Formic Acid (FA) is a viable carbon-neutral fuel that can be produced from biomass1 or via the reduction of CO22. Formic acid electro-oxidation (FAO) in low-temperature direct formic acid fuel cells (DFAFCs) has attracted considerable interest due to its numerous advantages compared to other similar fuel cells3. DFAFCs show an equilibrium cell voltage (1.40 V) that is higher than that shown by hydrogen (1.23 V) and methanol (1.21 V) fuel cells among others4. Additionally, FA is liquid (unlike hydrogen) and, compared to methanol, is less toxic4,5 and shows less crossover through Nafion® membranes in fuel cells6. Platinum (Pt) and Palladium (Pd) are the best monometallic catalysts for FAO, however, they suffer from stability issues in acidic media, are poisoned by CO during the reaction, and are too expensive to make DFAFCs economically viable 3. One way to mitigate these issues is to deposit pseudomorphic monolayers of Pt or Pd atop other less expensive metals 7. Such structures would reduce the amount of Pt or Pd in the catalyst (and thereby the cost), and could improve the stability and activity of Pt and Pd8-10. To this end, we present a first-principles, self-consistent periodic density functional theory (PW91-GGA) study of FAO on model (111) and (100) facets of Pt and Pd monolayers atop six fcc metals (Au, Ag, Pt, Pd, Ir, and Rh), and (0001) facets of Pt and Pd monolayers atop three hcp metals (Os, Ru, and Re). On all surfaces, we calculate the free energies of the adsorbed intermediates: carbon monoxide (CO), hydroxyl (OH), carboxyl (COOH), and formate (HCOO). Using these data, we develop thermochemical free energy diagrams and, thereby, calculate onset potentials of three mechanisms: direct oxidation via carboxyl, direct oxidation via formate, and the indirect mechanism leading to the formation of CO. Comparing these onset potentials on the surfaces studied reveals trends in the preference to either direct mechanisms, as well as trends governing their propensity towards CO poisoning. For a subset of the close-packed surfaces, we compare the calculated theoretical trends to experimental trends, as determined by electrochemical measurements. Finally, having studied both close-packed and open facets of fcc systems, we discuss the structure sensitivity of FAO on Pt and Pd monolayers on transition metals.
References
1. Bozell JJ, Petersen GR. Technology development for the production of biobased products from biorefinery carbohydrates-the US Department of Energy's "Top 10" revisited. Green Chem. Apr 2010;12(4):539-554.
2. Moret S, Dyson PJ, Laurenczy G. Direct synthesis of formic acid from carbon dioxide by hydrogenation in acidic media. Nature Commun. Jun 2014;5.
3. Jiang K, Zhang H-X, Zou S, Cai W-B. Electrocatalysis of formic acid on palladium and platinum surfaces: from fundamental mechanisms to fuel cell applications. Phys. Chem. Chem. Phys. 2014 2014;16(38):20360-20376.
4. Demirci UB. Direct liquid-feed fuel cells: Thermodynamic and environmental concerns. J. Power Sources. Jun 20 2007;169(2):239-246.
5. Tedsree K, Li T, Jones S, et al. Hydrogen production from formic acid decomposition at room temperature using a Ag-Pd core-shell nanocatalyst. Nature Nanotechnology. May 2011;6(5):302-307.
6. Rhee YW, Ha SY, Masel RI. Crossover of formic acid through Nafion((R)) membranes. J. Power Sources. May 15 2003;117(1-2):35-38.
7. Kibler LA, El-Aziz AM, Hoyer R, Kolb DM. Tuning reaction rates by lateral strain in a palladium monolayer. Angew. Chem. Int. Ed. 2005 2005;44(14):2080-2084.
8. Bligaard T, Nørskov JK. Ligand effects in heterogeneous catalysis and electrochemistry. Electrochim. Acta. May 10 2007;52(18):5512-5516.
9. Mavrikakis M, Hammer B, Nørskov JK. Effect of strain on the reactivity of metal surfaces. Phys. Rev. Lett. Sep 28 1998;81(13):2819-2822.
10. Sasaki K, Naohara H, Cai Y, et al. Core-Protected Platinum Monolayer Shell High-Stability Electrocatalysts for Fuel-Cell Cathodes. Angew. Chem. Int. Ed. 2010;49(46):8602-8607.
