Automotive drive cycles

Uchimura M, Sugawara S, Suzuki Y, Zhang J, Kocha SS (2008) Electrocatalyst durability under simulated automotive drive cycles. ECS Trans 16(2) 225-234... 

The United States Department of Energy (DOE) and the Freedom CAR Fuel Cell Technical Team have recently released a set of durability-test protocols that includes tests of electrocatalysts (see Table 23.1) and of electrocatalyst supports (see Table 23.2) as well as other components [44]. The documents describe testing protocols to assess the performance and durability of fuel cell components for automotive and stationary applications. These protocols are intended to establish a common approach for determining and projecting the durability of PEMFC components under simulated automotive drive cycle conditions. 

Calculations that are repeatedly made can be made more accurately, and at lower cost, by using a computer. If, for example, automotive emissions are continually tested over a standardized driving cycle, a computer program to analyze the data is a necessity. Otherwise, days would be spent calculating the data obtained in hours. 

The stability of electrocatalysts for PEMFCs is increasingly a key topic as commercial applications become nearer. The DoE has set challenging near-term durability targets for fuel cell technology (automotive 5,000 h by 2010 stationary 40,000 h by 2011) and has detailed the contribution of the (cathode) catalyst to these. In particular, for automotive systems as well as steady-state stability, activity after simulated drive cycles and start-stop transients has been considered. In practice, both these treatments have been found to lead to severe degradation of the standard state-of-the-art Pt/C catalyst, as detailed next. 

Maxoulis, C., Tsinoglou, D., Koltsakis, G. (2004). Modeling of automotive fuel cell operation in driving cycles. Energy Conversion Management 45,559-573. 

Durability is a fundamental and necessary feature for PEM fuel cells to see a wide diffusion as a practical power source in any application field. The requirements for fuel cell lifetime vary with the specific application, in particular a duration of at least 5000 h is mandatory for use on cars, while even longer periods are needed for bus and stationary employments (for most applications an acceptable degradation rate is considered to be comprised in the range 2-10 pV/h [51]). However, the wide variability of operative conditions usually encountered in automotive applications, such as dynamic driving cycles, startup/shutdown phases, and freeze/thaw, makes also the target for car very difficult to be met with the current technologies. 

In the direct dissolution pathway, the equilibrium potential of Pd dissolution is 0.2 V lower than that of PL In the chemical dissolution pathway, the Pd " equilibrium concentration is 5 orders of magnitude higher than Pt. Thus, Pd should have a much higher dissolution rate at the same operation potential and thus much less stable than Pt in the fuel cell environment. Consequently, Pd is not expected to meet the durability requirement of the PEMFC, especially for those in the automotive application due to intensive start-stop driving cycles. The very low stability of Pd/C during potential cycling was confirmed recently [93]. The stability of Pd in alkaline medium may not be a problem however, no such study is available in the literature. 

The targets currently accepted for automotive application are 5,000 h operation under drive cycle conditions or 40,000 h for stationary conditions with less than... 

Fig. 9 Typical automotive fuel ceU vehicle drive cycle, showing average cell voltage versus time...

In order to define the extent of emissions from automotive brakes and clutches, a study was carried out in which specially designed wear debris collectors were built for the dmm brake, the disk brake, and the clutch of a popular U.S. vehicle (1). The vehicle was driven through various test cycles to determine the extent and type of brake emissions generated under all driving conditions. Typical original equipment and aftermarket friction materials were evaluated. Brake relines were made to simulate consumer practices. The wear debris was analyzed by a combination of optical and electron microscopy to ascertain the asbestos content and its particle size distribution. It was found that more than 99.7% of the asbestos was converted to a nonfibrous form and... 

A good way to get a feel for the dynamic nature of automotive catalyst operation is to look at a plot of speed versus time during the U.S. Federal Test Procedure (FTP) as shown in Fig. 1. Clearly, steady-state conditions do not exist in this test, which simulates the acceleration and deceleration cycles of urban driving and which is similar to the European test procedure. 

---