Measurement of Net Water Drag Coefficients in Polymer Electrolyte Fuel Cells under Cathode-Dry Conditions

Abstract

For polymer electrolyte fuel cell (PEFC) systems in vehicle applications, removal of external humidifiers from the cathode is desirable to reduce system efficiency, cost, space and weight, and thus PEFCs should achieve continuous self-humidification operation under cathode-dry conditions. A critical index to judge the success or failure of self-humidification is net water drag coefficient (α NWD ). Self-humidification operation requires α NWD to be negative. Here, α NWD is experimentally evaluated using cells with different configurations of gas diffusion layer (GDL), membrane, and flow channel geometry. Experimental results confirmed that α NWD remains negative under cathode-dry conditions at a cell temperature of 60°C, and that current density -net water drag coefficient (i − α NWD ) characteristics are immune to configurations compared with the cell performance, when the flow rate of air at the cathode is low enough to keep α NWD low. In addition, under these conditions, the effect of flow velocity and pressure on the i − α NWD characteristics was limited. On the other hand, as expected, a thinner membrane promotes back-diffusion of water to the anode. In conclusion, the flow rate of air should be determined carefully so that the cell performance is maintained while self-humidification is achieved.

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

“…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).…”

“…).-The definition and the method of calculation of NWD α was the same as those in our previous studies. [41][42][43] ) was then calculated by using the measured gas flow rates, total pressures, and dew points as presented in Table II. At steady state, the rate of water amount is balanced between charged, generated and discharged water from the cell as follows:…”

“…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.…”

“…[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. Second, we evaluated the temperature dependence of i NWD α − characteristics in a T cell range from 80 °C to 120 °C, when either a Nafion or Aquivion membrane (50-μm thickness) was used as an electrolyte and when air γ = 2 or 4 at the cathode and the flow rate of H 2 at the anode was constant (0.4 NL min −1 ).…”

“…42 Our results clearly demonstrated that NWD α increased with increasing T cell and was positive in almost the entire i range at T cell = 120 °C. Although these two studies 41,42 provided a great deal of knowledge about , NWD α the gas supply conditions differed from those in a real system, because the flow rate and/or RH of supplied gases was too high. Third, the i NWD α − characteristics were measured at T cell = 80 °C under conditions similar to those of the gas supply in a real system, that is, standard conditions were 1.5 for both H2 γ and , Air γ and 30% for both RH A in and RH .…”