Denis Kramer
University of Southampton, Highfield, Southampton, SO17 1BJ
Supported catalyst technologies have made Pt-based Polymer-Electrolyte Membrane Fuel Cell (PEMFC) electrodes cost-competitive. It has, however, become clear that current standard supports (i.e., high surface area carbons) might not provide the corrosion resistance needed to achieve life-time targets for stationary and/or automotive applications. This has sparked strong interest in alternatives in recent years. Research ranges from exploring different allotropes of carbon (e.g., graphitised carbons, carbon-nano-tubes, and graphene) to new chemistries such as oxides. Especially, oxides in their highest valence state appear attractive for their likely stability under strongly oxidising conditions. Indeed a number of metal oxides promise thermodynamic stability over the full operating range of a PEFC; amongst them TiO2, Bi2O3, and SnO2 also appear economically attractive. Being fully oxidised, all of them are intrinsic wide band-gap semi-conductors or even insulators. Hence, achieving and retaining near metallic conductivity over the life-time of a PEFC is a major challenge for oxide supports.
We have investigated the possibility of extrinsic n-type doping to tailor the conductivity of SnO2 using Hybrid Density-Functional-Theory (DFT). We find that Ta-doping provides limited potential to tailor conductivity of SnO2 for two reasons: (1) the stoichiometric ternary SnTa2O6 competes for thermodynamic stability along the rutile SnO2-TaO2 pseudo-binary potentially limiting the solubility of Ta in SnO2, and (2) collaborative Jahn-Teller distortions tend to localise the Ta donor state leading to a freezing out of the donor state with increasing dopant concentration. Our calculations indicate that the maximum extrinsic carrier concentration should be around 1% Ta-doping.
Although the implied trade-off between stability and conductivity remains a challenge, the interaction between oxide supports and electrocatalysts (known as Strong-Metal-Support-Interactions in heterogeneous catalysis, SMSI) provides a novel route to tailor catalytic activity. We have investigated the electronic structure of Pt nano-particles on SnO2 using DFT and find a sensitivity of oxygen adsorption energy towards the support facet that is in agreement with a two-fold increase in ORR activity measured for Pt thin film electrodes supported on different SnO2 facets. Further, we will sketch ideas for a generalised theory of catalytic hetero-structures based on our findings that promises a rational design strategy for Pt ORR activity as a function of support material.