Black TiO₂ Photoanodes for Direct Methanol Photo Fuel Cells

Photocatalytic fuel cells (PFCs) are considered the next generation of energy converter devices, since they can harvest solar energy through relatively low-cost semiconductor material to convert the chemical energy of renewable fuels and oxidants directly into electricity. Here, we report black TiO₂ nanoparticle (NP) photoanodes for simple single-compartment PFCs and microfluidic photo fuel cells (μPFCs) fed by methanol. We show that Ti³⁺ and oxygen vacancy (O_V) defects at the TiO₂ NPs are easily controlled by annealing in a NaBH₄-containing atmosphere. This optimized noble-metal-free black TiO₂ photoanode shows superior PFC performance for methanol oxidation and O₂ reduction with a maximum power density (P_max) ∼2000% higher compared to the undoped TiO₂. At flow conditions, the black TiO₂ photoanode showed a P_max ∼90 times higher than the μFC equipped with regular TiO₂ in the dark. The PFC and μPFC operate spontaneously with little activation polarization, and black TiO₂ photoanodes are stable under light irradiation. The improved photoactivity of the black TiO₂ photoanode is a consequence of the self-doping with Ti³⁺/O_V defects, which significantly red-shifted the bandgap energy, induced intragap electronic states, and widened both the valence band and conduction band, enhancing the overall absorption of visible light and decreasing the interfacial charge transfer resistance.