Electrochemically assisted organosol method for Pt-Sn nanoparticle synthesis and in situ deposition on graphite felt support: Extended reaction zone anodes for direct ethanol fuel cells

Two electrochemically assisted variants of the Bönneman organosol method were developed for Pt-Sn nanoparticle synthesis and in situ deposition on graphite felt electrodes (e.g. thickness up to 2 mm). Tetraoctylammonium triethylhydroborate N(C₈H₁₇)₄BH(C₂H₅)₃ was employed as colloid stabilizer and reductant dissolved in tetrahydrofuran (THF). The role of the electric field at a low deposition current density of 1.25 mA cm⁻² was mainly electrophoretic causing the migration and adsorption of N(C₈H₁₇)₄BH(C₂H₅)₃ on the graphite felt surface where it reduced the PtCl₂-SnCl₂ mixture. Faradaic electrodeposition was detected mostly for Sn. Typical Pt-Sn loadings were between 0.4 and 0.9 mg cm⁻² depending on the type of pre-deposition exposure of the graphite felt: surfactant-adsorption and metal-adsorption variant, respectively. The catalyst surface area and Pt:Sn surface area ratio was determined by anodic striping of an underpotential deposited Cu monolayer. The two deposition variants gave different catalyst surfaces: total area 233 and 76 cm² mg⁻¹, with Pt:Sn surface area ratio of 3.5:1 and 7.7:1 for surfactant and metal adsorption, respectively. Regarding electrocatalysis of ethanol oxidation, voltammetry and chronopotentiometry studies corroborated by direct ethanol fuel cell experiments using 0.5 M H₂SO₄ as electrolyte, showed that due to a combination of higher catalyst load and Pt:Sn surface ratio, the graphite felt anodes prepared by the metal-adsorption variant gave better performance. The catalyzed graphite felt provided an extended reaction zone for ethanol electrooxidation and it gave higher catalyst mass specific peak power outputs compared to literature data obtained using gas diffusion anodes with carbon black supported Pt-Sn nanoparticles.