Sequential Degradation Mechanism of Stationary PEMFC Under Dehydration Condition

Abstract

With regard to increasing energy consumption, a polymer electrolyte membrane fuel cell (PEMFC) has received lots of attention due to its pollutant-free fuel and byproduct (water). There are lots of works to improve the performance and stability of PEMFC, however, there remains unsolved problem of long-term durability directly related to cost, which are caused by the corrosive environments inside the membrane-electrode assembly (MEA). In particular, both electrochemical corrosion and chemical deterioration result in a fast degradation of PEMFC, especially under the specific degradation conditions such as fuel starvation, flooding, dehydration, and on-off cycle. Among them, a study on dehydration condition becomes the most important as next-generation fuel cell system demands high temperature (low RH) in stack operation for better tolerance to CO and the simplified water management systems in PEMFC [1]. However, the previously reported degradation mechanisms under dehydration are mostly focused on the membrane decay and the electrochemical analyses were too conventional to elucidate the mechanism. Hence, in the present study we aim at performing sequential deconvolution of the degradation mechanism of PEMFC operating at 300 mA cm-2 under low humidity of RH 30%. During the dehydration degradation process, we conducted electrochemical measurements under both humidity condition: normal condition (RH 100%) and dry condition (RH 30%). In particular, electrochemical measurement under low humidity condition allows us to exclude the influence of the water as a proton transport medium (See Figure 1(a)) and subdivide specific resistance components from the profile of the Nyquist plots by increasing the ionic resistance deliberately in order to focus on only the intrinsic properties of membrane itself as shown in Figure 1(b). As shown in Figure 2(a-b), the current density at 0.7 V of cell voltage under RH 100 % condition decreases from 620.0 to 518.8 cm-2 during initial 300 h of degradation test and additional 300 h of procedure causes additional 17.2% decrement with the increase of charge transfer resistance. Even though these results show the degraded MEA under corrosive condition, these are not enough to identify the successive degradation components. As shown in Figure 2(c), the i-V curves measured under RH 30 % shows the performance also decrease during initial 300 h of degradation process with the increase of ionic resistance from 1.15 to 3.15 ohm cm2. However, after additional 300 h (total 600 h) the abnormal phenomenon showing the temporarily improved performance was confirmed with the decrease of ionic resistance from 3.15 to 1.9 ohm cm2 as shown in Figure 2(d). The unique characteristic could seem to be induced by a shorten proton diffusion path by the membrane thinning [2]. After operating at 300 mA cm-2 for more than 600 h at RH 30%, TEM surface analyses were conducted. As shown in Figure 3(a-e), the Pt particles were grown more largely on cathode than anode, which implies that reverse current decay (cathode carbon corrosion) occurs even in the dehydration degradation. In summary, the degradation mechanism under dehydration can be separately identified in sequential order: (1) ionomer degradation, (2) membrane thinning, and (3) reverse current decay.