C L E A N E N E R G Y T R A N S I T I O N A N D S U S T A I N A B L E T E C H N O L O G I E S

S C I E N T I F I C H I G H L I G H T S

1 0 2 H I G H L I G H T S 2 0 2 3 I

X-ray tomography unveils the contribution of different mechanisms to the instability of iron-based fuel cell catalysts

The stability of iron-based fuel cell catalysts can be affected by several mechanisms, whose impact on catalyst performance remains poorly understood. Different testing protocols were combined in order to decouple the contribution of each mechanism to the overall instability. X-ray fluorescence computed tomography was used to quantify the iron lost by a catalyst during one of the protocols.

The strong reliance on the use of fossil fuels for transportation applications renders this sector one of the major contributors to greenhouse gas emissions. As a result, the extended adaptation of environmentally sustainable transportation technologies is crucial to achieve the stringent societal decarbonisation targets required to mitigate climate change. Among these emerging technologies, proton exchange membrane fuel cells (PEMFCs) fuelled with green hydrogen (H2) (i.e., produced using renewable energy sources) are particularly appealing due to their fast refuelling times, extended driving ranges and suitability for heavy-duty applications, but their extensive commercialisation is hindered by their excessive cost.

The latter would be significantly reduced if the Pt-based materials currently used to catalyse the reduction of O2 (i.e., the oxygen reduction reaction, or ORR) in PEMFC cathodes could be substituted with inexpensive, iron (Fe)- based catalysts. However, these materials suffer from a fast performance decay that has been ascribed to the following mechanisms: (i) the dissolution (or demetallation) of the Fe in the active sites that catalyse the ORR; (ii) the corrosion of the carbon matrix hosting those sites; and (iii) the damage to the active sites and carbonaceous matrix caused by radicals derived from hydrogen peroxide produced as an ORR by-product. Most importantly, little is known regarding the relative contributions of these mechanisms to the overall instability endured by the catalyst during fuel cell operation.

To shed light on this issue, this work studied four PEMFC- testing protocols entailing different potential hold values and durations, along with the use of air vs. an inert gas (N2) as the PEMFC cathode gas feed. This made it possible

Fig. 80: From left to right: schematic representation of the three processes causing the instability of Fe-based catalyst layers (H2O2-related effects, electrochemical carbon oxidation and demetallation); their decoupling using a combination of gaseous cathode feeds (air vs. N2), potentials (0.5 vs. 0.8 V) and hold durations; and the relative contributions to the

overall kinetic current losses at 0.8 V (ikin,0.8V) estimated through these protocols.