Accelerated Degradation of High-Temperature Polymer Electrolyte Fuel Cells: Discussion and Empirical Modeling

Abstract

Accelerated stress tests are commonly applied in order to obtain a prediction of fuel cell life time within a short testing period. The stress test in this work considers a fuel cell which is constantly under load with frequent periods of very high current. Four different cells were operated each with a specific load profile. As a result severe performance degradation was observed in the region of high current densities. A short overview over the phenomena which may contribute to this specific fuel cell degradation is compiled from literature. Based on this overview major degradation modes are identified and combined with a simple polarization curve model. The modeling results allow for two different interpretations. Carbon corrosion reactions can explain the observed effects if cell voltage is assumed as the driving force for degradation. A better fit was obtained by using the overall heat flux as degradation criterion. In this case an increase in local temperature could lead to redistribution and loss of phosphoric acid from the MEA. The expected lifetime of a polymer electrolyte fuel cell (PEFC) ranges from 5 000 h for mobile application to 40 000 h for stationary application.1,2 In order to gain information about time dependent degradation in advance accelerated test protocols are used.3,4 Depending on the design of these tests valuable information about the nature of the main degradation mode under the applied operation conditions can be obtained. Despite the difference in operation conditions for PEFC, HT-PEFC and PAFC the structure and material of the electrodes are almost the same. Consequently, observed degradation phenomena show strong similarities.5 A good description of these phenomena can be found in several review papers 6-13 as well as in at least four books 1,2,14,15 where recent experimental facts and interpretations are summarized. A compact description can also be found in a recent review paper on modeling transport phenomena in PEFCs. 16 The purpose of this paper is to present the results of a specific accelerated degradation test for high temperature polymer electrolyte fuel cells (HT-PEFC) with phosphoric acid doped polybenzimidazole (PBI/H 3 PO 4 ) membranes. The fuel cell is constantly under load with frequent periods of very high current. In order to accelerate degradation effects the fuel cell is operated beyond it's point of maximum power. A simple polarization curve model is used as starting point for the analysis of data. Based on this first analysis a degradation model is proposed which allows for a straightforward physical interpretation. The complexity of degradation effects which occur simultaneousy usually leads to models which either resolve a specific mechanism in depth or describe more general effects. This paper aims at general effects which requires a broader understanding of degradation. In the following it is attempted to provide an overview over the phenomena which may contribute to fuel cell degradation. Based on this overview major degradation modes are identified and ...

Determination of Anion Transference Number and Phosphoric Acid Diffusion Coefficient in High Temperature Polymer Electrolyte Membranes

“…The effect of acid migration may also explain the observed increase in degradation of HT-PEMFC at very high current densities. 43 The coupled effect of acid migration and back diffusion was also discussed for the operation of fuel cells with poly-perflourosulfonic acid membranes. 44 In that case the very low diffusion coefficient of the acid is assumed to be responsible for the very poor performance.…”

Effect of accelerated ageing tests on PBI HTPEM fuel cells performance degradation

“…Indeed, carrying out ageing tests based on load cycling on high temperature PEM fuel cells is not intended to study their behavior in real applications, but it could be of great interest in order to isolate the effects of electrocatalyst degradation from other mechanisms that can be favored during other operation modes. Load cycling has been proved to be a significant stressor for catalyst degradation [24,25,38,39,40] and it is possible to perform this operation keeping constant other parameters such as temperature and reactants flow.…”

New approaches towards novel composite and multilayer membranes for intermediate temperature-polymer electrolyte fuel cells and direct methanol fuel cells

“…The routes to produce composite polymer membranes are similar to any other composite polymer. [14] However, composite membranes can be considered as one of the first attempts to develop something better and challenge the existing industry standards. [15] membrane prepared with NMP as the solvent showed a performance similar to that of Nafion 117.…”

“…However, Nafion ® still suffers from various drawbacks like degradation due to dry conditions during start stop cycles, and due to fuel crossover, especially in DMFC systems, leading to loss of cell performance as revealed by various long-term studies. Moreover, recent studies [14][15][16] investigating performance of membranes in intermediate temperature (100 to 120 °C) PEFC (or IT-PEFC), suggest the need to develop new membranes which could solve specific problems faced by PEMs in this kind of environment. Operating at higher temperatures not only enables faster reaction kinetics for hydrogen oxidation and oxygen reduction but also a) enables better catalytic activity due to reduced poisoning from CO and other gases and b) facilitates easier water management and elimination.…”