Preparation of a self-assembled organosilane coating on carbon black as a catalyst support in polymer electrolyte membrane fuel cells

Background

Electrochemical corrosion of the carbon support, in which carbon reacts with water to form CO2, is a major cause of performance loss in PEM fuel cells and becomes severe when the electrode potential rises during repeated start-up and shut-down or fuel starvation. Carbon black is widely used as a catalyst support because of its high surface area, low cost and electrical conductivity, but its amorphous carbon and surface oxygen groups make it prone to corrosion. Graphitized carbons such as nanofibers and nanotubes resist corrosion through their hydrophobicity, yet they require costly high-temperature treatment that also reduces surface area and enlarges the supported catalyst particles.

Design and approach

CB powder (Vulcan XC-72) was immersed in 3 mM DTS in toluene for 30 min at room temperature, filtered and baked at 150 °C for 1 h, giving a DTS content of 2 wt% of CB. The chlorosilane groups of DTS hydrolyze with water adsorbed on CB and condense with surface hydroxyl groups, so that the anchored alkyl chains lower the surface energy and turn CB hydrophobic. Pt was then loaded at 40 wt% by a modified polyol process; according to the authors, Pt deposits on uncoated CB sites, which keeps a direct electron path between Pt and carbon.

How it was evaluated

Hydrophobicity was assessed by dispersing CB and DTS-CB in a water/hexane mixture and by water contact angles on coated electrodes. Pt size and distribution were examined by high-resolution transmission electron microscopy, Pt content by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and Pt surface state by X-ray photoelectron spectroscopy (XPS). Membrane electrode assemblies (MEAs) of 5 cm2 with 0.4 mg cm−2 Pt on each electrode, a commercial Pt/C anode and the prepared catalysts as cathodes on Nafion 212 were tested by polarization curves at 75 °C with O2 or air and by impedance at 0.8 V. Corrosion was accelerated by holding 1.3 V for 30 min at 90 °C and 100% humidity while CO2 was measured by on-line mass spectrometry, and polarization and impedance were compared before and after this test.

Key results

Uncoated CB stayed in the water phase, whereas DTS-CB moved to the hexane phase, and the water contact angle of the electrode increased from 22° for CB to 153° for DTS-CB. Pt nanoparticles approximately 3 nm in diameter were dispersed homogeneously on CB, and similar sizes and dispersion were observed on DTS-CB; ICP-AES gave the target 40 wt% Pt for both catalysts. XPS showed a Si peak from DTS, while the Pt 4f peak position was the same with and without DTS, indicating no interaction between DTS and Pt, unlike thiol and amine groups that had poisoned Pt in the Kim group's earlier work.

With O2 at the cathode, cells with Pt/DTS-CB and Pt/CB gave almost identical polarization curves. With air, Pt/DTS-CB performed better at high currents where mass transfer overpotential dominates, which the authors attribute to the hydrophobic support repelling water and improving air diffusion in the electrode.

During the 1.3 V corrosion hold, CO2 from the Pt/CB cell rose to 330 ppm, and the total emitted CO2 was 291 μL for Pt/CB and 177 μL for Pt/DTS-CB. After the corrosion test, cell performance at 0.6 V decreased by approximately 59% for Pt/CB and by 17% for Pt/DTS-CB. The charge transfer resistance of Pt/CB increased significantly after corrosion, whereas that of Pt/DTS-CB changed little. The authors explain the higher corrosion resistance by the hydrophobic coating increasing the contact resistance with water, the reactant of carbon corrosion.

How it connects to other research

Corrosion-resistant supports such as carbon nanofibers and nanotubes rely on graphitization and require high-temperature treatment, whereas the DTS coating makes conventional carbon black hydrophobic through a solution-processed self-assembled organosilane layer while leaving uncoated sites for Pt deposition.