Nitrogen Blanketing and Hydrogen Starvation in Dead-Ended-Anode Polymer Electrolyte Fuel Cells Revealed by Hydro-Electro-Thermal Analysis

Abstract

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...

Analysis of the performance of a passive hybrid powerplant to power a lightweight unmanned aerial vehicle for a high altitude mission

“…To this end, several test were performed in order to optimize the frequency and opening time (duty cycle) of the solenoid valves used to purge both anode and cathode gas circulation circuits. When working in dead-end mode, purges contribute to eliminate the accumulated impurities (liquid and gaseous) improving the stack performance [30][31][32][33]. In Fig.…”

Design of experiments to generate a fuel cell electro-thermal performance map and optimise transitional pathways

“…Furthermore, lowering the cell temperature in a controlled manner reduces the risk of heat spikes and thermal runaway, alongside with limiting the thermal stress on the membrane, which can lower the fuel cell performance, and drastically reduce the lifespan of the system (Sutharssan et al, 2016) As such, an air-cooled, open-cathode fuel cell represents a system that generally benefits from having greater air supply from blowers (both reactant supply and cooling) at the expense of the increasing parasitic power required to deliver the air. Recent investigations have been carried out by the authors to map the current, temperature and water distribution in air-cooled, open-cathode fuel cells (Meyer et al, 2016;Meyer et al, 2015aMeyer et al, , 2015bMeyer et al, , 2015cMeyer et al, , 2015d, revealing that these parameters need to be considered in unison to understand how these complex systems operate. However, as the experiments are typically expensive in terms of planning, resource and time, a 'smart' reduction of the number of experiments, yet with identical relevance in the interpretation is desirable.…”

Effect of gas diffusion layer properties on water distribution across air-cooled, open-cathode polymer electrolyte fuel cells: A combined ex-situ X-ray tomography and in-operando neutron imaging study

“…In the first case, it is possible to differentiate between the water content in the cathode and the anode GDL [29][30][31][32][33][34][35][36]. In the second case, it enables investigations of the effect of different designs, components, and operating conditions on the water distribution across the lateral extent of the cell [37][38][39][40][41][42][43][44][45][46][47]. Neutron imaging has been combined with other modelling and experimental techniques, such as current mapping [48], computational fluid dynamics (CFD) model validation [34,49,50], optical imaging [51], neutron scattering [52] and localised electrochemical impedance spectroscopy (EIS) [53].…”

“…Neutron imaging has been combined with other modelling and experimental techniques, such as current mapping [48], computational fluid dynamics (CFD) model validation [34,49,50], optical imaging [51], neutron scattering [52] and localised electrochemical impedance spectroscopy (EIS) [53]. The authors have recently combined neutron imaging with current and temperature mapping in a single device [37,38], linking water formation / evaporation with current density and temperature under steady state and transient conditions.…”