Effect of Flow Conditions of Anode Hydrogen on Net Water Drag Coefficient of Polymer Electrolyte Fuel Cells

Abstract

The net water drag coefficient (αNWD) is supposedly an essential index in water management, and must be negative for system operation for polymer electrolyte fuel cell (PEFC) systems. In this study, αNWD is carefully examined by measurement of the water content in the gases supplied to and those discharged from the cell/stack under gas supply conditions similar to those in a real PEFC system. We focus on the effect of anode hydrogen (H2) flow conditions (i.e., flow rate and relative humidity) on αNWD under a constant flow condition of cathode air. Measured characteristics of current density (i) - αNWD in which the relative humidity of discharged H2 from the anode is constant at around 100%, show that αNWD is stable when i exceeds a certain threshold. The value of αNWD in this stable region is particularly sensitive to changes in the stoichiometric ratio of H2. These measurements also suggest that in the αNWD stable region, almost the entire area of the membrane is fully hydrated, and the effect of back diffusion on αNWD becomes negligible. Analysis of the observed relationships between αNWD and several variables indicate that increasing the H2 circulation flow rate effectively prevents anode dry-out.

Temperature Dependency of Net Water Drag Coefficient in Polymer Electrolyte Fuel Cells

“…Cell set-ups.-A single cell was used to measure the i− NWD α characteristics under various operating conditions. The cell used in this study was identical to those used in our previous studies 42,43 and had an electrode area of 25 cm 2 (50 × 50 mm, square). The bipolar plates (separators) of both sides anode and cathode were made of graphite for high-temperature applications, and the geometry of the flow field was "serpentine-dual" with a channel and rib width of 1 mm and a channel depth of 0.5 mm.…”

“…The flow direction of both the anode gas (H 2 ) and cathode gas (air) was set at a "counter-flow" configuration. The same commercial catalyst-coated membranes (CCMs) as in previous studies 41,43 were applied in this study. The membrane used in the CCMs was made of perfluorosulfonic acid (PFSA) polymer and reinforced with expanded polytetrafluoroethylene (ePTFE).…”

“…Target values for the humidifying gas dew point at each RH are summarized in Table SI. As for the dew point measurements at the outlet, our previous study using the same type of measurement system 43 showed that due to the high content of water, the condensation of water vapor could not be avoided at the cathode outlet even when the pipeline was heated, and thus, the dew point of discharged air at cathode (T _ dew C out ) could not be measured properly. Therefore, in this experiment, a DPM was not installed at the cathode outlet, but only at the anode outlet to measure the dew point of discharged H 2 (T _ dew A out ).…”

“…C in 43 Measured i NWD α − characteristics in which the relative humidity of discharged H 2 from the anode (RH A out ) was constant at around 100%, showed that NWD α was stable when i exceeded a certain threshold. The value of NWD α in this stable region was generally negative and particularly sensitive to changes in .…”

“…We previously conducted experimental studies to measure NWD α under various operating conditions using a single cell (25 cm 2 ). [41][42][43] First, we evaluated the i -NWD α characteristics under cathode dry conditions at T cell = 60 °C, when air γ = 2 or 3 at the cathode and at a constant H 2 flow rate of 0.5 NL min −1 at the anode. 41 The measured i NWD α − characteristics confirmed that NWD α could be kept negative in the measurement range of i (⩽1.4 A cm −2 ), though NWD α increased with increasing i.…”