductionPolymer electrolyte fuel cells (PEFC) fuelled with hydrogen are among the most promising energy conversion technologies for a broad range of applications, including portable, stationary and automotive power delivery. However, understanding the cell water management is crucial for performance optimisation.Flooding impedes reactant transport (water mainly concentrating at the cathode) and reduces the surface area of the catalyst, causing significant if not catastrophic decay in cell performance, and dehydration can lead to cracks and irreversible damage [1][2][3]. The gas diffusion layer (GDL) provides a pathway for electron transport, ensures even reactant delivery and helps water management within each cell. The water balance between flooding and membrane dehydration is a function of the GDL's structure, porosity and PTFE (hydrophobic) content. Here, two commercial GDLs with microporous layers are characterised ex-situ by capturing the design and structure via X-ray computed tomography (CT), along with its polytetrafluoroethylene (PTFE) / carbon distribution via SEM/EDS analysis and thermogravimetric analysis (TGA); in-situ 'visualisation' of the water distribution in the in-plane orientation was performed using neutron radiography. These techniques can be correlated with one another to gain new insights into the water management role of the GDL in fuel c...

The development of novel in-situ diagnostic techniques allows new insight into the internal working of polymer electrolyte fuel cells (PEFCs) so that improved performance can be realised. Electrochemical impedance spectroscopy (EIS) is a widely used characterisation technique that takes advantage of the dynamic relationship between current and voltage to deconvolute critical mechanisms and sources of performance loss occurring with different time constants. Here, we apply electrochemical pressure impedance spectroscopy (EPIS) which examines the transfer function relating reactant gas pressure modulation to the electrical response of the fuel cell. A sinusoidally oscillating perturbation is applied to the cathode backpressure using an innovative loudspeaker arrangement and the resulting voltage 2 perturbation is monitored across a frequency range while the fuel cell is operated in galvanostatic mode. It is shown that the technique can be used to separate the explicit effect of water management from reactant starvation when a PEFC is operated under different reactant humidification conditions. KeywordsElectrochemical pressure impedance spectroscopy; polymer electrolyte fuel cell; backpressure; transfer function analysis; water management; mass transport limitation.

Thermal imaging is an established technique for studying temperature distribution with high spatial resolution; however, lock-in thermography offers the ability to identify heat transfer mechanisms and internal defects. Here, for the first time, lock-in thermography is applied to Li-ion batteries to investigate the effect of periodic electro-thermal stimulation on the amplitude and phase response of both new and aged Li-ion pouch cells. We demonstrate the effectiveness of this technique in the detection of gas pockets formed during cell aging, and propose its application in the identification of internal defects in operational systems and validation of thermal models.

Dead-ended anode operation has a number of practical advantages that simplify system complexity and lower cost for polymer electrolyte fuel cells. However, deadended mode leads to performance loss over time which can only be reversed by performing intermittent purge events. This work applies a combined hydro-electrothermal analysis to an air-cooled open-cathode fuel cell, presenting experimental functional maps of water distribution, current density and temperature. This approach has allowed the identification of a 'nitrogen blanketing' effect due to nitrogen crossover from the cathode and a 'bypass' effect where a peripheral gap between the gasket and the GDL offers a hydrogen flow 'short circuit' to the border of the electrode. A consequence of high local current density at the margin of the electrode, and resulting high temperatures, may impact the lifetime of the cell in dead-end mode. KeywordsDead-ended anode; bypass effect; neutron imaging; nitrogen blanketing; current and temperature mapping. IntroductionPolymer electrolyte fuel cells (PEFC) fuelled with hydrogen are among the most promising energy conversion technologies for a broad range of applications, including portable, stationary and automotive power delivery. Dead-ended anode operation enables significant design simplification, with the replacement of humidifiers, and flow controllers by pressure regulators [1]. However, it causes 3 reversible performance decay, and intermittent purging of the anode chamber is required to sustain effective operation. Greater insight into the mechanism of fuel cell operation during dead-ended mode is required in order to optimise the purging programme and ensure that irreversible degradation does not result. In-situ diagnostic techniques provide one of the most effective ways of studying the performance of fuel cells; combined mapping of current density, temperature and water distribution is applied here to give an unprecedented level of understanding into dead-ended fuel cell operation. Current and temperature mapping in fuel cellsInitiated by Cleghorn et al. 4 Combined temperature and current mapping studies allow the impact and interactions of these two parameters on the overall performance to be assessed [17,20,[31][32][33]. However, capturing the water content may provide insights on how the temperature and current density fluctuate, and therefore should ideally be measured at unison. Neutron imaging in fuel cellsNeutron imaging can identify water in the in-plane orientation (with the membrane plane parallel to the beam) and through-plane orientation (with the membrane plane perpendicular to the beam), enabling differentiation of water content in the cathode and the anode [34][35][36] Dead-ended anode operations in an air-cooled open cathode fuel cell.Dead-ended anode operation is a common mode for operating fuel cells as it can simplify the fuel cell system, potentially avoiding flow meters, humidifiers, and drastically reducing hydrogen losses (slippage). It employs a single pressure regulator befor...

The development of in-situ diagnostic techniques is critical to ensure safe and effective operation of polymer electrolyte fuel cell systems. Infrared thermal imaging is an established technique which has been extensively applied to fuel cells; however, the technique is limited to measuring surface temperatures and is prone to errors arising from emissivity variations and reflections. Here we demonstrate that electro-thermal impedance spectroscopy can be applied to enhance infrared thermal imaging and mitigate its limitations. An open-cathode polymer electrolyte fuel cell is used as a case study. The technique operates by imposing a periodic electrical stimulus to the fuel cell and measuring the consequent surface temperature 2 response (phase and amplitude). In this way, the location of heat generation from within the component can be determined and the thermal conduction properties of the materials and structure between the point of heat generation and the point of measurement can be determined. By selectively 'locking-in' to a suitable modulation frequency, spatially resolved images of the relative amplitude between the current stimulus and temperature can be generated that provide complementary information to conventional temporal domain thermograms.