Formic acid has been studied as a promising fuel for its direct oxidation in fuel cells since, in comparison with other organic molecules, it is more easily oxidized to CO₂, has a high thermodynamic voltage and fewer problems associated with crossover [1]. It is accepted that the electro-oxidation of formic acid on platinum proceeds through parallel reaction pathways, including an indirect path through carbon monoxide poisoning, the active intermediate (or formate) pathway and the direct pathway [2]. From another point of view, the electro-oxidation of formic acid on Pt and on Pt-group metals is a typical system showing periodic oscillations. Basically, the mechanism that gives rise to this type of oscillatory dynamics is the interplay between the adsorption isotherm of a catalytic poison, for example θ_CO, and of oxygenated species, θ_OH, both of which are potential dependent [3]. A key question underlying this dynamics is how it can affect the efficiency of a DFAFC.

The purpose of this study was to evaluate the performance of a DFAFC under oscillatory regime. The experiments were performed using the typical setup of a unit cell, with a special approach (hydrogen on the cathode) that allows reading the anodic overvoltage directly. The conditions were: Pt/C (50 wt%) was used as anodic and cathodic catalysts with a metal loading of 1 mg_Pt cm^-2. The anode was fed with 8 mol L^-1 of HCOOH and the cathode was fed with H₂. The unit cell (≈ 4.6 cm²) was operated at 30^oC under atmosphere pressure.

Figure 1 exhibits the current-potential curves obtained under potentiostatic and galvanostatic control modes for the electro-oxidation of formic acid and the theoretical curve for oxygen reduction. It is worth noting that the high overpotential required for the oxygen reduction and for the formic acid oxidation on Pt drastically reduces the DFAFC voltage, as it does for all fuel cells, given the sluggish kinetics.

However, during the slow galvanodynamic sweep, spontaneous voltage oscillations emerge and this kinetic instability periodically decreases the anodic overpotential. This fact is interesting since the oscillations increase the average cell voltage and could result in the deceleration of long-term surface deactivation [4]. It also testifies to the importance of evaluating its dynamics to improve the fuel cell efficiency.

The occurrence of such oscillatory behavior during the operation of a Direct Methanol Fuel Cell (DMFC) has been report, however, due to the small amplitude and waveform of the oscillations that takes place during the methanol electro-oxidation, no net improvement was observed in this system [5]. Although, a decrease in long-term instabilities can still play an important role. Probably, the self-cleaning process is more efficient for formic acid oxidation than that for methanol due to its larger amplitude and also because during a cycle the anodic overpotential remains longer at lower values [4].

Here we report a self-organization process observed in a DFAFC and its influence on the device efficiency. It was observed that at certain moments the anodic potential is lower than that on a steady state and this phenomena together with a self-cleaning process could improve the overall fuel cell performance.

Acknowledgements

J.A.N. and H.V. acknowledge São Paulo Research Foundation (FAPESP) for the scholarship (grant #2015/09295-9) and financial support (grants #2012/21204-0, and #2013/16930-7). H.V. (#306151/2010-3) also acknowledges Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support.

References

[1] UHM, S.; LEE, H. J.; LEE, J. Phys. Chem. Chem. Phys. 11 (2009) 9326.

[2] CHEN, Y. X.; HEINEN, M.; JUSYS, Z.; BEHM, R. J. Angew. Chem. Int. Ed.45 (2006) 981.

[3] ZÜLKE, A. A.; VARELA, H. Scientific Reports 6 (2016) 24553.

[4] DELMONDE, M. V. F.; SALLUM, L. F.; PERINI, N.; GONZALEZ, E. R.; SCHLÖGL, R.; VARELA, H. J. Phys. Chem. C120 (2016) 22365.

[5] NOGUEIRA, J. A.; ARIAS, I. K. P.; HANKE-RAUSCHENBACH, R.; VIDAKOVIC-KOCH, T.; VARELA, H.; SUNDMACHER, K. Electrochim. Acta212 (2016) 545.

Figure 1. (potential) curves for formic acid oxidation obtained by either sweep the applied current (green curve) or the applied potential (black curve) and for oxygen reduction (blue curve).