Liquid Water Removal from a Polymer Electrolyte Fuel Cell

Abstract

Liquid water transport and removal from the gas diffusion layer ͑GDL͒ and gas channel of a polymer electrolyte fuel cell ͑PEFC͒ are studied experimentally and theoretically. In situ observations of the liquid water distribution on the GDL surface and inside the gas channel were made in an operating transparent PEFC. Liquid droplet formation and emergence from the GDL surface are characterized and two modes of liquid water removal from the GDL surface identified: one through droplet detachment by the shear force of the core gas flow followed by a mist flow in the gas channel, and the other by capillary wicking onto the more hydrophilic channel walls followed by the annular film flow and/or liquid slug flow in the channel. In the former regime, typical of high gas flow rates, the droplet detachment diameter is correlated well with the mean gas velocity in the channel. In the latter regime characteristic of low gas flow rates, liquid spreading over hydrophilic channel surfaces and drainage via corner flow were observed and analyzed. A theory is developed to determine what operating parameters and channel surface contact angles lead to sufficient liquid drainage from the fuel cell via corner flow. Under these conditions, the fuel cell could operate stably under a low flow rate ͑or stoichiometry͒ with only a minimum pressure drop required to drive the oxidizer flow. However, when the corner flow is insufficient to remove liquid water from the gas channel, it was observed that the annular film flow occurs, often followed by film instability and channel clogging. Channel clogging shuts down an entire channel and hence reduces the cell's active area and overall performance.Polymer electrolyte fuel cells ͑PEFCs͒ are presently regarded as a promising energy conversion system for future automobiles and stationary applications. A significant technical challenge in a PEFC is that the cell is prone to excess liquid water formation due to water production from oxygen reduction reaction ͑ORR͒ at the cathode. Liquid water may fill open pores of a gas diffusion layer ͑GDL͒, thereby blocking the transport of oxygen into a catalyst layer ͑CL͒, and may further cover the catalyst sites in the CL, rendering them electrochemically inactive. This is known as "GDL/CL flooding." Liquid water formation and subsequent flooding may also occur at low current densities under certain operating conditions, such as low temperatures and low gas flow rates, due to faster saturation of the gas phase with water vapor. If liquid water accumulation becomes excessive in a PEFC, a water lens or water band may form inside the gas channel, thereby clogging and shutting down the oxidizer flow. This latter condition is referred to as "channel flooding and clogging." In the presence of either GDL/CL flooding or channel flooding, the cell performance decreases and the longevity of PEFC materials and components suffers. Therefore, liquid water removal from a PEFC is of paramount importance for improving PEFC performance and durability.The need for modeling liqui...

Experimental study of two-phase flow in a proton exchange membrane fuel cell in short-term microgravity condition

“…Visualization method is an effective way to observe the two-phase flow, which has been used for tube [15] or minichannel [16] recently. Researchers have studied the two-phase flow caused by liquid water inside PEMFCs by using the direct visualization method and transparent PEMFCs [17][18][19][20][21][22][23][24][25][26][27]. Yang et al [19] applied an optical PEMFC to study the mechanics of liquid water transport.…”

Performance characteristics and internal phenomena of polymer electrolyte membrane fuel cell with porous flow field

“…In addition to the conventional serpentine and straight channels [1], a conventional and interdigitated-switchable gas flow field was proposed to improve the cell performance under flooded conditions [2], and a self-draining stirred tank reactor cell was proposed for operations with low relative humidity gases [3]. To develop a detailed understanding of PEFC dynamics, a number of studies on the diagnostics of PEFC, such as on the measurement of the current density distribution and visualization of the water production behavior, have been conducted [3][4][5][6][7][8][9], and an overview was provided on the distributions of current density, high-frequency resistance, gas species and temperature, and including a two-phase visualization [1]. The effects of the distribution of the produced water and the gas composition on the cell performance were discussed using the measured results of the current density distribution [4,5], and the relationships between the local cell impedance and current density have also been investigated [3].…”

“…The water production behavior in the cathode gas flow channels has been investigated using a transparent fuel cell, and the relation between liquid water behavior and cell performance was demonstrated [6]. Liquid water removal from gas channels has also been characterized in detail [7]. The authors have observed phenomena related to water production behavior inside a cell, analyzed the effects on the current and temperature distributions across the reaction area [8], and reported the specific phenomena in the flow of condensed water and gas using cells with serpentine and straight channels [9].…”

Thermal Conductivity and Contact Resistance of Compressed Gas Diffusion Layer of PEM Fuel Cell

“…On the other hand, deficiency of water decreases the proton conductivity of the membrane and thus humidification of reactants is typically needed. Water management has a considerable effect on fuel cell performance and the process is highly sensitive to operational and material parameters [9][10][11][12][13]. Water transport inside the fuel cell has a close link to temperature distribution since water and oxygen diffusivity, saturation parameters, and reaction kinetics are all temperature dependent, see, e.g.…”