Optimized Direct Formic Acid Fuel Cells Anodes

Efficient direct electro-oxidation of liquid fuels, such as methanol and formic acid for 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 anionic repulsion of the polarized formic acid dipole by the anionicly charged sulfonic groups in the proton exchange membrane (PEM).[2]  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 (Nafion^TM) in the catalyst layer has thus far been probed from 20wt% - 40 wt%. Optimal performance ca. 32wt% Nafion^TM. It has been shown that DFAFC performance is strongly impacted by catalyst layer porosity via either swelling of the catalyst layer[3] or using a pore former to create larger pore structures in the catalyst layer[4]. The pore-forming templating agent selected for these initial studies was micron-sized lithium carbonate (Li₂CO₃), due to facile removal upon acid treatment.[5]

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.

References:

1.     Parsons, R. and T. VanderNoot, 'The oxidation of small organic molecules: A survey of recent fuel cell related research', Journal of Electroanalytical Chemistry and interfacial Electrochemistry, 1988, 257, 9.

2.     Rhee, Y.-W., S.Y. Ha, and R.I. Masel, 'Crossover of formic acid through Nafion membranes', J. Power Sources, 2003, 117, 35.

3.     Ha, S., C.A. Rice, R.I. Masel, and A. Wieckowski, 'Methanol conditioning for improved performance of formic acid fuel cells', Journal of Power Sources, 2002, 112, 655.

4.     Bauskar, A.S. and C.A. Rice, 'Impact of anode catalyst layer porosity on the performance of a direct formic acid fuel cell', Electrochimica Acta, 2012, 62, 36.

5.     Pistono, A.O., C.S. Burke, J.W. Cisco, C. Wilson, B.G. Adams, and C.A. Rice, 'Inhibition of Bismuth Dissolution during Anode Catalyst Layer Pore Former Removal in a Direct Formic Acid Fuel Cell', ECS Electrochemistry Letters, 2014, 3, F65.