Conductivity Limits of Extrinsically Doped SnO2 Supports

Thursday, October 15, 2015: 10:40
211-A (Phoenix Convention Center)
M. Worsdale (University of Southampton), A. Rabis (Paul Scherrer Institut), E. Fabbri (Paul Scherrer Institut), T. J. Schmidt (ETH Zürich, Paul Scherrer Institut), and D. Kramer (University of Southampton)
Supported catalyst technologies 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 [1]. 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 [2].

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 [3]; amongst them TiO2, Bi2O3, and SnO2also 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 investigate the possibility of extrinsic n-type doping to tailor the conductivity of SnO2 using Hybrid Density-Functional-Theory (DFT) as implemented in Crystal09. 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 [4] 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.

The use of B3LYP as function provided a significantly more accurate description of the band gap of SnO2than semi-local approximations to DFT produce. This enabled accurate description of electron localisation of the donor state without erroneous resonances with the conduction band. Electron localisation is accompanied with a strong Jahn-Teller distortion. Therefore, Ta centres interact via long-ranged elastic effects providing relatively large stabilisation of ordered superlattices. A cluster expansion based search strategy was implemented to bias the search towards these low energy orderings. This can be seen as a collaborative distortion of the crystal structure that leads to a freezing out of the donor state with increasing Ta concentration. Further, the Ta donor state is predicted to be a deep donor state that can only contribute to conductivity at room temperature if secondary defects provide a significant lowering of the conduction band.

Using thin-film deposition techniques [5], we have synthesised doped SnOthin films. Consistent with our First Principles calculations, we find a declining conductivity in the range 1.0% to 7.1% Ta-doping. Interestingly, our experiments indicate that Nb as an alternative donor impurity shows an increase in conductivity up to at least 2.1% Nb-doping. Given the similarity between the two ions (same ionic radii, same d1 state in four-valent configuration, both are known for Jahn-Teller activity), we currently try to identify the underlying reason for the markedly different behaviour using the methodology described above.

This work was supported by CCEM Switzerland and Umicore AG & Co KG within the project DuraCat. The authors acknowledge the use of the ARCHER UK National Supercomputing Service.

[1] Tang et al., J. Power Sources 158 (2006) 1306-1312
[2] A. Rabis, P. Rodriguez, T.J. Schmidt, ACS Catalysis 2 (2012) 864-890
[3] K. Sasaki et al., ECS Transactions 33 (1) (2010) 473-482
[4] G. Gehring and K. Gehring, Reports on Progress in Physics 38 (1) (1975) 1-89
[5] A. Rabis, D. Kramer, E. Fabbri, M. Worsdale, R. Kötz, and T.J. Schmidt, The Journal of Physical Chemistry C 118 (2014) 11292-11302