Abstract
At present, despite the great advances in polymer electrolyte membrane fuel cell (PEMFC) technology over the past two decades through intensive research and development activities, their large-scale commercialization is still hampered by their higher materials cost and lower reliability and durability. In this review, water management is given special consideration. Water management is of vital importance to achieve maximum performance and durability from PEMFCs. On the one hand, to maintain good proton conductivity, the relative humidity of inlet gases is typically held at a large value to ensure that the membrane remains fully hydrated. On the other hand, the pores of the catalyst layer (CL) and the gas diffusion layer (GDL) are frequently flooded by excessive liquid water, resulting in a higher mass transport resistance. Thus, a subtle equilibrium has to be maintained between membrane drying and liquid water flooding to prevent fuel cell degradation and guarantee a high performance level, which is the essential problem of water management. This paper presents a comprehensive review of the state-of-the-art studies of water management, including the experimental methods and modeling and simulation for the characterization of water management and the water management strategies. As one important aspect of water management, water flooding has been extensively studied during the last two decades. Herein, the causes, detection, effects on cell performance and mitigation strategies of water flooding are overviewed in detail. In the end of the paper the emphasis is given to: (i) the delicate equilibrium of membrane drying vs. water flooding in water management; (ii) determining which phenomenon is principally responsible for the deterioration of the PEMFC performance, the flooding of the porous electrode or the gas channels in the bipolar plate, and (iii) what measures should be taken to prevent water flooding from happening in PEMFCs.
Highlights
Polymer electrolyte membrane fuel cells (PEMFCs) are considered a possible answer to environmental and energy problems, and are expected to soon become the most promising energy converters for automotive, stationary, and portable applications, because of their high-energy density at low operating temperatures, quick start-up and zero emissions
In the end of the paper the emphasis is given to: (i) the delicate equilibrium of membrane drying vs. water flooding in water management; (ii) determining which phenomenon is principally responsible for the deterioration of the PEMFC performance, the flooding of the porous electrode or the gas Energies 2009, 2 channels in the bipolar plate, and (iii) what measures should be taken to prevent water flooding from happening in PEMFCs
Besides output voltage losses of an operating PEM fuel cell, water flooding can cause changes in the characteristic properties, such as: (1) water condensation in a local site leads to a locally lower current density owing to water-hindered gas transport; (2) a locally elevated temperature leaded by the released enthalpy of the condensing water could cause an uneven local distribution of current density and temperature; and (3) the accumulation of liquid water in gas diffusion layer (GDL) or in the flow channels leads to the gas flow resistance to rise, which in turn results in an increase in the pressure drop between the inlet and outlet of the fuel cell [17,24,27]
Summary
Polymer electrolyte membrane fuel cells (PEMFCs) are considered a possible answer to environmental and energy problems, and are expected to soon become the most promising energy converters for automotive, stationary, and portable applications, because of their high-energy density at low operating temperatures, quick start-up and zero emissions. The operating principle of a PEMFC is as follows: at the anode, fuel H2 is oxidized liberating electrons and producing protons. Recent studies have shown that water management is of vital importance to achieve maximum performance and durability from PEMFCs [2,3]. A subtle equilibrium has to be maintained between membrane drying and liquid water flooding to prevent fuel cell degradation and guarantee a high performance level. This is the key to water management in PEMFCs. The purpose of the present review is to summarize the state-of-the-art studies of water management. It covers: (1) a brief introduction to the general issues of water management, where the stated focus is placed on the water transport in PEMFCs; (2) the consequences of flooding or drying caused by improper water management; (3) the experimental diagnosis and measurement of water management; (4) the mathematical models and experiments related to water management; (5) the water management strategies; and (6) a survey of prospects in future water management studies
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