(Invited) Direct Formic Acid Fuel Cells Anode Optimization: Flow Fields and Catalysts

Abstract

Direct formic acid fuel cells (DFAFCs) offer an efficient and compact electrochemical conversion technology to power portable electronic devices ranging from sensors to unmanned surveillance vehicles. Liquid fuel cells are desirable for portable applications due to their ease of liquid refueling and the minimal incorporation of system fail safes for handling, as compared to compressed gasses. Directly compared to other liquid fuels, formic acid has minimal parasitic losses due to (i) efficient anodic overpotentials and (ii) cathodic depolarization due to low fuel crossover.[1]The anodic overpotentials are reduced in the presence of efficient catalysts that enhance the fraction of formic acid that is electro-oxidized via the direct non-strongly adsorbed reaction intermediate pathway,[2] while most liquid fuels are plagued with strongly adsorbed carbon monoxide reaction intermediates that require high parasitic overpotentials. Fig 1 compares the formic acid overpotential on a platinum-ruthenium (PtRu) alloy catalyst through the indirect electro-oxidation pathway and carbon supported platinum catalyst decorated with a submonolayer of bismuth (Pt/C-Bi) promoting the direct electro-oxidation pathway. Two short comings incurred in a DFAFC are (Part 1) durability of the catalyst and (Part 2) efficient removal of the CO2-byproduct.[3] Part 1 – The Bi, at 54% of a monolayer coverage, promotes the direct electro-oxidation pathway by the ‘third-body’ effect and adsorption in the CH-down orientation. Alternative adsorbates to Bi are being sought that are strongly adsorbed to the Pt surface and enhance formic acid electro-oxidation through the direct reaction pathway. Stability of the adsorbate during operation and idle will be a key parameter in down selection. Part 2 – The removal of CO2 is imperative to maintain elevated formic acid concentration both in the anode flow field and at the catalyst interfaces within the 3-dimensional anode catalyst layer. The limited crossover of formic acid to the cathode permits the cell to be operated in the presences of neat fuel concentrations. Previously in the Rice group, it has been shown that DFAFC performance is strongly impacted by catalyst layer porosity via either swelling of the catalyst layer[4] or using a pore former to create larger pore structures in the catalyst layer[5]. As the DFAFC operates under higher loads, additional CO2 must be removed through a typical single serpentine flow field, resulting in non-uniformed fuel distribution at the catalyst interface due to two-phase flow. Recent work on advanced electrically conductive selectively gas permeable anode flow field (SGPFF) designs have resulted in efficient removal of CO2 perpendicular to the active area near the location where it is formed in the catalyst layer.[6] Fig 2 shows the potentiostatic hold performance (0.3V) in a dead-ended flow field operation for a standard flow field vs. the SGPFF using PtRu/C as the anode catalyst. Acknowledgements: 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.