The dynamic behavior of the DFAFC was initially studied via slow potential and current sweeps, and characteristic results are presented in figure 1A. When the potential is the control parameter, a typical stationary response is reached, and the cell voltage decreases monotonically with the increase of the current. However, under galvanostatic control, the DFAFC presented voltage oscillations above a certain current threshold, has been already reported for the cell fed with H2/CO and directly with methanol and formic acid. Additionally, it is observed in figure 1A that the oscillations during the electro-oxidation of formic acid start at approximately 0 V, have an amplitude of about 0.6 V and that an inversion in the cell voltage occurs. To better assess the dynamics of these oscillations, it was selected a current value where the system presents kinetic instability, 0.076 A cm-2, and a chronopotentiometry was performed to evaluate how the voltage evolves over time, figure 1B. It can be seen that between about 0.13 and -0.05 V there is a slow poisoning of the anode electrode due to carbon monoxide adsorbed (COad) from the dehydration of weakly adsorbed formic acid. Another stable intermediate that also contributes to the loss of electrode activity is the bridged-bonded formate (HCOOad) which, like COad, inhibits the direct oxidation pathway of formic acid. Eventually the electrode reaches a critical coverage of catalytic poisons and, in order to keep the current constant, the anode overpotential increases (therefore the cell voltage decreases) leading to the formation of oxygenated species on platinum. These species react with COad via the Langmuir-Hinshelwood mechanism, releasing free sites which causes the anodic overpotential to decrease and the new cycle begins again.
The emergence of oscillatory kinetics during the electro-oxidation of formic acid has been extensively studied with the aid of techniques such as infrared spectroscopy and numerical modeling. Mota et al. have showed kinetic instabilities in a DFAFC using H2 on the cathode instead of O2. An intriguing aspect to be understood in this contribution consists of the inversion in the DFAFC voltage observed during oscillations. This fact adds a difficulty in studying this system, since cells operating at negative voltages results in degradation of the carbon support contained in the catalytic layer as well as in the diffusion layer. Lopes et al. observed this polarity inversion during the oscillatory dynamics in a PEMFC with PdPt/C anodic catalyst. They assumed that this phenomenon is caused by the abrupt increase of the anodic overpotential due to the dynamic of the oscillations with H2/CO. Nevertheless, this voltage inversion may be better understood as follows.
Figure 1C shows a schematic description of the current-potential curves for the electro-oxidation of formic acid (anodic reaction), and for the oxygen reduction (cathodic reaction). The amplitude of the potential oscillations are comparable to that found in half-cell experiments. In the present case, i.e. a DFAFC fed with oxygen in the cathode, the sluggish kinetic of the cathodic reaction and its consequent huge overpotential, contributes to the voltage inversion. In fact, as presented in figure 1C, the overpotential (Ecell in Fig. 1A) remains positive for lower applied currents and when potential oscillations set in, the higher current required causes a higher overpotential in both electrodes, that is, anode and cathode, and consequently the voltage inversion.