Birss Group
New Materials for Proton Exchange Membrane Fuel Cells (PEMFCs)
Our group is working on many of the challenges related to PEMFC development. In addition to the modification of the surface of carbon-based materials to control their wettability and corrosion resistance (see Carbon section), we are working in the following project areas:
Replacement of carbon with alternative materials,
e.g., metal oxides, metal nitrides, and metal carbides
The catalyst layer within PEMFCs typically consists of Pt/Pt alloy nanoparticles (NPs) supported on carbon. However, the corrosion of the carbon support under normal operating conditions is a key problem. Our approach to address this problem is to investigate the replacement of carbon with alternative materials, such as metal oxides, metal nitrides, metal oxynitrides, etc. Our work currently focusses on replacing the carbon support completely or partially with Ta oxynitride (TaON) in the form of nanotubes. Although the conductivity of TaON is lower than carbon, it is very stable, and thus shows promise as a carbon replacement material.
SEM (a and b) and TEM (c and d) images showing the Tantalum Oxynitride (TaON) nanotubes that could be sued as an alternative support material for PEMFCs.
Development of non-noble metal or metal-free oxygen reduction catalysts to replace costly commercial Pt catalysts;
Pt is known as the most active catalyst for the oxygen reduction reaction (ORR), but it has some challenges, including scarcity and thus associated high cost, activity loss over time, CO poisoning, etc. Compared to the anode, a higher amount of catalyst is required in the cathode of PEMFCs to make a good cell performance. In addition to enhancing the cathode catalysts efficiency, one of our goals is to reduce their cost by developing non-noble metal/metal-free catalysts. In our group we developed a carbon black supported Co-phenylenediamine/Fe-phenylenediamine with a good ORR activity, leaving Co/Fe and N on the carbon surface after appropriate heat treatment. We are also works on the development metal-free catalysts by doping our nanoporous carbon materials with heteroatoms (e.g., N, Fe or B), which are believed to create active sites (e.g., pyridine-N, pyrrole-N, and graphite N) for ORR. This combination of nanostructures and doped active sites is expected to provide us a path to develop a novel low cost catalyst for ORR.
SEM images of Fe-polyphenylenediamine (Fe-PoPDA) as an example of non-noble ORR catalyst
that could be prepared using (a) electrochemical and (b) chemical methods. The proposed ORR active sites of Fe-PoPDA are shown in (c).
Synthesis and testing of core/shell nanoparticles to enhance activity and lower cost
The high utilization of Pt at PEMFC anodes and cathodes is a necessity in order to reduce production costs. Most Pt-based alloys have an unstable composition due to the dissolution (de-alloying) of the second metal during PEMFC operation. Although Pt-Ru is considered to be the best alloy catalyst for PEMFCs and direct alcohol fuel cells (DAFCs), this material is still limited in terms of its activity and durability. Thus, Rucore@Ptshell NPs, with a controlled Ptshell coverage and two different Rucore sizes (2 and 3 nm), were synthesized in our group to determine the precise effect of Ru on the Pt activity towards CO stripping and methanol oxidation. As Ir should be a more stable candidate than Ru, Pt-Ir alloy and Ir@Pt NPs are also being synthesized, with tunable and controllable surface compositions. Understanding the role of both Ir and Ru should allow for the better design of PtIrRu catalysts with high activity and durability. Other less costly candidate materials are also under investigation in our group using the core@shell NP approach.
Understanding the electrochemical activity of Ru core @Pt shell NPs (Pt shell coverage from sub monolayer to 2 monolayers) towards CO and methanol oxidation reactions, showing the effects of Ru on the activity at different Pt shell coverages.
Nanoparticle array development: Effect of size, composition, and support effects on electrocatalysis
Nanoparticle (NP) arrays have been fabricated by others using “top-down” approaches which are often costly and time consuming, or by “bottom-up” approaches which suffer from poor long range (> µm) order. Our work focuses on developing ordered NP arrays on “dimpled” Ta (DT) templates, with relatively large dimensions (e.g. cm2), by thermal (or laser, in collaboration with Dr. Y. Shi’s group) dewetting of thin metallic films on the air-formed Ta oxide on the DT surface. This approach has been used to form Au, Pt, and Ag NPs of controllable size, with a linear relationship observed between the initial metallic film thickness and the resulting NP diameter. We are also constructing arrays of alloy and core-shell NPs, with the goals being to understand particle size and support effects through the study of an ensemble of identical NPs.
Developing ordered NP arrays on “dimpled” Ta (DT) templates by using the bottom-up approach, in which dewetting a metallic thin film on the air-formed Ta oxide surface to form metallic NPs of controllable size. The SEM images show the DT, DT/Au thin films, and DT/Au NPs in sequence.