Pt/C Spontaneously Decorated with Citrate As a Catalyst for Formic Acid Electro-Oxidation

Abstract

Efficient direct electro-oxidation of liquid fuels, such as methanol and formic acid for low-temperature portable fuel cell applications, remains a challenge limiting commercialization. The major performance detractors are: (1) the formation of strongly adsorbed reaction intermediates, (2) fuel crossover through the membrane depolarizing the cathode, and (3) mass transport for fast fuel delivery into the anode catalyst layer and gaseous product removal. Direct formic acid fuel cells (DFAFCs) have the advantage over direct methanol fuel cells in that it is possible for formic acid to be electro-oxidized via a direct non-strongly adsorbed reaction intermediate pathway.[1] In addition to being less susceptible to fuel crossover due to electrostatic repulsion of the polarized formic acid dipole by the negatively charged sulfonic groups in the proton exchange membrane (PEM).[2] However, the catalyst selection is pivotal for enhanced performance. Previous research from our group has shown highly active carbon-supported platinum (Pt/C) catalyst layers spontaneously decorated with bismuth exhibiting low overpotential for formic acid electro-oxidation, shown in Fig 1. This enhancement of catalytic ability has been attributed to promotion of the direct oxidation pathway by the ‘third-body’ as well as an electronic effect to promote CH-down adsorption of formic acid on the unoccupied Pt/C surface.[3] However, adsorbed Bi is unstable under oxidizable potentials, as demonstrated in our previous studies, Figure 2.[4] Pore-former was incorporated into the catalyst layer to increase formic acid mass transport. During open potential acid wash removal of the pore-former the Bi was lost from the Pt surface and performance was also lost. Research by Antaño-López et al. has shown that the spontaneous adsorption of citrate, a common nanoparticle capping agent, suppresses the formation of platinum oxide.[5] This implies that citrate is a stable surface adsorbate in the oxide region. It is predicted that citrate will exhibit a similar ‘third-body’ effect as Bi with an optimal surface coverage fraction yet to be determined due to the chelation effect of citrate that Bi does not exhibit.