Mike L. Perry

Systems Strategies to Mitigate Carbon Corrosion in Fuel Cells

The oxidation of carbon used as catalyst support in state-of- the art fuel cells is a serious decay mechanism that must be mitigated in order to achieve acceptable performance stability. Although the corrosion of carbon is certainly not a new concern for fuel-cell developers, a number of fairly recent developments have brought this issue to the forefront. These concerns include the unique operating conditions of transportation applications, the discovery of a mechanism that results in higher-than-expected potentials (i.e., the reverse- current mechanism), and the use of certain materials (e.g., high surface area carbon supported catalysts with high Pt mass fractions). Since improvements in catalyst-support stability typically comes at the expense of performance (and/or cost), and these improvements alone will be insufficient for many applications, system-mitigation strategies are required. A number of system strategies that have been developed and demonstrated at UTC Power will be described.

A back-up power solution with no batteries

Fuel cells offer potentially attractive alternative backup power solutions for telecom applications. In order to intelligently assess different fuel-cell alternatives, one needs to understand the major types of fuel cells and their current state of development. Recent major developments in polymer-electrolyte membrane fuel cells (PEMFC) have resulted in multiple companies introducing PEMFC-based products. However, there are important differences in PEMFC technologies that clearly differentiates these products and their key attributes (e.g., lifetime and cost). UTC Fuel Cells has recently employed their proprietary PEMFC technology in a prototype 5-kW back-up power unit, which provides a seamless power transition without the use of batteries.

GDL Degradation in PEFC

Performance decay in a polymer-electrolyte fuel cell can result from changes in the gas-diffusion layer, particularly in the micro-porous layer. Examples and characteristics of this decay will be presented here. Performance decay is greater with exposure to potential cycles than with exposure to constant high potential holds. Presumably, electrochemical oxidation of the carbon within the micro-porous layer is primarily responsible for this performance decay; it is known that transient potentials can accelerate the electrochemical oxidation of carbon. The implication that these results have on the development of advanced gas-diffusion layer materials is also considered. Finally, a recommendation for a standardized accelerated stress test protocol focused on the gas-diffusion layer is proposed.

Operating Requirements for Durable Polymer-Electrolyte Fuel Cell Stacks

Successful developers of fuel cells have learned that the keys to achieving excellent durability are controlling potential and temperature, as well as proper management of the electrolyte. While a polymer-electrolyte fuel cell (PEFC) has inherent advantages relative to other types of fuel cells, including low operating temperatures and an immobilized electrolyte, PEFC stacks also have unique durability challenges owing to the intended applications. These challenges include cyclic operation that can degrade materials owing to significant changes in potential, temperature, and relative humidity. The need for hydration of the membrane as well as the presence of water as both liquid and vapor within the cells also present complications. Therefore, the development of durable PEFC stacks requires careful attention to the operating conditions and effective water management.

Fuel-Cell Based Back-Up Power for Telecommunication Applications: Developing a Reliable and Cost-Effective Solution

Fuel cells combine the best features of engine-driven generators and batteries, since they can operate for as long as fuel is available and they produce electricity directly from this fuel via electrochemistry, which is inherently more efficient than combustion and minimizes the adverse characteristics associated with combustion engines (e.g., excessive noise, emissions, and maintenance). Therefore, fuel cells offer a potentially attractive back-up power solution for telecom sites where extended run times are desired, but generators are considered unacceptable. As customer expectations for a variety of uninterrupted communication technologies increase and/or as the reliability of the electric grid decreases, these fuel-cell attributes will become increasing attractive. However, in order to be widely accepted for telecom-power applications, fuel cells must prove that they are more reliable and cost competitive than the incumbent solutions. This will require fuel-cell systems that are very simple, yet also very small, since added complexity and size results in inherently unreliable and expensive products. UTC Power has developed a back-up power product for telecom applications, which utilizes proprietary technology that requires minimal number of balance-of-plant components and has a polymer-electrolyte membrane fuel-cell (PEMFC) stack with unmatched power density. The fundamental PEMFC technology that enables this unique system will be explained, as well as a brief description of a complete 5-kW back-up power product

Experimental Diagnostics and Durability Testing Protocols

The purpose of this chapter is to discuss diagnostic tests for assessing the causes of performance decay in polymer-electrolyte fuel cells. Determining the causes of degradation can be challenging and often requires application of a myriad of investigative techniques. For example, a destructive analysis of the cell that includes sophisticated analytical tools like scanning-electron microscopy is usually required to identify the physical changes responsible for loss of performance. Effective diagnostic tests can provide insight into which components, portions of the active area, and cells within a stack deserve detailed examination. Therefore, effective execution and analysis of in-cell diagnostics are critical to determining the causes of performance changes in an efficient manner. The first step is to determine what types of overpotential are responsible for the changes in performance, namely: kinetic, ohmic, or mass transport. Because leaks within a cell can also contribute to changes in performance, or failure, this type of degradation is included as well. A polarization-change curve is presented here as a simple tool to assist in apportioning the decay to the different types of overpotential. Additional diagnostics can be selected to establish the sources of these changes, once the major types of overpotential have been identified. In-cell diagnostics for each of the major types of polarization are recommended and briefly described. The intent of this chapter is not to provide complete descriptions of test procedures, since most have been adequately described in references provided throughout the chapter, but instead to outline an effective and efficient protocol to help determine the root cause of performance degradation. An overview of accelerated testing, which is often part of any concerted effort to study and improve durability, is also included. In order to develop accelerated test protocols one must understand the parameters that affect decay rates. Advantages and disadvantages of different types of accelerated testing are discussed.