Although being an extremely promising technology, proton exchange membrane fuel cells (PEMFCs) commercialization is still hindered by several challenges, such as the cathode (where the oxygen reduction reaction – ORR – occurs) catalyst activity and stability. Pt nanoparticles homogeneously distributed over carbon-based supports are state of the art catalyst materials at the cathode. However, carbon supports are particularly susceptible to electrochemical corrosion at high potentials. Hence, support materials with an higher stability are needed [1]. Pt supported on TiO₂ shows an improved corrosion resistance for ORR compared to carbon supported catalysts [2]. However, the low intrinsic conductivity of TiO₂, as well as the high cost of Pt, require further materials optimization before these technologies can hit the market. Here, we present our results on the modification of the electronic structure of TiO₂ by doping with pentavalent metals such as Ta, Nb, W, and Mo to improve the conductivity of the support, as well as stabilize Pt on the support surface.

DFT calculations were performed to understand the band structure of doped M:TiO₂ (4 at%, M= Ta, Nb, W, Mo) which shows that these materials will be n-type semiconductors, characteristic of defective or nonstoichiometric oxides [3]. The density of d-states for the Pt supported on M:TiO₂ was calculated and compared to that of unsupported Pt as shown in Figure 1, evidencing a shift in the d-band center towards lower values (i.e. for M = Ta (-2.34 eV) < Nb (-2.32 eV) < W (-2.30 eV) < Mo (-2.28 eV)). Shift in the d-band center to lower values reduces the strength by which Pt supported on M:TiO₂ binds oxygen, hence increasing its reactivity toward the ORR. Based on these results, we predict Ta:TiO₂ will not only possess improved conductivity, but also facilitate ORR reduction similarly to Pt alloyed with Co and Ni [4]. Then, we applied those principles to synthesize, and characterize for the ORR, M:TiO₂ supported Pt electrocatalysts.

References:

[1] X. Yu and S. Ye, J. Power Sources., 172(1), 145–154, (2007).

[2] S. Bagheri, N. Muhd Julkapli, and S. Bee Abd Hamid, Sci. World J., 2014, 1–21, (2014).

[3] C. He, S. Sankarasubramanian, I. Matanovic, P. Atanassov, and V. Ramani, ChemSusChem., (2019).

[4] V. Stamenkovic et al., Angew. Chem. Int. Ed., 45(18), 2897–2901, (2006).