EFFECT OF MECHANICAL VIBRATION ON PLATINUM PARTICLE AGGLOMERATION AND GROWTH IN PROTON EXCHANGE MEMBRANE FUEL CELL CATALYST LAYER

Abstract

The objective of the current research is to study the effect of mechanical vibration on catalyst layer degradation via Platinum (Pt) particle agglomeration and growth in the membrane electrode assembly (MEA) of a proton exchange membrane fuel cell (PEM Fuel Cell). This study is of great importance, since many PEM fuel cells operate under a vibrating environment, such as the case of vehicular applications, and this may influence the catalyst layer degradation and fuel cell performance. Through extensive literature review, there are only few researches that have been studied the effect of mechanical vibration on PEM fuel cells. These studies focused only on PEM fuel cell performance under vibration for less than 50 hours and none of them considered the degradation of the fuel cell components, such as MEA and its catalyst layer. To study the effect of the mechanical vibration on the catalyst layer an accelerated test with potential cycling was specially designed to simulate a typical vehicle driving condition. The length of the accelerated test was designed to be 300 hour with potential cycling comprised of idle running, constant load, triangle (variable) load and overload running at various mechanical vibration conditions. These mechanical vibration conditions were as follows: 1g 20 Hz, 1g 40 Hz, 4g 20 Hz and 4g 40 Hz. No vibration tests were also conducted to study the influence of operating time and were used as a baseline for comparison study. The series of accelerated tests were followed by microscopy and spectroscopy analyses using environmental scanning electron microscopy (ESEM), transmission electron microscopy (TEM) and X-Ray diffraction (XRD). An ESEM was used to qualitatively analyze pristine and degraded catalyst. TEM and XRD were used to quantitatively analyze catalyst layer degradation via Pt agglomeration and growth in pristine and degraded states. For each test condition, PEM fuel cell performance by means of Voltage - Current (VI) curves was monitored and recorded. It was observed that the mean diameter of Pt particles tested under mechanical vibration is 10% smaller than the ones that were tested under no vibration conditions. The Pt particles in the order of 2 to 2.5 nm in the pristine state have grown to 6.14 nm (after 300 hour accelerated test at no vibration condition), to 5.64 nm (after 300 hours accelerated test under 4g 20 Hz vibration condition) and to 5.55 nm (after 300 hours accelerated test under 1g 20 Hz vibration condition). The mean Pt particle diameters, after 300 hour accelerated test under 1g 40 Hz and 4g 40 Hz vibration conditions, were 5.89 nm. With an increase of the mean Pt particle diameter, the active surface area of the catalyst layer of the MEA decreases and as a result, performance of MEA and PEM fuel also decreases. It was observed that performance of the MEA tested under no vibration condition is about 10% lower than the one tested under 1g 20 Hz. The VI curve showed that the lowest performance of the MEA after 300 hour accelerated test corresponded to no vibration conditions and equaled to 7.85 Watts at 0.5 V (Pt particle size ~ 6.14 nm) and highest performance, corresponded to the MEA tested under 1g 20 Hz, and equaled to 8.66 Watts at 0.5 V (Pt particle size ~ 5.55 nm).