Understanding the performance increase of catalysts supported on N-functionalized carbon in PEMFC catalyst layers

Applying nitrogen-modified carbon support in PEMFCs has been attracting arising interest due to the resulting performance enhancement. In the present study, we attempt to uncover the origin and gain a deeper understanding of the different N-modification processes, whose influences are responsible for the performance improvement. By utilizing chemically modified Ketjenblack supports comprising altered fraction of N-functionalities, we investigate the underlying mechanism of the drastically reduced voltage losses under fuel cell operation conditions. In all, we demonstrate the key role of support modification induced by ammonia in strengthened support/ionomer interactions and alter physico-chemical properties of the carbon support contributing towards enhanced MEA performance. With the use of X-ray photoelectron spectroscopy (XPS), we show unambiguous evidences that not all N modified surfaces yield the desired performance increase. Rather, the latter depends on a complex interplay between different electrochemical parameter and catalyst properties. We want to emphasize the ionomer/support interaction as one important factor for enhanced ionomer distribution and present a prove of a direct interaction between the ionomers´ sidechains and N-functional groups of the support

Property-reactivity relations of N-doped PEM fuel cell cathode catalyst supports

This study clarifies the effect of the nature of solid N-precursor molecules on the N-modification, more specifically unravels how the N-precursors affect the physiochemical and catalytic properties of the resulting carbon supports and final catalysts. Therefore, cyanamide and melamine were used as N-source. Utilizing such modified high surface area carbons, in situ measurements as humidity dependent performance, electrochemical surface area, proton resistivity and limiting current measurements were conducted to access the role and degree of ionomer coverage and transport resistances. Additional X-ray photoelectron spectroscopy (XPS) proves molecular interaction between acidic side chains and basic N-groups. Overall, we show the importance of the N-precursor and synthetic route determining which physiochemical parameter will be influenced in the resulting catalytic layer. Based on this, the pure presence of some N-moieties does not guarantee an improved ionomer distribution, but the modification process enables a tailoring effect of the carbon specie itself affecting transport phenomena

Ionomer distribution control in porous carbon-supported catalyst layers for high-power and low Pt-loaded proton exchange membrane fuel cells

The reduction of Pt content in the cathode for proton exchange membrane fuel cells is highly desirable to lower their costs. However, lowering the Pt loading of the cathodic electrode leads to high voltage losses. These voltage losses are known to originate from the mass transport resistance of O-2 through the platinum-ionomer interface, the location of the Pt particle with respect to the carbon support and the supports' structures. In this study, we present a new Pt catalyst/support design that substantially reduces local oxygen-related mass transport resistance. The use of chemically modified carbon supports with tailored porosity enabled controlled deposition of Pt nanoparticles on the outer and inner surface of the support particles. This resulted in an unprecedented uniform coverage of the ionomer over the high surface-area carbon supports, especially under dry operating conditions. Consequently, the present catalyst design exhibits previously unachieved fuel cell power densities in addition to high stability under voltage cycling. Thanks to the Coulombic interaction between the ionomer and N groups on the carbon support, homogeneous ionomer distribution and reproducibility during ink manufacturing process is ensured

Simulative Investigation on Local Hydrogen Starvation in PEMFCs: Influence of Water Transport and Humidity Conditions

Durability targets of automotive polymer electrolyte membrane fuel cells (PEMFCs) could be crucially threatened by local hydrogen starvation, typically induced by local blockage of gas channels. To gain a deep insight on the evolving of such starvation events and related carbon corrosion losses, we have developed a numerical model with transient nature that includes detailed transport phenomena and electrochemistry. Special focus is on water transport and sensitivity of relative humidity (RH) on both anode and cathode sides, whose influences were commonly neglected in starvation-related modeling studies. Utilizing the model, we show the dominating effect of in-plane hydrogen convection within the anode gas diffusion layer, which is again determined by the accumulation of other gas species including water vapor. We demonstrate how this is again linked with the water management throughout the fuel cell. Furthermore, water transport is shown to affect local current density and membrane oxygen permeability, both being critical influential factors regarding the severity of a local starvation event. The developed model is validated by conducting transient current density distribution measurements. As RH levels are crucial operational conditions within automotive PEMFCs, this work serves as useful input towards development of future operation strategies for better PEMFC durability.ISSN:0013-4651ISSN:1945-711