Enhanced Anodic Catalyst Support Using TiO₂‑Carbon Nanofibers for Direct Methanol Fuel Cells

Background

Direct methanol fuel cells (DMFCs) are attractive for portable and stationary power due to their high energy density, low weight, and ambient‑temperature operation. However, commercialization is limited by sluggish methanol oxidation kinetics, catalyst poisoning, methanol crossover, and durability challenges. Platinum remains the most effective catalyst for the methanol oxidation reaction (MOR), but its high cost and CO‑tolerance issues drive the search for more efficient supports and bimetallic systems.

PtRu alloys mitigate CO poisoning by promoting CO oxidation, yet the performance still depends heavily on the support material. Conventional carbon black provides high conductivity but suffers from limited surface area and weak metal–support interactions. Emerging nanostructured supports—such as carbon nanofibers (CNFs), carbon nanotubes (CNTs), and metal oxides—offer improved surface area and potential electronic synergy. Titanium dioxide (TiO₂) is chemically stable, non‑toxic, and can enhance catalytic activity via electron transfer pathways, but its low conductivity is a drawback that can be overcome by combining it with conductive carbon matrices.

In this work, we combine TiO₂ with CNFs to create a composite support that leverages the high surface area of CNFs and the catalytic benefits of TiO₂, aiming to reduce PtRu loading while boosting MOR activity.

Methods

Materials

Titanium isopropoxide, poly(vinyl acetate), dimethylformamide, and other reagents were used as received. PtRu (1:1 atomic ratio) was deposited on supports by chemical reduction with NaBH₄. Supports included laboratory‑synthesized TiO₂‑CNF and commercial C, CNF, and TiO₂ nanopowders.

Preparation of TiO₂‑CNF

A 11.5 wt % PVAc/DMF solution was mixed with 50 wt % TiPP, ethanol, and acetic acid. The homogeneous feed was electrospun (16 kV, 18 cm distance) at 0.1 mL h⁻¹, dried, then calcined at 600 °C for 2 h in N₂ to yield TiO₂‑laden CNFs.

PtRu Deposition

Supports were sonicated in a 1:1 water/IPA mixture, then Pt and Ru precursors were added and stirred. After pH adjustment to 8 and heating to 80 °C, NaBH₄ was introduced dropwise. The resulting PtRu/TiO₂‑CNF powder was washed, dried, and ground to a fine powder.

Characterization

XRD (D8 Advance, 40 kV/20 mA) revealed anatase TiO₂ and cubic PtRu phases. BET analysis (Micromeritics ASAP 2020) showed a surface area of 50.6 m² g⁻¹ for TiO₂‑CNF. SEM/TEM confirmed uniform PtRu dispersion (~7 nm particles) on the nanofiber skeleton. ECSA was calculated from CV in 0.5 M H₂SO₄, yielding 10.4 m² g⁻¹ PtRu for TiO₂‑CNF.

Electrochemical Testing

CV (20 mV s⁻¹, 0–1.1 V vs. Ag/AgCl) and chronoamperometry (0.5 V, 3600 s) were performed in 2 M methanol/0.5 M H₂SO₄. A Nafion 117 membrane and carbon cloth backing formed the MEA, with 2 mg cm⁻² catalyst loading. Single‑cell performance was measured at room temperature with a 3 M methanol feed.

Results and Discussion

Structural Characterization

TiO₂‑CNF exhibited characteristic anatase peaks at 25°, 38°, and 48°, confirming the preservation of the TiO₂ crystal structure after calcination. The PtRu/TiO₂‑CNF composite displayed additional peaks at 39.7°, 46.2°, 67.5°, and 81.3° corresponding to Pt, while Ru peaks appeared at 40.7°, 47°, 69°, and 83.7°. Crystallite sizes (Debye–Scherrer) were 4.6–9.8 nm for PtRu, 19–38 nm for TiO₂, and 10–19 nm for C.

BET data revealed a modest surface area for TiO₂‑CNF (50.6 m² g⁻¹) compared to PtRu/C (≈140 m² g⁻¹), but the mesoporous distribution (average pore diameter 22–33 nm) supports efficient reactant diffusion.

Electrochemical Performance

CV curves showed a peak current density of 345.64 mA mg⁻¹ PtRu for TiO₂‑CNF, 5.54 × higher than PtRu/C (62.5 mA mg⁻¹). The forward/reverse peak ratio (>4.7) indicates excellent CO tolerance. Tafel analysis yielded an exchange current density of 0.501 mA cm⁻², the highest among tested supports.

Chronoamperometry demonstrated only a 5 % drop after 3600 s, confirming good stability. In single‑cell tests, the TiO₂‑CNF anode achieved a peak power density of 3.8 mW cm⁻² versus 2.2 mW cm⁻² for commercial PtRu/C, representing a 1.66 × improvement.

Conclusions

The TiO₂‑CNF support, fabricated by sol‑gel/electrospinning and high‑temperature carbonization, delivers superior anodic catalyst performance in DMFCs. Its high surface area, uniform PtRu dispersion, and synergistic TiO₂/CNFs interaction yield current densities 5.5 × higher and power densities 1.7 × higher than commercial PtRu/C. These findings position TiO₂‑CNF as a promising, cost‑effective anode support for future DMFC applications.