Low-humidity and load-change operations of polymer electrolyte fuel cells (PEFCs) often tend to induce the excessive voltage drop called “undershoot”. Especially when the load current rapidly increases in steps, the strong electro-osmotic drag dehydrates the electrolyte membrane and deteriorates the proton conductivity, resulting in a temporal increase in ohmic loss. To identify a factor for causing undershoot, it is necessary to precisely grasp the transient water behavior within PEFCs operated under load-change conditions. In the previous works, several researchers vigorously explored the water vapor distribution along flow fields of working cells with micro gas chromatography (μGC) [1]. Although GC techniques provide more accurate species concentrations, it takes a long time (at least two minutes) to collect a gas sample and identify a species. As an alternative approach, the authors have developed a single-ended fiber-optic sensor to measure the in-situ concentration of water vapor within narrow channels of fuel cells at high speed and with high accuracy based on tunable diode laser absorption spectroscopy (TDLAS) [2]. TDLAS is one of the high-speed and high-sensitive absorption spectroscopy techniques which detect the absorbance of a chemical species and identify its concentration using infrared diode laser. This study became able to monitor the transient variation of water vapor concentration in the anode channel of a PEFC during low-humidity and load-change operations using the TDLAS-based fiber-optic sensor and discussed the correlation between the water behavior and the voltage fluctuation. Furthermore, the effects of load current and compression pressure were also investigated.

Figs. 1(a) and 1(b) show the dynamic responses of cell voltage and the transient water concentrations in the anode channel of the operating PEFC assembled under two different compression pressures of 1.0 and 2.0 MPa. The electrode area of the experimental cell is 50 mm². Two separators on both sides have a single-straight channel (1.0 mm width, 1.0 mm depth, 6.0 mm length). Hydrogen (stoichiometry: 2.3@1.0A/cm², 30%RH) and oxygen (stoichiometry: 9.1@1.0A/cm², 30%RH) were fed to the anode and cathode channel in the co-flow arrangement. The fiber-optic probe for water detection was positioned at the fractional location of x/L=0.5 on the anode side and the modulated beam emitted from a DFB laser diode (wavelength: 1392.5 nm) was directly injected into the channel. The cell temperature is 70^oC. The repetitive current cycling was performed between 0.2 A/cm² for 23 s and 0.6 A/cm² for 23 s. In the case of the compression pressure of 1.0 MPa, when the current density increases steeply from 0.2 to 0.6 A/cm², the strong electro-osmotic drag temporarily reduces the water content on the anode side of the electrolyte membrane and increases the ohmic loss, resulting in the voltage undershoot. After a step increase in the current density, the temporal drop of water vapor concentration in the anode channel can also be observed. When the compression pressure is increased to 2.0 MPa, the porosities of the diffusion media are reduced and the water concentration becomes high especially under the anode rib. Thus, even if the current density is increased stepwise after the repeated current cycles, the water shortage in the anode and the undershoot are not caused. It was noted that the strong compression effectively improves the water retaining on the anode side under low-humidity operations and alleviate the voltage undershoot.

References:

[1] M.M. Mench, et al., J. Power Sources, 124, 90 (2003).

[2] K. Nishida, et al., ECS Trans., 92(8), 119 (2019).