Optimized Direct Formic Acid Fuel Cells Anodes
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. In addition to being less susceptible to fuel crossover due to anionic repulsion of the polarized formic acid dipole by the anionicly charged sulfonic groups in the proton exchange membrane (PEM). However, catalyst selection is pivotal for enhanced performance as show by the low onset of formic acid electro-oxidation in Fig 1 for bismuth decorated Pt/C catalyst layer. The bismuth at 54wt% coverage promotes the direct electro-oxidation pathway by the ‘third-body’ effect and adsorption in the CH-down orientation.
Fig 1. Anodic linear sweep voltammetry (40°C) with 5M formic acid (2.5 ml min-1) at 10 mV sec-1 for catalyst layers of PtRu alloyed and Pt/C-Bi (54% of a monolayer).
The next component of anode catalyst layer optimization is selective structuring. This will be done in two ways (i) ionomer content and (ii) catalyst layer porosity. Ionomer content (NafionTM) in the catalyst layer has thus far been probed from 20wt% - 40 wt%. Optimal performance ca. 32wt% NafionTM. It has been shown that DFAFC performance is strongly impacted by catalyst layer porosity via either swelling of the catalyst layer or using a pore former to create larger pore structures in the catalyst layer. The pore-forming templating agent selected for these initial studies was micron-sized lithium carbonate (Li2CO3), due to facile removal upon acid treatment.
We gratefully acknowledge support of this work by the NSF-funded TN_SCORE program, NSF EPS-1004083, under Thrust 2 and the Center for Manufacturing Research at Tennessee Tech University.
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