Relative humidity effects current density

Fig. 2.25 Steady-state polarization curves at different humidification is observed at high current densities and relative humidities for high temperature polymer electro- low RH. Reproduced from [32] with permission of the lyte fuel cell based on Aquivion (Solvay) membrane American Chemical Society and CNR-ITAE catalysts. The effect of internal...

Tsushima et al. (2010) developed an MRI system to investigate the effects of relative humidity (RH) and current density on the transverse water content profile in a membrane under fuel cell operation at a practical PEMFC operating temperature. The MRI visualization revealed that in dry conditions (40% RH), the membrane hydration X number was 3, and the water content profile in the membrane was fiat because the diffusion process in the membrane was dominant in the water transport. In a standard condition (80% RH) the water content in the membrane was 8, and a partial dehydration at the anode was observed at a current density of 0.2 A/cm, indicating that electroosmosis was influential. At the higher RH level of 92%, the water content X within the membrane at 0.2 A/cm was around 22, corresponding to the eqnilibrium state of the membrane in liquid water, and the water content profile with the increase in current density became fiat. This indicates that the liquid water generated in the cathode catalyst layer permeated the membrane, where water transport plays a more dominant role. 

Since the temperature in the PEFC typically varies by at most 15°C, the effect of temperature distribution on the reaction kinetics will be relatively small compared to the temperature interaction with the liquid water distribution and relative humidity that controls membrane ionic conductivity. The local relative humidity in the anode phase typically controls the local current density of an underhumidified fuel cell because electro-osmotic drag exacerbates anode dryout, while water generation at the cathode diminishes any electrolyte dryout in the cathode catalyst layer [11]. That is, if the other parameters are constant, the local current distribution can be predicted with a knowledge of the anode in an underhumidified cell [11]. Anode dryout can be the result of the loss of only a few hundredths of a milligram of water per square centimeter active area in the catalyst layer... 

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