Methodology for the Design of Accelerated Stress Tests for Non-Precious Metal Catalysts in Alkaline Fuel Cell Cathodes

Tuesday, 3 October 2017: 14:00
National Harbor 14 (Gaylord National Resort and Convention Center)
L. Elbaz (Bar-Ilan University)
As the interest in alkaline fuel cells (AFCs) rises, the development of new materials to increase their activity and lower their cost, the question of their durability has become critical. The durability testing protocols suggested by agencies such as the US DOE is limited to the study of platinum on carbon in acid. These protocols have become the generic testing protocols to all fuel cell catalysts but the they possess one significant flaw: they do not necessarily take into account the vulnerabilities of other types of catalysts. When studying the durability of fuel cells, one can conduct a constant lifetime test; however, this method is time-consuming and expensive. Instead of long-term measurements, the fuel cells industry along with government agencies such as the US-DOE and researchers develop protocols and perform accelerated stress tests (ASTs). ASTs are methods for determining the durability of the whole cell or a particular component in the fuel cell system in short periods of time by exposing the cell to extreme, yet realistic, operating conditions that may cause some of its components to degrade rapidly (e.g. support, catalyst and membrane). Using such methods, the weaknesses of the fuel cell can be determined and then improved in order to increase its lifetime. The performance, degradation rate and component damage level under specific working conditions are examined during and after the ASTs. The aim of the AST is to predict the fuel cell's lifetime and explain the probable failure and degradation mechanisms.

So far, most of the AST protocols were designed for commercial polymer electrolyte membrane fuel cells (PEMFCs) which consist on Pt-based catalysts, carbon supports and Nafion membranes. In the case of other catalysts, supports and membranes, specifically in AFCs, these ASTs become irrelevant due to the different chemical and electrochemical properties of the different components. For example, in some of the US-DOE ASTs, the electroactive surface area (ECSA), an indication of the catalysts activity, of Pt-based catalysts is measured from the hydrogen adsorption/desorption peaks: this cannot be done with non-precious group metal catalysts since most of them do not adsorb hydrogen. Hence, ASTs need to be considered as case sensitive, or at least divided into categories of materials, since not all fuel cells are composed of the same materials and/or operate under the same conditions. Therefore, other methods and measurable parameters need to be found in order to assess the catalyst degradation in such cases. Here, we developed the methodology which will allow groups who are studying non-conventional materials for fuel cells to study the stability and durability of their catalysts and supports, develop case-sensitive ASTs for their fuel cells in general and for AFCs in particular.

In this work a wide array of methods are used for testing non-precious group metal catalysts and support degradation alkaline fuel cell cathodes. In this case study, we used a cathode composed of a pyrolyzed non-precious metal group catalyst (NPMGC) on activated carbon. The vulnerabilities of the cathode components were studied in order to develop the methodology and design an accelerated stress test for NPMGC-based cathode in alkaline environment. Cyclic voltammetry (CV), chronoamperometry (CA) and impedance spectroscopy (EIS) were used to characterize the electrochemical behavior of the cathode and to follow the changes that occur as a result of exposing the cathodes to extreme operating conditions. Rotating ring disk electrode (RRDE) was used to study the cathodes kinetics; Raman spectroscopy and X-ray fluorescence (XRF) were used to study the structural changes in the electrode surface as well as depletion of the catalysts’ active sites from the electrode. The changes in the composition of the electrode and catalyst were detected using X-ray diffraction (XRD). For the first time, we show that NPMC degrade rapidly at low operating potentials whereas the support degrades at high operating potentials. We developed a tailor-made AST for the studied electrodes based on real operating statistics and operating conditions.