In-situ diagnostics and degradation mapping of a mixed-mode accelerated stress test for proton exchange membranes

Abstract

h i g h l i g h t sA mixed-mode accelerated stress test for PEM fuel cells is proposed. High spatial resolution to diagnose PEM degradation is achieved via a segmented fuel cell. The experimental results have been validated using traditional diagnostic methods. a b s t r a c tWith increasing availability of more durable membrane materials for proton exchange membrane fuel cells, there is a need for a more stressful test that combines chemical and mechanical stressors to enable accelerated screening of promising membrane candidates. Equally important is the need for in-situ diagnostic methods with sufficient spatial resolution that can provide insights into how membranes degrade to facilitate the development of durable fuel cell systems. In this article, we report an accelerated membrane stress test and a degradation diagnostic method that satisfy both needs. By applying highamplitude cycles of electrical load to a fuel cell fed with low-RH reactant gases, a wide range of mechanical and chemical stressful conditions can be created within the cell which leads to rapid degradation of a mechanically robust Ion Power™ N111-IP membrane. Using an in-situ shorting/crossover diagnostic method on a segmented fuel cell fixture that provides 100 local current measurements, we are able to monitor the progression and map the degradation modes of shorting, thinning, and crossover leak over the entire membrane. Results from this test method have been validated by conventional metrics of fluoride release rates, physical crossover leak rates, pinhole mapping, and cross-sectional measurements.

Accelerated Stress Testing of Fuel Cell Membranes Subjected to Combined Mechanical/Chemical Stressors and Cerium Migration

“…Correspondingly, an increase in convective crossover is an indication that pinhole leaks have developed which can result from exposure to mechanical stress or at the last stage of chemical degradation. This approach of correlating diffusive and convective crossover currents with membrane thinning and pinhole leaks has been successfully proven in a previous study using a non-reinforced Nafion N111-IP membrane 18 which showed excellent agreement between the residual thicknesses measured from the SEM cross sectional imaging and the calculations from diffusive crossover measurements.…”

“…19 The strategies employed in these ASTs typically involved a combination of periods of OCV in hot/dry conditions and humidity cycling either sequentially [11][12][13][14]19 or simultaneously, 2,15,17,20 or simultaneous humidity and voltage cycling driven by current cycling. 18,21 These studies show that the combined stressors can significantly accelerate the membrane failure; 2,18 degrade the membrane's mechanical properties, water uptake behavior, or proton conductivity; [11][12][13][14] or increase the size of existing defects. 19 The synergistic pathways for membrane degradation by the combined mechanical and chemical stressors have been summarized in Kusoglu and Weber.…”

“…In the HAST, we have improved an approach developed in an earlier study 18 on a mechanically durable extruded membrane, Nafion N111-IP. In the test design, we aimed to induce a variable but known distribution of combined stressors within the cell and used a current distribution (CD) measurement tool to detect the initiation, monitor the progress, and distinguish the modes of local degradation.…”

“…For example, an early version of HAST using a traditional 50 cm 2 serpentine square flowfield also showed accelerated membrane thinning and pinhole formation correlated with the increased hydration cycling toward the gas outlet. 18 While the use of CD and RTD boards provide more precise information of the local stressors and degradation diagnostics, they are not necessary if the goal is to compare materials or understand the failure/degradation modes on the cell level. One can still use the proposed shorting/crossover diagnostic method to monitor the degradation modes over the entire cell and then perform postmortem analysis to confirm the failure mode at the end of the test.…”

Fuel Cell Durability Enhancement with Cerium Oxide under Combined Chemical and Mechanical Membrane Degradation

“…It was reported that the MEA, which experienced more mechanical fatigue damage of hygrothermal cycling, exhibited formation of gas-leak initiating pinholes at less membrane thinning of chemical degradation. 14 In practical point of view, a mitigation strategy of membrane degradation should meet the practical need of high power performance. During the AST operation, the HF impedance of the CeO 2 -MEA was evaluated to be about 0.26 ·cm 2 which is 1.5 times higher than the baseline (0.18 ·cm 2 ) as shown in Fig.…”