Improved Anode Catalyst for Formic Acid Fuel Cells

Abstract

The use of direct formic acid fuel cells (DFAFCs) in portable electronic devices can improve available power density and convenience, compared to today’s rechargeable batteries. Unfortunately, current DFAFC anode catalysts either have high parasitic overpotentials and/or deactivate over time. This deactivation causes a decrease in sustained performance of the catalyst over time.[1] A typical anode catalyst used in these DFAFC consists of carbon supported platinum nanoparticles with a sub-monolayer of bismuth (Pt/C-Bi).[2] Over time, bismuth is oxidized and lost from surface resulting in deactivation of the catalyst nanoparticles, which leads to the DFAFC losing performance.[3] Bismuth at moderate Pt surface coverages promotes formic acid adsorption in the CH-down orientation through a combination of the third-body (steric) and electronic effects. When formic acid adsorbs in the CH-down orientation the direct reaction pathway is promoted bypassing the formation of strongly adsorbed reaction intermediates. The overpotentials associated with the direct reaction pathway are minimal, thereby elevating the power output of the DFAFCs. The goal of this project is to explore stable alternative adsorbates to replace bismuth in order to maximize power density and durability of DFAFCs. Three-electrode electrochemical studies are performed on a polycrystalline Pt disk working electrode in 0.1M HClO4. Initial studies with citrate have demonstrated strong surface adsorption to Pt. The cyclic voltammograms in Figure 1 demonstrate that citrate blocks hydrogen adsorption/desorption sites on Pt and coverage is tailorable to that of a submonolayer of citrate. Linear sweep voltammetry in formic acid shows minimal electro-oxidation performance on pure Pt and a full monolayer of citrate. However, for a submonolayer of citrate the activation overpotential is reduced to 0.2 V vs. RHE due to the significant contribution to the direct reaction pathway.