Catalytic activities of Pt thin films electrodeposited onto Bi coated glassy carbon substrate toward formic acid electrooxidation

• Platinum coated Bi deposits on GC electrode prepared by a two-step process.
• Electrochemical treatment creates shell composed of Pt and Bi-oxide.
• Pt(Bi)/GC compare to Pt/GC exhibit enhanced activity in formic acid oxidation.
• Improved activity in formic acid oxidation with increasing Bi content.

Formic acid oxidation was studied on platinum-coated bismuth deposits on glassy carbon substrate. The catalyst was prepared by a two-step process using chronocoulometry, i.e. controlled amount of Bi was electrodeposited onto glassy carbon followed by electrodeposition of Pt layer. The amount of Pt was constant while the amount of Bi vary to correspond the molar ratio of Pt:Bi = 1:0.1 or 1:1 or 1:10. AFM characterization of the electrode surface indicates that Pt is deposited preferentially on previously formed Bi particles, but cyclic voltammetry revealed Bi leaching meaning that Bi was not completely occluded by Pt. In order to obtain stable electrode surface, deposits were subjected to potential cycling up to 1.2 V vs. SCE in supporting electrolyte, prior to use as catalysts for formic acid oxidation. On this way the electrodes composed of Bi core occluded by Pt and Bi-oxide surface layers were obtained. The Pt(Bi)/GC electrodes exhibit enhanced electrocatalytic activity in comparison to Pt/GC for formic acid oxidation which depends on composition and surface morphology. High currents and onset potential shifted to negative values indicate a significant increase in direct path what is explained through ensemble effect induced by Bi-oxide species interrupting Pt domains. Electronic modification of Pt both by surface and sub-surface Bi can play some role as well. Significantly prolonged potential cycling in supporting electrolyte of previously stabilized Pt(Bi)/GC electrodes by Bi oxide, led to considerably lower Bi leaching accompanied by dissolution and redeposition of Pt and the outcome of this treatment was Pt shell over Bi core. These Pt@Bi/GC catalysts also exhibit higher activity for HCOOH oxidation in comparison to Pt/GC depending on the quantity of Bi remained under Pt shell, but in this case the improvement is induced solely by electronic effect of under-laying Bi.