Bismuth-coated mesoporous platinum microelectrodes as sensors for formic acid detection

Mesoporous platinum microeletrodes (MPtEs) modified by sub-monolayers of irreversibly adsorbed bismuth (Bi-MPtE) were investigated for their potential use as sensors for the detection of formic acid in direct formic acid fuel cells. The mesoporous platinum films were prepared by electrodeposition of platinum on Pt microdisks substrates 25 m diameter, from hexachloroplatinic acid dissolved in the aqueous domain of the lyotropic liquid crystalline phase of octaethylene glycol monohexadecyl ether. The roughness factor (RF) of the MPtEs was about two orders of magnitude greater than those of the corresponding polished microelectrodes. Bismuth ad-atoms onto the platinum surface were deposited by under potential deposition from 1 mM Bi3+ ions in 0.5 M H2SO4 solutions. The catalytic activity of a series of Bi-MPtEs, characterized by different roughness and fractional bismuth coverage (Bi), towards the oxidation of HCOOH, was investigated by cyclic voltammetry and potential step experiments. Compared to MPtEs, Bi-MPtEs displayed enhanced electrooxidation currents at lower potentials. The stability of irreversibly adsorbed bismuth, and consequently the Bi-MPtEs catalytic activity, was found to depend on the high potential limit employed in the measurements. In general, both electrode stability and electrocatalytic performance were good, provided that the operational potential was kept  0.4 V vs. Ag/AgCl. Bi-MPtEs with Bi > 0.3 provided almost sigmoidal shaped waves with low hysteresis, as those expected for microelectrodes working under steady state. The effect of concentration of HCOOH was investigated over the range 0.01 – 5 M, and linearity between current and concentration depended on both roughness factor and bismuth coverage. A Bi-MPtE characterised by RF = 210 and Bi ≥ 0.6 provided linearity up to 2 M of formic acid. Reproducibility of the sensors was within 2% (r.s.d). The same sensor, under the optimized experimental conditions, could be employed for at least two months with negligible loss of the initial performance.