Development of supported bifunctional electrocatalysts for unitized regenerative fuel cells

Mixed metal catalysts containing Pt, Ir, Ru, Os, and Rh were synthesized on three different conductive oxide supports, Ebonex, which is a mixture of Ti₄O₇ and other phases, phase-pure microcrystalline Ti₄O₇, and Ti_0.9Nb_0.1O₂, a doped rutile compound. Ebonex-supported catalysts were prepared as arrays and screened combinatorially for activity and stability as bifunctional oxygen reduction/water oxidation catalysts. The highest activity and stability was found in the Pt-Ru-Ir ternary region at compositions near Pt₄Ru₄Ir₁. X-ray near edge absorption spectra indicated a significant electronic interaction between the catalyst and the support, and a substantial increase in catalyst utilization was observed, even though the support surface areas were relatively low. Both Ebonex and Ti₄O₇ have short-lived electrochemical stability under conditions of oxygen evolution at + 1.6 V vs. RHE in 0.5 M H₂SO₄. Current at these supported catalysts gradually decreases, and the decrease is attributed to loss of electronic conductivity. Ebonex and Ti₄O₇ are also thermally oxidized in air at temperatures above 400°C. In contrast, Ti_0.9Nb_0.1O₂, which has a nondefective oxygen lattice, is quite resistant to electrochemical and thermal oxidation. Conditioning of Ti_0.9Nb_0.1O₂-supported Pt₄Ru₄Ir₁ at positive potentials had little effect on the activity of the catalyst.